Vacancy-Driven Robust Metallicity of Structurally Pinned Monoclinic Epitaxial VO₂ Thin Films

Abstract

Vanadium dioxide (VO2) is a strongly correlated material with 3d electrons, which exhibits temperature-driven insulator-to-metal transition with a concurrent change in the crystal symmetry. Interestingly, even modest changes in stoichiometry-induced orbital occupancy dramatically affect the electrical conductivity of the system. Here, we report a successful transformation of epitaxial monoclinic VO2 thin films from a conventionally insulating to permanently metallic behavior by manipulating the electron correlations. These ultrathin (∼10 nm) epitaxial VO2 films were grown on NiO(111)/Al2O3(0001) pseudomorphically, where the large misfit between NiO and Al2O3 were fully relaxed by domain-matching epitaxy. Complete conversion from an insulator to permanent metallic phase is achieved through injecting oxygen vacancies (x ∼ 0.20 ± 0.02) into the VO2–x system via annealing under high vacuum (∼5 × 10–7 Torr) and increased temperature (450 °C). Systematic introduction of oxygen vacancies partially converts V4+ to V3+ and generates unpaired electron charges which result in the emergence of donor states near the Fermi level. Through the detailed study of the vibrational modes by Raman spectroscopy, hardening of the V–V vibrational modes and stabilization of V–V dimers are observed in vacuum-annealed VO2 films, providing conclusive evidence for stabilization of a monoclinic phase. This ultimately leads to convenient free-electron transport through the oxygen-deficient VO2–x thin films, resulting in metallic characteristics at room temperature. With these results, we propose a defect engineering pathway through the control of oxygen vacancies to tune electrical and optical properties in epitaxial monoclinic VO2.

Dislocation and oxygen-release driven delithiation in Li2MnO3

“…This situation is directly confirmed by the upward shift of O- K edge near the surface as indicated by the arrow in Fig. 1g 28 , suggesting that oxygen vacancies are introduced during delithiation, i.e., oxygen release. To estimate the local elemental concentrations, we evaluate the Li/Mn and O/Mn ratios by the integrated intensities of Li- K , Mn- M , O- K, and Mn- L edges, where the intensity integrations were performed through the yellow filled-regions in Fig.…”

Nanoscale Imaging and Control of Volatile and Non‐Volatile Resistive Switching in VO2

“…The observed evolution of the crystal lattice disorder suggests that the formation and relaxation of persistent switching can be explained by diffusion of defects, such as oxygen vacancies. Oxygen vacancies in VO 2 are capable of reducing the MIT temperature, [ 24,25 ] can migrate through the grain boundaries, and have a diffusion rate matching the observed time scales (see Figure 4b). Furthermore, the observed asymmetry, such as fast formation and slow relaxation, can be explained by trapping of vacancies inside the switched grains due to a reduced activation energy.…”

Stabilization of a honeycomb lattice of IrO6 octahedra by formation of ilmenite-type superlattices in MnTiO3

“…4). Although the Mn L-edge spectra of the two regions (i.e., Mn 2+ ) are in good agreement, a partial reduction of Ti 4+ to Ti 3+ occurred near the Mn−Ir−O layer 25,26 . Associated with this reduction, the presence of an oxygen deficiency is expected from the O K-edge spectra.…”