Defect Coupling and Sub-Angstrom Structural Distortions in W_1–xMo_xS₂ Monolayers

Abstract

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 WS 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 WS 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.

Monolayer Vanadium‐Doped Tungsten Disulfide: A Room‐Temperature Dilute Magnetic Semiconductor

“…[31] Comparing experimental STEM images to image simulations (simulation details in Experimental Section, Figure 2a-d and Figure S7, Supporting Information) clearly reveals elemental identities by contrast differences (W > 2S > V > 1S). At vanadium concentrations of 8 at% or above ( Figure S3e, Supporting Information), sulfur vacancies are more likely to be coupled to V atoms (written V W +S vac ), which is consistent with prior work on TMD alloys [18,32] and first-principles calculations described below.…”

Two-Dimensional MoxW1−xS2 Graded Alloys: Growth and Optical Properties

“…This mechanism corroborates and helps to explain the S vacancy-Mo coupling in W-rich Mo x W 1− x S 2 reported by Azizi et al . 39 .

Figure 3 DFT-calculated proposed reaction path and schematics of atomic configurations for inward diffusion of a Mo atom and outward diffusion of a W atom and S vacancy.

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Substitutional doping in 2D transition metal dichalcogenides

“…This affinity between Mo and S vacancies manifests in the splitting of partially-filled defect bands related to VS due to symmetry breaking when Mo is in the vicinity. According to DFT calculations, the Mo dopant splits the originally degenerate bands into two levels that are 0.15 eV apart in the band gap [100]. Band gap modulation and carrier doping effects of Mo1−xWxS alloy system remain unexplored.…”