Self-consistent semi-empirical transport models for molecular conductors

In this study we focus in the development of transport models for molecular conductors using only semi-empirical methods but with a rigorous self-consistent approach. In our models, an Extended Huckel Theoretical (EHT) treatment of the molecular chemistry is combined with a Non-equilibrium Green's function (NEGF) treatment of quantum transport. In our first model (Huckel I-V 2.0), a simple charging scheme is used for the description of the self-consistent potential where the potential profile across the molecule is assumed to be flat. In the next stage of our study, the self-consistent potential is modified by CNDO (complete neglect of differential overlap) with the electrostatic effects of metallic leads (bias and image charges) included through a 3-D finite element method (FEM). This new model (Huckel I-V 3.0) takes into account the spatial profile of the potential inside the molecule by incorporating both screening and charging effects. We apply this model to investigate recent experimental results on alkane dithiol molecules obtained from nanopore set-up and observe excellent agreement. We also present a study on single molecule transistors and identify electronic properties that control their performance by comparing the transistor action of two different types of molecules. Finally, we successfully explain and match experimental data on I-V asymmetry recently observed in a break junction set-up. It is shown that the asymmetry in the I-V is induced by charging effects.