Profiles of Work Function Shifts and Collective Charge Transfer in Submonolayer Metal–Organic Films



Vacuum level shifts Δ(d) at metal–organic (m–O) interfaces indicate the formation of surface dipoles for film thickness ddML up to a monolayer (ML). Shifts or profiles Δ(θ) of submonolayer films are nonlinear in the coverage θ = d/dML ≤ 1, which points to cooperative interactions between adsorbed molecules. Adsorption with weak nonspecific bonding is modeled as charge transfer (CT) between molecules M and localized surface states S of the metal. The dipole μ0 of ions SM+ or S+M gives upper bounds for the vacuum level shift ϕ0 and dipole–dipole repulsion V0 at θ = 1. Partial CT ρ(θ) < 1 is found self consistently and accounts for published profiles Δ(θ) of representative planar and nonplanar molecules with dML∼ 4 and ∼10 Å. Initial adsorption at θ ∼ 0 has considerable ionic character, ρ(0) ∼ 1/2, that decreases to ρ(1) ∼ 1/10 at θ = 1. Planar molecules with small μ0 and V0 have slightly nonlinear profiles while molecules with large μ0 and V0 have highly nonlinear Δ(θ). Collective CT is a phenomenological model for m–O interfaces with nonspecific bonding. The CT model is contrasted to fixed dipoles on the surface, to calculations of Δ(1) and to simulations of sub-ML films.