Wanying Geng

Rare-Earth-Free High-Efficiency Narrow-Band Red-Emitting Mg3Ga2GeO8:Mn4+ Phosphor Excited by Near-UV Light for White-Light-Emitting Diodes.

A series of novel red emission Mg3Ga2GeO8 (MGG):Mn(4+) phosphors under near-UV (NUV) excitation are synthesized successfully by traditional high-temperature solid-state reaction. The structure of Mg3Ga2GeO8 is investigated by high-resolution transmission electron microscopy, scanning electron microscopy, and powder X-ray diffraction (XRD) Rietveld refinement. It has one octahedral site and one tetrahedral site in the crystal structure. According to XRD and photoluminescence (PL) property analysis, Mn(4+) can occupy an octahedral (Mg(2+)/Ga(3+)) site. The PL properties are investigated by diffuse-reflectance, emission, excitation, and temperature-dependent spectroscopy and decay curves. It can emit red light peaking at 659 nm under NUV excitation. The critical quenching concentration of Mn(4+) was about 0.5 mol %. The concentration quenching mechanism could be a d-d interaction for the Mn(4+) center. The CIE chromaticity coordinates and full-width at half-maximum are (0.295, 0.677) and 24 nm, respectively. It demonstrated that MGG:Mn(4+) has high color purity. The PL intensity of MGG:0.5% Mn(4+) drops to 72% when the temperature is raised up to 150 °C. Furthermore, MGG:0.5% Mn(4+) exhibits outstanding quantum efficiency (64.7%). By tuning of the weight ratio of blue, green, and red phosphors, the fabricated white-light-emitting diodes using a 405 nm GaN NUV chip combined with a blend of blue phosphor BAM:Eu(2+), green phosphor Sr2SiO4:Eu(2+), and red-emitting phosphor MGG:Mn(4+) driven by 40 mA current can get white light with chromaticity coordinates (0.316, 0.375) and CCT = 3340 K. This demonstrates that MGG:Mn(4+) is a potential red phosphor matching NUV LED chips to get white light.

Recent development in rare earth doped phosphors for white light emitting diodes

As new light sources for next-generation illumination, white light-emitting diodes (WLEDs) have been extensively developed and are commercially available due to their excellent advantages, such as high efficiency, energy-saving, compactness, long operational lifetime and environmental friendliness. Currently, WLEDs with high color rendering are mainly based on wavelength conversion by one or more phosphor materials. In this review, the recent developments of phosphors for WLEDs were introduced combined with the relative work of our group. The common methods for generating white light for blue/ultraviolet (UV) WLEDs were summarized, including: (1) optimizing the commercially used phosphors; (2) developing some new phosphors based on UV LEDs chips; (3) realizing white light emission based on single host. Moreover, some typical new developed phosphors and their luminescence properties were introduced.

Novel blue and green phosphors obtained from K2ZrSi3O9:Eu2+ compounds with different charge compensation ions for LEDs under near-UV excitation

The structural properties of wadeite (K2ZrSi3O9) have been investigated using high-resolution transmission electron microscopy and X-ray powder diffraction refinement, and the luminescence properties of Eu2+ activated K2ZrSi3O9 have been studied to explore new material for phosphor-converted white light near-ultraviolet light-emitting diodes (NUV-LEDs). Eu2+ was introduced into the K2ZrSi3O9 host in a reducing atmosphere, and the special crystallographic positions of Eu2+ were determined based on XRD, photoluminescence emission spectra, temperature dependence properties, and time-resolved photoluminescence. The calculated band gap is about 4.7 eV. The CIE chromaticity coordinates and FWHM of the blue phosphor K2ZrSi3O9:1%Eu2+ are (0.1538, 0.1857) and 57 nm respectively. The photoluminescence properties of co-doped Eu2+–Al3+and Eu2+–Sc3+ charge compensation pair phosphors were investigated. The Eu2+ single-doped K2ZrSi3O9 phosphor shows blue emission with a broad band peaking at 465 nm upon 400 nm NUV excitation. With Eu2+–Al3+ as the charge compensation pair, the photoluminescence properties do not change distinctly, while the photoluminescence emission spectrum red shifts by about 39 nm and the emission becomes green with the Eu2+–Sc3+ pair. Different occupation locations of the different charge compensation pairs were discussed. Blue and green emissions can be obtained from K2ZrSi3O9:Eu2+ compounds by different charge compensation mechanisms. This reveals that K2ZrSi3O9:Eu2+ possesses remarkable optical properties and can be used in NUV-LEDs.

