Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials

Bi-magnetic Mn3O4@Ni core-shell binary superparticles: Self-assembly preparation and magnetic behaviors

Binary superparticles formed by self-assembling two different types of nanoparticles may utilize the synergistic interactions and create advanced multifunctional materials. Bi-magnetic superparticles with a core-shell structure have unique properties due to their specific spatial configurations. Herein, we built Mn3O4@Ni core-shell binary superparticles via an emulsion self-assembly technique. The superparticles are generated with a spherical morphology, and have a typical average size of about 240 nm. By altering the ratio of the two magnetic nanoparticles, the thickness of Ni shells can be adjusted. Oleic acid ligands are crucial for the formation of core-shell structure. Magnetic analysis suggests that core-shell superparticles display dual-phase magnetic interactions, contrasting with the single-phase magnetic behaviors of commonly core-shell magnetic nanoparticles. The calculation on the effective magnetic anisotropy constants indicates that the presence of Ni shell layers reduces the dipole interactions among the Mn3O4 core particles. Due to the presence of Ni nanoparticle shells, the blocking temperature of Mn3O4 is reduced, while the Curie temperature of Mn3O4 is independent on Ni content. Tunable magnetic properties can be achieved by modulating the Ni nanoparticle shell thickness. This study offers insights for the development of core-shell superparticles with varied magnetic characteristics.

Guest-guided anchoring patterns of cyclodextrin supramolecular microcrystals on droplet surfaces

The growth of cyclodextrin inclusion complexes (ICs) on oil/water interfaces represents a beautiful example of spontaneous pattern formation in nature. How the supramolecules evolve remains a challenge because surface confinement can frustrate microcrystal growth and give rise to unusual phase transitions. Here we investigate the self-assembly of ICs on droplet surfaces using microfluidics, which allows directly visualizing packing, wetting and ordering of the microcrystals anchored on the surface. The oil guests of distinct molecular structures can direct the assembly of the ICs and largely affect anchoring dynamics of the ICs microcrystals, leading to a range of behaviors including orientating, slipping, buckling, jamming, or merging. We discuss the behaviors observed in terms of the flexibility of the building blocks, which offers a new degree of freedom through which to tailor their properties and gives rise to a striking feature of anchoring patterns that have no counterpart in normal colloidal crystals.

Predictive Modeling of Nanocrystal Orientation in Superlattices: Insights from Ligand Entropy

The self-assembly of nanocrystals (NCs) into close-packed, ordered superlattices (SLs) is of broad, engineering interest. The coherent orientation of polyhedral nanocrystals within NC SLs enhances electronic, magnetic, and vibrational coupling, leading to a variety of emergent phenomena. Here, we show that coherent orientation of polyhedral NCs in many SLs can be understood simply by considering its effect on the conformational entropy of surface ligands. We report the predicted nanocrystal orientations and entropic driving force to orient for a broad range of nanocrystal shapes and superlattice unit cells, and we show that ligand entropy is sufficient to reproduce a host of reported experimental and computational observations. We additionally use this framework to predict the expected distribution of interstitial species such as solvent or unbound ligands in an oriented NC SL. This work offers intuition for understanding the orientation of NCs in superlattices and a future framework for analyzing multinary structures.

Surface Orthogonal Patterning and Bidirectional Self-Assembly of Nanoparticles Tethered by V-Shaped Diblock Copolymers

We investigated the surface orthogonal patterning and bidirectional self-assembly of binary hairy nanoparticles (NPs) constructed by uniformly tethering a single NP with multiple V-shaped AB diblock copolymers using Brownian dynamics simulations in a poor solvent. At low concentration, the chain collapse and microphase separation of binary polymer brushes can lead to the patterning of the NP surface into A- and B-type orthogonal patches with various numbers of domains (valency), n = 1-6, that adopt spherical, linear, triangular, tetrahedral, square pyramidal, and pentagonal pyramidal configurations. There is a linear relationship between the valency and the average ratio of NP diameter to the polymers' unperturbed root-mean-square end-to-end distance for the corresponding valency. The linear slope depends on the grafting density and is independent of the interaction parameters between polymers. At high concentration, the orthogonal patch NPs serve as building blocks and exhibit directional attractions by overlapping the same type of domains, resulting in self-assembly into a series of fascinating architectures depending on the valency and polymer length. Notably, the 2-valent orthogonal patch NPs have the bidirectional bonding ability to form the two-dimensional (2D) square NP arrays by two distinct pathways. Simultaneously patching A and B blocks enables the one-step formation of 2D square arrays via bidirectional growth, whereas step-by-step patching causes the directional formation of 1D chains followed by 2D square arrays. Moreover, the gap between NPs in the 2D square arrays is related to the polymer length but independent of the NP diameter. These 2D square NP arrays are of significant value in practical applications such as integrated circuit manufacturing and nanotechnology.

Coassembly of Complementary Polyhedral Metal-Organic Framework Particles into Binary Ordered Superstructures

Here we report the formation of a 3D NaCl-type binary porous superstructure via coassembly of two colloidal polyhedral metal-organic framework (MOF) particles having complementary sizes, shapes, and charges. We employed a polymeric-attenuated Coulombic self-assembly approach, which also facilitated the coassembly of these MOF particles with spherical polystyrene particles to form 2D binary superstructures. Our results pave the way for using MOFs to create sophisticated superstructures comprising particles of various sizes, shapes, porosities, and chemical compositions.

Organic Molecular Glues to Design Three-Dimensional Cubic Nano-assemblies of Magnetic Nanoparticles

Self-assembled magnetic nanoparticles offer next-generation materials that allow harnessing of their physicochemical properties for many applications. However, how three-dimensional nanoassemblies of magnetic nanoparticles can be synthesized in one-pot synthesis without excessive postsynthesis processes is still a bottleneck. Here, we propose a panel of small organic molecules that glue nanoparticle crystallites during the growth of particles to form large nanoassembled nanoparticles (NANs). We find that both carbonyl and carboxyl functional groups, presenting in benzaldehyde and benzoic acid, respectively, are needed to anchor with metal ions, while aromatic rings are needed to create NANs through π-π stacking. When benzyl alcohol, lacking carbonyl and carboxyl groups, is employed, no NANs are formed. NANs formed by benzoic acid reveal a unique combination of high magnetization and coercivity, whereas NANs formed by benzaldehyde show the largest exchange bias reported in nanoparticles. Surprisingly, our NANs show unconventional colloidal stability due to their unique nanoporous architectures.

