VO2 Phase Research Articles

Active optical switch and polarization-selective absorption in a VO2 based metasurface in THz region

Active metasurfaces with the capability of dynamic control enable versatile functions by applying external stimuli. In this paper, the two functions of switch and polarizer is realized by integrating vanadium dioxide (VO2) into a double-aperture array. With the VO2 phase transition from insulator to metallic phase, the absorption of VO2 based metasurface is switched from ∼0 to ∼1 at 2.68 THz, which behaves as a high-efficiency switch. Simultaneously, the same array at metallic VO2 state shows dual-band polarization selectivity. The giant linear dichroism (0.86) could be weakened to ∼0 by switching the phase of VO2 to insulator. In addition, the mechanism of the two-function switchable metasurface is discussed. The proposed VO2 based active metasurface possesses the merit of flexible dynamic control for both absorption strength and polarization selectivity in a fixed nanostructure, which will promote the development of THz system integration.

Thermochromic and temperature regulation properties of phase change materials derived from polymer composites loaded with VO2 particles

AbstractThis research work concerns the development of phase change materials based on poly(ethylene terephthalate) (PET) mixed with vanadium dioxide particles (VO2). Monoclinic VO2 particles were synthesized via a hydrothermal process followed by modification of the particle surface with a silane coupling agent, namely 3‐(aminopropyl) triethoxysilane (APTES). After that, the metal oxide particles were mixed with PET flakes, derived from used PET bottles, in an internal mixer. In this study, the effects of surface modification and concentrations of VO2 particles on thermomechanical and optical properties of the composites were investigated. From the DSC results, it was found that the percentage crystallinity of PET increased after loading with VO2. This indicates that VO2 can act as a nucleating agent inducing the crystallization of the PET chains. From the scanning electron microscopy with energy dispersive X‐ray spectroscopy (SEM–EDX) images, better dispersion of VO2 particles in the PET matrix was noticed when the surface‐modified VO2 particles were used as a replacement for the normal VO2 particles. This contributed to the greater percentage of visible light transmittance through the composite sheets. Thermogravimetric analysis (TGA) profiles of both PET and PET/VO2 specimens are similar, indicating that the VO2 did not accelerate the thermal decomposition of the PET. When the specimens were exposed to an IR lamp, it was found that the temperature behind the PET/VO2 sheets dropped when compared to that of the neat PET sheets. Some shrinkage and deformation of the PET/VO2 specimens were also noticed. The higher the VO2 loading, the greater the deformation. The effect was ascribed to the fact that the actual temperature was above the critical transition temperature of VO2. Consequently, the NIR transmittance was suppressed at the expense of the greater NIR absorbance by the VO2 phase in the composites. Finally, when the specimens were exposed to solar irradiation at ambient temperature, the temperature underneath the PET/VO2 specimens became greater than that of the control specimen (the neat PET). The result was discussed in the light of the emissivity of VO2 at ambient temperature, which is considerably low. Overall, this study demonstrates that PET/VO2 can exhibit both cooling and warming effects, depending on the ambient temperature in relation to the critical transition temperature (Tc) of VO2.

A facile green approach to the synthesis of Bi2WO6@V2O5 heterostructure and their photocatalytic activity evaluation under visible light irradiation for RhB dye removal

Bismuth tungsten oxide and vanadium pentoxide (Bi2WO6/V2O5) heterostructures are produced by a green synthesis approach using Azadirachta indica extract for photocatalytic performance. The hydrothermal method at temperatures between 120 °C and 140 °Cis used to synthesize Bi2WO6. Bi2WO6 and V2O5 phases are formed in pure orthorhombic wells according to the XRD pattern. The SEM displays V2O5 nanorods, Bi2WO6 hierarchical microspheres that resemble flowers at 120 °C, and particles with a particle-like character at 140 °C. In V2O5, the asymmetric stretching vibrations of the triplely coordinated oxygen (chain oxygen) bonds and the vibration of the doubly coordinated oxygen (bridge oxygen) bonds are responsible for a peak at 611 cm−1. In FTIR spectra between 600 and 1600 cm−1, the major absorption bands in Bi2WO6 are attributed to the W-O stretching, Bi-O stretching, and W-O-W bridging stretching modes. Bi2WO6@V2O5 at 120 °C has the lowest bandgap energy (2.32 eV) and optical electronegativity (0.62), as well as the highest refractive index (2.57), extinction coefficient (2.21), and dielectric constant (εr = 0.72 and εi = 11.4) among all samples, making it a suitable material for photocatalysis. Rhodamine blue (RhB) dye degradation is used to measure the photocatalytic activity (PCA) of certain materials. The results showed that heterostructure V2O5@Bi2WO6 synthesized at 120 °C is more attractive among all samples due to high degradation of RhB dye under sunlight irradiation in 90 min.

