Effect of Covalent Surface Functionalization of Si on the Activity of Trifluoromethanesulfonic Anhydride for Suppressing Surface Recombination

Abstract

An examination of the efficacy of combining physisorbed and chemisorbed passivation strategies on crystalline Si has been performed. This report compares the influence of a linear alkyl adsorbate tethered by either a Si−C or Si−Si linkage, prepared by reaction of Si(111) with organometallic Grignard reagents or organosilanes, respectively. These modified surfaces are first analyzed and compared by IR and X-ray photoelectron spectroscopies. Their behavior toward a known potent physisorbate, trifluoromethanesulfonic anhydride (Tf 2 O), is then examined. Microwave photoconductivity measurements were obtained which indicate that, while Tf 2 O shows a beneficial lowering of surface recombination on both surface types initially, only surfaces featuring Si−C linkages exhibit long-lasting suppressed surface recombination. The data for Grignard-treated Si after exposure to Tf 2 O in fact represent the longest known report of surface recombination suppression by a physisorbate. Conversely, the data for the Si surfaces prepared by dehydrogenative coupling suggest that these passivating groups themselves introduce defect states that cannot be ameliorated by Tf 2 O physisorption.

Multi-Electron Transfer at H-Terminated p-Si Electrolyte Interfaces: Large Photovoltages under Inversion Conditions

“…45,46 The data described herein establish the direct spectroelectrochemical detection of conduction band electrons as a general tool for distinguishing inversion from FLP. 61 Alternatively, in situ quantification of the surface recombination kinetics, accomplished herein by TRIR, as a function of the applied potential will certainly provide new insights. A decrease in the recombination rate is expected for an inversion layer, and it will be of particular interest to test the assertion made herein that the surface recombination rate will increase or stay constant as the Fermi energy of a photoelectrode under FLP conditions is raised toward the vacuum level.…”

“…TRIR experiments following pulsed bandgap excitation provided additional evidence for this assignment and allowed the electron–hole pair lifetimes to be quantified. Historically, the recombination of photogenerated carriers has been quantified by time-resolved microwave conductivity measurements. − An advantage of the TRIR approach is that it enables in situ measurements in a photoelectrochemical cell as a function of the applied potential. When plotted as a rate constant, k 3 = 1/τ 3 , and normalized to the maximum value, k 3 / k 3,max , a bell-shaped dependence on the applied potential results (Figure a).…”

“…The photoelectrochemical behavior for H-terminated p-Si reported by Bocarsly and Wrighton and attributed to FLP are likely instead due to inversion layer formation. ,, Likewise, the large photovoltages and Faradaic yields reported in Kubiak’s pioneering work on CO 2 photoreduction with Lehn’s catalyst are also almost certainly due to the presence of an inversion layer . The situation for III–V semiconductors and layered chalcogenide materials is less clear as a high density of surface states might be expected to underlie FLP behavior. ,, However, as was first pointed out by Dare-Edwards, Bard’s description of p-GaAs behaving as if it were coated with a monolayer of metal is more in line with behavior expected for an inversion layer than for electron transfer mediated by poorly defined surface states. , The data described herein establish the direct spectroelectrochemical detection of conduction band electrons as a general tool for distinguishing inversion from FLP . Alternatively, in situ quantification of the surface recombination kinetics, accomplished herein by TRIR, as a function of the applied potential will certainly provide new insights.…”

Absence of a Relationship between Surface Conductivity and Electrochemical Rates: Redox-Active Monolayers on Si(211), Si(111), and Si(110)

“…Starting from the seminal work of Sagiv in 1980, fundamental and applied research on the self-assembly of organic molecules onto electrode surfaces has continued gaining momentum . While most of the research on self-assembly to date has focused on gold surfaces by exploiting alkanethiol chemistry, the 1990s reports by Chidsey and co-workers of Si–C-bound monolayers have helped expand this research from metals to semiconductors. − Silicon remains the technologically most relevant semiconducting material, − and silicon electrodes that are functionalized with organic monolayers have broad application prospects in the fields of microscopy, , sensing, − chemical catalysis, (bio)­molecular electronics, ,, and information storage. ,, …”