Heat gets graphene moving
Atomic force microscopy image of three graphene ribbons formed from a fretted contact by self-assembly.
Atomic force microscopy image of three graphene ribbons formed from a fretted contact by self-assembly.

Single sheets of two-dimensional graphene can be prompted to slide, fold, peel, and tear into strips spontaneously, researchers have found.

James Annett and Graham L. W. Cross from Trinity College Dublin noticed that when a fold or pleat is created mechanically by pressing a sharp diamond tip into graphene, the sheet spontaneously tears into ribbon-like structures at room temperature [Annett and Cross, Nature (2016), doi: 10.1038/nature18304].

Wrinkles form in graphene’s perfectly smooth structure when it is heated, previous research has shown. But now the two researchers have demonstrated that graphene can be guided to move and reconfigure itself driven simply by ambient energy.

“We have created a simple, folded-over and self-adhering con- figuration of graphene that turns out to animate the material, inducing it into self-locomotion to tear into ribbons,” explains Cross.

The formation of the tapering nanoribbons, which can be of any dimension from 300 nm to 2000 nm wide and up to 5 microns in length, occurs spontaneously at room temperature but can be accelerated with heat or a laser. The self-assembly process can be controlled to produce different and more complicated shapes in quite a reliable manner, according to the researchers. Moreover, the reconfiguration of the graphene can occur simultaneously at multiple locations across a sheet.

“This self-animated kind of behavior, known in roughly analogous forms for molecules and polymers, has never been observed for ‘large scale’ material such as our graphene systems,” says Cross. “The effect is almost visible to the naked eye!”

The researchers suggest that the heat-activated behavior is driven by a thermodynamic mechanism under which isolated, two-dimensional material tends to take up a lower energy, three-dimensional structure.

If the phenomenon is common to other two-dimensional materials, as Annett and Cross believe, it could provide a simple self-assembly route to transforming two-dimensional materials into more complex three-dimensional architectures. For graphene, the approach could enable the construction of high quality, high frequency oscillators, nanoscale imaging systems, microfluidic valves, and even miniature heat engines.

“We see applications in the biomedical space for microfluidic diagnostic and drug delivery systems, as well as in communications and sensing via new opto-electromechanical device concepts,” suggests Cross.

The researchers are now exploring the effect in other two-dimensional materials and hoping to build prototype devices.

Changhong Ke of Binghamton University, the State University of New York believes that the work represents a major advance in the manufacturing of graphene ribbons in a controllable and potentially scalable manner.

“The manufacturing of graphene flakes with well-controlled size, shape, and conformation has been a critical and challenging step in the pursuit of graphene origami and kirigami and their applications,” says Ke. “Cross’ work provides a plausible solution.”

This article was originally published in Nano Today (2016), doi:10.1016/j.nantod.2016.08.003