Journal of Nanoparticle Research

, Volume 11, Issue 5, pp 1179–1183

Synthesis and morphology evolution of GaN/C nanocables

Authors

  • Xuefeng Du
    • Shanghai Institute of CeramicsChinese Academy of Sciences
    • College of Material Science and EngineeringShanghai University
    • Shanghai Institute of CeramicsChinese Academy of Sciences
  • Tao Yang
    • Shanghai Institute of CeramicsChinese Academy of Sciences
  • Yue Shen
    • College of Material Science and EngineeringShanghai University
  • Yi Zeng
    • Shanghai Institute of CeramicsChinese Academy of Sciences
  • Fangfang Xu
    • Shanghai Institute of CeramicsChinese Academy of Sciences
Research Paper

DOI: 10.1007/s11051-008-9519-4

Cite this article as:
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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Abstract

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.

Keywords

GaN/C nanocablesChemical vapour depositionCoatingsDecompositionNanostructure

Introduction

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.

Experiment

An induction furnace was used for the synthesis of GaN/C nanocables, as shown in Fig. 1. The Ga2O3 powder (1.00 g, 99.999%) was filled in a graphite crucible with graphite powders. The graphite crucible was enclosed in a graphite susceptor, and the susceptor was put in the work coil center of the furnace with a flowing N2 atmosphere (1,000 mL/min from the bottom and 500 mL/min from the top). The bottom flowing N2 acted as carrier gas to transfer Ga-contained clusters to the position where GaN was formed. The top flowing N2 acted as protective gas to keep the top optical window clean for temperature measurement. The furnace was heated with the temperature at work coil center up to 1,250 °C and kept 90 min under the continuous flow of NH3 (200 mL/min). There was a temperature gradient which decreases from 1,250 °C at the work coil center to 400 °C at the top of the graphite susceptor. Ga2O3 powders were evaporated at 1,250 °C at work coil center. A dark and wool-like product was deposited on the wall of the graphite reactor where the temperature was about 600–1,050 °C. Besides, GaN/C nanocables were mainly collected in the temperature region of 600–800 °C, while waved GaN nanowires and discontinuously ordered nanodots with carbon sheaths were collected in the temperature region of 900–1,050 °C. The synthesized products were characterized by using scanning electron microscopy (SEM, JSM-6700F) and transmission electron microscopy (TEM, JEM-2010).
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9519-4/MediaObjects/11051_2008_9519_Fig1_HTML.gif
Fig. 1

Schematic diagram of the vertical induction furnace utilized in the experiments

Results and discussion

Figure 2a shows a typical scanning electron microscopy image of the GaN/C nanocables. The nanocables have a mean diameter of 80 nm and a length of tens of micrometers. The core-shell structure of the nanocables is clearly presented by TEM image, as shown in Fig. 2b. The inset of Fig. 2b shows a nanocable with a round catalyst head, indicating that the nanocable is grown by a catalyst-assisted (Vapor–Liquid–Solid) VLS process. Figure 2c shows a magnified TEM image of a GaN/C nanocable. We can find that the nanocable is composed of GaN nanowire core and carbon nanotube shell with a clear interface. The thickness of the carbon nanotube is about 5 nm. The Fig. 2d depicts the high-resolution TEM image of the GaN/C nanocable. The inset of Fig. 2d gives the corresponding electron diffraction (ED) pattern from [1\( \overline{1} \)0] zone axis of the GaN nanocable. The measuring d-spacings of 0.26 nm and 0.16 nm correspond well to (002) and (110) planes of the wurtzite GaN, respectively. The Parallel carbon layers on the surface of GaN nanowire have an interplanar spacing of 0.33 nm, which matches well with d002 of the graphite. The perfect diffraction pattern and lattice fringes indicate that the GaN nanowire is a single crystal.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9519-4/MediaObjects/11051_2008_9519_Fig2_HTML.jpg
Fig. 2

a SEM image of GaN/C nanocables. b TEM image of GaN/C nanocables. Inset: a nanocable with a round tip. c High-magnification TEM image of a GaN/C nanocable. d HRTEM image of a GaN/C nanocable. Inset of d: the corresponding ED pattern taken from [1\( \overline{1} \)0] zone axis

The various morphologies evolved from GaN/C nanocables are shown in Fig. 3. GaN nanowire cores decompose at the high temperature region around 1,050 °C. According to the extent of decomposition, GaN/C nanocables gradually evolved into sawtoothlike-GaN nanocable, waved GaN nanocable and beanlike-GaN nanocable, as shown in Fig. 3a, b, and c, respectively. The higher magnification TEM images of wavelike-GaN nanocable are displayed in Fig. 3d and e. The gaps are vaulted and 20 nm in the depth. HRTEM analysis shows that waved GaN nanocables have crystalline GaN core and carbon sheath with the d-spacing as 0.33 nm, as displayed in Fig. 3f. The crystal structure in the center of the partially decomposed GaN nanowire remains, but the crystal structure of the edge is blurry.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9519-4/MediaObjects/11051_2008_9519_Fig3_HTML.jpg
Fig. 3

ac TEM images of waved GaN nanocables and ordered beanlike-GaN nanocables. d, e high-magnification TEM image of waved GaN nanocables. f HRTEM image of a waved GaN nanocable

A simple and apprehensible schematic model has been established for describing the growth and evolution process of GaN/C nanocables, as shown in Fig. 4. Firstly, the GaN nanowires grew with a VLS mechanism, in which liquid catalyst droplets attached to nanowire tips, as revealed by TEM observations in the inset of Fig. 2b. Following reactions may be involved in this step:
$$\hbox{Ga}_{2} \hbox{O}_{3} + \hbox{C} \to \hbox{Ga}_{2} \hbox{O} + \hbox{CO} $$
(1)
$$\hbox{Ga}_{2} \hbox{O} \to \hbox{Ga} + \hbox{Ga}_{2} \hbox{O}_{3} $$
(2)
$$ \hbox{Ga}_{2} \hbox{O}_{3} + 2\hbox{N}\hbox{H}_{3} \to 2 \hbox{Ga}\hbox{N} + 3\hbox{H}_{2} \hbox{O} $$
(3)
$$\hbox{Ga}_{2} \hbox{O} + 2\hbox{N}\hbox{H}_{3} \to \hbox{Ga}\hbox{N} + \hbox{H}_{2} \hbox{O} + 2\hbox{H}_{2} $$
(4)
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9519-4/MediaObjects/11051_2008_9519_Fig4_HTML.gif
Fig. 4

Schematic model for illustrating the growth and evolution of GaN/C nanocables

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.

Conclusion

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.

Acknowledgments

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.

Copyright information

© Springer Science+Business Media B.V. 2008