Using printing processes on paper or foil allows flexible circuits to be produced cheaply and enables futuristic designs with curved input elements or diodes. To achieve this, printable electronic materials that keep high levels of conductivity while being used, in spite of their curved surfaces, are needed. The new type of hybrid ink allows for example, electronic circuits to be applied directly from a pen to paper. No further processing is required and they are usable after drying.
The researchers combined the benefits of metallic nanoparticles and polymers to create their hybrid inks. Silver or gold nanoparticles are coated with conductive organic polymers. These nanoparticles are then suspended in mixtures of alcohol and water.
Kraus, a materials scientist, explained that electronics circuits are already printed today with metal nanoparticles with ligands. A sintering process was however required to remove the shells. Although the arrangement of the nanoparticles was controlled by the shells, they hamper conductivity. When carrier materials such as polymer or paper films are sensitive to temperature, it created difficulties, as these would be damaged during the sintering process. Kraus added that the new hybrid inks are mechanically flexible, conductive in the as-dried state and do not require sintering.
The organic compounds have three functions in the hybrid inks.
- They serve as ligands, keeping the nanoparticles suspended in the liquid mixture. Any cluster of particles would affect the printing process negatively.
- The organic ligands also ensure that while drying, the nanoparticles have a good arrangement.
- The organic compounds ultimately act as ´hinges´ by ensuring that the electrical conductivity is maintained if the material is bent. The electrical conductivity would be lost quickly on bending in a layer of metal particles that doesn’t have the polymer sheath.
Because both materials are combined, the electrical conductivity when bent is greater than in a layer made solely from metal nanoparticles, or a layer that is made from only conductive polymer.