Image: Ji Hye Hong
One of the most exciting areas of integrated circuit research involves the move off brittle silicon wafers and into the wild—smart fabrics, artificial muscles, flexible displays, and other smart materials prove to be a challenge for all aspects of the traditionally fragile materials used in semiconductor devices. Flexible transistors have seen their share of press lately, but the electrodes that move current to and from the actual transistors have to take their fair share of abuse as well. We take a look at two of the latest breakthroughs in producing transparent, flexible electrodes that could find their way into the consumer electronics of the future.
The inherent problem with producing flexible electronics is that you can’t have your cake and eat it too. Superior electrical properties are provided by semiconductors and metals, neither of which has been known for its flexibility, while polymers are willing to bend, but fail miserably in moving electrons. Physically combining the two classes of materials without adversely affecting the electrical or mechanical properties of either has proven to be quite challenging.
A recent attempt, published in Advanced Functional Materials, focused on use for artificial muscles, which requires large deformations and hundreds of thousands of use cycles. It uses the flexible polymer polydimethylsiloxane (PDMS) as a base, and implants nanoscale clusters of conductive metals using a technique called filtered cathode vacuum arc ion implantation. Metal ions are accelerated and stuck into the polymer near the surface, creating a conductive surface layer. The pockets of metal are close enough together that they can touch each other and conduct, but are not strongly bonded like a bulk metal, which would break during large deformations.
The technique lends itself to allowing thin films to be produced (about 20 microns), while various lithography and laser milling techniques can produce well-defined zones of metal implantation, and thus conductivity. By experimenting with several metals and ion implantation parameters, gold-implanted PDMS films were produced that were able to sustain 170% deformation before a loss in conductivity (a traditional metal thin-film manages only a few percent).
Meanwhile, research published in Nature also used a polymer film for robust mechanical properties, but instead draws upon continuous sheets of graphene for the electrical properties. We’ve been covering graphene and carbon nanotubes extensively; they have superlative electrical and mechanical properties, but production techniques has proven to be a limiting factor.
Researchers fine-tuned a chemical vapor deposition technique—where several gasses flow across a hot substrate and decompose into a solid material—to produce high-quality sheets of graphene only a few atoms thick. The importance of using chemical vapor deposition instead of niche laboratory techniques cannot be understated, and is a huge step towards the eventual scale up for any real applications.
Moving the graphene sheet from the nickel that it was deposited on and onto a polymer for eventual integration into an electronic device proved to be relatively simple. The researchers etched the nickel to release the graphene and stamped it directly onto the polymer. Electrical conductivity measurements confirmed the quality of the graphene produced, and demonstrated that it was significantly better than other flexible electrode technologies, including the metal ion implantation technique discussed earlier.
The trade-off came in the mechanical properties, where more-than-respectable deformations of 6.5 percent could be achieved before drop-offs in conductivity were seen; this is still a far cry from the massive deformations seen in gold-ion implanted PDMS. Graphene sheet-based electrodes are vastly superior in transmitting light though, with up to 93 percent of light transmitted in some cases.
Its clear that we don’t have a material that is capable of addressing all of the needs of a flexible electrode but, as new approaches emerge, the fact that we have a number of potential methods may allow engineers to tailor the technology used to the specific requirements of the application.
Advanced Functional Materials. DOI: 10.1002/adfm.200801218
Nature. DOI: 10.1038/nature07719