Fabricating Metallic Circuit Patterns on Polymer Substrates through

Nov 17, 2016 - Before the laser activation, the surface morphology of the sample is compact and smooth. As shown in Figure 3b, it can be clearly obser...
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Fabricating Metallic Circuit Patterns on Polymer Substrates through Laser and Selective Metallization Jihai Zhang, Tao Zhou,* Liang Wen, and Aiming Zhang State Key Laboratory of Polymer Materials Engineering of China, Polymer Research Institute, Sichuan University, Chengdu 610065, China S Supporting Information *

ABSTRACT: Nowadays, with the rapid development of portable electronics, wearable electronics, LEDs, microelectronics, and bioelectronics, the fabrication of metallic circuits onto polymer substrates with strong adhesion property is an ever-increasing challenge. In this study, the high-resolution and well-defined metallic circuits were successfully prepared on the polymer surface via laser direct structuring (LDS) based on copper hydroxyl phosphate [Cu2(OH)PO4], and the key mechanism of the selective metallization was systematically investigated. XPS confirmed that Cu0 (elemental copper) was formed through photochemical reduction reaction of Cu2(OH)PO4, after 1064 nm NIR pulsed laser irradiation. During the electroless plating, because it is the important active catalytic center, this newly formed Cu0 was the key factor to achieve the successful selective metallization. SEM revealed that after the electroless plating, the copper layer actually physically anchored into the polymer substrate, giving an excellent mechanical adhesion property of the obtained metallic patterns. In addition, the micro-Raman surface imaging approved the generation of the amorphous carbon on the polymer composites’ surface after NIR laser irradiation, and the chemical reaction region caused by the pulsed laser spot was found at approximately 40 μm. This environmentally friendly and effective strategy for fabricating circuit patterns on the polymer surface has a possible application in the printed circuit plate (PCB) industry. KEYWORDS: laser direct structuring, metallization, near-infrared pulsed laser, polymer, copper hydroxyl phosphate combination of inkjet printing and electroless plating.5 Wu et al. investigated the manufacture of copper conductive patterns by reduction-assisted sintering with ascorbic acid at room temperature.28 However, these traditional techniques typically require expensive equipment, toxic solvents, and complicated steps. Therefore, reducing the wet-chemical steps and avoiding or minimizing environmental and occupational risk as well as developing simple and cost-effective approaches for fabricating metallic patterns are highly desirable and of a great importance. Light amplification by stimulated emission of radiation (abbreviated to laser), as a clean energy, has been widely applied in the field of laser patterning, surface modification, medical treatment, laser-assisted metal deposition, and so on.29−33 For laser-assisted metal deposition technology, it is mainly divided into laser-enhanced electrode position (LEED), laser-induced metal deposition in electrolyte (LID), laserassisted chemical vapor deposition (LCVD) or laser-assisted physical vapor deposition (LPVD). Recently, as a new family of laser application, laser direct structuring (LDS) technology has

1. INTRODUCTION In the past decade, the fabrications of metallic circuit patterns on nonconductive materials have received increasing attention due to their wide applications, including portable electronics, wearable electronics, light-emitting displays (LEDs), automotive, microelectronics, bioelectronics, and so on.1−9 Numerous innovative methods for selective metallization have already been investigated and developed, such as photolithography,10 inkjet printing, 11−14 ligand-induced electroless plating (LIEP),15−18 and printed electronics.19−23 Commonly, photolithography combined with the physical vapor deposition (PVD) is suitable for metallic patterning on the surface of large-area polymers with a good alignment.24,25 However, the main shortcomings of PVD are a high equipment cost and high time consumption. In contrast, electroless plating is the most frequently used method for polymer substrates metallization in industry owing to its simplicity and low cost.26,27 It is worth mentioning that the surface-selective activation is essential for the manufacture of metallic patterns. According to the literature, Alexandre Garcia and co-workers successfully fabricated the micrometric metallic circuit patterns on polymer substrates based on the LIEP process.18 Liao et al. prepared conductive copper patterns on plastic substrates through the © XXXX American Chemical Society

Received: September 6, 2016 Accepted: November 17, 2016 Published: November 17, 2016 A

