Atoms on a hot graphene surface

The migration of carbon atoms over the surface of the two-dimensional material graphene has been measured for the first time. Although the atoms move too swiftly 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. The study by researchers in the Faculty of Physics at the University of Vienna in Austria is reported in a paper in Carbon.

Carbon is essential to all known life and exists in nature primarily as graphite or diamond. Over the past few decades, material scientists have created many novel 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 properties but also because it is particularly well-suited to experiments and modeling. However, 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.

Diffusion refers to the natural motion of particles such as atoms or molecules in gases, liquids and 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 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 higher the temperature, the faster the atoms migrate. In principle, by measuring this speed at different temperatures, scientists can determine the energy barrier that determines how easy it is for the atoms to hop from one site on the surface to the next. But this cannot be done by direct imaging if the atoms do not stay put for long enough, which is the case for carbon atoms on graphene.

As a consequence, our understanding of the migration of carbon atoms over graphene has, up to now, relied on computer simulations. The new study overcomes this difficulty by indirectly measuring the effect of carbon migration while heating graphene on a microscopic hot plate inside an electron microscope.

By visualizing the atomic structure of graphene with electrons while occasionally kicking out atoms from the material, the researchers could determine how fast carbon atoms on the surface must be moving to explain the filling of the resulting holes in the graphene 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 measured.

“After careful analysis, we pinpointed the value to 0.33 electronvolts, somewhat lower than expected,” said lead author Andreas Postl.

This 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,” said senior author Toma Susi.

This story is adapted from material from the University of Vienna, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.