  Two dimensional (2D) transition-metal dichalcogenide (TMD) based
semiconductors have generated intense recent interest due to their novel
optical and electronic properties, and potential for applications. In this
work, we characterize the atomic and electronic nature of intrinsic point
defects found in single crystals of these materials synthesized by two
different methods - chemical vapor transport and self-flux growth. Using a
combination of scanning tunneling microscopy (STM) and scanning transmission
electron microscopy (STEM), we show that the two major intrinsic defects in
these materials are metal vacancies and chalcogen antisites. We show that by
control of the synthetic conditions, we can reduce the defect concentration
from above $10^{13} /cm^2$ to below $10^{11} /cm^2$. Because these point
defects act as centers for non-radiative recombination of excitons, this
improvement in material quality leads to a hundred-fold increase in the
radiative recombination efficiency.
