Helium ion modified luminescence and robust valley polarization of atomically thin MoS$_{2}$

We show a systematic study of the impact of focused helium ions on the optical, vibrational, and valleytronic properties of atomically thin MoS$_{2}$. A quantitative analysis reveals significant shifts of the first-order Raman modes which are explained by a phonon confinement model that directly links the ion dose to an effective interdefect distance. A disorder-related luminescence peak is observed at low temperatures. We perform ab-initio density functional theory calculations on a variety of defect related morphologies to explain the additional peak by O$_{2}$ molecules chemisorbed at mono-sulphur vacancies. We further observe that an interdefect distance approaching the size of the free exciton Bohr radius results in a significant reduction of valley polarization, most likely caused by strong intervalley scattering. Our studies clearly reveal an operational range of helium ion doses which allows the fabrication of valleytronic circuits on the nanoscale.