Three-Dimensional, High-Resolution Printing of Carbon Nanotube

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Three-Dimensional, High-Resolution Printing of Carbon NanotubeLiquid Metal Composites with Mechanical and Electrical Reinforcement Young-Geun Park, Hyegi Min, Hyobeom Kim, Anar Zhexembekova, Chang Young Lee, and Jang-Ung Park Nano Lett., Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Three-Dimensional, High-Resolution Printing of Carbon Nanotube-Liquid Metal Composites with Mechanical and Electrical Reinforcement Young-Geun Park,1, 2† Hyegi Min, 3† Hyobeom Kim,1,2 Anar Zhexembekova, 3 Chang Young Lee, 3*

and Jang-Ung Park1,2*

1Nano

Science Technology Institute, Department of Materials Science and Engineering, Yonsei

University, Seoul 03722, Republic of Korea 2Center

for Nanomedicine, Institute for Basic Science (IBS), Seoul, 03722, Republic of Korea

3School

of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology

(UNIST), Ulsan 44919, Republic of Korea

†These

authors contributed equally to this work.

*Corresponding

author. E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT The formation of three-dimensional (3D) interconnections are essential in integrated circuit packaging technology. However, conventional interconnection methods, including wire-bonding, were developed for rigid structures of electronic devices, and they are not applicable to the integration of soft and stretchable electronic devices. Hence, there is a strong demand for 3D interconnection technology that is applicable to soft, stretchable electronic devices. Herein, we introduce the material and the processing required for stretchable 3D interconnections on the soft forms of devices and substrates with high resolutions. Liquid metal-based composites for use as stretchable interconnection materials were developed by uniformly dispersing Pt-decorated carbon nanotubes in a liquid metal matrix. The inclusion of carbon nanotubes in the liquid metal improves the mechanical strength of the composite, thereby overcoming the limitation of the liquid metal that has low mechanical strength. The composites can be 3D printed with various dimensions; the minimum diameters are about 5 µm and have a breakdown current density comparable to metal wires.


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KEYWORDS liquid metals, carbon nanotubes, stretchable electronics, 3D printing, printed electronics


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Interconnects are essential components in integrated electronic circuits for connecting individual components of devices electrically and transmitting signals1,2. Demands for high-performance electronic devices that can process high throughputs of information have required a drastic increase in the density of chips, resulting in advances in three-dimensional (3D) integration of the devices. The necessity of 3D interconnection technology has increased accordingly3–6. Flip-chip technology helps increase the density of the components of the device, but it has a low resolution as the pad size, and the pitch cannot be reduced to less than 100 µm3. The shapes of the chips and the substrates, which cannot be changed once predetermined, further limit the extensive application of flip-chip technology. However, in wire-bonding technology, two pads are connected by a thin metal wire that has the size of the lines and pads, i.e., about 15 µm. Since the metal wire can be adjusted, the technology copes flexibly with changes in the shapes or positions of chips and substrates2,3,5. Therefore, the wire-bonding method, which provides high resolution and flexibility, is well suited for 3D interconnections.

Recent studies actively have sought to develop materials and devices that have


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soft and stretchable forms7–13. Thus, the 3D interconnection technologies must be compatible with the elastic and elastomeric substrates in order to create 3D integrated circuits using such stretchable electronic devices. However, conventional interconnection technologies, performed with heat (~150 °C) and pressure on rigid metal pads1, are not compatible with soft substrates and devices that can be deformed by heat and pressure. Also, the junction between the metal and the soft materials is prone to failure due to fatigue14 and to the large difference in Young’s modulus of the two materials. Therefore, it is necessary to develop stretchable materials that are capable of forming 3D structures and processes that enable interconnections between soft materials.

