Researchers Design Innovative Sensor for Heat, Light, and Touch

Prostheses that respond to touch, robotics, and health monitoring are three areas in which scientists worldwide are working to create electronic skin. They want that skin to be flexible and to consist of some form of sensitivity. Scientists at the Laboratory of Organic Electronics at Linköping University have recently made some progress towards such a system by integrating a number of physical phenomena and materials. The outcome is a sensor that, akin to human skin, can sense temperature difference that originates from the touch of a warm object, in addition to the heat from solar radiation.

Mina Shiran Chaharsoughi and Magnus Jonsson. (Image credit: Thor Balkhed)

Inspiration from nature

“We have been inspired by nature and its methods of sensing heat and radiation”, says Mina Shiran Chaharsoughi, doctoral student in the Organic Photonics and Nano-optics group at the Laboratory of Organic Electronics.

Along with colleagues, she has built a sensor that integrates pyroelectric and thermoelectric effects with a nano-optical occurrence.

A voltage forms in pyroelectric materials when they are cooled or heated. It is the variation in temperature that gives a signal, which is fast and powerful, but that decays nearly as quickly.

In comparison, a voltage forms in thermoelectric materials when the material has one hot and one cold side. The signal here forms gradually, and a little time must pass before it can be measured. The heat may arise from the sun or from a warm touch; all that is necessary is that one side is colder than the other.

We wanted to enjoy the best of both worlds, so we combined a pyroelectric polymer with a thermoelectric gel developed in a previous project by Dan Zhao, Simone Fabiano and other colleagues at the Laboratory of Organic Electronics. The combination gives a rapid and strong signal that lasts as long as the stimulus is present.

Magnus Jonsson, Leader of the Organic Photonics and Nano-optics group, Laboratory of Organic Electronics, Linköping University

Moreover, it turned out that the two materials interact in a way that strengthens the signal.

Plasmons

The new sensor also uses another nano-optical entity called plasmons.

Plasmons arise when light interacts with nanoparticles of metals such as gold and silver. The incident light causes the electrons in the particles to oscillate in unison, which forms the plasmon. This phenomenon provides the nanostructures with extraordinary optical properties, such as high scattering and high absorption.

Magnus Jonsson, Leader of the Organic Photonics and Nano-optics group, Laboratory of Organic Electronics, Linköping University

In earlier work, he and his colleagues have shown that a gold electrode that has been perforated with nanoholes absorbs light well with the help of plasmons. The absorbed light is then converted to heat. With this kind of an electrode, a thin gold film with nanoholes, on the side that faces the sun, the sensor can also change visible light quickly to a stable signal.

As an extra advantage, the sensor is also pressure-sensitive.

“A signal arises when we press the sensor with a finger, but not when we subject it to the same pressure with a piece of plastic. It reacts to the heat of the hand”, says Magnus Jonsson.

Along with Mina Shiran Chaharsoughi and Magnus Jonsson, their colleagues Dan Zhao, Simone Fabiano and Professor Xavier Crispin at the Laboratory of Organic Electronics have also contributed to the research, the results of which have in recent times been published in the scientific journal Advanced Functional Materials.

The research has been funded by, among other sources, the Swedish Foundation for Strategic Research, the Swedish Research Council, the Wenner-Gren Foundations, and the Strategic Initiative in Advanced Functional Materials, AFM, at Linköping University.

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