White LEDs Grown On Paper
Imagine a room with wallpaper that lights up with just a touch. A simple chemical that's used as a pigment in breakfast cereals could make that possible by allowing LEDs to be grown right on the surface of paper, or even printed onto it like ink.
Gul Amin, a recent PhD grad from Linkoping University in Sweden, used zinc oxide, which is commonly used in many industrial processes and added to food. It's also a very good semiconductor, and has been proposed as a material for LEDs.
To make his printable ZnO diodes, he first had to grow tiny, nanometer-sized ZnO crystals. He started by coating paper in a polymer and then seeding the paper with a solution that contained zinc. After it was seeded, he coated it again with another polymer.
After that, he dipped the seeded paper into a mixture of other chemicals that induced the zinc to grow into tiny hollow rods. He added yet another polymer layer to make sure the tubes (which stand up like hairs on the paper) stayed isolated from each other. After that he etched away a portion of the very top polymer layer to expose the nanorods, and finally put down silver electrical contacts. When he sent an electrical charged through the paper, the zinc rods lit up.
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Not satisfied with that, however, Amin looked at how to make the ZnO nanorods printable, which would get him to the end result faster. For that, he had to get the nanorods off of the paper. He used ultrasound to "scrape" off the rods and then mixed them with a solution to create an ink. Using an ordinary silkscreen printing machine, he printed the ZnO onto a paper treated in a similar way to the one he used to grow the crystals. When he added a charge to the paper, the zinc ink lit up.
Patents on both methods for using ZnO have been filed, and university is continuing to explore uses for the nanomaterials.
“This is the first time anyone has been able to build electronic and photonic inorganic semiconducting components directly on paper using chemical methods,” said Amin's professor Magnus Willander in press release. Willander lead the research.