Influence of steps on the tilting and adsorption dynamics of ordered Pn films on vicinal Ag(111) surfaces

TL;DR

This study combines experimental He atom diffraction and DFT+vdW calculations to reveal how step edges on vicinal Ag(111) surfaces influence the adsorption, tilting, and growth of ordered pentacene films, highlighting the role of surface steps as nucleation sites.

Contribution

It provides new insights into the adsorption behavior of pentacene on vicinal Ag(111), emphasizing the importance of step edges in film growth and structure formation.

Findings

Step edges are preferred adsorption sites for pentacene.

Isolated molecules tilt and bind strongly at step edges with 1.4 eV energy.

Ordered monolayers form with flat terraces and tilted step edge molecules.

Abstract

Here we present a structural study of pentacene (Pn) thin films on vicinal Ag(111) surfaces by He atom diffraction measurements and density functional theory (DFT) calculations supplemented with van der Waals (vdW) interactions. Our He atom diffraction results suggest initial adsorption at the step edges evidenced by initial slow specular reflection intensity decay rate as a function of Pn deposition time. In parallel with the experimental findings, our DFT+vdW calculations predict the step edges as the most stable adsorption site on the surface. An isolated molecule adsorbs as tilted on the step edge with a binding energy of 1.4 eV. In addition, a complete monolayer (ML) with pentacenes flat on the terraces and tilted only at the step edges is found to be more stable than one with all lying flat or tilted molecules, which in turn influences multilayers. Hence our results suggest that step edges can trap Pn molecules and act as nucleation sites for the growth of ordered thin films with a crystal structure similar to that of bulk Pn.