Layer-by-Layer Assembly of Free-Standing Nanofilms by Controlled

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Interface-Rich Materials and Assemblies

Layer-by-Layer Assembly of Free-Standing Nanofilms by Controlled Rolling Sumin Kang, Jae-Bum Pyo, and Taek-Soo Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01063 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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Layer-by-Layer Assembly of Free-Standing Nanofilms by Controlled Rolling Sumin Kang, Jae-Bum Pyo, and Taek-Soo Kim* Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 34141, Republic of Korea KEYWORDS: adhesion, assembly, nanofilms, rolling, water surface

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ABSTRACT

A water surface not only provides a habitat to many living organisms but also opens up new possibilities to develop state-of-the-art technologies. Here, we show a technology for layer-bylayer assembly of free-standing nanofilms by controlled rolling. The water surface is exploited as an ideal platform for rolling a nanofilm, allowing adhesion control and frictionless feeding. The nanofilm floated on the water surface is attached to a tube by van der Waals adhesion, and is rolled up by rotation of the tube. This method can assemble diverse film materials including metals, polymers and two-dimensional materials, with easy control of the number of layers. Furthermore, heterogeneous and spiral structures of nanofilm are achieved. Various applications such as a stretchable tubular electrode, an electroactive polymer tube actuator and a super-elastic nanofilm tube are demonstrated. We believe this work can potentially lead a breakthrough in nanofilm assembly processes.

INTRODUCTION A water surface has been utilized for development of many advanced thin film technologies such as direct tensile testing of ultra-thin films1-4 and formation of high quality polymer films5-7. The surface tension of water, 72.8 mN/m, enables free-standing of nanofilms despite their high surface to volume ratio.8 Moreover, the surface tension of water can be controlled by addition of solvents or surfactants, thus adhesion between floating nanofilms and the water surface is also adjustable.9 The low viscosity of water, 1.002 mPa·s, allows easy and damage-free manipulation of nanofilms because the nanofilms floated on the water surface can slide on the platform


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without friction.10 We adopt the unique characteristics of the water surface to develop a novel approach for controllable and versatile rolling of the nanofilms. The technologies for rolling nanofilms to a tubular shape have received considerable attention, allowing novel designs and applications.11-20 However, the method for controllable and versatile rolling up of a nanofilm has not been reported yet because it is difficult to handle free-standing nanofilms. Although micro- and nanoscale tubular structures have been proposed using a straininduced self-rolling method21-23, the accurate design and scale-up of a structure are challenging because the number of rotations and the dimensions of the tube are dominated by a complex mismatched strain. A deposited nanofilm can be rolled up onto a tubular substrate by adhesioncontrolled transfer24-26, but the transfer methods are also limited by the difficulty of adhesion control without adhesives.27 Moreover, the delamination process can incur structural damage to the nanofilm. Therefore, the demand for an innovative roll-up technology for nanofilms is steadily increasing. In this work, we develop a method for layer-by-layer assembly of free-standing nanofilms by exploiting the water surface and rolling manner. The water surface is utilized as an ideal platform for rolling a nanofilm, allowing adhesion control and frictionless feeding. With this method, one can easily roll up various nanofilm materials (Figure 1a) onto diverse tubular materials with a diameter in the range of submillimeter to few millimeters (Figure 1b). Moreover, control of the number of layers is allowed by precision rolling apparatus, and unique structures of assembled nanofilm such as heterogeneous and spiral structures are attained. We demonstrate various applications including a stretchable tubular electrode, an electroactive polymer (EAP) tube actuator and a super-elastic nanofilm tube. The assembly method presented here would pave the way toward the development of many tubular applications based on thin films, such as compact


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electronic devices, high strength and toughness composite materials, actuators for a steerable microcatheter.

