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

A charge transfer (CT) model is introduced for strong acceptors like A = F4TCNQ that are ionized on surfaces at low coverage θ. Each A forms a CT dimer with a surface state S. Dipole–dipole repulsion grows as θ3/2 up to θ = 1 in a full monolayer. Electron transfer ρ(θ) within dimers is found self-consistently and decreases with increasing coverage. The surface dipole and work function shift ΔΦ are proportional to θρ(θ). The CT model has implications for photoemission and accounts for ΔΦ(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 ΔΦ(θ) of organic donors on metals and is contrasted to previous treatments of dipoles on a surface.