  Ultrathin transition metal dichalcogenides (TMDCs) of Mo and W show great
potential for digital electronics and optoelectronic applications. Whereas
early studies were limited to mechanically exfoliated flakes, the large-area
synthesis of 2D TMDCs has now been realized by chemical vapor deposition (CVD)
based on a sulfurization reaction. Since then, the optoelectronic properties of
CVD grown monolayer MoS$_{2}$ have been heavily investigated, but the influence
of stoichiometry on the electrical and optical properties has been largely
overlooked. Here we systematically vary the stoichiometry of monolayer
MoS$_{2}$ during CVD via controlled sulfurization and investigate the
associated changes in photoluminescence and electrical properties. X-ray
photoelectron spectroscopy is employed to measure relative variations in
stoichiometry and the persistence of MoO$_{x}$ species. As MoS$_{2-{\delta}}$
is reduced (increasing {\delta}), the field-effect mobility of monolayer
transistors increases while the photoluminescence yield becomes non-uniform.
Devices fabricated from monolayers with the lowest sulfur content have
negligible hysteresis and a threshold voltage of ~0 V. We conclude that the
electrical and optical properties of monolayer MoS$_{2}$ crystals can be tuned
via stoichiometry engineering to meet the requirements of various applications.
