Transition In VO2 Research Articles

Vanadium oxide metal-insulator phase transition in different types of one-dimensional photonic microcavities

The optical properties of vanadium dioxide (VO2) can be tuned via metal-insulator transition. In this work, different types of one-dimensional photonic structure-based microcavities that embed vanadium dioxide have been studied in the spectral range between 900 nm and 2000 nm. In particular, VO2 has been sandwiched between: i) two photonic crystals made of SiO2 and ZrO2; ii) two aperiodic structures made of SiO2 and ZrO2 that follow the Thue-Morse sequence; iii) two disordered photonic structures, made of SiO2 and ZrO2 in which the disorder is introduced either by a random sequence of the two materials or by a random variation of the thicknesses of the layers; iv) two four material-based photonic crystals made of SiO2, Al2O3, Y2O3, and ZrO2. The ordered structures i and iv show, respectively, one and two intense transmission valleys with defect modes, while the aperiodic and disordered structures ii and iii show a manifold of transmission valleys due to their complex layered configurations. The metal-insulator transition of VO2, controlled by temperature, results in a modulation of the optical properties of the microcavities.

Transient dynamics of the phase transition in VO2 revealed by mega-electron-volt ultrafast electron diffraction

Vanadium dioxide (VO2) exhibits an insulator-to-metal transition accompanied by a structural transition near room temperature. This transition can be triggered by an ultrafast laser pulse. Exotic transient states, such as a metallic state without structural transition, were also proposed. These unique characteristics let VO2 have great potential in thermal switchable devices and photonic applications. Although great efforts have been made, the atomic pathway during the photoinduced phase transition is still not clear. Here, we synthesize freestanding quasi-single-crystal VO2 films and examine their photoinduced structural phase transition with mega-electron-volt ultrafast electron diffraction. Leveraging the high signal-to-noise ratio and high temporal resolution, we observe that the disappearance of vanadium dimers and zigzag chains does not coincide with the transformation of crystal symmetry. After photoexcitation, the initial structure is strongly modified within 200 femtoseconds, resulting in a transient monoclinic structure without vanadium dimers and zigzag chains. Then, it continues to evolve to the final tetragonal structure in approximately 5 picoseconds. In addition, only one laser fluence threshold instead of two thresholds suggested in polycrystalline samples is observed in our quasi-single-crystal samples. Our findings provide essential information for a comprehensive understanding of the photoinduced ultrafast phase transition in VO2.

Evaluation of V<sub>1</sub><sub>-<i>x</i></sub>Cr<i><sub>x</sub></i>O<sub>2</sub> Thin Films Fabricated by MOD

Cr-doped vanadium dioxide, V1-xCrxO2 (x = 0-0.20), thin films were fabricated by metal-organic decomposition. From X-ray diffraction patterns of the films at room temperature, VO2(M2)(201) and VO2(M2)(201) diffraction peaks were observed for x = 0.03-0.20, while a VO2(M1)(011) peak was observed for x = 0-0.01, where M1 and M2 represent different monoclinic structures of VO2. Moreover, all the fabricated films showed a p-type conductivity, and the hole concentration in the films increased with increasing x. From R-T characteristics of the films at the temperature range from 20 to 90℃, abrupt resistance changes caused by the metal-insulator transition in VO2 with a hysteresis loop were gradually suppressed with increasing x; especially x≥0.15, they were completely suppressed, since the transition temperature of the film shifted to a temperature above 140℃. Relatively flat temperature dependence of the temperature coefficients of resistance (TCR) as high as -3.9%/K in the range of 20-90℃ was obtained for the V0.85Cr0.15O2 thin film. The TCR value of the film is almost same as that of a V0.75Ti0.25O2 thin film, whereas a resistivity of the former at room temperature is one order lower than that of the latter.

Terahertz phase modulator based on a metal-VO2 reconfigurable metasurface.

