Rings sliding on a honeycomb network: Adsorption contours, interactions, and assembly of benzene on Cu(111)

TL;DR

This study uses advanced ab initio calculations to analyze benzene adsorption on Cu(111), revealing how van der Waals interactions influence binding stability, diffusion, and self-assembly of benzene molecules on the surface.

Contribution

It demonstrates the importance of nonlocal vdW interactions in accurately modeling benzene adsorption and explains the formation of different benzene overlayer phases on Cu(111).

Findings

vdW-DF increases binding energy and alters stability of adsorption sites

Benzene molecules diffuse freely along specific surface corridors

Two distinct benzene overlayer phases are explained by direct and indirect interactions

Abstract

Using a van der Waals density functional (vdW-DF) [Phys. Rev. Lett. 92, 246401 (2004)], we perform ab initio calculations for the adsorption energy of benzene (Bz) on Cu(111) as a function of lateral position and height. We find that the vdW-DF inclusion of nonlocal correlations (responsible for dispersive interactions) changes the relative stability of eight binding-position options and increases the binding energy by over an order of magnitude, achieving good agreement with experiment. The admolecules can move almost freely along a honeycomb web of "corridors" passing between fcc and hcp hollow sites via bridge sites. Our diffusion barriers (for dilute and two condensed adsorbate phases) are consistent with experimental observations. Further vdW-DF calculations suggest that the more compact (hexagonal) Bz-overlayer phase, with lattice constant a = 6.74 \AA, is due to direct Bz-Bz vdW attraction, which extends to ~8 \AA. We attribute the second, sparser hexagonal Bz phase, with a = 10.24 \AA, to indirect electronic interactions mediated by the metallic surface state on Cu(111). To support this claim, we use a formal Harris-functional approach to evaluate nonperturbationally the asymptotic form of this indirect interaction. Thus, we can account well for benzene self-organization on Cu(111).