Doped Vanadium Dioxide Research Articles

Agile Inverse Design of Polarization-Independent Multi-Functional Reconfiguration Metamaterials Based on Doped VO2.

Increasing attention is being paid to the application potential of multi-functional reconfigurable metamaterials in intelligent communication, sensor networks, homeland security, and other fields. A polarization-independent multi-functional reconfigurable metasurface based on doped vanadium dioxide (VO2) is proposed in this paper. It can be controlled to switch its function among three working modes: electromagnetically induced absorption (EIA), electromagnetically induced transparency (EIT), and asymmetrical absorption. In addition, deep learning tools have greatly accelerated the design of relevant devices. Such devices and the method proposed in this paper have important value in the field of intelligent reconfigurable metamaterials, communication, and sensing.

Optical, Electrical, Structural, and Thermo-Mechanical Properties of Undoped and Tungsten-Doped Vanadium Dioxide Thin Films.

The undoped and tungsten (W)-doped vanadium dioxide (VO2) thin films were prepared by electron beam evaporation associated with ion-beam-assisted deposition (IAD). The influence of different W-doped contents (3-5%) on the electrical, optical, structural, and thermo-mechanical properties of VO2 thin films was investigated experimentally. Spectral transmittance results showed that with the increase in W-doped contents, the transmittance in the visible light range (400-750 nm) decreases from 60.2% to 53.9%, and the transmittance in the infrared wavelength range (2.5 μm to 5.5 μm) drops from 55.8% to 15.4%. As the W-doped content increases, the residual stress in the VO2 thin film decreases from -0.276 GPa to -0.238 GPa, but the surface roughness increases. For temperature-dependent spectroscopic measurements, heating the VO2 thin films from 30 °C to 100 °C showed the most significant change in transmittance for the 5% W-doped VO2 thin film. When the heating temperature exceeds 55 °C, the optical transmittance drops significantly, and the visible light transmittance drops by about 11%. Finally, X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to evaluate the microstructure characteristics of VO2 thin films.

Multilevel Information Encryption Mediated by Reconfigurable Thermal Emission in Smart Bilayer Material

AbstractSmart thermal emission, which is response to external stimuli, has provided an unparalleled approach to exploiting optical devices that allows for tunable photon radiance. Here, an approach of implementing information encryptions using a smart thermal emission material that exhibits non‐volatile, multifunctional, and on‐chip‐integrable properties is reported. The smart material is a bilayer structure, consisting of a tungsten (W)‐doped vanadium dioxide (VO2) thin film layer and a self‐assembled polystyrene (PS) microsphere layer. Significant differences between the phase transition temperatures of differently doped VO2 films enable reconfigurable thermal emission in response to heat. Levels of information recorded by the patterning of these VO2 films are thus encrypted thermo‐radiatively. Additionally, the structural color of PS layer, as a result of photonic crystal bandgap effect, shades the encoded patterns from visible observation, and provides an anti‐counterfeiting function. Notable observation angle independence and device‐friendly encoding processes are also demonstrated. Based on these thermo‐tunable emission characteristics, the concept of multilevel data encryption via infrared radiation detection, temperature dependence, and visible anti‐counterfeiting, is realized by the smart bilayer material.

Role of 4f electrons and 3d-4f hybridization in metal-insulator transition in RE (La, Nd, Sm, Eu, Dy and Er)-doped vanadium dioxide for thermochromic applications

Vanadium dioxide is a promising thermochromic material for novel applications to smart optoelectronic devices due to its reversible insulator-to-metal transition which is accompanied by rapid variation in its optical properties. In this work, rare-earth (La, Nd, Sm, Eu, Dy and Er) dopants substituting vanadium site were investigated in detail to control the phase transition temperature (TMIT) and optical properties of VO2. It is found that TMIT is mainly related to the nature of 3d-4f hybridization in RE-doped VO2(M1): the direct overlapping of 3d-4f orbitals contributes to V–V dimerization, but it is less pronounced to lower TMIT; the indirect 3d-4f interaction via O-2p makes 4f electrons itinerant to egπ states to assist MIT from M1 to R phase at lower temperatures. The dominating nature of 3d-4f hybridization varies with changing RE dopant's ionic radius and electronegativity. Er offered the lowest TMIT with a decreasing rate of 12.8 °C/at. % due to effective 4f-O2p-3d hybridization. RE dopants were also found effective in changing optical properties by scaling Eg1 and Eg2 bandgaps of VO2. Hence, the fashion of 3d-4f interaction in RE-doped VO2 is found to play important role in modulating thermochromic properties of VO2 for smart window applications and smart optoelectronics devices.

