Qiwu Shi

Giant Phase Transition Properties at Terahertz Range in VO2 films Deposited by Sol–Gel Method

VO(2) films were fabricated on high-purity single-crystalline silicon substrate by the sol-gel method, followed by rapid annealing. The composition and microstructure of the films were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). The results indicated a polycrystalline nature with high crystallinity and compact nanostructure for the films, and the concentration of +4 valence vanadium is 79.85%. Correlated with these, a giant transmission modulation ratio about 81% of the film was observed by terahertz time domain spectroscopy. The experimentally observed transmission characteristics were reproduced approximately, by a simulation at different conductivities across the phase transition. According to the effective-medium theory, we assumed that it is important to increase the concentration of +4 valence vanadium oxide phases and improve the compactness of the VO(2) films for giant phase transition properties. The sol-gel-derived VO(2) films with giant phase transition properties at terahertz range, and the study on their composition and microstructure, provide considerable insight into the fabrication of VO(2) films for the application in THz modulation devices.

Terahertz transmission characteristics across the phase transition in VO2 films deposited on Si, sapphire, and SiO2 substrates

Vanadium dioxide (VO2) films were deposited on high-purity Si, sapphire, and SiO2 substrates by an organic sol-gel method. The effect of the substrate on the structure, morphology, and phase transition properties of the VO2 films was demonstrated. We proposed that the film-substrate interaction induced the differences in the fraction of the +4 valence state vanadium oxide phase, surface morphology, and grain size for the VO2 films. The VO2 film on the Si substrate exhibited a switching property of about 2 orders of change in electrical resistivity. By contrast, the VO2 films on the sapphire and SiO2 substrates exhibited a switching property of about 3 orders of change in resistivity. The THz transmission across the phase transition in the VO2 films was quite different in the transmission modulation ratio, the width, and the slope of the hysteresis loop. In particular, the VO2 films on the sapphire and SiO2 substrates have the same reduction in THz transmission by about 46% comparing with about 35% in the ...

Synthesis and terahertz transmission properties of nano-porous vanadium dioxide films

Vanadium dioxide (VO2) films were prepared by the sol–gel method on high-purity silicon substrates, in which cetyltrimethyl ammonium bromide (CTAB) was used as a functional additive to form nano-porous structure in the VO2 films. The morphology, crystalline structure and stoichiometry of the films were investigated by field emission scanning electron microscopy, x-ray diffraction and x-ray photoelectron spectroscopy. Furthermore, the effects of nano-scaled grain size and pores on the THz transmission properties across the phase transition in the VO2 films were studied. The results indicated that the film modified with CTAB can form a nano-porous VO2 structure with a uniform grain size of about 30 nm. The nano-porous VO2 film exhibited significantly broader hysteresis loops and a slight decrease in THz transmission reduction across the phase transition, compared with the common film without a nano-porous structure. A tentative interpretation is given for these phenomena.

Nanostructured VO2 film with high transparency and enhanced switching ratio in THz range

We investigated the terahertz (THz) transmission characteristics of semiconductor VO2 film and its THz suppression behavior after the phase transition. The VO2 films were deposited by the sol-gel method, and an in situ growth with surface nanocrystallization occurring in the films with increasing thickness was presented. Morphology-induced percolation leads to high THz transparency in the semiconductor VO2 film, and the more compact nanostructure could account for the enhanced THz switching ratio in the metallic film. These results may offer insights into the artificial design of VO2 films for THz device applications.

Curing of polyester powder coating modified with rutile nano-sized titanium dioxide studied by DSC and real-time FT-IR

Polyester powder coating modified with 2 mass% of rutile nano-sized titanium dioxide (nano-TiO2) was prepared by melt-blend extrusion method. The nano-TiO2 dispersion state in the powder coating matrix was analyzed by field emission scanning electron microscopy (FE-SEM), which presented a well dispersion of modified nano-TiO2 in the polyester powder coating. The effect of nano-TiO2 on the curing of polyester powder coating was investigated by differential scanning calorimeter (DSC). Kissinger and Crane methods were used to study the activation energy (E) and reaction order (n) of the coating. The results indicated that 2 mass% of nano-TiO2 additive played a prompting role in the curing of polyester powder coatings, due to the hydroxyl functional groups existed on the surface of nano-TiO2. Furthermore, real-time Fourier transform infrared (FT-IR) spectroscopy with a heating cell was also employed to record the curing actions, of which results were consistent with the DSC experiments.

