# Accessing high optical quality of MoS2 monolayers grown by chemical vapor deposition

## TL;DR

This study demonstrates that CVD-grown MoS2 monolayers can achieve optical qualities comparable to exfoliated samples by removing substrate effects and encapsulating in hBN, enabling high-quality optical and valley physics studies.

## Contribution

The paper shows that encapsulating CVD-grown MoS2 monolayers in hBN reduces defects and dielectric disorder, resulting in optical quality comparable to exfoliated samples, and enables valley coherence and polarization.

## Key findings

- Optical transition linewidth of 5 meV at 4 K.
- High optical quality comparable to exfoliated samples.
- Observation of valley coherence and polarization.

## Abstract

Chemical vapor deposition (CVD) allows growing transition metal dichalcogenides (TMDs) over large surface areas on inexpensive substrates. In this work, we correlate the structural quality of CVD grown MoS$_2$ monolayers (MLs) on SiO$_2$/Si wafers studied by high-resolution transmission electron microscopy (HRTEM) with high optical quality revealed in optical emission and absorption from cryogenic to ambient temperatures. We determine a defect concentration of the order of 10$^{13}$ cm$^{-2}$ for our samples with HRTEM. To have access to the intrinsic optical quality of the MLs, we remove the MLs from the SiO$_2$ growth substrate and encapsulate them in hBN flakes with low defect density, to reduce the detrimental impact of dielectric disorder. We show optical transition linewidth of 5 meV at low temperature (T=4 K) for the free excitons in emission and absorption. This is comparable to the best ML samples obtained by mechanical exfoliation of bulk material. The CVD grown MoS$_2$ ML photoluminescence is dominated by free excitons and not defects even at low temperature. High optical quality of the samples is further confirmed by the observation of excited exciton states of the Rydberg series. We optically generate valley coherence and valley polarization in our CVD grown MoS$_2$ layers, showing the possibility for studying spin and valley physics in CVD samples of large surface area.