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International audience; The present study is dedicated to the development of aluminum borate luminescent powders which appear as promising and more environmental-friendly than conventional phosphors. The different steps of the luminescent powder synthesis, in particular the precursors (inorganic and organic) ratios and calcination conditions, are adjusted by studying the resulting optical features to get a broader and warmer white emission. As shown in previous works, the photoluminescence emission originates from polyaromatic hydrocarbons trapped in the inorganic aluminum borate matrix. Morphological and structural properties of the particles remain otherwise identical for every optimized parameter. The aluminum borate powder synthesized in these optimized conditions exhibits a wide and intense emission band under near-UV excitation. These structural and optical properties combined with time-resolved photoluminescence measurements demonstrate that the adjustments of the synthetic process allow the formation and trapping of more and different emitting centers. The modulation of the excitation wavelength (from 305 to 405 nm) leads to a tunable photoluminescence emission characterized by a large band lying between 400 and 700 nm. This feature, associated with the use of secure and abundant precursors makes aluminum borate powders very interesting phosphors for optical applications.

International audience; Improving phosphor photoluminescence efficiency is a key parameter to boost the performances of many optical devices. In this work, colloidal silver nanocubes, homogeneously spread on a luminescent surface, have proved to help both injecting and extracting light in and out of the photoluminescent layer and hence contributed significantly to the enhancement of the fluorescence. This approach has been applied to two materials: the well-known Y3Al5O12:Ce yellow phosphor and an optical quartz. The emission efficiency, for sol–gel derived YAG:Ce layers, has increased 80% in the presence of an optimal nanoparticle density, whereas for quartz, a weakly fluorescent material, the photoluminescence signal can be enhanced by a 200-fold factor. A physical analysis based on simulations shows that the disorder is an important factor and that the surface density of Ag nanoparticles is a crucial parameter.

International audience; In this study, we report on the enhancement of the light extraction efficiency of sol–gel-derived Y3Al5O12:Ce3+ (YAG:Ce) coatings using ZnO nanowire (NW) arrays. The ZnO NWs were grown by hydrothermal synthesis from a ZnO seed layer directly deposited on a YAG:Ce coating. Highly dense and vertically aligned ZnO NW arrays were evidenced on the top of the YAG:Ce coating by electron microscopy. A photoluminescence study showed that this original design leads to a different angular distribution of light together with an increase in emission efficiency of the YAG:Ce coating upon blue excitation, up to 60% more efficient compared to a non-structured YAG:Ce coating (without NWs). These improvements are ascribed to multi-scattering events for photons within the structure, allowing them to escape from the phosphor layer by taking optical paths different from those of the non-structured coating. This strategy of light extraction enhancement appears to be very promising, since it uses soft chemical processes and cheap ZnO NWs and is applicable to any sol–gel-derived luminescent coating.

International audience; Luminescent ZnO nanowires (NWs)/YAG:Ce heterostructure coatings have been elaborated by impregnating cheap hydrothermally-grown ZnO NWs array with ground commercial YAG:Ce nanoparticles (NPs). TEM and photoluminescence quantum yield measurements have been used to make YAG:Ce NPs suitable for soaking within the ZnO NWs. Structural, morphological and optical studies of functional nanocomposite coatings have evidenced an optimal amount of YAG:Ce allowing the visible photoluminescence signal of ZnO NWs nearly to triple whereas slight red shift accompanied by an enlargement of the long wavelength side has been observed for YAG:Ce emission. These phenomena have been explained through the existence of structural defects on ZnO NWs surface and on emission mechanisms implying either scattering effects or the passivation of NWs surface by YAG:Ce NPs. Furthermore, an energy diagram involving different structural defects of ZnO NWs as well as Ce 3+ ions energy levels has been built from excitation and emission spectra and from decay curves recorded monitoring the orange-red emission of ZnO. The elaborated heterostructure functional coatings have proved to exhibit tunable optical properties in terms of spectral distribution playing on the excitation source. Hence, they appear as a smart way for developing LEDs-based devices with flexible photometric parameters for a broader range of optical applications.

International audience; The use of rare-earth elements (Y, Ce, Eu.. .) in commercial phosphor-converted white light-emitting diodes (pc-WLEDs) raises environmental and geopolitical concerns. Lanthanide-free white emitting phosphors based on amorphous yttrium aluminium borate (YAB) have previously been synthesized by the polymeric precursor (PP) method, showing high internal quantum yields (iQY) and a broad emission band in the visible range. However, yttrium has still been largely present in the amorphous YAB matrix. Here we synthesize zinc aluminium borate (ZAB) powder by the PP method, a novel family of rare-earth-free phosphors producing broad and intense photoluminescence emissions when excited by near-UV from 300 to 400 nm, with high iQYs up to 60%. The photoluminescence band is easily tuneable in the visible range of the electromagnetic spectrum, from warm-to cold-white emissions by adjusting either the precursor ratio or the calcination temperature used to prepare the ZAB powders. Finally, we report spectroscopic arguments that reinforce the hypothesis of the molecular origin of the photoluminescence in ZAB phosphors. Our results demonstrate the essential role of the composition (Al/B ratio) and thermal treatment in the transformation and trapping of carbonaceous species (i.e. black carbon or luminescent molecular species) in an amorphous ZAB matrix. This efficient family of phosphors only composed of abundant and non-toxic elements opens the way for the integration of rare-earth-free single phosphors synthesized by wet chemistry into WLED lighting devices.

