Real-Time Atomic Scale Observation of Surface-Induced Crystallization of a Bismuth Nanodroplet by Stepwise Ordering Mechanism

Surface-induced crystallization of an isotropic Bi nanodroplet in three distinct steps was recorded at atomic resolution by in situ high resolution transmission electron microscopy. First, formation of a crystalline nucleus at the edge of the nanodroplet is induced by the liquid-state surface faceting, which initiates the formation of a prefreezing state with a distorted structure. Then, a periodic line pattern of the atomic columns with a periodicity of 0.49 nm that is relative to the interlayer spacing of the Bi crystal along the ⟨112⟩* direction (one-dimensional ordered structure) was formed by structure relaxation of the prefreezing nanoparticle, followed by a concerted growth and ordering process. Finally, the crystallization from the periodic structure was carried out by atom/atom interactions within and between the atomic layers to form the covalent Bi–Bi bonds that have the same arrangement as in the Bi crystal (crystallization along the ⟨11̅0⟩ direction). Motivated by the experimental observations, a stepwise ordering mechanism on the crystallization of a nanodroplet is revealed, which would provide new insight into the dynamics of phase transitions, crystal growth, and three-dimensional crystallization of supercrystals.