Models of electron transport through organic molecular monolayers self-assembled on nanoscale metallic contacts

We propose a model of electrical conduction through self-assembled monolayers (SAM's) of organic molecules that bridge the gap between a pair of nanoscale metallic contacts. In this model each molecule bonds chemically to only one of the contacts. For dense SAM's the dominant electric current path is through two overlapping molecules each bonded to a different metal contact whereas for dilute SAM's the current flows through a single molecule. The model accounts quantitatively for the experimental data of M. A. Reed et al. [Science 278, 252 (1997)] on gold break junctions containing benzene-dithiol (BDT) SAM's, including the magnitude of the measured differential conductance. It also accounts for the striking differences between the data on the Au/BDT system and the recent measurements of J. Reichert et al. [cond-mat/0106219, 2001 (unpublished)] on a somewhat different system, namely, the strong asymmetry of the current-voltage characteristic and the larger size of the differential conductance found in latter experiments. We also present calculations of electron transport through dense and dilute SAM's of SC₆H₄S-CH₃ molecules. Experiments on these systems should test the validity of the proposed model.