optical and structural properties of zinc oxide...
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OPTICAL AND STRUCTURAL PROPERTIES OF ZINC OXIDE
NANOPARTICLES SYNTHESIZED BY DIFFERENT SOL-GEL ROUTES
MOHAMMED ABDULSALAM ALRAHABI
UNIVERSITI TEKNOLOGI MALAYSIA
OPTICAL AND STRUCTURAL PROPERTIES OF ZINC OXIDE
NANOPARTICLES SYNTHESIZED BY DIFFERENT SOL-GEL ROUTES
MOHAMMED ABDULSALAM ALRAHABI
A dissertation submitted in fulfilment of the
requirements for the award of the degree of
Master of Science (Physics)
Faculty of Science
Universiti Teknologi Malaysia
JUNE 2014
iii
To my beloved mother and father
To my dear wife and son Osamah
iv
ACKNOWLEDGEMENT
Alhamdulillahirabbulalamin, all praise to Allah Almighty for fulfilling
everything I required for finishing this dissertation. It has been amazing experience
throughout the periods I studied at Universiti Teknologi Malaysia (UTM) and it
would not has been so without peoples who have been involving contributing in
many ways to this work. Therefore, I want to acknowledge them as to express my
heartfelt gratitude to them.
In particular, I wish to express my sincere appreciation to my supervisor,
Prof. Dr. Zulkafli Othaman, for encouragement, critics and friendship. I would have
never been able to complete this work without his guidance, comments and advices.
I am also indebted to Ministry of Higher Education in Yemen for funding my
master study also special thanks to Universiti Teknologi Malaysia UTM for their
assistance in supplying the relevant literatures as well as my fellow postgraduate
students should also be recognised for their support. My sincere appreciation also
extends to all my colleagues and others who have provided assistance at various
occasions. Their views and tips are useful indeed. Unfortunately, it is not possible to
list all of them in this limited space. I am grateful to all my family members and
friends.
v
ABSTRACT
Transformation into nanoscale from bulk structures involves an outstanding
modification in the properties of materials. It gives an opportunity to get a novel
behaviour such as size dependent in structural and optical properties. Zinc oxide
nanoparticle is a promising material for many applications especially optical devices.
There are many methods to synthesis these nanoparticles. In this work ZnO
nanoparticles are synthesized by sol-gel method using different routes with different
stirring temperatures. The structural, morphological and optical properties of ZnO
nanoparticles are studied. The result of the XRD shows that the average particle size
of ZnO particles decreases with increasing stirring temperature. By using
Williamson-Hall formula to calculate the average size of ZnO nanoparticles it
reveals that the Zinc Acetate precursor gives smaller nanoparticles than Zinc Nitrate
(at 70 o C ~22 and ~50 nm respectively). FESEM images display the semi-spherical
shaped of the synthesized nanoparticles and emphasise that the nanoparticles
synthesized by Zinc Acetate are finer and more homogenous than those synthesized
with Zinc Nitrate. Furthermore, the photoluminescence result shows that the band
energy of the zinc oxide nanoparticles synthesized by Zinc Nitrate is 3.02 eV while
for those synthesized with Zinc Acetate is 3.10 eV. The excellent features of the
result suggest that this work may constitute a basis for the tuneable synthesis of ZnO
nanoparticles suitable for optoelectronics devices.
vi
ABSTRAK
Transformasi kepada skala nano dari struktur berskala besar melibatkan
pengubahsuaian luar biasa (ketara) di dalam sifat-sifat suatu bahan. Ia memberi
peluang untuk memperolehi kelakuan novel seperti kebergantungan saiz di dalam
sifat struktur dan optik. Nanopartikel zink oksida merupakan bahan yang baik untuk
pelbagai aplikasi terutama peranti optik. Terdapat pelbagai kaedah boleh digunakan
dalam proses sintesis nanopartikel. Di dalam kajian ini, nanopartikel zink oksida
telah disintesis melalui kaedah sol-gel dengan menggunakan bahan kimia) berbeza
dengan suhu kacau yang berbeza. Sifat struktur, morfologi dan optik terhadap
nanopartikel zink oksida dikaji. Hasil pencirian XRD menunjukkan bahawa saiz
purata partikel ZnO berkurangan apabila suhu kacau ditambah. Dengan
menggunakan formula Williamson-Hall bagi kiraan saiz purata nanopartikel ZnO,
hasil menunjukkan bahawa pelopor zink acetat memberikan saiz nanopartikel lebih
kecil berbanding pelopor zink nitrat (pada suhu 70º iaitu 22 dan 50 nm). Imej
FESEM menunjukkan bahawa nanopartikel ZnO yang terbentuk adalah semi-sfera
serta menunjukkan permukaan nanopartikel yang disintesis menerusi zink acetat
lebih halus dan sekata berbanding zink nitrat. Tambahan lagi, hasil pencirian
kefotopendarcahayaan (fotoluminasi) menunjukkan bahawa jurang jalur tenaga bagi
sampel nanopartikel ZnO bagi zink nitrat adalah 3.02 eV manakala bagi zink acetat
adalah 3.10 eV. Hasil terbaik pencirian yang diperolehi dicadangkan bahawa kajian
ini mampu memberikan asas kepada sintesis nanopartikel ZnO sesuai bagi aplikasi
peranti optoelektronik.
