Selective Electroless Metallization of Micro- and Nanopatterns via Poly

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Functional Inorganic Materials and Devices

Selective Electroless Metallization of Micro- and Nanopatterns via Poly(dopamine) Modification and Palladium Nanoparticle Catalysis for Flexible and Stretchable Electronic Applications Jingxuan Cai, Cuiping Zhang, Arshad Khan, Liqiu Wang, and Wen-Di Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07411 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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ACS Applied Materials & Interfaces

Selective Electroless Metallization of Micro- and Nanopatterns via Poly(dopamine) Modification and Palladium Nanoparticle Catalysis for Flexible and Stretchable Electronic Applications Jingxuan Cai1,2, Cuiping Zhang1,2, Arshad Khan1, Liqiu Wang1,2, and Wen-Di Li1,2* 1

Department of Mechanical Engineering, The University of Hong Kong, Pokfulam, Hong Kong, China

2

HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), Hangzhou, Zhejiang, China

KEYWORDS: surface modification, electroless plating, flexible electronics, transparent electrode, flexible printed circuit

ACS Paragon Plus Environment

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Abstract

The authors report a new patterned electroless metallization process for creating micro- and nanoscale metallic structures on polymeric substrates, which are essential for emerging flexible and stretchable optical and electronic applications. This novel process features a selective adsorption of catalytic Pd nanoparticles (PdNPs) on a lithographically masked poly(dopamine) (PDA) interlayer in-situ polymerized on the substrates. The moisture-resistant PDA layer has excellent stability under harsh electroless plating bath, which enables electroless metallization on versatile substrate materials regardless of their hydrophobicity, and significantly strengthens the attachment of electroless plated metallic structures on the polymeric substrates.

Prototype

devices fabricated using this PDA-assisted electroless metallization patterning exhibit superior mechanical stability under high bending and stretching stress. The lithographic patterning of the PDA spatially confines the adsorption of PdNPs and reduces defects due to random adsorption of catalytic particles on undesired area. The high resolution of the lithographic patterning enables the demonstration of a copper micrograting pattern with a linewidth down to 2 µm and a silver plasmonic nanodisk array with a 500-nm pitch. Copper mesh is also fabricated using our new patterned electroless metallization process and functions as flexible transparent electrodes with > 80% visible transmittance and < 1 ohm sq−1 sheet resistance. Moreover, flexible and stretchable dynamic electroluminescent displays and functional flexible printed circuits (FPCs) are demonstrated to show the promising capability of our fabrication process in versatile flexible and stretchable electronic devices.


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1. Introduction Flexible and stretchable conductive components have been attracting increasing research interest because of their promising applications in many practical fields, such as wearable electronics,1-3 medical implants,4 and portable devices.5 For example, flexible printed circuits (FPC), which contain conductive circuitry patterns created on flexible substrates, are widely used in portable consumer electronic devices.6-7 Micro- and nanoscale metallic networks are also considered as promising replacement of indium tin oxide (ITO) for flexible and stretchable transparent conductors.8-15 Deformable metallic patterns have found unique applications as tunable antennas,16 plasmonic sensors,17-19 etc. Cost-effective and high-quality fabrication of these flexible and stretchable conductive components, which contain multi-scale features of various metals on versatile polymeric substrates, is of great importance but still remains challenging. The most commonly used fabrication approach in the FPC industry is based on a subtractive process featuring photolithography and wet etching of copper, which typically has high cost, high pollution, and high material wastage. New techniques that can overcome these limitations using additive fabrication strategies have been investigated. For example, a combined process of photolithography, electroplating and imprint transfer has been introduced to create metal mesh structures on thermoplastic substrates for flexible transparent electrodes.20 However, the electroplating process used in this process requires the metal patterns to be created on an intermediate conductive substrate and then transferred to the device substrates, which inevitably increases production complexity. Another widely researched technique in additive fabrication is inkjet printing of metallic nanoparticles conductive ink.21-24 This process often requires heatresistant substrates that can sustain the high sintering temperature of the conducive ink and the conductivity of the nanoparticle ink is usually much lower than bulk metal.25 Oxidization of metallic nanoparticles further limits their applications in the fabrication of practical devices.26


