Fractal Gold Nanoframework for Highly Stretchable Transparent Strain

Apr 30, 2018 - a high stretchability of 420% strain with negligible resistance changes under low-level .... added and the stirring speed is increased ...
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Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX

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Fractal Gold Nanoframework for Highly Stretchable Transparent Strain-Insensitive Conductors My Duyen Ho,†,‡ Yiyi Liu,† Dashen Dong,† Yunmeng Zhao,†,‡ and Wenlong Cheng*,†,‡ †

Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia The Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton, Victoria 3800, Australia

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S Supporting Information *

ABSTRACT: Percolation networks of one-dimensional (1D) building blocks (e.g., metallic nanowires or carbon nanotubes) represent the mainstream strategy to fabricate stretchable conductors. One of the inherent limitations is the control over junction resistance between 1D building blocks in natural and strained states of conductors. Herein, we report highly stretchable transparent strain-insensitive conductors using fractal gold (F−Au) nanoframework based on a one-pot templateless wet chemistry synthesis method. The monolayered F−Au nanoframework (∼20 nm in thickness) can be obtained from the one-pot synthesis without any purification steps involved and can be transferred directly to arbitrary substrates like polyethylene terephthalate, food-wrap, polydimethylsiloxane (PDMS), and ecoflex. The F−Au thin film with no capping agents leads to a highly conductive thin film without any posttreatment and can be stretched up to 110% strain without significantly losing conductivity yet with the optical transparency of 70% at 550 nm. Remarkably, the F−Au thin film shows the strain-insensitive behavior up to 20% stretching strain. This originates from the unique fractal nanomesh-like structure which can absorb external mechanical forces, thus maintaining electron pathways throughout the nanoframework. In addition, a semitransparent bilayered F−Au film on 100% prestrained PDMS could achieve to a high stretchability of 420% strain with negligible resistance changes under low-level strains. KEYWORDS: Gold electrodes, fractal structures, stretchable conductors, transparent conductors, stretchable electronics

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structures based on platinum (Pt)31 or gold32 could circumvent the aforementioned limitation of 1D nanomaterials-based percolation networks with much more reduced resistance change during mechanical deformations. However, fabrication processes of those mesh-like structures are typically a complex multistep procedure, which is very costly, involving in sputtering or lithography, high-temperature, and hazardous chemicals. It has also been noted that crackle-based approach emerges as a promising strategy to fabricate transparent conductors.33,34 This approach is usually based on self-forming cracking templates such as from a gel film33 or natural leaf.35 Despite the excellent performances of crackle-based transparent conductors, some shortcomings include complex multiple steps and materials waste in the nontemplating area which requires either recycle or trash. From material considerations, gold has some merits in stretchable transparent conductors. Unlike other metal materials such as silver or copper, gold has excellent chemical stability against oxidation. The work-function of gold excellently matches with most of the p-type organic semiconductor materials which can be used for various optoelectronic applications.36 As a model system, Lacour et al.

tretchable transparent electronics has been attracting significant research efforts due to their potential to achieve “invisible” wearable/implantable electronic devices.1−3 In this regard, a stretchable transparent conductor is one of crucial components in order to build up lots of stretchable and skinlike devices such as solar cells,4−7 light-emitting-diodes and displays,8−10 touch screens,11,12 and smart windows.13,14 Typically, there are two main approaches toward the design of stretchable conductors: “materials that stretch” (MTS) and “structures that stretch” (STS).2 In the MTS strategy, the percolation networks based on onedimensional (1D) materials such as silver nanowires (AgNWs),15−17 gold nanowires, 18−20 copper nanowires (CuNWs),21−24 and carbon nanotubes (CNTs)25,26 have been widely utilized in flexible/stretchable electronics due to their high electrical conductivity, mechanical stretchability, and/or good optical transparency. However, drawbacks such as high resistance at wire−wire junction areas and insulating passivation of long organic ligands significantly limit their conductivity. Additional post-treatment processes such as high temperature annealing,27 compression pressure,28 and plasmonic laser welding23,29,30 are typically used to improve the conductivity of the percolation networks which make fabrication processes costly and more complicated and may damage the mechanical property of the 1D percolation network.27 Alternatively, in the STS, metallic mesh-like © XXXX American Chemical Society

Received: February 17, 2018 Revised: April 30, 2018

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DOI: 10.1021/acs.nanolett.8b00694 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters

