Tunable Electrical Conductivity of Individual Graphene Oxide Sheets Reduced at Low Temperatures

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1 Supporting Information to Accompany: Tunable Electrical Conductivity of Individual Graphene Oxide Sheets Reduced at Low Temperatures Inhwa Jung, Dmitriy A. Dikin,*, Richard Piner, and Rodney S. Ruoff*, Department of Mechanical Engineering, University of Texas at Austin Department of Mechanical Engineering, Northwestern University SI-1. Description of the devices fabrication The devices were fabricated as follows. First, the SiO 2 thin film was grown on a silicon wafer (p-type, (100), resistivity: Ω-cm) by thermal oxidation at 1100 C. The SiO 2 thickness was measured with a spectroscopic ellipsometer (MV-2000; J. A. Woollam, Inc.) at 4 different locations of the wafer and was found to be ± 4.7 nm. Alignment marks were then photolithographically (MA6 Mask Aligner from SUSS MicroTec) patterned and deposited (1- nm-thick Ti followed by a 2-nm-thick Au film) on these substrates. Prior to deposition of graphene oxide sheets, the substrate was cleaned by acetone and isopropanol, and then treated in a plasma cleaner (Plasma Preen II-862, Plasmatic Systems, Inc.) with oxygen plasma for 3 min. at 1.2 Torr and approximately 350 Watts. A droplet of a water dispersion of graphene oxide (0.01mg/mL) was placed onto the substrate, and after about one minute the substrate was gently blown with nitrogen gas. This time interval and the graphene oxide concentration in water led to a sparse dispersion of the sheets on the substrate. A confocal microscope (Leica HC) was used to locate the individual graphene oxide sheets on the silica surface. Optimized imaging conditions (543 nm wavelength of laser light; other details are described in Ref. 1 ) allowed for easier detection of single layer sheets based on the enhanced image contrast, as was also discovered and discussed for a visibility of pristine graphene 2-6. Some of the (nominally, individual) sheets were scanned by AFM (Park Scientific AutoProbe CP/MT scanning probe microscope). The minimum measured thickness of the graphene oxide sheets was slightly less than 1 nm. Ambient imaging conditions in combination with the very hydrophilic nature of graphene oxide suggest the possibility of adsorbed water molecules. Based on the acquired optical images, a set of metal electrodes contacting an individual graphene oxide sheet was photolithographically patterned and deposited (5-nm-thick Ti layer 1

2 followed by 50-nm-thick Au). Thermal reduction of the wired up graphene oxide sheets and their simultaneous electrical measurements were performed inside the vacuum chamber of a scanning electron microscope (Nova NanoSEM600, FEI Co.) at a vacuum of approximately 10-5 Torr. For this purpose, a vacuum-compatible device holder with an integrated heater (UHV heater, model , Heatwave Labs) and thermometer (10 kω thermistor 103JG1F, NTC) was custom-built. The thermometer was calibrated against a thermocouple glued to a similar silicon substrate and placed at the device location (top of the hot plate) in the vacuum chamber. The device was mechanically clamped onto the hot plate by a ceramic screw and washer. A thermally conductive but electrically insulating epoxy bond (Duralco 128 Epoxy resin) was applied between the top of the heating plate and device to secure electrical isolation. Measurements were made by a 4-probe configuration using a current source (model 6221, output resistance >10 14 Ω, Keithley) and two electrometers (model 6514, input resistance >2x10 14 Ω Keithley 6514 electrometer). Using two electrometers and isolated probes eliminated the common-mode leakage current and made the high impedance measurements possible. The backgate potential was supplied by a DC power supply (Agilent 6544A or Agilent E3612A). SI-2. Description of devices and analysis of their conductivity The characteristics for ten devices are summarized in Table S-1. More than 10 devices were tested, and as shown in Table S, three different configurations of electrodes were used. Eight more devices are not shown in this table, from which two had two electrodes, five devices had 4 electrodes, and one device had 6 electrodes. Some of these devices were used for preliminary tests in ambient conditions, and some did not withstand the full set of tests performed, for various reasons. The conductivity of device 7, which was reduced by gas phase hydrazine followed by thermal annealing, was the highest among all measured devices. This result suggests additional studies, which will be published elsewhere. Thin metal electrodes (5-nm-Ti followed by 50-nm-Au) were deposited on naturally (irregularly) shaped sheets of graphene oxide. We have used all possible combinations of current and voltage probes for electrical measurement with the goal of taking into account the shape of the graphene oxide sheets and the electrode positions with respect to the sheet edges in order to get the best estimate the device conductivity. This imperfect van der Pauw configuration 7 did not allow the use of a simple analytical calculation, instead we have applied a 2

