Soft circuits — circuits based on flexible substrates — have been a hot topic for years now, in fields from medicine to consumer electronics. Graphene would be a nearly perfect material to use for these circuits, if only we could apply it effectively. While it’s extremely conductive, it’s flaky and difficult to manufacture at scale.
Unlike metal wiring, which you can melt and deposit, graphene doesn’t really melt. Most conductive ink or paint is made with finely powdered metal. When it dries, the metal particles are close enough to pass through a limited amount of current. Graphene sheets don’t self-assemble from a slurry of carbon atoms in solution, though, and if you try to start with big graphene sheets in solution, they tend to clog nozzles. Worse, they don’t dry flat, and that means a tiny conductive cross-section, high resistance, and terrible performance. Is Scotch tape really our best option for making graphene sheets?
There has to be a better way. A team of Chinese researchers just offered their take on the problem. They’ve come up with a way to print soft circuits using graphene ink — and the method keeps the graphene flakes in the same plane, making the printed graphene traces much more effective.
Graphene’s high conductivity makes it an attractive component of soft circuits, because a high-conductivity wire can be used to make circuits that work with a low driving voltage. This characteristic is also why graphene remains, thus far, tantalizingly out of reach as a semiconductor. Graphene isn’t really a semiconductor. It’s actually a fabulous conductor. Graphene has little to no band gap, which means that electrons don’t get bogged down in the system; they flow in an unrestricted manner through the flake, from source to sink.
When flakes of graphene are out of plane with one another, there’s just a tiny area for current to pass through. The researchers printed traces using graphene ink laid down through a bent needle. The bend nudged the flakes into coplanar alignment, so they all laid down flat on their mica or silicon substrates. The conductivity the researchers reported was among the highest ever confirmed for printed circuits.
But the electrical properties of graphene change depending on the size of the graphene flake. Graphene is hard to produce in big, contiguous molecules in the first place. In theory, graphene is a 2D molecule of indefinite size. In practice, though, it’s an achievement to get a single graphene flake visible to the naked eye. And the graphene will behave differently depending on which edge of the lattice is exposed: the “zigzag” edge, or the “armchair” edge.
See the top edge of the graphene nanoribbons in the photo to the right? That’s an “armchair” edge. (While the ribbon at right is doped with boron, that doesn’t change the configuration of atoms at the edge of the ribbon.) Like the rocky bank of a fast-flowing river, there’s turbulence around the edges of the graphene ribbon, even though the undulations are regularly spaced. If there’s too much turbulence in proportion to the smooth, laminar flow of electrons through the center of the graphene ribbon, things clog up and slow down again.
That’s not the only reason to try for larger contiguous pieces of graphene. Graphene’s planar structure makes it very reactive, since its atoms are accessible from both sides. Breaks in the lattice expose even more atoms to even more angles of attack. That means smaller flakes are still more reactive — and more susceptible to degradation. Graphene won’t be much use as a conductor, whether super, semi, or otherwise, if it breaks down too fast to be cost-effective.
The ability to print soft graphene circuits at a consistent conductivity doesn’t solve all of the problems related to graphene production, and it’s only useful in industries where flexible materials are themselves a suitable substrate. Despite these limitations, research like this is important to the long-term goal of creating graphene in useful quantities, and at sufficiently high quality, to allow for mass manufacturing.