Fabrication of Conductive Copper Films on Flexible Polymer Substrates by Low-Temperature Sintering of Composite Cu Ink in Air

Chemically Driven Sintering of Colloidal Cu Nanocrystals for Multiscale Electronic and Optical Devices

Emerging applications of Internet of Things (IoT) technologies in smart health, home, and city, in agriculture and environmental monitoring, and in transportation and manufacturing require materials and devices with engineered physical properties that can be manufactured by low-cost and scalable methods, support flexible forms, and are biocompatible and biodegradable. Here, we report the fabrication and device integration of low-cost and biocompatible/biodegradable colloidal Cu nanocrystal (NC) films through room temperature, solution-based deposition, and sintering, achieved via chemical exchange of NC surface ligands. Treatment of organic-ligand capped Cu NC films with solutions of shorter, environmentally benign, and noncorrosive inorganic reagents, namely, SCN- and Cl-, effectively removes the organic ligands, drives NC grain growth, and limits film oxidation. We investigate the mechanism of this chemically driven sintering by systemically varying the Cu NC size, ligand reagent, and ligand treatment time and follow the evolution of their structure and electrical and optical properties. Cl--treated, 4.5 nm diameter Cu NC films yield the lowest DC resistivity, only 3.2 times that of bulk Cu, and metal-like dielectric functions at optical frequencies. We exploit the high conductivity of these chemically sintered Cu NC films and, in combination with photo- and nanoimprint-lithography, pattern multiscale structures to achieve high-Q radio frequency (RF) capacitive sensors and near-infrared (NIR) resonant optical metasurfaces.

The Hydrogenation of Crotonaldehyde on PdCu Single Atom Alloy Catalysts

Recyclable PdCu single atom alloys supported on Al₂O₃ were applied to the selective hydrogenation of crotonaldehyde to elucidate the minimum number of Pd atoms required to facilitate the sustainable transformation of an α,β-unsaturated carbonyl molecule. It was found that, by diluting the Pd content of the alloy, the reaction activity of Cu nanoparticles can be accelerated, enabling more time for the cascade conversion of butanal to butanol. In addition, a significant increase in the conversion rate was observed, compared to bulk Cu/Al₂O₃ and Pd/Al₂O₃ catalysts when normalising for Cu and Pd content, respectively. The reaction selectivity over the single atom alloy catalysts was found to be primarily controlled by the Cu host surface, mainly leading to the formation of butanal but at a significantly higher rate than the monometallic Cu catalyst. Low quantities of crotyl alcohol were observed over all Cu-based catalysts but not for the Pd monometallic catalyst, suggesting that it may be a transient species converted immediately to butanol and or isomerized to butanal. These results demonstrate that fine-tuning the dilution of PdCu single atom alloy catalysts can leverage the activity and selectivity enhancement, and lead to cost-effective, sustainable, and atom-efficient alternatives to monometallic catalysts.

The effects of complex agent and sintering temperature on conductive copper complex paste

In this research study, the formulation of precursor-type Cu pastes was done by mixing copper (II) acetate and 2-amino-2-methyl-1-propanol (AMP). Accordingly, the influence of the complex agent amount on the Cu pastes stability was examined at diverse mole ratio ratios. The Cu paste's optimal formulation was then obtained with the copper acetate to AMP ratio of 0.5. The Cu paste with a CuA/AMP ratio at 0.5 was then coated onto glass substrates; they were then sintered under an N₂ environment at various temperatures including 140, 180 and 220 °C. Characterization of the copper complex paste was done by Fourier transform infrared (FTIR) spectroscopy, UV-vis spectroscopy as well as Thermal analysis. Pattern's characterization was also done through X-ray diffractometry (XRD), field emission scanning electron microscope (FE-SEM) as well as four-point probe method for the purpose of confirming the related crystal structure, microstructure and electrical conductivity, respectively. It is demonstrated how the sintering temperature could affect the Cu film. The printed patterns on the glass substrate which was cured at the temperature of 180 °C displayed metalized cooper with low resistivity (30 μΩ cm) and dense copper films.

