Conductance investigations of stretched molecules

Recent experiments have been able to measure the conductance of individual molecules by repeatedly raising and lowering a gold-plated atomic force microscope tip into a self-assembled monolayer on a gold substrate, as demonstrated by Xu and Tao and Xiao et al. Upon raising the tip after it has formed several metal-molecule-metal junctions and recording the current simultaneously, the experiments reveal descending steps in the trace, which indicate the detachment of individual junctions until a single molecular conductance is isolated. Interesting fluctuations in these steps indicate changes in the molecular conductance with stretching. We report theoretical calculations that employ a local orbital density functional theory code for a candidate molecule with varying degrees of stretching. An efficient self-consistent transfer matrix program is then used to determine the conductance as the molecule is stretched. Conductance peaks are observed despite the increased tunneling distance, indicating an enhanced coupling of extended gold states in the contacts to the molecular states. Two different bonding configurations are examined in order to compare the influence of the interface on the conductance behavior.