Effects of Hydrogen Partial Pressure in the Annealing Process on

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Effects of Hydrogen Partial Pressure in the Annealing Process on Graphene Growth Da Hee Jung, Cheong Kang, Minjung Kim, Hyeonsik Cheong, Hangil Lee, and Jin Seok Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp410961m • Publication Date (Web): 29 Jan 2014 Downloaded from http://pubs.acs.org on February 1, 2014

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The Journal of Physical Chemistry

Effects of Hydrogen Partial Pressure in the Annealing Process on Graphene Growth Da Hee Jung1, Cheong Kang1, Minjung Kim2, Hyeonsik Cheong2, Hangil Lee1*, and Jin Seok Lee1* 1

Department of Chemistry, Sookmyung Women's University, Seoul 140-742, Republic of Korea 2

Department of Physics, Sogang University, Seoul 121-742, Republic of Korea

AUTHOR INFORMATION Corresponding Author * Jin Seok Lee Address: Department of Chemistry, Sookmyung Women's University, Seoul 140742, Republic of Korea Tel: +82-2-2077-7464 E-mail: [email protected] *Hangil Lee Address: Department of Chemistry, Sookmyung Women's University, Seoul 140742, Republic of Korea Tel: +82-710-9409 E-mail: [email protected]

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ABSTRACT

Graphene domains with different sizes and densities were successfully grown on Cu foils using a chemical vapor deposition method. We investigated the effects of volume ratios of argon to hydrogen during the annealing process on graphene growth, especially as a function of hydrogen partial pressure. The mean size and density of graphene domains increased with an increase in hydrogen partial pressure during the annealing time. In addition, we found that annealing using only hydrogen gas resulted in snowflake-shaped carbon aggregates. Energy-dispersive X-ray spectroscopy (EDX) and high-resolution photoemission spectroscopy (HRPES) revealed that the snowflake-shaped carbon aggregates has stacked sp2 carbon configuration. With these observations, we demonstrate the key reaction details for each growth process and a proposed growth mechanism as a function of the partial pressure of H2 during the annealing process.

KEYWORDS: Graphene domain, Hydrogen partial pressure, Annealing process, Snowflakeshaped carbon aggregates, Raman spectroscopy, HRPES


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INTRODUCTION Graphene is a single-atom-thick planar sheet of sp2-bonded carbon atoms densely packed into a honeycomb crystal lattice. It has attracted great interest recently, mostly due to its unusual physical properties.1 It has an extremely high electron mobility and a tunable band gap, making graphene potentially useful for innovative approaches to electronics.2-5 In the efforts to industrialize graphene film as a carbon nano-material, a number of recent attempts have been made to improve its large-scale production by mechanical exfoliation of highly orientated polymeric graphite (HOPG), thermal treatment of SiC surface, solution-phase synthesis, and chemical vapor deposition (CVD).6-11 Of the above approaches, the CVD method is known to be a powerful and reproducible technique for the large-scale production of high-quality graphene films with controlled thicknesses.7 Recently, CVD growth of graphene on metal catalysts such as Ir,12 Ru,13Ni,14-16 and Cu17 foils has been reported. In particular, uniform single-layer deposition of graphene on Cu foil with low-carbon solubility has allowed the production of high-quality graphene for industrial applications.17 In spite of the complexity of CVD procedures involving different catalysts, different carbon sources, volume ratios of the gas mixture, and other variables, the physical principles underlying this method mainly rely either on a surface catalytic reaction13,17 for catalysts with low-carbon solubility or on bulk carbon precipitation onto the surface during cooling18,19 for catalysts with high-carbon solubility. In both cases, graphene nucleation on the catalyst surface is a critical step in the growth process. Various factors affect the initiation of the graphene nucleation process, including the surface microstructure of the metal catalyst,13,18 the carbon source,19 carbon segregation from metal-carbon melts,20 and the reaction parameters used during CVD growth.2123

Recently, it was reported that graphene growth is strongly dependent on the hydrogen (H2)


