VO2 Phase Research Articles

Deep neural network-enabled dual-functional wideband absorbers

The increasing interest in switchable and tunable wideband perfect absorbers for applications such as modulation, energy harvesting, and spectroscopy has significantly driven research efforts. In this study, we present a dual-function terahertz (THz) metamaterial absorber supported by deep neural networks (DNN). This absorber achieves dual-wideband perfect absorption through the use of graphene and vanadium dioxide (VO₂), enabling both switching and tuning functionalities. Simulation results show that, in the insulating phase of VO₂, a high-frequency wideband absorption ranging from 9.31 to 9.77 THz is achieved, with an absorption rate exceeding 90%. In contrast, in the metallic phase of VO₂, a full-band wideband absorption above 90% is observed from 8.44 to 9.75 THz. The corresponding fractional bandwidths are 61.3% and 174.6%, respectively. Additionally, electrical tuning of graphene’s Fermi level from 0.01 to 1 eV enables continuous modulation of absorption intensity between 48 and 100%. The absorber also exhibits polarization insensitivity to TE and TM waves due to its symmetric design and broad incidence angle. This design holds significant potential for various THz applications, including switching, electromagnetic shielding, stealth technology, filtering, and sensing.

Strong spherical V2O5/TiO2–SiO2 composites obtained by template combined with sol-gel method

This study is devoted to the preparation of strong spherical composites V2O5/TiO2–SiO2 by a combined template and sol-gel method. The composition, size and shape of the colloidal particles in butanol ash with tetrabutoxytitanium and tetraethoxysilane, as well as the physicochemical processes leading to the strengthening of the spherical agglomerates obtained using an anion exchanger with a gel structure, have been determined. Electrophoresis, small-angle X-ray scattering, and viscometry were used to demonstrate the presence in the sol of positively charged colloidal particles of lenticular and cylindrical shape, whose size, when the sol is stabilised, reaches 53 Å. The absorption of the sol by the anion exchanger in vanadium form is due to the equalisation of the osmotic pressure in the anion exchanger/sol system. Spherical composites with a diameter of 300 µm were obtained. It was shown by X-ray diffraction that the composites consist of V2O5 with an orthorhombic structure, TiO2 with an anatase structure, and amorphous silicon dioxide. The interaction at the interface between the phases of V2O5 with TiO2 and SiO2, which leads to the strengthening of the sphere of the V2O5/TiO2–SiO2 composite, has been demonstrated by IR and Raman spectroscopy. The results obtained can be used for the synthesis of MxOy/TiO2–SiO2 oxide composites with spherical agglomerates.

Switchable phase modulation and multifunctional metasurface with vanadium dioxide layer

Abstract Metasurface, comprising subwavelength unit cells, offers a flexible modulation of the optical phase. However, traditional metasurfaces are typically engineered to function solely in one mode, limiting their efficiency and adaptability. In this study, we proposed a switchable metasurface consisting of gold bars deposited on polyimide and vanadium dioxide (VO2) layer. Upon the phase transition of VO2, this switchable metasurface exhibits functionality in both transmissive and reflective modes. Specifically, it efficiently converts left circularly polarized (LCP) light into right circularly polarized (RCP) light. For the dielectric (metallic) phases of VO2, the design behaves as metalens with a focal length of 1000 (906) μm at working frequency of 1.2 THz, respectively. Furthermore, the vortex phase can be effectively manipulated with topological number from 1 to 4 through the analysis of the electric field distribution. A directional emission from 12.9° to 42.2° is obtained in the reflective mode and Airy beam paths way can also be well regulated at 1.49 THz. The phase modulation is further achieved by varying the inter-mediums and its thickness. Finally, the sensing ability is explored with different covered solution. Consequently, this multifunctional and adaptable metasurface offers valuable insights for the development of reflectors, modulators, lenses and sensors.

Sol-Gel Pt-VO2 Films as Selective Chemoresistive and Optical H2 Gas Sensors.

