Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 19–24

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Synthesis, structural and optical properties of ZnO and Ni-doped ZnO hexagonal nanorods by Co-precipitation method K. Raja a, P.S. Ramesh b,⇑, D. Geetha a a b

Department of Physics, Annamalai University, Annamalai Nagar 608002, Tamilnadu, India Department of Physics (DDE Wings), Annamalai University, Annamalai Nagar 608002, Tamilnadu, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 FTIR spectra, the stretching of band

appearing at (666 cm1) confirms the formation of rod shaped ZnO particles.  Presence of functional groups and the chemical bonding with Ni confirmed by FTIR.  The average crystalline size is reduced by Ni-doped ZnO.  Optical properties which are appreciable for the fabrication of nano-optoelectronic device.

a r t i c l e

i n f o

Article history: Received 26 July 2013 Received in revised form 9 September 2013 Accepted 26 September 2013 Available online 11 October 2013 Keywords: Ni-doped ZnO rods FTIR Co-precipitation Optical absorption

a b s t r a c t Ni doped ZnO (Zn1xNixO, x = 0.0, 0.03, 0.06 and 0.09) nanorods have been synthesized by Co-precipitation method. Zinc acetate dehydrate [Zn(CH3COO)22H2O], nickel nitrate [Ni(NO3)36H2O], sodium hydroxide and poly (vinyl pyrrolidone) (PVP) were mixed together. The morphology, optical and microstructure were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy dispersive spectrum (EDS), atomic force microscopy (AFM), UV-DRS spectrum, photoluminescence spectra (PL) and Fourier transformer infrared spectroscopy (FT-IR). The presence of functional groups and chemical bonding is confirmed by FTIR. PL spectra of the Zn1xNixO systems shows that the shift in near band edge (NBE) UV emission from 321 to 322 nm and a shift in red band (RB) emission from 620 to 631 nm which conforms the substitution of Ni into the ZnO lattice. The investigation conformed that the products were of the wurtzite structure of ZnO. The hexagonal nanorods have edge length 31 nm and thickness of 39 nm. EDS result showed that the amount of Ni in the product is about 9%, these Ni doped hexagonal nanorods exhibits a blue shifts and weak (UV) emission peak, compared with pure ZnO, which may be induced by the Ni-doping different concentrations 0.0, 0.3, 0.6 and 0.9 M. The growth mechanism of the doped hexagonal nanorods was also discussed. Ó 2013 Elsevier B.V. All rights reserved.

Introduction Zinc oxide (ZnO) is one of transparent conducting oxides (TCO). It has degenerate semiconducting properties with a wide band gap (3.3 eV) and transparent properties in visible range of spectrum. ⇑ Corresponding author. Tel.: +91 9842301700. E-mail address: [email protected] (P.S. Ramesh). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.09.103

It is used in the production of many optoelectronic devices, like solar cells or flat panel displays, etc. [1]. Usually ZnO is used doped with different types of metallic ions, like Sn, Ga, In, Sn, Al, Sc, and Y, in order to improve its TCO properties [2,3]. Transition metal doping into the ZnO lattice can modify its optical, electrical, and magnetic properties. A number of studies have reported on the improved optical and magnetic properties of ZnO with transition metal doping [4,5]. Unique chemical stability of nickel on zinc sites

