Synthesis of Visible Light-Sensitive ZnO Nanostructures

May 18, 2009 - Single crystalline ZnO nanowires and nanorods were synthesized by a new sol−gel method carried out through formation of liposome−Zn...
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Synthesis of Visible Light-Sensitive ZnO Nanostructures: Subwavelength Waveguides† Jooran Lee and Minjoong Yoon* Molecular/Nano Photochemistry and Photonics Lab, Department of Chemistry, Chungnam National UniVersity, Daejeon 305-764, Korea ReceiVed: April 6, 2009; ReVised Manuscript ReceiVed: April 29, 2009

Single crystalline ZnO nanowires and nanorods were synthesized by a new sol-gel method carried out through formation of liposome-ZnO nanocomposites with or without hydrothermal reaction, respectively, and they were characterized to be hexagonal wurtzite structure by measurements of XRD patterns and SEM and TEM images. The UV-visible absorption spectrum of the nanowires was observed to exhibit a very high absorption of visible light from 400 to 600 nm as compared to that of the nanorods. The photoluminescence (PL) images and spectra of the single nanowires and nanorods were measured upon excitation with a 415 nm femtosecondpulsed laser beam by a confocal scanning microscope-coupled PL spectral system, and the single nanowires were observed to exhibit waveguided red-emission at the tip whereas unguided emission only was observed by UV-excitation. Analysis of the dependence of nanowire length on the waveguide mode spacing indicated that the subwavelength waveguide is operated by the axial Fabry-Pe˙rot-type microcavity resonance. These results demonstrated that the ZnO nanowires can exhibit the subwavelength waveguide behavior as long as the defects are excited directly with visible light, suggesting that the present ZnO nanowires will be a promising nanomaterial for future visible light-sensitive lasing cavity and optoelectronic devices. Introduction One-dimensional ZnO nanostructures, especially ZnO nanowires, nanorods, and nanobelts, have received considerable attention due to their unique optoelectronic properties to be applied to photocatalysts,1 photodetectors,2-4 photochemical sensors,5 and photonic wire lasers and waveguides.6-8 ZnO is also biocompatible, biodegradable, and nontoxic for medical applications, and ZnO nanostructures have been recently applied to develop biosensors.9,10 Such optoelectronic applications have been mostly carried out by UV illumination, and their natures of different ZnO nanostructures usually have been explored by photoluminescence (PL) spectroscopy with UV excitation. Usually they have been known to exhibit two distinct types of detectable PL emission bands observed from single ZnO nanowires upon UV excitation: free exciton emission near 385 nm (UV emission) and defect emission near 510 nm (green emission). Further the PL image of the single nanowire has been reported to show strong waveguiding of the UV emission to the nanowire end with pulsed excitation, allowing UV lasing action to occur even at room temperature. However, waveguiding of the green emission has not been observed from pure ZnO nanowires in the UV-excitation regime as one generally expects the subwavelength waveguide to show a large optical loss as compared to near-UV emission, even though the waveguided emission in the subwavelength region can be observed from the ZnO nanowire coupled with SnO nanoribbon waveguides11 or with silica fibers.12 The lack of waveguiding behavior of the subwavelength emission also has been attributed to the polarization preference, which probably originates from a competition between an increase in the number of excitons migrating from shallow surface states to the deeper defects and a decrease in the number of defects as proved by saturation of the intensity †

Part of the “Hiroshi Masuhara Festschrift”. * To whom correspondence should be addressed. Phone: 82-42-821-6546. Fax: 82-42823-7008. E-mail: [email protected].

