Bringing touch to robots with piezo-photronic LEDs

R. Colin Johnson

August 12, 2013

R. Colin JohnsonAugust 12, 2013

PORTLAND, Ore. — The glow from light-emitting diodes may give robots artificial skin that can sense touch.

A new "piezo-phototronic" approach to the human-machine interface has been proposed by piezo-electric material expert Zhong Lin Wang, a professor at the Georgia Institute of Technology (Georgia Tech, Atlanta). First proposed by Wang as a new semiconductor-device category, his team has since produced prototypes that could act as artificial skin for robots to give them human-like touch sensitivity, for inkless fingerprint scanners, for biological imaging, and to enhance the performance of micro-electro-mechanical systems (MEMS).

The piezo-phototronic effect uses the charge polarization caused by deforming piezo-electric nanowires to enhance the charge transport and recombination of tiny light-emitting-diodes. The LEDs were formed by growing p-type zinc-oxide piezo-electric nanowires atop an n-type gallium nitride layer on an insulating sapphire substrate (see illustration). Every time the nanowire arrays feel strain from the touch of a finger -- human or robotic -- the p-n junction forming the LED glows with an intensity proportional to the applied pressure. That glowing image -- of an fingerprint, for instance -- can then be sensed with conventional optical techniques.

Imaging pressure with the piezo-phototronic effect arrays nanowire-LED sensors on a sapphire substrate (A) to enable a touch (B) to turn on zinc-oxide LEDs (bottom) in a character pattern.
 (Source: Georgia Tech)
Imaging pressure with the piezo-phototronic effect arrays nanowire-LED sensors on a sapphire substrate (A) to enable a touch (B) to turn on zinc-oxide LEDs (bottom) in a character pattern.
(Source: Georgia Tech)

In his recent demonstration, Wang's team grew 2.7 micron diameter zinc-oxide nanowires into arrays that were able to sense touch with a resolution of 6,300 dots per inch and a switching time of 90 milliseconds. A low-temperature chemical growth technique patterned the nanowires atop a gallium nitride film with a thermoplastic providing support between nanowires. Patterned nickel-gold electrodes formed the ohmic contact with the gallium nitride film on the bottom while transparent indium-tin oxide (ITO) was deposited on top as the common electrode. For the future, the team plans to increase the resolution further with a higher-temperature nanowire growth process.

Professor Zhong Lin Wang of the Georgia Institute of Technology has developed a sensor that converts mechanical pressure directly into an optical image. 
(Source: Georgia Tech)
Professor Zhong Lin Wang of the Georgia Institute of Technology has developed a sensor that converts mechanical pressure directly into an optical image.
(Source: Georgia Tech)

Wang's research team included postdoctoral researchers Caofeng Pan, Guang Zhu, and Qing Yang, visiting scholar Lin Dong, and doctoral candidates Simiao Niu, Ruomeng Yo, and Ying Liu.

Funding was provided by the US Department of Energy's Office of Basic Energy Sciences, the National Science Foundation, and the Knowledge Innovation Program of the Chinese Academy of Sciences.

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