Publication date: 15th May 2025
Colloidal semiconductor nanocrystals are prized in optoelectronics for their bandgap tunability, high photoluminescence quantum yield, and ease of colloidal processing. However, rapid nonradiative Auger recombination (AR) can negatively impact device efficiencies at high excitation intensities. In bulk semiconductors, AR is temperature-dependent, but in zero-dimensional quantum dots (QDs), it becomes temperature-independent due to the discretized band structure. The two-dimensional morphology of nanoplatelets (NPLs) complicates predictions of their photophysical behaviors.
We examined temperature dependent excited-state lifetime and fluence-dependent emission in CdSe NPLs, comparing them with QDs. As temperature decreases, the biexciton lifetime in NPLs surprisingly decreases, becoming shorter than trion emission, while emission intensity increases nearly linearly with fluence, indicating dominance of radiative recombination over AR. This contrasts fundamentally with core-only QDs.
Building on this, we found that photoexcitation of high-density 4 or 5 monolayer (ML) thick CdSe NPL films at temperatures below 200K results in pump-intensity-dependent, superlinear, and progressively red-shifted light output due to amplified spontaneous emission (ASE) from polyexcitonic species, distinct from biexcitonic ASE. These polyexcitonic species, known as "quantum droplets," form when multiexciton binding energy exceeds thermal energy fluctuations.
The ASE threshold for close-packed films decreases significantly with lower temperatures, attributed to extrinsic (trapping) and intrinsic (phonon-derived line width) factors. For pump intensities exceeding the ASE threshold, intense emission shifts to lower energy, particularly when the film temperature is ≤200 K. In contrast, NPLs diluted in an inert polymer suppress both biexcitonic ASE and low-energy emission, indicating reliance on high chromophore density and rapid, collective processes.
These findings enhance understanding of quantum droplet optical nonlinearities and the utility of photogenerated excitons and multiexcitons in optoelectronic applications.
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