Lateral and in-depth distribution of functional groups on diamond-like carbon after oxygen plasma treatments

LATERAL AND IN-DEPTH DISTRIBUTION OF FUNCTIONAL GROUPS ON DIAMOND-LIKE CARBON AFTER OXYGEN PLASMA TREATMENTS C. López-Santos*, F. Yubero, J. Cotrino, A.R. González-Elipe Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla). Avda. Américo Vespucio 49. E-41092 Sevilla. Spain. *e-mail: [email protected] ABSTRACT A diamond like carbon material has been exposed to a low pressure microwave and an atmospheric pressure plasma of oxygen to enhance its hidrophilicity and surface energy. For comparison data are also reported for the surface activation with a beam of neutral species of oxygen. The surface incorporation of oxygenated functional groups and the determination of the in-depth distribution of this element have been studied by means of the X ray photoemission spectroscopy (XPS). Atomic force microscopy (AFM) has been used to get information of the surface topography and, by recording friction maps of the surface, the lateral distribution of oxygenated functional groups formed after the different activation treatments. Differences in surface composition, topography and indepth and lateral distribution of oxygen have been correlated with the distinct characteristics of the plasma processes as characterized by Optical Emission Spectroscopy (OES). INTRODUCTION Plasma techniques, widely used for the surface functionalization of polymers [1], can be also applied for the surface treatment of a related family of materials, the so-called diamond-like carbon coatings (DLC). Due to its chemical inertness, its high hardness [2,3] and a good bio-compatibility [4,5]. DLC has been used for different applications in fields like silicon solar cells and coatings for the reduction of wear and corrosion [6,7]. Several procedures including sputtering [8], laser ablation [9], chemical vapor deposition (CVD) [10], and ion beam implantation (IB) [11], have been used to prepare DLC thin films. During friction experiments, DLC coatings are known to be extremely sensitive to the presence of oxidizing species (oxygen, water vapor), which can give rise to a noticeable tribo-oxidation of the surface and lead to an increase of the friction and wear.[12] The increase of the hydrophilic character and surface energy of DLC is a direct consequence of the surface activation processes required for many technological applications including its use as a biomaterial [13-15]. This is the case for example, of low-pressure oxygen plasma treatments employed for sterilization purposes [15-18]. Surface modification of DLC thin films by plasmas can alter the bond structure and other properties like roughness and hydrophilicity [13]. For example, surface treatments with argon or hydrogen plasmas are known to decrease roughness [19]. Recently, it has been found that new oxygen chemical groups can be anchored onto the DLC surface after its exposure to oxygen plasmas [16, 20]. However, in comparison with the ample literature dealing with the plasma modification of polymers like PET or LDPE [21, 22] less is known about the mechanisms of surface activation of DLC with plasmas techniques. Trying to contribute to this topic, the present work presents a study of the oxygen plasma treatment of DLC by means of two different plasmas: a low pressure microwave (MW) plasma and an atmospheric pressure dielectric barrier discharge (DBD) plasma. For comparison a source of neutral oxygen atoms has been also used. A primary objective of this study is to establish possible correlations between the different plasma conditions and the characteristics of the induced changes in surface properties of DLC thin films. For this purpose we have carried out a X-ray photoelectron spectroscopy (XPS) analysis of both the surface chemical composition and the in-depth profile distributions. To get such profiles, we have used the non-destructive XPS peak shape analysis developed by Tougaard [23]). This method has been employed before to characterize the oxygen in-depth distribution in several polymers like PET and