Shunlong Pan

Optical properties of δ-Bi2O3 thin films grown by reactive sputtering

The optical properties of δ-Bi2O3 thin films were investigated using spectroscopic ellipsometry and optical absorption spectrum. δ-Bi2O3 thin films were grown on Si and quartz substrates under different oxygen flow ratios (OFR) by radio frequency reactive magnetron sputtering. The Tauc-Lorentz dispersion method was adopted to model the optical dispersion functions of the thin films. The optical bandgap was obtained by three different methods. It was found that refractive index and extinction coefficient decrease, and the optical bandgap has a slight blue shift with increasing the OFR. Factors influencing the optical constants and optical bandgap are discussed.

Influence of annealing on the structural and optical properties of ZnO:Tb thin films

The influence of annealing on the morphological, structural, and optical properties of ZnO:Tb thin films on Si substrate grown by magnetron cosputtering is investigated. It has been found that the ZnO:Tb thin films with structures of tetrapod and screwlike nanorod are formed after annealing at temperature of 950°C. X-ray photoelectron spectroscopy, energy dispersive spectroscopy, and Raman analyses prove that the tetrapod-aiguille zinc oxide (T-A-ZnO) and the screwlike nanorods are composed of Zn, Tb, and O elements. The photoluminescence spectra of the ZnO:Tb thin films with the T-A-ZnO structure and the screwlike nanorods are featured with two ultraviolet emission peaks and one strong green emission band, and the photoluminescence intensity increases with increasing annealing temperature. The surface defects in the T-A-ZnO structure and the screwlike nanorods are considered to be responsible for enhanced green emission in the annealed ZnO:Tb thin films.

Spectroscopic ellipsometry characterization of nitrogen-incorporated HfO2 gate dielectrics grown by radio-frequency reactive sputtering

Spectroscopic ellipsometry with photon energy 0.75–6.5 eV at room temperature has been used to derive the optical properties of nitrogen-incorporated HfO2 films on Si(100) substrates grown by radio-frequency reactive sputtering using different N2∕(N2+O2+Ar) gas ratios from 20% to 50%. Excellent agreement has been found between the experimental and the simulated spectra, in which an empirical dielectric dispersion relation based on Tauc-Lorentz model has been successfully adopted. Increases in the refractive index n and the extinction coefficient. k, with increases in nitrogen-incorporation content are observed due to the nitrogen-incorporation-induced higher packing density. The change of the complex dielectric functions and reduction in the optical band gap with an increase in nitrogen concentration resulting from the effect of the nitrogen-incorporation on the structure are discussed in detail.

Efficient removal of Cr(VI) from wastewater under sunlight by Fe(II)-doped TiO2 spherical shell

Fe(II)-doped TiO2 spherical shell catalyst was synthesized by one-pot hydrothermal method. The photocatalytic removal of Cr(VI) from plating wastewater under sunlight of the catalyst was demonstrated. It was found that the removal effectiveness of about 99.99% for initial Cr(VI) concentration of 102.3 ppm and 99.01% for 153.4 ppm under 3h sunlight irradiation is realized. The Fe(II) ions serve not only as reducing agents for reducing the Cr(VI) to Cr(III) but also as an intermedium of a two-step reduction, in which the TiO2 photoreduces the Fe(II) ions to Fe atoms firstly, and then the Fe atoms reduce the Cr(VI) to Cr(III). The improved photocatalytic activity of the catalyst is considered due to the synergistic effect of a multi reducing process by Fe(II) doping. The extended optical response and effectively utilization of sunlight of the special spherical-shell-like morphology also contribute to the enhanced photocatalytic activity.

Localized exciton luminescence in nitrogen-incorporated SnO2 thin films

The photoluminescence properties of nitrogen-incorporated SnO2 thin films on Si (100) substrates deposited by reactive magnetron sputtering have been studied. A strong photoluminescence band centered at 3.696eV with full width at half maximum of 0.2eV has been observed at room temperature. The peak position of the emission shifts to higher energy with increasing excitation intensity or decreasing temperature. The emission is considered due to the localized exciton recombination, and the observed exciton localization and band-tail states are believed to originate from the potential fluctuation induced by spatial distribution fluctuations of nitrogen concentration and∕or grain boundary defects in the nanocrystalline film.The photoluminescence properties of nitrogen-incorporated SnO2 thin films on Si (100) substrates deposited by reactive magnetron sputtering have been studied. A strong photoluminescence band centered at 3.696eV with full width at half maximum of 0.2eV has been observed at room temperature. The peak position of the emission shifts to higher energy with increasing excitation intensity or decreasing temperature. The emission is considered due to the localized exciton recombination, and the observed exciton localization and band-tail states are believed to originate from the potential fluctuation induced by spatial distribution fluctuations of nitrogen concentration and∕or grain boundary defects in the nanocrystalline film.