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Selective electroless plating (ELP) of metallic patterns, which benefits from an autocatalytic redox reaction to deposit a thin-layer metal on a catalyst-preloaded substrate, is another promising fabrication method.27 The conductivity of the electroless plated metal can reach the same level of the bulk metal.28-29 However, the poor adhesion of the plated metallic patterns on bare polymeric substrates, due to the lack of binding sites for the catalyst, is one major challenge when applying ELP in flexible and stretchable electronic device fabrication. Practically, the modification of the bare polymeric surfaces to enhance the catalyst adsorption and improve the adhesion of deposited metal is necessary. The surface modification of polymeric substrates for ELP should enable effective adsorption of catalyst moieties, and the modified surface must be chemically resistant to the usually alkaline or acidic electroless plating bath. Many efforts have been devoted to investigating the surface modification processes. For example, chemical roughening,30 UV-ozone treatment,31 and oxygen plasma treatment,32 are the most commonly used methods in surface modification for ELP. However, important challenges still exist in these processes. Conventional chromium-containing etching agents for the surface modification of ELP, particularly used for FPC manufacturing, are harmful to the environment. The adhesive strength of the deposited metals on UV-ozone or oxygen plasma-treated plastic surface is still far below that of the industrial standard. Surface modification can also be realized by coating an extra active layer onto the plastic surface, typically by polymer grafting,33 surface silanization,34 deposition of polyelectrolytes,35 inkjet printing of ion-adsorption nanoparticles,36 matrix-assisted catalytic printing,37 and laser printing of a mask for the catalyst immobilization.38-39 However, most of the aforementioned methods have limitations when being adopted for practical manufacturing process, such as complicated


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processing steps or equipment, poor stability under long ELP process for thick metal layer, and insufficient patterning resolution for micro-scale metallic structures. Here, a new patterned electroless metallization process is developed for the fabrication of micro- and nanoscale metallic structures on various polymeric substrates. The fabrication is based on a novel selective adsorption of catalytic Pd nanoparticles (PdNPs) on a poly(dopamine) (PDA) interlayer masked by a lithographically patterned sacrificial resist layer. PDA has shown the capability for binding of metallic ions and nanoparticles for electroless deposition of metals.40-42 The mask prevents the random adsorption of PdNPs on undesirable area, hence improving the patterning resolution of the metallic structures. Using this method, a Cu micrograting pattern with a linewidth down to 2 µm and an Ag nanodisk array with a 500-nm pitch were successfully demonstrated. Moreover, the superior moisture-resistance adhesion of PDA enables electroless plating of metals on both hydrophilic and hydrophobic substrates with excellent adhesion strength. As applications of the proposed method, flexible and stretchable transparent electrodes (TEs) with electroless plated Cu micromesh were demonstrated on plastic films and elastomeric substrates, showing > 80% visible transmittance and < 1 ohm sq−1 sheet resistance. The PDA modification layer also enables excellent mechanical, chemical, and environmental stabilities for the fabricated devices. Finally, flexible and stretchable electroluminescent displays and functional FPCs were also fabricated to demonstrate the versatile applications of metallic structures made by this new fabrication process. 2. Results and Discussion 2.1.

Miro-

and

Nanoscale

Metallization

through

Poly(dopamine)

Modification,

Lithographic Patterning and Electroless Plating Our proposed universal fabrication process for metallic micro- and nanopatterns is schematically illustrated in Figure 1a–e. In a typical fabrication process, a substrate is first


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immersed in a dopamine polymerization solution triggered by CuSO4 and H2O2 at room temperature for 5 – 20 min.43 The dopamine is oxidatively self-polymerized to PDA and then deposited on the substrate (Figure 1a). Subsequently, the PDA-modified substrate is rinsed in de-ionized (DI) water and dried at 60 °C for 30 min. Then, a layer of resist is spin-coated onto the PDA-modified substrate, and a lithography step is conducted to create a micro- or nanoscale pattern in the resist layer (Figure 1b). In this work, we mainly use photolithography patterning but various lithography techniques, such as e-beam lithography and nanoimprint lithography, can also be applied for the patterning. Before the metallization of the micro- and nanopatterns, the PDA-modified substrate is activated by immersion in a 300 nM PdNPs solution, which contains 13-nm-diameter PVP-capsuled PdNPs (Figure S1a, Supporting Information), at 40 °C for 300 seconds. The PdNPs are selectively adsorbed onto the exposed PDA layer in the lithographically defined trenches (Figure 1c). The sacrificial resist layer is then dissolved to eliminate the contamination of decomposed resist in the ELP solution (Figure 1d). Then, during the following ELP step, the metal is selectively deposited on the PdNP-activated surface to form a uniform metallic pattern by immersing the sample in the electroless plating bath (Figure 1e). Finally, the film is rinsed in DI water and stored at 60 °C for 30 min to enhance the adhesion of the metal layer and complete the fabrication process. The entire fabrication process is solution-based, performed in the ambient environment and suitable for large-area fabrication. Additionally, this process can be readily standardized and used for industrial production. The long-term stability of the PdNPs has been examined by exposure to ambient conditions over 8 months. No obvious changes in the diameter of nanoparticles (Figure S1b, Supporting Information), appearance or the catalytic capability of the PdNPs have been observed, which confirms the stability of the PdNPs.


