Nanopaper semiconductors promising for sustainable electronics

Japanese researchers have developed wood-derived semiconductors that can be tuned for use in a range of sustainable electronic devices. Their findings have been published in the journal ACS Nano.

Semiconducting nanomaterials with 3D network structures have high surface areas and lots of pores that make them suitable for applications involving adsorbing, separating and sensing. However, simultaneously controlling the electrical properties and creating useful micro- and macro-scale structures, while achieving excellent functionality and end-use versatility, remains challenging. Osaka University researchers, in collaboration with The University of Tokyo, Kyushu University and Okayama University, have now developed a nanocellulose paper semiconductor that provides nano−micro−macro trans-scale designability of the 3D structures and wide tunability of the electrical properties.

Cellulose is a natural and easy-to-source material derived from wood. Cellulose nanofibres (nanocellulose) can be made into sheets of flexible nanocellulose paper (nanopaper) with dimensions like those of standard A4. Nanopaper does not conduct an electric current, but heating can introduce conducting properties; unfortunately, this exposure to heat can also disrupt the nanostructure. The researchers have therefore devised a treatment process that allows them to heat the nanopaper without damaging the structures of the paper from the nanoscale up to the macroscale.

“An important property for the nanopaper semiconductor is tunability because this allows devices to be designed for specific applications,” said study author Associate Professor Hirotaka Koga, from Osaka University. “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.”

The researchers used origami (paper folding) and kirigami (paper cutting) techniques to provide playful examples of the flexibility of the nanopaper at the macro level. A bird and box were folded, shapes including an apple and snowflake were punched out, and more intricate structures were produced by laser cutting. This demonstrated the level of detail possible, as well as the lack of damage caused by the heat treatment.

Examples of successful applications showed nanopaper semiconductor sensors incorporated into wearable devices to detect exhaled moisture breaking through facemasks and moisture on the skin. The nanopaper semiconductor was also used as an electrode in a glucose biofuel cell and the energy generated lit a small bulb.

“The structure maintenance and tunability that we have been able to show is very encouraging for the translation of nanomaterials into practical devices,” Koga said. “We believe that our approach will underpin the next steps in sustainable electronics made entirely from plant materials.”

Image caption: The origami-processed cellulose nanofibre paper (CNP) before and after pyrolysis. White and black papers indicate the original and pyrolysed CNPs, respectively. Scale bar: 10 mm. Image credit: 2022 Koga et al. Nanocellulose paper semiconductor with a 3D network structure and its nano−micro−macro trans-scale design. ACS Nano (under CC BY 4.0)

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