New Twin Structures in GaN Nanowires - The Journal of Physical

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New Twin Structures in GaN Nanowires Sheng Dai, Jiong Zhao, Mo-rigen He, Hao Wu, Lin Xie, and Jing Zhu* Beijing National Center for Electron Microscopy, The State Key Laboratory of New Ceramics and Fine Processing, Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Wurtzite-type gallium nitride (GaN) nanowires, with single crystalline and twin structures, were simultaneously synthesized via chemical vapor deposition (CVD) method. High-resolution transmission electron microscopy (HRTEM) was utilized to characterize different twin boundaries (TBs), (103) type TB in acute-angle twin structures, and (304) type TB in obtuse-angle twin structures. In special, the new (304) TB was reported and identified at atomic scale for the first time. With the assistance of molecular dynamics (MD) simulations, the growth mechanism to interpret the prevalence of these obtuse-angle twin nanowires with higher energy of TB is discussed.

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INTRODUCTION Gallium nitride (GaN), a significant wide band gap (3.4 eV) semiconductor, has attracted great research attention due to its application in short wavelength optoelectronics devices1 and high-temperature/high-power electronics.2,3 While down to one-dimensional scale, GaN nanowires have been demonstrated to be fundamental building blocks of photonic, optoelectronic, and electronic nanodevices4−7 because of their enhanced physical properties.8−10 According to the existing results, GaN nanowires have provided attractive opportunities for the applications in blue and ultraviolet light emitters, sensors, and lasers.11−13 The synthesis of GaN nanowires has been achieved by various techniques in the past decade, such as laser ablation,14 metal−organic chemical vapor deposition (MOCVD),15 hydride vapor phase epitaxy (HVPE),16 template-assisted organometallic vapor phase epitaxy (OMVPE),17 and plasma assistant molecular beam epitaxy (PAMBE).18,19 Compared to these methods, the regular chemical vapor deposition (CVD)20,21 is a simple and economical process with a low demand for experimental equipments. Unlike the high quality and well-oriented GaN nanowires obtained from MOCVD and PAMBE, the products of CVD synthesis have been found to be with different morphologies, structures, and defects. Nam et al. have reported the morphological evolution of GaN nanostructures by varying the ammonia (NH3) flow rate in the CVD process.22 They also characterized (001) stacking faults and the rotational twin structures in GaN nanowires.23 Needlelike bicrystalline GaN nanowires could also be synthesized via CVD method by Liu et al.24 The GaN nanowires, produced by the carbon nanotubes confined reaction in the CVD system, show discontinuous GaN crystallinity at the interface between the GaN crystal and the amorphous layer.25,26 According to the recent results, ordered twin domains in GaN nanowires have been observed to change the local band gap, acting as an inserted quantum well.19 Planar defects, such as stacking faults © XXXX American Chemical Society

and twin boundaries, are likely to introduce additional electronic states near the band edges27,28 and influence the luminescence, electrical, and field emission properties of GaN nanowires.19,24,27−29 Therefore, it is necessary to investigate the structures and reveal the planar defects, especially those unclear atomic configurations29,33 in GaN nanowires, which are synthesized via a CVD process. In this paper, three kinds of structures in GaN nanowires and different twin boundaries (TBs) were investigated by highresolution transmission electron microscopy (HRTEM) and molecular dynamics (MD) simulations. A new kind of TB and its growth mechanism has been disclosed. The study on TBs is a fundamental issue for understanding the properties and potential applications of GaN nanowires, especially in field emission display (FED) and optoelectronics.