Effectiveness of Oxygen during Sintering of Silver Thin Films Derived by Nanoparticle Ink

Silver nanoparticle (NP) inks have been widely used in the ink-jet printing field because of their excellent properties during low-temperature sintering. However, the organic dispersant used to prevent the aggregation and sedimentation of NPs can hinder the sintering process and result in the high resistivity of sintered films. In this study, silver thin films derived from silver NP ink with polyvinylpyrrolidone (PVP) dispersant were sintered in different atmospheres of pure nitrogen, air, and pure oxygen. The effect of the oxygen content in the sintering atmosphere on the thermal properties of the ink, the electrical resistivity and microstructure of the sintered films, and the amount of organic residue were studied by using differential scanning calorimetry, the four-point probe method, scanning electron microscopy, Fourier transform infrared spectroscopy, etc. The mechanism of optimizing the film resistivity by influencing the decomposition of the PVP dispersant and the microstructure evolution of the silver thin films through the sintering atmosphere was discussed. The results demonstrated that an oxygen-containing atmosphere could be effective for silver NPs in two ways. First, the oxygen content could enhance the diffusion ability of silver atoms, thus accelerating the stage transition of microstructural evolution at low temperatures. Second, the oxygen content could enable the PVP to decompose at a temperature much lower than in conditions of pure nitrogen, thus helping to finalize the densification of a silver film with a low resistivity of 2.47 μΩ·cm, which is approximately 1.5-fold that of bulk silver. Our findings could serve as a foundation for the subsequent establishment of ink-jet printing equipment and the optimization of the sintering process for printing silver patterns on flexible substrates.

Low-Temperature Sintering of l-Alanine-Functionalized Metallic Copper Particles Affording Conductive Films with Excellent Oxidative Stability

Here, the alpha amino acid l-alanine is employed as both a capping and stabilizing agent in the aqueous synthesis of submicron-sized metallic copper particles under ambient atmospheric conditions. The reduction of the copper(II) precursor is achieved using l-ascorbic acid (vitamin C) as the reducing agent. The nature of the complex formed between l-alanine and the copper(II) precursor, pH of the medium, temperature, and the relative proportion of capping agent are found to play a significant role in determining the size, shape, and oxidative stability of the resulting particles. The adsorbed l-alanine is shown to act as a barrier imparting excellent thermal stability to capped copper particles, delaying the onset of temperature-induced aerial oxidation. The stability of the particles is complemented by highly favorable sintering conditions, rendering the formation of conductive copper films at significantly lower temperatures (T ≤ 120 °C) compared to alternative preparation methods. The resulting copper films are well-passivated by residual surface l-alanine molecules, promoting long-term stability without hindering the surface chemistry of the copper film as evidenced by the catalytic activity. Contrary to the popular belief that ligands with long carbon chains are best for providing stability, these findings demonstrate that very small ligands can provide highly effective stability to copper without significantly deteriorating its functionality while facilitating low-temperature sintering, which is a key requirement for emerging flexible electronic applications.

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.

MODs vs. NPs: Vying for the Future of Printed Electronics

This Minireview compares two distinct ink types, namely metal-organic decomposition (MOD) and nanoparticle (NP) formulations, for use in the printing of some of the most conductive elements: silver, copper and aluminium. Printing of highly conductive features has found purpose across a broad array of electronics and as processing times and temperatures reduce, the avenues of application expand to low-cost flexible substrates, materials for wearable devices and beyond. Printing techniques such as screen, aerosol jet and inkjet printing are scalable, solution-based processes that historically have employed NP formulations to achieve low resistivity coatings printed at high resolution. Since the turn of the century, the rise in MOD inks has vastly extended the range of potentially applicable compounds that can be printed, whilst simultaneously addressing shelf life and sintering issues. A brief introduction to the field and requirements of an ink will be presented followed by a detailed discussion of a wide array of synthetic routes to both MOD and NP inks. Unindustrialized materials will be discussed, with the challenges and outlook considered for the market leaders: silver and copper, in comparison with the emerging field of aluminium inks.