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Figure 1. Schematic illustration of the fabrication of a metallic micromesh pattern using the proposed selective electroless plating process via PDA modification and PdNP catalysis. (a) Self-polymerization of PDA on a substrate. (b) Micromesh patterns formed in a resist layer coated on the PDA-modified substrate by (photo)lithography. (c) Selective adsorption of PdNPs by immersing into the 300 nM PdNP solution. (d) Removal of the resist in a 5% w.t. NaOH aqueous solution. (e) Electroless deposition of metal on the PdNP-activated surface to form a uniform metal micromesh. (f) A photograph of a fabricated Cu micromesh on a PET film. The scale bar represents 1 cm. 2.2. Adhesive Strength of the Electroless Deposited Metal Layer The catechol group and amino group of PDA can form chemical bonds with the polymeric substrate, as well as the PdNPs; thus, the adsorbed catalytic PdNPs are tightly bonded to the substrate through the intermediate PDA layer.44-45 Compared with other commonly used


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adhesive modification agents, PDA modification results in both greater stability and stronger adhesion on the deposited metal. We performed 90° peel strength tests on the Cu films electroless deposited on polyethylene terephthalate (PET) films using different surface modification approaches, including PDA (polymerization time: 5 min, 10 min, and 20 min), 3aminopropyltriethoxysilane (APTES), 3-mercaptopropyltriethoxysilane (MPTES), and oxygen plasma, as shown in Table 1. The presented data clearly reveal that the Cu layer on the PDAmodified PET film is much stronger than that on the APTES- and MPTES-modified PET films. The adhesive strength of plated Cu increased from 0.74 N mm-1 to 0.81 N mm-1, when the PDA modification time increased from 5 min to 20 min, which can be attributed to the increased thickness and coverage of the PDA film. The adhesive strength of plated Cu using the PdNP catalyst is also higher than that obtained from PDA-modified epoxy resin treated using a conventional Pd/Sn catalyst46. The adhesive strength of electroless deposited Cu could be further enhanced by increasing the surface roughness of the substrate by chemical or mechanical roughening.47 Because increasing PDA modification time will decrease the transmittance of the PET film (Figure S2, Supporting Information) but not significantly enhance the adhesive strength of deposited Cu layer, the modification time was held at 5 min in the following experiments. The excellent adhesive strength of the electroless plated Cu was also confirmed using an adhesive tape test, as shown in the Supporting Information Movie S1. No obvious Cu residual was observed on the adhesive tape after 10 times peeling tests on the electroless plated Cu film on a PET substrate under optical microscopy characterization.

Table 1. Comparison of the adhesive strength of the plated Cu on PET substrates modified with various agents and conditions.


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Surface modification agent

Adhesive strength [N mm-1] a)

Poly(dopamine), 5 min

0.74 ± 0.24


0.78 ± 0.18


0.81 ± 0.23

1% w.t. APTES alcohol solution, 30 min

0 b)

1% w.t. MPTES alcohol solution, 30 min

0.12 ± 0.01

Oxygen plasma, 1 min

~0.02 c)

a)

The adhesive strength of the Cu layer was measured on a 90° peeling test platform using a 10-mm width 3M adhesive tape. The electroless plating temperature was 40 ℃ and the plating time was 15 min.; b) The Cu layer was peeled off spontaneously because of the hydrolyzation of polysiloxane; c) A conventional Pd/Sn catalyst was used in this experiment. 2.3. Fabrication of Flexible and Stretchable Metal-mesh Transparent Electrodes (TEs) A flexible TE with Cu micromesh on the PDA-modified PET film was fabricated as a demonstration of our patterned ELP process. As displayed in Figure 1f, the prototype electrode shows excellent uniformity and transparency. The use of Cu in a TE as well as in other conductive components is advantageous because of Cu’s low resistivity and high abundance. PET was selected as the substrate material because of its high transparency, good chemical resistivity, low cost, and wide usage in the electronics industry. PET exhibits particularly favorable optical transparency (Figure S2, Supporting Information), which is desirable in many applications, including photovoltaics, OLED displays, etc. The morphology of bare PET and PDA-modified PET film are displayed in Figure S3 (Supporting Information). The thickness of the PDA coating on the film increases from 8.2 nm to 36.9 nm for polymerization time ranges from 5 min to 20 min (Table S1, Supporting Information). Figure S4 (Supporting Information) shows the SEM characterization of the PDA-coated film before and after PdNPs adsorption. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images in Figure 2a-b show the morphological characterization of the Cu TEs at different stages of the


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fabrication. Figure 2a displays the SEM and AFM images of the trenches in the photoresist layer created using photolithography. The photoresist mesh had a 50-µm pitch, and its trench width and depth were 2.7 µm and 635 nm, respectively. Figure 2b presents the electroless deposited Cu mesh on the PDA-modified PET film. The Cu mesh was deposited over a 3 × 3 cm2 area by immersion into a Cu electroless plating bath at 40 °C for 30 min. As evident from the SEM and AFM images, the Cu mesh had linewidth and thickness of ~3.2 µm and 945 nm, respectively. The selective electroless Cu deposition was also confirmed by energy-dispersive X-ray spectroscopy (EDS) analysis (Figure S5, Supporting Information).

