Sintering of ultrathin gold nanowires for transparent electronics

Insight into the morphological instability of metallic nanowires under thermal stress

HYPOTHESIS

Metallic nanowires, particularly polyol-grown silver nanowires, exhibit a morphological instability at temperatures significantly lower than their bulk melting point. This instability is commonly named after Rayleigh's description of the morphological instability of liquid jets, even though it has been shown that its quantitative predictions are not consistent with experimental measurements. In 1996, McCallum et al. proposed a description of the phenomenon assuming a solid wire lying on a substrate. It is assumed that the latter description depicts more accurately the reality.

EXPERIMENTS

Nanowires with varying diameters have been deposited on silicon wafers. Statistical analysis of their radius and the wavelength of their periodical instability have been performed.

FINDINGS

McCallum et al.'s model better aligns with experimental observations compared to Rayleigh's description. This validation provides a robust theoretical framework for enhancing the stability of nanowires, addressing a crucial aspect of their development.

Tailor-Made Gold Nanomaterials for Applications in Soft Bioelectronics and Optoelectronics

In modern nanoscience and nanotechnology, gold nanomaterials are indispensable building blocks that have demonstrated a plethora of applications in catalysis, biology, bioelectronics, and optoelectronics. Gold nanomaterials possess many appealing material properties, such as facile control over their size/shape and surface functionality, intrinsic chemical inertness yet with high biocompatibility, adjustable localized surface plasmon resonances, tunable conductivity, wide electrochemical window, etc. Such material attributes have been recently utilized for designing and fabricating soft bioelectronics and optoelectronics. This motivates to give a comprehensive overview of this burgeoning field. The discussion of representative tailor-made gold nanomaterials, including gold nanocrystals, ultrathin gold nanowires, vertically aligned gold nanowires, hard template-assisted gold nanowires/gold nanotubes, bimetallic/trimetallic gold nanowires, gold nanomeshes, and gold nanosheets, is begun. This is followed by the description of various fabrication methodologies for state-of-the-art applications such as strain sensors, pressure sensors, electrochemical sensors, electrophysiological devices, energy-storage devices, energy-harvesting devices, optoelectronics, and others. Finally, the remaining challenges and opportunities are discussed.

In Situ Growth of Gold Nanofilms with Branched Structures in the Presence of Organosulfur for High-Performance Flexible Electronics

Herein, a novel method is presented for the in situ growth of gold nanofilms with branched structures in the presence of organosulfur. The key feature in this approach is the Rayleigh instability of ultrathin gold nanowires (AuNWs) without oleylamine (OAm), which allows the ultrathin AuNWs to decompose into gold nanoparticles (AuNPs) and the AuNPs to in situ grow into branched structures for high-performance stability and electrical conductivity. The sheet resistance of the gold nanofilms initially sharply decreased, whereas it subsequently slightly increased with the concentration of CS(NH2)2 until it exceeded the optimal range. After undergoing a 10 min heat treatment at 150 °C, the sheet resistance of the nanofilms was further reduced to 18 Ω/sq, which could be maintained for more than five months. The internal structure becomes fully grown and denser, forming a branched structure after heat treatment. Only certain organosulfurs can improve the electrical properties of the gold nanofilms, and the mechanism of organosulfur in the in situ growth of gold nanofilms with branched structures has also been presented. Overall, this novel method provides a straightforward and convenient approach to obtaining gold nanomaterials with branched structures, holding great potential promise for applications in flexible electronics, catalysis, and energy fields.

