Low-frequency Raman scattering in WSe2−MoSe2 heterobilayers: Evidence for atomic reconstruction
Johannes Höller, Sebastián Meier, Michael Kempf, Philipp Nagler, Kenji Watanabe, Takashi Taniguchi, Tobias Korn, Christian Schüller
Abstract
The great appeal of van-der-Waals materials is the possibility to fabricate artificial multilayer structures, consisting of different materials, with arbitrary but well controlled relative crystal orientations. We investigate WSe2-MoSe2 heterobilayers with different twist angles q ± d between the two layers, by low-frequency Raman scattering. In sufficiently aligned samples with q = 0°, or q = 60° and d ≤ 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. Our findings are corroborated by the fact that the ISM is not observed in samples with twist angles, which deviate by d > 3° from 0° or 60°. 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.