Vanadium Dioxide Films Research Articles

VO2 films on flexible thin polyimide films: Fabrication and characterization of electrical and optical properties in insulator-metal transition

We report on the realization of thin polyimide films on which phase transition vanadium dioxide (VO2) films grow. Biased reactive sputtering achieved b-axis-oriented growth of VO2 films on ZnO-buffered polyimide films with a thickness of 8.5 μm. By peeling off the polyimide films from a quartz substrate, stand-alone VO2/ZnO/polyimide layered films that exhibited insulator-metal transition (IMT) with nearly three orders of resistivity change were fabricated. Dependence of IMT on a mechanical curvature was investigated for demonstrating the high flexibility. Temperature-dependent optical transmittance at 1.45 μm showed a high switching ratio for infrared light in VO2/ZnO/polyimide layered films. The proposed structure can be utilized for active metasurfaces that control terahertz waves with quite low reflection loss due to its small thickness.

Broadside-coupling–enabled insulator-to-metal transition in a terahertz metasurface

We theoretically demonstrate stacked-dipole-resonators–based (broadside near-field coupling configuration) multilayer metasurfaces separated by a vanadium dioxide film to achieve stronger field confinement in the spacer (VO2) region. Under relatively intense terahertz excitation (20 Vm−1) assisted by larger area electric field confinement, insulator-to-metal transition (IMT) in VO2 spacer is realized resulting in frequency (dipole mode) and amplitude (Fano mode) tunable metasurfaces. Enhancement in probing THz field triggers much stronger field confinement inside the spacer layer leading to increased VO2 conductivity (responsible for IMT) through the Poole-Frankel effect. Such broadside coupled IMT-based terahertz metamaterials can help in realizing active meta devices for THz domain.

Sensing enhancement of a Fabry-Perot THz cavity using switchable VO2 mirrors.

We experimentally investigate the sensing properties of an open cavity operating in the THz regime and realized by employing as mirrors two thin vanadium dioxide (VO2) films grown on silicon parallel plates and separated by a variable length. The phase transition of VO2 is used to control the behavior of the system between two different responses: a high transmission mode to the incident radiation (VO2 in the insulating state) and a high sensitivity to tiny changes in the cavity refractive index (VO2 in the conducting state). In the first state, the low loss regime enables to adjust the cavity length and easily optimize the resonances due to the Fabry-Perot (FP) effect in the Si plates and in the cavity volume. The activation of the metallic-like state instead, by damping the FP oscillations in the plates, promotes the onset of a comb-like spectrum that can be exploited as a versatile tool for accurate sensing applications. Using both an analytical model and full-wave simulations, we estimate the device response to variation in the refractive index of the cavity volume, showing that the proposed structure can achieve sensitivity values among the highest reported for THz sensors.

Behavior of the monoclinic order in VO2 thin films grown on sapphire near the metal-insulator transition

We report on the behavior of the monoclinic order near the metal–insulator phase transition (MIT) in vanadium dioxide (VO2) films grown on c-plane sapphire investigated by three-dimensional x-ray reciprocal space mapping (RSM). In the plane perpendicular to the film normal monoclinic b-axis [010] direction, pronounced diffuse scattering was observed in six specific directions whose origin was attributed to the rutile-like planar defects separating ordered monoclinic domains. The correlated region of the monoclinic domains was thin elliptical disk-shaped with the disk normal along the monoclinic [001] direction. As the MIT was approached, the diffuse peaks first moved away from the (020) Bragg peak progressively and then disappeared rapidly, which suggests that the monoclinic-to-rutile structural phase transition accompanying the MIT was progressed in two stages. In the first pre-transitional stage, the rutile-like defect boundaries separating monoclinic domains are nucleated and grow progressively, which crosses over to the transitional stage where the rutile phase grows rapidly within the domains leading to the transition.

Flexible VO2 Films for In‐Sensor Computing with Ultraviolet Light

AbstractWith their unique advantages in portability, shape adaptability, and human friendly surfaces, flexible electronics pave the way for the implementation of wearable electronic textiles and human–machine interfaces. Although organic materials are promising for flexible devices because of the low‐cost manufacturing and inherent flexibility, they meet challenges in harsh environments such as ultraviolet (UV) irradiation, which limits their applicability in UV sensors. Here, a flexible UV neuromorphic sensor is presented using inorganic vanadium dioxide (VO2) films grown on mica substrates. The flexible device shows UV photoinduced nonvolatile phase transition, and can be reversibly modulated using electrolyte gating. The optical responses remain almost unchanged after 10 000 bending cycles or at small bending radius, exhibiting high tolerance to the bending deformation. Besides, the variations in image recognition accuracy under different bending conditions keep within 1.6%, indicating that the device can be adapted to various deformation conditions. By constructing near‐/in‐sensor computing architectures using the flexible VO2 neuromorphic sensors with photoinduced nonvolatile phase transition, both static image processing and motion detection are realized without redundant and massive information transfer. This result lays the foundation for the development of flexible UV neuromorphic sensors.