Figure 1. Schematic illustration of the fusion process between adjacent nanoparticles after toluene injection (a), and the direct-transfer roomtemperate solution-process of F−Au thin film from the surface of water to flexible substrates (b).

the synthesis procedure is conducted by dropping a reduction agent of sodium borohydride (NaBH4) into an aqueous solution of gold(III) chloride hydrate (HAuCl4·xH2O) at about 5 °C. At this stage, the color of the solution changes from light yellow to wine red which indicates that gold nanoparticles (AuNPs) are successfully synthesized. Subsequently, toluene is added and the stirring speed is increased to 600 rpm. After about 10 min, the F−Au nanoframework forms spontaneously at the air/water interface. There is no capping agent used during the synthesis reaction. Unlike other synthesis approaches where small molecules or long-chain polymers are typically utilized as passivated ligands, the synthesis method in this report is simple, facile, and does not require harsh conditions. More importantly, we can obtain a highly conductive F−Au framework because no capping agent is added during the synthesis, and no postsynthesis purification is required. Figure 1a illustrates the crystalline connections between adjacent gold nanoparticales (Au NPs), resulting in the nanowire formation. The synthesized F−Au nanoframework spreads homogeneously over cm2 areas at the interface of water and toluene, which is potentially utilized for large-scale fabrication. Figure 1b illustrates the transfer process of the F−Au from the surface of water to flexible substrates by gently pressing the substrates on the F−Au film and subsequently lifting substrates up. The step-by-step synthesis of the F−Au and the fabrication process of the stretchable transparent F−Au conductors based on the low-temperature all-solution approaches can be found in Figure S1. Figure S2a−e reveals the field-emission scanning electron microscope (FE-SEM) images of a monolayer of the F−Au nanonetwork on ITO-glass, PET, food wrap, PDMS, and ecoflex surfaces, which indicates that the F−Au nanonetwork can be utilized for many affordable, bendable, and stretchable electronic devices as it can be formed easily on different types of substrates. Moreover, because most of the elastic stretchable substrates are hydrophobic, the F−Au film can be transferred easily on those substrates with a better adhesion as compared with most of hydrophilic materials such as AgNWs, CuNWs or CNTs which may require additional surface treatments. The as-prepared film is a nanomesh-like structure with toluene absorbs weakly on the surface of gold and has no junction area as observed in the FESEM (Figure 2a) and transmission electron microscopy (TEM)

pioneered initial studies on the stripes of 100 nm thick gold films fabricated on a compressive PDMS, which could be stretched up to 22% strain without losing conductivity.37 Later, the mechanism of gold film bonded to an elastomeric substrate that can be stretched repeatedly tens of percent strains without fatigue was revealed to be due to percolating microcracks in which metal films deform much less than elastomeric substrates.38 More recently, transparent conductors using ultrathin gold nanowires39 and gold nanobelts40 have also been reported for applications in stretchable electronics. Despite the encouraging progress in the literature, it remains challenging to simultaneously achieve high electrical conductivity, high optical transparency, and high mechanical stretchability in a simple yet efficient method. Herein, we exploit a self-assembled fractal gold nanostructure for the stretchable transparent conductors for the first time. The fractal patterns are ubiquitously present in Mother nature such as in trees, mountains, and coastlines and also found in nanoscale metallic nanostructures.41 The fractal geometry was mathematically studied by Mandelbrot in 1982.42 The well-defined fractal structures were also designed at the micrometer scale using topdown STS strategy to fabricate stretchable electronic devices.43,44 In contrast, this study introduces a one-pot lowtemperature self-assembly templateless wet-chemical approach to fabricate fractal gold-based stretchable conductors, which are in essence based on the MTS approach. The as-synthesized monolayered F−Au film is highly conductive with sheet resistance as low as 100 ohm/square (Ω/□) at 70% transparency, which can be achieved without any purification steps. The film can also be transferred directly onto arbitrary elastomeric substrates such as polyethylene terephthalate (PET), food-wrap, PDMS, and ecoflex without any post-treatments. The monolayered F−Au-based conductor can be stretched to a maximum 110% strain without losing its conductivity and is also able to withstand up to 10 000 cycles stretching or bending test. Furthermore, a bilayered F−Aubased film on a prestrained PDMS can be stretched up to 420% strain with negligible changes in conductivity under 20% strain. Results and Discussion. The synthesized F−Au nanoframework is formed at the interface of toluene and water. The synthesis procedure is based on the facile one-pot wetchemistry low-temperature approach.45 Details of the experiments are mentioned in the Supporting Information. Briefly, B

DOI: 10.1021/acs.nanolett.8b00694 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 2. F−Au nanomesh morphology characterization. (a,b) Top-view FE-SEM and TEM images of the F−Au network. (c) High-resolution TEM image showing crystalline structures between fused nanoparticles. (d) Electron diffraction pattern from a selected-area of F−Au. Red arrows indicate crystalline structure of the F−Au.

