Synthesis of carbon tubes using microwave plasma-assisted chemical vapor deposition
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Carbon tubes were successfully produced using microwave plasma-enhanced chemical vapor deposition on silicon, quartz, and ceramic substrates. The carbon tubes, about 80–100 nm in diameter and a few tens of microns in length, were formed under methane and hydrogen plasma at 720 °C with the aid of iron oxide particles. In this approach, an average tube density of about 109 cm−2 was obtained. The crooked and nonuniform diameters of some tubes suggested that they were composed of incompletely crystallized graphitic shells due to existing defects. The characteristic of the tubes grown upward on the silicon substrate accounted for a remarkably large electron field emission current of 0.1 mA/cm2 from the surface of the tube sample at a low turn-on field of 3 V/m.
I. INTRODUCTION
Since the initial research in carbon nanotubes by Iijima in 1991,1 an intensive investigation on carbon nanotubes has been made because of its fundamental research interest and potential application.2 Arc-discharge was first employed for producing carbon nanotubes. It is a common and efficient technique, especially for producing a high percentage of single-walled nanotubes.3– 6 Carbon vaporization process using laser,7–9 electron beam,10 and solar energy11 is another important technique for high production yield of carbon nanotubes. Pyrolysis was also reported to be able to prepare nanotubes.12,13 Electrolysis of graphite electrodes in molten ionic salts has been demonstrated to be capable of forming nanotubes in the liquid phase.14 Recently, several chemical vapor disposition (CVD) techniques have been developed for nanotube fabrication from methane.15,16 In contrast, the microwave plasma-assisted CVD technique, which is widely used for the preparation of diamond films, has seldom been reported for nanotube production so far. In this paper we shall give a detailed description of carbon tube production from a mixture of hydrogen and methane using the microwave plasma CVD. By this method, the carbon tubes can grow on the surfaces of almost any hightemperature solid materials (up to 750 °C), such as Si, quartz, ceramic plate, metallic plate, etc., with the aid of fine iron oxide catalyst particles.
surface of the substrates which could be silicon wafer, quartz, ceramic, or metallic plate. The substrate used in this study was an n+ type (100) orientated silicon wafer with a resistivity of 0.02 ⍀ cm−1. The gas flow ratio of H2/CH4, deposition pressure, microwave power, and deposition time were 40/1, 15 torr, 1.3 kW, and 10 min, respectively. The substrate temperature monitored using a thermal couple buried in the sample holder was kept at 720 °C. The scanning electron microscopy (SEM) and transmission electron microscope (TEM) images were obtained using the LEO982 and JEM-2010 system, respectively. The Ranishaw micro-Raman spectrometer was used to examine the Raman spectra excited by an Ar+ laser (514.5 nm line). X-ray diffraction (XRD) was performed using the vertical Siemens D5005 x-ray diffractometer. The diffraction data were collected using C
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