Scanning tunneling microscopy investigations of the surface structure and electronic properties of ternary graphite intercalation compounds

Scanning tunneling microscopy has been used to characterize the surface structure of the KHgC4 (stage 1) and KHgC8 (stage 2) graphite intercalation compounds. Images of the stage 1 and stage 2 materials exhibit a new commensurate 2×2 superlattice in addition to the centered hexagonal lattice observed in images of pristine graphite. Consideration of the intercalant layer structure and previous studies of stage 1 (MC8) and stage 2 (MC24) alkali metal graphite intercalation compounds (GICs) indicate that the 2×2 superlattice is due to a modulation of the surface carbon layer density of states by the periodic (2×2) potential of the potassium ions. In addition, a new orthorhombic superlattice, a=b=0.89 nm and <a−b≂89°, has been observed in images of the stage 1 KHgC4 material. Possible origins of this novel superstructure are discussed.