Transition In VO2 Research Articles

Periodic Arrays of 3D AuNP‐Capped VO2 Shells and Their Temperature‐Tunable SERS Performance

AbstractIn this work, periodic arrays of 3D composite structures consisting of gold nanoparticle (AuNP) capped VO2 shells on self‐assembled highly‐ordered SiO2 microspheres have been designed and fabricated, and their temperature‐tunable surface‐enhanced Raman scattering (SERS) performance is investigated. The results show that the SERS intensity enhancement factor (EF) for R6G molecules adsorbed on the AuNP‐capped VO2 shells increases from 8.3 × 105 to 3.9 × 106 when triggering the semiconductor‐to‐metal phase transition of VO2 by temperature rise, which strengthens the plasmonic coupling between the AuNPs and VO2 shells and consequently leads to a larger electric near‐field enhancement at the gaps between neighboring AuNPs. Such periodic arrays of AuNP‐capped VO2 shells with tunable plasmonic coupling can provide great opportunities for various optical applications, such as quantitative SERS detection of analytes, photothermal nano‐regulators, and biosensing in general.

Direct in situ observation of the percolation transition in VO2 thin film by peak-shift spectroscopy

When a thin film undergoes a percolative insulator-to-metal phase transition, the real part of the dielectric constant dramatically changes; it diverges close to the percolation threshold and becomes negative in the metallic phase. To date, the percolative nature of this phase transition has been monitored by near-field imaging or conventional spectroscopy. Although such methods are successful, they require laborious raster scanning or complicated theoretical fitting processes. In this study, using the strong coupling between slot antennas and a phase-transition material, a direct step-by-step investigation of the percolation transition can be demonstrated without any scanning or fitting processes. The percolation threshold can be identified in real time by monitoring the peak position, because the resonance transmission through the slot antenna array is highly affected by the dielectric environment. The work presented in this study could potentially contribute to the characterization of phase-transition materials to be used in memory devices or active metamaterials.

Dynamically controlled asymmetric transmission of linearly polarized waves in VO2-integrated Dirac semimetal metamaterials

We numerically demonstrated a novel chiral metamaterial to achieve broadband asymmetric transmission (AT) of linearly polarized electromagnetic waves in terahertz (THz) band. The proposed metamaterial unit cell exhibits no rotational symmetry with vanadium dioxide (VO2) inclusion embedded between Dirac semimetals (DSMs) pattern. The resonant frequency of AT can be dynamically tunable by varying the Fermi energy (E F) of the DSMs. The insulator-to-metal phase transition of VO2 enables the amplitude of the AT to be dynamically tailored. The transmission coefficient |Tyx | can be adjusted from 0.756 to nearly 0 by modifying the conductivity of VO2. Meanwhile, the AT parameter intensity of linearly polarized incidence can be actively controlled from 0.55 to almost 0, leading to a switch for AT. When VO2 is in its insulator state, the proposed device achieves broadband AT parameter greater than 0.5 from 1.21 THz to 1.80 THz with a bandwidth of 0.59 THz. When the incident wave propagates along the backward (–z) direction, the cross-polarized transmission | Tyx | reaches a peak value 0.756 at 1.32 THz, whereas the value of | Txy | well below 0.157 in the concerned frequency. On the other hand, the co-polarized transmission |Txx | and |Tyy | remained equal in the whole frequency range. This work provides a novel approach in developing broadband, tunable, as well as switchable AT electromagnetic devices.

Direct comparison of the electrical, optical, and structural phase transitions of VO2 on ZnO nanostructures

We examined the temperature-dependent electrical, optical, and structural properties of VO2 on ZnO nanorods with different lengths in the temperature range from 30 to 100 °C. ZnO nanorods with a uniform length were grown on Al2O3 substrates using a metal organic chemical vapor deposition, and subsequently, VO2 was ex-situ deposited on ZnO nanorods/Al2O3 templates using a sputtering deposition. The optical properties of the VO2/ZnO nanorods were measured simultaneously with direct current (DC) resistance using the reflectivity of an infrared (IR) laser beam with a wavelength of 790 nm. The local structural properties around V atoms of VO2/ZnO nanorods were simultaneously measured with the DC resistance using x-ray absorption fine structure at the V K edge. Direct comparison of the temperature-dependent resistance, IR reflectivity, and local structure reveals that an optical phase transition first occurs, a structural phase transition follows, and an insulator-to-metal transition finally appears during heating.

