Stable Thin Clathrate Layers

Abstract

We explore \emph{elemental} Si, Ge and Sn free-standing slabs in diamond, clathrate and other competing structures and evaluate their stability by comparing Kohn-Sham total energies at the same atoms/area coverage $\rho$. We find that within 3-6 ML range (in units of diamond monolayer), the surface energy stabilizes {\textit{clathrate}} thin films against the diamond slabs or other competitors lowering internal energy per atom, for all three elements. At~1.3-1.6 ML coverages, Si and Ge stable layers are adatomic decorations of the puckered honeycomb lattice, forming a dumbbell--decorated pattern with rectangular unit cell. Analogical Sn layer is unstable to a web-like net of interconnected Sn$_9$ clusters. The zoo of~stable thin-layer non-diamond structures is completed by a Sn {\textit metallic} bilayer at 2.3 ML coverage.