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EXPERIMENTAL SECTION Our GaN nanowires were synthesized via the CVD process in a horizontal tube furnace. Gallium oxide (Ga2O3) was chosen as the starting reactant with the flowing NH3 gas. Gold (Au) clusters, whose radii were controlled less than 200 nm, had been previously deposited on a cleaned silicon (Si) substrate with only a native silicon oxide (SiOx) layer. The experimental setup is described schematically in Figure 1. Ga2O3 powder was located at the middle of the horizontal tube, where the temperature was highest. The Si substrate with deposited catalysts was placed downstream 10−15 cm from the source. The flow rate of NH3 was maintained at 100−150 sccm. The pressure in the reactor was about 0.5−1.0 atm. The temperature in the furnace was raised from the room temperature to 1100 °C in the first 40 min and then kept for 3 h. Received: April 2, 2013 Revised: May 24, 2013

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are able to be found at the tips of these nanowires (Figure 2c,d). This indicates that the vapor−liquid−solid (V−L−S) growth mechanism30 was involved during the formation of our GaN nanowires. As shown in Figure 3a−f, two different morphology types are revealed by the high-magnification SEM images. Actually, this distinction is corresponding to the different cross-section shapes in Figure 2c,d. Here, the line scanning function of EDS is utilized to measure the projected thicknesses of these nanowires so as to present this distinction. During the EDS analysis, the characteristic X-ray intensity of one element (e.g., Ga) is proportional to its content in the homogeneous materials. This indicates that more numbers of X-rays would be emitted from the thicker area of the sample. In fact, the generation depth and width of the characteristic X-ray are about 1.5 and 1.0 μm, respectively, when the sample is irradiating by an incident electron beam at an accelerating voltage of 30 kV.34 Therefore, the GaN nanowires with diameters less than 300 nm are completely inside the excited area. With the aim of collecting more signals, the scanning results were acquired from the Lα peak of Ga, the heavier element compared to N. Then, according to the results, it is obvious that the nanowire in Figure 3a has a convex surface (Figure 3c) while another nanowire in Figure 3e has a concave one (Figure 3g). By means of the large-angle tilting in the SEM, we found that the crosssection shape of the nanowire in Figure 3a is triangular. The developed facets (shown in Figure 3d), similar to that reported in the literature,31 can be observed at the two base angles and attributed to fluctuations in reaction conditions that affected the stability of the growth of a certain plane. On the other hand, the nanowire with the concave surface (Figure 3g) seems to be bicrystalline, comprised of two parts. As presented in Figure 3f,h, the cross section of this nanowire is dumbbell-shaped. Further TEM observations would reveal the structural information on these different GaN nanowires. Single Crystalline GaN Nanowires. TEM characterizations, including the high-resolution transmission electron microscopy (HRTEM) and the selected area electron diffraction (SAED), were used for the structural analysis of GaN nanowires. The SAED pattern (Figure 4b), taken along the [101̅] zone axis, demonstrates that the GaN nanowire with a triangular cross section has a good single crystallinity of wurtzite structure and grows along the [120] direction. Figure 4c is the corresponding HRTEM image of the dashed square region in Figure 4a, showing exactly the (010) lattice plane perpendicular to the wire axis [120] in the hexagonal crystal system. These structural results are in agreement with the published data,23,31,32 which have made further study to reveal the isosceles triangular cross section with (2̅12̅), (21̅2̅), and (001) facets. Acute-Angle Twin Structures in GaN Nanowires. For the GaN nanowires with dumbbell-shaped cross sections, careful TEM characterizations reveal that these bicrystalline nanowires comprise two different structures. The first kind is the acute-angle twin (AT) structure, as presented in Figure 5. Figure 5a is a typical TEM image, where a boundary, corresponding to the concave reentrant in Figure 3e, could be clearly observed along the wire axis. The twin structure can be verified evidently in the SAED pattern. Two sets of diffraction patterns, both along [010] zone axis (the yellow one and the blue one in Figure 5b), coexist and constitute this structure with (103̅) facet as the twin plane.

Figure 1. Schematic experimental setup for the growth of GaN nanowires.

After 3 h reaction, the furnace was cooled to room temperature and the resulting nanowires were obtained on the surface of the Si substrate. The chemical composition, morphology, and crystal structure of the products were characterized by using scanning electron microscopes (SEMs, JEOL JSM-6301F at 15 kV and Hitachi S-5500 at 30 kV), an Xray energy dispersive spectrometer (EDS, Oxford INCAR 300), and a transmission electron microscope (TEM, JEOL JEM2010F).

