Coherent Multidimensional Spectroscopy Reveals Hot Exciton Cooling Landscapes in CsPbBr Quantum Dots.

Hot exciton relaxation dynamics is one of the main processes in quantum dots (QD), conferring their functions in optoelectronic devices spanning photovoltaics and solar fuel generation to light emitting diodes, lasers, and quantum light sources. The challenge has been to monitor energy relaxation dynamics in parallel with resolution of excitation or excess energy. Here, we exploit the unique capacity of Coherent Multi-Dimensional Spectroscopy (CMDS) to provide the first observation of the hot exciton cooling landscape of a large size range of CsPbBr lead halide perovskite QD, notable for their impact on optoelectronic devices, as well as their strong and unique exciton-lattice coupling. The CMDS data reveal that the hot exciton relaxation landscape is a complex function of the energy. Ab initio quantum dynamics simulations rationalize the observed behavior through energy dependent nonadiabatic exciton-phonon coupling. This first observation of cooling landscapes in QD suggests that materials science that either accelerates or slows hot exciton cooling can better be understood as a landscape to optimize for applications.