High-Resolution Transfer Printing of Graphene Lines for Fully Printed

Jul 7, 2017 - KEYWORDS: pristine graphene ink, hydrophobic molds, transfer printing, high-resolution graphene patterns, flexible electronics...
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High-Resolution Transfer Printing of Graphene Lines for Fully Printed, Flexible Electronics Donghoon Song,† Ankit Mahajan,† Ethan B. Secor,‡ Mark C. Hersam,‡ Lorraine F. Francis,† and C. Daniel Frisbie*,† †

Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208, United States

‡

S Supporting Information *

ABSTRACT: Pristine graphene inks show great promise for flexible printed electronics due to their high electrical conductivity and robust mechanical, chemical, and environmental stability. While traditional liquid-phase printing methods can produce graphene patterns with a resolution of ∼30 μm, more precise techniques are required for improved device performance and integration density. A highresolution transfer printing method is developed here capable of printing conductive graphene patterns on plastic with line width and spacing as small as 3.2 and 1 μm, respectively. The core of this method lies in the design of a graphene ink and its integration with a thermally robust mold that enables annealing at up to ∼250 °C for precise, highperformance graphene patterns. These patterns exhibit excellent electrical and mechanical properties, enabling favorable operation as electrodes in fully printed electrolyte-gated transistors and inverters with stable performance even following cyclic bending to a strain of 1%. The high resolution coupled with excellent control over the line edge roughness to below 25 nm enables aggressive scaling of transistor dimensions, offering a compelling route for the scalable manufacturing of flexible nanoelectronic devices. KEYWORDS: pristine graphene ink, hydrophobic molds, transfer printing, high-resolution graphene patterns, flexible electronics

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promise for versatile process integration while maintaining excellent performance metrics.34−37 In particular, the tunable ink rheology of these pristine graphene inks offers compatibility with various printing methods such as inkjet, gravure, and screen printing. The resulting graphene patterns offer electrical conductivity as high as ∼25,000 S/m and excellent tolerance to extreme bending stresses even for thick films (>1 μm), enabling bendable and foldable printed organic transistors.34−37 Moreover, this platform for ink development is cost-effective and scalable, with established relevance for commercial distribution. However, the demonstrated printing resolution for pristine graphene inks has to-date been limited to 30 μm,34−37 primarily due to lateral spreading of the functional inks which results in low aspect ratio features with suboptimal conductance. In addition, line edge roughness decreases print consistency and yield for high-resolution, closely spaced features such as transistor electrodes. As such, a reliable route to printed graphene patterns with line width and spacing of several

igh-resolution printing of electronically functional inks on plastic or paper substrates is a compelling manufacturing platform for large-area, low-cost, flexible electronic systems with applications in sensors, displays, energy devices, smart packaging, and radio frequency identification.1−13 A cornerstone of this technology is the design and integration of functional liquid inks and reliable, high-resolution patterning methods. Among emerging nanomaterial inks, graphene offers a suite of desirable properties for printed electronics, including high electrical conductivity, flexibility, and robust mechanical, chemical, and environmental stability.14,15 Graphene can be produced in a scalable manner by liquid-phase exfoliation, using methods such as ultrasonication,16 high-shear mixing,17 microfluidization,18 and other techniques.19−23 Proper engineering of the fluid properties, including solvent composition and dispersants or additives, enables the integration of graphene with a variety of printing methods.24−30 Moreover, these methods can in many cases be translated to additional nanomaterials, enabling the straightforward fabrication of heterostructures.31−33 One class of graphene inks, containing exfoliated graphene flakes stabilized by ethyl cellulose in organic solvent, has shown significant © 2017 American Chemical Society

