  Successful doping of single-layer transition metal dichalcogenides (TMDs)
remains a formidable barrier to their incorporation into a range of
technologies. We use density functional theory to study doping of molybdenum
and tungsten dichalcogenides with a large fraction of the periodic table. An
automated analysis of the energetics, atomic and electronic structure of
thousands of calculations results in insightful trends across the periodic
table and points out promising dopants to be pursued experimentally. Beyond
previously studied cases, our predictions suggest promising substitutional
dopants that result in p-type transport and reveal interesting physics behind
the substitution of the metal site. Doping with early transition metals (TMs)
leads to tensile strain and a significant reduction in the bandgap. The bandgap
increases and strain is reduced as the d-states are filled into the mid TMs;
these trends reverse are we move into the late TMs. Additionally, the Fermi
energy increases monotonously as the d-shell is filled from the early to mid
TMs and we observe few to no gap states indicating the possibility of both p-
(early TMs) and n- (mid TMs) type doping. Quite surprisingly, the simulations
indicate the possibility of interstitial doping of TMDs; the energetics reveal
that a significant number of dopants, increasing in number from molybdenum
disulfide to diselenide and to ditelluride, favor the interstitial sites over
adsorbed ones. Furthermore, calculations of the activation energy associated
with capturing the dopants into the interstitial site indicate that the process
is kinetically possible. This suggets that interstitial impurities in TMDs are
more common than thought to date and we propose a series of potential
interstitial dopants for TMDs relevant for application in nanoelectronics based
on a detailed analysis of the predicted electronic structures.
