In2O3

Nov 8, 2006 - Liang Xu,* Yong Su, Sen Li, Yiqing Chen, Qingtao Zhou, Song Yin, and Yi Feng. School of Materials Science and Engineering, Hefei UniVers...
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J. Phys. Chem. B 2007, 111, 760-766

Self-Assembly and Hierarchical Organization of Ga2O3/In2O3 Nanostructures Liang Xu,* Yong Su, Sen Li, Yiqing Chen, Qingtao Zhou, Song Yin, and Yi Feng School of Materials Science and Engineering, Hefei UniVersity of Technology, Hefei, Anhui, 230009 P.R. China ReceiVed: October 8, 2006; In Final Form: NoVember 8, 2006

We report on the realization of novel 3-D hierarchical heterostructures with 6-and 4-fold symmetries by a transport and condensation technique. It was found that the major core nanowires or nanobelts are singlecrystalline In2O3, and the secondary nanorods are single-crystalline monoclinic β-Ga2O3 and grow either perpendicular on or slanted to all the facets of the core In2O3 nanobelts. Depending on the diameter of the core In2O3 nanostructures, the secondary Ga2O3 nanorods grow either as a single row or multiple rows. The one-step growth of the unique Ga2O3/In2O3 heteronanostructures is a spontaneous and self-organized process. The simultaneous control of nanocrystal size and shape together with the possibility of growing heterostructures on certain nanocrystal facets opens up novel routes to the synthesis of more sophisticated heterostructures as building blocks for opto- and nanoelectronics.

1. Introduction

2. Experimental Section

A key motivation underlying research on nanoscale devices is the potential to achieve integration at a level not possible with conventional top-down approaches. To achieve this goal in future nanosystems will require the development and implementation of efficient and scalable strategies for assembly of nanoscale building blocks into increasingly complex architectures. Hierarchical assembly of nanoscale building blocks (nanocrystals, nanowires, and nanotubes) is a crucial step toward realization of functional nanosystems and represents a significant challenge in the field of nanoscale science. Nanocrystal heterostructures represent a convenient approach to the development of nanoscale building blocks.1,2 Recently, 3-D hierarchical structures at micro- and nanometer scale became increasingly important with the technological advances in micro- or nanoelectromechanical systems,3 micro- or nanofluidic devices,4,5 and micro- or nano-optics.6 Although significant advances have been made in the fabrication of nanostructures and to some extent of 3-D submicro- and nanometer structures, the fabrication of ordered 3-D hierarchical nanostructures remains challenging.7,8 Two key wide-band semiconducting oxides, Ga2O3 and In2O3, have distinctive properties and are now widely used as optoelectronic devices and gas sensors. Nanostructures of Ga2O3 and In2O3 have been studied widely in recent years.9-11 Here, we report that a variety of novel well-ordered 3-D hierarchical heteronanostructures in which the branches and core are singlecrystal Ga2O3 and In2O3, respectively, that are successfully synthesized by a thermal vapor transport and condensation technique. These nanostructures have basic 6- and 4-fold structural symmetries. The branches (Ga2O3 nanorods) and the core (In2O3 nanostructures) have definite crystallographic orientation and thus form a series of self-assembled hierarchical heteronanostructures. These hierarchical heteronanostructures may have important applications for nanoscale-based devices including diodes, gas sensors, and optical devices.

