Phase Transition Temperature Of VO2 Research Articles

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.

Thermally Controllable Infrared Absorption in Cylindrical Groove Array Covered by Phase Change Material

A perfect infrared spectrum absorber in a cylindrical groove array perforated in a metallic layer and followed by vanadium dioxide (VO2), a phase-change material, is introduced and analytically investigated. The phase-change material can change from insulator to metallic phase if proper stimuli, such as heat, voltage, and current, are applied, herein the phase change initiated by heating beyond the phase-transition temperature of VO2. The large refractive index variations between two phases enable the manipulation of optical properties. Simulation results reveal that in the metallic phase, the absorption is suppressed to one third, while VO2 works as a transparent medium for impinging light, and perfect absorption is observed in the insulation phase. The process is reversible in a picosecond time scale. Given the structure symmetry, the absorption is irreverent to impinging light polarization. The thermally controlled infrared absorber may have applications in various optical systems, like imaging and sensing, due to its simple and easy fabrication process.

Increase in Coercive Field by Magnetoelastic Coupling in Co/VO₂ Films

Herein, a ferromagnetic cobalt (Co) film was deposited on a VO₂ film exhibiting a structural phase transition from monoclinic to tetragonal at ~340 K to investigate the magnetoelastic effect caused by a phase-transition-induced strain. First, (100) and (101) VO₂ films with thicknesses of 11-43 nm were grown on Al₂O₃ using pulsed laser deposition, and 2.5-nm-thick Co films were deposited on top of them via sputtering. The magneto-optic Kerr effect hysteresis loops were measured with the temperature variation across the structural phase transition temperature of VO₂. Upon heating, an increase in the coercive field of the Co layer was observed at the structural phase transition temperature of VO₂, suggesting a magnetoelastic coupling between the Co and VO₂ layers. The coercive field increment diminished with decreasing VO₂ thickness, and it disappeared as the VO₂ became thinner than a critical value of ~11 nm. These results imply that a phase-transition-induced strain in the VO₂ layer can be used to change the coercive field of the neighboring magnetic layer.

Effects of heat treatment conditions and Y-doping on structure and phase transition temperature of VO2 powders

The VO2 powders were prepared by hydrothermal synthesis. The effects of heat treatment conditions and Y-doping on the structure and phase transition temperature of VO2 were studied. The XRD, SEM and TEM results show that the heat treatment temperature has a significant effect on the crystal transformation of VO2 precursor. Increasing temperature is conducive to the transformation of precursor VO2(B) to ultrafine VO2(M). The Y-doping affects the structure of VO2. Y3+ can occupy the lattice position of V4+ to form YVO4 solid solution, which can increase the cell parameters of VO2. Due to the lattice deformation caused by Y-doping, the aggregation of particles is prevented, and the grain is refined obviously. DSC curves show that Y-doping can reduce the phase transition temperature of VO2(M). After adding 9 at.% Y, the phase transition temperature can be reduced from 68.3 to 61.3 °C.

Influence of Ce–W co-doping on phase transition temperature of VO2 thin films deposited by ultrasonic nebulized spray pyrolysis of aqueous combustion mixture

Vanadium dioxide is widely explored as a viable material for switching applications and a robust progress has been achieved on laboratory-scale. To realise its commercialisation, industrially-viable synthetic techniques are highly required. Ultrasonic nebulised spray pyrolysis of aqueous combustion mixture (UNSPACM) is a simple, low-cost and scalable technique which offers high quality films for various energy applications. Herein, we report synthesis, characterization and influence of Ce–W co-doping on phase transition temperature of VO2(M1) thin films deposited on quartz substrates at 350 °C by this technique. The phase was verified by x-ray diffraction and Raman spectroscopic studies. FTIR studies at 2.5 µm to 14 µm IR frequencies showed 63% reversible reflectance change of the films from low reflectivity below 60 °C to high reflectivity above 60 °C. Electrical resistance measurements on the thin films showed a first order transition with resistance change of three orders of magnitude. Ce-doping (2 atomic% of Ce) resulted in increase in reflectance (up to 72%) and transition temperature (up to 75 °C) while retaining strength of the transition. Ce–W co-doping lowered transition temperature (up to 67 °C) and reflectance (up to 45%) while retaining the transition strength. Ce–W co-doping also reduced surface roughness of the films up to rms roughness value of 36 ± 0.7 nm. We believe this technique provides an easy and simple means to synthesize VO2 thin films on large-scale for multiple applications such as smart windows among others.

