VO2 Layer Research Articles

Tuning Spin‐Orbit Torques Across the Phase Transition in VO2/NiFe Heterostructure

AbstractThe emergence of spin‐orbit torques as a promising approach to energy‐efficient magnetic switching has generated large interest in material systems with easily and fully tunable spin‐orbit torques. Here, current‐induced spin‐orbit torques in VO2/NiFe heterostructures are investigated using spin‐torque ferromagnetic resonance, where the VO2 layer undergoes a prominent insulator‐metal transition. A roughly twofold increase in the Gilbert damping parameter, α, with temperature is attributed to the change in the VO2/NiFe interface spin absorption across the VO2 phase transition. More remarkably, a large modulation (±100%) and a sign change of the current‐induced spin‐orbit torque across the VO2 phase transition suggest two competing spin‐orbit torque generating mechanisms. The bulk spin Hall effect in metallic VO2, corroborated by the first‐principles calculation of the spin Hall conductivity , is verified as the main source of the spin‐orbit torque in the metallic phase. The self‐induced/anomalous torque in NiFe, with opposite sign and a similar magnitude to the bulk spin Hall effect in metallic VO2, can be the other competing mechanism that dominates as temperature decreases. For applications, the strong tunability of the torque strength and direction opens a new route to tailor spin‐orbit torques of materials that undergo phase transitions for new device functionalities.

Tunable and switchable bifunctional meta-surface for plasmon-induced transparency and perfect absorption

Tunable multi-function metasurfaces have become the latest research frontiers in planar optics. In this study, a dynamically tunable plasmon-induced transparency (PIT) structure based on a graphene split-ring resonator and graphene ribbon is proposed. The influences of the structural parameters and graphene Fermi energy on the PIT response were investigated both analytically and numerically simulations. The inclusion of an additional vanadium dioxide (VO2) substrate layer enables the metasurface to achieve dynamic switching between PIT and perfect absorption using the phase change property of VO2. The new metasurface device exhibits the PIT effect when the VO2 layer is in an insulating state and acts as a perfect absorber when it is in a metallic state. Moreover, the response of the two functions can be easily adjusted dynamically by changing the Fermi energy of graphene. In addition, both functions were highly sensitive to changes in the ambient refractive index. The results of this work have potential applications in slow-light devices, optical switches, modulators, perfect absorbers, highly sensitive sensors, and multifunctional devices.

Photoinduced Multistable Resonance Frequency Switching of Phase Change Microstring at Room Temperature

AbstractVanadium dioxide (VO2), a promising phase change material, exhibits insulator to metal transition at 68 °C, manifests a drastic change in multiple physical properties, such as electrical resistance, mechanical modulus, lattice parameters, etc. From technological perspective, the transition temperature can be reduced by precise strain engineering. Here a noncontact, all‐optical, and highly energy efficient platform is demonstrated to study macroscopic dynamics related to the localized structural rearrangements at room temperature. A thin layer (≈25 nm) of polycrystalline VO2 deposited on a platinum coated silicon nitride microstring resonator shows a fast controlled mechanical resonance frequency response upon variations in optical power and wavelength. It is shown multiple stable frequencies of the resonator, designated as different equilibrium states, can be activated at different optical powers (≈200 µW) and wavelength, i.e., 450, 520, and 635 nm. The observed multiple resonance states of the microstring are explained because of the generation of stress due to the interplay between thermal expansion and the temperature‐induced phase change of VO2. It is believed this change in frequency states under the controlled external optical excitation can have potential applications in ultrafast optical switching, intelligent temperature sensors, and neuromorphic devices operated at room temperature.

Layer-by-layer self-assembled vanadium dioxide and its temperature-dependent light interference

Vanadium Dioxide (VO2) has gained wide attention for various applications as it undergoes a crystallographic transition from the monoclinic phase to the tetragonal phase at 68 °C, accompanied by huge transmittance contrast in the near-infrared (NIR) range and negligible change in the visible range. In this paper, we present a unique Layer-by-Layer self-assembled approach to construct a VO2/spacing superstructure with controlled VO2 layers, spacing thicknesses, and repeating times. The simulation indicates that such structures give intriguingly temperature-dependent light interference phenomena, which was demonstrated its potential applications in smart windows and the calculated results suggest this approach outperforms existing approaches. This simple and versatile solution-based approach opens a new avenue to fabricate the controlled optical stack which could be explored in other applications.

