White Light Region Research Articles

Effects of La 3+ doping on the optical characteristics and color tunability of (Mg, Mn)(Y, Ce, La) 4Si 3O 13 phosphors

The doping effects of La 3+ ions on the photoluminescence properties and color tunability of (Mg, Mn)(Y, Ce, La) 4Si 3O 13 phosphors have been elaborated in this study. On increasing the La 3+ concentration in the host, tunability of excitation wavelength is achieved first, which in turn leads to the blue shift in the wavelength of Ce 3+ emission. The high-energy emission shift of Ce 3+ ions results from the change in the crystal field around Ce 3+ ions. With the incorporation of La 3+ ions, the chromaticity coordinates shift towards white-light region showing the tunable characteristics. Moreover, the correlated color temperature (CCT) change from warm white to cool white light region with different concentration of La 3+ ions. The variation of different excitation wavelength tunes the emission thus augmenting the probability of (Mg, Mn)(Y, Ce, La) 4Si 3O 13 phosphors for potential application in optical devices.

Identification of White Light Emission with Two Complementary Colors in Organic Light-Emitting Devices

Simple method of determining emission light color in organic light-emitting diodes (OLEDs) with two emissive materials from their electroluminescent (EL) spectra has been presented. The Commission Internationale de l'Eclairage (CIE) coordinates and two EL intensity rates of OLEDs can be simulated with this method. White light OLEDs were fabricated with a blue light emission from 4,4'-bis[N-1-napthyl-N-phenyl-amino]biphenyl (NPB) and a yellow light emission from bis[2-(4-tert-butylphenyl)benzothiazolato-N,C2’]iridium (acetylacetonate) [(t-bt)2Ir(acac)], and the device shows the CIE coordinates of (0.34, 0.32) at 10 V bias, which located in white light region.

White light emission from sonochemically synthesized rare earth doped ceria nanophosphors

Undoped CeO 2, and single and triple doped CeO 2:M (where M=Dy 3+, Tb 3+and Eu 3+) nanophosphors were synthesized through a simple sonochemical process and characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), EDS and photoluminescence (PL) spectrophotometry. The TEM micrographs show that resultant nanoparticles have flower-like shape. The doped samples showed multicolor emission on single wavelength excitation. Energy transfer was observed from host to the dopant ions. Characteristic blue emission from Dy 3+ ions, green from Tb 3+ ions and red from Eu 3+ ions were observed. The CIE coordinates of the triple doped Ce 0.86Dy 0.005Tb 0.055Eu 0.08O 2 nanoflowers lie in the white light region of the chromaticity diagram and show promise as good phosphor materials for new lighting devices.

White light generation in Dy^3+-doped oxyfluoride glass and transparent glass-ceramics containing CaF_2 nanocrystals

The radiative emission properties of the Dy3+ ions in an oxyfluoride glass and glass-ceramics have been studied for the generation of white light. The x-ray diffraction pattern of the glass-ceramics shows the formation of CaF2 fluorite-type nanocrystals in the glass matrix after a suitable thermal treatment of the precursor glass, whereas time-resolved optical measurements show the incorporation of the Dy3+ ions in the CaF2 nanocrystals. Intense white light has been observed when the samples are excited with 451 nm laser light. From the visible emission spectra, yellow to blue intensity ratios and the chromaticity color coordinates have been determined. All the color coordinates are found to lie in the white light region of the chromaticity color diagram.

Color control and white light generation of upconversion luminescence by operating dopant concentrations and pump densities in Yb3+, Er3+ and Tm3+ tri-doped Lu2O3 nanocrystals

We synthesized a series of Yb3+, Er3+ and Tm3+ tri-doped Lu2O3 nanocrystals with various dopant concentrations by the hydrothermal approach. Due to a unique electronic state at the top of the valence band, Lu2O3 based materials exhibit intense upconversion luminescence involving 1G4 → 3H6 of Tm3+ in blue, (2H11/2, 4S3/2) → 4I15/2 in green and 4F9/2 → 4I15/2 in red of Er3+ upon near infrared excitation at 980 nm. The variation of upconversion spectra and color points with dopant concentrations and pump densities are studied in detail on the basis of energy transfer processes. An ideal white upconversion light with color coordinates of (0.327, 0.339) is obtained by controlling the intensity of red, green, and blue emission in Lu1.906Yb0.08Er0.008Tm0.006O3nanocrystals under a pump density of 8 W cm−2. Based on the present experimental data, we may predict the dopant concentrations and pump densities for any color point within or around the white light region in the tri-doped Lu2O3 nanocrystals.