In this work, VO2 (M1/R) thin films were exploited as H2 gas sensors. A flat film morphology, obtained by furnace annealing, was compared with a laser-induced nanostructured one. The combination of the environmentally friendly sol-gel approach with the ultrafast laser crystallization allows for significant reductions in energy consumption and related emissions during the fabrication of VO2 sensors. By decorating the sensors' surface with Pt nanoparticles (NPs), the sensor response was enhanced exploiting the hydrogen spillover effect. The Pt/VO2 sensors, tested at operating temperatures between 20 and 200 °C and for concentration of H2 from few ppm to 50000 ppm, offered a dual chemoresistive and optical sensing mode. Low operating temperatures of 150 °C were achieved, along with a detection limit as low as 2 ppm and a perfect baseline recovery. Both sensors guaranteed specific selectivity toward H2, without response to NO2 or humidity, and long-term stability over 500 h. The H2 sensing mechanism, for both the monoclinic and rutile VO2 phases, was investigated through in operando X-ray Diffraction and in situ X-ray Photoelectron Spectroscopy tests. The interaction was found to be based on the reversible formation of HxVO2 bronze, along with the reversible variations in the oxidation state of V.

Synthesis of titanium-vanadium oxide thin films through thermal oxidation process for their use in CO gas sensing application

The thermal oxidation of evaporated titanium-vanadium thin films was carried out at different post-annealing temperatures. The structural, morphological, compositional, optical, electrical, and gas sensing properties of titanium-vanadium oxide thin films were primarily investigated. The results of the XRD study and Raman spectroscopy verified the presence of the orthorhombic V2O5, tetragonal TiO2, and monoclinic V2Ti3O9 phases. The crystallinity of TVO thin films was improved at higher annealing temperatures. The grain size (seen from SEM images) and oxygen compositions (obtained from EDS measurement) of TVO samples are also enhanced when the post-annealing temperature is increased from 500 to 575 °C. From the transmittance curves, the bandgap values for TVO samples were estimated and found in the range of 2.14–2.36 eV. The n-type conductivity of TVO films was confirmed by the negative Hall coefficient values. At last, the CO gas sensing response of TVO thin films was analyzed by monitoring the change in the surface electrical resistances at different operating temperatures and CO gas concentrations. The dynamic and static resistances were assessed at different operating temperatures varying from 100 to 300 °C. The TVO sample prepared at 550 °C showed the best conditions by examining the materials and CO sensing properties.

Electronic vs phononic thermal transport in Cr-doped V2O3 thin films across the Mott transition

Understanding the thermal conductivity of chromium-doped V2O3 is crucial for optimizing the design of selectors for memory and neuromorphic devices. We utilized the time-domain thermoreflectance technique to measure the thermal conductivity of chromium-doped V2O3 across varying concentrations, spanning the doping-induced metal–insulator transition. In addition, different oxygen stoichiometries and film thicknesses were investigated in their crystalline and amorphous phases. Chromium doping concentration (0%–30%) and the degree of crystallinity emerged as the predominant factors influencing the thermal properties, while the effect of oxygen flow (600–1400 ppm) during deposition proved to be negligible. Our observations indicate that even in the metallic phase of V2O3, the lattice contribution is the dominant factor in thermal transport with no observable impact from the electrons on heat transport. Finally, the thermal conductivity of both amorphous and crystalline V2O3 was measured at cryogenic temperatures (80–450 K). Our thermal conductivity measurements as a function of temperature reveal that both phases exhibit behavior similar to amorphous materials, indicating pronounced phonon scattering effects in the crystalline phase of V2O3.

Mechanisms of electrical transition using a conformal MoOx cap on Mo-doped VO2 thermochromics

Mo-doped VO2 thin-films are deposited on the soda-lime glass by a reactive high-power impulse magnetron co-sputtering technique (R-HiPIMS). Rapid thermal annealing at 500 °C for 3 min is performed to achieve the fabrication under low thermal budget. To prevent VO2 from further oxidation, a conformal and semi-conducting MoOx film is applied as the capping layer by the plasma-enhanced atomic layer deposition (PE-ALD). After MoOx film is deposited, an electrical transition in the resistance ratio of 6–10 orders of magnitude has been found, especially for MoOx thicknesses less than 20 nm. A secondary phase of V2O3 plays an important role in abrupt change of current transportation. Also, the hysteresis of optical transition in transmittance at a wavelength of 2500 nm shows that both the width and transition temperature (TC) decrease with increasing the thickness of the MoOx cap. Moreover, the effect of localized surface plasmonic resonance (LSPR) due to the Karst-like structure has been detected by the absorption spectrum. The variations in LSPR peak, such as intensity, blue-shifting, and red-shifting, originated from either excess carrier or structure change are discussed. The high aspect ratio of surface morphology is further examined by atomic force microscopy. In addition, the coverage of MoOx on Mo-doped VO2 concerning the friction behavior is evaluated by lateral force microscopy (LFM), which is helpful in clarifying the responsible mechanisms during TC transition.

