Concentration quenching,surface and spectral analyses of SrF2:Pr3+ prepared by different synthesis techniques

Pr³⁺ doped strontium fluoride (SrF₂) was prepared by hydrothermal and combustion methods. The phosphors were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL) spectroscopy. XRD patterns indicated that the samples were completely crystallized with a pure face-centred cubic (space group: Fm3m) structure. SEM images showed different morphologies which is an indication that the morphology of the SrF₂:Pr³⁺ phosphor strongly depends on the synthesis procedure. Both the SrF₂:Pr³⁺ samples exhibit blue–red emission centred at 488 nm under a 439 nm excitation wavelength (λ_exc) at room temperature. The emission intensity of Pr³⁺ was also found to be dependent on the synthesis procedure. The blue–red emission has decreased with an increase in the Pr³⁺ concentration. The optimum Pr³⁺ doping level for maximum emission intensity was 0.4 and 0.2 mol% for the hydrothermal and combustion samples, respectively. The reduction in the intensity for higher concentrations was found to be due to dipole–dipole interaction induced concentration quenching effects.     

Epitaxial growth of Fe nanodots on SrF2/Si (111)

Fe nanodots were grown on SrF₂ (111) surfaces deposited on Si (111) substrates in a molecular beam epitaxy (MBE) system. The crystallographic and surface morphological characters were studied by reflection high-energy electron diffractometry (RHEED) and atomic force microscopy (AFM). The triangular terraces and step edges of SrF₂ were formed at 600 °C, and its crystallinity was high in quality. Fe (111) layers with thicknesses between 0.5 and 5 nm were grown epitaxially on this SrF₂ layer at room temperature (RT) with the help of electron beam exposure. In 1-nm thick Fe layers deposited at RT, the number density was about 2 × 10¹² cm^− 2. At the end of this report, an epitaxial growth of the SrF₂/Fe/SrF₂ tri-layer on Si (111) is briefly described.     

Synthesis and characterization of ultrafine Ln2Ti2O7 (Ln = Sm,Gd, Dy,Er) pyrochlore oxides by stearic acid method

Stearic acid method (SAM) was developed to synthesize series of pyrochlore Ln₂Ti₂O₇ (Ln = Sm, Gd, Dy, Er) nanocrystals. The synthesis process was monitored by X-ray diffraction, Thermal–gravimetric–differential thermal analysis and Fourier Transform InfraRed methods. Comparing with traditional solid-state reaction (SSR), Ln₂Ti₂O₇ can be synthesized at relatively low temperature (700–800 °C) with shortened reaction time (2–4 h). The average particle size of Ln₂Ti₂O₇ was greatly reduced (ca. 40 nm) and the BET surface area was increased (ca. 12 m²/g) by using SAM. From the X-ray diffraction patterns, we found that Ln has an effect on the crystal structure of Ln₂Ti₂O₇, every lattice peak shifted to larger angle slightly with the increasing atomic number of Ln. Also, the lattice constant of Ln₂Ti₂O₇ was calculated by Jade.5 and found it decreased along with the decrease of ionic radius of Ln³⁺. The morphology of obtained Ln₂Ti₂O₇ was determined by transmission electron microscopy technique. Results showed that the obtained Ln₂Ti₂O₇ were all square-like and the interplanar distance of Ln₂Ti₂O₇ (Ln = Sm, Gd, Dy, Er) according to (111) plane was 0.65, 0.64, 0.63, and 0.62 nm respectively, which was measured from High Resolution Transmission Electron Microscopy images. Possible reason for this phenomenon was presented.     

Morphology-tailored synthesis of flower-like Y2O3:Eu3+ microspheres

Flower-like Y₂O₃:Eu³⁺ microspheres with strong red photoluminescent emission were successfully synthesized through a controlled solvothermal approach followed by a subsequent heat treatment. The experimental results showed that the flower-like microspheres were composed of nanopetals with the thickness of about 50 nm, and the solvent properties as well as the characteristics of the reactants were very crucial for the morphology-controlled process. Meanwhile, the formation mechanism study revealed a possible assembly and etching process. In addition, their photoluminescence property investigation indicated that the flower-like products exhibited the strongest red emission corresponding to ⁵D₀ → ⁷F₂ transition (609 nm) among the synthesized samples, implying better photoluminescence property provided by the assembled spheres with higher crystallinity and better size-distribution and suggesting their potential application in optoelectronics.     