Random close packing of semi-flexible polymers in two dimensions: Emergence of local and global order

Through extensive Monte Carlo simulations, we systematically study the effect of chain stiffness on the packing ability of linear polymers composed of hard spheres in extremely confined monolayers, corresponding effectively to 2D films. First, we explore the limit of random close packing as a function of the equilibrium bending angle and then quantify the local and global order by the degree of crystallinity and the nematic or tetratic orientational order parameter, respectively. A multi-scale wealth of structural behavior is observed, which is inherently absent in the case of athermal individual monomers and is surprisingly richer than its 3D counterpart under bulk conditions. As a general trend, an isotropic to nematic transition is observed at sufficiently high surface coverages, which is followed by the establishment of the tetratic state, which in turn marks the onset of the random close packing. For chains with right-angle bonds, the incompatibility of the imposed bending angle with the neighbor geometry of the triangular crystal leads to a singular intra- and inter-polymer tiling pattern made of squares and triangles with optimal local filling at high surface concentrations. The present study could serve as a first step toward the design of hard colloidal polymers with a tunable structural behavior for 2D applications.

DNA-mediated assembly of Au bipyramids into anisotropic light emitting kagome superlattices

Colloidal crystal engineering with DNA allows one to design diverse superlattices with tunable lattice symmetry, composition, and spacing. Most of these structures follow the complementary contact model, maximizing DNA hybridization on building blocks and producing relatively close-packed lattices. Here, low-symmetry kagome superlattices are assembled from DNA-modified gold bipyramids that can engage only in partial DNA surface matching. The bipyramid dimensions and DNA length can be engineered for two different superlattices with rhombohedral unit cells, including one composed of a periodic stacking of kagome lattices. Enabled by the partial facet alignment, the kagome lattices exhibit lattice distortion, bipyramid twisting, and planar chirality. When conjugated with Cy-5 dyes, the kagome lattices serve as cavities with high-density optical states and large Purcell factors along lateral directions, leading to strong dipole radiation along the z axis and facet-dependent light emission. Such complex optical properties make these materials attractive for lasers, displays, and quantum sensing constructs.

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.

Choreographing Oscillatory Hydrodynamics with DNA-Coated Gold Nanoparticles

Periodic responses to nonperiodic energy inputs, such as oscillations, are hallmarks of living systems. Nanoparticle-based systems have largely remained unexplored in the generation of oscillatory features. Here, we demonstrate a nanosystem featuring hierarchical response to light, where thermoplasmonic effects and reversible DNA-hybridization generate thermal convective forces and ultimately, oscillatory hydrodynamic flows. The slow aggregation of gold nanoparticles (AuNPs) serves as a positive feedback, while fast photothermal disassembly acts as negative feedback. These asymmetric feedback loops, combined with thermal hysteresis for time-delay, are essential ingredients for orchestrating an oscillating response.

Multi-Stimuli-Responsive Water-Dispersible Magnetite Nanoparticles Using Arylazopyrazole-Modified Polymer Ligands

In order to design new nanomaterials with improved functionalities, magnetite nanoparticles (MNP) modified with arylazopyrazole (AAP) molecular photoswitches are presented. Water dispersibility is achieved by using poly(acrylic acid) (pAA) as a multidentate ligand, which is modified with AAP by amide coupling. The polymer ligand stabilizes the MNP, allows for E-Z isomerization of the photoswitch, and provides pH responsiveness. Three different AAP are synthesized and attached to pAA via amide coupling giving pAA-AAP with photoswitches substituted statistically along the hydrophilic polymer backbone. MNP are synthesized by coprecipitation and pAA-AAP is introduced as a stabilizing agent in situ. Photoisomerization of pAA-AAP and pAA-AAP@MNP is investigated showing good photostationary states and cyclability. The MNP can be assembled and dispersed reversibly in water either by applying a magnetic field or by a change in pH.

Orientation-Dependent Photoconductivity of Quasi-2D Nanocrystal Self-Assemblies: Face-Down, Edge-Up Versus Randomly Oriented Quantum Wells

Here, strongly orientation-dependent lateral photoconductivity of a CdSe monolayer colloidal quantum wells (CQWs) possessing short-chain ligands is reported. A controlled liquid-air self-assembly technique is utilized to deliberately engineer the alignments of CQWs into either face-down (FO) or edge-up (EO) orientation on the substrate as opposed to randomly oriented (RO) CQWs prepared by spin-coating. Adapting planar configuration metal-semiconductor-metal (MSM) photodetectors, it is found that lateral conductivity spans ≈2 orders of magnitude depending on the orientation of CQWs in the film in the case of utilizing short ligands. The long native ligands of oleic acid (OA) are exchanged with short-chain ligands of 2-ethylhexane-1-thiol (EHT) to reduce the inter-platelet distance, which significantly improved the photoresponsivity from 4.16, 0.58, and 4.79 mA W-1 to 528.7, 6.17, and 94.2 mA W-1, for the MSM devices prepared with RO, FO, and EO, before and after ligands exchange, respectively. Such CQW orientation control profoundly impacts the photodetector performance also in terms of the detection speed (0.061 s/0.074 s for the FO, 0.048 s/0.060 s for the EO compared to 0.10 s/0.16 s for the RO, for the rise and decay time constants, respectively) and the detectivity (1.7 × 1010, 2.3 × 1011, and 7.5 × 1011 Jones for the FO, EO, and RO devices, respectively) which can be further tailored for the desired optoelectronic device applications. Attributed to charge transportation in colloidal films being proportional to the number of hopping steps, these findings indicate that the solution-processed orientation of CQWs provides the ability to tune the photoconductivity of CQWs with short ligands as another degree of freedom to exploit and engineer their absorptive devices.

Modulation of excited state intramolecular proton transfer and intramolecular charge transfer pathways of symmetrical azines through micellar medium

The photophysical studies of fluorescent probes in micellar medium can give a better insight about their interaction with biological membranes. This study attempts to access the photophysical properties of the dual emitting azine based probe diethylamino salicylidene azine dimer (DEASAD) in micellar media. DEASAD showed dual charge transfer emission due to the presence of open enol (480 nm) and closed enol (510 nm) forms in polar protic solvents. Upon increasing the concentration of ionic surfactants, there is a significant increase in the emission intensity of both the enol forms of DEASAD until premicellar concentration. After micellization, occurrence of a new anomalous keto form emission through excited state intramolecular proton transfer (ESIPT) was observed around 530 nm in ionic micelles and its intensity changes depend on the micellar surface charge. The emission studies revealed the position and interaction of DEASAD with the charge of micellar stern layer as confirmed through interaction of metal ion with the probe and control molecules with and without ESIPT and ICT moieties. In contrast, the new anomalous longer wavelength keto form of DEASAD emission was absent in neutral micelles like Triton X-100.

