Flexible arrays of tactile pixels improve touch sensing


GEORGIA Tech researchers have developed flexible piezoelectric “taxel” arrays capable of converting mechanical motion directly into electronic controlling signals.

Potential applications include robots with adaptive touch sensing, security for handwritten signatures, and new user interfaces for electronic devices.

The touch-sensitive arrays are constructed using bundles of vertical zinc oxide nanowires. They include more than 8000 functioning piezotronic transistors, each of which produces an electronic controlling signal when placed under mechanical strain.

The researchers call these touch-sensitive transistors “taxels” (tactile pixels), and say their sensitivity is on par with that of a human fingertip, providing significant improvements in resolution (better than 100 microns), sensitivity (as low as 10 kilopascals) and active/adaptive operations compared to current electronic touch sensing.

The vertically-aligned taxels operate with two-terminal transistors. Instead of a third gate terminal used by conventional transistors to control the flow of current passing through them, taxels control the current with a technique called “strain-gating”.

Unlike the pressure sensitivity of many conventional touch sensors, this technique utilises the electrical charges generated at the Schottky contact interface by the piezoelectric effect when the nanowires are placed under strain by the application of mechanical force.

The technique only works in materials that have both piezoelectric and semiconducting properties. These properties are seen in nanowires and thin films created from the wurtzite and zinc blend families of materials, which includes zinc oxide, gallium nitride and cadmium sulfide.

The laboratory prototype arrays are populated with 92 by 92 transistors. The researchers used a chemical growth technique at approximately 85 to 90 degrees Celsius, which allowed them to fabricate arrays of strain-gated vertical piezotronic transistors on substrates that are suitable for microelectronics applications.

The transistors are made up of bundles of approximately 1,500 individual nanowires, each nanowire between 500 and 600 nanometers in diameter.

The active strain-gated vertical piezotronic transistors are sandwiched between top and bottom electrodes made of indium tin oxide aligned in orthogonal cross-bar configurations. A thin layer of gold is deposited between the top and bottom surfaces of the zinc oxide nanowires and the top and bottom electrodes, forming Schottky contacts. A thin layer of the polymer Parylene is then coated onto the device as a moisture and corrosion barrier.

The arrays are transparent, which could allow them to be used on touch-pads or other devices for fingerprinting. They are also flexible and foldable.