Thermal Reactions of Methanethiol and Ethanethiol on Si(100)

Abstract

We investigated adsorption and thermal decomposition of methanethiol (CH 3 SH) and ethanethiol (C 2 H 5 SH) 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, CH 3 SH and C 2 H 5 SH 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. CH 3 generated from CH 3 S reacts with surface hydrogen to evolve CH 4, whereas the C 2 H 5 moiety of C 2 H 5 S undergoes β-hydride elimination to form C 2 H 4 . 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.