Hierarchical Ga2O3 nanorods have been synthesized on In2O3 core nanostructures (nanowires or nanobelts) by a vapor transport and condensation process. Ga2O3 (purity > 99%), In2O3 (purity > 99%), and graphite powders were mixed thoroughly with a molar ratio of 1.5:1:1 and then placed at the sealed end of a one-end sealed quartz tube. An n-type (100) Si wafer was used as the collector at the open end of the quartz tube to collect the nanostructures. The whole assembly was finally pushed into a horizontal tube furnace pumped by a rotary pump. The vacuum in the ceramic tube was kept at around 0.5 to 1.5 Torr. The mixed powders were rapidly heated to 1000 °C and held for 30 min, respectively. The temperature at which the nanostructures were grown was controlled to be about 800850 °C due to the temperature gradient in the ceramic tube. Argon was used as the carrier gas and flowed constantly at a rate of 50 standard cubic centimeters per minute (sccm). The chamber pressure was kept at 150 Torr during the whole experimental process. After the furnace slowly cooled to room temperature, substrate was taken out from the furnace tube. A yellow wool-like product was found on the full surface of the substrate. The morphologies and structures of the as-deposited products were characterized and analyzed by field-emission scanning electron microscopy (FE-SEM) (JEOL model JSM-6700F), X-ray diffraction (XRD) (MAC Science, model MXPAHF), high-resolution transmission electron microscopy (HRTEM) (JEOL model 2010, operating at 200 kV), and selected area electron diffraction (SAED). Their components were measured via energy-dispersive X-ray spectroscopy (EDS) attached in the HRTEM system. A photoluminescence (PL) spectrum was measured using a He-Cd laser (325 nm) as the excitation source at room temperature.

* Corresponding author. E-mail: [email protected]. Telephone: +86551-2901365. Fax: +86-551-2901362.

3. Results and Discussion Figure 1 shows the representative FE-SEM images of the 3-D hierarchical nanostructures at low and high magnifications, respectively. The low magnification image in Figure 1a reveals

10.1021/jp066609p CCC: $37.00 © 2007 American Chemical Society Published on Web 01/04/2007

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Figure 1. FE-SEM images of the Ga2O3/In2O3 hierarchical heteronanostructures synthesized by the vapor transport and condensation technique. (a) Low magnification SEM image showing the abundance of Ga2O3-based hierarchical nanostructures. (b) Medium magnification SEM image of the 4-fold hierarchical nanostructures. (c-e) High magnification SEM images of the 4-fold hierarchical nanostructures to show the various structural symmetries. (f) High magnification SEM image of the 6-fold hierarchical nanostructures.

that the products consist of a large quantity of such 3-D hierarchical heteronanostructures with a major core nanostructure and secondary branch nanorods aligned on the facets of major core. In general, the length of the major core nanobelts along the axis can be as long as tens of micrometers as and the width is about 200-600 nm, as shown in Figure 1b, whereas the length of the secondary nanorods grown on the major core nanobelts ranges from 200 nm to a few micrometers, with diameters ranging 20-200 nm. The high magnification images clearly show that there are two major structural symmetries, i.e., 4-fold (Figure 1b-e) and 6-fold (Figure 1f). From these images, we also found that the major core has two kinds of nanostructures, nanobelts (Figure 1b,c) and nanowires (Figure 1d), and the secondary nanorods grow highly ordered on the facets of the major core nanostructures. Closer examination of these hierarchical nanostructures reveals that there are subsymmetries associated with each major symmetry for the 4-fold nanostructures. The high-magnification FE-SEM observations indicate that there are at least three variations for the 4-fold

symmetry. We have identified the three sub-symmetries as 4S(Figure 1c,d), 4M- (Figure 1e), and 4M*1-fold (Figure 1b).12 When the diameter of the major core nanowire or the width of the major core nanobelt is small, the secondary nanorods (or nanoribbons) grow in a single row, as shown in Figure 1c,d. When the width of the major core nanobelt is large enough, multiple rows of secondary nanorods form on the major nanobelt, as shown in Figure 1e. Figure 1c,d shows the typical high magnification SEM image of the basic 4-fold to show the single row of the secondary nanorods perpendicular to the major core nanostructure. In addition, we have also observed that the secondary nanorods (or nanoribbons) are not always perpendicular to the major core nanostructure but grow at certain angles with the core nanostructures (Figure 1b,f). For the 6-fold symmetry (Figure 1f), it can be easily observed that the secondary nanorods grow perpendicular to the side of major core nanobelt but grow slanted to the facets of the core nanobelt. Furthermore, the time-dependent SEM images of the nanostructures also show that, as the as the deposition time increases,

