Thermal Barrier: Flash- Light-Sintered

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Ultrathin Plasmonic Optical/Thermal Barrier: Flash-Light-Sintered Copper Electrodes Compatible with Polyethylene Terephthalate Plastic Substrates Hye Jin Park, Min Kyung Cho, Young Woo Jeong, Dojin Kim, Su Yeon Lee, Youngmin Choi, and Sunho Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14654 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Applied Materials & Interfaces

Ultrathin Plasmonic Optical/Thermal Barrier: FlashLight-Sintered Copper Electrodes Compatible with Polyethylene Terephthalate Plastic Substrates Hye Jin Park,a,b Min Kyung Cho,c Young Woo Jeong,c Dojin Kim,b Su Yeon Lee,a,* Youngmin Choi,a,d,* Sunho Jeong a,d,*

a

Division of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT),

141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea b

Department of Materials Science and Engineering, College of Engineering, Chungnam National

University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Korea

c

Advanced Analysis Center, Korea Institute of Science and Technology (KIST), Seoul

02792, Republic of Korea

d

Department of Chemical Convergence Materials, Korea University of Science and Technology

(UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea

Keywords: plasmonic, flash, sinter, copper, electrode, PET

ACS Paragon Plus Environment

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Abstract In recent years, highly conductive, printable electrodes have received tremendous attention in various research fields as the most important constituent components for large-area, low-cost electronics. In terms of an indispensable sintering process for generating electrodes from printable metallic nanomaterials, a flash-light-based sintering technique has been regarded as a viable approach for continuous roll-to-roll processes. In this paper, we report cost-effective, printable Cu electrodes that can be applied to vulnerable polyethylene terephthalate (PET) substrates, by incorporating a heretofore-unrecognized ultrathin plasmonic thermal/optical barrier, which is composed of a 30 nm-thick Ag nanoparticle layer. The different plasmonic behaviors during a flash-light-sintering process are investigated for both Ag and Cu nanoparticles, based on a combined interpretation of experimental results and theoretical calculations. It is demonstrated that by a continuous printing process and a continuous flashlight-sintering process, the Cu electrodes are formed successfully on large PET substrates, with a sheet resistance of 0.24 Ω/square and a resistivity of 22.6 µΩ·cm.


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INTRODUCTION In recent years, as an alternative methodology to replace time-consuming, costly vacuum deposition and photholithography processes, printable nanomaterials have drawn tremendous attention in various microelectronics and energy applications.1-3 A variety of phase-pure metallic nanoparticles have been demonstrated in conjunction with new post-treatment techniques to easily fabricate highly conductive electrodes that comprise the main part of active device circuitries.4-6 Metal nanoparticles (NPs) are dispersed in a solvent medium in the form of ink, followed by a printing process to make particulate layers. The electrically-insulating particulate films are transformed into conductive dense films by a structural evolution under the provision of thermal/photon energies that trigger a mass transport between neighboring NPs.7,8 In particular, for the case of Ag nanoparticles, various low-temperature annealing processes have been demonstrated even on paper substrates.9-11 In comparison with conventional thermal sintering processes, flash-light-based sintering techniques have the following critical advantages: (i) the accessibility of a high-throughput roll-to-roll process by a short processing time12-14 and (ii) the possibility of kinetically-controlled microstructural evolution, which is required in annealing Cu NP-based electrodes in air.14 As a pioneer work, a laser annealing process was suggested for Au and Ag NP-based particulate layers, based on the fact that noble metal NPs have a characteristic plasmonic absorption at a specific wavelength.15,16 The efficiency of the laser sintering process is enhanced drastically by using a laser as a photon source with a wavelength of ~500 nm, at which the plasmonic absorption can be manifested greatly in noble metal NPs.17-19 Recently, the welding process for Ag nanowires was also reported to be achievable by a flash-light-sintering method based on ultraviolet (UV) and near-infrared (NIR) plasmonic absorption behaviors.20 Both laser


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and flash-light-sintering processes for silver nanomaterials (nanoparticles and nanowires) have been demonstrated even with the use of vulnerable polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) substrates.19,20 However, for practical applications, noble metal NPs should be replaced with inexpensive ones while maintaining electrical properties, and low-cost plastic substrates must be usable as carrier substrates for roll-to-roll processes. For the case of noble metal nanomaterials, the photonic annealing process is well-controllable to some extent, as it is only involved with the thermal decomposition of organic compartments and a photon-energy-driven microstructural evolution. However, Cu NPs, which have been researched intensively to replace expensive noble metal ones, have more complicated reaction mechanisms because of the photon-involved reduction of surface oxides and spontaneous thermal oxidation during annealing in air.21,22 Furthermore, the melting point (1085 oC) of Cu bulk is higher than that (961 oC) of Ag bulk, and Cu NPs require more excessive energy to induce the photonic reduction reaction of surficial oxides; this necessity of a higher energy dose tends to have undesirable impacts on the underlying substrates in the case of cheap and vulnerable PEN and PET substrates. To date, highly conductive electrodes have been formed mostly with the use of highly durable polyimide substrates, and flash-sintered Cu electrodes with a highly conductive nature have been rarely reported on PET substrates that are in great demand as a low-cost plastic substrate material in commercial industries.5,14,22 In this study, we have designed an ultrathin Ag-nanoparticle-based plasmonic optical/thermal barrier that can suppress the propagation of thermal damage toward underlying substrates, when highly conductive copper electrodes are formed from particulate layers composed of surface-oxide free Cu NPs on PET substrates. We use a flash-light-sintering


