Search Results (17)

Vanadium dioxide (VO₂) is a highly promising material for infrared laser protection due to the pronounced optical switching effect during its metal–insulator transition (MIT). However, due to the relatively high MIT temperature of VO₂ and the low transmittance contrast before and after the MIT, practical applications face challenges in modulation depth and response time. In this study, we address these issues using a wafer-scale VO₂/GaN/Al₂O₃ heterostructure fabricated by oxide molecular beam epitaxy. The conductive GaN interlayer enables local Joule heating of the VO₂ film, permitting direct control of the MIT via an external bias with a threshold of 4.7 V. This structure exhibits a substantial resistance change of four orders of magnitude and enables adaptive limiting of a 3.7 μm laser, reducing transmittance from 60% to 10%. Our work demonstrates a practical, wafer-scale laser-protection device and introduces a pre-excitation strategy via external biasing to enhance response performance. Full article

Terahertz (THz) metamaterials based on phase-change materials (PCMs) offer promising approaches to the dynamic modulation of electromagnetic responses. In this study, we design and experimentally demonstrate a tunable THz metamaterial composed of a symmetric split-ring resonator (SRR) pair, with the left halves covered by a 35 nm thick epitaxial vanadium dioxide (VO₂) film, enabling the simultaneous exploitation of both permittivity- and conductivity-induced modulation mechanisms. During the metal–insulator transition (MIT) of VO₂, cooperative changes in permittivity and conductivity lead to the excitation, redshift, and eventual disappearance of a quasi-bound state in the continuum (QBIC) resonance. Finite element simulations, using optical parameters of VO₂ film defined by the Drude–Smith model, predict the evolution of the transmission spectra well. These results indicate that the permittivity change originating from mesoscopic carrier confinement is a non-negligible factor in THz metamaterials hybridized with VO₂ film and also reveal the potential for developing reconfigurable THz metamaterials based on the dielectric modulation effects of VO₂ film. Full article

This study delves into the effects of titanium (Ti) doping on the optical properties of vanadium dioxide (VO₂), a material well known for its metal–to–insulator transition (MIT) near room temperature. By incorporating Ti into VO₂’s crystal lattice, we aim to uncover the resultant changes in its physical properties, crucial for enhancing its application in smart devices. Utilizing polarized infrared micro–spectroscopy, we examined Ti_xV_1−xO₂ single crystals with varying Ti concentrations (x = 0.059, x = 0.082, and x = 0.187) across different crystal phases (the conductive rutile phase and insulating monoclinic phases M1 and M2) from the far–infrared to the visible spectral range. Our findings reveal that Ti doping significantly influences the phononic spectra, introducing absorption peaks not attributed to pure VO₂ or TiO₂. This is especially notable with polarization along the crystal growth axis, mainly in the x = 0.187 sample. Furthermore, we demonstrate that the electronic contribution to optical conductivity in the metallic phase exhibits strong anisotropy, higher along the c axis than the a–b plane. This anisotropy, coupled with the progressive broadening of the zone center infrared active phonon modes with increasing doping, highlights the complex interplay between structural and electronic dynamics in doped VO₂. Our results underscore the potential of Ti doping in fine-tuning VO₂’s electronic and thermochromic properties, paving the way for its enhanced application in optoelectronic devices and technologies. Full article