of green emission in contrast to the linear increase of the UV emission upon increasing pump power.8 However, it should be noteworthy that such interpretations are based on the PL properties observed with excitation with UV light corresponding to a wide band gap energy of 3.37 eV, and excitation wavelength dependence of the subwavelength waveguiding nature has not been explored, particularly in the visible wavelength regime. This may be because most of the conventional ZnO nanowires used for the optoelectronic functions do not absorb visible light. Thus, synthesis of visible light-sensitive ZnO nanowires would be required. Further, the visible light-sensitive ZnO nanostructures would be useful to expand their application fields to efficient solar cells and nonphototoxic biomedicinal sensors. Most of the conventional ZnO nanowires are fabricated by the vapor-liquid-solid (VLS) growth13,14 and anodic alumina templating15 methods, which are known to exhibit no visible light absorption in spite of the green or red emission observed from surface defect levels. This might be because the density of inherent surface defects formed by the conventional methods is too low to be detected by the visible light absorption. Thus, the visible light-absorbing ZnO nanomaterials usually have been fabricated by extrinsic surface modification through implanting transition metal ions, nitrogen-doping, or coupling with dye sensitizers as applied to fabricate the visible light-sensitive TiO2 nanomaterials.16 However, the ZnO nanomaterials fabricated by such methods are not inherent, and it is not suitable to exhibit the intrinsic optoelectronic behaviors, and it is necessary to fabricate the pure visible light-absorbing ZnO nanostructures by intrinsic surface modification methods. Recently, as a first example of visible light-absorbing ZnO nanostructures, Wu et al.17 synthesized ZnO nanorods by hydrothermal reaction in the presence of cetyltrimethylammonium bromide (CTAB) surfactants, which are claimed to absorb visible light based on the measurement of their UV-visible absorption spectra. However, the absorption spectra were measured in the presence of CTAB solution, and the visible light absorptivity cannot be distin-

10.1021/jp903167x CCC: $40.75  2009 American Chemical Society Published on Web 05/18/2009

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Figure 2. SEM images of (a) ZnO nanowires and (b) ZnO nanorods on bare soft substrates.

Figure 1. Schematic flow chart for the synthesis of ZnO nanostructures.

guished clearly from scattering of the turbid particles suspended in the solution. Nevertheless, such a ZnO nanorod did not exhibit waveguiding properties, and they are not quite as useful as optoelectronic materials as ZnO nanowires. Further, no visible light-absorbing ZnO nanowires have been synthesized yet. Thus, it is necessary to synthesize well-defined visible light-absorbing ZnO nanowires and ZnO nanorods. Previously we had synthesized TiO2 nanodiscs through formation of liposome-TiO2 composites,18 which exhibit the surface-defect absorption beyond 400 nm. Thus, herein, we attempted to synthesize the visible light-sensitive ZnO nanostructures such as nanowires and nanorods with or without hydrothermal reaction of liposome-ZnO composites which may be expected to form more ZnO surface defects. Thereafter, we investigated the optical properties of the ZnO nanostructures including the subwavelength waveguiding behaviors as monitored by photoluminescence (PL) imaging and spectra of the single nanostructures using a confocal scanning microscopecoupled PL system. Experimental Section 1. Chemicals. The chemicals for the synthesis of ZnO nanostructures, zinc nitrate hexahydrate (98%, Aldrich), egg lecithin (L-R-phosphatidylcholine, >60%, Aldrich), chloroform (>99.8%, Samchun, Korea), sodium hydroxide (>93%, Duksan, Korea), and ethanol (200 proof, >99.8%, Aldrich), were used as received. All chemicals were analytic grade reagents without further purification. 2. Synthesis of ZnO Nanostructures. The ZnO Nanostructures were synthesized by using a sol-gel method and/or hydrothermal reaction through formation of liposome-ZnO composites as shown in Figure 1. In the beginning step, 100 mL of 0.6 M NaOH aqueous solution was added into 0.06 M Zn(NO3)2 · 6H2O in ethanol (20 mL) at room temperature with stirring. Next, to form the liposome-ZnO composites, a thin phospholipid film was prepared by evaporating a chloroform solution of egg licithin lipid (0.025 g/mL) to dryness in a roundbottomed flask under reduced pressure.18 The lipid films were mixed with precursory solution, followed by sonication for 60 min in an ultrasonic bath (Fisher Scintific model FS20, 70 W, 42 kHz) to produce turbid liposome-ZnO composites. The turbid solution of liposome-ZnO composites was centrifuged for 30 min by using an ultracentrifuge, and the transparent supernatant solution was transferred into a Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h of hydrothermal reaction. After the hydrothermal reaction, the Teflon-

Figure 3. X-ray diffraction patterns of (a) ZnO nanowires and (b) ZnO nanorods.

