Supercrystal Engineering of Nanoarrows Enabled by Tailored Concavity.

Self-assembly of nanoparticles into supercrystals represents a powerful approach to create unique and complex superstructures with fascinating properties and novel functions, but the complexity in spatial configuration, and the tunability in lattice structure are still quite limited compared to the crystals formed by atoms and molecules. Herein, shallowly concave gold nanoarrows with a unique concave-convex geometry are synthesized and employed as novel building blocks for shape-directed self-assembly of a wealth of complex 3D supercrystals with unprecedented configurations. The obtained diverse superstructures including six Interlocking-type supercrystals and three Packing-type supercrystals exhibit four types of Bravais lattices (i.e., tP, oI, tI, and oF) and six types of crystallographic space groups (i.e., Pmmm, I222, Pnnm, Ibam, I4/mmm, and Fmmm), which have not been documented in the mesoscale self-assembled systems. It has been revealed that the relative yield of different supercrystal structures is mainly determined by the packing density and deformability of the supercrystals, which are closely related to the tailored concavity of the nanoparticles and is affected by the particle concentration, thus allowing for programmable self-assembly into specific supercrystals through particle shape modulation. The concavity-enabled supercrystal engineering may open a new avenue toward unconventional nanoparticle superstructures with expanded complexity, tunability, and functionality.

Spontaneous Supercrystal Formation During a Strain-Engineered Metal-Insulator Transition.

Mott metal-insulator transitions possess electronic, magnetic, and structural degrees of freedom promising next-generation energy-efficient electronics. A previously unknown, hierarchically ordered, and anisotropic supercrystal state is reported and its intrinsic formation characterized in-situ during a Mott transition in a Ca2RuO4 thin film. Machine learning-assisted X-ray nanodiffraction together with cryogenic electron microscopy reveal multi-scale periodic domain formation at and below the film transition temperature (TFilm ≈ 200-250 K) and a separate anisotropic spatial structure at and above TFilm. Local resistivity measurements imply an intrinsic coupling of the supercrystal orientation to the material's anisotropic conductivity. These findings add a new degree of complexity to the physical understanding of Mott transitions, opening opportunities for designing materials with tunable electronic properties.

Ready-to-Use Free-Standing Super-Powder Made with Complex Nanoparticles for SERS.

This study presents a straightforward and efficient synthetic approach for producing high-yield, ready-to-use, free-standing super-powder. The synthesis protocol demonstrates versatility, enabling the creation of assemblies from various nanoparticle morphologies and compositions without the need for specific substrates. Au nanorings are employed as building blocks for fabricating the super-powder, which can be used in surface-enhanced Raman spectroscopy (SERS). The distinctive aspect ratio of the ring nanoframes allows the formation of densely packed columnar assemblies on the substrate, aligning the exposed gaps perpendicular to the laser beam. This arrangement significantly enhances the charge separation among nanorings, leading to a highly focused near-field that is applicable to SERS analysis. The SERS detection feasibility of this powder in both pre- and post-contamination conditions is demonstrated. Using a wide range of building blocks, encompassing various shapes (for instance, rods, hexagons, cubes, cuboctahedrons, elongated dodecahedrons, triangular rings, double-rings, elongated dodecahedra frames, cuboctahedra frames, and double-walled frames), the generalizability of the process for synthesizing super-powders with diverse morphologies is substantiated.

Multiscale Supercrystal Meta-atoms.

Meta-atoms are the building blocks of metamaterials, which are employed to control both generation and propagation of light as well as provide novel functionalities of localization and directivity of electromagnetic radiation. In many cases, simple dielectric or metallic resonators are employed as meta-atoms to create different types of electromagnetic metamaterials. Here, we fabricate and study supercrystal meta-atoms composed of coupled perovskite quantum dots. We reveal that these multiscale structures exhibit specific emission properties, such as spectrum splitting and polaritonic effects. We believe that such multiscale supercrystal meta-atoms will provide novel functionalities in the design of many novel types of active metamaterials and metasurfaces.

Metal Core-Shell Nanoparticle Supercrystals: From Photoactivation of Hydrogen Evolution to Photocorrosion.

Gas nanobubbles are directly linked to many important chemical reactions. While they can be detrimental to operational devices, they also reflect the local activity at the nanoscale. Here, supercrystals made of highly monodisperse Ag@Pt core-shell nanoparticles are first grown onto a solid support and fully characterized by electron microscopies and X-ray scattering. Supercrystals are then used as a plasmonic photocatalytic platform for triggering the hydrogen evolution reaction. The catalytic activity is measured operando at the single supercrystal level by high-resolution optical microscopy, which allows gas nanobubble nucleation to be probed at the early stage with high temporal resolution and the amount of gas molecules trapped inside them to be quantified. Finally, a correlative microscopy approach and high-resolution electron energy loss spectroscopy help to decipher the mechanisms at the origin of the local degradation of the supercrystals during catalysis, namely nanoscale erosion and corrosion.

