Heterogeneous Surface Orientation of Solution-Deposited Gold Films

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Heterogeneous Surface Orientation of Solution-Deposited Gold Films Enables Retention of Conductivity with High Strain – A New Strategy for Stretchable Electronics Yiting Chen, Yunyun Wu, Sara S. Mechael, and Tricia Breen Carmichael Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04487 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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Chemistry of Materials

Heterogeneous Surface Orientation of Solution-Deposited Gold Films Enables Retention of Conductivity with High Strain – A New Strategy for Stretchable Electronics

Yiting Chen, Yunyun Wu, Sara S. Mechael, and Tricia Breen Carmichael* Department of Chemistry & Biochemistry, University of Windsor, Windsor, Ontario, Canada, N9B 3P4

*e-mail: [email protected]

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Abstract Stretchable electronic devices rely on stretchable conductors to form device interconnects and electrodes that maintain electrical performance during deformation. Although the high conductivity of metals makes them desirable materials for these applications, the lack of intrinsic stretchability of metals is a fundamental problem in stretchable electronics. Research efforts to impart stretchability to metal films on elastomers have involved configuring the films into wavy features that unbend with strain or using high surface roughness to engineer how cracks form in metal films under strain. However, the topographies used in these approaches cause problems with integrating these metal films as electrodes in thin-film devices. This paper presents a new, simple, and low-cost strategy for the fabrication of stretchable gold films with planar topography that remain highly conductive to 95% elongation. Using solution-based electroless plating to deposit gold films on the elastomer poly(dimethylsiloxane) results in a heterogeneous crystalline surface texture with misoriented grains that are strong barriers to dislocation movement. Under strain, the misoriented grains cause the formation of unique nanoscale cracking pattern that is remarkably effective at preserving conductivity. We demonstrate that this performance, coupled with the planar topography of these gold films, makes them suitable as electrodes in intrinsically stretchable light-emitting devices.


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Introduction Soft, stretchable, and conformable devices made by combining functional electronic materials with elastomers tolerate mechanical deformations such as stretching, twisting, folding, and conformal wrapping, unlocking a future of exciting new technologies and applications. The array of soft electronics reported in the literature include skin-mounted sensors,1-3 transistors,4 light-emitting devices,5-10 supercapacitors,11 battery arrays,12 and solar cells.13 Stretchable conductors are essential elements of all stretchable electronics, providing electrical connections between discrete devices in stretchable circuits and acting as electrodes and contacts with other functional materials within devices that are intrinsically stretchable. A key parameter is high conductivity that persists under mechanical strain, which enables interconnects to behave like ideal wires with minimal signal flux and prevents problematic voltage drop across device electrodes by minimizing Ohmic losses. Metals are the materials of choice due to their low resistivities (~10-8 m),14 but the mechanical mismatch of hard metals with soft elastomers results in cracking of the metal film with stretching.15 Although alternative materials, such as carbon nanotubes,16 graphene,17 and conductive polymers18 may provide more compatible mechanical properties, none of these possess conductivities that rival those of metals. The need for stretchable metal films has provoked extensive research into various methodologies to preserve the conductivity of metals with stretching.19 Here, we describe a new approach to stretchable metal films that uses solution-based deposition to fabricate gold films with a heterogeneous crystalline surface texture on the elastomer poly(dimethylsiloxane) (PDMS). We demonstrate, for the first time, that the misoriented grains of these gold films impede the linear propagation of cracks and instead give rise to a unique nano-cracking pattern that enables the films to remain remarkably conductive to 3 ACS Paragon Plus Environment


