Andrea Hwang

Figure 1: Schematic representation of intermixing oxygen adatoms with different masses. Blue oxygen atoms were deposited first, indicated by a blue region on the bottom. Green oxygen atoms were deposited on top of the blue layer. The color gradient and the directions of the arrows represent the movement of oxygen atoms with different masses between the two layers. Reprinted (adapted) with permission from Tiffany C. Kaspar, Peter Hatton, Kayla H. Yano, Sandra D. Taylor, Steven R. Spurgeon, Blas P. Uberuaga, and Daniel K. Schreiber. Nano Letters 2022 22 (12), 4963-4969. Copyright 2022 American Chemical Society.

Scientists in the Fundamental Understanding of Transport Under Reactor Extremes (FUTURE) Energy Frontier Research Center (EFRC) have seen the intermixing of oxygen atoms between thin films as such atoms are deposited on top of each other. Since this phenomenon occurs on the surface of a thin film, the deposited atoms are also known as adatoms. This sheds light on how these oxygen-containing, or oxide, surfaces break down and corrode. Their investigation into the transport mechanisms of deposited oxygen atoms in oxide thin films has the potential to enhance our understanding of interfacial corrosion in materials exposed to extreme environments.

Imagine a stack of pancakes, alternating between chocolate chip and blueberry flavors. Now, imagine a hypothetical scenario where blueberries can migrate into chocolate chip pancakes and vice versa. Interestingly, researchers at FUTURE have discovered that atoms can move within a material as they are stacked layer by layer, analogous to the migration of ingredients in a stack of pancakes. By focusing on the transport behavior of intermixing oxygen atoms in oxide thin films, scientists have revealed that oxygen atoms of different masses, or isotopes, can intermix and diffuse into various layers of the thin film.

To conduct their study, scientists grew alternating thin film layers of chromium oxide (Cr2O3) and iron oxide (Fe2O3) on aluminum oxide (Al2O3) substrates. These films utilized naturally abundant oxygen isotopes with a mass of 16 atomic mass units (amu). An oxide layer (M218O3, where M = Cr, Fe) containing oxygen atoms with a mass of 18 amu, known as a tracer layer, was deposited on top of each film. Subsequently, a second layer of Cr2O3 or Fe2O3 was added. The purpose of the tracer layer with the oxygen isotope was to track the distribution of 16O as M2O3 was deposited onto M218O3, as well as the distribution of 18O as M218O3 was deposited onto M2O3. In order to trace the distribution of different oxygen isotopes within the samples, the scientists used a needle-like probe to detect the masses of the oxygen atoms at various depths in thin film layers.

The researchers observed that the oxygen adatoms, or atoms that are absorbed on the surface but not part of the lattice, tended to draw oxygen atoms from deeper layers within the thin film. Furthermore, the diffusion of oxygen isotopes was slower in chromium oxide films compared to the diffusion in iron oxide films. While the scientists observed the diffusion of oxygen atoms with different masses into different layers, they also performed computer simulations to further explain the experimental findings. These simulations involved calculating the overall total energy of atomic systems to minimize energy within a given system. Simulations of three oxide layers with alternating oxygen isotopes provided evidence supporting a three-layer intermixing model.

By tracking the movement of oxygen adatoms, researchers have gained critical fundamental insights into transport mechanisms near the surface of materials. These results will help scientists in FUTURE enhance their understanding of corrosion mechanisms under extreme environments.

More Information

1. Tiffany C. Kaspar, Peter Hatton, Kayla H. Yano, Sandra D. Taylor, Steven R. Spurgeon, Blas P. Uberuaga, and Daniel K. Schreiber. Nano Letters 2022 22 (12), 4963-4969

This work was supported as part of FUTURE (Fundamental Understanding of Transport Under Reactor Extremes), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences. A portion of the research was performed on a project award (10.46936/cpcy.proj.2021.60217/60007228) from the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility sponsored by the Biological and Environmental Research program and located at Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is a multiprogram national laboratory operated by Battelle for the U.S. DOE under Contract DE-AC0579RL01830. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is managed by Triad National Security, LLC for the U.S. Department of Energy’s NNSA, under contract 89233218CNA000001.

About the author(s):

Andrea Hwang is a PhD student in Materials Science and Engineering at the University of California, Berkeley, advised by Professor Mark Asta. She is part of the Fundamental Understanding of Transport Under Reactor Extremes (FUTURE) EFRC. Andrea uses molecular dynamics and ab initio methods to investigate systems that involve molten salts and metals. ORCID ID # 0000-0001-7043-817X.

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