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Figure 2. XRD spectrum of the Ga2O3/In2O3 hierarchical heteronanostructures.

the length of side branches and the density on individual core nanostructures increases with time, as shown in Figure 1e (grown for 1h) and Figure 1f (grown for 2h). Powder XRD measurements show that the products are mixtures of monoclinic Ga2O3 and cubic In2O3 (Figure 2). From the XRD spectra, lattice constants for Ga2O3 are derived as a ) 5.80 Å, b ) 3.04 Å, and c ) 12.23 Å (JCPDS 11-0307), consistent with the standard values for bulk Ga2O3, whereas the lattice constant for the cubic In2O3 is a ) 10.11 Å (JCPDS 06-0416), which is in good agreement with the reported bulk value. Detailed microstructures and composition information of these novel 3-D hierarchical nanostructures are further characterized by TEM. A typical TEM image of the basic 4-fold in Figure 3a reveals that each 3-D hierarchical nanostructure consists of a faceted In2O3 core nanobelt with aligned and packed Ga2O3 side branches (nanoribbons or nanorods) emanating from facets of the central nanobelt core. Each Ga2O3 nanoribbon has a uniform width along its entire length, and the typical widths of the nanribbons range 20-50 nm. They are self-assembled at a certain angle with respect to the In2O3 cores, forming orderly arrays of multiple junctions. The atomic-resolved images for the square-marked area of hierarchical nanostructures in Figure 3a are shown in Figure 3b (Ga2O3 nanribbon) and Figure 3c (In2O3 nanobelt), respectively. The selected-area electron diffraction (SAED) patterns are essentially identified over the entire major core nanobelt and the side nanoribbons, indicating the single-crystalline character of the cubic In2O3 core and the β-Ga2O3 side branches. Figure 3b shows that the Ga2O3 nanoribbon grows along the [010] direction, indicating that the (200) fringes parallel to the axis of nanoribbon are separated by 0.594 nm, which is consistent with that of the bulk β-Ga2O3 crystal. The SAED pattern, recorded perpendicular to the axis of the nanoribbon (inset Figure 3b), can be indexed for the [001] zone axis of β-Ga2O3. Diffraction analyses indicate that all Ga2O3 nanribbons grow in the [010] direction and the nanoribbons in the same row have the same crystallographic orientation. Figure 3c is an atomically resolved image for the In2O3 nanobelt, which reveals that the In2O3 nanobelt grows along the [100] direction and the marked interplanar d-spacing of 0.506 nm corresponding to the (200) lattice fringes of cubic In2O3. The inset SAED pattern can be indexed in accord with the [01h1] zone axis of cubic In2O3. The Energy-dispersive X-ray