Slipping-Free Halide Perovskite Supercrystals from Supramolecularly-Assembled Nanocrystals.

Supramolecularly assembled high-order supercrystals (SCs) help control the dielectric, electronic, and excitonic properties of semiconductor nanocrystals (NCs) and quantum dots (QDs). Ligand-engineered perovskite NCs (PNCs) assemble into SCs showing shorter excitonic lifetimes than strongly dielectric PNC films showing long photoluminescence (PL) lifetimes and long-range carrier diffusion. Monodentate to bidentate ligand exchange on ≈ 8 nm halide perovskite (APbX3 ; A:Cs/MA, X:Br/I) PNCs generates mechanically stable SCs with close-packed lattices, overlapping electronic wave functions, and higher dielectric constant, providing distinct excitonic properties from single PNCs or PNC films. From Fast Fourier Transform (FFT) images, time-resolved PL, and small-angle X-ray scattering, structurally and excitonically ordered large SCs are identified. An Sc shows a smaller spectral shift (<35 meV) than a PNC film (>100 meV), a microcrystal (>100 meV), or a bulk crystal (>100 meV). Also, the exciton lifetime (<10 ns) of an SC is excitation power-independent in the single exciton regime 〈N〉<1, comparable to an isolated PNC. Therefore, bidentate-ligand-assisted SCs help overcome delayed exciton or carrier recombination in halide perovskite nanocrystal assemblies or films.

Strengthening Engineered Nanocrystal Three-Dimensional Superlattices via Ligand Conformation and Reactivity.

Nanocrystal assembly into ordered structures provides mesostructural functional materials with a precise control that starts at the atomic scale. However, the lack of understanding on the self-assembly itself plus the poor structural integrity of the resulting supercrystalline materials still limits their application into engineered materials and devices. Surface functionalization of the nanobuilding blocks with organic ligands can be used not only as a means to control the interparticle interactions during self-assembly but also as a reactive platform to further strengthen the final material via ligand cross-linking. Here, we explore the influence of the ligands on superlattice formation and during cross-linking via thermal annealing. We elucidate the effect of the surface functionalization on the nanostructure during self-assembly and show how the ligand-promoted superlattice changes subsequently alter the cross-linking behavior. By gaining further insights on the chemical species derived from the thermally activated cross-linking and its effect in the overall mechanical response, we identify an oxidative radical polymerization as the main mechanism responsible for the ligand cross-linking. In the cascade of reactions occurring during the surface-ligands polymerization, the nanocrystal core material plays a catalytic role, being strongly affected by the anchoring group of the surface ligands. Ultimately, we demonstrate how the found mechanistic insights can be used to adjust the mechanical and nanostructural properties of the obtained nanocomposites. These results enable engineering supercrystalline nanocomposites with improved cohesion while preserving their characteristic nanostructure, which is required to achieve the collective properties for broad functional applications.

Double-Lattice Packing of Pentagonal Gold Bipyramids in Supercrystals with Triclinic Symmetry.

Pentagonal packing is a long-standing issue and a rich mathematical topic, brought to the fore by recent progress in nanoparticle design. Gold pentagonal bipyramids combine fivefold symmetry and anisotropy and their section varies along the length. In this work, colloidal supercrystals of pentagonal gold bipyramids are obtained in a compact arrangement that generalizes the optimal packing of regular pentagons in the plane. Multimodal investigations reveal a two-particle unit cell with triclinic symmetry, a lower symmetry than that of the building blocks. Monte Carlo computer simulations show that this lattice achieves the densest possible packing. Going beyond pentagons, further simulations show an odd-even effect of the number of sides on the packing: odd-sided bipyramids are non-centrosymmetric and require the double-lattice arrangement to recover inversion symmetry. The supercrystals display a facet-dependent optical response that is promising for sensing, metamaterials applications, and for fundamental studies of self-assembly processes.

Tunable Nanoscale Evolution and Topological Phase Transitions of a Polar Vortex Supercrystal.

