Growth and self-assembly of ultrathin Au nanowires into expanded hexagonal superlattice studied by in situ SAXS

Angle-Resolved Linear Dichroism to Probe the Organization of Highly Ordered Collagen Biomaterials

Controlling the assembly of high-order structures is central to soft-matter and biomaterial engineering. Angle-resolved linear dichroism can probe the ordering of chiral collagen molecules in the dense state. Collagen triple helices were aligned by solvent evaporation. Their ordering gives a strong linear dichroism (LD) that changes sign and intensity with varying sample orientations with respect to the beam linear polarization. Being complementary to circular dichroism, which probes the structure of chiral (bio)molecules, LD can shift from the molecular to the supramolecular scale and from the investigation of the conformation to interactions. Supported by multiphoton microscopy and X-ray scattering, we show that LD provides a straightforward route to probe collagen alignment, determine the packing density, and monitor denaturation. This approach could be adapted to any assembly of chiral (bio)macromolecules, with key advantages in detecting large-scale assemblies with high specificity to aligned and chiral molecules and improved sensitivity compared to conventional techniques.

A combined TEM and SAXS study of the growth and self-assembly of ultrathin Pt nanowires

Ultrathin Pt nanowires possess high activity for various electrocatalytic applications. However, little work has focused on understanding their growth mechanisms. Herein, we utilize a combination of time-dependent,ex situtransmission electron microscopy (TEM) and small angle x-ray scattering (SAXS) techniques to observe the growth process in addition to associated surfactant-based interactions. TEM images indicate that initially nanoparticles are formed within 30 s; these small 'seed' particles quickly elongate to form ultrathin nanowires after 2 min. These motifs remain relatively unchanged in size and shape up to 480 min of reaction. Complementary SAXS data suggests that the initial nanoparticles, which are coated by a surfactant bilayer, arrange into abccsuperlattice. With increasing reaction time, thebcclattice disappears as the nanoparticles grow into nanowires, which then self-assemble into a columnar hexagonal structure in which the individual nanowires are covered by a CTAB monolayer. The hexagonal structure eventually degrades, thereby leading to the formation of lamellar stacking phases comprised of surfactant bilayers. To the best of our knowledge, this is the first time that SAXS has been used to monitor the growth and self-assembly of Pt nanowires. These insights can be used to better understand and rationally control the formation of anisotropic motifs of other metallic nanostructures.

Entropy of Branching Out: Linear versus Branched Alkylthiols Ligands on CdSe Nanocrystals

Surface ligands of semiconductor nanocrystals (NCs) play key roles in determining their colloidal stability and physicochemical properties and are thus enablers also for the NCs flexible manipulation toward numerous applications. Attention is usually paid to the ligand binding group, while the impact of the ligand chain backbone structure is less discussed. Using isothermal titration calorimetry (ITC), we studied the effect of structural changes in the ligand chain on the thermodynamics of the exchange reaction for oleate coated CdSe NCs, comparing linear and branched alkylthiols. The investigated alkylthiol ligands differed in their backbone length, branching position, and branching group length. Compared to linear ligands, lower exothermicity and entropy loss were observed for an exchange with branched ligands, due to steric hindrance in ligand packing, thereby justifying their previous classification as "entropic ligands". Mean-field calculations for ligand binding demonstrate the contribution to the overall entropy originating from ligand conformational entropy, which is diminished upon binding mainly by packing of NC-bound ligands. Model calculations and the experimental ITC data both point to an interplay between the branching position and the backbone length in determining the entropic nature of the branched ligand. Our findings suggest that the most entropic ligand should be a short, branched ligand with short branching group located toward the middle of the ligand chain. The insights provided by this work also contribute to a future smarter NC surface design, which is an essential tool for their implementation in diverse applications.

