Early Stages of Plasma Oxidation of Graphite: Nanoscale

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Early Stages of Plasma Oxidation of Graphite: Nanoscale Physicochemical Changes As Detected by Scanning Probe Microscopies J. I. Paredes, A. Martıńez-Alonso,* and J. M. D. Tascoń Instituto Nacional del Carbo´ n, CSIC, Apartado 73, 33080 Oviedo, Spain Received October 9, 2001 Scanning tunneling and atomic force microscopies (STM/AFM) have been employed to investigate the initial stages of the surface modification of highly oriented pyrolytic graphite (HOPG) by oxygen plasma. Through a rigorous control of very short exposure times to the plasma it was determined, by means of STM, that the attack starts with the formation of single-atom vacancies (detected as 1 nm wide protrusions) randomly distributed all over the basal planes. The mobility of the active species from the plasma adsorbed on the basal planes was evidenced at following stages by the marked tendency of HOPG to develop multiatom vacancies and was estimated to be on the order of tens of nanometers. The multiatom vacancies appeared as large (up to 5 nm in diameter) protrusions in the STM images, although it could be established that their actual size (the physical region with missing carbon atoms) was not above 1 nm. The fact that very large atomic vacancies were developed by the present plasma treatments made it possible to study the physicochemical properties of this type of defect by AFM for the first time. Lateral force images obtained in the contact mode displayed enhanced friction due to the vacancies, the origin of which is discussed. Likewise, the structural and chemical heterogeneity of the defects could be revealed independently of one another by phase imaging in the tapping mode and choosing appropriate tip-sample interaction regimes. These results shed some light into the possibility of visualizing active sites on the surface of carbon materials at very high resolution, a question of scientific and technological relevance.

Introduction Graphite is a layered semimetal in which carbon atoms are linked by strong covalent sp2 bonds within each layer and by weak van der Waals interactions between neighbor layers. A small density of states near the Fermi level makes this material possess low surface free energy, lending the graphite basal plane in general chemically inert to adsorption of molecules or metal atoms.1,2 Owing to the well-defined structure, large crystallite size along the basal planes (typically a few tens of micrometers),3 and clean and atomically flat surfaces, highly oriented pyrolytic graphite (HOPG), a man-made type of graphite, is frequently employed, either as a direct object of study or as a substrate, for fundamental investigations on a wide variety of phenomena. These include metal-surface interactions,2 cluster deposition,4 electron or ion irradiation,5-7 molecular physisorption,8 or nanoscale and lowdimensional magnetism.9 In this respect, one particular topic of research which has a special relevance from both a scientific and a technological perspective is that of graphite oxidation. This interest arises in connection with issues regarding * Corresponding author. Telephone no.: (+34) 985 11 90 90. Fax no.: (+34) 985 29 76 62. E-mail: [email protected]. (1) Charlier, M.-C.; Charlier, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1987; Vol. 20, Chapter 2. (2) Ma, Q.; Rosenberg, R. A. Phys. Rev. B 1999, 60, 2827. (3) Ohler, M.; Sanchez del Rio, M.; Tuffanelli, A.; Gambaccini, M.; Taibi, A.; Fantini, A.; Pareschi, G. J. Appl. Crystallogr. 2000, 33, 1023. (4) Carroll, S. J.; Pratontep, S.; Streun, M.; Palmer, R. E.; Hobday, S.; Smith, R. J. Chem. Phys. 2000, 113, 7723. (5) Takeuchi, M.; Muto, S.; Tanabe, T.; Arai, S.; Kuroyanagi, T. Philos. Mag. A 1997, 76, 691. (6) Hahn, J. R.; Kang, H. Phys. Rev. B 1999, 60, 6007. (7) Habenicht, S. Phys. Rev. B 2001, 63, 125419. (8) Stawasz, M. E.; Sampson, D. L.; Parkinson, B. A. Langmuir 2000, 16, 2326. (9) Binns, C.; Baker, S. H.; Demangeat, C.; Parlebas, J. C. Surf. Sci. Rep. 1999, 34, 105.

carbon combustion and water-gas production,10-13 the durability of structural materials employed in spacecraft,14 the removal of carbon deposits from catalysts,15 the synthesis of graphite intercalation compounds,16 or the production of molecule corrals for their use in nanoscale science.17 It should also be noted that, due to its status as a model carbon material, many of the studies performed on HOPG are also relevant for other carbon-based materials of interest possessing the six-membered ring as a structural unit, such as activated carbon fibers (ACFs) and carbon nanotubes (CNTs). ACFs, and more generally porous carbons, show exceptionally high specific surface areas (up to 3000 m2/g), which render them very attractive as efficient molecular adsorbents, for instance in environmental applications.18-20 In this context, the oxidative modification, e.g. by oxygen plasma, of the HOPG basal plane presents interest, since it provides the graphite surface with structural defects that can be considered as model micropores for carbon materials.21 (10) Yang, R. T. In Chemistry and Physics of Carbon; Walker, P. L., Thrower, P. A., Eds.; Marcel Dekker: New York, 1984; Vol. 19, Chapter 3. (11) Chang, H.; Bard, A. J. J. Am. Chem. Soc. 1991, 113, 5588. (12) Tandon, D.; Hippo, E. J.; Marsh, H.; Sebok, E. Carbon 1997, 35, 35. (13) Lee, S. M.; Lee, Y. H.; Hwang, Y. G.; Hahn, J. R.; Kang, H. Phys. Rev. Lett. 1999, 82, 217. (14) Kinoshita, H.; Umeno, M.; Tagawa, M.; Ohmae, N. Surf. Sci. 1999, 440, 49. (15) Yang, Y. W.; Hrbek, J. J. Phys. Chem. 1995, 99, 3229. (16) Levi, M. D.; Levi, E.; Gofer, Y.; Aurbach, D.; Vieil, E.; Serose, J. J. Phys. Chem. B 1999, 103, 1499. (17) Zhu, Y. J.; Hansen, T. A.; Ammermann, S.; McBride, J. D.; Beebe, T. P. J. Phys. Chem. B 2001, 105, 7632. (18) Daley, M. A.; Tandon, D.; Economy, J.; Hippo, E. J. Carbon 1996, 34, 1191. (19) Dresselhaus, M. S. Annu. Rev. Mater. Sci. 1997, 27, 1. (20) Derbyshire, F.; Jagtoyen, M.; Andrews, R.; Rao, A.; MartinGulloń, I.; Grulke, E. A. In Chemistry and Physics of Carbon; Radovic, L. R., Ed.; Marcel Dekker: New York, 2001; Vol. 27, Chapter 1.