Herein, we introduce a carbon nanotube-liquid metal (CNT/LM) composite and its high-resolution 3D printing technology as stretchable electrical interconnections. Liquid metals are highly stretchable, self-healable, and have relatively high conductivities similar to those of solid metals15–17. These properties make liquid metals be suitable for interconnects in stretchable electronic devices. A solid-state surface oxide that forms on the outer surface makes the liquid metal structurally stable and prevents it from spreading


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in the air18. However, the interior remains in the liquid state, which often causes bulging and deformation by external forces, including gravity19. To improve the mechanical stability of the liquid metal, we prepared a CNT/LM composite in which the CNTs were dispersed uniformly in liquid metal up to 15 wt%. Because of high thermal conductivity of the CNTs, the interconnects prepared using the CNT/LM composite had a breakdown current density of 2.6 × 1010 A m-2, which is about five orders of magnitude higher than that of solder-based interconnects20. Using the direct printing method, the high-resolution 3D printing of this composite forms free-standing, wire-like interconnects with diameters of 5 µm. This is superior to the resolution of existing wire-bonding technology, and it demonstrates potential applications in creating various 3D electrical interconnect structures.

In this study, carbon nanotubes were used as a reinforcing material in a liquid metal matrix for a CNT/LM composite with enhanced mechanical and electrical properties. The uniform and homogeneous dispersion of CNTs in liquid metal requires appropriate surface treatment to minimize the aggregation of the nanotubes. The outer surface of the


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liquid metal is oxidized readily in the presence of only a few ppm of oxygen, and the oxide layer adheres strongly, even to materials with low surface energy18. However, oxides do not form in the interior of the liquid metal, so pristine CNTs cannot be dispersed. Thus, mixing untreated nanotubes with liquid metals results in a complete separation of the two components, as exemplified in liquid metal marbles21. Kim et al. reported a nanocomposite composed of CNTs and polydimethylsiloxane (PDMS), which they produced by coating the nanotubes with methyl group-terminated PDMS, which has high affinity for the PDMS elastomer22. Referring to this approach, we searched for a material that had a strong affinity to liquid metal, followed by functionalization of CNTs with the selected material. Figure 1a shows the experimental procedure for fabricating the composite. First, the CNTs were treated with 70% HNO3 to form about three carboxyl groups per 100 carbon atoms in the nanotube (Figures S1 and S2). The acid treatment also removed carbon impurities and the metal catalysts that were used to synthesize the nanotubes. Then, we explored an interface material that readily binds to both nanotubes and liquid metal by measuring the contact angle of liquid metal droplets on various substrates, such as Au, Pt, Ag, PDMS, glass, and a Si wafer (Figure S3). We selected Pt


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as the interface material because the contact angle was the lowest (79°) on Pt, whereas other substances had much higher contact angles, e.g., 132° for bare silicon, that indicated the affinity of Pt to the liquid metal (Figure 1b). By reacting with Pt(NH3)4(NO3)2, the carboxylated nanotubes were decorated with Pt nanoparticles after reduction in an H2 atmosphere (Figures 1c and S4)23. To prepare a CNT/LM composite with minimal aggregates of CNTs, The Pt-decorated nanotubes and liquid metal were dispersed ultrasonically in organic solvent in separate vials. Among various organic solvents, 1methyl-2-pyrrolidone (NMP) was the one that formed the most homogeneous and stable dispersions of both CNTs and LM. Mixing the two dispersants and removing the NMP by evaporation resulted in the CNT/LM composite. Figure 1d shows optical images of the pristine liquid metal (left), the CNT/LM without Pt nanoparticles (middle), and CNT/LM with Pt nanoparticles (right). Pristine nanotubes remained on the exterior of the liquid metal rather than partitioning into the matrix. However, the nanotubes that were decorated with Pt nanoparticles were dispersed into the liquid metal, forming a uniform and homogeneous composite, as described in greater detail in the following section. Without dispersing the powdery nanotubes in NMP and, instead, mixing them (at 5 wt%) directly


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with the liquid metal, we obtained a highly-viscous, paste-like CNT/LM composite (Figure S5). Obviously, the 5 wt% was much lower than 15 wt% shown in Figure 1d (right). Optical micrographs of the directly mixed CNT/LM composite clearly show large aggregates of nanotubes in the matrix. Therefore, we concluded that the initial dispersion of the nanotubes and the liquid metal matrix in NMP was appropriate to obtain a uniform and homogeneous CNT/LM composite.

