VO2 Films Research Articles

Technology for obtaining VO2 films from the gas phase during thermal decomposition

The paper considers the process of obtaining films of vanadium dioxide by the method of deposition of inorganic coatings and films from the vapor phase during the thermal decomposition of organometallic compounds. In recent years, there has been a growing interest in the practical use of organometallic compounds. This has led to the development of many devices that include various vapor deposition techniques. This technology is currently being used as the latest technology. The design of the technological installation for pyrometric film deposition is presented and the principle of operation is given. In the course of the study, film samples were obtained and experimental work was carried out. The results obtained are presented in the form of graphs.

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.

Bifunctional flexible metasurface based on graphene and vanadium dioxide for polarization conversion and absorption

A bifunctional flexible metasurface with high polarization conversion ratio (PCR) and absorptivity based on graphene and vanadium dioxide (VO2) is proposed in microwave band, which consists of metallic resonator with two VO2 films in the diagonal direction, graphene resistance film layer, two polyimide (PI) dielectric layers and copper ground plate. The proposed metasurface can be switched from a polarization converter to an absorber by employing insulator-to-metal transition in VO2, and the PCR and absorptivity are dynamically manipulated by the graphene. When the conductivity of VO2 (σvo2) is 10 S/m (room temperature) and the square resistance of graphene (Rg) is 900 Ω/sq., the metasurface becomes a broadband polarization converter with the PCR exceeding 90 % in 6.97–17.62 GHz and the corresponding relative bandwidth (RB) is 86.62 %. Once the temperature is high enough and VO2 is in its fully metallic state (σvo2 = 2 × 105 S/m and Rg = 30 Ω/sq), the metasurface efficiently absorbs normally incident electromagnetic (EM) waves in 13.33–19.47 GHz with the absorptivity exceeding 90 % (RB = 37.44 %). The influence of geometric and EM parameters and incident angles on its bifunctional features are also investigated. Moreover, the proposed metasurface exhibits polarization insensitivity under dual functions. The designed bifunctional metasurface is expected to be applied in the fields of wireless communication systems, radar and sonar systems and EM stealth technology.

The influence of rare earth (La, Er and Yb) doped V2O5 films on the structural, linear and nonlinear optical properties and optical limiting for optoelectronic applications

The influence of rare earth (La, Er and Yb) doped V2O5 films on the structural, linear and nonlinear optical properties and optical limiting for optoelectronic applications

High-performance p-type V2O3 films by spray pyrolysis for transparent conducting oxide applications.

High-quality epitaxial p-type V2O3 thin films have been synthesized by spray pyrolysis. The films exhibited excellent electrical performance, with measurable mobility and high carrier concentration. The conductivity of the films varied between 115 and 1079 Scm-1 while the optical transparency of the films ranged from 32 to 65% in the visible region. The observed limitations in thinner films' mobility were attributed to the nanosized granular structure and the presence of two preferred growth orientations. The 60nm thick V2O3 film demonstrated a highly competitive transparency-conductivity figure of merit compared to the state-of-the-art.

A review of the preparation, properties and applications of VO2 thin films with the reversible phase transition

The reversible phase transition of vanadium dioxide under thermal, electrical, and optical stimuli is the enabling concept for the functioning of smart materials and is the basis for the development of various device materials such as optical, electrical, thermal, and mechanical devices based on VO2 on rigid and flexible platforms. The phase transition temperature of VO2 near room temperature is considered an excellent choice and a potential candidate to replace traditional materials in a variety of applications. There is a growing interest in VO2 applications for a wide range of devices, and the use of VO2’s structure to manipulate and explore the functions of various application devices, as well as the modification of VO2 structures to improve performance in a variety of materials, can lead to extremely exciting innovations. A lot of effort has been put into the challenges of practical production and practical application, and it is necessary to find an industrially feasible manufacturing method for the preparation of VO2 films, which is the basis for the practical application of VO2-based equipment. Based on this background, we first briefly describe the structure of VO2, the phase transition mechanisms involved, and the factors and other properties induced by the phase transition of VO2. Then, the current status and advantages and disadvantages of VO2 thin film preparation technologies are introduced in detail, including pulsed laser deposition (PLD), magnetron sputtering, the sol-gel method, and chemical vapour deposition (CVD). In addition, we propose three strategies to improve the performance of VO2 thin films, including element doping, multi-layer composites, and surface structure. We also discussed the different applications of VO2 under thermal, electrical, and light stimulation, as well as the development trends and future challenges of VO2 thin films.