International audience; Cubic α-NaYF4 nanometer-sized crystals doped with Eu3+, Tb3+, Yb3+-Eu3+ or Yb3+-Tb3+ were synthesized by an original coprecipitation route. The obtained nanoparticles exhibited primary particles showing cubic shape with sizes ranging between 35 and 65 nm. The singly- or co-doped nanophosphors exhibited strong red (Eu3+) or green (Tb3+) fluorescence upon ultraviolet (UV) or near infrared (NIR) excitation, which resulted respectively from down- or up-conversion processes occurring in their structure. Spectroscopic properties were investigated on the basis of emission spectra as well as luminescence decays. From the emission spectra of Eu3+ doped samples, the Ω2 and Ω4 Judd-Ofelt intensity parameters were calculated. The concentration quenching of the Eu3+ or Tb3+ ion emissions in singly-doped or Yb3+ co-doped α-NaYF4 were ascribed to resonant cross-relaxations. The main derived interaction between the active ions was evidenced as an electric dipole-dipole one through fitting the decays curves with the Inokuti-Hirayama model. The critical distances and energy transfer microparameters for the transfer processes were given showing very short range interaction. The dependence of integral up-conversion intensity on the NIR energy of the beam power was measured. The results indicated a two-photon process based on a cooperative energy transfer.

International audience; Pure hexagonal β-NaGdF 4 and Eu 3+ or Pr 3+-doped β-NaGdF 4 nanocrystals in shape of nanorods of ∼140 nm of length have been synthesized using a simple and rapid coprecipitation method. XRD analysis evidenced pure single phases exhibiting characteristic luminescence of Eu 3+ or Pr 3+ ions when excited under UV or blue radiations. Samples gave rise to intense orange-red emission and quite white emission in the case of respectively Eu 3+ or Pr 3+ doped samples. Optical properties are discussed in the frame of Judd-Ofelt theory and considering that an energy transfer occurs between active ions. Notably it was found that the quantum-cutting by a two-step energy transfer from Gd 3+ to Eu 3+ can improve the red emission of Eu 3+ ions under near-VUV excitation. Such quantum-cutting mechanism has been also considered in the case of Pr 3+ doped β-NaGdF 4. However, the efficiency of both processes in comparison with literature suggest that only one part of the energy in the excited states within Gd 3+ can be transferred to Eu 3+ for its red emission. In the case of Pr 3+ ions, the energy is more probably released through the self-trapped exciton emission certainly due to the lack of resonant VUV excitation.

International audience; K2SiF6 and BaSiF6 nanoparticles are synthesized for the first time by thermal decomposition of a mixture consisting of oleic acid and 1-octadecene using ammonium fluoride as fluorine source instead of highly toxic hydrofluoric acid. The structural and morphological properties of the as-obtained nanocrystals are investigated.

International audience; Light extraction is a key parameter to improve the performances of optical devices. Nanopatterned Y3Al5O12:Tb3+ luminescent coatings usable in such devices have been elaborated and have shown enhanced emission efficiency compared to their unpatterned counterparts. These nanostructured films were obtained by using the colloidal lithography combined with the Langmuir-Blodgett technique. It is the first time to our knowledge that this patterning technique is directly applied on YAG matrix. Resulting nanostructuring is a hexagonal network capable of modifying the light travelling path within the substrate. Conventional and angular-resolved photoluminescence were investigated on both unpatterned and patterned samples. Due to the nanostructuring, the extraction efficiency is improved by 26% and 131% depending on the crystallinity of the sample. Noticeably, nanostructuring is found to have an influence on the angular distribution of photoluminescence whose intensity has been evaluated to its maximum normal to the film surface.

International audience; Nano-sized Eu 3+-doped GdBO 3 vaterite type nanophosphors have been synthesized successfully via acetylacetone assisted aqueous sol-gel route. Systematic investigation of the influence of Eu 3+ ion content and pH value of the suspension of precursors on structural, morphological and luminescence spectroscopic properties has been performed. X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, field emission-scanning electron microscopy (FE-SEM), Thermogravimetric and differential thermal gravimetry (TG/DTA), room temperature steady and time resolved photoluminescence (PL) spectroscopy techniques were employed to characterize the obtained powder samples. It has been found that the Eu 3+ ion content and pH value precursors suspension have important effect on the structural, morphology and emission properties. The XRD analysis reveals that GdBO 3 : 10% mol Eu 3+ synthesized at pH = 2 and pH = 10 exhibit the quasipurest P6 3 /mmc vaterite structure. The vibrational absorption of (BO 3) 3group is assigned to the GdBO 3 with calcite type or other vaterite structure namely triclinic and monoclinic. The morphologies of GdBO 3 particles size increases with increasing of pH value. All sample present only the orange-red colors emission from 5 D 0 levels to the 7 F J levels (J = 0-3) of Eu 3+ ion. Furthermore, it was found that 10% concentration of Eu 3+ presents the quenching of emission in GdBO 3. Also, it was that the emission intensity was affected by the pH value and sample with pH = 10 exhibits the highest emission intensity. The asymmetry ratio parameter R between red and orange emission intensities in function of Eu 3+ content and pH value is determinate and discussed.

International audience; With the goal of controlling the oxidation state of cerium ions in Ce-doped Y3Al5O12 nanocrystals (YAG:Ce), different gases (Ar/H2, O2, Ar) were bubbled inside the solution of precursors and solvent prior to the solvothermal synthesis. As assessed by X-ray absorption spectroscopy, this protocol modifies the Ce3+ : Ce4+ ratio in YAG:Ce nanocrystals, although it does not allow complete prevention of the Ce3+ → Ce4+ oxidation. In addition, gas bubbling strongly influences the size and crystal quality of YAG:Ce nanocrystals, as evidenced by X-ray diffraction and transmission electron microscopy. Indeed, the presence of bubbles in the precursor solution induces heterogeneous nucleation, leading to an earlier nucleation step. This favors the growth and the high crystallinity of YAG:Ce nanocrystals leading to record iQY at around 52% for 60 nm sized nanocrystals and 66% for those of 200 nm.

