Green Synthesis in Ambient of Photothermally Activatable Copper Nanoparticles for Additive Manufacturable 3D Electronics

Recently, three-dimensional (3D) printed electronics has gained a tremendous interest as a technology that can potentially address a critical economical issue in patterning designated functional materials in three-axis directions. A development of 3D printable conductive ink is highly prerequisite to ensure a realization of 3D printed electronics. The conventional silver nanoparticle-based conductive inks can be an excellent candidate in terms of electrical conductance and environmental stability, but they are limited in widespread use due to critical issues in cost and susceptibility to electromigration. The cost-effective copper nanoparticles have been recognized as a viable alternative owing to their capability of offering a high conductivity comparable that of silver nanoparticles. Herein, we introduce a green technique of synthesizing surface oxidation-suppressed copper nanoparticles in air. The copper nanoparticles with an ultrathin surface oxide layer are designed to be sintered by 3D surface-conformal green laser irradiation process to address the limitations of conventional heat treatment process. More importantly, we also suggest a surface modification technique of removing photothermally the ultrathin surface oxide layer upon a completion of green laser irradiation process. We demonstrate that a combinatorial strategy of combining the 3D green laser activation process and the surface modification technique enables effectively to achieve the widen processing window, long-term storage of copper nanoparticles (for over 30 days in air), and high electrical conductivity of over 20,000 S/cm in 3D shaped circuitries.