Figure 2. Fabrication of the 50 µm pitch Cu-mesh electrode prototype. Morphological characterization by SEM (left) and AFM (right) of a Cu electrode at different fabrication stages:


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(a) as-developed mesh pattern in the photoresist; and (b) Cu mesh on the PDA-modified PET substrate after 30 min electroless plating at 40 °C. (c) Plot of the Cu mesh thickness and width versus the electroless plating time at a constant temperature of 40 °C and a substrate size of 3 × 3 cm2. The fabrication process was further investigated by changing the electroless deposition time to fabricate Cu meshes with various thicknesses, in which the plating temperature (40 °C) and substrate size (3 × 3 cm2) were maintained. The dependence of the thickness and the width of the Cu mesh on the plating time is shown in Figure 2c. The curves indicate that the metal thickness increases nearly linearly with the plating time after the electroless deposition process is triggered by the catalytic PdNPs. Meanwhile, the Cu mesh width did not change significantly even after 30 min. It can be calculated from the curve that the electroless deposition speed of the Cu plating bath is approximately 2 µm/h at 40 °C. No obvious change in the electroless deposition speed of Cu on PDA-modified PET film with different polymerization time was observed. Figure 3a provides the transmittance of typical Cu-mesh electrodes with plating times of 10 min, 20 min, and 30 min in the 500-900 nm wavelength range. A small decrease in the transmittance over the measured spectral range was observed when the plating time increased from 10 min to 30 min, and this decrease is attributed to the increased random deposition of Cu on the non-activated PDA surface after prolonged plating. Meanwhile, the sheet resistance of the electrodes can be substantially reduced when the plating time is increased, as displayed in Figure 3b. A low sheet resistance of 0.52 ohm sq-1 was observed for the 30 min Cu-mesh electrode, and the transmittance at a wavelength of 550 nm was still above 70%.


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Figure 3. Performance characterization of the 50-µm-pitch Cu-mesh electrode prototype. (a) Transmittance spectra of the representative electrodes with plating times of 10 min, 20 min, and 30 min, respectively. (inset) Photograph of a Cu-mesh electrode. The scale bar represents 1 cm. (b) Plot of transmittance versus sheet resistance for the Cu-mesh electrodes at different plating times, with calculated FoMs shown in the inset. To quantitatively study how the plating time affects the overall performance of the TEs, the figure of merit (FoM) representing the ratio of the electrical conductance to the optical conductance (σdc/σopt), was calculated for all the electrodes displayed in Figure 3b by using the following expression48-50


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FOM =

σ dc = σ opt

188.5  1  Rs  − 1  T 

where T is the optical transmittance at a wavelength of 550 nm and Rs is the sheet resistance of the electrode. The inset of Figure 3b plots the FoM as a function of the plating time. The presented curve indicates that the plating time has considerable effects on the sheet resistance and hence on the values of the FoM, because of the increase in thickness of plated Cu mesh and enhancement of its electrical conductivity. Our Cu-mesh electrode achieved FoM of more than 2 × 103, which is much better than that of the commercial indium tin oxide (ITO).51 The excellent adhesive strength of PDA layer greatly enhances the stability of the plated metal layer under bending, stretching, heating, and chemical attack. Figure 4a provides the test results of the mechanical stability on the Cu-mesh electrode prototype under tensile bending stress with bending radii ranging from 2 mm to 10 mm. The results clearly indicate that for a bending radius larger than 6 mm, the variation in the sheet resistance (1.08 ohm sq-1, 15 min plating time) is negligible. The change in the sheet resistance under a 2 mm bending radius is still within 32% of its original value. Figure 4b shows the variation in the sheet resistance as a function of the number of bends for repeated tensile bending to a radius of 4 mm in comparison with a commercial ITO/PET film. The curves show that no significant change in the electrical conductivity occurs up to 1000 bending cycles, while as comparison, the sheet resistance on the ITO/PET increases more than 600 times after 400 bending cycles. Figure S6 in the Supporting Information presents the real-time variation in the sheet resistance under a repetitive tensile bending loading to a radius of 2 mm at 0.25 Hz. The maximum variation in the sheet resistance (1.08 ohm sq-1, 15 min plating time) is within 300% of its original value, and the sheet resistance


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could be recovered when the bending stress is released. These results prove the superior flexibility of our TEs, which is mainly attributed to the strong adhesion of the PDA coating.

Figure 4. Mechanical and environmental stability of the prototype flexible Cu-mesh electrode. (a) Plot of the variations in the sheet resistance versus the bending radii (tensile loading). (inset) Photograph of the electrode during bending. (b) Plots of variations in the sheet resistance versus the number of cycles of repeated bending (tensile loading) to radius of 4 mm for the Cu mesh and ITO/PET. (inset) Photograph showing the electrode lighting a red LED after 1000 bending cycles. (c) Plot of the variations in the sheet resistance versus the applied strain for the Cu-mesh electrode on a PDMS film. (inset) Photograph and SEM micrograph of the Cu-mesh electrode. Variations in the sheet resistance during (d) environmental and chemical stability tests in (e) acidic and alkaline and (f) various organic solvents. The scale bars represent 1 cm.


