Transition In Vanadium Dioxide Research Articles

Crystallographic Pathways to Tailoring Metal-Insulator Transition through Oxygen Transport in VO2.

The metal-insulator (MI) transition of vanadium dioxide (VO2) is effectively modulated by oxygen vacancies, which decrease the transition temperature and insulating resistance. Oxygen vacancies in thin films can be driven by oxygen transport using electrochemical potential. This study delves into the role of crystallographic channels in VO2 in facilitating oxygen transport and the subsequent tuning of electrical properties. A model system is designed with two types of VO2 thin films: (100)- and (001)-oriented, where channels align parallel and perpendicular to the surface, respectively. Growing an oxygen-deficient TiO2 layer on these VO2 films prompted oxygen transport from VO2 to TiO2. Notably, in (001)-VO2 film, where oxygen ions move along the open channels, the oxygen migration deepens the depleted region beyond that in (100)-VO2, leading to more pronounced changes in metal-insulator transition behaviors. The findings emphasize the importance of understanding the intrinsic crystal structure, such as channel pathways, in controlling ionic defects and customizing electrical properties for applications.

Defect‐Driven Light Perception and Memristor Storage with Phase Transition in Vanadium Dioxide

AbstractTunable optical information storage is crucial in artificial retinal systems for mimicking neurobiological visual characteristics. The perception and storage of light signals rely heavily on the regulation of the conductivity states of memristor materials (e.g., transition metal oxides). Controlling light memristor behavior via defects and polymorphic phases remains underexplored and differs from traditional plasticity training via repeated testing. In this study, defect‐driven ultraviolet light perception and memristor storage with phase transitions in vanadium dioxide (VO2) thin films are presented. The effects of oxygen defects and the corresponding polymorphic phases on ultraviolet light memristors are investigated. The dependence of phonon vibrations and insulator–metal transition behavior on defect levels are revealed. Self‐doping and polymorphs enable VO2 to exhibit distinct ultraviolet memristor performance. It is anticipated that defect‐driven light memristors significantly contribute to the realization of artificial synaptic devices and the implementation of advanced electronic neuron systems.

Anisotropic metal–insulator transition in strained VO2(B) single crystal

Mechanical strain can induce noteworthy structural and electronic changes in vanadium dioxide, imparting substantial scientific importance to both the exploration of phase transitions and the development of potential technological applications. Unlike the traditional rutile (R) phase, bronze-phase vanadium dioxide [VO2(B)] exhibits an in-plane anisotropic structure. When subjected to stretching along distinct crystallographic axes, VO2(B) may further manifest the axial dependence in lattice–electron interactions, which is beneficial for gaining insights into the anisotropy of electronic transport. Here, we report an anisotropic room-temperature metal–insulator transition in single-crystal VO2(B) by applying in-situ uniaxial tensile strain. This material exhibits significantly different electromechanical responses along two anisotropic axes. We reveal that such an anisotropic electromechanical response mainly arises from the preferential arrangement of a strain-induced unidirectional stripe state in the conductive channel. This insulating stripe state could be attributed to the in-plane dimerization within the distorted zigzag chains of vanadium atoms, evidenced by strain-modulated Raman spectra. Our work may open up a promising avenue for exploiting the anisotropy of metal–insulator transition in vanadium dioxide for potential technological applications.

Toward an Insulator-to-Metal Transition (IMT) in VOx Deposited by ALD: Huge Effect on Large IR Transmission Range

