Nanostructured morphology control and optical properties of ZnO thin film deposited from chemical solution

Materials Research Bulletin 52 (2014) 183–188

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Nanostructured morphology control and optical properties of ZnO thin film deposited from chemical solution Chen Chen, Haiyan Xu *, Fengjun Zhang, Guotian Wu School of Materials & Chemical Engineering, Anhui University of Architecture, Hefei 230022, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 30 August 2013 Received in revised form 31 December 2013 Accepted 12 January 2014 Available online 19 January 2014

Zinc oxide thin films with the sheet-like, rose-like to flower-like structures were directly synthesized on glass substrates through a simple, rapid, facile and one-step chemical bath deposition method at 80 8C. The formation mechanisms of these ZnO structures were proposed. It was revealed that the morphologies were controlled by anisotropic structure of ZnO and the contents of TSC and NH4OH. Particularly, the content of NH4OH played an important role in controlling the morphology of ZnO thin film. Moreover, optical properties of ZnO thin films with three morphologies were also investigated by the UV–vis transmittance spectra and photoluminescence spectra. ß 2014 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures A. Semiconductors A. Thin films B. Chemical synthesis D. Optical properties

1. Introduction Recent years, the morphology-controlled synthesis of metallic oxides has received much attention to produce highly functional materials due to their special morphology–structure–function relationship. Especially, because of the unique properties of zinc oxide (ZnO) such as wide band gap (3.37 eV), large exciton binding energy (60 meV), thermal stability, and low threshold intensity, various dimensional ZnO nanostructured materials have aroused great interest, which make it as an excellent candidate for the fabrication of electronic and optoelectronic devices [1–6]. ZnO nanomaterials with diverse structures and morphologies such as 1D nanowires, nanotubes and nanorods [7–9], 2D nanoplates and nanosheets [10,11], and 3D hierarchical architectures [12–14] have been designed and synthesized by varieties of methods. Generally, ZnO nano/microstructures were prepared through three methods: vapor phase process, melt growth, and solution phase synthesis [15–20]. In comparison with vapor phase and melt methods, solution phase method was remarkable for their cheap experimental setups, large productivity, and mild conditions. Chemical bath deposition (CBD) was one of the useful solution methods for film deposition from aqueous solution containing precursors, complexing agents, and pH buffers [21,22]. Compared to other solution phase methods like sol–gel process [18], hydrothermal methods [19], electro-deposition [20], the advantages of CBD included not only low processing temperature, not

* Corresponding author. Tel.: +86 551 63828152; fax: +86 551 63828106. E-mail addresses: [email protected], [email protected] (H. Xu). 0025-5408/$ – see front matter ß 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2014.01.014

requiring vacuum systems, allowing growth upon a variety of substrates, but also its compatibility with large area deposition. Moreover, CBD was low cost, energy saving, industrial-scale and environmentally benign [23]. Bai et al. [24] synthesized ZnO nanosheet thin films on the copper substrates through a solvothermal route and annealing processing. The star-like ZnO film consisted of needle-like spines was produced on TOF glass substrate via CBD [21]. Peng et al. [25] obtained the flower-like, spindle-like and rod-like ZnO film on ITO substrates by CBD with adjusting precursor concentration. Herein, we report one-step synthesis of morphology tunable ZnO thin film on glass substrates through CBD. The sheet-like, novel rose-like and flower-like ZnO structures were achieved by changing the NH4OH content. Detailed formation mechanisms of these ZnO structures were discussed. Moreover, optical properties of ZnO thin films with three morphologies were also investigated by the UV–vis transmittance spectra and photoluminescence spectra. 2. Experimental details 2.1. Materials All reagents were of analytical grade and used without further purification. Zinc nitrate hexahydrate (Zn(NO3)26H2O, 99+%), trisodium citrate (TSC, C6H5Na3O72H2O, 99+%) and ammonium hydroxide (NH4OH, 25–28 wt% in water, 99.99%) were used for the deposition of ZnO thin film in the aqueous medium. Deionized water was employed throughout the experiment. Films were obtained on the commercial microscope glass slides as the substrates (25.4 mm  20.0 mm  1.0 mm).

