exploring the potential of metal xanthate precursors for
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Exploring the Potential of Metal
Xanthate Precursors for the Synthesis of
Doped and Quaternary Metal Sulfides
A Thesis Submitted to the University of Manchester for
the Degree of Doctor of Philosophy in the Faculty of
Science and Engineering
2020
Abdulaziz M. Alanazi
Department of Chemistry
1
Table of contents
Table of contents ............................................................................................................................... 1
List of Figures .................................................................................................................................... 5
List of Tables ................................................................................................................................... 11
Abstract ............................................................................................................................................ 13
Declaration....................................................................................................................................... 14
Copyright Statement ....................................................................................................................... 15
Acknowledgment ............................................................................................................................. 16
Abbreviations .................................................................................................................................. 17
Chapter 1. Introduction .................................................................................................................. 19
1.1. Classification of solids ........................................................................................................... 19
1.2. Semiconductors ...................................................................................................................... 19
1.3. Intrinsic and extrinsic semiconductors ................................................................................... 20
1.3.1. n-type doping .................................................................................................................. 21
1.3.2. p-type doping .................................................................................................................. 22
1.3.3. p-n junction ..................................................................................................................... 23
1.4. Direct and indirect semiconductors ........................................................................................ 23
1.5. The semiconductor bandgap .................................................................................................. 25
1.6. Classification and applications of semiconductors ................................................................ 29
1.7. Nanoparticle materials ........................................................................................................... 30
1.8. Nanocrystals of semiconductors ............................................................................................ 31
1.8.1. Compound semiconductors ............................................................................................. 32
1.9. Transition metal chalcogenide semiconductors ..................................................................... 33
1.9.1. Binary TMCs .................................................................................................................. 35
1.9.2. Ternary TMCs ................................................................................................................. 39
1.9.3. Quaternary TMCs ........................................................................................................... 40
1.10. Doping ................................................................................................................................. 44
1.11. Synthesis of nanoparticle semiconductors ........................................................................... 48
1.11.1. Hot injection method ..................................................................................................... 51
1.11.2. The solvent-less thermolysis ......................................................................................... 53
2
1.12. Synthesis of thin films .......................................................................................................... 54
1.12.1. Spin coating method...................................................................................................... 55
1.12.2. The doctor blade method ............................................................................................... 56
1.13. Single source precursors (SSPs) .......................................................................................... 57
1.14. SSPs for metal sulfide nanostructures .................................................................................. 58
1.14.1. Xanthates: a general introduction .................................................................................. 59
1.15. Aims and objectives ............................................................................................................. 68
1.16. References ............................................................................................................................ 69
Chapter 2. Instruments section ...................................................................................................... 86
2.1. Measurement Methodologies ................................................................................................. 86
2.2. Elemental analysis ................................................................................................................. 86
2.3. Thermogravimetric analysis (TGA) ....................................................................................... 86
2.4. X-Ray crystallography ........................................................................................................... 87
2.5. Powder X-ray Diffraction (p-XRD) ....................................................................................... 87
2.6. Raman Spectroscopy .............................................................................................................. 89
2.7. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) .... 90
2.8. UV/Vis spectroscopy ............................................................................................................. 92
2.9. Magnetic measurements ......................................................................................................... 92
2.10. Reference ............................................................................................................................. 93
Chapter 3. Structural investigations of α-MnS nanocrystals and thin films synthesised from
single source precursors by hot injection, scalable solvent-less and doctor blade routes ......... 94
3.1. Introduction ............................................................................................................................ 94
3.2. Author distribution ................................................................................................................. 95
3.3. References .............................................................................................................................. 95
3.4. Structural investigations of α-MnS nanocrystals and thin films synthesised from single
source precursors by hot injection, scalable solvent-less and doctor blade routes ........................ 97
3.4.1. Abstract ........................................................................................................................... 97
3.4.2. Introduction ..................................................................................................................... 98
3.4.3. Experimental ................................................................................................................. 100
3.4.4. Results and discussion .................................................................................................. 104
3.4.5. Conclusion .................................................................................................................... 121
3
3.4.6. Acknowledgements ....................................................................................................... 122
3.4.7. References ..................................................................................................................... 123
3.4.8. Supporting Information ................................................................................................. 127
Chapter 4. The influence of single precursor on manganese incorporation into Mn-doped PbS
(Pb1-xMnxS) nanoparticles by solvent-less thermolysis. ............................................................. 139
4.1. Introduction .......................................................................................................................... 139
4.2. Author distribution ............................................................................................................... 140
4.3. References ............................................................................................................................ 140
4.4. The influence of single precursor on manganese incorporation into Mn-doped PbS (Pb1-
xMnxS) nanoparticles by solvent-less thermolysis. ..................................................................... 141
4.4.1. Abstract ......................................................................................................................... 141
4.4.2. Introduction ................................................................................................................... 141
4.4.3. Experimental ................................................................................................................. 143
4.4.4. Results and discussion .................................................................................................. 145
4.4.5. Conclusion .................................................................................................................... 154
4.4.6. Acknowledgements ....................................................................................................... 155
4.4.7. References ..................................................................................................................... 155
4.4.8. Supporting Information ................................................................................................. 159
Chapter 5. Effects of annealing temperature on the structural and optical properties of
CMTS (Cu2MnSnS4) nanoparticle prepared by solvent-less thermolysis ................................ 165
5.1. Introduction .......................................................................................................................... 165
5.2. Author distribution ............................................................................................................... 166
5.3. References ............................................................................................................................ 166
5.4. Effects of annealing temperature on the structural and optical properties of CMTS
(Cu2MnSnS4) nanoparticle prepared by solvent-less thermolysis ............................................... 167
5.4.1. Abstract ......................................................................................................................... 167
5.4.2. Introduction ................................................................................................................... 168
5.4.3. Experimental ................................................................................................................. 170
5.4.4. Results and dissections .................................................................................................. 174
5.4.5. Conclusions ................................................................................................................... 184
5.4.6. Acknowledgements ....................................................................................................... 185
5.4.7. References ..................................................................................................................... 185
5.4.8. Supporting Information ................................................................................................. 189
Chapter 6. A molecular precursor route to quaternary chalcogenide CFTS (Cu2FeSnS4)
4
powders as potential solar absorber materials ........................................................................... 196
6.1. Introduction .......................................................................................................................... 196
6.2. Author distribution ............................................................................................................... 197
6.3. Citation ................................................................................................................................. 197
6.4. References ............................................................................................................................ 197
6.5. Manuscript 1: A molecular precursor route to quaternary chalcogenide CFTS (Cu2FeSnS4)
powders as potential solar absorber materials ............................................................................. 199
6.5.1. Abstract ......................................................................................................................... 199
6.5.2. Introduction ................................................................................................................... 200
6.5.3. Materials and experimental ........................................................................................... 203
6.5.4. Result and discussion .................................................................................................... 206
6.5.5. Conclusions ................................................................................................................... 218
6.5.6. Acknowledgements ....................................................................................................... 218
6.5.7. References ..................................................................................................................... 218
6.5.8. Supporting Information ................................................................................................. 222
Chapter 7. Conclusion and Future Work ................................................................................... 229
7.1. Conclusion ........................................................................................................................... 229
7.2. Future work .......................................................................................................................... 233
7.3. References ............................................................................................................................ 234
Final Word Count: 43524
5
List of Figures
Figure 1. 1. Donor levels produced by n-type doping ...................................................................... 22
Figure 1. 2. Acceptor levels produced by p-type doping ................................................................. 23
Figure 1. 3. Schematic diagram illustrating the direct and indirect band gap of a semiconductor.46
..........................................................................................................................................................24
Figure 1. 4. Band structures of bulk, nanoparticles and molecule.60 ............................................... 32
Figure 1. 5. Schematic representation of the structure development tree for the formation of binary,
ternary and multinary semiconductors starting from a II–VI parent compound.65 ............................ 33
Figure 1. 6. The crystalline structures of cubic rock-salt (RS) α-MnS, a, b, c = 5.224 Å (ICDD 01-
089-4952), metastable cubic zincblende (ZB) β-MnS, a, b, c = 5.615 Å (ICDD 00-040-1288) and
hexagonal wurtzite (WZ) γ-MnS structures, a and b = 3.979 Å and c = 6.446 Å (ICDD 00-040-
1289). Color code: Mn, violet; S, yellow.129 ..................................................................................... 38
Figure 1. 7. Unit cell representations of Cu2FeSnS4; (a) the Stannite type structure a = 5.449 Å; c =
10.726 Å, α. β and γ= 90, ICDD: 0005838 (b) kesterite type structure a = 5.434 Å; c = 10.856 Å, α. β
and γ= 90 ICDD: 0005843.1 .............................................................................................................. 43
Figure 1. 8. setup of hot injection method.236 ................................................................................... 52
Figure 1. 9. setup of solvent-less thermolysis.243 ............................................................................. 53
Figure 1. 10. Basic diagram of the spin coating technique. 267 ......................................................... 55
Figure 1. 11. Schematic picture of doctor blade coating process for thin film deposition.270 .......... 56
Figure 1. 12. Some common ligands used in single source precursors to prepare metal sulfides. ... 58
Figure 1. 13. Synthesis of alkali metal xanthates ............................................................................. 61
Figure 1. 14. Bimolecular synthesis of potassium ethyl xanthate .................................................... 62
Figure 1. 15. Coordination behaviour of xanthate ligands. (A): monodentate; (B) isobidentate; (C)
anisobidentate; (D) and (E): bimetallic bridging through sulfur; (F) and (G): bridging to metal
through oxygen. ................................................................................................................................ 63
Figure 1. 16. The hydrolysis of dissolved the xanthate in water ...................................................... 63
Figure 1. 17. The complex of metal xanthate, where M(II) = different metals and R an alkyl group.
..........................................................................................................................................................64
Figure 2. 1. Schematic representation of Bragg’s Diffraction. ......................................................... 88
Figure 2. 2. Top; Energy level diagram of stimulated Raman scattering, down; Raman spectrum
showing the relative intensities of the different scattering processes ................................................ 90
Figure 3. 1. The crystalline structures of (a) cubic rock-salt (RS) α-MnS, a, b, c = 5.224 Å (ICDD
01-089-4952), (b) metastable cubic zincblende (ZB) β-MnS, a, b, c = 5.615 Å (ICDD 00-040-
6
1288) and (c) hexagonal wurtzite (WZ) γ-MnS structures, a and b = 3.979 Å and c = 6.446 Å
(ICDD 00-040-1289). Colour code: Mn, violet; S, yellow.41 .......................................................... 100
Figure 3. 2. The molecular structures of the manganese xanthates. [Mn(S2COMe)2.TMEDA] (1),
[Mn(S2COEt)2.TMEDA] (2), [Mn(S2COnPr)2.TMEDA] (3), [Mn(S2COnBut)2.TMEDA] (4),
[Mn(S2COnPen)2.TMEDA] (5), [Mn(S2COnHex)2.TMEDA] (6) and [Mn(S2COnOct)2.TMEDA] (7).
H atoms are omitted for clarity. Violet = Mn, yellow = S, red = O, blue= N, grey = C. ............. 107
Figure 3. 3. Thermogravimetric analysis (TGA) profiles of complexes (1-6) and inset picture for
complexes (5 and 6). ....................................................................................................................... 108
Figure 3. 4. P-XRD patterns of MnS prepared at 250 °C via hot injection from precursors 1-
6.The standard pattern ( black sticks) is cubic α–MnS (ICDD No. 03-065-0891).56 ............. 110
Figure 3. 5. Representative secondary electron SEM images (10 kV) of MnS samples prepared
using precursor (a-f) (1-6) prepared by hot injection thermolysis at 250 °C, taken at magnification
of 1µm ............................................................................................................................................. 111
Figure 3. 6. P-XRD patterns of MnS prepared at 350 °C via solvent-less thermolysis of precursors
(1-6). The standard pattern is cubic manganese sulfide, MnS (ICDD No. 03-065-0891).56 .......... 113
Figure 3. 7. Representative secondary electron SEM images (10 kV) of MnS samples using
precursor (a-f) (1-6) prepared by a solvent-less thermolysis at 350 °C .......................................... 114
Figure 3. 8. P-XRD patterns of MnS thin films prepared at 350 °C Deposition by the doctor blade
method from precursor (1-6). The standard pattern is cubic manganese sulfide, MnS (ICDD No.
03-065-0891).56 ...............................................................................................................................116
Figure 3. 9. Representative secondary electron SEM images (10 kV) of MnS thin films using
precursor (a-f) (1-6) deposited by the Doctor Blade method at 350 °C .......................................... 117
Figure 3. 10. X-band EPR spectrum of -MnS NCs obtained from complex 2 ............................ 118
Figure 3. 11. Thermal dependence of the magnetisation for -MnS NCs obtained from complex 2,
measured in zero-field cooled (ZFC) (red circles) and field-cooled (FC) (black squares) regimes,
with the difference MFC-MZFC plotted in blue. Insert: Plot of –d(MFC-MZFC)/dT for the same
nanocrystals..................................................................................................................................... 119
Figure 3. 12. Plot of 1/ versus temperature for -MnS NCs obtained from complex 2, measured
in zero-field cooled (ZFC) (red) and field-cooled (FC) (black) regimes, with a fit to the Curie law
= C/(T-) presented in blue (dashed lines) ...................................................................................... 120
Figure 3. 13. Magnetic hysteresis at 5 and 300 K for -MnS NCs obtained from 2. The inset shows
the region around zero fields. .......................................................................................................... 121
Figure 3.S 1. Crystal structures of 1, 2, 3, 4, 5, 6 and 7 showing intermolecular C–H⋯S non-
covalent contacts and S⋯S interactions. ......................................................................................... 129
Figure 3.S 2. IR spectra of manganese alkyl xanthate precursors (1-6) ......................................... 131
7
Figure 3.S 3. The XRD patterns of manganese sulfide nanoparticles prepared by hot-injection from
[Mn(S2COEt)2(TMEDA)] (2) complex heated at different temperature 200 °C for 30 min to
determine the optimum temperature for thermal decomposition. ................................................... 132
Figure 3.S 4. EDX spectra of MnS from precursors (a-f) (1-6) prepared by hot injection
thermolysis. ..................................................................................................................................... 133
Figure 3.S 5. Raman spectra of cubic rock-salt (RS) α-MnS from complexes (1-6) synthesised by
hot injection thermolysis. ................................................................................................................ 133
Figure 3.S 6. SEM images of MnS nanoparticles from complex (a-f) (1-6) prepared by hot
injection thermolysis at 250 °C, 5μm magnification. ...................................................................... 134
Figure 3.S 7. The XRD patterns of manganese sulfide nanoparticles prepared by solvent-less
thermolysis from [Mn(S2COEt)2(TMEDA)] (2) complex heated at different temperature 250, 300
and 350°C for 60 min to determine the optimum temperature for thermal decomposition. ........... 134
Figure 3.S 8. SEM images of MnS nanoparticles from complex (a-f) (1-6) prepared by solvent-less
thermolysis at 350 °C, 5μm magnification. ..................................................................................... 135
Figure 3.S 9. EDX spectra of MnS from precursors (a-f) (1 – 6) prepared by solvent-less
thermolysis. ..................................................................................................................................... 136
Figure 3.S 10. Raman spectra of cubic rock-salt (RS) α-MnS from complexes (1-6) synthesised by
solvent-less thermolysis. ................................................................................................................. 136
Figure 3.S 11. EDX spectra of MnS thin films from precursors (a-f) (1-6) prepared by doctor blade
method. ........................................................................................................................................... 137
Figure 3.S 12. Raman spectra of cubic rock-salt (RS) α-MnS from complexes (1-6) Deposition by
the doctor blade method. ................................................................................................................. 138
Figure 4. 1. TGA profile of (1) lead(II) ethylxanthate and (2) manganese(II) ethylxanthate.
TMEDA .......................................................................................................................................... 146
Figure 4. 2. XRD patterns and lattice parameters a, unit cell volume V and d(200) spacing of Pb1-
xMnxS (0≤ x ≤ 0.08) samples prepared by solvent-less thermolysis at 350 °C using lead and
manganese xanthate precursors with different mole fractions of manganese: (a) x = 0 (PbS), (b) x =
0.02, (c) x = 0.04, (d) x = 0.06 and (e) x = 0.08. ............................................................................ 148
Figure 4. 3. Unit cells of (a) PbS (ICDD: 03-065-0692) and (b) MnS (ICDD: 03-065-0891) along
with their bonds. .............................................................................................................................. 149
Figure 4. 4. Approximately linear correlation between the amounts of manganese in the precursor
feedstock and the mole % Mn found in Pb1-xMnxS samples from EDX spectroscopy ................... 150
Figure 4. 5. Representative SEM secondary electron SEM images (10 kV) of Pb1-xMnxS (0≤ x ≤
0.08) samples prepared by solvent-less thermolysis at 350 °C using lead and manganese xanthate
precursors with different mole fractions of manganese: (a) x = 0 (PbS), (b) x = 0.02, (c) x = 0.04,
(d) x = 0.06 and (e) x = 0.08. .......................................................................................................... 151
8
Figure 4. 6. Raman spectra of Pb1-xMnxS (0 ≤x≤ 0.08) samples prepared by solvent-less
thermolysis at 350 °C using lead and manganese xanthate precursors with different mole fractions
of Mn. .............................................................................................................................................. 152
Figure 4. 7. Relationship between Particle Size and band gap of undoped PbS and Pb1-xMnxS (0
≤x≤ 0.08) samples prepared by solvent-less thermolysis at 350 °C ................................................ 154
Figure 4. S 1. XRD for cubic PbS (ICDD: 03-065-0692) from lead(II) ethylxanthate at (a) 300 °C
and (b) 350 °C. ................................................................................................................................ 159
Figure 4.S 2. XRD for cubic MnS (ICDD: 03-065-0891) from Manganese(II)
ethylxanthate.TMEDA at (a) 300 °C and (b) 350 °C. .................................................................... 160
Figure 4. S 3. EDX spectra of of Pb1-xMnxS (0≤ x ≤ 0.08) samples prepared by solvent-less
thermolysis at 350 °C with different mole fractions of manganese: (a) x = 0 (PbS), (b) x = 0.02, (c)
x = 0.04, (d) x = 0.06 and (e) x = 0.08 ............................................................................................ 160
Figure 4. S 4. EDX elemental mapping (20 kV) of Pb, Mn and S for Pb1-xMnxS samples. (a) x =
0.02, (b) x = 0.04, (c) x = 0.06 and (d) x = 0.08 mole fractions of manganese .............................. 161
Figure 4. S 5. Particle size distribution histogram of the samples prepared Pb1-xMnxS by solvent-
less thermolysis at 350 °C with different mole fractions of Mn: (a) x = 0 (PbS), (b) x = 0.02, (c) x =
0.04, (d) x = 0.06 and (e) x = 0.08. ................................................................................................. 162
Figure 4. S 6. The UV-Vis-NIR absorbance spectra of undoped PbS and Pb1-xMnxS (0 ≤x≤ 0.08)
samples prepared by solvent-less thermolysis at 350 °C ................................................................ 163
Figure 4. S 7. Tauc plot (ahν)2 vs. hν showing the direct bandgaps of undoped PbS and Pb1-xMnxS
(0 ≤x≤ 0.08) samples prepared by solvent-less thermolysis at 350 °C............................................ 164
Figure 5. 1. Crystal structures for stannite Cu2MnSnS4, a = 5.449 Å; c = 10.726 Å, α. β and γ= 90°,
ICDD: 0005838.30 ........................................................................................................................... 169
Figure 5. 2. Illustration of the formation of Cu2MnSnS4 nanoparticles through thermal
decomposition of copper(II) ethylxanthates (1), manganese(II) ethylxanthates (2) and tin(II)
ethylxanthates (3) and reaction using the solvent-less thermolysis................................................. 173
Figure 5. 3. Thermogravimetric analysis of (1) bis(ethylxanthate) copper(II), (2) bis(ethylxanthate)
manganese(II).TMEDA and (3) bis(ethylxanthate) tin(II) .............................................................. 175
Figure 5. 4. P-XRD patterns of the CMTS nanoparticles prepared at different temperatures ........ 176
Figure 5. 5. Room temperature Raman spectra of the CMTS nanocrystals prepared at different
temperatures. ................................................................................................................................... 179
Figure 5. 6. SEM images of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500.
........................................................................................................................................................180
9
Figure 5. 7. Tauc plots of the of the CMTS nanoparticles prepared at different temperatures 350
°C, 400 °C, 450 °C and 500 °C ....................................................................................................... 182
Figure 5. 8. Variation of bandgap and grain size as a function of annealing temperature. ............ 183
Figure 5. S1. EDX spectra of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500.
........................................................................................................................................................189
Figure 5. S2. EDX elemental mapping of the CMTS nanoparticles prepared at different
temperatures, (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C. Scale bars represent 10 µm in all
cases. A secondary electron SEM image of the mapped area is included in each case, labelled as
SE. ................................................................................................................................................... 190
Figure 5. S3. Absorption spectra of the CMTS nanoparticles prepared at different temperatures 350
°C, 400 °C, 450 °C and 500 °C ....................................................................................................... 190
Figure 5. S4. XRD patterns of the CMTS films prepared by spin coating from 350 C to 500 C.
........................................................................................................................................................192
Figure 5. S5. Raman spectra of the CMTS films prepared by spin coating from 350 C to 500 C.
........................................................................................................................................................192
Figure 5. S6. SEM images of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500
thin films prepared by spin coating ................................................................................................. 193
Figure 5. S7. EDX spectra of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500
thin films prepared by spin coating ................................................................................................. 194
Figure 5. S8. EDX elemental mapping of the CMTS thin films prepared at different temperatures,
(a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C. Scale bars represent 5 µm in all cases. A
secondary electron SEM image of the mapped area is included in each case, labelled as SE. ....... 194
Figure 5. S9. Absorption spectra of the CMTS thin films prepared at different temperatures 350
°C, 400 °C, 450 °C and 500 °C ....................................................................................................... 195
Figure 5. S10. Tauc plots of the of the CMTS thin films prepared at different temperatures 350 °C,
400 °C, 450 °C and 500 °C ............................................................................................................. 195
Figure 6. 1. Unit cell representations of Cu2FeSnS4; (a) the Stannite type structure a = 5.449 Å; c =
10.726 Å, α. β and γ= 90o, ICDD: 0005838 (b) kesterite type structure a = 5.434 Å; c = 10.856 Å,
α. β and γ= 90o ICDD: 0005843.23 .................................................................................................. 202
Figure 6. 2. Thermogravimetric analysis of [Cu(S2COEt)2] (red colour), [Fe(S2COEt)3] (blue
colour), [Sn(S2COEt)2] (green colour) and [Sn(S2COEt)4] (black colour) precursors. ................... 208
Figure 6. 3. P-XRD patterns of Cu2FeSnS4 powder (1) and (2) synthesised at (a) 250°C; (b) 350°C
and (c) 450°C for 1 hour ................................................................................................................. 210
10
Figure 6. 4. Raman spectra of Cu2FeSnS4 powder (1) and (2) synthesized at a temperature of
450°C for 1 hour ............................................................................................................................. 211
Figure 6. 5. XPS spectra of Cu2FeSnS4 powder (1) and (2) synthesized at a temperature of 450°C
for 1 hour: (a) Fe 2p, (b) Cu 2p, (c) Sn 3d and (d) S 2p ................................................................. 213
Figure 6. 6. SEM images of Cu2FeSnS4 powder (1) and (2) synthesised at 450 °C for 1 hour. Scale
bar showing different magnifications. ............................................................................................. 214
Figure 6. 7. Elemental mapping of Cu2FeSnS4 powder (1) and (2) synthesised at 450 °C for 1 hour
showing the distribution of Cu, Fe, Sn and S. Scale bar represented 5 μm in all cases .................. 215
Figure 6. 8. Tauc plot (ahѵ)2 vs. hѵ showing the direct bandgap of Cu2FeSnS4 Powders (1) and (2).
........................................................................................................................................................217
Figure 6. S 1. The EDX plots of Cu2FeSnS4 powder (1) and (2) synthesised at a temperature of
450°C for 1 hour. The inset of figure 6. S1 shows the compositional data of Cu2FeSnS4 powder (1)
and (2) ............................................................................................................................................. 222
Figure 6. S 2. The UV-Vis-NIR absorbance spectra of Cu2FeSnS4 powder (1) and (2) synthesised
at a temperature of 450°C for 1 hour .............................................................................................. 223
Figure 6. S 3. P-XRD patterns of Cu2FeSnS4 thin films deposited using (3) and (4) and annealed at
450°C for 1 hour ............................................................................................................................. 224
Figure 6. S 4. Raman spectra of Cu2FeSnS4 thin films deposited using (3) and (4) and annealed at
450°C for 1 hour ............................................................................................................................. 224
Figure 6. S 5. SEM images of Cu2FeSnS4 thin films deposited using (3) and (4) and annealed at
450°C for 1 hour ............................................................................................................................. 225
Figure 6. S 6. EDX plots of Cu2FeSnS4 thin films deposited from (3) and (4) and annealed at a
temperature of 450°C for 1 hour. The inset image showing the atomic percent of Cu2FeSnS4 thin
films. ............................................................................................................................................... 225
Figure 6. S 7. Elemental mapping of Cu2FeSnS4 thin films deposited from (3) and (4) and annealed
at 450°C for 1 hour, showing the distribution of Cu, Fe, Sn and S ................................................. 226
Figure 6. S 8. The UV-Vis-NIR absorbance spectra of Cu2FeSnS4 thin films deposited from (3)
and (4) and annealed at 450°C for 1 hour ....................................................................................... 226
Figure 6. S 9. Tauc plot (αhѵ)2 vs. hѵ showing the direct bandgap of Cu2FeSnS4 thin films
deposited from (3) and (4) and annealed at 450°C for 1 hour ......................................................... 227
11
List of Tables
Table 1. 1. Band gap energies and electrical conductivity at room temperature for semiconductor
materials.4 .......................................................................................................................................... 20
Table 1. 2. Common semiconductors and their applications.43 ........................................................ 29
Table 3.S 1. X-ray crystallographic data and refinement details for (1-7) using Cu K radiation and
with H-atom parameters constrained. ............................................................................................. 127
Table 3.S 2. Selected Bond Lengths (Å) and Angles (o) for novel complexes (1-7) ...................... 128
Table 3.S 3. Details of selected intermolecular non-covalent contacts (Å) in the prepared
compounds (1-7) ............................................................................................................................. 128
Table 3.S 4. Elemental and thermal analyses of xanthates diaminemanganese(II) complexes 1 - 7.
........................................................................................................................................................130
Table 3.S 5. The unit cell parameters for the MnS synthesised by hot injection thermolysis from
precursors (1-6), with (ICDD No. 03-065-0891) as the MnS reference pattern, volume, crystallite
size, EDX measurements and Raman data from these samples ...................................................... 132
Table 3.S 6. The unit cell parameters for the MnS synthesised by solvent-less thermolysis from
precursors (1 – 6), with (ICDD No. 03-065-0891) as the MnS reference pattern, volume, crystallite
size and EDX measurements from these samples ........................................................................... 135
Table 3.S 7. The unit cell parameters for the MnS synthesised by doctor blade method from
precursors (1-6), with (ICDD No. 03-065-0891) as the MnS reference pattern, volume, crystallite
size and EDX measurements from these samples ........................................................................... 137
Table 4. 1. Lattice parameters a, unit cell volume (V), band gap (Eg) and grain size of Pb1-xMnxS
(0 ≤ x ≤ 0.08) with variations in Mn/Mn+Pb molar ratios. ............................................................. 149
Table 4. S 1. Composition of Pb1-xMnxS (0 ≤ x ≤ 0.08) ........................................................................... 159
Table 4. S 2. Summary of the required composition of Pb1-xMnxS (0 ≤ x ≤ 0.08) calculated from
the elements in the feed and analysis of final products by EDX spectroscopy ............................... 161
Table 5. 1. Lattice constants of the CMTS nanoparticles obtained from XRD patterns. ............... 178
Table 5. 2. Electrical properties of CMTS films prepared by spin coating from 350 C to 500 C.
........................................................................................................................................................184
Table 5. S1. Chemical composition and composition ratio from EDX spectra of the CMTS
nanoparticles prepared at different temperatures ............................................................................ 189
Table 5. S2. Chemical composition and composition ratio of the CMTS thin films prepared at
different temperatures ..................................................................................................................... 193
12
Table 6. 1. Reported band gaps of CFTS nanomaterials prepared by different methods ............... 201
13
Abstract
Metal sulfide nanomaterials thin films are important in photovoltaic applications due to their
exciting properties. Xanthate complexes are well known for the deposition of metals sulfide
films and nanomaterials. This produces suitable chemical and physical properties to deposit
films with a very low level or no contamination at low temperature. Different synthetic
protocols are available for preparation of metal sulfide nanomaterials and/or thin films.
Nonetheless, the use of single source precursors is usually advantageous, as they can be used
for the synthesis of metal chalcogenide nanocrystals, and are equally suitable for the
deposition of thin films. Furthermore, a better control over stoichiometry and phase can be
achieved, due to preformed bonds between metal and chalcogen atom.
The work shows the synthesis of a series of novel manganese complexes of xanthate ligands,
their spectroscopic characterization, crystal structures and thermal decomposition have been
studied. The complexes were used as single source precursors for the production of MnS
nanocrystals and thin films. MnS nanocrystals have been synthesised by hot injection and
solvent-less thermolysis at 230 °C and 350 °C, respectively. In addition, MnS thin films have
been synthesised by doctor blade method at 350 °C. The nanocrystals and films were
characterised by powder X-ray diffraction, Raman spectra, scanning electron microscopy
and energy dispersive X-ray spectroscopy.
Additionally, xanthate complexes of lead has been used for the production of Mn-doped PbS
nanocrystals using solvent-less thermolysis, adding with a low concentration of Mn source.
The nanocrystals were characterised by several techniques to study the formation behaviour,
structure and chemical composition.
Finally, we report the use of copper, manganese, iron and tin xanthates in solvent-less
thermolysis to produce stannite Cu2MnSnS4 (CMTS) nanoparticles and Cu2FeSnS4 (CFTS)
powders at temperatures between 250 and 500 °C. Higher temperatures give the normal
tetragonal phase CMTS and CFTS, while low temperatures are contaminated with cubic
phases.
14
Declaration
I hereby declare that no portion of the work referred to in the thesis has been submitted in
support of an application for another degree or qualification of the University of Manchester
or any other university or other institute of learning.
Abdulaziz Mohammed Alanazi
15
Copyright Statement
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has given The University of Manchester certain rights to use such Copyright,
including for administrative purposes.
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16
Acknowledgment
First and foremost, I would like to thank God Almighty for giving me the strength,
knowledge, ability and opportunity to undertake this research study and to persevere and
complete it satisfactorily. Without his blessings, this achievement would not have been
possible. I have also been supported by many good people to whom I would like to express
my deepest gratitude.
I would like to thank the Ministry of Education in Kingdom of Saudi Arabia and Islamic
university in Madinah for funding and support. I would like to thank and acknowledge my
supervisor, Professor Paul O’Brien who passed away for his excellent advices, and for giving
me the opportunity to research at the University of Manchester, and thanks for his keenness
and follow-up me in the first two years of my PhD stages. I would like also to thank my co-
supervisor Dr. David J. Lewis a person who has had a high impact on my improvement, he
has been instrumental in my success and has encouraged me during this long journey to
complete PhD. Dr. Lewis’s valuable advice, guidance and trust during my study made it
possible for me to complete this goal. Also many thanks for his patience, help and guidance
for writing papers and thesis. I would like to thank Prof. David Collison for his academic
and administrated roles to make sure I finish strong. I am really grateful to Post doc Dr Firoz
Alam Dr Paul McNaughter who I have worked with them on some projects and I have taken
useful comments and I appreciate their scientific advice. I’m very grateful to POB’s research
group members who have been of great help. Big thanks to technical staffs in the department
of Chemistry for their great help in the use of X-ray crystallography and analytical analysis.
Also thanks to technical staffs in the school of Materials for their help and support on the
SEM.
Finally, I would like to thank my parents for their support, prayers and encouragement
throughout my study. I would also like to thank my brothers, sisters and friends for their
assistance and supports. Last but not least, distinctive thanks to my friend Asil for sharing
this journey with kindness, cooperation, and encouragement and care. Also, Very special
thanks to my best friend Rosie, for her support and assistance in every step of my thesis.
Thank you for colouring my journey.
17
Abbreviations
1D 1- Dimensional
a, c Lattice parameter
AACVD Aerosol Assisted Chemical Vapour
Deposition
nBu n-Butyl
CCDC Cambridge Crystallographic Data Centre
CFTS Copper Iron Tin Sulfide
CMTS Copper Manganese Tin Sulfide
CVD Chemical Vapour Deposition
eV Electron Volt
EDX Energy Dispersive X-ray Spectroscopy
Eg Energy Gap
et al et alia
Et Ethyl
K Kelvin
KS kesterite
M.pt Melting Point
Me Methyl
MeOH Methanol
ml millilitres
mmol millimole
NCs Nanocrystals
nm nanometers
NPs Nanoparticles
OLA Oleylamine
p-XRD Powder X-ray Diffraction
Pr Propyl
SEM Scanning Electron Microscope
SSP Single Source precursor
ST stannite
TMEDA N,N,N′,N′-Tetramethylethylenediamine
THF Tetrahydrofuran
18
TGA Thermogravimetric Analysis
TOP Trioctylphosphine
UV/Vis ultra violet/ visible
WZ Wurtzite
ZB Zinc Blende
19
Chapter 1. Introduction
1.1. Classification of solids
Solids can be classified into two main categories: amorphous and crystalline. A material
will be referred to as an amorphous solid if the constituent atoms do not have a regular and
repeating arrangement. Examples of such structures are found in plastics, rubber and glass.
Some substances may adopt different arrangements when in a solid state; an example is
carbon that coexists as graphite crystalline or fullerenes. This phenomenon is referred to as
allotropy, the elements have allotropes.1
At the other end of the spectrum, a crystalline solid is a material containing particles that are
arranged in an orderly manner. Examples of such materials include sucrose, sodium chloride
and diamond. In general, a crystalline solid will have a sharp melting point which, when
reached, will cause the crystalline solid to become an isothermal liquid. After the cooling
process, the same arrangements can be observed. These types of element are also referred
to as true solids.1
Solids can also be classified according to the type of bond that holds them together.
According to this, crystalline materials are classified as molecular, covalent, ionic and
metallic. There are three types of solid-state materials depending on their electrical
conductivity, which are conductors, semiconductors and insulators. The distinction between
these types is made evident through band theory.2
1.2. Semiconductors
Semiconductors are solid substances that have conductivity between an insulator and most
of the metals. A material’s resistance depends on its purity and the temperature. The
resistance in semiconductors is significantly reduced owing to the addition of impurities.3
Also, when the semiconductor’s temperature is raised the resistance decreases significantly,
20
but the material does not have high conductivity. Conversely, the resistance increases by
reducing the temperature and becomes close to the resistance of the insulating materials.3
Devices made of semiconductors such as silicon, are important components of most
electronic circuits. The properties of certain common semiconductor materials are shown in
Table 1.1.4
Table 1. 1. Band gap energies and electrical conductivity at room temperature for semiconductor
materials.4
Material Band Gap (eV) Electrical Conductivity
[(Ω-m)-1]
Elemental
Si 1.11 4×10-4
Ge 0.67 2.2
III-V Compounds
GaAs 1.42 10-6
InSb 0.17 2×104
II-VI Compounds
CdS5 2.40 6.14×10-4
ZnO6 2.26 2×10-2
1.3. Intrinsic and extrinsic semiconductors
Intrinsic semiconductors are represented by elements such as silicon (Si) or germanium (Ge),
or compounds such as gallium arsenide (GaAs) or copper indium sulfide (CuInS2), which
contain no impurities in contrast to the number of thermal generated holes and electrons
present in a lattice. Although, in practical terms, these impurities do exist, their levels are
infinitely small and thus negligible.7
21
Extrinsic semiconductors represent conductors that, from the pure form, have had impurities
deliberately placed within them. Due to the intense degree of difficulty of obtaining pure
semiconductor material, these elements are not used. However, by adding small amounts of
impurities to these materials during the crystal growth and even in the later stages in selected
regions of the crystal, the process called doping takes place.8 This results in different
materials, by which the doping process becomes classified as an n-type or a p-type. In n-type
semiconductors, an extra electron in the conduction band is present while, in p-type
semiconductors, additional holes in the valence band are present.7
1.3.1. n-type doping
An n-type dopant is differentiated from the other types by the addition of negatively-charged
electrons to the semiconductor. This type represents the most common form of dopant and
is located in Group 15 of the Periodic Table. In some cases, n-type dopants are also referred
to as donors, due to the fact that they donate electrons to the semiconductor. These elements
are nitrogen, phosphorus, arsenic, antimony and bismuth. Any donor atom becomes easily
ionised, holding a free negatively-charged electron and leaving a positive ion core (Figure
1.1).9
22
Figure 1. 1. Donor levels produced by n-type doping.
1.3.2. p-type doping
p-type doping is generally produced by adding B, Ga or In to the silicon lattice of an initial
intrinsic semiconductor. In this scenario, each of the acceptor atoms will have three valence
electrons. Only three valence electrons will share the neighbouring silicon atoms in the
crystal lattice. This transformation will substantially increase the number of holes created
by the acceptor atoms, that are greater than the number of free electrons and holes that were
noted in the intrinsic semiconductor. As such, the majority of the carriers are represented
by the holes which also deliver a positive charge (Figure 1.2).10
23
Figure 1. 2. Acceptor levels produced by p-type doping.
1.3.3. p-n junction
p-n junctions are created by bringing together n and p-type semiconductor materials. In this
scenario, because of the high electron concentration in the n-type and high hole
concentration in the p-type, electrons pass from the n- to the p-type material. This is done
through the depletion region due to the non-uniform electron distribution. At an equilibrium
state, no electron continues to pass through the depletion region. The same effect is observed
regarding the holes in the valence band. In this regard, a uniform Fermi level is defined as
being flat throughout the p-n junction.11,12
1.4. Direct and indirect semiconductors
Direct and indirect semiconductors display complicated energy band diagrams, in which the
electron energy is set against the electron crystal momentum (k-vector). As shown in Figure
1.3 (from left to right), the conduction band minimum along with the valence band maximum
are placed within the same momentum value, implying that the electron can transit from the
valence to the conduction band with no change in momentum taking place. When this
24
phenomenon is possible, the material is referred to as a direct semiconductor. An example
of such material is gallium arsenide, GaAs.13
Figure 1. 3. Schematic diagram illustrating the direct and indirect band gap of a semiconductor.46
By contrast, as shown in the right hand image of Figure 1.3, the conduction band minimum
along with the valence band maximum are now moved from the momentum value, each with
its own momentum location. When this phenomenon occurs, the electron requires a change
in momentum and extra energy input. This is the case for indirect semiconductors, for which
group silicon is a representative material.13 In recent years, the potential for silicon to
become integrated and more widely applicable to the solar power industry has been
discussed, specifically due to its characteristics as an indirect semiconductor.14
In this regard, in a p-n junction photovoltaic (PV) cell, a photon of light produces an electron
hole pair if the energy of the photon is at least similar to the band gap of the material forming
the p-n junction. Consequently, materials with high carrier mobilities are preferable for high
efficiency.15 Thus, the p-type semiconductor can be utilised as light absorber in a p-n
junction solar cell duo to the electrons have higher mobility than the holes.15
In this regard, kesterite materials are seen as the main candidates for the creation of thin-film
solar cells, as they possess an increased absorption coefficient and a direct band gap; see
Section 1.9.3.2 for the structure of kesterite. An example of this is the Cu2ZnSnS4, for which
25
Chen and Chuang proposed a formation mechanism for crystals nanorods.16 Apart from the
quantum properties of such elements (e.g., Cd2+, Hg2+, S2- and Se2-, …) El-Sayed discussed
the potential of size to influence the energy distribution within a semiconductor material.17
He argued that colloidal semiconductor nanoparticles will display certain features that will
depend upon the electronic relaxation rates that occur when spherical nanoparticles are
transformed into nanorods. Size changes of small nanoparticles were also noted at a
structural level upon adsorbed strongly bound molecules.17 The implications of such
findings reveal a practical application in the photovoltaic industry.17 As this researcher
discusses, the atoms in a nanoparticle will behave quite differently to those in a particle
located in a bulk material owing to surface effects and quantum size effects that are not
present in bulk, but are notable at quantum levels.18
1.5. The semiconductor bandgap
The bandgap in solid materials is the energy difference between the unoccupied conduction
band and the fully-occupied valence band. It encompasses a range of energy values that are
forbidden to the electrons of the material.19 The high electrical conductivity of metals is due
to the presence of a partially filled (and, hence, partially unoccupied) conduction band at any
temperature. Meanwhile, semiconducting and electrically insulating materials contain a full
valence band and a more-or-less completely empty conduction band. However, in
semiconductors, an elevated temperature can supply enough energy to promote electrons
from the valence band into the conduction band.20 This gives the semiconductors electrical
conductivity properties intermediate between those of the conductors and the insulators,
since current can flow only when electrons are promoted to the conduction band. The
primary difference between semiconductors and insulators is the size of the bandgap, being
larger in insulators (e.g. 5.5 eV for diamond and 9 eV for silicon dioxide (SiO2)) than in
semiconductors (e.g. 1.1 eV for silicon (Si), 0.67 eV for germanium (Ge) and 1.43 eV for
26
gallium arsenide (GaAs)).19 The electrons cannot take on energy values intermediate
between the valence and conduction bands, but can be promoted from one to the other by
absorbing photons of sufficient energy. In general, materials with a bandgap of less than 3
eV are classified as semiconductors while those with a bandgap greater than 3 eV are
regarded as insulators because insufficient thermal energy is available at 300 K to promote
electrons to the conduction band in the latter case.19
The semiconductor bandgap is a key determinant of energy absorption in solar technology;
hence the various types of semiconductor that can be used in this field have been examined
in a wide range of studies. For instance, the bandgap in binary materials varies with
stoichiometry, e.g. Cu2S (~2.47 eV) and CuS (1.26 eV).21 As a result, the different forms
of CuxS behave distinctly, with Cu2S nanomaterials displaying p-type conduction and the
CuS nanomaterials displaying n-type conduction.21,22 See Section 1.3 for a discussion of n-
and p-type doping. Moreover, Al-Shakban et al. identified a bandgap of approximately 1.4
eV for Cu1.74S nanorods.23
Due to the combination of lower toxicity, lower cost and higher efficiency, solar cells
containing iron pyrite (FeS2) would be preferable to those containing cadmium compounds.
The bandgap of natural pyrite crystals was found to be 0.9 eV by Ennaoui et al., while single
crystals and synthetic polycrystals both displayed a bandgap of 0.95 eV.24 Moreover, FeS
thin films were produced on glass substrates by Akhtar et al. and shown to have a bandgap
of 1.87 eV.25
While the indirect bandgap of tin sulfide (SnS) is 1.1 eV, the direct bandgap of about 1.3 eV
is close to the ideal value of 1.5 eV for solar cells,26 see Section 1.4 for a discussion of direct
and indirect bandgaps. However, the literature reveals a variation in this bandgap energy
that is strongly correlated with the method of preparation. For instance, Al-Shakban et al.
used chemical vapour deposition (CVD) to produce SnS from a single-source precursor
27
under conditions of heating at various temperatures to indicate a decrease in bandgap (from
a maximum of 1.4 eV) with increasing temperature.27 Meanwhile, single-crystal p-type SnS
specimens were produced by Chamberlain and Merdan with a direct bandgap of 1.43 eV and
indirect bandgaps of 1.13 eV and 1.22 eV at 77K.28
The p-type semiconductor manganese sulfide (MnS) has a broad bandgap around 3 eV.29 A
decrease in the bandgap of thin MnS films with increased temperature of annealing was
demonstrated by Girish et al. Thus, heating at 300 ᵒC gave a bandgap of 3.95 eV, whereas
heating at 400 and 450 ᵒC gave bandgaps of 3.44 eV and 3.33 eV, respectively.30 Similarly,
Shi et al. reported a decrease in the bandgap of a CBD-produced MnS thin film from 3.18
eV to 3.15 eV after annealing.31
The quaternary semiconductors Cu2MSnS4 (M = Ni2+, Co2+, Fe2+, Mn2+) have potential
application in low-cost thin film for solar cells.32 In the experiment, Cu2CoSnS4, Cu2FeSnS4,
Cu2NiSnS4 and Cu2MnSnS4 nano-crystals have been synthesised via a solvothermal method.
Cu2MSnS4 has a band gap at 1.2–1.5 eV, which indicates its viability for application in solar
energy capture.32
Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 are suitable for low cost energy and stipulated that the
band gap of these materials is situated at 1.5 eV and 1.3 eV, respectively, which would thus
make these quaternary compounds suitable for application.33 In 2009, Chen and
collaborators analysed this potential via first principles calculations and noted that, because
of the dependence of the band structure on the Cu and Zn, the cation ordering is low and is
therefore a predictor that the band gap of Cu2ZnSnS4 is situated at 1.0 eV, which is a far
lower value than initially predicted.34
A facile, economical, environmentally sound and industry-scalable ball-milling technique
for the preparation of quaternary copper iron tin sulfide (Cu2FeSnS4) or CFTS powder with
a bandgap of 1.42 eV has been developed by Vanalakar et al. These workers confirmed the
28
identity of the pure product by means of Raman spectroscopy, X-ray diffraction (XRD) and
energy-dispersive X-ray spectroscopy (EDX) analysis.35 Meanwhile, a solution-based
approach was used by Zhang et al. to produce CFTS nanocrystals with oblate spheroidal
shapes (bandgap 1.54 eV) and triangular plate shapes (bandgap 1.46 eV).36 Moreover, CFTS
nanoparticles have been deposited onto FTO substrates by Dong et al. via spin coating to
give a bandgap of 1.53 eV.37 In addition, single phase CFTS samples with an optical bandgap
of 1.40 eV, and CZTS samples with a 1.48 eV optical bandgap, were prepared by Mokurala
et al. using thermal decomposition.38 Furthermore, the chemical spray pyrolysis technique
was used at a range of deposition temperatures by Nilang et al. to produce CFTS thin films
with an optical band gap of 1.54 eV.39
The literature also contains examples of research into the copper manganese tin sulfide
(CMTS) semiconductor. For example, Chen et al. (2015) used the sol–gel technique to
produce CMTS thin films with tuneable bandgaps ranging from 1.62 eV to 1.14 eV
depending upon the post-annealing temperature.40 Moreover, Nie et al. used chemical spray
pyrolysis to generate CMTS with a 1.19 eV bandgap,41 while other researchers used a
microwave-assisted solvothermal method to produce CMTS nanocrystals with optical
bandgaps of 1.11 eV.42
29
1.6. Classification and applications of semiconductors
There are several types of semiconductor materials that can be used within electronic
devices.
