Room-temperature ferromagnetism in monolayer WSe2

Abstract

Substitutional doping of magnetic impurities in monolayer transition metal dichalcogenides is a potentially promising route to create above room temperature 2D ferromagnets. Recent reports, however, reveal significant inconsistencies between multiple experimental observations and theoretical calculations. To unravel these apparent discrepancies, a comprehensive study is performed. It is found that while magnetic dopants (V and Fe) substitutionally replace the W for concentrations up to 35 %, no ferromagnetism is observed in these doped WSe₂ monolayers with minimal point defects. The suppression of the theoretically predicted ferromagnetic order at these doping levels is due to clustering of the dopants which is observed and quantified from plan-view transmission electron microscopy. Room-temperature ferromagnetism is observed when the doped monolayers contain a significant concentration of selenium vacancies (Se_vac). These vacancies were intentionally created via a post-growth annealing process and magnetism is seen to scale with annealing time/vacancy concentration. In fact, very similar ferromagnetism is seen even in undoped WSe₂ with Se_vac > 10¹⁴ cm⁻² suggesting an easier potential route to synthesize room-temperature 2D ferromagnets compared to metal substitution. These findings can explain all the discrepancies in the literature and suggest that no room-temperature ferromagnetism in WSe₂ has been reported that was not caused by high levels of Se vacancies.