Using small-angle scattering to guide functional magnetic nanoparticle design

Magnetic nanoparticles offer unique potential for various technological, biomedical, or environmental applications thanks to the size-, shape- and material-dependent tunability of their magnetic properties. To optimize particles for a specific application, it is crucial to interrelate their performance with their structural and magnetic properties. This review presents the advantages of small-angle X-ray and neutron scattering techniques for achieving a detailed multiscale characterization of magnetic nanoparticles and their ensembles in a mesoscopic size range from 1 to a few hundred nanometers with nanometer resolution. Both X-rays and neutrons allow the ensemble-averaged determination of structural properties, such as particle morphology or particle arrangement in multilayers and 3D assemblies. Additionally, the magnetic scattering contributions enable retrieving the internal magnetization profile of the nanoparticles as well as the inter-particle moment correlations caused by interactions within dense assemblies. Most measurements are used to determine the time-averaged ensemble properties, in addition advanced small-angle scattering techniques exist that allow accessing particle and spin dynamics on various timescales. In this review, we focus on conventional small-angle X-ray and neutron scattering (SAXS and SANS), X-ray and neutron reflectometry, gracing-incidence SAXS and SANS, X-ray resonant magnetic scattering, and neutron spin-echo spectroscopy techniques. For each technique, we provide a general overview, present the latest scientific results, and discuss its strengths as well as sample requirements. Finally, we give our perspectives on how future small-angle scattering experiments, especially in combination with micromagnetic simulations, could help to optimize the performance of magnetic nanoparticles for specific applications.

Oleylamine Impurities Regulate Temperature-Dependent Hierarchical Assembly of Ultranarrow Gold Nanowires on Biotemplated Interfaces

Nanocrystals are often synthesized using technical grade reagents such as oleylamine (OLAm), which contains a blend of 9-cis-octadeceneamine with trans-unsaturated and saturated amines. Here, we show that gold nanowires (AuNWs) synthesized with OLAm ligands undergo thermal transitions in interfacial assembly (ribbon vs. nematic); transition temperatures vary widely with the batch of OLAm used for synthesis. Mass spectra reveal that higher-temperature AuNW assembly transitions are correlated with an increased abundance of trans and saturated chains in certain blends. DSC thermograms show that both pure (synthesized) and technical-grade OLAm have primary melting transitions near -5 °C (20-30 °C lower than the literature melting temperature range of OLAm). A second, broader melting transition (in the previous reported melting range) appears in technical grade blends; its temperature varies with the abundance of trans and saturated chains. Our findings illustrate that, similar to biological membranes, blends of alkyl chains can be used to generate mesoscopic hierarchical nanocrystal assembly, particularly at interfaces that further modulate transition temperatures.

In Situ Small-Angle X-ray Scattering Studies on the Growth Mechanism of Anisotropic Platinum Nanoparticles

Shape-controlled platinum nanoparticles exhibit extremely high oxygen reduction activity. Platinum nanoparticles were synthesized by the reduction of a platinum complex in the presence of a soft template formed by organic surfactants in oleylamine. The formation of platinum nanoparticles was investigated using in situ small-angle X-ray scattering experiments. Time-resolved measurements revealed that different particle shapes appeared during the reaction. After the nuclei were generated, they grew into anisotropic rod-shaped nanoparticles. The shape, size, number density, reaction yield, and specific surface area of the nanoparticles were successfully determined using small-angle X-ray scattering profiles. Anisotropic platinum nanoparticles appeared at a low reaction temperature (∼100 °C) after a short reaction time (∼30 min). The aspect ratio of these platinum nanoparticles was correlated with the local packing motifs of the surfactant molecules and their stability. Our findings suggest that the interfacial structure between the surfactant and platinum nuclei can be important as a controlling factor for tailoring the aspect ratio of platinum nanoparticles and further optimizing the fuel cell performance.

Large-Scale Noncovalent Functionalization of 2D Materials through Thermally Controlled Rotary Langmuir-Schaefer Conversion

As two-dimensional (2D) materials are more broadly utilized as components of hybrid materials, controlling their surface chemistry over large areas through noncovalent functionalization becomes increasingly important. Here, we demonstrate a thermally controlled rotary transfer stage that allows areas of a 2D material to be continuously cycled into contact with a Langmuir film. This approach enables functionalization of large areas of the 2D material and simultaneously improves long-range ordering, achieving ordered domain areas up to nearly 10 000 μm². To highlight the layer-by-layer processing capability of the rotary transfer stage, large-area noncovalently adsorbed monolayer films from an initial rotary cycle were used as templates to assemble ultranarrow gold nanowires from solution. The process we demonstrate would be readily extensible to roll-to-roll processing, addressing a longstanding challenge in scaling Langmuir-Schaefer transfer for practical applications.

