Jinge Cheng

Crystal structure, luminescence properties, energy transfer and thermal properties of a novel color-tunable, white light-emitting phosphor Ca9−x−yCe(PO4)7:xEu2+,yMn2+

A series of Ca9-x-yCe(PO4)7:xEu2+,yMn2+ phosphors were synthesized by a high-temperature solid-state reaction method. The as-prepared samples were characterized by XRD and EDX measurements, which showed that Eu2+ and Mn2+ could be efficiently doped into the host. Ce3+ acts concurrently as activator and sensitizer in Ca9Ce(PO4)7, and the energy transfer mechanisms between Ce3+/Eu2+ and Ce3+/Mn2+ in Ca9Ce(PO4)7 were validated and proven to be a resonant type via dipole-quadrupole and dipole-dipole interactions, respectively. Besides, there is also energy transfer from Eu2+ to Mn2+ ions. The host, Ca9Ce(PO4)7, emits blue-white light and Ca9Ce(PO4)7:xEu2+,yMn2+ phosphors emit blue-green through white to orange-red light under near-ultraviolet radiation as a result of tuning the ratio of Eu2+/Mn2+. Ca9Ce(PO4)7:0.04Eu2+,0.08Mn2+ emits white light with CIE coordinates (0.333, 0.310), a CCT of 5446 K, and a high CRI of 81. The energy transfer efficiency between Ce3+ and Mn2+ increases significantly with temperature. These results reveal that Ca9Ce(PO4)7:Eu2+,Mn2+ may be a potential candidate for white light-emitting phosphors.

Synthesis, color-tunable emission, thermal stability, luminescence and energy transfer of Sm3+ and Eu3+ single-doped M3Tb(BO3)3 (M = Sr and Ba) phosphors

A series of new tunable emission phosphors of Sm3+ and Eu3+ single-doped M3Tb(BO3)3 (M = Sr and Ba) phosphors were synthesized by the solid-state reaction. Crystallization behavior and structure, reflectance spectral properties, luminescence properties, energy transfer, lifetimes, temperature-dependent luminescence properties and CIE chromaticity coordinate of M3Tb(BO3)3:Ln3+ (M = Sr and Ba, Ln = Sm and Eu) were systematically investigated. M3Tb(BO3)3 (M = Sr and Ba) crystallizes in a rhombohedral cell with space group R by Rietveld structure refinement of the obtained phosphors with the standard data of Ba3Dy(BO3)3. Sr3Tb(BO3)3 and Ba3Tb(BO3)3 emit yellowish-green emission with the main peak around 555 and 550 nm, respectively, which originates from the 5D4 → 7F4 transition of Tb3+, and M3Tb(BO3)3:Ln3+ (Ln = Sm and Eu) phosphors show intense yellowish-green, yellow, orange and red emission with increasing Ln3+ concentration under 274 and 286 nm excitation. The orange-red/red emissions show peak maxima at 613 nm (Sr3Tb(BO3)3:Sm3+), 627 nm (Sr3Tb(BO3)3:Eu3+), 607 nm (Ba3Tb(BO3)3:Sm3+) and 625 nm (Ba3Tb(BO3)3:Eu3+), and are due to the efficient energy transfer of Tb3+–Sm3+ and Tb3+–Eu3+. The temperature-dependent luminescence properties of M3Tb(BO3)3:Ln3+ are characterised by good thermal stabilities up to 150 °C, up to 72.8%. However, due to the defect in the host of the Sr3Tb0.99(BO3)3:0.01Sm3+ phosphor, an amazing and interesting phenomenon can be observed that the emission intensity is enhanced constantly with increasing temperature.

Relationships between luminescence properties and polyhedron distortion in Ca9−x−y−zMgxSryBazCe(PO4)7:Eu2+,Mn2+

Generally, the luminescence properties of phosphors can be tuned by cations substitutions (Mg2+, Ca2+, Sr2+ and Ba2+); however, the underlying mechanism is not clear. The luminescence properties are influenced by many factors, such as symmetry of the ligand polyhedron and the bond length between the luminescent centre and its ligand. Therefore, in this research, the phosphor Ca9−x−y−zMgxSryBazCe(PO4)7:Eu2+,Mn2+ was selected as the research object, and the reasons are investigated. Detailed data regarding the crystal space structure is acquired through the Rietveld refinement and used to systematically analyze the change of the two factors. It was found that the two factors are sometimes synergetic and sometimes antagonistic to the spectrum shift. The change of the luminescence properties is the comprehensive result of the two factors. It was found that the influence produced by the change of symmetry of the polyhedron is stronger than that by the change of bond length between the luminescence centre and its ligand. When changing the doping concentration of Sr2+ and Ba2+, the chromaticity coordinates of (Ca,Sr,Ba)8.78Ce(PO4)7:Eu2+,Mn2+ could change from cold white light to warm white light, and there is a larger distribution range in the white light region in the CIE 1931 chromaticity diagram. Cations substitutions tuning the luminescence thermal stability are also observed. When partial Ca2+ are substituted by other cations, the luminescence thermal stability of the luminescent centre was improved to some degree. It is expected that these methods can be generalized to analyze the changes in optical and other properties that are sensitive to local coordination environments for luminescent materials.

