Toluene interactions with the 7 × 7 and sputtered Si(111) surfaces: evidence of methyl-induced and oxygen-enhanced dissociation pathways

Abstract

The effects of methyl substitution on the room-temperature (RT) adsorption and reactivity of toluene and benzene on Si(111)7 × 7 have been investigated by thermal desorption spectrometry (TDS). In particular, the mass 100 (molecular) desorption profile for the low RT exposure of d 8-toluene to Si(111)7 × 7 consists of desorption peaks at 370 and 420 K, with the intensity of the peak at 420 K saturating at a lower exposure than the peak at 370 K. The similarities in the exposure dependence of the TDS profiles of toluene and those of benzene suggest a similar adsorption configuration, i.e. a π-bonded configuration with the ring (near-)parallel to the surface. In addition to the strong molecular desorption, the presence of a relatively weak mass 4 desorption peak at 800 K at higher exposure indicates a weak secondary surface process involving the deposition of D atoms on Si(111)7 × 7 as SiD. Furthermore, the observation of a similar mass 4 desorption peak at 800 K for the RT exposures of other selectively deuterated toluene isotopes CH 3C 6D 5 and CD 3C 6H 5 to Si(111)7 × 7 suggests that there is no exclusive D (H) abstraction at either the aromatic or aliphatic position of the adsorbed olefin molecule. The lack of a corresponding mass 4 desorption profile (at 800 K) for the RT exposure of d 6-benzene to Si(111)7 × 7 indicates that the deposition of D atoms on the Si surface originates from the complete dissociation of toluene into C and D atoms, which is likely induced by a methyl-to-surface interaction. Moreover, the overall increase in the mass 4 desorption for the RT exposure of d 8-toluene to a sputtered Si surface shows that the sputtered Si surface is more reactive than the 7 × 7 surface. The presence of a new broad mass 4 desorption peak at 540 K suggests an additional pathway by which D 2 molecules evolve directly from the dissociation of adsorbed toluene molecules on the sputtered Si surface. In addition, a RT O 2 post-exposure to d 8-toluene adsorbed on Si(111)7 × 7 is found to induce a large increase in the D 2 (mass 4) desorption at 800 K. The increase in the toluene dissociation can be attributed to either a surface-enhanced oxidation or an O 2-induced effect that stabilizes the adsorbed toluene molecules to a higher temperature (>; 350 K) where dehydrogenation and dissociation occur. It is, however, difficult to determine the mechanism for the enhanced dissociation from TDS alone.