Scientists at Graphene Flagship partner FORTH/ICE-HT, Greece, and the University of Petras, Greece, have wrinkled, crinkled and compressed exfoliated graphene at room temperature to introduce a unique, mosaic-like morphology. Creases and channels in the mosaic structure can form water channels, which could lead to the material's use in new nanofluidic technologies.
Graphene can be prepared by chemical vapor deposition at over 1000°C. This can be made to buckle and crinkle when cooled down. Unlike most materials, that expand when heated up and contract when cooled, graphene does the opposite – contracting when heated and expanding when cooled. This expansion can be used to induce biaxial compression, resulting in buckling and a mosaic-like structure, with crinkles in both lateral and longitudinal directions.
Costas Galiotis, based at Graphene Flagship partner FORTH/ICE-HT and the University of Petras, Greece, says that this behavior inspired his team to introduce this mosaic structure to graphene at room temperature by applying stretching and compressing forces. "We were investigating interface engineering using graphene supported on polymers in the framework of the Graphene Flagship," comments Galiotis. "It brought us to induce the formation of mosaic morphology via mechanical deformation, and we were driven to understand the mechanisms through which we can introduce this – and to find new technological applications."
If you were to stretch a thin membrane, like a piece of cling film, it would wrinkle parallel to the direction in which you stretch it. Conversely, if you were to compress it, it would buckle and crease in the transverse direction. The same can be said for graphene –a one-atom thick membrane, which wrinkles and buckles in the same way. "Very few research groups have approached this topic, so there's been minimal effort to investigate the morphological changes in graphene under mechanical loading," Galiotis continues.
To introduce this behavior, they first deposited a layer of graphene on a polymer substrate. After axially stretching the material at room temperature, creating lateral wrinkles, they quickly compressed it without allowing time for the wrinkles to relax, introducing longitudinal wrinkles as well. Using atomic force microscopy, they found that the intersection of these perpendicular wrinkles formed a 'mosaic' morphology, with junctions and channels caused by the overlapping and intersecting wrinkles.
To investigate the properties of the new, mosaic-structured graphene, Galiotis and colleagues introduced the material to a high-humidity environment. They found that water became trapped in the interstitial space between the nano-channels and the substrate, forming what Galiotis calls 'nano-blisters.' Galiotis explains: "The mosaic morphology of graphene could be seen as reservoirs interconnected by nano-channels. This can be adopted into nanofluidic technologies – researchers could manufacture graphitic nanoconduits to be applied in water desalination or purification, chemical separation, energy harvesting, or lab-on-a-chip applications – such as miniature platforms for biochemical analysis."
He says that the mosaic structure could also be useful in strain engineering, due to coupling between localized deformations, and in tissue engineering, in which the unique morphology could be used as a surface texturing agent to direct cell alignment.
Jonathan Coleman, Deputy of the Graphene Flagship's Enabling Materials Work Package, says: "This is an extremely interesting piece of work which nicely demonstrates the ability to control wrinkling in grown graphene. A number of authors have hypothesised that these wrinkles might be useful. This paper goes a step further and provides evidence that they can be used as channels for nano-fluidic liquid delivery."
Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "Graphene never ceases to surprise. This work shows how the understanding of fundamental properties can be translated and tailored to support novel applications."
'Mosaic pattern formation in exfoliated graphene by mechanical deformation,' Nature Communications, 10, 1572 (2019). Maria Giovanna Pastore Carbone, Anastasios C. Manikas, Ioanna Souli, Christos Pavlou and Costas Galiotis.