J. Phys. Chem. C 2010, 114, 17369–17373
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Facile Synthesis and Ultraviolet Lasing Properties of ZnO Microtubes Hongxing Dong,† Liaoxin Sun,‡ Wei Xie,† Weihang Zhou,† Xuechu Shen,† and Zhanghai Chen*,† State Key Laboratory of Surface Physics and Department of Physics, Laboratory of AdVanced Materials, Fudan UniVersity, Shanghai 200433, China, and National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China ReceiVed: May 25, 2010; ReVised Manuscript ReceiVed: August 25, 2010
Single-crystalline ZnO microtubes with a conical tip, microtube flowers, microtube arrays and diamond-like polyhedrons were synthesized by a simple oxidation-sublimation process. No catalysts, carrier gases, or templates were used in the experiment. The morphology, crystalline nature, and composition of the as-synthesized products were analyzed in detail. The growth mechanisms of ZnO microstructures with different shapes were also discussed on the basis of the kinetics of nucleation, oxidation, and sublimation. The excitation power and temperature dependence of photoluminescence (PL) properties of the ZnO microstructures were investigated by using a spatially resolved spectroscopic technique. The results reveal that the microtubes obtained on the silicon substrate can produce stable UV lasing under low excitation power. The observed laser modes can be well explained with a whispering gallery mode model. Applications of these novel microstructures will be in the area of UV microlasers. Introduction As a wide band-gap II-VI group semiconductor oxide with a large exciton binding energy (60 meV), ZnO has attracted a great deal of interest during the past decades due to its unique properties of near-UV emission, piezoelectricity, pyroelectricity, and photoconductivity. Structurally, ZnO has three fast growth directions of [0001], [101j0], and [21j1j0], which is beneficial for the realization of many kinds of novel nanostructures.1-3 Up to now, many ZnO nanostructures with different morphologies, such as nanobelts,1 nanowires,4 nanorods,5 nanonails,6 nanocombs,7 nanotubes,8 and nanoparticles,9 have been fabricated by various methods. Many of them have found applications in nanolasers, gas sensors, field emitters, solar cells, lightemitting diodes, and so on. Among them, a tubular structure is an important building block for nanoscale devices owing to its exceptional structural characteristics. However, the synthesis of a three-dimensional tubular structure of ZnO remains a challenge compared with other materials that have bulk lamellar structures, such as C, BC, WS2, and MoS2. To our knowledge, only a few methods were employed in the preparation of ZnO microtubes, including vapor-phase deposition,3,10 template-assisted,11 hydrothermal,12,13 and wet chemical methods.14,15 Despite great progress in the synthesis of ZnO micro/nanotubes, almost all the synthesized micro/nanotubes are amorphous or polycrystalline structures or tangled distributions. Although some singlecrystalline and isolated ZnO nanotubes can be obtained, a catalyst, special equipment, or low pressure is needed during the growth process. Therefore, an imperative and challenging issue is to develop a new and efficient method for controllable fabrication of high-quality single-crystalline ZnO microtubes. Besides its structural diversities, ZnO has been considered as being one of the promising candidates for UV-light-emitting devices. The investigation of the lasing characteristics of ZnO * To whom correspondence should be addressed. E-mail: zhanghai@ fudan.edu.cn. † Fudan University. ‡ Chinese Academy of Sciences.
nanostructures with different morphologies has become the topic of extensive research. Two important types of lasing emission, that is, Fabry-Pe´rot mode (FPM) lasing and whispering gallery mode (WGM) lasing, have been observed in many ZnO micro/ nanostructures with different morphologies.16-20 Among them, WGM lasing has attracted much attention due to its high quality factor and low threshold power. However, to the best of our knowledge, except for ZnO nanonails,18 microdisks,20,21 and microrods,22,23 the WGM lasing effect in one-dimensional ZnO microtubes has never been explored, and the temperaturedependent evolution of the lasing properties in ZnO materials is also rarely investigated. In this paper, we report the controlled growth of singlecrystalline ZnO microtubes with a conical tip, microtube flowers, microtube arrays, and diamond-like polyhedrons via a simple oxidation-sublimation process without using any catalysts, carrier gases, or templates. By controlling the quantity of raw materials and the position of the product growth, ZnO crystals with different morphologies can be obtained. The structure, composition, and formation mechanism of ZnO microtubes and polyhedrons are systematically studied. Furthermore, we demonstrate for the first time that such high-quality ZnO microtubes can also serve as a gain medium and exhibit UV WGM lasing under very low power excitation. Their light emission characteristics are also systematically studied in the temperature dependence of PL measurements carried out between 10 and 290 K by using a spatially resolved spectroscopic technique. These results may find important application in novel lasing or light-emitting diode devices. Experimental Section Synthesis of the single-crystalline ZnO microstructures was carried out in a conventional horizontal tube furnace. No catalysts, carrier gases, or templates were used in the experiment. In a typical experimental procedure, ZnO powders (0.03-0.06 g) were mixed with graphite powders (0.03-0.06 g) according to the weight ratio of 1:1 and put into a small quartz boat. A Si
10.1021/jp1047908 2010 American Chemical Society Published on Web 09/27/2010
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Figure 1. SEM images of the fabricated ZnO microtubes with a conical tip in various magniscales, (a) low and (b) high, and (c) an individual microtube. (d) TEM image of the tip region of one single microtube and (e) the corresponding SAED pattern.
