Highly luminescent lead halide perovskite nanocrystals: revisiting their synthesis and tailoring their surface chemistry
Maksym Kovalenko a b
a ETH Zurich, Laboratory of Inorganic Chemistry, Department of Chemistry & Applied Biosciences, Vladimir-Prelog-Weg, 1, Zürich, CH
b EMPA Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse, 129, Dübendorf, Switzerland
Proceedings of Sustainable Metal-halide perovskites for photovoltaics, optoelectronics and photonics (Sus-MHP)
València, Spain, 2022 December 12th - 13th
Organizers: Teresa S. Ripolles and Hui-Seon Kim
Invited Speaker, Maksym Kovalenko, presentation 003
DOI: https://doi.org/10.29363/nanoge.sus-mhp.2022.003
Publication date: 15th November 2022

Colloidal lead halide perovskite (LHP) nanocrystals (NCs), with bright and spectrally narrow photoluminescence (PL) tunable over the entire visible spectral range, are of immense interest as classical and quantum light sources. Severe challenges LHP NCs form by sub-second fast and hence hard-to-control ionic metathesis reactions, which severely limits the access to size-uniform and shape-regular NCs in the sub-10 nm range. We show that a synthesis path comprising an intricate equilibrium between the precursor (TOPO-PbBr2 complex) and the [PbBr3-] solute for the NC nucleation may circumvent this challenge [1]. This results in a scalable, room-temperature synthesis of monodisperse and isolable CsPbBr3 NCs, size-tunable in the 3-13 nm range. The kinetics of both nucleation and therefrom temporally separated growth are drastically slowed, resulting in total reaction times of up to 30 minutes. The methodology is then extended to FAPbBr3 (FA = formamidinium) and MAPbBr3 (MA = methylammonium), allowing for thorough experimental comparison and modeling of their physical properties under intermediate quantum confinement. In particular, NCs of all these compositions exhibit up to four excitonic transitions in their linear absorption spectra, and we demonstrate that the size-dependent confinement energy for all transitions is independent of the A-site cation.

   We then show that this synthesis – relying on the labile ligand capping with TOPO-phosphinic acid mixture – makes for a convenient platform for the subsequent surface functionalization with diverse capping ligands [2]. Robust surface functionalization of highly ionic surfaces, as is the case of LHP NCs, has remained a formidable challenge due to the inherently non-covalent weak surface bonding. Leveraging the vast and facile molecular engineering of phospholipids, we present their efficacy as surface capping ligands for LHP NCs. Molecular dynamics simulations and solid-state NMR confirm that the surface affinity of these zwitterionic molecules is primarily governed by the geometric fitness of their anionic and cationic moieties. Judicious selection of the ligands yielded colloidally robust FAPbBr3 and MAPbBr3 NCs and enabled colloids in a variety of solvents, from n-hexane to acetone. Robustness of the surface capping is also reflected in optical properties: NCs exhibit PL quantum yield (QY) above 96% after numerous purifications. NCs are essentially blinking-free at a single particle level.

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