SCANNING TUNNELING MICROSCOPY OF A STAGE-1 CUCL2 GRAPHITE-INTERCALATION COMPOUND

We report a scanning-tunneling-microscope (STM) study of a stage-1 graphite intercalation compound (GIC) of approximate stoichiometry C6CuCl2. When the sample bias is positive with respect to the tip, we observe a hexagonal symmetry in which all the atoms of the graphite surface plane are imaged. This is in contrast to the threefold 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 threefold symmetry is attributed to the ABAB stacking of the atomic layers in HOPG. In GICs, this stacking sequence is interrupted by the intercalate layer, so that for the stage-1 compound all carbon atoms in the plane become equivalent, and sixfold symmetry develops. When the sample is biased negatively with respect to the tip, we observe a strikingly different STM image for the GIC; we believe that the pattern in this case is mainly due to the intercalate layer and the change in the electronic structure of graphite resulting from the presence of the intercalate layer. We also observe a reduction in the corrugation amplitude, between carbon-atom sites and the centers of the hexagonal nets, on the GIC relative to the HOPG. © 1990 The American Physical Society.