Phase Transition Of VO2 Research Articles

Phase transition in thermochromic VO2 coatings grown by ac dual magnetron cathode sputtering

High quality of thermochromic VO2 coatings were grown onto ITO-coated quartz by thermal oxidation of ac magnetron sputtered vanadium oxide films. The thermochromic properties of VO2 coatings were investigated by means of IR transmittance and Raman scattering measurements as a function of temperature (up to 100 °C). It is observed that the IR transmittance of VO2 decreased by increasing temperature, that is the signature of the semiconductor-to-metallic transition (SMT). The associated transition temperature was found to be around 56 °C. At room temperature, the Raman analysis showed only the presence of vibrational modes of the pure VO2 phase. However, the temperature dependent Raman measurements showed that the scattered signals decreased by increasing the temperature and they disappeared completely when the VO2 switched to its metallic state. The structural phase transition from a monoclinic crystalline structure (at room temperature) to a tetragonal rutile type structure (at high temperature) appeared at relatively high temperature (~ 75 °C) compared to the SMT transition temperature (~ 56 °C). The evolution of vibrational Raman modes in the heating and cooling cycles showed relatively large hysteresis (~ 10 °C).

Influence of copper on electrical conductivity of glass-ceramics based on vanadium dioxide

The results of studying the electrical conductivity σ for the glass-ceramic systems VO2 ‒vanadium-phosphate glass (VPG) and VO2‒VPG‒SnO2 with the additions of Cu and Cu2O are presented. It is found that the jump of conductivity at the temperature of metal-semiconductor phase transition (MSPT) in VO2 (Tt = 341K) takes place only for glass-ceramics containing not more than 5 wt% of these additives. When their content exceeds 5 wt%, the VO2 content decreases sharply according to the data of differential thermal analysis. The reason for this is the oxidation-reduction reactions in the liquid phase between Cu and VO2 at ceramics synthesis. These reactions result in the appearance of Magneli phases in glass‒ceramics composition. The phase transitions in Magneli phases V4O7, V5O9, and V6O13 are indicated by the temperature dependence of σ, as bends of the straight lines in coordinates log10σ ~ 1/T at the phase transition temperatures Tt of these phases: 240K, 150K, and 130K. The activation energy of σ for T> Tt is lower than for T <Tt, which indicates a change in the structure of energy zones inherent to MSPT.

VO2 Phase-Transition-Based Vertical MEMS Microactuators

A vertical multilayer VO2 MEMS microactuator device is reported in this work. A 150-nm-thick VO2 layer is sandwiched between the top Pt and bottom indium tin oxide (ITO) electrodes. An equivalent thermal circuit is developed with a thermal response time of 0.39 ms. With the insulator-to-metal phase transition of VO2 at ~68 °C accompanied by large electrical and lattice changes, a 35- $\mu \text{m}$ -long VO2 actuator experiences 2- $\mu \text{m}$ deflection across the phase transition driven by heating and 0.22- $\mu \text{m}$ deflection when electrothermally actuated. The COMSOL finite element analysis is performed to determine the spatial distribution of the current flow and temperature profile and to verify the experimental measurements of strain-induced bending in this multilayer vertical structure. The obtained experimental result shows a good fit with the simulation analysis, which can in future be used to develop a generic understanding of such vertical MEMS devices.

On‐Demand Nanoscale Manipulations of Correlated Oxide Phases

AbstractControlling material properties at the nanoscale is a critical enabler of high performance electronic and photonic devices. A prototypical material example is VO2, where a structural phase transition in correlation with dramatic changes in resistivity, optical response, and thermal properties demonstrates particular technological importance. While the phase transition in VO2 can be controlled at macroscopic scales, reliable and reversible nanoscale control of the material phases has remained elusive. Here, reconfigurable nanoscale manipulations of VO2 from the pristine monoclinic semiconducting phase to either a stable monoclinic metallic phase, a metastable rutile metallic phase, or a layered insulating phase using an atomic force microscope is demonstrated at room temperature. The capability to directly write and erase arbitrary 2D patterns of different material phases with distinct optical and electrical properties builds a solid foundation for future reprogrammable multifunctional device engineering.

