Yingli Zhu

Highly efficient Sr3Y2(Si3O9)2:Ce3+,Tb3+/Mn2+/Eu2+ phosphors for white LEDs: structure refinement, color tuning and energy transfer

A series of highly efficient and emission-tunable Sr3Y2(Si3O9)2(SYSO):Ce3+,Tb3+/Mn2+/Eu2+ phosphors have been prepared via a solid-state reaction. The structure refinement indicates that the as-prepared phosphors crystallized in a monoclinic phase with a space group of C2/c (no. 15), and there are three kinds of cation sites in the host lattice for the doped ions to occupy. The Ce3+ emission at different lattice sites in the SYSO host has been firstly identified and discussed. When introducing other doping ions into these cation sites, there exist efficient energy transfers from Ce3+ ions to these doping ions (Tb3+, Mn2+, and Eu2+) under UV excitation. The corresponding energy transfer mechanisms from Ce3+ to Tb3+/Mn2+/Eu2+ in SYSO:Ce3+,Tb3+/Mn2+/Eu2+ systems have been studied systematically. These energy transfers not only can enhance the luminescent efficiencies and broaden the width of emission spectra of SYSO:Ce3+,Tb3+/Mn2+/Eu2+ phosphors but also can modulate their emission colors from blue to green, orange or cyan, respectively. For example, the maximum quantum yields (QYs) of as-prepared SYSO:0.15Ce3+,xTb3+/yMn2+ phosphors can reach 90.4% and 74% at x = 0.70, y = 0.01, respectively. Based on these experiment results, the as-prepared SYSO:Ce3+,Tb3+/Mn2+/Eu2+ phosphors can act as potential color-tunable and emission band-widened phosphors for possible applications in ultraviolet light based white LEDs.

Investigation of luminescence properties and the energy transfer mechanism of tunable emitting Sr3Y2(Si3O9)2:Eu2+,Tb3+ phosphors

A series of Eu2+ and Tb3+ singly-doped and co-doped Sr3Y2(Si3O9)2 (SYSO) phosphors have been synthesized via a conventional high-temperature solid-state reaction. The crystal structures, photoluminescence properties, fluorescence lifetimes, thermal properties and energy transfer of SYSO:Eu2+,Tb3+ were systematically investigated in detail. Rietveld structure refinement of the obtained phosphors indicated that the SYSO host crystallized in a monoclinic system with the space group C2/c (15) and there are three kinds of cation sites for the doped ions to occupy forming emission centers. The photoluminescence (PL) emission bands of SYSO:xEu2+ show a red-shift tendency with increasing Eu2+ content which should be attributed to more Eu2+ ions entering into Sr2/Y2 and Sr3/Y3 sites from the Sr1/Y1 site. For the co-doped SYSO:Eu2+,Tb3+ samples, tunable colors from cyan to green can be realized by varying the doping concentration of the Tb3+ ions. The intense green emission was realized in the SYSO:Eu2+,Tb3+ phosphors on the basis of the highly efficient energy transfer from Eu2+ to Tb3+ with an efficiency of over 89%. As a result, the emission intensity of SYSO:0.01Eu2+,0.21Tb3+ is about 2.5 times higher than that of SYSO:0.21Tb3+ under 340 nm UV excitation. The energy transfer mechanism from Eu2+ to Tb3+ in the SYSO host was ascribed to the quadrupole–quadrupole interactions. These results indicated that the SYSO:Eu2+,Tb3+ phosphors can act as single-phase green emitting phosphors for possible applications in ultraviolet light-based white light-emitting diodes (w-LEDs).

High thermal stability and quantum yields of green-emitting Sr 3 Gd 2 (Si 3 O 9 ) 2 :Tb 3+ phosphor by co-doping Ce 3+

Abstract A series of Tb3+ mono-doped and Ce3+-Tb3+ co-doped Sr3Gd2(Si3O9)2 phosphors with high thermal stability and quantum yields were successfully prepared via the solid state reaction. The as-prepared Sr3Gd2(Si3O9)2:Tb3+ samples showed broad excitation spectrum from 250 to 400 nm and presented characteristic emission transitions 5D4→7FJ (J=6, 5, 4, 3) of Tb3+ under 313 nm excitation, which were located at about 488, 541, 584 and 620 nm. The emission intensities of Tb3+ rose steadily in Sr3Gd2(Si3O9)2 host with the increase of Tb3+ concentration even though Gd3+ ions were completely replaced by Tb3+ ions. The Ce3+ ion as a sensitizer could efficiently improve the performance of Tb3+ ion. First, with Ce3+ co-doping, the excitation spectrum of Tb3+ monitored at 541 nm showed a similar band that responds to the violet emission of Ce3+ monitored at 416 nm. Second, the quantum yields of Sr3Gd2(Si3O9)2:Tb3+ phosphors could be enhanced from 26.6% to 80.2% by co-doping Ce3+. Finally, the co-doping of Ce3+ was also effective to improve the thermal stability of Sr3Gd2(Si3O9)2:Tb3+. As the temperature rose to 150 °C, the emission intensity of Tb3+ remained at about 83.6% of that measured at room temperature, which was better than the commercial YAG:Ce phosphor in terms of their thermal quenching properties. These results indicated that the as-prepared Sr3Gd2(Si3O9)2:Tb3+,Ce3+ samples could be used as green emission phosphors for possible applications in near ultraviolet based WLEDs.

