"We succeeded in scaling the thin-film deposition of chemically derived graphene to 12 inch wafer size," team leader Manish Chhowalla told nanotechweb.org. "Previously, deposition was only demonstrated on small (a few centimetres across) substrates."

Graphene oxide films are transparent and their electrical properties can be tuned from semiconducting to metallic by controlling their thickness. These and other interesting optoelectronic properties could be exploited by fabricating devices on various platforms.

Easy transfer
The films produced by Chhowalla's team are highly uniform over large areas and atomically thin – something that was confirmed by Raman spectroscopy and atomic force microscopy measurements. They are deposited by spin coating a concentrated aqueous solution of graphene oxide onto hydroxylated surfaces of Si/SiO2 while controlling the evaporation rate of the solvent used. The films can then be transferred onto any other substrate, or left freestanding if wished.

And that's not all: the researchers can control the film thickness, which varies from 1–2 layers to up to 30 layers, by simply changing the spin-coating speed, graphene oxide concentration or number of deposition cycles. "Because the thickness of the material determines its optoelectronic properties, our method allows us to tune these properties in a simple way," said Chhowalla.

The films produced are electrically active and have a sheet resistance of about 1 kΩ/sq. Field-effect transistors made using the films show high mobilities of around 15 cm2/(V s) and are approximately 70% transparent.

According to the team, the material could be used in transparent electrodes for optoelectronic devices, such as solar cells and light-emitting diodes. However, it must be improved further before it can compete with conventional transparent conductors, like indium tin oxide, in terms of performance.

Chhowalla and colleagues now hope to improve the transparent and conducting properties of the films by carefully tuning the conditions under which graphene oxide is reduced.

The results were published in ACS Nano.