Dark-state impact on the exciton recombination of WS2 monolayers as revealed by multi-time-scale pump-and-probe spectroscopy

TL;DR

This study investigates how long-lived dark states and Auger-type scattering affect exciton recombination and luminescence efficiency in WS2 monolayers, revealing their impact over multiple time scales through advanced pump-and-probe spectroscopy.

Contribution

It provides new insights into the role of dark states and Auger scattering in exciton dynamics in WS2 monolayers, with detailed time-resolved measurements.

Findings

Dark states have microsecond lifetimes affecting luminescence.

Auger scattering occurs between excitons and dark states.

Luminescence quenching linked to dark state dynamics.

Abstract

The luminescence yield of transition metal dichalcogenide monolayers frequently suffers from the formation of long-lived dark states, which include excitons with intervalley charge carriers, spin-forbidden transitions, and a large center-of-mass momentum located outside the light cone of dispersion relations. Efficient relaxation from bright exciton states to dark states suppresses the quantum yield of photon emission. In addition, the radiative recombination of excitons is heavily influenced by Auger-type exciton-exciton scattering, which yields another nonradiative relaxation channel at room temperature. Here, we show that Auger-type scattering is promoted not only between (bright) excitons but also between excitons and long-lived dark states. We studied the luminescence dynamics of monolayer WS2 capped with hexagonal BN over broad time ranges of picoseconds to milliseconds using carefully designed pump-and-probe techniques. We observed that luminescence quenching associated with Auger-type scattering occurs on 1-100 microsecond time scales, which thus correspond to the lifetimes of the relevant dark states. The broad distribution of the measured lifetimes implies the impact of various types of long-lived states on the exciton annihilation process.