Phase-transition-induced superior ultraviolet photodetection of a ZnO/VO2 bilayer

The photodetection performance of ZnO is significantly improved across the phase transition temperature of VO2.

In-situ stirring assisted hydrothermal synthesis of W-doped VO2 (M) nanorods with improved doping efficiency and mid-infrared switching property

Vanadium dioxide (VO2) is considered to be one of the most competitive thermochromic materials, due to its semiconductor-metal phase transition characteristics. And the doping of tungsten (W) into the lattice of VO2 can significantly reduce the phase transition temperature to near room temperature. However, the reduction efficiency dependent on doping amount is quite different in previous reports. In this work, we proposed an in-situ stirring assisted hydrothermal to synthesize W-doped VO2 nanorods with improved doping efficiency. The effects of in-situ stirring during the hydrothermal synthesis on phase composition, sample morphology, and phase-transition temperature of VO2 with different W doping amounts were studied systematically. The phase transition temperature reduction efficiency of W-doped VO2 prepared by hydrothermal synthesis without in-situ stirring was at % W. By contrast, the phase transition temperature of W-doped VO2 prepared by in-situ stirring hydrothermal synthesis was depressed as much as 24.55 °C/at % W when W/V ranged in 0.75–2.0 at %. Further research found that the phase transition temperature of W-doped VO2 (M) could be simply tuned by changing the in-situ stirring time. Then, the 1 at% W-doped VO2 (M) (with phase transition temperature of around 41.5 °C) nanorods and acrylic resin were mixed to prepare VO2 composite film. It exhibited suitable infrared transmission (>60%) and excellent mid-infrared transmission switching performance (23.77%). This work proposed an in-situ stirring hydrothermal method for high efficiency synthesis of W-doped VO2 (M) and provided significant insights for its application in thermochromic windows.

Thermally controllable perfect absorber at telecommunication spectrum based on phase change material and cavity grating

We proposed and demonstrated a high-efficiency active absorber based on surface plasmon polaritons and Fabry–Perot cavity operating at telecommunication regime. The absorber comprised a resonance-cavity grating sandwiched between a metal and vanadium dioxide (VO2), a phase-change material (PCM) widely used in optical devices. As a PCM, VO2 is characterized by a large difference in the refractive index between the insulator and metallic phases initiated by various stimuli, such as heat, voltage, and current, thereby enabling the tailoring of optical properties in different systems. Herein, heating caused VO2 to undergo a transition from insulator to metallic phase, resulting in changes in its optical properties. For transverse magnetic polarization at resonance wavelength, the cavities beneath the VO2 confined the impinging light when VO2 was in insulator phase. Our simulation results revealed that the confinement enhanced the absorption up to 99%; conversely, when the system was heated beyond the phase-transition temperature of VO2 (68 °C), the absorption intensity decreased from 99% to 30%. For transverse electric polarization, a similar behavior was observed but the absorption intensity was about 50% at telecommunication band. These findings indicated that the proposed absorber was active, intensity tunable and highly responsive and may have applications in sensors and imaging.

Designable Phase Transition Temperature of VO₂ Co-Doped with Nb and W Elements for Smart Window Application.

Monoclinic vanadium dioxide (VO₂ (M)) particles co-doped with niobium and tungsten, with potential application in smart windows, were synthesized by hydrolysis and subsequent thermal decomposition of vanadyl sulfate. All the doped VO₂ particles exhibited a monoclinic crystalline phase and the critical phase transition temperature (Tc) of VO₂ (M) was adjusted by Nb and W co-doping. The Tc of Nb-doped VO₂ (M) decreased at a rate of approximately 10 °C/at% Nb dopant, and the transition temperature could also be accurately controlled to room temperature (about 27 °C) by co-doping with Nb and W. A film prepared using co-doped VO₂ (M) particles showed a solar modulation ability of ~18% and a luminous transmittance of 40%, indicating that the co-doped VO₂ (M) particles represent suitable candidates for smart window applications.

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.