10.1021/la0115280 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/26/2002

Early Stages of Plasma Oxidation of Graphite

Concerning plasma oxidation, the studies on graphite surface modification by oxygen plasma reported so far22-25 have dealt with advanced stages of the attack, implying extensive changes in the structure of the material and where direct traces of the basic, individual events that drive the process have been lost. This holds true not only for graphite and oxygen plasma but also in general for other treatments (e.g., ultraviolet ozone oxidation or Ar plasma etching) and/or materials, such as polymers.26-30 As a consequence, for a better understanding of the plasma etching process, it would be desirable to investigate the initial stages of graphite oxidation by this medium. Another point of interest within this approach would be the possibility of probing the modified physicochemical properties of the oxygen plasma-treated surface at a very local scale, neither of which have been considered previously. In addition to being of fundamental scientific interest, the investigation of the first steps of graphite oxidation by plasmas has also been motivated by its relevance to the controlled sidewall functionalization of CNTs, a topic receiving a great deal of attention at present.31,32 Owing to their exceptional mechanical properties, CNTs are considered as the ultimate fibers for use in future ultrastrong composite materials.33 In this respect, one of the most crucial issues to confront is that of attaining a robust CNT-matrix interface, which could be accomplished through the functionalization of the nanotube surface, but with the constraint of largely preserving its structural integrity at the same time.34,35 Plasma treatment of the nanotubes could be employed as a route to their sidewall functionalization, and as a matter of fact, reports on this topic have very recently begun to emerge.36,37 For oxygen plasma, however, the treatments usually performed produced severe damage to the CNT structure,36 ruining the material for most applications. Likewise, it has very recently been shown that the electronic properties of single-walled CNTs are extremely sensitive to oxygen adsorption,38 so they could be possibly modulated by the amount of oxygen adsorbed. Thus, there is a need for a highly controlled oxidative functionalization of the CNT sidewall. With that final objective, it would be (21) Paredes, J. I.; Martıńez-Alonso, A.; Tascoń, J. M. D. Langmuir 2001, 17, 474. (22) Joshi, A.; Nimmagadda, R. J. Mater. Res. 1991, 6, 1484. (23) You, H.-X.; Brown, N. M. D.; Al-Assadi, K. F. Surf. Sci. 1993, 284, 263. (24) Lu, X.; Huang, H.; Nemchuk, N.; Ruoff, R. S. Appl. Phys. Lett. 1999, 75, 193. (25) Paredes, J. I.; Martıńez-Alonso, A.; Tascoń, J. M. D. J. Mater. Chem. 2000, 10, 1585. (26) You, H.-X.; Brown, N. M. D.; Al-Assadi, K. F. Surf. Sci. 1992, 279, 189. (27) Mahlberg, R.; Niemi, H. E.-M.; Denes, F. S.; Rowell, R. M. Langmuir 1999, 15, 2985. (28) Teare, D. O. H.; Emmison, N.; Ton-That, C.; Bradley, R. H. Langmuir 2000, 16, 2818. (29) Ton-That, C.; Campbell, P. A.; Bradley, R. H. Langmuir 2000, 16, 5054. (30) Dupont-Gillain, C. C.; Adriaensen, Y.; Derclaye, S.; Rouxhet, P. G. Langmuir 2000, 16, 8194. (31) Ni, B.; Sinnott, S. B. Phys. Rev. B 2000, 61, R16343. (32) Seifert, G.; Ko¨hler, T.; Frauenheim, T. Appl. Phys. Lett. 2000, 77, 1313. (33) Salvetat, J.-P.; Briggs, G. A. D.; Bonard, J.-M.; Bacsa, R. R.; Kulik, A. J.; Sto¨ckli, T.; Burnham, N. A.; Forro´, L. Phys. Rev. Lett. 1999, 82, 944. (34) Calvert, P. Nature 1999, 399, 210. (35) Ajayan, P. M.; Schadler, L. S.; Giannaris, C.; Rubio, A. Adv. Mater. 2000, 12, 750. (36) Ago, H.; Kugler, T.; Cacialli, F.; Salaneck, W. R.; Shaffer, M. S. P.; Windle, A. H.; Friend, R. H. J. Phys. Chem. B 1999, 103, 8116. (37) Chen, Q.; Dai, L.; Gao, M.; Huang, S.; Mau, A. J. Phys. Chem. B 2001, 105, 618. (38) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801.

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useful to first study it on planar graphenes (i.e., HOPG), since they differ from the CNT walls only in their curvature, an effect which has been suggested to play only a minor role in oxygen adsorption on the nanotubes.39 Consequently, this work is focused on the earliest stages of oxygen plasma modification of HOPG with a 2-fold objective: first, to increase our understanding of the plasma oxidation of a graphene; second, to investigate the changes brought about by this treatment in the physicochemical properties of the graphene at a very local scale. To this end, scanning probe microscopies (SPM) are employed. As will be shown, the modifications in the structure and properties of the HOPG samples studied here are in general so minute that the use of local, highresolution probes, such as those provided by SPM, becomes essential. Experimental Section The HOPG samples (grade ZYH) used in this study were obtained from Advanced Ceramics Corp. (Cleveland, OH). Just prior to plasma exposure the specimens were cleaved in air to obtain fresh surfaces. The plasma treatments were accomplished in a Technics Plasma 200-G (Kircheim bei Mu¨nchen, Germany) apparatus employing 2.45 GHz microwave (MW) radiation to activate the plasma. A magnetron was utilized to generate the MW radiation, which was transferred via a waveguide to the quartz reactor (batch type) where the plasma was produced and the HOPG samples were placed. The working pressure in the chamber during the treatments was kept at 1.0 mbar. Unless otherwise specified, all the treatments were carried out using O2 as the plasma gas. To check for the reproducibility of the results, several plasma-etched specimens were prepared and studied for every specific set of plasma processing conditions implemented. Carrying out this verification was particularly important in the present case since, as will be shown, the very short treatment times used implied that the active species from the plasma had just been formed during the attack or were not yet formed at all. Therefore, it could be possible that different samples prepared with exactly the same exposure times would lead to qualitatively different results due to instabilities or fluctuations in the generation of the active species at the very initial stages. However, this was not the case and a reproducible behavior was observed instead; i.e., identical treatments yielded essentially the same features. For the SPM investigations, the plasma-treated HOPG samples were transferred to a Nanoscope Multimode IIIa, from Digital Instruments (Santa Barbara, CA), where they were characterized by both scanning tunneling and atomic force microscopy (STM/AFM). STM imaging was performed in the constant current mode (variable height) using mechanically prepared Pt/Ir (80/20) tips. The tunneling parameters typically employed were 100 mV and 1 nA for the bias voltage and tunneling current, respectively. The quality and stability of the tips were first verified by obtaining atomic resolution images of the basal plane of untreated HOPG. AFM measurements were carried out in the contact as well as in the tapping modes of operation. In the former case, microfabricated triangular Si3N4 cantilevers (Digital Instruments) with nominal spring constants of 0.06 and 0.58 N/m and integrated pyramidal tips were used. Imaging was accomplished in the constant deflection (constant force) mode. Height and lateral (friction) force images were acquired simultaneously with the scan angle set to 90°, i.e., with the fast scan direction perpendicular to the main axis of the cantilever. As regards the tapping AFM studies, rectangular silicon cantilevers (also from Digital Instruments) were employed. These cantilevers had spring constants of about 40 N/m and resonance frequencies around 250 kHz. Scanning was performed at a fixed amplitude of cantilever oscillation, which is termed the setpoint amplitude, collecting height and phase images concurrently. In the Nanoscope, phase images are obtained by recording the change in phase angle of the interacting cantilever oscillation (39) Britto, P. J.; Santhanam, K. S. V.; Rubio, A.; Alonso, J. A.; Ajayan, P. M. Adv. Mater. 1999, 11, 154.

