Publication date: 5th November 2025
Quantum dots (QDs) are a class of semiconductor nanocrystals that exhibit quantum confinement effects. When they are excited by light, electron-hole pairs that are bounded by Coulombic interactions are generated, known as excitons. Since the spatial dimensions of QDs are smaller than the exciton Bohr radius of the material, the exciton wavefunctions are theoretically confined within the particles, forming localized Frenkel excitons. QDs can be categorized into strong, moderate, and weak confinement regimes based on the difference between the particle size and the exciton Bohr radius. This theoretical framework holds true when a single QD is completed isolated from other QDs or exciton acceptors, for example when they are placed in a vacuum, dispersed in solution (where QDs are at low concentration), or capped by shell materials with high-energy-barriers. However, in many practical applications, QDs are assembled into solid-state films. Compared to isolated QDs, the distance between them in films is reduced to the nanoscale or even shorter, allowing the exciton wavefunctions of excited QDs to leak into neighboring particles, leading to the formation of delocalized Wannier excitons. Unlike localized excitons, delocalized excitons can move more easily within QD thin films and exhibit more complex dynamics.
This presentation will introduce the visualization of early-time exciton diffusion in QD thin films by using femtosecond transient absorption microscopy. We discovered that exciton diffusion in QD films within strong confinement regime can be extremely fast, which is likely due to a large degree of exciton delocalization. In addition, this presentation will also cover our recent progress on elucidating the effect of exciton delocalization to other phenomena such as energy transfer to organic molecules, Auger recombination and trapping. The effect of exciton delocalization to inorganic – organic hybrid based up- and down-conversion will also be discussed.
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