BODIPY directed one-dimensional self-assembly of gold nanorods

The assembly of anisotropic nanomaterials into ordered structures is challenging. Nevertheless, such self-assembled systems are known to have novel physicochemical properties and the presence of a chromophore within the nanoparticle ensemble can enhance the optical properties through plasmon-molecule electronic coupling. Here, we report the end-to-end assembly of gold nanorods into micrometer-long chains using a linear diamino BODIPY derivative. The preferential binding affinity of the amino group and the steric bulkiness of BODIPY directed the longitudinal assembly of gold nanorods. As a result of the linear assembly, the BODIPY chromophores positioned themselves in the plasmonic hotspots, which resulted in efficient plasmon-molecule coupling, thereby imparting photothermal properties to the assembled nanorods. This work thus demonstrates a new approach for the linear assembly of gold nanorods resulting in a plasmon-molecule coupled system, and the synergy between self-assembly and electronic coupling resulted in an efficient system having potential biomedical applications.

Nanoparticle Superlattices Driven by Linker-Mediated Covalent Bonding Interaction

The stability of the nanoparticle superlattice (NPSL) is essential for realizing its broad spectrum of potential applications. Here, we report a linker-mediated covalent bonding interaction method for the synthesis of highly stable NPSLs. Adipic acid is used as a linker molecule which connects two Au NPs functionalized with 6-mercaptohexanol through esterification reactions in the presence of H2SO4. As-prepared NPSLs are mostly fcc Wulff polyhedra with a fairly narrow size distribution and are highly stable in solvents of different polarities and pHs (0-14) as well as in dry conditions and at temperatures as high as 175 °C. The formation of NPSLs involves random homogeneous nucleation simultaneously accompanied by growth, a gradual change of the growth mode from reaction-controlled to diffusion-controlled with time, and the oriented attachments of small crystals. The size of the NPSL can be easily tuned by the concentration of linker molecules and the reaction temperature.

Light-Driven, Dynamic Assembly of Micron-To-Centimeter Parts, Micromachines and Microbot Swarms

This work describes light-driven assembly of dynamic formations and functional particle swarms controlled by appropriately programmed light patterns. The system capitalizes on the use of a fluidic bed whose low thermal conductivity assures that light-generated heat remains "localized" and sets strong convective flows in the immediate vicinity of the particles being irradiated. In this way, even low-power laser light or light from a desktop slide projector can be used to organize dynamic formations of objects spanning four orders of magnitude in size (from microns to centimeters) and over nine orders of magnitude in terms of mass. These dynamic assemblies include open-lattice structures with individual particles performing intricate translational and/or rotational motions, density-gradient particle arrays, nested architectures of mechanical components (e.g., planetary gears), or swarms of light-actuated microbots controlling assembly of other objects.

"Nano-egg" superstructures of hydrophobic nanocrystals dispersed in water

In this feature article, we use hydrophobic ferrite (Fe3O4) nanocrystal shells filled with Au nanocrystals self-assembled into 3D superlattices and dispersed in water. These superstructures act as nano-heaters. The stability of such superstructures is very high, even for several years, when stored at room temperature. When subjected to an electron beam, the inverted structure of Fe3O4 structures is gradually dissolved due to the formation of hydrated electrons and hydroxyl radicals.

Nanoparticle Superlattices with Nonequilibrium Crystal Shapes

Nanoparticle assembly is a material synthesis strategy that enables precise control of nanoscale structural features. Concepts from traditional crystal growth research have been tremendously useful in predicting and programming the unit cell symmetries of these assemblies, as their thermodynamically favored structures are often identical to atomic crystal analogues. However, these analogies have not yielded similar levels of influence in programming crystallite shapes, which are a consequence of both the thermodynamics and kinetics of crystal growth. Here, we demonstrate kinetic control of the colloidal crystal shape using nanoparticle building blocks that rapidly assemble over a broad range of concentrations, thereby producing well-defined crystal habits with symmetrically oriented dendritic protrusions and providing insight into the crystals' morphological evolution. Counterintuitively, these nonequilibrium crystal shapes actually become more common for colloidal crystals synthesized closer to equilibrium growth conditions. This deviation from typical crystal growth processes observed in atomic or molecular crystals is shown to be a function of the drastically different time scales of atomic and colloidal mass transport. Moreover, the particles are spherical with isotropic ligand grafts, and these kinetic crystal habits are achieved without the need for specifically shaped particle building blocks or external templating or shape-directing agents. Thus, this work provides generalizable design principles to expand the morphological diversity of nanoparticle superlattice crystal habits beyond the anhedral or equilibrium polyhedral shapes synthesized to date. Finally, we use this insight to synthesize crystallite shapes that have never before been observed, demonstrating the ability to both predict and program kinetically controlled superlattice morphologies.

Strain to shine: stretching-induced three-dimensional symmetries in nanoparticle-assembled photonic crystals

Stretching elastic materials containing nanoparticle lattices is common in research and industrial settings, yet our knowledge of the deformation process remains limited. Understanding how such lattices reconfigure is critically important, as changes in microstructure lead to significant alterations in their performance. This understanding has been extremely difficult to achieve due to a lack of fundamental rules governing the rearrangements. Our study elucidates the physical processes and underlying mechanisms of three-dimensional lattice transformations in a polymeric photonic crystal from 0% to over 200% strain during uniaxial stretching. Corroborated by comprehensive experimental characterizations, we present analytical models that precisely predict both the three-dimensional lattice structures and the macroscale deformations throughout the stretching process. These models reveal how the nanoparticle lattice and matrix polymer jointly determine the resultant structures, which breaks the original structural symmetry and profoundly changes the dispersion of photonic bandgaps. Stretching induces shifting of the main pseudogap structure out from the 1st Brillouin zone and the merging of different symmetry points. Evolutions of multiple photonic bandgaps reveal potential optical singularities shifting with strain. This work sets a new benchmark for the reconfiguration of soft material structures and may lay the groundwork for the study of stretchable three-dimensional topological photonic crystals.

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.

