For over 80 years, tailored molecular assemblies (e.g., H-and J-aggregates) have been of interest for the emergence of collective phenomena in their optical spectra 1 , coherent long-range energy transport 2,3 and their conceptual similarity with natural light-harvesting complexes 4,5 . Another highly versatile platform for creating controlled, aggregated states exhibiting collective phenomena arises from the organization of colloidal semiconductor nanocrystals (NCs) into long-range ordered superlattices (SLs) 6 . Cesium lead halide perovskite NCs 7-9 have recently emerged as highly appealing building blocks, owing to their high oscillator strength 10 , slow dephasing (long coherence times of up to 80 ps) 11,12 , minimal inhomogeneous broadening of emission lines, and a bright triplet exciton character with orthogonal dipole orientation 10 , potentially enabling an efficient omnidirectional coupling. Here we present perovskite-type (ABO3) binary and ternary NC SLs by a shapedirected co-assembly of steric-stabilized, highly luminescent cuboid-shaped CsPbBr3 NCs (occupying B-and/or O-sites) with spherical Fe3O4 or NaGdF4 NCs (A-sites) and truncated-cuboid PbS NCs (B-site). Such ABO3 SLs, as well as other newly obtained SL structures (binary NaCl-and AlB2-types), exhibit a high degree of orientational ordering of CsPbBr3 nanocubes. These novel perovskite mesostructures exhibit superfluorescence (SF) -a collective emission resulting in a burst of photons. SF is 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.

Attaining thermodynamic stability of colloids in a broad range of concentrations has long been a major thrust in the field of colloidal ligand-capped semiconductor nanocrystals (NCs). This challenge is particularly pressing for the novel NCs of cesium lead halide perovskites (CsPbX3; X = Cl, Br) owing to their highly dynamic and labile surfaces. Herein, we demonstrate that soy lecithin, a mass-produced natural phospholipid, serves as a tightly binding surface-capping ligand suited for a high-reaction yield synthesis of CsPbX3 NCs (6–10 nm) and allowing for long-term retention of the colloidal and structural integrity of CsPbX3 NCs in a broad range of concentrations—from a few ng/mL to >400 mg/mL (inorganic core mass). The high colloidal stability achieved with this long-chain zwitterionic ligand can be rationalized with the Alexander–De Gennes model that considers the increased particle–particle repulsion due to branched chains and ligand polydispersity. The versatility and immense practical utility of such colloids is showcased by the single NC spectroscopy on ultradilute samples and, conversely, by obtaining micrometer-thick, optically homogeneous dense NC films in a single spin-coating step from ultraconcentrated colloids.

Ligand-capped nanocrystals (NCs) of lead halide perovskites, foremost fully inorganic CsPbX3 NCs, are the latest generation of colloidal semiconductor quantum dots. They offer a set of compelling characteristicslarge absorption cross section, as well as narrow, fast, and efficient photoluminescence with long exciton coherence timesrendering them attractive for applications in light-emitting devices and quantum optics. Monodisperse and shape-uniform, broadly size-tunable, scalable, and robust NC samples are paramount for unveiling their basic photophysics, as well as for putting them into use. Thus far, no synthesis method fulfilling all these requirements has been reported. For instance, long-chain zwitterionic ligands impart the most durable surface coating, but at the expense of reduced size uniformity of the as-synthesized colloid. In this work, we demonstrate that size-selective precipitation of CsPbBr3 NCs coated with a long-chain sulfobetaine ligand, namely, 3-(N,N-dimethyloctadecylammonio)-propanesulfonate, yields monodisperse and sizable fractions (>100 mg inorganic mass) with the mean NC size adjustable in the range between 3.5 and 16 nm and emission peak wavelength between 479 and 518 nm. We find that all NCs exhibit an oblate cuboidal shape with the aspect ratio of 1.2 × 1.2 × 1. We present a theoretical model (effective mass/k·p) that accounts for the anisotropic NC shape and describes the size dependence of the first and second excitonic transition in absorption spectra and explains room-temperature exciton lifetimes. We also show that uniform zwitterion-capped NCs readily form long-range ordered superlattices upon solvent evaporation. In comparison to more conventional ligand systems (oleic acid and oleylamine), supercrystals of zwitterion-capped NCs exhibit larger domain sizes and lower mosaicity. Both kinds of supercrystals exhibit superfluorescence at cryogenic temperaturesaccelerated collective emission arising from the coherent coupling of the emitting dipoles.

Advances in developing ultrafast coherent sources operating at extreme ultraviolet (EUV) and x-ray wavelengths allow the extension of nonlinear optical techniques to shorter wavelengths. Here, we describe EUV transient grating spectroscopy, in which two crossed femtosecond EUV pulses produce spatially periodic nanoscale excitations in the sample and their dynamics is probed via diffraction of a third time-delayed EUV pulse. The use of radiation with wavelengths down to 13.3 nm allowed us to produce transient gratings with periods as short as 28 nm and observe thermal and coherent phonon dynamics in crystalline silicon and amorphous silicon nitride. This approach allows measurements of thermal transport on the ~10-nm scale, where the two samples show different heat transport regimes, and can be applied to study other phenomena showing nontrivial behaviors at the nanoscale, such as structural relaxations in complex liquids and ultrafast magnetic dynamics.

Nanocrystal (NC) self-assembly is a versatile platform for materials engineering at the mesoscale. The NC shape anisotropy leads to structures not observed with spherical NCs. This work presents a broad structural diversity in multicomponent, long-range ordered superlattices (SLs) comprising highly luminescent cubic CsPbBr 3 NCs (and FAPbBr 3 NCs) coassembled with the spherical, truncated cuboid, and disk-shaped NC building blocks. CsPbBr 3 nanocubes combined with Fe 3 O 4 or NaGdF 4 spheres and truncated cuboid PbS NCs form binary SLs of six structure types with high packing density; namely, AB 2 , quasi-ternary ABO 3 , and ABO 6 types as well as previously known NaCl, AlB 2 , and CuAu types. In these structures, nanocubes preserve orientational coherence. Combining nanocubes with large and thick NaGdF 4 nanodisks results in the orthorhombic SL resembling CaC 2 structure with pairs of CsPbBr 3 NCs on one lattice site. Also, we implement two substrate-free methods of SL formation. Oil-in-oil templated assembly results in the formation of binary supraparticles. Self-assembly at the liquid–air interface from the drying solution cast over the glyceryl triacetate as subphase yields extended thin films of SLs. 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.