Letter pubs.acs.org/NanoLett
Enhancing the Scratch Resistance by Introducing Chemical Bonding in Highly Stretchable and Transparent Electrodes Chuan Fei Guo,† Yan Chen,‡ Lu Tang,† Feng Wang,† and Zhifeng Ren*,† †
Department of Physics and TcSUH, University of Houston, 3201 Cullen Blvd, Houston, Texas 77204, United States State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing, China 100190
‡
S Supporting Information *
ABSTRACT: Stretchable transparent electrodes are key elements in flexible electronics and e-skins. However, existing stretchable transparent electrodes, including graphene sheets, carbon nanotube, and metal nanowire networks, weakly adheres to the substrate by van der Waals forces. Such electrodes suffer from poor scratch-resistance or poor durability, and this issue has been one of the biggest problems for their applications in industry. Here we show that, by introducing a Au−S bond between a Au nanomesh (AuNM) and the underlying elastomeric substrate, the AuNM strongly adheres to the substrate and can withstand scratches of a pressure of several megapascals. We find that the strong chemical bond, on the other hand, leads to a stiffening effect and localized rupture of the AuNM upon stretching; thus the stretchability is poor. A prestraining process is applied to suppress the localized rupture and has successfully improved the stretchability: electrical resistance of the prestrained AuNM exhibits modest change by one-time stretching to 160%, or repeated stretching to 50% for 25 000 cycles. This conductor is an ideal platform for robust stretchable photoelectronics. The idea of introducing a covalent bond to improve the scratch-resistance may also be applied to other systems including Ag nanowire films, carbon nanotube films, graphene, and so forth. KEYWORDS: Scratch resistance, Au−S bond, stretchability, localized rupture, stretchable transparent electrodes lexible photoelectronics require highly flexible and transparent electrodes (FTEs).1−4 Indium tin oxide (ITO) films have dominated the field of photoelectronics for several decades. However, industry is seeking for alternatives to ITO due to the facts that indium is scarce and ITO films are too brittle to meet the demands for next generation flexible electronics. In the past decade, carbon nanotube networks, graphene sheets, solution-processed metal nanowire (NW) networks, and templated metal nanointerconnects have been intensively investigated as FTEs,5−15 and some devices based on such FTEs have been demonstrated.1−4,6,7 Metal NW networks show a low sheet resistance (Rsh) and a high transmittance (T) comparable or superior to those of commercial ITO films that typically exhibit a Rsh of ∼100 Ohms per square @ T = 90% or Rsh of 10 Ohms per square @ T = 80%.16 However, these novel electrodes suffer from the poor adhesion between the conducting materials and the flexible substrate. Robustness is crucial for an FTE in practical applications.17 A highly robust FTE is expected to withstand scratching or pressing from out-of-plane, as well as in-plane deformations such as cyclic stretching or bending. However, existing FTEs including CNT and metal NW networks are not scratchresistant because of the poor van der Waals interaction between the conductor and the substrate. Especially, solution-processed Ag NW networks are loosely deposited on the substrate so that they are easily to detach. The poor adhesion between the conductor and the substrate may cause delamination or
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© XXXX American Chemical Society
peeling-off of the electrode when scratched or pressed, resulting in poor durability, and such Ag NW electrodes also show a large increase of resistance when stretched to large strains. Several groups have tried to enhance the adhesion by surface modification,18 by applying strong conformal pressure,19,20 by encapsulation using a thin layer of polymer,10 by forming a composite,21,22 or by using spray deposition plus the protection of a poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) polymer layer.23 The conductors can withstand taping (on a stress level of ∼10 KPa) with these improvements.21,23 However, since the interaction between the metal and the substrate is still by the poor van der Waals forces, the adhesion is limited; scratches with sharp tips of hard materials or even repeated finger touch can completely wipe-off the metal. Some applications including artificial skins, pressure sensors, and outdoor wearable electronics are still seeking for solutions to make highly scratch-resistant electrodes. Covalent bond has been known to be a strong interaction that is 2 orders of magnitude higher than van der Waals forces and even stronger than the interaction between metal atoms. Here in this article, we show that, by introducing a strong covalent bond (Au−S bond) to replace the poor van der Waals forces between a Au nanomesh (AuNM) electrode and the underlying elastomeric substrate, we significantly improved the adhesion Received: October 21, 2015 Revised: December 14, 2015
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DOI: 10.1021/acs.nanolett.5b04290 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Formation of Au−S bond. (a) Schematic of the formation of chemical bonds between the AuNM and the underlying PDMS. (b) XPS spectrum of S 2p. The S 2p peaks at 162.3 (the blue solid line) and 163.5 eV (the cyan solid line) are a doublet belonging to Au−S.
