Transparent and electrically-conductive flexible composites.
Why should we care? Why do we need transparent and electrically-conductive flexible composites?
These are materials that conduct electricity, so they can be used for any specific application where power and actuation are involved, for example in an electrical circuit. They are at the same time transparent and see-through, enabling the passage of light. So we are talking an electrical circuit you could put on your window.
They are flexible so it could be around a light bulb without impairing its inner electrical conductivity nor the lightening of the room.
Finally, they are composites, meaning that they are composed of a least two different materials. Composites are primarily developed and used as replacement of more expansive materials, for the same, if not superior, properties: polymers with carbon fibers are now replacing heavy and costly metallic parts in cars and planes.
But we do not necessarily need light bulbs with electrical circuits around. (Or maybe but we don’t know it yet). However, if the light is outside and if the bulb is your greenhouse, electrically-conductive, flexible and transparent composites are very interesting. They could replace the expensive and polluting Indium-Tin-oxide brittle layer commonly used in solar cells. These are also very polluting because Indium is a rare metal and thus are expensive too and their brittleness not only reduces their life-time but also their potential applications beyond solar cells. What about wearable electronics, what about sensors directly attached to the skin to monitor specific medical treatments? With flexible functional composites that are at the same time transparent and electrically conductive, not only the cost for solar-cells could be reduced, but it would be beneficial for the environment by permitting recycling, elongating the life-time of solar-cells and diversifying their applications.
The challenge behind being electrically-conductive and transparent at the same time is that usually materials that conduct electricity are metals. And they are opaque. Also, if they can be shaped easily into various forms, metals are not so conformable. The idea is thus to use a polymer (or a “plastic”) that is transparent and flexible, and to incorporate in it conductive particles that transport electricity. Two different materials: we are making a composite. This strategy has already been adopted successfully but our addition is that we want to use graphene particles and put them at specific positions in the polymer.
Graphene is what you obtain with a scotch tape and a tip of a pencil: it is one single layer of carbon atoms assembled at the corners of hexagons. At the tip of your pencil, you have graphite, which is just layers of graphene stacked together through atomic weak interactions. Simply using scotch tape, you can remove one tiny layer of graphene. The problem with graphene is that since it is only made of carbon, it doesn’t like water and is difficult to mix as such. Therefore, graphene flakes (we talk about flakes because the hexagons form a small tiny sheet) recombine to form graphite again if they are in a situation where they don’t like to be separated, like in water. Except in very dilute situation and with very toxic solvents, it is very difficult to keep all the graphene flakes apart from each other. Unfortunately, graphite is black.
But graphene can be oxidized and made to like water. In that case, it is possible to attach or simply “glue” other soap-like chemicals or particles at the surface, such as proteins and iron oxide nanoparticles. In the oxidized form, graphene has mediocre electrical response. The complex graphene oxide-protein–iron oxide can nevertheless be turned back to graphene-protein-iron oxide by a reduction step. The graphene still doesn’t like water but since the protein remains attached to its surface, this is not a problem anymore: the proteins are shielding the surface.
We now have iron oxide nanoparticles attached to the surface of the graphene. The graphene is thus magnetic and we can use some tricks to attract the graphene, our conductive elements, on specific pathways. For example, simply using permanent fridge magnets, it is possible to concentrate the graphene-complex on a line that can replace an electric wire. Or it can be concentrated onto complex patterns to reproduce complex electrical circuit. By doing so, not only we can localize the conductivity but also increase the amount of graphene in these fictive wires and therefore reduce the amount of graphene in the rest of the polymeric matrix. This prevents the matrix from being black if eventual restacking of graphene takes place, and it also prevents the matrix from turning brittle. Adding hard particles into a soft matrix inevitably leads to an increase of the stiffness of the matrix. This is governed by the “rule of mixture” which is simply the average between the stiffness of all the particles and of the matrix. Reducing the concentration of graphene within some parts of the matrix permits the “recovery” of matrix-like properties in these areas.
In conclusion, by creating a composite of polymers and conductive particles, and by modifying those particles with iron oxide, it is possible to use magnetic fields to position and concentrate the electrically-conductive elements along as-designed specific paths. This generates flexible and optically transparent electrical circuits for multiple usages. The beauty of the strategy described here is that it can be applied for various matrices and particles, thus enabling the addition of other functionalities such as biodegradation or thermal conductivity.
H. Le Ferrand, S. Bolisetty, A. Demirors, R. Libanori, A.R. Studart, R. Mezzenga, Magnetic assembly of transparent and conducting graphene-based functional composites, Nature Communications 7 (2016), 12078.