The wonder material graphene gets many of its handy quirks from the fact that it exists in two dimensions, as a sheet of carbon only one atom thick. But to actually make use of it in practical applications, it usually needs to be converted into a 3D form. Now, researchers have developed a new and relatively simple way to do just that, using lasers to "forge" a three-dimensional pyramid out of graphene.
This isn't the first time graphene has been given an extra dimension. In 2015, researchers from the University of Illinois molded graphene into 3D structures by layering it onto shaped substrates, and early this year MIT scientists found that tubes of the stuff could be shaped into 3D coral-like structures 10 times stronger than steel but just five percent as dense. Rice University researchers have also recently made graphene foam and reinforced it with carbon nanotubes.
But this new technique, developed by researchers in Finland and Taiwan, might be an easier and faster method to make 3D graphene. By focusing a laser onto a fine point on a 2D graphene lattice, the graphene at that spot is irradiated and bulges outwards. A variety of three-dimensional shapes can be made by writing patterns with the laser spot, with the height of the shape controlled by adjusting the irradiation dose at each particular point.
The team illustrated that technique by deforming a sheet of graphene into a 3D pyramid, standing 60 nm high. That sounds pretty tiny, but it's 200 times taller than the graphene sheet itself.
"We call this technique optical forging, since the process resembles forging metals into 3D shapes with a hammer," says Mika Pettersson, co-author of the study. "In our case, a laser beam is the hammer that forges graphene into 3D shapes. The beauty of the technique is that it's fast and easy to use; it doesn't require any additional chemicals or processing. Despite the simplicity of the technique, we were very surprised initially when we observed that the laser beam induced such substantial changes on graphene. It took a while to understand what was happening."
The researchers initially assumed that the laser had "doped" the graphene, introducing impurities into the material, but after further examination they found that wasn't the case.
"When we first examined the irradiated graphene, we were expecting to find traces of chemical species incorporated into the graphene, but we couldn't find any," says Wei Yen Woon, co-author of the study. "After some more careful inspections, we concluded that it must be purely structural defects, rather than chemical doping, that are responsible for such dramatic changes on graphene."
The team says that the optically forged graphene is structurally sound, highlighting its potential for building 3D architectures out of the material for a wide range of applications. In this form, the graphene has different electronic and optical properties from its 2D counterpart.
The research was published in the journal Nano Letters.
Source: Academy of Finland
