Residual polymer stabiliser causes anisotropic electrical conductivity during inkjet printing of metal nanoparticles

ARTICLE https://doi.org/10.1038/s43246-021-00151-0 OPEN Residual polymer stabiliser causes anisotropic electrical conductivity during inkjet printing of metal nanoparticles 1234567890():,; Gustavo F. Trindade 1,2 ✉, Feiran Wang2, Jisun Im 2, Yinfeng He2, Adam Balogh2, David Scurr Ian Gilmore 3, Mariavitalia Tiddia3, Ehab Saleh2,4, David Pervan2, Lyudmila Turyanska 2, Christopher J. Tuck2, Ricky Wildman 2, Richard Hague2 & Clive J. Roberts1 ✉ 1, Inkjet printing of metal nanoparticles allows for design flexibility, rapid processing and enables the 3D printing of functional electronic devices through co-deposition of multiple materials. However, the performance of printed devices, especially their electrical conductivity, is lower than those made by traditional manufacturing methods and is not fully understood. Here, we reveal that anisotropic electrical conductivity of printed metal nanoparticles is caused by organic residuals from their inks. We employ a combination of electrical resistivity tests, morphological analysis and 3D nanoscale chemical analysis of printed devices using silver nanoparticles to show that the polymer stabiliser polyvinylpyrrolidone tends to concentrate between vertically stacked nanoparticle layers as well as at dielectric/ conductive interfaces. Understanding the behaviour of organic residues in printed nanoparticles reveals potential new strategies to improve nanomaterial ink formulations for functional printed electronics. 1 Advanced Materials and Healthcare Technologies, School of Pharmacy, University of Nottingham, University Park, Nottingham, UK. 2 Centre for Additive Manufacturing, Faculty of Engineering, University of Nottingham, Jubilee Campus, Nottingham, UK. 3 National Physical Laboratory, Teddington, UK. 4 Future Manufacturing Processes Research Group, Faculty of Engineering and Physical Sciences, University of Leeds, Leeds, UK. ✉email: ; COMMUNICATIONS MATERIALS | (2021)2:47 | https://doi.org/10.1038/s43246-021-00151-0 | www.nature.com/commsmat 1 ARTICLE COMMUNICATIONS MATERIALS | https://doi.org/10.1038/s43246-021-00151-0 D igitally printed electronics are a driver for novel research in various fields owing to their design flexibility as well as other advantages such as expedited time-to-market1–3. Ink jetting of inks containing colloidal materials, such as metal nanoparticles4–8, germania-silica9, semiconductor quantum dots10, intrinsically conductive polymer colloids11 and magnetic nanoparticles12, have been successfully employed in applications ranging from flexible and wearable electronics2,7,13–19 to quantum optoelectronic devices20–22 and fully printed perovskite solar cells23–26. However, the performance of printed parts is not competitive with those made by traditional manufacturing methods. This is due to challenges in both manufacturing techniques and choices of materials, of which there is a lack of comprehensive understanding. Metal nanoparticles are among the most commonly used conductive materials for printed electronics, and typically require consolidation via a two-step process: solvent evaporation upon printing (pinning) and subsequent low-temperature sintering (120–200 °C)4,15,27–30, enabling conductive tracks on polymer substrates2,4,7. More recently, inkjet-based three-dimensional (3D) printing has been used to enable the selective co-deposition of different functional materials contemporaneously (i.e. dielectric and/or conductive materials)4,5,14,27,31,32 to ultimately achieve the production of macroscopic multi-material objects with multiple functionalities. Despite significant interest in utilising metal nanoparticlebased materials in two-dimensional (2D) and 3D printed electronic devices, the lower and anisotropic intra-layer (planar) and inter-layer (vertical) conductivity of metal nanoparticle layers, compared to bulk metals, limits device performance and hence uptake in industry and products4. The conductivity of printed layers are known to be dependent on thermal treatment profiles and have been previously attributed to morphological changes and possible organic residues4,30,33,34. However, the detailed mechanism of low-temperature sintering of metal nanoparticles that leads to reduced conductivity remains to be fully understood. Organic molecules are used in inks as stabilisers or capping agents to enable nanoparticle dispersion in low-viscosity solvents; however, their residues are likely to hinder device performance33, even when present in very small amounts. Surface sensitive chemical analysis techniques of time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS), in combination with gas cluster ion beams (GCIBs), are a powerful toolset for depth profiling organic materials with high chemical specificity, sensitivity, and nanometre depth resolution35–43, chemical imaging of buried hybrid organic/ inorganic interfaces44,45 and characterisation of core–shell structures46–52. Here, we present a comprehensive study of the effect of residues of an organic stabiliser in inkjet-printed silver nanoparticles (AgNPs). We show that residues tend to concentrate between vertically stacked layers, which correlates with reduced and anisotropic electrical conductivity before and after low-temperature sintering. Our results provide new insights on routes to improve intra-layer AgNPs sintering and overcome functional anisotropy and hence improve uptake of this potential transformational technology. Our methodology is transferable to other nanomaterial-based inks and relevant for the development and exploitation of both 2D and 3D printed electronics. Results Distribution of polymer stabiliser upon printing of AgNPs. To investigate the interface of AgNPs during the inkjet printing and pinning process, we carried out high specificity chemical analysis of samples under a selection of printing conditions. As a result, we have found polyvinylpyrrolidone (PVP) on the surface of 2 printed layers of AgNPs by means of the unambiguous identification of characteristic PVP signal36,53 using a novel 3D orbiSIMS instrument54 and XPS (Fig. 1 and Supplementary Fig. 1). PVP is a commonly used stabiliser, reducing agent, and shapecontrolling agent for the synthesis of metal nanoparticles55,56. As a capping agent, it makes AgNPs disperse well in aqueous and organic solvents due to its amphiphilic characteristics derived from the highly polar amide group within the pyrrolidone ring and hydrophobicity from the methylene backbone. PVP thus plays a vital role in dispersing and stabilising AgNPs in solvents for stable ink formulation56. However, PVP does not completely decompose under typical multi-material jetting and sintering temperatures (up to 150 °C)5 and its residues are likely to remain. Understanding the state and distribution of PVP upon inkjet printing and post-deposition treatment of AgNPs is essential to develop solutions that minimise its impact on the performance of printed elec (...truncated)