If graphene is ever to mature as the material of choice for the microelectronics industry, there are certain disadvantages that must be overcome first. For instance, while it can be both insulator and conductor it cannot be a semiconductor as it has no requisite bandgap. However, by attaching hydrogen atoms to graphene it is possible to create one. Now researchers from Göttingen and Pasadena have shown just how quickly hydrogen atoms become attached to graphene [Jiang, H. et al., Science, (2019) 364(6438): 379-382; DOI: 10.1126/science.aaw6378].
When the researchers bombarded pristine, clean graphene with hydrogen atoms under ultra-high vacuum conditions, they were surprised at how the hydrogen atoms behaved. "Instead of immediately flying away, the hydrogen atoms 'stick' briefly to the carbon atoms and then bounce off the surface," explains team member Alec Wodtke. "They form a transient chemical bond." This was not the only surprise, the hydrogen atoms hit the graphene carrying a lot of energy but as they bounce off, they lose much of that energy, but the team was puzzled as to where that energy went.
Göttingen researcher Alexander Kandratsenka worked with colleagues at the California Institute of Technology to develop a theoretical model and to simulate the processes involved. The theoretical simulations agree well with experiment and the team could see that the transient chemical bonds between graphene and bombarding hydrogen atoms lasts about ten femtoseconds. "This makes it one of the fastest chemical reactions ever observed directly," Kandratsenka says. Nevertheless, in this short time, the hydrogen atom transfers almost all of its energy to the carbon sheet and induces a sound wave to travel outward from the point impact over the graphene surface. This energy dissipation explains how the hydrogen atoms could bond to the carbon atoms so readily, more readily than earlier theory predicted. If the excess energy were not dissipated the new bond would simply open up and the hydrogen atom become detached from the graphene once more.
The new study could lead to fundamental insights into the formation of chemical bonds. Such insights will be of great interest to industry not least because attaching hydrogen atoms to graphene can generate a bandgap in the material and so potentially lead the way to converting this material into a useful semiconductor with applications beyond those currently possible in electronics.