The Morphology of Graphene Sheets Treated in an Ozone Generator

ARTICLE pubs.acs.org/JPCC

The Morphology of Graphene Sheets Treated in an Ozone Generator Haihua Tao,*,†,‡ Joel Moser,† Francesc Alzina,† Q. Wang,‡ and C. M. Sotomayor-Torres†,§,z †

Catalan Institute of Nanotechnology (CIN2-CSIC), Campus UAB, 08193 Bellaterra, Spain Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, 200240, Shanghai, People's Republic of China § Department of Physics, Campus UAB, UAB, 08193 Bellaterra, Spain z Instituci
o Catalana de Recerca i Estudis Avanc-ats (ICREA), 08010 Barcelona, Spain ‡

ABSTRACT: Using atomic force microscopy and Raman spectroscopy, we characterize the highly disordered morphology of single-layer and multilayer graphene sheets that results from long exposures to ozone molecules and oxygen radicals in an ozone generator. We identify a crossover from a nanocrystalline phase to an amorphous carbon phase in single-layer graphene as the exposure time is increased. We then consider the nanocrystalline phase and compare the morphology of single-layer sheets, multilayer sheets, and graphite flakes. Finally, we find that salient structures, such as steps and folds, act as sinks for disorder.

1. INTRODUCTION Graphene is a giant organic molecule whose carbon atoms span a hexagonal lattice.1,2 The versatility of elemental carbon allows one to graft chemical groups onto the molecular scaffold of graphene, giving rise to new giant molecules, such as graphane3 and graphene oxide.4,5 This chemical functionalization locally alters the hexagonal lattice, resulting in a structural disorder whose characteristic length can be estimated by Raman scattering spectroscopy. Upon mildly exposing graphene to reactive species, point-like disorder can be created whereby an atom from the chemical environment covalently bonds to an atom of the carbon lattice. These pointlike defects, yet sparsely distributed, have a significant impact on the physical properties of graphene; this has been shown for graphene exposed to such diverse chemical species as hydrogen radicals,3,6 oxygen radicals,4,5 fluor radicals,7,8 and ozone molecules.9,10 When the exposure is stronger (at either a larger kinetic energy of the radicals, higher concentration of the reactive species, or simply longer exposure), chemical reactions may proceed beyond the formation of these covalent bonds. As a result, carbon atoms are removed from the graphene lattice, and cavities are formed.11
13 Here, we present a study of single-layer and few-layer graphene sheets strongly exposed to chemical species inside an ozone generator. We find that cavities are formed, and we characterize the damaged sheets by atomic force microscopy and Raman spectroscopy. First, we identify a crossover from a nanocrystalline phase to an amorphous carbon phase in single-layer graphene sheets. We then consider the nanocrystalline phase and compare the size of cavities in single-layer, few-layer, and many-layer graphene sheets. Finally, we show that trenches form at the bottom of high steps in graphite and at the bottom of folds in single-layer graphene sheets. 2. EXPERIMENTAL METHOD Below, we briefly describe our fabrication technique. Graphene sheets are prepared by micromechanical exfoliation of r 2011 American Chemical Society

HOPG flakes using the adhesive tape method.14 Substrates of silicon coated with 285 nm of thermally grown silicon oxide are exposed to an oxygen plasma to obtain a clean, hydrocarbon-free oxide surface. Shortly thereafter, graphene sheets are deposited onto the oxide surface. This procedure enhances the chance of obtaining relatively large (10
100 μm) single-layer graphene sheets. Samples are then annealed at 300 °C in a flow of Ar/H2 for 2 h to remove most of the adhesive tape residues that invariably contaminate graphene sheets after deposition. Singleand few-layer graphene sheets are identified by Raman spectroscopy.15 Finally, samples are placed inside a Novascan ozone generator, where ozone molecules are produced at room temperature by UV-induced dissociation of oxygen molecules. This entails that graphene sheets are exposed to ozone molecules and to oxygen radicals. Our measurement technique involves scanning force microscopy, using a Nanoscope atomic force microscope (AFM) in tapping mode, and micro-Raman measurements. The latter are carried out in the backscattering geometry using a T64000 JobinYvon spectrometer with a cooled charge-coupled device detector. The light is focused into a spot of ∼1 μm in diameter using the 100 objective of an optical microscope. The 514.5 nm emission line of an Ar+ laser is used for excitation with a power of less than 120 μW, a low enough value to prevent structural damage of graphene.16

