The processes of intrinsic and extrinsic luminescence excitation by synchrotron radiation of 4-40 eV or electron pulses have been studied in ␣-Al 2 O 3 single crystals at 8 K. The intrinsic A ͑7.6 eV͒ and E emissions ͑3.77 eV͒ can be effectively excited in the region of long-wavelength ͑8.85-9.1 eV͒ and short-wavelength ͑9.1-9.3 eV͒ components of exciton absorption doublet, respectively. Fast ͑ϳ6 and ϳ20 ns͒ and slow ͑ϳ150 ns͒ components of the A emission correspond to the creation of singlet and triplet p 5 s excitons. The efficiency of the A emission in the region of band-to-band transitions is low. The intensity of A emission sharply increases ͑approximately quadratically͒ with a rise of the excitation density by nanosecond electron pulses. In Al 2 O 3 :Sc, the 5.6-eV luminescence is caused by the decay of near-impurity electronic excitations ͑ϳ8.5 eV͒ as well as by the electron recombination with holes localized near Sc 3ϩ centers. The efficiency of 7.6-, 5.6-, and 3.8-eV emission sharply increases at the energy of exciting photons of hϾ25 eV. One photon of 26-29 and 30-37 eV causes the ionization of the 2p 6 or 2s 2 shell of the oxygen ion and provides the creation of two or three electron-hole pairs, respectively. Long-term investigations of ␣-Al 2 O 3 crystals did not lead to the detection of immobile self-trapped holes or electrons. The A emission excited at the direct photocreation of excitons or at the recombination of free electrons and free holes is interpreted by us as the radiative decay of self-shrunk excitons. The theoretical model of Sumi allows the existence of such immobile self-shrunk excitons even if an electron and a hole do not separately undergo the self-trapping. ͓S0163-1829͑99͒01825-1͔
The technological challenge imposed by the time resolution essential to achieve real-time molecular imaging calls for a new generation of ultrafast detectors. In this contribution, we demonstrate that CdSe-based semiconductor nanoplatelets can be combined with standard scintillator technology to achieve 80 ps coincidence time resolution on a hybrid functional pixel. This result contrasts with the fact that the overall detector light output is considerably affected by the loss of index-light-guiding. Here, we exploit the principle of 511 keV energy sharing between a high-Z, high stopping power bulk scintillator, and a nano-scintillator with sub-1 ns radiative recombination times, aiming at a breakthrough in the combined energy and time resolution performance. This proof-of-concept test opens the way to the design and study of larger size sensors using thin nanocomposite layers able to perform as efficient time taggers in a sampling detector geometry of new generation.
npj 2D Materials and Applications (2019) 3:37; https://doi.
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