AVS 47th International Symposium
    Organic Films and Devices Wednesday Sessions
       Session OF-WeA

Paper OF-WeA3
Charge Transfer in Alkanethiol Self-Assembled Monolayers on Au Surfaces

Wednesday, October 4, 2000, 2:40 pm, Room 313

Session: Self-Assembled Monolayers: Electron Transfer and Film Properties
Presenter: K. Son, Sandia National Laboratories
Authors: K. Son, Sandia National Laboratories
H.I. Kim, Sandia National Laboratories
J.E. Houston, Sandia National Laboratories
Correspondent: Click to Email
Charge transfer in self-assembled monolayers (SAM) has been studied extensively for molecular electronics applications, and Scanning Probe Microscopy studies have led to a general agreement on the role of force induced structural deformation on the conductivity of alkyl based SAMs. However, controversies about the level of tip/sample contact remain unresolved, and the evidence for structural deformation is vague. Here we investigate charge transfer in alkanethiol SAMs on Au(111) using Interfacial Force Microscopy (IFM). This technique enables the first simultaneous measurements of conductivity, normal-force, and friction over the entire range of tip/sample interaction. For alkanethiols (C@sub n@H@sub 2n+1@SH, n>10), we observe no current until the IFM tip makes physical contact with the SAM surface (down to the fA level). The current then increases exponentially with applied stress. Current and force profiles track each other from the initial conduction point, and both show a strong correlation with the rise in friction. I-V data taken at various force levels show both linear and resonance regions while the slope and the transition point are strongly dependent on the force level as well as molecular chain length. Based on these results, we conclude that the current flow through SAM films occurs by a tunneling process with necessary tip/film contact. We attribute the conductivity increase to the force-induced changes in film structure resulting in a redistribution of the film's electronic states, as evidenced by both the friction and the I-V characteristics. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Dept. of Energy under Contract DE-AC04-94AL85000.