In situ Observation of Structural Evolution and Phase Engineering of Amorphous Materials during Crystal Nucleation

The nucleation pathway determines the structures and thus properties of formed nanomaterials, which is governed by the free energy of the intermediate phase during nucleation. The amorphous structure, as one of the intermediate phases during nucleation, plays an important role in modulating the nucleation pathway. However, the process and mechanism of crystal nucleation from amorphous structures still need to be fully investigated. Here, in situ aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) is employed to conduct real-time imaging of the nucleation of ultrathin amorphous nanosheets (NSs). The results indicate that their nucleation contains three distinct stages, i.e., aggregation of atoms, crystallization to form lattice-expanded nanocrystals, and relaxation of the lattice-expanded nanocrystals to form final nanocrystals. In particular, the crystallization processes of various amorphous materials are investigated systematically to form corresponding nanocrystals with unconventional crystalline phases, including face-centered-cubic (fcc) Ru, hexagonal-close-packed (hcp) Rh, and a new intermetallic IrCo alloy. In situ electron energy-loss spectroscopy (EELS) analysis unveils that the doped carbon in the original amorphous NSs can migrate to the surface during the nucleation process, stabilizing the obtained unconventional crystal phases transformed from the amorphous structures, which is also proven by density functional theory (DFT) calculations.