3. RESULTS AND DISCUSSION We start by considering single-layer graphene sheets. Figure 1 displays AFM images showing the morphology of two separate graphene sheets after exposing one for 6 min (Figure 1a) and the other for 8 min (Figure 1b). The sheets have been transformed Received: May 31, 2011 Revised: August 4, 2011 Published: August 11, 2011 18257

dx.doi.org/10.1021/jp2050756 | J. Phys. Chem. C 2011, 115, 18257–18260

The Journal of Physical Chemistry C

Figure 1. AFM images of two graphene sheets treated in the ozone generator for 6 min (a) and for 8 min (b). The image fuzziness in (b) may be related to water molecules attached to the sample after treatment. Both scale bars are 500 nm.

into a dense network of carbon filaments (light color) separated by cavities (dark color). The carbon filaments appear continuous in Figure 1a; they are broken up into clusters and look enlarged in Figure 1b. Figure 2a displays Raman spectra taken on four separate graphene sheets, each exposed for a different time (from bottom to top: pristine graphene and graphene exposed for 6, 7, 8, and 8.5 min). The D peak at 1355 cm
1, a hallmark of disordered graphene, gradually develops upon exposure: its height reaches a maximum at 7 min, then it decreases as the peak broadens. As for the G peak at ∼1600 cm
1, its height is less affected, but it becomes asymmetric and develops a shoulder on its right-hand side (which has been shown to be also related to disorder17). The Raman spectra in Figure 2a convey important information on the structural damage induced in graphene by the treatment in the ozone generator. As the exposure time increases, graphene experiences a broad crossover from a nanocrystalline phase to a highly disordered phase of amorphous carbon. We recall that the nanocrystalline phase (nc-G) is composed of a patchwork of crystallites within which carbon atoms are arranged in a hexagonal lattice and that the amorphous carbon phase (a-C) features a low density of these closed hexagons. Such an interplay between nc-G to a-C phases has been observed upon ion irradiation of glassy carbon18,19 and sputtered amorphous carbon.20,21 We use the G and D peak height dependence on exposure time to reveal the nc-G to a-C crossover. The G peak corresponds to a vibration mode of the lattice characterized by the bond-stretching of pairs of sp2 carbon atoms. Pairs of sp2 carbon atoms are present even in disordered graphene, which may explain the relatively weak dependence on exposure time of the G peak height compared to that of the D peak height. The D peak corresponds to a vibration mode of the lattice that requires the presence of disorder for its activation. This may account for the initial increase of the D peak height up to the 7 min exposure, as graphene is broken up into crystallites. Moreover, because the D peak is due to a breathing mode of the six carbon atoms that form each hexagon of the honeycomb lattice, it is expected to vanish in the limit of exceedingly large disorder. In this context, it is likely that the density of closed hexagons decreases for long exposures,22 which, in turn, should lead to a reduction of the D peak height as seen experimentally. This process is summarized in Figure 2b, where we plot the ratio I(D)/I(G) of the D peak height over the G peak height. Please note that the D peak height is related to the density of closed hexagons (left intact by the treatment in the ozone generator), while the broadening of the D peak for long exposures is

ARTICLE

Figure 2. (a) Raman spectra taken on four graphene sheets placed in the ozone generator, one at a time. Each spectrum corresponds to a given sheet and a given exposure time. From bottom to top: pristine graphene and graphene exposed for 6, 7, 8, and 8.5 min (spectra are offset for clarity). (b) Ratio of the D peak height to the G peak height. Different data points for a given exposure time are taken at different locations on the graphene sheet. The dashed curve is a guide to the eye. The nanocrystalline phase is labeled “nc-G”, and the amorphous carbon phase is labeled “a-C” (shaded area).

