Enhanced charge transfer and coupled resonance in Ni-doped sub-stoichiometric tungsten oxide nanostructure for plasmon-free SERS sensing

Abstract

Recent advancements in metal-free semiconductor-based SERS substrates have attracted significant attention due to their ease of fabrication, tunable optical properties, and exceptional stability. Achieving metal-like SERS enhancement necessitates a detailed understanding of material engineering, particularly its impact on charge transfer mechanisms and dielectric properties. In this study, we demonstrate that Ni doping in sub-stoichiometric tungsten oxide (W₁₈O₄₉) nanoflowers substantially enhances its SERS sensitivity. This improvement is attributed to increased carrier generation and alterations in the electronic band structure, which promote the photoinduced charge transfer (PICT) process. We show that Ni doping shifts the energy requirement for PICT resonance from the UV to the visible-NIR region, enabling both molecular and PICT resonance under 632.8 nm laser excitation. This “coupled resonance” effect results in an exceptionally low detection limit of 10⁻¹⁰ M and an outstanding enhancement factor of 6.85 × 10⁸ for the detection of Methylene Blue molecules, one of the highest reported for metal-free semiconductor-based SERS substrates. Additionally, the unique flower-like morphology of the material contributes significantly to electromagnetic (EM) enhancement, which is further amplified by the presence of Ni atoms. These findings are supported by finite element method (FEM) simulations and density functional theory (DFT) calculations, providing critical insights into the synergistic effects of structural and compositional tuning. This work offers a promising framework for the rational design of plasmon-free, cost-effective SERS substrates with outstanding enhancement factors, paving the way for advanced applications in molecular detection.