Optical spectroscopy of excited exciton states in MoS2 monolayers in van der Waals heterostructures

TL;DR

This study investigates excited exciton states in MoS2 monolayers encapsulated in hBN, revealing their energies and oscillator strengths through optical spectroscopy and theoretical modeling, leading to an estimated exciton binding energy of 220 meV.

Contribution

It provides detailed experimental and theoretical analysis of excited exciton states in MoS2 monolayers in van der Waals heterostructures, highlighting deviations from ideal models.

Findings

Excited A-exciton states identified in encapsulated MoS2 monolayers.

Estimated exciton binding energy of approximately 220 meV.

Theoretical models reproduce energy separations and oscillator strengths.

Abstract

The optical properties of MoS2 monolayers are dominated by excitons, but for spectrally broad optical transitions in monolayers exfoliated directly onto SiO2 substrates detailed information on excited exciton states is inaccessible. Encapsulation in hexagonal boron nitride (hBN) allows approaching the homogenous exciton linewidth, but interferences in the van der Waals heterostructures make direct comparison between transitions in optical spectra with different oscillator strength more challenging. Here we reveal in reflectivity and in photoluminescence excitation spectroscopy the presence of excited states of the A-exciton in MoS2 monolayers encapsulated in hBN layers of calibrated thickness, allowing to extrapolate an exciton binding energy of about 220 meV. We theoretically reproduce the energy separations and oscillator strengths measured in reflectivity by combining the exciton resonances calculated for a screened two-dimensional Coulomb potential with transfer matrix calculations of the reflectivity for the van der Waals structure. Our analysis shows a very different evolution of the exciton oscillator strength with principal quantum number for the screened Coulomb potential as compared to the ideal two-dimensional hydrogen model.