Over the last several decades, colloidal nanoparticles have evolved into a prominent class of building blocks for materials design. Important advances include the synthesis of uniform nanoparticles with tailored compositions and properties, and the precision construction of intricate, higher-level structures from nanoparticles via self-assembly. Grasping the modern complexity of nanoparticles and their superstructures requires fundamental understandings of the processes of nanoparticle growth and self-assembly. In situ liquid phase transmission electron microscopy (TEM) has significantly advanced our understanding of these dynamic processes by allowing direct observation of how individual atoms and nanoparticles interact in real time, in their native phases. In this article, we highlight diverse nucleation and growth pathways of nanoparticles in solution that could be elucidated by the in situ liquid phase TEM. Furthermore, we showcase in situ liquid phase TEM studies of nanoparticle self-assembly pathways, highlighting the complex interplay among nanoparticles, ligands, and solvents. The mechanistic insights gained from in situ liquid phase TEM investigation could inform the design and synthesis of novel nanomaterials for various applications such as catalysis, energy conversion, and optoelectronic devices.

Nanotechnology has provided novel modalities for the delivery of therapeutic and diagnostic agents. In particular, nanoparticles (NPs) can be engineered at a low cost for drug loading and delivery. For example, silica NPs have proven useful as a controlled release platform for anti-inflammatory drugs. Despite the wide-ranging potential applications for NPs, robust characterization across all size ranges remains elusive. Electron microscopy (EM) is the conventional tool for measuring NP diameters. However, imitations in throughput and the inability to provide comprehensive information on physical properties, such as mass and density, without underlying assumptions, hinder a complete analysis. In addition, assessing sample heterogeneity, aggregation, or coalescence in solution by traditional EM analysis is not possible. Resistive-pulse sensing (RPS) provides a high throughput, solution-phase method for characterizing particle heterogeneity based on volume. Complementing these methods, charge detection mass spectrometry (CD-MS), a single particle technique, provides accurate mass information for heterogeneous samples including NPs. By combining EM, RPS and CD-MS, accurate volume, mass, and densities were obtained for silica NPs of various sizes. The results show that the density for 20 nm silica NPs is close to the density of fused silica (2.2 g/cm(3)). Larger silica NPs were found to have densities that were either smaller or larger, while also falling outside the range of densities usually found for silica colloids and NPs (1.9-2.3 g/cm(3)). Lower densities are attributed to pores (i.e., porous particles). For one sample, the mass distribution showed two components attributed to two populations of particles in the sample with different densities. The synergistic combination of EM, RPS, and CD-MS measurements outlined here for NP samples, allows much more extensive information to be obtained than from any of the techniques alone.

Arranging magnetic nanoparticles (MNPs) into highly ordered structures, so-called superlattices or mesocrys-tals, is of great interest from a fundamental point of view, as the employment of the corresponding coupled nanoentities introduces additional degrees of freedom to manipulate the overall magnetic characteristics of such hierarchical materials. Characterizing the associated magnetic interactions on the mesoscopic scale is indispens-able for obtaining a profound understanding of the relative strengths of the types of interactions involved, such as dipole-dipole interactions, which affect the collective response of a corresponding mesocrystal. In this paper, nanoparticles are deposited onto silicon substrates by spin coating, leading to two-dimensional monolayered structures showing a close-packed hexagonal arrangement. The MNPs consist of iron oxide (magnetite Fe3O4) and are coated with a nonmagnetic polymer (polystyrene). The MNPs are synthesized such that their diameters dMNP are tuned in a range between 9 and 18 nm. A precise manipulation of the shell thickness dshell is achieved by coating the MNPs with polystyrene of different molecular weights. In this fashion, the spacing between the MNPs, dspacer = 2dshell, is varied in a range between 6 and 14 nm. Within the investigated dspacer range, dipolar interactions govern the collective properties showing distinct distance-dependent characteristics. As dspacer increases, the dipolar coupling strength between the MNPs decreases, as deduced from the spectral features of ferromagnetic resonance experiments. These observations are further corroborated by numerical simulations of the dynamic properties of appropriate model systems. A comparison of the experimental and theoretical findings shows that material parameters, such as the magnetization MMNP and the magnetocrystalline anisotropy CMNP of the MNPs, are reduced compared to their bulk values.

