Thermochromic Vanadium Dioxide Research Articles

Coordination Compound Guided a Novel Composite Film with Zn2V2O7 Nanoflake and VO2@Zn2V2O7 Nanoparticle for Enhanced Thermochromism Smart Window.

Thermochromic vanadium dioxide (VO2) can intelligently modulate the transmittance of indoor solar radiation to reduce the energy consumption of air conditioning in buildings. Nevertheless, it remains a great challenge to simultaneously improve the luminous transmittance (Tlum) and solar modulation ability (ΔTsol) of VO2. In this study, a novel approach is employed utilizing a coordination compound to finely tune the growth of a VO2 based composite film, yielding a hierarchical film comprising Zn2V2O7 nanoflakes and VO2@Zn2V2O7 core-shell nanoparticles. Remarkably, the resulting composite films showcase exceptional optical performance, achieving a Tlum of up to 73.0% and ΔTsol of 15.7%. These outcomes are attributed to the antireflection properties inherent in the nanoflake structure and the localized surface plasmon resonance of well-dispersed VO2 nanoparticles. In addition, the Zn2V2O7-VO2 film demonstrates remarkable environmental durability, retaining 90% of its initial ΔTsol even after undergoing aging at 100°C under 50% relative humidity for a substantial period of 30 days - a durability equivalent to ≈20 years under ambient conditions. This work not only achieves a harmonious balance between Tlum and ΔTsol but also introduces a promising avenue for the design of distinctive composite nanostructures.

Plasmonic Resonance Shifts in Gold Nanoparticles‐Thermochromic VO2 Thin Film Hybrid Platforms: A Joint Experimental and Numerical Study

AbstractThe combination of the phase transition in thermochromic vanadium dioxide (VO2) with plasmonic nanoparticles paves the way for applications in various fields, including optical sensing, advanced coatings, and dynamic optical devices. This study presents a simple fabrication method to control both the size and surface coverage of NPs combined with VO2. First, a thermochromic VO2 coating with a phase transition at 68 °C is synthesized using reactive magnetron sputtering. Then, monodisperse 30 nm diameter gold NPs are bonded to the VO2 surface using (3‐aminopropyl)trimethoxysilane (APTMS) linkers, examining the effect of immersion duration on surface coverage. Two platforms are developed: a VO2 thin film with a monolayer of NPs and a configuration with NPs between two VO2 films. The temperature‐dependent plasmonic response of these platforms is measured by extinction spectroscopy, showing a significant wavelength resonance shift of approximately 10 nm for the first platform and 20 nm for the second. Optical simulations analyze this shift over various geometries, from isolated NPs to fully covered NPs, achieving a 60 nm shift for NPs embedded in a thin VO2 film. This study demonstrates an effective approach to synthesizing thermochromic VO2 coatings with gold NPs, offering insights into the plasmonic properties of hybrid platforms.

Preparation of Thermochromic Vanadium Dioxide Films Assisted by Machine Learning.

In recent years, smart windows have attracted widespread attention due to their ability to respond to external stimuli such as light, heat, and electricity, thereby intelligently adjusting the ultraviolet, visible, and near-infrared light in solar radiation. VO2(M) undergoes a reversible phase transition from an insulating phase (monoclinic, M) to a metallic phase (rutile, R) at a critical temperature of 68 °C, resulting in a significant difference in near-infrared transmittance, which is particularly suitable for use in energy-saving smart windows. However, due to the multiple valence states of vanadium ions and the multiphase characteristics of VO2, there are still challenges in preparing pure-phase VO2(M). Machine learning (ML) can learn and generate models capable of predicting unknown data from vast datasets, thereby avoiding the wastage of experimental resources and reducing time costs associated with material preparation optimization. Hence, in this paper, four ML algorithms, namely multi-layer perceptron (MLP), random forest (RF), support vector machine (SVM), and extreme gradient boosting (XGB), were employed to explore the parameters for the successful preparation of VO2(M) films via magnetron sputtering. A comprehensive performance evaluation was conducted on these four models. The results indicated that XGB was the top-performing model, achieving a prediction accuracy of up to 88.52%. A feature importance analysis using the SHAP method revealed that substrate temperature had an essential impact on the preparation of VO2(M). Furthermore, characteristic parameters such as sputtering power, substrate temperature, and substrate type were optimized to obtain pure-phase VO2(M) films. Finally, it was experimentally verified that VO2(M) films can be successfully prepared using optimized parameters. These findings suggest that ML-assisted material preparation is highly feasible, substantially reducing resource wastage resulting from experimental trial and error, thereby promoting research on material preparation optimization.

