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Carbonization and graphitization of Kapton-type polyimide film having boron-bearing functional groups
Carbon 39 (2001) 1731–1740
Carbonization and graphitization of Kapton-type polyimide film having boron-bearing functional groups Hidetaka Konno a , *, Keisuke Shiba a , Yutaka Kaburagi b , Yoshihiro Hishiyama b , Michio Inagaki c a
b
Graduate School of Engineering, Hokkaido University, Sapporo, 060 -8628, Japan Musashi Institute of Technology, Tamazutsumi, Setagaya-ku, Tokyo, 158 -8557, Japan c Aichi Institute of Technology, Yakusa, Toyota, 470 -0392, Japan Received 7 August 2000; accepted 26 November 2000
Abstract Kapton-type polyimide film having boron-bearing functional groups in the molecule was synthesized and its carbonization / graphitization behavior was investigated by XPS, SEM, XRD, Raman spectrometry, and the measurements of electronic properties. It was found that .B–N, bonds started to form in the films around 8008C, and these bonds were broken above 12008C and boron atoms started to substitute carbon atoms in the turbostratic structure. Graphitization was recognized at 26008C for both boron-doped and undoped films but the boron-doped film had smaller Lc by XRD and more disordered structure which was revealed by Raman spectroscopy. The electronic measurements confirmed that boron could be substituted even in turbostratic structure. The boron-doped film formed at 26008C showed two-carrier (holes and electrons) type conduction similar to HOPG, but hole carriers were predominant to electrons because of the doping of boron. The boron doping decreases d 002 of graphite film but it does not contribute to the development of graphite structure and reduces electric conduction. 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon precursor; Doped carbon; C. Raman spectroscopy, X-ray photoelectron spectroscopy; XPS; D. Electronic properties
1. Introduction Boron is known to dissolve into all allotropes of carbon. Quantitative aspects on the dissolution of boron into graphite were first reported by Lowell [1]. Boron has been regarded to promote the graphitization of carbon materials [1–3] but boron doping introduced a local distortion around the boron atoms substituted in the graphite layer plane [4]. In addition, substituted boron may change transport properties of pristine graphite because boron atoms in the graphite lattice are considered to function as electron acceptors [5–8]. Recently, Hishiyama et al. prepared boronated graphite by heating the 30008C treated grafoil in a mixture of B 4 C powder and graphite at 25008C for 30 min [9]. They also prepared boronated graphite from the 30008C treated carbon films derived from Kapton by sandwiching them between boron-containing graphite
*Corresponding author. Tel. / fax: 181-11-706-7114. E-mail address: [email protected] (H. Konno).
plates and by heating at 23508C for 5 min or 25008C for 5 h. The boronated graphite was characterized by electronic measurements [8,9] and X-ray diffraction (XRD) and Raman spectroscopy [10]. We are studying the borondoped carbon materials and have reported the carbonization behavior of two types of boron containing polyimide (Kapton type) film up to 12008C mainly by X-ray photoelectron spectroscopy (XPS) [11]. In that work one film was prepared by mixing polyamic acid and dihydroxyphenylborane, and the other by polymerizing pyromellitic dianhydride and 4,4-diaminodiphenylether having a boron containing functional group at 2-position. The latter sample contained boron atoms in a polyimide molecule homogeneously and even after carbonization at 12008C the film was uniform by SEM observation, while large voids were found to distribute in the former samples carbonized at the same temperature [11]. Therefore, the latter type may be useful to give information about the effect of boron on the carbonization and graphitization in a wide range of heat treatment temperature. It will also provide information whether two doping methods, namely a diffusion doping
0008-6223 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00304-3
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[8–10] and an intrinsic doping [11], result in the different effects. In the present study, the Kapton-type polyimide film containing boron in the molecule was heat-treated at a constant temperature of 1200 to 26008C. Formed carbon or graphite films were characterized by XPS, XRD, Raman spectroscopy, and measurements of electronic properties.
