AbstractScanning 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 surface graphene layer. When the sample is under positive bias, symmetry properties of the uppermost graphite planes are revealed. Images of the CuCl2 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 HOPG. In GICs, this stacking sequence is interrupted by the layer of 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 CoCl2) 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.