We characterized the composite by Raman spectroscopy at a laser excitation of 532 nm to confirm the uniform dispersion of the CNTs in the liquid metal matrix. Due to the high reflectance of the liquid metal at the wavelength of the laser, it was challenging to collect Raman scattering of nanotubes from the composite24. Therefore, the composite was spin-coated as a thin film on a silicon substrate, which created regions with uniform thicknesses that were suitable for characterization by Raman spectroscopy. We prepared composites of CNT/LM with three different nanotube contents, i.e., 2, 5, and 15 wt%. There were no visible particles on the surface of the pristine liquid metal, and only the wrinkles formed by the oxidation layer were observed (Figure 2a). However, in the


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CNT/LM composites (Figure 2b), we found black particles, which were identified as bundles of CNTs based on the tangential (G-band, ~1590 cm-1) and disorder (D-band, ~1350 cm-1) modes in the Raman spectra (Figure 2c, top). When the content of the CNTs increased from 2 wt% to 15 wt% (Figure 2b), the density of the particles also increased. To confirm the dispersion of CNTs, Raman maps for the G-band (1550 cm-1 – 1650 cm-1) were obtained, as shown in Figures 2d and 2e, and these maps corresponded to the yellow boxed regions in Figures 2a and 2b, respectively. The G-band was observed only in the presence of nanotubes, and the peak intensity was higher for the higher contents. Note that mapping the large area in a reasonable amount of time required a short exposure time of 1.5 s per spot. Hence, the high-intensity regions in the map correspond to where the black particles (i.e., the bundled CNTs exposed on the surface of the composite) are located in the optical micrographs. However, by increasing the exposure time to 300 s, we were able to detect the Raman features of the nanotubes (Figure 2c, middle). The results confirmed that the CNTs were dispersed throughout the liquid metal matrix. We also proved the presence of CNTs in LM matrix using scanning electron microscope (SEM) (Figure S6). After irradiating electron beam at 10 keV for ~ 30 sec to


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the thin film of the composite, the LM was slightly dewetted and slowly uncovered the CNTs. The exposed CNTs were clearly seen in the SEM images, and the results confirm the role of CNTs as a conductive structural support that reinforces the mechanical and electrical properties of the LM. Although the aggregation of the nanotubes could not be avoided during the evaporation of NMP, the dispersion of carbon nanotubes in NMP is a promising approach to create a CNT/LM composite with uniformly dispersed CNTs that have minimal bundle sizes.

We used a nozzle for direct printing the CNT/LM composites with various threedimensional (3D) structures. Figure 3a shows a schematic illustration of a 3D direct printing system for CNT/LM composites. The printing system consists of a nozzle connected to an ink reservoir, a pressure controller, and a five-axis movement stage with automatic movements in the x-, y-, z-axis and two tilting axes in the xy-plane. Nozzles with inner diameters of 5 to 140 µm were prepared using a glass pipette puller. Then, a nozzle was mounted to a syringe-type reservoir, and a substrate was placed on a fiveaxis stage. The distance between the nozzle and the substrate was controlled to be 2 to


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100 µm, depending on the inner diameter of the nozzles. Then, the ink was delivered from the reservoir to the tip of the nozzle by pneumatic pressure to contact the substrate. Then, controlling the movement of the z-axis and the pneumatic pressure enabled the growth of the CNT/LM filaments in the z-direction. Subsequently, the tip of the 3D filament of CNT/LM composites, which was connected to the nozzle, can be located at the desired point, where an electrical interconnection is formed. CNT/LM composites are highly stretchable and deformable, so the filament forms an adaptive structure for wiring. Figure 3b shows stereoscopic images of the printed 3D structures of CNT/LM composites on a soft substrate. The substrate was fabricated from PDMS cast in an acrylic mold, and the CNT/LM composites were 3D printed for wiring sequential steps, ranging from heights of 100 to 300 µm at 100-µm increments. While the conventional wire-bonding process cannot be used on soft substrates, the CNT/LM composites and their 3D patterning process enabled electrical wiring on soft and stretchable substrates. The smallest diameter of the nozzle that was capable of printing the CNT/LM composite was 5 µm (Figure 3c), suggesting that the CNTs were dispersed in the liquid metal matrix without aggregating into bundles larger than the size of the nozzle. To verify the inclusion of the


