Chapter 1. Introduction
1.3 Flexible and transparent AgNW electrodes and device applications
1.3.1 Touch screen panels
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AgNW network electrodes showed high transparency with superior robustness. One of the most important factor of touch screen panel is the uniformity of the sheet resistance of TCEs. However, the uniformity of conventional AgNW networks are not comparable to that of ITO films. To address this issue, dynamic heating method based on infrared light was reported for the uniform sheet resistance (Figure 1.8b).59 By overcoming the coffee ring effect during the drying process with dynamic heated method, highly uniform AgNW TCEs are fabricated for the realization of uniform touch screen panels with excellent stability. For the fabrication highly transparent and precise touch screen panels, Jang and co-workers reported high-performance TCE films using a silver grid (Ag grid)/AgNW hybrid structure (Ag/NW-GFRHybrimer) (Figure 1.8c).60 The fabricated Ag/NW-GFRHybrimer shows good optoelectronic performance (Rs of 13 Ω sq-1 at 87%) with superior thermal, chemical, and mechanical stability, enabling the four-wire resistive-type touch screen panel with stable operation. Recently, a healable touch screen panel was demonstrated using healable and flexible AgNW electrodes (Figure 1.8d).61 The fabricated four-wire resistive-type touch screen panels with healable ability shows good touch accuracy with outstanding performance recovery after healing process.
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Figure 1.8. Resistive-type touch screen panel applications using the AgNW networks. (a) Demonstration of applying a very long AgNW transparent conductor for a touch screen panel (J. Lee et al. Nanoscale 2012, 4, 6408). (b) Demonstration for exact writing input using the uniform AgNW TCE- based flexible touch screen panel (Y. Jia et al. ACS Appl. Mater. Interfaces 2016, 8, 9865). (c) Demonstration of touch screen panel using the Ag/NW-GFRHybrimer films (J. Jang et al. ACS Appl.
Mater. Interfaces 2016, 8, 27035). (d) Demonstration of healable touch screen panel using AgNW healable conductor on the LCD monitor (J.-S. Bae et al. Adv. Mater. Technol. 2018, 3, 1700364).
Compared to the resistive-type touch screen panels, a capacitive-type captures the pressure point by measuring the capacitance change. The capacitive-type touch screen panel is generally preferred over resistive because less pressure is required for the touch sensing via the human body’s electrical current. Another advantage of capacitive-type touch screen panel is good stability compared to the resistive-type. Moreover, capacitive-type touch screen panel provides higher quality of visibility than resistive-type for the application in the smart displays. However, the fabrication process of capacitive-type touch screen panel is so complicated that requires the fine patterning of a TCEs for the high touch sensing accuracy. The first AgNW-based capacitive-type touch screen panel was reported using spray coated random AgNW electrodes (Figure 1.9a).62 The fabricated capacitive-type touch screen panel detects the capacitance change in four different positions without any response of the touch
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during concave or convex flexions. For the fine patterning of AgNW networks, a laser ablation method is used for line patterning ranging from 5 to 50 µm for fabricating capacitive touch screen panels (Figure 1.9b).63 The diamond shaped capacitive touch screen panel composed of 2 × 2 pixels successfully detects the capacitance change upon finger touching. For the wearable multifunctional sensors, capacitive-type touch screen sensor composed of screen-printed AgNW electrode and Ecoflex as a dielectric was demonstrated (Figure 1.9c).64 This capacitive-type touch screen sensor can differentiate the two different kinds of touch modes such as proximity mode (no pressure) and pressing mode (with pressure) by measuring capacitance change upon finger movements. The stretchable capacitive-type AgNW-based touch screen panel was also demonstrated (Figure 1.9d).65 Here, the AgNW networks adhere permanently to the surface of PDMS with the sigma-donating ability and hydrophilicity of silane modified surfaces, resulting in a fabrication of robust and stable device applications. To recover the surface damage, healable capacitive touch screen sensor based on transparent composite electrodes consisting of AgNW and healable Diels-Alder cycloaddition copolymer was fabricated (Figure 1.9e).66 The healable touch screen sensor comprised of 8 × 8 pixels shows touch sensing performance before the cutting by a razor blade and after the healing processes at 80 ℃ for 30 s. Recently, stretchable and transparent touch screen sensors based on capacitive-type with 5 × 5 pixels was demonstrated with a simple structure of two AgNW/reduced GO (rGO) electrodes and polyurethane (PU) dielectric layer in between them (Figure 1.9f).67 Thanks to the strain-insensitivity of patterned AgNWs/rGO lines embedded in PU dielectrics on PDMS, the fabricated touch screen sensor shows touch sensing ability in accordance with capacitance change. A variety of developed capacitive-type touch screen panels are applicable to future wearable and human-machine interface applications with conformability and good mechanical robustness.
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Figure 1.9. Capacitive-type touch screen panel applications using the AgNW networks. (a) Spray coated flexible AgNW electrode-based capacitive touch screen panel (C. Mayousse et al.
Nanotechnology 2013, 24, 215501). (b) Capacitive touch screen panel with diamond patterns fabricated by selective laser ablation of AgNW percolation networks (S. Hong et al. J. Nanosci. Nanotechnol.
2015, 15, 2317). (c) Screen printed AgNW TCE-based capacitive touch screen panel (S. Yao & Y. Zhu Nanoscale 2014, 6, 2345). (d) Capacitive AgNW PDMS touch screen panel (H. Lee et al. Adv. Funct.
Mater. 2014, 24, 3276). (e) Healable transparent capacitive touch screen panel based on healable AgNW-polymer composite electrodes (J. Li et al. ACS Nano 2014, 8, 12874). (f) Transparent and stretch-unresponsive capacitive touch screen panel with selectively patterned AgNW/rGO electrodes (T.
Y. Choi et al. ACS Appl. Mater. Interfaces 2017, 9, 18022).
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