Anomalous emission behavior of excitons at low temperature in monolayer WS 2

We report on the anomalous emission behavior of excitons ( X ) in monolayer WS 2 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.