2. Experimental The Kapton-type polyimide film having boron-bearing functional groups in a molecule was prepared from a mixture of polyamic acid (PAA) and boron-doped polyamic acid (PAA-B) which was prepared by polymerizing pyromellitic dianhydride (PMDA) and 4,4diaminodiphenylether having a boron containing functional group at 2-position (DDE-B). The mixture was cast on a glass plate and thermally imidized at 2008C in a vacuum, in the same way as normal Kapton films. Details are described elsewhere [11]. Hereafter the symbol PI–SB is used for boron-containing films and PI for normal Kaptontype films. Boron content of PI–SB is 0.9 at% and film thickness is about 25 mm for both. The film was cut into a square shape, mostly with a size of 10310 mm 2 . Heat treatment at 1200, 1400, and 16008C was carried out for 1 h by an electric furnace with a heating rate of 300 K h 21 in a flow of pure Ar gas. Heat treatment in the range of 2000 to 26008C was done for 30 mins for specimens pretreated at 10008C for 1 h. The specimens were sandwiched between two polished artificial graphite plates and heated in a graphite resistance furnace in an Ar atmosphere. XPS measurements were carried out for as formed specimens with a VG Scientific ESCALAB MKII by MgKa irradiation (15 kV, 20 mA). Details of the calibration of binding energy, EB , the peak separation method, and the peak intensity measurements by area are described elsewhere [11–13]. The mole ratio of N / C was determined by using the peak intensity ratio of I[N 1s] /I[C 1s] by area and experimentally obtained relative sensitivity factor s[N 1s] /s[C 1s]51.75 [13]. The mole ratio of B / C was only approximately obtained with an empirical value of s[B 1s] /s[C 1s]50.52 [14] because of the lack of reliable sensitivity factors based on the standard samples. Scanning electron microscopic observation (SEM) was performed for the surface and a cross-section of films using JEOL JSM-6300F under acceleration voltage of 2.0 kV. Measurements and analysis of XRD patterns were carried out in conformity with the specification standardized by JSPS (Japan Society for Promotion of Science) [15]. CuKa radiation and a specially designed sample holder were used, and the 002, 004, 006 diffraction patterns were measured in the reflection mode. The values of the interlayer spacing d 002 and crystallite size along c-axis Lc002 were determined after correcting the diffrac-
tion profiles for Lorentz-polarization, atomic scattering and absorption factors, referring to the outer standard of thin HOPG specimen. The value of Lc was determined by the Scherrer’s formula with the shape factor K50.9. The Raman spectra were taken by using a JASCO NRS-2000 having a triple-type monochromator and CCD detectors under the irradiation of an argon–ion laser (514.5 nm) of 12 mW. The electric conductivity parallel to the film surface was measured by van der Pauw’s method [16] at room temperature: the method makes it possible to measure the dc conductivity of flat samples having irregular shapes. For these measurements, silver leads were fixed at four corners of the square-shaped film specimen by silver paste. Measurements of Hall coefficient, R H , and maximum transverse magnetoresistance, (Dr /r ) max , were carried out in the magnetic fields up to 4.2 T applied perpendicular to the film surface at a temperature of 3.0 K by immersing the specimens directly into coolant. Details of these measurements are described elsewhere [17,18].
3. Results and discussion
3.1. Chemical states of nitrogen and boron in the films after heat treatment N 1s XPS spectra for PI–SB heat-treated at 1200– 16008C are shown in Fig. 1. The N 1s spectrum for 12008C film can be separated to five components [11]. Three components from lower EB to higher EB are (i) terminal nitrogen like in pyridine (pyridine-type, EB 5398.660.3 eV), (ii) nitrogen bonding to both boron and carbon atoms (.B–N, type, EB 5398.860.3 eV), and (iii) nitrogen substituted for carbon in a hexagonal plane structure (substituted nitrogen, EB 5400.960.3 eV), respectively. Two components at much higher EB side are the nitrogen bonding to oxygen atoms and / or shake-up satellites. Previously, type (iii) nitrogen was referred to as ‘tertiary nitrogen (pyrrole type)’ [11], but it may mislead one into erroneous images on the structure. In our peak separation procedure, peak positions (EB ) are not fixed but changeable parameters. Accordingly, when a peak intensity is very low like the 12008C film in Fig. 1, it is inevitable that EB value drifts to some extent. For the spectrum of 12008C in Fig. 1, the drifts of type (i) and (ii) are apparently large but still they are within the experimental and calculated errors. With increasing heat treatment temperature, the peaks other than .B–N, type diminished and at 16008C, most of the nitrogen atoms were .B–N, type (Fig. 1). The surface composition of nitrogen and boron by XPS is plotted against temperature in Fig. 2: experimental points here are the averages of two to three data for different specimens, except for total B / C at 14008C. As reported previously [11], the total nitrogen on the surface of carbonized PI decreases drastically up to 10008C, while
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Fig. 2. Surface composition of nitrogen and boron for carbonized PI (broken line) and PI–SB (solid line) films by XPS. Points are the averages of two to three data for different specimens except for the point of the total B / C at 14008C. Fig. 1. N 1s XPS spectra for PI–SB.
