Manipulating Excitonic Quantum Droplets in Two-Dimensional CdSe Nanoplatelets via Thickness and Core/Shell Morphology

Recently, photoexcitation of high-density 4- or 5-monolayer (ML) thick CdSe nanoplatelet films at temperatures below ∼200 K was observed to produce pump-intensity-dependent, superlinear, progressively red-shifted light output due to amplified spontaneous emission (ASE) from polyexcitonic species, distinct from biexcitonic ASE. These polyexcitonic species comprise Coulombically bound multiexciton states, termed "quantum droplets", that form when multiexciton binding energy exceeds thermal energy fluctuations. Here, we investigate thinner nanoplatelet samples as well as core/shell nanoplatelet films. We find that the 3 ML sample supports quantum droplet ASE up to higher temperatures than the 4- or 5-ML samples, whereas core/shell structures can suppress this form of ASE even at very low sample temperatures despite observable biexcitonic ASE, suggesting that the reduced or repulsive Coulomb binding energy of multiple excitons suppresses quantum droplets. These findings help to improve our understanding of routes to harness quantum droplet optical nonlinearities or preserve biexcitonic ASE at high fluence.