Self-Assembly of Atomically Aligned Nanoparticle Superlattices from Pt-Fe3O4 Heterodimer Nanoparticles

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.

Emergent Properties from Three-Dimensional Assemblies of (Nano)particles in Confined Spaces

The assembly of (nano)particles into compact hierarchical structures yields emergent properties not found in the individual constituents. The formation of these structures relies on a profound knowledge of the nanoscale interactions between (nano)particles, which are often designed by researchers aided by computational studies. These interactions have an effect when the (nano)particles are brought into close proximity, yet relying only on diffusion to reach these closer distances may be inefficient. Recently, physical confinement has emerged as an efficient methodology to increase the volume fraction of (nano)particles, rapidly accelerating the time scale of assembly. Specifically, the high surface area of droplets of one immiscible fluid into another facilitates the controlled removal of the dispersed phase, resulting in spherical, often ordered, (nano)particle assemblies. In this review, we discuss the design strategies, computational approaches, and assembly methods for (nano)particles in confined spaces and the emergent properties therein, such as trigger-directed assembly, lasing behavior, and structural photonic color. Finally, we provide a brief outlook on the current challenges, both experimental and computational, and farther afield application possibilities.

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.

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.

Mesoscale self-organization of polydisperse magnetic nanoparticles at the water surface

In this study, we investigated the self-ordering process in Langmuir films of polydisperse iron oxide nanoparticles on a water surface, employing in situ x-ray scattering, surface pressure-area isotherm analysis, and Brewster angle microscopy. X-ray reflectometry confirmed the formation of a monolayer, while grazing incidence small-angle x-ray scattering revealed short-range lateral correlations with a characteristic length equal to the mean particle size. Remarkably, our findings indicated that at zero surface pressure, the particles organized into submicrometer clusters, merging upon compression to form a homogeneous layer. These layers were subsequently transferred to a solid substrate using the Langmuir-Schaefer technique and further characterized via scanning electron microscopy and polarized neutron reflectometry. Notably, our measurements revealed a second characteristic length in the lateral correlations, orders of magnitude longer than the mean particle diameter, with polydisperse particles forming circular clusters densely packed in a hexagonal lattice. Furthermore, our evidence suggests that the lattice constant of this mesocrystal depends on the characteristics of the particle size distribution, specifically the mean particle size and the width of the size distribution. In addition, we observed internal size separation within these clusters, where larger particles were positioned closer to the center of the cluster. Finally, polarized neutron reflectometry measurements provided valuable insights into the magnetization profile across the layer.

Dynamic Nanocrystal Superlattices with Thermally Triggerable Lubricating Ligands

The size-dependent and collective physical properties of nanocrystals (NCs) and their self-assembled superlattices (SLs) enable the study of mesoscale phenomena and the design of metamaterials for a broad range of applications. However, the limited mobility of NC building blocks in dried NCSLs often hampers the potential for employing postdeposition methods to produce high-quality NCSLs. In this study, we present tailored promesogenic ligands that exhibit a lubricating property akin to thermotropic liquid crystals. The lubricating ability of ligands is thermally triggerable, allowing the dry solid NC aggregates deposited on the substrates with poor ordering to be transformed into NCSLs with high crystallinity and preferred orientations. The interplay between the dynamic behavior of NCSLs and the molecular structure of the ligands is elucidated through a comprehensive analysis of their lubricating efficacy using both experimental and simulation approaches. Coarse-grained molecular dynamic modeling suggests that a shielding layer from mesogens prevents the interdigitation of ligand tails, facilitating the sliding between outer shells and consequently enhancing the mobility of NC building blocks. The dynamic organization of NCSLs can also be triggered with high spatial resolution by laser illumination. The principles, kinetics, and utility of lubricating ligands could be generalized to unlock stimuli-responsive metamaterials from NCSLs and contribute to the fabrication of NCSLs.

On-Demand Assembly of Nanocrystals into a Superstructure Library in Co(OH)2 Single-Walled Nanotubes

The hierarchical self-assembly of colloidal particles facilitates the bottom-up manufacturing of metamaterials with synergistically integrated functionalities. Here, we define a modular assembly methodology that enables multinary co-assembly of nanoparticles in one-dimensional confined space. A series of isotropic and anisotropic nanocrystals such as plasmonic, metallic, visible, and near-infrared responsive nanoparticles as well as transition-metal phosphides can be selectively assembled within the single-walled Co(OH)2 nanotubes to achieve various increasingly sophisticated assembly systems, including unary, binary, ternary, and quaternary superstructures. Moreover, the selective assembly of distinct functional nanoparticles produces different integrated functional superstructures. This generalizable methodology provides predictable pathways to complex architectures with structural programming and customization that are otherwise inaccessible.