Vanadium dioxide (VO₂) has been a promising energy-saving material due to its reversible metal-insulator transition (MIT) performance. However, the application of VO₂ films has been seriously restricted due to the intrinsic low solar-energy modulation ability (ΔT_sol) and low luminous transmittance (T_lum) of VO₂. In order to solve the problems, the surface structure of VO₂ particles was regulated by the quenching process and the VO₂ dispersed films were fabricated by spin coating. Characterizations showed that the VO₂ particles quenched in deionized water or ethanolreserved VO₂(M) phase structure and they were accompanied by surface lattice distortion compared to the pristine VO₂. Such distortion structure contributed to less aggregation and highly individual dispersion of the quenched particles in nanocomposite films. The corresponding film of VO₂ quenched in water exhibited much higher ΔT_sol with an increment of 42.5% from 8.8% of the original VO₂ film, because of the significant localized surface plasmon resonance (LSPR) effect. The film fabricated from the VO₂ quenched in ethanol presented enhanced thermochromic properties with 15.2% of ΔT_sol and 62.5% of T_lum. It was found that the excellent T_lum resulted from the highly uniform dispersion state of the quenched VO₂ nanoparticles. In summary, the study provided a facile way to fabricate well-dispersed VO₂ nanocomposite films and to facilitate the industrialization development of VO₂ thermochromic films in the smart window field. Full article

Phase-change materials (PCMs) and metal-insulator transition (MIT) materials have the unique feature of changing their material phase through external excitations such as conductive heating, optical stimulation, or the application of electric or magnetic fields, which, in turn, results in changes to their electrical and optical properties. This feature can find applications in many fields, particularly in reconfigurable electrical and optical structures. Among these applications, the reconfigurable intelligent surface (RIS) has emerged as a promising platform for both wireless RF applications as well as optical ones. This paper reviews the current, state-of-the-art PCMs within the context of RIS, their material properties, their performance metrics, some applications found in the literature, and how they can impact the future of RIS. Full article

Vanadium dioxide (VO₂) has attracted interest from researchers because it undergoes a metal–insulator phase transition (MIT), which is accompanied by a reversible and remarkable change in both electrical and optical properties. VO₂ exhibits numerous polymorphs and thus it is essential to control the growth of specific monoclinic VO₂ (M) and rutile VO₂ (R) phases. In this study, we developed a cost-effective and facile method for preparing VO₂ nanorods with a highly crystalline monoclinic phase by one-step hydrothermal synthesis, in which only V₂O₅ and H₂C₂O₄ are used as raw materials. The phase evolution of VO₂ during the hydrothermal process was studied. The obtained VO₂ nanorods were thoroughly mixed with fluorocarbon resin and homogeneous emulsifier in an ethanol solution to obtain a VO₂ dispersion. To prepare VO₂ films, screen printing was performed with a stainless steel screen mesh mask on glasses or fabric substrate. The VO₂ coating had good thermochromic performance; the infrared transmittance change was greater than 20% @1.5 μm whilst keeping the visible transmittance greater than 50%. Meanwhile, the polyester base coating on the fabric had an emissivity change of up to 22%, which provides a solution for adaptive IR camouflage. Full article

In this paper, a new type of terahertz (THz) metamaterial (MM) modulator has been presented with bifunctional properties based on vanadium dioxide (VO₂). The design consists of a VO₂ resonator, polyimide substrate, frequency selective surface (FSS) layer, and VO₂ film. Based on the metal-insulator transition (MIT) of VO₂, this structure integrated with VO₂ material can achieve the dynamic modulation on both transmission and reflection waves at 2.5 THz by varying the electrical conductivity value of VO₂. Meanwhile, it also exhibits adjustable absorption performance across the whole band from 0.5–7 THz. At the lower conductivity ($\sigma$ = 25 S/m), this structure can act as a bandpass FSS, and, at the high conductivity ($\sigma$ = 2 × 10⁵ S/m), it behaves like a wideband absorber covering 2.52–6.06 THz with absorption A > 0.9, which realizes asymmetric transmission. The surface electric field distributions are illustrated to provide some insight into the physical mechanism of dynamic modulation. From the simulated results, it can be observed that this design has the capability of controlling tunable manipulation on both transmission/reflection responses at a wide frequency band. This proposed design may pave a novel pathway towards thermal imaging, terahertz detection, active modulators, etc. Full article