Synthesis and characterization of micro/nano-structured BaHPO4/Ba3(PO4)2/Ba5(PO4)3OH phases and their luminescence

Microspheres covered with microcuboids/nanorods and nanoparticles of BaHPO4/Ba3(PO4)2/Ba5(PO4)3OH phases have been successfully synthesized by a facile hydrothermal (HT) method using the citric acid as a surfactant at different pH values. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and fluorescence spectrometry were used to characterize the samples. It was found that the pH value was a crucial factor for the phase formation and shape determination of the final products, which were discussed in detail. Attractively, the as-prepared BaHPO4/Ba3(PO4)2/Ba5(PO4)3OH samples emitted an intense blue light in a broad band from 380 to 625 nm, for which the mechanism was complex ions luminescence originating from the transition of 3T1 → 1A1 in PO43−. Meanwhile, an obvious red shift for the emission band was observed between nano- and bulk-Ba3(PO4)2 synthesized by HT and conventional solid-state (CSS) reactions, respectively, which was due to the effect of the product being nanosized. The same effect was also revealed by the fact that the decay time of the latter was about 2.5 times that of the former. Moreover, the decay mode of Ba5(PO4)3OH was different from those of BaHPO4 and Ba3(PO4)2, which was ascribed to the effect of the substitution of three OH− for one PO43− on their electronic structures.

Investigation of the luminescence properties and thermal stability of dysprosium, terbium, and europium ions singly- and co-doped strontium yttrium borate phosphors

ABSTRACT A series of trivalent rare-earth element ions (europium, terbium, dysprosium) singly- and co-doped strontium yttrium borate phosphors was synthesized via the sol–gel method. The phase formation, luminescence properties, decay times, and energy transfer behaviors from terbium ions to europium ions, the thermal stability, and the Commission Internationale de L’Eclairage coordinates were investigated. Under the excitation of ultraviolet light, the singly doped phosphors exhibited green emission of terbium ions, white emission of dysprosium ions, and red emission of europium ions, respectively. For the terbium and europium ions co-doped strontium yttrium borate samples, a white emission can be realized by blending the doping concentration of terbium and europium ions. The critical distance between terbium and europium ions has been calculated to be about 14.52 Å and the energy transfer from terbium to europium occurred through the dipole–quadrupole interaction. At 150°C, the emission intensity of terbium and europium in the 12 mol% terbium and 14 mol% europium co-doped strontium yttrium borate sample was maintained at about 74% and 87% of their corresponding initial values, respectively, and the dysprosium ions singly doped strontium yttrium borate sample showed about 70% of its initial emission intensity at room temperature. The above results suggested that europium, terbium, dysprosium ions singly- and co-doped strontium yttrium borate phosphors have potential applications as ultraviolet-convertible phosphors.

A strategy for realizing tunable luminescence and full-color emission in Sr3Gd2(Si3O9)2:Eu phosphors by introducing dual functional Mn2+

The exploration of new single-phase phosphors with tunable luminescence and full-color emission is of great importance due to their unique material properties and potential applications in the solid-state lighting field. By introducing Mn2+ into the SGSO:Eu system, on the one hand, the Mn2+ ion can act as an activator to emit green light together with the blue light corresponding to Eu2+ and the red light of Eu3+ due to the efficient energy transfer from Eu2+ and Gd3+ to Mn2+ and on the other hand, Mn2+ can also act as a structure regulator to tune the luminescence properties of Eu2+ and Eu3+ ions in the SGSO host by varying their site occupation, distribution and polyhedron distortions. As a result, the color hue of SGSO:Eu,Mn can be tuned from cyan to white and to yellow under the excitation of selective ultraviolet (UV) light. A white light-emitting diode (LED) device has been demonstrated by integrating the representative SGSO:Eu,0.09Mn with a UV LED light source, which shows a low corresponding color temperature (CCT = 4934 K), a high color rendering index (Ra = 86.6), and a CIE color coordinate (0.35, 0.34). In addition, at low Mn2+ concentrations, the as-prepared SGSO:Eu,Mn presents excellent thermal stability and a tiny chromaticity shift under the excitation of 274 nm light in the temperature range of 298–478 K due to the comparable thermal stability of Eu2+, Mn2+ and Eu3+, while at high Mn2+ concentrations, the SGSO:Eu,Mn shows a high temperature sensitivity (as high as 2.9% K−1 at 298 K) excited at 350 nm due to the increased thermal stability of Eu2+ and the decreased thermal stability of Mn2+. More importantly, the approach to achieve tunable luminescence and full-color emission via incorporating the activator and structure regulator Mn2+ ions demonstrated in this work is not limited to the SGSO:Eu material, but applies to other phosphor material systems.

Control of the photoluminescence in Ba0.97Y2Si3O10:Eu2+ phosphors via the intensification effect of the second luminescence centre

Cation substitution has been proven to be a prevailing strategy for efficiently tuning the luminescence properties of divalent europium (Eu2+). In this work, the isostructural Ba0.97Y2−xLuxSi3O10:0.03Eu2+ (BYSO:Eu,xLu) and Ba0.97Y2−yGdySi3O10:0.03Eu2+ (BYSO:Eu,yGd) (x/y = 0.00–2.00) solid solutions with two emission bands were designed based on the existence of a second potential luminescence center in the Ba0.97Y2Si3O10:0.03Eu2+via the conventional solid-state reaction. Combined with the variation of distortion and disorder, a new model named the intensification effect of the second luminescence center was built in view of the local and average structures. All the samples exhibit a series of asymmetric and broad-band emissions covering the blue and cyan or blue and green regions, due to the increased Eu2+ amounts in the second coordination zone, which is attributed to the increase in distortion and disorder by varying the Gd3+ or Lu3+ content. Moreover, the thermal properties of BYSO:Eu,yGd are superior to the commercial BaMgAl10O17:Eu2+, with T1/2 over 523 K and small reductions in emission intensity at 423 K. The present research aims to shed new light on the improvement of phosphor properties with broad-band emission and high luminous efficacy.