Control of Graphene's Properties by Reversible Hydrogenation: Evidence for Graphane

TLDR

Graphene’s robust hexagonal lattice can be chemically modified; while graphene oxide is disordered, periodic attachment such as hydrogenation could produce a new crystalline material like graphane, a wide‑gap semiconductor. The authors show that exposing graphene to atomic hydrogen converts the conductive semimetal into an insulator. Transmission electron microscopy reveals a shortened hexagonal lattice that reversibly returns to graphene upon annealing, restoring the quantum Hall effect and demonstrating that hydrogenation is a reversible, tunable method for creating new two‑dimensional crystals.

Abstract

Graphene - a monolayer of carbon atoms densely packed into a hexagonal lattice - has one of the strongest possible atomic bonds and can be viewed as a robust atomic-scale scaffold, to which other chemical species can be attached without destroying it. This notion of graphene as a giant flat molecule that can be altered chemically is supported by the observation of so-called graphene oxide, that is graphene densely covered with hydroxyl and other groups. Unfortunately, graphene oxide is strongly disordered, poorly conductive and difficult to reduce to the original state. Nevertheless, one can imagine atoms or molecules being attached to the atomic scaffold in a strictly periodic manner, which should result in a different electronic structure and, essentially, a different crystalline material. A hypothetical example for this is graphane, a wide-gap semiconductor, in which hydrogen is bonded to each carbon site of graphene. Here we show that by exposing graphene to atomic hydrogen, it is possible to transform this highly-conductive semimetal into an insulator. Transmission electron microscopy reveals that the material retains the hexagonal lattice but its period becomes markedly shorter than that of graphene, providing direct evidence for a new graphene-based derivative. The reaction with hydrogen is found to be reversible so that the original metallic state and lattice spacing are restored by annealing and even the quantum Hall effect recovers. Our work proves the concept of chemical modification of graphene, which promises a whole range of new two-dimensional crystals with designed electronic and other properties.