Received: May 30, 2017 Accepted: July 7, 2017 Published: July 7, 2017 7431

DOI: 10.1021/acsnano.7b03795 ACS Nano 2017, 11, 7431−7439

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Figure 1. (a,b) Schematics of graphene ink showing graphene sheets stabilized by ethyl cellulose. (c) Shear viscosity of the graphene ink measured at room temperature, ∼150 mPa·s at a shear rate of 0.1 s−1 (inset: photograph of graphene ink). (d) Representative AFM image of graphene flakes. Distribution of (e) flake thickness and (f) flake area for >500 flakes.

smooth surface and clean edges are obtained; notably, line width and spacing as small as 3.2 and 2.7 μm, respectively, were realized, with line edge roughness smaller than 25 nm. To the best of our knowledge, this is the highest resolution reported to date for printed graphene features. The excellent electrical and bending performance of these graphene electrodes are leveraged in electrolyte-gated transistors (EGTs)35,36,44−46 fabricated with correspondingly high mechanical robustness and demonstrating the great potential of this strategy for flexible electronics applications.

microns is highly desirable for the development of highperformance flexible electronic devices. Transfer printing, a rapid method for transferring patterns from a patterned template or mold to a receiving substrate,38 offers suitable prospects for achieving high-resolution patterning of electronic materials such as graphene.20 Compared to other very high-resolution printing methods, such as electrohydrodynamic jet printing,39 it is more straightforward for patterning thick features necessary for high conductance. However, only thin graphene films (1 μm.36 To integrate this graphene ink with transfer printing on flexible substrates, thermal annealing requirements of the inks must be addressed. In principle, transfer printing offers the opportunity to decouple thermal annealing processes of inks with the thermal tolerance of the receiving substrate, in that annealing can be performed in a suitable mold prior to transfer to the substrate. However, the development of a thermally stable mold with precise, highresolution features and properly engineered wetting properties presents a challenge in this regard. Herein, we demonstrate a high-resolution transfer printing method for graphene inks based on selective dewetting42,43 using robust silicon molds with an engineered low surface free energy (γ). Low viscosity (≤500 mPa·s) of graphene inks for successful transfer printing is essential. High-viscosity (>1000 mPa·s) graphene ink is not desired because such inks exhibit limited spreading on the mold. The high thermal stability of the mold allows annealing of the low-viscosity graphene inks prior to transfer at temperatures as high as 250 °C. This feature enables broad process compatibility by decoupling the thermal annealing requirement of printed materials from the thermal tolerance of the receiving substrate and marks a clear distinction from conventional transfer printing methods, in which annealing of high-performance materials is completed on the final substrate after transfer. Furthermore, the molds can be flexible and are thus well-suited to roll-to-roll processing (Figure S1, Supporting Information). Upon transfer printing, high-resolution conductive graphene patterns featuring a

RESULTS AND DISCUSSION The graphene ink for transfer printing contains graphene sheets that are well exfoliated and stabilized by the polymer stabilizer ethyl cellulose in a solvent mixture composed of terpineol and cyclohexanone (Figure 1a,b). Briefly, for the ink fomulation, a desired solid powder comprising graphene flakes and excess ethyl cellulose (2:3, w/w) was first produced by liquid-phase exfoliation and flocculation methods.35 Graphene ink was then produced with 20% w/v solids in terpineol, exhibiting a shear viscosity of ∼1000 mPa·s at a shear rate 0.1 s−1 (Figure S2). Owing to the direct dispersion of pristine graphene flakes exfoliated from natural graphite and the favorable effects of the polymer, excellent electrical properties are typical.34−37 A low viscosity ≤500 mPa·s is required for selective dewetting and mold filling,43 and thus, the as-prepared ink was diluted using 50% cyclohexanone, a preferred solvent for viscosity control of graphene inks.37 The desirable viscosity ∼150 mPa·s was yielded as shown in Figure 1c. This low-viscosity ink assures stable performance even after a year or more, using as necessary a facile redispersion under ultrasonic bath for about 10 min. Such stability is notable because the ink was prepared directly from natural graphite without damaging, oxidizing treatments typically used to ensure adequate dispersion. The graphene flakes used in the ink have a nanoscale dimension, typical thickness of ∼2 nm and lateral area of ∼65 × 65 nm as determined by atomic force mircoscopy (AFM) (Figure 1d−f), a desirable size for high-resolution printing. A schematic illustration of the high-resolution transfer printing process is depicted in Figures 2 and 3. First, a silicon 7432

