Growth, Optical, and Electrical Properties of In2S3 Zigzag Nanowires

Growth, Optical, and Electrical Properties of In2S3 Zigzag Nanowires Anuja Datta, Godhuli Sinha, Subhendu K. Panda, and Amitava Patra* Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata-700032, India

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 427–431

ReceiVed June 23, 2008; ReVised Manuscript ReceiVed September 16, 2008

ABSTRACT: Ultralong In2S3 zigzag nanowires (diameter ∼66 nm) of variable periodicities are fabricated by physical vapor deposition of indium and sulfur, using Au as the catalyst element. The morphologies of the zigzag nanowires are controlled by interplay of surface free energy minimization and self-stacking of the closest-packed (103) and (0012) planes along their axis. The resulting zigzag nanowires show an enhanced luminescence and a rectifying behavior, which can open up avenues for a host of potential device applications. Introduction In recent years, research interest has been focused on the synthesis and characterization of one-dimensional (1D) nanostructures due to their unique properties and potential applications in nanodevices.1-3 Widening the use of these 1D nanostructures into potential nanoelectronic and optoelectronic devices requires greater diversity of structures to be synthesized in bulk and as thin films for applications. The recent urge to fabricate the new geometric configurations of well-defined “smart materials” has made it crucial to assemble nanoscale building blocks in either radial or axial directions that provides an additional dimension of tunability in their properties.4-6 For this reason, zigzag nanowires (NWs) having breaks in the lattice periodicity at junctions represent an unusual group of nanoscale structures that can offer good lateral confinement effect and enhanced excitonic response, providing a multichoice of electronic and photon conductivity. Kinetic control growth of these zigzag 1D NWs is generally obtained by chemical catalysis whose size, structure, and shape determine the configuration of the NWs growing on them.6-8 Nevertheless, in spite of considerable advances, the various proposed growth mechanisms for zigzag NWs are still controversial and the subject of intense discussion. In2S3 is an important member of III-VI group semiconductors with a cation-ordered structure that exists in three phases: the defective cubic structure R-In2S3 (stable up to 693 K); a defect spinel structure, β-In2S3 (stable up to 1027 K); and a higher temperature layered structure, γ-In2S3 (above 1027 K).9 Among these, the defect spinel structure, which is a typical n-type semiconductor of bulk band gap of 2.0-2.3 eV, is obtained in either cubic or tetragonal form above 420 °C and has a large Bohr exciton radius (33.8 nm).10,11 The In2S3 unit cell is composed of octahedra and tetrahedra, where eight In atoms are in the center of regular sulfur octahedra, another 16 are in the center of sulfur octahedra with slightly distorted symmetry, and the remaining eight In atoms are placed in the center of sulfur tetrahedral (Figure 1a).12 The tetragonal form arises out of vacancy ordering in the tetrahedral cation sites, leading to formation of a supercell. One of the important aspects of In2S3 is that almost all properties of In2S3 rest with its intrinsic defect structure. This structural uniqueness has made In2S3 an * Corresponding author. Phone +91-33-2473-4971. Fax+91-33-2473-2805. E-mail: [email protected].

Figure 1. (a) Crystalline structure of defect spinel tetragonal In2S3. (b) Small-range XRD spectrum with the inset showing the large-range XRD spectrum. (c) EDAX of In2S3 zigzag nanowires.

attractive material for optoelectronic, radiation detectors, and electrical switching applications.13-20 Previously, there are few attempts to synthesize 1D In2S3 nanostructures by metal-organic chemical vapor deposition,21 solvothermal,22 high-temperature approach,23 and a combination of electrodeposition and the gas-solid method.24 In this work, we report the Au catalyst mediated synthesis of In2S3 zigzag NWs by the high-temperature thermal evaporation route. Different from the previous studies, our synthesis of In2S3 zigzag NWs is carried out through a combination of vapor-solid (VS) and vapor-liquid-solid (VLS) growth mechanism, during thermal evaporation. VS and VLS mechanisms have been used extensively for the growth of other 1D semiconductor nanostructures, in which Au nanoparticles act as universal catalysts.25-27 Au nanoparticles used for synthesis, along with substrate temperature and growth time, effectively influence the morphologies of the resulting In2S3 NWs. The novel zigzag In2S3 1D NWs are found to show good optical behavior and an excellent electronic conductivity, which we