Xu et al. spectroscopy (EDX) spectra recorded from the core nanobelt and the side nanoribbons further verify the structural chemical composition. The EDX spectra (Figure 3d,e) show the appearance of Ga and O X-ray signals for the side nanoribbon and In and O reflections for the core nanobelt, thus confirming that the side single crystalline nanoribbons are composed of Ga2O3 and the major core nanobelt is made of In2O3 (Cu signals originate from the microgrid mesh that supports the nanostructures). Figure 4a is a typical TEM image of the basic 6-fold nanostructures. Because only 2-fold nanostructures can be found in the TEM observations, it can be considered that the 3-D hierarchical heteronanostructures as shown in Figure 1 can be removed when preparing the TEM specimen using an ultrasonic dispersion method. The inset of Figure 4a shows the SAED pattern of the nanoheterojunction of the Ga2O3 nanoribbons and the In2O3 nanobelt. It presents two sets of diffraction spots, one from the major In2O3 core nanobelt and the other from the secondary Ga2O3 nanoribbons. The diffraction patterns can be indexed using the [001] zone axis of In2O3 and the [010] zone axis of Ga2O3. So, the crystallographic relationship is that the [001] zone axis of In2O3 nanobelt is parallel to the [010] zone axis of Ga2O3 nanoribbons for the 6-fold symmetry. The magnified direct nanoheterojunction structure of the squaremarked section of Figure 4a is also shown in Figure 4b. A clear and smooth interface between the Ga2O3 nanoribbon and In2O3 nanobelt is observed. From the HRTEM image, the orientation relationships between the two crystals can also be determined as follows: [001] In2O3 | [010] Ga2O3, (200) In2O3 | (002) Ga2O3, and (020) In2O3 | (100) Ga2O3. The HRTEM investigations are consistent with the SAED results. The heteroepitaxial nature of Ga2O3 nanorods (or nanoribbons) from In2O3 cores gives many possible crystal orientation relations between the cores and nanorods, thus resulting in many different Ga2O3 nanorod orientations with respect to the core. Therefore, the symmetry of these hierarchical nanostructures is dependent on the crystallographic orientation of the In2O3 core nanostructures. The orientation of the In2O3 nanobelt along the [100] direction creates all of the 4-fold symmetries, whereas along the [010] direction all the 6-fold symmetries were produced. The Ga2O3 nanorods have a crystallographic orientation that is consistent with the core In2O3 nanobelts, which is possibly due to the requirement of the heteroepitaxial growth. The lattice mismatch dislocation between two crystals will be small only at some definite orientations, and thus, heteroepitaxial growth will proceed easily. In the present study, the [010] zone axis of the Ga2O3 nanorods parallels the [001] zone axis of the In2O3 nanobelt, and the (002) and (100) facets of the Ga2O3 nanorods are parallel to the (200) and (020) facets of In2O3 nanobelt. Therefore, a slight lattice mismatch between the Ga2O3 and In2O3 crystals will induce strain during growth of the crystal on the nanostructure. It is known that these strain effects cause the formation of heterojunctions in this orientation.13,14 The one-step growth of the unique hierarchical heteronanostructures is a spontaneous and self-organized process. Although the synthesis procedure is very simple, the formation and assembly processes are complex and precisely self-controlled. Another important factor in the production of a direct nanojunction is mass transport of the reactants onto the substrate.7 During the growth of Ga2O3 nanorods on the In2O3 nanobelt, O2 gas was used as a reaction-limiting species. The dosage of O2 gas into the reactor was controlled using a leak valve. The morphology of the heterostructure was remarkably dependent on the concentration of O2. Only low O2 concentrations resulted

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Figure 3. (a) Typical TEM image of the 4-fold hierarchical heteronanostructure. (b) HRTEM image corresponding to the Ga2O3 nanorod (square marked section (b) in (a)); the SAED pattern of the Ga2O3 nanorod reveals the [010] growth direction (inset). (c) HRTEM image corresponding to In2O3 nanobelt (section c); the SAED pattern of the In2O3 nanobelt reveals the [100] growth direction (inset). (d, e) EDS spectra obtained from Ga2O3 nanorod and In2O3 nanobelt, respectively (Cu signals originate from the microgrid mesh that supports the nanostructures).

in a direct hierarchical heteronanostructure. For high concentrations, Ga2O3 coat layers formed on the In2O3 nanobelt surface, followed by a high density of Ga2O3 nanocrystal layers depositing on the Ga2O3 coated In2O3 nanostructures. In the TEM observations, a typical TEM image of hierarchical heteronanostructures in Figure 4c may provide further details of VLS growth in In2O3 nanobelt. The image shows a long nanobelt terminating in a polygonal-shaped particle at the tip. EDX analyses shown in Figure 4d,e indicated that the nanoparticle on the tip mainly consisted of In, Ga, and O but that the nanobelt (stem) was only composed of In and O. The molecular ratio of In/O of the nanowire calculated from the EDS data was closed to that of In2O3. It can be concluded from these analyses of the observed In2O3 nanobelt that the Ga nanoparticles play a crucial role in the growth of In2O3 nanobelts based on the VLS mechanism and the growth of nanobelts is governed by the presence of eutectic metal alloys at the end of nanobelts.8 In our experimental condition, the formation of the Ga metallic particles likely resulted from the following processes,

which is possible at high temperature. The starting material, Ga2O3 powder, will undergo the following reactions to produce Ga2O vapor at the given high temperature:

Ga2O3 + 2C f 2Ga2O + 2CO

(1)

Ga2O3 h Ga2O + 2CO

(2)

where Ga2O is a volatile metal oxide.15,16 The formed Ga2O vapor can be readily transported to the deposition zone by the carrier gas (Ar). Meanwhile, the Ga2O vapor will also react with CO simultaneously at a desired temperature:

Ga2O + 2CO f2Ga + C + CO2

(3)

, and then the newly formed Ga may be in the form of clusters.17 These Ga clusters are then transported by the Ar gas to the lowtemperature furnace zone, where they deposit on the Si substrate in the form of small droplets. These small droplets provided an energetically favored site for the absorption of incoming In2O3 vapor. The process leads to the formation of Ga-In-O alloys.

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Figure 4. (a) Typical TEM image of the 6-fold hierarchical heteronanostructure (the SAED pattern is shown in the inset, revealing the crystal orientation relations between the core and branches. (b) HRTEM image corresponding to heteronanojunction (square marked section in (a)). (c) Another typical TEM image of Ga2O3/In2O3 hierarchical heteronanostructures with a nanocluster at the tip of core nanostructure. (d, e) EDS spectra obtained from the tip of core nanostructure and its stem, respectively (Cu signals originate from the microgrid mesh that supports the nanostructures).

With the gradual absorption of In2O3 vapors, the Ga-In-O alloys become supersaturated and In2O3 was phase-separated and crystallized to form the nanobelt. Therefore, these liquid Ga droplets can serve as ideal nucleation sites for the preferential absorption of the evaporated In2O3 vapor. Continuous dissolution of In and O atoms in Ga-In2O3 eutectic alloy droplets will lead to the nucleation and growth of In2O3 nanobelts through the VLS process when the alloy droplets become saturated with reactant.18 A similar mechanism has been used to explain the growth process of highly aligned silica nanowires and Si/SiO2 and ZnS/SiO2 heteronanostructures.19,20 From the experimental results demonstrated above, it is possible that the core In2O3 nanostructures form first in the substrate. At the reaction temperature, Ga2O vapors are transported to the substrate by the carried gas because the Ga2O vapor in the reactants is volatile and metastable. The Ga2O vapors are subsequently oxidized by the residual or leaking oxygen. The as-deposited In2O3 nanostructures thus act as templates for the adsorption or deposition of the Ga2O3 in the furnace system. Gradual absorption of Ga2O3 causes the epitaxial growth of

Ga2O3 nanorods on the In2O3 nanostructures, leading finally to the formation of hierarchical Ga2O3/In2O3 heteronanostructures. The PL was used to further investigate the optical properties of the Ga2O3-based hierarchical nanostructure. The roomtemperature PL spectrum measured using a He-Cd laser line at 325 nm (3.815 eV) as the excitation source and a 399 nm filter wavelength is shown in Figure 5. Figure 5 compares the PL spectrum of the hierarchical Ga2O3/In2O3 nanostructures synthesized for 2 h to that for 1 h. The two spectra show the same blue-light emission peak at about 494 nm (2.51 eV). Compared with the PL features of pure Ga2O3 nanostructures,21,22 the PL of the Ga2O3-based hierarchical nanostructure shows a red shift of about 30 nm. This shows that In2O3 in the nanostructures has an effect on the position of the Ga2O3 PL spectrum. The luminescence properties of β-Ga2O3 have been extensively studied for several decades, and an acceptable model for blue emission has been put forward. Upon optical excitation through the band gap, β-Ga2O3 can exhibit up to three different emissions, UV, blue, and green, according to the sample

