Single-crystal nanorings of b-gallium oxide synthesized using a microwave plasma

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1 phys. stat. sol. (a) 203, No. 8, (2006) / DOI /pssa Single-crystal nanorings of b-gallium oxide synthesized using a microwave plasma F. Zhu *, Z. X. Yang, W. M. Zhou, and Y. F. Zhang National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Institute of Micro and Nano Science and Technology, Shanghai Jiaotong University, 1954 Huashan-Road, Shanghai , P.R. hina Received 1 November 2005, revised 3 pril 2006, accepted 5 pril 2006 Published online 6 June 2006 PS Dq, Hk, Lp, Hd, c In this study, a new method for synthesizing single-crystal ring-like nanobelts (here named nanorings), of only µm in diameter, is presented. microwave plasma method involving liquidphase gallium containing H 2 O in an r atmosphere, using silicon as the substrate, is used for the first time. The morphology and structure of the nanorings were analysed using scanning electron microscopy and X-ray diffraction. The morphology of nanobelts was also characterized using transmission electron microscopy (TEM) and high-resolution TEM in order to propose possible growth mechanisms of the nanorings. 1 Introduction The synthesis of novel single-crystalline semiconducting nanostructures such as nanorings, nanohelixes, nanospirals and nanobows has attracted much attention due to their size, special morphology-related properties and potential nanoscale applications [1 6]. ontrolled synthesis of these novel nanostructures is a key for the manufacturing of nanodevices. Therefore ring-like nanostructures with special morphology are regarded as an ideal system to describe clearly the fundamental growth mechanism of nanomaterials. Monoclinic gallium oxide ( ) is an important wide band gap (E g = 4.9 ev) semiconductor because of its good chemical and thermal stability. It has potential applications in optoelectronic devices including flat-panel displays, solar energy conversion devices, optical limiters for ultraviolet radiation and high-temperature stable gas sensors [7 12]. Recently, several reports have appeared on synthesis methods for crystalline gallium oxide nanostructures, for example, thermal evaporation, arc-discharge, laser ablation, carbon thermal reduction and metalorganic chemical vapour deposition [13 18]. However, studies of the synthesis of gallium oxide nanorings using microwave plasma have always been a blank area. In this paper, we demonstrate a synthesis process to produce nanorings of using a microwave plasma. This represents important progress towards structure control and nanostructure synthesis design. The products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). * orresponding author: zhuf2@sjtu.edu.cn, Phone: , Fax:

2 Original Paper phys. stat. sol. (a) 203, No. 8 (2006) Experimental The synthesis was carried out in a microwave plasma reactor with H 2 O. nalytical-grade gallium droplet (0.3 g, purity more than 99.99%) and a silicon substrate (6 mm 8 mm in size) were placed on a holder (40 mm in diameter) and positioned horizontally in the chamber of the microwave plasma reactor. fter the chamber of the microwave plasma reactor had been evacuated using a mechanical rotary pump to a pressure of 60 torr, H 2 O monomer and high-purity r carrier gas were kept flowing at a rate of 100 sccm (standard cubic centimetres per minute). The pump continually evacuated the system and the pressure inside the chamber remained at 40 torr during the experiment. The microwave power was adjusted according to the following three steps: (a) maintained at 200 W for 10 min; (b) maintained at 400 W for 30 min; (c) maintained at 600 W for 2 h to terminate the experiment. When the gallium droplet placed on the silicon substrate was exposed to the microwave plasma, the droplet forms intentionally nuclei which were placed on substrates and further synthesis experiments were carried out. The nanorings discussed in this paper were grown from these large gallium drops. The experiments were performed under the following conditions: microwave power of 600 W, pressure of torr, growth duration of 2 3 h, 100 sccm of H 2 O and 100 sccm of r. 3 Results fter the formation reaction, a white wool-like product was present on the surface of the silicon substrates around the residual gallium droplet. Figure 1 shows the XRD pattern taken from the products on the silicon substrates. Miller indices are indicated for each diffraction peak. Within experiment error, it can be seen that the whole spectrum can be indexed from peak positions to monoclinic crystalline, revealing the production of deposits. The lattice parameters of crystalline phase are a = Å, b = 3.04 Å, c = 5.8 Å and β = (JPDS: ). No crystalline phases were found within the detection limit except for silicon, which came from the substrate. Figure 2 shows SEM images of an as-prepared sample, revealing that these products are composed of some nanorings. The rings have diameters of µm. Figure 2 shows a low-magnification image of the nanorings deposited on the silicon substrate. The SEM images clearly record at high magnification the complete rings with uniform sharp and flat surfaces (Fig. 2 and ). The different width and thickness of the nanorings can be seen in the high-magnification SEM images in Fig. 2D F. The width and Intersity (arb. units) (-4 0 1) (-2 0 2) (-1 1 1) (4 0 1) (111) (2 0 2) (-112) (1 1 2) (5 1 0) (020) (-3 1 3) (-7 1 2) (-2 2 2) Si (0 0 4) (4 2 1) θ (degree) Fig. 1 XRD pattern of the as-deposited products. The silicon diffraction peaks come from the substrate.

