Researchers from AMBER and Trinity College in Dublin, in collaboration with TU Delft, have fabricated printed transistors consisting entirely of two-dimensional nanomaterials for the first time. These 2D materials combine promising electronic properties with the potential for low-cost production. This research could unlock the potential for applications such as food packaging that displays a digital countdown to warn you of spoiling, wine labels that alert you when your white wine is at its optimum temperature, next-generation banknote security and even flexible solar cells. The findings have been published in Science*
The Trinity College researchers, from the groups of profs. Jonathan Coleman and Georg Duesberg, used standard printing techniques to combine graphene nanosheets as the electrodes with two other nanomaterials, tungsten diselenide and boron nitride, as the channel and separator (two important parts of a transistor) to form an all-printed, all-nanosheet, working transistor. “This publication is important because it shows that conducting, semiconducting and insulating 2D nanomaterials can be combined together in a complex device”, Coleman said. “We felt that it was critically important to focus on printing transistors as they are the electric switches at the heart of modern computing.”
Testing without touching
Two-dimensional transistors have been made before with methods such as chemical vapour deposition. While devices created in this manner perform well, the costs of these methods are high. Printable electronics, on the other hand, have until now been mostly based on carbon-based molecules. These molecules can cheaply and easily be turned into printable inks, but such materials are somewhat unstable and have well-known performance limitations.
Collaborating with Toyota’s Dr. Sachin Kinge, Dr. Jannika Lauth from the Laurens Siebbeles group at TU Delft tested the electrical transport characteristics of the transistors, proving they combine the best of both worlds.. “By using terahertz spectroscopy, we were able to determine the conductivity of the semiconductor materials”, said Lauth. “Terahertz radiation is a special form of light with an energy between that of microwaves and infrared cameras. It allowed us to analyse the samples without touching them.”
Towards a single nanosheet
The material that makes up the team’s printed electronics consist of many different nanosheets (or ‘flakes’) of varying sizes. During the printing process, these flakes are randomly deposited on top of each another like Mikado sticks. Lauth found that, while the conductivity between the flakes leaves something to be desired, the conductivity within nanosheets is good.
A promising next step is to print 2D-structures that are made up of a single nanosheet, which will drastically improve the performance of the printed electronics. This may be achieved by building up the nanosheets from scratch using molecular or nanoparticle precursors, a method that also yields inks and that Lauth is pursuing at TU Delft.
*All-printed thin-film transistors from networks of liquid-exfoliated nanosheets, Science, 7th April 2017