Nanocrystal Assemblies: Current Advances and Open Problems.

We explore the potential of nanocrystals (a term used equivalently to nanoparticles) as building blocks for nanomaterials, and the current advances and open challenges for fundamental science developments and applications. Nanocrystal assemblies are inherently multiscale, and the generation of revolutionary material properties requires a precise understanding of the relationship between structure and function, the former being determined by classical effects and the latter often by quantum effects. With an emphasis on theory and computation, we discuss challenges that hamper current assembly strategies and to what extent nanocrystal assemblies represent thermodynamic equilibrium or kinetically trapped metastable states. We also examine dynamic effects and optimization of assembly protocols. Finally, we discuss promising material functions and examples of their realization with nanocrystal assemblies.

Tuning perovskite nanocrystal superlattices for superradiance in the presence of disorder.

The cooperative emission of interacting nanocrystals is an exciting topic fueled by recent reports of superfluorescence and superradiance in assemblies of perovskite nanocubes. Several studies estimated that coherent coupling is localized to a small fraction of nanocrystals (10-7-10-3) within the assembly, raising questions about the origins of localization and ways to overcome it. In this work, we examine single-excitation superradiance by calculating radiative decays and the distribution of superradiant wave function in two-dimensional CsPbBr3 nanocube superlattices. The calculations reveal that the energy disorder caused by size distribution and large interparticle separations reduces radiative coupling and leads to the excitation localization, with the energy disorder being the dominant factor. The single-excitation model clearly predicts that, in the pursuit of cooperative effects, having identical nanocubes in the superlattice is more important than achieving a perfect spatial order. The monolayers of large CsPbBr3 nanocubes (LNC = 10-20 nm) are proposed as model systems for experimental tests of superradiance under conditions of non-negligible size dispersion, while small nanocubes (LNC = 5-10 nm) are preferred for realizing the Dicke state under ideal conditions.

Halide Perovskite Artificial Solids as a New Platform to Simulate Collective Phenomena in Doped Mott Insulators.

The development of quantum simulators, artificial platforms where the predictions of many-body theories of correlated quantum materials can be tested in a controllable and tunable way, is one of the main challenges of condensed matter physics. Here we introduce artificial lattices made of lead halide perovskite nanocubes as a new platform to simulate and investigate the physics of correlated quantum materials. We demonstrate that optical injection of quantum confined excitons in this system realizes the two main features that ubiquitously pervade the phase diagram of many quantum materials: collective phenomena, in which long-range orders emerge from incoherent fluctuations, and the excitonic Mott transition, which has one-to-one correspondence with the insulator-to-metal transition described by the repulsive Hubbard model in a magnetic field. Our results demonstrate that time-resolved experiments provide a quantum simulator that is able to span a parameter range relevant for a broad class of phenomena, such as superconductivity and charge-density waves.

Collective Diffraction Effects in Perovskite Nanocrystal Superlattices.