Sol–gel fabrication of WO3/RGO nanocomposite film with enhanced electrochromic performance

Tungsten trioxide/reduced graphene oxide (WO3/RGO) nanocomposite film was fabricated by the sol–gel spin-coating technique, using a mixed colloidal dispersion of WO3 precursor and GO. The structure, surface morphology and optical properties of WO3 and WO3/RGO films were investigated by means of X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and UV-visible spectrophotometry. The electrochemical and electrochromic performances of the WO3 and WO3/RGO films were studied by chronoamperometry, in situ optical transmittance and cyclic voltammogram analysis. In the WO3/RGO nanocomposite film, monoclinic WO3 nanoparticles were grafted onto the surface of RGO sheets via C–W and C–O–W chemical bonds. The results showed that the WO3/RGO nanocomposite film exhibited shorter coloration–bleaching times (tc = 9.5 s and tb = 7.6 s), higher coloration efficiency (75.3 cm2 C−1 at 633 nm), larger optical modulatory range (59.6% at 633 nm) and better cyclic stability compared with WO3 films, which are attributed to faster Li+ ion diffusion and electron transfer rate. The methodology provides a facile and industrially scalable approach to obtain fast switching electrochromic materials.

One-Step Hydrothermal Synthesis of W-Doped VO2 (M) Nanorods with a Tunable Phase-Transition Temperature for Infrared Smart Windows

Vanadium dioxide (VO2), with reversible metal–semiconductor transition near room temperature, is a compelling candidate for thermochromic windows. Nanocomposite coatings derived from VO2 nanoparticles are particularly superior to VO2 films due to their advantages in large-scale preparation, flexible shaping, and regulation of optical properties. In this work, we developed a novel method for one-step hydrothermal synthesis of W-doped VO2 (M) nanorods and studied their application in large-scale infrared smart windows. On introducing tartaric acid as a new reductant, VO2 underwent a two-stage phase evolution from the pure phase comprising VO2 (A) nanobelts to VO2 (M) nanorods, instead of the conventional three-stage B–A–M phase evolution during hydrothermal synthesis. This transition is very favorable for the large-scale hydrothermal synthesis of VO2 (M). The phase-transition temperature of VO2 (M) nanoparticles can be regulated systematically by W doping, with a reduction efficiency of about 24.52 °C/atom % W. Moreover, VO2 (M) composite films were fabricated using a convenient roller coating method, which exhibited significant midinfrared transmission switching up to 31%, with a phase-transition temperature of about 37.3 °C. This work demonstrates the significant progress in the one-step hydrothermal synthesis of VO2 (M) nanorods and provides significant insights into their applications in infrared smart windows.

Ultrafast terahertz modulation characteristic of tungsten doped vanadium dioxide nanogranular film revealed by time-resolved terahertz spectroscopy

The ultrafast terahertz (THz) modulation characteristic during photo-induced insulator-to-metal transition (IMT) of undoped and tungsten (W)-doped VO2 film was investigated at picoseconds time scale using time-resolved THz spectroscopy. W-doping slows down the photo-induced IMT dynamic processes (both the fast non-thermal process and the slow metallic phase propagation process) in VO2 film and also reduces the pump fluence threshold of photo-induced IMT in VO2 film. Along with the observed broadening of phase transition temperature window of IMT in W-doped VO2, we conclude that W-doping prevents metallic phase domains from percolation. By further extracting carrier properties from photo-induced THz conductivity at several phase transition times, we found that the electron-electron correlation during IMT is enhanced in W-doped VO2.

Phase transition properties of vanadium oxide films deposited by polymer-assisted deposition

Vanadium dioxide (VO2) films were synthesized on mica substrates by a polymer-assisted deposition method, followed by rapid annealing with different annealing temperatures. The crystalline structure and morphology of the films were investigated by XRD and FE-SEM, and their phase transition properties were studied by in situ FT-IR. The results indicated that the annealing temperature affected the crystalline structure and morphology of the films remarkably, which then resulted in varied phase transition properties. In particular, the films annealed at higher temperature showed more polycrystalline structure, increased particle size and reduced phase transition intensity. But the films exhibited the similar hysteresis temperature width with increasing annealing temperature.

Fabrication of nanocrystalline λ-Ti3O5 with tunable terahertz wave transmission properties across a temperature induced phase transition

λ-Ti3O5 was a newly discovered material with intriguing phase transition characteristics, which exhibits huge potential in the application of memory and tunable optoelectronic devices. However, the fabrication of λ-Ti3O5 still presents a great challenge and its application needs further investigation. In this work, we developed a novel method to fabricate nanocrystalline λ-Ti3O5 by carbothermal reduction of nano-TiO2, and explored its terahertz transmission properties through a temperature induced phase transition. A second phase was introduced to inhibit the grain growth of titanium oxide during the carbothermal reduction, by performing a surface modification of the precursor nano-TiO2 particles with Al2O3. This process was proved to be critical for the formation of nanocrystalline λ-Ti3O5. An in situ XRD analysis combined with a first-principles calculation based on plane wave DFT indicated that the nanocrystalline λ-Ti3O5 exhibited a semimetallic λ phase to metallic α phase transition across a large temperature range. The phase transition was accompanied by continuous, slow and reversible tuning of the terahertz transmission amplitude. This work provides considerable insights into the synthesis of λ-Ti3O5 and opens up studies on the applications of λ-Ti3O5 in the THz range, such as but not limited to sensors and smart windows.