Temperature-responded tunable metalenses based on phase transition materials

Once the metalenses are fabricated, the functions of most metalenses are invariable. The tunability and reconfigurability are useful and cost-saving for metalenses in realistic applications. We demonstrate this tunability here via a novel hybrid metalens with the strategic placement of an ultra-thin VO2 layer. The hybrid metalens is capable of dynamically modulating the focusing intensity of transmitted light at a wavelength of 1550 nm, and demonstrate a 42.28% focusing efficiency of the incident light and 70.01% modulation efficiency. The hybrid metalens’ optothermal simulations show an optothermal conversion process of dynamic focusing, and a maximum laser density of 1.76×103 W/cm2 can be handled at an ambient temperature lower than 330 K. The hybrid metalens proposed in this work, a light-dose sensitive tunable smart metalens that can protect other instruments/systems or materials from being damaged, has its specific applications such as in anti-satellite blinding, bio-imaging, etc.

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.

Photoassisted Electron-Ion Synergic Doping Induced Phase Transition of n-VO2/p-GaN Thin-Film Heterojunction.

As a typical correlated metal oxide, vanadium dioxide (VO2) shows specific metal-insulator transition (MIT) properties and demonstrates great potential applications in ultrafast optoelectronic switch, resistive memory, and neuromorphic devices. Effective control of the MIT process is essential for improving the device performance. In the current study, we have first proposed a photoassisted ion-doping method to modulate the phase transition of the VO2 layer based on the photovoltaic effect and electron-ion synergic doping in acid solution. Experimental results show that, for the prepared n-VO2/p-GaN nanojunction, this photoassisted strategy can effectively dope the n-VO2 layer by H+, Al3+, or Mg2+ ions under light radiation and trigger consecutive insulator-metal-insulator transitions. If combined with standard lithography or electron beam etching processes, selective doping with nanoscale size area can also be achieved. This photoassisted doping method not only shows a facile route for MIT modulation via a doping route under ambient conditions but also supplies some clues for photosensitive detection in the future.

Hybrid vanadium dioxide-liquid crystal tunable non-reciprocal scattering metamaterial smart window for visible and infrared radiation control

Smart windows based on VO2 can control the infrared radiation entering the building based on the temperature, however, the visible part of the spectrum is not controlled. Liquid crystal (LC) based privacy windows, on the other hand, control the visibility either with temperature or applied voltage, however, the total transparency remains fixed as the scattering is mainly in the forward direction. To be able to control both the visibility and the temperature in the house, here we combine both layers in which the LC layer is made of a composite of nanoporous organic microparticles called Cochleates at small concentrations embedded in the LC matrix, thus acting as a tunable scattering metamaterial. The VO2-LC interface has less Fresnel reflectivity and therefore higher solar modulation is expected in an optimized window. In addition, being hidden under the LC layer, VO2 will be protected from oxidation. Electro-optic and thermo-optic properties of the device are investigated including the response time measurements. A non-reciprocity effect is observed showing better performance when the VO2 layer is facing the outside world, in which the window becomes more transparent from inside than from outside and also showing higher solar modulation. Response time is 1 ms for the rise and 10 ms fall time at 70 V. This approach opens up a new possibility of thermochromic VO2 and LC-based systems to satisfy the real-life requirements on smart window applications.