Infrared-to-green up-conversion in Er3+, Yb3+-doped monoclinic KGd(WO4)2 single crystals

Optical spectroscopy of the green emission of erbium in KGd(WO₄)₂ (KGW) single crystals codoped with ytterbium ions is investigated. To do this, we firstly grew good-optical-quality KGW single crystals doped with Er³⁺ and Yb³⁺ at several dopant concentrations by the Top-seeded-solution-growth slow-cooling method (TSSG). Green photoluminescence of Er³⁺ in KGW host was studied at room temperature (RT) and low temperature (10 K) by means of Yb³⁺ sensitization after infrared excitation at 981 nm (10194 cm⁻¹). We calculated the emission and gain cross-sections and compared these with those of other known Er³⁺-doped laser materials like LiYF₄ :Er (YLF:Er) and Y₃Al₅O₁₂:Er (YAG:Er) at RT. Our study also focused on determining the optimal concentration of ions for generating the most intense green emission. We measured the lifetime of the green emission after infrared pump at several Yb³⁺ concentrations. From the low-temperature emission experiments, we determined the energy position of the sublevels of the ground state of erbium.     

Optical and scintillation properties of SrI2:Yb2+

The luminescence and scintillation properties of SrI₂:0.5%Yb²⁺ have been investigated. SrI₂:Yb single crystals were grown by the vertical Bridgeman method from the melt. They showed a light yield of 38,400 ph/MeV and energy resolution of 12.5% for the 662 keV full absorption peak. Yb²⁺ photoluminescence intensity and decay time were studied between 78 and 600 K. Two emission bands centered at 418 and 446 nm were observed and ascribed to spin-allowed and spin-forbidden Yb²⁺ 5d-4f transitions, respectively. Their corresponding room-temperature decay time constants are 710 ns and 77 μs. Both, the emission intensities and the decay time constants vary with temperature. The obtained results were interpreted using a model of self-absorption of Yb²⁺ emission and a model of non-radiative relaxation of the electron from the low spin to the high spin 4f¹³5d Yb²⁺ excited states. The radiative lifetime of the low spin Yb²⁺ excited state was determined as 400 ns.     

Hydrothermal synthesis and 121Sb Mössbauer characterization of perovskite-type oxides: Ba2SbLnO6 (Ln = Pr,Nd, Sm,Eu)

A mild hydrothermal process to prepare Ba₂SbLnO₆ (Ln = Pr, Nd, Sm, Eu) perovskite-type oxides are presented. These perovskites were characterized on the basis of X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), inductively-coupled plasma spectra (ICP) techniques. Primary structure was confirmed using Rietveld method based on XRD data shows that the likely space groups of Ba₂SbLnO₆ are R-3 for Ln = Pr and Nd and F_m-3_m for Ln = Sm and Eu, respectively. The measurement of Mössbauer effect of the 37.2 keV γ transition of ¹²¹Sb indicates that the isomer shift of these perovskites falls in the region of the Sb⁵⁺ and reflects some hybridized-orbital behavior in Sb–O bonds.     

Enhancement of photoluminescence properties of CaAl2Si2O8:Eu2+ by SiO2 addition

SiO₂ added phosphors, CaAl₂Si₂O₈:Eu²⁺ + xSiO₂ (x = 0, 1, 2, 3, 4, 5, 6 and 13 mol) were synthesized by a novel liquid phase precursor (LPP) method. The photoluminescence properties of phosphor added by 5 mol of SiO₂ showed 110% enhancement in the emission intensity compared to the CaAl₂Si₂O₈:Eu²⁺ phosphor. A broad emission and excitation wavelength was observed approximately from 400 nm to 600 nm centered at 430 nm and from 280 nm to 400 nm centered at 365 nm, respectively. Photoluminescence intensity of the phosphors increased continuously by SiO₂ addition up to x = 5 mol and then it decreased with further addition of SiO₂. The observed photoluminescence properties of the phosphors were discussed related to their crystalline structure and morphology.     

