Structure of graphene grown on liquid Cu by X-ray scattering

Francesco La Porta, Oleg Konovalov

 ID10, European Synchrotron Radiation Facility (ESRF), Grenoble, France

Liquid metals are known to catalyse the dissociation of methane and favour the formation of graphene crystals. Information about the lattice parameters the thickness and crystalline quality of the graphene layer is extremely important as they are directly correlated to its optical and electrical properties. In this scenario, x-ray scattering techniques play an important role to characterize the 2D material.

We applied: 1) Grazing Incidence Diffraction (GID) to determine the lattice parameters of two dimensional graphene and 2) X-Ray Reflectivity (XRR) to obtain the information about roughness of interfaces, the graphene layer thickness and van-der-Waals gap between copper and graphene. Both techniques require to have a well-defined grazing angle, which is however difficult to obtain in our system because the liquid copper forms a drop on the flat W sample holder. A solution of this problem will be discussed.

The LMCat (Liquid Metal Catalyst) reactor is a perfect tool to perform X-ray in-situ measurements.  During the experiments at different synchrotron facilities, we obtained sharp Bragg rods coming from the diffraction of graphene crystals floating on the molten copper surface. These GID measurements provide us with the lattice parameters of presumably relaxed graphene crystals. From the XRR measurements, we deduce that the graphene layer is indeed a single atom thick, in agreement with the Raman and the optical microscope results. From the GID curves measured on the bare surface of liquid copper, the structure factor of molten copper has been extracted .  

Together with Raman spectroscopy, these results provide crucial information to better understand the catalytic graphene growth process.