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Figure 2. (a) Schematics for selective dewetting and annealing of graphene ink on the Cytop/Si mold. The trench depth is ∼3 μm. (b) SEM images of five groups of graphene lines in the mold, separated by 40 μm. Each group has three single lines, with different spacings of 2.7, 5.7, 8.7, 11.7, and 14.7 μm, from left to right. (c) Corresponding 3D images (not to scale). (d) Enlarged SEM images of graphene lines spaced by 2.7 μm.

Figure 3. (a) Schematic illustration showing pattern transfer of high-resolution graphene lines to PET, using the UV-curable NOA73 adhesive. (b) SEM images of five groups of transferred graphene lines on the PET, separated by 40 μm. Each group has three single lines, with different spacing of 2.7, 5.7, 8.7, 11.7, and 14.7 μm, from left to right. (c) Corresponding 3D images (not to scale). (d) Enlarged SEM images of printed graphene lines spaced by 2.7 μm.

coated on the Si, followed by annealing at 250 °C for 1 h to form a hydrophobic Cytop/Si composite mold, as shown in

wafer (Si) was patterned by conventional microlithography. A 20 nm thick Cytop (Asahi Glass, Co. Ltd.) film was then spin7433

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Figure 4. (a) Representative optical image of 10 μm wide graphene line pattern for electrical characterization. (b) Resistance per unit length as a function of graphene line width, for a trench depth of 3 μm. (c) Relative resistance of graphene lines as a function of bending radius (bottom axis) and tensile strain (top axis) measured during bending. (d) Relative resistance of graphene lines over 1000 bending cycles at 0.5% and 1% tensile strain.

Figure 2a. The Cytop fluorinated polymer coating exhibits a low γ of 19 mJ/m2,47 corresponding to a large water contact angle of 107° on our mold (Figure S3). This low surface energy is comparable to that for conventional transfer printing molds (25 mJ/m 2 for polyurethane acrylate, 20 mJ/m 2 for polydimethylsiloxane),42,43 indicating that Cytop-coated Si can provide a robust platform for transfer printing. The graphene ink with γ of ∼33 mJ/m2 is introduced using a pipet along the mold surface (Figure 2a). The ink selectively fills the trenches, leaving no detectable residue on the top surface of the hydrophobic mold due to its intermediate surface energy in the range of 25−70 mJ/m2 (see Supporting Information). Earlier research identified a trench aspect ratio (depth:width) of 1:5−1:20 for successful ink filling, with lower values leading to ink beading up to minimize surface area.42,43 As shown in Figure S4, however, the graphene ink effectively fills trenches with a very low aspect ratio of ∼1:2000, possibly due to the low surface energy of the ink, leading to broader design flexibility for electronic applications. With this reliable selective wetting and control of the print direction for trenches of arbitrary length, filling the mold with graphene ink is readily demonstrated (Movies S1−S5). The graphene ink in the thermally stable mold was then annealed on a hot plate in air at 250 °C for 30 min to produce a conductive graphene film. Because the nanosized graphene flakes are effectively stabilized by ethyl cellulose, partial decomposition of this insulating polymer during the annealing step yields a dense and continuous graphene network with an electrical conductivity exceeding 104 S/m.34 Selective dewetting and annealing steps were repeated four times to increase the graphene film thickness, thereby reducing the line resistance