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relative to the phase angle of the freely oscillating cantilever. To set suitable working parameters, amplitude and phase versus distance curves were acquired first. As shown previously,40 phase-distance curves reflect the nature of the tip-sample interaction and consist of two differentiated regimes, according to the sign of the phase angle. When the cantilever is far from the sample surface and there is no interaction between tip and sample, the phase angle is 0° (following the Digital Instruments convention). Upon approach of the surface, tip-sample interactions become appreciable and the tip experiences an attractive force which damps the cantilever oscillation and shifts the phase angle to negative values. This is the noncontact regime. As the cantilever is drawn nearer to the sample, the intermittent contact regime is established, whereby the tip begins to experience repulsive forces intermittently, damping further the cantilever oscillation amplitude and switching the phase angle from negative to positive values. In general, the correct interpretation of the phase contrast images requires first establishing unambiguously the tip-sample interaction regime under which imaging is to be performed by recording a phase-distance curve and setting the free and setpoint oscillation amplitudes in accordance.40 This point has been often overlooked in the literature, but it was particularly crucial in the present case, where very high resolution information about the modified physicochemical properties of the plasma-treated HOPG was sought. As a rule, the lower the free amplitude, the larger the noncontact part of the phasedistance curve, which facilitates imaging in this regime. Likewise, for a given free amplitude, large (close to the free amplitude) and small setpoint amplitudes are generally required to work in the noncontact and intermittent contact regimes, respectively. However, the exact range of amplitudes which fixed the interaction in any of the two regimes was observed to vary considerably depending on the tip employed. This finding has also been reported recently by other groups40,41 and was attributed to the effects of tip sharpness: blunter tips are subjected to higher attractive forces, and thus the noncontact part of the phasedistance curve is large even at relatively high free amplitudes. In this work, since both noncontact and intermittent contact imaging were pursued, those tips were used that displayed the former regime ample enough in the phase-distance curve to allow stable measurement but without compromising the tip sharpness. This would not be necessary if only the intermittent contact regime were to be employed. Furthermore, to check that the tip state did not change when imaging in a particular regime (which would have implied a change in the tip-sample interaction during the measurement, invalidating the data), phase-distance curves were obtained at intervals after collecting several images and compared to the curve recorded at the beginning to verify that it was not altered. In general, all the SPM investigations were performed in ambient laboratory conditions (at a relative humidity of 3540% and a temperature of 20-22 °C), except for some contact AFM measurements carried out under distilled and deionized water (Milli-Q) in the Nanoscope liquid cell, which will be explicitly indicated in the text. Likewise, to confirm the reproducibility of the measurements and rule out possible artifacts in the images, several different and previously unused probes were employed for both the STM and AFM investigations.

Results and Discussion Since one of the aims of this study was to identify and visualize the incipient processes of the oxygen plasma attack, the initial step in the investigation consisted in determining the plasma processing conditions (essentially the exposure time) under which the HOPG samples experienced their very first observable changes. With this object, the power of the MW field that generates the plasma was set to as low a stable value (40 W) as permitted by the plasma etching apparatus employed. Taking account of the fact that the density of reactive species in the plasma (40) James, P. J.; Antognozzi, M.; Tamayo, J.; McMaster, T. J.; Newton, J. M.; Miles, M. J. Langmuir 2001, 17, 349. (41) Bar, G.; Brandsch, R.; Whangbo, M.-H. Surf. Sci. 1999, 422, L192.

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Figure 1. STM images of the HOPG surface following an oxygen plasma treatment for 4 s at a MW power of 40 W. Individual 1 nm wide spots are observed at the nanometer scale (a) and with atomic resolution (b). In (c), an atomic-scale image of a two-spot ensemble is shown.

decreases steadily with decreasing power,25 this served the purpose of attaining very mild treatment conditions. Then, at a fixed MW power of 40 W, different very short plasma exposure times were tried out and their effect on the HOPG surface was inspected by STM. Counting the treatment time as the interval elapsed since the MW field was activated and the plasma gas was allowed to enter the chamber until the moment the magnetron was switched off, it was observed that the minimum time necessary to produce any perceptible change in the STM images was 4 s. Figure 1 shows some images of the HOPG surface following such a treatment time, both at a scale of several tens of nanometers (Figure 1a) and at the atomic scale (Figure 1b,c). It can be noticed that, due to the plasma


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Figure 2. STM images of an HOPG sample treated in oxygen plasma for 6 s at a MW power of 40 W: (a) general image at the nanometer scale; (b) detailed image of an area with neighboring small and large spots. The 1 nm spots coexist with larger (up to 5 nm) ones.

