Watching early stage nucleation
In classic nucleation theory, a metastable disordered dense liquid or amorphous solid cluster spontaneously and irreversibly transforms into a crystalline nucleus. Jeon et al. observed the formation of gold crystals on a graphene substrate through the reduction of a precursor using an electron beam. Rather than the classic view, they instead observed a nucleation pathway that involves dynamic and reversible fluctuations of developing nuclei between disordered and crystalline states. The lifetime in the disordered state decreases with increasing cluster size, and at sufficiently small sizes, the binding energy per atom is large enough relative to the energy required to induce melting that the heat imparted upon binding is enough to drive partial collapse of an ordered cluster into the disordered state.
Science, this issue p. 498
Nucleation in atomic crystallization remains poorly understood, despite advances in classical nucleation theory. The nucleation process has been described to involve a nonclassical mechanism that includes a spontaneous transition from disordered to crystalline states, but a detailed understanding of dynamics requires further investigation. In situ electron microscopy of heterogeneous nucleation of individual gold nanocrystals with millisecond temporal resolution shows that the early stage of atomic crystallization proceeds through dynamic structural fluctuations between disordered and crystalline states, rather than through a single irreversible transition. Our experimental and theoretical analyses support the idea that structural fluctuations originate from size-dependent thermodynamic stability of the two states in atomic clusters. These findings, based on dynamics in a real atomic system, reshape and improve our understanding of nucleation mechanisms in atomic crystallization.
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