Nucleation is the process in which tiny clusters of atoms or molecules (called “nuclei”) begin to coalesce. It plays a vital role in physical and biological phenomena ranges from everyday transitions (from one state to another – such as freezing, melting or evaporation) to the formation of clouds and the onset of neurodegenerative disease.
Scientists have gained a never-before-seen view of nucleation – capturing how the atoms rearrange at 4D atomic resolution, which is in three dimensions of space and across time.
A research team led by UCLA used a state-of-the-art electron microscope located at Berkeley Lab’s Molecular Foundry, which images a sample using electrons. The sample is rotated, and in much the same way a CAT scan generates a three-dimensional X-ray of the human body, atomic electron tomography creates stunning 3D images of atoms within a material.
“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 senior author Jianwei “John” Miao, a UCLA professor of physics and astronomy.
For their study, the team 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, the scientists heated the nanoparticles to 520 degrees Celsius, or 968 degrees Fahrenheit, and took images after 9 minutes, 16 minutes 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.
In a painstaking process, the team tracked the same 33 nuclei – some as small as 13 atoms – within one nanoparticle.
Surprisingly, the results of the research were against the classical theory of nucleation that has long appeared in textbooks, which says that nuclei are perfectly round. In the study, by contrast, nuclei formed irregular shapes. Also, as per theory, nuclei have a sharp boundary. Instead, researchers observed that each nucleus contained a core of atoms that had changed to the new, ordered phase. However, the arrangement became more and more jumbled closer to the surface of the nucleus.
Also, the theory 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.
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.
“By capturing atomic motion over time, this study opens new avenues for studying a broad range of material, chemical, and biological phenomena,” said National Science Foundation program officer Charles Ying, who oversees funding for the STROBE center. “This transformative result required groundbreaking advances in experimentation, data analysis, and modeling, an outcome that demanded the broad expertise of the center’s researchers and their collaborators.”
Along with UCLA, the research team includes collaborators from Lawrence Berkeley National Laboratory, the University of Colorado at Boulder, the University of Buffalo and the University of Nevada, Reno.