Wei-Ren Liu

A study on the luminescence and energy transfer of single-phase and color-tunable KCaY(PO4)2:Eu2+,Mn2+ phosphor for application in white-light LEDs.

Novel single-phased white light-emitting KCaY(PO(4))(2):Eu(2+),Mn(2+) phosphors for light-emitting diode (LED) applications were synthesized by conventional solid-state reaction. The emission hue could be controlled by tuning the Eu(2+)/Mn(2+) ratio via the energy transfer; the the emission hue of KCaY(PO(4))(2):Eu(2+),Mn(2+) varied from blue (0.1853, 0.2627) to white-light (0.3350, 0.3203) and eventually to purple (0.3919, 0.2867). The mechanism of energy transfer from a sensitizer Eu(2+) to an activator Mn(2+) in KCaY(PO(4))(2):Eu(2+),Mn(2+) phosphors was demonstrated to be an electric dipole-quadrupole interaction. Combining a NUV 405-nm chip and a white-emitting KCaY(PO(4))(2):1%Eu(2+),4%Mn(2+) phosphor produced a white-light NUV LED, demonstrating CIE chromaticity coordinates of (0.314, 0.329) and a color temperature of 6507 K.

High efficiency and high color purity blue-emitting NaSrBO3:Ce3+ phosphor for near-UV light-emitting diodes

A highly intense blue-emitting phosphor NaSrBO3:Ce3+, peaking at 400 nm was synthesized by a solid state reaction. The crystal structure, luminescence properties and quantum efficiency of NaSrBO3:Ce3+ phosphors, as well as their thermal quenching capabilities and the fabrication of a 370 nm UV-chip and R/G/B phosphors were investigated for the first time. The composition-optimized NaSrBO3:1%Ce3+ exhibited high external quantum efficiency of 89% of the blue-emitting, commercial compound, BaMgAl10O17:Eu2+ (BAM:Eu2+). The color purity of as-synthesized NaSrBO3:1%Ce3+ phosphor is much better than that of BAM:Eu2+. By using a GaN-based UV-LED (370 nm) and a mixture of blue-emitting NaSrBO3:Ce3+, green-emitting (Ba,Sr)2SiO4:Eu2+ and red-emitting CaAlSiN3:Eu2+ phosphors as light converters, we constructed an intense white light emitting diode. The LED device exhibited an excellent color-rendering index Ra of 93.13 at a correlated color temperature of 5763 K with CIE coordinates of (0.324,0.337) and a maximum lumen efficacy of 26.2 lm W−1. Based on the results, we are currently evaluating the potential application of NaSrBO3:Ce3+ as a blue-emitting UV convertible phosphor.

Ca(2)PO(4)Cl : Eu(2+): an intense near-ultraviolet converting blue phosphor for white light-emitting diodes

A blue phosphor Ca2PO4Cl:Eu2+ was synthesized by solid state reaction and evaluated as a candidate for white LEDs. The luminescent intensity of Ca2PO4Cl:Eu2+ was found to be 128% under excitation at 380 nm, 149% under 400 nm, and 247% under 420 nm, as high as that of BaMgAl10O17:Eu2+. Furthermore, Ca2PO4Cl:Eu2+ reveals high quantum efficiency and excellent thermal stability. By utilizing a mixture of blue-emitting Ca2PO4Cl:Eu2+, green-emitting (Ba,Sr)2SiO4:Eu2+ and red-emitting CaAlSiN3:Eu2+ as light converters, an intense white GaN-based n-UV-LED (400 nm) was fabricated to exhibit an excellent color-rendering index Ra of 93.4 at a correlated color temperature of 4590 K. Based on the results, we are currently evaluating the potential application of Ca2PO4Cl:Eu2+ as a blue-emitting near-UV convertible phosphor.