LDPE exposed to different oxygen / nitrogen plasmas [24, 25]. The topographic changes experienced by the DLC after the different plasma treatments have been characterized by means of atomic force microscopy (AFM), a technique that has been also used to characterize the spatial distribution of the oxygenated functional groups through the analysis of the friction maps obtained when recording the images in contact mode [26, 27]. Finally, the modification of the DLC surface energy after plasma activation is confirmed by the measurements of contact angles and the estimation of the surface free energy. EXPERIMENTAL Materials DLC thin films of approximately 150nm thickness have been deposited on glass substrates by plasma enhanced chemical vapor deposition (PECVD) in a parallel plate reactor using mixtures of acetylene, hydrogen and argon gases in a well defined ratio. Experimental details are collected in a previous work [28]. Figure 1 shows a Raman spectrum corresponding to the DLC thin films used in the present study. The apparatus used for recording this spectrum was an Olimpos BX41 Horiba Jobin y Vonn, HR 800 UV. The spectrum is characterized by the typical G (1601cm-1) and D (1338cm-1) bands displayed by this type of material.[29,30] Their intensity ratio, I(D)/I(G)=0.6, determined after fitting analysis of the spectrum, is characteristic of a DLC where carbon atoms are majority bonded through sp3 bonds.[29,30] This ratio also indicates that the specimens are relatively rich in hydrogen. A previous determination using Elastic Recoil Detection Analysis (ERDA) and Reflected Electron Energy Loss Spectroscopy (REELS) yielded a H/C ratio of approximately 0.28 (22% H).[31] Fig 1.- Raman spectrum corresponding to the DLC thin film, characterized by the typical G (1601cm-1) and D (1338cm-1) bands Plasma treatments Treatments by low pressure MW plasmas were carried out in a reactor consisting of a quartz tube with a funnel shape termination attached to a stainless steel chamber where the samples were placed. The reactor has a remote configuration and the samples were exposed downstream to the plasmas species generated in the quartz tube by a surfatron [32] launcher that was supplied with a microwave power of 60 W. A scheme of the apparatus can be found in ref. [33]. A typical density of ionic species of about 1011 cm-3 can be reached with this reactor [34]. A kinetic energy of the ionic species of about 8-12 eV is expected for this type of oxygen microwave plasma [35]. The plasma gas mixtures consisted of Ar as the majority component (25 sscm) and O2 (0.7 sscm) as minority component. A typical working pressure of 0.3 Torr was used for the experiments. A base pressure of 10-3 Torr was routinely reached in this reactor. Before igniting the plasma, the gas mixture was kept flowing for at least 30 min to ensure the purity of the plasma gas within the reactor. Exposure times of 1 min were used for these experiments. This time was enough to achieve steady state conditions. The DBD reactor consisted of two active electrodes made of stainless steel. The DLC coatings supported on a glass plate were placed on the bottom electrode acting as a dielectric barrier. The rear part of the samples was contacted with the metal piece by means of a conductive silver paste. The distance between the upper metallic electrode and the polymer was one millimetre. The discharge between the metal and the polymer surface was induced by an AC high voltage power supply working at a typical voltage of 8 kV and a frequency of 1.5 kHz. Under these conditions, formation of small filaments was evidenced by means of an oscilloscope connected in parallel between the two electrodes [24]. Mixtures of Ar (76 sscm) and O2 (2 sscm) were used for these treatments. Before the plasma was ignited, a continuous flow of the gas mixture was maintained through the reactor to ensure that no traces of air remain in the reaction chamber. This was checked by mass spectrometry analysis of the outlet gases. Typical treatment times of 1 min were used by this procedure. In this system, the density of ionic