ConspectusFor almost a decade now, lead halide perovskite nanocrystals have been the subject of a steadily growing number of publications, most of them regarding CsPbBr3 nanocubes. Many of these works report X-ray diffraction patterns where the first Bragg peak has an unusual shape, as if it was composed of two or more overlapping peaks. However, these peaks are too narrow to stem from a nanoparticle, and the perovskite crystal structure does not account for their formation. What is the origin of such an unusual profile, and why has it been overlooked so far? Our attempts to answer these questions led us to revisit an intriguing collective diffraction phenomenon, known for multilayer epitaxial thin films but not reported for colloidal nanocrystals before. By analogy, we call it the multilayer diffraction effect.Multilayer diffraction can be observed when a diffraction experiment is performed on nanocrystals packed with a periodic arrangement. Owing to the periodicity of the packing, the X-rays scattered by each particle interfere with those diffracted by its neighbors, creating fringes of constructive interference. Since the interfering radiation comes from nanoparticles, fringes are visible only where the particles themselves produce a signal in their diffraction pattern: for nanocrystals, this means at their Bragg peaks. Being a collective interference phenomenon, multilayer diffraction is strongly affected by the degree of order in the nanocrystal aggregate. For it to be observed, the majority of nanocrystals within the sample must abide to the stacking periodicity with minimal misplacements, a condition that is typically satisfied in self-assembled nanocrystal superlattices or stacks of colloidal nanoplatelets.A qualitative understanding of multilayer diffraction might explain why the first Bragg peak of CsPbBr3 nanocubes sometimes appears split, but leaves many other questions unanswered. For example, why is the split observed only at the first Bragg peak but not at the second? Why is it observed routinely in a variety of CsPbBr3 nanocrystals samples and not just in highly ordered superlattices? How does the morphology of particles (i.e., nanocrystals vs nanoplatelets) affect the appearance of multilayer diffraction effects? Finally, why is multilayer diffraction not observed in other popular nanocrystals such as Au and CdSe, despite the extensive investigations of their superlattices?Answering these questions requires a deeper understanding of multilayer diffraction. In what follows, we summarize our progress in rationalizing the origin of this phenomenon, at first through empirical observation and then by adapting the diffraction theory developed in the past for multilayer thin films, until we achieved a quantitative fitting of experimental diffraction patterns over extended angular ranges. By introducing the reader to the key advancements in our research, we provide answers to the questions above, we discuss what information can be extracted from patterns exhibiting collective interference effects, and we show how multilayer diffraction can provide insights into colloidal nanomaterials where other techniques struggle. Finally, with the help of literature patterns showing multilayer diffraction and simulations performed by us, we demonstrate that this collective diffraction effect is within reach for many appealing nanomaterials other than halide perovskites.

Highly Emitting Perovskite Nanocrystals with 2-Year Stability in Water through an Automated Polymer Encapsulation for Bioimaging.

Lead-based halide perovskite nanocrystals are highly luminescent materials, but their sensitivity to humid environments and their biotoxicity are still important challenges to solve. Here, we develop a stepwise approach to encapsulate representative CsPbBr3 nanocrystals into water-soluble polymer capsules. We show that our protocol can be extended to nanocrystals coated with different ligands, enabling an outstanding high photoluminescence quantum yield of ∼60% that is preserved over two years in capsules dispersed in water. We demonstrate that this on-bench strategy can be implemented on an automated platform with slight modifications, granting access to a faster and more reproducible fabrication process. Also, we reveal that the capsules can be exploited as photoluminescent probes for cell imaging at a dose as low as 0.3 µgPb/mL that is well below the toxicity threshold for Pb and Cs ions. Our approach contributes to expanding significantly the fields of applications of these luminescent materials including biology and biomedicine.

Red-emissive nanocrystals of Cs4MnxCd1-xSb2Cl12 layered perovskites.

Layered double perovskites are currently being investigated as emerging halide-based materials for optoelectronic applications. Herein, we present the synthesis of Cs4MnxCd1-xSb2Cl12 (0 ≤ x ≤ 1) nanocrystals (NCs). X-ray powder diffraction evidences the retention of the same crystal structure for all the inspected compositions; transmission electron microscopy revealed monodisperse particles with a mean size between 10.7 nm and 12.7 nm. The absorption spectra are mostly determined by transitions related to Sb3+, whereas Mn2+ induced a red emission in the 625-650 nm range. The photoluminescence emission intensity and position vary with the Mn2+ content and reach the maximum for the composition with x = 0.12. Finally, we demonstrate that the photoluminescence quantum yield of the latter NCs was increased from 0.3% to 3.9% through a post-synthesis treatment with ammonium thiocyanate. The present work expands the knowledge of colloidal layered double perovskite nanocrystals, stimulating future investigations of this emerging class of materials.

Structure and Surface Passivation of Ultrathin Cesium Lead Halide Nanoplatelets Revealed by Multilayer Diffraction.

The research on two-dimensional colloidal semiconductors has received a boost from the emergence of ultrathin lead halide perovskite nanoplatelets. While the optical properties of these materials have been widely investigated, their accurate structural and compositional characterization is still challenging. Here, we exploited the natural tendency of the platelets to stack into highly ordered films, which can be treated as single crystals made of alternating layers of organic ligands and inorganic nanoplatelets, to investigate their structure by multilayer diffraction. Using X-ray diffraction alone, this method allowed us to reveal the structure of ∼12 Šthick Cs-Pb-Br perovskite and ∼25 Šthick Cs-Pb-I-Cl Ruddlesden-Popper nanoplatelets by precisely measuring their thickness, stoichiometry, surface passivation type and coverage, as well as deviations from the crystal structures of the corresponding bulk materials. It is noteworthy that a single, readily available experimental technique, coupled with proper modeling, provides access to such detailed structural and compositional information.