Table 1. 2. Common semiconductors and their applications.43
Material chemical
symbol/formula
Group Applications
Germanium
Silicon
Silicon carbide
Ge
Si
SiC
IV
Microchips, solar
cells
Gallium arsenide
Gallium nitride
Gallium phosphide
GaAs
GaN
GaP
III-V
Light emitting
diodes
Cadmium sulfide
CdS
II-VI
Solar cells, solid
state lasers
Tin sulfide
SnS
IV-VI
Thermal imaging,
IR detectors
30
1.7. Nanoparticle materials
Nanoparticles are defined as materials that have a diameter ranging from 1 to 100 nm.44
These materials come in a variety of shapes and sizes and are distinguished from bulk
materials through the different physical and chemical characteristics that nanoparticles
display. Such differentiations include specific optical characteristics, a higher surface area
and magnetization properties as well as lower melting points.44 In recent years, nanoparticles
have been widely investigated by researchers specifically because of these attributes, which
makes their applicability relevant to several industries, including biological labeling,
electronic devices, drug delivery systems, quantum dot catalysis as well as environmental
remediation.45,46
In addition to the relatively wide applicability of these materials across several industries,
their manipulation and adjustment for different purposes also make nanoparticles a desired
area for further investigation. In this regard, nanoparticles can be fabricated to suit specific
needs and thus vary in shape and size, thereby facilitating the creation of new materials.
Moreover, nanoparticles can achieve different morphologies and types, which place these
materials into different categories such as semiconductors, metal oxides, metals and even
biomaterials.47
To synthesise nanoparticles, two main approaches are used. The first is the so-called top-
down approach, which involves several attrition methods including laser ablation or
lithography where nanoparticles are extracted from bulk materials.48 At the other end of the
spectrum, the bottom-up approach involves attaining the molecules from their component
parts, specifically from molecules and atoms.49 This approach is favoured in the fabrication
of nanoparticles due to cost and the fact that, through this method, nanoparticles with less
defects are obtained.50,51
31
1.8. Nanocrystals of semiconductors
Semiconductor materials generate nanocrystals that have a diameter of 1 to 20 nm. These
particles exhibit very distinctive properties in contrast to the bulk materials from which they
are obtained. Several researches have focused on semiconductors specifically due to these
properties, which include special optical characteristics, electrical properties and catalytic
properties.52
The properties of nanocrystals obtained from semiconductor materials are a direct result of
their size. Firstly, with the significant reduction in size, the surface properties effects are
noted. The same effect resulting from the small size of the nanoparticle results in the electric
properties of these materials. This effect is referred to as quantum confinement and dictates
that the electrical properties of materials will change in accordance with changes in the size
of the material.53 Notably, particles will exhibit different behaviour when examined on a
small scale and when observed in bulk. Thus, quantum confinement is the spatial
confinement of electron–hole pairs in one or more dimensions within a material, and also
electronic energy levels are discrete. In this context, the smaller the particle obtained, the
wider the band gap achieved. Such manipulations in the size of the material will thus result
in distinctive optical, electrical, mechanical, magnetic and chemical reactivity properties.54
Furthermore, if one dimension of a semiconductor is smaller than the Bohr exciton radius of
the material, the band structure will be modified and blue shifted to higher energy by the
quantum confinement effect.55–59 In the limit of very small particle size, the so-called strong
confinement regime, quantized levels appear, which is distinct from the continuous band of
bulk counterparts and shows characteristics of the discrete molecular semiconductors, Figure
1.4.
32
Figure 1. 4. Band structures of bulk, nanoparticles and molecule.60
1.8.1. Compound semiconductors
As well as the Group IV elements, semiconductors often consist of compounds of Group III
and Group V elements or Group II and Group VI elements, giving an average of four valence
electrons in each type of material. The stoichiometry of semiconductors like GaAs can be
varied to give doping effects involving either a small increase in the proportion of As (n-
type doping) or a small increase in the proportion of Ga (p-type doping).
As shown in Figure 1.5, the Group II-VI the transition metal chalcogenides (TMCs) can be
used as starting materials to generate various binary, ternary and multinary semiconductors
via cation mutation. Thus, a Group III cation can substitute for a Group II cation to give a
I-III-VI semiconductor; one Group II and one Group IV cation can substitute for two Group
III atoms to give a I2–II–IV–VI4 semiconductor; one Group I and one Group III cation can
replace two Group II cations to give a I–III–II2–VI4; or half of the Group II cations can be
replaced by a different Group II cation to give I2–II–II-III-VI4. Control of the atomic ratios
via the mutation process makes it possible to engineer, tailor and optimise the material’s
33
bandgap properties for specific applications, including solar cells,34 spintronics,61
thermoelectrics62 and novel categories of topological insulators.63 However, since each of
the semiconductor categories encompasses numerous possible compositions, appropriate
research and screening is necessary to identify the correct composition.64
Figure 1. 5. Schematic representation of the structure development tree for the formation of binary,
ternary and multinary semiconductors starting from a II–VI parent compound.65
1.9. Transition metal chalcogenide semiconductors
Interest in the transition metal chalcogenide (TMC) materials has recently shown a dramatic
increase, with numerous research groups concentrating on their attractive characteristics and
wide range of applications such as in field effect transistors, sensors, solar cells and water
34
splitting photocatalysis.66 Several TMCs possess layered structures that provide distinct
electronic and chemical properties from those of bulk semi-conductors.
In addition, the potential use of these compounds as earth abundant, cheap, non-toxic and
environmentally sustainable photovoltaic (PV) materials is another source of the rise in
interest and is the main emphasis of the present review. The primary advantage of TMCs
relative to other conventional PV materials (e.g. lead perovskites, organic photovoltaics
(OPVs)) is their higher stability. The OPVs experience bleaching due to oxidation of the
photoabsorbent organic molecules in the presence of oxygen,67 while the lead perovskites
are similarly susceptible to both oxygen and water.68,69 While the conventional TMC
photovoltaics are based on the Cd(S, Se) family, the newer materials incorporate arsenic
(As), gallium (Ga) or indium (In). However, interest into other chalcogenide materials is
motivated by severe international restrictions limiting the industrial use of cadmium along
with continuing concerns surrounding the global supply and sustainable availability of In,
Ga and As.70 Although PV devices are frequently described as 'green' energy sources, they
can only be genuinely sustainable and economically practicable if the device efficiency is
high and the material cost is low. The annual energy generation potential of a range of PV
materials was modelled and compared with the production costs by Wadia et al. to
demonstrate that materials like FeS2, Cu2S and Cu2ZnSnS4 have the highest energy
production potential relative to material cost.71 The present challenge is therefore to achieve
the full potential of these materials.
The range of TMC semiconductor materials suitable for PV devices is remarkably large,
with an Inorganic Crystal Structure Database (ICSD) search indicating at least 15000 distinct
compounds. These can be divided into three primary categories, namely the binary (MxEn),
ternary (MxM′yEn) and quaternary (MxM′yM″zEn) systems, where M is a transition metal, M′
and M″ is another transition metal or other type of metal and E can be sulfur (S), selenium
35
(Se) or tellurium (Te). These categories are frequently designated by Roman numerals
indicating the oxidation state of the metal and the group of the chalcogen or pnictogen, for
example II–VI (e.g. CdS), I–III–VI2 (e.g. CuInS2) or III–V (e.g. InP).
Since the efficiency of a single p-n junction solar cell is subject to a theoretical maximum
value, a suitable photoactive semiconductor must have a 1.0 to 1.5 eV gap between the
lower-energy (valence) band and the higher-energy (conduction) band.72,73
The TMC semiconductors have found several uses in PV devices, including photo-absorbent
layers, buffer layers and anodes in dye-sensitized solar cells (DSSCs),74 where they generally
take the form of either quantum dots or nanostructured thin films.75,76 However, while the
TMCs are starting to achieve their potential in these applications, the production of a material
with a high absorption coefficient and 1.0–1.5 eV bandgap from cheap and plentiful elements
remains a significant research challenge. Films can be produced by solution processing of
TMCs to form nanocrystalline materials or inks77,78 or by other methods including chemical
bath deposition (CBD).79,80 As well as methods such as solvothermal synthesis, the widely-
available hot-injection technique has been used to produce nanocrystalline TMCs and has
proven suitable for binary, ternary and quaternary systems.
1.9.1. Binary TMCs
A wide range of binary TMCs displays properties that are suitable for PV systems. Examples
include FeS2,81,82 CdS,83 CuxS,84,85 CuSe,86 MnS87 and SnS,88 among which Cd(S, Se), FeS2
and the range of copper sulfides are possibly the most familiar and have the most attractive
properties. Following the development of simple synthetic protocols in the 1990s, the Cd(S,
Se) quantum dots became all-pervasive during the early 2000s.89–91 Although these materials
possess optimal photoelectric and electronic properties (e.g. bandgaps) that are readily
tuneable by adjusting the proportion of sulfur/selenium,92 the high toxicity of cadmium has
been well demonstrated and has resulted in stringent EU restrictions.93,94 While this would
36
seem to relegate the PV applicability of cadmium chalcogenides to the context of the
laboratory-scale test, the development of Cd-based solar cells is continuing such that, in
2016, a record efficiency of 22.1% was achieved for CdTe in a thin film device developed
by First Solar.95
Iron pyrite (FeS2, also known as fool's gold) is easily synthesised,96 has an absorption
coefficient of 105 cm-1, a 0.95 eV bandgap and exceptionally low raw material costs, all of
which should make it an ideal choice for PV devices. However, while nanostructured FeS2
has found application as a photoconductor, in a p–n heterojunction, in bulk heterojunction
inorganic–organic hybrid solar cells and in DSSCs,97–100 the electronic properties can be
adversely compromised by surface defects due to sulfur vacancies. According to Steinhagen
et al., nanocrystal devices are especially susceptible to this because of the high proportion of
grain boundaries and the high proportion of atoms likely to reside at the surface of nanoscale
particles.81 Indeed, when Shukla et al. obtained photovoltages from pyrite nanocubes by
sulfurization of a deposited colloidal ink, they determined that surface defects were the
primary source of electron–hole recombination and that the efficiency could be enhanced by
using an optimised synthetic route to decrease either the concentration of grain boundaries
or the number of defects.82
A wide range of copper sulfide phases based on the stoichiometry of CuxS all have bandgaps
of around 1.2 to 2.0 eV.84 While x values of less than 2 correlate to bandgaps near 2.0 eV
and, hence, minimal PV applicability, the indirect bandgap semiconductor Cu2S has a bulk
bandgap of 1.21 eV85 and the indirect bandgap of its selenide counterpart (Cu2Se) is 1.4
eV.86 The production of a 1.6% efficient PV device was reported by Wu et al. who spin-
coated a layer of CdS nanorods with Cu2S nanocrystals synthesised by reaction of
ammonium diethyldithiocarbamate and bis(acetylacetonato)copper(II) in a mixed dodecane
thiol/oleic acid solvent.85 Although Cu2S found frequent use in combination with CdS from
37
the 1960s through the 1980s,101 the PV cell tended to degrade over time due to Cu+ diffusion
into the CdS layer.
Quantum dots such as CdS, CdSe, PbS and ZnS have been doped with manganese(II)
sulfide;102 for instance, Punnoose et al. demonstrated a PbS quantum dot DSSC with a PV
conversion efficiency of 4.25%.103 Finally, interest in the main group binary chalcogenide
tin mono sulfide (SnS) for PV applications has arisen due to its appropriate bandgap for solar
absorption (generally between 1.1 and 1.4 eV)88 and up to 24% theoretical power conversion
efficiency. However, the maximum efficiency for the SnS PV cell, reported by
Sinsermsuksakul et al., was 4.4%;104 hence, there is significant scope for improvement. The
efforts of the present author's group have concentrated on using aerosol-assisted chemical
vapour deposition (AACVD) to produce suitable thin-film SnS semiconductors for ultimate
application in PV device designs.105–107 It is particularly interesting to note that SnS is a
van der Waals layer structure and the present group has demonstrated that the bandgap
energy can be controlled in a foreseeable, layer-dependent way by thinning the material to
the 2D limit.108
1.9.1.1. Manganese sulfide
Manganese sulfide (MnS) displays three crystalline polymorphs with distinct morphological
and physical properties. The naturally-occurring, thermodynamically stable, green-coloured
alpha (α) phase (alabandite) displays an octahedrally coordinated rock salt structure (space
group Fm3m) and forms at relatively high temperatures. Meanwhile, the metastable beta
(β) and gamma (γ) phases are both pink in colour and form at low temperatures, with β-MnS
crystallizing in the tetrahedral zinc blende structure (space group F43m) and γ-MnS
crystallizing in the tetrahedral wurtzite structure (space group P6(3)mc) at low temperatures
as shown in Figure 1.6.109 Early experimental research examining the electrical, magnetic
and optical properties of the MnS phases has been reviewed in the literature.109 Recent
38
photo-electrochemical research has focused on manganese sulfide (MnS) owing to its
hypothesised role in prebiotic synthesis on early Earth.110 To avoid the introduction of
extraneous sources of carbon, such studies make use of crystalline MnS generated in the
absence of organo-sulfur compounds. Although room-temperature formation of the
thermodynamically stable α-phase is kinetically hindered, it will readily form under
solvothermal conditions at temperatures above approximately 200 °C. Various published
synthetic routes to MnS in one or more of the three phases include the high-temperature
reaction of sulfur and manganese in elemental form111–113 and deposition as the thin-
film29,114–120 generated in at least one example by decomposition of organometallic
precursors.121 Numerous other experimental studies involved the initial aqueous synthesis
of MnS from inorganic precursors followed by drying and subsequent transformation to the
α-phase by heating at temperatures around 1000 °C.122,123 Previous studies have also
described the solvothermal synthesis of both metastable (β and γ) forms as well as the stable
α-phase.124–128 A synthesis of pure α-MnS is presented in the present work and, in agreement
with the previous studies.129
Figure 1. 6. The crystalline structures of cubic rock-salt (RS) α-MnS, a, b, c = 5.224 Å (ICDD 01-
089-4952), metastable cubic zincblende (ZB) β-MnS, a, b, c = 5.615 Å (ICDD 00-040-1288) and
hexagonal wurtzite (WZ) γ-MnS structures, a and b = 3.979 Å and c = 6.446 Å (ICDD 00-040-1289).
Color code: Mn, violet; S, yellow.129
39
1.9.2. Ternary TMCs
The ternary TMCs category is generated by replacing the metal of a binary metal
chalcogenide system (MxEn) with two metals giving the same total charge (MxM′yEn). The
presence of two different metals opens up bandgaps that are not available to the isoelectronic
binary metal chalcogenides.65 A frequently encountered ternary system combines two
metals in the +1 and +3 oxidation states with a chalcogen pair in the −2 oxidation state, e.g.
CuInS2, and is denoted as I–III–VI2. The I–III–VI2 system is derived from the II–VI parent
binary system, e.g. CdSe. Systems incorporating two different chalcogens (MxM′yEnE′m)
also belong to the I–III–VI2 system and are regarded as ternary in spite of containing four
distinct elements.65 As with the parent binary compounds, the ternary systems experience
quantum confinement and function as quantum dots,130 enabling these materials to interact
with the full solar spectrum via the resultant energy modulation effects. This is extremely
useful for light harvesting, making the ternary TMCs a desirable substitute for toxic binary
compounds such as the cadmium chalcogenides.
The chalcopyrite phase of the ternary compound copper indium sulfide (CuInS2) has
attracted research interest as a possible component of heterojunction PV devices due to its
absorption coefficient >105 cm-1, direct band gap of 1.5 eV, high radiation hardness and
defect tolerance.131,132 Early devices and homojunction devices in the 1970s combined
CuInS2 with CdS or InP.133–135 The high concentration of defects results in beneficial
characteristics including bandgap tuning by controlling the number of defect sites along with
a high dopant capacity.136,137 However, these properties can result in compositional
differences between identically-sized nanocrystals within a batch, thereby broadening the
ensemble properties such as the luminescence peak for colloidal nanocrystals.138
Another I–III–VI2 ternary metal chalcogenide with the chalcopyrite structure is copper
gallium selenide (CuGaSe2). Although this has a high optical absorption coefficient of 105
cm-1 and a direct bandgap of 1.66 eV,139 its use in PV devices has been obstructed by the
40
challenges involved in generating a single-phase material. New colloidal routes for the
synthesis of phase-pure CuGaSe2 have been examined in order to address this problem.140,141
1.9.3. Quaternary TMCs
The most promising TMC systems with respect to maximum efficiency are the quaternary
TMC compounds, although they are also some of the most problematic to synthesise. They
have the general formula MxM′yM″zEn, where M is a transition metal, M′ and M″ are
additional (distinct) transition or other types of metal and E can be S, Se or Te. Among the
chief quaternary systems, copper zinc tin sulfide/selenide or CZTS (Cu2ZnSn(S,Se)4) and
copper indium gallium selenide or CIGS (Cu(In,Ga)Se2) have been extensively studied.142,143
While the earliest chalcopyrite-based solar cells used CuInSe2, with its 1.04 eV bandgap, as
the absorber, it was subsequently realised that the bandgap could be tuned by substitution of
gallium for some of the indium (giving a maximum bandgap of 1.68 eV for CuGaSe2).
Further studies have shown that the power conversion efficiency can be optimised by using
CuInGaSe2, giving a bandgap between 1.10 eV and 1.25 eV.144 The CIGS PV materials
have numerous desirable characteristics, such as harmless grain boundaries and lenient phase
characteristics that facilitate compositional variety without triggering a phase-transition.145
Notably, CIGS PV materials are among the few TMCs to be commercially produced, with
several firms producing devices with greater than 15% efficiency.144
Although the above-mentioned ease of manufacture, efficiency and commercial presence
make the CIGS materials unarguably successful, they do share one significant drawback
with the ternary CIS and related compounds, namely the low availability of indium. Indium
has been classified in the 2015 Risk List issued by the British Geological Survey ranks as
having a high supply risk.70 The focus of research has consequently switched to CZTS
(Cu2ZnSnS4) as an economical, environmentally friendly and abundantly available PV
material. CZTS has a stoichiometrically tuneable direct band gap of 1.45 eV and a high
absorption coefficient.146 The significant promise of this material is demonstrated by the
41
present admirable record efficiency of 12.6%.147 Like CIGS, it is frequently generated by a
vapour deposition, sputtering or a process followed by a high-temperature annealing stage.
This high-temperature stage presents two challenges that must be addressed if CZTS is to
become commercially practicable. Firstly, volatile compounds such as SnS can be lost
during annealing, which presents problems in controlling the composition of the desired
phase and, hence, the stoichiometrically-dependent photoconversion efficiency of the
absorber layers.148–152 In practice, solar cells produced from Cu-poor films display a notably
better performance than those using stoichiometric Cu2ZnSnS4.149 Secondly, an extreme
reduction in efficiency can arise due to reaction of sulfur with the molybdenum (Mo)
electrode onto which the CZTS is frequently deposited, forming an unwanted MoS2 layer
between the electrode and the absorber.153–155
Additional challenges in the underlying materials science of CZTS arise from the existence
of three possible stable phases (namely kesterite, stannite and a primitive mixed CuAu-like
structure)1,145,156, which can negatively impact upon the electronic and optical properties of
the material.
1.9.3.1. Kesterite and Stannite
Research into economical materials for highly efficient solar cells has gained major
importance in order to address the continuing need arising from recent rapid increases in
global energy consumption. The low bandgap energies and high absorption coefficients of
the Cu-based multinary chalcogenides gives them significant potential as effective next-
generation solar cell materials. Although the most outstanding of these is CuInxGa(1-x)Se2,
with the highest known energy conversion efficiency, reproducibility and flexibility towards
a range of growth process technologies,143 the use of rare and expensive elements such as
Ga and In is regarded as an obstruction to its commercial application. Moreover, while a
power conversion efficiency (PCE) of 21.7% has been achieved using a solar cell based on
a thin CdTe film,95 this cannot be effectively used due to the presence of toxic cadmium.
42
The Cu-based chalcogenide compounds such as CZTSe (Cu2ZnSnSe4) and CZTS
(Cu2ZnSnS4), with their abundant elemental components, close-to ideal bandgaps, large
absorption coefficients and 32.2% theoretical maximum PCE, have therefore been employed
by research scientists as a means of replacing Cd, Ga and In as absorber materials.157–159 For
example, a PCE of up to 12.6% has been reported for a thin-film solar cell (TFSC) based on
the p-type CZTS,147 although synthesis of the pure kesterite phase is problematic because of
the quantity of secondary and ternary phases present.160 Several further challenges to the
development of CZTS-based thin film solar cells, including: (i) the continuous variation in
bandgap (grading) for a given film thickness; (ii) the closely comparable atomic sizes of Zn
and Cu, which facilitates intermixing and gives rise to defects;161 and (iii) deterioration in
solar cell performance due to variations in the electrostatic potential of Cu and Zn in the
CuZn + ZnCu defect.162 There is therefore pressing need to develop alternatives to kesterite
CZTS and a method for substituting other elements for Zn or Cu. For instance, attention has
focused on the p-type CFTS (Cu2FeSnS4) materials with comparable properties to the CZTS
materials, including appropriate bandgaps of 1.28 eV – 1.50 eV and optical absorption
coefficients greater than 104 cm−1. Furthermore, CFTS is entirely composed of cheap,
abundant, relatively non-toxic elements and the optical bandgap energy is decreased by the
substitution of Zn by Fe. The higher solubility of Fe in the lattice results in enhanced
conductivity and enhanced efficiency at converting solar energy to electricity.163 Notably,
in addition to functioning as an absorber layer in TFSCs, the CFTS materials can function
as counter electrodes in DSSCs. Research in this direction has suggested that CFTS may
provide a more economical substitute for platinum in DSSCs. An additional potential use
of CFTS, in the form of nanoparticles, is in the photo-catalytic degradation of dyes.
Nevertheless, in spite of their photo conversion efficiency of around 8%,164 research into the
CFTS-based solar cells remains rare.
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1.9.3.2. Crystal structures of kesterite and stannite
The component atoms of CFTS are arranged in the unit cell as 4Cu, 2Fe, 2Sn and 8S. The
Cu-based chalcogenides CZTS (Cu2ZnSnS4) and CFTS (Cu2FeSnS4) are most frequently
found in the kesterite and stannite crystal structures, respectively. Although these structures
are closely related, they differ in the distribution of cations such as Cu+, Zn2+and Fe2+ and
are therefore assigned to different space groups. Thus, as indicated in Figure 1.7, the
kesterite structure has one Cu atom in the 2a (0, 0, 0) position, Zn in the 2c (0, ½, ¼) position
and the remaining Cu atoms occupying the 2d (0, ½, ¾) position, whereas the stannite
structure has Fe sited at the origin (2a), Cu at the 4d (0, ½, ¼) position and the Sn4+cation
remains on the 2b site in each case.165,166
Figure 1. 7. Unit cell representations of Cu2FeSnS4; (a) the Stannite type structure a = 5.449 Å; c =
10.726 Å, α. β and γ= 90, ICDD: 0005838 (b) kesterite type structure a = 5.434 Å; c = 10.856 Å, α. ,
β and γ= 90 ICDD: 0005843.1
44
Quintero et al. studied the crystallographic properties of the I2-Fe-IV-VI4 compounds and X-
ray powder diffraction indicated a tetragonal stannite structure I42m for Cu2FeSnS4 derived
from chalcopyrite by replacing half of the Fe atoms with Sn atoms and changing the
symmetry from I42d to I42m. The stannite phase remains stable within the temperature
range of 420 to 500 °C. Structural refinements by Hall et al. and other researchers support
these deductions.167–169
While the most frequently generated tetragonal structure arises from the ordered
arrangement of metal atoms in the cubic cell, cubic polymorphous adaptations with
disordered sphalerite-like structures are also noted at high temperature. The cubic crystal
structure variation of Cu2FeSnS4 was recently identified by Evstigneeva et al.170 who
reported the existence of the CFTS compound in a cubic phase belonging to the space group
I42m. The stannite prototype with a small cubic unit cell was also confirmed in the X-ray
study.
1.10. Doping
Semiconductors containing impurities or foreign atoms within the crystal structure are
referred to as doped semiconductors. Such impurities can arise unintentionally due to
inadequate control during growth of the semiconductor crystal, but are often added
deliberately to provide available charge carriers and enhance the electrical, magnetic and
optical properties (e.g. luminescence efficiency) that are essential for practical
applications.171 Rare-earth (RE) and transition metal (TM) ion dopants influence the
nanocrystal morphology and band structure as well as generating intense emissions in a
broad range of wavelengths depending upon the concentration, crystal dimensions and
specific type of impurity. Much recent research has therefore focused on doping
semiconductor nanostructures with RE and TM materials in order to discover possible
applications in photonics and bio-photonics.172 More detailed investigation into doped
45
nanocrystals is also valuable because the bandgap of the nanocrystalline host material can
be fine-tuned, and new luminescence generated, by matching the nanocrystalline shapes and
sizes to the energy levels of luminescent centres. A range of nanocrystalline particle systems
have been examined as part of a matrix or as a free-standing powder.56 Semiconductor
doping creates allowed energy states within the bandgap that are very close to the energy
band of the dopant type, with donor impurities creating states close to the conduction band
and acceptor impurities creating states near the valence band.173 By introducing isolated
energy levels between the host's valence and conduction band, such dopants frequently form
an emissive trap. Another valuable effect is a shift in the Fermi level of the host material
towards the energy band of the highest-concentration dopant.173
The surface properties of nanocrystals are expected to have a notable influence upon the
optical and structural properties of the material because a significant proportion of the atoms
are located at or close to the surface.174 Moreover, the material's chemical and physical
properties can be optimised with specific applications and requirements in mind by selecting
appropriately from a range of metallic dopant atoms. In general, the electrical, magnetic and
optical properties of metal sulfides can be significantly modified by metal-atom doping due
to the resulting changes in electronic structure.
Numerous studies have noted the exceptional optical properties of doped semiconductor
nanoparticles, regarding them as a new class of luminescent materials.172 The majority of
studies on doped semiconductor nanostructures have involved samples in which the particles
have a range of dopant numbers per particle, with the primary aim of elucidating the physical
properties of such powder nanostructures.
One particularly intriguing example of the recent progress in the semiconductor field is the
production of a dilute magnetic semiconductor (DMS) with possible spin-based electronics
and optical applications via doping with magnetic impurities capable of generating an
46
extremely large Zeeman splitting (more than two orders of magnitude larger than that of
standard semiconductors).175,176 In addition, the quantity of charge carriers (electrons and
holes) in a semiconductor can be controlled by doping with standard impurities. Thus, as
mentioned earlier dopant with one less valence electron than the host atom provides a hole
or positive charge carrier ("p-type"), while a dopant with one more valence electron provides
an electron or negative carrier ("n-type"). These forms of doping provided the basis for p-n
semiconductor devices like computer chips.177
Due to the similarities in chemical properties such as valance band and ionic radius, Mn2+
ions are relatively easy to incorporate into zinc (Zn2+) or cadmium (Cd2+) lattices.
Consequently, the distinctive and intricate properties of ZnS and CdS nanoparticles doped
with various concentrations of Mn2+ have been noted by several researchers. With an
electronic configuration of 1s2, 2s2, 2p6, 3s2, 3p6, 3d5, the Mn2+ ions in a range of luminescent
materials displays a d5 configuration.178–181 These ions produce a broad emission peak at a
position strongly influenced by the relationship between host lattice and crystal field
strength.182–184 The bulk ZnS:Mn material has found frequent application as a phosphor,
notably in AC thin-film electron luminescent devices.185–188 It has been suggested that the
Mn2+ d-electron states function as luminescence centres under external electronic excitation
via strong interaction with the s-p electronic states of the ZnS or CdS host lattice. One
possible mechanism is the generation of an excited state in the Mn2+ ion by the
recombination of an electron with a hole trapped by the Mn2+ ion.189,190
For instance, nearly monodispersed Mn-doped ZnS and CdS nanoparticles capped with
trioctylphosphine oxide (TOPO) were synthesised by Malik et al. via a single-source method
using manganese dichloride and bis(diethyldithiocarbamato)zinc(II) or
bis(methylhexyldithiocarbamato)cadmium(II).191 The resulting particles displayed distinct
optical properties compared to those of bulk ZnS or CdS. Their nanometre sizes and
47
predominantly hexagonal crystalline phases were demonstrated by electron microscopy and
X-ray diffraction, while the presence of the expected proportion of manganese dopant in the
ZnS or CdS nanoparticles was confirmed by electron paramagnetic resonance (EPR) spectra
and inductively coupled plasma mass spectrometry (ICP-MS).191
Meanwhile, metal diethyldithiocarbamate complexes with the general formula
[M(S2CN(Et)2)n] where M = Co (III), Cu(II), Fe(III), Ni(II) or Zn(II) and n = 2 or 3 were
used by Khalid et al.192 as single-source precursors for the aerosol-assisted chemical vapour
deposition (AACVD) of thin films of iron pyrite (FeS2) or transition metal-doped iron pyrite
(MxFe1-xS2) onto glass or indium tin oxide (ITO)-coated glass. Thermogravimetric analysis
(TGA) confirmed the decomposition of each complex to the corresponding metal sulfide
within comparable temperature ranges. The iron complex, [Fe(S2CNEt2)3], was shown to
deposit as a single-phase granular cubic crystalline FeS2 film at 350 °C and as a mixed-phase
pyrite/marcasite film at 450 °C. A shift in the powder X-ray diffraction (p-XRD) peaks
confirmed the generation of an MxFe1-xS2 solid solution, while incorporation of the TMs into
the pyrite for films deposited at various mole ratios of the TM complexes was confirmed by
EDX spectroscopy.192
Similarly, Al-Dulaimi et al. used the AACVD technique at 475 °C with various mole ratios
of [Re(µ-SiPr)3(SiPr)6] and [Mo(S2CNEt2)4] complexes to deposit thin films of Mo1-xRexS2
(0 ≤ x ≤ 0.06). SEM analysis indicated a change in the thin-film morphology with increased
levels of Re dopant in the MoS2; while pure MoS2 displayed a lamellar morphology, 1.79%
Re-doping produced MoS2 clusters and 3.60% Re generated feather-like crystals, which
were also present at 6.25% Re. The p-XRD analysis showed regular variations in the peak
intensity, shape and position of the (002) planes with varying rhenium content.193
Meanwhile, the spin-coating and melt technique was used by Bakly et al. to generate thin
Cd1-xZnxS (CZS) films from bis(ethylxanthato)zinc(II), [Zn(S2COEt)2], and
48
bis(ethylxanthato)cadmium(II), [Cd(S2COEt)2], complexes. For doping ratios between 0
and 0.15, p-XRD analysis showed that the thin films were hexagonal, while a shift in the
CdS peaks to higher angles with increasing Zn content demonstrated successful doping of
CdS with Zn.194
The production of PbS semiconductor nanocrystals doped with 0.05 and 0.52% Mn2+ via
simple chemical methods has been reported by Kripal and Tripathi. Nanocrystals of PbS:Mn
with an average crystallite size of 5 to 10 nm and a cubic structure belonging to space group
Fm3m were produced. The X-ray diffraction (XRD) analysis demonstrated a mild variation
in the lattice constant (a) with the concentration of Mn2+ ions. Since the level of Mn doping
in the PbS nanocrystal was quite low (less than 1%), the variations in peak intensity for 0.05,
0.26 and 0.52% Mn2+ were minor. Successful doping of the PbS nanocrystals with Mn2+ ions
was confirmed by EPR spectroscopy.195
Finally, the chemical bath deposition (CBD) approach was successfully used by Kumar et
al. to deposit thin films of Sb-doped PbS onto a cleaned glass substrate. Doping of the pure
PbS was achieved during film growth by addition of an aqueous solution of Sb3+ ions. While
all the deposited films displayed good quality, the characterization revealed a significant
influence of Sb-doping and annealing upon their physical properties. The XRD analysis
revealed a face centred cubic crystalline structure with a preferred (200) orientation and an
increasing crystallite size with increasing concentration of Sb dopant. Due to the quantum-
size effect, the optical absorption band edge displayed a blue-shift in the doped
nanostructured films relative to that of pure PbS. Moreover, the bandgap energy values were
significantly influenced by the concentration of dopant and annealing.196
1.11. Synthesis of nanoparticle semiconductors
Significant research endeavours have focused on synthetic routes to high quality, crystalline
semiconductor thin films and nanocrystals (NCs). These synthetic approaches fall into four
49
primary categories, depending upon the state of the reaction medium, namely: vapour-phase,
solid-phase, liquid-phase, and two-phase synthesis. The chief techniques for semiconductor
NC synthesis include chemical vapour deposition (CVD),197,198 molecular beam epitaxy
(MBE),198–202 magnetron sputtering,203 laser ablation,204 ball milling205 and metal-organic
vapour chemical deposition (MOCVD).206
Nanoscale materials, especially thin films, have often been successfully synthesised using
gas- or vapour-phase methods such as chemical vapour deposition (CVD). Several reviews
are available207,208 and just the key aspects will be recapitulated herein. The molecular
precursor(s) are initially vaporised under atmospheric or lower pressures before introducing
them (e.g. via an inert carrier gas) to the heated substrate, at which point decomposition
occurs to generate the thin film or particulate product.207,209 In the absence of a capping
agent, nanoparticle growth can lead to broad particle size distributions.209
Due to limitations relating to instrumentation, precursors and control of the synthetic process
and NC quality, the vapour-phase and solid-phase approaches have been found less efficient
than the liquid-phase approach for generating well-defined semiconductor NCs. The liquid-
phase synthesised NC semiconductors are dispersed in appropriate solvents to form stable
suspensions with the aid of surfactants or capping ligands; hence, they are also known as
colloidal semiconductor nanocrystals (CS-NCs).210 The liquid-phase approach has a number
of key benefits, including tuneable bandgap energies, high dipole moments, high optical
absorption coefficients and the potential to generate multiple excitons.211 Moreover, the
low production costs and extremely high throughput capability make solution-based
methods such as inkjet printing, spin-coating and roll-to-roll casting with stable colloidal
suspensions the preferred route for the large-scale manufacturing of devices.212,213
Liquid-phase syntheses can be further subdivided into three types according to the reaction
medium, namely: aqueous syntheses, organic syntheses and aqueous/organic syntheses. The
50
advantages of aqueous syntheses are the use of environmentally sound chemical
precipitation,214,215 hydrothermal techniques,214–217 mild reaction temperatures218 and
biocompatible solvents, while the primary disadvantage is lack of control of NC
morphology. By contrast, organic-based methods such as hot-injection219 and the
solvothermal autoclave method220–222 use high-boiling organic solvents and organic ligands
to generate NCs with well-controlled morphologies.223–226 Finally, the aqueous-organic
techniques (also referred to as interface-mediated or liquid-solid-solution (LSS)
techniques)227–231 involve the generation of NCs at the interface between an aqueous phase
and an organic phase, with the reactants divided between the phases. This approach,
pioneered by Wang et al.229 and Pan et al.230 have been successfully employed to
synthesise a range of NCs including polymer nanoparticles.229 Benefits include
stoichiometric control and mild reaction conditions.231 The resulting NCs are frequently
spherical or have regular shapes facilitated by their crystal structures.
The LaMer model considers the formation of NCs in two primary stages, namely nucleation
and crystal growth.232 When the precursors are dissolved in appropriate solvents, they
chemically react to form monomers which increase in concentration until super-saturation
triggers aggregation and self-nucleation of the monomers. Monomers then continue to
aggregate onto the pre-existing nuclei and NC growth occurs when the concentration of
monomers falls below a critical level. New nuclei continue to form during NC growth,
leading to a broadening of the NC size range; hence efforts to control the size of NCs are
primarily aimed at limiting this size distribution by adjusting reaction parameters such as
reaction time, reaction temperature, reactant injection temperature (for hot-injection
methods), precursor reactivity, precursor concentration, choice of solvent, choice of
surfactant and pH.233
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1.11.1. Hot injection method
An early strategy is to separate the nucleation and growth stages. For instance, nucleation
can be induced by rapid injection of the precursors into solvents at an elevated temperature,
with a subsequent reduction in reaction temperature to separate nucleation from growth.
This technique, termed the hot-injection method, was first employed by Murray et al. to
successfully synthesise high-quality monodisperse cadmium chalcogenide NCs using
dimethyl cadmium, Cd(CH3)2, as the source of Cd, bis(trimethylsilyl)sulfide, selenide or
telluride as chalcogenide sources, and tri-n-octylphosphine (TOP) and its oxide (TOPO) as
solvents.234 The precursors were injected at 300 °C, while CdX NCs (where X = S, Se, Te)
of average size were obtained at temperatures between 112 and 115 °C. The quality of NCs
obtained by this method can be enhanced by rapidly quenching the reaction mixture or by
introducing size-selective precipitation into the procedure. These additional strategies have
been applied to the aforementioned CdSe NC synthesis by dispersing the NCs in 1-butanol,
adding methanol until persistent opalescence was observed and then centrifuging.219
The hot injection technique has been used to synthesise ternary and quaternary chalcogenide
NCs such as CZTS and CIS. In this case, dispersal of the precursor mixture in an appropriate
solvent was followed by addition of a capping agent (e.g. oleylamine) and heating to a
selected temperature under an inert atmosphere. Control of the particle size is achieved by
varying the precursor concentrations and reaction times. For instance, CuInS2 NCs have
been synthesised by mixing CuI, In(OAc)3 and DDT in octadecene solvent, followed by
addition of oleylamine and heating to 200 °C in an Ar atmosphere with controlled particle
sizes of 3.5 to 7.3 nm being obtained at reaction times of 20 to 120 min. The apparatus used
in the hot injection approach is shown schematically in Figure 1.8.235
52
Figure 1. 8. setup of hot injection method.236
In recent years, researchers have shown an increased interest in metal sulfide production
using single source precursors (SSPs) and which are described in Section 1.13. For example,
the thermolytic synthesis of copper(I) sulfide from a copper stearate complex in alkanethiol
solution has been reported by Li et al.237
Nanoparticulate manganese(II) sulfides have also been prepared by such methods. In
particular, MnS NCs with well-defined crystal structures and shapes have also been obtained
by Peng et al. via thermolysis of the SSPs bis(diethyldithiocarbamato)manganese(II),
[Mn(DDTC)2], or [Mn(DDTC)2(1,10-phenanthroline)] in oleic acid, oleylamine and 1-
octadecene (ODE) solvent.238
Phase-pure tin(II) sulfide nanosheets have also been obtained by Khan et al. by hot injection
of the SSP dibutyl-bis(piperidinedithiocarbamato)tin(IV) in oleylamine at 230 °C, revealing
how the SnS nanosheets develop from spherical nanoparticles during the course of the
reaction.239 Meanwhile, Yousefi et al. used the simple, comparatively low-temperature,
53
hydrothermal reaction of tin(II) chloride with thioglycolic acid to synthesise SnS
nanoflowers.240 Various methods have also been applied to the synthesis of iron sulfide. For
example, Vanitha and O’Brien described the thermolysis of a single-source cubane-like Fe–
S cluster in octylamine at 180 °C to generate nanocrystalline pyrrhotite (Fe7S8), while
thermolysis in dodecylamine at 200 °C produced greigite (Fe3S4).241
1.11.2. The solvent-less thermolysis
Due to its favourable scalability, we have seen that hot-injection is the most frequently used
chemical technique for nanoparticle synthesis at the present time,234 with the heating-up
approach also being favoured for similar reasons. Nevertheless, reactions in the molten state
are not only less complicated and more easily scaled up, but are also more environmentally
and economically favourable due to the absence of any solvent.242 The practical advantages
also of solvent-free techniques also include facile control of the precursors and reaction
parameters (e.g. duration and temperature of thermolysis), along with the lack of need for
strong reagents (e.g. reducing or sulfonating agents). With respect to scaling up, the process
of heating a precursor in a furnace should be comparatively economical and
straightforward.34 The apparatus used in the melt method is shown schematically in Figure
1.9.
Figure 1. 9. setup of solvent-less thermolysis.243
54
The melting method has been successfully used to synthesise a broad range of nanomaterials,
including metals and their oxides and chalcogenides, in a wide variety of morphologies,
including nanoscale spheres, wires, disks, rods and fabrics.244–247 For example, the O’Brien
group reported a solvent-less, self-capping approach in which an asymmetric cadmium
dithiocarbamate precursor was heated to 150 – 300 °C under vacuum, with particles of ~ 5
– 7 nm being obtained at 250 °C.246 The bismuth counterpart of this precursor,
[Bi(S2CN(C18H37)(CH3))3], was subsequently used by the same group, with one hour of
heating to 150 – 300 °C under vacuum to generate spherical, crystalline Bi2S3.248 At 250 °C,
the particles obtained were ~ 8 nm in diameter with a moderately wide size distribution.
Meanwhile, a low-temperature solvent-less method was used by Khan et al. to produce a
range of ternary chalcogenides,249 while Almanqur et al. used the melt method to produce
iron sulfide nanostructures from iron(III) xanthate SSPs of the type [Fe(S2COR)3], where R
= ethyl, methyl, 1-propyl and isopropyl.250 Similarly, Alqahtani et al. produced Bi2S3 and
Sb2S3 by heating the precursors tris(O-ethylxanthate)bismuth(III) and tris(O-
ethylxanthate)antimony(III) to 200 – 300 °C under vacuum in the absence of any solvent.251
1.12. Synthesis of thin films
Since the first thin film of CZTS was deposited in 1988 via the sputtering method, various
other methods for depositing different types of thin film have been developed. Spray
pyrolysis is seen as a fast, cheap method, which is also suitable when large areas are
sprayed.252 An updated version of the sputtering method was subsequently investigated,
which showed great potential for the growth of different metal sulfide films. Additional
deposition techniques include pulsed laser deposition (PLD), photochemical deposition
(PCD), electrochemical deposition (ECD), chemical vapour deposition (CVD) and the spin
coating technique.253–255
55
The selection process of the method to be employed during the deposition stage depends on
various characteristics that the researchers attempt to analyse. Such selections will be
dictated by the type of film, the characteristics of the material and the potential applications
of the material.256–258 For example, CZTS was found to have a greater absorption coefficient
when it was deposited via the dip coating method ,259 while copper sulfide (Cu2S) was found
to produce better results via the chemical deposition technique.260 Tin sulfide and tin dioxide
nanocomposites were found to produce better results when electrochemical deposition
techniques were employed,261 while Cu2S thin films obtained via chemical bath deposition
required more parameters to be considered, such as the bath temperature and the solution
pH.262 Additional studies emphasise the importance of the deposition technique used,
particularly for the durability of the materials obtained, but also highlight the potential cost
of the materials used in deposition methods.263,264
1.12.1. Spin coating method
The spin coating technique is a simple and swift process that is used for the deposition of
material on flat surfaces. Rotatable fixtures hold the substrate in place while the coating
solution is dispersed onto the surface. The technique presents some defect prevention
properties and can be used extensively for a variety of solar cell materials.265 Chuang et al.
employed this technique for ZnO/PbS quantum dot (QD) solar cells. The authors noted the
prolonged performance and air stability of the cells obtained via this method,266 as shown in
Figure 1.10.
Figure 1. 10. Basic diagram of the spin coating technique. 267
56
1.12.2. The doctor blade method
Thin film deposition can be rapidly achieved at room temperature in the presence of non-
toxic (hence, environmentally friendly) solvents by means of the doctor blade method, which
is shown schematically in Figure 1.11.268,269 Due to its use of inexpensive high-throughput
equipment, along with low material wastage and high uniformity over large deposition areas,
this technique is highly facile, efficient, economical and scalable. Because of its
effectiveness, the doctor blade method has been used in the present work.
Figure 1. 11. Schematic picture of doctor blade coating process for thin film deposition.270
Furthermore, the process is reproducible and allows faster film growth relative to other
coating techniques such as electrochemical deposition, chemical bath deposition (CBD) or
spray coating. However, in spite of its efficiency at the laboratory and industrial scales, the
technique continues to face a range of challenges including the necessity for high-
temperature annealing in a reactive atmosphere, material loss at the annealing stage, additive
contamination, low film adhesion and undesired effects during drying, such as cracking and
increased porosity. Nevertheless, high-quality films can be generated by selecting
57
appropriate solvents and mild additives along with an optimal slurry preparation and
multiple coating protocols.
The doctor blade method has been used by Murtaza et al. to deposit thin films of antimony
sulfide (stibnite, Sb2S3) from the SSP tris(thiobenzoato)antimony(III) complex with heating
of the glass substrate for 1 h at 300, 350 and 400 °C. While the films deposited at low
temperature displayed a random distribution of thick sheet-like crystallites on the substrate
surface, those annealed at higher temperatures displayed uniform bundles of sticks.270 The
technique has also been used by Ayala et al. to deposit thin films of CZGS (Cu2ZnGeS4)
from a paste of binary metal sulfide precursors, which were generated by mixing suitable
quantities of CuS, GeS and ZnS with dilute acetic acid and triethanolamine.271
1.13. Single source precursors (SSPs)
A number of researchers have focused on the use of single source precursors, i.e. precursors
containing two or more of the constituent elements in a single molecule, in the synthesis of
nanoparticles and thin films.246,272–275 These studies have often found SSPs to be an efficient
approach to high-quality, monodispersed crystalline semiconducting nanoparticles,
affording a number of key benefits over other routes such as: it minimizes the use of toxic
gases; the decomposition temperature of the precursors can be easily controlled; ease of
purification; considerable volatility and moisture stability;276 ease of handling and
characterisation;274 the presence of pre-existing bonds that can lead to a material with
improved stoichiometry and/or fewer defects;277 and the potential contribution to a decreased
environmental effect of material processing .272 The benefits of single-source precursors
(SSPs) include the enhanced tunability and control of the resulting material structure due to
the fixed geometry and close proximity of the components within the precursor molecule.
58
Several authors have reported the synthesis of metal sulfide nanoparticles via such
techniques using metal-thiolate complexes.
