Boosting ultrasensitive SERS activity on quasi-metallic tungsten oxide through synergistic vibronic coupling and electromagnetic resonance

Metal oxide semiconductors have emerged as promising candidates for surface-enhanced Raman scattering (SERS) to replace conventional noble metal substrates. Here, we demonstrate an insulator to quasi-metallic phase transition in electron-rich WO 3 doped with group 1 elements (H, Li, Na, and K) to amplify the Raman response of adsorbed probe molecules. In this category, the lithium-doped WO 3 (LWO) exhibited excellent balance between SERS sensitivity and photostability, with enhancement factors (EFs) up to 4.3 × 10 6 and a limit of detection (LOD) of 10 −8 M. Based on theoretical simulations and advanced experimental characterizations, we identified its non-discrete band arrangement and high free carrier concentration (1.3 × 10 21 cm 3 ), which create not only efficient migration channels for interfacial charge-transfer (CT) enhancement, but also a surface localized surface plasmon resonance (LSPR) effect-induced strong electromagnetic field that further amplified the Raman response via an electromagnetic (EM) mechanism. Our strategy greatly refines and expands the scope of existing metal oxide materials, offering new insights for designing efficient and robust non-noble metal-based SERS substrates. Quasi-metallic-phase tungsten oxide materials doped by main-group IA elements are investigated for synergistic physical and chemical SERS enhancement.