3 2026 F. Zhu et al.: Single-crystal nanorings of β-gallium oxide 1 µm D E F Fig. 2 (online colour at: ) Low-magnification SEM image of as-prepared nanorings., ) High-magnification SEM images of single-crystal nanorings, showing their uniform sharpness. D F) High-magnification SEM images of nanorings of different width and thickness. The nanoring dimeter is µm, the thickness of the nanoring is nm and the width of the nanoring shell is nm. 0.20nm (112) [110] 0.29nm [112] 0.29nm 2 nm 2 nm Fig. 3 (online colour at: TEM and HRTEM images of as-prepared twisted nanobelts: ) low-magnification TEM image of twisted nanobelt; ) HRTEM image of the twisted nanobelt, the inset showing the Fourier diffractogram obtained from the HRTEM image; ) the legible flat plans of the nanobelt surface.

4 Original Paper phys. stat. sol. (a) 203, No. 8 (2006) 2027 O 2- (112) O2- O 2- (-1-10) _ D Fig. 4 (online colour at: ) Structure model of and corresponding crystal planes discussed in the text, showing the ± polar surfaces. D) The growth process and corresponding SEM results showing the initiation and formation of the single-crystal nanoring via self-coiling of a polar nanobelt. The nanoring is initiated by folding a nanobelt into loop with overlapped ends driven by electrostatic interaction among the polar charges. thickness of the nanorings could be deduced from these images: about nm in width and nm in thickness. Figure 3 shows TEM images of twisted nanobelts. Low-magnification TEM and HRTEM images of a twisted nanobelt are displayed in Fig. 3 and. The inset shows the Fourier diffractogram, which was obtained from the HRTEM image of Fig. 3. Figure 3 shows the legible flat plans of the nanobelt surface. The growth direction of the twisted nanobelts was [112] and the upright direction was [110], the arrows indicating each. The spacing of about 0.29 nm and 0.20 nm between the arrowheads corresponds to the distance between planes and between (112) planes.

