The thermal response of lead sensitized terbium emission in group II sulfide nanoparticles: importance of spatial proximity and band gap engineering

Abstract

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.