10.1021/cg800663t CCC: $40.75  2009 American Chemical Society Published on Web 12/08/2008

428 Crystal Growth & Design, Vol. 9, No. 1, 2009

Datta et al.

presume can be most suitable for future nanoelectronic/optical device applications.28-30 Experimental Procedures Preparation of In2S3 Zigzag NWs. The zigzag In2S3 NWs were grown by a thermal decomposition process in a horizontal two-zone heating furnace.27 The distance between the zones is 15 cm, and the temperature was independently maintained at 400 and 750 °C, respectively. Indium metal pellets (Aldrich, 99.999% pure) and sulfur powder (Merck, India) were used as the In and S sources, respectively. Properly cleaned Si(100) wafers sputter-coated with a thin (∼25 Å) layer of Au film was used as the substrate and was clipped to the quartz boat horizontally by keeping the Au coated face down, maintaining a vertical distance of 5 mm from the In source. In another quartz boat, sulfur powder was taken inside the quartz tube, which was then properly sealed with coupling systems. Prior to placing the quartz tube in the furnace, it was vacuumed to a base pressure of 60 mTorr and flushed with the carrier gas repeatedly to decrease oxygen contamination. From the S end of the quartz tube, argon (Ar) gas was passed through at a flow rate of 200 sccm. The quartz tube was then inserted inside the preheated furnace so that the In source with Si substrate was placed in the heating zone at 750 °C and the S source at the 400 °C heating zone. The reaction was continued for 10-30 min and then suddenly the tube was removed out from the furnace to room temperature and was set to cool down under the flowing Ar gas. The Si substrates with a thin yellow layer of the product were carefully removed out from the boat after the reaction and used for characterizations. Characterizations. The crystal structure of the product was examined by a X-ray diffractometer (Seifert 3000P) using Cu KR radiation (λ ) 1.54178 Å). The morphology of the products was characterized by a scanning electron microscope (SEM, Hitachi S-2300), and a detail structural investigation was carried out by a high-resolution transmission electron microscope (HRTEM; JEOL 2010). Compositional analyses of the products were carried out by energy dispersive X-ray analysis (EDAX) attached with the TEM. Room temperature photoluminescence studies were accomplished by a fluorescence spectrophotometer (PL, Hitachi F-2500). The samples for the measurement of current-voltage (I-V) curves were prepared with gold electrode arrays created on the as deposited nanowires by conventional optical lithography. The device was put in a vacuum chamber and a dc source was connected between the two gold electrodes. Bias was applied with respect to the Au electrode, and I-V characteristics were recorded at room temperature with a Yokogawa 7651 dc source and a Keithley 486 picoammeter. The instruments were controlled by a personal computer via a generalpurpose interface bus (GPIB).

Results and Discussion The phase purity of the as-obtained nanostructures was investigated by X-ray diffraction (XRD) analysis, and a typical pattern of the as-synthesized product is shown in Figure 1b. A very highly crystalline peak located at ∼69.2° is a contribution of the (400) plane from Si substrate. The inset shows the magnified XRD spectra of the NWs, which reveals the formation of highly crystalline and pure tetragonal In2S3 phase.31 The crystal parameters, calculated to be a ) 7.8 Å and c ) 33.1 Å, are well-matched with the reported data.31 Composition analysis (EDAX) confirmed the formation of stoichiometric In2S3 phase (Figure 1c). In the XRD spectrum a set of planes are clearly visible, i.e., (109), (206), (0012), (1013), and (1015), which are parallel to the b-axis. From the appearance of b-axis parallel planes of In2S3 in the XRD spectrum, it is suggested that the NWs have retarded growth along the b-axis. As the XRD peak from the Si substrate32 is also parallel to the b-axis, the In2S3 zigzag nanowires possibly have crystallographic preference with the substrate. The morphologies of the nanomaterials deposited at different time periods are shown in Figure 2a-e. A dense growth of the nearly straight nanowires is noticed after 10 min of deposition, as shown in Figure 2a. The average diameter of the nanowires

Figure 2. SEM images of (a) 1D In2S3 nanostructures after 10 min deposition. (b) In2S3 zigzag nanowires deposited after 20 min. (c) Zigzag nanowire with the Au tip. (d) In2S3 zigzag nanowires with a combination of V and W periodicity. (e) Magnified image of a zigzag nanowire showing the block-by-block growth and the origin of bending.

