Growth and characterization of aligned carbon nanotubes from patterned nickel nanodots and uniform thin films
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J.H. Wang Department of Chemistry, State University of New York, Buffalo, New York 14260
L.E. Calvet, J. Chen, J.F. Klemic, and M.A. Reed Departments of Applied Physics, Electrical Engineering and Physics, Yale University, New Haven, Connecticut 06520-8284 (Received 6 March 2001; accepted 4 September 2001)
Microstructures of well-aligned multiwall carbon nanotubes grown on patterned nickel nanodots and uniform thin films by plasma-enhanced chemical vapor deposition have been studied by electron microscopy. It was found that growth of carbon nanotubes on patterned nickel nanodots and uniform thin films is different. During growth of carbon nanotubes, a nickel particle sits at the tip of each nanotube, and its [220] is preferentially oriented along the plasma direction, which can be explained by a channeling effect of ions coming into nickel particles in plasma. The alignment of nanotubes is induced by the electrical field direction relative to substrate surface.
I. INTRODUCTION
Individual single-wall carbon nanotubes (SWNTs) and multiwall carbon nanotubes (MWNTs) have been demonstrated to be excellent electron emitters due to their large aspect ratio, high chemical stability, and small end radius.1–5 However, there has been no successful alignment of single-wall individual or bundle nanotubes. Fortunately, the growth of large arrays of well-aligned MWNTs in an area up to inches in dimension has been achieved on glass,6 nickel,7 and silicon8 by plasmaenhanced chemical vapor deposition (PECVD), and on silica,9 porous silicon,10 and porous alumina11,12 by chemical vapor deposition. However, the emission property of these arrays is not satisfactory, which is probably due to the high site density (>109/cm2) leading to small electrical enhancement at the tips. A site density of about 107/cm2 has been calculated to be the right number for optimal electron emission properties in the sense of both emission site and current density.13 The growth of arrays of well-aligned freestanding MWNTs with sharp tips and controlled diameter, length, and site spacing has been achieved through PECVD. This method provides a new approach for obtaining optimal emitter density of carbon nanotubes. For the aligned MWNTs grown on a uniform Ni layer, a catalyst particle at the tip of each nanotube was observed.6–8 Furthermore, the particle had a preferential a)
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J. Mater. Res., Vol. 16, No. 11, Nov 2001 Downloaded: 04 Apr 2015
orientation relative to the catalytically active surface,14 which was explained by a model that Ni surfaces except (220) will be quickly covered up and the only exposed surface is the catalytically active (220) surface. The authors also attributed the alignment of the subsequent carbon nanotubes to the catalytically active surface mechanism. If the catalytically active surface mechanism is correct, then orientation of aligned nanotubes should not be affected by plasma direction. However, recent observation of a titled
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