Low packing density self-assembled superstructure of octahedral Pt3Ni nanocrystals

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.

Diffusion-Mediated Nucleation and Growth of fcc and bcc Nanocrystal Superlattices with Designable Assembly of Freestanding 3D Supercrystals

Diffusion-mediated assembly of octahedral PbS nanocrystals (NCs) in a confined antisolvent environment displays a primary burst nucleation and Ostwald ripening growth of rhombic bcc supercrystals, followed by a secondary seed-based nucleation and oriented attachment growth of triangle fcc supercrystals. As the diffusion proceeds from ethanol across a sharp interface into NC-suspended toluene, a burst nucleation of supercrystal seeds occurs, and such supercrystals are quickly developed into rhombic grains that have a bcc structure. At a critical size of 10 μm, an Ostwald ripening event appears to guide the supercrystal growth. Upon grain growth above 30 μm, the fcc supercrystals start a nucleation at two symmetrical tips of individual rhombic crystals. Such fcc supercrystals are developed with a triangle shape, and two triangles are combined with one bcc rhombus in-between to form a butterfly-like bowtie stacking structure. The fcc triangle wings grow larger at a reduction of bcc rhombus cores. As the bcc cores gradually fade, such butterfly-like bowtie crystals aggregate and undergo an oriented attachment process, leading to the formation of freestanding 3D triangle crystals that have a single fcc lattice. Analysis of experimental observations and defined diffusion parameters reveals that fast solvent diffusion and high-NC concentration promote the growth of rhombic bcc supercrystals, while slow solvent diffusion and low-NC concentration accelerate the development of triangle fcc supercrystals. Upon succeeding in designable growth of 3D fcc supercrystals, this study provides designing principles for controlled fabrication of supercrystals with desired superlattices for additional engineering and applications.

Structural Diversity in Dimension-Controlled Assemblies of Tetrahedral Gold Nanocrystals

Polyhedron packings have fascinated humans for centuries and continue to inspire scientists of modern disciplines. Despite extensive computer simulations and a handful of experimental investigations, understanding of the phase behaviors of synthetic tetrahedra has remained fragmentary largely due to the lack of tetrahedral building blocks with tunable size and versatile surface chemistry. Here, we report the remarkable richness of and complexity in dimension-controlled assemblies of gold nanotetrahedra. By tailoring nanocrystal interactions from long-range repulsive to hard-particle-like or to systems with short-ranged directional attractions through control of surface ligands and assembly conditions, nearly a dozen of two-dimensional and three-dimensional superstructures including the cubic diamond and hexagonal diamond polymorphs are selectively assembled. We further demonstrate multiply twinned icosahedral supracrystals by drying aqueous gold nanotetrahedra on a hydrophobic substrate. This study expands the toolbox of the superstructure by design using tetrahedral building blocks and could spur future computational and experimental work on self-assembly and phase behavior of anisotropic colloidal particles with tunable interactions.

Unconventional-Phase Crystalline Materials Constructed from Multiscale Building Blocks

Crystal phase, an intrinsic characteristic of crystalline materials, is one of the key parameters to determine their physicochemical properties. Recently, great progress has been made in the synthesis of nanomaterials with unconventional phases that are different from their thermodynamically stable bulk counterparts via various synthetic methods. A nanocrystalline material can also be viewed as an assembly of atoms with long-range order. When larger entities, such as nanoclusters, nanoparticles, and microparticles, are used as building blocks, supercrystalline materials with rich phases are obtained, some of which even have no analogues in the atomic and molecular crystals. The unconventional phases of nanocrystalline and supercrystalline materials endow them with distinctive properties as compared to their conventional counterparts. This Review highlights the state-of-the-art progress of nanocrystalline and supercrystalline materials with unconventional phases constructed from multiscale building blocks, including atoms, nanoclusters, spherical and anisotropic nanoparticles, and microparticles. Emerging strategies for engineering their crystal phases are introduced, with highlights on the governing parameters that are essential for the formation of unconventional phases. Phase-dependent properties and applications of nanocrystalline and supercrystalline materials are summarized. Finally, major challenges and opportunities in future research directions are proposed.