Characterization of the VO2 thin films grown on glass substrates by MOD

AbstractThis paper describes the deposition of Nb doped vanadium dioxide (VO2) films on a glass substrate by metal organic decomposition (MOD) and their thermochromic properties. The difference in thermochromic properties of VO2 thin films on a glass substrate was investigated with and without a buffer layer of Hf0.5Zr0.5O2 (HZO). The phase transition temperature of VO2 thin film successfully reduced from 83°C to 43°C on a glass substrate with a buffer layer of HZO. Without a buffer layer of HZO, the thermochromic properties of VO2 thin films deteriorated comparing with a buffer layer of HZO. HZO buffer layer effectively suppresses the miniaturization of VO2 crystallite size of thin film due to Nb doping. Moreover, it would block out the diffusion of Al, Na, and Ca impurity ions from a glass substrate and the partial oxidation of VO2 thin films judging from XPS O1s spectra and XPS depth profile analysis. We conclude that the insertion of the HZO buffer layer is a useful technique for controlling the transition temperature of VO2 for the smart window applications by MOD.

Characterization of the VO<sub>2</sub> Thin Films Grown on Glass Substrates by MOD

This paper describes the deposition of Nb doped vanadium dioxide (VO2) films on a glass substrate by metal organic decomposition (MOD) and their thermochromic properties. The difference in thermochromic properties of VO2 thin films on a glass substrate was investigated with and without a buffer layer of Hf0.5Zr0.5O2 (HZO). The phase transition temperature of VO2 thin film successfully reduced from 83 to 43℃ on a glass substrate with a buffer layer of HZO. Without a buffer layer of HZO, the thermochromic properties of VO2 thin films deteriorated comparing with a buffer layer of HZO. HZO buffer layer effectively suppresses the miniaturization of VO2 crystallite size of thin film due to Nb doping. Moreover, it would block out the diffusion of Al, Na and Ca impurity ions from a glass substrate and the partial oxidation of VO2 thin films judging from XPS O1s spectra and XPS depth profile analysis. We conclude that the insertion of the HZO buffer layer is a useful technique for controlling the transition temperature of VO2 for the smart window applications by MOD.

VO2 Tungsten Doped Film IR Perfect Absorber

We investigated infrared reflectivity of undoped and Tungsten (W) doped Vanadium dioxide (VO2) films at varying temperatures. Undoped VO2 exhibited a clear phase transition at 100°C, achieving near 0% reflectivity, or perfect light absorption. As W doping concentration increased, the phase-transition temperature decreased, maintaining the zero-reflectivity condition. Only a 0.75% W doping enabled room temperature perfect absorption without heating the film.

Remote Monitoring of Skin Temperature Through a Wristband Employing a Printed VO Sensor

The need for highly sensitive, environmentally stable, mechanically flexible, and low-cost temperature sensors for on-body measurements has been increasing with the wide adoption of personal healthcare-Internet-of-Things (H-IoT) devices. Printed electronics (PE) is a good platform for such sensors because it enables the realization of flexible devices through simple and rapid methods at a relatively low cost. However, previously reported printed temperature sensors suffer from poor sensitivity and/or environmental instability. In this article, we report a custom tungsten (W)-doped vanadium dioxide (VO2) ink-based screen-printed temperature sensor having the highest temperature coefficient of resistance (TCR) of 2.78% C-1 with a resolution of 0.1 °C between 30 °C and 40 °C. To protect it from environmental effects, a fluoropolymer-basedpassivation layer is added for accurate temperature readings even in 90% relative humidity. The sensor is printed on a flexible substrate and shows minimal deterioration in performance over 1000 bending cycles. For wearability and remote monitoring, the sensor is integrated with a custom Bluetooth low energy (BLE) wireless readout in the form of wristband. The BLE readout comprises an ultrathin and flexible patch antenna optimized for both BLE bandwidth (BW) and human wearability. It demonstrates a minimal specific absorption rate (SAR) value of only 0.068 W/kg, making it safe to wear. Despite the antenna’s thin structure (0.004λ), it has a gain of 1.65 dBi, enabling an excellent communication range. The proposed wristband is tested on ten volunteers and under daily activities, which shows promising results with a maximum error of 0.16 °C with reference to those of a commercial thermometer.

