Investigation of indirect excitons in bulk $2H$-MoS$_2$ using transmission electron energy-loss spectroscopy

TL;DR

This study uses transmission electron energy-loss spectroscopy to investigate indirect excitons in bulk 2H-MoS2, revealing their energy-momentum dispersions, effective masses, and transition characteristics along specific Brillouin zone directions.

Contribution

It provides the first detailed measurement of indirect exciton dispersions and effective masses in bulk 2H-MoS2 using electron energy-loss spectroscopy.

Findings

Identification of multiple indirect excitons between specific Brillouin zone points.

Quadratic dispersion of certain excitons enabling effective mass calculation.

Linear dispersion observed for the K_v1-K'_c transition.

Abstract

We have investigated indirect excitons in bulk $2H$-MoS$_2$ using transmission electron energy-loss spectroscopy. The electron energy-loss spectra were measured for various momentum transfer values parallel to the $\Gamma$K and $\Gamma$M directions of the Brillouin zone. The results allowed the identification of the indirect excitons between the valence band K$_{\mathrm{v}}$ and conduction band $\Lambda_{\mathrm{c}}$ points, the $\Gamma_{\mathrm{v}}$ and K$_{\mathrm{c}}$ points as well as adjacent K$_{\mathrm{v}}$ and K$^{\prime}_\textrm{c}$ points. The energy-momentum dispersions for the K$_{\mathrm{v}}$-$\Lambda_{\mathrm{c}}$, $\Gamma_{\mathrm{v}}$-K$_{\mathrm{c}}$ and K$_{\mathrm{v1}}$-K$^{\prime}_\textrm{c}$ excitons along the $\Gamma$K line are presented. The former two transitions exhibit a quadratic dispersion which allowed calculating their effective exciton masses based on the effective mass approximation. The K$_\mathrm{v1}$-K$^{\prime}_\textrm{c}$ transition follows a more linear dispersion relationship.