1.14. SSPs for metal sulfide nanostructures
A metal sulfide SSP must meet the following requirements: (i) it must contain both sulfur
and the desired metal, preferably linked to each other chemically; (ii) it must be sufficiently
stable for ease of handling and not be susceptible to oxidation prior to decomposition; (iii)
it must decompose fairly readily to generate a pure metal sulfide product; and (iv) any side-
products must be inert and, ideally, volatile in order to avoid contaminating the target
material.278 A metal-dithiolate complex such as those presented in Figure 1.12 provides an
evident candidate with the capacity to stabilise metal centres, although the additional
heteroatoms contained by some of these ligands (e.g. phosphorus in the dithiophosphates
and oxygen in the xanthates) may generate impurities in the obtained material.278 The
prevention of oxidation is particularly important and is a frequent problem in metal sulfide
synthesis; hence, an aim of the present work is to avoid these difficulties by synthesising a
metal sulfide from xanthate in the presence of nitrogen gas.
Figure 1. 12. Some common ligands used in single source precursors to prepare metal sulfides.
59
1.14.1. Xanthates: a general introduction
As noteworthy members of the 1,1-dithiolate family, the xanthates are ligands capable of
forming metallic complexes of the form ROCS2−M+, where R is an alkyl or aryl group and
M+ = Na+, K+ etc. They were first synthesised by W. C. Zeise in 1815, who named them
after the Greek word for yellow (xanthos) due to the colour of the lead compounds.279,280
The presence of the -CS2 group enhances the reactivity of xanthates towards various metals
and informs a wide variety of applications. They have found a wide range of uses in classical
and organometallic chemistry, some of which will be cited here.
The potential use of transition metal xanthate complexes in nonlinear optical applications
has been investigated,281 with a wide range of studies over an extended period of time
examining O-alkyl dithiocarbonate (alkyl xanthate) ligands with various transition metals282–
285 and main group elements.286,287 These ligands have been shown to bond with a range of
metals in various modes including monodentate288,289 and bidentate.290 They are known to
act as chelating agents with the vast majority of transition elements, making them highly
versatile reagents in the extraction and separation of metals in mineral floatation and
analytical chemistry.291–293 The alkali metal xanthates, especially sodium and potassium, are
widely used to selectively separate sulfide minerals by the froth floatation method and as
industrial flotation agents for minerals containing thiophilic transition metals e.g. cobalt,
copper, gold, nickel and zinc.294–297 The xanthic acids have also been found to act as
reducing agents. Heavy metal ions have been extracted from aqueous solutions with the aid
of insoluble cellulose xanthate and the O-alkyl dithiocarbonate of cellulose.298 The xanthates
have also found widespread use in the chemical separation and quantitative analysis of
transition metals, alcohols and carbon disulfide.299 A technique for the direct
spectrophotometric determination of micro-quantities of Co(II), Cu(II), Mo(VI), Ni(II),
Pd(II) and Ru(III) in a range of alloys and in environmental samples (fly ash) was developed
60
by Malik et al. using sodium iso-amylxanthate in the presence of a surfactant as a solubilising
agent.300 Mercury, silver etc. are frequently extracted and separated with the aid of the alkali
metal xanthates, with the ethyl xanthates of sodium and potassium having known curative
properties in cases of acute mercury poisoning.281,301
Cellulose xanthates are widely employed for the production of rayon in the textile
industry.302 The bactericidal and bacteriostatic properties of cellulose dixanthate and
cuprous xanthate have been reported.303 Xanthate compounds are capable of inhibiting a
range of infective DNA and RNA types at concentrations that avoid harming the mitotic
activity of healthy cells.304 The potential anti-tumour effects of tin xanthate complexes have
also been demonstrated, while certain phosphine-gold(I)dithiocarbonate complexes have
displayed anti-arthritic properties.305,306
Recent studies have also described the synthesis of aryl xanthates of cobalt, [Co(S2COC6H2-
2,4,6-Me3)3], and nickel, [Ni(S2COC6H4-4-t-Bu)2].293 While earlier studies of these ligands
focused on the use of sulfur-donor ligands as analytical reagents, the recent increased interest
in their synthesis and characterisation is due to their possible biological activity307,308 and
practical uses in a diverse range of fields including electronics309 and rubber technology.310
Since the divalent transition metal bis(1,1-dithiolate) complexes are partially unsaturated,
they can form 1:1 adducts with electron donor ligands including neutral oxygen, nitrogen,
sulfur or phosphorus to give a range of coordination geometries from square pyramidal to
trigonal bipyramidal.311
61
1.14.1.1. Xanthate synthesis
As shown in Figure 1.13, the xanthate salts of alkali metals are generated by reaction of an
alcohol with sodium or potassium hydroxide and carbon disulfide:312,313
ROH + CS2 + MOH RO(C=S)SM + H2O
where R = an alkyl group; M = a monovalent metal (e.g. Na or K)
Figure 1. 13. Synthesis of alkali metal xanthates
The mechanism of this reaction is likely to be nucleophilic addition of the alkoxide ion to
carbon disulfide. A range of alcohols has been used in this reaction, with commercially
significant xanthate products including sodium ethyl xanthate (C2H5OCS2Na), sodium
isobutyl xanthate (C4H9OCS2Na), sodium isopropyl xanthate (C3H7OCS2Na), potassium
ethyl xanthate (C2H5OCS2K) and potassium amyl xanthate (CH3(CH2)4OCS2K).243 The
chemistry of such xanthate ligands has been extensively investigated and reviewed.314,315
1.14.1.2. Structure and bonding in xanthates
The xanthates are essentially salts or esters of dithiocarbonic (xanthic) acid RO(C=S)SH or
its O-ester; hence, the xanthic acids can be described as dithiocarbonates obtained by
replacing the two oxygen atoms of carbonic acid with two sulfur atoms and substituting an
alkyl group for one hydrogen atom:
62
The remaining hydrogen atom in xanthic acid is easily replaced by a metal ion or by another
alkyl group to give a dialkyl xanthate:
The rate constant indicated that the reaction followed the bimolecular reaction rate law,
giving the mechanism shown in Figure 1.14 for potassium ethyl xanthate. The second step
of this reaction proceeds slowly and is therefore the rate-limiting step.
Figure 1. 14. Bimolecular synthesis of potassium ethyl xanthate
Meanwhile, the ground-breaking work of Hoskins and Winter led to an increased interest in
the structural and synthetic chemistry of the xanthates, with more recent and wide-ranging
structural analyses by Tiekink and by Haiduc revealing the capacity of these ligands to
undergo monodentate, isobidentate or anisobidentate coordination with metal atoms.316–318
These are shown in Figure 1.15, along with other possible forms of bonding such as
bimetallic bridging via sulfur atoms or, occasionally, the oxygen atom, or (rarely) an
additional metal-oxygen atom.318
63
Figure 1. 15. Coordination behaviour of xanthate ligands. (A): monodentate; (B) isobidentate; (C)
anisobidentate; (D) and (E): bimetallic bridging through sulfur; (F) and (G): bridging to metal
through oxygen.
Heat promotes the decomposition of solid xanthates to generate products such as alcohols,
dixanthogens, dialkylxanthates, elemental sulfur, mercaptans, mercaptides and metallic
sulfides. As noted previously, the decomposition of xanthate is particularly important
primarily due to its use in mineral flotation. When dissolved in water, the xanthate salts
dissociate into xanthate anions and alkali metal cations to form strong electrolytes which
slowly hydrolyse to produce xanthic acid, which further decomposes into the alcohol and
carbon disulfide, Figure 1.16:
Figure 1. 16. The hydrolysis of dissolved the xanthate in water.
64
Numerous transition metal xanthates are known (e.g. Ag, Cr, Co, Cu, Fe, Mo, Ni, Zn) and
their importance in catalysis, metallurgy, metalloenzymes and material precursors has
stimulated significant interest in their chemistry.319 Due to their very low solubility in water,
TM xanthate complexes are produced by double decomposition of potassium or sodium
xanthate and a soluble salt of the heavy metal in solution, with precipitation of the desired
product. In the moist conditions, heavy metal xanthates are highly prone to decomposition
in the presence of carbon dioxide and oxygen, so the precipitation and filtration must be
performed under an atmosphere of nitrogen.
Figure 1. 17. The complex of metal xanthate, where M(II) = different metals and R an alkyl group.
1.14.1.3. Xanthate complexes as SSPs
As previously mentioned, the compositions and physical and optical properties of synthetic
materials can often more readily be controlled by the use of single-source precursor (SSP)
compounds, which contain the desired elements in one molecule, rather than separate
precursors for each component.320–322 Hence, while a wide range of metal (N,N-dialkyl
dithiocarbamates) [M(S2CNR2)n] have been used for a number of years as SSPs for the
synthesis of metal sulfide nanocrystals,53,88,89,105,275,323,324 a group of O-alkyl xanthate
(S2COR) complexes have been considered potentially useful as SSPs in this type of
synthesis. The metal-organic xanthates are known to decompose via the comparatively clean
and low-temperature Chugaev elimination reaction.325 Moreover, the xanthate ligand-based
SSPs have facilitated the synthesis of a wide range of metal sulfides at lower temperatures
65
than required by their corresponding N,N-dialklydithiocarbamato compounds, including
CdS,326 CZTS,327 MoS2,328 NiS and PdS329 to name but a few. The O’Brien group has
recently reported the synthesis of PbS/polymer composites via a melt process using both
lead(II) dithiocarbamate and lead(II) xanthate complexes.315 Pure cubic PbS nanocrystals
were obtained by the decomposition of [Pb(S2COnBu)2] in a polymer matrix at 150 °C, and
temperatures considerably below 275 °C were needed to decompose [Pb(S2CNnBu2)2].
Hence, the xanthate-based SSPs are useful for enhanced control of PbS nanocrystal size,
shape and preferred orientation across a wide range of temperatures. In addition, published
approaches to the synthesis of tin(II) sulfide nanomaterials have been included studies on
the use of Sn-SSPs including [SnII(S2CNR2)2] and [R'2SnIV(S2CNR2)2] for the production of
orthorhombic SnS nanoparticles and films.106,108
Alkyl xanthate complexes of copper(II), lead(II), nickel(II), tin(II) and zinc(II) have also
been synthesised. For example, although lead(II) alkylxanthates can be produced by reacting
lead(II) acetate with sodium xanthate in aqueous solution, McNaughter et al. synthesised a
range of lead(II) complexes with good solubility in organic solvents. Pure lead(II) sulfide
(PbS) was obtained from these xanthate SSPs via a melt reaction at 150, 175 or 200 °C.243
Meanwhile, bis(O-ethylxanthato)tin(II), [Sn(S2COEt)2], was synthesised and used as an SSP
in a spin coating method for the deposition of SnS thin films by Al-Shakban et al. The
[Sn(S2COEt)2] powder showed excellent solubility in a wide range of common organic
solvents, including THF, but had to be stored at −20 °C to minimise decomposition. After
spin-coating, the substrates were heated to temperatures of 150 to 400 °C to obtain pure
crystalline SnS films. While the composition and morphology were influenced by the
temperature of heating, the films were primarily orthorhombic with approximately spherical
structures along with some flakes.107
66
A one-pot synthetic protocol for the synthesis of pure, high-quality MoS2 nanosheets capped
by oleylamine was developed by Savjani et al. based upon the hot injection thermolytic
decomposition of [Mo2O2S2(S2COEt)2] as SSP. A highly-crystalline monolayer of randomly
oriented nanosheets was obtained with a combination of small flakes and high purity that
made it an optimal material for energy storage applications, e.g. supercapacitors.48
In addition, tris(xanthato)iron(III) complexes ([Fe(S2COR)3], where R = ethyl, methyl, 1-
propyl and isopropyl) SSPs have been used by Almanqur et al. to deposit iron sulfide thin
films and nanostructures via spin-coating and solid-state deposition. The potential of both
techniques for the low-temperature synthesis of iron sulfide materials with well-controlled
crystalline phase was demonstrated. When the spin coating and annealing method was used,
powder diffraction-XRD indicated the formation of troilite, while solvent-free pyrolysis was
similarly shown to generate primarily iron sulfide pyrrhotite or Fe1-xS, where x = 0 to 0.2.
The morphologies of these materials were similar, consisting of approximately spherical
crystals with a large range of particle sizes.250
Since a variety of distinct crystallographic phases and stoichiometric combinations of CuxS
has been identified, copper sulfide has gained much interest as a p-type semiconductor in a
wide range of optoelectronic devices. The phase-controlled synthesis of copper sulfide
nanoparticles from xanthato complexes by non-colloidal or colloidal methods is therefore
significant. Such an approach using bis(O-alkylxanthato)copper(II) complexes (where alkyl
= ethyl, hexyl or octyl) as SSPs was followed by Akhtar et al. via solid-state deposition,
thermolysis in oleylamine and the doctor blade method. A range of reaction times and
temperatures was used to demonstrate that the product phase depended directly upon the
method, temperature and alkyl chain length of the precursor.330
The spin-coating technique was used by Al-Shakban et al. to produce thin films of kesterite
(CZTS, Cu2ZnSnS4) from copper, zinc and tin xanthates, with the product phase depending
67
on the temperature of decomposition. Raman spectroscopy and X-ray diffraction analysis
indicated that tetragonal films were obtained by annealing between 375 and 475 °C, while
hexagonal films resulted at temperatures below 375 °C. The Cu/(Zn + Sn) ratio was
identified by EDX measurement to be between 1 and 0.64. The films heated at various
temperatures (225, 375 and 450 °C) were found to be moderately uniform according to
resistivity, carrier concentration, mobility and Hall coefficient measurements.331 In addition,
a very recent paper by the same author has described the syntheses of diphenyltin bis(2-
methoxyethylxanthate) and diphenyltin bis(iso-butylxanthate). These were characterised by
single-crystal X-ray diffraction and used as SSPs in the deposition of thin tin chalcogenide
films by AACVD. The films were characterised by scanning transmission electron
microscopy with elemental mapping and grazing incidence X-ray diffraction to indicate that
deposition from diphenyltin bis(iso-butylxanthate) produced orthorhombic SnS, while an
SnS/SnO2 nanocomposite was obtained by deposition from diphenyltin bis(2-methoxyethyl
xanthate) between 400 and 575 °C.27
Akhtar et al. used an AACVD method to deposit thin films of nickel sulfide onto silicon and
glass substrates at a range of temperatures from bis(O-alkylxanthato)nickel(II) precursor
complexes. A low deposition temperature of 250 °C was shown by p-XRD to produce a
pure nickel sulfide phase with irregular morphology, while higher deposition temperatures
resulted in mixed nickel sulfide nanophases with enhanced crystallinity.332
68
1.15. Aims and objectives
The purpose of this study is to synthesise xanthate single-source precursors for the formation
of nanomaterials and deposition of thin films. Metal xanthate complexes show interesting
thermal behaviour, and these complexes could be a good choice to produce binary, ternary
and quaternary metal sulfide at low temperatures. Furthermore, we hypothesise their
xanthate complexes may lead to low temperature synthesis of alkali metal chalcogenide
nanomaterials that have previously only been produced from a high temperature. Moreover,
the synthesis of xanthate complexes with different chain length and different annealing
temperatures could be useful to study the variations of physical properties of metal sulfide
compounds. For the growing interest in nanoparticles and thin films based solar energy
generation it is important to find cheap, nontoxic and environmentally friendly materials.
The metal sulfides that were used here are considered as the cheapest and non-toxic material
for photovoltaic cells. The other aim of this project is the development of a novel synthetic
technique for the synthesis of Pb1-xMnxS nanocrystals (x = 0 to 0.08) with detailed
compositional studies based on the p-XRD patterns and EDX. The optical properties of these
materials will be analysed by UV-Vis spectroscopy. Finally, the method which we propose
in this study is solvent-less thermolysis which has some advantages such as straight forward,
solvent free, inexpensive and single step utilizing single source precursors (SSPs).
69
1.16. References
1 A. Khare, B. Himmetoglu, M. Johnson, D. J. Norris, M. Cococcioni and E. S. Aydil, J.
Appl. Phys., 2012, 111, 083707.
2 D. B. Sirdeshmukh, L. Sirdeshmukh, K. G. Subhadra and C. S. Sunandana, Electrical,
Electronic and Magnetic Properties of Solids, Springer, Switzerland, 2014.
3 M. Grundmann, in The Physics of Semiconductors: An Introduction Including
Nanophysics and Applications, ed. M. Grundmann, Springer Berlin Heidelberg, Berlin,
Heidelberg, 2010, pp. 775–776.
4 H. T. Grahn, Introduction to Semiconductor Physics, World Scientific Publishing
Company, Singapore, 1999.
5 N. M. Megahid, M. M. Wakkad, E. K. Shokr and N. M. Abass, Phys. B Condens. Matter,
2004, 353, 150–163.
6 P. H. Miller, Phys. Rev., 1941, 60, 890–895.
7 G. Lutz, Semiconductor radiation detectors. Device physics, Springer, Berlin
(Germany), 1999.
8 D. A. Neamen and D. A. Neamen, Semiconductor Physics and Devices, CRC Press,
Homewood, IL, 1 edition., 1992.
9 Z. Salameh, Renewable Energy System Design - 1st Edition, Academic Press, London,
2014.
10 S. Zhuiykov, Nanostructured Semiconductor Oxides for the Next Generation of
Electronics and Functional Devices: Properties and Applications, Woodhead
Publishing, Cambridge, 2014.
11 Y. Fu and M. Willander, Physical Models of Semiconductor Quantum Devices, Kluwer
Academic, Massachusetts, 1999.
12 C. G. B. Garrett and W. H. Brattain, Phys. Rev., 1955, 99, 376–387.
13 N. Dasgupta and A. Dasgupta, Semiconductor Devices: Modelling and Technology, PHI
Learning Pvt. Ltd., New Delhi, 2004.
14 N. Costa and A. Cartaxo, Advances in Lasers and Electro Optics, InTech, Croatia, 2010.
15 P. S. Vasekar and T. P. Dhakal, Thin Film Solar Cells Using Earth-Abundant Materials,
InTech, London, 2013.
16 L.-J. Chen and Y.-J. Chuang, Mater. Lett., 2013, 91, 372–375.
17 M. A. El-Sayed, Acc. Chem. Res., 2004, 37, 326–333.
18 E. Roduner, Chem. Soc. Rev., 2006, 35, 583–592.
19 K. Takahashi, Wide Bandgap Semiconductors, Springer, Berlin, 2007.
70
20 S. Kal, Basic Electronics: Devices, Circuits and IT Fundamentals, PHI Learning Pvt.
Ltd., New Delhi, 2009.
21 I. Tsuji, H. Kato, H. Kobayashi and A. Kudo, J. Phys. Chem. B, 2005, 109, 7323–7329.
22 P. Kar, S. Farsinezhad, X. Zhang and K. Shankar, Nanoscale, 2014, 6, 14305–14318.
23 M. Al-Shakban, P. D. Matthews, G. Deogratias, P. D. McNaughter, J. Raftery, I.
Vitorica-Yrezabal, E. B. Mubofu and P. O’Brien, Inorg. Chem., 2017, 56, 9247–9254.
24 A. Ennaoui, S. Fiechter, W. Jaegermann and H. Tributsch, J. Electrochem. Soc., 1986,
133, 97–106.
25 M. Saeed Akhtar, A. Alenad and M. Azad Malik, Mater. Sci. Semicond. Process., 2015,
32, 1–5.
26 V. Steinmann, R. Jaramillo, K. Hartman, R. Chakraborty, R. E. Brandt, J. R. Poindexter,
Y. S. Lee, L. Sun, A. Polizzotti, H. H. Park, R. G. Gordon and T. Buonassisi, Adv.
Mater., 2014, 26, 7488–7492.
27 M. Al-Shakban, P. D. Matthews, E. A. Lewis, J. Raftery, I. Vitorica-Yrezabal, S. J.
Haigh, D. J. Lewis and P. O’Brien, J. Mater. Sci., 2019, 54, 2315–2323.
28 J. M. Chamberlain and M. Merdan, J. Phys. C Solid State Phys., 1977, 10, L571–L574.
29 C. D. Lokhande, A. Ennaoui, P. S. Patil, M. Giersig, M. Muller, K. Diesner and H.
Tributsch, Thin Solid Films, 1998, 330, 70–75.
30 M. Girish, T. Dhandayuthapani, R. Sivakumar and C. Sanjeeviraja, J. Mater. Sci. Mater.
Electron., 2015, 26, 3670–3684.
31 Y. Shi, F. Xue, C. Li, Q. Zhao and Z. Qu, Mater. Res. Bull., 2011, 46, 483–486.
32 Y. Cui, R. Deng, G. Wang and D. Pan, J. Mater. Chem., 2012, 22, 23136–23140.
33 L. Shi, C. Pei, Y. Xu and Q. Li, J. Am. Chem. Soc., 2011, 133, 10328–10331.
34 S. Chen, X. G. Gong, A. Walsh and S.-H. Wei, Appl. Phys. Lett., 2009, 94, 041903.
35 S. A. Vanalakar, S. M. Patil, V. L. Patil, S. A. Vhanalkar, P. S. Patil and J. H. Kim,
Mater. Sci. Eng. B, 2018, 229, 135–143.
36 X. Zhang, N. Bao, K. Ramasamy, Y.-H. A. Wang, Y. Wang, B. Lin and A. Gupta, Chem.
Commun., 2012, 48, 4956–4958.
37 C. Dong, G. Y. Ashebir, J. Qi, J. Chen, Z. Wan, W. Chen and M. Wang, Mater. Lett.,
2018, 214, 287–289.
38 K. Mokurala, P. Bhargava and S. Mallick, Mater. Chem. Phys., 2014, 147, 371–374.
39 S. G. Nilange, N. M. Patil and A. A. Yadav, Phys. B Condens. Matter, 2019, 560, 103–
110.
71
40 L. Chen, H. Deng, J. Tao, W. Zhou, L. Sun, F. Yue, P. Yang and J. Chu, J. Alloys
Compd., 2015, 640, 23–28.
41 L. Nie, J. Yang, D. Yang and S. Liu, J. Mater. Sci. Mater. Electron., 2019, 30, 3760–
3766.
42 H. Guan, X. Wang and Y. Huang, Chalcogenide Lett., 2018, 15, 435–440.
43 S. Adachi, Properties of Semiconductor Alloys: Group-IV, III-V and II-VI
Semiconductors, John Wiley & Sons, Chichester, 2009.
44 C. J. Murphy and N. R. Jana, Adv. Mater., 2002, 14, 80–82.
45 F. E. Kruis, H. Fissan and A. Peled, J. Aerosol Sci., 1998, 29, 511–535.
46 S. Horikoshi and N. Serpone, Microwaves in Nanoparticle Synthesis: Fundamentals and
Applications, Wiley.VCH, Germany, 2013.
47 D. Chen, R. Wang, I. Arachchige, G. Mao and S. L. Brock, J. Am. Chem. Soc., 2004,
126, 16290–16291.
48 N. Savjani, E. A. Lewis, M. A. Bissett, J. R. Brent, R. A. W. Dryfe, S. J. Haigh and P.
O’Brien, Chem. Mater., 2016, 28, 657–664.
49 N. Savjani, E. A. Lewis, R. A. D. Pattrick, S. J. Haigh and P. O’Brien, RSC Adv., 2014,
4, 35609–35613.
50 G. Cao and Y. Wang, Nanostructures and Nanomaterials: Synthesis, Properties, and
Applications, World Scientific Publishing Co., Singapore, 2011.
51 C. Buzea, I. I. Pacheco and K. Robbie, Biointerphases, 2007, 2, MR17–MR71.
52 N. L. Pickett and P. O’Brien, Chem. Rec., 2001, 1, 467–479.
53 T. Trindade, P. O’Brien and N. L. Pickett, Chem. Mater., 2001, 13, 3843–3858.
54 M. Bangal, S. Ashtaputer, S. Marathe, A. Ethiraj, N. Hebalkar, S. W. Gosavi, J. Urban
and S. K. Kulkarni, Semiconductor Nanoparticles, Springer, Berlin Heidelberg, 2005.
55 L. Brus, J. Chem. Phys., 1983, 79, 5566–5571.
56 L. Brus, J. Phys. Chem., 1986, 90, 2555–2560.
57 M. G. Bawendi, M. L. Steigerwald and L. Brus, Annu. Rev. Phys. Chem., 1990, 41, 477–
496.
58 M. L. Steigerwald and L. Brus, Acc. Chem. Res., 1990, 23, 183–188.
59 D. J. Norris and M. G. Bawendi, Phys. Rev. B, 1996, 53, 16338–16346.
60 A. Ashrafi, Quantum Confinement: An Ultimate Physics of Nanostructures, American
Scientific, California, 2011.
61 S. Chen, W.-J. Yin, J.-H. Yang, X. G. Gong, A. Walsh and S.-H. Wei, Appl. Phys. Lett.,
2009, 95, 052102.
72
62 C. Sevik and T. Çağın, Appl. Phys. Lett., 2009, 95, 112105.
63 S. Chen, X. G. Gong, C.-G. Duan, Z.-Q. Zhu, J.-H. Chu, A. Walsh, Y.-G. Yao, J. Ma
and S.-H. Wei, Phys. Rev. B, 2011, 83, 245202.
64 S. Chen, X. G. Gong, A. Walsh and S.-H. Wei, Phys. Rev. B, 2009, 79, 165211.
65 P. D. Matthews, P. D. McNaughter, D. J. Lewis and P. O’Brien, Chem. Sci., 2017, 8,
4177–4187.
66 A. A. Tedstone, D. J. Lewis and P. O’Brien, Chem. Mater., 2016, 28, 1965–1974.
67 W. R. Mateker and M. D. McGehee, Adv. Mater., 2017, 29, 1603940.
68 D. Li, P. Liao, X. Shai, W. Huang, S. Liu, H. Li, Y. Shen and M. Wang, RSC Adv., 2016,
6, 89356–89366.
69 J.-P. Correa-Baena, A. Abate, M. Saliba, W. Tress, T. J. Jacobsson, M. Grätzel and A.
Hagfeldt, Energy Environ. Sci., 2017, 10, 710–727.
70 British Geological Survey, Risk List, BGS, Nottingham, 2015.
71 C. Wadia, A. P. Alivisatos and D. M. Kammen, Environ. Sci. Technol., 2009, 43, 2072–
2077.
72 J. D. Jastram, U.S. Geological Survey Virginia and West Virginia Water Science Center,
U.S. Geological Survey, Reston, VA, 2017.
73 A. J. Nozik, G. Conibeer and M. C. Beard, Advanced Concepts in Photovoltaics, Royal
Society of Chemistry, Cambridge, 2014.
74 K. Meng, G. Chen and K. Ravindranathan Thampi, J. Mater. Chem. A, 2015, 3, 23074–
23089.
75 X. Liu, Y. Feng, H. Cui, F. Liu, X. Hao, G. Conibeer, D. B. Mitzi and M. Green, Prog.
Photovolt. Res. Appl., 2016, 24, 879–898.
76 I. Gur, N. A. Fromer, M. L. Geier and A. P. Alivisatos, Science, 2005, 310, 462–465.
77 J. Puthussery, S. Seefeld, N. Berry, M. Gibbs and M. Law, J. Am. Chem. Soc., 2011,
133, 716–719.
78 Q. Guo, H. W. Hillhouse and R. Agrawal, J. Am. Chem. Soc., 2009, 131, 11672–11673.
79 P. O’Brien and J. McAleese, J. Mater. Chem., 1998, 8, 2309–2314.
80 D. S. Boyle, A. Bayer, M. R. Heinrich, O. Robbe and P. O’Brien, Thin Solid Films, 2000,
361–362, 150–154.
81 C. Steinhagen, T. B. Harvey, C. J. Stolle, J. Harris and B. A. Korgel, J. Phys. Chem.
Lett., 2012, 3, 2352–2356.
82 S. Shukla, G. Xing, H. Ge, R. R. Prabhakar, S. Mathew, Z. Su, V. Nalla, T. Venkatesan,
N. Mathews, T. Sritharan, T. C. Sum and Q. Xiong, ACS Nano, 2016, 10, 4431–4440.
73
83 D. C. Reynolds, G. Leies, L. L. Antes and R. E. Marburger, Phys. Rev., 1954, 96, 533–
534.
84 I. Grozdanov and M. Najdoski, J. Solid State Chem., 1995, 114, 469–475.
85 Y. Wu, C. Wadia, W. Ma, B. Sadtler and A. P. Alivisatos, Nano Lett., 2008, 8, 2551–
2555.
86 S. Deka, A. Genovese, Y. Zhang, K. Miszta, G. Bertoni, R. Krahne, C. Giannini and L.
Manna, J. Am. Chem. Soc., 2010, 132, 8912–8914.
87 X. Yang, Y. Wang, K. Wang, Y. Sui, M. Zhang, B. Li, Y. Ma, B. Liu, G. Zou and B.
Zou, J. Phys. Chem. C, 2012, 116, 3292–3297.
88 D. J. Lewis, P. Kevin, O. Bakr, C. A. Muryn, M. Azad Malik and P. O’Brien, Inorg.
Chem. Front., 2014, 1, 577–598.
89 T. Trindade, P. O’Brien and X. Zhang, Chem. Mater., 1997, 9, 523–530.
90 B. O. Dabbousi, J. Rodriguez-Viejo, F. V. Mikulec, J. R. Heine, H. Mattoussi, R. Ober,
K. F. Jensen and M. G. Bawendi, J. Phys. Chem. B, 1997, 101, 9463–9475.
91 D. R. Larson, W. R. Zipfel, R. M. Williams, S. W. Clark, M. P. Bruchez, F. W. Wise
and W. W. Webb, Science, 2003, 300, 1434–1436.
92 P. K. Santra and P. V. Kamat, J. Am. Chem. Soc., 2013, 135, 877–885.
93 P. Das, S. Samantaray and G. R. Rout, Environ. Pollut., 1997, 98, 29–36.
94 J. Godt, F. Scheidig, C. Grosse-Siestrup, V. Esche, P. Brandenburg, A. Reich and D. A.
Groneberg, J. Occup. Med. Toxicol., 2006, 1, 22.
95 M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog. Photovolt.
Res. Appl., 2015, 23, 1–9.
96 P. D. Matthews, M. Akhtar, M. Azad Malik, N. Revaprasadu and P. O’Brien, Dalton
Trans., 2016, 45, 18803–18812.
97 D.-Y. Wang, Y.-T. Jiang, C.-C. Lin, S.-S. Li, Y.-T. Wang, C.-C. Chen and C.-W. Chen,
Adv. Mater., 2012, 24, 3415–3420.
98 Y.-C. Wang, D.-Y. Wang, Y.-T. Jiang, H.-A. Chen, C.-C. Chen, K.-C. Ho, H.-L. Chou
and C.-W. Chen, Angew. Chem. Int. Ed., 2013, 52, 6694–6698.
99 A. Kirkeminde, R. Scott and S. Ren, Nanoscale, 2012, 4, 7649–7654.
100 Z. Yang, M. Wang, S. Shukla, Y. Zhu, J. Deng, H. Ge, X. Wang and Q. Xiong, Sci. Rep.,
2015, 5, 11377.
101 K. W. Boer, J Cryst Growth, 1982, 59, 111–120.
102 Z. Deng, L. Tong, M. Flores, S. Lin, J.-X. Cheng, H. Yan and Y. Liu, J. Am. Chem. Soc.,
2011, 133, 5389–5396.
74
103 D. Punnoose, S. Srinivasa Rao, S.-K. Kim and H.-J. Kim, RSC Adv., 2015, 5, 33136–
33145.
104 P. Sinsermsuksakul, J. Heo, W. Noh, A. S. Hock and R. G. Gordon, Adv. Energy Mater.,
2011, 1, 1116–1125.
105 P. Kevin, D. J. Lewis, J. Raftery, M. Azad Malik and P. O’Brien, J. Cryst. Growth, 2015,
415, 93–99.
106 K. Ramasamy, V. L. Kuznetsov, K. Gopal, M. A. Malik, J. Raftery, P. P. Edwards and
P. O’Brien, Chem. Mater., 2013, 25, 266–276.
107 M. Al-Shakban, Z. Xie, N. Savjani, M. A. Malik and P. O’Brien, J. Mater. Sci., 2016,
51, 6166–6172.
108 J. R. Brent, D. J. Lewis, T. Lorenz, E. A. Lewis, N. Savjani, S. J. Haigh, G. Seifert, B.
Derby and P. O’Brien, J. Am. Chem. Soc., 2015, 137, 12689–12696.
109 C. N. R. Rao and K. P. R. Pisharody, Prog. Solid State Chem., 1976, 10, 207–270.
110 X. V. Zhang, S. T. Martin, C. M. Friend, M. A. A. Schoonen and H. D. Holland, J. Am.
Chem. Soc., 2004, 126, 11247–11253.
111 R. L. Clendenen and H. G. Drickamer, J. Chem. Phys., 1966, 44, 4223–4228.
112 C. Sombuthawee, S. B. Bonsall and F. A. Hummel, J. Solid State Chem., 1978, 25, 391–
399.
113 H. Wiedemeier and A. G. Sigai, J. Cryst. Growth, 1969, 6, 67–71.
114 O. Goede, W. Heimbrodt and V. Weinhold, Phys. Status Solidi B, 1986, 136, K49–K54.
115 O. Goede, W. Heimbrod, V. Weinhold, E. Schnürer and H. G. Eberle, Phys. Status Solidi
B, 1987, 143, 511–518.
116 E. Jahne, O. Goede and V. Weinhold, Phys. Status Solidi B, 1988, 146, K157–K160.
117 P. Pramanik, M. A. Akhter and P. K. Basu, Thin Solid Films, 1988, 158, 271–275.
118 B. J. Skromme, Y. Zhang, D. J. Smith and S. Sivananthan, Appl. Phys. Lett., 1995, 67, 2690–
2692.
119 S. Sivananthan, L. Wang, R. Sporken, J. Chen, B. J. Skromme and D. J. Smith, J. Cryst.
Growth, 1996, 159, 94–98.
120 M. Okajima and T. Tohda, J. Cryst. Growth, 1992, 117, 810–815.
121 R. Nomura, K. Konishi, S. Futenma and H. Matsuda, Appl. Organomet. Chem., 1990, 4,
607–610.
122 J. J. Banewicz and R. Lindsay, Phys. Rev., 1956, 104, 318–320.
123 L. D. Ahuja, D. Rajeshwar and K. C. Nagpal, J. Colloid Interface Sci., 1988, 123, 380–
390.
75
124 J. Lu, P. Qi, Y. Peng, Z. Meng, Z. Yang, W. Yu and Y. Qian, Chem. Mater., 2001, 13, 2169–
2172.
125 Y. Jun, Y. Jung and J. Cheon, J. Am. Chem. Soc., 2002, 124, 615–619.
126 Y. Zhang, H. Wang, B. Wang, H. Yan and M. Yoshimura, J. Cryst. Growth, 2002, 243,
214–217.
127 Y. Zhang, H. Wang, B. Wang, H. Xu, H. Yan and M. Yoshimura, Opt. Mater., 2003, 23, 433–
437.
128 S. W. Kennedy, K. Harris and E. Summerville, J. Solid State Chem., 1980, 31, 355–359.
129 O. Kavcı and S. Cabuk, Comput. Mater. Sci., 2014, 95, 99–105.
130 T. Omata, K. Nose and S. Otsuka-Yao-Matsuo, J. Appl. Phys., 2009, 105, 073106.
131 N. N. Syrbu, R. V. Cretu and V. E. Tezlevan, Cryst. Res. Technol., 1998, 33, 135–144.
132 M. Al-Shakban, P. D. Matthews, X. L. Zhong, I. Vitorica-Yrezabal, J. Raftery, D.
J. Lewis and P. O’Brien, Dalton Trans., 2018, 47, 5304–5309.
133 S. Wagner, J. L. Shay, P. Migliorato and H. M. Kasper, Appl. Phys. Lett., 1974, 25, 434–
435.
134 L. L. Kazmerski, F. R. White and G. K. Morgan, Appl. Phys. Lett., 1976, 29, 268–270.
135 L. L. Kazmerski and G. A. Sanborn, J. Appl. Phys., 1977, 48, 3178–3180.
136 J. J. M. Binsma, L. J. Giling and J. Bloem, J. Lumin., 1982, 27, 35–53.
137 H. Y. Ueng and H. L. Hwang, J. Phys. Chem. Solids, 1990, 51, 11–18.
138 B. Chen, H. Zhong, W. Zhang, Z. Tan, Y. Li, C. Yu, T. Zhai, Y. Bando, S. Yang and B.
Zou, Adv. Funct. Mater., 2012, 22, 2081–2088.
139 J. Tuttle, D. Albin, J. Goral, C. Kennedy and R. Noufi, Sol. Cells, 1988, 24, 67–79.
140 S. N. Malik, S. Mahboob, N. Haider, M. A. Malik and P. O’Brien, Nanoscale, 2011, 3, 5132–
5139.
141 J. Tang, S. Hinds, S. O. Kelley and E. H. Sargent, Chem. Mater., 2008, 20, 6906–6910.
142 P. Kevin, M. Azad Malik and P. O’Brien, J. Mater. Chem. C, 2015, 3, 5733–5741.
143 P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann and
M. Powalla, Prog. Photovolt. Res. Appl., 2011, 19, 894–897.
144 P. Reinhard, A. Chirilă, P. Blösch, F. Pianezzi, S. Nishiwaki, S. Buechelers and A. N.
Tiwari, in 2012 IEEE 38th Photovoltaic Specialists Conference (PVSC) PART 2, 2012,
pp. 1–9.
145 J. A. Frantz, J. D. Myers, R. Y. Bekele, V. Q. Nguyen, B. M. Sadowski, S. I. Maximenko,
M. P. Lumb, R. J. Walters and J. S. Sanghera, IEEE J. Photovolt., 2016, 6, 1036–1050.
76
146 M. Kumar, A. Dubey, N. Adhikari, S. Venkatesan and Q. Qiao, Energy Environ. Sci.,
2015, 8, 3134–3159.
147 W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu and D. B.
Mitzi, Adv. Energy Mater., 2014, 4, 1301465.
148 Y. Feng, B. Yu, G. Cheng, T. Lau, Z. Li, L. Yin, Q. Song, C. Yang and X. Xiao, J.
Mater. Chem. C, 2015, 3, 9650–9656.
149 M. C. Johnson, C. Wrasman, X. Zhang, M. Manno, C. Leighton and E. S. Aydil, Chem.
Mater., 2015, 27, 2507–2514.
150 J. J. Scragg, T. Ericson, T. Kubart, M. Edoff and C. Platzer-Björkman, Chem. Mater.,
2011, 23, 4625–4633.
151 K. Yu and E. A. Carter, Chem. Mater., 2015, 27, 2920–2927.
152 K. Yu and E. A. Carter, Chem. Mater., 2016, 28, 4415–4420.
153 J. J. Scragg, T. Kubart, J. T. Wätjen, T. Ericson, M. K. Linnarsson and C. Platzer-
Björkman, Chem. Mater., 2013, 25, 3162–3171.
154 F. Jiang, Gunawan, T. Harada, Y. Kuang, T. Minegishi, K. Domen and S. Ikeda, J. Am.
Chem. Soc., 2015, 137, 13691–13697.
155 B. Shin, O. Gunawan, Y. Zhu, N. A. Bojarczuk, S. J. Chey and S. Guha, Prog. Photovolt.
Res. Appl., 2013, 21, 72–76.
156 J. Paier, R. Asahi, A. Nagoya and G. Kresse, Phys. Rev. B, 2009, 79, 115126.
157 W. Shockley and H. J. Queisser, J. Appl. Phys., 1961, 32, 510–519.
158 S. A. Vanalakar, A. S. Kamble, S. W. Shin, S. S. Mali, G. L. Agawane, V. L. Patil, J. Y.
Kim, P. S. Patil and J. H. Kim, Sol. Energy, 2015, 122, 1146–1153.
159 S. A. Vanalakar, S. S. Mali, G. L. Agwane, A. Kamble, I. Y. Kim, P. S. Patil, J. Y. Kim
and J. H. Kim, Sol. Energy Mater. Sol. Cells, 2016, 157, 331–336.
160 S. A. Vanalakar, G. L. Agawane, S. W. Shin, M. P. Suryawanshi, K. V. Gurav, K. S.
Jeon, P. S. Patil, C. W. Jeong, J. Y. Kim and J. H. Kim, J. Alloys Compd., 2015, 619,
109–121.
161 G. Zhong, K. Tse, Y. Zhang, X. Li, L. Huang, C. Yang, J. Zhu, Z. Zeng, Z. Zhang and
X. Xiao, Thin Solid Films, 2016, 603, 224–229.
162 T. Gokmen, O. Gunawan, T. K. Todorov and D. B. Mitzi, Appl. Phys. Lett., 2013, 103,
103506.
163 P. Kevin, M. Azad Malik and P. O’Brien, New J. Chem., 2015, 39, 7046–7053.
164 R. R. Prabhakar, N. Huu Loc, M. H. Kumar, P. P. Boix, S. Juan, R. A. John, S. K.
Batabyal and L. H. Wong, ACS Appl. Mater. Interfaces, 2014, 6, 17661–17667.
77
165 S. Schorr, H.-J. Hoebler and M. Tovar, Eur. J. Mineral., 2007, 19, 65–73.
166 C. J. Bosson, M. T. Birch, D. P. Halliday, K. S. Knight, C. C. Tang, A. K. Kleppe and
P. D. Hatton, in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 0405–
0410.
167 S. R. Hall, J. T. Szymanski and J. M. Stewart, Can. Mineral., 1978, 16, 131–137.
168 F. Di Benedetto, G. P. Bernardini, D. Borrini, W. Lottermoser, G. Tippelt and G.
Amthauer, Phys. Chem. Miner., 2005, 31, 683–690.
169 M. Quintero, A. Barreto, P. Grima, R. Tovar, E. Quintero, G. S. Porras, J. Ruiz, J. C.
Woolley, G. Lamarche and A.-M. Lamarche, Mater. Res. Bull., 1999, 34, 2263–2270.
170 T. L. Evstigneeva and Y. K. Kabalov, Crystallogr. Rep., 2001, 46, 368–372.
171 F. Ochanda, K. Cho, D. Andala, T. C. Keane, A. Atkinson and W. E. Jones, Langmuir,
2009, 25, 7547–7552.
172 R. N. Bhargava, D. Gallagher, X. Hong and A. Nurmikko, Phys. Rev. Lett., 1994, 72,
416–419.
173 D. W. Snoke, Solid State Physics: Essential Concepts, Addison-Wesley, San Francisco,
1 edition., 2008.
174 P. Jaszczyn-Kopec, B. Canny and G. Syfosse, J. Lumin., 1983, 28, 319–326.
175 J. D. Bryan and D. R. Gamelin, in Doped Semiconductor Nanocrystals: Synthesis,
Characterization, Physical Properties, and Applications, John Wiley & Sons, Ltd,
Hoboken, New Jersey, 2005, vol. 54.
176 S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molnár, M. L.
Roukes, A. Y. Chtchelkanova and D. M. Treger, Science, 2001, 294, 1488–1495.
177 B. Sapoval and C. Hermann, Physics of Semiconductors, Springer-Verlag, New York,
1995.
178 S. Sapra, A. Prakash, A. Ghangrekar, N. Periasamy and D. D. Sarma, J. Phys. Chem. B,
2005, 109, 1663–1668.
179 L. B. Chandrasekar, R. Chandramohan, R. Vijayalakshmi and S. Chandrasekaran, Int.
Nano Lett., 2015, 5, 71–75.
180 C. Barglik-Chory, C. Remenyi, C. Dem, M. Schmitt, W. Kiefer, C. Gould, C. Rüster, G.
Schmidt, D. M. Hofmann, D. Pfisterer and G. Müller, Phys. Chem. Chem. Phys., 2003,
5, 1639–1643.
181 S. Chaure, Mater. Res. Express, 2018, 6, 025912.
182 R. N. Bhargava, D. Gallagher and T. Welker, J. Lumin., 1994, 60–61, 275–280.
78
183 G. Blasse and B. C. Grabmaier, in Luminescent Materials, eds. G. Blasse and B. C.
Grabmaier, Springer, Berlin, 1994, pp. 10–32.
184 A. A. Bol and A. Meijerink, J. Lumin., 2000, 87–89, 315–318.
185 B. A. Smith, J. Z. Zhang, A. Joly and J. Liu, Phys. Rev. B, 2000, 62, 2021–2028.
186 D. Gallagher, W. E. Heady, J. M. Racz and R. N. Bhargava, J. Cryst. Growth, 1994, 138, 970–
975.
187 J. S. Lewis, M. R. Davidson and P. H. Holloway, J. Appl. Phys., 2002, 92, 6646–6657.
188 J. F. Wager, J. C. Hitt, B. A. Baukol, J. P. Bender and D. A. Keszler, J. Lumin., 2002,
97, 68–81.
189 S. Gupta, J. C. McClure and V. P. Singh, Thin Solid Films, 1997, 299, 33–37.
190 T. Hoshina and H. Kawai, Jpn. J. Appl. Phys., 1980, 19, 279.
191 M. A. Malik, P. O’Brien and N. Revaprasadu, J. Mater. Chem., 2001, 11, 2382–2386.
192 S. Khalid, E. Ahmed, M. A. Malik, D. J. Lewis, S. A. Bakar, Y. Khan and P. O’Brien,
New J. Chem., 2015, 39, 1013–1021.
193 N. Al-Dulaimi, D. J. Lewis, X. Li Zhong, M. A. Malik and P. O’Brien, J. Mater. Chem.
C, 2016, 4, 2312–2318.
194 A. A. K. Bakly, B. F. Spencer and P. O’Brien, J. Mater. Sci., 2018, 53, 4360–4370.
195 R. Kripal and U. M. Tripathi, J. Mater. Sci. Mater. Electron., 2018, 29, 12195–12205.
196 R. Kumar, R. Das, M. Gupta and V. Ganesan, Superlattices Microstruct., 2014, 75, 601–
612.
197 M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, 395–398.
198 A. Urbieta, P. Fernández and J. Piqueras, J. Appl. Phys., 2004, 96, 2210–2213.
199 J.-J. Wu and S.-C. Liu, Adv. Mater., 2002, 14, 215–218.
200 X. Xing, K. Zheng, H. Xu, F. Fang, H. Shen, J. Zhang, J. Zhu, C. Ye, G. Cao, D. Sun
and G. Chen, Micron, 2006, 37, 370–373.
201 G. A. Ozin, M. R. Steele and A. J. Holmes, Chem. Mater., 1994, 6, 999–1010.
202 S. H. Xin, P. D. Wang, A. Yin, C. Kim, M. Dobrowolska, J. L. Merz and J. K. Furdyna,
Appl. Phys. Lett., 1996, 69, 3884–3886.
