Self-activated Phosphor Research Articles

Experimental and first principle investigation the electronic and optical properties of YNbO4 and LuNbO4 phosphors

LnNbO4 (Ln = Y, Gd and Lu) is a type of material that can be used as both outstanding self-activated phosphor and laser host has generated significant interests. However, their electronic structures and optical properties still require elucidation. To solve this intriguing problem, firstly, YNbO4 and LuNbO4 polycrystalline with monoclinic crystal structure were prepared by traditional high-temperature solid-state reaction method and their structure and luminescent properties were characterized in experiment. Additionally, their CIE chromaticity coordinate were presented to illustrate the luminescent color and the chromaticity coordinates for YNbO4 and LuNbO4 are x = 0.1581, y = 0.0725 and x = 0.1581, y = 0.0753, respectively. Secondly, systematic theoretical calculations based on density functional theory method were carried out on them and their band structures, density of states as well as optical properties were obtained in theoretical. The calculated band gap for YNbO4 and LuNbO4 are 4.101 and 4.214 eV, respectively. Lastly, a comparison between the experimental and calculated band gap and refractive index was conducted. The good coincident between them indicating the calculated results are reliable. Therefore, all of the obtained results could provide an essential understanding of LnNbO4 materials.

Broadband near-infrared downconversion luminescence in Yb3+-doped BaZn2(BO3)2

BaZn2(BO3)2 self-activated phosphors were prepared by the conventional high temperature solid-state method. The PL spectra of BaZn2(BO3)2 powders prepared under reductive and air atmosphere consist of a weak ultraviolet emission band (∼410 nm) and a broad emission band which were centered at ∼ 500 and 545 nm, respectively. According to the spectral analysis and EPR results, the green and yellow emissions may arise from the transitions of photo-generated electron close to the conduction band to the deeply trapped hole in single ionized oxygen vacancy (V+ o) centers and single negatively charged interstitial oxygen ion (O- i), respectively. An efficient broadband near-infrared (NIR) quantum cutting was demonstrated in Yb3+ doped BaZn2(BO3)2 phosphor. Upon excitation with an ultraviolet photon at 375 nm, the emissions of two NIR photons at 983 nm from Yb3+ ions were achieved. The dependences of the visible and NIR emissions, the decay lifetime, the energy transfer efficiency, and the quantum efficiency on the Yb3+ doping content were investigated in detail. The results indicated that the maximum energy transfer and the corresponding downconversion quantum efficiency could reach between 68.5% and 168.5%.

Experiment and density functional theory analyses of GdTaO4 single crystal

GdTaO4 is a type of excellent materials that can be used as scintillation, laser matrix as well as self-activated phosphor has generated significant interest. Whereas its band structure, electronic structure and optical properties are still need elucidation. To solve this intriguing problem, high-quality GdTaO4 single crystal (M-type) was grown successfully using Czochralski method. Its structure as well as optical properties was determined in experiment. Moreover, a systematic theoretical calculation based on the density function theory methods were performed on M-type and M′-type GdTaO4 and their band structure, density of state as well as optical properties were obtained. Combine with the performed experiment results, the calculated results were proved with high reliability. Hence, the calculated results obtained in this work could provide a deep understanding of GdTaO4 material, which also useful for the further investigation on GdTaO4 material.

Modified optical properties via induced cation disorder in self-activated NaMg2V3O10.

Cation disorder in the phosphor lattice could be one of the effective approaches to modify the luminescence efficiency. In this work, cation substitutions of (Mo6+→ V5+) and (Na+→ Mg2+) were conducted in the self-activated NaMg2V3O10. All the samples of Na1+xMg2-xV3-xMoxO10 (x = 0, 0.01, 0.05, 0.1, 0.2, 0.3) were prepared via solid-state reaction. The morphological properties were measured via SEM and EDS analyses. Structural Rietveld refinement was performed to investigate the microstructure in the lattices. The cation substitution brings about structural disorder in the phosphor, which exerts great modifications in the luminescence properties. NaMg2V3O10 presents an intrinsic indirect transition with a band gap of 3.22 eV. The incorporation of Mo6+ and Na+ in the lattices moves the optical absorption to a longer wavelength bringing about a narrower band gap. The luminescence intensity, thermal stability and corresponding lifetime were modified by the cation disorder in the self-activated phosphor.

