Laser Direct Writing and Selective Metallization of Metallic Circuits for

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Laser Direct Writing and Selective Metallization of Metallic Circuits for Integrated Wireless Devices Jinguang Cai, Chao Lv, and Akira Watanabe ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16558 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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ACS Applied Materials & Interfaces

Laser Direct Writing and Selective Metallization of Metallic Circuits for Integrated Wireless Devices Jinguang Cai,*ab Chao Lvab and Akira Watanabe*b a

Institute of Materials, China Academy of Engineering Physics, Jiangyou 621908, Sichuan, PR

China. b

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1

Katahira, Aoba-ku, Sendai 980-8577, Japan. *Corresponding authors: [email protected]; [email protected] KEYWORDS: Laser direct writing, Electroless Ni plating, Wireless charging and storage, Near-field communication tag, Integrated devices

ACS Paragon Plus Environment

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ABSTRACT:

Portable and wearable devices have attracted wide research attention due to their intimate relations with human daily life. As basic structures in the devices, the preparation of highconductive metallic circuits or micro-circuits on flexible substrates should be facile, costeffective, and easily integrated with other electronic units. In this work, high-conductive carbon/Ni composite structures were prepared by using a facile laser direct writing method followed by an electroless Ni plating process, which exhibit a three-order lower sheet resistance of less than 0.1 ohm/sq compared to original structures before plating, showing the potential for practical use. The carbon/Ni composite structures exhibited a certain flexibility and excellent anti-scratch property due to the tight deposition of Ni layers on carbon surfaces. Based on this approach, a wireless charging and storage device on a polyimide film was demonstrated by integrating an outer rectangle carbon/Ni composite coil for harvesting electromagnetic waves and an inner carbon micro-supercapacitor (MSC) for energy storage, which can be fast charged wirelessly by a commercial wireless charger. Furthermore, a near-field communication (NFC) tag was prepared by combining a carbon/Ni composite coil for harvesting signals and a commercial IC chip for data storage, which can be used as an NFC tag for practical application.


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Introduction The internet of things (IoT) have attracted much research attention and commercial development, because of their increasing relations with the development of science and technology, world economy, and human daily life.1 Besides of basic fixed facilities, IoT also involves many aspects related with our daily life, such as portable and wearable electronics, light-emitting displays (LEDs), various kinds of sensors, microelectronics, bioelectronics, and so on.2-19 At the same time, wireless working devices, which can transmit or exchange energy and information without cables, would make the city and life wireless, smart, efficient, and more convenient in the future.1, 20-21 Thence, wireless, portable, and wearable devices should be one of the important research targets in this field. For either wire or wireless devices, one of the commonly most important components is high-conductive metallic circuits, which constitute the basic structure of the devices. On the other hand, though some components of an integrated device such as photodetectors can be prepared directly on the same film with circuits,22-25 it would be a good option to integrate commercial small-size electronic components into circuits on flexible substrates to extend the application range, because such semiconductor components are highly commercialized and small enough, which would not influence the flexibility of final devices. This concept is also called flexible hybrid electronics, which consists of functional micro-components and interconnected high-conductive metallic patterns on a flexible polymer substrate.14-19 Therefore, it is challenging, but important to develop facile methods to prepare high-conductive metallic circuits on flexible substrates. Numerous innovative approaches including photolithography and printing methods, have been studied and developed for fabrication of metallic circuits on polymer films.26-39 Generally, photolithography combined with the vapor deposition method is a traditional but effective way


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for metallic patterning on polymer surfaces with a high quality,36-39 but it needs equipment with a high cost and multiple processing steps. Inkjet printing is an efficient approach to preparing high-conductive circuits from noble Au or Ag nanoparticle inks on flexible substrates,28-31, 34-35 but it is still a challenge in the preparation of stable precursor inks and precision of the printing. Besides, in the case of relatively low-cost Cu nanoparticle ink, additives such as plastic binders in the dispersion and the oxidation of Cu would reduce the conductivity of the printed patterns which may need a sintering process to reduce the resistance at a relatively high temperature or under light flash irradiation that may limit the option of polymer substrates.40-43 Electroless metal plating (Cu, Ni, etc.) , which is a well-known process and widely used in industry for growth of conductive or anti-corrosion coatings, has been reported for preparation of metallic circuits on polymer substrates due to its simplicity and low cost, but surface-selective activation or catalyst distribution are necessary and important for the fabrication of metallic circuit patterns.44-50 Laser direct writing is a non-contact, fast, single-step fabrication technique with no need for masks, post-processing, and complex clean room, and compatible with current electronic product lines for commercial use, thus it has potential to be employed in the surface-selective activation or catalyst distribution,48-50 fabrication of materials with specific patterns,51-58 even preparation of self-powered integrated devices.23 Recently, we have demonstrated the fabrication of highperformance flexible carbon micro-supercapacitors (MSCs) by laser induced carbonization from polyimide (PI) films in air or Ar, a flexible photo-rechargeable carbon/TiO2 composite MSC on a PI film prepared by combining laser direct writing technique and electrophoretic deposition, and a self-powered integrated photo-detective system on a PI film.23, 55-56 Transparent electrodes with grid metal lines can also be fabricated by laser direct writing on the substrate coated with metal nanoparticle inks through laser induced sintering.59-64 Recently, laser direct writing combining


