Apr 8, 2016 | By Alec

As any electrical engineer will tell you, the transistor is the core building block of all the electronics that surround us. They are used to build those circuits that amplify electrical signals and switch between those countless 0s and 1s that form digital computation. While they have steadily come down in size since the 1950s, they are still quite difficult to manufacture, as they depend on a complex production system involving high-temperature, high-vacuum equipment. All of that could change in the near future, however, as a team of engineers from the University of Pennsylvania have pioneered a new method for making transistors, which involves the 3D printing of liquid nanocrystal inks.

The new study, published in the journal Science, entitled Exploiting the colloidal nanocrystal library to construct electronic devices was led by Cherie Kagan, the Stephen J. Angello Professor in the School of Engineering and Applied Science, and Ji-Hyuk Choi, a researcher at the Korea Institute of Geoscience and Mineral Resources. It also included a host of students and other researchers, including Professor Christopher Murray. The research conducted by the team could potentially open the door for the incorporation of transistors into flexible wearables, as it features a lower-temperature process that is compatible with a far wider range of applications and materials.

How do you make a nanocrystal field effect transistors? Well, the team began by taking nanocrystals (spherical nanoscale particles) that possessed the electrical qualities necessary for building a transistor, and dispersed them into different types of printable liquids. Four inks were made, each of which has a unique function: a conductor (silver), an insulator (aluminum oxide), a semiconductor (cadmium selenide) and a conductor/dopant combination (silver and indium), which enables the device to transmit a positive or a negative charge.

These materials were patterned onto flexible plastic backings, initially using spin coating technique in the laboratory. For a large scale production setup, however, an inkjet-based 3D printer would reportedly work perfectly. “These materials are colloids just like the ink in your inkjet printer,” professor Kagan explained. “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.”

As Kagan explained, the question was whether or not these materials could simply be deposited in patterns to form a functional transistor – especially as several of the inks had never been combined into full devices before. “This is the first work,” Choi said of the success, “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.”

Their success obviously depended on very precise patterning. Firstly, the conductive silver nanocrystal ink was printed onto a flexible plastic surface (which had been treated with a photolithographic mask). The mask was removed to leave the ink in the shape of a transistor’s gate electrode. This was subsequently followed by a spin-coated layer of the aluminum oxide insulation ink, followed by the cadmium selenide nanocrystal-based semiconductor. This was topped with a layer of the indium/silver mixture, which acts as a transistor’s source and drain electrodes. At low temperatures, the indium dopant diffused from those electrodes.

As Kagan explained, precision was key. “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 said. “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.”

The technique does come with several advantages over existing vacuum-based techniques and is certainly a step in the right direction for 3D printed wearables. For instance, the researchers were able to produce several transistors on a single flexible plastic sheet simultaneously. “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 said. “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.”

Posted in 3D Printing Technology

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