substrate cleaned by sonication in ethanol and acetone was put on top of the boat to collect the products. The boat was then placed in the center of the quartz tube. The ends of the quartz tube were sealed by using flexible plastic switches that ensured the appropriate pressure formed in the quartz tube during the heating process, and the whole assembly was put into a horizontal tube furnace. The oxygen source came from the air kept in the quartz tube. The temperature of the tube furnace was raised to 1000 °C at a rate of 25 °C min-1 and maintained at 1000 °C for 60 min. After the reaction, the substrate was found coated with a thin layer of white powder, and a large amount of crystal-like products was obtained on the position where the raw materials were located. The morphologies and microstructures of the products were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images were recorded on a JEOL 6400 microscope operating at 10 kV. TEM images were taken by a JEOL 2010 electron microscope with an acceleration voltage of 200 kV. The samples for TEM measurements were first suspended in ethanol and then dispersed onto Cu/lacey-carbon TEM grids. X-ray diffraction (XRD) analyses were performed on a D/max-rB diffractometer with Cu KR radiation (λ ) 1.54 Å) at a scanning speed of 8° min-1 in the range from 30 to 70°. Optical studies of the ZnO microtubes and polyhedrons were carried out in a confocal microphotoluminescence (µ-PL) system (JY LabRAM HR UV), using a pulse laser of 355 nm as the excitation source. Results and Discussion Figure 1a-c shows the SEM images of the ZnO microtubes in the quartz boat when the weights of ZnO powders and graphite powders are both 0.03 g. It can be seen that the ZnO microtubes with a conical tip and an inerratic hexagonal cross section were grown with a high yield. The ZnO microtubes have lengths in the range of 100-300 µm with diameters continuously decreasing from 5-20 µm at the tip to several micrometers at the root. The detailed geometrical morphology is shown in Figure 1c. Figure 1c shows a typical ZnO microtube of approximately 12 µm in diameter and the wall thickness of about 950 nm. The hexagonal cross section is very clear. The internal six facets of the tube are all regular and smooth. To get further information about the microstructure of the ZnO microtube, TEM analysis was performed. Figure 1d shows the TEM image of the tip region of one single ZnO microtube. The corresponding selected area electron diffraction (SAED) pattern shown in Figure 1e illustrates its single-crystalline nature. On the silicon substrate, some interesting ZnO structures were observed, as shown in Figure 2a. Many diamond-like polyhe-
Figure 2. SEM, TEM, and SAED analyses of ZnO diamond-shaped microcrystals. (a, b) Typical SEM images with different magnifications of the as-grown microcrystals. (c, d) Top-view and side-view SEM images of one representative single microcrystal from the sample. (e) TEM image of one single microcrystal and (f) the corresponding SAED pattern.
Figure 3. SEM and TEM images of the microtubes obtained on the Si substrate. (a) The typical morphologies of the ZnO microtubes. (b) The microflowers made up of microtubes. (c) A microtube array grown on the Si substrate directly. (d) TEM image of one single microtube and the corresponding SAED pattern (inset).