vii
TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES x
LIST OF FIGURES xi
LIST OF ABBREVIATION xiv
LIST OF SYMBOLS xvi
1 INTRODUCTION 1
1.1 Background of the Study 1
1.2 Problem Statement 2
1.3 Research Objectives 3
1.4 Scope and Significance of Study 4
2 LITERATURE REVIEW 5
2.1 Introduction 5
2.2 Nanotechnology 7
viii
2.2.1 Properties of Nanomaterials 7
2.2.2 Quantum Confinement Effect 8
2.3 Crystal Structure of ZnO 10
2.3.1 Energy Band Structure 11
2.3.2 The Exciton 13
2.3.3 Defects 15
2.4 Optical Properties 16
2.4.1 Optical Properties of Semiconductor 16
2.4.2 Optical Properties of ZnO 17
2.4.3 Nonlinear Optics 18
2.5 ZnO Nanostructures Application 19
2.6 Sol-Gel Method 19
3 METHODLOGY 21
3.1 Introduction 21
3.2 Experimental Set-up 23
3.3 Experimental Design 23
3.4 Analysis Method 24
3.4.1 X-ray Diffraction 24
3.4.2 Field Emission Scanning Electron Microscope 27
(FESEM)
3.4.3 Photoluminescence Spectroscopy 29
4 RESULTS AND DISUSSION 31
4.1 The Effect of Stirring Temperature on ZnO 31
nanoparticles
4.1.1 Structural Properties 31
4.1.2 Morphological Properties 35
4.1.3 Optical Properties 37
4.2 Comparative Study of ZnO nanoparticles Synthesized 38
by Two Different Routes
4.2.1 Structural Properties 38
4.2.2 Morphological Properties 41
4.2.3 Optical Properties 42
ix
5 CONCLUSION AND RECOMMENDATIONS 46
5.1 Conclusion 46
5.2 Recommendation 47
REFFERENCES 48
x
LIST OF TABLES
TABLE NO. TITLE PAGE
Table 4.1 The diffraction angle (2θ), the FWHM (β) and the size of
nanoparticles for the synthesized ZnO with different
stirring temperatures calculated from XRD results.
34
Table 4.2 The diffraction angle (2θ) and the FWHM (β) of the peak (100)
that appears in XRD for ZnO nanoparticles synthesized by
different routes with different stirring temperatures.
39
Table 4.3 The particle size and strain of ZnO nanoparticles
synthesized by different sol-gel routes calculated from
Williamson-Hall plot.
40
Table 4.4 The calculated band energy for ZnO nanoparticles
synthesized by different sol-gel routes taken from
photluminescence spectrum.
44
xi
LIST OF FIGURES
FIGURE NO. TITLE PAGE
Figure 2.1 Schematic representations of the energy levels in bulk
material and the splitting in the energy levels in
nanomaterials due to the quantum confinement effect, the
band gap increases by decreasing the size of
nanoparticles.
9
Figure 2.2 Stick-and-ball representation of ZnO crystal structures:
(a) cubic rocksalt, (b) cubic zinc blende and (c) hexagonal
wurtzite. Shaded gray and black spheres denote Zn and O
atoms, respectively.
11
Figure 2.3 A schematic diagram representing the direct gap energy of
ZnO.
12
Figure 2.4 A graphical representation of an exciton energy level in
comparison to the valence and conductive band within a
semiconductor.
14
Figure 3.1 Flow chart of ZnO nanoparticles synthesized by different
sol-gel routes.
23
Figure 3.2 (a) A basic features of X-ray diffractometer. (b) X-ray
diffraction and bragg’s law.
25
Figure 3.3 The X-ray diffractometer Bruker D8 Advance that used to
characterize the samples, located at Ibnu Sina Institute-
UTM.
27
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Figure 3.4 (a) Schematic diagram of an FESEM set-up. (b) The
(FESEM) that used to observe the samples surfaces JSM-
6701F, located at Ibnu Sina Institute-UTM.
