11 research outputs found
Sodium atoms and clusters on graphite: a density functional study
Sodium atoms and clusters (N<5) 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 A 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 about 0.5 electrons in both cases.Comment: 20 pages, 6 figure