Copper inks for printed electronics: a review

Miniaturized Micrometer-Level Copper Wiring and Electrodes Based on Reverse-Offset Printing for Flexible Circuits

High-resolution reverse-offset printing (ROP) is developed for miniaturization of printed electronics, resulting in a notable decrease in material usage compared to conventional printing processes. Two alternative ROP processes for patterning of metal conductors are available that are comparable in their cost per sample: direct nanoparticle (NP) printing (e.g., Ag and Cu) and patterning of vacuum-deposited metal (Ag, Al, Au, Cu, Ti, etc.) films using ROP printed polymer resist ink and the lift-off (LO) process. In this work, we focus on ROP of Cu NP ink followed by intense pulsed light (IPL) sintering and vacuum-deposited Cu patterned by ROP lift-off (LO). The good large-scale uniformity of the two processes is demonstrated by a grid of 300 individual thickness, sheet resistance, and resistivity measurement points with low variation over the 10 cm × 10 cm printed sample area. Sheet resistances of 0.56 ± 0.03 and 1.23 ± 0.05 Ω/□ are obtained at 113 and 40 nm thickness for Cu NP and Cu LO, respectively. Both processes show <5% thickness variation over a large area. A line-space (L/S) resolution of 2 μm is obtained for ROP patterned vacuum-deposited Cu having very low line edge roughness (LER) (∼60 nm), whereas for direct ROP printed Cu NP ink, the L/S resolution (2-4 μm) is limited by LER (∼900 nm) and influenced by the printed layer thickness. Based on the two fabrication routes, a flexible chip component assembly process is presented. Preliminary bending resistance results indicate that both ROP-based patterning processes yield a robust electrical interconnection between the ultrathin polyimide (PI) 5 mm × 5 mm chip and thermoplastic polyurethane (TPU). ROP shows promise as a scalable and sustainable patterning method for flexible ICs/chips that are assembled on flexible, stretchable, or biodegradable substrates and used, e.g., in wearable, large-scale sensing, and in environmental monitoring.

On-Demand Sintering of Gold Nanoparticles via Controlled Removal of o-Nitrobenzyl Thiol Ligands Under Record-Low Power for Conductive Patterns

Metal nanoparticles-based nanoinks have shown potential for fabricating metallic components essential to the realization of innovative 3D-printed electronic devices. However, fabricating metallic patterns on flexible, heat-sensitive substrates remains challenging due to high temperature and high energy sources, such as intense pulsed light (IPL), involved in the sintering process. Here an efficient sintering method is presented using ultralow power UV by leveraging the photocleavable ligand, o-nitrobenzyl thiol (NT), - functionalized gold nanoparticles (AuNPs). The controlled removal of NT ligands upon UV irradiation enhances light absorption by reducing the filling factor of voids in the printed layer, increasing the layer temperature, and facilitating further ligand desorption. This positive feedback mechanism accelerates nanoparticle sintering at several orders of magnitude lower energy than IPL, achieving an electrical conductivity of 7.0 × 106 S m-1. This nanoink promises the parallel printing of multimaterial components through ultralow power photonic sintering for fabricating multifunctional 3D-printed electronic devices.

An interesting aggregation induced red shifted emissive and ESIPT active hydroxycoumarin tagged symmetrical azine: Colorimetric and fluorescent turn on-off-on response towards Cu2+ and Cysteine, real sample analysis and logic gate application

We report a newly synthesized 7-diethylamino-4-hydroxycoumarin tagged symmetrical azine derivative (SHC), with an interesting color transformation from yellowish green to orange via aggregation induced red shifted emissive (117 nm) feature in THF-H2O mixture. Interestingly, the single crystal X-ray analysis of this molecule demonstrates that two hydroxycoumarin moieties were present in azine unit, among them one of the coumarin units was exist as enol form and another one transferred to keto form via ground state proton transfer reaction. The optical responses of the compound in different solvents exposed the observation of dual emissive bands which corresponds to the presence of ESIPT phenomenon in SHC molecule. Further, this characteristic was confirmed by absorption, emission, solid state structure and time resolved fluorescence decay measurements. Furthermore, the fluorophore, SHC was exploited as a colorimetric and turn on-off-on fluorescent probe for detection of Cu2+ ions and Cysteine (Cys). The 1:1 binding ratio of the probe with Cu2+ and Cys with SHC-Cu2+, was established via Job plot analysis, mass spectral technique and the DFT calculations. The probe, SHC was employed for the detection of copper ions in the environmental real water samples. Finally, the reversible fluorescent turn on-off-on character of the probe, SHC was established to construct the IMPLICATION logic gate application.