Optimizing deposition regimes to fabricate VO2/TiO2/c-Al2O3 thin films for active metasurfaces

Decreasing the scale of vanadium dioxide (VO2) structures is one of the ways to enhance the switching speed of the material. We study the properties of VO2 films of altered thicknesses in the range of 20–170[Formula: see text]nm prepared on c-sapphire substrates with a TiO2 sublayer by pulsed laser deposition (PLD) method. The synthesis regime to design a TiO2 film was preliminarily optimized based on XRD data. XRD patterns reveal an epitaxial growth of the VO2 films with distortion of the monoclinic cell to hexagonal symmetry. The positions of the lattice vibration modes in Raman spectra are similar to those in bulk VO2 when the film thickness is greater than [Formula: see text][Formula: see text]nm. For VO2 films thicker that [Formula: see text][Formula: see text]nm, a lattice strain results in the modes’ positions and intensity change. However, the electrically triggered transition in a [Formula: see text][Formula: see text]nm thick VO2 film reveals forward and reverse switching times as short as 20[Formula: see text]ns and 400[Formula: see text]ns, correspondingly.

Spatially confined growth of VO2(M) nanoparticles with improved thermochromic property

In the pursuit of VO2-based energy efficient smart window, high-temperature annealing is a common way to prepare well-crystallized VO2 nanoparticles (NPs) with reduced defect density. However, it is usually accompanied by the formation of oversized VO2 NPs due to Ostwald ripening, making unsatisfactory optical performances of VO2-based films. In this work, we reported a spatial confinement route via incorporating commercial fumed silica (FS) as space barrier during the process of annealing ultrafine γ-VOOH NPs, and the size of the obtained VO2 NPs could be reduced to ∼50 nm. Moreover, VO2 films based on the multilevel spatial confinement strategy exhibit outstanding optical performances with balanced solar regulation efficiency (ΔTsol ∼ 17.82 %) and luminous transmittance (Tlum ∼ 51.74 %). A simple dipping process could be carried out to enable a hydrophobic self-cleaning surface of the obtained VO2 films, which was meaningful for smart window applications. VO2@SiO2 core-shell structure, referring to a strong spatial confinement strategy, was also prepared via coating γ-VOOH NPs with SiO2 followed by annealing treatment. The top-level optical performances with ΔTsol of 20.46 % and Tlum of 44.26 % of VO2@SiO2/PVP films were achieved successfully. The spatial confinement strategy may inspire more facile and efficient processes for development of high-performance VO2 films for various applications.

Enhanced electrical and optical properties of porous W-doped V2O5 films under thermally and electrically induced phase transition

Herein, the nanoscale porous W-doped V2O5 thin film was prepared by sol–gel method and post-annealing process on fluorine-doped SnO2 (FTO) conductive glass substrate using polyethylene glycol (PEG) as the structure guide agent. Then a metal–semiconductor-metal (MSM) structure device was fabricated based on the prepared film. The analysis of the surface morphology, elemental constituents, and photoelectric properties confirms the successful formation of a porous W-doped V2O5 film, which exhibited improved electrical and optical properties. Under thermally and electrically induced phase transition, the maximum transmittance modulation depth of the film reached 17.74% at 700 nm between 20 °C and 320 °C and 36.46% at 600 nm under 0.5 V bias voltage, respectively. In the range of 700–1500 nm, the average value of the maximum transmittance modulation depth of the film also increased from 2.74% to 16.9% compared with V2O5 film. With repeated temperature cycling, the film was able to maintain good electrical and optical properties, which is expected to be applied to novel integrated optoelectronic and electrochromic devices.

Hollow-cathode plasma deposited vanadium oxide films: Metal precursor influence on growth and material properties

Due to its different polymorphs, including vanadium pentoxide (V2O5) and vanadium dioxide (VO2), the vanadium oxide (VOX) compound is an immensely interesting material with many important applications. While atomic layer deposition (ALD) is among the possible VOX film synthesis methods, literature reports have majorly utilized thermal-ALD, which reveals as-grown amorphous VOX films. Further post-deposition annealing process is needed to crystallize these films. High-temperature crystallization indeed limits the use of low-temperature compatible materials, processes, and substrates. In this work, we report on the low-temperature crystalline VOX film growth in a hollow-cathode plasma-enhanced atomic layer deposition reactor using two different vanadium precursors, tetrakis(ethylmethylamino)vanadium and vanadium(V) oxytriisopropoxide. Oxygen plasmas were used as co-reactants at a substrate temperature of 150 °C. Along with the purpose of investing in the impact of metal precursors on VOX film growth, we also studied Ar-plasma in situ and thermal ex situ annealing to investigate possible structural enhancement and phase transformation. In situ Ar-plasma annealing was performed with 20 s, 20 SCCM Ar-plasma, while post-deposition ex situ annealing was carried out at 500 °C and 0.5 mTorr O2 pressure. In situ ellipsometry was performed to record instant film thickness variation and several ex situ characterizations were performed to extract the optical, structural, and electrical properties of the films. The outcomes of the study confirm that both metal precursors result in as-grown crystalline V2O5 films at 150 °C. On the other hand, post-deposition annealing converted the as-grown crystalline V2O5 film to VO2 film. Finally, we have also successfully confirmed the metal-to-insulator transition property of the annealed VO2 films via temperature-dependent structural and electrical measurements.