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2.2. Synthesis of ZnO thin film Before deposition, the substrates were degreased by ultrasonic treatment in toluene, acetone, and ethanol in turn, rinsed with deionized water, and etched in hydrofluoric acid (20%) for about 3 min. They were then cleaned with deionized water and dried in air. Typically, NH4OH was taken into the mixture of Zn(NO3)2 and TSC with the pretreated substrate suspended horizontally by a rubber tape in a 100 ml beaker. Then, the beakers containing the precursor solution were maintained at 80 8C for 8 h in order to obtain the proposed films (shown in Fig. 1). After deposition, the slide with film was taken out from the solution and rinsed with deionized water, and dried at 80 8C for the storage. 2.3. Characterization X-ray powder diffraction (XRD) patterns of the prepared ZnO thin film were analyzed by a Bruker Advance D8 diffractometer (D/ max 30 kV) using Cu Ka radiation (l = 0.15406 nm). The scan range of 2u was set between 208 and 808 with a step of 0.028. The morphology of the samples was investigated by field emission scanning electron microscopy (SEM) (Siron 200). The working voltage of SEM was 5.00 kV. UV–vis transmittance spectra were acquired with Shimadzu UV-3101PC spectrophotometer. The photoluminescence spectra (PL) were taken on a PTI-C-700 fluorescence spectrometer. A CW He–Cd laser (50 mW, wavelength at 325 nm, IK 3552R-G) was used as the excitation source to measure the photoluminescence. 3. Results and discussion 3.1. Morphologies and Structures of ZnO thin film Fig. 2 shows the XRD pattern of samples prepared with NH4OH of (a) 4.5 mL, (b) 4.8 mL and (c) 5.1 mL by keeping mole ratio

(112) (201)

(103)

(110)

(101)

(100)

(c)

(102)

Fig. 1. Flow chart of ZnO film preparation.

Intensity(a.u.)

(002)

184

(b) (a)

ZnO (JCPDS #36-1451)

20

30

40

50

60

70

80

2 θ (deg.) Fig. 2. XRD patterns of the ZnO thin film obtained with NH4OH of (a) 4.5 mL, (b) 4.8 mL and (c) 5.1 mL by keeping mole ratio (Zn(NO3)2 (0.20 M):C6H5Na3O7) of 5:2 at 80 8C for 8 h.

(Zn(NO3)2 (0.20 M):C6H5Na3O7) of 5:2 at 80 8C for 8 h. Three notable peaks i.e., (1 0 0), (0 0 2) and (1 0 1), appeared at 2u = 31.788, 34.428 and 36.258, respectively, which revealed that those samples could be indexed to the hexagonal wurtzite structure of ZnO crystal (JCPDS No. 36-1451). No characteristic peaks of other impurities were detected in the pattern. The sharp diffraction peaks indicated the good crystallinity of the prepared crystals. It was noted that the relative intensities of the peaks differed from the standard pattern of the bulk material, which should be caused by preferred orientation and distribution of the ZnO crystals on the substrate surface [26]. An EDS shown in Fig. 3 furthermore indicated that Zn and O were the elements of the sample (Fig. 2a), which was in agreement with the XRD analysis. Si, Ca, Mg and Pt elements were not expected to be in the film and were originated from the glass substrates. As for the other samples (Fig. 2b and c) prepared at different content of NH4OH, the EDS spectrum were almost identical to Fig. 3. SEM micrographs (shown in Fig. 4) were taken to investigate the surface morphologies of ZnO thin film obtained with NH4OH of (a–c) 4.5 mL, (d–f) 4.8 mL and (g–i) 5.1 mL by keeping mole ratio (Zn(NO3)2 (0.20 M):C6H5Na3O7) of 5:2 at 80 8C for 8 h. Fig. 4a–c

Fig. 3. EDS of the ZnO thin film obtained with NH4OH of 4.5 mL by keeping mole ratio (Zn(NO3)2 (0.20 M):C6H5Na3O7) of 5:2 at 80 8C for 8 h.