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with selective metallization has been reported to prepare Ni electrodes for high-performance MSCs on a textile using a laser-scribed masking route, but the conductivity may be not high enough for practical application in electronic devices.49 Therefore, new strategies for preparation of high-conductive metallic circuits as well as the demonstration of integrated devices for energy and data exchange on a flexible substrate are still required to promote the development for practical use. Here, we demonstrated the preparation of carbon/Ni composite structures by using a laser direct writing technique for patterning Pd catalysts and a following metallic growth process through electroless Ni plating on PI films, where Pd catalysts on the surface of the porous carbon layer activate the plating. After electroless Ni plating, the resistance of carbon/Ni composite structures was reduced remarkably by more than three orders compared to starting carbon structures, reaching a low resistance of less than 0.1 ohm/sq, which is low enough for practical devices. Metallic growth process was followed to investigate the Ni plating process on macroporous carbon structures, which involved a gradual deposition of Ni structures tightly on carbon surfaces catalyzed by laser-reduced Pd and deposited Ni. Moreover, the carbon/Ni composite structures exhibited a certain degree of flexibility and mechanical stability against scratching due to the intimate adhesion between carbon and plating Ni structures, which is important for longterm practical application. Subsequently, a wireless charging and storage device on a PI film was demonstrated by integrating an outer rectangle carbon/Ni composite coil for harvesting electromagnetic waves and an inner carbon MSC for energy storage, which can be fast charged by a commercial wireless charger. In addition, a near-field communication (NFC) tag was prepared by combining a carbon/Ni composite coil for harvesting signals and a commercial IC chip for data storage, which can be used as an NFC tag for practical application.


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Experimental Materials Polyvinylpyrrolidone (PVP, K29-32, Mw = 58000), PdCl2, NiCl2·6H2O, sodium citrate dihydrate (Na3C6H5O7·2H2O), NaH2PO2·H2O, NH4Cl, NH3·H2O, and H3BO3 were purchased from Aladdin company. HCl, H2SO4, and acetone were purchased from Tianjin Kermel chemicals. Poly(vinyl alcohol) (PVA, Mw = 89000–98000) was purchased from Sigma-Aldrich. All chemicals were of analytical grade and used without further purification. Polyimide (PI) films with a thickness of 125 µm (Kapton 500H) were received from Du Pont-Toray Co. Ltd. Preparation and characterization of carbon/Ni composite structures A piece of PI film was first cleaned with water, ethanol, and acetone, respectively. A mixed aqueous solution of 0.1 wt% of PdCl2, 0.1 wt% of HCl, and 5 wt% of PVP was drop-casted onto the PI film with an amount of 0.1 mL/cm2, and allowed dried in air to form a uniform PdCl2/PVP coated PI (PPC-PI) film. Then, laser direct writing on the PPC-PI film was carried out in air under ambient conditions using a same laser direct writing system as reported in our previous work.56 A same compact blue-violet continuous-wave (CW) semiconductor laser with a rated power of 500 mW and a wavelength of 405 nm (M-33A405-500-G, Shenzhen 91Laser Co., Ltd.) and a fused silica achromatic lens with a focal length of 39.6 mm (ETL-30-40P, SIGMAKOKI) were used in the laser direct writing system. A typical laser power was set at 236 mW, and the typical scan interval between two lines was set at 10 µm. The influence of the laser power and scan interval on the conductivity of final plating structures were also studied. After laser writing, as-prepared micro-patterns were first immersed in a water beaker to dissolve most of the PVP/PdCl2 which can be recovered in the following process, and then cleaned by acetone and water to remove them thoroughly. Some Pd materials were deposited and fixed in the laser


