AVS 52nd International Symposium
    Surface Science Wednesday Sessions
       Session SS+EM-WeM

Paper SS+EM-WeM2
Surface Bonding and Dynamical Behavior of CH@sub 3@SH on Au(111)

Wednesday, November 2, 2005, 8:40 am, Room 202

Session: Self-Assembled Monolayers
Presenter: P. Maksymovych, University of Pittsburgh
Authors: P. Maksymovych, University of Pittsburgh
D.C. Sorescu, U.S. Department of Energy
D.B. Dougherty, University of Pittsburgh
J.T. Yates, Jr., University of Pittsburgh
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Alkanethiols are often used for the growth of self-assembled monolayers, which are ordered 2-D molecular films covalently bonded to a substrate, typically a gold surface. The bonding of alkanethiols to gold has remained a controversial issue. In particular, the bonding of the undissociated alkanethiols has not been addressed at the atomic scale so far. We have studied adsorption and surface chemistry of the smallest alkanethiol, CH@sub 3@SH, on the Au(111) surface using the Scanning Tunneling Microscopy (STM) at 5K and Density Functional Theory (DFT). It has been established that CH@sub 3@SH adsorbs and desorbs without dissociation. Using the STM, it was found that CH@sub 3@SH adsorbs on top of an Au atom on the defect-free surface, and undergoes hindered rotation at 5K. The barrier to rotation obtained by DFT calculations is ~0.1 kcal*mol@super -@@super 1@. The STM image of the rotating molecule has a shape of a flower with six petals. The pattern suggests that the potential minima directions occur for the rotating molecule at the six hollow sites surrounding the atop adsorption site. Furthermore, selective site occupation was observed on the length-scale of the herringbone reconstruction of the Au(111) surface. At very low coverage, the CH@sub 3@SH molecule was found to preferentially adsorb on the intrinsic defective sites on the surface, such as the herringbone "elbows" and random atomic step sites. At higher coverage preferential adsorption occured on the fcc-stacked regions of the herringbone reconstruction. The latter is explained by the variation of the surface stress across the Au(111) surface due to the herringbone reconstruction, which causes the mobile CH@sub 3@SH molecules to select the most exothermic adsorption sites available at various coverages. We thank the W. M. Keck Foundation and NEDO (Japan) for financial support of this work.