Structural Reconstruction of Reduced Graphene

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C: Physical Processes in Nanomaterials and Nanostructures

Structural Reconstruction of Reduced Graphene Yu-Han Wang, Yi-Zhe Hong, Lo-Yueh Chang, Chia-Hao Chen, and Wei-Yen Woon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04937 • Publication Date (Web): 23 Jun 2018 Downloaded from http://pubs.acs.org on June 23, 2018

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The Journal of Physical Chemistry

Structural Reconstruction of Reduced Graphene Yu-Han Wang1,Yi-Zhe Hong1, Lo-Yueh Chang2, Chia-Hao Chen2, Wei-Yen Woon1* 1

Department of Physics, National Central University, Jungli, 32001, Taiwan, Republic of China

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National Synchrotron Radiation Research Center, Hsinchu, 30076, Taiwan, Republic of China

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Phone: +886 927374826.

ACS Paragon Plus Environment

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ABSTRACT

Micrometer sized oxidation patterns were created in chemical vapor deposition (CVD) grown multi-layered, non-Bernal stacked graphene through scanning probe lithography (SPL). The oxidized patterns were then reduced by irradiation using a focused x-ray beam. The topographical, structural, and chemical modification of the oxidized graphene were characterized through atomic force microscopy (AFM), micro-Raman (µ-RS) and micro-x-ray photoelectron spectroscopy (µ-XPS), respectively, before and after reduction of the oxidized patterns. For multilayered graphene, we found that oxidation only occurs for the first outermost layer. Furthermore, it was found that while the oxygen functional groups were almost completely removed by the reduction process, the restoration to two dimensional honeycomb structure of the reduced graphene were dependent on the number of layer. Notably, structural restoration is more effective for multilayered graphene, as compared to the single layered counterpart. The mechanism of structural restoration is discussed in light of solid phase epitaxial regrowth.


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INTRODUCTION

Since the emergence of two dimensional (2D) materials, there has been vast research activities round it, both from the perspectives of fundamental physics and applications.1 Among all the 2D materials, the mobility of graphene is the highest due to the unique delocalization feature of π-electron in the 2D honeycomb structure.2 Therefore, graphene is one of the most highly anticipated candidates to replace current bulk materials in electronics and energy storage applications.3,4 Nowadays, reduced graphene oxide (rGO) is the most commonly used form of graphene related materials in graphene related products, because rGO can be mass-produced in large quantity with relatively low costs.5 Nevertheless, contrary to pristine graphene acquired through mechanical exfoliation or chemical vapor deposition (CVD), rGO is regarded as quasigraphene because in most cases, rGO is usually severely disordered.6 The residual functional groups include carboxyl (COOH), carbonyl (C=O), and epoxy/ether (C-O), which are usually difficult to be removed by thermal, electrochemical, or other reduction methods.7 These residual oxide functional groups could disrupt the 2D honeycomb lattice and degrade the conductivity drastically, thereby limits its applications.8 Therefore, it is important to remove the functional groups efficiently in order to recover the characteristics of graphene. Recently, efficient removal of functional groups has been demonstrated by a number of research groups, through various different reduction methodologies.9–13 For example, Damien et al. showed effective removal of the oxide functional groups in graphene via microwave reduction.9 We have also demonstrated almost complete removal of oxygen functional groups in oxidized graphene through focused x-ray irradiation10–12, provided that the total oxygen coverage


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is not exceedingly high prior the reduction process. In the above works, we only performed oSPL on CVD grown single layered graphene (SLG) and found that the structural order of the reduced graphene was always worse than the oxidized graphene. Recently, we were able to grow multilayered graphene (MLG) with large grain size.14 The availability of the large grain MLG opens up the opportunity to study the local oxidation and reduction of graphene with different number of layers in one sample. If the MLG is oxidized using the similar electrochemical methodologies employed previously, would the oxidation occur for all the layers or just the top layer? Besides, would the reduction of the oxidized MLG be different from the SLG counterpart? The above issues are the main focuses of this paper.

