Aqueous-Phase Oxidation of Epitaxial Graphene on the Silicon Face

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Aqueous-Phase Oxidation of Epitaxial Graphene on the Silicon Face of SiC(0001) Md. Zakir Hossain,*,† Maisarah B. A. Razak,† Shinya Yoshimoto,‡ Kozo Mukai,‡ Takanori Koitaya,‡ Jun Yoshinobu,‡ Hayato Sone,§ Sumio Hosaka,§ and Mark C. Hersam⊥,∥,¶ †

Advanced Research Leaders Developments Unit, Advanced Engineering Research Team, Faculty of Science and Technology, Gunma University, Kiryu City, Gunma 376-8515, Japan § Division of Electronics and Informatics, Faculty of Science and Technology , Gunma University, Kiryu City, Gunma 376-8515, Japan ‡ The Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan ⊥ Department of Materials Science and Engineering, ∥Department of Chemistry, and ¶Department of Medicine, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: To explore the chemical and electronic states of oxidized epitaxial graphene (EG) grown on the Si face of SiC(0001), we employ the Hummers oxidizing agents (H2SO4 + NaNO3 + KMnO4) under different reaction conditions that oxidize the graphene layer. The resulting material is characterized with scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). A mild “drop-cast” procedure at 60 °C is found to be equally effective at oxidizing EG as the conventional Hummers procedure. This aqueous-phase oxidation reaction appears to proceed in an autocatalytic manner as indicated by the concurrent observation of patches of oxidized and clean graphene areas in atomically resolved STM images on partially oxidized EG. STS further reveals substantial changes in electronic structure for oxidized EG including the opening of a local band gap of ∼0.4 eV. The oxidation is confined to the graphene layers as verified by XPS characterization of the underlying SiC substrate. In contrast to EG oxidized in ultrahigh vacuum that contains only epoxy groups and can be fully reverted back to pristine EG following annealing at 260 °C, aqueous-phase oxidized EG possesses carbonyl and hydroxyl groups in addition to the dominant epoxy groups and thus remains partially oxidized even following annealing at 1000 °C.

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electronics,25−30 aqueous-phase oxidation of epitaxial graphene (EG) on the silicon face of SiC has not yet been reported. The Hummers method for producing single-layer GO involves the treatment of graphite with a strong oxidizing agent followed by dispersion in an aqueous medium.15,16,18,19,22 Thus-prepared GO possesses a variety of oxygen moieties such as epoxy, ether, carboxylic, hydroxyl, and carbonyl groups at the edge and on the basal plane of the GO sheet.17,20,21 In contrast, atomic oxygen is typically used when trying to oxidize singlelayer epitaxial graphene on its growth substrate. For example, atomic oxygen induced oxidation of epitaxial graphene on Ir(111) and Pt(111) surfaces followed by thermal reduction has revealed significant degradation of the graphene structure via the loss of carbon as CO, CO2, and related volatile byproducts.31,32 In addition, X-ray photoelectron spectroscopy (XPS) studies of Hummers oxidation of multilayer graphene on the carbon face of SiC have reported the slow intake of hydrogen that converts epoxy functional groups to hydroxyl

INTRODUCTION

Graphene, an atomically thin hexagonal network of sp2 carbon atoms, has promise for a variety of applications including highperformance electronics because of its extraordinary electrical, optical, mechanical, and chemical properties.1−5 Although a wide range of applications have been anticipated,1,5 pristine graphene cannot be utilized for many of those applications without altering its chemical and electronic properties. Consequently, significant effort has been devoted to chemical approaches for functionalizing graphene.6−13 Indeed, a wellknown, decades-old oxidation procedure known as the Hummers method for oxidizing graphite has been employed to chemically modify graphene, thereby allowing the resulting graphene oxide (GO) to be utilized in composites, sensors, energy storage, and optoelectronics.14−23 While the Hummers method applied to graphite typically results in micrometer-scale GO flakes in solution, the Hummers oxidizing agent, which is a mixture of H2SO4, NaNO3, and KMnO4, has also been utilized to oxidize multilayer graphene at the wafer-scale on the carbon face of SiC.24 However, despite its importance for growing wafer-scale graphene and its suitability for high-frequency © XXXX American Chemical Society