Revisiting the structure of [PdAu9(PPh3)8(CN)]2+ produced by atmospheric pressure plasma irradiation of [PdAu8(PPh3)8]2+ in methanol

Some of the authors of the present research group have previously reported mass spectrometric detection of [PdAu9(PPh3)8(CN)]2+ (PdAu9CN) by atmospheric pressure plasma (APP) irradiation of [MAu8(PPh3)8]2+ (PdAu8) in methanol and proposed based on density functional theory (DFT) calculations that PdAu9CN is constructed by inserting a CNAu or NCAu unit into the Au-PPh3 bond of PdAu8 [Emori et al., J. Chem. Phys. 155, 124312 (2021)]. In this follow-up study, we revisited the structure of PdAu9CN by high-resolution ion mobility spectrometry on an isolated sample of PdAu9CN with the help of dispersion-corrected DFT calculation. In contradiction to the previous proposal, we conclude that isomers in which an AuCN unit is directly bonded to the central Pd atom of PdAu8 are better candidates. This assignment was supported by Fourier transform infrared and ultraviolet-visible spectroscopies of isolated PdAu9CN. The simultaneous formation of [Au(PPh3)2]+ and PdAu9CN suggests that the AuCN species are formed by APP irradiation at the expense of a portion of PdAu8. These results indicate that APP may offer a unique method for transforming metal clusters into novel ones by generating in situ active species that were not originally added to the solution.

Plasma-based post-processing of colloidal nanocrystals for applications in heterogeneous catalysis

This review summarizes the work on the use of plasmas to post-process nanostructures, in particular colloidal nanocrystals, as promising candidates for applications of heterogeneous catalysis. Using plasma to clean or modify the surface of nanostructures is a more precisely controlled method compared to other conventional methods, which is preferable when strict requirements for nanostructure morphology or chemical composition are necessary. The ability of plasma post-processing to create mesoporous materials with high surface areas and controlled microstructure, surfaces, and interfaces has transformational potential in catalysis and other applications that leverage surface/interface processes.

Materials-Driven Soft Wearable Bioelectronics for Connected Healthcare

In the era of Internet-of-things, many things can stay connected; however, biological systems, including those necessary for human health, remain unable to stay connected to the global Internet due to the lack of soft conformal biosensors. The fundamental challenge lies in the fact that electronics and biology are distinct and incompatible, as they are based on different materials via different functioning principles. In particular, the human body is soft and curvilinear, yet electronics are typically rigid and planar. Recent advances in materials and materials design have generated tremendous opportunities to design soft wearable bioelectronics, which may bridge the gap, enabling the ultimate dream of connected healthcare for anyone, anytime, and anywhere. We begin with a review of the historical development of healthcare, indicating the significant trend of connected healthcare. This is followed by the focal point of discussion about new materials and materials design, particularly low-dimensional nanomaterials. We summarize material types and their attributes for designing soft bioelectronic sensors; we also cover their synthesis and fabrication methods, including top-down, bottom-up, and their combined approaches. Next, we discuss the wearable energy challenges and progress made to date. In addition to front-end wearable devices, we also describe back-end machine learning algorithms, artificial intelligence, telecommunication, and software. Afterward, we describe the integration of soft wearable bioelectronic systems which have been applied in various testbeds in real-world settings, including laboratories that are preclinical and clinical environments. Finally, we narrate the remaining challenges and opportunities in conjunction with our perspectives.

Consolidation and performance gains in plasma-sintered printed nanoelectrodes

We report on the unusual, advantageous ageing of flexible transparent electrodes (FTEs) that were self-assembled from oleylamine-capped gold nanospheres (AuNPs) by direct nanoimprinting of inks with different particle concentrations (cAu = 3 mg mL-1 to 30 mg mL-1). The resulting lines were less than 2.5 μm wide and consisted of disordered particle assemblies. Small-Angle X-ray Scattering confirmed that particle packing did not change with ink concentration. Plasma sintering converted the printed structures into lines with a thin, electrically conductive metal shell and a less conductive hybrid core. We studied the opto-electronic performance directly after plasma sintering and after fourteen days of storage at 22 °C and 55% rH in the dark. The mean optical transmittance T̄400-800 in the range from 400 nm to 800 nm increased by up to ≈ 3%, while the sheet resistance Rsh strongly decreased by up to ≈ 82% at all concentrations. We correlated the changes with morphological changes visible in scanning and transmission electron microscopy and identified two sequential ageing stages: (I) post-plasma relaxation effects in and consolidation of the shell, and (II) particle re-organization, de-mixing, coarsening, and densification of the core with plating of Au from the core onto the shell, followed by solid-state de-wetting (ink concentrations cAu < 15 mg mL-1) or stability (cAu ≥ 15 mg mL-1). The plating of Au from the hybrid core improved the FTEs' Figure of Merit FOM = T̄400-800·Rsh-1 by up to ≈ 5.8 times and explains the stable value of ≈ 3.3%·Ωsq-1 reached after 7 days of ageing at cAu = 30 mg mL-1.

