Liping Yu

Changing Ce3+ Content and Codoping Mn2+ Induced Tunable Emission and Energy Transfer in Ca2.5Sr0.5Al2O6:Ce3+,Mn2+

A series of color-tunable Ce3+ single-doped and Ce3+, Mn2+ codoped Ca2.5Sr0.5Al2O6 phosphors were synthesized by a high-temperature solid-state reaction. The crystal structure, luminescent properties, and energy transfer were studied. For Ca2.5Sr0.5Al2O6:Ce3+ phosphors obtained with Al(OH)3 as the raw material, three emission profiles were observed. The peak of photoluminescence (PL) spectra excited at ∼360 nm shifts from 470 to 420 nm, while that of the PL spectra excited at 305 nm stays unchanged at 470 nm with the increase of Ce3+ content. Furthermore, the peak of PL spectra is situated at 500 nm under excitation at ∼400 nm. The relationship between the luminescent properties and crystal structure was studied in detail. Ce3+, Mn2+ codoped Ca2.5Sr0.5Al2O6 phosphors also showed interesting luminescent properties when focused on the PL spectra excited at 365 nm. Obvious different decreasing trends of blue and cyan emission components were observed in Ca2.5Sr0.5Al2O6:0.11Ce3+,xMn2+ phosphors with the increase in Mn2+ content, suggesting different energy transfer efficiencies from blue- and cyan-emitting Ce3+ to Mn2+. Phosphors with high color-rendering index (CRI) values are realized by adjusting the doping content of both Ce3+ and Mn2+. Studies suggest that the Ca2.5Sr0.5Al2O6:Ce3+,Mn2+ phosphor is a promising candidate for near UV-excited w-LEDs.

Composition Screening in Blue-Emitting Li4Sr1+xCa0.97–x(SiO4)2:Ce3+ Phosphors for High Quantum Efficiency and Thermally Stable Photoluminescence

Photoluminescence quantum efficiency (QE) and thermal stability are important for phosphors used in phosphor-converted light-emitting diodes (pc-LEDs). Li4Sr1+xCa0.97-x(SiO4)2:0.03Ce3+ (-0.7 ≤ x ≤ 1.0) phosphors were designed from the initial model of Li4SrCa(SiO4)2:Ce3+, and their single-phased crystal structures were found to be located in the composition range of -0.4 ≤ x ≤ 0.7. Depending on the substitution of Sr2+ for Ca2+ ions, the absolute QE value of blue-emitting composition-optimized Li4Sr1.4Ca0.57(SiO4)2:0.03Ce3+ reaches ∼94%, and the emission intensity at 200 °C remains 95% of that at room temperature. Rietveld refinements and Raman spectral analyses suggest the increase of crystal rigidity, increase of force constant in CeO6, and decrease of vibrational frequency by increasing Sr2+ content, which are responsible for the enhanced quantum efficiency and thermal stability. The present study points to a new strategy for future development of the pc-LEDs phosphors based on local structures correlation via composition screening.

Color-tunable emission in Ce3+, Tb3+ co-doped Ca5(BO3)3F phosphor

In this paper, Ce3+ doped and Ce3+, Tb3+ co-doped Ca5(BO3)3F phosphors were synthesized by a high-temperature solid-state reaction. Upon excitation at 360 nm, the emission spectra of Ce3+ doped phosphors exhibit a broad emission band peaking at 392 nm, which originates from the 5d to 4f transition of Ce3+. The Ce3+, Tb3+ co-doped phosphors show strong energy transfer from Ce3+ to Tb3+, and the emission color can be tuned from purplish blue to green by changing the Tb3+ content. The excitation band in the 300–400 nm region broadens when monitored at 541 nm compared to that monitored at 392 nm. Furthermore, the co-doping of Tb3+ facilitates the appearance of green emitting Ce3+, which originates from Ce3+ on the Ca site other than that for purplish-blue Ce3+. The relationship between the luminescence properties of Ce3+ and its coordination environments, namely, different Ca sites, is discussed based on the calculations of centroid shift and crystal field splitting of 5d energy levels of Ce3+. Results suggest that Ce3+, Tb3+ co-doped Ca5(BO3)3F phosphors may be a candidate for near-UV chip based white light-emitting diodes.