Received: September 17, 2013 Revised: December 12, 2013

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Figure 1. STM, Raman, and XPS results demonstrating the aqueous-phase oxidation of epitaxial graphene grown on the Si face of SiC(0001). Ambient STM images of (a) clean graphene, (b) fully oxidized, and (c) partially oxidized surfaces. The inset of (a) shows the atomically resolved hexagonal lattice structure of single-layer graphene. The partially oxidized surface is typically observed near the edge of the Hummers solution droplet on the surface. Vtip = 0.5 V; Itunnel = 0.5 nA. (d) Raman spectra of (i) SiC substrate, (ii) graphene on SiC, and (iii) and (iv) oxidized surface obtained by the drop-cast and dipped-in procedures. (e) Typical high-resolution C 1s spectra of (i) clean and (ii) oxidized graphene on SiC by the drop-cast procedure. The corresponding O 1s spectrum of the oxidized surface is shown as (f). The colored filled peaks are deconvoluted by Gaussian fitting. Different peaks are identified and discussed in the text.

groups, leading to an equilibrium O/C ratio of ∼0.38.24 Furthermore, some of the graphene layers for multilayer graphene on the carbon face of SiC are lost through exfoliation into aqueous solution during the Hummers method oxidative treatment.24 On the other hand, atomic oxygen exposure of the silicon face of SiC in ultrahigh vacuum (UHV) results in a chemically homogeneous epoxidation of the epitaxial singlelayer graphene that is thermally reversible at 260 °C.33 Herein, we explore oxidation of graphene grown on the silicon face of SiC using Hummers oxidizing agents in aqueous solution. The resulting surface is characterized by scanning tunneling microscopy (STM) and spectroscopy (STS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). In contrast to the conventional harsh and destructive treatment in which the entire graphitized SiC substrate is submerged in the Hummers oxidation solution, a milder ‘drop-cast’ procedure using Hummers oxidizing agents is employed at 60 °C, which leads to oxidation of epitaxial graphene without exfoliation from the SiC substrate. The resulting oxidized surface is found to be atomically flat, possesses a local electronic band gap of ∼0.4 eV, and shows characteristics of a highly p-type doped surface. The oxidation reaction appears to proceed in an autocatalytic manner and forms epoxy, carbonyl, ether, and hydroxyl functional groups on the surface. Unlike the thermal

reversibility that was observed at temperatures as low as 260 °C for UHV atomic oxygen induced oxidation of epitaxial graphene on SiC, the aqueous-phase oxidized surface cannot be reversed back to pristine graphene even by annealing at 1000 °C. The chemical and electronic states of aqueous-phase oxidized epitaxial graphene suggest its compatibility with the subsequent processing that is required for electronic device fabrication.

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EXPERIMENTAL METHODS EG was prepared by annealing 6H-SiC(0001) samples at 1350 °C for 10−12 cycles of 30 s under UHV conditions (maximum pressure ∼5.0 × 10−9 torr). Thus-prepared EG on the Si face of SiC substrate usually contains 1−3 layers of graphene.12,34 The EG grown in UHV is oxidized ex situ using Hummers oxidizing agents in two different reaction procedures that will be referred to as the “dipped-in” and “drop-casting” procedures. In the case of the dipped-in procedure, the EG on SiC is placed into a conical flask followed by addition of 20 mL of concentrated H2SO4 and 0.1 g of NaNO3. Then, 0.6 g of KMnO4 is slowly added into the flask with constant stirring in an ice bath. In the case of the drop-cast procedure, a few drops of Hummer solution are carefully placed onto the surface so that the meniscus of the droplet does not touch the edge of the sample. B