Fabrication and Properties of Epitaxial VO2 Thin Film on m-Al2O3 Substrate

A thin film of thermochromic VO2 was prepared on m-Al2O3 substrate using a radio frequency (RF) magnetron sputtering technique. The epitaxial growth of the monoclinic M1 phase of VO2 on the m-Al2O3 substrate was confirmed through synchrotron X-ray diffraction (XRD) measurements. The transformation of this monoclinic M1 phase into a rutile phase at ~68 °C was reflected in the temperature-dependent XRD measurements of the VO2 thin film. The temperature-dependent electrical resistance measurements of this sample also revealed an abrupt metal-to-insulator transition at ~68 °C, which is reversible in nature. Temperature-dependent X-ray absorption (XAS) measurements at V L-edge and O K-edge were performed to study the electronic structure of the epitaxial VO2/m-Al2O3 thin film during the metal-to-insulator (MIT) transition.

Transverse Magnetic Surface Plasmons in Graphene Nanoribbon Qubits: The Influence of a VO2 Substrate

We study the influence of the phase-change material VO2 on transverse magnetic (TM) surface plasmon (SP) modes in metallic arm-chair graphene nanoribbon (AGNR) qubits in the Lindhard approximation. We assess the effects of temperature as a dynamic knob for the transition from the insulating to the metallic phase on the TM SP modes in single-band (SB) and two-band (TB) transitions. We show that a VO2 substrate leads to TM SP modes in both SB and TB transitions. In addition, we observe that the SP modes have a lower frequency than those for a substrate of constant permittivity. In addition, we study the influence of the substrate-induced band gap Δ' on SP modes in TB transitions for the insulating and metallic phases of VO2.

Constructing 3D hierarchical CNTs/VO2 composite microspheres with superior electromagnetic absorption performance

To achieve efficient electromagnetic wave absorption (EWA) performance with requirements of low density and thin thickness, three-dimensional (3D) hierarchical CNTs/VO2 composite microsphere was successfully synthesized though an ultrasonic atomization process in this work. The phase composition, morphology, EWA performance and corresponding mechanism were investigated. The VO2 microspheres (MS) was obtained via the complete reduction of V2O5 by oxalic acid, and the introduced CNTs were uniformly distributed around VO2 MS. The addition of CNTs both improved the impedance matching and provided multiple attenuation modes. The highly conductive CNTs enhanced conductivity further increasing conductance loss. Moreover, heterogeneous interfaces formed between CNTs and VO2 phase generated strong polarization loss. Furthermore, the construction of 3D structure not only benefited the stability of structure but also extended the paths of EW reflection and scattering thus consuming more EW energy. With the synergistic effect of favorable impedance matching and attenuation ability, the excellent EWA performance was achieved that the minimum reflection loss was −58.2 dB with the effective absorption bandwidth of 3.94 GHz in X band. This research plays a significant guiding role in structural construction and optimization of EW absorber.

