Fabrication of Embedded Silver Nanowires on ... - ACS Publications

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Fabrication of Embedded Silver Nanowires on Arbitrary Substrates with Enhanced Stability via Chemisorbed Alkanethiolate Gui-Shi Liu,† Jing-Shen Qiu,† Duo-Hua Xu,† Xianzhong Zhou,‡ Dingyong Zhong,‡ Han-Ping D. Shieh,§ and Bo-Ru Yang*,† †

School of Electronics and Information Technology and ‡School of Physics, Sun Yat-Sen University, Guangzhou 510006, P. R. China Department of Photonics and Display Institute, National Chiao Tung University, Taiwan 300, Republic of China

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S Supporting Information *

ABSTRACT: We propose a versatile yet practical transferring technique to fabricate a high performance and extremely stable silver nanowire (AgNW) transparent electrode on arbitrary substrates. Hydroxylated poly(ethylene glycol) terephthalate (PET) or poly(dimethylsiloxane) (PDMS) deposited with AgNWs was selectively decorated to lower its polar surface energy, so that the AgNWs were easily and efficiently transferred into an epoxy resin (EPR) as a freestanding film (AgNWs− EPR) or onto various substrates. The AgNWs−EPR capped with alkanethiolate monolayers exhibits high conductivity, low roughness, ultraflexibility, and strong corrosion resistance. Using the transferring process, AgNWs−EPR was successfully constructed on rough, adhesive, flimsy, or complex curved substrates, including PET, thin optically clear adhesive, papers, a beaker, convex spherical PDMS, and leaves. A flexible touch panel enabling multitouch and a curved transparent heater on a beaker were first fabricated by using the composite film. These demonstrations suggest that the proposed technique for AgNWs is a promising strategy toward the next generation of flexible/portable/wearable electronics. KEYWORDS: embedded silver nanowire, transfer, curved substrates, alkanethiolate, corrosion resistance

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INTRODUCTION Most of today’s optoelectronic devices, such as touch panels (TPs),1 liquid crystal displays,2 solar cells,3 and light emitting diodes,4 require at least one electrode that must be highly conductive and transparent. The predominant material of this electrode is indium tin oxide (ITO), which has excellent optoelectronic performance but suffers from the critical drawbacks of brittleness and scarcity of indium. Currently, AgNW-based transparent conductive electrodes (TCEs) have been put forward as an appealing alternative to ITO due to their good flexibility, high figure of merit (FoM), and compatibility with large-scale manufacturing, such as the rollto-roll (R2R) process.5,6 Solution-processed AgNWs provide various advantages, such as process simplicity and high-throughput; however, they are also accompanied by several issues, including (i) high contact resistance between silver nanowires,7 (ii) poor adhesion to substrates,8,9 (iii) a rough surface,10,11 and (iv) especially poor corrosion resistance.12 Researchers have proposed various methods to address issues4,8,11,13−24 such as mechanical pressing15,20,21 and composite structures.14,19 Embedding AgNWs into the polymer via a transferring process is a promising strategy to simultaneously overcome these issues.3,10,25−27 The matrix polymer could fill the voids of a © XXXX American Chemical Society

AgNW network to offer a smooth surface, tighten NW-to-NW junctions, and enhance adhesion simultaneously.25,27 The embedding structure also reduces the exposed area of AgNWs and improves their chemical stability.25−27 However, the traditional transferring approach, which generally uses rigid silicon or glass as the donor substrate, is difficult to apply in large-scale manufacturing (e.g., the R2R process) and not suitable for curved substrates. Moreover, the fabricated AgNWs are still vulnerable under long-term exposure of oxygen or sulfide compounds due to incomplete coverage of the matrix polymer.25,27 It is a great challenge to propose a scalable coating method that can not only fabricate AgNW TCEs on both planar and curved substrates but also simultaneously tackle the issues, as mentioned above. Herein, we report a transferring technique that can fabricate a AgNWs−EPR composite as a freestanding film in a scalable way or on arbitrary substrates with high performance and excellent stability. A modified flexible PET or soft and shapeable PDMS was used as the donor substrate in this technique, making it compatible with the R2R process and Received: February 19, 2017 Accepted: April 13, 2017 Published: April 13, 2017 A

