The Y-3(Al,Ga)(5)O-12:Ce3+,Cr3+,Nd3+ (YAGG) nano-phosphors with homogeneous particle-size distribution, low aggregation and average crystalline size of about 65 nm were obtained using a modified Pechini method. Only slight aggregation of the crystallites occurs after post-annealing at 1100 degrees C. The intense Ce3+ bands in the excitation spectra of the Ce3+,Cr3+,Nd3+ co-doped materials monitoring the Cr3+ emission at 690 nm indicate energy transfer from Ce(3+ )to Cr3+. Weak Nd3+ lines are observed, as well. In addition, the emission of Nd3+ at 1060 nm with excitation of Ce3+ and Cr3+ confirms the Ce3+/Cr3+ to Nd3+ energy transfer. The short average luminescence decay times for the Ce3+ emission indicate the Ce3+/Cr3+ to Nd3+ energy transfer. Eventually, the Y-3(Al,Ga)(5)O-12:Ce3+,Cr3+,Nd(3+ )nano-phosphors exhibit persistent luminescence originating from the 4f(3) -> 4f(3) transitions of Nd(3+ )which matches well to the first biological window to be used in bioimaging applications. (C) 2019 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.

Trivalent europium (Eu3+ ) and terbium (Tb3+ ) ions are important activator centers used in different host lattices to produce red and green emitting materials. The current work shows the design of new clay minerals to act as host lattices for rare earth (RE) ions. Based on the hectorite structure, nano-chlorohectorites and nano-fluorohectorites were developed by replacing the OH- present in the hectorite structure with Cl- or F- , thus avoiding the luminescence quenching expected due to the OH- groups. The produced matrices were characterized through X-ray powder diffraction (XPD), transmission electron microscopy (TEM), FT-IR, 29 Si MAS (magic angle spinning) NMR, nitrogen sorption, thermogravimetry-differential scanning calorimetry (TGA-DSC) and luminescence measurements, indicating all good features expected from a host lattice for RE ions. The nano-clay materials were successfully doped with Eu3+ and/or Tb3+ to yield materials preserving the hectorite crystal structure and showing the related luminescence emissions. Thus, the present work shows that efficient RE3+ luminescence can be obtained from clays without the use of organic 'antenna' molecules.

Persistent luminescence materials based on Ti-doped rare earth oxysulfides were obtained by a rapid, one-step process for application in self-sustained warm-light LEDs.

The major problem that limits solar cells’ efficiency is their insensitivity to the whole solar spectrum which is the so-called spectral mismatch. Therefore, several mechanisms have been explored based on photoluminescence (PL) to convert the solar cell spectrum where the spectral response of the solar cell is low to regions where the spectral response of the solar cell is high. Downconversion, up-conversion (UC) and downshifting are some of the mechanisms that may be applied to improve the spectral response. Upconversion nanoparticles (UCNPs) have shown some promising possibilities to be considered in this respect, however, low UC efficiency of UCNPs is still the most severe limitation of their applications. This study reports on the PL and cathodoluminescence (CL) behaviour of different phosphors. The vastly studied lanthanide pairs in UC are Er 3+ , Ho 3+ and Tm 3+ with Yb 3+ as a sensitizer. Whereas, very few UC studies have been carried out for Yb 3+ /Tb 3+ and Yb 3+ /Eu 3+ pairs. The reason behind these two categories of lanthanides is the way they transfer their energies. Er 3+ , Ho 3+ and Tm 3+ co-doped with Yb 3+ can be excited by ground state absorption (GSA), excited-state absorption (ESA) and energy transfer up-conversion (ETU) mechanisms whereas the energy transfer from Yb 3+ to Tb 3+ /Eu 3+ is due to co-operative energy transfer (CET). Since, Tb 3+ and Eu 3+ do not have energy levels which can absorb the near infra-red (NIR) light directly, Yb 3+ is the best choice in order to obtain the UC luminescence in Tb 3+ / Eu 3+ doped system. Yb 3+ has a single electronic transition within the 4f subshell. The transition from the lower level through ground state absorption can be easily achieved by using NIR radiation, and resulting energy from two or more excited Yb 3+ ions can be utilized in the excitation of a single Tb 3+ or Eu 3+ ion, for gaining improved UC emission. Doping different host materials with these lanthanide pairs were investigated and tested for possible increase in solar cell efficiency. Power tuneable visible UC and infrared emissions were detected upon excitation with a 980 nm diode laser.