Salt-Assisted, In Situ Current Nanowelding of an Interfacial Au Nanoparticle Film for a High-Performance Electrocatalyst

Interfacial self-assembly is a well-established method for the preparation of a two-dimensional (2D) metal nanofilm from nanoscale building blocks. However, the as-prepared nanofilm exhibits limited conductivity because of the large contact resistance at the junctions among its building blocks. Here, we report a salt-assisted, in situ current nanowelding strategy to weld an interfacial Au nanoparticle (NP) film for downstream applications, such as high-performance electrocatalysts. Particularly, we found that salt-assisted interfacial assembly can reduce the size of the nanogaps among neighboring Au NPs and, in turn, greatly improve the conductivity of the resultant Au NP film. Consequently, the Au NP film can be readily welded using current, and the welding extent can be monitored in real-time by looking at the passing current. The welding finally produces a nanoporous Au film (NPGF) with a network nanostructure, high conductivity, and abundant active sites so that it delivers a large current density of 86.96 μA·cm-2 (1.81 times higher than that from the pristine Au NP film) and shows improved cycling stability for methanol electrooxidation. Thus, these results offer a low-cost, solution-processable approach for the fabrication of a large-area, interconnected nanofilm from nanoscale building blocks beyond Au NPs, which may find diverse downstream applications.

Precision engineering of nano-assemblies in superfluid helium by the use of van der Waals forces

The ability to precisely engineer nanostructures underpins a wide range of applications in areas such as electronics, optics, and biomedical sciences. Here we present a novel approach for the growth of nanoparticle assemblies that leverages the unique properties of superfluid helium. Unlike viscous solvents at or near room temperature, superfluid helium provides an unperturbed and cold environment in which weak van der Waals interactions between molecular templates and metal atoms become significant and can define the spatial arrangement of nanoparticles. To demonstrate this concept, diol and porphyrin-based molecules are employed as templates to grow gold nanoparticle assemblies in superfluid helium droplets. After soft-landing on a solid surface to remove the helium, transmission electron microscopy (TEM) imaging shows the growth of gold nanoparticles at specific binding sites within the molecular templates where the interaction between gold atoms and the molecular template is at its strongest.

Co-Assembly of Soft and Hard Nanoparticles into Macroscopic Colloidal Composites with Tailored Mechanical Property and Processability

Colloidal composites, translating the great potential of nanoscale building bricks into macroscopic dimensions, have emerged as an appealing candidate for new materials with applications in optics, energy storage, and biomedicines. However, it remains a key challenge to bridge the size regimes from nanoscopic colloidal particles to macroscale composites possessing mechanical robustness. Herein, a bottom-up approach is demonstrated to manufacture colloidal composites with customized macroscopic forms by virtue of the co-assembly of nanosized soft polymeric micelles and hard inorganic nanoparticles. Upon association, the hairy micellar corona can bind with the hard nanoparticles, linking individual hard constituents together in a soft-hard alternating manner to form a collective entity. This permits the integration of block copolymer micelles with controlled amounts of hard nanoparticles into macroscopic colloidal composites featuring diverse internal microstructures. The resultant composites showed tunable microscale mechanical strength in a range of 90-270 MPa and macroscale mechanical strength in a range of 7-42 MPa for compression and 2-24 MPa for bending. Notably, the incorporation of soft polymeric micelles also imparts time- and temperature-dependent dynamic deformability and versatile capacity to the resulting composites, allowing their application in the low-temperature plastic processing for functional fused silica glass.

Induced Huge Optical Activity in Nanoplatelet Superlattice

Chiral superstructures with unique chiroptical properties that are not inherent in the individual units are essential in applications such as 3D displays, spintronic devices, biomedical sensors, and beyond. Generally, chiral superstructures are obtained by tedious procedures exploring various physical and chemical forces to break spatial symmetry during the self-assembly of discrete nanoparticles. In contrast, we herein present a simple and efficient approach to chiral superstructures by intercalating small chiral molecules into preformed achiral superstructures. As a model system, the chiral CdSe nanoplatelet (NPL) superlattice exhibits a giant and tunable optical activity with the highest g-factor reaching 3.09 × 10-2 to the excitonic transition of the NPL superlattice, nearly 2 orders of magnitude higher than that of the corresponding separated chiral NPLs. The theoretical analysis reveals that the chiral deformation in the NPL superlattice induced by the chiral perturbation of the small chiral molecules is critical to the observed huge optical activity. We anticipate that this research lays a foundation for understanding and applying chiral inorganic nanosystems.

Growing Gold Nanostars on 3D Hydrogel Surfaces

Nanocomposites comprising hydrogels and plasmonic nanoparticles are attractive materials for tissue engineering, bioimaging, and biosensing. These materials are usually fabricated by adding colloidal nanoparticles to the uncured polymer mixture and thus require time-consuming presynthesis, purification, and ligand-exchange steps. Herein, we introduce approaches for rapid synthesis of gold nanostars (AuNSt) in situ on hydrogel substrates, including those with complex three-dimensional (3D) features. These methods enable selective AuNSt growth at the surface of the substrate, and the growth conditions can be tuned to tailor the nanoparticle size and density (coverage). We additionally demonstrate proof-of-concept applications of these nanocomposites for SERS sensing and imaging. High surface coverage with AuNSt enabled 1-2 orders of magnitude higher SERS signals compared to plasmonic hydrogels loaded with premade colloids. Importantly, AuNSt can be prepared without the addition of any potentially cytotoxic surfactants, thereby ensuring a high biocompatibility. Overall, in situ growth becomes a versatile and straightforward approach for the fabrication of plasmonic biomaterials.

Mesoscale simulations of diffusion and sedimentation in shape-anisotropic nanoparticle suspensions

We determine the long-time self-diffusion coefficient and sedimentation coefficient for suspensions of nanoparticles with anisotropic shapes (octahedra, cubes, tetrahedra, and spherocylinders) as a function of nanoparticle concentration using mesoscale simulations. We use a discrete particle model for the nanoparticles, and we account for solvent-mediated hydrodynamic interactions between nanoparticles using the multiparticle collision dynamics method. Our simulations are compared to theoretical predictions and experimental data from existing literature, demonstrating good agreement in the majority of cases. Further, we find that the self-diffusion coefficient of the regular polyhedral shapes can be estimated from that of a sphere whose diameter is the average of their inscribed and circumscribed sphere diameters.

Dynamic Transformation of High-Architectural Nanocrystal Superlattices upon Solvent Molecule Exposure

The cluster-based body-centered-cubic superlattice (cBCC SL) represents one of the most complicated structures among reported nanocrystal assemblies, comprised of 72 truncated tetrahedral quantum dots per unit cell. Our previous report revealed that truncated tetrahedral quantum dots within cBCC SLs possessed highly controlled translational and orientational order owing to an unusual energetic landscape based on the balancing of entropic and enthalpic contributions during the assembly process. However, the cBCC SL's structural transformability and mechanical properties, uniquely originating from such complicated nanostructures, have yet to be investigated. Herein, we report that cBCC SLs can undergo dynamic transformation to face-centered-cubic SLs in response to post-assembly molecular exposure. We monitored the dynamic transformation process using in situ synchrotron-based small-angle X-ray scattering, revealing a dynamic transformation involving multiple steps underpinned by interactions between incoming molecules and TTQDs' surface ligands. Furthermore, our mechanistic study demonstrated that the precise configuration of TTQDs' ligand molecules in cBCC SLs was key to their high structural transformability and unique jelly-like soft mechanical properties. While ligand molecular configurations in nanocrystal SLs are often considered minor features, our findings emphasize their significance in controlling weak van der Waals interactions between nanocrystals within assembled SLs, leading to previously unremarked superstructural transformability and unique mechanical properties. Our findings promote a facile route toward further creation of soft materials, nanorobotics, and out-of-equilibrium assemblies based on nanocrystal building blocks.

