How orbitals and oxidation states determine apparent topographies in scanning tunneling microscopy: the case of fluorine on silver surfaces

TL;DR

This study uses density functional theory and STM simulations to analyze how fluorine adsorption affects the apparent topographies on silver surfaces, linking structural and oxidation state information to observed STM features.

Contribution

It introduces a minimal model connecting STM apparent topography to orbital and oxidation state effects of fluorine on silver surfaces, supported by DFT calculations.

Findings

Hollow site is most favorable for F on Ag(100).

Multiple sites are energetically similar on Ag(110).

Kinetics, not thermodynamics, dominate surface states during fluorination.

Abstract

We use density functional theory calculations to characterize the early stages of fluorination of silver's (100) and (110) surfaces. In the Ag(100) surface, the hollow site is the most favorable for F adatoms. In the Ag(110) surface, three adsorption sites, namely hollow, long bridge, and short bridge, exhibit similar energies. These locations are also more favorable than an F adatom occupying a vacancy site irrespectively of whether the vacancy was present or not in the pristine surface. The computed energy as a function of surface coverage is used to compute the equilibrium thermodynamics phase diagram. We argue that for the typical pressure and temperature of fluorination experiments, the state of the surface is not determined by thermodynamics but by kinetics. Combining these results with scanning tunneling microscopy (STM) topographic simulations, we propose assignments to features observed experimentally. We present a minimal model of the apparent topography of adatoms in different locations in terms of hydrogenic orbitals, explaining the observed trends. The model links the STM apparent topography to structural information and the oxidation states of the Ag atoms near the adatom.