that of carbonized PI–SB decreases monotonously with increasing heat treatment temperature. On the PI–SB films, .B–N, bonds started to appear in the surface layer from about 8008C and saturated in the range of 1000–14008C. Fig. 3a shows B 1s spectra for the films treated at 1200–16008C. Reproducibility of these spectra was not very high because the peak of B 2 O 3 and / or H 3 BO 3 appeared slightly above 193 eV and the peak intensity changed from specimen to specimen. Anomalous high value of the total B / C at 14008C in Fig. 2 is due to B 2 O 3 and / or H 3 BO 3. Boron oxides on the surface may be due to the sublimation of borate components originated from the boron-containing functional groups in the molecule. As XPS spectra are very sensitive to the surface contamination, only a slight difference in the contamination level affects the intensity, though we have no explanation why it was so large at 14008C. Surface treatment to remove the contamination was not carried out, since an appropriate treatment method is lacking at present: ion-sputtering seriously damages the surface and changes the chemical state of element. For 1400 and 16008C films, however, three other components were distinguishable in addition to B 2 O 3 and / or H 3 BO 3 ; type (i) boron substituted for carbon in a hexagonal plane structure (substituted boron, EB 5 188.160.3 eV), type (ii) boron bonding to both nitrogen
and carbon atoms (.B–N, type, EB 5190.060.3 eV), and type (iii) boron bonding to both oxygen and carbon atoms (EB 5191.060.3 eV). Different EB [B 1s] values, EB 5 188.5–191.4 eV, are reported for the boron atoms substituted in a hexagonal plane structure [19–21], but EB [B 1s]5188.1 eV reported by Shirasaki et al. [22] is the most reliable on account of the experimental techniques and supporting data. The B / C mole ratio for 14008C film was estimated to be 0.0054 for type (i) (boron substituted in carbon hexagonal layer) and 0.004 for type (ii) (.B–N, type) from XPS spectra (Fig. 2). The latter is smaller than the corresponding value for nitrogen in Fig. 2, probably due to the sensitivity factor used and the peak separation which has some arbitrary factors. For 14008C film, nearly a half of the total boron is attributed to oxides, and this temperature is not the average but single data. For the film treated at 22008C in Fig. 3b three types of boron components are still distinguishable, though rather marked noise is observed due to low concentration on the surface. The peak at 186.4 eV is assigned to boron clusters. The peaks around 188.3 and 190.3 eV correspond to type (i) (substituted boron) and type (ii) (.B–N, type). At 24008C the peak of substituted boron became more pronounced but that for .B–N, type is not distinguishable. This is consistent with the results for PI (film without boron) that nitrogen disappeared from the surface at around 23008C [23]. Though the information by XPS is confined only to the surface, the surface composition by XPS for PI
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Fig. 3. B 1s XPS spectra for PI–SB films heat-treated at (a) 1200–16008C and (b) 2200–24008C.
films heat-treated up to 12008C paralleled the matrix composition by elemental analysis [12,13]. Accordingly, the present results by XPS are considered to represent the changes in the film. The present results suggest that boron atoms start to substitute into carbon structure around 14008C. In the samples formed by chemical vapor deposition around 11008C, formation of substituted boron was reasonably indicated [22]. Therefore, it is possible that the boron substitution takes place around 14008C in the present sample, though it is not concluded exclusively by only one type of experimental evidence, namely XPS data.