EXPERIMENTAL METHODS

CVD growth of multilayered graphene. Prior to the thermal process, Cu foil (e-light Technology Inc., 99.9%, 25µm) was electro-polished in 98% H3PO4 under 2.5 V, 2.6 A for 1 min. Subsequently the Cu foil was cleaned with acetone and DI water to remove the organic contamination, before drying with N2 flow. The Cu foil then was placed on a SiC coated graphite boat for further thermal processing in a cold-wall RTA (As-Micro RTP System, ANNEALSYS).15 The major thermal process for CVD graphene growth includes 3 stages: A ramping (10 mins) to target temperature, a constant temperature period for graphene growth at 1040 ℃ for 1 hr, and a cooling to room temperature for 15 mins. During stage 1, The copper substrate was mildly oxidized by exposure to hot Ar (500 sccm, 870 mtorr) that contains low concentration of residual oxygen. After the ramping period, graphene film was grown with CVD involving CH4/H2 ratio of 37.5: 1 (H2 = 30 sccm, CH4 = 0.8 sccm) under low pressure (1.1 torr), maintained by the accompanying Ar flow. After graphene growth, a fast cooling is applied by


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switching off the heating lamp with continuous Ar, H2 and CH4 flows until cooling down to room temperature within 15 mins. To transfer the graphene onto 300 nm SiO2/Si wafer, a bubble-free transfer process was conducted in a home-made electrolytic cell with graphite plate as counter electrode and 1M NaOH as electrolyte solution.16 Oxidizing Scanning Probe Lithography (o-SPL). The transferred multilayered graphene was then locally modified through o-SPL using an atomic force microscope (AFM) (Bruker DInnova) equipped with conductive AFM probe (NSC14/Pt, 160kHz, 5.0N/m, MikroMasch).10–12,17,18 For better reproducibility, the o-SPL was conducted with contact mode under a constant set point, in a controlled relative humidity ~35%. A DC negative bias Vbias = -10V was applied to the tip during o-SPL. Under strong electric field, the tip-water bridge-graphene system acted as an electrolysis cell where oxidation occurred on graphene and reduction occurred at the metallic tip.19 Characterizations. Optical microscopy (OM) were acquired through an upright microscope (BX-51, Olympus). Structural characterization of graphene through Raman spectroscopy was acquired with a homemade micro-Raman spectrometer (excitation wavelength = 532 nm, spatial resolution = 1.5 µm, spectral resolution = 0.5 cm-1). Topographical information was acquired through AFM. The chemical bond profile was probed with micro-photoelectron spectroscopy (µXPS) through soft x-ray emission from a synchrotron radiation facility (beam line 09A1, National synchrotron radiation research center, Hsinchu). Chemical bonds associated with modification of the sp2 bond and functionalization of carbon were identified by taking the carbon 1s spectra around photoelectron energy at 284.5 eV.20 By focusing the intense x-ray beam down to < 0.1 um through a zone plate, and scanning the sample with a motorized stage, the chemical bond profile of the SPL defects was obtained with sub-micrometer resolution.21 While the X-rays


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involved in taking the spectra did not change the spectra of the pristine graphene, repeated exposure of the o-SPL treated samples resulted in significant changes in the C1s spectra after each round (20s) of acquisition. Thus only the first spectrum taken at each point was used to represent initial chemical bonding condition of o-SPL treated patterns prior to reduction. The XPS signal collected from each scanned spot was used to produce chemical mapping images (scanning photoelectron microscopy, SPEM).21 Reduction of oxidized graphene occurs with prolonged irradiation of focused x-ray on the oxidized patterns. The reduction dynamics was recorded in-situ by taking multiple high resolution C1s XPS (acquisition time = 20s) with continuous x-ray irradiation at target spots until the XPS remained unchanged (located according to the contrast shown in SPEM acquired immediately prior to high resolution XPS acquisition). The reduction was probably caused by bombardment from low energy photo-electrons generated from both graphene and the underneath SiO2 by the absorption of x-ray radiation.10,22

