Selective hydrogenation of graphene on Ir(111): An X-ray standing wave study

Normally, graphene has a metallic nature, but when it reacts with hydrogen, it becomes a semiconductor. On the (111) surface of iridium, the process can be controlled to produce a regular pattern of hydrogen clusters, which means the carbon should either bind to the metal surface or to more hydrogen beneath the graphene sheet. In the first case, the semiconducting sheet should be loosely bound and transferrable, while in the latter case, the sheet will be strongly bound, but also protect the surface.

To shed light on this question, we produced samples of graphene on Ir(111) with gradually increasing amounts of hydrogen and irradiated it with X-rays, which can be absorbed by the electrons in the material. These electrons will then be emitted themselves, if the X-rays have sufficiently high energy. When a certain X-ray energy is being used, the velocity of the emitted electrons will depend mostly on the element of the atom it was a part of, but the energy can change depending on the chemical bindings and the environment that the atom is a part of. By carefully tuning the x-ray energy, we could extract the distance from the metal to the carbon atoms as well.

For hydrogenated graphene on iridium, the resulting spectrum shows no less than four different electron energies, which makes the data difficult to analyze. In spectra from the different samples with different amounts of hydrogen, we have shown that the electron energies show up one after the other as the amount of hydrogen increases, and that those carbon atoms move closer to the metal surface. Therefore we have been able to determine the origin of three of the four components as well as the geometrical structures they originate from: Highly ordered clusters of carbon which bind to hydrogen on top and the iridium surface underneath.

This work has been published as part of the Faraday Discussions. An advance version of the article can be found here: https://pubs.rsc.org/en/content/articlehtml/2022/fd/d1fd00122a