Vanadium dioxide (VO2) exhibits an insulating-to-metallic transition (IMT) from an insulating monoclinic to a metallic rutile phase at a IMT temperature (TIMT) of about 68 °C. This transition is accompanied by a decrease in electrical resistivity of 5 orders of magnitude and a drastic change in optical properties. Until recently, several works described the fundamental nature of the phase transition in a contradictory way, attributing it either to a structure-induced electron-phonon interaction: the Peierls transition [1], or to a strong electron-electron correlation: the Mott transition [2]. Nowadays, theoretical and experimental results [3-6] tend to show that the phase transition is affected by the crystal and electronic structures according to a mechanism combining the two opposed mechanisms and thus called collaborative Mott-Peierls transition.In order to achieve the growth of high-quality and conformal VO2 thin films and with a low thermal budget, Atomic Layer Deposition (ALD) is one of the most suitable ones, among physical and chemical vapor deposition techniques [7]. In this aim, vanadium oxide (VOx) films were deposited on both silicon (100) and glass substrates (figure with and without VOx) at 240°C using vanadium tri-isopropoxide (VTIP) asV-precursor and water as oxydant reagent. Deposited films, with thicknesses ranging from 30 nm to 120 nm depending on the number of ALD cycles, were annealed during one hour under a reducing atmosphere (forming gas) at four temperatures ranging from 400°C to 550°C with 50°C steps. Thanks to X-ray diffraction (XRD), and transmission electron microscopy (TEM), unannealed films were found amorphous independently of the substrate. Annealed films were found polycrystalline with a mixture of both VO2 and V2O5 (or V6O13) phases with crystallite lateral size up to 300 nm. RBS analysis showed how the chemical stoichiometry evolves with annealing temperature. The V content remains constant with annealing temperature while the O content and the film density were both decreasing under reducing conditions. By means of XPS measurements, at a temperature below TIMT, different valence states of V element were identified with a majority of V4+ ions and a minority of V3+ and V5+ ions. At a temperature above TIMT, a valence state modification occurs with a reduction of the V4+ ion concentration in favor of the V3+ and V5+ones. Cooling back under TIMT led almost to the same valence states initial reorganization with a majority of V4+.The variation of electrical and optical properties of VOx films occurring during IMT was also investigated. It was found that the resistivity r decreased down to 10-1 Ω.cm with a rising temperature above TIMT, and the r rose again with decreasing temperature with a hysteresis by contrast to the first drop. Dramatically, the optical transmittances of VOx films on Si wafer, measured with FTIR, displayed an important reduction during IMT of almost 70% in a broad spectral range from 2 to 20 µm wavelength. The optical properties of VOx on a glass substrate in UV-visible spectral range at IMT evidenced a modification of dielectric constant at IMT. Raman spectroscopy of VOx films around IMT showed the disappearance of low-temperature VO2 peaks, characteristic for this transition.In conclusion, crystalline VOx films were successfully produced by an ALD process and subsequent annealing at 500°C leading to films with reversible and reproducible IMT occurring at ~70°C. The important variation of some electrical or optical properties in infrared range paves the way for the light and heat monitoring in many applications including thermal stealth, heat management, tunable thermochromic or electrochromic windows.[1] Goodenough, J. B. The two components of the crystallographic transition in VO2.[2] Zylbersztejn, A. & Mott, N. F. Metal–insulator transition in vanadium dioxide. Phys. Rev. B 11, 4383–4395 (1975).[3] Cocker, T. L.; Titova, L. V.; Fourmaux, S.; Holloway, G.; Bandulet, H.-C.; Brassard, D.; Kieffer, J.-C.; El Khakani, M. A.; Hegmann, F. A. Phase Diagram of the Ultrafast Photoinduced, Insulator-Metal Transition in Vanadium Dioxide. Phys. Rev. B 2012,85, No. 155120.[4] Yao, T. et al. Understanding the Nature of the Kinetic Process in a VO2 Metal-Insulator Transition. Phys. Rev. Lett. 2010, 105, No. 226405.[5] Weber, C. et al. Vanadium dioxide: A Peierls–Mott insulator stable against disorder. Phys. Rev. Lett. 108, 256402 (2012).[6] Koethe, T. et al. Transfer of spectral weight and symmetry across the metal–insulator transition in VO2. Phys. Rev. Lett. 97, 116402(2006).[7] Prasadam, V. P. et al. Atomic layer deposition of vanadium oxides: process and application review. Mater. Today Chem. 12, 396–423 (2019). Figure 1

Reconfigurable Cascaded Thermal Neuristors for Neuromorphic Computing.

