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Watch nucleation at an early stage
In the classical theory of nucleation, a dense, disordered metastable liquid or amorphous solid mass spontaneously and irreversibly transforms into a crystal nucleus. Jeon et al. observed the formation of gold crystals on a graphene substrate thanks to the reduction of a precursor using an electron beam. Rather than the classical 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 bond energy per atom is large enough compared to the energy required to induce fusion so that the heat transmitted during binding is sufficient to cause partial collapse of an ordered cluster in the disordered state.
Science, this issue p. 498
Abstract
Nucleation in atomic crystallization remains poorly understood, despite advances in the classical theory of nucleation. The nucleation process has been described to involve an unclassical mechanism that includes a spontaneous transition from disordered to crystalline states, but a detailed understanding of the dynamics requires further investigation. In situ electron microscopy of heterogeneous nucleation of individual gold nanocrystals with millisecond time resolution shows that the early stage of atomic crystallization passes through dynamic structural fluctuations between disordered and crystalline states, rather than a single transition irreversible. Our experimental and theoretical analyzes support the idea that structural fluctuations arise from the thermodynamic stability dependent on the size of the two states in atomic clusters. These findings, based on the dynamics in a real atomic system, reshape and improve our understanding of the nucleation mechanisms in atomic crystallization.
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