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Piezo-photronic ZnO nanowire LEDs for future mobile devices

Materials Science Department researchers at the Georgia Institute of Technology recently announced in a Nature Photonics journal publication that they're using thousands of nanometer-scale wires for a sensor device that converts mechanical pressure from a signature or a fingerprint directly into a light signal, which can be processed optically. The goal is to enhance the sensitivity of the sensor device so that it's comparable to that of human skin and eventually incorporate these ZnO nanowire LEDs into future mobile devices. What’s more, the technique could also be used in biological imaging and micro-electromechanical (MEMS) systems, as the latter is booming amid the growth in mobile devices.

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Individual zinc oxide (ZnO) nanowires, that are part of the device, operate as tiny energy-efficient and energy-harvesting LEDs when undergoing strain from mechanical pressure, allowing the device to provide detailed information regarding pressure gradients. This technology is referred to as piezo-phototronics, which was first discovered by Wang in 2009. It offers an innovative way to capture information about pressure applied at very high resolution- up to 6,300 dots per inch. This research was funded by the U.S. Department of Energy’s Office of Basic Energy Sciences, the National Science Foundation. The Knowledge Innovation Program of the Chinese Academy of Sciences.

Piezoelectric materials generate a charge polarisation in a strained condition. The piezo-phototronic devices are based on that fundamental to control the charge transport and recombination by the polarisation charges present at the ends of individual nanowires. The ZnO nanowires are grown on a gallium nitride (GaN) film layer to create pixeled light emitters, whose output changes with the pressure applied, creating an electroluminescent signal that can be integrated with on-chip photonics for data transmission, processing and recording. Furthermore, the efficiency increase in the LED is proportional to the strain generated. The light output varies with the strain from the base where the nanowires touch the gallium nitride film. Fabrication of the devices include: low-temperature deposition processes, nanowire formation, oxygen plasma treatments and metal-semiconductor contacts, all of which were the subject of my Ph.D. dissertation on ZnO a decade ago that led to numerous highly-cited publications and the basis for textbooks on this material.

When pressure is applied to the Georgia Tech device through handwriting, nanowires are compressed along their axial directions, creating a negative piezo-potential, while uncompressed nanowires are void of a potential at all. The researchers have pressed letters into the top of the device, which generates light from the bottom of the device. The characteristic of being sensitive to all emitters at the same time, accelerates the response time up to a million pixels in a microsecond for an optical fiber.

The researchers evaluated the repeatability of the sensor array by examining the light emitting intensity of 20,000 individual pixels under strain for 25 on-off cycles and found that the output changed by about five percent, which was significantly less than the total signal- indicating relatively low noise. A spatial resolution of 2.7 microns was recorded from the device samples tested in the published work. The group believes the resolution could be enhanced by reducing the nanowire diameter to boost their areal density on the GaN film layer and by using a high-temperature fabrication process, which would potentially decrease unwanted impurity levels as well.

This energy-harvesting piezo-photronic technology may be commercialized in future tablets, smartphones and smartwatches from the likes of Apple and Samsung, which are all touch-sensitive mobile devices seeking new applications and capabilities to wow consumers.

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