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recognizes it as one of the most efficient doping element to improve and tune the optical and electrical properties of ZnO. The primary emission peak of ZnO is characterized as a strong ultraviolet (UV) emission at 380 nm. The root cause of this emission is usually the recombination of free excitons corresponding to the nearband-edge emission of ZnO. Ni:ZnO reportedly exhibits a blue shift in UV emission because the Ni dopant provides more electrons that occupy the energy levels located at the bottom of the conduction band [6]. PL spectra can provide much information about the dopant atoms [7]. In order to fully understand these unique properties and explore the application of ZnO nanostructures on the design of high performed nanodevices, great efforts are dedicated to rational synthesis of ZnO nanostructures and a rich family of ZnO nanostructures has been developed, including nanowires, nanosheets, nanotubes, nanobelts and nanorings, etc. [8,9]. Various chemical methods have been developed to prepare nanoparticles of different materials of interest. Most of the ZnO crystals have been synthesized by traditional high temperature solid state method in which, it is difficult to control the particle properties and also energy consuming. ZnO nanoparticles can be prepared on a large scale at low cost by simple solution based method, such as chemical precipitation, sol–gel synthesis, and hydrothermal reaction [10,11]. Zinc oxide is a semiconductor used for various applications such as gas sensor, transparent conducting films, piezoelectric transducers and photo catalytic activity [12,13]. It has been synthesized with a variety of well defined nanostructures such as nanowires, nanorods, nanonails, nanotubes and nanobelts [14,15]. Nanosized nickel oxide has demonstrated excellent properties such as catalytic [16], magnetic [17], electrochromic [18], optical and electrochemical properties [19]. Furthermore, nickel oxides can be used as a transparent p-type semiconducting layer [20] and dye sensitized photo cathodes [21]. While to our knowledge, no such investigation on Zn1xNixO has been reported as yet. In this study, ZnO and Ni:ZnO NRs were fabricated by chemical deposition method. The crystal phase, morphology, optical properties and microstructure are studied in details. Crystal growth and aspect ratio of the ZnO nanorods were sensitive to Ni doping condition, which is very important for both fundamental and applied points of view. Experimental Materials Zinc acetate dehydrate [Zn(CH3COO)22H2O], nickel nitrate [Ni(NO3)36H2O], sodium hydroxide (NaOH) and poly (vinyl pyrrolidone) (PVP) were purchased from Merck. All the chemicals were used as received since they were of analytical reagent grade with 99% purity. The glass wares used in this experimental work were washed by acid. Ultrapure water was used for all dilution and sample preparation. Synthesis of Ni doped ZnO nanorods and undoped ZnO nanocrystals For the synthesis of Ni doped ZnO nanorods, analytical grade zinc acetate dehydrate [Zn(CH3COO)22H2O], nickel nitrate [Ni(NO3)36H2O], sodium hydroxide (NaOH) and poly (vinyl pyrrolidone) (PVP) were used. All the reagents were used as received (Merck) without further purification. In a typical reaction process for the growth of hexagonal nanorod Ni doped ZnO, 1.975 g zinc acetate dehydrate, 0.305 g of nickel nitrate and 3.0 g of PVP were dissolved in 100 ml ultra pure water and stirred for 30 min. Simultaneously, a 10 ml NaOH (10 M) was added drop wise into this aqueous zinc acetates, nickel nitrate and PVP solution under vigorous stirring for 2 h at room temperature to produce a white, gelatinous precipitate. The white precipitates were filtered and washed

with ethanol and ultra pure water for many times. The find precipitate was dried in oven at 80 °C for 4 h. The dried precipitates were collected and ground in an agate mortar. Finally the samples collected at 550 °C under air atmosphere for 2 h followed by furnace cooling. The same procedure is repeated to the remaining samples synthesized with nominal composition of Zn1xNixO with x = 0.00, 0.03, 0.06 and 0.09. Characterization details The crystal structures of the as prepared samples were analyzed using X-ray diffractometer (XRD, X’PERT PRO) with Cu Ka radiation. The morphology of Ni-doped ZnO rod arrays was characterized by scanning electron microscopy (SEM, JSM-5610LV, and Japan). The analysis of elements was conducted by energy-dispersive spectrum (EDS) was attached to the scanning electron microscopy (OXFORD-6587). The structure properties of the nanopowder were investigated by atomic force microscopy (AFM, XE-100E). The UV-DRS spectrum absorption was measured on PERKIN ELMER LAMBDA-35 spectrophotometer. Fluorescence measurements were performed on a VARIAN spectrophotometer. The presence of chemical bonding in Ni-doped ZnO nanoparticles (as pellets in KBr) was studied by FT-IR spectrometer (Model: iS No. 5) in the range of 6000–600 cm1. The measurement was carried out by preparing dilute solution of ZnO and Ni-doped ZnO nanopowders in ethanol. Result and discussion Structural studies Fig. 1 shows the XRD patterns of ZnO and Ni doped ZnO samples. The XRD patterns of Zn1xNixO (x = 0.00, 0.03, 0.06 and 0.09 M) powder samples at 550 °C temperatures. The grown nanostructures shows several peaks of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1), planes and all the peaks can be indexed to standard values (JCPDS Card No. 80.0075), no signal of the metallic Zn are detected by XRD. Also there are no peaks corresponding with the Ni or its oxides. Suggesting the Ni element may be doped into ZnO. The higher intensities of calcined nanostructure peaks with narrower width reveal a highly crystallized wurtzite structure. The size of the Ni doped ZnO nanoparticles were estimated by applying the Scherer Eq. (1) to the half intensity width of the (1 0 0), (0 0 2) and (1 0 1) peaks.

