AVS 46th International Symposium
    Surface Science Division Thursday Sessions
       Session SS1+AS+BI-ThM

Paper SS1+AS+BI-ThM3
Structure, Bonding and Reactivity of Self-assembled Monolayers

Thursday, October 28, 1999, 9:00 am, Room 606

Session: Self-Assembled Monolayers
Presenter: G.J. Leggett, UMIST, UK
Authors: G.J. Leggett, UMIST, UK
B.D. Beake, UMIST, UK
N.D. Brewer, UMIST, UK
E. Cooper, Glaxo-Wellcome, UK
D.A. Hutt, University of Loughborough, UK
Correspondent: Click to Email
Two approaches to the characterisation of the nature of the sulfur bonding environment in self-assembled monolayers (SAMs) have been explored: sulfur K-edge surface extended X-ray absorption fine structure (SEXAFS) and static secondary ion mass spectrometry (SIMS). S K-edge SEXAFS has proved highly effective for SAMs on Ag and has confirmed that the sulfur adsorbs with threefold coordination during both solution-phase adsorption onto polycrystalline Ag and gas-phase adsorption onto Ag(111) single crystal surfaces. There is no change in the S adsorption site with coverage. Static SIMS has yielded powerful insights into the effect of electron beam bombardment on SAM structure. A complete loss of all gold-molecular fragments from the spectrum is observed after small doses of electrons, indicating a rapid alteration of the sulfur bonding environment, due either to S-C bond scission or, more likely, to oxidation of thiolates to disulfides. Changes in Au-S bonding during the formation of the low coverage phase of butanethiol on Au have also been studied by static SIMS. Photo- and air-oxidation rates are influenced by the adsorbate alkyl chain length; rates of both processes decrease with increasing chain length due to increasing SAM ordering. The nature of the terminal group also affects rates of oxidation, and has a profound influence on the stability of the SAM. Hydrogen bonding between neighbouring terminal groups leads to significant stabilisation. Friction coefficients may be measured for SAMs using friction force microscopy. These decrease with increasing alkyl chain length, and are higher for adsorbates with polar terminal groups. Oxidation of methyl terminated SAMs leads to an increase in the coefficient of friction, interpreted in terms of the collapse of film order following head group oxidation. The rate of increase in the coefficient of friction is faster for short chain SAMs than long-chain SAMs, in agreement with the findings of static SIMS studies.