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contribution during growth time, where it seems to act both as an activator for the formation of surface-bound carbon and as an etching reagent affecting the morphology of the graphene domains.24 Thus, the growth rate and shape of graphene domains rely on the partial pressure of H2 mixed with Ar as the buffer gas. 24 Although the role of H2 during growth time was identified, its role during annealing was not stated in detail and remained unclear. In this research, we focused on understanding the role of H2 during the annealing process as well as the growth process in order to control the growth rate and shape of graphene domains. We chose atmospheric-pressure CVD, as it was technologically more accessible for the desired production of graphene. To elucidate the role of H2 in the mixture gas, we synthesized graphene domains on a Cu foil with various volume ratios of H2 to Ar gas and carefully investigated the size, morphology, and composition of the as-produced graphene domains using scanning electron microscopy (SEM), micro-Raman spectroscopy, energy-dispersive X-ray spectroscopy (EDX) analysis, and high-resolution photoemission spectroscopy (HRPES).25 Based on our observation and characterization of as-produced graphene domains, we present the key reaction details for each growth process and a proposed growth mechanism as a function of the partial pressure of H2 during the annealing process.

EXPERIMENTAL SECTION Graphene synthesis was done using chemical vapor deposition method. Cu foil (25µm thick, 99.999%, Alfa Aesar) was first cleaned with acetone, isopropyl alcohol, and deionized (DI) water. Then, they were put into a one inch quartz tube inside a horizontal furnace (Lindberg/Blue M, Thermo Scientific). The system was evacuated for 10 minutes. After this, the pump was shut down and the growth chamber was brought back to atmospheric pressure by introducing argon


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and hydrogen mixture. This process was repeated for several times. The sample was then heated to 1050 °C at a ramping rate of 17.5 °C/min with desired ratio of argon to hydrogen. For the initial Cu cleaning and annealing, the chamber was maintained at 1050 °C for another 1h with desired gas composition. During the annealing process, in order to clarify the effect of hydrogen, Ar flow rate was fixed to 300 standard cubic centimeters per minute (sccm) and H2 flow was set to 0, 10 and 15 sccm, respectively. In addition, different Ar flow was used to identify the role of Ar flow in the annealing process. For this experiment, H2 flow rate was fixed to 20 sccm and Ar flow was set to 100 and 0 sccm. Then, Ar was shut off and 300 sccm of the diluted methane (50 ppm in Ar) was allowed to flow into the tube for the graphene growth with 20 sccm of H2. The growth time was set to 10 minutes. Finally, the sample was cooled down to room temperature under the H2/Ar atmosphere but without CH4 supply. For the study of the effect of H2 and Ar in the annealing process, we always keep the same growth condition and cooling rates to minimize the effects from the change of them. The scanning electron microscope (SEM) images of graphene flakes were obtained from a FESEM (JEOL 7600F) at an acceleration voltage of 10 to 20 kV, which was performed on the assynthesized product on substrates. We obtained the Raman spectrum of graphene samples with a homemade micro-Raman spectroscopy system. In micro-Raman spectroscopy, the 514.5 nm line of an Ar ion laser was used as the excitation source with a power of ~1 mW. The heating effect can be neglected at this power range. The laser beam was focused onto the graphene sample by a 50× microscope objective lens (0.8 Numerical Aperture). The collected scattered light was dispersed by a Shamrock SR 303i spectrometer (1200 grooves/mm) and was detected with a CCD detector. EDX spectroscopy was used to perform the elemental analysis of the composites. HRPES experiments were performed at the 8A2 beamline at the Pohang Accelerator Laboratory


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(PAL), which is equipped with an electron analyzer (SES100, Gamma Data Scienta). The Si 2p, C 1s, and O 1s core level spectra were obtained using photon energies of 150, 340, and 600 eV, respectively. Secondary electron emission spectra (–20 V sample bias) were measured at photon energies of 80 eV. The binding energies of the core level spectra were determined with respect to the binding energies of the clean Au 4f core level and the valence band (Fermi energy) for the same photon energy. All spectra were recorded in the normal emission mode. The photoemission spectra were carefully analyzed using a standard nonlinear least squares fitting procedure with Voigt functions.25

RESULTS AND DISCUSSION

FIGURE 1. SEM images of graphene domains synthesized with Ar flow fixed at 300 sccm and H2 flow varied in the annealing process. (a) H2 = 0 (sccm), (b) 5 sccm, and (c) 20 sccm. Scale bar


