Synthesis, formulation and printing of nanoparticle-containing inks for electronics applications

Abstract

Printing of conductive ink based on silver nanoparticles (Ag NPs) can be extensively applied for various applications in printed electronics (PE) and flexible electronic (FE) due to its high conductivity. The work presented in this study concentrates on the synthesis and characterization of NPs, ink formulation for inkjet printing technology, printing of Ag NPs inks on different substrates (photo paper and modified plastic substrates polyethylene terephthalate (PET) or polyimide (PI)) and investigates the effect of spontaneous room temperature sintering on the microstructure and electrical properties of the printed films. In the present study, a number of methods have been proposed and tested for this purpose. Silver nanoparticles were successfully synthesized with a reasonable nano size about 4 nm. However, some of those synthesis methods did not produce nanoparticles with the desired size or narrow size dispersity, which caused serious problems in size distribution and the quality of ink formulation. AgNO3 was reduced in ethylene glycol (EG) or sodium borohydride (NaBH4) in the presence of a polymer, which was acting as capping and dispersing agent, such as polyacrylic acid (PAA), polyvinyl pyrrolidone (PVP) or polyvinyl alcohol (PVOH), to ensure the stability of the Ag NPs solution. The prepared Ag NPs were characterized using various experimental techniques: dynamic light scattering (DLS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA ) and these finding were then utilized to develop a stable aqueous printable ink formulation. Printing materials cost effectively, with adequate conductivity and printability, has been attracting plenty of researchers and scientists as an alternative to using pure silver nanoparticles, due to their relatively high price from a commercial perspective. Therefore, using nickel or copper combined with silver in nanoparticles synthesis was tested, using core- shell and alloy configurations, since they are the most promising candidates for use as a conductive material as the replacement of Ag. Promising results were achieved, especially from the printing of copper in a core-shell system, which had a good particle distribution and achieved our goal while being more economical (at least in material cost – the increased synthetic cost would need to be evaluated). Both nickel and copper ii nano inks are unstable, however, and are prone to oxidizing during the synthesis process. They usually require a reductive atmosphere or a special sintering method to ensure good conductivity. This contrasts with silver ink, where Ag does not require additional care during its formulation as it is chemically relatively stable, which is an advantage. Conductive inks with 10, 20 and 30 wt.% silver content were formulated with the Ag NPs capped with PAA, PVP, or PVOH by using a combination of simple solvent mixtures. Similarly, the same method was applied to the alloy ink. However, for the synthesized copper–silver core-shell nanoparticles, due to the hydrophobic surface capping ligands, 10 wt% inks were formulated by dispersing these in a non-polar (xylene) solvent using probe sonication for 30 minutes to accomplish the formulation of the conductive ink. The above formulations were then ready to print by using a Jet Lab 4 inkjet printer, with different nozzles (size 80 or 50 µm) on a suitable substrate. This approach offered simple, flexible, low cost and rapid prototyping alternatives, compared with other conventional printing techniques. The printed conductive ink is able to sinter at room temperature and that makes it possible to print onto a flexible paper or even a plastic substrate in order to get conductive patterns. The ink stored under ambient conditions was stable against aggregation for more than six months. The investigation of the printed tracks was carried out using different techniques such as optical microscopy and SEM for the quality and morphology of the printed silver layers, profilometry for thickness measurements, and a Beha Amprobe (Glotteral, Germany) Digital Multimeter for the electrical properties. The mechanical stability of tracks was also evaluated using a custom-designed bending rig. The results showed that in the case of Ag/PAA and Ag/PVP ink, excellent conductivity was observed, and the performance approaches a remarkable resistivity of only 2 times that of bulk silver. Using Ag Ni alloy-based ink, we obtained better resistivity compared with Ag @Ni CS NPs ink. For alloy inks, the lowest resistivity of (Ag 75: Ni 25), (Ag 40: Ni 60) w/w% inks were 2 times and 29 times bulk nickel, respectively after printing 5 layers on photo paper, resulting in spontaneous room temperature sintering. The formulated ink from the as iii synthesized Cu–Ag core–shell nanoparticles was printed onto a PVOH coated PET substrate using an inkjet printer. The film was sintered using chemical sintering, since the strongly bound hydrophobic capping ligands prevented spontaneous Cl-based sintering. The lowest resistivity of the Cu–Ag core–shell conductive inks was approximately 129 times the resistivity of bulk Cu. The second part of this thesis examines how a plastic substrate surface (PET or PI) can be developed by applying a PVOH coating formula with different Cl- concentrations, in order to improve its surface properties to make it suitable for inkjet printing and room temperature sintering. The printed silver patterns showed great mechanical stability on both plastic and paper substrates, in tensile and compressive mode bending after 500 bending cycles on the plastic substrate and up to 1000 bending cycles on photo paper, suggesting it may be suitable for flexible electronic applications. This work potentially provides a promising route toward the large-scale fabrication of low cost yet flexible printed electronic devices. Having successfully printed and investigated these materials, the nano silver ink was employed to fabricate a prototype pattern for a simple resistive temperature sensor device. This was generated and tested across a range of temperatures (20 -130 °C). The conducted tests demonstrate that the Ag printed sensor on photo paper, PET/PVOH and PI/PVOH was altered after being exposed to the oven heat. Generally, they showed good linear response range/resistance behaviour. However, the pre-heated devices that were printed on PI/PET were transformed into excellent devices by heating them again and showed much lower resistance. After heat treatment, the temperature sensor printed on PI/ PET showed clearly some cracks and voids formed within the track. This is attributed to the instability of the coated PVOH layer over the substrate under the printed layer (because it is chemically altered when heated to high temp) so that actually, the temperature sensor is still only stable to about 100 °C, even though the base substrates are stable at a much higher temperature.