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Material-Independent Nano-Transfer onto a Flexible Substrate Using Mechanical-Interlocking Structure Min-Ho Seo, Seon-Jin Choi, Sang Hyun Park, Jae-Young Yoo, Sung Kyu Lim, Jae-Shin Lee, Kwang-Wook Choi, Min-Seung Jo, Il-Doo Kim, and Jun-Bo Yoon ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00159 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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ACS Nano

Material-Independent Nano-Transfer onto a Flexible Substrate Using Mechanical-Interlocking Structure

Min-Ho Seo†, Seon-Jin Choi∥, Sang Hyun Park‡, Jae-Young Yoo†, Sung Kyu Lim‡, Jae-Shin Lee†, Kwang-Wook Choi†, Min-Seung Jo†, Il-Doo Kim§, and Jun-Bo Yoon†*

†

School of Electrical Engineering, Korea Advanced Institute of Science and Technology

(KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

∥

Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139,

USA

‡

National NanoFab Center (NNFC), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic

of Korea

§

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

Keywords: nanowire-array, transfer, amorphous carbon, flexible heater, flexible gas-sensor 1 ACS Paragon Plus Environment

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Abstract Nanowire-transfer technology has received much attention thanks to its capability to fabricate high-performance flexible nano-devices with high simplicity and throughput. However, it is still challenging to extend the conventional nanowire-transfer method to the fabrication of a wide range of devices since a chemical-adhesion-based nanowiretransfer mechanism is complex and time-consuming, hindering successful transfer of diverse nanowires made of various materials. Here, we introduce a materialindependent mechanical-interlocking

based nanowire-transfer (MINT) method,

fabricating ultralong and fully aligned nanowires on a large flexible substrate (2.5 × 2 cm2) in a highly robust manner. For the material-independent nano-transfer, we developed a mechanics-based nano-transfer method, which employs an dry-removable amorphous carbon sacrificial layer between a vacuum-deposited nanowire and the underlying master mold. The controlled etching of the sacrificial layer enables the formation of a mechanical-interlocking structure under the nanowire, facilitating peeling-off of the nanowire from the master mold robustly and reliably. Using the developed

MINT

method,

we

successfully

fabricated

various

metallic

and

semiconductor nanowire arrays on flexible substrates. We further demonstrated that the developed method is well suited to the reliable fabrication of highly flexible and high-performance nano-electronic devices. As examples, a fully aligned gold (Au) microheater array exhibited high bending stability (106 cycling) and ultrafast (~220 ms) heating operation up to ~100 °C. Ultralong Au heater-embedded cuprous-oxide (Cu2O) nanowire chemical gas sensor showed significantly improved reversible reaction kinetics toward NO2 with 10-fold enhancement in sensitivity at 100 °C.

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Nanowires (NWs) have received great attention as an indispensable element for flexible and wearable devices that require light-weight, bending, and elongation properties since they have small size, low mass, and mechanical stability as well as excellent physical properties.1–3 Using various shapes and materials of NWs, many studies have successfully demonstrated performance-enhanced flexible devices, such as high-efficiency energy generators,4 highly sensitive chemical sensors,5 and flexible/transparent tactile sensor devices.6 To widen the ranges of NW-applications at the device-level and achieve higher device performance, it is very important to control the composition,7 crystallinity,8 and geometry9 of NWs integrated on flexible substrates. In addition, recent applications of NWs require robust and reproducible fabrication of highly aligned NW-arrays defined at specific locations over large areas with minimal imperfections, such as material loss.10, 11 Therefore, a way to fabricate fully aligned, ultralong NW-arrays made of various materials on large-area flexible substrates is urgently needed to meet the stringent demands of high-performance flexible applications.12 Among various emerging nanofabrication processes,13–15 a master-mold–based nanotransfer method has received considerable attention because of its ability to fabricate highly ordered NW-arrays on flexible substrates with simplicity and cost-effectiveness.16–19 Using this method, a few NW-arrays are transferred onto a flexible substrate and used to demonstrate high-performance devices, such as flexible chemical sensors and flexible solar cells.20, 21 Despite its simplicity and utility, the master-mold based nano-transfer method still suffers from a limited material selection because of its chemistry-based NW-transfer mechanism. To achieve a successful transfer, adequate chemicals, able to control chemical interfacial adhesion between the master mold, NW, and flexible substrate, must be present. A weak bond is necessary between the NW and the master-mold, whereas a strong bond is required between the NW and the flexible substrate at the same time, through chemical 3 ACS Paragon Plus Environment

