Intrinsic control of interlayer exciton generation rate in van der Waals materials via Janus layers

TL;DR

This study shows how Janus layers in van der Waals heterostructures can be engineered to control interlayer exciton generation, enhancing charge separation efficiency through exciton-phonon interactions.

Contribution

It introduces a method to tune interlayer exciton energies in MoS2-based heterobilayers using Janus layers, revealing new pathways for efficient exciton generation.

Findings

Janus layers modify electronic band alignments via intrinsic electric fields.

Exciton-phonon coupling enables interlayer exciton generation upon light absorption.

Resonance between exciton energy separation and phonon modes enhances exciton formation.

Abstract

We demonstrate the possibility of engineering the optical properties of transition metal dichalcogenide heterobilayers when one of the constitutive layers has a Janus structure. This has important consequences for the charge separation efficiency. We investigate different MoS$_2$@Janus layer combinations using first-principles methods including electron-hole interactions (excitons) and exciton-phonon coupling. The direction of the intrinsic electric field from the Janus layer modifies the electronic band alignments and, consequently, the energy separation between interlayer exciton states -- which usually have a very low oscillator strength and hence are almost dark in absorption -- and bright in-plane excitons. We find that in-plane lattice vibrations strongly couple the two states, so that exciton-phonon scattering may be a viable generation mechanism for interlayer excitons upon light absorption. In particular, in the case of MoS$_2$@WSSe, the energy separation of the low-lying interlayer exciton from the in-plane exciton is resonant with the transverse optical phonon modes (40 meV). We thus identify this heterobilayer as a prime candidate for efficient electron-hole pair generation with efficient charge carrier separation.