Capturing In Situ Atomic-Scale Insights into the Growth of CdS Quantum Dots in Aqueous Media

Despite extensive research in the past, the fundamental formation mechanisms of chalcogenide quantum dots (QDs) have remained poorly understood. To gain precise control over synthesis pathways and produce more complex QDs with targeted optoelectronic properties in the future, an in-depth understanding of the QD formation reactions is urgently required. In situ liquid-phase (scanning) transmission electron microscopy (LP-(S)TEM) offers a powerful possibility to directly visualize atomic-scale particle formation processes in realistic liquid environments. However, electron beam-induced radiolysis of water produces reactive species that strongly etch chalcogenides, making their investigations by LP-(S)TEM challenging. While recent studies have provided valuable insights into the etching processes of chalcogenide QDs, it remains a significant challenge to invert dissolution into formation processes. In this work, by beneficially tuning the chemical environment, we achieve in situ conditions that enable CdS QD formation in an aqueous environment under electron irradiation with direct observation. Our LP-STEM recordings visualize the atomic, multistep mechanisms of nucleation, growth, and nanocrystallization of CdS QDs. Our results reveal predominantly nonclassical growth pathways via coalescence that dominate over classical growth. Subsequent structural and analytical analyses further confirmed the formation of CdS QDs. Moreover, we provide a detailed discussion of how radiation-related factors and precursor solution chemistry affect CdS stability during LP-STEM experiments. Our findings hence offer detailed insights into the complex growth processes of CdS QDs and demonstrate how tailoring the liquid environment enhances the stability of sensitive chalcogenide materials in LP-STEM.