A study on microstructure and luminescent properties of oxyfluoride silicate glass-ceramics with (Ho,Yb):NaYF4 crystallites

Glass-ceramics containing NaYF₄ nanocrystals were prepared by heat-treatment from oxyfluoride silicate-based glass doped with Ho³⁺ and Yb³⁺ ions. The formation of crystalline fluoride phase was confirmed by X-ray diffraction and transmission electron microscopy. Absorption and emission spectra revealed that a fraction of Ho³⁺ and Yb³⁺ ions is incorporated into the NaYF₄ ordered lattice influencing spectroscopic features of glass-ceramics in comparison with those of precursor glass. Green up-conversion emission (545 nm) originating in the ⁵S₂ level in glass-ceramics and up-converted red emission (650 nm) originating in the ⁵F₅ level in as-melted glass were observed under optical pumping into Yb³⁺ absorption band and analyzed. Although both emissions in both materials are achieved by two-photon excitations, the relation between green and red emission intensity in glass-ceramics and glass implies that processes relevant to up-conversion phenomena are different. Based on a careful analysis of relaxation dynamics of Ho³⁺ and Yb³⁺ excited states, the mechanisms involved in conversion of the infrared radiation into the visible emission in these materials are proposed and discussed.     

Broadband UV excitable near-infrared downconversion luminescence in Eu/Yb:CaF2 nanocrystals embedded glass ceramics

Cooperative downconversion was realized in glass ceramics containing Eu²⁺/Yb³⁺:CaF₂ nanocrystals with Eu²⁺ greatly absorbing ultraviolet photons. Upon excitation of Eu²⁺ ions to the 5d level with an ultraviolet photon at 320 nm, emission of two near infrared photons at 976 nm of Yb³⁺ were achieved. The dependence of the visible and near-infrared emissions, decay lifetime, and quantum efficiency on the Yb³⁺ doping content has been investigated. The maximum energy transfer efficiency and the corresponding downconversion quantum efficiency were estimated to be 51% and 151%, respectively.     

Growth of Mn, Co and Ni-doped near-stoichiometric LiNbO3 by Bridgman method using K2O flux

The near-stoichiometric LiNbO₃ (SLN) single crystals doped Mn²⁺, Co²⁺ and Ni²⁺ in 0.5 mol% concentration in the raw compositions were grown by the Bridgman method under the conditions of taking K₂O as flux, a high temperature gradient (90–100 °C/cm) for solid–liquid interface. The XRD, absorption spectra, excitation spectra and emission spectra have been carried out. From the absorption edges of Mn²⁺, Co²⁺ and Ni²⁺-doped SLN crystals, the molar ratio of [Li⁺]/[Nb⁵⁺] are estimated to be about 0.977. The absorption spectra of Mn²⁺:SLN have shown a broad absorption band centered at 571 nm (⁶A_1g(⁶S) → ⁴T_1g(⁴G)), three absorption peaks at 520, 549 and 612 nm (overlapping of the ⁴T₁(F)–⁴A₂(F), ⁴T₁(F)–⁴T₁(P)), and a wide absorption band at 1400 nm (⁴T₁(F) → ⁴T₂(F)) of Co²⁺:SLN, Ni²⁺:SLN, and five absorption peaks at 381 nm (³A_2g(F) → ³T_1g(P)), 733 nm (³A_2g(F) → ³T_1g(F)), 1280 nm (³A_2g(F) → ³T_2g(F)), 430 nm (³A_2g(F) → ¹T_2g(D)), and 840 nm (³A_2g(F) → ¹E(D)) of Ni²⁺:SLN were observed. A red emission at 612 nm (⁴T_1g(⁴G) → ⁶A_1g(⁶S)) for Mn²⁺:SLN, a red emission at 775 nm (⁴T₁(P) → ⁴T₁(F)) for Co²⁺:SLN, and a green emission at 577 nm (¹T_2g(D) → ³A_2g(F)) and a red emission at 820 nm (¹T_2g(D) → ³T_2g(F)) for Ni²⁺:SLN were observed under excited by 416, 520 and 550 nm lights, respectively. The concentration distribution of Mn²⁺, Co²⁺and Ni²⁺ ion in SLN crystals was investigated primarily from the absorption and emission spectra for various parts. The effective distribution coefficient for Mn²⁺ was less than 1. While, for Co²⁺ and Ni²⁺ were more than 1.     

