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. Attaining pure single-photon emission is key for many quantum technologies, from optical quantum computing to quantum key distribution and quantum imaging.  Across single CsPbX3 NCs (X: Br and I) of different sizes and compositions, we find that increasing quantum confinement is an effective strategy for maximizing single-photon purity due to the suppressed biexciton quantum yield. 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 [1]. 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), which are on par with the best values known for structurally rigid, colloidal II-VI quantum dots (20−60 meV) [2]. NC self-assembly is a versatile platform for materials engineering, particularly for attaining collective phenomena with perovskite NCs, such as superfluorescence [3, 4, 5]. The NC shape anisotropy leads to structures not observed with spherical NCs. We present a broad structural diversity in multicomponent, long-range ordered superlattices (SLs) comprising highly luminescent cubic CsPbBr3 NCs (and FAPbBr3 NCs) co-assembled with the spherical, truncated cuboid, and disk-shaped NC building blocks. CsPbBr3 nanocubes combined with Fe3O4 or NaGdF4 spheres and truncated cuboid PbS NCs form binary SLs of six structure types with high packing density; namely, AB2, quasi-ternary ABO3, and ABO6 types as well as structures already known in all-spheres systems [NaCl, AlB2, and CuAu types]. In these structures, nanocubes preserve orientational coherence. Combining nanocubes with large and thick NaGdF4 nanodisks results in the orthorhombic SL resembling CaC2 structure with pairs of CsPbBr3 NCs on one lattice site. Also, we implement two substrate-free methods of SL formation. 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. Co-assembly of CsPbBr3 nanocubes with very thin disk LaF3 nanodisks (9.2–28.4 nm in diameter, 1.6 nm in thickness) yields six columnar structures with AB, AB2, AB4, and AB6 stoichiometry, not observed before and in our reference experiments with NC systems comprising spheres and disks. 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] Chenglial Zhu et al. Nano Lett. 2022, 22, 3751−3760

[2] Gabriele Raino et al. Nat. Commun., 2022, 13, 2587

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

[4] Ihor Cherniukh et al. ACS Nano, 2021, 15, 10, 16488–16500

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