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185

Fig. 4. SEM images of the ZnO thin film obtained with NH4OH of (a–c) 4.5 mL, (d–f) 4.8 mL and (g–i) 5.1 mL by keeping mole ratio (Zn(NO3)2 (0.20 M):C6H5Na3O7) of 5:2 at 80 8C for 8 h.

demonstrates the SEM images of ZnO thin film grown with NH4OH of 4.5 mL and revealed that formed structures were composed of sheet-like morphology in high density (Fig. 4a and b). The high magnification SEM image (Fig. 4c) showed that the nanosheets were connected to each other and dispersed quasi-vertically on the glass substrate, finally forming networks. The average dimensions of the observed nanosheets were in the range of 0.4–1.4 mm with the typical thickness of 60 nm. Each sheet had almost the same thickness perpendicular to its 2D face, indicating the plate growth was strictly extended in the 2D plane throughout the whole growing process. A more interesting morphology, sphere-shaped structure composed of several ZnO thin nanosheets, was also observed as shown in Fig. 4a–c. The diameter of sphere-shaped structure was about 1.5–3 mm. With the content of NH4OH increased to 4.8 mL (Fig. 4d–f), the sheet-like structure of ZnO thin film was transformed into the hierarchical rose-like structure with the maximum diameter of 1.6 mm. The hierarchical rose-like structure was composed of many nanosheets with the thickness of 30 nm, which were curled and wrapped layer by layer. As compared to the dimension and thickness of nanosheet in Fig. 4a– c, the nanosheet dimension and thickness of hierarchical rose-like structure in Fig. 4d–f obviously became smaller. When the content of NH4OH was up to 5.1 mL, the flower-like ZnO structure was obtained as shown in Fig. 4g–i. Fig. 4i shows that the flower-like ZnO structure was composed of several sword-like nanorods with the bottom diameter of 140–200 nm and lengths of 360–700 nm. Therefore, the morphology of ZnO thin film (shown in Table 1) was transformed from the sheet-like, rose-like to flower-like ZnO Table 1 The morphology of ZnO thin film prepared with different contents of NH4OH. NH4OH

4.5 mL

4.8 mL

5.1 mL

Morphology

Sheet-like

Rose-like consisting of nanosheet

Flower-like consisting of sword-like nanorod

structure with the increased content of NH4OH via keeping mole ratio (Zn(NO3)2 (0.20 M):C6H5Na3O7) of 5:2 at 80 8C for 8 h. Fig. 5 demonstrates the SEM images of ZnO thin film grown with deposition duration by keeping mole ratio (Zn(NO3)2 (0.20 M):C6H5Na3O7) of 5:2 at 80 8C and 4.8 mL NH4OH. The low SEM images revealed that ZnO thin film was composed of rose-like structures. With the prolonged deposition duration, the number density of ZnO thin films was decreased while the diameter of roselike structure was increased. Fig. 5b, d and f shows that the hierarchical rose-like structures were consisted of many nanosheets. With the increased deposition duration, the size of pores (as the red arrowheads shown in Fig. 5) among the hierarchical rose-like structures continued to decrease. Meanwhile, rose-like structures became more compact and smooth, leading to the formation of 3D rose-like compact structure. The formation of rose-like compact and smooth structures could be explained by the Ostwald ripening mechanism [27,28]. 3.2. Growth mechanism To understand the morphological evolution of ZnO thin film (sheet-like, rose-like and flower-like structure), the involved chemical reactions were suggested below. Zn2þ þ 2=3TSC $ ½ZnðTSCÞ2=3 2þ

(1)

Zn2þ þ 4NH3 $ ZnðNH3 Þ4 2þ

(2)

Zn2þ þ 2OH $ ZnðOHÞ2 orZn2þ þ 4OH ! ZnðOHÞ4 2

(3)

ZnðOHÞ2 ! ZnO þ H2 O

(4)

Generally, based on the ZnO intrinsic structure and surface energy minimization in the solution system, the fastest growth of ZnO crystallite spurred it to grow along the c-axis. Thus, the

