Large-area single-crystal sheets of borophene on various metal surfaces*

Borophene, monolayer boron, as a new two-dimensional (2D) quantum material,
is predicted to feature exotic properties ranging from negative Poisson ratio to high-Tc superconductivity. We report the synthesis of borophene on various metal surfaces with micrometer-scale single-crystal domains. We have studied the crystal formation process and the resulting structure by low-energy-electron microscopy (LEEM) and diffraction (LEED), X-ray photoemission spectroscopy (XPS), scanning tunneling microscopy (STM), and ab initio calculations. We have discovered several new crystal structures of borophene, some with a small basic unit cell. First principle calculations indicate that direct charge transfer rather than covalent bonding binds 2D boron to the metal surfaces. The electronic band structure features multiple anisotropic tilted Dirac cones, indicating potential co-existence of type-I and type-II Dirac and Weyl fermions.

R. Wu et al., “Large-area single-crystal sheets of borophene on Cu(111) surfaces”, Nature Nanotechnology 14, 44 (2019).
R. Wu, A. Gozar and I. Bozovic, “Large-area borophene sheets on sacrificial Cu(111) films promoted by recrystallization from subsurface boron”, npj Quantum Materials 4, 40 (2019)