Vanadium Dioxide Films Research Articles

Recent Advances in Fabrication of Flexible, Thermochromic Vanadium Dioxide Films for Smart Windows.

Monoclinic-phase VO2 (VO2(M)) has been extensively studied for use in energy-saving smart windows owing to its reversible insulator–metal transition property. At the critical temperature (Tc = 68 °C), the insulating VO2(M) (space group P21/c) is transformed into metallic rutile VO2 (VO2(R) space group P42/mnm). VO2(M) exhibits high transmittance in the near-infrared (NIR) wavelength; however, the NIR transmittance decreases significantly after phase transition into VO2(R) at a higher Tc, which obstructs the infrared radiation in the solar spectrum and aids in managing the indoor temperature without requiring an external power supply. Recently, the fabrication of flexible thermochromic VO2(M) thin films has also attracted considerable attention. These flexible films exhibit considerable potential for practical applications because they can be promptly applied to windows in existing buildings and easily integrated into curved surfaces, such as windshields and other automotive windows. Furthermore, flexible VO2(M) thin films fabricated on microscales are potentially applicable in optical actuators and switches. However, most of the existing fabrication methods of phase-pure VO2(M) thin films involve chamber-based deposition, which typically require a high-temperature deposition or calcination process. In this case, flexible polymer substrates cannot be used owing to the low-thermal-resistance condition in the process, which limits the utilization of flexible smart windows in several emerging applications. In this review, we focus on recent advances in the fabrication methods of flexible thermochromic VO2(M) thin films using vacuum deposition methods and solution-based processes and discuss the optical properties of these flexible VO2(M) thin films for potential applications in energy-saving smart windows and several other emerging technologies.

Switchable Metamaterial With Terahertz Buffering and Absorbing Performance

A terahertz metamaterial with switching characteristics from optical buffering to absorbing performance is realized by incorporating a phase-change film of vanadium dioxide. By introducing the electromagnetically induced transparency behavior based on simple strip pairs, the slow light effect with group delay up to 3.5 ps is obtained. When vanadium dioxide is in the insulator state, the remarkable delay can be observed as the incident pulse transmits through the designed structure. Once the vanadium dioxide film is tuned to the metallic state, the metamaterial is switched to a terahertz absorber and the maximum absorption rate of 94% is observed at 1.04 THz. The switching mechanism is discussed by analyzing the electric field and power loss distributions, as well as the impedance matching principle. Moreover, the buffering capability and the absorption performance both remain noticeable within a wide range of the incidence angle. This work offers a strategy for the function-switching metamaterial which provides potential applications in terahertz detecting, switching and slow light devices.

Multi-controlled broadband terahertz absorber engineered with VO2-integrated borophene metamaterials

We demonstrated a dynamically controlled broadband terahertz (THz) metamaterials absorber, which composed of continuous vanadium dioxide (VO2) film, a silicon dioxide (SiO2) layer, and a structured borophene layer. When VO2 is in its metallic state and the armchair direction of borophene along x axis, the proposed absorber realizes an absorptivity peak value of 100% at 7.2 THz for y polarized normal incidence, and an absorptivity peak value of 79% at 8.9 THz for x polarized normal incidence. It is the anisotropic property of borophene that results in the absorptivity difference for x and y polarization in the whole frequency range. Simulated electric field distribution and surface current oscillation has been extracted to explain the physical mechanism of THz wave absorption. Through modifying the geometric parameters of metamaterials microstructure, the broadband absorption performance can be tailored passively. Additionally, the proposed metamaterials absorber has been actively controlled by manipulating the carrier density of borophene and the conductivity of VO2, respectively. The absorptivity can be switched from 45% to 100% at 7.2 THz by changing the carrier density of borophene, and from 22% to 100% at 7.2 THz by changing the conductivity of VO2. Moreover, the proposed absorber exhibits an excellent operation tolerance for oblique TE and TE polarized incidence from 0° to 60°. This work provides a novel approach to design dynamically controlled broadband THz absorbers, which reveals promising applications in the devices of optoelectronic switches, cloakings, filters, and sensors, etc.