The effect of doped glazing on thermal comfort: A case study of solar decathlon Africa

This work is devoted to the study of the optimization of the energy performance of a modern architectural building with large glass surfaces. Within the framework of this study, we were led to use intelligent glazing. It was decided that the glass surfaces would be covered with a tungsten (W)-doped vanadium dioxide (VO2) film. This film is characterized by a phase transition, allowing its transmission coefficient to be high at low temperatures and low at high temperatures. An optimization study of the influence of the doping on the transfer coefficient of the glazing allowed us to obtain a significant improvement in the comfort conditions of the studied building. The results of our study indicate that the VO2/W film coating has a great potential to increase the comfort conditions in buildings, both in winter and in summer. This is because the film can modulate the solar radiation from the outside and thus maintain an ideal temperature inside without the need for an energy-consuming air conditioning system. As a result, VO2/W cladding film offers buildings significant thermal benefits and improved energy efficiency, but also a better quality of life as it provides a comfortable and pleasant indoor environment.

Understanding the partial substitution effect of Mg in Zn-V aqueous batteries

Although vanadium dioxide with various valence states, high theoretical capacity and low cost have been demonstrated as the ideal cathode materials for aqueous zinc-ion batteries (AZIBs), its unstable structure inhibits its further development. Herein, a structurally stable magnesium doped vanadium dioxide (Mg0.16VO2·0.34H2O) was prepared by a simple hydrothermal reaction. The regular of pre-intercalation Mg2+ on the structural and electrochemical properties of cathode was further explored through varying the doping amount of Mg2+. According to the experimental results, a small amount of Mg2+ insertion is not sufficient to stabilize the cathode, while excessive Mg2+ insertion will reduce capacity. The optimal parameter was obtained when the doping content of Mg2+ was 2 mmol, MgHVO/Zn batteries get specific capacity (315.4 mAh/g at a current density of 1 A/g) and outstanding long circulation stability (the capacity retention of 86.36 % over 10,00 cycles even at 5 A/g). And the electrochemical mechanism was explained by a series of in situ tests. This work reveals the regulation of the cathode structure by pre-intercalation Mg2+ and provides some insights for future pre-intercalation cathode regulation.

Influence of germanium substitution on the structural and electronic stability of the competing vanadium dioxide phases

We present a density-functional theory (DFT) study of the structural, electronic, and chemical bonding behavior in germanium (Ge)-doped vanadium dioxide $({\mathrm{VO}}_{2})$. Our motivation is to explain the reported increase of the metal-insulator transition temperature under Ge doping and to understand how much of the fundamental physics and chemistry behind it can be captured at the conventional DFT level. We model doping using a supercell approach, with various concentrations and different spatial distributions of Ge atoms in ${\mathrm{VO}}_{2}$. Our results suggest that the addition of Ge atoms strongly perturbs the high-symmetry metallic rutile phase and induces structural distortions that partially resemble the dimerization of the experimental insulating structure. Our work, therefore, hints at a possible explanation of the observed increase in transition temperature under Ge doping, motivating further studies into understanding the interplay of structural and electronic transitions in ${\mathrm{VO}}_{2}$.

The Origin of the Thermochromic Property Changes in Doped Vanadium Dioxide.

Vanadium dioxide is a promising material for novel smart window applications due to its reversible metal-insulator transition which is accompanied by a change in its optical properties. The transition temperature (TMIT) can be controlled via elemental doping, but the reduction of TMIT is generally coupled with a decrease of the optical contrast between the two phases. To better understand how the contrast is fundamentally connected to TMIT, the thermochromic properties of doped VO2 were theoretically investigated across the metal-insulator transition from first principles. Different dopants and their interaction with the VO2 host structure as well as different modes of doping were studied in detail. It was found that the transition temperature change is mainly related to the stabilization of the high-temperature metallic phase due to lattice deformations which are caused by the presence of the dopant ion. Inherent limitations to the thermochromic performance of VO2 substitutionally doped by the replacement of vanadium cations with other species were found, and alternative approaches were proposed. Specifically, a charge-neutral substitution of oxygen or an oxygen substitution in combination with interstitial doping without net charge transfer between the dopant atoms and VO2 were identified as promising avenues to ensure a low TMIT and no loss of optical contrast in vanadia-based smart window materials.