is measured to be ∼66 nm and length in the range of 1-2 µm. A remarkable change in the NW morphology is noticed as the time of the deposition was increased to 30 min. The length of the nanowires is increased to several tens of microns with diameter remaining nearly uniform and constant (Figure 2b). The low-resolution SEM image reveals that a significant portion of the nanowires are with zigzag morphologies of “V”- and “W”-shaped forms (Figure 2c,d). Growth of the NWs is not restricted to a certain orientation. The presence of a catalyst droplet (Au, confirmed by EDAX) is observed at the tip of the nanowires (Figure 2c). To understand the nature of the bending in the zigzag NWs, a closer study of a single NW was carried out (Figure 2e). The NWs are found to be formed of tetrahedral blocks, and a change in the way of attaching these blocks gives rise to the V or W forms. All V and W zigzag NWs with asymmetric configuration have irregular periodicity with angular separation of nearly 110°, suggesting that the V and W periodicities are indeed the same morphology. To understand the structural characteristics of these morphologies, TEM studies were conducted. Figure 3a shows the low-resolution TEM image of the NWs, indicating the coexistence of the straight and the zigzag structures in the sample. The presence of dark Au droplets at the tips of the NWs is also noted from the TEM image, indicating the VLS growth. A zigzag NW of very short periodicity is shown in Figure 3b. The base of the NW is also shown, from which perhaps the basal nanocrystals have grown. Careful observation of the periodic NW structure indicates the block-by-block stacking of tetrahedral nanocrystals in the axial direction. This stacking is chainlike, interpenetrating type and forms the sawtooth appearance of the NW, which is different from the other stacking process. Figure 3c shows the TEM image of long and short periodicity of zigzag NW. No Au catalytic particle is found at the tip of the NW in this case, thereby suggesting that the VS mechanism is also dominant in this growth process. The SAED recorded from different portions of this NW (inset of Figure 3c) shows a single crystalline pattern, indicating that the whole NW is a single crystal, and hence it is confirmed that the NWs have similar crystal structure and the same long-axis orientation. A high-resolution image of the bending of this NW in Figure 3d did not reveal any block-by-block or interpenetrating

Growth and Properties of In2S3 Zigzag Nanowires

Crystal Growth & Design, Vol. 9, No. 1, 2009 429

Figure 4. (a) Room temperature PL spectrum of In2S3 zigzag nanowires. (b) Current-voltage (I-V) characteristics of In2S3 zigzag nanowires revealing the rectifying nature. The inset shows an image of the device.

Figure 3. TEM image showing (a) an assemblage of In2S3 straight and zigzag nanowires. (b) Low-resolution TEM of a zigzag nanowire with small periodicity. (c) A nanowire without Au tip exhibiting random zigzag patterns. (d) A close image of a zigzag angle with an angular separation of 110°. (e and f) HRTEM images showing the lattice separations at the two zones indicated as I and II. (g) Single crystalline SAED pattern of a zigzag nanowire.

attachment, indicating continuous growth without breaks. Figure 3e,f shows the lattice fringe pattern from different portions of the NW, which are marked as I and II. The lattice fringe pattern of zone I showed high crystallinity, with the atomic spacing of 0.62 nm corresponding to the plane (103) of tetragonal In2S3. The lattice fringe spacing from zone II, on the other hand, shows two distinct spacings of 0.62 and 0.27 nm, corresponding to the planes (103) and (0012), at an angle of 110° to each other. A selected area electron diffraction (SAED) pattern recorded from a zigzag NW (Figure 3g) reveals the single crystallinity of the NW. The SAED shows a preferential growth along the b-axis parallel planes of tetragonal In2S3. The zigzag patterned V and W NW growth has been recently observed for wurtzite ZnO, ZnS, CdSe, GaN, GaSe, and Fe3O4 NWs and nanobelts in case of catalyzed growth.6,7,33-37 It is suggested that the periodic shrinking and expansion of the catalyst droplet at the tip of the NWs result in the change of the growth direction due to a change in the geometry and surface free energies. However, this mechanism can only be implemented when the NW growth is symmetric with respect to the growth axis. There is no such symmetry axis in zigzag In2S3 NWs, and therefore this mechanism does not seem to act alone in the growth process. Moreover, the zigzag NWs are grown