Thermochromic dynamic cool materials present a reversible change of their properties wherein by increasing the temperature, the reflectance, conductivity, and transmittance change due to a reversible crystalline phase transition. In particular, vanadium (IV) dioxide shows a reversible phase transition, accompanied by a change in optical properties, from monoclinic VO₂(M1) to tetragonal VO₂(R). In this paper, we report on a systematic exploration of the parameters for the synthesis of vanadium dioxide VO₂(M1) via an easy, sustainable, reproducible, fast, scalable, and low-cost hydrothermal route without hazardous chemicals, followed by an annealing treatment. The metastable phase VO₂(B), obtained via a hydrothermal route, was converted into the stable VO₂(M1), which shows a metal–insulator transition (MIT) at 68 °C that is useful for different applications, from energy-efficient smart windows to dynamic concrete. Within this scenario, a further functionalization of the oxide nanostructures with tetraethyl orthosilicate (TEOS), characterized by an extreme alkaline environment, was carried out to ensure compatibility with the concrete matrix. Structural properties of the synthesized vanadium dioxides were investigated using temperature-dependent X-ray Diffraction analysis (XRD), while compositional and morphological properties were assessed using Scanning Electron Microscopy, Energy Dispersive X-ray Analysis (SEM-EDX), and Transmission Electron Microscopy (TEM). Differential Scanning Calorimetry (DSC) analysis was used to investigate the thermal behavior. Full article

Vanadium dioxide (VO₂) is a typical metal-insulator transition (MIT) material, which changes from room-temperature monoclinic insulating phase to high-temperature rutile metallic phase. The phase transition of VO₂ is accompanied by sudden changes in conductance and optical transmittance. Due to the excellent phase transition characteristics of VO₂, it has been widely studied in the applications of electric and optical devices, smart windows, sensors, actuators, etc. In this review, we provide a summary about several phases of VO₂ and their corresponding structural features, the typical fabrication methods of VO₂ nanostructures (e.g., thin film and low-dimensional structures (LDSs)) and the properties and related applications of VO₂. In addition, the challenges and opportunities for VO₂ in future studies and applications are also discussed. Full article

The electronic properties of strontium ruthenate ${\mathrm{SrRuO}}_3$ perovskite oxide thin films are modified by epitaxial strain, as determined by growing on different substrates by pulsed laser deposition. Temperature dependence of the transport properties indicates that tensile strain deformation of the ${\mathrm{SrRuO}}_3$ unit cell reduces the metallicity of the material as well as its metal-insulator-transition (MIT) temperature. On the contrary, the shrinkage of the Ru–O–Ru buckling angle due to compressive strain is counterweighted by the increased overlap of the conduction Ru-4d orbitals with the O-2p ones due to the smaller interatomic distances resulting into an increased MIT temperature, i.e., a more conducting material. In particular, in the more metallic samples, the core level X-ray photoemission spectroscopy lineshapes show the occurrence of an extra-peak at the lower binding energies of the main Ru-3d peak that is attributed to screening, as observed in volume sensitive photoemission of the unstrained material. Full article

Memristive devices are attracting a great attention for memory, logic, neural networks, and sensing applications due to their simple structure, high density integration, low-power consumption, and fast operation. In particular, multi-terminal structures controlled by active gates, able to process and manipulate information in parallel, would certainly provide novel concepts for neuromorphic systems. In this way, transistor-based synaptic devices may be designed, where the synaptic weight in the postsynaptic membrane is encoded in a source-drain channel and modified by presynaptic terminals (gates). In this work, we show the potential of reversible field-induced metal-insulator transition (MIT) in strongly correlated metallic oxides for the design of robust and flexible multi-terminal memristive transistor-like devices. We have studied different structures patterned on YBa₂Cu₃O_7−δ films, which are able to display gate modulable non-volatile volume MIT, driven by field-induced oxygen diffusion within the system. The key advantage of these materials is the possibility to homogeneously tune the oxygen diffusion not only in a confined filament or interface, as observed in widely explored binary and complex oxides, but also in the whole material volume. Another important advantage of correlated oxides with respect to devices based on conducting filaments is the significant reduction of cycle-to-cycle and device-to-device variations. In this work, we show several device configurations in which the lateral conduction between a drain-source channel (synaptic weight) is effectively controlled by active gate-tunable volume resistance changes, thus providing the basis for the design of robust and flexible transistor-based artificial synapses. Full article