Polymorphism of VO2 thin film: M1, T, and M2 single phase synthesis using pulsed laser deposition

Vanadium dioxide (VO2) can undergo a reversible insulator-to-metal transition (IMT) along with a structural phase transition from monoclinic M1 to rutile tetragonal R at about 68ºC, thereby making a range of optic and electronic applications possible. The structural phase transition from monoclinic M1 to rutile tetragonal R can occur via two other intermediate metastable phases of triclinic T and monoclinic M2. Due to their narrow thermodynamic stability, selective phase deposition of VO2 thin films is a major challenge. In the present study, apart from synthesizing the VO2 thin film with M1 phase, we also selectively elaborated the T and M2 phases using pulsed laser deposition (PLD) by simply controlling the repetition rate of the laser. Analysis of the structural, morphological, nanomechanical, thermochromic, and optical properties of each of these polymorphs demonstrated strong dependence of these properties on the VO2 phases, and hence on the laser repetition rate. The ability to stabilize different polymorphs of VO2 thin films paves the way for novel structures that are promising for new device functionalities.

Phase-engineered cathode for super-stable potassium storage

The crystal phase structure of cathode material plays an important role in the cell performance. During cycling, the cathode material experiences immense stress due to phase transformation, resulting in capacity degradation. Here, we show phase-engineered VO2 as an improved potassium-ion battery cathode; specifically, the amorphous VO2 exhibits superior K storage ability, while the crystalline M phase VO2 cannot even store K+ ions stably. In contrast to other crystal phases, amorphous VO2 exhibits alleviated volume variation and improved electrochemical performance, leading to a maximum capacity of 111 mAh g−1 delivered at 20 mA g−1 and over 8 months of operation with good coulombic efficiency at 100 mA g−1. The capacity retention reaches 80% after 8500 cycles at 500 mA g−1. This work illustrates the effectiveness and superiority of phase engineering and provides meaningful insights into material optimization for rechargeable batteries.

Fabrication of Flexible Poly(m-aminophenol)/Vanadium Pentoxide/Graphene Ternary Nanocomposite Film as a Positive Electrode for Solid-State Asymmetric Supercapacitors.

In this study, poly(m-aminophenol) (PmAP) has been investigated as a multi-functional conductive supercapacitor binder to replace the conventional non-conductive binder, namely, poly(vinylene difluoride) (PVDF). The kye benefits of using PmAP are that it is easily soluble in common organic solvent and has good film-forming properties, and also its chemical functionalities can be involved in pseudocapacitive reactions to boost the capacitance performance of the electrode. A new ternary nanocomposite film based on vanadium pentoxide (V2O5), amino-functionalized graphene (amino-FG) and PmAP was fabricated via hydrothermal growth of V2O5 nanoparticles on graphene surfaces and then blending with PmAP/DMSO and solution casting. The electrochemical performances of V2O5/amino-FG/PmAP nanocomposite were evaluated in two different electrolytes, such as KCl and Li2SO4, and compared with those of V2O5/amino-FG nanocomposite with PVDF binder. The cyclic voltametric (CV) results of the V2O5/amino-FG/PmAP nanocomposite exhibited strong pseudocapacitive responses from the V2O5 and PmAP phases, while the faradaic redox reactions on the V2O5/amino-FG/PVDF electrode were suppressed by the inferior conductivity of the PVDF. The V2O5/amino-FG/PmAP electrode delivered a 5-fold greater specific capacitance than the V2O5/amino-FG/PVDF electrode. Solid-state asymmetric supercapacitors (ASCs) were assembled with V2O5/amino-FG/PmAP film as a positive electrode, and their electrochemical properties were examined in both KCl and Li2SO4 electrolytes. Although the KCl electrolyte-based ASC has greater specific capacitance, the Li2SO4 electrolyte-based ASC delivers a higher energy density of 51.6 Wh/kg and superior cycling stability.

Thermochromic properties of vanadium oxide thin films prepared by reactive magnetron sputtering at different oxygen concentrations

