Thermal desorption spectroscopy of alkanethiol self-assembled monolayer on Au(111)

The adsorption and desorption of octadecanethiol (CH3(CH2)(17)SH: monomer) self-assembled monolayers (SAMs) on Au(lll) have been investigated by thermal desorption spectroscopy (TDS). Two desorption peaks of dioctadecyl disulfide ((CH3(CH2)(17)S)(2)(+): dimer(+)) exhibiting different sticking behaviors have been identified in the TDS spectra, while the desorption of octadecanethiol (CH3(CH)(17)SH+: monomer(+)) occurred at a lower temperature than that of the dimer(+). The desorption activation energy and the pre-exponential factor for each peak were estimated assuming first- and second-order reactions. The analysis results suggest that both alkyl thiolate (RS-Au) with strong chemisorption and dialkyl disulfide (RS-SR: dimer) with relatively weak adsorption co-exist in the alkanethiol SAM on An. The total amount of relatively weakly adsorbed dimers increased appreciably in a slow reaction step, while the monolayer thickness remained almost constant after SAM formation. This result suggests that dimer molecules are formed in the monolayer. This formation of weakly adsorbed dimers is strongly related to the self-exchange phenomena in SAM films reported by Schlenoff et al. [J. Am. Chem. Sec. 117 (1995) 12528].