Doping of Transparent Electrode Based on Oriented Networks of Nickel in Poly(3,4-Ethylenedioxythiophene) Polystyrene Sulfonate Matrix with P-Toluenesulfonic Acid

This work aimed to obtain an optically transparent electrode based on the oriented nanonetworks of nickel in poly(3,4-ethylenedioxythiophene) polystyrene sulfonate matrix. Optically transparent electrodes are used in many modern devices. Therefore, the search for new inexpensive and environmentally friendly materials for them remains an urgent task. We have previously developed a material for optically transparent electrodes based on oriented platinum nanonetworks. This technique was upgraded to obtain a cheaper option from oriented nickel networks. The study was carried out to find the optimal electrical conductivity and optical transparency values of the developed coating, and the dependence of these values on the amount of nickel used was investigated. The figure of merit (FoM) was used as a criterion for the quality of the material in terms of finding the optimal characteristics. It was shown that doping PEDOT: PSS with p-toluenesulfonic acid in the design of an optically transparent electroconductive composite coating based on oriented nickel networks in a polymer matrix is expedient. It was found that the addition of p-toluenesulfonic acid to an aqueous dispersion of PEDOT: PSS with a concentration of 0.5% led to an eight-fold decrease in the surface resistance of the resulting coating.

Flexible and transparent electrodes imprinted from Au nanowires: stability and ageing

We study the stability of flexible transparent electrodes (FTEs) that were self-assembled from ultra-thin gold nanowires (AuNW) by direct nanoimprinting of inks with different particle concentrations (1 to 10 mg mL-1). The resulting lines were less than 3 μm wide and contained bundles of AuNW with oleylamine (OAm) ligand shells. Small-angle X-ray scattering confirmed a concentration-independent bundle structure. Plasma sintering converted the wire assemblies into lines with a thin metal shell that contributes most to electrical conductivity and covers a hybrid core. We studied the relative change in sheet resistance and the morphology of the FTEs with time. The sheet resistance increased at all concentrations, but at different rates. The metal shell aged by de-wetting and pore formation. The hybrid core de-mixed and densified, which led to a partial collapse of the shell. Residual organics migrated through the shell via its pores. Lines formed at low concentration (c Au = 2 to 3 mg mL-1) contained less residual organics and aged slower than those formed at high c Au ≥ 5 mg mL-1. We passivated the conductive shell with thin, adsorbed layers of PEDOT:PSS and found that it decelerated degradation by slowing surface diffusion and hindering further rupture of the shell. Thick capping layers prevented degradation entirely and stopped pore formation.

High-Voltage Wave Induced a Unique Structured Percolation Network with a Negative Gauge Factor

Nanocomposite percolation networks have attracted increasing attention in the field of wearable devices. Generally, the large junction resistance caused by the small contact area in the percolation network is considered as the bottleneck in preparing high-performance electronics. In such electronics, an applied strain will lead to deformation on the fiber junction, subsequently increasing the sheet resistance. However, taking advantage of the dominant role of the contact resistance in the percolation network, the overall resistance of the network can be controlled by skillfully adjusting the contact area. Here, we designed a combined gold-polycaprolactone (Au-PCL) network with a unique buckling net structure. When the thickness of the gold nanolayer is 50 nm, the network shows typical percolation behavior with high transparency (93%), good conductivity (20 Ω/sq), and good ductility. Moreover, the networks show a unique positive relationship between the conductivity and strain due to the variation of contact resistance. By designing different waving angles, the network can be used as a dynamic strain sensor with a tunable gauge factor ranging from -0.8 to -1.8. Overall, these highly stretchable and transparent Au-PCL networks show promising applications in the field of high-performance electronic and optoelectronic nanodevices.

