Large area single crystals of borophene on square- and triangular-lattice metallic surfaces: synthesis and characterization

Borophene, a crystalline monolayer boron sheet, stands out from all other two-dimensional materials because of its extraordinarily rich polymorphism. The multitude of potentially stable structures fuels hopes for obtaining on-demand structures with applications in flexible electronics, energy storage or catalysis. We have used a unique ultra-high vacuum system for synthesis, by Molecular Beam Epitaxy, and characterization, by Low Energy Electron Microscopy and Diffraction, of micron-size borophene crystals on Cu(111) and Cu(100) substrates. Our in-situ imaging capabilities provide information about the growth of faceted islands up full monolayer coverage and also about phase-stability, evaporation and sub-surface dissolution. First-principles calculations indicate that charge transfer rather than covalent bonding, couples borophene to the underling Cu surfaces. The calculated electronic band structures host multiple anisotropic Dirac cones. Ex-situ scanning near-field optical microscopy data reveal dielectric contrast between borophene and substrates, showing that nano-optical tools provide new ways to access intrinsic electronic properties of these novel structures.