The graphite found in pencils is made of layers of carbon stacked on top of each other, atom-thin sheets known as graphene. Graphene possesses a variety of unique electrical, mechanical, thermal and optical properties, leading scientists worldwide to research whether it could find use in advanced circuitry and other devices. Now scientists reveal they can transfer graphene onto flexible surfaces, hinting flexible, rollable graphene electronics might be possible in the future, report findings detailed in the Proceedings of the National Academy of Sciences.
Most work exploring graphene’s properties is conducted on rigid surfaces such as quartz. However, graphene is inherently flexible, so chemical engineer Luiz Martins at the Federal University of Minas Gerais in Brazil, electrical engineer Yi Song at MIT and their colleagues investigated whether graphene could find applications on pliable supports.
Most strategies to transfer graphene onto flexible surfaces involves using acrylic glass — polymethylmethacrylate (PMMA) — as an intermediary. First graphene is synthesized on metal, and then it gets coated with PMMA. The metal is then chemically etched away, and the PMMA-graphene combination is placed atop the desired flexible surface. The PMMA is then removed either via solvent or high-temperature annealing. This strategy has several drawbacks — for instance, some PMMA may get left on the graphene, and many flexible surfaces cannot withstand either the solvents or high temperatures used to remove the PMMA.
The researchers developed a fast, simple way to directly transfer graphene onto a variety of flexible surfaces without using PMMA as an intermediary. This involved laminating the graphene onto the surfaces. Graphene is synthesized on both sides of a sheet of copper foil and then sandwiched between the flexible surface in question and a layer of protective paper. This sandwich is then placed between two plastic films. This entire combination is placed into a lamination machine, which bonds the graphene to the flexible surface. The plastic films and protective paper are then removed. The remaining material is floated in a solvent that dissolves the copper, removing the extraneous graphene as well.
The researchers tested their strategy on a variety of flexible materials, such as Teflon, PVC, paper and cloth. The technique worked best on water-repellant hydrophobic surfaces and on materials that can keep good contact with graphene. For surfaces that do not satisfy these requirements, such as paper and cloth, PMMA can be used as a glue or a surface modifier to ensure successful transfer of graphene onto that material.
This advance could help lead to “a new era of high-quality flexible touchscreens, flexible light emitting diodes (LEDs), flexible sensors, gas filters and solar cells,” says study co-author Paulo Araujo, a condensed matter physicist at the University of Alabama.
In the future, the researchers would like to explore their technique with other, layered materials, such as boron nitride, transition metal dichalcogenides and oxides, Araujo says. They also want to improve the resulting graphene quality, as their strategy can result in patchy graphene films.