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Another benefit of the PDA modification is that the selectivity of the substrate in our method is broad and not limited to flexible plastic films. We also fabricated stretchable TEs with 50-µmpitch Cu mesh on an elastomeric polydimethylsiloxane (PDMS) substrate and examined the mechanical stability of the fabricated stretchable electrode. The selective electroless Cu deposition on the PDMS substrate was also confirmed by EDS analysis (Figure S7, Supporting Information). The fabricated stretchable TE shows a transmittance of 72% at a wavelength of 550 nm and a sheet resistance of 2.01 ohm sq-1 (Figure S8, Supporting Information). Figure 4c provides a plot of the changes in the sheet resistance as a function of the applied strain. The presented data clearly reveal that the changes in the sheet resistance of the electrode under a strain less than 20% is within 100% of its original value. The sheet resistance rapidly increases to 1000% of its original value (from 2.01 ohm sq-1 to 20.81 ohm sq-1) when 45% of the strain is applied. The higher increases in the sheet resistance values presumably arose because of metal mesh cracking under larger strains.52 Figure S9 in the Supporting Information provides the realtime variation in the sheet resistance under rapid repetitive stretching conditions to a strain of 20% at 1.25 Hz. The maximum change in the sheet resistance (2.21 ohm sq-1, 15 min plating time) during the stretching process is within 270% of its original value, and the sheet resistance measured when the mesh is released gradually increases to 1.3 times of its original value after 125 stretching cycles, which could be attributed to the cracking of the metal mesh under rapid stretching conditions. The environmental stability of the as-fabricated flexible Cu-mesh electrode was evaluated by exposing the electrodes to elevated temperature and dipping them in a 0.1 M hydrochloric acid solution (pH = 1.14), DI water (pH = 5.86), 0.1 M sodium hydroxide solution (pH = 13.01), ethanol, methanol, and toluene. Figure 4d shows that after heating at 50 °C and 60 °C for 6 h,


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the sheet resistances of the electrode remained unchanged. Figure 4e reveals that after immersion in both neutral and highly alkaline environments for 30 min, no noticeable changes in the sheet resistances of the electrode were observed. The superior stability of the metal mesh is attributed to the excellent moisture-resistant adhesion of PDA, and the PDA was protected from decomposition in alkaline solutions by the top Cu mesh.53 On the other hand, in the highly acidic environment, the sheet resistance of the electrode changed by 300% from its original value, which is caused by the electrochemical corrosion of Cu. Figure 4f shows the variation in the sheet resistances of the electrode after dipping in ethanol, methanol, and toluene for 30 min. The results indicate that no obvious changes in the sheet resistances of the electrode were observed. The long-term stability of the flexible Cu-mesh electrode is estimated by storage the asfabricated electrode in air for 10 days. The sheet resistance of the electrode increases by only 40 % after 10 days (from 1.09 ohm sq-1 to 1.50 ohm sq-1), and no obvious changes in the morphology of the Cu-mesh electrode is observed (Figure S10, Supporting Information). 2.4. Material Versatility and Dimensional Scalability. A crucial advantage of our method over other PDA modification-based electroless plating techniques is the wide range of reducible metals due to the excellent catalytic capability of PdNPs. To demonstrate that our fabrication is versatile regarding metal choice, Figure 5a-d present the photos and the SEM images of Ni and Ag letters fabricated on PI (Figure 5a, 5c) and PTFE films (Figure 5b, 5d). The electroless deposition of Ni and Ag on the PI and PTFE films were further confirmed by EDS analysis on the as-fabricated Ni and Ag letters (Figure S11 and S12, Supporting Information). The grain size of electroless plated metals on PTFE films were smaller than those on PI films for both Ni and Ag, which maybe correlated to the nucleation


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densities of the terminate group of the PDA on different substrates.54 However, the mechanism underlying need to be further investigated.

Figure 5. Photographs (insets) and SEM images of Ni letter ‘H’ plated on PI (a) and PTFE (b) films, and Ag letter ‘H’ plated on PI (c) and PTFE (d) films. The scale bars in the photographs and SEM images represent 0.2 cm and 1 µm, respectively. Besides the demonstrated millimeter-scale patterns, our method is also capable of fabricating dense metallic patterns at micro- and nanoscale. A university emblem that consisted of 4-µmpitch Cu gratings, rather than continuous Cu film, was successfully plated on PDA-modified PET films (Figure 6a-b and S13a). The uniform diffracted colors in Figure S13a and individual Cu lines as seen from the SEM characterization confirmed the high-resolution electroless plating of Cu gratings. We have also demonstrated the nanoscale metallization using the presented process on a glass substrate. Figure 6c-d and S13b show a 500-nm-pitch nanohole array thermal-imprinted in a resist layer coated on a PDA-modified glass substrate and the corresponding Ag nanodisk array formed through the patterned ELP process. The transmittance spectra of the Ag nanodisk array is illustrated in Figure S14 (Supporting Information). Such nanoscale metallic structures have been widely used in plasmonic sensing and structural color applications. Our method provides a new solution-processed approach for the scalable production of these plasmonic structures.


