Effects of oxygen vacancies and interfacial strain on the metal–insulator transition of VO2 nanobeams

Abstract

The role of oxygen vacancies and interfacial strain on the metal–insulator transition (MIT) behavior of high-quality VO₂ nanobeams (NBs) synthesized on SiO₂/Si substrates employing V₂O₅ as a precursor has been investigated in this research. Selective oxygen vacancies have been generated by argon plasma irradiation. The MIT is progressively suppressed as the duration of plasma processing increases; in addition, the temperature of MIT (T_MIT) drops by up to 95 K relative to the pristine VO₂ NBs. Incorporating oxygen vacancies into VO₂ may increase its electron concentration, which might shift the Fermi levels upward, strengthen the electronic orbital overlap of the V–V chains, and further stabilize the metallic phase at lower temperatures, based on first-principles calculations. Furthermore, in order to evaluate the influence of substrate-induced strain in our situation, the MIT in two distinct types of VO₂ NB samples is examined without metal contacts by using the distinctive light scattering characteristics of the metal (M) and insulator (I) phases (i.e., M/I domains) by optical microscopy. It is found that the domain structures in the “clamped” NBs persisted up to ∼453 K, while the “released” NBs (transferred to a new substrate) did not exhibit any domain structures and turned into an entirely M phase with a dark contrast above ∼348 K. When combined with first-principles calculations, the electronic orbital occupancy in the rutile phase contributes to explaining the interfacial strain-induced modulation of MIT. The current findings shed light on how interfacial strain and oxygen vacancies impact MIT behavior. It also suggests several types of control strategies for MIT in VO₂ NBs, which are essential for a broader spectrum of VO₂ NB applications.