The temperature influence on the brightening of neutral and charged dark excitons in WSe$_2$ monolayer

TL;DR

This study investigates how temperature affects the optical activation and emission intensities of dark and bright excitons in WSe$_2$ monolayers, revealing significant quenching of dark states and increased bright emissions with rising temperature.

Contribution

It provides new insights into the temperature-dependent behavior of dark and bright excitons in WSe$_2$ monolayers, highlighting the role of exciton-phonon interactions and energy splittings.

Findings

Dark exciton emissions decrease by 3-4 orders of magnitude from 4.2 K to 100 K.

Bright exciton emissions increase by about two orders of magnitude over the same temperature range.

Energy splittings between dark and bright excitons vary with temperature, influenced by exciton-phonon coupling.

Abstract

The optically dark states play an important role in the electronic and optical properties of monolayers (MLs) of semiconducting transition metal dichalcogenides. The effect of temperature on the in-plane-field activation of the neutral and charged dark excitons is investigated in a WSe$_2$ ML encapsulated in hexagonal BN flakes. The brightening rates of the neutral dark (X$^D$) and grey (X$^G$) excitons and the negative dark trion (T$^D$) differ substantially at a particular temperature. More importantly, they vanish considerably by about 3 -- 4 orders of magnitude with the temperature increased from 4.2 K to 100 K. The quenching of the dark-related emissions is accompanied by the two-order-of-magnitude increase in the emissions of their neutral bright counterparts, $i.e.$ neutral bright exciton (X$^B$) and spin-singlet (T$^S$) and spin-triplet (T$^T$) negative trions, due to the thermal activations of dark states. Furthermore, the energy splittings between the dark X$^D$ and T$^D$ complexes and the corresponding bright X$^B$, T$^S$, and T$^T$ ones vary with temperature rises from 4.2 K to 100 K. This can be explained in terms of the different exciton-phonon couplings for the bright and dark excitons stemming from their distinct symmetry properties.