Current-Voltage Characteristics of Self-Assembled Monolayers by Scanning Tunneling Microscopy

This paper presents a comparison of the theoretical and experimental current-voltage (I-V) characteristics of a self-assembled monolayer of $\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}\mathrm{x}\mathrm{y}\mathrm{l}\mathrm{y}\mathrm{l}$ dithiol molecules on a gold substrate measured with a scanning tunneling microscope probe. Good quantitative agreement is obtained with the tip-molecule distance as the only ``fitting parameter.'' Several other thiol-coupled molecules that we have studied also show similar agreement. The conceptual picture presented in this paper could be useful for the interpretation of I-V measurements on molecular monolayers in general.