Continuously and reversibly electro-tunable optical nanoantennas based on phase transition of vanadium dioxide

Optical nanoantennas have attracted significant attention over the past decades, owing to their exceptional capabilities in terms of light manipulation and versatile optical applications. Recently, active nanoantennas have been developed by introducing phase change materials, to achieve specific tunable electromagnetic responses. However, most of these attempts only function with ‘ON’/‘OFF’ states or switch in a few discrete states, significantly restricting the application in dynamic tunability. Thus far, the continuous and reversible modulation of optical nanoantennas has not been sufficiently explored. In this article, we experimentally demonstrate a continuously and reversibly electro-tunable optical nanoantenna, by integrating an asymmetric gold nanodisk dimer array with a vanadium dioxide (VO2) film and graphene thin film. By accurately controlling the applied electrical current, the Joule heat generated in the graphene film excites the metal-insulator phase transition of VO2, and the refractive index of VO2 exhibits a relatively large variation. When VO2 is in the insulating phase, we observe an apparent resonance dip in the reflection spectrum, which is attributed to a hybrid mode originating from the gap plasmon in the dimers and localized surface plasmon (LSP) resonance excited at the edge of the nanodisks. Meanwhile, owing to the coupling between two asymmetric LSP resonances in the neighboring nanodisks, the reflected peak based on the Fano effect is realized. However, once VO2 is in the metallic phase, the hybrid mode becomes weaker and red-shifted, and the Fano effect disappears. Thereafter, the continuous and reversible electro-modulation of the nanoantenna features, including the resonant wavelength, resonant intensity, and quality factors (Q), are experimentally verified in the optical communication region, by varying the applied electrical current in the hybrid structure. To further increase the modulation range of these properties, we scan the gap size and structural asymmetry parameter of the nanodisk dimer. The results show that, with a smaller gap size, the resonant intensity of the nanoantenna is stronger. When the structural asymmetry parameter increases, the resonant wavelength is redshifted. We expect that such continuously and reversibly electro-tunable nanoantennas will stimulate various applications in optical communication systems, tunable photoelectric sensors, and beyond.

Azithromycin electrochemical detection using a VO2 thin film

Vanadium dioxide (VO2) films were successfully synthesized by hydrothermal treatment and used as a sensing material for the detection of azithromycin (AZI) antibiotics. The VO2 thin film was characterized by electrochemical studies at different scan rates, pH values, and AZI concentrations. The roughness factor results demonstrated that the electrode surface has a large active surface area per unit and elevated active sites influenced by the high porosity of the sensing material on the electrode. The VO2 thin film was investigated as a sensing material in the pH range of 5–8 and presented sensitivities of 61.5 and 60.0 mV pH–1 for cyclic voltammetry (CV) and square-wave voltammetry (SWV) studies, respectively, demonstrating high sensitivity and stability for AZI determination. Under different concentrations of AZI (from 1.0 to 80 μmol L−1), the LOD and LOQ values were 0.02 and 0.06 µmol L–1, respectively, for CV and 0.025 and 0.084 µmol L–1, respectively, for SWV. The excellent sensitivity and facile operation of this sensor give it the potential to serve as an alternative method for the analysis of pharmaceutical products and aquatic environments.

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.

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.

Back Cover: Quantum Sensing of Insulator‐to‐Metal Transitions in a Mott Insulator (Adv. Quantum Technol. 5/2021)

The back cover of this issue, referring to article number 2000142 by Chunhui Rita Du and co-workers, represents the authors' work on measuring insulator-to-metal transition in a Mott insulator using nitrogen-vacancy (NV) centers in diamond. An optically active NV center works as a nanoscale quantum sensor to locally probe the temperature and magnetic field environment of the current filaments between gold contacts on a film of vanadium dioxide, revealing the fundamental mechanism of insulator-to-metal transition. This research highlights the opportunities offered by NV centers in exploring nanoscale thermal and electrical behaviors in Mott materials which might have applications in developing cutting-edge neuromorphic computing platforms. [Cover design: Gerald Yan]

Annealing temperatures effect on the electrical and structural properties of nanocrystalline vanadium dioxide films prepared by Sol–Gel technique