Fast Intrinsic Emission Quenching in Cs4PbBr6 Nanocrystals.

Cs4PbBr6 (0D) nanocrystals at room temperature have both been reported as nonemissive and green-emissive systems in conflicting reports, with no consensus regarding both the origin of the green emission and the emission quenching mechanism. Here, via ab initio molecular dynamics (AIMD) simulations and temperature-dependent photoluminescence (PL) spectroscopy, we show that the PL in these 0D metal halides is thermally quenched well below 300 K via strong electron-phonon coupling. To unravel the source of green emission reported for bulk 0D systems, we further study two previously suggested candidate green emitters: (i) a Br vacancy, which we demonstrate to present a strong thermal emission quenching at room temperature; (ii) an impurity, based on octahedral connectivity, that succeeds in suppressing nonradiative quenching via a reduced electron-phonon coupling in the corner-shared lead bromide octahedral network. These findings contribute to unveiling the mechanism behind the temperature-dependent PL in lead halide materials of different dimensionality.

News in Nanocrystals Seminar: Self-Assembly of Early Career Researchers toward Globally Accessible Nanoscience.

In 2020, many in-person scientific events were canceled due to the COVID-19 pandemic, creating a vacuum in networking and knowledge exchange between scientists. To fill this void in scientific communication, a group of early career nanocrystal enthusiasts launched the virtual seminar series, News in Nanocrystals, in the summer of 2020. By the end of the year, the series had attracted over 850 participants from 46 countries. In this Nano Focus, we describe the process of organizing the News in Nanocrystals seminar series; discuss its growth, emphasizing what the organizers have learned in terms of diversity and accessibility; and provide an outlook for the next steps and future opportunities. This summary and analysis of experiences and learned lessons are intended to inform the broader scientific community, especially those who are looking for avenues to continue fostering discussion and scientific engagement virtually, both during the pandemic and after.

Relations between absorption, emission, and excited state chemical potentials from nanocrystal 2D spectra.

For quantum-confined nanomaterials, size dispersion causes a static broadening of spectra that has been difficult to measure and invalidates all-optical methods for determining the maximum photovoltage that an excited state can generate. Using femtosecond two-dimensional (2D) spectroscopy to separate size dispersion broadening of absorption and emission spectra allows a test of single-molecule generalized Einstein relations between such spectra for colloidal PbS quantum dots. We show that 2D spectra and these relations determine the thermodynamic standard chemical potential difference between the lowest excited and ground electronic states, which gives the maximum photovoltage. Further, we find that the static line broadening from many slightly different quantum dot structures allows single-molecule generalized Einstein relations to determine the average single-molecule linewidth from Stokes' frequency shift between ensemble absorption and emission spectra.

Multilayer Diffraction Reveals That Colloidal Superlattices Approach the Structural Perfection of Single Crystals.

Colloidal superlattices are fascinating materials made of ordered nanocrystals, yet they are rarely called "atomically precise". That is unsurprising, given how challenging it is to quantify the degree of structural order in these materials. However, once that order crosses a certain threshold, the constructive interference of X-rays diffracted by the nanocrystals dominates the diffraction pattern, offering a wealth of structural information. By treating nanocrystals as scattering sources forming a self-probing interferometer, we developed a multilayer diffraction method that enabled the accurate determination of the nanocrystal size, interparticle spacing, and their fluctuations for samples of self-assembled CsPbBr3 and PbS nanomaterials. The multilayer diffraction method requires only a laboratory-grade diffractometer and an open-source fitting algorithm for data analysis. The average nanocrystal displacement of 0.33 to 1.43 Å in the studied superlattices provides a figure of merit for their structural perfection and approaches the atomic displacement parameters found in traditional crystals.

Detection of Pb2+ traces in dispersion of Cs4PbBr6 nanocrystals by in situ liquid cell transmission electron microscopy.