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RESULTS AND DISCUSSION The low-magnification SEM image, shown in Figure 2a, reveals that the nanowires were grown densely on the Si substrate. The

Figure 2. (a) Low-magnification SEM image of the as-synthesized GaN nanowires. (b) EDS spectrum and the quantitative results taken from the square area in (a). (c, d) High-magnification SEM images of GaN nanowires with catalyst particles on their tips (indicated by the arrows).

chemical composition of these products is checked by the EDS analysis, collected from the square area in Figure 2a. It confirms that the as-synthesized nanowires are only composed of nitrogen (N) and gallium (Ga). According to the quantitative results, both the atomic % of N and Ga are fairly close to 50%, indicating the high purity and no variation of the chemical composition in the GaN nanowires. From a careful observation in SEM, it could be seen that the diameter of the products ranges from 60 to 300 nm and the length extends to several micrometers. In addition, Au particles B

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Figure 3. (a, e) SEM images of GaN nanowires with different morphology. (c, g) Projected thickness profiles measured by EDS, taken across the red lines in (a) and (e), respectively. (b, f) High-magnification SEM images that show the triangular and the dumbbell-shaped cross section of GaN nanowires. (d, h) Schematic diagrams illustrating typical cross-section shapes of nanowires in (b) and (f). The arrows indicate the geometric orientation of the electron beam while operating the line scanning of element analysis in the SEM/EDS system.

reflected in the HRTEM image (Figure 5c). The measured dspacing of 5.19 Å between the adjacent lattice fringes corresponds to the (001) planes in the wurtzite-type GaN. It is clear that two mirror crystals are tightly joined along the sharp (103̅) TB, which has only one atomic layer. According to our HRTEM results, periodical unit cells could be identified clearly along the TB. The measured right triangle is illustrated in Figure 5e, where the longer right-angle side is 8.28 Å = [(3√3)/2]a and the hypotenuse is 9.77 Å = [c2 + ((3√3)/2) a)2]1/2, considering the lattice constants a = b = 3.19 Å, c = 5.19 Å in wurtzite-type GaN. Figure 5d presents the projection of atomic models along [010] direction in single crystalline GaN, where the blue spheres are for N atoms and the gray ones are for Ga atoms. Undoubtedly, the highlighted triangle is consistent with the periodical unit cell in the experimental HRTEM image (Figure 5e). Obtuse-Angle Twin Structures in GaN Nanowires. In addition to the AT nanowires, large amounts of another kind of bicrystalline nanowires exist in our products. Their typical results of TEM characterizations are presented in Figure 6. Similar to the AT structure, the SAED picture (Figure 6b) is also composed of two sets of diffraction patterns, both along [010] zone axis. However, the main difference here is the angle between two sets of (001) planes, as illustrated in Figure 6c.

Figure 4. (a) Low-magnification TEM image of a GaN nanowire with a triangle-shaped cross section. (b) The corresponding SAED patterns of this GaN nanowire. (c) HRTEM image taken from the dashed-line region in (a).

The angle between two sets of (001) spots, as well as (001) planes in two crystals, is measured to be 64.0° so that we define this structure as the acute-angle twin (AT) structure. The symmetrical relationship of the AT nanowire is also well

Figure 5. (a) Low-magnification TEM image of an AT nanowire. (b) The corresponding SAED patterns showing the twin structure and the twin plane [103̅]. (c) HRTEM image taken along [010] zone axis of the AT nanowire in (a). (d) Projection of the atomic model along [010] direction in single crystalline GaN. Blue: N; gray: Ga. (e) Enlarged HRTEM image of the TB, illustrating the periodical unit cells along the twin plane. C

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Figure 6. (a) Low-magnification TEM image of an OT nanowire. (b) Acquired from the GaN nanowire in (a); SAED patterns are composed of two sets of the diffraction patterns along [010] zone axis. (c) HRTEM image taken from the square area in (a). (d, e) HRTEM image and the corresponding SAED patterns of another OT nanowire, showing the (304) TB in such structure.