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process with a processing time of ~1 msec for the following reasons: (i) a lamp-type apparatus is more appropriate for a high-throughput manufacturing process than a point-source laser equipment, (ii) an in-line roll-to-roll process is easily achievable when a substrate is moved continuously in a synchronized fashion with the irradiation of a flash-light lamp, and (iii) the unpredictable formation of surface oxides would be suppressed kinetically owing to an ultrafast microstructural evolution for less than ~ 1 msec. To clarify the role of an ultrathin (~30 nmthick) barrier layer, the different plasmonic behaviors were monitored for Ag and surface oxidefree Cu NPs under a flash-light, in conjunction with theoretical calculation. It is demonstrated that the simple introduction of an ultrathin barrier layer would facilitate the formation of Cu electrodes, with a sheet resistance of 0.24 Ω/square and a resistivity of 22.6 µΩ·cm, on largearea PET substrates by a continuous flash-light sintering process.

EXPERIMENTAL METHODS Synthesis of Cu and Ag nanoparticles. Ag and Cu NPs were synthesized by a wetchemical synthesis method, as reported in our previous papers.12,23 All chemicals were used as received without further purification. Cu acetate (Cu(CO2CH3)2, 98%), octylamine (C8H17NH2, 99%), oleic acid (C18H34O2, 90%), phenylhydrazine (C6H5NHNH2, 97%), and toluene (C6H5CH3, anhydrous, 99.8%) were purchased from Aldrich. Ag nitrate (AgNO3, 99.9%) was purchased from Kojima Chemicals. For synthesizing Ag NPs, 9.5 g of Ag nitrate and 25.4 ml of oleic acid were added into a three-neck round-bottomed flask containing 93.2 ml of octylamine. The prepared reacting solution was heated to 80 oC and stirred with a magnetic stirrer under refluxing conditions. When the temperature approached 80 oC, 87.4 g of phenylhydrazine was injected at an injection rate of 5 ml/min. The reaction was continued for 60 min and then cooled to room


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temperature. The synthesized Ag nanoparticles were washed with toluene by centrifugation. For synthesizing Cu NPs, 10.4 g of Cu acetate and 25.1 g of oleic acid were added into a three-neck round-bottomed flask containing 73.6 mL of octylamine. The flask was fitted with a reflux condenser and a mechanical stirrer. The solution was purged with nitrogen for 60 min and then heated to 150 oC. Then, phenylhydrazine was added dropwise and the reaction was continued for 30 min. After the completion of the synthesis reaction, the synthesized Cu nanoparticles were separated by centrifugation and washed with toluene. The synthesized Ag and Cu nanoparticles were kept in air without additional surface-passivation procedures. For the preparation of sprayprintable inks, both nanoparticles were dispersed in toluene with a solids loading of 0.7 wt%. The metal nanoparticle inks for a spin-coating process were prepared with a solids loading of 20 wt%. Printing Metal Nanoparticle Inks and Flash-Light-Sintering process. For forming general Cu and Ag nanoparticle layers, air-brush printing was carried out using metal nanoparticle inks with a solids loading of 0.7 wt%, on PI (thickness(t) = 75 µm, Kapton film 300HN, Teijin DuPont Films), PES (t = 120 µm, Glastic SCL120, I-Components), PEN (t = 125 µm, Teonex Q65HA, Teijin DuPont Films), PET (t = 125 µm, Tetoron KEL86W, Tejin Dupont Films) substrates heated at 100 oC at a speed of 0.3 mm/sec. The automated spray machine was equipped with a moving stage/nozzle. For forming ultra-thin Ag nanoparticle layers, Ag nanoparticle inks with a solids loading of 20 wt% were spin-coated with a rotation speed of 3,000 rpm on O2-plasma-treated plastic substrates and pre-formed bare Cu particulate layers. Flash-light-sintering was accomplished using a Xenon flash lamp system (Sinteron 2010, Xenon Corp.) in which an A type lamp was equipped with a broadband spectrum of 370 to 800 nm. The flash lamp dimensions were 0.75 inch in width and 12 inch in length, and the size of the moving


