A study published in Proceedings of the Royal Society A demonstrates mathematically what happens to stacks of graphene sheets under slight lateral compression — a gentle squeeze from their sides. Rather than forming smooth, gently sloping warps and wrinkles across the surface, the researchers show that layered graphene forms sharp, saw-tooth kinks with interesting electrical properties.
“We call these quantum flexoelectric crinkles,” says Professor Kyung-Suk Kim of Brown University. “What’s interesting about them is that each crinkle produces a remarkably thin line of intense electrical charge across the surface, which we think could be useful in a variety of applications.”
The charge, Prof Kim says, is generated by the quantum behaviour of electrons surrounding the carbon atoms in the graphene lattice. When the atomic layer is bent, the electron cloud becomes concentrated either above or below the layer plane. That electron concentration causes the bend to localise into a sharp point, and produces a line of electrical charge roughly 1nm wide and running the length of the crinkle. The charge is negative across the tip of an upraised ridge and positive along the bottom of a valley.
The researchers believe the electrical charge could, for example, be used to direct nanoscale self-assembly. The charged crinkles attract particles with an opposite charge, causing them to assemble along crinkle ridges or valleys.
Previous experiments in particle assembly along crinkles involved graphene sheets and buckyballs — soccer-ball-shaped molecules formed by 60 carbon atoms.
The Brown team explains that researchers “dumped” buckyballs onto different kinds of graphene sheets and observed how they dispersed. In most cases, the buckyballs spread out randomly on a layer of graphene like marbles dropped on smooth wooden floor. But, on one particular type of multilayer graphene known as HOPG, the balls would spontaneously assemble into straight chains stretching across the surface. Prof Kim thinks flexoelectric crinkles can explain that strange behaviour.
“We know that HOPG naturally forms crinkles when it’s produced,” Prof Kim said. “What we think is happening is that the line charge created by the crinkles causes the buckyballs, which have an electric dipole near the line charge, to line up.”
Similarly, strange behaviours have been seen in experiments with biomolecules like DNA and RNA on graphene. The molecules sometimes arrange themselves in peculiar patterns rather than flopping out randomly – and the Brown team think that these effects can be traced to crinkles as well. Most biomolecules have an inherent negative electrical charge, which causes them to line up along positively charged crinkle valleys.
It might be possible to engineer crinkled surfaces to take full advantage of the flexoelectric effect. Prof Kim envisions a crinkled surface that causes DNA molecules to be stretched out in straight lines making them easier to sequence.
“Now that we understand why these molecules line up the way they do, we can think about making graphene surfaces with particular crinkle patterns to manipulate molecules in specific ways,” Prof Kim concludes.