Figure 7. (a) TEM image of the TB in OT structure taken along [010] zone axis. (b) Experimental HRTEM image of (304) TB showing the atomic configuration. The inset is the simulated result. Blue: N; gray: Ga. (c) Projection of the atomic model along [010] direction in single crystalline GaN, illustrating the periodic unit cell along (304) TB.

After measuring the angles between two 001 spots in SAED patterns from a great deal of GaN nanowires with such structure, we are able to obtain the result of 109.2° by averaging, while the maximum deviation is merely 0.7°. Therefore, we define this new kind of bicrystalline structure as the obtuse-angle twin (OT) structure. It seems that the

coincident spot, corresponding to the boundary, is unable to be found in Figure 6b. However, in fact, the g vector, representing the twin plane, is in the high index area of the reciprocal space. Under a shorter camera length, a larger area of the diffraction pattern of an OT nanowire (Figure 6d) is shown in Figure 6e. If we pick up the equivalent point (O′) as the new origin of the D

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Table 1. Calculated TB Energy (mJ/m2) of Wurtzite-Type GaN

reciprocal lattice, the coincident spot 304′ could be identified. This means (304) facet is certainly the twin plane of the OT structure. According to the simple calculations, the angle between (001) plane and (304) plane is 54.6°, just half the angle of 109.2° in the wurtzite-GaN. These analyses convincingly demonstrate that the obtuse-angle GaN nanowires are indeed twin structures. Our results, different from some other bicrystalline GaN nanostructures,29,33 are the first time to identify the (304) TB in wurtzite-type GaN nanowires. Furthermore, HRTEM technique is utilized to investigate this new TB in great details. Figure 7a is an HRTEM image of an OT nanowire, whose atomic configuration of the boundary could be recognized clearly. HRTEM simulations were conducted by using multislice method35 as implemented in the Mac Tempas program,36 and the best-matched simulated image is presented as the inset in Figure 7b. In such focus condition identified by simulations, dark contrast here corresponds to the positions of Ga atoms (the gray spheres). We can also mark out the periodical unit cells along the (304) facet in Figure 7b. The longer right-angle side here is 15.6 Å = 3c, and the hypotenuse is 19.1 Å = [(2√3a)2 + (3c)2]1/2, consistent with the result in the atomic model (Figure 7c). It is worthwhile to note that the (304) TB (Figure 7a) was not as sharp as the (103) TB (Figure 5c). This is because the atomic configurations of (304) TB is more complex. The atomic model of the AT structure can be established by the simple reflection, using the monolayer of (103) plane as the mirror. However, the model of OT structure could not be derived only with the mirror symmetry. The (304) type TB would have a structural relaxation in order to minimize the energy, avoiding the scenario that near-boundary atoms are fairly close to others. This relaxation phenomenon could also be found in our HRTEM result, as shown in Figure 7b. The Growth Mechanism. With the aid of theoretical calculations, the predicted possible TBs in wurtzite structures are (101), (102), and (103) planes.39 Computer simulations revealed that the (103) twin plane has a relatively low energy so that this kind of TBs is the most common type and always being observed in GaN,24 ZnO,47 and ZnS48 nanowires/ nanobelts. Therefore, it is essential to calculate the energy of the new (304) TB with the intention to understand why the OT nanowires could be synthesized in our experiments. The molecular dynamics (MD) simulations were carried out using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code37,38 of Sandia National Laboratories. Referring to the GaN simulation works of Béré and Serra,39−41 the empirical potential of Stillinger−Weber type, ideal for calculating the atomic structures of semiconductors, is employed in our simulations. The atomic models of different twin structures were built according to the experimental HRTEM images. Our results show that the TB energy of (103) twin plane is 1364 mJ/m2, in agreement with the existing data (1302 mJ/m2) in the literature.39 Of greater significance, the energy of (304) twin plane is 3429 mJ/m2, which is even higher than other structures, such as the (101) and (102) twins.39 Our simulation results indicate that the formation of (304) twin structures is dependent not only on the thermodynamic factors but also on the kinetic effect. This will facilitate the understanding of the growth mechanism of OT nanowire, a new structure with a higher energy state. It is interesting that the single-crystalline nanowires and twin nanowires were produced simultaneously in our synthesis.

