Fabrication of belt-like VO2(M)@C core-shell structured composite to improve the electrochemical properties of VO2(M)

VO₂(M) nanobelts encapsulated into carbon core–shell structured composite (VO₂(M)@C) was successfully synthesized by the thermal treatment with the precursor V₃O₇·H₂O@C composite under the inert atmosphere. The as-obtained sample was characterized by XRD, EDS, EA, FT-IR, Raman, SEM and TEM measurements. The core exhibited monoclinic phase VO₂(M) and the carbon coated on the surface of VO₂(M) was amorphous. The average thickness of carbon was about 18.5 nm. The possible formation mechanism of VO₂(M)@C was proposed as that the reaction underwent the solid state reaction by the interface reaction between V₃O₇ core and carbon shell. Furthermore, VO₂(M) and VO₂(M)@C composite were explored as the cathode materials to apply in lithium-ion batteries, indicating that the VO₂(M)@C composite electrode exhibited the better electrochemical properties than that of pure VO₂(M), achieving the aim of improving the electrochemical properties of VO₂(M).     

On the Optical Properties of Thin‐Film Vanadium Dioxide from the Visible to the Far Infrared

The insulator‐to‐metal transition (IMT) in vanadium dioxide (VO₂) can enable a variety of optics applications, including switching and modulation, optical limiting, and tuning of optical resonators. Despite the widespread interest in VO₂ for optics, the wavelength‐dependent optical properties across its IMT are scattered throughout the literature, are sometimes contradictory, and are not available at all in some wavelength regions. Here, the complex refractive index of VO₂ thin films across the IMT is characterized for free‐space wavelengths from 300 nm to 30 µm, using broadband spectroscopic ellipsometry, reflection spectroscopy, and the application of effective‐medium theory. VO₂ films of different thicknesses are studied, on two different substrates (silicon and sapphire), and grown using different synthesis methods (sputtering and sol–gel). While there are differences in the optical properties of VO₂ synthesized under different conditions, these differences are surprisingly small in the ≈2–11 µm range where the insulating phase of VO₂ also has relatively low optical loss. It is anticipated that the refractive‐index datasets from this article will be broadly useful for modeling and design of VO₂‐based optical and optoelectronic components, especially in the mid‐wave and long‐wave infrared.     

Thermally tunable optical constants of vanadium dioxide thin films measured by spectroscopic ellipsometry

Smart materials with reversible tunable optical constants from visible to near-infrared wavelengths could enable excellent control over the resonant response in metamaterials, tunable plasmonic nanostructures, optical memory based on phase transition and thermally tunable optical devices. Vanadium dioxide (VO₂) is a promising candidate that exhibits a dramatic change in its complex refraction index or complex dielectric function arising from a structural phase transition from semiconductor to metal at a critical temperature of 70 °C. We demonstrated the thermal controllable reversible tunability of optical constants of VO₂ thin films. The optical/dielectric constants showed an abrupt thermal hysteresis which confirms clearly the electronic structural changes. Temperature dependence of dielectric constants as well as optical conductivity of sputtered VO₂ thin films was also reported and compared to previous theoretical and experimental reports.     

Direct comparison of the electrical,optical, and structural phase transitions of VO2 on ZnO nanostructures

We examined the temperature-dependent electrical, optical, and structural properties of VO₂ on ZnO nanorods with different lengths in the temperature range from 30 to 100 °C. ZnO nanorods with a uniform length were grown on Al₂O₃ substrates using a metal organic chemical vapor deposition, and subsequently, VO₂ was ex-situ deposited on ZnO nanorods/Al₂O₃ templates using a sputtering deposition. The optical properties of the VO₂/ZnO nanorods were measured simultaneously with direct current (DC) resistance using the reflectivity of an infrared (IR) laser beam with a wavelength of 790 nm. The local structural properties around V atoms of VO₂/ZnO nanorods were simultaneously measured with the DC resistance using x-ray absorption fine structure at the V K edge. Direct comparison of the temperature-dependent resistance, IR reflectivity, and local structure reveals that an optical phase transition first occurs, a structural phase transition follows, and an insulator-to-metal transition finally appears during heating.     

