J. Phys. Chem. C 2007, 111, 17437-17441
17437
Vibrational and Photoelectron Investigation of Amorphous Fluorinated Carbon Films Luisa D’Urso,*,† Giuseppe Compagnini,† Orazio Puglisi,† Antonino Scandurra,‡ and Rosario Sergio Cataliotti§ Dipartimento di Scienze Chimiche, Laboratorio Film Sottili e Nanostrutture, UniVersita` di Catania, Viale A. Doria 6, Catania 95125 Italy, Laboratorio Superfici ed Interfasi-Consorzio Catania Ricerche, c/o ST-Microelectronics, Stradale Primosole 50, Catania, Italy, and Dipartimento di Chimica, UniVersita` di Perugia, Via Elce di Sotto, 8, I 06100, Perugia, Italy ReceiVed: July 24, 2007; In Final Form: September 10, 2007
sp/sp2 amorphous carbon mixtures have been deposited by pulsed jet cooled supersonic carbon beams using graphite and polytetrafluoroethylene (PTFE) as target materials in a pulsed laser vaporization source. The investigated structure and composition reveal the formation of hydrogenated and fluorinated carbons, with the latter embedded in a PTFE-like matrix. There is a relevant structural difference between the two classes of samples in both the sp and sp2 subsystems, but a similar behavior in the degradation kinetics performed by dry air exposure was observed by Raman spectroscopy. Kinetic results are discussed with relation to the separate degradation of double- and triple-bonded carbon sp chains in the films.
Introduction Carbon is the element with the greatest number of known allotropes that are radically different from one another, due to its coordination chemistry which is flexible in allowing pure sp3 and sp2 structures or continuous mixtures of sp3, sp2, and sp1 hybridizations in one structure. Foreign species (hydrogen, halogens, nitrogen, etc.) added to carbon allotropes can furthermore have a critical influence on the structures and properties of carbon-based materials, leading to a variety of oneto three-dimensional structures. In this respect fluorinated amorphous carbon (a-C:F) is one of the most promising materials for interlevel dielectric films due to its low dielectric constant (lower than silicon).1-3 Most studies suggest wide potential applications of the a-C:F thin films due to the improvements achieved with respect to carbon films in the nonwetting4 and mechanical properties,5,6 not far from those of polytetrafluoroethylene (PTFE). Partially or totally fluorinated carbon films have been produced in a variety of ways, from radio frequency plasma deposition7 to plasma enhanced or hotwire chemical vapor deposition8-10 and magnetron sputtering.11 More recently the continuum and pulsed laser irradiation of PTFE was proposed not only to deposit polymer thin films (deposition of PTFE films by conventional wet chemical routes is precluded due to its scarce solubility in several solvents), but also as an alternative strategy toward doped carbon allotrope synthesis.12,13 In this last case the dynamics of the removal process involves chemical and physical phenomena (optical absorption, heat conduction, phase transition, plasma dynamics, etc.) within macromolecules which in turn strongly depend on several variables (material composition, laser pulse parameters, irradiation environment, etc.). Many studies have been performed in an effort to elucidate the laser-polymer interaction mechanisms by studying different polymers under different * To whom correspondence should be addressed. Telephone: +39 095 7385102. Fax: +39 095 580138. E-mail:
[email protected]. † Universita ` di Catania. ‡ Consorzio Catania Ricerche. § Universita ` di Perugia.