5 2028 F. Zhu et al.: Single-crystal nanorings of β-gallium oxide s the twisted nanobelt was one part of the nanoring, it might reveal the partial structure of the total nanoring. 4 Discussion From our point of view, the ring-like nanobelts could not be interpreted by the present growth mechanism of nanobelts. The growth mechanism of the nanorings was more similar to that of ZnO nanorings [1]. The growth of the nanoring structures could be understood on the basis of the polar surface of the nanobelt. The monoclinic Ga 2 crystal can be described as alternating planes composed of O 2 and ions, which are stacked alternately along the [110] axis (Fig. 4). The oppositely charged ions form positively charged -Ga and negatively charged -O polar surfaces on the top and bottom surfaces of the nanobelt (Fig. 4). Then the polar nanobelt can build up the nanorings and grow along [112]. s the nanobelt continues to lengthen, it needs to fold itself in order to minimize the area of the polar surface. The folded nanobelt is shown in Fig. 4. While the nanobelt further folds itself, it becomes to interface the positively charged -Ga plane (top surface) with the negatively charged -O plane (bottom surface), which results in neutralization of the local polar charges and reduction of surface area. Thus a loop with an overlapped end is formed (Fig. 4). s growth continues, the nanobelt might be naturally attracted to the rim of the nanoring by electrostatic interactions to neutralize the local polar charge and form a self-coiled nanoring (Fig. 4D). This is a possible mechanism for the formation of a nanoring. However, the exact causes for the formation of the nanorings are yet not clear. To understand fully the growth mechanism of the nanorings, further studies are needed and the growth process parameters should also be optimized. In summary, single-crystalline nanorings were synthesized by a microwave plasma method for the first time. The diameter of the nanorings is typically µm, the thickness is nm and the width of the nanoring shell is nm. possible growth mechanism of the nanorings is the polar charge inducing the formation of self-coiling ring-like nanobelts, which is similar to that for ZnO nanorings. The nanorings investigated here have potential applications in nanoscale smart devices. cknowledgements This work was supported by the Shanghai Municipal ommission for Science and Technology (Grant Nos. 03DZ14025 and 0452nm056), and the National asic Research Program of hina (No ). References [1] X. Y. Kong, Y. Ding, R. S. Yang, and Z. L. Wang, Science 303, 1348 (2004). [2] X. Y. Kong and Z. L. Wang, Nano Lett. 3, 1625 (2003). [3] X. Y. Kong and Z. L. Wang, ppl. Phys. Lett. 84, 975 (2004). [4] W. L. Hughes and Z. L. Wang, J. m. hem. Soc. 126, 6703 (2004). [5] P. X. Gao, Y. Ding, W. Mai, W. L. Hughes,. Lao, and Z. L. Wang, Science 309, 1700 (2005). [6] R. Yang and Z. L. Wang, J. m. hem. Soc. 128, 1466 (2006). [7] D. Edwards, T. O. Mason, F. Goutenoir, and K. R. Peoppel, ppl. Phys. Lett. 70, 1706 (1997). [8] M. O. Gita, N. Saika, Y. Nakanishi, and Y. Hatanaka, ppl. Surf. Sci. 142, 188 (1999). [9] Z. J. Hajnal, J. Miró, G. Kiss, F. Réti, P. Deak, R.. Herndon, and J. M. Kuperberg, J. ppl. Phys. 86, 3792 (1999). [10] L. inet and D. Gourier, J. Phys. hem. Solids 59, 1241 (1998). [11] M. Ogita, K. Higo, Y. Nakanishi, and Y. Hatanaka, ppl. Surf. Sci. 175/176, 721 (2001). [12] M. Fleischer, S. Kornely, T. Weh, J. Frank, and H. Meixner, Sens. ctuators 69, 205 (2000). [13] G. Gundiah,. Govindaraj, and. N. R. Rao, hem. Phys. Lett. 351, 189 (2002). [14] J. Q. Hu, Q. Li, X. M. Meng,. S. Lee, and S. T. Lee, J. Phys. hem. 106, 9536 (2002). [15] G. S. Park, W.. hoi, J. M. Kim, Y.. hoi, Y. H. Lee, and.. Lim, J. ryst. Growth 220, 494 (2000). [16] Z. X. Yang, F. Zhu, Y. J. Wu, W. M. Zhou, and Y. F. Zhang, Physica E 27, 351 (2005). [17] X. Xiang,.. ao, Y. J. Guo, and H. S. Zhu, hem. Phys. Lett. 378, 660 (2003). [18] K. W. hang and J. J. Wu, dv. Mater. 17, 241 (2005).