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Fast Plasmonic Laser Nanowelding for a Cu-Nanowire Percolation Network for Flexible Transparent Conductors and Stretchable Electronics Seungyong Han, Sukjoon Hong, Jooyeun Ham, Junyeob Yeo, Jinhwan Lee, Bongchul Kang, Phillip Lee, Jinhyeong Kwon, Seung S. Lee, Min-Yang Yang,* and Seung Hwan Ko* Nanomaterials offer many novel approaches for high-performance devices and fabrication processes for diverse applications, such as transparent conductor electrodes (TCEs),[1,2] organic light-emitting diodes (OLEDs),[3,4] thermo-electrics, sensors,[5] and thin-film solar cells.[6] In particular, nanomaterials have been actively pursued by researchers in hopes of developing novel transparent conductors, especially for flexible or stretchable electrodes for next-generation devices,[7,8] because the demands for thin-film and large-area devices are constantly increasing in the optoelectronics field. In the current transparent-conductor industry, indium tin oxide (ITO) film is most frequently used for optoelectronic devices, such as thin-film solar cells, flat-panel displays, and touch-screen panels. However, due to the unpredictable supply of indium, the large material waste involved in production,[9] and the slow production speed,[10] the price of ITO film is increasing rapidly. Most of all, although ITO can be made to coat flexible substrates, its brittle ceramic nature limits its application in flexible and stretchable devices. As an alternative to ITO, carbon-based nanomaterials (i.e., carbon nanotubes (CNT)[11,12] and graphene)[13,14] and noblemetal (Au, Ag) nanowires (NWs)[15–17] and microgrids[18] have been used extensively to make transparent conductors, but the S. Han,[+] Dr. S. Hong,[+] Dr. J. Yeo, Prof. S. H. Ko Applied Nano and Thermal Science Lab Department of Mechanical Engineering Seoul National University 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea E-mail: [email protected] S. Han, J. Ham, J. Lee, J. Kwon, Prof. S. S. Lee, Prof. M.-Y. Yang Department of Mechanical Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea E-mail: [email protected] Dr. B. Kang Department of Mechatronics Engineering Gyeongnam National University of Science and Technology Jinju 305-701, Korea Dr. P. Lee Micro & Nano System Lab Department of Mechanical Engineering Massachusetts Institute of Technology 77 Massachusetts Ave., Cambridge, MA 02139, USA [+]These authors contributed equally to this work.
DOI: 10.1002/adma.201400474
Adv. Mater. 2014, DOI: 10.1002/adma.201400474
carbon-based nanomaterials often cannot meet the requirements for an efficient transparent conductor for many applications, and the high price of noble metals (Au and Ag) makes them uncompetitive for large-area, low-cost transparent conductors even though it can exhibit superior conductivity and transmittance. Recently, Cu NWs have received considerable attention as an alternative to Ag NWs for future transparent conductors because the price of Cu is almost a hundred times cheaper than that of other noble metals, while the electrical conductivity is comparable to the metals with the highest conductivities.[9] Despite those advantages, the actual use of Cu NWs for transparent conductors has been limited mainly due to the fast oxidation problem upon the thermal annealing process in air. The oxidation rate of the Cu nanostructure is drastically higher than bulk Cu because the surface area is significantly increased compared with that in the bulk state. This inherent oxidation problem hinders the development of Cu-NW device fabrication processes carried out in ambient conditions. To prevent this oxidation problem, a great deal of research has involved maintaining the annealing under an inert-gas or vacuum environment. However, noble-gas or vacuum environments can result in a considerable increase in production cost and time, cancelling out the low price advantage of Cu. In addition, uniform heating and annealing at high temperature can damage the heat-sensitive flexible or stretchable substrates.[19] In this work, the flexible transparent conductor and stretchable electrode based on a Cu-NW percolation network is reported. Its fabrication involves ultrafast plasmonic nanoscale welding using a circularly polarized laser under ambient conditions and room temperatures are used to minimize the Cu oxidation problem, as anticipated from previous studies involving rapid thermal annealing (RTA) techniques.[20,21] A stretchable electrode and touch-screen panel are fabricated as demonstration devices, confirming the applicability of the resultant transparent conductor in practical devices. Cu NWs were firstly synthesized by a modified nucleationand-growth method in aqueous solution[10,22,23] (see Supporting Information, SI: Figure S1). We obtained reddish Cu NWs (SI: Figure S1b) with a length of 98%). After bulk heating (blue line), due to the existence of oxygen in ambient air that causes the oxidation, elemental Cu becomes CuO (56.0%) or Cu2O (41.2%), and the overall Cu-NW film completely loses its electrical conductivity. However, for the laser-plasmonic-nanowelded film (red line), quantitative XRD analysis revealed that composition change to copper oxides is less than 5 wt% (see SI: Section S5), so the overall resistance of the Cu-NW film is dramatically decreased.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201400474
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COMMUNICATION Figure 2. a) Sheet resistance of Cu-NW transparent conductors subject to bulk heating (black) and plasmonic laser nanowelding (red). Inset: FDTD simulation in the vicinity of the Cu-NW junction at a wavelength of 532 nm. b) Change in the sheet resistance of Cu-NW transparent conductor with multiple laser scanning at different laser powers. c) The sheet resistances of the Cu-NW transparent conductor at different concentrations of Cu-NW solution and following nanowelding using linear or circular polarization. This is the only case where linearly polarized light is employed throughout the study. d) The spectral transmittance of the Cu-NW transparent conductor originating from different Cu-NW solution concentrationsafter plasmonic laser nanowelding.
