preparation and properties of zn0.9ni0.1o diluted magnetic semiconductor nano particles 2010 journal...

8/8/2019 Preparation and Properties of Zn0.9Ni0.1O Diluted Magnetic Semiconductor Nano Particles 2010 Journal of Nano Par

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RESEARCH PAPER

Preparation and properties of Zn 0.9 Ni 0.1 O diluted magnetic

semiconductor nanoparticlesK. Srinivas S. Manjunath Rao

P. Venugopal Reddy

Received: 8 March 2010 / Accepted: 5 September 2010 Springer Science+Business Media B.V. 2010

Abstract With a view to study the structural,electronic, magnetic, and electrical properties of Zn0.9 Ni0.1 O diluted magnetic semiconductor nano-particles, systematic investigation has been under-taken. Samples were prepared for the rst time byhydrazine-assisted polyol method, and the powderswere annealed at various temperatures in order toobtain the samples with different grain sizes. Fromthe Rietveld rened XRD data, lattice parameters, theaverage crystallite size values, and r.m.s micro-strainvalues were computed. From the AFM and TEMstudies, the average particle sizes were obtained andare found to be in the range 1246 nm. XPSmeasurements clearly indicate that the chemicalstates as ? 2 for both Zn and Ni ions and are stablewith varying annealing temperature. Further, usingXPS and optical studies, the electronic structure of the materials was analyzed. A careful phase analysisof the Rietveld rened XRD data (at logarithmicscale) selected area electron diffraction patterns,FTIR, Raman, and XPS studies; it was concludedthat all the samples are having hexagonal wurtzite

structure without any detectable impurity phases. Theoptical band gap values are found to exhibit a clearblue shift. The inuence of oxygen vacancies on theemission spectra was studied by Photo Luminescencemeasurement. The magnetization studies were under-taken by VSM, MFM, and FMR techniques andconrmed the presence of clear room temperatureferromagnetism without any magnetic clusters. Thecarrier concentration ( n) values obtained from thethermo power studies are found to decrease withincreasing annealing temperature and depend onthe local defects which are critically inuenced bythe annealing temperature and crystallite size of thenanomaterials.

Keywords Nanoparticles Optical band gap RT ferromagnetism Diluted magnetic Semiconductors Carrier concentration Synthesis

Introduction

Diluted magnetic semiconductors (DMSs) play a vitalrole in the eld of spintronics due to their ability toaccommodate electron charge, spin degrees of free-dom, and their interplay (Ohno 1998 ; Matsumotoet al. 2001 ). Recent advances have led to theexploration of tunable properties by changing theirconstituent stoichiometries and synthesis conditions.As the lattice and conductivity mismatch at metal/ semiconductor interfaces critically inuences the

K. Srinivas P. Venugopal Reddy ( & )Department of Physics, Osmania University,Hyderabad 500007, Indiae-mail: [email protected]

S. Manjunath RaoCentral Instruments Laboratory, University of Hyderabad,Hyderabad 500046, India

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effective spin injection (Schmidt et al. 2000 ; vanWees 2000 ) and in search of high T c ferromagneticDMSs, interest has been focused on the developmentof wideband gap oxide-based diluted magnetic semi-conductors (ODMSs) (Sarma 2003 ; Chu et al. 2007 ;Sato and Yoshida 2001 ; Shi and Duan 2009 ).

Although, extensive study has been going on varioustypes of ODMSs with different dopants and withvarying compositions for the past decade, no deniteconclusion on the origin of intrinsic ferromagnetismhas been arrived at. In recent times, the studies of nanometric size effects on various properties of ODMSs further renewed the interest due to theirpotential applications in nanoscale spintronic devices,magneto-optical devices, magnetic sensors, and otherbio-medical applications (Matsumoto et al. 2001 ;Sarma 2003 ; Xu et al. 2008 ; Han et al. 2006 ;Punnoose et al. 2006 ). However, for sustainable RTferromagnetism, understanding and tuning of theirstructural, electronic, magnetic, and electrical prop-erties is crucial. Therefore, elaborative investigationsare needed in search of high T c ferromagnetic andcontrollable spin properties of real suitable materialsat nanoscale.

Among the ODMSs, as ZnO is an inexpensivealternative for tin-doped indium oxide (ITO), transi-tion metal (Co, Mn, Ni, Fe, Cr, etc.)-doped ZnO withwurtzite structure has attracted considerable attention,due to their above room temperature ferromagnetism(RT FM) and their suitability in variety of devices.Among the known TM-doped ZnO systems, Ni-basedmaterials are the most interesting and importantones due to their typical magnetic behavior. Aftertheoretical prediction of ferromagnetism above roomtemperature by Sato and Yoshida ( 2001 ), severalresearchers investigated their structural and magneticproperties. Recent investigations (Radovanovic andGamelin 2003 ; Pei et al. 2006 ; Cong et al. 2006 ; Thotaet al. 2006 ; Li et al. 2006 ; Cheng et al. 2007 ; Zhanget al., 2007 ; El-Hilo et al. 2009 ; Lojkowski et al. 2009 )based on Ni-doped ZnO nanoparticles are moreinteresting due to their possible applications inspintronics. Moreover, the study on magnetic nano-particles exhibiting variety of phenomena such assuperparamagnetism, quantum tunneling of magneti-zation, magnetism induced by surface effects, etc. isalso a motivating factor. Further, to avoid theprecipitation of TM impurities at higher dopingconcentrations, there is a need to develop a suitable

synthesis method with reproducible results. Loweringof the decomposition temperature has better possibil-ity to obtain the material with ne particle size andwithout any agglomeration. As a matter of fact, thecoarse nature of these submicron size particles mayaffect the magnetization and coercivity. Moreover, the

proper defect level is also crucial for making a goodferromagnetic DMS. Although several workers madeefforts to dope higher than 5% Ni in ZnO, a few of them (Liu et al. 2009 ; Zuo et al. 2009 , Lojkowski et al.2009 ) were successful in preparing single phasematerial exhibiting clear RT ferromagnetism. How-ever, with 10 mol% doping of Ni, Co, Cr, and Mn,except in Mn 2 ? -doped ZnO, all other samples arefound to exhibit unidentied secondary phases. It isimportant to know the possibility of increasing thesolubility of dopant by decreasing the nanometric sizeof the materials (Straumal et al. 2008 ; Straumal et al.2009 ). Moreover, the role of suitable synthesisconditions in avoiding the metallic precipitates iscrucial. Apart from this, improving the conductivityand magnetization of the materials is important fordevice applications. In fact, the manipulation of thesurface states of nanosized ZnO particles may alsoinuence the magnetic properties of these materials(Radovanovic and Gamelin 2003 ; Schwartz et al.2004 ). Therefore, the authors of present investigationprepared single phase Ni (10%)-doped ZnO nanopar-ticles for the rst time by a novel hydrazine-assistedpolyol method to investigate the inuence of nano-metric size on structural, optical, electrical, andmagnetic properties and their tunability vis-a-vis thenanosize. The results of such an investigation arepresented here.

Experimental

Nickel (10%)-doped zinc oxide nanoparticles (with-out forming metallic clusters or secondary phases)were prepared by hydrazine-assisted polyol chemicalroute, mainly to improve the solubility of Ni (10%)dopant and to control ZnNiO interactions. Thismethod facilitates the incorporation of a higherpercentage of dopant ions, far above the thermody-namic solubility limit of nickel and utilizes surfac-tants to the chemical nucleation and growth of nanoparticles at lower temperatures inuencing theequilibrium conditions. In this method, the hydrazine

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acts as a fuel for the combustion reaction and alsoacts as a foaming agent and polymerization catalyst,thus preventing the precipitation of hydroxylatedcompounds. Moreover, hydrazine is used as a con-venient antioxidant. Thus, hydrazine (N 2H4) inte-grates these materials easily and may also slow down

the oxidation process. The hydrazine released atlower temperatures reacts with oxygen in the atmo-sphere liberating enormous energy which is sufcientfor the oxidative decomposition of the complexes.This method is a simple approach for the preparationof nanomaterials with uniformly distributed particlesat lower temperatures. Further, the evolved nitrogenor hydrogen in the reaction of hydrazine with oxygenmay be trapped at the grain boundaries or inuencesthe oxygen vacancies in ZnO:Ni lattice and is usefulin enhancing the electronic properties (Tong et al.2010 ).

The starting materials, Zinc (ZnCl 26H2O, 99%,Aldrich) and Nickel (NiC1 26H20, 99% Aldrich)chlorides, were dissolved in an ice-cooled 0.5 mol/Laqueous solution of ethylene glycol (AR, Sd ne) andhigh purity deionized water with vigorous stirring.Later, citric acid (AR, Sd ne) was added to thissolution (citric acid:ethylene glycol = 1:2) to convertinto citrates. The mixed solution (pH = 2) in themolar ratio of Zn 2 ? /Ni2 ? = 0.9:0.1 was stirred with amagnetic stirrer for 30 min at room temperature andlater hydrazine monohydrate ((NH 2)2H2O, 80%) wasadded dropwise until pH value of 8 was reached. Inthis process, the solvent is crucial, which acts as thereaction medium that allows relatively high temper-ature required for the crystallization of inorganicmaterials and controls the size of the nanoparticles,whereas the addition of ethylene glycol also preventsthe formation of Ni clusters. The reaction mixture wasthen transferred to an oil bath maintained at 70 C andconcentrated slowly for about 12 h followed bycooling. The product was separated from the suspen-sion by centrifugation and then suspended in ethanoland subjected to vigorous agitation for 1 h to separatethe agglomerates formed. After the product wasltered and washed in hot water to remove adsorbedhydrazine and chlorides, it was dried at 120 C forabout 12 h. Thereafter, the powders were calcined at200 C for about 6 h to get crystalline ZnO:Nipowders. Finally, the product was annealed in airbetween 300 and 600 C for about 3 h to producenanocrystalline powders of varying particle sizes.

