Efficient Luminescence Properties Research Articles

Structural characterization, luminescence and electrochemical properties of the Schiff base ligands

In this study, we prepared two Schiff base ligands N-(4-hydroxy phenyl)-2,4-di-methoxy benzaldimine (TS1) and N-(4-hydroxy phenyl)-2,5-di-methoxybenzaldimine (TS2) which were characterized by structural, spectroscopic and analytical methods. The ligands TS1 and TS2 were obtained as single crystals from ethanol solution. X-ray diffraction data for two compounds showed that the bond lengths are within the normal ranges. The electrochemical properties of the Schiff base ligands were studied in different solvents and at various scan rates. The luminescence properties of the ligands TS1 and TS2 in different solvents and at different pH values have been investigated. The results show that the ligands exhibit more efficient luminescence properties in CH3CN and n-butanol.

Gold nanorods for intravital vascular imaging of preneoplastic oral mucosa

We explore the feasibility of using gold nanorods with efficient two-photon luminescence properties as contrast agents for intravital imaging of neoplasia. This investigation spanned ex vivo characterization in cells/tissue to in vivo implementation in an oral carcinogenesis model. GNRs were >40 times brighter than surrounding tissue. Intravital imaging revealed 3D microvasculature, and in dysplasia, abnormal vessels (dense and tortuous) compared to normal. GNRs were diffusely distributed in lesions after 24 hours. No known previous study has revealed abnormal vessel structure in dysplasia by imaging. Results suggest GNRs can function as high-contrast agents for in vivo visualization of carcinogenesis features.

Preparation and characterization of flower-like SrAl 2O 4:Eu 2+,Dy 3+ phosphors by sol-gel process

A flower-like Eu 2+ and Dy 3+ co-doped SrAl 2O 4 long-lasting phosphorescent (LLP) phosphor was synthesized via the inorganic- salt-based sol-gel method. The crystal structure, morphology and optical properties of the composite were characterized. X-ray diffraction diffusion (XRD) data and DSC-TG curves of the phosphor revealed that the SrAl 2O 4 crystallites have been formed after the precursor was calcined at 900 °C and to be single-phase SrAl 2O 4 at 1100 °C. The SEM photographs indicated that the sample exhibited a universal flower-like morphology with crystallite size of about 1-2 μm. After being irradiated with ultraviolet (UV) light, the flower-like phosphor emitted long-lasting green phosphorescence with an excitation peak at 365 nm and emission peak at 500 nm which was ascribed to the characteristic 5d-4f transition of Eu 2+. Both the PL spectra and the luminance decay curve revealed that this phosphor exhibited efficient luminescence and long lasting properties.

Monodisperse nanospheres of yttrium oxysulfide: Synthesis, characterization, and luminescent properties

A red long-lasting phosphorescent material, monodisperse Y 2O 2S: Eu 3+, Mg 2+, Ti 4+ nanospheres have been prepared successfully. Y(OH)(CO 3): Eu 3+ nanospheres were firstly synthesized via an urea-based homogeneous precipitation technique to serve as the precursor. Nanospheres long-lasting phosphors Y 2O 2S: Eu 3+, Mg 2+, Ti 4+ were obtained by calcinating the precursor in CS 2 atmosphere. XRD investigation shows a pure phase of Y 2O 2S, indicating no other impurity phase appeared. SEM observation reveals that the structures are nanosphere. The Y 2O 2S: Eu 3+, Mg 2+, Ti 4+ nanospheres with particle size about 100–150 nm show uniform size and well-dispersed distribution. After irradiation by ultraviolet radiation with 325 nm for 5 min, the phosphor emitted red color long-lasting phosphorescence corresponding to typical emission of Eu 3+ ion. The main emission peaks are ascribed to Eu 3+ ions transition from 5D J ( J = 0, 1, 2) to 7F J ( J = 0, 1, 2, 3, 4). Both the PL spectra and luminance decay revealed that this phosphor had efficient luminescent and long-lasting properties. It was considered that the red-emitting long-lasting phosphorescence was due to the persistent energy transfer from the traps to the Ti 4+ and Mg 2+ ions.