Synthesis, structure and luminescence properties of novel NIR luminescent materials Li2ZnGe3O8:xMn2+

A series of near-infrared (NIR) emitting Li2ZnGe3O8 (LZG):xMn2+ phosphors were synthesized via a conventional solid state reaction. They can be obtained by controlling the heating temperature (950 °C) or changing the doping concentration of Mn2+ (900 °C). In particular, due to the transition of the crystal from ZnO6 to ZnO4, the NIR phosphors perform with different spectral widths when the Zn/Mg ratio in LZG:xMn2+ is adjusted. The excitation spectrum is characterized by a broad band from 240 nm to 640 nm, centered at 475 nm and tailing on the low energy side. The emission spectrum presents a broad emission band from 650 to 900 nm with the main peak at 832 nm. Hence, the phosphors can convert visible and ultraviolet light to NIR emission. Whats more, this kind of NIR phosphor is based on the mechanism of down-conversion, and it possesses excellent thermal effect properties.

Improvement of emission intensity, colour rendering index and thermal stability of Ca9Ce(PO4)7:Eu2+,Mn2+ via H3BO3 doping

Ca8.78Ce (PO4)7:0.06Eu2+,0.16Mn2+,xH3BO3 phosphors were synthesized by a high-temperature solid-state reaction method. XRD patterns of the samples indicate that H3BO3 doping does not change the phase formation. However, H3BO3 doping can change the emission intensities of Ce3+, Eu2+, Mn2+ and the energy transfer efficiencies of Ce3+–Eu2+ and Ce3+–Mn2+. The reason for this is that H3BO3 doping changes the luminescent centre and the distance between different luminescent centres to some degree. When the ligand polyhedron volume decreases, the lifetime and emission intensity of the luminescent centre will increase. When the distance between Ce3+ and Eu2+/Mn2+ decreases, the energy transfer efficiency between them will increase. In Ca8.78Ce(PO4)7:0.06Eu2+,0.16Mn2+,xH3BO3, the average volume of an [(Eu/Mn)1/2/3–O8/9] polyhedron will decrease from 27.511 A3 to 21.847 A3, and the average distance between Ce and (Eu/Mn)1/2 will decrease from 3.702 A to 3.573 A with increased doping amounts of H3BO3 from 0 to 7. Meanwhile, the emission intensity and lifetime of Ce3+ decrease, and those of Eu2+/Mn2+ increase. Furthermore, the energy transfer efficiency from Ce3+ to Eu2+/Mn2+ and the quantum efficiency are improved. H3BO3 doping improves the ratio of red, green and blue colours of the spectrum, which improves the colour rendering index of materials from 80.5 to 90.7. H3BO3 doping also improves the thermal stability of the material to some degree. The quenching temperature of Eu2+ is improved from 75 °C to 100 °C.

Substitution priority of Eu2+ in multi-cation compound Sr0.8Ca0.2Al2Si2O8 and energy transfer

A blue phosphor was obtained by doping Eu2+ into a multi-cation host Sr0.8Ca0.2Al2Si2O8 through high temperature solid state reaction. The emission spectra show a continuous red-shift behavior from 413 nm to 435 nm with Eu2+ concentration increasing. The substitution priority of Eu2+ in Sr0.8Ca0.2Al2Si2O8 was investigated via x-ray diffraction (XRD) and temperature properties in detail: the Ca2+ ions are preferentially substituted by Eu2+at lower doping, and with the Eu2+ concentration increasing, the probability of substitution for Sr2+ is greater than that of replacing Ca2+. Accordingly, we propose the underlying method of thermal property to determine the substitution of Eu2+ in the multi-cation hosts. Moreover, the abnormal increase of emission intensity with increasing temperature was studied by the thermoluminescence spectra. The energy transfer mechanism between the Eu2+ ions occupying different cation sites was studied by the lifetime decay curves. A series of warm white light emitting diodes were successfully fabricated using the blue phosphors Sr0.8Ca0.2Al2Si2O8: Eu2+ with commercial red phosphor (Ca Sr)SiAlN3: Eu2+ and green phosphor (Y Lu)3Al5O12: Ce3+, and the luminescent efficiency can reach 45 lm/W.