Thermogravimetric and differential thermal anal-yses have been carried out using Shimadzu ThermalAnalysis System [model DTG-60H]. The X-raydiffraction (XRD) measurements were undertakenusing a Phillips (Xpert) diffractometer with Cu K aradiation ( k = 1.5406 A ). In the present investiga-

tion, the detection limit of X-ray diffractometeremployed was * 1.5%, which is below the dopedconcentration (10%) of nickel. The X-ray diffractionpatterns were analyzed by MAUD (Materials Anal-ysis Using Diffraction) program (Lutterotti 2000 ;Lutterotti et al. 1999 ) using Rietveld whole proletting method mainly to determine the latticeparameters, crystallite size, and the r.m.s strains of the materials. The peak heights and shapes were ttedto an isotropic sizestrain model. The instrumentalbroadening such as instrumental asymmetry andGaussianity of reections was estimated using a Sistandard sample. The surface topographical studieswere undertaken on pellet samples using Atomicforce microscope (Model-CP-II, Veeco Instruments).For this purpose, phosphorus ( n)-doped Si probe(Model: RTESPA-CP, MPP-11123) was used intapping mode. The micro-structural studies wereundertaken by a transmission electron microscope(TEM, JEOL JEM-200CX working at 160 kV). Thecomposition of the materials was estimated by EDSand X-ray photoelectron spectroscopy (XPS) [KRA-TOS X SAM 800]. Fourier transmission infrared(FTIR) spectra of the powders were recorded usingFTIR thermo Nicolet spectrometer (Model: AVA-TAR330) in the range 4000400 cm - 1 with a reso-lution of 0.52 cm - 1. Raman spectra were recordedat room temperature with an optical microscope usingthe line of an Ar ? laser as an excitation source. Thebackground and the Rayleigh line corrections weremade using a commercial high purity (99.99%) SnO 2powder. The spectral resolution is 2 cm - 1 . To studythe electronic state of elements, substitution, and alsoto understand the electronic structure of the materials,the X-ray photoelectron spectroscopic (XPS) mea-surements were carried out on a KRATOS AXIS165X-ray photoelectron spectrometer. Using excitationenergy of 1253.6 eV (Mg K a ), the spectra wererecorded with pass energy of 80 eV. The anglebetween the detector and the X-ray ux direction wasconstant at 90 , while the measurements were madeat an electron take off angle of 70 . The calibra-tion of the spectrometer was done using both Au

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4 f (at 83.96 eV) and C1 s (at 285 eV) features. Inorder to account for the binding energy values and tominimize the surface charge contributions, the bind-ing energies were corrected for the charging effectwith reference to the peak position of C 1 s at284.6 eV. The maximum error in our measurement

may be 0.01 eV. Optical absorbance measurementswere carried out at room temperature using UVVisible spectrophotometer (Model: Evolution 300,Thermo electron corporation) in the wavelengthrange 200600 nm to determine the optical bandgap of Zn 1 - x Ni x O, while the Photo Luminescencestudies were carried out using uorolog spectrouorimeter (Jyobin Vyon) with a 450 W Xenon lampsource. In order to study the ferromagnetic behaviorof the materials, magnetization measurements wereundertaken using a Vibrating Sample Magnetom-eter (Model: DMSADE-1660 MRS) by applyingmagnetic eld up to 15 kOe. To study the behaviorof Zeeman splittings and the exchange interactions,the ferromagnetic resonance (FMR) spectra werealso recorded at room temperature on a JOEL PE-3XX-band spectrometer equipped with 9153.593 MHzeld modulation unit. FMR was optimized formodulation amplitude, receiver gain, time constant,and scan time. The experimental conditions were keptconstant with equal quantity for all the samples. Allthe FMR measurements were carried out by usingDPPH ( g = 2.00455) as a standard. Error in evalu-ating geff is & 0.005. Magnetic force microscopy(MFM) studies were undertaken (AFM model CP-IIVeeco Instruments) using commercial CoCr-coatedSiN probe (model: MESP) at specic heights of liftmode. For all the MFM measurements air damping

effects are meticulously avoided. The MFM probewas magnetized along the tip direction for about160170 s using a commercial MFM tool kit (Model:PSIT-0005 for CP-II SPM). The room temperatureDC resistivity at room temperature was measuredusing a (Keithley 485 Autoranging) PicoAmmeter,

(Keithley 2000) DC multimeter, while thermoelectricpower was measured using the differential techniquemethod. The temperature gradient of both the sides of the sample was measured with the help of chromelalumel thermocouples monitored by two micro-voltmeters, while the thermo-electromotive force(D V ) developed across the sample was measured bya nanovoltmeter (Keithly 181).

Results and discussion

The decomposition process of the calcined precursorpowder was analyzed by TGDTA studies, and thecorresponding plot of the starting precursor is shownin Fig. 1a. The TGDTA curves show a minor weightloss step between 130 and 180 C along with a majorweight loss step from * 200 to 295 C. The minorweight loss is related to hydroxide, water, and carbondioxide, while the major weight loss of approxi-mately 21.5% with an enthalpy change of 3 kJ/gmight be due to combustion of citric acid andhydrazine. As no further weight loss has beenobserved beyond 300 C, one may conclude that thenanocrystalline ZnO:Ni compound might have beenformed at this temperature. In view of this, post-annealing was carried out in the temperature range

T r a n s m

i t t a n c e

( % )

Wavenumber (cm -1)

(1)-ZNO300(2)-ZNO400(3)-ZNO500(4)-ZNO600

(1)(2)(3)(4)

(b)

3500 3000 2500 2000 1500 1000 500100 200 300 400

4.0

4.5

5.0

5.5

6.0

-30

-20-20

-10

00

10

2020

30

D TA

W i e g h

t L o s s

( m g )

Temperature (C)

TGA

E n

d o

E x o

H e a

t F l o w

(a)

Fig. 1 a TG/DTA analysis of calcined precursor of Ni (10%)-doped ZnO powder and b FTIR spectra of nanocrystalline ZnO:Ni(10%) samples annealed at different temperatures

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300600 C mainly to vary the nanometric size of thematerials.

In order to understand the inuence of Ni substi-tution and annealing temperature, FTIR spectralstudies were carried out and the plot is shown inFig. 1b. A distinct characteristic intense broad band

in the vicinity of * 460506 cm - 1 was observed andis assigned to ZnO vibrations (Kwon et al. 2002 ;Silva and Zaniquelli 2002 ). The vibrational modes at* 667 cm - 1 might be due to the substitution of Ni 2 ?

for Zn 2 ? sites (Ghosh et al. 2008 ). Interestingly, withincreasing annealing temperature, the FWHM of thisvibrational mode is found to decrease. A broad IRpeak with less intensity observed in the range 3380 -3600 cm - 1 is attributed due to stretching vibration of hydrogen bond, indicating the existence of hydroxylgroup and water adsorbed on the surface.

The phase analysis has been carried out bycomparing with the standard PCPDFWIN data les,and the XRD data were analyzed by Rietveld rene-ment technique using MAUD software. Figure 2ashows the XRD patterns of all the samples, whileFig. 2b shows a typical Rietveld rened plot of ZNO600 sample. By comparing the XRD patternsof present investigation with the standard data of

undoped ZnO (JCPDS: 36-1451), it has been con-cluded that all the samples are having hexagonalwurtzite structure with space group of P63mc and thatthe wurtzite structure is not disturbed due to thesubstitution of Ni. No additional secondary phasessuch as NiO, Zn, or Ni metallic phases were observed

in all the samples suggesting that nickel occupies zincsites in Zn 1 - x Ni x O. The lattice constants were calcu-lated from the rened patterns (Table 1) and arecomparable with those of undoped ZnO (JCPDS: 36-1451). Interestingly there is a slight change in thelattice parameters with increasing annealing temper-ature. It is interesting to note that with increasingannealing temperature, c / a values are found toincrease gradually. This behavior may be due to therandom distribution of Ni ions or local defectsaffected by changing the annealing temperature. Thislattice distortion seems to be playing a crucial role incontrolling the structural properties and the diffusionbehavior of randomly substituted Ni ions therebyinuencing the distribution of local defects such asoxygen vacancies. The average crystallite sizes of thepowders obtained using the Rietveld renement of X-ray diffraction patterns and isotropic sizestrainmodel by eliminating the instrumental, grain, and line

20 30 40 50 60 70 80

(d) I n t e n s

i t y ( a

. u )

2 Theta (degrees)

(a)- ZCO-300(b)- ZCO-400(c)- ZCO-500(d)- ZCO-600 ( 1

0 0 )

( 0 0 2 )

( 1 0 1 )

( 1 0 2 ) ( 1

1 0 )

( 1 0 3 )

( 2 0 0 )

( 1 1 2 )

( 2 0 1 )

( 0 0 4 )

( 1 0 2 )

(a)(b)(c)

(a) (b)Fig. 2 a XRD spectra of Zn0.9 Ni0.1 0 nanoparticlesannealed at differenttemperatures and b Rietveldrenement plot of ZNO600 sample

Table 1 Rietveld rened XRD data of nanocrystalline Zn0.9

Ni0.1

0

Sample Averagecrystallitesize fromXRD

\ D[ nm

Average particlesize fromAFM (nm)

Averageparticlesize fromTEM (nm)

a A c A c / a r.m.s micro-strain \ e2[

Rw(%)

Rexp(%)

Rb(%)

Goodnessof t (S )

ZNO300 15 12 12 3.265 5.201 1.5929 0.001503 7.83 6.02 7.20 1.30

ZNO400 22 26 25 3.256 5.201 1.5974 0.001320 6.62 5.17 6.33 1.28

ZNO500 30 34 35 3.254 5.218 1.6035 0.001284 6.22 5.10 6.11 1.22

ZNO600 42 45 46 3.252 5.222 1.6058 0.001138 5.69 4.99 6.09 1.14

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broadening effects with respect to standard silicon andare given in Table 1. In general the strain would tendto be higher in nanomaterials, and in order to study theorigin of micro-strain due to high surface-to-volumeratio, the r.m.s micro-strain values were evaluatedusing the Rietveld rened data and are given inTable 1. It can be seen from the table that these valuesare found to decrease with increasing nanosize and theobserved behavior may be attributed to the relaxationof surface energy with decreasing surface-to-volumeratio.

The variation of grain size with annealing tem-perature has also been analyzed by atomic forcemicroscope, and the 2D topography images alongwith their 3D images are shown in Fig. 3. Theseimages clearly indicate the presence of well-devel-oped nano-particles with regular geometrical shapeand are found to be distributed randomly. It isinteresting to note that the distribution of grains andsurface condition of nanograins affect the surfacemorphology of the materials. Therefore, in modifying

the surface microstructure, the grain size and anneal-ing temperature of the materials might be playing acrucial role. From AFM topography images, theaverage sizes of the nanoparticles were computedusing the SPM Lab analysis software (Table 1) andare comparable with those obtained from the XRDstudies. Figure 4 shows the bright eld electronmicrograph of all the samples. The nanoparticles arefound to aggregate and distribute randomly, which iscrucial in defect generation. The estimated averageparticle sizes of these materials are given in theTable 1. These results are consistent with thosecalculated from AFM and XRD data. It can be seenfrom the TEM morphological images that withincreasing annealing temperature, the aggregation of nanoparticles is reduced indicating that the randomdistribution of nanograins critically inuences themicrostructural properties of the materials by chang-ing the local disorder and surface defects. Further, thesegregated small metallic precipitates of impurityphases at surfaces and at grain boundaries have not

Table 2 XPS data of Ni(10%)-doped ZnOnanomaterials annealed atdifferent temperatures

Sample code ZNO300 ZNO400 ZNO500 ZNO600Average particle size (nm) 12 26 34 45

Ni content

Value of Ni/(Zn ? Ni) 0.099 0.098 0.097 0.098

Binding energy (eV)

Ni 2 p3/2 855.282 855.311 855.345 855.575Satellite peak ( S 1) 861.872 861.150 861.089 860.89