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Fig. 5. SEM images of the ZnO thin film prepared for (a and b) 2 h, (c and d) 5 h and (e and f) 8 h by keeping mole ratio (Zn(NO3)2 (0.20 M):C6H5Na3O7) of 5:2 at 80 8C and 4.8 mL NH4OH. (For interpretation of the references to color near the citation of this figure, the reader is referred to the web version of this article.)

elongated rod-like shape was much more favorable in the thermodynamic equilibrium state [21]. Therefore, the released Zn2+ by the [Zn(TSC)2/3]2+ and Zn(NH3)42+ complexant (reactions (1) and (2)) reacted with the OH to form the ZnO nucleus (reactions (3) and (4)) via the heterogeneous nucleation on the activation sites of glass substrate. Especially, the formation of a large quantity of the growth units (Zn(OH)42) contributed to the fast growth of ZnO nucleus along orientation [0 0 1] under the condition of strong alkaline solution. As for the flower-like ZnO structure as shown in Fig. 4g–i, the sword-like nanorods with sharp tips could be explained by the different growth rate of the crystalline plane [29,30]. The variation in the orientation of the side branches raised in part from variations in the orientation of the primary rods in each bundle, and in part from the effect of substrate on the relative orientation between the primary and secondary crystals [31]. However, the growth of ZnO crystallite was also affected by the external conditions such as ligand, pH value, and temperature [21,32]. In our reaction system, TSC played multiform roles. First, it was an effective ligand that controlled the concentration of Zn2+ in solution. Second, TSC anions selectively adsorbed on the positively charged Zn2+ face on the (0 0 1) plane of the wurtzite ZnO structure as a structure-directing reagent [33,34]. Apart from the TSC, NH4OH not only played the modified pH agent, but also the complexing agent [35]. At the low pH value (4.5 mL NH4OH), the sheet-like structure of ZnO thin film had been synthesized in solution with TSC anions as structure-directing agent to be adsorbed selectively on ZnO basal planes. Because some negative TSC ions replaced OH dangling bond on ZnO positive polar faces (0 0 2), the growth along the c direction was suppressed, resulting in the formation of sheet-like ZnO structures. When the growth velocity along c axis was suppressed, the nanosheets increase in length and width but not in thickness until their final dimensions were reached. At the length limitation in the process of nanosheet growth, the sheet structure was curled, mingled or aggregated to a new structure (sphere-shaped structure). The reason was the spatial restriction on substrates, reducing the exterior free energy and long-range electrostatic interactions among the polar charges of the {0 0 1} planes [36,37]. With the increased pH (4.8 mL NH4OH), the nucleation and growth of ZnO were improved due to the enhanced supersaturation concentration and produced quantity of growth units (Zn(OH)42). Additionally, as the Zn2+-terminated and O2-terminated planes aligned alternatively along [0 0 1] direction in the crystals, the structure of ZnO possessed an intrinsic dipole moment along [001]