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= 10 µm. (d) Raman spectra corresponding to domains shown in panel a, b, and c. (e-f) Intensity mappings of the G and 2D peaks, respectively. (g) Image mapping of the full width at half maximum (FWHM) of the 2D peak. (h) Optical image of the graphene sample on a SiO2/Si substrate. Scale bar = 5 µm. We conducted the atmospheric-pressure CVD growth of graphene domains on Cu foil using various volume ratios of H2 and Ar during annealing in order to investigate the influence of the partial pressure of H2 on the growth rate and shape of the graphene domains. We fixed several factors, such as the flow rate (300 sccm) of Ar for the annealing process and the gas flow rate ratios (300:20 sccm) of diluted CH4 (50 ppm in Ar) and H2 for the growth process respectively, changing only the flow rate of H2 during the annealing process. Figure 1(a)-(c) show SEM images of graphene domains grown after annealing with different flow rates of H2: 0, 5, and 20 sccm. Interestingly, hexagonal graphene domains, albeit only a small quantity, were intermittently synthesized on a Cu foil even after annealing only with Ar gas (Figure 1(a)). Based on the fact that nucleation and growth of the graphene domains on a Cu foil are initiated by the super-saturation of carbon species dehydrogenated using surface-bound active hydrogen atoms as a catalyst,24 we attribute the growth phenomenon to surface-bound active hydrogen atoms originating from the flow of H2 during the growth process. The mean size of as-produced graphene domains gradually increased with the increasing flow rate of H2, from 5 to 20 sccm, during the annealing process, as shown in Figure 1(b) and 1(c). Therefore, if an abundant number of hydrogen molecules exist in the annealing process, formation of surface-bound, active hydrogen atoms, which can assist to dissociate methane into active carbon species, increase. Subsequently, when methane molecules are introduced to the reaction zone, more active carbon species form and react to produce graphene. Therefore, the above result suggests that the growth


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rate of graphene domains also increases with increasing partial pressure of H2 gas during the annealing process. In addition to a marked increase in the size of the graphene domains, the SEM images further revealed that, with the increasing flow rate of H2, the density of the as-produced graphene domains noticeably increased, indicating that the partial pressure of H2 gas for the annealing process significantly affects nucleation density. Generally, based on the fact that the initial nucleation of graphene takes place more frequently at step edges, folds, or other imperfections on the Cu foil, it is known that the presence and the number of graphene domains strongly depends on the topography of the Cu foil surface. Recently, it was reported that the density of graphene domains can be suppressed by increasing the growth temperature, because the number of step edges, folds, or other impurities is reduced at higher temperature and the smaller number of defects on the Cu surface results in a lower density of nucleation sites and fewer domains.26, 27 However, based on the above result, we believe that an abundant amount of surface-bound, active hydrogen atoms originating from the high partial pressure of H2 gas for the annealing process lead to an activation of defects on the Cu surface, and resulted in an increased density of graphene domains even at the same growth temperature. Thus, by increasing the partial pressure of H2 gas for annealing, densely distributed and larger graphene domains were synthesized in the same reaction time. To further evaluate the quality of as-produced graphene, we obtained the Raman spectra of graphene domains grown after annealing with different flow rates of H2, shown in Figure 1(d). In the Raman spectrum of graphene, the G-peak (1580 cm-1) and D-peak (1320 cm-1) correspond to the E2g phonon at the Brillouin zone center and the breathing mode of sp2carbon atoms, respectively. Particularly, the D-peak is activated by the existence of defects in as-produced graphene.28 The observed Raman spectra of all graphene samples have almost


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negligible D-peaks and a high intensity ratio (I2D/IG) above 2.1, indicating that as-produced graphene domains are single crystals of monolayer graphene20,21 regardless of the partial pressure of H2 gas in the annealing process. To further confirm the uniformity of as-produced graphene domains, we conducted Raman mapping of the experimental graphene domains. For a hexagonally shaped graphene domain, shown in Figure 1(c), the intensities of the characteristic Raman peaks, such as G-peak and 2D-peak (~2690 cm-1), and the width of 2D-peak were extracted and their spatial dependencies were plotted in Figure 1e-g; the corresponding optical image is shown in Figure 1(h). The Raman mapping images further revealed that the intensities of G-peak and 2D-peak, IG (Figure 1(e)) and I2D (Figure 1(f)), are fairly uniform over asproduced hexagonal graphene regions with a high I2D/IG ratio above 2.6. The full width at half maximum (FWHM) of a symmetric 2D peak (Figure 1(g)) is below 35 cm-1, confirming single crystals of monolayer graphene. There are a few spots with high G-peak intensities and large 2Dpeak widths, which may be due to the beginning of a second layer. Moreover, the uniformity of the graphene domains can be identified through the homogeneous contrast in the optical image (Figure 1(h)). As a control experiment, we carried out CVD growth of graphene domains after annealing with different flow rates of Ar for the fixed flow rate (20 sccm) of H2. We focused on the influence of partial pressure of Ar used as a buffer gas during the annealing process on the growth rate and shape of the graphene domains. Figure 2(a), (b) show SEM images of graphene domains grown after annealing with two different Ar flow rates: 100 and 0 sccm. When only hydrogen was used during annealing process, as opposed to annealing with 100 sccm of Ar (Figure 2(a)), the snowflake-shaped carbon aggregates(s-CA) were synthesized on a Cu foil as


