Non-metallic SERS on MoO _<i>x</i> /AZO heterostructures: role of charge transfer and polarons

In this study, a non-noble-metal SERS substrate based on MoO _<i>x</i> /Al-doped ZnO (AZO) heterostructures was successfully fabricated using a cost-effective DC magnetron sputtering method. The AZO thin film, optimized at a sputtering power of 45 W, provides a highly crystalline, textured surface, and optical characteristics that support both a chemical and electromagnetic enhancement mechanism. Upon deposition of a thin MoO _<i>x</i> layer for 7.5 minutes, the resulting heterostructure exhibits improved light absorption, enhanced defect-level emissions, and significant SERS activity. Spectroscopic analyses (UV-Vis, Raman, PL, and XPS) of the MoO _<i>x</i> /AZO heterostructures confirm the presence of oxygen vacancies and mixed-valence Mo⁵⁺/Mo⁶⁺ species, indicative of small polaron formation. These polarons, along with interfacial energy alignment, enable efficient charge transfer from the SERS substrate to the analyte, supporting the chemical enhancement mechanism. Meanwhile, localized field enhancement at surface protrusions and junctions contributes to electromagnetic effects. The optimized MoO _<i>x</i> /AZO substrate achieved a detection limit as low as 10^-7 M for Rhodamine 6G. This work underscores the critical impact of charge transfer and polaron-assisted processes in boosting Raman signals, highlighting the promise of oxide-based heterostructures for sensitive and scalable metal-free SERS applications.