Publication date: 22nd July 2026
The self-assembly of nanocrystals (NCs) into ordered mesocrystals enables collective phenomena driven by long-range inter-particle correlations. High order requires a delicate balance between sufficientlyhigh NC density to drive assembly and sufficient dynamic freedom allowing particles to find energetically favourable positions upon densification. Understanding how NC diffusivity evolves during densification is therefore essential for achieving reliable, high-yield outcomes. Accessing NC diffusivity at high particle densities remains challenging due to their fast motion, determined by a multitude of interconnected factors. Here, we map the spatio-temporal evolution of CsPbBr3 NC diffusion during evaporation-driven self-assembly into mesocrystals by X-ray photon correlation spectroscopy (XPCS) using a nanofocused high-flux X-ray beam at a fourth-generation synchrotron. Our results reveal that spatial heterogeneity in collective diffusivity directly determines local mesocrystal yield. Near the evaporation front, collective NC diffusivity slows during densification but stabilizes at a slowed-down-but-mobile plateau. In contrast, regions far from the evaporation front undergo premature agglomeration and kinetic arrest before long-range order can develop. These findings demonstrate that local diffusive dynamics determine whether assembly proceeds towards ordered mesocrystals or kinetically
arrested aggregates. By correlating the measured diffusivity directly with the NC volume fraction, structural evolution and predictions from theoretical models, we propose solvent mediated hydrodynamic interactions between NCs to be the dominant factor controlling collective NC diffusivity and agglomeration during evaporation. Our results highlight that long range hydrodynamic ligand-ligand and ligand-solvent interactions largely determine whether NCs agglomerate randomly or form orderd mesocrystals. This points to solvent viscosity, evaporation rate, and solvent mixtures as direct design criteria for improving mesocrystal yield and spatial homogeneity through tailored solvent and ligand choice.
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