  Excitonic states in monolayer transition metal dichalcogenides (TMDCs) have
been the subject of extensive recent interest. Their intrinsic properties can,
however, be obscured due to the influence of inhomogeneity in the external
environment. Here we report methods for fabricating high quality TMDC
monolayers with narrow photoluminescence (PL) linewidth approaching the
intrinsic limit. We find that encapsulation in hexagonal boron nitride (h-BN)
sharply reduces the PL linewidth, and that passivation of the oxide substrate
by an alkyl monolayer further decreases the linewidth and also minimizes the
charged exciton (trion) peak. The combination of these sample preparation
methods results in much reduced spatial variation in the PL emission, with a
full-width-at-half-maximum as low as 1.7 meV. Analysis of the PL line shape
yields a homogeneous width of 1.43$\pm$0.08 meV and inhomogeneous broadening of
1.1$\pm$0.3 meV.
