Apr 30, 2012
Fluorinating graphene the easy way
A new way to functionalize graphene using a solid fluorine source has been developed by researchers at the University of Texas at Austin in the US. The technique is a much more environmentally friendly and non-toxic process compared with conventional fluorination methods that rely on gaseous fluorine. Importantly, it can also be used to pattern the graphene surface in a single step, something that makes subsequent integration of the fluorinated material into fully functional devices easier.
Functionalizing pristine graphene is a good way to modify its electronic band structure – for example, by doping and bandgap opening. Normally, gaseous elements at high temperatures are used to synthesize halogenated graphene in a process that attaches fluorine or chlorine atoms to the basal plane of the material. However, there is a problem with such techniques because high-temperature plasma can damage the graphene lattice by ion bombardment.
Now, Rodney Ruoff and colleagues have overcome this problem by coming up with a new way to fluorinate graphene using a solid fluorine source and laser irradiation.
The researchers began by transferring CVD-grown monolayer graphene onto a silicon dioxide/silicon substrate. They then coated the fluoropolymer, CYTOP, onto the graphene surface. Finally, the ensemble was irradiated with light from a laser to pattern the material. The laser decomposes CYTOP into active intermediates, such as CFx and F radicals, which then react with sp2-hybridized graphene to form C-F sp3 bonds.
Ruoff and co-workers analysed the structure of the fluorinated graphene by looking at electron diffraction patterns of the material using a transmission electron microscope. They found that the basic skeleton of the carbon bonding network in graphene remains unchanged. However, subsequent electrical measurements revealed that the resistance of the material greatly increases after fluorination, so turning the material into an insulator. Pristine graphene, on the other hand, is a zero bandgap semiconductor, that is, a special type of semi-metal.
“The fluorination process we developed is an efficient way to isolate graphene devices because laser irradiating fluoropolymer-covered graphene produces fluorinated graphene with highly insulating properties in a single step,” team member Wi Hyoung Lee told nanotechweb.org. “Such an approach could also reduce production costs by avoiding complicated isolation patterning procedures such as photolithography, etching and lift off that are commonly used today.”
The team is now busy looking at whether the method developed here can be used to make semiconducting graphene.
The current work is detailed in Nano Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org