Collective Interactions of Quantum-Confined Excitons in Halide Perovskite Nanocrystal Superlattices

Abstract

Collective optical properties can emerge from an ordered ensemble of emitters due to interactions between the individual units. Superlattices of halide perovskite nanocrystals exhibit collective light emission, influenced by dipole-dipole interactions between simultaneously excited nanocrystals. This coupling changes both the emission energy and rate compared to the emission of uncoupled nanocrystals. We demonstrate how quantum confinement governs the nature of the coupling between the nanocrystals in the ensemble. The extent of confinement is modified by controlling the nanocrystal size or by compositional control over the Bohr radius. In superlattices made of weakly confined nanocrystals, the collective emission is red-shifted with a faster emission rate, showing the key characteristics of superfluorescence. In contrast, the collective emission of stronger quantum-confined nanocrystals is blue-shifted with a slower emission rate. Both types of collective emission exhibit correlative multiphoton emission bursts, showing distinct photon bunching emission statistics. The quantum confinement changes the preferred alignment of transition dipoles within the nanocrystal and switches the relative dipole orientation between neighbors, resulting in opposite collective optical behaviors. Our results extend these collective effects to relatively high temperatures and provide a better understanding of exciton interactions and collective emission phenomena at the solid state.

Keywords: lead halide perovskites; nanocrystal coupling; nanocrystals; quantum confinement; superfluorescence; superlattices.

Figure 1

Underlying mechanism of dipole–dipole coupling-influenced collective emission behavior. (a, b) Energy level diagrams for the Coulomb dipole–dipole interaction between transition dipoles for a dimer of (a) “head-to-tail” interaction, J-aggregate, and (b) “head-to-head” interaction, H-aggregate. The transition between the ground and excited coupled states is allowed only to the state with aligned transition dipole moments. This bright state is the lower/upper coupled state for J/H-aggregates, respectively, leading to either a spectral red shift or blue shift in the coupled emission.

Figure 2

Quantum-confinement tunable collective emission of halide perovskite nanocrystals. (a–c) TEM micrographs and size distribution histograms of the nanocrystal building blocks for (a) 9.3 nm, (b) 7.8 nm, and (c) 6.3 nm cuboid-shaped CsPbBr₃ nanocrystals. (d–f) PL spectrum at T = 80 K of superlattices made from (d) 9.3 nm, (e) 7.8 nm, and (f) 6.3 nm CsPbBr₃ nanocrystals. The coupled emission is (d, e) red-shifted or (f) blue-shifted relative to the uncoupled emission present in PL from nanocrystal glassy films (black dashed lines). (g–i) PL spectra at different temperatures of superlattices made from (g) 9.3 nm, (h) 7.8 nm, and (i) 6.3 nm CsPbBr₃ nanocrystals. In the larger nanocrystals, a red-shifted peak appears, and in smaller nanocrystals, a blue-shifted peak appears below a temperature of 180–200 K.

Figure 3

Temporal and correlative characteristics of the different collective emissions. (a, b) Time-resolved PL at 80 K of superlattices made from (a) 7.8 nm and (b) 6.3 nm CsPbBr₃ nanocrystals. Average emission rates show (a) accelerated red-shifted coupled emission and (b) slower emission rate for the blue-shifted coupled emission. (c–f) Second-order correlation function measurements, obtained with a Hanbury–Brown and Twiss setup using pulsed excitation with 10 MHz repetition rate, for SLs made from (c, e) 7.8 nm and (d, f) 6.3 nm CsPbBr₃ nanocrystals. Both red-shifted (e) and blue-shifted (f) collective emissions of coupled nanocrystals show photon bunching at a zero time delay, while emission of the uncoupled nanocrystals (c, d) shows Poissonian photon emission statistics.

Figure 4

Composition control over quantum confinement and the resulting collective emission. (a, b) PL spectrum at T = 80 K of superlattices made of anion-exchanged (a) 7.8 nm CsPb(Br_0.5I_0.5)₃ and (b) 6.3 nm CsPb(Br_0.4Cl_0.6)₃ nanocrystals. The compositional changes modify the extent of quantum confinement and switch the collective spectral behaviors shown previously. (c, d) PL spectra at different temperatures of superlattices made of (c) 7.8 nm CsPb(Br_0.5I_0.5)₃ and (d) 6.3 nm CsPb(Br_0.4Cl_0.6)₃ nanocrystals. In chloride-exchanged nanocrystals, a red-shifted peak appears, and in iodide-exchanged nanocrystals, a blue-shifted peak appears at cryogenic temperatures. (e) Time-resolved PL at 80 K of superlattice made of (right) 7.8 nm CsPb(Br_0.5I_0.5)₃ and (left) 6.3 nm CsPb(Br_0.4Cl_0.6)₃. (f) Measured coupled PL energy difference vs the extent of quantum confinement of the nanocrystal building blocks.

Figure 5

Quantum confinement effects on the transition dipole orientation and coupling type. (a, b) Angular resolved cathodoluminescence emission patterns measured from superlattices made of (a) 7.8 nm, and (b) 6.3 nm CsPbBr₃ nanocrystals. The upper panels show intensity vs polar angle θ along the azimuthal axis of φ = 120°/300° marked in dashed lines on the 2D projections. (c) Schematic of the coupled nanocrystal dimer with a nonplanar transition dipole conformation. (d) Dipole–dipole coupling conformation factor calculation for nonplanar transition dipoles as a function of inclination angle β and angle between transition dipole planes α.