High-throughput measurements of thermochromic behavior in V(1-x)Nb(x)O(2) combinatorial thin film libraries

We describe a high-throughput characterization of near-infrared thermochromism in V1-xNbxO2 combinatorial thin film libraries. The oxide thin film library was prepared with a VO2 crystal structure and a continuous gradient in composition with Nb concentrations in the range of less than 1% to 45%. The thermochromic phase transition from monoclinic to tetragonal was characterized by the accompanying change in near-infrared reflectance. With increasing Nb substitution, the transition temperature was depressed from 65 to 35 °C, as desirable for smart window applications. However, the magnitude of the reflectance change across the thermochromic transition was also reduced with increasing Nb film content. Data collection, handling, and analysis supporting thermochromic characterization were fully automated to achieve high throughput. Using this system, in 14 h, temperature-dependent infrared reflectances were measured at 165 arbitrary locations on a thin film combinatorial library; these measurements were analyzed for thermochromic transitions in minutes.

Conformal Coating of a Phase Change Material on Ordered Plasmonic Nanorod Arrays for Broadband All-Optical Switching

Actively tunable optical transmission through artificial metamaterials holds great promise for next-generation nanophotonic devices and metasurfaces. Plasmonic nanostructures and phase change materials have been extensively studied to this end due to their respective strong interactions with light and tunable dielectric constants under external stimuli. Seamlessly integrating plasmonic components with phase change materials, as demonstrated in the present work, can facilitate phase change by plasmonically enabled light confinement and meanwhile make use of the high sensitivity of plasmon resonances to the variation of dielectric constant associated with the phase change. The hybrid platform here is composed of plasmonic indium-tin-oxide nanorod arrays (ITO-NRAs) conformally coated with an ultrathin layer of a prototypical phase change material, vanadium dioxide (VO₂), which enables all-optical modulation of the infrared as well as the visible spectral ranges. The interplay between the intrinsic plasmonic nonlinearity of ITO-NRAs and the phase transition induced permittivity change of VO₂ gives rise to spectral and temporal responses that cannot be achieved with individual material components alone.

Sputtering Deposition of Sandwich-Structured V2O5/Metal (V, W)/V2O5 Multilayers for the Preparation of High-Performance Thermally Sensitive VO2 Thin Films with Selectivity of VO2 (B) and VO2 (M) Polymorph

For specific application to an uncooled infrared detector, VO2 thin films should have a series of characteristics including purposefully chosen polymorphs, accurate stoichiometry, phase stabilization, a high temperature-coefficient of resistance (TCR), and suitable square-resistance. This work reports controllable preparation of high-performance VO2 films via post annealing of a sandwich-structured V2O5/metal (V, W)/V2O5 multilayer precursor, which was deposited by RF magnetron sputtering. This sandwich structure can dynamically regulate oxygen contents and doping element levels in the films, enabling us to achieve accurate regulation of stoichiometry and polymorphs. The precursor films undergo a B to M phase transition depending on the quantity of the metal layers. At the thickness of the metal layer below a limitation, the resulting film after heat treatment was VO2 (B), and above the limitation, the product was VO2 (M). The optical modulation of the VO2 (M) in the near-infrared region can be tuned from 1.2 to 39.8% (ΔT2000 nm). TCR values can range from -1.89 to -4.29%/K and the square-resistances at room temperature (R0) from 69.68 to 12.63 kΩ. The simplicity in phase regulation of the present method and the superior optical and electrical properties of the films may allow its wide applications in thermo-opto-electro sensing devices.

A Universal Approach To Achieve High Luminous Transmittance and Solar Modulating Ability Simultaneously for Vanadium Dioxide Smart Coatings via Double-Sided Localized Surface Plasmon Resonances

Vanadium dioxide (VO₂)-based thermochromic coatings has attracted considerable attention in the application of smart windows as a result of their intriguing property of metal-insulator transition at moderate temperatures. However, the practical requirements of smart windows, i.e., the high luminous transmittance of T_lum > 60% and large solar modulating ability of ΔT_sol > 10%, are competing to a large extent and hardly satisfied simultaneously. Here, we proposed a facile and universal method to prepare VO₂ coatings for exceeding the criteria above using double-sided localized surface plasmon resonances (LSPRs), which are excited by the VO₂ nanoparticles dispersed evenly on both surfaces of the fused silica substrate. With subtle engineering of the sol-gel and heat treatment processes, the morphology of as-prepared VO₂ nanoparticles and corresponding LSPRs are controlled to achieve a high luminous transmittance (T_lum = 68.2%) and solar modulating ability (ΔT_sol = 11.7%) simultaneously. Further simulation suggests that the double-sided LSPRs can collectively enhance the performance of VO₂ smart coatings. Moreover, the double-sided VO₂ nanoparticle coatings demonstrate stable performance with no more than 1% degradation of T_lum and ΔT_sol after 1500 cycles. This study provides an alternative strategy to obtain high-quality VO₂ (M) solar modulating coatings.

