Phase Transition Temperature Of VO2 Research Articles

A review of the preparation, properties and applications of VO2 thin films with the reversible phase transition

The reversible phase transition of vanadium dioxide under thermal, electrical, and optical stimuli is the enabling concept for the functioning of smart materials and is the basis for the development of various device materials such as optical, electrical, thermal, and mechanical devices based on VO2 on rigid and flexible platforms. The phase transition temperature of VO2 near room temperature is considered an excellent choice and a potential candidate to replace traditional materials in a variety of applications. There is a growing interest in VO2 applications for a wide range of devices, and the use of VO2’s structure to manipulate and explore the functions of various application devices, as well as the modification of VO2 structures to improve performance in a variety of materials, can lead to extremely exciting innovations. A lot of effort has been put into the challenges of practical production and practical application, and it is necessary to find an industrially feasible manufacturing method for the preparation of VO2 films, which is the basis for the practical application of VO2-based equipment. Based on this background, we first briefly describe the structure of VO2, the phase transition mechanisms involved, and the factors and other properties induced by the phase transition of VO2. Then, the current status and advantages and disadvantages of VO2 thin film preparation technologies are introduced in detail, including pulsed laser deposition (PLD), magnetron sputtering, the sol-gel method, and chemical vapour deposition (CVD). In addition, we propose three strategies to improve the performance of VO2 thin films, including element doping, multi-layer composites, and surface structure. We also discussed the different applications of VO2 under thermal, electrical, and light stimulation, as well as the development trends and future challenges of VO2 thin films.

Microwave Hydrothermal Preparation of VO2

As a thermal phase transition material, VO2 is famous for its phase transition performance. When the temperature is lower than the phase transition temperature, VO2 behaves as a semiconductor with high infrared transmittance. When the temperature exceeds the phase transition temperature, VO2 exhibits a metallic state and has a high refractive index for infrared. Regulation of phase transition temperature is key to the application of VO2. In this article, microwave hydrothermal is used to prepare VO2. H+ concentration, microwave power, annealing temperature, and annealing time are used as variable conditions to find the best preparation conditions. The preparation of samples is studied by X‐ray diffractometer, scanning electron microscope, and differential scanning calorimeter. After comparative research, the optimum preparation process of vanadium dioxide by microwave hydrothermal method is found. In addition, by varying the acidic conditions and microwave power during the preparation process, the phase transition temperature of VO2 can be regulated.

Dynamically switchable tri-functional THz-integrated metamaterial absorber based on VO2-graphene

Based on the easily accessible phase transition temperature of VO2 and the electrically tunable properties of graphene, a tri-functional THz-integrated metamaterial absorber with dynamically switchable low-frequency broadband, high-frequency broadband, and multi-band absorption are proposed. This absorber is composed of a patterned VO2 layer, a SiO2 dielectric layer with a layer embedded patterned graphene, a thick unpatterned VO2 layer, both sides of which are coated with ML graphene, a SiO2 dielectric layer, and an Au substrate. When the patterned and unpatterned VO2 layers are both metallic, the proposed absorber behaves as a low-frequency broadband absorber from 2.7 to 6.2 THz with average absorptivity exceeding 90 %, with the value of absorption sensitive to the structure of the patterned VO2. When the patterned VO2 is insulating and the unpatterned VO2 is metallic, the absorber exhibits high-frequency broadband absorptivity, above 90 % from 5.8 to 7.6 THz. When the patterned and unpatterned VO2 are both insulating, the device acts as a multi-band absorber with 12 tunable resonance peaks. In addition, the proposed absorber is robust against the incidence angle and polarization. Furthermore, the electric field strength distributions and multiple interference theory are presented to explain the physical mechanisms of tri-functional near-100 % absorptivity. Our work may provide a promising path for the development of multifunctional THz-integrated metamaterial absorbers.

