  Single spin defects in 2D transition-metal dichalcogenides are natural
spin-photon interfaces for quantum applications. Here we report high-field
magneto-photoluminescence spectroscopy from three emission lines (Q1, Q2 and
Q*) of He-ion induced sulfur vacancies in monolayer $\text{MoS}_2$. Analysis of
the asymmetric PL lineshapes in combination with the diamagnetic shift of Q1
and Q2 yields a consistent picture of localized emitters with a wavefunction
extent of $\sim$ 3.5 nm. The distinct valley-Zeeman splitting in out-of-plane
$B$-fields and the brightening of dark states through in-plane $B$-fields
necessitates spin-valley selectivity of the defect states and lifted
spin-degeneracy at zero field. Comparing our results to ab-initio calculations
identifies the nature of Q1 and Q2 and suggests that Q* is the emission from a
chemically functionalized defect. Analysis of the optical degree of circular
polarization reveals that the Fermi level is a parameter that enables the
tunability of the emitter. These results show that defects in 2D semiconductors
may be utilized for quantum technologies.
