Silver inks reduced with low-temperature plasma are printable even on paper

Direct printing using metallic inks is a relatively new manufacturing process for electronic and high-tech industries. The approach enables (conductive) writing on arbitrary substrates, providing a process to manufacture flexible electronics. Typical existing systems use conductive, particle-based inks that require sintering at high temperatures after the inks are deposited on their substrate. In order to move to temperature-sensitive, flexible substrates, industry needs new inks that do not need thermal sintering which damages substrates in the process.

Research by Sui et al. has yielded a new, low temperature process to form metallic structures by inkjet printing. Instead of using high-temperature sintering of printed nanoparticles, the new process uses metal salts that are reduced to form solid metal structures using a low-temperature, inert gas plasma. The process, which the authors used to form silver structures from silver nitrate inks, allows fabrication on a variety of substrates including paper.

Importantly, this technique allows for the creation of variable resistances in the same device during writing by altering the plasma exposure conditions, specifically exposure time and power. As a result, both conductors and resistors in a device can be made on the fly using the same ink. Presumably, the voids, grain boundaries, or other microstructural features are responsible for the variable resistivity. Using selective masking to vary the plasma exposure over the entire substrate can create devices that exploit the tunable electrical properties.

Future work is geared towards better understanding the mechanisms of chemical reduction by the plasma, and how the volatile reactants behave after the reduction so that the process can be optimized for eventual high-yield, high-throughput processing.

Source: “Tunable resistivity in ink-jet printed electrical structures on paper by plasma conversion of particle-free, stabilizer-free silver inks,” by Yongkun Sui, Souvik Ghosh, Christopher Miller, Daphne Pappas, R. Mohan Sankaran, and Christian A. Zorman, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films (2018). The article can be accessed at https://doi.org/10.1116/1.5042459 .