Atomic-scale dynamic observation reveals temperature-dependent multistep nucleation pathways in crystallization†
Uncovering kinetic pathways of non-classical multistep nucleation at the atomic-scale is critical for understanding the complex microscopic mechanism of heterogeneous nucleation and crystallization. However, due to the intricacies in tackling such challenging topics experimentally, the structure of intermediate states and the temperature dependent effect on multistep nucleation at the atomic-scale are still not fully understood. Here, we conduct direct in situ atomic-scale observations of kinetic processes of interfacial multistep nucleation pathways using an aberration-corrected TEM. We provide direct evidence for temperature-dependent multistep nucleation pathways in a supported bismuth system at the atomic-scale: a droplet-crystal two-step nucleation pathway at high temperature close to the melting point of bulk bismuth, a droplet-local ordered structure-crystal three-step nucleation pathway at medium temperature between a critical temperature Tc1 and the lowest size-dependent melting temperature of bismuth nanoparticles, and a cluster-crystal two-step nucleation pathway at a temperature lower than the lowest melting point (Tl). The related critical temperature Tc1 and the lowest melting point in the three pathways are confirmed based on the size-dependent melting phase diagram of Bi nanoparticles. Statistical analyses imply that the nucleation pathway plays an important role in deciding the nucleation and crystallinity kinetics, and the critical size of crystal nucleation is controlled by temperature. The understanding of multiple intermediate steps in nucleation and the temperature effect on nucleation pathways has important implications for the synthesis and growth of clusters, amorphous and crystalline materials, and these findings may further enrich the nucleation theory.