Optimization of microstructure and optical properties of VO2 thin film prepared by reactive sputtering

VO2 (M) thin films with good optical switching properties have been grown by reactive sputtering method. The influence of sputtering parameters on the structural and optical properties of the as-grown VO2 thin films was investigated, and the correlation between the microstructure and optical switching properties were studied. It was found that the phase transition temperature, hysteresis width, and the amplitude of the transition depend on the sputtering gas pressure, and the amplitude of the transition can reach as high as 70% with an approximately zero infrared transmission in metal state at a wavelength of 2.5 μm. The anomalous optical properties of the VO2 thin films were analyzed and discussed together with the studies of the refractive index and optical band gap.

Recyclable magnetic photocatalysts of Fe2+TiO2 hierarchical architecture with effective removal of Cr(VI) under UV light from water

We report the synthesis and photocatalytic removal of Cr(VI) from water of hierarchical micro/nanostructured Fe(2+)/TiO(2) tubes. The TiO(2) tubes fabricated by a facile solvothermal approach show a three-level hierarchical architecture assembled from dense nanosheets nearly vertically standing on the surface of TiO(2) microtube. The nanosheets with a thickness of about 20 nm are composed of numerous TiO(2) nanocrystals with size in the range of 15-20 nm. Ferrous ions are doped into the hierarchical architecture by a reduction route. The Fe(2+)/TiO(2) catalyst demonstrates an effective removal of Cr(VI) from water under UV light and the removal effectiveness reaches 99.3% at the initial Cr(VI) concentration of 10 mg L(-1). The ferrous ion in the catalyst serves not as the photo-electron trap but as an intermedium of a two-step reduction. The TiO(2) photoreduces the Fe(2+) ions to Fe atoms firstly, then the Fe atoms reduce the Cr(VI) to Cr(III), and the later is removed by adsorption. The hierarchical architecture of the catalyst serves as a reactor for the photocatalytic reaction of Cr(VI) ions and an effective absorbent for the removal of Cr(III) ions. The catalyst can be easily magnetically separated from the wastewater after photocatalytic reaction and recycled after acid treatment.

Optical properties of MgO–TiO2 amorphous composite films

The microstructure and optical properties of MgO–TiO2 composite thin films prepared by radio frequency magnetron sputtering were studied. The composite films have an amorphous structure and the growth rate of the composite film increases with increasing Mg content. The refractive index almost linearly decreases with increasing Mg content in the composite films at wavelength larger than 300nm and can be adjusted in the wide range between the refractive index of TiO2 and that of MgO. The extinction coefficient decreases with Mg content and is nearly zero at visible light region. The optical band gap of the composite films determined from the absorption spectra increases with Mg content, and has a good agreement with that obtained from spectroscopic ellipsometry simulation. A broad photoluminescence band in the visible region was observed in the composite films with high Mg content and was considered resulted from the oxygen vacancies.

Facile controlled synthesis of highly ordered dendritic and flower-like cobalt superstructures

Dendritic and flower-like cobalt superstructures have been selectively prepared by a facile hydrothermal route. The synthetic process involves the chemical reduction of cobalt ions and the oriented growth of cobalt nanocrystals under kinetic control independent of surfactants or external magnetic field. The dendritic superstructures are found to be composed of well-aligned metallic cobalt dendrites radiating from the center. Upon adding 0.3 M of potassium sodium tartrate (C4H4KNaO6·4H2O) into the reaction system, flower-like cobalt superstructures consisting of spear-like petals attached to a mutual core can be prepared instead of the dendritic ones. The size and morphology of the products can be tailored by properly tuning the process conditions and the component of the reaction solution. The magnetic properties of the products have been studied as well and the results demonstrate that the products present ferromagnetic properties related to the corresponding microstructures. Based on a series of contrast experiments, the probable growth mechanism and fabrication process of the products have been proposed. This work provides a facile and effective strategy to fabricate self assembled cobalt hierarchical superstructures with tunable morphologies.

Facile preparation and characterization of glass/Fe3O4 core/shell composite hollow spheres

Novel glass/Fe3O4 core/shell composite hollow spheres with the shell layer assembled by multi-layered wafers were successfully synthesized by reduction of a glass/alpha-Fe2O3 precursor, which was obtained through a facile solvothermal process. The effects of some reaction parameters on the morphology of the shell layers and the magnetic properties of the products were investigated.