Transformative Dynamics: Self-Assembly of Iron Oxide Hydroxide Nanorods into Iron Oxide Microcubes for Enhanced Perfluoroalkyl Substance Remediation

We report the controlled synthesis of iron oxide microcubes (IOMCs) through the self-assembly arrays of ferric oxide hydroxide nanorods (NRs). The formation of IOMCs involves a complex interplay of nucleation, self-assembly, and growth mechanisms influenced by time, thermal treatment, and surfactant dynamics. The self-assembly of vertically aligned NRs into IOMCs is controlled by dynamic magnetism properties and capping agents like cetyltrimethylammonium bromide (CTAB), whose concentration and temperature modulation dictate growth kinetics and structural uniformity. These controlled structural growths were obtained via a hydrothermal process at 120 °C at various intervals of 8, 16, 24, and 32 h in the presence of CTAB as the capping agent. In this hydrothermal method, the formation of vertically oriented NR arrays was observed without the presence of ligands, binders, harsh drying techniques, and solvent evaporation. The formation of the self-assembly of NRs to IOMCs is obtained with an increase in saturated magnetization to attain the most stable state. The synthesized IOMCs have a uniform size, quasi-shape, and excellent dispersion. Due to its excellent magnetic and catalytic properties, IOMCs were employed to remove the various emerging pollutants known as per- and polyfluorinated substances (PFAS). Various microscopic and spectroscopic techniques were employed for the characterization and interaction studies of IOMCs with various PFAS. The interaction between IOMCs and perfluoroalkyl substances (PFAS) was investigated, revealing strong adsorption tendencies facilitated by electrostatic interactions, as evidenced by UV-vis and FT-IR spectroscopic studies. Furthermore, the higher magnetic and positive surface charge of IOMCs is responsible for an effective remediation eliminating any secondary pollution with ease of recovery after the sorption interaction studies, thereby making it practically worthwhile.

Structural interactions in polymer-stabilized magnetic nanocomposites

Superparamagnetic iron oxide nanoparticles (SPIONs) have attracted significant attention because of their nanoscale magnetic properties. SPION aggregates may afford emergent properties, resulting from dipole-dipole interactions between neighbors. Such aggregates can display internal order, with high packing fractions (>20%), and can be stabilized with block co-polymers (BCPs), permitting design of tunable composites for potential nanomedicine, data storage, and electronic sensing applications. Despite the routine use of magnetic fields for aggregate actuation, the impact of those fields on polymer structure, SPION ordering, and magnetic properties is not fully understood. Here, we report that external magnetic fields can induce ordering in SPION aggregates that affect their structure, inter-SPION distance, magnetic properties, and composite Tg. SPION aggregates were synthesized in the presence or absence of magnetic fields or exposed to magnetic fields post-synthesis. They were characterized using transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), superconducting quantum interference device (SQUID) analysis, and differential scanning calorimetry (DSC). SPION aggregate properties depended on the timing of field application. Magnetic field application during synthesis encouraged preservation of SPION chain aggregates stabilized by polymer coatings even after removal of the field, whereas post synthesis application triggered subtle internal reordering, as indicated by increased blocking temperature (TB), that was not observed via SAXS or TEM. These results suggest that magnetic fields are a simple, yet powerful tool to tailor the structure, ordering, and magnetic properties of polymer-stabilized SPION nanocomposites.

Seeing the Supracolloidal Assemblies in 3D: Unraveling High-Resolution Structures Using Electron Tomography

Transmission electron microscopy (TEM) imaging has revolutionized modern materials science, nanotechnology, and structural biology. Its ability to provide information about materials' structure, composition, and properties at atomic-level resolution has enabled groundbreaking discoveries and the development of innovative materials with precision and accuracy. Electron tomography, single particle reconstruction, and microcrystal electron diffraction techniques have paved the way for the three-dimensional (3D) reconstruction of biological samples, synthetic materials, and hybrid nanostructures at near atomic-level resolution. TEM tomography using a series of two-dimensional (2D) projections has been used extensively in biological science, but in recent years it has become an important method in synthetic nanomaterials and soft matter research. TEM tomography offers unprecedented morphological details of 3D objects, internal structures, packing patterns, growth mechanisms, and self-assembly pathways of self-assembled colloidal systems. It complements other analytical tools, including small-angle X-ray scattering, and provides valuable data for computational simulations for predictive design and reverse engineering of nanomaterials with the desired structure and properties. In this perspective, I will discuss the importance of TEM tomography in the structural understanding and engineering of self-assembled nanostructures with specific emphasis on colloidal capsids, composite cages, biohybrid superlattices with complex geometries, polymer assemblies, and self-assembled protein-based superstructures.

Solvent Isotherms and Structural Transitions in Nanoparticle Superlattice Assembly

We introduce a Molecular Theory for Compressible Fluids (MOLT-CF) that enables us to compute free energies and other thermodynamic functions for nanoparticle superlattices with any solvent content, including the dry limit. Quantitative agreement is observed between MOLT-CF and united-atom molecular dynamics simulations performed to assess the reliability and precision of the theory. Among other predictions, MOLT-CF shows that the amount of solvent within the superlattice decreases approximately linearly with its vapor pressure and that in the late stages of drying, solvent-filled voids form at lattice interstitials. Applied to single-component superlattices, MOLT-CF predicts fcc-to-bcc Bain transitions for decreasing vapor pressure and for increasing ligand length, both in agreement with experimental results. We explore the stability of other single-component phases and show that the C14 Frank-Kasper phase, which has been reported in experiments, is not a global free-energy minimum. Implications for precise assembly and prediction of multicomponent nanoparticle systems are discussed.