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treatment.19 Also, even when a chemical-material pair is found, the chemical treatment is sometimes time-consuming and complex, and it may only be possible on a limited area due to uniformity and consistency issues.22-25 Therefore, the method is limited to the fabrication of very few metallic NWs, hindering more diverse applications. To overcome the limited material selection issue, several research groups have improved the master-mold based transfer method by adopting various approaches. For example, Jung et al. proposed a solvent-assisted nanotransfer method, which transfers silver (Ag), gold (Au), and palladium (Pd) NWs through a solvent-dissolvable polymer master mold.22 More recently, Hwang et al. developed a covalent bonding-assisted nanotransfer technique, which employs an adhesion layer between the NW and flexible substrate to fabricate Au, aluminum (Al), and Ag NWs on a flexible substrate.23 Even though more variety NWs were successfully transferred by the reported studies, however, these methods are still based on a transfer-mechanism of the chemical interaction between the NW and the target flexible substrate; hence, an additional chemical treatment is unavoidable. Moreover, they cannot be used for a diverse range of materials, such as crystalline semiconductors, because they use polymeric master molds, which limits the ability to apply additional thermal treatments to control material properties, such as crystallinity, phase, and defect density.10 Herein, we report a material-independent nano-transfer method to fabricate ultralong, fully-aligned NWs on a flexible substrate via a dry-removable nano-sacrificial layer located between the master-mold and NWs. Owing to the selectively removable nano-sacrificial layer, mechanically controllable adhesion between the master-mold and NWs can be achieved, facilitating robust transfer of the NWs from the master-mold onto the flexible substrate. In particular, the mechanical adhesion-control enables the mechanical-interlocking based nanowire transfer (MINT) method to simply fabricate NWs from a diverse range of materials without any chemical consideration. Using the proposed MINT method, we demonstrated the 4 ACS Paragon Plus Environment

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fabrication of fully-aligned metal and metal-oxide NWs on a large-area flexible substrate with high yield. To demonstrate the robustness and versatility of MINT, we fabricated flexible Au NW heater arrays and Au NW heater-embedded chemical gas-sensor devices with copper oxide (Cu2O) NWs as a sensing structure. The developed heater arrays showed bending-insensitive, low-voltage, and ultrafast heating and cooling characteristics, which are essential for flexible mobile applications, with high uniformity and reproducibility. In addition, the Cu2O NW sensors exhibited reversible NO2 reaction with greatly enhanced reaction kinetics assisted by effective heat generation from the embedded Au NW heater.

Results and Discussion Figure 1a schematically presents the proposed MINT process, and Figure 1b shows the important steps in detail. The MINT method is based on a transfer method in which a NW is formed on the master mold by a vacuum-deposition technique, such as physical vapor deposition (PVD), and then transferred to a flexible substrate. The fabrication process begins with fabrication of a large-scale silicon (Si) master mold (Figure 1a i). The Si master mold has nanograting patterns of 400 nm (150 nm line and 250 nm space) in pitch, made by conventional KrF lithography. Then, sequential deposition of a sacrificial layer (Figure 1a ii) and the NW (Figure 1a iii) is carried out. The key concept of the MINT method is the use of amorphous carbon (a-C) as the nano-sacrificial layer between the master mold and the NW. We use a-C because it is a metastable solid having a nano-crystalline structure of hybridized carbons,26–28 so it is easily oxidized by oxygen plasma and vaporized into CO or CO2 gas phase.29, 30 It should be noted that those deposition and etching characteristics of a-C make it suitable as a sacrificial layer material, which cannot be easily achieved by typical organic and inorganic materials because of the insufficient fabrication stability and poor etching selectivity to others materials, respectively.10 In case of the MINT, the side-wall of the a-C 5 ACS Paragon Plus Environment

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sacrificial layer beneath the NW is exposed to the ambient atmosphere after the NW deposition, so it can be readily vaporized and removed by oxygen plasma (Figure 1a iv and Figure 1b iii–iv). Therefore, the undercut of the a-C sacrificial layer beneath the NW is easily made without any degradation of the NW or master mold (Figure 1b iv). In particular, since a-C vaporization is performed under dry conditions, the degree of undercut can be uniformly and precisely controlled over a large area of the specimen by controlling the plasma treatment time or power. The undercut further forms the mechanical interlocking structure of the NW when embedded in the flexible substrate in the following steps (Figure 1a v, vi and Figure 1b vi). A liquid-state poly-urethane acrylate (PUA) template polymer is poured on the oxygen plasmatreated specimen (Figure 1a v) and it is covered with a polyethylene terephthalate (PET) backing film (Figure 1a vi); the PUA and PET become the flexible template and backing layer, respectively, and they compose the monolithic flexible substrate which is finally formed by UV curing. The liquid-state PUA easily slides into the undercut of the a-C sacrificial layer. Thus, a unique mechanical-interlocking structure of NWs and a flexible substrate can easily be formed (Figure 1b iv). The NW grabbing force of the PUA template with the mechanical-interlocking structure is much stronger than the adhesion between the NWs and the master mold, generating a significantly larger critical energy release rate of ‘PUA template/NWs’ in comparison to that of the ‘NWs/master mold’. Thus, the NW-array is robustly transferred onto the flexible template when the backing PET substrate is simply peeled off without being affected by the peeling off velocity (Figure 1a vii, viii and Figure 1b vii).31-33 Note that the NW-array formed on the a-C deposited master mold can endure thermal treatment at an appropriate temperature because a-C is physically and chemically stable up to about 200 °C (Figure S1). This is important in the sense that we can thermally


