To mass-produce technologies that exploit graphene’s exciting electronic and optical properties, manufacturers will need to make large, high-quality films quickly. Turns out, fluorine can help. The gas speeds up the growth of graphene to more than 1,000 times as fast as without, researchers report (Nat. Chem. 2019, DOI: 10.1038/s41557-019-0290-1).
“The recorded growth rate of 1.2 mm per min could, in principle, allow us to grow a very large graphene wafer in less than 10 min,” says Feng Ding of the Institute for Basic Science in Ulsan, South Korea. Fluorine can also significantly accelerate the growth of two other promising 2-D electronic materials, tungsten disulfide and hexagonal boron nitride (h-BN).
Researchers usually make graphene with chemical vapor deposition (CVD), which involves injecting methane vapors into a heated vacuum chamber, where the gas decomposes and deposits as graphene on copper foil. Recently, researchers have found that adding oxygen or hydrogen into the reactor helps fine-tune graphene growth, speeding it up and helping create high-quality single-crystal graphene sheets.
Fluorine, which is highly reactive and used to trigger, direct, and accelerate many other chemical reactions, also holds promise for controlling graphene growth. But injecting it directly into CVD reactors is a challenge because it can corrode the vessels and form toxic hydrogen fluoride.
A team led by Ding, Jie Xiong of the University of Electronic Science and Technology of China, and Kaihui Liu of Peking University, devised a handy, safe way to introduce fluorine. They place an ultrathin film of barium fluoride about 10 µm under the copper foil. When heated, the film releases a small amount of fluorine that mostly stays in the narrow space between the barium fluoride film and the copper foil, aiding the graphene-forming reaction. But the concentration of fluorine in the rest of the reactor is too low to cause corrosion.
Fluorine reacts with methane to form fluoromethane, Ding explains. Although the decomposition of methane on copper is relatively slow because it is endothermic, the breakdown of fluoromethane is exothermic, “which speeds up the decomposition and leads to a fast releasing of carbon atoms for graphene growth.” In less than a minute, the side of the foil exposed to fluorine had 1 mm wide circular sheets of graphene while the other side without the fluorine had tiny star-like graphene flakes that were about 20 µm in size.
The researchers also tested the method to see how it affects the CVD growth of h-BN and WS2. Fluorine sped up both reactions, with h-BN samples growing 10 times as fast and WS2 growing 100 times as fast.
“The approach of introducing localized fluorine utilizing the decomposition of metal fluorides is brilliant,” says Dong Wang of the Chinese Academy of Sciences. Contamination of the 2-D materials with fluorine could be a risk on a large scale since fluorine is reactive, so researchers will need to carefully monitor sample quality. Nevertheless, this technique is an important advance, he says, and “may inspire other interesting ways to modulate 2-D materials growth.”