Transition In VO2 Research Articles

Embedding epitaxial VO2 film with quality metal-insulator transition to SAW devices

Epitaxial VO2 films grown by pulsed laser deposition (PLD) method with superior phase transition related switching characteristics are successfully embedded to SAW devices using concept of the “impedance loaded SAW sensor”. A resistance of VO2 sensor abruptly drops from 0.7 M[Formula: see text] to 90 [Formula: see text] when it is heated above [Formula: see text]65[Formula: see text]C and shows a narrow hysteresis loops when switching. Two designs of SAW devices are examined in which RF signal is reflected back from interdigital transducer (IDT) or a surface acoustic waves (SAW) is transferred through a coupler and the RF response is altered 2 and 3 times correspondingly upon the phase transition in VO2. In the proposed devices with external load, a SAW does not propagate via VO2 film and therefore is not attenuated which is beneficiary for wireless applications. Additionally, a SAW phase shift as great as 50[Formula: see text] is induced to the SAW transferred through the coupler due to the phase transition in VO2. The proposed approach may boost the development of remotely controlled autonomous sensors, including those based on VO2 metamaterials and hybrid plasmonic structures for near IR/middle IR and sub-THz/THz applications.

Room Temperature Phase Transition of W‐Doped VO2 by Atomic Layer Deposition on 200 mm Si Wafers and Flexible Substrates

AbstractThe unique structural transition of VO2 between dielectric and metallic phases has significant potential in optical and electrical applications ranging from volatile switches and neuromorphic computing to smart devices for thermochromic control and radiative cooling. Critical condition for their widespread implementation is scalable deposition method and reduction of the phase transition to near room temperature. Here, a W:VO2 process based on atomic layer deposition (ALD) is presented that enables precise control of W‐doping at the few percent level, resulting in a viable controllable process with sufficient W incorporation into VO2 to reduce the phase transition to room temperature. It is demonstrated that the incorporation of 1.63 at.% W through ALD growth leads to a state‐of‐the‐art phase transition at 32 °C with emissivity contrast between the low‐temperature and high‐temperature phase exceeding 40% in a metasurface‐based radiative cooling device configuration. The process is shown to be viable on 200 mm silicon substrates as well as flexible polyimide films. The full and self‐consistent temperature‐dependent characterization of the W‐doped VO2 using spectroscopic ellipsometry, electrical conductivity, mid‐wave infrared camera, and Fourier transform infrared emissivity, allows for a fully validated material model for the theoretical design of various smart and switchable device applications.

Function switchable broadband wave plate based on the Au-VO2 hybrid metasurface.

In recent years, the integration of active materials into a metasurface to achieve tunable devices has attracted much attention. Here, we design an Au-VO2 hybrid metasurface, which can switch between quarter-wave plate and half-wave plate due to the phase transition of VO2. At 298 K, the proposed structure acts as a quarter-wave plate in the 0.87-1.2 THz band, achieving the mutual conversion between linear polarization and circular polarization. Raising the temperature to 358 K, it works as a broadband half-wave plate in the range of 0.65-1.45 THz, with the reflective chirality preservation of circular polarization and the cross-polarization conversion of linear polarization. In the above cases, the response efficiencies are both above 90%. The switchable multifunction results from the tunable geometric phase of the metasurface, where the elaborately designed Au and VO2 blocks separately bring the phase of π/2. Furthermore, the electric field and current density distributions are employed to explain the physical mechanisms leading to the different functions. Such an active broadband metasurface is expected to find applications in tunable and multifunction devices manipulating the polarization and phase of terahertz waves.

Revealing the Role of Hydrogen in Electron-Doping Mottronics for Strongly Correlated Vanadium Dioxide.

Hydrogen-associated electron-doping Mottronics for d-band correlated oxides (e.g., VO2) opens up a new paradigm to regulate the electronic functionality via directly manipulating the orbital configuration and occupancy. Nevertheless, the role of hydrogen in the Mottronic transition of VO2 is yet unclear because opposite orbital reconfigurations toward either the metallic or highly insulating states were both reported. Herein, we demonstrate the root cause for such hydrogen-induced multiple electronic phase transitions by 1H quantification using nuclear reaction analysis. A low hydrogenation temperature is demonstrated to be vital in achieving a large hydrogen concentration (nH ≈ 1022 cm-3) that further enhances the t2g orbital occupancy to trigger electron localizations. In contrast, elevating the hydrogenation temperatures surprisingly reduces nH to ∼1021 cm-3 but forms more stable metallic H0.06VO2. This leads to the recognition of a weaker hydrogen interaction that triggers electron localization within VO2 via Mottronically enhancing the orbital occupancies.