Actively controlling the phase of a terahertz (THz) wave is of great significance for beaming, tunable focusing, and holography. We present a THz phase modulator based on an electrically triggered vanadium dioxide (V O 2) reconfigurable metasurface. The unit cell of the device consists of two split-ring resonators embedded with a V O 2 ribbon. By electrically triggering the insulator-to-metal transition of V O 2, the resonance mode and resonance intensity of the unit cell can be dynamically controlled. The simulation results show that the structure can achieve a phase shift of about 360° in the range of 1.03-1.13THz, and the reflection amplitude can reach 80%. The device has potential applications in THz imaging, radar, broadband wireless communications, and array phase control.

Noncontact evaluation of the interface potential in VO2/Si heterojunctions across metal–insulator phase transition

This study conducted laser-induced terahertz emission spectroscopy on a VO2/Si heterojunction. Consequently, rapid estimation of the local interface potential was realized and the work function of VO2 was obtained as 5.17–5.25 eV with increasing temperature from 320 to 380 K. Moreover, an obvious terahertz emission variation was observed across the metal–insulator phase transition of VO2, and the doping conditions of the Si substrate largely influenced the terahertz emission. These results imply a strong relationship between the terahertz emission amplitude and the interface electric field, which supports the rapid performance of terahertz emission spectroscopy in estimating the work function of VO2.

Tunable bi-direction terahertz vortex beam generator based on Dirac semimetals

We proposed a tunable bi-direction terahertz vortex beam generator based on Dirac semimetals (DSMs) and Vanadium dioxide (VO2). The insulator–metal phase transition of VO2 occurs by changing the external temperature, thus realizing a switch between the transmissive vortex beam and the reflective vortex beam. We design a vortex beam generator with topological charges −1, −2, ＋3 in transmission mode and achieve tunable terahertz vortex beam from 0.89 THz to 1.13 THz by changing the Fermi energy level of the DSMs. In addition, the proposed metasurface generates a broadband terahertz vortex beam with topological charges −1, −2, ＋3 from 1.0 THz to 1.8 THz when VO2 is in the metal state.

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).

An active tunable terahertz functional metamaterial based on hybrid-graphene vanadium dioxide.

In this paper, we propose a switchable and tunable functional metamaterial device based on hybrid graphene-vanadium dioxide (VO2). Using the properties of the metal-insulator transition in VO2, the proposed metamaterials can enable switching between tunable circular dichroism (CD) and dual-band perfect absorption in the terahertz region. When VO2 is in the insulator state, a polarization-selective single-band perfect absorption can be achieved for circularly polarized waves, thus resulting in a strong CD response with a maximum value of 0.84. When VO2 acts as a metal, there is a tunable dual-band perfect absorption for the designed metamaterial device under the illumination of x-polarization waves. The operation mechanism behind the phenomena can be explained by utilizing the electric field distribution and the coupled mode theory. Moreover, the influences of the Fermi energy of graphene and geometrical parameters on the CD and absorption spectra are discussed in detail. Our proposed switchable and tunable metamaterial can provide a platform for designing versatile functional devices in the terahertz region.

Terahertz state switching of holograms enabled by vanadium dioxide-based metasurfaces.

Holography is an important topic in optical research. Metasurface holography has attracted increasing attention in recent years. However, it is still challenging to achieve dynamic tuning of holograms in the terahertz band. As an excellent phase change material, vanadium dioxide (VO2) is widely employed to dynamically manipulate electromagnetic waves. Here, VO2 meta-atoms are designed to manipulate phase and amplitude by changing the state of VO2 at 3.0 THz. These meta-atoms are composed of a VO2 block, silica spacer, and gold substrate. As the metallic VO2 is involved, 360° phase coverage is achieved by changing the dimension of VO2. The phase difference between the VO2 meta-atoms is approximately equal to 90°. Holograms are generated by aligning these meta-atoms. By combining convolution operations, holograms are deflected and reproduced. As the insulating VO2 is involved, the phase difference between the VO2 meta-atoms vanishes and the reflection amplitudes of the meta-atoms almost reach 100%. Using the phase transition of VO2, three types of metasurfaces are designed to manipulate holograms and they realize state switching of the hologram generator, state switching of hologram deflection, and state switching of the multi-beam hologram. Our work may find applications in optical holography and information privacy.