Figure 3. Transmittance (λ = 550 nm) as a function of the sheet resistance, stretchability, and durability characterizations of the F−Au conductors. (a) Transmittance (λ = 550 nm) of the F−Au film with different layer numers (1 layer, 2 layers, 5 layers, and 10 layers) as a function of the sheet resistance. The FoM value is estimated about 14.3 by the numerial fitting method. (b) Stretchability of F−Au thin film. The inset shows the relative resistance change of the one layer of F−Au film at the strain up to 60% with the strain-insensitive up to 20% stretching strain. (c,d) Durability of the F−Au conductor at 30% stretching strain and 50% bending (∼1.7 mm bending radius). The inset is the digital image of the conductor at stretching state of 30% strain and bending state of 50% strain.

(Figure 2b) images. The TEM image in Figure 2c provides the clear evidence that the grain boundary between two gold

nanoparticles is disappeared. These gold nanoparticles are melded through nanoparticle coalescence to form nanowires C

DOI: 10.1021/acs.nanolett.8b00694 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 4. (a) AFM images of one layer of F−Au film on PDMS. Left, height mode; right, phase mode. (b) Optical images of the F−Au nanomesh on the PDMS substrates showing the morphology change at different strain levels (0%, 20%, 50%, 100%).

costly lithography (R/Ro ∼ 3.8 at 100% strain)32 and much better than those of the Pt-nanomesh (R/Ro > 400 at 20% strain) and the very long 1D nanowelded silver nanowires network (R/Ro > 100 at 90% strain).46 Additionally, there are only a few reports of transparent electrodes based on one single material that are able to reach the stretching strain beyond 100%. At the low-level strain of ∼20%, the monolayered F−Au film shows negligible changes in electrical conductivity (R/Ro ∼ 1.0) (inset of Figure 3b). As can be seen, a very slight increase in the ratio of R/Ro at 30% and 50% strains, only 1.1 and 1.3, respectively. Despite of encouraging progress in stretchable conductors over the past several years, the success in achieving the strain-insensitivity is still limited.36,47 Typically, those previous strain-insensitive conductors are nontransparent which are composed of a thick composite films. In contrast, our 20 nm thin monolayered F−Au film has an optical transparency of 70% and a maximum stretchability of up to 110% strain. The film exhibited an stretch-insensitive behavior with negligible change in resistance at different strain levels (3%, 5%, 10%, and 20%) as shown in Figure S4. This is attributed to robust fractal frameworks, free of strain-induced junction breakage which will be discussed in more details below. In case of bilayered F−Au film, it is able to achieve a higher stretchability of 130% strain. However, the relative resistance change in this case is higher than that of the monolayered film (Figure 3b). This can be explained because the contacts between the primary F−Au layer and the

with about 8 nm in diameter. An electron diffraction pattern shows the faced-centered cubic lattice structures (Figure 2d). Unlike conventional 1D nanomaterials percolation system, our F−Au films own the unique mesh-like fractal structure allowing for maintaining electron-transport pathways even under high strain. The F−Au transparent stretchable conductor is fabricated on the PDMS substrates and the optical transmittance, stretchability as well as durability are characterized as shown in Figure 3. Figure 3a reveals the plot of the transmittance (λ= 550 nm) of the F−Au film with different layer numers (1 layer, 2 layers, 5 layers, and 10 layers) as a function of the sheet resistance. By using the numerical fitting method, the value of the FoM is estimated about 14.3. Details of the relation between the transmittance (λ = 550 nm) and the sheet resistance of the transparent electrodes can be found in the Supporting Information. The transparency of a monolayer of the F−Au is approximately 70% (Rs ∼ 100 Ω/□) obtained at the visible wavelength of 550 nm (Figure S3). Nevertheless, the transparency of the bilayered F−Au film reduces to about 65.0% at 550 nm (Figure S3). The F−Au thin films exhibit excellent stretchabilities of 110% and 130% strains for one layer and two layers of F−Au, respectively (Figure 3b). One attribute of F−Au-based stretchable conductor is the strain-insensitive resistance changes. With one layer of F−Au, the relative resistance (R/Ro) is only 3.0 at 100% strain. This ratio of R/Ro is slightly better than the gold nanomesh fabricated by the D