experimental conditions.14-17 In particular, studies on PTFE laser ablation were reported with short and ultrashort pulsed lasers, with wavelengths ranging between 157 nm (excimer F2 laser)18 and 266 nm (third harmonic of a Nd:YAG laser),19 while only a limited number of studies are reported in the literature on the laser ablation of PTFE by visible light because of the transparency of the polymer in this wavelength range (PTFE has a significant absorption only below 300 nm).12 In the present work we report on the synthesis and characterization of thin films of sp and sp2 carbon mixtures obtained by the ablation of two classes of carbon materials: graphite or a PTFE target. The aim is to enlarge our knowledge on the structures and stabilities of the different carbon films, analyzed by Fourier transform infrared, Raman, and X-ray photoelectron spectroscopies. Experimental Methods Carbon cluster beams are generated either using a graphite target or a PTFE one by means of a pulsed laser vaporization source (a Nd:YAG operating at 532 nm and 10 Hz repetition rate) in a flow of helium buffer gas (5 bar). Details of the apparatus have been reported elsewhere.20 Different laser fluences have been used to induce ablation, ranging from 5 to 10 J/cm2. A collimated beam of these clusters was gently deposited (impinging energies ∼0.1 eV) with a rate of about 0.6-2.4 nm/min on a monocrystalline silicon substrate, kept at room temperature in a chamber with a background pressure of 10-8 mbar and situated at a distance of 0.8 m from the expansion nozzle. By this way, a-C films (graphite target) and a-C:F films (PTFE target) thin films with thicknesses of around 0.1 µm were deposited on a silicon wafer. In order to show the details of functional groups present in the a-C and a-C:F thin films, Fourier transform infrared (FTIR) spectra have been collected using a Bruker Equinox 55 spectrometer with a resolution of 2 cm-1. Composition of the deposited a-C and a-C:F at different laser fluences was further investigated with X-ray photoelectron spectroscopy (XPS). XPS spectra were performed with an AXIS HS Spectrometer by Kratos, using a Mg X-ray source (Mg
10.1021/jp075817u CCC: $37.00 © 2007 American Chemical Society Published on Web 10/30/2007
17438 J. Phys. Chem. C, Vol. 111, No. 46, 2007
Figure 1. FTIR spectra of a-C:F thin films deposited at different laser fluences: (a) 10 and (b) 5 J/cm2. Adsorption spectra of an a-C thin film (c) and of a PTFE foil (d) are also shown.
KR1,2) in ultrahigh vacuum (UHV) conditions (10 -9 mbar). The binding energy scales were referred to the C(1s) peak assigned to C-C and C-H chemical bonds, assumed as the internal standard at 285.0 eV. C(1s) peak components were deconvoluted using Gaussian curve fitting after linear background subtraction. The deconvolutions as well as quantitative data were obtained using Vision software (version 1.4.1) by Kratos Analytical Ltd. and experimentally derived atomic sensitivity factors. Raman scattering characterization has been performed by the 514.5 nm radiation from an Ar ion laser. The backscattered light was collected through suitable optics and analyzed by a confocal system. An ISA-Jobin Yvon single monochromator equipped with a notch filter and a CCD detector, cooled at liquid nitrogen temperature, was used (3 cm-1 spectral resolution). Results and Discussion In this section a comparison in terms of the structure and composition of the deposited films (a-C and a-C:F) obtained by means of FTIR, XPS, and Raman spectroscopy is presented. Fourier transform infrared spectra reported in Figure 1 evidence the differences in the bonding structures of a-C and a-C:F thin films. The spectrum of a PTFE target is reported for comparison. In the spectrum of the a-C film only two bands are present, and they refer to interesting features related to the chemical state of carbon. In the first region, located between 1500 and 1800 cm-1, we observe two weak bands centered at 1609 and 1713 cm-1, associated with the presence of the CdC stretching modes. The second one is located between 2800 and 3000 cm-1, and it refers to C-H stretching modes. The FTIR spectrum of PTFE shows a strong absorption in the range 1100-1400 cm-1 which is due to the CF2 (1120-1280 cm-1) and CF3 (12001400 cm-1) stretching vibration modes. Referring to standard assignments,21 the other observed peaks are attributed as follows: CF2 rocking, bending, and wagging, respectively, at 502, 553, and 625 cm-1, CF3 bending at 710 cm-1, and a characteristic band of the PTFE centered at about 2360 cm-1. Interestingly, the a-C:F films maintain almost the same features of the PTFE target with a relevant intensity difference if the laser fluence is changed. At low laser fluence (5 J/cm2), the film composition does not change drastically, in agreement with works of Tsuboi et al.19 In this case only CF2 and CF3 moieties are observed. If the fluence is increased to 10 J/cm2, a new band appears at 1100 cm-1 due to the formation of CF groups coming from the fragmentation and degradation of PTFE polymer chains. No CH signals have been detected, indicating
D’Urso et al.
