Thermochromic VO2 Research Articles

Atomic and electronic structures of thermochromic VO2 with Sb-doping

Vanadium dioxide (VO2) is an attractive transition metal oxide owing to its intriguing metal to insulator phase transition and has great potential for applications in smart windows. In this paper, first-principles calculations were conducted and the results revealed that Sb is as an efficient dopant in improving the thermochromic properties of VO2 by lowering the phase transition temperature and enhancing the transmission of visible light, as well as increasing the near-IR absorption of VO2. These changes in the optical properties of VO2 are closely correlated with the variations in the atomic and electronic structures, where the VV chains along the [001] direction present dimerization features in VO2(R) with Sb-doping; in addition, the OV bonds appear less ionic due to the OSb ionic bonds. The calculation results are confirmed by the subsequent experiments, suggesting that our findings would be useful for further understanding the physics of atomic doping in VO2, which may provide useful guidance for the fabrication of high-quality VO2 based smart windows.

Investigation on preparation and properties of quasicontinous AgNWs micromesh regulated thermochromic and low emissivity VO2 energy-efficient windows

Vanadium dioxide (VO2) based thermochromic smart windows are considered to be one of most promising hotspots in building energy conservation fields. However, to achieve higher energy efficiency throughout one total year, low thermal emission feature for VO2 coating is required. In this paper, silver nanowire (AgNW) layers with cross-linked micromesh structure were applied to modify the thermal emission properties. Investigations indicated that for AgNWs/VO2 multilayered films deposited on fused quartz substrate, both low emissivity ( ɛ ) and excellent soar modulation efficient (Δ T sol) could be obtained. The quasicontinous structure retained highest conservation rate for solar modulation, among the published reports of AZO and Pt coated VO2 thin films, when the emissivity degraded. And for one sample, ɛ of 0.21 and Δ T sol of 5.80% can be obtained. The results in this paper indicated that the current compositing structures may provide beneficial ways for the achievements high performance VO2-based energy efficient coatings.

Low temperature fabrication of thermochromic VO2 thin films by low-pressure chemical vapor deposition

High-performance thermochromic VO2 films were fabricated by LPCVD, and the mechanism for their low transition temperatures was studied using first-principles calculations.

Particle size, morphology and phase transitions in hydrothermally produced VO2(D)

An easy and reproducible method to synthesise thermochromic VO2[M] via VO2[D] at a low calcination temperature.

HiPIMS-deposited thermochromic VO2 films with high environmental stability

A comparative study of the environmental stability of high power impulse magnetron sputtered (HiPIMS) and radio frequency magnetron sputtered (RFMS) thermochromic vanadium dioxide thin films in a highly oxidizing environment has been performed. We observe that RFMS-deposited VO2 films quickly transform into V2O5 at 80°C and 100% relative humidity while HiPIMS-deposited films retain their thermochromic behaviour at least three times longer. Following a thorough analysis of the films, this increase in performance is ascribed to the films’ higher density and larger average grain size. The results indicate that the HiPIMS process provides sufficient durability for VO2 films without the need for a diffusion barrier used in typical everyday conditions. Still, when a 35nm thick SiNx diffusion barrier is added on top of the HiPIMS VO2 films, the thermochromic device shows less than 5% loss in thermochromic performance following extended accelerated ageing.

Characterization of thermochromic VO2 (prepared at 250 °C) in a wide temperature range by spectroscopic ellipsometry

The paper deals with thermochromic VO2 prepared by reactive high-power impulse magnetron sputtering and characterized by spectroscopic ellipsometry. We focus on the dispersion of optical constants in a wide temperature range and on the transmittance predicted using the optical constants. While the thermochromic behavior of VO2 in itself has been reported previously (particularly above the room temperature, RT), in this paper we present (i) optical properties achieved at a low deposition temperature of 250°C and without any substrate bias voltage (which dramatically increases the application potential of the coating) and (ii) changes of these properties not only above but also below RT (down to −30°C). The properties include very low (for VO2) extinction coefficient at RT (0.10 at 550nm), low transition temperature of around or even below 50°C (compared to the frequently cited 68°C) and high modulation of the predicted infrared transmittance (e.g. 39% at −30°C, 30% at RT and 3.4% above the transition temperature at 2000nm for a 100nm thick coating on glass). The results are important for the design of thermochromic coatings, and pathways for their preparation under industry-friendly conditions, for various technological applications.