Narrowband-to-broadband switchable and polarization-insensitive terahertz metasurface absorber enabled by phase-change material

A terahertz absorber with controllable and switchable bandwidth that is insensitive to polarization is of great interest. Here, we propose and demonstrate a metasurface absorber with switchable bandwidth based on a phase-change material of vanadium dioxide (VO2) and verify its performance by finite element method simulations. The metasurface absorber is composed of a hybrid cross fractal as a resonator separated from a gold ground plane by a polyimide spacer. Switching from narrowband to broadband absorber is achieved via connecting VO2 patches to the gold first-order cross fractal converting the resonator to a third-order cross fractal. In the insulator phase of VO2, the main narrowband absorption occurs at the frequency of 6.05 THz with a 0.99 absorption and a full-width half-maximum (FWHM) of 0.35 THz. Upon insulator-to-metal transition of VO2, the metasurface achieves a broadband absorption with FWHM of 6.17 THz. The simulations indicate that by controlling the partial phase transition of VO2, we can tune the bandwidth and absorption level of the absorber. Moreover, the designed absorber is insensitive to polarization due to symmetry and works well for a very wide range of incident angles. In the metallic state of VO2, the absorber has an absorption exceeding 0.5 in the 3.57–8.45 THz frequency range with incident angles up to 65°.

Large‐Area Plasmonic Switches Based on Crystalline Au Nanoparticles Between VO2 Layers for Enhanced Switching in Visible–Near‐Infrared Regime

Vanadium dioxide (VO2) undergoes a reversible phase change, from semiconducting to metallic, with a change in temperature. Herein, plasmonic switches–based on gold nanoparticles (Au NPs) embedded between layers of VO2 films–that exhibit enhanced switching of the optical transmission as the temperature is increased above the VO2 phase transition threshold are presented. The operating wavelength of the switching device can be tuned between 650 and 1000 nm using the localized surface plasmon resonance properties of the Au NPs. This is for the first time that plasmonic switches have been demonstrated in the visible spectral regime (i.e., for wavelengths shorter than 700 nm) when VO2 is employed as the phase change material. A higher switchability, i.e., a higher extinction ratio upon phase transition of VO2, than what has been shown earlier in the spectral regions less than 1000 nm is demonstrated experimentally and theoretically. All the device layers are grown at the same substrate temperature and laser intensity via a single‐vacuum pulsed laser deposition process, leading to a simplified large‐area fabrication process. It is also shown for the first time that the Au NPs embedded in the proposed Au‐VO2 nanocomposites are crystalline.

Influence of VO2 based structures and smart coatings on weather resistance for boosting the thermochromic properties of smart window applications.

Vanadium dioxide (VO2)-based energy-saving smart films or coatings aroused great interest in scientific research and industry due to the reversible crystalline structural transition of VO2 from the monoclinic to tetragonal phase around room temperature, which can induce significant changes in transmittance and reflectance in the infrared (IR) range. However, there are still some obstacles for commercial application of VO2-based films or coatings in our daily life, such as the high phase transition temperature (68 °C), low luminous transmittance, solar modulation ability, and poor environmental stability. Particularly, due to its active nature chemically, VO2 is prone to gradual oxidation, causing deterioration of optical properties during very long life span of windows. In this review, the recent progress in enhancing the thermochromic properties of VO2-hybrid materials especially based on environmental stability has been summarized for the first time in terms of structural modifications such as core-shell structures for nanoparticles and nanorods and thin-films with single layer, layer-by-layer, and sandwich-like structures due to their excellent results for improving environmental stability. Moreover, future development trends have also been presented to promote the goal of commercial production of VO2 smart coatings.