Chemical transformations of [MAu8(PPh3)8]2+ (M = Pt, Pd) and [Au9(PPh3)8]3+ in methanol induced by irradiation of atmospheric pressure plasma

The reaction processes of ligand-protected metal clusters induced by irradiating atmospheric pressure plasma (APP) were investigated using optical spectroscopy, mass spectrometry, and density functional theory (DFT) calculations. The target clusters were phosphine-protected gold-based clusters [MAu₈(PPh₃)₈]²⁺ (M = Pt, Pd) and [Au₉(PPh₃)₈]³⁺, which have a crown-shaped M@Au₈ (M = Pt, Pd, Au) core with an unligated M site at the central position. The APP irradiation of [MAu₈(PPh₃)₈]²⁺ (M = Pt, Pd) in methanol resulted in the selective formation of [PtAu₈(PPh₃)₈CO]²⁺ and [PdAu₉(PPh₃)₈CN]²⁺ via the addition of a CO molecule and AuCN unit, respectively, generated in situ by the APP irradiation. In contrast, the APP irradiation of [Au₉(PPh₃)₈]³⁺ in methanol yielded [Au₉(PPh₃)₇(CN)₁]²⁺ and [Au₁₀(PPh₃)₇(CN)₂]²⁺ as the main products, which were produced by sequential addition of AuCN to reactive [Au₈(PPh₃)₇]²⁺ formed by dissociation equilibrium of [Au₉(PPh₃)₈]³⁺. DFT calculations predicted that a unique chain-like {-(CNAu)_n-PPh₃} (n = 1, 2) ligand was formed via the sequential insertion of -CNAu- units into the Au-PPh₃ bond of [PdAu₈(PPh₃)₈]²⁺ and [Au₈(PPh₃)₇]²⁺. These findings open up a new avenue for developing novel metal clusters via the chemical transformation of atomically defined metal clusters by APP irradiation.

Precisely Engineered Supported Gold Clusters as a Stable Catalyst for Propylene Epoxidation

Designing a stable and selective catalyst with high H₂ utilisation is of pivotal importance for the direct gas-phase epoxidation of propylene. This work describes a facile one-pot methodology to synthesise ligand-stabilised sub-nanometre gold clusters immobilised onto a zeolitic support (TS-1) to engineer a stable Au/TS-1 catalyst. A non-thermal O₂ plasma technique is used for the quick removal of ligands with limited increase in particle size. Compared to untreated Au/TS-1 catalysts prepared using the deposition precipitation method, the synthesised catalyst exhibits improved catalytic performance, including 10 times longer lifetime (>20 days), increased PO selectivity and hydrogen efficiency in direct gas phase epoxidation. The structure-stability relationship of the catalyst is illustrated using multiple characterisation techniques, such as XPS, ³¹ P MAS NMR, DR-UV/VIS, HRTEM and TGA. It is hypothesised that the ligands play a guardian role in stabilising the Au particle size, which is vital in this reaction. This strategy is a promising approach towards designing a more stable heterogeneous catalyst.

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.

