ABSTRACT
We review the state-of-the-art of determining the electronic structure of nanocrystals in thin films by electrochemistry. Our core conclusion, the necessity of combining electrochemical with spectroscopic techniques, is illustrated with the holistic analysis of thin films of CdSe nanocrystals cross-linked with electroactive metal ß-tetraaminophthalocyanines by differential pulse voltammetry, optical spectroscopy and potential modulated absorption spectroscopy. We show that the same nanocrystals cross-linked with phthalocyanines of different metal centers exhibit rather similar electrochemical signatures, but behave distinctly different in spectroelectrochemical investigations. We argue that in one case, namely CdSe nanocrystals cross-linked with Co ß-tetraaminophthalocyanine, we find supporting evidence for the hybridization of energy levels at the organic/inorganic interface. This work suggests that spectroelectrochemistry is capable of revealing the electronic structure of complex nanomaterials, such as semiconductor nanocrystals functionalized with organic pi-systems.
ABSTRACT
We correlate spatially resolved fluorescence (-lifetime) measurements with X-ray nanodiffraction to reveal surface defects in supercrystals of self-assembled cesium lead halide perovskite nanocrystals and study their effect on the fluorescence properties. Upon comparison with density functional modeling, we show that a loss in structural coherence, an increasing atomic misalignment between adjacent nanocrystals, and growing compressive strain near the surface of the supercrystal are responsible for the observed fluorescence blueshift and decreased fluorescence lifetimes. Such surface defect-related optical properties extend the frequently assumed analogy between atoms and nanocrystals as so-called quasi-atoms. Our results emphasize the importance of minimizing strain during the self-assembly of perovskite nanocrystals into supercrystals for lighting application such as superfluorescent emitters.