(Figures S5, S6). High-resolution scanning electron microscopy (SEM) images of the graphene lines in the Cytop/Si mold are shown in Figure 2b,d, confirming that the graphene inks leave no visible residue on the mold surface, and the lines inside the trenches remain continuous. Higher magnification SEM images (Figure S7a) reveal a sharp graphene line edge and a flat, continuous film of graphene flakes on the Cytop/Si mold, with sub-10 nm line edge roughness (Figure S8). The annealed graphene lines were transferred to transparent, flexible PET substrates using a UV-curable adhesive, as shown schematically in Figure 3a. First, a liquid prepolymer of Norland Optical Adhesive (NOA73) with a viscosity of ∼130 mPa·s was coated on the annealed graphene lines in the Cytop/ Si mold, with a 10 min wait time to ensure reliable contact between the viscous NOA73 solution and the graphene surface. After bringing O2 plasma-treated PET into contact with the NOA73-coated mold, the adhesive was cured upon exposure to UV light (∼1000 W/m2) for 20 min. The annealed graphene lines, now bonded to the cured NOA73 adhesive, are readily peeled off from the Cytop/Si mold. The final, transfer-printed, high-resolution graphene lines on PET exhibit an inverted structure replicated from the mold, and the process is highly reproducible. Characterization of the transfer-printed graphene lines on PET is shown in Figure 3b−d. The large-area SEM images in Figure 3b confirm the successful transfer of graphene lines with no defects or breaks. Moreover, 3D optical imaging (Figure 3c) verifies the spatial uniformity and shows no curling in the NOA73 film. In the high-resolution SEM image of Figure 3d, the top surface of the high-resolution graphene patterns exhibits low roughness. AFM imaging confirms this, with a 7434

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Figure 5. (a) Photograph of selectively filled EGT patterns by the graphene ink. (b) Optical microscopy image of (a), with L and W of 20 and 400 μm, respectively. (c) Schematic drawing of aerosol jet printing for EGT fabrication: (i) transferred graphene electrodes on PET; (ii) P3HT semiconductor printing; (iii) ion gel dielectric printing; (iv) PEDOT:PSS gate connection printing. (d) Photograph of flexible EGTs printed on PET. (e) Optical image showing printed layers of source, drain, ion gel, and gate (the P3HT is difficult to see, but was printed between source and drain electrodes below the ion gel).

root mean squared surface roughness of 9 nm over a 4 × 4 μm2 area (Figure S9). This is considerably lower than the surface roughness (30−50 nm) resulting from gravure or screen printing34−36 and results from the templating of the exposed graphene surface on the smooth Cytop/Si mold. In addition, a sharp line edge (5 μF/cm2), which allows low voltage operation on flexible substrates.44 The hydrophobic mold for EGT source and drain electrodes was prepared with the same method used for the line patterns. As shown in Figure 5a,b, selective dewetting by graphene ink was successful with the EGT mold. After the annealing, the source and drain graphene electrodes were transferred to PET, mediated by the NOA73 adhesive. To complete the EGTs, aerosol jet printing was employed to deposit the remaining electronically functional inks (Figure 5c). These include poly(3-hexylthiophene) (P3HT) as the semiconductor, an ion gel as the gate dielectric (see Methods section for details), and poly(3,4ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT:PSS) as the gate electrode. The printed features on flexible PET are shown in Figures 5d,e, and the gate connection is shown in Figure S16. A p-type EGT was demonstrated with a P3HT semiconductor film for which a 3D conducting channel was created by electrochemical doping.44 Figure 6a presents the transfer characteristics of the EGTs with graphene electrodes, showing well-controlled transistor operation with negligible drain current (ID) hysteresis. When the graphene electrodes with good stability were replaced with the silver electrodes, the resulting EGTs were unstable and showed significantly degraded performance even during the second measurement

