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 ScienceUniversity of Minnesota 421 Washington Ave. SE , Minneapolis Minnesota 55455 , USA

E-mail: frisbie@umn.edu; lfrancis@umn.eduE. B. Secor, Prof. M. C. Hersam Department of Materials Science and Engineering Northwestern University

2220 Campus Drive , Evanston , Illinois 60208 , USAProf. M. C. Hersam Departments of Chemistry and Medicine Northwestern University Evanston , Illinois 60208 , USA

Adv. Mater. 2015, 27, 109–115, DOI: 10.1002/adma.201404133

Introduction

Description of high-resolution patterning of pristine graphene by screen printing using asilicon stencil is provided.

The screen printing stencil is prepared from a thin silicon wafer. Silicon is compatible withphotolithography processing and the silicon stencil does not have a mesh, which enableswell-defined and high-resolution stencil patterns.

In addition, viscous, concentrated, pristine graphene ink is developed, and the effects ofink properties and printing parameters are studied.

High-quality patterns with high electrical conductivity and excellent mechanical toleranceare produced.

To demonstrate the feasibility of this approach for printed electronics, they fabricated all-printed organic thin-film transistors on flexible substrates using screen printed graphenesource and drain electrodes, and charaterized the device performance and mechanicaldurability.

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.

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

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

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

OM images of a silicon stencil and graphenelines printed through the stencil on polyimidefilms. (a) The silicon stencil contains lineopenings of different widths (wscreen = 50, 40,30, 20, 15, 7.5, and 5 μm). The white lines areline openings and the orange curves are thereflection of the microscope lamp. (b-e) Tofigure out the printing capability, screenprinting was carried out from two inks withdifferent viscosities (Ink 1 and 2) in twodifferent printing directions (A and B). Theminimum printing capability in terms of wscreenwas 7.5 μm for the combination of Ink 1 andDirection A, 10 μm for Ink 1-B, 15 μm for Ink 2-A, and 20 μm for Ink 2-B. 8 of 10 screenprintings showed the similar printingcapability.

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

a) OM images of a screen-printed graphene line with wprinted 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 wprinted of the printed graphene linesthrough a 20 μm wide line opening with respect to the ink viscosity and the printing direction.

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

a) OM image of screen-printed graphenesource and drain electrodes on a polyimidesubstrate for EGTs ( W / L = 900 μm/90 μm). b)Schematic illustration for the EGT architecturefabricated on the graphene electrodes. c)Transfer and d) output characteristics of theprinted EGTs. The voltage sweep rate was 50mV s −1 . e) Stability of charge carrier mobility (μ ) and threshold voltage ( Vth ) for the EGTsduring repeated bending cycles with a bendingradius of 4 mm, corresponding to 1.0% strain.

To demonstrate a possible application of screen-printedgraphene electrodes, all printed electrolyte-gatedtransistors (EGTs) were fabricated and characterized.EGTs are promising for flexible printed electronics; thehigh capacitance of the electrolyte enables low voltageoperation, and the material offers broad processcompatibility for printing on flexible substrates withhigh tolerance to thickness variations

Application

Summary and ConclusionThey have demonstrated fine patterning of pristine graphene by screen printing using asilicon 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 photolithographyprocess, 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 graphenepatterns, achieving a resolution as high as 40 μm, which can be attributed to the fine lineopening as well as the tuned viscosity of the graphene ink.

The screen-printed graphene lines on polyimide films exhibited high electrical conductivity of≈1.86 × 104 S m−1 and outstanding mechanical flexibility, suitable for electronic applications.

With the high quality and flexible graphene patterns as source and drain electrodes, all-printed EGTs on flexible substrates showed desirable transfer and output characteristics, aswell as durable operation over many bending cycles.

Overall, this work establishes a scalable method for the facile and practical printing of highlyconductive graphene patterns for flexible and printed electronics.

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,.