exposure, the originally featureless basal planes have been decorated by several randomly distributed bright spots (protrusions), the rest of the surface retaining the atomically flat and perfectly ordered structure as observed in pristine HOPG (i.e., the well-known atomic scale triangular pattern). Below this etching time of 4 s (for instance, after a 3 s exposure), no sign of the spots could be detected anywhere on the surface, the basal planes therefore appearing completely unchanged when compared to untreated graphite. Such observation was supported on a large number of images (typically a few hundreds for each sample). Following 4 s exposures, many protrusions were seen in each one of the images, whereas for the 3 s samples no protrusions were detected at all, which can be considered as statistically significant. In the 4 s sample (Figure 1), the bright spots tended to display a more or less circular shape and rather similar lateral (between 0.8 and 1.2 nm) and vertical (around 0.1-0.25 nm) dimensions. Their uniform size, together with the fact that no features were ever found with smaller lateral sizes than those mentioned previously, indicate that the protrusions observed in Figure 1 are the STM signature (i.e., as can be detected by STM) of the very first individual events which lead to the modification of the material. Likewise, although the bright spots were normally found isolated from one another (Figure 1a,b), it was not uncommon to observe ensembles of two spots. This is illustrated in Figure 1c, where such an ensemble is recognized at the bottom part of the image. It can be clearly noticed that the two protrusions appear merged while keeping their individuality at the same time. Figure 2 shows the evolution of the HOPG samples when they were exposed to the oxygen plasma for slightly longer times (6 s). In this case, albeit the images are still dominated by the presence of a number of protrusions, some changes can be observed with respect to the 4 s treatment. The small bright spots characteristic of the 4 s sample (plus some occasional ensembles of two spots) now coexist alongside much larger protuberances, with diameters between 2 and 5 nm. For instance, in Figure 2b one such extensive protrusion (about 3.5 nm wide) can be seen on the top left corner together with a smaller spot, approximately 1.1 nm in diameter, located in the lower half of the image. Even though in this particular example both protrusions present similar heights (0.22-0.24 nm), in general the bigger ones tended to display also larger heights. This point can be best realized inspecting Figure 2a, where the large protrusions appear somewhat brighter than the small spots, with heights lying typically between

0.2 and 0.4 nm. Likewise, the overall density of features has increased considerably after the 6 s treatment compared to the 4 s one: for the 4 s samples the spot densities were in the range of 1000-2500 µm-2, whereas values of 5000-12000 µm-2 (counting both the small and the large protrusions) were normally measured after 6 s exposures. From the results presented beforehand, it is apparent that the atomic-sized features which decorate the HOPG surface upon the initial instants of exposure to the oxygen plasma are a consequence of few events coming about through the interaction between the samples and the plasma. In principle, two prime contributions to the graphite-plasma interaction leading to the surface modification of the former must be taken into account: a chemical and a physical one.25,42 The chemical interaction proceeds through the combination between the chemically active species from the plasma (mainly oxygen atoms in the present case)14,23 and the carbon atoms of the graphite surface. The physical interaction consists of a bombardment of positive ions formed in the plasma (such as O2+),23,43 giving rise to the sputtering of carbon atoms from the sample surface and/or ion implantation between the top graphite layers and, as a consequence, to the creation of surface defects. The predominance of anyone of these two processes, which is important to ascertain for the subsequent investigations, will generally be determined by the plasma processing conditions. For a MW plasma, the energy of the ions impinging upon the sample is thought to be typically around 10-20 eV.44 These figures lie below the minimum energy an ion must transfer to the surface to create stable defects on graphite, which is 34.5 eV,45 suggesting that purely physical effects are not a source of surface modification in the present case. To corroborate this point, pristine HOPG samples were submitted to another set of treatments, identical to those described previously, but in this case using Ar as the plasma gas. Since no chemically active species are expected to be formed in such a plasma, only the bombardment of the Ar+ ions created in the discharge could have an effect (42) Lieberman, M. A.; Lichtenberg, A. J. Principles of Plasma Discharges and Materials Processing; John Wiley and Sons: New York, 1994; Chapter 9. (43) Kitajima, M.; Kamioka, I.; Nakamura, K. G.; Hishita, S. Phys. Rev. B 1996, 53, 3993. (44) Paraszczak, J.; Heidenreich, J. In Microwave Excited Plasmas; Moisan, M., Pelletier, J., Eds.; Elsevier: Amsterdam, 1992; Chapter 15. (45) Marton, D.; Bu, H.; Boyd, K. J.; Todorov, S. S.; Al-Bayati, A. H.; Rabalais, J. W. Surf. Sci. 1995, 326, L489.

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Figure 3. Schematic representation of the two most favorable adsorption sites of atomic oxygen on the graphite basal surface: (1) bridge site; (2) top site.

on the graphite surface structure (provided that their impinging energy was high enough),26 which would then be detectable by STM.6 It should be noted that the ion energies attained in argon and oxygen plasmas are very similar,23,26 so the results obtained for the former concerning damage of graphite by ions will generally be valid for the latter. However, the graphite basal planes were not observed by STM to undergo any changes following the Ar plasma treatments under these conditions, i.e., no bright spots or other features were developed, indicating that the ion energies reached in the present plasma configuration are indeed too small to create any structural defects on graphite simply by a process of physical bombardment. Accordingly, the STM observations of the oxygen plasma-treated surfaces (Figures 1 and 2) relate for the most part to chemical effects and have to be interpreted in such terms. On the basis of the previous considerations, it can be inferred that the attack of HOPG by oxygen plasma under the conditions studied here starts when some of the oxygen atoms produced in the discharge approach the graphite surface and become chemisorbed at random locations. Recent theoretical calculations in the framework of the density functional formalism have indicated that the most favorable site for the chemisorption of atomic oxygen on the graphite basal plane is the bridge one (located just above the center of a C-C bond; see Figure 3).46,47 As a result of the strong adsorption over this site, the C-C bonds adjacent to the O atom suffer a significant reduction in their bond order,46,48 implying a weakening and a subsequent finite destruction cross section of these sp2 bonds, which eventually leads to the formation of gaseous oxidation products, such as CO. In turn, the release of the carbon oxide molecule leaves an atomic vacancy, i.e., a structural defect, on the HOPG surface. Now, it has been shown that a surface atomic vacancy on graphite (with one to several C atoms missing) produces an increase in the local electronic density at energies close to the Fermi level in the atoms surrounding the vacancy.6,49 This partial electronic density enhancement, which may also be interpreted as dangling bonds, can then be probed and recognized by STM as a protrusion in the images.21 Therefore, the bright spots observed after a 4 s treatment (Figure 1) are attributed to the atomic vacancies developed on the basal plane by the release of the first volatile molecules that form in the oxidation process. Furthermore, taking into account that these were the smallest protrusions ever found in the experiments and that their sizes were comparable to those of the theoretically calculated protrusions with just one atom missing,49 we conclude (46) Lamoen, D.; Persson, B. N. J. J. Chem. Phys. 1998, 108, 3332. (47) Incze, A.; Pasturel, A.; Chatillon, C. Appl. Surf. Sci. 2001, 177, 226. (48) Janiak, C.; Hoffmann, R.; Sjo¨vall, P.; Kasemo, B. Langmuir 1993, 9, 3427. (49) Hjort, M.; Stafstro¨m, S. Phys. Rev. B 2000, 61, 14089.

