Effect of Oxalic Acid Treatment on Conductive Coatings Formed by Ni@Ag Core-Shell Nanoparticles.

Low-cost metallic nanoink based on nickel-silver core-shell nanoparticles (Ni@Ag NPs) was used for the formation of conductive metallic coatings with low sintering temperature, which can be successfully applied for replacement of currently used silver-based nanoinks in printed electronics. The effect of oxalic acid (OA) on the sintering temperature and conductivity of coatings formed by Ni@Ag NPs was evaluated. It was found that the addition of OA to the ink formulation and post-printing treatment of deposited films with this acid provided a noticeable decrease in the sintering temperature required for obtaining conductive patterns that is especially important for utilizing the polymeric substrates. The obtained resistivity of metallic coatings after sintering at temperature as low as 100 °C was found to be 30 µΩ·cm, only ~4 times higher compared to the resistivity of bulk Ni that is promising for future application of such materials for fabrication of low-cost flexible printed patterns.

Metallic core-shell nanoparticles for conductive coatings and printing.

The review is focused on bimetallic nanoparticles composed of a core formed by low-cost metal having high electrical conductivity, such as Cu and Ni, and a protective shell composed of stable to oxidation noble metal such as Ag or Au. We present the chemical and physical approaches for synthesis of such particles, as well as the combination of the two, the stability to oxidation of core-shell nanoparticles at various conditions, and the formulation of conductive compositions and their application in conductive coatings and printed electronics.

Polydispersity vs. Monodispersity. How the Properties of Ni-Ag Core-Shell Nanoparticles Affect the Conductivity of Ink Coatings.

The effect of polydispersity of nickel-silver core-shell nanoparticles (Ni-Ag NPs) on the conductivity of ink coatings was studied. Ni-Ag NPs of various average diameters (100, 220, and 420 nm) were synthesized and utilized for the preparation of conductive inks composed of monodisperse NPs and their polydisperse mixtures. The shell thickness of synthesized Ni-Ag NPs was found to be in the range of 10-20 nm and to provide stability of a core metal to oxidation for at least 6 months. The conductivity of metallic films formed by inks with monodisperse Ni-Ag NPs was compared with those formed by polydisperse inks. In all cases, the optimal conditions for the formation of conductive patterns (weight ratio of monodisperse NPs for polydisperse composition, the concentration of the wetting agent, sintering temperature, and duration) were determined. It was found that metallic films formed by polydisperse ink containing 100, 220, and 420 nm Ni-Ag NPs with a mass ratio of 1:1.5:0.5, respectively, are characterized by the lowest resistivity, 10.9 µΩ·cm, after their thermal post-coating sintering at 300 °C for 30 min that is only 1.6 higher than that of bulk nickel.

3D Printing Materials for Soft Robotics.

Soft robotics is a growing field of research, focusing on constructing motor-less robots from highly compliant materials, some are similar to those found in living organisms. Soft robotics has a high potential for applications in various fields such as soft grippers, actuators, and biomedical devices. 3D printing of soft robotics presents a novel and promising approach to form objects with complex structures, directly from a digital design. Here, recent developments in the field of materials for 3D printing of soft robotics are summarized, including high-performance flexible and stretchable materials, hydrogels, self-healing materials, and shape memory polymers, as well as fabrication of all-printed robots (multi-material printing, embedded electronics, untethered and autonomous robotics). The current challenges in the fabrication of 3D printed soft robotics, including the materials available and printing abilities, are presented and the recent activities addressing these challenges are also surveyed.

Conductive nanomaterials for 2D and 3D printed flexible electronics.

This review describes recent developments in the field of conductive nanomaterials and their application in 2D and 3D printed flexible electronics, with particular emphasis on inks based on metal nanoparticles and nanowires, carbon nanotubes, and graphene sheets. We present the basic properties of these nanomaterials, their stabilization in dispersions, formulation of conductive inks and formation of conductive patterns on flexible substrates (polymers, paper, textile) by using various printing technologies and post-printing processes. Applications of conductive nanomaterials for fabrication of various 2D and 3D electronic devices are also briefly discussed.

Additive Manufacturing of Transparent Silica Glass from Solutions.

