Connecting structural defects and optical properties of core-shell quantum dots induced by interfacial lattice strain

by Omogo, Benard; Benamara, Mourad; Heyes, Colin D.

Type I core-shell quantum dots are continuing to receive great interest due to their superior optical properties compared to other types of fluorophores. High quantum yields and high photostability are easily achievable when the core material is coated with a higher-bandgap shell material which confines the excitons to the core and decreases charge carrier trapping at the surface. However, the shell material shows different lattice parameters from the core, thereby introducing interfacial lattice strain, which depends on both the material and its thickness. Furthermore, the interfacial lattice strain may cause the shell to grow anisotropically, so that controlling the distribution of the shell material around the core remains a challenge. Succesive ion layer adsorption and reaction (SILAR) has helped to alleviate these problems, however, the exact details of the shell growth are still far from understood. To address this, we have synthesized various core-shell and core-shell-shell quantum dots (e.g. CdSe/ZnS, CdSe/CdS, CdSe/CdS/ZnS) and analyzed both their optical and structural properties as a function of the shell thickness. By analyzing HR-TEM images and HAADF-STEM coupled with electron energy loss spectroscopy (EELS), we report on the effect of the interfacial lattice strain on their optical and structural properties, such as quantum yield, fluorescence lifetime, blinking, and shell distribution. These results are used to draw connections between structural defects and charge carrier relaxation pathways.