3 numerical method of finite elements (FEM) based on multi-physics modeling software (COMSOL), which we have already used for our high resistance polymer composite samples 8, 9. We have used a two-dimensional model. The conductivity of the electrodes was defined as the conductivity of Au, and the material conductivity was set as an arbitrary (isotropic) value. Insulating boundary conditions were applied to the perimeter of the graphene oxide sheet. The boundary conditions at the electrode cross-section were chosen as either ground, constant, or zero current flow depending on the experimental conditions. The current flow across the boundaries between metal electrodes and graphene oxide was assumed to be normal. Examples of the calculated distribution of electric potential are shown in Figure Sb1 and Sb2. The material sheet resistance was determined by dividing the potential difference between the two electrodes ( V=V1-V2) by the total current flow. The device resistivity was calculated based on an assumed graphene oxide thickness of 1 nm. The procedure for determining device resistivity is shown in the Figure S1c. Based on an assumed value of resistivity for a particular combination of electrodes, the device resistance was calculated and compared with the measured value. The material resistivity (conductivity) is defined to be correct when the error between measured and calculated values converges to within one percent. The measurements and the calculations were repeated for different combinations of current and voltage probes. For example, resistance for device S9 was determined in 12 directions. The combination of the current probes I 5,2 and the voltage probes V 8,11 as shown in Figure S1-(b-1), defines the resistance along the red arrow and the direction a in Figures S1-(b- 1) and S1-(d). The direction b is determined based on the combination of I 6,3 and V 9,12 electrodes, and so on. As one can see in Figure S1-(d), both measured and calculated resistances did not exactly matched along all directions, but they follow each other qualitatively. In the next to last column of the table S1, the ratios between the maximum resistance and the minimum resistance are shown. Although the measured and calculated ratio did not exactly matched, the direction of maximum ratio is aligned for all 4-electrodes devices and deviated only 30 for 12- electrodes device (S9). On the basis of the entire set of data, we have concluded that the apparent anisotropy is a result of the geometry of the sheet and electrodes and other artifacts rather than an intrinsic material property. The values of conductivity for each device reported in Table S1 were obtained as an average value for all calculated directions. 3

4 SI-3. Raman spectroscopy A scanning confocal Raman microscope (WITec, Ulm, Germany) was used to image a few individual graphene sheets before and after vacuum thermal treatment situated on SiO 2 (Figure S2) and on Si 3 N 4 substrates. Spectra were acquired in ambient conditions using a laser excitation of 532 nm delivered through a single-mode optical fiber. A very low incident power minimizes any heating effect. Under identical experimental conditions Raman spectra were also taken from graphene sheets prepared by the "scotch-tape" technique (Figure S2). In the Raman spectrum of graphene oxide sheets, there are two prominent peaks at 1350 and 1600 cm -1 corresponding to the D and G bands, respectively. Furthermore, one can see the absence of a well-defined 2D peak that is well-known for both graphene and graphite and referred to as the G' peak in other papers (e.g. Ref. 10 ); but there are two small and wide bumps between ~2500 and ~3100 cm -1. It is known 11 that the D-band, a breathing mode of A 1g symmetry, becomes Raman-active in graphite with finite-sized crystallites and is forbidden in perfect graphite, while the G-band peak corresponds to the Raman-allowed E 2g mode, in-plane bond-stretching motion of pairs of sp 2 -bonded carbon atoms 12. According to a modified model 13 of Ref. 11, the in-plane nanocrystalline size, L a, can be estimated by an empirical relation L a [nm] = (2.4x10-10 ) λ 4 (I G /I D ), where I G and I D is the intensity of the G- and D-band peaks respectively, and λ is a wave length of an irradiation in nm units. The graphitic domain size is thus found to be about 3-4 nm in our graphene oxide sheets. The fact is that G and D peaks of varying intensity, position and width are also present in the Raman spectra of disordered, amorphous, and diamond-like carbon (Ref. 14, and references therein). As a consequence of thermal treatment of our graphene oxide sheets, (Fig. 1d) the ratio I G /I D decreased. This might be interpreted as a decrease of the mean size of nanocrystals, L a, although the electrical conductivity increased at the same time by more than 4 orders of magnitude. Similar behavior was seen earlier, on thin films of graphene oxide 15 and reported by others 16, 17. Using XPS analysis, Becerril et al 18 also concluded that a relatively modest increase in the content of non-oxygenated carbon, causes a significant boost in electrical conductivity. Based on the study of various forms of carbon reviewed in Ref. 14 and the modeling discussed therein, one can conclude that: (i) the L a equation is not applicable for the early stages of structural transformation of graphene oxide, and (ii) the nanocrystalline graphite-like state of graphene oxide is created from the amorphous state of 4

