A process called nucleation plays a critical role in many physical and biological phenomena that range from crystallization, melting and evaporation to the formation of clouds and the initiation of neurodegenerative diseases. However, nucleation is a challenging process to study experimentally, especially in its early stages, when several atoms or molecules start to form a new phase from a parent phase. Now, a team of physicists led by the University of California, Los Angeles has used a method called atomic electron tomography to study early-stage nucleation in four dimensions (that is, in three dimensions of space and across time) at atomic resolution.
“This is truly a groundbreaking experiment — we not only locate and identify individual atoms with high precision, but also monitor their motion in 4D for the first time,” said University of California, Los Angeles Professor Jianwei ‘John’ Miao.
Professor Miao and colleagues examined an iron-platinum alloy formed into nanoparticles so small that it takes more than 10,000 laid side by side to span the width of a human hair.
To investigate nucleation, they heated the nanoparticles to 968 degrees Fahrenheit (520 degrees Celsius) and took images after 9, 16 and 26 minutes.
At that temperature, the alloy undergoes a transition between two different solid phases.
Although the alloy looks the same to the naked eye in both phases, closer inspection shows that the 3D atomic arrangements are different from one another.
After heating, the structure changes from a jumbled chemical state to a more ordered one, with alternating layers of iron and platinum atoms.
The team tracked the same 33 nuclei — some as small as 13 atoms — within one nanoparticle.
The results were surprising, as they contradict the classical theory of nucleation. That theory holds that nuclei are perfectly round. In the study, by contrast, nuclei formed irregular shapes.
The theory also suggests that nuclei have a sharp boundary. Instead, the physicists observed that each nucleus contained a core of atoms that had changed to the new, ordered phase, but that the arrangement became more and more jumbled closer to the surface of the nucleus.
Classical nucleation theory also states that once a nucleus reaches a specific size, it only grows larger from there. But the process seems to be far more complicated than that. In addition to growing, nuclei in the study shrunk, divided and merged; some dissolved completely.
“Nucleation is basically an unsolved problem in many fields. Once you can image something, you can start to think about how to control it,” said Dr. Peter Ercius, a staff scientist at the Lawrence Berkeley National Laboratory, Berkeley.
The findings, published in the journal Nature, offer direct evidence that classical nucleation theory does not accurately describe phenomena at the atomic level.
The discoveries about nucleation may influence research in a wide range of areas, including physics, chemistry, materials science, environmental science and neuroscience.
Jihan Zhou et al. 2019. Observing crystal nucleation in four dimensions using atomic electron tomography. Nature 570: 500-503; doi: 10.1038/s41586-019-1317-x