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treat NWs to obtain further enhancement of the material characteristics owing to the crystallization.10 To verify the MINT method, we fabricated ultralong, fully aligned Au NW-arrays on a flexible substrate using the proposed process. The detailed fabrication conditions are described in the Experimental Methods section. By optical imaging and X-ray photoelectron spectroscopy (XPS), we confirmed that a uniform and sp2/sp3 hybridized amorphous carbon was deposited on the Si master mold through our plasma-enhanced chemical vapor deposition (PECVD) system (Figure S2). Figure 1c shows the results of the transferred Au NW-array. Optically distinct yellow square domains of Au-NW on the flexible substrate were transferred from the master mold, and neither the transfer template nor the master mold contained any detectable residue or defects (Figure S3). Top views of the various scanning electron microscope (SEM) images (inset in Figure 1c and Figure S3a) revealed perfectly aligned Au line patterns on the large-area flexible substrate. A cross-sectional SEM image of the transferred sample confirms that a mechanical-interlocking structure of the Au NWs in PUA was successfully realized, and the method achieved the robust transfer of NWs with fully aligned configuration (Figure 1d). We also investigated the reusability of the master mold of the MINT process. Any residues in the used master mold were removed by subsequent processes of wet-chemical etching, oxygen plasma etching, and standard piranha cleaning. After the cleaning processes, we observed neither marked physical damage nor chemical contamination of the master mold (Figure S3b). These results indicate that we can recycle the master mold, which makes the MINT process more cost effective. To accomplish the MINT process successfully, the thickness and etch-distance of the a-C sacrificial layer should be optimized; since the undercut is formed according to the thickness of the a-C layer, and the mechanical-adhesion between the NW and master mold is determined by the etch-distance of the a-C layer. Thanks to the advanced CMOS-compatible 7 ACS Paragon Plus Environment

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MINT process, we could perform a process simulation first using Silvaco to determine the proper conditions for the formation of the sacrificial layer undercut prior to actual fabrication (Figure 2a). If the thickness of the deposited a-C sacrificial layer is not sufficient (20 nmthick), the sidewall of the a-C is fully covered by the subsequent deposition for the NWformation and is not exposed to the ambient atmosphere, hindering the undercut formation, which prevents successful transfer (Top in Figure 2a). On the other hand, as the thickness of the a-C layer increases (80 nm-thick), the a-C profile begins to have a laterally protruding shape. This is because of the anisotropic deposition characteristic of the performed PECVD method. When PECVD is performed on a three-dimensional substrate, such as the nanograting master mold, varying deposition-rates on the top and side surfaces occur, thus making a laterally protruding profile at the deposited a-C corner, which is called the “breadloafing effect”.34 The protruding a-C can generate a shadowing effect when additional material (for example, Au in Figure 2a) is subsequently deposited.35 As a result, the sidewalls of the a-C layer under the NW material are exposed to the ambient gas; thus, the a-C can be dry etched by oxygen plasma treatment (Bottom in Figure 2a). To ensure the shadowing effect, a 10 nm protrusion is designed in the a-C layer which corresponds to the deposition of an 80 nm-thick a-C layer, as shown in Figure 2b. The effects of various a-C layer thicknesses on the sidewall exposition are shown in Figure S4. The a-C etch-distance for the mechanical transfer was also optimized by a combination of a COMSOL finite-element method (FEM) simulation and an experiment. In the proposed MINT process, the NW transfer is performed by breaking the remaining a-C layer between the NW and the master mold; therefore, the optimized oxygen plasma (a-C etching) condition, forming the mechanically weak a-C sacrificial layer, defined as ‘support’, is essential. For the optimal condition, we calculated the induced mechanical stress transitions in the a-C layer with respect to the size of the a-C support (Figure 2c). For the simulation, we 8 ACS Paragon Plus Environment

