Tailoring the charge carrier dynamics in ZnO nanowires: the role of surface hole/electron traps

Post-fabrication thermal-annealed ZnO nanowires (NWs) in an oxidizing (or a reducing) ambient were investigated using transient photoluminescence and X-ray photoelectron spectroscopy. Our findings reveal an ultrafast hole-transfer process to the surface adsorbed oxygen species (e.g., O2−) occurring within a few hundred picoseconds (ps) in the air-annealed samples; and an ultrafast electron-transfer process to charged oxygen vacancies (i.e., VO2+) occurring within tens of ps in the H2-annealed samples. Contrary to the common perception that the band edge emission (BE) dynamics are strongly influenced by the carrier trapping to the green emission related defect states (i.e., VZn), these above processes compete effectively with the ZnO BE. Hole trapping by ionized VZn, which occurs in an ultrashort sub-ps-to-ps timescale (and hence limits its effective hole capture radius), however, has less influence on the BE dynamics. Importantly, our findings shed new light on the photoinduced charge transfer processes that underpins the novel properties of enhanced photocatalytic activity, photovoltaic performance, and photoconductivity response of ZnO NWs, thereby suggesting a strategy for tailoring the ultrafast carrier dynamics in ZnO NW-based devices.