Vanadium Dioxide Films Research Articles

Vanadium Dioxide by Atomic Layer Deposition: A Promising Material for Next-Generation Memory Devices.

The synthesis of a vanadium dioxide (VO2) film using atomic layer deposition (ALD) with vanadium tetrachloride (VCl4) as a precursor for the realization of programmable memory devices is reported. X-ray diffraction analysis revealed the epitaxial growth of VO2 on c-Al2O3. The phase transition was monitored using resistivity measurements across varying temperatures, demonstrating a decrease of >4 orders of magnitude at the transition temperature, thereby confirming the high quality of the material. From this material, memristive devices are fabricated as resistive random-access memory (RRAM). On the basis of spiking voltage inputs, these RRAM exhibited cycle stability over 512 cycles and state retention stability for >450 s, showing <2% drift. With respect to synaptic-like applications, the RRAM devices were piloted through step patterns to enable multilevel memory states. These ALD-grown VO2-based devices demonstrate potential for use as synaptic connections with multiweight synapses, advancing scalability in neuromorphic applications.

Flexible Vanadium Dioxide Films and Devices on Kapton Fabricated With Low-Temperature Atmospheric Oxidation of Vanadium

Flexible Vanadium Dioxide Films and Devices on Kapton Fabricated With Low-Temperature Atmospheric Oxidation of Vanadium

Dimension-Controlled VO2 Film for Optoelectronic Logic Gates and Information Encryption.

As the fields of photonics and information technology develop, a lot of novel applications based on VO2 material, such as optoelectronic computing and information encryption, have been developed. While the performance of these devices was not only closely associated with the VO2 phase transition properties but also depended on their dimensional characteristics. In the current study, we conducted the dimension-controlled vanadium dioxide (VO2) film growth, resulting in the epitaxial 2-dimensional (2D) VO2 film and well-distributed 3-dimensional (3D) VO2 crystal film deposition, respectively. It was revealed that, unlike the 2D film, the pronounced localized surface plasmon resonance dominated the near-infrared spectrum across the phase transition for the 3D VO2 film due to the naturally formed meta-surface structure, which showed a transmittance valley in the infrared spectrum after metallization. Based on this distinct infrared spectrum feature in the 3D VO2 film, we proposed an optoelectronic logic gate controlled by the input voltage and the probing Vis/IR light. By detecting the transmittance states of the probing light with different wavelengths, we achieved multistate encoding functions and demonstrated the information encryption application. This new conception device also showed great potential for some other applications such as optoelectronic coupled computing, information encryption, and optical near-field sensing computing.

Preparation of Thermochromic Vanadium Dioxide Films Assisted by Machine Learning.

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

Separation of terahertz and DC conductivity transitions in epitaxial vanadium dioxide films

Terahertz (THz) light, probing electronic responses near the Fermi level, is closely linked to DC transport. In this study, we investigate the in situ THz and DC conductivity of epitaxial VO2 films near the heating-induced insulator–metal transition. We find that the THz conductivity transition occurs at lower temperatures than the DC conductivity transition, with the separation increasing with film thickness and crystallization quality. Analysis of the complex conductivity spectra attributes this separation to released carrier confinement at THz frequencies. Our findings contribute to a deeper understanding of electron dynamics in VO2 films.

Inhomogeneous Photosusceptibility of VO_{2} Films at the Nanoscale.

Pump-probe nano-optical experiments were used to study the light-induced insulator to metal transition (IMT) in thin films of vanadium dioxide (VO_{2}), a prototypical correlated electron system. We show that inhomogeneous optical contrast is prompted by spatially uniform photoexcitation, indicating an inhomogeneous photosusceptibility of VO_{2}. We locally characterize temperature and time dependent variations of the photoexcitation threshold necessary to induce the IMT on picosecond timescales with hundred nanometer spatial resolution. We separately measure the critical temperature T_{L}, where the IMT onsets and the local transient electronic nano-optical contrast at the nanoscale. Our data reveal variations in the photosusceptibility of VO_{2} within nanoscopic regions characterized by the same critical temperature T_{L} where metallic domains can first nucleate.