DOI: 10.1021/acsami.6b11305 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

phosphate [Cu2(OH)PO4] was purchased from Merck Chemicals Technology Co., Ltd. The spectroscopy and morphology characterizations of Cu2(OH)PO4 used in our experiments are shown in Figure S1 and Figure S2 in the Supporting Information. Analytical reagent grade copper sulfate (powder), potassium sodium tartrate (powder), ethylene diamine tetra-acetic acid (powder), sodium citrate (powder), formaldehyde (37 wt % solution in water), methanol (liquid), sodium hydroxide (powder), and absolute ethanol (liquid) were purchased from by Chengdu Kelong Reagent, China. All the above-mentioned materials were used as received without any further purification. 2.2. Preparation of ABS/Cu2(OH)PO4 Composite. First, ABS resin incorporated with 5 wt % Cu2(OH)PO4 powder were mixed uniformly in a high mixing machine and melt-blended using a laboratory twin-screw extruder at 220 °C. Then, the ABS/Cu2(OH)PO4 composite sheets (78 mm × 65 mm × 3 mm) were obtained using a laboratory injection molding machine at 210 °C. The Cu2(OH)PO4 were thermally stable and had no decomposition during the processing (see Supporting Information Figure S3). Finally, the asprepared samples were put into an oven at 60 °C for 4 h to eliminate the inside stress. 2.3. Surface-Selective Laser Activation. The samples of ABS/ Cu2(OH)PO 4 composite sheets were immersed into sodium hydroxide (1 M) solution in an ultrasonic bath for 15 min to remove surface organic contaminants. Afterward, the surface selective laser activation was performed via MK-GQ10B optical fiber laser machining system (Mike Laser Technology Co., Ltd. Kunshan, China) equipped with EZCAD 2.0 software. The samples surface was line-by-line scanned by a 1064 nm NIR pulsed fiber laser at a scanning speed of 2000 mm/s in air atmosphere. It is noted that the distance of the adjacent lines was set to 30 μm in our experiments. The pulse width of the used laser is 100 ns, and the pulse frequency can be varied from 20 kHz to 100 kHz. In the experiments, the powers of laser beam and the laser pulse frequency were set to 8 W and 60 kHz, respectively. After selective laser activation, the as-prepared samples were cleaned with absolute ethanol in an ultrasonic bath for 10 min at ambient temperature. 2.4. Metallization. The metallization experiments were carried out at 50 °C in a homemade electroless copper plating bath which contained copper sulfate (8 g/L) as a source of Cu2+ ions, potassium sodium tartrate (30 g/L), ethylene diamine tetra-acetic acid (3 g/L), sodium citrate (3 g/L) as the complexing agent, formaldehyde (12 mL/L) as the reducing agent, and methanol (150 mL/L) as the stabilizer. The pH value was approximately to 12.5 via adjusting the content of sodium hydroxide. The deionized water was used for all experiments. The electrochemical reactions during the electroless copper plating using formaldehyde as a reducing agent were listed as follows: Overall electrochemical reaction:

exhibited a significant potential for fabricating electrically conductive patterns. Commonly, the LDS technology comprises merely three steps: (1) the injection molding of LDS materials contained special laser-sensitive additives; (2) the computer-controlled laser activation of as-desired patterns on the materials’ surface; and (3) the metallization by the electroless plating. As is previously reported,34 a laser could directly irradiate the surface of modified polymer and breaking the chemical bond of special laser-sensitive additives to create the catalytic centers. Subsequently, the activated regions have the capacity to selectively deposit copper, nickel, gold, and other metals through the electroless plating. As a result, the metallic patterns were achieved on the surface of the polymer substrate. In comparison with other selective metallization techniques, LDS technology has obvious advantages of design flexibility, short fabrication cycle, large-scale production, and environmentally friendly. However, the existing literature for the details of this technology is relatively sparse, and the related mechanism is still unclear. Generally speaking, the interaction between the laser and materials are determined by their intrinsic characteristics. Among the laser sources used in LDS technology, the 1064 nm near-infrared (NIR) laser has specific advantages that relate to cost-effectiveness and practicality. Unfortunately, for the pristine polymer resin, there is a weak absorption of the NIR pulsed laser, and basically, this has no practical value for LDS technique because the electroless plating should be carried out on the catalytic activity surface. Therefore, the selection of appropriate laser sensitizers having capacity to generate the catalytic active sites for the polymer substrate plays a crucial role in the developing of LDS materials. Copper hydroxyl phosphate [Cu2(OH)PO4] has attracted multiple investigations because of its intrinsic structure, photocatalytic activity, and chemical properties.35−40 Very recently, Wang et al.35 reported that Cu2(OH)PO4 exhibits a strong absorption in the nearinfrared region. In our work, the tentative study on Cu2(OH)PO4 as a laser sensitizer was carried out, and the unexpected finding was that Cu2(OH)PO4 can be used to prepare the LDS material with a very good performance. Specifically, the highresolution conductive patterns were successfully produced on the polymer surface on the basis of the acrylonitrile− butadiene−styrene (ABS) incorporated with Cu2(OH)PO4. Herein, we introduce a simple approach to fabricating electronic circuit patterns on polymer substrate. The mechanism of laser direct structuring (LDS) technology based on Cu2(OH)PO4 and ABS matrix is systematically investigated. After 1064 nm high-energy NIR pulsed laser irradiation, the Cu0 (elemental copper) is converted and released from Cu2(OH)PO4, which plays the key catalytic role for the metallic growth in electroless copper plating. The experimental results demonstrate that the laser etching microrough structure provides an excellent mechanical adhesion between the copper layer and polymer substrates, attributing to the penetrating anchorage effect. This study also provides an ideally versatile strategy to fabricate fine circuit patterns and decorative applications.

