Recent experimental explorations of non-classical nucleation
Nucleation, the early stage of crystallization, is a key step to produce functional materials, but nucleation processes have yet to be understood in detail. Recent advanced characterization techniques, especially atomic force microscopy, liquid-phase transmission electron microscopy (TEM) and cryo-TEM, enable scientists to observe previously inaccessible nucleation details at the nanoscale. In this review, we summarize newly suggested non-classical nucleation models with respect to mechanistic understanding from the experimental views, which highlight multi-steps nucleation mechanism. Different immediated phases such as clusters at pre-nucleation stage, liquid-like precursors, amorphous phase and even oligomers precursor have been revealed and they play vital roles in nucleation and structure control. Moreover, these immediated phases practically contribute in general to the structural variances of materials for nanoscience and nanotechnology. Overall, these studies ultimately enable us to control nucleation processes for materials synthesis.