Tunable insulator–metal transition in epitaxial VO2 thin films via strain and defect engineering†

The Metal to Insulator Transition (MIT) in materials, particularly vanadium dioxide (VO₂), has garnered significant research interest due to its potential applications in smart windows, memristors, transistors, sensors, and optical switches. The transition from an insulating, monoclinic phase to a conducting, tetragonal phase involves changes in optical and electrical properties, opening avenues in adaptive radiative coolers, optical memories, photodetectors, and optical switches. VO₂ exhibits MIT close to 68 °C, thereby requiring tuneable transition temperatures (T_c) in VO₂ thin films for practical device applications. In this work, we explore the role of strain and defect engineering in tuning the MIT temperature in epitaxial VO₂ thin films deposited on c-cut sapphire using Pulsed Laser Deposition (PLD). The study involves tuning the metal-to-insulator transition (MIT) by varying growth parameters, mainly temperature and oxygen partial pressure. Strain engineering along the b-axis helped tune the transition temperature from 65 °C to 82 °C with the out-of-plane b-strain varying from −0.71% to −0.44%. Comprehensive structural and property analyses, including X-ray diffraction (XRD), Reciprocal Space Mapping (RSM), X-ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, and resistivity–temperature (R–T) measurements, were performed to correlate structural properties with T_c. Additionally, density functional theory (DFT) calculations were performed using Quantum Espresso within the generalized gradient approximation of the revised Perdew–Burke–Ernzerhof (PBEsol) functional to provide theoretical validity to the experimentally obtained results. Our study provides critical insights into the interplay between strain and oxygen vacancies and their effect on the physical properties of VO₂ thin films with DFT calculations supporting the experimental findings.