  Precise tunability of electronic properties of 2D nanomaterials is a key goal
of current research in this field of materials science. Chemical modification
of layered transition metal dichalcogenides leads to the creation of
heterostructures of low-dimensional variants of these materials. In particular,
the effect of oxygen-containing plasma treatment on molybdenum disulfide
(MoS$_2$) has long been thought to be detrimental to the electrical performance
of the material. Here we show that the mobility and conductivity of MoS$_2$ can
be precisely controlled and improved by systematic exposure to oxygen:argon
plasma, and characterise the material utilising advanced spectroscopy and
microscopy. Through complementary theoretical modelling which confirms
conductivity enhancement, we uncover the role of a two-dimensional phase of
molybdenum trioxide (2D-MoO$_3$) in improving the electronic behaviour of the
material. Deduction of the beneficial role of MoO$_3$ will serve to open the
field to new approaches with regard to the tunability of 2D semiconductors by
their low-dimensional oxides in nano-modified heterostructures.