A green approach to green-conversion material and green-agriculture: alkaline-earth metal sulfide phosphors

In this paper, a novel intermediate phase transition metathesis strategy is adopted to prepare Eu2+-doped CaS-based materials on the basis of green chemistry. ZnS acts as a vulcanizing agent and contributes to the formation of CaS from CaO through thermal decomposition of an intermediate state of CaZnOS. The recycling of zinc content is realized by collections of zinc evaporation during the process at high temperature. Few atoms would be wasted and no hazardous auxiliaries are used or produced. Meanwhile, solid solubilities of Mg, Sr and Ba in the CaS compound are verified quite different from the previous results in this case. The effect of Sr/Ca ratios on the luminescence properties of (Ca,Sr)S:Eu2+ phosphors is also investigated. A solar energy conversion laminated glass is promoted to overcome the drawback of decomposition of phosphors to moisture and recycling of agriculture films, which provides a promising permanent functional green-to-red sunlight spectrum conversion greenhouse device for green agriculture.

Synthesis, crystal structure and luminescence of a near ultraviolet-green to red spectral converter BaY2S4:Eu2+, Er3+

Based on the idea that modification of light quality could realize the modulation of morphogenesis and growth rate of plants, we fabricate a near ultraviolet (NUV)-green to red spectral converting BaY2S4:Eu2+, Er3+ phosphor by a solid state reaction route. We herein investigated the crystal structure and photoluminescence properties of the compounds in details. The incorporation of Eu2+ ions does not only enhance the red emission of Er3+ tenfold, but also broadens the absorption region of phosphor. Photoluminescence and decay curves confirm the energy transfer from the host lattices and Eu2+ ions to Er3+ ions. The optimized compositions have strong absorption in the NUV-green region and show intense luminescence at 600 and 663 nm, which matches well with the absorption spectrum (red region) of chlorophyll a. On the basis of the experiment results, a possible luminescent mechanism for the red-emitting BaY2S4:Eu2+, Er3+ phosphor is proposed. We believe this new NUV-green to red spectral converter may open a new route to the design of advanced phosphors for agricultural applications.

Near-UV-to-red light conversion through energy transfer in Ca2Sr(PO4)2:Ce3+,Mn2+ for plant growth

A series of Ca2Sr(PO4)2:Ce3+,Mn2+,Na+ phosphors were synthesized by a high-temperature solid-state reaction. Under ∼320 nm excitation, the Ce3+ and Mn2+ co-doped phosphors exhibit two emission bands peaking at 370 and 645 nm, which originate from 5d–4f and 3d–3d transitions of Ce3+ and Mn2+, respectively. Luminescence properties, diffuse reflectance spectra, and decay curves indicate that energy transfer (ET) occurs from Ce3+ to Mn2+, and the ET efficiency reaches its maximum (91%) at an Mn2+ content of 0.35. The diffuse reflectance spectrum shows that the co-doped phosphors have strong absorption around 320 nm, which is good for the anti-aging ability of an agricultural film. The absolute quantum efficiency of the Ca1.6Sr(PO4)2:0.15Ce3+,0.10Mn2+,0.15Na+ phosphor is ∼94%. And the co-doped phosphors exhibit good stability in water. Therefore, Ca2Sr(PO4)2:Ce3+,Mn2+,Na+ phosphors have potential application as a light conversion material in agricultural films.