Theory and simulation of ligand functionalized nanoparticles - a pedagogical overview

Synthesizing reconfigurable nanoscale synthons with predictive control over shape, size, and interparticle interactions is a holy grail of bottom-up self-assembly. Grand challenges in their rational design, however, lie in both the large space of experimental synthetic parameters and proper understanding of the molecular mechanisms governing their formation. As such, computational and theoretical tools for predicting and modeling building block interactions have grown to become integral in modern day self-assembly research. In this review, we provide an in-depth discussion of the current state-of-the-art strategies available for modeling ligand functionalized nanoparticles. We focus on the critical role of how ligand interactions and surface distributions impact the emergent, pre-programmed behaviors between neighboring particles. To help build insights into the underlying physics, we first define an "ideal" limit - the short ligand, "hard" sphere approximation - and discuss all experimental handles through the lens of perturbations about this reference point. Finally, we identify theories that are capable of bridging interparticle interactions to nanoscale self-assembly and conclude by discussing exciting new directions for this field.

Self-Assembled Ring-Based Complex Colloidal Particles by Lock-And-Key Interaction and Their Self-Assembly into Unusual Colloidal Crystals

Creating hierarchical crystalline materials using simple colloids or nanoparticles is very challenging, as it is usually impossible to achieve hierarchical structures without nonhierarchical colloidal interactions. Here, we present a hierarchical self-assembly (SA) route that employs colloidal rings and anisotropic colloidal particles to form complex colloids and uses them as building blocks to form unusual colloidal columnar liquid crystals or crystals. This route is realized by designing hierarchical SA driving forces that is controlled by the colloidal shape and shape-dependent depletion attraction. Depletion-induced lock-and-key interaction is the first driving force, which ensures a high efficiency (>90%) to load colloidal particles of other shapes such as spheres, spherocylinders, and oblate ellipsoids into rings, providing high-quality building blocks. Their SA into ordered superstructures has to require a second driving force such as higher volume fraction and/or stronger depletion attraction. As a result, unusual hierarchical colloidal (liquid) crystals, which have previously been difficult to fabricate by simple binary assembly, can be achieved. This work presents a significant advancement in the field of hierarchical SA, demonstrating a promising strategy for constructing many unprecedented crystalline materials by the SA route.

Classification of complex local environments in systems of particle shapes through shape symmetry-encoded data augmentation

Detecting and analyzing the local environment is crucial for investigating the dynamical processes of crystal nucleation and shape colloidal particle self-assembly. Recent developments in machine learning provide a promising avenue for better order parameters in complex systems that are challenging to study using traditional approaches. However, the application of machine learning to self-assembly on systems of particle shapes is still underexplored. To address this gap, we propose a simple, physics-agnostic, yet powerful approach that involves training a multilayer perceptron (MLP) as a local environment classifier for systems of particle shapes, using input features such as particle distances and orientations. Our MLP classifier is trained in a supervised manner with a shape symmetry-encoded data augmentation technique without the need for any conventional roto-translations invariant symmetry functions. We evaluate the performance of our classifiers on four different scenarios involving self-assembly of cubic structures, two-dimensional and three-dimensional patchy particle shape systems, hexagonal bipyramids with varying aspect ratios, and truncated shapes with different degrees of truncation. The proposed training process and data augmentation technique are both straightforward and flexible, enabling easy application of the classifier to other processes involving particle orientations. Our work thus presents a valuable tool for investigating self-assembly processes on systems of particle shapes, with potential applications in structure identification of any particle-based or molecular system where orientations can be defined.

Three-body interaction of gold nanoparticles: the role of solvent density and ligand shell orientation

Molecular dynamics simulations are used to study the effective interactions of alkanethiol passivated gold nanoparticles in supercritical ethane at two- and three-particle levels with different solvent densities. Effective interaction is calculated as the potential of mean force (PMF) between two nanoparticles, and the three-body effect is estimated as the difference in PMFs calculated at the two- and three-particle levels. The variation in the three-body effect is examined as a function of solvent density. It is found that effective interaction, which is completely repulsive at very high solvent concentrations, progressively turns attractive as solvent density declines. On the other hand, the three-body effect turns out to be repulsive and increases exponentially with decreasing solvent density. Further, the structure of the ligand shell is analyzed as a function of nanoparticle separation, and its relationship with the three-body effect is investigated. It is observed that the three-body effect arises when the ligand shell begins to deform due to van der Waals repulsion between ligand shells. The study provides a deep insight into good understanding of the solvent evaporation-assisted nanoparticle self-assembly and can aid in experiments.

Low-Temperature Synthesis of Bi2S3 Hierarchical Microstructures via Co-Precipitation and Digestive Process in Aqueous Medium

Bismuth sulfide (Bi2S3) nanostructures have gained significant attention in the fields of catalysis, optoelectronics, and biomedicine due to their unique physicochemical properties. This paper introduces a simple and cost-effective method for producing Bi2S3 microstructures at low temperatures (25 and 70 °C). These microstructures are formed by the hierarchical self-assembly of Bi2S3 nanoparticles, which are typically 15-40 nm in size. The nanoparticles are synthesized by the co-precipitation of thioglycolic acid, thioacetamide, and bismuth nitrate in water. The study delves into the phase composition and morphological evolution of the microstructures, concerning the chemical composition of the solution and the synthesis temperature. X-ray analysis has confirmed the formation of single-phase bismuthinite Bi2S3. The synthesis process generates primary building blocks in the form of 15-40 nm Bi2S3 nanocrystals, which then go through a hierarchical self-assembly process to produce a range of micrometer-sized structures. A scanning electron microscopy examination revealed that the primary nanoparticles self-assemble into quasi-1D worm-like nanostructures, which then self-assemble to create sponge-shaped microstructures. These structures subsequently self-organize and refine into either flower- or dandelion-like microstructures, mostly depending on the synthesis temperature and the chemistry of the digestion medium.

Growth of Nanocrystal Superlattices from Liquid Crystals

The growth of superlattices (SLs) made from self-assembled nanocrystals (NCs) is a powerful method for creating new materials and gaining insight into fundamental molecular dynamics. Previous explorations of NCSL syntheses have mostly compared them to crystallization. However, NCSL synthesis has not broadly shown cooling crystallization from saturated solutions as a reversible crystallization-dissolution process. We demonstrate the reversible growth of NCSLs by dispersing NCs in liquid crystal (LC) "smart solvents," and harnessing the transitions between the isotropic and nematic phases of the LCs. The growth mode and morphology can be tuned. This process is a model platform for studying crystallization and demonstrates great potential in manufacturing NCSLs as colloidal crystals through liquid-phase epitaxy or colloidal synthesis.