Vanadium oxide (VO2) films glass substrate deposition was achieved at room temperature by the sol–gel method. The fabricated VO2 films were annealed for 1 h at 200, 250, and 300 °C in air. The annealing temperature’s effects on the electrical and structural properties of VO2 films were investigated by DC conductivity, dielectric, AC conductivity, and X-ray diffraction (XRD). XRD results of the annealed films reveal the presence of polycrystalline VO2 with a monoclinic structure. Using the Scherrer equation, an average grain size of VO2 films was estimated to be 2.1–10.8 nm. The density of annealed films was found to be 4.53–4.75 g/cm3. The DC-electrical conductivity reveals the semiconductor behavior of all samples. The activation energy of the prepared films was deduced as 0.22–0.38 eV. The DC conductivity for the VO2 films decreases at an annealed temperature higher than 200 °C, which may due to the increase in grain boundary scattering due to the increase in grain size. The dielectric measurements show that the dielectric loss and dielectric constant decline with the increasing frequency however of these parameters with the annealing temperature. Moreover, the increasing of the AC conductivity with the temperature and frequency is observed. These results suggest that the mobility of charge carriers was increased.

VO2–graphene-integrated hBN-based metasurface for bi-tunable phonon-induced transparency and nearly perfect resonant absorption

A bi-tunable hexagonal boron nitride (hBN)-based metasurface with bi-functional phonon-induced transparency (PIT) and nearly perfect resonant absorption features in the mid-infrared (MIR) range is proposed. The metasurface, that is composed of axially symmetric hBN rings, is separated from a uniform thin vanadium dioxide (VO2) film with a SiO2 spacing layer and is integrated with a top graphene sheet. For the insulating phase of VO2 (i-VO2), PIT with an 80% transmission contrast ratio is observed inside the reststrahlen (RS) band of hBN due to the support of hyperbolic phonon polaritons. A considerably large group delay of 9.5 ps and up to 1.8 THz RIU−1 frequency shift per refractive index unit is also achieved for the i-VO2 case. On the other hand, it is found that for the metallic phase of VO2 (m-VO2), light transmission is prohibited and nearly perfect resonant absorption peaks are appeared inside the RS band of hBN. Finally, by integrating the hBN-based metasurface into the graphene sheet on the top, a tunable PIT-like effect and nearly perfect light absorption is achieved duo to the hybridization of graphene plasmons and hBN phonons. This leads to a modulation depth as high as 87% in the transmission (i-VO2) and 62% in the absorption (m-VO2) responses. Our findings offer a tunable and bi-functional device that is practical for MIR slow-light, sensing, and thermal emission applications.

Spin injection into vanadium dioxide films from a typical ferromagnetic metal, across the metal–insulator transition of the vanadium dioxide films

A vanadium dioxide VO2 film shows metal-insulator transition (MIT) induced by changing environmental temperature. We report the temperature dependence of electromotive force properties generated in VO2/Ni80Fe20 bilayer junctions under the ferromagnetic resonance (FMR) of the Ni80Fe20 layer. An electromotive force generated in a VO2/Ni80Fe20 bilayer junction under the FMR showed a small change across the MIT temperature of the VO2 film, while the VO2 film resistance drastically changed. This behavior was not only explained with the temperature dependence of the electromotive force property generated in the Ni80Fe20 film itself under the FMR, but also with the generated electromotive forces due to the inverse spin-Hall effect (ISHE) in the VO2 film under the FMR of the Ni80Fe20 film. That is, we successfully demonstrated the spin injection from a Ni80Fe20 film into a VO2 film across the MIT temperature of the VO2 film.

Role of seed layer in van der Waals growth of vanadium dioxide film on mica prepared by chemical solution deposition

Vanadium dioxide (VO2) has unique electrical and optical properties, which have practical applications in many devices. However, preparing high-quality VO2 films and transferring them to flexible substrates by an economic and simple way are still challenging. Functional thin films can be grown on van der Waals (vdW) substrates and then transferred to flexible substrates due to the weak interaction between films and their substrates. Herein, VO2 thin films were grown on mica substrates by using the polymer-assisted deposition technique. By controlling the ion concentration in the polymer precursor solution, a flat seed layer was prepared at the initial growth stage. It was found that the introduction of the seed layer can improve the crystallinity, increase the grain size and reduce the defects of the VO2 thin films. It is believed that less nucleation density and layered growth at the initial growth stage are critical to the formation of large size grains. Our results confirmed that chemical solution deposition can prepared high-quality VO2 films on vdW substrates.

