Using thousands of nanometer-scale wires, researchers at the Georgia Institute of Technology have developed a sensor device that converts mechanical pressure – from a signature or a fingerprint – directly into light signals that can be captured and processed optically. The sensor device could provide an artificial sense of touch, offering sensitivity comparable to that of the human skin. Beyond collecting signatures and fingerprints, the technique could also be used in biological imaging and micro-electromechanical (MEMS) systems. Ultimately, it could provide a new approach for human-machine interfaces.
Individual zinc oxide (ZnO) nanowires that are part of the device operate as tiny light emitting diodes (LEDs) when placed under strain from the mechanical pressure, allowing the device to provide detailed information about the amount of pressure being applied. Known as piezophototronics, the technology – first described by Zhong Lin Wang, Regents’ professor and Hightower Chair in the School of Materials Science and Engineering at Georgia Tech, in 2009 – provides a new way to capture information about pressure applied at very high resolution: up to 6,300 dots per inch.
Piezoelectric materials generate a charge polarization when they are placed under strain. The piezo-phototronic devices rely on that physical principle to tune and control the charge transport and recombination by the polarization charges present at the ends of individual nanowires. Grown atop a gallium nitride (GaN) film, the nanowires create pixeled light emitters whose output varies with the pressure, creating an electroluminescent signal that can be integrated with on-chip photonics for data transmission, processing and recording.
Placing a zinc oxide nanowire under strain creates a piezoelectric charge at both ends, which forms a piezoelectric potential. The presence of that potential distorts the band structure in the wire, causing electrons to remain in the p-n junction longer and enhancing the efficiency of the LED. 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 gallium nitride film.
To fabricate the devices, a low-temperature chemical growth technique is used to create a patterned array of zinc oxide nanowires on a gallium nitride thin film substrate with the c-axis pointing upward. The interfaces between the nanowires and the gallium nitride 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, either through handwriting or another source of pressure, nanowires are compressed along their axial directions, creating a negative piezo-potential, while uncompressed nanowires have no potential. 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.
The ability to see all of the emitters simultaneously allows the device to provide a quick response. Up to a million pixels can be read in a microsecond. The nanowires stop emitting light when the pressure is relieved. Switching from one mode to the other takes 90 milliseconds or less.
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 and found that the output fluctuation was approximately five 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 microns has been recorded from the device samples tested so far, and it is believed that the resolution could be improved by reducing the diameter of the nanowires – allowing more nanowires to be grown in a given space – and by using a high-temperature fabrication process.
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