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Figure 6. (a) SEM characterization of a 4-µm-pitch grating formed onto the photoresist coated on a PDA-modified PET film. (b) SEM characterization of 4-µm-pitch Cu gratings electroless plated on a PDA-modified PET film. (c) SEM characterization of a 500-nm-pitch nanohole array imprinted on a resist layer coated on a PDA-modified glass substrate. (d) SEM characterization of a 500-nm-pitch Ag nanodisk array on a PDA-modified glass substrate. The scale bars in (a) and (b) represent 5 µm, and in (c) and (d) represent 200 nm. 2.5. Flexible and Stretchable Electroluminescent Displays and Flexible Printed Circuits. We constructed flexible and stretchable electroluminescent displays as practical applications of the metallic structures fabricated through our selective ELP method, as schematically illustrated in Figure 7a. A typical electroluminescent display consists of three layers: one transparent electrode layer, one electroluminescent light-emitting layer (e.g., Cu doped zinc sulfide particles), and one conductive layer as the bottom electrode. These layers are stacked in a


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sandwich structure. When an AC voltage is supplied across the electrodes, the electroluminescent particles will be excited and emit light. In our devices, the front electrode is a selectively electroless plated Cu micromesh on a PET film (in the flexible display) or a PDMS film (in the stretchable display). The light-emitting layer is either a mixture of ZnS:Cu particles and polybutene adhesive (in the flexible display) or a mixture of ZnS:Cu particles and PDMS (in the stretchable display). The back electrode is an electroless plated Cu pattern on the PET film (in the flexible display) or a Cu micromesh electrode on the PDMS film (in the stretchable display). With proper peripheral circuit including a programmed microcontroller, we further demonstrated a dynamic 7-segment digital display using the same device structures and the same fabrication process. Figure 7b shows a screenshot of this dynamic display and the video of the working device is shown in the Movie S2 (Supporting Information). Microscopic optical image of the illuminating electroluminescent display in Figure S15 (Supporting Information) shows individual light-emitting electroluminescent particles under the Cu micromesh electrode (dark grid in the image).


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Figure 7. Demonstration of the electroluminescent displays and FPCs. (a) Schematic illustration of

the

electroluminescent

display.

(b)

Screenshot

of

a

dynamic

seven-segment

electroluminescent display. (c) Photograph of a stretchable electroluminescent display (i) before and (ii) after stretching to 125%. (d) Photograph of the strechable electroluminescent display under bending. Photograph of the FPCs fabricated on (e) PET and (f) PI films. The scale bars represent 1 cm. The stretchable electroluminescent light emitter fabricated on a thin PDMS film is demonstrated in Figure 7c, which shows its illuminating images (i) before and (ii) after being stretched to 125% of its original width. Small defects observed in the display after stretching were attributed to the failure of the lines in the Cu micromesh and partial local delamination between the sandwiched layers. Figure 7d shows that such electroluminescent light emitter can still work properly when bended.


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One promising application of our patterned electroless plating process is in the fabrication of FPCs that are widely used in electronic devices. As a prototype demonstration, Figure 7e and Figure 7f show photos of the FPCs fabricated on PET and PI films, respectively. LEDs and resistors were welded on the FPCs by silver paste and connected to a peripheral circuit with a programmed microcontroller. Movie S3 (Supporting Information) displays the fast switching of LEDs. 3. Conclusion In summary, we have developed a new patterned electroless metallization process of microand nanoscale metallic structures on polymeric substrates for flexible and stretchable optical and electronic applications. This novel process utilizes a selective adsorption of catalytic PdNPs on a lithographically masked moisture-resistant, high-stability PDA interlayer in-situ polymerized on the substrates, enables the electroless metallization on versatile substrate materials regardless of their hydrophobicity, and strengthens the attachment of electroless plated metallic structures on the polymeric substrates. Versatile electronic applications, including transparent electrodes, electroluminescent light emitters, and flexible printed circuits, were successfully demonstrated on flexible and stretchable substrates using this method. Because of the strong adhesion of PDA interlayer, excellent mechanical, chemical, and environmental stabilities of metallic structures were observed on the fabricated devices. With the unique performance of high-throughput, lowcost, etching-free, and high-resolution fabrication comparing with conventional approaches in the current electronic industry, this process has broad application in the fields of optics and electronics. 4. Materials and Methods Materials: PET and PI films were purchased from Boyuan Plastics (Dongguan, China). PTFE films were purchased from Hongfu Insulating Materials (Dongguan, China). APTES (99%),