Thermochromic properties and surface chemical states of reactive magnetron sputtered vanadium oxide thin films at various deposition pressures

Vanadium oxide thin films were prepared using reactive magnetron sputtering method, varying the deposition pressures from 4 to 10 mTorr while maintaining the other sputtering parameters constant. Our investigations revealed sensitive dependence of deposition pressure, chemical states of the V and O atoms, crystal phases, and thermochromic (TC) optical modulation. Specifically, the film deposited at 8 mTorr displayed distinct TC characteristics with a prevalent monoclinic m-VO2 phase according to X-ray diffraction (XRD), supported by X-ray photoelectron spectroscopy (XPS). Optical transmittance modulation and electrical property measurements revealed a metal–insulator transition (MIT), confirming the intimate connection between TC characteristics and presence of the m-VO2 phase. Films deposited at pressures lower or higher than 8 mTorr exhibited typical semiconductor behavior without TC characteristics, further supported by in situ XRD, where (110) plane shifted to lower angle upon heating above τC. Chemical state analysis by XPS revealed a clear correlation between V−O peaks and the presence of a VO2 phase, with the film deposited at 8 mTorr displaying relatively large peak fitted area indicative of the VO2 phase. These findings highlight the critical and sensitive dependence of the deposition conditions in tailoring the properties of vanadium oxide thin films and provide valuable insights for potential applications in chromogenic films.

First-principles investigation of the metal-insulator transition in the high-temperature phase of VO2 (A) at 473 K

The structure, electronic and magnetic properties of the high-temperature phase of VO2 (A) [HT-VO2 (A)] are extensively investigated by employing first-principles electronic structure calculations. Structural distortion is observed due to the alternative formation of V-V square wells and V-V rectangular four-chain columns in the ab plane and extending along the c-direction. The system is found to be a ferrimagnetic metal due to sharing of the available single V-3d electron by the V-dxy, dyz, dxz states in the spin majority channel and exclusively by the V-dxy states in the spin minority channel. This material encounters metal-to-half-metal transition for an effective Coulomb interaction of Ueff = 2 eV, exhibiting ferromagnetism. The sharing of the single V-3d electron by the V-dxy, dyz and dxz states in the spin majority channel results in the half-metallic behaviour of HT-VO2 (A). The system eventually encounters metal-insulator transition (MIT) upon the application of Ueff = 2.5 eV. Strong electron correlation triggered by U completely breaks both the V-dxy and dyz states into occupied and unoccupied components and hence sets up complete orbital ordering that is responsible for the observed MIT in HT-VO2 (A). The cooperative effect of p-d hybridization and V-O antiferromagnetic coupling gives rise to ferromagnetism in both the half-metallic and insulating phases of HT-VO2 (A).

Unveiling Intercalation Chemistry via Interference-Free Characterization Toward Advanced Aqueous Zinc/Vanadium Pentoxide Batteries.

Aqueous Zn/V2O5 batteries are featured for high safety, low cost, and environmental compatibility. However, complex electrode components in real batteries impede the fundamental understanding of phase transition processes and intercalation chemistry. Here, model batteries based on V2O5 film electrodes which show similar electrochemical behaviors as the real ones are built. Advanced surface science characterizations of the film electrodes allow to identify intercalation trajectories of Zn2+, H2O, and H+ during V2O5 phase transition processes. Protons serve as the vanguard of intercalated species, facilitating the subsequent intercalation of Zn2+ and H2O. The increase of capacity in the activation process is mainly due to the transition from V2O5 to more active V2O5·nH2O structure caused by the partial irreversible deintercalation of H2O rather than the increase of active sites induced by the grain refinement of electrode materials. Eventually, accumulation of Zn species within the oxide electrode results in the formation of inactive (Zn3(OH)2V2O7·2H2O) structure. The established intercalation chemistry helps to design high-performance electrode materials.

