The Thermal Response of Lead Sensitized Terbium Emission in Group II Sulfide Nanoparticles: Importance of Spatial Proximity and Band Gap Engineering

Trivalent terbium (Tb3+) emission in Tb3+ doped zinc sulfide (Zn(Tb)S) nanoparticles (NPs) is benefitted by the introduction of lead (Pb2+) as a co-dopant, as both ZnS and Pb2+ act as sensitizers for Tb3+ emission brightening. For Pb2+ sensitization, key mechanistic questions on whether this interaction is dynamic in nature and the presence of any host dependence are yet to be answered. In order to gain insights into co-dopant electronic interactions in light of these questions, this work investigates the thermal response of lead sensitized Tb3+ emission in postsynthetically modified Zn(Tb)S/Pb NPs. The irreversible thermal response of this sensitization suggests the role of a static phenomenon with a spatial proximity between Tb3+ and Pb2+ to form an intimate pair. This effect is found to be in remarkable contrast to that of the other sensitized component from the host ZnS NPs, which is found to be thermally reversible. Further experiments with Cd(Tb)S NPs revealed the presence of host dependence. Thus, the results presented in this work point to the simultaneous importance of (i) the spatial proximity of Pb2+ and Tb3+ and (ii) the colocalization of electron-hole pairs at the Tb center via the Pb center in a type-I codopant energy level alignment in order to realize the Pb sensitized Tb3+ emission in group II sulfide NPs. Lead sensitized terbium emission in zinc sulfide nanoparticles is thermally irreversible, in which the spatial proximity of lead-terbium is important.