title: On the mechanism of silicon activation by halogen atoms.
abstract: Despite the widespread use of chlorinated silicon as the starting point for further functionalization reactions, the high reactivity of this surface toward a simple polar molecule such as ammonia still remains unclear. We therefore undertook a comprehensive investigation of the factors that govern the reactivity of halogenated silicon surfaces. The reaction of NH3 was investigated comparatively on the Cl-Si(100)-2 × 1, Br-Si(100)-2 × 1, H-Si(100)-2 × 1, and Si(100)-2 × 1 surfaces using density functional theory. The halogenated surfaces show considerable activation with respect to the hydrogenated surface. The reaction on the halogenated surfaces proceeds via the formation of a stable datively bonded complex in which a silicon atom is pentacoordinated. The activation of the halogenated Si(100)-2 × 1 surfaces toward ammonia arises from the large redistribution of charge in the transition state that precedes the breakage of the Si-X bond and the formation of the Si-NH2 bond. This transition state has an ionic nature of the form Si-NH3(+)X(-). Steric effects also play an important role in surface reactivity, making brominated surfaces less reactive than chlorinated surfaces. The overall activation-energy barriers on the Cl-Si(100)-2 × 1 and Br-Si(100)-2 × 1 surfaces are 12.3 and 19.9 kcal/mol, respectively, whereas on the hydrogenated Si(100)-2 × 1 surface the energy barrier is 38.3 kcal/mol. The reaction of ammonia on the chlorinated surface is even more activated than on the bare Si(100)-2 × 1 surface, for which the activation barrier is 21.3 kcal/mol. Coadsorption effects in partially aminated surfaces and in the presence of reaction products increase activation-energy barriers and have a blocking effect for further reactions of NH3.
authors.affiliation: Departamento de Fisicoquímica and ‡Departamento de Matemática y Física, Facultad de Ciencias Químicas and Instituto de Investigaciones en Fisicoquímica de Córdoba (INFIQC), Universidad Nacional de Córdoba , Córdoba, Argentina.
authors.forename: Federico A
authors.initials: FA
authors.lastname: Soria
authors.forename: Eduardo M
authors.initials: EM
authors.lastname: Patrito
authors.forename: Patricia
authors.initials: P
authors.lastname: Paredes-Olivera
mesh_terms: D000641:Ammonia / Q000737:chemistry; D006219:Halogens / Q000737:chemistry*; D011789:Quantum Theory; D012825:Silicon / Q000737:chemistry*
chemical_list: D006219:Halogens; D000641:Ammonia; D012825:Silicon