The role of pre-nucleation clusters in the crystallization of gold nanoparticles

The syntheses of metal nanoparticles by reduction in apolar solvents in the presence of long chain surfactants have proven to be extremely effective in the control of the particle size and shape. Nevertheless, the elucidation of the nucleation/growth mechanism is not straightforward because of the multiple roles played by surfactants. The nucleation stage, in particular, is very difficult to describe precisely and requires in situ and time-resolved techniques. Here, relying on in situ small angle X-ray scattering (SAXS), X-ray absorption spectroscopy (XAS) and high-energy X-ray diffraction (HE-XRD), we propose that ultra-small gold particles prepared by reduction of gold chloride in a solution of oleylamine (OY) in hexane with triisopropylsilane do not follow a classical nucleation process but result from pre-nucleation clusters (PNCs). These PNCs contain Au(iii) and Au(i) precursors; they are almost stable in size during the induction stage, as shown by SAXS, prior to undergoing a very fast shrinkage during the nucleation stage. The gold speciation as a function of time deduced from the XAS spectra has been analyzed through multi-step reaction pathways comprising both highly reactive species, involved in the nucleation and growth stages, and poorly reactive species acting as a reservoir for the reactive species. The duration of the induction period is related to the reactivity of the gold precursors, which is tuned by the coordination of OY to the gold complexes, while the nucleation stage was found to depend on the size and reactivity of the PNCs. The role of the PNCs in determining the final particle size and structure is also discussed in relation to previous studies. The multiple roles of OY, as the solubilizing agent of the gold salt, the ligand of the gold complexes determining both the size of the PNCs and the reactivity of the gold precursors, and finally the capping agent of the final gold particles as oleylammonium chloride, have been clearly established. This work opens new perspectives to synthesize metal NPs via metal-organic PNCs and to define new synthesis routes for nanoparticles that may present structure and morphologies different from those obtained by the classical nucleation routes.

Entropy Can Bundle Nanowires in Good Solvents

Surfaces with surface-bound ligand molecules generally attract each other when immersed in poor solvents but repel each other in good solvents. While this common wisdom holds, for example, for oleylamine-ligated ultrathin nanowires in the poor solvent ethanol, the same nanowires were recently observed experimentally to bundle even when immersed in the good solvent n-hexane. To elucidate the respective binding mechanisms, we simulate both systems using molecular dynamics. In the case of ethanol, the solvent is completely depleted at the interface between two ligand shells so that their binding occurs, as expected, via direct interactions between ligands. In the case of n-hexane, ligands attached to different nanowires do not touch. The binding occurs because solvent molecules penetrating the shells preferentially orient their backbone normal to the wire, whereby they lose entropy. This entropy does not have to be summoned a second time when the molecules penetrate another nanowire. For the mechanism to be effective, the ligand density appears to best be intermediate, that is, small enough to allow solvent molecules to penetrate, but not so small that ligands do not possess a clear preferred orientation at the interface to the solvent. At the same time, solvent molecules may be neither too large nor too small for similar reasons. Experiments complementing the simulations confirm the predicted trends.

In Situ Synchrotron X-ray Characterization Shining Light on the Nucleation and Growth Kinetics of Colloidal Nanoparticles

Rational synthesis of colloidal nanoparticles with desirable properties relies on precise control over the nucleation and growth kinetics, which is still not well understood. The recent development of in situ high energy synchrotron X-ray techniques offers an excellent opportunity to quantitatively monitor the growth trajectories of colloidal nanoparticles in real time under real reaction conditions. The time-resolved, quantitative data of the growing colloidal nanoparticles are unique to reveal the mechanism of nanoparticle formation and determine the corresponding intrinsic kinetic parameters. This review discusses the kinetics of major steps of forming colloidal nanoparticles and the capability of in situ synchrotron X-ray techniques in studying the corresponding kinetics.

The Sub-Nanometer Scale as a New Focus in Nanoscience

Size is one of the central issues in nanoscience. The practical meaning of the term "sub-nanometric material (SNM)" requires two aspects: (1) its size should be at the atomic level; (2) it shows unique (size-related) properties compared to its nano-counterparts with larger sizes. Here, SNMs in the form of wires (SNWs) and the unique properties arising from their special size are reviewed. First, their polymer-like behavior, including rheological behavior and self-assembly, is dicussed. Their origins may stem from the special size and the ligands around the wire. Even a slight increase in diameter would risk the polymer-like behavior. Meanwhile, the ligands on SNWs are proportional to the inorganic entity at this scale. Consequently, surface ligands should have a profound impact on the properties, like catalysis, self-assembly, optics, etc. To reveal more potential applications, their applications in energy conversion are comprehensively reviewed. To some extent, characterization can greatly influence the way things are observed. Thus, some appropriate characterization techniques are briefly introduced. Finally, another emerging part of SNWs (atomic chain material) is briefly introduced. It is hoped that this review can provide new insights to this special scale.

