Sensor device converts signatures into LEDs

Researchers at Georgia Institute of Technology have developed a sensor device that converts a signature or a fingerprint directly into light signals that can be captured and processed optically.

The technique that converts mechanical pressure into LEDs could also be used in biological imaging and microelectromechanical (MEMS) systems. Ultimately, it could provide a new approach for human-machine interfaces.

“You can write with your pen and the sensor will optically detect what you write at high resolution and with a very fast response rate,” said Zhong Lin Wang, Regents’ professor and Hightower Chair in the School of Materials Science and Engineering at Georgia Tech.

Known as piezophototronics, the technology provides a new way to capture information about pressure applied at very high resolution: up to 6300 dots per inch. Piezoelectric materials generate a charge polarisation when they are placed under strain. The piezophototronic devices rely on that physical principle to tune and control the charge transport and recombination by the polarisation 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.

“When you have a zinc oxide nanowire under strain, you create a piezoelectric charge at both ends which forms a piezoelectric potential,” Wang explained. 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 through handwriting or other source, nanowires are compressed along their axial directions, creating a negative piezo-potential, while uncompressed nanowires have no potential.

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.

The ability to see all of the emitters simultaneously allows the device to provide a quick response. The nanowires stop emitting light when the pressure is relieved. Switching from one mode to the other takes 90 milliseconds or less, Wang said.

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%, 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 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.