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CNTs in the printed composite, the CNT/LM composite that was printed in 5-µm line widths was analyzed by Raman spectroscopy, as shown in Figure 3d. The G-band was detected only in the printed CNT/LM composite. Figure 3e shows the resistances of the printed CNT/LM line patterns versus the lengths of the lines, which were printed with constant thicknesses and widths. Resistances were measured for at least three different lines that had the same lengths, i.e., from 2 to 16 mm with 2 mm increments. The resistance values increased linearly with the lengths, which meant that the resistance was dependent only on the dimensional factor. The results demonstrated that the CNTs were dispersed uniformly in the liquid metal matrix, achieving uniform electrical conductivity of the composite. Figure 3f shows that the CNT/LM composite can be printed with various line widths. The width of the printed line increases as the inner diameter of the nozzle increases at a fixed translation speed of the stage. The minimum printable line width was about 5 µm in both the 2D and 3D structures, whereas the maximum can exceed 100 µm. As the size of the nozzle increased, the increase in the width of the 3D line was smaller than the increase in the width of the 2D line because the line was pulled slightly by the tensile force during printing. By controlling the line width, it is possible to print


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interconnects or conductive wires at the adjustable current flux of each application. The line width also can be tuned by the translation speed of the stage, the distance between the nozzle and the substrate, and the pressure applied to the syringe. By the printing of CNT/LM composite ink, it is possible to form various 3D, soft, conductive structures. Figures 3g and 3h show the schematic images and SEM images of printed 3D structures of CNT/LM composites, respectively. The 3D printed arc-shaped structures can surpass the 2D lines or other 3D lines without electrical contact, which provides advantages in the integrity of the interconnection and electronics design.

We used tensile tests to measure the mechanical strength of the CNT/LM composites. Figure 4a shows the SEM image of a specimen for the tensile test (left) and the schematic image of the test (right). The CNT/LM composites were printed to form free-standing structures between two separated polyethylene (PET) plates for the test specimens. A nozzle with an inner diameter of 100 µm was used for the 3D printing of the specimen, and the free-standing filament was formed by horizontal translation of the substrate while applying a pressure of 1.4 psi to the ink. During printing, the z-axis was


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fixed with an initial distance of 70 µm between the substrate and the tip of the nozzle. In the case of a printed CNT/LM composite, the surface oxide has a greater elastic modulus than the liquid metal core, so the overall strength of the material can vary with its diameter. Therefore, the tensile test was conducted with specimens printed uniformly at a diameter of 85 ± 1.2 µm. Figure 4b shows the elastic modulus and tensile strength of the printed CNT/LM composite. The elastic modulus of the pristine liquid metal was about 204 kPa. The modulus and strength increased linearly as the CNT contents increased, reaching a factor of three at 15 wt%. Further increasing the CNT content to 20 wt%, however, was not feasible as the composite behaved like a thick paste and could not be printed due to the coagulation at the nozzle. The enhanced modulus and strength gave printed patterns mechanical stability against external forces and subsequent shape deformation. Relative changes in resistance and conductivity were investigated while the CNT/LM composite was being stretched and released uniaxially (Figure 4c). The relative change in conductivity was calculated by Δσ/σ0 = (R0/R)·(l/l0)·(A0/A)-1 and l/l0 = A0/A = ε, in which R is resistance, l is length, A is a cross-sectional area of the printed line at strain ε. For the measurement, the composite was printed and encapsulated with PDMS, and a Cu wire