While the complementary metal-oxide semiconductor (CMOS) technology is the mainstream for the hardware implementation of neural networks, an alternative route is explored based on a new class of spiking oscillators called "thermal neuristors", which operate and interact solely via thermal processes. Utilizing the insulator-to-metal transition (IMT) in vanadium dioxide, a wide variety of reconfigurable electrical dynamics mirroring biological neurons is demonstrated. Notably, inhibitory functionality is achieved just in a single oxide device, and cascaded information flow is realized exclusively through thermal interactions. To elucidate the underlying mechanisms of the neuristors, a detailed theoretical model is developed, which accurately reflects the experimental results. This study establishes the foundation for scalable and energy-efficient thermal neural networks, fostering progress in brain-inspired computing.

High-performance thermochromic YSZ/[formula omitted]/YSZ coatings for energy-saving smart windows

The reversible semiconductor-to-metal transition of vanadium dioxide (VO2) makes VO2-based coatings a promising candidate for thermochromic smart windows, reducing the energy consumption of buildings. We report on a scalable sputter deposition technique for fast preparation of strongly thermochromic YSZ/V0.986W0.014O2/YSZ coatings, where YSZ denotes Y-stabilized ZrO2, on conventional soda-lime glass at a relatively low substrate surface temperature (350 °C) and without any substrate bias voltage. The thermochromic V0.986W0.014O2 layers and the antireflection YSZ layers were deposited using a controlled high-power impulse magnetron sputtering of a single V–W and Zr–Y target, respectively. A coating design utilizing a second-order interference in the YSZ layers was applied to increase both the integral luminous transmittance (Tlum) and the modulation of the solar energy transmittance (ΔTsol). We present the phase composition (X-ray diffraction) and microstructure (high-resolution transmission electron microscopy) of the coatings and their optical properties (spectrophotometry and spectroscopic ellipsometry). The YSZ/V0.986W0.014O2/YSZ coatings exhibit a transition temperature of 33–35 °C with Tlum = 64.5% and ΔTsol = 7.8% for a V0.986W0.014O2 thickness of 37 nm, and Tlum = 46.1% and ΔTsol = 13.2% for a V0.986W0.014O2 thickness of 67 nm. The results constitute an important step to a cost-effective and high-rate preparation of large-area thermochromic VO2-based coatings for future smart-window applications.

Electro‐optic modulator based on vanadium dioxide epsilon‐near‐zero nanorods embedded in silicon slot waveguide

AbstractWe present an electro‐optic modulator exploiting a metamaterial made of an array of vanadium dioxide nanorods operating in epsilon‐near‐zero regime as the active switching material in a silicon waveguide. The modulator takes advantage of the insulator‐to‐metal transition of vanadium dioxide along with near‐zero effective permittivity to achieve a modulation depth of 19.7 dB µm–1 in a footprint of 1.6 µm × 1 µm over a broad range of wavelengths. Using simulations, we demonstrate how the effective permittivity of the metamaterial can be tuned to a near‐zero value by varying the nanorod geometry to increase the modulation depth. The paper further investigates a novel hexagonal array design using the metamaterial nanorods to obtain a lower insertion loss and high modulation depth. The results provide insight into the design of ultra‐compact epsilon‐near‐zero modulators with high operation frequencies and low insertion losses.

