Wave\nFunction Engineering in CdSe/PbS Core/Shell Quantum\nDots

Abstract

The\nsynthesis of epitaxial CdSe/PbS core/shell quantum dots (QDs)\nis reported. The PbS shell grows in a rock salt structure on the zinc\nblende CdSe core, thereby creating a crystal structure mismatch through\nadditive growth. Absorption and photoluminescence (PL) band edge features\nshift to lower energies with increasing shell thickness, but remain\nabove the CdSe bulk band gap. Nevertheless, the profiles of the absorption\nspectra vary with shell growth, indicating that the overlap of the\nelectron and hole wave functions is changing significantly. This leads\nto over an order of magnitude reduction of absorption near the band\ngap and a large, tunable energy shift, of up to 550 meV, between the\nonset of strong absorption and the band edge PL. While the bulk valence\nand conduction bands adopt an inverse type-I alignment, the observed\nspectroscopic behavior is consistent with a transition between quasi-type-I\nand quasi-type-II behavior depending on shell thickness. Three effective\nmass approximation models support this hypothesis and suggest that\nthe large difference in effective masses between the core and shell\nresults in hole localization in the CdSe core and a delocalization\nof the electron across the entire QD. These results show the tuning\nof wave functions and transition energies in CdSe/PbS nanoheterostructures\nwith prospects for use in optoelectronic devices for luminescent solar\nconcentration or multiexciton generation.