White Light Emission from Spin Coated Gd<SUB>2</SUB>O<SUB>3</SUB>:Dy Nano Phosphors Synthesized Using Polyol Technique

Water dispersible Gd2O3:Dy3+ (2%) nanophosphors were synthesized through a facile polyol process and characterized by using X-ray diffraction (XRD), transmission electron microscopy (TEM), Dynamic Light Scattering (DLS) and photoluminescence (PL) spectrophotometry. The results of XRD, TEM and DLS show that resultant nanoparticles are single phasic and have spherical shape with 17 to 22% dispersibility. An efficient energy transfer was observed from host to the dopant ions. Characteristic blue and yellow emissions from Dy3+ ions were observed. The CIE coordinates of the nanophosphor lie in the white light region of the chromaticity diagram. Spin coating of the nanophosphor was done on quartz substrate. Bright white luminescence of this film was observed under ultraviolet light with lamda exc = 310 nm.

Quasienergy band engineering and broadband dynamic localization in photonic lattices with long-range interaction

Polychromatic dynamic localization in tight-binding lattices with long-range interaction is theoretically proposed and experimentally demonstrated in curved-waveguide photonic lattices. Efficient suppression of discrete diffraction over the whole white-light spectral region ($450\ensuremath{-}750$ nm) has been demonstrated in femtosecond-laser-written triangular-waveguide lattices with first- and second-order coupling.

White Light Emission from NaCaPO4:Dy3+ Phosphor for Ultraviolet‐Based White Light‐Emitting Diodes

Trivalent dysprosium (Dy3+)‐doped NaCaPO4 phosphors were synthesized by using a conventional solid‐state reaction technique. The phase and the structure of the as‐prepared powders were characterized by using X‐ray diffraction analysis and revealed that the pure NaCaPO4 formed with orthorhombic structure. The photoluminescence excitation and emission spectra were measured to characterize the luminescent properties of NaCaPO4:Dy3+ phosphors. Sharp emission peaks were observed at 482 nm (blue) and 575 nm (yellow) upon 367 nm excitation, which are attributed to the characteristic 4F9/2→6HJ (J=15/2 and 13/2) transitions of trivalent Dy3+ ions, respectively. The suitable control of the blue/yellow intensity ratio is expected to realize a white luminescent system. The lifetime of 4F9/2 level was measured by exciting Dy3+ ions at 355 nm excitation. The chromaticity coordinates were calculated from the emission spectra and analyzed with Commission International de I'Eclairage (CIE) programs and diagrams. The CIE color coordinates fall in the white light region under different ultraviolet (UV) excitations. These results indicate that NaCaPO4:Dy3+ phosphor could be a potential candidate for UV‐based white light‐emitting diodes.

Upconversion luminescence of Yb 3+/Ho 3+/Er 3+/Tm 3+ co-doped KGd(WO 4) 2 powders