Ultrafast X-ray imaging of the light-induced phase transition in VO2

Using light to control transient phases in quantum materials is an emerging route to engineer new properties and functionality, with both thermal and non-thermal phases observed out of equilibrium. Transient phases are expected to be heterogeneous, either through photo-generated domain growth or by generating topological defects, and this impacts the dynamics of the system. However, this nanoscale heterogeneity has not been directly observed. Here we use time- and spectrally resolved coherent X-ray imaging to track the prototypical light-induced insulator-to-metal phase transition in vanadium dioxide on the nanoscale with femtosecond time resolution. We show that the early-time dynamics are independent of the initial spatial heterogeneity and observe a 200 fs switch to the metallic phase. A heterogeneous response emerges only after hundreds of picoseconds. Through spectroscopic imaging, we reveal that the transient metallic phase is a highly orthorhombically strained rutile metallic phase, an interpretation that is in contrast to those based on spatially averaged probes. Our results demonstrate the critical importance of spatially and spectrally resolved measurements for understanding and interpreting the transient phases of quantum materials.

The Role of Local DFT+U Minima in the First-Principles Modeling of the Metal-Insulator Transition in Vanadium Dioxide.

The DFT+U method is frequently employed to improve the first-principles description of strongly correlated materials. However, it is prone to deliver metastable electronic minima. While these local minima of the DFT+U method are often considered to be computational artifacts, their physical meaning and relationship to true excited states remains unclear. In this work, the possibility of theoretically modeling transformations in the solid state that require thermal or optical excitations of electrons is explored, taking into account the metastable states of the computationally undemanding DFT+U formalism. For this purpose, we choose to examine the example of the VO2 metal-insulator transition. Metastable states that are located on different electronic potential energy surfaces are found to correspond to experimentally observed VO2 phases. The identified metastable electronic states can be used to model the collapse of the VO2 band gap at elevated temperatures and upon photoexcitation as well as other monoclinic-monoclinic phase transformations. The results suggest that local DFT+U minima can indeed carry physical meaning, while they remain under-reported in theoretical literature on transition metal oxides like VO2.

Effect of ethanol vapour sorption on the semiconductor-metal phase transition in powdered vanadium dioxide

We studied the effect of ethanol vapour on the parameters of the semiconductor-metal transition in a polycrystalline vanadium dioxide powder of the stable monoclinic phase a-VO2. It was synthesised by reducing vanadium pentoxide with oxalic acid while heated in air. The DC electrical resistance of the samples was studied between room temperature and 100оС in a tubular heater, in a microcompressor-generated air stream with saturated ethyl alcohol vapour. It was found that in the presence of ethanol vapour, the hysteresis loop shifted to higher temperatures by ten degrees and its cooling branch (corresponding to the transition of vanadium dioxide from the metallic phase to the semiconductor phase) became stepped.

Ultrafast infrared nano-imaging of far-from-equilibrium carrier and vibrational dynamics

Ultrafast infrared nano-imaging has demonstrated access to ultrafast carrier dynamics on the nanoscale in semiconductor, correlated-electron, or polaritonic materials. However, mostly limited to short-lived transient states, the contrast obtained has remained insufficient to probe important long-lived excitations, which arise from many-body interactions induced by strong perturbation among carriers, lattice phonons, or molecular vibrations. Here, we demonstrate ultrafast infrared nano-imaging based on excitation modulation and sideband detection to characterize electron and vibration dynamics with nano- to micro-second lifetimes. As an exemplary application to quantum materials, in phase-resolved ultrafast nano-imaging of the photoinduced insulator-to-metal transition in vanadium dioxide, a distinct transient nano-domain behavior is quantified. In another application to lead halide perovskites, transient vibrational nano-FTIR spatially resolves the excited-state polaron-cation coupling underlying the photovoltaic response. These examples show how heterodyne pump-probe nano-spectroscopy with low-repetition excitation extends ultrafast infrared nano-imaging to probe elementary processes in quantum and molecular materials in space and time.

