Proceedings of MATSUS Fall 2024 Conference (MATSUSFall24)
DOI: https://doi.org/10.29363/nanoge.matsusfall.2024.003
Publication date: 28th August 2024
Core/shell (c/s) semiconductor nanocrystals (NCs) are key building blocks in modern optoelectronic devices. The specific core and the shell materials determine the alignment between valence- (VB) and conduction-band (CB) energy levels. Because photogenerated electrons (e-) and holes (h+) relax to their lowest and highest available energy levels, respectively, these carriers are confined to the core and away from the surface in type-I c/s NCs, resulting in high photoluminescence (PL) stability and efficiency. In contrast, c/s NCs with a reverse-type-I configuration are highly susceptible to the environment as both carriers localize on the shell and are easily extractable by charge scavengers. In a type-II configuration, one of the semiconductors has both higher (or lower) VB and CB values, resulting in one carrier being confined to the core and the other to the shell. The presence of physically separated e-–h+ pairs (excitons) gives type-II c/s NCs long PL lifetimes. In the presence of quantum confinement, the exact size of the core and shell open a continuum between quasi-type-II—with only partial delocalization of one of the carriers, for example—and true type-II configurations.
Here, we report the synthesis and structural characterization of PbCh/AeCh core/shell nanocrystals (Ae = Ca, Sr, Ba; Ch = S, Se). Using a new synthesis developed by us, we have successfully passivated PbS or PbSe cores with alkaline-earth (Ae) chalcogenide shells. For example, PbS/SrS and PbSe/SrS are near-IR active NCs with PL maxima ranging between ~1000–2000 and 1500–2300 nm, respectively. Colloidal epitaxy in this system is possible thanks to both core and shell materials adopting identical rock salt crystalline structures, with a lattice parameter mismatch of ≤ 5% for SrS, CaSe, and SrSe (Figure). We predict that these materials will be more robust and, because the lead-based core is buried inside the NCs, potentially less toxic and more biocompatible compared to other bare lead-based materials.
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