DOI: 10.1021/acs.nanolett.8b00694 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 5. Characterization of two layers of the F−Au conductors on the 100% prestrained PDMS. (a) I−V curves of the F−Au nanomesh at different stretching strain levels (from 0% to 40%) showing the strain-insensitive behavior of the F−Au conductor at up to 20% stretching strain. b) Relative resistance change of the F−Au conductor revealing highly stretchability (up to 420% strain). The inset shows the relative resistance change of the bilayered F−Au film at the strain up to 60%. (c) Transmittance of the F−Au conductor on the flat and prestrained PDMS at the visible range of wavelengths (350−1000 nm).

surfaces, exhibit promising applications in soft electronics, particularly for facile multilayered devices. The structural features and properties of the F−Au-based conductors are compared with those of literatures as shown in Table S1. The roughness of the monolayered F−Au film is comparable with those of AgNW films embedded in elastomer substrates (12− 20 nm in roughness) and graphene (6 nm in roughness), and much lower than that of the as-prepared gold nanosheet (400 nm in roughness). More importantly, the F−Au thin film shows better conductivity and stretchability at a similar transparency compared with those of carbon-based materials like CNTs and graphene. In comparison with metallic nanowire-based conductors, the F−Au thin film is comparable in terms of the transparency and conductivity but exhibits better stretchability. It has to be noted that the synthesis method of the F−Au materials and the fabrication process of the F−Au films are allsolution-based under ambient conditions, and there is no purification or post-treatment involved, which is advantageous for potential future commercialization. In order to gain a better understanding of the fractal geometrical change under dynamic stretching states at different strain levels, the morphology change of the F−Au thin film on PDMS is investigated. As observed in the Figure 4b, open mesh geometries deformed at various strain levels of 20%, 50%, and 100% are applied but no significant cracks can be found even at a high strain of 100%. Unlike the “out-of-plane” architecture,

secondary F−Au layer are loose, thus contact resistances are expected to increase more significantly under strains due to possible sliding between the two F−Au layers. The durability of the F−Au thin film is characterized by undertaking repeated stretching and bending cycles under the ambient condition. The monolayered F−Au film demonstrates an excellent resistance recovery after stretching at 30% strain or bending at 50% strain (bending radius ∼1.7 mm) for 10 000 cycles (Figure 3c,d). This indicates that the monolayered F−Au nanoframework has an excellent ability to absorb external mechanical deformation and to maintain good conductive pathways under strains. Remarkably, there is no noticeable change in the ratio of R/Ro (only ∼1.2) after 10 000 bending cycles. The insets in the Figure 3c,d shows digital pictures of the stretchable transparent F−Au based-conductor at 30% stretching state and 50% bending strain. Excellent electrical recovery of the F−Au film is very crucial because it indicates that the film is able to revert from the damages and thus maintain its conductivity after long time under strains. To further prove the robustness of the fractal nanoframework under deformations, structural characterizations are carried out by the atomic force microscope (AFM). Fractal structures are evident in both height and phase images as can be observed in Figure 4a. The monolayered film is about 20 nm in thickness and the root-mean-square roughness measured is only 10 nm. Stretchable transparent conductors, which are thin with smooth E

