Two dimensional phases of Ag on Ge(111): insights from first-principles calculations

Experimental observations using low-energy electron diffraction (LEED) of deposited Ag on the Ge(111) surface have revealed a number of two-dimensional Ag structures which depend on Ag coverage and sample temperature[1]. We have applied density functional theory based calculations to explore the Ag/Ge(111) phase diagram and to obtain insights into the electronic and geometric structures and the vibrational dynamics. In this comparative study of clean Ge(111) with (1x1), reconstructed c(2x8) and (2x1) geometries, and of sub-monolayer Ag covered Ge(111) with (4x4), (v3xv3)R30 and (1x1) overlayers, we find that Ge atoms on the surface prefer three-fold hollow sites and Ag adatoms prefer to form triangular structures. Furthermore, temperature-coverage dependent surface phase diagram constructed by minimizing the surface free energies shows that up to 37% Ag coverage, (4x4) is the stable phase beyond which up to 90% coverage, the phase coexists with (v3xv3)R30 and (1x1) phases at temperature below 600K and higher respectively. Beyond 90% coverage, isolated (v3xv3)R30 and (1x1) phase are the most stable with the same temperature boundary of 600K.
[1]. Jason Allan Giacomo (2009, Ph.D. Thesis, UC Davis).