Atomic displacements drive flat band formation and lateral electron and hole separation in near-60 degree twisted MoSe2/WSe2 bilayers

TL;DR

This study uses ab initio calculations to show that near-60 degree twisted MoSe2/WSe2 bilayers develop flat electronic bands and in-plane electron-hole separation due to atomic displacements, affecting exciton properties.

Contribution

It reveals how atomic displacements in near-60 degree twisted TMD bilayers induce flat bands and lateral electron-hole separation, a novel insight into moire superlattice effects.

Findings

Flat bands emerge at ~3 degree twist in MoSe2/WSe2 bilayers.

Atomic displacements create a polarization gradient and confining potential.

Electrons and holes are laterally separated, affecting exciton dipole moments.

Abstract

Transition metal dichalcogenide (TMD) bilayers with an interlayer twist exhibit a moire super-period, whose effects can manifest in both structural and electronic properties. Atomic displacements can lead to reconstruction into domains of aligned stacking, and flat bands can form that may host correlated electron states. In heterobilayers angular mismatch is nearly unavoidable, so understanding the consequences of an interlayer twist is essential. Using ab initio density functional theory, we find that in near-60 degree twisted MoSe2/WSe2 bilayers valence and conduction band flat bands emerge at ~3 degree twist. Despite relatively limited reconstruction at these angles, atomic displacement creates a polarization gradient that forms a confining potential, localizing and laterally separating electrons and holes within the moire supercell. Excitons formed from flat band electrons and holes should therefore have not only the out-of-plane dipole moment familiar from MoSe2/WSe2 interlayer excitons, but an in-plane dipole moment as well.