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that they are single-atom vacancies and, hence, the very first structural change arising upon the plasma exposure. The two-spot ensembles that were also observed in the 4 s sample (Figure 1c) provide an indication of the next step in the oxidation process, which implies a certain degree of mobility of the adsorbed O atoms across the surface. In this regard, it should be noted that the aforementioned theroretical computations46 also revealed that the adsorption of an O atom on a top site (lying directly above a C atom; see Figure 3) was energetically very similar to that of the bridge site, other sites being much less favorable. This suggests that O atoms can easily diffuse over the graphite surface jumping between neighboring brigded sites through the top sites. The random diffusion across the basal plane would take place until either (a) the C-C bonds adjacent to the O atom are destroyed at some point in its path, releasing a carbon oxide molecule and leaving behind an atomic vacancy, as described before, or (b) the O atom meets a vacancy site already formed by the action of the plasma as in (a), where the large electronic density at the Fermi level (probed by STM) strongly enhances the O atom adsorption and an oxygen functional group is created, eventually desorbing as CO/CO2 and producing another atomic vacancy next to the first one. In consequence, the two-spot ensembles can be attributed to the first O atoms which, in their arbitrary motion across the surface, encountered the enhanced electronic density region of a single-atom vacancy previously formed and became more strongly chemisorbed. Upon release of the gaseous oxidation products a new single-atom vacancy appeared next to the original one, thus creating the two neighboring vacancies observed by STM (Figure 1c). Therefore, plasma oxidation proceeds both at perfect sites of basal planes and defect sites (e.g., atomic vacancies), which is a situation rather different from that of thermal oxidation with molecular oxygen. At temperatures below 700 °C, oxidation with O2 takes place exclusively at preexisting defects on the graphite surface, such as step edges or atomic vacancies, exposing unsaturated sp2 bonds.11,12 Extrapolating the process of plasma attack it can be expected that, as time elapses, more O atoms will run across the vacancies formed earlier, becoming adsorbed and forming more oxygen functional groups in the vacancy region, which in turn will enlarge on the release of the functionalities as CO/CO2. This implies that multiatom vacancies (i.e., vacancies with several atoms missing) will develop following sufficiently long exposure times to the plasma and would explain the large protrusions observed in the 6 s samples (Figure 2): it has been proposed that multiatom vacancies induce an enhancement in the electronic density near the Fermi level encompassing up to several hundred atoms around the vacancy,6 which is consistent with the sizes of the large protrusions in Figure 2. These observations then provide direct atomic-scale evidence of a relative chemical selectivity of defects (e.g., atomic vacancies) compared to perfect sites of basal planes in the plasma oxidation of carbon materials, which has been observed previously on much larger scales.25,50 Likewise, they also give an approximate idea of the lateral distance an adsorbed O atom travels across the perfect graphite surface before creating an atomic vacancy. As long as the density of formed vacancies remains low, the distance separating them will be larger than the lateral distances typically traversed by an O atom. Therefore, multiatom vacancies will not appear yet, which is essentially the case of Figure (50) Brown, N. M. D.; Cui, N.; McKinley, A. Appl. Surf. Sci. 1998, 133, 157.


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Figure 4. STM images of HOPG treated in oxygen plasma for 5 min at a MW power of 100 W: (a) relatively ordered area at atomic resolution; (b) (x3 × x3)R30° superstructure observed in the neighborhood of a small pit.

1. When the density becomes large enough and the vacancies form closer to one another, the lateral distances traveled by the O atom will allow the formation of multiatom vacancies (Figure 2). In a comparison of both situations, it can be roughly estimated that an O atom wanders several tens of nanometers laterally over the pristine graphite surface before the breakup of the C-C bonds, the release of a carbon oxide molecule, and the subsequent formation of an atomic vacancy take place. As expected, much more agressive exposures to the plasma led to very high levels of disorder on the HOPG surface, where in general atomic scale features could no longer be clearly identified. However, some limited areas kept a relative degree of order. This is exemplified in Figure 4 for a sample treated at a MW power of 100 W for 5 min. In Figure 4a, a pit at the bottom left corner of the image is worthy of attention. Its size is only about 1.2 nm and its depth only one graphite monolayer (taking the small unperturbed region on the top left corner of the image as a reference). The pit can be attributed to the expansion of the atomic vacancies shown in Figures 1 and 2. Furthermore, from this and similar images it was observed that the minimum size of the monolayer pits was around 1 nm, implying that the mechanism driving the electronic density enhancement of the vacancy region disappears when the vacancy reaches such a size. In turn, this sets an upper limit for the real size of the multiatom vacancies (considered as the physical region with missing atoms) which are observed as STM protrusions in graphite (e.g., Figure 2). Figure 4b shows an example of a (x3 × x3)R30° superstructure, sometimes observed surrounding the mentioned pits and extending several nanometers away from them. It is believed that these superstructures reflect an electronic density perturbation near the Fermi level (and not a surface reconstruction) due to defects51 (the pits in this case). In Figure 4b, it is seen that one out of three spots of the superstructure lies ∼0.15 nm above the spots of the regular graphite lattice (which is recovered at the bottom right corner of the image), the other two retaining the height of the unperturbed graphite spots. As a result of this net increase in electronic density, the superstructure region should display enhanced adsorption compared to perfect, unperturbed graphite. Owing to the very nature of the etching process of graphite by oxygen plasma, described previously, it is expected that oxygen functionalities will be present to (51) Takeuchi, N.; Valenzuela-Benavides, J.; Morales de la Garza, L. Surf. Sci. 1997, 380, 190.

some extent on the plasma-generated defects (Figures 1 and 2). Although the oxygen concentration levels attained by the very short treatments reported here are below the detection limit of X-ray photoelectron spectroscopy (XPS), we have verified by this technique that longer treatments do in fact introduce oxygen groups (hydroxyl, carbonyl, carbonxyl) on the graphite surface. So, in the 4 and 6 s samples these functionalities are also assumed to be present, though their detection is a problem. In this context, the question arises of whether SPM is capable of detecting this presence or not. As concerns STM, it should be noted that Hahn and Kang6 did not observe any significant difference between the signature of defects, created by Ar+ bombardment of a graphite surface, in ambient and vacuum conditions of STM operation. As some oxygen must have chemisorbed from the ambient onto the defects in the former case,13 whereas none was expected to exist in the latter, it was concluded that the adsorption of impurity atoms did not modify significantly the appearance of the mentioned defects. Furthermore, very recently Giunta and Kelty52 have shown theoretically that the STM images of the graphite layer edges present little variation with their chemical termination (O, Cl). Accordingly, the STM images would only reflect the presence of structural (as opposed to chemical) heterogeneities on the graphite surface (i.e., atomic vacancies in this case) and the presence of oxygenated groups on the plasmainduced defects would not be ascertained by this technique. As a further step in the investigation, AFM measurements were carried out. Figure 5 shows typical height (a) and corresponding lateral force (b) images obtained in the contact mode for the 6 s samples. The topographical (height) image (Figure 5a) appears flat and featureless, although we know from STM that the surface is decorated with atomic vacancies which enhance the partial electronic density in their surroundings (Figure 2). This apparent contradiction has been reported before for Ar+-bombarded HOPG53 and is resolved by taking into account the facts that (a) the height images in contact mode AFM probe the total (rather than a partial) electron density, which is not enhanced in the region around the vacancy but remains nearly unchanged compared to defect-free areas,54 and (b) the hollow site of an atomic vacancy cannot be detected by contact AFM as a depression in the height image, since its size is smaller than the tip-sample contact diameter. (52) Giunta, P. L.; Kelty, S. P. J. Chem. Phys. 2001, 114, 1807. (53) Hahn, J. R.; Kang, H.; Song, S.; Jeon, I. C. Phys. Rev. B 1996, 53, R1725. (54) Lee, K. H.; Causa´, M.; Park, S. S.; Lee, C.; Suh, Y.; Eun, H. M.; Kim, D. THEOCHEM 2000, 506, 297.

