  Molybdenum disulfide is a novel two-dimensional semiconductor with potential
applications in electronic and optoelectronic devices. However, the nature of
charge transport in back-gated devices still remains elusive as they show much
lower mobility than theoretical calculations and native n-type doping. Here we
report transport study in few-layer molybdenum disulfide, together with
transmission electron microscopy and density functional theory. We provide
direct evidence that sulfur vacancies exist in molybdenum disulfide,
introducing localized donor states inside the bandgap. Under low carrier
densities, the transport exhibits nearest-neighbor hopping at high temperatures
and variable-range hopping at low temperatures, which can be well explained
under Mott formalism. We suggest that the low-carrier-density transport is
dominated by hopping via these localized gap states. Our study reveals the
important role of short-range surface defects in tailoring the properties and
device applications of molybdenum disulfide.