203 S. V. Ivanov, A. A. Toropov, S. V. Sorokin, T. V. Shubina, I. V. Sedova, A. A. Sitnikova,
P. S. Kop’ev, Z. I. Alferov, H.-J. Lugauer, G. Reuscher, M. Keim, F. Fischer, A. Waag
and G. Landwehr, Appl. Phys. Lett., 1999, 74, 498–500.
204 J. J. Berry, S. H. Chun, K. C. Ku, N. Samarth, I. Malajovich and D. D. Awschalom,
Appl. Phys. Lett., 2000, 77, 3812–3814.
205 C. Zhang, J. Zhong and J. Tang, Front. Optoelectron., 2015, 8, 252–268.
79
206 K. Hönes, E. Zscherpel, J. Scragg and S. Siebentritt, in Physica B Condensed Matter,
Elsevier, 2009, vol. 404, pp. 4949–4952.
207 M. Afzaal, M. Azad Malik and P. O’Brien, J. Mater. Chem., 2010, 20, 4031–4040.
208 N.-M. Hwang and D.-K. Lee, J. Phys. Appl. Phys., 2010, 43, 483001.
209 C. N. R. Rao, A. Müller and A. K. Cheetham, Eds., The Chemistry of Nanomaterials, 2
Volume Set: Synthesis, Properties and Applications, Wiley-VCH, Weinheim, 1 edition.,
2004.
210 Y. Jouane, S. Colis, G. Schmerber, P. Kern, A. Dinia, T. Heiser and Y.-A. Chapuis, J.
Mater. Chem., 2011, 21, 1953–1958.
211 H. Usui, Y. Shimizu, T. Sasaki and N. Koshizaki, J. Phys. Chem. B, 2005, 109, 120–
124.
212 J. Y. Kim, O. Voznyy, D. Zhitomirsky and E. H. Sargent, Adv. Mater., 2013, 25, 4986–
5010.
213 D. B. Mitzi, Adv. Mater., 2009, 21, 3141–3158.
214 D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang and S. Guha, Sol. Energy Mater. Sol.
Cells, 2011, 95, 1421–1436.
215 L. Brus, J. Chem. Phys., 1984, 80, 4403–4409.
216 L. Spanhel, M. Haase, H. Weller and A. Henglein, J. Am. Chem. Soc., 1987, 109, 5649–
5655.
217 M.-M. Titirici, M. Antonietti and A. Thomas, Chem. Mater., 2006, 18, 3808–3812.
218 D. L. Morgan, H.-Y. Zhu, R. L. Frost and E. R. Waclawik, Chem. Mater., 2008, 20,
3800–3802.
219 S. Mourdikoudis and L. M. Liz-Marzán, Chem. Mater., 2013, 25, 1465–1476.
220 M. D. Regulacio, C. Ye, S. H. Lim, M. Bosman, E. Ye, S. Chen, Q.-H. Xu and M.-Y.
Han, Chem. – Eur. J., 2012, 18, 3127–3131.
221 Y. Li, H. Liao, Y. Ding, Y. Fan, Y. Zhang and Y. Qian, Inorg. Chem., 1999, 38, 1382–
1387.
222 J. Xu, J.-P. Ge and Y.-D. Li, J. Phys. Chem. B, 2006, 110, 2497–2501.
223 A. Mamakhel, C. Tyrsted, E. D. Bøjesen, P. Hald and B. B. Iversen, Cryst. Growth Des.,
2013, 13, 4730–4734.
224 Z. Zhuang, Q. Peng and Y. Li, Chem. Soc. Rev., 2011, 40, 5492–5513.
225 Y. Yin and A. P. Alivisatos, Nature, 2005, 437, 664.
226 Z. Zhang, M. Lu, H. Xu and W.-S. Chin, Chem. – Eur. J., 2007, 13, 632–638.
80
227 H. G. Yang, G. Liu, S. Z. Qiao, C. H. Sun, Y. G. Jin, S. C. Smith, J. Zou, H. M. Cheng
and G. Q. (Max) Lu, J. Am. Chem. Soc., 2009, 131, 4078–4083.
228 X. Wang, J. Zhuang, Q. Peng and Y. Li, Nature, 2005, 437, 121.
229 X. Wang, Q. Peng and Y. Li, Acc. Chem. Res., 2007, 40, 635–643.
230 D. Pan, Q. Wang and L. An, J. Mater. Chem., 2009, 19, 1063–1073.
231 Q. Wang, D. Pan, S. Jiang, X. Ji, L. An and B. Jiang, Chem. – Eur. J., 2005, 11, 3843–
3848.
232 V. K. LaMer and R. H. Dinegar, J. Am. Chem. Soc., 1950, 72, 4847–4854.
233 J. Chang and E. R. Waclawik, RSC Adv., 2014, 4, 23505–23527.
234 C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. Soc., 1993, 115, 8706–
8715.
235 W. Zhang, H. Zhang, Y. Feng and X. Zhong, ACS Nano, 2012, 6, 11066–11073.
236 S. Mahajan, M. Rani, R. Dubey, J. Mahajan and H. Ece, 2013, 2, 518–521.
237 S. Li, H. Wang, W. Xu, H. Si, X. Tao, S. Lou, Z. Du and L. S. Li, J. Colloid Interface
Sci., 2009, 330, 483–487.
238 L. Peng, S. Shen, Y. Zhang, H. Xu and Q. Wang, J. Colloid Interface Sci., 2012, 377,
13–17.
239 M. Dilshad Khan, J. Akhtar, M. Azad Malik, M. Akhtar and N. Revaprasadu, New J.
Chem., 2015, 39, 9569–9574.
240 M. Yousefi, M. Salavati-Niasari, F. Gholamian, D. Ghanbari and A. Aminifazl,
Inorganica Chim. Acta, 2011, 371, 1–5.
241 P. V. Vanitha and P. O’Brien, J. Am. Chem. Soc., 2008, 130, 17256–17257.
242 E. Lewis, S. Haigh and P. O’Brien, J. Mater. Chem. A, 2014, 2, 570–580.
243 P. D. McNaughter, S. A. Saah, M. Akhtar, K. Abdulwahab, M. Azad Malik, J. Raftery,
J. A. M. Awudza and P. O’Brien, Dalton Trans., 2016, 45, 16345–16353.
244 A. Ghezelbash, M. B. Sigman and B. A. Korgel, Nano Lett., 2004, 4, 537–542.
245 K. Abe, T. Hanada, Y. Yoshida, N. Tanigaki, H. Takiguchi, H. Nagasawa, M. Nakamoto,
T. Yamaguchi and K. Yase, Thin Solid Films, 1998, 327–329, 524–527.
246 M. Lazell and P. O’Brien, Chem. Commun., 1999, 0, 2041–2042.
247 T. H. Larsen, M. Sigman, A. Ghezelbash, R. C. Doty and B. A. Korgel, J. Am. Chem.
Soc., 2003, 125, 5638–5639.
248 M. Lazell, S. J. Nørager, P. O’Brien and N. Revaprasadu, Mater. Sci. Eng. C, 2001, 16, 129–
133.
81
249 M. Dilshad Khan, G. Murtaza, N. Revaprasadu and P. O’Brien, Dalton Trans., 2018, 47, 8870–
8873.
250 L. Almanqur, I. Vitorica-yrezabal, G. Whitehead, D. J. Lewis and P. O’Brien, RSC Adv.,
2018, 8, 29096–29103.
251 T. Alqahtani, M. Dilshad Khan, D. J. Kelly, S. J. Haigh, D. J. Lewis and P. O’Brien, J.
Mater. Chem. C, 2018, 6, 12652–12659.
252 K. Ramasamy, M. A. Malik and P. O’Brien, Chem. Commun., 2012, 48, 5703–5714.
253 X. Zhang, N. Bao, B. Lin and A. Gupta, Nanotechnology, 2013, 24, 105706.
254 S. Ahmed, K. B. Reuter, O. Gunawan, L. Guo, L. T. Romankiw and H. Deligianni, Adv.
Energy Mater., 2012, 2, 253–259.
255 Z. Su, C. Yan, K. Sun, Z. Han, F. Liu, J. Liu, Y. Lai, J. Li and Y. Liu, Appl. Surf. Sci.,
2012, 258, 7678–7682.
256 S. Ahmadi, N. Asim, M. A. Alghoul, F. Y. Hammadi, K. Saeedfar, N. A. Ludin, S. H.
Zaidi and K. Sopian, Int. J. Photoenergy, 2014, 2014, 1–19.
257 D. M. Frigo, O. F. Z. Khan and P. O’Brien, J. Cryst. Growth, 1989, 96, 989–992.
258 N. M. Shinde, R. J. Deokate and C. D. Lokhande, J. Anal. Appl. Pyrolysis, 2013, 100,
12–16.
259 S. I. Swati, R. Matin, S. Bashar and Z. H. Mahmood, J. Phys. Conf. Ser., 2018, 1086,
012010.
260 H. M. Pathan, J. D. Desai and C. D. Lokhande, Appl. Surf. Sci., 2002, 202, 47–56.
261 H. Karami and S. Babaei, Int. J. Electrochem. Sci., 2013, 8, 12078–12087.
262 A. J. Varkey, Sol. Energy Mater., 1989, 19, 415–420.
263 A. Wangperawong, J. S. King, S. M. Herron, B. P. Tran, K. Pangan-Okimoto and S. F.
Bent, Thin Solid Films, 2011, 519, 2488–2492.
264 T. P. Dhakal, C. Peng, R. Reid Tobias, R. Dasharathy and C. R. Westgate, Sol. Energy,
2014, 100, 23–30.
265 M. Mennig, M. Schmitt, C. Fink-Straube and H. Schmidt, in Sol-Gel Technologies for
Glass Producers and Users, eds. M. A. Aegerter and M. Mennig, Springer US, Boston,
MA, 2004, pp. 161–168.
266 C.-H. M. Chuang, P. R. Brown, V. Bulović and M. G. Bawendi, Nat. Mater., 2014, 13,
796–801.
267 T. J. Rehg and G. Higgins, AIChE J., 1992, 38, 489–501.
268 I.-K. Ding, J. Melas-Kyriazi, N.-L. Cevey-Ha, K. G. Chittibabu, S. M. Zakeeruddin, M.
Grätzel and M. D. McGehee, Org. Electron., 2010, 11, 1217–1222.
82
269 S. Abermann, Sol. Energy, 2013, 94, 37–70.
270 G. Murtaza, M. Akhtar, M. Azad Malik, P. O’Brien and N. Revaprasadu, Mater. Sci.
Semicond. Process., 2015, 40, 643–649.
271 A. M. Ayala, N. R. Mathews, M. Pal, G. K. Gupta, A. Dixit and X. Mathew, Thin Solid
Films, 2019, 676, 68–74.
272 D. Fan, M. Afzaal, M. A. Mallik, C. Q. Nguyen, P. O’Brien and P. J. Thomas, Coord.
Chem. Rev., 2007, 251, 1878–1888.
273 M. Akhtar, J. Akhter, M. Azad Malik, P. O’Brien, F. Tuna, J. Raftery and M. Helliwell,
J. Mater. Chem., 2011, 21, 9737–9745.
274 J. Akhtar, M. Azad Malik, P. O’Brien and M. Helliwell, J. Mater. Chem., 2010, 20,
6116–6124.
275 M. A. Malik, M. Afzaal and P. O’Brien, Chem. Rev., 2010, 110, 4417–4446.
276 D. Barreca, A. Gasparotto, C. Maragno, R. Seraglia, E. Tondello, A. Venzo, V. Krishnan
and H. Bertagnolli, Appl. Organomet. Chem., 2005, 19, 59–67.
277 J. S. Ritch, T. Chivers, K. Ahmad, M. Afzaal and P. O’Brien, Inorg. Chem., 2010, 49, 1198–
1205.
278 M. D. Khan, M. A. Malik and N. Revaprasadu, Coord. Chem. Rev., 2019, 388, 24–47.
279 J. Wisniak, Educ. Quím., 2013, 24, 23–30.
280 R. F. Semeniuc, T. J. Reamer, J. P. Blitz, K. A. Wheeler and M. D. Smith, Inorg. Chem.,
2010, 49, 2624–2629.
281 S. Shahzadi, S. Ali, R. Jabeen and M. K. Khosa, Turk. J. Chem., 2009, 33, 307–312.
282 G. Exarchos, S. D. Robinson and J. W. Steed, Polyhedron, 2001, 20, 2951–2963.
283 M. J. Cox and E. R. T. Tiekink, Z. Für Krist. - Cryst. Mater., 2010, 211, 111–113.
284 S. Vastag, L. Markó and A. L. Rheingold, J. Organomet. Chem., 1990, 397, 231–238.
285 M. Farid Hussain, R. Kumar Bansal, B. Krishan Puri and M. Satake, Analyst, 1984, 109, 1151–
1153.
286 J. E. Drake, A. B. Sarkar and M. L. Y. Wong, Inorg. Chem., 1990, 29, 785–788.
287 S. Ghoshal and V. K. Jain, J. Chem. Sci., 2007, 119, 583–591.
288 P. F. R. Ewings, P. G. Harrison and T. J. King, J. Chem. Soc. Dalton Trans., 1976, 0, 1399–
1403.
289 K. Xu, W. Ding, W. Meng and F. Hu, J. Coord. Chem., 2003, 56, 797–801.
290 J. P. Fackler, D. Coucouvanis, J. A. Fetchin and W. C. Seidel, J. Am. Chem. Soc., 1968,
90, 2784–2788.
83
291 I. Ara, F. El Bahij, M. Lachkar and N. Ben Larbi, Transit. Met. Chem., 2003, 28, 908–
912.
292 M. Concepción Gimeno, E. Jambrina, A. Laguna, M. Laguna, H. H. Murray and R.
Terroba, Inorganica Chim. Acta, 1996, 249, 69–77.
293 H. W. Chen and J. P. Fackler, Inorg. Chem., 1978, 17, 22–26.
294 W. Ngobeni and G. Hangone, South Afr. J. Chem. Eng., 2013, 18, 41–50.
295 United States, US5308381A, 1994.
296 United States, US4083783A, 1978.
297 D. Coucouvanis, The Chemistry of the Dithioacid and 1,1-Dithiolate Complexes, John
Wiley & Sons, Inc, New York, 1970, vol. 11.
298 H.-T. Kim and K. Lee, Korean J. Chem. Eng., 1999, 16, 298–302.
299 A. W. DeMartino, D. F. Zigler, J. M. Fukuto and P. C. Ford, Chem. Soc. Rev., 2017, 46,
21–39.
300 A. K. Malik, K. N. Kaul, B. S. Lark, W. Faubel and A. L. J. Rao, Turk. J. Chem., 2001,
25, 99–105.
301 V. Pejchal, J. Holeček, M. Nádvorník and A. Lyčka, Collect. Czechoslov. Chem.
Commun., 1995, 60, 1492–1501.
302 C. F. Cross, E. J. Bevan and C. Beadle, J. Chem. Soc. Trans., 1895, 67, 433–451.
303 K. A. Malyshevskaya, N. A. Mazur and I. P. Dimitrenko, Fibre Chem., 1981, 12, 353–
354.
304 G. Sauer, E. Amtmann, K. Melber, A. Knapp, K. Müller, K. Hummel and A. Scherm,
Proc. Natl. Acad. Sci., 1984, 81, 3263–3267.
305 N. Donoghue, E. R. T. Tiekink and L. Webster, Appl. Organomet. Chem., 1993, 7, 109–
117.
306 M. W. Whitehouse, P. D. Cookson, G. Siasios and E. R. T. Tiekink, Met.-Based Drugs,
1998, 5, 245–249.
307 United States, US6583175B2, 2003.
308 W. Friebolin, G. Schilling, M. Zöller and E. Amtmann, J. Med. Chem., 2005, 48, 7925–
7931.
309 V. W.-W. Yam, C.-L. Chan, C.-K. Li and K. M.-C. Wong, Coord. Chem. Rev., 2001,
216–217, 173–194.
310 S. Palaty and R. Joseph, J. Appl. Polym. Sci., 2000, 78, 1769–1775.
311 A. V. Ivanov, V. I. Mitrofanova, M. Kritikos and O. N. Antzutkin, Polyhedron, 1999,
18, 2069–2078.
84
312 L. I. Victoriano, H. B. Cortés, M. I. S. Yuseff and L. C. Fuentealba, J. Coord. Chem.,
1996, 39, 241–251.
313 W. Mellert, E. Amtmann, V. Erfle and G. Sauer, AIDS Res. Hum. Retroviruses, 1988, 4,
71–81.
314 S. A. Alderhami, D. Collison, D. J. Lewis, P. D. McNaughter, P. O’Brien, B. F. Spencer,
I. Vitorica-Yrezabal and G. Whitehead, Dalton Trans., 2019, 48, 15605-15612.
315 E. A. Lewis, P. D. McNaughter, Z. Yin, Y. Chen, J. R. Brent, S. A. Saah, J. Raftery, J.
A. M. Awudza, M. A. Malik, P. O’Brien and S. J. Haigh, Chem. Mater., 2015, 27, 2127–
2136.
316 G. Winter and E. Tiekink, Inorganic Xanthates: A Structural Perspective, Walter de
Gruyter GmbH & Co, Berlin, 2011, vol. 12.
317 E. R. T. Tiekink and G. Winter, Rev. Inorg. Chem., 2011, 12, 183–302.
318 J. S. Casas, A. Castineiras, I. Haiduc, A. Sánchez, R. F Semeniuc and J. Sordo, J. Mol.
Struct., 2003, 656, 225–230.
319 B. Quiclet-Sire, A. Wilczewska and S. Z. Zard, Tetrahedron Lett., 2000, 41, 5673–5677.
320 M. Lazell, P. O’Brien, D. J. Otway and J.-H. Park, Dalton Trans., 2000, 4479–4486.
321 S. L. Castro, S. G. Bailey, R. P. Raffaelle, K. K. Banger and A. F. Hepp, J. Phys. Chem.
B, 2004, 108, 12429–12435.
322 L. Tian, H. Yao Tan and J. J. Vittal, Cryst. Growth Des., 2008, 8, 734–738.
323 P. O’Brien and R. Nomura, J. Mater. Chem., 1995, 5, 1761–1773.
324 K. Ramasamy, M. A. Malik, N. Revaprasadu and P. O’Brien, Chem. Mater., 2013, 25,
3551–3569.
325 G. Kociok-Köhn, K. C. Molloy and A. L. Sudlow, Can. J. Chem., 2014, 92, 514–524.
326 N. Pradhan and S. Efrima, J. Am. Chem. Soc., 2003, 125, 2050–2051.
327 A. Fischereder, A. Schenk, T. Rath, W. Haas, S. Delbos, C. Gougaud, N. Naghavi, A.
Pateter, R. Saf, D. Schenk, M. Edler, K. Bohnemann, A. Reichmann, B. Chernev, F.
Hofer and G. Trimmel, Monatshefte Für Chem. - Chem. Mon., 2013, 144, 273–283.
328 N. Savjani, J. R. Brent and P. O’Brien, Chem. Vap. Depos., 2015, 21, 71–77.
329 J. Cheon, D. S. Talaga and J. I. Zink, Chem. Mater., 1997, 9, 1208–1212.
330 M. Akhtar, Y. Alghamdi, J. Akhtar, Z. Aslam, N. Revaprasadu and M. A. Malik, Mater.
Chem. Phys., 2016, 180, 404–412.
331 M. Al-Shakban, P. D. Matthews, N. Savjani, X. L. Zhong, Y. Wang, M. Missous and P.
O’Brien, J. Mater. Sci., 2017, 52, 12761–12771.
85
332 M. Akhtar, N. Revaprasadu, M. A. Malik and J. Raftery, Mater. Sci. Semicond. Process.,
2015, 30, 368–375.
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Chapter 2. Instruments section
2.1. Measurement Methodologies
A Specac single reflectance ATR instrument (4000-400 cm-1, resolution 4 cm-1) was used to
record the infrared spectra and a Barloworld SMP10 melting point apparatus was used to
record the melting points.
2.2. Elemental analysis
Using a standard reference material and a calibrated Carlo Erba EA 1108 elemental analyser,
nitrogen, hydrogen, and carbon analysis was completed. The samples were contained within
a tin, which was placed into a furnace and burnt in oxygen at 1000 degrees Celsius. At this
heat, hydrogen is removed as a water vapour and carbon, nitrogen and sulfur react to become
dioxides. Gas chromatography columns were used to analyse the gases and identify the
amount of each element within the samples. In order to analyse the metal content of samples,
a weighed volume of the specific elements, i.e. copper, iron, nickel, zinc, cadmium, were
digested in acid by heating to the optimum temperature before transferring the residue to a
volumetric flask and creating a solution with water. Using a Fison's Horizon ICP-OES, the
solution was analysed and the individual element concentrations within the solution were
measured. The School of Chemistry microanalysis team completed all the elemental analysis
throughout the duration of the project.
2.3. Thermogravimetric analysis (TGA)
Chemical and physical properties of materials can be measured using the TGA thermal
analysis method, with these properties being assessed as a function of time at constant
temperature, or temperature at constant time. Phase transitions and other physical
phenomena, like vaporisation, sublimation, and absorption and desorption, and chemical
phenomena, like solid gas reactions, oxidation or reduction, decomposition and dehydration,
87
can all be assessed using TGA. The method can also be used to identify specific material
characteristics, which are correlated with a mass gain or loss resulting from oxidation,
decomposition or volatile loss, as seen with water. A sample’s organic and inorganic content,
its degradation mechanisms and reaction kinetics, and its decomposition patterns are
analysed during TGA to determine the material characteristics. A Seiko SSC/S200 model
was used to complete these measurements using a heating rate of 10 degrees Celsius per
minute under nitrogen. The instrument is calibrated using indium metal as a reference.
2.4. X-Ray crystallography
Using a Rigaku FRX diffractometer and graphite mono-chromated Mo-Kα or Cu-Kα
radiation, single crystal X-ray diffraction data was collected. The Sheldrick (2015) SHELXL
program was used to solve the structures.1 Anisotropic atomic displacement parameters were
used to refine the non-hydrogen atoms, whilst the hydrogen atoms were carefully positioned
and allocated with isotropic thermal parameters before being permitted to connect their
parent carbon atoms.1,2
2.5. Powder X-ray Diffraction (p-XRD)
A Panalytical X’Pert PRO diffractometer was used for the p-XRD diffraction studies using
Cu-Kα radiation. The sample nature dictated the various count rate used after they had been
mounted flat and scanned between 10 to 80o with a step size of 0.05. The sample’s diffraction
patterns and the ICDD index pattern were then compared. Crystalline materials can be
identified using the powerful powder X-ray diffraction (p-XRD) technique. The different
phases within a material can also be identified using this technique along with a
measurement of structural properties, including phase composition, defect structures, strain
state, preferred orientation, epitaxy, and grain size. The p-XRD technique is ideal for studies
88
of structure due to its non-destructive technique. Bragg's law dictates the diffraction beam
position (angle θ).
𝑛= 2𝑑s𝑖𝑛𝜃
where the order of diffraction is denoted as 𝑛, incident X-ray beam wavelength is denoted as
λ, the diffraction contribution dictated by the spacing between planes is denoted as 𝑑, and
the angle between the crystallographic plane and incident beam is denoted as 𝜃.
This equation shows that the path difference between the rays reflected by consecutive
planes within the lattice can be used to calculate the angle of incidence. Figure 2.1 shows
how the synthesised material can be determined by identifying the intensities and positions
of diffraction peaks and comparing these with the crystalline materials that are known to be
present in the database. Furthermore, the equation also allows the identification and
quantification of different sample phases and the average particle size can be estimated using
the p-XRD pattern line broadening.3
Figure 2. 1. Schematic representation of Bragg’s Diffraction.
89
2.6. Raman Spectroscopy
Raman spectroscopy is a technique that uses the interaction of light with a material to gain
understanding into the material’s characteristics, the same as IR spectroscopy. The data
provided by Raman spectroscopy results depend on light scattering, while those by IR
depend on light absorption. Both Raman and IR results provide the exact vibrations of a
molecule, called a molecular fingerprint, and are used to identify a substance. However,
Raman spectroscopy can provide additional information about frequency modes and
vibrations that give insight into crystal lattices.
When light interacts with particles, the majority of the photons are scattered at the same
energy as the incident photons. This is called Rayleigh scattering. When a small number of
these photons scatter at a different frequency than the incident photon, this is called Raman
scattering. When the change in energy of the incident photon is higher than that of the
scattered photon, the scattering is called Stokes scatter. However, some molecules may
start in a vibrational excited state and, when promoted to the higher energy, may relax to a
final energy state that is lower than the initial excited state. This scattering is called anti-
Stokes (Figure 2.2).
The structural characteristics of molecular bonds can be altered due to the frequency of the
light scattered from the molecule. These characteristics can provide data on a
semiconductor’s electrical and vibrational properties that show sensitivity to free carrier
density, alloy composition, strain, microstructure and crystalline quality. Over the past
decade, the vibrational spectra of polymer films, glasses and crystals have generally been
completed using Raman spectroscopy. The method is advantageous due its spatial
resolution being as low as 1 μm, which allows it to be used with microcrystalline samples.
For this study, a Renishaw 1000 micro-Raman system with a 514 nm laser was used to
perform Raman analysis at room temperature in backscattering mode.4
90
Figure 2. 2. Top; Energy level diagram of stimulated Raman scattering, down; Raman
spectrum showing the relative intensities of the different scattering processes.
2.7. Scanning electron microscopy (SEM) and energy dispersive X-ray
spectroscopy (EDX)
A TESCAN MIRA3 FEG-SEM was used to perform the SEM analysis. Following this, the
chemical composition of the samples was completed using energy dispersive X-ray
91
spectroscopy (EDX). A sample’s images are scanned via a focused beam of electrons during
SEM electron microscopy. There are a range of cryogenic or elevated temperatures that can
be used to observe the specimens, as well as high, medium and low vacuum conditions and
wet conditions in environmental SEM. The secondary electrons that the atom’s emitted
following excitation of the electron beam, allowed the analysis of the sample’s composition
and surface morphology. A resolution of 1nm or higher can be produced by SEM, however,
the quantity of secondary electrons that are produced depends on the angle that the surface
specimen (topography) and beam meet. Therefore, the surface topography is displayed as an
image as a special detector collects the secondary electrons that have scanned the sample.
Transmitted electrons, specimen current, light cathode luminescence (CL), characteristic X-
rays, back-scattered electrons (BSE), and secondary electrons (SE) are all SEM signals.
Whilst all SEMs have secondary electron detectors as standard equipment, it is unusual for
a single machine to contain detectors for all these signals.
Where there are atom and electron beam interactions close to or on the surface of the sample,
signals are created. The fine electron beam creates a large depth of field in SEM
micrographs. Elastic scattering reflects beam electrons from the sample, which are known
as BSE. The intensity of the BSE signal is closely correlated with the specimen’s atomic
number and are therefore, frequently used in analytical SEM, alongside characteristic X-Ray
spectra. A sample’s element distribution can be determined through the use of BSE images.
When a sample’s inner shell electron is removed by an electron beam, characteristic x-rays
are emitted. The shell is then filled with a higher energy electron, which therefore releases
energy. The abundance and composition of the sample’s elements can be identified using
characteristic x-rays, whilst the identification and quantification of these elements on a
micron scale is completed using EDX analysis. This can also be utilised to determine a
sample’s chemical composition and complete elemental mapping.5,6
92
2.8. UV/Vis spectroscopy
Any absorption spectroscopy occurring in the ultraviolet to visible regions of light is referred
to as UV/Vis spectroscopy. Electronic transitions within the compound vary the rate of
absorption and these transitions are measured from the ground to the excited state during
UV-vis spectroscopy techniques and from the excited state to the ground during fluorescence
spectroscopy.
As ligands and anions strongly influence the colour of metal ion solutions, any change in
maximum absorption wavelength (λmax) in transition metals where ligands and anions are
present, can be studied using UV/Vis spectroscopy. A system’s degree of conjugation can
also be assessed using this technique as there is an absorption of light in the UV or visible
region by any molecules that have a high degree of conjugation. Toluene, hexane, ethanol
and water can be used as solvents.
Optical absorbance spectra were used in the current study to measure the sample’s band gap
with these absorbance spectra being recorded with a Shimadzu UV-1800, double beam
UV/Vis NIR spectrophotometer. A wavelength range between 200 and 1100 nanometres
was used and a Tauc plot was created by plotting the absorption coefficient as a function of
photon energy to determine the optical band gap. This Tauc plot was correlated to the
semiconductor material’s absorption edge.7
2.9. Magnetic measurements
A Quantum Design MPMS-XL SQUID magnetometer fitted with a 7T magnet was used to
complete the magnetic measurements. For this, both field-cooled and and zero-field-cooled
magnetisation curves over a temperature range of 5 - 300 K and an applied magnetic field of
100 Oe were recorded.8
93
2.10. Reference
1 W. Clegg, Crystal Structure Determination, Oxford University Press, U.S.A., Oxford ;
New York, 1996.
2 M. . Woolfson, Introduction x ray crystallography., Cambridge ; New York, 2nd edn.
3 J. H. Robertson, Acta Crystallogr. A, 1979, 35, 350–350.
4 R. Singh, Phys. Perspect., 2002, 4, 399–420.
5 C. W. Oatley, W. C. Nixon and R. F. W. Pease, Adv. Electron. Electron Phys. 1966, 21,
181–247.
6 M. A. Haque and M. T. A. Saif, Exp. Mech., 2002, 42, 123–128.
7 P. Minutolo, G. Gambi and A. D’Alessio, Symp. Int. Combust., 1996, 26, 951–957.
8 A. F. Orchard, Magnetochemistry, Oxford University Press, Oxford, New York, 2003.
94
Chapter 3. Structural investigations of α-MnS nanocrystals and
thin films synthesised from single source precursors by hot
injection, scalable solvent-less and doctor blade routes.
3.1. Introduction
Considerable attention has been given to sulfide-based nanomaterials, due to their unique
properties; they have high conductivity, are low-cost, have low toxicity, high thermal
stability and catalytic ability.1,2 Its properties give it a significant range of possible
applications across a number of fields; it has potential to be used within supercapacitors,
batteries, dye-sensitized solar cells, drug delivery and electrocatalysis.3–6 Manganese sulfide
(MnS) is a magnetic semiconductive material (Eg=3.1 eV). It has potential within a number
of short wavelength opto-electronic applications, including solar selective coatings, solar
cells, sensors, photoconductors, and optical mass memories.7–10 MnS is one of the most
promising anode materials to be found among the sulfides. It has a wide range of variance
in its possible nanostructures, which includes nanorods, nanocubes, nanowires, nanosaws,
and nanospheres. Additionally, it has a high theoretical capacity.2,11–14 Typically, MnS thin
films and powders are found in one of several polymorphic forms. The most common form
it is found within is the rock salt type structure (α−MnS). This structure crystallizes into the
zincblende (β−MnS) or a wurtzite (γ-MnS) structure when exposed to a low temperature
growing technique.15,16
Hot injection, solvent-less thermolysis, and doctor blade processes were applied in order for
the formation of manganese sulfide nanomaterials from xanthate complexes to occur. A
series of novel manganese(II) xanthate single-source precursors [(TMEDA)Mn(S2COR)2 (R
= methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl and n-octyl) were investigated. Reports
were carried out on the crystal structures of these complexes. These complexes were tested
95
as single-source precursors for the formation of manganese sulfide nanomaterials and thin
films. Oleylamine was used for the thermal decomposition of manganese(II) xanthate
complexes, allowing manganese sulfide nanomaterials to be produced at a lower temperature
of 250 oC under hot injection. This differs from both the solvent-less thermolysis and the
doctor blade technique, which require a higher decomposition temperature of 350 °C.
3.2. Author distribution
In this work, I synthesised and then characterised xanthate complexes via IR, elemental
analysis and TGA. The experimental work to produce nanocrystals was carried out by me, I
characterised the samples by XRD, Raman, SEM, EDX and UV-Visible spectroscopy.
Floriana Tuna provided the magnetic measurements and analysis of the data. The
crystallographic data of the complexes has been collected by Inigo Vitorica-Yrezabal and
George Whitehead. Firoz Alam checked the characterization of complexes and materials.
David collison provided useful editing. The original idea was provided by Paul O’Brien.
David J. Lewis supporting me in the project and he provided as well a nice and useful
discussion, and also editing the manuscript. The experimental work was done in the
laboratory of Paul O’Brien.
3.3. References:
1 S. Biswas, S. Kar and S. Chaudhuri, J. Phys. Chem. B, 2005, 109, 17526–17530.
2 X. Yang, Y. Wang, K. Wang, Y. Sui, M. Zhang, B. Li, Y. Ma, B. Liu, G. Zou and B.
Zou, J. Phys. Chem. C, 2012, 116, 3292–3297.
3 A. Marques, M. Marin and M.-F. Ruasse, J. Org. Chem., 2001, 66, 7588–7595.
4 M. Govindasamy, S. Manavalan, S.-M. Chen, U. Rajaji, T.-W. Chen, F. M. A. Al-
Hemaid, M. A. Ali and M. S. Elshikh, J. Electrochem. Soc., 2018, 165, B370–B377.
5 Y. Tang, T. Chen and S. Yu, Chem. Commun., 2015, 51, 9018–9021.
6 Y. Tang, T. Chen, S. Yu, Y. Qiao, S. Mu, J. Hu and F. Gao, J. Mater. Chem. A, 2015, 3,
12913–12919.
96
7 D. Fan, X. Yang, H. Wang, Y. Zhang and H. Yan, Phys. B Condens. Matter, 2003, 337,
165–169.
8 B. Piriou, J. Dexpert-Ghys and S. Mochizuki, J. Phys. Condens. Matter, 1994, 6, 7317–
7327.
9 R. Tappero, P. D’Arco and A. Lichanot, Chem. Phys. Lett., 1997, 273, 83–90.
10 C. D. Lokhande, A. Ennaoui, P. S. Patil, M. Giersig, M. Muller, K. Diesner and H.
Tributsch, Thin Solid Films, 1998, 330, 70–75.
11 Y. Gui, L. Qian and X. Qian, Mater. Chem. Phys., 2011, 125, 698–703.
12 X. Yang, Y. Wang, Y. Sui, X. Huang, T. Cui, C. Wang, B. Liu, G. Zou and B. Zou,
Langmuir, 2012, 28, 17811–17816.
13 J. Beltran-Huarac, J. Palomino, O. Resto, J. Wang, W. M. Jadwisienczak, B. R. Weiner
and G. Morell, RSC Adv., 2014, 4, 38103–38110.
14 J. Beltran-Huarac, O. Resto, J. Carpena-Nuñez, W. M. Jadwisienczak, L. F. Fonseca, B.
R. Weiner and G. Morell, ACS Appl. Mater. Interfaces, 2014, 6, 1180–1186.
15 R. L. Clendenen and H. G. Drickamer, J. Chem. Phys., 1966, 44, 4223–4228.
16 M. Kobayashi, T. Nakai, S. Mochizuki and N. Takayama, J. Phys. Chem. Solids, 1995,
56, 341–344.
97
3.4. Structural investigations of α-MnS nanocrystals and thin films
synthesised from single source precursors by hot injection, scalable
solvent-less and doctor blade routes.
Abdulaziz M. Alanazi,a,c Firoz Alam,a,b Inigo Vitorica-yrezabal,a George Whitehead,a
Floriana Tuna,a David Collison,a Paul O’Briena,b and David J. Lewisb*
a, Department of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
b, Department of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
c, Department of Chemistry, Islamic university, Prince Naif Ibn Abdulaziz Rd, Madinah, 42351,
KSA
*Email: david.lewis-4@manchester.ac.uk
3.4.1. Abstract
Manganese (II) xanthate precursors [Mn(S2COR)2(TMEDA)] (R = methyl (1), ethyl (2), n-
propyl (3), n-butyl (4), n-pentyl (5), n-hexyl (6) and n-octyl (7), TMEDA =
tetramethylethylenediamine) have been synthesised and their crystal structures have been
determined using single crystal X-ray diffraction. The complexes were used as single source
precursors to synthesise manganese sulfide (MnS) nanocrystals and thin films using hot
injection, solvent-less and doctor blade thermolysis, respectively. The nanocrystals and thin
films were characterised by powder X-ray diffraction, scanning electron microscopy (SEM),
energy dispersive X-ray (EDX) and Raman spectroscopy. Analysis of MnS obtained from
all routes indicates that the hot injection thermolysis provides superior control over
composition. Also, the oleylamine (OLA), which is used as capping agent assists the
decomposition of the complexes at lower temperatures whereas the solvent-less thermolysis
and doctor blade technique requires higher decomposition temperature of 350 °C. The
magnetic measurements recommend that α-MnS nanocrystals depict a ferromagnetic
98
behaviour. Magnetic hysteresis measurements reveal that α-MnS nanocrystals have large
coercive field strength (e.g., 0.723 kOe for 8.2 nm nanocrystals), which is associated with
the size and self-assembly of the materials.
3.4.2. Introduction
Semiconductor nanocrystals (NCs) or 'quantum dots' have drawn significant interest in the
research community because of their size dependant optical and electronic properties.1–11
The unique properties of these nano-materials can also be altered using composition, shape
and surface states, and have strong potential to be used in many applications such as
optoelectronics (display, lighting and photovoltaics), as well as in biological imaging and in
photodetectors.12–19
For the synthesis of metal chalcogenide nanomaterials, the use of single source molecular
precursors has been extensively reported.20–22 For example, xanthate complexes with a metal
chalcogenide bond as their single source precursor (SSP) have been demonstrated to be
effective for the synthesis of metal sulfide nanocrystals and thin films.23–27 The xanthates
were developed as broadly adaptable ligands for generating an extensive variety of complex
organic and inorganic materials with desirable chemical and physical properties.28,29 Their
advantages arise from ease of synthesis and the presence of a sulfur donor atom which can
stabilize a broad range of elements and transition metals in a variety of oxidation states.23
Metal xanthates [M(S2COR)x] have been synthesised to be an excellent precursors to a range
of metal chalcogenides previously, including complex materials such as alkaline earth metal
sulfides.30–33
Manganese sulfide (MnS) is a p-type magnetic semiconductor with potential application in
short wavelength or high temperature optoelectronics, having a wide band gap of 2.7-3.7
eV.34–36 It also finds application as a magnetic semiconductor and in luminescent materials,
optical mass memory, photo-conductors, sensors and solar selective coatings.37–40
99
Manganese sulfide (MnS) is generally found in three crystal forms, which are shown in
Figure 3.1, namely the stable cubic rock-salt RS α-MnS, metastable cubic zincblende ZB β-
MnS, and hexagonal wurtzite WZ γ-MnS structures. The ZB and WZ phases are unstable or
metastable, being easily transformed into the stable octahedrally coordinated RS phase under
high temperature or pressure.41 A range of synthetic methods has been applied to the
generation of MnS nanostructures including chemical bath deposition (CBD), hydrothermal,
microwave, solvothermal and sonochemical methods.42–47 Synthetic routes for the controlled
synthesis of single-phase MnS NCs have also been reported.48–50 For example, Hyeon et al,
synthesised hexagonal MnS with the wurtzite structure by heating a mixture of sulfur and
MnCl2 in oleyamine at 280 °C.48 Moreover, the use of a Mn(II) dithiocarbamate complex in
the production of manganese sulfide has been reported in a study that examined the effect
of the counter anion upon the morphology and phase of the synthesized product.27 The
properties of the synthesised NCs are strongly dependent upon the specific method of
preparation. Solvent-less thermolysis has a facile and comparatively economical single-step
approach that uses xanthates as single source precursors and does not necessitate the use of
intricate apparatus. The technique is simple, cost-effective, solventless, atom efficient,
environmental friendly and has a significant potential for scaling up. This work examines
the synthesis of novel bis(O-alkylxanthato) manganese(II) (alkyl = Me, Et, n-Pr, n-But, n-Pen,
n-Hex and n-Oct) complexes stabilised by the bidentate N-donor ligand
tetramethylethylenediamine (TMEDA) and their use as precursors for the synthesis of MnS
NCs as well thin films by (i) a hot injection thermolysis, using oleylamine (OLA) as a
capping agent and trioctylphosphine (TOP) as the dispersion medium, (ii) Solvent-less
thermolysis and (iii) doctor blade technique for the deposition of thin films. To the best of
our knowledge, these approaches have not previously been applied to the synthesis of MnS
NCs and thin films using manganese(II) xanthate complexes.
100
Figure 3. 1. The crystalline structures of (a) cubic rock-salt (RS) α-MnS, a, b, c = 5.224 Å (ICDD
01-089-4952), (b) metastable cubic zincblende (ZB) β-MnS, a, b, c = 5.615 Å (ICDD 00-040-1288)
and (c) hexagonal wurtzite (WZ) γ-MnS structures, a and b = 3.979 Å and c = 6.446 Å (ICDD 00-
040-1289). Colour code: Mn, violet; S, yellow.41
3.4.3. Experimental
3.4.3.1. Materials and instrumentation
All chemicals were purchased from Sigma Aldrich and used as received. Melting points were
determined with a Stuart melting point apparatus (Cole-Palmer, UK). Infrared spectra (IR)
were recorded using a Nicolet iS5 Thermo Scientific ATR instrument (4000–400 cm−1,
resolution 4 cm−1). Elemental analyses (EA) and Thermogravimetric analyses (TGA) were
carried out by the Micro-elemental Analysis Service in the School of Chemistry at the
University of Manchester. EA was performed for all complexes using a Flash 2000 Thermo
Scientific elemental analyser and TGA data were obtained with Mettler Toledo TGA/DSC
stare system in the range 30–600 °C at a heating rate of 10 °C min−1 under nitrogen flow.
Powder X-ray diffraction (p-XRD) analyses were carried out using an X-Pert diffractometer
with a Cu-Kα1 source (λ = 1.54059 Å), the samples were scanned between 10° to 80°, the
applied voltage and current was 40 kV and 30 mA, respectively. Scanning electronic
microscopy (SEM) and energy dispersive X-ray spectroscopy analysis is carried out using
TESCAN MIRA3 FEG-SEM. The EDX was used to know the chemical composition of the
samples. Raman spectra were measured using a Renishaw 1000 Micro- Raman System
101
equipped with a 514 nm laser. Single crystal X-ray diffraction data for all the complexes
were obtained using Mo-Kα or Cu-Kα radiation on a Rigaku FR-X diffractometer. The
structures were solved by SHELXL (Sheldrick, 2015) program.51 Crystals were grown using
vapour diffusion of hexane in to a solution of precursor in acetone. Non-hydrogen atoms
were refined with anisotropic atomic displacement parameters. Hydrogen atoms were placed
in calculated positions, assigned isotropic thermal parameters and allowed to ride on their
parent carbon atoms.
3.4.3.2. Synthesis of [Mn(S2COMe)2.(TMEDA)], (1)
All the potassium xanthate reported herein are prepared according to the previously
published papers.31,52 Briefly, potassium hydroxide (0.76 g, 13.63 mmol) was dissolved in
excess methanol and stirred for 2 h at room temperature. The mixture was then cooled to 0
°C. Carbon disulfide (1.04 g, 0.83 mL, 13.63 mmol) was added drop-wise and the mixture
stirred for 1 h. 50 ml of an aqueous solution of Mn(CH3COO)2.4H2O (1.6 g, 6.8 mmol) was
added drop-wise to the reaction mixture, which was stirred for 30 min to form a
brown/yellow solution. TMEDA (0.79 g, 6.76 mmol) was added to the solution while stirring
for 1 h to form a brown precipitates. The solid residue was filtered off and washed with
deionised water. The final product was dried in a vacuum oven for overnight. Then the
product was recrystallized from acetone. Yield: 83.5% (3.5g). Melting point: 138 °C.
Elemental analysis: Calc (%): C, 31.17; H, 5.76; S, 33.22; N, 7.27; Mn, 14.27. Found (%):
C, 30.98; H, 5.56; S, 33.22; N, 7.02; Mn, 13.94. IR (νmax/cm-1): 2995 (w), 1140-1193 (s),
1037(s).
3.4.3.3. Synthesis of [Mn(S2COEt)2.(TMEDA)], (2)
The complex 2 was prepared in the same way as mentioned in the 1, using excess ethanol
instead of methanol. Yield: 88 % (3.7g). Melting point: 137 °C. Elemental analysis: Calc
102
(%): C, 34.86; H, 6.34; S, 30.96; N, 6.78; Mn, 13.30. Found (%): C, 34.94; H, 6.28; S, 31.26;
N, 6.70; Mn, 13.01. IR (νmax/cm-1): 2980 (w), 1142-1185(s), 1032(s).
3.4.3.4. Synthesis of [Mn(S2COn-Pr)2.(TMEDA)], (3)
The complex 3 was prepared in the same way as 1, using n-propanol. Yield: 77.1% (3.8g).
Melting point: 134 °C. Elemental analysis: Calc (%): C, 38.09; H, 6.86; S, 29.00; N, 6.35;
Mn, 12.46. Found (%): C, 37.88; H, 6.67; S, 29.37; N, 6.12; Mn, 12.18. IR (νmax/cm-1): 2968
(w), 1145-1179 (s), 1043(s).
3.4.3.5. Synthesis of [Mn(S2COn-But)2.(TMEDA)], (4)
The complex 4 was prepared in the same way as 1, using n-butanol. Yield: 76.3% (4.1g).
Melting point: 85 °C. Elemental analysis: Calc (%): C, 40.93; H, 7.30; S, 27.26; N, 5.97;
Mn, 11.71. Found (%): C, 40.78; H, 7.15; S, 27.58; N, 5.61; Mn, 11.55. IR (νmax/cm-1): 2958
(w), 1043 (s), 1142-1180 (s).
3.4.3.6. Synthesis of [Mn(S2CO n-Pent)2.(TMEDA)], (5)
The complex 5 was prepared in the same way as 1, using 1-pentanol. Yield: 78.3% (4.5g).
Melting point: 65 °C. Elemental analysis: Calc (%): C, 43.45; H, 7.70; S, 25.73; N, 5.63;
Mn, 11.05. Found (%): C, 43.41; H, 7.69; S, 25.98; N, 5.42; Mn, 10.86. IR (νmax/cm-1): 2952
(w), 1040 (s), 1145-1180(s).
3.4.3.7. Synthesis of [Mn(S2CO n-Hex)2.(TMEDA)], (6)
The complex 6 was prepared in the same way as 1, using 1-hexanol. Yield: 82.9% (5.1g).