Electrical Pulse Driven Insulator-Metal Transition of VO2 Devices As Leaky-Integrate-and-Fire Artificial Neurons

Both artificial neurons and synapses are needed to efficiently implement artificial neural networks directly in hardware. Memristors have been actively investigated to implement synaptic functions. In contrast, the implementation of artificial neurons are still limited to Si-based circuit components, while single device based artificial neuron has not been well explored. In this study, we show that VO2 two-terminal devices, driven under electrical pulses, can be used to implement the leaky, integrate and fire function of spiking neurons. Since the thermally driven metal-insulator transition of VO2 is volatile and electrical pulses can produce Joule heating, while the heating and cooling have delay times, VO2 devices exhibit characteristic firing time and relaxation time, corresponding to timing difference of the exciting pulse and the resistance switching. In addition, the integrating effect of several small pulses can also trigger the firing from high resistance to low resistance transition. These characteristics suggest the potential using the volatile phase transition of VO2 for artificial spiking neuron. Experimental measurements are further elaborated by a 2-D heterogeneous resistor network based modelling. The insulator-metal transition in VO2 under electrical bias was simulated to confirm the dynamic formation of current filament between two electrodes, and the formation of this path is a flashing process during a single pulse in the pulse train when the integrate effect of these pulses arrives the threshold.

Design of far-field thermal rectifiers using gold–vanadium dioxide micro-gratings

We design thermal emitters based on gold micrograting structures with VO2-filled slots for enhanced far-field thermal rectification. We numerically calculate the thermal rectification ratio for two different approaches, peak extinction and peak shift. In peak extinction, the VO2 phase transition switches the radiative coupling of the surface plasmon on and off. In peak shift, the phase transition shifts the wavelength of the radiatively-coupled surface plasmon. We vary the extinction coefficient of VO2 to determine the effect on rectification for each approach. In both cases, the rectification ratio can be increased by increasing the VO2 loss in the metallic state. This suggests that highly efficient, microstructured thermal rectifiers can be achieved via suitable manipulation of the optical properties of VO2.

Experimental Study of the Insulator-to-Metal Phase Transition in VO2 Thin Films in the Microwave Range

Temperature dependence of insulator-to-metal phase transition in VO2 epitaxial thin films was studied in the microwave range. Biaxially textured VO2 thin films were obtained on 3-inch sapphire substrates by the method of chemical vapour deposition (MCVD). The change in the electrical resistance with temperature reached approximately 3 orders of magnitude for the obtained films, measured in the metal-insulator phase transition region. Using a high-temperature quasi-optical setup, the microwave features of the phase transition in VO2 thin films were studied at temperature. The hysteresis of microwave properties of the studied samples was observed in the temperature range of VO2 phase transition.

Tunable broadband, wide angle and lithography-free absorber in the near-infrared using an ultrathin VO2 film

We demonstrate a tunable broadband near-infrared absorber composed of VO2 /Al2O3/ITO layers with a facile lithography-free method. Simulated and experimental spectra show that absorption bands with above 90% absorptance can be tuned from 1.62 ∼ 2.42 μm to 1.2 ∼ 1.5 μm by thermally induced VO2 phase transition. Furthermore, the absorber with VO2 in the intermediate state can yield a broadband perfect absorption over the wavelength range of 1.2 ∼ 2.5 μm. Besides high absorption performance can still be remained at the incident up to 70° for both polarizations due to the presence of ultrathin layer of VO2 (λ/52) in the absorber.

Localized surface plasmon resonance tunability of VO2/Au/VO2 composite nanotriangle sandwich array

A vanadium dioxide (VO2) and Gold nanotriangle composite sandwich structure (VO2/Au/VO2) was fabricated via nanosphere lithography technique. It exhibits reversible localized surface plasmon resonance tunability through external thermal stimuli VO2 phase transition by tuning the near-field permittivity of Au nanotriangle. Compared optical response to the Au/VO2 (bottom Au top VO2) and VO2/Au (bottomVO2 top Au) composite nanotriangle arrays, VO2/Au/VO2 structure shows a largest wavelength shift of 42 nm and a largest transmittance modulation of 13.2% between the hot-metallic and cold-insulating states. And the wavelength shift of Au/VO2 nanotriangle array is larger than that of VO2/Au array for the higher electric intensity on top surface of Au nanotriangle. The results suggest that this sandwich architecture is one path for designing a plasmonic structure with tunable resonance responses that can be tailored by varying the top and bottom phase materials properties of the hybrid structures.

Tunable and scalable broadband metamaterial absorber involving VO2-based phase transition

Optical absorbers with dynamic tuning features are able to flexibly control the absorption performance, which offers a good platform for realizing optical switching, filtering, modulating, etc. Here, we propose a thermally tunable broadband absorber applying a patterned plasmonic metasurface with thermo-chromic vanadium dioxide (VO2) spacers. An actively tunable absorption bandwidth and peak resonant wavelength in the region from the near- to mid-infrared (NMIR) are simultaneously achieved with the insulating–metallic phase transition of VO2. Moreover, the scalable unit cell, which is composed of multi-width sub-cells, provides a new freedom to further manipulate (i.e., broaden or narrow) the absorption bandwidth while maintaining a high relative absorption bandwidth and efficient absorbance at the same time. For both transverse-electric and transverse-magnetic polarizations, the proposed nanostructure exhibits a high absorption over a wide angular range up to 60°. This method holds a promising potential for versatile utilizations in optical integrated devices, NMIR photodetection, thermal emitters, smart temperature control systems, and so forth.