lined autoclaves were cooled naturally to room temperature. Then, the solid matter was washed with deionized water and ethanol several times, followed by calcinations at 823 K for 24 h to result in formation of nanowires. On the other hand, when the liposome-ZnO composites were directly calcinated at 823 K for 24 h, ZnO nanorods were formed. 3. Characterization of ZnO Nanostructures. X-ray Diffraction (XRD) Measurement. Crystal phases of the as-prepared nanostructures were characterized by XRD, using a rotating anode X-ray diffractometer (D/MAX-2200 Ultima/PC) with Cu KR radiation (λ ) 1.5405 Å). Scanning Electron Microscope (SEM) Measurement. The size and morphology of the synthesized ZnO nanostructures were examined by scanning electron microscope (JEOL, JSM-7000F). Field Emission Transmission Electron Microscope (FE-TEM) Measurement. Samples for field emission transmission electron microscope (Tecnai G2 F30) measurement were prepared by dip-coating Formvar/carbon film Cu grids with a nanocolloidal solution obtained by sonicating the produced powder material in ethanol. 4. Ensemble-Averaged Spectroscopic Measurements. Diffuse Reflectance UV-Visible Absorption Spectroscopic Measurements. Diffuse reflectance UV-vis absorption spectra were recorded by using a Shimadzu UV-3101PC spectrophotometer equipped with an integral sphere. Photoluminescence (PL) Measurements. Static photoluminescence emission spectra of the powders at all stages of the procedure were obtained at room temperature with UV light at 325 nm, using a He/Cd laser (Kimmon Co. IK3302R-E) or Ti: sapphire laser (Coherent model Mira 900) under second harmonic generation at 415 nm at room temperature. 5. PL Measurements of Single ZnO Nanostructures. PL images and spectra were measured by using a confocal scanning

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Lee and Yoon scope (Carl Zeiss Axiovert 200), being isolated from Rayleigh scattering by a filter.

Figure 4. (a) TEM image of ZnO nanowires. (b) High-resolution TEM image showing the lattice image of ZnO nanowires (lattice spacing ∼0.26 nm). Inset: Fast Fourier Transform (FFT) of the corresponding structures. (c) TEM image of ZnO nanorods. (d) High-resolution TEM image showing the lattice image of ZnO nanorods (lattice spacing ∼0.26 nm). Inset: FFT of the corresponding structures.

Figure 5. UV-visible reflectance absorption spectra of (a) ZnO nanowires, (b) ZnO nanorods, and (c) ZnO nanoparticles.

Results and Discussion 1. Structural Characterization of ZnO Nanostructures. The general morphologies of the as-prepared ZnO nanomaterials were examined by SEM. As shown in Figure 2a,b, a random mat of homogeneous ZnO nanowires and nanorods was obtained with a diameter of 400 nm and a length of 3-20 µm (aspect ratio: 7.5-50) and a diameter of about 100 nm and a length of about 200 nm (aspect ratio: 2), respectively. Figure 3 shows the XRD patterns of the as-prepared ZnO nanowires (a) and nanorod (b). All the diffraction peaks in the XRD patterns of both ZnO nanostructures can be indexed as Wurtzite hexagonal structure, which are consistent with the results in the standard card (JCPDS 05-0664). No characteristic peaks of other species were observed, indicating that all the ZnO precursors grew into pure ZnO single crystalline structures without any lipid contaminant left. The full width at halfmaximum (fwhm) of the (002) peak is obviously smaller than that of other diffraction peaks, indicating the oriented growth of the single ZnO nanostructures along the [0001] direction according to the Scherrer equation. Further detailed structural characterization of the ZnO nanowires and rods was performed by using FE-TEM combined with the Fast Fourier Transform (FFT). Panels a and c of Figure 4 show the low-resolution TEM images of the ZnO nanowire and rod, respectively, while panels b and d of Figure 4 show the high-resolution TEM images and FFT of the corresponding structures. All the observed lattice spacing was observed to be about 0.26 nm with the space between [0002] planes. These results unambiguously confirm that the ZnO nanowires and rods have hexagonal wurtzite single crystalline structure grown along the [0001] c-axis direction, which is in good agreement with the FFT results20,21 and the XRD results. Formation of a hexagonal phase crystalline ZnO nanowire may be due to the following hydrothermal reactions of Zn2+ under high alkaline solution as reported in the other reports.22-25 A suitable hydrothermal condition may be helpful for the nucleation