The theoretical background and mechanism of laser annealing will now be explained. It is well known that the electromagnetic wave can generate thermal heating that is proportional to the electric field intensity adjacent to the target material,[26] while the electric field can be significantly augmented at the junction of piled metallic NWs and produce a hot spot due to the plasmonic heating effect.[27] Such high field enhancement at the junction has been extensively studied in stacked NWs[28] and other nanostructures,[29,30] whereas its analytical solution revealed that the nano-focusing of light at the junction of two NWs is due to the surface plasmon modes that propagate towards the contact point between two NWs.[31] The 3D finite-different time-domain (FDTD) simulation confirms that a similar plasmonic nano-focusing phenomenon at the NW junction can also happen for the Cu-NW-based percolation network. The inset in Figure 2a shows the steady-state intensity distribution at the junction of two NWs stacked in perpendicular or parallel directions when a plane wave with wavelength of λ = 532 nm is incident from the top. The electric field intensity, as well as the corresponding photothermal heating, is calculated to have increased by ~1–2 orders of magnitude at the NW junctions (for more on the FDTD simulations, refer to SI: Section S6) once the laser polarization of the excitation source
Adv. Mater. 2014, DOI: 10.1002/adma.201400474
is perpendicular to the first NW since the top NW acts as a nano-antenna that directs light to the bottom NW,[32] while the enhancement becomes insignificant for the orthogonal polarization. The intensified electric field intensity at the Cu-NW junction consequently causes confined thermal heating at the NW contact point to achieve local nano-welding, which only affects the intersection and not other areas. The main advantage of the plasmonic laser nanowelding method is demonstrated by comparing the change in the electrical resistance as a result of conventional bulk annealing and laser nanowelding under ambient conditions. Figure 2a shows the real-time electrical properties of Cu-NW percolation networks at an area of 8 mm × 8 mm during bulk heating at 200 °C and during laser nanowelding at 100-mW power and a 100-mm/s scanning speed at a 5-µm hatch size. The sheet resistance of the Cu-NW percolation network undergoing bulk heating initially declines due to the removal of residual chemicals and the slight melting of the Cu-NW surface; however, it starts to rebound after ∼200 s on account of the Cu oxidation. As a result, the sheet resistance remains relatively high (~105–106 Ω/sq) at all times during bulk heating as long as the annealing is conducted under ambient conditions. In contrast, laser nanowelding instantaneously drops the resistance
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Figure 3. a) Optical photographs of Cu NWs transferred onto a PI film for bulk-heated (top), as-prepared (middle), and laser-nanowelded (bottom) samples and the cyclic bending test of the Cu-NW transparent conductor. Inset: schematic diagram of the bending test, maximum bending radius = 2 mm, 1.25 Hz) b) Repeated folding (complete bending) test of the Cu-NW transparent conductor.