Ni 2 p1 /2 872.862 872.67 872.551 872.335

Satellite peak ( S 2) 879.881 879.590 879.539 879.465

Zn 2 p3/2 1021.42 1021.38 1021.26 1021.17

Zn 2 p1/2 1044.15 1044.21 1044.33 1044.40

O1s (1) 530.16 530.34 530.392 530.268

O1s (2) 531.729 531.49 531.38 531.33

O1s (3) 532.789 532.52 532.472

Spinorbit splitting ( D B.E) (eV)D B.E of Ni 2 p3/2 and Ni 2 p1/2 17.58 17.359 17.206 17.28

D B.E of Zn 2 p3/2 and Zn 2 p1/2 22.73 22.83 23.07 23.23FWHM (eV)

Ni 2 p3/2 3.73 3.68 3.5 3.14

Ni 2 p1 /2 4.14 4.022 3.81 3.45

Zn 2 p3/2 2.67 2. 43 2.31 1.83

Zn 2 p1/2 3.63 2.75 2.53 2.43

O1s (1) 1.49 1.52 1.66 1.87

O1s (2) 1.64 1.30 1.15 0.9

O1s (3) 1.37 1.26 0.9

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been observed, indicating the formation of solidsolution. The selected area electron diffraction pat-terns (SAED) of all the samples are shown in Fig. 5and were indexed to wurtzite ZnO structure. Acareful analysis of these SAED patterns reveals thatbesides the diffraction rings of wurtzite structure of

ZnO, no additional rings were observed. Furthermore,the quantitative analysis of EDS results revealed thatnearly 10% Ni is doped into ZnO and the value of O/(Zn ? Ni) is found to be 0.99, while the ratio of Ni/(Zn ? Ni) is found to be 0.0999. With increasingannealing temperature oxygen stoichiometry is found

Fig. 3 AFM 2D and 3Dtopography images of nanocrystalline Ni (10%)-doped ZnO annealed atdifferent temperatures

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to increase slightly, indicating the diffusive behaviorof oxygen into the lattice from air. Although, Ni and

Zn stoichiometry values are comparable with thenominal values, they are found to uctuate slightlywith varying annealing temperature and the behaviormay be attributed to the random distribution of Niions or oxygen vacancies.

The room temperature Raman spectra of thesamples of present investigation is shown in Fig. 6.The improvement of crystallinity due to annealing isalso evidenced in these spectra. Further, the Ramanspectra of all the samples are found to exhibit most of the ZnO phonon modes (Serrano et al. 2004 )

indicating that Ni-doped ZnO nanoparticles mighthave not disturbed the crystal structure of microcrys-talline ZnO. A strong peak in the range* 429433 cm - 1 (E2 (High) mode) is attributed tohigh frequency E 2 mode whereas the low frequencyE2 mode is beyond the measurable range of presentinvestigation. The Raman bands at 380 cm - 1 (A1(TO) mode) and at 578 cm - 1 (E1 (LO) mode) mightbe ascribed to the transverse and longitudinal opticalphonon modes with A 1 symmetry, respectively. The

broadening of 578 cm - 1 peak might be due todisorder caused by the doping of Ni ions into ZnO

host lattice and is found to decrease with increasingannealing temperature. In the Raman spectra of allthe samples of present investigation, an impurity peak related to higher Ni content at * 690 cm - 1 (Gayenet al. 2010 ) could not be observed indicating theabsence of Ni-related secondary phases at micro-level. The peak in the range 32833 cm - 1 (2E 2(Low) mode) is the second-order vibration of ZnO.Interestingly, all the Raman frequencies are found toshift toward the low frequencies side. This shiftingdepends not only on the nanoparticle size but also on

residual stress, structural disorder, and local defectsof the samples. The doping of transition metals intoZnO lattice may introduce lattice defects and disorderinto the host ZnO. Therefore, the red shift of E 2(High) phonon cannot be totally ascribed to theoptical phonon connement by the nanocrystallineboundaries. Further study is needed to identify itsexact origin.

In order to investigate the chemical state of thehost and dopant elements and to quantify the doping

Fig. 4 TEM morphologyimages of a ZNO300,b ZNO400, c ZNO500,and d ZNO600 samples

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concentration, the XPS studies were undertaken. XPSanalysis shows the presence of Zn, Ni, and Oelements in Ni (10%)-doped ZnO. Further, thecompositional details were also analyzed and Ni/ (Zn ? Ni) values are found to vary between 0. 097and 0.099 which is the nominal composition (0.1) of

the starting material. These results are consistent withthe EDS results. The over-lapped bands were decon-voluted into separated peaks by Gaussian tting usingXPS PEAK41 software. The analyzed XPS core levelspectra of Zn 2 p, Ni 2 p and O 1s are shown in Figs. 7,8, and 9, respectively. The evaluated binding energiesand FWHM values are given in Table 2, and theresults are in agreement with the earlier reports (Iqbalet al. 2009 ; Ghosh C et al. 2008 ; Islam et al. 1996 ).From the observed spinorbital splitting ( D Zn ) valuesand the binding energy positions of two strong peaksof core level XPS Zn 2 p spectra (Fig. 7) and fromtheir line widths, it has been concluded that Zn ispresent as Zn 2? . When compared with that of Zn 2 p3/2 ,(10220.2 eV) in bulk ZnO, a slight change in thebinding energies of Zn 2 p3/2 (1021.171021.42 eV)in Ni-doped ZnO samples has been observed. Thisbehavior may be attributed to the reduction innanometric size of the materials. However, theobserved B.E values of Zn 2 p3/2 peaks are belowthose of metallic Zn wherein the XPS core level Zn

Fig. 5 Selected areaelectron diffraction patternsof a ZNO300, b ZNO400, c ZNO500, andd ZNO600 samples

250 300 350 400 450 500 550 600 650 700

A 1

( T O )

ZNO-600

ZNO-500

ZNO-400

I n

t e n s

i t y

( a . u

)

Raman shift (cm-1

)

ZNO-300

E 2 (High)

A 1

( L O )

2 E

2 ( L o w )

Fig. 6 Room temperature micro-Raman spectra of Ni (10%)-doped ZnO nanomaterials annealed at different temperatures

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2 p3/2 B.E position is 1021.5 eV. Thus, the formation

of Zn clusters is ruled out. As shown in Fig. 8, thebinding energies of Ni 2 p3/2 , N i 2 p1/2 , and thecorresponding satellite peaks are comparable withthose available in the literature (Pei et al. 2008 ; Wanget al. 2007 ). Comparing the binding energies of Niprimary and satellite peaks with those of Ni, Ni 2O3,and NiO (Moulder et al. 1992 ); Yu et al. 2001 ); Yinet al. 2005 ), the electronic state of Ni in the samplesof present investigation is concluded as ? 2. It hasalso been concluded that Ni is not bonded with

oxygen either in the form of Ni 2O3 or in the metallic

form. The B.E position of Ni 2 p3/2(855.282855.575 eV) is quite different from thatof metallic Ni (852.7 eV), NiO (853.8 eV), andNi2O3 (856.7 eV) (Moulder et al. 1992 ; Yu et al.2001 ; Yin et al. 2005 ; Yu et al. 2003 ). The spinorbital splitting value ( D Ni ) of Ni 2 p is found to be inthe range 17. 017.9 eV, which is less than that of NiO which value is 18.4 eV (Moulder et al. 1995 ).These results indicate Ni ions with valence ? 2 mighthave been substituted into the tetrahedral sites of ZnO

Fig. 7 XPS of the Zn 2 p3/2and Zn 2 p1/2 states of 10%Ni-doped ZnO annealed atdifferent temperatures

Fig. 8 XPS of the Ni 2 p3/2and Ni 2 p1/2 states of 10%Ni-doped ZnO annealed atdifferent temperatures

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without forming any detectable impurity phase. It isinteresting to note that the observed binding energyvalues of Zn 2 p3/2 and Ni 2 p1/2 are found to shifttoward high energy side and binding energy values of Ni 2 p3/2 and Zn 2 p1/2 XPS peaks are found to shifttoward lower energy side with decreasing particlesize indicating that nanometric size might haveinuenced the core level electronic structure as wellas the distribution of Ni ions. The FWHM of both Zn2 p and Ni 2 p values are found to decrease withincreasing annealing temperature indicating thatalong with annealing effects the nanometric size of the materials might have also inuenced the elec-tronic structure and local disorder.

The analysis of XPS O1 s spectra revealed threedistinct components as shown in Fig. 9. The peak centered in the range 530.16530.268 (O 1 s (1)) isattributed to O 2 - ions on wurtzite structure (Chenet al. 2000 ; Cebulla et al. 1998 ). The binding energyof the peaks in the range 531.729531.33 eV (O 1 s (2))may be associated with O 2 - ions in the oxygen-decient regions within the matrix of Ni-doped ZnO(Chung et al. 2005 ). The high binding energy of thepeak in the range 532.789532.472 eV (O 1 s (3)) isusually attributed to loosely bound oxygen on thesurface or micro-pores, belonging to hydrated oxidespecies such as adsorbedCO 3 , H2O, and O 2 (Chenet al. 2000 ; Chen et al. 2000 ; Islam et al. 1996 ).It is interesting to note that higher binding energy

O 1s (3) peak intensity and its FWHM are found todecrease with increasing annealing temperature andnot appeared in ZNO600 sample. Further FWHMvalues of O1 s (1) peak is found to increase while thatof O1s (2) peak is found to decrease with increasingannealing temperature indicating reduction of oxygenvacancies.

In order to study the electronic interactions nearthe optical band gap region due to substitution of Ni2 ? ions for Zn 2 ? , UVVis absorbance measure-ments were undertaken. The optical absorbancecoefcient ( a) of a semiconductor close to the bandedge may be expressed as a function of incidentphoton energy (h m) by an equation (Tau 1974 ),

a A=hmfhm E ggm 1

where E g is the absorption band gap, m depends onthe nature of the transitions, m may take values of 1/2, 2, 3/2, and 3 corresponding to allowed direct,allowed indirect, forbidden direct, and forbiddenindirect transitions, respectively. A is a constant andis different for different transitions. In the presentinvestigation, as the values of m evaluated from theslope of the plot of ln ( ahm) versus ln (h m- E g) arefound to be in the range 0.510.53, it has beenconcluded that direct allowed band gap transitionsmay be present in the samples of present investiga-tion. Therefore, by considering m = 1/2, the effectivedirect band gap values were obtained from the plots

Fig. 9 XPS spectra of theO 1s state of 10% Ni-dopedZnO annealed at differenttemperatures

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(ahm)2 versus h mby extrapolating the linear region tozero absorption and are given in Table 3. Figure 10ashows ( ahm)2 versus h mplots of all the samples. Whencompared with the microcrystalline undoped ZnO(* 3.26 eV), the band gap values are found to belesser, which is a clear indication of the incorporationof Ni2 ? into ZnO host lattice. It is interesting to note

that with decreasing crystallite size and annealingtemperature, the band gap values are found toincrease indicating a clear blue shift and may beattributed to the BursteinMoss effect (Kamat et al.1989 ). Therefore, it has been concluded that thenanometric size of the grains and their distributionssignicantly inuence the local electronic structure of Ni-doped ZnO lattice.