Ultrathin Gold Nanowires with the Polytetrahedral Structure of Bulk Manganese

Despite the intensive interest in thin gold nanowires for a variety of technologically important applications, key details of the mechanism of their formation and atomic-scale structure remain unknown. Here we synthesize highly uniform, very long, and ultrathin gold nanowires in a liquid-phase environment and study their nucleation and growth using in situ high-energy synchrotron X-ray diffraction. By controlling the type of solvents, reducing agents, and gold precursor concentration, it is shown that the nucleation and growth of gold nanowires involve the emergence and self-assembly of transient linear gold complexes, respectively. In sharp contrast with the face-centered-cubic bulk gold, the evolved nanowires are found to possess a tetrahedrally close packed structure incorporating distorted icosahedra and larger size coordination polyhedra of the type observed with the room-temperature phase of bulk manganese. We relate the complexes to synergistic effects between the selected precursor and reducing agents that become appreciable over a narrow range of their molar ratios. We attribute the unusual structural state of gold nanowires to geometrical frustration effects arising from the conflicting tendencies of assemblies of metal atoms to evolve toward attaining high atomic packing density while keeping the atomic-level stresses low, ultimately favoring the growth of cylindrical nanowires with a well-defined diameter and atomically smooth surface. Our work provides a roadmap for comprehensive characterization and, hence, better understanding of 1D metallic nanostructures with an unusual atomic arrangement and may have important implications for their synthesis and performance in practical applications.

Liquid-cell Transmission Electron Microscopy for Tracking Self-assembly of Nanoparticles

Drying a nanoparticle dispersion is a versatile way to create self-assembled structures of nanoparticles, but the mechanism of this process is not fully understood. We have traced the trajectories of individual nanoparticles using liquid-cell transmission electron microscopy (TEM) to investigate the mechanism of the assembly process. Herein, we present the protocols used for liquid-cell TEM studies of the self-assembly mechanism. First, we introduce the detailed synthetic protocols used to produce uniformly sized platinum and lead selenide nanoparticles. Next, we present the microfabrication processes used to produce liquid cells with silicon nitride or silicon windows and then describe the loading and imaging procedures of the liquid-cell TEM technique. Several notes are included to provide helpful tips for the entire process, including how to manage the fragile cell windows. The individual motions of nanoparticles tracked by liquid-cell TEM revealed that changes in the solvent boundaries caused by evaporation affected the self-assembly process of nanoparticles. The solvent boundaries drove nanoparticles to primarily form amorphous aggregates, followed by flattening of the aggregates to produce a 2-dimensional (2D) self-assembled structure. These behaviors are also observed for different nanoparticle types and different liquid-cell compositions.

Surface-Engineering of Ultrathin Gold Nanowires: Tailored Self-Assembly and Enhanced Stability

Gold nanowires with a mean diameter of 1.7 nm were synthesized by reduction of HAuCl₄ in a solution of oleylamine (OY) in hexane. A bilayer of oleylammonium chloride/oleylamine at the surface of the raw nanowires was evidenced by NMR and diffusion ordered spectroscopy (DOSY) experiments. After washing a monolayer of oleylammonium chloride remained at the surface of the nanowires. The oleylammonium chloride layer could be progressively replaced by a phosphine shell as evidenced with NMR and DOSY experiments, which are in good agreement with the adsorption energies given by density functional theory calculations. The nanowires crystallize into hexagonal superlattices with a lattice parameter that can be tailored depending on the ligand shell. Small-angle X-ray scattering showed the following lattice parameters: Au@OY⁺Cl^-(OY) (a = 7.2 nm) > Au@TOPO/OY (a = 6.6 nm) > Au@ OY⁺Cl^- (a = 4.1 nm) > Au@TOP (a = 3.75 nm). This is one of a few examples of surface modification of ultrathin nanowires that does not alter their morphology. Moreover, the nanowires coated with phosphines exhibited long time stability (at the opposite of other ligands like thiols) opening the way to more complex functionalization.

