Intense red-emitting phosphors for LED solid-state lighting

The phosphors Gd_2−xEu_x(MoO₄)₃ (x = 0.20, 0.40, 0.60, 0.80, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0), Gd_0.8−xY_xEu_1.2(MoO₄)₃ (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8) and Gd_0.2Y_0.6−xEu_1.2Sm_x(MoO₄)₃ (x = 0.02, 0.024, 0.028, 0.032, 0.036, 0.04) were prepared by solid-state reaction technique at 950 °C. The presence of the Y³⁺ and Sm³⁺ ions strengthen and broaden the absorption of the phosphors at ∼400 nm. The intense red-emitting phosphor Gd_0.2Y_0.572Eu_1.2Sm_0.028(MoO₄)₃ with orthorhombic structure was obtained. Both Eu³⁺ and Sm³⁺ f-f transition absorptions are observed in the excitation spectra, the main emission line is at 616 nm (⁵D₀ → ⁷F₂ transition of Eu³⁺) and the chromaticity coordinates (x = 0.66, y = 0.33) is very close to the NTSC standard values (x = 0.67, y = 0.33). It is considered to be an efficient red-emitting phosphor for GaN-based light emitting diode (LED).     

Upconversion emission from Tm:Yb and Tm:Er:Yb doped Y2SiO5 powders prepared by combustion synthesis

Tm³⁺:Er³⁺:Yb³⁺ doped Y₂SiO₅ powders were prepared by combustion synthesis with estimated as-prepared weight (wt.) % concentrations of 0.25:0.0:2.0, 0.25:0.5:2.0 and 0.25:1.0:2.0, respectively. Blue (Tm³⁺: ¹G₄ → ³H₆), green (Er³⁺: ⁴S_3/2, ²H_11/2 → ⁴I_15/2) and red (Er³⁺: ⁴F_9/2 → ⁴I_15/2) upconversion (UC) emissions were observed under 975 nm infrared diode laser excitation. The UC process took place via energy transfer from Yb³⁺ to Er³⁺ and Tm³⁺ ions. The CIE chromaticity coordinates of Tm³⁺:Er³⁺:Yb³⁺ doped Y₂SiO₅ powders were investigated as a function of the diode laser power and Er³⁺ concentration.     

White upconverted luminescence of Ho/Yb/Tm tri-doped Gd2Mo3O9 phosphors

Yb³⁺/Tm³⁺/Ho³⁺ tri-doped Gd₂Mo₃O₉ phosphors were synthesized by the high-temperature solid-state method. Under 980 nm near-infrared excitation, the white-light emission can be observed, which is consists of the blue, green, and red UC emissions. The green and red emission at 547 nm and 660 nm originated from the transition of Ho³⁺ (⁵S₂, ⁵F₄ → ⁵I₈ and ⁵F₅ → ⁵I₈) and the blue emission at 475 nm attributed to the transition of Tm³⁺ (⁵G₄ → ⁵H₆). In this experiment, we selected the optimum concentration ratio of the three rare earths for the bright white emission. The Commission internationale de L’Eclairage (CIE) coordinates for the samples were calculated, and chromaticity coordinates were very close to white light regions. We find that the calculated CIE color coordinates of the Yb³⁺/Tm³⁺/Ho³⁺ tri-doped Gd₂Mo₃O₉ phosphors changed with the incident pump power from 400 mW/cm² to 1000 mW/cm². The upconversion luminescence mechanism of the samples was discussed on its spectral. The white light may be proved to be a candidate material for applications in various fields.     

A red-emitting heavy doped phosphor Li6Y(BO3)3:Eu for white light-emitting diodes

A series of Eu³⁺ activated Li₆Y_1−xEu_x(BO₃)₃ (0.05 ? x ? 1) phosphors were synthesized by solid-state reaction method. The structures and photoluminescent properties of the phosphors were investigated at room temperature. The results of XRD patterns indicate that these phosphors are isotypic to the monoclinic Li₆Gd(BO₃)₃. The excitation spectra indicate that these phosphors can be effectively excited by near UV (370-410 nm) light. The red emission from transition ⁵D₀→⁷F₂ is dominant. The emission spectra exhibit strong red performance (CIE chromaticity coordinates: x = 0.65, y = 0.35), which is due to the ⁵D₀−⁷F_J transitions of Eu³⁺ ions. The relationship between the structure and the photoluminescent properties of the phosphors was studied. The concentration quenching occurs at x ≈ 0.85 under near UV excitation. Li₆Y(BO₃)₃:Eu³⁺ has potential application as a phosphor for white light-emitting diodes.     

