Surface dipole of F4TCNQ films: Collective charge transfer and dipole-dipole repulsion in submonolayers

TL;DR

This paper introduces a charge transfer model for F4TCNQ films that explains how surface dipoles and work function shifts depend on coverage, considering collective effects and dipole interactions.

Contribution

It presents a self-consistent CT model for strong acceptors on surfaces, incorporating dipole-dipole repulsion and electron transfer dynamics, advancing understanding of surface dipoles.

Findings

Electron transfer decreases with increasing coverage.

Surface dipole and work function shift are proportional to coverage and electron transfer.

The model explains photoemission profiles and surface dipoles of F4TCNQ on various substrates.

Abstract

A charge transfer (CT) model is introduced for strong acceptors like A = F4TCNQ that are ionized on surfaces at low coverage {\theta}. Each A forms a CT dimer with a surface state S. Dipole-dipole repulsion grows as {\theta}^(3/2) up to {\theta} = 1 in a full monolayer. Electron transfer {\rho}({\theta}) within dimers is found self-consistently and decreases with increasing coverage. The surface dipole and work function shift {\Delta}{\Phi} are proportional to {\theta}{\rho}({\theta}). The CT model has implications for photoemission and accounts for {\Delta}{\Phi}(d) profiles of F4TCNQ films of thickness d on Cu(1,1,1) or on hydrogenated diamond (1,0,0). The CT model also describes {\Delta}{\Phi}({\theta}) of organic donors on metals and is contrasted to previous treatments of dipoles on a surface.