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Two-wavelength Raman study of poly(ethylene terephthalate) surfaces modified by helium plasma-based ion implantation
Applied Surface Science 263 (2012) 423–429
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Two-wavelength Raman study of poly(ethylene terephthalate) surfaces modified by helium plasma-based ion implantation M. Veres a,∗ , A. Tóth b , M. Mohai b , I. Bertóti b , J. Szépvölgyi b , S. Tóth a , L. Himics a , M. Koós a a b
Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary
a r t i c l e
i n f o
Article history: Received 12 April 2012 Received in revised form 11 September 2012 Accepted 15 September 2012 Available online 2 October 2012 Keywords: Poly(ethylene terephthalate) Plasma-based ion implantation Two-wavelength micro-Raman spectroscopy
a b s t r a c t The surface of poly(ethylene terephthalate) (PET) was modified by helium plasma-based ion implantation (He PBII). The untreated and surface modified samples were characterised with optical absorption spectroscopy and two-wavelength micro-Raman spectroscopy excited with 488 nm and 785 nm light sources, allowing to examine the chemical bonding configuration of the surface layers on different depths and by selective enhancement of vibrations of different structural units. Upon treatment, simultaneously with the development of the broad D and G bands, a gradual decrease of the peaks corresponding to the C C stretching and C O stretching modes were observed with both excitations. Downshifting and broadening were detected for the C C peak with both excitations and also for the C O peak with the 488 nm excitation due to formation of condensed aromatic rings. Oppositely, upshifting was found with 785 nm excitation for the C O peak and especially for its broad shoulder newly developed at the high wavenumber side. The latter feature was assigned to C O groups attached to polymer chains without conjugation and the bands behaviour was interpreted by breaking of the C C bonds of the polymer, leading to the formation of a crosslinked, disordered and stressed structure with still intact C O groups, due to the increased nuclear damage at the end of the ion track. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Coating of polymeric substrates by diamond-like carbon (DLC) layers can be of great interest, due to the improved properties that these layers may impart to them. In particular, characteristics of polymers like gas permeability and friction can be decreased, chemical inertness, hardness and wear resistance can be increased and biocompatibility of polymer-based devices can be enhanced in various applications. In the literature, deposition of DLC coating onto several polymers has been reported, including polyethylene [1,2], polyamide [2], poly(tetrafluoro-ethylene) [2], polycarbonate [3], poly(ethylene terephthalate) [4,5], polyurethane [6,7] and poly(vinyl-chloride) [8]. A possible way of creating such coatings, alternative to deposition, can be the direct transformation of the surface layers of polymers themselves into DLC type layers, applying plasma-based ion implantation [9]. An advantage of the latter method is that its application results in the formation of a functionally graded material, characterised by the lack of a sharp substrate-coating interface, at which excessive stress and damage could be accumulated.
∗ Corresponding author. Tel.: +36 1 392 2222x3620; fax: +36 1 392 2215. E-mail address: [email protected] (M. Veres). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.072
Recently we studied the controlled formation of amorphous carbon type layers on the surface of poly(ethylene terephthalate) (PET) by helium plasma-based ion implantation (He PBII) and obtained relevant structure–property and composition–property relationships [10,11]. In addition to fully amorphised regions, parts of the original polymer framework, being only slightly modified by the implantation, can also affect the properties of the resulting structure. In this work, therefore, the He PBII-treated PET system was characterised by two-wavelength Raman spectroscopy, focusing mainly on structural units partially modified by the applied surface treatment. The use of more than a single wavelength for the excitation extends the possibilities of Raman spectroscopy. Firstly, by changing the excitation energy, the conditions of resonant scattering can be fulfilled for different structural units, which allows for selective enhancement of their characteristic vibrations—Raman spectroscopy on nanotubes is a successful utilisation of this approach [12]. Secondly, due to differences in light absorption at different wavelengths in a given sample, the excitation depth can also be different, and therefore structural information may be obtained for surface layers of different thickness. In case of amorphous carbons [13–15] and many polymers, the situation is similar to that of nanotubes, in the sense that these materials contain structural units (sp2 carbon clusters in amorphous carbon or chains in the polymer) of different size
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and topology, each with a different band gap and characteristic vibrations. Thus, the use of different photon energies is expected to enhance characteristic vibrations of different structural units–those fulfilling the conditions of resonant Raman scattering in the given case. In this case, comparison of the Raman spectra of the treated samples excited with different photon energies will give information on the bonding configuration of the formed amorphous carbon and polymer matrix (intact and modified by the ion bombardment) from different depths, allowing also to characterise the chemical changes taking place in the subsurface. This region cannot be studied by the frequently applied surface analytical techniques like XPS or static SIMS, while a combination with depth profiling introduces artefacts in case of organic substances. On the other hand, processes occurring in the subsurface region are known to be decisive in the preparation of DLC layers, see for instance the subsurface implantation (or subplantation) mechanism of DLC formation [16], confirmed also by various numerical and analytical simulations [17,18] 2. Experimental details 2.1. Material applied and sample preparation Poly(ethylene terephthalate) (PET) of the type Docapet (Ensinger), virgin grade was used, from which discs of 10 mm diameter and 2 mm thickness were machined. These were polished with SiC paper (grit number P1200 and P4000) and then with felt sheet. 2.2. Plasma-based ion implantation PET samples were treated by helium plasma based ion implantation. The experimental details are described in our previous paper [10]. The method of design of experiments was applied with three factors (acceleration voltage (U), fluence (F), and fluence rate (FR)). For clarity, the treatment parameters of the samples are reported in Table 1. As a guiding information, suitable for comparison purposes, the nominal dose per sample is also given in the right column of Table 1, in keV/PET unit (C10 O4 H8 ). These values were calculated from the energy loss of N+ ions in PET [19], considering the voltage U applied. The real dose cannot be calculated precisely, but is expected to be smaller than the nominal one, since ions are characterised by an energy distribution due to factors including acceleration of ions by the plasma sheath of varying potential, energy loss of ions due to collisions in the gas phase, etc. 2.3. Raman investigations Raman spectroscopic measurements were carried out using a Renishaw 1000 Raman spectrometer attached to a microscope. The 488 nm (2.54 eV) line of an Ar ion laser and a 785 nm (1.58 eV) diode laser served as excitation sources. The excitation beam was focused into a spot having a diameter of ∼1 m and the laser density was limited to 750 W/mm2 (785 nm excitation) and 1250 W/mm2 (488 nm excitation) in order to avoid the damage of the sample surface during the measurements. The integration time was 50 s in each case. The spectra were baseline corrected and fitted using Gaussian (broad bands) or Lorentzian (narrow peaks) functions. The resolution of the spectrometer is 0.9 cm−1 for 488 nm and 0.5 cm−1 for 785 nm excitation. 2.4. Optical measurements The optical absorption of the samples was determined using transmittance measurements in the 350–800 nm region with 1 nm
Fig. 1. Optical absorption spectra of selected irradiated PET samples: (a) PET7 (nominal dose = 21.0 keV/PET), (b) PET2 (47.4 keV/PET) and (c) PET9 (71.1 keV/PET). The Raman excitation wavelengths are indicated by arrows.
