Low-frequency Raman scattering in WSe 2 - MoSe 2 heterobilayers: Evidence for atomic reconstruction

We investigate WSe2-MoSe2 heterobilayers with different twist angles theta +/- delta between the two layers by low-frequency Raman scattering. In sufficiently aligned samples with theta = 0 degrees or theta = 60 degrees and delta less than or similar to 3 degrees, 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 18cm(-1) and 17cm(-1) for H-type and R-type reconstructions, respectively, independent of the exact value of theta +/- delta. Our findings are corroborated by the fact that the ISM is not observed in samples with twist angles, which deviate by delta > 3 degrees from 0 degrees or 60 degrees. This is expected since, in such incommensurate structures, with the possibility of Moire-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.