Inks containing metal nanoparticles (MNP) are among the most commonly-used conducive materials for 3D-printed electronics. Ink-jetting layers of MNP materials allow for design flexibility, rapid processing and 3D printing of devices such as sensors, solar panels, LED displays, transistors and smart textiles.
A two-step process is used for inkjet 3D printing of metals: solvent evaporation upon printing (pinning) and subsequent low-temperature consolidation of nanoparticles (sintering). Low temperature is important as the nanoparticles are often co-printed with other functional/structural organic materials that are sensitive to higher temperatures.
However, layers produced by inkjet printing of metal nanoparticles have different electrical conductivity between horizontal and vertical directions. This is known as functional anisotropy and is a long-standing problem for 3D-printed electronics.
Previously, reduced vertical conductivity was mainly thought to have been caused by shape and physical continuity problems at the interfaces of the constituent nanoparticles at micro and nanoscale.
Nottingham researchers have now used silver nanoparticles to show that it is caused by organic chemical residues in the inks. These are added to the inks to help stabilise the nanomaterials, leading to the formation of low-conducting, thin nanoscale layers which interfere with the electrical conductivity of the printed sample in the vertical direction.
“This new insight enables the development of routes to overcome functional anisotropy in inkjet-based nanoparticles, and will therefore improve uptake of this potentially transformational technology, making it competitive with conventional manufacturing,” said lead author, Centre for Additive Manufacturing (CfAM) research fellow Dr Gustavo Trindade.
Trindade said that the approach is transferable to other nanomaterial-based inks including those containing graphene and functionalised nanocrystals, and could enable development of both 2D- and 3D-printed electronics. Researchers now hope to define new techniques and develop new ink formulations to overcome functional anisotropy.
The study was carried out by the CfAM under a £5.85m EPSRC-funded grant. Researchers used Nottingham’s orbiSIMS instrument, which allows label-free 3D chemical imaging of materials with high resolution. Findings are published in the journal Communications Materials.