The everyday transition from one state of matter to another ̵
A team led by UCLA has gained an unprecedented view of nucleation – it has captured how the atoms rearrange at a 4 D atomic resolution (ie in three dimensions of space and over time). , The results published in the journal Nature differ from predictions based on classical nucleation theory, which has long been published in textbooks.
"This is truly a groundbreaking experiment – we not only locate and identify individuals with high precision atoms, but also monitor their movement in 4-D for the first time," said senior author Jianwei "John" Miao, a UCLA Professor of Physics and Astronomy, who is Deputy Director of the STROBE National Science Foundation Science and Science Technology Center and a member of the California NanoSystems Institute at UCLA.
Research by the team, which involves collaborators from the Lawrence Berkeley National Laboratory of the University of Colorado at Boulder, the University of Buffalo and the University of Nevada, Reno, are building on a powerful imaging technique previously developed by Miao's research group has been. This method, called "atomic electron tomography," uses a state-of-the-art electron microscope located in the Molecular Foundry of Berkeley Lab to image a sample with electrons. The sample is rotated, and in the same way as a CAT scan produces a three-dimensional X-ray of the human body, atomic-electron tomography produces stunning 3-D images of atoms within a material.
Miao and his colleagues studied an iron-platinum alloy that was formed into nanoparticles that are so small that more than 10,000 hairs adjacent to each other must span a human's breadth. To study nucleation, the scientists heated the nanoparticles to 520 degrees Celsius and took pictures after 9 minutes, 16 minutes, and 26 minutes. At this temperature, the alloy passes between two different solid phases.
Although the alloy looks similar to the naked eye in both phases, a closer look reveals that the 3-D atomic arrangements are different. After heating, the structure changes from a disordered chemical state to an ordered one with alternating layers of iron and platinum atoms. The change of the alloy can be compared to the dissolution of a magic cube: In the confused phase, all colors are mixed randomly, while in the ordered phase all colors are aligned.
In a meticulous process led by co-authors and postdoctoral researchers from UCLA Scientists Jihan Zhou and Yongsoo Yang have traced the same 33 nuclei – some of them only 13 atoms – within a nanoparticle.
"People think it's hard to find a needle in a haystack," Miao said. "How difficult would it be to find the same atom in more than a trillion atoms at three different times?"
The results were surprising, as they contradicted the classical theory of nucleation. This theory states that the cores are perfectly round. In contrast, nuclei in the study formed irregular shapes. The theory also suggests that nuclei have a sharp boundary. Instead, the researchers observed that each nucleus contained a nucleus of atoms that had turned into the new, ordered phase, but the arrangement was approaching the surface of the nucleus.
The classical nucleation theory also states that once a kernel reaches a certain size, it only becomes larger. But the process seems to be much more complicated: in addition to growing, the nuclei in the study shrank, dividing and merging; some completely dissolved.
"Nucleation is essentially an unsolved problem in many areas," said co-author Peter Ercius, a member of the Molecular Foundry, a nanoscience facility that provides users with cutting-edge tools and expertise for collaborative research. "Once you get an idea of something, you can think about how to control it."
The results directly show that classical nucleation theory does not accurately describe phenomena at the atomic level. The discoveries of nucleation can affect research in a variety of fields, including physics, chemistry, materials science, environmental science, and neuroscience.
"By capturing the movement of atoms over time, this study opens up new possibilities for studying a wide range of materials, chemical and biological phenomena," said Charles Ying, program officer of the National Science Foundation, who funded the STROBE center supervised. "This transformational result required groundbreaking advances in experimentation, data analysis and modeling, which required the broad expertise of the Center's researchers and their staff."
Visualized nucleation of liquids
Jihan Zhou et al. Observing nucleation in four dimensions using atomic electron tomography, Nature (2019). DOI: 10.1038 / s41586-019-1317-x
Atomic motion recorded for the first time in 4-D (2019, 27 June)
accessed June 27, 2019
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