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shown in Figure 2(b). Raman spectra of the s-CA revealed that their 2D-peak shifted from 2680 to 2678 cm-1and the FWHM of their 2D-peak increased from 33 to 44 cm-1 (Figure 2(c)).

FIGURE 2. (a) SEM image of graphene domains synthesized with Ar/H2 = 100:20 sccm in the annealing process. (b) SEM image of the snowflake-shaped carbon aggregates(s-CA) after hydrogen annealing. Scale bar = 5 µm. (Inset) Enlarged SEM image of the s-CA. Scale bar = 500 nm. (c) Raman spectra taken at the graphene sample in panel a and the s-CA in panel b. In addition, the corresponding I2D/IG ratio decreased from 2.0 to 1.1 for the s-CA. Based on the fact that a bilayer graphene has a much broader 2D-peak with respect to a monolayer graphene,28 the above results indicate that the s-CA consists of stacked sp2 carbon but not a monolayer graphene.


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Figure 3 shows EDX spectra and the HRPES spectra of the s-CA. We observed that almost every s-CA had a particle in the center, as shown in the left SEM image in Figure 3. The particles are believed to have been introduced from the quartz tube used in the CVD chamber, and the formation of the s-CA was initiated by aggregation of activated carbon species in locations where they were deposited.29,30 EDX spectroscopy was performed to test the composition of the s-CA and the corresponding particles. The EDX spectrum for the s-CA showed the peaks corresponding to C, O, Cu, and Si elements, confirming the existence of SiO2 and carbon on the surface of the copper film. (Figure 3(b)) Furthermore, EDX spectra of a particle inside a s-CA show strong silicon and oxygen peak (Figure 3(c)). It consists of 12.84 % (atomic percent) Si and 22.45 % O, suggesting Si- rich SiO2 particles.

FIGURE 3. (a) SEM image of the snowflake-shaped carbon aggregates(s-CA) grown with only hydrogen present during annealing process. Scale bar = 1 µm. (Inset) SiO2 particles on Cu foil after H2 annealing. Energy-dispersive X-ray (EDX) spectra of the area enclosed by the solid line (b) and the dotted line (c) in panel a. (d) HRPES spectra of a graphene flake and the s-CA.


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To further investigate the carbon, silicon, and oxygen content, HRPES was performed with a scanning X-ray microprobe on both typical graphene samples and the s-CA. For the normal graphene, the main peak of the C 1s core-level spectrum had a narrow band width centered at 284.7 eV, corresponding to the binding energy of the surface sp2 hybridized states, and no other peaks were observed in the spectrum.31,32 In contrast, the HRPES spectrum of the s-CA exhibited the main C 1s peak centered at 284.3 eV and a peak at 287.8 eV corresponding to the binding energies of the bulk sp2 carbon and the C=O bond, respectively. 31,32 Regarding the assignment of sp2 carbon, Balasubramanian et al.31 have previously investigated the surface and bulk components of HOPG using synchrotron radiation with various energies. They obtained two fitting curves for each spectrum using the DŠ function convoluted with a Gaussian function, and then assigned the curve at the higher binding energy to surface sp2 carbon and the curve at the lower binding energy to bulk sp2 carbon. 31 Moreover, the C 1s presence of the C=O bond is due to H2O adsorption. 33 We also conducted Si 2p core level spectra for two distinct conditions to compare the electronic properties of the graphene sample and the s-CA. As shown in Figure 3(d), we did not detect any Si-related bonding features in the sheet with the graphene flake. On the other hand, we could observe the two Si 2p core level peaks in the substrate with the s-CA, which can be assigned to Si 1 (SiO2: 102.2 eV) and Si 2 (Si: 99,1 eV). It can be surmised that these Sirich SiO2 particles come from the quartz tube used in the CVD chamber.29 The overall CVD process is shown in Figure 4. In general, graphene growth occurs as a multistep reaction involving (1) binding of active hydrogen atoms to the surface of the Cu foil, (2) transport and adsorption of the carbon species on the surface, (3) dissociation of surfacebound CH4 as a form of active carbon species, (4) graphene nucleation and carbon incorporation into the growing graphene layer, and (5) etching of the as-formed graphene (Figure 4(a)).24