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was contacting both ends. At 100% strain, the resistance was increased by a factor of four (i.e., ΔR/Ro ~ 3), but the conductivity was not significantly changed by the tensile strain. These results followed the change in resistance by geometry. In the case of 100% stretching at a constant volume, the length increased by a factor of two while the crosssectional area decreased by a factor of two, therefore the resistance became four times of initial value. The negligible change in conductivity means that the CNT/LM composite is stable without local cracks or disconnection under the tensile strain. After stretched to 100% in tensile strain, the resistance of the CNT/LM composite recovered to the initial value by its releasing (strain: 0%). To investigate the reversibility against repetitive stretching and releasing, the relative change of its resistance was measured during 10,000 cycles of stretching (100% tensile strain) and releasing. As shown in Figure S7, the resistance change was not significant, which indicates the good reversibility of this CNT/LM composite against repetitive stretching cycles. Figure S8 also presents its reversible stress-strain characteristic measured by the tensile test with 100%-strain stretching and releasing (strain rate: 10 µm s-1). Furthermore, when CNT/LM formed freestanding arc-shaped structure, this 3D geometrical configuration also enabled the


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recoverable deformation of this 3D structure (Figures S9 and S10). When stretched to 100% strain, there was no significant change in resistance. (Figure S11). The arc-shaped structure of the CNT/LM composite also had good reliability against 10,000 cycles of 100%-strain stretching and releasing, as shown in Figure S12. The CNT/LM composite is also electrically restorable after its physical disconnection by excessive stretching (tensile strain: 300%). Figure S13 presents the real-time electrical restoration of the CNT/LM composite under its disconnection-reconnection cycles by stretching and releasing. The degradation of its electrical current was less than 4% after 500 cycles, which indicated its good self-healing property. The electrical and thermal properties of the CNT/LM composites also were evaluated and compared with those of the pristine liquid metal. The electrical conductivity of the CNT/LM composite was determined by measuring the linear resistances and cross-sectional areas of the printed line patterns (Figure S14). Figure 4d shows the conductivities of the CNT/LM composites with different contents of CNTs. The conductivity was decreased only by 10% even at CNT contents of 15 wt%. This may be due to the uniform dispersion of CNTs in the liquid metal matrix with ohmic contact between the liquid metal and the CNTs25, as well as the negligible formation of additional


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oxides during the mixing procedure. Figure 4e shows the maximum current density of the CNT/LM composite printed as straight lines with line widths and lengths of 12 and 500 µm, respectively. The linear behaviors in the broad range of the electric field reflected the ohmic contact of the wires with the Cu pad. The nonlinear behaviors in high current density were thought to be due to the electromigration-induced deformation of liquid metals26. The nonlinear behavior of the current density began at 2.6 × 1010 A m-2 in the case of the liquid metal, but the CNT/LM composite showed stable linear behavior up to 3.3 × 1010 A m-2. The breakdown current density was improved by a factor of 1.3 by adding CNTs to the liquid metal. The higher breakdown current density allows carrying higher current with smaller line widths and increases the reliability of the wire against electrical loads7. Figure S15 presents the average values of the maximum electric field and maximum current density obtained from 50 samples of the CNT/LM composite or pristine liquid metal.

Since electronic circuits often must operate at high temperatures because of the dissipation of power27, the thermal stability of the electrical interconnects is important.


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Figure 4f shows SEM images of the 3D CNT/LM composite interconnects before and after the heat treatment. The printed 3D line formed an electrical connection between two indium-tin-oxide (ITO) pads with an arc-shaped, free-standing structure. Here, the 650µm-stepped substrate was prepared by stacking two layers of Si wafers. The printed 3D structure was placed on a hotplate at 340 °C for 70 minutes in ambient conditions, and the initial free-standing structure was preserved. In addition, the resistance of the printed CNT/LM composite showed a negligible change after the heat treatment, as shown in the current-voltage characteristics (Figure 4g). The vapor pressures of gallium-based liquid metals are extremely low (below 10-6 Pa at 500 °C), so they undergo very little evaporation or loss of volume, which enables mechanical and electrical stability at high temperatures21,28. Interconnects with high thermal conductivity can improve the reliability of the device by diffusing the heat that is concentrated in the chip. By increasing the CNT contents in the composite, the thermal conductivity was increased slightly, e.g., the thermal conductivity of the CNT/LM composite with 15 wt% CNTs increased from 18.4 to 21 W m-1 K-1 (Figure S16). The conductivity was comparable to the conductivities of other metals in circuits, e.g., lead: 35 W m-1 K-1 and tin: 62 W m-1 K-1.


