The improved graphene inks are more than 85% transparent to 550 nm wavelength light, a value that is 45% higher than that of films which have not been laser annealed. They also have a sheet resistance of around 30 KΩ/cm, which is 70% lower than that of unannealed films. They are much smoother than unannealed films too.

Graphene is a 2D sheet of carbon with unique electronic and mechanical properties and dispersions of the material could be ideal as inks for making printed electronics devices and surface coatings. And transparent conductors made from graphene dispersions might be as good as those traditionally made from metallic oxide thin films, while having the adding advantage of being flexible and chemically stable. Graphene is also good in this respect because it absorbs light over a broad range of light wavelengths.

Uniform, transparent and conductive graphene thin films

Researchers have already made transparent graphene conducting films before now using inkjet printing, vacuum filtration and spray coating. A team led by Max Lemme of the Institute of Graphene-based Nanotechnology at the University of Siegen in Germany, has now succeeded in producing uniform, transparent and conductive graphene thin films by simply drop-casting dispersions of the carbon sheet onto a glass surface and combining this drop-casting step with laser annealing. “The annealing part involves scanning a laser beam across the surface of the films, which distinctly improves their transparency and how well they conduct electricity,” says Lemme.

The researchers obtained their graphene inks using a solvent exchange technique that was developed by team member Jiantong Li of the KTH-Royal Institute of Technology in Kista, Sweden. They began by exfoliating (or shaving off) graphite powders in dimethylformamide (DMF) and then exchanged the DMF for terpineol to increase the amount of graphene in the inks and adjust how viscous they were. The graphene concentration in the final inks is as high as around 1 mg/L.

Laser scanning

To form multilayer graphene films, Lemme and colleagues dispersed the graphene inks by drop-casting 1μL droplets onto a glass slide. After drying the inks, they baked the films at 400 °C for 30 minutes to remove the stabilizing polymer (this step is crucial for preventing the graphene flakes from aggregating during drying). Finally, the team scanned the films with a continuous wave 500 mW, 532 nm laser beam during 1–4 milliseconds over a spot size of 1 µm. The laser system was also set up to characterize the sample in situ during the processes (albeit at a lower laser power).

“There are currently a number of potential applications envisioned for such flexible graphene films,” says Lemme. “In the energy storage sector, for example, we can cite batteries, supercapacitors and hybrid batteries/supercapacitors. Other possibilities include sensors and printed thin film transistors.”

The team says that it is now busy looking at inks made from other 2D materials. “We are also experimenting with different lasers to speed up processing times,” he tells nanotechweb.org.

The research is detailed in 2D Materials doi:10.1088/2053-1583/2/1/011003.