Two-dimensional materials offer a remarkably rich materials platform to study the origin of different material behaviors at the atomic level, and doping provides a key means of tailoring such materials' functional properties. The local atomic structure around such dopants can be critically important in determining the material's behavior as it could modulate scattering, catalytic activity, electronic and magnetic properties, and so forth. Here, using aberration-corrected scanning transmission electron microscopy (STEM) with sub-Ångstrom resolution in conjunction with density functional theory calculations, we demonstrate a strong coupling between Mo dopants and two types of defects in WS2 monolayers: sulfur monovacancies and grain boundaries. Although Mo does occupy a transition metal lattice site, it is not an ideal substitutional dopant: ∼80% of the S vacancies identified by STEM colocalize with Mo dopants, an affinity that appears to be enhanced by symmetry breaking of a partially occupied midgap defect state. Although a Mo dopant by itself does not considerably distort the WS2 lattice, it induces substantial lattice deformation by apparently facilitating the charging of a sulfur monovacancy paired with it, which is consistent with the results of first-principles calculations. This coupling of foreign substitutional dopants with vacancies could potentially be exploited to control the distribution and location of chalcogenide vacancies within transition metal dichalcogenides (TMD), by segregating vacancies into regions of high Mo concentration that are purposely placed away from active regions of TMD-based devices.
Keywords: Doped monolayer transition metal dichalcogenides; atomic and chemical structures; defect coupling; sub-Ångstrom structural distortion.