Moiré phonons in twisted MoSe₂–WSe₂ heterobilayers and their correlation with interlayer excitons

Abstract

We report about the investigation of twisted MoSe 2 homo-and MoSe 2 -WSe 2 heterobilayers by means of low-frequency Raman spectroscopy (LFRS) and low-temperature micro photoluminescence (µPL). In room-temperature LFRS experiments on both, twisted MoSe 2 homobilayers and twisted MoSe 2 -WSe 2 heterobilayers, we observe moiré phonons, i.e. folded acoustic phonon modes due to the moiré superlattice. In the heterobilayers, we can identify moiré phonons of both materials, MoSe 2 and WSe 2 . While the twist angles for the homobilayers are relatively precisely known from the applied tear-and-stack preparation method, the twist angles of the heterobilayers have to be determined via second-harmonic-generation microscopy on monolayer regions of the samples, which has significant uncertainties. We show that by the moiré phonons of the heterobilayers, the relative twist angles can be determined on a local scale with much higher precision. We apply our technique for the investigation of a large area H-type (twist angle θ = 60 • + δ) MoSe 2 -WSe 2 heterobilayer. These investigations show that spatial regions, which can be identified to be atomically reconstructed (i.e. δ = 0 • ) by the observation of an interlayer shear mode in LFRS experiments, exhibit a strong, momentum-allowed interlayer-exciton signal in low-temperature µPL. On the contrary, regions, where moiré phonons are observed, i.e. which can be identified to be rigidly twisted by a misalignment angle in the range of 5 • ≲ |δ| ≲ 6 • , exhibit no significant interlayer-exciton signals.

Interlayer exciton valley polarization dynamics in large magnetic fields

“…When the interlayer twist angle gets small enough, it becomes energetically favorable for the HS to slightly distort the constituent lattices, so that domains are formed in which there is a well-defined interlayer atomic registry. This so-called atomic reconstruction has been observed in recent experiments [26][27][28] , and regions of atomic reconstruction may coexist with regions with a moiré-type potential modulation within a single heterostructure sample 29 .…”

Two-color Kerr microscopy of two-dimensional materials with sub-picosecond time resolution

“…This provides a significant limitation especially in the field of 2D materials, where samples are often prepared in a nondeterministic way by exfoliation from bulk crystals, resulting in flakes with spatially inhomogeneous number of layers and typical lengths scales of a few microns. When two or more 2D materials are to be combined into a van der Waals (vdW) heterostructure (HS), this issue is exacerbated, as interlayer contact in a vdW HS is seldom homogeneous over more than a few square microns, even if the HS area itself can be made larger, and the relative alignment of the adjacent layers (twist angle) also varies locally [22]. Additionally, the use of nondegenerate pump and probe energies may give access to relaxation or up-conversion dynamics [23].…”

Chiral valley phonons and flat phonon bands in moiré materials

“…Our analysis shows that the polarization vectors of chiral phonons near 0 • and 60 • are qualitatively different from the those of the monolayer phonons. This suggests that commonly used approaches for calculating phonon properties in twisted bilayers that are based on a zone folding procedure of monolayer results are unreliable [63,64].…”