Resolving the controversy in biexciton binding energy of cesium lead halide perovskite nanocrystals through heralded single-particle spectroscopy

TL;DR

This work unambiguously determine the biexciton binding energy in single cesium lead halide perovskite nanocrystals at room temperature, enabled by the recently introduced single-photon avalanche diode array spectrometer, and shows that, within ensembles of both materials, single-nanocrystal biexiton binding energies are correlated with the degree of charge-carrier confinement.

Abstract

Understanding exciton-exciton interaction in multiply-excited nanocrystals is crucial to their utilization as functional materials. Yet, for lead halide perovskite nanocrystals, which are promising candidates for nanocrystal-based technologies, numerous contradicting values have been reported for the strength and sign of their exciton-exciton interaction. In this work we unambiguously determine the biexciton binding energy in single cesium lead halide perovskite nanocrystals at room temperature. This is enabled by the recently introduced SPAD array spectrometer, capable of temporally isolating biexciton-exciton emission cascades while retaining spectral resolution. We demonstrate that CsPbBr$_3$ nanocrystals feature an attractive exciton-exciton interaction, with a mean biexciton binding energy of 10 meV. For CsPbI$_3$ nanocrystals we observe a mean biexciton binding energy that is close to zero, and individual nanocrystals show either weakly attractive or weakly repulsive exciton-exciton interaction. We further show that within ensembles of both materials, single-nanocrystal biexciton binding energies are correlated with the degree of charge-carrier confinement.

Figures (4)

• Figure 1: Particles investigated in this work.a) Transmission electron micrograph of the CsPbBr3 NCs investigated in this work. b) Transmission electron micrograph of the CsPbI3 NCs investigated in this work. Both scale bars are 20nm. c) Ensemble emission (solid lines) and absorption (dashed dotted lines) of the CsPbBr3 (green) and CsPbI3 (red) NCs. Blue line marks the excitation wavelength (470nm).
• Figure 2: Heralded spectroscopy of a single particle.a) A schematic illustration of the heralded spectroscopy scheme. A linear SPAD array is placed at the output of a grating spectrometer such that each SPAD pixel detects a different wavelength. The data from each SPAD pixel consists of the absolute arrival times of photons. By identifying the first and second arriving photons in each coincidence detection (BX and 1X, respectively), their corresponding energies can be extratced ($E_{BX}$ and $E_{1X}$). b) 2D histogram of photon pairs following the same excitation pulse, from a 5-minute measurement of a single CsPbBr3 NC. Green dashed line is a guide to the eye marking both photons with the same energy (undetectable by the system). c) BX spectrum (red dots) and 1X spectrum (blue circles) extracted by full horizontal and full vertical binning of panel (b), respectively. Grey area is the 1X spectrum (normalized) extracted by summing over all detected photons. Red solid line and blue dashed line represent fit of the BX and 1X spectra, respectively, to Cauchy-Lorentz distributions. Binding energy for this specific NC, estimated as the difference between the spectral peaks of the two fits, is $\varepsilon_b = 13.5\pm1.8meV$.
• Figure 3: CsPbBr3 binding energy.a) Binding energy histogram for 60 NCs. Mean single-particle error is $\pm3.1meV$. b) Binding energy as a function of 1X emission peak. c) Binding energy as a function of $g^{(2)}(0)$. std - standard deviation, CC coeff - cross-correlation coefficient. p-value - p-value of the cross-correlation.
• Figure 4: CsPbI3 binding energy.a) Binding energy histogram for 20 NCs. Mean single-particle error is $\pm4.8meV$. b) Binding energy as a function of 1X emission peak. c) Binding energy as a function of $g^{(2)}(0)$. std - standard deviation, CC coeff - cross-correlation coefficient. p-value - p-value of the cross-correlation.