Figure 2. XPS C(1s) spectra of a-C:F thin films deposited at different laser fluences (a) 10 and (b) 5 J/cm2, and of (c) an a-C film and (d) a PTFE foil.
that fluorine atoms have been not replaced by hydrogen atoms, leading to the deposition of a H-free thin film. A quantitative evaluation by infrared absorption of the functional groups present inside each sample cannot be performed due to the unknown IR cross sections. Fortunately, such an evaluation can be obtained by X-ray photoelectron spectroscopy, which is extremely sensitive to the chemical state of differently bonded C atoms. For these reasons in Figure 2 we report the evolution of the atomic composition for the a-C:F films, compared with those of a-C and PTFE. The chemical state of differently bonded C atoms was studied by analyzing the binding energies of C(1s) photoelectrons that show large chemical shifts in fluoropolymers due to chemical bonds with fluorine. C(1s) spectra are the result of the overlapping of several peaks corresponding to C atoms organized in different chemical structures. By a Gaussian curve fitting of the C(1s) spectra, one can observe that in the case of lower fluence (5 J/cm2) the spectrum contains the components assigned to carbides (283 eV), C-C (284-285 eV), -(CH2-CH2) (285 eV), -(C*H2CF2) (286.4 eV), -(C*HF-CH2) (287.9 eV), -[C*F(CF3)CF2] (289.8 eV), -(C*F2-CH2) (290.9 eV), -(CF2-CF2) (292.5 eV), and -CF3 (293.9 eV) bonds. The same moieties are observed in the case of higher fluences with the exception of those centered at 283 and 284 eV.22 Table 1summarizes the relative intensities of these components. The XPS analysis clearly indicates that, at lower fluences, carbon atoms are mainly in the form of -CF2- in agreement with C-F features observed in the FTIR spectrum. The main difference with respect to the original polymer target is given by the presence of a relevant intensity of the CF3 (nearly absent in PTFE, where one can observe only CF2 moieties), confirming that during the vaporization a breaking process of polymer chains takes place leading to the formation of a higher amount of CF3 terminal groups. A fluorine to carbon atomic ratio of 1.6, obtained by the integrated intensities of XPS C(1s) (see Table 1), evidences, however, a slight loss of fluorine. In addition, while the intensities of CF3 and CF2 signals confirm the deposition of a polymer-like thin film with about 18 carbon atoms/chain, the presence of a low-intensity C-C/ C-H peak at 284.8 eV and other components of the type of -CF2-CH2- suggest that amorphous carbon structures are present and mixed into the polymer.
Spectroscopy of Amorphous Fluorinated Carbon Films
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17439
TABLE 1: Relative Intensities of Components of the C(1s) Spectra, Labeled from 1 to 9, Reported in Figure 2 components obtained by a Gaussian deconvolution procedure
binding energy (eV)
intensities (%) at 5 J/cm2
intensities (%) at 10 J/cm2
(1) -(CH2-CH2)(2) -(C*H2-CF2)(3) -(C*HF-CH2)(4) -[C*F(CF3)-CF2](5) -(CF2-CH2)(6) -(CF2-CF2)(7) -CF3 (8) carbides (9) -CC
284.8-285 286.4 287.9 289.8 290.9 292.5 293.9 283 284
6.4 4.3 4.8 2.1 12 49.7 7 7 6.7
51.7 11 8.4 5 7 9.4 7.5
Moreover, by increasing the laser fluence (10 J/cm2), a still lower amount of fluorinated species was found. In particular, the intensity of CF3, CF2, and CF bands decreases with respect to the C-C/C-H band when the fluence increases, and a very low F/C atomic ratio of 0.6 was obtained, indicating that during the irradiation the fluorine is eliminated as volatile species. In order to achieve more information about the structure of deposited films, a Raman investigation was performed in vacuum and after sample air exposure. Raman spectra of a-C and a-C:F (5 J/cm2) samples, before air exposure, are shown in Figure 3. In the same figure for comparison the spectrum of the PTFE target is also reported. The vibrational features allow us to observe two carbon bonding states, namely sp1-hybridized carbons and sp2-hybridized ones, both of them in the amorphous state. It is known that the first species gives signals in the region around 2000 cm-1 while the latter presents a broad structure between 1000 and 1700 cm-1, which can be decomposed into a high-frequency component (G line) and a low-frequency one (D line). As it is well-known, peak position and width of D and G bands are strongly associated with the structural order, in terms of size and number of graphitic domains.23 In this respect, a two-peak Gaussian fitting was chosen to separate the components and to calculate the so-called ID/IG ratio.23 While in the a-C sample the D band appears centered at about 1339 cm-1 in the form of a shoulder of the main G band (1526 cm-1), in the a-C:F film we observe two separated D (1353 cm-1) and G (1566 cm-1) peaks shifted toward higher frequencies. As generally reported in the literature, a blue shift of the G peak suggests the presence of a more ordered sp2 structure, similar to a nanocrystalline graphite architecture. The absence of C-H stretching modes in the FTIR spectrum of a-C:F films supports this view. In addition to the expected G and D Raman peaks,
Figure 3. Typical Raman spectra of a-C and a-C:F thin films and PTFE target. In the inset deconvolutions into D and G features are shown for a-C and a-C:F films. Arrows indicate the positions of the D and G bands.
