Infrared Response and Optoelectronic Memory Device Fabrication Based on Epitaxial VO2 Film

Investigation of Resistance Switching and Synaptic Properties of VO x for Neuromorphic Applications

The taking run on artificial intelligence in the last decades is based on the von Neumann architecture where memory and computation units are separately located from each other. This configuration causes a large amount of energy and time to be dissipated during data transfer between these two units, in contrast to synapses in biological neurons. A new paradigm has been proposed inspired by biological neurons in human brains, known as neuromorphic computing. Due to the unusual current-voltage characteristic of memristor devices such as pinched hysteresis loops, memristors are considered a key element of neuromorphic architecture. In this study, we report the basic current-voltage characteristic of the memristor devices in the form of Si/SiO2/Pt(30 nm)/VO x (3, 13, 25 nm)/Pt (30 nm) sandwich structure. Synaptic functions such as spike-time-dependent plasticity (STDP), paired-pulse facilitation (PPF), long-term potentiation (LTP), and long-term depression (LTD) of memristor devices were examined in detail. The oxide layer VO x has been grown by using the VO2 target in a pulsed laser deposition (PLD) chamber. The composition and oxidation states of the oxide layer were examined using the X-ray photoelectron spectroscopy (XPS) technique. The status of oxygen vacancies, which play an active role in the operation of the devices, was examined with a photoluminescence (PL) technique. The experimental results showed that the thickness of the oxide layer can significantly influence the synaptic and resistive switching properties of the devices.

Terahertz dynamic multichannel holograms generated by spin-multiplexing reflective metasurface

In recent years, metasurfaces have attracted considerable interest for their unprecedented capabilities to manipulate intensity, phase, and polarization of an electromagnetic wave. Although metasurface-based wavefront modulation has achieved numerous successful results, implementation of multifunctional devices in a single metasurface still meet significant challenges. Here, a novel multilayer structure is designed using properties of vanadium dioxide (VO2). Propagation phase and geometric phase are introduced in this structure to achieve multichannel holographic imaging in terahertz band. When the temperature is above 68°C, VO2 becomes a metal and it plays a role in wavefront modulation for terahertz wave. The left-handed channel realizes a hologram letter L and the right-handed channel realizes a hologram letter R. When the temperature is below 68°C, VO2 changes to an insulator, and electromagnetic wave is controlled by gold structures embedded inside a VO2 film. In this case, hologram number 2 is realized in the left-handed channel and hologram number 6 appears in the right-handed channel. Our structure has advantages of low crosstalk, multiple channels, and large bandwidth. This novel design paves a new road for multichannel imaging and information encryption.

Memristor-based neural networks: a bridge from device to artificial intelligence

Since the beginning of the 21st century, there is no doubt that the importance of artificial intelligence has been highlighted in many fields, among which the memristor-based artificial neural network technology is expected to break through the limitation of von Neumann so as to realize the replication of the human brain by enabling strong parallel computing ability and efficient data processing and become an important way towards the next generation of artificial intelligence. A new type of nanodevice, namely memristor, which is based on the variability of its resistance value, not only has very important applications in nonvolatile information storage, but also presents obsessive progressiveness in highly integrated circuits, making it one of the most promising circuit components in the post-Moore era. In particular, memristors can effectively simulate neural synapses and build neural networks; thus, they can be applied for the preparation of various artificial intelligence systems. This study reviews the research progress of memristors in artificial neural networks in detail and highlights the structural advantages and frontier applications of neural networks based on memristors. Finally, some urgent problems and challenges in current research are summarized and corresponding solutions and future development trends are put forward.

