Researchers at the University of Vienna have measured the migration of carbon atoms on the surface of graphene for the first time. Although the atoms move too quickly to be directly observed with an electron microscope, their effect on the stability of the material can now be determined indirectly while the material is heated on a microscopic hot plate.

Graphene has been the subject of intensive research for years, but it has not been possible to measure some fundamental processes, including the motion of carbon atoms on its surface. This random migration is the atomic origin of the phenomenon of diffusion - the natural motion of particles such as atoms or molecules in gases, liquids or solids. In the atmosphere and the oceans, this phenomenon ensures an even distribution of oxygen and salt. In the technical industries, it is of central importance for steel production, lithium-ion batteries, and fuel cells, to name a few examples. In materials science, diffusion at the surface of solids explains how certain catalytic reactions proceed and many crystalline materials including graphene are grown.

Surface diffusion rates generally depend on temperature: the warmer, the faster the atoms migrate. In principle, by measuring this speed at different temperatures, it is possible to determine the energy barrier that describes how easy it is for the atoms to hope from one site on the surface to the next. However, in graphene this is impossible by direct imaging as the atoms do not stay put for long enough. Thus, until now, scientifical understanding in this area has relied on computer simulations. This new study overcomes this difficulty by indirectly measuring their effect while heating the material on a microscopic hot plate inside an electron microscope.

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

"After careful analysis, we pinpointed the value to 0.33 electronvolts, somewhat lower than expected," lead author Andreas Postl states. The study is also an example of serendipity in research, as the team's original goal was to measure the temperature dependence of this irradiation damage. "Honestly, this was not what we initially set out to study, but such discoveries in science often happen by persistently pursuing small but unexpected details," senior author Toma Susi concludes.

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