DOI: 10.1021/acsami.7b02458 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Transferring process proposed for AgNWs. (a, g) As-coated loose AgNWs on the silica/PET or PDMS. (b, h) An antiadhesive SAM was dip-coated on the silica coated with loose AgNWs. (c) A UV adhesive was coated on the AgNWs/silica. (d, e) Delamination of the AgNWs−EPR and the donor substrate. (f, i) Schematic of the transferring process for curved substrates. (j) AgNWs−EPR fabricated on a plastic box (corner radius: 7 mm) and a leaf. The scale bar is 5 mm. (e, g) Scanning electron microscopy (SEM) images of the pristine and embedded AgNWs.

Figure 2. (a) Deposition process of a silica-like layer on PET or PDMS by dipping into an acetone solution with 12.8 wt % (3aminopropyl)trimethoxysilane (APTMS). (b) AgNWs were coated on the hydroxylated silica/PET or PDMS. (c) Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) spectra of PET, APTMS-treated PET (APTMS/PET), and UV/O3-treated APTMS/PET. (d) Schematic of the AgNWs−silica composite surface decorated with hexamethyldisilazane (HMDS) and trichloro(octadecyl)silane (OTS).

contact lens-like PDMS, beaker, and so forth. Finally, a projected capacitive TP and a curved heater on a beaker based on AgNWs−EPR were demonstrated.

suitable for curved substrates. By a selective decoration and the capillarity of the UV adhesive, the AgNWs on the decorated donor substrate can be easily and efficiently transferred onto easily fractured or various curved substrates. Besides the wrapping of the EPR, the AgNWs were also protected by alkanethiolate SAMs to offer long-term stability without sacrificing conductivity. By using the transferring technique, we have fabricated freestanding AgNWs−EPR films of high performance, and demonstrated their fabrication on flimsy, rough, or curved substrates, including thin optically clear adhesive (OCA, thickness: 25 μm), rough papers, curved leaf,

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RESULTS AND DISCUSSION

The overall transferring process, as shown in Figures 1 and 2a, mainly consists of three steps: (i) solution-depositing a silicalike layer on PET or oxidized PDMS (silica/PET, silica/ PDMS); (ii) coating AgNWs on the substrates and grafting antiadhesive functional groups on the silica surface; (iii) B


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Figure 3. (a) Surface energy γ of the as-prepared silica/PET, HMDS-, and OTS-treated silica vs the treating time of HMDS and OTS. Insets are the contact angles of the EPR adhesive on the silica/PET. (b) The polar component γP and the dispersive component γD of γ (γ = γP + γD according to Fowkes theory34) of the silica/PET, HMDS-treated silica/PET for 15 min, and OTS-treated silica/PET for 10 s, and the corresponding normalized sheet resistance RS of the transferred AgNWs. (c, e, g) OM images of these silica substrates after the transfer. (d, f, h) OM and atomic force microscopy (AFM) images of the corresponding AgNWs−EPR films, the blue curves in the AFM images are the contour profiles along the green lines.

