1D selective confinement and diffusion of metal atoms on graphene

The role of moiré graphene superstructures in favoring confined adsorption of different metal atoms is an intriguing problem not yet completely solved. Graphene (G) grown on Ni(100) forms a striped moiré pattern of valleys, where G approaches the nickel substrate and interacts with it rather strongly, and ridges, where G stays far away from the substrate and acts almost free-standing. Combining density functional theory (DFT) calculations and scanning-tunneling microscopy (STM) measurements, we show that this peculiar moiré constitutes a regular nanostructured template on a 2D support, confining in 1D trails single metal atoms and few atoms clusters. DFT calculations show that the confinement is selective and highly dependent on the atomic species, with some species preferring to adsorb on ridges and the other showing preference for valleys. Co and Au adsorbates, for instance, have opposite behavior, as predicted by DFT and observed by STM. The origin of such disparate behavior is traced back to the electrostatic interaction between the charged adsorbate and the nickel surface. Moreover, the selectivity is not restricted to the adsorption process only, but persists as adsorbate starts its diffusion, resulting in unidirectional mass transport on a continuous 2D support. These findings hold great promise for exploiting tailored nanostructured templates in a wide range of potential applications involving mass transport along element-specific routes.