Piezoelectric Lateral-Extensional Mode Resonators With Reconfigurable Electrode and Resonance Mode-Switching Behavior Enabled by a VO₂ Thin-Film.

This article presents the first two-port lateral-extensional mode zinc oxide (ZnO) piezoelectric resonator with a reconfigurable bottom electrode that is enabled by embedding a vanadium dioxide (VO2) thin film. The insulator-to-metal phase transition of VO2 is triggered by substrate heating that translates to abrupt changes in electric field patterns and piezoelectrically transduced modal vibrations, thus allowing mode-switching of piezoelectric resonators at specific frequencies. Finite element method (FEM) analysis was used to model the broadband frequency response, while frequency characteristics of the corresponding two-port resonator were measured over a temperature range between 20 °C and 95 °C with a specific focus on two resonances at 88 and 148 MHz. By leveraging the hysteretic behavior of VO2 thin film during a heating/cooling cycle, a change in both the capacitive feedthrough and resonance signal levels was observed, due to the abrupt change in the conductivity of VO2 during its phase transition. The unique switch- ON behavior of the resonance at 88 MHz starts at 70 °C during the heating cycle, while the switch- OFF transition begins at 60 °C during the cooling cycle. On the other hand, when the temperature is increased from 20 °C to 60 °C, a decrease in the insertion loss and resonance frequency of 12 dB and 0.28 MHz, respectively, were observed for the resonance at 148 MHz. Meanwhile, a resonance frequency increase of 0.42 MHz was observed during a temperature increase from 60 °C to 95 °C, which can be ascribed to VO2 phase transition from monoclinic to rutile phase. The hysteresis loops for insertion loss and resonance frequency indicate a different critical temperature for phase transition from the monoclinic (insulator) phase and rutile (metallic) phase and vice versa. The substantial variation in the temperature coefficient of frequency can be largely ascribed to electrode reconfiguration enabled by VO2 phase transition.

VO2-enabled transmission-reflection switchable coding terahertz metamaterials.

Coding metamaterials have offered unprecedented degrees of freedom to manipulate electromagnetic waves in time and frequency domains in terms of various coding sequences, however, it is still challenging to realize dynamic coding metamaterials in the terahertz range. Here, we propose VO2-enabled transmission-reflection switchable coding terahertz metamaterials consisting of multilayered gold and VO2 patterns. The insulator-to-metal transition of VO2 leads to switch between the refractive and reflective scattering beams by changing the temperature. The four 2-bit elements are used to construct coding metasurface-based OAM generator with l = 1. Remarkably, the transmission-reflection switching functionality of the coding metasurface can be achieved at different frequencies. In addition, the novel designs in our work can achieve EM waves manipulation and provide a useful method to dynamically switch transmission-reflection response in the THz frequency regime.

Anisotropic coding metasurfaces and their active manipulation based on vanadium dioxide for multifunctional applications in the terahertz region.

Various kinds of metasurfaces have been proposed because they can be tailored to achieve the desired modulations on electromagnetic wave that do not occur in nature. Compared to conventional metamaterials, coding metasurfaces integrated with information science theory possess numerous distinctive advantages - simple design, time-saving and compatibility with digital devices. Here we propose terahertz multifunctional anisotropic reflective metasurfaces with a metal-insulator-metal cavity structure whose top constructional layer consists of a pair of gold arc-rings and a gold cut-wire located between them. Two different functions of narrow-band absorption and broadband polarization conversion are realized based on different coding matrices using the binary codes '0' and '1'. Furthermore, we integrate a specific coding metasurface with vanadium dioxide (VO2) to realize a temperature-controlled active metasurface. Through the temperature change, dynamic functionalities switching between a narrow-band polarization converter with a polarization conversion ratio over 94% and an efficient low-pass filter are achieved under the phase transition of VO2, and the active metasurface is polarization independent. The proposed coding metasurfaces are verified numerically and experimentally, and have promising applications in terahertz modulation and functional devices.

