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Electronic Materials

Silver nanowires make transparent fingerprint sensor prototype for smartphones

Sensor material outperforms conventional material by combining high transparency and low resistance

by Cici Zhang
July 7, 2018 | A version of this story appeared in Volume 96, Issue 28

Detection of a fingerprint is achieved by capturing the capacitance difference between ridges and valleys of a human fingerprint.
Credit: Nat. Commun.
A transparent sensor identifies fingerprints via changes in capacitance signals, which are highest at the ridges of a print (red) and lowest in the valleys (green).

Fingerprint sensors allow us to unlock our phones with a tap. Yet their opaqueness is a problem for developing devices with edge-to-edge screens. To address that issue, researchers at Ulsan National Institute of Science & Technology (UNIST) used silver nanostructures to create a transparent fingerprint sensor that can detect fingerprints anywhere on a phone display (Nat. Commun. 2018, DOI: 10.1038/s41467-018-04906-1). One way to detect a fingerprint is to sense the capacitance difference created by air gaps between a print’s ridges and valleys. But the conventional transparent electrode material, indium tin oxide (ITO), cannot be used for this application because its resistance is too high, says Franklin Bien, an electrical engineer at UNIST. So Bien, Jang-Ung Park, and coworkers used an electrospinning method to form a highly transparent mesh from silver nano­fibers measuring roughly 350 nm in diameter. Then they decreased the mesh resistance by depositing ultrathin Ag nano­wires (20-nm diameter) on the mesh surface. The team showed that their sensor, which combines high transparency and low resistance, is 17 times as sensitive to variations in capacitance as comparable ITO-based sensors. The researchers also incorporated temperature and pressure sensors to make detecting human fingerprints more secure. Bien says the plan is to bring the technology to the hands of users in two years.


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