AVS 53rd International Symposium
    Surface Science Tuesday Sessions
       Session SS-TuP

Paper SS-TuP5
Gold Adatom-mediated Bonding in Self-assembly of Alkanethiols on the Au(111) Surface

Tuesday, November 14, 2006, 6:00 pm, Room 3rd Floor Lobby

Session: Surface Science Poster Session
Presenter: P. Maksymovych, University of Pittsburgh
Authors: P. Maksymovych, University of Pittsburgh
D.C. Sorescu, U. S. Department of Energy
J.T. Yates, Jr., University of Pittsburgh
Correspondent: Click to Email
The majority of self-assembled molecular monolayers (SAMs) are at present grown using the adsorption of alkanethiol molecules on the Au(111) surface. Despite decades of research in this field, the structural nature of the S-Au anchor bond of the thiolate to the Au(111) surface is still a matter of active debate. We addressed this issue by studying the decomposition of prototypical CH@sub 3@SH, CH@sub 3@SSCH@sub 3@ and C@sub 3@H@sub 7@SH molecules on the Au(111) surface using Scanning Tunneling Microscopy (STM) and Density Functional Theory. Initially, the CH@sub 3@S species were produced by STM-tip induced dissociation of CH@sub 3@SH or CH@sub 3@SSCH@sub 3@ molecules on the Au(111) surface at 5K. By comparing the experimental and calculated STM images of the CH@sub 3@S species produced in this way, we determined that the S-headgroup adsorbs on the two-fold coordinated bridge-site between two surface Au atoms, consistent with theoretical predictions for this system. In the next step, CH@sub 3@SSCH@sub 3@, CH@sub 3@SH and C@sub 3@H@sub 7@SH molecules were dissociated thermally in conditions typically used for SAM growth. Unexpectedly, we find that dissociation of the parent molecules in these conditions is mediated by gold adatoms, which populate the Au(111) surface at elevated temperatures. The product CH@sub 3@S (and C@sub 3@H@sub 7@S) species form self-assembled Au adatom-bound complexes, which have substantially higher calculated binding energy than CH@sub 3@S species bonded to the Au(111) surface. The involvement of Au adatoms in the self-assembly of alkanethiols resolves the current controversy around the headgroup-sulfur bonding, as we show for the case of the low-coverage stripe-phase SAM, and explains the morphological changes of the Au(111) surface that accompany self-assembly. We thank W. M. Keck Foundation and NEDO (Japan) for the financial support of this work.