Fabrication of Nano Copper Highly Conductive and Flexible Printed Electronics by Direct Ink Writing

Nanoscale metals have emerged as crucial materials for conductive inks in printed electronics due to their unique physical and chemical properties. However, the synthesis of high-precision and highly conductive copper ink remains a challenge. Herein, a high-precision, highly conductive, and oxidation-resistant nanocopper ink was synthesized to fabricate highly conductive and flexible printed electronic devices. Copper nanoparticles with a particle size of only 8.5 nm, a controllable structure, and excellent oxidation resistance were synthesized by the alcohol phase reduction method. The conductive ink was formulated with ethylene glycol, ethanol, and isopropanolamine (IPA) as the solvent, exhibiting excellent printability and sintering reducibility. Fluid dynamics simulations were employed to investigate the influence of printing parameters on the circuit forming performance, enabling precise control over the printing process. The sintering behavior of copper nanoparticles with varying particle sizes was investigated by combining experiments with molecular dynamics (MD) simulations. Highly conductive and flexible circuits were fabricated using direct ink writing (DIW) under low-temperature sintering, exhibiting a low resistance level as low as 1.9 μΩ·cm. Moreover, the circuit demonstrated an excellent adhesion performance and bending flexibility. The developed copper ink demonstrates outstanding printing potential for applications in flexible electronics, advancing the field of flexible printing and wearable electronic devices.

Tissue-Adhesive and Stiffness-Adaptive Neural Electrodes Fabricated Through Laser-Based Direct Patterning

Recently, implantable devices for treating peripheral nerve disorders have demonstrated significant potential as neuroprosthetics for diagnostics and electrical stimulation. However, the mechanical mismatch between these devices and nerves frequently results in tissue damage and performance degradation. Although advances are made in stretchable electrodes, challenges, including complex patterning techniques and unstable performance, persist. Herein, an efficient method for developing a tissue-adhesive, stiffness-adaptive peripheral neural interface (TA-SA-PNI) is presented employing mechanically and electrically stable ultrathin conductive micro/nanomembrane bilayer (UC-MNB) electrodes. A direct laser-patterning technique is utilized to anchor the UC-MNB, comprising a conductive Cu micromembrane encapsulated by a biocompatible Au nanomembrane, onto a tough self-healing polymer (T-SHP) substrate using the thermoplastic properties of T-SHP. The UC-MNB with a wavy structure exhibited strain-insensitive performance under strains of up to 60%. Furthermore, its dynamic stress-relaxation properties enable stiffness adaptation, potentially minimizing chronic nerve compression. Finally, the phenylboronic acid-conjugated alginate (Alg-BA) adhesive layer offers stable tissue adhesion and ionic conductivity, optimizing the TA-SA-PNI for seamless integration into neural applications. Leveraging these advantages, in vivo demonstrations of bidirectional neural pathways are successfully conducted, featuring stable measurements of sensory neural signals and feedback electrical stimulation of the sciatic nerves of rats.

