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Synthesis, characterization and optical properties of star-like ZnO nanostructures
Materials Letters 64 (2010) 898–900
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Synthesis, characterization and optical properties of star-like ZnO nanostructures Zhiwei Peng, Guozhang Dai, Peng Chen, Qinglin Zhang, Qiang Wan, Bingsuo Zou ⁎ State Key Lab of CBSC, Micro-Nano Technologies Research Center, Key Lab for Micro-Nano Optoelectronic Devices of MOE, Hunan University, Changsha 410082, China
a r t i c l e
i n f o
Article history: Received 13 December 2009 Accepted 10 January 2010 Available online 22 January 2010 Keywords: ZnO Thermal evaporation Nanomaterials Luminescence
a b s t r a c t Star-like ZnO nanostructures were synthesized in bulk quantity by thermal evaporation method. The morphologies and structure of ZnO nanostructures were investigated by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results demonstrated that the as-synthesized products consisted of star-like ZnO nanostructure with hexagonal wurtzite phase. The legs of the star-like nanostructures were preferentially grown up along the [0001] direction. A vapor–solid (VS) growth mechanism was proposed to explain the formation of the star-like structures. Photoluminescence spectrum exhibited a narrow ultraviolet emission at around 380 nm and a broad green emission around 491 nm. Raman spectrum of the ZnO nanostructures was also discussed. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction ZnO is a direct wide band gap (Eg = 3.37 eV) semiconductor with a large exciton binding energy (60 meV). As one of the most important II–IV group semiconductors, ZnO nanomaterials have been investigated extensively due to its unique optical and electronic properties. It has been demonstrated that ZnO nanostructures are suitable candidates for many applications, including magnetic, electronic, photonic, optoelectronic, piezoelectronic, nonlinear optical devices, chemical and biological sensors [1–5], etc. Since the novel properties and potential applications of nanomaterials depend sensitively on their shapes and sizes, much effort has been made to synthesize various types of ZnO nanostructures. Over the last few years, a wide variety of ZnO nanostructures with different morphologies have been successfully prepared, such as nanorods, nanobelts, nanosprings, nanorings, nanocombs, nanosaws and hierarchical nanostructures [5], nanotetrapods [6], nanodisk [7], nanopropellers [8], nanobridges and nanonails [9], tower-like, flower-like and tube-like nanostructures [10]. However, despite great progress in this field, the shape-controlled synthesis of ZnO nanostructures still remains a remarkable challenge. ZnO nanostructures have been synthesized by various methods such as thermal evaporation [11], molecular beam epitaxy (MBE) [12], pulsed laser deposition [13], chemical vapor deposition (CVD) [14], metal-organic chemical vapor deposition (MOCVD) [15], hydrothermal synthesis [16], etc. Among them, thermal evaporation is a simple and low-cost technique for the growth of metal oxide nanostructures. This technique has been commonly employed to fabricate various morphologies of ZnO nanostructures [17,18]. In this paper, we report a simple thermal
⁎ Corresponding author. Tel./fax: +86 0731 88822137. E-mail address: [email protected] (B. Zou).
evaporation method for the synthesis of star-like ZnO nanostructures. The morphologies and structure of the ZnO nanostructures are studied by FESEM, XRD and TEM. Growth process and optical properties of these ZnO nanostructures are also reported. 2. Experiment The experiment was carried out in a conventional horizontal tube furnace. High-purity Zn (0.26 g) and Al (0.02 g) powders were mixed as source material and were put in an alumina boat located at the center of the quartz tube. Several pieces of silicon wafer were placed downstream to collect products. For the sake of reducing the concentration of O2 in the tube, an Ar (99.99% purity) flow with a rate of 30 SCCM was turned on and kept for about 10 min before heating. Subsequently, the quartz tube was heated to 650 °C at a rate of 20 °C/min and held at this temperature for 30 min. After the furnace naturally cooled down to room temperature, white wool-like products were found on the Si substrates. The as-synthesized ZnO nanostructures were characterized by X-ray diffraction (XRD, Siemens D-5000), field emission scanning electron microscopy (FESEM, JEOL-JSM-6700F) equipped with an energy dispersive X-ray spectroscopy (EDS, Oxford) and high resolution transmission electron microscopy (HRTEM, JEOL-3010). Raman spectrum and Photoluminescence (PL) measurements were carried out on a scanning near-field optical microscope (Alpha SNOM, WITec) using the Ar+-ion laser (488 nm) and He–Cd laser (325 nm) as excitation source, respectively. 3. Results and discussion Fig. 1 shows a typical XRD pattern of the as-synthesized ZnO nanostructures. All the diffraction peaks can be well indexed to the hexagonal phase ZnO (JCPDS No.36-1451) with the measured lattice
0167-577X/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.01.029
Z. Peng et al. / Materials Letters 64 (2010) 898–900
899
Fig. 1. XRD pattern of the as-grown ZnO nanostructures. The inset shows an EDS spectrum.