lines, which is highly competitive with other printable graphenes.49 Such resistances are sufficiently low for demonstration of nano- or micron-scale electronic devices. For largearea applications such as extended interconnect lines, further enhancement in conductivity while maintaining the advantages of graphene electrodes could potentially be realized using a combination of graphene and silver.50 To assess the mechanical stability of the transfer-printed graphene lines, bending tests were performed. The adhered NOA73 film on flexible PET serves as an excellent supporting film for the graphene line patterns without noticeable delamination or other defects resulting from the bending tests. The relative resistance of R/R0, where R0 is the resistance without bending, is plotted with respect to strain (ε) in Figure 4c. The bending strain was estimated by the relation ε = d/2r, where d denotes substrate thickness (135 μm) and r is the bending radius, with the condition r ≫ d.51 The relative resistance of printed graphene lines gradually increased during bending for strain values up to 1%, with an abrupt increase to 1.4 at higher strains, at ε = 2.3%. The increase is apparently arising from nanoscale fracture between graphene flakes as shown in a high-resolution image (Figure S13). When the applied strain of 2.3% was removed, the relative resistance values partially recovered to 1.25 and remained consistent upon further cycling of the strain (Figure S14). To further assess the bending fatigue tolerance of the graphene patterns, cyclic bending tests were performed (Figure 4d). The rise in the resistance of graphene is minimal over 1000 cycles with a strain of 0.5%. Even at an increased strain of 1%, only a ∼5% increase of the resistance is observed, compared with a ∼300% increase in resistance for the Ag patterns (Figure S15). Considering the line thickness (∼1 μm), the obtained bending stability of graphene is remarkable. Further enhancement in the mechanical stability could potentially be realized by tailoring the amount and nature of polymer residue in the film.52 The transfer-printed graphene lines therefore exhibit good bending 7436

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METHODS

cycle (Figure S17), likely due to the oxidation reaction (e.g., Ag → Ag+ + e−). This result emphasizes the superior interface properties of the graphene electrode. The output characteristics (Figure 6b) show clear ID saturation, resulting from pinch-off at high drain voltages (VDS). Ohmic contact is evident by the linear I−V relationship at low VDS. The standard saturation regime equation was used to calculate hole mobility (μ) and threshold voltage (Vth) from eq 1:53 ID = μCi

