VO2 Phase Research Articles

Boosting the Electrochemical Performance of V2 O3 by Anchoring on Carbon Nanotube Microspheres with Macrovoids for Ultrafast and Long-Life Aqueous Zinc-Ion Batteries.

Zinc-ion batteries (ZIBs) are next-generation energy storage systems with high safety and environmental friendliness because they can be operated in aqueous systems. However, the search for electrode materials with ideal nanostructures and compositions for aqueous ZIBs is in progress. Herein, the synthesis of porous microspheres, consisting of V2 O3 anchored on entangled carbon nanotubes (p-V2 O3 -CNT) and their application as cathode for ZIBs is reported. From various analyses, it is revealed that V2 O3 phase disappears after the initial charge process, and Zn3+ x (OH)2+3 x V2- x O7-3 x ∙2H2 Oand zinc vanadate (Zny VOz ) phases undergo zinc-ion intercalation/deintercalation processes from the second cycle. Additionally, the electrochemical performances of p-V2 O3 -CNT, V2 O3 -CNT (without macrovoids), and porous V2 O3 (without CNTs) microspheres are compared to determine the effects of nanostructures and conductive carbonaceous matrix on the zinc-ion storage performance. p-V2 O3 -CNT exhibits a high reversible capacity of 237mA h g-1 after 5000 cycles at 10 A g-1 . Furthermore, a reversible capacity of 211mA h g-1 is obtained at an extremely high current density of 50 A g-1 . The macrovoids in V2 O3 nanostructure effectively alleviate the volume changes during cycling, and the entangled CNTs with high electrical conductivity assist in achieving fast electrochemical kinetics.

Dynamically tunable perfect absorber based on VO2–Au hybrid nanodisc array

The nanoscale modulation of light can be achieved by integrating plasmonic nanostructures with phase change materials. A dynamic and tunable perfect absorber is designed by combining VO2–Au hybrid nanodisc array with metal–insulator–metal structure. The finite-difference time-domain method is used to simulate the absorption curves under different structural parameters and temperatures. In the near-infrared band of 700 to 1800 nm, the average absorption rate and absorption bandwidth of the absorber in the insulating phase of VO2 are 87.6% and 939 nm, respectively. And the maximum absorption modulation depth before and after phase transition of VO2 reaches 63.8%. The absorber can tolerate a wide range of incident angles and exhibits polarization-independent features. We envisage that the overall advantages of this absorber will open new opportunities for optical modulation and optoelectronics.

Multifunctional tunable gradient metasurfaces for terahertz beam splitting and light absorption.

Obtaining functional devices with tunable features is beneficial to advance terahertz (THz) science and technology. Here, we propose multifunctional gradient metasurfaces that are composed of a periodic array of binary Si microcylinders integrated with VO2 and graphene. The metasurfaces act as transmittive (reflective) beamsplitters for the dielectric (metallic) phase of VO2 with a switchable characteristic. Moreover, by integrating the metasurfaces with graphene and modifying its chemical potential, one can tune the intensity of the split beam as well as obtain nearly perfect resonant absorptions. Consequently, the proposed metasurfaces can find potential applications in THz interferometers, multiplexers, and light absorbers.

Single-nanoantenna driven nanoscale control of the VO2 insulator to metal transition

Abstract The ultrafast concentration of electromagnetic energy in nanoscale volumes is one of the key features of optical nanoantennas illuminated at their surface plasmon resonances. Here, we drive the insulator to metal phase transition in vanadium dioxide (VO2) using a laser-induced pumping effect obtained by positioning a single gold nanoantenna in proximity to a VO2 thermochromic material. We explore how the geometry of the single nanoantenna affects the size and permittivity of the nanometer-scale VO2 regions featuring phase transition under different pumping conditions. The results reveal that a higher VO2 phase transition effect is obtained for pumping of the longitudinal or transversal localized surface plasmon depending on the antenna length. This characterization is of paramount importance since the single nanoantennas are the building blocks of many plasmonic nanosystems. Finally, we demonstrate the picosecond dynamics of the VO2 phase transition characterizing this system, useful for the realization of fast nano-switches. Our work shows that it is possible to miniaturize the hybrid plasmonic-VO2 system down to the single-antenna level, still maintaining a controllable behavior, fast picosecond dynamics, and the features characterizing its optical and thermal response.