resolution using a Jasco V550 spectrometer. The untreated sample was used as reference, so the measured data correspond to relative optical absorption of the structure formed during treatment. 2.5. TRIM calculations The details of TRIM calculation were as follows: formula: C10 O4 H8 , density = 1.41 g cm−3 , default values of binding energy (Edisp = 28 eV for C and O, 10 eV for H, Elatt = 3 eV, Esurf = 7.4 eV for C and 2 eV for O and H), and angle of incidence = 0◦ . 3. Results and discussion 3.1. Optical absorption Fig. 1 shows typical optical absorption spectra of the irradiated PET samples in the 350–800 nm region. The data were not corrected for sample thickness, so the spectra provide relative absorptions only. The results obtained for the different samples show similar features: the absorption has a maximum at around 400 nm and decreases when moving to higher wavelengths, reaching the lowest values above 700 nm. 3.2. Raman depth of excitation The Raman depth of excitation can be estimated from the depth of focus z = /(2 × NA2 ), where is the wavelength and NA is the numerical aperture [20]. In our studies the same experimental setup was used for Raman measurements with both excitation wavelengths, utilising an objective of 100× magnification with NA = 0.75. Accordingly, the depth of focus for 488 nm and 785 nm excitations are z488 = 433 nm and z785 = 697 nm. These values suggest that with near-infrared excitation the Raman scattering in a material having no optical absorption at this wavelength will be recorded from a depth of more than 1.5 times larger than for visible excitation. Considering also the trends of the curves in Fig. 1, where the absorption at 488 nm is much higher than at 785 nm, it can be concluded that there is a remarkable difference in the depth of excitation for the two energies and the 488 nm light excites a shallower surface region than the 785 nm light. This is supported also by the calculated penetration depths for the two wavelengths performed for the PET7 sample based on the absorbance data taken from Fig. 1. The values were found to be 210 nm and 1030 nm for 488 and 785 nm wavelengths, respectively. It should be noted that the above considerations give only estimation for the excitation depths. The real region from where the
M. Veres et al. / Applied Surface Science 263 (2012) 423–429
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Table 1 Treatment parameters of the PET samples. Sample
U (kV)
F × 1017 (cm−2 )
FR × 1013 (cm−2 s−1 )
Nominal dose (keV/PET unit)
Untreated PET1 PET2 PET3 PET4 PET5 PET6 PET7 PET8 PET9
– 15 30 22.5 15 15 30 22.5 22.5 30
– 2 2 2 1 3 1 1 3 3
– 3 5 7 5 7 7 3 5 3
– 35.6 47.4 41.9 17.8 53.4 23.7 21.0 62.9 71.1
scattered light comes from is affected by several factors, including surface roughness, local absorption at the point of the measurement, the angular distribution and absorption of the backscattered light in the sample. However, the obtained values show that the 488 nm light excites much shallower region than the 785 nm one. 3.3. Ion range distribution Fig. 2 shows the ion range distribution curves for He-ions, calculated by the TRIM program [19] for the different ion energies applied. A comparison of the ion range distributions with the excitation depths calculated in Section 3.2 suggests that the 488 nm excitation provides information mainly from the modified and partially modified regions, while the excitation with 785 nm also from deeper layers, including not only the interphase region, but also a significant portion of the unmodified polymer. 3.4. 488 nm excited Raman spectra Fig. 3 shows the Raman spectra of untreated and treated PET samples irradiated with He-ions at various intensities. According to the bottom spectrum in the figure, the Raman spectrum of PET is composed of a number of different peaks, the most intense of which in the given region are located at 853, 1281, 1609 and 1723 cm−1 and correspond to C C breathing, (ring and O C stretching), ring C C stretching and C O stretching modes, respectively [21]. When the energy deposited to the polymer is relatively small, only small changes can be seen in the spectrum: a lowintensity broad band appears between 1500 and 1600 cm−1 . With the increase of deposited energy, the intensity of this feature rises rapidly and a broad shoulder band shows up at lower wavenumbers. Peaks characteristic for PET decrease in intensity,
Fig. 2. Ion range distribution for the He PBII-treated PET samples.
Fig. 3. 488 nm excited Raman spectra of (a) untreated PET, (b) PET4 (nominal dose = 17.8 keV/PET), (c) PET5 (53.4 keV/PET) and (d) PET2 (47.4 keV/PET).
broaden and finally disappear from the spectrum of the severely treated sample. The two very broad peaks appearing as a result of ion bombardment are the so called D and G bands of amorphous carbon, corresponding to breathing vibrations of sp2 carbon rings and stretching C C vibrations of sp2 carbon atoms (both forming rings and chains), respectively. Their appearance in the spectra indicates the formation of amorphous carbon structure upon He PBII-treatment. Features of the Raman spectra related to the resulting amorphous carbon have been analysed in our previous paper [11].
Fig. 4. 785 nm excited Raman spectra of (a) untreated PET, (b) PET4 (nominal dose = 17.8 keV/PET), (c) PET5 (53.4 keV/PET), and (d) PET2 (47.4 keV/PET). Decomposition of the observed C C and C O peaks (inset).
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3.5. 785 nm excited Raman spectra The 785 nm excited Raman spectra of the same samples together with the decomposition of the C C and C O bands are depicted in Fig. 4. The evolution of the spectra upon treatment is similar to that seen in Fig. 3. A broad, composite band appears in the 1000–1700 cm−1 region, related to amorphous carbon, the intensity of which increases with the increase of deposited energy, while the intensities of the characteristic PET peaks decrease. In contrary to what we can observe in Fig. 3, PET peaks still can be seen in the spectrum of the severely treated sample, which is in agreement with the increased sampling depth of the near-infrared excitation: it excites deeper regions, with higher amounts of intact PET. A new feature in the spectra in Fig. 4 is the appearance of a shoulder at
the higher wavenumber side of the C O band, above 1740 cm−1 , which is missing in the 488 nm excited spectra. Its origin will be discussed later. This shoulder peak broadens and shifts to higher wavenumbers with the increasing amorphisation of the structure. 3.6. Effect of He-ion bombardment on PET The obtained Raman spectra are composite ones since they contain scattering contribution from the formed amorphous carbon and the intact and partially modified polymer structure. As seen from the appearance of D and G bands, the most significant effect of He-ion bombardment on the structure of PET is the formation of an amorphous carbon layer on its surface. However, characteristic PET peaks appear in the spectra, and their detailed analysis
Fig. 5. Dependence of peak width on peak position for (a) C C and (b) C O Raman peaks at 488 nm (, ) and 785 nm (, , ) excitation. (c) Correlation of the dose (upper panel) and the G peak position (lower panel) with the C C peak position in the 488 nm excited spectra.