二氧化钒纳米棒的相变行为

研究了VO₂(M)纳米棒的金属绝缘体相变(MIT)行为．在VO₂(M)纳米棒的DSC分析曲线上发现了两个MIT,分别位于低温和高温区．低温MIT总是伴随出现VO₂(B)纳米棒,而独立的高温MIT出现在纯VO₂(M)纳米棒．分析和讨论了这两个MIT的机制．     

VO2-WO3 nanocomposite thin films synthesized by pulsed laser deposition technique

Pure VO₂ and VO₂-WO₃ composite thin films were grown on quartz substrate by pulsed laser deposition (PLD) technique. The influence of varying WO₃ molar concentration in the range from x = 0.0 to x = 0.4 on structural, electrical and optical properties of VO₂-WO₃ nanocomposite thin films has been systematically investigated. X-ray diffraction studies reveal the single crystalline monoclinic VO₂ phase (m-VO₂) up to 10% of WO₃ content whereas both m-VO₂ as well as h-WO₃ (hexagonal WO₃) phases were present at higher WO₃ content (0.2 ≤ x ≤ 0.4). Optical transmittance spectra of the films showed blue shift in the absorption edge with increase in WO₃ content. Temperature dependence of resistivity (R-T) measurements indicates significant variation in metal-insulator transition temperature, width of the hysteresis, and shape of the hysteresis curve. Cyclic Voltammetry measurements were performed on VO₂-WO₃ thin films. A direct correlation between V/W ratio and structure-property relationship was established. The present investigations reveal that doping of WO₃ in VO₂ is effective to increase the optical transmittance and to reduce the semiconductor to metal phase transition temperature close to room temperature.     

Synthesis of V O2(A) nanobelts by the transformation of V O2(B) under the hydrothermal treatment and its optical switching properties

V O₂(A) nanobelts had been successfully synthesized by the transformation of V O₂(B) using H₂O as the solvent under the hydrothermal approach at 280 °C for 48 h. Some parameters, such as the reaction temperature and time, had been briefly discussed to reveal the transition from V O₂(B) to V O₂(A). It was found that H₂O played a crucial role in the transition from V O₂(B) to V O₂(A). The phase transition of V O₂(A) nanobelts was at 162 °C. The optical switching properties of V O₂(A) were studied by the variable-temperature infrared spectra for the first time. In addition, V O₂(A) nanobelts were calcined at 700 °C for 2 h under a high purity Ar (99.999%) atmosphere to obtain V O₂(M) which exhibited a strong crystallographic transition at around 65 °C.     

Metal insulator transition characteristics of macro-size single domain VO2 crystals

The metal insulator transition (MIT) characteristics of macro-size single-domain VO₂ crystal were investigated. At the MIT, the VO₂ crystal exhibited a rectangular shape hysteresis curve, a large change in resistance between the insulating and the metallic phases, in the order of ～10⁵, and a small transition width (i.e. temperature difference before and after MIT) as small as 10^?3°C. These MIT characteristics of the VO₂ crystals are discussed in terms of phase boundary motion and the possibility of controlling the speed of the phase boundary, with change in size of crystal, is suggested.     

Effect of oxygen partial pressure and VO<Subscript>2</Subscript> content on hexagonal WO<Subscript>3</Subscript> thin films synthesized by pulsed laser deposition technique

We report on the effect of oxygen partial pressure and vacuum annealing on structural and optical properties of pulsed laser-deposited nanocrystalline WO₃ thin films. XRD results show the hexagonal phase of deposited WO₃ thin films. The crystallite size was observed to increase with increase in oxygen partial pressure. Vacuum annealing changed the transparent as-deposited WO₃ thin film to deep shade of blue color which increases the optical absorption of the film. The origin of this blue color could be due to the presence of oxygen vacancies associated with tungsten ions in lower oxidation states. In addition, the effects of VO₂ content on structural, electrochemical, and optical properties of (WO₃)_1−x (VO₂) _x nanocomposite thin films have also been systematically investigated. Cyclic voltammogram exhibits a modification with the appearance of an extra cathodic peak for VO₂–WO₃ thin film electrode with higher VO₂ content (x ≥ 0.2). Increase of VO₂ content in (WO₃)_1−x (VO₂) _x films leads to red shift in optical band gap.     

