Bifunctional Fe3O4@C/YVO4:Sm composites with the core–shell structure

A novel Fe₃O₄@C/YVO₄:Sm³⁺ composites with magnetic and luminescent properties were reported. Firstly, the as-synthesized Fe₃O₄ nanoparticles were coated by carbon as a shell via a simple hydrothermal method. Furthermore, the Fe₃O₄@C nanoparticles were modified by YVO₄:Sm³⁺ phosphors through a simple sol–gel process to prepared the Fe₃O₄@C/YVO₄:Sm³⁺ microspheres. The characterization of as-prepared products were measured by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectra, and vibrating sample magnetometer (VSM). It is shown that Fe₃O₄@C/YVO₄:Sm³⁺ composites with well-crystallized and core–shell structure were prepared. Additionally, the Fe₃O₄@C/YVO₄:Sm³⁺ composites show excellent magnetic properties (13 emu/g) and luminescent properties, which made the composites useful for applications in biomedical devices such as magnetic bio-separation and drug/gene delivery.     

Color-tunable properties of Eu3+- and Dy3+-codoped Y2O3 phosphor particles

Rare-earth phosphors are commonly used in display panels, security printing, and fluorescent lamps, and have potential applications in lasers and bioimaging. In the present study, Eu³⁺- and Dy³⁺-codoped uniform-shaped Y₂O₃ submicron particles were prepared using the urea homogeneous precipitation method. The structure and morphology of the resulting particles were characterized by X-ray diffraction, field emission scanning electron microscope, and field emission transmission electron microscope, whereas their optical properties were monitored by photoluminescence spectroscopy. The room-temperature luminescence color emission of the synthesized particles can be tuned from red to yellow by switching the excitation wavelength from 254 to 350 nm. The luminescence intensities of red and yellow emissions could be altered by varying the dopant concentration. Strong quenching was observed at high Eu³⁺ and Dy³⁺ concentrations in the Y₂O₃ host lattice.     

Luminescence of mesoporous silicon powders treated by high-pressure water vapor annealing

We have studied the photoluminescence of nanocrystalline silicon microparticle powders fabricated by fragmentation of PSi membranes. Several porosities were studied. Some powders have been subjected to further chemical etching in HF in order to reduce the size of the silicon skeleton and reach quantum sizes. High-pressure water vapor annealing was then used to enhance both the luminescence efficiency and stability. Two visible emission bands were observed. A red band characteristic of the emission of Si nanocrystals and a blue band related to localized centers in oxidized powders. The blue band included a long-lived component, with a lifetime exceeding 1 sec. Both emission bands depended strongly on the PSi initial porosity. The colors of the processed powders were tunable from brown to off-white, depending on the level of oxidation. The surface area and pore volume of some powders were also measured and discussed. The targeted applications are in cosmetics and medicine.     

Ultralong zinc-blende ZnS nanowires grown on polar C face of 6H-SiC substrates at low temperatures by metalorganic chemical vapor deposition

Ultralong ZnS nanowires with high purity were grown on Au-coated polar C face of 6H-SiC substrates via metalorganic chemical vapor deposition at low temperatures. The ZnS nanowires have zinc-blende structure and the length is up to tens of micrometers. HRTEM investigations show that the nanowires are well crystalline single crystal grown along [1 1 1] and free of bulk defects. However, sparse straight and curved nanowires with poor crystalline nature are randomly grown on the Au-coated Si face of 6H-SiC substrates. We deduce that the growth of ZnS is related to the substrates and C face can enhance Au-catalytic VLS growth. The CL spectra of an individual nanowire grown on C and Si face reveal different optical properties. Intrinsic sulfur and zinc vacancies are the main reasons for the 458.1 nm and 459.2 nm blue emission detected in the nanowire grown on C face and Si face, respectively. Nevertheless, an unusual green emission at 565.1 nm is observed in the poor crystalline nanowire grown on Si face, which originates from the bulk defects.     

Green emission from Eu/Dy codoped SrO-Al2O3-B2O3 glass-ceramic by ultraviolet light and femtosecond laser irradiation

A spectroscopic investigation of Eu²⁺/Dy³⁺ codoped SrO-Al₂O₃-B₂O₃ glass-ceramic is presented. The sample exhibits green emission excited by ultraviolet (UV) light and near-IR femtosecond (fs) laser. The emission profile obtained by near-IR fs laser irradiation is similar to that by UV excitation, indicating that both of the emissions come from 5d → 4f transition of the Eu²⁺ ions. The relationship between the upconversion luminescence (UCL) intensity and pump power reveals a two-photon process in the conversion of near-IR radiation to the green emission. The possible mechanism of UCL from such glass-ceramic is proposed.     