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Figure 5. Contact AFM height (a) and lateral force (b) images of the HOPG surface exposed to the oxygen plasma for 6 s at a MW power of 40 W. (c) Equivalent lateral force image of the untreated HOPG surface.

Therefore, the absence of features in the contact AFM height images is consistent with the idea of atomic vacancies in the 6 s sample. On the other hand, a marked frictional contrast, which cannot have a topographical origin (as the height image is flat), was observed in the lateral force image (Figure 5b). For comparison, the equivalent friction image of the untreated graphite basal plane is shown in Figure 5c. It is noticed that whereas the former is decorated everywhere with bright features (revealing enhanced friction), these are completely absent from the latter. As the only difference between the two surfaces is the atomic vacancies of the 6 s sample, the bright features of Figure 5b have to be ascribed to these plasma-generated defects. The images of Figure 5 were obtained with a cantilever of spring constant ∼0.06 N/m at an external applied load ∼3 nN. Different cantilevers with this and different (0.58 N/m) spring constants and

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applying several different external loads, always within the wearless friction regime,55 yielded consistently the same finding: higher friction due to the defects compared to perfect graphite (the bright features often appeared smaller than observed in Figure 5b, depending on the cantilever used and due to variations in the actual tip radius between cantilevers). To the best of our knowledge, this is the first time that atomic vacancies in graphite have been detected by AFM (as regions of increased friction in the lateral force images), and we attribute it to the plasma chemical attack, which allows the expansion of originally single-atom vacancies into defects large enough to provide detectable frictional contrast while at the same time retaining their nature as atomic vacancies (STM protrusions). In fact, no frictional contrast was observed in the 4 s samples: in this case, the defects were too small to give rise to any perceptible contrast. Therefore, it is the large multiatom vacancies that are mainly responsible for the areas of enhanced friction observed in the lateral force images. Friction arises from the dissipation of energy at a sliding interface. Thus, to elucidate the origin of the increased friction in the vacancy regions of HOPG (Figure 5b), the possible mechanisms of energy dissipation must be considered. First, since the experiments were carried out in air, it could be possible that differences in AFM tip adhesion between perfect and defect areas of the plasmatreated surface would have led to corresponding differences in friction. As the vacancies developed by the plasma must possess a certain level of oxygen functionalities (rather hydrophilic), the defect regions must have a considerable amount of water molecules clustered over them, particularly when compared to the extremely hydrophobic nature of the perfect basal plane areas.56-59 Accordingly, higher tip-sample capillary forces (which would imply higher friction forces)60 should be expected on the vacancy regions. Unfortunately, no clear evidence supporting or excluding this hypothesis could be gathered from AFM forcedistance curves (measuring adhesion as the pull-off force from these curves). Owing to their very small size, locating the tip accurately just above the defects was rather impracticable. Also, comparing the adhesion forces obtained at many random points of untreated HOPG with those of the plasma-treated surface was not conclusive. To circumvent this difficulty, another approach was pursued: when water is the working medium between tip and sample, hydrophobic interactions (arising from the incompatibility of the water molecules with nonpolar surfaces) have been shown to come into play, with the consequence that higher adhesion (and higher friction) is measured on hydrophobic regions of the sample than on hydrophilic regions,61-63 just the reverse of the behavior in air. Therefore, if the dominant mechanism responsible for the friction observed in the present case resulted from (55) Riedo, E.; Chevrier, J.; Comin, F.; Brune, H. Surf. Sci. 2001, 477, 25. (56) Bradley, R. H.; Sutherland, I.; Sheng, E. J. Chem. Soc., Faraday Trans. 1995, 91, 3201. (57) Bradley, R. H.; Sutherland, I.; Sheng, E. J. Colloid Interface Sci. 1996, 179, 561. (58) Mu¨ller, E. A.; Hung, F. R.; Gubbins, K. E. Langmuir 2000, 16, 5418. (59) McCallum, C. L.; Bandosz, T. J.; McGrother, S. C.; Mu¨ller, E. A.; Gubbins, K. E. Langmuir 1999, 15, 533. (60) Sirghi, L.; Nakagiri, N., Sugisaki, K.; Sugimura, H.; Takai, O. Langmuir 2000, 16, 7796. (61) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. Rev. Mater. Sci. 1997, 27, 381. (62) Sinniah, S. K.; Steel, A. B.; Miller, C. J.; Reutt-Robey, J. E. J. Am. Chem. Soc. 1996, 118, 8925. (63) Papastavrou, G.; Akari, S.; Mo¨hwald, H. Europhys. Lett. 2000, 52, 551.