Sol-gel approaches to thermochromic vanadium dioxide coating for smart glazing application

Thermochromic VO2 has received great attention for smart window application because of its ability to control solar heat gain in response to the temperature variations. It goes through a first order reversible phase transition from the semiconductor to metallic state at critical temperature of 68°C, having higher infrared reflectance above this temperature. This paper summarizes some prominent theories behind this phase transition and presents a comprehensive review of recent progress in all-solution synthesis of VO2 thin film with a particular focus on sol-gel method. Emphasis is given to highlight different precursors exploited to prepare thin films with adjustable morphology and optimized optical properties. Actually, VO2-based coatings show good combination of integrated luminous transmittance (33.1–81.3%) and solar modulation ability (2.2–22.3%). The effect of various dopants such as tungsten, magnesium on thermochromic properties of VO2 thin films is also elaborated. These studies indicate that innovative solutions are essential to overcome the current shortcomings associated with practical application of VO2.

Scalable Production of Thermochromic Nb-Doped VO<SUB>2</SUB> Nanomaterials Using Continuous Hydrothermal Flow Synthesis

The synthesis of effective thermochromic Nb-doped VO2 nanoparticles, containing a range of Nb dopant concentrations (1, 5 and 15 at.% Nb), was carried out via continuous hydrothermal flow process under supercritical water regime. The synthesis method employed here is directly scalable for industrial purposes and the materials may be produced at a rate of kg h-1 in our pilot plant. The thermochromic properties of these materials were studied at variable temperature using UV/Vis spectroscopy. The structure, morphology and particle properties were characterised using powder X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The incorporation of Nb dopant into VO2 was studied X-ray photoelectron spectroscopy. The Nb-doped VO2 materials showed a substantial decrease in metal- To-semiconductor transition temperature (ca. 20 °C) and improved optical features (narrow hysteresis loop, 10 °C difference) compared to undoped VO2 nanoparticles synthesised under similar conditions. A reduction in the MST temperature of 3.6°C per at.% Nb incorporated was estimated from these studies.

Low-temperature deposition of thermochromic VO2 thin films on glass substrates

Highly dense and highly crystalline vanadium dioxide (VO2) thermochromic thin films were successfully sputter-deposited on glass substrates at a low substrate temperature of 300°C. By superimposition of RF and DC magnetron sputtering, we were able to fabricate VO2 films with high solar transition efficiency (△Tsol) of 12.8% at a substrate temperature of 300°C, which was comparable to the results attained at 450°C by conventional DC sputtering. The VO2 crystallization at a lower temperature of 300°C could be achieved using a high-energy and high-density plasma induced by RF-superimposed DC sputtering. The hysteresis curves of transmittance as a function of temperature revealed that the VO2 films by RF-superimposed DC sputtered at 300°C exhibited a reduced transition temperature of 59°C and a narrow hysteresis width of 3°C. The enhanced transition properties might result from the compressive stress in the b and c axes as well as the densification of the films due to ion bombardment effect.

Enhanced Visible Transmittance of Thermochromic VO₂ Thin Films by SiO₂ Passivation Layer and Their Optical Characterization.

This paper presents the preparation of high-quality vanadium dioxide (VO2) thermochromic thin films with enhanced visible transmittance (Tvis) via radio frequency (RF) sputtering and plasma enhanced chemical vapor deposition (PECVD). VO2 thin films with high Tvis and excellent optical switching efficiency (Eos) were successfully prepared by employing SiO2 as a passivation layer. After SiO2 deposition, the roughness of the films was decreased 2-fold and a denser structure was formed. These morphological changes corresponded to the results of optical characterization including the haze, reflectance and absorption spectra. In spite of SiO2 coating, the phase transition temperature (Tc) of the prepared films was not affected. Compared with pristine VO2, the total layer thickness after SiO2 coating was 160 nm, which is an increase of 80 nm. Despite the thickness change, the VO2 thin films showed a higher Tvis value (λ 650 nm, 58%) compared with the pristine samples (λ 650 nm, 43%). This enhancement of Tvis while maintaining high Eos is meaningful for VO2-based smart window applications.

HiPIMS-deposited thermochromic VO2 films on polymeric substrates

Thermochromic vanadium dioxide, VO2, is a semi-conductor of major interest for energy control applications. Generally, the fabrication of high-quality VO2 films requires a high deposition temperature over 400°C which limits its large scale implementation as well as its compatibility with temperature-sensitive substrates. In this work, we demonstrate the deposition of thermochromic VO2 thin films on flexible polymer substrates using High Power Impulse Magnetron Sputtering (HiPIMS); this process is known for its high degree of ionization, high plasma density and hence high ion flux towards the substrate that allows one to deposit crystalline films at lower temperatures in comparison with conventional sputtering techniques. We first describe the optimization approach of the HiPIMS process followed by the optical performance study of the resulting VO2 films on Kapton polyimide substrates. The films deposited at temperatures as low as 275°C are then analyzed by means of temperature-resolved spectrophotometry, XPS, ERD, RBS, Raman spectroscopy and spectroscopic ellipsometry. Optimized films show an infrared modulation ΔT2500 nm of up to 50% with a transition temperature Tc close to the bulk temperature of 68°C.