linked to the distribution of disorder.17 Thus, we elect to plot the ratio of the peak heights instead of the ratio of the peak areas. We propose that the nc-G phase crosses over to the a-C phase after an exposure of ∼7 min. In addition, the I(D)/I(G) ratio allows a rough estimate of the size L of the crystallites. In the nc-G phase, the Tuinstra
Koenig relation for graphite reads23,24
 IðDÞ
1 L½nm ¼ 2:4  10
10 λ4  ð1Þ IðGÞ where λ = 514.5 nm is the wavelength of the Ar+ laser. We find L = 25 nm for the 6 min exposure. In the a-C phase, the relationship between L and I(D)/I(G) reads17,25 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 IðDÞ  ð2Þ L½nm ¼ 0:55 IðGÞ We find L = 2 nm for exposure times beyond 7 min. It is, however, difficult to correlate these values for L with the size of filaments shown in Figure 1a,b. For the 6 min exposure, the width of filaments in Figure 1a is certainly of the order of 20
50 nm. For longer exposures, we suspect that graphene becomes overgrown with molecules present in the air, such as water. This is suggested by the fact that we have to heat some samples up to 200 °C for ∼1 h following a long exposure in order to be able to obtain a stable signal during AFM imaging. In this context, this overgrowth may be the result of bonding between molecules present in the air and epoxide groups that are strongly reactive and have been shown to form in graphene upon exposure to ozone molecules.26,27 Having identified a nc-G to a-C crossover, we now compare the morphology of single-layer graphene, multilayer graphene, and graphite in the nc-G phase. Figure 3a shows an AFM image of one-, two- and three-layer graphene sheets, and Figure 3b shows an AFM image of a graphite flake featuring multiple steps. Samples in both images were found on the same substrate and were thus subjected to the same experimental conditions. Qualitatively, cavities exhibit a rather broad distribution of their overall diameters. However, diameters do tend to increase with the thickness of the sample: the comparison between single-layer graphene and graphite is especially striking. 18258

dx.doi.org/10.1021/jp2050756 |J. Phys. Chem. C 2011, 115, 18257–18260

The Journal of Physical Chemistry C

ARTICLE

Figure 4. (a) High folds (∼10 nm) in single-layer graphene treated in the ozone generator. The white arrow points to a trench at the bottom of a fold. The scale bar is 500 nm. (b) Cross section perpendicular to the fold indicated by the arrow in (a), but measured prior to ozone treatment. R = 75 nm is the estimated radius of the curvature of the fold, and α = 20° is the angle between the plane and the bottom of the fold. Figure 3. (a) One-, two-, and three-layer graphene sheets and (b) graphite flake treated in the ozone generator. In (b), white arrows point to trenches along the bottom of steps. Cross sections along the white dotted lines (AFM scan direction) in (a) and (b) are shown in (c) and (d), respectively. All scale bars are 500 nm.

The mechanisms that contribute to the formation of cavities in single-layer graphene on one hand and in multilayer graphene and graphite on the other are different. Single-layer graphene annealed on SiO2 partially follows the roughness of the oxide surface.1,2 Surface roughness is usually invoked to explain the high chemical reactivity of single-layer graphene.3
10,12,13,28 Thicker samples are flatter and mechanically much less flexible, and their chemical reactivity depends on occasional pointlike defects. The flatness of thicker samples may also explain larger diameter cavities: chemical reactions responsible for the removal of carbon atoms may proceed faster, unhindered by variations in the orientation of pz orbitals of carbon atoms. Further insight into the formation of cavities can be gained by carefully examining their distribution along steps. In Figure 3a, cavities are continuous across the borders between one and two layers and between two and three layers. By contrast, cavities in Figure 3b are strongly affected by the presence of steps in graphite: they are not continuous across steps; instead, they nucleate at the bottom of steps and form extended trenches (white arrows). Figure 3c,d displays cross sections taken along the white dotted line (AFM scan direction) of Figure 3a,b, respectively. In both cases, the cavity depth is about 1 nm; yet an out-of-plane, steplike structure can only be discerned for graphite (∼1 nm high steps, Figure 3d). Interestingly, trenches following the bottom of steps are neither observed in graphite exposed to oxygen radicals11 nor in graphite exposed to ozone at high temperature.26,27,29 To verify the role of out-of-plane structures in the formation of elongated trenches, we selected graphene sheets with folds protruding ∼10 nm out of the plane, and we treated them in our ozone generator. Figure 4a displays an AFM image of one such graphene sheet after treatment. Interestingly, trenches are also seen at the bottom of the folds (see white arrow), while the top of the folds are essentially unaffected. In the following, we discuss the relevance of several mechanisms to the formation of these trenches. In the case of graphite, the contribution of dangling bonds present at the edge of steps does not appear to be significant, since the discontinued, upper edge of steps does not display any enhanced reactivity compared to areas that lay further away from steps.26 Folds in graphene sheets appear to be related to the details of the micromechanical

