Graphene was in the headlines this month again, after researchers from Korea demonstrated chemical vapor deposition (CVD) growth of single-crystal graphene on germanium.
Photo: Dry graphene transfer, courtesy of A. Castellanos-Gomez.
Typically, graphene is grown by CVD on metal films, such as copper for instance. After growth, the graphene is transferred from copper to a desired substrate, which is most commonly a silicon-on-insulator wafer. The insulator is an important component in transistor technology, acting to prevent a short circuit between the back gate used for switching and the current-carrying graphene. Transfer to insulating substrates has matured over the years, resulting in ever-increasing quality of the end product. Also, the copper layers have been getting thinner and thinner, resulting in a continuous drop in the cost of manufacturing graphene.
Now, the work from Korea shows monocrystalline graphene grown on a semiconductor. Although the advantage of growing graphene on a semiconductor is not very clear, according to some theories, monocrystalline graphene should in principle be of better quality than the typically obtained polycristalline graphene on copper; however the graphene grown on germanium displays mobility and sheet resistance similar to that obtained after transfer from a copper substrate. The main advantage of the current technique may thus lie in the ability to precisely control the crystal orientation of the graphene layer.
And although the traditional method of transferring graphene from copper, which involves coating the graphene with a polymer and etching away the metal, keeps improving, it is important to keep an eye out for new transfer methods. In that direction, researchers from Rice University have shown the growth of graphene on top of a network of carbon nanotubes. The nanotubes are first deposited on a copper substrate, after which the substrate is heated. The heat causes the top layer of nanotubes to unwrap and form a graphene layer, supported by the underlying network of remaining nanotubes. The network acts as a reinforcement to prevent graphene from breaking during transfer, much as a polymer does in the traditional method. The new approach gets rid of polymer residue which has been an issue in graphene fabrication for years. The utility of the approach is demonstrated by making flexible transparent all-carbon electrodes.
Finally, researchers at the Kavli Institute in the Netherlands have come up with a purely dry transfer of 2-D materials, including graphene. The new technique, which is quick, efficient and clean, makes use of viscoelastic stamps. As well as being much simpler than traditional wet transfer techniques, it could also be used to fabricate freely suspended 2D structures thanks to the fact that the samples are not subject to any capillary forces during the process.
The method follows a sequence of steps, starting by mechanically exfoliating graphene on a commercially available material called Gelfilm. Mechanical exfoliation, also known as the Scotch tape technique, was the way graphene was made originally for the Nobel prize winning work of Andre Geim and Konstantin Novoselov. The Gelfilm with graphene flakes is then attached to a transparent stamp. The stamp is made from sticky silicone rubber, similar to the “sticky-hands” toy. When silicone rubber comes into contact with a surface, after some time it flows away, leaving the graphene in contact with the desired transfer substrate. The Gelfilm is then gently peeled away. The technique can be applied to any of the exciting new “2D” materials.
We are excited to see the research of graphene transfer progressing further. Graphenea has so far specialized in the synthesis and transfer process which allows us to offer our customers graphene on many different substrates, and even custom user-provided ones. For more information, contact us at email@example.com.