Solving Quantum-Dot Excitonic Riddles with Absolute Pump–Probe Spectroscopy

Abstract

Absolute absorption changes in molecular flash photolysis experiments are routinely translated into molar extinction coefficients and oscillator strengths of reactive intermediates. These direct quantum chemical investigation and allow precise concentration readings in later experiments. In this Perspective we show how a similar approach can deliver crucial information for interpreting transient absorption spectra in colloidal semiconductor quantum dots. The intrinsic complexity of such samples stemming from the inhomogeneity of particle size, shape, and surface chemistry poses unique challenges to mechanistic assignment of ultrafast pump–probe measurements. We will describe applications of this approach to elucidate the photophysics of quantum confined nanocrystals made of various semiconducting materials. These case studies demonstrate how, faced with conflicting interpretations, it has pointed in the right direction in assessing single and multiple exciton generation and relaxation, in searches for ultrafast carrier trapping and scavenging, and in tests of band edge level structure and state degeneracies.