3.2. Structural changes in the film SEM images of the surface and cross-section of PI–SB films heat-treated at 2000–26008C are shown in Fig. 4. At 20008C a lot of pores less than 100 nm in diameter were observed but they are confined to the surface. These pores were formed below this temperature when boron atoms
were released as B 2 O 3 from the surface. Pores disappeared at 22008C and small flower-like clusters were observed instead. Above 24008C these clusters grew up and were formed even inside the film, as can be seen in the cross section. Considering the results by XPS in Fig. 3b, these may be boron clusters, though diffraction peaks corresponding to metallic boron were not observed in XRD patterns. In Fig. 5, d 002 and Lc002 determined from the XRD patterns of PI and PI–SB are plotted against heat treatment temperature. The d 002 of PI slowly decreased up to 24008C and then abruptly decreased to 0.3363 nm. With commercially available Kapton films, this change started around 21008C [23]. The difference may be due to the degree of the orientation of molecules in the film: our PI films were prepared by casting on a glass plate, so that it is less orientated. The d 002 of PI–SB decreased more steadily and the value at 26008C was 0.3359 nm, apparently indicating graphitization. The Lc002 of PI–SB was small up to 24008C but slightly larger than that of PI. At 26008C, however,
H. Konno et al. / Carbon 39 (2001) 1731 – 1740
Fig. 4. SEM images of the surface and cross-section of PI–SB films heat-treated at 2000–26008C.
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Fig. 5. d 002 and Lc002 calculated from XRD data for PI and PI–SB films heat-treated at 2000–26008C.
formed around 25008C by diffusion doping to have d 002 5 0.33527 nm and 0.4 at% B (estimated from XRD data) by Hishiyama et al. [10], except for R(5ID /IG in Raman spectra) value: R¯0.1 for 26008C PI–SB and R¯0.14 for the boronated graphite. Hishiyama et al. showed that R value increases with increasing boron content in the boronated graphite [10], suggesting that the substituted boron in 26008C PI–SB is smaller than 0.4 at%. The information depth by Raman spectroscopy is less than 1 mm but it is deep enough compared with the film thickness (15–17 mm). In addition, the cross-sectional views of film by SEM indicate no significant difference between the outer layer and the central part, except for the 20008C film (Fig. 4). The present data, XRD and Raman spectra, and those by Hishiyama et al. [10] clearly indicate that boron doping into graphite decreases d 002 but it does not mean the development of graphite structure, irrespective of the doping method. This can be singly attributed to the presence of substituted boron atoms in the matrix: they were detected on the surface by XPS. Disappearance of .B–N, type bonds from the surface between 22008C and 24008C by XPS (Fig. 3b) and the development of boron clusters at 24008C (Figs. 3b and 4) suggest that most of the nitrogen was released below this temperature, that is, .B–N, type bonds were broken and boron atoms remained. The results that no significant change was observed in XRD and Raman spectra between 22008C and 24008C do not deny the presence of substituted boron atoms in the matrix of 24008C film. There is a possibility that structural change could not catch up with compositional change due to the degree of the orientation of molecules in the film. Supporting evidences are given by the measurements of electric properties as follows.
3.3. Electromagnetic properties of the films both Lc002 increased suddenly and the value of PI was much larger than that of PI–SB. First and second order Raman spectra for PI and PI–SB are shown in Fig. 6a and b, respectively. In case of PI, the G line (around 1580 cm 21 ) became strong and sharp with increasing heat treatment temperature, while the D line (around 1360 cm 21 ) and the D’ line (around 1620 cm 21 ) became weak. At 26008C both D and D’ lines were very weak and a clear shoulder appeared on the lower wave number side of 2D line (around 2700 cm 21 ). Although PI film at 26008C was not completely graphitized from XRD (d 002 50.3363 nm), the above spectra indicate that the ordered structure was well developed. In contrast to PI, the D and D’ lines were clearly appeared up to 26008C for PI–SB and the split of 2D line was not very sharp, though the intensity of D line decreased suddenly at 22008C. It is clear that the disordered structure remained significantly in the PI–SB film at 26008C in spite of the small d 002 value (0.3359 nm). The feature of the Raman spectra for 26008C PI–SB is very similar to that of the boronated graphite
The electric conductivity parallel to the surface, s, is plotted against heat treatment temperature in Fig. 7. The conductivity for both films increases very slightly up to 24008C, that of PI–SB being a little larger than that of PI. At 26008C, however, the conductivity of PI increases to the value more than ten times of that at 24008C, while that of PI–SB remains low. The difference is not explained only by the difference in the crystalline structure (Fig. 5). It suggests that the boron atoms in the hexagonal carbon structure reduce the mobility of electrons. The results of Hall coefficient, R H , and magnetoresistance, (Dr /r ) max , measurements at 3.0 K for 24008C and 26008C films are shown as a function of applied magnetic field, B, in Fig. 8a and b, respectively. The values of R H for PI and PI–SB heat-treated at 24008C are positive and almost constant against B. (Dr /r ) max for both films are negative and decreasing with increasing B. These results in Fig. 8a indicate that 24008C films, both PI and PI–SB, are hole-carrier type, similar to the carbon films derived from
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Fig. 6. First and second order Raman spectra for (a) PI and (b) PI–SB films heat-treated at 2000–26008C.
