Woo Jin Hyun, Ethan B. Secor, Mark C. Hersam, C. Daniel Frisbie,* and Lorraine F. Francis*

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1 Woo Jin Hyun, Ethan B. Secor, Mark C. Hersam, C. Daniel Frisbie,* and Lorraine F. Francis* Dr. W. J. Hyun, Prof. C. D. Frisbie, Prof. L. F. Francis Department of Chemical Engineering and Materials Science University of Minnesota 421 Washington Ave. SE, Minneapolis Minnesota 55455, USA E. B. Secor, Prof. M. C. Hersam Department of Materials Science and Engineering Northwestern University 2220 Campus Drive, Evanston, Illinois 60208, USA Prof. M. C. Hersam Departments of Chemistry and Medicine Northwestern University Evanston, Illinois 60208, USA Adv. Mater. 2015, 27, , DOI: /adma

2 Introduction Description of high-resolution patterning of pristine graphene by screen printing using a silicon stencil is provided. The screen printing stencil is prepared from a thin silicon wafer. Silicon is compatible with photolithography processing and the silicon stencil does not have a mesh, which enables well-defined and high-resolution stencil patterns. In addition, viscous, concentrated, pristine graphene ink is developed, and the effects of ink properties and printing parameters are studied. High-quality patterns with high electrical conductivity and excellent mechanical tolerance are produced. To demonstrate the feasibility of this approach for printed electronics, they fabricated allprinted organic thin-film transistors on flexible substrates using screen printed graphene source and drain electrodes, and charaterized the device performance and mechanical durability.

3 a) Fabrication steps for a thin silicon stencil using conventional lithography techniques. b) Schematic process of screen printing using the silicon stencil and a pristine graphene ink. c) Cross-sectional illustration of the screen printing method with the flexible silicon stencil during printing.

4 (a) Flexible silicon wafer with a thickness of ~90 μm, thinned by a KOH wet etching process from a general 525 μm thick silicon wafer. (b) Silicon stencil with line openings fabricated from the thin silicon wafer by a photolithography process.

5 Schematic diagram of screen printing with a silicon stencil in a cross-sectional view. (a) The graphene ink was placed on the silicon stencil. (b) The squeegee moved the ink and pressed the stencil at the same time, which made a contact between the stencil and the substrate. (c) As the squeegee passed the openings, the ink was printed on the substrate. (d) When the squeegee was removed from the stencil, the stencil was separated from the substrate, leaving the ink on the substrate.

6 a) OM image of a thin silicon stencil showing line openings with different widths ( w screen ) of 20 (top), 30 (middle), and 40 μm (bottom) in a silicon stencil. b,c) High-resolution OM images for line openings with w screen of 20 and 5 μm, respectively. d) OM images of graphene lines printed on polyimide films through line openings with w screen of 20 (top), 30 (middle), and 40 μm (bottom). The printing was accomplished from two inks of different viscosities (Ink 1 and 2), in two different printing directions (A and B). e) Measured shear viscosity for Ink 1 and 2. f) Width ( w printed ) of screen-printed graphene lines measured by optical microscope on polyimide substrates with different inks and printing directions for varying w screen.

7 OM images of a silicon stencil and graphene lines printed through the stencil on polyimide films. (a) The silicon stencil contains line openings of different widths (w screen = 50, 40, 30, 20, 15, 7.5, and 5 μm). The white lines are line openings and the orange curves are the reflection of the microscope lamp. (b-e) To figure out the printing capability, screen printing was carried out from two inks with different viscosities (Ink 1 and 2) in two different printing directions (A and B). The minimum printing capability in terms of w screen was 7.5 μm for the combination of Ink 1 and Direction A, 10 μm for Ink 1-B, 15 μm for Ink 2- A, and 20 μm for Ink 2-B. 8 of 10 screen printings showed the similar printing capability.

8 Atomic force microscopy (AFM) characterization of graphene flakes. (a) Representative AFM image of graphene flakes dropcast on SiO2. Distributions of (b) flake thickness and (c) flake area for 715 particles, indicating typical flake dimensions of ~2 nm thickness and ~70 nm 70 nm area.

9 a) OM images of a screen-printed graphene line with w printed of 40 μm and (b e) graphene lines (dark double stripes) with different spacing of 30, 50, 70, and 90 μm on polyimide films. f) Thickness of screen-printed graphene lines for different inks and printing directions. g) Comparison of the aspect ratio (thickness/width) and w printed of the printed graphene lines through a 20 μm wide line opening with respect to the ink viscosity and the printing direction.

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11 a) OM image of screen-printed graphene to measure electrical properties of the graphene lines for different lengths and w printed. b) Scanning electron microscopy image of the graphene after annealing at a temperature of 300 C for 30 min. c) Resistance per unit length of the graphene as a function of w printed. d) Relative resistance of the screen-printed graphene lines on flexible substrates with two different thicknesses over 1000 bending cycles at a bending radius of 4 mm, corresponding to 1.0% tensile strain.

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14 Application To demonstrate a possible application of screen-printed graphene electrodes, all printed electrolyte-gated transistors (EGTs) were fabricated and characterized. EGTs are promising for flexible printed electronics; the high capacitance of the electrolyte enables low voltage operation, and the material offers broad process compatibility for printing on flexible substrates with high tolerance to thickness variations a) OM image of screen-printed graphene source and drain electrodes on a polyimide substrate for EGTs ( W / L = 900 μm/90 μm). b) Schematic illustration for the EGT architecture fabricated on the graphene electrodes. c) Transfer and d) output characteristics of the printed EGTs. The voltage sweep rate was 50 mv s 1. e) Stability of charge carrier mobility ( μ ) and threshold voltage ( V th ) for the EGTs during repeated bending cycles with a bending radius of 4 mm, corresponding to 1.0% strain.

15 Summary and Conclusion They have demonstrated fine patterning of pristine graphene by screen printing using a silicon stencil and a high conductivity ink based on graphene and EC in terpineol. The well-defined stencil was obtained from a thin silicon wafer by a photolithography process, which was produced with openings as fine as 5 μm on 90-μm-thick silicon wafers. The silicon stencil and ink formulation facilitated screen printing of high quality graphene patterns, achieving a resolution as high as 40 μm, which can be attributed to the fine line opening as well as the tuned viscosity of the graphene ink. The screen-printed graphene lines on polyimide films exhibited high electrical conductivity of S m 1 and outstanding mechanical flexibility, suitable for electronic applications. With the high quality and flexible graphene patterns as source and drain electrodes, allprinted EGTs on flexible substrates showed desirable transfer and output characteristics, as well as durable operation over many bending cycles. Overall, this work establishes a scalable method for the facile and practical printing of highly conductive graphene patterns for flexible and printed electronics.

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17 A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving, to form a patterned coating on a surface Material used: Poly(methyl methacrylate) (PMMA), Poly(methyl glutarimide) (PMGI) Phenol formaldehyde resin, etc,.