Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
In this work we describe the synthesis and spectroscopic characterization of colloidal ZnSe/ZnS/CdS nanocrystals with a potential barrier of ZnS. Core and barrier are made via cation exchange from CdSe/CdS followed by epitaxial growth of the CdS shell to yield samples with a narrow size distribution.
The system exhibits a Type-II electronic structure and wave function overlap that is strongly dependent on the thickness of the ZnS barrier. Barrier thickness is controlled by both the amount of deposited material and the reaction and annealing temperature of CdS shell growth.
Our results show that a single monolayer of ZnS mitigates charge carrier overlap significantly, while more than four monolayers effectively suppress band edge absorption and emission. Long exposure to high temperatures smoothes out the barrier and pushes the particles back to a quasi Type-II structure. This is accompanied by a strong increase of fluorescence efficiency and a decrease of trap emission. Transient absorption spectra reveal suppression of the biexciton effect and a broad distribution of excitons with mixed S and P symmetry, which become optically allowed due to alloy formation and contribute to charge carrier relaxation across the barrier. These states play a significant role for charge carrier dynamics in nanocrystal heterojunctions.
We present a model of the core/shell interface based on cation diffusion, which allows to estimate the extent of the diffusion layer from optical spectra. The model accounts for the observed, non-linear shifts of absorption and band edge photoluminescence with increasing temperature.
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