Density Functional Study of the Dissociative Adsorption of Aromatic Molecules on the Si(100) Surface: On the Way from Benzene to Larger Polycyclic Hydrocarbons

Abstract

Density functional calculations have been performed on possible mechanisms for the hypothetic C-H bond cleavage process of benzene chemisorbed on the Si(100) surface, in order to shed light on the analogous process on larger polycyclic aromatic hydrocarbons. We first identified the minima on the potential energy surface for the benzene adsorption on Si(100) and for the breaking of two C-H bonds, with formation of two Si-H bonds, and then we analyzed possible pathways for the C-H bond cleavage, looking for the transition states connecting the adsorption configurations to the final products of C-H breaking. We identified two adsorbed configurations of benzene from which the breaking of two C-H bonds can be accessible, i.e., the 1,2 tilted di-o bonded configuration on top of a single dimer (T) and the 1,4 di-σ bonded configuration where benzene bridges two dimer rows (BR). The kinetically most favorable reactive channel on the T configuration involves the abstraction of two hydrogen atoms on the sp 3 carbon atoms by the silicon atoms of an adjacent dimer, with an energy barrier of 22.0 kcal mol -1 . Although seemingly low, such an activation energy is not expected to be accessible at temperatures below the onset of benzene desorption from this configuration, which requires 15.9 kcal mol -1 The kinetically most favorable reactive channel on the BR configuration, which has not been experimentally detected for the benzene molecule, involves the rupture of one Si-C bond, passing through an energy barrier of 29.8 kcal mol -1 , and ends with the formation of a Si-H bond and a vertical phenyl unit anchored on a silicon dimer.