Researchers from the University of Nottingham and the National Physical Laboratory (NPL) have carried out a new study that concludes thin layers of organic stabilizer residue in metal nanoparticle (MNP) inks are the reason for a loss of conductivity in 3D printed materials and electronics.
While reduced vertical conductivity through 3D printed electronics has been attributed to shape and physical continuity issues between nanoparticles in the past, the researchers have used silver nanoparticles to show, reportedly for the first time, that reduced conductivity is actually caused by organic chemical residues in the inks used.
“The conductivity of inkjet-printed metal nanoparticles is known to be dependent on processing temperature and have previously been attributed to changes in the shape and porosity of clustered nanoparticles, the role of organic residues being only speculated,” said Dr. Gustavo Trindade, Lead Author of the study and Centre for Additive Manufacturing (CfAM) Research Fellow.
“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.”
Inkjet printing for electronics
Inks containing MNPs are among the most commonly-used conductive materials for 3D printed electronics. Typically, inkjet 3D printing is used to form solid objects through a two-step process involving solvent evaporation upon printing and low-temperature consolidation of nanoparticles via sintering.
Printing at low temperatures is crucial to the success of the process, as in many applications the nanoparticles are printed alongside other functional and structural organic materials that are sensitive to higher temperatures.
The inkjet 3D printing process allows for excellent design flexibility, rapid processing, and printing of functional electronic devices such as sensors, LED displays, transistors, and solar panels. However, printed layers of MNPs have different levels of electrical conductivity between their horizontal and vertical directions, an effect known as functional anisotropy, which has previously hindered the use of 3D printed electronics for advanced applications.
Last year, CfAM researchers reported a breakthrough in the inkjet 3D printing of electronic devices with graphene, which they slated as a promising step towards replacing single-layer graphene as a contact material for 2D metal semiconductors.
New insights into conductivity
The University of Nottingham and NPL study was carried out by the CfAM under a £5.85 million EPSRC-funded program grant, Enabling Next Generation Additive Manufacturing.
Generally, organic chemical residues are added to 3D printable MNP inks in order to stabilize the nanomaterials, which leads to the formation of very thin, low-conductive nanoscale layers that can interfere with the electrical conductivity of a 3D printed object along the vertical direction.
The researchers demonstrated that poor inter and intra-layer electrical conductivity in silver structures produced via 3D inkjet printing results from a combined chemistry and morphology interface evolution during the low-temperature sintering step of the printing process. The residual polymer led to an anisotropic conductivity reduction as it accumulated at the interface of vertically stacked printed layers.
To closely observe the conductivity of the printed structures, the researchers used a state-of-the-art 3D orbiSIMS instrument which enables label-free 3D chemical imaging of materials with very high resolution. Using the orbiSIMS, the researchers were able to identify and understand the distribution of residual organic additives within the printed layers and their effects.
Going forwards, the team hopes to use this information and apply it to the development of new techniques and ink formulations to overcome functional anisotropy of inkjet-based 3D printed electronics. One possible strategy they suggest in the study is to convert post-processing steps into in-situ processing steps, or the use of additional post-processing methods for the sintering stage of the inkjet printing process.
“Our approach is transferable to other nanomaterial-based inks including those containing graphene and functionalized 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,” said Trindade.
More information on the study can be found in the paper titled “Residual polymer stabiliser causes anisotropic conductivity in metal nanoparticle 2D and 3D printed electronics”, published in the Nature journal, Communications Materials. The study is co-authored by G. Trindade, F. Wang, J. Im, Y. He, A. Balogh, D. Scurr, I. Gilmore, M. Tiddia, E. Saleh, D. Pervan, L. Turyanska, C. Tuck, R. Wildman, R. Hague, and C. Roberts.
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Featured image shows thin layers of organic stabiliser residue in MNP inks are behind a loss of conductivity in 3D printed electronic materials. Image via University of Nottingham.