Chien-Hao Huang

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,Mg,Sr)9Y(PO4)7:Eu2+,Mn2+: Phosphors for white-light near-UV LEDs through crystal field tuning and energy transfer

Two series of single-composition (Ca,Mg,Sr)9Y(PO4)7:Eu2+ and (Ca0.5Sr0.5)9Y(PO4)7:Eu2+,Mn2+ phosphors were synthesized via high-temperature solid-state reactions. Their emission colors could be tuned from blue to green and eventually to red through tuning the crystal field splitting and energy transfer. On examining the Mn2+ concentration-dependent photoluminescence properties, we found that co-doping with Mn2+ would lead to a change in Eu2+/Eu3+ ratio. Moreover, an energy transfer from Eu2+ to Mn2+ occurs because of the spectral overlap between the emission band of Eu2+ and the excitation band of Mn2+. The resonance-type energy transfer via a dipole–quadrupole interaction mechanism was supported by decay lifetime data and the critical distance of energy transfer was calculated to be 11.09 A. A trichromatic white-light emitting diode was fabricated by integrating a 380 nm near-ultraviolet (n-UV) chip comprising yellow-emitting (Ca0.5Sr0.5)9Y(PO4)7:0.007Eu2+,0.02Mn2+ and blue-emitting (Ca0.5Mg0.5)9Y(PO4)7:0.007Eu2+ phosphors into a single package. Such a composite device emitted white light with a correlated color temperature of 6303 K, a color rendering index of 87.4, and color coordinates (0.314, 0.348) close to those of ideal white light. The results suggest that a phosphor blend of (Ca0.5Sr0.5)9Y(PO4)7:0.007Eu2+,0.02Mn2+ and (Ca0.5Mg0.5)9Y(PO4)7:0.007Eu2+ is potentially useful for white n-UV light-emitting diodes (LEDs).

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.

Eu2+-Activated Sr8ZnSc(PO4)(7): A Novel Near-Ultraviolet Converting Yellow-Emitting Phosphor for White Light-Emitting Diodes

The crystal structure of Eu(2+)-activated Sr(8)ZnSc(PO(4))(7):Eu(2+) phosphor was refined and determined from XRD profiles by the Rietveld refinement method using a synchrotron light source. This phosphor crystallizes in the monoclinic structure with the I2/a space group. The SZSP:xEu(2+) phosphors showed a broad yellow emission band centered at 511 and 571 nm depending on the concentration of Eu(2+), and the composition-optimized concentration of Eu(2+) in the Sr(8)ZnSc(PO(4))(7):Eu(2+) phosphor was determined to be 2 mol %. The estimated crystal-field splitting and CIE chromaticity coordinates of Sr(8)ZnSc(PO(4))(7):xEu(2+) (x = 0.001-0.05 mol) were 20181-20983 cm(-1) and (0.3835, 0.5074) to (0.4221, 0.5012), respectively, and the emission band showed a redshift from 547 to 571 nm with increasing Eu(2+) concentration. The nonradiative transitions between the Eu(2+) ions in the Sr(8)ZnSc(PO(4))(7) host were attributable to dipole-dipole interactions, and the critical distance was approximately 19.8 Å. The combination of a 400 nm NUV chip with a blend of Sr(8)ZnSc(PO(4))(7):0.02Eu(2+) and BAM:Eu(2+) phosphors (light converters) gave high color rendering indices between 79.38 and 92.88, correlated color temperatures between 4325 and 7937 K, and tuned CIE chromaticity coordinates in the range (0.381, 0.435) to (0.294, 0.310), respectively, depending on the SZSP:0.02Eu(2+)/BAM:Eu(2+) weight ratio. These results suggest that the Sr(8)ZnSc(PO(4))(7):0.02Eu(2+)/BAM:Eu(2+) phosphor blend has potential applications in white NUV LEDs.

BaZrSi3O9:Eu2+: a cyan-emitting phosphor with high quantum efficiency for white light-emitting diodes

In this paper, a cyan-emitting phosphor BaZrSi3O9:Eu2+ was synthesized and evaluated as a candidate for white light emitting diodes (WLEDs). This phosphor shows strong and broad absorption in 380–420 nm region, and the emission intensity of the optimized BaZrSi3O9:Eu2+ was found to be 90% and 198% of that of the commercial BaMgAl10O17:Eu2+ (BAM:Eu2+) under excitation at 405 nm and 420 nm, respectively. Upon excitation at 405 nm, the quantum efficiency of the optimized BaZrSi3O9:Eu2+ is 83% of that of BAM:Eu2+. The performance of this phosphor was further tested to fabricate white LED lamps. By coating BaZrSi3O9:Eu2+ with a green-emitting (Ba,Sr)2SiO4:Eu2+ and a red-emitting CaAlSiN3:Eu2+ on a near-ultraviolet (405 nm) LED chip, driven by a 350 mA forward bias current, intense warm white light with a color rendering index of 90 has been produced.

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.

Near UV-pumped yellow-emitting Sr8MgSc(PO4)7:Eu2+ phosphor for white-light LEDs with excellent color rendering index

An Eu2+-activated Sr8MgSc(PO4)7:Eu2+ yellow-emitting phosphor with strong luminescence was synthesized and its crystal structure has been refined and determined from the XRD profiles using synchrotron light source by Rietveld refinement method. Non-radiative transitions between Eu2+ ions in the Sr8MgSc(PO4)7 host have also been demonstrated to be attributable to dipole–dipole interactions, and the critical distance was estimated to be 24.83 A by using spectral overlap methods. In addition, white-light near-UV LEDs were fabricated by using a phosphor blend of composition-optimized yellow-emitting Sr8MgSc(PO4)7:0.01Eu2+ and blue-emitting BaMgAl10O17:Eu2+ commodity pumped with a 400 nm near-UV chip and the Ra value and reduced color temperature of were found to be 96.7 and 5,614 K, respectively. These results indicate that yellow-emitting Sr8MgSc(PO4)7:xEu2+ can serve as a promising candidate for application in white-light LEDs.

(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.

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.