  We systematically explore chemical functionalization of monolayer black
phosphorene via chemisorption of oxygen and fluorine atoms. Using the cluster
expansion technique, with vary- ing concentration of the adsorbate, we
determine the ground states considering both single- as well as double- side
chemisorption, which have novel chemical and electronic properties. The nature
of the bandgap depends on the concentration of the adsorbate: for fluorination
the direct bandgap first decreases, and then increases while becoming indirect,
with increasing fluorination, while for oxidation the bandgap first increases
and then decreases, while mostly maintaining its direct nature. Further we find
that the unique anisotropic free-carrier effective mass for both the electrons
and holes, can be changed and even rotated by 90 degrees, with controlled
chemisorption, which can be useful for exploring unusual quantum Hall effect,
and novel electronic devices based on phosphorene.
