Carrier Dynamics and Interactions for Bulk-Like Photoexcitation of Colloidal Indium Arsenide Quantum Dots

Abstract

The remarkable photonic and photochemical properties of colloidal quantum dots (QD) depend critically on the dynamics of carrier interactions and relaxation. Despite their importance, a quantitative experimental evaluation of these processes has proven elusive due to the inherent challenge of exactly separating single-exciton and multi-exciton dynamics, whose spectroscopic signatures overlap in time, spectrum, and excitation fluence. Here, we measure pump fluence-dependent absolute pump-probe transients of indium arsenide QDs, refreshing the sample using beam scanning to limit repetitive excitation. Focusing on the low fluence limit near the onset of bi-exciton formation, excitation conditions were precisely controlled and characterized by averaging Poisson-distributed excitation statistics over all three spatial dimensions of the pump and probe beam spatial profiles to determine the average excitation probability. A saturation model is developed to uniquely decompose the pump-probe signal into single-exciton and bi-exciton signals. This method harnesses the distinct pump-fluence scaling of absolute pump-probe signals from singly- and doubly-excited QDs without any assumptions regarding the relative timescales or amplitudes of single-exciton and bi-exciton signals. Probing in the bulk-like region of the QD absorption spectrum, the signal from bi-excitons is found to be 1.8 times the signal from single excitons at T = 0, consistent with the conventionally assumed factor of 2 within the 95% confidence intervals. The bi-exciton signal contains the same hot carrier relaxation dynamics as that from single excitons, but signal from a second exciton additionally exhibits a 26 ps exponential decay attributed to Auger recombination.