Elsevier

Carbon

Volume 46, Issue 6, May 2008, Pages 841-849
Carbon

Noncatalytic synthesis of carbon nanotubes, graphene and graphite on SiC

https://doi.org/10.1016/j.carbon.2008.02.013Get rights and content

Abstract

Graphene and carbon nanotubes (CNT) can be produced by vacuum decomposition of SiC, but discrepancies and conflicting data in the literature limit the use of this method for CNT synthesis. A systematic study of the effects of SiC surface morphology and carbon transport through the gas phase leads to reproducible and controlled growth of arrays of small-diameter (1–4 walls) nanotubes, which show pronounced radial breathing modes in Raman spectra, on either carbon (0001¯) or silicon (0 0 0 1) face of 6H SiC wafers at 1400–1900 °C. These nanotube arrays have a very high density and are catalyst-free with no internal closures. They show a higher oxidation resistance compared to CNTs produced by catalytic chemical vapor deposition (CVD). Their integration with graphite/graphene or silica layers on SiC wafers is possible in a simple 2-step process and opens new horizons in nanoscale device fabrication.

Introduction

Remarkable potential of CNTs [1] for applications including electronics, electrical energy storage, composites, gas sensors, and field emitters [2] led to development of many methods for their synthesis. However, the yield, purity, controlled synthesis and integration of CNTs on wafers for device applications remain challenging. It is known that both graphene [3] and CNTs can be produced by high temperature decomposition of SiC by the reaction [4]:SiCSi(g)+Cwith most attention directed towards graphene during the past year or two [5]. Kusunoki et al. [6], [7], [8] observed formation of CNTs growing normally to the carbon terminated (0001¯) C-face of hexagonal SiC with primarily zigzag chirality [9] and graphite growth on the Si terminated (0 0 0 1) Si-face. In contrary, Derycke et al. [10] reported single-wall carbon nanotubes (SWCNTs) with 1.2–1.6 nm diameter on the Si-face growing parallel to the crystal surface. Nagano et al. claimed CNT formation on both carbon and silicon faces of hexagonal 6H–SiC [11] and cubic 3C–SiC [12] after HF etching, with tubes growing normal to the surface. However, transmission electron microscope (TEM) images of “nanotubes” show anisotropic graphite lamellas instead [13]. While TEM evidence of small-diameter CNTs has been presented [6], [7], [8], only a single Raman spectrum containing two weak peaks which were assigned to RBM modes of nanotubes has been published [14], raising a question about the content of nanotubes in these films. Our recent work on vacuum decomposition of β-SiC whiskers showed well ordered graphene layers but no nanotube formation [15]. The most recent study [16] suggested the growth of CNTs at 1400–1700 °C on both C- and Si-faces of 4H SiC, but with different growth rates, following the reaction:SiC+1/2O2(g)SiO(g)+C(s)No satisfactory explanation of differences in the carbon growth on Si- and C-terminated surfaces of SiC has been offered so far. The mechanism of nanotube formation on SiC is not clear and results suffer from poor reproducibility.

Here, we report on a systematic experimental study of vacuum decomposition of 6H SiC single crystals that elucidates the nanotube formation mechanisms and conditions for the growth of nanotubes.

Section snippets

Experimental

6H-SiC single crystal wafers, 0.37 mm in thickness with epi-ready polished (0 0 0 1) Si-face and optical polished (0001¯) C-face on axis without any dopants (resistivity > 105 Ω cm) were obtained from Intrinsic Semiconductor Corporation (Fig. 1). The as-received samples were heated at six temperatures 1400, 1500, 1600, 1700, 1800, and 1900 °C for 4 h in a Solar Atmospheres (US) vacuum (10−6 and 10−4 Torr) furnace with an electric resistance carbon heater. The heating rate was 5 °C min−1. Arkema multi-wall

Results and discussion

Raman microspectroscopy analysis (Fig. 2a) showed that the decomposition rate of SiC on the C-face at 10−6 Torr is higher than that on the Si-face. G-band of graphite is clearly seen on the C-face after annealing at 1400 °C and carbon completely shields SiC bands in the samples annealed at 1600 °C and above (Fig. 2b). On the Si-face, G-band only becomes pronounced at 1600 °C, but formation of graphene layers can be observed on the Si-surface at 1400 °C by monitoring second order peaks of graphite

Summary and conclusions

Our work shows that the structure (graphene, graphite or CNT) and thickness of carbon coating on SiC can be controlled by changing the silica structure, its thickness or removal rate; Si evaporation rate, temperature of the substrate, composition of the environment or by producing a diffusion barrier for Si (graphene formation or pre-deposited coating). As a result, either graphene (or relatively thick graphite) films or CNT carpets can be produced on C- and Si-faces of SiC (Fig. 11). This

Acknowledgments

We thank Prof. J.E. Fischer, Dr. M. Nikiforov (University of Pennsylvania), and Mr. M. Kurtoglu for helpful discussions; Prof. C. Li (both at Drexel University) for providing access to the thermobalance, Solar Atmospheres for the vacuum furnace, Dr. E. Flahaut (Toulouse, France), Arkema and Superior Graphite for providing carbon samples, and Instrinsic for SiC wafers. Materials Characterization Facility of the A.J. Drexel Nanotechnology Institute provided access to microscopes and spectrometers

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