Direct observation of the moment of nucleation from a solution by TEM

Recent discoveries of various nucleation processes make understanding the initial processes of crystallization, where atoms and molecules come together to form a crystal nucleus, difficult. Herein, transmission electron microscopy is used to directly visualize the moment of nucleation from a solution and to show that the singular phenomena of nanoparticles are critical to understanding nucleation. Our work revealed that nanoparticles undergo precipitation and dissolution even in undersaturated and equilibrium conditions, that crystal polymorphs formed at different frequencies compete, that molecules follow various nucleation pathways in the same solution, and that the hydration layer plays a critical role in crystallization. The observations visually support the common belief that dehydration is the greatest barrier to crystallization in an aqueous solution. These results are expected to contribute to the development of a unified nucleation model.