An STM Study of Metal Nanoclusters and Molecular Fragments on Graphene/Cu(111)

Adsorption of Pt, Rh and Pd nanoclusters and molecular fragments on a graphene moiré pattern on Cu(111) was studied with ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Isolated graphene islands with different periodicities were successfully grown on the Cu surface. As a result of weak coupling to the Cu substrate, different graphene rotational domains were observed including one with a periodicity of 4 nm that has not been previously reported. Our results have shown that Pt and Rh form dispersed nanoclusters on graphene/Cu(111) with Pt forming more regular structures mostly trapped at the hollow sites. This variation in growth behavior is mainly attributed to differences in Pt–carbon and Rh-carbon interaction energies. Moreover, the shape, organization and structural evolution of the Pd nanoclusters on graphene/Cu(111) were investigated using two different growth methods, continuous and stepwise. The size and shape of the formed nanoclusters were found to greatly depend on the growth technique used. The size and density of spherical Pd nanoclusters increased with increasing coverage during stepwise deposition as a result of coarsening of existing clusters and continued nucleation of new clusters. In contrast, continuous deposition gave rise to well-defined triangular Pd clusters as a result of anisotropic growth on the graphene surface. Exposure to ethylene caused a decrease in the size of the Pd clusters. This is attributed to the exothermic formation of ethylidyne on the cluster surfaces and an accompanying weakening of the Pd-Pd bonding. Additionally, single methoxy molecules and oxygen atoms were successfully adsorbed on graphene/Cu(111) at low temperatures. Motion of a single methoxy molecule was induced by means of STM/AS and IETS with the aim of understanding the coupling between molecular motions and electronic/vibrational excitations of the molecule. C-H stretching and frustrated rotation of methyl groups were found to be responsible for the observed motion. The motion of molecules adsorbed on graphene was found to be too fast compared to previous single molecule studies conducted on metal surfaces. Although too-fast motion results in an “averaging out of the numerous stable adsorption configurations of the molecule” on the graphene, AS and IETS provided important information about the nature of the motion. The interaction between single methoxy molecules and graphene was found to be stable and the hopping probability was found to be only 6%. The combined properties of imaging and triggering molecular motion on surfaces make STM the perfect tool for exploring molecular rotors and motors in more detail. A key existing challenge is to obtain full control over the rate and/or direction of molecular motion on the graphene surface.