Superradiance and Exciton Delocalization in Perovskite Quantum Dot Superlattices

Abstract

Achieving superradiance in solids is challenging due to fast dephasing processes from inherent disorder and thermal fluctuations. Perovskite quantum dots (QDs) are an exciting class of exciton emitters with large oscillator strength and high quantum efficiency, making them promising for solid-state superradiance. However, a thorough understanding of the competition between coherence and dephasing from phonon scattering and energetic disorder is currently unavailable. Here, we present an investigation of exciton coherence in perovskite QD solids using temperature-dependent photoluminescence line width and lifetime measurements. Our results demonstrate that excitons are coherently delocalized over 3 QDs at 11 K in superlattices leading to superradiant emission. Scattering from optical phonons leads to the loss of coherence and exciton localization to a single QD at temperatures above 100 K. At low temperatures, static disorder and defects limit exciton coherence. These results highlight the promise and challenge in achieving coherence in perovskite QD solids.

Quantifying Structural Heterogeneity in Individual CsPbBr3 Quantum Dot Superlattices

“…We then correlated the low-temperature PL linewidth to the structural properties obtained from STEM-FFT analysis. In molecular J- and H-aggregates with exciton delocalization, PL linewidth is inversely proportional to the number of coupled emitters, a phenomenon known as exchange narrowing. − PL linewidths for CsPbBr 3 SLs also exhibit exchange narrowing below 100 K commensurate with their degree of coherence, though some convolution with inhomogeneous broadening is present (Figure S9e, Supporting Information Section S4). , Surveying our population of SLs, we find that PL linewidth has no correlation to FFT linewidth, the primary metric for disorder in the SL assembly (Figure b). In other words, the extent of exciton delocalization at low temperatures is not dictated by long-range structural order in the SL.…”

“…More recently, our group showed that CsPbBr 3 cubic SLs also exhibit single-photon superradiance arising from exciton delocalization at low excitation fluence. Unlike superfluorescence, superradiance does not require a light-induced phase-synchronization of emitters . However, we found that the number of coherent emitters in the superradiant state varied widely across SLs grown under the same conditions, which was reflected in their heterogeneous photoluminescence (PL) spectra and lifetimes at low temperatures.…”

“…Steady-state PL measurements were performed with the same home-built confocal micro-PL setup as described in our recent publication. 16 Briefly, the SLs were excited using a 447 nm picosecond-pulsed diode laser (LDH-P-C-450B, PicoQuant, FWHM ∼50 ps, repetition rate 5 MHz). A 40× objective with a numerical aperture of 0.6 was used in reflection mode to excite and collect sample emission.…”

“…Optical measurements were collected prior to STEM imaging since extended exposure to the electron beam during STEM imaging quenches the fluorescence of the SLs. Steady-state PL measurements were performed with the same home-built confocal micro-PL setup as described in our recent publication . Briefly, the SLs were excited using a 447 nm picosecond-pulsed diode laser (LDH-P-C-450B, PicoQuant, FWHM ∼50 ps, repetition rate 5 MHz).…”

“…Unlike superfluorescence, superradiance does not require a light-induced phase-synchronization of emitters. 16 However, we found that the number of coherent emitters in the superradiant state varied widely across SLs grown under the same conditions, which was reflected in their heterogeneous photoluminescence (PL) spectra and lifetimes at low temperatures. Dynamic disorder from phonon scattering becomes negligible at 11 K, which indicates that differences in PL properties stem strictly from structural heterogeneity among SLs.…”