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

Abstract

Vacuum level shifts Δ ( d ) at metal-organic (m-O) interfaces indicate the formation of surface dipoles for fi lm thickness d ≤ ≤ d ML up to a monolayer (ML). Shifts or profi les Δ ( θ ) of submonolayer fi lms are nonlinear in the coverage θ = d / d ML ≤ ≤ 1, which points to cooperative interactions between adsorbed molecules. Adsorption with weak nonspecifi c 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 profi les 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 profi les 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 nonspecifi c bonding. The CT model is contrasted to fi xed dipoles on the surface, to calculations of Δ (1) and to simulations of sub-ML fi lms.

Doping of C60(sub)monolayers by Fermi-level pinning induced electron transfer

“…Indeed, dipole-dipole repulsion is typically responsible for the formation of wormlike (or "labyrinth") structures and for large interadsorbate distances. 21,34,35 This interpretation is also in line with the phenomenological model recently put forward by Topham et al 36 Finally, we want to rationalize the presence of neutral C 60 molecules adjacent to C −1 60 . The dipoles created by C −1 60 give rise to an increase by several tenths of an electron volt in the local electrostatic potential, as evidenced by the shift of the 6T levels.…”

Interface dipoles of organic molecules on Ag(111) in hybrid density-functional theory

“…First, the position of the LUMO for α = 0. In contrast to the situation on Cu or Au, where complete filling only occurs at low coverages [128,133], the LUMO is almost completely filled for F4TCNQ even at full coverage on Ag (111). Further α-induced downward shifts can therefore not yield a significant change in charge transfer.…”

Role of charge transfer, dipole-dipole interactions, and electrostatics in Fermi-level pinning at a molecular heterojunction on a metal surface

“…We further remark that numerous similarities are found between the present experimental findings ( profile and varying proportion of charged and neutral molecules) and the predictions of the charge transfer model developed by Topham et al where dipole-dipole repulsion is a key point. 20 In a more general picture, it is also interesting to notice that the gain in energy due charge transfer, defined as the energy difference between E F and the energy position of the partially filled molecular orbital will be balanced by the electrostatic potential described above, so that the total energy of the system is minimized. For a comprehensive understanding of the energy level alignment, one has to further consider the potential drop in between two charges (in vertical direction).…”

“…[11][12][13][14][15][16][17][18][19] Apart from DFT, other approaches have been proposed, consisting of (nonspecific) phenomenological models to describe the electronic properties of these interfaces. [20][21][22] Despite the increasing modeling capabilities, no general rules have been established unequivocally yet to predict the energy level alignment, and it is still good advice today to address each specific interface experimentally.…”