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

Abstract

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 ( µ PL). 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 µ PL. 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mn>5</mml:mn> <mml:mo>°</mml:mo> </mml:msup> <mml:mo>≲</mml:mo> <mml:mo>|</mml:mo> <mml:mi>δ</mml:mi> <mml:mo>|</mml:mo> <mml:mo>≲</mml:mo> <mml:msup> <mml:mn>6</mml:mn> <mml:mo>°</mml:mo> </mml:msup> </mml:mrow> </mml:math> , exhibit no significant interlayer-exciton signals.

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