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Low Temperature Chemical Sintering of Inkjet-Printed Zn Nanoparticles for Highly Conductive Flexible Electronic Components

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Low Temperature Chemical Sintering of Inkjet-Printed Zn Nanoparticles for Highly Conductive Flexible Electronic Components www.nature.com/npjflexelectron ARTICLE OPEN Low temperature chemical sintering of inkjet-printed Zn nanoparticles for highly conductive flexible electronic components ✉ Subimal Majee 1 , Mikael C. F. Karlsson1,2, Anurak Sawatdee1, Mohammad Yusuf Mulla1, Naveed ul Hassan Alvi1, Valerio Beni1 and ✉ David Nilsson 1 This study illustrates an innovative way to fabricate inkjet-printed tracks by sequential printing of Zn nanoparticle ink and curing ink for low temperature in situ chemical sintering. Employing chemical curing in place of standard sintering methods leads to the advantages of using flexible substrates that may not withstand the high thermal budgets of the standard methods. A general formulation engineering method is adopted to produce highly concentrated Zn ink which is cured by inkjet printing an over-layer of aqueous acetic acid which is the curing agent. The experimental results reveal that a narrow window of acid concentration of curing ink plays a crucial role in determining the electrical properties of the printed Zn nanoparticles. Highly conductive (~105 S m−1) and mechanically flexible printed Zn features are achieved. In addition, from systematic material characterization, we obtain an understanding of the curing mechanism. Finally, a touch sensor circuit is demonstrated involving all-Zn printed conductive tracks. npj Flexible Electronics (2021) 5:14 ; https://doi.org/10.1038/s41528-021-00111-1 1234567890():,; INTRODUCTION Although non-metallic inkjet printable conductive inks are Inkjet-printing of electrodes and connectors using conductive inks available based on conductive polymers such as PEDOT:PSS, and has captured much attention in the last couple of decades in carbonaceous NPs (i.e., graphene); the majority of the conducting 2,7–13 various printed electronics applications owing to its non- inks available in the market rely on metallic NPs (Ag, Au, Cu, Ni) . contact1,2, additive, and high-resolution drop-on-demand fabrica- The state-of-the-art fabrication of conducting patterns in inkjet- tion ability3,4. Not least due to its compatibility with a broad range printed electronics is dominated by the use of Ag-based inks. In of substrates and functional inks1–5. Nanoparticle (NP) based order to provide electrical functionality to the Ag layer, the printed conductive inks are commonly used in inkjet printing because of film must be subjected to highly energetic curing methods such as 2,7 the particle size limitation for the inkjet nozzles, scalability of NP thermal, NIR, electrical heating, or microwave sintering . However, production, and dispersibility with high concentrations in the ink in the case of heat-sensitive substrates (paper, plastic, etc.), most of carrier fluid6. The unprocessed non-dispersed NPs can be sintered the earlier stated sintering techniques cannot be applied due to the at relatively lower temperatures compared to their bulk counter- risk of damage. The Ag ink printed on top of heat-sensitive parts due to the high surface area to volume ratio. However, most substrates is curable using laser or photo-sintering14.Inallthe NP-based inks require additional surfactants and binders to above cases the energy consumption is substantial. Another prevent their agglomeration inside the ink carrier fluid. These problem is related to the costs of production. Ag nanoparticle- additives should be removed from the printed films through based inks are widely used because of their high conductivity and elevated sintering temperature to achieve required electrical good leveling property. However, Ag is too expensive to be functionalities, hence, sometimes limiting their use to heat- employed for low-cost applications. In some cases, the costs of sensitive substrates2,6. In most cases, sintering is carried out by production may be lowered with the application of Cu ink instead transferring the printed samples from the printer stage to curing of Ag due to the lower material cost. Although roll-to-roll electroless machinery adding extra processing steps, and causing alignment deposition of Cu metallic films has been reported by Eastman issues for the subsequent printed layers. Thus, in situ low- Kodak which is suitable for deposition on polyethylene terephtha- temperature sintering of printed NPs on the same printer stage is late (PET) substrates15. However, the use of Cu deteriorates the exciting which leads to advantages of eliminating alignment performance of the printed tracks because of the presence of the issues, using flexible substrates that may not withstand the high resistive CuOx.Thissignificantly increases the contact resistance and thermal