species reaches a value of 109 cm-3, approximately [36]. After the samples were subjected to the different treatments, they were stored in air at room temperature in a diseccator until their analysis by XPS or the contact angle measurements. For comparison, experiments were also carried out with plasmas of pure Ar. The obtained results are commented in the text when appropriate. Treatments with neutral species These experiments were carried out “in situ” in the same XPS apparatus used for the chemical analysis of the surfaces treated by DBD and MW plasmas. The samples were exposed in the pre-treatment chamber to a beam of neutral atoms/species generated with an Oxford OSPrey source (Oxford, UK) supplied with oxygen at a pressure of 5x10-5Torr. The base pressure in this pre-treatment chamber was 2x10-8Torr. The source is an ECR MW system with an extraction grid. If that extraction source is switched off, the system produces a beam of thermalized atoms or neutral species (i.e., with a kinetic energy smaller than 1eV according to the supplier information). In comparison with the MW and DBD plasmas we will assume that neutral, non accelerated, species are impinging on the surface of the polymer. A way of estimating the dose of species arriving to the substrate is to measure the beam current through the substrate when selecting the ion option as an operating mode of the source. This measurement yielded an approximate value of 0.1 µA cm-2. Taking this value into account and the information of the supplier about the dissociation degree of oxygen, it is possible to make a rough estimation of the number of neutral species arriving to the surface of the sample. A calculated value of 5x1013 atoms cm-2 s-1 has been obtained for the sample situated 10 cm apart from the source. This flow of neutral species ensures that a steady state has been reached by the samples surface after the exposure times used in this experiment. Since the beam density provided by this source was relatively small, long exposure times of up to 30 min were used to reach steady state conditions in most experiments. Analysis methods Optical Emission Spectroscopy (OES) analysis of the plasma under different conditions of work was carried out by means of a CVI Digikron 480 monochromator with a onehalf meter path-length. The instrument operation mode was set for constant spectral resolution, whereby the slit width is automatically modified to compensate the change in dispersion with wavelength and to maintain a constant spectral band-pass. The resolution of the measurements was estimated in 0.5 nm when using a slit width of 0.125mm. The signals were collected with an optical fibre positioned in the plasma zone. In the MW reactor, the spectra were recorded for the glow region just after the ‘‘surfatron’’ launcher and as close as possible to the place where the samples were located. In the DBD reactor, the optical fibre was placed just in the middle of the interelectrode space (i.e., lateral recording of spectra). It was placed in this position by introducing a quartz tube with its closed end placed at approximately 2 cm of the discharge zone. It is important to realize that, since the geometries of observation were different for the MW and DBD plasmas, the absolute spectral intensities obtained for the two reactors cannot be compared. Therefore, the spectra are presented after normalizing their intensities to a common peak attributed to Ar excited species (ArI 2p9-1s5 transition at 811.5 nm). Surface energy and wetting properties of DLC thin films were determined by measuring the wetting contact angles (WCA) of water and iodomethane [37, 38]. Roughness was determined by AFM and chemical and in-depth composition by XPS, before and after the DLC was subjected to the different activation treatments. WCA measurements were carried out for the samples immediately (within few minutes) after the MW and DBD plasma treatments or after the analysis by XPS of the samples treated with the neutral beam. Regarding the XPS characterization, a time comprised between 30 to 60 minutes was needed to introduce