Supplementary Information

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1 Supplementary Information Plasma-assisted reduction of graphene oxide at low temperature and atmospheric pressure for flexible conductor applications Seung Whan Lee 1, Cecilia Mattevi 2, Manish Chhowalla 3, and R. Mohan Sankaran 1,* 1 Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 2 Department of Materials, Imperial College London, London, United Kingdom 3 Department of Materials Science and Engineering, Rutgers University, Piscataway, NJ *Address correspondence to: mohan@case.edu

2 1. Synthesis of graphene oxide (GO) Graphene oxide (GO) was prepared from natural graphite (Asbury Carbons) by a modified Hummers method reported by Kovtyukhova. 1 Briefly, graphite powder (10 g) was stirred and heated at 80 C in a solution of concentrated H 2 SO 4 (30 ml), K 2 S 2 O 8 (5 g), and P 2 O 5 (5 g). The mixture was thermally isolated for 6 hours and allowed to cool to room temperature. The preoxidized graphite was then carefully diluted with distilled water, filtered, and washed on the filter until the rinse water ph became neutral. The product was dried in air at ambient temperature overnight. This preoxidized graphite was then subjected to oxidation by Hummers method. The oxidized graphite powder (10 g) was placed in cold (0 C) concentrated H 2 SO 4 (230 ml). KMnO 4 (30 g) was added gradually with stirring and cooling, so that the temperature of the mixture was not allowed to reach 20 C. The mixture was then stirred at 35 C for 2 hours followed by the addition of distilled water (460 ml). After 15 minutes, the reaction was terminated by the addition of a large amount of distilled water (1.4 L) and 30 % H 2 O 2 solution (25 ml). The mixture was filtered and washed with 1:10 HCl solution (2.5 L) in order to remove metal ions. The GO product was finally suspended in distilled water (1L). 2. Temperature measurements The plasma process itself could heat the GO samples without heating by the furnace because of convective heat transfer (the gas is heated in the plasma then flows and transfers its heat to the substrate). To estimate this temperature, a thermometer was placed at the same distance (1.5 cm) as the GO films from the plasma anode. As shown in Fig. S1, the temperature measurements showed a steady-state value after approximately 10 min. The final steady-state

3 temperature was a function of discharge current and the gas atmosphere. For the typical conditions used in this study (Ar/H 2 =50/50, discharge current=10 ma), the temperature was estimated to be less than 70 o C. 3. XPS characterization of GO films XPS characterization of GO films was performed before and after thermal and plasma treatment at a series of temperatures. Carbon 1s core level spectra are included in the main text. Oxygen 1s core level spectra are shown in Fig. S2a and b. Survey spectra are shown in Fig. S3. From the survery spectra, we estimated the oxygen content in the films. 4. Micro Raman characterization of GO films Typical exposure times to collect Raman spectra were 2 min. To confirm that the laser did not cause reduction or other structural changes to the GO samples, we obtained a series of Raman spectra after continuous exposure to the same spot. Figure S4 shows that the laser did not cause any visible structural changes to the film for at least 30 min of exposure. Micro Raman spectra of GO films reduced on PET were obtained to compare with reduction on Au substrates. As shown in Fig. S5, the spectra show the D and G band corresponding to GO before and after reduction, consistent with results for GO reduction on Au.

4 5. Sheet resistance measurement Sheet resistance measurements were performed on GO films deposited on polyethylene terephthalate (PET) and reduced by thermal annealing alone or plasma exposure at 150 C. The average transmittance of the initial films was measured to be 43.8% at 520 nm which indicates that the films were thick. The transmittance varied little from sample to sample (<2%) confirming that the initial film thicknesses were comparable. We assumed that the reduction did not occur through the films (since the films were so thick) and that at these thicknesses, the sheet resistance is independent of film thickness. 2 To obtain the sheet resistance, we constructed a two-probe set up consisting of Pt foil pieces. During the resistance measurement, the pressure was kept constant (500 psi) (Fig. S6). The measured resistance (R) value was converted to sheet resistance (R S ) by the following equation: R ρ ρ where ρ is the resistivity, A is the cross-sectional area, and L is the gap between two electrodes. The cross-sectional area is estimated from the width (W) and the sheet thickness (t). Table S1 summarizes the resistance measurements and sheet resistance calculations. 6. Hydrogenation of graphene To assess whether hydrogenation of sp 2 domains in the GO film could occur during the plasma reduction process, we designed a test experiment based on exposure of monolayer graphene to our remote plasma. Graphene films were prepared by standard methods reported in