sp-hybridized carbon signals between 1900 and 2200 cm-1 were detected in both samples. According to Raman spectroscopy studies on carbynoid samples, these peaks are related to the presence of a mixture of both sp isomer chains, polyyne (energetically preferred because of a stabilization by the Peierls distortion)24-27 and polycumulene. The sp isomers have very similar Raman spectra; hence a clear distinction between them is very difficult and they are usually resolved by means of a two Gaussian band fitting. While in the a-C sample a typical broad and asymmetric peak was observed, in the a-C:F one a double peak suggests the presence of two narrower and better resolved sp structures: polycumulene centered at 2037 cm-1 and polyyne at 2165 cm-1.28 It is necessary to point out that the frequency of polyyne bands shifts toward lower values for longer sp chain so that an overlap of polycumulene and polyyne bands cannot be completely ruled out.29 A similar sp double peak was observed by Yasuda et al.30 and by Kavan et al.31 in sp carbon chains obtained from PTFE. While Yasuda et al. do not discuss the presence of the sp double peak, Kavan et al. hypothesize that the PTFE initially converts into short oligocumulenic segments, and finally into polyynes. In our case, we cannot detect any conversion from oligocumulenic to polyyne chains because the PTFE defluorination only takes place during the laser ablation process; consequently, after deposition no further chemical transformations could be observed. In order to study the stability of sp-hybridized carbons in our samples, Raman investigations were performed by allowing air to enter the chamber. It is well-known that sp chains embedded in carbon thin films easily degrade as a consequence of their interaction with atmospheric gases.28 In Figure 4 it is possible to observe a number of changes in the corresponding Raman regions when the a-C and the a-C:F deposited films are progressively exposed to air. By increasing the time of air exposure of a-C and a-C:F thin films to 70 min, both sp signals strongly decrease with respect
Figure 4. Raman spectra obtained from (a) a-C:F and (b) a-C sp-rich carbon thin films before (as deposited) and after air exposure with aging time increasing from top to bottom.
17440 J. Phys. Chem. C, Vol. 111, No. 46, 2007
Figure 5. Decay behavior of sp carbon chains (Ipln and Ipcn) with respect to the sp2 (IDG) for a-C:F thin film as a function of air exposure time. In the figure the overall sp signal to respect the sp2 is also represented (IC/IDG). The inset shows the linear fit of the inverse of the Ipln/IDG and Ipcn/IDG ratios as a function of air exposure time.
to the sp2 one (IC/IDG) due to the degradation of both components. In addition to the decrease of the sp signal, further changes are evident in the sp2 Raman region, where the G peak position has a blue shift and a slight increase in intensity, indicating a trend toward ordering of the sp2 phase. The correlation between the decrease of the overall sp signals and the increase of the graphite-like band in the a-C film has been reported in a previous work32 by considering the integrated intensity of the sp band (IC) with respect to the sp2 (IDG). The case of the a-C:F samples is particularly interesting because of the separation of the sp signal into two separated components. In this case, on the basis of the integrated intensity of the polycumulene (Ipcn) and polyyne (Ipln) components, it is possible to monitor in a separate way the decay behavior of double- and triple-bonded carbon sp chains (Ipcn/IDG and Ipln/IDG) in the films during controlled exposure to dry air (see Figure 5). In this respect, it is known that the integrated intensity ratio of the carbynoid signals (Ipcn, I pln) with respect to the graphite-like one (including both G and D lines, IDG) is proportional to the fraction of sp (polycumulene and polyyne) and sp2-hybridized bonded carbons as follows:
