Low-frequency Raman scattering in $WSe2−MoSe2$ heterobilayers: Evidence for atomic reconstruction

We investigate WSe₂–MoSe₂ heterobilayers with different twist angles $θ±δ$ between the two layers by low-frequency Raman scattering. In sufficiently aligned samples with $θ=0°$ or $θ=60°$ and $δ≲3°$⁠, 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 $θ±δ$⁠. Our findings are corroborated by the fact that the ISM is not observed in samples with twist angles, which deviate by $δ>3°$ from $0°$ or $60°$⁠. This is expected since, in such incommensurate structures, with the possibility of Moiré-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.