Growth, Structure, and Vibrational Properties of Few Layer Graphene Grown on Rh(111)

We examine the growth of supported multilayer graphene overlayers on a Rh(111) single crystal substrate. At elevated surface temperatures, ethylene dissociates on the surface and carbon is absorbed into the bulk of the Rh. When the crystal is slowly cooled, the carbon diffuses to the surface and forms multilayer graphene films. The thickness was determined by Auger electron spectroscopy and the surface ordering by He scattering. The top layer is azimuthally aligned with the Rh(111) surface, and the moire pattern due to the lattice mismatch between the Rh and graphite is still visible, though greatly attenuated, for thicknesses up to four layers. We also used He scattering to measure the low-energy vibrations of the surface for both monolayer and multilayer graphene. One set of experiments measured the thermal attenuation of the elastic He scattering. The result was that the monolayer graphene is much "stiffer" (a higher Debye temperature) than even similar to 2 layers. We also used time-and angle-resolved inelastic He scattering to measure the low-energy modes near the zone center. Unlike the graphite ZA acoustic mode, which goes to zero frequency at the zone center, the monolayer has a mode that is a nearly constant 7 meV near the zone center. On the other hand, multilayer graphene has a mode that much more closely resembles the ZA mode of graphite. For the low-lying phonons with some degree of polarization perpendicular to the surface, our observations indicate that there is a real difference in the surface dynamical properties as one transitions between a supported two-dimensional monolayer to three-dimensional films consisting of only a few atomic layers in thickness. These findings add important information on how the properties of supported graphene films evolve with dimensional crossover from 2D to 3D systems, knowledge that is needed for achieving targeted properties in thin films of 2D materials.