  The puckered surface of black phosphorus represents an ideal substrate for an
unconventional arrangement of physisorbed species and the resulting specific
two-dimensional chemistry of this system. This opens the way to investigate the
chemical and physical properties of locally confined areas of black phosphorus
without the necessity for further physical downscaling of the material. We have
evaporated TCNQ on top of black phosphorus under over-saturation
non-equilibrium conditions in vacuum. The evolution of linear density and
height of droplets formed through oxidation during exposure to air was studied
time-dependently by scanning-force microscopy. Our study suggests that the TCNQ
molecules spontaneously arrange in a thin layer of the order of a few nm
height, which, however, is fragmented with a periodicity of about 100 nm. It is
shown that within the confined space separating the layer fragments the
chemical dynamics of the oxidation process is remarkably different than on a
bare black phosphorus surface.