RESULTS

Before o-SPL, transferred MLG was characterized with OM, µ-RS, and AFM, to check the number of layer, structural properties, and morphology, respectively. Figure S1(a) shows optical image of a typical non-Bernal stacked MLG film with large grain sizes on SiO2/Si. MLG with layer number up to six layers can be clearly identified through OM. Under the same acquisition condition, Raman spectra show clear trend in increased G and 2D bands as number of layer increases (Figure S1(b)), with 2D/G ratio > 2 for all cases (Figure S1(c)), contrary to what is expected for Bernal-stacked MLG (Figures S1(d), S1(e), and S1(f)). The defect densities of the multilayer graphene grains are negligible as shown by the overall low D/G ratio (Figure S1(g)).23 Furthermore, the full width at half maximum (FWHM), peak area, and peak position of


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2D bands acquired for the MLG (Table S1) indicate that the twisting angles between layers are 20~30 degrees, i.e., confirming that the layer stacking is non-Bernal.24 Zoom-in views of the non-Bernal stacked MLG measured through AFM and OM, respectively, are shown in Figures 1(a) and 1(b). As shown in the cross sections (bold dark and light blue lines in Figure 1(a)) taken across different layers, it was found that each graphene layer is about 4Å, as expected. XPS taken at SLG, BLG, and TLG are shown in Figure S2. With the same acquisition condition, the peak intensity of C=C bond (284.4 eV) increases as graphene layer increases. Nevertheless, the concentrations of oxide related functional groups remain the same for SLG, BLG, and TLG. The data shows that the photoelectron detection sensitivity in our system is high enough to distinguish the difference in amount of photoelectrons emitted with precision down to one atomic layer.


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Figure 1. (a) AFM and (b) OM of a non-Bernal stacked multilayered graphene. The two light blue lines are the cross-sections across two layers, while the long dark blue line is the cross-section across three layers. As shown in the cross sections. (c) AFM and (d) OM of the same graphene area after o-SPL fabrication. After o-SPL with Vbias = -10V, oxidized patterns with size = 1.5 µm x 1.5 µm are fabricated. For better statistics, in total 70 patterns were fabricated on the SLG, BLG and trilayered (TLG) regions on the MLG film (Figure S3). As shown in Figures 1(c) and 1(d), clear contrast can be seen through OM because the effect of optical interference for oxidized region and the pristine CVD graphene region are different under white light illumination. The height of oxidized patterns is about 7 Å higher than the surrounding pristine regions. Figure 2 shows the chemical modification of MLG by o-SPL. Figure 2(a) shows the optical image of a region where SLG, BLG, and TLG are oxidized through O-SPL. SPEM images for channel 8 (~286.5 eV, C-O bonds) and channel 11 (~284.4 eV, C=C bonds) of the C1s spectrum are shown in Figures 2(b) and 2(c), respectively. From Figure 2(c), we note that SPEM images are able to show clear contrasts between SLG, BLG and TLG in term of peak intensity of the C=C bond (284.4 eV). It


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is evident that C=C bonds are converted into C-O related bonds after o-SPL process. The above characterization shows that o-SPL results in modifications of topographical, optical, and chemical properties.

Figure 2. (a) OM of a region where SLG, BLG, and TLG are oxidized through OSPL. (b) SPEM images for channel 8 (~286.5 eV, C-O bonds) and (c) channel 11 (~284.4 eV, C=C bonds) of the C1s spectrum. Previously, it was found that for graphene/Si or graphene/SiC cases17, 25, 26, o-SPL would not only oxidize the top graphene layer but also the underneath Si/SiC substrates. On the other hand, o-SPL process on exfoliated (Bernal-stacked) BLG also seems to oxidize more than one atomic layer27 as the 2D band is almost completely diminished after o-SPL. In our case, the nonBernal stacking features in the MLG transferred on SiO2/Si may cause differences in the result of o-SPL, compared to previous works. Therefore, we need to first figure out whether the o-SPL would have similar or different effects on the MLG system through closer looks at the structural and chemical characterizations using µ-RS and µ-XPS.