M. Veres et al. / Applied Surface Science 263 (2012) 423–429
indicates a gradual alteration in the polymer framework, modified, but not destroyed completely by the He-ion bombardment. The two different excitation wavelengths will probe the structure not only in different depths but also on the level of different structural units: vibrations of building blocks with band gaps close to the given excitation energy will be enhanced. Fitting of the bands at around 1609 and 1723 cm−1 shows their shifting and broadening, as well as changing of their intensity ratios upon treatment. In addition, the already mentioned new peak appears in the near-infrared excited spectra above 1740 cm−1 , which can be related to C O groups not conjugated with C C groups [22]. This indicates the elimination of the conjugated structure in the partially modified subsurface region, because of the breaking of the C C bonds of the aromatic rings, while C O ones tend to remain intact. Decomposition shows the presence of this peak shoulder also in the spectrum of the pristine sample, but with a very low intensity. Fig. 5a and b reflects the dependence of the widths of C C and C O stretching bands on their positions, for both excitations. Peak positions are different for the two excitations, even in the spectrum of the pristine sample, because of the dispersion of the Raman bands [23]. The position and width of the C C peak for the pristine sample are 1609 and 19 cm−1 , and 1619 and 9 cm−1 , for 488 nm and 785 nm excitations, respectively. In the He-ion bombarded samples, the C C peak shifts to lower wavenumbers and broadens for both excitations (Fig. 5a). Fig. 5c shows that this shift correlates with the nominal dose applied to the samples. The C O peak changes similarly to the C C band in the 488 nm excited spectra: from its original position and width of 1723 and 26 cm−1 it shifts to lower wavenumbers and broadens in the spectra of the He PBII-treated samples (Fig. 5b). However, the situation is entirely different for the corresponding data recorded with 785 nm excitation. Here the two C O peaks (original and shoulder) were decomposed, and both were found to shift to higher wavenumbers for the treated samples. While this change for the first peak is minimal, it is much more remarkable in the case of the broad shoulder, since it reaches values ranging up to 20 cm−1 (Fig. 5b). The decreasing position and broadening of the C C and C O peaks in the 488 nm excited Raman spectra of the He PBII-treated samples indicate the formation of an altered polymer structure upon treatment. Our previous results obtained by Raman spectroscopy (D and G bands), XPS and electrical conductivity measurements suggest the formation of enhanced portion of condensed aromatic sp2 carbon rings in the surface region [10,11]. While the observed C C and C O peaks in the spectra of implanted samples still arise from the polymer chains, the structural transformations – mainly scission and subsequent crosslinking – caused by ion bombardment result in the observed blueshift and broadening of these bands. The positions of the G band [10] and the C C peak analysed here correlate well in the 488 nm excited spectra (Fig. 5c). The closer the G band position to the 1582 cm−1 value of graphite, the higher the amount of aromatic rings in the sp2 clusters of the amorphous matrix; lower G band positions mean chain-like arrangement of the sp2 C atoms. The largest shifts in the C C peak position compared to its original value are observed for G bands with lowest wavenumbers, i.e. for amorphous structures with chain-like arrangement of sp2 C atoms—a topology being very similar to the crosslinked polymeric structure. Higher G band positions mean a different topology with aromatic rings, probably less interconnected with the intact polymer framework. In deeper regions, less affected by the He-ion bombardment (tested here by the 785 nm excitation), mainly the C C bonds (and thus the aromatic rings) break. This bond breaking causes loss of conjugation and, as a result, formation of a disordered, highly crosslinked polymer structure with internal stresses and
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Fig. 6. Dependence of peak intensity on the ion acceleration voltage for (a) C C and (b) C O Raman bands at 488 nm excitation.
bond angle distortions. As a consequence, the characteristic C O peaks shift in this case to higher wavenumbers. This shift is more pronounced for the shoulder band, since the corresponding C O groups are situated in the polymer chain close to the site of C C bond breakage. At the same time, the original C O peak shifts only slightly, since its chain section is essentially intact, only the distorted surrounding and crosslinking of the chains put some stress on it. The higher the amount of broken C C bonds, the higher the He-ion induced disorder in the structure. This effect results in increasing broadening and pronounced shifting of the C O shoulder peak pertaining to non-conjugated C O bonds. The above results suggest that during He-ion bombardment the chemical transformation in the subsurface region starts with the formation of new C C bonds through crosslinking, in the presence of relatively high amounts of carbonyl groups in the structure. The altered chemical transformation in the subsurface region, as compared to that in the surface region, can be explained by the increased nuclear damage usually observed at the end of the ion track [24]. In addition, this structure presumably represents an early stage of the polymer degradation due to He bombardment.
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Fig. 7. Dependence of peak width on the ion acceleration voltage for the C O Raman band at 488 nm excitation.
