Infrared transition properties of vanadium dioxide thin films across semiconductor-metal transition

Vanadium dioxide thin films were fabricated through annealing vanadium oxide thin films deposited by dual ion beam sputtering. X-ray diffraction （XRD）, atom force microscopy （AFM）, and Fourier transform infrared spectrum （FTIR） were employed to measure the crystalline structure, surface morphology, and infrared optical transmittance. The phase transition properties were characterized by transmittance. The results show that the annealed vanadium oxide thin film is composed of monoclinic VO2, with preferred orientation of （011）. The maximum of transmittance change is beyond 65% as the temperature increases from 20 to 80 C. The reversible changes in optical transmittance against temperature were observed. The change rate of transmittance at short wavelength is higher than that at long wavelength at the same temperature across semiconductor-metal phase transition. This phenomenon was discussed using diffraction effect.

Synthesis and Characterization of Tungsten Doped Vanadium Dioxide Nanopowders by Thermolysis

As far as we known, with white powdery tungstic acid （WPTA） used as the dopant for the first time, tungsten-doped vanadium dioxide （VO2） nanopowders were synthesized through thermolysis at low temperature. The products were characterized by XRD, TEM and DSC. When WPTA was added into V205 （with W/V = 2 atom %）, the phase transition temperature （Tc） of VO2 decreased markedly from 67.15℃ to 26.46℃ after annealing at 500℃. Such a low Tc is beneficial to the application and development of smart windows materials.

Surface oxidation of vanadium dioxide films prepared by radio frequency magnetron sputtering

This paper reports that the thermochromic vanadium dioxide films were deposited on various transparent substrates by radio frequency magnetron sputtering, and then aged under circumstance for years. Samples were characterized with several different techniques such as x-ray diffraction, x-ray photoelectron spectroscopy, and Raman, when they were fresh from sputter chamber and aged after years, respectively, in order to determine their structure and composition. It finds that a small amount of sodium occurred on the surface of vanadium dioxide films, which was probably due to sodium ion diffusion from soda-lime glass when sputtering was performed at high substrate temperature. It also finds that aging for years significantly affected the nonstoichiometry of vanadium dioxide films, thus inducing much change in Raman modes.

Wideband switchable dual-functional terahertz polarization converter based on vanadium dioxide-assisted metasurface

The terahertz technology has attracted considerable attention because of its potential applications in various fields.However,the research of functional devices,including polarization converters,remains a major demand for practical applications.In this work,a reflective dual-functional terahertz metadevice is presented,which combines two different polarization conversions through using a switchable metasurface.Different functions can be achieved because of the insulator-to-metal transition of vanadium dioxide(VO_(2)).At room temperature,the metadevice can be regarded as a linear-to-linear polarization convertor containing a gold circular split-ring resonator(CSRR),first polyimide(PI)spacer,continuous VO_(2) film,second PI spacer,and gold substrate.The converter possesses a polarization conversion ratio higher than 0.9 and a bandwidth ratio of 81%in a range from 0.912 THz to 2.146 THz.When the temperature is above the insulator-to-metal transition temperature(approximately 68℃)and VO_(2) becomes a metal,the metasurface transforms into a wideband linear-to-circular polarization converter composed of the gold CSRR,first PI layer,and continuous VO_(2) film.The ellipticity is close to-1,while the axis ratio is lower than 3 dB in a range of 1.07 THz-1.67 THz.The metadevice also achieves a large angle tolerance and large manufacturing tolerance.

Multi-functional vanadium dioxide integrated metamaterial for terahertz wave manipulation

We proposed a vanadium dioxide(VO2)-integrated multi-functional metamaterial structure that consists of three metallic grating layers and two VO2 films separated by SiO2 dielectric spacers.The proposed structure can be flexibly switched among three states by adjusting temperature,incident direction,and polarization.In state 1,the incident wave is strongly transmitted and perfectly converted to its orthogonal polarization state.In state 2,the incident wave is perfectly absorbed.In state 3,incident wave is totally reflected back.The working frequency of the multi-functional metamaterial can be arbitrarily tuned within a broad pass band.We believe that our findings are beneficial in designing temperature-controlled metadevices.