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In conclusion, the work described here demonstrates the high-resolution 3D printing of CNT/LM composites as stretchable interconnects for soft, elastomeric devices and substrates. CNTs were dispersed in the liquid metal matrix to improve the mechanical strength and structural stability of liquid metals. For this, platinum nanoparticles, which have a low contact angle with liquid metal, were decorated on the surfaces of CNTs, forming uniform and homogeneous composites with liquid metals. A direct printing method through a nozzle can form free-standing, wire-like, 3D structures of these CNT/LM composites with high resolutions (minimum diameter of the printed wire: 5 µm) superior to the diameters of the wires of provided by commercially-available wire-bonding techniques. The printed composites had high conductivity, i.e., up to 3 S µm-1, with a maximum current density of 3.5×1010 A m-2, which is comparable to conventional metal interconnections (the order of 1010 A m-2). The formation of CNT/LM composites and their ability to create diverse 3D structures for soft substrates and devices allow high-resolution 3D integrations of stretchable circuits and soft electronics, and they represent substantial progress towards next-generation electronics.


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FIGURES

Figure 1. Preparation of the CNT/LM composite. (a) Schematic illustration that shows the preparation of the CNT/LM composite using Pt as an interface material. (b) Contact angles of liquid metal droplets on a bare Si (top) and Pt-coated Si (bottom) substrates. (c) TEM image of the Pt-decorated CNTs. Scale bar, 10 nm. (d) Optical images of pristine liquid metal, CNT/LM composite using unfunctionalized CNTs, and CNT/LM composite using CNTs decorated with Pt nanoparticles.


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Figure 2. Characterization of the CNT/LM composite. (a) Optical micrograph of liquid metal spin-coated on the Si substrate. Scale bar, 100 μm. (b) Optical micrographs of CNT/LM composites with 2, 5, and 15 wt% of CNTs. Scale bars, 10 μm. (c) Representative Raman spectra of CNTs exposed on the surface of the composite, which appear as black particles in (b) (top), CNTs buried under the liquid metal (middle), and pristine liquid metal (bottom). (d) Raman map showing the intensity of G-band (1550 – 1650 cm-1) from the yellow boxed region in (a). Scale bar, 10 μm. (e) Raman maps showing the intensity of G-band from the yellow boxed regions in (b). Scale bars, 5 μm.


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Figure 3. Three-dimensional printing of CNT/LM composites. (a) Schematic illustration of the 3D direct printing system of CNT/LM composites. (b) Stereoscopic micrographs of CNT/LM composites printed on a soft 3D substrate. Scale bars, 100 µm. (c) SEM image of printed CNT/LM composites. Scale bar, 10 µm. (d) Raman spectra of CNT/LM composites (top) and pristine liquid metal (bottom). (e) Resistance of printed CNT/LM composites with different lengths. (f) Line widths of 2D- and 3D-printed CNT/LM composites according to nozzle diameters. (g, h) Schematic illustrations (left) and SEM images (right) of various 3D structures of printed CNT/LM composites. Scale bars, 100 µm.


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Figure 4. Mechanical, electrical, and thermal properties of CNT/LM composites. (a) SEM image of free-standing CNT/LM composites (left) and schematic illustration of the tensile test (right). Scale bar: 500 µm. (b) Elastic modulus and tensile strength of the composite as a function of CNT contents from 0 to 15 wt%. (c) Relative changes in resistance (top) and relative changes in conductivity (bottom) of CNT/LM composites during stretching and releasing as a function of tensile strain (CNT contents: 15 wt%). (d) Electrical conductivity of the composite as a function of CNT contents from 0 to 15 wt%. (e) Current density of pristine liquid metal (black) and CNT/LM composites (red). (f) Colorized SEM images of a 3D CNT/LM composite structure before (left) and after (right) the heat treatment. Blue color corresponds to liquid metal. Scale bars, 500 µm. (g) Current-voltage characteristics of CNT/LM composites before and after heat treatment.