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FIGURE 4. Schematic diagrams of the formation of graphene and the snowflake-shaped carbon aggregates(s-CA) under different annealing conditions. (a) Formation of graphene after Ar and H2 mixed annealing. (b) Formation of s-CA after hydrogen annealing. (Xs: surface-bound molecules or atoms of X) Molecular hydrogen more readily dissociates on copper and continuously forms active hydrogen atoms during ramping, annealing, and growth. These hydrogen atoms promote activation of physisorbed methane as shown by reaction (2).24 That is, if enough number of active hydrogen atoms exist during the annealing time, they will facilitate the efficient activation of surfacebound carbon, when a carbon source is introduced into the reaction zone, and will thereby accelerating the growth rate. As a result, larger and densely distributed graphene domains were synthesized in the same growth time, as we can see in Figure 1(c). However, if the Cu foil is annealed with only hydrogen (20 sccm, H2), the surface is fully covered by surface-bound active


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hydrogen atoms (Figure 4(b)). Moreover, it was reported that hydrogen used in the pretreatment can affect the subsequent CH4 chemisorption kinetics.34 In this regard, once hydrogen covers the copper surface, methane molecules are prevented from binding to the surface to form the active carbon species. Subsequently, the hydrogen atoms that cover the Cu surface prevent the transport and adsorption of active carbon species onto the copper surface. Thus, graphene is presumed not to form through surface migration of the activated carbon species. Instead, methane molecules react with surface bound hydrogen atoms to form activated carbon, and subsequently, the activated carbons on HS atoms aggregate around active sites, such as SiO2 particle,29,30 to form stacked graphene quickly (Figure 4(b)). On the other hand, Ar, which serves as a buffer gas, is mainly used to add pressure to a system, and it causes collisions with the other co-existing molecules such as hydrogen.35 In this case, if Ar co-exists with hydrogen in the reaction chamber, it is likely to collide with hydrogen frequently, preventing hydrogen molecules from binding to the surface (see the Supporting Information). As a result, there are increasing opportunities for CH4 to bind to the copper surface, and therefore, normal graphene synthesis can proceed as illustrated in Figure 4(a). We finally measured the work function changes of a pristine monolayer graphene and s-CA while monitoring the low kinetic energy cutoff region, as shown in Figure 5. The measured kinetic energies were converted after correcting for the applied bias and the analyzer work function, so that the sample work function was obtained from the intersection between the baseline of the spectrum and the linear fit to the tail of the sample secondary electron cutoff. Generally, electronic properties, such global doping effects, on monolayer graphene can be confirmed by measuring the difference between the secondary electron edges, which is the same as the work function difference.36,37


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FIGURE 5. Top panel: Secondary edge measurements as a function of aniline exposure of the (hexagonal graphene flakes annealed with Ar/H2 (a) 300:20 (graphene flake) and (b) 0:20 sccm (the snowflake-shaped carbon aggregates), respectively under a sample bias of -20 V. Bottom panel: Band diagrams of work function change. The spectrum displayed in Figure 5(a) was taken of the typical monolayer graphene with work function of 4.30 eV synthesized as we control the volume ratio between argon and hydrogen concentration, which was same to the previous reported pristine monolayer graphene reference value.38-40 To compare the work function difference between monolayer and multilayer graphene, we also measured the work function value of multilayer graphene with about 4.47 eV. On the other hand, the spectrum obtained without argon flow in the annealing process (Figure


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5(b)), we clearly observed that the secondary electron edge moved to the higher-kinetic-energy region with respect to pristine monolayer graphene. In other words, the s-CA (Figure 2(b)) grown with hydrogen annealing without argon flow increases the work function value. The increase in the work function can be explained by partially charge transfer from graphene to the adsorbed oxygen (acquired from quartz tube) indicating to p-type doping character. Through the results of work function values, we could confirm that the fabricated process of graphene needs Ar as we showed our suggested mechanism (Figure 4(a)).

