Phase Transition Of VO2 Research Articles

Optically tunable guide-mode resonance grating based on VO2 phase transition material

In this paper, highly efficient tunable guide-mode resonance (GMR) grating patterned on top of a film of the phase transition material, vanadium dioxide (VO2), is demonstrated. The optimal optical transmission and tuning can be achieved by optimizing the device structure. Based on the large insulator-to-metal transition (IMT) characteristic tuned by thermally driven of VO2 in the near-infrared range, we show that the full width at half maximum (FWHM) of about 100 nm of the transmittance resonance spectra of the dielectric GMR in VO2 grating offers remarkable extinction modulation depths (68%) at 1535 nm for tunability. The tunability is accompanied by a hysteresis-like behavior that can be exploited for versatile memory effects. In addition, we experimentally demonstrate tuning of the reflection phase over 20°. This work further improves the research topic of GMR and expands the design of novel reconfigurable devices for photonics.

Enable thermal insulation improvement of Al2O3 aerogels doped with graded oriented extinction phase-change fiber at large temperature differences

Phase change materials (PCMs) play essential roles in extending transient thermal insulation in the aerospace field; however, the extinction properties variations during the phase change process have seldom been investigated. The present study presents an extinction phase-change fiber (EPCF) design, i.e., composed of extinction material shell as well as the PCM core and dispersed in Al2O3 aerogel. The extinction performance (especially after the phase transition) and the insulation performances at steady and transient states according to specific infrared radiation wavelength modulation are numerically investigated. This study reveals that the extinction characteristic of VO2@SiO2 is superior to that of SiO2 and TiO2 fiber. The extinction performance is improved by ∼8.37% after the phase transition of VO2. Besides, the time constant of VO2@SiO2 increased by ∼2.17 times the pure aerogel at the insulation aspect. Similar conclusions could be self-evidently found in some other EPCFs; furthermore, multi-layer EPCFs design is studied for spectral modulation principle combined with the transient heat transfer characteristic. Our developmental work could inspire some novel ideas on insulation material design in the future and improve the understanding of phase change insulation.

Kinetically-decoupled electrical and structural phase transitions in VO2

Vanadium dioxide (VO2) has drawn significant attention for its near room temperature insulator to metal transition and associated structural phase transition. The underlying Physics behind the temperature induced insulator to metal and concomitant structural phase transition in VO2 is yet to be fully understood. We have investigated the kinetics of the above phase transition behaviors of VO2 with the help of resistivity measurements and Raman spectroscopy. Resistance thermal hysteresis scaling and relaxation measurements across the temperature induced insulator to metal transition reveal the unusual behaviour of this first-order phase transition, whereas Raman relaxation measurements show that the temperature induced structural phase transition in VO2 follows usual behaviour and is consistent with mean field prediction. At higher temperature sweeping rates decoupling of insulator to metal transition and structural phase transition have been confirmed. The observed anomalous first order phase transition behavior in VO2 is attributed to the unconventional quasi particle dynamics, i.e. significantly lowered electronic thermal conductivity across insulator to metal transition, which is confirmed by ultrafast optical pump-probe time domain thermoreflectance measurements.

Dynamically switchable self-focused thermal emission.

The ability to manipulate thermal emission is paramount to the advancement of a wide variety of fields such as thermal management, sensing and thermophotovoltaics. In this work, we propose a microphotonic lens for achieving temperature-switchable self-focused thermal emission. By utilizing the coupling between isotropic localized resonators and the phase change properties of VO2, we design a lens that selectively emits focused radiation at a wavelength of 4 µm when operated above the phase transition temperature of VO2. Through direct calculation of thermal emission, we show that our lens produces a clear focal spot at the designed focal length above the phase transition of VO2 while emitting a maximum relative focal plane intensity that is 330 times lower below it. Such microphotonic devices capable of producing temperature-dependent focused thermal emission could benefit several applications such as thermal management and thermophotovoltaics while paving the way for next-generation contact-free sensing and on-chip infrared communication.