Two-step current-temperature-induced electrical and optical modifications in VO2 films around the metal–insulator transition

We report on two-step current-induced effects on the electrical, optical, and structural properties of VO2 films around the Metal–Insulator Transition (MIT) in synergy with ambient temperature (T). Simultaneous electrical resistance and transmittance measurements of VO2 semitransparent thin films as a function of T show that the electric current modifies the MIT that takes place in two steps: an abrupt change that increases upon increasing current, implying the formation of larger metallic domains within the current path, accompanied by a smoother change that follows the temperature change. Resistance measurements of thicker bulk-like VO2 films have been also investigated exhibiting similar two-step behavior. By monitoring the specimen temperature (To) during resistance measurements, we show that the abrupt resistance step, accompanied by instantaneous heating/cooling events, occurs at temperatures lower than TMIT and is attributed to current-induced Joule heating effects. Moreover, by monitoring To during current–voltage measurements, the role of T in the formation of two-step current modified MIT is highlighted. X-ray diffraction with in situ resistance measurements performed for various currents at room temperature as a function of To has shown that the current can cause partially MIT and structural phase transition, leading to an abrupt step of MIT. The formation of a rutile metallic phase of VO2 under high applied currents is clearly demonstrated by micro-Raman measurements. By controlling current in synergy with T below TMIT, the VO2 film can be driven to a two-step current-induced MIT as gradually a larger part of the film is transformed into a rutile metallic phase.

Systematic Exploration of the Synthetic Parameters for the Production of Dynamic VO2(M1).

Thermochromic dynamic cool materials present a reversible change of their properties wherein by increasing the temperature, the reflectance, conductivity, and transmittance change due to a reversible crystalline phase transition. In particular, vanadium (IV) dioxide shows a reversible phase transition, accompanied by a change in optical properties, from monoclinic VO2(M1) to tetragonal VO2(R). In this paper, we report on a systematic exploration of the parameters for the synthesis of vanadium dioxide VO2(M1) via an easy, sustainable, reproducible, fast, scalable, and low-cost hydrothermal route without hazardous chemicals, followed by an annealing treatment. The metastable phase VO2(B), obtained via a hydrothermal route, was converted into the stable VO2(M1), which shows a metal–insulator transition (MIT) at 68 °C that is useful for different applications, from energy-efficient smart windows to dynamic concrete. Within this scenario, a further functionalization of the oxide nanostructures with tetraethyl orthosilicate (TEOS), characterized by an extreme alkaline environment, was carried out to ensure compatibility with the concrete matrix. Structural properties of the synthesized vanadium dioxides were investigated using temperature-dependent X-ray Diffraction analysis (XRD), while compositional and morphological properties were assessed using Scanning Electron Microscopy, Energy Dispersive X-ray Analysis (SEM-EDX), and Transmission Electron Microscopy (TEM). Differential Scanning Calorimetry (DSC) analysis was used to investigate the thermal behavior.

VO2-based micro-electro-mechanical tunable optical shutter and modulator.

VO2-based MEMS tunable optical shutters are demonstrated. The design consists of a VO2-based cantilever attached to a VO2-based optical window with integrated resistive heaters for individual mechanical actuation of the cantilever structure, tuning of the optical properties of the window, or both. Optical transmittance measurements as a function of current for both heaters demonstrates that the developed devices can be used as analog optical shutters, where the intensity of a light beam can be tuned to any value within the range of VO2 phase transition. A transmittance drop off 30% is shown for the optical window, with tuning capabilities greater than 30% upon actuation of the cantilever. Unlike typical mechanical shutters, these devices are not restricted to binary optical states. Optical modulation of the optical window is demonstrated with an oscillating electrical input. This produces a transmittance signal that oscillates around an average value within the range off VO2's phase transition. For an input current signal with fixed amplitude (fel= 0.28 Hz), tuned to be at the onset of the phase transition, a transmittance modulation of 14% is shown. Similarly, by modulating the DC-offset, a transmittance modulation of VO2 along the hysteresis is obtained.

