Enhancing structural coherence and optical coupling in quantum dot supercrystals

• Applicants: Professor Dr. Marcus Scheele; Professor Dr. Frank Schreiber
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 546072194

Advances in the synthesis of uniformly sized nanocrystals have enabled the growth of macroscopic super-crystals through the self-assembly of nanocrystals into highly ordered, three-dimensional lattices. Compared to classical atomic crystals, nanocrystals take the role of atoms in super-crystals, which has led to the hypothesis that nanocrystals behave "quasi-atomically" in such ordered ensembles. The project tests this hypothesis with regard to a possible correlation between the structural and optical properties in super-crystals. It is based on the well-known fact that the optical properties of atomic crystals are significantly affected by nanoscale structural defects, e.g. near their surface. The project will work out whether a similar correlation also exists in super-crystals, i.e. whether the optical properties of the ordered ensemble of nanocrystals are different from those of the individual components. This is implemented through the combination of diffraction-limited optical confocal microscopy with synchrotron-based X-ray scattering methods. Of central importance are nanodiffraction experiments, in which the X-ray beam is focused to 200 nm and less so that nanoscale defects in super-crystals can be detected with the same spatial resolution as the fluorescence, fluorescence lifetime and Raman properties in the confocal microscope. This way, correlations between the local optical properties and structural defects in the super-crystals will be revealed. In the second part of the project, the formation of super-crystals is studied in real time using X-ray scattering methods to elucidate the basic kinetics of the formation of defects. By varying the crystallization parameters (concentration, solvent, surface ligands, etc.), in-situ X-ray scattering is applied to identify conditions under which the formation of defects can be inhibited to afford super-crystals with fewer defects. In the third phase of the project, the previously optimized crystallization conditions are used to grow supercrystals with low defect concentration and determine their optical properties. If the initial hypothesis of an analogy between atoms and nanocrystals holds true, a significantly more homogeneous fluorescence behavior can be expected for such low-defect supercrystals. The results of the project are relevant for the application of supercrystals as superfluorescent emitters, e.g. in micro LEDs.

• DFG Programme: Research Grants