Red persistent luminescence in rare-earth codoped Ca2SiS4:Eu2+

Since the 1990s there is a large interest in persistent luminescent materials, for use in e.g. safety signs and UV-dosimeters. Several compounds have been identified since then, which mainly emit in the blue to green part of the visible spectrum. An efficient red persistent phosphor is still sought after, partly due to the reduced eye sensitivity for low light intensity levels. Recently we studied rare-earth doped thiosilicates (M2SiS4, with M = Ca, Sr, Ba) which allow broadband emission in the range from blue (Ba2SiS4:Ce3+) to red (Ca2SiS4:Eu2+) [1]. The emission of the latter compound consists of two bands, peaking at 565 and 660nm, with the relative intensity depending on the europium concentration [2]. Although Ca2SiS4:Eu2+ itself does not show any persistence luminescence, codoping with rare earth elements can cause persistent luminescence in this material. The Ca2SiS4:Eu2+ samples were codoped with different rare earth elements (Dy3+, Nd3+, Sm3+,...) with a concentration of 0.5 to 2 molar%. No significant changes in emission spectrum were observed upon codoping. Both the spectrum and the decay profile of the persistence luminescence was characterized. The colour of the persistence emission was strongly redshifted compared to the steady state emission due to a relatively stronger emission of the 660nm-band. Clear differences are observed between the different codoped rare-earth elements. Ca2SiS4:Eu,Sm does not show any persistent luminescence, while the persistence for Nd and Dy codoping is very similar. For Ca2SiS4:Eu2+,Nd3+ persistent emission of the Nd3+ lines at 0.89, 1.08 and 1.36µm was observed. Interestingly, the excitation spectra of the steady state and the persistence luminescence do not coincide. Although Ca2SiS4:Eu2+ is efficiently excited in the 375 to 500nm wavelength range, due to transitions from the 4f7 ground state to the lower crystal field split 4f65d excited state, for the persistence luminescence excitation below 350 nm is required. This implies that trapping of the 5d excited electron of Eu2+ by a nearby defect via the conduction band is not causing the persistence luminescence, in contrast to the mechanism proposed for e.g. CaAl2O4:Eu2+,RE3+ [3]. An alternative energy level scheme is put forward based on thermoluminescence measurements