W (VG − Vth)2 2L

Materials. Cytop (CTL-809M; Asahi Glass Co., Ltd.), its solvent (CT-SOLV180; Asahi Glass Co., Ltd.), and UV-curable adhesive of NOA73 (Norland Products Inc.) were used as received. PET was cleaned by 2-propanol and O2 plasma prior to use. Reactive silver ink was synthesized according to the literature.48 The inks used for EGT fabrication were prepared following the literature methods.35,36 In short, the semiconductor ink was prepared by dissolving P3HT in chloroform (1 mg mL−1). The ion gel dielectric ink consists of 1 wt % of a triblock copolymer, poly(styrene-b-methyl methacrylate-bstyrene) (PS-PMMA-PS) and 9 wt % of an ionic liquid, 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) in 90 wt % ethyl acetate. The commercial PEDOT:PSS ink (PH1000, Heraus) was further diluted by ethyl acetate at a blend ratio of 9:1 (w/ w). Graphene inks for line patterns and EGT electrodes were synthesized according to the literature.35 In short, liquid-phase exfoliation and flocculation methods yielded a graphene-ethyl cellulose powder (≈40 wt % graphene), and 2.0 g of this powder was directly dispersed in a solvent mixture of 10 mL terpineol (Aldrich, mixture of isomers, anhydrous) and 40 g ethanol (Koptec, 200 proof) under bath sonication for 120 min at 35 °C. A 3.1 μm glass fiber syringe filter (Acrodisc) was used to filter the dispersion, and mild heating on a hot plate resulted in removal of ethanol, thus producing a graphene ink, comprising 20% w/v solids in terpineol, with a graphene:ethyl cellulose ratio of 2:3 w/w. The shear viscosity of the ink was evaluated to be ∼1000 mPa·s at room temperature. For transfer printing, the ink was further diluted by 50% with cyclohexanone, yielding a viscosity of ∼150 mPa·s, below the threshold value (≤500 mPa·s) for selective dewetting.43 Preparation of Hydrophobic, Patterned Molds. The silicon wafer (100) was first cleaned by O2 plasma before prebaking at 115 °C for 1 min. The silicon wafer was spin-coated with photoresist (Shipley S1813) at 2000 rpm for 30 s, followed by postbaking at 115 °C for 1 min. Then, a Karl Suss MA/BA6 was used to expose the photoresistcoated wafer through a line or EGT patterned mask. After developing, the patterned silicon wafer was dry-etched by reactive ion etching (SLR 770 Deep Trench Etcher). The etched silicon wafer was immersed in acetone and subsequently in piranha solution (mixture of 96 wt % H2SO4 and 30 wt % H2O2 in a ratio of 50:50) to remove the photoresist. Caution! Piranha solution is a strong oxidizer, requiring extreme caution while handling, and should not be stored in closed containers. Then, O2 plasma was further applied to the patterned wafer, before spin-coating Cytop solution with the volumetric ratio of 10:1 CTL-809M:CT-SOLV180 at 2000 rpm for 1 min. Finally, the Cytopcoated, patterned silicon wafer was annealed in air at 250 °C for 1 h. Graphene Ink Printing. While dragging the graphene ink over the Cytop/Si mold, the rate was maintained at ∼1 cm/s or lower. Higher rates resulted in small droplets of ink remaining on the unetched surface regions of the mold. Device Fabrication. Aerosol-jet printing (AJ 200, Optomec) was employed to fabricate the transistor and inverter. The deposition of the P3HT semiconductor, ion gel dielectric, and PEDOT:PSS gate electrode was carried out on PET with printed graphene source, drain, and gate contact pads. A 150 μm diameter nozzle was used for aerosoljet printing, and the carrier gas/sheath gas rates were 15 sccm/40 sccm for P3HT and the ion gel and 20 sccm/40 sccm for PEDOT:PSS. During aerosol-jet printing, the stage was maintained at 60 °C. The printer was enclosed in a vented acrylic box to minimize exposure to the aerosol. Characterization. For AFM characterization, SiO2 wafers were cleaned prior to use by sonication in acetone and isopropanol, followed by O2-plasma cleaning. Further dilution of the graphene ink by cyclohexanone to yield ∼0.001 mg/mL solids was performed for sparsely and uniformly coated graphene film. The diluted ink was spincast onto the wafers for 1 min at a spin speed of 2000 rpm. Then, the graphene coated wafers were annealed in air at 400 °C for 2 h to completely remove the ethyl cellulose. A rheometer (AR-G2, TA Instruments) with a parallel plate assembly and a drop shape analyzer (Krüss DSA30) were used for shear viscosity measurement and for

(1)

where Ci is the capacitance of ion gel dielectric, L is the channel length, W is the channel width, and VG is the gate voltage. The capacitance was measured to be 57 μF cm−2 for low-frequency operation (Figure S18). The hole mobility and threshold voltage were calculated to be 0.3 cm2 V−1 s−1 and −0.3 V, respectively, with an on/off current ratio of ∼105, promising performance metrics for sub-1 V, fully printed EGTs. Cyclic bending of the EGTs to 1% strain over 1000 cycles resulted in small shifts of 0.02 cm2 V−1 s−1 in mobility and 0.03 V in threshold voltage, respectively (Figure 6c). Finally, a 1 MΩ resistor-loaded inverter was fabricated to evaluate the dynamic response of the devices (see circuit diagram in Figure 6d). This inverter switched well at 50 Hz, with the output voltage (Vout) swinging between −1 and 0 V in response to the dynamic input voltage (Vin). In addition to improved device performance, high-resolution printing offers increased packing density, with benefits for circuit integration for electronic applications. The device footprint using graphene electrodes, defined by the substrate area per transistor, is about 2.3 mm2, which is considerably larger than that of the transistor working region (0.016 mm2). This is mostly because contact pads (∼300 μm in diameter) were largely designed for measurement convenience. They can be easily reduced in size to ∼10 μm, resulting in a footprint of 0.043 mm2. This indicates that a high density of transistors (∼2000 devices cm−2) can be achieved by the transfer printing method compared to previous methods such as screen printing (∼50 devices cm−2).35 Furthermore, shorter channel length of ∼1 μm, enabled by the precision printing method with line edge roughness below 25 nm, can be transfer printed using the EGT mold to allow printing of ∼2500 transistors cm−2 (Figure S19), demonstrating the suitability of transfer-printed graphene electrodes for printed electronic systems.