Different lanthanide ions (Yb 3+/Ho 3+/Er 3+/Tm 3+) codoped KGd(WO 4) 2 phosphors were prepared by high-temperature solid-state reaction. The upconversion luminescence properties of two-ion and three-ion co-doped KGd(WO 4) 2 phosphors were investigated in detail. The concentration quenching effect of the two-ion co-doped KGd(WO 4) 2 phosphors was studied, and the optimum concentration of Ho 3+, Er 3+ and Tm 3+ are 2 mol.%, 2 mol.% and 3 mol.%, respectively. The Yb 3+/Ho 3+/Tm 3+ co-doped KGd(WO 4) 2 sample is the best white upconversion luminescence material among three-ion co-doped KGd(WO 4) 2 phosphors. The CIE coordinates for the samples were calculated, and chromaticity coordinates were close to white light regions. The upconversion luminescence mechanism of the samples was discussed. The upconversion luminescence of Ho 3+ and Er 3+ was both produced through two-photon process. The emissions of Ho 3+ at 546 and 654 nm originated from the transition of Ho 3+ ( 5S 2→ 5I 8 and 5F 5→ 5I 8), while those of Er 3+ at 530 and 551 nm were attributed to the transition of Er 3+ ( 2I 11/2→ 4I 15/2 and 4S 3/2→ 4I 15/2). The upconversion luminescence mechanism of Tm 3+ was a three-photon process. The blue emission of Tm 3+ was due to the transition of Tm 3+ ( 1G 4→ 3H 6).

Cool white light emission on Ca3−(l+n)MgSi2O8: $\mathrm{Ce}_{\mathrm{l}}^{3+}$ , $\mathrm{Eu}_{n}^{2+}$ phosphors and analysis of energy transfer mechanism

Rare earth ions (Ce3+, Eu2+) activated Ca3MgSi2O8 (CMSO) phosphors have been synthesized using solid-state reaction method in 95%N2+5%H2 reduction atmosphere at elevated temperatures by varying Eu2+ concentration from 0.0075 to 0.0300 at the fixed Ce0.03 concentration to study their photoluminescence (PL) properties. An energy transfer occurs from Ce3+ to Eu2+ through a significant overlap of Eu2+ excitation spectrum with Ce3+ emission spectrum in CMSO, together with the systematic relative decrease and increase in emission bands of Ce3+ and Eu2+, respectively, have been observed. To support the phenomenon, diffuse reflectance spectra show various absorption levels corresponding to Ce3+, Eu2+, and/or mixture of both rare earth ions. An optimum emission was realized at 0.0150 of Eu2+ via. energy transfer from Ce3+ ion. By utilizing the principle of energy transfer, the critical distance (Rc) between activator ions was found to be 18.64 A. The CIE chromaticity coordinates measured on the Ca3MgSi2O8:Ce3+, Eu2+ phosphors excited under ultraviolet (365 nm) source shows the values lie in cool white light region could be applied to solid state lighting.

Bright White Organic Light-Emitting Diode Mixed Blue and Green Emission

Double-layer and triple-layer organic light-emitting diodes (OLEDs) were fabricated using a novel star-shaped hexafluorenylbenzene organic material, 1,2,3,4,5,6-hexakis(9,9-diethyl-9H-fluoren-2-yl)benzene (HKEthFLYPh) as an energy transfer layer, N, N′-bis-(1-naphthyl)- N, N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) as a hole-transport layer (HTL) and blue emissive layer (EML), and tris(8-hydroxyquinoline)aluminum (Alq 3) as an electron-transport layer (ETL) and green light-emitting layer. Bright white light was obtained with a triple-layer device structure of indium-tin-oxide (ITO)/NPB (40 nm)/HKEthFLYPh (10 nm)/Alq 3 (50 nm)/Mg:Ag (200 nm). A maximum luminance of 8523 cd·m −2 at 15 V and a power efficiency of 1.0 lm·W −1 at 5.5 V were achieved. The Commissions Internationale de L′Eclairage (CIE) coordinates of the device were (0.29, 0.34) at 9 V, which located in white light region. With increasing film thickness of HKEthFLYPh, light emission intensity from NPB increased compared to that of Alq 3.

