Superradiance from Strongly Quantum-Confined Perovskite Quantum Dot Superlattices with Ligand-Tuned Electronic Coupling

Cooperative emission of photons as superradiance from an ensemble of quantum dots arises from the coupled quantum dots that prepare a coherent emitting state, unlike in superfluorescence which emits coherent photons via synchronization of the incoherently excited quantum dots. Strongly confined metal halide perovskite quantum dots, with their sizes significantly smaller than the exciton Bohr diameter, are particularly suitable for generating superradiance due to the stronger delocalization of the exciton wavefunction among the quantum dots in their superlattices. The ability to control the interfacial ligand structure further allows us to systematically vary the inter-quantum dot electronic coupling, which can directly affect the coherent photon-emitting behavior. Here, we have examined the superradiance from 2D and 3D superlattices of CsPbBr₃ quantum dots in a strongly quantum-confined regime (4 nm quantum dot size vs 7 nm Bohr diameter) using surface ligands of varying lengths that tune the electronic coupling. While 2D superlattice CsPbBr₃ quantum dots do not exhibit superradiance, even with the ligand that reduce the facet-to-facet distance to 0.5 nm, 3D quantum dot superlattices exhibit well-defined superradiance emission when the original long ligand (oleylammonium bromide) is replaced with a ligand with shorter carbon chains at cryogenic temperatures (e.g., below 100-150 K). The superradiance from 4 nm CsPbBr3 quantum dot superlattices exhibits a much larger redshift (<150 meV) from the incoherent exciton emission, a narrower linewidth (3 meV), and stronger photon anti-bunching (g⁽²⁾ of 1.6) compared to the weakly confined QD superlattices. These observations indicate a higher degree of exciton delocalization and electronic coupling of the state generating superradiance. The 4 nm CsPbBr₃ quantum dot superlattices also exhibit a preferred direction of linear polarization, indicating the anisotropic electronic coupling within the superlattice that may result from the lowered symmetry of the superlattice, especially when shorter ligands are used. These results demonstrate the potential of the superlattices of strongly confined perovskite quantum dots with controlled surface ligand structures as effective coherent multiphoton emitters.