Search Results (40)

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

The intrinsic metal–insulator transition (MIT) of VO₂ films near room temperature presents significant potential for reconfigurable metamaterials in the terahertz (THz) frequency range. While previous designs primarily focused on changes in electrical conductivity across the MIT, the accompanying dielectric changes due to the mesoscopic carrier confinement effect have been largely unexplored. In this study, we integrate asymmetric split-ring resonators on 35 nm epitaxial VO₂ film and identify a “dielectric window” at the early stages of the MIT. This is characterized by a redshift in the resonant frequency without a significant degradation in the resonant quality. This phenomenon is attributed to an inhomogeneous phase transition in the VO₂ film, which induces a purely dielectric change at the onset of the MIT, while the electrical conductivity transition occurs later, slightly above the percolation threshold. Our findings provide deeper insights into the THz properties of VO₂ films and pave the way for dielectric-based, VO₂ hybrid reconfigurable metamaterials. 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

Due to metal–insulator transitions occurring in those compounds, materials and devices based on vanadium (III) and (IV) oxides draw increasing scientific attention. In this paper, we observed the transitions in both oxides using contemporary laboratory equipment. Changes in the crystallographic structure were precisely investigated as a function of the temperature with a step of 2 °C. Thermal effects during transitions were observed using differential scanning calorimetry. The DC conductivity of the materials was measured quasi-continuously as a function of the temperature. All the experiments were consistent and showed considerable hysteresis of the metal–insulator transition in both vanadium oxides. 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

Our investigation focuses on the analysis of the conductive properties of high-mobility 2D-Si-MOSFETs as they approach the critical carrier density, $n_{sc}$ (approximately $0.72\times {10}^{11} {\mathrm{c}\mathrm{m}}^{-2}$), which marks the metal insulator transition (MIT). In close proximity to the $n_{sc}$, the conductivity exhibits a linear dependence on the temperature (T). By examining the extrapolated conductivity at the absolute zero temperature (T = 0), denoted as ${\sigma }_0$, as a function of the electron density $n_s$, we identify two distinct regimes with varying ${\sigma }_0\left(n_s\right)$ patterns, indicating the existence of two different phases. The transition from one of these two regimes to another, coinciding with $n_{sc}$, is abrupt and serves as the focus of our investigation. Our aim is to establish the possibility of a percolation type transition in the 2D-Si-MOSFETs’ sample. In fact, we observed that the model of percolation is applicable only for densities very close to $n_{sc}^{*}=n_2$ (where $n_2$ is the linear extrapolation of ${\sigma }_0$), indicating the percolation type transition essentially represents a phase transition at the zero temperature. 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

VO₂ is one of the most studied vanadium oxides because it undergoes a reversible metal-insulator transition (MIT) upon heating with a critical temperature of around 340 K. One of the most overlooked aspects of VO₂ is the band’s anisotropy in the metallic phase when the Fermi level is crossed by two bands: π* and d_||. They are oriented perpendicularly in one respect to the other, hence generating anisotropy. One of the parameters tuning MIT properties is the unbalance of the electron population of π* and d_|| bands that arise from their different energy position with respect to the Fermi level. In systems with reduced dimensionality, the electron population disproportion is different with respect to the bulk leading to a different anisotropy. Investigating such a system with a band-selective spectroscopic tool is mandatory. In this manuscript, we show the results of the investigation of a single crystalline 8 nm VO₂/TiO₂(101) film. We report on the effectiveness of linearly polarized resonant photoemission (ResPES) as a band-selective technique probing the intrinsic anisotropy of VO₂. Full article

A thin film of thermochromic VO₂ was prepared on m-Al₂O₃ substrate using a radio frequency (RF) magnetron sputtering technique. The epitaxial growth of the monoclinic M₁ phase of VO₂ on the m-Al₂O₃ substrate was confirmed through synchrotron X-ray diffraction (XRD) measurements. The transformation of this monoclinic M1 phase into a rutile phase at ~68 °C was reflected in the temperature-dependent XRD measurements of the VO₂ thin film. The temperature-dependent electrical resistance measurements of this sample also revealed an abrupt metal-to-insulator transition at ~68 °C, which is reversible in nature. Temperature-dependent X-ray absorption (XAS) measurements at V L-edge and O K-edge were performed to study the electronic structure of the epitaxial VO₂/m-Al₂O₃ thin film during the metal-to-insulator (MIT) transition. Full article