high (95%) elongations. We demonstrate the use of these films as electrodes in stretchable light-emitting devices. Extensive studies of gold films deposited on PDMS by physical vapor deposition (PVD) have shown that elongation causes incipient cracks to form at low elongations (< 5%) due to strain localization at defect sites in the gold film.20,21 Further elongation causes these cracks to lengthen, forming channel cracks that increase the resistance and eventually break the conductive pathway through the film at elongations typically around 20-30%.20,22 Researchers have established two methods to fabricate metal films that retain conductivity with stretching: The first method involves configuring metal films on elastomers into “wavy” configurations, such as in-plane serpentines23 or out-of-plane sinusoidal structures,24,25 to avoid crack formation by converting the tensile strain into less destructive bending strains of the individual architectural features. The stable resistance with elongation exhibited by such structures has inspired extensive use as stable device interconnects in sophisticated stretchable circuits, such as stretchable and wearable displays,9 stretchable batteries,12 and implantable bioelectronics.26 These metal structures have found less utility as device electrodes, however, due to the limited surface area of serpentines and considerable topography of out-of-plane structures. The second method engineers how cracks form in metal films under strain by adding topographical features to the PDMS surface, which create defect sites in the overlying metal film. With elongation, strain localizes at these topographical defect sites to generate numerous micron-scale cracks in the metal film, distributing strain relief and limiting crack propagation. Although the resistance increases with strain due to cracking, this approach can preserve the conduction pathway to strains as high as 65%.27,28 The high surface roughness necessary to achieve these results, however, can create problems for the 4 ACS Paragon Plus Environment

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integration of these metal films as electrodes with other device materials to form thin-film devices. Consequently, there remains a need for flat metal films that remain highly conductive with stretching and possess sufficient surface area to be used as electrodes in thin-film stretchable devices. Recent research efforts to address this shortcoming have radically altered the elastomeric substrate or the metal film itself to change the electromechanical performance. For example, new layered elastomeric composites bury a rough SiO2 layer at the interface between the two elastomers to alter crack formation in metal films coated on the membrane surface through stress transfer while preserving planarity of the metal surface.29 In another approach, depositing the liquid metal gallium on the surface of the solid metal film creates a hybrid solid-liquid conductor that maintains electrical continuity with stretching.30 Here, we report a new, simple approach to the fabrication of stretchable gold films on PDMS that uses the crystalline surface texture of the gold to impede the linear propagation of cracks, providing planar gold films that remain remarkably conductive with a low normalized resistance change (R/R0) of 17.8 ± 2.1 at high (95%) elongations. Instead of depositing gold films using PVD, which generates polycrystalline gold films with a dominant (111) texture, we use solution-based electroless deposition (ELD) to fabricate gold films with a heterogeneous crystalline texture. In PVD, the metal vapor that condenses on the substrate surface has sufficient energy to rearrange and grow crystal grains that expose the thermodynamically favorable (111) crystallographic plane at the surface.31 In contrast, ELD generates a primary (111) crystalline texture accompanied by other highly expressed orientations, including (200), (220), and (311) due to its much lower deposition temperatures that limit atomic rearrangement during the deposition.32 Although it is well known that misoriented grains of polycrystalline 5 ACS Paragon Plus Environment


aggregates are strong barriers to dislocation movement and consequently change the direction of propagating cracks,33,34 to the best of our knowledge this effect has not been investigated in stretchable electronics to mitigate channel cracking. Despite substantial work on the development of novel ELD processes to deposit metal films on PDMS, these reports have typically relied on the standard methods of in-plane patterning into serpentines35 or out-of-plane buckling deformation36 to produce metal films that retain conductivity to high elongations. Experimental Section All chemicals were purchased commercially and used as received. PDMS stamps and flat substrates were prepared by casting PDMS prepolymer (Sylgard 184, Dow Corning, prepared using 10:1 ratio) against patterned photolithographic masters or polystyrene Petri dishes according to published procedures.37 Surface Modification of PDMS Substrates: Flat PDMS substrates were oxidized for 40 s in an air plasma with an air pressure of 10 psig (flow rate of 32 mL/min, Harrick Plasma), immersed in a 1% v/v APTES solution in deionized water for 10 min, and then rinsed with deionized water and dried under a stream of nitrogen gas. Selective Deactivation of PDMS Substrates: The surface of a PDMS stamp bearing a surface relief pattern was flooded with a solution of POMA in acetone (2 mg/mL). After 10 s, a stream of nitrogen was used to blow off the excess solution and dry the stamp. The stamp was then placed on the surface of the APTES-terminated PDMS substrate for 1 min and then removed. ENIG Procedure: Modified PDMS substrates were immersed in a Pd/Sn solution prepared as directed by the manufacturer (Cataposit 44 and Cataprep 404, Shipley) for 2 min, rinsed with water, and then immersed in an accelerator solution (6 M HCl) for 1 6 ACS Paragon Plus Environment

