Self-activated afterglow luminescence of un-doped Ca2ZrSi4O12 material and explorations of new afterglow phosphors in a rare earth element-doped Ca2ZrSi4O12 system

Un-doped Ca₂ZrSi₄O₁₂ material is prepared using a solid state reaction and it emits intense green afterglow luminescence peaking at 490 nm. The afterglow luminescence can be recorded for about 4800 s (0.32 mcd m⁻²). The thermoluminescence revealed that at least four types of traps existed in the Ca₂ZrSi₄O₁₂ material and the depths of these traps were calculated. The contributions of these traps on the afterglow luminescence were also investigated in detail. Accordingly, a possible afterglow mechanism of Ca₂ZrSi₄O₁₂ was proposed. Moreover, Ca₂ZrSi₄O₁₂ was also doped by rare earth or metal ions for developmental purpose and we arrived at some useful conclusions according to the experimental results.     

White-emitting phosphors Ca6La2Na2(PO4)6F2:Dy and luminescence enhancement through Ce → Dy energy transfer

Ce³⁺ and Dy³⁺ activated fluoro-apatite Ca₆La₂Na₂(PO₄)₆F₂ with chemical formulas Ca₆La_2−xLn_xNa₂(PO₄)₆F₂ (Ln = Ce³⁺, Dy³⁺) were prepared by a solid state reaction technique at high temperature. The vacuum-ultraviolet (VUV) and ultraviolet (UV) spectroscopic properties are investigated. The results indicate that Ce³⁺ ions show the lowest 5d excitation band at ∼305 nm and a broad emission band centered at ∼345 nm. Dy³⁺ ions exhibit intense absorption at VUV and UV range. White-emitting under 172 nm excitation is obtained based on two dominant emissions from Dy³⁺ ions centered at 480 and 577 nm. In addition, the energy transfer from Ce³⁺ to Dy³⁺ in the co-doped samples are observed and discussed.     

Near-ultraviolet excitable Ca4Y6(SiO4)6O: Ce,Tb white phosphors for light-emitting diodes

The present investigation aims to demonstrate the potentiality of Tb³⁺ and Ce³⁺ co-doped Ca₄Y₆(SiO₄)₆O phosphors. By incorporation of Ce³⁺ into Ca₄Y₆(SiO₄)₆O: Tb³⁺, the excitation band was extended from short-ultraviolet to near-ultraviolet region. The energy transfer from Ce³⁺ to Tb³⁺ in Ca₄Y₆(SiO₄)₆O host was investigated and demonstrated to be a resonant type via a dipole–dipole mechanism with the critical distance of 10.2 Å. When excited by 352 nm, Ca₄Y₆(SiO₄)₆O: Ce³⁺, Tb³⁺ exhibited a brighter and broader violet-blue emission (421 nm) from the Ce³⁺ and an intense green emission (542 nm) from the Tb³⁺. Combining the two emissions whose intensities were adjusted by changing the doping levels of the co-activator, an optimized white light with chromaticity coordinates of (0.278, 0.353) is generated in Ca₄Y₆(SiO₄)₆O: 2% Ce³⁺, 8% Tb³⁺, and this phosphor could be potentially used in near-ultraviolet light-emitting diodes.     

Photoluminescence properties of emission-tunable Ca9Y(PO4)7: Tm,Dy phosphor for white light emitting diodes

A white-emitting Ca₉Y(PO₄)₇: Tm³⁺, Dy³⁺ phosphor has been successfully prepared by conventional high-temperature solid-state reaction. X-ray diffraction (XRD) and fluorescence spectrophotometer were used to characterize the as-synthesized phosphors. The excitation and emission spectra show that all the Tm³⁺ and Dy³⁺ co-doped Ca₉Y(PO₄)₇ samples can be effectively excited by UV light and then emit blue and yellow light simultaneously. Furthermore, the emission and color coordinate of as-obtained samples pumped by 365 nm are able to be adjusted around white light by varying the doping concentrations of Tm³⁺ and Dy³⁺. So, the as-fabricated single-composition Ca₉Y(PO₄)₇: Tm³⁺, Dy³⁺ phosphor will have a promising application in the area of white light emitting diodes.     

