AVS 53rd International Symposium
    Surface Science Monday Sessions
       Session SS2-MoM

Paper SS2-MoM2
Influence of Film Thickness on the Reactions of Gas Phase Radicals with Self Assembled Monolayers

Monday, November 13, 2006, 8:20 am, Room 2004

Session: Gas-Surface Reaction Dynamics
Presenter: H. Fairbrother, Johns Hopkins University
Authors: H. Fairbrother, Johns Hopkins University
J. Gorham, Johns Hopkins University
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The influence of chain length in the reactions of thin film assemblies with reactive gas phase species has been identified in studies designed to probe the modification of alkanethiolates by atomic hydrogen (AH). In these studies the change in chemical composition of four alkanethiolate self assembled monolayers (SAMs), octadecanethiolate, hexadecanethiolate, dodecanethiolate and nonathiolate, as a result of exposure to atomic hydrogen have been studied in situ, using X-ray Photoelectron Spectroscopy. Reactions of atomic hydrogen with the native thiolate S atoms at the film/substrate interface are responsible for sulfur desorption and the formation of new C-S-C species. For each alkanethiolate, exposure to atomic hydrogen results in a loss of sulfur and carbon. The rate of sulfur loss from each SAM are well described by first order kinetics with a rate constant that increases with increasing chain length. This is consistent with the idea that sulfur desorption is controlled by the rate of atomic hydrogen permeation through the hydrocarbon overlayer. In the two shorter chain SAMs, the initial loss of carbon and sulfur is strongly correlated, indicative of a dominant role for the desorption of intact adsorbate chains and large alkyl sulfur fragments. In contrast, for octadecanethiolate and hexadecanethiolate SAMs the rate of carbon loss is dramatically reduced and the carbon and sulfur desorption kinetics are uncorrelated. In these longer chain SAMs the variation in the filmâ?Ts chemical composition as a function of AH exposure is consistent with the formation of a disordered carbonaceous overlayer, due to cross-linking reactions between adjacent hydrocarbon chains, before atomic hydrogen permeates to the film/substrate interface. Results from this investigation highlight the important role that film thickness can play in moderating the reaction kinetics in organized thin film assemblies.