Naturally formed graphene has been discovered for the first time, in 3.2-billion-year old rocks underneath a South African gold mine. The find could lead to more energy-efficient ways to produce the material, which has a number of useful electronic properties.
Graphene, a one-atom-thick sheet of carbon, was first discovered in 2004 by scientists using sticky tape to take layers off pencil graphite, but scaling up its production has proved difficult. Current industrial methods to make large quantities of graphene require temperatures exceeding 1000掳C (1800掳F).
Now, at Hokkaido University in Japan and her colleagues have found natural graphene that appears to have been formed in a gold-rich region of South Africa known as the Barberton Greenstone Belt, in conditions with temperatures less than 300掳C.
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鈥淚f we can understand the formation process at lower temperatures, it will be much better for saving energy,鈥� says Ohtomo, who presented the work at Goldschmidt geochemistry conference in Lyon, France, on 10 July.
Ohtomo and her team took 24 samples of rock from the Sheba gold mine in South Africa and analysed them using an electron microscope. They found unusual and rare carbon structures, including thin carbon filaments of a few micrometres long and flakes of a few micrometres across. The graphene appears to form a thin film around larger crystal nanoparticles embedded in the rock.
The researchers also looked at the sample using a spectroscope to measure the levels of different carbon isotopes. Isotopes are atoms of the same element that differ by the amount of neutrons they contain, which can be used to track how a material was formed. They found elevated levels of a particular carbon isotope that has biological origins. Ohtomo says this suggests the carbon must have come from bacteria near the ocean surface that died and fell to the sea floor.
This detritus, after settling, would have undergone chemical reactions with byproducts of iron hydroxide in the ocean to form carbon-containing compounds. It then would have been subject to high pressure and elevated temperatures, eventually turning it into exotic forms of carbon.
The exact structure and form of the graphene-containing structures still have to be worked out, says Ohtomo, as they have unusual properties. For example, the graphene film appears to be transparent, rather than black as it usually appears under a microscope, which suggests it has other elements in its structure.