Upconversion luminescence in Er/Yb codoped Y2CaGe4O12

Calcium yttrium tetrametagermanates Y₂CaGe₄O₁₂ doped with Er³⁺ and Er³⁺/Yb³⁺ reveal upconversion emission in visible spectral range under near-infrared excitation, λ_ex = 980 nm. For the solid solution Er_xY_2−xCaGe₄O₁₂ concentration dependencies for the green and red lines of the visible emission around 526 nm (²H_11/2 → ⁴I_15/2), 545 nm (⁴S_3/2 → ⁴I_15/2) and 670 nm (⁴F_9/2 → ⁴I_15/2) show the optimal value for the sample x = 0.2. The power dependence of the visible luminescence measured at room temperature in the low-power limit indicates two-photon upconversion process. Direct intensification of the upconversion emission signals has been achieved by ytterbium sensitizing. The other upconversion excitation mechanism in Y₂CaGe₄O₁₂:Er³⁺ is discussed for an 808 nm incident laser irradiation. A scheme of excitation and emission routes involving ground/excited state absorption, energy transfer upconversion, nonradiative multiphonon relaxation processes in trivalent lanthanide ions in Y₂CaGe₄O₁₂:Er³⁺ and Y₂CaGe₄O₁₂:Er³⁺, Yb³⁺ has been proposed. Conditions for visible emission occurrence under quasi-resonance λ_ex = 1064 nm excitation depending on pump power values are considered. In the low-power regime only near-infrared emission caused by the transition ⁴I_13/2 → ⁴I_15/2 in erbium ions has been detected.     

Effect of Yb concentration on the structures and upconversion luminescence properties of Y2O3:Er ultrafine phosphors

Y₂O₃:Er³⁺ ultrafine phosphors with a varying Yb³⁺ ion concentration were prepared by a urea homogeneous precipitation method. The results of XRD show that all the samples are of a pure cubic structure and the average crystallite sizes can be calculated as 45, 34, and 28 nm for Y₂O₃:Er³⁺ ultrafine phosphors with Yb³⁺ ion concentrations of 0, 10%, and 20%, respectively. The lattice constant and cell volume of the ultrafine phosphors decrease with enhancing Yb³⁺ ion concentration. The upconversion luminescence spectra of all the samples were studied under 980 nm laser excitation. The strong green and red upconversion emission were observed, and attributed to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, respectively. The intensity of red emission increases with increasing Yb³⁺ ion concentration. The effect of Yb³⁺ ion concentration on the structures and upconversion luminescence mechanism were discussed.     

Preparation, structure and photoluminescence of nanoscaled-Nd: Lu3Al5O12

Nd:Lu₃Al₅O₁₂ (Nd:LuAG) nano-crystalline was synthesized by co-precipitation method. Its phase transformation, structure, absorption and photoluminescence properties were studied. The Nd:LuAG polycrystalline phase is formed above 900 °C and its particle sizes are in the range of 18-36 nm. The structure of Nd:LuAG was refined by Rietveld method. The lattice parameters and the distortion of Lu³⁺-O²⁻ polyhedron in Nd:LuAG are larger than that of in pure LuAG. Because the distortion of Lu³⁺-O²⁻ polyhedron is larger than that of Y³⁺-O²⁻ polyhedron in YAG and the distance of Lu³⁺-O²⁻ is smaller than that of Y³⁺-O²⁻ in YAG, Nd³⁺ in LuAG experiences a stronger crystal field effect, which is proved by the crystal field strength and the chemical environment parameter. The absorption spectrum shows that Nd:LuAG has a broad absorption band at 808 nm with FWHM above 6 nm, which is favorable for improving laser efficiency. The fluorescence lifetime from ⁴F_3/2 → ⁴I_11/2 transition is 320 μs and longer than that of Nd:YAG. The longer lifetime is propitious to energy storage. The emission cross section at 1064 nm is 2.89 × 10⁻¹⁹ cm², taking into account the Boltzmann distribution of the excited state. The emission cross section in Nd:LuAG is also larger than that of Nd:YAG, which is useful for laser operation. All results indicate that Nd:LuAG is a promising crystal material to apply in high energy lasers.     