the samples in the XPS apparatus after the plasma treatments. XPS spectra were acquired 6-8 hours afterwards (time needed to achieve the minimum vacuum conditions required for perform measurements). Besides these studies with the freshly treated samples, other analyses were carried out with the samples kept in a diseccator for different periods of time. Thus, WCA measurements were done after one day, one week and one month storage. XPS spectra at normal take-off angles were recorded with a PHOIBOS-100 5MCD (SPECS) electron spectrometer working in the constant pass energy mode fixed at a value of 20 eV. An un-monochromatized Mg Kα radiation was used as excitation source which, in part, was responsible for a relatively low resolution of the spectra. Pressure during acquisition was about 5x10-9 mbar. For calibration of the binding energy (BE) scale, a value of 284.5 eV was assigned to the C 1s component attributed to C-H and C-C bonds. Average surface composition was estimated by calculating the area behind the C1s and O1s peaks and by correcting the obtained values with the corresponding sensitivity factors. The in-depth distribution of O atoms within the surface region (up to 6-10 nm depth) of the samples before and after plasma treatments was assesed by XPS peak shape analysis using the QUASES software [39]. The XPS peak shape analysis method relies on the fact that the inelastic background appearing in the low kinetic energy side of a photoelectron peak carries information on the depth of origin of the detected electrons [24]. Thus, wide scan spectra of O1s peaks (i.e., including their inelastic background) were analysed according to this procedure. Clear changes in the in-depth distribution of oxygen were found after the different activation treatments. This procedures have been tested previously for PET surfaces treated with similar plasma systems [25], by comparing the obtained profiles with angle resolved analysis of the samples Static WCA measurements were carried out by the Young method by measuring the contact angle of small water and iodomethane droplets (5l) with a CAM100 instrument (KSV Instruments Ltd, Finland). Carefully deionized and bidistilled water was used for these experiments. The reported values are an average of a minimum of seven measurements taken for each examined surface. Although a straightforward analysis of the surface energy of plasma treated polymer surface requires the measurement of both the advancing and receding wetting angles [40], in this work we just present static contact angles because our purpose is the comparative assessment of the effects of three different methods of surface activation. Contact angles of iodomethane were also measured to determine the polar and non-polar contributions to the surface tension by the Owens-Wendt-Kaelble approximation [41]. Topographic images of the surface were taken with a Nanotec AFM microscope and a Dulcinea electronics, working in tapping mode and using high frequency levers. AFM images were processed with the WSxM software. For each treated sample, more than five good quality images were taken at different regions of the sample after the different treatments. The derived RMS values were determined for a representative image of the surface and agree well with the average values of all images. On the other hand, when contact mode is employed, besides normal forces due to topographic response, friction forces producing a lateral torsion appear on the tip when moving onto the surface [26,27]. Calibration force constants of AFM system is extracted from the interaction between the cantilever tip and the surface in a F-Z curve (applied tip force versus depth).