Unveiling the Dynamical Assembly of Magnetic Nanocrystal Zig-Zag Chains via In Situ TEM Imaging in Liquid

The controlled assembly of colloidal magnetic nanocrystals is key to many
applications such as nanoelectronics, storage memory devices, and nanomedicine.
Here, the motion and ordering of ferrimagnetic nanocubes in water via
liquid-cell transmission electron microscopy is directly imaged in situ. Through
the experimental analysis, combined with molecular dynamics simulations and
theoretical considerations, it is shown that the presence of highly competitive
interactions leads to the formation of stable monomers and dimers, acting
as nuclei, followed by a dynamic growth of zig-zag chain-like assemblies. It
is demonstrated that such arrays can be explained by first, a maximization of
short-range electrostatic interactions, which at a later stage become surpassed
by magnetic forces acting through the easy magnetic axes of the nanocubes,
causing their tilted orientation within the arrays. Moreover, in the confined
volume of liquid in the experiments, interactions of the nanocube surfaces with
the cell membranes, when irradiated at relatively low electron dose, slow down
the kinetics of their self-assembly, facilitating the identification of different
stages in the process. The study provides crucial insights for the formation
of unconventional linear arrays made of ferrimagnetic nanocubes that are
essential for their further exploitation in, for example, magnetic hyperthermia,
magneto-transport devices, and nanotheranostic tools.