Spectator excitons teach new things about nanocrystal photophysics

Much of what is known about the dynamics of excitons in nanocrystals (NC) has been learned by following their ultrafast changes in absorption after excitation. Femtosecond pump-probe experiments have been interpreted in terms of two main mechanisms underlying these changes, state filling whereby Pauli exclusion blocks the states responsible for the ultra-intense band edge absorption feature, or bi-exciton shifting which changes the resonance energy of the probe pulse due to presence of pump induced excitations. In this talk we will describe a new approach involving “spectator excitons” to test these accepted views. It consists of comparing pump-probe experiments on pristine samples, with equivalent scans conducted on the same sample after it has been saturated in cold mono-excitons. We will show how this method has uncovered previously unrecognized spin blockades in the relaxation of hot multi-exciton states in iconic CdSe NCs, has proven that the assumed linearity in the band edge bleach with the number of absorbed photons in lead Chalcogenide NCs is incorrect, and questions the mechanisms behind unusual pump-probe spectra obtained in quantum confined perovskite NCs.