Thermal reactions of methanethiol and ethanethiol on Si(100)

We investigated adsorption and thermal decomposition of methanethiol (CH₃SH) and ethanethiol (C₂H₅SH) on a Si(100) surface by means of temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) with synchrotron radiation. At an adsorption temperature of 115 K, CH₃SH and C₂H₅SH dissociate to form thiolates and hydrogen at a small coverage (<0.2 monolayer), whereas molecular chemisorption occurs at a greater coverage; all chemisorbed molecules either deprotonate to form thiolate or desorb intact up to 400 K. Adsorption and decomposition of thiols occur on the dangling bonds of a dimer without breaking the Si-Si dimer bond, resulting in preservation of a 2 × 1 LEED pattern. Thiolates further decompose to evolve hydrocarbons via scission of the C-S bond to form a sulfur adatom on the surface at a temperature above 550 K. Maximum desorption of surface sulfur as SiS occurs at 820 K. CH₃ generated from CH₃S reacts with surface hydrogen to evolve CH₄, whereas the C₂H₅ moiety of C₂H₂S undergoes β-hydride elimination to form C₂H₄. To a small extent, the alkyl moiety transfers onto the surface and undergoes dehydrogenation, resulting in desorption of hydrogen and deposition of carbon on the surface.