Researchers have, for the first time, used a conventional, high-throughput printing process to create sheets of high-quality carbon nanotube transistors on flexible plastic (PET) sheets. These flexible sheets, made from the same kind plastic as the Coke bottle you just took a swig from, could be used to create flexible, low-power displays for applications as small as a smartphone or as large as your living room wall — or touch-sensitive electronic skin for robots and prosthetic limbs.
This development comes from the Ali Javey and colleagues at Berkeley, the same group that previously produced rugged carbon nanotube-based e-skin back in 2011. While the end result of both studies is fairly similar — polymer sheets with thin-film carbon nanotube transistors baked in — the main difference is that the first study used a very slow and laborious process. These new sheets of single-walled carbon nanotube thin-film transistors (SWCNT TFTs) are produced using inverse gravure printing — a continuous, rotary printing process that’s typically used for magazines, catalogs, and packaging (cardboard, plastic, etc.) This is exciting because gravure printing is massively scalable: Commercial gravure printing presses can churn its way through a three-meter-wide reel of paper or plastic at speeds of up to 14 meters (46 feet) per second.
To perform the gravure printing, the research team first have to create an “ink” of 99% semiconducting SWCNTs to act as the channel (like graphene, one of the main issues of using CNTs is their lack of a natural bandgap). They then need two more inks to make up the other portions of the transistors — a silver ink for the source, drain and gate electrodes, and a high-k barium titanate ink for the dielectric.
tion of SWCNTs and is allowed to dry, effectively creating a solid, uniform forest of CNTs — the transistor channel. This sheet is then run through the gravure printer multiple times, first to deposit the source and drain electrodes (silver ink), then to deposit the high-k dielectric between the source and drain, and then finally another silver gate electrode above the dielectric.
The end result is SWCNT TFTs that have much higher electron mobility than any previous example of printed, flexible SWCNT TFTs. Higher mobility means that less operating voltage is required, and thus these sheets of SWCNT TFTs could realistically be used as the basis of wall-sized OLED displays. Due to the top-gate approach, which protects the underlying CNTs, these printed transistors also show good resistance against ambient conditions, too.