Luminescent β-NaYF4:Yb3+,Er3+ (xYb: 0.17, xEr: 0.03) nanomaterials were synthesized for use as labels for biomedical applications with high temperature co-precipitation synthesis in 1-octadecene and oleic acid. The effect of the synthesis conditions (e.g. argon flow, cooling and stirring rates) on the products’ up-conversion luminescence intensity, particle size and morphology were studied. The factors contributing to these properties were analysed. It was observed that an efficient inert gas flow is essential to the formation of the preferred highly-luminescent hexagonal structure. Furthermore, the flow rate, together with the stirring rate, crucially affect the Er:Yb molar ratio of the products. The optimization of this ratio is essential when strong up-conversion emission is required from small particles, whereas the morphology and uniformity of the nanoparticles can be controlled with the cooling rate. These results emphasize the importance of controlling the synthesis conditions, especially when nanoparticles need to have a specific morphology because of their use e.g. as luminescent labels in medical diagnostics.

Highly luminescent rare earth (R3+) doped alkaline-earth tungstates MWO4:R3+ (M2+: Ca, Sr and Ba, R3+: Eu, Tb, Gd) were prepared with a room temperature coprecipitation method. The phosphors were characterized by X-ray powder diffraction (XPD), thermal analysis (TG), infrared absorption spectroscopy (FTIR) and UV excited photoluminescence. The as-prepared MWO4:R3+ particles belong to the tetragonal scheelite phase, and are well crystallized and are of the average size of 16–48nm. The excitation and emission spectra of the materials were recorded at 300 and 77K temperatures. The luminescent materials exhibit intense red (Eu3+) and green (Tb3+) colors under UV excitation. The excitation spectra of the Eu3+ doped materials show broad bands arising from the ligand-to-metal charge transfer transitions (O2−→WVI and O2−→Eu3+) as well as narrow bands from 4f–4f intraconfigurational transitions of Eu3+. 4f–4f emission data of the Eu3+ and Tb3+ in the MWO4 host matrices as well as the values of emission quantum efficiencies of the 5D0 level and the 4f–4f experimental intensity parameters of Eu3+ ion are presented and discussed.

The up-conversion luminescence where the absorption of two or more low-energy photons results in emission of a higher-energy photon has been highly effective in certain applications such as bioanalytical assays. Currently one of the most efficient up-converting materials is the NaYF4:Yb3+,Er3+. However, some applications (e.g. solar cells) still require improvement of the up-conversion efficiency in order to have actual practical use. Therefore, it is important to enhance the performance of the materials. In this work, the effect of Mn and Cr doping on the up-conversion luminescence of NaYF4:Yb3+,Er3+ was studied. The materials were prepared using the co-precipitation method and the ratios of the doping ions were optimized for best up-conversion intensity. The as-prepared materials showed very similar sizes and morphologies. Cr co-doping was observed to increase the up-conversion luminescence whereas the addition of Mn resulted in quenching. Up-conversion luminescence was also measured with different excitation power densities and the results suggested that Cr co-doping enables the up-conversion in lower power densities than without Cr co-doping. Furthermore, the reasons for the changes in up-conversion luminescence intensity were discussed. (C) 2016 Elsevier B.V. All rights reserved.

Up-converting NaYF4:Yb3+,Er3+ (x(yb): 0.20, x(Er): 0.02) nanomaterials were prepared with a microwave assisted solvothermal synthesis to study how the synthesis parameters affect the structure and up conversion luminescence of the materials and thus their usability as labels in biomedical applications. The purity of the materials was studied with Fourier transform infra-red (FT-IR) spectroscopy and the particle size and morphology with transmission electron microscopy (TEM). The crystal structure was characterized with X-ray powder diffraction (XPD) and the crystallite sizes were calculated with the Scherrer formula. Up-conversion luminescence and luminescence decays were studied with near infrared (NIR) laser excitation at 970 nm.The presence of the oleic acid was observed in the FT-IR spectra. The TEM images showed small quasi spherical nanoparticles as well as long nanorods. The XPD measurements revealed that both cubic and hexagonal forms of NaYF4 were present in the materials. The crystallite sizes ranged from ca. 20 to over 150 nm for the cubic and hexagonal phases, respectively. The characteristic up-conversion luminescence of Era(3+) in red (640-685 nm; F-4(9/2) -> I-4(15/2)) and green (515-560 nm; H-2(11/2), S-4(3/2) -> I-4(15/2) transitions) wavelengths was observed. The most intense luminescence and the longest luminescence emission lifetime were obtained with the material annealed for 12 hat 177 degrees C with 1.8 MPa pressure due to the predominance of the well-crystallized hexagonal form of NaRF4 (R: Y, Yb, Er). (C) 2016 Elsevier B.V. All rights reserved.

