Tailoring the surface reactivity of silicon surfaces by partial halogenation

F. A .SORIA; E. M. PATRITO; P. PAREDES-OLIVERA

San Francisco

Congreso; 2012 Materials Research Society Spring Meeting; 2012

Materials Research Society

Halogen terminated surfaces are more reactive alternatives to the hydrogenated silicon surface for functionalization reactions. However, despite the widespread use of chlorinated silicon as the starting point for further functionalization reactions, the high reactivity of this surface towards simple polar molecules such as NH3, H2O, H2S still remains unclear. Therefore, we undertook a comprehensive density functional theory investigations to elucidate the factors that govern the reactivity of fully and partially halogenated Si(100) and Si(111) surfaces. We considered Cl and Br. Both electronic and steric factors explanin the observed reactivity trends. We show that steric effects (more important for increasing halogen coverage or for increasing halogen size) may be used to tailor the surface reactivity. The halogenated surfaces show a considerable activation with respect to the hydrogenated surface. The reaction on the halogenated surfaces proceeds via the formation of a stable dative bonded complex in which a silicon atom is pentacoordinated [1]. The activation of the halogenated Si(100)-2×1 surfaces towards the polar molecules arises from the large redistribution of charge in the transition state which precedes the breakage of the Si-X bond. Steric effects also play an important role on the surface reactivity, making brominated surfaces less reactive than the chlorinated surfaces. The voluminous bromine atoms prevent the nucleophilic attack of ammonia to the silicon atom of a SiBr group, which leads to an increase in the activation energy barrier. Therefore, the surface reactivity can be tailored by either changing the nature of the halogen atom or by changing its surface coverage. These results explain the fact that activation energy barriers are higher on the chlorinated Si(111) surface than on the chlorinated Si(100) surface because the 111 surface has a higher density of chlorine atoms than the 100 surface. [1] F. A. Soria, E. M. Patrito, P. Paredes-Olivera. Langmuir, 2011, 27, 2613.