Off-State Quantum Yields in the Presence of Surface Trap States in CdSe Nanocrystals: The Inadequacy of the Charging Model To Explain Blinking

In the charging model, the fluorescence intermittency observed in semiconductor nanocrystals is explained in terms of an Auger-mediated quenching of the photoluminescence in the presence of a single additional charge delocalized in the dot core. The validity of this description has however been challenged by recent experimental results. A realistic scenario that is often proposed to explain the origin of the extra charge portrays the latter as the result of photogeneration followed by trapping of one of the particles at the surface. The effects of the presence of a surface-localized charge were however not considered in any of the models that led to the challenge. This study addresses this fundamental issue. Using ab initio-quality wave functions within the semiempirical pseudopotential approach, the predictions of the charging model are here thoroughly tested for all possible configurations that could exhibit efficient Auger recombination and, therefore, be responsible for the quenching of the PL in the off-state. The results, although confirming the conclusion that the presently accepted charging model is inadequate to explain blinking in NCs, suggest multiple charging as a possible origin of off-states with very low quantum yield. A modification to one of the recently proposed phenomenological blinking models is suggested to account for this feature.