Millimeter-wave to near-terahertz sensors based on reversible insulator-to-metal transition in VO2

In the quest for low power bio-inspired spiking sensors, functional oxides like vanadium dioxide are expected to enable future energy efficient sensing. Here, we report uncooled millimeter-wave spiking detectors based on the sensitivity of insulator-to-metal transition threshold voltage to the incident wave. The detection concept is demonstrated through actuation of biased VO2 switches encapsulated in a pair of coupled antennas by interrupting coplanar waveguides for broadband measurements, on silicon substrates. Ultimately, we propose an electromagnetic-wave-sensitive voltage-controlled spike generator based on VO2 switches in an astable spiking circuit. The fabricated sensors show responsivities of around 66.3 MHz.W-1 at 1 μW, with a low noise equivalent power of 5 nW.Hz-0.5 at room temperature, for a footprint of 2.5 × 10-5 mm2. The responsivity in static characterizations is 76 kV.W-1. Based on experimental statistical data measured on robust fabricated devices, we discuss stochastic behavior and noise limits of VO2 -based spiking sensors applicable for wave power sensing in mm-wave and sub-terahertz range.

Vanadium Oxide: Phase Diagrams, Structures, Synthesis, and Applications

Vanadium oxides with multioxidation states and various crystalline structures offer unique electrical, optical, optoelectronic and magnetic properties, which could be manipulated for various applications. For the past 30 years, significant efforts have been made to study the fundamental science and explore the potential for vanadium oxide materials in ion batteries, water splitting, smart windows, supercapacitors, sensors, and so on. This review focuses on the most recent progress in synthesis methods and applications of some thermodynamically stable and metastable vanadium oxides, including but not limited to V₂O₃, V₃O₅, VO₂, V₃O₇, V₂O₅, V₂O₂, V₆O₁₃, and V₄O₉. We begin with a tutorial on the phase diagram of the V-O system. The second part is a detailed review covering the crystal structure, the synthesis protocols, and the applications of each vanadium oxide, especially in batteries, catalysts, smart windows, and supercapacitors. We conclude with a brief perspective on how material and device improvements can address current deficiencies. This comprehensive review could accelerate the development of novel vanadium oxide structures in related applications.

Strain-Induced Modulation of Resistive Switching Temperature in Epitaxial VO2 Thin Films on Flexible Synthetic Mica

The resistive switching temperature associated with the metal-insulator transition (MIT) of epitaxial VO₂ thin films grown on flexible synthetic mica was modulated by bending stress. The resistive switching temperature of polycrystalline VO₂ and V₂O₅ thin films, initially grown on synthetic mica without a buffer layer, was observed not to shift with bending stress. By inserting a SnO₂ buffer layer, epitaxial growth of the VO₂ (010) thin film was achieved, and the MIT temperature was found to vary with the bending stress. Thus, it was revealed that the bending response of the VO₂ thin film depends on the presence or absence of the SnO₂ buffer layer. The bending stress applied a maximum in-plane tensile strain of 0.077%, resulting in a high-temperature shift of 2.3 °C during heating and 1.8 °C during cooling. After 10⁴ bending cycles at a radius of curvature R = 10 mm, it was demonstrated that the epitaxial VO₂ thin film exhibits resistive switching temperature associated with MIT.

Vanadium Dioxide Nanosheets Supported on Carbonized Cotton Fabric as Bifunctional Textiles for Flexible Pressure Sensors and Zinc-Ion Batteries

Flexible pressure sensors and aqueous batteries have been widely used in the rapid development of wearable electronics. The synergistic functionalities of versatile materials with multidimensional architectures are recognized to have a significant impact on the performance of flexible electronics. Herein, a facile hydrothermal strategy was demonstrated to conformally grow vanadium dioxide nanosheets on carbonized cotton fabrics (VO₂/CCotton), which is a candidate material used in flexible piezoresistive sensors. As a result, the VO₂/CCotton-based pressure sensor behaved with high sensitivity (S = 7.12 kPa^-1 in the pressure range of 0-2.0 kPa) and a stable sensing ability in a wide pressure scale of 0-120 kPa. Further practical applications were performed in monitoring delicate physiological signals as well, such as twisting, blowing, and voice vibration recognitions. In addition, another application for energy storage was investigated as well. A quasi-solid-state aqueous zinc-ion battery was assembled with VO₂/CCotton as the cathode and a film of Zn nanosheets/carbon nanotube as the anode. A capacity as high as 301.5 mAh g^-1 and remarkable durability of 88.7% capacity retention after 5000 cycles at 10 A g^-1 were found. These exceptional outcomes are attributed to the unique three-dimensional architecture and the prominent synergetic effects of CCotton and VO₂ and allow for the proposal of novel guidelines for next-generation multifunctional flexible electronics.