Transfer of the sputter technique for deposition of strongly thermochromic VO2-based coatings on ultrathin flexible glass to large-scale roll-to-roll device

The reversible semiconductor-to-metal transition of vanadium dioxide (VO2) makes VO2-based coatings a promising candidate for thermochromic smart windows, reducing the energy consumption of buildings. We report on transfer of the sputter technique for deposition of strongly thermochromic ZrO2/W-doped VO2/ZrO2 coatings on ultrathin (0.1 mm) flexible glass from a laboratory-scale device with three (V, W and Zr) planar magnetron targets to a large-scale roll-to-roll device with two (W-doped V and ZrO2) rotatable magnetron targets. The depositions were performed at a relatively low substrate surface temperature (330–350 °C) and without any substrate bias voltage. The W-doped VO2 layers were deposited using a reactive high-power impulse magnetron sputtering with a pulsed O2 flow control. We compare the process parameters used in both deposition devices and explain the basic principle of this sputter technique using the discharge characteristics measured during a large-scale roll-to-roll deposition. We characterize the design, structure (X-ray diffraction) and optical properties (spectrophotometry and spectroscopic ellipsometry) of the three-layer coatings. The coatings prepared on ultrathin flexible glass using the large-scale roll-to-roll device at a temperature close to 350 °C exhibit a low transition temperature of 22 °C, an integral luminous transmittance over 45% and a modulation of the solar energy transmittance approaching 10%. This is a promising first step to a cost-effective and high-rate preparation of large-area thermochromic VO2-based coatings for future smart-window applications.

Quantitative hyperspectral coherent diffractive imaging spectroscopy of a solid-state phase transition in vanadium dioxide.

Solid-state systems can host a variety of thermodynamic phases that can be controlled with magnetic fields, strain, or laser excitation. Many phases that are believed to exhibit exotic properties only exist on the nanoscale, coexisting with other phases that make them challenging to study, as measurements require both nanometer spatial resolution and spectroscopic information, which are not easily accessible with traditional x-ray spectromicroscopy techniques. Here, we use coherent diffractive imaging spectroscopy (CDIS) to acquire quantitative hyperspectral images of the prototypical quantum material vanadium oxide across the vanadium L 2,3 and oxygen K x-ray absorption edges with nanometer-scale resolution. We extract the full complex refractive indices of the monoclinic insulating and rutile conducting phases of VO2 from a single sample and find no evidence for correlation-driven phase transitions. CDIS will enable quantitative full-field x-ray spectromicroscopy for studying phase separation in time-resolved experiments and other extreme sample environments where other methods cannot operate.

Single-nanoantenna driven nanoscale control of the VO2 insulator to metal transition

Abstract The ultrafast concentration of electromagnetic energy in nanoscale volumes is one of the key features of optical nanoantennas illuminated at their surface plasmon resonances. Here, we drive the insulator to metal phase transition in vanadium dioxide (VO2) using a laser-induced pumping effect obtained by positioning a single gold nanoantenna in proximity to a VO2 thermochromic material. We explore how the geometry of the single nanoantenna affects the size and permittivity of the nanometer-scale VO2 regions featuring phase transition under different pumping conditions. The results reveal that a higher VO2 phase transition effect is obtained for pumping of the longitudinal or transversal localized surface plasmon depending on the antenna length. This characterization is of paramount importance since the single nanoantennas are the building blocks of many plasmonic nanosystems. Finally, we demonstrate the picosecond dynamics of the VO2 phase transition characterizing this system, useful for the realization of fast nano-switches. Our work shows that it is possible to miniaturize the hybrid plasmonic-VO2 system down to the single-antenna level, still maintaining a controllable behavior, fast picosecond dynamics, and the features characterizing its optical and thermal response.

