I will discuss my group’s recent progress on “electron videography” based on liquid-phase transmission electron microscopy (TEM) in studying the assembly pathways of nanoparticles into unconventional metamaterials. The first example is a Maxwell lattice—a class of idealized frames with marginally constrained degrees of freedom. In contrast to closely packed tiles, Maxwell lattices can exhibit isoenergetic structural degeneracy, topological mechanical modes, and frustrated quantum states. We experimentally realized Maxwell lattices from nanoparticle assembly for the first time, observed low energy penalty phonon excitation modes, which match quantitatively with a modified bead-spring model. In our analysis, we observed an intriguing dependence of the modes and structural randomness on the lattice size due to entropy effect and also due to the conceptual shift that in such a lattice, the building blocks interact collectively. The conventional picture of pairwise interaction no longer applies. In the second example, we achieved a large-scale chiral lattices from assembly of gold nanoparticles both within a liquid-phase TEM chamber and ex-situ on a silicon wafer. The collapsing of a low packing density assembled structure leads to variants of symmetry-breaking chiral assemblies of high packing density. These structures demonstrate chiroptical properties in experimental characterization and finite-difference time-domain (FDTD) simulations. In both examples, electron videography provides unprecedented insights into the dynamics involved in the formation, relaxation, and reconfiguration of nanoparticle assemblies, which allows us to exploit previously underexplored collective functions.

Electron videography of self-assembling nanoparticles into Maxwell mechanical frames and symmetry-breaking chiral lattices

ACS Spring 2022

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