Phase management in single-crystalline vanadium dioxide beams

A systematic study of various metal-insulator transition (MIT) associated phases of VO2, including metallic R phase and insulating phases (T, M1, M2), is required to uncover the physics of MIT and trigger their promising applications. Here, through an oxide inhibitor-assisted stoichiometry engineering, we show that all the insulating phases can be selectively stabilized in single-crystalline VO2 beams at room temperature. The stoichiometry engineering strategy also provides precise spatial control of the phase configurations in as-grown VO2 beams at the submicron-scale, introducing a fresh concept of phase transition route devices. For instance, the combination of different phase transition routes at the two sides of VO2 beams gives birth to a family of single-crystalline VO2 actuators with highly improved performance and functional diversity. This work provides a substantial understanding of the stoichiometry-temperature phase diagram and a stoichiometry engineering strategy for the effective phase management of VO2.

Plasmonic nanodiscs on vanadium dioxide thin films for tunable luminescence enhancement.

We propose an alternative method to dynamically tune luminescence enhancement in the near infrared spectral range using noble metal nanostructures on top of phase change material vanadium dioxide (VO2) thin films. The VO2 phase change is used to tune the nanodisc plasmon resonance providing a luminescence modification mechanism. We employ a model to calculate the emission of quantum emitters, such as dye molecules, in hybrid systems comprising single silver (Ag) nanodiscs on top of a thin layer of VO2. The model considers different dipole orientations and positions with respect to the nanostructure-VO2 film and determines the degree of observable luminescence modification. In the NIR spectral region, the observable photoluminescence of Alexa Dyes in the hybrid systems at room temperature is enhanced by more than a factor of 2.5 as compared to the same system without plasmonic particles. An additional photoluminescence enhancement by more than a factor of 2 can be achieved with the Ag nanodisc-VO2 film systems after the phase transition of the VO2. These systems can be used for tunable luminescence modification and for compensation of thermally induced luminescence quenching. Through optimization of the Ag nanodisc-VO2 film system, luminescence enhancements of up to a factor of 4 can be seen in the metallic VO2 compared to the semiconducting phase and would therefore compensate for a thermal quenching of up to 70% between room temperature and 70° C, rendering the hybrid systems as promising candidates for improved photon management in optoelectronic devices where elevated temperatures minimize the efficiencies of such devices.

Semiconductor-to-metal transition in atomic layer deposition (ALD) of VO2 films using VCl4 and water

The semiconductor-to-metal transition of vanadium dioxide (VO2) films is studied using temperature-dependent Raman, optical, and electrical measurements. The VO2 films are deposited via an atomic layer deposition (ALD) process using alternate pulses of vanadium tetrachloride and H2O at 350 °C. A growth rate of 0.021 nm/cycle and a thickness of 33 nm of VO2 are obtained for all films studied. The phase of the film is determined using x-ray diffraction. The as-deposited films are amorphous and are transformed to the monoclinic phase with a post-deposition, forming gas anneal at temperatures ≥ 500 °C for 60 min. The purity of the films is determined using x-ray photoelectron spectroscopy and no evidence of residual chlorine is detected. The temperature-dependent Raman Ag mode of the monoclinic VO2 phase is observed to monotonically decrease from 25 °C to 78 °C; where no evidence of the Ag peak is observed in the film beyond 68 °C. The refractive index and extinction coefficient extracted from temperature-dependent ellipsometry confirm that, beyond 68 °C, free carriers are generated in the film. Electrical measurements performed on a fabricated p++Si/VO2/Ti/Au device show a semiconductor-to-metal transition behavior with a high resistance of 14701 ± 2284 Ω at 62 °C and a low resistance of 1064.1 ± 143 Ω at 67 °C. This work demonstrates that a halide-based ALD process provides a clean and robust approach to synthesizing high-quality VO2 films.

