CONDUCTANCE SPECTRA OF MOLECULAR WIRES

A relatively simple and straightforward procedure for characterizing m olecular wires is to measure the conductance spectrum by forming a sel f-assembled ordered monolayer (SAM) on a metallic surface and using a high scanning-tunneling microscope resolution (STM) tip as the other c ontact. We find that the conductance spectrum (dI/dV vs. V) can be und erstood fairly well in terms of a relatively simple model, provided th e spatial profile of the electrostatic potential under bias is propert y accounted for. The effect of the potential profile is particularly s triking and can convert a symmetric conductor into a rectifier and vic e versa. The purpose of this paper is to (1) describe the theoretical model in detail, (2) identify the important parameters that influence the spectra and show how these parameters can be deduced directly from the conductance spectrum, and (3) compare the theoretical prediction with experimentally measured conductance spectra for xylyl dithiol and phenyl dithiol. (C) 1998 American Institute of Physics.