5 initially very disordered graphene oxide (see the phase diagram in Fig.1 and detailed discussion in Ref. 14 ). Table S1. Summary of the measured graphene oxide devices which survived all tests. No Confocal image Distance between I probes (µm) Steps of reduction, Max temperature, Duration (Step - 0 C - min) Final measured resistance (MΩ) Calculated conductivity (S/m) Measured R max/r min Calculated R max/r min Resistance saturation S1 33 (1) (2) (3) (4) step 1 S2 15 (1) step 1 S3 21 (1) (2) step 2 S4 15 (1) (2) not reached S5 39 (1) not reached S6 27 (1) (2) not reached S7* 33 (1) Hydrazine (2) step 2 S8 27 (1) not reached S9 33 (1) (2) step 2 S12 37 (1) (2) (3) (4) step 4 * This graphene oxide sheet was treated first by hydrazine vapor and then thermally heated. The distance between the current, I, probes is determined as an edge-to-edge distance between: two neighboring metal lines for devices S1-S8; two lines, as shown in Fig.S1-b1, for device 9; and two wide metal lines for device 12. Time of temperature increase to its maximum value. Device cooling was based on turning off the heater. 5

6 (a) (b-1) (b-2) g h i j f e k l d c a b µm V2 V1 V2 V1 (c) Measure resistance (R M ) Define resistivity in FEM, calculate resistance (R C ) (d) g 1.0 h 0.5 i - Measured - Calculated j R M R - R M C < 0.01 Obtain resistivity 1.0 f e d k l 1.0 Change direction c b a Average for all directions 1.0 Figure S1. The method for determining device resistivity. (a) Confocal microscope image (at 543 nm wavelength) of S12 device. (b) Examples of the calculated distribution of electric potential for the indicated combination of current and voltage probes for device S12 (b-1) and for device S9 (b-2). (c) Procedure for determining device resistivity (conductivity) based on measured values and FEM fitting. (d) Normalized relative resistance for S12 device in 12 directions as indicated in the panel (b-1): experiment ( - red), calculation ( - blue). 6

7 (a) (c) D-band 2D-band G-band (b) Figure S2: Raman imaging. (a) Optical microscope image of the portion of the graphene oxide sheet with two metal leads. (b) Scanning confocal Raman image of the corner area of the sheet shown in panel (a) collected in the integrated range cm -1. (c) Raman spectra collected from: single sheet of graphene oxide deposited from water solution (NR-SGO), thermally reduced single sheet of graphene oxide (tr-sgo), and single sheet of pristine graphene. 7

8 References 1. Jung, I.; Pelton, M.; Piner, R.; Dikin, D. A.; Stankovich, S.; Watcharotone, S.; Hausner, M.; Ruoff, R. S. Nano Lett. 2007, 7, Ni, Z. H.; Wang, H. M.; Kasim, J.; Fan, H. M.; Yu, T.; Wu, Y. H.; Feng, Y. P.; Shen, Z. X. Nano Letters 2007, 7, Blake, P.; Hill, E. W.; Neto, A. H. C.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Appl. Phys. Lett. 2007, 91, Roddaro, S.; Pingue, P.; Piazza, V.; Pellegrini, V.; Beltram, F. Nano Letters 2007, 7, Casiraghi, C.; Hartschuh, A.; Lidorikis, E.; Qian, H.; Harutyunyan, H.; Gokus, T.; Novoselov, K. S.; Ferrari, A. C. Nano Lett. 2007, 7, Abergel, D. S. L.; Russell, A.; Fal'ko, V. I. Appl. Phys. Lett. 2007, 91, van der Pauw, L. J. Philips Res. Rep. 1958, 13, Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, Zimney, E. J.; Dommett, G. H. B.; Ruoff, R. S.; Dikin, D. A. Meas. Sci. Technol. 2007, 18, Cancado, L. G.; Reina, A.; Kong, J.; Dresselhaus, M. S. Physical Review B 2008, Tuinstra, F.; Koenig, J. L. J Chem. Phys. 1970, 53, Nemanich, R. J.; Solin, S. A.; Gúerard, D. Phys. Rev. B 1977, 16, Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Cancado, L. G.; Jorio, A.; Saito, R. Phys. Chem. Chem. Phys. 2007, 9, Ferrari, A. C.; Robertson, J. Philos. Trans. R. Soc. Lond. Ser. A-Math. Phys. Eng. Sci. 2004, 362, Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Nano Lett. 2007, 7, Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B. Nano Lett. 2007, 7, Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. ACS Nano 2008, 2,