Anomalous Temperature-Dependent Exciton-Phonon Coupling in Cesium Lead Bromide Perovskite Nanosheets

The role of exciton-phonon coupling in light emission in cesium lead bromide (CsPbBr3) nanosheets is investigated with combined photoluminescence spectroscopy and the multimode Brownian oscillator model. A good agreement between theory and experiment in the low temperature range of 5-40 K enables us to determine several key parameters, including the dimensionless Huang-Rhys factor characterizing the exciton-longitudinal-optical phonon coupling strength and the damping constant accounting for the phonon bath (quasi-continuous acoustic phonons) dissipation. It is found that the Huang-Rhys factor of the free excitons peculiarly tends to diminish upon increasing the temperature in the interested low temperature range. However, the damping constant shows a linear increase with temperature in the interested temperature range. These new findings may deepen the understanding of the exciton-phonon coupling in CsPbBr3 nanosheets and relevant solids.