W-doped VO2 Thin Film Research Articles

Developing an Analysis Procedure and Dispersion Model for Pristine and W-Doped VO2 Thin Films Using Density Functional Theory and Spectroscopic Ellipsometry.

Vanadium dioxide (VO2) and its unique phase transition from semiconductor to metal near room temperature (TIMT = 68 °C) offer significant potential for applications in smart materials and advanced technologies. This transition is accompanied by a drastic modulation of VO2's optical properties in the near- and far-infrared regions. Tungsten (W) has been successfully used as a dopant to lower the transition to room temperature. VO2 is highly dependent on the synthesis method, as for each fabrication protocol, the optical properties differ. Therefore, the optical properties of VO2 must be determined frequently. In this work, a universal analysis procedure to accurately determine the optical properties of all pristine VO2 thin films is presented. Density functional theory is employed to create a dispersion model specifically catered to VO2, a novel approach that justifies the oscillator center energies. This dispersion model explicates the four different contributions to the absorption of VO2 between 2 and 5 eV. We showcase the versatility of our dispersion model by applying it to data sets from the literature, correctly fitting each one. We then further illustrate the robustness of the analysis procedure by successfully applying it to W-doped VO2 thin films. This allows for a direct comparison of the optical properties of pristine and W-doped VO2 well below and well above the transition temperature for the first time. We show that W-doping affects the optical properties almost exclusively in the low-temperature phase for near-infrared photon energies. Indeed, the optical absorption of the doped films is higher than that of the pristine films for photon energies below 1 eV, and the onset of optical absorption is lowered to near 0 eV as opposed to 0.4 eV for pristine VO2.

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.

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.

Stabilization of the VO2(M2) Phase and Change in Lattice Parameters at the Phase Transition Temperature of WXV1-XO2 Thin Films.

Various methods have been used to fabricate vanadium dioxide (VO2) thin films exhibiting polymorph phases and an identical chemical formula suited to different applications. Most fabrication techniques require post-annealing to convert the amorphous VO2 thin film into the VO2 (M1) phase. In this study, we provide a temperature-dependent XRD analysis that confirms the change in lattice parameters responsible for the metal-to-insulator transition as the structure undergoes a monoclinic to the tetragonal phase transition. In our study, we deposited VO2 and W-doped VO2 thin films onto silica substrates using a high repetition rate (10 kHz) fs-PLD deposition without post-annealing. The XRD patterns measured at room temperature revealed stabilization of the monoclinic M2 phase by W6+ doping VO2. We developed an alternative approach to determine the phase transition temperatures using temperature-dependent X-ray diffraction measurements to evaluate the a and b lattice parameters for the monoclinic and rutile phases. The a and b lattice parameters versus temperature revealed phase transition temperature reduction from ∼66 to 38 °C when the W6+ concentration increases. This study provides a novel unorthodox technique to characterize and evaluate the structural phase transitions seen on VO2 thin films.

Tunable IR perfect absorbers enabled by tungsten doped VO2 thin films

The temperature tunability of complex dielectric constants of vanadium dioxide (VO2) makes it a promising phase-change material for use in active, dynamic, tunable photonics applications. Specifically, the semiconductor-to-metal phase transition in VO2 enables reversible, broadband, and large complex refractive index variation and paves the way for a plethora of applications. Although the critical temperature for phase-transition is 68 °C for VO2 films, its transition temperature can be reduced to room temperature by tungsten-doping of vanadium dioxide. Such a degree of freedom in controlling the critical temperature through tungsten doping provides further tunability of the thermochromic behavior. In this work, we investigate a variety of W-doped VO2 thin films deposited by laser ablation of targets with increasing W doping content and report detailed infrared characterization together with numerical simulations. Our experimental results indicate that the perfect absorption can be achieved at different temperatures, within the VO2 insulator-to-metal phase transition process, as a function of W doping content. Tunable subwavelength layers allow perfect absorption under different temperature conditions around λ = 12 µm. We show that a high dynamic range of reflectivity can be achieved when the temperature is increased above the phase transition temperature. Furthermore, we observe perfect absorption at 11.8 µm at room temperature for a W content of 0.75%. We believe that W-doped VO2 thin films with tunable and controllable perfect absorption will open the way for a class of promising thermo-optical devices including thermos-photovoltaics, infrared filters, radiative cooling devices, and thermal emitters.

The Feasibility of Tungsten-Doped VO2 Films on Soda-Lime Glass with Low Thermal Budget by High Density Plasma Source for Smart Windows

Thermochromic W-doped VO2 thin films on soda-lime glass are successfully fabricated by co-sputtering technique using a high density plasma source, which is equipped by high power impulse magnetron sputtering (HIPIMS). A post-rapid thermal annealing of 500 °C is performed for the purpose of low thermal budget. The effect of doping amount of tungsten on thermochromic properties such as transmittance and transition temperature are addressed. The transition temperature can be lower down to 30 °C with a small amount of 4.5% tungsten added in VO2 nanocrystals. A solar regulation efficiency, ΔTsol = 10% is achieved at thicker TiO2 thickness. The competition of secondary phase of V2WO7.5 and oxygen-rich phase of V2O5 under different O2/Ar ratio is studied. The crystalline behavior of monoclinic phase is examined by X-ray diffraction (XRD) pattern and high resolution transmission electron microscope (HR-TEM). The good endurance property ensures the feasible use for the energy-saving applications.

