Unraveling the Role of Dopant Clustering in Magnetic Impurity Doped Monolayers of Transition Metal Dichalcogenides

Efforts to achieve above room temperature ferromagnetism in monolayers of transition metal dichalcogenides (TMDs) through substitutional doping with magnetic impurities are actively being pursued for energy-efficient logic and memory devices. However, the current limitations stem from phase separation and multi-layered growth at heavy doping levels, restricting the doping in monolayers to levels well below the threshold established by density functional theory (DFT) for above room temperature Curie temperature. On the other hand, room temperature magnetism has been frequently observed at significantly lower doping levels (0.1-1%), but this magnetism arises from a combination of substitutional dopants, point defects, contaminants, interstitials, or edge states. As a result, the origin of purely substitutional doping-induced ferromagnetism remains a subject of debate.<br/>Towards this end, this study employs molecular beam epitaxy to achieve up to 30% substitutional doping of vanadium (V) and iron (Fe) in a monolayer of tungsten diselenide, surpassing the doping requirements (&gt;15%) indicated by DFT for room temperature ferromagnetism. Magnetometry measurements, however, reveal the absence of ferromagnetism down to a temperature of 4 K in these phase-pure films, with only the phase-separated films exhibiting any room temperature ferromagnetic behavior at Fe doping levels exceeding 30%. Structural characterization utilizing plan-view transmission electron microscopy reveals significant dopant clustering, even at modest doping levels (~5%), which serves as the primary factor responsible for the absence of ferromagnetism in phase-pure films. Remarkably, these observations align with DFT calculations, which predict a low formation energy for dopant clustering, leading to a weakened exchange interaction that subsequently suppresses ferromagnetism. The insights gained from this exploratory study offer a promising pathway to attain high doping densities in monolayer TMDs while emphasizing the influence of dopant clustering on the magnetic properties of the films.