Topological States in Artificial Lattices of Graphene by STM

Artificially constructed lattices on a surface can create 2D materials with novel quantum properties, for example, quantum corals, or that mimic elemental 2D materials. Directed construction can offer larger lattice spacings which facilitate characterization and greater control to create unique components and geometries. Powerful examples of imitation include hexagonal arrays of quantum dots on semiconductors or manipulation of CO molecules on a Cu(111) surface to create artificial graphene. The otherwise free 2D electron surface states on Cu(111) are restricted locally by adsorbed CO and an appropriate assembly results in an electronic structure mimicking graphene. We have used cryogenic scanning tunneling microscopy to assemble CO molecules into islands of modified honeycomb lattices and scanning tunneling spectroscopy to identify the electronic states, exploring boundary geometries and fractal dimensions. By restricting the width of the CO islands, graphene-like nanowires are formed, with associated conducting or semiconducting properties. Assisted by machine learning, construction of junctions between nanowires of different widths are formed where 1D topological states are expected.