Synthetic Insights into Surface Functionalization of Si(111)–R Photoelectrodes: Steric Control and Deprotection of Molecular Passivating Layers

We report the utility of
controlled spacing of molecular monolayers on Si(111) surfaces by
the use of sterically bulky silanes. The steric bulk of a 3,5-diphenolic
linker of type Ph–diO–SiR3 (R = hexyl, phenyl, iPr)as well as the smaller Ph–diOMeis
shown to control the surface coverage on Si(111). The para substituent
was also changed from −F (small) to −OTf (triflate,
large) to modulate the conformation of a selected bulky silane (SiR3; R = hexyl) to further control the steric environment of
the monolayer. The surface coverage values are found to vary systematically
from 57 → 21 → 15 → 11% for the series CH3 → hexyl → iPr →
phenyl. Substitution at the para position (F → OTf) decreased
the packing density for R = hexyl to as low as 8% (from 21%). The
molecular coverage was also found to control the rate and extent of
surface oxidation when unfunctionalized sites were allowed to oxidize.
Following attachment, facile deprotection of the silanes was achieved
by treatment with BBr3 to afford the diphenolic −OH
groups. To electronically characterize the monolayers, voltammetry
was performed in contact with liquid Hg to determine the barrier height,
which was decreased by 70 mV as the coverage is increased. This study
provides a synthetic rationale for controlling the packing density
of surface linkers using electroless chemistry at semiconductor interfaces,
thus providing further tunability and functionality of photoelectrochemical
devices.