Geometric and electronic structure of commensurate4Ar∕Ag(111)−(7×7)R19.1°by density functional theory

Abstract

We report the optimization of a $4\mathrm{Ar}∕\mathrm{Ag}(111)\text{\ensuremath{-}}(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.1\ifmmode^\circ\else\textdegree\fi{}$ monolayer using first-principles density functional theory. The adsorption energies resulting from our ground-state minimizations are similar for all three anchor sites. A commensurate adsorbate cell is comprised of four argon atoms which settle into three different configurations. We refer to these sites as top, threefold hcp hollow, and threefold fcc hollow sites. Each of these structures has one Ar atom located at one of the base sites of the Ag(111) surface. The remaining three Ar atoms are found near the bridge sites of the silver surface. In both of the hollow site structures, the bridge locations are measurably off of the exact geometric bridge positions while maintaining the $\sqrt{7}R19.1\ifmmode^\circ\else\textdegree\fi{}$ unit cell. Our results show that both the hcp and fcc hollow sites have slightly lower adsorption energies than the top site, by only 1.5 and $4.5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, respectively (per cell of four Ar atoms). The vertical harmonic vibration frequencies were computed for the top, hcp, and fcc hollow sites. They measured 5.22, 5.01, and $5.21\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, respectively. We studied the possible cause for this high-order commensurate monolayer. We have concluded that neither steric repulsion nor hybridization of the electrons of the Ar atoms is the reason for the commensurability in our model. We propose that the causal mechanism is the Novaco-McTague [Phys. Rev. Lett. 38, 1286 (1977)] theory for orientational epitaxy.