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

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Abstract

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.

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