Yunchuan Xin

Electron transfer induced thermochromism in a VO2–graphene–Ge heterostructure

As a marvelous thermochromic material, vanadium dioxide (VO2) holds great promise for applications in smart devices due to its semiconductor–metal transition (SMT), which is an active response to external temperature stimuli and near-infrared irradiation. Herein, we demonstrate for the first time that the phase transition temperature of a VO2 film can be effectively manipulated using graphene as an interlayer. A high quality graphene monolayer was deposited using chemical vapor deposition on a Ge underlayer, followed by the growth of a VO2 film onto the graphene to form a semimetal–semiconductor contact, namely, a VO2–graphene–Ge junction. The thermochromic properties of the VO2 film were demonstrated with ∼20% infrared reflectance contrast. Furthermore, the transition temperature of the VO2 film was effectively reduced from 340 K to 330 K. On the basis of the Mott–Hubbard phase transition theory, a plausible mechanism is proposed here for the first time from the perspective of charge transfer to elucidate the experimental phenomenon, which indicates that the electron transfer can increase the electron concentration in the VO2 film and destabilize the semiconductor phase of the VO2 film, thus decreasing the SMT temperature of the thermochromic VO2 film. In addition to stimulating scientific interest, this study may also contribute to thermochromic VO2-based applications in sensors, optical and electrical switches, and other nanodevices.

The optical properties of low infrared transmittance WO3−x nanocrystal thin films prepared by DC magnetron sputtering under different oxygen ratios

Low infrared transmittance WO3−x (0 < x < 1) ratios, as transparent conductors with high transmittance in the visible range and UV blocking. The infrared shielding properties could be adjusted by tuning the oxygen ratio during the sputtering process. The intrinsic defects, such as oxygen vacancy () and special atom (W5+), determine the main optical properties by localized states originated from the electron caused by interband transition and the surface dipole of the plasmon oscillation of the nanoparticle with optical scattering. The structures, morphology, defects and chemical states were investigated. With respect to oxygen ratio, it has little effect on the structure and morphology, yet the optical shielding performance, defects and chemical states can be controlled linearly by considering the oxygen ratio during the sputtering process. The WO3−x thin films have great potential for application in infrared shielding and energy conservation.

Thermochromic multilayer films of WO3/VO2/WO3 sandwich structure with enhanced luminous transmittance and durability

A novel thermochromic WO3/VO2/WO3 sandwich structure was deliberately designed and deposited by a reactive magnetron sputtering technique. The double layer of WO3 not only functions as an antireflection (AR) layer to enhance the luminous transmittance (Tlum) of VO2, but also performs as a good protective layer for thermochromic VO2. Basically, the bottom WO3 layer functions as a buffer beneficial for the formation of the intermediate VO2 layer and serves as an AR layer while the intermediate VO2 layer with primary monoclinic phase acts as an automatic solar/heat control for energy saving. The top WO3 layer acts as another AR layer, and provides protection in a complex environment. An obvious increase in Tlum by 49% (from 37.2% to 55.4%) is found for VO2 films after introducing double-layer WO3 AR coating. The VO2 deposited on glass exhibits good thermochromism with an optical transition at 54.5 °C, which decreases to 52 °C in WO3/VO2/WO3 sandwich structure, and the hysteresis loop is sharper around the transition temperature, which may be ascribed to the strain and interfacial diffusion. In comparison with single-layer VO2, the durability in automatic solar/heat control of sandwich-structure VO2 films is improved nearly 4 times in high temperature and humidity conditions. This multilayer will open a new avenue for the design and integration of advanced thermochromic heterostructures with controllable functionalities for intelligent window and sensing system applications.