the differences in hydrophilicity between perfect and defect areas of the plasma-treated sample, then a contrast reversal would be observed in the lateral force images when performing the measurements under water compared to air. However, the measurements made under water on the 6 s sample yielded again bright (and not dark) spots in the lateral force images, i.e., there was no contrast reversal, so local variations in hydrophilicity are not the most influential factor in the frictional contrast of the sample. In any case, this observation does not mean that there is no contribution of the mentioned effect to friction but, in the contact mode, it is not the dominant one and other mechanisms must be much more important. On the atomic scale, and in the wearless regime, frictional energy is dissipated via two main channels.55,64,65 One is the sliding-induced excitation of atomic lattice vibrations (phonons), which propagates to the rest of the lattice and finally dissipates as heat. Within this mechanism, the fact that atomic vacancies are present on the graphite surface implies the existence of carbon lattice sites (those which are immediately surrounding the vacancy), where the translational symmetry of the solid is broken. In turn, this means that vibrational modes that were forbidden by symmetry in defect-free graphite are now allowed on the defects; i.e., there is an increased number of vibrational modes available for excitation (and for energy dissipation) on the defects compared to the perfect areas of the sample. Consequently, more energy will dissipate when the tip slides over a defect than over a defect-free region, giving rise to higher friction upon the former. In studying alkylsilane monolayers on mica, Xiao et al.66 showed that an increased number of energy dissipation modes (rotational, vibrational, etc.), arising from the presence of defects, was responsible for the larger friction observed for monolayers with shorter chains. This pathway of energy dissipation is thus consistent with the lateral force images of the 6 s sample (Figure 5b). Further indication of the presence of new vibrational modes on the defects was obtained from Raman measurements of the 6 s sample. For untreated HOPG, only the G peak (corresponding to E2g vibrational mode) at ∼1580 cm-1 was observed in the first-order spectrum. On the other hand, a faint, but detectable and highly reproducible over different regions of the sample, band at ∼1360 cm-1 (D band) appeared for the 6 s sample. The D band is attributed to the A1g vibrational mode, forbidden in perfect graphite but allowed when defects are present.67 The ratio between the intensities of the D and G bands was ID/IG ≈ 0.02. The second energy dissipation channel referred to above involves the excitation of electrons at the surface. The magnitude of its contribution to friction is largely dependent on the number of electrons within the conduction bands of both tip and sample. Thus, a larger number of conduction band electrons implies greater opportunities for energy dissipation by virtue of their excitation.65 Now, the STM observations of the present work (Figure 2) reveal an increase in the number of electrons near the Fermi level in the region around the vacancies in the 6 s samples, also supported by theoretical calculations in the literature.49,54 Accordingly, more electrons are available for excitation on defect areas compared to perfect regions in the plasma-treated graphite surface, so more friction can be expected on the former as a result of this mechanism. Thus, it is concluded that both phononic and electronic (64) Krim, J. Langmuir 1996, 12, 4564. (65) Merrill, P. B.; Perry, S. S. Surf. Sci. 1998, 418, 342. (66) Xiao, X.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235. (67) Ferrari, A. C.; Robertson, J. Phys. Rev. B 2000, 61, 14095.

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mechanisms contribute to the observed increased friction over the defects of the 6 s samples. At present, though, it is not clear which is the dominant one. In this respect, an indication could be obtained considering the number of atoms involved in the enhanced dissipation processes around the vacancies. It is expected that only the atoms immediately surrounding the vacancy contribute with more vibrational modes, whereas up to a few hundred atoms increase their electronic density near the Fermi level (Figure 2), suggesting that it would be the electronic mechanism that dominates the contrast. However, irrespective of whether its origin is electronic or phononic, the frictional contrast in Figure 5b reflects again, as with STM, the structural (not the chemical) heterogeneity of the surface. Figure 6 presents several tapping AFM images of the HOPG surface after a 6 s treatmemt. Figure 6a,b displays height and corresponding phase images, respectively, obtained in the noncontact regime (free amplitude ∼40 nm, setpoint amplitude ∼36 nm). Figure 6c,d presents again height and phase images, respectively, recorded in exactly the same location as that of Figure 6a,b, but in this case in the intermittent contact regime, with ∼80 (free) and ∼40 nm (setpoint) amplitudes. Similarly to the previous observations, the most distinctive feature of the 6 s samples was the ubiquitous presence of small spots on the basal planes, being bright or dark depending on the type of image and operating regime. Thus, they were brighter than their surroundings for the phase images collected in noncontact (Figure 6b) and for the height images in intermittent contact (Figure 6c), whereas they appeared darker for the phase images of this latter regime (Figure 6d) or were almost indiscernible in the noncontact height images (Figure 6a). They appeared best defined in the intermittent contact height image (Figure 6c), so their position in this image can be used as a guide to the eye to locate their counterparts on the phase images (Figure 6b,d). Also, to assist in the location of the spots, one of such features is marked by white arrows in Figure 6. Again in this case, the small spots were never observed under any operating regime in the height or phase images of the basal planes of either untreated or 4 s plasma-treated HOPG, so their presence on the 6 s samples must be attributed mainly to the largest defects developed after this etching time. The difference between the spots and their surroundings, albeit reproducible, was observed to be rather meager (0.1-0.2 nm in height for Figure 6c and 1-2° in phase angle for Figure 6b,d). Moreover, their lateral sizes were also considerably small by tapping mode standards (∼5 nm), suggesting that these features were near the limit of tapping mode detection, which would explain why the single-atom vacancy defects of the 4 s samples were not detected. In addition to the spots, Figure 6a,c reveals a shallow (0.1-0.2 nm) trench running more or less diagonally. This was not a feature developed by the treatment (it is found occasionally in pristine HOPG), but its observation alongside the spots allows several remarks regarding the origin of the measured phase contrast to be made. As of its nature, we believe that it is a near-surface structural defect covered by a few perfect graphite layers. Its depth is clearly below that of a graphene (0.335 nm); therefore, it cannot be a onemonolayer-deep strip of missing atoms on the very surface. Also, whereas steps, grain borders, and other defects exposing unsaturated sp2 bonds on the HOPG surface are distinctively etched upon exposure to air at 650 °C,11,12 we verified by STM/AFM that this type of shallow trenches remained unchanged following such a treatment, supporting the idea of a defect lying only very few layers

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Figure 6. Tapping AFM images of the oxygen plasma-treated HOPG surface (6 s, 40 W). The four images were obtained in the same location of the sample: (a) height image and (b) corresponding phase image acquired in the noncontact regime; (c) height image and (d) corresponding phase image obtained in the intermittent contact regime. White arrows indicate the location of one of the many spots observed in the images of this sample.

below the surface, the very surface being (slightly curved) perfect graphite without unsaturated bonds. As has been shown in the literature, tapping mode phase contrast images can be interpreted in terms of differences in the energy dissipated by the tip-sample interaction according to the expression40,68

cos φ )

QED ωΑ + ω0A0 πkAA0

(1)

where φ is the phase angle (in the Digital Instruments phase convention), ω and ω0 are the working and resonance frequencies, respectively, A and A0 are the setpoint and free amplitudes, respectively, Q is the quality factor of the cantilever, ED is the energy dissipated by the tipsample interaction, and k is the cantilever spring constant. Note that in the usual phase convention (phase ) 90° for free cantilever, > 90° in noncontact regime, and < 90° in intermittent contact regime) the cosine is replaced by a sine, but both expressions are equivalent. Equation 1 indicates that surface regions dissipating more energy will appear brighter in the phase contrast images of the noncontact regime (φ < 0°) and darker in the intermittent contact regime (φ > 0°). Consequently, the phase images in both interaction regimes shown in Figure 6b,d reveal that the defects created by the plasma are more dissipative than the unchanged, perfect areas. The question to elucidate is how these spatial variations in energy dissipation arise in both regimes. As regards noncontact, (68) Tamayo, J.; Garcıá, R. Appl. Phys. Lett. 1998, 73, 2926.