drons were fabricated, and some of them have a hollow cavity, as indicated by the red arrowheads (Figure 2b). Figure 2c shows a top-view SEM image of a typical ZnO polyhedron, which exhibits a hexagonal shape and ends with a dodecagonal pyramid. Figure 2d shows the side-view SEM image of the single ZnO polyhedron. It can be seen that twelve facets of the pyramid form alternating concave- and convex-type surfaces. This unique morphology is distinct from the normal and idealized hexagonal pyramid widely reported in previous literature.24-26 Figure 2e shows a TEM image of a single ZnO polyhedron. The corresponding SAED pattern taken from the polyhedron (Figure 2e) shown in Figure 2f illustrates its singlecrystalline nature. The amount of raw materials is very important for the morphology of the products. Figure 3a,b shows the general morphologies of the obtained products on the Si substrate, when the weights of ZnO powders and graphite are both increased from 0.03 to 0.06 g. It can be seen that a high density of microcrystals uniformly grew over the entire substrate. On the microcrystals, some microtube-based flower-like structures can also be observed. A high-magnification SEM image (Figure 3b) reveals that the microflowers are made up of microtubes. The
Synthesis and UV Lasing Properties of ZnO Microtubes
Figure 4. XRD pattern of the ZnO microstructures with different morphologies. The inset is an enlarged view of the diffraction peaks between 35 and 45°. The presence of the Zn diffraction peaks indicates the formation of Zn in the synthesis process.
microtubes grow out from the center and have a hexagonal cross section, forming flower-like structures as a whole. The lengths of the microtubes are 15-25 µm, and their diameters are 3-6 µm. Figure 3c shows the high-magnification SEM image of the microcrystals grown on the Si substrate directly. It can be seen that large-scale quasialigned microrods with a hexagonal cross section are produced. The microrods with diameters ranging from 2 to 4 µm and lengths of about 50 µm are separated from each other, but a close examination reveals that some of the microrods have an obvious tubular structure (as indicated by the red arrowhead). Furthermore, the TEM image (Figure 3d) and the corresponding SAED pattern (inset) of a single microtube show that the ZnO microtube is single-crystalline with [0001] as its growth direction. The X-ray diffraction (XRD) patterns of these samples are shown in Figure 4. It is found that all as-prepared samples are highly crystalline, and the major peaks in every pattern can be indexed as hexagonal wurtzite-type ZnO (JCPDS No. 65-3411). The predominant ZnO peak from (002) planes indicates that the fabricated microstructures were grown with c-axis orientation. An enlarged view of the diffraction peaks between 35 and 45° is shown in the inset of Figure 4. Very weak Zn(002), (100), and (101) diffraction peaks were observed (JCPDS No. 653358), which indicates that the Zn metal was formed during the synthesis process. The formation of ZnO microtubes is considered to be a process composed of the formation of a Zn/ZnOx (x < 1) embryo, surface oxidation, and sublimation of the nuclei core. Graphite acts as a reducing agent, resulting in ZnO powder being reduced to Zn vapor at high temperature. With the increase of the concentration of the Zn vapor, Zn atoms condense and form liquid clusters on the Si substrate and in the quartz boat. At the same time, the formed liquid clusters were reoxidized in the oxygen environment at the nucleation sites. In the closed system, the oxygen in the air sealed in the quartz tube is not sufficient as the oxygen source. Without additional oxygen, the Zn/Zn suboxide nanostructures formed at the beginning of the reaction, which is crucial for the formation of the tubelike microstructures. Both Zn and ZnO have the hexagonal crystal structure with lattice parameters of a ) 0.2665, c ) 0.4947 and a ) 0.3249, c ) 0.5206, respectively. They both have three fast growth directions of [0001], [101j0], and [21j1j0]. These unique structural properties are conducive to the formation of nanorods with a hexagonal cross section. Once formed, the nuclei rod will react continuously with the oxygen to form a stable outer shell of ZnO. The oxidation rate varies with the crystal surfaces. The Zn {0001} surfaces have lower energy than the {101j0}/ {21j1j0}surfaces, which tend to be the most stable to resist oxidation.27 Therefore, the sidewall of a hexagonal nanorod is oxidized into ZnO shells first. At the same time, the remaining
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Figure 5. Schematic setup for microphotoluminescence experiments of microstructures. It allows spatially resolved PL measurements.