Structural and optical properties of tungsten-doped vanadium dioxide films

Thin films of tungsten （W）-doped thermochromic vanadium dioxide （VO2） were deposited onto soda-lime glass and fused silica by radio frequency magnetron sputtering. The doped VO2 films were characterized by X-ray diffraction, optical transmittance measurement, and near field optical microscopy with Raman spectroscopy. X-ray diffraction patterns show that the （011） peak of W-doped thermochromic VO2 film shifts to a lower diffraction angle with the increase of W concentration. The optical measurements indicated that the transmittance change （AT） at wavelength of 2500 nm drops from 65% （AT at 35 ℃ and 80 ℃ for undoped VO2 film） to 38% （AT at 30 ℃ and 42 ℃ for the doped VO2 film）. At the same time, phase transition temperature drops from 65 ℃ to room temperature or lower with the increase of W concentration. Near field optical microscopy image shows that the surface of W-doped VO2 film is smooth. Raman results show that the main Raman modes of W-doped VO2 are centered at 614 cm 1, the same as that of undoped VO2, suggesting no Raman mode changes for lightly W-doped VO2 at room temperature, due to no phase transition appearing under this condition.

Terahertz Metamaterial Modulator Based on Vanadium Dioxide

We present a design of terahertz modulator based on metamaterial absorber structure withvanadium dioxide （VO2）, which can be controlled by optical-pumping or temperature variation. With the state change of VO2 from an insulator to a metal, the absorption has an abrupt increase from zero to 88.5%. In particular, the VO2 layer here is used to not only provide the modulating character, but also replace the metal ground plane to join the resonance operating as a metamaterial absorber. This work demonstrates a feasibility of VO2 in metamaterial perfect absorber, and exhibits potential applications in controllable terahertz devices.

Probing thermal properties of vanadium dioxide thin films by time-domain thermoreflectance without metal film

Vanadium dioxide(VO_(2))is a strongly correlated material,and it has become known due to its sharp metal-insulator transition(MIT)near room temperature.Understanding the thermal properties and their change across MIT of VO_(2)thin film is important for the applications of this material in various devices.Here,the changes in thermal conductivity of epitaxial and polycrystalline VO_(2)thin film across MIT are probed by the time-domain thermoreflectance(TDTR)method.The measurements are performed in a direct way devoid of deposition of any metal thermoreflectance layer on the VO_(2)film to attenuate the impact from extra thermal interfaces.It is demonstrated that the method is feasible for the VO_(2)films with thickness values larger than 100 nm and beyond the phase transition region.The observed reasonable thermal conductivity change rates across MIT of VO_(2)thin films with different crystal qualities are found to be correlated with the electrical conductivity change rate,which is different from the reported behavior of single crystal VO_(2)nanowires.The recovery of the relationship between thermal conductivity and electrical conductivity in VO_(2)film may be attributed to the increasing elastic electron scattering weight,caused by the defects in the film.This work demonstrates the possibility and limitation of investigating the thermal properties of VO_(2)thin films by the TDTR method without depositing any metal thermoreflectance layer.

Preparation of Vanadium Dioxide Films for Protection from High-energy Laser Hits

Vanadium dioxide(VO 2)thin films are used for protection from high-energy laser hits due to their semiconductor-to-metal phase transition experienced during heating at temperature of approximately 68 ℃,which followed by a abrupt change of optical behavior, namely from transparent semiconductor state below 68 ℃ to highly reflective metallic state beyond 68 ℃.The preparation and properties of the films are described as well as the primary principle of the device for protection from high energy laser hits. An ion-beam-sputtering system is used to deposit VO 2 thin films.The technique is reactive ion beam sputtering of vanadium at temperature of 200 ℃ on Si, Ge and Si 3N 4 substrates in a well controlled atmosphere of argon with a partial pressure of O 2, followed by a post annealing at 400-550 ℃ with argon gas.The optical transmittance changes from 60% to 4% are obtained within the temperature range from 50 ℃ to 70 ℃. X-ray diffraction (XRD) shows that the films are of single-phase VO 2.

Manipulation of Terahertz Radiation Using Vanadium Dioxide

Vanadium dioxide （VO2） is a phase transition material which undergoes a reversible metal-insulator transition （MIT） when triggered by thermal, photo, electrical, and even stress. The huge conduction change of VO2 renders it a promising material for terahertz （THz） manipulation. In this paper, some interesting works concerning the growth and characteristics of the VO2 film are selectively reviewed. A switching of THz radiation by photo-driven VO2 film is demonstrated. Experiments indicate an ultrafast optical switching to THz transmission within 8 picoseconds, and a switching ratio reaches to over 80%during a wide frequency range from 0.3 THz to 2.5 THz.