Efficient green up-conversion emission in Yb3+/Ho3+ co-doped CaIn2O4

Intense green up-conversion (UC) emissions from ⁵F₄/⁵S₂ → ⁵I₈ transitions of Ho³⁺ in Yb³⁺/Ho³⁺ co-doped CaIn₂O₄ have been observed with excitation at 980 nm. The optimal processing parameters were determined by investigating emission intensities as a function of annealing temperature, duration time and Ho³⁺/Yb³⁺ dopant concentrations. It has been confirmed that the green UC luminescence was generated via a two-photon process from the quadratic dependence of the emission intensity on the pump power. A UC mechanism was proposed and the lifetime of the green emission was measured to be ～204 μs. The infrared-to-visible UC efficiency of the optimized CaIn₂O₄: 0.005Ho³⁺, 0.1Yb³⁺ sample increases to ～5.5% with the excitation power and saturates at 1.5 W. The chromaticity coordinates (0.281, 0.708) of the samples are located in the green region and hardly changed due to the negligible red emission. The results indicate that CaIn₂O₄: Yb³⁺, Ho³⁺ could act as an effective UC green light emitter and CaIn₂O₄ is an ideal oxide host for UC luminescence.     

Efficient first-order resonant near-infrared quantum cutting in β-NaYF4:Ho,Yb

We demonstrated an efficient two-photon near-infrared (NIR) quantum cutting (QC) in Ho³⁺-Yb³⁺ co-doped hexagonal β-NaYF₄, which could efficiently convert an incident high-energy photon in the wavelength region of 300-550 nm into two NIR photons. Underlying mechanism for the two-photon NIR-QC process is analyzed in terms of static and dynamic photoemission and monitored excitation spectra. It is found that NIR-QC can occur through two possible energy transfer (ET) approaches: (i) the excited Ho³⁺:⁵F₃ state may simultaneously excite two Yb³⁺ neighbors via a cooperative ET process, and (ii) the NIR-QC can be feasibly induced by a first Ho³⁺(⁵S₂,⁵F₄) + Yb³⁺(²F_7/2) → Ho³⁺(⁵I₆) + Yb³⁺(²F_5/2) resonant ET process and a sequential ⁵I₆ → ⁵I₈ transition of Ho³⁺. This novel NaYF₄:Ho³⁺,Yb³⁺ NIR-QC phosphor, may explore a new approach to maximize the performance of solar cells.     

Infrared emission properties and energy transfer in ZnO-SiO2:Yb composites

Intense near-infrared emission at 1 μm has been obtained in ZnO-SiO₂:Yb³⁺ composites via a facile sol-gel method upon broadband ultraviolet light excitation. Systematic optical measurements including static and time-resolved photoluminescence have been performed to elucidate the energy transfer from ZnO quantum dots to Yb³⁺ ions. The dependence of energy transfer efficiency on Yb³⁺ concentration has been investigated in detail. Codoping with Li⁺ ions leads to about twice enhancement of the near-infrared luminescence intensity around 1 μm at room temperature. The enhancement in the luminescence intensity could be mostly attributed to the modification of the local symmetry around Yb³⁺ ions by codoping with Li⁺ ions.     

A novel red-emitting phosphors K2Ba (MoO4)2: Eu, Sm and improvement of luminescent properties for light emitting diodes

The novel red-emitting phosphors K₂Ba(MoO₄)₂: Eu³⁺, Sm³⁺ were prepared by solid-state reaction and their crystal structures, photo luminescent characteristics were investigated. The results show that all samples can be excited efficiently by UV (397 nm) and blue (466 nm) light, which are coupled well with the characteristic emission from UVLED and blue LED, respectively. A small amount of Sm³⁺, acting as a sensitizer, increased the energy absorption around 400 nm. In the Eu³⁺-Sm³⁺ co-doped system, both Eu³⁺ and Sm³⁺ f-f transition absorptions are observed in the excitation spectra, the intensities of the main emission line (⁵D₀ → ⁷F₂ transition of Eu³⁺ at 616 nm) are strengthened because of the energy transition from Sm³⁺ to Eu³⁺. The doping concentration of Eu³⁺-Sm³⁺ was optimized. The approach to charge compensation was used: Ba²⁺ → Eu³⁺/Sm³⁺ + X⁻ (X = F, Cl, and Br), and the influence of charge compensation on the luminescent intensity of phosphors is investigated.     

