Unveiling the Atomic and Electronic Structure of Stacked-Cup Carbon Nanofibers.

We report results of comprehensive experimental exploration (X-ray photoemission, Raman and optical spectroscopy) of carbon nanofibers (CNFs) in combination with first-principles modeling. Core-level spectra demonstrate prevalence of sp2 hybridization of carbon atoms in CNF with a trace amount of carbon-oxygen bonds. The density functional theory (DFT)-based calculations demonstrated no visible difference between mono- and bilayers because σ-orbitals are related to in-plane covalent bonds. The influence of the distortions on π-peak is found to be significant only for bilayers as a result of π-π interlayer bonds formation. These results are supported by both experimental Raman and XPS valence band spectra. The combination of optical measurements with a theoretical modeling indicates the formation of optically active graphene quantum dots (GQDs) in the CNF matrix, with a radiative relaxation of the excited π* state. The calculated electronic structure of these GQDs is in quantitative agreement with the measured optical transitions and provides an explanation of the absence of visible contribution from these GQDs to the measured valence bands spectra.

Thermoluminescence of new Al2O3-BeO ceramics after exposure to high radiation doses.

New Al2O3-BeO ceramics were synthesized from alumina nanopowder in BeO crucibles by high-temperature treatment in reducing conditions. It was found that the TL curve of these ceramics is characterized by a dosimetric peak at 520-530 K for a heating rate of 2 K/s. The integrated TL of the peak is not influenced by the heating rate of the samples, unlike that in anion-defective alumina and beryllia crystals. The presence of an energy distribution of the traps which cause dosimetric TL peak is verified by Tm-Tstop and initial rise methods. Linear dependence on dose of the intensity of TL peak at 520-530 K is observed in the 10-300 Gy gamma-dose range. The diversity of TL sensitivity of the ceramic samples synthesized in one cycle does not exceed 10%. The results obtained show that the newly synthesized Al2O3-BeO ceramics are promising materials for TL dosimetry of high doses of ionizing radiation.