Exposing the trion's fine structure by controlling the carrier concentration in hBN-encapsulated MoS$_2$

TL;DR

This study demonstrates how tuning the carrier concentration in hBN-encapsulated MoS$_2$ monolayers reveals detailed excitonic fine structures, including three distinct negatively charged excitons, through advanced optical spectroscopy.

Contribution

It introduces a method to control carrier concentration via hBN thickness modification, enabling detailed observation of MoS$_2$ excitonic states not previously resolved.

Findings

Carrier concentration can be tuned by changing hBN thickness.

Resolved three distinct negatively charged excitons in MoS$_2$.

Observed subtle optical and spin-valley properties of excitonic complexes.

Abstract

Atomically thin materials, like semiconducting transition metal dichalcogenides, are highly sensitive to the environment. This opens up an opportunity to externally control their properties by changing their surroundings. In this work, high-quality van der Waals heterostructures assembled from hBN-encapsulated monolayer MoS$_2$ are studied with the aid of photoluminescence, photoluminescence excitation, and reflectance contrast experiments. We demonstrate that carrier concentration in MoS$_2$ monolayers, arising from charge transfer from impurities in the substrate, can be significantly tuned within one order of magnitude by the modification of the bottom hBN flake thickness. The studied structures, characterized by spectral lines approaching the narrow homogeneously broadened limit enabled observations of subtle optical and spin-valley properties of excitonic complexes. Our results allowed us to resolve three optically-active negatively charged excitons in MoS$_2$ monolayers, which are assigned to the intravalley singlet, intervalley singlet, and intervalley triplet states.