Papers by Keyword: Luminescence

Abstract: We report synthesis of well-grown blue phosphor powder, CaMgSi2O6:Eu2+, from new vapor phase method (Flash creation method) derived nanopowder precursor. Single phase phosphor sample was obtained by post-annealing at 1100 oC. Luminescence properties were compared with that of the sample synthesized by a conventional solid state reaction. The post-annealed sample showed intense blue emission under UV excitation.

Abstract: We report the effect of solvent on the rhodamine 6G encapsuled into channels of mesoporous silica, synthesized by two-step process that gives intermediary stable hybrid micelles. Mesoporous materials have been obtained by the method that involves surfactant micelles (mainly cationic) and inorganic precursor of the structure to be obtained. MSU-X type mesoporous silica has been synthesized with polyethylene oxide surfactant as the directing-structure agent and tetraethyl orthosilicate Si(OEt)4 as the silica source. The influence of the solvent on the encapsulation of rhodamine dye was systematically explored, specially its influence on the luminescence properties. Rhodamine 6G encapsuled into mesoporous silica channel was characterized by UV-Vis and luminescence spectroscopies, scanning electron microscopy, small angle x ray scattering and N2 sorption-desorption. The pore size and the solvent effects into luminescence dye encapsuled into mesoporous silica channels are observed in the visible absorption and emission spectra of rhodamine 6G. The intense photoluminescence band of rhodamine 6G dye is in 500 to 600 nm region. The observed shift of the absorption and emission bands can be assigned to the effect of the solvents dielectric constant and pore size of mesoporous silica.

Abstract: The synthesis of fluorescent nanocrystals is receiving a lot of attention for potential application in biological labeling as well as phosphors for field emission devices. Zinc sulphide doped with manganese (ZnS:Mn2+) is one of the most efficient electroluminescent phosphor displaying a wide emission band centred around 590 nm resulting from the intra-ionic transition in Mn2+ ions. We report a unique synthesis of zinc sulphide nanoparticles doped with manganese using a biocompatible passivating agent ‘chitosan’, with bright luminescence peaking at 590 nm. This high luminescence efficiency of the synthesized nanocrystals are ideal for quantum dot based bio-labeling applications. Synthesis of the nanoparticles was carried out by precipitation reaction in aqueous media of zinc acetate and sodium sulphide where manganese acetate was added as the dopant. The obtained nanoparticles were around 4 to 6 nm in size and were found to be stable for months of shelf life. The photoluminescence intensity did not degrade when the colloid was heated up to 65 oC for prolonged periods.

Abstract: Threading dislocations (TDs) in (Al,In,Ga)N semiconductors are known to affect the luminescence efficiency of near-band-edge (NBE) emissions in bulk films and quantum structures. However, the principal role of point defects such as vacancies on the luminescent properties has not been fully understood. In this article, impacts of point defects on the luminescence quantum efficiency of NBE emissions and on the intensity of deep emission bands will be described, based on the results of steady-state and time-resolved photoluminescence (TRPL) and positron annihilation measurements. The room temperature nonradiative lifetime (τNR) of the NBE excitonic photoluminescence (PL) peak in polar (0001) and (000-1) , nonpolar (11-20) and (10-10), and zincblende (001) GaN layers prepared by various growth techniques was shown to increase with the decrease in concentration or size of Ga vacancies (VGa) and with the decrease in gross concentration of point defects including complexes, leading to an increase in the NBE PL intensity. As the edge TD density decreased, the concentration or size of VGa tended to decrease and τNR tended to increase. However, there existed remarkable exceptions. The results indicate that the nonradiative recombination process is governed not by single point defects, but by certain defects introduced with the incorporation of VGa, such as VGa-defect complexes. Similar relations were found in AlxGa1-xN alloy films grown by metalorganic vapor phase epitaxy: i. e. τNR at room temperature increased with the decrease in the concentration of cation vacancies (VIII) and with the decrease in gross concentration of point defects. In addition to nonradiative processes, the VIII concentration was found to correlate with the intensity ratio of characteristic deep emission band to the NBE emission (Ideep/INBE). For example, Ideep/INBE at low temperature for the deep emission bands at 4.6, 3.8, and 3.1 eV of AlN epilayers grown by NH3-source molecular beam epitaxy had a linear correlation with the concentration or size of Al vacancies (VAl). Since the relative intensities of 3.1 eV and 3.8 eV bands increased remarkably with lowering the supply ratio of NH3 to Al (V/III ratio) and growth temperature (Tg), they were assigned to originate from VAl-O as well as VAl-shallow donor complexes. The VAl concentration could be decreased by adjusting the V/III ratio and Tg. In the case of AlxGa1-xN alloys, the concentration or size of VIII and Ideep/INBE at 300 K increased simultaneously with the increase in x up to approximately 0.7. Similar to the case for GaN and AlN, the deep emission band was assigned as being due to the emission involving VIII-O complexes.

