Self-catalyzed copper-silver complex inks for low-cost fabrication of highly oxidation-resistant and conductive copper-silver hybrid tracks at a low temperature below 100 °C

Self-Organizing, Environmentally Stable, and Low-Cost Copper-Nickel Complex Inks for Printed Flexible Electronics

Cost-effective copper conductive inks are considered as the most promising alternative to expensive silver conductive inks for use in printed electronics. However, the low stability and high sintering temperature of copper inks hinder their practical application. Herein, we develop rapidly customizable and stable copper-nickel complex inks that can be transformed in situ into uniform copper@nickel core-shell nanostructures by a self-organized process during low-temperature annealing and immediately sintered under photon irradiation to form copper-nickel alloy patterns on flexible substrates. The complex inks are synthesized within 15 min via a simple mixing process and are particle-free, air-stable, and compatible with large-area screen printing. The manufactured patterns exhibit a high conductivity of 19-67 μΩ·cm, with the value depending on the nickel content, and can maintain high oxidation resistance at 180 °C even when the nickel content is as low as 6 wt %. In addition, the printed copper-nickel alloy patterns exhibit high flexibility as a consequence of the local softening and mechanical anchoring effect between the metal pattern and the flexible substrate, showing strong potential in the additive manufacturing of highly reliable flexible electronics, such as flexible radio-frequency identification (RFID) tags and various wearable sensors.

Highly Conductive, Flexible, and Oxidation-Resistant Cu-Ni Electrodes Produced from Hybrid Inks at Low Temperatures

Recently, Ni and Ni-Cu nanoparticle-based inks have gained considerable research interest because of their high corrosion resistance as conductors in electronic devices. However, reported inks based on Cu-Ni nanoparticles need to be sintered at high temperatures above 300 °C to obtain electrodes with high conductivity on the order of 10^-5 Ω·cm. This study proposes a new conductive Cu-Ni-based hybrid ink that could be sintered at only 150-180 °C for producing Cu-Ni electrodes with low electrical resistance, high oxidation resistance, and flexibility. The hybrid ink contains Cu flakes and a complex of nickel formate and 1-amino-2-propanol (NiF-AmIP complex). At 150-180 °C, the Cu flakes catalyze the self-reduction of the NiF-AmIP complex, and Cu-Ni electrodes with high conductivity (on the order of 10^-5 Ω·cm) are formed on flexible polymer substrates at temperatures exceeding 150 °C. Analysis indicates that metallic Ni was decorated on the Cu flakes (especially on the edge) to improve the electrode's conductivity, oxidation resistance, and flexibility by forming bridging interconnections between the Cu flakes. The Cu-Ni electrodes demonstrated high stability against oxidation up to approximately 400 °C in air, as well as at 80 °C and 80% RH after 7 days. In addition to the excellent oxidation stability, the Cu-Ni electrode showed high durability under mechanical bending stress. Such sintered Cu-Ni electrodes obtained from hybrid inks have great potentials in printed/flexible devices due to their oxidation resistance and cost-effectiveness.

Mechanistic Insight into Bis(amino) Copper Formate Thermochemistry for Conductive Molecular Ink Design

Increasing attention has been given to amine-copper formate complexes for their use as low-cost printable conductive inks. The structure of amine ligands coordinated to copper centers has been reported to dictate the properties of copper molecular inks, such as stability and printability, thereby influencing the copper reduction pathway during the thermolysis. Yet, the underlying mechanism by which formate is oxidized when complexed with amine ligands is still not fully understood. Here, we propose a mechanistic pathway of copper formate dehydrogenation and decarboxylation and examine the critical role that amine ligands play in their thermal decomposition by employing first-principles electronic structure computations and experimental analyses of thermolysis reactions. Based on the computational characterization of the relevant reaction pathways for a number of primary and secondary amines as well as pyridine ligand complexes, we are the first to show that the hydrogen bonds formed between the amine ligand and formate are the key factors governing the activation energy, providing a design principle for the synthesis of organic ligands that can tune the height of the reaction barriers of the dehydrogenation and decarboxylation reactions. The calculations, confirmed by NMR studies, show that the reduction of Cu(II) to Cu(I) occurs in concert with the release of H₂ via the dimerization of Cu(II) hydride. This result suggests that the monomeric elimination of H₂ is not favorable for the Cu(II) to Cu(I) reduction and thus identifies dimeric amino copper formate as an important intermediate for copper reduction whose thermodynamic stabilities are also dictated by the nature of the amine ligands.

Large-Scale and Galvanic Replacement Free Synthesis of Cu@Ag Core-Shell Nanowires for Flexible Electronics

Copper nanowires (CuNWs) are considered a promising alternative to indium tin oxide due to their cost-effectiveness as well as high conductivity and transparency. However, the practical applications of copper-based conductors are greatly limited due to their rapid oxidation in atmosphere. Herein, a facile adsorption and decomposition process is developed for galvanic replacement free and large-scale synthesis of highly stable Cu@Ag core-shell nanowires. First, Ag-amine complex ([Ag(NH₂R)₂]⁺) as silver source adsorbs on CuNWs surface, and Cu@Ag-amine complex core-shell structure is formed. After that, Ag-amine complex is easily decomposed to pure Ag shell through a simple thermal annealing under air. By adjusting the concentration of Ag-aminein CuNWs solution, Cu@Ag core-shell nanowires with different thickness of silver shell can be easily obtained. The obtained core-shell nanowires exhibit high stability for at least 500 h at high temperature (140 °C) and high humidity (85 °C, 85% RH) due to the protection of Ag shell. More importantly, the conductivity and transparency of Cu@Ag nanowires-based conductors is similar to that of pure CuNWs. The large-scale and facile synthesis of Cu@Ag core-shell nanowires provides a new method to prepare stable metallic core-shell nanowires.