Seed-mediated synthesis is a versatile method to prepare multimetallic nanocrystals for diverse applications. However, many fundamental questions remain on how the structural and chemical properties of nanocrystal seeds control the reaction pathways, especially for nonaqueous synthesis at elevated temperatures. Herein, we elucidate the role of surface ligands and crystallinity of Au nanocrystal seeds on the heterometallic seeded growth of Cu-based nanocrystals. We found that weakly coordinating ligands are critical to facilitate the diffusion between Au and Cu, which enables subsequent one-dimensional growth of Cu. Replacing multiple-twinned Au seeds with single-crystalline ones switched the growth pathway to produce heterostructured nanocrystals. Our work illustrates the importance of precise control of seed characteristics for the predictive synthesis of structurally complex multimetallic nanocrystals.

The self-assembly of shape-anisotropic nanocrystals intolarge-scalestructures is a versatile and scalable approach to creating multifunctionalmaterials. The tetrahedral geometry is ubiquitous in natural and manmadematerials, yet regular tetrahedra present a formidable challenge inunderstanding their self-assembly behavior as they do not tile space.Here, we report diverse supracrystals from gold nanotetrahedra includingthe quasicrystal (QC) and the dimer packing predicted more than adecade ago and hitherto unknown phases. We solve the complex three-dimensional(3D) structure of the QC by a combination of electron microscopy,tomography, and synchrotron X-ray scattering. Nanotetrahedron vertexsharpness, surface ligands, and assembly conditions work in concertto regulate supracrystal structure. We also discover that the surfacecurvature of supracrystals can induce structural changes of the QCtiling and eventually, for small supracrystals with high curvature,stabilize a hexagonal approximant. Our findings bridge the gap betweencomputational design and experimental realization of soft matter assembliesand demonstrate the importance of accurate control over nanocrystalattributes and the assembly conditions to realize increasingly complexnanopolyhedron supracrystals.

Cu is an inexpensive alternative plasmonic metal with optical behaviour comparable to Au but with much poorer environmental stability. Alloying with a more stable metal can improve stability and add functionality, with potential effects on the plasmonic properties. Here we investigate the plasmonic behaviour of Cu nanorods and Cu-CuPd nanorods containing up to 46 mass percent Pd. Monochromated scanning transmission electron microscopy electron energy-loss spectroscopy first reveals the strong length dependence of multiple plasmonic modes in Cu nanorods, where the plasmon peaks redshift and narrow with increasing length. Next, we observe an increased damping (and increased linewidth) with increasing Pd content, accompanied by minimal frequency shift. These results are corroborated by and expanded upon with numerical simulations using the electron-driven discrete dipole approximation. This study indicates that adding Pd to nanostructures of Cu is a promising method to expand the scope of their plasmonic applications.

Crystallization is a universal phenomenon underpinning many industrial and natural processes and is fundamental to chemistry and materials science. However, microscopic crystallization pathways of nanoparticle superlattices have been seldom studied mainly owing to the difficulty of real-time observation of individual self-assembling nanoparticles in solution. Here, using in situ electron microscopy, we directly image the full self-assembly pathway from dispersed nanoparticles into ordered superlattices in nonaqueous solution. We show that electron-beam irradiation controls nanoparticle mobility, and the solvent composition largely dictates interparticle interactions and assembly behaviors. We uncover a multistep crystallization pathway consisting of four distinct stages through multi-order-parameter analysis and visualize the formation, migration, and annihilation of multiple types of defects in nanoparticle superlattices. These findings open the door for achieving independent control over imaging conditions and nanoparticle assembly conditions and will enable further study of the microscopic kinetics of assembly and phase transition in nanocolloidal systems.

