In the last decade, methods for producing electrical components and devices by inkjet printing them onto paper, plastics and other materials have advanced, opening the door for enhanced functionality and a new generation of electronics made from sustainable materials.
But so far it is still not possible to inkjet print— using the same inkjet-printing process that drops ink on paper — high-performing electronics, such as computers or mobile phones.
Now, researchers in the Cockrell School of Engineering at The University of Texas at Austin are working to bridge the gap in performance between conventionally manufactured and printed electronics. Their goal is to develop a printing method that could produce electronics and devices that perform as good or better than conventionally manufactured electronics, but use fewer resources and are more easily recyclable.
To this end, the researchers have developed the first inkjet-printed small-channel, single-walled carbon nanotube (SWCNT) field-effect transistors (FETs), a basic semiconductor building block and a primary component of electronic circuits. A SWCNT, which has a tube wall that is 1 carbon atom thick, exhibits very high thermal and electrical conductivity properties. SWCNTs are still a relatively new electronic component used to build transistors, semiconductor devices and electronic circuits, but they have the potential to make printed electronics smaller and faster.
The researchers built SWCNT FETs that achieved conductivity properties that are 10 times faster than other inkjet-printed carbon nanotubes FETs. Significantly, the team found that the performance of their carbon nanotube transistors is approaching the performance of SWCNT FETs made from more complex and expensive non-printing methods.
Professor Ananth Dodabalapur and graduate students Seonpil Jang and Bongjun Kim, in collaboration with researchers from Northwestern University, describe their method for inkjet printing carbon nanotubes in the Nov. 4 issue of Small, a journal of nano and microelectronics.
“We think our carbon nanontube transistor is an important step toward high-performing, low-cost printed electronics, such as smart labels, TV screens, sensors and green electronics,” said Dodabalapur, a professor in the Department of Electrical and Computer Engineering.
The team’s breakthrough is the development of a carbon nanotube transistor structure for inkjet printing with small channel lengths (by the standards of printed electronics), which allow electrons to travel along the length of the tube and conduct electricity faster. The device’s geometry, with channel lengths of 150-250 nanometers, is the shortest channel length reported thus far in which the active material is deposited by inkjet printing.
The researchers’ process for inkjet printed SWCNT FETs starts by using electron-beam lithography (EBL) to pattern a rectangular metal line structure in addition to source and drain (S/D) electrodes. Next, a single drop of SWCNT ink is printed on the pre-patterned area. Confining the droplet in a set area allows the team to use a minimal amount of SWCNT ink, which saves resources and potentially lowers costs.
In this work, researchers used EBL as a proof-of-concept substrate pre-patterning method. However, the team believes that the EBL pattern can be fabricated by roll-to-roll processing methods (nanoimprint lithography) in the near future. Their ultimate goal is to print an entire SWCNT device, layer by layer.
This research was part of a Multidisciplinary University Research Initiative funded by the Office of Naval Research (N00014-11-1-0690) that will continue through fall of 2016. The study is part of an ongoing series of papers on inkjet-printed nanotube circuits published by Dodabalapur’s group.