upon exposure to the focused laser beam, and the final sheet resistance reaches ∼20 Ω/sq after a single scan over the sample is complete. The total processing time required for the 8 mm × 8 mm area was ∼1 min in this example, but the processing time can be reduced down to ∼2 s/cm2 or even shorter (see SI: Section S7). According to the recent study on the sintering of Cu nanomaterials under ambient conditions, the heating time at each point should be less than milliseconds in order to obtain an electrode at low resistivity and having undergone an insignificant amount of oxidation. Plasmonic laser nanowelding together with fast scanning (
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Fast Plasmonic Laser Nanowelding for a Cu-Nanowire Percolation Network for Flexible Transparent Conductors and Stretchable Electronics Seungyong Han, Sukjoon Hong, Jooyeun Ham, Junyeob Yeo, Jinhwan Lee, Bongchul Kang, Phillip Lee, Jinhyeong Kwon, Seung S. Lee, Min-Yang Yang,* and Seung Hwan Ko* Nanomaterials offer many novel approaches for high-performance devices and fabrication processes for diverse applications, such as transparent conductor electrodes (TCEs),[1,2] organic light-emitting diodes (OLEDs),[3,4] thermo-electrics, sensors,[5] and thin-film solar cells.[6] In particular, nanomaterials have been actively pursued by researchers in hopes of developing novel transparent conductors, especially for flexible or stretchable electrodes for next-generation devices,[7,8] because the demands for thin-film and large-area devices are constantly increasing in the optoelectronics field. In the current transparent-conductor industry, indium tin oxide (ITO) film is most frequently used for optoelectronic devices, such as thin-film solar cells, flat-panel displays, and touch-screen panels. However, due to the unpredictable supply of indium, the large material waste involved in production,[9] and the slow production speed,[10] the price of ITO film is increasing rapidly. Most of all, although ITO can be made to coat flexible substrates, its brittle ceramic nature limits its application in flexible and stretchable devices. As an alternative to ITO, carbon-based nanomaterials (i.e., carbon nanotubes (CNT)[11,12] and graphene)[13,14] and noblemetal (Au, Ag) nanowires (NWs)[15–17] and microgrids[18] have been used extensively to make transparent conductors, but the S. Han,[+] Dr. S. Hong,[+] Dr. J. Yeo, Prof. S. H. Ko Applied Nano and Thermal Science Lab Department of Mechanical Engineering Seoul National University 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea E-mail: [email protected] S. Han, J. Ham, J. Lee, J. Kwon, Prof. S. S. Lee, Prof. M.-Y. Yang Department of Mechanical Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea E-mail: [email protected] Dr. B. Kang Department of Mechatronics Engineering Gyeongnam National University of Science and Technology Jinju 305-701, Korea Dr. P. Lee Micro & Nano System Lab Department of Mechanical Engineering Massachusetts Institute of Technology 77 Massachusetts Ave., Cambridge, MA 02139, USA [+]These authors contributed equally to this work.
DOI: 10.1002/adma.201400474
Adv. Mater. 2014, DOI: 10.1002/adma.201400474
carbon-based nanomaterials often cannot meet the requirements for an efficient transparent conductor for many applications, and the high price of noble metals (Au and Ag) makes them uncompetitive for large-area, low-cost transparent conductors even though it can exhibit superior conductivity and transmittance. Recently, Cu NWs have received considerable attention as an alternative to Ag NWs for future transparent conductors because the price of Cu is almost a hundred times cheaper than that of other noble metals, while the electrical conductivity is comparable to the metals with the highest conductivities.[9] Despite those advantages, the actual use of Cu NWs for transparent conductors has been limited mainly due to the fast oxidation problem upon the thermal annealing process in air. The oxidation rate of the Cu nanostructure is drastically higher than bulk Cu because the surface area is significantly increased compared with that in the bulk state. This inherent oxidation problem hinders the development of Cu-NW device fabrication processes carried out in ambient conditions. To prevent this oxidation problem, a great deal of research has involved maintaining the annealing under an inert-gas or vacuum environment. However, noble-gas or vacuum environments can result in a considerable increase in production cost and time, cancelling out the low price advantage of Cu. In addition, uniform heating and annealing at high temperature can damage the heat-sensitive flexible or stretchable substrates.[19] In this work, the flexible transparent conductor and stretchable electrode based on a Cu-NW percolation network is reported. Its fabrication involves ultrafast plasmonic nanoscale welding using a circularly polarized laser under ambient conditions and room temperatures are used to minimize the Cu oxidation problem, as anticipated from previous studies involving rapid thermal annealing (RTA) techniques.[20,21] A stretchable electrode and touch-screen panel are fabricated as demonstration devices, confirming the applicability of the resultant transparent conductor in practical devices. Cu NWs were firstly synthesized by a modified nucleationand-growth method in aqueous solution[10,22,23] (see Supporting Information, SI: Figure S1). We obtained reddish Cu NWs (SI: Figure S1b) with a length of 98%). After bulk heating (blue line), due to the existence of oxygen in ambient air that causes the oxidation, elemental Cu becomes CuO (56.0%) or Cu2O (41.2%), and the overall Cu-NW film completely loses its electrical conductivity. However, for the laser-plasmonic-nanowelded film (red line), quantitative XRD analysis revealed that composition change to copper oxides is less than 5 wt% (see SI: Section S5), so the overall resistance of the Cu-NW film is dramatically decreased.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201400474
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COMMUNICATION Figure 2. a) Sheet resistance of Cu-NW transparent conductors subject to bulk heating (black) and plasmonic laser nanowelding (red). Inset: FDTD simulation in the vicinity of the Cu-NW junction at a wavelength of 532 nm. b) Change in the sheet resistance of Cu-NW transparent conductor with multiple laser scanning at different laser powers. c) The sheet resistances of the Cu-NW transparent conductor at different concentrations of Cu-NW solution and following nanowelding using linear or circular polarization. This is the only case where linearly polarized light is employed throughout the study. d) The spectral transmittance of the Cu-NW transparent conductor originating from different Cu-NW solution concentrationsafter plasmonic laser nanowelding.
