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.     

Effects of Bi doping on the optical properties of Er:Y2O3

The influences of Bi³⁺ doping on the optical properties of Er³⁺:Y₂O₃ are investigated under UV and IR excitations. The emission intensity of Er³⁺ is remarkably enhanced by the introduction of Bi³⁺ under both two excitations. The emission enhancement under UV excitation originates from the energy transfer from Bi³⁺ to Er³⁺, while under IR excitation it can be attributed to the modification of the local crystal field around the Er³⁺.     

Synthesis and characterization of Er doped CaF2 nanoparticles

Er³⁺ doped CaF₂ nanoparticles were synthesized by a chemical co-precipitation method. Effect of the dopant concentrations on the structure and optical properties of the CaF₂ nanoparticles was investigated. The X-ray powder diffraction and transmission electron microscopy analysis was used to characterize the structure and morphology of the nanoparticles. The nanoparticles with different dopant concentration exhibited a sphere-like morphology with diameters of about 8-36 nm. The incorporation of Er³⁺ ions into CaF₂ resulted in the decrease in grain size and deterioration of crystallinity, but enlarged the lattice constants of CaF₂. Additional annealing treatment at 400 °C to the prepared CaF₂ removed the NO₃⁻ and OH⁻ groups adsorbed on the particles’ surfaces, and improved the optical properties of the nanoparticles. The fluorescence intensity, with a maximum at approximately 0.4 mol%, decreased with the increase in doping concentration because of concentration quenching.     

White light emission in Tm/Er/Yb tri-doped Y2O3 transparent ceramic

Tm³⁺/Er³⁺/Yb³⁺ triply doped Y₂O₃ transparent ceramics were fabricated by solid state reaction and characterized from the point of view of white light upconversion luminescence. All the samples exhibited high transparency not only in near-infrared band but also in visible region. Strong red (Er³⁺: ⁴F_9/2 → ⁴I_15/2), green (Er³⁺: ²H_11/2, ⁴S_3/2 → ⁴I_15/2) and blue (Tm³⁺: ¹G₄ → ³H₆) upconversion emissions have been observed under 980 nm excitation at room temperature. By varying the concentration of Er³⁺ ion, various colors of upconversion luminescence (pure blue, bluish green, pure green and yellowish green), including white light with CIE-X = 0.295 and CIE-Y = 0.312, can be easily achieved.     

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.     

Preparation and luminescent characteristics of Sr3RE2(BO3)4:Dy (RE = Y, La, Gd) phosphors for white LED

Dysprosium-activated Sr₃RE₂(BO₃)₄ (RE = Y, La, Gd) phosphors were synthesized by a high temperature solid-state reaction method. The phase uniformity of the phosphors was characterized by X-ray powder diffraction (XRD) and the luminescence characteristics were investigated. The excitation spectra at 575 nm emission show strong spectral bands in the region of 300-500 nm. The emission spectra of the phosphors with 365 nm excitation show three bands centered at 484 nm, 575 nm and 680 nm, which originate from the transitions of ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 of Dy³⁺, respectively. The effect of Dy³⁺ concentration on the emission intensity of the phosphors was investigated. The fluorescence decay curves for ⁴F_9/2 → ⁶H_13/2 excited at 365 nm and monitored at λ_em of 575 nm were measured. The decay times decreased slowly with increasing Dy³⁺ doping concentration due to a trap capturing to resonance fluorescence transfer of the activated ions and due to the exchange interactions between activated ion pairs. In order to determine the type of interaction between activated ions, the concentration dependence curves (lg(I/x) versus lg x) of Sr₃RE₂(BO₃)₄:Dy³⁺ (RE = Y, La, Gd) were plotted. The concentration quenching mechanism of the ⁴F_9/2 → ⁶H_13/2 (575 nm) transition of Dy³⁺ is the d-d interaction. All results indicate these phosphors are promising white-color luminescent materials.     

