May 16, 2014
Dry transfer technique improves GFETs
Researchers at the Pohang University of Science and Technology in Korea have come up with a new way to transfer graphene grown by chemical vapour deposition (CVD) onto substrates without using water. Most conventional graphene transfer methods require wet chemistry steps, so materials that are water sensitive cannot be used as substrates. The new technique overcomes this problem and devices made from graphene transferred this way also appear to perform better because they contain fewer charged impurities and “wrinkles".
Graphene was first isolated in 2004 using the now famous sticky tape method (in which individual atomic layers of graphene are “shaved off” from bulk graphite). Although this technique produces high-quality samples, it cannot be used to produce large quantities of graphene, so researchers have developed methods such as CVD that can.
In CVD, graphene is usually grown on a catalytic metal substrate such as copper or nickel, so an additional transfer step is needed to place the graphene on its final substrate (usually a silicon wafer). This step is usually done using wet transfer techniques, in which a polymer supporting layer is first coated onto the graphene/metal layers. The catalytic metal layer is etched away and then the polymer/graphene floated on water is “scooped” onto the target substrate. This simple process produces graphene with few cracks, but the downside is that the graphene cannot be transferred onto substrates that are sensitive to water – such as flexible organic semiconductors.
The new method, developed by a team led by Kilwon Cho, overcomes this problem because it does not require any wet steps for transfer onto the target substrate. The researchers have already used their technique to transfer CVD-grown graphene onto a water-sensitive substrate and have even made flexible, air-stable, low-voltage FETs from the transferred materials.
The team began by coating CVD-grown graphene with polymeric bilayers made of polybutadiene (PBU) and poly(methylmethacrylate) (PMMA). After removing the catalytic metal below the graphene, the polymers and graphene are placed on a sample holder (the red-coloured square in the image above). The sample holder is moved onto the desired substrate and nitrogen gas is used to “break” the edges of the polymers/graphene sample. The sample layers are then laminated onto the target substrate.
“The polymeric bilayers we used not only provide robust support during the transfer process, but also effectively passivate the graphene, which protects the carbon sheet from undesirable charged impurities that are invariably introduced in ambient air,” team member Yoonyoung Chung told nanotechweb.org.
Outstanding carrier mobility
“The polymer layers are so good at their task that transistors made from material transferred this way appear to have very good properties,” he added. “For example, our graphene FETs have outstanding carrier mobility values of around 2575 and 3075 cm2/V·s for holes and electrons, respectively. Carrier mobility is a measure of how fast electrons and holes move through a material. The FETs also have an average Dirac point (which indicates how ‘clean’ a device is) of around 0.38 V.”
These values appear to be better than those seen in FETs made from graphene transferred using wet chemistry processes, he says.
Flexible, high-performance FETs
And that is not all: the FETs are flexible and remain high-performance even after they have been bent and released more than 5000 times or exposed to 80% relative humidity. Such properties will be required for future flexible electronics applications, such as flexible smartphones, displays and smart textiles, says Chung.
The team says that it is now busy making flexible graphene circuits that contain graphene transferred using its dry technique.
The current work is published in Advanced Materials DOI: 10.1002/adma.201305940.
About the author
Belle Dumé is contributing editor at nanotechweb.org