Polarization-sensitive tunable extraordinary terahertz transmission based on a hybrid metal-vanadium dioxide metasurface.

Thermally tunable extraordinary terahertz transmission in a hybrid metal-vanadium dioxide (VO2) metasurface is numerically demonstrated. The metasurface consists of a metal sheet perforated by square loops, while the loops are connected with strips of VO2. The frequency and amplitude of the transmission resonance are modulated by controlling the conductivity of VO2. For a y-polarized incident field, the resonance transmission peak redshifts from 0.88 to 0.81 THz upon insulator-to-metallic phase transition of VO2. For an x-polarized incident field, the transmission resonance at 0.81 THz is observed in the insulator phase. However, in the metallic phase of VO2, the electromagnetic field is effectively reflected in the 0.5-1.1 THz range with a transmission level lower than 0.14. The proposed metasurface can be utilized as a terahertz modulator, reconfigurable filter, or switch.

Nonvolatile Control of Metal-Insulator Transition in VO2 by Ferroelectric Gating.

Controlling phase transitions in correlated materials yields emergent functional properties, providing new aspects to future electronics and a fundamental understanding of condensed matter systems. With vanadium dioxide (VO2 ), a representative correlated material, an approach to control a metal-insulator transition (MIT) behavior is developed by employing a heteroepitaxial structure with a ferroelectric BiFeO3 (BFO) layer to modulate the interaction of correlated electrons. Owing to the defect-alleviated interfaces, the enhanced coupling between the correlated electrons and ferroelectric polarization is successfully demonstrated by showing a nonvolatile control of MIT of VO2 at room temperature. The ferroelectrically-tunable MIT can be realized through the Mott transistor (VO2 /BFO/SrRuO3 ) with a remanent polarization of 80 µC cm-2 , leading to a nonvolatile MIT behavior through the reversible electrical conductance with a large on/off ratio (≈102 ), long retention time (≈104 s), and high endurance (≈103 cycles). Furthermore, the structural phase transition of VO2 is corroborated by ferroelectric polarization through in situ Raman mapping analysis. This study provides novel design principles for heteroepitaxial correlated materials and innovative insight to modulate multifunctional properties.

On/Off Ultra‐Short Spin Current for Single Pulse Magnetization Reversal in a Magnetic Memory Using VO2 Phase Transition

AbstractUltrafast magnetization reversal of magnetic thin films induced by one single femtosecond laser light pulse, without any applied magnetic nor electrical field, known as all‐optical switching has become one of the most attractive technologies for prospective ultrafast and low‐energy magnetic devices. A single femtosecond laser pulse switching of a ferromagnet in specific spin valves, driven by electron pulse originating from ultrafast demagnetization of a ferrimagnetic layer, has been recently demonstrated. This opens the way to ultrafast and energy‐efficient writing for magnetic memory. Here, a new functionality is presented through introducing a thin VO2 into the specific spin valve, where a switchable ultra‐short polarized electron pulse by thermally triggering the phase transition of VO2 to dynamically control the switching of the ferromagnet is experimentally achieved. It is demonstrated that the generated ultra‐short electron pulse can be free to pass through the VO2 in its metallic state while it is blocked in its insulating state. Additionally, the change on electrical and optical properties of VO2 during the phase transition leads to various possible magnetic states. On/off ultra‐short spin current controlled by phase transition paves the way to address specific ultra‐fast spintronic devices in large array devices.

Reconfigurable high-Q terahertz filtering of VO2-based metamaterials using optical tunneling

Up to date, reconfigurable terahertz (THz) band-pass filters based on the design of metamaterials with vanadium dioxide (VO2) have been investigated extensively. However, VO2-based THz metamaterials with high-quality tunable narrowband transmission and good stop-band rejection have yet to be reported, which are essential for THz wave filtering components or systems. Here, we propose a reconfigurable filter in combination with a dielectric metastructure and a VO2 thin film. Utilizing the optical tunneling effects of a cavity layer, the filter has a high band-pass transmission up to 98% at 0.246 THz with a narrow bandwidth of 0.0009 THz, an ultra-high Q-factor of 273, and two broad nearly-suppressed sidebands. The transmission intensity can be modulated by the phase transition of VO2, and the modulation depth is as high as 96.4%. By efficiently tuning the cavity layer thickness, the filter can be designed for a series of transmission wavelengths and further accomplish the switch between the narrow band-pass filtering and wideband reflection. Moreover, the manipulation of the incident angle and polarization can be adopted as an additional degree to achieve a similar switching function. Our study implies a potential for developing various THz applications, where reconfigurable high-Q THz filtering is required.