Understanding the phase transitions and domain evolutions of mesoscale topological structures in ferroic materials is critical to realizing their potential applications in next-generation high-performance storage devices. Here, the behaviors of a mesoscale supercrystal are studied with 3D nanoscale periodicity and rotational topology phases in a PbTiO3 /SrTiO3 (PTO/STO) superlattice under thermal and electrical stimuli using a combination of phase-field simulations and X-ray diffraction experiments. A phase diagram of temperature versus polar state is constructed, showing the formation of the supercrystal from a mixed vortex and a-twin state and a temperature-dependent erasing process of a supercrystal returning to a classical a-twin structure. Under an in-plane electric field bias at room temperature, the vortex topology of the supercrystal irreversibly transforms to a new type of stripe-like supercrystal. Under an out-of-plane electric field, the vortices inside the supercrystal undergo a topological phase transition to polar skyrmions. These results demonstrate the potential for the on-demand manipulation of polar topology and transformations in supercrystals using electric fields. The findings provide a theoretical understanding that may be utilized to guide the design and control of mesoscale polar structures and to explore novel polar structures in other systems and their topological nature.

From Molecules to Frameworks to Superframework Crystals.

Building chemical structures of complexity and functionality approaching the level of biological systems is an ongoing challenge. A general synthetic strategy is proposed by which progressive levels of complexity are achieved through the building block approach whereby molecularly defined constructs at one level serve as constituent units of the next level, all being linked through strong bonds-"augmented reticular chemistry". Specifically, current knowledge of linking metal complexes and organic molecules into reticular frameworks is applied here to linking the crystals of these frameworks into supercrystals (superframeworks). This strategy allows for the molecular control exercised on the molecular regime to be translated into higher augmentation levels to produce systems capable of dynamics and complex functionality far exceeding current materials.

Deformation Behavior of Cross-Linked Supercrystalline Nanocomposites: An in Situ SAXS/WAXS Study during Uniaxial Compression.

With the ever-expanding functional applications of supercrystalline nanocomposites (a relatively new category of materials consisting of organically functionalized nanoparticles arranged into periodic structures), it becomes necessary to ensure their structural stability and understand their deformation and failure mechanisms. Inducing the cross-linking of the functionalizing organic ligands, for instance, leads to a remarkable enhancement of the nanocomposites' mechanical properties. It is however still unknown how the cross-linked organic phase redistributes applied loads, how the supercrystalline lattice accommodates the imposed deformations, and thus in general what phenomena govern the overall material's mechanical response. This work elucidates these aspects for cross-linked supercrystalline nanocomposites through an in situ small- and wide-angle X-ray scattering study combined with uniaxial pressing. Because of this loading condition, it emerges that the cross-linked ligands effectively carry and distribute loads homogeneously throughout the nanocomposites, while the superlattice deforms via rotation, slip, and local defects generation.

Surface-Enhanced Raman Scattering and Surface-Enhanced Infrared Absorption by Plasmon Polaritons in Three-Dimensional Nanoparticle Supercrystals.

Surface-enhanced vibrational spectroscopy strongly increases the cross section of Raman scattering and infrared absorption, overcoming the limited sensitivity and resolution of these two powerful analytic tools. While surface-enhanced setups with maximum enhancement have been studied widely in recent years, substrates with reproducible, uniform enhancement have received less attention although they are required in many applications. Here, we show that plasmonic supercrystals are an excellent platform for enhanced spectroscopy because they possess a high density of hotspots in the electric field. We describe the near field inside the supercrystal within the framework of plasmon polaritons that form due to strong light-matter interaction. From the polariton resonances we predict resonances in the far-field enhancement for Raman scattering and infrared absorption. We verify our predictions by measuring the vibrations of polystyrene molecules embedded in supercrystals of gold nanoparticles. The intensity of surface-enhanced Raman scattering is uniform within 10% across the crystal with a peak integrated enhancement of up to 300 and a peak hotspot enhancement of 105. The supercrystal polaritons induce pairs of incoming and outgoing resonances in the enhanced cross section as we demonstrate experimentally by measuring surface-enhanced Raman scattering with multiple laser wavelengths across the polariton resonance. The infrared absorption of polystyrene is likewise enhanced inside the supercrystals with a maximum enhancement of 400%. We show with a coupled oscillator model that the increase originates from the combined effects of hotspot formation and the excitation of standing polariton waves. Our work clearly relates the structural and optical properties of plasmonic supercrystals and shows that such crystals are excellent hosts and substrates for the uniform and predictable enhancement of vibrational spectra.

Spontaneous Formation of Highly Stable Nanoparticle Supercrystals Driven by a Covalent Bonding Interaction.

