Surface dominant photoresponse of multiferroic BiFeO3 nanowires under sub-bandgap illumination.

A surface dominant sub-bandgap photo-carrier generation has been observed in multiferroic BiFeO3 (BFO) nanowires, which is mainly attributed to the depopulation of surface states that exist within the bandgap. Mapping of surface potential using Kelvin probe force microscopy (KPFM) further supports the depopulation of surface states in BFO nanowires under sub-bandgap illumination. The mechanism of photovoltage generation in BFO nanowires is investigated by measuring the photoresponse with local illumination of visible laser pulses at different positions of the BFO nanowires. Interestingly, large photovoltage signals were observed when the laser spot was focused close to contact electrodes, showing a position dependent effect of photoresponse in the BFO nanowires. The sub-bandgap excitation of surface states in multiferroic nanowires offers potential new strategies for application in photovoltaic devices.