Zn inside the nanorods will be sublimated through the open ends at the {0001} surface because of the low melting point of Zn (419.5 °C), leading to the formation of hexagonal ZnO microtubes. Moreover, we note that the density of the Zn liquid clusters and the quantity of oxygen are crucial for the shape of the ZnO microstructures (Figures 1-3). It is likely that some Zn liquid droplets were well dispersed on the substrate when the density of Zn vapor was not very high. Thus, some individual, freestanding microtubes formed. For diamond-like polyhedrons, the growth of the dodecagonal pyramid top may be controlled by thermodynamics at high temperature due to the lattice mismatch between ZnO/Zn and the Si substrate. The microstructures synthesized on the substrate will be affected by the interface energy. To minimize the interface energy, the growth tendency of the ZnO nuclei will be altered with the changing of the reactant concentration in the growth process. Thus, the structural transformation from microrods with a uniform radius to polyhedrons with a pyramid would occur. A similar situation was also found in the synthesis of other ZnO microstructures.28,29 On the other hand, by increasing the concentration of Zn vapor, several Zn liquid droplets will coalesce together to form a large polycrystalline Zn nucleus, leading to the formation of microtube flowers. Optical studies of individual ZnO microtubes were carried out with a confocal microphotoluminescence spectrometer. The experimental setup is shown schematically in Figure 5. The excitation laser was focused by a microscope objective (15×) to a 2 µm spot on the synthesized samples. The PL signals were recorded by a silicon CCD detector. It allows spatially resolved PL measurements and thus the observation of emitting light from single microcrystals. Figure 6a-c shows the excitation-powerdependent PL spectra of the obtained ZnO microstructures in the UV range at room temperature. It can be seen that all the ZnO samples exhibit a UV emission peak centering at ∼386 nm. The origin of this UV emission is generally related to exciton transitions and their phonon replicas.30,31 For each sample, there is a slight red shift of the spectral maxima with increasing excitation power and this may be attributed to the band-gap renormalization. From Figure 6c, we can see that the emission intensity increased rapidly and several sharp peaks abruptly appeared above the spontaneous emission background when the excitation power exceeded the threshold of ∼0.31 µW. When the excitation power is furthered increased, the intensity of the sharp peaks exponentially increases and new peaks arise on the low-energy side. This indicates that a transition from spontaneous emission to stimulated emission occurs in this sample and electron-hole plasma arises from the spectrum under high excitation power. However, no lasing effect was stimulated in the other two samples (microtubes with a conical
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Figure 7. PL of one single ZnO microtube separated from the ZnO microtube flowers: (a) excitation-power-dependent PL spectra measured at 10 K and (b) temperature-dependent PL spectra in the range of 30-290 K under a fixed excitation power of 160 nW.
corresponding mode number NTE for TE polarization can be deduced from the WGM model
Figure 6. Excitation-power-dependent PL spectra of the ZnO samples with different morphologies: (a) microtubes with a conical tip, (b) diamond-like polyhedrons, and (c) microtube flowers. The left inset in (c) shows an SEM image of the ZnO microtube investigated in the PL measurement. A schematic of a microtube with a hexagonal cross section is shown in the right inset. All the PL spectra were measured at room temperature.
tip and polyhedrons) even with the maximum output of the excitation light source. The main reason for this may be attributed to the insufficient optical gain due to the exciton nonradiative recombination in these two samples.32 To quantify the lasing observed in the ZnO microtube, optical mode calculations were performed using a classical plane-wave model. The left inset in Figure 6c shows an SEM image of the microtube that was used for the spectroscopic measurements. It can be seen that the microtube has flat and smooth hexagonal facets. The microtube has a length (D) of about 58 µm with the outside radius (R) of about 3.2 µm, and the wall thickness (d) is about 1.8 µm. From the point of view of geometrical optics, two kinds of resonant cavities are formed in this microstructure: (i) an FPM cavity formed between the two ends of the microtube and (ii) a WGM cavity formed in the hexagonal cross section, as shown in the right inset in Figure 6c. For the WGM cavity, the light wave travels along closed paths due to the multiple total internal reflections at the interface between ZnO microtube walls and air when d g (3/8)R, forming a WGM. In optical microcavities, the mode spacing ∆λ is given by
∆λ )
λ2
(
Ln-λ
dn dλ
)
(1)
