Infrared and X-ray photoelectron spectroscopic studies of the reactions of hydrogen-terminated crystalline Si(111) and Si(100) surfaces with Br₂, I₂, and ferrocenium in alcohol solvents

The reaction chemistry of H-terminated crystalline Si(111) and Si(100) surfaces in CH₃OH, CD₃OD, CF₃(CH₂)₃OH, C₄H₉OH, and C₄D₉OD solutions containing ferrocenium (Fc⁺)-BF₄, I₂, or Br₂ was monitored using X-ray photoelectron (XP) spectroscopy and infrared (IR) spectroscopy. Addition of the one-electron oxidant Fc⁺, or addition of the oxidizing species I₂ or Br₂, produced diagnostic changes in the IR spectra that clearly indicated formation of surficial Si-OR groups. XPS data confirmed the conclusions of the IR studies: Under our reaction conditions, no detectable reaction occurred without the presence of the oxidant. The data are consistent with oxidative activation of the surficial Si-H bonds toward nucleophilic attack by the alcohols. The reaction chemistry was generally similar on (111)-and (100)-oriented Si surfaces, although some differences were observed in the ratio of reaction products on the two different surface orientations. Alkoxylated surfaces were also prepared by a two-step process in which the surface was first chlorinated and then reacted with LiOCH₃, LiOCD₃, or LiO(CH₂)₃CF₃. The data indicate that formation of silicon-halogen bonding alone is not sufficient to provide a robust correlation between the electronic and chemical properties of such crystalline Si surfaces and that formation of silicon-alkoxyl bonds is a common motif for surfaces often used in electronic and electrochemical studies of Si.