Semiconductor-to-metal transition in atomic layer deposition (ALD) of VO2 films using VCl4 and water

The semiconductor-to-metal transition of vanadium dioxide (VO2) films is studied using temperature-dependent Raman, optical, and electrical measurements. The VO2 films are deposited via an atomic layer deposition (ALD) process using alternate pulses of vanadium tetrachloride and H2O at 350 °C. A growth rate of 0.021 nm/cycle and a thickness of 33 nm of VO2 are obtained for all films studied. The phase of the film is determined using x-ray diffraction. The as-deposited films are amorphous and are transformed to the monoclinic phase with a post-deposition, forming gas anneal at temperatures ≥ 500 °C for 60 min. The purity of the films is determined using x-ray photoelectron spectroscopy and no evidence of residual chlorine is detected. The temperature-dependent Raman Ag mode of the monoclinic VO2 phase is observed to monotonically decrease from 25 °C to 78 °C; where no evidence of the Ag peak is observed in the film beyond 68 °C. The refractive index and extinction coefficient extracted from temperature-dependent ellipsometry confirm that, beyond 68 °C, free carriers are generated in the film. Electrical measurements performed on a fabricated p++Si/VO2/Ti/Au device show a semiconductor-to-metal transition behavior with a high resistance of 14701 ± 2284 Ω at 62 °C and a low resistance of 1064.1 ± 143 Ω at 67 °C. This work demonstrates that a halide-based ALD process provides a clean and robust approach to synthesizing high-quality VO2 films.

Wideband switchable dual-functional terahertz polarization converter based on vanadium dioxide-assisted metasurface

The terahertz technology has attracted considerable attention because of its potential applications in various fields. However, the research of functional devices, including polarization converters, remains a major demand for practical applications. In this work, a reflective dual-functional terahertz metadevice is presented, which combines two different polarization conversions through using a switchable metasurface. Different functions can be achieved because of the insulator-to-metal transition of vanadium dioxide (VO2). At room temperature, the metadevice can be regarded as a linear-to-linear polarization convertor containing a gold circular split-ring resonator (CSRR), first polyimide (PI) spacer, continuous VO2 film, second PI spacer, and gold substrate. The converter possesses a polarization conversion ratio higher than 0.9 and a bandwidth ratio of 81% in a range from 0.912 THz to 2.146 THz. When the temperature is above the insulator-to-metal transition temperature (approximately 68 °C) and VO2 becomes a metal, the metasurface transforms into a wideband linear-to-circular polarization converter composed of the gold CSRR, first PI layer, and continuous VO2 film. The ellipticity is close to −1, while the axis ratio is lower than 3 dB in a range of 1.07 THz–1.67 THz. The metadevice also achieves a large angle tolerance and large manufacturing tolerance.

Experimental Dynamics of Temperature-Initiated Metal–Insulator Transition in Vanadium Dioxide

We present an analytical review of key experiments concerning the dynamics of reversible temperature-initiated metal–insulator transition in vanadium dioxide, which results in anomalous physical phenomena in optical, electric, thermal, and other properties of this material. We highlight the analysis of temperature hysteresis curves, which are the main source of information on the phase transition, and changes in the temperature position of phase transition. In the metal–dielectric phase transition, the electric, optical, thermal, and magnetic properties undergo anomalously large and rapid changes, which opens radically new possibilities for applying unique properties of this phase transition in special-purpose instrument making.

Switchable diurnal radiative cooling by doped VO<sub>2</sub>

This paper presents design and simulation of a switchable radiative cooler that exploits phase transition in vanadium dioxide to turn on and off in response to temperature. The cooler consists of an emitter and a solar reflector separated by a spacer. The emitter and the reflector play a role of emitting energy in mid-infrared and blocking incoming solar energy in ultraviolet to near-infrared regime, respectively. Because of the phase transition of doped vanadium dioxide at room temperature, the emitter radiates its thermal energy only when the temperature is above the phase transition temperature. The feasibility of cooling is simulated using real outdoor conditions. We confirme that the switchable cooler can keep a desired temperature, despite change in environmental conditions.

