Insights into the Semiconductor SERS Activity: The Impact of the Defect-Induced Energy Band Offset and Electron Lifetime Change

oxide semiconductor. By modulating the density of oxygen vacancy defects using ultraviolet (UV) light irradiation, SERS is found to be improved with irradiation time in the first place, but such improvement later deteriorates for prolonged irradiation even if more defects are generated. Insights into the observed SERS evolution are provided by ultraviolet photoelectron spectroscopy and femtosecond time-resolved transient absorption spectroscopy measurements. Results reveal that (1) a suitable offset between the energy band of the substrate and the orbitals of molecules is facilitated by a certain defect density and (2) defect states with relatively long electron lifetime are essential to achieve optimal SERS performance.