Decoupling the metal-insulator transition temperature and hysteresis of VO2 using Ge alloying and oxygen vacancies.

The metal-to-insulator transition of VO2 underpins applications in thermochromics, neuromorphic computing, and infrared vision. Ge alloying is shown to elevate the transition temperature by promoting V-V dimerization, thereby expanding the stability of the monoclinic phase to higher temperatures. By suppressing the propensity for oxygen vacancy formation, Ge alloying renders the hysteresis of the transition exquisitely sensitive to oxygen stoichiometry.

Tunable and reflective polarization converter based on single-layer vanadium dioxide-integrated metasurface in terahertz region

A single-layer and actively tunable reflective polarization converter is proposed, which is composed of periodical two-dimensional unit cell of inclined split circular ring filled into by vanadium oxide (VO2) and central rod. Through numerical simulation obtained by the CST Microwave Studio, we find that the broad bandwidth of reflective cross-polarization conversion above 90% is normally obtained from 2.03 to 5 THz at the temperature about 25 °C for both the linearly and circularly polarized wave incidence. At the same time, the polarization conversion ratio (PCR) and energy conversion ratio (ECR) have been furtherly investigated, which reflect high cross-polarization conversion efficiency with nearly no energy loss at room temperature of 25 °C. Thereafter, namely, the cross-polarization conversion can be achieved by theoretical analysis from the views on qualitative variables of polarization azimuth angle (θ), ellipticity (η) and symmetrical polarization decomposition. What is more, the device is sensitive to the incident angles which result into the split reflected frequency spectrum. Afterwards, the actively dynamic responses for reflective frequency spectrum depending on the insulator-to-metal phase transition of VO2 have been investigated. To be demonstrated, the physical mechanism of changeable polarization conversion has been discussed by the distributions of current densities. Lastly, also, the influences of structural parameter variation can be further studied. According to the interesting results, the method provides a strategy that manipulate active and passive metadevices for polarization converter, wireless communication in THz wavelength domain.

Photothermal polydopamine coated VO2 nanoparticle thin film with enhanced optical property and stability

Vanadium dioxide (VO2) exhibits great potential in smart windows due to its phase-transition property. Herein, polydopamine (PDA), coated on VO2 nanoparticles, is employed to replace doping to trigger the phase transition of VO2 at ambient temperature based on its photothermal effect. The VO2@PDA thin films not only can avoid optical property degradation caused by doping, but also exhibit better optical property than VO2 films due to the improved dispersion after coating PDA that the solar energy modulation (ΔTsol) is improved from 11.2 to 14.5% when the luminous transmittance (Tlum) remains unchanged. In addition, VO2@PDA nanoparticles remain stable under acidic and oxygenic condition, which is attributed to the stable PDA shell and the interfacial interaction between VO2 core and PDA shell. This work provides an novel method to replace doping treatment to promote the phase transition of VO2.

Probing Relaxation Dynamics and Stepped Domain Switching in Boron‐Alloyed VO2

AbstractThe characteristic metal–insulator phase transition (MIT) in vanadium dioxide results in nonlinear electrical transport behavior, allowing VO2 devices to imitate the complex functions of neurological behavior. Chemical doping is an established method for varying the properties of the MIT, and interstitial dopant boron has been shown to generate a unique dynamic relaxation effect in individual B‐VO2 particles. This paper describes the first demonstration of an electrically stimulated B‐VO2 proto‐device which manifests a time‐dependent critical transformation temperature and switching voltage derived from the coupling of dopant diffusion dynamics and the metal–insulator transition of VO2. During quasi‐steady current‐driven transitions, the electrical responses of B‐VO2 proto‐devices show a step‐by‐step progression through the phase transformation, evidencing domain transformations within individual particles. The dynamic relaxation effect is shown to increase the critical switching voltage by up to 41% (ΔVcrit = 0.13 V) and also to increase the resistivity of the M1 phase of B‐VO2 by 14%, imbuing a memristive response derived from intrinsic material properties. These observations demonstrate the dynamic relaxation effect in B‐VO2 proto‐devices whose electrical transport responses can be adjusted by electronic phase transitions triggered by temperature but also by time as a result of intrinsic dynamics of interstitial dopants.

