  Chemical vapor deposition (CVD) has been established as a versatile route for
the large-scale synthesis of transition metal dichalcogenides, such as tungsten
disulfide (WS2). Yet, the role of the precursor composition on the efficiency
of the CVD process remains largely unknown and yet to be explored. Here, we
employ Pulsed Laser Deposition (PLD) in a two-stage process to tune the oxygen
content in tungsten oxide (WO3-x) precursors and demonstrate that the presence
of oxygen vacancies in the precursor films leads to a more facile conversion
from WO3-x to WS2. Using a joint study based on ab initio density functional
theory (DFT) calculations and experiments, we unravel that the oxygen vacancies
in WO3-x can serve as niches through which sulfur atoms enters the lattice and
may facilitate an efficient growth of WS2 crystals. By solely modulating the
precursor stoichiometry, the photoluminescence emission of WS2 can be greatly
enhanced, while the size of WS2 domains increases significantly. Atomic
resolution scanning transmission electron microscopy (STEM) reveals that
tungsten vacancies are the dominant intrinsic defects in mono- and bilayers
WS2. Moreover, our data reveal that grain boundaries in bilayer WS2 emerge upon
the coalescence of AA' and AB-oriented crystals, while turbostatic moir\'e
patterns originate upon formation of distinct grain boundaries between the
bottom and bottom layers. The atomic resolution images show local strain
buildup in bilayer WS2 due to competing effects of complex grain boundaries.
Our study provides a means to tune the precursor composition in order to
control the lateral growth of TMDs, while revealing insights into the different
pathways for the formation of grain boundaries in bilayer WS2.