A sol, aqueous solution-based ink is presented for fabrication of 3D transparent silica glass objects with complex geometries, by a simple 3D printing process conducted at room temperature. The ink combines a hybrid ceramic precursor that can undergo both the photopolymerization reaction and a sol-gel process, both in the solution form, without any particles. The printing is conducted by localized photopolymerization with the use of a low-cost 3D printer. Following printing, upon aging and densifying, the resulting objects convert from a gel to a xerogel and then to a fused silica. The printed objects, which are composed of fused silica, are transparent and have tunable density and refractive indices.

Evaporation of Nanosuspensions on Substrates with Different Hydrophobicity.

Liquid drop evaporation on surfaces is present in many industrial and medical applications, e.g., printed electronics, spraying of pesticides, DNA mapping, etc. Despite this strong interest, a theoretical description of the dynamic of the evaporation of complex liquid mixtures and nanosuspensions is still lacking. Indeed, one of the aspects that have not been included in the current theoretical descriptions is the competition between the kinetics of evaporation and the adsorption of surfactants and/or particles at the liquid/vapor and liquid/solid interfaces. Materials formed by an electrically isolating solid on which a patterned conducting layer was formed by the deposits left after drop evaporation have been considered as very promising for building electrical circuits on flexible plastic substrates. In this work, we have done an exhaustive study of the evaporation of nanosuspensions of latex and hydrophobized silver nanoparticles on four substrates of different hydrophobicity. The advancing and receding contact angles as well as the time dependence of the volume of the droplets have been measured over a broad range of particle concentrations. Also, mixtures of silver particles and a surfactant, commonly used in industrial printing, have been examined. Furthermore, the adsorption kinetics at both the air/liquid and solid/liquid interfaces have been measured. Whereas the latex particles do not adsorb at the solid/liquid and only slightly reduce the surface tension, the silver particles strongly adsorb at both interfaces. The experimental results of the evaporation process were compared with the predictions of the theory of Semenov et al. (Evaporation of Sessile Water Droplets: Universal Behavior in the Presence of Contact Angle Hysteresis. Colloids Surf. Physicochem. Eng. Asp. 2011, 391 (1-3), 135-144) and showed surprisingly good agreement despite that the theory was developed for pure liquids. The morphology of the deposits left by the droplets after total evaporation was studied by scanning electronic microscopy, and the effects of the substrate, the particle nature, and their concentrations on these patterns are discussed.

Plasma-Induced Decomposition of Copper Complex Ink for the Formation of Highly Conductive Copper Tracks on Heat-Sensitive Substrates.

The use of Cu-formate-2-amino-2-methyl-1-propanol ink and low-pressure plasma for the formation of highly conductive patterns on heat sensitive plastic substrates was studied. It was found that plasma results in decomposition of copper complex to form metallic copper without heating at high temperatures. Ink composition and plasma parameters (predrying conditions, plasma treatment duration, gas type, and flow rate) were optimized to obtain uniform conductive metallic films. The morphology and electrical characteristics of these films were evaluated. Exposing the printed copper metallo-organic decomposition (MOD) ink to 160 W plasma for 8 min yielded resistivity as low as 7.3 ± 0.2 µΩ cm, which corresponds to 23% bulk copper conductivity. These results demonstrate the applicability of MOD inks and plasma treatment to obtain highly conductive printed patterns on low-cost plastic substrates and 3D printed polymers.

Conductive nanomaterials for printed electronics.

This is a review on recent developments in the field of conductive nanomaterials and their application in printed electronics, with particular emphasis on inkjet printing of ink formulations based on metal nanoparticles, carbon nanotubes, and graphene sheets. The review describes the basic properties of conductive nanomaterials suitable for printed electronics (metal nanoparticles, carbon nanotubes, and graphene), their stabilization in dispersions, formulations of conductive inks, and obtaining conductive patterns by using various sintering methods. Applications of conductive nanomaterials for electronic devices (transparent electrodes, metallization of solar cells, RFID antennas, TFTs, and light emitting devices) are also briefly reviewed.

Transparent conductors composed of nanomaterials.

