New study pinpoints loss of conductivity in metal nanoparticle inks

Inks containing metal nanoparticles are among the most commonly-used conductive materials for printed electronics. However, A study carried out by the University of Nottingham and National Physical Laboratory has found that very thin layers of organic stabiliser residue in metal nanoparticle (MNP) inks lead to a loss of conductivity in 3D printed materials and electronic devices.

Using inkjet to produce layers of metal nanoparticles, or MNP, materials allows for a great deal of design flexibility, rapid processing and 3D printing of functional electronic devices such as sensors, solar panels, LED displays, transistors and smart textiles. However, layers produced by inkjet printing of metal nanoparticles have different electrical conductivity between horizontal and vertical directions. This effect is known as functional anisotropy and is a long-standing problem for the 3D printing of functional electronic devices, preventing its use for advanced applications.

Previously, it was thought that this was down to reduced vertical conductivity, which itself was caused by shape and physical continuity problems at the interfaces of the constituent nanoparticles. 

However, the researchers from Nottingham used silver nanoparticles to demonstrate that it is actually caused by organic chemical residues in the inks. These residues, which are added to the inks to help stabilise the nanomaterials, lead to the formation of low-conducting, very thin nanoscale layers which interfere with the electrical conductivity of the printed sample in the vertical direction. This discovery should allow for new techniques and new ink formulations to be developed in order to overcome this functional anisotropy for inkjet-based 3D printed electronics.

Dr Gustavo Trindade, Research Fellow at the Centre for Additive Manufacturing and the lead author of this study, commented: “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. Our approach is transferable to other nanomaterial-based inks including those containing graphene and functionalised nanocrystals, and will enable the development and exploitation of both 2D and 3D printed electronics like flexible and wearable sensors, solar panels, LED displays, transistors and smart textiles.”

The study was carried out by the Centre for Additive Manufacturing (CfAM), under the £5.85m EPSRC-funded Programme Grant, Enabling Next Generation Additive Manufacturing. Their findings are published in a new paper ‘Residual polymer stabiliser causes anisotropic conductivity in metal nanoparticle 2D and 3D printed electronics’ in the Nature journal Communications Materials. You can find more information about CfAM from nottingham.ac.uk.