Orientational Growth of Carbon Nanotube by Thermal CVD
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Orientational Growth of Carbon Nanotube by Thermal CVD Shen Zhu1, Ching-Hua Su2, J. C. Cochrane1, S. Gorti2, S. Lehoczky2, Y. Cui3, and A. Burger3 1
USRA, NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA Microgravity Science and Applications Department, Science Directorate, SD47, NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA 3 Center for Photonic Materials, Fisk University, Nashville, TN 37208, USA 2
ABSTRACT Carbon nanotubes are synthesized using thermal chemical vapor deposition. Various temperature and pressure are used to fabricate carbon nanotubes. It is found that the nanotubediameter distribution mainly depends on the growth-temperature. With the substrate surface normal either along or against the gravity vector, different growth orientations of multi-walled carbon nanotubes are observed by scanning electron microscopy although the Raman spectra are similar for samples synthesized at different locations. The sizes of these carbon nanotubes in each sample are quite uniform and the length of the tube is up to hundreds of micrometers. These results suggest the gravitation effects in the growth of long and small diameter CNT.
INTRODUCTION Due to the superior properties of carbon nanotubes (CNT), which could enhance the performance in numerous devices such as electronics and sensors, many efforts have been engaged in synthesizing particular structural or dimensional CNT [1-3]. Of the techniques available, chemical vapor deposition (CVD) has demonstrated several advantages of growing carbon nanotubes. For examples, it is able to fabricate not only aligned but also single-walled or multi-walled CNT by using different hydrocarbon gases [1-7]. In CVD growth, the CNT can be synthesized in different environments using a variety of growth systems. For a number of gaseous environments of different chemical compositions and concentrations, the growth conditions include a variety of temperatures and pressures ranging from 650 to 1000 °C, or higher, and several tens of mTorr to one atmosphere, respectively. The acetylene gas environment is considered to be associated with the synthesis of multi-walled CNT, while single-walled CNT are preferentially fabricated using the methane gas [3 – 8]. Dependent on the method used, two growth modes, named top-mode and base-mode, have been observed in CNT growth. In the top-mode, the growth of CNT is considered to occur at the top of nanotubes where the catalyst is found. On the base-mode, the catalyst from which CNT grow is adhered to the surface of the substrate. The former mode is better for fabricating long CNT because carbon vapor species interact more readily with the catalysts. The CVD method has also demonstrated the advantages in controlling growth orientations and in producing massive quantities of CNT [1, 3, 5 9, 10]. When a plasma source is used in the CVD system, CNT are usually grown in the direction where plasma applies. However, many defects were embedded in the grown carbon nanotubes because the catalyst particles were etched away constantly
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