Synthesis and photoluminescence properties of yellow-emitting Ca5(Zn1-xMgx)4(VO4)6 self-activated phosphors

Novel yellow-emitting Ca5(Zn1-xMgx)4(VO4)6 self-activated phosphors were synthesized via a sol-gel method. The X-ray diffraction showed their typical cubic garnet structure. The photoluminescence excitation spectra of Ca5(Zn1-xMgx)4(VO4)6 phosphors exhibited a broadband ranging from 250 to 430nm, which was originated from the [VO4]3− group charge transfer transition. For the emission spectra, Ca5Zn4(VO4)6 and Ca5Mg4(VO4) phosphors exhibited the characteristic broadband emissions between 420 and 750nm with highest intensities at 538 and 533nm, respectively. The calculated CIE coordinates were (0.331, 0.476) and (0.318, 0.463) for Ca5Zn4(VO4)6 and Ca5Mg4(VO4)6 yellow phosphors, respectively. The results hint that these luminescent powders may be good candidates for their potential use in the fabrication of rare-earth elements free WLEDs.

Synthesis and photoluminescence properties of multicolor tunable GdNbO4: Tb3+, Eu3+ phosphors based on energy transfer

A color-tunable phosphor based on Tb[Formula: see text]/Eu[Formula: see text] co-doped GdNbO4 were synthesized by a traditional solid-state reaction method. X-ray powder diffraction (XRD), diffuse reflectance spectra, photoluminescence spectra and decay curves were utilized to characterize the as-prepared phosphors. XRD result indicated that various concentrations Tb[Formula: see text]/Eu[Formula: see text] single-doped and co-doped phosphors were well indexed to the pure GdNbO4 phase. The GdNbO4 host was proved to be a self-activated phosphor with broad absorption range from 200 nm to 325 nm. When Tb[Formula: see text] ions were added into the host lattice, the energy transferring from host to Tb[Formula: see text] was identified. And the broad absorption in the UV region was changed and enhanced. Therefore, we selected Tb[Formula: see text] as the sensitizer ion, and adjusted red component from Eu[Formula: see text] to control the emission color. The energy transfer from Tb[Formula: see text] to Eu[Formula: see text] was confirmed based on the luminescence spectra and decay curves. Furthermore, the energy transmission mechanism was deduced to be the dipole–quadrupole interaction. On the whole, the obtained GdNbO4, GdNbO4:Tb[Formula: see text], and GdNbO4:Tb[Formula: see text], Eu[Formula: see text] phosphors may have potential application in the UV white-light-emitting diodes (w-LEDs) and display devices.

Multicolor Tuning in Room-Temperature Self-Activated Ca2 Nb2 O7 Submicroplates by Lanthanide Doping.

Self-activated phosphors are capable of generating optical emissions from the internal ion groups of host lattice before externally introducing luminescent ions. However, numerous self-activated phosphors only show luminescence at low temperature due to the thermally activated energy migration among ion groups at room temperature, severely confining their application conditions. In this letter, we propose a strategy to converting the low-temperature luminescence to a room-temperature one through changing the synthesis conditions to induce structural distortions and thus to limit energy migration. Room-temperature self-activated luminescence of Ca2 Nb2 O7 was accordingly achieved in submicroplates synthesized using the sol-gel method. By further coupling the blue broadband emission from Ca2 Nb2 O7 submicroplates with the characteristic luminescence of Ln3+ (Pr3+ , Sm3+ , and Dy3+ ) dopants, multicolor emissions were successively tuned through adjusting the concentration of Ln3+ . Our results are expected to expand the scope of designing room-temperature self-activated phosphors and tuning multicolor emission.

