Nucleation and growth are universally important in systems from the atomic to the micrometer scale as they dictate the structural and functional attributes of crystals. However, at the nanoscale, the pathways toward crystallization have been largely unexplored owing to the challenge of resolving the motion of individual building blocks in a liquid medium. Here we address this gap by directly imaging the full transition of dispersed nanoparticles to a superlattice at the single-particle level with liquid-phase transmission electron microscopy. Physical parameters including surface tension, chemical potential, and phase coordinates are quantitatively measured, charting the nonclassical nucleation pathway involving a dense, amorphous intermediate. We elucidate the origin of this two-step crystallization by experimentally measuring the energy barrier for nucleation, which is lower in the presence of the amorphous precursor. We demonstrated the versatility of our approach by studying a growing nanoparticle superlattice with fluctuating interfaces due to thermal agitations, which defines the final shape of the superlattice. Our real space nanoscopic imaging platform not only offers a new understanding of crystallization kinetics but also opens opportunities to other nanoscale fluctuating systems, from biofouling, lubrication, melting, phonon transport, to dendrite formation.

Unraveling nonclassical crystallization and growth pathways of nanoparticle superlattices via liquid-phase TEM imaging

ACS Fall 2021

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