Charge transfer states at the interface of the pentacene monolayer on TiO$_2$ and their influence on the optical spectrum

TL;DR

This study uses density functional theory to analyze charge-transfer excitations at the pentacene/TiO2 interface, revealing how molecular states influence optical properties and polarization dependence.

Contribution

It provides a detailed ab initio analysis of charge-transfer states at the pentacene/TiO2 interface, linking molecular orientation to optical spectra and excitonic behavior.

Findings

Charge transfer excitations occur from pentacene HOMO to TiO2 conduction band.

Optical spectrum exhibits strong polarization dependence.

Excitons are characterized by their symmetry and k-space location.

Abstract

The full monolayer of pentacene adsorbed on rutile TiO$_2$(110) provides an intriguing model to study charge-transfer excitations where the optically excited electrons and holes reside on different sides of the internal interface between the pentacene monolayer and the TiO$_2$ surface. In this work we investigate the electronic properties of this system with density functional theory, and compute its excitonic and optical properties making use of \emph{ab initio} matrix elements. The pentacene molecules are found to lie flat on the surface, head to tail, and slightly tilted towards the troughs of the oxygen rows of the surface --- in agreement with experiment. Molecular states appear in the band gap of the clean TiO$_2$ surface which enable charge transfer excitations directly from the molecular HOMO to the TiO$_2$ conduction band. The calculated optical spectrum shows a strong polarization dependence and displays excitonic resonances corresponding to the charge-transfer states. We characterize the computed excitons by their symmetry and location in k-space and use this information to explain the polarization dependence of the optical spectrum.