Struttura della materia

Tales told by "spectator excitons" in nanocrystals

by Sandy Ruhman (The Hebrew University of Jerusalem, Institute of Chemistry)

Europe/Rome
Aula Conversi (Dip. di Fisica - Edificio G. Marconi)

Aula Conversi

Dip. di Fisica - Edificio G. Marconi

Description

Paradigms for interpretation of femtosecond pump-probe experiments in nanocrystals concentrate on variations to their prominent band edge absorption band. They assume that excited holes and electrons cool within ~1 ps to the lowest available levels in the valence and conduction manifolds, and that the resulting bleach of the 1S1S absorption band progresses linearly with additional excitations until the underlying states are filled. Spectral shifts due to bi-exciton interactions on order 10 meV to absorption of electronically excited particles complete this conceptual framework. To test these, spectator exciton experiments follow stepwise changes in nanocrystal absorption with the accumulated number of excitons by comparing pump-probe spectra in pristine nanocrystal samples with that from samples excited with a progressive number of cold excitons. Case studies of using this approach will be presented for CdSe, PbS, and CsPbBr3 nanocrystals, in all showing that these paradigms are not adhered to an any of these materials. For instance, 50% of hot electrons are blocked from cooling directly to the band edge in CdSe nano-dots already populated with a single cold exciton due to spin orientation conflicts. Furthermore, contrary to expectations, PbS nano-dots exhibit a show no contributions of stimulated emission to their pump-probe signals, and that contrary to the attractive nature of exciton-exciton interactions in most semiconductor nanocrystals, in CsPbBr3 they are repulsive and unusually strong. Together these case studies demonstrate the potential of ths spectator exciton approach in deepening our understanding of nanocrystal photophysics.

Organised by

Tullio Scopigno