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surface as the dipped-in procedure). The extent of oxidation is further confirmed by both XPS and high-resolution XPS measurements on different samples. Taking into consideration the photoemission cross section of C 1s and O 1s electrons, the O/C ratio is estimated by both area intensities of the graphene C 1s and O 1s peaks in the narrow-scan spectra and the corresponding peak heights in the survey spectrum obtained by low-energy X-rays. The estimated values are found to be 0.30− 0.35 and 0.36−0.38 for the drop-cast and dipped-in oxidized samples, respectively. These estimated O/C ratios are in agreement with the reported values for oxidation of graphene on the carbon face of SiC (0.38)24 as well as for graphene oxide obtained from graphite (0.25−0.50).37 A typical STM image of the oxidized surface obtained by the drop-cast procedure is shown in Figure 1b. Compared to the clean surface, the oxidized surface appears to be inhomogeneous, which is also supported by the disorder-induced broadening of the G-band in the Raman spectrum [Figure 1d(iii)].36 Unlike STM studies reported earlier for reduced graphene oxide,38,39 no atomically resolved graphene-like feature is observed in the high-resolution STM image for oxidized EG (Supporting Information). Despite this increased inhomogeneity, the atomic step of the substrate is still observable in the STM image of the oxidized sample, suggesting that the surface is nearly atomically flat. The surface morphologies of the clean and oxidized graphene regions can be directly compared in the partially oxidized sample shown in Figure 1c. The observation of patches of clean and oxidized areas on the same surface suggests that the oxidation reaction proceeds in autocatalytic manner (i.e., once a graphene C atom reacts with oxygen, the C atoms surrounding the reacted one become more susceptible to further reaction). Note that this partially oxidized surface is observed near the edge of the droplet of the Hummers oxidizing solution. To further characterize the surface chemistry and any subsurface oxidation, high-resolution XPS measurements were performed. The C 1s and O 1s spectra of the clean and oxidized epitaxial graphene samples are shown in Figures 1e and 1f. Although the C 1s spectrum of epitaxial graphene on SiC is complicated by the substrate and buffer layer C peaks,40 the dominant graphene C 1s peaks can be resolved. For the clean epitaxial graphene sample [Figure 1e(i)], the best fitting of the experimental spectrum resolves four individual peaks as marked by g1, g2, C6, and S. The C6 at 283.7 eV and S at ∼285.0 eV are ascribed to the bulk SiC and buffer layer carbon atoms.40 Indeed, the S peak should consist of two components S1 and S2 corresponding to the carbon atoms bonded and nonbonded to the substrate atoms.40 Note that EG grown on SiC usually consists of monolayer, bilayer, and trilayer graphene, which are separated from the SiC by a buffer layer of C atoms.28,34,40 The g1 and g2 peaks are due to the sp2 C of graphene layer. The relatively smaller and higher binding energy component (g2) arises from the graphene layer directly attached to the buffer layer.40 Following oxidation of graphene, the spectral shape is drastically changed from that of clean graphene as shown in [Figure 1e(ii)]. The main peak at ∼284.7 eV is broadened, and two new peaks at 286.7 and 288.0 eV are now present. In addition to S and C6 related to the substrate, five more components marked as C1, C2, C3, C4, and C5 can be resolved by the best fitting of the experimental spectrum. Fitting parameters for individual peak position and area intensity are given in the Supporting Information. On the basis of previous

The sample is then heated to the desired temperature by placing the sample on a hot plate. After oxidation, the sample is cleaned by DI water and H2O2 repeatedly. Ambient STM and STS measurements are performed using a Nanosurf easyScan 2 STM system with Pt−Ir tips. EG on SiC was mounted on the magnetic steel disc using silver paste, which was attached to the sample holder for STM measurements. During STS and constant current STM measurements, the bias was employed to the tip. STS data were obtained by sweeping the tip bias from negative to positive values with constant tip−surface distance defined by the set point. Twoprobe surface conductivity measurement was performed using a homemade probing system with W probes and LabVIEW system (National Instruments). The distance between the two probes was adjusted to 1 mm. Raman measurements were performed by Nicolet Almega XR Raman with a 532 nm laser. The XPS data were obtained with an AXIS-NOVA XPS system using an AlKα X-ray source and synchrotron facilities in KEK, Japan. In KEK, the highresolution core-level XPS measurements were performed using a hemispherical electron analyzer (Scienta SES-200) at the Photon Factory BL13A (KEK-PF PAC 2009-S2-007), Japan. The incident and emission angles for the photon and photoelectron were 65° and 0°, respectively, with respect to the surface normal. The total energy resolution was ∼70 meV. The electron binding energy was referenced to the C 1s peak of bulk SiC at 283.7 eV.34

