Unveiling carbon dimers and their chains as precursor of graphene growth on Ru(0001)

Carbon precursor that forms on the catalyst surface by the dissociation of feedstock gas plays an important role in the controllable growth of graphene on metal substrates. However, the configuration about the precursor has so far remained elusive. Here, we report the direct observation of uniformly structured precursor units and their chain formation at the nucleation stage of graphene growing on Ru(0001) substrate by using scanning tunneling microscopy. Combining this experimental information with density function theory calculations, the atomic-resolved structures of carbon precursor are characterized as adsorbed CH₂ segments on the substrate. The dissociated carbon feedstock molecules or radicals further react to form nonplanar -[C₂H₄]- chains adsorbed on hexagonal-close-packed hollow sites of the Ru(0001) substrate before incorporating into the graphene island. These findings reveal that CH₂ and nonplanar -[C₂H₄]- segments act as precursors in graphene growth and are helpful to improve the quality and the domain size of desired graphene by precursor or feedstock control.