Thermochromic light scattering from particulate VO2 layers

Particulate layers of thermochromic (TC) VO2 were made by reactive DC magnetron sputtering of vanadium onto In2O3:Sn-coated glass. The deposits were characterized by scanning electron microscopy, atomic force microscopy, and X-ray diffraction. Specular and diffuse optical transmittance and reflectance were recorded in the 300–2500-nm wavelength range and displayed pronounced TC effects. These properties could be reconciled with a semi-quantitative model based on Lorentz–Mie theory applied to the distribution of particle sizes and accounting for particle shapes by the Grenfell–Warren approach with equal-volume-to-area spheres.

Thermochromic VO2 films from ammonium citrato-oxovanadate(iv) with excellent optical and phase transition properties

Thermochromic VO2 film with excellent optical and phase transition properties was successfully fabricated from newly synthesized ammonium citrato-oxovanadate(iv).

Use of ZnO as antireflective, protective, antibacterial, and biocompatible multifunction nanolayer of thermochromic VO2 nanofilm for intelligent windows

A multifunctional VO2/ZnO bilayer film is designed and deposited by magnetron sputtering apparatus. The integration of the antireflective, antioxidative and anti-corrosion functions, and antibacterial performance makes the heterostructure film a promising candidate in the energy-saving smart window. The ZnO thin film as the antireflection layer can markedly boost the solar regulation efficiency (ΔTsol) from 7.7% to 12.2% and possesses excellent luminous transmittance (Tlum-L=50.3%) in the low-temperature semiconductor phase. The ZnO layer as the protection barrier can not only protect VO2 thin film from oxidation to much toxic V2O5, but also decrease the release of V ions. Besides, the synergistic effect of releasing killing by Zn2+ ions and contact killing by ZnO NPs makes ZnO thin film an outstanding antibacterial coating. In terms of the biological safety, ZnO coating with appropriate film thickness can effectively attenuate the cytotoxicity of VO2 on human HIBEpiC cells. We hope this work can provide new insights for better designing of novel multifunctional VO2-based intelligent energy-saving windows.

Thermochromic VO2 films by thermal oxidation of vanadium in SO2

Thermochromic films of VO2 were prepared by a two-step procedure: Sputtering was first used to deposit metallic vanadium, and such layers were subsequently oxidized in SO2 at a temperature in the 600–650°C range. X-ray diffraction, Raman spectroscopy, measurements of temperature-dependent electrical resistance, and spectrophotometric transmittance data at different temperatures were employed to demonstrate that the films consisted of polycrystalline VO2 with good thermochromism, especially when oxidized at the highest temperature. Oxidation in SO2 is able to produce VO2 without the stringent process control that can be an obstacle for making VO2 by oxidation in O2.

Study of low temperature rf-sputtered Mg-doped vanadium dioxide thermochromic films deposited on low-emissivity substrates

Undoped VO2 was fabricated by RF sputtering from metallic target on low emissivity glass substrates at a record low growth temperature. The structure of the films was examined by X-Ray Diffraction whereas temperature dependence transmittance was employed for monitoring the film's thermochromic properties. Thermochromic VO2 could be achieved at the substrate temperature of 300°C which is the lowest growth temperature of VO2 phase by sputtering reported in literature without any additional post-deposition annealing treatment of the films. The thermochromic properties obtained for undoped VO2 were: luminous transmittance of 36.2%, metal-to-insulator transition temperature at 55.7°C and infrared transmittance modulation, ΔTIR, (transmittance difference in the infrared, at λ=2000nm, when heated from 25°C to 90°C) by 20.5% and solar transmittance modulation ΔTsol=5.2%. Subsequently, magnesium (Mg) was introduced in the structure of thermochromic VO2 up to 2.4at.%, and the Mg-induced changes in thermochromisity were examined. By increasing the amount of Mg in VO2 structure the visible transmittance increased up to 70%. Improvement in thermochromic characteristics was observed for atomic percentage of Mg up to 0.3% above which the films tend to be non-thermochromic. The optimum atomic percentage of Mg (0.3at.%) leads to thermochromic VO2 with the characteristic lowest phase transition temperature at 49.2°C, luminous transmittance ΤIum=46.6% and solar transmittance modulation ΔΤsol=2.8% and ΔΤIR=13%. Overall, 70nm thick thermochromic VO2 films deposited at the low substrate temperature of 300°C and low deposition rate 0.75nm/min when doped with Mg up to 0.3at.% resulted in vanadium dioxide with improved thermochromic properties is regarded as particularly promising for smart windows applications on flexible substrates.