Optical properties of Yb-doped phosphate laser glasses

Ytterbium-doped phosphate glasses have been prepared and studied their spectroscopic properties through absorption, emission and Fourier transform infrared (FTIR) spectral studies and time-resolved luminescence decay curves. The absorption cross-section has been found to vary with the variation of Yb₂O₃ concentration. The results of the FTIR spectra show that the OH⁻ content is increasing with increase of the Yb₂O₃ concentration in these glasses. The decay curves of the ²F_5/2 level of Yb³⁺ ions exhibit a single exponential nature for all the concentrations. The lifetimes of the ²F_5/2 level of Yb³⁺ ions decreases from 1.04 to 0.27 ms when the Yb₂O₃ concentration is increased from 0.1 to 6.0 mol%. The quenching of lifetimes has been found to vary directly with the inter-ionic distance between the Yb³⁺ ions. The concentration quenching of the lifetime has been analyzed using different energy transfer processes and no evidence of cooperative luminescence of Yb³⁺ ions has been found in these glasses, which reveals that the present glasses are useful for photonic device applications. The laser performance properties have also been evaluated for these glasses and compared with those of other reported Yb³⁺-doped glass systems.     

Influence of niobium doping on phase composition and defect-mediated photoluminescence properties of Eu-doped TiO2 nanopowders synthesized in Ar/O2 thermal plasma

Nb⁵⁺:Eu³⁺-codoped TiO₂ nanopowders for chemical composition adjustment have been synthesized via Ar/O₂ radio-frequency thermal plasma. X-ray diffraction (XRD) results reveal that all the resultant powders exhibited mixture polymorphs of anatase (mean size: ∼45 nm) as the major phase and rutile (mean size: ∼71 nm). Rutile formation was promoted by the Eu³⁺ doping but suppressed by the Nb⁵⁺ addition. Combined observation using FE-SEM and TEM indicates that all the plasma-synthesized powders had a majority of facet-shaped particles (several nanometers) and a small proportion of nearly spherical crystals (∼150 nm). For the defect-mediated photoluminescence (PL) emission through the energy transfer from the TiO₂ host to the Eu³⁺ activator, the PL intensity originating from the ⁵D₀ → ⁷F₂ electronic transition weakened but that from the ⁵D₀ → ⁷F₁ electronic transition strengthened with increasing Nb⁵⁺ content. This may be a result of the decrease in the oxygen vacancy defects in the TiO₂ host lattice, as revealed by the joint means of UV-vis absorption spectra and excitation and emission spectra.     

Synthesis and luminescent properties of GdSrAl3O7:Tb phosphor under VUV/UV excitation

Superfine powder of Tb³⁺ ion-doped aluminates phosphors, GdSrAl₃O₇:Tb³⁺ was synthesized with a precursor prepared by an EDTA-sol–gel method at 900 °C. Field-emission scanning electron-microscopy (FE-SEM) observation indicated a narrow size-distribution of about 150–300 nm for the particles with elliptical shape. Upon excitation with vacuum ultraviolet (VUV) and near UV light excitation, the phosphors show a strong emission line at around 542 nm corresponding to the ⁵D₄ → ⁷F₅ transition of Tb³⁺, and the highest PL intensity at 542 nm was found at a content of about 12 mol% Tb³⁺. As the Tb³⁺ concentration increases, Tb ions strongly cross-relaxation interact resulting in a decrease of the lifetime. The results reveal that GdSrAl₃O₇:Tb³⁺ would be a promising green phosphor for PDP application.     