Nanotextured Dynamics of a Light-Induced Phase Transition in VO2.

We investigate transient nanotextured heterogeneity in vanadium dioxide (VO2) thin films during a light-induced insulator-to-metal transition (IMT). Time-resolved scanning near-field optical microscopy (Tr-SNOM) is used to study VO2 across a wide parameter space of infrared frequencies, picosecond time scales, and elevated steady-state temperatures with nanoscale spatial resolution. Room temperature, steady-state, phonon enhanced nano-optical contrast reveals preexisting "hidden" disorder. The observed contrast is associated with inequivalent twin domain structures. Upon thermal or optical initiation of the IMT, coexisting metallic and insulating regions are observed. Correlations between the transient and steady-state nano-optical textures reveal that heterogeneous nucleation is partially anchored to twin domain interfaces and grain boundaries. Ultrafast nanoscopic dynamics enable quantification of the growth rate and bound the nucleation rate. Finally, we deterministically anchor photoinduced nucleation to predefined nanoscopic regions by locally enhancing the electric field of pump radiation using nanoantennas and monitor the on-demand emergent metallicity in space and time.

Room Temperature Insulator‐to‐Metal Transition of VO2/TiO2 Epitaxial Bilayer Films Grown on M‐plane Sapphire Substrates

AbstractSince the optoelectronic properties of VO2 are modulated around the critical temperature (Tc) of insulator‐to‐metal transition, VO2 is a promising candidate material for smart window. However, the Tc of bulk VO2 is rather high 341 K (68 °C) and therefore, needs to be decreased down to near temperature for practical applications. Although Tc can be reduced to room temperature if rutile TiO2 crystal is used as the substrate, it is not technologically viable for large‐scale applications because of the size limitation of available TiO2 crystal. Here, this work shows that the Tc can be reduced to near room temperature using M‐plane sapphire as the substrate. VO2/TiO2 bilayer films are fabricated with varied thicknesses on M‐plane sapphire [(100) α‐Al2O3] substrates. The 5.5‐nm‐thick VO2 film on 200‐nm‐thick TiO2 buffered sapphire substrate shows clear insulator‐to‐metal transition at ≈305 K (32 °C). It is also found that the insulator‐to‐metal transition is sensitive to the in‐plane lattice distortion, which induces carrier generation. The systematic study on the effect of in‐plane lattice distortion on the insulator‐to‐metal transition of VO2 will be useful for the practical application of VO2 as an active material of smart window.

Quantum cluster algorithm for data classification

We present a quantum algorithm for data classification based on the nearest-neighbor learning algorithm. The classification algorithm is divided into two steps: Firstly, data in the same class is divided into smaller groups with sublabels assisting building boundaries between data with different labels. Secondly we construct a quantum circuit for classification that contains multi control gates. The algorithm is easy to implement and efficient in predicting the labels of test data. To illustrate the power and efficiency of this approach, we construct the phase transition diagram for the metal-insulator transition of VO2, using limited trained experimental data, where VO2 is a typical strongly correlated electron materials, and the metallic-insulating phase transition has drawn much attention in condensed matter physics. Moreover, we demonstrate our algorithm on the classification of randomly generated data and the classification of entanglement for various Werner states, where the training sets can not be divided by a single curve, instead, more than one curves are required to separate them apart perfectly. Our preliminary result shows considerable potential for various classification problems, particularly for constructing different phases in materials.

