Skip to content

Illuminating Invisible Grain Boundaries in Coalesced Single-Orientation WS2 Monolayer Films

MOCVD 2H-WS₂ on c-plane sapphire shows near-single-orientation SAED/BF-TEM, yet dark-field TEM reveals faceted defect arrays; ADF-STEM identifies translational grain boundaries. ReaxFF MD at 300 K relaxes type A/B mismatch models consistent with experiment; facet statistics link nm-scale motifs to μm-scale morphology.

Summary

Electron microscopy (BF/DF-TEM, SAED, ADF-STEM) characterizes wafer-scale monolayer WS₂ transferred to TEM grids. Apparent single-crystal diffraction hides translational GBs visible in DF-TEM. Atomic models distinguish type A (single vacancy-line-like mismatch with compressed planes yet hexagonal 3|W metal rows) versus type B (sub-unit-cell offset yielding rectangular 4|W motifs). ReaxFF MD equilibrates these GB structures at 300 K, reproducing out-of-plane tilt upon freestanding relaxation. >1300 facet traces support a picture of microstructure built from nanoscale units spanning Å-to-μm scales. The study addresses optoelectronic yield: large-area monolayers can still harbor hidden GBs that scatter carriers and lower mobility, so diffraction-only characterization is insufficient for quality metrics.

Methods

1 — Experiments and microscopy. MOCVD 2H-WS₂ on 2 in. c-plane Al₂O₃; transfer to TEM grids (Quantifoil). BF/DF-TEM, SAED (including ±15° tilt series), and ADF-STEM on translational grain boundaries; >1300 facet traces connect local STEM motifs to mesoscale texture on wafers.

2 — MD application. ReaxFF LAMMPS MD (potential from 20210000-0002-3621-2481-x-reaxff-force, same DOI family) NVT equilibration at 300 K of type A and B GB supercells (including freestanding relaxations with in-plane PBC as in the paper). Sub-1 fs timestep; Nose–Hoover (or Langevin) thermostat; ps-scale equilibration runs. NPT / barostat—N/A for the static tilt-recovery tests if the article holds cell vectors fixed. External electric field; rare-event (umbrella)—N/A in the described protocol.

3 — Force-field training on this page. N/A—the JPCC study defines the WS₂+sapphire ReaxFF; this Nano Lett. work applies it to GB models.

4 — Galley. The registered pdf_path is a galley; use the VOR for final values.

Findings

Outcomes and mechanisms (experiment + MD). Wafer-scale MOCVD WS₂ can show SAED consistent with a single orientation while DF-TEM still reveals translational grain boundaries hidden to diffraction-averaging views. ADF-STEM distinguishes type A and B mismatches (e.g. (3|W) hex-row vs (4|W) rect-row motifs in the paper’s language). ReaxFF NVT equilibration at 300 K relaxes these GB supercells, including out-of-plane tilt upon freestanding-like release, in qualitative structural agreement with STEM (not electronic structure). Comparisons pair imaging with ReaxFF-relaxed models; >1300 facet traces link local STEM motifs to mesoscale texture on wafers.

Sensitivity and limitations. The study emphasizes that apparent single-crystal diffraction is an insufficient quality metric for optoelectronic yield if GBs that scatter carriers are present. ReaxFF addresses mechanics and morphology, not band structure. Corpus note: the local pdf_path is a galley; verify values and locators on the version of record PDF. Strain from transfer and substrate-induced defects may add mechanisms beyond the two translational classes highlighted; kinetic pathways during CVD may not match static after-the-fact relaxations.

Limitations

Galley PDF. ReaxFF does not predict electronic GB states. Transfer wrinkles and substrate strain during CVD may introduce defect types beyond the two translational classes emphasized in the main text. Growth kinetics on sapphire may favor metastable GB arrays that differ from equilibrium structures after long anneals.

Relevance to group

Joint STEM + ReaxFF paper linking synthesis conditions to defect arrays in TMD films.

Citations and evidence anchors

Reader notes (navigation)

  • Pair with paper:20210000-0002-3621-2481-x-reaxff-force for the WS₂ ReaxFF definition.
  • paper_keywords includes keyword:galley-or-proof-pdf.
  • After VOR PDF ingest, refresh locators and figure references that may shift vs the galley text.