28
Figure 3.5 (a) Diagram of PL experiment set-up. (b) Luminescence
Spectrophotometer LS55 (Perkin Elmer) located at
Material Characterization Laboratory at Physics
department-UTM.
29
Figure 4.1 XRD of ZnO nanoparticles with different stirring
temperatures 50, 70, 90, 110 and 130 o C.
32
Figure 4.2 Standard JCPDS card of bulk ZnO with hexagonal
structure.
33
Figure 4.3 Plot between particles size (d) and temperature, which
show that d decreases when stirring temperature is
increased.
34
Figure 4.4 Image of FESEM confirm the semi-spherical shape of the
synthesized ZnO nanoparticles.
36
Figure 4.5 FESEM images for ZnO nanoparticles synthesized by
different temperature: a) 70o C, b) 90
o C.
36
Figure 4.6 Photoluminescence spectrum of ZnO nanoparticles
synthesized with different temperatures.
37
Figure 4.7 XRD of ZnO nanoparticles using different routes at 90 oC
stirring temperature.
38
Figure 4.8 Williamson-Hall plot for ZnO nanoparticless for different
routes. The strain is extracted from the slope and the
crystalline size is extracted from the y-intercept of the fit.
40
Figure 4.9 FESEM images shows the ZnO nanoparticles synthesized
at the same temperature using different precursor (a) Zinc
Nitrate, (b) Zinc Acetate.
41
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Figure 4.10 FESEM images with different magnifications for ZnO
nanoparticles synthesized by different precursor at the
same temperature: (a) Zinc Nitrate (10,000X, 25,000X
and 50,000X), (b) Zinc Acetate (10,000X, 25,000X and
50,000X).
42
Figure 4.11 Photoluminescence of ZnO nanoparticles synthesized by
sol-gel with different routes: (a) Zinc Nitrate precursor.
(b) Zinc Acetate precursor it shows the head peak occurs
at 400 nm and 410 nm respectively, as well as the big
shoulder peak for both around 445 nm.
43
Figure 4.12 Schematic representation of the electron-hole
recombination (EHR) energy 3.10 and 3.02 eV for ZnO
nanoparticles synthesized by Zinc Nitrate and Zinc
Acetate respectively and Position of the defect level 2.79
and 2.41 eV attributed to Oxygen vacancies (VO) in
singlet (SO) and triplet (T1) states respectively. Finally the
Zinc interstitial (Zni) which caused the defect level at 2.18
eV.
45
xiv
LIST OF ABBREVIATION
1D - One Dimensional
2D - Two Dimensional
3D - Three Dimensional
Å - Angstrom (=10-10
m)
arb. u. - Arbitrary Unit
C.B - Conduction Band
DC - Direct Current
DOS - Density of State
EHR - Electron-Hole Recombination
eV - Electron Volt
FESEM - Field Emission Scanning Electron Microscopy
FWHM - Full Width Half Maximum
GaAs - Gallium Arsenide
Ge - Germanium
IR - Infra Red
JCPDS - Joint Committee on Powder Diffraction Standard
nm - Nanometer
NPs - Nanoparticles
LED - Light Emitting Diodes
Oi - Oxygen Interstitial
PL - Photoluminescence
QD - Quantum Dot
Rad. - Radian
Si - Silicon
So - Singlet State
T - Temperature
xv
T1 - Triplet State
UV - Ultra-Violet
V.B - Valence Band
Vo - Oxygen Vacancy
XRD - X-Ray Diffraction
W-H - Williamson-Hall
Zni - Zinc Interstitial
ZnO - Zinc Oxide
xvi
LIST OF SYMBOLS
A/V - Surface to Volume Ratio
d - Size of Particles
E - Energy
Eg - Band Gap Energy
ξ - Electric Field
k - Shape Factor
β - Full Width Half Maximum
λ - Wavelength
θ - Angle of Diffraction
ε - Strain of Particles
1
CHAPTER 1
INTRODUCTION
1.1 Background of the Study
Recently design, characterization, production, and application of materials,
which involves the manipulation of matter at the smallest scale (from 1 to 100 nm),
have been widely used as current meaning of nanotechnology rather than just
materials. Three distinct aspects can be considered for evolution of nanotechnology:
indirect, direct, and conceptual. The advanced miniaturization of obtainable
technologies, which open new areas of application for those technologies, can be
explained by indirect aspect. Direct refers to the application of novel nanoscale
artifacts to improve the performance of presented process and materials or for
completely novel purposes. Finally, there is a conceptual aspect of nanotechnology,
in which all materials and process considered from molecular or even atomic
viewpoint especially in living system and biology. Now a few areas of technology
are exempt from the advantages of nanotechnology. The information and
communication systems such as novel semiconductor and optoelectronic device,
environment (filtration), energy (reduction of energy, consumption increasing, the
efficiency of energy production nuclear accident cleanup and waste storage), heavy
industry (aerospace and catalysis), and consumer goods are some applications of
nanotechnology (Capper et al., 2011; Jagadish et al., 2011).