Synthesis and luminescence properties of clew-like CaMoO4:Sm, Eu

Eu³⁺ and Sm³⁺ co-doped CaMoO₄ microclews have been successfully synthesized via a facile hydrothermal method directly in surfactant-free environment. The as-prepared phosphor present clew-like agglomerates composed of 40 nm nanosheets under the moderated reaction temperature. The red phosphor CaMoO₄:Eu³⁺, Sm³⁺ can generate a strong absorption line at 405 nm, originating from ⁶H_5/2 → ⁶P_5/2 transition of Sm³⁺, which is suitable for the emission of the near-ultraviolet light-emitting diodes (∼400 nm). Energy transfer between Sm³⁺ and Eu³⁺ is detected from the varied photoluminescence spectra with different Eu³⁺ concentrations and the energy transfer mechanism is clarified via the photoluminescence spectra. When Sm³⁺ is excited (405 nm), the electron is excited from ⁶H_5/2 to ⁶P_5/2, and then relaxed to ⁴G_5/2. It jumps from ⁴G_5/2 to the lower levels corresponding to the emissions of Sm³⁺; meanwhile, the transfers from ⁴G_5/2 state of Sm³⁺ ion to ⁵D₀ state of Eu³⁺ ion come out. The transition of ⁵D₁ → ⁷F_J (J = 0, 1, 2) does not appear indicating that the transfer from ⁴G_5/2 state of Sm³⁺ to ⁵D₀ state rather than ⁵D₁ state of Eu³⁺ is the energy transfer pathway.     

Luminescence properties of Ca10K(PO4)7:RE (RE = Ce, Tb, Dy, Tm and Sm) under vacuum ultraviolet excitation

A series of RE³⁺ (RE = Ce, Tb, Dy, Tm and Sm) activated Ca₁₀K(PO₄)₇ were synthesized by conventional state reaction and their photoluminescence properties under vacuum ultraviolet excitation were investigated. The PO₄³⁻ absorption lies within the range from 125 to 180 nm in all the excitation spectra of Ca₁₀K(PO₄)₇:RE³⁺. The first f-d transition of Ce³⁺ is observed at 316 nm, and the Ce³⁺ emission is located at about 350 nm. Both the first spin-allowed and spin-forbidden f-d transitions of Tb³⁺ are situated at 232 and 263 nm, respectively. The emission spectrum of Ca₁₀K(PO₄)₇:Tb³⁺ exhibits typical Tb³⁺ emissions with the predominant peak at 544 nm. The O²⁻-Dy³⁺ charge transition band was calculated and identified around 173 nm, the CIE chromaticity coordinates of Dy³⁺ were calculated to be 0.364 and 0.392. The Ca₁₀K(PO₄)₇:Tm³⁺ demonstrates the strongest excitation at about 182 nm assigned to O²⁻-Tm³⁺, and gives the predominant emission at 453 nm. The ⁴G_5/2-⁶H_7/2 transition of Sm³⁺ at 601 nm is the most intensive in the emission spectrum.     

Tunable red-green-blue multicolor luminescence in Yb/Tm/Ho:Gd3Ga5O12 nano-crystals

The Yb³⁺/Tm³⁺/Ho³⁺: Gd₃Ga₅O₁₂ nano-crystals have been successfully prepared via a citric acid complex procedure. The luminescence spectra were measured and the up-conversion processes were discussed. By means of adjusting the doping concentrations of Yb³⁺/Tm³⁺/Ho³⁺, the red-green-blue up-conversion luminescence changed obviously. Results indicated that the ratio of red-green-blue up-conversion emissions enhanced heavily with the increasing concentrations of Tm³⁺ doped in the Yb³⁺/Tm³⁺/Ho³⁺:Gd₃Ga₅O₁₂ nano-crystals, which was rooted in the three-photon resonant cross relaxation processes(¹G₄ (Tm) + ⁵I₇ (Ho) → ³H₅ (Tm) + ⁵S₂ (Ho)). The tunable red-green-blue luminescence could be used in the fields of display, illumination, and photonics such as the white light generation.     