Gold nanowire growth through stacking fault mechanism by oleylamine-mediated synthesis

Tadpole-like gold nanowires were obtained by the oleylamine-mediated synthesis presenting an unusual mixture of fcc and hcp phases. Structural analyses were performed to understand their structure and growth using aberration-corrected high-resolution scanning transmission electron microscopy and electron diffraction at the tail region of tadpoles showing that the anisotropic shape occurred due to stacking fault defects. Stacking faults and twin defects are responsible for the hcp phase inferring a defect dependent growth. The stacking fault model used in X-ray diffraction (XRD) refinement resulted in 60% of hcp stacking sequences. Temperature-dependent XRD analyses showed that the faults become unstable around 120 °C, and it is completely converted to the fcc phase at 230 °C. We attribute the nanowire formation to a stacking fault mechanism of growth that begins in the later stage of nanoparticle growth. The UV-Vis spectrum presented two localized surface plasmon resonance bands at 500 nm and from 800 nm extending to near-infrared, associated with transverse and longitudinal modes, respectively. A surprising ferromagnetic behavior is also observed with a blocking temperature near 300 K.

Multiscale Soft-Hard Interface Design for Flexible Hybrid Electronics

Emerging next-generation soft electronics will require versatile properties functioning under mechanical compliance, which will involve the use of different types of materials. As a result, control over material interfaces (particularly soft/hard interfaces) has become crucial and is now attracting intensive worldwide research efforts. A series of material and structural interface designs has been devised to improve interfacial adhesion, preventing failure of electromechanical properties under mechanical deformation. Herein, different soft/hard interface design strategies at multiple length scales in the context of flexible hybrid electronics are reviewed. The crucial role of soft ligands and/or polymers in controlling the morphologies of active nanomaterials and stabilizing them is discussed, with a focus on understanding the soft/hard interface at the atomic/molecular scale. Larger nanoscopic and microscopic levels are also discussed, to scrutinize viable intrinsic and extrinsic interfacial designs with the purpose of promoting adhesion, stretchability, and durability. Furthermore, the macroscopic device/human interface as it relates to real-world applications is analyzed. Finally, a perspective on the current challenges and future opportunities in the development of truly seamlessly integrated soft wearable electronic systems is presented.

Fabrication of Flexible and Transparent Conductive Nanosheets by the UV-Irradiation of Gold Nanoparticle Monolayers

Conductive films that are highly transparent and flexible are extremely attractive for emerging optoelectronic applications. Currently, indium-doped tin oxide films are the most widely used transparent conductive films and much research effort is devoted to developing alternative transparent conductive materials to overcome their drawbacks. In this work, a novel and facile approach for fabricating transparent conductive Au nanosheets from Au nanoparticles (AuNPs) is proposed. Irradiating an AuNP monolayer at the air-water interface with UV light results in a nanosheet with ≈3.5 nm thickness and ≈80% transparency in the UV-visible region. Further, the so-fabricated nanosheets are highly flexible and can maintain their electrical conductivity even when they are bent to a radius of curvature of 0.6 mm. Fourier-transform infrared and X-ray photoelectron spectroscopy characterizations reveal that the transformation of the monolayer of AuNPs into the nanosheet is induced by the photodecomposition and/or photodetachment of the dodecanethiol ligands capping the AuNPs. Further, the UV-irradiation of a hybrid monolayer consisting of AuNPs and silica particles affords the patterning of Au nanosheets with periodic hole arrays.

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.

On the kinetics of the removal of ligands from films of colloidal nanocrystals by plasmas

This paper describes the kinetic limitations of etching ligands from colloidal nanocrystal assemblies (CNAs) by plasma processing. We measured the etching kinetics of ligands from a CNA model system (spherical ZrO₂ nanocrystals, 2.5-3.5 nm diameter, capped with trioctylphosphine oxide) with inductively coupled plasmas (He and O₂ feed gases, powers ranging from 7 to 30 W, at pressures ranging from 100 to 2000 mTorr and exposure times ranging between 6 and 168 h). The etching rate slows down by about one order of magnitude in the first minutes of etching, after which the rate of carbon removal becomes proportional to the third power of the carbon concentration in the CNA. Pressure oscillations in the plasma chamber significantly accelerate the overall rate of etching. These results indicate that the rate of etching is mostly affected by two main factors: (i) the crosslinking of the ligands in the first stage of plasma exposure, and (ii) the formation of a boundary layer at the surface of the CNA. Optimized conditions of plasma processing allow for a 60-fold improvement in etching rates compared to the previous state of the art and make the timeframes of plasma processing comparable to those of calcination.