[42] The forward and backward recording of lateral forces makes possible to obtain a friction force map of the surface according to: FF = 0.5[FLfor – FLback] This procedure removes the topographic contribution and isolates the friction response. In this work, friction maps are obtained with Au-covered SiN tips (Olympus) with 40nm of diameter. Width and length of the cantilever are 40m and 200m respectively, whereas the resonance frequency is 20kHz and the spring constant 0.09Nm-1. AFM friction measurements are carried out with a load of 28nN. RESULTS Analysis of plasma species Figure 2 shows the OES spectra recorded for the MW and DBD reactors under the working conditions used for surface functionalization of DLC. It is apparent in these plots that the detected bands/peaks and, particularly, their relative intensities are different depending on the plasma. Table 1 summarizes the main detected features with their attribution to specific electronic transitions according to the literature [43-50]. Fig. 2.- Selected zones of OES spectra recorded for the MW and DBD plasmas used for functionalization of DLC. From the plots in Figure 2 and the data in Table 1, it is apparent the development in the MW Ar+O2 plasma of bands attributed to OH* (2  2 system) and N2* (second positive system C3  B3) species. [43,44] O2+ ionic species (second negative system: 2  2) and [46] argon (5p4s system), [47] hydrogen (H, H and H bands) and oxygen (4p-3s and 3p3s transitions) [48] atomic lines are also detected in this plasma. By contrast, no ionic species of oxygen can be observed in the DBD Ar+O2 discharge, where H2* rotational bands [44] and argon atomic lines (4p4s system) could be detected in this case. [47] It is likely that in the two reactors, the lines of hydrogen and the molecular bands of nitrogen are generated from the water vapour and other residual molecules present in the plasma chambers and / or as surface contamination on their walls. [45]. XPS analysis of functional groups after plasma and beam treatments XPS has been used to assess the chemical changes that occur at the DLC surface exposed to the different plasmas and/or neutral beams. Table 2 reports, expressed in the form of atomic percents, the surface composition of DLC coating subjected to the different plasma and beam treatments. These data indicate that the percentage of oxygen increases slightly after the plasma / beam treatments. Due to the small increase in the oxygen content, no significant changes could be appreciated in the shape of the C1s spectra [25] which, in all cases, is characterized by a main peak at 284.5 eV (this peak is used as a reference to calibrate the energy scale of the spectra) and a tail spreading up to 290.0 eV attributed to the superposition of different C-O functional groups.[28] (Fig. 3 left). Since in this tail there are no well defined peaks/shoulders a fitting analysis has not been intended. Figure 3 also shows the O1s spectra recorded after the plasma/beam treatments and after the storage of the samples for one month. The oxygen present in the original sample must be due to the adsorption of this element during the handling in air of the DLC samples after its preparation. The surface character of this oxygen was confirmed by its readily removal by a gentle cleaning by Ar+ sputtering. After the plasma treatments, the oxygen peak changed slightly both in intensity and in position, shifting from 531eV to 533eV. In polymer materials well defined peaks of oxygen at 531.5 eV and 533.2 eV have been attributed to C-O-C and C-O species, respectively [49, 50] Fig 2 and table 3 presents the fitting of the O1s XPS level with two elemental bands and the atomic compositions deduced from the intensity of the different photoemission peaks. This fitting reveals that the band at higher binding energies has a higher intensity, indicating that C-O species (i.e., C-OH, C-O-C, etc. functional groups) are preferentially formed on the DLC surface, particularly after MW Ar+O2 plasma and oxygen atoms beam treatments. This type of functional groups has been blame for the surface hydrophilicity induced in other plasma treated materials [24, 28]. The observed shift in the BE of the main O1s peak was reversed in DLC treated samples aged for one month. Fig 3.