Zn2+ + 2OH- f Zn(OH)2

(1)

Zn(OH)2 + 2OH- f [Zn(OH)4]2-

(2)

[Zn(OH)4]2- f ZnO + Η2O + 2OH-

(3)

19

microscope-coupled PL system previously built in our lab. The scanning confocal microscope is based on an inverted optical microscope (Axiovert, 135, Zeiss) and a piezoelectric x-y sample scanner (Physik Instrument, Polytech) driven by an independent homemade scanning controller. The samplecoated cover slide glass on a scanning piezo-electric X-Y stage (Physik Instrumente, P517.3CL) was illuminated with 415 nm light from a second harmonic generated self-mode-locked Ti: sapphire laser (Coherent model Mira 900) pumped by a Nd: YVO4 laser (Coherent Verdi diode pumped laser) (200 fs pulse width with repetition rate of 3800 kHz). The excitation light was passing through the single mode optic fiber and then incident on the back of a 100 × 1.3 NA oil immersion objective lens (Carl Zeiss Plan-NEOfluor). Different excitation power was adjusted by using neutral density filters. The emission signals were collected through an inverted confocal scanning micro-

and subsequent 1D preferential growth of ZnO crystals. Particularly, the molar ratio of Zn2+ to OHsvarying from 1:4 to 1:30shad been reported to play a key role in the formation of the nanowires.24,25 Therefore, it is obvious that the ratio of Zn2+: OH- ) 1:10 used in the present experiment gave rise to perfect single-crystalline ZnO nanowires after removal of lipids by calcination. However, ZnO nanorods were formed only by calcination of liposome-ZnO composites without hydrothermal reaction, indicating the nanorods are formed probably by templating cylindrical liposomes similarly to the formation of the liposometemplated TiO2 nanodiscs.18 2. Ensemble Averaged UV-Visible Absorption and PL Spectral Properties. The diffuse reflectance UV-visible absorption spectra of the as-prepared ZnO nanowires and

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Figure 6. PL emission spectra of (a) ZnO nanowires and (b) ZnO nanorods under excitation with UV light at 325 nm, using a He/Cd laser at room temperature. The intensity of (a) ZnO nanowires is 7 times higher than the intensity of (b) ZnO nanorods. Room temperature PL emission spectra of (c) ZnO nanowires and (d) ZnO nanorods under excitation with visible light at 415 nm, using a Ti/sapphire laser. The intensity of (c) ZnO nanowires is 14 times higher than the intensity of (d) ZnO nanorods.

nanorods were measured at room temperature as represented as curves a and b respectively in Figure 5. They exhibit welldefined visible light-absorption bands around 450 nm which are much red-shifted as compared to UV exciton absorption of ZnO nanoparticles (curve c, Figure 5). Considering that some native defects of the conventional ZnO nanostructures would be responsible for the green emission,26-28 the visible light absorption around 450 nm must be due to electronic transition to deep surface states by some defects from the ground state of the ZnO nanostructures. In general, native surface defects in the ZnO crystals are formed through reaction of Zn (g) with an optimum amount of oxygen during heat treatment under the thermal equilibrium condition,26-28 and their densities are increased by reducing excessive oxygen pressure at a growth temperature.27 Both theory29 and experiment30 have analyzed structures and energies of many fundamental defects such as oxygen vacancies (VO, VO•) and zinc vacancies (VZn, VZn•), suggesting that crystal grown under a reducing atmosphere forms VO• containing one electron as the most abundant defects. The energy level of VO• is located at about 2.5 eV29 below the conduction band edge as analyzed by Kroger.30 Since this energy is close to the absorption maximum wavelength, the VO• would be responsible for the visible light absorption of the present ZnO nanostructures. Incidentally, the conventional ZnO nanocrystals grown under a reducing oxygen pressure have been reported to have increased density of defects so that they exhibit enhanced visible emission but rarely absorption. This implies that defect densities of the conventional ZnO nanostructures are still too low to be detected

