Oct 29, 2009
Graphene transfer for transparent electronics
Researchers in Texas have found a new way to transfer large-area graphene thin films grown on copper foils onto other substrates. The material could be used to make components for photovoltaic devices and even be an alternative to traditional conductive metal oxides, like indium tin oxide, in transparent electrodes for liquid crystal displays (LCDs), for example.
"Wonder material" graphene consists of a planar single sheet of carbon arranged in a honeycombed lattice. It could be useful for a host of device applications from transistors to single-molecule detectors thanks to its exceptionally high room-temperature electron and hole mobility, extremely high thermal conductivity and interesting mechanical and optical properties.
For applications such as transparent conductive electrodes (in LCDs, for instance) researchers need to be able to grow large-area thin films of graphene or multilayer graphene and place them onto other materials. A new technique developed by Rod Ruoff and Xuesong Li, both of the University of Texas at Austin, together with Luigi Colombo of Texas Instruments, allows such films to be transferred from the copper surface they were grown on while minimizing the formation of cracks and tears in the graphene.
"We now have a way of transferring graphene layers that are much larger than those produced by other methods, such as exfoliating natural or synthetic graphite," Ruoff told nanotechweb.org. "The resulting graphene will enable new devices such as the bilayer pseudospin field-effect transistor (BisFET) and transparent conductive electrodes applications in photovoltaic devices and display technology.
The graphene could also replace indium tin oxide (ITO), which is expensive.
The new transfer process is an improvement on a previous technique developed by the same group. It involves using polymethylmethacrylate (PMMA) to carry graphene to the target substrate, dissolving the first PMMA layer by a PMMA solution to form a second PMMA layer, and then removing the PMMA by acetone. The second PMMA layer relaxes the graphene and produces a material with fewer cracks and tears.
The graphene material produced on copper substrates can be made in very large areas of up to 20 centimetres squared and transferred, so that when four individual layers are stacked onto each other a sheet resistance of only about of 350 ohm/square is possible. This value indicates a continous film. The film also has a high optical transmission of 90% -- significantly higher than the 82–85% of standard ITO -- and is mechanically flexible.
"These properties fulfil the requirements for applications like cathode ray tubes, touch screens and flat panel displays," said the researchers. "And although the sheet resistance of the films is still perhaps higher than required for photovoltaic cells, we believe we can improve this quality."
The team is continuing to optimize its synthesis technique by better understanding the fundamental physical properties of graphene.
The work was reported in Nano Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org