Unconventional Janus Properties of Enokitake-like Gold Nanowire Films

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Unconventional Janus Properties of Enokitake-like Gold Nanowire Films Yan Wang,†,¶ Shu Gong,†,¶ Daniel Gómez,§ Yunzhi Ling,† Lim Wei Yap,† George P. Simon,‡,∥ and Wenlong Cheng*,†,‡,⊥ †

Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia New Horizon Research Centre, Monash University, Clayton, Victoria 3800, Australia § Applied Chemistry and Environmental Sci, RMIT University, Melbourne, Victoria 3000, Australia ∥ Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia ⊥ The Melbourne Centre for Nanofabrication, Clayton, Victoria 3800, Australia

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‡

S Supporting Information *

ABSTRACT: We report on unconventional Janus material properties of vertically aligned gold nanowire films that conduct electricity and interact with light and water in drastically different ways on its two opposing sides. These Janus-like properties originate from enokitake-like nanowire structures, causing the nanoparticle side (“head”) to behave like bulk gold, yet the opposing nanowire side (“tail”) behaves as discontinuous nanophases. Due to this Janus film structure, its head side is hydrophilic but its tail side is hydrophobic; its head side reflects light like bulk gold, yet its tail side is a broadband superabsorber; its tail side is less conductive but with tunable resistance. More importantly, the elastomer-bonded Janus film exhibits unusual mechatronic properties when being stretched, bent, and pressed. The tail-bonded elastomeric sheet can be stretched up to ∼800% strain while remaining conductive, which is about 10-fold that of head-bonded film. In addition, it is also more sensitive to bending forces and point loads than the corresponding tail-bonded film. We further demonstrate the versatility of nanowire-based Janus films for pressure sensors using bilayer structures in three different assembly layouts. KEYWORDS: gold nanowire, metallic Janus film, Janus property, stretchable, soft electronics film, the Janus film is composed of a closely packed nanoparticle layer (“head side”) on the top and a densely aligned nanowire layer (“tail side”) below. The way that the film conducts electrons, interacts with light and water, and responds upon external force (stretching, bending, and pressing) are entirely dependent on which side is exposed. The head side exposed film has exceptional stretchability up to 800% strain, while tail side exposed film loses its conductivity with only 83% strain. In addition, the freestanding film can construct versatile head/tail configurations and multilayered nanoarchitectures. We anticipate the results will help reshape the understanding of metal/polymer interface design, which suggests many possibilities to form sophisticated classes of assemblies for future optoelectronic devices.

etallic thin films are ubiquitous in everyday products ranging from ornaments to optoelectronic devices, which are typically obtained by sputtering, 1 evaporation,2−4 electroplating,5 electroless deposition,6−8 or recent nanomaterial films.9−15 Conventional metallic thin films, whether in continuous bulk phase1,2,5−8 or in discontinuous percolating/heterogeneous nano/microphase,3,4,9−15 have no gradient structures in the normal direction and hence exhibit identical materials properties on either side of the film.16,17 Advances in the field of nanotechnology have fueled the vision of future devices spawned from tiny functional components.18 In this concept, Janus materials could be fabricated based on anisotropic building blocks in the form of particles,18,19 fibers,20 and films.21 To date, fabrication strategies including self-assembly,22 polymerization,23 and phase separation24 have been applied for this purpose. Herein, we report on unexpected Janus optical, wetting, electrical, and mechatronic properties of the vertically aligned gold nanowire films. Unlike the existing conventional metallic

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© XXXX American Chemical Society

Received: June 22, 2018 Accepted: July 20, 2018

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DOI: 10.1021/acsnano.8b04748 ACS Nano XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION We begin with growth of standing gold nanowire films on a silicon wafer by a modified seed-mediated electroless plating method.25 The film formation process is illustrated in Figure S1. First, a silicon wafer is treated with air plasma and modified by (3-aminopropyl)trimethoxysilane (APTMS) to make it favorable for gold seed attachment. Then the amine-functionalized wafer was immersed into a gold seed solution for 1 h before further immersion into a growth solution containing gold precursors, surfactants, and reducing agents. Longer growth times lead to longer nanowires but reach a plateau height of ∼14 μm after 20 mins of growth (Figure S2). Further cross-sectional characterizations by scanning electron microscopy (SEM) revealed enokitake-like Janus film structures (Figure 1a), in which the top layer (“head” side) consists of