direction [38,39]. In order to counterbalance the dipolar field, two ZnO slices with opposite polar directions assembled together for charge neutralization and plane stabilization, resulting in the formation of ZnO twinned crystals [39–41]. This resulted in the alignment of ZnO nanocrystallines in slices ranges in three dimensions along the dipole field [40]. Eventually, the rose-like networks were formed and served as a substrate for newly growing ZnO slices. The twinned ZnO slices grew and assembled simultaneously due to charge interactions between the (0 0 1) ¯ plane of another one. Thus, plane of one ZnO slice and the (0 0 1) the rose-like ZnO structure was obtained. In addition, the hexagonal faceted structure of ZnO slice transformed to a more circled shape to minimize total energy [42]. As for the high pH (5.1 mL NH4OH), the rod-like ZnO structure was formed instead of sheet-like or rose-like structure, even if the TSC occurred in solution. The reason was that the OH played an important role in the growth process of ZnO thin film. According to the reactions (3) and (4), the produced tremendous amount of growth units (Zn(OH)42) steeply adsorbed on the (0 0 1) plane of the wurtzite ZnO structure to promote the formation of 1D ZnO structure. Besides, the effect of TSC on the morphology was weakened mainly due to the reduced relative concentration ratio between [TSC] and [OH] [33,36]. Therefore, the morphology of ZnO thin film from the sheet-like, rose-like to flower-like structure was affected by the TCS and NH4OH. The possible growth routes of three morphologies of ZnO thin film could be schematically summarized in Fig. 6. 3.3. UV–vis and photoluminescence properties of different morphologies of ZnO thin film The UV–vis transmittance spectra of sheet-like ZnO thin film (a), rose-like ZnO thin film (b) and flower-like ZnO thin film (c) were shown in Fig. 7. It could be seen that the absorption edges of the as-prepared ZnO thin film respectively appeared at 372 nm (a), 351 nm (b) and 380 nm (c). The results showed that the absorption edge of sheet-like ZnO thin film (a) and rose-like ZnO thin film (b) was blue-shifted compared to that of flower-like ZnO thin film (c). Particularly, the blue-shifted level of rose-like ZnO thin film (b) was the most. The absorption edge shift could be explained by the surface effect (grain boundaries) [43] and the particle energy (E) [44]. Grain boundaries effect (GB), mentioned by Straumal [43], could be approximately quantified by the specific GB area (sGB). The GB area

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187

Fig. 6. Schematic illustration of the possible formation process for sheet-like, rose-like and flower-like structures.

to volume ratio is sGB = 1.65a/D, where D is the mean grain width. In case of elongated grains, a < 1 is the aspect ratio (ratio of grain width to grain height). In the case of flattened grains, a > 1 is the ratio of grain width D to grain height. Therefore, the sGB value of sheet-like ZnO thin film (a), rose-like ZnO thin film (b) and flowerlike ZnO thin film (c) was 2.8  107 m2/m3, 5.5  107 m2/m3 and 2.4–4.6  106 m2/m3, respectively. These sGB values showed that the sGB of sheet structure (sheet-like (a) and rose-like ZnO thin film (b)) was greater than that of sword-like rod structure (flower-like ZnO thin film (c)). Besides, the particle energy relation of threedimensional single-particle box is E ¼ Aðn2x =X 2 þ n2y =Y 2 þ n2z =Z 2 Þ, where A was a constant; nx, ny, nz, are the quanta number; X, Y, Z were the scales in each dimension [44]. The equation was used to approximately quantify three morphologies energy (E). For the thin ZnO nanosheets, taking the direction perpendicular to the sheet plane as the z-axis direction, the sheet plane as the x–y plane, and considering the energy ground state (i.e. nx = ny = nz = 1), then X, Y could be approximately infinite and the above energy relation could be simplified as E = A/Z2 [45]. Apparently, a decreased Z value (the thickness of ZnO sheets from 60 nm (a) to 30 nm (b)) leaded to an increased energy (E). The increased energy (E) and sGB with the reduction of sheet thickness from Fig. 7a and b produced a blueshift of the band edge. However, for the flower-like ZnO thin film, the above energy relation could be simplified as E = A (1/X2 + 1/Y2) [46]. The bottom diameter of sword-like nanorod (140–200 nm) was larger than that (60 nm) in the sheet-like ZnO thin film. The energy (E) of sheet-like ZnO thin film (a) was increased with compared to that of flower-like ZnO thin film (c). Moreover, the sGB value of sheet-like ZnO thin film (2.8  107 m2/m3) was greater than that of flower-like ZnO thin film (2.4–4.6  106 m2/m3).