Flexible smart photovoltaic foil for energy generation and conservation in buildings

Building-integrated solar cells not only generate electricity but also impact the envelope thermal characteristics, thus changing the micro-climate of buildings. Therefore, the collective effects of energy conservation and generation on buildings should be considered in one design. In this work, a smart photovoltaic window foil with near-infrared (NIR) modulation and low long-wavelength IR emissivity has been fabricated by combining organic perovskite and inorganic tungsten doped vanadium dioxide nanoparticles (W-VO2 NPs). The W-VO2 offers solar modulation and promotes extraction and transport of photogenerated charges, giving peak power conversion efficiency of 15.4% at 25 °C and 16.1% at 45 °C and average visible transmission of 25.5%, comparable to the best reported semitransparent perovskite solar cell with an additional favorable NIR modulation of 10.7%. This new strategy to integrate energy-saving functionality into photovoltaic foil initiates a new design rule for smart window development.

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.

Infrared stealth nanofibrous composites with thermal adaptability and mechanical flexibility

Thermal adaptability is one of the key parameters for infrared stealth materials regarding the applications in infrared camouflage and energy saving aircrafts and buildings. By employing polyacrylonitrile (PAN) as the polymer matrix, antimony doped tin oxide (ATO) and vanadium dioxide (VO2) as the fillers, fibrous membrane composites of ATO/PAN-xVO2 (x =0, 20, 40, and 60 mol %) are prepared by electrospinning followed by sintering. The infrared stealth performances including the emissivity and infrared images, as well as the uniaxial stress-strain relationship of the fibrous membranes are characterized. At the temperature range of 25–90 °C, the infrared emissivity (ε) decreases continuously for ATO/PAN (ATO/PAN-0VO2) membrane, while there is an abrupt drop around 68 °C for ATO/PAN-xVO2 (x = 20, 40, and 60 mol %) membranes. The infrared images taken at different temperatures further verifies the infrared stealth performance for the four fibrous composites. Due to the thermistor behavior of ATO, with the temperature rising, increase in the electronic conductivity renders the composite enhanced infrared stealth performance. The abrupt drop near 68 °C for composites with VO2 is related to the phase transition of VO2 from monoclinic (semiconductor) to rutile structure (metal). The stress-strain behavior under the uniaxial tension reveals that the fracture strength and strain decrease monotonically with VO2 content increasing. Our results offer a selection of thermal adaptive materials with combined infrared stealth and mechanical behavior.

Effect of monolayer graphene on the performance of near-field radiative thermal rectifier between doped silicon and vanadium dioxide

We investigate near-field radiative thermal rectifiers (NFRTRs) comprising an asymmetric nanostructure with and without graphene coatings. The asymmetric nanostructure consists of n-type doped silicon (D-Si) and vanadium dioxide (VO2) plates separated by a vacuum gap. On the basis of the stochastic Maxwell equations and fluctuation-dissipation theorem, we analyse the effect of graphene on the near-field radiative heat transfer (NFRHT) and the performance of the NFRTR. We find that the total thermal rectification factor (TTRF) of an NFRTR composed of n-type D-Si and VO2 plates can be significantly enhanced by the presence of graphene, depending on the doping concentration of Si, the chemical potential value of the graphene, and the vacuum gap. When both n-type D-Si and VO2 plates are covered by a layer of graphene, the TTRF of the NFRTR whose n-type D-Si and VO2 plates are separated by a 10 nm vacuum gap improves from 4.38 to 7.79 for a doping concentration of 1019 cm−3 and a chemical potential of 0.25 eV. We attribute this to the strong interaction among the p-polarized surface modes of graphene-covered n-type D-Si with the doping concentration of 1019 cm−3, p-polarized surface modes of graphene-covered insulating VO2, and p-polarized hyperbolic modes (HMs) of insulating VO2. This work is important for near-field radiative thermal management and the application of NFRHT-based thermal devices.

Modelling the enthalpy change and transition temperature dependence of the metal-insulator transition in pure and doped vanadium dioxide.

We compare various calculation methods to determine the electronic structures and energy differences of the phases of VO2. We show that density functional methods in the form of GGA+U are able to describe the enthalpy difference (latent heat) between the rutile and M1 phases of VO2, and the effect of doping on the transition temperature and on the band gap of the M1 phase. An enthalpy difference of ΔE0 = -44.2 meV per formula unit, similar to the experimental value, is obtained if the randomly oriented spins of the paramagnetic rutile phase are treated by a non-collinear spin density functional calculation. The predicted change in the transition temperature of VO2 for Ge, Si or Mg doping is calculated and is in good agreement with the experiment data.