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designed the PUA to cover an 80 nm-thick Au NW, an 80 nm-thick a-C sacrificial layer, on a 150 nm-wide Si nanograting master mold, and the peeling off force is applied upward on the PUA. Without the a-C etching, a uniform stress is induced in the entire structure when the peeling off force is applied (upper panel of Figure 2c). However, a considerably high stress is induced when the a-C etching is done and the force is concentrated to the narrow a-C support (bottom panel). In particular, when the width of the a-C support becomes about 10 nm, the calculated von Mises stress (σ) induced in the a-C support reaches more than 30 GPa, which is much higher than the well-known fracture stress of the a-C of 20 GPa,36 guaranteeing successful mechanics-based NW transfer. We experimentally verified the simulated results. Figure 3d shows the etched distance (d) of the a-C with respect to the oxygen plasma etch time. The oxygen plasma treatment was performed with 80 W and a 50 sccm oxygen flow rate. The evaluated etch rate was about 0.8 nm/min so that we conducted oxygen plasma etching for 90 min to achieve a 10 nm-wide a-C support as shown in Figure 2e. To verify the importance of the a-C layer etching time, we performed the transfer process with different aC supports made with different etching times, and the results are shown in Figure 2f. The upper and lower panels of Figure 2f show optical microscope images and their cross-sectional SEM images, respectively. Without etching of the a-C layer, no mechanical-interlocking structure was formed, which resulted in complete failure of the NW transfer to the flexible substrate. As a result, a blue-colored PUA which did not have any Au nanowires was observed. In contrast, the transfer was gradually more successful as the plasma etching time increased, and perfect NW transfer was achieved when the a-C layer was etched for 90 min. We performed the transfer test with respect to the a-C etching time using three units of samples, and the transfer-yield results are presented quantitatively in Figure 2g. When no a-C layer etching was conducted, no NW was transferred (0 % yield). However, the transfer yield became higher as the a-C etching time was increased. Finally, 100 % yield transfer was 9 ACS Paragon Plus Environment

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achieved in the samples subjected to 90 min of oxygen plasma treatment (bottom panel in Figure 2g). These result can be easily explained by the previously mentioned fact that the stress induced in the a-C support should exceed the fracture stress of the a-C layer to increase the transfer yield, which is also shown in the upper-side graph in Figure 2g. The yield inspection of the transferred Au NW is also shown in Figure S5 in detail. In this study, the MINT process was performed using the 150 nm-wide nanograting master mold, but the MINT process can also be used to fabricate wider NWs because oxygen ions can be smoothly diffused under the NWs based on the high diffusivity under a dry condition.37 The developed MINT process provides facile and reliable transfer of a variety of NWs onto flexible substrates since the formation of NWs is based on the PVD method; any material that can be deposited with PVD is suitable for NW fabrication. In addition, it is possible to transfer thermally-treated crystalline semiconductor NWs, such as metal oxides, onto a flexible substrate by controlling the material properties of the NWs since the a-C sacrificial layer is physically and chemically stable for heat treatment up to 200 °C.10, 38 To demonstrate a wide range of material-applicability of the proposed method, we fabricated various metallic (platinum Pt, copper Cu) and semiconductor (cuprous-oxide Cu2O) NWarrays on flexible substrates as shown in Figure 3a-d. Note that the Cu2O NWs were formed on the a-C deposited master mold by 200 °C annealing of Cu NW, and its high-quality polycrystallinity was confirmed by X-ray diffraction (XRD) analysis (Figure 3d). Figure 3a-c show SEM and optical (inset) images of the fabricated NW-arrays on flexible substrates, respectively. From top surface SEM images, we confirmed that fully aligned Pt, Cu, and Cu2O NW-arrays were successfully formed on flexible substrates without significant failure, such as material-loss. It should be noted that the roughness of sample surfaces originated from the Pt sputtering treatment for SEM measurement. Moreover, defect-sites such as particles and cracks, were caused by contaminants, such as dust in air ambient and the 10 ACS Paragon Plus Environment