by observing visible light absorption. Thus, the strong visible light absorption from the present ZnO nanostructures formed by using liposome-ZnO composites indicates that liposome plays an important role in the formation of an extra large amount of surface defects, enough to exhibit the visible light absorption. The exact role of liposome may not be unambiguously understood. Nevertheless, it can be speculated that liposome consumes excess oxygen to keep the optimum pressure of oxygen to react with Zn (g) because it consists of unsaturated egg-lecithin lipids which are easily oxidized. This may be the reason why more oxygen vacancies would be formed on the ZnO nanostructures by the heat treatment of the liposome-ZnO composites. Particularly, the ZnO nanowires exhibit a significantly broadened band extended to longer wavelengths from 400 to 600 nm in the visible region, with much higher absorptivity as compared to the nanorod. This may be due to additional heat treatment such as the hydrothermal reaction in addition to calcinations for the formation of the ZnO nanowire, which results in increasing density of surface defects. Thus, the surface state energy level of electron transition in the crystal field of the ZnO nanowire is narrowed by the additional defect levels. PL spectra of the as-prepared ZnO nanostructure powders were measured with UV excitation at 325 nm at room temperature, as shown in Figure 6a,b. They exhibit two subwavelength emission bands around 500 and 630 nm as well as the UV emission band at 385 nm in contrast to the conventional ZnO nanostructures11-13 which usually exhibit one green emission around 500 nm with the UV emission. The subwavelength and

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Lee and Yoon

Figure 7. (a) Epi-PL microscopy images of ZnO nanowires. (b) A representative image of a selected single ZnO nanowire. (c) Map of polarization with the tip PL of a single ZnO nanowire. Emission spectra collected from the tip (d) and body (e) of the selected single ZnO nanowire upon excitation with visible light at 415 nm, using femtosecond Ti/sapphire pulse laser.

the UV emissions are generally attributed to the electronic transition from lower energy levels originating from defects in the ZnO crystal field and the direct recombination of the free excitons near band edge, respectively.31,32 It is noteworthy that the UV emission intensity of the nanowire is relatively lower than that of the nanorod, indicating that density of defects is larger in the nanowire than in the nanorod as discussed above. The nanostructures were also excited with visible light at 415 nm, and it was found that the PL spectra of ZnO nanowires (rods) only exhibited a broad emission band with maximum intensity around 650 nm (Figure 6, panels c and d). The intensity of the red emission of the nanowires was observed to be about 14 times larger than that of ZnO nanorods, indicating again that the defect density seems to increase more in the nanowire than in the nanorod. 3. PL Spectral Properties of Single Nanostructures: Subwavelength Waveguiding. To investigate waveguiding properties of the as-prepared ZnO nanostrucures, PL properties of

individual ZnO nanostructures were examined by exciting the nanostuctures, using the confocal scanning microscope-coupled PL system. As shown in an epi-PL micrograph of a random ZnO nanowire mat (Figure 7a), the nanowires exhibited characteristic red emission, with bright PL spots at the tips and comparatively weaker emission from the bodies upon excitation with visible light at 415 nm, implying that the propagation of PL emission along the nanowire axis results in bright tips as the nanowires are excited. This unique PL behavior was not observed upon UV excitation as in the case of the conventional nanowires. These results indicate that the present ZnO nanowires have visible light-sensitive red-waveguiding behaviors which have not been observed before from the conventional ZnO nanowires. Such subwavelength waveguiding behavior was observed from more than 90% of 100 isolated nanowires. However, it is not observed from the nanorods even though the nanorods can absorb the visible light. This is probably because the ZnO nanorod has too small a diameter (100 nm) to confine

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Figure 8. (a) Plot of mode spacing measured at 650 nm versus inverse nanowire length for 11 different nanowires. (b) Plot of the tip emission peak intensity versus excitation pump power for the selected nanowire shown in Figure 7b.

the fundamental mode of subwavelength emission within its core as predicted from the equation m < (2d/λ)(n2ns - no)0.5, where m is the integer number of higher order guided modes, d is the diameter, and n2nw and no are the refractive indices of ZnO nanostructure (∼2.2) at 415 nm and air (1.0), respectively.33 To explore the waveguiding behavior of the nanowire in detail, several isolated ZnO nanowires of different lengths were excited to measure the PL emission spectra. A PL image of one representative ZnO nanowire of 15 µm length is shown in Figure 7b. It was observed that the enhanced red emission at the tip is preferred for the polarization along the wire major axis oriented at Ø ) 0° as seen from the polarization map of the emission intensity (Figure 7c), indicating the strong red waveguiding is evident. Panels d and e of Figure 7 show the PL emission spectra acquired from the tip and body of the representative single ZnO nanowire upon excitation with visible light at 415 nm, using a femtosecond Ti/sapphire pulse laser. The single nanowire body emission spectra were in general agreement with the ensembleaveraged red emission, but the tip emission spectra exhibited

