Publication date: 15th May 2025
Synthetic advancements have enabled the production of III-V quantum dots with highly tunable composition and excellent optoelectronic properties. One novel approach is to disperse indium pnictide (e.g., InP1-yAsy) nanocrystals in a gallium-containing molten salt, wherein gallium exchanges with indium to yield indium-gallium solid-solution nanocrystals (e.g., In1-xGaxP1-yAsy). This process yields high quality nanocrystals with high photoluminescence quantum yield. However, the atomistic details of how the cation exchange occur are still not well understood, limiting our understanding of how the compositional complexity of the nanocrystals relates to their optoelectronic properties. In this work, we use scanning transmission electron microscopy (STEM) to evaluate the chemical and structural evolution of the nanocrystals with high spatial resolution. We overcome key technical challenges of beam damage and poor signal collection by performing extensive optimization of beam parameters and developing an analysis method to sum signal from many particles, which yields a reconstruction of the elemental distribution within the nanocrystals. We corroborate stacked measurements with energy dispersive x-ray spectroscopy (STEM-EDS) of individual particles, verifying that the heterogeneous elemental distribution is not an artifact of the analysis, and by high-angle annular dark-field (HAADF-STEM) imaging, which shows local variations in the lattice that are consistent with the variations in elemental distribution. Overall, we develop the use of STEM to provide a coherent understanding of the composition and atomic structure resulting from molten salt annealing. This insight into the synthesis may enable the composition of III-V nanocrystals to be tuned for superior optoelectronic performance.
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