RESULTS AND DISCUSSION The layer-by-layer assembly of a free-standing nanofilm was performed as follows. First, a nanofilm was floated on a deionized (DI) water surface (See details in the Supporting Information, Section S1) and the water level was adjusted to reach the top of a container. Second, the floating nanofilm was gently attached to a tubular substrate by van der Waals adhesion (Figure 1c). Moreover, the process did not require any intervention of an adhesive and pre-wetting of the substrate. Third, the substrate was rotated to roll up the floating nanofilm onto the tube (Figure 1e). The low viscosity of water permitted frictionless feeding of the nanofilm during the entire process, therefore damage-free assembly and facile manipulation of the nanofilm were possible. The assembled nanofilm on the tubular substrate is shown in Figure 1f. The nanofilm assembly process is governed by the competition between the adhesion energy for the substrate-film interface, GcSubstrate/Film, and the adhesion energy for the water-film interface, GcWater/Film, (Figure 1d). The quantitative GcWater/Film and GcSubstrate/Film values can be estimated by calculating the work of adhesion9,28,29 which is defined as the required energy for disjoining a unit area of the interface (See the Supporting Information, Section S2). We calculated the work of adhesion of the Au nanofilm to various substrate materials and the water surface (Figure S2). The results indicate that many substrate materials adhere strongly to the Au nanofilm with higher adhesion energy than that of the water-Au interface. Moreover, GcWater/Film can be controlled by surface energy modulation of the water surface via addition of solvents or


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surfactants. Therefore, successful assembly of Au nanofilm is allowed. This investigation not only clarifies the mechanism of the nanofilm assembly but also suggests a design guide for integrating nanofilm layers onto diverse tubular substrates. The nanofilm, which was transferred by the adhesion competition, was rolled up by using the precision rolling apparatus described below. The experimental setup for implementation of the nanofilm rolling method is shown in Figure 2a. To support a pliable polymer tube without deflection, a rigid rod was inserted into the tube, and one end of the rod was coupled with a micro-motor which was mounted on a z-axis stage. After mounting the tubular substrate, the nanofilm floated on the water surface was gently attached with the tubular substrate by controlling the z-position (height) of the substrate. Finally, the nanofilm was rolled up onto the tubular substrate by the rotation transferred from the micro-motor (Movie S1). Figure 2b indicates the rolling-up process of 27-nm-thick Au film, and Figure 2c indicates the assembled 27-nm-thick Au film on a nafion tube that had a submillimeter size diameter. The rolling method enables both the integration of diverse nanofilm materials onto a tube and the control of the number of assembled layers, which are unattainable with conventional methods such as chemical vapor deposition, spin, immersive and spray coatings. Figures 2d and 2e confirm that Au and polystyrene (PS) nanofilms were rolled up on the nafion tube, respectively. Multilayer graphene which is a two-dimensional material, was also assembled by rolling (Figure 2f). Moreover, the number of assembled layers was controlled in the range of one to tens of layers by the rotation number of the micro-motor (Figures 2d-f, and 4c). The assembly processes were conducted in an extremely short time (approximately one second to assemble one layer), while conventional assembly methods consume several minutes to hours.30


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Furthermore, diverse designs of the rolled-up nanofilm structures were accomplished. First, a heterogeneous structure can be obtained by rolling up disparate nanofilm materials. We rolled up Au, poly (methyl methacrylate) (PMMA), and Au nanofilms in sequence (Figure 2g). The assembled nanofilm with a heterogeneous structure is shown in Figure 2h. By utilizing the heterogeneous structure, supercapacitors and composite materials can be developed. A tubular supercapacitor that has high capability and compactness would be fabricated by rolling up both electrode and electrolyte membranes. In addition, a composite material that has high strength and toughness would also be achieved by rolling up both organic and inorganic layers.31 Second, a spiral structure was attained by rolling up a tilted nanofilm strip (Figure 2i). Figure 2j represents the assembled 50-nm-thick Au film strip on a polydimethylsiloxane (PDMS) cylinder and nafion tube. The spiral electrode can be used for inductors and strain sensors due to its geometrical characteristics. The rolling method provides opportunities to develop novel applications by integrating a nanofilm on a functional polymer tube. To the best of our knowledge, integration of a stretchable nanofilm electrode onto a tubular substrate have not been reported yet. In this respect, we achieved a stretchable tubular electrode by rolling up a 50-nm-thick Au film onto a pre-strained nafion tube. To introduce pre-strain of the nafion tube, it was swelled into a mixture of DI water and isopropyl alcohol (IPA) for an hour (Figure 3a). The mixtures of DI water and IPA created a uniform swelling strain of the nafion tube in all directions, and the amount of pre-swelling strain was controlled by the weight percentage of IPA which has less polarity.32 The floating 50-nmthick Au film was assembled on the pre-swelled nafion tube (Figure 3b). Finally, the swelled nafion tube was dried at room temperature, and wrinkles were formed in the rolled-up Au electrode by shrinkage of the nafion tube (Figure 3c).