First‐Principles Studies of Electronic Structures and Optical Properties of Cobalt‐Doped VO2

First‐principles calculations based on density functional theory (DFT) are used to investigate the phase transition characteristics, electronic structures, and optical properties of pure and Co‐doped VO2 (M1 and R phase). Studies show that the metal‐to‐insulator phase transition temperature of VO2 is significantly reduced after Co doping, which is correlated to the decrease of bandgap value. Besides, the decrease of the energy required for electron transition of M1‐phase Co‐doped VO2 corresponds to the imaginary part of the dielectric peak moving to the low‐energy region. For both the M1‐ and R‐phase VO2, the visible light transmissivity of the Co‐doped VO2 is increased than that of pure VO2, which is beneficial to the application of VO2 film as visible windows. In addition, the absorptivity and reflectivity of Co‐doped R‐phase VO2 in the infrared light range are larger than those of M1‐phase VO2, indicating that the Co‐doped VO2 can block more infrared light at higher temperature to fulfill the purpose of lowering temperature. Overall, these results give new insights for the application of Co‐doped VO2 as a photoenergy material to regulate the room temperature.

Simultaneous tuning of the phase transition temperature and infrared optical properties of Mo-doped VO2 powders for intelligent infrared stealth materials

The development of an intelligent infrared camouflage material whose infrared emissivity can actively adapt to environmental changes is a key frontier in the field of infrared stealth. In this study, Mo-doped VO2 powder was prepared via a hydrothermal method, which led to an intelligent infrared camouflage material whose infrared radiation characteristic can adaptively change with the environmental temperature. The samples were characterized by XRD, SEM, DSC, FTIR and infrared thermal imaging. Combined with the results of the first-principles calculation, the coupling effect mechanism of Mo6+ doping concentration on the phase transition temperature and infrared photoelectric properties of VO2 material was systematically analyzed. The results showed that Mo6+ impurities had significant effects on the structure, morphology, composition, phase transition temperature and infrared reflectivity of VO2 powder. The doping process effectively reduced the phase transition temperature of VO2 and expanded the change range of infrared emissivity (△ε) before and after the metal-to-insulator (MIT) transition. With the increasing amount of Mo6+ doping, the infrared reflectance of VO2(M) gradually decreased at low temperatures, while the infrared reflectance of VO2(R) increased at high temperatures. The MIT transition temperature of Mo-doped VO2 versus undoped VO2 reduced to 31.5 °C, and the △ε increased by 153%, this is expected to meet the performance requirements of intelligent infrared stealth materials.

Regulating the phase transition temperature of VO2 films via the combination of doping and strain methods

Phase change materials have the potential for reversible modulation of the physical and chemical properties of other materials, making them suitable for a wide range of applications. Among these phase change materials, VO2 is particularly attractive for electronic applications due to its ultrafast reversible phase transition at near room temperature (68 °C). Regulating the phase transition temperature of VO2, however, remains a challenge. In this study, two factors, i.e., film thickness and buffer layer, that can effectively regulate the phase transition temperature of VO2 films were introduced, and the effect of doping on the phase transition temperature was also investigated. The interfacial strain between the VO2 film and substrate was modulated by adjusting the film thickness and doping concentration in the buffer layer. This was to explore the effect of strain on the phase transition temperature of the film. Changes in VO2 lattice parameters were reflected by the shifts of XRD diffraction peaks. Test and measurement results show that the phase transition temperature of VO2 gradually increased as the interfacial strain increased. For instance, the phase transition temperature (Th) of the GeVO/AlGeO-3 sample reached 91.2 °C. Moreover, some samples exhibited increased abrupt parameters, such as phase transition amplitude and thermal hysteresis width. These findings have important implications for the use of VO2 materials in applications, including switches, sensors, and amnesic resistors.

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.

Effect of W-Mo doping on the phase transition temperature of VO2 synthesized by hydrothermal microwave method

In this study, the combination of hydrothermal microwave technology and high-temperature method was used to efficiently control the formation of M-phase vanadium dioxide (VO2) nanoparticles, which were promising materials for optoelectronic switches and smart windows due to their excellent optoelectronic properties during the phase transition. The phase state and structure of VO2depended on its synthesis parameters, and the results showed that the optimal conditions for VO2(B) synthesis in a hydrothermal microwave were 120 °C for 2 h, which was a novel method for efficiently preparing VO2(B) at a low temperature. By vacuum annealing, VO2(B) could be transformed into monoclinic VO2(R), where VO2(R) converts into VO2(M) on cooling to room temperature. Furthermore, the phase transition temperature of W-Mo co-doped VO2(M) decreased by 14.8 °C, showing that the incorporation of W-Mo elements into the VO2-based structure affects the material’s phase transition temperature.

