  Electric and thermoelectric properties of strictly monolayer MoS$_2$ films,
which are grown using a novel micro-cavity based CVD growth technique, have
been studied under diverse environmental and annealing conditions. Resistance
of a thermoelectric device that is fabricated on a continuous monolayer MoS$_2$
layer using photolithography technique has been found to reduce by about six
orders of magnitudes upon annealing in vacuum at 525 K. Seebeck coefficient of
the layer also reduces by almost an order of magnitude upon annealing. When the
sample is exposed to oxygen atmosphere, these parameters return to their
previous values. In fact, it has been found that the electron concentration,
mobility as well as the thermoelectric power of the material can be tuned by
controlling the temperature of annealing and oxygen exposure. Once established,
these values are maintained as long as the layer is not exposed to oxygen
environment. This can offer a unique way to control doping in the material
provided an effective encapsulation method is devised. Such control is an
important step forward for device application. The effect has been attributed
to the passivation of di-sulfur vacancy donors present in the MoS$_2$ film by
physisorbed oxygen molecules. Band structural calculations using density
functional theory have been carried out, results of which indeed validate this
picture.