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The degree of wrinkle and stretchability of the rolled-up Au electrode were investigated by scanning electron micrographs and tensile testing, respectively. The pre-strain conditions of the nafion tube were controlled as a no pre-strain, a 4.8 % pre-strain and an 18.4 % pre-strain (See the Supporting Information, Section S4). Figure 3d shows the degree of wrinkle of the rolled-up Au electrode with respect to pre-strain conditions of the nafion tube. The rolled-up Au electrode on a dried nafion tube had smooth surfaces, whereas the degree of wrinkle increased gradually as the pre-strain of the nafion tube increased. Moreover, we measured the in situ electrical resistance of the rolled-up Au electrode under tension in order to evaluate stretchability (See the Supporting Information, Section S5). The results in Figure 3e indicate that the electrode that was fabricated with the 18.4% pre-strain condition maintained its electrical performance even when severe tensile strain was applied. In contrast, the electrode that was fabricated with no pre-strain conditions was continuously damaged with an increasing applied tensile strain. Furthermore, we realized fast fabrication of an EAP tube actuator with a submillimeter diameter, because the electrode integration process was significantly simplified by using the rolling method.33 An ionic liquid-containing polymer was used for the tubular substrate (See the Supporting Information, Section S6), and a 50-nm-thick Au film was rolled up as the electrode. To introduce the electrical potential difference in the EAP tube actuator, the electrode was separated as two parts using femtosecond laser pulses. When the voltage was applied, the bending motion of the EAP tube actuator occurred by migration of the ions inside the tube (Figure 3f).34 The demonstrated EAP tube actuator is shown in Figure 3g and Movie S2. The EAP tube actuator which has submillimeter diameter can be exploited for advanced biomedical applications such as a steerable micro-catheter for cerebral vascular surgery.


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So far, two unique applications have been introduced by assembling nanofilms onto functional polymer tubes. We also developed a tubular material which is composed of stacked nanofilm layers. The tubular material, namely a free-standing nanofilm tube, was manufactured by rollingup a large area 50-nm-thick Au film on a heat-shrink tube. After nanofilm assembly, the heatshrink tube was contracted in a 215 ºC convection oven for 15 minutes and the contracted tube was pulled out from the rolled-up nanofilm (Figure 4a). Even when the supporting substrate was removed, the remaining rolled-up nanofilm maintained the tubular shape because the large number of stacked layers provided sufficient rigidity (Movie S3). The obtained free-standing nanofilm tube and an enlarged view of the stacked twenty-two layers of 50-nm-thick Au film are shown in Figures 4b and 4c, respectively. The developed tubular material can be utilized to obtain nanofilm stacked structures by cutting in longitudinal direction and opening it (Figure S6). This method can reduce the process time significantly than conventional thin film stacking methods which are performed by multiple deposition or transfer.35,36 Interestingly, we found out the free-standing nanofilm tube has super-elastic properties even if it was composed of the Au film which is easy to undergo permanent deformation. This outstanding elastic property was verified by applying compression to the free-standing nanofilm tube via a steel rod (Figure 4d). Although, compressive loading deformed the nanofilm tube severely, the tube recovered its original shape as the compression was removed (Figure 4f; see also Movie S4). The mechanism of this phenomenon was revealed by considering strain decoupling effects between the stacked nanofilm layers.37 When the compression was applied to the free-standing nanofilm tube, sliding between the rolled-up nanofilm layers occurred because they were attached by weak van der Waals adhesion. The interlayer sliding introduced strain decoupling of rolled-up nanofilm (Figure 4e). As a result, each layer underwent a small


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deformation in the elastic region and super-elastic properties were assigned. In contrast, an interlayer bonded structure deformed permanently because the compressive loading caused a large strain to the tubular structure (see Supporting Information, Figure S8). The free-standing nanofilm tube is a macroscale material, but it also exhibits nanoscale characteristics. Accordingly, the nanofilm tube material possesses extraordinary performance which originated from its multi-scale nature.

CONCLUSIONS We have developed a powerful way to assemble free-standing nanofilms onto tubular substrates by utilizing the water surface and controlled rolling manner. The nanofilms floated on the water surface were assembled on the tubular materials by van der Waals adhesion. The method enabled not only layer-by-layer stacking of diverse nanofilm materials, but also heterogeneous and spiral structures of rolled-up nanofilms. We have further demonstrated a stretchable electrode and an EAP actuator by using functional polymer tubes, and have revealed that a rolled-up nanofilm tube possesses extraordinary mechanical properties. We expect these results to enable novel designs and applications by utilizing a tubular shape in a wide variety of fields such as composite materials, supercapacitors, sensors, low-loss transmission lines, bio-medical and wearable devices.