Weak light absorption of common Ir(III) complexes (e. g., using phenylpyridine as the ligand) has hindered their applications in photocatalytic hydrogen generation from water as an efficient photosensitizer. To address this issue, a series of cyclometalated Ir(III) complexes (Ir1-Ir5), featuring different electron-donating substituents to enhance the absorptivity, have been synthesized and studied as photosensitizers (PSs) for light-driven hydrogen production from water. Ir6-Ir7 were prepared as fundamental systems for comparisons. Electron donors, including 9-phenylcarbazole, triphenylamine, 4,4 '-dimethoxytriphenylamine, 4,4 '-di(N-hexylcarbazole)triphenylamine moieties were introduced on 6-(thiophen-2-yl)phenanthridine-based cyclometalating (C<^>N) ligands to explore the donor effect on the hydrogen evolution performance of these cationic Ir(III) complexes. Remarkably, Ir4 with 4,4 '-dimethoxytriphenylamine achieved the highest turn-over number (TON) of 12 300 and initial turnover frequency (TOFi) of 394 h(-1), with initial activity (activity(i)) of 547 000 mu mol g(-1) h(-1) and initial apparent quantum yield (AQY(i)) of 9.59 %, under the illumination of blue light-emitting diodes (LEDs) for 105 hours, which demonstrated a stable three-component photocatalytic system with high efficiency. The TON (based on n(H-2)/n(PSr)) in this study is the highest value reported to date among the similar photocatalytic systems using Ir(III) complexes with Pt nanoparticles as catalyst. The great potential of using triphenylamine-based Ir(III) PSs in boosting photocatalytic performance has also been shown.

The conversion of CO2 into value-added products is a compelling way of storing energy derived from intermittent renewable sources and can bring us closer to a closed-loop anthropogenic carbon cycle. The ability to synthesize nanocrystals of well-defined structure and composition has invigorated catalysis science with the promise of nanocrystals that selectively express the most favorable sites for efficient catalysis. The performance of nanocrystal catalysts for the CO2 reduction reaction (CO2RR) is typically evaluated with nanocrystal ensembles, which returns an averaged system-level response of complex catalyst-modified electrodes with each nanocrystal likely contributing a different (unknown) amount. Measurements at single nanocrystals, taken in the context of statistical analysis of a population, and comparison to macroscale measurements are necessary to untangle the complexity of the ever-present heterogeneity in nanocrystal catalysts, achieve true structure-property correlation, and potentially identify nanocrystals with outlier performance. Here, we employ environment-controlled scanning electrochemical cell microscopy to isolate and investigate the electrocatalytic CO2RR response of individual facet-defined gold nanocrystals. Using correlative microscopy approaches, we conclusively demonstrate that {110}-terminated gold rhombohedra possess superior activity and selectivity for CO2RR compared with {111}-terminated octahedra and high-index {310}-terminated truncated ditetragonal prisms, especially at low overpotentials where electrode kinetics is anticipated to dominate the current response. The methodology framework described here could inform future studies of complex electrocatalytic processes through correlative single-entity and macroscale measurement techniques.

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.

Polymer-inorganic nanocomposites based on polymer-grafted nanocrystals (PGNCs) are enabling technologically relevant applications owing to their unique physical, chemical, and mechanical properties. While diverse PGNC superstructures have been realized through evaporation-driven self-assembly, this approach presents multifaceted challenges in experimentally probing and controlling assembly kinetics. Here, we report a kinetically controlled assembly of binary superstructures from a homogeneous disordered PGNC mixture utilizing solvent vapor annealing (SVA). Using a NaZn13-type superstructure as a model system, we demonstrate that varying the solvent vapor pressure during SVA allows for exquisite control of the rate and extent of PGNC assembly, providing access to nearly complete kinetic pathways of binary PGNC crystallization. Characterization of kinetically arrested intermediates reveals that assembly follows a multistep crystallization pathway involving spinodal-like preordering of PGNCs prior to NaZn13 nucleation. Our work opens up new avenues for the synthesis of multicomponent PGNC superstructures exhibiting multifunctionalities and emergent properties through a thorough understanding of kinetic pathways.

