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The efficiency increase in the LED is proportional to the strain created.
Differences in the amount of strain applied translate to differences in light emitted from the root where the nanowires contact the GaN film.
To fabricate the devices, a low-temperature chemical growth technique is used to create a patterned array of ZnO nanowires on a GaN thin film substrate with the c-axis pointing upward.
The interfaces between the nanowires and the GaN film form the bottom surfaces of the nanowires. After infiltrating the space between nanowires with a PMMA thermoplastic, oxygen plasma is used to etch away the PMMA enough to expose the tops of the zinc oxide nanowires.
A nickel-gold electrode is then used to form ohmic contact with the bottom gallium-nitride film, and a transparent indium-tin oxide (ITO) film is deposited on the top of the array to serve as a common electrode.
When pressure is applied to the device through handwriting, nanowires are compressed along their axial directions, creating a negative piezo-potential, while uncompressed nanowires have no potential.
The ability to see all of the emitters simultaneously allows the device to provide a quick response. “The response time is fast, and you can read a million pixels in a microsecond,” says Wang. “When the light emission is created, it can be detected immediately with the optical fibre.”
The nanowires stop emitting light when the pressure is relieved. Switching from one mode to the other takes 90 milliseconds or less, Wang says.
The researchers studied the stability and reproducibility of the sensor array by examining the light emitting intensity of the individual pixels under strain for 25 repetitive on-off cycles. They found that the output fluctuation was approximately 5 percent, much smaller than the overall level of the signal. The robustness of more than 20,000 pixels was studied.
A spatial resolution of 2.7 µm was recorded from the device samples tested so far. Wang believes the resolution could be improved by reducing the diameter of the nanowires - allowing more nanowires to be grown - and by using a high-temperature fabrication process.
The researchers’ study is described in detail in the paper, “High-resolution electroluminescent imaging of pressure distribution using a piezoelectric nanowire LED array,” by Caofeng Pan et al in Nature Photonics (2013), published online on 11th August 2013. DOI:10.1038/nphoton.2013.191
This research was sponsored by the U.S. Department of Energy›s Office of Basic Energy Sciences, the National Science Foundation, and the Knowledge Innovation Program of the Chinese Academy of Sciences.
PhotonStar reshuffles board
The III-nitride LED manufacturer has appointed a senior executive who worked at GE Lighting
PhotonStar LED Group plc, a British designer and manufacturer of smart LED lighting solutions, has appointed Philip Marshall as a non-executive director with immediate effect.
Schematic showing a device for imaging pressure distribution by the piezo-phototronic effect. The illustration shows a nanowire-LED based pressure sensor array before (a) and after (b) applying a compressive strain. A convex character pattern, such as “ABC,” molded on a sapphire substrate, is used to apply the pressure pattern on the top of the indium-tin oxide (ITO) electrode.
The researchers have pressed letters into the top of the device, which produces a corresponding light output from the bottom of the device. This output - which can all be read at the same time - can be processed and transmitted.
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www.compoundsemiconductor.net August/September 2013 Philip Marshall
Marshall most recently served as the President and Chief Executive Officer at GE Lighting EMEA, the $600 million turnover division of General Electric specialising in lighting
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