Toward a Mechanistic Understanding of Exciton-Mediated Hydrosilylation on Nanocrystalline Silicon

White-light initiated hydrosilylation of nanocrystalline porous silicon was found to be far more efficient (in terms of both kinetics and yield) in the presence of electron-accepting molecules with suitably high reduction potentials, particularly halocarbons. It is known that absorption of visible light by nanocrystalline silicon results in the formation of excitons (electron/hole pairs) and that this exciton can be harnessed to drive a hydrosilylation reaction with an alkene; the Si–C bond forms as a result of attack of the π-electrons of the alkene on the positively charged holes. In order to better understand the white-light initiated mechanism through which this reaction takes place, and to compare with UV-mediated photoemission on Si(111)–H, a series of electron acceptors were screened for their effect on surface alkene hydrosilylation. A very strong correlation between reduction potentials (<i>E</i>_red) of the oxidant and reaction efficiency was observed, with a minimum “turn-on” <i>E</i>_red required for an increase to take place. The oxidant appears to accept, or remove, the electron from the nanocrystallite-bound exciton, favoring attack by the alkene on the positively charged Si nanocrystallite, leading to Si–C bond formation. Radical reactions were discounted for a number of reasons, including lack of effect of radical traps, no apparent Si–Cl bond formation, lack of oxidation of the surfaces, and others. Unlike with other oxidants such as nitro-aromatics, halocarbons do not cause additional surface reactions and promote very clean, fast, and selective hydrosilylation chemistry

Nanoscale Plasmonic Stamp Lithography on Silicon

Nanoscale lithography on silicon is of interest for applications ranging from computer chip design to tissue interfacing. Block copolymer-based self-assembly, also called directed self-assembly (DSA) within the semiconductor industry, can produce a variety of complex nanopatterns on silicon, but these polymeric films typically require transformation into functional materials. Here we demonstrate how gold nanopatterns, produced <i>via</i> block copolymer self-assembly, can be incorporated into an optically transparent flexible PDMS stamp, termed a plasmonic stamp, and used to directly functionalize silicon surfaces on a sub-100 nm scale. We propose that the high intensity electric fields that result from the localized surface plasmons of the gold nanoparticles in the plasmonic stamps upon illumination with low intensity green light, lead to generation of electron–hole pairs in the silicon that drive spatially localized hydrosilylation. This approach demonstrates how localized surface plasmons can be used to enable functionalization of technologically relevant surfaces with nanoscale control