Graphene is celebrated as an extraordinary material. It's made of pure carbon, only a single atomic layer thick. Nevertheless, it is extremely stable, strong and even conductive. However, graphene still has significant disadvantages for electronics. It can not be used as a semiconductor because there is no bandgap. By adhering hydrogen atoms to graphene, such a band gap can be formed. Now researchers from Göttingen and Pasadena (USA) have produced a film on an atomic scale, showing how hydrogen atoms bind to graphene in one of the fastest reactions ever studied.
The international research team bombarded graphene with hydrogen atoms. "The hydrogen atom behaved quite differently than we expected," says Alec Wodtke, head of the dynamics department of Surfaces at the Max Planck Institute (MPI) for biophysical chemistry and professor at the Institute of Physical Chemistry, University of Göttingen. "Instead of flying away immediately, the hydrogen atoms" stick "to the carbon atoms and then bounce off the surface, forming a temporary chemical bond," says Wodtke. And another thing the scientists were surprised about: The hydrogen atoms have a lot of energy before they hit the graph, but there's not much left over when they take off. Hydrogen atoms lose most of their energy in collision, but where does it go?
To explain these surprising experimental observations, the Göttingen MPI researcher Alexander Kandratsenka, in collaboration with colleagues from the California Institute of Technology, developed theoretical methods that they used simulated on the computer and then compared with their experiments. With these theoretical simulations, which agree well with the experimental observations, the researchers were able to reproduce the ultrafast motions of atoms forming the transient chemical bond. "This bond lasts only about ten femtoseconds ̵
"During these ten femtoseconds, almost all the hydrogen atoms transfer their energy to the carbon atoms of the graphene, triggering a sound wave that propagates outward from the impingement of the hydrogen atom across the surface of the graphene, much like a stone falling into the water causes a wave, "says Kandratsenka. The sound wave helps the hydrogen atom bind to the carbon atom more easily than the scientists had expected and predicted earlier models.
The results of the research team provide fundamentally new insights into chemical bonding. In addition, they are of great interest to the industry. The adhesion of hydrogen atoms to graphene can lead to a band gap that makes it a useful semiconductor and is much more versatile in electronics.
The effort involved in setting up and carrying out these experiments was enormous, explained Oliver Bünermann, project group leader at the University of Göttingen. "We had to do it in ultra-high vacuum to keep the graphene surface perfectly clean." The scientists also had to use a variety of laser systems to prepare the hydrogen atoms before the experiment and detect them after the collision. According to Bünermann, the outstanding technical staff in the workshops at the MPI for Biophysical Chemistry and at the University of Göttingen were essential for the success of the project.
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