  Ultraviolet-ozone (UV-O3) treatment is a simple but effective technique for
surface cleaning, surface sterilization, doping and oxidation, and is
applicable to a wide range of materials. In this study, we investigated how
UV-O3 treatment affects the optical and electrical properties of molybdenum
disulfide (MoS2), with and without the presence of a dielectric substrate. We
performed detailed photoluminescence (PL) measurements on 1-7 layers of MoS2
with up to 8 minutes of UV-O3 exposure. Density functional theory (DFT)
calculations were carried out to provide insight into oxygen-MoS2 interaction
mechanisms. Our results showed that the influence of UV-O3 treatment on PL
depends on whether the substrate is present, as well as the number of layers.
The PL intensity of the substrate-supported MoS2 decreased dramatically with
the increase of UV-O3 treatment time and was fully quenched after 8 mins.
However, the PL intensity of the suspended flakes was less affected. 4 minutes
of UV-O3 exposure was found to be optimal to produce p-type MoS2, while
maintaining above 80% of the PL intensity and the emission wavelength, compared
to pristine flakes (intrinsically n-type). Our electrical measurements showed
that UV-O3 treatment for more than 6 minutes not only caused a reduction in the
electron density but also deteriorated the hole-dominated transport. It is
revealed that the substrate plays a critical role in the manipulation of the
electrical and optical properties of MoS2, which should be considered in future
device fabrication and applications.
