STRUCTURAL AND ELECTRONIC-PROPERTIES OF GRAPHITE AND GRAPHITE-INTERCALATION COMPOUNDS MC(8) (M=K, RB, CS) GOVERNING THEIR SCANNING-TUNNELING-MICROSCOPY IMAGES

Several puzzling observations in the scanning tunneling miucroscopy (STM) and atomic force microscopy (AFM) studies of highly oriented pyrolytic graphite (HOPG) and its intercalation compounds MC(8) (M = K, Rb, Cs) were investigated on the basis of atom-atom potential and Coulombic interaction energy calculations. The charge or spin density wave state of a graphite monolayer is found inconsistent with an identical peak registry of the HOPG STM images obtained at plus and minus bias voltages. Simultaneous STM/AFM measurements of HOPG show the STM and AFM images to have an identical peak registry, which implies that the local hardness of the surface monolayer is larger at the B-site than at the A-site. We confirm this implication by estimating the local hardness in the surface monolayer of a graphite bilayer in terms of atom-atom potential calculations. The essential characteristics of the Moire STM images of HOPG are correctly predicted by the local hardness map obtained for the surface monolayer of a graphite bilayer in terms of atom-atom potential calculations. This supports the notion that the tip-force-induced topography change in the surface monolayer is generally responsible for Moire STM patterns in layered materials. It is most likely that the surface charge density wave (CDW) of MC(8) (M = K, Rb, Cs) observed by STM is associated with the pi band electrons of the surface graphite monolayer and is not caused either by a Fermi surface nesting driven electronic instability or by a possible topography change induced by the tip force. We calculate the Coulombic interaction energy of the surface KC8 blayer for several different negative charge (transferred from K) distributions in the graphite monolayer. This energy is increasingly lowered when the charge distribution becomes more nonuniform, thereby suggesting that the surface CDW of MC(8) (M = K, Rb, Cs) occurs most likely to lower the Coulombic interaction energy in the surface MC(8) bilayer.