constants of a = 3.25 Å, c = 5.21 Å. The sharp diffraction peaks reveal that the products have a high crystal quality. No diffraction peaks from Zn, Al or other impurities are detected in the spectrum, indicating that the as-synthesized products are pure wurtzite ZnO structure. The composition of the samples was analyzed by EDS. A typical EDS spectrum (inset in Fig. 1) shows that only O and Zn elements are detected, confirming the formation of pure ZnO. Fig. 2a shows a typical SEM image of the as-grown ZnO nanostructures. Many star-like ZnO nanostructures are observed. The leg of the star-like structures generally takes the form of a hexagonal pyramid with very smooth side faces, indicating that it can grow along the [0001] direction [19]. The length of the legs is about 600–800 nm. The diameter of the legs gradually becomes smaller along the growth direction, leading to a tapered structure with a sharp tip. The diameter varies from 500 nm at the bottom to about 60 nm at the tip. Furthermore, a core can be clearly seen at the intersection of the legs. The size and shape of the star-like ZnO nanostructures strongly depend on the deposition temperature. Fig. 2b shows a SEM image of the as-grown product obtained at lower deposition temperature region. Star-like structures with cone-shaped legs are observed. The length of the legs becomes longer and the diameter becomes smaller. Fig. 2c shows a TEM image of the star-like ZnO nanostructures. The diameter of the leg gradually decreased from the root to form a sharp tip, in agreement with the results from SEM observations. The structural details of the star-like ZnO nanostructures were analyzed using HRTEM. The HRTEM image (shown in Fig. 2d) displays clear lattice fringes, which reveal the single crystalline nature of the legs. The measured lattice spacing is about 0.26 nm, corresponding to the (0002) fringes, confirming [0001] as the preferred growth direction for the legs. Based on the above analysis, the growth mechanism of ZnO nanostructures was thought to be vapor–solid (VS) rather than conventionally vapor–liquid–solid (VLS) model because no catalyst particles were found at the tips of the legs. It is generally considered that the formation of the ZnO nanostructure has two stages: nucleation and growth. The vapor of Zn or ZnOx (x b 1) [20] generated at the high temperature region was transported to the low temperature region and condensed into liquid droplets on the substrate. Those droplets migrated and merged with the original Zn droplets, then the incorporated liquid droplet would be oxidized and nucleated into a larger ZnO nucleus. It has been proposed that the addition of alloy element such as Ni [21,22] and Cu [23] in the Zn powders leads to the formation of polyhedral ZnO cores, which
Fig. 2. (a–b) Typical SEM images of the as-grown ZnO nanostructures. (c) TEM image and (d) HRTEM image of the ZnO nanostructures.
are ideal nuclei for the multipod-like ZnO structures' growth [23]. In our experiment, we suggest that the addition of Al to the source materials would be similar to that of Ni and Cu. The polyhedral ZnO cores would lead to the formation of star-like ZnO nanostructures. According to the group theory, hexagonal wurtzite ZnO belongs to the C46v space group, near the Brillouin zone, there are eight sets of optical phonons: A1, 2B2, E1, and 2E2 [24]. Among these, A1, E1, and 2E2 modes are Raman active. The Raman spectrum of the ZnO nanostructures is shown in Fig. 3a. A dominated and strong intensity peak at 436 cm− 1 was observed, which is the E2 (high) mode of the non-polar optical phonons, indicating that the ZnO nanostructures are excellent single crystals with hexagonal wurtzite structure [24]. A small and suppressed peak at 584 cm− 1 attributed to the E1 (LO) mode has also been observed. In general, the E1 (LO) mode is associated with the structural defects such as oxygen vacancies and zinc interstitials in ZnO [24,25]. In addition, the peak at 384 cm− 1 corresponds to the A1 (TO) mode and the peak at 332 cm− 1 is due to multiple phonon scattering processes [26,27]. The room temperature PL measurement result of the star-like ZnO is shown in Fig. 3b. The PL spectrum shows a narrow UV emission peak at 380 nm and a broad green emission band centering at 491 nm. The UV emission is commonly attributed to the near-band-edge emissions of ZnO [6,24]. The broad green emission band at about 491 nm is generally attributed to the radiative recombination of a photo-generated hole with an electron occupying the oxygen vacancy [6]. However, surface states have also been identified as a possible
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Z. Peng et al. / Materials Letters 64 (2010) 898–900
4. Conclusion In summary, star-like ZnO nanostructures have been successfully synthesized via thermal evaporation technique. XRD pattern and Raman measurement confirm that the as-grown products have good crystallinity with hexagonal wurtzite phase. A vapor–solid (VS) growth mechanism has been proposed for the formation of the starlike structures. The room temperature PL spectrum shows a narrow near-band-edge peak at 380 nm and a broad defect related emission band around 491 nm. This specific structure of star-like ZnO may have potential application in optical and electrical nanodevices. The simple synthesis method discussed in the present work might be useful for the fabrication of other semiconductor nanostructures.
Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Grant Nos 90606001, 20873039 and 90406024), and Hunan Provincial Natural Science Foundation (No. 07jj4002).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Fig. 3. (a) Raman spectrum and (b) room temperature PL spectrum of the as-grown ZnO nanostructures.
cause of the visible emission in ZnO nanomaterials[28]. It is reasonable that there are some defects in the star-like ZnO nanostructures at the surface and subsurface due to their fast reaction formation process and large surface-to-volume ratio.
[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Jayakumar OD, Sudakar C, Vinu A, Asthana A, Tyagi AK. J Phys Chem C 2009;113:4814. Kind H, Yan HQ, Messer B, Law M, Yang PD. Adv Mater 2002;14:158. Bai XD, Wang EG, Gao PX, Wang ZL. Nano Lett 2003;3:1147. Ning TY, Zhou YL, Shen H, Lu H, Sun ZH, Cao LZ, et al. Appl Surf Sci 2008;254:1900. Wang ZL. J Phys Condens Matter 2004;16:R829. Qiu YF, Yang SH. Adv Funct Mater 2007;17:1345. Long T, Yin S, Tkabatake K, Zhnag P, Sato T. Nanoscale Res Lett 2009;4:247. Pu XG, Wang ZL. Appl Phys Lett 2004;84:2883. Lao JY, Huang JY, Wang DZ, Ren ZF. Nano Lett 2003;3:235. Wang Z, Qian XF, Yin J, Zhu ZK. Langmuir 2004;20:3441. Deng R, Zou Y, Chen Z, Jiang G, Tang H. Mater Lett 2007;61:3582. Heo YW, Varadarajan V, Kaufman M, Kim K, Norton DP, Ren F, et al. Appl Phys Lett 2002;81:3046. Kawakami M, Hartanto AB, Nakata Y, Okada T. Jpn J Appl Phys 2003;42:L33. Chang PC, Fan ZY, Wang DW, Tseng WY, Chiou WA, Hong J, et al. Chem Mater 2004;16:5133. Maejima K, Ueda M, Fujita SZ, Fujita SG. Jpn J Appl Phys 2003;42:2600. Yuan MX, Fu WY, Yang HB, Yu QJ, Liu SK, Zhao Q, et al. Mater Lett 2009;63:1574. Comini E, Faglia G, Ferroni M, Sberveglieri G. Appl Phys A 2007;88:45. Yao BD, Chan YF, Wang N. Appl Phys Lett 2002;81:757. Li J, Srinivasan S, He GN, Kang JY, Wu ST, Ponce FA. J Cryst Growth 2008;310:599. Cheng WD, Wu P, Zou XQ, Xiao T. J Appl Phys 2006;100:054311. Gao T, Huang YH, Wang TH. J Phys Condens Matter 2004;16:1115. Gao T, Wang TH. Appl Phys A 2005;80:1451. Sun XM, Chen X, Li YD. J Cryst Growth 2002;244:218. He FQ, Zhao YP. Appl Phys Lett 2006;88:193113. Chen SJ, Wang GR, Liu YC. J Lumin 2009;129:340. Fauteux C, Longtin R, Pegna J, Therriault D. Inorg Chem 2007;46:11036. Wang RP, Xu G, Jin P. Phys Rev B 2004;69:113303. Hsu NE, Hung WK, Chen YF. J Appl Phys 2004;96:4671.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Synthesis, characterization and optical properties of star-like ZnO nanostructures Zhiwei Peng, Guozhang Dai, Peng Chen, Qinglin Zhang, Qiang Wan, Bingsuo Zou ⁎ State Key Lab of CBSC, Micro-Nano Technologies Research Center, Key Lab for Micro-Nano Optoelectronic Devices of MOE, Hunan University, Changsha 410082, China
a r t i c l e
i n f o
Article history: Received 13 December 2009 Accepted 10 January 2010 Available online 22 January 2010 Keywords: ZnO Thermal evaporation Nanomaterials Luminescence
a b s t r a c t Star-like ZnO nanostructures were synthesized in bulk quantity by thermal evaporation method. The morphologies and structure of ZnO nanostructures were investigated by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results demonstrated that the as-synthesized products consisted of star-like ZnO nanostructure with hexagonal wurtzite phase. The legs of the star-like nanostructures were preferentially grown up along the [0001] direction. A vapor–solid (VS) growth mechanism was proposed to explain the formation of the star-like structures. Photoluminescence spectrum exhibited a narrow ultraviolet emission at around 380 nm and a broad green emission around 491 nm. Raman spectrum of the ZnO nanostructures was also discussed. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction ZnO is a direct wide band gap (Eg = 3.37 eV) semiconductor with a large exciton binding energy (60 meV). As one of the most important II–IV group semiconductors, ZnO nanomaterials have been investigated extensively due to its unique optical and electronic properties. It has been demonstrated that ZnO nanostructures are suitable candidates for many applications, including magnetic, electronic, photonic, optoelectronic, piezoelectronic, nonlinear optical devices, chemical and biological sensors [1–5], etc. Since the novel properties and potential applications of nanomaterials depend sensitively on their shapes and sizes, much effort has been made to synthesize various types of ZnO nanostructures. Over the last few years, a wide variety of ZnO nanostructures with different morphologies have been successfully prepared, such as nanorods, nanobelts, nanosprings, nanorings, nanocombs, nanosaws and hierarchical nanostructures [5], nanotetrapods [6], nanodisk [7], nanopropellers [8], nanobridges and nanonails [9], tower-like, flower-like and tube-like nanostructures [10]. However, despite great progress in this field, the shape-controlled synthesis of ZnO nanostructures still remains a remarkable challenge. ZnO nanostructures have been synthesized by various methods such as thermal evaporation [11], molecular beam epitaxy (MBE) [12], pulsed laser deposition [13], chemical vapor deposition (CVD) [14], metal-organic chemical vapor deposition (MOCVD) [15], hydrothermal synthesis [16], etc. Among them, thermal evaporation is a simple and low-cost technique for the growth of metal oxide nanostructures. This technique has been commonly employed to fabricate various morphologies of ZnO nanostructures [17,18]. In this paper, we report a simple thermal
⁎ Corresponding author. Tel./fax: +86 0731 88822137. E-mail address: [email protected] (B. Zou).
evaporation method for the synthesis of star-like ZnO nanostructures. The morphologies and structure of the ZnO nanostructures are studied by FESEM, XRD and TEM. Growth process and optical properties of these ZnO nanostructures are also reported. 2. Experiment The experiment was carried out in a conventional horizontal tube furnace. High-purity Zn (0.26 g) and Al (0.02 g) powders were mixed as source material and were put in an alumina boat located at the center of the quartz tube. Several pieces of silicon wafer were placed downstream to collect products. For the sake of reducing the concentration of O2 in the tube, an Ar (99.99% purity) flow with a rate of 30 SCCM was turned on and kept for about 10 min before heating. Subsequently, the quartz tube was heated to 650 °C at a rate of 20 °C/min and held at this temperature for 30 min. After the furnace naturally cooled down to room temperature, white wool-like products were found on the Si substrates. The as-synthesized ZnO nanostructures were characterized by X-ray diffraction (XRD, Siemens D-5000), field emission scanning electron microscopy (FESEM, JEOL-JSM-6700F) equipped with an energy dispersive X-ray spectroscopy (EDS, Oxford) and high resolution transmission electron microscopy (HRTEM, JEOL-3010). Raman spectrum and Photoluminescence (PL) measurements were carried out on a scanning near-field optical microscope (Alpha SNOM, WITec) using the Ar+-ion laser (488 nm) and He–Cd laser (325 nm) as excitation source, respectively. 3. Results and discussion Fig. 1 shows a typical XRD pattern of the as-synthesized ZnO nanostructures. All the diffraction peaks can be well indexed to the hexagonal phase ZnO (JCPDS No.36-1451) with the measured lattice
0167-577X/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.01.029
Z. Peng et al. / Materials Letters 64 (2010) 898–900
899
Fig. 1. XRD pattern of the as-grown ZnO nanostructures. The inset shows an EDS spectrum.