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Figure 3. (a) Raman spectra acquired at the oxidized patterns in SLG, BLG, and TLG, respectively. Inset shows the D peak areas of pristine and oxidized graphene with different number of layers. (b) XPS spectra acquired at the oxidized patterns in SLG, BLG, and TLG region, respectively. Inset shows the peak area of C-O related bonds of pristine and oxidized graphene with different number of layers. Figure 3(a) shows the Raman spectra acquired at the oxidized patterns in SLG, BLG, and TLG, respectively. It can be clearly seen from the spectra that blue-shift, along with the decrease/increase in intensity/width of G band is only significantly for SLG. The inset of Figure 3(a) shows the statistical result of Raman measurement averaged over all available measurable patterns. Initially, SLG (black bars in the inset) possesses negligible D band as the CVD graphene quality is good. The D band area slightly increases as the number of graphene layer increases due to the 20~30 degrees twisting angle between layers.28 After o-SPL (blue bars in the inset), the increases of the integrated D band areas are found to be almost identical for SLG, BLG and TLG. Moreover, comparing to the Raman spectra in previous SPL work on exfoliated MLG, where G bands decreased significantly for both SLG and BLG, the G bands in our o-SPL treated MLG remains high and narrow. The observation suggests that in our case the sp2 C-C


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bonds are still highly ordered for MLG, while for SLG the oxidation leads to severe deterioration in structure, in spite of the fact that the amounts of defect generation are similar. We further examine the chemical modification of the oxidized patterns in SLG, BLG and TLG, through XPS. Figure 3(b) shows the XPS spectra acquired at the oxidized patterns in SLG, BLG, and TLG region, respectively. The inset in Figure 3(b) shows the statistical data averaged over all available oxidized patterns. Prior to o-SPL, there are almost no measurable differences between the integrated peak areas of oxide related functional groups (COOH + C=O + C-O) for SLG, BLG, and TLG. After o-SPL, the integrated peak areas of the oxide function groups all increase ten folds but the differences between SLG, BLG, and TLG are still within error bars (blue bars in inset). Combining the above observations from µ-RS and µ-XPS, we conjecture that o-SPL resulted in the same extent of oxidation for graphene with different number of layers but the oxidation only occurs at the top outermost layer. We cannot give a definite answer on why it is so at the moment but it is the most plausible scenario we can think of. Perhaps the non-Bernal stacking essence of the MLG, in which the interactions between conductive SLG layers are negligible, limits the oxidation only at the surface layer, contrary to the Bernal stacked exfoliated MLG. The MLG cases (defected oxidized C-C network layer on a crystalline C-C network layer) may be viewed as the one atomic-layer version of amorphous/crystalline interface found in the bulk solid state system. On the other hand, the SLG/SiO2 case may be viewed as the amorphous/amorphous interface. Will the result of structural reconstruction be different for the two cases under the same treatment? This is the main aim for this work.


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Upon prolonged irradiation of focused soft x-ray (beam energy = 400 eV), the oxidized graphene areas were reduced (Figure S4). The reduction dynamics were recorded and there were no significant differences found for SLG, BLG, and TLG (Figure S5). The reduction dynamics is a step-by-step type, i.e, the oxide functional groups with higher binding energies convert to the one with lower binding energies subsequently. The finding is in accordance to our previous work in which step-by step reduction is expected when total oxygen coverage is lower than 70%.12 Figure 4(a) shows the XPS spectra acquired at the reduced oxidized patterns in graphene regions with different number of layers. The XPS spectra of pristine CVD graphene are shown in the inset of Figure 4(a). While in both cases, the C1s signals of MLG are stronger than SLG due to stronger photoelectron emission, slight blue shift of the C1s peak can only be found for reduced SLG. Figure 4(b) shows that the amount of oxide related functional groups in the reduced oxidized patterns slightly decreases as the number of layer increases. On the other hand, the composition of C-C related bonds is slightly different for different number of layers. In particular, the ratio of C=C to C-C increases as number of layer increases (Figure 4(c)). The above observations imply that removal of oxide related functional groups is more complete, and formation of the more stable C=C bonding is more favored in the bond reconstruction process, for reduced oxidized graphene with graphene flake as the template underneath.