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induced stress during the peeling-off step (Figure 1a vii), respectively. The detailed fabrication conditions of each NW are given in the Experimental Methods section, and a wide range of top-view SEM images of the transferred Pt and Cu2O NWs are shown in Figure S6. The proposed MINT process also enables the fabrication of other nanostructures, such as nanodot-arrays, on a flexible substrate as shown in Supporting Information Figure S7. So far, we verified that the proposed MINT process can be used to fabricate a wide material-range of NWs on a flexible substrate in terms of manufacturing simplicity and robustness. Despite the successful results, the a-C sacrificial layer and oxygen plasma treatment used in this study would not be suitable for the fabrication of a few types of NWs that can be damaged by oxygen plasma treatment. However, as the MINT process is based on micro-/nano-electromechanical systems (M/NEMS) fabrication, there is a high potential to solve this issue if an appropriate sacrificial layer material is secured that meets the following requirements: i) anisotropic deposition with high uniformity, ii) selective etching, and iii) thermal stability. Among the various issues in nanomaterial-based electronics, the fabrication of NW devices with high yield, fabrication-reliability, and repeatability is a major concern.39, 40 The aligned and dense NW-arrays, made by the proposed MINT process, enable the fabrication of various devices in a reliable manner. To demonstrate the method’s manufacturing reliability, we applied it to fabricate a high-performance flexible micro-heater array, which has a wide range of possible applications, such as lab on a chip,41 mobile gas sensors,42 and braille display devices.43 The proposed heater has a fully aligned metallic NWs heating part, as shown in Figure 4a, which allows ultrafast and low voltage heating operation with bendinginsensitivity, in comparison with a conventional NW-heater based on randomly networked NWs.44–47 To fabricate the proposed flexible micro-heater array, we formed an electrode-set at both ends of the aligned NW-array as shown in Figure 4b. Note that the embedded Au 11 ACS Paragon Plus Environment

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NWs in the PUA polymer were exposed by additional oxygen plasma so that they could make an ohmic contact with the electrodes (See Supporting Information Figure S8). Figure 4c shows the fabricated 100 units of the micro-heater array on a 1 cm2 area. An optical image of the fabricated heater array with 300 µm in the electrode distance and its SEM image are shown in Figure 4c and inset, respectively. Supporting Information Figure S9 shows the SEM image of the device array in detail. We first evaluated the uniformity/yield of the fabricated devices by means of measuring on-resistance of the heaters using a programmable electrometer (Keithley 6514). Thanks to the fact that each heater device had the same length (length d=300 µm) and density of NWs, the measured 100 units of the devices showed highly uniform electrical characteristics (Figure 4d). The average on-resistance was about 5.75 Ω, and its standard deviation was 0.52. The MINT process also shows high manufacturing repeatability. We also fabricated another device array (batch) under the same conditions and confirmed their electrical characteristics. The devices from two different batches showed almost the same electrical I-V characteristic (Figure 4e), indicating that the MINT method has very high repeatability. The robust transfer result of ultralong NWs enables the fabrication of heater devices of various sizes, having different lengths (d=400, 700, 1100, and 1400 µm). As shown in Figure 4f, the fabricated devices exhibited linearly increased electrical resistance with respect to their lengths, indicating that scalable devices can be fabricated by the MINT process. We evaluated the electro-thermal performance of the fabricated flexible micro-heater device. Using a programmable electrometer, variable electrical potential was applied to the micro-heater, made of a 300 µm-long Au NW-array. The induced Joule-heating characteristics of the NWs were measured using a customized infrared detecting system. Figure S10 in the Supporting Information gives detailed information about the measurement 12 ACS Paragon Plus Environment

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set-up and measured IR data. Figure 5a shows the measured temperatures of the heater with respect to various static-voltages; no hysteresis was observed between temperature rise and fall. The measured steady-state temperatures were 26, 29, 39, 54, and 100 °C when the applied voltages were 0.2, 0.4, 0.6, 0.8, and 1.0 V, respectively. As an important parameter of the flexible heater, the response time of the device was evaluated by applying a voltage of 1.0 V. It was revealed that the temperature reached a steady-state temperature within 220 ms in the temperature range from 27 to 100 °C. The developed device maintained low-voltage and ultrafast heating operation with high reliability. During repeated on-off operation (100 cycles) from room temperature to 100 °C, the device showed stable heating characteristics and a fast response time of 100 °C and 220 ms, respectively (Figure 5c). The heating stability under continuously applied voltage was evaluated. As shown in Figure 5d, the on-state temperature of the device was stably maintained for 5 minutes. Until 24 hours, the on-resistance stayed within 5.4 % of its original value when 1.0 V was applied, which corresponds to 100 °C (inset in Figure 5d). To investigate its mechanical stability and bending-insensitivity, the heater was operated under various bending states. To confirm its bending-insensitive and stable heating operation, the device was mounted on various curved surfaces and bent in the NW alignment direction. Then the on-resistance of the device was measured under constant applied voltage at 1.0 V (see inset in Figure 5e). The on-resistance with respect to the radius of curvature (ROC) of the curved surface revealed that the device did not show significant resistance change in the range from ROC=∞ (flat-state) to ROC=2.5 mm (highly bent), indicating the on-temperature of 100 °C in all bending situations (Figure 5e). Note that the bend device performance along the length direction of NW was also evaluated, but the performance was not significantly changed because of the ductility of the Au NWs and their low piezo-resistive


