University of Vienna: “Hot” graphene shows migration of carbon atoms

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

Carbon is an essential element for all known life and occurs in nature primarily as graphite or diamond. Over the past few decades, materials scientists have created many new forms of carbon, including fullerenes, carbon nanotubes, and graphene. Graphene in particular is the subject of intense research, not only because of its salient properties, but also because it is particularly well suited for experimentation and modelling. However, some fundamental processes have not yet been measured, including the movement of carbon atoms on its surface. This random movement is the atomic origin of the phenomenon of diffusion.

Diffusion (from the Latin “diffundere”: spreading out, scattering) refers to the natural movement of particles such as atoms or molecules in gases, liquids or solids. This phenomenon ensures an even distribution of oxygen and salt in the atmosphere and in the oceans. In the technical industry, it is of central importance for steel production, lithium-ion batteries and fuel cells, to name just a few. In materials science, diffusion at the surface of solids explains how certain catalytic reactions occur and how many crystalline materials, including graphene, are grown.

The diffusion rate at the surface generally depends on the temperature: the warmer it is, the faster the particles diffuse. By measuring this speed at different temperatures, one can in principle determine the energy barrier that describes how easily (in this case) atoms can move from one place on the surface to the next. However, this is not possible if the atoms do not stay in place long enough, as is the case with carbon atoms on graphene. Therefore, our understanding has so far relied on computer simulations. The new study overcomes this difficulty by indirectly measuring their effect while heating the material on a microscopic hot plate in an electron microscope.

By visualizing the atomic structure of graphene with electrons, and occasionally ejecting atoms, the researchers were able to determine how fast the carbon atoms on the surface must be moving to explain the filling of the resulting holes at elevated temperatures. Thus, by combining electron microscopy, computer simulations and an understanding of the interaction between image formation and diffusion, an estimate for the energy barrier could be determined. “After careful analysis, we were able to determine a value of 0.33 electron volts, slightly lower than expected,” says first author Andreas Postl.

The study is also an example of a happy accident in research. The team’s initial goal was to measure the temperature dependence of radiation damage from electrons. “Honestly, it wasn’t what we originally set out to study. However, such discoveries in science often happen when you persistently investigate small but unexpected details,” summarizes senior author Toma Susi.

The work of the Vienna team was supported by the European Research Council (ERC) within the framework of the European Union’s research and innovation program Horizon 2020 (Grant agreement No. 756277-ATMEN) and by the Vienna Doctoral School in Physics (VDS-P).