Ipcn σpcn xpcn ) Isp2 σsp2 xsp2 Ipln σpln xpln ) Isp2 σsp2 xsp2 Here σpcn, σpln, and σ sp2 are respectively the polycumulene, polyyne, and sp2 Raman scattering cross sections (corrected for the presence of any resonance effect) and xpcn, xpln, and xsp2 are the number fraction of the polycumulene, polyyne, and sp2hybridized carbon atoms. In this case σsp/σsp2 is a constant which depends on the excitation wavelength. It is important to observe that both cross sections can be influenced by the structure of the graphite-like domains or by the length of the linear carbon chains. For this reason σpcn, σpln, and σsp2 are to be considered as average for this specific material. By plotting (Ipcn/IDG)-1 and (Ipln/IDG)-1 as a function of the air exposure time (see Figure 5), we can obtain kinetic information about the change of the sp-hybridized components (xpcn and xpln) with time. Specifically, by a best fit of the inverse of sp-hybridized components (1/ (Ipcn/IDG) and 1/(Ipln/IDG)) in both cases, we observe a linear trend, typical of a second-order kinetic mechanism of conversion from sp to sp2. The observed second-order law suggests the necessity of a reaction between two sp moieties present in linear chains, unique reactants in our systems, by excluding any
D’Urso et al. involvement in the reaction of the extremely stable sp2 component. The idea of an interchain cross-linking during polyyne degradation was first formulated by Jansta33 and successively confirmed by Sprinborg and Kavan25 by comparing experimental results with first-principles calculations on two interacting, parallel chains of polyyne. They showed the existence of a metastable configuration, with dangling bonds, of two cross-linked chains and interpreted the presence of these structures as a transition from sp-bonded to sp2-bonded carbon atoms. The necessity of an interaction between sp chains was also recently reported by Tabata et al.34 by investigating the polyyne concentration effect on the IC/IDG ratio. The authors suggest that a high polyyne dilution leads to a low probability of interaction between sp chains and consequently to a low sp to sp2 conversion. The decay behavior sp f sp2 is then governed by the encounter probability of sp chains. In addition, from kinetic data reported in Figure 5 it is possible to observe that the decay behavior is faster for the polycumulene structure than for polyyne by a factor of 2. This result confirms that the polyyne configuration is energetically preferred to the polycumulene one, as widely reported in the literature. Conclusions In this work, we explored the possibility of producing carbon allotropes by low-energy cluster beam deposition starting from a PTFE target. Photoinduced dehydrohalogenation, with wavelength below 300 nm, of halogenated polymers is a known technique for the production of carbon chains. Here it was shown that a pulsed visible (532 nm) laser vaporization source can be also successfully used to deposit sp-rich carbon thin films from the polymer with tunable composition and structure depending on the experimental conditions. Fluorinated carbon thin films with F/C ratio ranging from 0.6 to 1.6 have been synthesized by changing the laser fluence respectively from 5 to 10 J/cm2. From a structural investigation by Raman spectroscopy, it results that our films are essentially composed of sp chains (polycumulene and polyyne structures) embedded in a polymer-like matrix at different degrees of fluorination. Spectroscopic studies conducted in a vacuum before and after the degradation have been of fundamental importance for the understanding of the conversion mechanism of polycumulene and polyyne to graphenelike species. Kinetic data show that the sp carbon chains tend to cross-link to sp2 hybridizations by a second-order kinetic law, suggesting the necessity of a reaction between sp linear chains. The obtained cross-linking rate is 2 times faster for the polycumulene structure than for polyyne, confirming the higher stability of the polyynic configurations. More detailed experiments are underway in our laboratory to clarify the role of fluorine atoms in the polyyne and polycumulene formation and stability. Acknowledgment. We are grateful to Giuseppe Indelli, Consorzio Catania Ricerche, for his technical support. This work has been partially supported by the Ministero Istruzione, Universita` e Ricerca (MIUR). References and Notes (1) Endo, K.; Tatsumi, T. J. Appl. Phys. 1995, 78 (2), 1370-1372. (2) Theil, J. A. J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct. 1999, 17 (6), 2397-2410. (3) Rastogi, A. C.; Desu, S. B. Appl. Phys. A: Mater. Sci. Process. 2006, 83, 57-66. (4) Lim, M. S.; Yun, Y.; Gellman, A. J. Langmuir 2006, 22, 10861092.