constants of a = 3.25 Å, c = 5.21 Å. The sharp diffraction peaks reveal that the products have a high crystal quality. No diffraction peaks from Zn, Al or other impurities are detected in the spectrum, indicating that the as-synthesized products are pure wurtzite ZnO structure. The composition of the samples was analyzed by EDS. A typical EDS spectrum (inset in Fig. 1) shows that only O and Zn elements are detected, confirming the formation of pure ZnO. Fig. 2a shows a typical SEM image of the as-grown ZnO nanostructures. Many star-like ZnO nanostructures are observed. The leg of the star-like structures generally takes the form of a hexagonal pyramid with very smooth side faces, indicating that it can grow along the [0001] direction [19]. The length of the legs is about 600–800 nm. The diameter of the legs gradually becomes smaller along the growth direction, leading to a tapered structure with a sharp tip. The diameter varies from 500 nm at the bottom to about 60 nm at the tip. Furthermore, a core can be clearly seen at the intersection of the legs. The size and shape of the star-like ZnO nanostructures strongly depend on the deposition temperature. Fig. 2b shows a SEM image of the as-grown product obtained at lower deposition temperature region. Star-like structures with cone-shaped legs are observed. The length of the legs becomes longer and the diameter becomes smaller. Fig. 2c shows a TEM image of the star-like ZnO nanostructures. The diameter of the leg gradually decreased from the root to form a sharp tip, in agreement with the results from SEM observations. The structural details of the star-like ZnO nanostructures were analyzed using HRTEM. The HRTEM image (shown in Fig. 2d) displays clear lattice fringes, which reveal the single crystalline nature of the legs. The measured lattice spacing is about 0.26 nm, corresponding to the (0002) fringes, confirming [0001] as the preferred growth direction for the legs. Based on the above analysis, the growth mechanism of ZnO nanostructures was thought to be vapor–solid (VS) rather than conventionally vapor–liquid–solid (VLS) model because no catalyst particles were found at the tips of the legs. It is generally considered that the formation of the ZnO nanostructure has two stages: nucleation and growth. The vapor of Zn or ZnOx (x b 1) [20] generated at the high temperature region was transported to the low temperature region and condensed into liquid droplets on the substrate. Those droplets migrated and merged with the original Zn droplets, then the incorporated liquid droplet would be oxidized and nucleated into a larger ZnO nucleus. It has been proposed that the addition of alloy element such as Ni [21,22] and Cu [23] in the Zn powders leads to the formation of polyhedral ZnO cores, which
Fig. 2. (a–b) Typical SEM images of the as-grown ZnO nanostructures. (c) TEM image and (d) HRTEM image of the ZnO nanostructures.
are ideal nuclei for the multipod-like ZnO structures' growth [23]. In our experiment, we suggest that the addition of Al to the source materials would be similar to that of Ni and Cu. The polyhedral ZnO cores would lead to the formation of star-like ZnO nanostructures. According to the group theory, hexagonal wurtzite ZnO belongs to the C46v space group, near the Brillouin zone, there are eight sets of optical phonons: A1, 2B2, E1, and 2E2 [24]. Among these, A1, E1, and 2E2 modes are Raman active. The Raman spectrum of the ZnO nanostructures is shown in Fig. 3a. A dominated and strong intensity peak at 436 cm− 1 was observed, which is the E2 (high) mode of the non-polar optical phonons, indicating that the ZnO nanostructures are excellent single crystals with hexagonal wurtzite structure [24]. A small and suppressed peak at 584 cm− 1 attributed to the E1 (LO) mode has also been observed. In general, the E1 (LO) mode is associated with the structural defects such as oxygen vacancies and zinc interstitials in ZnO [24,25]. In addition, the peak at 384 cm− 1 corresponds to the A1 (TO) mode and the peak at 332 cm− 1 is due to multiple phonon scattering processes [26,27]. The room temperature PL measurement result of the star-like ZnO is shown in Fig. 3b. The PL spectrum shows a narrow UV emission peak at 380 nm and a broad green emission band centering at 491 nm. The UV emission is commonly attributed to the near-band-edge emissions of ZnO [6,24]. The broad green emission band at about 491 nm is generally attributed to the radiative recombination of a photo-generated hole with an electron occupying the oxygen vacancy [6]. However, surface states have also been identified as a possible
900
Z. Peng et al. / Materials Letters 64 (2010) 898–900
4. Conclusion In summary, star-like ZnO nanostructures have been successfully synthesized via thermal evaporation technique. XRD pattern and Raman measurement confirm that the as-grown products have good crystallinity with hexagonal wurtzite phase. A vapor–solid (VS) growth mechanism has been proposed for the formation of the starlike structures. The room temperature PL spectrum shows a narrow near-band-edge peak at 380 nm and a broad defect related emission band around 491 nm. This specific structure of star-like ZnO may have potential application in optical and electrical nanodevices. The simple synthesis method discussed in the present work might be useful for the fabrication of other semiconductor nanostructures.
Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Grant Nos 90606001, 20873039 and 90406024), and Hunan Provincial Natural Science Foundation (No. 07jj4002).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Fig. 3. (a) Raman spectrum and (b) room temperature PL spectrum of the as-grown ZnO nanostructures.
cause of the visible emission in ZnO nanomaterials[28]. It is reasonable that there are some defects in the star-like ZnO nanostructures at the surface and subsurface due to their fast reaction formation process and large surface-to-volume ratio.
[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
Jayakumar OD, Sudakar C, Vinu A, Asthana A, Tyagi AK. J Phys Chem C 2009;113:4814. Kind H, Yan HQ, Messer B, Law M, Yang PD. Adv Mater 2002;14:158. Bai XD, Wang EG, Gao PX, Wang ZL. Nano Lett 2003;3:1147. Ning TY, Zhou YL, Shen H, Lu H, Sun ZH, Cao LZ, et al. Appl Surf Sci 2008;254:1900. Wang ZL. J Phys Condens Matter 2004;16:R829. Qiu YF, Yang SH. Adv Funct Mater 2007;17:1345. Long T, Yin S, Tkabatake K, Zhnag P, Sato T. Nanoscale Res Lett 2009;4:247. Pu XG, Wang ZL. Appl Phys Lett 2004;84:2883. Lao JY, Huang JY, Wang DZ, Ren ZF. Nano Lett 2003;3:235. Wang Z, Qian XF, Yin J, Zhu ZK. Langmuir 2004;20:3441. Deng R, Zou Y, Chen Z, Jiang G, Tang H. Mater Lett 2007;61:3582. Heo YW, Varadarajan V, Kaufman M, Kim K, Norton DP, Ren F, et al. Appl Phys Lett 2002;81:3046. Kawakami M, Hartanto AB, Nakata Y, Okada T. Jpn J Appl Phys 2003;42:L33. Chang PC, Fan ZY, Wang DW, Tseng WY, Chiou WA, Hong J, et al. Chem Mater 2004;16:5133. Maejima K, Ueda M, Fujita SZ, Fujita SG. Jpn J Appl Phys 2003;42:2600. Yuan MX, Fu WY, Yang HB, Yu QJ, Liu SK, Zhao Q, et al. Mater Lett 2009;63:1574. Comini E, Faglia G, Ferroni M, Sberveglieri G. Appl Phys A 2007;88:45. Yao BD, Chan YF, Wang N. Appl Phys Lett 2002;81:757. Li J, Srinivasan S, He GN, Kang JY, Wu ST, Ponce FA. J Cryst Growth 2008;310:599. Cheng WD, Wu P, Zou XQ, Xiao T. J Appl Phys 2006;100:054311. Gao T, Huang YH, Wang TH. J Phys Condens Matter 2004;16:1115. Gao T, Wang TH. Appl Phys A 2005;80:1451. Sun XM, Chen X, Li YD. J Cryst Growth 2002;244:218. He FQ, Zhao YP. Appl Phys Lett 2006;88:193113. Chen SJ, Wang GR, Liu YC. J Lumin 2009;129:340. Fauteux C, Longtin R, Pegna J, Therriault D. Inorg Chem 2007;46:11036. Wang RP, Xu G, Jin P. Phys Rev B 2004;69:113303. Hsu NE, Hung WK, Chen YF. J Appl Phys 2004;96:4671.