Article Cite This: J. Phys. Chem. C 2017, 121, 27915−27922
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Laterally Selective Oxidation of Large-Scale Graphene with Atomic Oxygen Olesya O. Kapitanova,†,‡ Elmar Yu. Kataev,†,‡ Dmitry Yu. Usachov,§ Anna P. Sirotina,∥ Alina I. Belova,† Hikmet Sezen,⊥ Matteo Amati,⊥ Mohamed Al-Hada,⊥ Luca Gregoratti,⊥ Alexei Barinov,⊥ Hak Dong Cho,# Tae Won Kang,# Gennady N. Panin,#,∇ Denis Vyalikh,○,◆ Daniil M. Itkis,† and Lada V. Yashina*,† †
Lomonosov Moscow State University, Leninskie gory, 119991 Moscow, Russia St. Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg 199034, Russia ∥ Institute of Nanotechnology of Microelectronics RAS, Nagatinskaya str., 16A/11, 115487 Moscow, Russia ⊥ Elettra - Sincrotrone Trieste S.C.p.A., Area Science Park, I-34012 Basovizza, Trieste, Italy # Quantum-Functional Semiconductor Research Center, Nano Information Technology Academy, Dongguk University, Seoul 100-715, Republic of Korea ∇ Institute of Microelectronics Technology and High-Purity Materials Russian Academy of Sciences, Chernogolovka, Moscow district 142432, Russia ○ Donostia International Physics Center (DIPC), Departamento de Fisica de Materiales and CFM-MPC UPV/EHU, 20080 San Sebastian, Spain ◆ IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain §
S Supporting Information *
ABSTRACT: Using X-ray photoemission microscopy, we discovered that oxidation of commercial large-scale graphene on Cu foil, which typically has bilayer islands, by atomic oxygen proceeds with the formation of the specific structures: though relatively mobile epoxy groups are generated uniformly across the surface of single-layer graphene, their concentration is significantly lower for bilayer islands. More oxidized species like carbonyl and lactones are preferably located at the centers of these bilayer islands. Such structures are randomly distributed over the surface with a mean density of about 3× 106 cm−2 in our case. Using a set of advanced spectromicroscopy instruments including Raman microscopy, X-ray photoelectron spectroscopy (μ-XPS), Auger electron spectroscopy (nano-AES), and angle-resolved photoelectron spectroscopy (μ-ARPES), we found that the centers of the bilayer islands where the second layer nucleates have a high defect concentration and serve as the active sites for deep oxidation. This information can be potentially useful in developing lateral heterostructures for electronics and optoelectronics based on graphene/graphene oxide heterojunctions
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forth.3 Among these tools covalent functionalization by H, O, or F4 enables control over the band gap in a wide range. Foreign atoms in this case are covalently bonded to carbon, which, in turn, becomes sp3 hybridized (for H or halogens) or forms C−O−C bridges without destruction of C−C bonds and graphene layer. Although such systems are less thermodynamically stable in comparison with those obtained by lattice doping, the ordering of chemisorbed species is not required for gap opening in contrast to the case of substitutional doping. Another benefit of covalent functionalization is the ability to
INTRODUCTION
Being a material that can be potentially included into a broad range of high-speed electronic and optical devices, graphene currently appears to be the most promising platform for nearfuture electronics, in particular for creating flexible and transparent digital devices.1 One of the main obstacles that limits the performance of such devices is the absence of a band gap in the graphene electronic structure.2 Within the past decade several physical and chemical approaches have been proposed to open the gap and to control its width. These methods include dimensional narrowing, application of a strong electric field, incorporation of foreign atoms into the graphene lattice during growth or postgrowth treatment (i.e., lattice doping with N, B, S, P), covalent functionalization, and so © 2017 American Chemical Society
Received: August 7, 2017 Revised: November 16, 2017 Published: November 16, 2017 27915
DOI: 10.1021/acs.jpcc.7b07840 J. Phys. Chem. C 2017, 121, 27915−27922
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Table 1. Summary of C 1s Binding Energies and Chemical Shifts (eV) for Graphene Grown on Different Substrates and Oxidized under Different Conditions chemical shift, eV sample
oxidation treatment
sp2
Gr/Cu Gr/Ir Gr/Cu Gr/Au/Ni Gr/SiC Gr/Cu Gr/Ir, Gr/Pt Gr/Ir HOPG HOPG
RF plasma O3/UV O3/UV O2 cracking O2 cracking O3/UV O2 cracking RF plasma RF plasma RF plasma
284 284.16 284.5 284.3 284.5 284.7 284.14 284.14 284.3 284.5
β-sp2 0.6 0.53 0.6 0.6
0.3−0.6 0.46 0.3 0.6
epoxy
carbonyl
1.8−1.9 1.86 1.87 1.74 1.8 1.8 1.5, 1.8 1.66−1.9 1.9−2.2 1.8