A Thermal-Switchable Metamaterial Absorber Based on the Phase-Change Material of Vanadium Dioxide.

This article presents a thermal-switchable metamaterial absorber (TSMA) based on the phase-change material of vanadium dioxide (VO2). VO2 thin film was deposited on sapphire substrate by magnetron sputtering followed by vacuum annealing treatment. Then, the prepared VO2 film was sliced into tiny chips for thermal-switchable elements. The surface structure of TSMA was realized by loading four VO2 chips into a square metallic loop. The absorption frequency of TSMA was located at 7.3 GHz at room temperature and switched to 6.8 GHz when the temperature was heated above the critical phase transition temperature of VO2. A VO2-based TSMA prototype was fabricated and measured to verify this design. The design is expected to be used in metasurface antennas, sensors, detectors, etc.

A universal approach to fabricating infrared-shielding smart coatings based on vanadium dioxide

Vanadium dioxide (VO2) is identified as one of the most promising candidates in smart windows due to its metal-to-insulator transition. Most reported studies concentrated on the improvement of their optical properties in sunlight wavelength, while the significant effect of their emissivity in the mid infrared region (MIR) was rarely studied. Here, we proposed a universal approach to fabricating W - doped VO2 based smart coatings with excellent infrared - shielding ability and good thermochromic performance. The W dopant is used to decrease the phase transition temperature of VO2, and promote the practicability. We believe that the method will attract more attention to MIR properties of VO2 based smart coatings and improve the dynamic temperature management ability significantly.

Role of the double-glow plasma pre-sputtering in the growth mechanisms and metal–insulator transition of VO2 film

High phase transition temperature and low utilization rate of sunlight of vanadium dioxide (VO2) have been an important obstacle to expand its application in smart semiconductor materials. In this paper, we propose a novel method by the double-glow plasma pre-sputtering to induce to generate a lot of surface vacancy defects/residual stresses, which can promote the high-quality growth of VO2 thin films on quartz substrates. The resultsshowed that the plasma pretreatment and annealing temperature affected the stability of phase structureto cause the formation of surface residual stresses of VO2films. The optical-electrical properties of the treated VO2 film indicated that the incident light at 2.5 μm has a great transmittance contrast of 63.25% and the solar transmittance modulation efficiency (ΔTsol) of 14.9 %before and after the phase transition. It is found that the metal–insulator phase transition temperature of VO2 after plasma treatment is 39.7℃, which is 20.4℃ lower than that of conventional VO2 film, greatly increases its application in intelligent windows and photoelectric fields. Our research provides an effective and low-cost approach to prepared high-quality VO2 thin films with low phase transition temperature and large-phase-transition amplitude on quartz substrate.

Synthesis, microstructure, and phase transition characteristics of Gd/Nd-doped nano VO2 powders

Abstract The nano VO2 powders were prepared by hydrothermal synthesis. The effects of Gd and Nd element doping on the structure and phase transition temperature of VO2 were studied. The X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), and Transmission electron microscopy (TEM) results showed that Gd element and Nd element will affect the structure of VO2. Gd3+ and Nd3+ can occupy partial position of V4+ lattice and form solid solution, increasing the lattice parameters of VO2. Both the doped and un-doped VO2 powders exhibit a monoclinic structure at room temperature. Due to the lattice deformation caused by Gd or Nd doping, the aggregation of particles is prevented, and the grain is refined obviously. Differential scanning calorimetry curves showed that both Gd doping and Nd doping can reduce the phase transition temperature of VO2(M). When the Gd doping concentration is 6 at%, the phase transition temperature can be reduced from 71.7°C to 60.3°C, and the infrared transmittance before and after the phase transition also changes significantly, reaching more than 40%. Nd doping is similar, and the phase transition temperature decreased to 55.6°C with the addition of 9 at% Nd.

