Sensor captures signatures using tiny LEDs

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.

"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.

"This is a new principle for imaging force that uses parallel detection and avoids many of the complications of existing pressure sensors."

In research published in the journal Nature Photonics yesterday, the researchers explain how 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 piezo-phototronics, the technology – first described by Wang in 2009 – provides a new way to capture information about pressure applied at very high resolution: up to 6,300 dots per inch.

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. "The presence of the 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.

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 and 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," said 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 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 five per cent, 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.