VO₂ Thermochromic Films on Quartz Glass Substrate Grown by RF-Plasma-Assisted Oxide Molecular Beam Epitaxy

Vanadium dioxide (VO₂) thermochromic thin films with various thicknesses were grown on quartz glass substrates by radio frequency (RF)-plasma assisted oxide molecular beam epitaxy (O-MBE). The crystal structure, morphology and chemical stoichiometry were investigated systemically by X-ray diffraction (XRD), atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses. An excellent reversible metal-to-insulator transition (MIT) characteristics accompanied by an abrupt change in both electrical resistivity and optical infrared (IR) transmittance was observed from the optimized sample. Remarkably, the transition temperature (T_MIT) deduced from the resistivity-temperature curve was reasonably consistent with that obtained from the temperature-dependent IR transmittance. Based on Raman measurement and XPS analyses, the observations were interpreted in terms of residual stresses and chemical stoichiometry. This achievement will be of great benefit for practical application of VO₂-based smart windows.

Room-Temperature Synthesis of Inorganic-Organic Hybrid Coated VO2 Nanoparticles for Enhanced Durability and Flexible Temperature-Responsive Near-Infrared Modulator Application

Vanadium dioxide is one kind of desirable infrared modulator for sensors because of its remarkable temperature-responsive infrared modulation ability via autogeneic metal-insulator transition. However, the detriments of poor chemical stability and narrow scope of extensive-researched application (e.g., smart windows) restrict its mass production. Here, we propose a VO₂@MgF₂@PDA inorganic-organic hybrid coated architecture for greatly enhancing the optical durability more than 13 times in contrast to pristine VO₂ and the transmittance difference between room and high temperature changed within 20% (decreasing from 25 to 20.1%) at λ = 1200 nm after the ageing time of 1000 h at constant temperature (60 °C) and relative humidity (90%). Furthermore, based on the as-synthesized durability-enhanced nanoparticles, we fabricated a flexible sensor for temperature-field fluorescence imaging by integrating the VO₂-based near-infrared modulator with the upconversion fluorescence material. Additionally, the formation mechanism of VO₂@MgF₂ core-shell nanoparticles was studied in detail. The inorganic-organic combination strategy paves a new way for improving the stability of nanoparticles, and the use of VO₂-based flexible temperature-fluorescence sensors is a promising technique for remote and swift temperature-field distribution imaging on complicated and campulitropal surfaces.

Tuning carrier density and phase transitions in oxide semiconductors using focused ion beams

We demonstrate spatial modification of the optical properties of thin-film metal oxides, zinc oxide (ZnO) and vanadium dioxide (VO2) as representatives, using a commercial focused ion beam (FIB) system. Using a Ga+ FIB and thermal annealing, we demonstrated variable doping of a wide-bandgap semiconductor, ZnO, achieving carrier concentrations from 1018 cm-3 to 1020 cm-3. Using the same FIB without subsequent thermal annealing, we defect-engineered a correlated semiconductor, VO2, locally modifying its insulator-to-metal transition (IMT) temperature by up to ∼25 °C. Such area-selective modification of metal oxides by direct writing using a FIB provides a simple, mask-less route to the fabrication of optical structures, especially when multiple or continuous levels of doping or defect density are required.

Polarization and Incident Angle Independent Multifunctional and Multiband Tunable THz Metasurface Based on VO2

Aiming at the limitations of single-functionality, limited-applicability, and complex designs prevalent in current metasurfaces, we propose a terahertz multifunctional and multiband tunable metasurface utilizing a VO2-metal hybrid structure. This metasurface structure comprises a top VO2-metal resonance layer, a middle polyimide dielectric layer, and a gold film reflective layer at the bottom. This metasurface exhibits multifunctionality, operating independently of polarization and incident angle. The varying conductivity states of the VO2 layers, enabling the metasurface to achieve various terahertz functionalities, including single-band absorption, broadband THz absorption, and multiband perfect polarization conversion for linear (LP) and circularly polarized (CP) incident waves. Finally, we believe that the functional adaptability of the proposed metasurface expands the repertoire of options available for future terahertz device designs.

Ultrathin VO2 Films on Functional Substrates

The metal-insulator transition (MIT) in vanadium dioxide (VO2) thin films is strongly affected by grain size, thickness, and interfacial properties. Typically, a minimum thickness around 50 nm is required for VO2 to exhibit a significant MIT when functional substrates like sapphire and silicon are used. Several works have shown that thin films below 20 nm, with up to 2-3 decades of change in the resistance across the MIT, can be achieved but require complex pre- or postprocessing of the samples. We show that predeposition substrate condition control facilitates the direct growth of VO2 ultrathin 15 nm films, exhibiting a resistance change between 3 and 4 decades across the MIT. Our findings indicate that the interface between the film and the substrate is crucial in determining the initial growth layers and the structural evolution. With appropriate substrate surface treatment, the desired VO2 MIT can be enhanced regardless of the substrate crystallographic orientation. Moreover, we propose a novel approach to obtain large resistance changes across the MIT in ultrathin VO2 films by incorporating a predeposited 1.5 nm vanadium oxide buffer layer, thereby eliminating the need to use different materials or complex pre- or postprocessing of the samples. We also demonstrate that this method improves the transition of 25-50 nm VO2 thin films on silicon substrates. Our study reveals a simple approach for direct growth of ultrathin VO2 films exhibiting a significant MIT, which is commonly accepted unattainable over substrates of technological importance, such as sapphire and silicon.

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.

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.