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stage was 900 cm2. The air cooled xenon linear flash lamp, located 2 inches away from the substrate stage, delivered optical energy of from 0.26 to 3.49 J/cm2 as a function of the electrical voltage and duration time for generating the electrical pulse energies. The optical energies of flash lamp irradiation were measured by a radiometer (ITL1700, International Light Technologies) with a detector (SED033, 200-1100 nm, International Light Technologies). Characterization. The size and shape of nanoparticles and the microstructures and elemental analysis of conductive layers were examined by transmission electron microscopy (TEM, Talos F200X, FEI) with Super-X EDS. The crystal structures of nanoparticles and conductive layers were analyzed using an X-ray diffractometer (XRD, D/MAX-2200V, Rigaku). The resistivities of conductive layers were analyzed by a four point probe (FPP-HS8, Dasol Eng.). The absorption spectra of the spin-coated Ag and Cu nanoparticle layers were measured by UV-Vis-NIR spectroscopy (Cary 5000 UV-vis-NIR, Agilent). Finite-Difference Time-Domain (FDTD) simulation. The model structure consisted of hexagonally ordered metal nanoparticles with 9 layers. The diameters of Ag and Cu nanoparticles were set to be 15 and 90 nm, respectively. A plane array of dipole sources was used to simulate the linearly polarized plane waves that impinge on metal nanoparticle layers. The near-field profiles of surface plasmon were investigated by continuously driving respective plasmon resonances using plane-wave dipole sources.

RESULTS AND DISCUSSION Figure 1a shows a schematic of the experimental procedures used in this study. By a spray-printing technique, metal NP inks were printed on target substrates loaded on the stage


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heated to 100 oC. The semi-dried droplets land on the substrate and pile up with complete solvent evaporation, resulting in well-packed particulate films. Then, the flash-light was irradiated with an on/off signal sequence, while the sample-loaded stage moves continuously at a specific speed such that all regions are irradiated. Both Ag and surface oxide-free Cu NPs were synthesized through a wet-chemical synthetic method, as reported previously by our group.12,23 Either Ag nitrate or Cu acetate was dissolved in octylamine at elevated temperatures with the presence of a surface capping agent, oleic acid. Metal cations were reduced into metallic species by adding a reducing agent, phenylhydrazine. The diameters of the synthesized Ag and Cu NPs were ~15 and ~90 nm, respectively (Figure S1). The phase purity was confirmed by X-ray diffraction (XRD) analysis for both types of nanoparticles (Figure S2). In the case of the Cu nanoparticles, the absence of surface oxides was monitored more with X-ray photoelectron spectroscopy (XPS) analysis. As seen in the Cu 2p3/2 spectra (Figure S3), the sub-peaks attributable to oxide phases are not observable. It was demonstrated that an introduction of oleic acid as a surface capping agent in the proposed synthetic scheme completely passivates energetically-unstable surface Cu atoms, effectively suppressing the formation of surface oxides.23 Both Cu and Ag nanoparticles are lack of oxide phases and were surrounded by an identical organic capping molecule. Thus, it is speculated that the trends in resistivity for both metallic layers could be not distinctively different; however, the results were unlike our expectation. Figures 1b and c show the resistivity evolution in Cu and Ag NP-based layers on various plastic substrates, as a result of single-pulse and continuous-pulse flash sintering. The variations in energy dose in relation to operation conditions, voltage and time, are summarized in Figure S4. For single-pulse sintering, the flash sintering process was carried out in a static condition in which the stage did not move. Polyimide (PI), polyethersulfone (PES), PEN, and PET substrates


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were used in this study. The glass transition temperatures (Tg) are 400, 225, 155, and 110 oC for PI, PES, PEN and PET substrates, respectively.24 As seen in Figure 1b, the resistivities of the Cu electrodes increased gradually, from 7.6 to 55.5 µΩ·cm, in a single-pulse process, as the Tg of plastic substrates is reduced by varying the substrate from PI to PET. During the flash sintering process, optical energy is absorbed by metallic NPs and some of this energy is dissipated into a thermal energy, some of which transverses down to the underlying substrates. Such substrate damage is commonly observable in photonic sintering processes, resulting in more resistive electrodes on vulnerable plastic substrates.12,14,24 In a continuous pulse process, the increment in resistivity is clearly manifested with the insulating nature of Cu electrodes on a PET substrate, because of overlapped irradiation in some parts. In contrast, in the case of Ag NPs, resistivities below 10 µΩ·cm were measured regardless of the kind of plastic substrate and the way the flash sintering process was carried out. A conductivity around 10 µΩ·cm is comparable to the previously-reported values obtainable with flash-sintered, Ag NP-based electrodes.12 Such a discrepancy for both metallic layers could be associated with a size-dependent decrease of the melting point of metal NPs. It has been wellknown that the melting point is decreased proportionally by reducing the size of metal NPs, in general, below a few tens of nanometers.25,26 The 15 nm-sized Ag NPs could need a lower energy input in triggering a sintering-based microstructural transformation, which consequently diminishes the possibility of thermal damage of the underlying substrates. However, as a size of metal NPs decreases on a nanoscale, the surface area increases exponentially, and the volume fraction of the surface capping molecule increases correspondingly. Thus, more energy input is required to completely eliminate many capping molecules, with which the advantage of extremely small metal NPs would be compensated. In fact, the minimum energy dose to initiate