3.1 2.9 3.1 2.7
ether/lactone −/4.5 1.1/4.2 −/4.2 −/3.8 −/3.9
2.66 2.4−2.7
1.16/3.86 0.9−1.2/3.7−4.2 1.4/−
ref this work; for details see also Figure S4
7 11 4 16 28 29
the graphene−metal interaction is relatively weak compared to typical segregation energies for carbon, and therefore, the growth is unstable toward making mounds rather than uniform multilayers on metals that bind graphene weakly. Thus, twolayer regions are unavoidably formed on Cu substrates. Typically, they consist of few parts of differently oriented grains20 nucleated from a single central region that evidently has a high defect concentration; the dominant defects are domain boundaries and vacancies.21 Local oxidation of graphene presents an interesting case because one needs to create lateral graphene/graphene oxide heterojunctions, which are reported recently6,22 to have outstanding electronic and optical properties. Local oxidation can be achieved, for instance, using anodic processing of graphene in atomic force microscope. It can be used to oxidize graphene locally with a tunable oxidation level providing local electronic properties from semiconducting to semi-insulating graphene oxide.6 Here, using X-ray photoemission spectromicroscopy, we show that oxidation of large-scale CVD-grown graphene on Cu foil, which has a certain density of second layer islands underneath graphene, proceeds in laterally nonuniform manner. This is accompanied by formation of specific structures distributed over the surface with a mean density of about 3 × 106 cm−2. Each structure includes the most oxidized 1 μm center surrounded by a less oxidized bilayer region, with relatively mobile epoxy groups being generated uniformly across the surface of single-layer graphene. We conclude that the more oxidized carbon species are preferably located at the central sites, surrounded by bilayer regions, which presumably acted as nucleation centers for the second graphene layer underneath graphene during graphene growth. By comparing the Raman and XPS maps of pristine and oxidized graphene, we reveal that these centers have the high concentration of domain boundaries and vacancies being the most vulnerable toward oxidation by atomic oxygen.
obtain materials with a high number of foreign atoms. Up to 25% coverage can be reached in the case of hydrogen or even more for deuterium.5 Graphene functionalization with oxygen offers even higher degree of freedom for the control of electronic structure.6 Oxygen-modified graphene can be obtained by using a variety of methods including treatment by atomic oxygen,4,7 oxygen plasma8−10 or ozone,11 photocatalytic oxidation12,13 or by chemical exfoliation of graphite using Hummers’ method.14,15 The latter technique leads to so-called graphene oxide, which forms micron-sized flakes whereas large-scale uniform films are often required for practical applications in electronics. Another drawback of this approach is the generation of various functional groups simultaneously (hydroxyl, epoxy, carbonyl, carboxyl, etc.). In contrast to chemical exfoliation, oxygen plasma and atomic oxygen exposure or ozone treatment under UV light of planar large-scale graphene results in more controllable introduction of specific oxygen functionalities containing no hydrogen. Oxygen plasma, although it is easy for technology, also induces vacancies to a significant degree.8−10 Therefore, atomic oxygen is perhaps the most promising agent for specific chemical functionalization of graphene, as it acts safely without significant damage of the graphene structure.4,7 Numerous studies were focused on the graphene oxidation.4,7,11,16 It is now generally acknowledged that epitaxial graphene oxidation with atomic oxygen results in generation of isolated epoxy groups at low coverage as it was demonstrated for the layers obtained on SiC (0001),7 Ir (111),4,16 and Pt (111)4 substrates. Graphene synthesized on precious metals is corrugated4,16 and oxidizes nonuniformly at nanometer scale due to preferential adsorption of epoxy groups at valleys than at ridges.4 However, at larger scale, oxygen is distributed uniformly.4 Large-scale graphene production for practical applications cannot be implemented using metal single-crystal substrates and is usually associated with synthesis of graphene by chemical vapor deposition (CVD) mainly on copper or, more rarely, on nickel foils.17,18 At the same time much less is known about oxidation pathways for such samples, which typically contain numerous inhomogeneities. Such inhomogeneities are mainly two-layer regions, which can appear, in most cases, due to segregation of carbon dissolved in metal substrate at high temperatures.19 This mechanism typically makes fabrication of single-layer graphene difficult, but it is still achievable for copper having relatively low bulk solubility of carbon at synthesis temperature. Meanwhile, even copper− graphene debonding driven by segregation is possible because
