  Atomic structure of single-crystalline black phosphorus was studied by high
resolution synchrotron-based photoelectron diffraction (XPD). The results show
that the topmost phosphorene layer in the black phosphorus is slightly
displaced compared to the bulk structure and presents a small contraction in
the direction perpendicular to the surface. Furthermore, the XPD results show
the presence of a small buckling among the surface atoms, in agreement with
previously reported scanning tunneling microscopy results. The contraction of
the surface layer added to the presence of the buckling indicates an uniformity
in the size of the sp3 bonds between P atoms at the surface.