Spectroscopy of Amorphous Fluorinated Carbon Films (5) Lamperti, A.; Ossi, P. M. Appl. Surf. Sci. 2003, 205, 113-120. (6) Bottani, C. E.; Lamperti, A.; Nobili, L.; Ossi, P. M. Thin Solid Films 2003, 433, 149-154. (7) Sah, R. E.; Dischler, B.; Bubenzer, A.; Koidl, P. Appl. Phys. Lett. 1985, 46 (8), 739-741. (8) Freire, F. L., Jr.; Maia da Costa, M. E. H.; Jacobsohn, L. G.; Franceschini, D. F. Diamond Relat. Mater. 2001, 10, 125-131. (9) Yu, G. Q.; Tay, B. K.; Sun, Z. Surf. Coat. Technol. 2005, 191, 236-241. (10) Lau, K. K. S.; Pryce, Lewis, H. G.; Limb, S. J.; Kwan, M. C.; Gleason, K. K. Thin Solid Films 2001, 395, 288-291. (11) Jiang, M.; Ning, Z. Surf. Coat. Technol. 2006, 200, 3682-3686. (12) Shibagaki, K.; Kamiya, Y.; Sasaki, K. Appl. Phys. A 2004, 79, 1161-1163. (13) Ravagnan, L.; Siviero, F.; Casari, C. S.; Li Bassi, A.; Lenardi, C.; Bottani, C. E.; Milani, P. Carbon 2005, 43, 1337-1339. (14) Tolstopyatov, E. M. J. Phys. D: Appl. Phys. 2005, 38, 19931999. (15) Okoshi, M.; Inoue, N. Appl. Phys. A 2004, 79, 841-844. (16) Lippert, T.; Dickinson, J. T. Chem. ReV. 2003, 103, 453-485. (17) Zhu, X. L.; Liu, S. B.; Man, B. Y.; Xie, C. Q.; Chen, D. P.; Wang, D. Q.; Ye, T. C.; Liu, M. Surf. Sci. 2007, 253, 3122-3126. (18) Huber, N.; Heitz, J.; Ba¨uerle, D. Eur. Phys. J.: Appl. Phys. 2005, 29, 231-38. (19) Tsuboi, Y.; Kuro-Oka, T.; Irie, K.; Itaya, A. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 339-342. (20) D’Urso, L.; Scalisi, A. A.; Altamore, C.; Compagnini, G. J. Mater. Res. 2006, 21 (7), 1638-1644.
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17441 (21) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, 1991. (22) Beamson, G.; Briggs, D. The XPS of Polymers Database; Surface Spectra: Manchester, 2000. (23) Ferrari, A. C.; Robertson, J. Phys. ReV. B 2000, 61 (20), 1409514107. (24) Kertesz, M.; Koller, J.; Azˇman, A. J. Chem. Phys. 1978, 68 (6), 2779-2782. (25) Springborg, M.; Kavan, L. Chem. Phys. 1992, 168, 249-258. (26) Karpfen, A. J. Phys. C: Solid State Phys. 1979, 12, 3227. (27) Rice, M. J.; Phillpot, S. R.; Bishop, A. R.; Campbell, D. K. Phys. ReV. B 1986, 34 (6), 4139-4149. (28) Casari, C. S.; Li Bassi, A.; Ravagnan, L.; Siviero, F.; Lenardi, C.; Piseri, P.; Buongiorno, G.; Bottani, C. E.; Milani, P. Phys. ReV. B 2004, 69, 075422-1-7. (29) Lucotti, A.; Tommasini, M.; Del Zoppo, M.; Castiglioni, C.; Zerbi, G.; Cataldo, F.; Casari, C. S.; Li Bassi, A.; Russo, V.; Bogana, M.; Bottani, C. E. Chem. Phys. Lett. 2006, 417, 78-82. (30) Yasuda, A.; Kawase, N.; Matsui, T.; Shimidzu, T.; Yamaguchi, C.; Matsui, H. React. Funct. Polym. 1999, 41, 13-19. (31) Kavan, L.; Zukalova, M.; Kalbac, M.; Osawa, E.; Dunsch, L. Carbon 2006, 44, 3113-3148. (32) D’Urso, L.; Compagnini, G.; Puglisi, O. Carbon 2006, 44 (10), 2093-2096. (33) Jansta, J.; Dousek, F. P. Carbon 1980, 18 (6), 433-437. (34) Tabata, H.; Fujii, M.; Hayashi, S. Chem. Phys. Lett. 2006, 420, 166-170.