Cu 2 + + 2HCHO + 4OH− → Cu + 2HCOO− + 2H 2O + H 2↑ Positive electrode reaction:

2HCHO + 4OH− → 2HCOO− + 2H 2O + H 2 ↑ +2e− Negative electrode reaction: Cu 2 + + 2e− → Cu 2.5. Characterizations. 2.5.1. Fourier Transform Infrared (FTIR) Spectroscopy. The FTIR spectra of Cu2(OH)PO4 in the region of 4000−400 cm−1 was collected using a Nicolet iS50 spectrometer equipped with a deuterated triglycine sulfate detector. The experiment was run at 4 cm−1 of the resolution, and the number of scans of each FTIR spectrum were 32. 2.5.2. X-ray Photoelectron Spectroscopy (XPS). The XPS spectra of the Cu2(OH)PO4 and ABS/Cu2(OH)PO4 composites sheet before and after laser irradiation were recorded on a Kratos XASAM 800 spectrometer (Kratos analysis, U.K.) with an Al Kα X-ray source (1486.6 eV) and an X-ray beam of around 1 mm. The binding energy scale was calibrated from the CH2 by assigning 284.6 eV to the C 1s peak.

2. EXPERIMENTAL SECTION 2.1. Materials. Acrylonitrile−butadiene−styrene (ABS, technical quality, PA-747, density: 1.03 g/cm3, melt flow rate: 1.2 g/10 min, 200 °C, 5 kg), which was synthesized via emulsion polymerization between acrylonitrile, butadiene, and styrene, was produced by Chi Mei Corporation (Taiwan). Analytical reagent grade copper hydroxyl B