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sintering was almost identical with values of 1.2 and 1.6 J/cm2 for 15 nm-sized Ag and 90 nmsized Cu NPs, respectively. To clarify the opto-electronic behaviors for both NP layers, the finite-difference timedomain (FDTD) method was used to perform electromagnetic simulations (Figure 2a and b). In the simulations, the spatial mesh size (∆x = ∆y = ∆z) was set to 1.0 nm to ensure that the rapidly varying electric fields at the metal/dielectric boundaries could be well-represented, while the time step ∆t ≅ 3.34×10-18 s was chosen to satisfy the Courant stability condition. The electric field intensity distributions (|E|2) were obtained by continuously driving respective plasmon resonances using plane-wave dipole sources. Then, |E|2 were normalized by the incident wave intensity (|E0|2) to obtain the intensity enhancement factors (η=|E|2/|E0|2). According to ultraviolet-visible absorption data for the Ag and Cu NP layers (Figure S5), both layers have specific absorption peaks at 550 and 675 nm, respectively. Thus, to understand light absorption behaviors inside nanoparticle-stacked layers during the flash-sintering process by a white light with wavelength ranging from 370 to 800 nm, we conducted a simulation based on the FDTD method at wavelengths of 550 and 675 nm for both nanoparticle layers. The spectra of white light used in the flash-sintering process is shown in Figure S6. The maximum η550nm and η675nm values were calculated to be 103.8 and 102.9 for the Ag NP layers, respectively, whereas the maximum η550nm and η675nm values of 101.7 and 101.7 were obtained for the Cu NP layers, respectively. This indicates that, for the case of Ag NP layers, strong light absorption occurs when an incident light with a specific wavelength for plasmon resonance is provided, and the Cu NP layers do not have characteristic plasmon resonance behaviors, compared with their Ag


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counterparts. Such a strong plasmon resonance behavior in the Ag NP layer affects the film thickness of the NP layers that can be flash-sintered. As shown in Figure 2a, an incident light with a wavelength of 550 nm (that can activate a strong plasmon resonance) does not transverse deeply by a strong absorption in shallow upper layers, which might lead to a highly efficient flash-sintering process in the limited number of particles placed in top positions. An incident light with wavelengths out of 550 nm (that only can active a weak plasmon resonance; representatively 675 nm) would lead to a moderate flash-sintering process in the overall films. In the case of the Cu NP layers, a more mild light absorption occurs deeply in the overall films, regardless of the wavelength of incident light. As shown in Figures 2c and d, the flash-sintered Ag NP layers exhibited a conductive nature below a thickness of 475 nm, while the Cu NP layers even with a thickness of 5 µm were flash-sintered with an evolution of resistivity ranging from 5 to 15 µΩ·cm. A resistivity increment in thin nanoparticle layers with a thickness below ~200 nm was attributable to restricted inter-particular connections in spray-printed nanoparticle layers. These different plasmonic behaviors during flash-light-sintering were carefully monitored for bi-layered structures comprising Ag and Cu NP layers (Figure 3). At first, the Cu(lower)/Ag(upper) bi-layered structures were fabricated by spray-printing Cu-NP inks and sequentially spin-coating Ag-NP inks on PET substrates (Figure 3a). The thicknesses of the Cu and Ag layers were 1.9 µm and 35 nm, respectively. According to the theoretical and experimental interpretation discussed above, it is expected that the upper Ag NP layer would vigorously absorb energetic photons and undergo an instantaneous transformation into a bulk Ag layer. With this event, a thermal energy generated predominantly inside the upper Ag layers is dissipated outward, as the thermal conductivity of the densified Ag bulk layer is much higher than that of the particulate Cu layers only with inter-particular point-contacts between