An electrical/thermal dual-controlled quad-functional terahertz metasurface absorber.

Although the design of graphene-based tunable broadband terahertz (THz) absorbers has attracted much attention, improving the functionality of the absorbers to adapt to different scenarios is still worth studying. This paper presents an innovative design of a quad-functional metasurface absorber (QMA) in the THz region, which can switch the absorption frequency/band by means of dual voltage/thermal manipulation. By electrically manipulating the chemical potential of graphene, the QMA can switch freely between the narrowband absorption mode ("NAM") and the broadband absorption mode ("BAM"), while thermally manipulating the phase transition of VO2 allows switching between the low-frequency absorption mode ("LAM") and the high-frequency absorption mode ("HAM"). Detailed mechanistic analysis shows that the "NAM" and "BAM" are due to the switching of the fundamental and second order graphene surface plasmon polariton (SPP) resonances, respectively, and the switching between "LAM" and "HAM" is due to the phase transformation of VO2. Furthermore, the QMA is polarization insensitive in all absorption modes and maintains excellent absorption performance at large angular incidence of TE- and TM-polarized waves. All the results indicate that the proposed QMA has great potential for stealth, sensing, switching, and filtering applications.

Metal-dielectric phase transition of VO2 assisted broadband and high-efficiency bifunctional metasurface in the terahertz frequency

The integration of multiple varied functionalities into a single and compact EM-based device is greatly demanded in EM integration due to their miniaturized configurations. In this paper, a broadband and high-efficiency bifunctional metasurface employing vanadium dioxide (VO2) is proposed for the terahertz (THz) frequencies. Due to the dielectric-to-metal transition of VO2, the metasurface can be dynamically tuned from a reflecting surface to a broadband absorber under low-temperature conditions. When VO2 is in the dielectric phase, the designed metasurface shows excellent reflection (> 96 %) in a broad frequency range from 0.5 THz to 4.5 THz. Once VO2 is heated up and transited to its metal phase, the proposed metasurface structure efficiently absorbs normally incident EM waves in the frequency range of 1.29 THz to 3.26 THz with an average absorption of 94.3 %. Moreover, the high absorption characterization of the proposed metasurface is maintained with a wide incident angle and is polarization-insensitive due to its symmetric structure, which makes it suitable for THz applications.

Ultrafast X-ray imaging of the light-induced phase transition in VO2

Using light to control transient phases in quantum materials is an emerging route to engineer new properties and functionality, with both thermal and non-thermal phases observed out of equilibrium. Transient phases are expected to be heterogeneous, either through photo-generated domain growth or by generating topological defects, and this impacts the dynamics of the system. However, this nanoscale heterogeneity has not been directly observed. Here we use time- and spectrally resolved coherent X-ray imaging to track the prototypical light-induced insulator-to-metal phase transition in vanadium dioxide on the nanoscale with femtosecond time resolution. We show that the early-time dynamics are independent of the initial spatial heterogeneity and observe a 200 fs switch to the metallic phase. A heterogeneous response emerges only after hundreds of picoseconds. Through spectroscopic imaging, we reveal that the transient metallic phase is a highly orthorhombically strained rutile metallic phase, an interpretation that is in contrast to those based on spatially averaged probes. Our results demonstrate the critical importance of spatially and spectrally resolved measurements for understanding and interpreting the transient phases of quantum materials.

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.