Porous silicon-VO2 based hybrids as possible optical temperature sensor: Wavelength-dependent optical switching from visible to near-infrared range

Morphological properties of thermochromic VO2—porous silicon based hybrids reveal the growth of well-crystalized nanometer-scale features of VO2 as compared with typical submicron granular structure obtained in thin films deposited on flat substrates. Structural characterization performed as a function of temperature via grazing incidence X-ray diffraction and micro-Raman demonstrate reversible semiconductor-metal transition of the hybrid, changing from a low-temperature monoclinic VO2(M) to a high-temperature tetragonal rutile VO2(R) crystalline structure, coupled with a decrease in phase transition temperature. Effective optical response studied in terms of red/blue shift of the reflectance spectra results in a wavelength-dependent optical switching with temperature. As compared to VO2 film over crystalline silicon substrate, the hybrid structure is found to demonstrate up to 3-fold increase in the change of reflectivity with temperature, an enlarged hysteresis loop and a wider operational window for its potential application as an optical temperature sensor. Such silicon based hybrids represent an exciting class of functional materials to display thermally triggered optical switching culminated by the characteristics of each of the constituent blocks as well as device compatibility with standard integrated circuit technology.

Printed Smart Photovoltaic Window Integrated with an Energy‐Saving Thermochromic Layer

A smart photovoltaic window is designed and constructed by solution-processing a layer of thermochromic VO2 nanoparticles on top of a semitransparent organic solar cell. The prepared smart window not only produces electricity using the visible part of the solar spectrum but also saves energy via intelligently modulating the amount of NIR radiation passing through the device in response to ambient temperature.

Temperature-dependent multiangle FTIR NIR–MIR ellipsometry of thermochromic VO2 and V1−xWxO2 films

VO2 and W-doped VO2 thin films were prepared by Radio Frequency (RF) reactive magnetron sputtering using V-metal target. The amount of W doping was quantified by Rutherford Backscattering Spectrometry (RBS). The optical constants of VO2 and V1−xWxO2 films were inferred above and below the transition temperature by temperature-dependent multiangle ellipsometry in the Near InfraRed (NIR) spectral range and by temperature-dependent multiangle Fourier Transform InfraRed ellipsometry (FTIR) in the Near InfraRed–Middle InfraRed (NIR–MIR) spectral range up to 20,000nm. The effect of the doping concentration on their optical constants was studied. A validation of the results was obtained comparing the optical constants determined by point-by-point fitting with those determined by the Lorentz–Drude model and the empirical Lorentz–Cauchy dispersion formula.

Influence of doping with alkaline earth metals on the optical properties of thermochromic VO2

Thin films of doped VO2 were deposited, analyzed, and optimized with regard to their solar energy transmittance (Tsol) and visible/luminous light transmittance (Tlum) which are important parameters in the context of smart window applications in buildings. The doping with alkaline earth metals (AEM) like Mg, Ca, Sr, or Ba increased both Tsol and Tlum due to a bandgap widening and an associated absorption edge blue-shift. Thereby, the brown-yellowish color impression of pure VO2 thin films, which is one major hindrance limiting the usage of VO2 as thermochromic window coating, was overcome. Transparent thin films with excellent switching behavior were prepared by sputtering. Highly doped V1−xMexO2 (Me = Ca, Sr, Ba) kept its excellent thermochromic switching behavior up to x(Me) = Me/(Me + V) = 10 at. % doping level, while the optical bandgap energy was increased from 1.64 eV for undoped VO2 to 2.38 eV for x(Mg) = 7.7 at. %, 1.85 eV for x(Ca) = 7.4 at. %, 1.84 eV for x(Sr) = 6.4 at. % and 1.70 eV for x(Ba) = 6.8 at. %, as well as the absorption edge is blue shifted by increasing AEM contents. Also, the critical temperature ϑc, at which the semiconductor-to-metal transition (SMT) occurs, was decreased by AEM doping, which amounted to about −0.5 K/at. % for all AEM on average. The critical temperature was determined by transmittance-temperature hysteresis measurements. Furthermore, Tsol and Tlum were calculated and were found to be significantly enhanced by AEM doping. Tlum increased from 32.0% in undoped VO2 to 43.4% in VO2 doped with 6.4 at. % Sr. Similar improvements were found for other AEM. The modulation of the solar energy transmittance ΔTsol, which is the difference of the Tsol values in the low and high temperature phase, was almost constant or even slightly increased when the doping level was increased up to about 10 at. % Ca, Sr, or Ba.