Photoluminescence properties of MgAl2O4:Dy powder phosphor

Trivalent dysprosium (Dy³⁺) activated magnesium alluminate phosphors were synthesized by high temperature solid state reaction method. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FT-IR), photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting phosphors. The results show that the obtained MgAl₂O₄:Dy³⁺ phosphors have good crystallinity, spherical morphology with sizes ranged from 120 to 140 nm and strong blue emission under an excitation of 258 nm. The emission spectrum of this phosphor consists of two emission bands: blue band and yellow band, and the emission intensity of the former is stronger than that of the later. Luminescence quenching is explained and the corresponding luminescence mechanisms have been proposed.     

Persistent luminescence of SrAl2O4: Eu,Dy, Cr phosphors in the tissue transparency window

We report on the persistent luminescence of SrAl₂O₄: Eu²⁺, Cr³⁺ phosphor centered at 760 nm. The phosphor was prepared by sol-gel-combustion method. Persistent luminescence from Cr³⁺ lasted for hundreds of seconds, comparable to the long afterglow from Eu²⁺ ions in the visible region based on the continuous energy transfer from Eu²⁺ ions to Cr³⁺ ions. The introduction of Dy³⁺ ions into the phosphor further prolonged the afterglow time of Eu²⁺ and Cr³⁺ ions through the depth control of the charge traps. The optimum doping concentrations for Eu²⁺, Cr³⁺ and Dy³⁺ were 1%, 2% and 1.5%, respectively.     

Synthesis and optical properties of erbium-doped BaF2 nanoparticles

Erbium-doped BaF₂ nanoparticles were prepared using the ultrasonic surge method. The particles were characterized by X-ray diffraction, transmission electron microscope, photoluminescence and absorption. The results show that when Er³⁺ ions were introduced into the BaF₂ lattice, cubic fluorite-type Ba_1−xEr_xF_2+x solid solution, x = 0.42 was formed. Furthermore, the crystalline phase of Ba_1−xEr_xF_2+x solid solution was largely influenced by the annealing temperature.     

Novel Dy-doped Ca2Gd8(SiO4)6O2 white light phosphors for Hg-free lamps application

The luminescent properties of Ca₂Gd_8(1−x)(SiO₄)₆O₂:xDy³⁺ (1% ≤ x ≤ 5%) powder crystals with oxyapatite structure were investigated under vacuum ultraviolet excitation. In the excitation spectrum, the peaks at 166 nm and 191 nm of the vacuum ultraviolet region can be assigned to the O²⁻ → Gd³⁺, and O²⁻ → Dy³⁺ charge transfer band respectively, which is consistent with the theoretical calculated value using Jφrgensen's empirical formula. While the peaks at 183 nm and 289 nm are attributed to the f-d spin-allowed transitions and the f-d spin-forbidden transitions of Dy³⁺ in the host lattice with Dorenbos's expression. According to the emission spectra, all the samples exhibited excellent white emission under 172 nm excitation and the best calculated chromaticity coordinate was 0.335, 0.338, which indicates that the Ca₂Gd₈(SiO₄)₆O₂:Dy³⁺ phosphor could be considered as a potential candidate for Hg-free lamps application.     

Synthesis and luminescent properties of ZnNb2O6 nanocrystals for solar cell

The phase formation, morphology and luminescent properties of ZnNb₂O₆ nanocrystals by the sol-gel method were investigated at a lower temperature than that of the traditional solid-state reaction method. The products were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), photoluminescence spectroscopy (PL) and absorption spectra. The activation energy of ZnNb₂O₆ grain growth is obtained about 18.4 kJ/mol. The diameters of the nanocrystals are in the range of 20-40 nm. The PL spectra excited at 276 nm have a broad and strong blue emission band maximum at 450 nm, corresponding to the self-activated luminescence of the niobate octahedra group [NbO₆]⁷⁻. The optical absorption spectrum of the sample at a calcination temperature of 800 °C has a band gap energy of 3.68 eV.     

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.     

