Recent advancements in next-generation displays, lighting, and lasers rely on the development of materials with exceptional optical properties. Colloidal semiconductor quantum dots (QD) are key materials for these technologies due to their high spectral tunability and pure light emission. QDs are typically synthesized as core/shell heteronanocrystals (HNC) in order to achieve the desired brightness and stability. Moreover, this design enables the maximization of key performance factors, such as near-unity photoluminescence quantum yield (PLQY) and narrow emission linewidths. However, the core/shell structure that enables this high performance introduces a critical challenge. Lattice strain originating from the crystal lattice mismatch between the materials can significantly diminish the optical performance of the QDs. At the same time, the particle synthesis conditions promote the formation of an interface alloy, which can effectively mitigate lattice strain. However, the precise nature of this interface alloy is still under debate. It remains unclear what its structural composition and spatial extent are, and whether it forms a sharp or a graded interface. Therefore, the first study of this thesis investigated the nature of the core/shell interface in CdSe/CdS QDs. An extensive and systematic sample library with varied core sizes and shell thicknesses was analyzed using Raman spectroscopy. This analysis reveals that the core/shell boundary is a graded CdSe1–xSx interface alloy. This structure plays a key role in minimizing lattice strain and preventing non-radiative recombination, which enables a high PLQY. Furthermore, transient absorption spectroscopy (TA) measurements demonstrated that biexciton Auger recombination (AR) could be mitigated, linking the interface structure to the enhanced optical properties required for lasing applications. The second part of the thesis focused on strain engineering in multi-shell InP/ZnSe/ZnS QDs to overcome the system’s large lattice mismatch. Building on benchmark samples, the thicknesses of both the ZnSe intermediate and ZnS outer shells could be systematically varied. Raman spectroscopy was used to analyze the lattice strain development for different shell thicknesses, and it was confirmed that the lattice strain could be effectively minimized. Through this systematic optimization, QDs with nearunity PLQY were synthesized, establishing the design principles required for high optical performance in this system. In summary, this work provides fundamental insights into the critical role of the core/shell interface in HNCs. It demonstrates that moving beyond simple structural models to precisely understand and control both interface alloying and lattice strain is essential for optimizing the optical performance of QDs for optoelectronic applications.
Sandra Zech (Thu,) studied this question.