Design of vanadium-dioxide-based resonant structures for tunable optical response.

Phase change materials are suitable for tunable photonic devices where the optical response can be altered under external stimuli, such as heat, an electrical or an optical signal. In this scenario, we performed numerical simulations to study the optical properties of a flat unpatterned resonant structure and a grating, both coated with a thin film of vanadium dioxide (VO2). Our results suggest that it is possible to modulate broadband and narrowband reflectance spectra of the resonant structures in the visible to near-infrared range by more than 40 % when the VO2 undergoes an insulator-to-metal phase transition. Resonant devices with a tunable spectral response may find application in sensors, filters, absorbers, and detectors.

Implicit method for solving building heat transfer model and its application in energy-saving materials

In this work, implicit method is used to solve the building heat transfer equations in order to improve the speed and stability of calculation. It only takes 161.906 s to calculate the annual data when the time step is 6 min in the specific calculation example. In addition, the effective heat capacity method is used to independently model the building module containing phase change materials (PCMs), which can accurately simulate the temperature distribution of each node inside PCMs. Therefore, the temperature changes of different nodes in PCM and the utilization efficiency of PCM can be analyzed. Radiative heat transfer in buildings can be considered separately so that it can be distinguished from convective heat transfer. The radiation characteristics of the envelopes can also be modified, which means that smart windows can be studied by changing the radiation characteristics of the transparent envelope. Combined with these calculation characteristics, a software called BuildingEnergy is proposed. At present, the reliability of the software has been proved by ANSI/ASHRAE Standard 140–2001. A set of experimental device was set up in Hefei (China), and through the comparison with the simulation results, it was found that the software has high reliability under both active and passive mode. Moreover, it has a high accuracy in simulating the performance of phase change materials and vanadium dioxide films in buildings.

Reversible Nanomodulation of Thin Vanadium Dioxide Films by a Tip-Induced Electric Field

Reversible Nanomodulation of Thin Vanadium Dioxide Films by a Tip-Induced Electric Field

Surface Plasmons Excited by X-rays in the Surface Layers of Solids

The phenomenon of total external reflection of X-rays at a sliding angle of incidence of a beam of incident X-rays is investigated. For metals, a quantitative law of direct dependence of the refractive index decrement on the interplane distance is obtained. The excitation of surface plasmons by X-rays that have experienced complete external reflection is detected. For surface plasmons, a dimensional effect was observed, expressed in an increase in the energy of plasmons and the concentration of conduction electrons with an increase in the depth of the output of surface plasmons. By the method of dispersion of surface plasmons, internal mechanical micro-stresses and spontaneous polarization of the surface layers of glassy dielectrics and in thin layers of vanadium dioxide were determined. The absence of micro-stresses in the lithium fluoride ionic single crystal was found out, and the polarization observed in it is due to the large dipole moment of the molecules of this crystal. In thin films of vanadium dioxide, the dependence of micro-stresses on the stresses in the substrates was found.

Design and energy analysis of tunable nanophotonic infrared filter based on thermochromic vanadium dioxide

In this work, a mid-infrared tunable nanophotonic filter is designed by placing a silicon spacer layer between two nanometric vanadium dioxide films on a thick infrared-transparent substrate. Transfer matrix formulation incorporated with the ray-tracing method is used for the optical modeling, and a transmittance peak near 4 µm wavelength is achieved based on Fabry–Perot resonance effect. Thermochromic vanadium dioxide allows the filter to be switched between a semi-transparent state when cooled below its phase transition temperature and an opaque state when heated above its phase transition temperature. The underlying mechanism is elucidated by calculating the total phase shift of electromagnetic waves through the spacer layer, and the effect of layer thicknesses on the spectral transmittance peak is discussed. The filter can find potential applications in energy dissipation where it can actively promote or suppress radiative cooling, and in thermal camouflage where the apparent temperature of a hot surface through the filter can be greatly reduced. Detailed energy analysis is conducted for both applications with the proposed tunable filter, whose structure is further optimized through the thicknesses of the thin-film layers to achieve maximum performance.

