  Atomically thin semiconductors have dimensions that are commensurate with
critical feature sizes of future optoelectronic devices defined using
electron/ion beam lithography. Robustness of their emergent optical and
valleytronic properties is essential for typical exposure doses used during
fabrication. Here, we explore how focused helium ion bombardment affects the
intrinsic vibrational, luminescence and valleytronic properties of atomically
thin MoS$_{2}$. By probing the disorder dependent vibrational response we
deduce the interdefect distance by applying a phonon confinement model. We show
that the increasing interdefect distance correlates with disorder-related
luminescence arising 180 meV below the neutral exciton emission. We perform
ab-initio density functional theory of a variety of defect related
morphologies, which yield first indications on the origin of the observed
additional luminescence. Remarkably, no significant reduction of free exciton
valley polarization is observed until the interdefect distance approaches a few
nanometers, namely the size of the free exciton Bohr radius. Our findings pave
the way for direct writing of sub-10 nm nanoscale valleytronic devices and
circuits using focused helium ions.
