Surface structures of black phosphorus investigated with scanning tunneling microscopy

The atomic and electronic structures of the black phosphorus surface are investigated by means of scanning tunneling microscopy (STM) combined with density functional theory (DFT) calculations for the first time. STM measurements reveal that the atoms on the surface occupy almost the same sites that they would have in bulk, except for a little relaxation perpendicular to the surface, which causes the atoms to take two types of positions (P1 and P2) with a contrast visible in the STM images. DFT calculations indicate that the small difference in height of 0.02 Å between atoms at P1 and P2 position breaks the symmetry, leading to an apparent discrimination of electronic structures. The bias-dependent variation of the contrast between P1 and P2 in the STM images is consistent with the DFT simulations.