Material scientists have found a way to apply the ancient art of kirigami – a way of building complex structures by cutting and folding paper – to the wonder material graphene. The experiment shows that ripples in a graphene sheet can increase the bending stiffness of the material significantly more than expected – a discovery that could lead to new types of sensors, stretchable electrodes or tools for use in nanoscale robotics.
Graphene is a single layer of graphite, a naturally occurring mineral with a layered structure. The material, first produced in the lab in 2003, has impressive electrical, thermal and mechanical properties, which makes it potentially useful in applications ranging from new electronic devices to additives in paints and plastics.
The promising material is made up of carbon atoms structured in a series of interconnecting hexagons, similar to chicken wire. It is made by pulling apart the layers in graphite in what scientists call a “top-down” approach (where we take something big and make it smaller). This can be done using adhesive tape; chemical reagents; or by sheer force such as those generated in a kitchen blender or mixer. Although this sounds quite simple it is not suitable for producing large sheets of graphene.
To do this, a “bottom-up” approach is needed, where graphene is assembled by decomposing a carbon-containing molecule such as methane over a hot metal surface, typically copper. This is the technique the researchers in the new study used to produce a sheet of graphene that they then could manipulate using a version of kirigami.
Paper and graphene versions of kirigami. Graphene image taken using a transmission white-light optical microscope. McEuen Group, Cornell University
Art meets science
Kirigami (“kiri” means cut and “gami” means paper) is a type of origami (“ori” means fold) which has been practised for centuries to produce beautifully complex shapes and patterns. Lots of us have probably tried our hand at these techniques as children, making snowflakes out of scrap paper.
The researchers used gold pads as “handles” to crumple a graphene sheet like paper – in a process that is entirely reversible. Like paper, the graphene folds and crumples but does not noticeably stretch.
A gold pad (the dark square which measures a few 10’s of microns) is being pushed by a micromanipulator and is attached to a graphene spring
By using a sophisticated measuring technique, where an infrared laser is used to apply pressure to the gold pad on the graphene film, it is possible to measure the level of displacement of the graphene film. This displacement can then be used to workout the elastic properties of the graphene sheet. Wrinkling of the graphene sheet improves its mechanical properties, similar to how a crumpled sheet of paper is more rigid than a flat one.
In fact it was such mechanical similarities that enabled the researchers to translate ideas directly from paper models to graphene devices. Using photolithography, a method of transferring geometric shapes on a mask to a surface, as the “cutting scissors”, the team showed that it is possible to create a series of springs, hinges and stretchable graphene transistors.
The research has opened the door to manipulating two-dimensional graphene sheets to create new material structures with movable parts. The possibility of stretchable transistors is extremely interesting as there is a growing demand to develop flexible and even wearable electronics.
When stretching a material you would normally expect the electrical resistance to change. In the stretchable transistors developed in the new study, a graphene spring is sandwiched between a source and a drain electrode. When stretched to over twice its original size no noticeable change in electrical properties was detected. Repeated stretching and un-stretching also had little effect either.
Working in a water and soap solution, large sheets of graphene can dramatically crumpled like soft paper, and return to their original shape
This ability to maintain graphene’s electrical properties is down to its lattice structure, which does not undergo much change during the stretching of the spring. It even proved possible to take the kirigami devices to the next level, moving or folding the graphene without using direct contact. For example, by replacing the gold pads with a ferromagnetic material, such as iron, the sheets could be manipulated in a magnetic field, creating complex motions such as twists. The technique could be used to create devices that respond to light, magnetic fields or temperature.
The concept of manipulating two-dimensional materials to generate more complex structures on the macro-, micro- and nanoscale is genuinely exciting. Being able to create new metamaterials, engineered to have properties not usually found in natural materials, could open the door to many new types of tools. Possibilities include new sensors, stretchable electrodes that could be used in robotics or nanomanipulators, tiny machines that can move things around with nanometer precision.
Stretchable electrodes would allow highly conformable or flexible electronics and sensors to be incorporated into synthetic skin or flesh, such as in robots or artificial limbs, while retaining full functionality. We could even visualise such flexible electrodes and sensors being used in wearable electronics incorporated into clothing for realtime personal health monitoring – the ultimate personal healthcare.