By means of temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy (XPS) with synchrotron radiation, we investigated the adsorption and thermal decomposition of alkanethiols (RSH, R = CH3, C2H5, and C4H9) on a GaAs(1 0 0) surface. All chemisorbed alkanethiols can deprotonate to form thiolates below 300 K via dissociation of the sulfhydryl hydrogen (-SH). Two types of thiolates species are observed on GaAs(1 0 0), according to adsorption on surface Ga and As sites. The thiolates adsorbed on a Ga site preferentially recombine with surface hydrogen to desorb as a molecular thiol at 350-385 K. The thiolate on the As site exhibits greater thermal stability and undergoes mainly dissociation of the C-S bond at ∼520 K, independent of the alkyl chain length. The decomposition of CH3S either directly desorbs CH3 or transfers the CH3 moiety onto the surface. The surface CH3 further evolves directly from the surface at 665 K. The dissociations of C2H5S and C4H9S yield surface C2H5 and C4H9, which further decompose to desorb C2H4 and C4H8, respectively, via β-hydride elimination. The complete decomposition of alkanethiol leads to the formation of surface S without deposition of carbon. Adsorption of CH3SSCH3 results in the formation of surface CH3S at initial exposures via scission of the S-S bond. Compared with the adsorption of CH3SH, the CH3S on the Ga site exhibits greater thermal stability because surface hydrogen is absent. At a high exposure, CH3SSCH3 can absorb molecularly on the surface and decompose to desorb CH3SCH3 via formation of a CH3SS intermediate.
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