Role of stress on charge transfer through self-assembled alkanethiol monolayers on Au

We have studied charge transfer through alkanethiol molecules self-assembled on Au(111) substrates using interfacial force microscopy. Simultaneous measurement of the tip-substrate current and the normal interfacial force reveals the critical role of tip-film contact. Measurable currents are seen only for tip-applied stresses above about 20 MPa, after which the current rises exponential with stress. We suggest that charge transfer results from stress-induced band-gap states near the Fermi level in these normally highly insulating molecular films.