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Vacuum ultraviolet (VUV) excitation spectroscopy under synchrotron irradiation is a powerful tool for the investigation of luminescence properties of wide bandgap nanocrystalline phosphor materials. The advantages of selective excitation in a wide spectral range present the possibility to explore the different aspects of luminescence processes in nanophosphors separately. It is shown that direct excitations of emission centers in wide bandgap materials need excitations in VUV spectral range and such excitations provide valuable information about f–f, f–d transitions, defects, and excitonic transitions in wide bandgap nanophosphors. It is also demonstrated that size‐dependent luminescence properties of nanophosphors are significant if the electron thermalization length becomes larger than the size of the nanoparticles. In this case, multiplication of electronic excitations processes well‐known in bulk materials are suppressed strongly in nanophosphors. [ABSTRACT FROM AUTHOR]

The BaGa2O4 ceramics doped with Eu3+ ions (1, 3 and 4 mol%) are obtained by solid‐phase sintering. The evolution of defect‐related extended free volumes in the BaGa2O4 ceramics due to the increase of the content of Eu3+ ions has been studied using the positron annihilation lifetime (PAL) spectroscopy technique. It is established that the increase in the number of Eu3+ ions in the basic BaGa2O4 matrix leads to the agglomeration of free‐volume defects with their subsequent fragmentation. The presence of Eu3+ ions results in the expansion of nanosized pores and an increase in their number with their future fragmentation. [ABSTRACT FROM AUTHOR]

Extended free‐volume positron trapping defects (positron trapping channel) and nanopores (positronium decaying channel) in the initial and modified MgAl2O4 ceramics sintered at 1200, 1300, and 1400 °C are investigated using positron annihilation lifetime spectroscopy within four‐component fitting procedure. It is established that the number and size of extended defects near grain boundaries (described by the second component) decrease with increasing sintering temperature of ceramics, which correlates with the content of additional phases in the materials. The agglomeration of nanopores occurs in the initial ceramics (described by the third component) and the decrease of number and size of nanopores in the modified ceramics. The corresponding processes of the evolution of nanopores described by the fourth component are reflected mainly in their reduction, except for ceramics sintered at 1400 °C. Such changes are associated with the evolution of porous structure and increased contact area between the grains of ceramics. [ABSTRACT FROM AUTHOR]

Herein, the optical absorption, X‐ray diffraction (XRD) analysis data, and mechanical properties along the ion path are analyzed for gadolinium gallium garnet (Gd3Ga5O12 or GGG) single crystals irradiated with fast 84Kr ions to fluences of 1013–1014 ion cm−2. It is found that the optical absorption spectra of Czochralski‐grown unirradiated GGG crystal consist of relatively narrow lines in the UV spectral range associated with the 4f–4f transitions in Gd3+ ions. Transitions from the 6S7/2 ground state to the 6P, 6J, and 6D states in the Gd3+ cation are visible. An additional absorption band is also observed at 350 nm, which is associated with an uncontrolled Ca impurity. In irradiated GGG single crystals, a shift of the fundamental absorption edge by ≈30 nm to the long‐wavelength part of the spectrum is observed. The observed changes are caused by structural disturbances caused by depletion of the surface layer and an increase in the number of displaced atoms accompanied by an increase in the crystal lattice parameter. XRD analysis shows the presence of size effects and deformation of interplanar distances due to the displacement of atoms from the sites of the crystal lattice with subsequent migration. Hardness measurements show ion‐induced softening, which may be due to ion‐induced amorphization. [ABSTRACT FROM AUTHOR]

Corundum (α‐Al2O3) is a technologically important material and, in particular, widely used in optical applications such as luminescent radiation dosimeters, cryogenic scintillators, and is being considered as a promising candidate for windows in future fusion reactors. Its optical and mechanical properties are controlled by the presence of radiation‐induced (in particular, by fast neutrons) defects. Herein, the thermal stability and recombination kinetics of primary anion Frenkel defects—the F and F+ electronic centers and oxygen interstitials—in fast‐neutron‐irradiated α‐Al2O3 single crystals are analyzed. Theory is developed considering the formation of both neutral and charged oxygen Frenkel defect pairs; defect migration, interaction and recombination. Based on ab initio calculations and new theoretical kinetics analysis, for the first time, a coexistence is demonstrated, in comparable concentrations, of two interstitial types—neutral O atoms and negatively charged O− ions (with attributed optical absorption band maxima at 6.5 eV and 5.6 eV, respectively); and their diffusion parameters, necessary for the prediction of secondary defect‐induced reactions and defect/material thermal stability, are obtained. [ABSTRACT FROM AUTHOR]

This article describes a simple synthesis of two types of carbon dots (CDs) from glucose (g‐CDs) and citric acid (ca‐CDs) using the hydrothermal treatment method. At the synthesis the water solution of ammonia was used for doping of the g‐CDs and ca‐CDs by N for improving their electronic and transport properties. These synthesized CDs consist mostly of 1–4 layers having a uniform size of about 14 nm, and the nitrogen atoms were successfully introduced into the lattice of CDs. The resulting CDs have a crystalline structure with functional groups on the surface and exhibit bright photoluminescence in the violet–blue region of the spectrum. The g‐CDs exhibit an excitation‐dependent blue emission, in the case of ca‐CDs their emission dependence on excitation is not clearly observed. The obtained CDs have a number of advantages and can be widely used for the creation of photodetectors, detectors for chemical elements and various optoelectronic applications. [ABSTRACT FROM AUTHOR]