Kapton at 1600–22008C as reported by Hishiyama et al. [24]. The electric conductivity, s, for one carrier type conductor is inversely proportional to R H so that the results of Fig. 8a are parallel to Fig. 7 that 24008C PI–SB is more conductive than 24008C PI. The carrier density, n, and
mean carrier mobility, m, for one carrier type conduction can be determined by n 5 1 /R H q and
(1)
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On the contrary, R H value for PI–SB remains positive and decreases with increasing B, and (Dr /r ) max value is small and negative up to B52.6 T, and then increases slightly to positive value. This behavior of the field dependence of (Dr /r ) max was also observed for the boronated graphite with low boron concentration by diffusion doping [9]. These results suggest that the electric conduction in the 26008C treated PI–SB film is also two-carrier type, but hole carriers are predominant to electrons because of the doping of boron.
4. Conclusion
Fig. 7. Electric conductivity of PI and PI–SB films measured at room temperature.
m 5 uR H us
(2)
where q is the charge of carriers and 1e for holes and 2e for electrons. From R H in Fig. 8a the hole concentration in 24008C films is calculated to be n PI – SB 55.7310 25 m 23 and n PI 52.5310 25 m 23. Similarly, mPI – SB ¯8310 23 m 2 V 21 s 21 and mPI ¯1.3310 22 m 2 V 21 s 21 . The values of n PI and mPI are nearly the same with those of 22008C Kapton film [24] but the mean mobility in PI–SB is much smaller than that in PI. These differences in the electronic properties between PI and PI–SB cannot be attributed to the clusters formed in the films (Fig. 4). The small mPI – SB suggests that substituted boron atoms act as strong scattering centers. If the hole concentration is assumed to be attributed to boron atoms, the boron concentration in 24008C PI–SB is estimated at about 0.03 at%. The negative magnetoresistance observed for 24008C PI is characteristic of carbon materials with turbostratic structure, and is ascribed to a two dimensional weak localization of carrier [25]. In contrast, the behavior of 24008C PI–SB resembles that of boronated graphite obtained by diffusion doping [7–9], and it can be attributed to a three dimensional weak localization of carriers [8]. The results for 24008C PI–SB in Fig. 8a, XRD (Fig. 5) and Raman data (Fig. 6) support that boron atoms can substitute for carbon even in turbostratic structure. After the heat treatment at 26008C, however, R H value for PI shifts to negative and become less negative with increasing B, while (Dr /r ) max value shifts to positive and increases with increasing B, as shown in Fig. 8b. It implies that 26008C treated PI films have two carriers, holes and electrons. This conduction behavior is common to graphite.