Melting point: 63 °C. Elemental analysis: Calc (%): C, 45.70; H, 8.06; S, 24.35; N, 5.33;
Mn, 10.46. Found (%): C, 45.30; H, 7.99; S, 24.32; N, 5.01; Mn, 10.20. IR (νmax/cm-1): 2952
(w), 1038 (s), 1142-1182 (s).
103
3.4.3.8. Synthesis of [Mn(S2CO n-Oct)2.(TMEDA)], (7)
The complex 7 was prepared in the same way as 1, using 1-octanol. Yield: 80.9% (5.5g).
Melting point: 60 °C. Elemental analysis: Calc (%): C, 49.55; H, 8.67; S, 22.00; N, 4.82;
Mn, 9.45. Found (%): C, 49.05; H, 8.42; S, 21.91; N, 4.65; Mn, 9.28.
3.4.3.9. Synthesis of MnS nanocrystals using hot injection thermolysis
The MnS nanocrystals were synthesized by dispersing (0.2 g) of manganese alkylxanthate
in 2.0 mL of trioctylphosphine (TOP) and then injected into 8.0 ml of pre-heated oleylamine
(OLA) at 230 °C with continuous stirring under nitrogen atmosphere. The temperature was
maintained at 230 oC for 30 min, after which the reaction mixture was removed from the
heating source for cooling. Then, precipitation was done by adding methanol (12.0 ml) into
the reaction mixture and then solid material was washed using methanol and separated by
centrifugation.
3.4.3.10. Synthesis of MnS nanocrystals using solvent-less thermolysis
Solvent-less thermolysis were performed by placing 0.4 g of the manganese alkylxanthate
in a ceramic boat under a stream of argon (300 cm3 min-1) in a furnace tube which was then
heated to 350 °C and the heating is continued at this temperature for 1 h. After cool down to
room temperature the MnS nanocrystals were collected for characterizations.
3.4.3.11. Deposition of MnS thin films using doctor blade technique
Thin films MnS were deposited on pre cleaned glass substrates using doctor blade technique.
In a typical deposition process, 0.02g of manganese alkylxanthate was be slurry in 0.2 ml of
Tetrahydrofuran (THF). As prepared complex slurry was pasted on the cleaned glass
substrate and distributed uniformly on the glass substrates using a sharp blade made up of
stainless steel to form wet thin films of MnS. The films were then placed in to furnace tube
which was then heated to 350 °C for 1h.
104
3.4.4. Results and discussion
3.4.4.1. Precursor crystal structures
We report here the synthesis and single-crystal structures of six novel manganese xanthates:
[Mn(S2COMe)2.TMEDA] (1), [Mn(S2COEt)2.TMEDA] (2), [Mn(S2COnPr)2.TMEDA] (3),
[Mn(S2COnBut)2.TMEDA] (4), [Mn(S2COnPen)2.TMEDA] (5), and
[Mn(S2COnHex)2.TMEDA] (6). We also prepared [Mn(S2COnOct)2.TMEDA] (7), which
was characterised only by X-ray diffraction analysis, elemental analyses and melting point.
These complexes were prepared from the reaction of a previously prepared potassium
alkylxanthate (from the insertion of CS2 into the relevant potassium alkoxide) and
manganese(II) acetate tetrahydrate with the subsequent addition of TMEDA. All the
complexes were soluble in common organic solvents such as chloroform, THF and toluene.
These complexes were stored at − 20°C to avoid premature decomposition. Crystals suitable
for X-ray analysis were grown from the slow evaporation of chloroform solution at room
temperature. The structures of the complexes are shown in Figure 3. 2. All 2, 3, 4, 6 and 7
adopt monoclinic crystal systems with space groups C2/c, P21/c, P21/c, I2/a and I2/a,
respectively, while 1 orthorhombic Pbca and 5 is triclinic P1.
In all cases, the central Mn ions were coordinated by 6 atoms, bound by two xanthate ligands
and single TMEDA ligand, with N and S donors arranged in a distorted octahedron.
Furthermore, no differences in Mn–S or Mn–N bond distances were observed in the
structures of 2, 6 and 7; therefore, the ligands were considered to be in a symmetric
(isobidentate) mode. However, the Mn–N bond distances in 5 were significantly different,
and a relatively small difference was observed in the cases of 1, 3 and 4, therefore, the ligands
were considered to be in an asymmetric mode.
The molecular structure of (2) [Mn(S2COCH2CH3)2 (TMEDA)] is shown in Figure 3. 2, and
the selected geometric parameters are presented in Table 3.S1. The shorter Mn–S and the
longest Mn–S bond lengths involving the xanthate ligands were 2.5645 and 2.6750 Å,
105
respectively, as listed in Table 3.S2, and were in good agreement with those reported for
other analogous 1:1 adducts of Mn–dithiocarbonato (xanthate) complexes.23
The symmetric mode of coordination of the xanthate ligands was reflected in the near
equivalence of the associated C–S bond distances. Within each of the xanthate ligands, the
shorter Mn–S bond had the S atom approximately trans to a N atom, and the two S atoms
forming the longer Mn–S bonds were approximately trans to each other. Another difference
between the structures was that the shortest Mn–N bond distance was observed in complex
6, and thus, the N–Mn–N angle was the shortest angle compared with other complexes.
As the alkyl chain length increased in the symmetrical structures, the difference in the
bonding modes of the two ligands became more obvious. The difference in the Mn–S bond
distances in the symmetrical binding decreased with an increase in the length of the alkyl
chain. The ∆(Mn–S) = (longer Mn–S bond distance − shorter Mn–S distance) values for the
remaining ligand were 0.11, 0.08 and 0.03 Å for 2, 6 and 7, respectively. In contrast, the
difference in the Mn–S bond distances in the asymmetric binding of 1, 3, 4 and 5 varied with
an increase in the length of the alkyl chain.
The relatively short C–O bond distances of 1.333 (2) Å for one ligand in complex 1, 2, 3, 4
and 5 were almost the same. However, in 6 and 7, the C–O bond distances were 1.361 (5)
and 1.341 (9) Å, respectively, which were longer than those of the other complexes. The
data shown in Table 3.S2 are consistent with a significant contribution of the resonance form
of the xanthate anion that features a formal C=O bond and the negative charges on each of
the S atoms. In the case of compound 2 the bidentate N-donor ligands had the same Mn–N1
and Mn–N2 distances (2.293 (15) Å). Because of the restricted ligand bite, the angles N–
Mn–N and S–Mn–S were lower than 90° in a regular octahedron. The N–Mn–N angles
averaged at approximately 79.22° (8) and S–Mn–S angles at 69.10° (15), as shown in Table
3.S2. The molecular structures of other novel complexes are shown in Figure 3. 2, and
selected bond distances and angles are given in Table 3.S2.
106
The most distinct difference between these compounds was how the ligand frameworks and
the presence of hydrogen bonds affected the crystal packing in the extended solid state for
all of these complexes. As shown in Figure 3.S1, all the complexes displayed intermolecular
hydrogen bonds through the sulfur atoms of the neighbouring molecules (C–HS), except
complex 5, wherein the (C–HS) interaction was not observed. The distances of these
interactions were slightly shorter than the sum of the contact radii (van der Waals radii),53
as shown in Table 3.S3.
Furthermore, 2, 3 and 6 had two main modes of association between molecules, one of them
was the H from the adduct contact with the S from the other molecule (N–C–HS) and the
H from the alkyl group contact with S from the other molecule (C–C–HS), as shown in
Figure 3.S1. In contrast, 1 and 7 had one mode of association between molecules, which was
the (N–C–HS) interaction in 1 and 4 and the (C–C–HS) interaction in complex 7. The
complex 5 also exhibited interchelate distances between S from the molecule and S from
another molecule (3.491 Å).
107
Figure 3. 2. The molecular structures of the manganese xanthates. [Mn(S2COMe)2.TMEDA] (1),
[Mn(S2COEt)2.TMEDA] (2), [Mn(S2COnPr)2.TMEDA] (3), [Mn(S2COnBut)2.TMEDA] (4),
[Mn(S2COnPen)2.TMEDA] (5), [Mn(S2COnHex)2.TMEDA] (6) and [Mn(S2COnOct)2.TMEDA] (7).
H atoms are omitted for clarity. Violet = Mn, yellow = S, red = O, blue= N, grey = C.
108
3.4.4.2. Thermogravimetric analysis of molecular precursors 1–7
Thermal analyses of all the complexes were conducted up to 600°C under a nitrogen
atmosphere. Complexes 2, 3 and 4 decomposed cleanly in one step to form MnS at 150–
350°C, while 1 decomposed around 200 °C. With increasing alkyl chain length, the TGA
profiles for 5 and 6 changed from a single step to a two-step breakdown. The inset picture
in figure 3.3 shows the first thermal decomposition step for 5 and 6 at approximately 150
°C, which was similar to the single decomposition of 2. In the case of 5 and 6 precursors,
the mass residue obtained from the TGA profiles for the first decomposition stage (57.5%)
agrees with the theoretical value calculated for the removal of one molecule of xanthate and
half from another one (58%). While in the secondary step, there is a mass loss in the
temperature range of 250°C to 350 °C that is consistent with the theoretical value for
production of MnS. All the six complexes gave the final solid residue amounts that matched
with the calculated value for MnS. The comparison for the final experimental residues and
the calculated values for MnS are given in Table 3.S4.
Figure 3. 3. Thermogravimetric analysis (TGA) profiles of complexes (1-6) and inset picture for
complexes (5 and 6).
109
3.4.4.3. MnS nanocrystals using hot-injection thermolysis
MnS nanocrystals were synthesized using hot-injection of dispersing manganese
alkylxanthate in trioctylphosphine (TOP) and then injected into pre-heated oleylamine
(OLA) at 230 °C. The OLA is used as a coordinating agent and often catalyses the
degradation of complexes at lower temperatures than the other methods.54,55 During the
optimization and the case of complex 2 we have observed that the complex showed no sign
of decomposition below 190°C, whereas at 200°C, complex 2 decomposed partially, leading
to poor crystallinity in the resulting materials as shown in Figure 3.S3. For this reason a
higher temperature has been applied for the decomposition of all complexes.
At 250°C, the complete decomposition of the complexes occurred yielding products with
good crystallinity, as shown by the p-XRD measurements. The p-XRD pattern of all the
samples prepared using hot-injection are shown in Figure 3.4. Diffraction peaks at 2θ values
of 29.62, 34.33, 49.35, 58.62, 61.45, and 72.37 correspond to the (111), (200), (220),
(311), (222) and (400) planes, respectively, of the cubic α–MnS (JCPDS 03−065−0891).56
The intensity profile of the p-XRD pattern also matched well with the standard pattern, with
the highest intensity peak being the (200) plane with the 2θ value of 34.33°. The pattern
showed significant changes in the intensity of peaks depending on the chain length. In the
case of complexes from 2 to 4 the change in intensity was more obvious along the (220)
plane, which showed a decrease in the peak intensity with an increase in the chain length.
The Scherrer equation was used to estimate the crystallite size of the MnS nanocrystals as
shown in Table 3.S5. Kan et al. have successfully synthesized a cubic α-MnS nanocrystals
of different sizes ranging from 20 to 80 nm by a colloidal synthesis route through the reaction
of MnCl2 and S[Si(CH3)3]2 in trioctylphosphineoxide.57 The lattice parameters were
calculated using the p-XRD data, unit cell, volume (V) of the cells for all the samples are
given in Table 3.S5.
110
Figure 3. 4. P-XRD patterns of MnS prepared at 250 °C via hot injection from precursors 1-
6.The standard pattern ( black sticks) is cubic α–MnS (ICDD No. 03-065-0891).56
The atomic percentage of all the elements present in the sample synthesized using the hot-
injection was determined by EDX spectroscopy (Figure 3.S4) and the atom percentages
observed from the EDX spectra suggest manganese sulfide was formed in agreement with
the XRD measurements (ESI Figure 3.S4,Table 3.S5).
The morphology of the synthesized MnS nanocrystals was observed by the SEM analysis
and images are shown in Figure 3.5. All the images are obtained at the same magnification
for comparison. The MnS nanocrystals obtained from 1-5 are irregularly shapes (Figure
3.5.(a to e)), while those obtained from complex 6 have spherical morphology (Figure 3.5.f).
111
Figure 3. 5. Representative secondary electron SEM images (10 kV) of MnS samples prepared using
precursor (a-f) (1-6) prepared by hot injection thermolysis at 250 °C, taken at magnification of 1µm.
Figure 3.S5 shows micro-Raman spectra of the MnS nanocrystals prepared by the hot-
injection. The Raman spectra revealed that the MnS synthesised from precursor 1 exhibited
a single band at 635.89 cm−1 which is corresponding to the strong photoluminescence band
as earlier reported. This peak is approximately at the same energy when the chain length was
increased in the cases of 2, 3, 4, 5 and 6, as shown in Figure 3.S5 (Table 3.S5). Similar
results have been previous reported in the literature.58,59
3.4.4.4. MnS nanocrystals using solvent-less thermolysis
MnS nanocrystals NCs were also synthesized using solvent-less thermolysis. This route is
an unexpansive and simple toward the production of nanocrystals semiconductor materials.
Generally, this method produces high yields in comparison with other method for instance
hot injection. The procedure involved the placement some grams of the complexes in a
112
ceramic boat in a furnace tube which was then heated for 1 h. The α-MnS NCs were obtained
after cool down to room temperature.
The NCs obtained from complex (2) [Mn(S2COEt)2.(TMEDA)] at different temperatures
were analysed using XRD to identify the best temperature for obtaining the good
crystallinity of MnS. At low temperatures of 250 and 300 °C, we found that the complete
conversion of the precursor had not occurred as evidenced by XRD (Figure 3.S7), while at
350 °C, complete conversion of the complex 2 occurred giving products with good
crystallinity.
Therefore, all the precursors were decomposed at 350°C, which resulted in highly
crystalline products. With longer heating times (1 h), all six precursors (1−6) generated MnS,
with a pattern that matched that of the cubic α–MnS (JCPDS 03−065−0891, Figure 3.6).56
The diffraction peaks observed at 2 values of 29.6°, 34.4°, 49.3°, 56.8°, 61.5° and 72.6°
correspond to the (111), (200), (220), (311), (222) and (400) planes of cubic α–MnS,
respectively. The unit cell parameters for the NCs are in good agreement with the previously
reported values for the bulk phase.49 The size of the crystalline was estimated using the
domains Debye–Scherrer equation as shown in Table 3.S6. These sizes are smaller than the
sizes of the nanocrystals obtained by using the hot-injection thermolysis.
113
Figure 3. 6. P-XRD patterns of MnS prepared at 350 °C via solvent-less thermolysis of precursors
(1-6). The standard pattern is cubic manganese sulfide, MnS (ICDD No. 03-065-0891).56
The SEM images of the MnS nanocrystals grown from the precursors using solvent-less
thermolysis at a scale bar of 1-μm are shown in Figure 3.7. For precursors 1 and 2, the
nanocrystals were found to be well-defined and quasi-spherical (Figures 7(a, b)). For
precursors 3 to 6 the products are irregular in appearance with some agglomeration (Figure
3.7.(c–f)). Magnified images (scale bar of 5 μm) are shown in Figure 3.S8. The EDX
spectroscopy of the nanocrystals has been done to quantity atomic percentage. The atomic
percentage of the NCs obtained from EDX from all the precursors (1-6) are shown in Figure
3.S9, Table 3.S6.
114
Figure 3. 7. Representative secondary electron SEM images (10 kV) of MnS samples using precursor
(a-f) (1-6) prepared by a solvent-less thermolysis at 350 °C.
Figure 3.S10 shows the micro-Raman spectra of the MnS nanocrystals prepared using the
solvent-less thermolysis. The Raman spectra revealed that the MnS synthesised from
precursor 1 exhibited a band at 635.18 cm−1 which is corresponding to the strong
photoluminescence band as earlier reported. This peak was almost the same as that obtained
by increasing the chain length of 2, 3, 4, 5 and 6 (Table 3.S6). The similar observation for
raman data has been previously reported in the literature.58,59
115
3.4.4.5. MnS thin films by Doctor Blading
MnS thin films were deposited using doctor’s blade using manganese alkylxanthate
complexes. For the preparation of MnS thin films, MnS thin films was prepared by the
incremental addition of THF to the precursor to make a uniform and lump-free paste. Thus
prepared uniform slurry was coated onto a glass substrate by a doctor blade technique. After
natural drying at room temperature, the thin films were annealed at 350°C for 1 h.
The p-XRD pattern of MnS thin films obtained from all the precursors are shown in Figure.
8. The diffraction peaks of the thin films prepared from complexes 1–6 were indexed to the
cubic manganese sulfide α–MnS (ICDD # 03-065-0891)56. The pattern showed significant
changes in the intensity of the peaks depending on the chain length. This change in the
intensity was indicated that the preferred orientation along the (200) and (220) plane at 2θ =
34.4° and 49.4°, respectively, which showed an increase in the peak intensity with an
increase in the chain length. Crystallites that have preferred orientation have been observed
in other thin films grown by chemical bath deposition technique.59 This suggests that the
substrate or molecular precursor structure may apply control over the nucleation and growth
kinetics of manganese sulfide thin films under these conditions. The grain size of the
crystallites was estimated as shown in Table 3.S7. These sizes of the crystallites are almost
the same as the sizes of the nanocrystals obtained by using the hot injection thermolysis but
are larger than those of the NCs obtained using the solvent-less thermolysis.
116
Figure 3. 8. P-XRD patterns of MnS thin films prepared at 350 °C Deposition by the doctor blade
method from precursor (1-6). The standard pattern is cubic manganese sulfide, MnS (ICDD No. 03-
065-0891).56
The SEM images of the MnS thin films obtained from all the precursors are shown in Figure
3.9. (a-d). For all precursors, the deposited thin films consisted of cube-like MnS crystals
deposited randomly on the surface of the glass substrate at the 1-µm magnification. The
morphology shown by the films deposited by using the doctor blade method was clearly
different than that of the samples obtained by the hot-injection or the solvent-less
thermolysis. The compositional analyses performed using EDX spectroscopy (Figure 3.S11)
revealed the presence of manganese and sulfur in all thin film samples. Table 3.S7 lists the
atomic percentage of each element, the cubic lattice parameter and the crystallite size.
117
Figure 3. 9. Representative secondary electron SEM images (10 kV) of MnS thin films using
precursor (a-f) (1-6) deposited by the Doctor Blade method at 350 °C.
The Figure 3.S12 shows the micro-Raman spectra of MnS thin films prepared using the
doctor blade technique. The Raman spectra revealed that the MnS thin films obtained from
the precursors 1, 2, 3, 4, 5 and 6 exhibited bands at 635.89 cm−1, 636.60 cm−1, 636.21 cm−1,
635.18 cm−1, 635.89 cm−1 and 636.92 cm−1, respectively (Table 3.S7). These peak positions
are corresponding to the strong photoluminescence band as earlier reported.58,59
3.4.4.6. Magnetic properties of MnS nanocrystal
The successful synthesis of α-MnS nanocrystals lets us to study their magnetic properties
and the α-MnS which was obtained from complex 2 was the only one that used to study the
magnetic properties because of the similarity of the other samples. The room temperature X-
band EPR spectrum of -MnS NCs obtained from complex 2 displays a strong signal with
g = 2.003, characteristic of magnetic nanocrystals (Figure 3.10). The magnetisation of the
nanocrystals was measured as a function of temperature, in field cooled (FC) and zero-field-
cooled (ZFC) regimes, under the applied field of 100 Oe (Figure 3.11). To compare with
118
different reports, the magnetic properties of α-MnS NCs have been investigated by Kan et
al. where different sizes between of 20 and 80 nm, α-MnS NCs were antiferromagnetic AFM
with reduced interaction strength in smaller NCs. However, those NCs actually were
aggregates with smaller of particles, which led to that their hysteresis loop is closed.57 Puglisi
et al. reported the magnetic properties of single-crystal octahedral α-MnS NCs of different
size (14, 20, and 29 nm). Below 50 K the NCs showed increased in (FC) magnetization and
a maximum of the (ZFC) magnetization at 25 K, which are both confirmed of a transition
between a superparamagnetic (SPM) and ferromagnetic (FM) type.49
Figure 3. 10. X-band EPR spectrum of -MnS NCs obtained from complex 2.
119
Figure 3. 11. Thermal dependence of the magnetisation for -MnS NCs obtained from complex 2,
measured in zero-field cooled (ZFC) (red circles) and field-cooled (FC) (black squares) regimes, with
the difference MFC-MZFC plotted in blue. Insert: Plot of –d(MFC-MZFC)/dT for the same nanocrystals.
Irreversible magnetic behaviour is observed below 40 K, which marks a transition from the
SPM to FM, the latter characterised by blocking of the magnetisation. The presence of FM-
like regions in the material is also evident in the T-dependence of the magnetisation
difference in Figure 3.11 (blue triangles). Above 70 K, the ZFC and FC magnetisation curves
fully superpose and data could be fitted to a Curie-Weiss law, = C/(T-), providing a Curie-
Weiss constant = -254 K (Figure 3.12).
The negative sign of indicates that the α-MnS nanocrystals obtained from 2 is
antiferromagnetic. The value is less negative than the bulk value of -465 K,60 and close to
the value that reported by Puglisi et al where = -272 K for 29 nm.49 Then, the AFM
interactions become less effective for smaller NCs, in approximately agreement with
previous results. The existence of the FM structure at the surface of the α-MnS NCs is
additionally supported by the hysteresis measured at 5 and 300 K, as shown in Figure 3.13.
At 300 K the saturated magnetization was smaller than 5 K, and there was no hysteresis
loops. The hysteresis curve recorded at 5 K shows that the magnetisation does not saturate
120
up to the magnetic field of 70 kOe, indicative of large anisotropy. Cycling of the
magnetisation between 70 kOe and -70 kOe reveals a hysteresis loop with a coercive Hc field
of 0.723 KOe. This field is larger than observed for similar nanocrystals. Puglisi et al.
confirmed that α-MnS NCs samples were obtained by the isothermal magnetization at 5 K
showed an open loop with size-dependent Hc = 0.009 kOe (14 nm), 0.081 kOe (20 nm),
0.180 kOe (29 nm).49 Yang et al. reported that at low-temperature hysteresis loops was
presented in the FM region since they display open loops withsize-dependent Hc ranging
from 0.01 kOe (14 nm) to 1.265 kOe (40 nm).61 So, this result is to the best of our knowledge,
the first demonstration of a large coercive field (0.723 kOe at 5 K with small size of 8.2 nm)
in α-MnS nanocrystals. It is noted that the magnetization of FM materials is depends on the
size, shape, and structure of these materials.62
Figure 3. 12. Plot of 1/ versus temperature for -MnS NCs obtained from complex 2, measured in
zero-field cooled (ZFC) (red) and field-cooled (FC) (black) regimes, with a fit to the Curie law =
C/(T-) presented in blue (dashed lines).
121
Figure 3. 13. Magnetic hysteresis at 5 and 300 K for -MnS NCs obtained from 2. The inset shows
the region around zero fields.
3.4.5. Conclusion
The synthesis and the single-crystal X-ray structure of seven novel
tetramethylethylenediamine manganese(II) bis(alkylxanthate) complexes [methylxanthate
(1), ethylxanthate (2), n-propylxanthate (3), n-butylxanthate (4), n-pentylxanthate (5), n-
hexylxanthate (6) and n-octylxanthate (7)] were reported. Complexes 2, 3, 4, 6 and 7 adopted
a monoclinic crystal system, while 1 was orthorhombic and 5 was triclinic. All the
compounds displayed intermolecular hydrogen bonds through the sulfur atoms of the
neighbouring molecules (C–HS), except complex 5, wherein the (C–HS) interaction was
not observed. The distances of these interactions were slightly shorter than the sum of the
contact radii (van der Waals radii). Furthermore, 4 and 5 exhibited intramolecular S-S
distances of 3.491 Å and 3.565 Å, respectively. The decomposition of the complexes was
studied using TGA measurements. The series of alkyl–xanthato manganese (II) complexes
were found to change from a single-step decomposition pathway to a two-step pathway with
an increase in the alkyl chain length. The two-step pathway implied that the decomposition
122
of one ligand occurred before that of the other. These six complexes were tested as single-
source precursors for the formation of MnS nanocrystals. The OLA used as the capping
agent in the hot injection thermolysis helped in the decomposition of the complexes at lower
temperatures, whereas the solvent-less thermolysis and the doctor blade technique required
a high decomposition temperature of 350°C. The XRD studies showed that all the precursors
broke down cleanly at 250 and 350°C by the hot-injection, solvent-less and doctor blade
methods, respectively, to form cubic rock-salt (RS) α-MnS. In comparison, the pattern
obtained by using the hot-injection and the solvent-less thermolysis were showed significant
changes in the intensity of the peaks depending on the chain length. This change in intensity
was more obvious along the (220) plane, which indicated a decrease in the peak intensity
with an increase in the chain length. In contrast, the peak intensity obtained by using the
doctor blade increased with an increase in the chain length. Moreover, the size of the α-MnS
nanocrystals synthesised using the hot-injection and the doctor blade methods was higher
than that of the nanocrystals synthesised by using the solvent-less thermolysis. The Raman
peaks were almost the same when the chain length was increased and was in good agreement
with that reported by previous studies. The magnetic measurements display the nanocrystals
have the ferromagnetic behaviour and large coercive field (0.723 kOe for 8.2 nm
nanocrystals). This report provides easy approaches to combine α-MnS nanocrystal with
suitable magnetic properties, which might have potential applications for the short
wavelength magneto-optical nanocrystal in the future.
3.4.6. Acknowledgements
A. Alanazi is thankful to the Ministry of Higher Education in Saudi Arabia for funding and
the University of Islamic, Saudi Arabia for permission to study in the United Kingdom. We
thank Dr P.D. McNaughter and Salman Alanazi for useful comments. We acknowledge the
123
EPSRC National EPR Facility at the University of Manchester for support with magnetic
and EPR measurements.
3.4.7. References
1 X. Wang, X. Ren, K. Kahen, M. A. Hahn, M. Rajeswaran, S. Maccagnano-Zacher, J.
Silcox, G. E. Cragg, A. L. Efros and T. D. Krauss, Nature, 2009, 459, 686–689.
2 M. Li, S. K. Cushing, Q. Wang, X. Shi, L. A. Hornak, Z. Hong and N. Wu, J. Phys.
Chem. Lett., 2011, 2, 2125–2129.
3 C. J. Murphy and N. R. Jana, Adv. Mater., 2002, 14, 80–82.
4 H. M. Joshi, Y. P. Lin, M. Aslam, P. V. Prasad, E. A. Schultz-Sikma, R. Edelman, T.
Meade and V. P. Dravid, J. Phys. Chem. C, 2009, 113, 17761–17767.
5 L. Qu, Z. A. Peng and X. Peng, Nano Lett., 2001, 1, 333–337.
6 M. Y. Guo, A. M. C. Ng, F. Liu, A. B. Djurišić, W. K. Chan, H. Su and K. S. Wong, J.
Phys. Chem. C, 2011, 115, 11095–11101.
7 R. E. Algra, M. Hocevar, M. A. Verheijen, I. Zardo, G. G. W. Immink, W. J. P. van
Enckevort, G. Abstreiter, L. P. Kouwenhoven, E. Vlieg and E. P. A. M. Bakkers, Nano
Lett., 2011, 11, 1690–1694.
8 E. Muthuswamy, P. R. Kharel, G. Lawes and S. L. Brock, ACS Nano, 2009, 3, 2383–
2393.
9 J. Li and Wang, Nano Lett., 2003, 3, 1357–1363.
10 J. Joo, S. G. Kwon, T. Yu, M. Cho, J. Lee, J. Yoon and T. Hyeon, J. Phys. Chem. B,
2005, 109, 15297–15302.
11 D.-S. Wang, W. Zheng, C.-H. Hao, Q. Peng and Y.-D. Li, Chem. Eur. J., 2009, 15,
1870–1875.
12 V. L. Colvin, M. C. Schlamp and A. P. Alivisatos, Nature, 1994, 370, 354.
13 N. Tessler, V. Medvedev, M. Kazes, S. Kan and U. Banin, Science, 2002, 295, 1506–
1508.
14 B. Sun, E. Marx and N. C. Greenham, Nano Lett., 2003, 3, 961–963.
15 W. Han, L. Yi, N. Zhao, A. Tang, M. Gao and Z. Tang, J. Am. Chem. Soc., 2008, 130,
13152–13161.
16 W. C. W. Chan and S. Nie, Science, 1998, 281, 2016–2018.
17 Q. Wang, Y. Xu, X. Zhao, Y. Chang, Y. Liu, L. Jiang, J. Sharma, D.-K. Seo and H. Yan,
J. Am. Chem. Soc., 2007, 129, 6380–6381.
124
18 M. Bruchez, M. Moronne, P. Gin, S. Weiss and A. P. Alivisatos, Science, 1998, 281,
2013–2016.
19 W. U. Huynh, J. J. Dittmer and A. P. Alivisatos, Science, 2002, 295, 2425–2427.
20 M. A. Malik, M. Afzaal and P. O’Brien, Chem. Rev., 2010, 110, 4417–4446.
21 J. Akhtar, M. Afzaal, M. A. Vincent, N. A. Burton, J. Raftery, I. H. Hillier and P.
O’Brien, J. Phys. Chem. C, 2011, 115, 16904–16909.
22 R. A. Hussain, A. Badshah, M. D. Khan, N. Haider, B. lal, S. I. Khan and A. Shah,
Mater. Chem. Phys., 2015, 159, 152–158.
23 M. V. Câmpian, I. Haiduc and E. R. T. Tiekink, J. Chem. Crystallogr., 2010, 40, 1029–
1034.
24 M. Al-Shakban, P. D. Matthews, X. L. Zhong, I. Vitorica-Yrezabal, J. Raftery, D.
J. Lewis and P. O’Brien, Dalton Trans., 2018, 47, 5304–5309.
25 A. A. K. Bakly, B. F. Spencer and P. O’Brien, J. Mater. Sci., 2018, 53, 4360–4370.
26 M. Al-Shakban, P. D. Matthews, N. Savjani, X. L. Zhong, Y. Wang, M. Missous and P.
O’Brien, J. Mater. Sci., 2017, 52, 12761–12771.
27 C. A. Téllez S., A. C. Costa, M. A. Mondragón, G. B. Ferreira, O. Versiane, J. L. Rangel,
G. M. Lima and A. A. Martin, Spectrochim. Acta. A. Mol. Biomol. Spectrosc., 2016, 169,
95–107.
28 M. Lazell, P. O’Brien, D. J. Otway and J.-H. Park, Dalton Trans., 2000, 4479–4486.
29 S. L. Castro, S. G. Bailey, R. P. Raffaelle, K. K. Banger and A. F. Hepp, J. Phys. Chem.
B, 2004, 108, 12429–12435.
30 A. M. Alanazi, F. Alam, A. Salhi, M. Missous, A. G. Thomas, P. O’Brien and D.
J. Lewis, RSC Adv., 2019, 9, 24146–24153.
31 S. A. Alderhami, D. Collison, D. J. Lewis, P. D. McNaughter, P. O’Brien, B. F. Spencer,
I. Vitorica-Yrezabal and G. Whitehead, Dalton Trans., 2019, 48, 15605-15612.
32 L. Almanqur, F. Alam, G. Whitehead, I. Vitorica-yrezabal, P. O’Brien and D. J. Lewis,
J. Cryst. Growth, 2019, 522, 175–182.
33 T. Alqahtani, R. J. Cernik, P. O’Brien and D. J. Lewis, J. Mater. Chem. C, 2019, 7,
5112–5121.
34 C. An, K. Tang, X. Liu, F. Li, G. Zhou and Y. Qian, J. Cryst. Growth, 2003, 252, 575–
580.
35 S. Biswas, S. Kar and S. Chaudhuri, Mater. Sci. Eng. B, 2007, 142, 69–77.
36 S. P. Freidman and V. A. Gubanov, J. Phys. Chem. Solids, 1983, 44, 187–194.
125
37 D. Fan, X. Yang, H. Wang, Y. Zhang and H. Yan, Phys. B Condens. Matter, 2003, 337,
165–169.
38 B. Piriou, J. Dexpert-Ghys and S. Mochizuki, J. Phys. Condens. Matter, 1994, 6, 7317–
7327.
39 R. Tappero, P. D’Arco and A. Lichanot, Chem. Phys. Lett., 1997, 273, 83–90.
40 X. Yang, Y. Wang, K. Wang, Y. Sui, M. Zhang, B. Li, Y. Ma, B. Liu, G. Zou and B.
Zou, J. Phys. Chem. C, 2012, 116, 3292–3297.
41 O. Kavcı and S. Cabuk, Comput. Mater. Sci., 2014, 95, 99–105.
42 X. Yu, C. Li-yun, H. Jian-feng, L. Jia, F. Jie and Y. Chun-yan, J. Alloys Compd., 2013,
549, 1–5.
43 Y. Zhang, H. Wang, B. Wang, H. Yan and M. Yoshimura, J. Cryst. Growth, 2002, 243,
214–217.
44 C. Gümüş, C. Ulutaş, R. Esen, O. M. Özkendir and Y. Ufuktepe, Thin Solid Films, 2005,
492, 1–5.
45 Y. Ren, L. Gao, J. Sun, Y. Liu and X. Xie, Ceram. Int., 2012, 38, 875–881.
46 J. Jiang, R. Yu, J. Zhu, R. Yi, G. Qiu, Y. He and X. Liu, Mater. Chem. Phys., 2009, 115,
502–506.
47 S. Wang, K. Li, R. Zhai, H. Wang, Y. Hou and H. Yan, Mater. Chem. Phys., 2005, 91,
298–300.
48 J. Joo, H. B. Na, T. Yu, J. H. Yu, Y. W. Kim, F. Wu, J. Z. Zhang and T. Hyeon, J. Am.
Chem. Soc., 2003, 125, 11100–11105.
49 A. Puglisi, S. Mondini, S. Cenedese, A. M. Ferretti, N. Santo and A. Ponti, Chem.
Mater., 2010, 22, 2804–2813.
50 Q. Tian, M. Tang, F. Jiang, Y. Liu, J. Wu, R. Zou, Y. Sun, Z. Chen, R. Li and J. Hu,
Chem. Commun., 2011, 47, 8100–8102.
51 G. M. Sheldrick, Acta Crystallogr. Sect. C Struct. Chem., 2015, 71, 3–8.
52 P. D. McNaughter, S. A. Saah, M. Akhtar, K. Abdulwahab, M. Azad Malik, J. Raftery,
J. A. M. Awudza and P. O’Brien, Dalton Trans., 2016, 45, 16345–16353.
53 A. Bondi, J. Phys. Chem., 1964, 68, 441–451.
54 M. D. Khan, M. Akhtar, M. A. Malik, N. Revaprasadu and P. O’Brien, ChemistrySelect,
2018, 3, 2943–2950.
55 N. Mntungwa, M. D. Khan, S. Mlowe and N. Revaprasadu, Mater. Lett., 2015, 145,
239–242.
126
56 J. Ning, D. Zhang, H. Song, X. Chen and J. Zhou, J. Mater. Chem. A, 2016, 4, 12098–
12105.
57 S. Kan, I. Felner and U. Banin, Isr. J. Chem., 2001, 41, 55–62.
58 N. S. Arul, J. I. Han and D. Mangalaraj, J. Mater. Sci. Mater. Electron., 2018, 29, 1636–
1642.
59 T. Dhandayuthapani, M. Girish, R. Sivakumar, C. Sanjeeviraja and R. Gopalakrishnan,
Appl. Surf. Sci., 2015, 353, 449-4585.
60 L. Corliss, N. Elliott and J. Hastings, Phys. Rev., 1956, 104, 924–928.
61 X. Yang, Y. Wang, Y. Sui, X. Huang, T. Cui, C. Wang, B. Liu, G. Zou and B. Zou,
Langmuir, 2012, 28, 17811–17816.
62 J. Lian, X. Duan, J. Ma, P. Peng, T. Kim and W. Zheng, ACS Nano, 2009, 3, 3749–3761.
127
3.4.8. Supporting Information
Table 3.S 1. X-ray crystallographic data and refinement details for (1-7) using Cu K radiation and
with H-atom parameters constrained.
Complex (1) (2) (3) (4) (5) (6) (7)
Chemical
formula
C10H22MnN2O2S4 C12H26MnN2O2S4 C14H30MnN2O2S4 C16H34MnN2O2S4 C18H38MnN2O2S4 C20H42MnN2O2S4 C24H50MnN2O2S4
Mr 385.47 413.53 441.58 469.63 497.68 525.73 581.84
Crystal
system,
space group
Orthorhombic,
Pbca
Monoclinic,
C2/c
Monoclinic,
P21/c
Monoclinic,
P21/c
Triclinic,
P¯1
Monoclinic,
I2/a
Monoclinic,
I2/a
Temperature
(K)
293 100 100 150 100 240 150
a, (Å)
b, (Å)
c, (Å)
15.2336 (8)
16.3399 (7)
13.8528 (7)
20.8959 (13)
8.0893 (4)
15.3732 (9)
11.8338 (5)
11.9042 (4)
15.5003 (6)
12.5433 (3)
21.2751 (5)
9.3308 (2)
7.5898 (4)
11.8599 (5)
16.0804 (7)
15.6601 (3)
8.15454 (13)
23.5223 (5)
13.8999 (11)
8.5486 (6)
27.052 (3)
(°)
(°)
(°)
90
90
90
132.491 (7)
90
90
106.751 (2)
90
102.885 (2)
70.704 (4),
78.932 (4),
72.591 (4)
108.507 (2)
97.371 (10)
V (Å3) 3448.2 (3) 1916.1 (2) 2090.90 (14) 2427.32 (10) 1296.60 (11) 2848.48 (10) 3187.9 (5)
Z 8 4 4 4 2 4 4
(mm-1) 10.75 9.71 8.94 7.73 7.26 6.64 5.98
Crystal size
(mm)
0.3 × 0.1 × 0.04 0.22 × 0.14 ×
0.06
0.24 × 0.13 ×
0.06
0.16 × 0.11 ×
0.01
0.20 × 0.07 ×
0.01
0.46 × 0.31 ×
0.02
0.4 × 0.35 × 0.1
Tmin, Tmax 0.282, 1.000 0.504, 0.593 0.469, 0.616 0.353, 1.000 0.743, 1.000 0.445, 1.000 0.664, 1.000
No. of
measured,
independent
and
observed [I
> 2(I)]
reflections
15125, 3288,
2271
6616, 1868, 1670
16722, 4099,
3538
11697, 4724,
3860
14258, 4999,
4661
29808, 2598,
2422
10324, 3236,
2083
Rint 0.096 0.045 0.053 0.039 0.034 0.042 0.065
(sin /)max
(Å-1)
0.623 0.617 0.617 0.617 0.617 0.602 0.628
R[F2 >
2(F2)],
wR(F2), S
0.070, 0.223,
1.05
0.026, 0.063,
1.05
0.028, 0.066,
1.02
0.037, 0.101,
1.06
0.031, 0.079,
1.06
0.068, 0.183,
1.10
0.099, 0.334,
1.13
No. of
reflections
3288 1868 4099 4724 4999 2598 3236
No. of
parameters
178 99 214 291 250 191 180
max, min
(e Å-3)
1.78, -0.73 0.34, -0.22 0.34, -0.24 0.54, -0.45 0.30, -0.47 1.34, -0.45 0.66, -0.80
128
Table 3.S 2. Selected Bond Lengths (Å) and Angles (o) for novel complexes (1-7).
Table 3.S 3. Details of selected intermolecular non-covalent contacts (Å) in the prepared compounds
(1-7).
Complexes N–C–HS interactions
distance
C–C–HS interactions
distance
1 2.886 a) -
2 2.840 a) 2.842 a)
3 2.894 a) 2.970 a)
4 2.997 a) -
5 - -
6 2.927 a) 2.885 a)
7 - 2.756 a)
Sum of the contact radii = 3.00 53
Reference:
53. A. Bondi, J. Phys. Chem., 1964, 68, 441–451.
129
Figure 3.S 1. Crystal structures of 1, 2, 3, 4, 5, 6 and 7 showing intermolecular C–H⋯S non-covalent
contacts and S⋯S interactions.
130
Table 3.S 4. Elemental and thermal analyses of xanthates diaminemanganese(II) complexes 1 - 7.
Complexes Elements analysis : Calc
(found) %
M. Pt
(oC)
Temperature
of TGA (oC)
Mass loss
(%)
(1)
(2)
(3)
(4)
(5)
(6)
(7)
3.4.8.1. Infra-red spectroscopy
The Figure 3.S2 shows the FTIR spectra of all the complexes. The vC=S and vC–O–C are
the two important bands of the xanthate moiety because the additional π–electron flows from
the oxygen atom to the sulfur atoms via a planar delocalized π–orbital system. The IR spectra
of the as-synthesized complexes [Mn(S2COMe)2.TMEDA] (1), [Mn(S2COEt)2.TMEDA]
(2), [Mn(S2COnPr)2.TMEDA] (3), [Mn(S2COnBut)2.TMEDA] (4),
[Mn(S2COnPFen)2.TMEDA] (5) and [Mn(S2COnHex)2.TMEDA] (6) revealed that the vC=S
vibration was at approximately 1034‒1046 cm−1, while the band around 1140‒1190 cm−1
was attributable to the stretching vibrations of the v(C–O–C) asymmetric group, as shown
in Figure 3. S2. Moreover, as reported by Bonati and Ugo et al. for analogous
dithiocarbamate complexes, the vC–S stretching frequencies may be used to distinguish
between the monodentate and the bidentate behaviours of the 1,1-dithiolate ligands. In the
C H S N Mn (Calc.)
Found
31.17 5.76 33.22 7.27 14.27 138 200 - 350 (22.6)
(30.98) (5.56) (33.22) (7.02) (13.94) 24.9
34.86 6.34 30.96 6.78 13.30 137 200 - 350 (21.1)
(34.94) (6.28) (31.26) (6.70) (13.01) 24.4
38.09 6.86 29.00 6.35 12.46 134 200 - 350 (19.7)
(37.88) (6.67) (29.37) (6.12) (12.18) 20.5
40.93 7.30 27.26 5.97 11.71 85 200 - 350 (18.5)
(40.78) (7.15) (27.58) (5.61) (11.55) 19.9
43.45 7.70 25.73 5.63 11.05 65 200 - 350 (17.5)
(43.41) (7.69) (25.98) (5.42) (10.86) 17.77
45.70 8.06 24.35 5.33 10.46 63 200 - 350 (16.5)
(45.30) (7.99) (24.32) (5.01) (10.20) 17.36
49.55 8.67 22.00 4.82 9.45 60 - - (49.05) (8.42) (21.91) (4.65) (9.28)
131
case of monodentate dithiolate ligands, a doublet peak appeared around 1000 cm−1 separated
by ≥20 cm−1, which could be attributed to the non-equivalence of two C=S stretching
vibrations.54 In contrast, in the case of bidentate dithiolate ligands, a strong singlet was
observed in the ~1000 cm−1 region, which was indicative of a symmetrically bound
dithiolate moiety. In the present series of manganese complexes, we observed only one
strong band at approximately 1030 cm−1, which indicated that all the xanthate ligands were
bidentate and symmetrically bonded.
Figure 3.S 2. IR spectra of manganese alkyl xanthate precursors (1-6).
132
3.4.8.2. Manganese sulfide nanoparticles by the hot injection thermolysis:
Figure 3.S 3. The XRD patterns of manganese sulfide nanoparticles prepared by hot-injection from
[Mn(S2COEt)2(TMEDA)] (2) complex heated at different temperature 200 °C for 30 min to
determine the optimum temperature for thermal decomposition.
Table 3.S 5. The unit cell parameters for the MnS synthesised by hot injection thermolysis from
precursors (1-6), with (ICDD No. 03-065-0891) as the MnS reference pattern, volume, crystallite
size, EDX measurements and Raman data from these samples.
MnS from Complexes
Lattice constant a
Volume (Å3)
Crystallite size
EDX measurements Raman shift (cm-1)
(Å) (nm) Mn (at%) S (at%)
(1) 5.214 141.75 19.5 ± 2.01 48.82 51.18 635.89
(2) 5.216 141.91 17.8 ± 2.12 48.65 51.35 636.21
(3) 5.227 141.99 17.0 ± 1.84 48.73 51.27 635.89
(4) 5.220 142.24 14.9 ± 1.75 49.01 50.99 635.90
(5) 5.221 142.32 10.0 ± 1.62 48.93 51.07 635.87
(6) 5.224 142.56 9.18 ± 1.52 48.32 51.68 634.88
133
Figure 3.S 4. EDX spectra of MnS from precursors (a-f) (1-6) prepared by hot injection thermolysis.
Figure 3.S 5. Raman spectra of cubic rock-salt (RS) α-MnS from complexes (1-6) synthesised by
hot injection thermolysis.
134
Figure 3.S 6. SEM images of MnS nanoparticles from complex (a-f) (1-6) prepared by hot injection
thermolysis at 250 °C, 5μm magnification.
3.4.8.3. Manganese sulfide nanoparticles by the solvent-less thermolysis:
Figure 3.S 7. The XRD patterns of manganese sulfide nanoparticles prepared by solvent-less
thermolysis from [Mn(S2COEt)2(TMEDA)] (2) complex heated at different temperature 250, 300
and 350°C for 60 min to determine the optimum temperature for thermal decomposition.
135
Table 3.S 6. The unit cell parameters for the MnS synthesised by solvent-less thermolysis from
precursors (1 – 6), with (ICDD No. 03-065-0891) as the MnS reference pattern, volume, crystallite
size and EDX measurements from these samples.
MnS from Complexes
Lattice constant a
Volume (Å3)
Crystallite size
EDX measurements Raman shift (cm-1)
(Å) (nm) Mn (at%) S (at%)
(1) 5.225 142.65 8.2 ± 1.35 48.44 51.56 635.18
(2) 5.219 142.15 6.8 ± 1.21 48.72 51.28 635.89
(3) 5.223 142.48 6.3 ± 1.14 48.59 51.41 637.02
(4) 5.211 142.50 8.9 ± 1.08 49.27 50.73 636.21
(5) 5.223 142.48 7.6 ± 1.23 50.08 49.92 633.98
(6) 5.210 142.42 8.7 ± 1.85 48.68 51.32 634.52
Figure 3.S 8. SEM images of MnS nanoparticles from complex (a-f) (1-6) prepared by solvent-less
thermolysis at 350 °C, 5μm magnification.