Large-Scale Synthesis of Colloidal Si Nanocrystals and Their Helium Plasma Processing into Spin-On, Carbon-Free Nanocrystalline Si Films

This paper describes a simple approach to the large-scale synthesis of colloidal Si nanocrystals and their processing into spin-on carbon-free nanocrystalline Si films. The synthesized silicon nanoparticles are capped with decene, dispersed in hexane, and deposited on silicon substrates. The deposited films are exposed to nonoxidizing room-temperature He plasma to remove the organic ligands without adversely affecting the silicon nanoparticles to form crack-free thin films. We further show that the reactive ion etching rate in these films is 1.87 times faster than that for single-crystalline Si, consistent with a simple geometric argument that accounts for the nanoscale roughness caused by the nanoparticle shape.

Emerging Novel Metal Electrodes for Photovoltaic Applications

Emerging novel metal electrodes not only serve as the collector of free charge carriers, but also function as light trapping designs in photovoltaics. As a potential alternative to commercial indium tin oxide, transparent electrodes composed of metal nanowire, metal mesh, and ultrathin metal film are intensively investigated and developed for achieving high optical transmittance and electrical conductivity. Moreover, light trapping designs via patterning of the back thick metal electrode into different nanostructures, which can deliver a considerable efficiency improvement of photovoltaic devices, contribute by the plasmon-enhanced light-mattering interactions. Therefore, here the recent works of metal-based transparent electrodes and patterned back electrodes in photovoltaics are reviewed, which may push the future development of this exciting field.

Ligand-Dependent Nanoparticle Assembly and Its Impact on the Printing of Transparent Electrodes

Metal grids with submicron line diameters are optically transparent, mechanically flexible, and suitable materials for transparent and flexible electronics. Printing such narrow lines with dilute metal nanoparticle inks is challenging because it requires percolation throughout the particle packing. Here, we print fully connected submicron lines of 3.2 nm diameter gold nanoparticles and vary the organic ligand shell to study the relation between colloidal interactions, ligand binding to the metal core, and conductivity of the printed lines. We find that particles with repulsive potentials aid the formation of continuous lines, but the required long ligand molecules impede conductivity and need to be removed after printing. Weakly bound alkylamines provided sufficient interparticle repulsion and were easy to remove with a soft plasma treatment after printing, so that grids with a transparencies above 90% and a conductivity of 150 Ω sq^-1 could be printed.

The role of ligands in coinage-metal nanoparticles for electronics

Coinage-metal nanoparticles are key components of many printable electronic inks. They can be combined with polymers to form conductive composites and have been used as the basis of molecular electronic devices. This review summarizes the multidimensional role of surface ligands that cover their metal cores. Ligands not only passivate crystal facets and determine growth rates and shapes; they also affect size and colloidal stability. Particle shapes can be tuned via the ligand choice while ligand length, size, ω-functionalities, and chemical nature influence shelf-life and stability of nanoparticles in dispersions. When particles are deposited, ligands affect the electrical properties of the resulting film, the morphology of particle films, and the nature of the interfaces. The effects of the ligands on sintering, cross-linking, and self-assembly of particles in electronic materials are discussed.

