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Unimolecular rectifiers and Coulomb blockade effects in electron transport calculations

Seminar by:

Robert Stadler

Laboratory for the Chemistry of Novel Materials
Bat. Materia Nova,
University of Mons-Hainaut
Belgium  

The presentation will be divided into two topics:

Unimolecular rectifiers: The 1974 proposal for molecular rectifiers by Aviram and Ratner has been the starting point for molecular electronics. We present a systematic study of rectification ratios (RR) for a range of molecules with nitro- and amino-groups as substituents. In our study we use both, the quantum chemical Austin model (AM1) and a non-equilibrium Green’s function approach for electron transport within the framework of density functional theory (NEGF-DFT). The trends in RR as a function of the molecular structure agree very well when these two methods are compared and can be explained in terms of the relative position of the nitro group within the generated electrostatic potential. These findings allow us to derive some general conclusions about the physical mechanisms behind unimolecular rectification.

Coulomb blockade: We present a model emphasizing the role of many-body effects in the general description of electron transport through single molecule junctions. We show numerically and derive analytically that a closed-shell ansatz, the usual ingredient of mean-field methods, does not properly describe the step-like electron transfer characteristics of I/V curves in Coulomb blockade (CB) systems. This can even lead to an erroneous prediction of the nature of the transport regime, describing it as coherent when it should be CB due to a negligible coupling between the molecule and the leads. The problem is general to closed-shell approaches and not related to the self- interaction errors in density functional theory.