  We report a straightforward chemical methodology for controlling the
thickness of black phosphorus flakes down to the monolayer limit by
layer-by-layer oxidation and thinning, using water as solubilizing agent.
Moreover, the oxidation process can be stopped at will by two different
passivation procedures, namely the non-covalent functionalization with perylene
diimide chromophores, which prevents the photooxidation, or by using a
protective ionic liquid layer. The obtained flakes preserve their electronic
properties as demonstrated by fabricating a BP field-effect transistor (FET).
This work paves the way for the preparation of BP devices with controlled
thickness