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.

Photonic bandgap engineering in (VO2)n/(WSe2)nphotonic superlattice for versatile near- and mid-infrared phase transition applications

The application of the photonic superlattice in advanced photonics has become a demanding field, especially for two-dimensional and strongly correlated oxides. Because it experiences an abrupt metal-insulator transition near ambient temperature, where the electrical resistivity varies by orders of magnitude, vanadium oxide (VO₂) shows potential as a building block for infrared switching and sensing devices. We reported a first principle study of superlattice structures of VO₂as a strongly correlated phase transition material and tungsten diselenide (WSe₂) as a two-dimensional transition metal dichalcogenide layer. Based on first-principles calculations, we exploit the effect of semiconductor monoclinic and metallic tetragonal state of VO₂with WSe₂in a photonic superlattices structure through the near and mid-infrared (NIR-MIR) thermochromic phase transition regions. By increasing the thickness of the VO₂layer, the photonic bandgap (PhB) gets red-shifted. We observed linear dependence of the PhB width on the VO₂thickness. For the monoclinic case of VO₂, the number of the forbidden bands increase with the number of layers of WSe₂. New forbidden gaps are preferred to appear at a slight angle of incidence, and the wider one can predominate at larger angles. We presented an efficient way to control the flow of the NIR-MIR in both summer and winter environments for phase transition and photonic thermochromic applications. This study's findings may help understand vanadium oxide's role in tunable photonic superlattice for infrared switchable devices and optical filters.

Active and Smart Terahertz Electro-Optic Modulator Based on VO2 Structure

Modulating terahertz (THz) waves actively and smartly through an external field is highly desired in the development of THz spectroscopic devices. Here, we demonstrate an active and smart electro-optic THz modulator based on a strongly correlated electron oxide vanadium dioxide (VO2). With milliampere current excitation on the VO2 thin film, the transmission, reflection, absorption, and phase of THz waves can be modulated efficiently. In particular, the antireflection condition can be actively achieved and the modulation depth reaches 99.9%, accompanied by a 180° phase switching. Repeated and current scanning experiments confirm the high stability and multibit modulation of this electro-optic modulation. Most strikingly, by utilizing a feedback loop of "THz-electro-THz" geometry, a smart electro-optic THz control is realized. For instance, the antireflection condition can be stabilized precisely no matter what the initial condition is and how the external environment changes. The proposed electro-optic THz modulation method, taking advantage of strongly correlated electron material, opens up avenues for the realization of THz smart devices.

Voltage triggered near-infrared light modulation using VO2 thin film

Development of compact and fast modulators of infrared light has garnered strong research interests in recent years due to their potential applications in communication, imaging, and sensing. In this study, electric field induced fast modulation near-infrared light caused by phase change in VO₂ thin films grown on GaN suspended membranes has been reported. It was observed that metal insulator transition caused by temperature change or application of electric field, using an interdigitated finger geometry, resulted in 7% and 14% reduction in transmitted light intensity at near-infrared wavelengths of 790 and 1550 nm, respectively. Near-infrared light modulation has been demonstrated with voltage pulse widths down to 300 µs at 25 V magnitude. Finite element simulations performed on the suspended membrane modulator indicate a combination of the Joule heating and electric field is responsible for the phase transition.