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min. The substrate was then metallized in a nickel ELD solution (0.08 M nickel (II) sulfate hexahydrate, 0.14 M sodium pyrophosphate decahydrate, and 0.07 M dimethylamine borane; pH adjusted to 8) at room temperature for 10 min. After rinsing with water, the nickel-coated PDMS substrates were immersed in the IG solution (Uyemura Gobright TAM-55) for 20 min at 60 °C. E-beam Evaporated Gold Films: PDMS substrates were oxidized for 40 s in air plasma with an air pressure of 10 psig (flow rate of 32 mL/min), and then e-beam evaporation was employed to deposit a 30-Å-thick titanium adhesion layer at a deposition rate of 1.0 Å/s, followed by a 570-Å-thick gold layer at a deposition rate of 1.0 Å/s. ACEL Device Fabrication: ZnS:Cu microparticles (Shanghai KPT Company) were mixed with PDMS prepolymer (weight ratio of base to curing agent = 10:1) in a 1:1 (w:w) ratio. The mixture was spin coated onto an ENIG film on PDMS at 500 rpm for 1 min and cured in a 60 °C oven overnight. The emissive layer was treated with air plasma for 1 min, and then a PEDOT:PSS aqueous dispersion (CleviosTM PH1000, Heraeus, USA) with 2 wt% Triton X-100 was spin-coated on the emissive layer at 2000 rpm for 1 min followed by annealing in a 60 °C oven overnight. Results and Discussion We deposited gold films on PDMS from solution using the electroless nickelimmersion gold (ENIG) process to avoid the short plating bath lifetimes, high sensitivity to contamination, and low plating rates associated with the direct ELD of gold.38 The ENIG process uses two solution-based plating steps, beginning with the ELD of a nickel film followed by an immersion gold (IG) process. In the nickel ELD process, a catalyst chemisorbed on the PDMS surface initiates the deposition of a nickel film. We used palladium-tin colloids as the ELD catalyst, which consist of a Pd-rich core, a hydrolyzed 7 ACS Paragon Plus Environment


Sn2+/Sn4+ shell, and associated chloride ions that give the colloids a negatively charged surface, enabling them to be electrostatically bound to cationic functional groups.38 To deposit a uniform ENIG film over the entire PDMS surface, we first modified the PDMS surface by plasma oxidation and reaction with 3-aminopropyltriethoxysilane (APTES) to generate an amine-terminated surface. Subsequent immersion in the acidic Pd/Sn solution protonates the amine groups to form positively charged ammonium groups that electrostatically bind the Pd/Sn colloids.39 Etching the Sn2+/Sn4+ layer in 6 M HCl exposes the Pd core, which catalyzes the initial ELD of nickel, with additional nickel depositing autocatalytically as a dimethylamine borane reducing agent in the plating solution is consumed. We found that a nickel ELD time of 10 minutes produced uniform nickel films (Figure S1); increasing the deposition time to 15 min or 20 min resulted in cracking and delamination of the nickel film likely due to hydrogen accumulation in the film and blistering.38 Subsequently immersing the nickel-coated PDMS substrate in a solution of potassium gold cyanide deposits a gold film by galvanic displacement, as Ni atoms in the film reduce Au+ ions from solution, forming a metallic gold film and releasing Ni2+ ions into the solution.40,41 To selectively deposit an ENIG film within a defined pattern on the PDMS surface, we used a patterning process we previously reported for selective copper ELD on PDMS in which microcontact printing the polymeric resist poly(octadecenyl-alt-maleic anhydride) (POMA) onto an APTESmodified PDMS surface blocks the binding of Pd/Sn colloids (Figure 1a).39 As previously reported, the anhydride groups of POMA rapidly react with the amine groups on the PDMS surface during the microcontact printing process to form amide groups that cannot easily be converted to a cationic form, tethering the hydrophobic POMA resist to the PDMS surface to block Pd/Sn colloid adsorption (Figure 1b, S2). Immersing the 8 ACS Paragon Plus Environment