Broadband and efficient terahertz helicity inversion by a reconfigurable reflective metasurface based on vanadium dioxide

We experimentally demonstrate the broadband and highly efficient inversion of the rotation direction of the terahertz circular polarization achieved by a dynamic metasurface with vanadium dioxide (VO2). This metasurface converts linear polarization into circular polarization. The rotational direction of the output circular polarization can be reversed based on the VO2 phase transition. The VO2 state can be dynamically switched by injecting direct current into an on-chip circuit. To improve the terahertz-wave utilization efficiency, the metasurface is operated at an incident angle of 45°, thereby eliminating the beam splitter typically used to separate the outgoing wave from the incident wave at normal incidence. To achieve broadband operation, we employ a dispersion-cancelation strategy that cancels the dispersive phase responses between orthogonal linear polarizations. This strategy was previously developed for static metasurfaces; we have applied it to the design of dynamic metasurfaces. At a center frequency of 0.99 THz, the experimental evaluation demonstrated helicity switching with conversion efficiencies of 92% and 87% for the insulating and metallic states of VO2, respectively. Dispersion cancelation is simultaneously achieved for the insulating and metallic states of VO2, resulting in a relative bandwidth of 0.26, which is 3.2 times broader than that of a previously developed efficient dynamic metasurface.

Enhancing phase and morphology control of vanadium oxide films for resistive memory applications: A water-assisted chemical approach

This study explores the influence of precursor solution composition and solvent selection on the synthesis of vanadium oxide (VO2) thin films tailored for resistive memory applications. The vanadium pentoxide and oxalic acid molar ratio and the dispersing medium (ethanol, water, and their mixture) were varied. Water content significantly influenced the VO2 phase, morphology, and film thickness. Ethanol yielded mixed vanadium oxide phases VO2 − Tetragonal, V2O5 − Orthorhombic, and Monoclinic) and a film with minor imperfections. Conversely, the active layer prepared using an aqueous medium showed a predominant VO2 − Monoclinic phase, albeit with a non-uniform film. An ethanol–water mixture produced the most uniform morphology and a mixture of vanadium oxide phases (VO2 − Tetragonal, V2O5- Orthorhombic). All devices displayed resistive switching behavior. VO2-containing devices exhibited a more pronounced transition from an ohmic conduction state to a space-charge-limited conduction mechanism. ON-state currents remained consistent, while OFF-state currents varied notably.

Single-site DFT+DMFT for vanadium dioxide using bond-centered orbitals

We present a combined density-functional theory and single-site dynamical mean-field theory (DMFT) study of vanadium dioxide (VO2) using an unconventional set of bond-centered orbitals as the basis of the correlated subspace. VO2 is a prototypical material undergoing a metal-insulator transition (MIT), hosting both intriguing physical phenomena and the potential for industrial applications. With our choice of correlated subspace basis, we investigate the interplay of structural dimerization and electronic correlations in VO2 in a computationally cheaper way compared to other state-of-the-art methods, such as cluster DMFT. Our approach allows us to treat the rutile and M1 monoclinic VO2 phases on an equal footing and to vary the dimerizing distortion continuously, exploring the energetics of the transition between the two phases. The choice of basis presented in this work hence offers a complementary view on the long-standing discussion of the MIT in VO2 and suggests possible future extensions to other similar materials hosting molecular-orbital-like states. Published by the American Physical Society 2024

Thermotunable mid-infrared metamaterial absorption material based on combined hollow cylindrical VO2 structure