Therefore, the increased energy (E) and sGB of sheet-like ZnO thin film leaded to a blue-shift of the band edge relative to the flowerlike ZnO thin film. In summary, the sGB and increased energy (E) of sheet-like ZnO thin film (a) and rose-like ZnO thin film (b) (shown in Table 2) contributed to a blue-shift of the band edge compared to that of flower-like ZnO thin film (c). The photoluminescence spectra of sheet-like ZnO thin film (a), flower-like ZnO thin film (b) and rose-like ZnO thin film (c) were illuminated in Fig. 8. It could be seen that a UV emission peak of the as-prepared ZnO thin film respectively appeared around 395 nm (a), 405 nm (b) and 374 nm (c) [35,47,48]. The results of PL spectra were consistent with the results of UV–vis transmittance spectra. The surface effect (grain boundaries) and the particle energy (E) were applied to explain the peak blue-shift of rose-like ZnO thin film and sheet-like ZnO thin film compared to the UV emission peak of flower-like ZnO thin film. However, the PL of rose-like ZnO thin film not only had the UV emission peak, but also a green emission band centered at around 550 nm range from 450 to 750 nm. It was known that the UV band-gap emission was related to the radiative recombination of an excited electron in the conduction band with the valence band hole. The green emission band corresponding to the single ionized oxygen vacancy in ZnO

Table 2 The specific GB area and particle energy of three morphologies of ZnO thin films. Morphology

Flower-like

Sheet-like

Rose-like

sGB E

2.4–4.6  106 m2/m3 A/20,000  A/9800

2.8  107 m2/m3 A/3600

5.5  107 m2/m3 A/900

80 395

(a)

70

50 40 30

(b)

20

PL Intensity (a. u.)

Transmittance (%)

60

405 374

(a)

(c)

10

(b) (c)

0 200 250 300 350 400 450 500 550 600 650 700 750 800

Wavelength (nm) Fig. 7. UV–vis transmittance spectra of sheet-like ZnO thin film (a), rose-like ZnO thin film (b) and flower-like ZnO thin film (c).

350

400

450

500

550

600

650

700

750

800

Wavelength (nm) Fig. 8. Photoluminescence spectra of sheet-like ZnO thin film (a), flower-like ZnO thin film (b) and rose-like ZnO thin film (c).

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resulted from the recombination of photogenerated hole with the single ionized charge state of this defect [49]. 4. Conclusions The ZnO thin film with hierarchical sheet-like, rose-like and flower-like structures by changing the NH4OH content were obtained through a simple, rapid, facile and one-step chemical bath deposition method at 80 8C. The hierarchical rose-like and flowerlike structures of ZnO thin film respectively were composed of many nanosheets and several sword-like nanorods. The formation mechanisms of these ZnO structures revealed that the morphologies were controlled by anisotropic structure of ZnO and the contents of TSC and NH4OH. Particularly, the content of NH4OH played an important role in controlling the morphology of ZnO thin film. Besides, the rose-like compact and smooth structures was formed at deposition duration of 8 h via the Ostwald ripening mechanism. The UV–vis and PL results showed the relationship between optical properties and morphology, which the blue-shift of the band edge and PL peak were relied on grain boundaries (sGB) and the particle energy (E) of ZnO structures. To our understanding, the ZnO thin film with hierarchical sheet-like, rose-like and flowerlike structures represents an ideal candidate for photon collectors, photoelectric converter, laser, catalysts, and electronic applications. In addition, this simple and low-cost chemical bath deposition method is expected to allow the large-scale production of other oxides with controllable morphologies, which is advantageous for practical application. Acknowledgement Financial support of Education Department of China (No. 2011075) is gratefully acknowledged. References [1] V. Subramanian, E.E. Wolf, P.V. Kamat, J. Phys. Chem. B 107 (2003) 7479–7485. [2] D. Weissenberger, D. Gerthsen, A. Reiser, G.M. Prinz, M. Feneberg, K. Thonke, H. Zhou, J. Sartor, J. Fallert, C. Klingshim, H. Kalt, Appl. Phys. Lett. 94 (2009) 042107– 042110. [3] Z.L. Wang, X.Y. Kong, Y. Ding, P. Gao, W.L. Hughes, R. Yang, Y. Zhang, Adv. Funct. Mater. 14 (2004) 943–956. [4] Z.L. Wang, ACS Nano 2 (2008) 1987–1992. [5] K.M. Shafi, R. Vinodkumar, R.J. Bose, V.N. Uvais, V.P.M. Pillai, J. Alloys Compd. 551 (2013) 243–248. [6] (a) T.H. Guo, Y. Liu, Y.C. Zhang, M. Zhang, Mater. Lett. 65 (2011) 639–641; (b) S.Z. Liu, Y.C. Zhang, T.X. T.X. Wang, F.X. Yang, Mater. Lett. 71 (2012) 154–156; (c) Y.C. Zhang, X. Wu, X.Y. Hu, R. Guo, J. Cryst. Growth 280 (2005) 250–254. [7] G. Zhong, A. Kalam, A.S. Al-Shihri, Q. Su, J. Li, G.H. Dua, Mater. Res. Bull. 47 (2012) 1467–1470. [8] R. Ranjusha, R. Sreeja, P.A. Mini, K.R.V. Subramanian, S.V. Nair, A. Balakrishnan, Mater. Res. Bull. 47 (2012) 1887–1891. [9] D. Liu, Y.F. Liu, R.L. Zong, X.J. Bai, Y.F. Zhu, Mater. Res. Bull. 49 (2014) 665–671. [10] J.X. Wang, C.M.L. Wu, W.S. Cheung, L.B. Luo, Z.B. He, G.D. Yuan, W.J. Zhang, C.S. Lee, S.T. Lee, J. Phys. Chem. C 114 (2010) 13157–13161.

