Molecular dynamics with combined quantum and empirical potentials: C2H2 adsorption on Si(100)

Abstract

Classical trajectory calculations were employed to study the reaction of acetylene with dimer sites on the Si(100) surface at 105 K. Two types of potential energy functions were combined to describe interactions for different regions of the model surface. A quantum mechanical potential based on the semiempirical AM1 Hamiltonian was used to describe interactions between C2H2 and a portion of the silicon surface, while an empirically parametrized potential was developed to extend the size of the surface and simulate the dynamics of the surrounding silicon atoms. Reactions of acetylene approaching different sites were investigated, directly above a surface dimer, and between atoms from separate dimers. In all cases, the outcome of C2H2 surface collisions was controlled by the amount of translational energy possessed by the incoming molecule. Acetylene molecules with high translational energy reacted with silicon dimers to form surface species with either one or two Si–C bonds. Those molecules with low translational energy either rebounded away from the surface or became trapped in a physisorbed state as evidenced by their bouncing motion above the surface. The reaction of C2H2 to form a bridge between dimers within the same dimer row was found to occur, while bridging between adjacent dimer rows appeared to be unlikely, the C2H2 molecule preferring to migrate to either of the dimers for direct reaction. A mechanism is proposed for chemisorption in which C2H2 first bonds to a dimer site in a mono-σ structure, subsequently attaining the more stable di-σ bonded state through radical–radical recombination. The simulations are consistent with C2H2 adsorption on Si(100) occurring through a mobile precursor mechanism.