Urea Glass Route as a Way to Optimize YAGG:Ce3+,Cr3+,Pr3+ Nanocrystals for Persistent Luminescence Applications.

A new approach for the synthesis of Y3Al2Ga3O12 (YAGG) nanophosphors allowing the preparation of crystallites with sizes starting from 45 nm is presented. The controllability of the energy and trap density of the resulting material samples by annealing temperature was confirmed by thermoluminescence (TL) measurements. It has been shown that the annealing of samples at temperatures up to 1300 °C does not cause any substantial growth of crystallites, still remaining below 100 nm, but leads to changes in the activation energy of the persistent luminescence (PersL) process. On the other hand, annealing above 1400 °C results in grain growth on the submicron scale, which was confirmed by X-ray powder diffraction (XRPD) and electron transmission microscopy (TEM) measurements. In addition, with an increase in the molar ratio of urea to the total amount of metals used (R), qualitative changes are observed in the PersL process occurring from the excited states of Cr3+ and Pr3+ ions. This proves the influence of the synthesis process, in particular of the metal complexation at its initial stage, on the final structure ordering in the annealed materials. These observations are linked to previously reported defects in the YAGG structure, leading to PersL.

Size-Dependent Persistent Luminescence of YAGG:Cr3+ Nanophosphors.

In the current work, YAGG:Cr3+ nanophosphors were synthesized by the Pechini method and then annealed at different temperatures in the range 800-1300 °C. The structure and morphology of the samples were characterized by X-ray Powder Diffraction (XRPD). The lattice parameters and average crystalline sizes as site occupation by Al3+ and Ga3+ ions were calculated from the Rietveld refinement data. To investigate the effect of crystalline size of the materials on their optical properties: excitation and emission spectra were recorded and analyzed. Finally, the effect of crystalline size on the probability of carrier recombination leading to PersL was determined experimentally with thermoluminescence analyses. The Tmax-Tstop method was applied to determine the trap type and particle size (calcination temperature) effect on their redistribution. A correlation between structural changes and trap redistribution was found. In particular, the extinction of high-temperature TL maximum with increasing annealing temperatures is observed, while low-temperature TL maximum increases and reaches a maximum when the lattice parameter reaches saturation.

Effect of Graphene Addition on the Thermal and Persistent Luminescence Properties of Gd2.994Ce0.006Ga3Al2O12 and Gd2.964Ce0.006Dy0.03Ga3Al2O12 Ceramics.

The gadolinium, gallium, aluminum garnet doped with cerium and co-doped with dysprosium ions were prepared using sol gel method. The SEM images show that after synthesis, the grains are below 100 nm. The powders were ultrasonically mixed with graphene nanoflakes and ceramics were prepared using the high pressure low temperature sintering technique. A series of the ceramics was prepared using different graphene content. The structure of the samples was examined using X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman techniques. The spectroscopic properties were checked using conventional and persistent luminescence spectra measurements. The thermoluminescence glow curves and fading time of persistent luminescence measurements were performed to check how the graphene presence affects the electron traps number and depth. It was found that the addition of graphene improved the thermal conductivity of co-doped samples. This resulted in faster release of deeper traps and an increase in fading of persistent luminescence. The possibility of releasing energy from deep traps without additional stimulation may allow the use in different applications, the matrices and luminescent ions, which so far did not show persistent luminescence at room temperature.

Graphene Coating Obtained in a Cold-Wall CVD Process on the Co-Cr Alloy (L-605) for Medical Applications.

Graphene coating on the cobalt-chromium alloy was optimized and successfully carried out by a cold-wall chemical vapor deposition (CW-CVD) method. A uniform layer of graphene for a large area of the Co-Cr alloy (discs of 10 mm diameter) was confirmed by Raman mapping coated area and analyzing specific G and 2D bands; in particular, the intensity ratio and the number of layers were calculated. The effect of the CW-CVD process on the microstructure and the morphology of the Co-Cr surface was investigated by scanning X-ray photoelectron microscope (SPEM), atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Nanoindentation and scratch tests were performed to determine mechanical properties of Co-Cr disks. The results of microbiological tests indicate that the studied Co-Cr alloys covered with a graphene layer did not show a pro-coagulant effect. The obtained results confirm the possibility of using the developed coating method in medical applications, in particular in the field of cardiovascular diseases.

Particle size-related limitations of persistent phosphors based on the doped Y3Al2Ga3O12 system.

Co-doped Ce3+, Cr3+ and Pr3+ yttrium-aluminium-gallium garnet powders of various sizes were obtained by co-precipitation method. The microstructure and morphology were investigated by XRPD, TEM and gas porosimetry. The luminescence properties were studied by excitation and emission spectra, quantum yield and decay times. Thermoluminescence measurements were performed to evaluate the activation energy, traps redistribution and frequency factor. Limitation in the energy transfer between dopant ions in the small particles, traps depth and surface defects were considered and investigated as responsible for the quenching of persistent luminescence. The phosphors annealed at 1100 °C show the optimal persistent luminescence and nano-particle size.

Phospholipid distribution in the cytoplasmic membrane of Gram-negative bacteria is highly asymmetric, dynamic, and cell shape-dependent.

The distribution of phospholipids across the inner membrane (IM) of Gram-negative bacteria is unknown. We demonstrate that the IMs of Escherichia coli and Yersinia pseudotuberculosis are asymmetric, with a 75%/25% (cytoplasmic/periplasmic leaflet) distribution of phosphatidylethanolamine (PE) in rod-shaped cells and an opposite distribution in E. coli filamentous cells. In initially filamentous PE-lacking E. coli cells, nascent PE appears first in the periplasmic leaflet. As the total PE content increases from nearly zero to 75%, cells progressively adopt a rod shape and PE appears in the cytoplasmic leaflet of the IM. The redistribution of PE influences the distribution of the other lipids between the leaflets. This correlates with the tendency of PE and cardiolipin to regulate antagonistically lipid order of the bilayer. The results suggest that PE asymmetry is metabolically controlled to balance temporally the net rates of synthesis and translocation, satisfy envelope growth capacity, and adjust bilayer chemical and physical properties.

Angular shaping of fluorescence from synthetic opal-based photonic crystal.

Spectral, angular, and temporal distributions of fluorescence as well as specular reflection were investigated for silica-based artificial opals. Periodic arrangement of nanosized silica globules in the opal causes a specific dip in the defect-related fluorescence spectra and a peak in the reflectance spectrum. The spectral position of the dip coincides with the photonic stop band. The latter is dependent on the size of silica globules and the angle of observation. The spectral shape and intensity of defect-related fluorescence can be controlled by variation of detection angle. Fluorescence intensity increases up to two times at the edges of the spectral dip. Partial photobleaching of fluorescence was observed. Photonic origin of the observed effects is discussed.