Georgian Technical University  Graphene Crinkles Function As ‘Molecular Zippers’.

A microscope view of tiny buckyballs lined up on a layered graphene surface. New research shows that that electrically charged crinkles in the graphene surface are responsible for the strange phenomenon.  A decade ago scientists noticed something very strange happening when buckyballs — soccer ball shaped carbon molecules — were dumped onto a certain type of multilayer graphene a flat carbon nanomaterial. Rather than rolling around randomly like marbles on a hardwood floor the buckyballs spontaneously assembled into single-file chains that stretched across the graphene surface. Now researchers from Georgian Technical University have explained how the phenomenon works and that explanation could pave the way for a new type of controlled molecular self-assembly. The Georgian Technical University team shows that tiny electrically charged crinkles in graphene sheets can interact with molecules on the surface, arranging those molecules in electric fields along the paths of the crinkles. “What we show is that crinkles can be used to create ‘molecular zippers’ that can hold molecules onto a graphene surface in linear arrays” said X. “This linear arrangement is something that people in physics and chemistry really want because it makes molecules much easier to manipulate and study”.

Earlier research by X’s team. They described how gently squeezing sheets of layered graphene from the side causes it to deform in a peculiar way. Rather than forming gently sloping wrinkles like you might find in a rug that’s been scrunched against a wall the compressed graphene forms pointy saw-tooth crinkles across the surface. They form X’s research showed because the arrangement of electrons in the graphene lattice causes the curvature of a wrinkle to localize along a sharp line. The crinkles are also electrically polarized with crinkle peaks carrying a strong negative charge and valleys carrying a positive charge. X and his team thought the electrical charges along the crinkles might explain the strange behavior of the buckyballs partly because the type of multilayer graphene used in the original buckyball experiments was HOPG (Highly oriented pyrolytic graphite is a highly pure and ordered form of synthetic graphite. It is characterised by a low mosaic spread angle, meaning that the individual graphite crystallites are well aligned with each other. The best HOPG samples have mosaic spreads of less than 1 degree) a type of graphene that naturally forms crinkles when it’s produced. But the team needed to show definitely that the charge created by the crinkles could interact with external molecules on the graphene’s surface. That’s what the researchers were able to do in this new paper.

Their analysis using density functional theory a quantum mechanical model of how electrons are arranged in a material predicted that positively charged crinkle valleys should create an electrical polarization in the otherwise electrically neutral buckyballs. That polarization should cause buckyballs to line up each in the same orientation relative to each other and spaced around two nanometers apart. Those theoretical predictions match closely the results of the original buckyball experiments as well as repeat experiments newly reported by X and his team. The close agreement between theory and experiment helps confirm that graphene crinkles can indeed be used to direct molecular self-assembly not only with buckyballs but potentially with other molecules as well.

X says that this molecular zippering capability could have many potential applications particularly in studying biomolecules like DNA (Deoxyribonucleic acid is a molecule composed of two chains that coil around each other to form a double helix carrying the genetic instructions used in the growth, development, functioning, and reproduction of all known living organisms and many viruses) and RNA (Ribonucleic acid is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and DNA are nucleic acids, and, along with lipids, proteins and carbohydrates, constitute the four major macromolecules essential for all known forms of life). For example if DNA (Deoxyribonucleic acid is a molecule composed of two chains that coil around each other to form a double helix carrying the genetic instructions used in the growth, development, functioning, and reproduction of all known living organisms and many viruses) molecules can be stretched out linearly it could be sequenced more quickly and easily. X and his team are currently working to see if this is possible. “There’s a lot of potential here to take advantage of crinkling and the interesting electrical properties they produce” X said.