DOI: 10.1021/acsami.6b11305 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces 2.5.3. Powder X-ray Diffraction (PXRD). The PXRD pattern of the Cu2(OH)PO4 was characterized by a Japan Rigaku D/max-2500 X-ray powder diffractometer diffractometer using Cu Kα radiation (λ = 0.15418 nm) and operating at 40 kV and 40 mA. 2.5.4. Ultraviolet−visible-Infrared (UV−vis-IR). The near-infrared (NIR) absorbance of Cu2(OH)PO4 was investigated at room temperature by a UV−vis-NIR spectrophotometer (UV-3600, Shimadzu) in the range from 400 to 1800 nm. 2.5.5. Metalloscope. The surface morphology of ABS/Cu2(OH)PO4 composites sheet before and after laser irradiation and after metallization were observed using a VHX-1000C digital microscope (Keyence, Japan). 2.5.6. Scanning Electron Microscopy (SEM). SEM micrographs of the Cu2(OH)PO4, as well as ABS/Cu2(OH)PO4 composites sheet before and after laser irradiation and after metallization, were conducted on a JEOL JSM-7500F field-emission scanning electron microscope equipped with an X-act electron microprobe for energydispersive X-ray spectroscopy (EDX) at 15 kV. For morphology observation, the samples were coated with a thin layer of gold to reduce charging. 2.5.7. Transmission Electron Microscopy (TEM). TEM characterization of the Cu2(OH)PO4 was performed using a JEOL JEM-2011 microscope with an accelerating voltage of 200 kV. The sample was prepared by dripping diluted Cu2(OH)PO4 solution onto copper grids. 2.5.8. Thermal Gravimetric Analysis (TGA). The thermal behaviors of Cu2(OH)PO4 was measured with a NETZSCHTG 209F1 thermal gravimetric analyzer at a heating rate of 10 °C/min in the range of 30− 800 °C under an air atmosphere. The sample size was 10 mg. 2.5.9. Micro-Raman Spectroscopy and Raman Surface Imaging. The Raman surface imaging (100 μm × 100 μm) and the corresponding micro-Raman spectra at different surface region after laser irradiation were carried out on a DXRxi micro-Raman spectrometer (Thermo Fisher Scientific, U.S.A.) equipped with a diode laser of excitation of 532 nm and a 25 μm confocal pinhole. The scan spacing of Raman surface imaging was 10 μm. 2.5.10. Contact Angle (CA) Measurement. The contact angle of a 2 μL water droplet on the sample surface was measured using a sessile drop method by a Krüss K100 contact-angle machine (Germany). Double-distilled water was used for contact angle measurements. The average CA values were obtained by measuring the same samples in at least five different areas of the surface. 2.5.11. Mechanical Adhesion Test. The mechanical adhesion property between the ABS substrate and the copper layer was evaluated by a qualitative Scotch tape test according to the ASTM D3359 standard. The test consists of applying and suddenly removing the pressure-sensitive adhesive tape over crosshatched squares of 1 × 1 mm2. According to the percentage of the area removed, the classification of adhesion scale are divided into 5B (no film square removed by the adhesive tape), 4B (less than 5% removed), 3B (5%− 15% removed), 2B (15%−35% removed), 1B (35%−65% removed), and 0B (more than 65% removed). 2.5.12. Electrical Properties. The electrical properties of the obtained copper layers were analyzed by Keithley (U.S.A.) model no. 6487 picoammeter/voltage source with a constant voltage of 0.2 mV at room temperature. To evaluate the conductivity of the copper layer, we fabricated a series of rectangular circuit lines, whose length and width are 21 mm and 1.8 mm, respectively. The resistance of the obtained copper layer was measured at least three times for the average values. The conductivity of a circuit can be calculated by the following equation:

ρ=

3. RESULTS AND DISCUSSION 3.1. Laser Direct Structuring (LDS) Process and Applications. Figure 1a shows the schematic illustration of

Figure 1. (a) Schematic illustration of the laser direct structuring (LDS) process, where a 1064 nm NIR pulsed laser is used to activate the polymer surface. (b) Photograph of ABS/Cu2(OH)PO4 composite sheet sample after the laser activation (left, 2 cm × 2 cm area) and the corresponding magnified metalloscope image (right), scale bar: 100 μm. (c) Photograph of sample after electroless copper plating (left, 2 cm × 2 cm area) and the corresponding magnified metalloscope image (right), scale bar: 100 μm.

the laser direct structuring (LDS) process. In our experiment, ABS/Cu2(OH)PO4 composite sheet samples containing 5 wt % Cu2(OH)PO4 were prepared using a laboratory twin-screw extruder and injection-molding machine. It is noted that the laser activation was performed in air atmosphere by scanning the sample surface line by line with a fiber pulsed laser (λ = 1064 nm). The laser energy parameters were set at 8 W (laser power) and 60 kHz (laser frequency) with a scanning speed of 2000 mm/s. After that, the samples of the laser activated sheet were immersed in the solution of electroless copper plating to complete the selective metallization. In Figure 1b, it can be seen that a clear contrast solid square pattern (2 cm × 2 cm) with a different color is fabricated on the surface of the ABS/ Cu2(OH)PO4 composite sheet after the laser irradiation. Moreover, the metalloscope image indicates that a series of etched pits appear in the laser irradiation area. As shown in Figure 1c, a successful metallization (copper layer) is achieved within the laser-etched area after electroless copper plating, and at the same time, no copper layer can be observed in the area without laser irradiation. That is to say, the copper layer is only selectively deposited in the area of NIR laser irradiation. The morphology of the ABS/Cu2(OH)PO4 composite sheet after electroless copper plating was also characterized by the metalloscope. The results also demonstrate that the welldefined copper layer is obtained on the laser-irradiated surface of the sample. In Figure 2a, for example, the computerized vector images of circuit patterns and the panda pattern are accurately patterned onto the polymer surface at desired positions through NIR pulsed laser. In particular, to illustrate the practical application of LDS, the fabrication of accurate, high-resolution, and welldefined metallic patterns are shown in Figure 2b. These two photographs are conductive circuit patterns and the panda pattern after electroless copper plating, respectively. Apparently, after electroless copper plating, the copper layer is selectively deposited on the activated areas. To understand the