We have designed a metamaterial absorption material based on the VO2 phase transition effect. The aim is to realize thermally tunable broadband absorption in the mid-infrared band. The metamaterial absorption material is a three-layer structure. From top to bottom, the combined hollow cylindrical VO2 resonant layer, the SiO2 dielectric layer and the Ti substrate. In this work, we perform numerical simulations using the three-dimensional time-domain finite element difference method. The simulation shows that the absorption material's average absorption intensity is adjustable by temperature between 0.09 and 0.95. At a temperature of 342 K, the absorption material has an absorption bandwidth of 17.3 μm (6.43 μm to 23.73 μm) with an absorption rate above 90%. And the average absorption in this band range is 94.95%. And this band covers the wavelength of the atmospheric transparency window (8 μm∼14 μm), which suggests that our absorption material has great application prospects. In this work, we first analyze the electromagnetic field at the absorption material's resonant wavelength and explain the absorption material's absorption mechanism. Then, we investigated the absorption effect of the absorption material under the VO2 structure with different temperatures. And we explain temperature's effect on the absorption material's absorption effect by analyzing the electric field's change on the absorption material's surface at different temperatures for the same wavelength. The effect of the absorption material structural parameters on the absorption material absorption performance to determine the optimal structural parameters was then investigated. Finally, we investigate the absorption curves of the absorption material under different columnar VO2 layers and demonstrate the innovative and unique nature of the designed absorption material. The absorption material we designed has a simple structure and is easy to configure. And the absorption material has high average absorption rate and a wide absorption bandwidth. We therefore believe that the metamaterial absorption material has great application prospects in optoelectronic detection, infrared imaging and infrared stealth.

Dimension-Controlled VO2 Film for Optoelectronic Logic Gates and Information Encryption.

As the fields of photonics and information technology develop, a lot of novel applications based on VO2 material, such as optoelectronic computing and information encryption, have been developed. While the performance of these devices was not only closely associated with the VO2 phase transition properties but also depended on their dimensional characteristics. In the current study, we conducted the dimension-controlled vanadium dioxide (VO2) film growth, resulting in the epitaxial 2-dimensional (2D) VO2 film and well-distributed 3-dimensional (3D) VO2 crystal film deposition, respectively. It was revealed that, unlike the 2D film, the pronounced localized surface plasmon resonance dominated the near-infrared spectrum across the phase transition for the 3D VO2 film due to the naturally formed meta-surface structure, which showed a transmittance valley in the infrared spectrum after metallization. Based on this distinct infrared spectrum feature in the 3D VO2 film, we proposed an optoelectronic logic gate controlled by the input voltage and the probing Vis/IR light. By detecting the transmittance states of the probing light with different wavelengths, we achieved multistate encoding functions and demonstrated the information encryption application. This new conception device also showed great potential for some other applications such as optoelectronic coupled computing, information encryption, and optical near-field sensing computing.

Near-Field Nanoimaging of Phases and Carrier Dynamics in Vanadium Dioxide Nanobeams.

The stable coexistence of insulating and metallic phases in strained vanadium dioxide (VO2) has garnered significant research interest due to the intriguing phase transition phenomena. However, the temporal behavior of charge carriers in different phases of VO2 remains elusive. Herein, we employ near-field optical nanoscopy to capture nanoscale alternating phase domains in bent VO2 nanobeams. By conducting transient measurements across the different phases, we observed a prolonged carrier recombination lifetime in the metallic phase of VO2, accompanied by an accelerated diffusion process. Our findings reveal nanoscale carrier dynamics in VO2 nanobeams, offering insights that can facilitate further investigations into phase-change materials and their potential applications in sensing and microelectromechanical devices.

Nanoscale optical nonreciprocity with nonlinear metasurfaces

Optical nonreciprocity is manifested as a difference in the transmission of light for the opposite directions of excitation. Nonreciprocal optics is traditionally realized with relatively bulky components such as optical isolators based on the Faraday rotation, hindering the miniaturization and integration of optical systems. Here we demonstrate free-space nonreciprocal transmission through a metasurface comprised of a two-dimensional array of nanoresonators made of silicon hybridized with vanadium dioxide (VO2). This effect arises from the magneto-electric coupling between Mie modes supported by the resonator. Nonreciprocal response of the nanoresonators occurs without the need for external bias; instead, reciprocity is broken by the incident light triggering the VO2 phase transition for only one direction of incidence. Nonreciprocal transmission is broadband covering over 100 nm in the telecommunication range in the vicinity of λ = 1.5 µm. Each nanoresonator unit cell occupies only ~0.1 λ3 in volume, with the metasurface thickness measuring about half-a-micron. Our self-biased nanoresonators exhibit nonreciprocity down to very low levels of intensity on the order of 150 W/cm2 or a µW per nanoresonator. We estimate picosecond-scale transmission fall times and sub-microsecond scale transmission rise. Our demonstration brings low-power, broadband and bias-free optical nonreciprocity to the nanoscale.

