Véronique Jubera

Luminescent properties of Eu3+-activated lithium rare earth borates and oxyborates

Abstract The luminescence characteristics of Eu 3+ in the borates Li 6 Y(BO 3 ) 3 and Li 3 La 2 (BO 3 ) 3 and the recently discovered oxyborates LiLa 2 O 2 BO 3 , LiLn 6 O 5 (BO 3 ) 3 (Ln=Y,Gd) and Li 2 Lu 5 O 4 (BO 3 ) 3 have been investigated and compared to those in the borates LnBO 3 (Ln=La,Y) and the oxyborates “Ln 3 BO 6 ” (Ln=La,Gd). The probability of electric dipole f–f transitions is generally markedly higher in oxyborates than in borates. This is ascribed to the highly anisotropic environment of rare earth ions and the low position of the charge transfer states. Consequently, the Eu 3+ -activated oxyborates are characterized by a deeper red emission and faster decay.

Europium-Doped Mesoporous Titania Thin Films: Rare-Earth Locations and Emission Fluctuations under Illumination

Herein, Eu(III)-doped 3D mesoscopically ordered arrays of mesoporous and nanocrystalline titania are prepared and studied. The rare-earth-doped titania thin films-synthesized via evaporation-induced self-assembly (EISA)-are characterized by using environmental ellipsoporosimetry, electronic microscopy (i.e. high-resolution scanning electron microscopy, HR-SEM, and transmission electron microscopy, HR-TEM), X-ray diffraction, and luminescence spectroscopy. Structural characterizations show that high europium-ion loadings can be incorporated into the titanium-dioxide walls without destroying the mesoporous arrangement. The luminescence properties of Eu(III) are investigated by using steady-state and time-resolved spectroscopy via excitation of the Eu(III) ions through the titania host. Using Eu(III) luminescence as a probe, the europium-ion sites can be addressed with at least two different environments within the mesoporous framework, namely, a nanocrystalline environment and a glasslike one. Emission fluctuations ((5)D(0)-->(7)F(2)) are observed upon continuous UV excitation in the host matrix. These fluctuations are attributed to charge trapping and appear to be strongly dependent on the amount of europium and the level of crystallinity.

ZnAl2O4 as a potential sensor: variation of luminescence with thermal history

ZnAl2O4 spinel powders were prepared using the Pechini or co-precipitation synthetic route and were then treated at different temperatures (600–1350 °C). These powders were characterised by X-ray diffraction, scanning electron microscopy (SEM), diffuse reflectance and luminescence measurements. SEM investigations and the X-ray patterns showed that the spinel crystallite size was dependent on the synthetic route and the treatment temperature. In addition, the structural evolution was investigated by Rietveld refinements. The inversion rate decrease was correlated with the temperature, leading to a direct spinel phase for the sample treated at high temperature. Furthermore, luminescence measurements showed various emissions linked to the presence of defects in the matrix structure. The two main emissions observed were attributed to oxygen vacancy and Zn in the interstitial positions (as revealed by differential Fourier maps). The luminescence spectra exhibited strong differences between 1200 °C and 1350 °C. At the higher temperature, the characteristic emission spectra can be attributed to the direct spinel phase. The indirect–direct spinel transformation can be monitored through the change in the optical properties and correlated to the thermal history of the sample.

A facile building-block synthesis of multifunctional lanthanide MOFs

We report a building blocks approach providing a direct route to multifunctional MOFs, that display photoluminescent properties, robustness, porosity and in some cases unique magnetic properties. The self-assembly of [Mo(CN)8]4− with several in situ prepared lanthanide building blocks gives 3D robust porous networks with open channels. This approach solves the coordination problem, allowing exact placement of the lanthanide ions within the structure. Our MOFs feature good thermal stability and permanent porosity thanks to the strong carboxylate and cyanide linkages. The fact that we have both nitrogen-containing ligand and a π-system means that these MOFs can be excited using low-energy photons. Efficient visible emission was observed for MOFs containing Eu(III) and Tb(III). Surprisingly, the Tb-MOF shows ferromagnetic behavior, proving that the magnetic interaction between Tb(III) ions is strong enough to compensate the ligand field effects.

Lithium ion as growth-controlling agent of ZnO nanoparticles prepared by organometallic synthesis

ZnO nanoparticles were synthesized by adding solid Zn(c-C6H11)2 to a THF solution of the lithium (sodium) precursor and octylamine (OA) as stabilizer. The proportion of Li (Na) was varied from 1 to 10 mol% compared to Zn. Two different lithium precursors namely Li[N(CH3)2] (series 1) and Li[N(Si(CH3)3)2] (series 2) and one sodium precursor namely Na[N(Si(CH3)3)2] (series 3) were used. Interestingly, Li precursors induce a modification of the growth of the particles while, no effect is observed when Na precursors is used. Indeed, isotropic nanoparticles were obtained when Li precursors were used while nanorods were formed with Na precursor as already observed in the same experimental conditions without alkali-metal precursor. Observations by TEM show that as the Li/Zn molar ratio increases, the mean diameters of the nanoparticles vary from 3.7 ± 0.7 nm to 2.5 ± 0.4 nm, and from 4.3 ± 1.0 nm to 3.1 ± 0.8 nm for series 1 and series 2, respectively, while the length and the diameter of the nanorods are ca. 11 × 4 nm, for series 3. Interestingly, the consequence of the lithium induced size variation leads to a shift of the emission band in the visible range, from yellow to blue through white as a function of increasing concentration of lithium precursor. The intensity of this emission is strong enough to be clearly seen by the human eye.

