  Atomic scale defects in semiconductors enable their technological
applications and realization of novel quantum states. Using scanning tunneling
microscopy and spectroscopy complemented by ab-initio calculations we determine
the nature of defects in the anisotropic van der Waals layered semiconductor
ReS$_2$. We demonstrate the in-plane anisotropy of the lattice by directly
visualizing chains of rhenium atoms forming diamond-shaped clusters. Using
scanning tunneling spectroscopy we measure the semiconducting gap in the
density of states. We reveal the presence of lattice defects and by comparison
of their topographic and spectroscopic signatures with ab initio calculations
we determine their origin as oxygen atoms absorbed at lattice point defect
sites. These results provide an atomic-scale view into the semiconducting
transition metal dichalcogenides, paving the way toward understanding and
engineering their properties.