Crystal structure and luminescence properties of self-activated phosphor CsDyP2O7

A new phosphor CsDyP2O7 has been prepared using a high temperature flux method and structurally characterized by single crystal X-ray diffraction analyses. Its structure can be characterized as a three-dimensional anionic framework of [DyP2O7]∞ that is constructed by PO4 tetrahedra and DyO6 octahedra, which delimited an 1D infinite channel running along the c-axis. Compound CsDyP2O7 was characterized by X-Ray diffraction (XRD), UV–vis absorption spectrum and Photoluminescence (PL) analyses. The excitation spectrum covers a wide range from 310 to 410nm, which suggests that the CsDyP2O7 phosphor can be effectively excited by a near-UV light source. The emission of CsDyP2O7 (348nm excitation) was observed at 484nm (blue) and 574nm (yellow) due to the 4F9/2→6H15/2 transitions and 4F9/2→6H13/2 transitions, respectively. The CIE chromaticity color coordinates (x=0.2882, y=0.3105) were found to be in the white light region. We may expect that self-activated phosphor CsDyP2O7 is a new candidate for pure white-light applications.

Photoluminescence enhancement of self-activated vanadate NaMg4(VO4)3 by cation substitutions

A novel self-activated phosphor Na1−xCsxMg4–4xCa4x(VO4)3 (x=0–0.05) was prepared by a solid-state reaction. The phase structure was verified by Rietveld refinements. Luminescence characteristics such as photoluminescence excitation (PLE) and emission (PL) spectra, decay curves and thermal quenching were measured. The phosphor can be efficiently excited by near UV-light, and exhibits a broad emission, which is close to white color region. The detailed luminescence quantum and thermal stability were investigated on the Cs/Ca substitution for Na/Mg in the NaMg4(VO4)3 lattices. The experiments concluded that the big cation substitution for the smaller one in the lattices enhanced the luminescence efficiency and thermal stability of the self-activated vanadate.

Luminescence Properties of Self-Activated Mm(VO4)2 (M = Mg, Ca, Sr, and Ba) Phosphors Synthesized by Solid-State Reaction Method.

In this paper, M3(VO4)2 (M = Mg, Ca, Sr, and Ba) self-activated phosphors were prepared by a solid-state reaction method at 1,000 °C for 5 h. The phase formation and micrographs were analyzed by X-ray diffraction and scanning electron microscopy. The Ca3(VO4)2 phosphor does not show any emission peaks under excitation with ultraviolet (UV) light. However, the M3(VO4)2 (M = Mg, Sr, and Ba) samples are effectively excited by UV light chips ranging from 200 nm to 400 nm and exhibit broad emission bands due to the charge transfer from the oxygen 2p orbital to the vacant 3d orbital of the vanadium in the VO4. The color of these phosphors changes from yellow to light blue via blue-green with increasing ionic radius from Mg to Sr to Ba. The luminescence lifetimes and quantum yield decrease with the increasing unit cell volume and V-V distance, in the order of Mg3(VO4)2 to Sr3(VO4)2 to Ba3(VO4)2. The emission intensity decreases with the increase of temperatures, but presents no color shift. This confirms that these self-activated M3(VO4)2 phosphors can be suggested as candidates of the single-phase phosphors for light using UV light emitting diodes (LEDs).

Preparation, structure and luminescence properties of Ba2V2O7 microrods

Self-activated phosphor Ba2V2O7 microrods were prepared by the hydrothermal reaction method.

Photoluminescence properties of La3+ ion-doped YInGe2O7 self-activated phosphor

A color-tunable La3+ ion-doped YInGe2O7 self-activated phosphor has been synthesized using a high energy vibrating milled solid state reaction with metal oxides and calcined at 1200°C for 10h in air. The structure and optical properties of (Y1−xLax)InGe2O7 phosphors have been investigated. XRD results show that all the diffraction peaks keep the monoclinic YInGe2O7 phase when the La3+ ion concentration is increased up to 3mol%. The UV–visible absorption spectra analysis indicated that the intensities of absorption band at 280–450nm corresponding to the oxygen deficient center of the GeO4 anion were increased and then decreased with increasing the La3+ ion concentrations. Under an excitation of 263nm, the emission intensities of 487 and 545nm corresponding to the single ionized Vo defect emission are increased and then decreased as the La3+ ion concentration is increased. The maximum intensity for these two emission peaks was observed when the La3+ ion concentration is 0.3mol%. This is due to the lattice distortion and induced an increase in the degree of tensile strain, which causes the amount of oxygen vacancies to increase when the Y3+ ion was substituted by the La3+ ion. In addition, the CIE color coordinates changed from a blue→greenish→bluish region as the La3+ ion concentration increased from 0 to 3mol%.