Structural analysis of micrometer-long gold nanowires using a wormlike chain model and their rheological properties

The recent growing interest in the applications of gold nanowires (AuNWs) as flexible materials has raised the fundamental issue of how their mechanical properties are related to their morphology. In this work, to address this issue, the systematic synthesis of AuNWs, their structural analysis, and their rheological investigation were demonstrated. The structural analysis of AuNWs was performed based on TEM observations and light-scattering experiments. From these observations, it was found that the length of AuNWs varies from nanometer to micrometer depending on the reaction time while a constant width of 1.6 nm is maintained. On the basis of static light-scattering experiments and a wormlike chain model, the structural parameters of AuNWs during their growth were successfully obtained. When the contour length of AuNWs reached around 5 μm, the AuNW solution showed non-Newtonian behavior and appeared to behave as a gel. Dynamic viscoelasticity measurements indicated that such viscous behavior is responsible for entanglement between AuNWs. It is concluded that AuNWs are analogous with conventional polymers in terms of both their structure and their rheological behavior.

Spinning Hierarchical Gold Nanowire Microfibers by Shear Alignment and Intermolecular Self-Assembly

Hierarchical structures lend strength to natural fibers made of soft nanoscale building blocks. Intermolecular interactions connect the components at different levels of hierarchy, distribute stresses, and guarantee structural integrity under load. Here, we show that synthetic ultrathin gold nanowires with interacting ligand shells can be spun into biomimetic, free-standing microfibers. A solution spinning process first aligns the wires, then lets their ligand shells interact, and finally converts them into a hierarchical superstructure. The resulting fiber contained 80 vol % organic ligand but was strong enough to be removed from the solution, dried, and mechanically tested. Fiber strength depended on the wire monomer alignment. Shear in the extrusion nozzle was systematically changed to obtain process-structure-property relations. The degree of nanowire alignment changed breaking stresses by a factor of 1.25 and the elongation at break by a factor of 2.75. Plasma annealing of the fiber to form a solid metal shell decreased the breaking stress by 65%.

Evidence of contact epitaxy in the self-assembly of HgSe nanocrystals formed at a liquid-liquid interface

The grazing incidence x-ray scattering results presented here show that the self-assembly process of HgSe nanocrystals formed at a liquid-liquid interface is quite different along the in-plane direction and across the interface. In situ x-ray reflectivity and ex situ microscopy measurements suggest quantized out-of-plane growth for HgSe nanoparticles of a size of about [Formula: see text] nm initially. Grazing incidence small-angle x-ray scattering measurements for films transferred from the water-toluene interface at various stages of reaction show that these nanoparticles first form random clusters with an average radius of 2.2 nm, giving rise to equally spaced rings of several orders. Finally, these clusters self-organize into face-centered cubic superstructures, giving sharp x-ray diffraction peaks oriented normal to the liquid-liquid interface with more than 100 nm-coherent domains. We also observed the x-ray diffraction pattern of the HgSe crystalline phase, with the superlattice peaks in these grazing incidence measurements of the transferred films. The electron microscopy and atomic force microscopy results support the x-ray observation of the self-organization of HgSe nanocrystals into close-packed superlattices. These results show that capillary wave fluctuation promotes the oriented attachment of clusters at the liquid-liquid interface, giving direct experimental evidence of contact epitaxy.

Liquid Cell Electron Microscopy of Nanoparticle Self-Assembly Driven by Solvent Drying

Drying a colloidal solution of nanoparticles is a versatile method to construct self-assembled structures of nanoparticles. However, mechanistic understanding has mostly relied on empirical knowledge obtained from the final structures of self-assembly as relevant processes during solvent drying are likely kinetic and far from equilibrium. Here, we present in situ TEM studies of nanoparticle self-assembly under various conditions, including the concentrations of the initial solution and the types of nanoparticles and substrates. The capability of tracking trajectories of individual nanoparticles enables us to understand the mechanisms of drying-mediated self-assembly at the single-nanoparticle level. Our results consistently show that a solvent boundary primarily affects nanoparticle motions and the resulting self-assembly processes regardless of different conditions. The solvent boundary drives nanoparticles to form two-dimensional assembly mainly through two pathways, transporting scattered nanoparticles by lateral dragging and flattening aggregated nanoparticles by vertical pressing.