Functional Zonation Strategy of Heterodimer Nanozyme for Multiple Chemiluminescence Imaging Immunoassay

Although nanozymes with intrinsic enzyme-like characteristics have aroused great interest in the biosensing field, the challenge is to keep high enzyme-like activity of the nanozyme after the modification of biomolecules onto nanozymes. Herein, a functional zonation strategy of a heterodimer nanozyme was proposed to tackle the challenge and further construct a multiple chemiluminescence (CL) imaging immunoassay. Here Fe3O4-Au as a heterodimer nanozyme model was divided into two zones, in which Fe3O4 nanoparticles (NPs) were regarded as a nanozyme zone and AuNPs were defined as an antibody immobilization zone. A signal amplification probe (Fe3O4-Au-Ab2) was prepared by modifying the secondary antibody (Ab2) on AuNPs of the Fe3O4-Au heterodimer owing to the Au-S bond. The exposed Fe3O4 of the Fe3O4-Au-Ab2 probe shows very high peroxidase-like activity and can efficiently catalyze H2O2-luminol to produce strong CL imaging signals for multiple antigens detection. Using chicken interleukin-4 (ChIL-4) and chicken gamma interferon (ChIFN-γ) as models, the proposed CL imaging immunoassay shows wide linear ranges (0.005-0.10 ng/mL for both ChIL-4 and ChIFN-γ) and low detection limits (0.58 pg/mL for ChIL-4, 0.47 pg/mL for ChIFN-γ) with the characteristics of high sensitivity, high specificity, and good stability. This work provides a promising functional zonation concept for nanozymes to construct new types of nanozyme probes for immunoassay of multiple biomolecules.

Annealing effects on the magnetic and magnetotransport properties of iron oxide nanoparticles self-assemblies

In magnetic tunnel junctions based on iron oxide nanoparticles the disorder and the oxidation state of the surface spin as well as the nanoparticles functionalization play a crucial role in the magnetotransport properties. In this work, we report a systematic study of the effects of vacuum annealing on the structural, magnetic and transport properties of self-assembled ∼10 nm Fe3O4nanoparticles. The high temperature treatment (from 573 to 873 K) decomposes the organic coating into amorphous carbon, reducing the electrical resistivity of the assemblies by 4 orders of magnitude. At the same time, the 3.Fe2+/(Fe3++Fe2+) ratio is reduced from 1.11 to 0.13 when the annealing temperature of the sample increases from 573 to 873 K, indicating an important surface oxidation. Although the 2 nm physical gap remains unchanged with the thermal treatment, a monotonous decrease of tunnel barrier width was obtained from the electron transport measurements when the annealing temperature increases, indicating an increment in the number of defects and hot-spots in the gap between the nanoparticles. This is reflected in the reduction of the spin dependent tunneling, which reduces the interparticle magnetoresistance. This work shows new insights about influence of the nanoparticle interfacial composition, as well their the spatial arrangement, on the tunnel transport of self-assemblies, and evidence the importance of optimizing the nanostructure fabrication for increasing the tunneling current without degrading the spin polarized current.

Temperature-Controlled Reversible Formation and Phase Transformation of 3D Nanocrystal Superlattices Through In Situ Small-Angle X-ray Scattering

For decades, the spontaneous organization of nanocrystals into superlattices has captivated the scientific community. However, achieving direct control over the formation of the superlattice and its phase transformations has proven to be a grand challenge, often resulting in the generation of multiple symmetries under the same experimental conditions. Here, we achieve direct control over the formation of the superlattice and its phase transformations by modulating the thermal energy of a nanocrystal dispersion without relying on solvent evaporation. We follow the temperature-dependent dynamics of the self-assembly process using synchrotron-based small-angle X-ray scattering. When cooled below -24.5 °C, lead sulfide nanocrystals form micrometer-sized three-dimensional phase-pure body-centered cubic superlattices. When cooled below -35.1 °C, these superlattices undergo a collective diffusionless phase transformation that yields denser body-centered tetragonal phases. These structural changes can be reversed by increasing the temperature of the dispersion and may lead to the direct modulation of the optical properties of these artificial solids.