Emergent Moire Phonons Due to Zone Folding in WSe2-WS2 Van der Waals Heterostructures

Bilayers of 2D materials offer opportunities for creating devices with tunable electronic, optical, and mechanical properties. In van der Waals heterostructures (vdWHs) where the constituent monolayers have different lattice constants, a moire superlattice forms with a length scale larger than the lattice constant of either constituent material regardless of twist angle. Here, we report the appearance of moire Raman modes from nearly aligned WSe2-WS2 vdWHs in the range of 240-260 cm(-1), which are absent in both monolayers and homobilayers of WSe2 and WS2 and in largely misaligned WSe2-WS2 vdWHs. Using first-principles calculations and geometric arguments, we show that these moire Raman modes are a consequence of the large moire length scale, which results in zone-folded phonon modes that are Raman active. These modes are sensitive to changes in twist angle, but notably, they occur at identical frequencies for a given small twist angle away from either the 0-degree or 60-degree aligned heterostructure. Our measurements also show a strong Raman intensity modulation in the frequency range of interest, with near 0 and near 60-degree vdWHs exhibiting a markedly different dependence on excitation energy. In near 0-degree aligned WSe2-WS2 vdWHs, a nearly complete suppression of both the moire Raman modes and the WSe(2)A(1g) Raman mode (similar to 250 cm(-1)) is observed when exciting with a 532 nm CW laser at room temperature. Temperature-dependent reflectance contrast measurements demonstrate the significant Raman intensity modulation arises from resonant Raman effects.

Automated summary

The appearance of moire Raman modes from nearly aligned WSe2-WS2 van der Waals heterostructures in a certain frequency range indicates sensitivity to twist angle and strong Raman intensity modulation dependent on excitation energy and temperature. These modes exhibit identical frequencies for a given small twist angle and are a consequence of the large moire length scale resulting in zone-folded phonon modes that are Raman active.