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

Vacuum level shifts Delta(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 Delta(theta) of submonolayer films are nonlinear in the coverage theta = 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 mu(0) of ions S-M+ or S+M- gives upper bounds for the vacuum level shift Phi(0) and dipole-dipole repulsion V-0 at theta = 1. Partial CT rho(theta) < 1 is found self consistently and accounts for published profiles Delta(theta) of representative planar and nonplanar molecules with d(ML)similar to 4 and similar to 10 angstrom. Initial adsorption at theta similar to 0 has considerable ionic character, rho(0) similar to 1/2, that decreases to rho(1) similar to 1/10 at theta = 1. Planar molecules with small mu(0) and V-0 have slightly nonlinear profiles while molecules with large mu(0) and V-0 have highly nonlinear Delta(theta). 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 Delta(1) and to simulations of sub-ML films.