In Situ Synthesis of Nitrogen-Doped Carbon Coated Copper: Boosting Superhydrophobicity, Conductivity, and Oxidation Resistance

Copper (Cu), known for its excellent electrical and thermal conductivity, faces significant challenges due to its susceptibility to oxidation, which leads to the formation of nonconductive oxide layers that impair its performance. We present an in situ thermal reduction method to synthesize nitrogen-doped carbon coated copper (NC@Cu) with enhanced oxidation and corrosion resistance. Using a stable, nontoxic, and cost-effective dopamine derivative, catechol (CA), and phenylenediamine, we developed a polydopamine-like (PDL) coating on copper oxide (CuO). Upon pyrolysis under an inert atmosphere, this coating transforms into a nitrogen-doped carbon layer, while simultaneously reducing CuO to metallic Cu in a stepwise process, initially forming Cu2O and then fully reducing to Cu. The resulting NC@Cu exhibits remarkable superhydrophobicity, enhanced conductivity, and exceptional resistance to oxidation and corrosion, attributed to the protective dense carbon layer. This study provides insights into the synergistic processes of PDL conversion into nitrogen-doped carbon and CuO reduction to Cu, offering a straightforward and practical passivation method for producing electrically conductive and oxidation-resistant copper with potential applications in harsh environments.

Pressure-constrained sonication activation of flexible printed metal circuit

Metal micro/nanoparticle ink-based printed circuits have shown promise for promoting the scalable application of flexible electronics due to enabling superhigh metallic conductivity with cost-effective mass production. However, it is challenging to activate printed metal-particle patterns to approach the intrinsic conductivity without damaging the flexible substrate, especially for high melting-point metals. Here, we report a pressure-constrained sonication activation (PCSA) method of the printed flexible circuits for more than dozens of metal (covering melting points from room temperature to 3422 °C) and even nonmetallic inks, which is integrated with the large-scale roll-to-roll process. The PCSA-induced synergistic heat-softening and vibration-bonding effect of particles can enable multilayer circuit interconnection and join electronic components onto printed circuits without solder within 1 s at room temperature. We demonstrate PCSA-based applications of 3D flexible origami electronics, erasable and foldable double-sided electroluminescent displays, and custom-designed and large-area electronic textiles, thus indicating its potential for universality in flexible electronics.

Bayesian Optimization of Low-Temperature Nonthermal Plasma Jet Sintering of Nanoinks

In response to the escalating demand for flexible devices in applications such as wearables, sensors, and touch panels, there is a need for innovative fabrication approaches for devices made from nanomaterial-based inks. Subsequent to ink deposition, a pivotal stage in device manufacturing typically involves high-temperature sintering, posing challenges for heat-sensitive substrates. Nonthermal plasma jet sintering utilizing an atmospheric pressure dielectric barrier discharge (DBD) plasma jet enables sintering at room temperature and standard pressure, facilitating the sintering of printed nanoparticle films without compromising substrate or film surface integrity. However, determining optimal plasma jet sintering conditions can be challenging due to multiple processing variables with intricate interrelationships. This work employed Bayesian optimization (BO) and machine learning (ML) to identify optimal values for seven primary plasma jet sintering variables. Optimization yielded a 99.2% increase in the measured electrical conductivity for plasma jet-sintered indium tin oxide (ITO) films after five rounds of experiments. Moreover, the optimal sintering conditions achieved an electrical conductivity that was 81.4% of conventional furnace sintering at 300 °C, but was three times faster and with a peak substrate temperature below 47 °C. This result demonstrates the prospect of applying BO to optimize processing techniques for emerging low-temperature requirements.

A compact tri-notched flexible UWB antenna based on an inkjet-printable and plasma-activated silver nano ink

