Band dispersion and bonding character of potassium on graphite

The band dispersion of potassium on highly oriented pyrolytic graphite (HOPG) has been studied by angle-resolved photoemission spectroscopy (ARPES). The ARPES around the K point shows shifts of the π and π∗ bands toward higher binding energies upon K adsorption in the submonolayer regime, showing charge transfer from the adsorbed potassium to the graphite π∗ band and an ionic bond between potassium and graphite. In addition, upon the K adsorption, gap opening between the π and π∗ bands, which degenerate at the Brillouin zone corner in graphite, is confirmed by the k∥-band map around the K point. This change in the dispersion of the π and π∗ bands upon the K adsorption is consistent with calculations reported for bilayer graphite under an electric field, while the K adsorption involves increase in the carrier density. The concentration of electron charge near the surface results in enhanced two-dimensionality of the π band. It is found from the ARPES at the zone center that electron charge resides at the potassium atoms in the ionic bond between K and graphite even at low K coverage below the saturation of the charge condensation at the graphite surface. In multilayer K/HOPG, a metallic bond among the adsorbed K atoms stabilizes and leads to reduced charge transfer to graphite.