Luminescence properties of nanocrystalline YVO4:Eu3+ under UV and VUV excitation

Single phase of Eu³⁺-doped YVO₄ nanophosphors at different pH values were synthesized by a mild hydrothermal method. Their photoluminescence were evaluated under UV and VUV region, respectively. Monitoring by 619 nm emission, broad bands at around 143 nm, 200 nm, 260 nm were observed in the excitation spectrum of YVO₄:5 mol%Eu³⁺. These peaks could be assigned to host absorption, the overlap of the VO₄³⁻ host absorption and charge transfer transition between Eu³⁺ and O²⁻, respectively. Both 254 nm and 147 nm excitations, the emission spectra were identical, they were all composed of Eu³⁺ emission transitions arising mainly from the ⁵D₀ level to the ⁷F_J (J = 1, 2, 3, 4) manifolds. With the pH values ranging from 7 to 11, the relative intensity of the emission spectra were decreasing, and the position of the predominant peak (⁵D₀ → ⁷F₂) was changed from 619 nm to 615 nm when the pH values changed from 7 to 11.     

Luminescence and scintillation properties of rare-earth-doped LuF3 scintillation crystals

The Nd-doped and Er-doped LuF₃ single crystals were grown by the micro-pulling-down method to study their scintillation properties in the vacuum-ultraviolet (VUV) region. The doubly Nd–Er codoped single crystal was grown to study possibility of scintillation performance improvement by energy transfer from Er³⁺ to Nd³⁺ ions. The LiF flux was to avoid phase transition below melting temperature. The 1%Nd-doped sample showed the highest overall scintillation efficiency under X-ray excitation which was 7 times as high as that of the LaF₃:Nd 8% standard. The leading Nd³⁺ 5d–4f emission was situated at 176 nm, while the Er³⁺ 5d–4f emission for Er-doped samples was observed at 163 nm, which better matches the sensitivity of some VUV-sensitive photodetectors. The optimum Er concentration was determined to be around 1–3 mol%. No Er³⁺ 5d–4f emission was observed for the doubly Er,Nd-codoped sample due to energy transfer from the Er³⁺ to Nd³⁺ ions. Slight improvement of the light yield was observed in the doubly-doped sample with respect to the Nd-only doped one.     

Hydrothermal synthesis of Eu3+-doped Y2Sn2O7 nanocrystals

Fine powders of Y₂Sn₂O₇ nanocrystals with pyrochlore structure have been successfully synthesized by the hydrothermal method in an alkaline system. The samples were characterized by X-ray diffraction, Fourier transform infrared and Raman spectroscopy. Furthermore, photoluminescence characterization of the Y₂Sn₂O₇ nanocrystals doped with 5 mol% Eu³⁺ was carried out, and the results show that there were some intense and prevailing emission peaks located at 580–635 nm.     

Photoluminescence properties of Sr3(PO4)2: Eu2+, Dy3+ double-emitting blue phosphor for white LEDs

Double-emitting blue phosphor Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ was synthesized by solid state reaction under H₂ atmosphere. XRD exhibited the pure hexagonal phase of the prepared phosphor. The photoluminescence results showed that all samples had intense broad absorption band between 250 and 450 nm, which matched well with the near-UV (350–420 nm) emission band of InGaN-based chips. The emission spectrum of Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ consisted of two broad bands, peaked at 485 nm and 410 nm, which originated from two luminescent centers, related to 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ in six-coordinated Sr(I) and ten-coordinated Sr(II) sites respectively. The intensity ratio of two emission bands could be easily tuned by adjusting Dy³⁺ co-doping content, which resulted in color-tunable luminescence in bluish green region to purplish blue region.     

Luminescence properties of Eu2+-activated Sr5(PO4)2(SiO4) for green-emitting phosphor

A green-emitting phosphor of Eu²⁺-activated Sr₅(PO₄)₂(SiO₄) was synthesized by the conventional solid-state reaction. It was characterized by photoluminescence excitation and emission spectra, and lifetimes. In Sr₅(PO₄)₂(SiO₄):Eu²⁺, there are at least two distinguishable Eu²⁺ sites, which result in one broad emission situating at about 495 nm and 560 nm. The phosphor can be efficiently excited in the wavelength range of 250–440 nm where the near UV (～ 395 nm) Ga(In)N LED is well matched. The dependence of luminescence intensities on temperature was investigated. With the increasing of temperature, the luminescence of the phosphor shows good thermal stability and stable color chromaticity. The luminescence characteristics indicate that this phosphor has a potential application as a white light emitting diode phosphor.     

Vacuum-UV excitation and visible luminescence of nano-scale and micro-scale NaLnF4:Pr3+ (Ln = Y,Lu)

The UV–Vis luminescence of NaLnF₄:Pr³⁺ (Ln = Y, Lu) materials can be efficiently excited by vacuum UV radiation (VUV) such as the 172 nm emission of mercury-free Xe-discharge lamps. In this work, the optical properties of the cubic α-phase and the hexagonal β-phase of NaLnF₄:Pr³⁺ (Ln = Y, Lu) powders are compared regarding particle sizes in the nano- and micrometer regime. Upon VUV excitation, the emission spectra of both crystal phases are found to be dominated by intraconfigurational [Xe]4f²–[Xe]4f² transitions, which is explained by the chemical properties of the ternary fluorides. Furthermore it is observed that the emission and excitation spectra of nano- and micro-scale powders are very similar, but that the luminescence intensity is affected by the average particle size.     

Synthesis,characterization and luminescence of Sr3Al2O6 phosphor with trivalent rare earth dopant

The luminescent properties of Sr₃Al₂O₆ doped and co-doped with the rare earths (Ln³⁺ = Eu³⁺, Dy³⁺, Eu³⁺ and Dy³⁺) have been studied. The material was synthesized by reflux method and fired up to 900 °C for 16 h. The X-ray diffraction pattern confirms that the synthesized material consists of Sr₃Al₂O₆ as main phase. The photoluminescence study gives a clear evidence of europium stabilizing in trivalent form and surprisingly with no presence of europium in the divalent state. The addition of Dy³⁺ as co-dopant in the Sr₃Al₂O₆:Eu³⁺ matrix shows the quenching effect in the photoluminescence (PL) spectra. The photoluminescence intensity of Eu³⁺ falls gradually on increasing the concentration of the co-dopant in the range from 0.1 mole% to 2.0 mole%. The significantly intense thermoluminescence (TL) glow peak was obtained for Sr₃Al₂O₆:Eu³⁺, Dy³⁺ (1% and 0.1%) at around 194 °C when irradiated with 10 kGy dose from Sr-90 β source.     

Spectroscopic properties and energy transfer in Er–Tm co-doped bismuth silicate glass

In this paper, we investigate the spectroscopic properties of and energy transfer processes in Er–Tm co-doped bismuth silicate glass. The Judd–Ofelt parameters of Er³⁺ and Tm³⁺ are calculated, and the similar values indicate that the local environments of these two kinds of rare earth ions are almost the same. When the samples are pumped at 980 nm, the emission intensity ratio of Tm:³F₄ → ³H₆ to Er:⁴I_13/2 → ⁴I_15/2 increases with increased Er³⁺ and Tm³⁺ contents, indicating energy transfer from Er:⁴I_13/2 to Tm:³F₄. When the samples are pumped at 800 nm, the emission intensity ratio of Er:⁴I_13/2 → ⁴I_15/2 to Tm:³H₄ → ³F₄ increases with increased Tm₂O₃ concentration, indicating energy transfer from Tm:³H₄ to Er:⁴I_13/2. The rate equations are given to explain the variations. The microscopic and macroscopic energy transfer parameters are calculated, and the values of energy transfer from Er:⁴I_13/2 to Tm:³F₄ are found to be higher than those of the other processes. For the Tm singly-doped glass pumped at 800 nm and Er–Tm co-doped glass pumped at 980 nm, the pumping rate needed to realize population reversion is calculated. The result shows that when the Er₂O₃ doping level is high, pumping the co-doped glass by a 980 nm laser is an effective way of obtaining a low-threshold ∼2 μm gain.     