The PL emission measurements were also under-taken to understand the behavior of the surfacedefects such as oxygen vacancies. The room temper-ature PL emission spectra of all the samples wererecorded at an excitation wavelength ( kex ) of 325 nmand are shown in Fig. 10b. A strong emission peak at* 400 nm has been observed for all the samples of Ni

(10%)-doped ZnO nanoparticles. Interestingly, thispeak is found to exhibit a red shift when comparedwith that of emission peak of bulk ZnO ( * 392 nm)and the observed behavior may be due to thesubstitution of Ni 2 ? at Zn2 ? site (He et al. 2005 ).As the band gap energy is found to be in the range2.952.78 eV, the observed peak at * 400 nm(* 3.1 eV) is ascribed to the free exciton emission.

(4)(3)(2) I n

t e n s

i t y ( a

. u )

Wavelength (nm)

(1)- ZNO-300(2)- ZNO-400(3)- ZNO-500(4)- ZNO-600

(1)

(b)

300 400 500 600 7002.5 3.0 3.5 4.0 4.5 5.0

(4)(3)

(2)

( h ) 2 / a r b . u

n i t s

(1)-ZNO-300(2)-ZNO-400(3)-ZNO-500(4)-ZNO-600

h /eV

(1)

(a)Fig. 10 a Plot of ( ahm) 2

versus h mb PL emissionspectra (at kex = 325) of the Ni (10%)-doped ZnOnanoparticles annealed atdifferent temperatures

Table 3 Results of optical,electrical, and magneticstudies of Zn 0.9 Ni0.1 0annealed at differenttemperatures

Description Samples

ZNO300 ZNO400 ZNO500 ZNO600

Optical band gap values ( E g) eV 2.95 2.88 2.84 2.78

Magnetization studies

M s (emu/g) 0.35 0.18 0.13 0.11

H c (Oe) 33.12 56.33 68.11 76.24

M r(emu/g) 0.0249 0.0075 0.0065 0.0047

FMR studies

g-factor 2.427 2.415 2.378 2.325

RT resistivity

q (k X m) 215 187 147 127

Conductivity vs. temperature studies

Activation energies

E aL (eV) 0.815 0.756 0.671 0.614

E aH (eV) 1.470 1.432 1.405 1.390

Thermo electric power studies

Carrier concentration ( n)/cm 3 6.19 x 10 18 5.0 x 1017 1.03 x 10 17 3.54 x 10 16

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An additional low-intensity bluegreen luminescencepeak at * 472 nm ( * 2.63 eV) was also observed inall the PL spectra. Interestingly, this peak is invariantin all the samples with varying annealing temperatureand may be attributed to Zn interstitial defects oroxygen vacancies introduced due to larger ionic

radius of Ni 2 ? than that of Zn 2 ? (rNi2 ? = 0.69 A ;r Zn

2 ? = 0.60 A). The PL peak of ZnO at * 483 nm isassumed to originate from the transition betweenoxygen vacancy and interstitial oxygen (Mahamuniet al. 1999 ; Gayen et al. 2010 ). A low-intensity peak at 560 nm was also observed. This green emissionhas been attributed to surface defects, as well as thedefects just below the crystallite surface (Cheng et al.2008 ; Wang et al. 2006 ; Yuvaraj and Narasimha Rao2010 ; Studenikin et al. 1998 ). Interestingly, a peak at630 nm has also been observed and may be attributedto the surface defects or oxygen and zinc anti-sites(Gayen et al. 2010 ; Van Dijken et al. 2000 ; Konget al. 2006 ). These defects might be originated duringthe annealing process, where in oxygen may diffuse

into the nanoparticles and react with the surface.Therefore, it is possible to have large number of surface defects (oxygen vacancies) at the surface of nanomaterials. It has also been found that withincreasing annealing temperature, the intensity of theemission peaks reduced gradually and the behavior

may be due to the reduction in the concentration of oxygen vacancies which in turn decrease the con-centration of recombination centers.

The magnetization versus magnetic eld ( M H )plots of all the samples (maximum eld of 15 kOe) areshown in Fig. 11 , and the magnetization results aregiven in Table 3. It is interesting to note that all thesamples of the present investigation are found toexhibit a clear room temperature ferromagnetic behav-ior along with nite values of remanence and coerc-ivities. Although RT ferromagnetism was reportedearlier in the case of Ni ( B 5%)-doped ZnO DMSs(Schwartzet al. 2004 ; Radovanovic andGamelin 2003 ;Pei et al. 2006 ; Cong et al. 2006 ; Li et al. 2006 ; Chenget al. 2007 ; Zhang et al. 2007 ), the same could not be

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

Applied Field (in Oe)

0

-0.008

0.000

0.008


ZNO-400

M a g n e

t i z a t

i o n

( e m u / g )

ZNO-400

M a g n e

t i z a t

i o n ( e m u / g )

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4


-200 0 200

0.00

0.02

M a g n e

t i z a t

i o n

( e m u / g

)


ZNO-300

M a g n e

t i z a t

i o n

( e m u / g )

ZNO-300

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0

0.00

M a g n e

t i z a t

i o n

( e m u / g )


ZNO-500

M a g n e

t i z a t

i o n

( e m u / g )


ZNO-500

-15000 -10000 -5000 0 5 00 0 1 00 00 1 500 0-15000 -10000 -5000 0 500 0 1000 0 15000

-15000 -10000 - 5000 0 5000 10000 1 5 00 0 -15000 -10000 - 5000 0 5 00 0 10000 15000-0.12-0.10-0.08-0.06-0.04-0.020.000.02

0.040.060.080.100.12

-30 0 -20 0 -100 0 100 200 300

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008

M a g n e

t i z a t

i o n

( e m u / g )


ZNO-600

M a g n e

t i z a t i o n

( e m u / g )


ZNO-600

Fig. 11 Room temperature M H curves of nanocrystalline Ni-doped ZnO nanoparticles annealed at different temperatures. Inset images show their corresponding coercivity and remanence close examination plots

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obtained in 10% Ni-doped ZnO nanopowders withsingle phase. In contrast with these reports, in thepresent investigation the room temperature ferromag-netism has been observed in ZnO doped with 10%nickel without any impurity phases indicating a clearimprovement in the solubility of dopant. This is

possible due to decrease in the crystallite size of thematerials andby adopting suitable synthesis conditionsin the synthesis method. It is interesting to note that,with decreasing particle size, the specic magnetiza-tion and remanence values are found to increase whilethe coercivities are decreasing. Further, the nanopar-ticles with an average size of 12 nm are found toexhibit highest specic magnetization (0.35 emu/g)and low coercivity (33.12 Oe). However, with increas-ing annealing temperature and crystallite size, themagnetization values arefound to decrease.This mightbe due to the decrease in participated ferromagneticexchange couplings between the bound and the ferro-magnetic exchange interactions.

Earlier, Herzer ( 1992 ) reported the variation of H cwith grain size for several magnetic materials andfound that for materials with grain sizes less than theferromagnetic exchange length ( L ex ), the coercivityincreases as D 6 , whereas at higher grain sizes, thecoercivity decreases as D - 1 . When the grain size isless than L ex , the exchange interaction dominates theanisotropy energy, resulting in alignment of magne-tization vectors parallel to each other over severalgrains. This averages out the effective anisotropyresulting in good soft magnetic properties. Most of the results published so far used this model to explainthe soft magnetic properties of nanocrystalline mate-rials. However, the results obtained in the presentinvestigation do not follow this trend and the

observed behavior may be attributed to the presenceof internal strain (Kuhrt and Schultz 1993 ). Theinternal strain may inhibit Bloch wall movement sothat the number of magnetic domains within theexchange length is not high enough to average out themagnetocrystalline anisotropy, resulting in higher

coercivity values. Kuhrt and Schultz ( 1993 ) observeda similar reduction in coercivity with increasing grainsize in Fe62 Co38 powders. Therefore, it has beenconcluded that the coercivity and magnetization of the samples of present investigation depend onnanometric size and magnetic interactions amongthe particles. These results indicate that the magneticproperties of nanocrystalline ODMSs might havebeen controlled by the native oxygen vacancies at thesurface of nanoparticles.

Figure 12a shows the variation of magnetization M (T ) of zero eld cooled (ZFC) nanocrystallineZn0.9 Ni0.1 O samples at 500 Oe. For ZFC measure-ments, rst the samples were cooled to 80 K withoutany magnetic eld, and the magnetization was mea-sured in the heating mode at a constant rate of 10 K/ min and in a small uniform external eld of 500 Oe.With increasing temperature, thermal vibrationsbecome strong enough to overcome Zeeman interac-tions and initiate randomization of the magneticmoments, which in turn decreases in the magnetizationvalue. In view of this, it has been concluded that theCurie temperatures of the samples of present investi-gations might be above 300 K. The curve obtained in amagnetic eld of 500 Oe shows a non-zero magneti-zation up to room temperature and the behavior is inaccordance with that exhibited by M H curves. Fur-ther, M T behavior of the present samples is inagreement with the presence of nanosized species.

0.04

0.06

0.08

0.30

M a g n e

t i c M o m e n

t ( e m u / g )

Temperature (K)

ZNO300

ZNO400ZNO500ZNO600

100 150 200 250 300 0 1000 2000 3000 4000 5000 6000 7000 8000

(d)(c)

(b)

d I / d B

Magnetic Field (Oe)

(a)- ZNO-300

(b)- ZNO-400

(c)- ZNO-500(d)- ZNO-600

(a)

3240 Oe

Fig. 12 a M T curves andb FMR of nanocrystalline

Ni (10%)-doped ZnOannealed at differenttemperatures

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From M T plots, the net magnetization is found todecrease gradually with increasing crystallite size. Itwas reported earlier that the magnetic behavior of hexagonal close-packed (hcp) metastable Ni nanopar-ticles are anti-ferromagnetic while the stable fcc Ninanoparticles are ferromagnetic (Jeon et al. 2006 ).

However, as the synthesis of the DMS nanocrystals of present investigation was performed under oxidizingconditions, metallic nickel might have not beenformed. If isolated nickel ions are present, duringannealing in air they might have converted into nickeloxides easily. In such an eventuality, the antiferro-magnetic behavior or robust ferromagnetic behavior orsuperparamagnetic behavior may be exhibited. More-over, fcc Ni nanoclusters are not detected in XPS core-level spectra and SAED patterns.

In order to comprehend the reasons for theobserved weak ferromagnetism and to understandthe oxygen vacancies the FMR spectral measure-ments were also carried out. Figure 12b shows thetypical room temperature FMR spectra of Ni-(10%)doped ZnO nanoparticles obtained at differentannealing temperatures. The spectra show a broadFMR signal without splitting into additional peaks,and are a clear signature of ferromagnetic behavior of the materials. The broad signal may be attributed toFMR which arises due to the long range exchangeinteractions and transition within the ground state of ferromagnetic domain. A shift in the broad FMRsignal toward lower applied magnetic eld sideclearly indicates the inuence of annealing temper-ature and the local disorder on long range ferromag-netic exchange interactions. The g-factors are foundto be in the range, 2.432.33 (Table 3). As the g-value of Ni metal species is centered at 2.2 (Selimet al. 2005 ; Stocker et al. 2000 ; Pawelec et al. 1996 )the presence of Ni metallic phase has been ruled outin the samples of present investigation. These resultsclearly indicate the incorporation of Ni 2 ? ions into Znsites in the interior of Ni-doped ZnO nanostructureand also indicate that Ni (10%) doped ZnO samplesare ferromagnetic at room temperature without anyNi precipitates.

In order to conrm the absence of Ni clusters andto understand the behavior of the distribution of magnetic domains, MFM measurements were alsocarried out at room temperature by adjusting themagnetized cantilever at a lift height of 50 nm. Atsuch tipsample separation, the topography signals

were disappeared and only long-range forces such asmagnetic forces are expected and can be ascribed thecontrast only to the magnetic properties of thesample. Figure 13 shows MFM phase images of allthe samples and exhibited a clear dark contrastcorresponds to the ferromagnetic phase indicating the

presence of ferromagnetism in the samples. It can beseen from the MFM phase images that the magneticdomains are found to be non-uniform, smaller in sizeand the domain sizes are found to increase graduallywith increasing particle size and annealing tempera-ture. As most of the domains are interconnected anddistributed randomly and also the oxygen vacanciesare present, it is difcult to understand the inuenceof the inter-particle distances. Moreover, it has beenobserved that the dark contrast is decreasing whiledomain size is increasing with increasing annealingtemperature. It is interesting to note that, in thesamples annealed above 500 C the domains areseparated uniformly, indicating that the samples arequalitatively homogeneous. The variations in dark contrast clearly signify the room temperature ferro-magnetic behavior without any magnetic clusters.

In the case of Ni-doped ZnO samples, the origin of ferromagnetism is very complex. Although differentmechanisms might be responsible, an effort has beenmade to explain the phenomenon using the boundmagnetic polarons (BMP) model (Liu et al. 2007 ;Wolff et al. 1996 ; Bhatt et al. 2002 ; Liu et al. 2009 ). Infact same theory was used by a few groups (Raebigeret al. 2008a , b; Coey et al. 2008 ; Coey and Chambers2008 ) to explain the ferromagnetism in some ODMSmaterials. According to this model, the localized spinsof the dopant ion interact with the charge carriers suchas oxygen vacancies, resulting in a magnetic polari-zation of the surrounding local moments. In the presentinvestigation, due to the random substitution of Ni 2 ?

and incorporated hydrogen ions, a number of freecharge carriers and oxygen vacancies might haveintroduced to maintain the charge neutrality leading tothe formation of bound magnetic polarons (BMPs),and overlapping of BMPs may create spinsplitimpurity band. The exchange interactions betweenthese BMPs (Liu and Yu 2007 ), which are coupledwith the randomly distributed neighboring Ni 2 ? , maybe responsible for the observed ferromagnetism.Further, the decrease of specic magnetization withincreasing particle size and annealing temperaturemay be attributed to the less number of BMPs and their

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overlapping behavior by decreasing the availableoxygen vacancies surrounded by Ni 2 ? ions, besidesmany isolated polarons may not contribute to theferromagnetic interactions. These results suggest that

the volume occupied by BMPs inuences the proba-bility of overlapping of large number of Ni ions intoferromagnetic domains leading to the ferromagneticcoupling more effectively. Apart from this, weak ferromagnetism observed in pristine ZnOsamples maybe attributed to the effective exchange interactionsbetween the unpaired electron spins originated fromthe surface defects such as oxygen vacancy clustersinstead of single neutral oxygen vacancies associatedwith the nanosize (Banerjee et al. 2007 ; Zhou et al.2008 ). Further, as the samples of present investigation

exhibited considerable magnetization values, the con-tribution of the surface ferromagnetism is very less.Therefore, the surface ferromagnetism has beenneglected in the present investigation. However, themechanism of exchange coupling induced by defectstates or due to inhomogeneous dopant distributionswhich are responsible for the ferromagnetism is stillnot clear due to the fact that localized electrons fromother sources may also contribute to the ferromagne-tism and requires further studies.

It was reported earlier that based on rst-principlesof pseudo-potential calculations and Monte carlosimulations, hydrogen present in ZnNiO may betrapped at the interfaces between the fused nanocrys-

tals during material preparation and provide neces-sary n-type character to activate the long-rangemagnetic ordering. Further, the incorporation of hydrogen may also directly mediate a strong spinspin interaction leading to high temperature ferro-magnetism (Sato and Yoshida 2001 ; Park et al. 2005 ).Apart from this, hydrogen incorporation in ZnO isalso expected to introduce a shallow donor state(Tong et al. 2010 ). Moreover, the studies on N-dopedZnO crystals indicate the presence of an acceptorstate due to nitrogen substitution (Garces et al. 2002 ).

A theoretical study of Zn 1 - x Mn x O DMS materials co-doped with N (Maouche et al. 2007 ) indicated that theprocess may change the ground state from anti-ferromagnetic to ferromagnetic. Earlier, Van deWalle et al. ( 2000 ) has reported that hydrogen hasgone into ZnO unintentionally due to the preparationconditions. As the samples of present investigationwere prepared by the hydrazine-assisted polyolmethod, there may be a possibility of the presenceof hydrogen or nitrogen in the interstitial positions or

Fig. 13 MFM phaseimages of a ZNO300,b ZNO400, c ZNO500,and d ZNO600 samples

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at the grain boundaries thereby increasing the con-ductivity of the samples. Therefore, electrical prop-erties have also been studied to understand theconductivity behavior, and the room temperatureresistivity values are given in Table 3. It can be seenfrom the table that the resistivity values are found to

decrease with increasing particle size of the materials(Fig. 14a) and that the behavior may be due toincrease in the electron transport upon decreasing thegrain boundary scattering and the effective electronscattering originated from different grain orientationswith increasing annealing temperature and particlesize of the materials.

The variation of electrical conductivity withtemperature is shown in Fig. 14b. It can be seenfrom the gure that the conductivity increases withincreasing temperature and the behavior may beattributed to increasing number of charge carriers byovercoming the energy barrier. The Arrhenius graphis also found to exhibit a change of slope at 476 K,indicating two different conduction mechanismsmight be present and the activation energies calcu-lated are given in Table 3. The variation of conduc-tivity ( r ) may be explained by equation,

r r L expE aLK BT & ' r H exp E aHK BT & ' 2

where r L and r H are the exponential factors, E aL andE aH are the activation energy of low and hightemperature conducting regions, respectively. The

low temperature activation energy of the samplesmay be associated with the oxygen vacancies ( V O )(Simpson and Cordero 1988 ). In the low temperatureregion, a donor level ( * 0.6140.815 eV) might havebeen formed below the conduction band indicatingthe extrinsic behavior. Interestingly, the high tem-perature activation energies are comparable with thehalf of optical energy band gap values, indicatingthe intrinsic behavior of the samples. However, as thehigh temperature activation energies may also beassociated with desorption of O 2

- species (Fujitsu

et al. 1999 ), the mechanism has not been clearlyunderstood. The decrease of activation energies withincreasing annealing temperature and nanometric sizeof the materials may be due to the modication of local electronic structure of the materials. Seebeck coefcient ( a) versus temperature T (K) plots of allthe samples are shown in Fig. 14c. The sign of

-150

-100

-50

0

50

(

V / K )

Temperature (K)

ZNO-300ZNO-400ZNO-500ZNO-600

(c)

10 16

10 17

10 18

1019

C a r r i e r

C o n c e n t r a

t i o n

( n ) c m

- 3

Temperature (K)

ZNO-300ZNO-400ZNO-500ZNO-600

(d)

-12

-10

-8

-6

-4

-2

0

2

4

l n

1000/T K-1

ZNO600ZNO500ZNO400ZNO300

(b)

320 340 360 380 400 420 440 460 480 500 300 350 400 450

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.215 20 25 30 35 40 45120

140

160

180

200

220

R e s

i s t i v i t y ( ) ( k O h m - m

)

Crystallite size (nm)

(a)Fig. 14 Electricalcharacteristics of Ni (10%)-doped ZnO nanomaterials.a RT resistivity, b Arrheniusplot, c Seebeck coefcientversus temperature plot, andd carrier concentration plotof Ni-doped ZnO

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Seebeck coefcient ( a) is negative, indicating thatelectrons are the majority charge carriers. Further,from the TEP data the carrier concentration ( n) wasalso estimated using the semi-classical Mott relation(Cutler et al. 1964 )

s p2

k 2

m

3p22=3"h2 ej j:T n2=3 3

where n is the carrier concentration and m is theeffective mass of the material. Equation 3 is valid forthe samples of present investigation because theirmean free path, lm B 3 nm, is much larger than theinteratomic distance and that this semi-classicalapproach is appropriate without quantum correctionsince k F lm * 1, where k F is Fermi wave vector andsmall-gap semiconductor. By assuming the effectivemass, m & 0.28 m e (Ohtomo et al. 1999 ), the carrierconcentration ( n) values have been estimated. Fig-ure 14d shows the carrier concentration versus tem-perature plots of all the samples. The carrierconcentration values are found to increase withdecreasing crystallite size, which is in conformitywith the blue shift of the optical band gap behavior.These results suggest that with increasing annealingtemperature, the oxygen vacancies which are thesource of free charge carriers might have inuencedthe carrier concentration thereby the magneticproperties.

Conclusion

In summary, nanocrystalline Zn 0.9 Ni0.1 O DMSs havebeen synthesized by a hydrazine-assisted polyolchemical route and investigated their structural,electronic, magnetic, and electrical properties as afunction of annealing temperature and nanometricsize. From these studies the following conclusionshave been arrived at.

1. The aggregation of the nanoparticles and theoxygen vacancies along with Ni ions signi-cantly affected the local disorder without chang-ing the macro-structure.

2. The optical band gap values are found to increasewith decreasing nanometric size indicating aclear blue shift.

3. The binding energy and spin-orbital splitting(D B.E) values clearly inuenced by the nanometric

size of the materials and local defect density thematerials.

4. All the samples are found to exhibit clear RTferromagnetism without any magnetic clusters.The specic magnetization values are found toincrease with decreasing particle size and anneal-

ing temperature due to increase in carrierconcentration.

5. The distribution of oxygen vacancies along withtheir concentration is playing a crucial role ininuencing the ferromagnetic exchange media,and the presence of free carriers might be aprerequisite for the appearance of ferromagne-tism in these materials. In addition, the magneticproperties depend on electronic structural mod-ications and the distribution of nanocrystallites.

6. Finally, the studies of the present investigation

may be useful in tuning the structural, optical,magnetic, and electrical properties of nanocrys-talline Zn 0.9 Ni0.1 O for possible applications inmagneto-optical and spintronic devices whichdepend on the ability of preparing the materialswith stable and controlled defect distributions.

Acknowledgments The authors thank DRDO (ER & IPR),New Delhi, for providing nancial assistance through aresearch project and also thank Materials Research Center,Indian Institute of Technology of Madras, Chennai for

providing facilities TEM, Raman, and XPS studies.

References

Banerjee S, Mandal M, Gayathri N, Sardar M (2007)Enhancement of ferromagnetism upon thermal annealingin pure ZnO. Appl Phys Lett 91:18250. doi: 10.1063/1.2804081

Bhatt RN, Berciu M, Kennett MP, Wan X (2002) Dilutedmagnetic semiconductors in the low carrier densityregime. J Supercond 15:7183. doi: 10.1023/A:1014031327996

Cebulla R, Weridt R, Ellmer K (1998) Al-doped zinc oxidelms deposited by simultaneous rf and dc excitation of amagnetron plasma: relationships between plasma param-eters and structural and electrical lm properties. J ApplPhys 83:1087. doi: 10.1063/1.366798

Chen M, Wang X, Yu YH, Pei ZL, Bai XD, Bai XD, Sun C,Huang FF, Wen LS (2000) X-ray photoelectron spec-troscopy and auger electron spectroscopy studies of Al-doped ZnO lms. Appl Surf Sci 158:134. doi: 10.1016/ S0169-4332(99)00601-7

Cheng ZX, Wang XL, Dou SX, OzawaK, Kimura H, FabricationMP (2007) Raman spectra and ferromagnetic properties of

J Nanopart Res

123
http://dx.doi.org/10.1063/1.2804081http://dx.doi.org/10.1063/1.2804081http://dx.doi.org/10.1023/A:1014031327996http://dx.doi.org/10.1023/A:1014031327996http://dx.doi.org/10.1063/1.366798http://dx.doi.org/10.1016/S0169-4332(99)00601-7http://dx.doi.org/10.1016/S0169-4332(99)00601-7http://dx.doi.org/10.1016/S0169-4332(99)00601-7http://dx.doi.org/10.1016/S0169-4332(99)00601-7http://dx.doi.org/10.1063/1.366798http://dx.doi.org/10.1023/A:1014031327996http://dx.doi.org/10.1023/A:1014031327996http://dx.doi.org/10.1063/1.2804081http://dx.doi.org/10.1063/1.2804081


19/21

the transition metal doped ZnO nanocrystals. J Phys D40:65186521. doi: 10.1088/0022-3727/40/21/008

Cheng C, Guoyue X, Zhang H, Luo Y (2008) Hydrothermalsynthesis Ni-doped ZnO nanorods with room-temperatureferromagnetism. Mater Lett 62:16171620. doi: 10.1016/ j.matlet.2007.09.035

Chu D, Zeng Y-P, Jiang D (2007) Synthesis and growthmechanism of Cr-doped ZnO single crystalline nanowires.Solid State Commun 143:308312. doi: 10.1016/j.ssc.2007.05.036

Chung YM, Moon CS, Jung MJ, Han JG (2005) The lowtemperature synthesis of Al doped ZnO lms on glass andpolymer using magnetron co-sputtering: working pressureeffect. Surf Coatings Technol 200:936939. doi: 10.1016/ j.surfcoat.2005.02.197

Coey JMD, Chambers SA (2008) Theme articleoxide dilutemagnetic semiconductorsfact or ction. MRS Bull33:10531058

Coey JMD, Wongsaprom K, Alaria J, Venkatesan M (2008)Charge-transfer ferromagnetism in oxide nanoparticles.J Phys D 41:134012-1134012-6. doi: 10.1088/0022-3727/

41/13/134012Cong CJ, Hong JH, Liu QY, Liao L, Zhang KL (2006) Syn-thesis, structure and ferromagnetic properties of Ni-dopedZnO nanoparticles. Solid State Commun 138:511515.doi:10.1016/j.ssc.2006.04.020

Cutler M, Leavy JF, Fitzpatrick RL (1964) Electronic transportin semimetallic cerium sulde. Phys Rev 133:A1143A1152. doi: 10.1103/PhysRev.133.A1143

El-Hilo M, Dakhel AA, Ali-Mohamed AY (2009) Roomtemperature ferromagnetism in nanocrystalline Ni-dopedZnO synthesized by co-precipitation. J Magn Magn Mater321:22792283. doi: 10.1016/j.jmmm.2009.01.040

Fujitsu S, Koumoto K, Yanagida H, Watanabe Y, Kawazoe H(1999) Change in the oxidation state of the adsorbedoxygen equilibrated at 25 C on ZnO surface during roomtemperature annealing after rapid quenching. Jpn J ApplPhys 38:15341538. doi: 10.1143/JJAP.38.1534

Garces NY, Giles NC, Halliburton LE, Cantwell G, Eason DB,Reynolds DC, Look DC (2002) Production of nitrogenacceptors in ZnO by thermal annealing. Appl Phys Lett80:13341336. doi: 10.1063/1.1450041

Gayen RN, Rajaram A, Bhar R, Pal AK (2010) Ni-dopedvertically aligned zinc oxide nanorods prepared by hybridwet chemical route. Thin Solid Films 518:16271636. doi:10.1016/j.tsf.2009.11.067

Ghosh CK, Malkhandi S, Mitra MK, Chattopadhyay KK (2008) Effect of Ni doping on the dielectric constant of ZnO and its frequency dependent exchange interaction.J Phys D 41:245113-1245113-9. doi: 10.1088/0022-3727/ 41/24/245113

Han S, Zhang D, Zhou C (2006) Synthesis and electronicproperties of ZnO/CoZnO coreshell nanowires. ApplPhys Lett 88:133109-1133109-3. doi: 10.1063/1.2187435

He V Jr, Lao CS, Chen LJ, Davidovic D, Wang ZL (2005)Large-scale Ni-doped ZnO nanowire arrays and electricaland optical properties. J Am Chem Soc 127:1637616377.doi:10.1021/ja0559193

Herzer G (1992) Nanocrystalline soft magnetic materials.J Magn Magn Mater 112:258262. doi: 10.1016/0304-8853(92)91168-S

Iqbal J, Wang B, Liu X, Yu D, He B, Yu R (2009) Oxygenvacancy-induced green emission and room-temperatureferromagnetism in Ni-doped ZnO nanorods. New J Phys11:063009-1063009-14. doi: 10.1088/1367-2630/11/6/063009

Islam MN, Gjpsj TB, Chopra KL, Acharya HN (1996) XPS andX-ray diffraction studies of aluminum-doped zinc oxidetransparent conducting lms. Thin Solid Films 280:2025.doi:10.1016/0040-6090(95)08239-5

Jeon YT, Moon JY, Lee GH, Park J, Chang Y (2006) Com-parison of the magnetic properties of metastable hexago-nal close-packed Ni nanoparticles with those of the stableface-centered cubic Ni nanoparticles. J Phys Chem B110:11891191. doi: 10.1021/jp054608b

Kamat PV, Dimitrijevic NM, Nozik AJ (1989) Dynamic Bur-steinMoss shift in semiconductor colloids. J Phys Chem93:28732875. doi: 10.1021/j100345a003

Kong DH, Choi WC, Shin YC, Park JH, Kim TG (2006) Roleof oxygen in green emission from ZnO thin lms.J Korean Phys Soc 48:12141217

Kuhrt C, Schultz L (1993) Magnetic properties of nanocrys-

talline mechanically alloyed FeCo and FeNi. J ApplPhys 73:65886590. doi: 10.1063/1.352573Kwon YJ, Kim KH, Lim CS, Shim KB (2002) Characterization

of ZnO nanopowders synthesized by the polymerizedcomplex method via an organo chemical route. J CeramProc Res 3:146149

Li BB, Xiu XQ, Zhang R, Tao ZK, Chen L, Xie ZL, ZhengYD, Xie Z (2006) Study of structure and magnetic prop-erties of Ni-doped ZnO-based DMSs. Mater Sci SemicondProcess 9:141145. doi: 10.1016/j.mssp.2006.01.074

Liu SH, Hsu HS, Lin CR, Lue CS, Huang JCA (2007) Effectsof hydrogenated annealing on structural defects, conduc-tivity, and magnetic properties of V-doped ZnO powders.Appl Phys Lett 90:222505-1222505-3. doi: 10.1063/1.2745642

Liu SH, Huang JCA, Lin CR, Qi X (2009a) Electrical transportand ac conductivity properties of hydrogenated annealingV-doped ZnO. J Appl Phys 105:07C502-107C502-3. doi:10.1063/1.3055274

Liu XF, Yu RH (2007) Mediation of room temperature ferro-magnetism in Co-doped SnO 2 nanocrystalline lms bystructural defects. J Appl Phys 102:083917-1083917-5.doi:10.1063/1.2801375

Liu XJ, Zhu XY, Song C, Zeng F, Pan F (2009b) Intrinsic andextrinsic origins of room temperature ferromagnetism inNi-doped ZnO lms. J Phys D 42:035004-1035004-7.doi:10.1088/0022-3727/42/3/035004

Lojkowski W, Gedanken A, Grzanka E, Opalinska A, Stra-chowski T, Pielaszek R, Tomaszewska-Grzeda A, Ya-tsunenko S, Godlewski M, Matysiak H, Kurzydowski KJ(2009) Solvothermal synthesis of nanocrystalline zincoxide doped with Mn 2 ? , Ni2 ? , Co2 ? and Cr3 ? ions.J Nanopart Res 11:19912002. doi: 10.1007/s11051-008-9559-9

Lutterotti L (2000) MAUD CPD, Newsletter (IUCr) 24Lutterotti L, Matthies S, Wenk H-R (1999) MAUD (Material

Analysis Using Diffraction): a user friendly Java programfor Rietveld Texture Analysis and more. Proceeding of theTwelfth International Conference on Textures of Materi-als (ICOTOM12), vol 1, p 1599

