Metal oxide semiconductor SERS-active substrates by defect engineering

A general route to transform metal oxide semiconductors from non-SERS active to SERS-active substrates based on defect engineering is reported. The SERS enhancement factor (EF) of metal oxide semiconductors like α-MoO₃ and V₂O₅ can be greatly enhanced and the SERS performance can be optimized according to the detecting analyte and activating laser wavelength by introducing oxygen vacancy defects. The EF of R6G on α-MoO_3-x nanobelts can be as high as 1.8 × 10⁷ with a detection limit of 10^-8 M, which is the best among metal oxide semiconductors and comparable to noble metals without a "hot spot". A model, named "effective electric current model", was proposed to describe the photo-induced charge transfer process between the absorbed molecules and semiconductor substrates. The EF of 4-MBA, R6G and MB on α-MoO_3-x nanobelts with different oxygen vacancy concentrations calculated based on the model matches very well with experimental results. As an extension, some potential metal oxide semiconductor SERS-active substrates were predicted based on the model. Our results clearly demonstrate that, through defect engineering, the metal oxide semiconductors can be made SERS-active substrates with high stability and high biocompatibility.