Electrochemical gating-induced reversible and drastic resistance switching in VO2 nanowires

Recent Progress on Vanadium Dioxide Nanostructures and Devices: Fabrication, Properties, Applications and Perspectives

Vanadium dioxide (VO₂) is a typical metal-insulator transition (MIT) material, which changes from room-temperature monoclinic insulating phase to high-temperature rutile metallic phase. The phase transition of VO₂ is accompanied by sudden changes in conductance and optical transmittance. Due to the excellent phase transition characteristics of VO₂, it has been widely studied in the applications of electric and optical devices, smart windows, sensors, actuators, etc. In this review, we provide a summary about several phases of VO₂ and their corresponding structural features, the typical fabrication methods of VO₂ nanostructures (e.g., thin film and low-dimensional structures (LDSs)) and the properties and related applications of VO₂. In addition, the challenges and opportunities for VO₂ in future studies and applications are also discussed.

Reduced Transition Temperature in Al:ZnO/VO2 Based Multi-Layered Device for low Powered Smart Window Application

The metal-to-insulator transition (MIT) closest to room temperature of 68–70 °C as shown by vanadium oxide (VO₂), compared with other transition metal oxides, makes it a potential candidate for smart window coating. We have successfully fabricated a potential smart window device after the optimum design of a multilayered thin film structure made out of transparent conducting oxide (aluminum doped zinc oxide) and pure VO₂ using pulsed laser deposition technique. This comprehensive study is based on two different configurations for multi-layered structure approach, with the intention to reduce the transition temperature, as well as to maintain the MIT properties that would strengthen the potential of the structure to be used for a smart window device. By creating a multi-layered structure, we were able to create a low powered device that can operate less than 15 V that leads to significant decline in the infrared transmission by a magnitude of over 40% and provided sufficient heat to trigger the MIT at a temperature around 60 °C, which is almost 10 °C lower than its bulk counterpart. This finding would positively impact the research on VO₂ thin films, not only as smart windows but also for numerous other applications like bolometers, infrared detectors, Mott transistors and many more.

Nonvolatile ferroelectric field effect transistor based on a vanadium dioxide nanowire with large on- and off-field resistance switching

We fabricate a ferroelectric field effect transistor (FeFET) based on a semiconducting vanadium dioxide (VO₂) nanowire (NW), and we investigate its electron transport characteristics modulated by the ferroelectric effects.

Long-range propagation of protons in single-crystal VO2 involving structural transformation to HVO2

Vanadium dioxide (VO₂) is a strongly correlated electronic material with a metal-insulator transition (MIT) near room temperature. Ion-doping to VO₂ dramatically alters its transport properties and the MIT temperature. Recently, insulating hydrogenated VO₂ (HVO₂) accompanied by a crystal structure transformation from VO₂ was experimentally observed. Despite the important steps taken towards realizing novel applications, essential physics such as the diffusion constant of intercalated protons and the crystal transformation energy between VO₂ and HVO₂ are still lacking. In this work, we investigated the physical parameters of proton diffusion constants accompanied by VO₂ to HVO₂ crystal transformation with temperature variation and their transformation energies. It was found that protons could propagate several micrometers with a crystal transformation between VO₂ and HVO₂. The proton diffusion speed from HVO₂ to VO₂ was approximately two orders higher than that from VO₂ to HVO_2. The long-range propagation of protons leads to the possibility of realizing novel iontronic applications and energy devices.

Gate-Tunable Thermal Metal-Insulator Transition in VO2 Monolithically Integrated into a WSe2 Field-Effect Transistor

Vanadium dioxide (VO₂) shows promise as a building block of switching and sensing devices because it undergoes an abrupt metal-insulator transition (MIT) near room temperature, where the electrical resistivity changes by orders of magnitude. A challenge for versatile applications of VO₂ is to control the MIT by gating in the field-effect device geometry. Here, we demonstrate a gate-tunable abrupt switching device based on a VO₂ microwire that is monolithically integrated with a two-dimensional (2D) tungsten diselenide (WSe₂) semiconductor by van der Waals stacking. We fabricated the WSe₂ transistor using the VO₂ wire as the drain contact, titanium as the source contact, and hexagonal boron nitride as the gate dielectric. The WSe₂ transistor was observed to show ambipolar transport, with higher conductivity in the electron branch. The electron current increases continuously with gate voltage below the critical temperature of the MIT of VO₂. Near the critical temperature, the current shows an abrupt and discontinuous jump at a given gate voltage, indicating that the MIT in the contacting VO₂ is thermally induced by gate-mediated self-heating. Our results have paved the way for the development of VO₂-based gate-tunable devices by the van der Waals stacking of 2D semiconductors, with great potential for electronic and photonic applications.

In Situ Visualization of Fast Surface Ion Diffusion in Vanadium Dioxide Nanowires

We investigate in situ ion diffusion in vanadium dioxide (VO₂) nanowires (NWs) by using photocurrent imaging. Alkali metal ions are injected into a NW segment via ionic liquid gating and are shown to diffuse along the NW axis. The visualization of ion diffusion is realized by spatially resolved photocurrent measurements, which detect the charge carrier density change associated with the ion incorporation. Diffusion constants are determined to be on the order of 10^-10 cm²/s for both Li⁺ and Na⁺ ions at room temperature, while H⁺ diffuses much slower. The ion diffusion is also found to occur mainly at the surface of the NWs, as metal contacts can effectively block the ion diffusion. This novel method of visualizing ion distribution is expected to be applied to study ion diffusion in a broad range of materials, providing key insights on phase transition electronics and energy storage applications.

Enhanced electronic-transport modulation in single-crystalline VO2 nanowire-based solid-state field-effect transistors

Field-effect transistors using correlated electron materials with an electronic phase transition pave a new avenue to realize steep slope switching, to overcome device size limitations and to investigate fundamental science. Here, we present a new finding in gate-bias-induced electronic transport switching in a correlated electron material, i.e., a VO₂ nanowire channel through a hybrid gate, which showed an enhancement in the resistive modulation efficiency accompanied by expansion of metallic nano-domains in an insulating matrix by applying gate biases near the metal-insulator transition temperature. Our results offer an understanding of the innate ability of coexistence state of metallic and insulating domains in correlated materials through carrier tuning and serve as a valuable reference for further research into the development of correlated materials and their devices.

Resistive Switching Memory of TiO2 Nanowire Networks Grown on Ti Foil by a Single Hydrothermal Method

The resistive switching characteristics of TiO2 nanowire networks directly grown on Ti foil by a single-step hydrothermal technique are discussed in this paper. The Ti foil serves as the supply of Ti atoms for growth of the TiO2 nanowires, making the preparation straightforward. It also acts as a bottom electrode for the device. A top Al electrode was fabricated by e-beam evaporation process. The Al/TiO2 nanowire networks/Ti device fabricated in this way displayed a highly repeatable and electroforming-free bipolar resistive behavior with retention for more than 104 s and an OFF/ON ratio of approximately 70. The switching mechanism of this Al/TiO2 nanowire networks/Ti device is suggested to arise from the migration of oxygen vacancies under applied electric field. This provides a facile way to obtain metal oxide nanowire-based ReRAM device in the future.