Structural deviation of hydrogenated graphene and multi-layer graphene under high pressure

Abstract

Graphene, a layer of graphite, has marvelous properties that can enhance the performance of its composite materials. To broaden the capability of its applications, the adatoms are introduced onto graphene to create such distinguish property. Hydrogenated graphene is a hydrogen deposition on graphene forming sp3-hybridization between carbon and hydrogen atoms and deviating the structure of graphene. Partial hydrogenation induces the energy gap and the magnetization of graphene which opens the door to many applications, e.g. semiconducting and spintronic devices. The properties of hydrogenated graphene can be tuned by either plasma bombarding or thermal annealing to raise or reduce, respectively, the number of hydrogen atoms on a graphene surface. Therefore, the hydrogen concentration plays a central role in tunable properties and phase stability of hydrogenated graphene. In this thesis, we studied the phase stability and the phase transition pathway of hydrogenated single-layer graphene and hydrogenated bilayer graphene on theoretical and experimental bases. In the theoretical part, the formation energies of the hydrogenated graphenes with various hydrogen concentrations were obtained using density functional theory (DFT). The phase stabilities are portrayed by the convex hull which is based on the thermodynamic principle. Most of the phases in this study are not energetically favorable, while some of the phases can be stabilized by compressive strain. However, many of them are still dynamically stable. The vibrational analysis was performed to show the fingerprint of possible Raman and infrared (IR) spectrum for guiding the experiment. In the experimental part, the highly hydrogenated graphene was synthesized using laser-heating at high pressure. As a result, the hydrogenation could occur more when the sample was compressed at a higher pressure in the hydrogen environment presenting the hydrogenation pathway toward fully hydrogenated graphene. We also model the possible slippery of graphene from its substrate and show the possible hole-doping of hydrogenated graphene at high pressure.