interface, where the surface-binding ligands are absent. This enables lifting of seed particles by forming vertical nanowires. The nanowire thin film was then soaked in DI water for half an hour to lift it off the silicon wafer, forming free-standing Janus thin films (Figure 1b,c and Movie S1) in water. The Janus film could be transferred either head or tail side exposed to other soft/hard substrates (e.g., glass slides, silicon wafer, polyethylene terephthalate, polydimethylsiloxane) by slowly pulling the target substrate out of the aqueous subphase from the bottom of the Petri dish. It is even possible to transfer the versatile head/tail configurations and multilayered nanoarchitectures (Figure 1d−f) onto soft/hard substrates without deterioration or rupture of the film. Unlike conventional continuous bulk metallic films or discontinuous percolation systems, the Janus gold films exhibit distinct optical, wetting, electrical, and mechanical properties differing from head to tail side. We compare the optical absorbance of both head and tail side thin film on polydimethylsiloxane (PDMS) substrates. In particular, the head side of the film reflects light in a way akin to bulk gold, while their tail side displays high (near-complete) absorption of light (Figure 2a). The observed near-perfect absorption of the tail side can be understood by considering Fresnel’s law of reflection26 (Supplementary Section S2), which states that the reflectance R for a normally incident beam of light from a medium characterized with a refractive index ninc (for air ninc ≈ 1) onto a surface of refractive index ns is given by ji n − ninc zyz R = jjj s z j ns + ninc zz k {

2

(1)

which clearly approaches R = 0 (i.e., perfect absorption, given that there is no transmission of light) as the refractive index of the surface matches that of the incident medium (or ns → ninc) in the equation. This refractive index matching originates from the spatial sparseness of the gold nanowires on the tail side of the films (Figure S3). The Janus gold films also exhibit Janus wetting properties: their head side is hydrophilic, but their tail side is hydrophobic. For a typical film, the contact angle of its head side was 21 ± 3 degrees, whereas its tail side was 101 ± 8 degrees (Figure 2b). The head side is hydrophilic because of chemical modification, whereas the tail side is hydrophobic because of porous nanowire structures. The low wettability of the tail side is due to air pockets trapped within the nanowire matrix, and the experimentally measured angle can be well predicted by the Cassie−Baxter model27 (Supplementary Section S3). The high

Figure 1. Versatility of standing enokitake-like gold nanowirebased Janus films. (a) Cross-sectional SEM image demonstrating enokitake-like morphologies of gold nanowires. Scale bar: 500 nm. (b) Schematic of structures of standing enokitake nanowire-based films. (c) Photograph of a free-standing Janus film, showing golden reflection from the nanoparticle side but complete darkness for nanowire side. Scale bar: 1 cm. (d−f) Cross-sectional SEM images of versatile assembly of Janus films: (d) head-to-head, (e) tail-totail, (f) layer-by-layer. Scale bar: 500 nm.

closely packed gold nanoparticles. The bottom layer (“tail” side) is composed of nanowires standing normal to the silicon wafer. The vertical growth process resembles the well-known chemical vapor deposition but occurs in solution. The formation of vertical nanowires is possible because of sitespecific nucleation and growth at the seed particle/substrate

Figure 2. “Janus” properties of standing enokitake nanowire-based films. (a) Distinct optical properties from nanoparticle head side to nanowire tail side. The film was obtained after 5 min of growth time. (b) Different contact angles from head side to tail side. The film was obtained after 5 min of growth time. (c) Measured sheet resistance as a function of nanowire height. Red square: measurement on the tail side; black square: measurement on the head side. B

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Figure 3. Janus stretchability of standing enokitake nanowire-based films. (a) Comparison of stretchability between tail-side-bonded (red) and head-side-bonded (black) Janus films. (b) Scheme illustrating possible structural changes of tail-side-bonded (top) and head-sidebonded (bottom) films during the stretching/releasing process.