twin boundaries present simulations calculations by Béré et al.39,41

(103) 1364 1302

(304) 3429

(101)

(102)

2709−3567

1954−2809

Then the statistical analysis was performed on the structure distribution of the products. Based on the TEM observation of over 50 nanowires, the results in Figure 8a show that OT structure with (304) TB accounts for more than 60% while the single-crystalline structure for 30%. The AT structure with (103) TB only occupied less than 5% of all the samples. Therefore, when we come to discuss the growth mechanism, the AT nanowires would not be included because the quantity is so few that this kind of structure is not representative for analysis. During the chemical vapor deposition, atoms from the vapor phase begin to condense on the substrate. The initial nuclei may be perfect (free of planar defects) or having stacking faults/twins,42 depending on the total free energy ΔGperfect and ΔGtwin estimated as ΔGperfect = 2πrhγsurface + πr 2γcatalyst + πr 2γsubstrate − πr 2hΔGv

(1)

′ ΔGtwin = 2πrhγsurface + πr 2γcatalyst + πr 2γsubstrate + 2rhγtwin − πr 2hΔGv

(2)

In eqs 1 and 2, ΔGperfect is the total free energy of a disk-shaped perfect nucleus with the radius r and height h and ΔGtwin is the total free energy of a twin nucleus with the same size. γsurface is the surface energy and γcatalyst is the interface energy at the nucleus/catalyst interface. γsubstrate and γ′substrate are the interface energy between the nuclei and substrate in the two cases, perfect nucleus and twin nucleus, respectively. ΔGv is the bulk free energy per unit volume driving the nucleation. γtwin is the twin boundary energy. Considering the same value of (2πrhγsurface + πr2γcatalyst − 2 πr hΔGv) in the two equations above, if the value of πr2γsubstrate in eq 1 is close to that of (πr2γsubstrate ′ + 2rhγtwin) in eq 2, then the difference between ΔGperfect and ΔGtwin will be negligible small and the twin nuclei may occur. In fact, although the twin nucleus will add the boundary energy 2rhγtwin, it could release the mismatch energy at the substrate/nucleus interface to reduce πr2γ′substrate so that ΔGtwin can be close to ΔGperfect. Hence, the probability of formation of twin nuclei would increase. In addition, the high flow rate in a CVD system, as close as our condition, could also induce the twin nuclei in the synthesis of GaN nanowires.22−24,29 The high flow rate will destroy the balance of the system energy for crystal growth and overcome the excess twin boundary energy 2rhγtwin. Therefore, on the basis of these analyses, it is plausible that the twin nuclei appeared at the nucleation stage in our CVD synthesis. In the growing stage of GaN nanowires, Nam et al. have proposed a defect-mediated V−L−S growth model to explicate the prevalence of defected nanowires.23 The results in our experiments can also be interpreted by this explanation. According to the conventional theories,23,42,43 the planar defects, such as TBs and stacking faults, which are higher energy states, could afford more nucleation sites during crystal E

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Figure 8. (a) Statistic result of the structure distribution on the as-synthesized GaN nanowires. (b) Schematic diagram showing the prevalence of the OT nanowires in the products.

Li Shu, and Mr. Yueliang Li for their helpful discussions and technical support.

growth. Here, the OT structure with a twinning plane could expose more nucleation sites at the catalyst/nanowire growing interface than a single crystalline GaN nanowire with fewer or no defects (as the schematic diagram in Figure 8) and therefore grow more quickly and to greater radii and lengths. Moreover, from the viewpoints of the Wagner−Hamilton− Seidensticker (W−H−S) theory,44,45 the reentrant groove at the twin boundary also plays a crucial role during the crystal growth in that it provides more energetically favorable sites on morphology than a flat facet. Fu et al. have reported a preferential growth of the boron carbide twin nanowires and proved the validity of this reentrant-groove-assistant mechanism in V−L−S synthesis.46 Therefore, taking into account these two factors, the twin boundary in structure and the reentrant groove in morphology, it is reasonable that the OT nanowires grow faster than the single crystalline ones and dominate the production in our experiments.