Layered Silicates (LS) such as hectorite clays have been used as host matrices for optically functional species due to their tunable structure. Luminescence has been reported for different phosphors using LS as a host host intercalated with rare earth containing compounds. However, the LS composition has OH- ions which may result in severe luminescence quenching. In the present work, a new LS material based on the hectorite structure was synthesized by replacing OH- with F-. This LS produced without doping strong blue/green emission. Also persistent luminescence was observed for this material with the main emission lifetime about 321 ms. It is the first time that such kind of luminescence has been reported for trioctahedral smectites. The origin of the luminescence was proved to be a Ti3+ impurity - originally as TiIV in the SiO2 precursor. Both conventional and persistent luminescence mechanisms were suggested for this “non-doped” system.

A series of terbium doped TiO2 (TiO2:Tb3+) nanophosphors (NPr) were synthesized by the solution combustion method with varying the concentration of Tb3+. The X-ray diffraction results confirmed that the polycrystalline tetragonal structure of TiO2 NPr was formed. The X-ray photoelectron spectroscopy and electron paramagnetic resonance measurements confirmed the presence of oxygen and TO3+ defects. The blue emission from the TiO2:Tb3+ NPr was tuned when the concentration of Tb3+ was varied. These TiO2:Tb3+ NPr have potential applications as sources of blue light in light emitting devices. (C) 2015 Elsevier B.V. All rights reserved.

Praseodymium doped ZrO2 materials were prepared via sol–gel route and structurally characterized by X ray powder diffraction (XPD) technique as well as Rietveld refinements. The addition of the Gd3+ co-dopant gradually changes the zirconia structure from monoclinic to tetragonal, and then to cubic. Intensification of the Pr3+ luminescence was observed with the increasing Gd3+ co-dopant concentration. Emission spectra of the Zr0.99−xGdxPr0.01O2 materials show an initial strengthening of the red emission of Pr3+ (1D2→3H4 transition) with increasing Gd3+ co-doping. However, the luminescence is quenched at the highest Gd3+ concentration-possibly due to strongly increased concentration of the charge compensation defects. The valence change (PrIV→Pr3+) is supported by the XANES results on the LIII edge of Pr. Although predominantly in the Pr3+ form irrespective of the Gd3+ concentration, the contribution from PrIV is clearly visible at low (or zero) Gd3+ concentrations leading to the loss of Pr3+ and to poor luminescence output. Though enhancing the emission intensity of Pr3+, the defect clusters engender short Pr3+–Pr3+ distances enhancing the cross-relaxation process coupling the 3P0→1D2 relaxation with the 3H4→3H6 excitation. This process leads to the high red/blue–green emission ratio by quenching the 3P0→3H4 transition in blue–green. Eventually, the increased Gd3+ co-doping dilutes the Pr3+ ions and, the cross-relaxation process becomes non-operational; the quenching of the 3P0→3H4 transition is reversed.

Photon upconverting luminescent hexagonal NaRF4:Tm (0.5 mol%) (R:Y3+,Yb3+) crystals with Yb3+concentrations between 20 and 99.5 mol% were synthesized by a modified thermal coprecipitation method.

In this paper, we discuss the impact of the temperature and the duration of the melting on the persistent luminescence properties of phosphate glasses within the P2O5-Na2O-CaO and P2O5-Na2O-SrO systems prepared using a standard melting process in normal atmosphere by adding Sr(4)Al(14)O25:Eu2+,Dy3+ microparticles in the glass batch before melting. Glasses with persistent luminescence properties can be successfully prepared if the melting conditions are carefully controlled. (C) 2015 Elsevier B.V. All rights reserved.

