Nanomaterials For Conductive Inks In Printed Electronics
With an estimated $300 (€200) billion market value over the next 20 years. Printed Electronics (PE) emerges as an innovative and cheaper alternative to traditional production methods, like lithography or vacuum deposition, which require expensive equipment and the use of toxic chemical. In contrast to these multistaged, expensive, and wasteful methods, PE offers a rapid, cheap and eco-friendly way of producing circuitry with commodity printers and off-the-shelf materials.
Various methods of PE production exist, among these methods, digital inkjet printing is an emerging technique, with the advantage of being digital and therefore very attractive for small batches and very short lead times as it facilitates initial explorations of various aspects in a laboratory setting, on account of its minimal waste generation and efficient handling of expensive materials. As a non-contact, additive patterning approach, inkjet printing allows for deposition of versatile thin films, the designs of which can be changed with ease from batch to batch. An attractive aspect of inkjet printing is its scalability to large area manufacturing, that enable roll-2-roll (R2R) production.
Conductive Inks Inkjet printing requires very specific inks. These inks are made of a conducting nanomaterial dispersed in an aqueous or organic solution. These nanomaterial range from metal nanoparticles (NP) too more recently carbon based inks using graphene or carbon nanotubes (CNT). Metal Based InksThe main factors in choosing which metal to use for the inks are related bulk resistivity, price and ease of handling. Being that an ideal metal would be cheap, easy to use and have a low resistivity post-processing. Nowadays the most used and studied NP are silver based nanoparticles, as they achieve a balance between all three requirements despite it still being an expensive material. The replacement of silver based nanoparticles still poses a challenge, as cheaper noble metal like copper of aluminum are still susceptible to oxidation. M. Grouchko et al. present an alternative by encapsulating copper nanoparticles inside silver shells. The approach appears to be stable as no oxides were found to have formed on the inkjet printed copper patterns after several months.
Another challenge in using metal nanoparticles is the need for a sintering step, which is required to obtain a continuous metallic phase, as these inks contain stabilizing agents and several other organic components that prevent a good conductivity. The sintering step usually involves heat treatment, however in case of a heat sensitive substrate like paper or plastic severe damage to the substrate can occur as seen in Figure 1, a non-destructive sintering method is therefore required. Perelaer et al. report using microwaves as an alternative to traditional sintering methods and achieving success in their experiment.
Carbon Based InksCarbon based nanomaterials, in the form of graphene or carbon nanotubes [CNT], present many desirable attributes for printed electronics due to their advantageous optical, and mechanical properties and having an electrical conductivity close to that of metals, as so they can provide a good alternative to metal based inks. However, there are still a number of challenges when using these material, as when using CNTs, due to their large surface area, wan der Waals forces can cause them to bundle together and form large ropes, which may cause clogging of the inkjet print heads. It has also been reported, by Stahl et al. , that for these aggregated CNTs, electric current flows only on the outermost tubes in the bundle, observing almost no contribution from the inner tubes to the conductivity.