Figure 2. Scratch resistance. (a) Scratching demonstration of A-AuNM and S-AuNM. Red arrows indicate the scratching directions. (b) The change of transmittance after repeated scratches with a steel ruler. The transmittance of the S-AuNM remains unchanged after 500 times of scratches, while the A-AuNM becomes fully transparent after 50 times of scratches. (c) Schematic of a scratching demonstration in which the S-AuNM is covered by a solution-processed Ag NW network. (d) Low- and (e) high-magnification SEM images of a S-AuNM with a Ag NW network on top scratched by tweezers, showing that the S-AuNM still adheres on substrate while the Ag NWs are mostly removed. The residual Ag NWs in panel e are seriously deformed (false-colored in aquamarine). (f) A S-AuNM scratched by a micropipette tip. White arrows indicate the scratching directions.
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DOI: 10.1021/acs.nanolett.5b04290 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. Stretchability and mechanism of the rupture. (a) Rs/R0 as a function of tensile strain for S-AuNM, A-AuNM, and the embedded AuNM. (b) Strain fatigue in S-AuNM (cycled at 20% and 50% strains) and A-AuNM (cycled at a 50% strain). (c) SEM image shows localized rupture of SAuNM, which breaks into isolated segments at a strain of 80%. (d) SEM image of delocalized rupture of A-AuNM at a strain of 100%. (e and f) Finite elemental analysis of the stress and the deformation of the ligaments of a perfectly bonded AuNM (e) and a nonbonded AuNM (f) supported on a PDMS substrate. (g and h) Out-of-plane deformation of the perfectly bonded AuNM (g) and the nonbonded AuNM (h) at a strain of 24%.
between the metal conductor and the substrate. The AuNM is applied because it consists of well-interconnected serpentines so that the stretchability is exceptionally large: it has ITO level transmittance and sheet resistance when stretched to a large strain of 160%.13 Such a well-bonded electrode can withstand scratches from plastic tips or even metal tweezers at a pressure of megapascal level. The enhancement of adhesion does not cause a change in transmittance; however, we find that it has a dramatically adverse effect on stretchability and fatigue resistance. The strong adhesion causes a “stiffening effect” and localized rupture upon stretching. Fortunately, the stretchability of the Au nanomesh can be significantly improved by applying a prestrain. Resistance (R) of the AuNM on prestretched and well-bonded substrate increases by only 130% after stretching to 160%, or about four times after stretching to 50% for 25 000 cycles. This work provides a path to robust stretchable photoelectronics.
We improve the adhesion between the AuNM and the polydimethylsiloxane (PDMS) substrate by introducing a strong Au−S bond. The bond energy of the chemical bonds is typically 2 orders of magnitude higher than that of the van der Waals forces, and thus the AuNM is expected to strongly adhere to the substrate with a chemical bond. A rough estimation indicates that the strength of the chemical bond is larger than 10 GPa, much higher than the strength of Au NWs (∼5 GPa),24 indicating that the Au−S bond may not break even if Au ruptures.25 3-mercaptopropyltrimethoxysilane (MPTMS) molecules are selected to form a Au−S bond between the PDMS substrate and the AuNM. The as-cured PDMS consists of fully methylated linear siloxane polymers containing repeating units of the formula (CH3)2SiO, with trimethylsiloxy end-blocking units of the formula (CH3)3SiO−.26 The methyl groups need to be oxidized to provide silicon hydroxyl groups so that the MPTMS molecules C
DOI: 10.1021/acs.nanolett.5b04290 Nano Lett. XXXX, XXX, XXX−XXX
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AuNM breaks locally into isolated segments when stretched to 80%, and this morphology can explain why the stretched SAuNM is not conducting (Figure 3c). By contrast, for AAuNM, although distributed cracks are generated at a 100% strain, the NWs in the stretched A-AuNM are still interconnected, forming a nested self-similar network (Figure 3d), and is still conducting. If we further decrease the adhesion between the AuNM and the PDMS substrate, the size of the cracks increases (Figure S5a−d). The large crack size is not desired to improve stretchability because once cracks are formed the stress concentrates at the tips of the cracks, and the cracks become very long so that resistance increases sharply. Therefore, immoderate decrease of adhesion may not lead to improvement of stretchability (Figure S5e). In an extreme case: if the mesh is free-standing, once a crack forms it propagates throughout the mesh, and the crack breaks the mesh into two halves. This is similar to the case of stretching a piece of paper, in which we can always see only one crack forms, and this crack will go throughout the entire paper and it breaks into two halves. Although we are