Bistable absorption in a 1D photonic crystal with a nanocomposite defect layer

We investigate the nonlinear absorption properties of a defective one-dimensional photonic crystal at the near-infrared range using the nonlinear transfer matrix method. The defect is a nanocomposite layer containing vanadium dioxide nanoparticles sandwiched between two nonlinear dielectric layers. The linear absorption spectrum of the designed structure has three resonant absorption lines at the bandgap region of the photonic crystal. We can reconfigure the structure in the linear regime from nearly transparent to absorbent or vice versa in multiple resonant wavelengths by adjusting the temperature. Moreover, the system shows absorptive bistability by adjusting the intensity and incident angle of the input light. We discuss the tunability of the nonlinear absorption in detail. In the nonlinear regime, we show that, besides the temperature, the structure can be reconfigured from absorbent to transparent and vice versa by adjusting the incident optical power and the incident angle. We validate the results by examining the electric field distribution throughout the structure.

Transverse barrier formation by electrical triggering of a metal-to-insulator transition

Application of an electric stimulus to a material with a metal-insulator transition can trigger a large resistance change. Resistive switching from an insulating into a metallic phase, which typically occurs by the formation of a conducting filament parallel to the current flow, is a highly active research topic. Using the magneto-optical Kerr imaging, we found that the opposite type of resistive switching, from a metal into an insulator, occurs in a reciprocal characteristic spatial pattern: the formation of an insulating barrier perpendicular to the driving current. This barrier formation leads to an unusual N-type negative differential resistance in the current-voltage characteristics. We further demonstrate that electrically inducing a transverse barrier enables a unique approach to voltage-controlled magnetism. By triggering the metal-to-insulator resistive switching in a magnetic material, local on/off control of ferromagnetism is achieved using a global voltage bias applied to the whole device.

Piston-Type Optical Modulator for Dynamic Thermal Radiation Tuning Applications

This study introduces a movable piston-like structure that provides a simple and cost-effective avenue for dynamically tuning thermal radiation. This structure leverages two materials with dissimilar optical responses-graphite and aluminum-to modulate from a state of high reflectance to a state of high absorptance. A cavity is created in the graphite to house an aluminum cylinder, which is displaced to actuate the device. In its raised state, the large aluminum surface area promotes a highly reflective response, while in its lowered state, the expanded graphite surface area and blackbody cavity-like interactions significantly enhance absorptance. By optimizing the area ratio, reflectance tunability of over 30% is achieved for nearly the entire ultraviolet, visible, and near-infrared wavelength regions. Furthermore, a theoretical analysis postulates wavelength-dependent effectivenesses as high as 0.70 for this method, indicating that tunabilities approaching 70% can be achieved by exploiting near-ideal absorbers and reflectors. The analog nature of this control method allows for an infinitely variable optical response between the upper and lower bounds of the device. These valuable characteristics would enable this material structure to serve practical applications, such as reducing cost and energy requirements for environmental temperature management operations.

Flexible Phase Change Materials for Electrically-Tuned Active Absorbers

Phase change materials (PCMs), such as GeSbTe (GST) alloys and vanadium dioxide (VO₂ ), play an important role in dynamically tunable optical metadevices. However, the PCMs usually require high thermal annealing temperatures above 700 K, but most flexible metadevices can only work below 500 K owing to the thermal instability of polymer substrates. This contradiction limits the integration of PCMs into flexible metadevices. Here, a mica sheet is chosen as the chemosynthetic support for VO₂ and a smooth and uniformly flexible phase change material (FPCM) is realized. Such FPCMs can withstand high temperatures while remaining mechanically flexible. As an example, a metal-FPCM-metal infrared meta-absorber with mechanical flexibility and electrical tunability is demonstrated. Based on the electrically-tuned phase transition of FPCMs, the infrared absorption of the metadevice is continuously tuned from 20% to 90% as the applied current changes, and it remains quite stable at bending states. The metadevice is bent up to 1500 times, while no visible deterioration is detected. For the first time, the FPCM metastructures are significantly added to the flexible material family, and the FPCM-based metadevices show various application prospects in electrically-tunable conformal metadevices, dynamic flexible photodetectors, and active wearable devices.