Color tunable vanadium dioxide/fluorescent organic small molecule two-layer composite films with thermochromic property

Vanadium dioxide (VO2), a typical thermochromic material, can be used for smart windows, which can effectively reduce the building energy consumption. However, the unpopular color of the VO2-based smart windows has become one of the issues restricting its wide applications. Herein we report on VO2-based fluorescent two-layer composite films, which contain the thermochromic VO2 layer and the photoluminescent organic small molecule layer. Under sunlight, the fluorescent organic small molecule layers of 4,4′-bis[2-(2-methoxyphenyl)ethenyl]biphenyl (CBS-127) and 4,4′-bis[2-(2-sodiumsulfophenyl)- ethenyl]biphenyl (CBS-X) absorb ultraviolet irradiation and emit blue light, thus the strong absorption effect of the pure VO2 film in the blue region (hence the yellow brown color) can be effectively compensated, improving the color of the films. Compared with the VO2 film, the two-layer composite films not only maintain high luminous transmittance (Tlum,average > 70%) and solar regulation efficiency (ΔTsol > 8%), but also undergo reversible color change from yellow brown to blue under sunlight, and the UV blocking property is significantly improved as well. These properties are beneficial to the promotion and application of the VO2-based smart windows.

Flexible dynamic structural color based on an ultrathin asymmetric Fabry-Perot cavity with phase-change material for temperature perception.

Dynamic structural color has attracted considerable attentions due to its good tunable characteristics. Here, an ultrathin asymmetric Fabry-Perot (FP)-type structural color with phase-change material VO2 cavity is proposed. The color-switching performance can be realized by temperature regulation due to the reversible monoclinic-rutile phase transition of VO2. The various, vivid structural color can be generated by simply changing the thickness of VO2 and Ag layers. Moreover, the simple structural configuration enables a large-scale, low-cost preparation on both rigid and flexible substrates. Accordingly, a flexible dynamic structural color membrane is adhered on a cup with a curved surface to be used for temperature perception. The proposed dynamic structural color has potential applications in anti-counterfeiting, temperature perception, camouflage coatings among other flexible optoelectronic devices.

Plasmonic nanodiscs on vanadium dioxide thin films for tunable luminescence enhancement.

We propose an alternative method to dynamically tune luminescence enhancement in the near infrared spectral range using noble metal nanostructures on top of phase change material vanadium dioxide (VO2) thin films. The VO2 phase change is used to tune the nanodisc plasmon resonance providing a luminescence modification mechanism. We employ a model to calculate the emission of quantum emitters, such as dye molecules, in hybrid systems comprising single silver (Ag) nanodiscs on top of a thin layer of VO2. The model considers different dipole orientations and positions with respect to the nanostructure-VO2 film and determines the degree of observable luminescence modification. In the NIR spectral region, the observable photoluminescence of Alexa Dyes in the hybrid systems at room temperature is enhanced by more than a factor of 2.5 as compared to the same system without plasmonic particles. An additional photoluminescence enhancement by more than a factor of 2 can be achieved with the Ag nanodisc-VO2 film systems after the phase transition of the VO2. These systems can be used for tunable luminescence modification and for compensation of thermally induced luminescence quenching. Through optimization of the Ag nanodisc-VO2 film system, luminescence enhancements of up to a factor of 4 can be seen in the metallic VO2 compared to the semiconducting phase and would therefore compensate for a thermal quenching of up to 70% between room temperature and 70° C, rendering the hybrid systems as promising candidates for improved photon management in optoelectronic devices where elevated temperatures minimize the efficiencies of such devices.

Combining Switchable Phase‐Change Materials and Phase‐Transition Materials for Thermally Regulated Smart Mid‐Infrared Modulators

AbstractPhase‐change materials (PCMs) and phase‐transition materials (PTMs) both show a large contrast in their respective optical properties upon switching, enabling compact optical components with diverse functionalities like sensing, thermal imaging, and data recording. However, their switching properties differ significantly, that is, the switching is non‐volatile for PCMs while volatile for PTMs. Here, new‐generation smart mid‐infrared modulators with switchable transmission, reflection, and absorption are demonstrated conceptually and experimentally, which combine one PCM (Ge3Sb2Te6 or In3SbTe2) with one PTM (VO2) as two active layers. The bottom VO2 layer is employed as a thermally regulated (modulated) dynamic mirror, facilitating the switching of transmission between “on” state (using VO2 in its semiconducting state at temperatures below its phase transition temperature Tc) and “off” state (metallic VO2 at temperatures above Tc). The PCM layer on top of the metallic VO2 layer is used either for continuously adjusting the absorption peak spectrally (by up to 1.8 µm using different phases of Ge3Sb2Te6) or for switching between absorption mode (A = 0.99 with amorphous In3SbTe2) and reflection mode (R = 0.85 with crystalline In3SbTe2). The presented concept of merging static, non‐volatile thermal switching (via PCMs) with dynamic, volatile thermal modulation (via PTMs) empowers a new generation of optical devices for smart optical switching, for example in spectrally tunable safety optical switches.