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MPTES (95%), silver nitrate, ammonium chloride, and palladium chloride were all analytical reagents (AR) grade and purchased from Sinopharm Chemical Reagent (Shanghai, China). ARgrade copper sulfate pentahydrate, nickel sulfate heptahydrate, potassium sodium tartrate, ethylenediaminetetraacetic acid disodium, sodium hydroxide, 2,2’-bipyridyl, and potassium ferrocyanide were purchased from Acros Organics (New Jersey, USA). AR-grade dopamine hydrochloride, formaldehyde, hydrogen peroxide (30%), tris hydrochloride (Tris-HCl), sodium hypophosphite, sodium citrate, polyvinylpyrrolidone (PVP, M.W. 8000) and ammonium chloride were purchased from J&K Chemicals (Shenzhen, China). Zonyl FSN was purchased from Dupont (Wilmington, USA). Preparation of PdNPs: 900 mg ammonium chloride was first dissolved in 50 mL DI water by magnetic stirring, then 300 mg palladium chloride was added to the solution. Afterwards, 940 mg PVP was added in the solution until a homogenous solution was formed (termed Solution A). 600 mg ascorbic acid was dissolved in another 50 mL DI water (termed Solution B). Finally, solution B was added dropwise into solution A at a rate of 5 mL min-1 under stirring, the mixture was stirred for another 4 h. Preparation of PDMS films: The preparation of PDMS (Dow Corning, Sylgard 184) is conducted in accordance with the recommended recipe. A ratio of 10 parts of the base to 1 part of the curing agent were mixed thoroughly for 5 min. Then the mixed uncured PDMS was degassed in a vacuum desiccator for 30 min to obtain bubble-free mixtures. The degassed uncured PDMS was then poured on a silicon wafer and placed in the vacuum desiccator for another 30 min to evacuate any remaining bubbles in the uncured PDMS. Next, the vacuum desiccator was sealed to cure the PDMS at room temperature for 24 h. Finally, the cured PDMS was peeled off from the silicon wafer and cut to 3×3 cm2 pieces.


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Poly(dopamine) modification process: After ultrasonic cleaning in ethanol and DI water, PET films, glass slides, PTFE films and PI films were dipped into a dopamine solution (2 mg mL-1 in 50 mM Tris-HCl buffer), triggered by 5 mM copper sulfate and 19.6 mM hydrogen peroxide at room temperature for 5 – 20 min (PDMS films were modified by PDA without any other pretreatment). After rinsing with DI water, the samples were dried under nitrogen flow. The samples were stored at 60 °C for 30 min before use. APTES and MPTES modification process: PET films were first cleaned in ethanol and DI water in an ultrasonic bath. After drying in nitrogen flow, the films were treated in an oxygen plasma cleaner (Tonsontec, Shenzhen, China) to generate hydroxyl group on their surfaces. Thereafter, PET films were immersed into 1% w.t. APTES and MPTES ethanolic solutions for 30 min for the surface modification. After rinsing in ethanol and DI water, the samples were baked at 60 °C for 30 min before use. Photolithography process: PDA-modified substrates (PET, glass, PTFE, PI, and PDMS) were used without further treatment. A 2:1 diluted AZ 1500 (Clariant, Muttenz, Switzerland) photoresist in propylene glycol methyl ether acetate (PGMEA) (J&K Chemical, Shenzhen, China) was spin-coated at 3000 – 4000 rpm for 60 s to form a film thickness of about 945 nm on the substrates. Glass, PTFE, PI, and PDMS substrates were then baked on a hotplate at 95 °C for 60 s, and the PET substrate were baked in an oven at 60 °C for 15 min. Thereafter, the photoresist was exposed using a URE-2000/35 UV mask aligner (Chinese Academy of Sciences, China) with an exposure dose of 20 mJ cm-2 through a photomask. The photoresist was then developed in a 1:4 diluted AZ 351B developer (Clariant, Switzerland) for 60 s. The samples were finally rinsed in DI water and blow-dried under nitrogen flow.


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Thermal nanoimprint lithography process: PDA-modified glass substrates were used without further treatment. An imprint resist (4% w.t. poly(BMA-co-MMA) dissolved in anisole) was first spin-coated onto the substrate to form a film thickness of approximately 85 nm. After baking at 150 °C for 60 s, the substrate was imprinted at 100 °C for 300 s under a pressure of 1.2 MPa using a flexible imprint mold with 500 nm pitch nanodisk arrays. The substrate was released from the mold after cooling down to room temperature to complete the imprint process. Electroless plating of Cu, Ni and Ag: Lithographically patterned substrates were first immersed in 300 nM PdNPs solution at 40 °C for 300 s and rinsed by DI water. Then the samples were rinsed in a 5% w.t. aqueous solution of sodium hydroxide (for photoresist) or ethanol (for imprint resist) for 30 s to remove the resist. Subsequently, the samples were rinsed by DI water and immersed immediately in the Cu electroless plating bath at 40 °C for 5 – 30 min, Ni electroless plating bath at 55 °C for 5 min, or Ag electroless plating bath at room temperature for 30 s to 3 min for the metallization. Afterwards, the samples were rinsed in DI water and dried under nitrogen flow. The samples were heat-treated at 60 °C for 30 min to release the internal stress, leading to a better adhesion and stability of the metals. The Cu electroless plating bath comprised of copper sulfate pentahydrate (12 g L-1), nickel sulfate