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irradiated parts. Subsequently, the film was immersed in the plating solution at a certain temperature for electroless Ni plating, which was catalyzed by Pd materials on the carbon surface. The original plating solution contained 8.7 wt% of NiCl2·6H2O, 8.6 wt% of Na3C6H5O7·2H2O, 8.6 wt% of NaH2PO2·H2O, 2.6 wt% of NH4Cl, 0.8 wt% of NH3·H2O, 0.4 wt% of H3BO3 and 70.3 wt% of distilled water. The solution was diluted by 5 times with distilled water for electroless Ni plating. The samples were characterized by scanning electron microscopy (SEM, Hitachi S4800, 10 kV), powder X-ray diffractometry (XRD, Rigaku Rint-Ultima III, Cu Kα radiation), and X-ray photoelectron spectroscopy (XPS, Perkin Elmer PHI5600). Raman spectra were measured on a micro-Raman spectrometer equipped with an optical microscope (Olympus BX51), a 532 nm DPSS CW laser (MGL-H-532nm-1W, CNI), a CCD camera (DV401, Andor Technology), and a monochromator (MS257, Oriel Instruments Co.). The characterizations were conducted directly on the laser writing films. Square resistances were obtained by four-probe conductivity measurement, and the resistances of lines were measured with a SATA multimeter by contacting the probes on the line ends. The mechanical strain was evaluated by tightly bending a PI film with the line on stainless steel tubes with different curvatures. The scratch experiments were conducted on carbon/Ni lines by using pencils with different hardness and an applied weight of about 1 kg. Preparation and characterization of an integrated wireless charging and storage device An integrated wireless charging and storage device on a PI film was designed with an outer carbon/Ni composite circuit coil for harvesting electromagnetic waves and an inner carbon MSC for energy storage. The outer circuit coil has a size of 4 cm × 3 cm and 7 spiral cycles of carbon/Ni lines with a width of 600 µm and a gap of 400 µm between two adjacent lines. The


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MSC was fabricated at the center of the circuit. The preparation of such an integrated device needs two-step laser direct writing and an electroless Ni plating process. A piece of PI film with a size of 5 cm × 4 cm was first drop-casted with PVP/PdCl2 solution, and then irradiated under the laser direct writing system controlled by the programmed pattern. The laser power and scan interval here were set at 236 mW and 10 µm, respectively. Then, the film was cleaned by acetone and water, and immersed in the plating solution for metallic growth. After electroless Ni plating of the outer circuit, a second laser direct writing process for the preparation of inner MSC pattern at the center of the circuit was conducted in Ar using the same laser direct writing system as reported in our previous work.23 After cleaning the surface dusts with acetone and water, the integrated device was prepared by connecting each part with Ag paste, Cu tape, and PI tape. PVA-H2SO4 solution (150 µL), which was prepared by stirring 1 g of PVA in 10 mL of DI water and 1 mL of sulfuric acid for 4 hours at 95 °C, was dropped onto the active area of the inner MSC and allowed dried in air. Here, PVA-H2SO4 acts as electrolyte in the MSC to form allsolid-state carbon-based MSC after water evaporation. Cyclic voltammetry (CV), galvanostatic charge–discharge (CC), and electrochemical impedance spectra (EIS) measurements were carried out using an electrochemical workstation (VersaSTAT 4, Princeton Applied Research). The calculation of the specific capacitances (CA, mF/cm2) of the MSC from CV curves was based on the following equation:56 ଵ

‫ܥ‬A = ଶ×஺×௩×(௏

f

௏

f ‫ܸ݀)ܸ(ܫ ׬‬ ି௏ ) ௏ i

i

(1)

where A is the area of the active electrodes (cm2), v is the scan rate (V/s), Vf and Vi are the vertex ௏

potentials of the CV scan, and I(V) is the current as a function of potential (A). ‫׬‬௏ f ‫ ܸ݀)ܸ(ܫ‬is the i

numerically integrated area of the CV curve.


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The calculation of the specific capacitances (CA, mF/cm2) of the MSC from CC curves was based on the following equation:56 ூ

‫ܥ‬஺ = ஺×ௗ௏/ௗ௧

(2)

where I is the discharge current (A), A is the projected area of active electrodes (cm2), and dV/dt is the slope of the galvanostatic discharge curves. For the wireless charging and storage measurement, the circuit was placed on a Qi-certified wireless charger (output 5V/1A, EC Technology), and the potential change of the MSC was monitored and recorded by an oscilloscope (Tektronix TBS 1052B). A digital LCD clock was connected to show the charging status. A commercial wireless charging receiver (input 5 V, output 850 mA, Yoobao Technology) bought from the market was measured with the same method for comparison. Preparation and characterization of a near-field communication tag A passive near-field communication (NFC) tag on a PI film was designed by combining a carbon/Ni composite coil circuit as the NFC antenna for communicating with an NFC machine and a commercial electronic component for data storage. The antenna has an outer diameter of 3.5 cm and 6 cycles of carbon/Ni spiral lines with a width of 600 µm and a gap of 400 µm between two adjacent lines. A gap was left for connecting the commercial electronic component. Laser direct writing and electroless Ni plating processes were as same as above procedure except for different patterns. After Ni plating, a commercial electronic IC chip for data storage (NTAG216, NXP Semiconductors) was carefully connected into the circuit, forming an NFC tag. The resonant frequencies of the antenna before and after connecting the IC chip were measured using a spectrum analyzer equipped with a tracking generator (RIGOR DSA815), where a Cu coil with a diameter of 40 mm was connected to the tracking generator and covered by an acryl


