Testing the fate of nascent holes in CdSe nanocrystals with sub-10 fs pump–probe spectroscopy

Abstract

Numerous studies have reported that transient absorption spectra in core CdSe nanocrystals do not register state filling in 1S_h, an absence which has profound consequences in light-emitting applications. It has been assigned alternatively to rapid hole trapping, or to distribution over a dense degenerate valence band manifold which includes dark states. Here we attempt to observe early contributions of nascent holes to the bleaching of the band edge exciton transition by conducting 1S_e1S_h pump–1S_e1S_h probe spectroscopy with <10 fs laser pulses on organic ligand passivated CdSe crystals. The results show no rapidly hole-state filling effects in transient absorption measurements even at the earliest delay, despite the use of pulses which are capable of resolving all dissipation mechanisms reflected in the homogeneous 1S_e1S_h bandwidth. This proves that neither hole trapping nor rapid redistribution of the nascent hole over energetically available valence band states can explain the absence of hole contributions to band edge bleaching, calling for a mechanistic review of this phenomenon.