Exciton dynamics and annihilation in WS2 2D semiconductors

Abstract

We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS₂ two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 ± 37 ps, 401 ± 25 ps, and 332 ± 19 ps for WS₂ monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS₂ exhibits a strong excitation density-dependence, which can be described using an exciton–exciton annihilation (two-particle Auger recombination) model. The exciton–exciton annihilation rate for monolayered, bilayered, and trilayered WS₂ was determined to be 0.41 ± 0.02, (6.00 ± 1.09) × 10⁻³ and (1.88 ± 0.47) × 10⁻³ cm² s⁻¹, respectively. Notably, the exciton–exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton–exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton–exciton annihilation of indirect excitons.