by stacking of tetrahedral blocks in catalyst-assisted VLS mechanism. We believe that the formation of the stacking blocks is governed by the minimization of the energy of the system and is thus favored by the enclosure of the planes with the lowest surface energy. As a result, the zigzag shaped In2S3 NWs grow by the simple stacking of the closest-packed (103) and (0012) planes and layering of the unit cells along their axis. In case of NWs grown by the VS process, the unique structure is likely determined by growth kinetics. The shear stress is generally high in thin films and nanocrystals.38 During the growth of the In2S3 NWs from the nanocrystal nuclei on the film, shear strain may be invoked at the time of formation of the NWs and the zigzag structure formation may be initiated. Fluctuations in the rate of the reaction and the rapid growth far from the thermodynamic equilibrium may also cause the zigzag shapes to be created. This is a very complex growth process and further studies may be needed to confirm these matters. The room temperature photoluminescence investigation was carried out on the as-deposited films of the zigzag In2S3 NWs as shown in Figure 4a. When excited at 220 nm, a broad peak centered at ∼302 nm is observed, which can be resolved into two peaks at 300 nm and another at a higher wavelength at 350 nm, as shown in the figure. The peaks are largely shifted from the bulk value of the In2S3 band gap energy of 2.4 eV, which indicates that there is strong confinement effect in the zigzag NWs. Previously, such effects have been seen in In2S3 nanoparticles.39-41 The UV emission in In2S3 is referred to as near-band edge excitonic emission, which is generally sharp.42 In this case, the broadness of the emission is related to the transitions involving different electronic levels in close vicinity to the bottom of the lowest empty band and the top of the highest

430 Crystal Growth & Design, Vol. 9, No. 1, 2009

occupied band, as in the case of several other chalcogenide nanocrystals, attributed to the exciton recombination from charge carrier traps and surface states.42,43 Self-modulated zigzag NW exhibits unique structural characteristics and is an interesting system to know how electrons behave when they are transported through a zigzagshaped junction and encounter the effect of quantum size and are also scattered by its periodically changing growth direction. In order to judge the potentiality of forming functional devices using these NWs, we have carried out an electrical study of the NWs, as shown in Figure 4b. The resulting I-V curve of the device and an image of the resulting connection device through the In2S3 NWs are shown in Figure 4b. The I-V curve clearly exhibits a rectifying nature and represents the n-type Schottky diode characteristics of In2S3 NWs with a turn-on voltage of ∼1.7 V. No reverse bias breakdown voltage was observed upon increasing the measured voltage up to 3 V. The rectifying ratio [RR ) ratio of the current at a large voltage V divided by the negative of the current at the corresponding negative potential V] is calculated to be ∼1.25 × 102 at 2 V for this rectifying curve, which is found to be highly stable. The value is quite high when compared to the many other semiconductor rectifying junctions.44-47 The crystal structure of In2S3 has a considerable number of cation vacancies.10,18 Previously, it has been observed that a small amount of energy is required to transfer an In atom from a lattice site to an ordered vacancy.18 Therefore, a small fraction of In atoms may easily leave their ordered positions and occupy crystallographically ordered vacancies, resulting in quasi-interstitial cations and an equal amount of disordered cation vacancies. Previously, Rehwald and Harbeke calculated the dependence of the equilibrium concentrations of electrons, holes, ionized donors, and acceptors on sulfur vapor pressure according to Kroeger and Vink’s formulas for Frenkel disorder.18 They showed that at relatively low sulfur vapor pressure the difference in concentration of ionized donors and acceptors increased, ensuing a higher electron concentration and n-type conductivity of In2S3 crystals. Consequently, the electron concentration should be reduced at high sulfur vapor pressure. An important point is that Rehwald and Harbeke always observed n-type conductivity in In2S3 crystallites. Actually, the difference between the work functions of In2S3 and Au electrodes creates a potential barrier across the junction. For the In2S3/Au junction, as in here, the forward bias corresponds to an electric field from Au to In2S3 NW. This is justified from the fact that Au has a high work function, φAu ) 5.2 eV,48 and probably receives electrons from In2S3 NW. This kind of electrical behavior is very new to the In2S3 system, and carrier injection into NWs has yet to be completely explained, which can impede the effectiveness of device design. Conclusions In conclusion, we reported the synthesis of In2S3 zigzag NWs of diameter nearly ∼66 nm by Au catalyst assisted physical vapor growth for the first time. The densely grown NWs are obtained with varieties of zigzag structures in V and W forms, with or without the catalyst droplet at the tips. An interplay of geometry and surface free energy of the NWs is found to be the force behind the change in the growth direction of the NWs. Mainly, two processes of growth, VLS and VS, were active in controlling the shapes and structural varieties of the In2S3 zigzag NWs. The self-modulated zigzag NWs exhibit good lumines-

