Rapid Optical Mapping of Defect Formation in Monolayer WSe₂: Guiding Optimal CVD Growth

Introduction

Two‑dimensional TMDCs such as MoS₂, WS₂, and WSe₂ have emerged as versatile platforms for photodetectors, transistors, photovoltaics, sensors, and electrocatalysts (refs. 1–11). While mechanical exfoliation delivers pristine crystals, CVD offers scalable growth, controllable morphology, and reproducible heterostructures (refs. 12–18). However, the inevitable formation of vacancies, grain boundaries, and interstitials during CVD degrades carrier mobility, induces non‑uniform PL, and compromises lattice stability (refs. 19–25). For instance, CVD‑grown WSe₂ field‑effect transistors typically exhibit mobilities an order of magnitude below theoretical limits, and PL mapping routinely reveals patchy emission linked to defect clusters (refs. 20–24).

Traditional defect‑diagnostics such as transmission electron microscopy (TEM) and scanning tunneling microscopy (STM) are destructive and probe only micrometer‑scale areas, making them impractical for rapid growth optimization (refs. 26–27). Raman spectroscopy, sensitive to lattice vibrations, strain, and crystallinity, and PL spectroscopy, which directly probes excitonic dynamics, offer nondestructive, high‑throughput alternatives (refs. 28–36). Notably, the intensity, peak position, and full width at half maximum (FWHM) of the E¹_2g Raman mode correlate with defect density and strain, while PL intensity inversely tracks defect‑induced non‑radiative recombination (refs. 21, 30, 37).

Growth parameters—particularly temperature—are the dominant variables governing nucleation density, domain size, and defect incorporation in CVD‑grown TMDCs (refs. 38, 42). This work focuses on how temperature modulates defect chemistry and crystal stability in monolayer WSe₂. We employed confocal Raman and PL mapping to assess crystal quality, identify optimal synthesis conditions, and elucidate the mechanistic link between defect density and long‑term air degradation.

Methods

Synthesis of Monolayer WSe₂

High‑purity Se (Alfa‑Aesar 99.999 %) and WO₃ (Aladdin 99.99 %) powders were loaded into separate quartz boats within a 2‑inch furnace. Se (30 mg) was positioned in the first heating zone; WO₃ (100 mg) resided 25 cm downstream. c‑plane (0001) sapphire substrates were cleaned and placed 5–10 cm downstream of the WO₃ source. After evacuating the chamber for 10 min, an O₂‑free Ar flow (200 sccm, 99.9999 %) purged the system. The growth atmosphere was then switched to 10 % H₂/Ar (50 sccm, ambient pressure). The second zone was ramped to target temperatures (860–940 °C) at 20 °C min⁻¹, held for 6 min, while the first zone remained at 320 °C. Rapid furnace cooling followed the growth step.

Characterization

Optical microscopy (NPLANEPi100X) assessed morphology. Raman and micro‑PL were collected with a Renishaw inVia Qontor system using a 532 nm laser (× 100 objective, 1800 lines mm⁻¹ grating). AFM (Agilent 5500, tapping mode) measured thickness, and SEM (TESCAN MIRA3 LMU) examined long‑range morphology.

Results and Discussion

Growth temperatures between 860 °C and 940 °C were evaluated. Statistical analysis of optical images and PL spectra pinpointed 920 °C as the optimum (Fig. 1a,c). At 920 °C, triangular domains exhibited a uniform edge length of ~35 µm and a thickness of ~0.9 nm (AFM, Fig. 1b). Raman spectra revealed the characteristic E¹_2g (~249.5 cm⁻¹) and A_1g (~260 cm⁻¹) modes, confirming monolayer thickness (no B_2g at 308 cm⁻¹). Below or above 920 °C, domain density and size declined, likely due to insufficient precursor reaction or accelerated decomposition, respectively.

PL intensity mirrored the Raman trend, peaking at 920 °C (Fig. 1c). Raman analysis showed a minimum FWHM and maximum E¹_2g intensity at this temperature, indicating superior crystal quality (Fig. 1e,f). The E¹_2g peak shifted from 251.5 cm⁻¹ (860 °C) to a minimum of 249.5 cm⁻¹ (920 °C) before rising again, consistent with reduced strain and defect scattering.

PL mapping further elucidated defect distribution (Fig. 2). Monolayers grown at 920 °C exhibited homogeneous emission except for a modest dip at the center—attributed to Se‑deficient nucleation sites (WO_3‑xSe_y) (refs. 46–48). Growth at 900 °C produced weaker PL along the armchair direction, whereas 940 °C samples displayed strongest emission at the center, decreasing toward the edges. These patterns suggest that defect density—not strain or edge effects—drives PL inhomogeneity.