Further treatment causes detachment of oxygen and hydrogen atoms from the polymer framework and formation of the amorphous carbon network characterised by the D and G bands. Some peak parameters of the C C and C O peaks analysed show significant correlation with the treatment conditions used. In particular, as seen in Fig. 6, at 488 nm excitation the intensities of the C C and C O peaks decrease with the increase of the ion acceleration voltage. Furthermore, at 488 nm excitation, the width of the C O peak increases with the increase of ion acceleration voltage (Fig. 7). A similar tendency is characteristic also for the corresponding C C peak (not shown), but the scatter of experimental points is much stronger in this case. The decreasing intensity of the characteristic polymer peaks in Fig. 6 reflects an increasing level of degradation of the C C and C O bonds of the polymer chains. The increasing width of the C O peak (Fig. 7), caused by the increasing distortions in the structure, supports this conclusion. Considering the almost undetectable C C peak intensity for sample PET2 (see also Fig. 3), a practically complete destruction of the polymer structure can be deduced in the surface region of this sample under the conditions applied (U = 30 kV, F = 2 × 1017 cm−2 ). The relationships seen in Figs. 6 and 7 were observed for the spectra recorded with 488 nm excitation only and reflect the connection between the treatment conditions and structural changes in the surface region of the modified samples. No similar correlation was observed in case of the 785 nm excited Raman spectra, what can be explained by the larger excitation depth in this case, resulting in a remarkable contribution to the scattering intensity from the interphase and untreated regions. At this point we remind that a comparison of the maximum ion range of 30 kV He ions (see Fig. 2) with the calculated Raman excitation depths (z488 = 433 nm and z785 = 697 nm) shows comparable values for modification depth and sampling depth in case of excitation with 488 nm, and prevalence of sampling depth over modification depth for excitation with 785 nm. 4. Conclusions From the results of investigation of the He PBII-modified PET samples by two-wavelength micro-Raman spectroscopy it can be concluded that
• Excitation with 488 nm and 785 nm light sources allows to characterise the chemical bonding configuration of the structure in different depths and by selective enhancement of vibrations of different structural units. 785 nm photons have larger depth of excitation than 488 nm ones. • Upon treatment, simultaneously with the development of the broad D and G bands, a steady decrease of the peaks corresponding to the C C stretching and C O stretching modes can be observed with both excitations. • Downshifting and broadening takes place for the C C peak with both excitations and also for the C O peak with the 488 nm excitation caused by the formation of condensed aromatic rings. • Upshifting and broadening occurs with 785 nm excitation for the C O peak and especially for its broad shoulder, newly developed at the high wavenumber side. The latter arises from C O groups attached to distorted polymer chains without conjugation. • Analysis of the 785 nm excited spectra indicates breaking of the aromatic C C bonds in the subsurface region, leading to the formation of a crosslinked, distorted and stressed polymer structure with intact C O groups. Such structure can be explained by the increased nuclear damage at the end of the ion track. • Comparison of results obtained with the two excitations show that removal of oxygen atoms takes part in later stages of the structural evolution with subsequent formation of amorphous structure with condensed aromatic rings. • Treatment parameter–spectral change type relationships were found at 488 nm excitation: inverse correlation between the intensities of the C C and C O peaks and ion acceleration voltage, and direct correlation between the width of the C O peak and the ion acceleration voltage. Acknowledgements This work was co-sponsored by the National Scientific Research Fund (OTKA) and the National Development Agency (NFÜ) through project K-67741. References [1] I.Sh. Trakhtenberg, O.M. Bakunin, I.N. Korneyev, S.A. Plotnikov, A.P. Rubshtein, K. Uemura, Substrate surface temperature as a decisive parameter for diamondlike carbon film adhesion to polyethylene substrates, Diamond Relat. Mater. 9 (2000) 711–714. [2] K. Baba, R. Hatada, Deposition of diamond-like carbon films on polymers by plasma source ion implantation, Thin Solid Films 506–507 (2006) 55–58. [3] N.K. Cuong, M. Tahara, N. Yamauchi, T. Sone, Diamond-like carbon films deposited on polymers by plasma-enhanced chemical vapor deposition, Surf. Coat. Technol. 174–175 (2003) 1024–1028. [4] G.A. Abbas, J.A. McLaughlin, E. Harkin-Jones, A study of ta-C, a-C:H and Si-a:C:H thin films on polymer substrates as a gas barrier, Diamond Relat. Mater. 13 (2004) 1342–1345. [5] G.A. Abbas, P. Papakonstantinou, T.I.T. Okpalugo, J.A. McLaughlin, J. Filik, E. Harkin-Jones, The improvement in gas barrier performance and optical transparency of DLC-coated polymer by silicon incorporation, Thin Solid Films 482 (2005) 201–206. [6] Y. Ohgoe, K.K. Hirakuri, K. Tsuchimoto, G. Friedbacher, O. Miyashita, Uniform deposition of diamond-like carbon films on polymeric materials for biomedical applications, Surf. Coat. Technol. 184 (2004) 263–269. [7] J.M. Lackner, R. Major, L. Major, Th. Schöberl, W. Waldhauser, RF deposition of soft hydrogenated amorphous carbon coatings for adhesive interfaces on highly elastic polymer materials, Surf. Coat. Technol. 203 (2009) 2243–2248. [8] E.C. Rangel, E.S. de Souza, F.S. de Moraes, N.M.S. Marins, W.H. Schreiner, N.C. Cruz, Development of amorphous carbon protective coatings on poly(vinyl)chloride, Thin Solid Films 518 (2010) 2750–2756. [9] A. Tóth, K. Kereszturi, M. Mohai, I. Bertóti, Plasma based ion implantation of engineering polymers, Surf. Coat. Technol. 204 (2010) 2898–2908 (and references therein). [10] A. Tóth, M. Veres, K. Kereszturi, M. Mohai, I. Bertóti, J. Szépvölgyi, Formation of amorphous carbon on the surface of poly(ethylene terephthalate) by helium plasma based ion implantation, Nucl. Instrum. Methods B 269 (2011) 1855–1858.
M. Veres et al. / Applied Surface Science 263 (2012) 423–429 [11] A. Tóth, M. Veres, K. Kereszturi, M. Mohai, I. Bertóti, J. Szépvölgyi, Structure–property and composition–property relationships for poly(ethylene terephthalate) surfaces modified by helium plasma-based ion implantation, Appl. Surf. Sci. 257 (2011) 10815–10820. [12] M.S. Dresselhaus, G. Dresselhaus, A. Jorio, A.G. Souza Filho, R. Saito, Raman spectroscopy on isolated single wall carbon nanotubes, Carbon 40 (2002) 2043–2061. [13] M. Veres, S. Tóth, M. Koós, New aspects of Raman scattering in carbon-based amorphous materials, Diamond Relat. Mater. 17 (2008) 1692–1696. [14] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys. Rev. B 61 (2003) 14095–14107. [15] A.C. Ferrari, J. Robertson, Resonant Raman spectroscopy of disordered, amorphous, and diamondlike carbon, Phys. Rev. B 64 (2001) 075414. [16] Y. Lifshitz, S.R. Kasi, J.W. Rabalais, W. Eckstein, Subplantation model for film growth from hyperthermal species, Phys. Rev. B 41 (1990) 10468–10480. [17] H.P. Kaukonen, R.M. Nieminen, Atomic-scale modeling of the ion-beaminduced growth of amorphous carbon, Phys. Rev. B 61 (2000) 2806–2811.
