Abstract

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 moiré phonons, i.e. folded acoustic phonon modes due to the moiré superlattice. In the heterobilayers, we can identify moiré 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 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) MoSe2–WSe2 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 , exhibit no significant interlayer-exciton signals.

Highlights

  • One of the most fascinating properties of van der Waals materials is the possibility to stack different materials on top of each other with arbitrary, but well-controlled relative crystal orientations [1]

  • 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

  • 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

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Summary

Introduction

One of the most fascinating properties of van der Waals materials is the possibility to stack different materials on top of each other with arbitrary, but well-controlled relative crystal orientations [1]. In the past few years heterobilayer structures, like MoSe2–WSe2 heterobilayers, which exhibit a staggered type-II band-edge alignment [10,11,12] have been in the focus within this research avenue In these structures, an ultrafast charge separation of photoexcited charge carriers into the two constituent layers takes place, and interlayer excitons (ILEs) form [13,14,15,16]. This would only be true, if it would be the energetically most favorable state Very recently it has been shown experimentally via conductive atomic force microscopy [33] and transmission electron microscopy [34] that in TMDC heterobilayers [33, 34] and homobilayers [34] for small twist angle deviations δ from θ = 0◦ + δ or θ = 60◦ + δ, atomic reconstruction takes place, where the atoms in the two constituent layers arrange like in R- or H-type homobilayers. A noninvasive and simple method, by which samples can be characterized on a local scale with regard to atomic reconstruction and/or a precise determination of the local twist angle, i.e. the moiré periodicity, is highly desireable

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