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

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${}_{\text{c}}^{\prime }$ points. The energy-momentum dispersions for the K${}_{\mathrm{v}}$-${\Lambda }_{\mathrm{c}}$, ${\Gamma }_{\mathrm{v}}$-K${}_{\mathrm{c}}$ and K${}_{v1}$-K${}_{\text{c}}^{\prime }$ 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${}_{v1}$-K${}_{\text{c}}^{\prime }$ transition follows a more linear dispersion relationship.

8 pages, 4 figures