Graphene recently became the focus of many research projects because of its excellent electronic properties. However, until now, folding the material into well-defined structures has proved difficult for three reasons: there are energy potential barriers to folding, folding needs to be guided, and the resulting structure must be stabilized.
Petr Král and his team have now performed molecular simulations to show that nanodroplets of water wrinkle graphene sheets, making them more pliable. In fact, when two water droplets are placed on a 15 x 12 nm2 sheet of graphene, they form hollows with a radius of curvature of 5 nm in a process driven by van der Waals interactions. The droplets are highly mobile and advance towards each other by diffusion, until they eventually couple together. This formation of hollows minimizes the energy barrier to bending and simulations indicate that nanodroplets have the capability to activate and guide folding, such that conformational changes can occur in graphene.
“The dynamics always starts with placing a water nanodroplet on the graphene and observing what will follow. Sometimes the actual place of the nanodroplets determines the dynamics,” explains Král. “We are currently exploring this issue more and self-assemble the systems from a gas phase.”
Complex structures can be obtained from flakes or nanoribbons of graphene, including capsules, knots, rings and other formations through rapid bending, folding, sliding, rolling, and zipping of the material.
“We wanted to see how nanodroplets will modify graphene flakes and were surprised that they can self-assemble them according to the actual conditions (sizes of flakes and droplets, type of nanodroplets, temperature, etc.). We have seen all kinds of different scenarios that can be obtained under these conditions, where different structures have been assembled,” Král tells Materials Today.
Král suggests guiding the process by atomic force microscopy to control the sites at which water droplets are delivered to the graphene surface. If such experiments prove successful, they could potentially lead to devices with good mechanical, electrical, and optical properties, as Král explains. “Graphene is potentially a very interesting material for electronics. The self-assembly can be done in principle on a massive scale right on the chip. We believe that the future lies in this option.”