429
[18] K. Kohary, S. Kugler, Growth of amorphous carbon: Low-energy molecular dynamics simulation of atomic bombardment, Phys. Rev. B 63 (2001) 193404. [19] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985, Program available from: http://www.srim.org [20] M. Born, E. Wolf, Principles of Optics, Pergamon Press, Oxford, 1986. [21] Th. Lippert, F. Zimmermann, A. Wokaun, Surface analysis of excimerlaser-treated polyethylene-terephthalate by surface-enhanced Raman scattering and X-ray photoelectron spectroscopy, Appl. Spectrosc. 47 (1993) 1931–1942. [22] P.J. Larkin, IR and Raman Spectroscopy. Principles and Spectral Interpretation, Elsevier, 2011. [23] J.L. Brédas, R.J. Silbey, Conjugated Polymers: The Novel Science and Technology of Highly Conducting and Nonlinear Optically Active Materials, Springer, 1991. [24] I. Okur, P.D. Townsend, Waveguide formation by He+ and H+ ion implantation in filter glass containing nanoparticles, Nucl. Instrum. Methods B 124 (1997) 76–80.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Two-wavelength Raman study of poly(ethylene terephthalate) surfaces modified by helium plasma-based ion implantation M. Veres a,∗ , A. Tóth b , M. Mohai b , I. Bertóti b , J. Szépvölgyi b , S. Tóth a , L. Himics a , M. Koós a a b
Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary
a r t i c l e
i n f o
Article history: Received 12 April 2012 Received in revised form 11 September 2012 Accepted 15 September 2012 Available online 2 October 2012 Keywords: Poly(ethylene terephthalate) Plasma-based ion implantation Two-wavelength micro-Raman spectroscopy
a b s t r a c t The surface of poly(ethylene terephthalate) (PET) was modified by helium plasma-based ion implantation (He PBII). The untreated and surface modified samples were characterised with optical absorption spectroscopy and two-wavelength micro-Raman spectroscopy excited with 488 nm and 785 nm light sources, allowing to examine the chemical bonding configuration of the surface layers on different depths and by selective enhancement of vibrations of different structural units. Upon treatment, simultaneously with the development of the broad D and G bands, a gradual decrease of the peaks corresponding to the C C stretching and C O stretching modes were observed with both excitations. Downshifting and broadening were detected for the C C peak with both excitations and also for the C O peak with the 488 nm excitation due to formation of condensed aromatic rings. Oppositely, upshifting was found with 785 nm excitation for the C O peak and especially for its broad shoulder newly developed at the high wavenumber side. The latter feature was assigned to C O groups attached to polymer chains without conjugation and the bands behaviour was interpreted by breaking of the C C bonds of the polymer, leading to the formation of a crosslinked, disordered and stressed structure with still intact C O groups, due to the increased nuclear damage at the end of the ion track. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Coating of polymeric substrates by diamond-like carbon (DLC) layers can be of great interest, due to the improved properties that these layers may impart to them. In particular, characteristics of polymers like gas permeability and friction can be decreased, chemical inertness, hardness and wear resistance can be increased and biocompatibility of polymer-based devices can be enhanced in various applications. In the literature, deposition of DLC coating onto several polymers has been reported, including polyethylene [1,2], polyamide [2], poly(tetrafluoro-ethylene) [2], polycarbonate [3], poly(ethylene terephthalate) [4,5], polyurethane [6,7] and poly(vinyl-chloride) [8]. A possible way of creating such coatings, alternative to deposition, can be the direct transformation of the surface layers of polymers themselves into DLC type layers, applying plasma-based ion implantation [9]. An advantage of the latter method is that its application results in the formation of a functionally graded material, characterised by the lack of a sharp substrate-coating interface, at which excessive stress and damage could be accumulated.
∗ Corresponding author. Tel.: +36 1 392 2222x3620; fax: +36 1 392 2215. E-mail address: [email protected] (M. Veres). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.072
Recently we studied the controlled formation of amorphous carbon type layers on the surface of poly(ethylene terephthalate) (PET) by helium plasma-based ion implantation (He PBII) and obtained relevant structure–property and composition–property relationships [10,11]. In addition to fully amorphised regions, parts of the original polymer framework, being only slightly modified by the implantation, can also affect the properties of the resulting structure. In this work, therefore, the He PBII-treated PET system was characterised by two-wavelength Raman spectroscopy, focusing mainly on structural units partially modified by the applied surface treatment. The use of more than a single wavelength for the excitation extends the possibilities of Raman spectroscopy. Firstly, by changing the excitation energy, the conditions of resonant scattering can be fulfilled for different structural units, which allows for selective enhancement of their characteristic vibrations—Raman spectroscopy on nanotubes is a successful utilisation of this approach [12]. Secondly, due to differences in light absorption at different wavelengths in a given sample, the excitation depth can also be different, and therefore structural information may be obtained for surface layers of different thickness. In case of amorphous carbons [13–15] and many polymers, the situation is similar to that of nanotubes, in the sense that these materials contain structural units (sp2 carbon clusters in amorphous carbon or chains in the polymer) of different size
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and topology, each with a different band gap and characteristic vibrations. Thus, the use of different photon energies is expected to enhance characteristic vibrations of different structural units–those fulfilling the conditions of resonant Raman scattering in the given case. In this case, comparison of the Raman spectra of the treated samples excited with different photon energies will give information on the bonding configuration of the formed amorphous carbon and polymer matrix (intact and modified by the ion bombardment) from different depths, allowing also to characterise the chemical changes taking place in the subsurface. This region cannot be studied by the frequently applied surface analytical techniques like XPS or static SIMS, while a combination with depth profiling introduces artefacts in case of organic substances. On the other hand, processes occurring in the subsurface region are known to be decisive in the preparation of DLC layers, see for instance the subsurface implantation (or subplantation) mechanism of DLC formation [16], confirmed also by various numerical and analytical simulations [17,18] 2. Experimental details 2.1. Material applied and sample preparation Poly(ethylene terephthalate) (PET) of the type Docapet (Ensinger), virgin grade was used, from which discs of 10 mm diameter and 2 mm thickness were machined. These were polished with SiC paper (grit number P1200 and P4000) and then with felt sheet. 2.2. Plasma-based ion implantation PET samples were treated by helium plasma based ion implantation. The experimental details are described in our previous paper [10]. The method of design of experiments was applied with three factors (acceleration voltage (U), fluence (F), and fluence rate (FR)). For clarity, the treatment parameters of the samples are reported in Table 1. As a guiding information, suitable for comparison purposes, the nominal dose per sample is also given in the right column of Table 1, in keV/PET unit (C10 O4 H8 ). These values were calculated from the energy loss of N+ ions in PET [19], considering the voltage U applied. The real dose cannot be calculated precisely, but is expected to be smaller than the nominal one, since ions are characterised by an energy distribution due to factors including acceleration of ions by the plasma sheath of varying potential, energy loss of ions due to collisions in the gas phase, etc. 2.3. Raman investigations Raman spectroscopic measurements were carried out using a Renishaw 1000 Raman spectrometer attached to a microscope. The 488 nm (2.54 eV) line of an Ar ion laser and a 785 nm (1.58 eV) diode laser served as excitation sources. The excitation beam was focused into a spot having a diameter of ∼1 m and the laser density was limited to 750 W/mm2 (785 nm excitation) and 1250 W/mm2 (488 nm excitation) in order to avoid the damage of the sample surface during the measurements. The integration time was 50 s in each case. The spectra were baseline corrected and fitted using Gaussian (broad bands) or Lorentzian (narrow peaks) functions. The resolution of the spectrometer is 0.9 cm−1 for 488 nm and 0.5 cm−1 for 785 nm excitation. 2.4. Optical measurements The optical absorption of the samples was determined using transmittance measurements in the 350–800 nm region with 1 nm
Fig. 1. Optical absorption spectra of selected irradiated PET samples: (a) PET7 (nominal dose = 21.0 keV/PET), (b) PET2 (47.4 keV/PET) and (c) PET9 (71.1 keV/PET). The Raman excitation wavelengths are indicated by arrows.
