Promising oxonitridosilicate phosphor host Sr3Si2O4N2: synthesis, structure, and luminescence properties activated by Eu2+ and Ce3+/Li+ for pc-LEDs.

A novel oxonitridosilicate phosphor host Sr(3)Si(2)O(4)N(2) was synthesized in N(2)/H(2) (6%) atmosphere by solid state reaction at high temperature using SrCO(3), SiO(2), and Si(3)N(4) as starting materials. The crystal structure was determined by a Rietveld analysis on powder X-ray and neutron diffraction data. Sr(3)Si(2)O(4)N(2) crystallizes in cubic symmetry with space group Pa ̅3, Z = 24, and cell parameter a = 15.6593(1) Å. The structure of Sr(3)Si(2)O(4)N(2) is constructed by isolated and highly corrugated 12 rings which are composed of 12 vertex-sharing [SiO(2)N(2)] tetrahedra with bridging N and terminal O to form three-dimensional tunnels to accommodate the Sr(2+) ions. The calculated band structure shows that Sr(3)Si(2)O(4)N(2) is an indirect semiconductor with a band gap ≈ 2.84 eV, which is close to the experimental value ≈ 2.71 eV from linear extrapolation of the diffuse reflection spectrum. Sr(3-x)Si(2)O(4)N(2):xEu(2+) shows a typical emission band peaking at ~600 nm under 460 nm excitation, which perfectly matches the emission of blue InGaN light-emitting diodes. For Ce(3+)/Li(+)-codoped Sr(3)Si(2)O(4)N(2), one excitation band is in the UV range (280-350 nm) and the other in the UV blue range (380-420 nm), which matches emission of near-UV light-emitting diodes. Emission of Sr(3-2x)Si(2)O(4)N(2):xCe(3+),xLi(+) shows a asymmetric broad band peaking at ~520 nm. The long-wavelength excitation and emission of Eu(2+) and Ce(3+)/Li(+)-doped Sr(3)Si(2)O(4)N(2) make them attractive for applications in phosphor-converted white light-emitting diodes.

[Photoluminescence from bis-t-butylbenzoxazolylthiophene doped silica films].

Thin nano-porous silica films doped with high concentrations of fluorescent material, 2, 5-bis (5-tert-butyl-2-benzoxazolyl)-thiophene (BBOT) were prepared via a sol-gel process. Uniform and bright blue fluorescence was observed. Light emission properties of these organic molecule doped inorganic silica films, i.e., hybrid films, were measured using ultraviolet-visible (UV-Vis) absorption spectroscopy, steady and time-resolved fluorescence spectroscopy as well as optical microscopy. Features of these materials were revealed in this investigation: Firstly, photoluminescence intensity from BBOT doped silica films increased linearly as the concentrations of BBOT increased if the dopant concentration was relatively low and below 6 x 10(-3) mol x L(-1); Secondly, no molecular aggregation or phase separation was observed using optical microscopy when the BBOT concentration was below 6 x 10(-3) mol x L(-1) in BBOT doped silica films. Thirdly, the fluorescence lifetimes of BBOT in the doped silica films were longer than that in a dilute dioxane solution (1.957 ns), which was contradicted to our general understanding that the fluorescence lifetime may be reduced in a condensed matter due to molecular interactions or quenching. It was further found that the fluorescence lifetime also varied with the gelation conditions. Taking a BBOT concentration of 6 x 10(-3) mol x L(-1) for an example, the lifetime of BBOT in doped silica films was about 2.45 ns for a specimen polymerized at 50 degrees C; while the lifetime was increased to 3.04 ns for a specimen polymerized at 90 degrees C. This work demonstrates no concentration quenching when the BBOT dopant concentrations increased to as high as 6 x 10(-3) mol x L(-1) in the silica matrix. In comparison with the changes in time-resolved photoluminescence of BBOT in dioxane solution and that of the BBOT doped nano-porous silica in relation to their concentration dependence and the gelation conditions, it was found that concentration quenching can be effectively suppressed by the nano-porous silica matrix. A stable fluorescent organic-inorganic hybrid material is thus obtained.

A promising yellow phosphor of Ce3+/Li+ doped CaSiN(2-2δ/3)Oδ for pc-LEDs.

Single-phased orthorhombic nitridosilicate CaSiN2 powders were prepared at 1550 °C in a N2/H2 (6%) atmosphere by solid-state reaction using Ca3N2 and Si3N4 as the starting materials. The crystal structure refinements were carried out by the Rietveld method based on the X-ray and neutron powder diffraction data collected at 297 K, respectively. The CaSiN2 phase crystallizes in an orthorhombic unit cell with the space group Pbca (No. 61) and cell parameters a = 5.1334(3) Å, b = 10.3090(5) Å, c = 14.5756(5) Å, Z = 16 (XRD data). Instead of the ideal formula CaSiN2, small amounts of oxygen could be detected in the samples. The O/N composition of orthorhombic CaSiN2-2δ/3Oδ was analyzed by EDX and Rietveld refinement on the neutron powder diffraction data. SAED and HREM characterizations of the crystallites were conducted before the EDX analysis in order to select proper crystallites and exclude other minor second phases of (oxo)nitridosilicates such as cubic CaSiN2 and monoclinic CaSi2O2N2. The luminescence measurements indicate that Ce(3+)/Li(+) co-doped CaSiN2-2δ/3Oδ emits a typical shouldered yellow band peaking at 530 nm (FWHM ∼ 120 nm), which originates from the 5d → 4f transition of Ce(3+). The emission band shifts to longer wavelengths with an increment of Ce(3+) concentration. The strong excitation band appears in the range of 350-470 nm, which is very favorable for the applications in near-UV and blue phosphor-converted light-emitting diodes (pc-LEDs).