Demystifying the Semiconductor-to-Metal Transition in Amorphous Vanadium Pentoxide: The Role of Substrate/Thin Film Interfaces

The precise mechanism governing the reversible semiconductor-to-metal transition (SMT) in V₂O₅ remains elusive, yet its investigation is of paramount importance due to the remarkable potential of V₂O₅ as a versatile “smart” material in advancing optoelectronics, plasmonics, and photonics. In this study, distinctive experimental insights into the SMT occurring in amorphous V₂O₅ through the application of highly sensitive, temperature-dependent, in situ analyses on a V₂O₅ thin film deposited on soda-lime glass are presented. The ellipsometry measurements reveal that the complete SMT occurs at ≈340 °C. Remarkably, the refractive index and extinction coefficients exhibit reversible characteristics across visible and near-infrared wavelengths, underscoring the switch-like behavior inherent to V₂O₅. The findings obtained from ellipsometry are substantiated by calorimetry and in situ secondary ion mass spectrometry analyses. In situ electron microscopy observations unveil a separation of oxidation states within V₂O₅ at 320 °C, despite the thin film retaining its amorphous state. The comprehensive experimental investigations effectively demonstrate that alterations in electronic state can trigger the SMT in amorphous V₂O₅. It is revealed for the first time that the SMT in V₂O₅ is solely contingent upon electronic state changes, independent of structural transitions, and importantly, it is a reversible transformation within the amorphous state itself.