Nanoparticle supercrystals (NPSCs) are of great interest as materials with emergent properties. Different types of intermolecular forces, such as van der Waals interaction and hydrogen bonding, are present in the NPSCs fabricated to date. However, the limited structural stability of such NPSCs that results from the weakness of these intermolecular forces is a challenge. Here, we report a spontaneous formation of NPSCs driven by covalent bonding interactions, a type of intramolecular force much stronger than the above-mentioned intermolecular forces. A model solution-phase anhydride reaction is used to form covalent bonds between molecules grafted on the surface of gold nanoparticles, resulting in three-dimensional NPSCs. The NPSCs are very stable in different solvents, in dried conditions, and at temperatures as high as 160 °C. In addition to this, the large library of covalent-bond-forming reactions available and the low cost of reactants make the covalent bonding approach highly versatile and economical.

Integrating Plasmonic Supercrystals in Microfluidics for Ultrasensitive, Label-Free, and Selective Surface-Enhanced Raman Spectroscopy Detection.

Surface-enhanced Raman spectroscopy (SERS) microfluidic chips for label-free and ultrasensitive detection are fabricated by integrating a plasmonic supercrystal within microfluidic channels. This plasmonic platform allows the uniform infiltration of the analytes within the supercrystal, reaching the so-called hot spots. Moreover, state-of-the-art simulations performed using large-scale supercrystal models demonstrate that the excellent SERS response is due to the hierarchical nanoparticle organization, the interparticle separation (IPS), and the presence of supercrystal defects. Proof-of-concept experiments confirm the outstanding performance of the microfluidic chips for the ultradetection of (bio)molecules with no metal affinity. In fact, a limit of detection (LOD) as low as 10-19 M was reached for crystal violet. The SERS microfluidic chips show excellent sensitivity in the direct analysis of pyocyanin secreted by Pseudomonas aeruginosa grown in a liquid culture medium. Finally, the further integration of a silica-based column in the plasmonic microchip provides charge-selective SERS capabilities as demonstrated for a mixture of positively and negatively charged molecules.

Simultaneous Photonic and Excitonic Coupling in Spherical Quantum Dot Supercrystals.

Semiconductor nanocrystals, or quantum dots (QDs), simultaneously benefit from inexpensive low-temperature solution processing and exciting photophysics, making them the ideal candidates for next-generation solar cells and photodetectors. While the working principles of these devices rely on light absorption, QDs intrinsically belong to the Rayleigh regime and display optical behavior limited to electric dipole resonances, resulting in low absorption efficiencies. Increasing the absorption efficiency of QDs, together with their electronic and excitonic coupling to enhance charge carrier mobility, is therefore of critical importance to enable practical applications. Here, we demonstrate a general and scalable approach to increase both light absorption and excitonic coupling of QDs by fabricating hierarchical metamaterials. We assemble QDs into crystalline supraparticles using an emulsion template and demonstrate that these colloidal supercrystals (SCs) exhibit extended resonant optical behavior resulting in an enhancement in absorption efficiency in the visible range of more than 2 orders of magnitude with respect to the case of dispersed QDs. This successful light trapping strategy is complemented by the enhanced excitonic coupling observed in ligand-exchanged SCs, experimentally demonstrated through ultrafast transient absorption spectroscopy and leading to the formation of a free biexciton system on sub-picosecond time scales. These results introduce a colloidal metamaterial designed by self-assembly from the bottom up, simultaneously featuring a combination of nanoscale and mesoscale properties leading to simultaneous photonic and excitonic coupling, therefore presenting the nanocrystal analogue of supramolecular structures.

Highly Stable Perovskite Supercrystals via Oil-in-Oil Templating.

Inorganic perovskites display an enticing foreground for their wide range of optoelectronic applications. Recently, supercrystals (SCs) of inorganic perovskite nanocrystals (NCs) have been reported to possess highly ordered structure as well as novel collective optical properties, opening new opportunities for efficient films. Here, we report the large-scale assembly control of spherical, cubic, and hexagonal SCs of inorganic perovskite NCs through templating by oil-in-oil emulsions. We show that an interplay between the roundness of the cubic NCs and the tension of the confining droplet surface sets the superstructure morphology, and we exploit this interplay to design dense hyperlattices of SCs. The SC films show strongly enhanced stability for at least two months without obvious structural degradation and minor optical changes. Our results on the controlled large-scale assembly of perovskite NC superstructures provide new prospects for the bottom-up production of optoelectronic devices based on the microfluidic production of mesoscopic building blocks.

Template-Free, Surfactant-Mediated Orientation of Self-Assembled Supercrystals of Metal-Organic Framework Particles.

