F. Pellé

Synthesis, characterisation and luminescent properties of a new three-dimensional lanthanide trimesate: M((C6H3)–(CO2)3)(M = Y, Ln) or MIL-78

A new three-dimensional europium-doped yttrium 1,3,5-benzenetricarboxylate has been obtained under hydrothermal conditions. The structure of MIL-78(Y, Eu) or Y1−xEux((C6H3)–(CO2)3) (x ∼ 0.024) has been solved from X-ray powder diffraction data (a = 6.941(1) A, b = 14.700(2) A, c = 8.488(1) A, β = 107.73(1)°, V = 824.9(4) A3, space group C2/m (no. 12)). Its structure is built up from chains of edged-sharing eight-coordinated monocapped square antiprism polyhedra and trimesate anions, creating a three-dimensional structure. The same structure is formed by replacing yttrium by a rare-earth element. The thermal behaviour of MIL-78(Y, Eu) has been investigated using TGA and X-ray thermodiffractometry and indicates that MIL-78(Y, Eu) is stable up to 450 °C. The study of the optical properties of samples doped with different rare-earths reveals strong fluorescence under VUV radiation.

Toward laser emission of epitaxial nanorod arrays of ZnO grown by electrodeposition

Arrays of epitaxial, vertically oriented nanorods of ZnO are grown at low temperature (80°C) on the (0002) plane of GaN single crystals by electrodeposition in an aqueous solution. The freestanding nanocolumns are prepared by a template-free method based on simple solution chemistry. At room temperature, the nanorods present an amplified ultraviolet emission centered at 381nm with an excitation threshold at 4.4MWcm−2. The amplified emission of the ZnO columns is dominated by the radiative recombination of excitons.

Strong 1.53 μm to NIR-VIS-UV upconversion in Er-doped fluoride glass for high-efficiency solar cells

Optical spectra of Er-doped modified ZBLAN glasses are studied at room temperature. Radiative quantum yields of the 4I11/2 and 4I13/2 levels are estimated from the experimentally measured lifetimes and from the spontaneous emission probabilities calculated from the Judd-Ofelt theory. The spectra of upconversion (UC) luminescence excited with 1.53 μm cw Er fiber laser are investigated in a wide spectral domain [from the near-infrared (NIR) to UV]. Absolute UC efficiency (i.e., the ratio of UC luminescence power to the absorbed pump power) is experimentally measured; efficiency of up to 12.7% is obtained. A conclusion is made about perspectives of use of the studied glasses as upconverter material for solar cells of enhanced efficiency.

Synthesis, structure and properties of a three-dimensional porous rare-earth carboxylate MIL-83(Eu): Eu2(O2C-C10H14-CO2)3

A new three-dimensional rare-earth(III) dicarboxylate, MIL-83(Eu) or Eu2(O2C-C10H14-CO2)3, has been obtained under hydrothermal conditions from aqueous mixtures of europium nitrate and 1,3-adamantanedicarboxylic acid. Its three-dimensional structure consists of chains built up from face-sharing nine-coordinated rare-earth(III) polyhedra connected by the 1,3-adamantanedicarboxylate linkers, delimiting anhydrous porous channels. This structure is understood by comparison with the already known hybrid rare-earth(III)carboxylates. The thermal behavior of the solid has been investigated using TGA and X-ray thermodiffractometry and shows that the title solid is stable up to 300 °C. Finally, the study of the luminescence properties of the yttrium(III) form of MIL-83(Y/Eu) doped with europium(III) reveals a significant red-emission upon VUV radiation.

Cooperative luminescence as a defining process for RE-ions clustering in glasses and crystals

Abstract The presence of ions clustering is an important to be solved in glass fiber amplifiers even at a low doping concentration of active ions because it reduces the pumping efficiency. However ion-ion interaction can take place in a given sample by different processes which have different ranges of interaction: some of them from a few Angstrom to several tens of Angstrom (non-radiative transfer), others at centimetre distance (radiative transfer), so the problem arises: how to define an ion cluster? We propose to consider cooperative luminescence as a defining process for ions cluster which can help to distinguish them from energy transfer processes and so to separate processes occurring at different distances. In this work Yb 3+ is used as a clustering probe. Results obtained on halide glasses and crystals are presented and compared with results from literature for silica glasses.