Templated Self-Assembly of Ultrathin Gold Nanowires by Nanoimprinting for Transparent Flexible Electronics

We fabricated flexible, transparent, and conductive metal grids as transparent conductive materials (TCM) with adjustable properties by direct nanoimprinting of self-assembling colloidal metal nanowires. Ultrathin gold nanowires (diameter below 2 nm) with high mechanical flexibility were confined in a stamp and readily adapted to its features. During drying, the wires self-assembled into dense bundles that percolated throughout the stamp. The high aspect ratio and the bundling yielded continuous, hierarchical superstructures that connected the entire mesh even at low gold contents. A soft sintering step removed the ligand barriers but retained the imprinted structure. The material exhibited high conductivities (sheet resistances down to 29 Ω/sq) and transparencies that could be tuned by changing wire concentration and stamp geometry. We obtained TCMs that are suitable for applications such as touch screens. Mechanical bending tests showed a much higher bending resistance than commercial ITO: conductivity dropped by only 5.6% after 450 bending cycles at a bending radius of 5 mm.

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.

A Universal Wet-Chemistry Route to Metal Filling of Boron Nitride Nanotubes

We present a facile wet-chemistry method for efficient metal filling of the hollow inner cores of boron nitride nanotubes (BNNTs). The fillers conform to the cross-section of the tube cavity and extend in length from a few nm to hundreds of nm. The methodology is robust and is demonstrated for noble metals (Au, Pt, Pd, and Ag), transition metals (Co), and post-transition elements (In). Transmission electron microscopy and related electron spectroscopy confirm the composition and morphology of the filler nanoparticles. Up to 60% of BNNTs of a given preparation batch have some degree of metal encapsulation, and individual tubes can have up to 10% of their core volume filled during initial loading. The growth, movement, and fusing of metal nanoparticles within the BNNTs are also examined.

Multivalent bonds in self-assembled bundles of ultrathin gold nanowires

We describe solvent effects in the self-assembly of ultrathin gold nanowires and highlight the role of intermolecular ligand–solvent interactions.

Unconventional seed-mediated growth of ultrathin Au nanowires in aqueous solution

Ultrathin Au nanowires have garnered increasing attention in recent years because of their potential use in a range of applications due to their unique optical properties, conductivity, chemical activity, and discrete plasticity. Herein, we report an unconventional seed-mediated growth of ultrathin Au nanowires induced by hydrophobic molecules. Quite intriguingly, by adding a trace amount of hydrophobic molecules (i.e., toluene or chloroform) to the Au growth solution conventionally used for the growth of Au nanorods with cylindrical CTAB micelles as templates, CTAB-capped ultrathin Au nanowires (i.e., water-soluble ultrathin Au nanowires) were crafted. Similarly to the growth of Au nanorods, silver ions and Au seeds were crucially required to yield the water-soluble ultrathin Au nanowires. The growth mechanism of these ultrathin nanowires was also explored.

Stabilization of 4H hexagonal phase in gold nanoribbons

Gold, silver, platinum and palladium typically crystallize with the face-centred cubic structure. Here we report the high-yield solution synthesis of gold nanoribbons in the 4H hexagonal polytype, a previously unreported metastable phase of gold. These gold nanoribbons undergo a phase transition from the original 4H hexagonal to face-centred cubic structure on ligand exchange under ambient conditions. Using monochromated electron energy-loss spectroscopy, the strong infrared plasmon absorption of single 4H gold nanoribbons is observed. Furthermore, the 4H hexagonal phases of silver, palladium and platinum can be readily stabilized through direct epitaxial growth of these metals on the 4H gold nanoribbon surface. Our findings may open up new strategies for the crystal phase-controlled synthesis of advanced noble metal nanomaterials.

Sintering of ultrathin gold nanowires for transparent electronics

Ultrathin gold nanowires (AuNWs) with diameters below 2 nm and high aspect ratios are considered to be a promising base material for transparent electrodes. To achieve the conductivity expected for this system, oleylamine must be removed. Herein we present the first study on the conductivity, optical transmission, stability, and structure of AuNW networks before and after sintering with different techniques. Freshly prepared layers consisting of densely packed AuNW bundles were insulating and unstable, decomposing into gold spheres after a few days. Plasma treatments increased the conductivity and stability, coarsened the structure, and left the optical transmission virtually unchanged. Optimal conditions reduced sheet resistances to 50 Ω/sq.