CONCLUSIONS Overall, we have demonstrated here a transfer printing method based on a hydrophobic Cytop/Si mold suitable for highresolution patterning of graphene inks. The robust thermal stability of the mold (∼250 °C) allows broad process compatibility, enabling printing of conductive, flexible, sub-5 μm graphene patterns. Excellent conductivity and flexibility of the graphene electrodes are demonstrated and utilized for fully printed EGTs tolerant to tensile strains of 1% over 1000 bending cycles. By leveraging the high-fidelity printing process, capable of achieving line edge roughness below 25 nm, the transistor channel length could be scaled to ∼1 μm, offering a compelling advantage for progress in the scalable fabrication of nanoelectronic devices. This promising platform for transfer printing offers significant potential for expanding the design space to integrate functional inks with precise, high-resolution patterning methods, ultimately advancing the development of high-performance, flexible, printed electronic systems. 7437

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ACS Nano surface energy measurement, respectively. These measurements were carried out at room temperature. The printed graphene lines were characterized with a multimeter, optical microscope (KH-7700, HIROX), and SEM (JSL-6500, JEOL). The I−V performance of EGTs was measured using two source meters (Keithley 236, 237) and an electrometer (6517A, Keithley) in a nitrogen atmosphere at room temperature. In order to record inverter performance, the input signal of inverters was generated by an Agilent 33220 arbitrary-waveform generator, and the dynamic response was collected by a Tektronix TDS1002B digital oscilloscope.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03795. Additional experimental results, including Figures S1− S19 (PDF) Movie S1: Line trench filling by vertically dragging graphene ink at (droplet size) > (trench size) (AVI) Movie S2: Line trench filling by horizontally dragging graphene ink at (droplet size) > (trench size) (AVI) Movie S3: No line trench filling by vertically dragging graphene ink at (droplet size) < (trench size) (AVI) Movie S4: Line trench filling by horizontally dragging graphene ink at (droplet size) > (trench size) (AVI) Movie S5: No line trench filling with water droplet dragging (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Donghoon Song: 0000-0003-0914-1507 Mark C. Hersam: 0000-0003-4120-1426 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Multi-University Research Initiative (MURI) program (N00014-11-1-0690) sponsored by the Office of Naval Research. Support from the Air Force Research Laboratory under agreement number FA8650-15-25518 is also acknowledged. Parts of this work were performed at the Characterization Facility and the Nano-Fabrication Center of the University of Minnesota. The authors thank Fazel Zare Bidoky for experimental assistance and Chang-Hyun Kim and Krystopher Jochem for helpful discussions. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the sponsors. REFERENCES (1) Berggren, M.; Nilsson, D.; Robinson, N. D. Organic Materials for Printed Electronics. Nat. Mater. 2007, 6, 3−5. (2) Zhao, W.; Rovere, T.; Weerawarne, D.; Osterhoudt, G.; Kang, N.; Joseph, P.; Luo, J.; Shim, B.; Poliks, M.; Zhong, C.-J. Nanoalloy Printed and Pulse-Laser Sintered Flexible Sensor Devices with Enhanced Stability and Materials Compatibility. ACS Nano 2015, 9, 6168−6177. 7438

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DOI: 10.1021/acsnano.7b03795 ACS Nano 2017, 11, 7431−7439