The researchers began by taking nanocrystals, or roughly spherical nanoscale particles, with the electrical qualities necessary for a transistor and dispersing these particles in a liquid, making nanocrystal inks: a conductor (silver), an insulator (aluminium oxide), a semiconductor (cadmium selenide) and a conductor combined with a dopant (a mixture of silver and indium).
"These materials are colloids just like the ink in your inkjet printer," Cherie Kagan, Professor in the School of Engineering and Applied Science, said. "But you can get all the characteristics that you want and expect from the analogous bulk materials, such as whether they're conductors, semiconductors or insulators. Our question was whether you could lay them down on a surface in such a way that they work together to form functional transistors."
The electrical properties of several of these nanocrystal inks had been independently verified, but the researched say that they had never been combined into full devices. Such a process entails layering or mixing them in precise patterns.
First, the conductive silver nanocrystal ink was deposited from liquid on a flexible plastic surface that was treated with a photolithographic mask then rapidly spun to draw it out in an even layer. The mask was then removed, leaving the silver ink in the shape of the transistor's gate electrode. The researchers followed that layer by spin-coating a layer of the aluminium oxide nanocrystal-based insulator, then a layer of the cadmium selenide nanocrystal-based semiconductor and finally another masked layer for the indium/silver mixture, which forms the transistor's source and drain electrodes. Upon heating at low temperatures, the indium dopant diffused from those electrodes into the semiconductor component.
"The trick with working with solution-based materials is making sure that, when you add the second layer, it doesn't wash off the first, and so on," Prof Kagan said. "We had to treat the surfaces of the nanocrystals, when they're first in solution and after they're deposited, to make sure they have the right electrical properties and that they stick together in the configuration we want."
The researchers claim they were able to make several transistors on the same flexible plastic backing at the same time because this entirely ink-based fabrication process works at lower temperatures than existing vacuum-based methods. Prof Kagen added that these nanocrystal-based field effect transistors could eventually be constructed by additive manufacturing systems, like 3D printers.
The study opens the door for electrical components to be built into flexible or wearable applications, as the lower-temperature process is compatible with an array of materials and can be applied to larger areas.
