A Fabrication Method for Highly Stretchable Conductors with Silver Nanowires

Summary

A simple synthesis method is used to chemically solder silver nanowire thin film to fabricate highly stretchable and conductive metal conductors.

Abstract

Stretchable electronics are identified as a key technology for electronic applications in the next generation. One of the challenges in fabrication of stretchable electronic devices is the preparation of stretchable conductors with great mechanical stability. In this study, we developed a simple fabrication method to chemically solder the contact points between silver nanowire (AgNW) networks. AgNW nanomesh was first deposited on a glass slide via spray coating method. A reactive ink composed of silver nanoparticle (AgNPs) precursors was applied over the spray coated AgNW thin films. After heating for 40 min, AgNPs were preferentially generated over the nanowire junctions to solder the AgNW nanomesh, and reinforced the conducting network. The chemically modified AgNW thin film was then transferred to polyurethane (PU) substrates by casting method. The soldered AgNW thin films on PU exhibited no obvious change in electrical conductivity under stretching or rolling process with elongation strains up to 120%.

Introduction

Deformable electronic devices with large stretchability have been identified as critical parts to the realization of wearable and portable electronics in the next generation.¹ Those stretchable electronic devices not only show great flexibility as those electronic devices on plastic sheets,^{2, 3} but also exhibit excellent performance under severe stretching or twisting conditions.⁴ To realize the stretchable electronics, materials with great electric performance under large deformation is needed. Recent advancements in material sciences have shown the possibility to synthesize such functional materials and have used them to design stretchable optoelectronic devices ^5-9 with great tolerance to complex shape deformations. Among all the electronic functional materials, stretchable conductors are necessary to supply electrical power to those optoelectronic devices and thus are of critical importance for device performance. Because regular conducting materials, like metal or indium tin oxide, lack of mechanical robustness under large deformation, interconnects made of these materials are unable to exhibit good electrical conductivity under stretching process. Thus, elastic substrates covered with a thin layer of flexible conductive materials, such as carbon nanotube,¹ graphene,¹⁰ or AgNWs,^11-14 are designed for conductors with excellent stretchability. Because of the high bulk conductivity, AgNW thin films have been shown to be the most promising material for composite stretchable conductors.¹³ The percolating networks of AgNW thin films can effectively accommodate large elastic deformations in stretching process with great electrical conductance, and are considered as a promising stretchable electrode candidate. To implement AgNW thin films as stretchable conductors, it is necessary to have effective electrical contacts between AgNWs. After liquid deposition and drying on substrate surfaces, AgNWs regularly stack together to form a percolating mesh with loose contact points, which yield in large electrical resistances. Thus, one needs to anneal the contacts between nanowires by high-temperature or high-pressure annealing methods^15-20 to reduce the contact resistances.

In contrast to these annealing processes in the literature, here, we will demonstrate a simple chemical method to anneal AgNW network connections under regular laboratory conditions.²¹ The fabrication process is shown in Figure 4A. A reactive ink is used to sinter the spray coated AgNW thin films on a glass plate. After reaction, the contacts between nanowires are covered with silver and hence the AgNW network is soldered chemically together. A cast-and-peel method is then used to transfer the soldered AgNW network to a stretchable PU substrate to form a composite conductor, which can exhibit no obvious change in electrical conductivity even at large tensile strain of 120%.

Protocol

1. Preparation of Silver Precursor Ink Add 1.85 g of diethanolamine (DEA) in 3.15 ml deionized water. Dissolve 0.15 g of silver nitrate in 5 ml deionized water. Mix the aqueous silver nitrate solution with DEA at a 1:1 volume ratio to have 10 ml silver precursor ink right before use. 2. Fabrication of Stretchable Conductive Thin Films Preparation of AgNW ink Dilute 2 ml of 0.5 wt% AgNWs in isopropanol with 18 ml de…

Representative Results

The morphology of the AgNW thin film after chemical soldering process is shown in Figure 4B. Recovered AgNPs preferentially grow on the surface of AgNWs and wrap over the wire/wire junctions. Figure 5 shows the variation in sheet resistance with applied elongation strains for the unsoldered and the soldered thin films containing different amount of AgNWs. After the chemical soldering process, AgNW thin film conductors can maintain high conductivity under …

Discussion

The chemical soldering process can help reinforce the contact between silver nanowires. As shown in Figure 4b, the wire/wire junctions are covered with silver after applying the reactive silver ink over the spray coated AgNW thin film. The silver recovery relies strongly on the formaldehyde generated from DEA degradation, and thus the soldering process or silver reduction can be accelerated with increasing temperature.²² Because the metal surfaces of AgNWs provide effective electron exchange s…

Materials

Silver nanowire	Sigma-Aldrich	778095-25ML	AgNW, 120 nm in diameter and 20-50 mm in length, 0.5 wt% in IPA
Silver nitrate crystal	Macron Fine Chemicals	MK216903
Diethanolamine	Sigma-Aldrich	D8885-500G
Polyurethane emulsion	First Chemical	20130326036	35 wt% water-based anionic polyester-polyurethane emulsion
Airbrush	Taiwan Airbrush & Equipment	AFC-sensor
Desktop robot	Dispenser Tech	DT-200
Digital dispenser controller	Dispenser Tech	9000E
Auto-spraying program	Dispenser Tech	Smart robot edit version 3.0.0.5
Air compressor	PUMA Industrial	NCS-10
Linear motorized stage	TANLIAN E-O	Customized
Stage control software	TANLIAN E-O	Customized
Digital multimeter	HILA INTERNATIONAL	DM-2690TU
Digital multimeter software	HILA INTERNATIONAL	NA
Power supply	CHERN TAIH	CT-605
LED	PChome	M08330766	http://www.pcstore.com.tw/sun-flower/M08330766.htm