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ASSOCIATED CONTENT

Supporting Information.

The following file is available free of charge.

Supporting Methods and Figures (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]

Author Contributions

Y.-G. Park and H. Min contributed equally to this work.

ACKNOWLEDGMENT This work was supported by the Ministry of Science & ICT and the Ministry of Trade, Industry and Energy (MOTIE) of Korea through the National Research Foundation (2016R1A2B3013592 and 2016R1A5A1009926), the Bio & Medical Technology


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Development Program (2018M3A9F1021649), the Nano Material Technology Development Program (2015M3A7B4050308 and 2016M3A7B4910635), and the Industrial Technology Innovation Program (10080577), the Basic Science Research Program (NRF-2017R1A2B4008226). Also, the authors thank financial support by the Institute for Basic Science (IBS-R026-D1) and the Research Program (2018-22-0194) funded by Yonsei University.


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ToC Graphic:


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Figure 1. Preparation of the CNT/LM composite. (a) Schematic illustration that shows the preparation of the CNT/LM composite using Pt as an interface material. (b) Contact angles of liquid metal droplets on a bare Si (top) and Pt-coated Si (bottom) substrates. (c) TEM image of the Pt-decorated CNTs. Scale bar, 10 nm. (d) Optical images of pristine liquid metal, CNT/LM composite using unfunctionalized CNTs, and CNT/LM composite using CNTs decorated with Pt nanoparticles.


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Figure 2. Characterization of the CNT/LM composite. (a) Optical micrograph of liquid metal spin-coated on the Si substrate. Scale bar, 100 μm. (b) Optical micrographs of CNT/LM composites with 2, 5, and 15 wt% of CNTs. Scale bars, 10 μm. (c) Representative Raman spectra of CNTs exposed on the surface of the composite, which appear as black particles in (b) (top), CNTs buried under the liquid metal (middle), and pristine liquid metal (bottom). (d) Raman map showing the intensity of G-band (1550 – 1650 cm-1) from the yellow boxed region in (a). Scale bar, 10 μm. (e) Raman maps showing the intensity of G-band from the yellow boxed regions in (b). Scale bars, 5 μm.


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Figure 3. Three-dimensional printing of CNT/LM composites. (a) Schematic illustration of the 3D direct printing system of CNT/LM composites. (b) Stereoscopic micrographs of CNT/LM composites printed on a soft 3D substrate. Scale bars, 100 µm. (c) SEM image of printed CNT/LM composites. Scale bar, 10 µm. (d) Raman spectra of CNT/LM composites (top) and pristine liquid metal (bottom). (e) Resistance of printed CNT/LM composites with different lengths. (f) Line widths of 2D- and 3D-printed CNT/LM composites according to nozzle diameters. (g, h) Schematic illustrations (left) and SEM images (right) of various 3D structures of printed CNT/LM composites. Scale bars, 100 µm.


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Figure 4. Mechanical, electrical, and thermal properties of CNT/LM composites. (a) SEM image of freestanding CNT/LM composites (left) and schematic illustration of the tensile test (right). Scale bar: 500 µm. (b) Elastic modulus and tensile strength of the composite as a function of CNT contents from 0 to 15 wt%. (c) Relative changes in resistance (top) and relative changes in conductivity (bottom) of CNT/LM composites during stretching and releasing as a function of tensile strain (CNT contents: 15 wt%). (d) Electrical conductivity of the composite as a function of CNT contents from 0 to 15 wt%. (e) Current density of pristine liquid metal (black) and CNT/LM composites (red). (f) Colorized SEM images of a 3D CNT/LM composite structure before (left) and after (right) the heat treatment. Blue color corresponds to liquid metal. Scale bars, 500 µm. (g) Current-voltage characteristics of CNT/LM composites before and after heat treatment.


Three-Dimensional, High-Resolution Printing of Carbon Nanotube

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