Quantifying Structural Heterogeneity in Individual CsPbBr₃ Quantum Dot Superlattices

Abstract

A thorough understanding of the structural heterogeneity in CsPbBr3 quantum dot superlattices (SLs) is necessary for the realization of exciton coherence in these systems. Scanning transmission electron microscopy (STEM) coupled to fast-Fourier transform (FFT) analysis is utilized to characterize the structural properties of individual SLs. For each SL, the average constituent quantum dot size, size dispersity, and number of crystalline domains are quantified. Analysis of 40 individual SLs across eight growth experiments reveals that SLs are structurally heterogeneous but tend to have a narrower size distribution than the precursor solution due to size selection that occurs during evaporative self-assembly. We directly correlate STEM-FFT structural properties to low-temperature photoluminescence spectra for individual SLs, demonstrating that the substructure in the photoluminescence peak arises from multiple, locally ordered domains within the SL. In addition, we show that long-range structural disorder in an SL does not necessarily impact short-range phenomena such as exciton delocalization.

Tuning the Colloidal Softness of CsPbBr3 Nanocrystals for Homogeneous Superlattices

“…21 Recently, Clark et.al. 19 showed a coupled emission dependency on the structural heterogeneity of the CsPbBr 3 SLs. The coupled emission spectra at low temperatures changed due to the presence of multiple NC cubic-packed domains within the SL.…”

“…13 Observations of these SL domains were made using scanning transmission electron microscopy (STEM) and optically characterized by PL. 19 However, the spatial resolution of PL characterization is diffraction limited and probes an averaged emission from subdomains of the SL. Therefore, PL characterization lacks fine structural and optical correlative information about the heterogeneities in CsPbBr 3 SLs.…”

“…The coupled emission spectra at low temperatures changed due to the presence of multiple NC cubic-packed domains within the SL . Observations of these SL domains were made using scanning transmission electron microscopy (STEM) and optically characterized by PL . However, the spatial resolution of PL characterization is diffraction limited and probes an averaged emission from subdomains of the SL.…”

“…16−18 Inconsistencies in reported SF measurements were attributed to these structural heterogeneities of the CsPbBr 3 SLs and may be the main cause for reduced coupled emission. 8,19,20 Lapkin et al 21 mapped the structural and optical heterogeneities of single CsPbBr 3 SLs, using spatially resolved X-ray diffraction and scattering techniques along with photoluminescence (PL) measurements. A PL blue shift from the SL edges compared to their center was observed and attributed to residual uniaxial compressive strains from the SL formation.…”

“…This leads to the characteristics of SF emission such as accelerated decay rates and superlinear emission intensity. , As a result, the SF emission characteristics are highly dependent on the number of coupled NC emitters and are therefore highly sensitive to changes in NC coupling as a result of irregular ordering of the NCs. SL crystallographic defects such as grain boundaries, NC vacancies, and steps and kinks, along with size inhomogeneity and segregation, are all contributors to a reduced SL structural coherence. − Inconsistencies in reported SF measurements were attributed to these structural heterogeneities of the CsPbBr 3 SLs and may be the main cause for reduced coupled emission. ,, Lapkin et al mapped the structural and optical heterogeneities of single CsPbBr 3 SLs, using spatially resolved X-ray diffraction and scattering techniques along with photoluminescence (PL) measurements. A PL blue shift from the SL edges compared to their center was observed and attributed to residual uniaxial compressive strains from the SL formation .…”