Atomic motion captured in 4D
Image: Latest results from Fe-Pt nanoparticle study are set to re-write classic nucleation theory.
Using atomic electron tomography, US-based researchers have imaged how atoms rearrange during nucleation in 4D at atomic resolution, producing results that contradict classical nucleation theory.
By capturing the structure and dynamics of iron-platinum nanoparticles in early-stage nucleation, Professor Jianwei “John” Miao from the University of California, Los Angeles, and colleagues, have opened the door to better understanding fundamental problems in materials science, nanoscience, condensed matter physics and more.
“This is truly a groundbreaking experiment,” says Miao, also Deputy Director of the STROBE National Science Foundation Science and Technology Center. “We not only locate and identify individual atoms with high precision, but also monitor their motion in 4D for the first time.”
To observe crystal nucleation in 4D, the researchers deposited Fe-Pt nanoparticles on 5 nm thick silicon nitride membranes.
They then heated the samples to 520 degrees Celsius - the temperature at which the alloy undergoes a transition between different solid phases – for nine minutes in vacuum.
Using the TEAM 0.5 transmission electron aberration-corrected microscope – a modified FEI Titan 80-300 microscope - at the National Center for Electron Microscopy (NCEM) at Lawrence Berkeley National Laboratory, they acquired a set of tomographic tilt series from several FePt nanoparticles.
Images were collected at 200 kV in annular dark-field scanning transmission electron microscopy mode.
To study nucleation dynamics, the researchers then removed the nanoparticles from the microscope and further annealed samples acquiring sets of tomographic tilt series along the way.
Analyses revealed that early-stage nuclei were irregularly shaped, conflicting with perceived wisdom that nuclei are perfectly round.
The researchers observed that each nucleus contained a core of atoms that had changed to the new, ordered phase, but that the arrangement became less ordered closer to the surface of the nucleus.
In contrast, theory has suggested that nuclei have a sharp boundary.
Furthermore, results indicated that in addition to growing, nuclei in the study also shrunk, divided and merged while some dissolved completely.
Classical nucleation theory has stated that once a nucleus reaches a specific size, it only grows larger from there.
The findings offer direct evidence that classical nucleation theory does not accurately describe phenomena at the atomic level and are set to influence research in a wide range of areas, including physics, chemistry, materials science, environmental science and neuroscience.
Research is published in Nature.