Infrared and X-ray Photoelectron Spectroscopic Studies of the Reactions of Hydrogen-Terminated Crystalline Si(111) and Si(100) Surfaces with Br_2, I_2, and Ferrocenium in Alcohol Solvents

The reaction chemistry of H-terminated crystalline Si(111) and Si(100) surfaces in CH_3OH, CD_3OD, CF_3(CH_2)_3OH, C_4H_9OH, and C_4D_9OD solutions containing ferrocenium (Fc^+)−BF_4, I_2, or Br_2 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 I_2 or Br_2, 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 LiOCH_3, LiOCD_3, or LiO(CH_2)_3CF_3. 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