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The crystallization mechanism of sol–gel-derived NaYF4:(Yb,Er) up-converting phosphors has been studied by differential scanning calorimetry analysis using both model-free and model fitting approaches. Structural and optical data have shown that the hexagonal NaYF4:(Yb,Er) phase crystallization process occurs at around 315 °C as a result of the thermal decomposition of the metal trifluoroacetates. As the annealing temperature increases, sphere-like microcrystals of about 1–2 μm size (at 300 °C) break up into smaller ones (400–500 nm size) and finally collapse at higher temperatures (600 °C); the up-conversion luminescence signal intensity increases due to the crystallinity improvement and dehydration process. The crystallization process can be described as an autocatalytic-type reaction where the accompanying cubic NaYF4 phase played a catalytic role by reducing the energy barrier against the crystallization of the hexagonal NaYF4 phase, causing its fast self-accelerated crystallization. The energy resulting from the disintegration process of the initial NaYF4 microcrystals contributed to the growth and agglomeration processes and finally the collapse of the crystalline fragments with increasing temperature. [ABSTRACT FROM AUTHOR]

Sol-gel template method has been used to prepare BaFBr:Eu2+ nanophosphor-SiO2 hybrid entrapped within the nanopores array (of about 200 nm size) of a comercial anodized alumina (AA) membrane. Structural and morphological measurements using electron microscopy (SEM) and X-ray diffraction (XRD) have shown the presence of the BaFBr:Eu2+ nanophosphor in the silica xerogel entrapped within the nanopores array; photoluminescence and X-ray excited luminescence measurements have shown Eu2+ luminescence at 395 nm accompanied by a broad band due to AA membrane. The method assures a relatively uniform spreading of the BaFBr nanophosphor into the AA membrane pores array without the nanoparticles agglomeration. Preliminary imaging tests have shown a spatial resolution in the micrometer range and even in the submicrometer range can be expected. As BaFBr:Eu2+ is a very efficient X-ray phosphor the material might be used as X-ray micro-imaging detector. [ABSTRACT FROM AUTHOR]

Matrix-assisted pulsed laser evaporation (MAPLE) has been investigated as an alternative to the pulsed laser deposition (PLD) technique for Eu3+-doped crystalline LiYF4 thin-films deposition. MAPLE assumes laser ablation of a frozen target made of the material of interest diluted in a solvent, rather than that of a bulk target, of either pressed powder or single crystal, used in the case of PLD. Our approach stems from the assumption that laser ablation of a frozen dilute target would result in thin films with improved morphology, as compared to PLD. Indeed, we find that roughness values of samples obtained by the MAPLE technique are four times lower than in the case of PLD. A lower transmittance was noticed for PLD obtained layers with respect to those grown by MAPLE due to strong scattering of light by the morphological defects. Photoluminescence spectra are showing characteristic Eu3+-ion luminescence bands at 578, 591, 612, 650 and 698 nm (5D0 → 7FJ); crystal field splitting of the bands indicates dopant ions incorporation in the host material during transfer by either PLD or MAPLE. [ABSTRACT FROM AUTHOR]