Research published in Nature Nanotechnology shows that diamond and graphite are just a few chemical reactions apart. A research team led by Rodney S. Ruoff of Ulsan National Institute of Science and Technology in South Korea has demonstrated conversion of large-area bilayer graphene to F-diamane—an ultrathin, fluorinated diamond-like material—under non-extreme temperature and pressure conditions upon fluorination.

“The conversion from bilayer AB-stacked graphene to the thinnest possible diamond-like material is indeed possible without using extreme conditions, like high pressure,” says Pavel Bakharev, a research fellow in physics at the Center for Multidimensional Carbon Materials (CMCM) at the Institute for Basic Science in Ulsan, South Korea, and lead author of the findings. “[There have been] multiple theoretical papers on this conversion, but until now, it [has never been shown] experimentally.”

Previously, researchers managed to convert very small regions of bilayer graphene into diamond-like two-dimensional (2D) material, but only by applying very high pressure in a diamond anvil cell or by using an atomic force microscopy tip at the micro- or nanoscale, respectively. As soon as the high pressure was removed, the films converted back to the original bilayer graphene phase. Other attempts at such conversions used hydrogenation through gas activation by plasma and hot filament chemical vapor deposition, but the results, Ruoff says, “don’t provide any evidence of having produced diamane.”

Theoretical studies have suggested that it is possible to create a stable, diamond-like film without applying extreme conditions. But until now, these ideas have remained untested. “We were inspired by these papers, but there are issues in our area of materials science about what can be theoretically modeled and what can actually be made in the lab,” says Ruoff. “There are many interesting questions about actually doing the chemistry.”  

Ruoff and his colleagues had previously succeeded at making large-area single-crystal metal foils, including a Cu(111) version. Subsequent work led to electroplating nickel onto the Cu(111) foil and annealing it to yield perfect single crystal Cu/Ni(111) foils. In this latest work, the researchers used their single crystal CuNi(111) foil to grow high quality (AB)-stacked bilayer graphene over a large area. The team decided to try to obtain diamane through fluorination of this bilayer graphene rather than hydrogenation because fluorine can be more easily characterized than hydrogen. Bakharev realized that high energy resolution x-ray photoelectron spectroscopy (XPS) can be used for attempting to identify fluorinated diamane as a way of elucidating the formation of interlayer C-C bonding and for observing the peak regions associated with that bonding, which are in a different region of the spectrum from the C-F peak. This was helpful in that the XPS could be used as the primary method for determining that interlayer bonding was occurring. It also allowed the researchers to get at the stoichiometry of C₂F, or that the top and bottom layers of the AB-stacked graphene were fluorinated. Had they used hydrogenation, it would not have been possible to use XPS to make this distinction, because in the critical region of the spectrum, the peak for C-H lies essentially on top of the peak for interlayer C-C bonding. The peaks, in other words, are too overlapped to distinguish the types of chemical bonds by XPS. 

“For hydrogen, it’s very hard, if at all possible, to get the stoichiometry of the base materials to prove that you really obtained this material,” Bakharev says. “Fluorine allows us to really see the material.”

The fluorination system the team built allowed them to control pressure, temperature, and duration of exposure, which they optimized to obtain F-diamane. Notably, they made the F-diamane at low pressure and also near room temperature with a 12-hour exposure. The resulting several square centimeter film—which they characterized through electron microscopy, optical spectroscopy, and other techniques—is composed of a single-layer, highly sp³-bonded diamond-like 2D film that contains carbon atoms from the two different layers of the bilayer graphene film used to make it. “Diamond has also been functionalized to have perfect termination with fluorine at its surface, but of course that is a bulk material,” Ruoff says.

“Professor Ruoff’s group was able to remove all previous hurdles and has shown that just by flowing XeF₂ gas over bilayer graphene for longer hours, it is possible to force carbon atoms in bilayer graphene to bond with fluorine atoms in an sp³ configuration, which is remarkable,” says Anirudha Sumant, a materials scientist at Argonne National Laboratory, who was not involved in the study. “This opens the next door to be able to test electronic, mechanical, and optical properties of 2D diamane and explore its application potential in nanoelectronics.”

Boris Yakobson, a professor of materials science, nano-enigneering, and chemistry at Rice University, who was also not involved in the new research, agrees that diamane’s creation “is quite important and promising.” He hopes to see further research creating a material with greater thickness, including up to 10 layers. “To attempt this in an experiment may prove difficult, but time will show,” Yakobson says.

The researchers believe the diamond film could eventually find applications in novel electronic devices. “Diamond is just a mechanically superb material if one can make it in the right way,” Ruoff says. He and his colleagues were able, for example, to use an electron beam to defunctionalize local regions of the film to convert them back to bilayer graphene, paving the way for using diamond-like films as electrodes. This also opens up possibilities for its use as the active region in electronic devices such as field-effect transistors, in which the neighboring bilayer graphene regions could be the electrical contacts and the F-diamane the “active region.”

Sumant adds that if the diamane layer is stable enough, then it could have potential use as a seeding material for growing single-crystal diamond over a large area. “That would be another major breakthrough that will impact [the fields of] MEMS [microelectromechanical systems], quantum computing and the high-power electronics industry in a big way,” he says. Bakharev agrees that the team is interested in exploring a tri-layer film, as well as using the bilayer as a substrate for growing diamane vertically, to try to obtain thicker diamond films.

As a next step, Bakharev, Ruoff and their colleagues plan to work further on stabilizing the material, which, for now, can be easily defunctionalized when exposed to air. “Once fluorine detaches locally, the diamane converts back to the most energetically stable configuration without functional groups, which is the bilayer graphene,” Bakharev says.

“What’s exciting about this is we’re sort of piggy-backing on the worldwide graphene effort,” Ruoff says. This latest work, which required a series of steps and collaboration that spanned more than four years, “is just one wonderful example of how, if scientists are given time and proper tools and levels of support, we can deliver some very important fundamental achievements.”

Read the abstract in Nature Nanotechnology.