Enhancement of the WS$_2$ A$_{1 ext{g}}$ Raman Mode in MoS$_2$/WS$_2$ Heterostructures

When combined into van der Waals heterostructures, transition metal dichalcogenide monolayers enable the exploration of novel physics beyond their unique individual properties. However, for interesting phenomena such as interlayer charge transfer and interlayer excitons to occur, precise control of the interface and ensuring high-quality interlayer contact is crucial. Here, we investigate bilayer heterostructures fabricated by combining chemical-vapor-deposition-grown MoS$_2$ and exfoliated WS$_2$ monolayers, allowing us to form several heterostructures with various twist angles within one preparation step. In case of sufficiently good interfacial contact, evaluated by photoluminescence quenching, we observe a twist-angle-dependent enhancement of the WS$2$ A${1g}$ Raman mode. In contrast, other WS$_2$ and MoS$2$ Raman modes (in particular, the MoS$2$ A${1g}$ mode) do not show a clear enhancement under the same experimental conditions. We present a systematic study of this mode-selective effect using nonresonant Raman measurements that are complemented with ab-initio calculations of Raman spectra. We find that the selective enhancement of the WS$2$ A${1g}$ mode exhibits a strong dependence on interlayer distance. We show that this selectivity is related to the A${1g}$ eigenvectors in the heterolayer: the eigenvectors are predominantly localized on one of the two layers; yet, the intensity of the MoS$_2$ mode is attenuated because the WS$_2$ layer is vibrating (albeit with much lower amplitude) out of phase, while the WS$_2$ mode is amplified because the atoms on the MoS$_2$ layer are vibrating in phase. To separate this eigenmode effect from resonant Raman enhancement, our study is extended with near-resonant Raman measurements.

💡 Research Summary

This paper investigates how interlayer coupling and twist angle affect Raman signatures in van der Waals heterostructures composed of transition‑metal dichalcogenide (TMDC) monolayers, specifically MoS₂ and WS₂. By placing a large exfoliated WS₂ flake onto chemical‑vapor‑deposition (CVD) grown MoS₂ monolayers, the authors generate multiple heterostructure regions with twist angles ranging from 0° to 60° in a single fabrication step. After annealing at 300 °C, photoluminescence (PL) of the WS₂ A‑exciton is strongly quenched in the heterostructure areas, indicating efficient charge transfer and a reduction of the interlayer distance, i.e., improved interfacial contact.

Raman spectroscopy performed with a non‑resonant 532 nm laser reveals a striking, mode‑selective enhancement: the out‑of‑plane WS₂ A₁g phonon (~420 cm⁻¹) becomes significantly more intense in the heterostructure compared with isolated WS₂ monolayers, while other modes—including the MoS₂ A₁g, the WS₂ in‑plane E₂g, and the combined WS₂ 2LA(M)/E₂g peak—show no systematic change. The enhancement factor (heterostructure intensity divided by monolayer intensity) reaches ≈2 for twist angles close to 0°, drops to near unity around 30°, and rises again modestly near 60°. This angular dependence mirrors previously reported variations of the equilibrium interlayer spacing: the layers are closest at 0° and 60°, farther apart near 30°.

To elucidate the microscopic origin, the authors perform density‑functional‑theory (DFT) calculations of non‑resonant Raman intensities for several high‑symmetry stacking configurations (R‑type and H‑type). By varying the interlayer distance d in the simulations, they compute enhancement factors EF = I(d)/I(monolayer) for both WS₂ and MoS₂ A₁g and E₂g modes. The WS₂ A₁g EF rises sharply as d decreases, reaching ≈2 at the equilibrium distance and exceeding 3.5 when the layers are compressed by ~10% beyond equilibrium—quantitatively matching the experimental trend. In contrast, the MoS₂ A₁g and all other modes exhibit only modest changes (EF ≤ 2), confirming the selectivity.

Analysis of the phonon eigenvectors shows that each A₁g mode is largely localized on its own layer, but the interlayer coupling introduces a phase relationship between the two layers. For the WS₂ A₁g mode, the atoms in the underlying MoS₂ layer vibrate in phase with the WS₂ atoms, effectively amplifying the overall dipole change and thus the Raman intensity. Conversely, for the MoS₂ A₁g mode the WS₂ atoms move out of phase, partially canceling the dipole variation and attenuating the Raman signal. This phase‑dependent constructive or destructive interference provides a clear physical picture for the observed mode‑selective enhancement.

The authors also verify that the effect is non‑resonant: the enhancement appears on both Stokes and anti‑Stokes sides of the spectrum, and near‑resonant Raman measurements (633 nm excitation) show a uniform increase of all WS₂ modes, as expected for electronic resonance, whereas the selective WS₂ A₁g boost persists only under non‑resonant conditions. Additional experiments demonstrate that the phenomenon is not limited to CVD‑MoS₂/exfoliated‑WS₂ stacks; it also occurs in fully exfoliated heterostructures and in MoSe₂/WS₂ heterostructures, underscoring its generality.

In summary, the work establishes the WS₂ A₁g Raman mode intensity as a sensitive, non‑destructive probe of interfacial quality and interlayer spacing in TMDC heterostructures. The twist‑angle dependence highlights the importance of controlling rotational alignment to tune interlayer distance, and the phase‑interference mechanism uncovered by DFT provides a fundamental understanding of mode‑selective Raman enhancement. These insights advance the toolbox for characterizing and engineering van der Waals heterostructures for optoelectronic applications.