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F1.10 - Small Bright Charged Colloidal Quantum Dots 
April 22, 2014   11:45am - 12:00pm

Charged or multiply-excited colloidal quantum dots (QDs) have typically low emission because of fast nonradiative Auger process and this impedes the development of QD based lasers, light emitting diodes, and photovoltaics. However, in the last few years, strongly reduced Auger process has been observed in ultra-thick-shell CdSe/CdS nanocrystals and dot-in-rod structures with large particle sizes (~20 nm) where the electron confinement was minimal. This has been qualitatively attributed to various mechanisms, surface effects or interfacial alloying, but it remained very unclear whether Auger suppression could be achieved in small colloidal QDs (~5 nm) where the quantum confinement effect is preserved. We therefore investigated the photoluminescence of charged colloidal dots with varying core/shell structures in order to design and tune the wavefunctions. With two electrons and one hole, the negative trion is arguably the simplest system to study the Auger process in colloidal QDs and, using confocal microscopy and controlled charging, we could investigate single dots in a defined -1 charge state at room temperature. In summary, type I CdSe/ZnS exhibit very short-lived trions, while with the weakly type II CdSe/CdS, the trion lifetime lengthens slowly with increasing shell thickness in accord with other results. However, the first study of the strongly type-II CdTe/CdSe QDs revealed that a long trion lifetime is obtained for very small particles and, uncharacteristically, that the trion lifetime is maximum (~4.5 ns) for an intermediate shell thickness. This leads to the smallest particles (~4.5 nm) with the brightest negative trion to date. At the single dot level, the negative trion exhibit non-blinking behavior and can reach almost the same brightness as the single exciton.The unprecedented Auger suppression in such small dots is a significant departure from prior results with much larger nanoparticles. This leads us to a new proposal, whereas the optimum shell thickness in the type II structure is when the electron energy is exactly at the bottom of the conduction band of the core. Therefore, the Bloch functions for the electron and hole would have different central symmetries, leading to the vanishment of the transition matrix in the Auger process. We propose that this is also the main mechanism for the Auger suppression in the weakly type II CdSe/CdS, those needing a much larger shell to achieve the same effect. This study is a significant improvement in the understanding of multiexciton recombination in colloidal quantum dots, and it may impact the design of widely tunable small bright charged QDs and help develop efficient light-emitting devices.

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