Photoinduced Broad-band Tunable Terahertz Absorber Based on a VO2 Thin Film

The demand for terahertz (THz) communication and detection fuels continuous research for high performance of THz absorption materials. In addition to varying the materials and their structure passively, an alternative approach is to modulate a THz wave actively by tuning an external stimulus. Correlated oxides are ideal materials for this because the effects of a small external control parameter can be amplified by inner electronic correlations. Here, by utilizing an unpatterned strongly correlated electron oxide VO₂ thin film, a photoinduced broad-band tunable THz absorber is realized first. The absorption, transmission, reflection, and phase of THz waves can all be actively controlled by an external pump laser above room temperature. By varying the laser fluence, the average broad-band absorption can be tuned from 18.9 to 74.7% and the average transmission can be tuned from 9.2 to 69.2%. Meanwhile, a broad-band antireflection is obtained at 5.6 mJ/cm², and a π-phase shift of a reflected THz wave is achieved when the fluence increases greater than 5.7 mJ/cm². Apart from other modulators, the photoexcitation-assisted dual-phase competition is identified as the origin of this active THz multifunctional modulation. Our work suggests that advantages of controllable phase separation in strongly correlated electron systems could provide viable routes in the creation of active optical components for THz waves.

Hydrothermal epitaxy growth of self-organized vanadium dioxide 3D structures with metal–insulator transition and THz transmission switch properties

The hydrothermal method is an effective approach for the synthesis of VO₂ films with unique crystallites morphology and sharp electrical and optical switch properties.

Low-cost VO2(M1) thin films synthesized by ultrasonic nebulized spray pyrolysis of an aqueous combustion mixture for IR photodetection

We report detailed structural, electrical transport and IR photoresponse properties of large area VO2(M1) thin films deposited by a simple cost-effective two-step technique. Phase purity was confirmed by XRD and Raman spectroscopy studies. The high quality of the films was further established by a phase change from low temperature monoclinic phase to high temperature tetragonal rutile phase at 68 °C from temperature dependent Raman studies. An optical band gap of 0.75 eV was estimated from UV-visible spectroscopy. FTIR studies showed 60% reflectance change at λ = 7.7 μm from low reflectivity at low temperature to high reflectivity at high temperature in a transition temperature of 68 °C. Electrical characterization showed a first order transition of the films with a resistance change of four orders of magnitude and TCR of -3.3% K-1 at 30 °C. Hall-effect measurements revealed the n-type nature of VO2 thin films with room temperature Hall mobility, μ e of 0.097 cm2 V-1 s-1, conductivity, σ of 0.102 Ω-1 cm-1 and carrier concentration, n e = 5.36 × 1017 cm-3. In addition, we fabricated a high photoresponsive IR photodetector based on VO2(M1) thin films with excellent stability and reproducibility in ambient conditions using a low-cost method. The VO2(M1) photodetector exhibited high sensitivity, responsivity, quantum efficiency, detectivity and photoconductive gain of 5.18%, 1.54 mA W-1, 0.18%, 3.53 × 1010 jones and 9.99 × 103 respectively upon illumination with a 1064 nm laser at a power density of 200 mW cm-2 and 10 V bias voltage at room temperature.

Dynamic Electronic Doping for Correlated Oxides by a Triboelectric Nanogenerator

The metal-insulator transition of vanadium dioxide (VO₂ ) is exceptionally sensitive to charge density and electron orbital occupancy. Thus three-terminal field-effect transistors with VO₂ channels are widely adopted to control the phase transition by external gating voltage. However, current leakage, electrical breakdown, or interfacial electrochemical reactions may be inevitable if conventional solid dielectrics or ionic-liquid layers are used, which possibly induce Joule heating or doping in the VO₂ layer and make the voltage-controlled phase transition more complex. Here, a triboelectric nanogenerator (TENG) and a VO₂ film are combined for a novel TENG-VO₂ device, which can overcome the abovementioned challenges and achieve electron-doping-induced phase modulation. By taking advantage of the TENG structure, electrons can be induced in the VO₂ channel and thus adjust the electronic states of the VO₂ , simultaneously. The modulation degree of the VO₂ resistance depends on the temperature, and the major variation occurs when the temperature is in the phase-transition region. The accumulation of electrons in the VO₂ channel also is simulated by finite element analysis, and the electron doping mechanism is verified by theoretical calculations. The results provide a promising approach to develop a novel type of tribotronic transistor and new electronic doping technology.

