Schematic illustration of the process of opening and closing holes on few-layer graphene nanosheets to fabricate the highly dense and defect-free 2D graphene films and 3D graphene assemblies.A simple means of making tiny, nanoscale holes into graphene sheets – and removing them again – could hold the key to manufacturing high-quality structures, according to researchers [Chen et al., Materials Today (2018), https://doi.org/10.1016/j.mattod.2018.09.001].
Perfect two-dimensional sheets of graphene promise a combination of excellent electrical and thermal conductivity with chemical resistance, low density, and high mechanical strength. But in practice, the performance of graphene fibers, films, and more complex shapes is hampered by the presence of defects introduced during manufacturing.
“Pristine, high-quality graphene, with its inherent impermeability, poses challenges in fabricating dense films and thick assemblies with high electrical conductivity because of the difficulty in removing trapped air and/or solvents used in various fabrication methods,” explains Liangbing Hu of University of Maryland College Park, who led the research with colleagues from NASA Langley Research Center and the National Institute of Aerospace.
To get around this problem, Hu and his colleagues used a simple oxidation in hot air process to generate nanoscale holes in graphene nanosheets. During the process, defective sites in commercial graphene are oxidized into tiny perforations. The ‘holey graphene’ can then be fabricated into dense films using solution-based processes such as vacuum filtration.
“The holey graphene can even be directly compressed into a dense assembly under completely solvent-free conditions, opening up a new strategy for fast manufacturing of graphene based architectures,” adds Hu.
To remove the holes, the holey graphene films or structures need to be simply heated to a high temperature (2700 K). The Joule heating process creates hole-free, highly crystalline graphene films and structures with exceptional electrical and thermal conductivity, as well as high mechanical strength, in a fast, low-cost, and scalable manner. The treated graphene films show a 17-fold improvement in electrical conductivity and thermal conductivity twice as high as copper.
“Existing methods for fabricating graphene, including solution processing and high-temperature annealing, are time-consuming, expensive, and have limitations with respect to film thickness,” says Hu. “Our manufacturing process is not limited to solution processing and could be completed in dry conditions, is fast and scalable without limitations in size.”
The researchers believe that the repair process involves the scavenging of carbon atoms or radicals from the surroundings to recombine and fill in the holes. The resulting near-perfect carbon lattice resembles an ideal graphene structure.
“Our strategy of creating and repairing holes on graphene nanosheets using thermal treatment provides new tools to tune defects in graphene for applications such as separation, sensors, and transistors,” says Hu.
The work could also lead to a new, rapid manufacturing route for graphene materials with ultrahigh electrical and thermal conductivity ideal for next generation integrated circuits and high-power batteries for smartphones, tablets, and even electric vehicles and aerospace applications.