embedding the AgNWs into an epoxy resin (AgNWs−EPR). PET or PDMS instead of glass or silicon were used as the donor substrate, offering several advantages: (a) the flexible PET makes the transferring method compatible with the R2R process to fabricate the AgNWs−EPR film; (b) the flexible PET or soft and shapeable PDMS with AgNWs can be effectively attached to the curved substrates, which enables the fabrication of the AgNWs onto nonplanar surfaces (e.g., cylindrical surfaces), as shown in Figure 1f,j. In particular, the thin and soft PDMS film could conformably adhere to irregular surfaces via the capillarity of the EPR liquid (Figure 1i), so that the AgNWs−EPR could be fabricated on substrates with a complex curved surface, such as a leaf (Figure 1j). In addition, PDMS could be casted into different shapes with molds, enabling the AgNWs−EPR to be shaped into a contact lens-like film or transferred onto a convex spherical substrate (see below and Figure S1, Supporting Information). Steps (i) and (ii) play a crucial role in the transferring process. As the EPR adhesive shows strong adhesion to polar polymers (PET, oxidized PDMS), a silica-like layer with reactive −OH groups was deposited on the PET or oxidized PDMS to graft an antiadhesive SAM (Figure 1b,h) to easily exfoliate the donor substrates and efficiently transfer the AgNWs into the EPR. The embedding step includes mechanically pressing (Figure S2) or heating the AgNWs, blade-coating the EPR adhesive, UV curing, and a peeling off process. To improve corrosion resistance without sacrificing the FoM, the AgNWs were

deposited with an octadecanethiol (ODT) SAM before or after the embedding step (iii). Modification of PET and PDMS. Pristine PET does not have reactive groups, such as −OH, that can be used to graft hydrophobic chains,28 so the PET was modified using liquid deposition combined with UV/O3 treatment, as indicated in Figure 2a. As for hydrophobic PDMS, a highly hydroxylated surface can be directly achieved by using plasma or long-term UV/O3 treatment to uniformly coat AgNWs and graft hydrophobic chains on the PDMS, but the treatments lead to a wrinkled surface (Figure S3a).29 So PDMS was briefly treated with UV/O3 and subjected to liquid deposition to obtain a planar hydroxylated surface (Figure S3b). The liquid was a mixture of acetone and APTMS that had aged for 10 days (Figure S4). During the aging period, the carbonyl bond in the acetone reacts with the amine group in the APTMS to generate dimethyliminopropyltrimethoxysilane (DIPTMS) and water, which subsequently facilitates the hydrolysis of methoxy groups in the DIPTMS to produce silanol groups.30,31 The organosilicone layer was dip-coated on the PET or PDMS. For example, the ATR-FTIR spectrum of the modified PET (Figure 2c) indicates that the silanol groups are condensed into the siloxane Si−O−Si (at 1034 and 1145 cm−1)32 after heat treating at 120 °C for 3 min. To produce hydroxyl groups on the surface, the APTMS-treated PET (APTMS/PET) or PDMS was further irradiated by UV/O3. As shown in the spectrum of UV/O3-treated APTMS/PET, the signals corresponding to −CH3 (1382, 2870, 2960 cm−1), −CH2 (1487, C


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Figure 4. (a) RS−T performance of the AgNWs−EPR composite film and previously reported AgNW-based TCEs: AgNWs@iongel,42 AgNW/ CaAlg,8 patterned AgNWs,41 AZO/AgNW/AZO/ZnO,19 AgNWs/graphene,14 AgNWs/PEDOT:PSS,9 and Ag network.1 (b) Sheet resistance variation (RS/RS0) of AgNWs−EPR and AgNWs/PET during the adhesive tape test. (c) Resistance variations (R/R0) during the in situ bending test for the AgNWs−EPR and commercial ITO/PET films. (d) Resistance variations (R/R0) of AgNWs−EPR and AgNWs/PET as a function of the bending cycles. The radius of tensile bending is ∼1 mm. (e) RS before and after crumpling AgNWs−EPR. The thickness of the AgNWs−EPR in (c− e) is 15−20 μm.

