Radical formation at the gallium nitride nanowire-electrolyte interface by photoactivated charge transfer.

We investigated the transfer of photogenerated charge carriers from GaN nanowires into a surrounding electrolyte by electron paramagnetic resonance (EPR) and fluorescence spectroscopy. Using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap we find that the formation of hydroxyl radicals dominates in acidic, neutral and moderately basic environments, while in an electrolyte with a pH of 13.5 the superoxide formation becomes detectable. We explain the two processes considering the redox potentials for radical formation in the electrolyte as well as the positions of the conduction and valence bands. The role of surface band bending and surface states in the semiconductor is discussed.

Electrochemical properties of GaN nanowire electrodes--influence of doping and control by external bias.

We report on the electrochemical characteristics of GaN nanowire (NW) ensembles grown by plasma-assisted molecular beam epitaxy on Si111 substrates and on the influence of Si and Mg doping. The NW electrochemical properties in terms of surface capacitance (C(S)), surface resistance (R(S)) are extracted from electrochemical impedance spectra. While Mg doping of GaN NWs does not cause a significant variation of these quantities, an increase of the Si concentration leads to an increase of C(S) and a simultaneous decrease of R(S), indicating the presence of charge carriers in the NWs. According to the extracted values for R(S) and C(S) the NWs are classified into resistive and conductive. For conductive NWs charge transfer to a ferricyanide redox couple in the electrolyte is demonstrated and the ensemble average of the flatband voltage was determined. Variation of the lateral surface potential due to application of an external bias via the electrolyte is demonstrated.