Design and fabrication of frequency-responsive terahertz transmittance modulator based on vanadium dioxide spherical films

In this paper, a spherical film array structure based on vanadium dioxide (VO2) was designed and fabricated aimed to enhance the frequency response of transmittance in terahertz (THz) band of VO2 films. Vanadium dioxide and vanadium pentoxide composite spherical films were grown on the surface of two-dimensional single layer array of SiO2 sphere by magnetron sputtering and rapid thermal annealing method, and the frequency response of transmittance of spherical films was studied. The THz transmittance modulation depth (MD) of spherical films increased significantly in the 1.5–3 THz range as frequency increased, while MD of planar films only increased slightly throughout the frequency band. We attributed the frequency response of the MD of spherical films to the absorption by electric dipole resonance. The numerical simulations and electric field distributions illustrate that the regulating effect of resonance absorption would increase as the width of spherical gaps increase. When the width of the spherical gaps was 0.05μm, the absorptivity would increase by 120% from 1.5 to 3 THz, showing the possibility of further improving the THz MD of spherical films. This research would provide a good candidate material in the active modulation in terahertz band which broaden a path for dynamic terahertz radiation manipulation devices.

A patterned phase-changing vanadium dioxide film stacking with VO2 nanoparticle matrix for high performance energy-efficient smart window applications

A vanadium dioxide (VO2) based solid-to-solid phased changing material has been attracting great interest in smart window applications. However, achieving high solar modulation and high transparency simultaneously in visible light is the major challenge for the practical application of this smart material. To resolve this issue, in this paper, a smart film composed of a VO2 nanoparticle matrix and a patterned VO2 film is presented. Numerical modeling and electromagnetic simulation are carried out to characterize the performance in terms of solar modulation and luminous transmittance, and a parametric study is carried out to optimize the proposed smart window film. Compared with the VO2 nanoparticle matrix, the proposed structure can obtain 23% solar modulation and 57% luminous transmittance but with a much thinner thickness, which will significantly reduce the cost and fabrication complexity and extend the environment stability.

Casimir-Polder Interaction of an Atom with a Cavity Wall Made of Phase-Change Material out of Thermal Equilibrium

We consider the out-of-thermal-equilibrium Casimir-Polder interaction between atoms of He*, Na, Cs, and Rb and a cavity wall made of sapphire coated with a vanadium dioxide film which undergoes the dielectric-to-metal phase transition with increasing wall temperature. Numerical computations of the Casimir-Polder force and its gradient as the functions of atom-wall separation and wall temperature are made when the latter exceeds the temperature of the environment. The obtained results are compared with those in experiment on measuring the gradient of the Casimir-Polder force between 87Rb atoms and a silica glass wall out of thermal equilibrium. It is shown that the use of phase-change wall material significantly increases the force magnitude and especially the force gradient, as opposed to the case of a dielectric wall.

Phase Transition-Induced Temperature-Dependent Phonon Shifts in Molybdenum Disulfide Monolayers Interfaced with a Vanadium Dioxide Film.

We report the optical phonon shifts induced by phase transition effects of vanadium dioxide (VO2) in monolayer molybdenum disulfide (MoS2) when interfacing with a VO2 film showing a metal-insulator transition coupled with structural phase transition (SPT). To this end, the monolayer MoS2 directly synthesized on a SiO2/Si substrate by chemical vapor deposition was first transferred onto a VO2/c-Al2O3 substrate in which the VO2 film was prepared by a sputtering method. We compared the MoS2 interfaced with the VO2 film with the as-synthesized MoS2 by using Raman spectroscopy. The temperature-dependent Raman scattering characteristics exhibited the distinct phonon behaviors of the E2g1 and A1g modes in the monolayer MoS2. Specifically, for the as-synthesized MoS2, there were no Raman shifts for each mode, but the enhancement in the Raman intensities of E2g1 and A1g modes was clearly observed with increasing temperature, which could be interpreted by the significant contribution of the interface optical interference effect. In contrast, the red-shifts of both the E2g1 and A1g modes for the MoS2 transferred onto VO2 were clearly observed across the phase transition of VO2, which could be explained in terms of the in-plane tensile strain effect induced by the SPT and the enhancement of electron-phonon interactions due to an increased electron density at the MoS2/VO2 interface through the electronic phase transition. This study provides further insights into the influence of interfacial hybridization for the heterogeneous integration of 2D transition-metal dichalcogenides and strongly correlated materials.