not able to identify the exact value of the adhesion of the random AuNM and the substrate, our observations illustrates that the adhesion level can significantly affect the stretchability of the Au nanomesh. The strong adhesion, in principle, causes a “stiffening effect” of the AuNM. The ligaments in this case are imposed a homogeneous displacement field: they could not slide or rotate freely to relax stress. The limited in-plane deformation of the ligaments will in return constrain their out-of-plane deformation, which has been proven critical to the stretchability.13 Our finite element analyses (FEA) show that a perfectly bonded AuNM under strain has an even stress distribution and a much higher stress level than that in a nonbonded AuNM (Figure 3e and f). The homogeneous stress in the S-AuNM will cause collective rupture of the Au ligaments and form isolated segments (SEM image in Figure 3c and FEA in Figure 3e). The rupture of the ligaments, however, can only relax the strain very locally (on nanoscale or submicron scale, see area “A” in Figure S6); further stretching will cause new ruptures that are close to the existing ruptures (area “B” in Figure S6). As a result, the SAuNM breaks into small and isolated islands, which is not conducting. We call this failure mode “localized rupture”. For the localized rupture, the topology of the FTE has little influence on the stretchability: a continuous film, a mesh with straight lines, and a mesh with serpentine lines may not be substantially different. The localized rupture is also found in the AuNM embedded in PDMS, in which all the Au ligaments are fixed as well. This is evidenced by the fact that the Rs/R0 curve of the embedded AuNM overlaps with that of the S-AuNW (Figure 3a). The large and homogeneous stress in the ligaments can explain the serious fatigue under strain cycling (Figure 3b) as well. In addition, we verify that for the well-bonded AuNM there is no out-of-plane deformation (Figure 3g). When the mesh is weakly bonded or nonbonded, however, the ligaments can slide and rotate allowing for out-of-plane deformation (Figure 3h). The stress in a weakly bonded mesh is in general nonhomogenous depending on the local geometry (Figure 3f), and thus the mesh breaks delocalizedly. We call this failure mode “delocalized rupture” or “distributed rupture”. In the delocalized rupture, the average distance among the cracks is far larger than the hole size in the AuNM, so that the entire mesh is still interconnected and conducting. Since the local geometry can determine the stress state, the topology will be a key factor
can form a self-assembled monolayer (SAM) on it with a chemical bond (−Si−O−Si− unit). We use a solution of HNO3, H2O2, and water to hydroxylate PDMS (Figure S1). After oxidation, PDMS changes from hydrophobic to hydrophilic (Figure S2). A MPTMS SAM is then assembled on the PDMS. A PDMS substrate without hydroxylation could not form a chemical bond with the SAM of MPTMS (Figure S3). Next, a AuNM, which is made by using grain boundary lithography13,27,28 with a sheet resistance of 20−30 Ohms per square and a transmittance close to 90% (comparable to those of commercial ITO films), is transferred onto the surfacemodified PDMS and kept at 35 °C for 3 d, forming a strong Au−S bond with the MPTMS SAM (Figure S4). Figure 1 shows a schematic for the formation of Au−S bond. In our experiment, the Au−S bonds are covered by 40 nm-thick Au, so that many surface analysis techniques or spectrum analysis techniques that can detect chemical bonds are inapplicable. To verify the Au−S bond we deposited a 1-nm-thick Au film instead of a AuNM on the SAM and detected the Au−S bond by using X-ray photoelectron spectroscopy (XPS) (Figure 1b). We observed an XPS doublet at binding energies of ∼162.3 and 163.5 eV corresponding to the Au−S bond.29 Although we cannot exclude possible residual S−H bond which has an XPS doublet at 163.5 and 165 eV, the result can at least indicate the formation of Au−S bond, and it has been the dominant chemical state of S. The AuNM on as-cured PDMS (A-AuNM) is easily removed with a pair of stainless steel tweezers as the bonding between the metal and the elastomer is poor (Figure 2a, upper panel), while the AuNM on sticky PDMS (S-AuNM, with a chemical bond) can withstand this scratch (Figure 2a, lower panel). In Figure 2b, we show that the transmittance of the chemically bonded AuNM is quite close to that of the AuNM on as-cured PDMS, and it does not significantly change after 500 times of scratches with a steel ruler under an estimated pressure of 2 MPa, while the A-AuNM can be completely removed. Note that the AuNM used here are prestrained in order to achieve a larger stretchability, which will be discussed later. The S-AuNM exhibits a much better scratch-resistance than that of solutionprocessed Ag NW networks. For a S-AuNM covered by a Ag NW network (Figure 2c), the Ag NWs