A new breakthrough brings semiconductors made of wood closer to reality
The idea of making semiconductors from wood isn't as far-out as it sounds. That's because cell walls in plants are made of a material called cellulose, and cellulose can be made to conduct electricity when heated to relatively high temperatures under certain conditions.
But there's a problem with this renewable nanomaterial, which is a form of nanopaper. That burning process — called carbonization — can easily destroy the three-dimensional structures that would make cellulose-derived semiconductors so useful.
That's why a new process developed by researchers in Japan is a big deal. In a paper published Tuesday in ACS Nano, the researchers describe a treatment process that makes it possible "to heat the nanopaper without damaging the structures of the paper from the nanoscale up to the macroscale."
Designing 3D structures and tuning electrical properties
The researchers behind this breakthrough had to balance two competing challenges. First, the process needs to allow manufacturers to "tune" the nanopaper to have electrical properties suitable for the specific application. Second, the process needs to be gentle enough so that manufacturers can design structures with a lot of surface area and many pores, depending on the application. The solution was a multi-step process that offers a great deal of control over the final product.
“We applied an iodine treatment that was very effective for protecting the nanostructure of the nanopaper. Combining this with spatially controlled drying meant that the pyrolysis treatment did not substantially alter the designed structures and the selected temperature could be used to control the electrical properties,” says material scientist Hirotaka Koga, a co-author on the paper.
Early tests show promising results
The researchers used their new technique to create two relatively simple proof-of-concept devices. In one case, they used the nanopaper semiconductor as a sensor to monitor the flow of water vapor through two different types of masks. When attached to a washable mask made of cloth, the sensor was able to record pulses that were synchronized with exhalations. The water molecules in the wearer's breath temporarily lowered the electrical resistance of the sensor. When attached to a surgical mask, the sensor didn't record such pulses. "Only a gradual decrease in sensor resistance was observed, indicating the effective water vapor capture of the surgical mask," the researchers wrote.
When the researchers attached the nanopaper semiconductor to a glucose biofuel cell, the material demonstrated a power density 14 times higher than that of a commercial graphite sheet.
“The structure maintenance and tunability that we have been able to show is very encouraging for the translation of nanomaterials into practical devices,” Koga says. “We believe that our approach will underpin the next steps in sustainable electronics made entirely from plant materials.”