  Monolayers of molybdenum disulfide (MoS2) are the most studied
two-dimensional (2D) transition-metal dichalcogenides (TMDs), due to its
exceptional optical, electronic, and opto-electronic properties. Recent studies
have shown the possibility of incorporating a small amount of magnetic
transition metals (e.g., Fe, Co, Mn, V) into MoS2 to form a 2D dilute magnetic
semiconductor (2D-DMS). However, the origin of the observed ferromagnetism has
remained elusive, due to the presence of randomly generated sulfur vacancies
during synthesis that can pair with magnetic dopants to form complex
dopant-vacancy configurations altering the magnetic order induced by the
dopants. By combining high-angle annular dark-field scanning transmission
electron microscopy (HAADF-STEM) imaging with first-principles density
functional theory (DFT) calculations and magnetometry data, we demonstrate the
critical effects of sulfur vacancies and their pairings with vanadium atoms on
the magnetic ordering in V-doped MoS2 (V-MoS2) monolayers. Additionally, we
fabricated a series of field effect transistors on these V-MoS2 monolayers and
observed the emergence of p-type behavior as the vanadium concentration
increased. Our study sheds light on the origin of ferromagnetism in V-MoS2
monolayers and provides a foundation for future research on defect engineering
to tune the electronic and magnetic properties of atomically thin TMD-based
DMSs.