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

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

Flexible VO2 films grown on ZnO-nanorod buffered polyimide sheets with large insulator metal transition: Evaluation of flexible performance

We fabricated stand-alone flexible vanadium dioxide (VO2) films on 10 μm-thick polyimide (PI) sheets with large insulator metal transition (IMT). Zinc oxide (ZnO)-nanorods grown by the chemical synthesis method as a buffer layer between VO2 and PI realized IMT with resistance change nearly three orders of magnitude. Highly bm axis oriented VO2 films on ZnO_NR buffered PI realized switching of 1450 nm infrared-light more than 40%. The transmittance values at a metallic phase below 2% at temperatures higher than 70 °C were quite low, suggesting high potential for various applications in the infrared and terahertz wavelength region. Number densities of cracks in the VO2 films were estimated from the scanning electron microscopy (SEM) images. Quantitative relation between crack density and the bias voltage suggested the strain-induced formation of cracks in the VO2 films prepared by biased-sputtering. As for flexible performance, the stand-alone VO2 films on PI were directed to bending examinations up to 2000 times. The ZnO-nanorods buffered VO2 films on PI showed high durability for maintaining the superior IMT characteristics. The results obtained in this study show a way to realize practical flexibility of VO2 stand-alone sheets which are able to apply for a variety of fields utilizing switching of VO2.

Tailoring intrinsic chiroptical responses via twisted bilayer α-MoO3 separated by a VO2 film

Flexible control of intrinsic chiroptical responses within compact nanostructures is crucial for flat optics, topological photonics, and chiroptics. However, previous approaches require complicated patterns with both in-plane and out-of-plane mirror symmetry breaking to achieve intrinsic chirality, and their chiroptical responses cannot be dynamically controlled as well. Herein, we demonstrated that near-perfect intrinsic circular dichroism (CD) can be achieved within a lithography-free structure consisting of the twisted bilayer α-MoO3 separated by a vanadium dioxide (VO2) film. By twisting the bilayer α-MoO3, dual-band intrinsic chiroptical responses can be realized due to the excitations of the hyperbolic phonon polaritons modes in the mid-infrared. It is the spin-selected average electric-field enhancement instead of the chiral absorption that is responsible for the intrinsic CD of the device. In addition, the chiroptical responses are insensitive to the variation of the thickness of the structure as well as the incident angle, and high contrast CD can be dynamically tuned by varying the volume fraction of VO2.

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.

Switchable and Tunable Salisbury Screen Microwave Absorber based on a Uniform Vanadium Dioxide Film

Switchable and Tunable Salisbury Screen Microwave Absorber based on a Uniform Vanadium Dioxide Film

Selective growth of vanadium dioxide on patterned Al/SiO2 substrates by metal–organic chemical vapor deposition

The area-selective growth of vanadium dioxide can provide a valuable self-aligned process promising for novel oxide electronics compatible with silicon technology.

Optothermal characterization of vanadium dioxide films by Infrared Thermography

The thickness of vanadium dioxide (VO2) films is a crucial parameter for the study of their optical and thermal properties. In this paper we studied the effect of the film thickness on the thermal hysteresis loop during the phase transition of VO2 deposited on a sapphire substrate by pulsed laser deposition (PLD), by the application of the Infrared Thermography technique. We measure the main thermal hysteresis parameters of VO2 samples with different thicknesses in the LWIR range (8–14 μm) showing how the transition temperature during the heating and cooling cycles, and the width of the hysteresis loop, may change with thickness. We analyzed and compared the obtained results with, in situ Grazing Incidence X-Ray Diffraction (GI-XRD). A good agreement between the results obtained with the two techniques was found demonstrating the reliability of the IR Thermography as a quantitative characterization tool.The results show that the structural and IR emissivity properties of the VO2 layer exhibit a dynamic range dependent on the layer thickness due to a correlation with the crystalline grain size. This has important effects in view of a tailored energy management for the use of those materials as smart radiators or smart windows.