International audience; Undoped and Au-doped ZnO thin films were prepared on glass substrates by sol-gel process and dip-coating technique. The effects of Au concentration (10–30 at%) on structural, morphological and optical properties of the ZnO films were studied using X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), UV–Vis–NIR spectrometry, Hall-effect measurements and photoluminescence (PL) spectroscopy. XRD results show that all films crystallized in a hexagonal wurtzite structure and are highly c-axis oriented. The crystal quality of the films is found to decrease with the increase in Au doping. A weak diffraction peak related to Au cubic phase is also observed at Au concentration higher than 10 at%. The Raman studies confirmed that the film's crystallinity degraded with increasing Au content. FE-SEM micrographs and AFM images show that both morphology and surface roughness are affected by the Au doping concentration. UV–Vis–NIR optical spectra reveal that increasing Au doping into ZnO films enhances their transparency. Photoluminescence measurements at room temperature put into evidence the presence of two intense bands at 373.6 and 385 nm in the UV spectra and four very weak emission bands in the visible spectral range. All the visible emissions are found to increase with Au content. The emission at 373.6 is maximal for the film doped at 20 at%, whereas the one at 385 nm reaches its maximum intensity in the film doped at 10 at%.

International audience; Orthoborate nanopowder phosphor, with formulation Gd0.90Eu0.1BO3 has been synthesized by aqueous sol-gel method, and then annealed at 1000, 1100 and 1200 °C. The crystal structure and the morphology of the as prepared materials were cheeked using several techniques such as: IR, XRD, and SEM. The experimental results show that the synthesized nanoparticles have a primary average diameter of approximately 150–250 nm. The samples crystallize mainly in the vaterite structure. Despite that the signature of triangular borate group BO33- more characteristics of the calcite form is observed in the FTIR spectra indicating that some Eu3+ ions are embedded in parasitic phase such as calcite form. The VUV excitation spectra of the samples were recorded at room temperature and lead to selectivity for the VUV excited emission confirming that the overall emission come from the Eu3+ ions embedded in at least two crystallographic structures. However, the contribution from Eu3+ ions embedded in parasitic phase contribute weakly to the overall emission. The luminescence properties of the prepared phosphors were characterized also by its kinetic decay. The results show that luminescence decaying correlated the observations made for both excitation and emission spectra.

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International audience; This paper addresses the formulae-related enhancement of the GdxY1-xPO4:Tb3+ nanopowders when submitted to (VUV) vacuum ultraviolet. Structural characterizations of GdxY1-xPO4:Tb3+ nanocrystals were conducted by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In particular, for the first time, in this work, the photoluminescence upon VUV excitation of GdxY1-xPO4:Tb3+ powders synthesized by the solvothermal method, is reported, and the optical study reveals that the photoluminescent properties of nanophosphors are strongly dependent on their crystalline structures, as well as on gadolinium-yttrium composition. The highest emission intensity upon 160 nm radiation was observed for the Gd0.5Y0.5PO4:Tb3+ powders which exhibited a two-fold increase in luminescence intensity comparing with YPO4:Tb3+.

This work aims at achieving homogeneous luminescent films from Ce-doped yttrium aluminium garnet (YAG:Ce) phosphors dispersed in aqueous suspensions. Colloidal luminescent suspensions of YAG:Ce nanoparticles have been successfully prepared by a solvothermal process. Powder resulting from the dried as-synthesized suspension was submitted to different thermal treatments and a thorough study of nanoparticles aqueous suspensions has been carried out, for each thermal treatment. The influence of these treatments on the structural, morphological and optical properties of YAG:Ce nanopowders were studied by means of X-ray diffraction, infrared spectroscopy and transmission electron microscopy. Besides, particles size distribution and zeta potential of YAG:Ce aqueous suspensions in different pH conditions were investigated by dynamic light and electrophoretic light scatterings, respectively, in order to characterize suspensions stability. This study enabled us to optimize heat treatment and redispersion parameters in order to achieve very homogeneous luminescent films which can be used for display or mood lighting.

This paper presents some encouraging features on Tb3+-doped LaPO4, used for the first time as nanoparticles (NPs) for photo-structured composite material. First, LaPO4:Tb3+ nanorods are hydrothermally synthesized. TEM analysis point out that their size distribution and their morphology are well defined. Then, a small amount of NPs (2 or 3.9 wt%) is dispersed in acrylate monomers mixture and the photo-patterning is performed by using holography, a technique among the others. It is interesting to note that even if the loading of the NPs is weak, these last ones significantly influence the physical properties of the gratings. Indeed, the observed increase in the refractive index modulation depth is ascribed to both phenomena: the sequestration of LaPO4:Tb3+ nanorods within the periodic structure and the change in the kinetics of photopolymerization. The localization of NPs in the non-illuminated zones, evidenced by fluorescence microscopy, is favoured by the decrease in the rate of polymerization, revealed by infrared spectroscopy.

International audience; Blue-emitting BaMgAl 10 O 17 :Eu 2þ (BAM:Eu), suitable for applications in a next generation of Hg-free lamps based on UV LEDs, was prepared by a microwave induced solution combustion synthesis, using urea as combustion fuel and nitrates as oxidizers. Purity control of the as-synthesized blue phosphor was undertaken by a washing step followed by a reduction one. Structural and morphological properties of the outcoming phosphors have been considered. Synthesis process allows producing a well-crystallized and nanostructured BAM phase within only few minutes. The influence of reduction treatment on the relative amounts of Eu 2þ /Eu 3þ in our samples has been investigated through an original study by magnetization and M€ ossbauer spectroscopy. Furthermore, a complete optical study has been carried out and allowed us to determine the europium localization in the three possible sites in BAM matrix. The percentage of Eu 2þ increased twofold after the reduction treatment, entailing an increase in the lumi-nescence efficiency upon UV excitation. Finally, temperature-dependent luminescence of combustion-derived powders has been studied till 170 C and compared to that of commercial BAM:Eu. MISCS-derived phosphors present a higher thermal stability than commercial one: whereas the emission efficiency of this last was reduced by 64%, the one of combustion-derived BAM:Eu experienced an only 12% decline. Furthermore, while commercial BAM suffered from a severe blue-shift with increasing temperature , our phosphors keep its color quality with a good stability of the photometric parameters.