Datta et al.

cence property and stable electrical conductivity similar to a rectifying type, which is a novel observation in In2S3. The measured current-voltage graph revealed a low threshold voltage of ∼1.7 eV and a rectification ratio of 1.25 × 102 at 2 V. The results presented here successfully demonstrated the utilization of the physical evaporation technique for modulating the anisotropy of the resulting NWs. The zigzag-shaped In2S3 NWs also represent an unusual group of nanoscale structures that is an additional attribute to tune the functional properties for the applications in nanoscale optoelectronic devices. Acknowledgment. A.D. is thankful to CSIR, Govt. of India for providing a research fellowship. G.S. acknowledges DST, Govt. of India for providing financial assistance as a research stipend. The authors thank Prof. A. J. Pal, Department of Solid State Physics, I.A.C.S. for providing the electrical measurement facility and Mr. Bikas C. Das for kind help.

References (1) Characterization of Nanophase Materials; Wang, Z. L., Ed.; WileyVCH, Weinheim, Germany; 2000. (2) Yang, S. Nanowires and Nanobelts, Materials, Properties and DeVices, Vol 2: Nanowires and Nanobelts of Functional Materials; Wang, Z. L., Ed.; Kluwer Academic Publishers: New York, 2003; pp 209-238. (3) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (4) Mathur, S.; Barth, S. Small 2007, 3, 2070. (5) Zhan, J.; Bando, Y.; Hu, J.; Xu, F.; Golberg, D. Small 2005, 1, 883. (6) Peng, H.; Meister, S.; Chan, C. K.; Zhang, X. F.; Cui, Y. Nano Lett. 2007, 7, 199. (7) Zhou, X. T.; Sham, T. K.; Shan, Y. Y.; Duan, X. F.; Lee, S. T. J. Appl. Phys. 2005, 97, 104315. (8) Zhai, T.; Gu, Z.; Yang, W.; Zhang, X.; Huang, Z. J. Y.; Yu, D.; Fu, H.; Ma, Y.; Yao, J. Nanotechnology 2006, 17, 4644. (9) Diehl, R.; Nitsche, R. J. Cryst. Growth 1975, 28, 306. (10) Herrero, J.; Ortega, J. J. Sol. Energy. Mater. 1988, 17, 257. (11) Chen, W.; Bovin, J. O.; Joly, A. G.; Wang, S.; Su, F.; Li, G. J. Phys. Chem. B 2004, 108, 11927. (12) Kulin′ska, A.; Uhrmacher, M.; Dedryve’re, R.; Lohstroh, Hofsa¨ss, A. H.; Lieb, K. P.; Picard-Garcia, A.; Jumas, J. C. J. Solid State Chem. 2004, 177, 109. (13) Kim, W. T.; Kim, C. D. J. Appl. Phys. 1986, 60, 2631. (14) Kamoun, N.; Belgacem, S.; Amlouk, M.; Bennaceur, R.; Bonnet, J.; Touhari, F.; Nouaoura, M.; Lassabatere, L. J. Appl. Phys. 2001, 89, 2766. (15) El Shazly, A. A.; Abd Elhady, D.; Metwally, H. S.; Seyam, M. A. M. J. Phys.: Condens. Mater. 1998, 10, 5943. (16) Choe, S. H.; Bang, T. H.; Kim, N. O.; Kim, H. G.; Lee, C. I.; Jin, M. S.; Oh, S. K.; Kim, W. T. Semicond. Sci. Technol. 2001, 16, 98. (17) Amlouk, M.; Ben Said, M. A.; Kamoun, N.; Belgacem, S.; Brunet, N.; Barjon, D. Jpn. J. Appl. Phys 1999, 38, 26. (18) Rehwald, W.; Harbeke, G. J. Phys. Chem. Solids 1965, 26, 1309. (19) Giles, J. M.; Hatwell, H.; Offergeld, G.; Van Cakenberghe, J. J. Phys. Status Solidi 1962, 2, K73. (20) Dalas, E.; Sakkopoulos, S.; Vitoratos, E.; Maroulis, G. J. Mater. Sci. 1993, 28, 5456. (21) Afzaal, M.; Malik, M. A.; O’Brien, P. Chem. Commun. 2004, 334. (22) Xiong, Y. J.; Xie, Y.; Du, G. A.; Tian, X. B. J. Mater. Chem. 2002, 12, 98. (23) Liang, C.; Shimizu, Y.; Sasaki, T.; Umeharab, H.; Koshizaki, N. J. Mater. Chem. 2004, 14, 248. (24) Datta, A.; Panda, S. K.; Gorai, S.; Ganguli, D.; Chaudhuri, S. Mater. Res. Bull. 2008, 4, 983. (25) Lieber, C. M. MRS Bull. 2003, 28, 486. (26) Lieber, C. M.; Wang, Z. L. MRS Bull. 2007, 32, 99. (27) Panda, S. K.; Datta, A.; Sinha, G.; Chaudhuri, S.; Chavan, P. G.; Patil, S. S.; More, M. A.; Joag, D. S. J. Phys. Chem. C 2008, 112, 6240. (28) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (29) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; van Schalkwijk, W. Nat. Mater. 2005, 4, 366. (30) Duan, J. H.; Yang, S. G.; Liu, H. W.; Gong, J. F.; Huang, H. B.; Zhao, X. N.; Zhang, R.; Du, Y. W. J. Phys. Chem B 2005, 109, 3701. (31) JCPDS Card No. 25-0390.