Semiconductor–Metal Phase Transition and “Tristable” Electrical Switching in Nanocrystalline Vanadium Oxide Films on Silicon

Temperature dependences of the ac conductivity of nanocrystalline mixed vanadium oxide films on silicon revealed a multistep shape of the hysteresis loop observed during the semiconductor–metal phase transition in VO2, which was not manifested in the case of dc measurements. These peculiarities are related to the size effect, heterophase character of vanadium oxide films, and different types of charge carriers in the bulk of nanocrystallites and on their surfaces. The appearance of steps is explained by the phase transition taking place in separate groups of crystallites with close dimensions. The phenomenon of “tristable” electrical switching in these vanadium oxide films was observed for the first time.

Electrically Triggered Tunable Terahertz Band-Pass Filter Based on VO2Hybrid Metamaterial

We demonstrate an electrically triggered tunable terahertz (THz) band-pass filter based on vanadium dioxide (VO2) embedded hybrid metamaterials. The unit cell of the filter consists of two cross resonators and a central bar. The phase transition of VO2 is induced by the ohmic heating of the central bar. The transmission can be modulated by the applied currents. The mechanism of tunability is that the conductivity of the VO2 film changes with the applied currents. The relation between the upper cut-off frequency and the geometries is investigated using finite integration time domain method. The sample is fabricated by a thin film process, and characterized using a THz time domain spectroscopy system. The results show that, when the bias current rises from 0 to 0.27 A, the transmission decreases from 0.85 to 0.01. The maximum modulation depth reaches 96%, and the full width at half maximum is about 0.44 THz. This tunable THz band-pass filter has potential applications in THz communication, imaging, and spectroscopy systems.

Active tunable terahertz resonators based on hybrid vanadium oxide metasurface

Hybrid terahertz (THz) metamaterial resonators have exhibited superior reconfigurable resonant response enabled by active materials, such as liquid crystals, graphene, and semiconductors. However, the tunable range of constitutive parameters of materials is still limited, which leads to the low modulation depth of THz devices. Unlike other phase change materials, vanadium dioxide (VO2) exhibits an insulator-to-metal transition characteristic and the conductivity can be increased by 4–5 orders of magnitude under external stimulus including electric fields, optical, and thermal pumps. Here, we propose an active tunable THz resonator based on a hybrid VO2 metasurface for thermal control. The simulated results show that by external thermal stimulation, we realize the tuning between the single and double resonant modes. The resonant peak at high frequency disappears while the low frequency peak is enhanced with the increasing temperature. The simulated surface electric fields confirm the physical mechanism of the excellent tunable performance that the L-shaped and cross resonances play a dominant role at the low and high temperature, respectively, due to the VO2 phase transition. Such a hybrid VO2 metasurface resonator with tunable characteristics will greatly promote the practical application of THz functional devices, such as modulators, sensors and filters.

Thermally Controllable High-Efficiency Unidirectional Coupling in a Double-Slit Structure Filled With Phase Change Material

We present the realization of a high-efficiency, compact, and thermally tunable unidirectional launcher of surface plasmon polaritons (SPPs) in a double-slit structure filled with phase change materials (PCMs). PCMs such as VO2 provide a feasible way for tailoring optical properties, particularly refractive index as a function of heat. By introducing a narrow hybrid dielectric layer beneath the slits, SPPs couple efficiently into waveguide mode and propagate within the slot waveguide with considerably high intensity. Our simulation results revealed up to 80% coupling efficiency into the slot waveguide with extinction ratio of 1:800 between the left and the right slots can be achieved at telecommunication wavelength for normally incident P-polarized light. By heating the coupler above the VO2 phase transition temperature, the coupling and consequently the crosstalk approaches zero. Propagation length and other properties of the structure are characterized to confirm the feasibility of the coupler. This approach of unidirectional coupling into slot waveguides may lead to the development of integrated plasmonic circuits and on-chip applications.

Hysteresis in the spontaneous emission induced by VO2 phase change

We investigate the spontaneous emission of a two-level quantum emitter in the vicinity of a vanadium dioxide (VO2) thin film. By taking advantage of effective medium techniques to describe the dielectric constant of the VO2, we demonstrate that, in the near-field regime, the spontaneous emission rate of both electric and magnetic dipole emitters can be maximized at the insulator-to-metal phase transition temperature of the film. We find that VO2’s thermal hysteresis curve produces clear fingerprints in the emitter’s decay process, offering a new degree of freedom to dynamically control light emission at the nanoscale. We also find that the spontaneous emission rate is a non-monotonic function of the emitter transition wavelength, presenting a pronounced peak at infrared frequencies. Altogether, our results reveal that VO2 phase transition is a promising mechanism to achieve on-demand tuning of an emitter’s lifetime.

