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Acta Cryst. (2014). A70, C744
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Glass ceramics containing fluoride crystals such as BaF2 or CaF2 with crystallite sizes in the range from 5 to 100 nm are potential candidates for numerous photonic applications. Glass ceramics containing rare-earth-doped fluoride crystals are candidates for laser materials. The size distribution plays an important role and often a narrow size distribution is required for photonic applications. In the last years a hindered growth effect leading to a more narrow size distribution was observed during the crystallization of BaF2 as well as CaF2 nanocrystals in oxy-fluoride glasses. The aim of this study is a detailed quantitative structural and nano-chemical analysis of the formation of BaF2 or CaF2 in two glass ceramics by Anomalous Small-Angle X-ray Scattering (ASAXS) to reveal and understand the mechanism of hindered growth. Nanocrystals of BaF2 precipitate during heat treatment of a silicate glass of composition 69.6SiO2-7.52Al2O3-15.04K2O-1.88Na2O-4BaF2-2BaO. X-ray diffraction measurement proved the formation of BaF2 crystals in the glass matrix. High resolution TEM showed the formation of spherical particles of sizes in range from 10-40 nm surrounded by a layer enriched with SiO2. SAXS reveal the growth of nanocrystals with increasing annealing time and temperatures. ASAXS experiments are done at four energies close to the Ba-L3 X-ray absorption edge (5247eV). The ASAXS curves for the sample annealed at 5400C for 20h revealed a spherical core-shell model. It turned out that the layer surrounding the BaF2 crystals is enriched with SiO2. Sizes and compositions of these layers are analyzed quantitatively. Furthermore, the ASAXS analysis reveals the presence of very small nucleates of size of about 3 nm in the as melted glass sample already [1]. A precipitation of CaF2 nanoparticles takes place during heat treatment of glasses of composition 7.65Na2O-7.69K2O-10.58CaO-12.5CaF2-5.77Al2O3-55.8SiO2 up to 40 hours. SAXS experiments and especially ASAXS near the Ca-K edge proves the formation of CaF2 nanoparticle surrounded with SiO2 enriched layers, quantitatively. The ASAXS effect is very pronounced at this untypical low energy for ASAXS studies at the Ca-K edge. The ASAXS result reveals crystal sizes between 10-20 nm surrounded by a shell of lower electron density. Additional very small heterogeneities are found after long annealing with diameters of about 1.6 nm [2].

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Acta Cryst. (2014). A70, C891
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Self-assembled metallic nanoparticles are attractive candidates for plasmonic heating, non-linear optical switching [1], bio-analytical, chemical [2], catalytic , and surface enhanced RAMAN scattering (SERS) [3]. These applications are strongly dependent on the shape, size, composition, size distribution and volume fraction of nanoparticles. Here, self-assembly of gold nanoparticles (AuNPs) was obtained by low energy sputter deposition on Deep Eutectic Solvent (DES ; choline chloride and urea) surfaces and elucidated by Small Angle X-ray Scattering (SAXS), Cryogenic Transmission Electron Microscopy (Cryo-TEM) and UV-Vis. Data analysis shows the formation of spherically shaped AuNPs of 5 nm in diameter with narrow size distributions. Moreover, analysis reveals that prolongation of gold-sputtering time has no effect on the size of the particles and only the concentration of AuNPs increases linearly. The growth of the maxima in evaluated structure factor S(q) and the distance distribution function G(r) at higher concentrations of AuNPs is caused by the interference effects. Moreover, it indicates that the particles are not arranged in random but have a self-assembly in short-range order. Prolonged gold-sputtering time leads to increase in the ordering of the AuNPs with strong interactions. It is proposed that the self-assembly of AuNPs is due to the ionic liquid template effects of DES and the balancing physical forces. Moreover, a disulfide based stabilizer bis ((2-Mercaptoethyl) trimethylammonium) disulfide dichloride was applied to supress the self-assembly. The stabilizer even reverses the self-assembled or agglomerated AuNPs back to stable 5 nm individual particles. The templating effect of DES is compared with the non-templating solvent Castor oil. Our results will also pave a way to understand and control self-assembly of metallic and bimetallic nanoparticles.

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Acta Cryst. (2014). A70, C1583
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The chemical synthesis of core/shell colloidal nanocrystals (NCs) have lead to an pronounced improvement in the optical properties and the chemical stability of semiconducting NCs [1]. The main topic of this work is the structural characterisation of core/shell NCs with anomalous small angle x-ray scattering (ASAXS) in combination with diffraction techniques (XRD) at laboratory- and synchrotron sources (HZB-BESSY and ESRF). Furthermore we complete these findings with complementary microscopy techniques (TEM). The detailed knowledge of the structural properties of the core/shell NCs allows to study the impact of the nanometer sized dimensions on their optical properties. The infrared emission of lead chalcogenide nanocrystals (NCs) in the size range of 5 - 10 nm can be drastically increased stabilising the core with a hard protective shell [1,2]., e.g., PbS/CdS NCs shows a higher efficiency and stability [2] with respect to pure PbS-NCs. In contrast to a shell growth on top of a core, we investigated in this study the CdS-shell growth on PbS NCs driven by Cd for Pb cation exchange [2]. Especially, we studied three different final shell thicknesses of 0.9, 1.5 and 2 nm. The chemical composition profile of the CdS-shell as a function of reaction time are derived from ASAXS experiments in sub-nanomter resolution. The crystal structure of the shell was derived by XRD combined with TEM measurements, respectively. We relate the chemical and structural information to the measured PL intensities of the core/shell NCs. We reveal the existence of two different crystalline phases, i.e. the metastable rock salt and the equilibrium zinc blende phase within the chemically pure CdS-shell. The highest improvement in the PL emission was achieved for 0.9 nm shells depicting a large metastable rock salt phase fraction matching the crystal structure of the PbS core. These results could be only achieved using ASAXS that gieves a mean chemical profile of a large ensemble of single core/shell NCs, but in sub-nanometer resolution [3].
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