A novel synthetic route to Y2O3:Tb phosphors by bicontinuous cubic phase process

This study applies a novel approach to prepare the terbium-doped yttrium oxide phosphors (Y₂O₃:Tb³⁺) using the bicontinuous cubic phase (BCP) process. The experimental results show that the prepared precursor powder was amorphous yttrium hydroxide Y_1−xTb_x(OH)₃ with a spherical shape and primary size 30–50 nm. High crystallinity phosphors with body-centered cubic structures were obtained after heat treatment above 700 °C for 4 h. The primary size of the phosphors grew to 100–200 nm, and dense agglomerates with a size below 1 μm were formed during the calcination. The obtained Y₂O₃:Tb³⁺ phosphor had a strong green emitting at 542 nm. The optimum Tb³⁺ concentration was 1 mol% to obtain the highest PL intensity. This study indicates that the calcining temperature of 700 °C needed for high luminescence efficiency in this work is much lower than 1000 °C or above needed for the conventional solid-state method.     

A promising red phosphor MgMoO4:Eu for white light emitting diodes

Motivated by the need for new red phosphors for solid-state lighting applications Eu³⁺-doped MgMoO₄ was prepared by solid-state reaction and its excitation and emission spectra were measured at room temperature. In addition, the effects of firing temperature and Eu³⁺ doping concentration on the PL intensities were also investigated. Compared with Y₂O₂S:0.05Eu³⁺, the obtained Mg_0.80MoO₄:Eu³⁺_0.20 phosphor shows a stronger excitation band near 400 nm and intensely red-emission lines at 616 nm correspond to the forced electric dipole ⁵D₀ → ⁷F₂ transitions on Eu³⁺ under 394 nm light excitation. The CIE chromaticity coordinates (x = 0.651, y = 0.348) of Mg_0.80MoO₄:Eu³⁺_0.20 close to the NTSC (National Television Standard Committee) standard values, and therefore may find application on near UV InGaN chip-based white light emitting diodes.     

NIR-to-visible and NIR-to-NIR upconversion in lanthanide doped nanocrystalline GdOF with trigonal structure

Codoped Er³⁺/Yb³⁺, Tm³⁺/Yb³⁺, Ho³⁺/Yb³⁺ and triply doped Er³⁺/Tm³⁺/Yb³⁺ gadolinium oxyfluoride nanoparticles were prepared in aqueous solution by a simple coprecipitation method and a suitable heat treatment at 500 °C. From the experimental X-Ray powder diffraction patterns, a Rietveld analysis was carried out and it was determined that the nanoparticles are single phase trigonal GdOF. Electron microscopy images show that the average particle size is approximately 25 nm, even though a certain degree of agglomeration is evidenced. The spectroscopic properties of the lanthanide doped nanoparticles are investigated in terms of emission spectra. For proper lanthanide concentrations, the nanoparticles show visible upconversion upon excitation at 980 nm, making them useful as luminescent nanomaterials for photonic applications.     

White light upconversion of nanocrystalline Er/Tm/Yb doped tetragonal Gd4O3F6

Er³⁺, Tm³⁺ and Yb³⁺ codoped gadolinium oxyfluoride nanoparticles were prepared in aqueous solution by a simple coprecipitation method, under alkaline conditions. After a suitable heat treatment at 500 °C, the nanocrystalline powders were found to be single phase tetragonal Gd₄O₃F₆ after a structural characterization using X-ray powder diffraction. Transmission electron microscopy images showed that the average size of the nanoparticles was approximately 50 nm. Following appropriate lanthanide ion doping, the nanocrystals show bright white light upconversion upon excitation at 980 nm using a diode laser as the excitation source.     