Eu 2+ -activated silicon-oxynitride Ca 3 Si 2 O 4 N 2 : a green-emitting phosphor for white LEDs

The green-emitting phosphor Ca3Si2O4N2:Eu2+ was synthesized using a solid-state reaction. The luminescence properties, diffuse reflection spectrum, and thermal quenching were firstly studied, and a white light-emitting diode (wLED) was fabricated using the Eu2+-activated Ca3Si2O4N2 phosphor. Eu2+-doped Ca3Si2O4N2 exhibited a broad green emission band centered between 510 and 550 nm depending on the concentration of Eu2+. The optimal doping concentration of Eu2+ in Ca3Si2O4N2 was 1 mol%. The energy transfer between Eu2+ ions proceeds by an electric multipolar interaction mechanism, with a critical transfer distance of approximately 30.08 Å. A wLED with an color-rendering index Ra of 88.25 at a correlated color temperature of 6029 K was obtained by combining a GaN-based n-UV LED (380 nm) with the blue-emitting BaMgAl10O17:Eu2+, green-emitting Ca3Si2O4N2:Eu2+, and red-emitting CaAlSiN3:Eu2+ phosphors. The results present Ca3Si2O4N2:Eu2+ as an attractive candidate for use as a conversion phosphor for wLED applications.

Crystal structure of blue–white–yellow color-tunable Ca4Si2O7F2:Eu2+,Mn2+ phosphor and investigation of color tunability through energy transfer for single-phase white-light near-ultraviolet LEDs

The crystal structure of Ca4Si2O7F2:Eu2+,Mn2+ was refined and determined from X-ray diffraction (XRD) profiles obtained using a synchrotron light source by the Rietveld refinement method. It was found to crystallize into a monoclinic structure with the P21/c(14) space group. On examining the Mn2+-concentration-dependent photoluminescence properties, we found that the emission colors could be tuned from blue (0.152, 0.112) to white-light (0.351, 0.332) and eventually to yellow (0.430, 0.423) through energy transfer by changing the Eu2+/Mn2+ ratio. Moreover, energy transfer from a sensitizer Eu2+ to an activator Mn2+ occurs via a resonance-type dipole–quadrupole interaction mechanism, and the critical distances of the energy transfer were calculated to be 11.66 A and 12.61 A using concentration quenching and spectral overlap methods, respectively. Combining a 400 nm near-ultraviolet (NUV) chip and a single-phase white-emitting (Ca0.96Eu0.01Mn0.03)4Si2O7F2 phosphor produced a white-light NUV LED with CIE chromaticity coordinates of (0.347, 0.338) and a warm color temperature of 4880 K.

Luminescence and Energy Transfer Mechanism in Ca10K ( PO4 ) 7 : Eu2 + , Mn2 + Phosphor

A series of Ca 10 K(PO 4 ) 7 :Eu 2+ , Mn 2+ (CKP:Eu 2+ , Mn 2+ ) phosphors was synthesized by solid-state reaction. Under an optimal excitation of 347 nm, we have observed a blue emission band at 467 nm and a red emission band at 634 nm, which resulted from Eu 2+ and Mn 2+ , respectively. The energy transfer from Eu 2+ to Mn 2+ in CKP host matrix is of a resonant type via a dipolequadrupole mechanism. By properly varying the ratio of Eu/Mn, the emission hue can be tuned from blue (0.18,0.21), to white (0.30,0.24), and finally to red (0.35,0.24). The intensity of composition-optimized CKP:Eu 2+ is about 30% of BaMgAl 10 O 17 :Eu 2+ . The measurement of thermal quenching for CKP:Eu 2+ , Mn 2+ and yttrium aluminum garnetCe 3+ was also compared and investigated in the study. The (CKP:Eu 2+ , Mn 2+ ) phosphor may serve as a candidate for UV light-emitting diode application.