The rapid development of ultrawideband (UWB) communication systems has resulted in increasing performance requirements for the antenna system. In addition to a wide bandwidth, fast propagation rates and compact dimensions, flexibility, wearability or portability are also desirable for UWB antennas, as are excellent notch characteristics. Although progress has been made in the development of flexible/wearable antennas desired notch properties are still rather limited. Moreover, most presently available flexible UWB antennas are fabricated using environmentally not attractive subtractive etching-based processes. The usage of facile additive sustainably inkjet printing processes also utilizing low temperature plasma-activated conductive inks is rarely reported. In addition, the currently used tri-notched flexible UWB antenna designs have a relatively large footprint, which poses difficulties when integrated into miniaturized and compact communication devices. In this work, a silver nano ink is used to fabricate the antenna via inkjet printing and an efficient plasma sintering procedure. For the targeted UWB applications miniaturized tri-notched flexible antenna is realized on a flexible polyethylene terephthalate (PET) substrate with a compact size of 17.6 mm × 16 mm × 0.12 mm. The antenna operates in the UWB frequency band (2.9-10.61 GHz), and can shield interferences from WiMAX (3.3-3.6 GHz), WLAN (5.150-5.825 GHz) and X-uplink (7.9-8.4 GHz) bands, as well as exhibits a certain of bendability. Three nested "C" slots of different sizes were adopted to achieve notch features. The simulation and test results demonstrate that the proposed antenna can generate signal radiation in the desired UWB frequency band while retaining the desired notch properties and having acceptable SAR values on-body, making it a viable candidate for usage in flexible or wearable communication transmission devices. The research provides a facile and highly efficient method for fabricating flexible/wearable UWB antennas, that is, the effective combination of inkjet printing processing, flexible substrates, low temperature-activated conductive ink and antenna structure design.

The damage mechanism in copper studied using in situ TEM nanoindentation

Copper (Cu) has a soft-plastic nature, which makes it susceptible to damages from scratching or abrasive machining, such as lapping and polishing. It is a challenge to control these damages as the damage mechanism is elusive. Nonetheless, controlling damages is essential, especially on the atomic surfaces of Cu. To interpret the damage mechanism, in situ transmission electron microscopy (TEM) nanoindentation was performed using a cube-corner indenter with a radius of 57 nm at a loading speed of 5 nm s-1. Experimental results showed that damages originate from dislocations, evolve to stack faults, and then form broken crystallites. When the indentation depth was 45 nm at a load of 20 μN, damages comprised dislocations and stacking faults. After increasing the depth to 67 nm and load to 30 μN, the formation of broken crystallites initiated; and the critical depth was 67 nm. To validate the damage mechanism, fixed-abrasive lapping, mechanical polishing, and chemical mechanical polishing (CMP) were conducted. Firstly, a novel green CMP slurry containing silica, hydrogen peroxide, and aspartic acid was developed. After CMP, a surface roughness Ra of 0.2 nm was achieved with a scanning area of 50 μm × 50 μm; and the thickness of the damaged layer was 3.1 nm, which included a few micro-stacking faults. Lapping and mechanical polishing were carried out using a silicon carbide plate and cerium slurry, with surface roughness Ra values of 16.42 and 1.74 nm, respectively. The damaged layer of the former with a thickness of 300 nm comprised broken crystallites, dislocations, and stacking faults and that of the latter with a thickness of 33 nm involved several stacking faults. This verifies that the damage mechanism derived from in situ TEM nanoindentation is in agreement with lapping and polishing. These outcomes propose new insights into understanding the origin of damages and controlling them, as well as obtaining atomic surfaces using a novel green CMP technique for soft-plastic metals.

Insight into the Types of Alkanolamines on the Properties of Copper(II) Formate-Based Conductive Ink

Conductive inks are one of the most important functional materials for printed flexible electronic devices, and their properties determine the methods of subsequent patterning and metallization. In comparison with copper nanoparticle or nanowire inks, copper particle-free inks employing copper(II) formate (Cuf) as a precursor have attracted the interest of researchers due to their flexibility in preparation, excellent stability, and lower conversion temperature. Alkanolamines can provide Cuf with excellent solubility in alcohols while being less toxic and having a certain reducibility, making them preferable ligands in comparison with aliphatic amines and pyridine. However, there have been few studies on the effects of the alkanolamine types on the performance of Cuf inks. Also, the decomposition mechanism of copper-alkanolamine complex inks is not clear. In this work, different kinds of alkanolamines were chosen as ligands to formulate Cuf inks to address the mentioned issues. The influences of amine types on the stability, wettability, thermal decomposition behavior, and electrical performance of the formulated Cuf particle-free inks were investigated in detail. The results show that the utilization of alkanolamines could provide Cuf with excellent solubility in alcohols, resulting in an ink with good stability and favorable wetting properties. The thermal decomposition temperature and electrical performance of the formulated copper ink are largely dependent on the amine used. When amines with a longer carbon chain and more branches were utilized to prepare the ink, a decreased decomposition temperature was observed on the derived inks because of the steric hindrance effect. Copper films with good morphology and conductivity could be obtained at low temperatures by selecting the appropriate alkanolamine. Copper particle-free conductive ink from 2-amino-2-methyl-1-propanol demonstrated better morphology and electrical performance (16.09 μΩ·cm) and was successfully used for conductive circuits by direct-writing.