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METHODS Reference samples of graphene/Au/Ni(111)/W(110) were prepared in situ under ultrahigh-vacuum (UHV) conditions by chemical vapor deposition of propylene at 1 × 10−6 mbar on ∼10 nm thick Ni(111) films on W(110) single crystals at 600 °C for 10 min. This procedure was followed by intercalation of one monolayer (ML) Au at 550 °C for 20 min in accordance with the protocol described elsewhere.23 To grow graphene on Cu foil (99.999%, 10 × 30 cm2, 25 μm thick, Alfa Aesar), the latter was preliminarily annealed at 1060 27916
DOI: 10.1021/acs.jpcc.7b07840 J. Phys. Chem. C 2017, 121, 27915−27922
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The Journal of Physical Chemistry C °C under a hydrogen flow of 300 sccm and an argon flow of 2000 sccm at a pressure of 30 (I(G)/ I(D)≤4) 30−39
37, 47, 48
20, 38, 39
30−39
8, 36, 37
>39
8, 36, 37
24−30
38, 40
Figure 3. Evolution of the surface composition upon atomic oxygen treatment (RF plasma, 22 ± 2 °C) of pristine graphene on Cu foil: (a−d) C 1s spectra, (e−h) O 1s maps, and (i−l) C 1s maps. (m) Typical Raman 2D/G band intensity ratio (in red/blue) and D (in yellow/mustard) width and (n) G/D map of the sample oxidized for 7.5 min and (o) representative Raman spectra for the single-layer area, double-layer regions, and their central parts. (p) Variation of the relative intensity of the C 1s spectral features. (q) Schematic structure of the oxidation center (side view). Scale bar corresponds to 5 μm for all maps.
Although influenced by some other factors, the 2D/G intensity ratio is a main indicator of bilayer formation. In our case the copper surface is mainly covered by a single-layer graphene with a 2D/G intensity ratio between 2.5 and 4. Double-layer regions are marked with blue (2D/G < 2.5) and red (2D/G > 4). Though the former is typically associated with AA- and AB-stacked8,36,37 or slightly rotated layers,20,38,39 the latter is frequently assigned to highly misoriented layers.38,40 The corresponding typical spectra for single- and double-layer regions are exhibited in Figure 2b,c. Other parameters such as 2D peak position, peak width, and the D/D′ ratio are found to
be typical for graphene. The results of its mapping are shown in Figure S5 of the Supporting Information.27,36 It is worth noting that the G/D intensity ratio reveals highly defective areas in the centers of double-layer regions.41 According to the D/D′ ratio in these areas of 3.5−7 (Figure S5d) the dominant defects are domain boundaries and vacancies.21 So second layer islands appearing with a mean density of about 6 × 106 cm−2 have a complex structure including two or more crystallites of different orientation with rather high defect concentration in the center, which is believed 27919
DOI: 10.1021/acs.jpcc.7b07840 J. Phys. Chem. C 2017, 121, 27915−27922
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graphene on Au/Ni(111). The number of oxygen-containing groups for graphene on Cu first increases and then reaches saturation that is clearly observed in Figure 3p. Saturation on the curves means that either all active sites are already oxidized or further oxidation is accompanied by CO2 emission, which is typically detected at high temperatures only.16,28 CO/CO2 emission should be accompanied by increase of the defect concentration. The spectra presented in Figure 3 were acquired with lower energy resolution than those in Figure 1 for epitaxial graphene on Au/Ni(111), and the defect concentration cannot be extracted. To solve this issue, we have measured highresolution C 1s spectra for the graphene treated with atomic oxygen generated by O3/UV. According to these data (Figure S13) pristine graphene grown on Cu foil has 9% defects, which is in good agreement with the number of defects estimated from Raman spectrum (∼5%). Upon the treatment, the defect concentration deduced from XPS does not change, whereas the D mode in the Raman spectra increases drastically (not due to defects but due to C−O bonds in epoxy groups covering the surface; they are not stable in UHV and under X-ray beam, and therefore, the epoxy group concentration is always underestimated by XPS), which is typical for graphene covered with epoxy groups. Moreover, estimating the C/Cu atomic ratio before and after treatment, we can conclude that CO/CO2 species are not evolved. This conclusion is also supported by the direct STM observation reported previously,4 where the treatment of a graphene/Ir sample with atomic oxygen does not result in