Chemical stability of porous silicon surfaces electrochemically modified with functional alkyl species

The chemical stability of electrochemically alkylated porous silicon is studied. The hydride-terminated surface of p-type or p(++)-type porous silicon is stabilized by electrochemical reduction of organohalides in acetonitrile solutions. Reduction of 6-iodo-ethylhexanoate, 1-iodo-6-(trifluoroacetylamino)hexane, iodomethane, 1-bromohexane, or ethyl 4-bromobutyrate at a porous Si cathode results in removal of the halogen and attachment of the organic fragment to the porous Si surface via a Si-C bond. A two-step procedure involving attachment of the functional group of interest followed by attachment of methyl groups (by reduction of iodomethane) to residual, more sterically inaccessible sites on the porous Si surface is found to yield a more stable material. Three tests of the chemical stability of the modified surfaces are performed: treatment with dimethyl sulfoxide (a chemical oxidant for porous Si), treatment with aqueous Cu2+, and exposure to 10% ethanol in a solution of phosphate buffered (pH = 7.4) aqueous saline. The reactions are characterized by atomic force microscopy, Fourier transform infrared (FT-IR) and optical reflectivity spectroscopies. The data indicate that electrochemical alkylation greatly improves the stability of porous Si against oxidation and corrosion, and that the methyl capping procedure provides the most stable material.