Relaxation of the dielectric response in thin films of vanadium dioxide

The results of studying the processes of dielectric relaxation in thin nanocrystalline films of vanadium dioxide are presented. The existence of a non-Debye relaxation process was revealed, which is due to the presence of a distribution of relaxators over relaxation times according to the Cole-Davidson model. The activation energy of the dielectric relaxation process was found to be Ep = (0.9 ± 0.1) eV. The observed regularities are explained by a model which views the system as a set of relaxators whose physical parameters have different numerical values due to the Gaussian size distribution of nanocrystallites of the VO2 film. The temperature change in the parameters of the dielectric relaxation process, detected at T = 340 K, indicates that a complex Mott-Peierls semiconductor-metal phase transition occurs at a given temperature in the VO2 nanocrystalline film.

Terahertz absorption modulator with largely tunable bandwidth and intensity

A dual-tunable metamaterial absorption modulator is proposed based on a hybrid vanadium dioxide-graphene configuration. When vanadium dioxide is in the state of metal, the design serves as a narrowband absorber. It is composed of vanadium dioxide patch, topas spacer, and vanadium dioxide film. When vanadium dioxide is in the state of dielectric, the design serves as a broadband absorber. It includes vanadium dioxide patch, topas spacer, graphene layer, vanadium dioxide film, topas spacer, metal substrate. By adjusting Fermi level of graphene from 0.1 eV to 0.7 eV, absorptance of the proposed absorber can be dynamically tuned from 45.3% to 94.5% at 0.4 THz and from 31% to 96.3% at 1.0 THz, and absorption bandwidth gradually widens. Because of the unique dielectric to metal transition properties of vanadium dioxide, the tunable behavior of absorption frequency and intensity can be achieved through external stimuli. Thus our design can realize adjustable absorption of intensity and frequency in terahertz range. Besides, the designed narrowband and broadband absorbers are polarization-insensitive for small incident angle due to the symmetry, and absorptions of the design keep good performance within the angle range of 50°.

VO2 antennas with metallic inclusions for low‐loss reconfigurable cognitive radios

AbstractIn this article, we demonstrate a reconfigurable antenna concept based upon a film of vanadium dioxide (VO2) with embedded metallic surface inclusions to increase radiation efficiency. We fabricate a monopole antenna with the proposed film, demonstrate a good match between measurement and simulation, and propose methods for increasing performance. We show that the addition of metallic inclusions in the VO2 film results in a 37‐fold increase in the radiation efficiency. This concept, if paired with a spatially addressable micro‐heater array, could enable extremely flexible and low‐loss frequency‐agile cognitive radio antennas for dynamic spectrum applications as well as polarization and pattern reconfiguration for operation in 5G and beyond systems where spectrum is congested and coexistence is compulsory.

Metal–insulator switching of vanadium dioxide for controlling spin-wave dynamics in magnonic crystals

The present work focuses on the effect of vanadium dioxide (VO2) films exhibiting a metal–insulator transition (MIT) on the performance characteristics of the magnetic multilayers. It has been shown that the MIT provides a novel mechanism for controlling the microwave spin-wave dynamics in the yttrium iron garnet (YIG) films. In particular, the low and high levels of microwave attenuation of spin waves transmitted through the YIG–VO2 bilayer has been observed due to a variation of the VO2 conductivity within a narrow temperature range. This effect has been utilized to realize fully reconfigurable magnonic crystals composed of the thickness-modulated YIG and regular VO2 films. A promising functionality of the proposed waveguiding structures arises from a controllability of wave intensity, which provides an altering of the frequency response from an original band structure to a full rejection of spin waves. Numerical simulations taking into account both the YIG film saturation magnetization and the VO2 film conductivity have confirmed the experimentally observed spin-wave dynamics. An interest in ferrite–VO2 bilayers arises not only from possible practical applications but also from a variety of fundamental scientific problems devoted to the physics of wave phenomena in planar thin-film magnetic multilayers.