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Figure 4. (a) XPS spectra acquired at the reduced oxidized patterns in graphene regions with different number of layers. Inset shows the XPS spectra of pristine CVD graphene as comparison. (b) The peak area of C-O related bonds of pristine and reduced graphene. (c) The peak area ratio of C=C/C-C of pristine and reduced graphene. Figures 5(a), 5(b) and 5(c) show the Raman spectra prior to o-SPL, after o-SPL, and after X-ray irradiation induced reduction, acquired on SLG, BLG, and TLG, respectively. Broadening of G bands can be found for SLG, BLG and TLG, after X-ray reduction. On the other hand, for BLG and TLG, D/2D bands increase after reduction. However, for SLG, 2D band decreases significantly after reduction. Detailed analysis on Raman spectra for the reduced oxidized patterns on graphene with different number of layers are shown in Figures 5(d) and 5(e). Figure 5(d) shows the integrated D″ peak areas for pristine CVD graphene, o-SPL treated, and X-ray irradiation reduced oxidized graphene. The D″ band was found to be a good indicator for graphene crystallinity, i.e., D″ decreases as crystallinity of graphene improves.29 While there is hardly any D″ signal for pristine and o-SPL treated graphene, significant D″ appears after the X-


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Figure 5. Raman spectra prior to o-SPL, after o-SPL, and after X-ray irradiation induced reduction, acquired on (a) SLG, (b) BLG, and (c) TLG, respectively. (d) The D” peak area. (e) The peak area of 2D band. ray irradiation induced reduction, probably due to the dynamical nature of the bond reconstruction process that caused further extended structural defects. It is clear that D ″ band area decreases as graphene layer increases. Figure 5(e) shows the evolution of the integrated 2D band area. Contrary to the SLG cases, in which the 2D band decreases after reduction, the 2D band measured at reduced oxidized patterns on BLG and TLG increase after the reduction process. The above observations suggest the honeycomb lattice of the underneath graphene layer


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could support more ordered structural reconstruction process of graphene during the x-ray irradiation induced reduction, and result in better crystallinity (lower D″) and 2D honeycomb structural order (higher 2D).

DISCUSSION

In previous works on SLG, Raman spectroscopy revealed that the structures of SLG were still defective after reduction, as evidenced by significantly higher D/G ratio, broader G band, and lower 2D band. It was understood that the above measurements indicated removal of oxygen functional groups led to degradation in 2D planar structure, and generation of nanometer graphene domains. There are two major reasons that lead to structural degradation in the reduced graphene. First, generation of C-O related volatile products during the reduction process inevitably results in vacancies in the graphene plane.30 Second, the reconstruction of C-C bonds during reduction may be a random kinetic process because there is no lattice matched crystalline template that could guide the reconstruction of honeycomb structure during the reduction process. Namely, the structural reconstruction is non-epitaxial. In bulk crystal, solid phase epitaxial regrowth (SPER) is routinely employed to form epitaxial film from amorphous solids. For instance, recrystallization of amorphous Si film could occur on crystal Si, starting at the amorphous/crystalline interface.31, 32 If the amorphous Si film was deposited on an amorphous substrate, the reconstructed bond structure in the film would still remained disordered even under the same thermal annealing condition because the bond reconstruction directions are not energetically favored along a particular direction. On the contrary, in SPER, the originally disordered Si-Si bond in the amorphous film reconstructed to form ordered lattices according to the crystalline structure underneath, during thermal annealing.


