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

The significant boost in surface-enhanced Raman scattering (SERS) bythe chemical enhancement of semiconducting oxides is a pivotal finding. It offers aprospective path toward high uniformity and low-cost SERS substrates. However, adetailed understanding of factors that influence the charge transfer process is stillinsufficient. Herein, we reveal the important role of defect-induced band offset andelectron lifetime change in SERS evolution observed in a MoO3 oxide semiconductor.By modulating the density of oxygen vacancy defects using ultraviolet (UV) lightirradiation, SERS is found to be improved with irradiation time in the first place, butsuch improvement later deteriorates for prolonged irradiation even if more defects aregenerated. Insights into the observed SERS evolution are provided by ultravioletphotoelectron spectroscopy and femtosecond time-resolved transient absorptionspectroscopy measurements. Results reveal that (1) a suitable offset between theenergy band of the substrate and the orbitals of molecules is facilitated by a certaindefect density and (2) defect states with relatively long electron lifetime are essential to achieve optimal SERS performance.