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Figure 5. The PL spectrum of Ga2O3/In2O3 hierarchical heterostructures grown for 1 and 2 h. The excitation wavelength is 325 nm (He-Cd laser).

preparation and the nature of the dopant.23 The blue luminescence is produced only in conducting samples, either as-grown or doped with In3+,23 which demonstrates that it is related to the presence of oxygen vacancies. The blue emission can also be selectively excited at liquid helium temperature with photon energies slightly smaller than the band gap, indicating that it originates from the excitation of defects in the forbidden gap.23 It has been suggested that this blue emission at 494 nm could be produced by a tunnel recombination of an electron on a donor (VOx) with a hole on an acceptor, which could be either VGa or a pair of charged vacancy (VO, VGa).21-23 It is expected that the present preparation method would easily produce considerable quantities of oxygen vacancies (VO) and gallium-oxygen vacancy pairs (VO, VGa) due to high temperature and oxygendeficient ambient. The blue emission centered at 494 nm (2.51 eV) of Ga2O3/ In2O3 heteronanostructures synthesized for 2 h increases more in intensity and line width than that of 1 h, which is consitent with the relative density of Ga2O3 nanorods. It can result from the oxygen deficiencies at the increased surface area of Ga2O3 nanorods and more interfaces with In2O3 core nanostructures because doping with In, which increases the concentration in gallium vacancies, also enhances the blue luminescence.23 4. Conclusion In summary, the self-assembly of Ga2O3 nanorods and In2O3 nanostructures into novel 3-D hierarchical heteronanostructures with 4- and 6-fold symmetries was achieved on a large scale by a simple one-step “thermal evaporation and condensation” method. The core In2O3 nanostructures were grown by a Gaactalyzed vapor-liquid-solid process and then served as a template for the growth of Ga2O3 nanorods by the oxide-assisted growth process. As-grown Ga2O3/In2O3 hierarchical heteronanostructures exhibit an intense PL in the blue-light emission, with a peak at about 494 nm (2.51 eV). Hierarchical heteronanostructures represent unique systems in which the optical and electronic properties can be tuned by varying the chemical composition of their components and their mutual distance. This approach has the potential to facilitate the fabrication of heterostructures with tailored optical and electronic properties.

Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (NSFC, Grant No. 20671027) and by the Nature Science Foundation of Anhui province of China (Grant No. 050440904). Supporting Information Available: Nanostructure figures, XRD spectrum of the product, and image of the product. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Lauhon, L. J.; Gudiksen, M. S.; Wang, D.; Lieber, C. M. Nature 2002, 420, 57. (2) Gudiksen, M. S.; Lauhon, L. J.; Wang, J. F.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (3) Craighead, H. G. Science 2000, 290, 1532. (4) Wu, H.; Odom, T. W.; Chiu, D. T.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 554. (5) Khandurina, J.; Guttman, A. J. Chromatogr., A 2002, 943, 159. (6) Vlasov, Y. A.; Bo, X. Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289. (7) (a) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287. (b) Wen, J. G.; Lao, J. Y.; Wang, D. Z.; Kyaw, T. M.; Foo, Y. L.; Ren, Z. F. Chem. Phys. Lett. 2003, 372, 717. (c) Shen, G. Z.; Chen, Di.; Lee, C. J. J. Phys. Chem. B 2005, 109, 10779. (d) Shen, G. Z.; Chen, Di.; Lee, C. J. J. Phys. Chem. B 2006, 110, 15689. (8) (a) Bae, S. Y.; Seo, H. W.; Choi, H. C.; Park, J. J. Phys. Chem. B 2004, 108, 12318. (b) Gao, P. X.; Wang, Z. L. Appl. Phys. Lett. 2004, 84, 2883. (c) Lu, W. G.; Ding, Y.; Chen, Y. X.; Wang, Z. L.; Fang, J. Y. J. Am. Chem. Soc. 2005, 127, 10012. (d) Moore, D.; Ding, Y.; Wang, Z. L. Angew. Chem., Int. Ed. 2006, 45, 5150. (9) Choi, Y. C.; Kim, W. S.; Park, Y. S.; Lee, S. M.; Bae, D. J.; Lee, Y. H.; Park, G. S.; Choi, W. B.; Lee, N. S.; Kim, J. M. AdV. Mater. 2000, 12, 746. (10) Sharma, S.; Sunkara, M. K. J. Am. Chem. Soc. 2002, 124, 12288. (11) Nguyen, P.; Ng, H. T.; Yamada, T.; Smith, M. K.; Li, J.; Han, J.; Meyyappan, M. 2004, 4, 651. (12) Definition of the symmetry symbols: for example, in 4S*1-, the first character, such as 4 or 6, means the basic 4- and 6-fold symmetry of the major core nanowires, respectively. The second character S or M indicates the single or multiple rows of the secondary Ga2O3 nanorods. If there is nothing after S or M, all of the Ga2O3 nanorods are perpendicular to the major In2O3 core nanostructure. The third symbol, *, means that the secondary nanorods have an angle with the major core nanowire. The number 1 after the symbol * means that only one orientation of all the

