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

We report about the investigation of twisted MoSe2 homo- and MoSe2–WSe2 heterobilayers by means of low-frequency Raman spectroscopy (LFRS) and low-temperature micro photoluminescence (µPL). In room-temperature LFRS experiments on both, twisted MoSe2 homobilayers and twisted MoSe2–WSe2 heterobilayers, we observe moire phonons, i.e. folded acoustic phonon modes due to the moire superlattice. In the heterobilayers, we can identify moire phonons of both materials, MoSe2 and WSe2. 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 moire 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 = 60compfn + δ) MoSe2–WSe2 heterobilayer. These investigations show that spatial regions, which can be identified to be atomically reconstructed (i.e. δ = 0compfn) 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 moire phonons are observed, i.e. which can be identified to be rigidly twisted by a misalignment angle in the range of $5^\circ \lesssim |\delta | \lesssim 6^\circ$, exhibit no significant interlayer-exciton signals.