CsxWO3 nanoparticle-based organic polymer transparent foils: low haze, high near infrared-shielding ability and excellent photochromic stability

Well-dispersed cesium-doped WO3 (CWO) nanocrystals are an attractive candidate as hosts that exhibit excellent near infrared-shielding ability owing to two kinds of absorptions: localized surface plasmon resonance absorption around 900 nm and small polaron absorption near 1800 nm. In this work, a simple thermal reduction method was introduced to synthesize a CWO powder, which was further dispersed by bead milling to obtain a CWO nanodispersion liquid. The CWO nanodispersion liquid can be employed to make a unique nonlinear optical foil with 70% visible transmittance and a near infrared transmittance less than 10%, while maintaining a high transparency with less than 1.0% haze. However, the main issue concerning CWO nanoparticle-based polymer foils is the detrimental photochromic effect under strong ultraviolet (UV) irradiation, which hampers their extensive application in energy-saving windows, such as in automobiles and architecture. Herein, the photochromic effect is largely relieved by embedding CWO nanoparticles into inert polyethylene terephthalate (PET) instead of mixing with resins like polyurethane or acrylic resin, and the final organic–inorganic nonlinear optical foils exhibit almost no coloration under strong UV illumination. Therefore, this study provides a new insight into making hybrid foils with high stability, transparency and multifunctionality in the optical and magnetic fields.

Optical switching properties of Pd-Ni thin-film top-capped switchable mirrors

Switchable mirrors based on magnesium-nickel alloy thin films capped with catalytic Pd-Ni alloy thin films were prepared by a DC magnetron sputtering method. Their composition, structure and surface morphology were studied by XPS, XRD and AFM. Herein, the optical switching properties and durability of the switchable mirrors were investigated by varying the Ni content in the Pd-Ni alloys. Comparing pure Pd catalyst with Pd-Ni top-capped switchable mirrors, the latter show better hydrogenation and dehydrogenation kinetics, and the speed of hydrogen desorption is obviously improved with increasing Ni content in the Pd-Ni alloy. The Pd-Ni capped switchable mirrors also have better optical switching durability. The catalytic Pd layer with the addition of Ni does not influence the transmittance (hydride state) and reflectance (metallic state) of the switchable mirrors. In addition, replacing Pd with Pd-Ni alloy decreases the cost of the switchable mirrors: employing nickel in the alloy Pd89.2Ni10.8 can save about 11% use of Pd. Therefore, the Pd-Ni alloy can provide a cheaper catalytic thin film, and it is expected to have applications in energy-saving windows, hydrogen sensors and hydrogen storage materials.

Two-step fabrication of NaxWO3 thin film via oxygen-vacancy-induced effect for energy efficient applications

Developing solar modulation materials such as alkali tungsten bronzes with high-performance and low-cost fabrication is extremely important for energy-saving project applications in our modern lives. In this paper, sodium tungsten bronze (NaxWO3) film was fabricated by a novel approach using a two-step method. Amorphous sub-stoichiometric WO3−x (0 < x < 1) film was deposited on glass using magnetron sputtering and then formed NaxWO3 after post-annealing treatment. Oxygen vacancies and tunnels existing in WO3−x receive ions such as Na+, O2−, etc. that diffuse from the glass substrate. The Na+ ion enters tunnels among WO6 octahedra. The O2− ion combines with an oxygen vacancy. The NaxWO3 film exhibits excellent infrared shielding performance with less than 10% infrared transmittance and high transmittance for visible light, as well as completely blocking UV light. Compared with traditional wet chemistry methods, this approach is much more concise with low cost, thus having huge potential applications for energy-saving projects.

Preparation of microstructure-controllable superhydrophobic polytetrafluoroethylene porous thin film by vacuum thermal-evaporation

The three-dimensional porous network polytetrafluoroethylene (PTFE) thin films were achieved by a vacuum technique through evaporating the pure PTFE powders. The surfaces of PTFE thin films showed various morphologies by adjusting the evaporation temperature and the corresponding contact angle ranging from 133° to 155°. Further analyses of surface chemical composition and morphology by FTIR and FE-SEM revealed that the origin of hydrophobicity for the PTFE thin films could be ascribed to the fluorine-containing groups and the surface morphologies, indicating that abundant -CF2 groups and network structures with appropriate pore sizes played a vital role in superhydrophobicity. By characterization of UV-Vis, the films also showed high transmittance and antireflection effect. The films prepared by this simple method have potential applications such as waterproof membrane and self-cleaning coating.