Metallisation of Textiles and Protection of Conductive Layers: An Overview of Application Techniques

The rapid growth in wearable technology has recently stimulated the development of conductive textiles for broad application purposes, i.e., wearable electronics, heat generators, sensors, electromagnetic interference (EMI) shielding, optoelectronic and photonics. Textile material, which was always considered just as the interface between the wearer and the environment, now plays a more active role in different sectors, such as sport, healthcare, security, entertainment, military, and technical sectors, etc. This expansion in applied development of e-textiles is governed by a vast amount of research work conducted by increasingly interdisciplinary teams and presented systematic review highlights and assesses, in a comprehensive manner, recent research in the field of conductive textiles and their potential application for wearable electronics (so called e-textiles), as well as development of advanced application techniques to obtain conductivity, with emphasis on metal-containing coatings. Furthermore, an overview of protective compounds was provided, which are suitable for the protection of metallized textile surfaces against corrosion, mechanical forces, abrasion, and other external factors, influencing negatively on the adhesion and durability of the conductive layers during textiles' lifetime (wear and care). The challenges, drawbacks and further opportunities in these fields are also discussed critically.

Cu@Ni core-shell nanoparticles prepared via an injection approach with enhanced oxidation resistance for the fabrication of conductive films

Building core-shell structures is a valuable method of enhancing the oxidation-resistance performance of Cu nanoparticles for practical applications in the field of printed circuit boards. In this study, Cu@Ni core-shell nanoparticles are synthesized via an injection solution approach utilizing Cu seeds produced during the reactions to induce the epitaxial growth of Ni shells. The thickness of the Ni shell can be controlled by varying the Cu:Ni molar ratios in the injected precursor solution, whereas changing the injection rate of the Cu precursor solution affects the size of the Cu seeds and thus controls the eventual size of the core-shell nanoparticles. Thermogravimetric analysis reveals a superior thermal stability against oxidation for Cu@Ni core-shell nanoparticles, as compared with Cu nanoparticles. The oxidation resistance of Cu@Ni conductive films increases with an increase in the Ni:Cu ratio, while the conductivity increases with a decrease in the Ni:Cu ratio. A relatively low resistivity of 27.4 µΩ cm is achieved for Cu@Ni conductive films. The results demonstrate that coating Cu nanoparticles with Ni shells via epitaxial growth can form closed shells with smooth surfaces which are valuable for Cu nanoparticles in applications where oxidation resistance is a requirement .

Surface and Interface Designs in Copper-Based Conductive Inks for Printed/Flexible Electronics

Silver (Ag), gold (Au), and copper (Cu) have been utilized as metals for fabricating metal-based inks/pastes for printed/flexible electronics. Among them, Cu is the most promising candidate for metal-based inks/pastes. Cu has high intrinsic electrical/thermal conductivity, which is more cost-effective and abundant, as compared to Ag. Moreover, the migration tendency of Cu is less than that of Ag. Thus, recently, Cu-based inks/pastes have gained increasing attention as conductive inks/pastes for printed/flexible electronics. However, the disadvantages of Cu-based inks/pastes are their instability against oxidation under an ambient condition and tendency to form insulating layers of Cu oxide, such as cuprous oxide (Cu2O) and cupric oxide (CuO). The formation of the Cu oxidation causes a low conductivity in sintered Cu films and interferes with the sintering of Cu particles. In this review, we summarize the surface and interface designs for Cu-based conductive inks/pastes, in which the strategies for the oxidation resistance of Cu and low-temperature sintering are applied to produce highly conductive Cu patterns/electrodes on flexible substrates. First, we classify the Cu-based inks/pastes and briefly describe the surface oxidation behaviors of Cu. Next, we describe various surface control approaches for Cu-based inks/pastes to achieve both the oxidation resistance and low-temperature sintering to produce highly conductive Cu patterns/electrodes on flexible substrates. These surface control approaches include surface designs by polymers, small ligands, core-shell structures, and surface activation. Recently developed Cu-based mixed inks/pastes are also described, and the synergy effect in the mixed inks/pastes offers improved performances compared with the single use of each component. Finally, we offer our perspectives on Cu-based inks/pastes for future efforts.