The effect of Eu-activated InVO4 phosphors prepared by sol-gel method

Indium orthovanadate (InVO₄) doped with Eu³⁺ ions had been synthesized by sol-gel method. The precursor of InVO₄:Eu³⁺ powders were heated at 950 °C for 6 h in air, and the crystal structure, surface morphologies and photoluminescence properties were also investigated. XRD patterns indicated that the crystallinity of InVO₄:Eu³⁺ powders decreases with increasing Eu³⁺ ion concentrations. From the SEM micrographs, the shapes of the InVO₄ particles are uniform and like pebbles. With increasing Eu³⁺ ion concentrations, the shapes of the InVO₄:Eu³⁺ particles become smaller and irregular. In the PL studies, the sharp excitation peaks between 300 and 600 nm correspond to the Eu³⁺ intra-4f transitions. Excitation at 326 nm in terms of Eu³⁺ concentrations in (In_1−xEu_x)VO₄ powders shows that the (In_1−xEu_x)VO₄ phosphors display bright red luminescence at about 615 nm belonging to the ⁵D₀ → ⁷F₂ electric dipole transition, and the time-resolved ⁵D₀ → ⁷F₂ transition presents a single exponential decay behavior. The concentration quenching is active when the Eu³⁺ concentration is larger than 30 mol%, and the critical distance is about 8.024 Å.     

Synthesis and VUV luminescent properties of LaPO4:Eu nanowires

A series of different concentrations of LaPO₄:Eu³⁺ phosphors with the morphology of nanowires were successfully prepared by a precipitation method. It was observed that the integrated intensity of ⁵D₀ → ⁷F₂ transition of Eu³⁺ was higher than its ⁵D₀ → ⁷F₁ transition when the sintering temperature was at 600 °C. The most interesting phenomenon was that the doping concentrations of LaPO₄:Eu³⁺ nanowires were changed in a large range (5–15%), the photoluminescence intensities were maintained in a certain value. This phenomenon was not observed in the bulk LaPO₄:Eu³⁺ phosphors. A detailed explanation to this phenomenon was given.     

Synthesis and upconversion luminescence properties of Yb/Er codoped BaGd2(MoO4)4 powder

Synthesis and upconversion luminescence properties of the new BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor were reported in this paper. The phosphor powder was obtained by the traditional high temperature solid-state method, and its phase structure was characterized by the XRD pattern. Based on the upconversion luminescence properties studies, it is found that, under 980 nm semiconductor laser excitation, BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor exhibits intense green upconversion luminescence, which is ascribed to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺. While the observed much weaker red emission is due to the non-radiative relaxation process of ⁴S_3/2 → ⁴F_9/2 and ⁴F_9/2 → ⁴I_15/2 transition originating from the same Er³⁺. The concentration quenching effects for both Yb³⁺ and Er³⁺ were found, and the optimum doping concentrations of 0.5 mol% Yb³⁺ and 0.08 mol% Er³⁺ in the new BaGd₂(MoO₄)₄ Gd³⁺ host were established.     

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.     

Sol-gel synthesis and green upconversion luminescence in BaGd2(MoO4)4:Yb,Er phosphors

Yb³⁺/Er³⁺ codoped BaGd₂(MoO₄)₄ phosphor powders were prepared by the Sol-gel method and the upconversion luminescence properties were investigated in detail. Under 980 nm semiconductor laser excitation, BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor exhibits green upconversion luminescence with peaks at 530 and 550 nm, which are due to the transitions of Er³⁺ (²H_11/2) → Er³⁺ (⁴I_15/2) and Er³⁺ (⁴S_3/2) → Er³⁺ (⁴I_15/2), respectively. Both of the two green emission lines are produced by populating Er³⁺ ions to the excited state through a two-photon process. By monitoring the intensities of the green upconversion luminescence, the optimum conditions for the Sol-gel synthesis were determined when the molar ratio of citric acid to total chelate metal cations was 2:1 and the sintering temperature was at 1073 K. The concentration quenching effect for Er³⁺ was found at the optimum doping concentration of 6 mol%, and the critical distance for the neighboring Er³⁺ was determined to be about 21.5 Å.     