DOI: 10.1021/acs.nanolett.8b00694 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters for example, the “wavy layouts”, the open mesh structures tend to rotate in “in-plane” deformations like the motion of scissors. The concept of the mesh-like geometries in stretchable electronics has been proven in previous reports.1,31,32 In the concept, the mesh-sizes extend longer but narrower as larger strains are applied. The mesh-like structure changes its morphology to adapt to deformations, therefore no breakage occurs at certain strain levels. As a result, the electrical pathways still keep maintaining original conductivity of the whole nanoframework. This can explain why the F−Au film shows its stretch-insensitivity by stretching up to 20% strain, and then the ratio of R/Ro slowly increases to only 1.1 and 1.3 at 30% and 50% strain, respectively (see Figure 3b). Typically, the electrical failure is observed only as the mechanical breakage occurs. However, in comparison with 1D nanomaterials or bulk metallic materials, the cracks of our F−Au films are less significant. This is the reason why the F−Au nanomesh can withstand the high strain level (>100%) while the ratio of R/Ro is still relatively low (only 3.0) (see Figure 3b). Using the boxcounting method,48 the fractal dimension (D) of the F−Au material is calculated (D ∼ 1.91 at 0% strain). This is done by virtue of the Photoshop software at a resolution of 10 pixels/ cm. Fitting of the pixel number against logarithmic surface area allows for determination of fractal dimension as detailed in the Supporting Information (Figure S5). The morphological changes of the bilayered F−Au film are also characterized at 0% strain, 50% strain, 70% strain, and released to 0% strain (Figure S6), which indicates that the electron connectivity can be well-remained and can recover to its original morphology without forming any disconnected areas like in 1D materials. The conductivity of the F−Au film can be further enhanced by increasing the number of layers. With additional annealing treatment, the sheet resistance can be reduced because of the toluene decomposition is under high temperature.45 In comparison with silver- or copper-based conductors, the F− Au-based film exhibits good thermal stability against oxidation as it can maintain its conductivity at up to 400 °C (Figure S7). The much higher-strain tolerant strain-insensitive conductor can be obtained by depositing films onto a 100% prestrained PDMS substrate. The resistance of the F−Au film can withstand up to 20% stretching strain without any significant change in the current as shown in I−V curves (Figure 5a). At 30% stretching strain, the current of the bilayered F−Au film is dropped very slightly by only 7.7% compared to the initial current. The conductor is able to withstand to a large strain of up to 420% (Figure 5b). The relative resistance change rises significantly after 100% strain. The inset shows the change in relative resistance at the strain up to 60%. At 400% strain, a drop in the relative resistance shows that the compression force is dominant rather than the tensile strain due to know Poisson effect. This promotes the contacts in the F−Au nanoframework, resulting in an increase in the current. The I−V curves of the F−Au conductor at the different strain ranges (from 0% to 120% and from 200% to 420%) can be found in Figure S8 a-b. At all the levels of strains, the ohmic behavior is observed. The transparency of the two-layered F−Au conductor is dropped from 65% (on the flat PDMS) to 35% (on the prestrained PDMS) at the visible wavelength of 550 nm (Figure 5c) due to the light-scattering effect. To the best of our knowledge, the performance of the semitransparent stretchable conductor report in this work is the best in terms of stretchability (420% strain) and strain-insensitive (20% strain) which is potentially applied for many stretchable electronics that not

require very high transparency but good in stretchability as well as tensile-strain stability. In conclusion, for the first time the fractal nanostructures of gold are successfully exploited for constructing the stretchable strain-insensitive transparent conductors. The entire fabrication process is all-solution-based and scalable without the needs of purification steps or expensive equipment involved. The monolayered F−Au film has a typical sheet resistance of 100 Ω/□, an optical transparency of 70% at the visible wavelength of 550 nm, and a high stretch-insensitive range of 20% strain with a maximum stretchability of 110% strain. The F−Au film conductivities can be further improved by increasing the number of layers but at a sacrifice of the optical transparency. The maximum stretchability can be also enhanced by combining intrinsic fractal structures with prestrain elastomeric substrates. The outstanding performances of the F−Au films originate from their unique fractal topological structure which is free from strain-induced junction sliding, which is a common phenomenon in percolation nanowire system. The facile lowtemperature, low-cost fabrication in conjunction with the outstanding performances indicate that fractal nanostructure of gold offers a great promise for future soft electronics.

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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.8b00694. Experimental details, a comparison table of a singlematerial-based stretchable conductors for stretchable transparent electrodes, a schematic illustration of lowtemperature synthesis of F−Au material and fabrication of F−Au thin film for stretchable transparent conductors, FE-SEM images of F−Au monolayer on different substrates (ITO-glass, PET, food-wrap, PDMS, ecoflex), the transparency of the one-layer and two-layer F−Au thin film, fractal dimension calculated by the boxcounting method, FE-SEM images demonstrated morphology change of two layers of F−Au on PDMS, thermal stability of F−Au thin film, and the I−V curve of the F−Au on the prestrain 100% PDMS substrate at low and high strain (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wenlong Cheng: 0000-0002-2346-4970 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by ARC discovery projects DP170102208 and DP180101715. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF).

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DOI: 10.1021/acs.nanolett.8b00694 Nano Lett. XXXX, XXX, XXX−XXX