Stability of Thermochromic Vanadium Dioxide Thin Films in Harsh Environments of Temperature and Humidity

Stability of Thermochromic Vanadium Dioxide Thin Films in Harsh Environments of Temperature and Humidity

Room-temperature tuning of mid-infrared optical phonons and plasmons in W-doped VO2 thin films

The development of mid-infrared nanophotonics relies on the availability of new materials combining metal- and insulator-like optical features. We systematically studied the phase transition of thermochromic vanadium dioxide (VO2) using substitutional tungsten doping at room temperature. Our results reveal that vanadium dioxide thin films, doped with tungsten and grown via pulsed laser deposition techniques on sapphire substrates, can be precisely engineered to exhibit tailored infrared phonon and plasmon polaritonic responses. By controlling the extent of tungsten concentration, starting from VO2-WO3 cold-pressed powder targets, we demonstrate the ability to continuously adjust both the amplitude and frequency of optical phonon resonances. Furthermore, we observe tunable free-electron response due to varying tungsten concentrations. The adopted fabrication technique makes it possible to create multilayer structures by alternating layers with different concentrations of tungsten. Our results pave the way for the development of tunable mid-infrared metamaterial devices operating at room-temperature.

Chemical and physical effects of the addition of thermochromic VO2 particles to cement

This work aims to deepen the knowledge of systems formed by cement and thermochromic vanadium dioxide particles. Cement paste specimens with 9 % addition of monoclinic VO2 particles show a significant delay of the hydration at early ages with respect to plain cement specimens, seemingly due to the partial inhibition of hydrates precipitation during the first 24 h of reaction. Upon further hydration, a re-activation of the hydration reactions gives rise to a higher mechanical strength and a more refined pore structure in the paste with VO2 addition at 28 days, attributed to the enhanced formation of C–S–H. The stability of the VO2 particles within the matrix is confirmed along the hydration process and provides the cement paste with the associated thermochromic behavior that is demonstrated by a change in the reflection in the near infrared wavelength range upon cooling from 70 °C. The results confirm the viability of producing VO2-based thermochromic cementitious materials for building envelopes. The next steps in their development should target the enhancement of the solar reflectance modulation in order to produce a significant impact in the energy efficiency of buildings and the urban overheating effect.

2D ordered porous VO2-based composite film with enhanced thermochromism performance and high stability for smart windows

Thermochromic vanadium dioxide (VO2) can autonomously adjust near-infrared light transmittance in response to the external temperature changes, offering a significant reduction in building energy consumption when applied it in smart window. However, it is a big challenge for simultaneous optimization of visible light transmittance, solar modulation ability, and stability in VO2. To address such a big challenge, this study successfully prepared a 2D ordered porous VO2@Zn3V3O8 composite film. The as-obtained film exhibits exceptional optical performance with a visible light transmittance (Tlum) of 62.75 % and the solar modulation ability (ΔTsol) of 10.62 %, based on the anti-reflective effects of porous structure and the localized surface plasma resonance (LSPR) absorption of independently distributed VO2 nanoparticles. In addition, the composite structural film showcases excellent environmental durability that 80 % of ΔTsol could be retained after 30 days aging under the conditions of 50 % humidity and 100 °C. This work propels the practical application of VO2 smart windows and offers valuable insights for the design of composite structures.

Enhanced Post Deposition Annealing Conditions on the Fabrication of High Quality Thermochromic Vanadium Dioxide Films

Enhanced Post Deposition Annealing Conditions on the Fabrication of High Quality Thermochromic Vanadium Dioxide Films

High‐Performance Thermochromic VO2‐Based Coatings Deposited by Roll‐to‐Roll High‐Power Impulse Magnetron Sputtering

Thermochromic vanadium dioxide (VO2) undergoes a reversible semiconductor‐to‐metal transition, a property that can be exploited for reduction of the energy consumption in buildings using smart windows. Herein, high‐performance thermochromic three‐layer ZrO2/V0.98W0.02O2/ZrO2 coatings prepared on ultrathin flexible glass (0.1 mm) without any post annealing in a pilot‐scale roll‐to‐roll deposition device are presented. The V0.98W0.02O2 layers are deposited using a reactive high‐power impulse magnetron sputtering with an oxygen flow control at a substrate temperature of approximately 350 °C. The optimized coatings (width of 0.3 m and length over 2.6 m) exhibit a transition temperature of 28 °C, an integral luminous transmittance over 50%, and a modulation of the solar energy transmittance of about 10%. The deposition process is optimized, and the elemental composition (Rutherford backscattering spectrometry), design (field‐emission scanning electron microscopy), phase composition (X‐ray diffraction), and thermochromic properties (spectrophotometry) of these high‐performance VO2‐based coatings in the context of potential for commercial applications are characterized.