Highly Sensitive, Temperature-Independent Oxygen Gas Sensor Based on Anatase TiO2 Nanoparticle Grafted, 2D Mixed Valent VO x Nanoflakelets

Herein, we report a facile approach for the synthesis of TiO₂ nanoparticles tethered on 2D mixed valent vanadium oxide (VO _x/TiO₂) nanoflakelets using a thermal decomposition assisted hydrothermal method and investigation of its temperature-independent performance enhancement in oxygen-sensing properties. The material was structurally characterized using XRD, TEM, Raman, DSC, and XPS analysis. The presence of mixed valent states, such as V₂O₅ and VO₂ in VO _x, and the metastable properties of VO₂ have been found to play crucial roles in the temperature-independent electrical conductivity of VO _x/TiO₂ nanoflakelets. Though pristine VO _x exhibited characteristic semiconductor-to-metal transition of monoclinic VO₂, pure VO _x nanoflakelets exhibited poor sensitivity toward sensing oxygen. VO _x/TiO₂ nanoflakelets showed a very low temperature coefficient of resistance above 150 °C with improved sensitivity (35 times higher than VO _x for 100 ppm) toward oxygen gas. VO _x/TiO₂ nanoflakelets exhibited much higher response, faster adsorption and desorption toward oxygen as compared to pristine VO _x beyond 100 °C, which endowed the sensor with excellent temperature-independent sensor properties within 150-500 °C. The faster adsorption and desorption after 100 °C led to shorter response time (3-5 s) and recovery time (7-9 s). The results suggest that 2D VO _x/TiO₂ can be a promising candidate for temperature-independent oxygen sensor applications.

Bolometric photodetection using plasmon-assisted resistivity change in vanadium dioxide

Vanadium oxide is a key sensing material for bolometric photodetection, thanks to its strong temperature dependence of resistivity close to room temperature. Here we demonstrate the photodetection of a stoichiometric vanadium dioxide thin film integrated with silver nanorods. The nanorods convert light into heat, consequently suppressing the resistivity of vanadium dioxide via localised surface plasmon resonance. Incorporation of this thermo-plasmonic effect into bolometric photodetection allows for wavelength and polarisation sensitivity. This work opens the path to a broad family of photodetection functionalities for vanadium dioxide-based microbolometers.

Hydrothermal Synthesis of VO2 Polymorphs: Advantages, Challenges and Prospects for the Application of Energy Efficient Smart Windows

Vanadium dioxide (VO₂ ) is a widely studied inorganic phase change material, which has a reversible phase transition from semiconducting monoclinic to metallic rutile phase at a critical temperature of τ_c ≈ 68 °C. The abrupt decrease of infrared transmittance in the metallic phase makes VO₂ a potential candidate for thermochromic energy efficient windows to cut down building energy consumption. However, there are three long-standing issues that hindered its application in energy efficient windows: high τ_c , low luminous transmittance (T_lum ), and undesirable solar modulation ability (ΔT_sol ). Many approaches, including nano-thermochromism, porous films, biomimetic surface reconstruction, gridded structures, antireflective overcoatings, etc, have been proposed to tackle these issues. The first approach-nano-thermochromism-which is to integrate VO₂ nanoparticles in a transparent matrix, outperforms the rest; while the thermochromic performance is determined by particle size, stoichiometry, and crystallinity. A hydrothermal method is the most common method to fabricate high-quality VO₂ nanoparticles, and has its own advantages of large-scale synthesis and precise phase control of VO₂ . This Review focuses on hydrothermal synthesis, physical properties of VO₂ polymorphs, and their transformation to thermochromic VO₂ (M), and discusses the advantages, challenges, and prospects of VO₂ (M) in energy-efficient smart windows application.