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RESULTS AND DISCUSSION Epitaxial graphene grown on the silicon face of SiC by hightemperature annealing in UHV was oxidized with Hummers oxidizing agents in ambient conditions as described above. The pristine and oxidized graphene were then characterized by ex situ ambient STM, Raman spectroscopy, and XPS as shown in Figure 1. Note that epitaxial graphene on SiC remains clean in air for time scales up to 1 year. Large-area and zoomed-in STM images (Figure 1a), Raman spectrum [Figure 1d(ii)], and survey XPS (Supporting Information) confirm the surface cleanliness. In particular, the hexagonal structure of clean monolayer graphene is observed in the inset of Figure 1a. The Raman spectrum of epitaxial graphene on SiC shows the characteristic 2D and G bands at 2748 and 1605 cm−1.35 The G band is ascribed to the in-plane vibration of sp2 carbon atoms, which is a doubly degenerate phonon mode (E2g symmetry) at the Brillouin zone center.35,36 The 2D band is a two-phonon process, which is allowed only for a perfect lattice of graphene.36 The small D peak at 1380 cm−1 is a defect-induced band, which appears in the case of clean graphene on SiC due to the interaction of graphene with the substrate.35 Note that no such bands are observed in the Raman spectrum of bulk SiC [Figure 1d(i)]. The absence of any other peaks except C and Si in the survey XPS spectrum (Supporting Information) confirms that epitaxial graphene on SiC is unreactive to air alone. Following oxidation by both the dipped-in and drop-cast procedures, the 2D band of the Raman spectra almost disappears, and the D band is significantly enhanced because of the creation of defect sites by chemisorption of oxygen. The increased intensity of the G band in the case of oxidized graphene is perhaps related to Raman enhancement due to increased roughness of the surface caused by the chemisorption of oxygen. The D/2D ratio of the Raman spectra suggests that the extents of oxidation by these two methods are almost equal (i.e., the drop-cast procedure is equally effective at oxidizing the C

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Figure 2. (a) Typical current (I)−voltage (V) curves for clean and oxidized surfaces obtained by scanning tunneling spectroscopy (STS). The blue and red curves represent clean and oxidized epitaxial graphene surfaces. The reproducibility of the STS I−V curves is shown in the inset of (a). The green spectra are obtained for the dipped-in oxidized surface. Each STS curve is the average of 128 measurements acquired by the STM data acquisition software. The initial set point for all STS I−V measurements is Vtip = 0.5 V and Itunnel = 0.5 nA. (b) and (c) are the STS dI/dV for clean and oxidized epitaxial graphene on SiC. The curve for the clean surface is extracted by differentiating the polynomial fit to the experimental current (I)−voltage (V) curve. The curves shown in (c) are extracted by direct differentiation of the raw experimental I−V curves. (d) I−V curves obtained by two-probe surface conductivity measurements.

experimental and theoretical investigations of graphene and HOPG,31,32,41 we have ascribed the C2 to epoxy and hydroxyl, C5 to carbonyl, and C4 to ether functional groups. The C3 is ascribed to the second neighbor to the epoxy carbon.41 The presence of different oxygen functional groups is further indicated by the O 1s spectra that are shown in Figure 1f. Although O 1s peaks characteristic to different functional groups are not experimentally resolved, the best Gaussian fitting to the experimental curve suggests four components marked as O1 (531.9 eV), O2 (531.0 eV), O3(533.1 eV), and O4 (530.1 eV). Note that for clean epitaxial graphene on SiC, no trace of oxygen is observed in XPS. The major peak at 531.9 eV (O1), which is ∼60% of the total O1s peak area, is ascribed to the epoxy group.31,32,41 The relatively small components O2, O3, and O4 are ascribed to oxygen in ether, hydroxyl, and carbonyl groups, respectively.31,32,42 These peak assignments are supported by temperature-dependent measurements that will be discussed later. Aqueous-phase oxidation of epitaxial graphene on SiC induces significant electronic property changes. In particular, scanning tunneling spectroscopy (STS) provides information about both the occupied and unoccupied density of states near the Fermi level.43 A typical STS I−V curve obtained before and after oxidation of epitaxial graphene on SiC is shown in Figure 2a. The STS I−V curves measured at wider range of negative tip biases for oxidized surface are shown in the Supporting

