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

TL;DR

This work addresses how interlayer coupling and twist angle in MoS2/WS2 van der Waals heterostructures influence high-frequency Raman responses. Using a hybrid fabrication to access multiple twist angles in one batch, combined with nonresonant and near-resonant Raman, PL, and white-light reflectance measurements plus DFT/DFPT calculations, the authors reveal a twist-angle dependent, mode-selective enhancement of the WS2 A1g Raman mode. The enhancement correlates with reduced interlayer distance and a specific in-phase sulfur vibration pattern across the interface, while other modes remain largely unaffected; resonant conditions modulate the overall intensity but do not overturn the mode selectivity. The findings provide a practical, noninvasive diagnostic for high-quality interfacial contact in TMDC heterostructures and highlight the complex interplay of phonons across the van der Waals gap.

Abstract

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.

Figures (11)

• Figure 1: (a) Room temperature PL spectra of a MoS$_2$/WS$_2$ heterostructure. After annealing, PL quenching of the WS$_2$ A exciton emission occurs in the heterostructure region, indicating good contact between the constituent layers. Corresponding PL maps are shown in Supplementary Information S1. (b) Initially, Raman spectra of the same spots do not show intensity differences for the WS$_2$ A$_{1g}$ mode in the monolayer (red line) and heterostructure region (blue area). However, the WS$_2$ A$_{1g}$ mode is enhanced after annealing. Raman spectra are normalized to the intensity of the Si phonon on the bare Si/SiO$_2$ substrate. Both Raman and PL spectra were acquired with an excitation wavelength of 532 nm.
• Figure 2: (a) Optical microscope image of a sample consisting of CVD-grown MoS$_2$ monolayers which were covered with a large exfoliated WS$_2$ flake. This yields different heterostructures with various twist angles in the same sample. Some CVD-grown monolayers were marked for illustration purposes. (b) Raman scan (532 nm excitation) of the same sample showing the intensity of the WS$_2$ A$_{1g}$ Raman mode. (c) Averaged Raman spectra of the marked heterostructure with 0$^\circ°$ twist angle. Compared to other Raman modes, the WS$_2$ A$_{1g}$ is clearly enhanced. Averaged spectra for all heterostructure regions are shown in Supplementary Information S3. (d) The enhancement appears on the Stokes and Anti-Stokes side of the Raman spectrum. The spectrum was measured in the center of the 0$^\circ°$ twist angle heterostructure. (e,f) Maximum and average enhancement factors of the WS$_2$ A$_{1g}$ mode obtained for heterostructures with different twist angles. For comparison, 2LA(M)/E$^1_{2g}$ average enhancement factors are included, underlining the mode selectivity of the enhancement process. Error bars represent statistical uncertainties determined from all selected individual WS$_2$ spectra.
• Figure 3: Calculated Enhancement factors of WS$_2$ and MoS$_2$ (blue and red) A$_{1g}$ and E$_g$ (solid and hollow points) Raman modes in WS$_2$/MoS$_2$ heterostructures and $H_h^h$ homobilayers as a function of the interlayer distance. The range of equilibrium interlayer distances for each heterostructure is marked by a gray vertical bar. The insets depict the side views of atomic structures of the heterobilayers, where larger (smaller) blue (red) circles represent metal (sulfur) atoms in WS$_2$ (MoS$_2$) layer, and the gray dashed lines connect atoms that are vertically aligned in the given stackings.
• Figure 4: (a,b) Integrated Raman intensities for the WS$_2$ out-of-plane and in-plane Raman modes in a WS$_2$ monolayer (red) and several heterostructures (blue). Raman spectra were obtained from various spots on different heterostructures (symbols) shown in Figure \ref{['Panel_2_Raman_Enhancement']}a, triangles indicate data originating from the 0$^\circ°$ heterostructure. Data points marked with square and circle represent measurements from the 9$^\circ°$ and 55$^\circ°$ heterostructure, respectively. A laser of tunable wavelength was used for excitation. The lines are fitted resonance curves for the isolated monolayer and the 55$^\circ°$ heterostructure. (c,d) Exemplary Raman spectra excited below (1.98 eV) and above (2.07 eV) the WS$_2$ A exciton energy. The energy range used for intensity determination via numerical integration is highlighted in gray.
• Figure 5: (a) Temperature-dependent Raman spectra of a MoS$_2$/WS$_2$ heterostructure at 532 nm excitation. All spectra are normalized to the Si phonon. Raman modes of interest are the WS$_2$ A$_{1g}$ mode at 420 cm$^{-1}$ and the combined 2LA(M)/ E$^1_{2g}$ peak at 355 cm$^{-1}$. (b,c) Low-temperature white-light reflectance contrast (RC) measurements of the same sample. $RC=(R_{Sample}-R_{Ref})/R_{Ref}$ was used for normalization, with $R_{Ref}$ being the reflectance of the bare Si/SiO$_2$ substrate. Furthermore, all spectra were normalized to the minimum of the WS${_2}$ A exciton for better illustration of energetic shifts. The dotted line indicates the WS$_2$ monolayer's A and B exciton energies at 4 K.