Sensor converts mechanical pressure to light signals

A new sensor device uses thousands of nanowires to convert mechanical pressure into light signals that can be captured and processed optically. Applications for the sensor could include biological imaging and microelectro-mechanical systems. The device also could provide a new approach for human-machine interfaces, as it has the potential to produce an artificial sense of touch as sensitive as that of human skin.

“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, a professor at the Georgia Institute of Technology. “This is a new principle for imaging force that uses parallel detection and avoids many of the complications of existing pressure sensors.”

This schematic of the device 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. (SU8 = an epoxy-based negative photoresist; n-ZnO NWs = n-type zinc oxide nanowires; p-GaN = p-type doped gallium nitride; PMMA = poly(methyl methacrylate), also known as acrylic glass or the trade name Plexiglas). Photo courtesy of Zhong Lin Wang

In the device, individual zinc oxide nanowires operate as tiny LEDs, providing detailed information when placed under strain from mechanical pressure. When they are compressed along their axial direction – which is whenever pressure is applied to the device – a negative piezo potential results; uncompressed nanowires have no potential. Differences in the amount of pressure applied translate to differences in light emitted from the root, where the nanowires contact gallium nitride film.

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.

Because all of the emitters can be seen simultaneously, the device works quickly. “You can read a million pixels in a microsecond,” Wang said. “When the light emission is created, it can be detected immediately with the optical fiber.”

To fabricate the device, a low-temperature chemical growth technique creates a patterned array of zinc oxide nanowires on a gallium nitride thin-film substrate. After the space between nanowires is infiltrated with a poly(methyl methacrylate) (PMMA) thermoplastic, oxygen plasma is used to etch away the PMMA enough to expose the tops of the zinc oxide nano-wires. A nickel-gold electrode forms ohmic contact with the bottom gallium-nitride film, and a transparent indium-tin oxide film is deposited on top of the array to function as a common electrode.

The research was published in Nature Photonics (doi: 10.1038/nphoton.2013.191).