Self-assembly of anisotropic nanoparticles into functional superstructures

Self-assembly of colloidal nanoparticles (NPs) into superstructures offers a flexible and promising pathway to manipulate the nanometer-sized particles and thus make full use of their unique properties. This bottom-up strategy builds a bridge between the NP regime and a new class of transformative materials across multiple length scales for technological applications. In this field, anisotropic NPs with size- and shape-dependent physical properties as self-assembly building blocks have long fascinated scientists. Self-assembly of anisotropic NPs not only opens up exciting opportunities to engineer a variety of intriguing and complex superlattice architectures, but also provides access to discover emergent collective properties that stem from their ordered arrangement. Thus, this has stimulated enormous research interests in both fundamental science and technological applications. This present review comprehensively summarizes the latest advances in this area, and highlights their rich packing behaviors from the viewpoint of NP shape. We provide the basics of the experimental techniques to produce NP superstructures and structural characterization tools, and detail the delicate assembled structures. Then the current understanding of the assembly dynamics is discussed with the assistance of in situ studies, followed by emergent collective properties from these NP assemblies. Finally, we end this article with the remaining challenges and outlook, hoping to encourage further research in this field.

Interaction between capped tetrahedral gold nanocrystals: dependence on effective softness

Atomistic molecular dynamics simulations are performed to explore the interaction between two alkylthiol-capped tetrahedral gold nanocrystals (NCs) in a vacuum. The results highlight the influential role of the effective softness of the ligated NCs, i.e. the ratio of the ligand length to the core size. For sufficiently large softness, the relatively long ligand molecules round the shape of the NCs, causing their interaction to be nearly isotropic. For small effective softness, the relative shortness of the ligand molecules leads to a geometrically asymmetric morphology of the NCs, so that the interaction is orientation-dependent and is the strongest when the two NCs face each other with (111) facets. These findings are helpful for the understanding of interaction and structure formation in superlattices self-assembled from non-spherical ligand-capped NCs.

Tin-based nanomaterials: colloidal synthesis and battery applications

Tin-based nanomaterials have been of increasing interest in many fields such as alkali-ion batteries, gas sensing, thermoelectric devices, and solar cells. Finely controllable structures and compositions of tin-based nanomaterials are crucial to improve their performances. The solution-based colloidal synthesis of these compounds offers a promising path toward controlling their structures and components. This feature article summarizes the progress in recent studies on the colloidal synthesis of tin-based nanomaterials (such as metallic tin, alloys, oxides, chalcogenides, and phosphides) and their applications in alkali-ion batteries including our own recent contributions to this subject. The challenges and future outlook of the controllable synthesis and practical development of tin-based anode materials are also addressed.

Au nanocrystal superlattices: nanocrystallinity, vicinal surfaces, and growth processes

Vicinal Au supracrystal surfaces were prepared from Ausingle single domain nanocrystals (NCs), whereas by replacing Ausingle with their polycrystalline counterparts common low-energy supracrystal surfaces were produced. By analogy to atomic crystalline surfaces, we propose a mechanism to explain the formation of such unexpected supracrystal vicinal surfaces, composed of only Ausingle NCs which are non-compact (bct structure) with a coherent alignment of the atomic planes oriented along the [111] superlattice axis and a slight tilt configuration (8.1°) of NCs. In the presence of Co(ε) NCs, these Ausingle supracrystals lose both the slightly tilted configuration of NCs and their orientational order leading to a superlattice transition from bct to fcc. In contrast, supracrystals of Aupoly NCs are insensitive to the presence of Co(ε) NCs. These intriguing structural changes obtained with Ausingle compared to Aupoly supracrystals in the absence and presence of Co(ε) NCs could explain the formation of vicinal surfaces. Note that the solvent used to disperse the nanocrystals plays a key role in the formation of supracrystal vicinal surfaces. Here, a new analogy between supracrystals and atomic crystals is presented.

Nonisotropic Self-Assembly of Nanoparticles: From Compact Packing to Functional Aggregates

Quantum strongly correlated systems that exhibit interesting features in condensed matter physics often need an unachievable temperature or pressure range in classical materials. One solution is to introduce a scaling factor, namely, the lattice parameter. Synthetic heterostructures named superlattices or supracrystals are synthesized by the assembling of colloidal atoms. These include semiconductors, metals, and insulators for the exploitation of their unique properties. Most of them are currently limited to dense packing. However, some of desired properties need to adjust the colloidal atoms neighboring number. Here, the current state of research in nondense packing is summarized, discussing the benefits, outlining possible scenarios and methodologies, describing examples reported in the literature, briefly discussing the challenges, and offering preliminary conclusions. Penetrating such new and intriguing research fields demands a multidisciplinary approach accounting for the coupling of statistic physics, solid state and quantum physics, chemistry, computational science, and mathematics. Standard interactions between colloidal atoms and emerging fields, such as the use of Casimir forces, are reported. In particular, the focus is on the novelty of patchy colloidal atoms to meet this challenge.