2852, 2930 cm−1), and −NH2 (∼3300 cm−1) significantly decreased or completely disappeared, meanwhile the absorption bands due to Si−OH (943 cm−1) and Si−O−Si (1050 and 1162 cm−1) were observed.33 The two observations suggest that the residual APTMS and the alkyl chains were removed to form silanol groups by UV/O3 irradiation, and a layer of silicalike material was formed on the PET (Figure S5 and Table S1), the thickness of the silica layer is ∼200 nm (Figure S6). Transfer of the AgNWs into the EPR. The conductivity of the transferred AgNWs significantly varies with the different conditions of the transfer process. An efficient transfer requires well-defined adhesion of the silica to the EPR and AgNWs. The adhesion of the silica can be characterized using the Yang− Dupré equation35 WLS = γL(1 + cos θ )

AgNWs can still be decorated with HMDS and OTS, which replaces the hydrophilic −OH groups with hydrophobic −CH3 terminal groups on the surface of the silica, as shown in Figure 2d.36,37 The treated silica becomes hydrophobic (Figure S8) and is partially wetted by the EPR adhesive (insets of Figure 3a). Because of the high surface energy of silver (>1000 mJ m−2),38 the adhesive of EPR can still be completely wetted by the AgNWs−silica composite surface. The Yang−Dupré equation is not suitable for characterizing the adhesion for this case and the cured EPR. The work of adhesion WA between the cured EPR and the silica can be obtained by combining the Yang−Dupré equation and the Owens−Wendt model39 WA = 2 γSDγED + 2 γSPγEP

(1)

(2)

where the subscripts E and S correspond to the AgNWs−EPR and the silica after the transfer process, and the superscripts D and P refer to the dispersive and polar components of the surface energy γ, respectively. As shown in Figure 3b, the polar component γPS of silica significantly decreased after the HMDS and OTS treatments, which can be attributed to the decrease in the number of polar −OH groups40 that were replaced with trimethylsilyl groups or alkanes. The WA values between the HMDS- and OTS-treated silica and AgNWs−EPR are, thus, reduced from 69.6 to 56 and 48.1 mJ m−2, respectively. The adhesion of the HMDS-treated silica is still too large to leave residual NWs or segments of NWs on the silica in a striped pattern (Figure 3e), which increases sheet resistance RS by 40% after the transfer. The AFM image in Figure 3f also demonstrates that some AgNWs or segments were detached

where WLS is the work of adhesion, θ is the contact angle of the liquid on the silica, and γL is the surface tension of the liquid. The EPR adhesive was well wetted by the hydroxylated silica with a contact angle 90% only cover less than 21% of the substrate area (Figure S7), and many of the AgNWs are loosely deposited on the substrate (Figure 1g). Therefore, the hydrophilic silica coated with D


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thickness were easily exfoliated from the silica/PET due to low peeling strain (Figures 4e and S9). The embedded structure also endowed the composite film with an exceptionally smooth surface (Figure 3h). The root-mean-square roughness was significantly reduced from 25 (AgNWs on PET, Figure S12) to 3 nm, making the AgNWs−EPR suitable for use in thin-film devices. Stability Improvement. The chemical stability of AgNWs must be addressed for practical applications. Environmental oxygen, humidity, and sulfide have been reported to significantly degrade the conductivity of AgNWs.12 All previous research enhanced AgNWs’ reliability (mainly in terms of thermal oxidation stability) using a physical barrier, such as burying them into poly(vinyl alcohol)25 or covering them with graphene oxide,4 which more or less sacrifices the film performance. In contrast, we deposited a monolayer of methyl-terminated alkanethiolate on the AgNWs via stable Ag−S bonding (Figure S13).48 The ODT decoration achieved by dip-coating has little effect on the conductivity of the AgNWs, as shown in Figure 5a, because the ODT SAM is only

