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

Abstract

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.

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