Polyvinylpyrrolidone (PVP)-assisted hydrothermal synthesis of luminescent YVO4:Eu3+ microspheres

Spherical YVO₄:Eu³⁺ microstructures were hydrothermally synthesized by the reaction of NH₄VO₃, Y₂O₃, and Eu₂O₃ at 180 °C for 24 h with the assistance of polyvinylpyrrolidone (PVP) as a surfactant. The resulting products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. The experimental results showed that ball-like YVO₄:Eu³⁺ microspheres with a diameter of about 4–5 μm, corresponding to the SEM observations, formed at 180 °C for 24 h using 0.2 g PVP with the molecular weight of 20,000 g mol^?1. The amount of PVP and the reaction time of hydrothermal processing were found to play a key role in the formation of YVO₄:Eu³⁺ microspheres. It has been observed that the relative luminescence intensities of the as-synthesized samples increased with increasing hydrothermal reaction times due mainly to the increase of crystallinity.     

Effect of processing parameters on structural,morphological and optical Y2O3:Yb3+/Ho3+ powders characteristics

Rod-like and flake-like up-converting Y₂O₃:Yb³⁺/Ho³⁺ particles which are composed of nanoparticles with size less than 100 nm, are prepared by a simple hydrothermal processing at 473 K (3 h) followed by additional thermal treatment at 1373 K (3 and 12 h). The effect of precursor pH value on the formation of Y₂O₃:Yb³⁺/Ho³⁺ is followed through X-ray powder diffractometry (XRPD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Structural refinement confirms formation of the cubic bixbyte structure (S.G. Ia-3) with the non-uniform accommodation of dopants at C₂ and S₆ cationic sites. Under 978 nm laser excitation, strong green (530–570 nm) up-conversion is observed in all samples. The emission shows a decrease in intensity with an increase in external temperature, indicating FIR (fluorescence intensity ratio) based temperature sensing behavior of 0.52% for the ⁵F₄ → ⁵I₈/⁵S₂ → ⁵I₈ transitions.     

Synthesis and photoluminescence properties of Eu2+-doped Ca2AlSi3O2N5 green phosphors

Novel Eu²⁺-doped Ca₂AlSi₃O₂N₅ phosphors with a general formula of Eu_xCa_2?xAlSi₃O₂N₅ were successfully prepared via a solid-state reaction method under a nitrogen atmosphere. The produced phosphors were effectively excited by UV–vis light in the wavelength range between 250 and 400 nm, and featured an intense green emission band which peaked at about 500 nm. The emission spectra featured a red-shift over increasing Eu²⁺ content and the temperature of heat treatment. The maximum intensity of emission was obtained for x = 0.014 and heat treatment at 1450 °C. The photoluminescence properties of the produced Ca₂AlSi₃O₂N₅:Eu²⁺ phosphors qualify them for consideration in potential use as green phosphors in UVLED-based white LED.     

Intense blue-light emission from hydrothermally synthesized lead zirconate titanate platelets

Well-crystalline Pb(Zr_0.52Ti_0.48)O₃ square platelets, with dimensions of about 1 × 0.65 × 0.16 μm³, have been synthesized by hydrothermal method at low temperature. X-ray diffraction, micro-Raman spectrometry, and field emission scanning electron microscope were employed to study the crystal structure and morphologies of the products. Energy dispersive X-ray spectroscopy was used to analyze the elemental composition of the products. The photoluminescence of the products showed a strong narrow blue-light emission at 453 nm and a weak one at 468 nm at room temperature. The possible mechanisms for the blue-light emission are proposed in this report. The photoluminescence of this ferroelectric material is one more interesting property for technological applications.