Tunable optical anisotropy in epitaxial phase-change VO2 thin films

Abstract We theoretically and experimentally demonstrate a strong and tunable optical anisotropy in epitaxially-grown VO2 thin films. Using a combination of temperature-dependent X-ray diffraction, spectroscopic ellipsometry measurements and first-principle calculations, we reveal that these VO2 thin films present an ultra-large birefringence (Δn > 0.9). Furthermore, leveraging the insulator-to-metal transition of VO2, we demonstrate a dynamic reconfiguration of optical properties from birefringent to hyperbolic, which are two distinctive regimes of anisotropy. Such a naturally birefringent and dynamically switchable platform paves the way for multi-functional devices exploiting tunable anisotropy and hyperbolic dispersion.

Infrared Transmission Characteristics of Phase Transitioning VO2 on Various Substrates.

Infrared transmission characteristics of VO2 thin films synthesized on multiple substrates, using a low-pressure direct oxidation technique, have been characterized. Material characterization of these films indicates high material quality, which resulted in large variation of electrical and optical properties at phase transition. A change in optical transmissivity greater than 80% was observed for these films utilizing infrared (IR) laser illumination at 1550 nm. Phase transition enabled by temperature change induced by a pulsed high-power laser beam resulted in modulated IR laser transmission with a low time constant in VO2 on transparent quartz and muscovite substrates. Investigation of the effect of mechanical strain on phase transition in VO2 grown on flexible muscovite substrate indicate shift in transition temperature to higher for tensile and lower for compressive strains.

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.

Coupling of Phase Transition and Self-Powered Photodetection of a NbSe2/VO2 Heterojunction

An endeavor to design a self-powered photodetector that can work in a broad temperature range is required for everyday applications. Herein, we present a self-powered NbSe2/VO2 bilayer photodetector with boosted performance stimulated by the phase transition of VO2 after heating. The NbSe2/VO2 heterojunction can be converted into a Schottky junction from a staggered heterojunction thanks to the semiconductor-to-metal phase transition (SMT) of VO2, resulting in a controlled built-in electric field at the bilayer junction. Its origin comes from a sharp increase in carrier transport, which results in a boosted photoresponse in the bilayer. Our photovoltaic detector has shown a fast response time of 122 μs at the VO2 metallic phase for a visible wavelength (405 nm), featuring a self-powered, high-speed, and energy-efficient device.

Artificial Multisensory Neurons with Fused Haptic and Temperature Perception for Multimodal In‐Sensor Computing

The human receives and transmits various information from the outside world through different sensory systems. The sensory neurons integrate various sensory inputs into a synthetical perception to monitor complex environments, and this fundamentally determines the way how we perceive the world. Developing multifunctional artificial sensory elements that can integrate multisensory perception plays a vital role in future intelligent perception systems, whereas prior spiking neurons reported can only handle single‐mode physical signals. Herein, a bioinspired haptic‐temperature fusion spiking neuron based upon a serial connection of piezoresistive sensor and VO2 volatile memristor is presented. The artificial sensory neuron is capable of detecting and encoding pressure and temperature inputs based on the voltage dividing effect and the intrinsic thermal sensitivity of metal–insulator transition in VO2. Recognition of Braille characters is achieved through multiple piezoresistive sensors, taking advantage of the spatial integration capabilities of such spiking neurons. Notably, the traditionally separate haptic and temperature signals can be fused physically in the sensory neuron when synchronizing the two sensory cues, which is able to recognize multimodal haptic/temperature patterns. The artificial multisensory neuron thus provides a promising approach toward e‐skin, neurorobotics, and human–machine interaction technologies. A preprint version of the article can be found at: https://www.authorea.com/doi/full/10.22541/au.164668806.60849882.