Optical energy storage materials can store energy when exposed to radiation and subsequently release it as light after thermal or optical stimulation. Such materials are thus employed in, e.g., detectors, dosimetry, self-lit signs, and imaging. In, e.g., dosimetry, the response of the material is correlated with the absorbed energy, but no distinction of different radiation energies can be achieved. In this work, Sr3MgSi2O8:Eu2+,Dy3+ was studied with thermoluminescence (TL) initiated by irradiating the material with photon energies between 2.6 (480) and 5.4 eV (230 nm). The TL glow curves revealed that the material has two main traps. Both the overall TL intensity and the TL intensity ratio between the two traps strongly depend on the photon energy of the irradiation. A mechanism of energy storage and charge carrier release in this material was constructed from the results obtained.

The NaYF4:Yb3+,R3+ (R: none, Pr, Nd, Sm, Eu, Tb or Dy) materials were prepared with selected dopant concentrations, using co-precipitation synthesis, to study the effect of the dopant and its concentration on the structure of these materials. The thermal behaviour of the as-prepared materials was studied with differential scanning calorimetry (DSC) and thermogravimetry. The structures prior to and after annealing were identified with X-ray powder diffraction. The materials were mainly hexagonal with occasional slight cubic impurity. The DSC curves revealed the cubic-to-hexagonal phase transition at 400–450 °C which temperature changes irregularly with the R3+ dopant and its concentration. The specific enthalpy of this transition varies also in a complicated way but may be correlated with the completeness of the transition. The hexagonal-to-cubic transition temperature (ca. 670 °C) is rather constant regardless of the R3+ dopant or its concentration. The temperatures and specific enthalpies of the phase transitions are useful when choosing the optimum dopant concentrations. It is also possible to estimate the structure prior to annealing with significant savings in use of resources. Only with Sm3+ and Dy3+ doping, no visible up-conversion luminescence was observed—in addition to the Er3+ and Tm3+ impurity emission. Eventually, it was found that the hexagonal form gives much stronger up-conversion luminescence and changes in the rare earth concentration allows the formation of this form.

Similar to many other Eu2+,RE3+ -co-doped persistent luminescence materials, for Sr2MgSi2O7:Eu2+,RE3+ the initial intensity and duration of persistent luminescence was also found to depend critically on the rare-earth (RE) co-doping. An enhancement of 1- 2 orders of magnitude in these properties could be obtained by Dy3+ co-doping whereas total quenching of persistent luminescence resulted from the use of Sm3+ and Yb3+. To solve this drastic disparity, the effects of the individual RE3+ ions were studied with thermoluminescence (TL) spectroscopy to derive information about the formation of traps storing the excitation energy. The charge compensation defects were concluded to be the origin of the complex TL glow curve structure. The tuning of the band gap of the Sr2MgSi2O7 host and especially the position of the bottom of the conduction band due to the Eu2+,RE3+ co-doping was measured with the synchrotron radiation vacuum UV (VUV) excitation spectra of the Eu2+ dopant. The model based on the evolution of the band gap energy with RE3+ co-doping was found to explain the intensity and duration of the persistent luminescence.

The synthesis conditions of the Yb3+ and Tb3+ co-doped NaYF4 were optimized by reducing the number of washings to include only ethanol. The avoidance of the loss of amorphous NaF prior to post-annealing of the as-prepared materials resulted in the enhancement of the otherwise rather weak up-conversion from Tb3+ by 1–2 orders of magnitude. At the same time, the temperature of formation of the hexagonal NaRF4 phase with high up-conversion could be lowered by 100°C down to 350°C. This improvement in up-conversion was concluded to result from the better stoichiometry of the material without washing with water. The deficit of Na+ would result in the excess of fluoride which, although not as fatal to the luminescence as the fluoride vacancies, has serious implications to the up-conversion intensity. A further enhancement in the up-conversion luminescence was observed to be due to the Er3+ ion impurity frequently associated with high-concentration Yb3+ materials. The mechanism involving the unintentional Er3+ sensitizer and the resonance energy transfer in the Yb3+–Er3+–Tb3+ co-doped NaYF4 were discussed based on the energy level schemes of the Yb3+, Er3+, and Tb3+ ions in NaYF4.