Thermal chemistry of toluene and benzene on Si(1 0 0)2×1 and modified surfaces

Abstract

The effects of methyl substitution on the room-temperature (RT) adsorption and reactivity of toluene on Si(1 0 0)2×1 have been investigated by thermal desorption spectrometry (TDS), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). The similarities in the exposure dependence of the molecular desorption profiles of d 8-toluene and those of d 6-benzene suggest that both molecules have similar adsorption configurations on the (2×1) surface. In particular, three molecular desorption features are found and can be attributed to adsorption on the single-dimer, double-dimer, and defect sites. However, the higher coverage and much lower molecular desorption of toluene than those of benzene suggest that the majority of adsorbed toluene has undergone other surface reactions during the TDS experiment. In addition to the weak molecular desorption, strong recombinative hydrogen desorption at 820 K, characteristic of hydrogen coming from mono-hydride (Si–H) surface species, is observed for toluene. The lack of a correspondingly strong D 2 desorption peak for the RT exposure of d 6-benzene to Si(1 0 0)2×1 indicates that the deposition of D atoms results from hydrogen abstraction from the methyl group of d 8-toluene. Since a RT post-exposure of atomic hydrogen to Si(1 0 0)2×1 saturated with d 8-toluene does not appear to increase molecular desorption, the observed hydrogen abstraction therefore occurs upon adsorption and appears to be an irreversible process at RT. The difference in the D 2 desorption profiles between toluene and D 2 on Si(1 0 0)2×1 suggests, in addition to hydrogen desorption, other surface processes including condensation polymerization and/or dissociative desorption that occur at 750–950 K. The Si(1 0 0)2×1 surface is found to be more active in hydrogen abstraction and toluene dissociation than Si(1 1 1)7×7. For a RT exposure of d 8-toluene (d 6-benzene) to a sputtered Si(1 0 0) surface, the presence of an additional broad mass 4 desorption band at ∼600 K suggests the presence of a range of pathways by which D 2 evolves directly from the dissociation of adsorbed d 8-toluene. A RT exposure of O 2 to Si(1 0 0)2×1 pre-adsorbed with d 8-toluene (d 6-benzene) is found to induce a substantial increase in the mass 4 desorption. The increase in the dissociation can be attributed to either surface-mediated oxidation or an O 2-induced process that stabilizes the adsorbed molecules to a higher temperature at which dehydrogenation and dissociation occur.