are mostly removed when scratched with tweezers (Figure 2d and e) or a plastic pipet tip (Figure 2f). A few Ag NWs are left but get seriously deformed after the scratch. By contrast, the underlying SAuNM firmly adheres on PDMS and keeps the original morphology. We find that the improved adhesion has an adverse effect on stretchability. We show in Figure 3a that, for the S-AuNM, Rs/ R0 (where Rs is the resistance under stretched state, R0 is the original resistance) increases quickly with the increasing of strain, and the AuNM becomes nonconducting at a strain of ∼70%. By contrast, the A-AuNM remains conducting when stretched to a much larger strain of 200%. In addition, the SAuNM exhibits serious strain fatigue (Figure 3b). Under strain cycling to a 20% strain, Rs/R0 for S-AuNM increases from 1.2 for the first cycle to 3.5 for the 1000th cycle. When cycled to 50%, Rs/R0 increases from 6.5 for the first cycle to 141 for the 50th cycle, and the sample becomes completely nonconducting after only 100 cycles. In comparison, the A-AuNM has a much smaller increase of resistance after 1000 cycles of stretches to 50%. The huge difference in stretchability between S-AuNM and A-AuNM is also reflected in our scanning electron microscopy (SEM) observations. We can see that the SD
DOI: 10.1021/acs.nanolett.5b04290 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 4. Prestraining and improvement of stretchability and fatigue resistance. (a) Schematic of the fabrication of prestretched S-AuNM. (b) SEM image of a prestretched (100% prestrain) S-AuNM. (c) Rs/R0 as a function of tensile strain for S-AuNM with a 100% prestrain, and embedded AuNM with a 100% prestrain. Rs/R0 of nonpre-strained S-AuNM and A-AuNM are also shown as references. (d) Normalized sheet resistance of a SAuNM with a 100% pre-strain as a function of tensile strain. (e) Strain fatigue in prestrained S-AuNM under cyclic stretches to 50% and bends at a bending radius of 1 mm (with a compressive strain of 4%). Here Rb is the resistance at a bent state, and Rr is the resistance after releasing. Inset shows the morphology of prestrained S-AuNM after stretching to 50% for 10 000 cycles. Note that the resistance does not change after being released from a strained state so that corresponding curves overlap.
bending radius of 1 mm for 40 000 cycles. Such modest changes in resistance are acceptable for many photoelectronic applications and for e-skins. We can see that the resistance changes quite slowly after repeated stretching or bending for 10 000 cycles, and the morphology is not dramatically changed. Especially for bending, resistance increases only ∼4.4% from the 10 000th to the 40 000th bends. The continued slow increase of the resistance stems from the accumulation of residual stresses since the strongly bonded mesh cannot relax stress effectively until failure happens. The large stretchability and low fatigue of the prestrained S-AuNM is superior to that of the nonpre-strained S-AuNM or A-AuNM, or some other reported stretchable transparent electrode including Ag nanowire−graphene oxide composite and embedded Ag NW film by the Pei group,34 prestrained carbon nanotube film by the Bao group,2 and metal nanotroughs by the Cui group,14 but inferior to the AuNM on a slippery substrate.35 In summary, by introducing a strong chemical bond between the AuNM and the underlying elastomer, we get highly scratchresistant transparent conductors. We also find that improving adhesion is a double-edged sword: it significantly improves scratch-resistance, but on the other hand it also dramatically decreases the stretchability due to “localized rupture” of the stretchable electrode. The localized rupture stems from the fact that in-plane and out-of-plane deformations of the well-bonded Au NWs are not allowed, leading to sharp stress concentrations. This contradiction between scratch-resistance and stretchability can be solved by using a prestraining method. The prestrained S-AuNM exhibits combined features of high scratch-resistance, large stretchability, and low fatigue. The improvement of scratch resistance and stretchability by introducing a covalent
for the stretchability of an FTE that is weakly bonded on the substrate. The stretchability of the scratch-resistant AuNM can be significantly improved by applying a prestrain. We need to point out that the prestretching of a AuNM is quite different from that of a continuous film or a network of Ag NWs or CNTs which generates wrinkles:30−33 it causes an in-plane collapse of the holes, without generating periodic wrinkles or folds in the AuNM so that the transparency does not dramatically decrease, and this will be discussed in detail elsewhere. Figure 4a shows the preparation of prestrained SAuNM, and Figure 4b is a corresponding SEM image, showing that the S-AuNM is compressed in-plane but has no wrinkles. Prestraining has been proven to be effective to improve