Seedless wet synthesis of copper-twinned nanocrystals

The wet synthesis of copper (Cu)-twinned nanostructures often requires the addition of noble metal seeds, as twinned Cu seeds are prone to oxidative etching, which inevitably introduces other metal species. In this study, a universal and seedless wet method is proposed for the synthesis of various Cu-twinned nanostructures, such as large Cu decahedrons (with sizes up to 300 nm), singly twinned Cu right bipyramids, and Cu nanorods. The amount of chloride ions (Cl-) and oleylamine and an optimal heating rate at the initial stage were proven to be crucial in this synthesis. Theoretical results revealed that the amount of Cl- could adjust the Gibbs free energy of Cu seeds by promoting the dissociation of oleylamine, which, in turn, determined the structure of thermodynamically favorable seeds based on the thermodynamic model. To the best of our knowledge, this is the first report on large Cu decahedrons and singly twinned Cu right bipyramids. Moreover, they both showed strong localized surface plasmon resonance in the near-infrared region. The photothermal conversion efficiency of large Cu decahedrons increased up to 52.9% upon 808 nm laser irradiation, which is the highest value ever reported for Cu nanocrystals.

Rapid Fabrication of Flexible Cu@Ag Flake/SAE Composites with Exceptional EMIS and Joule Heating Performance

High electromagnetic interference shielding (EMIS) effectiveness and good thermal management properties are both required to meet the rapid development of integrated electronic components. However, it remains challenging to obtain environmentally friendly and flexible films with high EMIS and thermal management performance in an efficient and scalable way. In this paper, an environmentally friendly strategy is proposed to synthesize multifunctional waterborne Cu@Ag flake conductive films using water as the solvent and silicone-acrylic emulsion (SAE) as a matrix. The obtained films show high electrical conductivity and exceptional EMI SE and electrothermal conversion properties. The EMI SE in the X-band is higher than 76.31 dB at a thickness of 60 μm owing to the ultrahigh electrical conductivity of 1073.61 S cm-1. The film warms up quickly to 102.1 °C within 10 s under a low voltage of 2.0 V. In addition, the shielding coating is sufficiently flexible to retain a conductivity of 93.4% after 2000 bending-release cycles with a bending radius of 3 mm. This work presents an alternative strategy to produce high EMIS effectiveness and Joule heating films for highly integrated and flexible electronic components in a green, scalable, and highly efficient way.

Photonic sintering of copper for rapid processing of thick film conducting circuits on FTO coated glass

Copper potentially provides a cost-effective replacement for silver in printed electronic circuitry with diverse applications in healthcare, solar energy, IOT devices and automotive applications. The primary challenge facing copper is that it readily oxidizes to its non-conductive state during the sintering process. Photonic sintering offers a means of overcoming the oxidation by which rapid conversion from discrete nano-micro particles to fully or partially sintered products occurs. An experimental study of flash lamp sintering of mixed nano copper and mixed nano/ micro copper thick film screen printed structures on FTO coated glass was carried out. It shows that there may be multiple energy windows which can successfully sinter the thick film copper print preventing detrimental copper oxidation. Under optimum conditions, the conductivities achieved in under 1 s was (3.11-4.3 × 10^-7 Ω m) matched those achieved in 90 min at 250 °C under reducing gas conditions, offering a significant improvement in productivity and reduced energy demand. Also present a good film stability of a 14% increase in line resistance of 100 N material, around 10% for the 50N50M ink and only around 2% for the 20N80M.