Programming Hierarchical Self-Assembly of Patchy Particles into Colloidal Crystals via Colloidal Molecules

Colloidal self-assembly is a promising bottom-up route to a wide variety of three-dimensional structures, from clusters to crystals. Programming hierarchical self-assembly of colloidal building blocks, which can give rise to structures ordered at multiple levels to rival biological complexity, poses a multiscale design problem. Here we explore a generic design principle that exploits a hierarchy of interaction strengths and employ this design principle in computer simulations to demonstrate the hierarchical self-assembly of triblock patchy colloidal particles into two distinct colloidal crystals. We obtain cubic diamond and body-centered cubic crystals via distinct clusters of uniform size and shape, namely, tetrahedra and octahedra, respectively. Such a conceptual design framework has the potential to reliably encode hierarchical self-assembly of colloidal particles into a high level of sophistication. Moreover, the design framework underpins a bottom-up route to cubic diamond colloidal crystals, which have remained elusive despite being much sought after for their attractive photonic applications.

Shape-dependent ordering of gold nanocrystals into large-scale superlattices

Self-assembly of individual building blocks into highly ordered structures, analogous to spontaneous growth of crystals from atoms, is a promising approach to realize the collective properties of nanocrystals. Yet the ability to reliably produce macroscopic assemblies is unavailable and key factors determining assembly quality/yield are not understood. Here we report the formation of highly ordered superlattice films, with single crystalline domains of up to half a millimetre in two dimensions and thickness of up to several microns from nanocrystals with tens of nanometres in diameter. Combining experimental and computational results for gold nanocrystals in the shapes of spheres, cubes, octahedra and rhombic dodecahedra, we investigate the entire self-assembly process from disordered suspensions to large-scale ordered superlattices induced by nanocrystal sedimentation and eventual solvent evaporation. Our findings reveal that the ultimate coherence length of superlattices strongly depends on nanocrystal shape. Factors inhibiting the formation of high-quality large-scale superlattices are explored in detail.

Entropy-Driven Pt3Co Nanocube Assembles and Thermally Mediated Electrical Conductivity with Anisotropic Variation of the Rhombohedral Superlattice

Understanding the shape-dependent superlattices and resultant anisotropies of both structure and property allows for rational design of materials processing and engineering to fabricate transformative materials with useful properties for applications. This work shows the structural evolution from square lattice of two-dimensional (2D) thin film to rhombic lattice of large three-dimensional (3D) assembles of Pt₃Co nanocubes (NCs). Synchrotron-based X-ray supercrystallography determines the superlattice of large 3D supercrystal into an obtuse rhombohedral (Rh) symmetry, which holds a long-range coherence of both NC translation and atomic crystallographic orientation. The Rh superlattice has a trigonal cell angle of 104°, and the constitute NCs orient their atomic Pt₃Co(111) planes to the superlattice Rh[111] direction. The temperature-dependent in situ small and wide-angle X-ray scattering (SAXS/WAXS) measurements reveal a thermally induced superlattice contraction of supercrystal, which maintains translational ordering but slightly develops orientational disordering. The observed increases of both the packing density and the rotation magnitude of NCs indicate a rational compromise between configurational and rotational entropies of NCs. The resultant minimization of the total free energy is responsible for the formation and stability of the obtuse Rh superlattice. The temperature-dependent in situ measurements of SAXS and electrical resistance reveal that, in conjunction with the thermally induced sharp contraction of superlattice at 160 °C, the supercrystal becomes measurable of electrical resistance, which was followed by a temperature-dependent linear increase. Upon rapid annealing from 250 °C, the supercrystal remains almost constant in both structure and electrical resistance. The heating-enabled electrical conductivity of the supercrystal at high temperature implies the formation of a NC-interconnected architecture. The experiments and overall analysis provide solid evidence and essential information for the use of shape-dependent structural anisotropies of supercrystal to create nanobased novel architecture with desired properties.

