Anomalous emission behavior of excitons at low temperature in monolayer WS2

Abstract

We report on the anomalous emission behavior of excitons (X) in monolayer WS2 using temperature dependent photoluminescence spectroscopy. In general, PL emission from excitons enhances with decreasing temperature due to suppression of phonon mediated non-radiative transitions. Here, we observe that excitonic PL is temperature independent, although with decreasing temperature, the emission intensity for trion (X −) and biexciton or defect-bound excitons (XX/L 1) increases up to 123 K and then decreases. Analysis of experimental data with a model derived from the Boltzmann distribution and Saha equation reveal conversion of excitons into trions, biexcitons or defect-bound excitons and an increase of spin forbidden dark state with reduction in temperature. These findings could provide better strategies for designing future quantum devices.