# Defect healing and charge transfer mediated valley polarization in MoS$_2$/MoSe$_2$/MoS$_2$ trilayer van der Waals heterostructures

## TL;DR

This study demonstrates that sandwiching CVD-grown MoSe2 in MoS2/MoSe2/MoS2 trilayers heals defects and enhances optical quality through defect healing and charge transfer, enabling scalable production of high-quality TMDCs.

## Contribution

It reveals defect healing via S atom donation in trilayer heterostructures and shows charge-transfer induced valley polarization without resonant excitation.

## Key findings

- Optical quality of CVD MoSe2 is restored in trilayer structures.
- Defect healing occurs through S atom donation from MoS2 layers.
- Charge transfer induces valley polarization in MoSe2.

## Abstract

Monolayer transition metal dichalcogenides (TMDC) grown by chemical vapor deposition (CVD) are plagued by a significantly lower optical quality compared to exfoliated TMDC. In this work we show that the optical quality of CVD-grown MoSe$_2$ is completely recovered if the material is sandwiched in MoS$_2$/MoSe$_2$/MoS$_2$ trilayer van der Waals heterostructures. We show by means of density-functional theory that this remarkable and unexpected result is due to defect healing: S atoms of the more reactive MoS$_2$ layers are donated to heal Se vacancy defects in the middle MoSe$_2$ layer. In addition, the trilayer structure exhibits a considerable charge-transfer mediated valley polarization of MoSe$_2$ without the need for resonant excitation. Our fabrication approach, relying solely on simple flake transfer technique, paves the way for the scalable production of large-area TMDC materials with excellent optical quality.