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characteristic (see Supporting Information Figure S11). The bending reliability was also characterized using a custom-made dynamic bending machine under the repeated bending by ROC=3 mm and an applied voltage of 0.1 V (Figure 5f). During 1,000,000 cycles, the device did not show severe resistance change or structural damage. We summarized the performance of the heater and compared it with that of other nanostructure-based flexible heaters (see Supporting Information Table S1). Owing to the unique fully-aligned NW structure, our device showed much better performance in terms of operation voltage, response-time, flexibility, and ease of manufacturing, which are essential for the operation of low-power mobile and wearable applications. To further demonstrate the versatility of the MINT technique, we developed semiconductor metal oxide NWs-based flexible gas sensors integrated with the low-voltage operating Au NW heater. The flexible gas sensors were fabricated using Au and Cu2O NWs as the heating and sensing parts, respectively, to detect nitrogen-dioxide (NO2) with improved sensitivity and reversibility. The schematics of the device architecture and the real gas sensor image are presented in Figure 6a and Figure S12 in the Supporting Information, respectively. The resistance transitions of the Cu2O NW-based sensor at various heater operation temperatures was investigated with a measurement setup shown in Supporting Information Figure S13. Owing to the large-area robust MINT transfer, gradually increasing temperature was observed under applied voltage to the centimeter-scale Au NW heater in the range from 0.0 V to 0.6 V (upper panels in Figure 6b). The fully connected and aligned NWs also enable the temperature to be increased to about 100 °C by a bias voltage of 0.6 V with a fast response-time of 0.2 s. Accordingly, step-like decreasing resistance was observed using the Cu2O NW at various operating temperatures controlled by the Au NW heater (lower panel in Figure 6b). While the resistance of the Cu2O NWs was about 700 kΩ at room temperature without the heater operation, the resistance decreased down to about 40 kΩ when the heater 14 ACS Paragon Plus Environment

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generated 100 °C, which is a typical behavior of semiconductor metal oxide. The real-time resistance changes corresponding to the heater operation confirmed the effective heat delivery to the sensing part, which leads to acceleration of reaction kinetics toward chemical species. To demonstrate temperature-controlled NO2 adsorption and desorption properties, the resistance transitions of the Cu2O NWs were evaluated at various heater temperatures (Supporting Information Figure S14). The heater temperature was controlled by the application of voltages of 0 V (room temperature), 0.3 V (~50 °C), 0.5 V (~80 °C), and 0.6 V (~100 °C) to the Au NW heater. To compare the temperature-controlled NO2 sensing performance with that of other devices, we further calculated the normalized resistance changes as defined by sensitivity (S), i.e., S=[(Rair-Rgas)/Rair]×100 %, where Rair is the initial baseline resistance in air ambient, and Rgas is the resistance of the sensor in NO2 ambient. At room temperature, the measured resistance of the Cu2O NWs sensing part did not show appreciable changes upon cyclic NO2 exposure. However, the sensor device gradually exhibited a noticeable response and recovery characteristics toward NO2 as the applied voltage increased. The Cu2O NWs at 0.6 V (~100 °C) exhibited the highest sensitivity of about 35% at 20 ppm of NO2, and this value represents a 10-fold enhanced sensitivity in comparison to the sensitivity at room temperature. These results can be attributed to the improved reaction kinetics of the Cu2O NWs and the accelerated adsorption and desorption processes toward NO2 molecules due to the external heat source. Based on the sensitivity values of the device at the first reaction toward NO2, the adsorption and desorption kinetics were evaluated with Equations (1) and (2):42 = exp [− ∙ ], = ∙

1 − ! "−

(1)


∙ ∙ #$,

(2)

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where S0 is the sensitivity when NO2 is removed, Smax is the maximum sensitivity, Ca is the NO2 concentration, kdes is the desorption rate constant, kads is the adsorption rate constant, and K=kads/kdes is the equilibrium constant. The rate constants and the equilibrium constants were obtained by fitting the sensitivity curves to Equations (1) and (2) as shown in Figure 6d and Supporting Information Figure S15. All results including the rate constants and equilibrium constants are summarized in Table 1. It was revealed that 1.22-fold improvements were achieved for the adsorption kinetic. In particular, a greatly enhanced (4.69-fold) desorption kinetic was obtained during the recovery process operating at 100 °C (0.6 V) as compared to the room temperature (0.0 V), which demonstrates improved reversible NO2 reaction.