136
Figure 3.S 9. EDX spectra of MnS from precursors (a-f) (1 – 6) prepared by solvent-less thermolysis.
Figure 3.S 10. Raman spectra of cubic rock-salt (RS) α-MnS from complexes (1-6) synthesised by
solvent-less thermolysis.
137
3.4.8.4. Deposition of manganese sulfide thin films by the doctor blade method:
Table 3.S 7. The unit cell parameters for the MnS synthesised by doctor blade method from
precursors (1-6), with (ICDD No. 03-065-0891) as the MnS reference pattern, volume, crystallite
size and EDX measurements from these samples.
MnS from
Complexes
Lattice
constant a
Volume
(Å3)
Crystallite
size
EDX measurements Raman shift
(cm-1)
(Å) (nm) Mn (at%) S (at%)
(1) 5.20 140.61 20.8 ± 2.38 51.89 48.11 635.89
(2) 5.22 142.24 14.2 ± 2.18 51.38 48.62 636.60
(3) 5.20 140.61 13.4 ± 1.98 50.23 49.77 636.21
(4) 5.22 142.24 17.6 ± 2.03 50.36 49.64 635.18
(5) 5.21 142.42 16.5 ± 2.08 50.60 49.40 635.89
(6) 5.22 142.24 16.5 ± 1.86 50.34 49.66 636.92
Figure 3.S 11. EDX spectra of MnS thin films from precursors (a-f) (1-6) prepared by doctor blade
method.
138
Figure 3.S 12. Raman spectra of cubic rock-salt (RS) α-MnS from complexes (1-6) Deposition by
the doctor blade method.
139
Chapter 4. The influence of single precursor on manganese
incorporation into Mn-doped PbS (Pb1-xMnxS) nanoparticles by
solvent-less thermolysis.
4.1. Introduction
The rare properties of nanocrystalline materials, when compared with their large-grained
analogs, have led to the rapid development of nanotechnology. Significant evidence suggests
that reduction of chalcogenides’ (such as lead sulfide) particle sizes to several tens of
nanometers or below, will change their properties notably.1–7 Doped nanocrystals with
magnetic metal ions, such as transition ions or Mn2+, can result in an array of spectroscopic
and magnetic properties which might have utility in practical application. If a structure is
impure, containing magnetic metal ions, its optical and magnetic properties will be
dependent upon the concentration of these magnetic metal ions.8 Lead sulfide (PbS) is an
important IV–VI semiconductor. Its relatively small bulk band gap (Eg=0.41 eV)9 and a
large exciton Bohr radius of 18 nm gives it a range of applications: it may be used as an
infrared detector, photometer, nonlinear element or sensor.10–12 Contrastingly, PbS
nanocrystals doped with Mn2+ ions produce a material which displays abnormal
magnetooptical and switching properties. Additionally, the doping of Mn2+ ions may be
efficiently used in nano-spintronics, spin-photonics and magneto-electronics.13–16
Within this chapter, the uses of [Pb(S2COEt)2] (1) and [Mn(S2COEt)2.TMEDA] (2)
(TMEDA= Tetramethylethylenediamine) as single source precursors for the synthesis of
undoped PbS and doped Pb1−xMnxS (0 ≤ x ≤ 0.08) with the use of a facile solvent-less
thermolysis has been investigated. The thermogravimetric analysis (TGA) reveals that both
precursors are capable of successful decomposition within a similar temperature range. The
materials produced by this method were further investigated with the use of powder X-ray
140
diffraction (p-XRD), scanning electron microscopy (SEM), energy dispersive X-ray
spectroscopy (EDX), Raman spectroscopy and UV-Vis absorption spectroscopy.
4.2. Author distribution
In this work, I synthesised and then characterised xanthate complexes via IR, elemental
analysis and TGA. The experimental work to produce nanocrystals was carried out by me, I
characterised the samples by XRD, Raman, SEM, EDX and UV-Visible spectroscopy. The
original idea was provided by Paul O’Brien. David J. Lewis supporting me in the project and
he provided as well a nice and useful discussion, and also editing the manuscript. The
experimental work was done in the laboratory of Paul O’Brien.
4.3. References
1 S. I. Sadovnikov, N. S. Kozhevnikova, V. G. Pushin and A. A. Rempel, Inorg. Mater., 2012, 48,
21–27.
2 H. Cao, G. Wang, S. Zhang and X. Zhang, Nanotechnology, 2006, 17, 3280–3287.
3 D. D. W. Grinolds, P. R. Brown, D. K. Harris, V. Bulovic and M. G. Bawendi, Nano Lett., 2015,
15, 21–26.
4 A. Pimachev and Y. Dahnovsky, J. Phys. Chem. C, 2015, 119, 16941–16946.
5 J. K. Wu, L. M. Lyu, C. W. Liao, Y. N. Wang and M. H. Huang, Chem. Eur. J., 2012, 18,
14473–14478.
6 H. Zhao, H. Liang, F. Vidal, F. Rosei, A. Vomiero and D. Ma, J. Phys. Chem. C, 2014, 118,
20585–20593.
7 J. J. Peterson and T. D. Krauss, Nano Lett., 2006, 6, 510–514.
8 Y. T. Nien, K. H. Hwang, I. G. Chen and K. Yu, J. Alloys Compd., 2008, 455, 519–523.
9 D. J. Asunskis, I. L. Bolotin and L. Hanley, J. Phys. Chem. C, 2008, 112, 9555–9558.
10 I. Moreels, K. Lambert, D. Smeets, D. De Muynck, T. Nollet, J. C. Martins, F. Vanhaecke, A.
Vantomme, C. Delerue, G. Allan and Z. Hens, ACS Nano, 2009, 3, 3023–3030.
11 R. Kripal and U. M. Tripathi, J. Mater. Sci. Mater. Electron., 2018, 29, 12195–12205.
12 R. Kripal, C. Rudowicz and U. M. Tripathi, Appl. Magn. Reson., 2019, 50, 785–795.
13 R. Beaulac, P. I. Archer, S. T. Ochsenbein and D. R. Gamelin, Adv. Funct. Mater., 2008, 18,
3873–3891.
14 V. Proshchenko and Y. Dahnovsky, J. Phys. Chem. C, 2014, 118, 28314–28321.
15 D. A. Bussian, S. A. Crooker, M. Yin, M. Brynda, A. L. Efros and V. I. Klimov, Nat. Mater.,
2009, 8, 35–40.
16 P. I. Archer, S. A. Santangelo and D. R. Gamelin, Nano Lett., 2007, 7, 1037–1043.
141
4.4. The influence of single precursor on manganese incorporation into
Mn-doped PbS (Pb1-xMnxS) nanoparticles by solvent-less thermolysis.
Abdulaziz. M. Alanazi,a,c David J. Lewis*b and Paul O’Briena,b
a, Department of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
b, Department of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
c, Department of Chemistry, Islamic university, Prince Naif Ibn Abdulaziz Rd, Madinah, 42351,
KSA
*Email: david.lewis-4@manchester.ac.uk
4.4.1. Abstract
[Bis(O-ethylxanthate) lead(II)] (1) and [bis(O-ethylxanthate) manganese(II).(TMEDA)] (2)
were synthesized and used as single source precursors for the preparation of Pb1-xMnxS (x =
0, 0.02, 0.04, 0.06 and 0.08) nanoparticles using solvent-less thermolysis at 350 °C. P-XRD
revealed a cubic crystal structure, with lattice parameter a decreasing linearly as a function
of Mn content. For all samples the elemental compositions and stoichiometries were
determined by EDX spectroscopy. Raman spectroscopy indicates that the intensity of the
weak band observed at 270 cm-1 and 430 cm-1 increased with increasing amounts of
manganese. Incorporation of Mn2+ into PbS led to an increase in the band gap from 0.87 eV
to 0.89 eV, while the particle sizes decreases in the range of 24.80 to 22.07 nm.
4.4.2. Introduction
Recently, much effort has been made to the research of doped metal chalcogenide
nanoparticle materials. These kind of nanoparticles show different physical and chemical
properties in comparison with their bulk materials, such as size-dependent difference of the
band gap energy.1,2 Additionally, impurity ions doped into these nanoparticles can affect the
142
electronic structure and transition probabilities.3 In particular, when doped with magnetic
ions (e.g. Mn2+), these materials can produce unique magnetic and magneto-optical
properties and provide opportunities for the new field of photonic and spintronics.4–6
Lead sulfde (PbS) is an important II–VI semiconductor material, with a rather small bulk
band gap energy (Eg=0.41 eV at 300 K)7,8 and a large exciton Bohr radius of 18 nm,9 which
has applications such as optical switches, sensors, infrared detectors, photovoltaic solar cells,
and storage batteries.10–13 Several approaches have been reported for the preparation of PbS
and MnS including solvothermal,14,15 hydrothermal,16,17 hot injection18,19 and melt
techniques.20
Solvent-less thermolysis has benefits over other routes, such as it is a simple technique in
which a solid state precursor is decomposed and is carried out by thermal treatment under
inert conditions. This technique has confirmed to be an effective way of producing metal
chalcogenide nanomaterials with a varied range of morphologies for example nanorods,21
nanowires,22 nanospheres,23 and nanodisks.24 In comparison with the other chemical
techniques Solvent-less thermolysis offers a simple, an economic, environmental-friendly
and unexpansive way to scale up production.25
The use of single-source precursors provides significant benefits which are useful precursors
for preparation of a range of metal sulfide nanomaterials or thin films.26–31 These precursors
can be prepared simply in large quantities, are generally air-stable, easy to handle, purify
and characterise.32,33 Indeed, great success has been achieved using the thermal
decomposition of lead/manganese complexes of thiobenzoate,34,35
diethyldithiocarbamate36,37 as single-source precursors. The use of metal xanthate precursors
for the preparation of PbS38 and MnS is promising owing to the low decomposition
temperature of metal xanthate complexes (100–350 °C).
143
To our current knowledge, the synthesis of Mn2+ incorporated PbS nanoparticles using
solvent-less thermolysis from the single source precursors has not been reported. In this
study, we synthesised and characterized two precursors [Pb(S2COEt)2] (1) and
[Mn(S2COEt)2.TMEDA] (2) (TMEDA= tetramethylethylenediamine) in solvent-less
thermolysis for the preparation of undoped PbS and doped Pb1−xMnxS (0 ≤ x ≤ 0.08)
nanoparticles at 350 °C. The thermogravimetric analysis (TGA) reveals that both precursors
exhibit successful decomposition in a similar temperature range. The materials produced are
investigated by using powder X-ray diffraction (p-XRD), scanning electron microscopy
(SEM), energy dispersive X-ray (EDX) spectroscopy, Raman spectroscopy and UV-Vis
absorption spectroscopy.
4.4.3. Experimental
4.4.3.1. Chemicals
Acetone (≥ 99.8%, Sigma-Aldrich), potassium hydroxide (≥ 85%), carbon disulfide (≥ 99%),
manganese(II) acetate tetrahydrate (≥ 99%, Sigma-Aldrich), lead(II) acetate trihydrate (≥
99.9%, Sigma-Aldrich) and N,N,N′,N′-Tetramethylethylenediamine (≥ 99%, Sigma-
Aldrich) were used as received with no further purification.
4.4.3.2. Instrumentation
Melting points were determined with a Stuart melting point apparatus (Cole-Palmer, UK);
infrared spectra (IR) were recorded using a Nicolet iS5 Thermo Scientific ATR instrument
(4000–400 cm−1, resolution 4 cm−1). Elemental analyses (EA) and Thermogravimetric
analyses (TGA) were carried out by the Micro-elemental Analysis Service in the School of
Chemistry at the University of Manchester. EA was performed for all complexes using a
Flash 2000 Thermo Scientific elemental analyser and TGA data were obtained with Mettler
Toledo TGA/DSC stare system in the range 30–600 °C at a heating rate of 10 °C min−1 under
144
nitrogen flow. Powder X-ray diffraction (p-XRD) analyses were carried out using an X-Pert
diffractometer with a Cu-Kα1 source (λ = 1.54059 Å), the samples were scanned between
10° and 80°, with an applied voltage of 40 kV and a current of 30 mA. Scanning electron
microscopy (SEM) was carried out using a Philips XL 30 FEG. The voltage used was 40
kV. EDX spectroscopy (Philips EDAX DX4 X-ray micro-analyser SEM) was used to
determine elemental composition as well used for elemental mapping in order to know the
spatial distribution of elements in the sample. Raman spectra were measured using a
Renishaw 1000 Micro Raman System equipped with a 514 nm laser and UV-Vis spectra
were collected on a Lambda 1050, using 3.09 mM solution of Pb1−xMnxS nanoparticles in
ethanol.
4.4.3.3. Synthesis of [bis(O-ethylxanthato) Lead(II)] (1)
Synthesis of [Pb(S2COEt)2] was carried out by following literature.38 Briefly, [Pb(S2COEt)2]
(1) was prepared by a chemical reaction between an aqueous solution of potassium
ethylxanthate ligand (1.6 g, 9.9 mmol) and an aqueous solution of lead(II) acetate
(Pb(CH3COO)2. 4H2O, (0.5 g, 3.3 mmol)) at room temperature while stirring for 60 min to
form a white precipitate. The powder was filtered off and washed with water, and the crude
product was dried in a vacuum oven. The product was recrystallizesed from acetone. Yield:
84% (1.8 g). Melting point: 135 °C. Elemental analysis: Calc (%): C, 16.03; H, 2.24; S,
28.47; Pb, 46.13. Found (%): C, 16.36; H, 2.19; S, 28.62; Pb, 46.53%. IR (νmax/cm-1): 2969
(w), 1121-1138 (s), 1056(s).
4.4.3.4. Synthesis of [ bis(O-ethylxanthato) Manganese(II). (TMEDA)](2)
Potassium hydroxide (0.76 g, 13.63 mmol) was dissolved in 20 ml methanol and
stirred for 2 h at room temperature. Carbon disulfide (1.04 g, 0.83 ml, 13.63 mmol)
was added drop-wise at 0 °C and the mixture stirred for 1 h. 50 ml of an aqueous
145
solution of Mn(CH3COO)2.4H2O (1.60 g, 6.80 mmol) was added drop-wise to the
reaction mixture, which was stirred for 0.5 h to form a brown/yellow solution.
TMEDA (0.79 g, 6.76 mmol) was added to the solution while stirring for 60 min to
form a brown precipitate. The solid residue was isolated by filtration and washed with
water, and the product was dried in a vacuum oven. The product was crystallized from
acetone. Yield: 88% (11.2 g). Melting point: 137 °C. Elemental analysis: Calc (%):
C, 34.86; H, 6.34; S, 30.96; N, 6.78; Mn, 13.30. Found (%): C, 34.94; H, 6.28; S,
31.26; N, 6.70; Mn, 13.01. IR (νmax/cm-1): 2980 (w), 1142-1185(s), 1032(s).
4.4.3.5. Synthesis of Pb1−xMnxS nanoparticles by the solvent-less thermolysis
Metal sulfides were prepared by the thermal decomposition of the complexes (1) and (2),
mixed in different mole fractions of Mn (x = 0, 0.02, 0.04, 0.06 and 0.08) (Table 4. S1).
Both complexes were mixed in the required molar ratios, and the mixture was ground in air
using a pestle and mortar. The powder placed in a ceramic boat under a stream of argon
(300 cm3 min-1), and at 350 °C for 30 min. After this time the heating was turned off and
the combustion boat was allowed to cool naturally to room temperature, and the results
powder were collected for analysis.
4.4.4. Results and discussion
The thermal stability of Pb and Mn complexes was analysed using thermogravimetric
analysis (TGA) (Figure. 4.1). The TGA profile for both complexes displayed decomposition
in a single step, and complete decomposition occurred at around 200 °C. In the case of Pb
complex, one-step decomposition was observed with rapid mass loss of 53%, which is
consistent with the calculated value to produce PbS (53%). Similarly, the TGA for Mn
complex indicated a residual mass of 24%, which was close to calculated value to produce
MnS (21%).
146
Figure 4. 1. TGA profile of (1) lead(II) ethylxanthate and (2) manganese(II) ethylxanthate. TMEDA.
The thermal decomposition of (1) or (2) was performed at 300 °C and 350 °C, under Ar.
Decomposition of the complexes produced black residues which were structurally
characterised using p-XRD. The p-XRD pattern of the powder (Figure. 4.S1) prepared from
(1) alone could be indexed to cubic PbS (ICDD: 03-065-0692).39 The relatively intense (200)
plane indicates the preferred orientation in the pattern. Peaks observed at 2θ value of 25.86°,
29.96°, 42.97°, 50.84°, 53.31°, 62.43°, 68.77°, 70.83°, and 78.81°, correspond to the (111),
(200), (220), (311), (222), (400), (331), (420), and (422) Bragg planes of PbS, with no
indication of secondary phases. P-XRD patterns obtained from the decomposition of (2) at
300 °C and 350 °C are shown in Figure. 4.S2, with diffraction peaks indexing to cubic MnS
(ICDD: 03-065-0891).40 The diffraction peaks observed at 2θ values of 29.6°, 34.4°, 49.3°,
56.8°, 61.5° and 72.6° correspond to the (111), (200), (220), (311), (222) and (400) planes
of cubic α–MnS.
Furthermore, TGA profile indicated that both complexes decomposed completely in the
temperature range of 320–350 °C, and therefore, the temperature of 350 °C was used to
147
ensure the congruent decomposition of the complexes. Figure 4.2 shows p-XRD patterns of
undoped and Mn-doped PbS nanoparticles with various Mn concentrations (0, 2, 4, 6, and
8%), which were synthesised using solvent-less thermolysis. The introduction of manganese,
where x = 0.02, 0.04, 0.06 and 0.08, shifts the reflections associated with cubic PbS to larger
2θ implying a contraction of the lattice. The peaks were sharp for every sample, and the shift
in the peaks was also observed, thereby verifying their crystalline nature.
The diffraction peaks for the Pb1-xMnxS nanoparticles were observed at 2θ values between
those found for PbS and MnS. The peaks displayed a gradual shift with the change in the
ratio of the mole fraction of Mn. The shift in the peaks is consistent with Vegardian
behaviour where compositional result in a changes linear change in unit cell parameters. The
peaks shifted towards a higher theta value with an increase in manganese content, which can
be attributed to a reduction of the lattice parameters with the substitution of the larger Pb2+
(ionic radius 1.33 Å) for smaller Mn2+ (ionic radius 0.80 Å).41,42 The unit cells of both the
cubic PbS and MnS, along with their bond distances are shown in Figure 4.3.
The influence of the Mn2+ content on the lattice constants of the Pb1-xMnxS nanoparticles
indicated a linear decrease in the lattice parameter with an increase in the mole fraction of
Mn. The observed linearity provided further evidence in support of the successful
incorporation of Mn into the nanoparticles. As shown in Figure 4.2, lattice parameters were
plotted against the variations in Mn/Mn + Pb molar ratios, and it can be clearly seen that unit
cell volume is a linear function with respect to Mn2+ content, in the Pb1-xMnxS nanoparticles.
Table 4. 1 illustrates the unit cell lattice parameters ɑ and the unit cell volume V for PbS and
Pb1-xMnxS (0 ≤ x ≤ 0.08). X-ray-diffraction data was used to calculate these parameters,
which involved using the lattice relation for cubic structures: namely, 1/d2 = (h2 + k2 + l2)/ɑ2
and V = ɑ 3, where d represents the space between adjacent lattice planes and (hkl) are the
Miller indices of the plane.43
148
Figure 4. 2. XRD patterns and lattice parameters a, unit cell volume V and d(200) spacing of Pb1-
xMnxS (0≤ x ≤ 0.08) samples prepared by solvent-less thermolysis at 350 °C using lead and
manganese xanthate precursors with different mole fractions of manganese: (a) x = 0 (PbS), (b) x =
0.02, (c) x = 0.04, (d) x = 0.06 and (e) x = 0.08.
149
Figure 4. 3. Unit cells of (a) PbS (ICDD: 03-065-0692) and (b) MnS (ICDD: 03-065-0891) along
with their bonds.
Table 4. 1. Lattice parameters a, unit cell volume (V), band gap (Eg) and grain size of Pb1-xMnxS
(0 ≤ x ≤ 0.08) with variations in Mn/Mn+Pb molar ratios.
X Composition Lattice
parameter
a (Å)
V (Å)3 d spacing
(Å) (200)
Band
gap (eV)
Grain size (nm) estimated
from
XRD data SEM micrograph
0 PbS 5.947 ± 0.003 210.33 ±0.09 2.979 0.87 24.62 ± 3.45 24.80 ±1.29
2 Pb0.98Mn0.02S 5.946 ±0.003 210.22 ±0.09 2.977 0.87 23.94 ± 2.84 24.05 ± 1.83
4 Pb0.96Mn0.04S 5.945 ±0.002 210.11 ±0.08 2.975 0.88 23.89 ± 2.85 23.51 ± 1.39
6 Pb0.94Mn0.06S 5.944 ±0.002 210.01 ±0.08 2.973 0.88 23.67 ± 2.93 22.82 ± 0.86
8 Pb0.92Mn0.08S 5.943 ±0.002 209.90 ±0.08 2.970 0.89 22.29 ± 2.17 22.07 ± 0.73
EDX spectroscopy was used to determine the percentage of each element contained within
the synthesised nanoparticles (Figure. 4. S3). Qualitative analysis indicated the existence
only of requirement elements (i.e., Pb, S, and Mn), and so the quantitative analysis showed
that an increase in manganese concentration was accompanied by a decrease in lead content
when Mn/(Mn + Pb) composition ranged between x = 0 to x = 0.08. A linear trend between
manganese and lead was identified, whereas in the case of sulfur, the percentage was almost
the same. The stoichiometry of the nanomaterials was found to be close to the expected
value. There is an approximately quantitative linear relationship between the amount of
manganese in the precursor powders and the amount of manganese determined by the EDX
(a)
(b)
150
in the Pb1-xMnxS products (Figure. 4. 5). The spatial distribution of all elements in the
materials was analysed using EDX elemental mapping. The results indicated a uniform
distribution of Pb, S, and Mn throughout the samples (Figure.4. S4) at the microscale, and
gives credence to the p-XRD data which suggests that this is a true solid solution.
Figure. 4.6 Shows the morphologies of the synthesised Pb1-xMnxS nanoparticles, as observed
using SEM. The SEM image of pure PbS (x = 0) revealed cube-shaped particles (Figure.
4.6a), which transformed into cube-shaped particles with slightly sharper edges as the mole
fraction of Mn content increased. In accordance with the XRD result, close monitoring of
the SEM images of nanoparticles with different Mn concentration. SEM images have also
been utilized to estimate the particle size of the samples. (Table 4.1 and Figure 4.S5) show
the particle size distribution histogram of samples prepared by solvent-less thermolysis at
350 °C with different mole fractions of Mn. It can be seen that the particles size ranges from
24.8 to 22.1 nm depending on the Mn content. It was observed that the particles size of the
nanoparticles are decreased with increase in Mn doping.
Figure 4. 4. Approximately linear correlation between the amounts of manganese in the precursor
feedstock and the mole % Mn found in Pb1-xMnxS samples from EDX spectroscopy.
151
Figure 4. 5. Representative SEM secondary electron SEM images (10 kV) of Pb1-xMnxS (0≤ x ≤
0.08) samples prepared by solvent-less thermolysis at 350 °C using lead and manganese xanthate
precursors with different mole fractions of manganese: (a) x = 0 (PbS), (b) x = 0.02, (c) x = 0.04, (d)
x = 0.06 and (e) x = 0.08.
152
Raman spectroscopy was used to investigate the Pb1-xMnxS (0 ≤x≤ 0.08) samples prepared
from mixtures of the lead and manganese ethyl xanthates at 350 °C (Figure. 4.7). The pure
PbS sample displayed a strong peak at 136 cm-1, and it was assigned to a combination of
longitudinal and transverse acoustic modes [LO(L) + TO(L)].44 The other two small broad
bands were located at 270 cm-1 and 430 cm-1, which corresponded to a two-phonon process
and first overtones, respectively.45–47 The strong peak at 967 cm−1 can be attributed to the
photodegradation of PbS.48 For Pb1-xMnxS (x = 0.02, 0.04, 0.06, and 0.08), a significant
change was observed in the Raman spectra with an increase of manganese in the
nanoparticles. The intensity of the weak band observed at 270 cm-1 and 430 cm-1 increased
with the introduction of greater amounts of manganese, but no other significant effects were
observed.
Figure 4. 6. Raman spectra of Pb1-xMnxS (0 ≤x≤ 0.08) samples prepared by solvent-less thermolysis
at 350 °C using lead and manganese xanthate precursors with different mole fractions of Mn.
153
4.4.4.1. Optical properties
The changes in the optical properties of the undoped and doped lead chalcogenides were
observed by UV–Vis–NIR spectroscopy (Figure. 4. S6). The incorporation of manganese
into PbS resulted in the red shift of the absorption spectrum. The estimated energy band gaps
were calculated by Tauc plots to be found 0.87, 0.87, 0.88, 0.89 and 0.90 eV for undoped
PbS and doped Pb1-xMnxS (x = 0.02, 0.04, 0.06, and 0.08) respectively, which correspond to
wavelengths from 1100–1500 nm in the near infrared region in the spectrum (Figure. 4. S7).
Hence, in conclusion we can finally say that the band gap of synthesized PbS NPs is about
0.87 eV as determined from UV-Vis-NIR data which is about 2 times higher than its bulk
value of PbS (0.41eV).19,49–52 From Table 4.1 it is clearly seen that within the quantum
confinement regime band gap becomes a function of the grain size and increases with
decreasing particle size.53–56 Thus, with the subsequent doping of Mn2+ the particle size
decreases but remains comparable to the Bohr radius (18 nm). Thus absorption edge shifts
to shorter wavelength (blue shift) again as a result of decrease in particle size due to which
the optical band gap increases.56–58 Similar changes in the band gap energy for PbS
nanoparticles with smaller crystallite sizes have been reported for PbS nanoparticles by Saah
and Akhtar et al.19,50 The value of the band gap was found to vary from 0.88 to 1.71 eV,
depending on the particles size.
154
Figure 4. 7. Relationship between Particle Size and band gap of undoped PbS and Pb1-xMnxS (0 ≤x≤
0.08) samples prepared by solvent-less thermolysis at 350 °C.
4.4.5. Conclusion
[Bis(O-ethylxanthate) lead(II)] (1) and [bis(O-ethylxanthate) manganese(II).(TMEDA)] (2)
complexes have been used as single source precursors (SSPs) for the synthesis of single
phase Pb1-xMnxS (0 ≤x≤ 0.08) nanoparticles over the entire range of composition by the
solvent-less thermolysis. The TGA curve for both complexes shows decomposition in a
single step, and the successfully decomposition occurs around 200 °C. The p-XRD shows
the shift in peaks, the change in the lattice parameter and the change in the composition
indicate the successful integration of Mn in the crystal lattice of PbS. SEM images exhibited
slightly changes in the morphology as the amount of Mn2+ was increased in the samples. The
elemental compositions of all the samples were examined via EDX spectroscopic mapping,
with the latter technique revealing uniform spatial distribution of elements in all samples.
The Raman spectroscopy indicates that the intensity of the weak band observed at 270 cm-1
and 430 cm-1 increased with the increase amounts of manganese. The band gaps of Pb1-
xMnxS nanoparticles were found to vary from 0.87 eV to 0.89 eV with increasing Mn2+ mole
155
fraction (x). The particle sizes reduces due to the incorporation of Mn2+, while the band gap
increases. The estimated sizes of the nanoparticles found from the SEM are consistent with
the XRD results. This study shows that the use of SSPs is possibly advantageous for the
synthesis of nanoparticles and can be used to other systems for the tuning of their properties.
4.4.6. Acknowledgements
A. Alanazi is thankful to the Ministry of Higher Education in Saudi Arabia for funding and
the University of Islamic, Saudi Arabia for permission to study in the United Kingdom. We
acknowledge the EPSRC National Facility at the University of Manchester.
4.4.7. References
1 D. J. Norris, N. Yao, F. T. Charnock and T. A. Kennedy, Nano Lett., 2001, 1, 3–7.
2 W. Chen, J.-O. Malm, V. Zwiller, Y. Huang, S. Liu, R. Wallenberg, J.-O. Bovin and L.
Samuelson, Phys. Rev. B, 2000, 61, 11021–11024.
3 Y. L. Soo, Z. H. Ming, S. W. Huang, Y. H. Kao, R. N. Bhargava and D. Gallagher, Phys.
Rev. B, 1994, 50, 7602–7607.
4 S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molnár, M. L.
Roukes, A. Y. Chtchelkanova and D. M. Treger, Science, 2001, 294, 1488–1495.
5 S. J. Pearton, C. R. Abernathy, M. E. Overberg, G. T. Thaler, D. P. Norton, N.
Theodoropoulou, A. F. Hebard, Y. D. Park, F. Ren, J. Kim and L. A. Boatner, J. Appl.
Phys., 2002, 93, 1–13.
6 S. A. Chambers, T. C. Droubay, C. M. Wang, K. M. Rosso, S. M. Heald, D. A. Schwartz,
K. R. Kittilstved and D. R. Gamelin, Mater. Today, 2006, 9, 28–35.
7 D. E. Aspnes and M. Cardona, Phys. Rev., 1968, 173, 714–728.
8 T. Trindade and P. O. Brien, Chem. Vap. Depos., 1997, 3, 75–77.
9 J. L. Machol, F. W. Wise, R. C. Patel and D. B. Tanner, Phys. Rev. B, 1993, 48, 2819–
2822.
10 S.-M. Lee, Y. Jun, S.-N. Cho and J. Cheon, J. Am. Chem. Soc., 2002, 124, 11244–11245.
11 X. Zhao, I. Gorelikov, S. Musikhin, S. Cauchi, V. Sukhovatkin, E. H. Sargent and E.
Kumacheva, Langmuir, 2005, 21, 1086–1090.
12 N. B. Pendyala and K. S. R. Koteswara Rao, Colloids Surf. Physicochem. Eng. Asp.,
2009, 339, 43–47.
156
13 L. F. Koao, F. B. Dejene and H. C. Swart, Int J Electrochem Sci, 2014, 9, 11.
14 J. Lu, P. Qi, Y. Peng, Z. Meng, Z. Yang, W. Yu and Y. Qian, Chem. Mater., 2001, 13,
2169–2172.
15 R. Jin, G. Chen and J. Pei, J. Phys. Chem. C, 2012, 116, 16207–16216.
16 S. Wang, A. Pan, H. Yin, Y. He, Y. Lei, Z. Xu and B. Zou, Mater. Lett., 2006, 60, 1242–
1246.
17 S. G. Mohamed, S. Y. Attia, Y. F. Barakat, H. H. Hassan and W. A. Zoubi,
ChemistrySelect, 2018, 3, 6061–6072.
18 H. Zhang, B.-R. Hyun, F. W. Wise and R. D. Robinson, Nano Lett., 2012, 12, 5856–
5860.
19 S. Ama Saah, M. Dilshad Khan, P. D. McNaughter, J. A. M. Awudza, N. Revaprasadu
and P. O’Brien, New J. Chem., 2018, 42, 16602–16607.
20 P. D. McNaughter, S. A. Saah, M. Akhtar, K. Abdulwahab, M. Azad Malik, J. Raftery,
J. A. M. Awudza and P. O’Brien, Dalton Trans., 2016, 45, 16345–16353.
21 T. H. Larsen, M. Sigman, A. Ghezelbash, R. C. Doty and B. A. Korgel, J. Am. Chem.
Soc., 2003, 125, 5638–5639.
22 J. Chen, L. Chen and L.-M. Wu, Inorg. Chem., 2007, 46, 8038–8043.
23 K. Abe, T. Hanada, Y. Yoshida, N. Tanigaki, H. Takiguchi, H. Nagasawa, M. Nakamoto,
T. Yamaguchi and K. Yase, Thin Solid Films, 1998, 327–329, 524–527.
24 Y.-B. Chen, L. Chen and L.-M. Wu, Inorg. Chem., 2005, 44, 9817–9822.
25 E. Lewis, S. Haigh and P. O’Brien, J. Mater. Chem. A, 2014, 2, 570–580.
26 Z. Tshemese, M. D. Khan, S. Mlowe and N. Revaprasadu, Mater. Sci. Eng. B, 2018,
227, 116–121.
27 M. D. Khan, S. Hameed, N. Haider, A. Afzal, M. C. Sportelli, N. Cioffi, M. A. Malik
and J. Akhtar, Mater. Sci. Semicond. Process., 2016, 46, 39–45.
28 M. D. Khan, J. Akhtar, M. A. Malik and N. Revaprasadu, ChemistrySelect, 2016, 1,
5982–5989.
29 M. D. Khan, M. A. Malik, J. Akhtar, S. Mlowe and N. Revaprasadu, Thin Solid Films,
2017, 638, 338–344.
30 M. Dilshad Khan, G. Murtaza, N. Revaprasadu and P. O’Brien, Dalton Trans., 2018, 47,
8870–8873.
31 J. M Clark, G. Kociok-Köhn, N. J Harnett, M. S Hill, R. Hill, K. C Molloy, H. Saponia,
D. Stanton and A. Sudlow, Dalton Trans., 2011, 40, 6893–6900.
32 N. L. Pickett and P. O’Brien, Chem. Rec., 2001, 1, 467–479.
157
33 M. A. Malik, M. Afzaal and P. O’Brien, Chem. Rev., 2010, 110, 4417–4446.
34 R. D. Adams and L. Zhu, J. Mol. Struct., 2008, 890, 130–132.
35 L. Tian, L. Y. Yep, T. T. Ong, J. Yi, J. Ding and J. J. Vittal, Cryst. Growth Des., 2009,
9, 352–357.
36 H. Iwasaki and H. Hagihara, Acta Crystallogr. B, 1972, 28, 507–513.
37 C. A. Téllez S., A. C. Costa, M. A. Mondragón, G. B. Ferreira, O. Versiane, J. L. Rangel,
G. M. Lima and A. A. Martin, Spectrochim. Acta. A. Mol. Biomol. Spectrosc., 2016, 169,
95–107.
38 P. D. McNaughter, S. A. Saah, M. Akhtar, K. Abdulwahab, M. Azad Malik, J. Raftery,
J. A. M. Awudza and P. O’Brien, Dalton Trans., 2016, 45, 16345–16353.
39 E. A. Benjamin, B. Ezekoye, E. Tochukwu and K. O. Ighodalo, Int. J. Phys. Sci., 2015,
10, 385–390.
40 J. Ning, D. Zhang, H. Song, X. Chen and J. Zhou, J. Mater. Chem. A, 2016, 4, 12098–
12105.
41 M. El-Hagary, M. Emam-Ismail, E. R. Shaaban, A. Al-Rashidi and S. Althoyaib, Mater.
Chem. Phys., 2012, 132, 581–590.
42 K. O. Adebowale, E. I. Unuabonah and B. I. Olu-Owolabi, Chem. Eng. J., 2008, 136,
99–107.
43 S. R. Stock, MicroComputed Tomography : Methodology and Applications, CRC Press,
2008.
44 G. D. Smith, S. Firth, R. J. H. Clark and M. Cardona, J. Appl. Phys., 2002, 92, 4375–
4380.
45 H. Du, C. Chen, R. Krishnan, T. D. Krauss, J. M. Harbold, F. W. Wise, M. G. Thomas
and J. Silcox, Nano Lett., 2002, 2, 1321–1324.
46 K. K. Nanda, S. N. Sahu, R. K. Soni and S. Tripathy, Phys. Rev. B, 1998, 58, 15405–
15407.
47 H. Cao, G. Wang, S. Zhang and X. Zhang, Nanotechnology, 2006, 17, 3280–3287.
48 J.-P. Ge, J. Wang, H.-X. Zhang, X. Wang, Q. Peng and Y.-D. Li, Chem. Eur. J., 2005,
11, 1889–1894.
49 K. P. Mubiayi, N. Revaprasadu, S. S. Garje and M. J. Moloto, J. Saudi Chem. Soc., 2017,
21, 593–598.
50 J. Akhtar, M. A. Malik, P. O’Brien, K. G. U. Wijayantha, R. Dharmadasa, S. J.
O. Hardman, D. M. Graham, B. F. Spencer, S. K. Stubbs, W. R. Flavell, D. J. Binks, F.
Sirotti, M. E. Kazzi and M. Silly, J. Mater. Chem., 2010, 20, 2336–2344.
158
51 S. I. Sadovnikov, N. S. Kozhevnikova and A. I. Gusev, Semiconductors, 2011, 45, 1559–
1570.
52 Z. A. Motlagh and M. E. A. Araghi, Semicond. Sci. Technol., 2016, 31, 025017.
53 K. K. Nanda and S. N. Sahu, Adv. Mater., 2001, 13, 280–283.
54 C. T. Tsai, S. H. Chen, D. S. Chuu and W. C. Chou, Phys. Rev. B, 1996, 54, 11555–
11560.
55 G. Su, C. Liu, Z. Deng, X. Zhao and X. Zhou, Opt. Mater. Express, 2017, 7, 2194–2207.
56 S. Thangavel, S. Ganesan, S. Chandramohan, P. Sudhagar, Y. S. Kang and C.-H. Hong,
J. Alloys Compd., 2010, 495, 234–237.
57 B. Touati, A. Gassoumi, S. Alfaify and N. Kamoun-Turki, Mater. Sci. Semicond.
Process., 2015, 34, 82–87.
58 K. S. Babu, C. Vijayan and R. Devanathan, Mater. Lett., 2004, 58, 1223–1226.
159
4.4.8. Supporting Information
Table 4. S 1. Composition of Pb1-xMnxS (0 ≤ x ≤ 0.08).
Composition (x)
[Mn]/[Mn]+[Pb]
[Pb(S2COEt)2] (1) [Mn(S2COEt)2.(TMEDA)] (2)
0 0.22 mmol 0
0.02 5.93 mmol 0.24 mmol
0.04 2.91mmol 0.24 mmol
0.06 1.89 mmol 0.24 mmol
0.08 1.39 mmol 0.24 mmol
Figure 4. S 1. XRD for cubic PbS (ICDD: 03-065-0692) from lead(II) ethylxanthate at (a) 300 °C
and (b) 350 °C.
160
Figure 4.S 2. XRD for cubic MnS (ICDD: 03-065-0891) from Manganese(II) ethylxanthate.TMEDA
at (a) 300 °C and (b) 350 °C.
Figure 4. S 3. EDX spectra of of Pb1-xMnxS (0≤ x ≤ 0.08) samples prepared by solvent-less
thermolysis at 350 °C with different mole fractions of manganese: (a) x = 0 (PbS), (b) x = 0.02, (c)
x = 0.04, (d) x = 0.06 and (e) x = 0.08.
161
Table 4. S 2. Summary of the required composition of Pb1-xMnxS (0 ≤ x ≤ 0.08) calculated from
the elements in the feed and analysis of final products by EDX spectroscopy.
Composition
[Mn]/[Mn]+[Pb]
Target
composition
Required
Stoichiometry
EDX
Pb:Mn:S
Material
stoichiometry
0 PbS 50:50 51:0:49 PbS
0.02 Pb0.98Mn0.02S 49:1:50 46.83:1.03:52.14 Pb0.978Mn0.022S
0.04 Pb0.96Mn0.04S 48:2:50 46.51:2.07:51.42 Pb0.957Mn0.042S
0.06 Pb0.94Mn0.06S 47:3:50 45.30:3.14:51.56 Pb0.936Mn0.064S
0.08 Pb0.92Mn0.08S 46:4:50 45.10:3.91:50.99 Pb0.921Mn0.079S
Figure 4. S 4. EDX elemental mapping (20 kV) of Pb, Mn and S for Pb1-xMnxS samples. (a) x =
0.02, (b) x = 0.04, (c) x = 0.06 and (d) x = 0.08 mole fractions of manganese.
162
Figure 4. S 5. Particle size distribution histogram of the samples prepared Pb1-xMnxS by solvent-
less thermolysis at 350 °C with different mole fractions of Mn: (a) x = 0 (PbS), (b) x = 0.02, (c) x =
0.04, (d) x = 0.06 and (e) x = 0.08.
163
Figure 4. S 6. The UV-Vis-NIR absorbance spectra of undoped PbS and Pb1-xMnxS (0 ≤x≤ 0.08)
samples prepared by solvent-less thermolysis at 350 °C.
164
Figure 4. S 7. Tauc plot (ahν)2 vs. hν showing the direct bandgaps of undoped PbS and Pb1-xMnxS
(0 ≤x≤ 0.08) samples prepared by solvent-less thermolysis at 350 °C.
165
Chapter 5. Effects of annealing temperature on the structural
and optical properties of CMTS (Cu2MnSnS4) nanoparticle
prepared by solvent-less thermolysis
5.1. Introduction
Solar cells which have their basis in quaternary materials have a large photoelectric
conversion efficiency, of over 20%.1,2 Materials such as the quaternary chalcogenide
Cu2ZnSnS4 (CZTS) and the naturally occurring mineral Cu2MnSnS4 (CMTS) have a direct
band gap energy of 1.0–1.4 eV and a substantial absorption coefficient of over 104 cm-1.
These properties one of the absorbent materials which has the greatest potential for use in
sustainable and efficient solar cells.3–5 In comparison with CZTS, CMTS may have greater
potential to produce low-cost solar cells, as it is composed of earth-abundant and low-cost
Mn and Sn. Therefore, the development of an inexpensive, straightforward, solvent-free and
non-toxic fabrication method for high quality and single phase CMTS materials is desirable.
Development of a such a method is critical in order for CMTS to meet photovoltaic
technology requirements.
Firstly, the synthesis and characterisation of M(S2COR)2, M(S2COR)3 and (R´3P)2CuS2COR
R´ = Ph, R =Et M = Cu, Mn , Sn complexes were described. As [Cu(S2COEt)2],
[Mn(S2COEt)2.TMEDA] and [Sn(S2COEt)2] precursors annealed, the temperature-
dependent phase of Cu2MnSnS4 nanoparticles and thin films was successfully completed.
Through the X-ray diffraction patterns and Raman spectra conducted on the samples
annealed between 350 and 500 °C, the samples were revealed as having a tetragonal structure
with a space group of I42m.
166
5.2. Author distribution
In this work, I synthesised and then characterised xanthate complexes via IR, elemental
analysis and TGA. The experimental work to produce nanomaterials and thin films was
carried out by me, I characterised the samples by XRD, Raman, SEM, EDX and UV-Visible
spectroscopy. Mohamed Missous and Abdelmajid Salhi provided conductivity
measurements and analysis of the data. The original idea was provided by Paul O’Brien.
David J. Lewis supporting me in the project and he provided as well a nice and useful
discussion, and also editing the manuscript. The experimental work was done in the
laboratory of Paul O’Brien.
5.3. References:
1 P. Jackson, D. Hariskos, R. Wuerz, O. Kiowski, A. Bauer, T. M. Friedlmeier and M.
Powalla, Phys. Status Solidi RRL – Rapid Res. Lett., 2015, 9, 28–31.
2 K. Ito and T. Nakazawa, Jpn. J. Appl. Phys., 1988, 27, 2094.
3 M. Quintero, A. Barreto, P. Grima, R. Tovar, E. Quintero, G. S. Porras, J. Ruiz, J. C.
Woolley, G. Lamarche and A.-M. Lamarche, Mater. Res. Bull., 1999, 34, 2263–2270.
4 L. Chen, H. Deng, J. Tao, W. Zhou, L. Sun, F. Yue, P. Yang and J. Chu, J. Alloys
Compd., 2015, 640, 23–28.
5 X. Liang, P. Guo, G. Wang, R. Deng, D. Pan and X. Wei, RSC Adv., 2012, 2, 5044–
5046.
167
5.4. Effects of annealing temperature on the structural and optical
properties of CMTS (Cu2MnSnS4) nanoparticle prepared by solvent-less
thermolysis
Abdulaziz M. Alanazi,a,d Abdelmajid Salhi,c Mohamed Missousc and David J. Lewisb*
a, Department of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
b, Department of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
c, Department of Electrical and Electronic Engineering, The University of Manchester, Sackville
Street, Manchester, M13 9PL, UK.
d, Department of Chemistry, Islamic university, Prince Naif Ibn Abdulaziz Rd, Madinah, 42351,
KSA.
E-mail: david.lewis-4@manchester.ac.uk
5.4.1. Abstract
Earth-abundant Cu2MnSnS4 (CMTS) nanoparticles were prepared through a cheap, simple
and non-toxic solvent-less technique by using a mixture of copper(II), manganese(II) and
tin(II) ethylxanthate molecular precursors. The effect of annealing temperatures on the
structure, composition, morphology, and optical properties of the nanoparticles has been
studied. Characterization by X-ray diffraction, Raman spectroscopy, scanning electron
microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy and UV-Vis absorption
spectroscopy confirm that the nanoparticles are stoichiometric stannite CMTS. XRD reveals
that at high annealing temperatures (500°C) CMTS is produced as a single phase, whereas
samples annealed at lower temperatures (especially at 350°C) are contaminated with MnS.
Scherrer’s formula shows that there is a correlation between annealing temperature and grain
size. Elemental mapping of the CMTS nanoparticles reveals uniform elemental distributions
of Cu, Mn, Sn and S for every sample tested. The estimated band gap energies of CMTS are
168
in the range 1.75 to 1.40 eV and decrease with increase the annealing temperature. Champion
high-purity CMTS thin films exhibit a band gap energy of 1.16 eV, carrier concentrations of
4.89×1018, carrier mobility of 16.85 cm2 v-1 s-1, and electrical resistivity of 7.57×10-2 Ω cm.
5.4.2. Introduction
The very low price of energy generation with Si modules (<$1/W)has had an adverse
effect upon thin-film solar cell manufacturers.1 In efforts to counteract this,
researchers have concentrated upon very low cost or very high efficiency materials.