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plate with a thickness of 2 mm. An antenna was put on the acrylic plate adjusting the centers of the Cu coil and the antenna. The resonant frequency was determined from the energy loss caused by the electromagnetic induction between the Cu ring and the antenna. The resonant frequencies of a commercial NFC tag (Smartrac bullseye 320_3) without and with the IC chip were also measured for comparison. Results and discussion Fabrication of carbon/Ni composite structures The approach to preparing carbon/Ni composite patterns is schematically shown in Figure 1. PVP/PdCl2 aqueous solution was first drop-casted on a piece of clean PI film, forming a thin layer of PVP/PdCl2 coated on the surface of PI film (PPC-PI). Laser direct writing process was conducted on a PPC-PI film in air, producing carbon/Pd patterns in the PI film. After removing PVP/PdCl2 coating at unirradiated parts on the PI film, selective metallization through electroless Ni plating was taken place at laser-written carbon/Pd patterns, resulting in carbon/Ni composite structures.

Figure 1. Schematic illustration for preparation of carbon/Ni composite circuits by using laser direct writing technique and following electroless Ni plating process. Figure S1 shows SEM images of as-prepared structures obtained by laser direct writing in air at a laser power of 236 mW and scan interval of 10 µm and removing PVP/PdCl2 coating at other parts. SEM images in Figure S1a, b indicate that as-prepared structures are uniform in a large


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area with structures and pores in micrometer scale, which are very similar with the carbon structures obtained by laser direct writing in air on pure PI films,56 suggesting the PVP/PdCl2 coating layer has a light influence on the laser writing structures. An enlarged SEM image in Figure S1c suggests that the micrometer-size structures are comprised of nanoparticles and nanopores. The holes on the surface may be caused by the rhombohedral PdCl2 micro-crystals formed in the PVP/PdCl2 coating layer. During drop-casting the PVP/PdCl2 solution, the PdCl2 was crystallized into micro-crystals with typical rhombohedral shapes enwrapped by PVP molecules (Figure S2). Under laser irradiation, PVP molecules may be carbonized surrounding PdCl2 micro-crystals which act as templates, accompanying with partial reduction of PdCl2. After washing with water, there will be some holes of rhombohedral shapes remained on the surface. The thickness of the structures determined from the cross-sectional SEM image in Figure S1d is about 65 µm. Raman spectrum of as-prepared structures in Figure S3 clearly indicates that the structures are mainly amorphous or graphitic carbon materials, but no signals of Pd materials were detected probably due to its small remaining amount. However, the XPS spectrum in Figure 2 shows two small peaks related to Pd3d and Pd3p1 electrons of Pd material, confirming a small amount of Pd material existing on the surface of carbon structures. It can be proposed that laser irradiation induced the reduction of Pd ions to metallic Pd atoms deposited on the carbon surface. Therefore, carbon/Pd structures with specific patterns can be easily prepared by laser direct writing in air on a PPC-PI film, which can be further used in the metallic deposition through catalytic electroless plating process.


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Figure 2. XPS spectra of the structures obtained by laser direct writing on pure PI and PVP/PdCl2 coated PI film, and carbon/Ni structures after electroless Ni plating.

Figure 3. Top-view (a-d) and cross-sectional (e, f) SEM images of the structures obtained by electroless Ni plating at 60 °C for 8 hours.


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The typical electroless Ni plating experiment was conducted by immersing a PI film with patterned carbon/Pd structures in an electroless Ni plating solution at 60 °C for 8 hours. It should be noted that hydrogen was evolved during the plating process. SEM images of the structures obtained after electroless Ni plating are shown in Figure 3. SEM image in Figure 3a indicates a uniform plating structure in a large area, but the surface is not perfectly smooth, but quite rough in the scale of tens of micrometers. The pore size on the surface becomes rather small compared to carbon/Pd structures before Ni plating, which can be further confirmed by the SEM image in Figure 3b. It implies that Ni structures may be grown tightly on the carbon surfaces. Figure 3c shows that Ni structures have very smooth surfaces and are contacted with each other at some parts, which is beneficial for high conductivity of the carbon/Ni structures. An enlarged SEM image in Figure 3d shows some black nanopores on the surface, which may be caused by hydrogen evolving during electroless Ni plating. The cross-sectional SEM image in Figure 3e shows a dense film with rare micrometer-size pores, suggesting micrometer-size pores in the starting carbon/Pd structures have been almost filled with Ni structures during metallic deposition process. It should be noted that the detachment of the carbon/Ni layer from the substrate was caused by the strong transverse shear stress when the film was cut with a pair of scissors, although the adhesion of the carbon/Ni layer to the substrate was very good. An enlarged SEM image at a cracked part in Figure 3f clearly shows that carbon structures are surrounded by Ni structures with a thickness of about several micrometers, suggesting the electroless Ni plating reaction took place tightly on carbon surfaces. XRD spectrum after electroless Ni plating for 8 hours shows no apparent diffraction peaks of Ni crystals, but a broadening bulge at around 44.5° (Figure S4), indicating an amorphous crystal form of Ni structures. XPS spectrum of Ni plating structures in Figure 2 shows that not only Ni element but