D¼

0:9 k b cos h

ð1Þ

where D is the crystalline size, k is the wavelength of the incident 0 X-ray (1.54 A Å), h is the Bragg’s angle and b is the full width at half maximum (FWHM). From the X-ray line broading the crystalline size of ZnO and Ni-doped ZnO are estimated clearly shows the average particle size is reduced from 37.4 to 31 nm by the addition of Ni from 0.0 to 0.06 whereas Zn0.91Ni0.09O sample has 39.1 nm. It clearly shows the presences of nano-sized particles in the samples. A similar Ni doping concentration limit around x = 0.03 was reported in the Ni doped ZnO system [6]. The micro-strain (e) can be calculated using the formula [22].

ðeÞ ¼

b cos h 4

ð2Þ

Table 1 shows the 2h value, the full width at half maximum (FWHM, b) value, d-value, cell parameters ‘a’ and ‘c’, c/a ratio, average crystal size a (D) and micro-strain of different Zn1xNixO nanoparticles. At the lower Ni concentration (Ni = 0.06), the substitution of Ni2+ ions instead of Zn2+ ions at their lattice sites increases the lattice constant ‘a’ and ‘c’ and the interplanar distance ‘d’ which

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However, the decrease of strain causes the decrease of lattice constants and increase of particle size.

d

Relative intensity

Scanning electron microscope (SEM) – microstructural studies c

20

(101)

30

40

50

(103)

60

(200) (112)

(110)

a (102)

(100) (002)

b

70

80

2θ (degree) Fig. 1. XRD patterns of Ni doped ZnO samples (a) undoped ZnO, (b) Zn0.91Ni0.03O, (c) Zn0.94Ni0.06O and (d) Zn0.91Nix0.09O.

would lead to the decrease of the diffraction angle compared with undoped ZnO. The similar trend of lattice constant was reported by Kulyk et al. [23], by the Ni doping this is consistent with our experimental result with increase in Ni concentration. The resultant compound maintains wurtizite structures, but the lattice parameters ‘a’ and ‘c’ are slightly increase up to x = 0.06 and then decreases for x = 0.09. The change in the lattice parameters can be ascribed by the substituted of Ni2+ ion. Which has a smaller ionic radius than the Zn2+ (ionic radius = 0.074 nm) sites [24] in their tetrahedral coordinates. The slight shift in the XRD spectrum and the change in the d-value and the lattice constants with Ni doping indicated that Ni has really doped into ZnO lattice. It is observed from Table 1. That micro-strain of the Ni doped sample is increased up to x = 0.06 (0.8725  103), then slowdown and reaches minimum (0.8145  103) for x = 0.09. At x = 0.09, the atom trapped in the non-equilibrium position could shift to a more equilibrium position, which could release the strain and here it is decreases. The value of dislocation density (d) is calculated using the relation [25].

d¼

1

SEM is one of the techniques for the topography study of the samples and it gives important information regarding the growth mechanism, shape and size of the particles. The surface morphology of the undoped and Ni doped ZnO nanoparticles are as shown in Supplementary file Fig. 2a–c. The entire SEM picture clearly shows the average size of the nanoparticles is the order of nanometer size. The undoped ZnO nanoparticles Supplementary file (Fig. 2a) shows the synthesized nanoparticles are homogeneous, uniformly distributed over the surface and good connectivity between the particles containing the mixer of spheroid-like and rod-like particles. The grain size was decreased with the increase of Ni concentrations. Supplementary file Fig. 2b shows the surface morphology of the Zn0.94Ni0.06O sample which has slightly lesser grain size than the undoped ZnO. The shape of the particles is turnover from spheroid-like into the rod-like. The further increase of Ni, x = 0.09 Supplementary file (Fig. 2c) shows the uneven surface morphology with large grain size. The increase of Ni doped concentrations causes the more defects and deformed lattice structures. The existence of more defects greatly degenerate the particles size and shape and hence the grain size and shape is larger for x = 0.09 than the other undoped and Ni doped ZnO samples. A good correlation is found to exist between mathematical calculations from XRD analysis and those obtained from SEM studies. Energy-dispersive analysis X-ray spectra The energy dispersive analyses X-ray (EDX) is used to analysis the amount of Ni element in Zn1xNixO sample. The chemical compositional analysis of the pure and Ni-doped ZnO nanoparticles has been carried out using EDX as shown Fig. 3(a–c). It is clearly shown that the intensity of Ni increases with increasing Ni incorporation in the solution. Therefore, the addition of Ni induces a dominant effect on the optical, structural and morphological properties of ZnO. The element compositions of the pure Ni doped ZnO nanoparticles given in Table. 2. The EDS analysis confirms the presence of Ni in the ZnO system and wt% in very nearly equal to their nominal stoichiometry with in the experimental error. Therefore, the EDS spectra show well agreement with the experimental concentration used for Zn1xNixO system.