formation of holes in graphene. Holes in graphene are generated only after postannealing of this sample in vacuum conditions. Spatially resolved data for the oxidized graphene on Cu foil are collected in Figure 3e−l and Figures S9 and 10. At the beginning of the process, when only epoxy groups are present at the surface as follows from the C 1s spectrum in Figure 3a, both C 1s and O 1s maps in Figure 3e,f suggest uniform distribution of epoxy groups over the surface of single-layer graphene. It is strongly supported by Raman maps in Figure 3m,n, where a sharp increase of D-mode related to oxidation is observed for both the single-layer region (initially defect-free) and the centers of double-layer regions (initially very defective). The corresponding spectra are shown in Figure 3o. At the same time, the elemental contrast appears in the O 1s map in Figure 3f, indicating the fact that bilayer regions are less oxidized. Indeed, Raman spectra for these areas show a very weak Dmode. The lower reactivity of bilayer regions was observed earlier and explained by several reasons such as better stability of bilayer graphene, etc.;50 meanwhile, to interpret this effect in our case, we assume the decisive role of substrate in the stability of epoxy groups at the graphene surface. After a certain exposure, photoemission maps reveal stronger elemental contrast due to nonuniform oxygen distribution. The O 1s maps in Figure 3f−h demonstrate three different intensity levels. Bright 1 μm spots of maximal intensity are randomly distributed over the field with medium intensity, which correspond to single-layer graphene uniformly covered by epoxy groups. These highlighted areas indicate that the most oxidized regions are located in the centers of bilayer areas, and, as seen from the C KLL Auger map in Figure 2f, comprise only one layer shown to be quite defective. As expected, an inverted contrast is observed in the C 1s maps in Figure 3j−l. Moreover, maps of oxygen-containing carbon species distribution (Figures S9 and 10, Supporting Information) fit well to the oxygen intensity distribution. In contrast to the thermal oxidation of
to be a starting point of their growth. Total coverage of the bilayer graphene, however, does not exceed 10%. XPS and Auger mapping directly confirmed that the contrast in 2D/G Raman maps is caused by two-layer regions in graphene. XPS C 1s and Cu 3p maps in Figure 2d,e, as expected, demonstrate inverted contrast. With Cu 3p peak attenuation, the carbon layer thickness was estimated to correspond to single- and double-layer graphene (see the Supporting Information, Figure S7 for details). The C KLL Auger intensity map provides a better resolution image and reveals a structure of the central part of bilayer regions in detail (Figure 2f). It can be clearly seen that the C KLL intensity in the center, which is highly defective, is close to that for a singlelayer graphene region. As it was shown, such bilayer islands can nucleate at the defects on the metal surface, e.g., dislocations.42,43 The mismatch in neighboring grain orientation at the same time does not affect the orientation of the first graphene layer, which appears to be of a high structural perfection that is confirmed by a sharp ARPES pattern along with the absence of the D mode in the Raman spectra and low width of C 1s peak (close to that of graphene on Au/Ni(111)). μ-ARPES map in Figure 2g demonstrates photoemission intensity distribution for the energy corresponding to Cu 3d states. Comparison of ARPES patterns collected in points A and B, both representing singlelayer graphene but located over different copper crystallites (Figure 2h,i), evidences that graphene orientation is not directly related to the crystallite orientation of the Cu foil. For both points we observe a characteristic Dirac cone with the Dirac point slightly below the Fermi level, pinpointing no graphene−substrate interaction. At the same time Cu substrate sp bands with parabolic dispersion near the Fermi level44,45 show up at different positions relative to the graphene K-point, which is in line with epitaxy between Cu surface and graphene.45 μ-ARPES further helped in deeper understanding of the layer stacking in double-layer regions. In these areas (see point C in Figure 2g and corresponding dispersion in Figure 2j) we observe a double-Dirac cone as schematically illustrated in the inset. The Dirac point is shifted by 0.42 eV toward higher binding energies with respect to the π-band of single-layer graphene showing n-doping. The doping is also clearly seen in the induced current maps presented in Figure S8 of the Supporting Information. Besides, a band gap of 0.26 eV is seen at the K-point. The separation between two Dirac points (0.34 