Ronghui Liu

Study on co-precipitation synthesized Y3Al5O12:Ce yellow phosphor for white LED

A precursor of the Y3Al5O12:Ce (YAG:Ce) phosphor was obtained by co-precipitation of the solution of high purity nitrates with ammonium bicarbonate solution. The precipitation process of the precursor was studied in this work. YAG:Ce yellow phosphors with fine morphology was synthesized by annealing the precursor at a reducing atmosphere. The crystal phase, microstructure of the phosphors and their photoluminescence were investigated. The results indicated that the pure phase of YAG:Ce could be obtained at 1673 K annealing temperature. The particles of phosphor were basically spherical in shape with the mean size less than 2 μm. The excitation spectrum showed a broad and strong absorption band at about 460 nm, compatible with the excited wavelength of commercial blue light emitting diode (LED) for white LED lighting. Bright yellow-emission located at 540 nm was observed.

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.

Luminescence properties of Eu2+-doped Ba3Si6O12N2 green phosphor: concentration quenching and thermal stability

Abstract The efficient Eu 2+ -doped Ba 3 Si 6 O 12 N 2 green phosphors were prepared by a traditional solid state reaction method under N 2 /H 2 atmosphere at a temperature up to 1350 °C for 12 h. Photoluminescence (PL) properties showed a broad emission band with a peak of 525 nm and the full width of half-emission maximums (FWHM) of 70 nm under 460 nm light irradiation. The X-ray diffraction patterns (XRD) and scanning electron microscope (SEM) images of the synthesized powder demonstrated its pure phase and excellent crystallization. Quenching concentration in this phosphor was found to be 0.3. The mechanisms of concentration quenching and redshift of emission peak with increasing concentration of Eu 2+ were studied. The temperature dependence measurement of this green phosphor revealed excellent thermal quenching property compared to silicate green phosphor. It is believed that Ba 3 Si 6 O 12 N 2 :Eu 2+ is an excellent green phosphor for UV or blue chip based white LEDs.

Color tunable emission and energy transfer in LaSi3N5:Ce3+,Tb3+ phosphors for UV white LEDs

A series of new blue-green emitting LaSi3N5:Ce3+,Tb3+ phosphors were obtained through a high temperature solid-state method. The crystal structure, photoluminescence properties and energy transfer from Ce3+ to Tb3+ were investigated. The doped ions Ce3+ and Tb3+ were confirmed to occupy the same crystallographic sites as La3+ using a Rietveld structure refinement method. The emission spectra of these phosphors was composed of a blue emission band attributed to the d–f transition of Ce3+, together with several sharp lines ascribed to the emission of Tb3+ under UV light excitation. Thus a color tunable emission from blue to green was obtained via controlling the doping concentration of Tb3+. In addition, the mechanism of energy transfer, efficiency of energy transfer, chromaticity coordinates and thermal stability of these phosphors were investigated. The results indicated that the Ce3+,Tb3+ co-activated LaSi3N5 phosphors with color-tunable blue-green emission and high quantum efficiency may have potential for UV white LEDs.

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 Y3+ on the local structure and luminescent properties of La3−xYxSi6N11:Ce3+ phosphors for high power LED lighting

A series of Ce3+-activated La3−xYxSi6N11 yellow phosphors were synthesized by high temperature solid-state reaction; their structures and luminescent properties were systematically investigated. The results of structural and spectral characterization showed that with the increase of Y3+ replacing La3+ in La3−xYxSi6N11:Ce3+ (0 < x ≤ 0.9), the space group remains P4bm but with Y3+ occupying two crystallographic sites in different ratio, while the emission spectra exhibit a red-shift from 535 nm to 552 nm. Moreover, the thermal quenching properties of La2.86−xYxSi6N11:0.14Ce3+ exhibit somewhat of a decrease with the increase of x value, but still remains at 95% (x = 0.9) of the initial emission intensity measured at 200 °C. In addition, the light quality is significantly improved by direct packaging with a blue LED chip. The above results indicate that La3−xYxSi6N11:Ce3+ may be a promising phosphor for high power white-light LEDs.

Photoluminescent properties of tunable green-emitting oxynitride (Ba3−xSrx)Si6O12N2:Eu2+ phosphor and its application in white LEDs

Abstract Green emitting Eu2+-doped (Ba3−xSrx)Si6O12N2 solid solutions were synthesized through solid state reaction at 1350 °C for 10 h under a N2/H2 atmosphere. The XRD patterns revealed that the solid solution series of (Ba3−x−ySrx)Si6O12N2:yEu2+ with x value ranging from 0–0.6 were established. An efficient and intense tunable green light was observed by varying the cation Sr/Ba ratio. The emission spectra exhibited an entire shift towards long wavelength with increasing of x value, which was caused by large crystal field splitting and Stokes shift. The x value dependence of emission intensity was discovered and explained by the enhanced probability of electron from excited 4f state to 5d ground state via nonradioactive transition. Highly thermal stability and feasible color coordinates were verified. White LEDs with excellent photochromic properties were fabricated by packing GaN based blue chips and (Ba,Sr)3Si6O12N2:Eu2+ phosphors. All results indicated that the (Ba3−xSrx)Si6O12N2:Eu2+ phosphors were confirmed to be a promising candidate for pc-white LEDs in solid state lighting.

Synthesis and Photoluminescence Properties of a Blue-Emitting La3Si8N11O4:Eu2+ Phosphor

Eu2+-doped La3Si8N11O4 phosphors were synthesized by the high temperature solid-state method, and their photoluminescence properties were investigated in this work. La3Si8N11O4:Eu2+ exhibits a strong broad absorption band centered at 320 nm, spanning the spectral range of 300-600 nm due to 4f7 → 4f65d1 electronic transitions of Eu2+. The emission spectra show a broad and asymmetric band peaking at 481-513 nm depending on the Eu2+ concentration, and the emission color can be tuned in a broad range owing to the energy transfer between Eu2+ ions occupying two independent crystallographic sites. Compared to the Ce3+-doped La3Si8N11O4, the Eu2+-doped one shows a larger thermal quenching, predominantly owing to photoionization. Under 320 nm excitation, the internal and external quantum efficiencies are 44 and 33%, respectively.

Synthesis and luminescence properties of Sr3–z(AlxSi1x)O5–xFx:zCe3+ phosphors

Abstract Sr 3– z (Al x ,Si 1– x )O 5– x F x : z Ce 3+ phosphors were synthesized by high-temperature solid-state reaction. The structure and luminescence properties of phosphors with various Al/Si ratios and Ce 3+ concentrations were characterized using various methods such as X-ray diffraction, photoluminescence excitation and photoluminescence spectra. XRD result displayed that a complete solid solution between Sr 3 AlO 4 F and Sr 3 SiO 5 was formed. With the increasing of x value, a broader excitation band and stronger absorption appeared in the blue light region. Moreover, the emission band shifted to a shorter wavelength and the emission intensity reached a maximum at x =0.6. By adjusting the concentration of Ce 3+ , a widely tunable range of emission wavelength under the excitation of 460 nm was obtained from the green to yellow regions. In addition, the concentration and thermal quenching were also discussed.