Influence of thickness of functional layer on performance of organic salt-doped OLED with ITO/PVK:PBD:TBAPF6/Al structure

The effect of thickness of functional layer on the electrical and electroluminescence (EL) properties of single-layer OLED with ITO/PVK:PBD:TBAPF 6 /Al structure were investigated where indium tin oxide (ITO) was used as anode, poly(9-vinylcarbazole) (PVK) as polymeric host, 2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD) as electron-transporting molecule, tetrabutylammonium hexafluorophosphate (TBAPF 6 ) as organic salt dopant and aluminium (Al) as cathode. A unique transition phenomenon at high bias voltage in the devices was observed and the transition was reversible. The transition voltage and turn on voltage decreased with the decrease of functional layer thickness. The turn on voltage was approximately 5.5 V and 6.5 V for 55-nm-thick and 95-nm-thick devices, respectively. However, the current efficiency of 95-nm-thick device was higher than the 55-nm-thick device. More interestingly, the Commission Internationale d’Eclairage (C.I.E.) coordinates of EL spectra of 95-nm-thick device at bias voltage ranging from 7 V to 13 V located in the white light region even without any dye doping. The PL and EL spectra were found completely different. PBD electromer was proposed to dominate the EL spectrum, but the contribution from PVK–PBD electroplex cannot be completely ruled out.

The VUV‐VIS Spectroscopic Properties of Dy3+ Ion in Phosphors M5‐2XDyXNaX(PO4)3F (M=Ca, Sr, Ba) and Their Potential Applications in Mercury‐Free Lamps

The phosphors, fluorapatites M5(PO4)3F (M=Ca, Sr, Ba) activated with Dy3+ ions, were prepared by a high‐temperature solid‐state reaction technique. The VUV‐UV excitation spectra and emission spectra under VUV/UV excitation were measured. The phosphors show broad and strong absorption near 172 nm and intensive emission with the chromaticity coordinates entering the white light region. Hence, the phosphors may be considered as suitable candidates for Hg‐free lamps.

White upconverted luminescence of rare earth ions codoped Gd 2(MoO 4) 3 nanocrystals

The synthesis and upconversion luminescence (UL) of combustion method derived multiple rare earth ions doped gadolinium molybdate Gd 2(MoO 4) 3 nanocrystals have been presented. The monoclinic and orthorhombic Gd 2(MoO 4) 3 nanocrystals have been achieved by sintering the complex precursors respectively at 600 and 900 °C. The room temperature normal emission spectra and excitation spectra of Er 3+, Tm 3+, Ho 3+ codoped monoclinic Gd 2(MoO 4) 3 nanocrystals with sensitizer Yb 3+ excited by xenon source have been investigated. The UL properties of Er 3+, Tm 3+, Ho 3+ respectively doped monoclinic Gd 2(MoO 4) 3 nanocrystals pumped by 980 nm laser diode have been examined. White light is produced in these multiple rare earth ions doped gadolinium molybdates when pumped either by 808 nm or 980 nm laser diode (LD). The CIE color coordinates of these white lights are close to the white light region.

Using BCP layer to control the chroma of white phosphorescent organic light-emitting device

Three types of double emitting layer white organic light-emitting devices (POLEDs) were fabricated. The efficiency of the device can be increased largely by the introduction of phosphorescent dye (Ir(ppy)3) and the chroma of the device can be tuned by inserting a BCP layer between the two light-emitting layers. The device with the configuration of: ITO/PVK:NPB (30nm)/BALq:3.0 wt%TBPe(30nm)/BCP(5.0nm)/CBP:5.0 wt%Ir(ppy)3:2.0 wt%rubrene(15nm)/BCP(10nm)/Mg:Ag shows white emission and high efficiency. A white light with Commission Internationale de l'Eclairage (CIE) coordinates of (0.31, 0.37) and brightness of 4670 cd/m2 are obtained at 17V. The device shows a maximum external efficiency of 1.4%. The CIE coordinates are well within the white-light region when the applied voltage varies between 5.0 to 17V.