from the EPR during the transfer step. In contrast, almost no AgNWs were found on the OTS-treated silica, and RS only increased by 10% (Figure 3b,g,h). It should be noted that OTS decoration is more suitable, also, because of the liquid deposition and fast reaction rate (only requiring 10 s, Figure 3a). Besides the efficient transfer of AgNWs, the OTS decoration also facilitates the delamination of the EPR film and the treated silica/PET or PDMS, so a thin AgNWs−EPR film can be easily exfoliated from the PET (e.g., 15 μm, Figures 4e and S9) or transferred onto flimsy substrates without any fractures (see below). Performance of the AgNWs−EPR. The efficient transfer facilitates high performance of the AgNWs−EPR composite film. The film exhibits an RS of 29.7 Ω sq−1 with 96.2% transmittance, which is better than the reported TCEs, including Ag mesh,1 ordered AgNWs,41 and AgNW-based composite films,8,9,14,41,42 as indicated in Figure 4a. To judge the performance of the AgNW TCE, the trade-off relationship between RS and T can be quantified by −2 ⎡ 188.5 σOp ⎤ ⎥ T = ⎢1 + R S σDC,B ⎥⎦ ⎣⎢

(3)

where σOp is the optical conductivity, and σDC,B is the bulk DC conductivity of the AgNWs.43 The ratio of the DC to optical conductivity has been frequently used as the FoM. A high value of FoM is desirable to achieve low RS and high T, but this value decreases as the transmittance increases. The fitting results give high FoMs of 300−450 at high transmittance (92−96%). The values are among the highest reported values in TCEs with transmittance >90%. Besides the high efficient transfer, the good performance can be also attributed to the good welding of NW junctions by using roller compression and the slight alignment of the AgNWs, which is achieved using Meyer rodcoating (Figures S9 and S10).13 The roller compression reduced RS by more than 1 order of magnitude (Figure S2) and resulted in most transferred AgNWs embedding on the outer surface of the EPR but not being fully buried (Figure 1e), and the aligned AgNWs have been reported to possess much higher performance than those of random networks.44 The RS values of the aligned AgNWs show little variation in different measurement directions (i.e., parallel and perpendicular to the AgNW alignment) when the average RS is below 30 Ω sq−1. The RS variation in different measurement directions becomes apparent (e.g., >10%) when the average RS increases beyond 100 Ω sq−1 (Figure S11). It should be noted that greater FoMs have also been obtained using copper (Cu) nanotroughs45 and a AgNWs−CuMWs composite.46 However, the easy oxidation of copper might impede their application. The AgNWs−EPR composite with a high FoM also had excellent electro-mechanical stability (adhesion and flexibility). As shown in Figure 4b, RS only increased by 23% after 100 adhesive tests, which is attributed to the embedded structure and large dispersive surface energy of the EPR, promoting strong adhesion to Ag.47 In addition, the AgNWs−EPR film remains highly conductive with little change (ΔR/R0 ≈ 13.4%) after 3000 bending cycles with a bending radius of ∼1 mm, and its conductivity only slightly degraded when bended to a bending radius of 0.5 mm (ΔR/R0 ≈ 11.8%) or even after crumpling (Figure 4c−e, also see Video S1). The remarkable flexibility is ascribed to the strong bonding strength and thinness of the matrix EPR, which leads to low tensile strain under bending. The AgNWs−EPR composites with 15 μm

Figure 5. (a) Sheet resistance of the AgNWs−EPR vs immersion time of 50 mmol/mL ODT solution. (b) Transmittance spectra of AgNWs−EPR before and after ODT treatment for different times.

deposited on the surface of the AgNWs and not present at the NW-to-NW junctions. In addition, as for AgNWs−EPR, ODT is preferentially deposited on the AgNWs during the dipcoating, because the EPR film is hydrophobic with a water contact angle of 98°, whereas the AgNWs are superhydrophilic. So the transmittance of the AgNWs is negligibly affected (Figure 5b). The antioxidant ability of the ODT-decorated AgNWs (ODT-AgNWs) was investigated using an accelerated aging test at 85 °C and a relative humidity (RH) of 85% for a month. E


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Figure 6. (a) SEM images and (b) Ag 3d XPS spectra of the pristine AgNWs and ODT-AgNWs on silicon after high temperature and humidity testing (85 °C, 85 RH %) for a month. (c) Normalized RS change of AgNWs on glass, AgNWs−EPR, and ODT-AgNWs−EPR during the accelerated aging test (85 °C, 85 RH %). Insets are the schematic of the protection mechanism. (d) Normalized R change during a corrosion test using a 5 wt % Na2S solution.

