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

TL;DR

This study uses density functional theory to analyze how alkali metal atoms and monolayers adsorb, bind, and interact electronically with a graphite (0001) surface, revealing binding preferences, energies, and charge transfer characteristics.

Contribution

It provides detailed computational insights into the adsorption behavior, energies, and electronic effects of alkali metals on graphite, including charge transfer and diffusion barriers.

Findings

Li binds closest to the surface at 1.84 Å

Adsorption energies range from 0.55 to 1.21 eV

Charge transfer of 0.4-0.5 e per atom to graphite

Abstract

Alkali metal atoms (Li, Na, K, Rb, Cs), dimers and (2$\times$2) 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 \AA) and Cs farthest (3.75 {\AA}) from the surface. The adsorption energies range between $0.55-1.21$ eV, and the energy ordering of the alkali adatoms is Li$>$Cs$\ge$Rb$\ge$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 (2$\times$2) monolayers range between 0.55-0.81 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 (2$\times$2) monolayers are $\sim 0.1 e$ per atom. The transferred charge resides mostly in the $\pi$-bands (atomic $p_z$-orbitals) of the outermost graphene layer.