Moiré Phonons in Twisted Bilayer MoS₂

Abstract

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 moiré patterns in vdWHs create a periodic potential for electrons and excitons to yield many interesting phenomena, such as Hofstadter butterfly spectrum and moiré excitons. Here, in the as-grown/transferred twisted bilayer MoS (tBLMs), one of the simplest prototypes of vdWHs, we show that the periodic potentials of moiré patterns also modify the properties of phonons of its monolayer MoS constituent to generate Raman modes related to moiré phonons. These Raman modes correspond to zone-center phonons in tBLMs, which are folded from the off-center phonons in monolayer MoS. However, the folded phonons related to crystallographic superlattices are not observed in the Raman spectra. By varying the twist angle, the moiré phonons of tBLM can be exploited to map the phonon dispersions of the monolayer constituent. The lattice dynamics of the moiré phonons are modulated by the patterned interlayer coupling resulting from periodic potential of moiré patterns, as confirmed by density functional theory calculations. The Raman intensity related to moiré 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, moiré phonons, lattice dynamics, excitonic effects, and interlayer coupling.

Sodium‐Mediated Epitaxial Growth of 2D Ultrathin Sb2Se3 Flakes for Broadband Photodetection

“…As evidenced in Figure 1e, the maximum lateral size of as‐synthesized Sb 2 Se 3 crystals can reach up to ≈24 µm. It is particularly noteworthy that the growth of second layer is observed on top of the first layer, revealing a typical layered growth mode as demonstrated for other 2D materials, such as MoS 2 , MoSe 2 , WS 2 , WSe 2 42–47. The previous reports have revealed that the reduced surface mobility on the top of the flakes may play roles in the layered growth mode 42,43.…”

Precise control of the interlayer twist angle in large scale MoS2 homostructures

“…4a). This Raman mode can be assigned as Moiré phonon related with the A 1g phonon branch (here we denote it as FA 1g ), which stems from the offcenter phonons of monolayer linked with the lattice vectors of Moiré reciprocal space 42 . FA 1g peaks exhibit a sine-like behavior with twist angle (Fig.…”

Hybridized intervalley moiré excitons and flat bands in twisted WSe2 bilayers

“…Here, an exciton in state (ζ′, ν′) scatters to a virtual state within the light cone (Q ≈ 0) via emission or absorption of a phonon and subsequently emits a photon with the remaining energy. The exciton-phonon scattering strength is determined by the effective matrix D, [48][49][50] which is integrated over all possible moiré Umklapp processes, 51 cf. the ESI.…”