Thickness-driven phase transitions have been widely observed in many correlated transition metal oxides materials. One of the important topics is the thickness-driven metal to insulator transition in half-metal La_2/3Sr_1/3MnO₃ (LSMO) thin films, which has attracted great attention in the past few decades. In this article, we review research on the nature of the metal-to-insulator (MIT) transition in LSMO ultrathin films. We discuss in detail the proposed mechanisms, the progress made up to date, and the key issues existing in understanding the related MIT. We also discuss MIT in other correlated oxide materials as a comparison that also has some implications for understanding the origin of MIT. Full article

Vanadium pentoxide thin films were deposited onto insulating support by means of rf reactive sputtering from a metallic vanadium target. Argon-oxygen gas mixtures of different compositions controlled by the flow rates were used for sputtering. X-ray diffraction at glancing incidence (GIXD) and Scanning Electronic Microscopy (SEM) were used for structural and phase characterization. Thickness of the films was determined by the profilometry. It has been confirmed by GIXD that the deposited films are composed of V₂O₅ phase. The gas sensing properties of V₂O₅ thin films were investigated at temperatures from range 410–617 K upon NO₂ gas of 4–20 ppm. The investigated material exhibited good response and reversibility towards nitrogen dioxide. The effect of metal-insulator transition (MIT) on sensor performance has been observed and discussed for the first time. It was found that a considerable increase of the sensor sensitivity occured above 545 K, which is related to postulated metal-insulator transition. Full article

Vanadium dioxide (VO₂) with reversible metal–insulator transition (MIT) is one of the most promising energy-efficient materials. Especially for VO₂-based smart windows, the visible transmittance and solar modulation ability are the most critical parameters. However, VO₂ thin films that are directly deposited onto glass substrates are of poor crystallinity and MIT performance, limiting the practical applications of VO₂/glass heterostructures. In this paper, a buffer layer of Cu₅₀Zr₅₀ was introduced to build a novel Zr-based thin film metallic glass (VO₂/Cu₅₀Zr₅₀/glass) with multilayer structures for thermochromic applications. It is observed that the insertion of a Cu₅₀Zr₅₀ buffer layer with appropriate thickness results in a clear enhancement of crystalline quality and MIT performance in the VO₂/Cu₅₀Zr₅₀/glass thin films, compared with the single-layer VO₂/glass thin films. Moreover, the VO₂/Cu₅₀Zr₅₀/glass bi-layer films exhibit better optical performance with enhanced solar modulation ability (ΔT_sol = 14.3%) and a high visible transmittance (T_vis = 52.3%), which represents a good balance between ΔT_sol and T_vis for smart window applications. Full article

Advanced understanding of the physics makes phase change materials (PCM) and metal-insulator transition (MIT) materials great candidates for direct current (DC) and radio frequency (RF) switching applications. In the literature, germanium telluride (GeTe), a PCM, and vanadium dioxide (VO₂), an MIT material have been widely investigated for DC and RF switching applications due to their remarkable contrast in their OFF/ON state resistivity values. In this review, innovations in design, fabrication, and characterization associated with these PCM and MIT material-based RF switches, have been highlighted and critically reviewed from the early stage to the most recent works. We initially report on the growth of PCM and MIT materials and then discuss their DC characteristics. Afterwards, novel design approaches and notable fabrication processes; utilized to improve switching performance; are discussed and reviewed. Finally, a brief vis-á-vis comparison of resistivity, insertion loss, isolation loss, power consumption, RF power handling capability, switching speed, and reliability is provided to compare their performance to radio frequency microelectromechanical systems (RF MEMS) switches; which helps to demonstrate the current state-of-the-art, as well as insight into their potential in future applications. Full article