Impact of anharmonic effects on two phases of VO2 thin-film phase transition materials

Vanadium dioxide (VO2) is a strong correlation material, which can undergo transition from the monoclinic phase to the rutile phase near 340 K at ambient pressure. This phase transition can be controlled by extreme changes of optical, electrical and thermodynamic properties, and thus VO2 film possesses various potential device applications. Physically, lattice dynamics play a critical role in the investigation of the phase mechanism and properties of VO2. Given that widely adopted methods such as the density-functional perturbation theory and the quasiharmonic approximation (QHA) are typically only suitable to calculate the VO2 monoclinic phase, in this work we employ the temperature-dependent effective potential (TDEP) method to calculate the phonon properties of the two phases of VO2 by considering the anharmonicity of the lattice vibrations. Assisted by the experimental data, a resistance–temperature phase diagram has been constructed. The results show that the monoclinic phase is unstable because of the existence of the imaginary frequency. Compared with the thermal expansion, calculated by the QHA and TDEP methods, it is demonstrated that the TDEP method is reliable for the stable rutile phase, in which the imaginary frequency disappears. Simultaneously, it is shown that the calculated Gibbs free energy of VO2 agrees with the experimental results. Our work provides robust evidence tthat the anharmonicity in the lattice dynamics of VO2 cannot be ignored.

Publisher’s Note

Abstract Vanadium dioxide can exist in several polymorphs and among these the layered polymorph VO2 (B) with monoclinic symmetry has numerous applications. In this work, VO2 (B) phase thin film was prepared on quartz substrate via sputtering technique and its temperature dependent structural, electrical, and electronic properties were investigated. We have witnessed a broader structural phase transition around 220 K; which encounter significant changes in the lattice constants yet the monoclinic symmetry is retained over a temperature range from 100 K to 380 K. Temperature dependent resistance measurement also exhibited a semi-metal to insulator like transition near 220K displaying over 2 order of magnitude change in resistance across the transition. Small changes in the oxygen K-edge x-ray absorption spectrum were seen with change in temperature. At low temperature, an additional peak (d|| band) has emerged in the XAS spectra at energy higher than the σ* peak. The appearance of d|| band density of states is associated with the enhanced electron correlation effects driven by the localization of V–V pair's interactions at low temperature.

Low-Temperature Synthesis of Vanadium Dioxide Thin Films by Sol-Gel Dip Coating Method

The vanadium dioxide (VO2) thin films were synthesized by sol-gel dipping on a glass slide substrate at low temperature of 500°C in a vacuum tube furnace at a pressure of 2 × 10−3 mbar by 2-step calcination without an intermediate gas purging. Synthesis conditions, including temperature, vacuum pressure, and calcination steps in the vacuum tube furnace, were investigated to find the optimum condition that promoted the formation of VO2 phase. It was found that the 2nd calcination step was very important in realizing the monoclinic vanadium dioxide (VO2 (M)). The results of the valence electron analysis revealed the outstanding phase of VO2 and a small amount of V2O5 and V2O3 phases. The small crystallites of the VO2 were homogeneously distributed on the surface, and the grain was of an irregular shape of ∼220−380 nm in size. The film’s thickness was in a range of 69−74 nm. The film exhibited a metal-to-insulator transformation temperature of ∼68oC and good thermochromic property. Visible optical transmittance remained at ∼40−50% when the sample’s temperature changed from 25 to 80°C for a near infrared (NIR) region.

