FTIR Studies of CH 3 OH on Porous Silicon

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John A. Glass, Jr., Edward A. Wovchko, and John T. Yates, Jr. University of Pittsburgh, Surface Science Center, Department of Chemistry, Pittsburgh, PA, 15260. ABSTRACT Fourier transform infrared spectroscopy (FTIR) was used to investigate the reaction of methanol with porous silicon and hydrogen passivated porous silicon. At 300 K methanol adsorbs onto hydrogen free porous silicon by cleavage of the O-H bond. Both of the resulting surface species, Si-H and Si-OCH 3, were determined to be stable up to -500 K. Above 500 K the Si-OCH 3 moiety decomposes by breakage of the C-O and C-H bonds. The resulting carbon and oxygen were incorporated into the porous layer and additional Si-H surface species were detected. Further heating to 900 K removed the Si-H surface species. Adsorption of methanol onto hydrogen-passivated porous silicon did not occur until 600 K. At temperatures beyond 600 K, oxygen and carbon incorporation into the porous layer and Si-OCH 3, Si-CH 3, and Si-H surface species were seen. The previously unseen Si-CH 3 surface species is believed to be stabilized by oxygen incorporation.

INTRODUCTION Two important technologies - electronic device fabrication and catalytic silane and siloxane production [1-3] - use silicon and silicon containing materials. It is surprising that the surface reaction chemistry of organic and aqueous solvents with both silicon (100) crystals and amorphous silicon has not been more widely studied. Clearly, interactions between solvents employed in the cleaning, production, and growth protocols, and silicon (100) surfaces [4, 5] will become more critical with increasing demand for more efficient removal of contaminants. Initial studies of H20 on Si(100) have given insights into the dissociative adsorption of H20 and resulting surface species [6, 7]. Organic solvent decomposition on Si(100), however, has only just recently begun to receive attention[8, 9]. Research on silicon optoelectronics has focused primarily on porous silicon and its photoluminescence properties. However, the cause of the photoluminescent process has been the subject of much debate [10, 11]. This technology would probably involve solvent use on porous silicon. The Direct Synthesis of silane and siloxanes has recently begun to be studied [3, 12]. The overall goal in these studies has been to determine the mechanistic aspects of the process. The reaction of organic molecules, such as methanol, with silicon is a first step in understanding this technologically important process. Transmission Fourier Transform Infrared Spectroscopy (FTIR) is sensitive to surface species and reactions provided the material under study has a high surface area (~1018 399 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society

sites / cm2 ). Single crystals of silicon have insufficient surface site density to apply transmission FTIR spectroscopy. Porous silicon, a high surface area material, makes an excellent silicon surface for gathering transmission FTIR spectra from molecules adsorbed on silicon. Concerns relative t

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