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(1) Nakamura, S.; Pearton, S.; Fasol, G. The Blue Laser Diode: The Complete Story; Springer-Verlag: Berlin, 2000. (2) Pearton, S.; Ren, F. GaN Electronic. Adv. Mater. 2000, 12, 1571− 1580. (3) Nakamura, S. The Roles of Structure Imperfections in InGaNBased Blue Light-Emitting Diodes and Laser Diodes. Science 1998, 281, 956−961. (4) Huang, Y.; Lieber, C. M. Integrated Nanoscale Electronics and Optoelectronics: Exploring Nanoscale Science and Technology through Semiconductor Nanowires. Pure Appl. Chem. 2004, 76, 2051−2068. (5) Qian, F.; Li, Y.; Gradecak, S.; Wang, D. L.; Barrelet, C. J.; Lieber, C. M. Gallium Nitride-Based Nanowire Radial Heterostructures for Nanophotonics. Nano Lett. 2004, 4, 1975−1979. (6) Yang, P. D. The Chemistry and Physics of Semiconductor Nanowires. MRS Bull. 2005, 30, 85−91. (7) Huang, Y.; Duan, X. F.; Cui, Y.; Lieber, C. M. Gallium Nitride Nanowire Nanodevices. Nano Lett. 2002, 2, 101−104. (8) Uenoyama, T. Excitonic Enhancement of Optical Gain in Quantum Wells. Phys. Rev. B 1995, 51, 10228−10231. (9) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. One-Dimensional Nanostructures: Synthesis, Characterization, and Applications. Adv. Mater. 2003, 15, 353−389. (10) Kim, J. R.; So, H. M.; Park, J. W.; Kim, J. J.; Kim, J.; Lee, C. J.; Lyu, S. C. Electrical Transport Properties of Individual Gallium Nitride Nanowires Synthesized by Chemical-Vapor-Deposition. Appl. Phys. Lett. 2002, 80, 3548−3550. (11) Lee, S. K.; Kim, T. H.; Lee, S. Y.; Choi, K. C.; Yang, P. D. HighBrightness Gallium Nitride Nanowire UV-Blue Light Emitting Diodes. Philos. Mag. 2007, 87, 2105−2115. (12) Johnson, J. C.; Choi, H. J.; Knutsen, K. P.; Schaller, R. D.; Yang, P. D.; Saykally, R. J. Single Gallium Nitride Nanowire Lasers. Nat. Mater. 2002, 1, 106−110. (13) Han, S.; Jin, W.; Zhang, D.; Tang, T.; Li, C.; Liu, X.; Liu, Z.; Lei, B.; Zhou, C. Photoconduction Studies on GaN Nanowire Transistors under UV and Polarized UV Illumination. Chem. Phys. Lett. 2004, 389, 176−180. (14) Duan, X. F.; Lieber, C. M. Laser-Assisted Catalytic Growth of Single Crystal GaN Nanowires. J. Am. Chem. Soc. 2000, 122, 188−189. (15) Kuykendall, T.; Pauzauskie, P. J.; Zhang, Y.; Goldberger, J.; Sirbuly, D.; Denlinger, J.; Yang, P. D. Crystallographic Alignment of High-Density Gallium Nitride Nanowire Arrays. Nat. Mater. 2004, 3, 524−528. (16) Kim, H.; Kim, D. S.; Kim, D. Y.; Kang, T. W.; Cho, Y. H.; Chung, K. S. Growth and Characterization of Single-Crystal GaN Nanorods by Hydride Vapor Phase Epitaxy. Appl. Phys. Lett. 2002, 81, 2193−2195.