where L is the path length of a round trip, n ≈ 2.3 is the refractive index of ZnO, and λ is the lasing wavelength. In the case of FPM lasing, L is twice the length of the two opposite octahedral facets (11 6000 ( 100 nm). The calculated mode spacing ∆λ is about 0.17 nm, which is much smaller than the observed value of ∼1.3 nm in our experiment. However, in the WGM lasing, L is 16600 ( 100 nm and the calculated mode spacing ∆λ is about 1.2 nm, which is consistent with the experimental results. This indicates that the WGM lasing indeed occurred in our ZnO microtube. Generally, ZnO WGM lasings are preferentially TE-polarized; TM-polarized lasing is weak and broad and is difficult to be detected.22,23 Thus, we focus on the TE-polarized mode here. The lasing wavelength λ and the
NTE )
3√3nTER 6 2 -4 - tan-1 nTE√3nTE λ π
(
)
(2)
The refractive index for TE polarization is described by Sellmeier’s dispersion function33
(
nTE ) 1 +
2.4885λ2 0.2150λ2 + 2 + 2 λ - 102.30 λ - 372.602 0.2550λ2 2 λ - 18502 2
)
(3)
From eqs 2 and 3, one can determine the lasing mode numbers NTE ) 102-107, which were labeled in the optical spectra as shown in Figure 6c. To further investigate the UV WGM lasing property of the ZnO microtubes, the excitation-power-dependent PL spectra measured at 10 K are shown in Figure 7a. It shows that the UV emission peaks, compared with those taken at room temperature, have an obvious blue shift and are located at ∼373 nm. What is interesting is that the lasing occurs at a very low excitation power of ∼18 nW, which is much lower than that at room temperature. To get a better understanding of this behavior, we performed the temperature-dependent PL measurement ranging from 30 to 290 K with a fixed excitation power, under which the lasing had already been detected. Figure 7b shows the temperature-dependent PL spectra of 160 nW. With increasing temperature, the peak position shows an obvious red shift, and the emission intensity also decreases. The red shift with the increase of temperature is usually attributed to the temperatureinduced lattice dilatation and electron-lattice interaction.34 The decrease in intensity is due to the effects of the thermalization.35 For ZnO material, according to the conservation laws of energy and momentum, at low temperature, the radiative recombination usually occurs near the center of the Brillouin zone, where the electrons are close to the bottom of the conduction band and the holes are at the top of the valence band. However, the average kinetic energy and momentum of electron-hole pairs (and excitons) will increase with increasing temperature. These excitons with certain kinetic energies would be easily trapped by surface states or defect centers and enter the nonradiative recombination channels. In this case, the density of excitons close to the center of the Brillouin zone decreases, resulting in a reduced efficiency of radiative recombination. Thus, we can see that the lasing disappeared when the temperature rose to ∼260 K in the emission spectra. It is also easy to understand
Synthesis and UV Lasing Properties of ZnO Microtubes why the lasing can occur under very low excitation power at 10 K, as shown in Figure 7a. These results provide us with useful information to further understand the lasing behavior in ZnO microcrystals. Conclusion We have demonstrated the synthesis of ZnO microtubes and diamond-like polyhedrons through a simple oxidation-sublimation process without catalysts, templates, carrier gases, or low pressure. Microtubes with a conical tip, microtube flowers, microtube arrays, and diamond-like polyhedrons can be achieved by controlling the amount of raw materials and the position of product growth. The quantity of oxygen is crucial for the formation of ZnO microtubes. A possible growth mechanism based on the kinetics of nucleation, oxidation, and sublimation is proposed. The UV PL properties of the ZnO products were characterized by the excitation power and temperature dependence of PL spectra using the spatially resolved spectroscopic technique. The UV lasing was observed from the ZnO microtubes formed on the Si substrate at room temperature, and the lasing behaviors at the temperature range from 10 to 290 K were investigated. The plane-wave model of the whispering gallery mode describes well the observed lasing spectra. The results indicate that the ZnO microtubes with high crystalline quality may be a good candidate for the development of novel UV microlasers. Acknowledgment. This work is funded by the National Natural Science Foundation of China and 973 Projects of the Ministry of Science and Technology of China (Grant Nos. 2006CB921506 and 2004CB619004). References and Notes (1) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (2) Fan, D. H.; Shen, W. Z.; Zheng, M. J.; Zhu, Y. F.; Lu, J. J. J. Phys. Chem. C 2007, 111, 9116. (3) Gao, P. X.; Lao, C. S.; Ding, Y.; Wang, Z. L. AdV. Funct. Mater. 2006, 16, 53. (4) Hang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (5) Su, Y. K.; Peng, S. M.; Ji, L. W.; Wu, C. Z.; Cheng, W. B.; Liu, C. H. Langmuir 2010, 26, 603. (6) Lao, J. Y.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F. Nano Lett. 2003, 3, 235. (7) Pan, Z. W.; Mahurin, S. M.; Dai, S.; Lowndes, D. H. Nano Lett. 2005, 5, 723. (8) Guo, H. H.; Lin, Z. H.; Feng, Z. F.; Lin, L. L.; Zhou, J. Z. J. Phys. Chem. C 2009, 113, 12546.
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