Information. The results shown in Figure 2a are highly reproducible as demonstrated in the inset. Furthermore, the reproducibility of the STS I−V characteristics is confirmed by measuring the clean and oxidized areas of the same surface as shown in Figure 1c. The oxidized surfaces produced by the drop-cast and dipped-in procedures show almost identical STS I−V characteristics, which suggest similar levels of oxidation as previously deduced from Raman and XPS measurements. The STS I−V curve for the oxidized surface shows almost zero current around zero-tip bias (i.e., no density of states near the Fermi level).43 The dI/dV spectra extracted from the I−V curves for the clean and oxidized surfaces are shown in Figure 2b and 2c. The curve for clean graphene (Figure 2b) indicates zero band gap and the Dirac point at ∼0.3 V, which is characteristic of clean epitaxial graphene on SiC.12 For the oxidized surface (Figure 2c), the differential tunneling conductance is nearly zero between +0.1 and −0.3 V, suggesting a local band gap of ∼0.4 eV is opened when epitaxial graphene is oxidized by the Hummers reagents. Furthermore, the asymmetry of the STS I−V curve suggests that the oxidized surface is p-type doped. In addition, the twoprobe surface conductivity measurement shown in Figure 2d indicates a significant increase in the surface resistivity (1.15 × 10−5 ohm/square for clean and 1.89 × 10−3 ohm/square for oxidized graphene) following oxidation. D

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Figure 3. XPS and Raman spectra demonstrating the effect of annealing the oxidized surface. The C 1s (a) and O 1s (b) are measured after annealing the oxidized surface (i) at 300 °C (ii), 600 °C (iii), and 1000 °C (iv). (c) The Raman spectra are measured after annealing the oxidized surface (ii) at 1000 °C (iii) and 1300 °C (iv). For comparison, the Raman spectrum of clean epitaxial graphene on SiC is also included (i).

by the reappearance of the 2D band in the Raman spectrum (Figure 3c). Since the SiC substrate is susceptible to oxidation, it is of interest to see if the oxidation of epitaxial graphene also affects the underlying substrate. Toward this end, the Si 2p spectra are also measured. The Si 2p spectra in the case of clean and oxidized graphene on SiC are nearly identical as shown in Figure 4. Similar to that of the apparently decreased C 1s peak intensities of S and C6 (Figure 1e and Supporting Information), the decrease of the Si 2p peak intensity following oxidation perhaps relates to the increased thickness of the graphene layer by oxidation. Note that the thickness of the graphene layer is increased from 0.34 nm to 0.65−0.75 nm upon oxidation.38,39 From the intensity decreases of the Si 2p spectra, we have estimated ∼0.8 nm inflation of the graphene layer following the drop-cast oxidation based on the TPP-2 M equation.46 Indeed this value is in reasonable agreement with the 2−3 layer EG on the Si face of the SiC substrate. The major peak (101.3 eV) with a shoulder at higher binding energy (101.9 eV) can be deconvoluted into two peaks with area intensity ratio of 2:1 for both the clean and oxidized surfaces. The two components of the peaks arise due to the spin−orbit coupling of the 2p electrons. The identical appearance of the Si 2p peaks for both clean (Figure 4a) and oxidized (Figure 4b) surfaces indicates that the substrate Si atoms beneath the graphene layers do not undergo any significant oxidation. Therefore, we conclude that the aqueous-phase oxidation process is confined to only the graphene layers.