Polarized spectral analysis of Er ions in Er:LiGd(MoO4)2 crystal

The Er³⁺:LiGd(MoO₄)₂ crystal with Ø21 × 33 mm³ was grown by the Czochralski technique, and the absorption spectra, the fluorescence spectra and the fluorescence decay curves were measured at room temperature. Some spectroscopic parameters, such as the parameters of oscillator strengths, the spontaneous transition probabilities, the fluorescence branching ratios, the radiative lifetimes and the emission cross-sections were estimated based on Judd-Ofelt theory and Füchtbauer-Ladenburg method. The infrared emission at 1450-1650 nm, due to ⁴I_13/2 → ⁴I_15/2 transition and the visible emission at 520-569 nm corresponding to ²H_11/2,⁴S_3/2 → ⁴I_15/2 transition were observed in Er³⁺:LiGd(MoO₄)₂ crystals under 979 nm excitation at room temperature. The emission cross-sections are 4.37 × 10⁻²⁰ cm² at 553 nm and 0.584 × 10⁻²⁰ cm² at 1561 nm for π-polarization, and the following measured lifetimes are 4.57 ms and 10.74 μs. The upconversion emissions were attributed to energy transfer between Er³⁺ ions and the excited state absorption.     

Cr doping optimization in CaAl2O4:Eu blue phosphor

A new composition of phosphor material CaAl₂O₄:Eu²⁺ co-doped with Cr³⁺ was investigated. Various compositions with Eu²⁺ (1 and 2 mol%) and Cr³⁺ (0.05–0.1 mol%) were prepared by solid-state reaction method. These compositions show high brightness and longer persistent luminescence. Phase and crystallinity were investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Excitation and emission spectra were taken to investigate the luminescence characteristics. Broad band UV excited luminescence of the CaAl₂O₄:Eu²⁺, Cr³⁺ was observed in the blue region (λ_max = 440 nm) due to transitions from 4f⁶5d¹ to the 4f⁷ configuration of the Eu²⁺ ion. Cr³⁺ ion co-doping generates deep traps which results in longer afterglow phosphorescence compared to parent phosphor.     

Upconversion luminescence of Yb/Tb/Er-doped fluorosilicate glass ceramics containing SrF2 nanocrystals

Yb³⁺/Tb³⁺/Er³⁺-doped transparent alkaline earth fluorosilicate glass ceramics containing SrF₂ nanocrystals were prepared and their microstructures and spectroscopic properties were investigated. The formation of SrF₂ nanocrystals enriched with Yb³⁺/Tb³⁺/Er³⁺ in the glass ceramics was confirmed by XRD, HRTEM and SAED. The glass ceramics exhibited intense upconversion luminescence behaviors, owing to efficient energy transfers from Yb³⁺ to Er³⁺ and Tb³⁺ and low phonon assistant non-radiative transition probabilities of excited Er³⁺ and Tb³⁺. The upconversion could be enhanced significantly by increasing the annealing temperature and the luminescence color could be adjusted by varying the Yb³⁺/Tb³⁺/Er³⁺ ratio. Upconversion of Er³⁺ could be described as sequential energy transfer from single Yb³⁺ to single Er³⁺, while upconversion of Tb³⁺ described as cooperative energy transfer from double Yb³⁺ to single Tb³⁺.     

Structure and upconversion luminescence properties of BaYF5:Yb, Er nanoparticles prepared by different methods

BaYF₅:Yb³⁺, Er³⁺ (BYF) upconversion (UC) luminescence nanoparticles have been prepared using co-precipitation and hydrothermal techniques, respectively. Two different fluoride sources were used to synthesize BYF by the hydrothermal method, and the sizes of the as-prepared spherical particles were about 30 nm (NH₄BF₄ as a fluoride source) and 100 nm (NH₄HF₂ as a fluoride source), respectively. While the nanoparticles prepared by the co-precipitation method are irregular, many clusters and agglomerates can be seen. The UC fluorescence has been realized in all the as-prepared BYF samples upon 980 nm excitation. It is found that their luminescence spectra depend strongly upon the preparation method. Factors affecting the upconversion fluorescent intensity have been also studied. The UC emission transitions for ⁴F_9/2-⁴I_15/2 (red), ²H_11/2-⁴I_15/2 (green) and ⁴S_3/2-⁴I_15/2 (green) in the Yb³⁺/Er³⁺ codoped BYF nanoparticles depending on pumping power have also been discussed.