  Using Hubbard U corrected density functional theory calculations, lattice
Monte-Carlo, and spin-Monte-Carlo simulations, we investigate the impact of
dopant clustering on the magnetic properties of WSe2~doped with period four
transition metals. We use manganese (Mn) and iron (Fe) as candidate n-type
dopants and vanadium (V) as the candidate p-type dopants, substituting the
tungsten (W) atom in WSe2. Specifically, we determine the strength of the
exchange interaction in the Fe-, Mn-, and V-doped WSe2~ in the presence of
clustering. We show that the clusters of dopants are energetically more stable
than discretely doped systems. Further, we show that in the presence of dopant
clustering, the magnetic exchange interaction significantly reduces because the
magnetic order in clustered WSe2~becomes more itinerant. Finally, we show that
the clustering of the dopant atoms has a detrimental effect on the magnetic
interaction, and to obtain an optimal Curie temperature, it is important to
control the distribution of the dopant atoms.