Investigation of near-infrared-to-ultraviolet upconversion luminescence of Tm doped NaYF4 phosphors by Yb codoping

2 mol% Tm³⁺ doped NaYF₄ phosphors with 0–98 mol% Yb³⁺ codoping were synthesized by sol–gel method. The phase transition from the mixture of hexagonal and cubic phases to single cubic phase of Tm³⁺–Yb³⁺:NaYF₄ phosphors was investigated with increasing of Yb³⁺ concentration. Near-infrared, red, blue, violet and ultraviolet upconversion emissions of Tm³⁺ were observed from the Tm³⁺–Yb³⁺:NaYF₄ phosphors under 976 nm laser diode excitation, with the strongest near-infrared to ultraviolet emissions at 20 mol% Yb³⁺ codoping. The violet and blue emissions for the ¹D₂ → ³F₄ and ¹G₄ → ³H₆ transitions of Tm³⁺ can be tuned by varying Yb³⁺ codoping concentration, which was elucidated using steady-state equations. The intensity ratio of red emissions for the ³F₂ → ³H₆ and ³F₃→³H₆ transitions of Tm³⁺ was strongly related to the Yb³⁺ codoping concentration and temperature, implying a potential application of Tm³⁺–Yb³⁺:NaYF₄ phosphors for optical temperature sensing.     

Optical characterization of YAl3(BO3)4:Dy3+–Tm3+ phosphors under near UV excitation

Dy³⁺ and Tm³⁺ co-doped YAl₃(BO₃)₄ (YAB) phosphors were prepared by solid-state reaction method at 1200 °C/3 h. The average crystallite size was determined as 52.09 nm from the X-ray diffraction measurements. Upon 352 and 359 nm near ultra violet excitation, the YAB:Dy³⁺–Tm³⁺ phosphors exhibit Dy³⁺:⁴F_9/2 → ⁶H_J (J = 15/2, 13/2, 11/2) and Tm³⁺:¹D₂ → ³F₄ transitions with different luminescence intensity. The photoluminescence emission and decay measurements revealed the energy transfer from Dy³⁺ to Tm³⁺ ions under 359 nm excitation only. The energy transfer between Dy³⁺ and Tm³⁺ takes place in Dy³⁺–Tm³⁺ clusters through exchange interaction mechanism. The Commission International de I’Eclairage chromaticity coordinates of YAB:Tm³⁺ phosphor (λ_ex = 359 nm) were found very close to the European Broadcasting Union and National Television Standard Committee illuminants. The emission color of the studied phosphors could be tuned from blue-to-white as a function of excitation wavelength. The YAB:Dy³⁺–Tm³⁺ phosphors can be used as potential candidates in display technology.     

Bright white upconversion luminescence from Er-Tm-Yb doped CaSnO3 powders

Bright white upconversion luminescence from Er³⁺-Tm³⁺-Yb³⁺ doped CaSnO₃ powders is obtained under the diode laser excitation of 980 nm. It is composed of three primary colors of red, green and blue emissions, which originate from the transitions of ⁴F_9/2 → ⁴I_15/2, (²H_11/2, ⁴S_3/2) → ⁴I_15/2 of Er³⁺ ions and ¹G₄ → ³H₆ of Tm³⁺ ions, respectively. The efficient upconversion emission is attributed to the energy transfer between Yb³⁺ and Er³⁺ or Tm³⁺ions. Moreover, it is observed that Tm³⁺ acts as the quenching center for the green upconversion luminescence from Er³⁺ ions, and the sensitizer for the red and blue luminescence when the Tm³⁺ doping content is less than 0.3 mol%. This is interpreted in terms of the efficient energy transfer between Tm³⁺ and Er³⁺ ions. The calculated color coordinates fall within the white region in the standard 1931 CIE chromaticity diagram, indicating the potential applications of Er³⁺-Tm³⁺-Yb³⁺ doped CaSnO₃ in the field of displaying and lasers, etc.     

Green-emitting Yb-doped α-SiAlON phosphors prepared by spark plasma sintering

Divalent ytterbium-doped Ca-α-SiAlON phosphors were synthesized by a spark plasma sintering (SPS) method without a reducing gas. A single phase Ca-α-SiAlON:Yb²⁺ was successfully achieved at relatively low temperature of 1600 °C. Its photoluminescence excitation spectrum showed a broad band peaking at 450 nm due to the 4f¹³5d¹ → 4f¹⁴ transition of Yb²⁺ ions, while a corresponding green emission band was observed at 545 nm. The post-annealing in a reduced atmosphere significantly improved the luminescent properties by increasing the crystallinity, reducing the carbon contamination, and changing the residual Yb³⁺ to Yb²⁺ in as-synthesized samples. The post-annealed powders, compared to commercial YAG:Ce³⁺ and silicate phosphors, showed a low thermal quenching due to the rigid crystal structure.     

