Electronic structure and kinetics of K on graphite

Abstract

The K/graphite adsorption system is studied in a cluster model using ab initio density-functional methods. From the investigation of the potential energy surface a lower bound for the potassium atom binding energy 1.5 eV is obtained, and a surface diffusion barrier of 0.2 eV. To simulate experimentally reported thermal desorption spectra, a two-phase kinetic model is investigated and a desorption energy of 1 eV is found. The thermally activated surface diffusion of K atoms leads to intercalation at defects or steps, which is followed by desorption when further heating the sample. A normal mode analysis yields a K-graphite in-phase and out-of-phase vibrational mode with an energy split of 8 meV, which indicates a relatively strong dynamical coupling between the adsorbed K atom and the graphite substrate. The calculated electron density distribution is verified by an accurate reproduction of the measured dipole moment. From a projected density of state analysis we find a K 4s and an antibonding K 4p resonance located slightly above and 2.6 eV above the Fermi level, respectively. The location of the K 4s resonance, with a lower occupied tail, is consistent with an incomplete charge transfer, and the location of the K 4p resonance is consistent with a proposed hot-electron model to explain recent photodesorption data. The new assignment of the K-induced states near the Fermi level resolves previous apparent discrepancies of the charge state of the dispersed K atom.

Band dispersion and bonding character of potassium on graphite

“…It shows that incomplete charge transfer in the ionic bond between K and graphite also at the low K coverage in the dispersed phase. This result is consistent with the most of theoretical studies [7,21,23,24]. These theoretical studies predict that the bond between potassium and graphite in the low K coverage is not covalent but still ionic even though the charge transfer is incomplete.…”

Density functional study of alkali-metal atoms and monolayers on graphite (0001)

“…3 The preference of alkali metals for the hollow site is well known. 21,23,25,26,43 Fig . 1 shows the formation and adsorption energies (∆E and ∆E ⊥ ) as well as the separation from the surface (d ⊥ ) for alkali metal adatoms and (2×2) monolayers (see also Table I).…”

Tailoring graphene with metals on top

“…The hopping energy of the electrons in graphene is t ∼ 3 eV. As in fullerenes 30 , if a Hubbard U ∼ 3t can be taken as a reasonable estimate for the order of magnitude of U in graphene, then the LDA+U calculation may lower considerably the amount of charge transfer predicted by a pure GGA calculation 29 . On the other hand, since the LDA+U method does not take polarization effects into account, the local charging energy U can be additionally screened by the pocket of electrons in graphene.…”