periodic intensity variations clearly corresponding to waveguiding modes. Such variations were never apparent in the body spectra, suggesting that they were due to axial Fabry-Pe˙rottype cavity resonances. Such behavior has been reported for the UV emission waveguides in the conventional ZnO nanowire, too.8 If the red waveguiding is really operated by the axial Fabry-Pe˙rot-type microcavity resonances, the mode space (∆λ) should be given by (λ2/2L)[n - λ(dn/dλ)]-1, where n is the refractive index and dn/dλ is the dispersion relation. The mode spacing at 650 nm was plotted versus the inverse wire length for 11 nanowires as shown in Figure 8a, and was shown to exhibit a linear dependence, confirming the axial Fabry-Pe˙rottype spacing in agreement with longitudinal mode spacing. From this plot, [n - λ(dn/dλ)] were determined to be 2.24, which approximately gives the refractive index, n ) 2.2, assuming λ(dn/dλ) ≈ 1.0,33 which is close to the reported value (2.2 at 415 nm).34 Johnson et al.8 observed the polarization preference at Ø ) 90° for the green PL of the ZnO nanowire in contrast to the polarization preference at Ø ) 0° for the red PL. This

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observation was attributed to a competition between an increase in the number of charge carriers (electron and holes) which can migrate to the defects and a subsequent decrease in the number of defects as the green PL intensity was observed to saturate as the excitation power increases, suggesting this to be poor coupling of the green PL to the waveguide modes. This observation was performed by higher energy excitation with UV light, and one can expect that the density of defects would not be decreased by the charge carriers migrated from the band edges if the nanowire is excited with visible light. The electronic transitions would be more probable in the defect sites as the visible light excitation power is increased. As expected, the overall PL intensity was observed to be almost linearly increased without saturation as the excitation power was increased as shown in Figure 8b, indicating that the defect emission can be coupled to the subwavelength waveguide modes unless the defects are not disturbed by higher energy excitation. Nevertheless, the threshold could not be observed from the excitation power dependence of the red emission under our laser power limit, and lasing action is currently open for investigation with a more powerful laser system. Conclusions Visible light-sensitive single crystalline ZnO nanowires and nanorods were synthesized by a new sol-gel method carried out through formation of liposome-ZnO composites with or without hydrothermal reaction, respectively. The individual nanowires having a very high visible light absorptivity from 400 to 600 nm exhibited the PL images and spectra characteristic of subwavelength (red-emission) waveguide behavior upon excitation with 415 nm, which was analyzed to be operated by the axial Fabry-Pe˙rot-type microcavity resonance. Observation of the linear excitation power dependence of the red-emission intensity indicates that the defect emission from ZnO nanowire can be coupled to the subwavelength waveguide modes as long as the defects are excited with visible light excitation. These results suggest that the present ZnO nanowires will be a promising nanomaterial for future subwavelength lasing cavity or visible light-sensitive optoelectronic devices including biosensor. Acknowledgment. This work has been financially supported by the Korea Research Foundation (KRF-C00340) and BK 21 Program of the Korea Ministry of Education, Science and Technology. The authors thank Mr. Seokmin Yoon, a Ph.D. candidate at POSTECH, for his help on analysis of TEM data.