- C1s and O1s XPS spectra of DLC coating surface without treatment and after the exposition to different plasmas and beam of oxygen atoms Oxygen in-depth distributions from XPS peak shape analysis Atomic in-depth concentration profiles by XPS peak shape analysis have been previously used by us to assess the depth distribution of oxygen in plasma treated polymers [24,25], showing that oxygen can penetrate up to several nanometres in the interior of the treated materials [51]. Here we have proceeded with a systematic analysis of the shape of XPS photoelectron peaks with the help of the QUASES software according to the details commented in the experimental section. A schematic representation of the profiles of oxygen obtained in each case is presented in figure 4. For the original sample we assume a uniform profile which, after the DBD and beam activation treatments, changes in shape revealing an enrichment of oxygen at the topmost surface layers of the samples. By contrast, the profile of the sample exposed to the MW Ar+O2 plasma indicates that the oxygen in-depth distribution has not been altered significantly with respect to the original material. Fig. 4.- Oxygen in-depth distribution profiles determined by the QUASES method for the DLC thin films subjected to the different oxygen plasma and beam activation treatments. [33] M. Moisan, Z. Zakrzewski, J. Phys. D: Appl. Phys. 24, (1991) 1025. [34] Y. A. Lebedev, I. L. Epstein, A. V. Tatarinov, V. A. Shakhatov, J. Phys.: Confe. Ser. 44 (2006) 30. [35] J. E. Heidenreich, J. R. Paraszczak, M. Moisan, G. Sauve, Micro. Eng. 5 (1986) 363. [36] J. H. Kim, Y. H. Choi, Y. S. Hwang, Phys. Plasma 13, (2006) 093501. [37] C. van Oss, J. Interfacial Forces in Aqueous Media; Marcel Decker: New York, 1994. [38] B. Janczuk, M. L. Kerkeb, T. Bialopiotrowicz, F. Gonzalez- Caballero, J. Colloid Interface Sci. 151 (1992) 333. [39] S. Tougaard, QUASES, Software Package for Quantitative XPS/AES of Surface Nanostructures by Inelastic Peak Shape Analysis; see www.QUASES.com [40] M. Strobel, V. Jones, Ch. S. Lyons, M. Ulsh, M. J. Kushner, R. Dorai, M. Branch, Plasmas Polym. 8 (2008) 61. [41] D. K. Owens, R. C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741. [42] B. Cappella, P. Baschieri, C. Frediani, P. Miccoli, C. Ascoli, IEEE Eng. Med. Biol. 97 (1997) 0739. [43] G. Placinta, F. Arefi-Khonsari, M. Gheorghiu, J. Arouroux, G. Popa, J. Appl. Polym. Sci. 66 (1997) 1367. [44] N. Krstulovic, I. Lazaban, S. Milosevic, U. Cvelbar, A. Vesel, M. Mozetic, J. Phys. D: Appl. Phys. 39 (2006) 3799. [45] N. Dumitrascu, I. Topala, G. Popa, IEEE Trans. Plasma Sci. 33 (5) (2005) 1710. [46] R.W.B. Pearse, The identification of molecular spectra, Chapman &Hall LTD, London (1950). [47] G.-B. Zhao, M.D. Argyle, M. Radosz, J. Appl. Phys. 101 (2007) 033303. [48] H. Kumagai, D. Hiroki, N. Fujii, T. Kobayashi, J. Vacuum Sci. Technol. A 22 (2004) 1. [49] L. J. Gerenser, J. M. Grace, G. Apai, P. M. Thompson, Surf. Interface Anal. 29 (2000) 12. [50] G. Borcia, N. Dumitrascu, G. Popa, Surf. Coat. Technol. 197 (2005) 316. [51] V. Svorcik, K. Kolarova, P. Slepicka, A. Mackova, M. Novotna, V. Hnatowicz, Polym. Degration Stab. 91 (2006) 1219. [52] R.K. Roy, H.W. Choi, S.J. Park, K.R. Lee, Diamond Relat. Mater. 16 (2007) 1732. [53] P. Yang, J.Y. Chen, Y.X. Leng, H. Sun, N. Huang, P.K. Chu, Surf. Coat. Technol. 186 (2004) 125. [54] S.C.H. Kwok, J. Wang, P.K. Chu, Diamond Relat. Mater. 14 (2005) 78. [55] C.D. Frisbie, L.F. Rozsnyai, A. Noy, M.S. Wrighton, C.M. Lieber, Science 265 (1994) 2071. [56] Y. Yin, L. Hang, J. Xu, D.R. McKenzie, M.M.M. Bilek, Thin Solid Films 516 (2008) 5157. [57] M. Tokoro, Y. Aiyama, M. Masuko, A. Suzuki, H. Ito, K. Yamamoto, Wear 267 (2009) 2167. [58] Y. Yin, L. Hang, J. Xu, D.R. McKenzie, M.M.M. Bilek, Thin Solid Films 516 (2008) 5157. [59] S. Kvasnica, J. Schalko, C. Eisenmenger-Sittner, J. Benardi, G. Vorlaufer, A. Pauschitz, M. Roy, Diamond Relat. Mater. 15 (2006) 1743. [60] H.-S. Zhang, J.L. Endrino, A. Anders, Appl. Surf. Sci. 255 (2008) 2551. [61] R.M. Overney, E. Meyer, J. Frommer, D. Brodbeck, R. Luthi, L. Howald, H.-J. Guntherodt, M. Fujihara, H. Takano, Y. Gotoh, Nature 359 (1992) 133. [62] J.-B.D. Green, M.T. McDermott, M.D. Porter, J. Phys. Chem. 99 (1995) 10960. [63] G.J. Leggett, N.J. Brewer, K.S.L. Chong, Phys. Chem. Chem. Phys. 7 (2005) 1107. [64] A. Gangopadhyay, Tribology Letters 5 (1998) 25. [65] W. Tillmann, E. Vogli, F. Hoffmann, Surf. Coat. Technol. 