Precision engineering of luminescent lead-halide quantum dots: from single photons to coherent collective states
Maksym Kovalenko a b
a ETH Zürich, Department of Chemistry and Applied Biosciences, Switzerland, Switzerland
b Laboratory for Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, Dübendorf 8600, Switzerland
Invited Speaker, Maksym Kovalenko, presentation 068
Publication date: 21st 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 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 discuss the size-dependent single-photon emission across the LHP NC compositions. We achieve 98% single-photon purity (g(2) (0) as low as 2%) from a cavity-free, nonresonantly excited single 6.6 nm CsPbI3 NCs, showcasing the great potential of CsPbX3 NCs as room-temperature highly pure single-photon sources for quantum technologies [2]. In another study, we address the linewidth of the single-photon emission from perovskite NCs at room temperature. By using ab-initio molecular dynamics for simulating exciton-surface-phonon interactions in structurally dynamic CsPbBr3 NCs, followed by single quantum dot optical spectroscopy, we demonstrate that emission line-broadening in these quantum dots is primarily governed by the coupling of excitons to low-energy surface phonons. Mild adjustments of the surface chemical composition allow for attaining much smaller emission linewidths of 35−65 meV (vs. initial values of 70–120 meV) [3]. NC self-assembly is a versatile platform for materials engineering, particularly for attaining collective phenomena with perovskite NCs, such as superfluorescence [4, 5]. Collective electronic states arise at low temperatures from the dense, periodic packing of NCs, observed as sharp red-shifted bands at 6 K in the photoluminescence and absorption spectra and persisting up to 200 K. Perovskite SLs exhibit superfluorescence, characterized, at high excitation density, by emission pulses with ultrafast (22 ps) radiative decay and Burnham-Chiao ringing behaviour with a strongly accelerated build-up time.

 

1. Q. Akkerman et al. Science, 2022, 377, 1406-​1412

2. Chenglial Zhu et al. Nano Lett. 2022, 22, 3751−3760

3. Gabriele Raino et al. Nat. Commun., 2022, 13, 2587

3. Ihor Cherniukh et al. Nature, 2021, 593, 535–542

5. Ihor Cherniukh et al. ACS Nano, 2022, 16, 5, 7210–7232

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