High-Throughput Computational Screening of Solid-Binding Peptides

Inspired by biomineralization, a naturally occurring, protein-facilitated process, solid-binding peptides (SBPs) have gained much attention for their potential to fabricate various shaped nanocrystals and hierarchical nanostructures. The advantage of SBPs over other traditionally used synthetic polymers or short ligands is their tunable interaction with the solid material surface via carefully programmed sequence and being solution-dependent simultaneously. However, designing a sequence with targeted binding affinity or selectivity often involves intensive processes such as phage display, and only a limited number of sequences can be identified. Other computational efforts have also been introduced, but the validation process remains prohibitively expensive once a suitable sequence has been identified. In this paper, we present a new model to rapidly estimate the binding free energy of any given sequence to a solid surface. We show how the overall binding of a polypeptide can be estimated from the free energy contribution of each residue based on the statistics of the thermodynamically stable structure ensemble. We validated our model using five silica-binding peptides of different binding affinities and lengths and showed that the model is accurate and robust across a wider range of chemistries and binding strengths. The computational cost of this method can be as low as 3% of the commonly used enhanced sampling scheme for similar studies and has a great potential to be used in high-throughput algorithms to obtain larger training data sets for machine learning SBP screening.

Mechanism of DNA origami folding elucidated by mesoscopic simulations

Many experimental and computational efforts have sought to understand DNA origami folding, but the time and length scales of this process pose significant challenges. Here, we present a mesoscopic model that uses a switchable force field to capture the behavior of single- and double-stranded DNA motifs and transitions between them, allowing us to simulate the folding of DNA origami up to several kilobases in size. Brownian dynamics simulations of small structures reveal a hierarchical folding process involving zipping into a partially folded precursor followed by crystallization into the final structure. We elucidate the effects of various design choices on folding order and kinetics. Larger structures are found to exhibit heterogeneous staple incorporation kinetics and frequent trapping in metastable states, as opposed to more accessible structures which exhibit first-order kinetics and virtually defect-free folding. This model opens an avenue to better understand and design DNA nanostructures for improved yield and folding performance.

4D-STEM Mapping of Nanocrystal Reaction Dynamics and Heterogeneity in a Graphene Liquid Cell

Chemical reaction kinetics at the nanoscale are intertwined with heterogeneity in structure and composition. However, mapping such heterogeneity in a liquid environment is extremely challenging. Here we integrate graphene liquid cell (GLC) transmission electron microscopy and four-dimensional scanning transmission electron microscopy to image the etching dynamics of gold nanorods in the reaction media. Critical to our experiment is the small liquid thickness in a GLC that allows the collection of high-quality electron diffraction patterns at low dose conditions. Machine learning-based data-mining of the diffraction patterns maps the three-dimensional nanocrystal orientation, groups spatial domains of various species in the GLC, and identifies newly generated nanocrystallites during reaction, offering a comprehensive understanding on the reaction mechanism inside a nanoenvironment. This work opens opportunities in probing the interplay of structural properties such as phase and strain with solution-phase reaction dynamics, which is important for applications in catalysis, energy storage, and self-assembly.

Prismatic Confinement Induces Tunable Orientation in Plasmonic Supercrystals

Throughout history scientists have looked to Nature for inspiration and attempted to replicate intricate complex structures formed by self-assembly. In the context of synthetic supercrystals, achieving such complexity remains a challenge due to the highly symmetric nature of most nanoparticles (NPs). Previous works have shown intricate coupling between the self-assembly of NPs and confinement in templates, such as emulsion droplets (spherical confinement) or tubes (cylindrical confinement). This study focuses on the interplay between anisotropic NP shape and tunable "prismatic confinement" leading to the self-assembly of supercrystals in cavities featuring polygonal cross sections. A multiscale characterization strategy is employed to investigate the orientation and structure of the supercrystals locally and at the ensemble level. Our findings highlight the role of the mold interface in guiding the growth of distinct crystal domains: each side of the mold directs the formation of a monodomain that extends until it encounters another, leading to the creation of grain boundaries. Computer simulations in smaller prismatic cavities were conducted to predict the effect of an increased confinement. Comparison between prismatic and cylindrical confinements shows that flat interfaces are key to orienting the growth of supercrystals. This work shows a method of inducing orientation in plasmonic supercrystals and controlling their textural defects, thus offering insight into the design of functional metasurfaces and hierarchically structured devices.

Self-assembled soft alloy with Frank-Kasper phases beyond metals

Soft building blocks, such as micelles, cells or soap bubbles, tend to adopt near-spherical geometry when densely packed together. As a result, their packing structures do not extend beyond those discovered in metallic glasses, quasicrystals and crystals. Here we report the emergence of two Frank-Kasper phases from the self-assembly of five-fold symmetric molecular pentagons. The μ phase, an important intermediate in superalloys, is indexed in soft matter, whereas the ϕ phase exhibits a structure distinct from known Frank-Kasper phases in metallic systems. We find a broad size and shape distribution of self-assembled mesoatoms formed by molecular pentagons while approaching equilibrium that contribute to the unique packing structures. This work provides insight into the manipulation of soft building blocks that deviate from the typical spherical geometry and opens avenues for the fabrication of 'soft alloy' structures that were previously unattainable in metal alloys.

Randomly Layered Superstructure of In2O3 Truncated Nano-Octahedra and Its High-Pressure Behavior

This study outlines the preparation and characterization of a unique superlattice composed of indium oxide (In2O3) vertex-truncated nano-octahedra, along with an exploration of its response to high-pressure conditions. Transmission electron microscopy and scanning transmission electron microscopy were employed to determine the average circumradius (15.2 nm) of these vertex-truncated building blocks and their planar superstructure. The resilience and response of the superlattice to pressure variations, peaking at 18.01 GPa, were examined using synchrotron-based wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) techniques. The WAXS data revealed no phase transitions, reinforcing the stability of the 2D superlattice composed of random layers in alignment with a p31m planar symmetry as discerned by SAXS. Notably, the SAXS data also unveiled a pressure-induced, irreversible translation of octahedra and ligand interaction occurring within the random layer. Through our examination of these pressure-sensitive behaviors, we identified a distinctive translation model inherent to octahedra and observed modulation of the superlattice cell parameter induced by pressure. This research signifies a noteworthy advancement in deciphering the intricate behaviors of 2D superlattices under a high pressure.

Direct observation of phase transitions in truncated tetrahedral microparticles under quasi-2D confinement

Colloidal crystals are used to understand fundamentals of atomic rearrangements in condensed matter and build complex metamaterials with unique functionalities. Simulations predict a multitude of self-assembled crystal structures from anisotropic colloids, but these shapes have been challenging to fabricate. Here, we use two-photon lithography to fabricate Archimedean truncated tetrahedrons and self-assemble them under quasi-2D confinement. These particles self-assemble into a hexagonal phase under an in-plane gravitational potential. Under additional gravitational potential, the hexagonal phase transitions into a quasi-diamond two-unit basis. In-situ imaging reveal this phase transition is initiated by an out-of-plane rotation of a particle at a crystalline defect and causes a chain reaction of neighboring particle rotations. Our results provide a framework of studying different structures from hard-particle self-assembly and demonstrates the ability to use confinement to induce unusual phases.