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also a certain amount of P element exists in the plating structures, because the reaction (6) will also take place besides of the reactions (3-5) when the pH of the plating solution becomes higher.44-45 Therefore, amorphous Ni structures incorporated with a little P alloy can be easily prepared intimately on the carbon surfaces through a facile electroless Ni plating process. H2PO2- + H2O → H2PO32- + 2H+ + 2e- (3) Ni2+ + 2e- → 2Ni (4) H+ + e- → 1/2H2 (5) H2PO2- + 2H+ + e- → P + 2H2O (6) The electroless Ni plating process was investigated by observing the structures obtained by electroless plating at 60 °C for different reaction times. It is observed that after Ni plating for 30 min the carbon surface has been uniformly covered by a thin layer of Ni structures, and micrometer-size pores still exist between carbon/Ni structures (Figure S5a, b). From the enlarged SEM image at a cracked part (Figure S5c), it can be clearly seen that Ni particles with a size of hundreds of nanometers were grown on carbon surfaces tightly and uniformly, with almost all carbon surfaces covered. It suggests that the Ni plating reaction took place just on the carbon surface under the catalysis of Pd catalysts. The resistance was reduced to 56 ohm/sq after Ni plating at 60 °C for 30 min from about 105 ohm/sq for starting carbon/Pd structures (Figure 4a). When Ni plating time was increased to 2 hours, carbon/Ni structures becomes larger and the gap between carbon/Ni structures becomes smaller (Figure S5d, e). The Ni particles obtained at 30 min have connected with each other, forming a uniform and continuous Ni film on the carbon surface (Figure S5f). Consequently, the resistance was reduced largely to 3.7 ohm/sq after Ni plating at 60 °C for 2 hours probably due to the connection of Ni particles (Figure 4a). When the Ni plating reaction lasted for 4 hours, carbon/Ni composite structures with relatively smooth


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surfaces are formed and the space inside the films become smaller (Figure S5g, h). The Ni structures on carbon surfaces become thicker and tend to connect with each other (Figure S5i), and the resistance was reduced to 0.58 ohm/sq after Ni plating at 60 °C for 4 hours (Figure 4a). When the Ni plating process was further increased to 8 hours, the typical structures were obtained (Figure 3), and the resistance was further reduced to 0.14 ohm/sq, nearly three orders lower than original carbon/Pd structures. According to the above results, the Ni plating reaction may be proceeded as following steps: (1) electroless Ni plating firstly occurred at Pd catalytic sites on the carbon surfaces, forming isolated Ni particles; (2) Ni particles were grown gradually and connected to each other, forming a uniform and continuous Ni film with a thickness of several hundred nanometers; (3) Ni films were grown further and became a thicker layer and connected with each other, forming a dense film with almost no pores inside. It should be noted that the Ni structures obtained in step 2 and 3 also act as catalysts for further self-catalytic electroless Ni plating. Correspondingly, the resistance of the film was lowered gradually due to the growth of Ni structures. The effect of Ni plating temperature was investigated by measuring the morphologies and resistance change of the film and lines with different widths. Figure 4a clearly shows the resistance change of the structures obtained at different temperatures as the Ni plating time increases. At a lower temperature of 55 °C, the resistance of the obtained structures was decreased more slowly than that of the structures obtained at 60 °C, arriving at 0.45 ohm/sq after 10 hours. Corresponding SEM images in Figure S6a-d indicate that there are still many micrometer-size pores between carbon/Ni structures, and the thickness of the Ni layer deposited on the carbon surface is lower than that of the structures obtained at 60 °C (Figure S6d), although the Ni surface seems as smooth as the Ni surface obtained at 60 °C with several


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identified nanoholes (Figure S6c). The relatively high resistance can be attributed to the thin plating Ni layer caused by the slow plating process at a relatively low temperature. In contrast, when the plating temperature was increased by 5 °C to 65 °C, the reaction was accelerated significantly, and the resistance was decreased to 0.08 ohm/sq after 4 hours. SEM images in Figure S6e-h indicate that the plating Ni structures have connected with each other forming a continuous Ni layer, and the Ni layer has a higher thickness than that obtained at 55 °C for 10 hours and 60 °C for 8 hours. The surface is smooth, and there are no visible black nanopores (Figure S6g). It should be noted that the resistance couldn’t be further reduced largely by extending the plating time to even 8 or 10 hours at this condition due to the decreasing plating rate and the relatively low conductivity of amorphous Ni layer. The sheet resistance of a commercial Ni foil with a thickness of 80 µm was measured for comparison. The Ni foil shows a sheet resistance of about 0.009 ohm/sq, which is about one order lower than the carbon/Ni structures due to high crystallization degree of the bulk Ni foil. The resistance change of lines with a length of 2 cm and different widths obtained at different plating temperatures was also studied. For carbon/Ni composite structures with different line widths obtained at different temperatures, all the resistance was reduced gradually as the plating process proceeded (Figure 4b-d). As the temperature increased, the resistance was reduced faster and became lower. Taking carbon/Ni composite lines with a width of 1 mm for example, at 55 °C, the resistance was reduced to 4.21 ohm from original 1840 ohm after plating for 10 hours (Figure 4d), but it was reduced to 1.23 ohm after plating for 8 hours at 60 °C (Figure 4b) and 1.3 ohm after plating for 4 hours at 65 °C (Figure 4c). Therefore, it can be expected that the Ni plating time can be further reduced to get high-conductive carbon/Ni composite structures by increasing the plating temperature.