To study the thermal stability of the oxidized surface, the C 1s and O 1s spectra were measured after annealing the oxidized surface at different temperatures in UHV. The temperaturedependent evolution of the C 1s and O 1s spectra are shown in Figure 3. The O 1s component (O1) corresponding to the epoxy group has disappeared by annealing at 300 °C, and the main peak that is centered at ∼531 eV and the component O3 become visible as shown in Figure 3b(ii). The complete desorption of the epoxy functional group by annealing at 300 °C is also supported by the disappearance of the C2 and C3 components of the C1s spectrum [Figure 3a(ii)], which is in agreement with previous reports.32,33,44,45 Since the C 1s component C5 has also disappeared and the edge of the lower binding energy side of the O 1s spectrum becomes sharper by annealing at 300 °C, we suggest that the oxygen of the carbonyl group either desorbs or is converted to another functional group. By further annealing the oxidized surface at 700 and 1000 °C, a complex reaction involving the graphene C atoms and remaining oxygen functionalities is expected to occur. Recent infrared (IR) studies on the thermal reduction of graphene oxide obtained from graphite have indicated the simultaneous loss and formation of various oxygen moieties up to 800 °C.44 The O 1s spectra shown in Figure 3b suggest that even after annealing at 1000 °C, a minor amount of O remains on the surface. UHV annealing the oxidized surface above 300 °C leads to the desorption of oxygen in the forms of CO and CO2, leaving various types of undefined C vacancies and distorted graphene networks.31,32 Hence, the interpretation of the C 1s spectra after annealing the oxidized surface becomes complicated by the several C components with similar binding energies such as C−O−C, C−OH, C vacancies, and sp3 C.41 However, after annealing the surface at 1000 °C, the C 1s peak related to the sp2 C with a shoulder at lower (substrate SiC) and higher binding energy side is clearly observed. The shoulder at higher binding energy side can be ascribed to the remaining C−O−C or C−OH and other sp3 C species. Although the sp2 C peak appears at the same binding energy as that of graphene C, the 2D band characteristic of pristine graphene is not observed after annealing the surface at 1000 °C (Figure 3c). Hence, we suggest that the long-range graphene structure is destroyed by annealing the oxidized surface at 1000 °C. However, pristine graphene can be partly recovered by annealing the surface at the graphene formation temperature of 1350 °C, as indicated

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CONCLUSION To alter the chemical and electronic properties of graphene, we investigated oxidation of epitaxial graphene on the silicon face of SiC(0001) using the Hummers oxidizing agents (H2SO4 + NaNO3 + KMnO4) in two different reaction conditions. The pristine and chemically modified EG are characterized by STM, STS, Raman spectroscopy, XPS, and two-probe surface conductivity measurements. In contrast to the conventional harsh and destructive Hummers oxidation procedure, a milder drop-cast procedure at 60 °C using the Hummers oxidizing agents can be employed for effectively oxidizing the EG without exfoliation from the SiC substrate. Concurrent observation of atomically flat patches of oxidized and nonoxidized areas in atomically resolved STM images of partially oxidized EG suggest that the oxidation reaction proceeds in an autocatalytic manner. Furthermore, oxidation of EG induces substantial E

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ACKNOWLEDGMENTS

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REFERENCES

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The work is supported by the Program to Disseminate TenureTrack System of the Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT), Element Innovation Project granted to Gunma University and KAKENHI (grant no. 25390013). M.C.H. acknowledges support from the Office of Naval Research (N00014-11-10463), the Department of Energy (award no. DE-FG0209ER16109), and the W. M. Keck Foundation. S.Y., K.M., T.K., and J.Y. acknowledge Dr. K. Mase at KEK-PF for the assistance at BL13A (KEK-PF PAC 2009S2007).

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Figure 4. Si 2p core-level spectra of (a) clean and (b) oxidized epitaxial graphene on the silicon face of the SiC substrate. The best Gaussian fitting resolves two peaks, namely 2p1/2 and 2p3/2 as indicated.

changes in the electronic structure of graphene as revealed by STS measurements including a local band gap of ∼0.4 eV and p-type doping. High-resolution XPS characterization of the underlying subsurface region reveals that the oxidation is confined to the graphene layers leaving the underlying silicon carbide substrate intact. In contrast to ultrahigh-vacuum oxidized EG, the aqueous-phase oxidized EG is chemically inhomogeneous consisting of carbonyl and hydroxyl groups in addition to the dominant epoxy groups and cannot be reverted back to pristine graphene even following annealing at 1000 °C.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Survey XPS spectra, high-resolution STM images, XPS fitting parameters, and wider range STS I−V curves. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Tel. +81-277-30-1625. Notes

The authors declare no competing financial interest. F

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