Electron-Transfer Processes at Semiconductor/Liquid Interfaces and Metal/Nanogap Junctions

It is shown that n-ZnO/H₂O-A/A⁻ junctions (A/A⁻ = [Co(bpy)₃]³⁺/²⁺ or [OsL₂L']³⁺/²⁺) display energetic and kinetic behavior of unprecedented ideality. The rate constant of the junction with the highest driving force increased when the driving force was lowered, which indicates that the junction operated in the inverted regime. The driving force was varied by shifting the conduction-band edge of the semiconductor with pH. The contact with the lowest driving force was found to operate in the normal regime of charge transfer. These results provide the first experimental indication that semiconductor/liquid contacts can operate in the inverted regime. Junctions having a similar driving force but different reorganization energies show the expected dependence of the rate constant on the reorganization energy. Low surface-recombination velocities (SRVs) were observed for systems with an accumulation of holes or electrons at the Si surface. Formation of the charge-carrier accumulation layer was confirmed by a solution-gated transistor method. Digital simulations revealed that SRVs < 10 cm s⁻¹ can be produced by surfaces with trap densities as large as 10¹² cm⁻² provided that the surface is in accumulation or inversion. The degree of band bending and SRVs of Si(111) in contact with a variety of aqueous fluoride solutions were determined for the first time at open circuit. An accumulation of electrons at the surface is responsible for the low effective SRVs in NH₄F and buffered HF solutions. The protonation of basic defect sites is important for the low SRV of Si(111)/H₂SO4(aq) and Si(111)/HF(aq) contacts. The J-E characteristics of electron-tunnel junctions formed by the electromigration of metal nanowires without a molecule bridging the gap were explored in detail. The low-temperature J-E curves of some junctions showed regions of zero conductivity near zero bias, while such features were absent in the data collected for other junctions. A common pattern was discerned in that the low-bias resistances of all junctions decreased by at least an order of magnitude with increasing temperature according to Abeles' model for electron tunneling in granular metal junctions. These findings were consistent with the Coulomb blockade effect and can be attributed to metal islands in the gap.