In Situ Atomic-Scale Study of Particle-Mediated Nucleation and Growth in Amorphous Bismuth to Nanocrystal Phase Transformation.

Understanding classical and nonclassical mechanisms of crystal nucleation and growth at the atomic scale is of great interest to scientists in many disciplines. However, fulfilling direct atomic-scale observation still poses a significant challenge. Here, by taking a thin amorphous bismuth (Bi) metal nanosheet as a model system, direct atomic resolution of the crystal nucleation and growth initiated from an amorphous state of Bi metal under electron beam inside an aberration-corrected transmission electron microscope is provided. It is shown that the crystal nucleation and growth in the phase transformation of Bi metal from amorphous to crystalline structure takes place via the particle-mediated nonclassical mechanism instead of the classical atom-mediated mechanism. The dimension of the smaller particles in two contacted nanoparticles and their mutual orientation relationship are critical to governing several coalescence pathways: total rearrangement pathway, grain boundary migration-dominated pathway, and surface migration-dominated pathway. Sequential strain analyses imply that migration of the grain boundary is driven by the strain difference in two Bi nanocrystals and the coalescence of nanocrystals is a defect reduction process. The findings may provide useful information to clarify the nanocrystal growth mechanisms of other materials on the atomic scale.