PbSe/PbSexS1−x core-alloyed shell nanocrystals

This work describes a novel synthetic method for the formation of PbSe core-shell nanocrystals. The PbSe show a special interest due to their strong quantum size effect and activity in the near IR spectral regime. Common colloidal procedure of semiconductor core-shell structures includes an initial preparation of the core, isolation of the NCs from the mother solution, dissolving in a new solution with a post injection of the shell constituents. This paper discusses an alternative procedure, utilizing a single-injection of the core and shell constituents, simultaneously. The faster reactivity of selenium to a lead precursor, directs a nucleation of a PbSe core, while the sulfur reagent participate at a later stage as a PbSexS1−x shell. PbSe core-shell NCs, prepared by the discussed single-injection process, were compared with those prepared via a two-step injection, containing equimolar reagents and NCs size. Both cases showed a strong fluorescence (with quantum efficiency 30-60%), but with a pronounced red-shift of the band edge (30-120 meV), with respect to the core, upon an increase of the shell thickness. This red-shift was substantially larger for core-shell NCs prepared via a one-step injection. The red-shift corresponds to the formation of type II transition between Ec,PbSe and Ev,PbS. While the extra red-shift, upon preparation by a single injection, corresponds to the formation of shells with a tunable alloy composition.