Portable electronics users tend to upgrade their devices frequently as new technologies offering more functionality and more convenience become available. A report published by the U.S. Environmental Protection Agency in 2012 showed that about 152 million mobile devices are discarded every year, of which only 10 percent is recycled -- a legacy of waste that consumes a tremendous amount of natural resources and produces a lot of trash made from expensive and non-biodegradable materials like highly purified silicon. Now researchers from the University of Wisconsin-Madison have come up with a new solution to alleviate the environmental burden of discarded electronics. They have demonstrated the feasibility of making microwave biodegradable thin-film transistors from a transparent, flexible biodegradable substrate made from inexpensive wood, called cellulose nanofibrillated fiber (CNF). This work opens the door for green, low-cost, portable electronic devices in future. In a paper published this week in the from AIP Publishing ("Microwave Flexible Transistors on Cellulose Nanofibrillated Fiber Substrates"), the researchers describe the biodegradable device. 
An array of microwave silicon transistors sitting on a wood-derived CNF substrate. (Image: Jung-Hun Seo, Shaoqin Gong and Zhenqiang Ma/University of Wisconsin-Madison) "We found that cellulose nanofibrillated fiber based transistors exhibit superior performance as that of conventional silicon-based transistors," said Zhenqiang Ma, the team leader and a professor of electrical and computer engineering at the UW-Madison. "And the bio-based transistors are so safe that you can put them in the forest, and fungus will quickly degrade them. They become as safe as fertilizer." Nowadays, the majority of portable electronics are built on non-renewable, non-biodegradable materials such as silicon wafers, which are highly purified, expensive and rigid substrates, but cellulose nanofibrillated fiber films have the potential to replace silicon wafers as electronic substrates in environmental friendly, low-cost, portable gadgets or devices of the future. Cellulose nanofibrillated fiber is a sustainable, strong, transparent nanomaterial made from wood. Compared to other polymers like plastics, the wood nanomaterial is biocompatible and has relatively low thermal expansion coefficient, which means the material won't change shape as the temperature changes. All these superior properties make cellulose nanofibril an outstanding candidate for making portable green electronics. To create high-performance devices, Ma's team employed silicon nanomembranes as the active material in the transistor -- pieces of ultra-thin films (thinner than a human hair) peeled from the bulk crystal and then transferred and glued onto the cellulose nanofibrill substrate to create a flexible, biodegradable and transparent silicon transistor. But to make portable electronics, the biodegradable transistor needed to be able to operate at microwave frequencies, which is the working range of most wireless devices. The researchers thus conducted a series of experiments such as measuring the current-voltage characteristics to study the device's functional performance, which finally showed the biodegradable transistor has superior microwave-frequency operation capabilities comparable to existing semiconductor transistors. "Biodegradable electronics provide a new solution for environmental problems brought by consumers' pursuit of quickly upgraded portable devices," said Ma. "It can be anticipated that future electronic chips and portable devices will be much greener and cheaper than that of today." Next, Ma and colleagues plan to develop more complicated circuit system based on the biodegradable transistors.
read more "Biodegradable, flexible silicon transistors"
An array of microwave silicon transistors sitting on a wood-derived CNF substrate. (Image: Jung-Hun Seo, Shaoqin Gong and Zhenqiang Ma/University of Wisconsin-Madison) "We found that cellulose nanofibrillated fiber based transistors exhibit superior performance as that of conventional silicon-based transistors," said Zhenqiang Ma, the team leader and a professor of electrical and computer engineering at the UW-Madison. "And the bio-based transistors are so safe that you can put them in the forest, and fungus will quickly degrade them. They become as safe as fertilizer." Nowadays, the majority of portable electronics are built on non-renewable, non-biodegradable materials such as silicon wafers, which are highly purified, expensive and rigid substrates, but cellulose nanofibrillated fiber films have the potential to replace silicon wafers as electronic substrates in environmental friendly, low-cost, portable gadgets or devices of the future. Cellulose nanofibrillated fiber is a sustainable, strong, transparent nanomaterial made from wood. Compared to other polymers like plastics, the wood nanomaterial is biocompatible and has relatively low thermal expansion coefficient, which means the material won't change shape as the temperature changes. All these superior properties make cellulose nanofibril an outstanding candidate for making portable green electronics. To create high-performance devices, Ma's team employed silicon nanomembranes as the active material in the transistor -- pieces of ultra-thin films (thinner than a human hair) peeled from the bulk crystal and then transferred and glued onto the cellulose nanofibrill substrate to create a flexible, biodegradable and transparent silicon transistor. But to make portable electronics, the biodegradable transistor needed to be able to operate at microwave frequencies, which is the working range of most wireless devices. The researchers thus conducted a series of experiments such as measuring the current-voltage characteristics to study the device's functional performance, which finally showed the biodegradable transistor has superior microwave-frequency operation capabilities comparable to existing semiconductor transistors. "Biodegradable electronics provide a new solution for environmental problems brought by consumers' pursuit of quickly upgraded portable devices," said Ma. "It can be anticipated that future electronic chips and portable devices will be much greener and cheaper than that of today." Next, Ma and colleagues plan to develop more complicated circuit system based on the biodegradable transistors.