High brightness OLED with dual emitting layers

The bright white organic light-emitting diode can be obtained by using two major organic layers which emit blue and orange-yellow light, respectively. The two emitting organic materials 4,4-Bis(2,2-diphenyl-ethen-1-yl)-biphenyl (DPVBi) and 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminost-yry)-4H-pyran (DCM) doped into Tris(8-quin olinolato) aluminum (DCM:Alq 3) are used as a blue emitting and a orange-yellow emitting layer, respectively. The ratios of co-evaporated rate for DCM and Alq 3 could be controlled and the thickness of DPVBi was changed to get optimum result for white light. We got the maximum luminance 12,000 cd/m 2 at 15 V. The CIE coordinates were (0.3520, 0.2956) operated at 12 V entirely within the white light region in CIE coordinates. The current efficiency at 15 V was 6.7 cd/A. The optimum co-evaporated rate for DCM relative to Alq 3 to get orange-yellow emission in co-evaporation was 0.01:1 and the thickness of DPVBi was 35 Å.

Colour Tuning of Single Host Phosphors for White Light by AlAddition

We report a newly found strong disturbing effect of Al addition tothe intensity ratios of blue and green emissions from single hostfull-colour phosphor ofBa3MgSi2−xAlxO8:0.02Eu2+, 0.1Mn2+ fornear-UV excited white light. The phase-pure silicatephosphor in the size of around 4 μm is prepared by salt-assisted spraypyrolysis route, having three-colour emissions at thewavelength peak values of 437 nm, 500 nm and 608 nm simultaneously underthe excitation of 375 nm. The amount variation of Al ion added to thephosphor host results in a drastic change of the intensity ratiobetween green and blue emissions, while the intensity of red lightkeeps unchanged. As a consequence, the combination of three colourslies in the white light region and can be tuned by precisely controlledaddition of Al content. The Raman spectroscopy verifies the modifyingeffect of Al ion to the tetrahedral network in silicate hosts. Weassume that by addition of Al ion, the amount of those two kinds ofEu2+ substituting for those two kinds of Ba2+ lattice siteswith distinct energy levels of blue and green featured emissions can beadjusted to contribute to diversification of the ratios of blue and greenemissions.

Luminescent materials and spectroscopic properties of Dy 3+ ion

Dy 3+ ion can be used as a dopant for electron trapping materials such as long-lasting phosphorescence (LLP) or solid dosimeter. Due to the special electron trapping and thermoluminescent properties of the materials doped with Dy 3+, LLP such as SrAl 2O 4: Eu 2+,Dy 3+ or glass ceramic SrO–Al 2O 3–B 2O 3: Eu 2+,Dy 3+ and solid dosimeter such as BaB 4O 7:Dy 3+ and SrB 4O 7:Dy 3+ were prepared. Dy 3+ ion can also be used for lighting by mixing its yellow 4F 9/2→ 6H 13/2 and blue 4F 9/2→ 6H 15/2 emission bands as in high-pressure mercury lamp phosphor Y 0.79Gd 0.20V 0.5P 0.5O 4: 0.01Dy. In this paper, the VUV excitation spectra and visible emission spectra of Sr 3Gd 0.94(PO 4) 3:0.06Dy, Ba 3Gd 0.94(PO 4) 3:0.06Dy and Ca 4.99(PO 4) 3F: 0.01Dy with chromaticity coordinates in the white-light region are described. They have potential application for mercury-free fluorescence lamp.

White organic light-emitting diode comprising of blue fluorescence and red phosphorescence

A white organic light-emitting diode with the structure of ITO∕NPB30nm∕TCTA+2%TPP20nm∕BCP+0.4%Ir(piq)320nm∕Alq340nm∕Mg:Ag was fabricated and characterized, where 2,5,7,10-tetra-phenylpyrene and tris(1-phenylisoquinoline) iridium (III) [Ir(piq)3] were used as a blue fluorescent dye and a red phosphorescent dye respectively. The I–V characteristics of the device showed a turn-on voltage of 2.6 V. The electroluminescent spectra of the device consisted of blue fluorescent and red phosphorescent emissions. The intensity of the blue emission increased gradually relative to the red emission with increasing voltage. The emissions of the device were in the white-light region between 10 and 15 V. A maximum white light luminance of 1076cd∕m2 with CIE coordinates of (x, y=0.27, 0.24) was reached at 15 V with an efficiency of 1.35cd∕A. The white light emission is related to the simultaneous exciton formation on both sides of the TCTA/BCP interface.