Infrared Switching of Self-Heating VO<sub>2</sub>/ITO Films for Smart Window

The unique metal to insulator transition (MIT) of vanadium dioxide (VO2) makes it receiving extensive attention in the application of smart window. As for VO2-based smart window, the critical transition temperature (Tc) is required to be reduced to near room temperature for practical applications. In this paper, we fabricated VO2 films on ITO glass by hydrothermal method and applied voltage to ITO, therefore, the joule heat generated by ITO triggered the complete MIT of VO2 at room temperature in very short time ~3 s with applied voltage of 12 V. The VO2 film on ITO substrate shows obviously widened hysteresis behavior in the reversible transition process with a thermal hysteresis width of ~33 °C. The widened hysteresis loop makes it possible to stabilize the rutile phase (R) of VO2 at room temperature via applying a low holding voltage of 6 V. The proposed VO2/ITO film exhibits promising application in active smart window, and possesses advantages of simple structure, easy-fabricated and low-cost.

Influence of annealing on structure, phase and electrophysical properties of vanadium oxide films

The aim of this work was to study the effect of the parameters of deposition process and subsequent annealing on the properties of vanadium oxide VO x films deposited by the pulsed reactive magnetron sputtering of a V target in an Ar/O 2 gas mixture. The dependences of the structure, phase, temperature coefficient of resistance (TCR), resistivity p , band gap Egof the films on the oxygen concentration in Ar/O 2 gas mixture during the deposition Г O2 , and the temperature of annealing in an O 2 atmosphere were obtained. The films were found to have an amorphous structure after deposition. Crystallization processes are observed at temperatures above 275 °C. In this case, depending on the temperature, polycrystalline films with a monoclinic, cubic or mixed crystal lattice are formed and a transition occurs from the intermediate oxide V 4 O 9 to the mixed phase VO 2 /VO x /V 2 O 5 and then to the higher oxide V 2 O 5 . The character of changes in p , TCR and Egof films coming from the change in the annealing temperature is complex and largely determined by Г O2 . It was established that with the view of using VO x films as thermosensitive layers, the following conditions of deposition and annealing would be preferable: films deposited at the oxygen concentration 25 % in Ar/O 2 gas mixture and annealed at a temperature of 250–275 °C in an O 2 atmosphere for 10 min. Under these conditions VO x films with the following properties were obtained: p = (1.0 – 3.0) . 10 -2 Ohm . m, TCR = 2.05 %/°C, and E g = 3.76–3.78 eV.

Phase Transitions in Correlated Oxides Modulated through Electrochemical Gating

Strongly correlated oxides have many unusual properties, such as high-temperature superconductivity, colossal magnetoresistance, spin polarization, and metal-insulator phase transitions (MIT) that can be induced by various types of external stimuli, such as temperature, strain, pressure, electron and chemical doping, and light. VO2 undergoes an electronic and structural phase transition at 67°C from a non-magnetic monoclinic (M1) phase to a paramagnetic rutile (R) phase. Modulation of the transition temperature through oxygen over-stoichiometry is difficult to achieve in VO2 through annealing due to nucleation of the V2O5 phase within the VO2 lattice. However, electrochemical doping at room temperature using suitable electrolyte allows for systematic modulation of oxygen stoichiometry in VO2 through the regulation of charge carrier injection. Using this method, the oxygen stoichiometry in VO2 was modulated from 1.86 to 2.44 and its effects on the band structure and MIT was studied through detailed electrical and photoemission studies. In oxygen over-stoichiometric VO2 the TIMT was shifted from 67°C to 118°C, as shown in Figure 1.The results offer a different perspective on the temperature- and doping-induced IMT process. They suggest that charge fluctuation in the metallic phase of intrinsic VO2 results in the formation of e− and h+ pairs that lead to delocalized polaronic V3+ and V5+ cation states. The metal-to-insulator transition is linked to the cooperative effects of changes in the V–O bond length, localization of V3+ electrons at V5+ sites, which results in the formation of V4+–V4+ dimers, and removal of π* screening electrons. It is shown that the nature of phase transitions is linked to the lattice V3+/V5+ concentrations of stoichiometric VO2 and that electronic transitions are regulated by the interplay between charge fluctuation, charge redistribution, and structural transition. Figure 1

