The synthesis of garnet-type Gd3Sc2Al3O12 (GSAG) nanoparticles doped with Ce3+ ions under solvothermal conditions in 1,4-butanediol is reported in this work. X-ray diffraction with LeBail fitting, Electron microscopy images and photoluminescence spectra were used to characterize the samples. A detailed Transmission Electron Microscopy study shows strong modification of particle morphology upon reaction temperature. Performing the reaction at high temperature and high pressure leads to a clear increase of the particle crystallinity and allows the formation of 100-nm sized nanocrystals with a small size dispersion (± 20 nm). The formation mechanism of these particles is through the self-orientation of primary crystalline grains. Yellow-orange luminescence of Ce3+-doped GSAG nanoparticles is observed upon 457-nm excitation. Photoluminescence intensity drastically increases when the particles are synthesized at high temperature, which is directly correlated to their crystal quality.
Majority of w-LEDs (white light emitting diodes) are based on InGaN blue-LED associated with phosphors exhibiting complementary color emissions to generate required white light output. However, such complementary color mixing with the lack of violet and mainly orange-red color results in poor color-rendering index (CRI). To achieve better resolution of natural white color, recent trends on w-LEDs are focused on broadband visible emitting phosphors excitable with near UV or violet LEDs. Here, we present novel amorphous yttrium-aluminum-borate (g-YAB) phosphors exhibiting broad visible emission under UV-violet excitations. These g-YAB phosphors show high internal quantum yields (~90 %) under 365 nm excitation, possess good thermal stability over 200°C and maintains the performance for continuous and repeated operations that are ideal for the development of near UV-violet excitable w-LEDs with high color rendering ability.
This work is devoted to a new family of highly emissive white phosphors for solid-state lighting (SSL) applications. The phosphors, based on yttrium aluminoborates (g-YAB) compositions, were prepared from solutions by the polymeric precursor (PP) method (modified Pechini process), involving non-toxic and low cost precursors. The resulting resins were then dried at moderate temperatures. Then, a two-step annealing treatment with controlled atmospheres (pyrolysis under nitrogen and calcination under oxygen) favored the gradual oxidation of organic moieties coming from the starting materials. This allowed avoiding the formation of impurities, which are detrimental to photoluminescence (PL) emission such as pyrolytic carbon (visible light absorber) or carbonates (PL quenchers). Thus, we have synthesized glassy yttrium aluminoborate powders exhibiting intense PL emissions extended in the whole visible range leading to warm light emissions. These PL properties arise from structural defects (non-bridging oxygen or carbon impurities such as carbonyl radicals), whose energy levels are widely extended within the large bandgap of these g-YAB powders. Moreover, these samples exhibit good thermal and chemical stabilities. We determined their internal quantum yields using near ultraviolet excitations, which reached high promising values, around 80 - 90%. This new family of lanthanide-free phosphors exhibiting intense warm white emissions, with high Color Rendering Indexes (CRI), is promising for SSL through the development of phosphor coated -LED devices.
We review our work on several strategies to elaborate multifunctional nanoparticules for two-photon imaging or/and
photodynamic therapy. Our first strategy is based on the incorporation of two-photon hydrophobic fluorophors in bio-compatible
pluronic micelles using the mini-emulsion technique. Our second strategy is based on fluorescent organic
nanocrystal grown in silicate spheres. These core-shell hybrid nanoparticles are obtained by a spray-drying process from
sol-gel solutions. Our third strategy consists in the encapsulation of hydrophilic molecules in the water core of gold
nanospheres. They are obtained by a stabilized emulsion in biphasic liquid-liquid medium without surfactant.
Polyaromatic dyes were used to synthesize molecular nanocrystals in sol-gel thin films for sensor applications. Fluorescence Confocal Microscopy (FCM) and Transmission Electron Microscopy (TEM) experiments showed the advantages of our nanocrystallization process compared with microcrystallization in free solutions. Indeed, we obtained well-dispersed and spherical nanocrystals with a narrow size distribution, exhibiting a good crystallinity. Time-resolved fluorescence spectroscopy allowed us to measure fluorescence lifetimes of nanocrystals in presence of molecular probes. Then, chemical sensoring properties of these molecular nanocrystals were demonstrated.
We have developed a new type of hybrid organic-inorganic materials constituted of organic nanocrystals embedded in silicate matrices. They are prepared by a generic process based on the confined nucleation of molecular crystals in the pores of sol-gel matrices, bulk or thin films. In this study, we have controlled the spatial distribution of organic nanocrystals in sol-gel thin films prepared by spin-coating. This spatial control of nucleation is obtained by assisting the nanocrystallisation process through nanostructured substrates. Substrates exhibit arrays of wells where the dye nucleates when the initial solution is deposited. The 3-D spatial control is achieved through a multilayer process. This nucleation control gives an opportunity to design new 3-D optical data storage or 2-D arrays of luminescent crystals for chemical or biological sensor applications.
The nanosecond nonlinear absorption of organic molecules is a three-photon process, i.e. a two-photon initiating step
followed by a transient absorption during the pulse duration. Order of magnitude calculations show that it is realistic to
consider two-photon absorbing molecules for optical limiting applications at visible wavelengths. In this work we have
investigated two molecular engineering approaches to optimize molecules: internal charge transfer molecules and
conjugated oligomers. The oligomer approach is the most promising due the enhancement of nonlinearities that results from
cooperative effects between monomers.
We present a new type of composite materials that we have designed for optical applications: sol-gel glasses highly doped
with organic nanocrystals. The nanosized control of crystallization allows us to obtain optical grade samples with very high
concentrations of active molecules. Here, we report on the optical limiting properties of bulk glasses doped with stilbene 3
nanocrystals, and on the second-harmonic generation of thin films doped with oriented nonlinear nanocrystals.
The optical limiting properties of 17 organic molecular crystals (natural faces) have been characterized with 250-ps pulses
at &=532 nm. The best nonlinear absorption coefficients are f3=120 cm/OW and f3=400 cm/GW. Such high nonlinear absorptions are explained by the efficient ciystal packing of molecular two-photon absorptions. The ciystal phase allows high molecule concentration, efficient molecule orientation, and enhanced local field factor. Using the frequency-dependent molecular hyperpolarizability and the oriented-gas model one predicts that these high nonlinearities cover the whole visible spectrum.
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