Mesoscale self-assembly of particles into supercrystals is important for the design of functional materials such as photonic and plasmonic crystals. However, while much progress has been made in self-assembling supercrystals adopting diverse lattices and using different types of particles, controlling their growth orientation on surfaces has received limited success. Most of the latter orientation control has been achieved via templating methods in which lithographic processes are used to form a patterned surface that acts as a template for particle assembly. Herein, a template-free method to self-assemble (111)-, (100)-, and (110)-oriented face-centered cubic supercrystals of the metal-organic framework ZIF-8 particles by adjusting the amount of surfactant (cetyltrimethylammonium bromide) used is described. It is shown that these supercrystals behave as photonic crystals whose properties depend on their growth orientation. This control on the orientation of the supercrystals dictates the orientation of the composing porous particles that might ultimately facilitate pore orientation on surfaces for designing membranes and sensors.

Light Driven Design of Dynamical Thermosensitive Plasmonic Superstructures: A Bottom-Up Approach Using Silver Supercrystals.

When narrowly distributed silver nanoparticles (NPs) are functionalized by dodecanethiol, they acquire the ability to self-organize in organic solvents into 3D supercrystals (SCs). The NP surface chemistry is shown to introduce a light-driven thermomigration effect, thermophoresis. Using a laser beam to heat the NPs and generate steep thermal gradients, the migration effect is triggered dynamically, leading to tailored structures with high density of plasmonic hot spots. This work describes how to manipulate the hot spots and monitor the effect by holography, thus providing a complete characterization of the migration process on a single object basis. Extensive single object tracking strategies are employed to measure the SCs trajectories, evaluate their size, drift velocity magnitude and direction, allowing the identification of the physical chemical origins of the migration. The phenomenon is shown to happen as a result of the combination of thermophoresis (at short length scales) and convection (long-range), and does not require a metallic substrate. This constitutes a fully optical method to dynamically generate plasmonic platforms in situ and on demand, without requiring substrate nanostructuration and with minimal interference on the chemistry of the system. The importance of the proof-of-concept herein described stems from the numerous potential applications, spanning over a variety of fields such as microfluidics and biosensing.

Revealing Driving Forces in Quantum Dot Supercrystal Assembly.

The assembly of semiconductor nanoparticles, quantum dots (QDs), into dense crystalline nanostructures holds great promise for future optoelectronic devices. However, knowledge of the sub-nanometer scale driving forces underlying the kinetic processes of nucleation, growth, and final densification during QD assembly remains poor. Emulsion-templated assembly has recently been shown to provide good control over the bulk condensation of QDs into highly ordered 3D supercrystals. Here, emulsion-templated assembly is combined with in situ small-angle X-ray scattering to obtain direct insight into the nanoscale interactions underlying the nucleation, growth, and densification of QD supercrystals. At the point of supercrystal nucleation, nanoparticles undergo a hard-sphere-like crystallization into a hexagonal-close-packed lattice, slowly transforming into a face-centered-cubic lattice. The ligands play a crucial role in balancing steric repulsion against attractive van der Waals forces to mediate the initial equilibrium assembly, but cause the QDs to be progressively destabilized upon densification. The rich detail of this kinetic study elucidates the assembly and thermodynamic properties that define QD supercrystal fabrication approaching single-crystal quality, paving the way toward their use in optoelectronic devices.

Spontaneous Self-Assembly of Perovskite Nanocrystals into Electronically Coupled Supercrystals: Toward Filling the Green Gap.

Self-assembly of nanoscale building blocks into ordered nanoarchitectures has emerged as a simple and powerful approach for tailoring the nanoscale properties and the opportunities of using these properties for the development of novel optoelectronic nanodevices. Here, the one-pot synthesis of CsPbBr3 perovskite supercrystals (SCs) in a colloidal dispersion by ultrasonication is reported. The growth of the SCs occurs through the spontaneous self-assembly of individual nanocrystals (NCs), which form in highly concentrated solutions of precursor powders. The SCs retain the high photoluminescence (PL) efficiency of their NC subunits, however also exhibit a redshifted emission wavelength compared to that of the individual nanocubes due to interparticle electronic coupling. This redshift makes the SCs pure green emitters with PL maxima at ≈530-535 nm, while the individual nanocubes emit a cyan-green color (≈512 nm). The SCs can be used as an emissive layer in the fabrication of pure green light-emitting devices on rigid or flexible substrates. Moreover, the PL emission color is tunable across the visible range by employing a well-established halide ion exchange reaction on the obtained CsPbBr3 SCs. These results highlight the promise of perovskite SCs for light emitting applications, while providing insight into their collective optical properties.