Vanadium dioxide-based terahertz metasurfaces for manipulating wavefronts with switchable polarization

As a unique phase transition material, vanadium dioxide (VO2) has been widely applied in switchable and modulating metasurfaces. In this work, VO2-based reflective meta-atoms are designed to achieve wavefront reconfiguration in terahertz frequency. When VO2 is metal, four optimized meta-atoms with 90o phase difference provide a full phase coverage, and they achieve high polarization conversion efficiency. After phase transition of VO2, the capability in the metallic state is lost. Thus, the corresponding phase coverage is switched from 360° to 0°, and the phenomenon of polarization conversion almost vanishes. Assembled by these meta-atoms, three metasurfaces are numerically presented. When VO2 is in the state of metal, they are anomalous reflector, metalens, and vortex beam generator. The presented metasurfaces own the abilities of wavefront manipulation and polarization conversion. When VO2 is in the insulating state, three metasurfaces are switched to specular reflector. Simulations are conducted to verify their abilities of wavefront modulation and polarization control. The proposed metasurfaces may have potential applications in the fields of terahertz switching, communication, and focusing.

Thermally reconfigurable topological protection routes for the 2D phononic system

Tunable topological insulators (TIs) with active tunability of frequency or path and better adaptability to the dynamic environment than conventional TIs have attracted great interest recently. The research directions of tunable TIs are mainly in the frequency domain and spatial domain. However, the most current research is mainly based on the former. Obviously, the tunable spatial path can bring more degrees of freedom to the application of TIs. Here, we present a topological protection system realized by a thermally tunable elastic film composed of VO2 covered on a patterned substrate. The topological phase transition is induced by breaking the symmetry of the unit cell on the substrate, and we pre-define the topological protection paths in each direction on the substrate. The conduction or blocking of elastic waves is achieved by using the metal-insulator phase transition of VO2, which enables intelligent TIs with reconfigurable propagation paths and reconfigurable interfaces. In addition, the reconfigurable topological path and the thermal modulation are presented by numerical simulations. Such elastic topological materials and their corresponding devices will significantly promote the development of TIs and are expected to enable the coupling with communication band electromagnetic waves and the realization of light-induced non-contact thermal modulation.

Harnessing the Metal-Insulator Transition of VO2 in Neuromorphic Computing.

Future-generation neuromorphic computing seeks to overcome the limitations of von Neumann architectures by colocating logic and memory functions, thereby emulating the function of neurons and synapses in the human brain. Despite remarkable demonstrations of high-fidelity neuronal emulation, the predictive design of neuromorphic circuits starting from knowledge of material transformations remains challenging. VO2 is an attractive candidate since it manifests a near-room-temperature, discontinuous, and hysteretic metal-insulator transition. The transition provides a nonlinear dynamical response to input signals, as needed to construct neuronal circuit elements. Strategies for tuning the transformation characteristics of VO2 based on modification of material properties, interfacial structure, and field couplings, are discussed. Dynamical modulation of transformation characteristics through in situ processing is discussed as a means of imbuing synaptic function. Mechanistic understanding of site-selective modification; external, epitaxial, and chemical strain; defect dynamics; and interfacial field coupling in modifying local atomistic structure, the implications therein for electronic structure, and ultimately, the tuning of transformation characteristics, is emphasized. Opportunities are highlighted for inverse design and for using design principles related to thermodynamics and kinetics of electronic transitions learned from VO2 to inform the design of new Mott materials, as well as to go beyond energy-efficient computation to manifest intelligence.

High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films.

Vanadium dioxide (VO2) is a popular candidate for electronic and optical switching applications due to its well-known semiconductor-metal transition. Its study is notoriously challenging due to the interplay of long- and short-range elastic distortions, as well as the symmetry change and the electronic structure changes. The inherent coupling of lattice and electronic degrees of freedom opens the avenue toward mechanical actuation of single domains. In this work, we show that we can manipulate and monitor the reversible semiconductor-to-metal transition of VO2 while applying a controlled amount of mechanical pressure by a nanosized metallic probe using an atomic force microscope. At a critical pressure, we can reversibly actuate the phase transition with a large modulation of the conductivity. Direct tunneling through the VO2-metal contact is observed as the main charge carrier injection mechanism before and after the phase transition of VO2. The tunneling barrier is formed by a very thin but persistently insulating surface layer of the VO2. The necessary pressure to induce the transition decreases with temperature. In addition, we measured the phase coexistence line in a hitherto unexplored regime. Our study provides valuable information on pressure-induced electronic modifications of the VO2 properties, as well as on nanoscale metal-oxide contacts, which can help in the future design of oxide electronics.

