A supersonic molecular beam study of the reaction of tetrakis(dimethylamido)titanium with self-assembled alkyltrichlorosilane monolayers

Abstract

The reaction of a transition metal coordination complex, Ti[N(CH 3) 2] 4, with self-assembled monolayers (SAMs) possessing-OH, -NH 2, and -CH 3 terminations has been examined using supersonic molecular beam techniques. The emphasis here is on how the reaction probability varies with incident kinetic energy (E i = 0.4-2.07 eV) and angle of incidence (θ i=0°-60°). The most reactive surface is the substrate underlying the SAMs-SiO 2 with a high density of -OH(a) (>5 × 10 14 cm -2), "chemical oxide." On chemical oxide, the dynamics of adsorption are well described by trapping, precursor-mediated adsorption, and the initial probability of adsorption depends only weakly on E i and θ i,. The dependence of the reaction probability on substrate temperature is well described by a model involving an intrinsic precursor state, where the barrier for dissociation is approximately 0.2-0.5 eV below the vacuum level. Reaction with the SAMs is more complicated. On the SAM with the unreactive, -CH 3, termination, reactivity decreases continuously with increasing E i while increasing with increasing θ i. The data are best interpreted by a model where the Ti[N(CH 3) 2] 4 must first be trapped on the surface, followed by diffusion through the SAM and reaction at the SAM/SiO 2 interface with residual -OH(a). This process is not activated by E i and most likely occurs in defective areas of the SAM. On the SAMs with reactive end groups, the situation is quite different. On both the-OH and -NH 2 SAMs, the reaction with the Ti[N(CH 3) 2] 4 as a function of E i passes through a minimum near E i ∼1.0 eV. Two explanations for this intriguing finding are made - one involves the participation of a direct dissociation channel at sufficiently high E i. A second explanation involves a new mechanism for trapping, which could be termed penetration facilitated trapping, where the Ti[N(CH 3) 2] 4 penetrates the near surface layers, a process that is activated as the molecules in the SAM must be displaced from their equilibrium positions. © 2006 American Institute of Physics.