This work deals with phosphors suitable for applications in a new generation of Hg-free lamps based on deep ultraviolet (DUV), ultraviolet (UV) and blue light emitting diodes (LEDs). First, two synthesis routes, solid state reaction and Pechini method, have been used for preparing an original red phosphor, Y3BO6:Eu3+. Phase purity was confirmed by XRD analysis for Pechini derived sample after washing the powder with an acidic solution while the synthesis dependence of the morphological and optical characteristics was evidenced by SEM analysis and photoluminescence respectively. Y3BO6:Eu3+ powders derived from Pechini process exhibited high photoluminescence quantum efficiency upon excitation at wavelengths corresponding to afore mentioned sources, in comparison with commercial Y2O3:Eu3+ red phosphor. Furthermore, the Pechini method was used for elaborating Y3BO6:Eu3+ spray-coated thin film. Its structural and optical features were also investigated. Finally, colorimetric simulations have pointed out that Y3BO6:Eu3+ could be efficiently combined with blue and green phosphors to produce white light from DUV LEDs. Regarding blue LEDs sources, a mixture constituted of Y3Al5O12:Ce3+ and Y3Al5O12:Pr3+ powders has been investigated and resulted in promising results for providing white light with suitable features for indoor or outdoor lighting.

Luminescent Ce3+-doped and Ce3+/Tb3+-codoped Y3Al5O12 powders and spray-coated films have been elaborated by the sol–gel technique. Their purity has been assessed by X-ray diffraction whereas X-ray Absorption Spectroscopy has evidenced the partial oxidation of Ce3+ into Ce4+ in samples containing or not Tb3+. Film thicknesses have been determined by profilometry. Powder and film morphology has been studied using Scanning Electron and Transmission Electron Microscopies, evidencing nanostructured well-densified phosphors. Scintillation properties of both kinds of samples have been investigated upon X-ray radiation and optimal cerium concentration has been determined for crystallized powders. Substitution of one part of yttrium ions by terbium ions in Y3Al5O12:Ce samples has entailed a significant enhancement of scintillation efficiency for both powders and films. These improvements will be discussed regarding in particular the changes in the X-ray absorption by photoelectric effect related to the matrix modification. In addition to this enhancement, emission maximum of co-doped samples is red-shifted to 544 nm, that matches more suitably with the optical photon detectors used in X-ray imaging.

Blue-emitting Eu2+-doped barium magnesium aluminate (BaMgAl10O17:Eu2+) for advanced displays and lighting devices was prepared by a microwave induced solution combustion synthesis using urea as combustion fuel and nitrates as oxidizer. Purity control of as-synthesized blue phosphor particles was undertaken by modifying the fuel to oxidizer molar ratio. X-ray diffraction, scanning electron microscopy and photoluminescence were used to investigate powders crystallinity, particles size, morphology and luminescent properties, respectively. Fuel-rich urea reactions preferentially lead to pure phases compared to the powders synthesized with a stoichiometric fuel to oxidizer ratio. In both cases, we produce a nearly pure well-crystallized and nanostructured BaMgAl10O17:Eu2+. Photoluminescence measurements exhibit the characteristic blue emission of Eu2+ under UV light excitation however a weak red emission associated to Eu3+ is also detected.

Luminescent Tb 3+ yttrium-based garnets powders have been prepared by the sol–gel route. Optical properties of samples have been studied. Excitation spectra have revealed several excitation bands between 200 and 500 nm, which match very well with newly developed AlGaN-based deep-ultraviolet (DUV) or near-UV LEDs. Excitation profiles are dependent on the matrix formula. Emission spectral features studied upon different UV excitation wavelength have revealed that luminescence efficiency can be significantly improved by tuning the matrix composition. Thus, optical properties can be tuned to optimize the lighting source features, according to the kind of UV LEDs employed.

Multi-layer luminescent Y3Al5O12:Tb3+ coatings have been prepared by the sol–gel route from alkoxide precursors. Highly transparent and crack-free 700–800 nm thick films were produced by the dip-coating technique from sols stabilized with acetylacetone as a chelating agent. High-temperature X-ray diffraction analysis shows that the films start to crystallize at 850 °C. On the basis of atomic force and scanning electron microscopies, it has been observed to have uniform and smooth surfaces. Waveguiding properties have been demonstrated by m-lines spectroscopy giving refractive index values of transverse electric and magnetic modes very close each other and the same thickness for both modes. These properties have only been observed for amorphous films, crystallization entailing the vanishment of waveguiding properties. Xe-lamp induced emission spectra of Tb3+ ions were recorded from amorphous and crystallized dip-coated films respectively annealed at 400 °C and 1000 °C. Spectra reveal the characteristic green emission arising from the Tb3+ 5D4 manifold for both samples.

International audience; Eu3+ doped GdPO4 orthophosphates with hexagonal and monoclinic crystal structures were prepared by a solvothermal process at 160 °C. Nanoparticles (NPs) and nanowires (NWs) were obtained using a mixture of water and glycerol as solvents. Luminescence properties of 5% Eu3+ doped GdPO4 NPs and NWs were investigated. As-obtained luminescent NWs:Eu and NPs:Eu powders were post-annealed at 750 °C since this temperature did not change neither their initial morphology nor their crystalline structure and led to luminescence efficiency intense enough to suit optical applications. A post-heating treatment of the samples at higher temperature (1000 °C) resulted in a change in both the crystalline structure from hexagonal to monoclinic form and the morphology. Excitation and emission spectra as well as luminescence decays were recorded on both post-annealed luminescent NW:Eu and NP:Eu powders. A comparative study of the optical results obtained for the different morphologies and structures was carried out by calculating the asymmetry ratio (A21) of Eu3+ from emission spectra. The symmetry of the local environment of Eu3+ activators has been described in the frame of Judd–Ofelt theory, using Ω2 and Ω4 intensity parameters derived from emission spectra data analysis, adopting a standard procedure. It was observed that Ω2 value for the NPs is different from that of the NWs indicating that the environment surrounding the active ion changes depending on the sample morphologies. The radiative lifetimes of the 5D0 excited state and the corresponding branching ratios for different emission transitions were also estimated and compared to the recorded luminescence decay rates at room temperature.