The Cs4PbBr6 nanocrystals are often used as a starting material for the preparation of green-emitting CsPbBr3 perovskite nanocrystals by means of chemical and physical transformations. Herein, we probe the Cs4PbBr6 nanocrystals dispersed in a solvent by liquid cell transmission electron microscopy (LCTEM). The nanocrystal dispersion in toluene is placed between two electron-transparent membranes separated by a gold spacer in a liquid cell and studied in a high angular annular dark-field scanning TEM mode with a fixed electron dose rate. We observe the spontaneous nucleation and growth of round and dendrite-shaped nanoparticles under electron beam illumination in the areas of solution where no Cs4PbBr6 nanocrystals are seen. These newly-formed nanoparticles show high contrast and contain Pb as the only heavy element, suggesting that they are made from metallic lead and indicating Pb2+-containing species in solution as their precursor. Also, a small amount of Au0 nanoparticles are formed, most likely due to the dissolution of the gold spacer by free Br-containing species in the nanocrystal dispersion and a subsequent reduction of the leached species under the electron beam. The analysis of the UV-Vis absorption spectra of Cs4PbBr6 nanocrystals and the supernatant isolated from the synthesis points to mixed lead(ii) oleate/bromide species as the likely residue, corroborating LCTEM results. The identification of the residual precursors in Cs4PbBr6 nanocrystal samples after the post-synthetic isolation is an important task because the residues may alter the subsequent reactivity of the nanocrystals.

Metamorphoses of Cesium Lead Halide Nanocrystals.

Following the impressive development of bulk lead-based perovskite photovoltaics, the "perovskite fever" did not spare nanochemistry. In just a few years, colloidal cesium lead halide perovskite nanocrystals have conquered researchers worldwide with their easy synthesis and color-pure photoluminescence. These nanomaterials promise cheap solution-processed lasers, scintillators, and light-emitting diodes of record brightness and efficiency. However, that promise is threatened by poor stability and unwanted reactivity issues, throwing down the gauntlet to chemists.More generally, Cs-Pb-X nanocrystals have opened an exciting chapter in the chemistry of colloidal nanocrystals, because their ionic nature and broad diversity have challenged many paradigms established by nanocrystals of long-studied metal chalcogenides, pnictides, and oxides. The chemistry of colloidal Cs-Pb-X nanocrystals is synonymous with change: these materials demonstrate an intricate pattern of shapes and compositions and readily transform under physical stimuli or the action of chemical agents. In this Account, we walk through four types of Cs-Pb-X nanocrystal metamorphoses: change of structure, color, shape, and surface. These transformations are often interconnected; for example, a change in shape may also entail a change of color.The ionic bonding, high anion mobility due to vacancies, and preservation of cationic substructure in the Cs-Pb-X compounds enable fast anion exchange reactions, allowing the precise control of the halide composition of nanocrystals of perovskites and related compounds (e.g., CsPbCl3 ⇄ CsPbBr3 ⇄ CsPbI3 and Cs4PbCl6 ⇄ Cs4PbBr6 ⇄ Cs4PbI6) and tuning of their absorption edge and bright photoluminescence across the visible spectrum. Ion exchanges, however, are just one aspect of a richer chemistry.Cs-Pb-X nanocrystals are able to capture or release (in short, trade) ions or even neutral species from or to the surrounding environment, causing major changes to their structure and properties. The trade of neutral PbX2 units allows Cs-Pb-X nanocrystals to cross the boundaries among four different types of compounds: 4CsX + PbX2 ⇄ Cs4PbBr6 + 3PbX2 ⇄ 4CsPbBr3 + PbX2 ⇄ 4CsPb2X5. These reactions do not occur at random, because the reactant and product nanocrystals are connected by the Cs+ cation substructure preservation principle, stating that ion trade reactions can transform one compound into another by means of distorting, expanding, or contracting their shared Cs+ cation substructure.The nanocrystal surface is a boundary between the core and the surrounding environment of Cs-Pb-X nanocrystals. The surface influences nanocrystal stability, optical properties, and shape. For these reasons, the dynamic surface of Cs-Pb-X nanocrystals has been studied in detail, especially in CsPbX3 perovskites. Two takeaways have emerged from these studies. First, the competition between primary alkylammonium and cesium cations for the surface sites during the CsPbX3 nanocrystal nucleation and growth governs the cube/plate shape equilibrium. Short-chain acids and branched amines influence that equilibrium and enable shape-shifting synthesis of pure CsPbX3 cubes, nanoplatelets, nanosheets, or nanowires. Second, quaternary ammonium halides are emerging as superior ligands that extend the shelf life of Cs-Pb-X colloidal nanomaterials, boost their photoluminescence quantum yield, and prevent foreign ions from escaping the nanocrystals. That is accomplished by combining reduced ligand solubility, due to the branched organic ammonium cation, with the surface-healing capabilities of the halide counterions, which are small Lewis bases.