budgets, reducing the processing steps and overall cost in shortens the product life due to the progressive oxidation of the Cu the device fabrication and opening up opportunities for roll-to-roll film2,10,16,17. Inkjet printable Au and Ni NP inks are corrosion inkjet printing possibilities. The important and decisive criteria for resistant with high electrical conductivity. Nevertheless, Au inks are the choice of suitable NP conductive inks for specific printed costly compared to the other metallic inks and they need an electronics applications are electrical conductivity, oxidation elevated sintering temperature (200–400 °C) which prevents their stability, costs, and ink formulation methods. Similarly, sintering use for heat-sensitive substrates. Recently inkjet printable Au NP conditions and processing steps are also determinative factors7. inks are commercially available that can be cured by photonic These parameters form the context for the present study. sintering process allowing usage on PET substrates, which is, 1RISE Research Institutes of Sweden, Norrköping, Sweden. 2Energy and Environment Science and Technology, Idaho National Laboratory, Idaho Falls, ID, United States. ✉ email: [email protected]; [email protected] Published in partnership with Nanjing Tech University S. Majee et al. 2 Fig. 1 Formulation and printing of Zn NP ink and curing ink. a Inkjet printing of Zn ink followed by curing ink on PET substrates. b Drop formations of the Zn ink and the curing ink from the nozzles, respectively. c The dimensionless Z values of Zn and curing inks in the Weber number vs Reynolds number plot. d Splat diameter histogram for Zn ink and the droplets on PET substrate (inset). e Variation of the Zn layer dry thickness with a number of printing layers (Inset: ink consumption vs drop-spacing estimation). Standard deviations for the thickness measurements are shown as error bars. Lines are guides for the eye. 1234567890():,; however, not an energy-efficient process18. While commercial Ni concentrated (~40 wt%) inkjet printable metallic Zn NP ink is inkjet ink is cheaper than Ag and Au inks, Ni NP inks often tend to reported in this study. The sintering method results in electrical agglomerate and clog the inkjet printheads7. Moreover, curing of Ni conductivity of ~105 Sm−1 for the printed Zn tracks which is NP ink requires either a higher temperature (>350 °C) or a photonic comparable with previously reported values20,23. The entire sintering method12. Therefore, formulation of inkjet printable process is suitable for thermosensitive substrates, cost-effective, alternative conductive metallic NP inks, such as Zn NP ink, is rapid, minimizes alignment issues in the printing of successive necessary that is inexpensive, curable at low temperature, and layers, shortens production steps, and most importantly is an highly conductive. The environmental footprint of Zn layers is energy-efficient method. The construction of an all-Zn printed negligible when comparing with Ag. Zn oxidizes under wet touch sensor circuit serves as a demonstration of the utilization of environmental conditions turning into harmless ZnO which is the the Zn NPs in prospective printed electronics applications. main component of paints, toothpaste, and many other daily used products. The chemical sintering method of metallic NP inks is an interesting non-conventional route to activate the coalescence of RESULTS AND DISCUSSION NPs. This method firstly allows us to cure the NPs at low Printing and curing of Zn NPs 19 temperature and secondly is an energy-efficient process . Room Figure 1a summarizes the process to obtain conductive Zn printed temperature electrochemical sintering of Zn NP ink by weak acidic features following a four-step process: (i) printing of Zn ink; (ii) treatment has been reported previously by Lee et al.20. Utilizing a fi 21 drying the lms to evaporate off the solvent: typically at 60 °C for similar mechanism Jayasayee et al. reported a cost-effective ~1 min is sufficient to remove a liquid carrier from the film and the method of preparing porous Zn electrodes for rechargeable zinc–air resulting film has sufficient mechanical stability and adhesion to batteries. In all these cases, a mixture of CH3COOH and distilled the substrate but no electrical functionally at this point; (iii) over- water (1:10 by volume) was drop-casted either on screen-printed Zn printing of the curing ink on the dried Zn films and finally (iv) ink or on Zn slurry to enhance the electrical conductivity and other drying the films which give rise to shinny metallic color and fi desired functionality. While those reports give a detailed scienti c electrical conductivity of the films depending upon the film insight into the sintering mechanism, however, the described thickness. sintering technique (drop-casting) is unsuitable for large area device Formulating inkjet printable ink of nanomaterials is very manufacturing
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