Nitrogen-doped graphene nanosheet-supported non-precious iron nitride nanoparticles as an efficient electrocatalyst for oxygen reduction

Nitrogen-doped graphene-supported carbon-containing iron nitride (FeCN/NG) was synthesized by the chemical impregnation of iron and nitrogen-containing precursors in the presence of ammonia under thermal treatment. The resultant graphene-based material acted as an electrode with a much higher electrocatalytic activity in the catalysis via a 4-electron pathway in fuel cells. The results of X-ray diffraction, scanning electron microscopy, Fourier transform infrared and X-ray photoelectron spectroscopy indicated that graphite oxide was successfully reduced to nitrogen-doped graphene. X-Ray absorption spectroscopy further confirmed that carbon was incorporated into iron nitride, demonstrating that Fe–N–C catalytic active sites may be responsible for the oxygen reduction reaction. To the best of our knowledge, this is the first report of the combination of N-doped graphene with non-precious metal for oxygen reduction in fuel cells, and may open up a new possibility for preparing graphene-based nanoassemblies for intensive applications.

(Ba,Sr)Y2Si2Al2O2N5 : Eu2+: a novel near-ultraviolet converting green phosphor for white light-emitting diodes

A highly intense green-emitting phosphor BaY2Si2Al2O2N5 : Eu2+ (BYN : Eu2+) peaking at 511 nm was synthesized by a solid state reaction. The structure refinement, luminescence properties of BYN : Eu2+ phosphors as well as their thermal quenching and the fabrication of white-light-emitting diodes (W-LEDs) were firstly investigated. By partially substituting Ba by Sr, namely (Ba,Sr)Y2Si2Al2O2N5 : Eu2+ (BSYN : Eu2+), the emission peak was shifted to 565 nm, giving a yellow-orange color, due to the splitting of the 5d energy level. By utilizing a mixture of blue-emitting BaMgAl10O17 : Eu2+, green-emitting BYN : Eu2+ and red-emitting CaAlSiN3 : Eu2+ phosphors as light converters, an intense white GaN-based n-UV-LED (380 nm) was fabricated to a exhibit good color-rendering index Ra of 84.5 at a correlated color temperature of 5089 K and CIE coordinates of (0.3425, 0.3478). Based on these results, we are currently evaluating the potential application of BYN : Eu2+ as a green-emitting near-UV convertible phosphor.

ZnB 2 O 4 :Bi 3+ ,Eu 3+ :a highly efficient, red-emitting phosphor

The novel red phosphor of Eu(3+)-Bi(3+) co-activated ZnB(2)O(4) was prepared by a solid-state reaction. The composition-optimized (Zn(0.9)Eu(0.1))B(2)O(4) phosphor exhibits a dominant emission peak at 610 nm ((5)D(0)-(7)F(2)) with CIE coordinates of (0.63, 0.36) under the excitation at 393 nm. By co-doping Bi(3+) ions in ZnB(2)O(4):Eu(3+), the emission intensity and quantum efficiency can be efficiently enhanced by an increment of 14% and 6%, respectively. The luminescence performance and thermal stability of (Zn(0.8)Bi(0.1)Eu(0.1))B(2)O(4) phosphor were found to be superior to that of the commodity phosphor, La(2)O(2)S:Eu(3+). The red-emitting borate phosphor may be potentially useful in the fabrication of white light-emitting diodes (LEDs).

Single-phased white-light-emitting KCaGd(PO4)2:Eu2+,Tb3+,Mn2+ phosphors for LED applications

Single-phased white light-emitting KCaGd(PO4)2:Eu2+,Tb3+,Mn2+ phosphors were synthesized by a solid state reaction. By changing the doping contents of Eu2+, Tb3+ and Mn2+, the emission hue could be precisely controlled via the energy transfer mechanism. The structure refinement, luminescence properties as well as their thermal quenching and energy transfer mechanism were firstly investigated. The mechanism of transferring energy between Eu2+, Mn2+ and Tb3+ ions was also discussed in this study. The optimal-composition for white-light is K(Ca0.89Eu0.01Mn0.1)(Gd0.9Tb0.1)(PO4)2, which gives the CIE coordinates of (0.2984, 0.3171). These results indicate that the KCaGd(PO4)2:Eu2+,Mn2+,Tb3+ phosphor could be a promising single-composition phosphor for applications involving white-light NUV LEDs.