Near-infrared down-conversion and energy transfer mechanism in Yb(3+)-doped Ba2LaV3O11 phosphors.

Yb(3+)-doped Ba2LaV3O11 vanadate phosphors with near-infrared (NIR) emission are synthesized via the sol-gel method. The phase purity and structure of samples are characterized by X-ray diffraction (XRD). The electronic structure of the self-activated phosphor host Ba2LaV3O11 is estimated by density functional theory (DFT) calculation, and the host absorption is mainly ascribed to the charge transition from the O-2p to V-3d states. Photoluminescence emission (PL) and excitation (PLE) spectra, decay curves, absorption spectra and theoretical quantum yields of samples are also investigated. Results indicate that Ba2LaV3O11:Yb(3+) phosphors have strong broad band absorption and efficient NIR emission, which matches well with the spectral response of the Si-based solar cells. The energy transfer processes from [VO4](3-) to Yb(3+) and possible transfer models are proposed based on the concentration of Yb(3+) ions. Results demonstrate that Ba2LaV3O11:Yb(3+) phosphors might act as a promising NIR DC solar spectral converter to enhance the efficiency of the silicon solar cells by utilizing broad band absorption of the solar spectrum.

Systematic Tuning of the Luminescent Properties of Self-Activated ZnGa2O4 Phosphors by Cd Ion Substitution

The effect of host compositions on the luminescent properties of spinel-type ZnGa2O4 phosphors was investigated by systematic substitution of Cd2+ for Zn2+ in the host lattice. A continuous solid solution with the composition (Zn1−xCdx)Ga2O4 (0≤x≤1.0) was synthesized at 950°C as self-activated phosphors, as indicated by X-ray diffraction (XRD) analysis. With increasing substitution of Cd2+ the emission and excitation wavelengths (λem and λexc) of self-activated (Zn1−xCdx)Ga2O4 were observed to exhibit a red shift, as indicated by photoluminescence spectral analysis. Similar results were also confirmed by cathodoluminescence data. Our observations are attributed to the systematic narrowing of the energy gap of the host attributed to expanding lattice dimensions induced by Cd2+ substitution. A CIE chromaticity diagram manifests the effect of Cd2+ doping on variation of hue for the (Zn1−xCdx)Ga2O4 gallate phosphors.

The luminescence properties of AlN with manganese and rare earth activators under ultraviolet and cathode-ray excitation

The emission spectra of self-, Mn-, Eu- and Sm-activated AlN luminophores under ultraviolet and cathode ray excitation as well as the excitation spectra for Mn and self-activated phosphors are measured. Basic information about the kinetics, excitation density dependence and energy transfer mechanism is given. The possibility of the direct excitation mechanism of Eu centres is discussed.

Cryoluminescence of ZnS Phosphors

Cooling previously excited ZnS phosphors to temperatures of \ensuremath{\sim}50\ifmmode^\circ\else\textdegree\fi{}K results in an increase of luminescence when the material is exposed to infrared irradiation (from 0.7 to 4 \ensuremath{\mu}). This effect can be explained by assuming that the radiation originates from discrete emission levels close to the conduction band and that the lifetime of electrons in these levels increases with decreasing temperature. This effect is seen with self-activated, Cu-green, Cu-blue, and Mn-activated phosphors. The energy gap between the emission levels and the conduction band for the Cu-green material was found to be \ensuremath{\sim}${10}^{\ensuremath{-}2}$ eV.

Photoelectric polarization of ZnS and CdS self-activated phosphors

Photoelectric polarization of ZnS and CdS self-activated phosphors