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Figure 4. Plating time-dependent sheet resistance change of carbon/Ni composite structures obtained at different temperatures (a), plating time-dependent resistance change of carbon/Ni composite lines with a length of 2 cm and different widths obtained at 55 °C (b), 60 °C (c), and 65 °C (d), respectively, resistance change of the carbon/Ni composite line with a length of 2cm and a width of 0.2 mm at different curvatures (e), and resistance change of carbon/Ni composite lines with a length of 2 cm and different widths after scratching with pencils of different hardness (f). Besides, the influence of the laser power and scan interval between two lines on the final carbon/Ni structures was also studied. As laser power increased from 64 to 238 mW, the width of single scan line was increased gradually from about 27 to 134 µm (Figure S7), which can be attributed to the heat diffusion under laser irradiation. A series of carbon/Ni lines with a length of 2 cm and a width of 1 mm were prepared by laser direct writing at scan intervals of 10, 20, 50, and 100 µm under three laser powers of 236, 110, and 64 mW in combination with electroless Ni


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plating at 60 °C for 8 hours. It can be clearly seen from Figure S8 that the carbon/Ni structures obtained at different laser powers and scan intervals showed very similar resistance of around 1 ohm, which suggests that the laser power from 64 to 236 mW at a scan interval between 10 and 100 µm can be used to fabricate high-conductive carbon/Ni circuits in electronic devices. The reason can be attributed to the laser deposition of metallic Pd materials on the carbon surfaces which act as active catalysts for electroless Ni plating. The mechanical stability including flexibility and anti-scratch property of carbon/Ni composite lines was studied, which is important for practical use. The flexibility of one line with a width of 200 µm obtained by Ni plating at 60 °C for 8 hours was tested by bending the line tightly on stainless steel tubes with different diameters. As shown in Figure 4e, the resistance shows no apparent increase until the radius of 8 mm, and increases slightly from 6.6 ohm for the radius of 7 mm to 9 ohm for the radius of 5 mm. It should be noted that although the carbon/Ni composite structures show a certain flexibility, they cannot tolerate very high folded angles such as 180°, which will make them cracked. It is interesting that the carbon/Ni composite structures are very stable against scratching. Figure 4f shows the resistance change of the carbon/Ni structures with different widths obtained by Ni plating for at 60 °C 8 hours under hard scratching by using pencil tips with different hardness. The resistance of all carbon/Ni lines with different widths shows almost no apparent change under hard scratching by using pencil tips with hardness from 4B to 6H. SEM images of as-prepared carbon/Ni composite structures without scratching (Figure S9a) and the structures after scratching (Figure S9b-d) indicate that there is no damage on Ni structures by scratching, suggesting an excellent anti-scratch performance. It should be noted that the carbon/Ni lines with different widths obtained by electroless Ni plating at 55 °C for 10 hours and 65 °C for 4 hours exhibited the same anti-scratch performance (Figure S10). The flexibility


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and anti-scratch property can be attributed to the good adhesion between carbon structures and plating Ni structures, as well as the hardness of Ni structures. An integrated wireless charging and storage device An integrated wireless charging and storage device was prepared by combining laser direct writing technique and selective metallization through electroless Ni plating. The process was schematically shown in Figure 5a. The device is comprised of two parts, an outer coil based on carbon/Ni composite structures for harvesting electromagnetic waves and an inner carbon MSC for energy storage. The outer carbon/Ni composite coil was prepared by conducting electroless Ni plating process on laser-written carbon/Pd coil patterns, and carbon electrodes for inner MSC were prepared by another laser direct writing process at 157 mW in Ar. A photograph of an asprepared real device before sealing and adding polymer electrolyte is shown in Figure 5b. The resistance of the outer coil was reduced from 105 kohm of starting carbon/Pd structures to 42.4 ohm of carbon/Ni composite structures after electroless Ni plating at 60 °C for 8 hours (Figure 5c), which is low enough for use in device. Although the inner MSC was prepared using the same method as reported previously, the capacitive performance of the specifically integrated MSC was first evaluated by CV and CC measurements. The quasi-rectangle CV curves and triangular CC curves indicate a capacitive property of the MSC (Figure S11a, S11c), and corresponding specific capacitances calculated from CV curves at different scan rates and CC curves at different current densities are plotted in Figure S11b and Figure S11d, respectively. The typical specific capacitances at a scan rate of 10 mV/s and current density of 0.05 mA/cm2 are 14.3 mF/cm2 and 25.4 mF/cm2, separately, which are similar with the capacitances reported in our previous work using the same preparation condition.23 At the same time, the MSC shows an excellent cycling performance without capacitance degradation even after 20,000 CV cycles