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patterned PDMS substrates in the Pd/Sn colloidal solution results in catalyst binding only in POMA-free areas, producing ENIG films with features ranging from 10 - 100 m (Figure 1c) with a rough line edge typical of ELD nickel films (Figure 1d).42 ENIG films exhibit robust adhesion to the PDMS substrate, indicated by mechanical adhesion testing using the tape test (Figure S3), and present uniform gold surfaces (Figure 1e) composed of granular, nanoscale gold particles typical of ENIG films (Figure 1f).40 Energydispersive X-ray spectroscopy (EDX) analysis shows both nickel and gold in the film composition (Figure S4). Cross-sectional SEM of an ENIG film deposited on a silicon wafer – a mechanically rigid model for the oxidized PDMS surface – shows a total film thickness of ~60 nm (Figure S5).



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a c

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O H 2 N H2 N

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Figure 1. Fabrication of ENIG films on PDMS. a) Schematic of the process steps used to fabricate patterned ENIG films on PDMS. b) Reaction of APTES-terminated PDMS surface with POMA. c) Optical and d) SEM micrographs of patterned ENIG films on


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PDMS (light region corresponds to the ENIG film). e) Optical and f) SEM micrographs of ENIG films on PDMS.

We compared the properties of ENIG films on PDMS with a benchmark gold film of similar thickness fabricated by e-beam evaporation of 3 nm of Ti as an adhesion layer followed by 57 nm of Au on PDMS. This benchmark system, termed EBAu, exhibited a consistent, uniform, and smooth surface without cracks or buckles, comprising nanoscale gold grains similar to the surface of ENIG films (Figure S6), with excellent adhesion to the PDMS surface (Figure S7). The sheet resistances of ENIG and EBAu films on PDMS were comparable (3.6 ± 0.7 /sq and 2.9 ± 0.1 /sq, respectively). Atomic force microscopy (AFM) showed that while ENIG and EBAu films exhibit similar root-meansquare roughness values (5.0 ± 0.8 nm and 3.2 ± 0.5 nm, respectively) (Figure 2a, b), analysis of the grain sizes comprising the two film types are distributed differently (Figure 2c, d, S8, S9). ENIG films comprise a higher proportion of small grains compared to EBAu films: The majority (67%) of ENIG grains have areas up to 3000 nm2, with the remaining 33% falling between 3000 – 10000 nm2 (Figure 2c). In contrast, 35% of EBAu grains have areas up to 3000 nm2, with the remaining 65% falling between 3000 - 9000 nm2 (Figure 2d). Along with the difference in how the grain sizes are distributed, ENIG and EBAu films differ significantly in the orientation of the grains (Figure 2e). Analysis using X-ray diffraction (XRD) of EBAu gold films revealed a dominant (111) texture typical of face-centered cubic (fcc) metal films deposited by PVD, corresponding to a peak at 38.3° with a small second-order diffraction (222) peak at 81.8°. In contrast, the growth mechanism of the solution-based ENIG process generates films with a heterogeneous orientation, with a primary (111) orientation accompanied by peaks at 11 ACS Paragon Plus Environment


44.6°, 64.8°, 77.8°, and 81.9°, corresponding to (200), (220), (311), and (222) orientations, respectively. a

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(200) (311)

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Figure 2. Morphology and crystallinity of ENIG and EBAu films. AFM images of a) ENIG and b) EBAu films on PDMS substrates. Histogram of grain size distribution of c)


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ENIG and d) EBAu films. e) XRD spectra of ENIG (blue) and EBAu (red) films on glass substrates. XRD spectrum of isotropic gold shown in black.

The differences in orientation and grain size distribution profoundly affect how ENIG and EBAu films on PDMS respond to strain and retain electrical conductivity. We measured the change in electrical resistance of 3.0 cm x 1.5 cm ENIG and EBAu films on PDMS at 5% strain intervals (Figure 3a). The ENIG films exhibited an initial resistance (R0) of 8.4 ± 0.3  and remained conductive to a remarkable 95% elongation with a normalized resistance (R/R0) increase of only 17.8 ± 2.1; the PDMS substrate fractured beyond 95% strain. To the best of our knowledge, this is the first report of planar gold films that remain highly conductive with stretching without the use of serpentine designs, wavy features fabricated using compressive strain, or high surface roughness. The conductivity with stretching of ENIG films far surpasses that of the benchmark EBAu films on PDMS, which undergo electrical failure (R/R0 > 106) at only 5% elongation, as well as other planar EBAu films on PDMS reported in the literature, which undergo electrical failure at strains of 22 – 60% depending on film thickness and morphology.20,22,25 The best performance of planar EBAu films on PDMS previously reported remained conductive to 60% strain with a resistance of 2550 .25 ENIG films on PDMS are also durable to cycles of 40% strain, with R/R0 remaining < 10 in the strained state and recovering to ~1 each of 200 cycles (Figure S10).



