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Hydrogenation and defect formation control the strength and ductility of MoS\(_2\) nanosheets: Reactive molecular dynamics simulation

Summary

Reactive MD (ReaxFF) in LAMMPS studies uniaxial tension of single-layer 2H-MoS\(_2\) with variable H coverage and point defects (vacancy motifs aligned with experimental CVD defect taxonomy). Hydrogenation and defects strongly alter elastic modulus, strength, ductility, and failure mode (including S–S dimerization after yield in pristine sheets, suppressed by full hydrogenation). The work connects mechanical design of TMD nanosheets to hydrogen from processing or environment, where partial coverage can nonmonotonically tune modulus while full hydrogenation tends to embrittle by blocking S–S bond formation pathways.

Methods

  • Code / potential: LAMMPS; visualization OVITO; ReaxFF for Mo–S–H from Ostadhossein et al. (as cited in the paper).
  • Pristine sheet: 8280 atoms; box ~25 nm × 9.5 nm planar; periodic in-plane; timestep 0.25 fs; energy minimization (10⁻⁶ kcal/mol and 10⁻⁶ kcal/mol·Å criteria).
  • Equilibration: NPT at room temperature; Nosé–Hoover thermostat and barostat with damping 62.5 fs (T) and 625 fs (P).
  • Loading: Uniaxial engineering strain rate 1×10⁹ s⁻¹; NPT on width direction to approximate uniaxial stress; stress from virial, averaged 250 fs intervals; effective thickness 6.1 Å (pristine), 8.9 Å (full MoS\(_2\)H\(_2\)), interpolated by H% for partial coverage.
  • Defects: Monovacancy motifs V\(_S\), V\(_{S2}\), V\(_{MoS3}\), V\(_{MoS6}\), Mo\(_{S2}\), S\(_2\)Mo per Zhou et al. taxonomy.

Findings

  • Pristine MoS\(_2\) shows yield then noisy stress–strain behavior tied to phase-like transformation and S–S pairing enabled by ReaxFF cutoffs.
  • Hydrogenation reduces the phase-transformation window; fully hydrogenated sheets lose the yield anomaly and embrittle by blocking S–S bond formation.
  • Elastic modulus vs H% shows a slight initial drop, then rise after ~5% H; tensile strength drops sharply then plateaus near ~40% H.
  • Defects degrade mechanical metrics relative to pristine; trends are interpreted for nanodevice mechanical design.
  • Comparisons across V\(_S\), V\(_{S2}\), and larger chalcogen/metal vacancy motifs illustrate how local stoichiometry shifts the failure pathway even when global strain rate and system size are held fixed.

Limitations

High strain rates, finite defect sampling, and ReaxFF transferability for mechanical vs electronic applications. The S–S dimerization pathway after yield is a force-field-dependent feature that should be cross-checked when translating stress–strain trends to device failure criteria beyond the monotonic tension protocol used here. Thickness assumptions for stress conversion follow the article’s effective monolayer thickness conventions; adjust consistently when comparing to experimental nanoindentation or bulge tests.

Relevance to group

Direct van Duin-group collaboration on TMD ReaxFF mechanics.

Citations and evidence anchors

  • DOI: 10.1016/j.eml.2018.05.008.