Defect-Engineered\nNb<sub>2</sub>O<sub>5</sub> Nanoparticles\nfor SERS Sensing through Suppressed Phonon-Assisted Recombination\nat Cryogenic Temperature

Surface-enhanced Raman scattering (SERS) from semiconductor-based\nsubstrates is often limited by its low enhancement factor (EF). Here,\nwe demonstrate the ultrahigh SERS sensitivity of defect-engineered\nNb<sub>2</sub>O<sub>5</sub> nanoparticles through cryogenic temperature\nmeasurements. High-energy ball milling was used to produce Nb<sub>2</sub>O<sub>5</sub> nanoparticles with abundant surface defects,\nspecifically oxygen vacancy defects. Owing to these oxygen vacancy\ndefects, these nanoparticles show extraordinary SERS sensitivity toward\nthe detection of methylene blue (MeB) dye as well as other cationic\ndye molecules such as malachite green (MG) and crystal violet (CV).\nFurther, reducing the substrate temperature to a cryogenic regime\n(77 K) greatly enhances the photoinduced charge transfer (PICT) efficiency\nby lowering the phonon-assisted recombination rate, which in turn\namplifies the SERS signal by almost 2.5 times. By reducing the lattice\nthermal vibrations at sufficiently low temperatures, the defect states\nare populated with a large number of photoinduced electrons, which\nis reflected in the massive increase in the intensity of the photoluminescence\n(PL) signal. With this remarkable enhancement, we have detected down\nto 10<sup>–8</sup> M of MeB with an extraordinary SERS EF of\n8.38 × 10<sup>7</sup>, which is exceptional for a semiconductor-based\nSERS substrate. Additionally, other cationic dye molecules (MG, CV)\nand some volatile organic compounds (VOCs) also exhibited significant\nenhancement. Further, with the aid of density functional theory (DFT)\ncalculations and finite element method (FEM) based simulations, we\nsought to investigate the individual contributions of the chemical\nand electromagnetic enhancement mechanisms to the overall SERS enhancement.\nThese findings are crucial for developing metal-free, high-efficiency\nSERS substrates.