Aging of Self-Assembled Lead Halide Perovskite Nanocrystal Superlattices: Effects on Photoluminescence and Energy Transfer.

Excitonic coupling, electronic coupling, and cooperative interactions in self-assembled lead halide perovskite nanocrystals were reported to give rise to a red-shifted collective emission peak with accelerated dynamics. Here we report that similar spectroscopic features could appear as a result of the nanocrystal reactivity within the self-assembled superlattices. This is demonstrated by studying CsPbBr3 nanocrystal superlattices over time with room-temperature and cryogenic micro-photoluminescence spectroscopy, X-ray diffraction, and electron microscopy. It is shown that a gradual contraction of the superlattices and subsequent coalescence of the nanocrystals occurs over several days of keeping such structures under vacuum. As a result, a narrow, low-energy emission peak is observed at 4 K with a concomitant shortening of the photoluminescence lifetime due to the energy transfer between nanocrystals. When exposed to air, self-assembled CsPbBr3 nanocrystals develop bulk-like CsPbBr3 particles on top of the superlattices. At 4 K, these particles produce a distribution of narrow, low-energy emission peaks with short lifetimes and excitation fluence-dependent, oscillatory decays. Overall, the aging of CsPbBr3 nanocrystal assemblies dramatically alters their emission properties and that should not be overlooked when studying collective optoelectronic phenomena nor confused with superfluorescence effects.

Transforming colloidal Cs4PbBr6 nanocrystals with poly(maleic anhydride-alt-1-octadecene) into stable CsPbBr3 perovskite emitters through intermediate heterostructures.

The preparation of strongly emissive CsPbBr3 perovskite nanocrystals with robust surface passivation is a challenge in the field of lead halide perovskite nanomaterials. We report an approach to prepare polymer-capped CsPbBr3 perovskite nanocrystals by reacting oleylammonium/oleate-capped Cs4PbBr6 nanocrystals with poly(maleic anhydride-alt-1-octadecene) (PMAO). PMAO contains succinic anhydride units that are reactive towards the oleylamine species present on the Cs4PbBr6 nanocrystals' surface and produces polysuccinamic acid, which, in turn, triggers the Cs4PbBr6 to CsPbBr3 conversion. The transformation occurs through the formation of Cs4PbBr6-CsPbBr3 heterostructures as intermediates, which are captured because of the mild reactivity of PMAO and are investigated by high-resolution electron microscopy. The Cs4PbBr6-CsPbBr3 heterostructures demonstrate a dual emission at cryogenic temperature with an indication of the energy transfer from Cs4PbBr6 to CsPbBr3. The fully-transformed CsPbBr3 NCs have high photoluminescence quantum yield and enhanced colloidal stability, which we attribute to the adhesion of polysuccinamic acid to the NC surface through its multiple functional groups in place of oleate and alkylammonium ligands. The PMAO-induced transformation of Cs4PbBr6 NCs opens up a strategy for the chemical modification of metal halide NCs initially passivated with nucleophilic amines.

Characterization of gap-plasmon based metasurfaces using scanning differential heterodyne microscopy.

Optical phase-gradient metasurfaces, whose unique capabilities are based on the possibility to arbitrarily control the phase of reflected/transmitted light at the subwavelength scale, are seldom characterized with direct measurements of phase gradients. Using numerical simulations and experimental measurements, we exploit the technique of scanning differential heterodyne microscopy (SDHM) for direct phase and amplitude characterization of gap-plasmon based optical metasurfaces. Two metasurface configurations utilizing the third-order gap surface plasmon (GSP) resonance, representing a binary grating and linear phase gradient, are experimentally characterized with the SDHM operating at the light wavelength of 633 nm. Comparing the experimental performances of these GSP metasurfaces with those expected from the phase and amplitude profiles reconstructed from the SDHM measurements, we verify the efficiency and accuracy of the developed SDHM characterization approach for direct inspection of GSP reflective metasurfaces.