High-speed photothermally triggered bidirectional current switching in VO2/AlN/Si-based heterostructure device

The bidirectional current switching (BCS) of a two-terminal device based on a vanadium dioxide (VO2) thin film can be realized by harnessing the photothermally induced phase transition of VO2, triggered by laser pulses. Although this photothermally controlled BCS can achieve a large on-state current and high on/off current ratio compared to those obtained by other implementation approaches, a low repetition rate (< 10 Hz) and slow response time (tens of milliseconds) still restrict its practical use. To tackle these issues, we analyzed the fundamental cause of the performance limit of the BCS through simulations using COMSOL Multiphysics and found that the thermal conductivity of the substrate and the critical temperature of the VO2 film were mainly responsible for the switching speed of the BCS. On the basis of this analysis, we fabricated a VO2-based heterostructure device on a silicon substrate with a buffer layer of aluminum nitride. We measured the transient responses of the device current switched by directly irradiating the device with near-infrared laser pulses under various conditions. The highest-ever repetition rate (100 Hz) could be attained in a 50 mA BCS with 5 ms laser pulses. The response time was also remarkably improved to < 1 ms.

VOx Phase Mixture of Reduced Single Crystalline V2O5: VO2 Resistive Switching.

The strongly correlated electron material, vanadium dioxide (VO2), has seen considerable attention and research application in metal-oxide electronics due to its metal-to-insulator transition close to room temperature. Vacuum annealing a V2O5(010) single crystal results in Wadsley phases (VnO2n+1, n &gt; 1) and VO2. The resistance changes by a factor of 20 at 342 K, corresponding to the metal-to-insulator phase transition of VO2. Macroscopic voltage-current measurements with a probe separation on the millimetre scale result in Joule heating-induced resistive switching at extremely low voltages of under a volt. This can reduce the hysteresis and facilitate low temperature operation of VO2 devices, of potential benefit for switching speed and device stability. This is correlated to the low resistance of the system at temperatures below the transition. High-resolution transmission electron microscopy measurements reveal a complex structural relationship between V2O5, VO2 and V6O13 crystallites. Percolation paths incorporating both VO2 and metallic V6O13 are revealed, which can reduce the resistance below the transition and result in exceptionally low voltage resistive switching.

Metal-Insulator Transition Detection of Vanadium Dioxide Thin Films by Visible Light Reflection.

Vanadium dioxide (VO2)-based thin films have received considerable attention in recent years due to their superior performance in creating next-generation color-rendering materials. The near-room-temperature metal-insulator transition of VO2 promises the advantage of active color tuning in the visible wavelength range. Although various results of dynamic color generation combined with plasmonic nanostructures are currently being investigated, so far, very few studies have addressed the visible-light optical performance of pure VO2 thin films prepared on conventional substrates. This article shows in detail the phase-transition behavior of VO2 thin films in the visible wavelength range of 400-750 nm prepared on glass with subsequent annealing at temperatures of 350, 400, 450, and 500 °C. The results show an anomalous phase transition reducing the overall RGB reflectivity correlated with the crystallization behavior of the VO2 phase and scattering effect. The sample annealed at 350 °C shows the smallest phase transition at 47 °C, correlating with a crystallite size of 7 nm. The blue band reflectivity anomaly after annealing at 450 °C was considered an effect of the secondary reflection. The results of this research could play a huge role in the production of active-switching photonic devices, color-managed reflectors, and temperature indicators.