Thermal and energy performance investigation of a smart double skin facade integrating vanadium dioxide through CFD simulations

Double skin facades (DSFs) are known by their capacity to generate a natural heating energy during winter by increasing the greenhouse effect in the air cavity between the inner and the outer facade. However, this effect provides an undesirable overheating during summer. In this study, a smart DSF configuration able to control the greenhouse effect generation was numerically simulated using a two dimensional Computational Fluid Dynamic (CFD) method. The aim of these simulations is to investigate the thermal and the energetic behavior of a DSF integrating Tungsten (W) doped Vanadium dioxide (VO2) as an optically smart thin material and a high absorbing aluminum nitride (AlN) coating. A parametric study was carried out in order to analyze the impact of the air cavity thickness on the smart DSF behavior. Four thicknesses (10, 25, 50 and 75 mm) were inspected and compared with an uninsulated massive wall configuration. Results have shown that the smart configuration reduces significantly both heating and cooling loads. Indoor surface temperature has been decreased by around 2 °C during summer and increased by around 3 °C during a sunny winter day. The air velocity results have shown that the natural convection-radiation interaction during winter is higher than during summer. Thus, the new smart DSF configuration presented in this paper can effectively control the greenhouse effect depending on external climatic conditions.

Gradual tuning of the terahertz passband using a square-loop metamaterial based on a W-doped VO2 thin film

An electrically controllable terahertz (THz) square-loop metamaterial based on tungsten (W)-doped vanadium dioxide (VO2) films was designed and evaluated. The structure acts as both the bandpass filter and micro-heater. The various W concentrated VO2 films used to integrate with the metamaterial were successfully grown through the sol-gel method. The transition temperature decreased by ∼14 K/at% and the resistance–temperature hysteresis curves broadened with higher doping concentrations. By utilizing the designed device, the transmitted THz wave can be tuned precisely, and the modulation depth can reach up to 0.72 in the 0.3–0.7 THz passband with a bias voltage below 2 V.

Influence of the electrochemical properties of vanadium oxides on specific capacitance by molybdenum doping

Molybdenum (Mo)-doped vanadium dioxide (\(\hbox {VO}_{2}\)(B)) nanobelts were successfully synthesized using commercial vanadium pentoxide (\(\hbox {V}_{2}\hbox {O}_{5}\)) as the starting material and ammonium molybdate as the dopant by a simple hydrothermal route. Then, Mo-doped \(\hbox {VO}_{2}\)(B) nanobelts were transformed to Mo-doped \(\hbox {V}_{2}\hbox {O}_{5}\) nanobelts by calcination at \(400{^{\circ }}\hbox {C}\) under an air atmosphere. The samples were characterized by X-ray powder diffraction, energy-dispersive X-ray spectrometer, elemental mapping, X-ray photoelectron spectroscopy, X-ray fluorescence and transmission electron microscopy techniques. The results showed that Mo-doped \(\hbox {VO}_{2}\)(B) and \(\hbox {V}_{2}\hbox {O}_{5}\) solid solution with high purity were obtained. The electrochemical properties of Mo-doped \(\hbox {VO}_{2}\)(B) and \(\hbox {V}_{2}\hbox {O}_{5}\) nanobelts as supercapacitor electrodes were measured using cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD). The specific capacitance of \(\hbox {VO}_{2}\)(B) nanobelts slightly declines with Mo doping, however, the specific capacitance of \(\hbox {V}_{2}\hbox {O}_{5}\) nanobelts greatly improves with Mo doping. Mo-doped \(\hbox {V}_{2}\hbox {O}_{5}\) nanobelts exhibit the specific capacitance as high as 526 F \(\hbox {g}^{-1}\) at the current density of 1 A \(\hbox {g}^{-1}\). Both CV and GCD curves show that they have good rate capability and retain 464, 380, 324 and 273 F \(\hbox {g}^{-1}\) even at a high-current density of 2, 5, 10 and 20 A \(\hbox {g}^{-1}\), respectively. It turns out that Mo-doped \(\hbox {V}_{2}\hbox {O}_{5}\) nanobelts are ideal materials for supercapacitor electrodes in the present work.