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CONCLUSIONS To summarize, wurtzite-type GaN nanowires have been synthesized in our experiments via a simple CVD method. Three kinds of GaN nanostructures were carefully characterized by electron microscopy. A new twin structure, OT nanowire with (304) TB, has been identified at atomic scale for the first time. In addition, MD simulations have also been conducted to help us understand the growth mechanism of nanostructures with higher energy states. It is revealed that twin boundary plays a particular role during the growth of GaN nanorwires. Our results, especially the new OT nanowires, may give new insights into the formation of twin structures and have potential applications in field emission display and optoelectronics of GaN nanowires.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax +8610-62772507; Tel +8610-62794026 (J.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by National 973 Project of China (2009CB623701) and Chinese National Nature Science Foundation (50831001, 50971075). The research made use of the resources of the Beijing National Center for Electron Microscopy. The authors thank Dr. Ye Dai, Mr. Fuzhi Dai, Mr. F

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(17) Deb, P.; Kim, H.; Rawat, V.; Oliver, M.; Kim, S.; Marshall, M.; Stach, E.; Sands, T. Faceted and Vertically Aligned GaN Nanorod Arrays Fabricated without Catalysts or Lithography. Nano Lett. 2005, 5, 1847−1851. (18) Furtmayr, F.; Vielemeyer, M.; Stutzmann, M.; Arbiol, J.; Estradé, S.; Peiró, F.; Morante, J. R.; Eickhoff, M. Nucleation and Growth of GaN Nanorods on Si (111) Surfaces by Plasma-Assitant Molecular Beam Epitaxy − The Influence of Si- and Mg-Doping. J. Appl. Phys. 2008, 104, 034309. (19) Arbiol, J.; Estrade, S.; Prades, J. D.; Cirera, A.; Furtmayr, F.; Stark, C.; Laufer, A.; Stutzmann, M.; Eickhoff, M.; Gass, M. H.; et al. Triple-Twin Domains in Mg Doped GaN Wurtzite Nanowires: Structural and Electronic Properties of this Zinc-Blende-Like Stacking. Nanotechnology 2009, 20, 145704. (20) Chen, C. C.; Yeh, C. C. Large-Scale Catalytic Synthesis of Crystalline Gallium Nitride Nanowires. Adv. Mater. 2000, 12, 738− 741. (21) Chen, C .C; Yeh, C. C.; Chen, C. H.; Yu, M. Y.; Liu, H. L.; Wu, J. J.; Chen, K. H.; Chen, L. C.; Peng, J. Y.; Chen, Y. F. Catalytic Growth and Characterization of Gallium Nitride Nanowires. J. Am. Chem. Soc. 2001, 123, 2791−2798. (22) Nam, C. Y.; Tham, D.; Fischer, J. E. Effect of the Polar Surface on GaN Nanostructure Morphology and Growth Orientation. Appl. Phys. Lett. 2004, 85, 5676−5678. (23) Tham, D.; Nam, C. Y.; Fischer, J. E. Defects in GaN Nanowires. Adv. Funct. Mater. 2006, 16, 1197−1202. (24) Liu, B.; Bando, Y.; Tang, C.; Xu, F.; Hu, J.; Golberg, D. Needlelike Bicrystalline GaN Nanowires with Excellent Field Emission Properties. J. Phys. Chem. B 2005, 109, 17082−17085. (25) Han, W.; Fan, S.; Li, Q.; Hu, Y. Synthesis of Gallium Nitride Nanorods through a Carbon Nanotube-Confined Reaction. Science 1997, 277, 1287−1289. (26) Zhu, J.; Fan, S. J. Nanostructure of GaN and SiC Nanowires Based on Carbon Nanotubes. Mater. Res. 1999, 4, 1175−1777. (27) Bandic, Z. Z.; McGill, T. C.; Ikonic, Z. Electronic Structure of GaN Stacking Faults. Phys. Rev. B 1997, 56, 3564−3566. (28) Chu, W. H.; Chiang, H. W.; Liu, C. P.; Lai, Y. F.; Hsu, K. Y. Defect-Induced Negative Differential Resistance of GaN Nanowires Measured by Conductive Atomic Force Microscopy. Appl. Phys. Lett. 2009, 94, 182101. (29) Chen, Z.; Cao, C.; Zhu, H. Oriented Bicrystalline GaN Nanowire Arrays Suitable for Field Emission Applications. Chem. Vap. Deposition 2007, 13, 527−532. (30) Wagner, R. S.; Ellis, W. C. Vapor-Liquid-Solid Mechanism of Single Crystal Growth. Appl. Phys. Lett. 1964, 4, 89−90. (31) Nam, C. Y.; Jaroenapibal, P.; Tham, D.; Luzzi, D. E.; Evoy, S.; Fischer, J. E. Diameter-Dependent Electromechanical Properties of GaN Nanowires. Nano Lett. 2006, 6, 153−158. (32) Kuykendall, T.; Pauzauskie, P.; Lee, S.; Zhang, Y.; Goldberger, J.; Yang, P. Metalorganic Chemical Vapor Deposition Route to GaN Nanowires with Triangular Cross Sections. Nano Lett. 2003, 3, 1063− 1066. (33) Zhou, S. M.; Zhang, X. H.; Meng, X. M.; Fan, X.; Zou, K.; Wu, S. K. The Novel Bicrystalline GaN Nanorods. Mater. Lett. 2004, 58, 3578−3581. (34) Bethe, H. Handbook of Physics; Springer: Berlin, 1933. (35) Cowley, J. M. Diffraction Physics; North-Holland: New York, 1975. (36) O’Keefe, M. A.; Kilaas, R. Advances in High-Resolution Image Simulation. Scanning Microsc. 1988, 2, 225−244. (37) Plimpton, S. J. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1−19. (38) http://lammps.sandia.gov. (39) Béré, A.; Serra, A. Atomic Structures of Twin Boundaries in GaN. Phys. Rev. B 2003, 68, 033305. (40) Béré, A.; Serra, A. Structure of [0001] Tilt Boundaries in GaN Obtained by Simulation with Empirical Potentials. Phys. Rev. B 2002, 66, 085330.

