Low-frequency Raman scattering in WSe${}_2$-MoSe${}_2$ heterobilayers: Evidence for atomic reconstruction

Abstract

We investigate WSe${}_2$-MoSe${}_2$ heterobilayers with different twist angles $\theta \pm \delta$ between the two layers, by low-frequency Raman scattering. In sufficiently aligned samples with $\theta =0^{\circ }$, or $\theta =60^{\circ }$, and $\delta \lesssim 3^{\circ }$, we observe an interlayer shear mode (ISM), which is a clear sign of a commensurate bilayer structure, i.e., the layers must undergo an atomic reconstruction to form R-type or H-type stacking orders. We find slightly different ISM energies of about 18~cm${}^{-1}$ and 17~cm${}^{-1}$ for H-type and R-type reconstructions, respectively, independent of the exact value of $\theta \pm \delta$. Our findings are corroborated by the fact that the ISM is not observed in samples with twist angles, which deviate by $\delta >3^{\circ }$ from $0^{\circ }$ or $60^{\circ }$. This is expected, since in such incommensurate structures, with the possibility of Moir$\acute{\text{e}}$-lattice formation, there is no restoring force for an ISM. Furthermore, we observe the ISM even in sufficiently aligned heterobilayers, which are encapsulated in hexagonal Boron nitride. This is particularly relevant for the characterization of high-quality heterostructure devices.

5 pages, 4 figures

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• Raman spectroscopy of the interlayer shear mode in few-layer MoS2 flakes
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• Interlayer exciton valley polarization dynamics in large magnetic fields
• Signatures of electric field and layer separation effects on the spin-valley physics of MoSe${}_2$/WSe${}_2$ heterobilayers: from energy bands to dipolar excitons
• The optical response of artificially twisted MoS${}_2$ bilayers
• First-principles Calculations of MoSeTe/WSeTe Bilayers: Stability, Phonons, Electronic Band Offsets, and Rashba Splitting
• Phonons in MoSe2/WSe2 van der Waals heterobilayer
• Enhancement of the WS${}_2$ A${}_{1\text{g}}$ Raman Mode in MoS${}_2$/WS${}_2$ Heterostructures
• Phonon Renormalization in Reconstructed MoS${}_2$ Moiré Superlattices