the stretchability of some conductors including carbon nanotubes and Ag NW networks.30,31 Here we show that the prestretched S-AuNM exhibits improved stretchability: for a S-AuNM or an embedded AuNM with a prestrain of 100%, Rs/R0 only increases by ∼1.3 times when stretched to a large strain of 160% (Figure 4c). For thin film electrodes, sheet resistance is a key parameter that determines the overall performance. We can see from Figure 4d that the sheet resistance of the prestrained S-AuNM decreases until stretched to larger than 120%, and the normalized sheet resistance is even less than 0.5 in the strain range of 70% to 140%. The decrease of sheet resistance upon stretching lies in the relatively stable resistance but dramatic change in geometry (elongation). The prestrained S-AuNM also performs well under cyclic stretches or bends. In Figure 4e we show that resistance increases about four times after stretching to a 50% strain for 25 000 cycles, or increases by 50% after cyclic bending at a E
DOI: 10.1021/acs.nanolett.5b04290 Nano Lett. XXXX, XXX, XXX−XXX
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bond and a prestrain can also be applied to other metal FTEs such as Ag or Cu grids or nanowire networks because other metals also form a strong chemical bond with thiol.36 Other methods that form strong covalent bonds (not only metal−S bond) between the conducting layer and the substrate may also be applied to improve the scratch resistance. Carbon nanotube films and graphene may also find a surface treatment to bridge a strong bonding with the substrate. This work provides a new path toward the commercialization of robust stretchable electronics. Experimental Section. Fabrication and Characterization. The Au nanomesh was fabricated by using grain boundary lithography.13 For introducing a Au−S bond between the Au nanomesh and the underlying PDMS, as-made PDMS substrate was first immersed in blended solution of nitric acid (70%), hydrogen peroxide (35%), and deionized water with a volumetric ratio of 1:1:2 for 30 min, forming an atomic oxide layer. A self-assembled monolayer of MPTMS molecules was assembled on the oxidized PDMS substrate,37,38 and a AuNM was then transferred on the surface-treated PDMS and strongly bonded on the substrate via a Au−S bond. The Au−S bond was detected by using X-ray photoelectron spectroscopy (Physical Electronics, PHI 5700). Morphology of the electrodes was taken by using a LEO 1525 scanning electron microscope. Transmittance was recorded with an Agilent Cary 4/5/6000i spectrometer. Finite Elemental Analysis. The stress and deformation analysis of the AuNM was conducted by using the finite element program ABAQUS, and the model of AuNM was created based on an SEM image of a real AuNM. The mesh of the AuNM comprises of a total of 34646 linear hexahedral elements of type C3D8R and 62565 nodes. For the perfect adhesion case, the rotation of the AuNM was completely restricted, and a distributed tensile velocity which is gradually increasing along the x-direction was applied on the AuNM. For the zero adhesion case, the AuNM was set to be free and the tensile velocity load was imposed on the left and the right boundaries. An elasto-plastic constitutive relationship was introduced for Au according to ref 24., and the values of the material parameters involved in the analysis are as follows: Young’s modulus 78 GPa, Poisson’s ratio 0.42, density 19320 kg·m−3, yield stress 4.8 GPa.
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Funding for this work was provided by the US Department of Energy under Contract Number DOE DE-SC0010831/DEFG02-13ER46917 and National Natural Science Foundation of China under Grant No. 11202221 and 11572324. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work performed at University of Houston was funded by the US Department of Energy under Contract Number DOE DE-SC0010831/DE-FG02-13ER46917. The work performed at Institute of Mechanics, Chinese Academy of Sciences was funded by the National Natural Science Foundation of China under Grant No. 11202221 and 11572324.
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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/acs.nanolett.5b04290. Hydroxylation of PDMS and the chemical reactions in the formation of the Au−S bond, experimental demonstration of the strong covalent bonding, the effect of adhesion on stretchability, and simulations of the delocalized rupture (PDF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
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Author Contributions
C. F. Guo and Z. Ren conceived the idea. C. F. Guo conducted experiments, analyzed data, and wrote the paper. Y. Chen did mechanical simulations. L. Tang did XPS. All authors discussed the results. Z. Ren directed the whole work. F
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DOI: 10.1021/acs.nanolett.5b04290 Nano Lett. XXXX, XXX, XXX−XXX