Li+/Al3+ Composite Solid Electrolyte Based Electro‐Thermal Dual Responsive Devices with Optimized Optical Contrast and Cycle Durability

AbstractThe limited optical contrast and band adjustment range are the main factors that restrict chromic materials. In this study, Li+/Al3+ composite solid electrolyte is incorporated between VO2 and WO3 chromic functional layers to construct a new type of VO2 based electrothermal dual responsive devices. The dual responsive devices deliver optical contrast over 50% and independent modulation of visible and near infrared band. In addition to displaying four reversible optical states in response to environmental temperature and external voltage, the devices effectively utilize the synergistic effect of Li+/Al3+ hybrid ions. Furthermore, the optimized Li+/Al3+ composite electrolyte is proved to effectively control phase transition of VO2 and significantly improve the electrochromic properties of WO3 as well. Finally, the electro‐thermal dual responsive devices show excellent service stability in simulated outdoor environment of over 10 000 cycles. Combined with the above advantages, it is expected to develop smart windows and derivative products with large size, long life, remarkable energy saving effect and pleasant comfort.

Multifield Controlled Terahertz Modulator Based on Silicon‐Vanadium Dioxide Hybrid Metasurface

AbstractDespite flourishing and deepening trend in the terahertz (THz) field, there are still challenges in high‐performance active system components, such as THz modulators. Present THz modulators are typically limited to a single external driving field and the fixed switching speed, hindering the ability to flexibly manipulate THz waves. Here, an optically and thermally controlled THz modulator based on silicon (Si) and vanadium dioxide (VO2) hybrid metasurface is proposed to overcome these limitations. The modulator enables the dynamical control of the transmitted amplitude in the range of 0.4–1.8 THz. Under thermal management, a maximum modulation depth (MD) of 97.2% at 0.9 THz can be acquired, thanks to the phase transition of VO2. Once excited with an 800 nm pump light at 1600 µJ cm−2, the device achieves a maximum MD of 91.5% at 0.87 THz accomplished with an ultrafast speed of 2 ps, resulting from photogenerated carriers in the Si substrate. Furthermore, the findings of this study propose a promising strategy for developing multifield controlled active THz components, which would be a new paradigm to design switchable metasurface devices.

Spin Engineering of VO2 Phase Transitions and Removal of Structural Transition.

Vanadium dioxide undergoes a metal-to-insulator transition, where the energy of electron-electron, electron-lattice, spin-spin, and spin-lattice interactions are of the same order of magnitude. This leads to the coexistence of electronic and structural transitions in VO2 that limit the lifetime and speed of VO2-based devices. However, the closeness of interaction energy of lattice-electron-spin can be turned into an opportunity to induce some transitions while pinning others via external stimuli. That is, the contribution of spin, charge, orbital, and lattice degrees of freedom can be manipulated. In this study, spin engineering has been exploited to affect the spin-related interactions in VO2 by introducing a ferromagnetic Ni layer. The coercivity in the Ni layer is engineered by controlling the shape anisotropy via kinetics of growth. Using spin engineering, the structural pinning of the monoclinic M2 phase of VO2 is successfully achieved, while the electronic and magnetic transitions take place.

VO2-metallic hybrid metasurfaces for agile terahertz wave modulation by phase transition

The combination of VO2 and metasurfaces has opened an attractive route to dynamically control terahertz (THz) waves based on the giant conductivity change. However, the high-precision control of microfabrication and single performance of conductivity change limit the multifunctional application of VO2-based metasurfaces. Here, we proposed a VO2-metallic hybrid metasurface by in situ depositing high-quality VO2 thin films onto a metasurface composed of asymmetric Fano resonance units. It exhibits agile frequency and amplitude modulation for THz transmission across tuning the dielectric constant and conductivity of VO2. The metallic metasurface is designed as a matrix to achieve high transmission at 0.61 and 0.78 THz due to the split-ring resonance. During the thermally triggered phase transition of VO2, we found that the resonance frequency and amplitude can be tuned dominantly by the change of dielectric constant and conductivity, respectively. In particular, the increased dielectric constant enables red shift of the frequency by around 0.48 THz and the conductivity increases lead to a giant THz amplitude modulation of 88%. These results provide a route for developing VO2-based THz smart devices combined with functional metasurfaces and hold great promise for applications in THz sensor and modulation.