It is well known that graphene is a two-dimensional form of carbon. So, the question arises might materials scientists be able to convert these graphite-like monolayers into synthetic diamond layers? We might dub such a material, by analogy with graphite and graphene, diamane. [Bakharev, P.V., et al. Nature Nanotechnol. (2019); DOI: 10.1038/s41565-019-0582-z]

Researchers in South Korea have now made the first experimental observation of a chemically induced conversion of large-area bilayer graphene to the thinnest possible diamond-like material, under moderate conditions of pressure and temperature. This resulting material is both flexible and strong and has a potentially useful wide band-gap in terms of its semiconductor properties.

There have been earlier efforts to convert double and multiple layers of graphene into diamane but these generally involved high-pressure conditions or the addition of hydrogen. The results were not entirely controllable and characterization was difficult. Moreover, in the high pressure experiments the diamane reverted to graphene once the pressure was released from the system.

The team from South Korea, based at the Institute for Basic Science in Seoul, used chemical vapor deposition to create a graphene bilayer on a cupro-nickel foil. They then exposed this to xenon difluoride gas instead of hydrogen, which triggered the formation of a diamane-like substance. The resulting product is an ultra-thin diamond-like material, specifically a fluorinated diamond monolayer, F-diamane. Rather than being pure carbon there are fluorine atoms between the layers of carbon atoms as well as on the material's external surfaces. Spectroscopy and transmission electron microscopy, reveal the nature of the new material in detail and show that the synthesis is reproducible always giving an interlayer space between the graphene sheets of between 1.93 and 2.18 angstroms; as predicted by theoretical calculations on the material.

"This simple fluorination method works at near-room temperature and under low pressure without the use of plasma or any gas activation mechanisms, hence reduces the possibility of creating defects," explains first author Pavel Bakharev.

The team also demonstrated that their F-diamane film can be suspended freely. They were able to obtain a free-standing monolayer diamond by transferring F-diamane from the CuNi(111) substrate to the transmission electron microscope grid.

The potential of these ultrathin diamond films will arise through their unique electronic and mechanical properties. The properties might be tuned by changing the surface terminations using nanoscale patterning or chemical substitution reactions that swap fluorine atoms for functional chemical groups.