Determination of the effective thermal conductivity of particulate composites based on VO2 and SiO2

The effective thermal conductivity of composites made up of VO2 (SiO2) spherical particles randomly distributed and embedded in a SiO2 (VO2) matrix are numerically studied in a range of temperatures around the metal-insulator transition of VO2. This is done by means of three-dimensional finite element simulations for different concentrations and sizes of the particles as well as various interface thermal resistances. Our results are validated against the Mori-Tanaka analytical model. In addition, we develop a numerical method to calculate the heat storage capacity for composites with VO2 particles dispersed into a SiO2 matrix. It is shown that: i) The effective thermal conductivity of VO2/SiO2 composites increases with the VO2 particles' size, while the one of SiO2/VO2 composites is pretty much independent of the SiO2 particles' radius. ii) At the VO2 transition temperature (342.5 K), the effective thermal conductivity of VO2/SiO2 composites increases significantly at a rate of 2.7 × 10−3 Wm−1K−2, such that its value doubles up the SiO2 matrix thermal conductivity at the particle concentration of 40.2%. By contrast, the effective thermal conductivity of SiO2/VO2 composites decreases at a rate of 8.6 × 10−3 Wm−1K−2. iii) The effective thermal conductivity is strongly affected by the thermal resistance in VO2/SiO2 composites, by contrast the resistance effect does not play an important role for particle volume fractions of SiO2 up to 34.1% in SiO2/VO2 composites. The Mori-Tanaka model and our simulations predict the same trend of the effective thermal conductivity values of VO2/SiO2 composites. However, the analytical model fails when the matrix is made up of VO2 and the volumetric fraction of SiO2 exceeds 34.1%. The latent heat storage capacity of VO2/SiO2 composites increases with the VO2 particles’ concentration, such that at 40.2%, it takes the value of 24553 J kg−1 (486.7 cal mol−1), which is about half that of the pure VO2.

Reversible modulation of metal–insulator transition in VO2 via chemically induced oxygen migration

Metal-insulator transition (MIT), an intriguing correlated phenomenon induced by the subtle competition of the electrons' repulsive Coulomb interaction and kinetic energy, is of great potential use for electronic applications due to the dramatic change in resistivity. Here, we demonstrate a reversible control of MIT in VO2 films via oxygen stoichiometry engineering. By facilely depositing and dissolving a water-soluble yet oxygen-active Sr3Al2O6 capping layer atop VO2 at room temperature, oxygen ions can reversibly migrate between VO2 and Sr3Al2O6, resulting in a gradual suppression and a complete recovery of MIT in VO2. The migration of the oxygen ions is evidenced in a combination of transport measurement, structural characterization, and first-principles calculations. This approach of chemically induced oxygen migration using a water-dissolvable adjacent layer could be useful for advanced electronic and iontronic devices and studying oxygen stoichiometry effects on the MIT.

Tunable dual-band and high-quality-factor perfect absorption based on VO2-assisted metasurfaces.

Perfect absorbers with high quality factors (Q-factors) are of great practical significance for optical filtering and sensing. Moreover, tunable multiwavelength absorbers provide a multitude of possibilities for realizing multispectral light intensity manipulation and optical switches. In this study, we demonstrate the use of vanadium dioxide (VO2)-assisted metasurfaces for tunable dual-band and high-quality-factor perfect absorption in the mid-infrared region. In addition, we discuss the potential applications of these metasurfaces in reflective intensity manipulation and optical switching. The Q-factors of the dual-band perfect absorption in the proposed metasurfaces are greater than 1000, which can be attributed to the low radiative loss induced by the guided-mode resonances and low intrinsic loss from the constituent materials. By utilizing the insulator-metal transition in VO2, we further proved that a continuous tuning of the reflectance with a large modulation depth (31.8 dB) can be realized in the designed metasurface accompanied by a dual-channel switching effect. The proposed VO2-assisted metasurfaces have potential applications in dynamic and multifunctional optical devices, such as tunable multiband filters, mid-infrared biochemical sensors, optical switches, and optical modulators.