L RS

where ρ is the conductivity, and R is the resistance. The L is the length of the circuit, and S is the cross-sectional area of the circuit. C

DOI: 10.1021/acsami.6b11305 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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surface due to the irradiation of high-energy NIR pulsed laser. Each etching pit corresponds to an exact irradiation location of the pulsed laser spot, and the typical diameter of these pits is 27.2−27.6 μm. This microscale pits structure probably has benefits to improve the mechanical adhesion between the polymer substrate and the copper layer. Surprisingly, the decrease of the surface wettability after NIR laser irradiation is observed (see Supporting Information Figure S4), which is probably attributed to the formation of microscale rough structures on the surface of the ABS/Cu2(OH)PO4 composite. Furthermore, the nondestructive micro-Raman surface imaging analysis is illustrated in Figure 3c. The analysis surface region is 100 μm × 100 μm areas with the scan spacing of 10 μm. For carbon-based materials, the micro-Raman surface imaging is a powerful analytical method for revealing the structural change in chemistry.41 The micro-Raman surface image actually shows the differences of chemical composition in the region under NIR pulsed laser irradiation. It is necessary to say that the basic reference spectrum is required to collect before the micro-Raman surface imaging. As shown in Figure 3c, the color in different regions depends on the similarity of the Raman spectra at the given position with the basic reference spectra. For a high similarity, the regions are red; for a low similarity, the regions are green. The color of their intermediate regions is green. In Figure 3c, from a micro perspective, the chemical reaction region caused by the pulsed laser spot is approximately at 40 μm after the laser irradiation. As shown in Figure 3d, a broad diffusion band appears in the region of 1000−2000 cm−1 (assigned to the amorphous carbon), which is in agreement with the previous study.29 That is to say, the blue

Figure 2. Photographs of the circuit patterns and the panda pattern on the ABS/Cu2(OH)PO4 composite. (a) After NIR laser activation; (b) after electroless copper plating.

mechanism of this phenomenon, we carried out further investigations through a series of characteristic methods. 3.2. Characterizations of Laser Activation. To clearly observe the topography and morphology of the ABS/ Cu2(OH)PO4 composite sheet before and after NIR pulsed laser irradiation, the scanning electron microscopy (SEM) characterization was carried out. Before the laser activation, the surface morphology of the sample is compact and smooth. As shown in Figure 3b, it can be clearly observed that a series of regular microscale etching pits are achieved on the sample

Figure 3. SEM images of the surface morphologies of ABS/Cu2(OH)PO4 composite sheet sample (a) before NIR pulsed laser activation; (b) after NIR pulsed laser activation, scale bar: 10 μm; (c) the image of micro-Raman surface analysis after NIR pulsed laser patterning; (d) the typical Raman spectra in the red, green, and blue regions, respectively. The bottom black curve is the typical Raman spectrum before laser patterning. D

DOI: 10.1021/acsami.6b11305 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. High-resolution C1s XPS spectra of the ABS/Cu2(OH)PO4 composite sheet. (a) Before NIR laser irradiation; (b) after NIR laser irradiation. The high-resolution Cu2p XPS spectra of the ABS/Cu2(OH)PO4 composite sheet. (c) Before NIR laser irradiation; (d) after NIR laser irradiation. The XPS spectra are fitted using curve-fitting software (XPSPEAK v4.0).