Conclusions In summary, we demonstrated a mechanical-interlocking based nanowire transfer (MINT) method to obtained ultralong (1-inch), large-area (2.5 × 2 cm2), and fully aligned NWs on flexible substrates with a variety of NW material choices. Importantly, this method employs mechanical-structure based adhesion control between the NWs and the master mold, enabling robust, simple, uniform, and quick NW transfer onto a flexible substrate with diverse NW materials. Using the MINT method, we demonstrated ultralong, dense, laterally aligned, inch-scale Au, Pt, Cu, and Cu2O NW-arrays on a flexible substrate. This technology also enabled us to perform high-temperature thermal treatment to induce crystallization, which is essential to improve the material properties of NWs, without any material contamination or adhesion issues. Consequently, we realized an ultralong, perfectly aligned, semiconductor metal-oxide NW (Cu2O) array that was crystallized at 200 °C temperature and then transferred onto a flexible substrate. To demonstrate the versatility of the MINT technique, flexible micro-heater arrays and heater-embedded chemical gas sensor devices were fabricated using centimeter-scale transferred Au NWs and Cu2O NWs, respectively. 16 ACS Paragon Plus Environment

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The flexible heater showed bending-insensitive, ultrafast, and low-voltage heating performance (25 to 100 °C operation within 220 ms) with high manufacturing reliability and repeatability. The heater-embedded gas sensor device also showed drastically improved performance, achieving 10-fold, 1.22-fold, and 4.69-fold enhancement in sensitivity, adsorption, and desorption kinetic characteristics, respectively, toward 20 ppm NO2. The high-performance heater and the sensor device confirm that the developed MINT process is not only robust, regardless of materials, but it also has practical utility for various device applications. We expect that our developed MINT process is suitable for application in various wearable and flexible electronics, such as sensors, transistors, and displays, with variety of material selection, including metal and semiconductor NWs.

Experimental Methods Experimental conditions of Au NW transfer. Amorphous carbon (a-C; 80 nm) and gold (Au; 80 nm) layers were deposited by PECVD and thermal evaporation (Korea vacuum tech.), respectively, on a pre-fabricated Si nanograting master mold (pitch: 400 nm, line-width: 150 nm, line-space: 250 nm) as the sacrificial layer and nanowires, respectively. The oxygen plasma treatment for a-C etching was performed using an ICP asher (Korea vacuum tech.) at a power of 80 W and an oxygen flow rate of 50 sccm, and with an exposure time of 90 min. Then, a flexible substrate, composed of a commercial UV-curable resin containing polyurethane acrylate (PUA; HC11M-J5, Minuta Technology Co., Ltd.) as a flexible template and polyethylene terephthalate (PET; 50 µm) as a backing layer, was formed. The PUA was cured by 400-mJ I-line (365 nm wavelength) UV exposure, and the PET was peeled off gently to transfer the NW-array to the flexible substrate.


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Master mold cleaning after the Au NW transfer. To clean the Si master mold, Au wetchemical etching, oxygen plasma treatment, and standard piranha cleaning were sequentially conducted on the master mold after Au NW transfer. Au wet-chemical etching was performed using 10:1 diluted Au etchant (Au etchant, CNCTECH Co.). After the Au etching, the specimen was rinsed with deionized (DI) water for 5 min to remove the chemical solution residue. Oxygen plasma treatment was performed at a power of 200 W and oxygen flow rate of 50 sccm for 20 min.

Conditions for the transfer of Pt, Cu, and Cu2O NWs. For the Pt and Cu NW transfer, all process details, including the thicknesses of the a-C sacrificial layer and flexible template, substrate formation, and peeling-off were the same for all materials except for the a-C etching time. The etch rate of a-C can be varied according to the upper NW material because of the catalyst reaction of a-C and nanowire-materials. The oxygen plasma treatment times were 7 min and 60 min for the Pt and Cu NW fabrication, respectively, and it was conducted using an ICP asher at a power of 80 W and an oxygen flow rate of 50 sccm. The Cu2O NW transfer process was similar to that of the Cu NW transfer. However, an additional annealing process was performed after Cu NW deposition on the a-C-deposited Si master mold at 200 °C in air ambient for 1 h using our home-made furnace. Then, a-C etching was performed at the same power and oxygen flow rate with an etching time of 3 min.