Inherent stochasticity during insulator–metal transition in VO2

Vanadium dioxide (VO2), which exhibits a near-room-temperature insulator-metal transition, has great potential in applications of neuromorphic computing devices. Although its volatile switching property, which could emulate neuron spiking, has been studied widely, nanoscale studies of the structural stochasticity across the phase transition are still lacking. In this study, using in situ transmission electron microscopy and ex situ resistive switching measurement, we successfully characterized the structural phase transition between monoclinic and rutile VO2 at local areas in planar VO2/TiO2 device configuration under external biasing. After each resistive switching, different VO2 monoclinic crystal orientations are observed, forming different equilibrium states. We have evaluated a statistical cycle-to-cycle variation, demonstrated a stochastic nature of the volatile resistive switching, and presented an approach to study in-plane structural anisotropy. Our microscopic studies move a big step forward toward understanding the volatile switching mechanisms and the related applications of VO2 as the key material of neuromorphic computing.

Switchable bifunctional metasurface based on VO2 for ultra-broadband polarization conversion and perfect absorption in same infrared waveband

Conventional metasurfaces cannot adjust the electromagnetic responses once the structures are determined. In this paper, we numerically demonstrate a thermally switchable bifunctional metasurface based on the phase change material vanadium dioxide (VO2) applied to the mid-infrared band. The dual-layer ring resonator consists of an Ag split ring and a VO2 ring, which can achieve ultra-broadband polarization conversion and perfect absorption through the insulator-to-metal phase transition of VO2. When VO2 is in the insulating state, the designed metasurface acts as polarization converter with polarization conversion ratio of over 95% in the wavelength range of 2500 nm to 3500 nm. When VO2 is in the metallic state, the metasurface behaves as perfect absorber with over 90% absorptivity in the region from 2900 nm to 3700 nm. And the operating bandwidths of the two functions are highly overlapped. The ultra-broadband polarization conversion properties are mainly derived from the closed multiple electric and magnetic resonances. Single localized magnetic resonance results in a flat absorption spectrum. The proposed metasurface may provide new ideas for the design of tunable integrated optical devices.

Multifunctional Flexible Vanadium Dioxide Films

ConspectusAs a typical phase transition material, vanadium dioxide (VO2) has attracted much attention due to its amazing metal–insulator transition (MIT) at the critical temperature of 68 °C, which could be driven by multiple stimuli, including electricity, thermal irradiation, THz waves, strain, etc. In the MIT process, VO2 exhibits significant changes in its structure from monoclinic structure at low temperature to rutile structure at high temperature, accompanied by significant modulation of physical properties, such as the infrared transmittance from high to low and a resistivity drop over 5 magnitudes. Based on these features, VO2 has functioned as thermochromic coatings, sensors, switches, electronic devices, actuators, etc. However, the vanadium element possesses multiple valent states and can produce complicated thermodynamics of vanadium oxides. The fabrication of tetravalence VO2 with desirable phase-transition features usually requires rigorous fabrication conditions with a precisely controlled atmosphere for stoichiometric components and high temperatures for good crystallinity. Under these circumstances, it is difficult to directly fabricate crystalline VO2 films on flexible polymer substrates because most of them could not stand such high temperatures (usually >400 °C), which will lead to the loss of the substrate functionality. However, the featured phase transition of VO2 makes it a promising candidate for future flexible devices, while VO2 owns great potential as flexible optical coatings, flexible sensors, flexible electronics, etc. Hence, research on flexible VO2 films is necessary and could largely pave the way to the application field of VO2 material. In this Account, we start with a brief introduction to phase transition properties of VO2 to reveal the intrinsic advantages as key materials in flexible devices. Next, multifunctional devices based on flexible VO2 films with different forms and characteristics are presented, including (1) flexible VO2-film-based thermochromic smart windows for energy-saving function depending on the change of environmental temperatures, (2) flexible VO2 films optical devices based on the changing emissivity of VO2, (3) flexible VO2 films with compatibility as multifunctional sensors, (4) next-generation flexible electronics based on VO2 films, and (5) flexible VO2 actuators with significant mechanical motions under external stimuli. Meanwhile, various fabrication technologies of flexible VO2 films have been introduced and discussed. Finally, we end the Account with an overview of the remaining challenges and new opportunities that have been opened up for vanadium dioxide in new forms of flexible optical and electronic devices. We hope this Account will inspire new innovative designs, fabrication approaches, and more possible functions of flexible VO2 films in future work.