Theoretical investigation of the optical spectra and local lattice structure for Mn5+ in a Sr10(VO4)6F2 crystal

In this paper, the relationships between the optical spectra and local lattice structure for Mn⁵⁺ in a Sr₁₀(VO₄)₆F₂ crystal are established by the crystal- and ligand-field theory. The effect of spin–orbital coupling between the central 3d² ions and ligand ions has been considered in the full energy matrix. Using the matrix and superposition model formula, we have calculated the optical spectra and local lattice structure parameters of Mn⁵⁺ in Sr₁₀(VO₄)₆F₂ with a C_3v system. The calculated results are in good agreement with the observed values. In addition, the trigonal compressed distortions of the (MnO₄)^3? centers in Sr₁₀(VO₄)₆F₂ crystals are also obtained from the calculations.     

Formation mechanisms and crystallographic characteristics of metastable VO2(A) nanofiber hydrothermally synthesised in V2O5–H2C2O4–H2O system

There are many similarities between VO₂(B) and VO₂(A) from crystallographic view. However, missing of VO₂(A) during the preparation of VO₂ polymorph confused many researchers. Here, the preparation of VO₂(A) was studied systemically via a hydrothermal method in V₂O₅–H₂C₂O₄–H₂O system. As a metastable phase, it can be transferred from VO₂(B) by assembling process. Usually, poly-crystal VO₂(A) nano-fibers are formed by this process. On contrast, owing to the small energy gap between meta-stable VO₂(A) and stable VO₂(R), single crystal VO₂(A) with regular shape can also be obtained by exfoliating some parts of VO₂(R) during non-equilibrium cooling process. VO₂(A) has higher phase transition temperature than stable VO₂(R). The hysteresis in this phase transition can be observed by DSC measurement and the phase transition temperature of VO₂(A) can be tuned down by tungsten doping.     

Comparison of VO<Subscript>2</Subscript> thin films prepared by inorganic sol-gel and IBED methods

VO₂ thin films with semiconductor-to-metal phase-transition properties were prepared by inorganic sol-gel and IBED (ion-beam-enhanced deposition) methods on SiO₂/Si substrate. The crystalline phase and the shape and width of the hysteresis curves of these VO₂ films were significantly different. For sol-gel VO₂ films, the transition started at close to 62 °C upon heating. The temperature interval needed to complete the phase transition was 8 °C, the ratio of resistance (R_{20 °C}/R_{80 °C}) reached three orders and the hysteresis width was 6 °C. However, the IBED film post-annealed in Ar at 700 °C underwent a phase transition from 45 °C to 80 °C, the ratio of resistance was more than two orders and the hysteresis width was 2 °C. In addition, the TCR (temperature coefficient of resistance) at 22 °C of the IBED film was 3.5%/K, much larger than the 0.7%/K TCR of the sol-gel film. PACS 73.50.F; 73.66.E; 81.20; 81.05.Z; 81.15     

The characteristics of Au:VO2 nanocomposite thin film for photo-electricity applications

Au nanoparticles have been fabricated on normal glass substrates using nanosphere lithography (NSL) method. Vanadium dioxide has been deposited on Au/glass by reactive radio frequency (rf) magnetron sputtering. The structure and composition were determined by X-ray diffraction and X-ray photoelectron spectroscope. Electrical and optical properties of bare VO₂ and Au:VO₂ nanocomposite thin films were measured. Typical hysteresis behavior and sharp phase transition were observed. Nanopartical Au could effectively reduce the transition temperature to 40 °C. The transmittance spectrum for both Au:VO₂ nanocomposite thin film shows high transmittance under transition temperature and low transmittance above transition temperature. The characteristics present the Au:VO₂ nanocomposite thin film can be used for applications, such as “smart window” or “laser protector”.     

Role of surface defects and microstructure in infrared optical properties of thermochromic VO2 materials

