The photoelectrochemical behavior of methyl-terminated p-type and n-type Si(111) surfaces was determined in contact with a series of one-electron, outer-sphere, redox couples that span >1 V in the Nernstian redox potential, E(A/A -), of the solution. The dependence of the current vs potential data, as well as of the open-circuit photovoltage, V oc, on E(A/A -) was compared to the behavior of H-terminated p-type and n-type Si(111) surfaces in contact with these same electrolytes. For a particular E(A/A -) value, CH 3-terminated p-Si(111) electrodes showed lower V oc values than H-terminated p-Si(111) electrodes, whereas CH 3-terminated n-Si(111) electrodes showed higher V oc values than H-terminated n-Si(111) electrodes. Under 100 mW cm -2 of ELH-simulated Air Mass 1.5 illumination, n-type H-Si(111) and CH 3-Si(111) electrodes both demonstrated nonrectifying behavior with no photovoltage at very negative values of E(A/A -) and produced limiting V oc values of >0.5 V at very positive values of E(A/A -). Illuminated p-type H-Si(111) and CH 3-Si(111) electrodes produced no photovoltage at positive values of E(A/A -) and produced limiting V oc values in excess of 0.5 V at very negative values of E(A/A -). In contact with CH 3CN- octamethylferrocene +/0, differential capacitance vs potential experiments yielded a -0.40 V shift in flat-band potential for CH 3-terminated n-Si(111) surfaces relative to H-terminated n-Si(111) surfaces. Similarly, in contact with CH 3CN-1,1′- dicarbomethoxycobaltocene +/0, the differential capacitance vs potential data indicated a -0.25 V shift in the flat-band potential for CH 3-terminated p-Si(111) electrodes relative to H-terminated p-Si(111) electrodes. The observed trends in V oc vs E(A/A -), and the trends in the differential capacitance vs potential data are consistent with a negative shift in the interfacial dipole as a result of methylation of the Si(111) surface. The negative dipole shift is consistent with a body of theoretical and experimental comparisons of the behavior of CH 3-Si(111) surfaces vs H-Si(111) surfaces, including density functional theory of the sign and magnitude of the surface dipole, photoemission spectroscopy in ultrahigh vacuum, the electrical behavior of Hg/Si contacts, and the pH dependence of the current-potential behavior of Si electrodes in contact with aqueous electrolytes.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films