Correction: Transforming colloidal Cs4PbBr6 nanocrystals with poly(maleic anhydride-alt-1-octadecene) into stable CsPbBr3 perovskite emitters through intermediate heterostructures.

[This corrects the article DOI: 10.1039/D0SC00738B.].

Cs3Cu4In2Cl13 Nanocrystals: A Perovskite-Related Structure with Inorganic Clusters at A Sites.

An effort to synthesize the Cu(I) variant of a lead-free double perovskite isostructural with Cs2AgInCl6 resulted in the formation of Cs3Cu4In2Cl13 nanocrystals with an unusual structure, as revealed by single-nanocrystal three-dimensional electron diffraction. These nanocrystals adopt a A2BX6 structure (K2PtCl6 type, termed vacancy ordered perovskite) with tetrahedrally coordinated Cu(I) ions. In the structure, 25% of the A sites are occupied by [Cu4Cl]3+ clusters (75% by Cs+), and the B sites are occupied by In3+. Such a Cs3Cu4In2Cl13 compound prepared at the nanoscale is not known in the bulk and is an example of a multinary metal halide with inorganic cluster cations residing in A sites. The stability of the compound was supported by density functional theory calculations that also revealed that its bandgap is direct but parity forbidden. The existence of the Cs3Cu4In2Cl13 structure demonstrates that small inorganic cluster cations can occupy A sites in multinary metal halides.

Investigation into the Photoluminescence Red Shift in Cesium Lead Bromide Nanocrystal Superlattices.

The formation of cesium lead bromide (CsPbBr3) nanocrystal superlattices (NC SLs) is accompanied by a red shift in the NC photoluminescence (PL). The values of the PL red shift reported in the literature range from none to ∼100 meV without unifying explanation of the differences. Using a combination of confocal PL microcopy and steady-state optical spectroscopies we found that an overall PL red shift of ∼96 meV measured from a macroscopic sample of CsPbBr3 NC SLs has several contributions: ∼ 10-15 meV from a red shift in isolated and clean SLs, ∼ 30 meV from SLs with impurities of bulklike CsPbBr3 crystals on their surface, and up to 50 meV or more of the red shift coming from a photon propagation effect, specifically self-absorption. In addition, a self-assembly technique for growing micron-sized NC SLs on the surface of perfluorodecalin, an inert perfluorinated liquid and an antisolvent for NCs, is described.

Green-Emitting Powders of Zero-Dimensional Cs4PbBr6: Delineating the Intricacies of the Synthesis and the Origin of Photoluminescence.

A detailed investigation into the synthesis of green-emitting powders of Cs4PbBr6 and CsPbBr3 materials by antisolvent precipitation from CsBr-PbBr2 precursor solutions in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) is reported. Various solvated lead bromide and polybromide species (PbBr2, [PbBr3]-, [PbBr4]2-, and possibly [PbBr5]3-or [PbBr6]4-) are detected in the precursor solutions by optical absorbance and emission spectroscopies. The solvodynamic size of the species in solution is strongly solvent-dependent: ~1 nm species were detected in DMSO, while significantly larger species were observed in DMF by dynamic light scattering. The solvodynamic size of the lead bromide species plays a critical role in determining the Cs-Pb-Br composition of the precipitated powders: smaller species favor the precipitation of Cs4PbBr6, while larger species template the formation of CsPbBr3 under identical experimental conditions. The powders have been characterized by 133Cs and 207Pb solid-state nuclear magnetic resonance, and 133Cs sensitivity toward the different Cs environments within Cs4PbBr6 is demonstrated. Finally, the possible origins of green emission in Cs4PbBr6 samples are discussed. It is proposed that a two-dimensional Cs2PbBr4 inclusion may be responsible for green emission at ~520 nm in addition to the widely acknowledged CsPbBr3 impurity, although we found no conclusive experimental evidence supporting such claims.