Formation and reactivity of alkylsilane-based monolayers on gold.

Abstract

Chemisorbed monolayers of alkylsilanes on gold have been synthesized under ultrahigh vacuum (UHV) conditions and characterized using synchrotron and conventional X-ray photoemission spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIRS) and scanning tunneling microscopy (STM). Hexylsilane (C₆H₁₃SiH₃), octylsilane (C₈H₁₇SiH₃), and octadecylsilane (C₁₈H₃₇SiH₃) were employed. Monolayer were synthesized by exposing freshly evaporated Au samples to the chosen silane in UHV. Saturation occurred after exposure to &sim;400 Langmuir. Multilayer formation is not observed. Monolayer formation proceeds through the reductive elimination of the Si-H bonds and formation of three Si-Au bonds. Based on the narrow full width at half maximum of the Si 2p core level (&sim;0.4 eV), the Si atoms of the monolayer are in a chemically homogeneous environment. X-ray photoemission valence band spectra indicate the alkyl chains are intact in the monolayer. The alkylchains are oriented nearly perpendicular to the surface and &sim;96% of the surface is covered by the monolayer, as determined by variable energy XPS. Upon exposure to ambient, rapid oxidation (<15 minutes) of the Si head group produces a physisorbed siloxane monolayer. Exposure to O₂ and H₂O <italic> in vacuo</italic> do not result in oxidation of the monolayer. Exposure to ozone generates the physisorbed siloxane monolayer. Further ozone exposure results in oxidation of the alkylchain, consistent with the formation of alcohol and carboxylic acid species. Displacement of chemisorbed molecules has been observed through the reaction of octylsilane with a monolayer of H₈Si₈O₁₂ on Au. The resulting monolayer composition is &sim;70/30 (octylsilane/H₈Si₈O₁₂). The displacement reaction is self limiting at this ratio. Analogous monolayers have been synthesized for octylgermane (C₈H₁₇GeH₃) on gold.