Structural Stability and Electronic Properties of the I41/amd Vanadium Dioxide

By using LDA＋U approach based on the density functional theory, the structural stability of I41/amd VO2 is investigated. According to the phonon dispersion and stability criteria, the I41/amd is suggested to be another possible and stable structure for the VO2. Lattice parameters of the I41/amd VO2 are determined by geometry optimization. The energy band structure shows that the I41/amd VO2 should be a metal. Furthermore, the upper valence band has dominant 2p-orbital characters, but the lower conduction band shows distinctive 3d-orbital characters. Obvious hybridization between the O-2p and V-3d orbitals is observed.

Anisotropic metal–insulator transition in strained VO_(2)(B) single crystal

Mechanical strain can induce noteworthy structural and electronic changes in vanadium dioxide, imparting substantial scientific importance to both the exploration of phase transitions and the development of potential technological applications. Unlike the traditional rutile(R) phase, bronze-phase vanadium dioxide [VO_(2)(B)] exhibits an in-plane anisotropic structure. When subjected to stretching along distinct crystallographic axes, VO_(2)(B) may further manifest the axial dependence in lattice–electron interactions, which is beneficial for gaining insights into the anisotropy of electronic transport.Here, we report an anisotropic room-temperature metal–insulator transition in single-crystal VO_(2)(B) by applying in-situ uniaxial tensile strain. This material exhibits significantly different electromechanical responses along two anisotropic axes.We reveal that such an anisotropic electromechanical response mainly arises from the preferential arrangement of a straininduced unidirectional stripe state in the conductive channel. This insulating stripe state could be attributed to the in-plane dimerization within the distorted zigzag chains of vanadium atoms, evidenced by strain-modulated Raman spectra. Our work may open up a promising avenue for exploiting the anisotropy of metal–insulator transition in vanadium dioxide for potential technological applications.

Determining Hubbard U of VO_(2) by the quasi-harmonic approximation

Vanadium dioxide VO_(2) is a strongly correlated material that undergoes a metal-to-insulator transition around 340 K.In order to describe the electron correlation effects in VO_(2), the DFT+U method is commonly employed in calculations.However, the choice of the Hubbard U parameter has been a subject of debate and its value has been reported over a wide range. In this paper, taking focus on the phase transition behavior of VO_(2), the Hubbard U parameter for vanadium oxide is determined by using the quasi-harmonic approximation(QHA). First-principles calculations demonstrate that the phase transition temperature can be modulated by varying the U values. The phase transition temperature can be well reproduced by the calculations using the Perdew–Burke–Ernzerhof functional combined with the U parameter of 1.5eV. Additionally,the calculated band structure, insulating or metallic properties, and phonon dispersion with this U value are in line with experimental observations. By employing the QHA to determine the Hubbard U parameter, this study provides valuable insights into the phase transition behavior of VO_(2). The findings highlight the importance of electron correlation effects in accurately describing the properties of this material. The agreement between the calculated results and experimental observations further validates the chosen U value and supports the use of the DFT+U method in studying VO_(2).

Engineered nitrogen doping on VO_(2)(B)enables fast and reversible zinc-ion storage capability for aqueous zinc-ion batteries

Vanadium-based compounds with high theoretical capacities and relatively stable crystal structures are potential cathodes for aqueous zinc-ion batteries(AZIBs).Nevertheless,their low electronic conductivity and sluggish zinc-ion diffusion kinetics in the crystal lattice are greatly obstructing their practical application.Herein,a general and simple nitrogen doping strategy is proposed to construct nitrogen-doped VO_(2)(B)nanobelts(denoted as VO_(2)-N)by the ammonia heat treatment.Compared with pure VO_(2)(B),VO_(2)-N shows an expanded lattice,reduced grain size,and disordered structure,which facilitates ion transport,provides additional ion storage sites,and improves structural durability,thus presenting much-enhanced zinc-ion storage performance.Density functional theory calculations demonstrate that nitrogen doping in VO_(2)(B)improves its electronic properties and reduces the zinc-ion diffusion barrier.The optimal VO_(2)-N400 electrode exhibits a high specific capacity of 373.7 mA h g^(-1)after 100 cycles at 0.1 A g^(-1)and stable cycling performance after 2000 cycles at 5 A g^(-1).The zinc-ion storage mechanism of VO_(2)-N is identified as a typical intercalation/de-intercalation process.

Phase Transition Behaviour of VO2 Nanorods

The metal-insulator transition （MIT） of VO2 （M） nanorods was studied. It was found that there were two MITs in the differential scanning calorimetry （DSC） curves of the VO2（M） nanorods, one situated at low temperature from -3 ℃ to 19 ℃ and the other was at high temperature of 65-74℃. The low temperature MIT was always accompanied with VO2（B） nanorods, and the high temperature MIT existed singly only in pure VO2（M） nanorods. The mechanisms of these two MITs were analyzed and discussed.