Tunable and switchable multifunctional terahertz meta-mirror based on graphene and vanadium dioxide.

We design a multifunctional THz polarization modulation meta-mirror integrated with polarization conversion and dichroism functions switched by temperature and voltage. The meta-mirror is composed of two-layered graphene metasurfaces and a layer of vanadium dioxide (VO2) on a gold film substrate. Linear-to-linear polarization conversion and linear dichroism (LD) can be switched by temperature control in the VO2 film and Fermi level adjustments in the graphene metasurfaces, where the polarization conversion ratio (PCR) is higher than 0.9 in the range of 2.89 THz to 4.02 THz, LD value reached a maximum of 0.6 at 3.84 THz, and linear-to-circular polarization conversion and circular dichroism (CD) can also be tuned with ellipticity higher than 0.9 in the range of 2.32 THz to 2.69 THz and CD value as high as 0.71 at 2.45 THz. The proposed meta-mirror is the first THz metamaterial device integrating four switchable functions, including linear-to-linear polarization conversion, linear-to-circular polarization conversion, linear dichroism and circular dichroism. The meta-mirror is a promising design for compact system integration in THz imaging, sensing and biological detection applications.

Dynamic multi-color switching using ultrathin vanadium oxide on aluminum-based asymmetric Fabry–Pérot resonant structure

Vanadium dioxide (VO2) exhibits strong infrared optical switching due to its insulator–metal phase-transition property. However, in the visible wavelengths, its intrinsic optical switching is quite low. Current research explores solutions like multilayering, intricate structural patterning, high thermal budget processes, and costly metals for improved color switching. Nonetheless, the color gamut coverage with these methodologies remains notably limited. This work overcomes these limitations and demonstrates dynamic multi-color switching covering a large color gamut using a simple, unpatterned, ultrathin (∼ λ14, where wavelength λ is taken as 575 nm at the center of the visible spectrum) asymmetric Fabry–Pérot structure of VO2 on aluminum (Al). We use the transfer matrix method to design the VO2/aluminium (Al)/sapphire structure for maximum visible reflectance switching. VO2 films are synthesized using a simple, low thermal budget atmospheric oxidation of vanadium (V). With varying oxidation durations, different colors of the oxidized samples are observed. Consistent and reversible color-switching is observed visibly and in reflectance measurements with the change in temperature from low (RT ∼ 30 °C) to high (HT ∼ 100 °C) or vice versa due to the phase transition property of the VO2 layer in the structure. Compared to the existing studies, this work shows a significant change in chromaticities and covers a large color gamut when plotted on the CIE chromaticity diagram. This work has potential applications in the fields of display, thermochromic structures, and visible camouflage.

In-situ characterization of the semiconductor-metal phase transition in vanadium dioxide thin films

Vanadium dioxide (VO2) exhibits a reversible first-order semiconductor-metal phase transition (SMT) near 68 °C at ambient pressure, consisting in a structural transformation from a low-temperature semiconducting monoclinic phase to a high-temperature metallic rutile phase. This phenomenon is investigated on thin films of VO2, with thickness ranging from 15 to 300 nm, which are deposited on a silica substrate by magnetron sputtering. The films are systematically characterized at the morphological, structural, and optical level by using Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Grazing Incidence X-ray Di↵raction (GIXRD), Raman Spectroscopy, Spectrophotometry, and Spectroscopic Ellipsometry. The SMT is investigated in-situ by Optical Spectroscopy in the VIS-NIR spectral range and by Grazing Incidence X-ray Di↵raction (GIXRD). Compared to their bulk counterpart, thin films display broader phase transitions upon thermal excitation. This is evidenced by monitoring the temperature-dependent transmittance at specific wavelengths which reveals a hysteretic behaviour, whose thermal width and amplitude depends on the film thicknesses. Additionally, changes in peak positions and intensities in in-situ GIXRD di↵raction spectra further elucidate the phase transition dynamics.