Single entity electrochemical (SEE) studies that can probe activities and heterogeneity in activities at nanoscale require samples that contain single and isolated particles. Single, isolated nanoparticles are achieved here with electrospray deposition of colloidal nanoparticle solutions, with simple instrumentation. Role of three electrospray (ES) parameters, viz. spray distance (emitter tip-to-substrate distance), ES current and emitter tip diameter, in the ES deposition of single Au nano-octahedra (Au ODs) is examined. The ES deposition of single, isolated Au ODs are analyzed in terms of percentage of single NPs and local surface density of deposition. The local surface density of ES deposition of single Au ODs was found to increase with decrease in spray distance and emitter tip diameter, and increase in ES current. While the percentage of single particle ES deposition increased with increase in spray distance and decrease in emitter tip size. No significant change in the single Au ODs ES deposition percentage was observed with change in ES current values included in this study. The most favourable conditions in the ES deposition of Au ODs in this study resulted in the local surface density of 0.26 ± 0.05 single particles per μm2 and observation of 96.3% single Au OD deposition.

Visualization of dynamic interlinking networks which respond and adapt to the constantly changing environment would be highly beneficial in developing new composite materials and active/responsive materials. Here, optically and structurally stabilized plasmo‐bio interlinking networks (PBINs) free from photobleaching for high resolution, long term visualization are reported. Necessary for structural and optical stability, a new stability algorithm to comprehensively quantify stability and detect minute instability undetectable by traditional methods is introduced. Biocompatible plasmonic gold nanorods (Bio‐AuNRs) are synthesized for high resolution, long term imaging by utilizing bromide‐free alternatives to achieve CTA+ free. Systematic physical, chemical, and biological characterizations reveal the structural and optical stability of Bio‐AuNRs required for constructing PBIN. Lastly, with actin as a model of interlinking networks of the cytoskeleton, optically and structurally stable PBIN (100% CTA+ free, 97% crosslinking rate) in applications as active/responsive materials, are demonstrated.

Cetyltrimethylammonium bromide (CTAB) is a widely used surfactant that aids the aqueous synthesis of colloidal nanoparticles. However, the presence of residual CTAB on nanoparticle surfaces can significantly impact nanoparticle applications, such as catalysis and sensing, under hydrated conditions. As such, consideration of the presence and quantity of CTAB on nanoparticle surfaces under hydrated conditions is of significance. Herein, as part of an integrated material characterization framework, we demonstrate the feasibility of in situ atomic force microscopy (AFM) to detect CTAB on the surface of Au nanocubes (Au NCs) under hydrated conditions, which enabled superior characterization compared to conventional spectroscopic methods. In situ force-distance (FD) spectroscopy and Kelvin probe force microscopy (KPFM) measurements support additional characterization of adsorbed CTAB, while correlative in situ AFM and scanning electron microscopy (SEM) measurements were used to evaluate sequential steps of CTAB removal from Au NCs across hydrated and dehydrated environments, respectively. Notably, a substantial quantity of CTAB remained on the Au NC surface after methanol washing, which was detected in AFM measurements but was not detected in infrared spectroscopy measurements. Subsequent electrochemical cleaning was found to be critically important to remove CTAB from the Au NC surface. Correlative measurements were also performed on individual nanoparticles, which further validate the method described here as a powerful tool to determine the extent and degree of CTAB removal from nanoparticle surfaces. This AFM-based method is broadly applicable to characterize the presence and removal of ligands from nanomaterial surfaces under hydrated conditions.

Ligands play a central role for the energetics and kinetics of nanocrystal assembly. Yet, the precise and simultaneous manipulation of ligands to dictate assembly outcome has proven difficult. Here, we present macromolecular ligand-engineering strategies to control, characterize, and model four molecular parameters of grafted polymer chains: chain length, chain dispersity, grafting density, and chain distribution. Direct ligand-exchange between nanoprisms and polymers functionalizes facets selectively and produces patchy nanocrystals. We develop a generalizable two-step ligand-exchange approach for the independent control of the two emergent brush parameters, brush thickness and brush softness. The resultant polymer-grafted prismatic nanocrystals with programmable ligand brushes self-assemble into thin-film superstructures of different wallpaper symmetries and faceted supracrystals. Our experiments are complemented by coarse-grained computer simulations of nanoprisms with directional, facet-specific interactions. This work paves the way for the precision synthesis of polymer-nanocrystal hybrid materials and enables the further refinement of theoretical models for particle brush materials.