The theoretical background and mechanism of laser annealing will now be explained. It is well known that the electromagnetic wave can generate thermal heating that is proportional to the electric field intensity adjacent to the target material,[26] while the electric field can be significantly augmented at the junction of piled metallic NWs and produce a hot spot due to the plasmonic heating effect.[27] Such high field enhancement at the junction has been extensively studied in stacked NWs[28] and other nanostructures,[29,30] whereas its analytical solution revealed that the nano-focusing of light at the junction of two NWs is due to the surface plasmon modes that propagate towards the contact point between two NWs.[31] The 3D finite-different time-domain (FDTD) simulation confirms that a similar plasmonic nano-focusing phenomenon at the NW junction can also happen for the Cu-NW-based percolation network. The inset in Figure 2a shows the steady-state intensity distribution at the junction of two NWs stacked in perpendicular or parallel directions when a plane wave with wavelength of λ = 532 nm is incident from the top. The electric field intensity, as well as the corresponding photothermal heating, is calculated to have increased by ~1–2 orders of magnitude at the NW junctions (for more on the FDTD simulations, refer to SI: Section S6) once the laser polarization of the excitation source
Adv. Mater. 2014, DOI: 10.1002/adma.201400474
is perpendicular to the first NW since the top NW acts as a nano-antenna that directs light to the bottom NW,[32] while the enhancement becomes insignificant for the orthogonal polarization. The intensified electric field intensity at the Cu-NW junction consequently causes confined thermal heating at the NW contact point to achieve local nano-welding, which only affects the intersection and not other areas. The main advantage of the plasmonic laser nanowelding method is demonstrated by comparing the change in the electrical resistance as a result of conventional bulk annealing and laser nanowelding under ambient conditions. Figure 2a shows the real-time electrical properties of Cu-NW percolation networks at an area of 8 mm × 8 mm during bulk heating at 200 °C and during laser nanowelding at 100-mW power and a 100-mm/s scanning speed at a 5-µm hatch size. The sheet resistance of the Cu-NW percolation network undergoing bulk heating initially declines due to the removal of residual chemicals and the slight melting of the Cu-NW surface; however, it starts to rebound after ∼200 s on account of the Cu oxidation. As a result, the sheet resistance remains relatively high (~105–106 Ω/sq) at all times during bulk heating as long as the annealing is conducted under ambient conditions. In contrast, laser nanowelding instantaneously drops the resistance
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Figure 3. a) Optical photographs of Cu NWs transferred onto a PI film for bulk-heated (top), as-prepared (middle), and laser-nanowelded (bottom) samples and the cyclic bending test of the Cu-NW transparent conductor. Inset: schematic diagram of the bending test, maximum bending radius = 2 mm, 1.25 Hz) b) Repeated folding (complete bending) test of the Cu-NW transparent conductor.
upon exposure to the focused laser beam, and the final sheet resistance reaches ∼20 Ω/sq after a single scan over the sample is complete. The total processing time required for the 8 mm × 8 mm area was ∼1 min in this example, but the processing time can be reduced down to ∼2 s/cm2 or even shorter (see SI: Section S7). According to the recent study on the sintering of Cu nanomaterials under ambient conditions, the heating time at each point should be less than milliseconds in order to obtain an electrode at low resistivity and having undergone an insignificant amount of oxidation. Plasmonic laser nanowelding together with fast scanning (