This is a review on recent developments in the field of transparent conductive coatings (TCCs) for ITO replacement. The review describes the basic properties of conductive nanomaterials suitable for fabrication of such TCCs (metallic nanoparticles and nanowires, carbon nanotubes and graphene sheets), various methods of patterning the metal nanoparticles with formation of conductive transparent metallic grids, honeycomb structures and 2D arrays of interconnected rings as well as fabrication of TCCs based on graphene and carbon nanotubes. Applications of TCCs in electronic and optoelectronic devices, such as solar cells, electroluminescent and electrochromic devices, touch screens and displays, and transparent EMI shielders, are discussed.

Nanostructured electrochromic films by inkjet printing on large area and flexible transparent silver electrodes.

Printed electrochromic flexible films were obtained by combining transparent silver grid electrodes formed by self-assembly and inkjet printed WO3 nanoparticles. Concentrated dispersions of WO3 nanoparticles were inkjet printed on transparent plastic silver grid electrodes with a high transparency of 83% in the spectral range of 400-800 nm, and a low sheet resistance in the range of 1-5 Ω sq(-1). These electrodes were used for electrochromic applications for the first time. The resultant patterned nanostructured electrochromic films maintained their coloring and bleaching performance after bending of the flexible films.

Merging of metal nanoparticles driven by selective wettability of silver nanostructures.

The welding and sintering of nanomaterials is relevant, for example, to form electrical contacts between metallic particles in printed electronic devices. Usually the welding of nanoparticles is achieved at high temperatures. Here we find that merging of two different metals, silver and gold nanoparticles, occurs on contact at room temperature. The merging process was investigated by experimental and molecular dynamics simulations. We discovered that the merging of these particles is driven by selective wettability of silver nanoparticles, independent of their size and shape (spheres or rods); silver behaves as a soft matter, whereas gold as a hard surface being wetted and retaining its original morphology. During that process, the silver atoms move towards the surface of the Au nanoparticles and wrap the Au nanoparticles in a pulling up-like process, leading to the wetting of Au nanoparticles.

Plasma and microwave flash sintering of a tailored silver nanoparticle ink, yielding 60% bulk conductivity on cost-effective polymer foils.

A combination of plasma and microwave flash sintering is used to sinter an inkjet-printed and tailored silver nanoparticle formulation. By using two sintering techniques sequentially, the obtained conductivity is 60%, while keeping the processing temperature well below the glass transition temperature (T(g)) of the used polymer substrate. This approach leads to highly conductive features on cost-effective polymer substrates in relatively short times, which are compatible with roll-to-roll (R2R) production. An electroluminescence device is prepared as an example.

Conductive inks with a "built-in" mechanism that enables sintering at room temperature.

At present there is no metallic ink that enables formation of conductive patterns at room temperature by a single printing step. Printing conductive features by metallic nanoparticle-based inks must be followed by sintering while heating to elevated temperatures, thus preventing their utilization on most plastic substrates used in plastic electronics. In this report we present a new silver nanoparticle-based conductive ink, having a built-in sintering mechanism, which is triggered during drying of the printed pattern. The nanoparticles that are stabilized by a polymer undergo self-sintering spontaneously, due to the presence of a destabilizing agent, which comes into action only during drying of the printed pattern. The destabilizing agent, which contains Cl(-) ions, causes detachment of the anchoring groups of the stabilizer from the nanoparticles' surface and thus enables their coalescence and sintering. It was found that the new metallic ink leads to very high conductivities, by a single printing step: up to 41% of the conductivity of bulk silver was achieved, the highest reported conductivity of a printed pattern that is obtained from nanoparticles at room temperature.

Triggering the sintering of silver nanoparticles at room temperature.

A new approach to achieve coalescence and sintering of metallic nanoparticles at room temperature is presented. It was discovered that silver nanoparticles behave as soft particles when they come into contact with oppositely charged polyelectrolytes and undergo a spontaneous coalescence process, even without heating. Utilizing this finding in printing conductive patterns, which are composed of silver nanoparticles, enables achieving high conductivities even at room temperature. Due to the sintering of nanoparticles at room temperature, the formation of conductive patterns on plastic substrates and even on paper is made possible. The resulting high conductivity, 20% of that for bulk silver, enabled fabrication of various devices as demonstrated by inkjet printing of a plastic electroluminescent device.