Following the Assembly of Iron Oxide Nanocubes by Video Microscopy and Quartz Crystal Microbalance with Dissipation Monitoring

We have studied the growth of ordered arrays by evaporation-induced self-assembly of iron oxide nanocubes with edge lengths of 6.8 and 10.1 nm using video microscopy (VM) and quartz crystal microbalance with dissipation monitoring (QCM-D). Ex situ electron diffraction of the ordered arrays demonstrates that the crystal axes of the nanocubes are coaligned and confirms that the ordered arrays are mesocrystals. Time-resolved video microscopy shows that growth of the highly ordered arrays at slow solvent evaporation is controlled by particle diffusion and can be described by a simple growth model. The growth of each mesocrystal depends only on the number of nanoparticles within the accessible region irrespective of the relative time of formation. The mass of the dried mesocrystals estimated from the analysis of the bandwidth-shift-to-frequency-shift ratio correlates well with the total mass of the oleate-coated nanoparticles in the deposited dispersion drop.

Mild synthesis of monodisperse tin nanocrystals and tin chalcogenide hollow nanostructures

A mild but robust synthetic strategy was developed to synthesize monodisperse Sn nanocrystals with tunable size by using tungsten hexacarbonyl as the reducing agent, and novel tin chalcogenide nanostructures have also been prepared using Sn nanocrystals as templates.

Orientationally Ordered Silicon Nanocrystal Cuboctahedra in Superlattices

Uniform silicon nanocrystals were synthesized with cuboctahedral shape and passivated with 1-dodecene capping ligands. Transmission electron microscopy, electron diffraction, and grazing incidence wide-angle and small-angle X-ray scattering show that these soft cuboctahedra assemble into face-centered cubic superlattices with orientational order. The preferred nanocrystal orientation was found to depend on the orientation of the superlattices on the substrate, indicating that the interactions with the substrate and assembly kinetics can influence the orientation of faceted nanocrystals in superlattices.

Self-assembly of a space-tessellating structure in the binary system of hard tetrahedra and octahedra

We report the formation of a binary crystal of hard polyhedra due solely to entropic forces. Although the alternating arrangement of octahedra and tetrahedra is a known space-tessellation, it had not previously been observed in self-assembly simulations. Both known one-component phases - the dodecagonal quasicrystal of tetrahedra and the densest-packing of octahedra in the Minkowski lattice - are found to coexist with the binary phase. Apart from an alternative, monoclinic packing of octahedra, no additional crystalline phases were observed.

Tuning the structure and habit of iron oxide mesocrystals

A precise control over the meso- and microstructure of ordered and aligned nanoparticle assemblies, i.e., mesocrystals, is essential in the quest for exploiting the collective material properties for potential applications. In this work, we produced evaporation-induced self-assembled mesocrystals with different mesostructures and crystal habits based on iron oxide nanocubes by varying the nanocube size and shape and by applying magnetic fields. A full 3D characterization of the mesocrystals was performed using image analysis, high-resolution scanning electron microscopy and Grazing Incidence Small Angle X-ray Scattering (GISAXS). This enabled the structural determination of e.g. multi-domain mesocrystals with complex crystal habits and the quantification of interparticle distances with sub-nm precision. Mesocrystals of small nanocubes (l = 8.6-12.6 nm) are isostructural with a body centred tetragonal (bct) lattice whereas assemblies of the largest nanocubes in this study (l = 13.6 nm) additionally form a simple cubic (sc) lattice. The mesocrystal habit can be tuned from a square, hexagonal to star-like and pillar shapes depending on the particle size and shape and the strength of the applied magnetic field. Finally, we outline a qualitative phase diagram of the evaporation-induced self-assembled superparamagnetic iron oxide nanocube mesocrystals based on nanocube edge length and magnetic field strength.

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

Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micrometer colloids and block copolymer assembly. We outline the extensive catalog of superlattices prepared to date using hydrocarbon-capped nanocrystals with spherical, polyhedral, rod, plate, and branched inorganic core shapes, as well as those obtained by mixing combinations thereof. We also provide an overview of structural defects in nanocrystal superlattices. We then explore the unique possibilities offered by leveraging nontraditional surface chemistries and assembly environments to control superlattice structure and produce nonbulk assemblies. We end with a discussion of the unique optical, magnetic, electronic, and catalytic properties of ordered nanocrystal superlattices, and the coming advances required to make use of this new class of solids.