Bayesian optimization of periodic multilayered slabs for passive absorptivity control

A vanadium dioxide (VO2) film grown on a titanium oxide crystal shows a metal–insulator transition at room temperature with drastically changed optical properties. A multilayered slab with a sub-micron scale VO2 film was proposed to utilize its unique properties for passive intensity control of sunlight absorption and radiative cooling. Its optimal geometries were numerically explored using the Bayesian optimization (BO) method. BO was applied for three types of multilayered slabs, those having one, two, or three isolated slabs of different widths. For each type of multilayered slab, BO could optimize geometric variables with practical calculation times considering the total number of possible combinations of variables, which is subsequently referred to as the total number of candidates. Optimization results revealed that two isolated slabs had the most suitable spectral absorptivity in both hot and cold environments. The infrared absorptivity of the double slab was kept low in cold conditions to suppress radiative cooling. However, the double slab exhibited good radiative cooling performance under hot conditions. Electromagnetic energy density surrounding the slab illustrated that metallic VO2 and gold placed in a parallel manner excited the coupled mode of surface plasmon polaritons to enhance absorptivity. Radiative cooling faded for the triple slab because each slab could couple with radiation propagating only across a portion of the cross-sectional area. Through three BO trials, improvement of the VO2 visible reflectivity was recognized as a future issue for further development of passive sunlight absorption control.

Purifying VO2 phase synthesized by sol-gel method via annealing process

Vanadium dioxide (VO2) film has significant interesting phase transition properties called metal-semiconductor transition (MST) which occurs at 68 °C. This transition behavior makes it suitable for various applications, including thermochromic materials for smart windows, sensors, and electronics devices. In this study, we aimed to focus on the purification of VO2 through the sol-gel synthesis method from vanadium alkoxide (VO(OC3H7)3. To analyze the effects of annealing temperature (500 °C-600 °C) on the resulting structural, morphology, and electrical properties, x-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), temperature dependent resistance, and infrared (IR) thermal image were considered. The resistance as a function of the temperature shows an abrupt change confirming the phase transition. However, XRD shows an impurity of other vanadium oxides such as V6O13. In addition, the sample annealed at 600 °C shows a non-uniform surface filled with island clusters, while the sample annealed at 500 °C has a smooth and uniform surface. Raman spectra further supports the formation of VO2 (M) clusters mixed with V6O13 in smooth area, resulting in a decrease in electrical properties, significant reduced resistance change at the excessive annealing temperature.

Temperature-Controlled Switchable Photonic Nanojet Generated by Truncated Cylindrical Structure.

We propose a novel micro-nano structure that can realize a photonic nanojet (PNJ) switch by adjusting the temperature, which is composed of a truncated cylinder coated with a thin vanadium dioxide (VO2) film. The influence of temperature on the maximum strength, full width at half maximum (FWHM), working distance, and focal length of the PNJ were studied by finite-difference time-domain (FDTD) method. The results demonstrate that the structure can adjust the open and close state of the PNJ by changing the temperature. A PNJ with varying characteristics can be obtained at both high and low temperatures, and the maximum intensity ratio of the PNJ can reach up to 7.25. This discovery provides a new way of optical manipulation, sensing and detection, microscopy imaging, optoelectronic devices, and other fields.

High-quality VO2 films synthesized on polymer substrates using room-temperature pulsed laser deposition and annealing

With the growing trend towards portable and adaptable devices in various fields, the demand for flexible electronics and coatings utilizing phase transition materials like VO2 as a functional layer is rising. However, depositing high-quality crystalline VO2 films directly onto polymer substrates remains challenging due to the thermosensitivity of such materials being barely compatible with usual VO2 deposition methods. In this study, we present a simple method for directly growing VO2 on Kapton substrates. VOx films were first deposited onto the Kapton substrate at room temperature using pulsed laser deposition and subsequently annealed in an oxygen atmosphere at 390 °C. The electrical and optical properties of the VO2/Kapton samples with different film thicknesses were investigated. The 200 nm-thick vanadium dioxide films exhibit the best resistivity contrast with 3.1 orders of magnitude (OOM), while the 50 nm-thick films show the best transmittance change of 54 % at 2500 nm. The switching performances are comparable or even superior to those achieved previously using more complicated synthesis processes such as film transfer, introduction of buffer layers, and ink injection. Additionally, by coupling W elemental doping and thin film strain, we successfully tuned the VO2 transition temperature to room temperature with a dopant concentration of only 1.1 at.% while maintaining a high electrical contrast (2 OOM). The impressive electrical and optical transition performance of the VO2/Kapton samples, combined with their tunable transition temperature, makes them promising candidates for flexible electronic devices and thermochromic smart coatings.