Vanadium dioxide (VO2) is a thermochromic (TC) material, exhibits a reversible transmittance change in the near-infrared (NIR) region upon heating above a critical temperature τc, due to an insulator-to-metal phase transition. Numerous studies on the V–O system can be found in the literature, including the synthesis of VO2 films. The phase diagram of the V–O system involves complicated suboxide phases that prevent pure VO2 formation and obscures quantitative analysis. Here, VO2 thin films were systematically prepared with various oxygen flow ratios (Γ) using reactive magnetron sputtering. Thin films prepared with Γ = 4.1% show dominant VO2 phase and the highest TC performance. The Γ-range producing dominant VO2 phase is found to be narrow: 3.7% < Г<4.2%. At lower and higher Γ, TC characteristics are shown to be accompanied, as expected, by electrical conductivity changes, but also by microstructure transformations. The latter producing textured films that gradually develop upon cycling around τc. In particular, a change of texture, yielding oriented VO2 crystallites, is observed by in-situ XRD around τc. Our results shed light on the VO2 formation and associated structural, chemical and electrical properties under various oxidizing conditions during magnetron sputtering, which calls for careful preparation and strategies to stabilize the VO2 phase.

Light-Sensing Properties of Amorphous Vanadium Oxide Films Prepared by RF Sputtering.

In this study we analyzed the structure and light-sensing properties of as-deposited vanadium oxide thin films, prepared by RF sputtering in different Ar:O2 flow rate conditions, at low temperature (e.g., 65 °C). X-ray diffraction (XRD), Scanning Electron Microscopy (SEM-EDX), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were employed to analyze the film microstructure, composition and the oxidation states of vanadium ions. The SEM micrographs evidence VxOy films with smooth surfaces, whereas the XRD patterns show their amorphous structure. Raman spectra indicate an increased structural disorder in the films deposited in Ar:O2 flow comparatively with those deposited solely in Ar flow. The XPS data suggest the modification of the oxidation state from V4+ to V5+, thus proving the formation of the V2O5 phase when increasing the oxygen content, which further affects the films' optical properties. We observed a good stability of the photogenerated current in Si/SiO2/VxOy/TiN heterostructures upon excitation with pulses of UV (360 nm), VIS (white light) and NIR (860 nm) light. The responsivity, detectivity and linear dynamic range parameters increase with the O/V ratio in the VxOy films, reaching comparable values with photodetectors based on crystalline V2O5 or VO2.

Controllable Phase Transition Properties in VO2 Films via Metal-Ion Intercalation.

VO2 has shown great promise for sensors, smart windows, and energy storage devices, because of its drastic semiconductor-to-metal transition (SMT) near 340 K coupled with a structural transition. To push its application toward room-temperature, effective transition temperature (Tc) tuning in VO2 is desired. In this study, tailorable SMT characteristics in VO2 films have been achieved by the electrochemical intercalation of foreign ions (e.g., Li ions). By controlling the relative potential with respect to Li/Li+ during the intercalation process, Tc of VO2 can be effectively and systematically tuned in the window from 326.7 to 340.8 K. The effective Tc tuning could be attributed to the observed strain and lattice distortion and the change of the charge carrier density in VO2 introduced by the intercalation process. This demonstration opens up a new approach in tuning the VO2 phase transition toward room-temperature device applications and enables future real-time phase change property tuning.

Structural transition and photoluminescence behavior of (V2O5)1-x (Ag0.33V2O5)x (x = 0 to 0.1) nanocomposites

The two phase (V2O5)1-x(Ag0.33V2O5)x (x = 0, 0.05, 0.075 and 0.1) nanocomposites were synthesized through one step hydrothermal process. The X-ray diffraction followed by two phase Rietveld refinement confirms the crystallization of V2O5 and Ag0.33V2O5 phases in the Pmmn (orthorhombic) and C21 (monoclinic) symmetry respectively without any impurity phase. Furthermore, the FE-SEM, EDX, PL, UV–vis and XPS characterization were carried out for microstructural, optical and chemical studies. The formation of Ag0.33V2O5 nano-belts with average length and width of 3–4 μm and 40–50 nm respectively, were observed in the electron micrograph. Moreover, the XPS study confirms the constituent element are in expected oxidation states with Vanadium in only + 5 (V+5) for V2O5 and in mixed states (V+5 and V+4) for 10 % Ag0.33V2O5 nanocomposite. The photoluminescence spectra of these nanocomposite depicts two broad emission peaks located around 570 nm and 707 nm due to indirect inter-band and mid-state energy states transitions, respectively. PL also prove the increase in oxygen vacancies by loading of Ag in V2O5 matrix. The optical absorption of Ag0.33V2O5 represents a decrease in the energy band gap (2.10–1.97 eV) by adding Ag concentration in the V2O5, which also proved the semiconductor to metal transition (SMT).