Controllable synthesis of star-shaped FeCoMnOx nanocrystals and their self-assembly into superlattices with low-packing densities

We present a novel method for synthesizing monodisperse, star-shaped FeCoMnOx nanocrystals with tunable concavity. Through liquid-air interfacial assembly, these colloidal nanostars can form two-dimensional superlattices, which are characterized by low packing densities. Notably, the ability to adjust the degree of concavity of nanostars allows for the tuning of the packing symmetry of the assembled superlattices.

All-Perovskite Multicomponent Nanocrystal Superlattices

Nanocrystal superlattices (NC SLs) have long been sought as promising metamaterials, with nanoscale-engineered properties arising from collective and synergistic effects among the constituent building blocks. Lead halide perovskite (LHP) NCs come across as outstanding candidates for SL design, as they demonstrate collective light emission, known as superfluorescence, in single- and multicomponent SLs. Thus far, LHP NCs have only been assembled in single-component SLs or coassembled with dielectric NC building blocks acting solely as spacers between luminescent NCs. Here, we report the formation of multicomponent LHP NC-only SLs, i.e., using only CsPbBr3 NCs of different sizes as building blocks. The structural diversity of the obtained SLs encompasses the ABO6, ABO3, and NaCl structure types, all of which contain orientationally and positionally locked NCs. For the selected model system, the ABO6-type SL, we observed efficient NC coupling and Förster-like energy transfer from strongly confined 5.3 nm CsPbBr3 NCs to weakly confined 17.6 nm CsPbBr3 NCs, along with characteristic superfluorescence features at cryogenic temperatures. Spatiotemporal exciton dynamics measurements reveal that binary SLs exhibit enhanced exciton diffusivity compared to single-component NC assemblies across the entire temperature range (from 5 to 298 K). The observed coherent and incoherent NC coupling and controllable excitonic transport within the solid NC SLs hold promise for applications in quantum optoelectronic devices.

Transformable Superisostatic Crystals Self-Assembled from Segment Colloidal Rods

Transformable mechanical structures can switch between distinct mechanical states. Whether this kind of structure can be self-assembled from simple building blocks at microscale is a question to be answered. In this work, we propose a self-assembly strategy for these structures based on a nematic monolayer of segmented colloidal rods with lateral cutting. By using Monte Carlo simulation, we find that rods with different cutting degrees can self-assemble into different crystals characterized by bond coordination z that varies from 3 to 6. Among these, we identify a transformable superisostatic structure with pgg symmetry and redundant bonds (z = 5). We show that this structure can support either soft bulk modes or soft edge modes depending on its Poisson's ratio, which can be tuned from positive to negative through a uniform soft deformation. We also prove that the bulk soft modes are associated with states of self-stress along the direction of zero strain during uniform soft deformation. The self-assembled transformable structures may act as mechanical metamaterials with potential applications in micromechanical engineering.

Direct Synthesis of CuPd Icosahedra Supercrystals Studied by In Situ X-Ray Scattering

Nanocrystal self-assembly into supercrystals provides a versatile platform for creating novel materials and devices with tailored properties. While common self-assembly strategies imply the use of purified nanoparticles after synthesis, conversion of chemical precursors directly into nanocrystals and then supercrystals in simple procedures has been rarely reported. Here, the nucleation and growth of CuPd icosahedra and their consecutive assembly into large closed-packed face-centered cubic (fcc) supercrystals are studied. To this end, the study simultaneously and in situ measures X-ray total scattering with pair distribution function analysis (TS-PDF) and small-angle X-ray scattering (SAXS). It is found that the supercrystals' formation is preceded by an intermediate dense phase of nanocrystals displaying short-range order (SRO). It is further shown that the organization of oleic acid/oleylamine surfactants into lamellar structures likely drives the emergence of the SRO phase and later of the supercrystals by reducing the volume accessible to particle diffusion. The supercrystals' formation as well as their disassembly are triggered by temperature. The study demonstrates that ordering of solvent molecules can be crucial in the direct synthesis of supercrystals. The study also provides a general approach to investigate novel preparation routes of supercrystals in situ and across several length scales via X-ray scattering.

Printing 3D Metallic Structures in Porous Matrix

The fabrication of metallic micro/nanostructures has great potential for advancing optoelectronic microdevices. Over the past decade, femtosecond laser direct writing (FsLDW) technology has played a crucial role in driving progress in this field. In this study, silica gel glass is used as a supporting medium, and FsLDW is employed to reduce gold and palladium ions using 7-Diethylamino-3-thenoylcoumarin (DETC) as a two-photon sensitizer, enabling the printing of conductive multilayered and 3D metallic structures. How the pore size of the silica gel glass affects the electrical conductivity of printed metal wires is systematically examined. This 3D printing method is versatile and offers expanded opportunities for applying metallic micro/nanostructures in optoelectronic devices.

Multi-Scale Engineered 2D Carbon Polyhedron Array with Enhanced Electrocatalytic Performance

Electrocatalyst engineering from the atomic to macroscopic level of electrocatalysts is one of the most powerful routes to boost the performance of electrochemical devices. However, multi-scale structure engineering mainly focuses on the range of atomic-to-particle scale such as hierarchical porosity engineering, while catalyst engineering at the macroscopic level, such as the arrangement configuration of nanoparticles, is often overlooked. Here, a 2D carbon polyhedron array with a multi-scale engineered structure via facile chemical etching, ice-templating induced self-assembly, and high-temperature pyrolysis processes is reported. Controlled phytic acid etching of the carbon precursor introduces homogeneous atomic phosphorous and nitrogen doping, as well as a well-defined mesoporous structure. Subsequent ice-templated self-assembly triggers the formation of a 2D particle array superstructure. The atomic-level doping gives rise to high intrinsic activity, while the well-engineered porous structure and particle arrangement addresses the mass transport limitations at the microscopic particle level and macroscopic electrode level. As a result, the as-prepared electrocatalyst delivers outstanding performance toward oxygen reduction reaction in both acidic and alkaline media, which is better than recently reported state-of-the-art metal-free electrocatalysts. Molecular dynamics simulation together with extensive characterizations indicate that the performance enhancement originates from multi-scale structural synergy.