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TECH FRONT


Penn Engineers Develop Transistors Made Entirely of Nanocrystal “Inks”


T


ransistor fabrication is a highly complex process, requiring high-temperature, high-vacuum equipment. Thanks to engineers at the University of Pennsylvania, a new process is opening the door for electrical components to be built into fl exible or wearable applications. Their new process, which involves sequentially depositing their components in the form of liquid nanocrystal “inks,” is detailed in a new paper published in Science. Researchers from Pennsylvania’s School of Engineer- ing and Applied Science began by taking nanocrystals, or roughly spherical nanoscale particles, with the electri- cal qualities necessary for a transistor and dispersing these particles in a liquid, making nanocrystal inks. The team developed a library of four of these inks: a conductor (silver), an insulator (aluminum oxide), a semi-


conductor (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.


Because this process works at relatively low temperatures, many transistors can be made on a fl exible backing at once.


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“These materials are colloids just like the ink in your inkjet printer,” said Cherie Kagan, who lead the study, “but you can get all the characteristics that you want and expect from the analogous bulk materials, such as whether they’re conductors, semicon- ductors 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.” Though the electrical properties of the nanocrystal inks had been inde- pendently verifi ed, they had never been combined into full devices.


First, the conductive silver nanocrys- tal ink was deposited from liquid on a fl exible 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 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


28 AdvancedManufacturing.org | June 2016


Photo courtesy of the University of Pennsylvania


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