exfoliation procedure and to the environment (such as the humidity ratio in the air during exfoliation). This entails that highly reactive grain boundaries,12 transferred from a graphite flake, are unlikely to play any role in the formation of graphene folds. Alternatively, a large curvature 1/R of the graphene sheet, where R denotes the radius of curvature, is expected to enhance the chemical reactivity of carbon atoms by facilitating their sp3 hybridization. This requires that R be sufficiently small for the angle between the σ orbitals and the π orbital of such carbon atoms to be larger than 90°.30,31 For example, fullerenes covalently bonded to a carbon nanotube have been shown to be more reactive than the nanotube as their curvature is largest.32 In our case as well, a breaking of the sp2 symmetry is central to the reaction between graphene and ozone molecules since carbon atoms covalently bond to oxygen atoms and form epoxide groups.10,26,27 To explain the lack of reactivity of the top of graphene folds, Figure 4b displays a cross section perpendicular to the fold, indicated by the arrow in Figure 4a, and measured before ozone treatment. The estimated radius of the curvature of the fold is R = 75 nm so that 1/R is presumably small enough for the sp2 symmetry to be locally preserved at the top of folds. By contrast, the bottom of graphene folds may have a larger curvature: indeed, Figure 4b shows that folds emerge from the graphene sheet discontinuously and protrude out of the sheet by an angle α = 20°. There, the surface of graphene exposed to ozone molecules is concave, which, in principle, does not favor sp3 bonding. However, chemical reactions between ozone molecules and carbon atoms may still be energetically favorable to the graphene sheet, provided that these reactions are accompanied by a lowering of the elastic energy stored within the bent sheet that constitutes the bottom of a fold. Finally, we evoke a simple mechanism that may also contribute to the formation of trenches both in graphene and in graphite. We note that the nucleation of cavities at the bottom of steps in graphite and at the bottom of folds in graphene bears some similarities with the growth of Al atomic layers at the bottom of steps on vicinal GaAs subtrates.33 There as well, Al adatoms become trapped along the bottom corner of steps, because more atomic bonds (i.e., both along the plane and along the step riser) are available per bonding site than on the step terraces. For a similar reason, ozone molecules and oxygen radicals are more likely to interact with carbon atoms within a concave, corner-shaped region.

4. CONCLUSION In summary, we have presented a characterization of the highly disordered morphology of single-layer and multilayer graphene 18259

dx.doi.org/10.1021/jp2050756 |J. Phys. Chem. C 2011, 115, 18257–18260

The Journal of Physical Chemistry C sheets treated in an ozone generator. We have identified a crossover from nanocrystalline to amorphous carbon in singlelayer graphene. We also have shown that the morphology of exposed samples varies with their thickness. Finally, we have shown that long trenches are formed at the bottom of salient structures, such as steps and folds. These trenches may result from a larger number of bonding sites available within cornershaped structures; in the case of graphene folds, the formation of trenches may also be aided by a curvature effect.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 86-21-34207287. Fax: 86-21-34206883.

’ ACKNOWLEDGMENT We are grateful to M. Lira-Cant
u for the use of the ozone generator. Support from the Spanish Ministry of Science and Innovation (projects FIS2008-06830 and FIS2009-10150) and the EU project NANOPACK (project no. 216176) is gratefully acknowledged. ’ REFERENCES

ARTICLE

(18) McCulloch, D. G.; Prawer, S.; Hoffman, A. Phys. Rev. B 1994, 50, 5905. (19) McCulloch, D. G.; Prawer, S. J. Appl. Phys. 1995, 78, 3040–3047. (20) Parmigiani, F.; Klay, E.; Seki, H. J. Appl. Phys. 1988, 64, 3031–3036. (21) Li, F.; Lannin, J. S. Appl. Phys. Lett. 1992, 61, 2116. (22) Li, F.; Lannin, J. S. Phys. Rev. Lett. 1990, 65, 1905. (23) Tuinstra, F.; Koening, J. L. J. Chem. Phys. 1970, 53, 1126–1130. (24) Canc-ado, L. G.; Takai, K.; Enoki, T.; Endo, M.; Kim, Y. A.; Mizusaki, H.; Jorio, A.; Coelho, L. N.; Magalh~aes-Paniago, R.; Pimenta, M. A. Appl. Phys. Lett. 2006, 88, 163106. (25) Chhowalla, M.; Ferrari, A. C.; Robertson, J.; Amaratunga, G. A. J. Appl. Phys. Lett. 2000, 76, 1419. (26) Lee, B.; Park, S.-Y.; Kim, H.-C.; Cho, K.; Vogel, E. M.; Kim, M. J.; Wallace, R. M.; Kim, J. Appl. Phys. Lett. 2008, 92, 203102. (27) Lee, G.; Lee, B.; Kim, J.; Cho, K. J. Phys. Chem. C 2009, 113, 14225–14229. (28) Koehler, F. M.; Jacobsen, A.; Ensslin, K.; Stampfer, C.; Stark, W. J. Small 2010, 6, 1125–1130. (29) Tracz, A.; Wegner, G.; Rabe, J. P. Langmuir 2003, 19, 6807–6812. (30) Park, S.; Srivastava, D.; Cho, K. Nanotechnology 2001, 12, 245–249. (31) Park, S.; Srivastava, D.; Cho, K. Nano Lett. 2003, 3, 1273–1277. (32) Raula, J.; Makowska, M.; Lahtinen, J.; Sillanp€a€a, A.; Runeberg, N.; Tarus, J.; Heino, M.; Sepp€al€a, E. T.; Jiang, H.; Kauppinen, E. I. Chem. Mater. 2010, 22, 4347–4349. (33) Tsuchiya, M.; Petroff, P. M.; Coldren, L. A. Appl. Phys. Lett. 1989, 54, 1690.