ð3Þ

D2

Atomic force microscope (AFM) – microstructural study

It is observed the dislocation density Ni doped Zno sample up to increased x = 0.06 (8.163  104) then slowdown and reaches minimum (6.544  103) for x = 0.09. The intrinsic stress is changed monotonically with increasing the Ni concentration due to the change in the microstructure, size and shape of the particles. Mean while, the crystal quality of the system with Ni = 0.09 remarkably improved by Ni doping. It is evident that the increase of constants of strain causes the increased of lattice constants, reduction in the particle size, and the broadening and a small shift in XRD peaks.

Supplementary file Fig. 4 show an AFM image of the seed catalyst layer. Based on the AFM image, the root mean square (RMS) roughness of the Ni-doped ZnO nanoparticle was 25.4 nm over an area of 1 lm2. The analysis revealed that at a precursor Ni concentration x = 0.06 the prepared samples exhibited very smooth surface and good crystal structures. It is characterization technique for the examination of nanopowder materials in contact mode by atomic force microscopy (AFM). The AFM after significant

Table 1 The 2h value, cell parameters ‘a’ and ‘c’, c/a ratio, average crystal size (D), micro-strain (e) and dislocation density (d) of different Zn1xNixO nanoparticles using XRD spectra. Samples

2h values (0)

ZnO Zn0.97Ni0.03O Zn0.94Ni0.06O Zn0.91Ni0.09O

36.213 36.201 36.139 36.141

Cell parameter (A0) a=b

c

3.264 3.253 3.242 3.264

5.209 5.215 5.209 5.219

c/a ratio

Average crystalline size (D) (nm)

Micro-strain (e) (103)

Dislocation density (d) (104)

1.6012 1.6031 1.5958 1.5989

37 36 31 39

0.909 0.848 0.872 0.814

8.163 7.763 7.153 6.544

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advantage of probing in high details the surface topography qualitatively (by surface image) due to its nanometer scale spatial resolution, both lateral and vertical. AFM has provided to be very helpful for the determination and verification of various morphological features and parameter. The AFM image Ni-doped nanopowder sample as shown in Supplementary file Fig. 4. The surface image of Ni-doped ZnO size of the particle was observed 25.4 nm.

Table 2 The quantitative analysis of the compositional elements present for the different Zn1xNixO nanoparticles using EDS spectra. Samples

ZnO Zn0.97Ni0.03O Zn0.94Ni0.06O Zn0.91Ni0.09O

Weight percentage of the elements (%) O

Zn

Ni

.42 .56 .59 .54

.79 1.01 .97 .89

– 0.01 0.04 0.07

Optical absorption studies The optical absorption spectra of undoped and Ni doped ZnO (Zn1xNixO where x = 0.00, 0.03, 0.06 and 0.09) samples by UVDRS spectrum in the range of 200–800 nm were presented. From Fig. 5 it can be seen that the excitonic absorption peak of as prepared undoped and Ni doped ZnO samples appears around 230 nm which is fairly blue shifted from the absorption edge (i.e. much below the band gap wavelength of 365 nm, Eg = 3.4 eV) of bulk ZnO. The excitonic absorption peak of as prepared undoped and Ni doped samples become broad as the molar ratio increases. The sharp excitonic peak in the absorption spectra of pure ZnO is indicative of the small size distribution of nanocrystal in the samples and broading of peaks at Ni doped ZnO clearly indicates the increase in size of nanocrystals with the increase of molar ratio concentrations. It can be observed clearly from Fig. 5 that absorbance increases from x = 0.00 to x = 0.06 but it decreases the molar ratio of x = 0.09. In nickel doped zinc oxide at x = 0.06 an absorption band localized between 560 nm and 750 nm corresponding