Thermochromic vanadium dioxide VO₂ exhibits a semi-conducting to metallic phase transition at T_c=68 °C, involving strong variations in optical transmittance, reflectance and emissivity. However, the optical contrasts observed in thin films or nanostructured compacted samples seem to depend on both surface microstructure and surface crystal texture. In the case of opaque materials, surface defects might play a drastic role in optical reflectivity. As the high temperature metallic phase of VO₂ is opaque for infrared radiations, we used aluminum samples as standards allowing us to correlate reflectivity responses with porosity and surface defects. Then, various polycrystalline and nanostructured VO₂ samples compacted at various pressures and presenting variable surface roughness were prepared. Thin films were deposited by radio frequency sputtering process. The samples were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Optical properties (reflectance and emissivity) were analyzed above and below the transition temperature, making use of specific FTIR equipments. In thin films, the deposited VO₂ phase was systematically oriented and surface porosity was very weak. In polycrystalline samples, as the compaction pressure increased, surface porosity decreased, and infrared optical contrast increased. In such samples, preferred orientations were favored for low applied pressures. These features clearly show that the main parameters conditioning the optical contrast should be the surface defects and porosity, not the preferred crystal orientations. As an additional interesting result, the surfaces formed from compacted nanocrystalline VO₂ powders present improved optical contrast for reflectance and emissivity properties.     

Optical limiting in pulsed laser deposited VO2 nanostructures

Being a Mott type oxide, at a temperature of ~ 68 °C and ambient pressure, stoichiometric VO₂ undergoes a first order metal-insulator transition, which is accompanied by a reversible abrupt change in the band gap opening. From an optical point of view, this metal-insulator transition manifests itself by a significant and reversible variation of the refractive index under either a thermal stimuli or by photo-induction. This contribution reports on the ultrafast optical limiting in the IR regime of pulse laser deposited VO₂ nanostructures.     

Phase Transition VO2 Thin Films for Optical Switches

This paper presents a method to make vanadium dioxide (VO₂) crystallites on silicon substrates by reactive ion beam sputtering. The thickness of the thin film is about 100nm. The phase transition temperature of VO₂ is 65°C. The transmittance of the semiconducting phase VO₂ is about 50% and it is reduced to as low as 3% in metal phase at the infrared wavelenghth spectrum. The extinction ratio of the optical switches is 12dB. and the insertion loss is of 1-2dB. The switching time is about 1ms.     

A chemical lithography route to Bi2S3 nanotubes

Two different semiconducting bismuth sulfide (Bi₂S₃) nanostructures (feather-like Bi₂S₃ nanotubes and fiber-like Bi₂S₃ nanotubes) with diameters around 50-60 nm and lengths about tens of micrometers were prepared successfully by a chemical lithography route. The results indicated that the employment of polyvinylpyrrolidone led to the precursor with feather-like morphology and the acid had ripening effect on and etching action to the ultimate formation of the fiber-like Bi₂S₃ nanotubes. The photoluminescence spectra of two different Bi₂S₃ nanostructures revealed that the relative position of emission peaks was influenced by the thin edges of the feather-like nanotubes due to the quantum-confinement effect.     

Electrical and structural properties of VO2 in an electric field

We examined the electrical and local structural properties of a VO₂ film at different electric fields using electrical resistance and x-ray absorption fine structure (XAFS) measurements at the V K edge in the temperature range of 30–100 °C. The T_c value of the metal-to-insulator transition (MIT) during both heating and cooling decreases with electric field. When the electric field exceeds a certain value, the MIT becomes sharper due to Joule heating. The MIT, the structural phase transition (SPT), and the pre-edge peak transition of the VO₂ do not congruently occur at a uniform temperature. A metallic VO₂ is observed in only the rutile (or M2) symmetry. An electric field induces a substantial amount of conduction electrons in insulating VO₂. Simultaneously measured resistance and XAFS reveal that Joule heating caused by an external electric field significantly affects the MIT and SPT of VO₂.     

Spectral characterization and energy levels of Cr:Sc2(MoO4)3 crystal

This paper reports the spectral properties and energy levels of Cr³⁺:Sc₂(MoO₄)₃ crystal. The crystal field strength Dq, Racah parameter B and C were calculated to be 1408 cm⁻¹, 608 cm⁻¹ and 3054 cm⁻¹, respectively. The absorption cross sections σ_α of ⁴A₂→⁴T₁ and ⁴A₂→⁴T₂ transitions were 3.74×10⁻¹⁹ cm² at 499 nm and 3.21×10⁻¹⁹ cm² at 710 nm, respectively. The emission cross section σ_e was 375×10⁻²⁰ cm² at 880 nm. Cr³⁺:Sc₂(MoO₄)₃ crystal has a broad emission band with a broad FWHM of 176 nm (2179 cm⁻¹). Therefore, Cr³⁺:Sc₂(MoO₄)₃ crystal may be regarded as a potential tunable laser gain medium.