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Figure 1. Schematic of layer-by-layer assembly of nanofilms on a tubular substrate by the rolling method. Representative materials for (a) nanofilms and (b) tubular substrates. (c) Contacting of the nanofilm floated on water with the tubular substrate. (d) Mechanism of nanofilm rolling. Successful assembly occurs when GcSubstrate/Film is higher than GcWater/Film. (e) Rolling-up process of the floating nanofilm. (f) The assembled nanofilm on the tubular substrate.


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Figure 2. Implementation of nanofilm rolling and its versatility. Photographs of the actual implementation process for a 27-nm-thick Au film assembly onto a nafion tube (diameter of 0.9 mm); (a) the precision rolling apparatus, (b) rolling-up process and (c) assembled nanofilm. (d), (e) Optical micrographs of the assembled 27-nm-thick Au and 46-nm-thick PS films onto the nafion tube, respectively. The number of rolled-up layers and the boundary between layers (white dotted line) are indicated. (f) Scanning electron micrographs of the assembled multilayer graphene onto the nafion tube and its enlarged view. The inset graph is the Raman spectra of multilayer graphene on the nafion tube. (g) Schematic of the nanofilm assembly process for a heterogeneous structure. (h) Photograph of the assembled heterogeneous structure, which is fabricated by rolling up (1) 50-nm-thick Au, (2) 180-nm-thick PMMA and (3) 50-nm-thick Au films in sequence. The left schematic represent the cross section of the structure. (i) Schematic of the process to assemble the nanofilm strip with a spiral structure. (j) Photographs of the assembled spiral nanofilm strip onto a PDMS cylinder (diameter of 3 mm; left) and a nafion tube (right). Scale bars, 5 mm.


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Figure 3. Development of a stretchable electrode and an actuator. Schematic of the fabrication process for the stretchable tubular electrode; (a) the nafion tube is pre-swelled in a selective solution; (b) the nanofilm is rolled up on a swelled nafion tube and (c) the wrinkles in the nanofilm are formed by dehumidification of the nafion tube. (d) Scanning electron micrographs of the degree of surface wrinkles of rolled-up Au nanofilm with respect to the pre-swelling strain. Scale bar, 10 µm. (e) Normalized electrical resistance-tensile strain curves of the tubular electrode with various pre-swelling strain conditions. (f) Schematic of the fabrication and operation processes for the EAP tube actuator. (g) Photographs of the designed tubular EAP actuator (diameter of 0.8 mm). The initial state without bending motion (left) and actuation by applying 18 V of electric potential (right).


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Figure 4. Free-standing nanofilm tube. (a) Schematic of the fabrication process for a freestanding nanofilm tube; see the main text for a detailed description. (b) Scanning electron micrographs of the free-standing nanofilm tube and (c) its enlarged view. (d) Photographs of compressed the free-standing Au nanofilm tube via a rigid steel rod. (e) Schematic of the shape recovery mechanism; interlayer sliding and strain decoupling. (f) Photograph of the fully recovered free-standing Au nanofilm tube after unloading.


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ASSOCIATED CONTENT The Supporting Information is available free of charge. - Processes for nanofilm floating on a water surface, calculation of work of adhesion, preparation and thickness measurement of nanofilms, swelling strain of a nafion tube, in situ electrical resistance measurement of a tubular electrode under tension, fabrication of an ionic liquidcontaining polymer tube, stacking of nanofilms, interlayer bonded tube (PDF) - Supplementary Movies S1 – S5 (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions T.-S.K. and S.K. conceived and developed the project. S.K. performed the nanofilm rolling experiments, characterized the samples, analyzed the results, and demonstrated the stretchable tubular electrode and the free-standing nanofilm tube. J.-B.P. designed and fabricated electroactive polymer tube actuator. T.-S.K. supervised the research. T.-S.K. and S.K. wrote the paper. All authors commented on the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work was supported by the Wearable Platform Materials Technology Center (2016R1A5A1009926), the Graphene Materials and Components Development Program of MOTIE/KEIT (10044412, Development of basic and applied technologies for OLEDs with graphene), the Basic Science Research Program (2015R1A1A1A05001115) funded by the National Research Foundation (NRF) under the Ministry of Science, ICT, and the KAIST High Risk High Return Project (HRHRP).

ABBREVIATIONS EAP, electroactive polymer.

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Layer-by-Layer Assembly of Free-Standing Nanofilms by Controlled

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