Density Functional Theoretical Study of Perfluoropentacene/Noble Metal Interfaces with van der Waals Corrections: Adsorption States and Vacuum Level Shifts

We have studied the electronic structures of perfluoropentacene (PFP) on Cu(111), Ag(111), and Au(111) by means of density functional theory with a semiempirical van der Waals method (DFT-D). We show that DFT-D yields accurate equilibrium PFP-metal distances, thereby making an accurate prediction of the work-function change (Δϕ) possible. In order to investigate the nature of the interface dipole layer, we calculated Δϕ as a function of PFP-substrate distance and found that in contrast to pentacene/metal interfaces, the molecular distortion has a significant influence on Δϕ at a short distance. However, by subtracting the contribution of the molecular distortion from the total work-function change, we show that the work-function change does not depend on the substrate work-function at a long distance, while the work-function change varies linearly with the substrate work-function at a short distance. Our results indicate a transition from Schottky to Bardeen limits as a PFP molecule approaches the substrate metal surface, as in pentacene/metal interfaces.