resolution using a Jasco V550 spectrometer. The untreated sample was used as reference, so the measured data correspond to relative optical absorption of the structure formed during treatment. 2.5. TRIM calculations The details of TRIM calculation were as follows: formula: C10 O4 H8 , density = 1.41 g cm−3 , default values of binding energy (Edisp = 28 eV for C and O, 10 eV for H, Elatt = 3 eV, Esurf = 7.4 eV for C and 2 eV for O and H), and angle of incidence = 0◦ . 3. Results and discussion 3.1. Optical absorption Fig. 1 shows typical optical absorption spectra of the irradiated PET samples in the 350–800 nm region. The data were not corrected for sample thickness, so the spectra provide relative absorptions only. The results obtained for the different samples show similar features: the absorption has a maximum at around 400 nm and decreases when moving to higher wavelengths, reaching the lowest values above 700 nm. 3.2. Raman depth of excitation The Raman depth of excitation can be estimated from the depth of focus z = /(2 × NA2 ), where is the wavelength and NA is the numerical aperture [20]. In our studies the same experimental setup was used for Raman measurements with both excitation wavelengths, utilising an objective of 100× magnification with NA = 0.75. Accordingly, the depth of focus for 488 nm and 785 nm excitations are z488 = 433 nm and z785 = 697 nm. These values suggest that with near-infrared excitation the Raman scattering in a material having no optical absorption at this wavelength will be recorded from a depth of more than 1.5 times larger than for visible excitation. Considering also the trends of the curves in Fig. 1, where the absorption at 488 nm is much higher than at 785 nm, it can be concluded that there is a remarkable difference in the depth of excitation for the two energies and the 488 nm light excites a shallower surface region than the 785 nm light. This is supported also by the calculated penetration depths for the two wavelengths performed for the PET7 sample based on the absorbance data taken from Fig. 1. The values were found to be 210 nm and 1030 nm for 488 and 785 nm wavelengths, respectively. It should be noted that the above considerations give only estimation for the excitation depths. The real region from where the
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Table 1 Treatment parameters of the PET samples. Sample
U (kV)
F × 1017 (cm−2 )
FR × 1013 (cm−2 s−1 )
Nominal dose (keV/PET unit)
Untreated PET1 PET2 PET3 PET4 PET5 PET6 PET7 PET8 PET9
– 15 30 22.5 15 15 30 22.5 22.5 30
– 2 2 2 1 3 1 1 3 3
– 3 5 7 5 7 7 3 5 3
– 35.6 47.4 41.9 17.8 53.4 23.7 21.0 62.9 71.1
scattered light comes from is affected by several factors, including surface roughness, local absorption at the point of the measurement, the angular distribution and absorption of the backscattered light in the sample. However, the obtained values show that the 488 nm light excites much shallower region than the 785 nm one. 3.3. Ion range distribution Fig. 2 shows the ion range distribution curves for He-ions, calculated by the TRIM program [19] for the different ion energies applied. A comparison of the ion range distributions with the excitation depths calculated in Section 3.2 suggests that the 488 nm excitation provides information mainly from the modified and partially modified regions, while the excitation with 785 nm also from deeper layers, including not only the interphase region, but also a significant portion of the unmodified polymer. 3.4. 488 nm excited Raman spectra Fig. 3 shows the Raman spectra of untreated and treated PET samples irradiated with He-ions at various intensities. According to the bottom spectrum in the figure, the Raman spectrum of PET is composed of a number of different peaks, the most intense of which in the given region are located at 853, 1281, 1609 and 1723 cm−1 and correspond to C C breathing, (ring and O C stretching), ring C C stretching and C O stretching modes, respectively [21]. When the energy deposited to the polymer is relatively small, only small changes can be seen in the spectrum: a lowintensity broad band appears between 1500 and 1600 cm−1 . With the increase of deposited energy, the intensity of this feature rises rapidly and a broad shoulder band shows up at lower wavenumbers. Peaks characteristic for PET decrease in intensity,
Fig. 2. Ion range distribution for the He PBII-treated PET samples.
Fig. 3. 488 nm excited Raman spectra of (a) untreated PET, (b) PET4 (nominal dose = 17.8 keV/PET), (c) PET5 (53.4 keV/PET) and (d) PET2 (47.4 keV/PET).
broaden and finally disappear from the spectrum of the severely treated sample. The two very broad peaks appearing as a result of ion bombardment are the so called D and G bands of amorphous carbon, corresponding to breathing vibrations of sp2 carbon rings and stretching C C vibrations of sp2 carbon atoms (both forming rings and chains), respectively. Their appearance in the spectra indicates the formation of amorphous carbon structure upon He PBII-treatment. Features of the Raman spectra related to the resulting amorphous carbon have been analysed in our previous paper [11].
Fig. 4. 785 nm excited Raman spectra of (a) untreated PET, (b) PET4 (nominal dose = 17.8 keV/PET), (c) PET5 (53.4 keV/PET), and (d) PET2 (47.4 keV/PET). Decomposition of the observed C C and C O peaks (inset).
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3.5. 785 nm excited Raman spectra The 785 nm excited Raman spectra of the same samples together with the decomposition of the C C and C O bands are depicted in Fig. 4. The evolution of the spectra upon treatment is similar to that seen in Fig. 3. A broad, composite band appears in the 1000–1700 cm−1 region, related to amorphous carbon, the intensity of which increases with the increase of deposited energy, while the intensities of the characteristic PET peaks decrease. In contrary to what we can observe in Fig. 3, PET peaks still can be seen in the spectrum of the severely treated sample, which is in agreement with the increased sampling depth of the near-infrared excitation: it excites deeper regions, with higher amounts of intact PET. A new feature in the spectra in Fig. 4 is the appearance of a shoulder at
the higher wavenumber side of the C O band, above 1740 cm−1 , which is missing in the 488 nm excited spectra. Its origin will be discussed later. This shoulder peak broadens and shifts to higher wavenumbers with the increasing amorphisation of the structure. 3.6. Effect of He-ion bombardment on PET The obtained Raman spectra are composite ones since they contain scattering contribution from the formed amorphous carbon and the intact and partially modified polymer structure. As seen from the appearance of D and G bands, the most significant effect of He-ion bombardment on the structure of PET is the formation of an amorphous carbon layer on its surface. However, characteristic PET peaks appear in the spectra, and their detailed analysis
Fig. 5. Dependence of peak width on peak position for (a) C C and (b) C O Raman peaks at 488 nm (, ) and 785 nm (, , ) excitation. (c) Correlation of the dose (upper panel) and the G peak position (lower panel) with the C C peak position in the 488 nm excited spectra.