Made available in DSpace on 2018-12-11T17:28:07Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-01-01 Undoped and Tb3+-doped precursor solutions of Y3Al5O12 (YAG) have been prepared by the sol-gel process using alkoxide precursors stabilized by a chelating agent: acetylacetone (acacH), with various complexation ratios RC = 0, 1, 2 and 3 defined as the ratio between [Al(OPri)3] and [acacH]. Combining in situ UV-visible investigation of the hydrolysis process of these sols with X-ray diffraction study on the powders extracted after water addition to the sols, drying and subsequent heating, an optimal RC value has been determined. It leads to sols stable over a long period of time and resulting in a pure YAG phase. Structural and optical properties of Tb3+-doped precursor sols characterized by RC = 0 (as a reference for the influence of acacH) and RC = 1 (determined optimal value) were studied by means of X-ray Absorption Spectroscopy (XAS) and photoluminescence, respectively. Y K and Tb L3 edges XAS results show the presence of double heterometallic Y or Tb/Al alkoxides for the sol with RC = 1 involving pre-nuclei cluster with YAG structure. Emission and excitation spectra as well as decay curves were recorded and compared. Results revealed that the stabilized sol is much more efficient upon UV excitation than its unmodified counterpart thanks to an efficient energy transfer from acetylacetonate groups to active ions. Upon a 485 nm excitation, acac-modified samples remain the most efficient ones. It could be related to the dissimilar Tb local environments of the sols suggested by both UV-visible and photoluminescence excitation results. Université Clermont Auvergne Sigma Clermont Institut de Chimie de Clermont-Ferrand, BP 10448 CNRS UMR 6296 ICCF Synchrotron SOLEIL l'Orme des Merisiers, Saint-Aubin-BP48 Université Clermont Auvergne Université Blaise Pascal Institut de Chimie de Clermont-Ferrand, BP 10448 Instituto de Quimica UNESP, PO BOX 355 Instituto de Quimica UNESP, PO BOX 355

A soft synthesis route based on the sol–gel process was used for preparing rare-earth tetrafluoride powders from alkoxide precursors. In-situ fluorination was performed by decomposition of a fluorine containing organic compound named 1,1,1-trifluoro-5-methyl-2,4-hexanedione when sintering the as-prepared xerogel to produce crystallized samples. Both to insure complete departure of organic residues as well as to avoid any oxidation into oxyfluoride, annealing treatment was carried out under fluorine atmosphere. Free-oxygen content of resulting samples was evidenced by infrared and Raman spectroscopies. X-ray absorption spectroscopies (XAS) and 19F nuclear magnetic resonance (NMR) studies showed that samples heat treated at 300 °C are already crystallized but for a full crystallization in LiGdF4 and LiYF4 a thermal treatment at 550 °C is needed. Temperature dependence of powder morphology was analyzed by scanning electron microscopy (SEM).

Luminescent Y3(1-x)Al5O12:Ce3+3x (YAG: Ce3+) (x = 0.001 to 0.05), Lu2.97Al5O12:Ce3+0.03 (LuAG: Ce3+) and Y3-0.03-3yAl5O12:Ce3+0.03, Tb3y (YAG: Ce3+,Tb3+) (y = 0.1 and 0.2 phosphor powders have been successfully prepared by the sol-gel route from alkoxide precursors. Purity of derived materials was investigated by X-ray diffraction (XRD): Pure YAG phase was obtained without any intermediate whatever the Ce3+ doping rate between 0.1 and 5 mol% and LuAG phase was also effectively achieved. Luminescent properties of samples have been recorded. Excitation spectra of YAG:Ce3+ crystallized powders have revealed a broad excitation band around 470 nm, which matches very well with GaN-based blue LED. On the basis of the emission spectra study under blue excitation, the Ce3+ optimal concentration of powders has been determined. Eventually, effects of codoping by Tb3+ and substituting Y3+ by Lu3+ have been analysed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; This report describes the first synthesis of a highly crystalline YAG:Tb nanophosphor inside the pores of a mesoporous silica monolith (MSM). A simple wet impregnation procedure using a YAG:Tb sol and a MSM platform prepared by a sol–gel method was adopted to prepare highly homogeneous MSM–YAG:Tb luminescent composites. The morphological, structural and luminescence properties of in situ generated YAG:Tb nanocrystals, investigated using XRD, TEM, HRTEM, EDS, FTIR, nitrogen gas sorption and photoluminescence measurements, were compared to those of the bulk YAG:Tb phosphor. The specific surface area and the pore volume of the MSM host material were found to be lower when YAG:Tb nanoparticles were grown in the pores of the MSM material. The particle diameter of the nano-sized YAG:Tb phosphor was estimated to be about 23 nm. The MSM–YAG:Tb composite exhibited a strong green fluorescence emission with the characteristic main emission band of Tb3+ located at 548 nm. The prepared composite also showed a shorter photoluminescence (PL) lifetime and excitation bands shifted to lower wavelengths compared to the neat YAG:Tb phosphor. The as-prepared MSM–YAG:Tb material could be integrated into optoelectronic devices and holds good potential to be applied in decorative lighting

Chemical modifications are widely used in sol-gel method in order to control the mechanisms involved during hydrolysis-condensation and polymerization processes. In this paper, the influence of acetylaceton (acac) used as a chelating agent in the synthesis of undoped and Tb3+-doped Y3Al5O12 (YAG) powders and sols, has been investigated. Its effects on samples crystallization and morphology have been studied by means of high-temperature X-ray diffraction (HTXRD), thermal analysis (DTA-TG), infrared (IR) and Raman spectroscopies, transmission electron microscopy (TEM) and laser granulometry. It was shown that acetylaceton enhances the structural organization of YAG compound. Moreover, acac-modified powders exhibit much smaller particles than unmodified ones.