Synthesis and luminescent properties of LaPO4:Eu microspheres

LaPO₄:Eu³⁺ microspheres were synthesized, using LaCl₃, EuCl₃ and (NH₄)₂HPO₄ as starting materials. The morphology, formation mechanism, and luminescent property of samples were systemically studied. X-ray diffraction (XRD) and infrared spectroscopy (IR) show that LaPO₄:Eu³⁺ microspheres have a pure monoclinic phase. Cetyltrimethyl ammonium bromide (CTAB) usually forms spherical micelles above a critical micelle concentration, which plays an important role in the formation of LaPO₄:Eu³⁺ microspheres. The excitation spectrum of LaPO₄:Eu³⁺ microspheres consists of several sharp lines due to the direct excitation of the Eu³⁺ cations from the ground state to higher levels of the 4f-manifold. The emission intensity of microspheres is higher than irregular particles because of the lowlier surface area. The lifetimes of Eu³⁺ ions in the LaPO₄:Eu³⁺ microspheres are determined to be 2.41 ms.     

Single crystalline Co3O4: Synthesis and optical properties

Crystals of Co₃O₄ have been prepared from thermal decomposition of molecular precursors derived from salicylic acid and cobalt (II) acetate or chloride at 500 °C. A cubic phase Co₃O₄ micro- and nanocrystals have been obtained. The as-synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The images of electron microscopes showed octahedral crystals of Co₃O₄. The volume and polarizability of the optimized structures of molecular precursors have been calculated and related to the particle size. The optical band gap of the obtained crystals has been measured. The results indicated two optical band gaps with values 2.65 and 2.95 eV for (E_g1) (E_g2), respectively.     

Luminescent properties of long lasting phosphor Ca2MgSi2O7:Eu

Long afterglow phosphors (Ca_1−xEu_x)₂MgSi₂O₇ (0.002 ≤ x ≤ 0.02) were prepared by solid-state reactions under a weak reductive atmosphere. X-ray diffraction pattern, photoluminescence spectra, decay curve, afterglow spectra and thermoluminescence curves were investigated. The phosphors showed two emission peaks when they were excited by 343 nm, due to two types of Eu²⁺ centers existing in the Ca₂MgSi₂O₇ lattice. However, only one emission peak can be found in their afterglow spectra. Energy transfer between Eu²⁺ ions in inequivalent sites was found. A possible mechanism was presented and discussed. The afterglow decay time of Ca_1.998MgSi₂O₇:Eu_0.002 was nearly 12.5 h which means it was a good long lasting phosphor.     

Luminescence properties of Ce and/or Mn activated Ca10K(PO4)7 under ultraviolet and vacuum ultraviolet excitation

New Ce³⁺ and/or Mn²⁺ activated Ca₁₀K(PO₄)₇ phosphors were prepared by solid-state reaction, and their photoluminescence properties upon ultraviolet and vacuum ultraviolet excitation were investigated. Under 254 nm excitation, a series of Ca₁₀K(PO₄)₇:xMn²⁺ samples exhibit two emission bands at 463 and 650 nm, which could be attributed to oxygen defects and ⁴T₁–⁶A₁ transition of Mn²⁺, respectively. And an energy transfer from defects to Mn²⁺ has been observed. With the Mn²⁺ content increased, the emitting hues of Ca₁₀K(PO₄)₇:Mn²⁺ can range from blue to red. By co-doping Ce³⁺ to Ca₁₀K(PO₄)₇:Mn²⁺, the emission intensity of Mn²⁺ is strongly enhanced due to an efficient energy transfer by [Ce³⁺ → Mn²⁺] and [defects → Ce³⁺ → Mn²⁺]. But under 147 nm excitation, the emission intensity of Mn²⁺ in Ca₁₀K(PO₄)₇:0.25Mn²⁺ decreases slightly compared with that in Ca₁₀K(PO₄)₇:025Mn²⁺, 0.1Ce³⁺, 0.1K⁺ due to the host sensitization competition between Ce³⁺ and Mn²⁺.     

NIR to VIS frequency upconversion luminescence properties of Er-doped YPO4 phosphors

Different concentrations of Er³⁺-doped YPO₄:Er powder phosphors have been synthesized by the conventional solid state reaction method and are characterized by X-ray diffraction (XRD), field emission scanning electronic microscopy (FESEM), and upconversion emission measurements. An intense red emission band and a weak green emission band are observed under NIR excitation at 975 nm in case of samples with high dopant concentration while no upconversion emission was observed at lower Er³⁺ ion concentrations. The possible mechanisms involved in the upconversion process have been discussed in comparison to results with similar reported works.