Decay behavior of Eu2+-Activated sulfide phosphors: The Pivotal role of unique activator sites

We present a systematic decay analysis of four sulfide phosphors, namely SrS:Eu2+, CaAl2S4:Eu2+, KGaS2:Eu2+, and NaBaAlS3:Eu2+. In SrS and NaBaAlS3, there is only one activator site, whereas KGaS2 has two, and CaAl2S4 has three. We confirm the typical concentration quenching behavior, where the decay rate increases with the Eu2+ activator concentration. The decay curve for these sulfide phosphors consists of two components. The fast decay component remains unaffected by the Eu2+ activator concentration, while the slow component exhibits the typical concentration quenching behavior. The initial sharp decline in the decay curve, which is concentration-independent, is associated with direct quenching occurring in close proximity to the excited activator before inter-activator energy transfer takes place. The subsequent concentration quenching results from active inter-activator energy transfer. An intriguing observation is termed “wavelength quenching”, where the decay rate increases as the emission probe wavelength decreases. This phenomenon is active for multi-site phosphors but inactive for single-site phosphors, which can be attributed to the active inter-activator energy transfer between distinct crystallographic sites within the host.