Novel red phosphors Na2CaSiO4:Eu for light-emitting diodes

Novel red phosphors Na₂CaSiO₄:xEu³⁺ were synthesized using high temperature solid-state reaction and their luminescence characteristics were investigated for the first time. The excitation spectra indicate that the Na₂CaSiO₄:xEu³⁺ phosphors can be effectively excited by ultraviolet (393 nm) light. The emission spectra of Na₂CaSiO₄:xEu³⁺ phosphors invariably exhibit four peaks assigned to the ⁵D₀-⁷F_J (J = 1, 2, 3 and 4) transitions of Eu³⁺ under 393 nm excitation. The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates and quantum efficiency (QE) are (0.66, 0.34) and 58.9%, respectively. The good color saturation and high quantum efficiency indicate that Na₂CaSiO₄:Eu³⁺ phosphors are potential candidate for light-emitting diodes.     

Uniform lanthanide-doped Y2O3 hollow microspheres: Controlled synthesis and luminescence properties

Lanthanide-doped uniform pure cubic phase Y₂O₃ hollow microspheres have been successfully synthesized via a facile, high yield urea-based coprecipitation route with assistant of carbon spheres templates. The diameter and shell thickness of the microspheres can be manipulated by adjusting carbon sphere templates. Under a 980 nm excitation, Yb³⁺/Er³⁺, Er³⁺, Yb³⁺/Tm³⁺-doped Y₂O₃ hollow microspheres emit bright upconversion red, green, blue light with high purity, respectively, while Eu³⁺, Eu³⁺/Tb³⁺-doped Y₂O₃ hollow microspheres exhibit intense downconversion red light under the excitation of 254 nm ultraviolet light. Especially, the 610 nm emission intensity of Eu³⁺ in the Eu³⁺/Tb³⁺-codoped Y₂O₃ hollow microspheres is almost 5 times of that in the Y₂O₃:Eu³⁺ hollow microspheres indicating the occurring of the energy transfer from Tb³⁺ to Eu³⁺ ions.     

Growth of Yb-doped Y2O3 single crystal rods by the micro-pulling-down method

The rare-earth sesquioxides (RE₂O₃, RE = Lu, Y and Sc) are very promising host crystals for advanced laser diode (LD)-pumped Yb³⁺-doped solid-state lasers due to unusual combination, almost unique of favourable structural, thermal and spectroscopic properties which are described. In spite of these favourable properties, the bulk single crystal growth technology for the rare-earth sesquioxides has not been established yet. The extremely high melting temperature at around 2400 °C has prevented it. However, we shall show that yttrium oxide crystals (Yb_xY_1−x)₂O₃, x = 0.0, 0.005, 0.05, 0.08 and 0.15 of cylindrical shape as laser rods with 4.2 mm in diameter and 15-20 mm in length have been grown from rhenium crucibles by the micro-pulling-down method. The crystal quality characterisation of undoped Y₂O₃ crystal was determined using X-ray rocking curve (XRC) analysis. Yb were homogeneously distributed in Y₂O₃ host crystal.     

Frequency upconversion fluorescence studies of Er/Yb-codoped KNbO3 phosphors

Different concentrations of Er³⁺ and Yb³⁺ ions-doped potassium niobate (K_0.9NbO₃:Yb_(x)Er_{(0.1 − x)} for x = 0, 0.01, 0.05, 0.09 and 0.1) polycrystalline powder phosphors were prepared by the conventional solid state reaction method and were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Energy transfer and upconversion fluorescence properties of the Yb³⁺ and Er³⁺-codoped phosphors have been discussed. The XRD data has shown mono-phase for pure KNbO₃ while the doped samples represented additional phase formation. The SEM micrographs represented the rectangular crystal growth habit for the KNbO₃ phosphors when doped with 0.1 mol of Er³⁺ ions. An intense green emission at 557 nm along with a red emission at 674 nm was observed when the doped samples were excited with 975 nm IR radiation. The upconversion mechanism has been discussed based on the excited state absorption and energy transfer mechanisms.     