5 the literature: graphene was initially grown by chemical vapor deposition on Cu foil 3, 4, 5 then transferred onto 300 nm SiO 2 on Si substrates by spin-coating polymethylmethacrylate (PMMA) and dissolving the Cu foil in FeCl 3. 6,7,8 The graphene films were exposed to atomic hydrogen under the same conditions as the GO films (Ar/H 2 =50/50, discharge current=10 ma, substrate temperature=150 o C, exposure time=30 minutes). The Raman spectra in Fig. S7 indicate that the chemical structure of the graphene before and after the plasma treatment was unchanged. This result is consistent with a recent report 9 that showed the mechanism for hydrogenation of graphene is electron irradiation of water adsorbates which is avoided in our process since electrons do not interact with the film. Thus, we infer that hydrogenation of the sp 2 groups in the GO films does not occur. Table S1. Electrical properties of GO films before and after thermal treatment and plasma treatment in an Ar/H 2 atmosphere at 150 o C and 1 atm. As-prepared Thermal reduction Plasma-assisted reduction Length (L, cm) Width (W, cm) Resistance (R, Ω) Sheet resistance (R S, Ω/sq)

6 Figure S1. Temperature measurement as a functionn of distance from the microplasma. Photographs of (a) Ar microplasma and (b) Ar/H 2 plasma (50/50). (c) Temperature as a function of time at a distance of 1.5 cm from the plasma anode (same as substrate in GO experiments).

7 (a) (b) Intensity [A.U.] GO 70 O C 150 O C Intensity [A.U.] GO 70 O C 150 O C 300 O C 300 O C 450 O C 450 O C Binding Energy (ev) Binding Energy (ev) Figure S2. O 1s XPS spectra of GO films after (a) thermal treatment and (b) plasma treatment as a function of temperature. The gas atmosphere was Ar/H 2 (50/50) and the pressure was 1 atm in all cases.

8 (a) GO (b) GO Intensity [A.U.] 70 O C 150 O C Intensity [A.U.] 70 O C 150 O C 300 O C 300 O C 450 O C 450 O C Binding Energy (ev) Binding Energy (ev) Figure S3. XPS survey spectra of GO films after (a) thermal treatment and (b) plasma treatment as a function of temperature. Reduction was performed in a gas atmosphere of Ar/H 2 (50/50) at atmospheric pressure for 30 min in both cases.

9 30 minutes Intensity (A.U) 20 minutes 10 minutes 0 minute Wavelength (cm -1 ) Figure S4. Micro Raman spectra (λ=633 nm) of GO thin films on Si substrate as function of exposure time of Raman laser. Raman spectra was obtained with 17 mw of power and total 2 minutes of integration time.

10 H 2 /Ar plasma Intensity (A.U.) Thermal GO PET Wave length (cm -1 ) Figure S5. Micro Raman spectra (λ=633 nm) of GO films reduced on PET substrate. Spectra of blank PET substrate (black curve), original GO film (orange curve), GO reduced by thermal annealing in Ar/H 2 atmosphere at 150 o C (pink curve), and GO reduced by H 2 /Ar plasma at 150 o C (green curve) are shown. Reduction was performed in a gas atmosphere of 50/50 H 2 /Ar at atmospheric pressure for 30 min in both cases. The plasma was operated at a discharge current of 10 ma.

11 Figure S6. Photographs of setup used to measure sheet resistance. (a) As-prepared GO film, (b) thermally-reduced GO film, and (c) plasma-reduced GO film. The gap between the electrodes (0.5 cm) and contact pressure (500 psi) were kept constant during the measurements. Photographs of GO films on PET (e) before treatment, (f) after thermal annealing at 150 ºC and (g) after plasma treatment at 150 ºC. All treatments weree performed in an Ar/H 2 atmospheree at 1 atm.

12 Intensity (A.U.) Before plasma-assisted reduction After plasma-assisted reduction Wave length (cm -1 ) Figure S7. Micro Raman spectra (λ=633 nm) of pristine CVD-grown graphene (green curve) and graphene after exposure to remote plasma process (orange curve). Plasma exposure was carried out under typical conditions: 50/50 Ar/H 2, discharge current=10 ma, substrate temperature=150 o C, and process time=30 min.

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