Frequency Tuning of In-Plane Comb Drive MEMS Resonators Due to VO2 Phase Transition

This letter reports the thermal-mechanical tuning capability of an in-plane comb drive resonator using VO2 phase transition material. By inducing the insulator-to-metal transition (IMT) using a heat conduction method, a shift of approximately 2% was observed. The frequency tuning was attributed to the induced stresses during the IMT. [2022-0200]

Intelligent shielding material based on VO2 with tunable near-field and far-field electromagnetic response

Intelligent electromagnetic interference (EMI) shielding materials, which can generate reversible and real-time EM responses to external stimuli, have an attractive prospect in smart wearable electronics. Herein, we design and fabricate flexible and intelligent vanadium dioxide (VO2)/cellulose nanofiber (CNF) shielding composites, showing a controllable EM response based on the reversible metal-to-insulator transition characteristic of VO2 by thermal stimuli. With the phase transition of VO2 between the insulating M phase and metallic R phase at ∼68 °C, the electrical conductivity of VO2/CNF composites is reversibly changed between ∼0.3 and ∼446.5 S/m, leading to a tunable shielding effectiveness (SE) between ∼18.7 and ∼52.8 dB in the frequency of 8.2–12.4 GHz. The lattice strain of VO2 driven by thermal stimuli radically changes the EM parameters and impedance matching of VO2/CNF composites, which dominates the attenuation mechanism of EM waves. Uniquely, we demonstrate the VO2/CNF composites with a proof of concept to reversibly control the on and off of the wireless transmission. Moreover, the VO2/CNF composites exhibit an outstanding near-field shielding performance, which is potential to be applied in the electronic packaging field. This intelligent EMI shielding material has a broad prospect in the smart EM devices of next generation.

Photothermal Effect and Phase Transition in VO2 Enhanced by Plasmonic Particles

Phase change metasurfaces based on VO2, which are pre-heated with electric current and optically addressed by projected structured light hologram, are considered to become a new paradigm in programmed THz/middle IR flat optics. Macroscopic quasi-homogeneous arrays of Au nanoparticles show large near IR absorption and a significant photothermal effect capable of boosting a light-triggered switching of VO2 and are to be carefully examined. We propose a new approach to simultaneously probe the altered temperature and electric conductivity of a hybrid Au particle-VO2 film composite by monitoring a phase shift and attenuating a surface acoustic wave in a YX128° cut LiNbO3 substrate. The method shows a temperature resolution of 0.1 °C comparable with the best existing techniques for studying nanoobjects and surfaces. The laser-induced photothermal effects were characterized in a macroscopic array of Au nanostars (AuNSts) with different surface coverage. In a monolayer of 10 nm Au, coupled plasmonic nanoparticles were deposited on the LiNbO3 substrate. An optically triggered insulator-metal transition assisted by photothermal effect in AuNSts/VO2/TiO2/LiNbO3 composites was studied at varied light power. We believe that the proposed SAW-based method is of significant importance for the characterization and optimization of radiation absorbing or/and electrically heated elements of metasurfaces and other devices for lab-on-chip and optical communication/processor technology.

Tunable metasurface for independent controlling radar stealth properties via geometric and propagation phase modulation.

Metasurfaces have been verified as an ideal way to control electromagnetic waves within an optically thin interface. In this paper, a design method of a tunable metasurface integrated with vanadium dioxide (VO2) is proposed to realize independent control of geometric and propagation phase modulation. The reversible conversion of VO2 between insulator phase and metal phase can be realized by controlling the ambient temperature, which enables the metasurface to be switched quickly between split-ring and double-ring structures. The phase characteristics of 2-bit coding units and the electromagnetic scattering characteristics of arrays composed of different arrangements are analyzed in detail, which confirms the independence of geometric and propagation phase modulation in the tunable metasurface. The experimental results demonstrate that the fabricated regular array and random array samples have different broadband low reflection frequency bands before and after the phase transition of VO2, and the 10 dB reflectivity reduction bands can be switched quickly between C/X and Ku bands, which are in good agreement with the numerical simulation. This method realizes the switching function of metasurface modulation mode by controlling the ambient temperature, which provides a flexible and feasible idea for the design and fabrication of stealth metasurfaces.