Luminescent Y 3 Al 5 O 12 :Tb 3+ (YAG:Tb 3+ ) phosphor thin films and powders have been successfully prepared by the sol–gel route from alkoxide precursors. Advanced coatings were produced by spray and dip-coating from stabilized sols. X-ray diffraction (XRD), attenuated total reflectance/Fourier transform infrared spectroscopy (ATR/FTIR) and atomic force microscopy (AFM) were used to investigate the structure and morphology of layers. YAG phases of high purity were obtained for both powders and films. AFM study revealed coatings of homogeneous topography, whatever the deposition technique. However, dip-coated samples exhibited a much smoother topography, characterized by a root mean square (rms) roughness much lower than that of spray-coated ones. Laser induced emission spectra and decay times of Tb 3+ ions in a spray-coated film were recorded. Measurements exhibit the characteristic green emission of Tb 3+ . Effectiveness of the Tb 3+ doped coatings under UV excitation is also illustrated by pictures.

Pure and Tb3+-doped Y3Al5O12 materials have been successfully synthesized from sol-gel based alkoxide precursors. Their formation process, structure and microstructure were examined by means of X-ray diffraction (XRD), thermal analysis combined with infrared spectroscopy (TG-IR / TDA), infrared spectroscopy (IR), scanning electron microscopy (SEM) and solid-state NMR study (27Al MAS NMR). Pure highly crystalline YAG powders were obtained from 900°C. Laser induced luminescence spectra as well as decay times versus Tb3+ concentration were also studied. The well known 5D3 → 5D4 cross-relaxation and concentration quenching phenomena were observed. Decay times of the green emission (λem = 544 nm) were mainly found to be exponential with time constant lower than 5 ms. Finally, the optimal terbium concentration for the luminescence efficiency of YAG:Tb3+ powders was determined.

International audience; In this work, we report the solvothermal synthesis of Ce-doped YAG (YAG:Ce) nanoparticles (NPs) and their association with a free-Cd CuInS2/ZnS (CIS/ZnS) core/shell QDs for application into white light emitting diode (WLED). 1500 °C-annealed YAG:Ce NPs and CIS/ZnS core/shell QDs exhibited intense yellow and red emissions band with maxima at 545 and 667 nm, respectively. Both YAG:Ce nanophosphor and CIS/ZnS QDs showed high photoluminescence quantum yield (PL QY) of about 50% upon 460 nm excitation. YAG:Ce nanophosphor layer and bilayered YAG:Ce nanophosphor-CIS/ZnS QDs were applied on blue InGaN chip as converter wavelength to achieve WLED. While YAG:Ce nanophosphor converter layer showed low color rendering index (CRI) and cold white light, bilayered YAG:Ce nanophosphor-CIS/ZnS QDs displayed higher CRI of about 84 and warm white light with a correlated color temperature (CCT) of 2784 K. WLED characteristics were measured as a function of forward current from 20 to 1200 mA. The white light stability of bilayered nanophosphor-QDs-based WLED operated at 200 mA was also studied as a function of operating time up to 40 h. Interestingly, CRI and CCT of such device tend to remain constant after 7 h of operating time suggesting that layer-by-layer structure of YAG:Ce phosphor and red-emitting CIS/ZnS QDs could be a good solution to achieve stable warm WLED, especially when high current density is applied.

International audience; This work focuses on the study of BaMgAl10O17:Eu2+ (BAM:Eu) nanophosphors prepared by a microwave-assisted combustion procedure and more especially on the polymer/BAM:Eu nanocomposite film suitable for optical devices such as solid-state-lighting. Powder presented a specific nanomorphology, highly friable and thus easily ground into fine particles. They were then homogeneously dispersed into a polymer solution (poly(N-vinylpyrrolidone) or PVP) to elaborate a polymer phosphor nanocomposite. The structural, morphological and optical features of the nanocomposite film have been studied and compared to those of a pristine PVP film and BAM:Eu powder. All the characterizations (XRD, SEM, SAXS, etc.) proved that the blue phosphor nanoparticles are well incorporated into the polymer nanocomposite film which exhibited the characteristic blue emission of Eu2+ under UV light excitation. Furthermore, the photostability of the polymer/phosphor nanocomposite film has been studied after exposure to accelerated artificial photoageing at wavelengths above 300 nm.

This paper reports the initial results of an original and simple method to elaborate flexible, self-standing, and thick luminescent films suitable for optical devices. PVP/Y(3)Ga(5)O(12):Tb(3+) nanocomposite films have been successfully achieved from a sol-gel derived Y(3)Ga(5)O(12):Tb(3+) powder and an alcoholic solution of poly-N-vinylpyrrolidone (PVP). The structural, morphological, and optical properties of these nanocomposite films have been studied and compared to those of a pristine PVP film and Y(3)Ga(5)O(12):Tb(3+) powder. The nanocomposite films were characterized by infrared and Raman spectroscopies as well as scanning and transmission electron microscopies (SEM and TEM) and demonstrated good dispersion of the phosphor particles within the polymer matrix via an alveolar mesostructure. The optical properties of these nanocomposites were fully characterized, and both their excitation and emission spectra and decay curves were recorded. Furthermore, photostability of the nanocomposite films and of the luminescent raw powder has been studied after exposure to an accelerated artificial photoageing at wavelengths higher than 300 nm. The elaboration process used is both tunable and applicable to a large variety of powders and polymers because it does not require any additive to form homogeneous and easily shapeable phosphor/polymer nanocomposites applicable in a large variety of optical devices such as solid-state-lighting.

Due to the current environmental directives concerning the use of hazardous mercury in lighting devices and the necessity to save global energy, new economical and eco-friendly light sources are being developed. In this context, phosphor-converted light emitting diodes (LEDs) appear as the most interesting candidates to replace energy-greedy incandescent light bulbs and fluorescent lamps. Indeed, it is now a mature technology that can compete with the former ones, especially thanks to their long lifetime, low-power consumption, high energy efficiency and robustness (Hoppe 2009; Huang et al. 2009; Smet et al. 2011). However, some drawbacks remain, such as the lack of high colour rendering devices.