Whilst there is little difference in efficiency between Cu(In, Ga)Se2 (CIGS) thin-film
solar cells and established silicon-based PV technology, the quantities of gallium and
indium in the Earth’s crust are limited, and as they become scarcer, they will
inevitably become more expensive.2–5 Because these devices contain the rare and
expensive metals, In and Ga, the application of these devices commercially, is
constrained. However, due to the low cost, nontoxicity and abundance of elements,
thin film and nanoparticles solar cells have started being constructed using sulfide
minerals such as kesterite copper zinc tin sulfide (CZTS),6 copper zinc tin selenide
(CZTSe)7 and the sulfur-selenium alloy (CZTSSe)8–10 as substitutes to chalcogenide
materials, like CdTe and Cu(In, Ga)Se2.11,12
In addition to thin CZTS films, researchers have also concentrated their efforts on the
Cu2FeSnS4 (CFTS), which has a high optical absorption coefficient and a wide
bandgap energy. However, these features are preparation-method dependent and are
influenced by the resulting film’s crystalline structures.13–17
Copper manganese tin sulfide (Cu2MSnS4, CMTS where M= Mn) is another
compound in this group of materials, offering potential for use as a p-type
semiconductor. The absorption coefficient of CMTS, which crystallizes in the stannite
structure (space group: I42m), is high (≈104 cm-1) and it has a direct band gap energy
169
in the range of 1.0 eV to 1.4 eV; these are favourable qualities suited to PV
applications, and the crystal structure of CMTS is shown in Figure 5.1.13,18,19 CMTS’s
ferroelectric, magnetoelectric and optical properties have become the subject of
investigation in recent years.20–22 In addition, studies have explored various methods
of synthesising CMTS compounds including dip coating, electrospinning, hot
injection, microwave irradiation and spray pyrolysis techniques.18,23–26 However, each
of these synthesis methods demands special reaction conditions and involves a
complex solution phase using toxic organic solvents. The majority of research is
dedicated to synthesising CMTS thin films24,27–29
Figure 5. 1. Crystal structures for stannite Cu2MnSnS4, a = 5.449 Å; c = 10.726 Å, α. β and γ= 90°,
ICDD: 0005838.30
Xanthates have recently been used as single source precursors for metal sulfide
nanoparticles.31–35 The general chemical formula for xanthates is [M(S2COR)n],
where R is an alkyl group. Because of the pre-formed M–S bonds, xanthates are good
precursors to deposit metal sulfide thin films.36,37 Also, in comparison to other
precursors, their decomposition occurs at lower temperatures.38 A report has
described the synthesis of a number of xanthates and dithiocarbamates for uses as
170
metal sulfides; using thermogravimetric analysis (TGA) for investigated thermal
decomposition profiles of the compounds. The results of their studies indicate that in
both thin-film and nanoparticle forms, metal xanthates to be practical precursors for
Cu2ZnSnS4.39
Recently, we described the synthesis of CFTS from single source precursors using
solvent-less thermolysis which gave materials with optical band gap energy of 1.5
eV.40 Solventless thermolysis has advantages over other methods, as it is a simple
method in which solid state decomposition of a precursor is accomplished by thermal
treatment under inert conditions.41,42 This approach has confirmed to be an active way
of producing metal chalcogenide nanomaterials with a extensive range of
morphologies for example nanodisks,43 nanospheres,44 nanowires,45 and nanorods.46
Compared to the other solution based chemical approaches solvent-less thermolysis
offers a simple and economical way to scale up production. Furthermore, it offers
environmental benefits, removes the need for harsh reactants and the yields are
usually high.32
This paper considers the synthesis of several copper, manganese and tin O-
ethylxanthates and assessed their suitability as precursors in mixed complexes
reactions to form the quaternary sulfide CMTS.
5.4.3. Experimental
5.4.3.1. Chemicals
Potassium ethyl xanthogenate (96%, Sigma-Aldrich), Tin(II) chloride (99.9%),
copper(II) sulphate (98%) and manganese(II) acetate tetrahydrate (≥99%, Sigma-
Aldrich) N,N,N′,N′-Tetramethylethylenediamine (≥99%, Sigma-Aldrich) were used
with no further purification.
171
5.4.3.2. Instrumentation
Elemental analysis of the precursors was carried out by the chemistry microanalysis
laboratory at the University of Manchester. TGA was conducted from 25 °C to 600
°C under nitrogen using Mettler Toledo TGA/DCS system. Fourier transform infrared
(FTIR) spectra were obtained using a Specac single reflectance ATR and melting
points were obtained using a Barloworld SMP10 apparatus. Powder-X-ray diffraction
(p-XRD) of all the samples was carried out using a Bruker X-pert diffractometer. The
samples were scanned between 20° and 80° using CuKa radiation, the applied voltage
was 40 kV and the current 30 mA. Scanning electronic microscopy (SEM) and energy
dispersive X-ray spectroscopy analysis is carried out using TESCAN MIRA3 FEG-
SEM. The EDS was used to know the chemical composition of the samples. Raman
spectra were measured using a Renishaw 1000 Micro-Raman System equipped with
a 514 nm laser. UV-Vis spectra were collected on a Shimadzu UV-1800, using 3.09
mM solution of CMTS nanoparticles in ethanol.
5.4.3.3. Synthesis of bis (O-ethylxanthato) copper(II) (1)
The synthesis of [Cu(S2COEt)2] was carried out according to the literature.47 Briefly,
a 20 ml aqueous solution of potassium ethylxanthate (1.6 g, 9.9 mmol) and
CuSO4.5H2O (1.2 g, 4.9 mmol) were mixed at room temperature and stirred for 60
minutes. An orange precipitate was isolated by filtration and washed with deionised
water. The product was dried in a vacuum oven overnight at 30 °C. Yield: 87% (1.8
g). Melting point: 187 °C. Elemental analysis: Calc (%): C, 23.58; H, 3.30; S, 41.85;
Cu, 20.80. Found (%): C, 23.67; H, 3.21; S, 41.39; Cu, 21.09. IR (νmax/cm-1): 2979.63-
2932.92 (w), 1462.34-1367.20 (s), 1239.20(s), 1119.11(s), 846 (w).
5.4.3.4. Synthesis of bis (O- ethylxanthato) manganese(II).(TMEDA) (2)
Potassium hydroxide (0.76 g, 13.63 mmol) was dissolved in 20 ml methanol and
stirred for 2 h at room temperature. Carbon disulfide (1.04 g, 0.83 ml, 13.63 mmol)
172
was added drop-wise at 0 °C and the mixture stirred for 1 h. 50 ml of an aqueous
solution of Mn(CH3COO)2.4H2O (1.60 g, 6.80 mmol) was added drop-wise to the
reaction mixture, which was stirred for 0.5 h to form a brown/yellow solution.
TMEDA (0.79 g, 6.76 mmol) was added to the solution while stirring for 60 min to
form a brown precipitate. The solid residue was isolated by filtration and washed with
water, and the product was dried in a vacuum oven. The product was crystallized from
acetone. Yield: 88% (11.2 g). Melting point: 137 °C. Elemental analysis: Calc (%):
C, 34.86; H, 6.34; S, 30.96; N, 6.78; Mn, 13.30. Found (%): C, 34.94; H, 6.28; S,
31.26; N, 6.70; Mn, 13.01. IR (νmax/cm-1): 2980 (w), 1142-1185(s), 1032(s).
5.4.3.5. Synthesis of bis (O- ethylxanthato) tin(II) (3)
The synthesis of [Sn(S2COEt)2] was carried out by following the literature.36 Briefly,
tin(II) ethylxanthate was produced by adding an aqueous solution of K(S2COEt) (5.0
g, 31.1 mmol) into an aqueous solution of tin(II) chloride (2.95 g, 15.5 mmol) in 50
ml deionised water while stirring which continued for 60 min and resulted in a black
precipitate. The precipitate was filtered, and the product was dried at room
temperature. Yield: 88% (6.9 g). Melting point: 47 °C. Elemental analysis: Calc (%):
C, 19.98; H, 2.79; S, 35.45; Sn, 32.91. Found (%): C, 20.04; H, 2.72; S, 35.19; Sn,
32.87. IR (νmax/cm-1): 2977.2-2935.8 (w), 1462.8-1364.9 (s), 1272.8 (s), 1113.8 (s),
860 (w).
5.4.3.6. Synthesis of (O-ethylxanthato) copper(I) triphenylphosphine (4)
The synthesis of [(Ph3P)2CuS2COEt] was carried out by following the literature.48 A
mixture of triphenylphosphine (2.1 g, 8.0 mmol) and CuCl (0.4 g, 4.0 mmol) was
dissolved in 40 ml of chloroform wich was subsequently added to potassium
ethylxanthate (0.6 g, 4.0 mmol) in 40 ml of chloroform. The precipitate was filtered
to obtain a clear yellow solution after 1 h stirring. At 20 C yellow crystals of O-
ethylxanthato copper(I) triphenylphosphine were obtained. Yield: 85% (2.6 g).
173
Melting point: 185–191 C. Elemental analysis: calc. (%): C, 66.1; H, 4.97; S, 9.02;
P, 8.74; Cu, 8.96. Found (%): C, 65.7; H, 5.08; S, 8.77; P, 8.44; Cu, 8.74. IR (νmax/cm-
1): 3048 (w), 2992 (w), 1478 (m) 1433 (m), 1290 (s), 1142 (m), 1041 (m), 1009 (s),
849.5 (s), 740.8 (m), 617.7 (s), 559.2 (s).
5.4.3.7. Synthesis of Copper manganese tin sulfide quaternary system (Cu2MnSnS4)
using solvent-less thermolysis
For the synthesis of Cu2MnSnS4 nanoparticles, 2 mmol copper(II) ethylxanthates (1),
1 mmol manganese(II) ethylxanthates (2) and 1 mmol tin(II) ethylxanthates (3) were
mixed together. Then the mixture placed into a ceramic boat in a tube furnace and
annealed at 350, 400, 450 and 500 °C under nitrogen for 30 min, and the digram of
this process is shown in Figure 5.2. The obtained black residue was then characterised
by p-XRD, EDX, Raman spectroscopy, SEM and UV spectroscopy.
Figure 5. 2. Illustration of the formation of Cu2MnSnS4 nanoparticles through thermal
decomposition of copper(II) ethylxanthates (1), manganese(II) ethylxanthates (2) and tin(II)
ethylxanthates (3) and reaction using the solvent-less thermolysis.
174
5.4.4. Results and dissections
Ethylxanthate complexes of copper [Cu(S2COEt)2] (1), manganese
[Mn(S2COEt)2.TMEDA] (2) ; (TMEDA= N,N,N′,N′-tetramethylethylenediamine)
and tin [Sn(S2COEt)2] (3) were used in combination to produce CMTS at different
temperatures. The thermal stability of the complexes in the solid state was studied by
thermogravimetric analysis (TGA) in the range of 30 to 600 °C (10 °C min−1 in N2)
and resulting profiles are shown in Figure 5.3. The copper xanthate complex (1) is
stable up to 150 °C, after which there are two decomposition steps that involve mass
loss.
The first decomposition starts in the range of 150 to 180 °C and 58% of the original
mass is lost which corresponds to the calculated value of 58% for one ethyl xanthate
and half of another one. The solid residue left after decomposition was ca. 31% (calc.
31% for CuS) in the temperature range of 200 to 450 °C. The manganese xanthate
complex (2) is stable up to 150 °C, after which there is sharp single step
decomposition with major mass loss in the range of 150 to 180 °C. The TGA profile
shows that the solid residue left after decomposition was ca. 22% (calc. 21% for MnS)
in the temperature range of 200 to 350 °C. Similarly, the decomposition of the tin
xanthate complex (3) initiates at an even lower temperature of 120 °C and
decomposition is completed around 150–155 °C in a single step. The TGA profile
shows that the residual mass was ca. 42% (calc. for SnS 42%).
We therefore conclude that the decomposition of the complexes leads to the formation
of metal sulfides at temperatures, higher than 200 °C under the TGA conditions
employed here.
175
Figure 5. 3. Thermogravimetric analysis of (1) bis(ethylxanthate) copper(II), (2) bis(ethylxanthate)
manganese(II).TMEDA and (3) bis(ethylxanthate) tin(II).
5.4.4.1. XRD Characterisation
Figure 5.4 displays the XRD patterns exhibited in CMTS produced at between 350-
500°C. Prominent diffraction peaks observed at 2θ = 18.10°, 27.75°, 28.30°, 32.15°,
47.03°, 55.70° and 75.89° correspond to the (101), (110), (112), (200), (204), (312)
and (316) planes of stannite CMTS. This data corresponds with standard PDF data
(JCPDS no. 51-0757).49,50
When the temperature for annealing is increased, there is a resultant significant
increase in the (112) peak intensity, so this peak is considered as preferred orientation
as well. On the other hand, the remaining diffraction peaks remain distinct and do not
exhibit evidence of any impurities. Lattice parameters of a=b=5.50 Ǻ and c=10.85 Ǻ
were calculated. These are in accordance with the results obtained in previous
studies.49,50
176
When the temperature was lowered, diffraction peaks from a different crystalline
phase were observed at 2θ= 26.83° (marked with #), 2θ= 34.33°and 49.35° (marked
with *). We attribute these to cubic CuS2 (JCPDS no 00-019-0381) and MnS (JCPDS
no 03-065-0891). There were no observable peaks from other crystalline entities from
materials produced at higher temperatures (450-500°C).
Table 5.1 reports the full-width at half-maximum (FWHM) of the (112) peak. This
Table demonstrates that as the annealing temperature is increased there is a reciprocal
effect on the FWHM: an increase in temperature from 350°C to 500°C was shown to
cause a FWHM decrease from 0.46 to 0.36. This data can be used to estimate
relative average grain size using Scherrer’s formula51 as diffraction peak width is
directly related to internal strain, grain size and structural defects.
Figure 5. 4. P-XRD patterns of the CMTS nanoparticles prepared at different temperatures.
* Corresponds to peaks attributed to cubic MnS and # Corresponds to peaks attributed to cubic CuS2
177
Using Scherrer’s formula, the average size of CMTS grains were estimated to be 9.96
± 2.66, 10.47 ± 3.88, 14.26 ± 3.48 and 16.26 ± 2.12 nm at annealing temperatures
350, 400, 450 and 500 °C respectively. Therefore, it is evident that there is a positive
correlation between temperature and CMTS nanoparticle grain size. On the other
hand, the increase in annealing temperature did not affect the interplanar spacing of
CMTS samples.
Due to internal stresses, dislocations can sometimes occur. Dislocations are a
significant type of crystal defect and affect the material’s properties including
strength, rigidity and malleability. Therefore, it is important to mitigate the effects of
dislocations in crystal formation. Dislocation rate can be measured as the length of
dislocation lines occurring in each unit volume of crystal.52 The expression of
dislocation can vary contingent on structural properties, grain size and crystallite
formation. To estimate any resulting dislocation, the XRD line profile analysis
method can be used. This has been expressed in the Williamson-Smallman equation
as:52–54
δ = 1/D2
where D is the estimated crystal size. The dislocation density (δ) estimations
calculated using this method is displayed in Table 5.1. As evident from the values in
Table 5.1, CMTS samples annealed at lower temperatures typically exhibit more
defects (represented by decreased δ values). Chen et al. has reported that the smallest
value of δ obtained in 580°C confirms the good crystallinity of the film.28
Furthermore, the cell structure tetragonal distortion (c/2a) of the stannite
nanoparticles samples were calculated by reference to the lattice parameter ratio.
Calculations showed that at temperatures of 350, 400, 450 and 500 °C, the
corresponding c/2a were
178
0.9864, 0.9862, 0.9868 and 0.9887. This demonstrates that all stannite samples
displayed value around 1, or just less than 1and probably therefore stannite. This is
supported by existing literature.55
Table 5. 1. Lattice constants of the CMTS nanoparticles obtained from XRD patterns.
Sample
ID
d-
spacing
of
(112) (A)
Lattice
constant
a (A)
Lattice
constant
c (A)
Volume
of
crystal
(A3)
Tetragonal
distortion
(c/2a)
FWHM
of the
(112)
peak
(degree)
Crystallite
size D
(nm)
Lattice
Strain
10-3
Dislocation
density δ
(line per
m2)
CMTS-
350 °C
3.1508 5.5001 ± 0.05
10.8508 ± 0.06
328.246 ± 3.81
0.9864 0.4653 9.96 ± 2.66
8.1 10.08×1013
CMTS-
400 °C
3.1498 5.5073 ± 0.01
10.8626 ± 0.07
329.466 ± 2.30
0.9862 0.4344 10.47 ± 3.88
7.5 9.12×1013
CMTS-
450 °C
3.1508 5.4999 ± 0.02
10.8554 ± 0.04
328.363 ± 1.89
0.9868 0.3952 14.26 ± 3.48
6.8 4.92×1013
CMTS-
500 °C
3.1533 5.5063 ± 0.04
10.8889 ± 0.06
330.144 ± 1.63
0.9887 0.3638 16.26 ± 2.12
6.3 3.78×1013
5.4.4.2. Raman spectroscopy
The phase purity of the CMTS nanoparticles formed can be further examined using
Raman spectroscopy (λexc=514 nm). All the samples across all the temperatures
exhibited similar Raman spectra with a prominent peak at 327 cm-1. This peak
represents the A phonon mode which is typically the strongest mode in closely-
connected chalcopyrite crystals.24 Aside from this A mode peak, the spectra of CMTS-
350 °C and CMTS-400 °C also show a weak peak at 471 cm-1. This is likely to arise
from Cu2-xS.56
However, materials that were synthesised at lower temperatures exhibit no peaks at
292 cm-1 or 635 cm-1 as would be expected to indicate the MnS phase.57,58 This may
be due to the low concentration of MnS in both samples.
179
At the higher temperatures of 450 and 500°C, the Raman spectra display one major
peak at 328 cm-1, with no peaks representing MnS and Cu2-xS. This is further evidence
that there are fewer impurities when the crystals are annealed at a higher temperature.
Figure 5. 5. Room temperature Raman spectra of the CMTS nanocrystals prepared at different
temperatures.
5.4.4.3. Nanoparticles composition and morphology
The CMTS nanoparticles were analysed using energy dispersive x-ray (EDX)
spectrometry to determine elemental composition and structural configurations.
These results are displayed in Table 5.S1 and Figure 5.S1. As annealing temperature
was increased, a general decrease in Cu and Mn concentration was evident, which
may be attributed to their evaporation as chalcogenides. Nonetheless, all samples
examined display Mn-rich and Cu-rich composition as Mn/Sn and Cu/(Mn+Sn) > 1.
180
SEM images were also produced to explore the surface morphology of the materials.
These are shown in Figure 5.6 and display a positive correlation between temperature
and average particle size and particle accumulation.
For instance, the CMTS-350 and CMTS-400°C samples, the crystals were spherical
with small crystals, in accordance with the XRD peaks observed. Figure 5.6 (C)
shows the nanoparticles at 450°C, these crystals have a noticeably larger grain size.
At 500°C, the crystals showed agglomerates of spheres.
EDX elemental mapping of CMTS was employed to investigate the elemental
homogeneity in a spatial sense. As is clear from Figure 5.S2, there appears to be a
uniform distribution of Cu, Mn, Sn and S using a scale bar of 10 μm as the image
displays an even dispersion of colour of all four elements. This imaging analysis was
Figure 5. 6. SEM images of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500.
181
conducted on more than five different areas of each sample, and there was no
significant variation between the sites, indicating that the imaging is reliable. This
therefore further suggests that the CMTS produced are high quality with low levels
of impurity.
5.4.4.4. Optical properties
Figure 5.S3 shows the optical absorption spectra of these samples in the wavelength
range of 200–800 nm. The band gap (Eg) can be calculated by reference to the UV-
Visible spectra using the Tauc relation (αhѵ)2= A(hѵ-Eg).59 Where A is the energy
independent constant, α is the absorpation coefficient, h is Planck's constant, ѵ is the
photon frequency and Eg is the band gap energy.
The optical band gap energy can therefore be obtained from extrapolation of the
straight line portion of the (αhѵ)2 versus hѵ and reading the point of interception on the
horizontal photon energy axis, as shown in Figure 5.7. The band gaps were calculated
to be 1.75, 1.64, 1.60 and 1.40 eV for CMTS produced at temperatures 350, 400, 450
and 500 °C, respectively. These values are in the same range as those obtained in
previous studies.50,55 Chen et al. reported the changes of the optical bandgap in CMTS
thin films by spin-coating which found to be between 1.69 to 1.18 eV.50 Moreover,
Cui et al. has been successfully synthesized CMTS nanocrystal by a
182
Figure 5. 7. Tauc plots of the of the CMTS nanoparticles prepared at different temperatures 350 °C,
400 °C, 450 °C and 500 °C.
solvothermal method, and note that the band gap of this material was 1.28 eV which
indicating a potential applications in solar cells.13
Thus, increases in annealing temperature have the effect of gradually decreasing band
gap values. This may be attributed to the increased crystalline purity and
consequential compositional changes as the annealing temperature increases.60,61
As discussed above, there is a positive correlation between annealing temperature and
grain size. Therefore, as annealing temperature is increased, grain boundary density
will decrease, resulting in less electron scattering at grain boundaries.62 Thus, band
gap values will decrease as it becomes less difficult for electronic transitions to occur
between the valence band and the conduction band. This is evident as the sample with
the smallest grain size as 9.96 nm and the largest dislocation density at (10.08×1013
lines per m2), CMTS-350, also exhibits the largest band gap value at 1.75 eV. This is
shown in figure 5.8.
183
Alternatively, the changes in Eg may be attributable to the presence of secondary
phases in CMTS such as MnS. Thus, in the samples annealed at 350 and 400 °C, MnS
(Eg = 3.1 eV) was present. This may have resulted in greater Eg values.63 Furthermore,
the CMTS-500 optical gap is significantly smaller than the other samples investigated.
This may be attributed to sample purity and the lack of secondary phases.
Figure 5. 8. Variation of bandgap and grain size as a function of annealing temperature.
5.4.4.5. Electrical properties
The optical properties that we measure for these materials recommend that they may
well be valuable for applications in the absorber layers in solar cells because of the
strong optical transitions between the energy bands and high absorption coefficient (a
> 104 cm-1). We therefore studied the electronic properties of these materials as thin
films deposited using spin coating. Electrical properties of CMTS thin films deposited
at various temperatures are investigated on Hall measurements. All the CMTS films
184
deposited at various substrate temperatures exhibit the p-type conductivity, a
desirable requirement for the fabrication of heterojunction solar cells. The resistivity
(ρ), carrier mobility (μ) and carrier concentration (p) are shown in Table 5.2.
Table 5. 2. Electrical properties of CMTS films prepared by spin coating from 350 C to 500 C.
T 350 C 400 C 450 C 500 C References
ρ ( Ω cm ) 3.62×10-2 2.59×10-1 1.69×10-1 7.57×10-2 64
μ (cm2 v-1 s-1) 38.39 4.17 849.42 16.85 64,65
p (cm-3) 4.02×1018 4.40×1018 3.99×1014 4.89×1018 48
Conductivity p-type p-type p-type p-type 40,48,64
The resistivity (ρ) increased from 0.0362 Ω cm to around 0.0757 Ω cm for the
temperatures from 350 C to 500 C, respectively. In addition, we found that the carrier
concentration (p) in these films is increased from 4.02×10+18 cm-3 to 4.89×10+18 cm-3
for the from 350 C to 500 C, respectively. However, the carrier mobility (μ)
decreased from 38.39 cm2 v-1 s-1 for 350 C to 16.85 cm2 v-1 s-1 for 500 C. The same
finding for CMTS samples have been reported by Nie et al.19 The obtained values of
hole mobility and carrier density suggest that CMTS could be a potential material for
photovoltaic applications.66 Full details of the CMTS thin films preparation,
characterisation and electronic measurements are given in the ESI.†
5.4.5. Conclusions
A simple, inexpensive and non-toxic solvent-less thermolysis is introduced to prepare
Cu2MnSnS4 (CMTS) nanoparticles using single source precursors. The influence of
annealing temperatures on the grown nanoparticles was optimized. Specifically, the
secondary phase was existed with CMTS nanoparticles at low temperature. However,
185
at high annealing temperatures produced nanoparticles of CMTS with an increase
degree of crystallinity, large particles size and low dislocation density. Pure CMTS
was obtained with increasing the annealing temperatures to 450 and 500 °C. The SEM
morphology displays that increasing the annealing temperatures usually leads to
improved nanoparticle morphology of CMTS. The EDX data showed that the Mn
ratio decreased with the increase the annealing temperatures. Analysis of UV spectra
for the annealed CMTS nanoparticles displays that the band gap energy shifts toward
lower energies gradually with increasing the annealing temperatures from 1.75 to 1.40
eV; in specific, the CMTS-350 sample displays the largest band gap of 1.75 eV. The
decrease of band gap values with increasing annealing temperatures which could be
attributed to the joint effects of the upgrading of the crystalline value and the reducing
of the Mn ratio in the CMTS nanoparticles.
5.4.6. Acknowledgements
A. Alanazi is thankful to the Ministry of Higher Education in Saudi Arabia for funding and
the University of Islamic, Saudi Arabia for permission to study in the United Kingdom. We
acknowledge the EPSRC National Facility at the University of Manchester.
5.4.7. References
1 V. Bermudez, Sol. Energy, 2017, 146, 85–93.
2 S. N. Malik, S. Mahboob, N. Haider, M. A. Malik and P. O’Brien, Nanoscale, 2011, 3,
5132–5139.
3 P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann and
M. Powalla, Prog. Photovolt. Res. Appl., 2011, 19, 894–897.
4 O. Poncelet, R. Kotipalli, B. Vermang, A. Macleod, L. A. Francis and D. Flandre, Sol.
Energy, 2017, 146, 443–452.
5 C. S. Tao, J. Jiang and M. Tao, Sol. Energy Mater. Sol. Cells, 2011, 95, 3176–3180.
6 K. Ramasamy, M. A. Malik and P. O’Brien, Chem. Commun., 2012, 48, 5703–5714.
7 P. Kevin, S. N. Malik, M. A. Malik and P. O׳Brien, Mater. Lett., 2015, 152, 60–64.
8 P. Kevin, M. Azad Malik and P. O’Brien, J. Mater. Chem. C, 2015, 3, 5733–5741.
186
9 M. P. Suryawanshi, G. L. Agawane, S. M. Bhosale, S. W. Shin, P. S. Patil, J. H. Kim
and A. V. Moholkar, Energy Mater., 2013, 8, 98–109.
10 S. Tombolato, U. Berner, D. Colombara, D. Chrastina, M. Widenmeyer, S. O. Binetti
and P. J. Dale, Sol. Energy, 2015, 116, 287–292.
11 S. Siebentritt, Thin Solid Films, 2013, 535, 1–4.
12 J. Tao, J. Liu, L. Chen, H. Cao, X. Meng, Y. Zhang, C. Zhang, L. Sun, P. Yang and J.
Chu, Green Chem., 2016, 18, 550–557.
13 Y. Cui, R. Deng, G. Wang and D. Pan, J. Mater. Chem., 2012, 22, 23136–23140.
14 L. Ai and J. Jiang, Nanotechnology, 2012, 23, 495601.
15 X. Jiang, W. Xu, R. Tan, W. Song and J. Chen, Mater. Lett., 2013, 102–103, 39–42.
16 H. Guan, H. Shen, B. Jiao and X. Wang, Mater. Sci. Semicond. Process., 2014, 25, 159–
162.
17 X. Zhang, N. Bao, K. Ramasamy, Y.-H. A. Wang, Y. Wang, B. Lin and A. Gupta, Chem.
Commun., 2012, 48, 4956–4958.
18 X. Liang, P. Guo, G. Wang, R. Deng, D. Pan and X. Wei, RSC Adv., 2012, 2, 5044–
5046.
19 L. Nie, J. Yang, D. Yang and S. Liu, J. Mater. Sci. Mater. Electron., 2019, 30, 3760–
3766.
20 H.-J. Koo, Solid State Commun., 2012, 152, 1683–1685.
21 T. Fukushima, K. Yamauchi and S. Picozzi, Phys. Rev. B, 2010, 82, 014102.
22 G. Nénert and T. T. M. Palstra, J. Phys. Condens. Matter, 2009, 21, 176002.
23 X. Wang, X. Gu, H. Guan and F. Yu, Chalcogenide Lett., 2015, 12, 99–103.
24 L. Chen, H. Deng, J. Tao, H. Cao, L. Sun, P. Yang and J. Chu, Acta Mater., 2016, 109,
1–7.
25 F. Ozel, J. Alloys Compd., 2016, 657, 157–162.
26 R. R. Prabhakar, S. Zhenghua, Z. Xin, T. Baikie, L. S. Woei, S. Shukla, S. K. Batabyal,
O. Gunawan and L. H. Wong, Sol. Energy Mater. Sol. Cells, 2016, 157, 867–873.
27 S. Marchionna, A. Le Donne, M. Merlini, S. Binetti, M. Acciarri and F. Cernuschi, J.
Alloys Compd., 2017, 693, 95–102.
28 L. Chen, H. Deng, J. Tao, W. Zhou, L. Sun, F. Yue, P. Yang and J. Chu, J. Alloys
Compd., 2015, 640, 23–28.
29 A. Le Donne, S. Marchionna, M. Acciarri, F. Cernuschi and S. Binetti, Sol. Energy,
2017, 149, 125–131.
187
30 P. Dai, G. Zhang, Y. Chen, H. Jiang, Z. Feng, Z. Lin and J. Zhan, Chem. Commun.,
2012, 48, 3006–3008.
31 E. A. Lewis, P. D. McNaughter, Z. Yin, Y. Chen, J. R. Brent, S. A. Saah, J. Raftery, J.
A. M. Awudza, M. A. Malik, P. O’Brien and S. J. Haigh, Chem. Mater., 2015, 27, 2127–
2136.
32 E. Lewis, S. Haigh and P. O’Brien, J. Mater. Chem. A, 2014, 2, 570–580.
33 P. D. Matthews, M. Akhtar, M. Azad Malik, N. Revaprasadu and P. O’Brien, Dalton
Trans., 2016, 45, 18803–18812.
34 D. J. Lewis, A. A. Tedstone, X. L. Zhong, E. A. Lewis, A. Rooney, N. Savjani, J. R.
Brent, S. J. Haigh, M. G. Burke, C. A. Muryn, J. M. Raftery, C. Warrens, K. West, S.
Gaemers and P. O’Brien, Chem. Mater., 2015, 27, 1367–1374.
35 N. Savjani, E. A. Lewis, M. A. Bissett, J. R. Brent, R. A. W. Dryfe, S. J. Haigh and P.
O’Brien, Chem. Mater., 2016, 28, 657–664.
36 M. Al-Shakban, Z. Xie, N. Savjani, M. A. Malik and P. O’Brien, J. Mater. Sci., 2016,
51, 6166–6172.
37 K. Ramasamy, M. A. Malik, N. Revaprasadu and P. O’Brien, Chem. Mater., 2013, 25,
3551–3569.
38 N. Alam, M. S. Hill, G. Kociok-Köhn, M. Zeller, M. Mazhar and K. C. Molloy, Chem.
Mater., 2008, 20, 6157–6162.
39 G. Kociok-Köhn, K. C. Molloy and A. L. Sudlow, Can. J. Chem., 2014, 92, 514–524.
40 A. M. Alanazi, F. Alam, A. Salhi, M. Missous, A. G. Thomas, P. O’Brien and D.
J. Lewis, RSC Adv., 2019, 9, 24146–24153.
41 T. Alqahtani, M. Dilshad Khan, D. J. Kelly, S. J. Haigh, D. J. Lewis and P. O’Brien, J.
Mater. Chem. C, 2018, 6, 12652–12659.
42 L. Almanqur, I. Vitorica-yrezabal, G. Whitehead, D. J. Lewis and P. O’Brien, RSC Adv.,
2018, 8, 29096–29103.
43 Y.-B. Chen, L. Chen and L.-M. Wu, Inorg. Chem., 2005, 44, 9817–9822.
44 K. Abe, T. Hanada, Y. Yoshida, N. Tanigaki, H. Takiguchi, H. Nagasawa, M. Nakamoto,
T. Yamaguchi and K. Yase, Thin Solid Films, 1998, 327–329, 524–527.
45 J. Chen, L. Chen and L.-M. Wu, Inorg. Chem., 2007, 46, 8038–8043.
46 T. H. Larsen, M. Sigman, A. Ghezelbash, R. C. Doty and B. A. Korgel, J. Am. Chem.
Soc., 2003, 125, 5638–5639.
47 M. Akhtar, Y. Alghamdi, J. Akhtar, Z. Aslam, N. Revaprasadu and M. A. Malik, Mater.
Chem. Phys., 2016, 180, 404–412.
188
48 M. Al-Shakban, P. D. Matthews, N. Savjani, X. L. Zhong, Y. Wang, M. Missous and P.
O’Brien, J. Mater. Sci., 2017, 52, 12761–12771.
49 H. Guan, H. Hou, M. Li and J. Cui, Mater. Lett., 2017, 188, 319–322.
50 L. Chen, H. Deng, J. Tao, H. Cao, L. Huang, L. Sun, P. Yang and J. Chu, RSC Adv.,
2015, 5, 84295–84302.
51 Z. Su, K. Sun, Z. Han, H. Cui, F. Liu, Y. Lai, J. Li, X. Hao, Y. Liu and M. A. Green, J.
Mater. Chem. A, 2014, 2, 500–509.
52 A. A. Akl and A. S. Hassanien, Superlattices Microstruct., 2015, 85, 67–81.
53 A. A. Akl, S. A. Mahmoud, S. M. AL-Shomar and A. S. Hassanien, Mater. Sci.
Semicond. Process., 2018, 74, 183–192.
54 G. K. Williamson and R. E. Smallman, Philos. Mag. J. Theor. Exp. Appl. Phys., 1956,
1, 34–46.
55 J. Yu, H. Deng, L. Chen, J. Tao, Q. Zhang, B. Guo, L. Sun, P. Yang, X. Zheng and J.
Chu, Mater. Chem. Phys., 2018, 211, 382–388.
56 J. Tao, J. Liu, J. He, K. Zhang, J. Jiang, L. Sun, P. Yang and J. Chu, RSC Adv., 2014, 4,
23977–23984.
57 A. Anastassiadou, E. Liarokapis, E. Anastassakis and S. Stoyanov, Phys. Scr., 1988, 38,
444–447.
58 N. S. Arul, J. I. Han and D. Mangalaraj, J. Mater. Sci. Mater. Electron., 2018, 29, 1636–
1642.
59 D. B. Khadka and J. Kim, J. Phys. Chem. C, 2014, 118, 14227–14237.
60 K. Tanaka, Y. Fukui, N. Moritake and H. Uchiki, Sol. Energy Mater. Sol. Cells, 2011,
95, 838–842.
61 J. He, L. Sun, Y. Chen, J. Jiang, P. Yang and J. Chu, RSC Adv., 2014, 4, 43080–43086.
62 J. Tao, K. Zhang, C. Zhang, L. Chen, H. Cao, J. Liu, J. Jiang, L. Sun, P. Yang and J.
Chu, Chem. Commun., 2015, 51, 10337–10340.
63 C. D. Lokhande, A. Ennaoui, P. S. Patil, M. Giersig, M. Muller, K. Diesner and H.
Tributsch, Thin Solid Films, 1998, 330, 70–75.
64 S. K. Swami, A. Kumar and V. Dutta, Energy Procedia, 2013, 33, 198–202.
65 A. Tang, Z. Li, F. Wang, M. Dou and W. Mao, J. Mater. Sci. Mater. Electron., 2018,
29, 7613–7620.
66 D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang and S. Guha, Sol. Energy Mater. Sol.
Cells, 2011, 95, 1421–1436.
189
5.4.8. Supporting Information
5.4.8.1. Solvent-less thermolysis
Table 5. S1. Chemical composition and composition ratio from EDX spectra of the CMTS
nanoparticles prepared at different temperatures.
Figure 5. S1. EDX spectra of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500.
190
Figure 5. S2. EDX elemental mapping of the CMTS nanoparticles prepared at different
temperatures, (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C. Scale bars represent 10 µm in all
cases. A secondary electron SEM image of the mapped area is included in each case, labelled as SE.
Figure 5. S3. Absorption spectra of the CMTS nanoparticles prepared at different temperatures 350
°C, 400 °C, 450 °C and 500 °C.
191
5.4.8.2. Spin coating technique
The deposition of CMTS thin films using spin coating technique Glass substrates were cut
to 20 mm × 15 mm, and cleaned by acetone and water and allowed to dry. The solutions
were prepared by dissolving the mixture of 2 mmol triphenylphosphine copper(I)
ethylxanthates, 1 mmol TMEDA.manganese(II) ethylxanthates and 1 mmol tin (II)
ethylxanthates in tetrahydrofuran (Chloroform, 6 ml). A clear black solution was obtained.
The solution was used to deposit CMTS thin films on cleaned glass substrates using spin
coating techniques (Ossila, 24 V DC, 2.01 A) at 700 rpm for 30 s and allowed to dry. The
resulting films were placed into a tube furnace and heated at 450 oC for 60 min, under an
inert atmosphere. After that the furnace was turned off and the tube was allowed to cool
down to room temperature.
5.4.8.3. Electrical properties of CMTS thin films
The electrical properties of the CMTS thin films were characterized using Hall measurement
in a four-probe configuration. Conductive silver paste was used to form the four contact
electrodes. The Hall measurements performed on all CMTS samples of dimension
7mm×7mm shows that the majority carriers are holes, indicating the p-type conductivity in
the CMTS films deposited at different temperatures.
192
Figure 5. S4. XRD patterns of the CMTS films prepared by spin coating from 350 C to 500 C.
Figure 5. S5. Raman spectra of the CMTS films prepared by spin coating from 350 C to 500 C.
193
Figure 5. S6. SEM images of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500
thin films prepared by spin coating.
Table 5. S2. Chemical composition and composition ratio of the CMTS thin films prepared at
different temperatures.
194
Figure 5. S7. EDX spectra of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500
thin films prepared by spin coating.
Figure 5. S8. EDX elemental mapping of the CMTS thin films prepared at different temperatures,
(a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C. Scale bars represent 5 µm in all cases. A secondary
electron SEM image of the mapped area is included in each case, labelled as SE.
195
Figure 5. S9. Absorption spectra of the CMTS thin films prepared at different temperatures 350 °C,
400 °C, 450 °C and 500 °C.
Figure 5. S10. Tauc plots of the of the CMTS thin films prepared at different temperatures 350 °C,
400 °C, 450 °C and 500 °C.
196
Chapter 6. A molecular precursor route to quaternary
chalcogenide CFTS (Cu2FeSnS4) powders as potential solar
absorber materials
6.1. Introduction
This chapter was published as the article “a molecular precursor route to quaternary chalcogenide
CFTS (Cu2FeSnS4) powders as potential solar absorber materials” in Journal of Royal Society
Chemistry Advances (2019).1 In recent years, both the demand for and the rates of energy
consumption have steeply and rapidly increased on a global scale. Due to this increase, the research
of low-cost and high-efficiency solar cell material has become a matter of crucial importance. Cu
based multinary chalcogenides have great potential to be the new generation of solar cell materials;
their high absorption coefficient and low band gap energy increase their potential.2 There is a
newfound interest in photovoltaics at present, stimulating research into new, alternative materials
and the approaches that might be for fabricating low-cost thin-film solar cells. There are various
semiconductive materials that have been researched in this line, including CdTe/CdSe, Cu(InxGa1-
x)Se2 (CIGS), dyesensitized TiO2, organic materials, and so on.3–6 As indium and gallium are in
limited availability and as cadium is high in toxicity, interest in identifying and resarching low-cost,
non-toxic, earth-abundant photovoltaic materials has risen.6–9 One potential alternative is the
quaternary semiconductor Cu2FeSnS4 (CFTS), a strong candidate due to its optimal band gap p
(1.28–1.50 eV) and its optical absorption coefficients (> 104 cm−1). Additionally, CFTS is formed of
inexpensive, relatively non-toxic and earth-abundant elements.5,10,11 For the purposes of this
research, CFTS has been produced using a metal xanthate precursor and synthesised using a
straightforward and cost-efficient solvent-free thermolysis.
In this work, the synthesis and characterisation of M(S2COR)2, M(S2COR)3, M(S2COR)4 and
(R´3P)2CuS2COR [R´ = Ph, R =Et; M = Cu, Fe , Sn complexes are described. As [Cu(S2COEt)2],
[Fe(S2COEt)3], [Sn(S2COEt)2] or [Sn(S2COEt)4] precursors were annealed, the temperature-
dependent phase of a Cu2FeSnS4 powder was successfully prepared. As the X-ray diffraction patterns
197
and Raman spectra of powder annealed between 250 and 450 °C, it was shown that the powders were
organised in a tetragonal structure with a space group of I42m. The four probes method was used to
test for the carrier mobility and carrier density of the films when heated at 450 °C. The film’s
conduction type was found to be p-type.
6.2. Author distribution
In this work, I synthesised and then characterised xanthate complexes via IR, elemental
analysis and TGA. The experimental work to produce nanomaterials and thin films was
carried out by me, I characterised the samples by XRD, Raman, SEM, EDX and UV-Visible
spectroscopy. Firoz Alam checked the characterization of complexes and materials.
Mohamed Missous and Abdelmajid Salhi provided conductivity measurements and analysis
of the data. Andrew Thomas provided XPS measurements and analysis of the data. The
original idea was provided by Paul O’Brien. David J. Lewis supporting me in the project and
he provided as well a nice and useful discussion, and also editing the manuscript. The
experimental work was done in the laboratory of Paul O’Brien.
6.3. Citation
A. M. Alanazi, F. Alam, A. Salhi, M. Missous, A. G. Thomas, P. O’Brien and D. J. Lewis, RSC
Adv., 2019, 9, 24146–24153.
6.4. References:
1 A. M. Alanazi, F. Alam, A. Salhi, M. Missous, A. G. Thomas, P. O’Brien and D.
J. Lewis, RSC Adv., 2019, 9, 24146–24153.
2 Y. Zhang, X. Sun, P. Zhang, X. Yuan, F. Huang and W. Zhang, J. Appl. Phys., 2012,
111, 063709.
3 I. Gur, N. A. Fromer, M. L. Geier and A. P. Alivisatos, Science, 2005, 310, 462–465.
4 P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann and
M. Powalla, Prog. Photovolt. Res. Appl., 2011, 19, 894–897.
5 X. Zhang, N. Bao, K. Ramasamy, Y.-H. A. Wang, Y. Wang, B. Lin and A. Gupta, Chem.
Commun., 2012, 48, 4956–4958.
198
6 C. Wadia, A. P. Alivisatos and D. M. Kammen, Environ. Sci. Technol., 2009, 43, 2072–
2077.
7 H. Katagiri, K. Jimbo, W. S. Maw, K. Oishi, M. Yamazaki, H. Araki and A. Takeuchi,
Thin Solid Films, 2009, 517, 2455–2460.
8 D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang and S. Guha, Sol. Energy Mater. Sol.
Cells, 2011, 95, 1421–1436.
9 D. A. R. Barkhouse, O. Gunawan, T. Gokmen, T. K. Todorov and D. B. Mitzi, Prog.
Photovolt. Res. Appl., 2012, 20, 6–11.
10 F. Ozel, M. Kus, A. Yar, E. Arkan, M. Can, A. Aljabour, N. M. Varal and M. Ersoz, J.
Mater. Sci., 2015, 50, 777–783.
11 S. A. Vanalakar, P. S. Patil and J. H. Kim, Sol. Energy Mater. Sol. Cells, 2018, 182,
204–219.
199
6.5. Manuscript 1: A molecular precursor route to quaternary
chalcogenide CFTS (Cu2FeSnS4) powders as potential solar absorber
materials
Abdulaziz M. Alanazi,a,d Firoz Alam,a,b Abdelmajid Salhi,c Mohamed Missous,c Andrew G.
Thomas,b Paul O’Briena,b and David J. Lewis.b*
a School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
b School of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
cSchool of Electrical and Electronic Engineering, The University of Manchester, Sackville
Street, Manchester, M13 9PL, UK.
d School of Chemistry, Islamic university, Prince Naif Ibn Abdulaziz Rd, Madinah, 42351,
KSA.
E-mail: david.lewis-4@manchester.ac.uk
6.5.1. Abstract
In the present work we report on the synthesis of tetragonal phase of stannite Cu2FeSnS4
(CFTS) powder from Sn(II) and Sn(IV) using a solvent free melt method using a mixture of
Cu, Fe, Sn(II) / Sn(IV) O-ethylxanthates and annealed at different temperatures. The as-
synthesized powders were characterized by powder X-ray diffraction (p-XRD), Raman
spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-Vis absorption spectroscopy,
scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy,
which confirm the successful synthesis of stannite CFTS. Optical measurements shows that
Cu2FeSnS4 powders have visible light absorption onsets in the far red with direct band gap
energies in the range 1.32 eV - 1.39 eV which are suitable for acting as efficient absorber
layers in solar cells. Electronic characterisation of these materials deposited as thin films by
200
spin coating show that they are p type semiconductors with respectable carrier mobilities of
ca. 60 cm2 V-1 s-1 with carrier densities in the order of 1014 cm-1.
6.5.2. Introduction
Among inorganic semiconductors, quaternary metal chalcogenides materials have attracted
interest as light absorbers in photovoltaic applications.1–8 Copper iron tin sulfide
(Cu2FeSnS4) has drawn considerable attention in photovoltaics because of its p-type
conductivity, suitable band-gap 1.2-1.5 eV (Table 6.1) and high absorption coefficient
(> 104 cm−1).9–12 The structure of Cu2FeSnS4 is similar to the zinc blende structure. The
structures are adopted which depend on the configuration of the tetrahedral holes which are
called stannite (CFTS) and kesterite (CZTS), respectively.13 The stannite is tetragonal with
unit cell parameters a = 5.449 Å, c = 10.726 Å with a space group I42m as shown in Figure
6. 1(a), and the kesterite is tetragonal with unit cell parameters a = 5.434 Å, c = 10.856 Å
Figure 6. 1(b) with a space group I42m.13 Therefore, it also consists of inexpensive, non-
toxic and earth-abundant materials. However, the commercialized solar cells technologies
such as CdTe and Cu2InGaS4 (CIGS) have commonly used p-type semiconductors, which are
expensive, rare and toxic such as In, Ga and Cd.14 Therefore, the development of low-cost,
nontoxic and environmental friendly alternatives are needed to make high-efficiency solar
cells. Copper based chalcogenides such as Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe)
have been used as solar absorber materials in thin film solar cells.15,16
201
Table 6. 1. Reported band gaps of CFTS nanomaterials prepared by different methods
Method Band gap (eV) Refs.
