Letter pubs.acs.org/NanoLett
Stretchable, Transparent Electrodes as Wearable Heaters Using Nanotrough Networks of Metallic Glasses with Superior Mechanical Properties and Thermal Stability Byeong Wan An,† Eun-Ji Gwak,‡ Kukjoo Kim,† Young-Cheon Kim,‡ Jiuk Jang,† Ju-Young Kim,*,‡ and Jang-Ung Park*,†,§ †
School of Materials Science and Engineering, Wearable Electronics Research Group, Center for Smart Sensor Systems, ‡School of Materials Science and Engineering, and §School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan Metropolitan City 44919, Republic of Korea S Supporting Information *
ABSTRACT: Mechanical robustness, electrical and chemical reliabilities of devices against large deformations such as bending and stretching have become the key metrics for rapidly emerging wearable electronics. Metallic glasses (MGs) have high elastic limit, electrical conductivity, and corrosion resistance, which can be promising for applications in wearable electronics. However, their applications in wearable electronics or transparent electrodes have not been extensively explored so far. Here, we demonstrate stretchable and transparent electrodes using CuZr MGs in the form of nanotrough networks. MG nanotroughs are prepared by electrospinning and cosputtering process, and they can be transferred to various desired substrates, including stretchable elastomeric substrates. The resulting MG nanotrough network is first utilized as a stretchable transparent electrode, presenting outstanding optoelectronic (sheet resistance of 3.8 Ω/sq at transmittance of 90%) and mechanical robustness (resistance change less than 30% up to a tensile strain of 70%) as well as excellent chemical stability against hot and humid environments (negligible degradation in performance for 240 h in 85% relative humidity and 85 °C). A stretchable and transparent heater based on the MG nanotrough network is also demonstrated with a wide operating temperature range (up to 180 °C) and excellent stretchability (up to 70% in the strain). The excellent mechanical robustness of these stretchable transparent electrode and heater is ascribed to the structural configuration (i.e., a nanotrough network) and inherent high elastic limit of MGs, as supported by experimental results and numerical analysis. We demonstrate their real-time operations on human skin as a wearable, transparent thermotherapy patch controlled wirelessly using a smartphone as well as a transparent defroster for an automobile side-view mirror, suggesting a promising strategy toward next-generation wearable electronics or automobile applications. KEYWORDS: Metallic glasses, transparent electrodes, stretchable electronics, stretchable heaters, wearable electronics
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despite their submicron scale. In spite of these outstanding chemical stability and mechanical stretchability, their potential has not been studied for use in wearable electronics, which has been rapidly emerging with novel applications, such as stretchable displays,19 smart sensors,20−22 and portable healthcare devices,23 beyond the conventional rigid-type electronics. Sufficient levels of flexibility and stretchability of the electronic devices are essential to realize the practical comfort against wearing. In this respect, various conductive materials, including carbon nanotubes,24−28 conducting polymers,24 graphene,29−35 metal meshes,36−38 metal nanowires,25,39−44 and metal nano-
etallic glasses (MGs), amorphous metal alloys, have attracted great interest due to their extraordinary properties, such as high elastic limit, high resistance to corrosion and wear, soft-magnetic properties, and superconducting behavior,1−4 originating from their noncrystalline nature. A large number of MG systems have been reported using Cu-, Zr-, Ti-, Mg-, Fe-, Ni-, Pd-, Pt-, Al-, and Au-based alloys since 1960s.5−7 In recent decades, the development of processing techniques of MGs for submicron scale thin films and patterns has opened a new chapter in studies and applications of MGs, for example, in microelectromechanical system (MEMS) devices.8−13 Various micro- and nanoscale layouts of MGs have been introduced by lithography, ion etching, thermal imprinting, local laser heating, and nanomolding process,10,14−18 maintaining their unique properties, such as high corrosion resistance and mechanical deformability, © XXXX American Chemical Society
Received: October 10, 2015 Revised: December 2, 2015
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DOI: 10.1021/acs.nanolett.5b04134 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 1. Fabrication process and optoelectronic properties of the CuZr nanotrough network. (a) Schematic illustration of the fabrication process of the CuZr nanotrough network. (b) SEM image of a CuZr nanotrough (left, scale bar is 1 μm) and TEM image with fast Fourier transformationfiltered electron diffraction pattern (right, blue dashed line, scale bar is 4 nm). (c) Photograph of the CuZr nanotrough network on a PDMS substrate. Scale bar, 1 cm. Inset shows a magnified optical microscope image of the CuZr nanotrough network. Scale bar, 100 μm. (d) Sheet resistance of the CuZr nanotrough network as a function of polymer electrospinning time. (e) Transmittance versus sheet resistance of the CuZr nanotrough network. (f) Haze of the CuZr nanotrough network as a function of polymer electrospinning time. Inset shows the haze spectra of 5 s electrospun samples.
troughs,45,46 have been widely explored for application in stretchable, transparent conductors for electrodes or heaters. Herein, we report a novel application to stretchable, transparent electrodes and heaters in the form of nanotrough networks using MGs. To the best of our knowledge, this is the first attempt to utilize MGs for stretchable transparent electrodes or heaters. In this work, a CuZr alloy is chosen due to its superb glass-forming ability,47 excellent mechanical properties,48 and processability in sputtering. Electrospinning a sacrificial polymer web, cosputtering of Cu and Zr, and then selectively removing the polymer provide an amorphous CuxZr1−x alloy as forms of nanotrough networks within a composition range in 35 ≤ x ≤ 70.49 The resulting CuZr nanotrough network is utilized as a stretchable, transparent electrode with outstanding optoelectronic (sheet resistance of 3.8 Ω/sq at transmittance of 90%) and mechanical properties (minimum bending radius of ∼3.8 μm and maximum stretching up to 70% in tensile strain) as well as excellent chemical stability against hot and humid environments (stable for 240 h in 85% relative humidity (RH) and 85 °C). This sheet resistance value of the CuZr nanotrough electrode is much lower than those of ITO (