Nano-structured VO2 based films for space application and radiative environment

Recently, there has been a growing interest in the development of small satellites with more up-to-date technologies comparable to large conventional satellites in interplanetary space missions. Future development of these small satellites requires the application and use of passive thermochromic layers to achieve good thermal control for longer mission life and cost-effective launches. Thermochromic vanadium dioxide (VO2) thin films are being considered as an imminent prospect as a passive smart radiator coating (SRCs) for heat regulation in nano and microsatellites. Materials used in space missions can be subjected to various types of radiation including neutrons, gamma, gases, and heavy ions. The damage sustained can be quite complex and simultaneously degrade the characteristic properties of materials. The context of this mini review is detailed in two aspects, (i) overview of the meta-materials properties of VO2, synthesis, progress, and recent advances (ii) complemented by the radiation tolerance studies since spacecrafts interacts with radiation species of different energy and projectile doses.

Low Solar Absorptance, High Emittance Performance Thermochromic VO2-Based Smart Radiator Device.

Thermochromic vanadium dioxide (VO2)-based smart radiator devices (SRDs) display emittance variation with changes in temperature, making them very promising for energy-efficient thermal control of spacecrafts in general, and nanosatellites in particular. However, the high solar absorptance of the VO2-based SRDs remains too high for their intended application. Based on an approach combining optical simulation and experimental work, I demonstrate that an additional top stack layer alternating between high and low refractive indices made of a-Si(25 nm)/SiO2(67 nm) reduces the solar absorptance of a VO2-based SRD by 35% (from 0.43 to 0.28) while keeping the emittance performance of the SRD within the requirements for the intended application (low-temperature emittance εL = 0.35, high-temperature emittance εH = 0.81 and emittance tuneability with temperature Δε = 0.46). I also discuss factors to consider while designing additional top stack layers alternating between high and low refractive indices to further decrease the SRD's solar absorptance without affecting its emittance performance.

Phase Transition and Optical Properties of VO2 and Al: ZnO/VO2 Thin Films

Thermochromic Vanadium dioxide (VO2) has strong potential for smart window applications but its commercial scale usage is limited due to low visible light transmission. To address this issue, aluminum doped zinc oxide (AZO) anti-reflecting layer is integrated with VO2 layer in the present work. VO2 single layer and AZO/VO2 bilayer thin film samples were deposited by sputtering technique on quartz substrate. The single-phase growth of VO2 and AZO in single layer and bilayer thin film samples is confirmed by X-ray diffraction measurements. Monoclinic M1 phase of VO2 is detected in VO2 and AZO/VO2 thin film samples at room temperature. Monoclinic to rutile structural phase transition (SPT) in these samples is studied by performing temperature-dependent X-ray diffraction measurements. SPT in VO2 thin film samples is close to 68 °C and SPT temperature appears slightly lower in AZO/VO2 sample as compared to VO2 sample. Spectral transmittance measurement at room temperature showed significant improvement in the visible transmittance of AZO bilayer film than that of single layer VO2 thin film. These results demonstrate the possibility of integration of anti-reflecting layers such as AZO with VO2 layer for better visible transmittances suitable for large-scale smart window applications.

Adaptive gold/vanadium dioxide periodic arrays for infrared optical modulation

Localized surface plasmon resonances (LSPR) make systems capable of concentrating and amplifying light intensity at their near surface. Applications ranging from energy harvesting to biological sensors depend on the modulation of LSPR. Through the synthesis and shape modifications of Au quasi-triangular nanoplatelets (QTP) arrays, LSPR modulation from near- to mid- infrared (∼1.5 μm to ∼ 4.5 μm) is revealed. Au QTP arrays are then associated to a thermochromic vanadium dioxide (VO2) layer leading to a “smart” nanocomposite exhibiting modulated absorptions. The VO2 layer acts as a phase change material with a tunable dielectric function vs. temperature and represents an active matrix. The dynamics of the geometric changes in QTP arrays and the phase transition of the matrix are directly correlated to the shift of the LSPR position (ΔλLSPR ∼ 675 nm). The experimental data are supported by a theoretical approach via the finite difference time domain (FDTD) method that provides the LSPR characteristics in the various Au QTP array and nanocomposite configurations. The experimental and modelling investigations prove that the red-shift resonance modulation originates from the creation of a temperature-dependent core–shell structure of Au QTP (core) surrounded by a thin metallic VO2 layer (shell) and embedded into the VO2 dielectric matrix.