Tungsten doped M-phase VO2 mesoporous nanocrystals with enhanced comprehensive thermochromic properties for smart windows

The phase transition temperature (~68 °C) of M-VO2 film can be lowered significantly by tungsten (W) doping into the crystal lattice of VO2 due to the reduction of the strength of V-V pair interaction. However, W doping was always coupled with a serious weakening of luminous transmittance and solar modulation efficiency because W dopants can increase the electron concentration of VO2 film. Herein, the simultaneous introduction of W dopants and mesopores into M-VO2 nanocrystals was employed to prepare VO2 film. Interestingly, the obtained 0.4 at% W-doped mesoporous VO2 nanocrystals based composite films exhibited enhanced comprehensive thermochromic performance with excellent solar modulation efficiency (ΔTsol = 11.4%), suitable luminous transmittance (Tlum = 61.6%) and low phase transition temperature around 43 °C, much lower than 65.3 °C of undoped VO2. It was demonstrated that the lower phase transition temperature of VO2 can be primarily attributed to abundant lattice distortion after W doping, whereas the mesoporous structure can facilitate the uniform distribution of W dopants in VO2 nanocrystals, enhance the luminous transmittance and guarantee enough VO2 nanocrystals in the composite film to keep relatively high solar modulation efficiency. Therefore, this work can provide a new way to balance the three important parameters for the thermochromic performance of VO2 film (ΔTsol, Tlum and Tc) and probably promote the application of VO2 nanocrystals in the energy efficient windows.

Dynamic infrared thin-film absorbers with tunable absorption level based on VO2 phase transition

Phase change materials exhibit tunable electrical and optical response, providing rich potential to build active devices with tunable properties. Here, we propose and demonstrate a tunable infrared absorber based on vanadium dioxide (VO2) thin films. Compared with conventional absorbers relying on either nanostructures or Fabry-Perot cavities, our proposed device shows near perfect absorption while having a subwavelength thick absorbing film. Moreover, the absorption intensity can be controlled dynamically around the phase transition temperature of VO2. We model the optical response of the VO2 intermediate states with an effective medium theory to help fitting and understanding the phase change behavior during the phase transition. The calculated electric field distribution as well as the absorption maps are presented to show how the light is absorbed in the thin film platform. The proposed device has the potential for many applications including thin photodetectors, modulators and tunable emitters.

Near-field thermal rectification devices using phase change periodic nanostructure.

We theoretically analyze two near-field thermal rectification devices: a radiative thermal diode and a thermal transistor that utilize a phase change material to achieve dynamic control over heat flow by exploiting metal-insulator transition of VO2 near 341 K. The thermal analogue of electronic diode allows high heat flow in one direction while it restricts the heat flow when the polarity of temperature gradient is reversed. We show that with the introduction of 1-D rectangular grating, thermal rectification is dramatically enhanced in the near-field due to reduced tunneling of surface waves across the interfaces for negative polarity. The radiative thermal transistor also works around phase transition temperature of VO2 and controls heat flow. We demonstrate a transistor-like behavior wherein heat flow across the source and the drain can be greatly varied by making a small change in gate temperature.

A simple method to prepare V1-xWxO2 (x = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) controllable phase transition temperature powder

V1-xWxO2 (x = 0, 0.01, 0.02,0.03, 0.04, and 0.05) controllable phase transition powders have been synthesized by a simple carbothermal reduction method using the commercial vanadium pentoxide as the raw material, ammonium paratungstate as the doping resource, and graphite powders as the reducing agent, respectively. The synthesized powders have been characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope, X-ray photoelectron spectroscopy, differential scanning calorimetry, and vibrating sample magnetometer, respectively. Results showed that the prepared pure VO2 was a monoclinic structure and the W-doped VO2 was a tetragonal structure. The phase transition temperature of VO2 decreased gradually with increasing W-doping content. The function between phase transition temperature of VO2 and W-doping content and the phase transition mechanism have been presented.

One-Step Hydrothermal Synthesis of W-Doped VO2 (M) Nanorods with a Tunable Phase-Transition Temperature for Infrared Smart Windows.

Vanadium dioxide (VO2), with reversible metal–semiconductor transition near room temperature, is a compelling candidate for thermochromic windows. Nanocomposite coatings derived from VO2 nanoparticles are particularly superior to VO2 films due to their advantages in large-scale preparation, flexible shaping, and regulation of optical properties. In this work, we developed a novel method for one-step hydrothermal synthesis of W-doped VO2 (M) nanorods and studied their application in large-scale infrared smart windows. On introducing tartaric acid as a new reductant, VO2 underwent a two-stage phase evolution from the pure phase comprising VO2 (A) nanobelts to VO2 (M) nanorods, instead of the conventional three-stage B–A–M phase evolution during hydrothermal synthesis. This transition is very favorable for the large-scale hydrothermal synthesis of VO2 (M). The phase-transition temperature of VO2 (M) nanoparticles can be regulated systematically by W doping, with a reduction efficiency of about 24.52 °C/atom % W. Moreover, VO2 (M) composite films were fabricated using a convenient roller coating method, which exhibited significant midinfrared transmission switching up to 31%, with a phase-transition temperature of about 37.3 °C. This work demonstrates the significant progress in the one-step hydrothermal synthesis of VO2 (M) nanorods and provides significant insights into their applications in infrared smart windows.

