Oscillatory Phase Transition and Rotation in the Liquid-Solid Binary-Phase Zone: In-Situ Visualization of Nanoscale Alloying Dynamics.

Challenges in predicting and tuning nanoscale crystal growth stem from the fact that there are plenty of individual elementary steps occurring in the liquid-solid two phase zone involving phase transitions and that these steps are highly dynamic in nature. Direct visualization with high spatiotemporal resolution is indispensable to unveiling the important intermediate states underlying those alloying processes. Here, our in situ observations reveal that additional unexpected phase transformation between the crystalline and amorphous states also occurs periodically, even after the typical transition from the liquid to the precipitated solid nanoparticles (NPs) in the two-phase zone. The NPs demonstrate interesting oscillatory rotation in the liquid, followed by the anomalous Brownian motion type due to the nanoscale confinement. Such rotation further enhances the intermediate crystalline ↔ amorphous phase transition. The NPs finally become crystalline with a fixed orientation once the liquid region is nearly depleted. The underlying mechanism is discussed. The findings offer a fundamental understanding of the phase behaviors with dynamic intermediate structures in the complex liquid-solid zone under the input of nanoscale confinement and pave the way for optimizing the nanoalloy nucleation and growth with the desired properties.