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 d ≤ d ML up to a monolayer (ML). Shifts or profiles Δ ( θ ) of submonolayer films are nonlinear in the coverage θ = d / d ML ≤ 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 μ 0 of ions S − M + or S + M − gives upper bounds for the vacuum level shift ϕ 0 and dipole–dipole repulsion V 0 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 d ML ∼ 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 μ 0 and V 0 have slightly nonlinear profiles while molecules with large μ 0 and V 0 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.