Zn2+ for Cd2+ Cation Exchange in ZnSe: towards ZnSe/CdSe heteronanocrystals and alloy nanocrystals

Abstract

In this work cation exchange of Zn2+ for Cd2+ in ZnSe nanocrystals (NCs) was studied. The goal was to explore the possibilities of controlling the ion distribution within the NCs utilizing cation exchange. To obtain reproducible results low polydispersity parent ZnSe NCs were synthesized. A novel synthesis was developed to sustain a source of size controllable parent ZnSe NCs. ZnSe NCs were prepared via the hot-injection method and subsequent addition of a second, less reactive Zn precursor. Heating the isolated parent NCs in the presence of a Cd2+ precursor resulted in ZnSe-CdSe NCs.

The ion distribution within the NCs was investigated by energy-dispersive X-ray spectroscopy (EDS) in combination with optical absorption measurement. This showed that particles with different ion distributions have been synthesized. With electron energy loss spectroscopy (EELS) mapping this has been proven unambiguously. The results show that it is possible to control the ion distribution by chosing the proper reaction conditions, e.g. the reaction temperature has a strong effect on both the composition and ion distribution. The ion distribution can be anything in between the extremes ZnSe-CdSe core-shell hetero-nanocrystals (HNCs) (at 150 degree C) and (Zn,Cd)Se homogeneous alloy NCs (at 240 degree C). NCs with intermediate ion distributions are gradient alloy NCs (at 200 degree C and 220 degree C). Upon increasing temperature (220 degree C) after exchange at 150 degree C the ion distribution does not change from core shell to alloy, however the shell thickens to expense of the core.

We propose a mechanism for cation exchange of Zn2+ for Cd2+ in ZnSe NCs. Fast ion exchange occurs at the NC surface. Cd2+ ions diffuse inward, aided by outward diffusion of Zn2+ ions through Frenkel defects. This relatively slow diffusion is thermally activated.