Self-reducible copper complex inks with aminediol and OH-based solvent for the fabrication of a highly conductive copper film by calcination at low temperature under an air atmosphere

A simple fabrication of copper conductive film was accomplished in one step at low temperature under an air atmosphere. The strategy for solving the problem was the use of both aminediol and OH-based solvent for self-reducible copper complex inks.

Effectiveness of Ligand Denticity-Dependent Oxidation Protection in Copper MOD Inks

The recent cost-driven transition from silver- to copper-based inks for printing on flexible substrates is connected with new key challenges. Given the high oxidation sensitivity of copper inks before, during, and after the curing process, the conductivity and thereby the device performance can be affected. Strategies to limit or even avoid this drawback include the development of metal organic decomposition (MOD) inks with selected "protective" ligands. In this study, the influence of the ligand on the oxide formation during the ink decomposition process is described using a wide variety of in situ characterization techniques. It is demonstrated that bidentate ligands provide an improved oxidation barrier, although the copper preservation mechanism has its limits: oxygen can interfere in every reduction pathway depending on the curing duration and atmospheric conditions. The generated insights can be applied in the further evolution toward ambient-curable copper MOD inks.

Nano oxide intermediate layer assisted room temperature sintering of ink-jet printed silver nanoparticles pattern

Sintering of metallic nanoparticles (NPs) at low temperature is highly wanted in the manufacturing of flexible electronics. And for ink-jet printing, the metallic NPs after printing usually need thermal or chemical post-treatment to remove stabilizing agents and achieve conductivity. Here, we reported a facile method to realize one-step printed sintering of silver nanoparticle (AgNP) ink at room temperature by using intermediate coated layers composed of oxide NPs and polyvinyl alcohol (PVA) mixture. We found that the detachment of the stabilizer (citrate) from the AgNPs was caused by hydroxyl groups on the surface of the oxide NPs, which enabled the coalescence and sintering of the AgNPs. With the aid of SiO₂ NPs based intermediate layer, the patterns showed resistivity as low as 3.45 μΩ cm after sintering. Moreover, the mixed PVA could ensure the forming quality of patterns owing to its adsorption of ink and the high adhesiveness of PVA with substrates. So, we envision that this approach could serve as an adaptive method for sintering of AgNPs based conductive patterns on various substrates at room temperature and promote the manufacture of printed electronics.

Formulation of Screen-Printable Cu Molecular Ink for Conductive/Flexible/Solderable Cu Traces

Screen printing is the most common method used for the production of printed electronics. Formulating copper (Cu) inks that yield conductive fine features with oxidation and mechanical robustness on low-temperature substrates will open up opportunities to fabricate cost-effective devices. We have formulated a screen-printable Cu metal-organic decomposition (MOD) ink comprising Cu formate coordinated to 3-(diethylamino)-1,2-propanediol, a fractional amount of Cu nanoparticles (CuNPs), and a binder. This simple formulation enables ∼70-550 μm trace widths with excellent electrical [∼8-15 mΩ/□/mil or 20-38 μΩ·cm] and mechanical properties with submicron-thick traces obtained by intense pulse light (IPL) sintering on Kapton and poly(ethylene terephthalate) (PET) substrates. These traces are mechanically robust to flexing and creasing where less than 10% change in resistance is observed on Kapton and ∼20% change is observed on PET. Solderable Cu traces were obtained only with the combination of the Cu MOD precursor, CuNP, and polymer binder. Both thermally and IPL sintered traces showed shelf stability (<10% change in resistance) of over a month in ambient conditions and 10-70% relative humidity, suitable for day-to-day fabrication. To demonstrate utility, light-emitting diodes (LEDs) were directly soldered to IPL sintered Cu traces in a reflow oven without the need for a precious metal interlayer. The LEDs were functional not only during bending and creasing of the Cu traces but even after 180 min at 140 °C in ambient air without losing illumination intensity. High definition television antennas printed on Kapton and PET were found to perform well in the ultrahigh frequency region. Lastly, single-walled carbon nanotube-based thin-film transistors on a silicon wafer were fabricated with a screen-printed Cu source and drain electrodes, which performed comparably to silver electrodes with mobility values of 12-15 cm² V^-1 s^-1 and current on/off ratios of ∼10⁵ and as effective ammonia sensors providing parts per billion-level detection.

