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

Alkali-metal atoms (Li, Na, K, Rb, Cs), dimers, and (2x2) monolayers on a graphite (0001) surface have been studied using density functional theory, pseudopotentials, and a periodic substrate. The adatoms bind at the hollow site (graphite hexagon), with Li lying closest to (1.84 A) and Cs farthest (3.75 A) from the surface. The adsorption energies range between 0.55 and 1.21 eV, and the energy ordering of the alkali-metal adatoms is Li > Cs >= Rb >= K > Na. The small diffusion barriers (0.02-0.21 eV for the C-C bridge) decrease as the atom size increases, indicating a flat potential energy surface. The formation (cohesion) energies of (2x2) monolayers range between 0.55 and 0.81 eV, where K has the largest value, and increased coverage weakens the adsorbate-substrate interaction (decoupling) while a two-dimensional metallic film is formed. Analysis of the charge density redistribution upon adsorption shows that the alkali-metal adatoms donate a charge of (0.4-0.5)e to graphite and the corresponding values for (2x2) monolayers are similar to 0.1e per atom. The transferred charge resides mostly in the pi bands (atomic p(z) orbitals) of the outermost graphene layer.