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Figure 3. Stretching ENIG and EBAu films on PDMS.

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resistance of ENIG films as a function of elongation. b, c) Optical micrographs of an 14 ACS Paragon Plus Environment

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ENIG film at (b) 5% and (c) 80% elongation. d, e) SEM images of an ENIG film at 30% elongation. f, g) Optical micrographs of an EBAu film at (f) 5% and (g) 80% elongation. h, i) SEM images of an EBAu film at 30% elongation. Samples in b – i were stretched in the horizontal direction.

The considerable difference in conductivity with elongation between ENIG and EBAu films can be attributed to dramatic differences in how cracks in the films evolve with stretching. Optical images of benchmark EBAu films on PDMS at 5% strain (Figure 3f) show prominent channel cracks; these cracks widen and proliferate with additional elongation to 80% strain (Figure 3g). SEM images at 30% strain reveal cracks that are ~7 m in width, with the EBAu film slightly delaminated at the crack edges (Figure 3h, i). This delamination is characteristic of the channel cracking process, in which strain localization causes traction and subsequent delamination at the EBAu/PDMS interface.21 The processes of strain localization and traction coevolve, leading to the propagation of channel cracks that break the conductive pathway through the EBAu film at low strains.21 In contrast, ENIG films do not exhibit channel cracks, and furthermore appear surprisingly unchanged through 5 - 80% elongation according to optical micrographs (Figure 3b, c). SEM images at 30% strain reveal that ENIG films respond to strain by forming an unusual array of jagged nanocracks with widths of ~35-250 nm (Figure 3d). Higher magnification SEM images (Figure 3e) show no evidence of delamination at the crack edges typical of channel cracking; rather, ENIG films retain narrow (~50-230 nm) ENIG bridges that span the nanocracks, preserving tortuous conductive pathways that retain film conductivity under strain. The pronounced differences in the cracking of EBAu and ENIG films can be 15 ACS Paragon Plus Environment


attributed to the different crystallographic textures of the two film types, which are an indicator of important differences in the character of the grain boundaries (GBs) that play a critical role in the motion of dislocations and the nucleation and propagation of cracks in response to stress.34 Film texture correlates with the extent of misalignment present at the GBs, quantified by the angle of misorientation.43-45 Crystallographic orientation mapping of thin gold films have shown that films with a dominant (111) orientation comprise low-angle GBs, whereas films with a heterogeneous surface texture incorporate significant high-angle GBs.43,44 The dominant (111) texture of EBAu is consistent with low-angle GBs, which provide little impedance to crack propagation. Both experimental and computational studies have shown that dislocations emitting from cracks approaching low-angle GBs transmit into the GB region and to the neighboring grain, permitting the growing crack to easily penetrate without significant changes to its propagation direction.34 This process permits the formation of the channel cracks observed in EBAu films. In contrast, the heterogenous surface texture of ENIG films is consistent with high angle GBs, which act as strong barriers to dislocation movement, causing dislocation absorption and pile-ups in the GB region that change the direction of crack propagation. The high stresses and dislocation densities at the GB regions can also be micro-crack nucleation sites, consistent with the jagged array of nanoscale cracks observed in ENIG films.34 Furthermore, the smaller grain sizes of ENIG films compared to EBAu films may exacerbate the differences in crack formation and propagation due to the higher density of GBs in ENIG films. The planar topography of ENIG films coupled with the nanoscale cracks that preserve conductivity with stretching make these films suitable as electrodes in intrinsically stretchable light-emitting devices. Previous work on intrinsically stretchable 16 ACS Paragon Plus Environment