Theoretical study of the electronic and optical properties of rare-earth (RE = La, Ce, Pr, Nd, Eu, Gd, Tb)-doped VO2 nanoparticles

Vanadium dioxide (VO2) is an important thermochromic material due to its reversible metal-insulator transition at 340 K. However, the high phase transition temperature of VO2 prevents its practical application in smart windows. Here, we use first-principles calculations to study the effect of rare-earth (RE = La, Ce, Pr, Nd, Eu, Gd, Tb) dopants on the variation of the local octahedral volumes, lattice distortion of VO2 and dimerization of the V-V chain in VO2. Interestingly, Tb and La dopants with a concentration of 3.1 at.% decreased the phase transition temperature of VO2 to 206 K and 225 K, respectively. Moreover, three factors, including the V-V chain change, distortion of the REO6 octahedron, and formation of impurity states contributed by RE-f states, assist the phase transition of VO2. Additionally, the incorporation of an RE dopant scaled the band gap of VO2 and further tuned its near-IR absorption. These findings provide novel insights and guidance on chemically tailoring the phase transition of VO2.

Recent advances in fabrication strategies, phase transition modulation, and advanced applications of vanadium dioxide

Vanadium dioxide (VO2), with the first-order metal-insulator phase transition at near room temperature, has attracted increasing attention in the past few decades. With rapid electrical switching, the phase transition in VO2 also triggers the colossal property changes in various aspects, such as optical properties, magnetic properties, and strain, and, thus, enables a wide range of modern applications. In this review, we present a complete picture of the latest advances of VO2, including the fabrication strategies, property modulation, and advanced applications. This review summarizes several typical fabrication methods of VO2 crystals as well as some common problems and their possible solutions. The strategies for the fabrication of single-crystalline VO2 arrays are also discussed to meet the requirements of the high-performance devices at the macro-scale. This review concerns the typical approaches for the modulation of (metal-insulator transition) MIT and emphasizes on the domain study of VO2 single crystals at the nanoscale. We aim at a clear explanation of the effect of various inhomogeneities on the MIT behavior of VO2 and the importance of the accurate control of MIT at the domain level. After summarizing the preparation and modification of VO2, we focus on the applications of this amazing smart material in various aspects, including strain engineering, optical modulation, electrical switching, and multi-responsive sensing.

Gate-controlled VO2 phase transition for high-performance smart windows.

Vanadium dioxide (VO2) is a promising material for developing energy-saving "smart windows," owing to its infrared thermochromism induced by metal-insulator transition (MIT). However, its practical application is greatly limited by its relatively high critical temperature (~68°C), low luminous transmittance (<60%), and poor solar energy regulation ability (<15%). Here, we developed a reversible and nonvolatile electric field control of the MIT of a monoclinic VO2 film. With a solid electrolyte layer assisting gating treatment, we modulated the insertion/extraction of hydrogen into/from the VO2 lattice at room temperature, causing tristate phase transitions that enable control of light transmittance. The dramatic increase in visible/infrared transmittance due to the phase transition from the metallic (lightly H-doped) to the insulating (heavily H-doped) phase results in an increased solar energy regulation ability up to 26.5%, while maintaining 70.8% visible luminous transmittance. These results break all previous records and exceed the theoretical limit for traditional VO2 smart windows, making them ready for energy-saving utilization.

Polarized Tip-Enhanced Raman Spectroscopy in Understanding Metal-to-Insulator and Structural Phase Transition in VO2

Polarized Raman spectroscopy is capable of confirming the crystalline phase as well as the crystallographic orientation of the sample. At the same time, tip-enhanced Raman spectroscopy (TERS) is ca...

Tunable circular dichroism of composite metamaterial on the basis of the phase transition of VO2

The circular dichroism (CD) effect characterized by different optical responses between left and right circularly polarized lights is widely applied for polarization-resolved detection and imaging. The tunable CD effect is of substantial importance in improving the detection sensitivity and imaging resolution. In this paper, we show that planar Z-shaped composite metamaterial embedded with VO2 that exhibits insulator-metal transition (IMT) can enable thermally tunable chirality. Simulated by the finite element method, the tunable CD effect can be achieved by changing the environment temperature to initiate the IMT of VO2. We also demonstrate that the underlying mechanism of the CD effect generation is the electric multipole oscillation response in the vicinity of the VO2 gap in vertical direction. These findings not only provide a new strategy to change chirality and tune the effect of CD dynamically but also broaden its potential applications in polarization-resolved detection and imaging.