Yunsheng Hu

Synthesis, structure and luminescence properties of new blue-green-emitting garnet-type Ca3Zr2SiGa2O12:Ce3+ phosphor for near-UV pumped white-LEDs

A new blue-green-emitting phosphor Ca3Zr2SiGa2O12:Ce3+ has been synthesized by a conventional high temperature solid-state reaction method from the viewpoint of exploring new luminescent materials for white light-emitting diodes. The crystal structure of Ca3Zr2SiGa2O12 was investigated by the powder X-ray diffraction refinement and verified to be garnet-type with the Iad space group and lattice constant a = b = c = 12.5730(7) A. The luminescence properties, concentration quenching, fluorescence lifetime, thermal quenching, quantum efficiency, chromaticity coordinates and related mechanisms of the Ca3Zr2SiGa2O12:Ce3+ phosphor were investigated in detail. The optimized phosphor shows two main broad excitation bands with peaks at 330 nm and 400 nm, respectively, in the region of 300–450 nm, and exhibits intense blue-green emission with a peak wavelength at 478 nm under 400 nm excitation. The above results indicated that the phosphor can be effectively excited by near ultraviolet light and may have potential application as a near UV-convertible phosphor for white light-emitting diodes.

Structure, photoluminescence, and thermal quenching properties of Eu doped Sr2AlxSi5−xN8−x/3 red phosphors

Eu2+ doped Sr2AlxSi5−xN8−x/3 red phosphors have been synthesized by a solid state reaction method, with the aim of improving their thermal quenching properties. The XRD patterns confirm that Al3+ can efficiently substitute Si4+ in the Eu2+ doped Sr2Si5N8 host. The crystal structure is studied by Rietveld refinement, selected-area electron diffraction (SAED) and solid-state NMR analysis. With an increase in x, the emission spectra show no obvious shift and the full width at half maximum (FWHM) increases from 86 to 94.2 nm. Remarkably, the thermal quenching properties are obviously improved as appropriate Al3+ substituted Si4+. The corresponding mechanism is discussed in detail based on the influence of size mismatch and the configurational coordinate model.

Synthesis, structure and luminescent properties of a new blue-green-emitting garnet phosphor Ca2LuScZrAl2GeO12:Ce3+

A series of new garnet phosphors, Ca2Lu1−xZrScAl2GeO12:xCe3+ (0.005 ≤ x ≤ 0.1), have been successfully synthesized through a conventional high-temperature solid-state reaction method. The crystal structure and electronic structure of the host, and the morphology, luminescence property, diffuse reflectance spectra, fluorescence lifetime and thermal stability of the phosphors are investigated in this article. The crystal structure was characterized by X-ray diffraction Rietveld refinement and it belongs to the Iad(230) space-group. The band gap of this matrix is about 5.15 eV, which is much narrower than that of YAG, enhancing the possibility of photo-ionization. The optimized phosphor can be excited by UV or near-UV light in the range of 320–420 nm and exhibits a broad blue-green emission centering at 472 nm under 405 nm excitation. In addition, the concentration quenching mechanism and differentiation of dodecahedral sites were discussed in detail.

Effect of the replacement of Al–O by Si–N on the properties of Sr6Si25.6Al6.4N41.6O4.4:Eu2+ phosphors for white light-emitting diodes

Oxynitride phosphors Sr6Si25.6−xAl6.4+xN41.6−xO4.4+x:Eu2+ are synthesized by gas pressure sintering of powder mixture of SrCO3, AlN, Si3N4 and Eu2O3 at 1800 °C and 0.5 MPa of N2 for 10 h, and their photoluminescence properties are investigated. Sr6Si25.6−xAl6.4+xN41.6−xO4.4+x:Eu2+ can be efficiently excited over a broad spectral range between 300 and 500 nm, and exhibit an intense green emission centered at about 515 nm with a full width at half maximum of 65 nm due to the 4f65d1–4f7 transition of Eu2+ ions. The influence of the replacement of Si–N by Al–O on luminescence properties and crystal structure is reported in a series of Sr6Si25.6−xAl6.4+xN41.6−xO4.4+x:Eu2+ phosphors. Different Sr sites are found in the crystal structure. The thermal quenching properties of the green phosphors are better than that of typical orthosilicate Ba2SiO4:Eu2+ phosphor. Their interesting photoluminescence properties indicate that the Sr6Si25.6Al6.4N41.6O4.4 phosphor is a promising green emitting candidate for white LEDs applications.

A broad-band orange-yellow-emitting Lu2Mg2Al2Si2O12: Ce3+ phosphor for application in warm white light-emitting diodes

A new garnet-type Lu2Mg2Al2Si2O12 compound was successfully synthesized via high-temperature solid–state reaction. The crystal structure was confirmed by the Rietveld refinement method and belongs to a cubic system with a space group of Iad (230) and cell parameters of a = b = c = 11.88310(1) A. There are two different crystallographic positions for Mg2+ with coordination numbers of eight and six, respectively. A relatively large band gap (4.83 eV) was estimated by the diffuse reflectance spectrum, indicating that Lu2Mg2Al2Si2O12 may be a promising host matrix for luminescent materials. The synthetic Ce3+-doped Lu2Mg2Al2Si2O12 phosphor shows intense and broad-band orange-yellow emission peaking at about 575 nm under near-ultraviolet (n-UV) and blue light excitation. Additionally, the quantum efficiency, thermal stability, and packing performance were investigated to evaluate the practical use in white LEDs. The results indicate that Lu2Mg2Al2Si2O12: Ce3+ may be a promising orange-yellow phosphor for warm white LEDs.

Site occupancy and photoluminescence tuning of La3Si6−xAlxN11−x/3:Ce3+ phosphors for high power white light-emitting diodes

A series of blue light excitable yellow-emitting La3Si6−xAlxN11−x/3:Ce3+ (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) phosphors were prepared by a high temperature solid state reactions method. The variations in micro-structure and luminescent properties induced by substituting the Al3+/Si4+ ratio in La3Si6−xAlxN11−x/3:Ce3+ and the related mechanism were investigated. Interestingly, the emission spectrum displayed a red-shift by strengthening the longer wavelength component of the spectrum instead of shifting the emission peak, which was verified to be ascribed to the changed site occupancy of Ce3+. The inducement of redistribution of Ce3+ by introduction of Al3+ as well as its influence on local structures were explored. A tunable spectrum could be obtained through controlling the entering rate of Ce3+ at two different sites. The color rendering index of this phosphor system could be greatly improved with sufficient emission efficiency and thermal stability for applications in high power white light-emitting diodes.