Optical Forces on an Oscillating Dipole Near VO2 Phase Transition

We investigate optical forces on oscillating dipoles close to a phase change vanadium dioxide (VO2) film, which exhibits a metal-insulator transition around 340 K and low thermal hysteresis. This configuration emulates the interaction between an illuminated nanosphere and an interface and we employ a classical description to capture its important aspects. We consider both electric and magnetic dipoles for two different configurations, namely with the dipole moments parallel and perpendicular to the VO2 film. By using Bruggeman theory to describe the effective optical response of the material, we show that the thermal hysteresis present in the VO2 transition clearly shows up in the behavior of optical forces. In the near-field regime, the force on both dipoles can change from attractive to repulsive just by heating (or cooling) the film for a selected frequency range. We also verified that the optical forces are comparable to the Casimir-Polder force in a similar system, revealing the possibility of modulating or even changing the sign of the resultant force on an illuminated nano-object due to the presence of a thermochromic material. We hope that this work contributes to set the grounds for alternative approaches to control light-matter interactions using phase-change materials.

VO2 thin films fabricated by reduction of thermal evaporated V2O5 under N2 flow

VO2 thin films were fabricated by thermal evaporation of V2O5 on quartz and glass and subsequent reduction in nitrogen (N2). This Physical Vapor Deposition (PVD) method resulted in high-quality single phase VO2(M1) films with sharp changes in resistance (3–4 orders of magnitude) and transmittance (40–45%) at λ= 1550 nm accompanied by narrow hysteresis loops (~ 4–5 K) around the Semiconductor-to-Metal Transition (SMT).

Effect of annealing and swift heavy ions irradiation on vanadium oxide thin films

In this work, thin films (∼200 nm) of vanadium oxide were deposited on Si (100) substrate by e-beam evaporation method with low-cost V2O5 powder. Some of the as-deposited samples were annealed in the argon atmosphere at 500°C for 1 h and cooled at a natural rate in the argon atmosphere. The as-deposited samples were also irradiated with 100 MeV Ag ions beam at different fluences. Grazing incidence X-ray diffraction (GIXRD) and Raman spectroscopy revealed that as-deposited films were composed of the V6O13 phase (mixed valance state). This V6O13 phase was transformed into polycrystalline V2O5 phase after post-annealing. Raman spectra of irradiated films had a new peak of the V6O13 phase, and no change was observed at higher fluence, and in the FTIR spectra, bands shifted toward lower wavenumber side at higher fluence. This band shifting toward the lower wavenumber side may be due to the weakening of vanadium oxygen bonds, which create oxygen vacancies in the film. The creation of defects in films was due to the pressure applied by the molten zone, which is formed by the passage of SHIs in the material.

Surfaces of VO2 -Polymorphs: Structure, Stability and the Effect of Doping.

Vanadium dioxide is an interesting and frequently applied material due to its metal‐insulator phase transition. However, there are only few studies of the catalytic activity and surface properties of different VO2 polymorphs. Therefore, we investigated the properties of the surfaces of the most stable VO2 phases theoretically at density‐functional theory level using a self‐consistent hybrid functional which has demonstrated its accuracy for the prediction of structural, electronic and energetic properties in a previous study. We found that the surfaces of the rutile R phase of VO2 are not stable and show a spontaneous phase transition to the monoclinic M1 phase. Doping with Mo stabilizes the surfaces with rutile structure even for small dopant concentrations (6.25 %). Both M1 and R surfaces strongly relax, with and without doping. In particular the metal‐metal distances in the uppermost layers change by up to 0.4 Å. Mo segregates in the topmost layer of both R and M1 phases. The electronic structure is only slightly changed upon doping.