Capturing Nucleation at 4D Atomic Resolution

Abstract

Nucleation plays a critical role in many physical and biological phenomena ranging from crystallization, melting and evaporation to the formation of clouds and the initiation of neurodegenerative diseases. However, nucleation is a challenging process to study especially in the early stage when several atoms/molecules start to form a new phase from its parent phase. Here, we advance atomic electron tomography to study early stage nucleation at 4D atomic resolution. Using FePt nanoparticles as a model system, we reveal that early stage nuclei are irregularly shaped, each has a core of a maximum order parameter, and an order parameter gradient points from the core to the boundary of the nucleus. We capture the structure and dynamics of the same nuclei undergoing growth, fluctuation, dissolution, merging and/or division, which are regulated by the distribution of the order parameter and its gradient. These experimental results differ from classical nucleation theory (CNT) and to explain them we propose an order parameter gradient model, which is more general and thermodynamically more favourable than CNT. We further corroborate this model using molecular dynamics simulations of heterogeneous and homogeneous nucleation in the liquid-solid phase transition of Pt. Here, we anticipate that the order parameter gradient model is applicable to different kinds of nucleation processes and our experimental method opens the door to study the structure and dynamics of materials with 4D atomic resolution.