Lee and Yoon References and Notes (1) King, D. S.; Nix, R. M. J. Catal. 1996, 160, 76. (2) Minami, T. Mater. Res. Soc. Bull. 2000, 25, 38. (3) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (4) Kind, H.; Yan, H.; Law, M.; Messer, B.; Yang, P. AdV. Mater. 2002, 14, 158. (5) Law, M.; Kind, H.; Kim, F.; Messer, B.; Yang, P. Angew. Chem. 2002, 41, 24505. (6) Bagnall, D. M.; Chen, Y. F.; Zhu, Z.; Yao, T.; Koyama, S.; Shen, M. Y.; Goto, T. Appl. Phys. Lett. 1997, 70, 2230. (7) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (8) Johnson, J. C.; Yan, H.; Yang, P.; Saykally, R. J. J. Phys. Chem. B 2003, 107, 8816. (9) Li, Z.; Yang, R.; Yu, M.; Bai, F.; Li, C.; Wang, Z. L. J. Phys. Chem. C 2008, 112, 20114. (10) Liu, J.; Guo, C.; Li, C. M.; Li, Y.; Chi, Q.; Huang, X.; Liao, L.; Yu, T. Electrochem. Commun. 2009, 11, 202. (11) Law, M.; Sirbuly, D. J.; Hohnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. Science 2004, 305, 1269. (12) Voss, T.; Svacha, G. T.; Mazur, E.; Miller, S.; Ronning, C.; Konjhodzic, D.; Marlow, F. Nano Lett. 2007, 7, 3675. (13) Wu, Y.; Yang, P. J. Am. Chem. Soc. 2001, 123, 3165. (14) Park, W. I.; Kim, D. H.; Jung, S. W.; Yi, G. G. Appl. Phys. Lett. 2002, 80, 4232. (15) Zheng, M. J.; Zhang, L. D.; Li, G. H.; Shen, W. Z. Chem. Phys. Lett. 2002, 363, 123. (16) Li, D.; Hajime, H. Chemosphere 2004, 54, 1099. (17) Wu, L.; Wu, Y. J. Mater. Sci. 2007, 42, 406. (18) Yoon, M.; Seo, M.; Jeong, C.; Jang, J. H.; Jeon, K. S. Chem. Mater. 2005, 17, 6069. (19) Jeon, K.-S.; Oh, S.-D.; Suh, Y. D.; Yoshikawa, H.; Masuhara, H.; Yoon, M. Phys. Chem. Chem. Phys. 2009, 11, 534. (20) Jiang, P.; Zhou, J. J.; Fang, H. F.; Wang, C. Y.; Wang, Z. L.; Xie, S. S. AdV. Funct. Mater. 2007, 17, 1303. (21) Wang, C.; Mao, B.; Wang, E.; Kang, Z.; Tian, C. Solid State Commun. 2007, 141, 620. (22) Liu, B.; Zeng, H. C. Langmuir 2004, 20, 4196. (23) Hu, H.; Huang, X.; Deng, C.; Chen, X.; Qian, Y. Mater. Chem. Phys. 2007, 106, 58. (24) Gao, Y. J.; Zhang, W. C.; Wu, X. L.; Xia, Y.; Huang, G. S.; Xu, L. L.; Shen, J. C.; Siu, G. G.; Chu, P. Appl. Surf. Sci. 2008, 255, 1982. (25) Yi, R.; Zhang, N.; Zhou, H.; Shi, R.; Qiu, G. Mater. Sci. Eng. B 2008, 153, 25. (26) Xu, F.; Lu, Y.; Xie, Y.; Liu, Y. Mater. Des. 2009, 30, 1704. (27) He, Y.; Shang, S.; Cui, W.; Li, X.; Zhu, C.; Hou, X. Microelectron. J. 2009, 40, 517. (28) Mazloumi, M.; Zanganeh, S.; Kajbafvala, A.; Ghariniyat, P.; Taghavi, S.; Lak, A.; Mohajerani, M.; Sadrnezhaad, S. Ultrason. Sonochem. 2009, 16, 11. (29) Mackrodt, W. C.; Stewart, R. F.; Campbell, J. C.; Hillier, I. H. J. Phys. (Paris) 1980, 41, C6–64. (30) Kroger, F. A. The Chemistry of Imperfect Crystals; North-Holland Publishing Co.: Amsterdam, The Netherlands, 1964; p 691. (31) Sun, Y.; George Ndifor-Angwafor, N.; Jason Filey, D.; Ashfold, N. R. Chem. Phys. Lett. 2006, 431, 352. (32) Kahn, M. L.; Cardinal, T.; Bousquet, B.; Monge, M.; Jubera, V.; Chaudret, B. Chem. Phys. Chem. 2006, 7, 2392. (33) Srikant, V.; Clarke, D. R. J. Appl. Phys. 1998, 83, 5447. (34) Sun, X. W.; Kwok, H. S. J. Appl. Phys. 1999, 86, 408.

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