stability under atmospheric conditions, which have very low concentrations of sulfide compounds (Figure S14). Transferring AgNWs onto Various Substrates. The proposed transferring technique is suitable for various substrates. Firstly, the UV adhesive of EPR has a strong bonding strength with various substrates, including metal, glass, polar polymers, and paper. Secondly, the antiadhesive decoration of the donor silica layer significantly reduced the strains introduced during the peeling off step. Low adhesion could avoid the fractures of some flimsy target substrates, the donor silica, or PDMS. Therefore, AgNWs−EPR was successfully transferred onto thin PDMS (100−200 μm thickness), pan paper and typing paper, OCA with a thickness of 25 μm, and leaves, as shown in Figures 7a,d,e and S15. AgNWs−EPR integrated with OCA makes it a transferable conductive transparent film, so the AgNW film could be constructed on a glass rod with a diameter as low as 5 mm, as shown in Figure 7c. Lastly, the flexible PET or soft and shapeable PDMS, as a donor substrate, enables AgNWs to be transferred onto curved substrates, including cylinders, spheres, and free-form surfaces, as indicated in Figures 1f,j and 7d−f. The EPR liquid coated on the donor substrate formed a capillary bridge to the target substrate, the attractive capillary force induced by the curved liquid surface allowed the tight attachment between the two substrates (Figure 1i). Especially, the conformal attachment between the thin PDMS and a complex curved surface was easily achieved due to the high softness of PDMS of 100−180 μm thickness. AgNWs−EPR was thus successfully fabricated on the leaves and could replicate the structures of the leaf veins (Figures 1j and 7d). In addition, by using a concave PDMS mold as the donor substrate, the AgNWs spun-over the mold were transferred

Figure 6a shows that the pristine AgNWs break and migrate into clumpy shapes, whereas the morphologies of the ODTAgNWs were almost unchanged. The presence of two components in the Ag 3d XPS signals (near 368.4, 374.4 eV) demonstrates that the pristine AgNWs were severely oxidized after the one-month accelerated aging test (85 °C, 85 RH%), but no signal from Ag2O was found in the Ag 3d peaks of the ODT-AgNWs (Figure 6b). As shown in Figure 6c, the normalized change in RS of the AgNWs/glass increased by 169% after only a 5 day test. AgNWs−EPR, which had a smaller exposed area, enhanced the thermal oxidation stability, but RS still increased more than two fold. For ODT-AgNWs−EPR, the SAM with long hydrophobic chains and strong Ag−S bonds protected the exposed AgNW surface from oxygen (inset of Figure 6c), and the RS only increased 18% after a long-term thermal oxidation test. Actually, the conductivity of ODTAgNWs−EPR remained unchanged under atmospheric conditions for 2 months (Figure S14). The alkanethiolate layer also enhanced the resistance of the AgNWs against chemical attacks. ODT-AgNWs−EPR can maintain a high conductivity after chemical attacks using 5% Na2S solution for 120 s, whereas the conductivity of the pristine AgNWs and the embedded AgNWs suddenly broke down at 14 and 25 s, respectively, as shown in Figure 6d. Relatively poor resistance of ODT-AgNWs−EPR against the sulfide solution may arise from some defects of the ODT SAM, for the decoration time of ODT (30 min) may be too short to form a compact SAM.48 Sulfide attacks do not tend to form uniform thin films of Ag2S but a number of Ag2S nanoparticles on the AgNW surface or disconnections of the AgNW,12,26,49 so the AgNWs decorated with the ODT SAM can still be corroded severely at some tiny sites by sulfide. Nevertheless, ODT-AgNWs−EPR still has excellent long-term F


