Well-controlled modification of emission kinetics of colloidal semiconductor quantum wells

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Bilkent University
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Colloidal quantum wells (CQWs) belong to an important quasi-2-dimensional sub-family of semiconductor nanocrystals. Thanks to their uniquely tight quantum confinement of only few monolayers extending across their vertical thickness, CQWs possess giant oscillator strength, substantially increasing their absorption cross-section along with their large lateral size expanded over tens to hundreds of nm’s on one side. These together make CQWs excellent candidates for light-harvesting applications. In this thesis, to utilize CQWs’ superior light-harvesting capability, we investigated the alteration and control of photoluminescence de-cay lifetimes of the CQWs in a variety of hybrid absorbing systems. In particular, we proposed and demonstrated the nonradiative energy transfer from strongly-absorbing CQWs to indirect-bandgap bulk semiconductors as weak ab-sorbers, e.g., bulk silicon. To this end, we systematically studied and showed the well-controlled modification of the emission kinetics of these CQWs that are self-assembled into a single-layer all-face-down oriented ensemble in the vicinity of silicon with a fine-tuned dielectric separator (of thickness d). We found the Förster resonance energy transfer (FRET) to be the chief underlying mechanism for the observed modifications in the emission kinetics of the CQWs, which we further modeled and explained using full electromagnetic solutions. We showed that the rate of the resultant energy transfer from these CQWs to the bulk silicon scales slowly with d−1 in space. Finite element method (FEM) based computation revealed that this inverse relationship is caused by the delocalization of the electric field in the CQW layer and the substrate due to strong in-plane dipoles present in the CQWs. To address the shortcomings of silicon-based photo-detecting platforms, in a proof-of-concept hybrid device we fabricated, we experimentally demonstrated that the photosensitization using such a single-layer CQW-film enhances the photocurrent collected in the silicon by up to 3-folds. The findings in this thesis are expected to help further exploit the amazing light-harvesting potential of CQWs in optoelectronic applications.

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Semiconductor nanocrystals, Colloidal quantum wells, Nanoplatelets, Energy transfer
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