Thermal kinetic analysis of metal–insulator transition mechanism in W-doped VO2

Tungsten (W) doping can decrease the phase transition temperature of VO2, but underlining reasons are not clear. In this study, differential scanning calorimetry was employed to investigate the kinetics of the solid–solid transition in hydrothermal-synthesized W (X = 0.50, 1.09, 2.58 and 3.81 %)-doped VO2 nanoparticles. We firstly revealed the W-doping mechanism by combining the classical nucleation kinetics model with isoconversional kinetic analysis and applied them on the solid–solid transition taking place in doping. The experimentally observed large lag in the cooling stage and asymmetry effects of the decreasing temperature on insulator–metal transition and metal–insulator transition can be rationally explained. In the heating stage, W doping decreases free energy barrier (ΔG*) for homogenous nucleation and reduces geometrical factor (f(Θ)) and both factors promote the transition and thus lower the phase transition temperature quickly. However, in cooling stage, the free energy barrier (ΔG het * ) for heterogeneous nucleation was much larger than that of heating stage due to lacking of proper nucleation sites. The effect of decreasing geometrical factor was accompanied with the effect of increasing free energy barrier for homogenous nucleation by doping W. Such a competition mechanism slows down the trend of reducing temperature. It is important to unravel interaction mechanisms of doped W on different VO2 phases, which is helpful to further tailor kinetic properties of VO2 phase transition.

Controlling the Temperature and Speed of the Phase Transition of VO2 Microcrystals

We investigated the control of two important parameters of vanadium dioxide (VO2) microcrystals, the phase transition temperature and speed, by varying microcrystal width. By using the reflectivity change between insulating and metallic phases, phase transition temperature is measured by optical microscopy. As the width of square cylinder-shaped microcrystals decreases from ∼70 to ∼1 μm, the phase transition temperature (67 °C for bulk) varied as much as 26.1 °C (19.7 °C) during heating (cooling). In addition, the propagation speed of phase boundary in the microcrystal, i.e., phase transition speed, is monitored at the onset of phase transition by using the high-speed resistance measurement. The phase transition speed increases from 4.6 × 10(2) to 1.7 × 10(4) μm/s as the width decreases from ∼50 to ∼2 μm. While the statistical description for a heterogeneous nucleation process explains the size dependence on phase transition temperature of VO2, the increase of effective thermal exchange process is responsible for the enhancement of phase transition speed of small VO2 microcrystals. Our findings not only enhance the understanding of VO2 intrinsic properties but also contribute to the development of innovative electronic devices.

Phase transition characteristics in the conductivity of VO₂(A) nanowires: size and surface effects.

Transition-metal oxides have fascinating characteristics, and have been exploited for various applications, such as Mott transistors, optical switches and strain sensors, etc. Vanadium dioxide is a special and important transition-metal oxide, and exhibits the significant behavior of metal-insulator transition. In this work, single crystalline VO2(A) nanowires have been synthesized by a facile hydrothermal method. Due to the size and surface effects, the nanowires with different widths show great disparities in their hysteresis loops, phase transition temperatures and electrical conductivities. Our results show that the phase transition temperature is linearly dependent on the inverse of the nanowire widths, and a similar relationship between the electrical conductivity and the width of the nanowires has also been found. More interestingly, the first-order phase transition of the nanowire even coverts into high-order continuous phase transition when the width is below a critical size. To explore the intrinsic influence of the size and surface effects, the analysis of the transmission electron microscopy measurements showed that the rough surface structure of the nanowire is very different to the internal structure, and the thickness of this rough surface structure almost remains unchanged as the with of the nanowire decreases. Our results indicated that the surface structure has a remarkable effect on the phase transition characteristics decreasing nanowire width, and the suitable heterogeneous nucleation originating from the rough surface structure should play a crucial role in properties of the VO2(A) nanowires. Size-dependent phase transition features of the VO2(A) nanowires also suggest that the size and surface effects must be taken into consideration when designing VO2 nanodevices.

Effects of annealing ambient on oxygen vacancies and phase transition temperature of VO2 thin films

VO2 thin films are prepared on Si substrates by direct-current (DC) magnetron sputtering at room temperature and annealed in vacuum at different argon pressures.