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indicates a gradual alteration in the polymer framework, modified, but not destroyed completely by the He-ion bombardment. The two different excitation wavelengths will probe the structure not only in different depths but also on the level of different structural units: vibrations of building blocks with band gaps close to the given excitation energy will be enhanced. Fitting of the bands at around 1609 and 1723 cm−1 shows their shifting and broadening, as well as changing of their intensity ratios upon treatment. In addition, the already mentioned new peak appears in the near-infrared excited spectra above 1740 cm−1 , which can be related to C O groups not conjugated with C C groups [22]. This indicates the elimination of the conjugated structure in the partially modified subsurface region, because of the breaking of the C C bonds of the aromatic rings, while C O ones tend to remain intact. Decomposition shows the presence of this peak shoulder also in the spectrum of the pristine sample, but with a very low intensity. Fig. 5a and b reflects the dependence of the widths of C C and C O stretching bands on their positions, for both excitations. Peak positions are different for the two excitations, even in the spectrum of the pristine sample, because of the dispersion of the Raman bands [23]. The position and width of the C C peak for the pristine sample are 1609 and 19 cm−1 , and 1619 and 9 cm−1 , for 488 nm and 785 nm excitations, respectively. In the He-ion bombarded samples, the C C peak shifts to lower wavenumbers and broadens for both excitations (Fig. 5a). Fig. 5c shows that this shift correlates with the nominal dose applied to the samples. The C O peak changes similarly to the C C band in the 488 nm excited spectra: from its original position and width of 1723 and 26 cm−1 it shifts to lower wavenumbers and broadens in the spectra of the He PBII-treated samples (Fig. 5b). However, the situation is entirely different for the corresponding data recorded with 785 nm excitation. Here the two C O peaks (original and shoulder) were decomposed, and both were found to shift to higher wavenumbers for the treated samples. While this change for the first peak is minimal, it is much more remarkable in the case of the broad shoulder, since it reaches values ranging up to 20 cm−1 (Fig. 5b). The decreasing position and broadening of the C C and C O peaks in the 488 nm excited Raman spectra of the He PBII-treated samples indicate the formation of an altered polymer structure upon treatment. Our previous results obtained by Raman spectroscopy (D and G bands), XPS and electrical conductivity measurements suggest the formation of enhanced portion of condensed aromatic sp2 carbon rings in the surface region [10,11]. While the observed C C and C O peaks in the spectra of implanted samples still arise from the polymer chains, the structural transformations – mainly scission and subsequent crosslinking – caused by ion bombardment result in the observed blueshift and broadening of these bands. The positions of the G band [10] and the C C peak analysed here correlate well in the 488 nm excited spectra (Fig. 5c). The closer the G band position to the 1582 cm−1 value of graphite, the higher the amount of aromatic rings in the sp2 clusters of the amorphous matrix; lower G band positions mean chain-like arrangement of the sp2 C atoms. The largest shifts in the C C peak position compared to its original value are observed for G bands with lowest wavenumbers, i.e. for amorphous structures with chain-like arrangement of sp2 C atoms—a topology being very similar to the crosslinked polymeric structure. Higher G band positions mean a different topology with aromatic rings, probably less interconnected with the intact polymer framework. In deeper regions, less affected by the He-ion bombardment (tested here by the 785 nm excitation), mainly the C C bonds (and thus the aromatic rings) break. This bond breaking causes loss of conjugation and, as a result, formation of a disordered, highly crosslinked polymer structure with internal stresses and
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Fig. 6. Dependence of peak intensity on the ion acceleration voltage for (a) C C and (b) C O Raman bands at 488 nm excitation.
bond angle distortions. As a consequence, the characteristic C O peaks shift in this case to higher wavenumbers. This shift is more pronounced for the shoulder band, since the corresponding C O groups are situated in the polymer chain close to the site of C C bond breakage. At the same time, the original C O peak shifts only slightly, since its chain section is essentially intact, only the distorted surrounding and crosslinking of the chains put some stress on it. The higher the amount of broken C C bonds, the higher the He-ion induced disorder in the structure. This effect results in increasing broadening and pronounced shifting of the C O shoulder peak pertaining to non-conjugated C O bonds. The above results suggest that during He-ion bombardment the chemical transformation in the subsurface region starts with the formation of new C C bonds through crosslinking, in the presence of relatively high amounts of carbonyl groups in the structure. The altered chemical transformation in the subsurface region, as compared to that in the surface region, can be explained by the increased nuclear damage usually observed at the end of the ion track [24]. In addition, this structure presumably represents an early stage of the polymer degradation due to He bombardment.
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Fig. 7. Dependence of peak width on the ion acceleration voltage for the C O Raman band at 488 nm excitation.