Design and energy analysis of tunable nanophotonic infrared filter based on thermochromic vanadium dioxide

In this work, a mid-infrared tunable nanophotonic filter is designed by placing a silicon spacer layer between two nanometric vanadium dioxide films on a thick infrared-transparent substrate. Transfer matrix formulation incorporated with the ray-tracing method is used for the optical modeling, and a transmittance peak near 4 µm wavelength is achieved based on Fabry–Perot resonance effect. Thermochromic vanadium dioxide allows the filter to be switched between a semi-transparent state when cooled below its phase transition temperature and an opaque state when heated above its phase transition temperature. The underlying mechanism is elucidated by calculating the total phase shift of electromagnetic waves through the spacer layer, and the effect of layer thicknesses on the spectral transmittance peak is discussed. The filter can find potential applications in energy dissipation where it can actively promote or suppress radiative cooling, and in thermal camouflage where the apparent temperature of a hot surface through the filter can be greatly reduced. Detailed energy analysis is conducted for both applications with the proposed tunable filter, whose structure is further optimized through the thicknesses of the thin-film layers to achieve maximum performance.

Absorptivity Control Over the Visible to Mid-Infrared Range Using a Multilayered Film Consisting of Thermochromic Vanadium Dioxide

Absorptivity Control Over the Visible to Mid-Infrared Range Using a Multilayered Film Consisting of Thermochromic Vanadium Dioxide

Embedding Quaternary V1–x–ySrxWyO2 into Multilayer Systems to Enhance Its Thermochromic Properties for Smart Glass Applications

Smart window glazing technology possesses a great potential for reducing the overall energy demand in the building sector. Thin films based on thermochromic vanadium dioxide (VO2) are ideally suited as functional layers of thermochromic smart windows. VO2 itself exhibits a significant change in optical transparency and reflectivity in the infrared spectrum. However, the optimization of VO2-based thin films for application as intelligent window glazing is still ongoing. The improvement is rather tedious because the key parameters set by the application such as transmittance modulation ΔT2500, luminous transmittance Tlum, solar transmittance modulation ΔTsol, and transition temperature ϑc affect each other. Codoping of VO2 with strontium (Sr) and tungsten (W) drives the parameters close to the desired range. However, the requirements set for the use as smart window coating cannot be fully met. Here, we show by a systematic comparison of simulations and experiments that embedding a layer of thermochromic V1–x–ySrxWyO2 between an optimized TiO2 buffer and a TiO2 antireflection layer already adds additional degrees of freedom to simultaneously fulfill all smart window requirements for the optical parameters ΔT2500, Tsol, Tlum, and ΔTsol. In particular, the luminous transmittance Tlum is improved by not less than ≈10% in such a trilayer structure.

Controlled grain-size thermochromic VO2 coatings by the fast oxidation of sputtered vanadium or vanadium oxide films deposited at glancing angles

An original, simple and cost-effective oxidation strategy to attain thermochromic vanadium dioxide thin films is reported. This two-step procedure comprises the initial deposition of DC magnetron-sputtered vanadium or vanadium oxide films by the combination of glancing angle deposition and, if needed, reactive gas pulsing process, followed by fast oxidation of such layers in air atmosphere at high temperatures. Thanks to the careful control of the thermal treatment parameters, and taking advantage of the superior reactivity of the high surface-to-volume porous deposited structures, the formation of pure VO2 (M1) layers was achieved. The comprehensive characterization of such oxidized systems by means of scanning electron microscopy, Raman spectroscopy and scanning-transmission electron microscopy techniques such as electron energy-loss spectroscopy, not only confirmed the presence of the VO2 (M1) phase, but also allowed to shed light on the key role that reaction time plays in the selective formation of vanadium dioxide films of adjustable grain size and crystallinity. The optimal conditions to stabilize thermochromic VO2 consists in using large deposition angles (85 °) and short oxygen pulses (≤ 2 s) during the growth, followed by fast and short thermal treatments (≤ 45 s with a heating rate of 42 °C s−1) in air atmosphere at 550 °C. The metal-to-insulator response of the accomplished VO2 layers was finally evaluated by means of temperature dependent Kelvin probe force microscopy measurements, evidencing surface potential drops at heating, greater than those reported in the literature to date for VO2 thin films.