Heat accumulation and phase transition induction in a VO2 thin film by a femtosecond pulse-periodic radiation.

The kinetics of optical switching due to the insulator-metal phase transition in a VO2 thin film is studied experimentally at different laser pulse repetition frequencies (PRFs) in the NIR range and compared with temperature kinetics obtained through the thermal conductance calculations. Two switching processes have been found with characteristic times <2 ms and <15 ms depending on the PRF; the former is explained by the accumulation of metallic domains remaining after a single-pulse phase transition, and the latter is referred to the heat accumulation in the film. Consequently, the dynamics of the microscopic domains is leading in the initiation of phase transition under pulse-periodic conditions compared to the macroscopic heat transfer. The reverse transition at the radiation turn-off depends on the PRF with a time coefficient of 17.5 µs/kHz and is determined by the metallic domains' decay in the film. The results are important for understanding the nature of the insulator-metal transition in thin films of VO2 as well as using them in all-optical switches of pulse-periodic laser radiation.

Study of rectifying properties and true Ohmic contact on Sn doped V2O5 thin films deposited by spray pyrolysis method

In this paper, V2O5 thin-films were deposited on glass substrates by using the ultrasonically nebulized spray pyrolysis technique. Doping concentration of 1 %, 2 %, and 7 % of Sn, in V2O5 thin-films is studied by using metal–semiconductor-metal (MSM) based device structure. X-ray diffractometer (XRD) and field emission scanning electron microscope (FESEM) were used to analyze the surface morphology of the thin films. The XRD spectroscopic analysis shows the crystal size of above-mentioned samples, to be respectively 39 nm, 58 nm, 60 nm and 72 nm. The FESEM images showed the enhancement of crystal size with increase in Sn doping concentration. The optical properties of Sn doping on V2O5 thin film were studied by using UV–Vis spectrometer. The UV–Vis spectroscopic analysis shows that the absorption coefficient values decrease with increase in concentration of Sn metal doping in V2O5 thin film. The activation energies of all thin-film samples were calculated from Arrhenius plots and were found to be 0.938 eV, 1.101 eV, 1.11 eV and 1.169 eV, respectively, for all the thin-film samples mentioned above. The MSM based structure was fabricated by using a shadow mask and thermal evaporation. Later, the I-V characteristics of all the thin films were obtained by using semiconductor parameter analyzed at a biased voltage between −50 V and + 50 V with a step size of 1 V. Rectification ratio of the V2O5 films is significantly enhanced as the doping concentration increases. It was found that the rectification ratio of undoped V2O5 thin films increased linearly from 1.018 to 1.059 with an increase in temperature from room temperature to 130 °C. Similar trend was followed for 1 % (from 1.079 to 1.198), 2 % (from 1.081 to 1.224) and 7 % (from 1.095 to 1.311) Sn doped films. These results show the potential application of V2O5 thin films in the field of optoelectronics and thin film gas sensors.

Impact of impedance on electrochromic properties of W-doped V2O5 films

Two series of W-doped V2O5 films, planar films, and inverse opal films, were synthesized. The evolution of their electrochromic properties, including switching time, cycling stability, and color efficiency (CE), were systematically investigated as a function of electrochemical impedance. Both charge transfer resistance (Rct) and ion diffusion ability (KΩ) increase with the W:V atomic ratio in both planar films and inverse opal films series. KΩ plays a decisive role on coloration switching time, for instance, there exists a range of KΩ where switching time vastly decreases with an increase of KΩ and beyond this range, switching time is independent of KΩ and remains constant. The introduction of macroporosity, however, greatly reduces the low critic KΩ. CE is co-affected by both KΩ and Rct. KΩ and Rct collaboratively impact on EC cycling stability: KΩ plays a key role on cycling stability since the best cycling stability corresponds to highest KΩ. Rct plays its role only when it is low enough to reduce the possibility of chemical dissolution of the intermediate product of vanadium-containing ions: the present critical value could be around 76 Ω.