Realization of high luminous transmittance and solar modulation ability of VO2 films by multistep deposition and in-situ annealing method

Novel VO2(x nm)/VO2(x nm)/VO2(x nm) (x = 10, 14, 16, 20) trilayer films are proposed and prepared on quartz glass substrates via multistep devotion and in-situ annealing by pulsed laser deposition. The bottom, intermediate and top VO2 layers are distinguished by different thickness and annealing time. The characterization of FESEM and AFM showed that grain size and surface roughness of VO2 films increase gradually from the bottom to top layer. The trilayer structure has little effect on phase transition temperature of VO2, but hysteresis width can be tuned in the range of 11–27 °C. The luminous transmission and solar modulation ability of VO2(16 nm)/VO2(16 nm)/VO2(16 nm) trilayer film were remarkably enhanced to 55.7% and 8.0%. The results suggested that such an enhancement can be attributed to the well matching of grain size of the bottom, intermediate and top VO2 layers. The study in this work provide a simple and promising strategy for designing high-quality VO2-based smart windows without additional functional layers.

Controllable morphology, structure and optical properties of W–Eu Co-doped VO2 with a tunable phase transition temperature and band gap

Uncontrollable morphology, high phase transition temperature and vague phase transition mechanism of vanadium dioxide (VO2) hinder its large-scale application in smart windows and optoelectronic switches. In this paper, controllable VO2(B) powders co-doped with W–Eu were synthesized via a hydrothermal method using CH3COOH·2H2O and H2O2 as the reductant and assistant reductant, where W and Eu were derived from H2WO4 and Eu(NO3)3·6H2O, respectively. Most VO2(B) crystals transformed into VO2(M) after annealing. Pristine VO2 had a regular rectangular shape, and it would change to be fabric-like, spongy and coralloid after doping with W and Eu. Furthermore, the surface of W–Eu co-doped samples presented an urchin-like structure and nanorods appeared after annealing. Importantly, doping and annealing also have significant effects on the performances of VO2 samples. Upon W–Eu co-doping, the phase transition temperature of VO2 (M) samples decreased by an average of approximately 15 °C, and the energy band gap decreased from 0.70 eV to 0.33 eV corresponding to the change of phase transition temperature. Not only the thermochromic performances of VO2 were optimized, but also the mechanisms were explained accordingly in this paper.

Terahertz perfect absorber based on flexible active switching of ultra-broadband and ultra-narrowband

Metamaterial perfect absorbers in the terahertz band are attracting more and more attention. Pure narrowband absorbers as well as broadband absorbers have been proposed one after another in recent years. However, absorbers that can achieve both narrow-band absorption and broadband absorption have hardly been reported. To meet more practical needs, we propose a terahertz metamaterial perfect absorber that combines ultra-broadband and narrowband based on the phase transition properties of vanadium dioxide (VO2). Its main structure consists of a metal ring and four VO2 discs, and the absorber can be flexibly switched between ultra-broadband and narrowband absorption by adjusting the ambient temperature. The resonator consisting of metal rings and VO2 discs are mainly responsible for the formation of absorption peaks. A detailed explanation is given by means of magnetic resonance theory and the impedance matching principle. Compared to recent reports, our design offers a significant improvement in absorption rate and bandwidth and is also flexible in terms of tuning. Moreover, as the phase transition temperature of VO2 is only slightly higher than room temperature, there are almost no limitations for experimental and practical applications. Therefore, our design will have significant applications in modulation, sensing, energy harvesting, switching devices, etc.

Doubling of the Phase Transition Temperature of VO2 by Fe Doping.

Vanadium dioxide (VO2) undergoes a fully reversible first-order metal-insulator transition from the M1 monoclinic phase (P21/c) to a high-temperature tetragonal phase (P42/mnm) at around 68 °C. Modulation of the phase transition of VO2 by chemical doping is of fundamental and technological interest. Here, we report the synthesis of highly crystallized Fe-doped VO2 powders by a carbo-thermal reduction process. The impact of Fe doping on the structural and phase transition of VO2 is studied. The as-prepared Fe-doped VO2 samples crystallize in the M2 monoclinic form (C2/m), which is linked to segregation of the doping ions in the V2 zigzag chains. A large increase in the transition temperature to 134 °C is observed, which does correspond to a breakthrough in VO2-type thermochromic materials.

