Electron transport through π-stacked molecular multilayers: Metal-semiconductor transition

摘要

We present a theoretical study of electron transport in π-stacked polythiophene multilayers (with varying number of layers L) sandwiched between Au(111) electrodes in a geometry of flat molecular layers lying parallel to the electrode surface, as suggested by recent experiments. We use a method based on the local density approximation of density functional theory and implemented in the framework of the tight-binding linear muffin-tin orbital approach in its atomic sphere approximation. A fully atomistic description of the electrodes and the nanosystem is used, and the self-consistent charge and electrostatic potential for the system under applied bias are calculated using the nonequilibrium Green's function approach. For a small number of layers, metal-induced gap states render the molecular film metallic, while for thicker films, the conductance decreases exponentially with the number of layers, σ ≈ G_0 exp[-0.81(L-3.4)], indicating semiconductorlike behavior; the transition between these two regimes occurs at a film thickness of 20 A (L ≈ 6). This length scale originates from the decay length of the gap states (4.24 A) and the "band bending" in the multilayer. For L = 1, the current depends linearly on applied voltage, while at L > 1, current is nonlinear, reflecting strong bias and energy dependence of the transmission function due to formation of the band gap near the Fermi energy.