Figure 4. “Janus” mechanoelectrical properties of standing enokitake-like nanowire films. (a) Electrical responses of tail-side-bonded Janus film and head-side-bonded Janus film under tensile and compressive strains. The substrate used is a PET sheet. The nanowire height is about 1.5 μm. (b) Comparison of electrical responses to repeated point load force (0.056 N) for four kinds of gold films deposited onto a 1 mm thick PDMS sheet. (c) Versatile pressure sensors based on three types of bilayer assembly strategies. The Janus film size is 3 × 3 mm2. The films are assembled into head-to-head (black squares), head-to-tail (red circles), and tail-to-tail (blue triangles) layouts. PDMS films with a thickness of 1 mm were used as substrates.

contact angle than bulk gold surfaces. We also measured contact angles of Janus film under 50% strain (Figure S4). It shows that contact angles for both sides increase. This is due to the film crack reducing solid surface fraction value enabling

wettability of the head side is attributed to chemical modification of gold nanoparticle surfaces by 4-mercaptobenzoic acid (MBA) molecules, rendering a surface rich in carboxyl groups; thus the head side exhibits an even smaller C

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ACS Nano more air trapped in the film. However, the tail side still exhibits higher contact angle than the head side, consistent with enokitake-like film structures. In addition, the measured sheet resistance from the head side is about ∼6.6 Ω sq−1, which is independent of nanowire height. However, the resistance measured from the tail side decreases with nanowire height but remains greater than the head side (Figure 2c). This result is due to the top closely packed nanoparticle layer that promotes efficient electron hopping; in contrast, well-spaced nanowires inhibit efficient electron transfer from wire to wire. Each individual nanowire may be assumed as 1D zero-gap nanocube arrays without junction resistance. This allows applying the Volger model28 to fit our experimentally measured resistance (Supplementary Section S4, Figures S5−S7), which leads to a value of 15 000 for the dimensionless fitting parameter β. This large value indicates combined tunneling and ohmic contacts between nanowires.28 Furthermore, we show that elastomer-bonded Janus gold films exhibit entirely different mechatronic responses depending on whether it is head-bonded or tail-bonded. When Janus films were directly grown on Ecoflex substrates with the tail side chemically bound to surfaces, the films exhibit exceptionally high stretchability up to 800% strain without completely losing their conductivity (Figure 3a, red solid line). The original conductivity can be recovered upon stress release (Figure 3a, red dashed line). In contrast, if the Janus film is simply transferred onto Ecoflex with the nanoparticle side in contact with elastomers (tail side film), the conductivity is lost permanently with only 83% strain applied (Figure 3a, black line). Visual film delamination and cracks are evident for the head-bonded film but not for the tail-bonded one (Figure S8). We further establish that strong adhesion between the nanowire side and Ecoflex substrate and the “accordion-fanlike” V-shaped cracking process are responsible for the exceptionally high stretchability observed from the tail-sidebonded film. The adhesion test clearly shows that our tail-sidetethered Janus film could survive in the normal Scotch tape test with almost no conductivity loss, but not for head-side-bonded counterparts (Supplementary Movie S2). The strong adhesion may be due to the use of APTMS, which serves as a bifunctional molecular glue. Its amine side strongly interacts with gold nanowires, and its silane sides covalently bond to Ecoflex surfaces. The above structural characterizations may reveal the following mechanistic insights. In the tail-bonded nanowire thin film, cracks initiate from the head side, which serve as unzipping points for strongly bundling nanowire arrays, yet the interacting nanowire tail ends deform conformally to the substrate without cracking, owing to strong nanowire− substrate adhesion. At the point when substrate elongation commences, the mechanically relatively rigid top gold nanoparticle layer (head side) cracks, which triggers the formation of “V-shaped” cracks as the strain level is increased by unzipping them from the top side (Figure 3b, top). No delamination occurred between substrates and our Janus gold film at this stage during the whole stretching process, and the film recovered fully upon releasing (Figure S8a−d). On the contrary, transferred head-side-bonded thin film exhibited weak surface adhesion. Consequently, “U-shaped” cracks formed under a tensile strain of only 5% (Figure S8e−h). As the tensile strain further increases, “U-shaped” cracks propagate as a result of the Janus film sliding/delaminating

