Exciton storage in CdSe/CdS tetrapod semiconductor nanocrystals: electric field effects on exciton and multiexciton states

CdSe/CdS nanocrystal tetrapods are interesting building blocks for excitonic circuits, where the flow of excitation energy is gated by an external stimulus. The physical morphology of the nanoparticle, along with the electronic structure, which favors electron delocalization between the two semiconductors, suggests that all orientations of a particle relative to an external electric field will allow for excitons to be dissociated, stored, and released at a later time. While this approach, in principle, works, and fluorescence quenching of over 95% can be achieved electrically, we find that discrete trap states within the CdS are required to dissociate and store the exciton. These states are rapidly filled up with increasing excitation density, leading to a dramatic reduction in quenching efficiency. Charge separation is not instantaneous on the CdS excitonic antennae in which light absorption occurs, but arises from the relaxed exciton following hole localization in the core. Consequently, whereas strong electromodulation of the core exciton is observed, the core multiexciton and the CdS arm exciton are not affected by an external electric field.