Kapton-type polyimide film having boron-bearing functional groups in the molecule was synthesized. The films carbonized and graphitized up to 26008C were characterized by XPS, SEM, XRD, Raman spectrometry, and the measurements of electronic properties. Results are summarized as follows: 1. .B–C, type bonds started to form in the films around 8008C and these bonds were broken above 12008C and boron atoms started to substitute carbon atoms in the turbostratic structure. 2. Graphitization was recognized at 26008C for the present films but the boron-doped film had smaller Lc (by XRD) and more disordered structure (confirmed by Raman spectroscopy), which implies that the boron doping decreases d 002 of graphite film but it does not contribute to the development of graphite structure. 3. Electronic property measurements revealed that boron could be substituted even in turbostratic structure. 4. The boron-doped film formed at 26008C showed twocarrier (holes and electrons) type conduction but hole carriers were predominant to electrons because of the doping of boron, leading to less conductive than the undoped film. 5. Boron doping to turbostratic carbon film and graphite film was found to disturb electric conduction. 6. Below 26008C, two doping methods, a diffusion doping and a intrinsic doping, provided no significant difference in the structure and electronic properties. 7. The results of surface analysis by XPS and Raman spectroscopy were consistent with those by XRD and electric properties (Hall coefficient and magnetoresistance), which represent the matrix nature.
Acknowledgements This work was partly supported by the ‘Research for the Future’ program of JSPS (JSPS RFTF96R11701). We thank Prof. Norio Miyaura (Graduate School of Engineer-
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Fig. 8. Hall coefficient, R H , and magnetoresistance, (Dr /r ) max measured at 3.0 K for (a) 24008C films and (b) 26008C films.
ing, Hokkaido University) who has synthesized DDE-B for this work.
References [1] [2] [3] [4]
Lowell CE. J Am Ceram Soc 1967;50:142–4. Murty HM, Biederman DL, Heintz EA. Fuel 1977;56:305. Oya A, Yamashita R, Otani S. Fuel 1979;58:495. Hagio T, Nakamizo M, Kobayashi K. Carbon 1989;27:259– 63. [5] Marinkovic S. In: Thrower PA, editor, Chemistry and physics of carbon, Vol. 19, New York: Dekker, 1984, pp. 1–63. [6] Soule DE. In: Proceedings of 5th conference on carbon, Vol. 1, New York: Pergamon, 1963, pp. 13–21. [7] Hishiyama Y, Kaburagi Y, Kobayashi K, Inagaki M. Mol Cryst Liq Cryst 1998;310:279–84.
[8] Sugihara K, Hishiyama Y, Kaburagi Y. Mol Cryst Liq Cryst 2000;340:367–71. [9] Hishiyama Y, Kaburagi Y, Sugihara K. Mol Cryst Liq Cryst 2000;340:337–42. [10] Hishiyama Y, Kaburagi Y, Soneda Y. In: Eurocarbon 2000, Berlin, Germany, 2000, pp. 831–2, Extended abstracts. [11] Konno H, Oka H, Shiba K, Tachikawa H, Inagaki M. Carbon 1999;37:887–95. [12] Konno H, Nakahashi T, Inagaki M. Carbon 1997;35:669–74. [13] Konno H, Inagaki M. Anal Sci 1999;15:799–801. [14] Wagner CD. In: Briggs D, Seah MP, editors, Practical surface analysis, New York: Wiley, 1990, p. 635. [15] JSPS 117 Committee. Tanso 1963;36:25–34. [16] van der Pauw LJ. Philips Res Repts 1958;13:1–9. [17] Kaburagi Y. Tanso 1998;185:332–5. [18] Hishiyama Y, Kaburagi Y, Inagaki M. In: Thrower PA, editor, Chemistry and physics of carbon, Vol. 23, New York: Dekker, 1991, pp. 13–20. [19] Cermignani W, Paulson TE, Onneby C, Pantano CG. Carbon 1995;33:367–74.
1740
H. Konno et al. / Carbon 39 (2001) 1731 – 1740
[20] Jacques S, Guette A, Bourrat X, Langlais F, Guimon C, Labrugere C. Carbon 1996;34:1135–43. ´ C, Trinquecoste M, Chadeyron [21] Maqin B, Derre´ A, Labrugere ´ P. Carbon 2000;38:145–56. P, Delhaes ´ ´ [22] Shirasaki T, Derre´ A, Menetrier M, Tressaud A, Flandrois S. Carbon 2000;38:1461–7. [23] Inagaki M, Tachikawa H, Nakahashi S, Konno H, Hishiyama Y. Carbon 1998;36:1021–5.