from the elastomeric substrate (Figure 3b, bottom). As the strain increases further to a certain threshold, percolated conductive pathways could not be maintained, while the cracked film becomes completely nonconductive (Figure 3a). The tail-bonded film is very robust and durable, as evidenced by stable resistance changes for the 15 000 cycling tests (Figure S9). This is attributed to strong nanowire−elastomer adhesion force described earlier. Besides stretchability, the Janus films exhibit bimodal responses to compressive and tensile bending forces, depending on which side is bonded. When the nanowire side is tethered to an elastomeric substrate, the tensile and compressive bending sensitivities (see bending sensitivity definition in Supplementary Section S6) are 0.98 and −0.54 rad−1, respectively (Figure S10). In contrast, for the film with the head side attached to the elastomeric substrate, the corresponding values reduce to 0.66 and −0.34 rad−1, respectively. As illustrated in Figure 4a, the reason for high tensile and compressive sensitivities for the tail-bonded film is because of relatively large deformation for head sides, which are akin to bulk gold phase and are electrically more conductive. Note that traditional bulk metallic film or discontinuous percolation gold nanowire film only shows a single modal response to compressive and tensile strains (Figure S11). The way these Janus gold films respond electrically to a point load is also fundamentally different from conventional gold films (Figure 4b). As shown in Figure S12, the sensitivity of the point load to head side is ∼2.71 N−1 (see point load definition in Supplementary Section S7); under the same condition, the point load sensitivity to the tail side decreased to ∼0.73 N−1. Importantly, the electrical responses are reversible and conductivities recover fully upon load release for both sides of the Janus film. In contrast, neither bulk gold film nor nanowire percolation films show any discrimination for point load sensitivity, and no conductivity recovery was observed for bulk film upon load release (Figure 4b). This result is attributed to the elastic nature of tail-bonded Janus film, yet rigid gold films will experience irreversible crack/delamination under a point load. The sensitivity sequence of three types of gold nanowire film toward point load is tail-bonded nanowires > head-bonded nanowires > lying down nanowires (Figure 4b). In the standing nanowire system, the high sensitivity of the nanoparticle side may be due to its relatively more rigid nature, compared with the underlying nanowire, which has a greater tendency to crack under point load, thus leading to higher sensitivity.29 The reason that the film of lying down nanowires shows the lowest sensitivity is because the point load is applied to them in the transverse direction. The degree of deformation of nanowires lying parallel to the supporting substrate is much smaller compared to the point load applied to free-standing nanowires in the longitudinal direction. The Janus film structures and distinct properties demonstrated above enable the design of versatile pressure sensors by altering layer number and assembly strategies. As a proof of concept, we fabricated three bilayer films with head-to-head, head-to-tail, and tail-to-tail layouts and compared their sensitivities in the elastic deformation range of 0−1000 Pa. The measured sensitivity sequence is head-to-tail > head-tohead > tail-to-tail (Figure 4c). When the single nanoparticle layer is sandwiched between nanowire layers (i.e., head-to-tail assembly), its tendency to crack for the “floating” rigid metallic sheet is greatest. With an additional nanoparticle layer added in D

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solution for 1 h. APTMS-modified substrates were immersed in an excess citrate-stabilized Au seed (3−5 nm) solution for 2 h to ensure the saturated adsorption of gold seeds, followed by rinsing with water four times to remove the weakly bound seed particles. Finally, seedparticle-anchored substrates were contacted with a growth solution containing 980 μM MBA, 12 mM HAuCl4, and 29 mM L-ascorbic acid, leading to the formation of standing nanowire films. The length of the nanowires depended on the growth reaction time. Typical nanowire heights of ∼1.5, ∼3.5, ∼5, ∼7, and ∼14 μm were obtained by adjusting the growth time to 2, 4, 5, 8, to 15 min, respectively. Characterization. SEM imaging was carried out using a FEI Helios Nanolab 600 FIB-SEM operating at a voltage of 5 kV. The sheet resistances of the standing enokitake nanowire-based Janus films were carried out on a Jandel four-point conductivity probe by using a linear arrayed four-point head. To test the electromechanical responses for strain and bending sensing, the two ends of the samples were attached to motorized moving stages (THORLABS model LTS150/M). Uniform stretching/bending cycles were applied by a computer-based user interface (Thorlabs APT user), while the current changes were measured by the Parstat 2273 electrochemical system (Princeton Applied Research). For the analysis of detailed point load or pressure responses, a computer-based user interface and a force sensor (ATI Nano17 force/torque sensor) and a Maxon Brushless DC motor using a high-resolution quadrature encoder (15 μm linear resolution) were used to apply an external point load or pressure. Ecoflex with a thickness of 500 μm was chosen as the substrate of the standing Janus film in the strain test. PET with a thickness of 125 μm was chosen as the substrate of the standing Janus film in the bending test. PDMS with a thickness of 1 mm was chosen as the substrate of the standing Janus film in the point load/pressure test. The reflectance (R) data were collected from a PerkinElmer UV−vis−NIR spectrophotometer Lambda 1050 with an integrating sphere setup. Given that T ≈ 0 or below the detection limit, the absorbance (A) spectra were calculated to be A = 1 − R.