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at a scan rate of 50 mV/s (Figure S11e). The vertical slope of the Nyquist plot in Figure S11f in the low-frequency region suggests a typical capacitive behavior, which is similar as our previous reported data. 23

Figure 5. Schematic illustration for the fabrication of a wireless charging and storage device integrated with a carbon/Ni composite coil and a carbon MSC (a), a photograph of a real wireless charging and storage device integrated with an outer carbon/Ni composite coil circuit for harvesting electromagnetic waves and an inner carbon MSC for energy storage (b), resistance change of the carbon/Ni coil obtained at different plating time (c), potential change of the integrated MSC charged by the wireless carbon/Ni coil circuit placed on a commercial wireless charger (d), and pulse information of the wireless charger (e). The wireless charging and energy storage property of the integrated device was evaluated by putting the device on a commercial Qi-certified wireless charger (output power 5W), and the potential between two MSC electrodes was monitored and recorded. It can be clearly observed


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from Figure 5d that the MSC can be easily charged to above 1 V after several wireless pulses within one minute, demonstrating the effective charging and storage performance of the integrated device. It should be noted that the wireless charger works in a pulse mode with one pulse lasting for 100 ms in a period of 5 s (Figure 5e, S12). A video was recorded to show the wireless charging and storage process in the supporting information (Video S1), which clearly indicates that the MSC can be charged to above 1 V within some pulses, and can drive a digital LCD clock working. A commercial wireless charging receiver bought from the market was used for comparison. It consists of a copper-wire coil, a small control board, and an adapter, which can be used to charge an iPhone (Figure S13a, b). The copper-wire coil was taken from the receiver and used for measurement. Compared to the carbon/Ni coil, the copper-wire coil showed a higher pulse voltage of about 5.7 V for the cellphone charging (Figure S13c), because the turn of the copper-wire coil is 15, more than the 7 of the carbon/Ni coil, and the induction voltage is proportionate to the coil turns. Therefore, it suggests that the carbon/Ni coil showed a comparable performance to the commercial copper-wire coil. It should be noted that the wireless charging to the MSC with the commercial copper-wire coil receiver was not provided because the high voltage of 5.7 V will easily destroy the MSC with PVA/H2SO4 electrolyte. In our previous work, we have demonstrated a self-powered photodetector by combining an integrated photodetector and an MSC in the same PI film with a commercial solar panel,23 so it is reasonably expected that such a wireless charging and storage device can be easily integrated with different sensors such as photodetectors, humidity sensors, and gas sensors, to form an integrated wireless working system. Moreover, such a carbon/Ni composite coil can be directly used as a wireless charging receiver to charge the batteries, cellphones, or other electronic devices.


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A near-field communication tag based on carbon/Ni composite structures Besides the wireless device for energy harvesting and storage, a passive near-field communication (NFC) tag for data exchange was also demonstrated by integrating a carbon/Ni composite coil as the NFC antenna for receiving signals and a commercial IC chip for data processing. The preparation process is schematically shown in Figure 6a. The carbon/Ni composite coil was first fabricated using the same approach as described above, i.e. preparing carbon/Pd patterns by laser direct writing followed by electroless Ni plating. Then, an IC chip taken from a commercial NFC tag was connected into the circuit, forming the new NFC tag (Figure 6b). It should be noted that the size of the commercial IC chip is very small, only about 700 µm (Figure S14), which will not influence the flexibility of the device. The resonant frequencies of the carbon/Ni composite coil before and after connecting the IC chip were measured. As shown in Figure 6c, the carbon/Ni composite coil showed a resonant frequency of about 101.2 MHz, but the resonant frequency was shifted to around 19.1 MHz after connecting the IC chip. A commercial NFC tag bought from the market, which contains a metal coil and an IC chip, was also measured for comparison (Figure S15). The resonant frequency peak of the metal coil without the IC chip is at 52.7 MHz, and shifted to 13.7 MHz with the IC chip, which is near the standard working frequency of 13.56 MHz (Figure S15). Although the carbon/Ni NFC tag showed a little shift at the peak resonant frequency, it still responded as fast as the commercial one due to the broad resonant frequency peak. It can be expected that the position of resonant frequency peak can be tuned by optimizing the coil size and shape. The video provided in the supporting information clearly indicates the NFC tag can sensitively communicate with a Blackberry cellphone via NFC, and the data in the NFC tag can be easily written, read, and erased by the cellphone interface (Video S2). Therefore, an NFC tag has been demonstrated by


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integrating a carbon/Ni composite coil and a small commercial IC chip, which can be readily used in practical application. It can be expected that in future the combination of circuits prepared by laser direct writing techniques, printing methods, or other approaches with commercial electronic components of ultra-small sizes would be an effective way to produce practical flexible portable and wearable devices.