Reflective and transmissive cross-polarization converter for terahertz wave in a switchable metamaterial

Utilizing the phase transition characteristic of vanadium dioxide, we present a metamaterial configuration to achieve both reflective and transmissive cross-polarization converters. When vanadium dioxide is metal, the design behaves as a reflective cross-polarization converter. It consists of metallic grating, topas spacer, and vanadium dioxide film. Polarization conversion ratio is more than 90% in the frequency range from 4.80 THz to 13.13 THz. When vanadium dioxide is insulator, the design behaves as a transmissive cross-polarization converter using cascaded metallic gratings with rotation angle 45°. High-efficiency broadband cross-polarization wave conversion is achieved in the frequency band of 0.50–4.75 THz. Effect of oblique incidence is studied on polarization conversion. Results tell that cross-polarization conversion is better when incident angle is in the range of 0°–40°. The designed metamaterial may have a certain inspiration for the research of terahertz multifunctional polarization converter.

A VO2 film-based multifunctional metasurface in the terahertz band

We proposed a multifunctional terahertz metasurface based on a double L-shaped pattern and a vanadium dioxide (VO2) film separated by polyimide. When the VO2 film is an insulator, a dual-band electromagnetically induced transparency effect is obtained, and the physical mechanism is investigated based on the current distribution and “two-particle” model. When the VO2 film is a metal, a dual-band linear-to-circular polarization converter, in which the y-polarized linear wave can be effectively converted to left-handed circularly polarized (LCP) and right-handed circularly polarized simultaneously in different bands, can be achieved. By arranging the metal pattern rotating 30°, a multifunctional antenna can be obtained. When the VO2 is an insulator, the radiation of the LCP wave is divided into four beams, with two beams reflected and two beams transmitted. When the VO2 is in the metallic state, we can only get the co-polarized reflected wave with a 21° angle. Moreover, in our design, the VO2 film does not need lithography to obtain certain patterns, which improves the convenience of fabrication and experiment. Our design opens a new way for the development of multifunctional terahertz devices and has potential applications in the terahertz communication field.

Reflection of microvawes from thin film of vanadium dioxide

Reflection of microvawes from thin film of vanadium dioxide

Cluster-coalesced defects induced by gamma irradiation on pulsed laser deposited VO2 thin films

This contribution reports on the radiation tolerance of Vanadium dioxide thin films subjected to different projectile doses of gamma irradiation (3 kGy-100 kGy). Thin films of vanadium dioxide were deposited by pulsed laser deposition method on silicon substrate. Radiation modifications were investigated by studies on the morphology and electrical properties of vanadium dioxide. Gamma irradiation thermally altered the structural morphology of VO2 thin films, inducing defects like ball clusters characterized by the localized heating effect. At higher doses, cluster defects coalesce to large ball structures, consequently decreasing the overall current density of VO2 thin films.

Metal-Insulator phase transition in thin films of vanadium dioxide doped with indium

The electrical conductivity of thin polycrystalline films V(1-x)InxO2 has been studied in a wide temperature range covering the regions of existence of both the metallic and insulator phases. It is shown that with increasing indium concentration, the temperature of the metal-insulator phase transition decreases, and the width of the temperature region of phase coexistence monotonically increases. To explain the temperature dependence of the electrical conductivity of the insulator phase V(1-x)InxO2, a model of hopping conductivity is applied, taking into account the effect of thermal vibrations of atoms on the resonance integral. Calculated the values of the parameter ε depend on the degree of doping of VO2. Keywords: phase transition, vanadium oxides, thin films, polaron, electrical conductivity.

Thermal tuning of terahertz metamaterial absorber properties based on VO2.

We present a novel, structurally simple, multifunctional broadband absorber. It consists of a patterned vanadium dioxide film and a metal plate spaced by a dielectric layer. Temperature control allows flexible adjustment of the absorption intensity from 0 to 0.999. The modulation mechanism of the absorber stems from the thermogenic phase change properties of the vanadium dioxide material. The absorber achieves total reflection properties in the terahertz band when the vanadium dioxide is in the insulated state. When the vanadium dioxide is in its metallic state, the absorber achieves near-perfect absorption in the ultra-broadband range of 3.7 THz-9.7 THz. Impedance matching theory and the analysis of electric field are also used to illustrate the mechanism of operation. Compared to previous reports, our structure utilizes just a single cell structure (3 layers only), and it is easy to process and manufacture. The absorption rate and operating bandwidth of the absorber are also optimised. In addition, the absorber is not only insensitive to polarization, but also very tolerant to the angle of incidence. Such a design would have great potential in wide-ranging applications, including photochemical energy harvesting, stealth devices, thermal emitters, etc.