heptahydrate

(1.28

g

L-1),

potassium

sodium

tartrate

(11.2

g

L-1),

ethylenediaminetetraacetic acid disodium (15.6 g L-1), sodium hydroxide (14 g L-1), 2,2’bipyridyl (0.4 mg L-1), potassium ferrocyanide (0.6 mg L-1), Dupont Zonyl FSN (0.5 mL L-1), and formaldehyde (15 mL L-1). The Ni electroless plating bath comprised of nickel sulfate heptahydrate (30 g L-1), sodium hypophosphite (28 g L-1), sodium citrate (35 g L-1), ammonium chloride (30 g L-1), Dupont Zonyl FSN (0.5 mL L-1), the pH value of the plating bath was adjusted by ammonia to be approximately 8. The Ag electroless plating bath comprised of silver


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nitrate (2 g L-1), ethylenediaminetetraacetic acid disodium (0.2 g L-1), and formaldehyde (1 mL L-1). Silver nitrate and ethylenediaminetetraacetic acid disodium were first dissolved in DI water, then silver nitrate solution was gently poured into ethylenediamine tetra acetic acid disodium solution while stirring. Afterwards, 25% w.t. ammonia was added into the mixed solution until it became clear again. Then 1 mL L-1 formaldehyde was added into the solution and the volume of the solution was adjusted to be 1 L by DI water. Fabrication of static and dynamic electroluminescent displays: The seven-segment digit patterns were fabricated by photolithography on the photoresist coated on PDA-modified PET and PDMS films. After PdNP adsorption, the patterned samples were immersed in Cu electroless plating bath for 15 min for the metallization. Electroluminescent ZnS:Cu particles (Shanghai KPT, China) were mixed with a polybutene glue with a mass ratio of 1.2:1. Then the mixture was spin-coated onto the patterned substrate at 500 rpm for 15 s and baked in an oven at 60 °C for 10 min to partially solidify the emission layer. Afterwards, a Cu TE was attached to the emission layer with a pressure and the assembled stack was left in an oven at 60 °C for 6 h to complete the fabrication of electroluminescent displays. The electroluminescent displays were driven by a DG2-3-T AC driver (Shanghai KPT, China) connected to a control circuit with a programmed microprocessor. Fabrication of FPCs: Photoresist circuitry patterns were created on the PDA-modified PI and PET films through a photolithography process. Then, patterned samples were activated by PdNPs and then immersed in a Cu electroless plating bath for 15 min for the metallization. Subsequently, LEDs and resistors were welded on the circuits using silver paste. Finally, the fabricated FPCs were connected to an external control circuit with a programmed microprocessor.


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Morphological characterizations: The morphology of the samples was characterized using scanning electron microscopes (LEO-1530 Gemini, Zeiss, Germany; S-4800N and S-3400N, Hitachi, Japan), and an atomic force microscope (Multimode-8, Bruker, USA). EDS analysis was performed in S-3400N and S-4800N scanning electron microscopes. A CM-100 tunneling electron microscope (Philips, Netherlands) was used to investigate the morphology of PdNPs. Performance measurements of TEs: The sheet resistances of the TEs were measured using a four-probe method to eliminate the contact resistance. During the measurement, four probes were placed on two silver paste-covered edges of a square sample, and the resistance was recorded with a Keithley 2400 sourcemeter (Keithley, USA). During the measurement for the repetitive bending or stretching process, the sample was fixed to a home-built moving stage. Optical transmission spectra were recorded using an HR2000+ ultraviolet/visible/near-infrared spectrometer (Ocean Optics, USA). All transmittance values presented in this paper are normalized to the absolute transmittance through the bare PET or PDMS film substrate. The adhesive strength was measured using a home-built 90° peeling force testing platform with a force gauge (Aiyili, China) installed.

ASSOCIATED CONTENT Supporting Information. TEM image of PdNPs, optical transmittance of PET and PDA-modified PET, SEM and EDS images of the electroless plated metallic structures, mechanical characterization of transparent electrodes (PDF) AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT This work was partially supported by the Research Grants Council of Hong Kong (Grant No. 27205515, 17246116, 17237316, 17211115, 17207914 and 717613E), the Innovation and Technology Commission of Hong Kong (Grant No. ITS/297/17), the University of Hong Kong (URC 201511159175, 201511159108, 201411159074 and 201311159187), the Department of Science and Technology of Zhejiang Province (Grant No. 2017C01058). This work was also supported in part by the Zhejiang Provincial, Hangzhou Municipal and Lin’an County Governments.

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Selective Electroless Metallization of Micro- and Nanopatterns via Poly

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