Luminescence switch of Mn-Doped ZnAl2O4 powder with temperature

Manganese-doped ZnAl2O4 phosphors were prepared by the Pechini synthesis route and treated at various temperatures from 600 to 1350 °C. The samples were characterized by TEM-EDX, XRD, EPR, and their diffuse reflectance and luminescence properties were investigated. The structural analysis showed the high solubility limit of manganese in this spinel matrix and allowed the determination of the global inversion rate, which characterizes the cation distribution in the A and B sites of the spinel structure. As the annealing temperature increased, this factor decreased leading to a more direct matrix. EPR analysis showed that, besides Mn3+ to Mn2+ reduction, the local environment of Mn2+ cations changed with the annealing temperature, which was also reflected in the evolution of the optical properties. As the annealing temperature increased, the red luminescence related to the presence of divalent manganese in octahedral sites faded and was replaced by a new green emission due to Mn(II) ions located in tetrahedral sites within the spinel structure. For 0.5% Mn-doped ZnAl2O4, this red to green luminescence switch occurred for samples treated between 1200 and 1350 °C. Moreover, the Al-overstoichiometric samples (Mn:ZnAl2.2O4+δ) showed that it is possible to modify the temperature range and the kinetics of this variation in emission wavelength. These tuneable properties suggest that Mn-doped spinels are potential candidates for developing stable and highly sensitive thermal sensors.

Optical spectroscopic study of Eu 3+ crystal field sites in Na3La9O3(BO3)8 crystal

Time-resolved line-narrowed fluorescence spectroscopy of Eu(3+) ions in a new oxyborate Na(3)La(9)O(3)(BO(3))(8) crystal shows the existence of four independent symmetry crystal field sites for the rare-earth ion. A crystal field analysis and simulation of the experimental results have been performed in order to parametrize the crystal field at the Eu(3+) sites. A plausible argument about the crystallographic nature of these sites is given.

Continuous supercritical synthesis of high quality UV-emitting ZnO nanocrystals for optochemical applications

ZnO nanocrystals (NCs) are of increasing interest in many industrial applications especially in the field of optochemical sensors. We recently demonstrated the synthesis of excitonic luminescent ZnO NCs using the perfectly controlled environment offered by supercritical microfluidics. However, the limited production rates of such a microscale synthesis approach make it essential to develop scaled-up continuous supercritical synthesis processes. Here, we investigate the influence of the scale-up effect from continuous microfluidic up to millifluidic systems over the synthesized ZnO NCs’ optical properties. We also propose to discuss the influence of reactor’s dimensions, as a consequence of different hydrodynamics regimes, on the characteristics of ZnO NCs. The obtained ZnO NCs have various shapes depending on the considered set-up. A strong excitonic-only emission in the UV region in room temperature photoluminescence (PL) spectra is obtained from all types of reactors, demonstrating the success of the scale-up process for higher production rates (up to 100 mg per hour).

Hydrothermal synthesis, structural characterization and thermal, spectroscopic and magnetic properties of the inorganic–organic hybrid phosphate, [(C2N2H9)VF(PO4)]

The [(C 2 N 2 H 9 )VF(PO 4 )] compound has been synthesized by hydrothermal techniques under autogeneous pressure at 170°C. The compound was characterized from X-ray powder diffraction data with the Rietveld method. Its crystal structure consists of sheets linked by the ethylenediammonium cations and constructed by chains of VO 3 F 2 N octahedra. The thermal study indicates that the compound is stable up to 290°C. In the IR spectrum the bands of both the ethylenediammonium and phosphate ions are observed. From the reflectance diffuse spectrum the Dq and Racah parameters have been calculated. The magnetic measurements indicate the presence of antiferromagnetic interactions.

New luminescent rare earth activated oxynitridosilicates and oxynitridogermanates with the apatite structure

New luminescent oxynitrides based on the apatite structure doped with Eu2+ and Ce3+ have been prepared by ammonolysis of oxysilicate or oxygermanate precursors. The host compounds have the general formula La10−xSrx(Si/Ge)6O27−x/2−3y/2Ny. When doped with divalent europium or trivalent cerium they present blue, green, yellowish green or orange luminescence under UV/blue excitation. Remarkable long wavelength emissions centred beyond 500 nm are observed for the Ce3+ activated oxynitridosilicates and oxynitridogermanates, with compositions La8.9Ce0.1SrSi6N0.37O25.95, La8.9Ce0.1SrSi6N1.06O24.91 (λem = 545 nm) and La9.9Ce0.1Ge6N2.46O23.31 (λem = 590 nm). The treatment of precursor oxides with the required cationic ratios in NH3 gas was used as a convenient general method to prepare the oxynitride materials at moderate temperatures. This allowed decreasing the nitriding temperature from 1600–1400 °C to 1050–600 °C with respect to the method generally used for the synthesis of luminescent (oxy)nitride silicates, the solid state reaction between oxides, nitrides and carbonates under a N2 or N2–H2 flow.