eV) indicates the AB stacking of the graphene layers.46 We assume that bilayer regions are formed by an upper continuous layer and lower graphene islands, which was further found to be in line with the oxidation behavior and was also supported by earlier LEEM studies.42 The oxidation behavior, which we are now discussing, can help in understanding the structure of bilayer islands. If the fragments of the second layer grow on top of the first continuous layer, one could expect faster oxidation of their edges by analogy with the results on hydrogen etching obtained in ref 49. However, these are the single-layer spots in the center of bilayer islands that demonstrate increased reactivity toward oxygen. For pristine graphene, the spectrum is described well with a single spectral feature positioned at 284.3 eV and assigned to the sp2 network. Panels a−d of Figure 3 show areaintegrated C 1s spectra of graphene in the course of oxidation by atomic oxygen. The evolution of surface composition during oxidation was found to be generally similar to that of epitaxial 27920
DOI: 10.1021/acs.jpcc.7b07840 J. Phys. Chem. C 2017, 121, 27915−27922
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graphene on Cu foil in air, when Cu is oxidizing under defect places of graphene,31 we observe exclusively graphene oxidation rather than oxidation of Cu substrate, which stays intact according to Cu 3p spectra (Figure S11). We assume that although epoxy species are more or less uniformly distributed over the surface, carbonyl and lactone species are located mainly in the centers of two-layer islands. This assumption is additionally supported by the Raman maps, and spectra presented in Figure 3m−o and Figure S12. Despite the similar G/D intensity ratios for the centers of double-layer islands and single-layer graphene, the nature of defects is not the same. In detail, the spectra for centers are more than twice as broad (it is close to the value typical for amorphous Hummers graphene oxide14,15) due to a high defect concentration before oxygen treatment.51
‡
Olesya O. Kapitanova and Elmar Yu. Kataev contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Elettra and Helmholtz-Zentrum Berlin (HZB) for the allocation of synchrotron radiation beamtimes at ESCA miscroscopy, spectromicroscopy, and Russian-German beamlines. The work is performed within the joint project of the Russian Science Foundation (16-42-01093) and DFG (LA65517/1). The work of O.O.K. was supported by the Russian Foundation of Basic Researches (individual project 16-3360229). We thank to Dr. A. A. Eliseev for discussion of Raman measurements.
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CONCLUSIONS In conclusion, graphene oxidation is one of the ways of controllable modification of its electronic structure. In contrast to the epitaxial graphene on Au/Ni(111), where the atomic oxygen exposure leads to a chemically homogeneous modification, we observed oxidation patterns on graphene grown on Cu foil (a rather popular material for near future electronics) as a result of atomic oxygen treatment. In the course of such treament, epoxy groups are generated uniformly over the whole surface of single-layer graphene, similar to the case of epitaxial graphene on single-crystalline substrate. More oxidized species are preferably located at the centers of bilayer regions. These regions are surrounded by less oxidized areas of bilayer graphene, where the second-layer fragments are always between the first continuous layer and Cu substrate. As a result, oxidized islands of about 1 μm are randomly distributed over the surface with a mean density of about 3 × 106 cm−2. Such nonuniformities have never been observed before. We believe that our results can be useful to develop lateral heterostructures for electronics and optoelectronics based on graphene/ graphene oxide heterojunctions.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07840. Additional XPS and Raman measurements of pristine and oxidized graphene grown on Ni, Ir, and Cu, analysis of Cu 3p photoemission spectra acquired from pristine and O2 plasma treated graphene on Cu, details of electron beam-induced current (EBIC) profile of graphene on SiO2/Si, data for the influence of laser power on Raman spectra of graphene (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Elmar Yu. Kataev: 0000-0003-1016-946X Dmitry Yu. Usachov: 0000-0003-0390-0007 Hikmet Sezen: 0000-0002-5438-3305 Denis Vyalikh: 0000-0001-9053-7511 Daniil M. Itkis: 0000-0002-6363-6669 Lada V. Yashina: 0000-0002-8370-9140 27921
DOI: 10.1021/acs.jpcc.7b07840 J. Phys. Chem. C 2017, 121, 27915−27922
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DOI: 10.1021/acs.jpcc.7b07840 J. Phys. Chem. C 2017, 121, 27915−27922