Synthesis of (Bi0.5Na0.5)TiO3 (BNT) and Pr doped BNT using the soft combustion technique and its properties

In this work, bismuth sodium titanate (Bi_0.5Na_0.5)TiO₃ (BNT) and praseodymium (Pr)-doped BNT were successfully produced using the soft combustion technique. The effects of Pr doping on stoichiometry, microstructure, density and dielectric properties were studied. Pure Pr-doped BNT was obtained in all samples containing 5, 10 and 20 mol% Pr after calcination at 800 °C for 3 h. The produced powders were then pressed into pellets and sintered at 1100 °C for 3 h. The very similar ionic radii of Pr³⁺ with Bi³⁺ and Na⁺ made it possible to substitute both Bi and Na. The crystallite size and grain size decreased with increasing Pr amount because Pr acted as grain growth inhibitor, both for calcined powders and for sintered pellets. Maximum density was obtained in 5 mol% Pr-doped BNT, beyond which density decreased. The maximum dielectric constant of 756 was obtained in 5 mol% Pr-doped BNT and decreased at higher levels of Pr doping. Pr doped into BNT also caused a decrease in dielectric loss.     

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.     

Near cathode optical emission spectroscopy in N2-H2 glow discharge plasma

Optical emission spectroscopy (OES) is a non-intrusive diagnostic technique, widely used to study different kinds of plasmas. In the present work, a locally resolved OES technique was used to obtain near cathode (substrate) emission spectra for N₂-H₂ glow discharges. It was observed that, along with N₂⁺ and N₂ lines, the characteristic atomic nitrogen lines at 742.3 nm (3p ⁴S⁰_3/2 → 3s ⁴P_1/2), 744.2 nm (3p ⁴S⁰_3/2 → 3s ⁴P_3/2), 746.8 nm (3p ⁴S⁰_3/2 → 3s ⁴P_5/2) and H_α (656.3 nm) were the main emissions coming from the sheath region that shrouded the cathode. A qualitative analysis of the spectral lines near the cathode has been done in order to understand the mechanism of plasma nitriding and the role played by the hydrogen in the nitriding process. The decrease in local intensity of these atomic lines with hydrogen composition suggests that the effect of hydrogen is to enhance the sticking/adsorption of N on the cathode surface.     

Preparation of Dy-activated strontium orthosilicate (Sr2SiO4:Dy) phosphors and its photoluminescent properties

Dy³⁺-activated β/α′-Sr₂SiO₄ phosphors were successfully prepared by solid-state reaction method with ammonium chloride (NH₄Cl) as the flux. The influences of calcination temperatures, amounts of NH₄Cl and the concentrations of Dy³⁺ on phase composition, morphology and the photoluminescent properties of as-prepared powders were investigated in detail. The β and α′ phases of Sr₂SiO₄ were obtained with 1 wt% and 2-5 wt% NH₄Cl, respectively, as the sintered condition was at 1000 °C for 4 h. With increasing the amount of NH₄Cl, the morphology of phosphors changed from needlelike to regular polyhedron shape and the colors of the Sr₂SiO₄:Dy³⁺ phosphors changed from blue-green to white. The luminescence intensity of ⁴F_9/2 → ⁶H_15/2 transition was slightly higher than that of ⁴F_9/2 → ⁶H_13/2 (ΔL = 2, ΔJ = 2) transition owing to the low-symmetry around Dy³⁺ ions. The optimum concentration of Dy³⁺ was 2.0 mol% and the concentration quenching were caused by the d-d interaction and a cross relaxation. The yellow-to-blue intensity ratio (Y/B) of Dy³⁺ emission did not to change with varying the Dy³⁺ concentration using Li⁺ ions for charge compensation. These indicate that this phosphor can be used as a potential candidate for the phosphor-converted white LEDs with a UV chip.     