Figure 5. (a) SEM image of the surface morphology of ABS/Cu2(OH)PO4 composite sheet after 30 min of electroless copper plating, scale bar: 10 μm. (b) The corresponding magnified SEM image, scale bar: 1 μm. (c) The corresponding EDX analysis. (d) SEM image of the sample surface after 60 min electroless copper plating, scale bar: 10 μm. (e) Cross-sectional morphologies of the sample after 60 min electroless copper plating, scale bar: 10 μm.

analyzed by the high-resolution scans of C1s XPS spectra. As shown in Figure 4a, before laser irradiation, in the region of 292−280 eV, the main peak of C1s appears at 284.7 eV, which is attributed to C−C, C−H, and CC in ABS resin. In addition, one small shoulder peak at 286.5 eV is also observed, which is attributed to the −CN groups in acrylonitrile repeating units.42 As shown in Figure 4b, however, besides the abovementioned two peaks, a new peak at 288.8 eV, which is attributed to −CO groups is clearly observed after NIR laser irradiation. This result demonstrates that the sample surface occurs a slight thermal oxidation under NIR pulsed laser irradiation. To further analyze the chemical state of Cu element

and green regions in the micro-Raman surface image contain a number of the newly formed amorphous carbon after NIR pulsed laser irradiation. Thus, beyond the irradiation damage shown from the SEM image, the micro-Raman image directly proves the surface carbonization of ABS/Cu2 (OH)PO 4 composite caused by the laser. X-ray photoelectron spectroscopy (XPS) was used to further investigate the possible photochemical reactions on the surface of the polymer composite during NIR pulsed laser irradiation. Commonly, XPS can provide the element information within the depth of a few nanometers from the sample surface. The variation of chemical groups on the sample surface was E

DOI: 10.1021/acsami.6b11305 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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layer covering on the polymer surface is approximately at 5.6 μm. This penetrating anchorage effect will endow a good adhesion property between ABS resin and the copper layer. Very recently, Zhou et al. fabricated conductive silver electrodes with a high-bonding strength on transparent flexible substrates through a novel laser-direct writing.43 They found that the PVP as a “bridge” could easily form the chemical bonding between silver nanoparticles and polymer substrate, and a good adhesion was gained. The mechanical adhesion property, which is an important characteristic for circuit patterns, can be evaluated by the wellused industrial scotch-tape test. The cross-hatched squared copper layer (1 × 1 mm2) is made by Cross Hatch cutter, as shown in Figure S6 in the Supporting Information. After suddenly removing a high-performance transparent scotch-tape, the squared copper layer is fully undamaged after the scotchtape test, showing an excellent mechanical adhesion property. This good performance is certainly contributed by the copper layer physically anchoring into the ABS substrate, which gives a good prospect in industrial applications to fabricate metallic circuit patterns on the polymer substrate. To evaluate the resistivity of the obtained copper layer, we fabricated a series of circuit lines with the length and the width of 21 mm and 1.8 mm, respectively. To be sure, through the multimeter measurement, we found that the resistance of the polymer composite surface after initial laser activation was too large to be measured. Therefore, the ABS/Cu2(OH)PO4 composite surface after initial NIR laser irradiation is insulated. The resistances of the different electroless plating time are shown in Figure S7a in the Supporting Information. It can be observed that the resistance of the obtained copper layer obviously decreases with the increase of the electroless plating time. After 60 min of electroless copper plating, the resistance of the copper layer is around 0.06 Ω. In order to calculate the conductivity, the average thickness of the obtained copper layer is 17.85 μm estimated from Figure 5e. According to the resistance measurement, the conductivity of the copper layer is approximately at 1.09 × 107 Ω−1·m−1, which is within a factor of 5.5 of the bulk copper (5.96 × 107 Ω−1·m−1). In other words, the resistivity of the obtained copper layer is 9.18 μΩ·cm. In our work, the main reason for the electrical properties difference from the bulk copper is contributed to the loosely packed copper particles in the copper layer. La Fratta et al.7 developed PVP contained Ag microparticles that are conductively activated through electroless deposition, and the conductivity value of the obtained copper circuit is within a factor of 6.0 of the solid copper metal. Wu et al.28 fabricated copper conductive patterns with resistivity of around 774 μΩ· cm through reduction-assisted sintering with the ascorbic acid. Farraj et al.6 developed a copper-complex-based ink which was suitable for inkjet printing, yielding a conductive pattern with a resistivity of 10.5 μΩ·cm. Recently, Li et al. have reported Ag nanoplate44 and silver nanowire45 inks for fabricating flexible electronics with the resistivity range from 103 μΩ·cm to 1015 μΩ·cm via direct writing. Moreover, Chang et al.4 and Liao et al.5 fabricated copper patterns with an excellent conductivity, which was almost equivalent to bulk copper and had a good adhesion on polymer substrate through inkjet printing combined with the electroless plating. In addition, in Figure S7b in the Supporting Information, the resistance ratio (R/R0) of the obtained copper circuit (electroless copper plating of 60 min) with the storage days

