All-electron local density functional study of metallic monolayers. I. Alkali metals

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Abstract
Based on the all-electron local density functional theory, the electronic structures and cohesive energies of close-packed hexagonal monolayers of the alkali atoms Li-Cs have been calculated using the self-consistent full-potential linearised augmented plane wave method for thin films. The energy band structures of the monolayers of the 'light' alkali atoms Li and Na exhibit wide occupied bands of almost pure s character and do not show any sharp spectral features in the unoccupied part of their densities of states up to the vacuum level. In contrast, the monolayers of the 'heavy' alkali atoms K, Rb and Cs are found to have markedly smaller widths of their occupied bands and increasingly sharper structures in their densities of states above the Fermi energy. These structures originate from d-like states which, in the case of Cs, are present even at the Fermi energy. The theoretical work functions obtained for the monolayers are consistently larger by about 0.5 eV than the experimental values for clean metal surfaces, but about 2 eV smaller than the first ionisation potentials of the free atoms. In accordance with the trend observed for bulk alkali metals, the cohesive energies in the monolayers decrease from 1.38 eV for Li to 0.71 eV for Cs and are found to be surprisingly similar to the theoretical values for the bulk metals, if the number of nearest neighbours is taken into account.

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