Polymer-Assisted Metal Deposition (PAMD) for Flexible and Wearable Electronics: Principle, Materials, Printing, and Devices

The rapid development of flexible and wearable electronics favors low-cost, solution-processing, and high-throughput techniques for fabricating metal contacts, interconnects, and electrodes on flexible substrates of different natures. Conventional top-down printing strategies with metal-nanoparticle-formulated inks based on the thermal sintering mechanism often suffer from overheating, rough film surface, low adhesion, and poor metal quality, which are not desirable for most flexible electronic applications. In recent years, a bottom-up strategy termed as polymer-assisted metal deposition (PAMD) shows great promise in addressing the abovementioned challenges. Here, a detailed review of the development of PAMD in the past decade is provided, covering the fundamental chemical mechanism, the preparation of various soft and conductive metallic materials, the compatibility to different printing technologies, and the applications for a wide variety of flexible and wearable electronic devices. Finally, the attributes of PAMD in comparison with conventional nanoparticle strategies are summarized and future technological and application potentials are elaborated.

Printable Thick Copper Conductors from Optically Modulated Multidimensional Particle Mixtures

Printing techniques that enable the formation of arbitrarily designed architectures have been implemented in various research fields owing to their characteristic advantages in processing over other techniques. In particular, low-cost, printable conductors are of paramount importance in the production of highly functioning printed electronics. Among various candidates, copper (Cu) particle-based printable fluid has been regarded as the most promising constituent material in conjunction with the use of the flash-light-sintering (FLS) process in air. In this study, we synthesized surface-oxidation-suppressed Cu nanoparticles, sub-micronparticles, and flakes to regulate the optical absorption characteristics in FLS-processed, Cu-based printed conductors. Our results revealed clearly that the critical issues in FLS-processed conductors, namely, undesirable crack formation and a limitation of thickness, are resolved by adjusting the optical behaviors of particulate layers by variation of the composition of multidimensional mixture particles. It is suggested that crack-free, 13.2 μm thick printed Cu conductors can be generated with a resistivity of 11.4 μΩ cm by printing and FLS processes in air. The proposed alternative approach is demonstrated with electrical circuits comprising electrodes and interconnections.

Structure Inheritance in Nanoparticle Ink Direct-Writing Processes and Crack-Free Nano-Copper Interconnects Printed by a Single-Run Approach

When nanoparticle conductive ink is used for printing interconnects, cracks and pores are common defects that deteriorate the electrical conductivity of the printed circuits. Influences of the ink solvent, the solid fraction of the ink, the pre-printing treatment and the sintering parameters on the interconnect morphology and conductivity were investigated. It was found that the impacts of all these factors coupled with each other throughout the whole procedure, from the pre-printing to the post-printing processes, and led to a structure inheritance effect. An optimum process route was developed for producing crack-free interconnects by a single-run direct-writing approach using home-made nano-copper ink. A weak gel was promoted in the ink before printing in the presence of long-chain polymers and bridging molecules by mechanical agitation. The fully developed gel network prevented the phase separation during ink extrusion and crack formations during drying. With the reducing agents in the ink and slow evaporation of the ink solvent, compact packing and neck joining of copper nanoparticles were obtained after a two-step sintering process. The crack-free interconnects successfully produced have a surface roughness smaller than 1.5 μm and the square resistances as low as 0.01 Ω/□.

