Effects of Vanadium Doping on the Optical Response and Electronic Structure of WS₂ Monolayers

2D dilute magnetic semiconductors have been recently reported in transition metal dichalcogenides doped with spin-polarized transition metal atoms, for example vanadium-doped WS₂ monolayers, which exhibit room-temperature ferromagnetic ordering. However, a broadband characterization of the electronic band structure of these doped WS₂ monolayers and its dependence on vanadium concentration is still lacking. Therefore, power-dependent photoluminescence, resonant four-wave mixing, and differential reflectance spectroscopies are performed here to study optical transitions close to the A exciton energy of vanadium-doped WS₂ monolayers at three different doping levels. Instead of a single A exciton peak, vanadium-doped samples exhibit two photoluminescence peaks associated with transitions from a donor-like level and the conduction band minima. Moreover, resonant Raman and second-harmonic generation experiments reveal a blueshift in the B exciton energy but no energy change in the C exciton after vanadium doping. Density functional theory calculations show that the band structure is sensitive to the Hubbard U correction for vanadium, and several scenarios are proposed to explain the two photoluminescence peaks around the A exciton energy region. This work provides the first broadband optical characterization of these 2D dilute magnetic semiconductors, shedding light on the novel and tunable electronic features of V-doped WS₂ monolayers.