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 110 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.