Scanning-tunneling-microscopy investigations of ternary graphite intercalation compounds

The surface structures of the stage-1 ternary graphite intercalation compounds K${\mathit{M}}_{\mathit{x}}$${\mathrm{C}}_{4}$ (${\mathit{M}}_{\mathit{x}}$=Hg,${\mathrm{H}}_{0.8}$) have been investigated using scanning tunneling microscopy. Constant-current and constant-height images of both materials exhibit superstructures in addition to the graphite atomic lattice. In the case of ${\mathrm{KHgC}}_{4}$, a commensurate 2\ifmmode\times\else\texttimes\fi{}2 and an incommensurate superstructure with a period of 8.9\ifmmode\pm\else\textpm\fi{}0.2 \AA{} have been observed. Images of ${\mathrm{KH}}_{0.8}$${\mathrm{C}}_{4}$ exhibit a one-dimensional superstructure with a 12.4\ifmmode\pm\else\textpm\fi{}0.2 \AA{} period and a two-dimensional superlattice with a 14.5\ifmmode\pm\else\textpm\fi{}0.5 \AA{} period. The 2\ifmmode\times\else\texttimes\fi{}2 structure in ${\mathrm{KHgC}}_{4}$ has been attributed to a modulation of the surface density of states by the periodic potential of the underlying intercalant. Possible explantions for these long-period superstructures observed in ${\mathrm{KHgC}}_{4}$ and ${\mathrm{KH}}_{0.8}$${\mathrm{C}}_{4}$ will be discussed.