Evaluation of Flexible Electrochromic Device Based on V2 O5 Film Prepared by Electrophoresis

The red V2O5 sol was prepared by facile alternating stirring and ultrasonic dispersion route. A V2O5 film fabricated on ITO-PET by electrophoresis method was used to demonstrate a flexible electrochromic device. The effects of applied voltage and electrophoresis time for V2O5films were investigated in detail. When the electrophoresis time was kept 30 s, the applied voltage was above 3 V to deposit uniform V2O5-PET film. The electrophoresis time could more significantly regulate the deposition of V2O5 on ITO-PET film. A flexible electrochromic device with “sandwich” structure was fabricated using V2O5 film, and its electrochromic performance of the device was evaluated. The flexible V2O5 electrochromic devices show the multicolor electrochromic performance of yellow, yellow-green, green and orange-red. The coloration efficiency values of V2O5 devices were 21.6 cm2C[Formula: see text] (from initial yellow to green) and 26.9 cm2C[Formula: see text] (from green to orange-red), respectively, and the highest transmittance modulation range was 51% at 741 nm. The work provided a facile and green method for fabrication of flexible V2O5 electrochromic device.

Fabrication of V1−x Ti x O2 microbolometers on Si3N4/SiO2 membranes

V1−x Ti x O2 thin films have been prepared on Si3N4/SiO2/Si substrates by the two-step process of (1) growth of VO2/TiO2 thin films by metal-organic decomposition with carbothermal reduction, and (2) the interdiffusion between V and Ti atoms at the interface between the VO2 and TiO2 thin films during the growth of the VO2 thin film. The abrupt resistance change and the thermal hysteresis due to the metal-insulator transition were gradually suppressed with increasing the firing temperature, and they completely disappeared for the V0.65Ti0.35O2 thin film prepared at 650 °C. A relatively high TCR of −4.2%/K was obtained from the film over a wide temperature range of 20 °C–90 °C. V0.65Ti0.35O2 microbolometers with a size of 5 μm × 15 μm were fabricated by photolithography and Ar ion milling, and a Si3N4/SiO2 membrane under the microbolometer was formed with an aqueous KOH solution. The DC sensitivity of the fabricated microbolometer on the membrane was estimated to be 9876 W−1 and 20 times higher than that of the microbolometer on the Si3N4/SiO2/Si substrate. The V0.65Ti0.35O2 microbolometer on the membrane is available for a terahertz detector operating over the wide temperature range.

Modulation of insulator metal transition of VO₂ films grown on Al2O3 (001) and TiO2 (001) substrates by the crystallization of capping Ge2Sb2Te5 layer

We demonstrate modulation of insulator metal transition (IMT) of VO2 films grown on single crystalline substrates through the effect of in-plane compression with crystallization of capping chalcogenide layer on the targeted VO2 films. Chalcogenide germanium–antimony–telluride (Ge2Sb2Te5: GST), which shows large volume reduction of 6.8% with its phase change from amorphous to crystal, was deposited on VO2 films grown on Al2O3 (001) and TiO2 (001) substrates, where V–V atoms along the cR-axis in the tetragonal VO2 phase align parallel and perpendicular to the substrate surfaces, respectively. As a result, counter shifts in temperature-dependence of resistance characteristics, to lower and higher directions, were observed for VO2 films on Al2O3 (001) and TiO2 (001), consistent with the lattice modulation of VO2 films by the in-plane compression introduced by GST crystallization. The obtained results open a way to realize large resistance change of IMT under constant temperature by controlling GST phases.

Phase Transition Engineering of Vanadium Dioxide Induced by Oxygen Vacancies

A reconfigurable optical functional system is a pioneering and comprehensive approach for portable future flexible electronics, irrespective of traditional silicon-based technology. VO2 shows tremendous temperature-dependent structural and electrical phase transitions as a complex correlated metal oxide, presenting it as a potential candidate for bolometers, thermal camouflage, and optoelectronics applications. However, the structural stability and high transition temperature restrict room-temperature applications. The present study reports the oxygen vacancies-assisted phase transition engineering in the VO2/muscovite heterostructure. The ion bombardment alters the oxygen stoichiometry of VO2/muscovite thin film at room temperature. A significant decrease in lattice constant has been observed during the transition of the insulating M1 (monoclinic) phase into the metallic R (tetragonal) phase, which was confirmed through high-resolution XRD. The change in oxygen vacancy concentration determined using X-ray photoelectron spectroscopy (XPS) actively suppressed and stabilized the epitaxial thin film of VO2 near room temperature. We investigated whether applying increasing power-mediated ion bombardment for a few minutes affects the oxygen vacancy formation on the VO2/muscovite film’s surface. Consequently, a change in KPFM and 4 orders of magnitude change in resistance of VO2 thin film have been observed. This report reveals the fundamental understanding of controlling the MIT in epitaxial VO2 thin film via facile and clean phase transition engineering techniques for advanced flexible electronic devices.