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neighboring nanoparticles. This indicates that a flash sintering process would be restricted to the vicinity of the upper Ag layer. In fact, the color of the top surface was changed when irradiation from a flash-light was partly applied, as seen in a top-view image of a flash-sintered sample; however, most of the lower copper layers did not undergo any microstructural evolution, with a clear observation of individual Cu NPs in a cross-sectional SEM image. The sheet resistance was measured to be 16.4 Ω/square, from which a resistivity of 57.4 µΩ·cm was calculated with an assumption that only the upper Ag layer was sintered and contributed to the electrical property of the sample. This value corresponds well to those of the thin Ag-NP electrodes observed in Figure 2c. Such a role of ultrathin Ag NP layer as an optical/thermal blocking layer would also be valid for bi-layered structures with Ag (lower)/Cu (upper) architectures (Figure 3b). The topview SEM image is shown in Figure S7. The irradiated photons transverse entirely through the upper Cu layer and arrive at the bottom Ag NP layer. With a simultaneous event of instantaneous densification of both metallic layers, the lower Ag layer enables effective heat-dissipation toward the outer surroundings through a thermally-conductive dense Ag layer, minimizing thermal damage of the underlying PET substrate. As seen in a cross-sectional image, it is clear that the entire film was completely sintered with a fully-sintered microstructural morphology. The film thickness was reduced greatly to 730 nm, in comparison with the Cu (lower)/Ag (upper) layeredstructure, which is indicative of a densification reaction in the overall films. The sheet resistance was measured to be 0.11 Ω/square, and the resistivity of 8.6 µΩ·cm was obtained when it was calculated with a full thickness of 730 nm. This value in sheet resistance is lower by a factor of 149 than that of the Cu (lower)/Ag (upper) bi-layered structure.


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Note that, for the case of single layered, Cu NP-based electrodes, the high resistivity of 55.5 µΩ·cm was measured with a single-pulse annealing process and their conductive nature was not evolved with a continuous-pulse annealing process. To clarify the critical role of an ultrathin Ag NP layer as a plasmonic optical/thermal blocking layer, we measured the resistivities for Ag (lower)/Cu (upper) bi-layered electrodes prepared on PI, PEN and PET substrates after both single-pulse and continuous-pulse sintering processes (Figure 4a). For the case of the singlepulse sintering process, resistivities around 7 µΩ·cm were well maintained with PI and PEN substrates, and a slightly increased resistivity of 18 µΩ·cm was measured for on a PET substrate. On PET substrates, comparing bi-layered electrodes with Cu electrodes, the resistivity diminished significantly by a factor of 3 merely by inserting ultrathin Ag lower layers. Most importantly, for bi-layered sintered electrodes, the trend in resistivity observable did not vary distinguishably even in continuous-pulse-sintered ones. By a continuous flash-light-sintering process, the Cu electrodes are formed on PET substrates, with a sheet resistance of 0.24 Ω/square and a resistivity of 22.6 µΩ·cm. To date, for the case of laser sintering of films comprising surface-oxide free Cu NPs, a resistivity of 87 µΩ·cm was reported with sophisticatedlycontrolled processing conditions to prevent oxidation during laser-sintering on PET substrates.27 Recently, Cu/Cu10Sn3 core/shell nanoparticles were reported as surface-oxide free Cu nanoparticles surrounded by a conductive alloy phase with a low-melting point, activating a flash-light-sintering process by lower optical energy; however, with these well-designed nanoparticles, conductive electrodes were not successfully fabricated on a PET substrate by a continuous flash-sintering process.5 Another advantage of bi-layered electrodes suggested in this study is an extremely low consumption of noble metal species. As shown in Figure 4b, in flash-sintered, bi-layered


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electrodes, the thicknesses of the Cu and Ag NP layers were 640 and 32 nm, respectively. Taking into consideration the densities of the Cu and Ag elements, the weight ratio of Ag to Cu is calculated only to be 0.059. When Cu-based electrodes are formed on PET substrates from a mixture of Cu and Ag NPs, a resistivity of around 30 µΩ·cm, which is still higher than that of bilayered electrodes, was evolved even with incorporating a sufficient amount, 80 wt%, of Ag nanoparticles (Figure 4c). A characteristic feature in material combination of Cu and Ag NPs is that both metal elements have little solid solubility at low temperatures below 300 oC, as seen in the phase diagram of Cu and Ag (Figure S8). This indicates that even when a vigorous mass transport (for a microstructural transformation into dense films) is induced by a highly intensive flash-light-sintering process, neither material is miscible, performing their own critical roles in bi-layered electrodes. As seen in Figure S9, even after flash-light-sintering, the XRD peaks were present evidently for Cu and Ag pure phases. There was no visible indication of inter-diffusions between the lower Ag and upper Cu layers, as seen in the HRTEM image of flash-sintered bilayered electrodes (Figure 4b). Figure 5a shows the sheet resistance distribution for large-area Ag/Cu bi-layered electrodes fabricated by both of continuous printing and continuous flash-light-sintering on PET substrates. As shown in Figure 5b, in fabricating electrodes with a size of 6.5 cm x 6.5 cm, 9times continuous pulses were applied, while the stage moved at a speed of 220 mm/min, not leaving behind the non-sintered regions. The average sheet resistance was measured to be 0.38 Ω/square. In practical applications, the moving speed of the stage can be manipulated by enlarging the width of the flash lamp and increasing the number of installed flash lamps. The large-area bi-layered electrodes also exhibited a stable electrical performance when they were bent with an outer bending radius even down to 2 mm (Figure 5c). The normalized resistance


