  Black Phosphorus (bP) has emerged as an interesting addition to the category
of two-dimensional materials. Surface-science studies on this material are of
great interest, but they are hampered by bP's high reactivity to oxygen and
water, a major challenge to scanning tunneling microscopy (STM) experiments. As
a consequence, the large majority of these studies were performed by cleaving a
bulk crystal in situ. Here we present a study of surface modifications on
exfoliated bP flakes upon consecutive annealing steps, up to 550 C, well above
the sublimation temperature of bP. In particular, our attention is focused on
the temperature range 375 C - 400 C, when sublimation starts, and a controlled
desorption from the surface occurs alongside with the formation of
characteristic well-aligned craters. There is an open debate in the literature
about the crystallographic orientation of these craters, whether they align
along the zigzag or the armchair direction. Thanks to the atomic resolution
provided by STM, we are able to identify the orientation of the craters with
respect to the bP crystal: the long axis of the craters is aligned along the
zigzag direction of bP. This allows us to solve the controversy, and, moreover,
to provide insight in the underlying desorption mechanism leading to crater
formation.
