Sodium atoms and clusters on graphite by density functional theory -: art. no. 205404

Sodium atoms and clusters (Nless than or equal to5) on graphite (0001) are studied using density functional theory, pseudopotentials and periodic boundary conditions. A single Na atom is observed to bind at a hollow site 2.45 Angstrom above the surface with an adsorption energy of 0.51 eV. The small diffusion barrier of 0.06 eV indicates a flat potential energy surface. Increased Na coverage results in a weak adsorbate-substrate interaction, which is evident in the larger separation from the surface in the cases of Na(3), Na(4), Na(5), and the (2x2) Na overlayer. The binding is weak for Na(2), which has a full valence electron shell. The presence of substrate modifies the structures of Na(3), Na(4), and Na(5) significantly, and both Na(4) and Na(5) are distorted from planarity. The calculated formation energies suggest that clustering of atoms is energetically favorable, and that the open shell clusters (e.g., Na(3) and Na(5)) can be more abundant on graphite than in the gas phase. Analysis of the lateral charge density distributions of Na and Na(3) shows a charge transfer of similar to0.5 electrons in both cases.