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

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.