Tailoring ZnSe-CdSe colloidal quantum dots via Cation Exchange: From core/shell to alloy nanocrystals

摘要

We report a study of Zn~(2+) by Cd~(2+) cation exchange (CE) in colloidal ZnSe nanocrystals (NCs). Our results reveal that CE in ZnSe NCs is a thermally activated isotropic process. The CE efficiency (i.e., fraction of Cd~(2+) ions originally in solution, Cd_(sol), that is incorporated in the ZnSe NC) increases with temperature and depends also on the Cd_(sol)/ZnSe ratio. Interestingly, the reaction temperature can be used as a sensitive parameter to tailor both the composition and the elemental distribution profile of the product (Zn,Cd)Se NCs. At 150 C ZnSe/CdSe core/shell hetero-NCs (HNCs) are obtained, while higher temperatures (200 and 220 C) produce (Zn_(1-x)Cd_x)Se gradient alloy NCs, with increasingly smoother gradients as the temperature increases, until homogeneous alloy NCs are obtained at T ≥ 240 C. Remarkably, sequential heating (150 C followed by 220 C) leads to ZnSe/CdSe core/shell HNCs with thicker shells, rather than (Zn_(1-x)Cd_x)Se gradient alloy NCs. Thermal treatment at 250 C converts the ZnSe/CdSe core/shell HNCs into (Zn_(1-x)Cd_x)Se homogeneous alloy NCs, while preserving the NC shape. A mechanism for the cation exchange in ZnSe NCs is proposed, in which fast CE takes place at the NC surface, and is followed by relatively slower thermally activated solid-state cation diffusion, which is mediated by Frenkel defects. The findings presented here demonstrate that cation exchange in colloidal ZnSe NCs provides a very sensitive tool to tailor the nature and localization regime of the electron and hole wave functions and the optoelectronic properties of colloidal ZnSe-CdSe NCs.