Observing crystal nucleation in four dimensions using atomic electron tomography

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 1-3. However, nucleation is a challenging process to study in experiments especially in the early stage when several atoms/molecules start to form a new phase from its parent phase. Over the years, a number of experimental and computational methods have been used to investigate nucleation processes 4-17 , but it remains unachievable to experimentally determine the 3D atomic structure and dynamics of early stage nuclei. Here, we develop 4D atomic electron tomography (AET) to study early stage nucleation at atomic resolution. Using FePt nanoparticles as a model system, we reveal that early stage nuclei are irregularly shaped, each has a core of one to a few atoms 2 with the maximum order parameter, and the 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 order parameter distribution and its gradient. These experimental observations are corroborated by molecular dynamics simulations of heterogeneous and homogeneous nucleation in liquid-solid phase transitions of Pt. Our experimental and molecular dynamics results differ from classical nucleation theory (CNT) 1,2,18 , indicating a theory beyond CNT is needed to describe early stage nucleation at the atomic scale. Looking forward, we anticipate that 4D AET opens the door to study many fundamental problems in materials science, nanoscience, condensed matter physics and chemistry such as phase transition, atomic diffusion, grain boundary dynamics, interface motion, defect dynamics and surface reconstruction with 4D atomic resolution. AET is a powerful method to determine the 3D atomic structure of materials without the assumption of crystallinity 19 and has been applied to study dislocations, stacking faults, grain boundaries, atomic displacement, strain tensor, chemical order/disorder and point defects with unprecedented 3D detail 20-26. But all of these studies were of static structures. To probe the 4D atomic structure of early stage nucleation, we have tracked the same nuclei at different times and applied AET to determine their 3D atomic coordinates and species at each time (Methods). We used FePt nanoparticles as a model system because binary alloys have been widely used to study phase transitions 2 and FePt is a very promising material for next generation magnetic recording media 25,27. As-synthesized FePt nanoparticles form a chemically disordered face-centred cubic (fcc) structure (A1 phase) 27. With annealing, the A1 phase Author contributions J.M. conceived and directed the project; F.S. and H.Z. prepared the samples; J.Z.,

Nanoscale x-ray and electron tomography

“…24 Figure 3e shows how electron tomography can not only resolve atomic features inside nanomaterials, but is also sensitive enough to differentiate heavy versus medium heavy elements. 25 This information is extremely important for understanding how materials restructure at the single atom scale. Figure 3f shows a different type of 3D imaging technique in the TEM, the single-particle cryoEM approach.…”

“…(g) A newly proposed deep-learning approach that utilizes two generative adversarial models to jointly fill the missing-wedge information and remove artifacts for electron tomography reconstructions. [21][22][23][24][25][26]47 Understanding the failure mechanisms of a lithium-ion battery is a long-standing problem that limits the battery's capacity, reliability, and lifetime. To tackle this challenge, understanding the evolving compositional and chemical heterogeneity of the materials in the battery and their connection to the morphology of the electrode upon cycling are crucial.…”

“…Over the past few years, the resolution of electron tomography has seen dramatic improvements. 25,36,37 Though powerful, the current way of conducting electron tomography data collection and reconstruction is still far from optimal. The learning curve of electron tomography is still steep, making it a specialized technique that is far from being routinely used by the wider materials community.…”

Thermal reactions involving solids: a personal view of selected features of decompositions, thermal analysis and heterogeneous catalysis

“…Currently, we know little about if or how such surface reactions participate in or contribute to the subsequent nucleation step(s). A recent study [72] appears particularly promising.…”

“…This later stabilises into the (more robust) advancing interface of a growth nucleus. Techniques, including microscopy, much improved since the early studies [5,6,8,72], may now enable insights into these reaction-initiating steps. Again, different types of change may occur in different reactants and three, or more, stages may require characterisation.…”