Hot-injection 1.28 23
Solvothermal 1.33 9
Microwave irradiation 1.71 24
Electrospinning 1.24 25
a liquid reflux 1.32 26
solution-based 1.46 27
One of the challenges in the synthesis of CZTSSe materials is to obtain pure and
stoichiometric kesteritic materials materials as the optoelectronic properties seem to be
sensitive to in particular the Cu and Zn ratios.10,17–22
One of the alternative to CZTS is Cu2FeSnS4 (CFTS) , which has been used as an Pt-free counter
electrode in Dye-sensitized solar cells (DSSCs) as well as an absorber material in thin film solar
cells.23
CZTS and CFTS have suitable optical band gaps of around 1.4 eV and good absorption
coefficients (typically α > 104 cm-1) in the visible spectral range which is comparable to
CIGS materials, making them favourable candidates for photovoltaic applications.24,25
Hence, CZTS and CFTS thin film solar cells have reached power conversion efficiencies of
12.6% and 8.03%, respectively, where these materials are used as part of the absorber layer.26
In addition CZTS and CFTS show p-type conductivities which can be useful for pairing to
n-type materials in cell architectures.27
Up to now a range of methods have been reported for the synthesis of CFTS materials of
different shapes and sizes.28–30 Some reports focus on developing solution based processes
as an alternate to vacuum deposition. This offers the advantage of high productivity and low
processing temperatures.31 Other methods such as solvothermal process,29 hot injection28 and
202
microwave irradiation32 have been used for solution based synthesis of CFTS. However, the
solvothermal and hot injection processes have certain conditions, which give low yield, use
toxic chemicals (Ethylenediamine and Oleylamine) and require more steps like heat
treatment for 18-24 h, centrifugation and vacuum drying. Thus, it is necessary to design
inexpensive approaches for synthesis of CFTS materials. In this paper, we produce CFTS
from direct thermal decomposition of metal xanthate precursors. To the best of our
knowledge, the synthesis of CFTS powders using solvent free thermolysis has not been
reported so far. The method which we propose has many advantages and has been used to
prepare CFTS powders in large quantities in the laboratory. The technique is straight
forward, solvent free, inexpensive and single step utilizing single source precursors (SSPs)
such as xanthates and dithiocarbamate.33–35 Here, we use metal xanthate precursors because
their decomposition happens at a lower temperature and the by-products are gaseous.34–39
Figure 6. 1. Unit cell representations of Cu2FeSnS4; (a) the Stannite type structure a = 5.449 Å; c =
10.726 Å, α. β and γ= 90o, ICDD: 0005838 (b) kesterite type structure a = 5.434 Å; c = 10.856 Å, α.
β and γ= 90o ICDD: 0005843.23
203
6.5.3. Materials and experimental
6.5.3.1. Materials
Tin(II) chloride (99.9%), tin(IV) chloride (98%), carbon disulfide (99.9%), Iron(III) chloride
(97%)], copper(II) sulphate (98%), chloroform (99.8%), hexane (97%), toluene (99.7%) and
ethanol (99.8%) were purchased from Sigma-Aldrich or Alfa Aesar and used as received.
A Phillips X-PERT PRO with CuKa incident beam (λ=1.54059 Å) was used to record X-ray
diffraction patterns. The samples were scanned in the 2θ range of 10° to 80° for a period of
1 h. Scanning electron microscopy (SEM) was carried out using a Philips XL 30 FEG. The
voltage used was 40 kV. Carbon coating was carried out using an Edwards E306A coating
unit. EDX spectroscopy (Philips EDAX DX4 X-ray micro-analyser SEM) was used to
determine elemental composition as well used for elemental mapping in order to know the
spatial distribution of elements in the sample. The optical properties of the CFTS powders
were characterized by UV–Vis-NIR absorption spectroscopy recorded on a Shimadzu UV-
1800. Raman spectra were collected using a Renishaw 1000 Micro-Raman system equipped
with a 50× objective and a 514 nm laser. X-ray photoelectron spectroscopy (XPS)
measurements were performed using either a Kratos Axis Ultra or SPECS XPS instrument.
Both facilities are equipped with monochromated Al Kα X-ray sources with a photon energy
of 1486.6 eV. Emitted photoelectrons were collected using either a 165 mm hemispherical
energy analyser (Kratos) or a 150 mm hemispherical energy analyser (Phoibos 150 SPECS),
respectively. The peaks were calibrated through referencing C 1s to 284.8 eV. Infrared
spectra were recorded on a Specac single reflectance ATR instrument (4000-400 cm-1,
resolution 4 cm-1). Melting points were determined using a Barloworld SMP10 device and
the Elemental analyses of complexes were done using a Flash 2000 Thermo Scientific
elemental analyser. Thermogravimetric analysis (TGA) was performed using a Mettler
Toledo TGA/DSC 1 system under an atmosphere of dry nitrogen.
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6.5.3.2. Synthesis of metal xanthate complexes
6.5.3.3. Synthesis of potassium ethylxanthate
The synthesis of the potassium ethylxanthate was conducted in the following way. Potassium
hydroxide (11.29 g, 0.2 mmol) was dissolved in ethanol (75 ml) and cooled in an ice bath.
Carbon disulfide (15.32 g, 12.16 ml, 0.2 mmol) was added dropwise while stirring. The
ethanol was evaporated at room temperature to obtain the product, 71.8% yield.
6.5.3.4. Synthesis of bis (O- ethylxanthato) copper(II)
The synthesis of [Cu(S2COEt)2] was carried out according to the literature.40 Briefly, an
aqueous solution of potassium ethylxanthate (1.596 g, 9.9 mmol) and CuSO4.5H2O (1.242
g, 4.9 mmol) mixed at room temperature while stirring and the stirring was continue for 60
minutes. Orange precipitate was obtained and washed with deionised water. The precipitate
was filtered, and then product was finally dried in a vacuum oven overnight at room
temperature. Yield: 87%. Melting point: 187 oC. Elemental analysis: Calc (%): C, 23.58; H,
3.30; S, 41.85; Cu, 20.80%. Found (%): C, 23.67; H, 3.21; S, 41.39; Cu, 21.09%. IR
(νmax/cm-1): 2979.63-2932.92 (w), 1462.34-1367.20 (s), 1239.20(s), 1119.11(s), 846 (w).
6.5.3.5. Synthesis of (O-ethylxanthato)copper(I) triphenylphosphine
The synthesis of [(Ph3P)2CuS2COEt] was carried out by following the literature.16 A mixture
of triphenylphosphine (2.09 g, 0.008 mol) and CuCl (0.40 g, 0.0040 mol) was dissolved in
40 ml of chloroform and later it was added to the potassium ethylxanthate (0.641 g, 0.0040
mol) that was dissolved in 40 ml of chloroform. After stirring for 1 h at room temperature a
white precipitate was obtained. The precipitate was filtered to obtain a clear yellow solution.
At -20 oC the yellow crystals of O-ethylxanthato copper(I) triphenylphosphine was obtained.
Yield: 85%. Melting point: 185-191 oC. Elemental analysis: Calc (%): C, 66.1; H, 4.97; S,
9.02; P, 8.74; Cu, 8.96. Found (%): C, 65.7; H, 5.08; S, 8.77; P, 8.44; Cu, 8.74. IR (νmax/cm-
205
1): 3048 (w), 2992 (w), 1478 (m) 1433 (m), 1290 (s), 1142 (m), 1041 (m), 1009 (s), 849.5
(s), 740.8 (m), 617.7 (s), 559.2 (s).
6.5.3.6. Synthesis of tris (O- ethylxanthato) Iron(III)
The synthesis of [Fe(S2COEt)3] was carried out by following the literature.41 Briefly, an
aqueous solution of potassium ethylxanthate (1.596 g, 9.9 mmol) and an aqueous solution
of FeCl3 (0.538 g, 3.3 mmol) mixed at room temperature while stirring and the stirring was
continue for 60 minutes. The black precipitates was obtained and washed with deionised
water. The precipitate was filtered using whatman paper, and the product was finally dried
in a vacuum oven overnight at room temperature. Yield: 85%. Melting point: 118 oC.
Elemental analysis: Calc (%): C, 25.79; H, 3.61; S, 45.81; Fe, 13.34%. Found (%): C, 25.59;
H, 3.35; S, 45.36; Fe, 12.70%. IR (νmax/cm-1): 2987.63-2979.92 (w), 1458.55-1425.36 (s),
1233.18(s), 1059.25 (s), 856 (w).
6.5.3.7. Synthesis of bis (O- ethylxanthato) tin(II)
The synthesis of [Sn(S2COEt)2] was carried out by following the literature.42 Briefly, tin(II)
ethylxanthate was produced by adding an aqueous solution of K(S2COEt) (5 g, 31.1 mmol)
into an aqueous solution of tin(II) chloride (2.95 g, 15.5 mmol) in 50 ml deionised water
while stirring and the stirring was continue for 60 minutes that results in a black precipitates.
The precipitate was filtered using whatman paper, and the product was finally dried at room
temperature. Yield: 87.5%. Melting point: 47 oC. Elemental analysis: Calc (%): C, 19.98; H,
2.79; S, 35.45; Sn, 32.91. Found (%): C, 20.04; H, 2.72; S, 35.19; Sn, 32.87 IR (νmax/cm-1):
2977.24-2935.84 (w), 1462.82-1364.91 (s), 1272.76(s), 1113.75 (s), 860 (w).
6.5.3.8. Synthesis of tetrakis (O- ethylxanthato) tin(IV)
The synthesis of [Sn(S2COEt)4] was carried out using a technique that was modified from
literature.43 Briefly, SnCl4 (1 g, 3.8 mmol) was dissolved in 50 ml of toluene and added drop
206
by drop to the potassium ethylxanthate (2.5g, 15.3 mmol) in toluene at room temperature.
The reaction mixture was stirred for 60 minutes then precursor solution was evaporated
under reduced pressure and then oily residue was shaken by adding 50 ml of hexane. The
yellow crystals of [Sn(S2COEt)4] were extracted from the solution. Yield: 91.3%. Melting
point = 73 °C. Elemental analysis: Calc (%):C, 23.91; H, 3.34; S, 42.43; Sn, 19.69. Found:
C, 23.98; H, 3.29; S, 42.01; Sn, 20.11. IR (νmax/cm-1): 2987.29 (w), 1462.48-1365.84 (s),
1247.83 (s), 1025.47 (s), 860 (w).
6.5.3.9. Synthesis of Cu2FeSnS4 powders
For the synthesis of CFTS powders, 2 mmol copper(II) ethylxanthates, 1 mmol iron(III)
ethylxanthates and 1 mmol tin (II) or (IV) ethylxanthates were mixed together. Then the
mixture was heated in a furnace at 250 oC, 350 oC and 450 oC, for 1 hour under a nitrogen
atmosphere. The CFTS powders were allowed to cool-down to room temperature in the inert
atmosphere. The CFTS powder synthesised using Sn(II) and Sn(IV) are named as (1) and
(2), respectively. In addition to the synthesis of CFTS powders, we also deposited the CFTS
thin films using spin coating technique from Sn(II) and Sn(IV), which are named as (3) and
(4), respectively. Full details on the synthesis and characterisation of these thin film samples
can be found in the Supporting Information.
6.5.4. Result and discussion
6.5.4.1. Thermogravimetric analysis (TGA) of precursors
The synthesis of [Cu(S2COEt)2], [Fe(S2COEt)3], [Sn(S2COEt)2] and [Sn(S2COEt)4]
complexes were performed and their suitability for melt reactions was measured through
thermal stability measurement in a nitrogen atmosphere. Figure 6. 2 shows the TGA profiles
of [Sn(S2COEt)2], [Cu(S2COEt)2], [Sn(S2COEt)4] and [Fe(S2COEt)3], respectively. The
[Cu(S2COEt)2] and [Sn(S2COEt)4] complexes display a two- step decomposition pattern. In
case of [Cu(S2COEt)2] precursor, the mass residue obtained from the TGA profiles for the
207
first decomposition stage (58%) agreed with the theoretical value calculated for the removal
of one molecule of xanthate and half from another one (58%). While in the second step, there
is a mass loss of 31.15% in the temperature range of 200 to 450 °C that is agreed with
theoretical value (31.3%) of CuS. In the case of [Sn(S2COEt)4] the first step involves a
degradation of the mass loss 58.6% in the temperatures range of 45.61 to 120 oC obtained
from the TGA profile, which corresponds and agreed with the theoretical value calculated
for the removal of three molecules of xanthate (60.1%), and the final decomposition residue
obtained after 150°C was found to be SnS2 which is almost in conformity with the mass loss
data obtained from the TGA profile (32%) and the theoretical value (33.4). On contrast, the
[Sn(S2COEt)2] and [Fe(S2COEt)3] precursor complexes have a single step decomposition.
The single step decomposition of [Sn(S2COEt)2] occurred in the temperature range of 304-
396 °C with a mass loss of 41.6% and for [Fe(S2COEt)3] is 27% in the temperature range
of 73.19-400.70°C which are in good agreement with the theoretical values of SnS (41.7%)
and FeS2 (28.6%), respectively. Other researchers have been observed with often metal
xanthates.35,36 For instance, Almanqur et al, have successfully synthesised a series of iron
alkyl xanthate complexes to deposit iron sulfide thin films and nanostructures using the spin
coating and the solventless pyrolysis methods. The TGA profiles of these complexes showed
approximately the same with a rapid residue loss within the temperature range of 120 to 300
°C, and final step occurred between 320 to 500 °C. All complexes showed the final solid
residue amounts that matched with the calculated values for FeS2 or FeS.34
Al-Shakban et al, have synthesised the SnS thin films from diphenyltin bis(iso-
butylxanthate) complexes using aerosol-assisted chemical vapor deposition (AACVD). The
TGA profile of this complex showed two-step decomposition, the first step of which
involves elimination of the alkyl groups, followed by carbonyl sulfide (SCO). Then, the
final step may involve the loss of another carbonyl sulfide.44
208
Figure 6. 2. Thermogravimetric analysis of [Cu(S2COEt)2] (red colour), [Fe(S2COEt)3] (blue colour),
[Sn(S2COEt)2] (green colour) and [Sn(S2COEt)4] (black colour) precursors.
6.5.4.2. Bulk Structural characterisation of CFTS powders
The powder XRD patterns of CFTS synthesized at different temperatures using Sn(II) and
Sn(IV) precursors are shown in Figure 6. 3. The diffraction peaks observed at 2θ values of
28.50, 32.85, 33.36, 36.97, 47.15, 47.50, 50.93, 56.66, 70.04 and 76.69 correspond
to the (112), (200), (004), (202), (220), (204), (301), (116), (008) and (316) planes of the
tetragonal phase of stannite, respectively. The calculated lattice parameters for powder (1)
and (2) are a = 5.4501 Å, c = 10.7468 and a = 5.4467 Å, c = 10.7510 Å, respectively which
are in good agreement with the reported literature values for CFTS.29
The XRD peaks intensities increased with increasing the temperature without affecting the
phase of the powder. The average domain size of both powder (1) and (2) are approximately
13 ± 1.15 nm calculated using Scherrer’s formula. The comparison between kesterite and
stannite with experimentally determined and calculated value is represented by the tetragonal
209
distortion (deviation of the c/2a ratio from 1, where, c and a are the lattice parameters). The
tetragonal distortion parameter is important for the resulting electronic structure of the
material,45 for example, strong deviations away from the ideal structure caused by a changes
in crystal field can lead to non-degenerate valence band maxima.46–48 Therefore, it is
important to look into the tetragonal distortion values found experimentally and
theoretically. In kesterite CZTS, the c/2a ratio has been reported to be greater than 1 in a
neutron diffraction study done on powder samples.45 However, in stannite CFTS this ratio
has been reported to be less than 1, as estimated using XRD studies. In our study, the ratio
of stannite CFTS was determined to be 0.99, which is slightly less than 1 and thus this value
is in good agreement with the values in the literature.13,45
In order to prove the pristine nature of the synthesized powders and to rule out the existence
of secondary phases that were not distinguished by the XRD, Raman spectroscopy was
performed. Figure 6. 4 shows the Raman spectra of CFTS, which exhibits a large peak at
312.22 cm−1 and 317.25 cm−1 corresponding to tetragonal CFTS in both (1) and (2),
respectively. It is reported in the literature that this is the A1 symmetric vibrational motion
of sulfur atoms in CFTS.49–51
We also note the absence of Raman peaks corresponding to FeS (214 and 282 cm−1) and
Cu2SnS3 (267, 303 and 356 cm−1) which are common contaminants of CFTS,52 and is
consistent with the XRD patterns of the powders being a single crystalline phase (Figure 6.
3).
210
Figure 6. 3. P-XRD patterns of Cu2FeSnS4 powder (1) and (2) synthesised at (a) 250°C; (b) 350°C
and (c) 450°C for 1 hour.
211
Figure 6. 4. Raman spectra of Cu2FeSnS4 powder (1) and (2) synthesized at a temperature of 450°C
for 1 hour.
Figure 6. 5(a-d) show the Fe 2p, Cu 2p, Sn 3d and S 2p X-ray photoelectron spectra recorded
from powders prepared using a Sn(II) and Sn(IV) precursor. The Sn 3d spectra show no
difference between the two samples, with peaks at 486.9 eV and 495.3 eV arising from the
spin orbit split 3d5/2 and 3d3/2, respectively. It is difficult to determine the Sn oxidation state
from the Sn 3d XPS spectrum since the literature reports both Sn(II) and Sn(IV) compounds
with binding energies in the region. It is possible there is some surface oxidation for both
synthesis methods. The S 2p spectra in figure 6. 5d are fitted with three spin orbit split
doublets from S2p3/2 and S2p1/2. Both samples show significant surface oxidation with a
substantial sulphate derived peak with the S 2p3/2 at a binding energy of 168.8 eV. There is
a clear sulfide derived doublet with the 2p3/2 at a binding energy of 161.6 eV and some
residual contamination at the surface attributed to S-O, S-H or S-C at the surface.53 We found
that the Fe 2p and Cu 2p spectra in Figure 6. 5(a, b) are difficult to fit. It is well established
212
that the delectrons in these transition metals lead to a range of multiplet split features, and
complex shake up structures.54,55 Simple analysis of the binding energies of the features in
the Cu 2p3/2 region are consistent with the presence of CuO at the surface.
The strong, narrow peak at a binding energy of 932.2 eV, however, is similar to that of the
mineral chalcopyrite (CuFeS2) and more intense than would be expected for CuO.55 The Fe
2p spectra also suggest some oxidation at the surface. The binding energy of the lowest
energy peak of 711.7 eV is often attributed to the presence of FeSO4 and Fe2O3. The former
is consistent with the binding energy of sulphate derived S.54 The satellite feature at a binding
energy of 725.22 eV is 2p peak in figure 6.5a and the lower energy satellite at 716.5
indicative of the presence of Fe(III) seemingly confirming the oxidation of Fe to Fe2O3. It is
clear that the high binding energy satellite and the 711.7 eV peaks are much more
pronounced for the material synthesized from the Sn(IV) precursor, but the reasons for this
are unclear. Unfortunately, a lack of high quality XPS spectra from FeCuSnS2 standards
means it is difficult to determine contributions from this material.
213
Figure 6. 5. XPS spectra of Cu2FeSnS4 powder (1) and (2) synthesized at a temperature of 450°C
for 1 hour: (a) Fe 2p, (b) Cu 2p, (c) Sn 3d and (d) S 2p.
214
6.5.4.3. Microscopic Characterisation of CFTS powders
The scanning electron microscopy (SEM) images of the CFTS powders at different
magnifications are shown in Figure 6.6. The CFTS (2) show that the quaternary
chalcogenides particles were largely agglomerated with variation in their size, while the
agglomerated (same size) particles are obtained in CFTS (1). In both cases agglomeration of
crystals is seen without any definite shape. The compositional data and EDS spectra of CFTS
powder synthesized at 450 oC are shown in Figure 6. S1. The atomic % of Cu, Fe, Sn2+ and
S were 27.76, 13.19, 13.73 and 45.32, respectively in (1), while in (2) the atomic % of Cu,
Fe, Sn4+ and S were 25.41, 14.09, 15.40 and 45.10, respectively which indicates that both
Cu2FeSnS4 powders have the required stoichiometry. Elemental mapping of CFTS is used
to investigate the homogeneity in terms of material composition at the microscale. Figure
6.7 shows the elemental mapping CFTS powders. It is clear from the Figure 6.7 that the
distributions of Cu, Fe, Sn and S elements in the sample are uniform in a scale bar of 5µm.
Figure 6. 6. SEM images of Cu2FeSnS4 powder (1) and (2) synthesised at 450 °C for 1 hour. Scale
bar showing different magnifications.
215
Figure 6. 7. Elemental mapping of Cu2FeSnS4 powder (1) and (2) synthesised at 450 °C for 1 hour
showing the distribution of Cu, Fe, Sn and S. Scale bar represented 5 μm in all cases.
216
6.5.4.4. Optical properties
The UV-Vis-NIR absorbance spectra of the CFTS powders dissolved in ethanol are shown
in Figure 6. S2. In order to quantify the band gap energy of stannite Cu2FeSnS4 powders
synthesized at 450 oC for 1 hour, the optical absorption measurements were done in the
wavelength range of 400-1100 nm. Figure 6.8 shows (ahѵ)2 versus hѵ with a straight line
fitting, indicating the direct bands gaps of 1.32eV and 1.39eV for (1) and (2), respectively,
which are in good agreement with the literature values.29,56 Ideally, the absorber material of
an efficient solar cell should be a direct bandgap semiconductor because of strong optical
transitions between the energy bands and high absorption coefficient (α >104 cm-1). The
calculated limiting efficiency for a single band gap solar cell of Eg = 1.3 - 1.4 eV in a
simulated solar spectrum (AMG 1.5, i.e. fixed incident light) is around 30%. Hence, the
optical properties that we measure for these materials suggest that they may well be useful
for applications in the absorber layers in solar cells. The optimum thickness of absorber
layers is inversely proportional to the absorption coefficient. Hence these materials would
potentially be suitable as absorber layers for thin film cells in particular. We therefore studied
the electronic properties of these materials as thin films deposited using spin coating. Four-
probe Hall measurements performed on CFTS thin films revealed that the majority carriers
are holes (p-type), whilst the carrier mobility ranged between 58 – 60 cm2 V-1 s-1. The
estimated carrier densities in these films are of the order of 1014 cm-3. Full details of the
CFTS thin film preparation, characterisation and electronic measurements are given in the
Supporting Information.
217
Figure 6. 8. Tauc plot (ahѵ)2 vs. hѵ showing the direct bandgap of Cu2FeSnS4 Powders (1) and (2).
218
6.5.5. Conclusions
Copper, iron and tin O-ethylxanthate complexes have been successfully synthesized. The
complexes were found to decompose in the temperature range of 150 - 450°C to give the
metal sulfide as the final product in conformity with the mass loss data and were used for
the synthesis of CFTS powders. The CFTS powder (1) and (2) have been successfully
synthesised from both Sn(II) and Sn(IV) precursors respectively using pyrolysis in the
temperature range of 250 to 450 °C. The stannite phase is obtained for both CFTS powders,
which was ascertained from a tetragonal distortion parameter c/2a of less than 1 in all cases.
Absorption measurements confirm that Cu2FeSnS4 powder (1) and (2) are direct band gap
semiconductors having bandgap energies of 1.32 eV and 1.39 eV, respectively and thus are
suitable for photovoltaic absorber layer applications.
6.5.6. Acknowledgements
A. Al-A. thankful to Ministry of Higher Education in Saudi Arabia for funding and the
University of Islamic, Saudi Arabia for permission to study in the United Kingdom. DJL and
FA are funded by EPSRC grant EP/R020590/1.
6.5.7. References
1 P. D. Matthews, P. D. McNaughter, D. J. Lewis and P. O’Brien, Chem. Sci., 2017, 8,
4177–4187.
2 K. Woo, Y. Kim and J. Moon, Energy Environ. Sci., 2012, 5, 5340–5345.
3 Y.-C. Wang, D.-Y. Wang, Y.-T. Jiang, H.-A. Chen, C.-C. Chen, K.-C. Ho, H.-L. Chou
and C.-W. Chen, Angew. Chem. Int. Ed., 2013, 52, 6694–6698.
4 D. J. Lewis, P. Kevin, O. Bakr, C. A. Muryn, M. Azad Malik and P. O’Brien, Inorg.
Chem. Front., 2014, 1, 577–598.
5 A. A. Tedstone, D. J. Lewis and P. O’Brien, Chem. Mater., 2016, 28, 1965–1974.
6 Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman and M. S. Strano, Nat.
Nanotechnol., 2012, 7, 699–712.
7 D. B. Mitzi, Adv. Mater., 2009, 21, 3141–3158.
8 S.-L. Li, K. Tsukagoshi, E. Orgiu and P. Samorì, Chem. Soc. Rev., 2016, 45, 118–151.
219
9 M. Cao, C. Li, B. Zhang, J. Huang, L. Wang and Y. Shen, J. Alloys Compd., 2015, 622,
695–702.
10 D. Aldakov, A. Lefrançois and P. Reiss, J. Mater. Chem. C, 2013, 1, 3756–3776.
11 F.-J. Fan, L. Wu and S.-H. Yu, Energy Environ. Sci., 2014, 7, 190–208.
12 A. H. Reshak, K. Nouneh, I. V. Kityk, J. Bila, S. Auluck, H. Kamarudin and Z. Sekkat,
Int. J. Electrochem. Sci., 2014, 9, 955–974.
13 P. Bonazzi, L. Bindi, G. P. Bernardini and S. Menchetti, Can. Mineral., 2003, 41, 639–
647.
14 S. S. Mali, P. S. Patil and C. K. Hong, 2014, 6, 1688–1696.
15 P. Kevin, S. N. Malik, M. A. Malik and P. O׳Brien, Mater. Lett., 2015, 152, 60–64.
16 M. Al-Shakban, P. D. Matthews, N. Savjani, X. L. Zhong, Y. Wang, M. Missous and P.
O’Brien, J. Mater. Sci., 2017, 52, 12761–12771.
17 B. Ananthoju, J. Mohapatra, M. K. Jangid, D. Bahadur, N. V. Medhekar and M. Aslam,
Sci. Rep., 2016, 6, 35369.
18 S. Chatterjee and A. J. Pal, Sol. Energy Mater. Sol. Cells, 2017, 160, 233–240.
19 S. Chen, X. G. Gong, A. Walsh and S.-H. Wei, Phys. Rev. B, 2009, 79, 165211.
20 S. A. Vanalakar, P. S. Patil and J. H. Kim, Sol. Energy Mater. Sol. Cells, 2018, 182,
204–219.
21 C. Huang, Y. Chan, F. Liu, D. Tang, J. Yang, Y. Lai, J. Li and Y. Liu, J. Mater. Chem.
A, 2013, 1, 5402–5407.
22 C. Dong, W. Meng, J. Qi and M. Wang, Mater. Lett., 2017, 189, 104–106.
23 P. Dai, G. Zhang, Y. Chen, H. Jiang, Z. Feng, Z. Lin and J. Zhan, Chem. Commun.,
2012, 48, 3006–3008.
24 Q. Guo, H. W. Hillhouse and R. Agrawal, J. Am. Chem. Soc., 2009, 131, 11672–11673.
25 G. El Fidha, N. Bitri, S. Mahjoubi, M. Abaab and I. Ly, Mater. Lett., 2018, 215, 62–64.
26 W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu and D. B.
Mitzi, Adv. Energy Mater., 2014, 4, 1301465.
27 M. Adelifard, J. Anal. Appl. Pyrolysis, 2016, 122, 209–215.
28 C. Yan, C. Huang, J. Yang, F. Liu, J. Liu, Y. Lai, J. Li and Y. Liu, Chem. Commun.,
2012, 48, 2603–2605.
29 X. Jiang, W. Xu, R. Tan, W. Song and J. Chen, Mater. Lett., 2013, 102–103, 39–42.
30 L. Ai and J. Jiang, J. Mater. Chem., 2012, 22, 20586–20592.
31 T. Todorov and D. B. Mitzi, Eur. J. Inorg. Chem., 2010, 2010, 17–28.
220
32 F. Cao, W. Hu, L. Zhou, W. Shi, S. Song, Y. Lei, S. Wang and H. Zhang, Dalton Trans.,
2009, 0, 9246–9252.
33 E. A. Lewis, P. D. McNaughter, Z. Yin, Y. Chen, J. R. Brent, S. A. Saah, J. Raftery, J.
A. M. Awudza, M. A. Malik, P. O’Brien and S. J. Haigh, Chem. Mater., 2015, 27, 2127–
2136.
34 L. Almanqur, I. Vitorica-yrezabal, G. Whitehead, D. J. Lewis and P. O’Brien, RSC Adv.,
2018, 8, 29096–29103.
35 T. Alqahtani, M. Dilshad Khan, D. J. Kelly, S. J. Haigh, D. J. Lewis and P. O’Brien, J.
Mater. Chem. C, 2018, 6, 12652–12659.
36 M. Dilshad Khan, G. Murtaza, N. Revaprasadu and P. O’Brien, Dalton Trans., 2018, 47,
8870–8873.
37 F. Ozel, J. Alloys Compd., 2016, 657, 157–162.
38 J. Zhou, Z. Ye, Y. Wang, Q. Yi and J. Wen, Mater. Lett., 2015, 140, 119–122.
39 X. Zhang, N. Bao, K. Ramasamy, Y.-H. A. Wang, Y. Wang, B. Lin and A. Gupta, Chem.
Commun., 2012, 48, 4956–4958.
40 M. Akhtar, Y. Alghamdi, J. Akhtar, Z. Aslam, N. Revaprasadu and M. A. Malik, Mater.
Chem. Phys., 2016, 180, 404–412.
41 M. Akhtar, M. Azad Malik, F. Tuna and P. O’Brien, J. Mater. Chem. A, 2013, 1, 8766–
8774.
42 M. Al-Shakban, Z. Xie, N. Savjani, M. A. Malik and P. O’Brien, J. Mater. Sci., 2016,
51, 6166–6172.
43 C. L. Raston, P. R. Tennant, A. H. White and G. Winter, Aust. J. Chem., 1978, 31, 1493–
1500.
44 M. Al-Shakban, P. D. Matthews, E. A. Lewis, J. Raftery, I. Vitorica-Yrezabal, S. J.
Haigh, D. J. Lewis and P. O’Brien, J. Mater. Sci., 2019, 54, 2315–2323.
45 T. Shibuya, Y. Goto, Y. Kamihara, M. Matoba, K. Yasuoka, L. A. Burton and A. Walsh,
Appl. Phys. Lett., 2014, 104, 021912.
46 J. L. Shay and J. H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electronic
Properties, and Applications: International Series of Monographs in The Science of The
Solid State, Elsevier, 2017.
47 M. I. Alonso, M. Garriga, C. A. Durante Rincón, E. Hernández and M. León, Appl. Phys.
A, 2002, 74, 659–664.
48 K. Hönes, M. Eickenberg, S. Siebentritt and C. Persson, Appl. Phys. Lett., 2008, 93,
092102.
221
49 D. B. Khadka and J. Kim, J. Alloys Compd., 2015, 638, 103–108.
50 K. Mokurala, S. Mallick and P. Bhargava, J. Power Sources, 2016, 305, 134–143.
51 X. Meng, H. Deng, J. Tao, H. Cao, X. Li, L. Sun, P. Yang and J. Chu, J. Alloys Compd.,
2016, 680, 446–451.
52 B. Zhou, X. Yan, P. Li, L. Yang and D. Yu, Eur. J. Inorg. Chem., 2015, 2015, 2690–
2694.
53 D. J. H. Cant, K. L. Syres, P. J. B. Lunt, H. Radtke, J. Treacy, P. J. Thomas, E. A. Lewis,
S. J. Haigh, P. O’Brien, K. Schulte, F. Bondino, E. Magnano and W. R. Flavell,
Langmuir, 2015, 31, 1445–1453.
54 A. P. Grosvenor, B. A. Kobe, M. C. Biesinger and N. S. McIntyre, Surf. Interface Anal.,
2004, 36, 1564–1574.
55 M. C. Biesinger, Surf. Interface Anal., 2017, 49, 1325–1334.
56 L. Li, X. Liu, J. Huang, M. Cao, S. Chen, Y. Shen and L. Wang, Mater. Chem. Phys.,
2012, 133, 688–691.
222
6.5.8. Supporting Information
6.5.8.1. Powder from solvent-less thermolysis
Figure 6. S 1. The EDX plots of Cu2FeSnS4 powder (1) and (2) synthesised at a temperature of
450°C for 1 hour. The inset of figure 6. S1 shows the compositional data of Cu2FeSnS4 powder (1)
and (2).
223
Figure 6. S 2. The UV-Vis-NIR absorbance spectra of Cu2FeSnS4 powder (1) and (2) synthesised at
a temperature of 450°C for 1 hour.
6.5.8.2. The deposition of CFTS thin films using spin coating technique
Glass substrates were cut to 20 mm × 15 mm, and cleaned by acetone and water and allowed
to dry. The solutions were prepared by dissolving the mixture of 2 mmol triphenylphosphine
copper(I) ethylxanthates, 1 mmol iron(III) ethylxanthates and 1 mmol tin (II) ethylxanthates
or tin (IV) ethylxanthates in tetrahydrofuran (THF, 6 ml). A clear black solution was
obtained. The solution was used to deposit CFTS thin films on cleaned glass substrates using
spin coating techniques (Ossila, 24 V DC, 2.01 A) at 700 rpm for 30 s and allowed to dry.
The resulting films were placed into a tube furnace and heated at 450 oC for 60 min, under
an inert atmosphere. After that the furnace was turned off and the tube was allowed to cool
down to room temperature. The CFTS thin films deposited from Sn(II) and Sn(IV) are named
as (3) and (4), respectively.
224
Figure 6. S 3. P-XRD patterns of Cu2FeSnS4 thin films deposited using (3) and (4) and annealed at
450°C for 1 hour.
Figure 6. S 4. Raman spectra of Cu2FeSnS4 thin films deposited using (3) and (4) and annealed at
450°C for 1 hour.
225
Figure 6. S 5. SEM images of Cu2FeSnS4 thin films deposited using (3) and (4) and annealed at
450°C for 1 hour.
Figure 6. S 6. EDX plots of Cu2FeSnS4 thin films deposited from (3) and (4) and annealed at a
temperature of 450°C for 1 hour. The inset image showing the atomic percent of Cu2FeSnS4 thin
films.
226
Figure 6. S 7. Elemental mapping of Cu2FeSnS4 thin films deposited from (3) and (4) and annealed
at 450°C for 1 hour, showing the distribution of Cu, Fe, Sn and S.
Figure 6. S 8. The UV-Vis-NIR absorbance spectra of Cu2FeSnS4 thin films deposited from (3) and
(4) and annealed at 450°C for 1 hour.
227
Figure 6. S 9. Tauc plot (αhѵ)2 vs. hѵ showing the direct bandgap of Cu2FeSnS4 thin films deposited
from (3) and (4) and annealed at 450°C for 1 hour.
228
6.5.8.3. Electrical properties of CFTS thin films
The electrical properties of the CFTS thin films were characterized using Hall measurement
in a four-probe configuration. Conductive silver paste was used to form the four contact
electrodes. The Hall measurements performed on both CFTS samples of dimension
7mm×7mm shows that the majority carriers are holes, indicating the p-type
conductivity in the CFTS films deposited using (3) and (4). The carrier mobility in
CFTS thin films obtained from (3) and (4) are 58 cm2/V.s and 60 cm2/V.s,
respectively. The estimated carrier densities are 8.2×1014 cm-3 and 4.6×1014 cm-3 for
CFTS thin films obtained from (3) and (4), respectively. The same finding for CFTS
samples have been reported by Prabhakar et al.1 The obtained values of hole mobility
and carrier density suggest that CFTS could be a potential material for photovoltaic
applications.2
6.5.8.4. References
1. R.R. Prabhakar, N.H. Loc, M.H. Kumar, P.P. Boix, S. Juan, R.A. John, S.K.
Batabyal,L.H. Wong, ACS Appl. Mater. Interfaces, 2014, 6, 17661–17667.
2. D. B. Mitzi, O. Gunawan, T. K.Todorov, K. Wang, S. Guha, Sol. Energy Mater.
Sol. Cells, 2011, 95, 1421-1436.
229
Chapter 7. Conclusion and Future Work
7.1. Conclusion
An increasingly popular choice for semiconducting materials is the nanoscale rather than
bulk. In this area layered transition chalcogenides (TMCs) are drawing increased attention
due to optical, electric, magnetic properties of the materials that change at a large rate in
going to the nanoscale from that of bulk scale. The modification of both transition metals
and chalcogenide components through intercalation and replacement may result in the
production of novel properties. TMCs are non-poisonous, of negligible cost and abundant
semiconductor materials that can be used for several applications.
TMCs attract interest as a result of their impressive array of characteristics, in addition to
the broad array of uses, including solar cells, sensors, field effect transistors, and water
splitting photocatalysts. A key property of TMCs, and the main topic of this study, is their
photovoltaic (PV) potential. Since some of these compounds are relatively affordable,
plentiful and non-toxic, their capacity to be used in sustainable energy production is
unrivalled.
These valuable properties have resulted in detailed investigation and application of these
compounds. TMCs can be described in accordance with the following formula MXn, (M =
transition metal, X = chalcogenide S, Se or Te). While there are multiple methods for the
synthesis of metal chalcogenides at the nanoscale, it is clear, that to facilitate industrial
processes, an uncomplicated procedure is the favoured choice, such as the solvent-less
thermolysis, which is relatively inexpensive and nontoxic. Furthermore, this process is
economic, environmental-friendly and typical yields are usually high.
Single source precursors for the synthesis of metal sulfide nanomaterials provide many
additional benefits, which include consistency in thermal responses, and improved stability
230
both in exposure to air and in response to moisture. Furthermore, using a single source
precursor enables precise design to manage the decomposition temperature, enabling purer
nanoparticle production, with lower rates of imperfection. Downstream processing is
enhanced by using a single source precursor, as impurity levels are reduced and purification
is easier whilst the metal stoichiometry is preserved. In addition, single source precursors
are appropriate for production of powdered and thin-film nanomaterials. Lastly, but of vital
importance, using single source precursors, as compared to dual or multiple sources,
facilitates stringent stoichiometry, phase and morphology management, with greater ease.
Researchers including our team, have elaborated the application of metal xanthates
[M(S2COR)x] (M = transition metal, R = alkyl chain), using AA-CVD to produce thin films
metal sulfides and solvent-less and hot-injection thermolysis to produce nanostructured
metal sulfides.1–5 The selection of metal xanthates as a single source precursor for metal
sulfides is favourable due to their clean decomposition, observed at low temperatures.
In this study, a series of metal (Fe, Mn, Cu, Pb and Sn) complexes of organoxanthates were
synthesised. These complexes were characterized by IR spectroscopy, elemental analysis
and single crystal X-ray diffraction. Thermal stability of the complexes was analysed by
TGA. The complexes were employed as potential single source precursors for annealing and
depositing of metal sulfide nanomaterials and thin films by doctor blade deposition, hot
injection and solvent-less thermolysis, respectively. The syntheses of the complexes are
simple and use low cost chemicals.
Chapter 3 presented the synthesis of several novel bis(O-alkylxanthato)manganese(II) (alkyl
= Me (1), Et (2), nPr (3), nBut (4), nPen (5), nHex (6) and nOct (7)) complexes stabilised by
the bidentate N-donor ligand tetramethylethylenediamine (TMEDA), and their use as single
source precursors to produce Mn-S materials. They are characterised by elemental analysis,
infrared spectroscopy and thermogravimetric analysis. The X-ray single crystal structures of
231
2, 3, 4, 6 and 7 are based on a monoclinic crystal system with space groups C2/c, P21/c,
P21/c, I2/a and I2/a, respectively, while 1 is orthorhombic Pbca and 5 is triclinic P1. In all
the cases, the central Mn ions were coordinated by six atoms, bound by two chelating
xanthate ligands and a chelating TMEDA ligand in a distorted octahedral manner. Moreover,
the coordination of each of the bidentate ligands was symmetric for 2, 6 and 7 and
asymmetric for 1, 3, 4 and 5. Most of the compounds displayed intermolecular hydrogen
bonds through the sulfur atoms of the neighbouring molecules (C–HS). The distances of
these interactions were slightly shorter than the sum of the contact radii (van der Waals radii).
Hot injection was the first method used to produce manganese sulfide nanocrystals from
these complexes. Thermolysis of these complexes in oleylamine at 250 °C was also chosen,
and the temperature and capping agent were important factors in controlling the phase and
shape of the deposited material. Moreover, α-MnS nanomaterials were also annealed from
these complexes at temperature of 350 °C by using solvent-less thermolysis, which is
straightforward, solvent-free and inexpensive. Finally, α-MnS thin films were also deposited
from these complexes on glass substrates at temperature of 350 °C by using the doctor blade
method. The Mn-S materials were characterised using powder X-ray diffraction (p-XRD),
Raman spectroscopy, Scanning Electronic Microscopy (SEM) and Energy Dispersive X-ray
Spectroscopy analysis (EDS). The XRD studies showed that all the precursors broke down
cleanly by these methods to form cubic rock-salt (RS) α-MnS. The Raman peaks were almost
the same when the chain length of the precursor was increased in all methods. The difference
was in the morphologies, which were spherical, irregular and cube-shaped from hot
injection, solvent-less thermolysis and doctor blade route, respectively.
Chapter 4 describes nanomaterials of Mn doped in PbS, which were synthesised from the
precursors [Pb(S2COEt)2] (1) and [Mn(S2COEt)2(TMEDA)] (2) , using solvent-less
thermolysis. Different molecular ratios of Pb:Mn precursors were used to obtain
232
nanomaterials of stoichiometry Pb1-xMnxS (0 ≤x≤ 0.08) and showed the enhancement of the
morphology of the materials. The samples were characterised by powder XRD, Raman
spectroscopy, SEM and EDX. The p-XRD shows the shift in peaks, the change in the lattice
parameter and the change in the composition, which indicate that the successful integration
of Mn in the crystal lattice of PbS. The characterisation by powder XRD, SEM and EDX
indicated that increase in mole fraction of Mn2+ results in decrease of cell constant a and
volume of unit cell. Incorporation of Mn2+ into PbS led to an increase in the band gap from
0.87 eV to 0.89 eV, while the particle sizes decrease in the range of 24.80 to 22.07 nm.
Copper and tin based chalcogenides are widely employed for their potential use in solar
energy applications. The band gap of these materials can be tuned between 1.0 to 2.0 eV by
addition of elements such as Zn, Fe, Mn, In and Ga. Chapters 5and 6 of the thesis mainly
targeted the development of nanomaterials of Cu2MnSnS4 (CMTS) and Cu2FeSnS4 (CFTS)
by solvent-less thermolysis using a mixed precursor approach.
In Chapter 5 the synthesis of Cu2MnSnS4 nanocrystals from simple ethylxanthate complexes
of copper, manganese and tin, which were used in combination to produce a pure phase at
different temperatures, was reported. The effect of annealing temperatures on the structure,
composition, morphology, and optical properties of the processed precursor nanocrystals has
been studied. Characterization by X-ray diffraction, Raman spectroscopy, SEM, EDX
spectroscopy and UV-Vis absorption spectroscopy confirm that the nanocrystals are
nominally stoichiometric stannite, CMTS. The estimated band gap energies of CMTS
gradually decreased from 1.67 to 1.38 eV with increase in the annealing temperatures, and
this range of band gaps is suitable for photovoltaic applications.
Chapter 6 presented the synthesis of Cu2FeSnS4 powders from mixtures of ethylxanthate
complexes of copper, iron and tin(II) or tin(IV). The CFTS powders synthesised using Sn(II)
and Sn(IV) are named as (1) and (2), respectively. The CFTS powders (1) and (2) have been
233
successfully synthesised from Sn(II) and Sn(IV), respectively using the solvent-less
thermolysis in the temperature range of 250 to 450 °C. Pure stannite phase is obtained for
both CFTS powders. The strong Raman peaks clearly indicate the purity of CFTS powders.
The average domain size of both powders (1) and (2) are approximately 13 ± 1.15 nm
calculated using Scherrer’s formula. Moreover, absorption measurements confirm that
CFTS powders (1) and (2) are direct band gap semiconductors, having bandgaps of 1.32 eV
and 1.39 eV, respectively, suitable for photovoltaic applications.
To summarise, xanthate precursors have been proved to be good candidates for the synthesis
of crystalline and pure phase metal sulfide nanoparticles in high yields, short duration of
time and in low temperatures. The reagents used in synthesis were also comparatively non-
toxic, high natural abundance and very low cost. The unexpansive, non-toxic,
straightforward and solvent-less thermolysis of the xanthate precursors could produce
nanoparticles with different morphologies, particle size and band gap by controlling the
reaction conditions, viz. precursor’s concentration, growth temperature and different doping
ratios.
7.2. Future work
In order to further the role of xanthate complexes as single source precursors, future work
could focus on exploring more of the different complexes and using different methods. Four
areas suggested by the work in this thesis for early study are listed below.
Synthesis of single source precursors for manganese telluride and selenide to
compare the chemical, physical, optical and magnetic properties with those of
manganese sulfide.
234
The synthesis of Mn doped in different transition metal sulfides using single source
precursor such as Cd bis(ethylxanthato)cadmium(II) and bis(ethylxanthato)zinc(II)
to enhance the chemical, physical and optical properties of the host materials.
Deposition of ternary or quaternary materials using different combination of these
metal complexes and different methods, which may lead to novel phases and hence
novel properties.
Preparation of alloys of the composition Cu2Zn1−xFexSnS4 and Cu2Mn1−xFexSnS4
from ethylxanthate complexes as single source precursors using solvent-less
thermolysis to study the chemical, physical, optical and electrical properties for
photovoltaic applications.
7.3. References
1 M. Al-Shakban, P. D. Matthews, E. A. Lewis, J. Raftery, I. Vitorica-Yrezabal, S. J. Haigh,
D. J. Lewis and P. O’Brien, J. Mater. Sci., 2019, 54, 2315–2323.
2 S. C. Masikane, P. D. McNaughter, D. J. Lewis, I. Vitorica‐Yrezabal, B. P. Doyle, E.
Carleschi, P. O’Brien and N. Revaprasadu, Eur. J. Inorg. Chem., 2019, 2019, 1421–1432.
3 T. Alqahtani, R. J. Cernik, P. O’Brien and D. J. Lewis, J. Mater. Chem. C, 2019, 7, 5112–
5121.
4 L. Almanqur, F. Alam, G. Whitehead, I. Vitorica-Yrezabal, P. O’Brien and D. J. Lewis,
J. Cryst. Growth, 2019, 522, 175–182.
5 E. A. Lewis, P. D. McNaughter, Z. Yin, Y. Chen, J. R. Brent, S. A. Saah, J. Raftery, J. A.
M. Awudza, M. A. Malik, P. O’Brien and S. J. Haigh, Chem. Mater., 2015, 27, 2127–
2136.
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