Synthesis of monodispersed ultrafine Bi2S3 nanocrystals

Monodispersed ultrafine Bi₂S₃ nanocrystals of ∼3 nm were synthesized via a facile and mild method, in which thioacetamide and bismuth oleate complex were used as the sulfur and bismuth precursors, respectively. The obtained Bi₂S₃ nanocrystals possessed a high surface area of 305 m² g⁻¹. The nanostructures of Bi₂S₃ nanocrystals were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and selective area electron diffraction (SAED) techniques. The optical property of the Bi₂S₃ nanocrystals was studied by photoluminescence spectroscopy. A remarkable blue shift and a band gap of ∼1.5 eV were observed. The shape of the Bi₂S₃ nanocrystals could be tuned by adjusting the initial Bi/S molar ratio and reaction temperature, respectively. A possible burst nucleation mechanism for this monodispersed ultrafine Bi₂S₃ nanocrystals was proposed.     

Nano-sized Y2O3:Eu hollow spheres with enhanced photoluminescence properties

Nano-sized Y₂O₃:Eu³⁺ hollow spheres were fabricated via a facile strategy including preparation of the hollow precursor and a later calcination. Moreover, the growth process of these hollow spheres was monitored by time-dependent experiments and their luminescence properties were also intensively studied. The products exhibit strong red emitting at 613 nm under ultraviolet excitation and control experiments were carried out to optimize the synthetic conditions. It was found 850 °C calcination with 9 mol% doping level could give out the best photoluminescence performance. Moreover, a possible mechanism for the enhanced PL performance was also proposed based on the FT-IR investigation.     

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.     

Influence of indium doping on the properties of spray deposited CdS0.2Se0.8 thin films

Polycrystalline indium doped CdS_0.2Se_0.8 thin films with varying concentrations of indium have been prepared by spray pyrolysis at 300 °C. The as deposited films have been characterized by XRD, AFM, EDAX, optical and electrical resistivity measurement techniques. The XRD patterns show that the films are polycrystalline with hexagonal crystal structure irrespective of indium doping concentration. AFM studies reveal that the RMS surface roughness of film decreases from 34.68 to 17.76 with increase in indium doping concentration up to 0.15 mol% in CdS_0.2Se_0.8 thin films and further it increases for higher indium doping concentrations. Traces of indium in CdS_0.2Se_0.8 thin films have been observed from EDAX studies. The optical band gap energy of CdS_0.2Se_0.8 thin film is found to decrease from 1.91 eV to 1.67 eV with indium doping up to 0.15 mol% and increase after 0.15 mol%. The electrical resistivity measurement shows that the films are semiconducting with minimum resistivity of 3.71 × 10⁴ Ω cm observed at 0.15 mol% indium doping. Thermoelectric power measurements show that films exhibit n-type conductivity.     

Enhancement of upconversion luminescence of Y2O3:Er nanocrystals by codoping Li-Zn

The upconversion (UC) luminescence in sol-gel synthesized Li⁺, Zn²⁺, or Li⁺-Zn²⁺ codoped Y₂O₃:Er³⁺ nanocrystals were investigated under the excitation of a 970 nm diode laser. Compared to undoped Y₂O₃:Er³⁺ samples, proper doping of Li⁺-Zn²⁺ leads to an drastic increase of the UC luminescence centered at 560 nm by a factor of 28. The UC luminescence enhancement is a result of the increased lifetime of the intermediate state ⁴I_11/2 (Er). The intensity ratio of the green over red emissions (green/red) is also affected by the codoping of Zn²⁺, Li⁺ and Li⁺-Zn²⁺ ions. Our results demonstrated that the Li⁺-Zn²⁺ codoping in Y₂O₃:Er³⁺ phosphors produced remarkable enhancement of the UC luminescence and green/red ratio, making this nanocrystal a promising candidate for photonic and biological applications.     

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.     