Single crystals of the perovskite nickelate NdNiO₃ with dimensions of up to 50 μm on edge have been successfully grown using the flux method at a temperature of 400 °C and oxygen pressure of 200 bar. The crystals were investigated by a combination of techniques, including high-resolution synchrotron X-ray single-crystal and powder diffraction and physical property measurements such as magnetic susceptibility and resistivity. Resistivity measurements revealed a metal-insulator transition (MIT) at T_MIT~180 K with apparent thermal hysteresis; however, no superlattice peaks or peak splitting below T_MIT, which corresponds to a structural transition from Pbnm to P2₁/n, was observed. The successful growth of NdNiO₃ crystals at relatively low temperatures and oxygen pressure provides an alternative approach for preparing single crystals of interesting perovskites such as RNiO₃ (R = Sm-Lu) and parent phases of superconducting square planar nickelates. Full article

In this work, NbOx-based selector devices were fabricated by sputtering deposition systems. Metal-to-insulator transition characteristics of the device samples were investigated depending on the oxygen flow rate (3.5, 4.5, and 5.5 sccm) and the deposition time. The device stack was scanned by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). The yields, including MIT, nonlinear, and Ohmic, in working devices with different deposition conditions were also evaluated. Moreover, we observed the trend in yield values as a function of selectivity. In addition, the current–voltage (I–V) curves were characterized in terms of DC and pulse endurance. Finally, the switching speed and operating energies were obtained by applying a triangular pulse on the devices, and the recovery time and drift-free characteristics were obtained by the paired pulses. 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

Vanadium oxide (VO₂) is considered a Peierls–Mott insulator with a metal–insulator transition (MIT) at T_c = 68° C. The tuning of MIT parameters is a crucial point to use VO₂ within thermoelectric, electrochromic, or thermochromic applications. In this study, the effect of oxygen deficiencies, strain engineering, and metal tungsten doping are combined to tune the MIT with a low phase transition of 20 °C in the air without capsulation. Narrow hysteresis phase transition devices based on multilayer VO₂, WO₃, Mo_0.2W_0.8O_3, and/or MoO₃ oxide thin films deposited through a high vacuum sputtering are investigated. The deposited films are structurally, chemically, electrically, and optically characterized. Different conductivity behaviour was observed, with the highest value towards VO_1.75/WO_2.94 and the lowest VO_1.75 on FTO glass. VO_1.75/WO_2.94 showed a narrow hysteresis curve with a single-phase transition. Thanks to the role of oxygen vacancies, the MIT temperature decreased to 35 °C, while the lowest value (T_c = 20 °C) was reached with Mo_0.2W_0.8O₃/VO₂/MoO₃ structure. In this former sample, Mo_0.2W_0.8O₃ was used for the first time as an anti-reflective and anti-oxidative layer. The results showed that the MoO₃ bottom layer is more suitable than WO₃ to enhance the electrical properties of VO₂ thin films. This work is applied to fast phase transition devices. Full article

We successfully demonstrated a transition from a metallic InO_x film into a nondegenerate semiconductor InO_x:H film. A hydrogen-doped amorphous InO_x:H (a-InO_x:H) film, which was deposited by sputtering in Ar, O₂, and H₂ gases, could be converted into a polycrystalline InO_x:H (poly-InO_x:H) film by low-temperature (250 °C) solid-phase crystallization (SPC). Hall mobility increased from 49.9 cm²V⁻¹s⁻¹ for an a-InO_x:H film to 77.2 cm²V⁻¹s⁻¹ for a poly-InO_x:H film. Furthermore, the carrier density of a poly-InO_x:H film could be reduced by SPC in air to as low as 2.4 × 10¹⁷ cm⁻³, which was below the metal–insulator transition (MIT) threshold. The thin film transistor (TFT) with a metallic poly-InO_x channel did not show any switching properties. In contrast, that with a 50 nm thick nondegenerate poly-InO_x:H channel could be fully depleted by a gate electric field. For the InO_x:H TFTs with a channel carrier density close to the MIT point, maximum and average field effect mobility (μ_FE) values of 125.7 and 84.7 cm²V⁻¹s⁻¹ were obtained, respectively. We believe that a nondegenerate poly-InO_x:H film has great potential for boosting the μ_FE of oxide TFTs. Full article