Effect of phase transition of vanadium dioxide on resonance characteristics of terahertz anti-resonant fiber and its applications

Terahertz (THz) wave is an electromagnetic wave with frequency in a range of 0.1–10 THz, which possesses excellent photonic and electronic properties. THz wave has higher penetration and lower photon energy to non-polar materials, which makes it possess great academic value in medical, non-destructive testing and other related fields. In addition, the features such as wide bandwidth and large communication capacity of THz wave allow it to be widely used in communication, radar detection and other applications. Despite its rapid development in recent years, THz technology is used still mainly in free space currently and it is difficult to control the transmission direction of THz wave over a long distance in free space. What is more, the transmission of THz waves in free space is affected usually by the dust and water vapor. For achieving the efficient transmission of THz waves, researchers have proposed a variety of THz waveguides, including plastic fiber, Bragg fiber, photonic crystal fiber and anti-resonant fiber (ARF). The ARF confines the incident beam within the air hole of fiber center by the anti-resonance effect, which has aroused great interest because of its simple structure, low transmission loss, high damage threshold, low dispersion, and high transmission bandwidth. At present, adjustable THz fiber devices based on ARF are still reported rarely. In the near-infrared band, researchers have combined ARF with vanadium dioxide (VO<sub>2</sub>) to realize the exceptional modulation effects. The VO<sub>2</sub> is a metal oxide with insulator-metal phase transition when the ambient temperature is near 68 ℃, in which its electrical conductivity, dielectric constant and other properties will change drastically. In this paper, the VO<sub>2</sub> is coated on the inner wall of the THz ARF cladding tubes, and the effect of the phase transition of VO<sub>2</sub> on the propagation characteristics of the ARF is studied. Simulation results indicate that in the THz band, the phase transition of VO<sub>2</sub> will cause the anti-resonance period of the ARF to change greatly, in which the confinement effect of the ARF cladding tubes on the incident beam is converted from anti-resonant state to resonant state. Without changing the structure of the ARF, the effective modulation on the THz wave in the core of the ARF can be achieved only by controlling the phase transition of VO<sub>2</sub>, which has a wide application prospect in the field of THz adjustable devices. In this paper, a THz optical switch and a polarization controller based on VO<sub>2</sub>-coated ARF are proposed. With the optical switch being on and off, the corresponding losses are 0.5 dB/m and 110 dB/m respectively at 120 μm. If phase transition of VO<sub>2</sub> is induced by the excitation laser, it is expected to realize a fast-optical switch. Regarding the polarization controller, the polarization state and polarization direction of the THz wave in the core of the ARF can be controlled, and the birefringence coefficient of the ARF in the polarization state is more than 1.4 × 10<sup>–4</sup>.

Pulsed Magnetron Sputtering of Strongly Thermochromic VO2-Based Coatings with a Transition Temperature of 22 °C onto Ultrathin Flexible Glass

The reversible semiconductor-to-metal transition of vanadium dioxide (VO2) makes VO2-based coatings a promising candidate for thermochromic smart windows, reducing the energy consumption of buildings. This paper deals with maximizing the application potential of these coatings in terms of their performance, an industry-friendly preparation technique, and an industrially relevant substrate. We present a scalable sputter deposition technique for the preparation of strongly thermochromic ZrO2/V0.984W0.016O2/ZrO2 coatings on ultrathin flexible glass and standard glass at a relatively low substrate surface temperature (330 °C) and without any substrate bias voltage. The V0.984W0.016O2 layers were deposited by a controlled high-power impulse magnetron sputtering of a V target, combined with a simultaneous pulsed dc magnetron sputtering of a W target. We explain the fundamental principles of this technique using the discharge characteristics measured for both discharges. We characterize the coating structure (X-ray diffraction) and a wide range of optical properties (spectrophotometry and spectroscopic ellipsometry). We find that the coatings combine a transition temperature of 22 °C, a luminous transmittance approaching 50%, a modulation of the solar energy transmittance over 10% and a temperature-independent color. The results in general, and the successful transfer from a standard glass to the ultrathin flexible glass in particular, are crucial for future applications of the coatings on smart windows.