on the sample surface after NIR laser irradiation, we also studied the high-resolution XPS spectra of Cu2p. As shown in Figure 4c, before laser irradiation, four peaks within 965−925 eV are observed, which are contributed by Cu2+ in Cu2(OH)PO4 (incorporated into the ABS matrix). Surprisingly, after laser irradiation, the characteristic peaks (Cu2p1/2 and Cu2p3/2) of Cu0 (elemental copper) appear at 952.8 and 932.9 eV, indicating that a part of Cu2(OH)PO4 undergoes a complex chemical reaction to reduce elemental copper during the NIR laser irradiation (Figure 4d). During the laser irradiation, the Cu2(OH)PO4 absorbs laser energy to produce enormous heat, resulting in an instantaneous high temperature on the polymer composite surface. This temperature is high enough to cause the polymer carbonization. In Figure 3c,d, the micro-Raman image also reveals the generation of the amorphous carbon of ABS caused by laser. In our opinion, this generated amorphous carbon plays a role as a reducing agent, which provides an environment to reduce Cu2(OH)PO4 to element copper at high temperature. Generally speaking, the carbonization temperature of ABS is above 600 °C. So, the estimation of the temperature induced by the laser is at least above 600 °C. Here, we can conclude that this newly generated Cu0 is the key reason to achieve a successful selective metallization in the electroless plating. As is known, the elemental copper is very effective as the catalytic active center during the electroless plating. Once the initial reaction is initiated by Cu0, the cooper is deposited continuously due to the autocatalytic characteristics. 3.3. Characterizations of Metallization. In electroless copper plating, the copper ions in aqueous solutions are selectively deposited on the surface of the laser-activated plastic by autocatalytic redox reactions. It is worth mentioning that the laser activation has significant effects on the final metallization morphology and performance. As shown in Figure 5a,b, the morphologies of the ABS/Cu2(OH)PO4 composites sheet sample after electroless copper plating are characterized by SEM. Clearly, after 30 min of electroless plating, the sample surface appears a thin copper layer which is formed from the aggregated copper particles. Additionally, the size of copper particles is approximately around 1−1.5 μm. To further identify the elemental composition, the copper layer is investigated by energy dispersive X-ray (EDX) analysis. As shown in Figure 5c, two K peaks (8.0 and 8.9 keV) and one L peak (at 0.9 keV) are appeared in the EDX spectra, which are only the characteristic peaks of copper. In Figure 5a, although the experiment of electroless copper plating is conducted for 30 min, the pulsed laser etched pits still not covered by the copper layer. In Figure 5d, after 60 min electroless copper plating, the copper layer becomes thicker and completely covers the laser etched pits. In addition, the XRD analysis is carried out to confirm the high quality of the copper layer, and the result is shown in Figure S5 in the Supporting Information. The signals of Cu metal are detected in XRD, indicating the deposition of pure Cu species on the surface of ABS/Cu2(OH)PO4 composites. The cross-sectional microstructure of the sample after 60 min electroless copper plating is also characterized by SEM. Before the experiment, the cross sections of the samples were repeatedly polished to be smooth using an abrasive paper with grain size of 7000 mesh. In Figure 5e, it can be seen that a series of “V-shaped” copper layer (just like a rivet) anchor into the ABS substrate, which increases the contact area between the copper layer and the polymer substrate. The total depth of the rivet structure reaches 35.7 μm, and the thickness of the copper F

DOI: 10.1021/acsami.6b11305 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces at room temperature is also provided. In Figure S7b, R0 is the resistance at 0 day, and R is the resistance at the given day. The results show that the resistance slightly enhances with the increase of the storage days. The resistance value reaches to a plateau and remains stable after 12 days, which possibly attributes to the generation of the protective oxidation layer on the surface.5 3.4. Mechanism of LDS. For the laser direct structuring (LDS), the Cu0 species as the catalytic center are formed directly from the photochemical reduction reaction of laser sensitizers [Cu2(OH)PO4] induced by NIR pulsed laser irradiation. During electroless copper plating, these generated Cu0 immediately initiate the autocatalytic deposition of copper in elecrtroless plating bath, and the selective metallization on polymer surface is successfully achieved. According to the results of Figures 3, 4, and 5, we propose a simple schematic illustration of LDS mechanism. As illustrated in Figure 6, under