to the spin orbit 3T1(F) ? 3T1(P) ligand field transition of Ni2+ in the tetrahedral symmetry is observed for the Zn1xNixO from that of undoped ZnO samples [26,27]. The energy band gap is determined by using the relationship: a = A(hmEg)n, where hm = photon energy, a = Absorption co-efficient (a = pk/k; k is the absorption index (or) absorbance, k is wavelength in nm), Eg = band gap energy, A = constant, n = 1/2 for allowed direct band gap. Exponent n depends on the type of transition and it may have the values 1/2, 2, 3/2 and 3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions respectively [28]. This absorption shift is due to quantum size effect, representing a change is band gap along with exciton feature, which can be used as measure of particle size and size distribution [29]. The band gap energy of the nanocrystals was calculated from a simple energy wave equation, E = hc/k and the determined values are 4.3, 4.13, 3.2 and 3.39 eV for undoped and Ni doped ZnO, respectively.

Fig. 3. EDS spectra of undoped and Ni doped ZnO nanoparticles for different Ni concentration.

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322nm

631nm

Zn0.91Ni0.09O

Zn0.91Ni0.09O

Zn0.97Ni0.03O

ZnO

PL intensity (a.u.)

Absorbance (a.u)

Zn0.94Ni0.06O

318nm

630nm

Zn0.94Ni0.06O 624nm

321nm Zn0.97Ni0.03O

200

300

400

500

600

700

620nm

800

321nm

Wavelenght (nm)

ZnO

Fig. 5. UV-DRS spectra of undoped and Ni doped ZnO nanoparticles for different Ni concentration.

350

This result reveals that the size of the ZnO nanocrystals has been decreased on doping.

400

450

500

550

600

650

700

Wavelength (nm) Fig. 6. Room temperature photoluminescence spectra of undoped and Ni doped ZnO nanoparticles.

PL spectra of the pure ZnO and Ni-doped ZnO

860

672

1019 1448

3424

882 877

671

1018 1014

1384

3434

1617

2849

2920

Zn0.96Ni0.06O

3500

2500

2000

1500

666 664

1120 1019

1419

1627

3000

1375

2852

3432

1617

2920

2852

3431

2922

Zn0.97Ni0.03O

ZnO

4000

1383

1639

2923

Zn0.91Ni0.09O

Transmittance (a.u)

Fig. 6 PL spectra of the pure ZnO and Ni-doped ZnO hexagonal nano-rod excited at room temperature. The PL spectra of pure ZnO nanorods have a strong UV band peak at 321 nm. Besides, a relatively strong and broad red band centered at about 620 nm occurred. The UV emission resulted from excitonic recombination corresponding to the near band-edge emission of ZnO. The green emission peak is originated due to deep-level or trap-state emission [30,31]. In fact, there are different mechanisms have been proposed for the visible light emission of ZnO. Oxygen vacancies occur in three different states: the neutral oxygen vacancy, the singly ionized oxygen vacancy and the doubly ionized oxygen vacancy [32]. Vanheusden et al. [33] found that only the singly ionized oxygen vacancies are responsible for the green luminescence in the ZnO. Compared with the PL spectrum of ZnO nanorods, the peak position of UV emission, for Ni-doped ZnO nanorods, exhibits blue shift while the red emission band is sharply suppressed. For Ni doped ZnO nanorod, more electrons contributed by nickel dopants would take up the energy levels located at the bottom of conduction band. When they were excited by the laser of 322 nm, the excitons take up higher energy levels at the bottom of conduction band. Radioactive recombination of these excitons will lead to a blue shift and broaden of UV emission peak [34]. The lattice strain induced by the lattice distance would also lead to some shift in band gap but may not play a major role in the determination of band gap as the small deformation of the lattice distances. Decreased UV emission was considered due to the increase of the nano-radiative defects and decrease of ZnO nano-disc size [35]. A blue shift of the band edge revealed from PL points to the incorporation of at least a part of Ni on the lattice sites. This supports the X-ray results on the incorporation of Ni into the nano-rod structure. When Ni ions are incorporated into ZnO and become donors, multi-emission centers are formed, such as the emission of the electron–hole plasma (EHP), the emission of the donors to the valence band, and the intrinsic transition of Ni3+ ions [36]. So the green emission of Ni doped ZnO nanorod was more broad than the undoped. The strong UV and weak green bands imply good crystal surface [37]. The increased ratio suggests the good crystal of Ni-doped ZnO nanorods. These results showed a great promise for the Ni-doped ZnO nanorod with applications in optoelectronic devices.

