Nov 27, 2009
Making graphene ultraflat
Researchers at Columbia University in the US have made the first samples of completely ripple-free graphene. Such ultraflat samples could now be used to study how ripples, previously thought to be unavoidable in graphene, affect the material's physical and chemical properties.
"Wonder material" graphene consists of a planar single sheet of carbon arranged in a honeycombed lattice. It is currently attracting a flurry of interest thanks to its unique electronic and mechanical properties that make it ideal for a host of device applications. The material could even replace silicon as the electronic material of choice in the future thanks to the fact that charge carriers, electrons and holes travel ballistically through the material at extremely high speeds.
One important feature of graphene is that it contains ripples, similar to those seen on tightly pulled plastic cling film. These occur because of pre-existing strains in the material, possibly caused by the contours of underlying substrates. The corrugations are roughly 1 nm high and spread over distances of between 10 and 25 nm. They can strongly affect the material's electronic properties by inducing effective magnetic fields and changing local potentials that limit how fast charge carriers can travel.
Many scientists argue that graphene ripples are unavoidable, and even necessary for thermally stabilizing graphene sheets. Ripples are also thought to be responsible for several observed properties of graphene, such as higher chemical reactivity.
Tony Heinz and colleagues obtained their ultraflat samples by exfoliating graphite onto a novel substrate that has an atomically flat surface – freshly cleaved mica. The researchers, who published their result in Nature, believe that such a perfectly flat substrate stops the graphene from rippling.
"The ultraflat graphene samples could be used as a reference for controlled studies of many observed physical phenomena associated with rippling," Heinz told nanotechweb.org. "For instance, we could carry out various chemical reactions on both ultraflat and rippled graphene under the same conditions to compare the two. The results will tell us more about the role rippling really plays in enhancing chemical reactivity."
Other, physical, properties that could now be studied include "electron-hole puddles", decreased charge carrier speeds and enhanced spin-orbit coupling.
The Columbia team is currently planning a set of experiments in which electrical transport and chemical reactivity of ultraflat and rippled graphene can be directly compared.
About the author
Belle Dumé is contributing editor at nanotechweb.org