Absorption and emission spectral studies of Sm-doped lead tungstate tellurite glasses

The present work reports the effect of concentration on photoluminescence properties of Sm³⁺ ions doped lead tungstate tellurite (LTTSm) glasses by using the absorption, emission and decay measurements. The Judd-Ofelt theory has been used to evaluate the three Judd-Ofelt intensity parameters (Ω_2,4,6) and calculated oscillator strengths (f_c). LTTSm glasses exhibited intense reddish-orange emission when excited with 477 nm wavelength. Concentration quenching has been noticed beyond 1.0 mol% of Sm³⁺ ion concentration. The decay curves of ⁴G_5/2 level exhibited single exponential behavior for all the concentrations and the measured lifetimes are found to depend strongly on Sm³⁺ concentration. From the emission characteristic parameters of ⁴G_5/2 level, it is concluded that the LTTSm glasses could be useful for photonic devices like visible lasers, fluorescent display devices and optical amplifiers.     

Synthesis and conductivity of GdPO4 nanorods: Impacts of particle size and Ca doping

A series of Gd_1−xCa_xPO₄·nH₂O nanorods were prepared using a simple hydrothermal reaction which was optimized by tuning the pH values of the precursor. The resulted nanorods were characterized by X-ray diffraction, transmission electron microscopy, Fourier transformation infrared spectroscopy, and alternative current impedance technique. It is demonstrated that all Gd_1−xCa_xPO₄·nH₂O nanorods crystallized in a pure hexagonal structure. For x = 0, the particle dimension decreased with increasing the pH value. For x > 0, the solid solution limit of Ca²⁺ in GdPO₄·nH₂O nanorods was about 3 mol%, below which the lattice volume increased with increasing the doping level of Ca²⁺. The conductivities of nanorods were highly dependent on both the particle size and Ca²⁺ concentration, as indicated by the increased conductivity as particle size reduces or Ca²⁺ doping level increases. These observations were understood in terms of the dehydration and the introduction of HPO₄²⁻ defects by Ca²⁺ doping.     

Preparation and luminescent properties of Eu doped Sr3La(PO4)3 phosphor

Eu²⁺-doped Sr₃La(PO₄)₃ phosphors were synthesized by solid-state reaction method. Their luminescent properties were investigated. The phosphor could be excited by ultraviolet light effectively. The emission spectra exhibit two emission peaks located at 418 nm and 500 nm, respectively. These two peaks originated from two different luminescent centers, respectively. One is nine-coordinated Eu(I) center, other is six-coordinated Eu(II) center. It was found that the doping concentration of Eu²⁺ ions affected the shape of emission spectra. As the doping concentration increasing, Eu²⁺ ions are more likely to form Eu(I) luminescent centers and emit purple light.     

Effects of Ce concentration, beam voltage and current on the cathodoluminescence intensity of SiO2:Pr-Ce nanophosphor

SiO₂:Pr³⁺-Ce³⁺ phosphor powders were successfully prepared using a sol-gel process. The concentration of Pr³⁺ was fixed at 0.2 mol% while that of Ce³⁺ was varied in the range of 0.2-2 mol%. High resolution transmission electron microscopy (HRTEM) clearly showed nanoclusters of Pr and Ce present in the amorphous SiO₂ matrix, field emission scanning electron microscopy (FE-SEM) indicated that SiO₂ clustered nanoparticles from 20 to 120 nm were obtained. Si-O-Si asymmetric stretching was measured with Fourier transform-IR (FT-IR) spectroscopy and it was also realized that this band increased with incorporation of the activator ions into the SiO₂ matrix. The broad blue emission from the Ce³⁺ ions attributed to the 5d¹-4f¹ transition was observed from the SiO₂:0.2 mol% Pr³⁺-1 mol% Ce³⁺ phosphor. This emission was slightly enhanced compared to that of the singly doped SiO₂:1 mol%Ce³⁺ phosphor. Further investigations were conducted where the CL intensity was measured at different beam voltages and currents from 1 to 5 kV and 8.5 to 30 μA, respectively, in order to study their effects on the CL intensity of SiO₂:0.2 mol% Pr³⁺-1 mol% Ce³⁺. The electron-beam dissociated the SiO₂ and as a result an oxygen-deficient surface dead or non-luminescent layer of SiO_x, where x < 2 on the surface, was formed.