only the presence of a water layer on the surface is believed to cause energy dissipation, so that over a more hydrophilic region a thicker layer will be formed and more energy will dissipate in this regime as a result.40 In the present case, this would imply a higher hydrophilicity on the plasmagenerated defects which, in turn, would be consistent with the idea of oxygenated functionalities (where water adsorption would preferentially take place)57,59 introduced by the plasma on the vacancies observed by STM. This is further supported by a previous study by Brandsch et al.,69 who, in a tapping mode investigation of patterned self-assembled monolayers on Au, noted that in noncontact the phase angle was more negative in CH3-terminated regions than in COOH-terminated regions, the difference being just a few degrees and attributed to the different hydrophilicity of the two regions. In terms of hydrophilicity, a CH3-terminated region is very similar to perfect graphite (both are highly hydrophobic), whereas polar, hydrophilic groups (such as that of the COOH region) must be present on the defects. This parallel is reflected in the noncontact phase images, as the phase angle of the 6 s sample is also slightly more negative on perfect areas than on defects; i.e., the defects appear slightly brighter (Figure 6b). However, in the mentioned work the CH3 and COOH regions were several micrometers in size. By contrast, the noncontact phase images of the present work reveal the chemical heterogeneity of the graphite surface on a very local scale. Concerning the intermittent contact regime (Figure 6d), it is clear that mechanisms other than (69) Brandsch, R.; Bar, G.; Whangbo, M.-H. Langmuir 1997, 13, 6349.


spatial variations in hydrophilicity must be responsible for the contrast. This assertion is based on the observation of the shallow trench of Figure 6: while it appears more dissipative than perfect HOPG in intermittent contact (Figure 6d), it is completely transparent in noncontact (Figure 6b). As the trench is thought to reveal a subsurface defect with no unsaturated bonds on its very surface, it must be as hydrophobic as a perfect basal plane, so both dissipate the same amount of energy in noncontact. Since there is no chemical heterogeneity over the trench, its detection in intermittent contact (Figure 6d) must necessarily be due to its nature as a structural heterogeneity. It is also noted that not all the spots detected in intermittent contact (Figure 6c,d) are necessarily observed in noncontact or they are observed with varying degrees of contrast (Figure 6b). This implies the following: (a) The spots are observed in intermittent contact mainly due to their nature as structural defects (like the trench), rather than as chemical heterogeneities, so once contact between tip and sample is established, energy dissipates principally by channels different to that of the water layer. This is consistent and supports the interpretation for the observed frictional contrast in contact AFM discussed before. (b) The defects present different degrees of chemisorbed oxygen, giving rise to different amounts of adsorbed water molecules around them.57,59 Finally, the fact that the defects appear as bright (high-lying) features in the intermittent contact height images (Figure 6c) does not contradict the contact AFM results (Figure 5a): it is known that anomalous height images inevitably arise in tapping mode due to differences in amplitude damping over heterogeneous surfaces.69 This is the case of the spots in Figure 6c, but although artifactual, their occurrence is useful to reveal the defects. As a last remark, the relevance of the previous results for the visualization of active sites on carbon surfaces should be stressed. This is an issue of particular interest but yet unresolved, mainly due to their tiny size. For instance, Miranda-Hernańdez et al. have very recently underlined the enormous difficulty in visualizing active sites on carbon electrodes, hindering progress in certain areas of electrochemistry.70 In the present work, active sites (as oxygenated atomic vacancies) were created on the HOPG basal plane by oxygen plasma etching. Although an active site is essentially a chemical heterogeneity on the carbon surface (oxygen functionalities), it also displays structural heterogeneity (a vacancy in this case). The results presented here demonstrate that noncontact phase imaging in tapping AFM, in contrast to the cases of STM, friction imaging in contact AFM and even intermittent contact phase imaging, is able to detect chemical (rather than structural) heterogeneity on the graphite surface on an extremely local scale, thus revealing the probable (70) Miranda-Hernańdez, M.; Gonza´lez, I.; Batina, N. J. Phys. Chem. B 2001, 105, 4214.

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nature as active (oxygenated) sites of many of the defects created by the oxygen plasma as well as their lateral distribution. Conclusions The use of scanning probe microscopies has allowed one to track the very first signs of the surface modification of highly oriented pyrolytic graphite (HOPG) by oxygen plasma. Scanning tunneling microscopy (STM) revealed that the structural modification of the material started with the formation of isolated single-atom vacancies (detected as 1 nm wide STM protrusions) at random locations on the basal planes, this being a consequence of the adsorption of the first active species from the plasma (O atoms) onto the HOPG surface, most probably at bridge sites, and their subsequent release as gaseous oxidation products. When the density of single-atom vacancies reached a certain level, multiatom vacancies developed, which appeared as protrusions up to 5 nm in diameter, providing an indication of the mobility of the adsorbed O atoms across the basal planes (on the order of tens of nanometers). Likewise, the observation of monolayer pits formed at longer exposure times permitted us to conclude that the real size of the multiatom vacancies (the physical region with missing C atoms) was below 1 nm. If one takes advantage of this plasma etching behavior, which allowed the formation of relatively large defects while preserving their nature as atomic vacancies (STM protrusions) at the same time, the physicochemical properties of this type of defect could be investigated for the first time using atomic force microscopy (AFM), both in contact and tapping mode. The lateral force images obtained in the former mode displayed enhanced friction arising from the vacancies. The increased friction was attributed to structural, rather than chemical, causes; i.e., it was not ascribed to the presumably large hydrophilicity over the vacancies which originates from the presence of oxygen functional groups on the vacancy regions. Intermittent contact phase imaging in tapping mode was also sensitive for the most part to the structural heterogeneity of the defects. By contrast, noncontact phase imaging was able to reveal the chemical heterogeneity associated with the vacancies through variations in hydrophilicity between defects and perfect areas, thus providing a means of detecting and visualizing active sites on carbon surfaces. Finally, it is believed that the knowledge generated by the present results will be helpful in tailoring the sidewall functionalization of carbon nanotubes nondestructively at the atomic scale, an important and necessary step for their use in several applications (e.g., carbon nanotube composites). Acknowledgment. The authors acknowledge financial support from the DGICYT (Project PB98-0492). LA0115280

Early Stages of Plasma Oxidation of Graphite: Nanoscale

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