J Nanopart Res

123
http://dx.doi.org/10.1088/0022-3727/40/21/008http://dx.doi.org/10.1016/j.matlet.2007.09.035http://dx.doi.org/10.1016/j.matlet.2007.09.035http://dx.doi.org/10.1016/j.ssc.2007.05.036http://dx.doi.org/10.1016/j.ssc.2007.05.036http://dx.doi.org/10.1016/j.surfcoat.2005.02.197http://dx.doi.org/10.1016/j.surfcoat.2005.02.197http://dx.doi.org/10.1088/0022-3727/41/13/134012http://dx.doi.org/10.1088/0022-3727/41/13/134012http://dx.doi.org/10.1016/j.ssc.2006.04.020http://dx.doi.org/10.1103/PhysRev.133.A1143http://dx.doi.org/10.1016/j.jmmm.2009.01.040http://dx.doi.org/10.1143/JJAP.38.1534http://dx.doi.org/10.1063/1.1450041http://dx.doi.org/10.1016/j.tsf.2009.11.067http://dx.doi.org/10.1088/0022-3727/41/24/245113http://dx.doi.org/10.1088/0022-3727/41/24/245113http://dx.doi.org/10.1063/1.2187435http://dx.doi.org/10.1021/ja0559193http://dx.doi.org/10.1016/0304-8853(92)91168-Shttp://dx.doi.org/10.1016/0304-8853(92)91168-Shttp://dx.doi.org/10.1088/1367-2630/11/6/063009http://dx.doi.org/10.1088/1367-2630/11/6/063009http://dx.doi.org/10.1016/0040-6090(95)08239-5http://dx.doi.org/10.1021/jp054608bhttp://dx.doi.org/10.1021/j100345a003http://dx.doi.org/10.1063/1.352573http://dx.doi.org/10.1016/j.mssp.2006.01.074http://dx.doi.org/10.1063/1.2745642http://dx.doi.org/10.1063/1.2745642http://dx.doi.org/10.1063/1.3055274http://dx.doi.org/10.1063/1.2801375http://dx.doi.org/10.1088/0022-3727/42/3/035004http://dx.doi.org/10.1007/s11051-008-9559-9http://dx.doi.org/10.1007/s11051-008-9559-9http://dx.doi.org/10.1007/s11051-008-9559-9http://dx.doi.org/10.1007/s11051-008-9559-9http://dx.doi.org/10.1088/0022-3727/42/3/035004http://dx.doi.org/10.1063/1.2801375http://dx.doi.org/10.1063/1.3055274http://dx.doi.org/10.1063/1.2745642http://dx.doi.org/10.1063/1.2745642http://dx.doi.org/10.1016/j.mssp.2006.01.074http://dx.doi.org/10.1063/1.352573http://dx.doi.org/10.1021/j100345a003http://dx.doi.org/10.1021/jp054608bhttp://dx.doi.org/10.1016/0040-6090(95)08239-5http://dx.doi.org/10.1088/1367-2630/11/6/063009http://dx.doi.org/10.1088/1367-2630/11/6/063009http://dx.doi.org/10.1016/0304-8853(92)91168-Shttp://dx.doi.org/10.1016/0304-8853(92)91168-Shttp://dx.doi.org/10.1021/ja0559193http://dx.doi.org/10.1063/1.2187435http://dx.doi.org/10.1088/0022-3727/41/24/245113http://dx.doi.org/10.1088/0022-3727/41/24/245113http://dx.doi.org/10.1016/j.tsf.2009.11.067http://dx.doi.org/10.1063/1.1450041http://dx.doi.org/10.1143/JJAP.38.1534http://dx.doi.org/10.1016/j.jmmm.2009.01.040http://dx.doi.org/10.1103/PhysRev.133.A1143http://dx.doi.org/10.1016/j.ssc.2006.04.020http://dx.doi.org/10.1088/0022-3727/41/13/134012http://dx.doi.org/10.1088/0022-3727/41/13/134012http://dx.doi.org/10.1016/j.surfcoat.2005.02.197http://dx.doi.org/10.1016/j.surfcoat.2005.02.197http://dx.doi.org/10.1016/j.ssc.2007.05.036http://dx.doi.org/10.1016/j.ssc.2007.05.036http://dx.doi.org/10.1016/j.matlet.2007.09.035http://dx.doi.org/10.1016/j.matlet.2007.09.035http://dx.doi.org/10.1088/0022-3727/40/21/008


20/21

Mahamuni S, Borgohain K, Bendre BS, Leppert VJ, Risbud SH(1999) Spectroscopic and structural characterization of electrochemically grown ZnO quantum dots. J Appl Phys85:2861-12861-5. doi: 10.1063/1.369049

Maouche D, Ruterana P, Louail L (2007) Carrier-mediatedferromagnetism in N co-doped (Zn, Mn)O-based dilutedmagnetic semiconductors. Phys Lett A 365:231234. doi:10.1016/j.physleta.2007.01.014

Matsumoto Y, Murakami M, Shono T, Hasegawa T, FukumuraT, Kawasaki M, Ahmet P, Chikyow T, Koshihara S,Koinuma H (2001) Room-temperature ferromagnetism intransparent transition metal-doped titanium dioxide. Sci-ence 291:854856. doi: 10.1126/science.1056186

Moulder JF, Stickle WF, Sobol PE and Bomben KD (1992)Handbook of X-ray photoelectron spectroscopy. PhysicalElectronics Division, Perkin-Elmer Corp., Eden PrairieMN, ISBN 0962702625

Moulder JF, Stickle WF, Sobol PE, Bomben KD, Chastain JJ,King RC (eds) (1995) Handbook of X -ray photoelectronspectroscopy, physical electronics. Perkin-Elmer Corp,Eden Prarie, MN

Ohno H (1998) Making nonmagnetic semiconductors ferro-magnetic. Science 281:951956. doi: 10.1126/science.281.5379.951

Ohtomo A, Kawasaki M, Ohkubo I, Ohkubo I, Koinuma H,Yasuda T, Segawa Y (1999) Structure and optical prop-erties of ZnO/Mg 0.2 Zn0.8 O superlattices. Appl Phys Lett75:980982. doi: 10.1063/1.124573

Park CH, Chadi DJ (2005) Hydrogen-mediated spinspininteraction in ZnCoO. Phys Rev Lett 94:127204-1127204-4. doi: 10.1103/PhysRevLett.94.127204

Pawelec B, Daza L, Fierro JLG, Anderson JA (1996) Regen-eration of NiUSY catalysts used in benzene hydrogena-tion. Appl Catal A 145:307322. doi: 10.1016/0926-860X(96)00134-2

Pei G, Xia C, Cao S, Zhang J, Wu F, Xu J (2006) Synthesis andmagnetic properties of Ni-doped zinc oxide powders.J Magn Magn Mater 302:340342. doi: 10.1016/j.jmmm.2005.09.029

Pei GQ, Wu F, Xia C, Zhang JG, Li X, Xu J (2008) Inuencesof Al doping concentration on structural, electrical andoptical properties of Zn 0.95 Ni0.05 O powders. Curr ApplPhys 8:1823. doi: 10.1016/j.cap.2007.04.003

Punnoose A, Reddy KM, Hays J, Thurber A, Engelhard MH(2006) Magnetic gas sensing using a dilute magneticsemiconductor. Appl Phys Lett 89:112509-1112509-3.doi:10.1063/1.2349284

Radovanovic PV, Gamelin DR (2003) High-temperature ferro-magnetism in Ni 2 ? -Doped ZnO aggregates prepared fromcolloidal diluted magnetic semiconductor quantum dots.Phys Rev Lett 91:1572021-11572021-4. doi: 10.1103/ PhysRevLett.91.157202

Raebiger H, Lany S, Zunger A (2008a) Charge self-regulationupon changing the oxidation state of transition metals ininsulators. Nature (London) 453:763766. doi: 10.1038/ nature07009

Raebiger H, Lany S, Zunger A (2008b) Control of ferromag-netism via electron doping in In 2O3 :Cr. Phys Rev Lett101:027203-1027203-4. doi: 10.1103/PhysRevLett.101.027203

Sarma SD (2003) Ferromagnetic semiconductors: a giantappears in spintronics. Nat Mater 2:292294. doi: 10.1038/ nmat883

Sato K, Yoshida HK (2001) Stabilization of ferromagneticstates by electron doping in Fe-, Co- or Ni-doped ZnO.Japan J Appl Phys 40:L334L336. doi: 10.1143/JJAP.40.L334

Schmidt G, Ferrand D, Molenkamp LW, van Wees BJ (2000)Fundamental obstacle for electrical spin injection from aferromagnetic metal into a diffusive semiconductor. PhysRev B 62:R4790R4793. doi: 10.1103/PhysRevB.62.R4790

Schwartz DA, Kittilstved KR, Gamelin DR (2004) Above-room-temperature ferromagnetic Ni 2 ? -doped ZnO thinlms prepared from colloidal diluted magnetic semicon-ductor quantum dots. Appl Phys Lett 85:13951397. doi:10.1063/1.1785872

Selim MM, Islam H, El-Maksoud A (2005) Spectroscopic andcatalytic characterization of Ni nano-size catalyst foredible oil hydrogenation. Microporous Mesoporous Mater85:273278. doi: 10.1016/j.micromeso.2005.06.027

Serrano J, Romero AH, Manjo n FJ, Lauck R, Cardona M, Rubio

A (2004) Pressure dependence of the lattice dynamics of ZnO: an ab initio approach. Phys Rev B 69:094306-1094306-14. doi: 10.1103/PhysRevB.69.094306

Shi H, Duan Y (2009) First-principles study of magneticproperties of 3d transition metals doped in ZnO nano-wires. Nanoscale Res Lett 4:480484. doi: 10.1007/ s11671-009-9260-7

Silva RF, Zaniquelli MED (2002) Morphology of nanometricsize particulate aluminium-doped zinc oxide lms. Col-loids Surf A 198200:551558. doi: 10.1016/S0927-7757(01)00959-1

Simpson JC, Cordero JF (1988) Characterization of deep levelsin zinc oxide. J Appl Phys 63:17811783. doi: 10.1063/ 1.339919

Stocker M, Tangstad E, Aasand N, Myrstad T (2000) Quanti-tative determination of Ni and V in FCC catalysts moni-tored by ESR spectroscopy. Catal Lett 69:223229. doi:10.1023/A:1019094611940

Straumal BB, Mazilkin AA, Protasova SG, Myatiev AA,Straumal PB, Baretzky B (2008) Increase of Co solubilitywith decreasing grain size in ZnO. Acta Mater56:62466256. doi: 10.1016/j.actamat.2008.08.032

Straumal B, Baretzky B, Mazilkin A, Protasova S, Myatiev A,Straumal P (2009) Increase of Mn solubility withdecreasing grain size in ZnO. J Eur Ceram Soc29:19631970. doi: 10.1016/j.jeurceramsoc.2009.01.005

Studenikin SA, Golego N, Cocivera M (1998) Fabrication of green and orange photoluminescent, undoped ZnO lmsusing spray pyrolysis. J Appl Phys 84:22872294. doi:10.1063/1.368295

Tau J (1974) Amorphous and liquid semiconductor. PlenumPress, New York, p 159

Thota S, Dutta T, Kumar J (2006) On the solgel synthesis andthermal, structural, and magnetic studies of transitionmetal (Ni, Co, Mn) containing ZnO powders. J PhysCondens Matter 18:24732486. doi: 10.1088/0953-8984/ 18/8/012