Large optical and structural enhancement by designing VOx nanostructures for smart windows applications

An investigation of the Optical and Structural behavior of VOx Nanostructures for smart windows applications is performed. The study is done by generating an optical model by using COMSOL Multiphysics commercial package. The optimized model suggests a sandwich structure of ZnSThF-4/VO2/Si3N4 on fused quartz SiO2 glass, in order to enhance the targeted behavior, by adding two antireflection coatings below and above the VO2 layer. A sweep is performed on the layers’ thicknesses with a total number of 729 structures. It was concluded that the best structure has the following thicknesses: ZnSThF-4 (201 nm)/VO2 (45 nm)/Si3N4 (198 nm). The best selected structures achieve luminous transmission in semiconducting state Tlum(s) = 63.8%, luminous transmittance in metallic state Tlum(m) = 56.5%, solar transmittance in semiconducting state Tsol(s) = 55.5%, solar transmission in metallic state Tsol(m) = 45.1% and solar modulation ΔTsol = 10.4%.

Atomic layer deposition of vanadium oxides using vanadyl acetylacetonate as the precursor

Thermochromic VO2 thin films are fabricated by atomic layer deposition with vanadyl acetylacetonate as the precursor. The crystallization process has been performed by post-annealing process, i.e. rapid thermal annealing. The effect of annealing temperature on thermochromic properties is investigated. Though the transmittance increases with increasing annealing temperature, a moderate temperature is suggested by the consideration of atomic inter-diffusion between TiO2 and VO2 layers. It is found that an oxygen-deficient phase of V2O3 appears at low annealing temperatures, while an oxygen-ambient phase of V2O5 presents at high temperatures. Additionally, a ternary compound, Ti3V6O17 is dominant beyond 700°C. The crystalline behavior of monoclinic phase of VO2 and Ti3V6O17 observed from the X-ray diffraction pattern is further examined by high resolution transmission electron microscope. The depth profiling in binding energies of V 2p, O 1s and Ti 2p orbits are compared.

Thermally activated resonant grating using a vanadium dioxide waveguide

In this work, we report on the design of a one-dimensional subwavelength resonant grating comprised of a fused silica substrate and a bi-layer waveguide, consisting of a solgel synthetized anatase TiO2 layer followed by a thin VO2 layer that is applied using pulsed laser deposition and rapid thermal annealing. A TE waveguide mode is excited under normal incidence in the VO2/TiO2 bi-layer via a positive photoresist based grating printed on top, leading to high resonant reflection at room temperature. Increasing the temperature to about 68°C causes the VO2 to undergo a dielectric to metallic transition accompanied by optical modifications in the IR region, canceling the resonance effect. This thermally triggered absorber/emitter tunable configuration enabling the on and off switching of optical resonant excitation in a reversible manner is proposed for passive Q-switching self-protecting devices for high power lasers in the IR wavelength range. Modeling of the optimized temperature dependent resonant waveguide and preliminary experimental results are presented.

Digital Tuning of the Transition Temperature of Epitaxial VO2 Thin Films on MgF2 Substrates by Strain Engineering

AbstractStraining the vanadium dimers along the rutile c‐axis can be used to tune the metal‐to‐insulator transition (MIT) of VO2 but has thus far been limited to TiO2 substrates. In this work VO2/MgF2 epitaxial films are grown via molecular beam epitaxy (MBE) to strain engineer the transition temperature (TMIT). First, growth parameters are optimized by varying the synthesis temperature of the MgF2 (001) substrate (TS) using a combination of X‐ray diffraction techniques, temperature dependent transport, and soft X‐ray photoelectron spectroscopy. It is determined that TS values greater than 350 °C induce Mg and F interdiffusion and ultimately the relaxation of the VO2 layer. Using the optimized growth temperature, VO2/MgF2 (101) and (110) films are then synthesized. The three film orientations display MITs with transition temperatures in the range of 15–60 °C through precise strain engineering.