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was not varied to a bending radius of 8 mm and the variation in normalized resistance at a bending radius of 2 mm was only 1.3. In repeated bending tests during 10,000 cycles at a bending radius of 8 mm, the variations in normalized resistance were merely 1.1 and 1.3 for inner and outer bending conditions (Figure S10). It is well known that well-formed metallic films can survive with unchanged electrical properties under bending tests, as long as they are thin enough to sustain the stress applied during bending tests. In general, the metallic films with a few hundred of nanometers could accommodate the mechanical strain stress on plastic substrates with a thickness below around 100 um. It has been demonstrated that the electrical resistance is not altered significantly in phonic-sintered thin metallic films when the dense film structure evolves after an instantaneous sintering process by a vigorous interparticular mass transport.12,14 As seen in Figure 5d, when flash-sintered, bi-layered electrodes were wrapped along a stick with a diameter of 7 mm, the light emitting diode (LED) turn on with an unchangeable electrical conduction through electrodes.

CONCLUSIONS In summary, we have designed an ultrathin plasmonic optical/thermal barrier to facilitate large-area Cu electrodes on PET substrates. Based on experimental results and theoretical calculations, it was revealed that both Cu and Ag nanoparticle layers exhibit distinctively different plasmonic behaviors during flash-light sintering. It was demonstrated that even ultrathin Ag nanoparticle layers (with a strong plasmonic characteristic under an intense white light) act as a metallic layer that does not transfer thermal energy to adjacent layers, minimizing thermal damage to underlying vulnerable PET substrates. By incorporating this ultrathin optical/thermal blocking layer, highly-conductive, low-cost Cu electrodes, with a sheet resistance of 0.24


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Ω/square and a resistivity of 22.6 µΩ·cm, were fabricated successfully on large-area PET substrates by continuous printing and flash-light-sintering processes.

ASSOCIATED CONTENT Supporting Information SEM image and XRD result of Cu and Ag nanoparticles, XPS spectra for Cu nanoparticles, the variation of optical energy generated during flash-light-sintering process, UV-visible spectra for Ag and Cu nanoparticle layers, the spectra of white light used in flash-light-sintering process, top-view SEM image and XRD result for bi-layered electrode, Cu-Ag phase diagram, and the variation in resistance during repeated bending tests.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S. Lee) *E-mail: [email protected] (Y. Choi) *E-mail: [email protected] (S. Jeong)

ACKNOWLEDGMENT This research was supported by Global Research Laboratory Program of the National Research Foundation (NRF) funded by Ministry of Science, Information and Communication


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Technologies and Future Planning (NRF-2015K1A1A2029679), and partially supported by the Nano·Material Technology Development Program through the National Research Foundation of Korea funded by the Ministry of Science, Information and Communication Technologies and Future Planning (NRF-2015M3A7B4050306).

REFERENCES (1) Yang, C.; Wong, C. P.; Yuen, M. F. Printed Electrically Conductive Composites: Conductive Filler Designs and Surface Engineering. J. Mater. Chem. C 2013, 1, 4052-4069. (2) Fu, K.; Yao, Y.; Dai, J.; Hu, L. Progress in 3D Printing of Carbon Materials for EnergyRelated Applications. Adv. Mater. 2017, 29, 1603486. (3) Farahani, R. D.; Dubé , M.; Therriault, D. Three-Dimensional Printing of Multifunctional Nanocomposites: Manufacturing Techniques and Applications. Adv. Mater. 2016, 28, 57945821. (4) Kamyshny, A.; Magdassi, S. Conductive Nanomaterials for Printed Electronics. Small 2014, 10, 3515-3535. (5) Oh, S.–J.; Kim, T. G.; Kim, S.-Y.; Jo, Y.; Lee, S. S.; Kim, K.; Ryu, B.-H.; Park, J.-U.; Choi, Y.; Jeong, S. Newly Designed Cu/Cu10Sn3 Core/Shell Nanoparticles for Liquid PhasePhotonic Sintered Copper Electrodes: Large-Area, Low-Cost Transparent Flexible Electronics. Chem. Mater. 2016, 28, 4714-4723. (6) Nittaynen, J.; Sowade, E.; Kang, H.; Baumann, R. R.; Ma ntysalo, M. Comparison of Laser and Intense Pulsed Light Sintering (IPL) for Inkjet-Printed Copper Nanoparticle Layers. Sci. Rep. 2015, 5, 8832. (7) Yong, Y.; Yonezawa, T.; Matsubara, M.; Tsukamoto, H. The Mechanism of AlkylamineStabilized Copper Fine Particles towards Improving the Electrical Conductivity of Copper Films at Low Sintering Temperature. J. Mater. Chem. C 2015, 3, 5890-5895. (8) Ahn, B. Y.; Duoss, E. B.; Motala, M. J.; Guo, X.; Park, S.-I.; Xiong, Y.; Yoon, J.; Nuzzo, R. G.; Rogers, J. A.; Lewis, J. A. Omnidirectional Printing of Flexible, Stretchable, and Spanning Silver Microelectrodes. Science 2009, 323, 1590-1593.


