First Transistors Made Entirely of Nanocrystal "Inks" Developed

University of Pennsylvania engineers have developed a new approach for fabricating transistors: sequentially depositing their components in the form of liquid nanocrystal “inks.” This method opens the door for electrical components to be built into flexible or wearable applications, as the lower-temperature process is compatible with a wide array of materials and can be applied to larger areas.

Researchers led by Cherie Kagan, professor in the School of Engineering and Applied Science, 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.

Kagan’s group developed four of these inks: a conductor (silver), an insulator (aluminum oxide), a semiconductor (cadmium selenide) and a conductor combined with a dopant (a mixture of silver and indium). “Doping” the semiconductor layer of the transistor with impurities controls whether the device transmits a positive or negative charge.

Four nanocrystal inks that comprise the transistor are deposited on a flexible backing. Image credit: UPenn.

Four nanocrystal inks that comprise the transistor are deposited on a flexible backing. Image credit: UPenn.

“These materials are colloids just like the ink in your inkjet printer,” Kagan says, “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 they had never been combined into full devices. “This is the first work showing that all the components, the metallic, insulating and semiconducting layers of the transistors, and even the doping of the semiconductor, could be made from nanocrystals,” says fellow researcher Ji-Hyuk Choi, then a member of Kagan's lab and now a senior researcher at the Korea Institute of Geoscience and Mineral Resources.

The process entails layering or mixing the inks in precise patterns. First, the conductive silver nanocrystal ink was deposited from liquid onto a flexible plastic surface that was treated with a photolithographic mask and then rapidly spun to draw it out in an even layer. The mask was then removed to leave the silver ink in the shape of the transistor’s gate electrode.

The researchers followed that layer by spin coating a layer of the aluminum oxide nanocrystal-based insulator, followed by 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 relatively 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,” Kagan says. “We had to treat the surfaces of the nanocrystals, both 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.”

Because this entirely ink-based fabrication process works at lower temperatures than existing vacuum-based methods, the researchers were able to make several transistors on the same flexible plastic backing at the same time.

“Making transistors over larger areas and at lower temperatures have been goals for an emerging class of technologies—when people think of the Internet of things, large-area flexible electronics and wearable devices,” Kagan says. “We haven’t developed all of the necessary aspects so they could be printed yet, but because these materials are all solution-based, it demonstrates the promise of this materials class and sets the stage for additive manufacturing.”