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Figure 7. (a) AgNWs−EPR transferred onto oxidized PDMS (thickness: ∼100 μm), OCA (thickness: 25 μm) with a release film, pan paper, and PET. (b) The projected capacitive TP with a 2 inch circular structure, the TP consists of a patterned ODT-AgNWs−EPR/PET film (70 × 70 mm2), an anisotropic conductive film, and a flexible printed circuit containing a driven IC. Multitouch and writing “SYSU” were demonstrated on the TP. (c) AgNWs−EPR transferred on OCA film was adhered to a glass rod. (d) Fabrication of AgNWs−EPR on a leaf by peeling off the silica/PDMS. (e) AgNWs−EPR fabricated on a convex spherical PDMS film and a convex AgNWs−EPR film without supporting film, both radii are ∼14 mm. LEDs were lit up by using AgNWs−EPR as an electrode. (f) A curved heater on a beaker with or without water and corresponding infrared images. The scale bars in (d, e, f) are 10, 7, and 15 mm, respectively.

onto a convex oxidized PDMS film with a radius of ∼14 mm, or AgNWs−EPR was directly casted into a contact lens-like film. The conductivity of the transferred AgNWs−EPR was demonstrated by lighting up LEDs (Figures 7 and S15). Device Applications. To verify the applicability of the transferred AgNWs−EPR composite, the films were used to construct a flexible projected capacitive TP and a curved heater on a glass beaker. Because of the good mechanical robustness of the film, the TP only consisted of one layer with patterned AgNWs, which was obtained using laser ablation. Multitouch using fingers and the letters “SYSU” were demonstrated on the prototype, as shown in Figure 7b (also see Video S2). The TP verified that the large-area AgNWs (70 × 70 mm2) could be efficiently transferred onto target substrates by the proposed process. A curved heater of 22 × 22 mm2 was also directly fabricated on a beaker by using the transferring technique. The temperature of the beaker near the heater was increased up to ∼70 °C at 7 V. To further demonstrate the curved heater, 60 mL of water with an initial temperature of 19 °C was added into the beaker. The water near the heater was heated up to ∼30 °C in 10 min at 7 V.

stability. It should be noted that the alkanethiolates chemisorbed onto the AgNW surface together with the wrapping EPR offers long-term environmental stability without sacrificing conductivity or transmittance. Furthermore, by using the transfer technique, the AgNW composite films have been successfully transferred onto various substrates, such as papers, a beaker, convex PDMS, and leaves. Finally, a flexible projected capacitive TP and a heater fabricated on a beaker were demonstrated using the AgNWs−EPR composite film. This transferring method paves a way for forming deformable devices onto various surfaces, making things digitalized for future internet of things environments.

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EXPERIMENTAL SECTION

PET and PDMS Treatment. APTMS (Sigma-Aldrich) was mixed with anhydrous acetone (12 wt %), and the mixture was aged for 10 days. PDMS was treated using UV/O3 for 5−15 min. PET and PDMS were immersed in the mixture for 5 min and 30 s, respectively, and lifted at a rate of 2 mm/s (Dip coater, SYDC-100). The treated PET was heated on a hotplate at 120 °C for 3 min. Then the APTMStreated PET and PDMS were treated with UV/O3 (18 mW cm−2, SEN, PL17-110) for 5 min to remove the organic groups. AgNW Film Preparation. AgNWs dispersed in ethanol were acquired from KECHUANG Inc. The average diameter and length of the AgNWs were 43.5 nm and 21.2 μm, respectively (Figure S10). The solution was diluted to the desired concentration with deionized water, the weight ratio of the AgNWs was in the range of 0.1−0.75%. The surfactant Triton X-100 (Sigma-Aldrich) was added to the diluted

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CONCLUSIONS In summary, we have demonstrated a facile and practical transferring process to fabricate an ODT-AgNWs−EPR composite film, which exhibits an overall enhancement in conductivity, smoothness, mechanical robustness, and chemical G



