Atomic-Resolution Vibrational Mapping of Bilayer Borophene

The\nsuccessful synthesis of borophene beyond the monolayer limit\nhas expanded the family of two-dimensional boron nanomaterials. While\natomic-resolution topographic imaging has been previously reported,\nvibrational mapping has the potential to reveal deeper insight into\nthe chemical bonding and electronic properties of bilayer borophene.\nHerein, inelastic electron tunneling spectroscopy (IETS) is used to\nresolve the low-energy vibrational and electronic properties of bilayer-α\n(BL-α) borophene on Ag(111) at the atomic scale. Using a carbon\nmonoxide (CO)-functionalized scanning tunneling microscopy tip, the\nBL-α borophene IETS spectra reveal unique features compared\nto single-layer borophene and typical CO vibrations on metal surfaces.\nDistinct vibrational spectra are further observed for hollow and filled\nboron hexagons within the BL-α borophene unit cell, providing\nevidence for interlayer bonding between the constituent borophene\nlayers. These experimental results are compared with density functional\ntheory calculations to elucidate the interplay between the vibrational\nmodes and electronic states in bilayer borophene.