Scanning Tunneling Microscopy and Spectroscopy of GuCl 2 and CoCl 2 Graphite Intercalation Compounds
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SCANNING TUNNELING MICROSCOPY AND SPECTROSCOPY OF CuCl 2 AND CoCl 2 GRAPHITE INTERCALATION COMPOUNDS C.H. OLK*, J. HEREMANS*, M.S. DRESSELHAUS**, J.S. SPECK", J.T. NICHOLLS** *Physics Department, General Motors Research Laboratories, Warren, MI 48090 Institute of Technology, Cambridge, MA 02139 "**Massachusetts ABSTRACT Scanning tunneling microscopy (STM) is used to visualize the structure of copper chloride and cobalt chloride graphite intercalation compounds (GICs). When the samples are biased negatively with respect to the tip, the images show details of the structure of the intercalant layers, and of its effects on the When the sample is under positive bias, symmetry surface graphene layer. Images of the CuCl2 properties of the uppermost graphite planes are revealed. stage 1 GIC display a hexagonal symmetry in which all the atoms of the graphite surface plane appear. This is in contrast to the three-fold symmetry usually seen in atomic resolution STM images of highly oriented pyrolytic graphite (HOPG), which we also observe on a reference sample of HOPG. The three-fold symmetry is attributed to the ABAB stacking of the atomic layers in In GICs, this stacking sequence is interrupted by the layer of HOPG. intercalate, so that for the stage 1 compound all carbon atoms in the plane become equivalent, and six-fold symmetry develops. For a stage 2 (CuC12 or CoCl 2) GIC three-fold symmetry is expected to persist. Images of the CoCl2 stage 1 GIC, taken with the sample bias is positive with respect to the tip, display a mixed trigonal and hexagonal symmetry, and may be attributable to the fact that the surface of sample is of mixed stage. INTRODUCTION The ability of the scanning tunneling microscope (STM) to probe both geometric and spectroscopic features of surfaces in real space was recognized Of the wide variety of materials by its inventors [1] and is well known [2]. that have come under investigation by the STM [3], graphite is one of great interest. Graphite displays a number of unique physical features and has been the subject of many experimental [4-8] and theoretical [9-16] STM studies. The two most notable features of the surface of graphite, as observed by STM, are the threefold (rather than sixfold) symmetry of the STM pattern and the large corrugation amplitude (CA) between the atomic sites and the centers of the hexagons. The former is the result [9] of the ABAB stacking sequence of the three dimensional layered structure, producing two inequivalent atomic sites per planar unit cell. Three of the atoms in a single surface hexagonal net, a sites, lie directly above three atoms in the layer below and are electronically coupled to them. The other three atoms in the surface net, p sites, lie above the The origin of the anomalously large corrugation centers of the nets below. feature often observed [6] with STM of graphite is not fully understood and may be a combination of physical and electronic effects. Several authors [10,12,14-16] of theoretical works on graphite have suggested that STM images of graphite
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