Silicon Valley may want to look for another new name: Paper Valley. In another response to the dwindling silicon supply that has had researchers looking for alternatives to silicon, one of the main ingredients in transistors (which is, in turn, one of the essential components that power all our gadgets), the University of Maryland is researching transistors that you can print on paper, which might revolutionize the transistor market. Why? Paper is a renewable resource, and will allow us to mass produce devices for years to come, while replenishing the supply at the same time.
Researchers can build electric connections on paper because paper readily absorbs inks, whether those inks contain pigments used to print the newspaper, or electronic materials like carbon nanotubes and semiconducting polymers. Materials scientists envision fabricating circuits like publishers print periodicals. There are some challenges that have yet to be overcome, however. One of the big challenges is that previous paper transistors performed poorly because the surface of regular paper is bumpy and uneven. For a transistor to perform optimally, electrons have to move easily through super thin layers of conducting and semiconducting materials. These layers are only a couple hundred nanometers thick, while the bumps on the surface of regular paper are tens of micrometers tall. The paper bumps disrupt the flat layers of electronic materials and interrupt the device’s electron flow. In addition to its rough surface, regular paper’s other limitation is its opaqueness. To produce electronics for transparent displays, researchers need a transparent material, like plastic or glass.
To overcome these challenges, Liangbing Hu, a research scientist at the university, has been trying to create a new type of paper, nanopaper, that will accommodate the paper transistors. Instead of the micrometer-sized cellulose fibers found in regular paper, sheets of this material contain nanoscale fibers that produce an even surface and allow light to pass through. The method of creating nanopaper involves treating paper pulp with oxidizing chemicals. The nanopaper has cellulose fibers with an average diameter of 10 nm. “It’s as flat as plastic,” Hu says.
Hu and his team built transistors on the paper by depositing a layer each of three materials: first carbon nanotubes, next an insulating organic molecule, and then a semiconducting organic molecule. To complete the device, the team topped it with electrodes, also made by laying down carbon nanotubes. While serving as the electrodes for the transistors, the nanotubes also provide a structural backbone, preventing excessive wrinkling in the paper after the solvents used in the fabrication process evaporate.
“These nanopaper-based transistors could be another step down the road to renewable printed electronics,” said Jeffrey Youngblood, a materials engineer at Purdue University. However, as with any new technological innovation, it also has to be practical so the cost doesn’t get passed on to consumers. “The researchers will have to find a way to produce the transistors via a scalable process, such as roll-to-roll printing, instead of the tedious layer-by-layer process described in the report,” Youngblood added.