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(9) Zhang, X.-Y.; Xu, J.-J.; Wu, J.-Y.; Shan, F.; Ma, X.-D.; Chen, Y.-Z.; Zhang, T. Seeds Triggered Massive Synthesis and Multi-Step Room Temperature Post-Processing of Silver Nanoink-Application for Paper Electronics. RSC Adv. 2017, 7, 8-19. (10) Hui, Z.; Liu, Y.; Guo, W.; Li, L.; Mu, N.; Jin, C.; Zhu, Y.; Peng, P. Chemical Sintering of Direct-Written Silver Nanowire Flexible Electrodes under Room Temperature. Nanotechnology 2017, 28, 285703. (11) Li, R.-Z.; Hu, A.; Zhang, T.; Oakes, K. D. Direct Writing on Paper of Foldable Capacitive Touch Pads with Silver Nanowire Inks. ACS Appl. Mater. Interfaces 2014, 6, 21721-21729. (12) Jo, Y.; Oh, S.-J.; Lee, S. S.; Seo, Y.-H.; Ryu, B.-H.; Moon, J.; Choi, Y.; Jeong, S. Extremely Flexible, Printable Ag Conductive Features on PET and Paper Substrates via Continuous Millisecond Photonic Sintering in a Large Area. J. Mater. Chem. C 2014, 2, 9746-9753. (13) Jo, Y.; Oh, S.-J.; Lee, S. S.; Seo, Y.-H.; Ryu, B.-H.; Yoon, D. H.; Choi, Y.; Jeong, S. Crystalline Structure-Tunable, Surface Oxidation-Suppressed Ni Nanoparticles: Printable Magnetic Colloidal Fluids for Flexible Electronics. J. Mater. Chem. C 2015, 3, 4842-4847. (14) Oh, S.-J.; Jo, Y; Lee, E. J.; Lee, S. S.; Kang, Y. H.; Jeon, H,-J.; Cho, S. Y.; Park, J.-S.; Seo, Y.-H.; Ryu, B.-H.; Choi, Y.; Jeong, S. Ambient Atmosphere-Processable, Printable Cu Electrodes for Flexible Device Applications: Structural Welding on a Millisecond Timescale of Surface Oxide-Free Cu Nanoparticles. Nanoscale 2015, 7, 3997-4004. (15) Chung, J.; Ko, S.; Bieri, N. R.; Grigoropoulos, C. P.; Poulikakos, D. Conductor Microstructures by Laser Curing of Printed Gold Nanoparticle Ink. Appl. Phys. Lett. 2004, 84, 801. (16) Ko, S. H.; Pan, H.; Grigoropoulos, C. P.; Luscombe, C. K.; Frechet, J. M. J.; Poulikakos, D. Air Stable High Resolution Organic Transistors by Selective Laser Sintering of Ink-Jet Printed Metal Nanoparticles. Appl. Phys. Lett. 2007, 90, 141103. (17) Son, Y.; Yeo, J.; Moon, H.; Lim, T. W.; Hong, S.; Nam, K. H.; Yoo, S.; Grigoropoulos, C. P.; Yang, D.-Y.; Ko, S. H. Nanoscale Electronics: Digital Fabrication by Direct Femtosecond Laser Processing of Metal Nanoparticles. Adv. Mater. 2011, 23, 3176-3181. (18) An, K.; Hong, S.; Han, S.; Lee, H.; Yeo, J.; Ko, S. H. Selective Sintering of Metal Nanoparticle Ink for Maskless Fabrication of an Electrode Micropattern Using a Spatially Modulated Laser Beam by a Digital Micromirror Device. ACS Appl. Mater. Interfaces 2014, 6, 2786-2790. (19) Hong, S.; Yeo, J.; Kim, G.; Kim, D.; Lee, H.; Kwon, J.; Lee, H.; Lee, P.; Ko, S. H. Nonvacuum, Maskless Fabrication of a Flexible Metal Grid Transparent Conductor by LowTemperature Selective Laser Sintering of Nanoparticle Ink. ACS Nano 2013, 7, 5024-5031.