Synthesis and luminescence properties of needle-like SrAl 2O 4:Eu, Dy phosphor via a hydrothermal co-precipitation method

Needle-like SrAl 2O 4:Eu 2+, Dy 3+ phosphors had been prepared by calcining the precursors obtained from hydrothermal process at the temperature of 1100 °C in a weak reductive atmosphere of active carbon. The crystal structure, morphology and optical properties of the composites were characterized. X-ray diffraction (XRD) patterns illustrated that the single-phase SrAl 2O 4 was formed at 1100 °C, which is much lower than that prepared by the traditional method. The transmission electron microscope (TEM) observation revealed the precursors and the resulted SrAl 2O 4:Eu 2+, Dy 3+ phosphors had well-dispersed distribution and needle-like morphology with an average diameter about 150 nm at the center and the length up to 1 μm. After irradiation by ultraviolet radiation with 350 nm for 5 min, the phosphors emit green color long-lasting phosphorescence corresponding to the typical emission of Eu 2+ ion, both the PL spectra and luminance decay revealed that the phosphors had efficient luminescent and long lasting properties.

A transparent surface-crystallized Eu 2+, Dy 3+ co-doped strontium aluminate long-lasting phosphorescent glass-ceramic

A high-brightness Eu 2+ and Dy 3+ co-doped strontium aluminate-based transparent surface-crystallized long-lasting phosphorescent (LLP) glass-ceramic was synthesized using solid-state sintering method in CO atmosphere. The crystal structure, morphology and optical properties of the composite were characterized. By the X-ray diffraction diffusion (XRD) data, SrAl 2O 4 was demonstrated to be the crystallite in the glass-ceramic. The TEM photographs indicated that the sample had a universal small crystallite size about 10–20 nm on its surface. The SrAl 2O 4:Eu 2+, Dy 3+ crystallites were produced during the quenching process. After irradiated with ultraviolet (UV) light, the glass-ceramic emitted green long-lasting phosphorescence with an excitation peak at 375 nm and emission peak at 520 nm ascribed to the characteristic 5d–4f transition of Eu 2+. Both the PL spectra and luminance decay revealed that this phosphor had efficient luminescence and long-lasting properties. By comparing with the heat-treated sample, the obtained transparent surface-crystallized LLP glass-ceramic puts up higher transparency and long phosphorescence, and could be obtained more easily.

Host Size Effects on Optical Properties of Y<sub>2</sub>O<sub>3</sub>:Eu<sup>3+</sup> and Gd<sub>2</sub>O<sub>3</sub>:Eu<sup>3+</sup> Nanoparticles Synthesized by Laser Pyrolysis

Rare earth doped materials have many familiar applications (TV screens, solid lasers, scintillators…) thanks to their efficient and robust luminescence properties. In recent years, growing interest has been focused on the changes in their optical properties with the size of the host particles. In this work, nanomaterials were produced for the first time by using laser pyrolysis. Y, Gd, and Eu nitrates were dissolved in water and used as precursors. Cubic phases of Y2O3:Eu3+ and Gd2O3:Eu3+ were obtained with sizes ranging from 3 to 40 nm. The spectroscopic properties revealed a new and nanostructure-specific broad band in the excitation spectrum. The emission spectrum was found to be characteristic of nanostructured sesquioxides only when excited in this new band, which was finally assumed to be a new charge transfer band for the smallest nanoparticles in the sample.

Synthesis and luminescence properties of a novel red long lasting phosphor Y 2O 2S: Eu 3+, Si 4+, Zn 2+

A novel red long lasting phosphor Y 2O 2S: Eu 3+, Si 4+, Zn 2+ was synthesized by solid state reaction. XRD and photoluminescence (PL) spectra as well as the luminance decay were used to characterize the long lasting phosphor. The results of XRD indicate that the product synthesized under 1200 °C for 2 h has a good crystallization without any detectable amount of impurity phase. Both the PL spectra and luminance decay reveal that this phosphor has efficient luminescent and good long lasting properties.