Cooperative Bilayer of Lattice-Disordered Nanoparticles as Room-Temperature Sinterable Nanoarchitecture for Device Integrations

Decreasing the interconnecting temperature is essential for 3D and heterogeneous device integrations, which play indispensable roles in the coming era of "more than Moore". Although nanomaterials exhibit a decreased onset temperature for interconnecting, such an effect is always deeply impaired because of organic additives in practical integrations. Meanwhile, current organic-free integration strategies suffer from roughness and contaminants at the bonding interface. Herein, a novel bilayer nanoarchitecture simultaneously overcomes the drawbacks of organics and is highly tolerant to interfacial morphology, which exhibits universal applicability for device-level integrations at even room temperature, with the overall performance outperforming most counterparts reported. This nanoarchitecture features a loose nanoparticle layer with unprecedented deformability for interfacial gap-filling, and a compact one providing firm bonding with the component surface. The two distinct nanoparticle layers cooperatively enhance the interconnecting performance by 73-357%. Apart from the absence of organics, the internal abundant lattice disorders profoundly accelerate the interconnecting process, which is supported by experiments and molecular dynamics simulation. This nanoarchitecture is successfully demonstrated in diversified applications including paper-based light-emitting diodes, Cu-Cu micro-bonding, and SiC power modules. The strategy proposed here can open a new paradigm for device integrations and provide a fresh understanding on interconnecting mechanisms.

Laser-Induced Selective Metallization on Polymer Substrates Using Organocopper for Portable Electronics

Our work proposed a facile strategy for selective fabrication of the precise metalized patterns onto polymer substrates through the laser direct structuring (LDS) technology using organocopper compounds. Copper oxalate (CuC₂O₄) and copper acetylacetonate [Cu(acac)₂] which can be used as laser sensitizers were first introduced into an acrylonitrile-butadiene-styrene (ABS) matrix for preparing LDS materials. After the activation with 1064 nm pulsed near-infrared laser, the Cu⁰ (metal copper) was generated from CuC₂O₄ and Cu(acac)₂ and then served as catalyst species for the electroless copper plating (ECP). A series of characterizations were conducted to investigate the morphology and analyze the surface chemistry of ABS/CuC₂O₄ and ABS/Cu(acac)₂ composites. Specially, the X-ray photoelectron spectroscopy analysis indicated that 58.3% Cu²⁺ in ABS/CuC₂O₄ was reduced to Cu⁰, while this value was 63.9% for ABS/Cu(acac)₂. After 30 min ECP, the conductivities of copper circuit on ABS/CuC₂O₄ and ABS/Cu(acac)₂ composites were 1.22 × 10⁷ and 1.58 × 10⁷ Ω^-1·m^-1, respectively. Moreover, the decorated patterns and near-field communication circuit were demonstrated by this LDS technology. We believe that this study paves the way for developing organocopper-based LDS materials, which have the potential for industrial applications.

High-performance stretchable conductive nanocomposites: materials, processes, and device applications

Highly conductive and intrinsically stretchable electrodes are vital components of soft electronics such as stretchable transistors and circuits, sensors and actuators, light-emitting diode arrays, and energy harvesting devices. Many kinds of conducting nanomaterials with outstanding electrical and mechanical properties have been integrated with elastomers to produce stretchable conductive nanocomposites. Understanding the characteristics of these nanocomposites and assessing the feasibility of their fabrication are therefore critical for the development of high-performance stretchable conductors and electronic devices. We herein summarise the recent advances in stretchable conductors based on the percolation networks of nanoscale conductive fillers in elastomeric media. After discussing the material-, dimension-, and size-dependent properties of conductive fillers and their implications, we highlight various techniques that are used to reduce the contact resistance between the conductive filler materials. Furthermore, we categorize elastomer matrices with different stretchabilities and mechanical properties based on their polymeric chain structures. Then, we discuss the fabrication techniques of stretchable conductive nanocomposites toward their use in soft electronics. Finally, we provide representative examples of stretchable device applications and conclude the review with a brief outlook for future research.

PVP-Mediated Galvanic Replacement Synthesis of Smart Elliptic Cu-Ag Nanoflakes for Electrically Conductive Pastes

Elliptic Cu-Ag nanoflakes were syntheszied via facile in situ galvanic replacement between prepared Cu particles and Ag ions. Alloy nanoflakes with high purity and uniformity present a size of 700 × 500 nm, with a thinness of 30 nm. Nontoxic and low-cost polyvinyl pyrrolidone was used as a dispersant and structure-directing agent, promoting the formation of the remarkable structure. Synthesized nanoflakes were utilized as a filler for conductive paste in an epoxy resin matrix. Conductive patterns on flexible substrates with a resistivity of 3.75 × 10^-5 Ω·cm could be achieved after curing at 150 °C for 2 h. Compared with traditional silver microflakes, smart alloy nanoflakes provide much improved conductive interconnection, whose advantage could be attributed to their nanoscale thicknesses. It is also noteworthy that the conductive patterns are able to tolerate multiple bendings at different angles, having good conductivity even after 200 repeated bendings. Therefore, alloy nanoflakes could be a promising candidate conductive filler for flexible printing electronics, electronic packaging, and other conductive applications.