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(20) Park, J. H.; Hwang, G.-T.; Kim, S.; Seo, J.; Park, H.-J.; Yu, K.; Kim, T.-S.; Lee, K. J. Flash-Induced Self-Limited Plasmonic Welding of Silver Nanowire Network for Transparent Flexible Energy Harvester. Adv. Mater. 2017, 29, 1603473. (21) Kang, B.; Han, S.; Kim, J.; Ko, S.; Yang, M. One-Step Fabrication of Copper Electrode by Laser-Induced Direct Local Reduction and Agglomeration of Copper Oxide Nanoparticle. J. Phys. Chem. C 2011, 115, 23664-23670. (22) Hwang, H.-J.; Oh, K.-H.; Kim, H.-S. All-Photonic Drying and Sintering Process via Flash White Light Combined with Deep-UV and Near-Infrared Irradiation for Highly Conductive Copper Nano-Ink. Sci. Rep. 2016, 6, 19696. (23) Jeong, S.; Lee, S. Y.; Jo, Y.; Lee, S. S.; Seo, Y.-H.; Ahn, B. W.; Kim, G.; Jang, G.-E.; Park, J.-U.; Ryu, B.-H.; Chol, Y. Air-Stable, Surface-Oxide Free Cu Nanoparticles for Highly Conductive Cu Ink and Their Application to Printed Graphene Transistors. J. Mater. Chem. C 2013, 1, 2704-2710. (24) Kim, I.; Woo, K.; Zhong, Z.; Lee, E.; Kang, D.; Jeong, S.; Choi, Y.-M.; Jang, Y.; Kwon, S.; Moon, J. Selective Light-Induced Patterning of Carbon Nanotube/Silver Nanoparticle Composite To Produce Extremely Flexible Conductive Electrodes. ACS Appl. Mater. Interfaces 2017, 9, 6163-6170. (25) Buffat, P.; Borel, J.-P. Size Effect on the Melting Temperature of Gold Particles. Phys. Rev. A 1976, 13, 2287. (26) Yeshchenko, O. A.; Dmitruk, I. M.; Alexeenko, A. A.; Dmytruk, A. M. Size-Dependent Melting of Spherical Copper Nanoparticles Embedded in a Silica Matrix. Phys. Rev. B 2007, 75, 085434. (27) Park, J. H.; Jeong, S. H.; Lee, E. J.; Lee, S. S.; Seok, J. Y.; Yang, M.; Choi, Y.; Kang, B. Transversally Extended Laser Plasmonic Welding for Oxidation-Free Copper Fabrication toward High-Fidelity Optoelectronics. Chem. Mater. 2016, 28, 4151-4159.


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Figure 1. (a) Schematics of printing metal nanoparticle inks and continuous flash-light-sintering process. Variations in resistivity for flash-sintered (b) Cu and (c) Ag nanoparticle-based electrodes on various plastic substrates. The flash-light-sintering process was carried out under optimized irradiation conditions: (Cu nanoparticle-based electrodes) 2.3 kV/1.5 msec, 2.0 kV/1.5 msec, 1.8 kV/2.0 msec, and 2.0 kV/1.5 msec for PI, PES, PEN, and PET substrates, respectively; (Ag nanoparticle-based electrodes) 2.3 kV/1.5 msec for all plastic substrates.


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Figure 2. Calculated electric field intensity (|E|2) distributions of the two plasmon resonances (λ= 550, 675 nm) for (a) Ag and (b) Cu nanoparticle layers. Variations in resistivity in relation to film thickness for (c) Ag and (d) Cu nanoparticle-based electrodes. Both electrodes were prepared on PI substrates. Ag and Cu nanoparticle layers were flash-sintered at 2.3 kV for 1.5 msec.


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Figure 3. The proposed mechanism for bi-layered electrodes, and photographs and SEM images showing microstructures for (a) Cu(lower)/Ag(upper) and (b) Ag(lower)/Cu(upper) bi-layered electrodes. The Cu(lower)/Ag(upper) and Ag(lower)/Cu(upper) bi-layered electrodes were single-flash-sintered at 2.0 kV for 2.0 msec.


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Figure 4. (a) Variations in resistivity for flash-sintered Ag/Cu bi-layer electrodes prepared on PET, PEN, and PI substrates. Continuous-pulse flash-light-sintering was carried out under optimized irradiation conditions: 2.3 kV/1.5 msec, 2.3 kV/1.5 msec and 2.0 kV/2.0 msec for PI, PEN, and PET substrates, respectively, while the substrate stage moves with a moving speed and an interval time of 330 mm/min and 2 sec, 380 mm/min and 2 sec, 220 mm/min and 2 sec for PI, PEN and PET substrates, respectively. (b) TEM image and EDS analysis data for flash-sintered Ag/Cu bi-layer electrodes prepared on PET substrate. The scale bar is 200 nm. (c) Variation in resistivity for flash-sintered electrodes prepared from Cu-Ag nanoparticle mixture inks.


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Figure 5. (a) Sheet resistance and (b) photograph of large-area Ag/Cu bi-layered electrodes fabricated by a continuous printing technique and continuous flash-light-sintering process on PET substrates. The film thickness was 600 nm. (c) Variation of normalized resistance of flashsintered Ag/Cu bi-layered electrodes depending on an outer bending radius; (d) Photograph showing an electrical conduction through Ag/Cu bi-layered electrodes wrapped along a stick with a dimeter of 7 mm.


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Thermal Barrier: Flash- Light-Sintered

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