Recent Advances in Fabrication of Flexible, Thermochromic Vanadium Dioxide Films for Smart Windows.

Monoclinic-phase VO2 (VO2(M)) has been extensively studied for use in energy-saving smart windows owing to its reversible insulator–metal transition property. At the critical temperature (Tc = 68 °C), the insulating VO2(M) (space group P21/c) is transformed into metallic rutile VO2 (VO2(R) space group P42/mnm). VO2(M) exhibits high transmittance in the near-infrared (NIR) wavelength; however, the NIR transmittance decreases significantly after phase transition into VO2(R) at a higher Tc, which obstructs the infrared radiation in the solar spectrum and aids in managing the indoor temperature without requiring an external power supply. Recently, the fabrication of flexible thermochromic VO2(M) thin films has also attracted considerable attention. These flexible films exhibit considerable potential for practical applications because they can be promptly applied to windows in existing buildings and easily integrated into curved surfaces, such as windshields and other automotive windows. Furthermore, flexible VO2(M) thin films fabricated on microscales are potentially applicable in optical actuators and switches. However, most of the existing fabrication methods of phase-pure VO2(M) thin films involve chamber-based deposition, which typically require a high-temperature deposition or calcination process. In this case, flexible polymer substrates cannot be used owing to the low-thermal-resistance condition in the process, which limits the utilization of flexible smart windows in several emerging applications. In this review, we focus on recent advances in the fabrication methods of flexible thermochromic VO2(M) thin films using vacuum deposition methods and solution-based processes and discuss the optical properties of these flexible VO2(M) thin films for potential applications in energy-saving smart windows and several other emerging technologies.

Reconfigurable, vivid reflective colors based on solution-processed Fabry–Perot absorber using thermochromic vanadium dioxide

Artificial structural color generation has attracted much attention in developing ink-free color technology for applications such as security devices, reflective displays, and functionalized color decoration. An asymmetric Fabry–Pérot (F–P) cavity-based absorber can play this role because of its advantages of an ultra-thin structure, lithographic-free manufacturing, and applicability to a large area. However, the optical response of F–P absorbers, which is determined by the structural parameters and compositions of the individual layers, is fixed at a single frequency, and only a static color can be passively implemented. In this study, we propose a new active metamaterial-based F–P absorber that exhibits dynamically tunable optical responses with temperature change. An active F–P absorber is fabricated by incorporating lossy nanoporous Ag nanoparticles (NPs) and a phase change material—vanadium dioxide (VO2)— interlayer via a solution process. Coupled with finite-difference time-domain simulations and systematic experiments, we demonstrate that F–P absorbers generate enhanced reflective color purity due to their closely-coupled Ag NPs. The reflective colors were dynamically modulated by temperature changes owing to the variation of optical constants between the phase transition of the monoclinic VO2. Furthermore, the demostrated tunable color image with micro-patterned absorbers opens the way for designing thermo-optical devices operating in the visible wavelength.

VO2 nanostripe-based thin film with optimized color and solar characteristics for smart windows

This study presents a structuration approach to reach a high luminous transmittance (Tlum) and solar modulation efficiency (ΔTsol) of thermochromic vanadium dioxide (VO2) thin films. Before starting optical simulations, we confirm that the optical properties of monoclinic VO2 continuous films deposited by reactive magnetron sputtering show a good correspondence with numerical results. Then, calculations on ordered VO2 nanostripes demonstrate an enhanced transmittance due to the presence of vertical openings, leading to a variety of photonic effects. A series of optimizations by varying the column width, period, and film thickness establishes that 20 nm wide and 10 nm separated nanostripes have a ΔTsol of 14.2% and a Tlum of 47.6% for a film thickness of 250 nm. In comparison to a dense film without nanostructuring, the film transparency (related to Tlum) is significantly enhanced, while ΔTsol remains unchanged. Furthermore, this also translates into a favorable, less opaque color, so the geometry could be useful for various thermochromic applications, such as smart windows.