[11] M. Fu, J. Zhou, Q.F. Xiao, B. Li, R.L. Zong, W. Chen, J. Zhang, Adv. Mater. 18 (2006) 1001–1004. [12] P. Ramasamy, J. Kim, Mater. Lett. 93 (2013) 52–55. [13] H. Li, E.T. Liu, F.Y.F. Chan, Z. Lu, R. Chen, Mater. Lett. 65 (2011) 3440–3443. [14] C.Y. Jiang, X.W. Sun, G.Q. Lo, D.L. Kwong, J.X. Wang, Appl. Phys. Lett. 90 (2007) 263501–263503. [15] S. Mandal, R.K. Singha, A. Dhar, S.K. Ray, Mater. Res. Bull. 43 (2008) 244–250. [16] M.M. Chen, Q.L. Zhang, L.X. Su, Y.Q. Su, J.S. Cao, Y. Zhu, T.Z. Wu, X.C. Gui, C.L. Yang, R. Xiang, Z.K. Tang, Mater. Res. Bull. 47 (2012) 2673–2675. [17] K. Jacobs, D. Schulz, D. Klimm, S. Ganschow, Solid State Sci. 12 (2010) 307–310. [18] R. Vettumperumal, S. Kalyanaraman, B. Santoshkumar, R. Thangavel, Mater. Res. Bull. 50 (2014) 7–11. [19] B.M. Wen, Y.Z. Huang, J.J. Boland, J. Phys. Chem. C 112 (2008) 106–111. [20] B.Q. Cao, W.P. Cai, G.T. Duan, Y. Li, Q. Zhao, D.P. Yu, Nanotechnology 16 (2005) 2567–2574. [21] K. Govender, D.S. Boyle, P.B. Kenway, P. O’Brien, J. Mater. Chem. 14 (2004) 2575– 2591. [22] M. Kokotov, G. Hodes, J. Mater. Chem. 19 (2009) 3847–3854. [23] H.Y. Xu, C. Chen, L. Xu, J.K. Dong, Thin Solid Films 527 (2013) 76–80. [24] W. Bai, X. Zhu, Z. Zhu, J. Chu, Appl. Surf. Sci. 254 (2008) 6483–6488. [25] W. Peng, S. Qu, G. Cong, Z. Wang, Cryst. Growth Des. 6 (2006) 1518–1522. [26] T. Andelman, Y. Gong, M. Polking, M. Yin, I. Kuskovsky, G. Neumark, S. O’Brien, J. Phys. Chem. B 109 (2005) 14314–14318. [27] X.P. Wu, Y.Q. Wang, S.M. Zhou, X.Y. Yuan, T. Gao, K. Wang, S.Y. Lou, Y.B. Liu, X.J. Shi, Cryst. Growth Des. 13 (2013) 136–142. [28] Y.F. Xu, D.K. Ma, X.A. Chen, D.P. Yang, S.M. Huang, Langmuir 25 (2009) 7103–7108. [29] H. Zhang, D.R. Yang, Y.J. Ji, X.Y. Ma, J. Xu, D.L. Que, J. Phys. Chem. B 108 (2004) 3955–3958. [30] J.B. Liang, J.W. Liu, Q. Xie, S. Bai, W.C. Yu, Y.T. Qian, J. Phys. Chem. B 109 (2005) 9463–9467. [31] T.L. Sounart, J. Liu, J.A. Voigt, J.W.P. Hsu, E.D. Spoerke, Z. Tian, Y.B. Jiang, Adv. Funct. Mater. 