A Transparent and Flexible Capacitive‐Force Touch Pad from High‐Aspect‐Ratio Copper Nanowires with Enhanced Oxidation Resistance for Applications in Wearable Electronics

Abstract

Copper nanowires are widely utilized for flexible electronics applications due to their excellent electrical conductivity, mechanical flexibility, and optical transparency with very low material cost. While many previous studies are dedicated to developing effective synthesis routes for copper nanowires, most of them have focused on the control of the morphology including length and diameter rather than synthesis yields. Although many postprocessing methods have been established to make use of the copper nanowires, there still remains crucial weakness in the nanowires against oxidation stability. In this study, a new synthesis method for the morphology control of copper nanowires as well as synthesis yields is introduced. After optimizing of the copper nanowire synthesis, a copper‐nanowire‐based flexible transparent conductor is fabricated as a highly robust electrode by using UV‐curable polyurethane acrylate resin. As a proof‐of‐concept, a flexible transparent capacitive‐force‐detection touch pad is demonstrated. The developed flexible transparent copper nanowire electrode with enhanced oxidation resistance is expected to be applied in various flexible and wearable electronics applications.

Aqueous Synthesis, Degradation, and Encapsulation of Copper Nanowires for Transparent Electrodes

“…A large drawback of CuNWs that cannot be omitted is their gradual oxidation at ambient conditions, which is readily increased for temperatures above 100 °C [ 49 ]. As tested by a few groups with success, this oxidation can be slowed by encapsulating the CuNWs with following approaches: (i) applying polymer coatings composed of polydimethylsiloxane (PDMS) [ 49 ], polymethylmethacrylate (PMMA) [ 49 ], PEDOT:PSS [ 50 ], and polyurethane acrylate resin [ 51 ] (ii) atmospheric pressure spatial atomic layer deposition (AP-SALD) of aluminum oxide (Al 2 O 3 ) and (iii) the electrodeposition of zinc, tin and indium shells onto the nanowires, followed by their oxidation [ 30 ]. Besides encapsulation, there are also works that report on the laser-induced nanowelding of CuNWs [ 52 ] and even the photothermochemical reduction [ 53 , 54 ] of oxidized and non-conductive CuNW networks to highly conductive ones.…”

Buckled Structures: Fabrication and Applications in Wearable Electronics

“…Sometimes, buckled electrodes of pressure sensors were divided by extra spacers such as electrospun mats to increase the sensitivity (Figure e) . Compared with resistive pressure sensors, capacitive pressure sensors have been more widely used for touch panel and positioning using metal nanowires as flexible electrodes . The buckling geometry is the key to making sensors stretchable; thorough reviews on this topic are available …”

70‐2: Low Temperature Process and Material Development for Flexible/Stretchable Transparent Conductor

“…The transparent conductor made from nanoparticle can be flexible but usually does not work at high stretchable condition. More advanced applications for transparent flexible/transparent conductor can be found in metal nanowires including silver [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] and copper [29][30][31][32][33] nanowire percolation networks. Most metal nanowire percolation network research as the ITO replacement started for flexible transparent conductor but recently it was found that the similar metal percolation network can be applied for highly stretchable transparent electrode.…”