Generation of circular and hexagonal microholes in a graphite surface
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Circular and hexagonal microholes, ranging in size from several nanometers to micrometers, were generated in a graphite surface by electron beam irradiation. The depth of the holes was always 0.34 nm, indicating that only the uppermost layer was modified. The shape of the holes was switched from circular to hexagonal when the size reached about 500 nm. Although the hexagons appeared at random on the surface, they were aligned in one direction, implying that the formation and growth of the holes were determined by the atomic structure of graphite. The results described in this study will be used to modify the surface structure of graphite for many applications.
I. INTRODUCTION
A single piece of graphite sheet, also known as graphene, has a two-dimensional hexagonal honeycomb structure consisting of sp2-hybridized carbon atoms. If one could control the geometry of graphene on a nano- or micrometer scale, one could obtain unique structures with well-defined physical and chemical properties.1,2 This possibility is well illustrated by the properties of fullerenes and carbon nanotubes. For instance, carbon nanotubes, which are made of cylindrical graphitic sheets,3 were predicted by theoretical calculations to have unique electronic properties depending on their helicity and diameter.4–7 Experimental efforts to identify the electronic properties of multi-walled carbon nanotubes have been carried out extensively.8–11 Present nano-fabrication technology has realized true molecularscale transistors using single-walled carbon nanotubes.12–15 Recently, nanometer-size graphitic ribbons have been found on the surface of graphite by atomic force microscopy (AFM) and scanning tunneling microscopy (STM),1,2 and are also expected to have unique properties. The folding and tearing of one or more graphite sheets along one of their symmetrical axes leads to geometrical structures that are reminiscent of folding paper, “origami” as shown in Fig. 1. However, such graphite origami is formed unintentionally by the STM or AFM probe tip. In seeking methods to systematically modify the structures and properties of graphite sheets, we found that micropatterns of circular and hexagonal holes were generated in individual graphite sheets by an electron exposure in vacuo. It has been already reported that monolayer-deep, flat-bottomed etch pits can be created in graphite by chemical modification using oxidative gases in the temperature range of about 675–1275 K.16–20 J. Mater. Res., Vol. 16, No. 5, May 2001
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However, the method reported here is very simple, and thereby highly homogeneous microholes can be generated in an uppermost layer of graphite. These random antidot structures were analyzed in detail by STM and the most important features are reported here. The results in this study indicate that our simple electron irradiation method will be useful for modifying the graphite surface in a controlled manner. II. EXPERIMENT
A flat, square, highly oriented pyrolitic graphite (HOPG; Union
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