CPM Seminar

First-principles studies of single-molecule conductance

Jeffrey Neaton

The Molecular Foundry
Lawrence Berkeley National Laboratory

A fundamental challenge in nanoscience is to understand and control current flow through individual molecules and nanostructures. Recently, transport properties of small organic molecules have been reported using multi-probe break-junction and scanning probe techniques. In these molecular junctions, the contacts are a significant fraction of the conducting element, and contact chemistry will strongly influence measured conductance. In this talk, I will describe studies using first-principles theoretical approaches, based on density functional theory (DFT) and beyond, to investigate the transport properties of specific single-molecule transport measurements. First, a DFT-based scattering-state approach is used to investigate the influence of contacts on the conductance of benzene-diamine-Au junctions. We find that while certain structural trends are in agreement with experiment, the calculated conductance is found to be ~7x larger than experiment. We show that this overestimate is due to specific exchange and correlation effects missing from standard DFT approaches; a simple model of correlation in the junction can quantitatively explain the experimental conductance. Second, I will summarize recent calculations, using a many-electron self-energy approach (the GW approximation), of cyclopentene and related molecules chemisorbed on p-type Si(001), where we reexamine a widely-accepted, previously proposed resonant tunneling mechanism for negative differential resistance in these systems.