Nucleation, Growth and Shape Transformation of Nanoparticles by In Situ TEM

Molecular or atomistic mechanistic understanding of nucleation, growth, and structural transformations of materials have not achieved enough at the nanoscale or below. It is mainly because of a lack of appropriate analytical methods that can obtain in-situ structural information with a spatial resolution at such small length scale along with sub-msec temporal resolution. The in situ, both in liquid phase and dry state, transmission electron microscopy (TEM) offers an opportunity to directly observe diverse classes of chemical reactions. In liquid phase TEM, there are two-types of liquid cell that are widely used: silicon nitride window liquid cell and graphene liquid cell. The former allows real time observation in relatively low spatial resolution but it is possible to incorporate electrical biasing or heating conditions into imaging. The latter suggests an opportunity for high-resolution in situ imaging of materials reactions occurring in thin liquid media. Here we present applications of in situ TEM to study nucleation, crystallization, and shape transformation of diverse types of nanoparticles. We reveal that the early stage of crystal growth is driven by reversible transition between disordered and ordered phases before crystalline phase is stable above a certain size. It is also frequently observed that different types of non-classical pathway, including two-step nucleation, amorphous-to-crystalline transition, and coalescence of clusters, are heavily involved in different conditions of crystallization of materials. We also present a new development using liquid phase TEM to investigate 3D atomic structures of nanoparticles directly in the colloidal synthesis batch.