16 (2006) 335–344. [32] D.S. Boyle, K. Govender, P. O’Brien, Chem. Commun. (2002) 80–81. [33] Z.R. Tian, J.A. Voigt, J. Liu, B. Mckenzie, M.J. Mcdermott, M.A. Rodriguez, H. Konishi, H. Xu, Nat. Mater. 2 (2003) 821–826. [34] T. Kawano, H. Imai, Cryst. Growth Des. 6 (2006) 1054–1056. [35] H.X. Li, M.X. Xia, G.Z. Dai, H.C. Yu, Q.L. Zhang, A.L. Pan, T.H. Wang, Y.G. Wang, B.S. Zou, J. Phys. Chem. C 112 (2008) 17546–17553. [36] A. Umar, Y.B. Hahn, Nanotechnology 17 (2006) 2174–2180. [37] Q. Ahsanulhaq, J.H. Kim, N.K. Reddy, Y.B. Hahn, J. Ind. Eng. Chem. 14 (2008) 578– 583. [38] G. Bruno, M.M. Giangregorio, G. Malandrino, P. Capezzuto, I.L. Fragala, M. Losurdo, Adv. Mater. 21 (2009) 1700–1706. [39] Z. Liu, X.D. Wen, X.L. Wu, Y.J. Gao, H.T. Chen, J. Zhu, P.K. Chu, J. Am. Chem. Soc. 131 (2009) 9405–9412. [40] Y.H. Tseng, H.Y. Lin, M.H. Liu, Y.F. Chen, C.Y. Mou, J. Phys. Chem. C 113 (2009) 18053–18061. [41] L. Jia, W. Cai, H. Wang, H. Zeng, Cryst. Growth Des. 8 (2008) 4367–4371. [42] K.S. Krishna, U. Mansoori, N.R. Selvi, M. Eswaramoorthy, Angew. Chem. 119 (2007) 6066–6069. [43] (a) B.B. Straumal, A.A. Mazilkin, S.G. Protasova, A.A. Myatiev, P.B. Straumal, G. Schu¨tz, P.A. van Aken, E. Goering, B. Baretzky, Phys. Rev. B 79 (2009) 205206(6); (b) B.B. Straumal, S.G. Protasova, A.A. Mazilkin, G. Schu¨tz, E. Goering, B. Baretzky, P.B. Straumal, JETP Lett. 97 (2013) 367–377; (c) B.B. Straumal, A.A. Mazilkin, S.G. Protasova, P.B. Straumal, A.A. Myatiev, G. Schu¨tz, E. Goering, T. Tietze, B. Baretzky, Philos. Mag. 93 (2013) 1371–1383. [44] J.Y. Zeng, Introduction to Quantum Mechanics, Peking University Press, Beijing, 1991. [45] A. Pan, R. Yu, S. Xie, Z. Zhang, C. Jin, B. Zou, J. Cryst. Growth 282 (2005) 165–172. [46] M. Salavati-Niasari, M.R. Loghman-Estarki, F. Davar, J. Alloys Compd. 475 (2009) 782–788. [47] Y.C. Lu, L.L. Wang, D.J. Wang, T.F. Xie, L.P. Chen, Y.H. Lin, Mater. Chem. Phys. 129 (2011) 281–287. [48] Y.X. Wang, X.Y. Li, N. Wang, X. Quan, Y.Y. Chen, Technology 62 (2008) 727–732. [49] D.S. Bohle, C.J. Spina, J. Am. Chem. Soc. 129 (2007) 12380–12381.