Moire Phonons in Twisted Bilayer MoS2

The material choice, layer thickness, and twist angle widely enrich the family of van der Waals heterostructures (vdWHs), providing multiple degrees of freedom to engineer their optical and electronic properties. The moire patterns in vdWHs create a periodic potential for electrons and excitons to yield many interesting phenomena, such as Hofstadter butterfly spectrum and moire excitons. Here, in the as-grown/transferred twisted bilayer MoS2 (tBLMs), one of the simplest prototypes of vdWHs, we show that the periodic potentials of moire patterns also modify the properties of phonons of its monolayer MoS2 constituent to generate Raman modes related to moire phonons. These Raman modes correspond to zone-center phonons in tBLMs, which are folded from the off-center phonons in monolayer MoS2. However, the folded phonons related to crystallographic superlattices are not observed in the Raman spectra. By varying the twist angle, the moire phonons of tBLM can be exploited to map the phonon dispersions of the monolayer constituent. The lattice dynamics of the moire phonons are modulated by the patterned interlayer coupling resulting from periodic potential of moire patterns, as confirmed by density functional theory calculations. The Raman intensity related to moire phonons in all tBLMs are strongly enhanced when the excitation energy approaches the C exciton energy. This study can be extended to various vdWHs to deeply understand their Raman spectra, moire phonons, lattice dynamics, excitonic effects, and interlayer coupling.