White light generation in Al2O3:Ce:Tb:Mn films deposited by ultrasonic spray pyrolysis

Aluminium oxide (Al₂O₃) films doped with CeCl₃, TbCl₃ and MnCl₂ were deposited at 300 °C with the ultrasonic spray pyrolysis technique. The films were analysed using the X-ray diffraction technique and they exhibited a very broad band without any indication of crystallinity, typical of amorphous materials. Sensitization of Tb³⁺ and Mn²⁺ ions by Ce³⁺ ions gives rise to blue, green and red simultaneous emission when the film activated by such ions is excited with UV radiation. The overall efficiency of such energy transfer results to be about 85% upon excitation at 312 nm. Energy transfer from Ce³⁺ to Tb³⁺ ions through an electric dipole-quadrupole interaction mechanism appears to be more probable than the electric dipole-dipole one. A strong white light emission for the Al₂O₃:Ce³⁺(1.3 at.%):Tb³⁺(0.2 at.%):Mn²⁺(0.3 at.%) film under UV excitation is observed. The high efficiency of energy transfer from Ce³⁺ to Tb³⁺ and Mn²⁺ ions, resulting in cold white light emission (x = 0.30 and y = 0.32 chromaticity coordinates) makes the Ce³⁺, Tb³⁺ and Mn²⁺ triply doped Al₂O₃ film an interesting material for the design of efficient UV pumped phosphors for white light generation.     

Flame synthesis and effects of host materials on Yb/Er co-doped upconversion nanophosphors

The upconversion nanophosphors (UCNPs) of Yb³⁺/Er³⁺ co-doped into Y₂O₃, La₂O₃, and Gd₂O₃ were synthesized via the combustion method and characterized by powder X-ray diffractometer (XRD), scanning electron microscopy (SEM) and upconversion fluorescence spectroscopy. The characterization results showed that at the same flame temperature (2705 K) and precursor concentration (0.1 M), pure monoclinic and cubic-phase phosphors were achieved on Gd₂O₃ and Y₂O₃ hosted UCNPs, respectively; while the mixed phases were observed on La₂O₃ hosted UCNPs. Further annealing process at 850 °C produced pure cubic-phase La₂O₃:Yb³⁺,Er³⁺ UCNPs; while there was no phase transition observed on Gd₂O₃:Yb³⁺,Er³⁺ UCNPs. The dependence of upconversion luminescence on precursor concentrations and host materials was then examined. The La₂O₃ and Gd₂O₃ hosts were shown to be the promising alternates for the commonly used Y₂O₃ hosts for rare-earth doped phosphors.     

Mid-infrared luminescence and energy transfer of Tm/Yb doped fluorophosphate glass

Tm³⁺/Yb³⁺-codoped fluorophosphate glass is prepared by conventional melt-quenching method and the thermal stability of the glass is analyzed. The Judd–Ofelt parameters, radiation emission rates, radiative lifetime and branching ratios of prepared samples are calculated based on the absorption spectra. Upon excitation of a conventional 980 nm laser diode, 1.8 μm emission is obtained from Yb³⁺/Tm³⁺ codoped fluorophosphate. Additionally, the energy transfer between Yb³⁺ and Tm³⁺ ions are quantitatively analyzed. Hence, this Yb³⁺/Tm³⁺ codoped fluorophosphate glass possessing high energy transfer efficiency and excellent thermal stability is a good candidate for efficient 1.8 μm laser.     

Synthesis and spectral analysis of Yb/Tm/Ho-doped Na0.5Gd0.5WO4 phosphor to achieve white upconversion luminescence

Yb³⁺/Tm³⁺/Ho³⁺-doped Na_0.5Gd_0.5WO₄ phosphors were synthesized by the high-temperature solid-state method. Bright white luminescence upon 980 nm near-infrared excitation can be observed for the sample at the optimum chemical composition of Na_0.5Gd_0.5WO₄:10%Yb³⁺/1%Tm³⁺/0.4%Ho³⁺, which is produced via an upconversion (UC) process by tuning the dopant ions concentration. The measured white light consists of the blue, green, and red UC emissions which correspond to the transitions ¹G₄ → ³H₆ of Tm³⁺, ⁵F₄(⁵S₂) → ⁵I₈, and ⁵F₅ → ⁵I₈ of Ho³⁺ ions, respectively. The calculated color coordinates display that white light can be achieved in a wide range of dopant concentrations. The UC mechanisms were also proposed based on their spectral and pumping power dependence analyses.