Metal-insulator alternating behavior in VO2/TiO2 supercells

The interface between band insulator TiO2 and correlated insulator VO2 is a fertile ground for new behaviors. In the present paper, we conducted the first-principles calculations to study the interfaces of VO2(m)/TiO2(n) superlattices (referred as m/n for simplicity, where m and n denote the number of rutile VO2 and TiO2 layers, respectively). Interestingly, the VO2/TiO2 superlattices show insulating states in the 2/6, 4/6 and 6/6 systems with the oscillation of V-V distances, while the superlattices present metallic states in the 1/6, 3/6 and 5/6 systems as the oscillation of V-V distances is broken. Accordingly, we reported an alternating metal-insulator behavior in VO2/TiO2 superlattices with the increasing number of VO2 layers. These findings may help to better understand the Peierls structural transition in correlated insulator, and may bring a light to the Schottky barrier homojunction composed of different VO2(m)/TiO2(n) superlattices.

A multi-band metamaterial absorber based on VO2 layer

In this paper, a multi-band tunable metamaterial absorber with vanadium dioxide (VO2) layer is proposed and verified in terahertz (THz) band. At Room temperature, two absorption peaks (P1 and P2) are achieved at 5.08 and 34.42 THz, respectively. The P1 peak is results by the local surface plasmon (LSP) modes resonance, while the P2 peak is excited by the surface plasmon polarization (SPP) modes resonance. With the increase of temperature, new absorption resonance behaviors are excited based on the phase transition properties of the VO2 layer. Two new absorption peaks P3 and P4 are revealed at frequencies 8.3 and 28.46THz, respectively. Both P1 and P2 peaks remain stable even when the temperature is increased from 300 K to 400 K. It is found that the P3 peak is revealed by the hybridization of LSP modes, while the P4 peak is excited by the Fabry-Perot resonance. This proposed metamaterial absorber demonstrates two characteristics: P1 and P2 peaks are insensitive to temperature, while P3 and P4 peaks are temperature tunable.

Switchable wavefront control using an all-dielectric metasurface mediated by VO2

Active metasurfaces using metal–insulator transition materials such as vanadium dioxide (VO2) have been demonstrated recently. As most of them are based on plasmonic metasurfaces, it is difficult to realize transmissive devices at optical frequencies. In this study, we theoretically propose and demonstrate the transmission type of an active all-dielectric metasurface mediated by VO2. We numerically study the optical properties of a cylindrical Mie resonator consisting of crystalline silicon and VO2 layers, and we find that it achieves a 2π phase shift of approximately 1550 nm by tuning the radius. The proposed structure can be applied to non-mechanical beam steering, polarizers, metalenses, etc.

Exceptional adaptable MWIR thermal emission for ordinary objects covered with thin VO2 film

Monotonous thermal radiation emitted from an ordinary object can be brought into a dynamic and versatile form that can be shaped according to the application area with the ingenious design of the surface coatings. Building the coatings with phase change materials provides exceptional and surprising properties in terms of tunability, adaptability and multifunctionality. In this paper, we investigate the thermal radiation properties in the MWIR band through comprehensive thermographic measurements and theoretical methods while a thin (~90 nm thick) vanadium dioxide (VO2) layer on the sapphire substrate (VO2 thin film) is placed on different ordinary objects under heating/cooling conditions. It is indicated that the emission of the metal object (low emittance) can be boosted and the emission of the black-body-like object (high emittance) can be suppressed at the relevant temperatures. The thermal emission of the objects covered with thin VO2 film at high temperatures (>75 °C) is determined by only the VO2 thin film, since the VO2 layer is completely metallized and the MWIR radiation of the underlying object is masked. When the actual temperature of the object behaving like a blackbody rises up to 95 °C, the temperature detected in the MWIR thermal camera is reduced by more than 20% to approx. 75 °C due to the VO2 thin film on this object, providing thermal camouflage. It is experimentally and theoretically revealed that the underlying physical mechanism on these strange results is associated with the drastic change in the infrared optical parameters of the VO2 as a result of the applied temperature.