Boosting the efficiency of Cu2ZnSnS4 solar cells with VO2 phase transition photonic crystal

Photonic crystal (PhC) has been studied for their potential to improve the efficiency of Cu2ZnSnS4 solar cells by increasing the generated photocurrent by integrating it as a back reflector with almost zero transmission through the absorption active zone of the solar cell. It was found that the thickness of PhC layers greatly affects the width of the photonic bandgap and that increasing the thickness of VO2 causes it to shift to a higher wavelength range. The PhC layers were added at the back side of the solar cell in two different configurations: (Monoclinic (M) VO2/TiO2) and (Tetragonal (T) VO2/TiO2) via SCAPS model. The study found that the (M VO2/TiO2) configuration led to an enhancement of the device's efficiency from 11.02 to 12.79%, while the (T VO2/TiO2) reaches 16.88%. The study concluded that the PhC layers enhance the light-matter coupling and photonic coupling and improvement in the device's performance.

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.

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

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

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

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

Effect of surface modification by Ar+ ion irradiation on thermal hysteresis of VO2

Vanadium dioxide (VO2) undergoes a metal–insulator phase transition at ∼70 °C. As this is a first-order phase transition, VO2 exhibits thermal hysteresis. The reflectivity and electrical resistivity of VO2 drastically change at insulator-to-metal (TIMT) and metal-to-insulator (TMIT) transition temperatures during heating and cooling, respectively. For smart glass and thermal memory applications employing VO2, the origin and control factor of thermal hysteresis must be investigated. Additional elemental doping and nano-structuring of VO2 affect the thermal hysteresis width. However, the factors determining TIMT and TMIT remain unclear. TIMT and TMIT can be modified by irradiating Ar+ on the surface of VO2 nanostructures with varying Ar+ irradiation doses (nAr+) at 1 keV. The temperature-dependent reflectivity against IR light is evaluated. For VO2, TIMT decreases with nAr+ = 3.9 × 1014 cm−2; TMIT increases with nAr+ > 3.9 × 1015 cm−2. Ar+ irradiation decreases the thermal hysteresis width. Because the expected penetration depth of Ar+ at 1 keV into the VO2 surface is <6 nm, the VO2 chemical state at the outermost surface is investigated using x-ray absorption spectroscopy with soft x-ray irradiation. The V L-edge peak energy decreases with increasing nAr+ . Ar+ irradiation reduces V only at the outermost surface state. TIMT is more sensitive than TMIT to the reduction of V. The reduction of only a small fraction at the surface affects the phase transition of the entire VO2. These results are beneficial for understanding the cause of thermal hysteresis width and improving the performance of devices using VO2.

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.

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.

Temperature controlled yolk-shell-like VO2(M)/Ti3C2Tx composite films for high-performance electromagnetic interference shielding

With the rapid development of communication technology, it will lead to many electromagnetic wave (EMW) pollutions affecting the normal operation of precision instruments in industry. Then need smart electromagnetic interference (EMI) shielding material to protect the instrument from electromagnetic radiation. In this paper, multi-dimensional architectures yolk-shell-like VO2(M)/Ti3C2Tx MXene (MXene) composite films (VM) were successfully prepared by hydrothermal treatment and electrostatic self-assembly. Yolk-shell-like VO2(M) microspheres were riveted onto few-layer MXene nanosheets as a smart EMI shielding material. Above 65 °C, the metal-insulator phase transition (MIT) of VO2(M) changes from the semiconductor state to the metallic state. VM composite films possess temperature controlled EMI shielding properties, which have not been reported before. When the thickness is 29.06 µm, the composite sample exhibits an EMI SE of 43.5 dB at 85 °C, which is 8 dB higher than room temperature. Besides, the few-layer structure and the unique yolk-shell-like structure of VM composite films exhibit excellent dielectric loss. The introduction of few-layer MXene nanosheets improves the EMI shielding performance of yolk-shell-like VO2(M) microspheres and achieves optimized impedance matching. Therefore, VM composite films are expected to provide new ideas for smart EMI shielding materials.