Tunable terahertz multi-functional hybrid-patterned metasurface arrays based on vanadium dioxide

This study investigates a nine-layer multi-functional periodic array with active broadband tuning in the terahertz (THz) band. The device comprises symmetrical vanadium dioxide (VO2) films and polypropylene (PP) layers, along with silicon dioxide (SiO2) layers, hybrid-patterned metasurfaces, and a central VO2 layer. Through detailed analyses of the electric field distribution, equivalent circuit, and effective impedance, we have performed a thorough investigation of the resonance modes present in the device and meticulously optimized various parameters. Leveraging the insulator-to-metal transition of VO2, a remarkable device capable of seamlessly switching between extraordinary terahertz transmission and bi-directional perfect absorption was obtained. These characters exhibit limited susceptibility to incident angle of the incoming wave. By incorporating bow-tie apertures within the “vacuum region” of the hybrid-patterned metasurfaces, a significant improvement in field enhancement has been achieved, all while effectively eliminating any adverse effects on transmission and absorption performance. This device presents a novel and effective approach in the development of adjustable and multifunctional THz metasurface devices.

A dynamically switchable multifunctional meta-device in terahertz based on vanadium dioxide and photosensitive silicon

In this study, we propose a dynamically switchable multifunctional device operating in the terahertz range. The device is designed by employing a multilayered planar structure that incorporates vanadium dioxide and photosensitive silicon films. We analyze the transmission characteristics and multifunctional attributes of the device. The device exhibits the capacity to transition among diverse functionalities, including linear polarization conversion, field intensity modulation, frequency selection, and wave absorption. The polarization conversion ratio exceeds 90%, and precise control over the amplitude ratio between cross-polarized and co-polarized fields is achievable. Field intensity modulation can be realized through any of three distinct methods. Upon activation of the frequency selection function, the device can effectively operate as a band-stop filter, yielding a -20dB bandwidth of 0.57 THz. Furthermore, in absorber mode, the device demonstrates a peak absorption efficiency of 90%.

A theoretical study on the effect of protective layer on the solar absorption and infrared emittance of spacecraft smart thermal control devices

With the arrival of the space economy, the miniaturization of spacecraft and their multi-functional trends have put a greater demand on space thermal control systems. Smart radiation devices (SRDs) are gaining attention as a low-cost, light-weight material for the thermal control of spacecraft. Previous works have focused on how to enhance IR emittance tunability of SRDs. In fact, the SRDs performance and lifetime may be affected by the external space environment. Thus, researchers proposed to cover the SRDs with a protective layer to resist oxidation and other problems. However, the influence of protective layer on the performance of SRDs, including IR emittance tunability Δε and solar absorption αs, has not been studied systematically yet. In this paper, the impact of protective layer on the performance of SRDs is comprehensively evaluated in theory. Firstly, a three-layer SRD is composed of a vanadium dioxide (VO2) film, BaF2, and a silver (Ag) substrate, which is optimized using the transfer matrix method. The IR emittance tunability and solar absorption can reach 0.64 and 0.29, respectively. Secondly, the smart thermal control device with three protection materials (BaF2, Al2O3 and Si) is investigated. The results show that the performance of SRD can be enhanced or deteriorated, depending on the types of protection materials and the thickness. When the protection material is Si, the IR emittance tunability can be enhanced to 0.65, while the solar absorption can be decreased to 0.18. The performance of the proposed SRD meets the requirement of the spacecraft thermal control devices. We believe that this work provides a theoretical basis for the design of more reasonable spacecraft thermal control devices.