Enhancing visible-light transmittance while reducing phase transition temperature of VO2 by Hf–W co-doping

In order to enhance the visible-light transmittance while reducing the insulator–metal transition (IMT) temperature, Hf–W co-doping is designed for modification of VO2. We grow high-quality HfxWyV1−x−yO2 (HfWVO2) alloy films on c-plane sapphire substrates by pulsed laser deposition, and test structural, electrical, and optical properties of the films by various techniques. The Hf–W co-doped VO2 films exhibit outstanding thermochromic performances with a high luminous transmittance up to 41.1%, a fairly good near-infrared modulation capacity of 13.1%, and a low phase transition temperature of 38.9 °C. The enhanced luminous transmittance along with reduced IMT temperature in HfWVO2 is attributed to the co-doping synergetic effect of Hf and W, which effectively improves the optical bandgap and donates extra electrons into the system, respectively, while largely retaining the near-infrared modulation capacity of VO2. Our work provides an effective strategy in tailoring VO2 toward practical smart-coating applications by Hf–W co-doping.

Correlation between Symmetry and Phase Transition Temperature of VO2 Films Deposited on Al2O3 Substrates with Various Orientations

AbstractThe structural aspects of the insulator–metal transition (IMT) characteristics of VO2 are sensitive to the octahedral symmetry variation of VO6. By varying substrate orientation (c‐, a‐, and m‐plane Al2O3), the correlation between IMT temperature and local symmetry is investigated. For a VO2 film deposited on m‐plane Al2O3, which has high symmetry due to fewer domain boundaries induced by m‐plane Al2O3, the IMT temperature is low (326.47 K). In contrast, for a film deposited on c‐plane Al2O3 (having lower symmetry), the IMT temperature is the highest (336.74 K) among the films used in this work. Furthermore, temperature‐dependent Raman spectra reveals that the structural phase transition temperature decreases in the order of the VO2 film deposited on c‐, a‐, and m‐plane Al2O3, suggesting that the symmetrical structure reduces the activation energy for IMT by decreasing thermodynamic energy. These results demonstrate that structural symmetry plays a crucial role in lowering the transition temperature.

Improving phase transition temperature of VO2 via Ge doping: a combined experimental and theoretical study

Enhancing the semiconductor–metal phase transition temperature (TSMT) of VO2 is of great consequence for further exploring the potential applications of VO2 at elevated temperatures. In this study, Ge4+-doped VO2 (GexV1−xO2) samples were prepared by the hydrothermal and annealing approach. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), differential scanning calorimetry (DSC) and resistivity–temperature (R-T) analyses were used to investigate the influence of Ge doping on the lattice structures and phase transition properties of GexV1–xO2 samples. We found that the lattice parameter of GexV1−xO2 decreased with the Ge concentration increasing from 2 at% to 18 at%, which was further supported by density functional theory (DFT)-based first-principle simulations. TSMT firstly increased from 64.5 to 73.0 °C at 8 at% Ge and then decreased to 71.5 °C at higher Ge concentration. Furthermore, DFT analysis revealed that the impact of lattice distortion induced by Ge doping rather than the changes in electronic structure is more pronounced on modulating TSMT of GexV1−xO2. The present work has pointed out the direction that the TSMT could be enhanced and illustrated the physical reason behind the regulation of TSMT in ions-doped VO2 systems. The d (logρ)/dT vs T curves are plotted for GexV1−xO2 (0≤x≤0.18) samples (a) un-doped VO2 ; (b) 2%; (c) 8%; (d) 18%, the transition temperatures upon heating, Th, and cooling, Tc. The difference between Th and Tc gives the hysteresis width, ΔTt, while the FWHM determines the sharpness of the semiconductor-to-metal transition

Phase-change regulation criterion based on size-dependent lattice distortion rate and born theory for VO2 nanomaterials

Vanadium dioxide (VO2) is a promising thermally induced phase transition material because of the abrupt changes in electrical and optical properties. However, the high phase transition temperature of VO2 and its unspecified modulation relationship need to be resolved urgently. Herein, we proposed a simple and precise regulation criterion for VO2 materials based on size-dependent lattice distortion rate and Born theory. The results indicated that the application of a tensile stress changed the elastic properties of the VO2, which promoted VO2 phase transition, and regulated the phase transition temperature. Moreover, the specific modulation relationship between the stress and phase transition temperature of VO2 was confirmed experimentally. These results show that our criterion provides theoretical guidance to regulate VO2 thermally induced phase transition materials.