(1) Stolyarova, E.; Rim, K. T.; Ryu, S.; Maultzsch, J.; Kim, P.; Brus, L. E.; Heinz, T. F.; Hybertsen, M. S.; Flynn, G. W. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 9209–9212. (2) Ishigami, M.; Chen, J. H.; Cullen, W. G.; Fuhrer, M. S.; Williams, E. D. Nano Lett. 2007, 7, 1643–1648. (3) Elias, D. C.; Nair, R. R.; Mohiuddin, T. M. G.; Morozov, S. V.; Blake, P.; Halsall, M. P.; Ferrari, A. C.; Boukhvalov, D. W.; Katsnelson, M. I.; Geim, A. K.; Novoselov, K. S. Science 2009, 323, 610–613. (4) G
omez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Nano Lett. 2007, 7, 3499–3503. (5) Kaiser, A. B.; G
omez-Navarro, C.; Sundaram, R. S.; Burghard, M.; Kern, K. Nano Lett. 2009, 9, 1787–1792. (6) Bostwick, A.; McChesney, J. L.; Emtsev, K. V.; Seyller, T.; Horn, K.; Kevan, S. D.; Rotenberg, E. Phys. Rev. Lett. 2009, 103, 056404. (7) Cheng, S.-H.; Zou, K.; Okino, F.; Gutierrez, H. R.; Gupta, A.; Shen, N.; Eklund, P. C.; Sofo, J. O.; Zhu, J. Phys. Rev. B 2010, 81, 205435. (8) Hong, X.; Cheng, S.-H.; Herding, C.; Zhu, J. Phys. Rev. B 2011, 83, 085410. (9) Moser, J.; Tao, H.; Roche, S.; Alzina, F.; Sotomayor-Torres, C. M.; Bachtold, A. Phys. Rev. B 2010, 81, 205445. (10) Leconte, N.; Moser, J.; Ordejon, P.; Tao, H.; Lherbier, A.; Bachtold, A.; Alsina, F.; Sotomayor-Torres, C. M.; Charlier, J.-C.; Roche, S. ACS Nano 2010, 4, 4033–4038. (11) Wong, C.; Yang, R. T.; Halpern, B. L. J. Chem. Phys. 1983, 78, 3325–3328. (12) Liu, L.; Ryu, S.; Tomasik, M. R.; Stolyarova, E.; Jung, N.; Hybertsen, M. S.; Steigerwald, M. L.; Brus, L. E.; Flynn, G. W. Nano Lett. 2008, 8, 1965–1970. (13) Yang, R.; Zhang, L.; Wang, Y.; Shi, Z.; Shi, D.; Gao, H.; Wang, E.; Zhang, G. Adv. Mater. 2010, 22, 4014–4019. (14) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451–10453. (15) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Phys. Rev. Lett. 2006, 97, 187401. (16) Krauss, B.; Lohmann, T.; Chae, D. H.; Haluska, M.; von Klitzing, K.; Smet, J. H. Phys. Rev. B 2009, 79, 165428. (17) Ferrari, A. C.; Robertson, J. Phys. Rev. B 2000, 61, 14095. 18260

dx.doi.org/10.1021/jp2050756 |J. Phys. Chem. C 2011, 115, 18257–18260