Graphene’s unique combination of electrical and physical properties marks it out as a potential candidate for transparent, stretchable electronics, which could enable a new generation of sophisticated displays, wearable health monitors, or soft robotic devices. But, although graphene is atomically thin, highly transparent, conductive, and more stretchable than conventional indium tin oxide electrodes, it still tends to crack at small strains.
Now researchers from Stanford University believe they have found a way to overcome this shortcoming and have created the most stretchable carbon-based transistors to date [Liu et al., Science Advances 3 (2017) e1700159].
“To enable excellent strain-dependent performance of transparent graphene conductors, we created graphene nanoscrolls in between stacked graphene layers,” explains first author of the study, Nan Liu
Illustration of the stacked graphene MGG structure.
The team led by Zhenan Bao dub their combination of rolled up sheets of graphene sandwiched in between stacked graphene layers ‘multi-layer G/G scrolls’ or MGG. The scrolls, which are 1–20 microns long, 0.1–1 microns wide, and 10–100 nm high, form naturally during the wet transfer process as graphene is moved from one substrate to another.
“By using MGG graphene stretchable electrodes (source/drain and gate) and semiconducting carbon nanotubes, we were able to demonstrate highly transparent and highly stretchable all-carbon transistors,” says Liu.
The all-carbon devices fabricated by the team retain 60% of their original current output when stretched to 120% strain (parallel to the direction of charge transport). This is the most stretchable carbon-based transistor reported to date, believe the researchers.
The graphene scrolls are key to the stretchable electrode’s remarkable properties because they seem to provide a conductive path even when graphene sheets start to crack at high strain levels.
“Taking into account the electronic and optical properties as well as the cost, our MGG exhibits substantial strengths over other conductors, such as carbon nanotubes and metal nanowires,” says Liu.
Transparent, stretchable graphene electrodes could be useful as contacts in flexible electronic circuits such as backplane control units for displays, as well as functional sensors and digital circuits for electronic skin.
“This is a very important area of research with a variety of possible applications,” comments Andrea C. Ferrari of the University of Cambridge. “The approach taken by Bao et al. is an interesting one that could be quite general.”
The concept of using a mixture of graphene scrolls and platelets to enable an electrode to stretch without significant losses in transmittance or conductivity is a good and should, in principle, not be too complicated to scale up for real devices, he adds.
“We are now seeking to extend this method to other two-dimensional materials, such as MoS2, to enable stretchable two-dimensional semiconductors,” says Liu.