1000

-1

Wavenumber (cm ) Fig. 7. FT-IR spectra of undoped and Ni doped ZnO nanoparticles for different Ni concentration.

FT-IR studies In the FT-IR spectra shown in Fig. 7, the broad absorption band at 3437 cm1 corresponds to the O–H stretching vibration of water present in ZnO and the other transmission band at 2922 cm1 is assigned to a residual organic component. The band at 1617 cm1 can be associated with the bending vibration of H2O molecules. The transmission band at 1534 cm1 and 1458 cm1 in both the samples is due to the carbonyl group of the carboxylate ions which might remain adsorbed on the surface of ZnO. The stretching of band appearing at 666 cm1 confirms the formation of rod shaped ZnO particles. The peaks appearing between 400 and 600 cm1 are assigned to the Meta-Oxygen (M-H)

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Table 3 IR peak and their assignments for the Zn. Assignments

O–H stretching Additional weak band H–O–H bending Vibration Zn–O stretching

Wave number (cm1) ZnO

Zn0.97Ni0.03O

Zn0.94Ni0.06O

Zn0.91Ni0.09O

3432 2920 1617 664

3431 2922 1627 666

3434 2920 1617 671

3424 2923 1639 672

stretching mode from Table 3. The appearance of peaks in three different position depends on shapes of ZnO have reported by Verges et al. [38]. The shape affects the position and intensity of the peaks [38]. As Fig. 7 shows the FT-IR spectra of Ni doped ZnO which are almost similar to that of the ZnO nanorods. The vibration mode at wave number 543 cm1 slightly changes. The vibration modes at 671 and 1014 cm1 are associated with Ni2+ occupation at Zn2+ sites. Because of ionic radiation mismatch between Ni2+ and Zn2+ intrinsic host lattice defects are activated. These types of activated impurity cause a shift in the vibration mode [39]. Conclusions Zn1xNixO (x = 0.0, 0.03, 0.06, 0.09) nanoparticles have been successfully synthesized by a Co-precipitation method and annealed under Ar atmosphere at 550 °C. The XRD and SEM + EDS measurement results conclude that ZnO and Ni doped ZnO are rod shaped particles with single phase hexagonal structure. The presence of functional groups and the chemical bonding with Ni is confirmed by FT-IR spectra. PL spectra of the Zn1xNixO system described the shift in near band edge (NBE) UV emission from 321 to 322 nm and a shift in red band (RB) emission from 620 to 631 nm which confirms the substitution of Ni into the ZnO lattice. The AFM analysis revealed that at a precursor Ni concentration x = 0.06 the prepared films exhibited very smooth surface and good crystal structures. Hydrogenated samples have good crystalline structure and better optical properties which are appreciable for the fabrication of nano-optoelectronic device like tunable light emitting diode in the near future. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.09.103. References [1] K. Ellmer, A. Klein, B. Rech (Eds.), Springer Series in Materials Science, 104 (2008) ISBN 978-3-540–73611-0. [2] T. Moriga, Y. Hayashi, K. Kondo, Y. Nishimura, J. Vac, Sci. Technol. A 22 (2004) 705–1710. [3] H. Huang, Y. Ou, S. Xu, G. Fang, M. Li, X.Z. Zhao, Appl. Surf. Sci. 254 (2008) 2013–2016. [4] Y. Zhang, E.W. Shi, Z.Z. Chen, Mater. Sci. Semicond. Proc. 13 (2010) 132–136. [5] C.C. Vidyasagar, Y. Arthoba Naik, T.G. Venkatesh, R. Viswanatha, Powder Technol. 214 (2011) 337–343.

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