NIR to visible up-conversion, infrared luminescence, thermoluminescence and defect centres in Y2O3?:?Er phosphor

Er3+ doped Y2O3 phosphor was prepared by the solution combustion method and characterized using powder x-ray diffraction and energy-dispersive analysis of x-ray mapping studies. Room temperature near infrared (NIR) to green up-conversion (UC) emissions in the region 520–580 nm {(2H11/2, 4S3/2) →4I15/2} and red UC emissions in the region 650–700 nm (4F9/2 →4I15/2) of Er3+ ions have been observed upon direct excitation to the 4I11/2 level using ~972 nm laser radiation of nanosecond pulses. The possible mechanisms for the UC processes have been discussed on the basis of the energy level scheme, the pump power dependence as well as based on the temporal evolution. The excited state absorption is observed to be the dominant mechanism for the UC process. Y2O3 : Er exhibits one thermally stimulated luminescence (TSL) peak around 367 °C. Electron spin resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the centres responsible for the TSL peak. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of at least three distinct centres. One of them (centre I) with principal g-values g|| = 2.0415 and g⊥ = 2.0056 is identified as centre while centre II with an isotropic g-factor 2.0096 is assigned to an F+-centre (singly ionized oxygen vacancy). Centre III is also assigned to an F+-centre with a small g-factor anisotropy (g|| = 1.974 and g⊥ = 1.967). Additional defect centres are observed during thermal annealing experiments and one of them appearing around 330 °C grows with the annealing temperature. This centre (assigned to an F+-centre) seems to originate from an F-centre (oxygen vacancy with two electrons) and the F-centre appears to correlate with the observed TSL peak in Y2O3 : Er phosphor. The trap depth for this peak has been determined to be 0.97 eV from TSL data.

Concentration and excitation effects in multiphonon non-radiative transitions of rare-earth ions

Abstract Non-radiative transitions in rare-earth ion doped systems are usually considered to be concentration and excitation intensity independent. Here we consider theoretically and experimentally a saturation in the capacity for the modes of the lattice to dissipate energy. Some first experimental results of this new effect in the case of Yb3+ doped borate glasses are presented showing that at excited ion densities larger than 1018 cm−3, non-radiative multiphonon transitions can be completely quenched well before amplification by stimulated emission is reached except when concentration rises above 8 × 1020 cm−3. The experiment is well described at lower concentration by considering that the excited ions can share a common phnon density of states. At higher concentration, the common accepting modes diffusion length is reduced from 23 to 8 A and a non-saturable radiative relaxation rate background of about 575 s−1 is observed which can be viewed as an increase in the effective accepting mode coupling strength by a factor 1.33.

Lanthanide doped ZnS quantum dots dispersed in silica glasses: an easy one pot sol–gel synthesis for obtaining novel photonic materials

Silica glasses containing both ZnS quantum dots (QDs) and luminescent lanthanide ions are attractive candidates to develop new lighting displays, sensor devices or laser emitters. This work reports an easy sol–gel method to prepare Eu3+-doped and Eu3+,Mn2+-codoped ZnS nanocrystals dispersed in a transparent silica matrix. Semiconductor nanocrystals with an average size of 5–6 nm and exhibiting both cubic and hexagonal phases were obtained at low temperature. The luminescent interactions between ZnS QDs, Eu3+ and Mn2+ ions provided materials with different optical responses but also gave information about the organization of the different species in the nanocomposite. Indeed, Eu ions were found to be both dispersed within the silica and located at the surface of the nanochalcogenide, the latter providing a ZnS → Eu3+ energy transfer. Incorporation of Mn2+ into the ZnS lattice induced the appearance of defect states that enhance the blue luminescence of the nanocomposite. These results underline the sensitivity of optical processes to the nature and organization of the active species, which is of vital importance for the design of photonic materials.

Colloidal and monocrystalline Ln3+ doped apatite calcium phosphate as biocompatible fluorescent probes

Ultrafine individualised mono crystalline Ca(10-x)(PO4)(6-x)(HPO4)x(OH)(2-x) deficient calcium hydroxyapatite nanocrystals displaying fluorescence under visible excitation are proposed for utilisation as biocompatible biological probes.

RED TO BLUE UP-CONVERSION EMISSION OF TM3+ IONS IN YB3+-DOPED GLASS-CERAMIC

A detailed study of the spectroscopic properties of PbF2+GeO2+WO3 glass ceramics doped with Tm3+ and codoped with Tm3+ and Yb3+ ions upon 680 nm dye laser light excitation has been made. The absorption, up‐conversion emission, excitation, and time‐resolved spectra were measured as a function of TmF3 and YbF3 concentrations at room temperature. An enhancement of blue emission centered at 478 nm in the codoped sample was observed. The optium concentrations of TmF3 and YbF3 for this blue emission are about 0.2 and 15 mol %, respectively. It is suggested that the results are due to energy transfer between Yb3+ and Tm3+ ions. Measured oscillator strengths and radiative rates for several transitions are compared with calculated values using the Judd–Ofelt theory with reasonable agreement between theory and experiments for sample doped with Tm3+ ions.