Switchable Induced-Transmission Filters Enabled by Vanadium Dioxide.

An induced-transmission filter (ITF) uses an ultrathin metallic layer positioned at an electric-field node within a dielectric thin-film bandpass filter to select one transmission band while suppressing other bands that would have been present without the metal layer. We introduce a switchable mid-infrared ITF where the metal can be "switched on and off", enabling the modulation of the filter response from a single band to multiband. The switching is enabled by the reversible insulator-to-metal phase transition of a subwavelength film of vanadium dioxide (VO2). Our work generalizes the ITF─a niche type of bandpass filter─into a new class of tunable devices. Furthermore, our fabrication process─which begins with thin-film VO2 on a suspended membrane─enables the integration of VO2 into any thin-film assembly that is compatible with physical vapor deposition processes and is thus a new platform for realizing tunable thin-film filters.

Wafer-scale freestanding vanadium dioxide film.

Vanadium dioxide (VO2), with well-known metal-to-insulator phase transition, has been used to realize intriguing smart functions in photodetectors, modulators, and actuators. Wafer-scale freestanding VO2 (f-VO2) films are desirable for integrating VO2 with other materials into multifunctional devices. Unfortunately, their preparation has yet to be achieved because the wafer-scale etching needs ultralong time and damages amphoteric VO2 whether in acid or alkaline etchants. Here, we achieved wafer-scale f-VO2 films by a nano-pinhole permeation-etching strategy in 6 min, far less than that by side etching (thousands of minutes). The f-VO2 films retain their pristine metal-to-insulator transition and intrinsic mechanical properties and can be conformably transferred to arbitrary substrates. Integration of f-VO2 films into diverse large-scale smart devices, including terahertz modulators, camouflageable photoactuators, and temperature-indicating strips, shows advantages in low insertion loss, fast response, and low triggering power. These f-VO2 films find more intriguing applications by heterogeneous integration with other functional materials.

Tunable Infrared Optical Switch Based on Vanadium Dioxide.

A tunable infrared optical switch based on a plasmonic structure consisting of aluminum nanoarrays with a thin film of vanadium dioxide is proposed. This optical switch can realize arbitrary wavelength-selective optical switching in the mid-infrared region by altering the radii of the aluminum nanoarrays. Furthermore, since vanadium dioxide transforms from its low-temperature insulator phase to a high-temperature metallic phase when heated or applied voltage, the optical switch can achieve two-way switching of its “ON” and “OFF” modes. Finite-difference time-domain software is used to simulate the performance of the proposed infrared optical switch. Simulation results show that the switch offers excellent optical performances, that the modulation depth can reach up to 99.4%, and that the extinction ratio exceeds −22.16 dB. In addition, the phase transition time of vanadium dioxide is on the femtosecond scale, which means that this optical switch based on a vanadium dioxide thin film can be used for ultrafast switching.

Controlled grain-size thermochromic VO2 coatings by the fast oxidation of sputtered vanadium or vanadium oxide films deposited at glancing angles

An original, simple and cost-effective oxidation strategy to attain thermochromic vanadium dioxide thin films is reported. This two-step procedure comprises the initial deposition of DC magnetron-sputtered vanadium or vanadium oxide films by the combination of glancing angle deposition and, if needed, reactive gas pulsing process, followed by fast oxidation of such layers in air atmosphere at high temperatures. Thanks to the careful control of the thermal treatment parameters, and taking advantage of the superior reactivity of the high surface-to-volume porous deposited structures, the formation of pure VO2 (M1) layers was achieved. The comprehensive characterization of such oxidized systems by means of scanning electron microscopy, Raman spectroscopy and scanning-transmission electron microscopy techniques such as electron energy-loss spectroscopy, not only confirmed the presence of the VO2 (M1) phase, but also allowed to shed light on the key role that reaction time plays in the selective formation of vanadium dioxide films of adjustable grain size and crystallinity. The optimal conditions to stabilize thermochromic VO2 consists in using large deposition angles (85 °) and short oxygen pulses (≤ 2 s) during the growth, followed by fast and short thermal treatments (≤ 45 s with a heating rate of 42 °C s−1) in air atmosphere at 550 °C. The metal-to-insulator response of the accomplished VO2 layers was finally evaluated by means of temperature dependent Kelvin probe force microscopy measurements, evidencing surface potential drops at heating, greater than those reported in the literature to date for VO2 thin films.