Large-Area WS₂ Film with Giant Single Domains Grown by Atmospheric‑Pressure CVD

Background

Two‑dimensional (2D) materials are pivotal for next‑generation electronics and photonics, offering quantum confinement, high carrier mobility, and flexibility. While graphene excels in conductivity, its zero bandgap limits semiconductor applications. Transition‑metal dichalcogenides (TMDCs), such as WS₂, provide a direct bandgap, strong spin‑orbit coupling, and robust light‑matter interaction, making them attractive for transistors, photodetectors, and photovoltaics. Fabrication routes—mechanical exfoliation, sulfurization, thermal decomposition, and CVD—each present trade‑offs. CVD, in particular, offers large‑area, uniform films with controllable thickness, yet achieving high‑quality monolayers remains challenging due to precursor chemistry, temperature control, and gas dynamics. This work addresses these challenges by optimizing AP‑CVD growth parameters to produce millimetre‑scale, single‑domain WS₂ films suitable for device integration.

Methods

High‑purity WO₂.₉ (99.99 %) and S (99 %) powders served as tungsten and sulfur sources, respectively. SiO₂/Si wafers (300 nm SiO₂) were cleaned sequentially in ethanol, isopropanol, and deionized water (15 min ultrasonication each). A 0.1 g WO₂.₉ pellet was placed in a quartz crucible, with the wafer positioned face‑down above it. A quartz boat containing sulfur was positioned upstream. The quartz tube (60 mm diameter) was evacuated to 100 mTorr, then purged with Ar (500 sccm) for 30 min. Growth temperatures ranged from 750 to 950 °C, ramped at 10 °C min⁻¹, held for 5–30 min, and then cooled naturally. The tube pressure was maintained at atmospheric pressure during growth. After synthesis, films were characterized by SEM (Hitachi S4800, 5–10 kV), Raman (Nanophoton Raman‑11, Si 520 cm⁻¹ reference), AFM (Bruker Multimode 8, tapping mode), PL (532 nm laser, confocal micro‑PL), TEM (JEOL JEM‑2100F, 200 kV), and EDS.

Results and Discussion

We investigated how each growth parameter influences WS₂ morphology and quality, then identified the optimal conditions yielding the largest, most crystalline monolayer domains.

Precursor Choice (WO₃ vs WO₂.₉)

SEM imaging revealed that WO₂.₉ produces abundant triangular WS₂ domains, whereas WO₃ yields negligible WS₂ growth. The reduced W⁶⁺ content in WO₂.₉ facilitates early sulfur incorporation, promoting nucleation and growth.

Tube Pressure

Comparing low pressure (<100 mTorr) with atmospheric pressure, the latter yielded triangular domains >100 µm, while low pressure produced only WO₃ leaf‑like flakes. Higher pressure shortens mean free paths, increases collision frequency, and lowers the nucleation energy barrier, thus enhancing WS₂ nucleation density and domain size.

Growth Temperature

Domain size initially increases with temperature (optimal at 900 °C), then declines beyond 950 °C due to thermal cracking. Temperature balances precursor diffusion, surface reaction rates, and defect formation.

Holding Time

Extending the growth period from 5 min to 10 min expands domain size from ~30 µm to ~120 µm; longer times (20–30 min) show diminishing returns due to defect accumulation and impingement effects.

Sulfur Quantity

Optimal sulfur loading is 0.7 g. Lower amounts (<0.5 g) produce incomplete sulfides; higher amounts (>0.9 g) generate WOₓSᵧ particles and irregular edges, reducing crystallinity. The 0.7 g target balances supersaturation and nucleation density, yielding smooth, triangular domains.

Gas Flow Rate

Increasing Ar flow from 50 to 160 sccm enlarges domains (~60 µm at 120 sccm to ~100 µm at 160 sccm) and sharpens edges by reducing W:S ratios. Flow above 160 sccm introduces turbulence and defects, halting size growth.

Substrate Position

Positioning the wafer directly above the WO₂.₉ source (substrate A) yields ~200 µm domains due to higher local temperature and precursor concentration; downstream placement (substrate B) results in ~10 µm domains.

Optimization and Characterization

Optimal conditions: 0.1 g WO₂.₉, 0.7 g S, substrate A, 900 °C, 10 min hold, 160 sccm Ar, atmospheric pressure. SEM shows ~400 µm triangular domains. Raman mapping reveals characteristic E¹₂g (~352 cm⁻¹) and A₁g (~419 cm⁻¹) peaks; the 71 cm⁻¹ separation confirms a 2–3 layer film, while 67 cm⁻¹ indicates monolayer regions. AFM height of 0.82 nm matches monolayer WS₂. PL shows a strong 627 nm emission (direct bandgap ~2 eV) with FWHM ~47 meV, confirming high optical quality. TEM and SAED confirm hexagonal symmetry with d‑spacing 0.271 nm (100) and 0.155 nm (110), matching bulk WS₂.

Conclusions

Systematic exploration of AP‑CVD parameters—precursor type, pressure, temperature, time, sulfur loading, gas flow, and substrate positioning—enabled the fabrication of millimetre‑scale, high‑quality monolayer WS₂ films with single‑domain sizes up to 400 µm. The films exhibit pristine crystallinity, layer‑dependent Raman signatures, strong PL, and atomically flat surfaces, establishing a scalable route for TMDC‑based optoelectronics, photovoltaics, photocatalysis, and energy storage devices.

Abbreviations

2D:

Two‑dimensional

AFM:

Atomic force microscopy

APCVD:

Atmospheric‑pressure chemical vapor deposition

EDS:

Energy‑dispersive spectroscopy

FWHM:

Full width at half maximum

LPCVD:

Low‑pressure chemical vapor deposition

PL:

Photoluminescence

SAED:

Selected‑area electron diffraction

SEM:

Scanning electron microscopy

TEM:

Transmission electron microscopy

TMDCs:

Transition metal dichalcogenides