Detailed Raman and PL spectra from the center and edge of 900 °C samples (Fig. 3) revealed a defect‑related emission (D) at ~1.53 eV, alongside neutral exciton (A, 1.624 eV) and trion (A⁺, 1.60 eV) peaks. The D emission saturated with excitation power, confirming its defect origin. Low‑temperature (77 K) PL further amplified the defect peak, with binding energies of ~24 meV for trions and ~100 meV for defects—consistent with literature (refs. 33, 35, 51, 52).

Aging experiments over 90 days in ambient air demonstrated a stark contrast between growth temperatures. PL intensity for 900 °C and 940 °C samples decayed rapidly, while 920 °C monolayers retained significant emission (Fig. 4). Raman mapping revealed that PL quenching began at defect‑rich regions, confirming that defects accelerate decomposition via O₂ and OH intercalation (refs. 25, 64, 65). SEM images (Fig. 5) tracked the morphological evolution, showing progressive erosion from the center to the corners over 180 days, culminating in complete degradation.

Collectively, these observations establish a direct link between growth temperature, defect density, and long‑term air stability. The 920 °C optimum balances sufficient precursor reaction with minimal decomposition, yielding monolayer WSe₂ with the highest optical quality and durability.

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The growth optimization of monolayer WSe₂ on sapphire substrate. a Optical and b the corresponding AFM images of triangular monolayer WSe₂ grown at 920 °C. c The average domain size and integrated PL intensity. d Raman spectra. e The E¹_2g frequency and intensity together with f FWHM of E¹_2g peak for monolayer WSe₂ grown from 860 °C to 940 °C. All the Raman and PL spectra were taken from the similar region from the triangle monolayer WSe₂, as pointed out by a red point in a

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PL integral (725–785 nm) mapping of the monolayer WSe₂ grown under different temperatures together with the corresponding optical images. a, d 900 °C. b, e 920 °C. c, f 940 °C. The inset in a is an atomic illustration of the WSe₂ layer showing the armchair direction. The excitation power for the PL mapping is 50 μW

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a Raman spectra obtained from the center region and edge region at 50 μW excitation laser power levels. PL spectra confirm the existence of crystal defects in WSe₂ grown at 900 °C. Room temperature PL spectra from the b center and c edge of the WSe₂ together with fitted spectra using voigt (50% Gaussian, 50% Lorentzian) equation. d Low temperature (77 K) PL spectra from the center position and the edge position showing a strong defect-related peak from the center region. The PL spectrum at 77 K from the center region is fitted with three peaks

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The direct correlation between crystal stability and lattice defect of WSe₂. PL mapping of WSe₂ monolayer grown at a 900 °C, b 920 °C, and c 940 °C, respectively, after placing in the air for 90 days. Optical images of WSe₂ grown at 900 °C d before and e after 90 days. f Raman and g PL spectra comparison from the center and the edge of the WSe₂ sample grown at 900 °C before and after 90 days. The excitation power for PL measurements is 50 μW

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SEM images of a fresh monolayer WSe₂ grown at 900 °C, placing in the air for b 30 days, c 90 days, and d 180 days, respectively. The enlarged view of the center and angle f in d. All the samples were stored in 25 °C. e, f Enlarged views of the center and vertex of monolayer d, respectively

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These results confirm that defect density directly governs both optical quality and environmental stability. By targeting the 920 °C growth window, researchers can reliably produce monolayer WSe₂ with minimal defects, leading to stronger neutral‑exciton PL, higher carrier mobilities, and extended air‑life—critical parameters for next‑generation optoelectronic devices.

Conclusion

We demonstrated that the CVD growth temperature of monolayer WSe₂ on sapphire decisively controls defect formation, optical performance, and air‑stability. A temperature of 920 °C optimizes nucleation, suppresses vacancy clustering, and yields triangular domains with uniform PL dominated by neutral excitons. Deviations below or above this temperature increase defect density, alter PL spatial patterns, and accelerate degradation. These findings provide a clear, experimentally validated pathway for synthesizing high‑quality TMDC monolayers, facilitating their deployment in reliable optoelectronic devices.

Availability of Data and Materials

All data supporting these findings are fully available without restriction.

Abbreviations

2D:

two‑dimensional

AFM:

Atomic force microscope

CVD:

Chemical vapor deposition

FWHM:

Full width at half maximum

PL:

Photoluminescence

sccm:

standard‑state cubic centimeter per minute

SEM:

Scanning electron microscope

STM:

Scanning tunneling microscopy

TEM:

Transmission electron microscopy

TMDCs:

Transition metal dichalcogenides