Defect-Engineered Nb2O5 Nanoparticles for SERS Sensing through Suppressed Phonon-Assisted Recombination at Cryogenic Temperature

Surface-enhanced Raman scattering (SERS) from semiconductor-based substrates is often limited by its low enhancement factor (EF). Here, we demonstrate the ultrahigh SERS sensitivity of defect-engineered Nb2O5 nanoparticles through cryogenic temperature measurements. High-energy ball milling was used to produce Nb2O5 nanoparticles with abundant surface defects, specifically oxygen vacancy defects. Owing to these oxygen vacancy defects, these nanoparticles show extraordinary SERS sensitivity toward the detection of methylene blue (MeB) dye as well as other cationic dye molecules such as malachite green (MG) and crystal violet (CV). Further, reducing the substrate temperature to a cryogenic regime (77 K) greatly enhances the photoinduced charge transfer (PICT) efficiency by lowering the phonon-assisted recombination rate, which in turn amplifies the SERS signal by almost 2.5 times. By reducing the lattice thermal vibrations at sufficiently low temperatures, the defect states are populated with a large number of photoinduced electrons, which is reflected in the massive increase in the intensity of the photoluminescence (PL) signal. With this remarkable enhancement, we have detected down to 10–8 M of MeB with an extraordinary SERS EF of 8.38 × 107, which is exceptional for a semiconductor-based SERS substrate. Additionally, other cationic dye molecules (MG, CV) and some volatile organic compounds (VOCs) also exhibited significant enhancement. Further, with the aid of density functional theory (DFT) calculations and finite element method (FEM) based simulations, we sought to investigate the individual contributions of the chemical and electromagnetic enhancement mechanisms to the overall SERS enhancement. These findings are crucial for developing metal-free, high-efficiency SERS substrates.