S. Hamacher, B. Bachmann, A. Yakushenko
Keywords: low sintering temperature, gold conductive ink, inkjet printing
Summary:Inkjet printing is a widely used and a well-known technique, which has been recently used for deposition of functional materials, such as conductive inks for the fabrication of low-cost electronic systems. Nowadays, a variety of conductive inks (e.g. silver, gold and copper inks) are commercially available. While copper is a relatively cheap material, it has the disadvantage of being prone to oxidation in air, thereby, changing its conductive properties over time. On the other hand, silver is less susceptible to oxidation, however, the electric properties of printed structures are usually also altered upon formation of silver oxides. Additionally, silver is significantly more expensive than copper. While bearing an even higher price tag than silver, gold has the advantage of being resistant to oxidation and therefore, being useful for printed functional systems, in which electrode oxidation might pose an issue. Such applications include devices exposed to harsh environments, such as wearables that can see high salt concentrations from sweat or electrochemical sensors. In these cases, noble gold often serves as a standard electrode material. The down side of currently available commercial gold inks is the high sintering temperature required for good conductivity. As opposed to common silver inks having a curing temperature of 120-150 °C, available gold inks require curing temperatures of >190 °C. Such high curing temperatures limit the application to substrates with a glass transition temperature >200°C, i.e. leaving out many of the common cost-saving polymeric substrates. In this context, a functional gold nanoparticle based conductive ink with an extremely low sintering temperature that matches that of standard silver inks was developed. The ink is based on a suspension of gold nanoparticles, produced with a Brust-Schiffrin method, in a mixture of organic solvents. The sintering temperature mainly depends on the size of the nanomaterial and the capping agent. Here, nanoparticles stabilized with branched thiols with diameters of 2- 3 nm were synthesized. Owing to a small size and a specific capping agent, the sintering temperature could be decreased to 120°C, which is 70-100 °C lower than any known gold ink. Most importantly, it is sufficiently low for printing on low-cost polymer substrates used in printed electronics. The amount of gold nanoparticles within the ink is varied between 25-30 wt%, and this results in conductivities 7-50 times less than that of bulk gold. The ink can be printed with Dimatix cartridges and has a latency time of 5 min at 1 kHz, which promises a stable and consistently printable ink. Moreover, when using this ink, it is possible to print very small and defined structures with line widths of 40 µm and thicknesses of 70 nm. The aforementioned properties of the functional gold ink, especially the low curing temperature and the fact that the ink is not prone to oxidation, render the material interesting for several applications, where oxidation processes may take place and flexible low-cost substrates are required. This is true for wearables in contact with the human body, as well as electrochemical sensors used in point-of-care diagnostics.