Low temperature solid-state wetting and formation of nanowelds in silver nanowires

This article focuses on the microscopic mechanism of thermally induced nanoweld formation between silver nanowires (AgNWs) which is a key process for improving electrical conductivity in NW networks employed for transparent electrodes. Focused ion beam sectioning and transmission electron microscopy were applied in order to elucidate the atomic structure of a welded NW including measurement of the wetting contact angle and characterization of defect structure with atomic accuracy, which provides fundamental information on the welding mechanism. Crystal lattice strain, obtained by direct evaluation of atomic column displacements in high resolution scanning transmission electron microscopy images, was shown to be non-uniform among the five twin segments of the AgNW pentagonal structure. It was found that the pentagonal cross-sectional morphology of AgNWs has a dominant effect on the formation of nanowelds by controlling initial wetting as well as diffusion of Ag atoms between the NWs. Due to complete solid-state wetting, at an angle of ∼4.8°, the welding process starts with homoepitaxial nucleation of an initial Ag layer on (100) surface facets, considered to have an infinitely large radius of curvature. However, the strong driving force for this process due to the Gibbs-Thomson effect, requires the NW contact to occur through the corner of the pentagonal cross-section of the second NW providing a small radius of curvature. After the initial layer is formed, the welded zone continues to grow and extends out epitaxially to the neighboring twin segments.

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%.

Assembly of Ultrathin Gold Nanowires into Honeycomb Macroporous Pattern Films with High Transparency and Conductivity

Because of its promising properties, honeycomb macroporous pattern (HMP) film has attracted increasing attention. It has been realized in many artificial nanomaterials, but the formation of these HMPs was attributed to templates or polymer/supermolecule/surfactant assistant assembly. Pure metal HMP film has been difficult to produce using a convenient colloidal template-free method. In this report, a unique template-free approach for preparation of Au HMP film with high transparency and conductivity is presented. Ultrathin Au nanowires, considered a linear polymer analogue, are directly assembled into HMP film on various substrates using a traditional static breath figure method. Subsequent chemical cross-linking and oxygen plasma treatment greatly enhance the stability and conductivity of the HMP film. The resulting HMP film exhibits great potential as an ideal candidate for transparent flexible conductive nanodevices.

Metallic Nanowire-Based Transparent Electrodes for Next Generation Flexible Devices: a Review

Transparent electrodes attract intense attention in many technological fields, including optoelectronic devices, transparent film heaters and electromagnetic applications. New generation transparent electrodes are expected to have three main physical properties: high electrical conductivity, high transparency and mechanical flexibility. The most efficient and widely used transparent conducting material is currently indium tin oxide (ITO). However the scarcity of indium associated with ITO's lack of flexibility and the relatively high manufacturing costs have a prompted search into alternative materials. With their outstanding physical properties, metallic nanowire (MNW)-based percolating networks appear to be one of the most promising alternatives to ITO. They also have several other advantages, such as solution-based processing, and are compatible with large area deposition techniques. Estimations of cost of the technology are lower, in particular thanks to the small quantities of nanomaterials needed to reach industrial performance criteria. The present review investigates recent progress on the main applications reported for MNW networks of any sort (silver, copper, gold, core-shell nanowires) and points out some of the most impressive outcomes. Insights into processing MNW into high-performance transparent conducting thin films are also discussed according to each specific application. Finally, strategies for improving both their stability and integration into real devices are presented.

Gold nanorods with conjugated polymer ligands: sintering-free conductive inks for printed electronics

Metal-based nanoparticle inks for printed electronics usually require sintering to improve the poor electron transport at particle-particle interfaces. The ligands required for colloidal stability act as insulating barriers and must be removed in a post-deposition sintering step. This complicates the fabrication process and makes it incompatible with many flexible substrates. Here, we bind a conjugated, electrically conductive polymer on gold nanorods (AuNRs) as a ligand. The polymer, poly[2-(3-thienyl)-ethyloxy-4-butylsulfonate] (PTEBS), provides colloidal stability and good electron transport properties to stable, sintering-free inks. We confirm that the polymer binds strongly through a multidentate binding motif and provides superior colloidal stability in polar solvents over months by IR and Raman spectrometry and zeta potential measurements. We demonstrate that the developed ligand exchange protocol is directly applicable to other polythiophenes such as poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). Films of AuNRs coated with above polymers reached conductivities directly after deposition comparable to conventional metal inks after ligand removal and retained their conductivity for at least one year when stored under ambient conditions.

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.