Growth and Properties of In2S3 Zigzag Nanowires (32) JCPDS Card No. 27-1402. (33) Wang, Z. L.; Kong, X. Y.; Zuo, J. M. Phys. ReV. Lett. 2003, 91, 185502. (34) Moore, D; Ronning, C; Ma, C; Wang, Z. L. Chem. Phys. Lett. 2004, 385, 8. (35) Ma, C.; Ding, Y.; Moore, D.; Wang, X. D.; Wang, Z. L. J. Am. Chem. Soc. 2004, 126, 708. (36) Ma, C.; Ding, Y.; Moore, D.; Wang, X. D.; Wang, Z. L. Nature 2004, 427, 497. (37) Mathur, S.; Barth, S.; Werner, U.; Hernandez-Ramirez, F.; RomanoRodriguez, A. AdV. Mater. 2008, 20, 1550. (38) Fang, X. S.; Ye, C. H.; Zhang, L. D.; Xie, T. AdV. Mater. 2005, 17, 1661. (39) Ai, Z. P. Opt. Mater. 2003, 24, 589. (40) Vigneashwari, B.; Ravichandran, V.; Parameswaran, P.; Dash, S.; Tyagi, A. K. J. Nanosci. Nanotechnol. 2008, 8, 689. (41) Datta, A.; Gorai, S.; Chaudhuri, S. J. Nanopart. Res. 2006, 8, 919.

Crystal Growth & Design, Vol. 9, No. 1, 2009 431 (42) Nagesha, D. K.; Liang, X. R.; Mamodov, A.; Gainer, G.; Eastman, M. A.; Giersig, M.; Song, J.; Kotov, N. A. J. Phys. Chem. B 2001, 105, 7490. (43) Bawendi, M. G.; Caroll, M.; Wilson, W. L.; Brus, L. J. Chem. Phys. 1992, 96, 946. (44) Chai, Y.; Zhou, X. L.; Li, P. J.; Zhang, W. J.; Zhang, Q. F.; Wu, J. L. Nanotechnology 2005, 16, 2134. (45) Cheng, G.; Kolmakov, A.; Zhang, Y.; Moskovites, M.; Munden, R.; Reed, M. A.; Wang, G.; Mosses, D.; Zhang, J. Appl. Phys. Lett. 2003, 83, 1578. (46) Jang, S.; Chen, J. J.; Kang, B. S.; Ren, F.; Norton, D. P.; Pearton, S. J.; Lopata, J.; Hobson, W. S. Appl. Phys. Lett. 2005, 87, 222113. (47) Ryu, Y. R.; Lee, S. T.; Leem, J. H.; White, H. W. Appl. Phys. Lett. 2003, 83, 4032. (48) Hansen, W. N.; Johnson, K. B. Surf. Sci. 1994, 316, 373.

CG800663T