Tong L-N, He X-M, Han H-B, Hu J-L, Xia A-L, Tong Y(2010) Effects of annealing on ferromagnetism of Ni-

J Nanopart Res

123
http://dx.doi.org/10.1063/1.369049http://dx.doi.org/10.1016/j.physleta.2007.01.014http://dx.doi.org/10.1126/science.1056186http://dx.doi.org/10.1126/science.281.5379.951http://dx.doi.org/10.1126/science.281.5379.951http://dx.doi.org/10.1063/1.124573http://dx.doi.org/10.1103/PhysRevLett.94.127204http://dx.doi.org/10.1016/0926-860X(96)00134-2http://dx.doi.org/10.1016/0926-860X(96)00134-2http://dx.doi.org/10.1016/j.jmmm.2005.09.029http://dx.doi.org/10.1016/j.jmmm.2005.09.029http://dx.doi.org/10.1016/j.cap.2007.04.003http://dx.doi.org/10.1063/1.2349284http://dx.doi.org/10.1103/PhysRevLett.91.157202http://dx.doi.org/10.1103/PhysRevLett.91.157202http://dx.doi.org/10.1038/nature07009http://dx.doi.org/10.1038/nature07009http://dx.doi.org/10.1103/PhysRevLett.101.027203http://dx.doi.org/10.1103/PhysRevLett.101.027203http://dx.doi.org/10.1038/nmat883http://dx.doi.org/10.1038/nmat883http://dx.doi.org/10.1143/JJAP.40.L334http://dx.doi.org/10.1143/JJAP.40.L334http://dx.doi.org/10.1103/PhysRevB.62.R4790http://dx.doi.org/10.1063/1.1785872http://dx.doi.org/10.1016/j.micromeso.2005.06.027http://dx.doi.org/10.1103/PhysRevB.69.094306http://dx.doi.org/10.1007/s11671-009-9260-7http://dx.doi.org/10.1007/s11671-009-9260-7http://dx.doi.org/10.1016/S0927-7757(01)00959-1http://dx.doi.org/10.1016/S0927-7757(01)00959-1http://dx.doi.org/10.1063/1.339919http://dx.doi.org/10.1063/1.339919http://dx.doi.org/10.1023/A:1019094611940http://dx.doi.org/10.1016/j.actamat.2008.08.032http://dx.doi.org/10.1016/j.jeurceramsoc.2009.01.005http://dx.doi.org/10.1063/1.368295http://dx.doi.org/10.1088/0953-8984/18/8/012http://dx.doi.org/10.1088/0953-8984/18/8/012http://dx.doi.org/10.1088/0953-8984/18/8/012http://dx.doi.org/10.1088/0953-8984/18/8/012http://dx.doi.org/10.1063/1.368295http://dx.doi.org/10.1016/j.jeurceramsoc.2009.01.005http://dx.doi.org/10.1016/j.actamat.2008.08.032http://dx.doi.org/10.1023/A:1019094611940http://dx.doi.org/10.1063/1.339919http://dx.doi.org/10.1063/1.339919http://dx.doi.org/10.1016/S0927-7757(01)00959-1http://dx.doi.org/10.1016/S0927-7757(01)00959-1http://dx.doi.org/10.1007/s11671-009-9260-7http://dx.doi.org/10.1007/s11671-009-9260-7http://dx.doi.org/10.1103/PhysRevB.69.094306http://dx.doi.org/10.1016/j.micromeso.2005.06.027http://dx.doi.org/10.1063/1.1785872http://dx.doi.org/10.1103/PhysRevB.62.R4790http://dx.doi.org/10.1143/JJAP.40.L334http://dx.doi.org/10.1143/JJAP.40.L334http://dx.doi.org/10.1038/nmat883http://dx.doi.org/10.1038/nmat883http://dx.doi.org/10.1103/PhysRevLett.101.027203http://dx.doi.org/10.1103/PhysRevLett.101.027203http://dx.doi.org/10.1038/nature07009http://dx.doi.org/10.1038/nature07009http://dx.doi.org/10.1103/PhysRevLett.91.157202http://dx.doi.org/10.1103/PhysRevLett.91.157202http://dx.doi.org/10.1063/1.2349284http://dx.doi.org/10.1016/j.cap.2007.04.003http://dx.doi.org/10.1016/j.jmmm.2005.09.029http://dx.doi.org/10.1016/j.jmmm.2005.09.029http://dx.doi.org/10.1016/0926-860X(96)00134-2http://dx.doi.org/10.1016/0926-860X(96)00134-2http://dx.doi.org/10.1103/PhysRevLett.94.127204http://dx.doi.org/10.1063/1.124573http://dx.doi.org/10.1126/science.281.5379.951http://dx.doi.org/10.1126/science.281.5379.951http://dx.doi.org/10.1126/science.1056186http://dx.doi.org/10.1016/j.physleta.2007.01.014http://dx.doi.org/10.1063/1.369049


21/21

doped ZnO powders. Solid State Commun 150:11121116. doi: 10.1016/j.ssc.2010.03.029

Van de Walle CG (2000) Hydrogen as a cause of doping inzinc oxide. Phys Rev Lett 85:10121015. doi: 10.1103/ PhysRevLett.85.1012

Van Dijken A, Meulenkamp EA, Vanmaekelbergh D, Meijer-ink A (2000) Identication of the transition responsiblefor the visible emission in ZnO using quantum sizeeffects. J Lumin 90:123128. doi: 10.1016/S0022-2313(99)00599-2

van Wees BJ (2000) Comment on observation of spin injectionat a ferromagnet semiconductor interface. Phys Rev Lett84:50235024. doi: 10.1103/PhysRevLett.84.5023

Wang ZG, Zu XT, Zhu S, Wang LM (2006) Green lumines-cence originates from surface defects in ZnO nanoparti-cles. Physica E 35:199202. doi: 10.1016/j.physe.2006.07.022

Wang H, Chen Y, Wang HB, Zhang C, Yang FJ, Duan JX,Yang CP, Xu YM, Zhou MJ, Li Q (2007) High resolutiontransmission electron microscopy and Raman scatteringstudies of room temperature ferromagnetic Ni-doped ZnO

nanocrystals. Appl Phys Lett 90:052505-1052505-3. doi:10.1063/1.2435606Wolff PA, Bhatt RN, Durst AC (1996) Polaronpolaron

interactions in diluted magnetic semiconductors. J ApplPhys 79:5196-15196-3. doi: 10.1063/1.361338

Xu Q, Hartmann L, Zhou S, Mcklich A, Potzger K, Helm M,Biehne G, Hochmuth H, Lorenz M, Grundmann M,Schmidt H (2008) Spin manipulation in Co-doped ZnO.

Phys Rev Lett 101:07660110766014. doi: 10.1103/PhysRevLett.101.076601

Yin ZG, Chen NF, Yang F, Song SL, Chai CL, Zhong J, QianHJ, Ibrahim K (2005) Structural, magnetic properties andphotoemission study of Ni-doped ZnO. Solid StateCommun 135:430433. doi: 10.1016/j.ssc.2005.05.024

Yu GH, Zeng LR, Zhu FW, Chai CL, Lai WY (2001) Magneticproperties and X-ray photoelectron spectroscopy study of NiO/NiFe lms prepared by magnetron sputtering. J ApplPhys 90:4039140395. doi: 10.1063/1.1401804

Yu GH, Zhu FW, Chai CL (2003) X-ray photoelectron spec-troscopy study of magnetic lms. Appl Phys A 76:4547.doi:10.1007/s003390201292

Yuvaraj D, Narasimha Rao K (2010) Photo catalytic activity of ZnO nanostructured lm grown by activated reactiveevaporation. Mater Res Soc Symp Proc 1217:1217-Y03050. doi: 10.1557/PROC-1217Y03-50

Zhang XL, Qiao R, Li Y, Qiu R, Kang YS (2007) Synthesisand characterization of nickel-doped ZnO nanocrystals.Mater Res Soc Symp Proc 957: 0957-k10-24

Zhou X, Ge S, Yao D, Yalu Z, Yuhua X (2008) Preparation,

magnetic and optical properties of ZnO and ZnO:Co rodsprepared by wet chemical method. J Alloy Compd463:L9L11. doi: 10.1016/j.jallcom.2007.11.099

Zuo Y, Ge S, Chen Z, Zhang L, Zhou X, Yan S (2009) Mor-phology, optical and magnetic properties of Zn 1 - x Ni x Onanorod arrays fabricated by hydrothermal method.J Alloy Compd 470:4750. doi: 10.1016/j.jallcom.2008.03.010

J Nanopart Res

123
http://dx.doi.org/10.1016/j.ssc.2010.03.029http://dx.doi.org/10.1103/PhysRevLett.85.1012http://dx.doi.org/10.1103/PhysRevLett.85.1012http://dx.doi.org/10.1016/S0022-2313(99)00599-2http://dx.doi.org/10.1016/S0022-2313(99)00599-2http://dx.doi.org/10.1103/PhysRevLett.84.5023http://dx.doi.org/10.1016/j.physe.2006.07.022http://dx.doi.org/10.1016/j.physe.2006.07.022http://dx.doi.org/10.1063/1.2435606http://dx.doi.org/10.1063/1.361338http://dx.doi.org/10.1103/PhysRevLett.101.076601http://dx.doi.org/10.1103/PhysRevLett.101.076601http://dx.doi.org/10.1016/j.ssc.2005.05.024http://dx.doi.org/10.1063/1.1401804http://dx.doi.org/10.1007/s003390201292http://dx.doi.org/10.1557/PROC-1217Y03-50http://dx.doi.org/10.1016/j.jallcom.2007.11.099http://dx.doi.org/10.1016/j.jallcom.2008.03.010http://dx.doi.org/10.1016/j.jallcom.2008.03.010http://dx.doi.org/10.1016/j.jallcom.2008.03.010http://dx.doi.org/10.1016/j.jallcom.2008.03.010http://dx.doi.org/10.1016/j.jallcom.2007.11.099http://dx.doi.org/10.1557/PROC-1217Y03-50http://dx.doi.org/10.1007/s003390201292http://dx.doi.org/10.1063/1.1401804http://dx.doi.org/10.1016/j.ssc.2005.05.024http://dx.doi.org/10.1103/PhysRevLett.101.076601http://dx.doi.org/10.1103/PhysRevLett.101.076601http://dx.doi.org/10.1063/1.361338http://dx.doi.org/10.1063/1.2435606http://dx.doi.org/10.1016/j.physe.2006.07.022http://dx.doi.org/10.1016/j.physe.2006.07.022http://dx.doi.org/10.1103/PhysRevLett.84.5023http://dx.doi.org/10.1016/S0022-2313(99)00599-2http://dx.doi.org/10.1016/S0022-2313(99)00599-2http://dx.doi.org/10.1103/PhysRevLett.85.1012http://dx.doi.org/10.1103/PhysRevLett.85.1012http://dx.doi.org/10.1016/j.ssc.2010.03.029

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