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‘Hot’ graphene reveals migration of carbon atoms

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A carbon atom (marked in orange) that migrates on the surface of graphene at elevated temperature towards a vacancy, racing against a scanning electron beam (green-yellow glow) approaching the same position. Credit: Concept: Toma Susi / Uni. Vienna, Graphic design: Ella Maru Studio

The migration of carbon atoms on the surface of the nanomaterial graphene has recently been measured for the first time. Although the atoms are moving too fast to be directly observed with an electron microscope, their effect on the stability of the material can now be determined indirectly as the material is heated on a microscopic hot plate. The study by researchers from the Faculty of Physics at the University of Vienna was published in the journal Carbon

Carbon is an element essential to all known life and occurs in nature mainly as graphite or diamond. In recent decades, materials scientists have created many new forms of carbon, including fullerenes, carbon nanotubes and graphene. Graphene in particular has been the subject of intensive research, not only because of its superlative nature, but also because it is particularly suitable for experiments and modelling. However, it was not possible to measure some fundamental processes, including the movement of carbon atoms on the surface. This random migration is the atomic origin of the diffusion phenomenon.

Diffusion refers to the natural movement of particles such as atoms or molecules in gases, liquids or solids. In the atmosphere and oceans, this phenomenon ensures an even distribution of oxygen and salt. In the engineering industry, it is critical for steelmaking, lithium-ion batteries and fuel cells, to name just a few examples. In materials science, diffusion at the surface of solids explains how certain catalytic reactions proceed and how many crystalline materials, including graphene, are grown.

Surface diffusion rates generally depend on temperature: the hotter, the faster the atoms migrate. By measuring this velocity at different temperatures, we can basically determine the energy barrier that describes how easy it is for the atoms to pile up from one place on the surface to another. However, this is impossible by direct imaging if they don’t sit long enough, which is the case for carbon atoms on graphene. So until now, our understanding has depended on computer simulations. The new study overcomes this difficulty by indirectly measuring their effect as the material is heated on a microscopic hot plate in an electron microscope.

By visualizing the atomic structure of graphene with electrons and occasionally kicking out atoms, the researchers were able to determine how fast carbon atoms on the surface must move to explain the filling of the resulting holes at elevated temperatures. By combining electron microscopy, computer simulations and understanding of the interplay of the imaging process with the diffusion, an estimate for the energy barrier could be measured.

“After careful analysis, we determined the value to be 0.33 electron volts, slightly lower than expected,” said lead author Andreas Postl. The study is also an example of serendipity in research, as the team’s original goal was to measure the temperature dependence of this radiation damage. “Frankly, this wasn’t what we initially wanted to study, but such discoveries in science often happen by constantly pursuing small but unexpected details,” concludes senior author Toma Susi.

Tracing the diffusion of carbon isotopes using atomic-scale vibrational spectroscopy

More information:
Andreas Postl et al, Indirect measurement of the carbon-adatom migration barrier on graphene, Carbon (2022). DOI: 10.116/j.carbon.2022.05.039

Provided by the University of Vienna

Quote: ‘Hot’ graphene reveals carbon migration (2022, June 24) retrieved June 24, 2022 from https://phys.org/news/2022-06-hot-graphene-reveals-migration-carbon.html

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