2
Zinc oxide (ZnO) is a chemical compound which commonly grows in a white
powder form that is insoluble in water. This powder is mostly used as an additive
into numerous materials and products such as ceramics, plastics, glass etc. Although
ZnO exists as a mineral zincite in the Earth crust, most of the commercially usage is
produced synthetically. Zinc and oxygen belong to the second and sixth groups in
the periodic table respectively (Morkoç et al., 2008).
ZnO nanoparticles have been used extensively as the preferable
semiconductor because of their potential applications, as piezoelectric transducers,
optical waveguides, acousto-optic media, surface acoustic wave devices, conductive
gas sensors, transparent conductive electrodes, solar cell windows, and varistors for
about two decade (Kim et al., 2005; Look et al., 2004; Schwenzer et al., 2006;
Shaoqiang et al., 2005). ZnO is a II–VI semiconductor which has wide-band gap of
~3.37 eV and large exciton binding energy of 60meV (Yamamoto et al., 2001)
therefore, it is now known as a promising candidate for blue and ultraviolet light-
emitting diodes (Goyal and Kachhwaha, 2012), laser diodes, and LEDs for general
illuminations such as traffic signals, backlight for liquid-crystal display (LCD),
mobile phone, and automotive lighting. The large exciton binding energy allows
excitonic absorption and recombination even at room temperature, which makes this
material interesting for optoelectronic devices (Schwenzer et al., 2006; Yamamoto et
al., 2001).
1.2 Problem Statement
Controlled synthesis and fabrication of semiconductor nanostructure by
simplicities and cost effectiveness method is challenging for electronic,
optoelectronic and sensor application. Research shows that, through metal oxide
semiconductor nanoparticles, the ZnO semiconductor is a preferred engineering
material for its advantages including its wide bandgap (3.37 eV). ZnO presents itself
as one of the potential and important material for the fabrication of light emitting
diodes, laser diodes, optical switches and so on (Jagadish et al., 2011; Takahashi et
3
al., 2007; Zhang et al., 2012). Moreover, the high exciton binding energy (60 meV)
makes it as an appropriate choice for the room temperature ultraviolet laser diodes
(Lim et al., 2006).
Remarkably, a variety of techniques has been used to control the size of ZnO
particles in the nanometer range. Experimental observation requires further
verification and surface modification require further studies. The results generated
from sol-gel method need to be compared with other techniques. However it can be
observed from the previous studies that to attain ZnO nanostructure usually catalyst
or additives with a high temperature and/or low pressure is required (Umar et al.,
2008).
Consequently, there is a need to improve an effective and simple method to
synthesis these types of nanoparticles in a large quantity with a high quality at low-
temperature without using catalyst or additives. The easy and economic nature of our
method is suitable for the fabrication of varieties of other nanostructures to
understand the fundamental physics of nanoscale structure under different synthesis
condition is also important.
1.3 Research Objectives
The aims of this research will focus on the following points:
To determine the structural properties of ZnO nanoparticles synthesized by
two different sol-gel routes with different stirring temperatures using X-Ray
diffraction (XRD).
To investigate the surface morphology of the nanoparticles by Field Emission
Scanning Electron Microscopy (FESEM).
4
To characterize the optical behaviour using photoluminescence spectroscopy.
1.4 Scope and Significance of Study
Sol-gel method with two different routes is employed to synthesis ZnO
nanoparticles. X-ray diffraction and Field emission scanning electron microscopy
(FESEM) are used to characterize the structural properties and surface morphology
of the samples. The effect of synthesis parameters on optical behaviour are studied
by photoluminescence (PL) spectroscopy.
Nanostructuring of semiconductors is a novel means of developing new
electronic and optoelectronic devices. In particular, the discovery of room-
temperature visible photoluminescence (PL) from ZnO nanostructures has stimulated
much interest in these particular kinds of nanostructures and in small semiconductor
particles. Easy and economic fabrication technique would be developed. The
instrumentation for large-scale fabrication has social economic impact. The
fundamental physics behind the synthesis would be understood.
ZnO nanoparticles can be exploited extensively for the use in optoelectronic
devices such as high bright full color light emitting diodes (LED), high quality flat
panel displays, high efficiency blue laser and low threshold optical pumped laser
applications at room temperature (Wang et al., 2003).
48
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