Publication date: 28th August 2024
The ability to efficiently up-convert broadband, low-intensity light would be an enabling technology for volumetric 3D printing, background-free biomedical imaging, and sensitizing silicon-based cameras to the short-wave infrared. Our approach uses colloidal quantum dots to absorb low-energy photons and sensitize the spin-triplet excitonic states of nearby conjugated molecules.[1-3] Once there, pairs of these long-lived excitations can combine via triplet fusion to generate shorter-wavelength fluorescence.
We recently harnessed high-quality, ultra-small (d:1.7-2.8 nm) PbS quantum dots[4] to generate photochemically active blue light (λ~420nm) from continuous-wave red (λ: 635nm) excitation.[5] However, this performance was somewhat unanticipated, because the large ‘Stokes shift’ in most ultra-small nanocrystals appears to herald an unacceptable loss of incident photon energy. Intriguingly, we inferred from the quasi-equilibrium dynamics of triplet energy transfer that the chemical potential of photoexcited, ultra-small PbS quantum dots is surprisingly high—completing an advantageous suite of photophysical properties, but reinforcing fundamental questions regarding their emissive state(s).[5,6]
Accordingly, I will present a photophysical effort to relate these anomalous behaviours to the long-discussed, surface-linked ‘trap’ emission from canonical Cd-chalcogenide quantum dots. We show that this non-ideal emission can be observed, intermittently, from individual nanocrystals, and is consistent with the occasional formation of defect states, shallow within the gap, on timescales that align with rare, photoinduced structural reorganization.
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