Synthesis and morphology evolution of GaN/C nanocables
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- Du, X., Zhu, Y., Yang, T. et al. J Nanopart Res (2009) 11: 1179. doi:10.1007/s11051-008-9519-4
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GaN/C nanocables were synthesized via a thermochemical process. The GaN/C nanocables were composed of single crystalline GaN nanowire cores with a mean diameter of 80 nm and parallel carbon sheathes with a thickness of several nanometers. We find that GaN nanocables were partially evolved into waved GaN nanowires and discontinuously ordered nanodots within the carbon sheaths due to the decomposition of GaN at high temperature regions. Both the carbon sheathes and GaN nanowire cores show a high degree of crystalline perfection. This method may be applied to coat a wide range of nanostructures with carbon sheathes and prepare various hetrostructures, which may serve as potential building blocks in nanodevices.
KeywordsGaN/C nanocablesChemical vapour depositionCoatingsDecompositionNanostructure
Gallium nitride(GaN), as an important wide band-gap semiconductor, is an intriguing target for nanostructure research because of its extensive use in Utlraviolet (UV) or blue photon emitters, photodetectors, high-speed field-effect transistors, and high temperature/high power electronic devices (Ren et al. 1998; Nakamura 1998; Suenaga et al. 1997; Son et al. 2006; Kim et al. 2004; Huang et al. 2002). GaN nanostructures with various morphologies have been reported, such as nanobelts (Bae et al. 2002; Li et al. 2001), nanotubes (Yin et al. 2004a, b; Liu et al. 2006), and nanowires (Xiang et al. 2006; Cai et al. 2006; Sekiguchi et al. 2004; Seryogin et al. 2005; Kuykendall et al. 2003; Duan and Lieber 2000; Nam et al. 2004; Kim et al. 2003; Zhou et al. 2005; Chang and Wu 2003). Various methods have been used to fabricate GaN nanowires, such as chemical vapor deposition (Yin et al. 2004a, b; Liu et al. 2006; Xiang et al. 2006; Cai et al. 2006; Sekiguchi et al. 2004), catalytic hydride vapor phase epitaxy (Seryogin et al. 2005), metal organic vapor deposition (MOCVD) (Kuykendall et al. 2003), and laser-assisted catalytic growth (Duan and Lieber 2000). Nanostructures, including nanowires and nanotubes, often display high chemical reactivity due to their low dimensionality and a high surface-to-volume ratio. The reactivity may lead to oxidation and contamination and dramatic changes in morphologies and properties of nanostructures. Thus, it is extremely important to have a protective sheath made of thermally and chemically stable materials on growing semiconductor nanowires to enhance their performances (Suenaga et al. 1997; Hu et al. 1999; Yin et al. 2004a, b; Liao et al. 2007). Graphite coatings could act as chemically inert protecting layers for GaN nanowires, and several groups have coated GaN nanowires with graphite layer by various methods, such as microwave plasma-enhanced chemical vapor deposition (Zhi et al. 2003), arc discharge in nitrogen atmosphere (Han et al. 2000), two-step catalytic reaction (Chen et al. 2001), and annealing experiment based on surface decoration with small metal clusters (Sutter et al. 2007). In this letter, we reported the preparation of GaN/C nanocables by a carbon-assisted chemical vapor deposition, a convenient catalyst-free process.
Results and discussion
The Ga liquid droplets were produced from the decompositions of starting reactants at 1,250 °C, which would be carried to the deposition region by flowing gas. Ga-contained clusters were carried by flowing gas to deposition region and adsorbed by Ga liquid droplets. These clusters reacted with NH3 forming GaN nanowires under the catalysis of Ga droplets. Secondly, carbon nanotubes sheathed on GaN nanowires via a (Vapor–Solid) VS process. When the reaction of NH3 and graphite provided a continuous C source, C layers deposited on GaN nanowires. The adsorption of carbon species on the surface of GaN nanowires may decrease the energy barrier for the formation of carbon nanotube sheathing (Yin et al. 2004a, b). Thirdly, GaN nanowires decomposed and evolved into wavelike-GaN nanocables and beanlike-GaN nanocables with the increasing of the temperature at the deposition region. Surface melting and decomposition is the general feature in nanosized materials. Surface atoms generally melt and decompose first at temperatures just below the melting and decomposition temperature of bulk material. Besides, studies of molecular dynamics simulations indicate that the thermal stability of GaN nanowires is strongly size dependent (Wang et al. 2007). It is estimated that the GaN core began to decompose nearly 1,050 °C (Park et al. 2005). Nanowires generally display high chemical reactivity due to their low dimensionality and high surface-to-volume ratio. Thus, the carbon sheaths may thermally and chemically increase the stability of the GaN cores. With the carbon sheaths coated on the GaN cores, the nanowire cores decomposed partially, which led to the formation of waved GaN nanowires and discontinuously ordered nanodots within the carbon sheaths. The HRTEM image of wavelike-GaN nanocable in Fig. 3f reveals that the decomposition of the GaN nanowires was initiated at the surface edges and then spreads across the nanowire surface.
GaN/C nanocables were synthesized by a carbonitridation thermal-chemical process. GaN nanowires in the carbon nanotube turned into waved nanowires and ordered nanodots due to the decomposition of the GaN nanowire cores at 1,050 °C. GaN/carbon hetrostructures with GaN cores of different morphologies can be synthesized by controlling the temperature at the deposition region and the reaction time. The method could be applied to prepare various carbon-related core-shell semiconductor hetrostructures for potential building blocks in nanodevices.
The research was partially financially supported from the Nation Natural Science of China (20571082, 50772125), the Science and Technology Commission of Shanghai, and the National High Technology Research and Development Program of China.