The laser beam induces small changes in the lattice structure of graphene by producing defects in it, explains team leader Mika Pettersson of the University of Jyväskylä. This, in turn, slightly expands the lattice, which leads to the material bulging. By drawing patterns on the graphene with a tightly focused beam, we can build structures with variable shapes and can accurately control their height by controlling the irradiation dose.

     The process is quite simple and uses standard laser sources, he tells nanotechweb.org. We call it optical forging since it is much like the way a blacksmith uses a hammer to forge a metal into 3D shapes. In our case, the laser beam is the hammer for the graphene sheet.

    One example of a shape that the researchers can make using their technique is a pyramid that is 60 nm high, which is roughly 200 times bigger than the thickness of the graphene sheet itself.
Difficult to controllably shape graphene into 3D shapes

    Graphene is the most widely studied of all 2D materials. It boasts excellent carrier mobility, has a high strength, is flexible and transparent, and absorbs light over a broad range of wavelengths. It could be ideal for making novel electronics, photonics and optoelectronics devices and has already been used to make sensors, field-effect transistors, supercapacitors and photodetectors.

    Although graphene is a 2D sheet of carbon, it is not, strictly speaking, flat but contains corrugations, wrinkles, ripples and other out-of-plane deformations. These structures can be exploited to modify the electronic properties of the material, but controlling them is no easy task. Until now, researchers have modified the surface of graphene using techniques such as spot blistering, substrate moulding and strain-induced periodic modulation, or by cutting the material and connecting graphene flakes to functional groups. They have not, however, been able to controllably shape graphene into more complex, custom-made 3D architectures.
Femtosecond laser pulses induce local strain

     Pettersson and colleagues have now succeeded in forging graphene into free-standing 3D shapes by exploiting the fact that femtosecond laser pulses induce local strain into the material when applied in an inert atmosphere. Although the researchers knew that laser irradiation in air functionalizes graphene with oxygen-containing groups, they say that irradiation in an inert atmosphere has a fundamentally different effect and produces structural defects rather than chemically-doped ones.

    We do not yet know what the exact atomic-level mechanism behind the structural deformation is, but preliminary evidence (from atomic force microscopy images, for example), indicates that it comes from the expansion of the graphene lattice caused by photo-induced structural modification, says Pettersson. Graphene is highly promising for applications in a wide range of different fields, such as transparent and flexible electronics, optoelectronics and sensors, and the 3D version of material has many properties that will allow for the development of new kinds of devices.

    The team, reporting its work in Nano Letters DOI: 10.1021/acs.nanolett.7b03530, says that it is now busy trying to understand the mechanisms behind the laser-induced structural deformation in graphene. The key to this is the atomic-level structure of the 3D material, says Pettersson and once we have understood this, it will be time to work on developing applications for it.