Tb(3+)-doped Y(3)Al(5)O(12) powders have been synthesized from alkoxide precursors using, or not using, acetylacetone as a chemical modifier. The terbium oxidation state and local environment of amorphous and crystallized powders have been comparatively investigated by means of X-ray absorption near-edge structures (XANES) and extended X-ray absorption fine structure (EXAFS) while laser-induced luminescence was used to study their optical properties. Excitation and emission spectra have been recorded and a relative luminescence yield has been assessed upon 277 nm excitation. Whatever the sample, the terbium oxidation state remains +III and, for a similar heating treatment until 900 degrees C, its local environment appears to be better organized in powders modified by acetylacetone. Structural parameters obtained during XAS study are consistent with the YAG structure in which Tb atoms are in solid solution with Y ones. Besides, Tb(3+) characteristic green luminescence has been recorded for all samples, even those amorphous. It has been evidenced that the use of acetylacetone leads to more efficient luminescent materials for calcination temperatures ranging from 400 degrees C to 900 degrees C upon 277 nm excitation. Discrepancies between optical properties will be discussed with regards to the terbium local environment and expected luminescence features.

Resume. La necessite de developper de nouveaux dispositifs de visualisation, d’affichage, d’eclairage ou susceptibles de convertir efficacement l′energie solaire, a la fois performants, respectueux de l’environnement et possedant un encombrement reduit a permis de relancer fortement l’industrie des materiaux fluorescents a vocation de luminophores. Un des challenges a relever reside dans la realisation de materiaux a la “demande” possedant, en plus des caracteristiques physico-chimiques, une versatilite de forme et de morphologie. Meme si une recherche intense reste toujours consacree au developpement de nouvelles familles de materiaux compatibles avec l′une des applications visees, la tendance actuelle est toutefois fortement orientee vers l’amelioration des proprietes emissives des luminophores deja existants. Applique aux materiaux inorganiques ou aux materiaux hybrides organiques-inorganiques, actives par des ions de terres rares, le procede sol-gel permet a la fois d’obtenir des poudres a granulometrie controlee et des films de qualite en utilisant des techniques de depot comme le spray ou le dip-coating. Tout en conservant des caracteristiques structurales et des proprietes spectrales intactes, les materiaux solgel presentent souvent des proprietes ameliorees par rapport a leurs homologues issus de techniques de preparation classiques comme la ceramisation, malgre des temperatures de preparation plus basses. On peut attribuer ces specificites a la methode de synthese elle-meme qui permet de structurer les materiaux en utilisant des mecanismes reactionnels a l’echelle moleculaire, conduisant a des materiaux plus homogenes et plus purs. Quelques exemples de composes sont presentes pour illustrer l′utilisation de cette technique pour preparer de facon originale des materiaux excitables sous UV-VUV ou sous RX.

A soft synthesis route based on the sol–gel process was used for preparing rare-earth tetrafluoride powders from alkoxide precursors. In-situ fluorination was performed by decomposition of a fluorine containing organic compound named 1,1,1-trifluoro-5-methyl-2,4-hexanedione when sintering the as-prepared xerogel to produce crystallized samples. Both to insure complete departure of organic residues as well as to avoid any oxidation into oxyfluoride, annealing treatment was carried out under fluorine atmosphere. Free-oxygen content of resulting samples was evidenced by infrared and Raman spectroscopies. X-ray absorption spectroscopies (XAS) and 19F nuclear magnetic resonance (NMR) studies showed that samples heat treated at 300 °C are already crystallized but for a full crystallization in LiGdF4 and LiYF4 a thermal treatment at 550 °C is needed. Temperature dependence of powder morphology was analyzed by scanning electron microscopy (SEM).

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Solvothermal methods to synthesize cerium-doped YAG nanoparticles (YAG:Ce3+ NPs) generally require relatively long heating times (th ≥ 1 h) and temperatures close to 300 °C. In this work, we report a rapid solvothermal synthesis (th = 5 min) leading to well-crystallized and highly monodisperse YAG:Ce3+ NPs of about 30 nm diameter by heating mixed yttrium, aluminium and cerium carbonate precursors in an autoclave at 300 °C using a 1,4-butylene glycol–water mixture as solvent. The as-prepared YAG:Ce3+ NPs showed a broad emission band in the green–yellow range having maximum intensity at 550 nm upon blue or UV excitation. The effect of heating time and Ce3+ concentration on structural, morphological and optical properties of YAG:Ce3+ NPs has been systematically studied by X-ray powder diffraction, transmission electron microscopy, FT-IR spectroscopy, energy dispersive spectroscopy and room temperature photoluminescence experiments. The combination of low Ce3+ doping concentration and short heating time plays a crucial role in the good optical properties of the obtained YAG:Ce3+ NPs. The particles hold good potential to be applied in solid-state lighting or used as coating for display system applications.

Acetylacetone has been used as a chemical modifier for the synthesis of undoped and Tb(3+)-doped Y(3)Al(5)O(12) powders. A systematic investigation concerning its influence on the structural and morphological properties of amorphous and crystallized samples has been carried out. These properties have been comparatively studied by means of X-ray diffraction, infrared spectroscopy, SEM, XAS and SAXS. (27)Al NMR and EPR experiments have been performed to complete the study. The combined results have evidenced that acetylacetone promotes organic groups departure during calcination, entailing a better structural organization at lower temperatures compared with unmodified powders. Structuration has been proven to occur at short-scale range until a 600 degrees C heating treatment before being extended by coalescence at higher temperatures. Finally, the presence of acac ligands on the alkoxides leads to a monomer-cluster aggregation process, and thus to a more open network.

Optical properties in the vacuum ultraviolet (VUV) region of cerium and terbium activated Y3Al5O12 powders obtained by the sol-gel process were investigated at room and low temperatures. Both Tb3+ and Ce3+ doped samples exhibited a broad absorption band within the conventional Xe-Ne plasma emission range (140–190 nm). Optimal doping rates have been determined for both rare-earth ions. Emission spectra from cerium doped samples were recorded at 8.2 K and uncovered the splitting of the ground state of Ce3+ ions. Decay times of Ce3+ fluorescence can be fitted by double exponential curves, which can be ascribed to a retrapping phenomenon. Eventually, optical characteristics of codoped Y3Al5O12:(Tb,Ce) and Y3Al5O12:(Tb,Gd) were studied.

