Micrometer-scale single-crystalline borophene on a square-lattice Cu(100) surface*

Borophene, a crystalline monolayer boron sheet, is a new two-dimensional (2D) quantum material, predicted to feature tunable structure, intriguing physics and to find applications in flexible electronics, energy storage, and catalysis. Nanoscale borophene flakes have been synthesized on noble-metal surfaces, but for device fabrication, one needs large single-crystal domains. We report the synthesis of borophene on a square lattice Cu(100) surface and show that incommensurate coordinations could reduce the borophene-substrate interactions and alter the borophene structures in interesting ways. Micrometer-scale single-crystal domains can form as isolated faceted islands or merge together to achieve full monolayer coverage. We have discovered a new crystal structure of borophene, with ten boron atoms and two hexagonal vacancies in the unit cell. First-principle calculations indicate that charge transfer rather than covalent bonding binds 2D boron to the copper surface, and confirm its integrity and uniformity. The electronic band structure features multiple anisotropic tilted Dirac cones, heralding emergent quantum fermions.