Structural and spectroscopic characterization of a new series of Ba2RE2Ge4O13 (RE = Pr, Nd, Gd, and Dy) and Ba2Gd2-xEuxGe4O13 tetragermanates.

A new series of Ba2RE2Ge4O13 (RE = Pr, Nd, Gd, Dy) germanates and Ba2Gd2-xEuxGe4O13 (x = 0.1-0.8) solid solutions have been synthesized using the solid-state reaction technique and characterized by X-ray powder diffraction. All compounds crystallize in the monoclinic system, space group C2/c, Z = 4. The crystal lattice consists of RE2O12 dimers, zigzag C2-symmetric [Ge4O13]10- tetramers, and ten-coordinated Ba atoms located in voids between polyhedra. The density-functional theory (DFT) calculations performed on a rich set of Ba2RE2Ge4O13 compounds have confirmed the high thermodynamic stability of monoclinic modification. Under ultraviolet (UV) light excitation Ba2Gd2-xEuxGe4O13 phosphors exhibit an orange-red emission corresponding to the characteristic f-f transitions in Eu3+ ions. The highest intensity of lines at 580 nm (5D0→7F0), 582-602 nm (5D0→7F1), 602-640 nm (5D0→7F2), 648-660 nm (5D0→7F3), and 680-715 nm (5D0→7F4) is observed for the samples with x = 0.4-0.6. The possibility of their application has been assessed by studying their color characteristics, quantum efficiency, and thermal stability. The obtained data indicate that Ba2Gd2-xEuxGe4O13 solids can be considered as promising materials for UV-excited phosphor-converted light-emitting diodes (LEDs) if an aluminum nitride substrate (λex = 255 nm) is used as a semiconductor chip.

Luminescence and energy transfer mechanisms in photo-thermo-refractive glasses co-doped with silver molecular clusters and Eu3.

Silver molecular clusters were synthesized in photo-thermo-refractive glasses using the Na+-Ag+ ion exchange technique followed by heat treatment. Comprehensive study of cluster emission reveals the presence of spectrally separated fluorescence and phosphorescence with nanosecond and microsecond lifetime. Co-doping of glasses with Eu3+ was shown to results in quenching of cluster luminescence caused by energy transfer. The monitoring of silver cluster luminescence quantum yield and lifetime in the presence of Eu3+ indicates the presence of two different mechanisms of energy transfer. The first one affects the decay kinetics of cluster fluorescence and manifests at long distances, while the second one leads to static quenching of cluster emission at shorter distances and becomes prominent at higher doping Eu3+ concentration.

Fluorescence enhancement of monodisperse carbon nanodots treated with aqueous ammonia and hydrogen peroxide.

The treatment of monodisperse carbon nanodots (MCNDs) with a combination of aqueous ammonia and hydrogen peroxide is found to result in a prominent enhancement of their fluorescence efficiency. Depending on the hydrogen peroxide concentration, an increase of the MCNDs quantum yield of up to seven-fold has been achieved. Considering the absence of prominent changes in fluorescence lifetime and fluorescence spectra upon the treatment it is suggested that the observed rise of fluorescence efficiency originates from additional formation of new isolated sp2 domains surrounded by defect sites. The structural modification of MCNDs induced by their treatment with combination of aqueous ammonia and hydrogen peroxide is indicated by both transmission electron microscopy images and infrared spectra. The applied method has insignificant effect on the aggregation properties and size distribution of the studied MCNDs. Taking into account the proposed mechanism, the applied treatment procedure can serve as a basis for a facile approach for modification of emissive properties of various nanocarbon structures.

Controllable spherical aggregation of monodisperse carbon nanodots.

Monodisperse carbon nanodots (MCNDs) having an identical composition, structure, shape and size possess identical chemical and physical properties, making them highly promising for various technical and medical applications. Herein, we report a facile and effective route to obtain monodisperse carbon nanodots 3.5 ± 0.9 nm in size by thermal decomposition of organosilane within the pores of monodisperse mesoporous silica particles with subsequent removal of the silica template. Structural studies demonstrated that the MCNDs we synthesized consist of ∼7-10 defective graphene layers that are misoriented with respect to each other and contain various oxygen-containing functional groups. It was demonstrated that, owing to their identical size and chemical composition, the MCNDs are formed via coagulation primary aggregates ∼10-30 nm in size, which are, in turn, combined into secondary porous spherical aggregates ∼100-200 nm in diameter. The processes of coagulation of MCNDs and peptization of their hierarchical aggregates are fully reversible and can be controlled by varying the MCND concentration or the pH value of the hydrosols. Submicrometer spherical aggregates of MCNDs are not disintegrated as the hydrosol is dried. The thus obtained porous spherical aggregates of MCNDs are promising for drug delivery as a self-disassembling container for medicinal preparations.