Abstract: In the present study the distribution of nanofillers in the polyurethane matrix and the composite properties were investigated. As a nanofiller, zirconium oxide doped with 10% Eu3+ was used. The nanofiller was added at 0.1 wt%. Different ways of nanofiller incorporation were investigated. The microstructure of the obtained materials was examined by atomic force microscopy in force modulation. The size analysis of the nanofiller was investigated with HRSEM. The thermal (DSC, TGA) properties of polyurethane nanocomposites were also investigated in addition to the analysis of transmittance and luminescence of obtained materials. The results obtained indicate a possibility of fabrication of polymeric nanocomposites for optoelectronic applications via a relatively inexpensive processing route.

Abstract: Compositions and heat treatment conditions were determined at which absorption and luminescence properties of aluminosilicate transparent glass-ceramics are defined mainly by tetrahedrally coordinated Co2+ ions located in spinel nanocrystals. Optical properties of cobaltdoped aluminosilicate transparent glass-ceramics have been studied in visible – near infrared (IR) spectral range. It was demonstrated that the concentration of tetrahedrally coordinated Co2+ ions in the spinel nanocrystals is determined by the CoO content in the initial glass and is independent of the heat-treatment schedule. Absorption saturation and bleaching relaxation under excitation of the 4A2→4T1(4F) transition of tetrahedrally coordinated Co2+ ions were studied. The value of groundstate and excited state absorption cross-sections of tetrahedrally coordinated Co2+ ions was estimated. Comparative study of output pulse parameters of Q-switched Er:glass laser using cobaltdoped glass-ceramics and Co2+:MgAl2O4 single crystal is presented.

Abstract: Hexagonal NaYF4:Ln3+ (Ln3+=Yb3+ and Tm3+) was prepared via solid-state synthesis route. The synthesized powders were identified with X-ray diffraction patterns. Hexagonal phase was obtained by heating the powders at 550°C in reducing atmosphere, and the ions of Yb3+ and Tm3+ were completely incorporated into the lattice of hexagonal NaYF4 in the doping concentration range. Microstructure of the phosphor was observed by scanning electron microscopy (SEM). Intense blue (1G4→3H6, 1D2→3F4) and weak red (1G4→3F4) upconversion emission of Tm3+ ions with the energy transferred from Yb3+ ions were observed in the phosphor excited with a 980nm continuous wave laser diode. The intensity of blue emission was effectively enhanced by suitable doping of Tm3+ and Yb3+ ions, and strongest blue emission was obtained with the concentration of 0.2mol% Tm3+ and 40mol% Yb3+ doped.

Abstract: Rare earth phosphors of Ca0.85(W1-xMox)O4:Eu0.15 (x=0.00-0.50) were synthesized via solidstate reaction in air. The lattice parameters strongly depended on the x value, which was analyzed by the XRD patterns. Microstructures of the powders were observed by SEM. The phosphors could be effectively excitated by ultraviolet (UV) light (250-300nm) and blue light (464nm), which were showed with the excitation and emission spectra. With the increasing of x value, moreover, another emission peak of the sample which was excitated by UV light appeared and shifted to short wavelength, from 597 to 545 nm. The luminescence properties of the phosphors and energy transfer mechanism were also discussed.

Abstract: The luminescent properties of SrAl2O4 based materials were studied systematically, which emit long persisting phosphorescence with high brightness and show greater chemical stability than the traditional ZnS:Cu,Co phosphor. In order to obtain the better dispersed powders, SrAl2O4:Eu2+, Dy3+ was synthesized by a dipping process. The as-received samples are yellow-green in color and display better dispersion than those synthesized by the co-deposition method. The optimized conditions of this dipping process were explored. The luminescent properties and the crystal structure were studied in detail.

Abstract: New phosphors of Tm3+ doped Re2SiO4 (Re=Mg, Ba, Sr) were prepared by the solid-state reaction and their luminescent properties were investigated. X-ray powder diffraction analysis confirmed the formation of Re2SiO4: Tm3+ (Re=Mg, Ba, Sr). The excitation spectra indicated that this phosphor can be effectively excited by 161nm. It consists of peaked at 175nm and a broad excitation band from 184nm to 300nm, corresponded to the absorption of the host lattice and the O−Tm3+ charge transfer transitions. The emission peaks at about 357nm, 459nm, 485nm is separately corresponds to the 1D2 → 3H6, 1D2 → 3H4, 1G4 → 3H6 transitions of Tm3+. As the radius of alkali earth ion in Re site increases, the main emission peaks changed to 478nm from 459nm.