On Conductance and Interface Effects in Molecular Devices

Abstract

In this dissertation we discuss ab initio studies of quantum transport through single-molecule devices, using a combination of techniques from quantum chemistry and many-body physics. The molecular transport calculations discussed here are based on the DFT+NEGF approach to molecular transport. We first outline the density functional theory (DFT) and non-equilibrium Green's functions (NEGF) formalisms. We discuss the assumptions behind their combined use, as well as consequent limitations to the computational results we have obtained. This has been implemented as a custom, scalable extension of the ADF/BAND quantum chemistry package originally developed in the theoretical chemistry group at the VU University Amsterdam, and currently developed and commercialized by Scientific Computing and Modelling N.V. Using this code, we study chains and more complex chemical systems such as multi-ring phenyl junctions and a class of porphyrin-derivative devices recently studied experimentally. To address the polarization effects suspected to be behind the mechanical gating effects observed in the experiments, we extend our transport method with a complementary approach which accounts for image-charge effects at a metal-molecule interface. We close with a further study of the performance of our main method in relation to approximate transport methods based on simplified treatments of the leads in molecular devices.