Infrared sensitive mixed phase of V7O16 and V2O5 thin-films.

We report an infrared (IR) sensitive mixed phase of V7O16 and V2O5 thin films, grown by cathodic vacuum arc-deposition on glass substrates at relatively low temperatures. We have found that the mixed phase of V7O16 and V2O5 can be stabilized by post-annealing of amorphous VxOy between 300-400 °C, which gets fully converted into V2O5 after annealing at higher temperatures ∼450 °C. The local conversion from VxOy to V2O5 has also been demonstrated by applying different laser powers in Raman spectroscopy measurements. The optical transmission of these films increases as the content of V2O5 increases but the electrical conductivity and the optical bandgap decrease. These results are explained by the role of defects (oxygen vacancies) through the photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. The IR sensitivity of the mixed phase is explained by the plasmonic absorption by the V7O16 degenerate semiconductor.

Heterostructured V2O5/FeVO4 for enhanced liquid-phase epoxidation of cyclooctene.

Efficient cyclooctene epoxidation process under mild reaction conditions highly relies on the rational design and synthesis of high-performance heterogeneous catalysts. Herein, we report the facile one-pot synthesis of V2O5/FeVO4 heterostructures featured with heterointerfaces for the boosted epoxidation of cyclooctene. The intensive interfacial electronic interaction between the V2O5 and FeVO4 phases is versatile in the modulation of coordination microenvironment and formation of abundant oxygen vacancies, contributing to the performance enhancement. Under the optimal reaction conditions, a high yield of 87.0% can be achieved with the cyclooctene conversion of 96.5% (initial reaction rate of 55.1mmolgcat-1h-1) and cyclooctene oxide selectivity of 90.2%. Additionally, the V2O5/FeVO4 catalyst is stable and recyclable, endowing it a promising prospect for practical applications. This study demonstrates that the application of interface engineering strategy can be an appealing avenue towards the development of high-performance catalysts for epoxidation of cyclooctene and beyond.

Manipulating the metal-to-insulator transitions of VO2 by combining compositing and doping strategies.

Vanadium dioxide (VO2) exhibits the most abrupt metal-to-insulator transition (MIT) property near room temperature among the representative 3d-orbital correlated oxides, and its structural variation during the MIT usually results in poor mechanical properties as bulk pellets. Moreover, compositing with highly resistive oxides has been reported to improve the mechanical strength of bulk VO2 since the generation and propagation of microcracks is suppressed upon thermocycling across the MIT; further, their respective impacts on electrical transportation are yet unclear. Herein, we demonstrate the role of these highly resistive oxide composites (e.g., HfO2, CoO and Al2O3) in reducing charge leakage along the microcracks within the insulating phase of VO2, leading to more abrupt MIT properties from the perspective of electrical transportation. This enables the possibility of simultaneously regulating the critical temperature and abrupt MIT transition, as well as the mechanical properties of the VO2 bulk pellets via compositing with oxides with different melting points using spark plasma-assisted reactive sintering (SPARS).

Switchable optics based on guided mode resonance in lithographically patterned vanadium dioxide with integrated heating layer

Vanadium dioxide (VO2) has promising applications in smart windows and active micro-optical devices due to its thermochromic properties. However, the successful fabrication and patterning of VO2 thin films with the correct stoichiometry and phase are challenging. In this study, we investigated lithographically patterned and non-patterned VO2 thin films fabricated by reactive ion beam deposition, using variable angle spectroscopic ellipsometry, Raman spectroscopy, and transmission and reflection measurements. The results show that the refractive index and extinction coefficient exhibit significant changes for near-infrared wavelengths when heated above 68 °C, confirming its thermochromic properties. The Raman spectroscopy results indicate the formation of the monoclinic phase VO2(M) after annealing, which was not changed by reactive ion etching. Lithographically structured VO2-layers were successfully realized demonstrating the potential of VO2 as a material for active micro-optical devices, such as guided mode resonance filters with switchable reflectance. The results suggest that VO2 has great potential as a promising material for actively switched optical elements and micro-optical devices.