Copper Nanoparticles for Printed Electronics: Routes Towards Achieving Oxidation Stability.

In the past few years, the synthesis of Cu nanoparticles has attracted much attention because of its huge potential for replacing expensive nano silver inks utilized in conductive printing. A major problem in utilizing these copper nanoparticles is their inherent tendency to oxidize in ambient conditions. Recently, there have been several reports presenting various approaches which demonstrate that copper nanoparticles can resist oxidation under ambient conditions, if they are coated by a proper protective layer. This layer may consist of an organic polymer, alkene chains, amorphous carbon or graphenes, or inorganic materials such as silica, or an inert metal. Such coated copper nanoparticles enable achieving high conductivities by direct printing of conductive patterns. These approaches open new possibilities in printed electronics, for example by using copper based inkjet inks to form various devices such as solar cells, Radio Frequency Identification (RFID) tags, and electroluminescence devices. This paper provides a review on the synthesis of copper nanoparticles, mainly by wet chemistry routes, and their utilization in printed electronics.

Coalescence of silver nanoparticles at room temperature: unusual crystal structure transformation and dendrite formation induced by self-assembly.

It was found that during the evaporation of water from a droplet of a silver nanoparticles dispersion a self-assembly process leads to the coalescence of the nanoparticles at room temperature and eventually results in a 3D, micrometer-sized dendrite. Direct in situ HR-TEM observation of coalescence events of individual nanoparticles revealed that during this process a transformation of the nanoparticles' crystal structure takes place, from the common fcc silver structure to the unusual hcp structure. It was found that even-though a majority of the nanoparticles in the dispersion have the fcc structure the obtained dendrites are characterized by the hcp structure, reflecting the crystal structure transformation due to the coalescence process.

Polymer-surfactant interactions: binding mechanism of sodium dodecyl sulfate to poly(diallyldimethylammonium chloride).

The binding mechanism of poly(diallyldimethylammonium chloride), PDAC, and sodium dodecyl sulfate, SDS, has been comprehensively studied by combining binding isotherms data with microcalorimetry, zeta potential, and conductivity measurements, as well as ab initio quantum mechanical calculations. The obtained results demonstrate that surfactant-polymer interaction is governed by both electrostatic and hydrophobic interactions, and is cooperative in the presence of salt. This binding results in the formation of nanoparticles, which are positively or negatively charged depending on the molar ratio of surfactant to PDAC monomeric units. From microcalorimetry data it was concluded that the exothermic character of the interaction diminishes with the increase in the surfactant/polymer ratio as well as with an increase in electrolyte concentration.

Ring stain effect at room temperature in silver nanoparticles yields high electrical conductivity.

We demonstrate that metallic rings formed spontaneously at room temperature via evaporation of aqueous drops containing silver nanoparticles (20-30 nm in diameter) exhibit high electrical conductivity (up to 15% of that for bulk silver). The mechanism underlying this self-assembly phenomena is the "ring stain effect", where self-pinning is combined with capillary flow to form a ring consisting of close-packed metallic nanoparticles along the perimeter of a drying droplet. Our macroscopic and microscopic (applying conductive atomic force microscopy) transport measurements show that the conductivity of the ring, which has a metallic brightness, is orders of magnitude larger than that of corresponding aggregates developed without the ring formation, where high conductivity is known to appear only after annealing at high temperature.

Water-soaked evidence: detectability of explosive traces after immersion in water.

Various factors governing the detectability of explosive traces after being soaked in water were studied. The variables are: the type of the surface (surfaces liable to be found in aircraft were chosen), the type of explosive, the type of water (tap or seawater), and movement of the immersed surface in the water. The maximal immersion times (tmax) after which explosive detection was possible were evaluated. This datum was found to depend on the type of explosive (one of the important factors is solubility in water), the surface material and the environmental conditions (tap or seawater movement). Detection of PETN on high-density polyethylene, linoleum, glass and aluminum, by the chemical Explosive Testing Kit (ETK), was possible even after a month of soaking in seawater. In addition, it has been found that movement of bulk water around the samples with deposited explosives considerably decreases tmax values. It is, therefore, recommended to retrieve samples for explosive analysis as soon as possible and in areas where the currents of water is minimal.