Ultrahigh Conductivity and Superior Interfacial Adhesion of a Nanostructured, Photonic-Sintered Copper Membrane for Printed Flexible Hybrid Electronics

Inkjet-printed electronics using metal particles typically lack electrical conductivity and interfacial adhesion with an underlying substrate. To address the inherent issues of printed materials, this Research Article introduces advanced materials and processing methodologies. Enhanced adhesion of the inkjet-printed copper (Cu) on a flexible polyimide film is achieved by using a new surface modification technique, a nanostructured self-assembled monolayer (SAM) of (3-mercaptopropyl)trimethoxysilane. A standardized adhesion test reveals the superior adhesion strength (1192.27 N/m) of printed Cu on the polymer film, while maintaining extreme mechanical flexibility proven by 100 000 bending cycles. In addition to the increased adhesion, the nanostructured SAM treatment on printed Cu prevents formation of native oxide layers. The combination of the newly synthesized Cu ink and associated sintering technique with an intense pulsed ultraviolet and visible light absorption enables ultrahigh conductivity of printed Cu (2.3 × 10^-6 Ω·cm), which is the highest electrical conductivity reported to date. The comprehensive materials engineering technologies offer highly reliable printing of Cu patterns for immediate use in wearable flexible hybrid electronics. In vivo demonstration of printed, skin-conformal Cu electrodes indicates a very low skin-electrode impedance (<50 kΩ) without a conductive gel and successfully measures three types of biopotentials, including electrocardiograms, electromyograms, and electrooculograms.

Filtration-induced production of conductive/robust Cu films on cellulose paper by low-temperature sintering in air

Cellulose paper is an attractive substrate for paper electronics because of its advantages of flexibility, biodegradability, easy incorporation into composites, low cost and eco-friendliness. However, the micrometre-sized pores of cellulose paper make robust/conductive films difficult to deposit onto its surface from metal-nanoparticle-based inks. We developed a Cu-based composite ink to deposit conductive Cu films onto cellulose paper via low-temperature sintering in air. The Cu-based inks consisted of a metallo-organic decomposition ink and formic-acid-treated Cu flakes. The composite ink was heated in air at 100°C for only 15 s to give a conductive Cu film (7 × 10^-5 Ω cm) on the cellulose paper. Filtration of the Cu-based composite ink accumulated Cu flakes on the paper, which enabled formation of a sintered Cu film with few defects. A strategy was developed to enhance the bending stability of the sintered Cu films on paper substrates using polyvinylpyrrolidone-modified Cu flakes and amine-modified paper. The resistance of the Cu films increased only 1.3-fold and 1.1-fold after 1000 bending cycles at bending radii of 5 mm and 15 mm, respectively. The results of this study provide an approach to increasing the bending stability of Cu films on cellulose paper.

Synthesis, Spray Deposition, and Hot-Press Transfer of Copper Nanowires for Flexible Transparent Electrodes

We report a solution-phase approach to the synthesis of crystalline copper nanowires (Cu NWs) with an aspect ratio >1000 via a new catalytic mechanism comprising copper ions. The synthesis involves the reaction between copper(II) chloride and copper(II) acetylacetonate in a mixture of oleylamine and octadecene. Reaction parameters such as the molar ratio of precursors as well as the volume ratio of solvents offer the possibility to tune the morphology of the final product. A simple low-cost spray deposition method was used to fabricate Cu NW films on a glass substrate. Post-treatment under reducing gas (5% H₂ + 95% N₂) atmosphere resulted in Cu NW films with a low sheet resistance of 24.5 Ω/sq, a transmittance of T = 71% at 550 nm (including the glass substrate), and a high oxidation resistance. Moreover, the conducting Cu NW networks on a glass substrate can easily be transferred onto a polycarbonate substrate using a simple hot-press transfer method without compromising on the electrical performance. The resulting flexible transparent electrodes show excellent flexibility ( R/ R_o < 1.28) upon bending to curvatures of 1 mm radius.

