Fluorescence from core shell quantum dots

[avkr]

What is the reason for enhancement in the intensity of emission due to the introduction of a shell in quantum dots? I do understand the blue shift in quantum dots but how does a shell enhance it?

 

[Ygggdrasil]

Core-Shell Quantum Dots
The luminescent properties of quantum dots arise from recombination of electron-hole pairs (exciton decay) through radiative pathways. However, the exciton decay can also occur through nonradiative methods, reducing the fluorescence quantum yield. One of the methods used to improve efficiency and brightness of semiconductor nanocrystals is growing shells of another higher band gap semiconducting material around them. These quantum dots with small regions of one material embedded in another with a wider band gap are known as core-shell quantum dots (CSQDs) or core-shell semiconducting nanocrystals (CSSNCs). For example, quantum dots with CdSe in the core and ZnS in the shell (Product Nos. 748056, 790192) available from Sigma-Aldrich Materials Science exhibit greater than 50% quantum yield. Coating quantum dots with shells improves quantum yield by passivizing nonradiative recombination sites and also makes them more robust to processing conditions for various applications. This method has been widely explored as a way to adjust the photophysical properties of quantum dots.8-10

https://www.sigmaaldrich.com/techni...ce/nanomaterials/quantum-dots.html#core-shell

tl;dr: Quantum dots can interact with their surroundings to provide non-radiative pathways for exciton decay. Shells reduce these interactions, increasing the quantum yield of the dots.