Influences of molecular orientations on stimulated emission characteristics of oligofluorene films

Abstract Efficient luminescence and interesting thermal properties of oligofluorenes provide the opportunity to investigate influences of molecular orientational distributions on characteristics of stimulated emission in organic semiconducting films. While vacuum-deposited terfluorene films exhibit preferential in-plane molecular orientation, they are reoriented and become isotropic with annealing around or above its glass transition temperature. Studies of stimulated emission (amplified spontaneous emission) show that the anisotropic terfluorene films exhibit higher gain and lower threshold than the isotropic films, due to larger confinement of the waveguided mode and larger cross section of stimulated emission. A net gain as high as 78 cm−1 (at a pump intensity of ∼30 kW/cm2) and a threshold as low as 0.7 kW/cm2 were observed. The results suggest that the gain and threshold of stimulated emission in organic semiconducting films could be substantially improved beyond the isotropic case if molecules and their transition dipole moments can be well aligned with the polarization of stimulated emission yet without causing significant loss and emission quenching.

Theoretical investigation of electronic structures of the ground and excited states of pyrene and its derivatives

The low-lying electronic states of pyrene and substituted pyrenes have been studied in detail by high level ab initio and density functional methods to aid in the development of the efficient luminescent properties of pyrenes. The electronic structures of the ground and the low-lying singlet excited states have been examined by the complete active space SCF (CASSCF) method. The energy gaps between the ground state and the low-lying excited states have been estimated by means of the multireference perturbation theory based on CASSCF wavefunctions as well as time-dependent density functional theory (TDDFT). The changes in electronic structure of pyrene, in particular, the excitation energies of the excited states by several different substituents have been investigated by the TDDFT method. Both mono-substituted and di-substituted pyrene derivatives have been considered, and the effects on the energy gap between HOMO and LUMO by electron donor and electron acceptor groups have been discussed.

Luminescence Of A New Material: GaN Grown On NdGaO3

AbstractIt is shown that heteroepitaxial GaN layers grown on NdGaO3, in spite of a very high conductivity (˜ 10 Ω−1cm−1) have very efficient luminescence properties. It is shown that a high electrical conductivity is caused by contamination of GaN layers with oxygen. Efficient emission due to donor bound excitons (at hv = 3.475 eV), free excitons and free electron – hole recombination have been identified. The total PL emission in the exciton region exceeds the intensity from the homoepitaxial GaN layers. It is argued that a high oxygen concentration eliminates nonradiative channels connected with point defects, leading to efficient radiative recombination.

Morphological effects on the electronic band structure of porous silicon.

Porous silicon presents a fascinating pore morphology, which could be relevant for its efficient luminescent properties in the visible spectrum. This work attempts to give some insight into the understanding of its optical properties, by studying the electronic band structure. The porous structure is modeled as empty columns of different sizes and shapes, produced into an otherwise perfect silicon crystal. The columns are passivated with hydrogen atoms. A tight-binding Hamiltonian on an ${\mathit{sp}}^{3}$${\mathit{s}}^{\mathrm{*}}$ basis set is applied to supercells of 8, 32, 128, and 256 atoms. Due to the simplicity of the model, morphology effects can be analyzed in detail, even in the case where the column diameter oscillates is included. The results show that the band gap broadens and the conduction-band minimum shifts towards the \ensuremath{\Gamma} point, producing an almost direct band gap, as the porosity increases. A strong splitting of the originally degenerate states at the top of the valence band is also observed, for certain morphologies. Finally, the implications of quasiconfinement, where electrons can find ways out through the necks between the pores, are discussed. \textcopyright{} 1996 The American Physical Society.