Photochemical Reaction of Diazomethane with Hydrogen-Terminated Silicon Surfaces

Abstract

Diazomethane reacts with hydrogen-terminated porous or single crystal silicon surfaces under irradiation\nwith the 365 nm line from a mercury lamp. The surface Si−H<i>_x</i> groups are lost and an HF-resistant, polymeric\nhydrocarbon species is formed. The mechanism is proposed to commence with insertion of singlet methylene\ninto the Si−H bond as in the analogous reaction of molecular hydrosilanes. However, the infrared spectra\nindicate that −CH₂− groups rather than terminal −CH₃ are dominant on the surface after reaction. This is\nattributed to an oligomerization which may proceed via attack of singlet methylene on C−H bonds or via a\nradical process. In the case of porous silicon, significant oxidation occurs in parallel to the reaction with\ndiazomethane due to the presence of trace water from the reagents used to generate diazomethane. The oxidized\nsilicon species can, however, be removed selectively by washing in aqueous 48% HF without affecting the\nintegrated intensity of the C−H stretching bands observed by FTIR spectroscopy. In combination with an\nanalysis of the IR spectroscopic data, we conclude that the hydrocarbon species on the surface are anchored\nvia robust Si−C bonds rather than the labile Si−O−C linkage. This is further supported by the stability of\ndiazomethane-treated, polished single crystal wafers to in-air STM and electrochemical cycling in aqueous\nmedia. The diazomethane treatment also shifts the flatband potential in a negative direction in aqueous NaCl\ncompared to Si(111)−H surfaces.