White LEDs grown on paper

Imagine a room with wallpaper that lights up with the touch of a hand. A zinc oxide and polymer solution could soon make this dream a reality by allowing LEDs to be grown directly onto the surface of paper, or even printed onto wallpaper like ink.

Gul Amin, who recently earned his doctorate from Linköping University, showed in his thesis how it is possible to grow white LEDs directly onto paper. His technique for wallpaper printing has a patent pending.

Amin’s colleague, Naved ul Hassan Alvi, reviewed his summer 2011 thesis, which presented various methods for growing different zinc oxide nanostructures on a variety of semiconducting materials.

Nanostructures of zinc oxide have many characteristics that make them suitable for manufacturing white LEDs – among them a large bandgap and electrons that move easily and give off relatively large amounts of energy once they have bounced back toward the nucleus. In addition, the energy these structures emit is perfect white light.

Methods for growing different zinc oxide (ZnO) nanostructures on a variety of semiconducting materials could allow white LEDs to be printed on paper. Shown here are scanning electron microscope images of ZnO nanostructures: (a) nanorods on a silicon substrate at a neutral pH=6.6; (b) starlike nanostructures at pH=10; (c) flowerlike nanostructures at pH=11; and (d) “urchinlike” nanostructures at pH=12. Courtesy of Linköping University.

Amin has now gone a step further and succeeded in growing white LEDs directly on paper. The active components are nanothreads of zinc oxide on a thin layer of polydiethylfluorene, a conducting polymer. First, however, the paper must be coated with a thin, water-repellent, protective and leveling layer of a resin called cyclotene.

“This is the first time anyone has been able to build electronic and photonic inorganic semiconducting components directly on paper using chemical methods,” said professor Magnus Willander, who is leading the research.

Amin also shows how it is possible to grow nanothreads on paper, blow them off the surface using ultrasound and collect them in the form of a powder, which can be used to print the zinc oxide nanothreads and LEDs on paper or plastic in a traditional printing press.

Because zinc oxide is a natural semiconductor of the n-type, due to defects in the material, Amin combined the semiconductor with copper oxide, a p-type material, to create a few types of electrochemical sensors.

His research group is exploring the potential of zinc oxide when combined with graphene, copper oxide and other materials.

The work appeared in physica status solidi - Rapid Research Letters.