Physical basis of persistent luminescence: The case of europium doped Ca1−xSrxS

The physical basis of persistent luminescence in terms of the color of emission as well as the useful afterglow time is discussed for a commercial red-emitting Ca1−xSrxS:Eu phosphor. The discussion is based on the results of excitation/emission spectra and critical analysis of thermoluminescence (TL) data. The work focuses on: (i) the well known tuning of emission in Ca1−xSrxS:Eu phosphor (ii) control of intrinsic trapping parameters that decides the lifetime (τ) of charge in traps relevant to the phenomenon of persistent luminescence. XRD data is used to characterize the commercial phosphor. The emission spectra (λex = 275 nm) reveals the prominent well known broad emission (FWHM = 0.20 eV) at 1.92 eV/647 nm of Eu2+ along with weak transitions at 1.63 eV/761 nm, 1.75 eV/710 nm, 2.13 eV/581 nm and 2.56 eV/485 nm indicating presence of europium in Eu3+ state. Rigorous analysis of TL shows that non-first order TL peaks having τ300≈mins to hours are key to persistent luminescence in Ca1−xSrxS:Eu.