Remarkably Enhancing Green-Excitation Efficiency for Solar Energy Utilization: Red Phosphors Ba2ZnS3:Eu2+, X– Co-Doped Halide Ions (X = Cl, Br, I)

Eu2+-activated Ba2ZnS3 has been reported as a red phosphor with a broad emission band peaking at 650 nm under blue excitation for white-LED. In this study, Ba2ZnS3:Eu2+, X- (X = F, Cl, Br, I) phosphors doped with halide ions were prepared by traditional high-temperature solid-state reaction. Phase identification of powders was performed by X-ray powder diffraction analysis, confirming the existence of single-phase Ba2ZnS3 crystals without dopant. The corresponding excitation spectra showed an additional broad band in the green region peaking at 550 nm when the phosphor was halogenated except by the smallest F-. It was proved that the green-excitation efficiency successively strengthened from Cl-, to Br-, to I-, which suggested larger halide ions made a greater contribution to the further splitting of the t2g energy level of the doped Eu2+ ions in the host Ba2ZnS3, and the optimized formula Ba1.995ZnS2.82:Eu2+0.005, I-0.18 showed a potential application in solar spectral conversion for agricultural greenhouse and solar cell. Defect chemistry theory and crystal field theory provided insights into the key role of halide ions in enhancing green-excitation efficiency.

Cation vacancy repair for the enhancement of orange-yellow luminescence in Sr9Mg1.5−xKx(PO4)7:Eu2+ phosphors

Many approaches have contributed to improving the luminescence properties of phosphors, including surface coating, direct reduction and charge compensation, which are involved in reducing the inner or surface defects of luminescent particles. In this study, we propose a cation vacancy repair strategy to successfully improve the orange-yellow luminescence properties of the Sr9Mg1.5(PO4)7:Eu2+ phosphor. The main idea of the strategy is that introducing K+ repairs the vacancy at the Mg(1) site according to the formula . The preferred substitution of the large Mg(1) site by K+ is supported by theoretical calculations. As a result, the luminescence of the repaired phosphate phosphor was enhanced by 2.5 times, and the internal and external quantum efficiency significantly increased by 50.5% and 55.4%, respectively. Additionally, the thermal stability was also a little improved. The enhancement mechanisms for luminescence are discussed in detail based on investigations of the microstructure, luminescence spectra, decay curves and concentration using various analysis techniques. We find that low luminescence efficiency is due to the existence of vacancy defects in the phosphor. Through using the repaired Sr9MgK(PO4)7:Eu2+ phosphor, a white LED package demonstrated a low correlated color temperature (∼2900 K), high color rendering index (∼90) and R9 > 91. We expect that the strategy proposed herein would improve luminescence for other phosphors with vacancy defects.

Fine-Tunable Self-Activated Luminescence in Apatite-Type (Ba,Sr)5(PO4)3Br and the Defect Process

Intrinsic defect-related luminescence has recently been attracting more research interest for the modification of phosphors. However, the connection between defect formation and crystal structure has never been considered. In this work, we report that in the absence of an impurity activator, under a reducing atmosphere, apatite-type compound M5(PO4)3X (M = Ca, Sr, or Ba; X = F, Cl, or Br) can emit tunable colors ranging from blue to orange depending on the content of M and X. To better understand the cause, Ba5- mSr m(PO4)3Br (BSPOB; m = 0-5) solid solutions were analyzed in detail. The dependency of self-activated luminescence on atmospheric conditions and solid solution compositions was investigated by combining experimental characterizations and theoretical calculations using density functional theory. Crystal structures of these solid solutions were verified by X-ray diffraction patterns as well as Rietveld refinements. With the defect formation energy and electron paramagnetic resonance measurement, we propose that an oxygen vacancy (VO) should be mainly responsible for the peculiar super wide band emission. Moreover, the enhanced distortion of solid solution crystal structures augments VO concentrations and leads to luminescence intensities in solid solutions that are higher than that in end point compounds. Variations of the electronic structure of BSPOB matrices with gradual tuning of the Sr/Ba ratio were also investigated. As a result, the introduction of VO defect levels within the band gap leads to the formation of donors and acceptors, allowing for a modulation of the photoluminescence throughout the visible part of the spectrum. As the first report in the literature to demonstrate fine-tunable emissions over a wide wavelength range as a consequence of native defective levels in a series of continuous apatite-type solid solutions, our results illustrate the feasibility of defect-meditated systems by carefully tailoring defect chemistry and nonstoichiometric chemical composition under controlled conditions to engineer phosphor properties.