Helical structures are ubiquitous in natural and synthetic materials across multiple length scales. Excellent and sometimes unusual chiral optical, mechanical, and sensing properties have been previously demonstrated in such symmetry-breaking shape, yet a general principle to realize helical structures at the sub-100 nm scale via colloidal synthesis remains underexplored. In this work, we describe the wet-chemical synthesis of monodisperse nanohelices based on gadolinium oxide (Gd2O3). Aberration-corrected electron microscopy revealed that individual nanohelices consist of a bilayer structure with the outer and inner layers derived from the {111} and {100} planes of bulk Gd2O3, respectively. Distinct from existing inorganic nanocoils with flexible bending geometries, the built-in lattice misfit between two adjacent crystal planes induces continuous helical growth yielding three-dimensional rigid nanohelices. Furthermore, the presence of water in the reaction was found to suppress the formation of nanohelices, producing nanoplates expressing predominantly {111} planes. Our study not only provides a bottom-up synthetic route and mechanistic understanding of nanohelices formation but may also open up new possibilities for creating chiral plasmonic nanostructures, luminescent biological labels, and nanoscale transducers.

Electrocatalytic reduction reactions (i.e., the hydrogen evolution reaction (HER) and oxygen reduction reaction) at individual, faceted Au nanocubes (NCs) and nano-octahedra (ODs) expressing predominantly {100} and {111} crystal planes on the surface, respectively, were studied by nanoscale voltammetric mapping. Cyclic voltammograms were collected at individual nanoparticles (NPs) with scanning electrochemical cell microscopy (SECCM) and correlated with particle morphology imaged by electron microscopy. Nanoscale measurements from a statistically informative set of individual NPs revealed that Au NCs have superior HER electrocatalytic activity compared to that of Au ODs, in good agreement with macroscale cyclic voltammetry measurements. Au NCs exhibited more particle-to-particle variation in catalytic activity compared to that with Au ODs. The approach of single-particle SECCM imaging coupled with macroscale CV on well-defined NPs provides a powerful toolset for the design and activity assessment of nanoscale electrocatalysts.

Six donor−π−acceptor (D–π–A) organic dyes based on thieno[3,2-b]thiophene π-linker comprising different alkyl side chains were synthesized and characterized. Their photophysical and electrochemical properties were studied and their corresponding performances as photosensitizers (PSs) in dye-sensitized solar cells (DSSCs) and photocatalytic hydrogen production systems were also investigated. A power conversion efficiency (PCE) of 5.25 % in DSSCs and a decent H2 turnover number (TON) of 5170 (48 h) in platinized TiO2 hydrogen production system were demonstrated. The results indicated that alkyl chain engineering is one of the crucial steps in designing a superior photosensitizer.

We report a heterometallic seed-mediated synthesis method for monodisperse penta-twinned Cu nanorods using Au nanocrystals as seeds. Elemental analyses indicate that resultant nanorods consist predominantly of copper with a gold content typically below 3 atom %. The nanorod aspect ratio can be readily adjusted from 2.8 to 13.1 by varying the molar ratio between Au seeds and Cu precursor, resulting in narrow longitudinal plasmon resonances tunable from 762 to 2201 nm. Studies of reaction intermediates reveal that symmetry-breaking is promoted by rapid nanoscale diffusion in Au-Cu alloys and the formation of a gold-rich surface. The growth pathway features coevolving shape and composition whereby nanocrystals become progressively enriched with Cu concomitant with nanorod growth. The availability of uniform colloidal Cu nanorods with widely tunable aspect ratios opens new avenues toward the synthesis of derivative one-dimensional metal nanostructures, and applications in surface-enhanced spectroscopy, bioimaging, and electrocatalysis, among others.

Three metal-free molecular photosensitizers (S1-S3) featuring a starburst triarylamine donor moiety have been synthesized. They show attractive photocatalytic performance in visible light-driven H2 production from water in their platinized TiO2 composites. A remarkable H2 turnover number (TON) of 10 200 (48 h) was achieved in an S1-anchored system.