Effect of the doped nitrogen on the optical properties of β-Ga2O3 nanowires

In this study, optical properties of the nitrogen-doped β-Ga₂O₃ nanowires (N-doped β-Ga₂O₃ NWs) were synthesized by exposing β-Ga₂O₃ NWs under high input power nitrogen plasma (2 kW), using a microwave plasma enhanced chemical vapor deposition (MPECVD) system. The nitrogen contents in the NWs were as-prepared about 7.4, 8.9, 9.7, 13.9, 19.3, and 26.6 at.%, respectively. Low temperature (10 K) cathodoluminescence (CL) spectra exhibit significantly different optical properties for the different nitrogen contents. The CL result of the N-doped β-Ga₂O₃ NWs (210 s N₂ plasma treatment) exhibited four distinct emission peaks at 378, 516, 759, and 970 nm. The possible light emission mechanism including the effect of the nitrogen dopant was discussed.     

Sol-gel synthesis of yellow-emitting La4Ca(SiO4)3O:Ce phosphors for white-light emitting diodes

Ce³⁺ doped La₄Ca(SiO₄)₃O phosphors with silicate oxyapatite structure were synthesized by the sol-gel process. The X-ray diffraction (XRD) patterns showed that a pure phase was formed when sintering temperature was higher than 1300°C. The optical properties of La₄Ca(SiO₄)₃O:Ce³⁺ phosphors with varying sintering temperature and concentration were investigated by examining their excitation and emission spectroscopy. The phosphors exhibited a broad emission band centered at 550nm which could be attributed to the 5d-4f transition of Ce³⁺ and a stronger excitation peak around 467nm as well as several shoulder bands, nicely matching with the widely applied blue LED chips. Higher emission intensity was observed when firing temperature above 1300°C, due to increasing crystallinity of the powders. When Ce³⁺ concentration was equal to 5 at%, the sample exhibited the optimum excitation and emission efficiency. The results indicate that La₄Ca(SiO₄)₃O:Ce³⁺ is a promising candidate in the application of blue chip excited white light emitting diodes (LEDs).     

Synthetics of ZnO nanostructures by thermal oxidation in water vapor containing environments

Metallic Zn films deposited on glass were wet or dry oxidized at 390 °C in pure N₂ or O₂ to understand the effects of water vapor in different oxygen partial pressure on growth of ZnO nanostructure during thermal oxidation. As-prepared ZnO oxides were characterized by a scanning electron microscope (SEM), an X-ray diffractometer (XRD), and a transmission electron microscope (TEM). Optical and electric properties of these ZnO films were characterized by photoluminescence (PL) and resistance measurements, respectively. It was found that the oxygen partial pressure and water vapor of environment significantly affect the morphologies of ZnO nanostructures. Decreasing oxygen partial pressure in dry oxidation can enhance a green light peak at 500 nm on PL spectra arising from defect-related emission and reduce the resistivity of the oxide films. High H₂O^(g)/O₂ ratio in wet oxidation will significantly increase the intensity of a green light peak and reduce the resistivity of the oxide films. The effect of oxygen partial pressure and H₂O^(g)/O₂ ratio on the PL spectra and resistivity of ZnO films are explained by the theory of defects equilibrium during oxidation.     

Luminescent mesoporous colloidal silica: A nanoporous substrate for photosensitization of lanthanide ions

Nearly monodisperse, mesoporous, colloidal silica spheres were treated with aminopropyltriethoxysilane and subsequently calcined to generate luminescent mesoporous colloidal silica (LMCS), with broad bimodal visible emission and high surface areas exceeding 700 m²/g. The combination of high surface area and localization of luminescence centers at the surface provides opportunities for exploring new efficient photophysical/photochemical relaxation processes with the metal-free/dye-free luminescent silica sol-gel materials. Addition of LMCS to solutions of TbCl₃ or EuCl₃ results in efficient lanthanide sensitization. The enhancement in lanthanide emission is accompanied by a quenching of LMCS emission.     

Photoluminescence of Tb/Ce co-doped aluminum oxynitride powders

Aluminum oxynitride(AlON) phosphors co-doped by Tb³⁺ and Ce³⁺ were synthesized by nitridation of the precursor which was co-precipitated from Al(NO₃)₃ solution and nanosized carbon black at 1750 °C for 2 "hrs" in flowing nitrogen atmosphere. The obtained AlON based powders were composed of polycrystalline spinel typed particles with sizes in the range of 1-3 μm. Under an excitation of 275 nm, it was found that co-doping of Ce³⁺ could drastically enhance the luminescence of AlON:Tb³⁺ powder by energy transfer. The product with 0.5 mol% Ce³⁺ and 0.67 mol% Tb³⁺ exhibited a strong broad green emission at 540 nm. The critical quenching concentration of Tb³⁺ in AlON:0.5 mol% Ce³⁺/xmol% Tb³⁺ phosphor was determined to be 0.67 mol%. It was supposed that the mechanism of concentration quenching of Tb³⁺ in AlON:0.5 mol% Ce³⁺ xmol% Tb³⁺ phosphor was dipole-dipole interaction.