Tunable mid-infrared broadband absorption in the atmospheric window of wavelength in 3–5 μm using VO2 phase transition in a three-layer metamaterial

Tunable metasurfaces have garnered considerable attention for their remarkable ability to manipulate electromagnetic waves within carefully designed nanostructures. Achieving resonant absorption requires precise control over the geometry and size of micro-nano structures. This study introduces a highly versatile metasurface platform that leverages the unique phase transition properties of vanadium dioxide (VO2) for controlled absorption. Our design features a planar layered structure with a 0.1 μm thick VO2 top layer, enabling tunability in both thermal and intensity aspects. We demonstrate significant tuning of broadband light absorption across the mid-infrared spectrum, with absorption rates ranging from approximately 30 %–98 % in the critical atmospheric infrared window of 3–5 μm. This range is crucial for infrared sensing, imaging, and communication applications. Notably, the absorption rate increases as the temperature decreases, indicating a positive shift in dynamics, for which we provide a physical explanation. Numerical simulations highlight the ability to control absorption bandwidth by adjusting the thickness and temperature of VO2 layer. These adaptable broadband absorbers hold significant promise for a diverse range of applications, including absorption filters, thermal emitters, thermos photovoltaics, and thermal imaging. Furthermore, the introduced thermally switchable, spectrally selective metamaterials can find applications in dynamic infrared camouflage and active radiative thermal management, opening new horizons in the realm of metamaterial-based technologies.

Temperature-switchable anti-reflective structure based on vanadium dioxide phase transition in the visible and near-infrared wavelength regions

Nowadays, the anti-reflective (AR) structures are essential in many applications like display screens, photovoltaic structures and light detection and ranging. Traditionally, the AR surfaces are almost multilayer (ML) structures to minimize the reflection value by producing the destructive interference of reflected light beams at the layers’ interfaces. In the new and advanced AR surfaces, nanostructures (NS) are proposed and used for minimizing the reflection. In this paper, we propose a temperature-switchable AR-ML-NS, based on vanadium dioxide (VO2) phase transition from semiconductor to metallic state around the critical temperature of 68 °C. Here, a pyramidal NS of VO2 is considered on top surface of a ML which minimizes the light reflection of the structure. While some AR structures may work in some restricted light wavelengths, here our proposed structure’s AR wavelength region can be tuned between the visible and near-infrared (NIR) region through the thermal phase transition of VO2. VO2 phase control leads to a temperature-switchable AR structure, which is of great importance for investigating different switchable AR structures.

Irregular α-V2O5 nanodiscs fabrication for highly sensitive and responsive ethanol vapours detection

The current study presents a template-free hydrothermal approach for synthesizing nanodiscs resembling vanadium pentoxide (V2O5) nanostructures, highlighting their potential as ethanol gas sensing materials. X-ray diffraction (XRD) analysis confirms the presence of a highly crystallized orthorhombic phase of V2O5 (JCPDS file no 85–0601). The morphological features of V2O5, resembling irregular nanodiscs, were investigated using field-emission scanning electron microscopy (FE-SEM). Fourier-transform infrared (FTIR) spectroscopy was employed to explore the chemical structure and bonding of V2O5. High-resolution transmission electron microscopy (HRTEM) confirmed the crystalline nature of the material, with lattice spacing at 0.57 nm corresponding to the (020) crystal planes. Elemental chemical composition analysis of V2O5 was conducted via energy-dispersive X-ray spectroscopy (EDX). The V2O5 sample exhibited a BET surface area of 15.32 m2/g and a BJH desorption average pore width of 28.76 nm, providing essential insights into its surface availability for gas adsorption. In ethanol vapor environments, the sensing capabilities of the V2O5 sensor were evaluated at room temperature. The prepared V2O5-based sensor demonstrated exceptional selectivity towards ethanol over other volatile organic compounds (VOCs), with a sensing response of 74.88 % towards 1000 ppm ethanol at room temperature (30 °C). These findings underscore the efficacy of the V2O5-based sensor in ethanol atmospheres, exhibiting rapid response and recovery times of 36s and 12s, respectively, at room temperature.