A multifunctional system was developed to be used as a dual ultraviolet (UV) and near infrared (NIR) light harvester and photo-converters by Down (DS-DC) and Up Conversions (UC) in visible light to enhance the light trapping properties of thin-film solar cells. Nanorods ( 140 nm x 60 nm) of β-NaGdF4 singly doped by Eu3+, Pr3+, Er3+ and co-doped by Yb3+ were synthesized by co-precipitation method (optimized conditions: 150°C, 60 min). The structural and morphological properties were characterized by X-ray diffraction, infrared spectroscopy, Raman and electron microscopy (TEM) confirming the quality of the elaborated materials. Using the structural probe properties of the Eu3+ ion, local order was identified and discussed on the basis of Judd-Ofelt theory. The analysis of the fluorescence properties was carried out under pulsed and cw excitations. The radiative de-excitation and energy transfer channels for both DC and UC processes, occurring in each system, have been identified and discussed based on the evolution of fluorescence intensities as a function of continuous pump power and on the kinetics of luminescence decays under UV and NIR excitations. Particular attention was paid to the Er3+/Yb3+ system for which a coupling by plasmonic effect was considered with metallic nanoparticles (NPMs) of noble metals gold and silver. Depending on the nature, shape and size of the NPMs, enhancements or attenuations of fluorescence intensities, in addition in some cases to an increase in decaying rates have been observed. They are discussed considering two main processes, scattering and the Purcell effects., Un système multifonctionnel a été élaboré pour être utilisé comme double capteur de lumière ultraviolette (UV) et proche infrarouge (NIR) et permettant de convertir ces rayonnements par des mécanismes Stokes (DS-DC) et anti-Stokes (UC) en lumière visible susceptible d’accroître le rendement des cellules photovoltaïques. Des nano-bâtonnets ( 140 nm x 60 nm) de β-NaGdF4 monodopés par Eu3+, Pr3+, Er3+ et codopés par Yb3+ ont été synthétisés de façon reproductible par co-précipitation (conditions optimisées : 150°C, 60 min). Les propriétés structurales et morphologiques ont été caractérisées par diffraction des rayons X, spectroscopie infrarouge, Raman et microscopie électronique (MET) confirmant la qualité des matériaux synthétisés. L’utilisation des propriétés de sonde structurale de l’ion Eu3+ a permis d’identifier et de discuter de l’ordre local en s’appuyant sur la théorie de Judd-Ofelt. L’analyse des propriétés de fluorescence a été menée sous excitations impulsionnelle et continue. Les canaux de désexcitation radiative et de transfert d’énergie pour les deux processus DC et UC, pour chaque système, ont été identifiés et discutés en s’appuyant sur l’évolution des intensités de fluorescence en fonction de la puissance de pompe en régime continu et sur les cinétiques de décroissances temporelles sous excitations UV et NIR. Une attention particulière a été accordée au système Er3+/Yb3+ pour lequel un couplage par effet plasmonique a été envisagé avec des nanoparticules métalliques (NPMs) de métaux nobles or et argent. Selon la nature, la forme et la taille des NPMs, des exaltations ou des atténuations des intensités de fluorescence, s’accompagnant dans certains cas d’un accroissement des taux de décroissance temporelle ont été observées. Elles sont discutées en considérant deux processus principaux, la diffusion et l’effet Purcell.

In this work, the peculiar features (high specific surface, aspect ratio…) of ZnO nanowires (NWs) have been exploited to elaborate nanocomposite coatings combining them either with YAG:Ce or with TiO2. These composites exhibit improved optical or photocatalytic properties respectively compared to the neat YAG:Ce or TiO2 compounds. Via an optimized hydrothermal method, NWs have been synthesized as coatings on the full substrate or photoetched micrometric diffracting gratings. Besides, synthesis or grinding processes have led to suspensions/solutions made of undoped or Ag+/Fe3+-doped TiO2 nanoparticles on one hand and YAG:Ce on the other hand, in order to perform the composites.Photocatalytic measurements carried out on TiO2/ZnO composites have highlighted the synergy between the two photocatalysts ZnO and TiO2 since a significant improvement of the photocatalytic decomposition of methylene blue in basic medium in comparison with that obtained for TiO2 and ZnO alone has been observed. Meanwhile, the association ZnO/YAG:Ce provides enhanced photoluminescence properties when compared with those of YAG:Ce alone, whether it is in terms of spectral profile or emission intensity, which allows foreseeing new applicative directions in the field of LED-based lighting., Dans ce travail de thèse, nous avons mis à profit les spécificités de nanofils (NFs) de ZnO (surface spécifique, facteur de forme…) pour réaliser des revêtements nanocomposites Y3Al5O12 :Ce (YAG :Ce)/NFs ZnO et TiO2/NFs ZnO présentant des propriétés respectivement optiques et photocatalytiques améliorées par rapport à celles des matrices YAG :Ce et TiO2 seules.Ces NFs de ZnO ont été synthétisés selon un protocole optimisé par voie hydrothermale sous forme de revêtements pleine plaque ou sous forme de réseaux micrométriques auto-organisés obtenus par photogravure. Par ailleurs, des protocoles soit de synthèse, soit de broyage, ont permis d’élaborer des suspensions de nanoparticules (NPs) de TiO2 dopées ou non par les ions Ag+ et Fe3+ et de YAG :Ce afin de les coupler avec les NFs de ZnO.Les mesures photocatalytiques réalisées sur les composites TiO2/ZnO ont mis en évidence la synergie entre les deux photocatalyseurs ZnO et TiO2 puisqu’on aboutit à une nette amélioration de la décomposition photocatalytique du bleu de méthylène en milieu basique comparée à celle obtenue pour ZnO ou TiO2 seuls. L’association NFs ZnO/YAG :Ce conduit quant à elle à des propriétés de luminescence améliorées par rapport au YAG :Ce seul, que ce soit en termes de profil spectral ou d’intensité d’émission, sous excitation bleue ou UV, ce qui permet d’envisager de nouvelles orientations applicatives dans le domaine de l’éclairage à LED.