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

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 (TMIT) 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.

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.

Nano-Particle VO2 Insulator-Metal Transition Field-Effect Switch with 42 mV/decade Sub-Threshold Slope

The possibility of controlling the insulator-to-metal transition (IMT) in nano-particle VO2 (NP-VO2) using the electric field effect in a metal-oxide-VO2 field-effect transistor (MOVFET) at room temperature was investigated for the first time. The IMT induced by current in NP-VO2 is a function of nano-particle size and was studied first using the conducting atomic force microscope (cAFM) current-voltage (I-V) measurements. NP-VO2 switching threshold voltage (VT), leakage current (Ileakage), and the sub-threshold slope of their conductivity (Sc) were all determined. The cAFM data had a large scatter. However, VT increased as a function of particle height (h) approximately as VT(V) = 0.034 h, while Ileakage decreased as a function of h approximately as Ileakage (A) = 3.4 × 10−8e−h/9.1. Thus, an asymptotic leakage current of 34 nA at zero particle size and a tunneling (carrier) decay constant of ~9.1 nm were determined. Sc increased as a function of h approximately as Sc (mV/decade) = 2.1 × 10−3eh/6 and was around 0.6 mV/decade at h~34 nm. MOVFETs composed of Pt drain, source and gate electrodes, HfO2 gate oxide, and NP-VO2 channels were then fabricated and showed gate voltage dependent drain-source switching voltage and current (IDS). The subthreshold slope (St) of drain-source current (IDS) varied from 42 mV/decade at VG = −5 V to 54 mV/decade at VG = +5 V.

Nanoscale Behavior and Manipulation of the Phase Transition in Single-Crystal Cu2 Se

Phase transition is a fundamental physical phenomenon that has been widely studied both theoretically and experimentally. According to the Landau theory, the coexistence of high- and low-temperature phases is thermodynamically impossible during a second-order phase transition in a bulk single crystal. Here, the coexistence of two (α and β) phases in wedge-shaped nanosized single-crystal Cu₂ Se over a large temperature range are demonstrated. By considering the surface free-energy difference between the two phases and the shape effect, a thermodynamic model is established, which explicitly explains their coexistence. Intriguingly, it is found that with a precise control of the heating temperature, the phase boundary can be manipulated at atomic level. These discoveries extend the understanding of phase transitions to the nanoscale and shed light on rational manipulation of phase transitions in nanomaterials.

VO2(D) hollow core–shell microspheres: synthesis, methylene blue dye adsorption and their transformation into C/VOxnanoparticles

Hollow core–shell VO₂(D) microspheres were fabricated and they exhibited excellent MB adsorption ability; and the regenerated C/VO_xnanoparticles showed enhanced adsorption performance and good reusability.

Hydrothermal encapsulation of VO2(A) nanorods in amorphous carbon by carbonization of glucose for energy storage devices

This work developed a new route to synthesize VO₂(A)@C composites and explore their application as a promising material for SCs.

Scaling behavior of oxide-based electrothermal threshold switching devices

Materials exhibiting insulator to metal transition (IMT) and transition metal oxides showing threshold switching behavior are considered as promising candidates for selector devices for crossbar non-volatile memory application. In this study, we use an electrothermal model to simulate the behavior of nanoscale selectors based on several different functional oxides (TaO_x, VO₂ and NbO₂). We extract the device characteristics, such as threshold voltage (V_TH), leakage current, device temperature in the ON state, and the size of the conductive filament as a function of selector diameter and functional layer thickness. In addition, we benchmark these devices in a 1 selector/1 resistor (1S1R) cell with a generic phase change-like memory element. These findings provide an insight into how device performance changes with scaling and help with material selection and design of selectors.