plating after 1064 nm NIR pulsed laser irradiation. For the polymer composites incorporated with Cu2(OH)PO4, the mechanism of the selective metallization for LDS was also systematically investigated. XPS confirmed that, after NIR pulsed laser irradiation, the Cu0 (elemental copper) was generated on the polymer composites surface via the photochemical reduction reaction of Cu2(OH)PO4. In our work, during the electroless plating, this newly formed Cu0 as the active catalytic center is the key factor to achieve successful selective metallization. SEM and the metalloscope results demonstrated that a series of regularly microscale etching pits (27.2−27.6 μm) were formed on the polymer composites surface after laser irradiation. SEM also revealed that, after the electroless plating, the copper layer actually physically anchors into the polymer substrate, which is well explained by the excellent mechanical adhesion property (the scotch-tape test) of the obtained metallic patterns. Specifically, the thickness of the copper layer covering the polymer surface was approximately at 5.6 μm, and total depth of the rivet structure was 35.7 μm. The deposited copper layer is actually formed from the aggregated copper particles around 1−1.5 μm. In addition, the micro-Raman surface imaging and the corresponding Raman spectroscopy approved the generation of the amorphous carbon on the polymer composites surface after NIR laser irradiation, and the chemical reaction region caused by the pulsed laser spot was found approximately at 40 μm. This environmentally friendly and effective strategy for fabricating circuit patterns on the polymer surface provides a possible method in the printed circuit plate (PCB) industry. For the application in PCB substrates, more research needs to be done in the near future. It is very important to mention here that, as a fast-growing new field, the LDS technology is worthy of long-term research. Moreover, ABS resin is a common manufacturing plastic for fused deposition molding on 3D printers, and therefore, the ABS/Cu2(OH)PO4 composite can be also designed as 3D printing materials with special functions. The desired circuits and patterns on the surface of 3D plastic parts prepared by 3D printers can be easily produced by the LDS technology. This also has a promising prospect in the development of functional 3D printing materials.

Figure 6. Schematic illustration of the mechanism of the laser direct structuring (LDS) based on Cu2(OH)PO4.

the 1064 nm NIR pulsed laser irradiation, the Cu2(OH)PO4 strongly absorbs the laser energy, and the active Cu0 species are generated; meanwhile, the high-energy laser also fabricates a series of regular etching pits on the ABS/Cu2(OH)PO4 composites surface, resulting in a certain degree of surface roughness. In the electroless plating process, the copper layer in situ anchors into the ABS substrate through autocatalytic redox reaction. Finally, the conductive copper patterns are selectively formed on the polymer surface. It is inevitable that, after the laser irradiation, a small amount of reduced Cu0 species exist on the edge of the craters due to the thermal effect and the ejecting effect. In contrast, the number of active species in craters is more than that of on edges. In the electroless plating process, the copper layer is generated and deposited continuously through autocatalytic redox reaction. However, it is noted that the speed of copper deposition in the craters is far faster than that of on the edges. Therefore, a uniformly copper layer is finally formed between craters.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11305. FTIR spectra, XPS full spectrum, PXRD pattern, and UV−vis−IR spectroscopy of Cu2(OH)PO4; SEM and TEM images of Cu2(OH)PO4; TG and DTG curves of Cu2(OH)PO4; surface contact angles of the ABS/ Cu2(OH)PO4 composite before and after NIR pulsed laser irradiation; XRD pattern of the copper layer; mechanical adhesion test between polymer substrate and copper layer; electrical properties of the copper circuits (PDF)

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4. CONCLUSIONS In summary, to obtain the accurate, high-resolution, and welldefined metallic patterns on the polymer surface, the laser direct structuring (LDS) technology based on Cu2(OH)PO4 was successfully proposed. The core of LDS for polymers is the selective metallization on the surface during the electroless

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-28-85402601. Fax: +86-28-85402465. Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acsami.6b11305 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51473104), and State Key Laboratory of Polymer Materials Engineering (Grant Nos. sklpme2014-3-06, sklpme2016-3-10).

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DOI: 10.1021/acsami.6b11305 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX