Atomic Displacements Drive Flat Band Formation and Lateral Electron and Hole Separation in Near-60° Twisted MoSe2/WSe2 Bilayers.

Transition metal dichalcogenide (TMD) bilayers with an interlayer twist exhibit a moiré superperiod, 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, an 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° twisted MoSe2/WSe2 bilayers, spectrally isolated valence and conduction band flat bands emerge at ∼3° 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 moiré 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 moment as well.