766 J. Phys. Chem. B, Vol. 111, No. 4, 2007 secondary Ga2O3 nanorods branches has an angle with the core nanostructure. (13) Lin, Y. R.; Tseng, Y. K.; Yang, S. S.; Wu, S. T.; Hsu, C. L.; Chang, S. J. Cryst. Growth Des. 2005, 5, 579. (14) Liu, X.; Wu, X.; Cao, H.; Chang, R. P. J. Appl. Phys. 2004, 95, 3141. (15) Han, W. Q.; Fan, S. S.; Li, Q. Q.; Hu, Y. D. Science 1997, 277, 1287. (16) Hu, J. Q.; Li, Q.; Meng, X. M.; Lee, C. S.; Lee, S. T. J. Phys. Chem. B 2002, 106, 9536. (17) (a) Gao, Y. H.; Bando, Y. Appl. Phys. Lett. 2002, 81, 4133. (b) Hu, J. Q.; Bando, Y.; Liu, Z. W. AdV. Mater. 2003, 15, 1000. (18) Xu, L.; Su, Y.; Chen, Y. Q.; Xiao, H. H.; Zhu, L.A.; Zhou, Q. T.; Li, S. J. Phys. Chem. B 2006, 110, 6637.

Xu et al. (19) Pan, Z. W.; Dai, Z. R.; Ma, C.; Wang, Z. L. J. Am. Chem. Soc. 2002, 124, 1817. (20) (a) Hu, J. Q.; Bando, Y.; Zhan, J. H.; Yuan, X. L.; Sekiguchi, T.; Golberg, D. AdV. Mater. 2005, 17, 971. (b) Shen, G. Z.; Bando, Y.; Tang, C. C.; Golberg, D. J. Phys. Chem. B 2006, 110, 7199. (21) Xiang, X.; Cao, C. B.; Zhu, H. S. J. Cryst. Growth 2005, 279, 122. (22) Binet, L.; Gourier, D. J. Phys. Chem. Solids 1998, 59, 1241. (23) (a) Harwig, T.; Kellendonk, F. J. Solid State Chem. 1978, 24, 255. (b)Vasil’tsiv, V. I.; Zakharko, Ya. M.; Prim, Ya. I. Ukr. Fiz. Zh. 1988, 33, 1320. (c) Liang, C. H.; Meng, G. W.; Wang, G. Z.; Zhang, L. D.; Zhang, S. Y. Appl. Phys. Lett. 2001, 78, 3202. (d) Gundiah, G.; Govindaraj, A.; Rao, C. N. R. Chem. Phys. Lett. 2002, 351, 189.