Spectator Exciton Effects on Nanocrystal Photophysics II: PbS

The spectator exciton method follows stepwise changes in a nanocrystal's absorption with the accumulated number of excitons. It involves comparison of pump-probe spectra in pristine samples with that from nanocrystals excited with a progressive number of cold excitons. Using this approach, 50% of hot electrons were shown to be blocked from cooling directly to the band edge in CdSe nanodots already excited with a single cold exciton due to electron spin orientation conflicts. Similar experiments which are reported here in quantum-confined PbS nanodots have not detected spin-related relaxation blockades. Instead, a progressive broadening of the 1S_e1S_habsorption band with loading of cold excitons is manifest in the difference spectra, demonstrating the inadequacy of any single-probe wavelength to quantify band-edge state filling. Comparing data for single and double spectator states proves that the sub band gap induced absorption characteristic of hot exciton states in all quantum dots is not the low-energy half of biexciton shifting to the 1S_e1S_habsorption band. Instead, it is part of a broad induced absorption characteristic of hot excitons regardless of the spectator count. Finally, the unique capability of spectator excitons (SXs) to detect contributions of stimulated emission to pump-probe signals is demonstrated in singly excited PbS nanocrystals. Contrary to recent literature reports of such contributions, none are observed. These outcomes call for revisions to the conventional interpretation of excited state spectroscopy in quantum confined nanocrystals.