Essential to the realization of superstructure-by-design is to dissect assembly pathways and understand the kinetics of phase transitions. The process of nanocrystal assembly, analogous to a chemical reaction, usually traverses a complex free-energy landscape before reaching the final dry state. Therefore, we must begin to think of assembly as a reaction pathway connecting multiple nonequilibrium intermediates that constitute a comprehensive phase diagram. The commonly used drying-mediated method presents a significant challenge to real-space observation of crystallization. Meanwhile, the kinetic pathways of self-assembly are dictated by how nanoscale building blocks diffuse and interact in a solvated state. Fully understanding these pathways requires real-space, real-time characterization with meaningful spatiotemporal resolution, which is not by possible with existing ex-situ characterization or scattering-based methods. In this talk, I will our recent advances in direct imaging of nanocrystal assembly and crystallization using in-situ liquid cell transmission electron microscopy (LCTEM). We demonstrate that the interparticle interaction can be readily tuned by changing the nature of solvent, enabling the observation of a multistep crystallization pathways of two-dimensional nanocrystal superlattices. Machine-learning-based approaches were applied for LCTEM image segmentation and particle tracking, which allows subsequent quantitative and statistical analysis of the crystallization pathways. Our study paves the way to study complex crystallization pathways of multicomponent nanocrystals in non-aqueous solution using LCTEM.

Understanding the formation and reconfiguration of complex nanocrystal superlattices using in-situ electron microscopy

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ACS Spring 2023

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