Towards Room Temperature Phase Transition of W-Doped VO2 Thin Films Deposited by Pulsed Laser Deposition: Thermochromic, Surface, and Structural Analysis.

Vanadium dioxide (VO2) with an insulator-to-metal (IMT) transition (∼68 °C) is considered a very attractive thermochromic material for smart window applications. Indeed, tailoring and understanding the thermochromic and surface properties at lower temperatures can enable room-temperature applications. The effect of W doping on the thermochromic, surface, and nanostructure properties of VO2 thin film was investigated in the present proof. W-doped VO2 thin films with different W contents were deposited by pulsed laser deposition (PLD) using V/W (+O2) and V2O5/W multilayers. Rapid thermal annealing at 400-450 °C under oxygen flow was performed to crystallize the as-deposited films. The thermochromic, surface chemistry, structural, and morphological properties of the thin films obtained were investigated. The results showed that the V5+ was more surface sensitive and W distribution was homogeneous in all samples. Moreover, the V2O5 acted as a W diffusion barrier during the annealing stage, whereas the V+O2 environment favored W surface diffusion. The phase transition temperature gradually decreased with increasing W content with a high efficiency of -26 °C per at. % W. For the highest doping concentration of 1.7 at. %, VO2 showed room-temperature transition (26 °C) with high luminous transmittance (62%), indicating great potential for optical applications.

Novel laser texturing of W-doped VO2 thin film for the improvement of luminous transmittance in smart windows application

Vanadium dioxide is a promising material for application in smart windows, which undergoes a metal-to-insulator transition, MIT, at 68 °C accompanied by a structural phase transformation, SPT, from VO2(M1) to VO2(R). In this study, two VO2 thin films, one W6+-doped and another one undoped, have been prepared. Both were synthesized via polymer-assisted sol–gel route. The effect of doping on thermochromic parameters was evaluated, giving rise to a reduction of the phase transition temperature by 21 °C with an IR modulation capability of 20 % in the doped sample. Additionally, doping effect on the thermochromic parameters and nanomechanical properties has been analyzed by Spectrophotometry and Atomic Force Microscopy. With the aim of recovering the luminous transmittance that decreased because of doping, the sample has been subjected to an innovative laser texturing process. This laser treatment is able to increase the luminous transmittance by almost 40 % without negatively affecting the other thermochromic parameters. Results show that the combination of both strategies, doping and laser texturing, leads to the production of VO2 thin films with low critical temperature, high solar and IR modulation capacity and enhanced luminous transmittance, with values close to those required for their real application in smart windows.

Preparation and optical properties of W-doped VO2/AZO bilayer composite film

In order to improve its visible light transmittance, W-doped VO2 thin film was prepared with direct current (DC) reactive magnetron sputtering on the surface of Al-doped ZnO (AZO) thin film deposited on quartz glass substrate in advance with radio frequency (RF) magnetron sputtering. The effects of sputtering power for AZO film were investigated on the crystal structures, surface morphologies and optical properties of AZO thin film and W-doped VO2/AZO bilayer composite film. The results show that the crystallinity of both AZO monolayer film and the bilayer film first increases and then decreases with the increase of sputtering power. As the sputtering power increases, the film thickness increases. The integral visible luminous transmittance (Tlum) of the W-doped VO2/AZO bilayer film decreases continuously, and the solar modulation efficiency (ΔTsol) increases first and then decreases. When the sputtering power is 150 W, Tlum and ΔTsol of W-doped VO2/AZO bilayer film are 30.14% and 11.95%, 2.77% and 1.71% higher than those of W-doped VO2 monolayer film, respectively.

Tungsten-doped vanadium dioxide thin film synthesis by alternate layer-by-layer growth and post-deposition annealing

Intrinsic vanadium dioxide (VO2) encounters a metal–insulator transition (MIT) sharply at ~67 °C. Incorporating W6+ into VO2 enables lowering the MIT temperature effectually. Here we have developed a novel two-step synthesis process of high-quality W-doped VO2 thin films along with a delicate doping control: deposition of nanoscale alternately-layered precursor films composed of V and V–W layers by successive sputtering and subsequent annealing. The produced films have been characterized comprehensively to understand the effects of W-doping and process parameters into MIT functionality and structural properties in detail. With W-doping, the change rate of the MIT temperature was determined as ~ −14.5 °C/at%W while the resistance change across MIT decreases considerably. This work would be of close relevance to rendering VO2-based devices operable in the wider temperature range.

