Scientists have made considerable progress in the field of flexible electronics in the last few years using various types of semiconductor materials. These include organic semiconductors, silicon with a buckling structure, carbon nanotubes, inorganic nanowires and metal oxides. Ideally, the materials employed to fabricate the flexible circuits should have a high charge-carrier mobility. They should also be easy and cheap to process at room temperature.

Thin films of carbon nanotubes could be ideal in this respect. Indeed, researchers have already made sophisticated integrated flexible circuits, including flip-flops and decoders, using carbon nanotubes directly grown from chemical vapour deposition.

Compatible with roll-to-roll?

“Unlike these previous reports, however, ours is the first study on using purified semiconducting-only carbon nanotube ink for fabricating integrated circuits on mechanically flexible substrates,” team member Chuan Wang told nanotechweb.org. “The solution-based process we employed could potentially be compatible with large-scale roll-to-roll printing processes and the technology we describe is simple, scalable, low-cost, robust and produces highly reproducible and uniform transistors with good performance.”

Team leader Ali Javey and colleagues Wang, Kuniharu Takei, and Toshitake Takahashi made nanotube thin films using a room-temperature drop-casting process. The structures produced are very uniform, with a high density of nanotubes throughout the sample, which makes for transistors with very small device-to-device variations. “Unlike organic semiconductor materials, our devices are not sensitive to oxygen or moisture either and thanks to the use of very thin flexible substrates, the devices are highly flexible and can be bent and unbent a number of times without suffering any damage,” explained Wang.

The as-made transistors have on-currents of 15 µA/um, transconductances of 4 µS/µm and mobilities of around 50 cm2/V/s. What is more, devices with relatively long channel lengths (of 4 µm) have cut-off frequencies of 170 MHz, a value that is already high enough for certain wireless communication applications.

The proof-of-concept platform should have a wide range of applications in electronic skin, wearable devices, prostheses and robotics, says Javey. “More specifically, it could be adapted for use in flexible active-matrix backplanes for display and sensor applications, circuits that can directly integrated with multifunctional sensor arrays for on-chip signal processing and wireless data transfer.”

The team is now busy developing some of these applications.

The work was reported in Nano Letters.