Hydrothermal Synthesis of VO2 Polymorphs: Advantages, Challenges and Prospects for the Application of Energy Efficient Smart Windows

Vanadium dioxide (VO₂ ) is a widely studied inorganic phase change material, which has a reversible phase transition from semiconducting monoclinic to metallic rutile phase at a critical temperature of τ_c ≈ 68 °C. The abrupt decrease of infrared transmittance in the metallic phase makes VO₂ a potential candidate for thermochromic energy efficient windows to cut down building energy consumption. However, there are three long-standing issues that hindered its application in energy efficient windows: high τ_c , low luminous transmittance (T_lum ), and undesirable solar modulation ability (ΔT_sol ). Many approaches, including nano-thermochromism, porous films, biomimetic surface reconstruction, gridded structures, antireflective overcoatings, etc, have been proposed to tackle these issues. The first approach-nano-thermochromism-which is to integrate VO₂ nanoparticles in a transparent matrix, outperforms the rest; while the thermochromic performance is determined by particle size, stoichiometry, and crystallinity. A hydrothermal method is the most common method to fabricate high-quality VO₂ nanoparticles, and has its own advantages of large-scale synthesis and precise phase control of VO₂ . This Review focuses on hydrothermal synthesis, physical properties of VO₂ polymorphs, and their transformation to thermochromic VO₂ (M), and discusses the advantages, challenges, and prospects of VO₂ (M) in energy-efficient smart windows application.

Atomic scale observation of a defect-mediated first-order phase transition in VO2(A)

The study of first-order structural transformations has attracted extensive attention due to their significant scientific and industrial importance. However, it remains challenging to exactly determine the nucleation sites at the very beginning of the transformation. Here, we report the atomic scale real-time observation of a unique defect-mediated reversible phase transition between the low temperature phase (LTP) and the high temperature phase (HTP) of VO₂(A). In situ Cs-corrected scanning transmission electron microscopy (STEM) images clearly indicate that both phase transitions (from the HTP to the LTP and from the LTP to the HTP) start at the defect sites in parent phases. Intriguingly, the structure of the defects within the LTP is demonstrated to be the HTP of VO₂ (A), and the defect in the HTP of VO₂(A) is determined to be the LTP structure of VO₂(A). These findings are expected to broaden our current understanding of the first-order phase transition and shed light on controlling materials' structure-property phase transition by "engineering" defects in applications.

Presence of Peierls pairing and absence of insulator-to-metal transition in VO2 (A): a structure–property relationship study

Based on structure–property relationships, we propose a two-step semiconductor-to-semiconductor phase transition in VO₂ (A).

One-Step Hydrothermal Synthesis of W-Doped VO2 (M) Nanorods with a Tunable Phase-Transition Temperature for Infrared Smart Windows

Vanadium dioxide (VO₂), with reversible metal-semiconductor transition near room temperature, is a compelling candidate for thermochromic windows. Nanocomposite coatings derived from VO₂ nanoparticles are particularly superior to VO₂ films due to their advantages in large-scale preparation, flexible shaping, and regulation of optical properties. In this work, we developed a novel method for one-step hydrothermal synthesis of W-doped VO₂ (M) nanorods and studied their application in large-scale infrared smart windows. On introducing tartaric acid as a new reductant, VO₂ underwent a two-stage phase evolution from the pure phase comprising VO₂ (A) nanobelts to VO₂ (M) nanorods, instead of the conventional three-stage B-A-M phase evolution during hydrothermal synthesis. This transition is very favorable for the large-scale hydrothermal synthesis of VO₂ (M). The phase-transition temperature of VO₂ (M) nanoparticles can be regulated systematically by W doping, with a reduction efficiency of about 24.52 °C/atom % W. Moreover, VO₂ (M) composite films were fabricated using a convenient roller coating method, which exhibited significant midinfrared transmission switching up to 31%, with a phase-transition temperature of about 37.3 °C. This work demonstrates the significant progress in the one-step hydrothermal synthesis of VO₂ (M) nanorods and provides significant insights into their applications in infrared smart windows.