204 (2009) 1040. [66] A. N. Bhoj, M.J. Kushner, Plasma Sources Sci. Technol. 17 (2008) 035024. [67] C.Z. Liu, J.Q. Wu, L.Q. Ren, J. Tong, J.Q. Li, N. Cui,N.M.D. Brown, B.J. Meenan Mat. Chem. Phys. 85 (2004) 340. List of Tables : Table 1.- Identification of detected species by means of OES for the different DBD and MW plasmas reactor  (nm) species / transitions ref DBD/MW Ar+O2 740 - 850 Ar atomic lines (4p - 4s) system 4 4 280-315 OH molecular band (A 2+ ,v  X 2 i ,v’) 43, 45 MW Ar+O2 394 - 470 Ar atomic lines (5p - 4s) system 4 4 656.5 486.1 434.0 H atomic lines H H H  45 300-450 SSP N 2 molecular band (C 3 ,v  B 3 ,v’) 43 394.7 436.8 777.2 844.6 O I atomic lines 3s5S0 - 4p5P 3s3S0 - 4p3P 3s5S0 - 3p5P 3s 3 S 0 - 3p 3 P 45 603-642 PSN O 2 + molecular band ( 4- g ,v  4 u ,v’) 43 282-306 SSN O 2 + molecular band ( 2 i ,v  2 i ,v’) 43 DBD Ar+O 2 470-640 H2 molecular band (Fulcher system) 43 Table 2.- carbon (C) and oxygen (O) surface atomic concentration of DLC original sample, treated with different plasmas and beam of oxygen atoms and aged after 1 month of storage sample / treatment 1 hour (%) (  1%) C O DLC original 86 14 MW Ar+O2 84 16 DBD A+O2 83 17 Ox species 83 17 Table 3: O1s spectra fitting of the DLC original surface, just after the different plasma and beam of atoms treatments and after one month of storage sample / treatment 1 hour (%) (  3%) -CO-, -COH -C=O, -COO DLC original 32 68 MW Ar+O2 58 42 DBD Ar+O2 49 51 Ox species 60 40 Table 4.- Surface roughness of DLC coatings after different treatments by means of roughness quadratic coefficient RMS treatment Rms (nm) (  0.2) (5  mx5  m) Rms (nm) (  0.2) (1  mx1  m) DLC original 2.4 2.3 MW Ar+O2 2.0 2.1 DBD A+O2 2.5 2.4 Ox species 3.0 2.7 List of figures: Fig 1.- Raman spectrum corresponding to the DLC thin film, characterized by the typical G (1601cm-1) and D (1338cm-1) bands 800 1000 1200 1400 1600 1800 0.0 5.0x103 1.0x104 I(D) / I(G) = 0.6 Raman Intensity (a.u.) Wavenumbers /cm-1 Fig. 2.- Selected zones of OES spectra recorded for the MW and DBD plasmas used for functionalization of DLC. 200 250 300 350 400 0.0 0.2 0.4 0.6 0.8 1.0 N2 SSP intensity (a.u.)  (nm) MW Ar+O2 DBD Ar+O2 O2 + SSN OH 22 600 650 700 750 800 850 0.0 0.2 0.4 0.6 0.8 1.0 CO (a'2-a3) intensidad (u.a.)  (nm) O2 + PSN OI 3p - 3s H Ar I 4p - 4s H2 Fig 3.- C1s and O1s XPS spectra of DLC coating surface without treatment and after the exposition to different plasmas and beam of oxygen atoms Fig. 4.- Oxygen in-depth distribution profiles determined by the QUASES method for the DLC thin films subjected to the different oxygen plasma and beam activation treatments. Fig. 5.- Relative variations in the wetting contact angle with water energy and polar and dispersive components for DLC just after being subjected to the indicated treatment. original MW Ar+O2 DBD Ar+O2 Ox species 0 10 20 30 dispersive c. (mJ/m -2 ) Dispersive Component original MW Ar+O2 DBD Ar+O2 Ox species 0 20 40 60 80 surfaceenergy (mJ/m -2 ) Surface energy original MW Ar+O2 DBD Ar+O2 Ox species 0 10 20 30 40 50 polar c. (mJ/m -2 ) Polar Component 0 50 100 150 200 600 800 -1.0 -0.8 -0.6 -0.4 -0.2 -0.0 MW Ar + O2 DBD Ar + O2 Ox species  i time / h original MW Ar+O2 DBD Ar+O2 Ox species 0 10 20 30 dispersive c. (mJ/m -2 ) Dispersive Component original MW Ar+O2 DBD Ar+O2 Ox species 0 20 40 60 80 surfaceenergy (mJ/m -2 ) Surface energy original MW Ar+O2 DBD Ar+O2 Ox species 0 10 20 30 40 50 polar c. (mJ/m -2 ) Polar Component 0 50 100 150 200 600 800 -1.0 -0.8 -0.6 -0.4 -0.2 -0.0 MW Ar + O2 DBD Ar + O2 Ox species  i time / h Fig. 6.- AFM images of the surface of DLC before and after being subjected to the indicated treatments. Bearing plots of the different surfaces are also included Fig. 7.- friction maps by AFM of the surface of DLC before and after being subjected to the indicated treatments. 0.01 nN - 0.01 nN original O x species MW Ar + O 2 DBD Ar + O 2 0.01 nN - 0.01 nN 0.01 nN - 0.01 nN original O x species MW Ar + O 2 DBD Ar + O 2