Competing Interactions between Various Entropic Forces toward Assembly of Pt3Ni Octahedra into a Body-Centered Cubic Superlattice

Anisotropic nanocrystal assembled supercrystals with open superlattices (SLs) manifest novel and unique properties, but poor understanding of the nucleation/growth mechanisms limits their design and fabrication for practical applications. Using highly anisotropic Pt3Ni octahedral nanocrystals, we have grown large single supercrystals with an open body-centered cubic (bcc) superlattice that has a low filling factor of 26.8%. Synchrotron-based X-ray structural reconstruction fully revealed the coherence of translational and orientational orderings and determined that the constituent octahedra arrange themselves with the vertex-to-vertex and face-to-face configurations along the SL[100] and SL[111] directions, respectively. The large face-to-face separation and flexible vertex-to-vertex elastic contact provided the rattle space and supporting axis for local rotations of Pt3Ni octahedra within the bcc superlattice. Development of orientational disordering along with robust preservation of translational ordering during the heating process of a supercrystal in the oleic acid wetting environment confirmed the dominance of rotational entropy of hard octahedra in the formation of the open bcc superlattice. Ultimate achievement of dynamic equilibrium between the vertex-oriented elastic repulsions and the face-oriented attractions of surface-coating ligands governs the structural and mechanical stability of the supercrystal. This discovery provides significant insights into the design and control of geometrical shapes for the fabrication of highly anisotropic nanocrystals into desired open superlattices with tailored optical and electronic properties.

Small Angle X-ray Scattering for Nanoparticle Research

X-ray scattering is a structural characterization tool that has impacted diverse fields of study. It is unique in its ability to examine materials in real time and under realistic sample environments, enabling researchers to understand morphology at nanometer and angstrom length scales using complementary small and wide angle X-ray scattering (SAXS, WAXS), respectively. Herein, we focus on the use of SAXS to examine nanoscale particulate systems. We provide a theoretical foundation for X-ray scattering, considering both form factor and structure factor, as well as the use of correlation functions, which may be used to determine a particle's size, size distribution, shape, and organization into hierarchical structures. The theory is expanded upon with contemporary use cases. Both transmission and reflection (grazing incidence) geometries are addressed, as well as the combination of SAXS with other X-ray and non-X-ray characterization tools. We conclude with an examination of several key areas of research where X-ray scattering has played a pivotal role, including in situ nanoparticle synthesis, nanoparticle assembly, and operando studies of catalysts and energy storage materials. Throughout this review we highlight the unique capabilities of X-ray scattering for structural characterization of materials in their native environment.

Shape-controlled synthesis of nanopyramids and nanoprisms of nickel sulfide (Ni3S4)

Two different nickel precursors (NiCl2 or Ni(CH3COCH2COCH3)2) in the presence of 1-dodecanethiol and a mixture of oleylamine and oleic acid were used for a one-pot colloidal synthesis to produce high purity Ni3S4 nanoparticles with controlled shapes in high yields and narrow size distributions. By simply changing the nickel precursors, the shape of Ni3S4 nanocrystals can be readily tuned from triangular nanoprisms to tetrahedra (nanopyramids). The produced nanocrystals were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction, selected area electron diffraction, ultraviolet-visible spectroscopy and superconducting quantum interference device (SQUID). TEM tomography (3D-TEM) was employed to determine details of the particle shapes. SQUID measurements confirmed that particle shape and domain size could dramatically impact their magnetic properties, where tetrahedral nanopyramids of Ni3S4 nanoparticles showed ferromagnetic properties while the Ni3S4 nanoprisms showed random antiferromagnetic interactions between magnetic centers.

Structural diversity in iron oxide nanoparticle assemblies as directed by particle morphology and orientation

The mesostructure of ordered arrays of anisotropic nanoparticles is controlled by a combination of packing constraints and interparticle interactions, two factors that are strongly dependent on the particle morphology. We have investigated how the degree of truncation of iron oxide nanocubes controls the mesostructure and particle orientation in drop cast mesocrystal arrays. The combination of grazing incidence small-angle X-ray scattering and scanning electron microscopy shows that mesocrystals of highly truncated cubic nanoparticles assemble in an fcc-type mesostructure, similar to arrays formed by iron oxide nanospheres, but with a significantly reduced packing density and displaying two different growth orientations. Strong satellite reflections in the GISAXS pattern indicate a commensurate mesoscopic superstructure that is related to stacking faults in mesocrystals of the anisotropic nanocubes. Our results show how subtle variation in shape anisotropy can induce oriented arrangements of nanoparticles of different structures and also create mesoscopic superstructures of larger periodicity.