Directed assembly of single colloidal gold nanowires by AFM nanoxerography

Ultrathin gold nanowires (NWs) dispersed in hexane were prepared by chemical reduction of HAuCl4 in oleylamine, along with nanospheres (NSs), side products of the reaction. X-ray photoelectron spectroscopy and small-angle X-ray scattering evidenced a stabilization of these nano-objects by oleylammonium chloride surfactants. The directed assembly of these nano-objects on surfaces was performed by atomic force microscopy (AFM) nanoxerography in a few seconds. Selective assembly of gold NWs only occurred on positively charged patterns, while NSs assembled more specifically on the negatively charged ones. This sorting suggests that the strong electric field generated by the charge patterns induced a negative effective charge on the gold NWs and a weak positive effective charge on the NSs. Such difference could be explained by the ion organization at the colloid surface, monolayered in the case of NWs, and bilayered in the case of NSs. By adjusting the design of the positive patterns and the experimental conditions of development, single gold nanowires were successfully assembled by AFM nanoxerography on predefined sites of surfaces without damaging them, opening the way for future electrical and mechanical characterizations.

Organic-free synthesis of ultrathin gold nanowires as effective SERS substrates

Ultrathin gold nanowires (Au NWs) were synthesized in the absence of organic reagents. The obtained organic-free ultrathin Au NWs present great potential for surface-enhanced Raman scattering (SERS) applications.

Room temperature growth of ultrathin Au nanowires with high areal density over large areas by in situ functionalization of substrate

Although ultrathin Au nanowires (∼2 nm diameter) are expected to demonstrate several interesting properties, their extreme fragility has hampered their use in potential applications. One way to improve the stability is to grow them on substrates; however, there is no general method to grow these wires over large areas. The existing methods suffer from poor coverage and associated formation of larger nanoparticles on the substrate. Herein, we demonstrate a room temperature method for growth of these nanowires with high coverage over large areas by in situ functionalization of the substrate. Using control experiments, we demonstrate that an in situ functionalization of the substrate is the key step in controlling the areal density of the wires on the substrate. We show that this strategy works for a variety of substrates ranging like graphene, borosil glass, Kapton, and oxide supports. We present initial results on catalysis using the wires grown on alumina and silica beads and also extend the method to lithography-free device fabrication. This method is general and may be extended to grow ultrathin Au nanowires on a variety of substrates for other applications.

New insights into electronic and geometric effects in the enhanced photoelectrooxidation of ethanol Using ZnO nanorod/ultrathin Au nanowire hybrids

Oxidation of small organic molecules in a fuel cell is a viable method for energy production. However, the key issue is the development of suitable catalysts that exhibit high efficiencies and remain stable during operation. Here, we demonstrate that amine-modified ZnO nanorods on which ultrathin Au nanowires are grown act as an excellent catalyst for the oxidation of ethanol. We show that the modification of the ZnO nanorods with oleylamine not only modifies the electronic structure favorably but also serves to anchor the Au nanowires on the nanorods. The adsorption of OH(-) species on the Au nanowires that is essential for ethanol oxidation is facilitated at much lower potentials as compared to bare Au nanowires leading to high activity. While ZnO shows negligible electrocatalytic activity under normal conditions, there is significant enhancement in the activity under light irradiation. We demonstrate a synergistic enhancement in the photoelectrocatalytic activity of the ZnO/Au nanowire hybrid and provide mechanistic explanation for this enhancement based on both electronic as well as geometric effects. The principles developed are applicable for tuning the properties of other metal/semiconductor hybrids with potentially interesting applications beyond the fuel cell application demonstrated here.

Dynamic HAADF-STEM observation of a single-atom chain as the transient state of gold ultrathin nanowire breakdown

Ultrathin chemically grown gold nanowires undergo irremediable structural modification under external stimuli. Thanks to dynamic high-angle annular dark-field imaging, electron-beam-induced damage was followed, revealing the formation of linear chains of gold atoms as well as reactive clusters on the side, opening fascinating prospects for applications in both catalysis and electronic transport.