Further treatment causes detachment of oxygen and hydrogen atoms from the polymer framework and formation of the amorphous carbon network characterised by the D and G bands. Some peak parameters of the C C and C O peaks analysed show significant correlation with the treatment conditions used. In particular, as seen in Fig. 6, at 488 nm excitation the intensities of the C C and C O peaks decrease with the increase of the ion acceleration voltage. Furthermore, at 488 nm excitation, the width of the C O peak increases with the increase of ion acceleration voltage (Fig. 7). A similar tendency is characteristic also for the corresponding C C peak (not shown), but the scatter of experimental points is much stronger in this case. The decreasing intensity of the characteristic polymer peaks in Fig. 6 reflects an increasing level of degradation of the C C and C O bonds of the polymer chains. The increasing width of the C O peak (Fig. 7), caused by the increasing distortions in the structure, supports this conclusion. Considering the almost undetectable C C peak intensity for sample PET2 (see also Fig. 3), a practically complete destruction of the polymer structure can be deduced in the surface region of this sample under the conditions applied (U = 30 kV, F = 2 × 1017 cm−2 ). The relationships seen in Figs. 6 and 7 were observed for the spectra recorded with 488 nm excitation only and reflect the connection between the treatment conditions and structural changes in the surface region of the modified samples. No similar correlation was observed in case of the 785 nm excited Raman spectra, what can be explained by the larger excitation depth in this case, resulting in a remarkable contribution to the scattering intensity from the interphase and untreated regions. At this point we remind that a comparison of the maximum ion range of 30 kV He ions (see Fig. 2) with the calculated Raman excitation depths (z488 = 433 nm and z785 = 697 nm) shows comparable values for modification depth and sampling depth in case of excitation with 488 nm, and prevalence of sampling depth over modification depth for excitation with 785 nm. 4. Conclusions From the results of investigation of the He PBII-modified PET samples by two-wavelength micro-Raman spectroscopy it can be concluded that
• Excitation with 488 nm and 785 nm light sources allows to characterise the chemical bonding configuration of the structure in different depths and by selective enhancement of vibrations of different structural units. 785 nm photons have larger depth of excitation than 488 nm ones. • Upon treatment, simultaneously with the development of the broad D and G bands, a steady decrease of the peaks corresponding to the C C stretching and C O stretching modes can be observed with both excitations. • Downshifting and broadening takes place for the C C peak with both excitations and also for the C O peak with the 488 nm excitation caused by the formation of condensed aromatic rings. • Upshifting and broadening occurs with 785 nm excitation for the C O peak and especially for its broad shoulder, newly developed at the high wavenumber side. The latter arises from C O groups attached to distorted polymer chains without conjugation. • Analysis of the 785 nm excited spectra indicates breaking of the aromatic C C bonds in the subsurface region, leading to the formation of a crosslinked, distorted and stressed polymer structure with intact C O groups. Such structure can be explained by the increased nuclear damage at the end of the ion track. • Comparison of results obtained with the two excitations show that removal of oxygen atoms takes part in later stages of the structural evolution with subsequent formation of amorphous structure with condensed aromatic rings. • Treatment parameter–spectral change type relationships were found at 488 nm excitation: inverse correlation between the intensities of the C C and C O peaks and ion acceleration voltage, and direct correlation between the width of the C O peak and the ion acceleration voltage. Acknowledgements This work was co-sponsored by the National Scientific Research Fund (OTKA) and the National Development Agency (NFÜ) through project K-67741. References [1] I.Sh. Trakhtenberg, O.M. Bakunin, I.N. Korneyev, S.A. Plotnikov, A.P. Rubshtein, K. Uemura, Substrate surface temperature as a decisive parameter for diamondlike carbon film adhesion to polyethylene substrates, Diamond Relat. Mater. 9 (2000) 711–714. [2] K. Baba, R. Hatada, Deposition of diamond-like carbon films on polymers by plasma source ion implantation, Thin Solid Films 506–507 (2006) 55–58. [3] N.K. Cuong, M. Tahara, N. Yamauchi, T. Sone, Diamond-like carbon films deposited on polymers by plasma-enhanced chemical vapor deposition, Surf. Coat. Technol. 174–175 (2003) 1024–1028. [4] G.A. Abbas, J.A. McLaughlin, E. Harkin-Jones, A study of ta-C, a-C:H and Si-a:C:H thin films on polymer substrates as a gas barrier, Diamond Relat. Mater. 13 (2004) 1342–1345. [5] G.A. Abbas, P. Papakonstantinou, T.I.T. Okpalugo, J.A. McLaughlin, J. Filik, E. Harkin-Jones, The improvement in gas barrier performance and optical transparency of DLC-coated polymer by silicon incorporation, Thin Solid Films 482 (2005) 201–206. [6] Y. Ohgoe, K.K. Hirakuri, K. Tsuchimoto, G. Friedbacher, O. Miyashita, Uniform deposition of diamond-like carbon films on polymeric materials for biomedical applications, Surf. Coat. Technol. 184 (2004) 263–269. [7] J.M. Lackner, R. Major, L. Major, Th. Schöberl, W. Waldhauser, RF deposition of soft hydrogenated amorphous carbon coatings for adhesive interfaces on highly elastic polymer materials, Surf. Coat. Technol. 203 (2009) 2243–2248. [8] E.C. Rangel, E.S. de Souza, F.S. de Moraes, N.M.S. Marins, W.H. Schreiner, N.C. Cruz, Development of amorphous carbon protective coatings on poly(vinyl)chloride, Thin Solid Films 518 (2010) 2750–2756. [9] A. Tóth, K. Kereszturi, M. Mohai, I. Bertóti, Plasma based ion implantation of engineering polymers, Surf. Coat. Technol. 204 (2010) 2898–2908 (and references therein). [10] A. Tóth, M. Veres, K. Kereszturi, M. Mohai, I. Bertóti, J. Szépvölgyi, Formation of amorphous carbon on the surface of poly(ethylene terephthalate) by helium plasma based ion implantation, Nucl. Instrum. Methods B 269 (2011) 1855–1858.
M. Veres et al. / Applied Surface Science 263 (2012) 423–429 [11] A. Tóth, M. Veres, K. Kereszturi, M. Mohai, I. Bertóti, J. Szépvölgyi, Structure–property and composition–property relationships for poly(ethylene terephthalate) surfaces modified by helium plasma-based ion implantation, Appl. Surf. Sci. 257 (2011) 10815–10820. [12] M.S. Dresselhaus, G. Dresselhaus, A. Jorio, A.G. Souza Filho, R. Saito, Raman spectroscopy on isolated single wall carbon nanotubes, Carbon 40 (2002) 2043–2061. [13] M. Veres, S. Tóth, M. Koós, New aspects of Raman scattering in carbon-based amorphous materials, Diamond Relat. Mater. 17 (2008) 1692–1696. [14] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys. Rev. B 61 (2003) 14095–14107. [15] A.C. Ferrari, J. Robertson, Resonant Raman spectroscopy of disordered, amorphous, and diamondlike carbon, Phys. Rev. B 64 (2001) 075414. [16] Y. Lifshitz, S.R. Kasi, J.W. Rabalais, W. Eckstein, Subplantation model for film growth from hyperthermal species, Phys. Rev. B 41 (1990) 10468–10480. [17] H.P. Kaukonen, R.M. Nieminen, Atomic-scale modeling of the ion-beaminduced growth of amorphous carbon, Phys. Rev. B 61 (2000) 2806–2811.
429
[18] K. Kohary, S. Kugler, Growth of amorphous carbon: Low-energy molecular dynamics simulation of atomic bombardment, Phys. Rev. B 63 (2001) 193404. [19] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985, Program available from: http://www.srim.org [20] M. Born, E. Wolf, Principles of Optics, Pergamon Press, Oxford, 1986. [21] Th. Lippert, F. Zimmermann, A. Wokaun, Surface analysis of excimerlaser-treated polyethylene-terephthalate by surface-enhanced Raman scattering and X-ray photoelectron spectroscopy, Appl. Spectrosc. 47 (1993) 1931–1942. [22] P.J. Larkin, IR and Raman Spectroscopy. Principles and Spectral Interpretation, Elsevier, 2011. [23] J.L. Brédas, R.J. Silbey, Conjugated Polymers: The Novel Science and Technology of Highly Conducting and Nonlinear Optically Active Materials, Springer, 1991. [24] I. Okur, P.D. Townsend, Waveguide formation by He+ and H+ ion implantation in filter glass containing nanoparticles, Nucl. Instrum. Methods B 124 (1997) 76–80.