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solution at a weight ratio of 0.0125% to reduce the agglomeration of the AgNWs. The AgNW dispersion was Meyer rod-coated on the silica/PET, or spun-over the silica/PDMS with a coverage area of 7.5− 37.8% (Figure S7). Hydrophobic Treatment of the Silica. The hydroxylated silica coated with AgNWs (AgNWs/silica) was treated with HMDS or OTS. HMDS (Sigma-Aldrich) was vaporized at 80 °C in a vacuum oven containing the AgNWs/silica samples. OTS (Sigma-Aldrich) was dissolved in n-hexane at 0.2 wt %. The antiadhesive SAM was formed by dipping the AgNWs/silica into the OTS solution for ≤15 s. The samples were then rinsed with pure n-hexane and dried on a hotplate. Transfer Process. The freestanding AgNWs−EPR was fabricated using the silica/PET as a donor substrate. The prepared AgNWs/ silica/PET was mechanically pressed by two rollers (ACSL-M25E, Aokai Co.). The speed, temperature, and pressure of the rollers was 0.15 mm/s, 55 °C, and 7 kg/cm2, respectively. To prevent the transfer of AgNWs onto the upper roller, there is a release film between the roller and AgNWs. Then, a UV adhesive of the modified EPR (DELOKATIOBOND AD VE 110543) was blade-coated (ERICHSEM Model 510 MC) on the pressed AgNWs/silica with a thickness of 15−80 μm. The sample was cured under UV irradiation (15 mW cm−2) for 5 min. The cured EPR film with embedded AgNWs was peeled off from the silica/PET. As for the curved surface, thin PDMS (thickness: 100−180 μm) was preferred to PET as a donor substrate. The AgNWs/silica/PDMS was firstly heated on a 70 °C hotplate for 20 min. Then, the AgNWs were coated with the EPR adhesive and laminated with the target surface. After the curing and peeling off step, AgNWs−EPR was transferred onto curved substrates. The transferring process of the AgNWs onto a contact lens-like surface is described in the Supporting Information in detail (Figure S1). ODT Decoration. ODT (Powder, Aladdin Co.) was dissolved in ethanol (50 mmol dm−3) and agitated on a magnetic stirrer for 3 h. Alkanethiolate−SAM was prepared by dipping the AgNWs into ODT solution for 10−30 min. The sample was then rinsed with pure ethanol and dried on a hotplate. Characterization. ATR-FTIR (Thermo Scientific Nicolet 6700Continuum) was performed to characterize the surface modification. Multidip coatings were conducted to obtain a thicker film on PET for the ATR-FTIR measurements. To obtain the surface energy of various substrates, the contact angles of the polar (water) and dispersive (diiodomethane) liquid was determined by a contact angle analyzer (Dataphysics OCA 15EC). The surface energy was calculated by the Owens−Wendt−Rabel−Kaelble model.50 The morphologies of the AgNWs after the accelerated aging test (85 °C, 85 RH %, a month, Yashilin GDJS-225) were observed using SEM (Carl Zeiss SUPRA 60). Silicon was used as the substrate to avoid electron charging. Scotch tape (3M, width: 12.7 mm) was used to characterize the adhesion of the AgNWs to substrates. The bending test was conducted by using a custom-made motorized stage. The resistance change during the bending test was monitored by a Keithley 2400 Sourcemeter.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gui-Shi Liu: 0000-0003-2507-4092 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program of China (863 Program: 2015AA033408), the National Natural Science Foundation of China (61307027), the Science and Technology Program (2014B090914001, 2015B090915003), and the Economic and Information Industry Commission of Guangdong Province, P. R. China (20140401).

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REFERENCES

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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.7b02458. Fabrication of the projected capacitive TP and the contact lens-like AgNWs−EPR films, supplementary figures (including schematic, photo, AFM and SEM images, XPS spectra, and sheet resistance variation under different experiments), table summarizing surface composition of various modified PET from XPS analysis (PDF) Two supplementary videos of the bending test and the flexible TP (AVI) (AVI) H


Research Article

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Fabrication of Embedded Silver Nanowires on ... - ACS Publications

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