Abstract
The photoluminescence emission (PLE) mapping and infrared-stimulated luminescence (ISL) spectra of CaS:Eu,Sm demonstrated that Eu is formed in Eu2+ as a luminescent centre, whereas Sm is formed in Sm3+ as an electron-trapping centre by replacing Ca2+ in the CaS host lattice. It was found that electron trapping occurred in the photoluminescence excitation (PLX) process. A series of concentrations of Eu with a fixed Sm concentration and a series of concentrations of Sm with a fixed Eu concentration in doubly doped CaS:Eu,Sm were studied by the time-related PLE of Eu2+ in the PLX process. According to the electronic energy level(s) of Eu2+ and Sm3+, excitation-energy transfer is realized by translocation of excited electrons from Eu2+ to Sm3+, causing simultaneous ionization of Eu2+ (leaving Eu3+) and capture of excited electrons at Sm3+ (forming Sm2+). ISL is then produced due to the energy released from recombination of de-trapped electrons from the occupied electron-trapping sites (Sm2+) and the previously ionized luminescent centres (Eu3+) under infrared irradiation.
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