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Could the above SPER concept be borrowed and applied to reconstruct 2D crystal structure such as the honeycomb lattice in graphene from initially disordered C-C network? The Raman and XPS measurement suggest similar physical mechanism may be involved in the 2D version of SPER. Similar epitaxial growth of 2D materials such as MoS2 grown on graphene was found in previous work33. The Van der Waals interactions between the MoS2 film and the underneath graphene template resulted in rotationally commensurate epitaxial MoS2 with lower defect density than other lattice mismatch templates. In short, based on our study, we propose a promising way to recover the 2D honeycomb structure of the reduced oxidized graphene through providing a lattice-matched template underneath, similar to the SPER concept of 3D bulk crystal.

CONCLUSION

We study the chemical reduction and structural reconstruction of a locally oxidized graphene on a perfectly lattice matched template, i.e., graphene itself. CVD grown non-Bernal stacked multilayered graphene was oxidized by SPL under ambient condition. It turns out that only the top outermost graphene layer was oxidized by SPL while the underneath graphene layers remained intact. Chemical reduction and structural reconstruction of the defected and oxidized top graphene were found under prolonged focused x-ray irradiation. While the effectiveness of oxide functional groups removal was similarly for the SLG and MLG, the structural reconstruction showed drastic differences. It is found that the recovery to 2D planar structure is by far better for the case with graphene as the underneath template.


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SUPPORTING INFORMATION

Figure S1 shows the OM and Raman spectroscopies of typical non-Bernal and Bernal stacked MLG graphene. Table S1 shows the details of the characteristic Raman peaks acquired from the non-Bernal stacked MLG graphene. Figure S2 shows the XPS taken at the non-Bernal stacked SLG, BLG, and TLG. Figure S3 shows the OM of all SPL-oxidized patterns used in this work. Figure S4 shows the SPEM images of the SPL patterns before and after the x-ray irradiation induced reduction. Figure S5 shows the waterfall figures and corresponding heatmaps of reduction processes of x-ray irradiation induced reduced SPL graphene for SLG, BLG, TLG. Figure S6 shows the detailed XPS spectra acquired during the reduction of the oxidized graphene. Also shown are the typical fitting parameters used for the XPS spectrum acquired for an oxidized graphene. Figure S7 shows the typical fitting parameters used for a Raman spectrum acquired from a defected graphene. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT

This work is supported by the Ministry of Science and Technology of Taiwan under contract MOST106-2112-M008-003-MY3


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Figure 1. (a) AFM and (b) OM of a non-Bernal stacked multilayered graphene. The two light blue lines are the cross-sections across two layers, while the long dark blue line is the cross-section across three layers. As shown in the cross sections. (c) AFM and (d) OM of the same graphene area after o-SPL fabrication. 85x74mm (300 x 300 DPI)


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Figure 2. (a) OM of a region where SLG, BLG, and TLG are oxidized through O-SPL. (b) SPEM images for channel 8 (~286.5 eV, C-O bonds) and (c) channel 11 (~284.4 eV, C=C bonds) of the C1s spectrum. 85x33mm (300 x 300 DPI)



Figure 3. (a) Raman spectra acquired at the oxidized patterns in SLG, BLG, and TLG, respectively. Inset shows the D peak areas of pristine and oxidized graphene with different number of layers. (b) XPS spectra acquired at the oxidized patterns in SLG, BLG, and TLG region, respectively. Inset shows the peak area of CO related bonds of pristine and oxidized graphene with different number of layers. 175x65mm (300 x 300 DPI)


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Figure 4. (a) XPS spectra acquired at the reduced oxidized patterns in graphene regions with different number of layers. Inset shows the XPS spectra of pristine CVD graphene as comparison. (b) The peak area of C-O related bonds of pristine and reduced graphene. (c) The peak area ratio of C=C/C-C of pristine and reduced graphene. 175x84mm (300 x 300 DPI)



Figure 5. Raman spectra prior to o-SPL, after o-SPL, and after X-ray irradiation induced reduction, acquired on (a) SLG, (b) BLG, and (c) TLG, respectively. (d) The D” peak area. (e) The peak area of 2D band. 175x166mm (300 x 300 DPI)


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Structural Reconstruction of Reduced Graphene

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