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.

Tungsten-doped vanadium dioxide thin film based tunable antenna

Since vanadium dioxide (VO2) is a phase change material that shows a metal to insulator transition behavior at a temperature near 68℃, it has many potential applications in the areas of science and engineering. In this study, tungsten (W) -doped VO2 thin films were synthesized by pulsed laser deposition (PLD) technique with a metallic vanadium target and W-wires which were simply attached on the surface of the vanadium target. Also, bowtie RF antenna structures were fabricated by using the films to check the frequency tunability. The experimental results show a decrease of the transition temperature and a broadening of the phase transition as the atomic percentage of W increases in the film structure. The results also reveal that the W-doped VO2 thin film based RF antenna was fully functional with the successful frequency tuning capability.

Thermochromic properties of W-doped VO2 thin films deposited by aqueous sol-gel method for adaptive infrared stealth application

The W doped VO2 thin films with various W contents were successfully deposited by aqueous sol-gel method followed by a post annealing process. The derived thin films were characterized by X-ray diffraction, Raman spectra, scanning electron microscopy and atomic force microscopy. Besides, the resistance-temperature relationship and infrared emissivity in the waveband 7.5–14μm were analyzed, and the effects of W doping on the thermochromic properties of VO2 thin films were studied. The results show that W atoms enter the crystal lattice of VO2 and the transition temperature decreases gradually with increasing doping amount of W. The emissivity of VO2-W-4% thin films has dropped to 0.4 when its real temperature is above 30°C. The thermal infrared images were also examined under different temperature by thermal imager. The results indicate that the temperature under which W doped VO2 thin films begin to have lower emissivity decreases gradually with increasing doping amount of W. W doped VO2 thin films can control its infrared radiation intensity actively at a lower temperature level of 30°C, which has great application prospects in the adaptive infrared stealth technology.

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.

Infrared stealth property based on semiconductor (M)-to-metallic (R) phase transition characteristics of W-doped VO2 thin films coated on cotton fabrics

The infrared stealth fabric was prepared using W-doped VO2 (M) paints by the coating technology. The thermochromic W-doped VO2 (M) was synthesized through hydrolysis method and two-step calcinations under N2 atmosphere. The powders were evaluated by scanning electron microscopy, X-ray diffraction patterns and differential scanning calorimetry. The infrared emissivity of coated cotton fabric was measured by IR-2 Infrared Emissometer, which was as low as 0.752. The low infrared radiation intensity for coated cotton fabric was detected using the infrared imaging systems above the phase temperature of W-doped VO2 (M). The results indicated that the W-doped VO2 (M) thin films exhibited thermal infrared stealth performance, which could adapt to the surroundings spontaneously.

Preparation of W-doped VO2/FTO composite thin films by DC magnetron sputtering and characterization analyses of the films

In order to obtain low phase transition temperature and superior thermochromic optical material, W-doped VO2/FTO composite thin films are prepared by depositing metallic vanadium on FTO (F:SnO2) conductive glass substrate in argon atmosphere at room temperature and then annealed in air ambient. XPS, XRD and SEM are used for analyzing the structures and surface morphologies of the films. The results indicate that no mixed oxides of V, W and F are produced during high-temperature thermal oxidation. W is doped by replacing V atoms. Compared with the pure VO2/FTO composite thin film prepared using the same process, the crystal orientation of W-doped VO2 thin film is not changed and still retains preferred crystal orientation in the (110) direction. The phase transition temperature drops down to about 35 ℃, and the thermal hysteresis loop narrows to 4 ℃. The variation of IR transmittance between the high temperature and the low temperature reaches 28%. SEM results show that the crystallinity of the thin film is improved significantly, showing smooth, compact and uniform surface morphology. This brings about many new opportunities for optoelectronic devices and industrial production.

Annealing Effects on W-Doped VO<sub>2</sub> Thin Films Prepared from Magnetron Sputtering

W-doped VO2 thin films were prepared by magnetron sputtering after annealing in vacuum. The structure, morphology, infrared transmittance and phase transition were characterized by X-ray diffractometer, atomic force microscopy(AFM), infrared spectrometer (IR) and differential thermal analysis(DTA), respectively. The results show that after vacuum annealing at 500 °C for 2h, the major phase of W doped films is VO2. Dopant reduce the phase transition temperature of VO2 thin films to 21.9°C.

High Temperature-Coefficient of Resistance at Room Temperature in W-Doped VO2 Thin Films on Al2O3 Substrate and Their Thickness Dependence

The temperature coefficient of resistance (TCR) in V1-xWxO2 (VWO) with various W-doping levels and thicknesses were investigated on Al2O3(0001) single crystal substrates. The VWO thin films with an appropriate doping level (x=0.015) showed high TCR over 10%/K just at room temperature (300 K), which was larger than that in other reported high TCR materials. Moreover, no significant change of the TCR property was found based on their thickness dependence, meaning effective enhancement of the sensing performance for bolometric application.