Insight from First-Principles Calculations into the Interactions between Hydroxybenzoic Acids and Alkali Chloride Surfaces

Tuning of effective band gaps at insulator surfaces by adsorbed molecules is of fundamental interest but also technologically relevant for contact charging induced by adsorbed molecules like hydroxybenzoic acids. Our studies by density functional theory of the adsorption of benzoic acid (BA), salicylic acid (SA), and para-salicylic acid (p-SA) on perfect KCl and NaCl(100) surfaces as well as on polar and nonpolar step edges reveal the importance of polar defects in this context. We also found that the van der Waals dispersion contributes to about half of the adsorption energy in most cases. Therefore, the van der Waals interaction must be explicitly taken into account even for the interaction between molecules and wide band gap insulator surfaces, respective of the adsorption on flat or stepped surfaces. On the other hand, the adsorption geometry is still determined by short-range forces, that is, by the weak chemical bonds and electrostatic interactions. These short-range interactions are mostly mediated by resonant coupling between HOMO-1 and HOMO-2 molecular states and the valence band of the solid, which results in very similar adsorption energies of these molecules on the KCl and NaCl surfaces. The interaction strength is significantly modified only at defects with large dipole moments (e.g., pairs of polar steps) resulting in split-off edge states below the valence band. Resonant coupling to these states leads to an almost rigid down-shift of all molecular orbitals that efficiently reduces the effective band gap at the Surface. As a result, an effective band gap of 0.9 eV has been achieved oil the polar KCl [011] stepped Surface thanks to the relatively small HOMO-LUMO gap of SA.