Structural Studies of Submonolayers of Carbon Atoms on Graphite
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STRUCTURAL STUDIES OF SUBMONOLAYERS OF CARBON ATOMS ON GRAPHITE
M.Ge, K.Sattler, J. Xhie, and N.Venkateswaran University of Hawaii, Department of Physics &Astronomy 2505 Correa Road Honolulu, Hawaii 96822, USA ABSTRACT
Submonolayer coverages of carbon adsorbed on highly-oriented pyrolytic graphite were examined by scanning tunneling microscopy under ultra-high vacuum condition. Linear carbon wires were found on atomically flat graphite surfaces. The wires had different thicknesses, from single atomic width to about lnm. The long wires extended to over several hundred nanometers. Two directions, graphite p3-p direction and 300 rotated, were preferred for the long wire orientation. Parallel wire alignment, with several nanometers of inter-wire spacings were observed. Carbon particles, from 0.7 to 2 nm in diameter were found to be attached to the carbon wires. Particles from different wires formed parallel linear chains about perpendicular to the wire direction. INTRODUCTION Graphite and diamond are considered as the two basic carbon crystal forms. They have been extensively studied for a long time. Since the recent discovery of solid C60 and other fullerenes [1-3], new forms of carbon have been the focus of intense interest. Carbon tubules with a few nanometers in diameter were observed on a carbon-arc electrode [4]. Theoretical calculation of a fullerene tubule structure predicted that the carbon tube has carrier density similar to that of metals [5]. A negative-curvature fullerene structure with 168 carbon atoms in one unit cell was also proposed [6]. One approach for understanding novel carbon forms is to study their formations, especially the early growth stages, from atoms to clusters and to nanoscale crystallites. Properties of free carbon cluster have been investigated by mass spectrometry and other related techniques [7-9]. Different size carbon clusters have been theoretically proposed [10-15]. Calculations [10] show that Cn clusters favor one-dimensional structures for n20, closed fullerene cages have greater stability than both one- and two-dimensional structures (such as graphite flakes and buckled fullerene 'caps'). Linear and 'fullerene' structures should coexist for the critical cluster size n=20. It has also been predicted that Cn clusters change from linear to monocyclic rings at n=10 [14]. Although many experiments and theoretical studies have been carried out, structures of carbon clusters are still far from being well understood. Among the previous studies of carbon clusters, almost no experiment has been done to directly determine their structures on atomic scale. However such direct study should play an important role in carbon cluster research. Scanning Tunneling Microscopy (STM) is a perfect tool to determine atomic configurations of supported carbon clusters. We performed an experiment to investigate carbon clusters supported on graphite by STM under ultra-high vacuum (UHV) condition. Our samples were prepared by vapor deposition of carbon atoms on highly-oriented pyrolytic graphite Mat. Re
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