For the first time, scientists have combined light sensors and polymer-based electronics to create a flexible sensor that lights up when touched. The more pressure put on the sensor, the brighter it gets.
It seems like a simple thing to do, but in fact it's hugely challenging to combine light sensors and electronics made of different crystalline materials onto the same foundation, or substrate. Crystalline materials have microscopic structures that work against each other when put down on the same surface.
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But Ali Javey, an associate professor of computer sciences at the University of California at the University of California, Berkeley, and his colleagues found a way. They came up with two printing methods to attach nanowire circuitry and photosensors to a flexible surface, and the two solutions could open the door on exotic technologies, such as motion-sensitive walls that light up when a person comes near.
To make their flexible, paper-thin sensor, Javey's team started with an ordinary layer of silicon as the bed. Over that, they laid a thin sheet of polymer, called polymide. The sheet is like paper, Javey said - delicate to work with, but strong enough when it has support. Next, they tried two different techniques to "print" the electronics to the silicon bed.
One involved a roller method. They grew two different kinds of wiry nanostructures on the surface of a cylinder: cadmium selenide nanowires for use as the light sensors gernamium-silicon nanowires for use as transistors. Next, then rolled the cylinder across the substrate, essentially painting on the nanowires.
In the other method, they grew the two different kinds of wiry nanostructures on a flat substrate, inverted it and pressed them onto the substrate.
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In both cases, they covered the printed surfaces with another layer of polyamide to protect them. And in the final step, they peeled the silicon substrate away from the printed part, leaving behind the painted nanowire circuitry. The circuitry can even be made stretchable, if it's designed to have lots of tiny holes, taking on a mesh-like structure.
When a person presses on the sensor, the pressure alters the amount of current going to each nanowire "pixel," causing it to light up. The more pressure, the brighter the light.
This is different from touchscreen displays on tablets or phones, which sense changes in the tiny electrical force emitted by a person's finger.
Javey said that although his sensor lights up, it doesn't have to. He integrated the photosensors so that he could demonstrate the sensor's sensitivy in a visual way. The signal from a finger press could be sent to any device or devices. Instead of merely a passive sensor, it is interactive, combining sensing and processing. "It's a whole new type of electronic component," he said.
The research was published in this week's Nature Materials.
Credit: Chuan Wang and Ali Javey