Acknowledgements This research was supported by the Basic Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (MSIT) (NRF2017R1A2B3005221). This research was also supported by the Open Innovation Lab Cooperation Project funded by the National NanoFab Center (NNFC). 18 ACS Paragon Plus Environment

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Supporting Information Cross sectional SEM images of PECVD-deposited a-C and its microstructural changes after 200 ºC annealing; optical image and XPS analysis of the deposited a-C; SEM images of the transferred Au nanowires and the master mold re-usability; various thicknesses of a-C layer; transfer-yield; transferred Pt and Cu2O nanowires; transferred Cu nanodot-array; oxygen plasma treatment effect on the PUA; the fabricated micro-heater array; set-up and measured thermal images of heater; I-V curves of the bent heater; optical photograph of the gas sensor; set-up for the NO2 gas sensing; temperature-controlled NO2 reaction; evaluated reaction kinetics; and performance comparisons of the developed NW-based micro-heater array (PDF)

Corresponding Author *E-mail: [email protected]

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Figure 1. a, b) Schematic illustrations of the proposed MINT process for wafer-scale fully aligned and ultralong nanowires on a flexible substrate without limitation of material selection. c) Photograph of the fabricated wafer-scale fully aligned and ultralong Au nanowire-array on a flexible substrate (Inset: surface SEM image. Scale bar=1 µm). d) Cross-


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sectional SEM image of the transferred Au nanowire-array. Mechanically interlocked Au nanowire with PUA polymer is indicated by red dashed line.

Figure 2. a) Silvaco simulation results of side-wall exposure in relation to a-C nanosacrificial layer thickness (80 nm-thick Au NW deposited on 20 nm-thick and 80 nm-thick aC nano-sacrificial layers as shown in top and bottom panels, respectively, and scale bar represents 100 nm). b) Experimental result of protrusion (r) formation with respect to a-C deposition thickness (t). c) von Mises stress COMSOL simulation result induced in the a-C 26 ACS Paragon Plus Environment

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nano-sacrificial layer with respect to a-C etching. (Scale bar represents 50 nm) d) a-C etchdistance experimental results with respect to etching time. Time linear etch rate of 0.8 nm/min was obtained from the linear fit. e) Cross-sectional SEM images of the a-C etching results. Top and bottom panels show states before and after 90 min a-C etching, respectively. f) Experimental influence of a-C etching on Au nanowire transfer onto flexible substrate. Optical microscope and cross-sectional SEM images are shown in upper and lower panels, respectively. Scale bar in the bottom pane represents 1 µm. g) Simulation result of the induced von Mises stress in the a-C with respect to the a-C support width (Top) and the experimental transfer yield with respect to a-C etching time (Bottom). The previously reported a-C fracture stress is indicated with the red-dashed line in the top panel.


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Figure 3. Material-versatility of the developed MINT process. Surface SEM images of the transferred Cu, Pt, and Cu2O NWs are shown in a, b, and c, respectively. Insets show optical photograph images of the transfer results. d) XRD analysis result of the fabricated Cu2O nanowires (Top: as Cu deposited, middle: after oxidation, and bottom: JCPDS cards on Cu (red squares) and Cu2O (blue solid circles).


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Figure 4. a,b) Schematic illustrations of the proposed flexible electro-thermal micro-heater (a) and its fabrication (b). The fully aligned ultralong Au nanowire-structure provides various advantages as a high-performance heater, including ultrafast, low-voltage, and bendinginsensitive operation. c) Photograph of the fabricated heater array (Inset: Magnified SEM image of two devices). d) Measured resistances (R) of the fabricated heaters (Inset shows the measured 100 units with red-shadow). The resistance was measured at 0.1 V static voltage. e) Measured I-V curves of devices selected from two different batches. f) Measured electrical resistance (R) of the fabricated heater with respect to the length (d) of the heating part (Inset: schematic illustration of the measurement set-up).


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Figure 5. a) Measured temperature of the 300 µm-length heater with respect to applied voltages. b) Dynamic response of the heater (top: applied voltage, bottom: temperature response). c, d) Electrical reliability of the fabricated heater. Heaters showed stable heating up and down operations under 100-times dynamically repeated bias-voltage of 1.0 V (c) and static electrical stresses of 1.0 V for 24 h (d). e) Measured electrical resistance (R) of the 30 ACS Paragon Plus Environment

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device, bent to various ROC (1.0 V bias-voltage). f) Measured electrical resistance (R) during 1 million dynamic bending and release motions.


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Figure 6. a) Schematic illustrations of the proposed Au nanowire heater-embedded Cu2Onanowir- based NO2-gas sensor device (right) and its structure (left). b) Measured temperature of the heater with various applied voltages (upper panel) and electrical resistance response of the sensing part (Cu2O nanowires) in relation to heater operations. c) Measured sensitivity of the sensor device with various heater temperature operations against NO2 concentration transitions. d) Evaluated adsorption and desorption kinetics of the device by NO2 of 20 ppm. Improved adsorption and desorption kinetics were confirmed for 100 ºC heater operation (upper panel) in comparison to those of 25 ºC heat operation (lower panel).


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Table 1. Adsorption rate constant (kads), desorption rate constant (kdes), and equilibrium constant (K=Kads/Kdes) of the heater-embedded sensor device toward 20 ppm of NO2 at various temperature (applied voltages).

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