Figure 6. Schematic illustration for the fabrication of a near-field communication (NFC) tag with a carbon/Ni composite coil and a commercial IC chip (a), a photograph of a real NFC tag integrated with a carbon/Ni composite coil as the NFC antenna for receiving signals and a commercial IC chip for data processing (b), and signal curves of the carbon/Ni coil circuit before and after connecting a commercial IC chip (c). Conclusions In summary, the preparation of high-conductive carbon/Ni composite patterns have been demonstrated by combining laser direct writing technique and selective metallization through electroless Ni plating. The resistance of the carbon/Ni composite structures was lowered by more than three orders to less than 0.1 ohm/sq compared to the starting carbon/Pd structures before electroless Ni plating, which is low enough for practical devices. The investigation of the electroless Ni plating process elucidates that Ni deposition took place tightly on the carbon


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surface, forming carbon/Ni composite structures with a certain flexibility and excellent antiscratch property. A wireless charging and storage device including an outer carbon/Ni composite coil for harvesting electromagnetic waves and an inner carbon MSC for energy storage in the same PI film was prepared by laser direct writing and electroless Ni plating, which can be used in practical wireless charging. Furthermore, an NFC tag was prepared by integrating a carbon/Ni composite coil as an NFC antenna for receiving signals with a commercial IC chip for data processing, showing a practical application in data writing, reading, and erasing via NFC. This work paves new avenue for the preparation of practical integrated devices by laser direct writing, electroless Ni plating, as well as employing commercial electronic components. It is reasonably expected that the combination of facile low-cost techniques such as laser direct writing, printing methods, electroless plating or electrochemical deposition, even lithographic approaches for high-conductive circuits and commercial ultra-small semiconductor components for functions will be an effective way to produce flexible portable and wearable devices in future. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 21603201), a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element Blocks (No.2401)” (JSPS KAKENHI Grant Number JP24102004) and JSPS KAKENHI Grant Number JP15H04132, and China Academy of Engineering Physics (item no. TP02201303). The authors thank Mr. Eiji Aoyagi of the Electron Microscopy Center in Tohoku University and Mr. Changda Zhang in Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences for help with SEM measurements. The authors also thank Ms. Sayaka Ogawa for help with the XPS measurement. Supporting Information


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SEM images and Raman spectrum of carbon/Pd structures, optical microscopy (OM) image of PdCl2 micro-crystals formed in PVP/PdCl2 layer on the PI substrate, XRD patterns of the structures before and after electroless Ni plating at different plating times, SEM images of the carbon/Ni structures obtained at different plating times, SEM images of the structures obtained by electroless Ni plating at 55 °C and 65 °C, single line width at different laser power, resistance of carbon/Ni lines obtained at different laser power and scan interval, SEM images of the carbon/Ni structures without and after scratching, resistance change of carbon/Ni lines obtained at 55 °C and 65 °C towards scratching, characterizations on capacitive performance of the integrated MSC in the wireless device, pulse information produced from the wireless charger, OM image of an IC chip in the NFC tag, and photographs and performance of the commercial wireless charging receiver and NFC tag. Supporting information is available free of charge via the Internet at http://pubs.acs.org. References (1) Gubbi, J.; Buyya, R.; Marusic, S.; Palaniswami, M. Internet of Things (IoT): A vision, architectural elements, and future directions. Future Gener. Comp. Syst. 2013, 29, 16451660. (2) Wang, X.; Liu, Z.; Zhang, T. Flexible Sensing Electronics for Wearable/Attachable Health Monitoring. Small 2017, 1602790. (3) Zhang, B.; Chen, J.; Jin, L.; Deng, W.; Zhang, L.; Zhang, H.; Zhu, M.; Yang, W.; Wang, Z. L. Rotating-Disk-Based Hybridized Electromagnetic–Triboelectric Nanogenerator for Sustainably Powering Wireless Traffic Volume Sensors. ACS Nano 2016, 10, 6241-6247. (4) Pu, J.; Wang, X.; Xu, R.; Komvopoulos, K. Highly Stretchable Microsupercapacitor Arrays with Honeycomb Structures for Integrated Wearable Electronic Systems. ACS Nano 2016, 10, 9306-9315. (5) Wang, T.; Guo, Y.; Wan, P.; Zhang, H.; Chen, X.; Sun, X. Flexible Transparent Electronic Gas Sensors. Small 2016, 12, 3748-3756.


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