Electrocatalytic and Conductive Vanadium Oxide on Carbonized Bacterial Cellulose Aerogel for the Sulfur Cathode in Li-S Batteries

Many transition-metal-oxide-based catalysts have been investigated to chemically bind soluble lithium polysulfides and accelerate their redox kinetics in lithium-sulfur (Li-S) battery chemistry. However, the intrinsic poor electrical conductivities of these oxides restrict their catalytic performance, consequently limiting the sulfur utilization and the rate performance of Li-S batteries. Herein, we report a freestanding electrocatalytic sulfur host consisting of hydrogen-treated VO2 nanoparticles (H-VO2) anchored on nitrogen-doped carbonized bacterial cellulose aerogels (N-CBC). The hydrogen treatment enables the formation and stabilization of the rutile VO2(R) phase with metallic conductivity at room temperature, significantly enhancing its catalytic capability compared to the as-synthesized insulative VO2(M) phase. Several measurements characterize the electrocatalytic performance of this unique H-VO2@N-CBC structure. In particular, the two kinetic barriers between S8, polysulfides, and Li2S are largely reduced by 28.2 and 43.3 kJ/mol, respectively. Accordingly, the Li-S battery performance, in terms of sulfur utilization and charge/discharge rate, is greatly improved. This work suggests an effective strategy to develop conductive catalysts based on a typical transition metal oxide (VO2) for Li-S batteries.

THz broadband and dual-channel perfect absorbers based on patterned graphene and vanadium dioxide metamaterials.

This paper proposes a novel and perfect absorber based on patterned graphene and vanadium dioxide hybrid metamaterial, which can not only achieve wide-band perfect absorption and dual-channel absorption in the terahertz band, but also realize their conversion by adjusting the temperature to control the metallic or insulating phase of VO2. Firstly, the absorption spectrum of the proposed structure is analyzed without graphene, where the absorption can reach as high as 100% at one frequency point (f = 5.956 THz) when VO2 is in the metal phase. What merits attention is that the addition of graphene above the structure enhances the almost 100% absorption from one frequency point (f = 5.956 THz) to a wide frequency band, in which the broadband width records 1.683 THz. Secondly, when VO2 is the insulating phase, the absorption of the metamaterial structure with graphene outperforms better, and two high absorption peaks are formed, logging 100% and 90.7% at f3 = 5.545 THz and f4 = 7.684 THz, respectively. Lastly, the adjustment of the Fermi level of graphene from 0.8 eV to 1.1 eV incurs an obvious blueshift of the absorption spectra, where an asynchronous optical switch can be achieved at fK1 = 5.782 THz and fK2 = 6.898 THz. Besides, the absorber exhibits polarization sensitivity at f3 = 5.545 THz, and polarization insensitivity at f4 = 7.684 THz with the shift in the polarization angle of incident light from 0° to 90°. Accordingly, this paper gives insights into the new method that increases the high absorption width, as well as the great potential in the multifunctional modulator.

Femtosecond laser-induced phase transition in VO2 films.

VO2 is a very promising material due to its semiconductor-metal phase transition, however, the research on fs laser-induced phase transition is still very controversial, which greatly limits its development in ultrafast optics. In this work, the fs laser-induced changes in the optical properties of VO2 films were studied with a variable-temperature Z-scan. At room temperature, VO2 consistently maintained nonlinear absorption properties at laser repetition frequencies below 10 kHz while laser-induced phase transition properties appeared at higher repetition frequencies. It was found by temperature variation experiments at 100 kHz that the modulation depth of the laser-induced VO2 phase transition was consistent with that of the ambient temperature-induced phase transition, which was increased linearly with thickness, further confirming that the phase transition was caused by the accumulation of thermal effects of a high-repetition-frequency laser. The phase transition process is reversible and causes substantial changes in optical properties of the film, which holds significant promise for all-optical switches and related applications.