Temperature and chemical bonding-directed self-assembly of cobalt phosphide nanowires in reaction solutions into vertical and horizontal alignments

The preparation of vertically or horizontally aligned self-assemblies of CoP nanowires is demonstrated for the first time by aging them in the reaction solution for a sufficient time at 20 or 0 °C. This strategy opens up a way for exploring the controlled self-assembly of various highly anisotropic nanostructures into long-range ordered structures with collective properties.

Tilted face-centered-cubic supercrystals of PbS nanocubes

We demonstrate a direct fabrication of PbS nanocube supercrystals without size-selection pretreatment on the building blocks. Electron microscopic and synchrotron small angle X-ray scattering analyses confirm that nanocubes pack through a tilted face-centered-cubic (fcc) arrangement, that is, face-to-face along the <110>(super) direction, resulting in a real packing efficiency of as high as ∼83%. This new type of superstructure consisting of nanocubes as building blocks, reported here for the first time, is considered the most stable surfactant-capped nanocube superstructure determined by far.

3D quantitative analysis of platinum nanocrystal superlattices by electron tomography

The work reported herein focuses on the 3D relative arrangement of individual platinum nanocrystals with a size of about 5 nm, and on the structure of the superlattices, they spontaneously form. Electron tomography was systematically used in this study because it allows obtaining quantitative 3D information in real space. Performing tomography in the bright-field TEM mode allowed investigating the short and long-range orderings of the nanoparticles packed as self-organized supercrystals. Systematic fcc pilings were observed with a mean lattice parameter measured to be 19.5 nm, the nature of the arrangement being controlled by the truncated octahedral morphology of platinum nanocrystals and the associated steric effects. A numerical 3D quantitative analysis of the ordering characteristics of the superlattice with a nanometer resolution has been performed that, for the first time, showed a direct correlation between single entities' characteristics and their ordering in periodic arrays. It has been shown that the lattice parameter is different in two orthogonal directions of the fcc structure, which indicates the presence of a slightly compressed superlattice. Inside the superstructure, vacancies and axial defects were observed that would blur the occurrence of potential collective effects from the supercrystals.

Crystalline assemblies and densest packings of a family of truncated tetrahedra and the role of directional entropic forces

Polyhedra and their arrangements have intrigued humankind since the ancient Greeks and are today important motifs in condensed matter, with application to many classes of liquids and solids. Yet, little is known about the thermodynamically stable phases of polyhedrally shaped building blocks, such as faceted nanoparticles and colloids. Although hard particles are known to organize due to entropy alone, and some unusual phases are reported in the literature, the role of entropic forces in connection with polyhedral shape is not well understood. Here, we study thermodynamic self-assembly of a family of truncated tetrahedra and report several atomic crystal isostructures, including diamond, β-tin, and high-pressure lithium, as the polyhedron shape varies from tetrahedral to octahedral. We compare our findings with the densest packings of the truncated tetrahedron family obtained by numerical compression and report a new space-filling polyhedron, which has been overlooked in previous searches. Interestingly, the self-assembled structures differ from the densest packings. We show that the self-assembled crystal structures can be understood as a tendency for polyhedra to maximize face-to-face alignment, which can be generalized as directional entropic forces.

Synthesis of In2O3@SiO2 core-shell nanoparticles with enhanced deeper energy level emissions of In2O3

In(2)O(3)@SiO(2) core-shell nanoparticles were prepared using an organic solution synthesis approach and reverse-microemulsion technique. In order to explore the availability of various silica encapsulations, a partial phase diagram for this ternary system consisting of hexane/cyclohexane (1:29 wt), surfactant (polyoxyethylene(5)nonylphenyl ether, i.e., Igepal CO-520), and aqueous solution containing ammonium hydroxide was also established. It is realized that the shell-thickness can be tuned by several parameters such as the concentration of In(2)O(3) nanocrystal suspension and the dose of the Si-precursor, tetraethyl orthosilicate. It was observed that the deeper energy level emissions of In(2)O(3) were apparently enhanced when In(2)O(3) was confined by the silica-shell in such core-shell nanoparticles. However, this enhancement could be degraded by increasing the shell-thickness.