Highly efficient cyan-emitting garnet Ca3Hf2SiAl2O12: xCe3+ phosphor for solid state white lighting

In this paper, we have synthesized a new garnet structure compound Ca3Hf2SiAl2O12 and a series of Ce3+-doped Ca3Hf2SiAl2O12: xCe3+ phosphors. The crystal structure and luminescence properties of the Ca3Hf2SiAl2O12: xCe3+ are investigated in detail. Transmission electron microscopy analysis and XRD Rietveld refinement shows that Ca3Hf2SiAl2O12 is body-centered cubic and belongs to Iad (230) space-group with a = 12.3666 A. Ca3Hf2SiAl2O12: xCe3+ has broad excitation bands at 330 and 400 nm attributed to the characteristic Ce3+ 5d–4f transition and can produce cyan emission under 400 nm UV light excitation. The emission intensity of CHSA: 1%Ce3+ phosphor can reach 68.9% of YAG: Ce3+ (commercial) and 72.1% of Ca8Mg(SiO4)4Cl2: Eu2+ (commercial). The average decay lifetime of Ca3Hf2SiAl2O12: 1%Ce3+ is 48.91 ns, which is consistent with the nanosecond peculiarity of Ce3+. In the process of increasing the temperature from 25 °C to 250 °C, the integrated emission intensity of Ca3Hf2SiAl2O12: 0.5%Ce3+ only decreases to 57.8% (250 °C) and shows nice thermal stability compared with commercial YAG: Ce3+(P46-Y3). Furthermore, Ca3Hf2SiAl2O12: 0.5%Ce3+ exhibits outstanding quantum efficiency (74.7%). It reveals that Ca3Hf2SiAl2O12: xCe3+ possesses remarkable optical properties and can be utilized in UV-LEDs.

Structure, luminescence property and abnormal energy transfer behavior of color-adjustable Ca3Hf2SiAl2O12:Ce3+,Mn2+ phosphors

A series of color-adjustable phosphors Ca3Hf2SiAl2O12:Ce3+,Mn2+ were synthesized through a high temperature solid-state method. Ca3Hf2SiAl2O12 belongs to body-centered cubic crystal system and Iad (230) space-group. It was found that three different cation sites in the Ca3Hf2SiAl2O12 phase were occupied evenly by Ce3+ and Mn2+ ions. Different situations of Mn2+ occupying sites and energy transfer from Ce3+ to Mn2+ can appear with different Mn2+ content and the critical distance was calculated to be Rc1 = 10.8 A, Rc2 = 10.1 A and Rc3 = 12.6 A after calculation of energy transfer from the Ce3+ to Mn2+ by using the concentration quenching method. Ca3Hf2SiAl2O12:Ce3+,Mn2+ phosphors exhibited a broad excitation band ranging from 300 to 450 nm and two broad asymmetric emission bands upon 400 nm excitation. The emission colors of Ca3Hf2SiAl2O12:Ce3+,Mn2+ could be tuned from blue-green (0.2303, 0.3265) to white (0.3350, 0.3388) by changing the ratio of Ce3+/Mn2+. The correlated color temperature can be adjusted from 12 763 K to 5379 K. It indicated that Ca3Hf2SiAl2O12:Ce3+,Mn2+ possesses potential applications in white-LEDs.

K4CaGe3O9:Mn2+,Yb3+: a novel orange-emitting long persistent luminescent phosphor with a special nanostructure

In this work, a novel germanate-based orange-emitting long persistent luminescent (PersL) phosphor, K4CaGe3O9(KCGO):Mn2+,Yb3+, has been successfully synthesized via a solid-state reaction. The special nanostructures of KCGO:Mn2+ and KCGO:Mn2+,Yb3+ were indicated by transmission electron microscope (TEM) images, where the block particles of these samples are shown to consist of many irregular spherical nanoparticles and the size of these spherical particles is less than 10 nm. This special nanostructure contains many defects and could provide the preconditions for the PersL properties of KCGO:Mn2+. Furthermore, after co-doping with Yb3+, the PersL properties are effectively improved due to the high concentration of new traps caused by the non-equivalent substitution of Yb3+. With the optimum doping concentration and sufficient excitation with UV light, the afterglow of KCGO:0.02Mn2+,0.015Yb3+ can persist over 5 h above the recognizable intensity level (≧0.32 mcd m−2). Both the fluorescence and phosphorescence spectra of KCGO:Mn2+,Yb3+ exhibit only one broad emission band, which belongs to the 4T1(G)–6A1(S) transition of Mn2+. In addition, with the help of thermoluminescence (TL) curves, the processes and possible mechanism are studied and discussed.

Potential single-phased white-emitting phosphor (Ca0.33Sr0.67)7(SiO3)6Cl2: Ce3+, Eu2+ for ultraviolet light-emitting diode

A series of white-emitting and color temperature-tunable phosphors (Ca0.33Sr0.67)7(SiO3)6Cl2: Ce3+, Eu2+ have been synthesized by high temperature solid-state reaction. The photoluminescence emission and excitation spectra, and the lifetime are investigated in detail. The samples could display varied color emission from violet (0.1696, 0.0693) towards standard white (0.3314, 0.334) and ultimately to warm white (0.3773, 0.396) under excitation by 340 nm light by adjusting the relative proportion of Ce3+/Eu2+. The energy transfer mechanism from Ce3+ to Eu2+ in the (Ca0.33Sr0.67)7(SiO3)6Cl2 phosphors is dominated by the electric dipole–dipole interaction. These results indicate that the (Ca0.33Sr0.67)7(SiO3)6Cl2: Ce3+, Eu2+ phosphor might be promising as a single-phased white-light-emitting phosphor for UV LEDs.