CONCLUSION In this study, we conducted atmospheric-pressure CVD growth of graphene domains on Cu foil with various volume ratios of mixtures of hydrogen and argon during annealing, to investigate the influence of the partial pressure of H2 on the growth rate and morphology of as-produced graphene domains. With increased partial pressure of H2 during the annealing process, densely distributed and larger graphene domains were synthesized in the same reaction time. In particular, the annealing process with pure hydrogen resulted in the increase in the graphene growth rate, which led to the formation of s-CA. Their composition and atomic configuration were confirmed by EDX and HRPES spectra. These results suggest that SiO2 particles from the quartz tube used in the CVD chamber acted as a seed for the formation of the bulk sp2 s-CA. Based on our results, we present the key reaction details for each growth process and a proposed growth mechanism as a function of the partial pressure of H2 during annealing process. Additionally, we clarified the role of hydrogen in the annealing process as well as in graphene growth, and furthermore, we could control the growth rate of graphene material by using various gas volume ratios during the annealing process.


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FIGURE 1. SEM images of graphene domains synthesized with Ar flow fixed at 300 sccm and H2 flow varied in the annealing process. (a) H2 = 0 sccm, (b) 5 sccm, and (c) 20 sccm. Scale bar = 10 µm. (d) Raman spectra corresponding to domains shown in panel a, b, and c. (e-f) Intensity mappings of the G and 2D peaks, respectively. (g) Image mapping of the full width at half maximum (FWHM) of the 2D peak. (h) Optical image of the graphene sample on a SiO2/Si substrate. Scale bar = 5 µm. FIGURE 2. (a) SEM image of graphene domains synthesized with Ar/H2 = 100:20 sccm in the annealing process. (b) SEM image of the snowflake-shaped carbon aggregates(s-CA) after hydrogen annealing. Scale bar = 5 µm. (Inset) Enlarged SEM image of the s-CA. Scale bar = 500 nm. (c) Raman spectra taken at the graphene sample in panel a and the s-CA in panel b. FIGURE 3. (a) SEM image of the snowflake-shaped carbon aggregates(s-CA) grown with only hydrogen present during annealing process. Scale bar = 1 µm. (Inset) SiO2 particles on Cu foil after H2 annealing. Energy-dispersive X-ray (EDX) spectra of the area enclosed by the solid line (b) and the dotted line (c) in panel a. (d) HRPES spectra of a graphene flake and the s-CA. FIGURE 4. Schematic diagrams of the formation of graphene and the snowflake-shaped carbon aggregates(s-CA) under different annealing conditions. (a) Formation of graphene after Ar and H2 mixed annealing. (b) Formation of s-CA after hydrogen annealing. (Xs: surface-bound molecules or atoms of X) FIGURE 5. Top panel: Secondary edge measurements as a function of aniline exposure of the (hexagonal graphene flakes annealed with Ar/H2 (a) 300:20 (graphene flake) and (b) 0:20 sccm (the snowflake-shaped carbon aggregates), respectively under a sample bias of -20 V. Bottom panel: Band diagrams of work function change.


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Author Contributions D.H.J. and J.S.L. designed the experiments with the additional support from and H.L. for controlled experiment design. D.H.J. carried out most of the experiments, and C.K., M.K., and H.C. helped with the controlled experiments. D.H.J., H.L., and J.S.L. wrote the paper. All authors analyzed the data, discussed the results, and commented on the manuscript. ACKNOWLEDGMENT We are grateful to Duhee Yoon for assistance with the setup of micro Raman spectroscopy and helpful discussions of synthesis of graphene. This work was supported by Nano·Material Technology Development Program (2012M3A7B4034986) funded by the National Research Foundation and the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2012-0009562). Additionally, it was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A2022597, 2013-021127). ASSOCIATED CONTENT Supporting Information More details on SiO2 particles and on the size distribution of s-CA. This material is available free of charge via the Internet at http://pubs.acs.org.


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Table of Contents

H2

CH4

Active sites

C Slow

Fast HS atoms

Formation of graphene and snowflake-shaped carbon aggregates


25

Effects of Hydrogen Partial Pressure in the Annealing Process on

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