the middle (i.e., head-to-head assembly), the tendency of both layers to crack is reduced due to enhanced strength. In contrast, when both nanoparticle layers are in contact with the elastomer (i.e., tail-to-tail assembly), no cracking results, and thus the film conductivity is insensitive to applied pressure. The hysteresis of all three layouts of pressure sensors is evaluated by measuring their resistance changes during loading and unloading at a uniform pressure of 100 Pa (Figure S13). The response times of three sensors are in the range of 100− 160 ms (Figure S13b,d,f). Interestingly, both sensors of headto-head and head-to-tail layouts exhibit full recovery in conductivity immediately after pressure is removed (Figure S14a,c, while the tail-to-tail layout shows much slower recovery capability and it cannot be fully recovered in the 5 s unloading interval (Figure S13e). The high hysteresis of the tail-to-tail layout may be due to the soft nature of nanowires, which need longer time to recover to their original state. It has to be noted that the stretching- and pressure-sensing performances with our Janus nanowire film are comparable with those of conventional percolation nanowire films30−34 or an interlocked system.35 As an example, we demonstrate small pressure forces from a human artery pulse can be easily identified with our Janus nanowire sensors (Figure S14a). It is evidenced that both systolic peak and reflective peak could be well detected by our sensor (Figure S14b). However, the Janus materials’ properties described above have not yet been reported in the literature,30−35 which may lead to specific sensors for selective biometric data collection that is being explored in our lab.

CONCLUSIONS In conclusion, we demonstrate the emergence of unexpected Janus optical, wetting, electrical, and mechatronic properties of standing gold nanowire films, due to their enokitake-like Janus structures. Such Janus properties are previously unknown and not observed for any conventional bulk phase or nanophase metallic thin films, which are in essence isotropic with identical materials properties on either side. Our findings may be extended to other metallic films or 2D materials to form heterogeneous, multifunctional optoelectronic devices, indicating broad application potentiality in next-generation soft electronics.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b04748. Video of a free-standing Janus gold film in water (AVI) Video of adhesion test of tail-bonded standing gold nanowire film on Ecoflex and transferred head-bonded standing gold nanowire film on Ecoflex (AVI) Figures including a schematic illustration the fabrication process of standing nanowires, change of nanowires height as a function of growth time, the calculated optical absorbance from the nanowire side of a Janus film, and wettability of a Janus film under 50% strain, theoretical calculation of conductivity from the nanowire side of a Janus film, optical images of standing nanowires, and robustness, bending, and point load performance of a standing nanowire thin film (PDF)

METHODS Chemicals. Gold(III) chloride trihydrate (HAuCl4·3H2O, 99.9%), triisopropylsilane (99%), 4-mercaptobenzoic acid (90%), (3aminopropyl)trimethoxysilane, sodium citrate tribasic dihydrate (99.0%), L-ascorbic acid, and ethanol (analytical grade) were purchased from Sigma-Aldrich. All solutions were prepared using deionized water (resistivity >18 MΩ·cm−1). All chemicals were used as received unless otherwise indicated. Conductive wires were purchased from Adafruit. Synthesis of Standing Gold Nanowires. A modified seedmediated approach was used, as described in the literature.8 First, 3−5 nm seed gold nanoparticles were synthesized. Briefly, 0.147 mL of 34 mM sodium citrate was added into a conical flask with 20 mL of H2O under vigorous stirring. After 1 min, 600 μL of ice-cold, fresh prepared 0.1 M NaBH4 solution was added with stirring. The solution turned brown immediately. The solution was then stirred for 5 min and stored at 4 °C until needed. To grow standing nanowires on substrates (e.g., Si wafer, Ecoflex), an oxygen plasma was applied to render the surfaces hydrophilic. Depending on the types of substrates, the plasma treatment time varied from 2 to 17 min. Then the substrates were functionalized with an amino group by a silanization reaction with a 5 mM APTMS

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Lim Wei Yap: 0000-0003-3072-6307 Wenlong Cheng: 0000-0002-2346-4970 Author Contributions ¶

Y. Wang and S. Gong contributed equally to this article.

Notes

The authors declare no competing financial interest. E

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DOI: 10.1021/acsnano.8b04748 ACS Nano XXXX, XXX, XXX−XXX