Development of Highly Conductive and Stable Direct Writeable Metal-Free Inks Based on Carbon Nanotube and Functionalized Graphene Oxide.

In this study, we report our development of a new highly conductive and extremely stable 3D printable (or directly writable) metal-free ink for printed electronics applications. The ink consists of carbon nanotubes (CNTs) and functionalized (with p-phenylenediamine or PPD) graphene oxide and exhibits conductivity that is nearly 1 order of magnitude greater than that of the previously reported CNT-GO inks. Comprehensive characterization of the CNT-GO-PPD ink reveals its optimal rheological properties (making it 3D printable or direct writable), uniform deposition characteristics, and remarkably high conductivity (among the very best in metal-free carbon-based inks). We conduct molecular dynamics (MD) simulations and, from the equilibrium structure of the CNT-GO-PPD system (revealed by such simulations), explain the manner in which the PPD and CNT molecules simultaneously adsorb on the GO flakes to ensure significantly large conductivity of the CNT-GO-PPD ink. The ink can be directly written on various substrates and for creating complex geometries. Additionally, the traces printed with this ink demonstrate temperature sensing and humidity sensing capabilities and at the same time show excellent reliability and stability across a broad range of temperatures and humidity levels, establishing the ink's potential for advancing the field of printed electronics.