Mechanistic insights into the oxidative reaction of hydrogen-terminated Si(111) surfaces with liquid methanol

Abstract

The reactivity of nucleophiles at H-terminated Si surfaces has been widely exploited to tailor the electronic and mech. properties of Si interfaces for targeted applications. In select cases, the addn. of nucleophiles to Si surfaces has no analogous reaction in mol. silane chem. To investigate this phenomenon, H-Si(111) surfaces were reacted with liq. methanol (CH_3OH) in the absence or presence of a series of one-electron oxidants and /or illumination. Oxidant-activated methoxylation of H-Si(111) surfaces was obsd. in the dark after exposure to CH_3OH solns. contg. the oxidants acetylferrocenium, ferrocenium, or 1,1'-dimethylferrocenium. The oxidantactivated reactivity of intrinsic and n-type H-Si(111) surfaces toward CH_3OH increased upon exposure to ambient light. The results suggest that oxidant-activated methoxylation requires that two conditions be met: (1) the position of the quasi-Fermi levels must energetically favor oxidn. of the H-Si(111) surface and (2) the position of the quasi-Fermi levels must energetically favor redn. of an oxidant in soln. The results are described by a mechanistic framework that analyzes the positions of the quasi-Fermi levels relative to the applicable energy levels for each system. Thus, the reactions of nucleophiles at H-terminated Si surfaces implicates the bulk charge carriers, allowing for reactions to occur at Si surfaces that are not accessible to mol. silanes.