Second-order bright-dark exciton thermal mixing in single CsPbBr3 nanocrystals
Mohamed-Raouf Amara a b, Caixia Huo a, Carole Diederichs b, Qihua Xiong a
a Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore , Singapore, Nanyang Link, 21, Singapore, Singapore
b Laboratoire de Physique de l'Ecole Normale Supérieure, Rue Lhomond, 24, Paris, France
Proceedings of nanoGe Fall Meeting 2021 (NFM21)
#PerNC21. Perovskites II: Synthesis, Characterization, and Properties of Colloidal
Online, Spain, 2021 October 18th - 22nd
Organizers: Maksym Kovalenko, Ivan Infante and Lea Nienhaus
Poster, Mohamed-Raouf Amara, 289
Publication date: 23rd September 2021

Low-dimensional lead-halide perovskite nanocrystals (pNCs) have emerged in the recent years as new emitters with remarkable optoelectronic properties, all the while being easily synthesized and processed. Recent reports of optical coherence time measurements have further shown that, although new and unprocessed, pNCs readily show favourable characteristics as a quantum light source [1,2]. The emission nonetheless remains impacted by solid-state decoherent processes such as carrier-phonon scattering and interaction with the electrostatic environment.

Here, we focus on all-inorganic CsPbBr3 NCs in the weak confinement regime (L≥2aB∼7 nm) dispersed in a polystyrene matrix. They exhibit stable and bright emission at cryogenic temperatures comprised of two- or 3-peak spectra with almost linear polarisation attributed to the bright exciton states [3]. The emission arising from this bright exciton manifold shows strong photon antibunching and fast sub-100 ps decay times. We use a combination of steady-state photoluminescence (PL) and time-resolved PL to study the emission of individual CsPbBr3 NCs from liquid-helium temperature to ~100 K and gain further understanding of the mechanisms at play, namely exciton-phonon coupling and its interaction with the exciton fine structure in this material.

As previously reported [4], the temperature-dependence of the PL reveals that the emission exhibits a blueshift and a thermal broadening induced mainly by optical phonon scattering. We identify the optical phonon mode at play in the Stokes optical phonon sidebands, together with other lower-energy optical phonon modes that do not seem to play a role in the thermal broadening.

Further information is sought from the temperature dependence of the time-resolved PL. While at the lowest temperatures, the emission is fast and mono-exponential, as temperature is increased a longer decay component gains weight and shortens, to become the main decay channel at ~80 K.

This behaviour reveals that the emission results from the interplay between thermally mixed exciton fine structure states (bright and dark). which enables us to estimate the characteristic energies of the phonons involved in this process and rationalize these findings in light of the recent literature on other hybrid and all-inorganic pNCs [5, 6].

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