(41) Béré, A.; Serra, A. On the Atomic Structures, Mobility and Interactions of Extended Defects in GaN: Dislocations, Tilt and Twin Boundaries. Philos. Mag. 2006, 86, 2159−2192. (42) Zhang, X.; Anderoglu, O.; Hoagland, R. G.; Misra, A. Nanoscale Growth Twins in Sputtered Metal Films. JOM 2008, 60, 75−78. (43) Xiao, R. F.; Alexander, J. I. D.; Rosenberg, F. Growth Morphologies of Crystal Surfaces. Phys. Rev. A 1991, 43, 2977−2992. (44) Wagner, R. S. On the Growth of Germanium Dendrites. Acta Metall. 1960, 8, 57−60. (45) Hamilton, D. R.; Seidensticker, R. G. Propagation Mechanism of Germanium Dendrites. J. Appl. Phys. 1960, 31, 1165−1168. (46) Fu, X.; Jiang, J.; Liu, C.; Yu, Z. Y.; Lea, S.; Yuan, J. Re-EntrantGroove-Assisted VLS Growth of Boron Carbide Five-Fold Twinned Nanowires. Chin. Phys. Lett. 2009, 26, 086110. (47) Ding, Y.; Wang, Z. L. Structures of Planar Defects in ZnO Nanobelts and Nanowires. Micron 2009, 40, 335−342. (48) Liu, B. D.; Bando, Y.; Liao, M. Y.; Tang, C. C.; Mitome, M.; Golberg, D. Bicrystalline ZnS Microbelts. Cryst. Growth Des. 2009, 9, 2790−2793. (49) Shan, X.; Zhang, X.; Gao, J.; You, L.; Xu, H.; Xu, J.; Yu, D.; Ye, H. Self-Assembled [2-1-10] Twin Junctions Formed by Intercrossing of ZnO Nanowires. J. Phys. Chem. C 2009, 113, 18014−18019.

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