Electroless Deposition Metals on Poly(dimethylsiloxane) with Strong Adhesion As Flexible and Stretchable Conductive Materials

A new surface modification method is developed for electroless deposition of robust metal (copper, nickel, silver) layers on poly(dimethylsiloxane) (PDMS) substrate with strong adhesion. Under the synergies of the polydopamine (PDA), the plasma process enhances Ag⁺ reduction, and a thin Ag film is capable of tightly attaching to the PDMS surface, which catalyzes electroless deposition (ELD) to form robust metal layers on the PDMS surface with strong adhesion. Subsequently, a flexible and stretchable Cu-PDMS conductor is obtained through this method, showing excellent metallic conductivity of 1.2 × 10⁷ S m^-1, even at the longest stretch strain (700%). This process provides a successful strategy for obtaining good robust metal layers on PDMS and other polymer substrate surfaces with strong adhesion and conductivity.

Enhanced Oxidation-Resistant Cu@Ni Core-Shell Nanoparticles for Printed Flexible Electrodes

In this work, the fabrication and application of highly conductive, robust, flexible, and oxidation-resistant Cu-Ni core-shell nanoparticle (NP)-based electrodes have been reported. Cu@Ni core-shell NPs with a tunable Ni shell thickness were synthesized by varying the Cu/Ni molar ratios in the precursor solution. Through continuous spray coating and flash photonic sintering without an inert atmosphere, large-area Cu@Ni NP-based conductors were fabricated on various polymer substrates. These NP-based electrodes demonstrate a low sheet resistance of 1.3 Ω sq^-1 under an optical energy dose of 1.5 J cm^-2. In addition, they exhibit highly stable sheet resistances (ΔR/R₀ < 1) even after 30 days of aging at 85 °C and 85% relative humidity. Further, a flexible heater fabricated from the Cu@Ni film is demonstrated, which shows uniform heat distribution and stable temperature compared to those of a pure Cu film.

Chemical formation of soft metal electrodes for flexible and wearable electronics

Efficient chemical approaches to fabricating soft metal electrodes aiming at wearable electronics are summarized and reviewed.

Textured ZnO films from evaporation-triggered aggregation of nanocrystal dispersions and their use in solar cells

Due to its high electron mobility, good stability and potential for low-temperature synthesis ZnO has received considerable attention for use in solar cells, photodetectors and sensors. Whilst there have been reports involving the formation ZnO films with porous morphologies the majority of those have involved elaborate or time-consuming preparation methods. In this study we investigate a simple new method for preparing textured porous ZnO (tp-ZnO) films. We used colloidal instability triggered by the evaporation of a volatile stabilising ligand during spin-coating of a ZnO nanocrystal (NC) dispersion to deposit crack-free tp-ZnO films. The porosity of the tp-ZnO films was 56% and they could be prepared using amine-based ligands with boiling points less than or equal to 78 °C. To demonstrate the ability to use the tp-ZnO films as electron acceptors they were infiltrated with poly(3-hexylthiophene) (P3HT) and solar cells prepared. The power conversion efficiencies of the tp-ZnO/P3HT devices reached values that were three times higher than a control bilayer ZnO/P3HT device prepared using a sol-gel derived ZnO film. Because our method used a low temperature treatment and ZnO films are used in a wide variety of third-generation solar cells, the new tp-ZnO films introduced here may offer a low cost method for improving the efficiency of other solar cells.

Inkjet printed nanomaterial based flexible radio frequency identification (RFID) tag sensors for the internet of nano things

The Internet of Things (IoT) has limitless possibilities for applications in the entire spectrum of our daily lives, from healthcare to automobiles to public safety.