Dipole moment The work will be of interest to those considering graphene elements in flexible touchscreens or memories that store bits by controlling electric dipole moments of carbon atoms, the researchers said. Perfect graphene – an atom-thick sheet of carbon – is a conductor, as its atoms’ electrical charges balance each other out across the plane. But curvature in graphene compresses the electron clouds of the bonds on the concave side and stretches them on the convex side, thus altering their electric dipole moments, the characteristic that controls how polarized atoms interact with external electric fields. The researchers who published their results this month in the American Chemical Society’s ("Flexoelectricity in Carbon Nanostructures: Nanotubes, Fullerenes, and Nanocones") discovered they could calculate the flexoelectric effect of graphene rolled into a cone of any size and length. The researchers used density functional theory to compute dipole moments for individual atoms in a graphene lattice and then figure out their cumulative effect. They suggested their technique could be used to calculate the effect for graphene in other more complex shapes, like wrinkled sheets or distorted fullerenes, several of which they also analyzed. “While the dipole moment is zero for flat graphene or cylindrical nanotubes, in between there is a family of cones, actually produced in laboratories, whose dipole moments are significant and scale linearly with cone length,” Yakobson said. Carbon nanotubes, seamless cylinders of graphene, do not display a total dipole moment, he said. While not zero, the vector-induced moments cancel each other out. That’s not so with a cone, in which the balance of positive and negative charges differ from one atom to the next, due to slightly different stresses on the bonds as the diameter changes. The researchers noted atoms along the edge also contribute electrically, but analyzing two cones docked edge-to-edge allowed them to cancel out, simplifying the calculations. Yakobson sees potential uses for the newly found characteristic. “One possibly far-reaching characteristic is in the voltage drop across a curved sheet,” he said. “It can permit one to locally vary the work function and to engineer the band-structure stacking in bilayers or multiple layers by their bending. It may also allow the creation of partitions and cavities with varying electrochemical potential, more ‘acidic’ or ‘basic,’ depending on the curvature in the 3-D carbon architecture.”
Carbon fiber composites are typically prized for their high strength and low weight, and carbon nanotube (CNT) composites (or nanocomposites), which have more and smaller carbon filaments, show promise for high strength as well as other properties such as the ability to conduct heat and electricity. However, according to NIST's Alex Liddle, an author on the study, while researchers previously could reliably measure a nanocomposite's bulk properties, they didn't know exactly why various formulations of the composite had different properties. "Figuring out why these materials have the properties they do requires a detailed, quantitative understanding of their complex 3-D structure," says Liddle. "We need to know not only the concentration of nanotubes but also their shape and position, and relate that to the properties of the material." Seeing the arrangement of carbon nanotubes in a composite material is tough, though, because they're surrounded by an epoxy resin which also is mostly carbon atoms. Even with sophisticated probes the contrast is too low for software image processors to pick them out easily. In such research situations, you turn to graduate students and postdocs like NIST's Bharath Natarajan, because humans generally make great image processors. But marking thousands of carbon nanotubes in an image is mighty boring, so Natarajan designed an image-processing algorithm that can distinguish CNTs from an epoxy resin as well as he can. It paid off. According to Liddle, a CNT expresses its full potential in strength and thermal and electrical conductivity when it is stretched out and straight, but … "When CNTs are suspended in an epoxy resin, they spread out, bundle and twist into different shapes," Liddle says. "Our analysis revealed that the benefits of CNTs increases in a non-linear fashion as their concentration increases. As the concentration raises, the CNTs come into contact, increasing the number of intersections, which increases their electrical and thermal conductivity, and the physical contact causes them to conform to one another, which straightens them, increasing the material's strength." The fact that increasing the concentration of CNTs enhances properties is not particularly surprising, but now researchers know how this affects the materials' properties and why earlier models of nanocomposite materials' performance never quite matched how they performed in practice. "We've really only seen the tip of the iceberg with respect to this class of material," says Liddle. "There are all sort of ways other researchers might slice and dice the data to model and eventually manufacture optimal materials for thermal management, mechanical reinforcement, energy storage, drug transport and other uses."