Review on thermochromic vanadium dioxide based smart coatings： from lab to commercial application

With an urgent demand of energy efficientcoatings for building fenestrations, vanadium dioxide（VO2）-based thermochromic smart coatings have beenwidely investigated due to the reversible phase transition ofVO2 at a critical transition temperature of 68 ℃, which isaccompanied by the modulation of solar irradiation, espe-cially in the near-infrared region. As for commercialapplications in our daily life, there are still some obstaclesfor VO2-based smart coatings, such as the high phasetransition temperature, optical properties （luminous trans-mittance and solar modulation ability）, environmental sta-bility in a long-time period, as well as mass production. Inthis review, recent progress of thermochromic smart coat-ings to solve above obstacles has been surveyed. Mean-while, future development trends have also been given topromote the goal of commercial production of VO2 smartcoatings.

Thin film design for advanced thermochromic smart radiator devices

This paper describes the research on the materials and design methods for advanced smart radiator devices （SRDs） on large-area flexible substrates utilized on spacecraft. The functional material is thermochromic vanadium dioxide. The coating design of SRD is similar to the design of broadband filter coatings in a mid-infrared region. The multilayer coatings have complex structures. Coating materials must be highly transparent in a required spectrum region and also mechanically robust enough to endure the influence from the rigorous environments of outer space. The number of layers must be very small, suitable for the deposition on large-area flexible substrates. All the coatings are designed initially based on optical calculation and practical experience, and then optimized by the TFCALC software. Several designs are described and compared with each other. The results show that the emittance variability of the designed SRDs is great than 400%, more advanced than the reported ones.

Study of nanocrystalline VO_2 thin films prepared by magnetron sputtering and post-oxidation

Nanocrystalline VO2 thin films were deposited onto glass slides by direct current magnetron sputtering and postoxidation. These films undergo semiconductor-metal transition at 70 ℃, accompanied by a resistance drop of two magnitude orders. The crystal structures and surface morphologies of the VO2 films were characterized by x-ray diffraction （XRD） and atomic force microscope （AFM）, respectively. Results reveal that the average grain size of VO2 nanograins measured by XRD is smaller than those measured by AFM. In addition, Raman characterization indicates that stoichiometric VO2 and oxygen-rich VO2 phases coexist in the films, which is supported by x-ray photoelectron spectroscopy （XPS） results. Finally, the optical properties of the VO2 films in UV-visible range were also evaluated. The optical band gap corresponding to 2p-3d inter-band transition was deduced according to the transmission and reflection spectra. And the deduced value, Eopt2p-3d ： 1.81 eV, is in good agreement with that previously obtained by theoretical calculation.

Synthesis and Characterization of VO_2/Mesoporous Carbon Composites for Hybrid Capacitors

VO2/ordered mesoporous carbon （CMK-3） composites were prepared by solid-state reaction process. The microstructures were characterized by X-ray diffraction （XRD）, nitrogen adsorption and desorption, field-emission scanning electron microscopy （FE-SEM） and transmission electron microscopy （TEM）. The experimental results showed that the vanadium oxide in the composites was vanadium dioxide （VO2） with monoclinic structure, which was artificially loaded on the outer surface of CMK-3. VO2/ordered mesoporous carbon composites present a significantly improved capacitive performance （131 F/g） increased by 40.86% compared to that of CMK-3 carbon （93 F/g）. Therefore, as-prepared VO2/mesoporous carbon composites suggest promising applications in hybrid capacitors.

Numerical investigations of an optical switch based on a silicon stripe waveguide embedded with vanadium dioxide layers

A novel scheme for the design of an ultra-compact and high-performance optical switch is proposed and investigated numerically. Based on a standard silicon(Si) photonic stripe waveguide, a section of hyperbolic metamaterials(HMM) consisting of 20-pair alternating vanadium dioxide (VO_2)∕Si thin layers is inserted to realize the switching of fundamental TE mode propagation. Finite-element-method simulation results show that, with the help of an HMM with a size of 400 nm × 220 nm × 200 nm(width × height × length), the ON/OFF switching for fundamental TE mode propagation in an Si waveguide can be characterized by modulation depth(MD) of5.6 d B and insertion loss(IL) of 1.25 dB. It also allows for a relatively wide operating bandwidth of 215 nm maintaining MD > 5 dB and IL < 1.25 dB. Furthermore, we discuss that the tungsten-doped VO_2 layers could be useful for reducing metal-insulator-transition temperature and thus improving switching performance. In general, our findings may provide some useful ideas for optical switch design and application in an on-chip all-optical communication system with a demanding integration level.