Researchers now have ability to stack graphene sheets on top of each other which should make possible an entirely new class of devices that exploit previously inaccessible physics. The new advanced approach targets specific flakes of graphene and carves them into any desired shape and then picks them up individually so that they can be transferred and placed in a new environment. The process is relatively straightforward. It begins by identifying the graphene flake to be removed and then carving it into shape with a laser. This is instead of using scotch tape to lift flakes.
The next step is to cover the entire substrate, including all the unwanted graphene flakes, with photoresist and then use the laser to expose the photoresist above the patterned graphene to light. The exposed photoresist is then dissolved to reveal the graphene below.
The final step is to pick up the exposed graphene using the ordinary Scotch tape technique. Since all the unwanted graphene flakes are covered by undissolved photoresist, this only transfers the desired graphene sheet.
Graphene is an emerging class of two-dimensional (2D) material with unique electrical properties and a wide range of potential practical applications. In addition, graphene hybrid structures combined with other 2D materials, metal microstructures, silicon photonic crystal cavities, and waveguides have more extensive applications in van der Waals heterostructures, hybrid graphene plasmonics, hybrid optoelectronic devices, and optical modulators. Based on well-developed transfer methods, graphene grown by chemical vapor deposition (CVD) is currently used in most of the graphene hybrid applications. Although mechanical exfoliation of highly oriented pyrolytic graphite provides the highest-quality graphene, the transfer of the desired microcleaving graphene (MG) to the structure at a specific position is a critical challenge, that limits the combination of MG with other structures. Herein, we report a new technique for the selective transfer of MG patterns and devices onto chosen targets using a bilayer-polymer structure and femtosecond laser microfabrication. This selective transfer technique, which exactly transfers the patterned graphene onto a chosen target, leaving the other flakes on the original substrate, provides an efficient route for the fabrication of MG-based microdevices. This method will facilitate the preparation of van der Waals heterostructures and enable the optimization of the performance of graphene hybrid devices.
There are limitations, of course. The new technique requires the hands-on involvement of a researcher at every stage. So it is clearly unsuited to mass production. But that matters little at this stage when physicists are merely attempting to understand the properties and capabilities of this new class of device. The problems of mass production can safely be saved for later.
The promise is huge. Physicists have known for some time now that graphene has extraordinary electronic and mechanical properties. They’ve now spent a decade or so getting to grips with this.
The big question now is what becomes possible when these monolayers are stacked. For example, physicists know that high-temperature superconductivity comes about because of the way that layers of copper oxides are stacked and that the temperature at which superconductivity kicks in is particularly sensitive to the distance between each layer. However the mechanisms involved are unknown.
Graphene is itself a reasonable superconductor so an interesting question is whether its superconducting temperature could be raised by stacking graphene sheets in certain ways or by alternating the sheets with other materials.
It’s this kind of thinking that raises the tantalising prospect that van der Waals heterostructures will be able to exploit previously inaccessible physics.
And that is just the beginning. Physicists are intensely interested in the growing number of other two-dimensional crystals that have been discovered in recent years. These include hexagonal boron nitride, molybdenum disulphide, tungsten diselenide and so on. Just what could be possible with pancakes structures created with these materials is an exciting question to ponder.