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light-emitting devices using gold films deposited on PDMS by PVD as stretchable electrodes resulted in device failure at low elongations (27%) due to channel cracking.6 Consequently, other studies focused on developing new stretchable electrodes of ionic conductors,8 carbon nanotubes,10 or silver nanowires5,7 to support device function to high strains. We fabricated intrinsically stretchable alternating current electroluminescent (ACEL) devices by depositing a film of ZnS:Cu phosphor microparticles blended with PDMS onto an ENIG film on PDMS, and then depositing a film of the conductive polymer PEDOT:PSS plasticized with the surfactant Triton X-100 as the transparent electrode (Figure 4a).46 Operating the devices at 165 V AC and a frequency of 37 kHz resulted in the emission of uniform blue light that persisted with bending and stretching to 40% strain (Figure 4b-d), with insignificant variance in radiance (Figure 4e). Previous studies of stretchable ACEL devices have demonstrated that the light emission intensity under strain depends on the effects of reducing the emissive layer thickness and increasing the device area that occur with stretching and is insensitive to resistance changes exhibited by the electrodes.7,8 Reducing the emissive layer thickness with stretching increases the electric field and thus contributes to increasing the device brightness; at the same time, increasing the device area with stretching reduces the density of the phosphor in the emissive matrix and thus contributes to reducing the brightness. The consistent radiance exhibited by our stretchable ACEL devices with stretching is consistent with a balance of these two factors. Furthermore, since ACEL devices are capacitively coupled to the external electrical bias, the emission intensity of ACEL devices is mainly determined by the bias voltage. Increases in the resistance of the electrodes with strain (R/R0 of gold = 5.4 and R/R0 of PEDOT:PSS/Triton X-100 = 49.6 at 40% strain) (Figure S11) remain small compared to the resistance of the device, 17 ACS Paragon Plus Environment


enabling stable light emission with stretching. Stretching the ACEL devices beyond 40% elongation, however, caused device failure due to the loss of conductivity of the PEDOT:PSS/Triton X-100 electrode. The stretchability of the ACEL devices may be further improved by integration of a transparent electrode that retains conductivity to higher elongations; this study is currently under investigation in our lab. a

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Figure 4. Stretchable ACEL devices fabricated using ENIG films on PDMS. a) Diagram of ACEL device. b, c) Photographs of ACEL devices (3 cm x 3 cm) bent to (b) 15% and (c) 40% strain. d) Photographs of ACEL devices (1.5 cm x 3 cm) stretched to 0%, 10%, 20%, 30% and 40% strain. e) Normalized change in maximum radiance of ACEL devices at 0 – 40% elongation.


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Conclusions Using the heterogenous crystalline texture of solution-deposited ENIG films is a new approach to the fabrication of stretchable metals that produces favorable nanoscale cracking patterns that enable the retention of conductivity to high strain. This simple, solution-based approach avoids the complexity of other methods that involve lithographic patterning to produce serpentine designs and eliminates rough surface topographies that interfere with integration into thin-film devices. The ability to integrate ENIG films into thin-film devices, coupled with the high conductivity of the metal films, unlocks new opportunities in large-area, soft, and conformable devices, such as wearable lighting panels and displays, biomedical applications, and “smart skins”. Furthermore, we are continuing to explore the use of metal films with heterogenous surface texture in stretchable electronic systems to gain deeper insight into the limits and parameters of this approach, using both experimental and computational methods. Acknowledgments This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through Discovery grant 312167-2012-RGPIN. Y. W. was supported by a MITACS Globalink Graduate Scholarship, and S. S. M. was supported by an Ontario Graduate Scholarship. We thank R. S. Carmichael for photography. Supporting Information The Supporting information is available free of charge on the ACS Publications website. Characterization details; optical microscope images of ELD nickel, adhesion testing of ENIG films; EDX spectrum of ENIG film; cross-sectional SEM image of ENIG film; optical and SEM images of EBAu films; adhesion testing of EBAu films; grain size analysis of ENIG films; grain size analysis of EBAu films; resistance change with cyclic 19 ACS Paragon Plus Environment


strain of ENIG films; resistance change with strain of PEDOT:PSS/Triton-X 100 film. References (1)

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Heterogeneous Surface Orientation of Solution-Deposited Gold Films

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