[24] Hishiyama Y, Igarashi K, Kanaoka I, Fujii H, Kaneda T, Koidesawa T, Shimazawa Y, Yoshida A. Carbon 1997;35:657–68. [25] Bayot V, Piraux L, Michnaud J-P, Issi J-P, Lelaurain M, Moore A. Phys Rev B 1990;41:11770–9.
Carbonization and graphitization of Kapton-type polyimide film having boron-bearing functional groups Hidetaka Konno a , *, Keisuke Shiba a , Yutaka Kaburagi b , Yoshihiro Hishiyama b , Michio Inagaki c a
b
Graduate School of Engineering, Hokkaido University, Sapporo, 060 -8628, Japan Musashi Institute of Technology, Tamazutsumi, Setagaya-ku, Tokyo, 158 -8557, Japan c Aichi Institute of Technology, Yakusa, Toyota, 470 -0392, Japan Received 7 August 2000; accepted 26 November 2000
Abstract Kapton-type polyimide film having boron-bearing functional groups in the molecule was synthesized and its carbonization / graphitization behavior was investigated by XPS, SEM, XRD, Raman spectrometry, and the measurements of electronic properties. It was found that .B–N, bonds started to form in the films around 8008C, and these bonds were broken above 12008C and boron atoms started to substitute carbon atoms in the turbostratic structure. Graphitization was recognized at 26008C for both boron-doped and undoped films but the boron-doped film had smaller Lc by XRD and more disordered structure which was revealed by Raman spectroscopy. The electronic measurements confirmed that boron could be substituted even in turbostratic structure. The boron-doped film formed at 26008C showed two-carrier (holes and electrons) type conduction similar to HOPG, but hole carriers were predominant to electrons because of the doping of boron. The boron doping decreases d 002 of graphite film but it does not contribute to the development of graphite structure and reduces electric conduction. 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon precursor; Doped carbon; C. Raman spectroscopy, X-ray photoelectron spectroscopy; XPS; D. Electronic properties
1. Introduction Boron is known to dissolve into all allotropes of carbon. Quantitative aspects on the dissolution of boron into graphite were first reported by Lowell [1]. Boron has been regarded to promote the graphitization of carbon materials [1–3] but boron doping introduced a local distortion around the boron atoms substituted in the graphite layer plane [4]. In addition, substituted boron may change transport properties of pristine graphite because boron atoms in the graphite lattice are considered to function as electron acceptors [5–8]. Recently, Hishiyama et al. prepared boronated graphite by heating the 30008C treated grafoil in a mixture of B 4 C powder and graphite at 25008C for 30 min [9]. They also prepared boronated graphite from the 30008C treated carbon films derived from Kapton by sandwiching them between boron-containing graphite
*Corresponding author. Tel. / fax: 181-11-706-7114. E-mail address: [email protected] (H. Konno).
plates and by heating at 23508C for 5 min or 25008C for 5 h. The boronated graphite was characterized by electronic measurements [8,9] and X-ray diffraction (XRD) and Raman spectroscopy [10]. We are studying the borondoped carbon materials and have reported the carbonization behavior of two types of boron containing polyimide (Kapton type) film up to 12008C mainly by X-ray photoelectron spectroscopy (XPS) [11]. In that work one film was prepared by mixing polyamic acid and dihydroxyphenylborane, and the other by polymerizing pyromellitic dianhydride and 4,4-diaminodiphenylether having a boron containing functional group at 2-position. The latter sample contained boron atoms in a polyimide molecule homogeneously and even after carbonization at 12008C the film was uniform by SEM observation, while large voids were found to distribute in the former samples carbonized at the same temperature [11]. Therefore, the latter type may be useful to give information about the effect of boron on the carbonization and graphitization in a wide range of heat treatment temperature. It will also provide information whether two doping methods, namely a diffusion doping
0008-6223 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00304-3
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[8–10] and an intrinsic doping [11], result in the different effects. In the present study, the Kapton-type polyimide film containing boron in the molecule was heat-treated at a constant temperature of 1200 to 26008C. Formed carbon or graphite films were characterized by XPS, XRD, Raman spectroscopy, and measurements of electronic properties.
2. Experimental The Kapton-type polyimide film having boron-bearing functional groups in a molecule was prepared from a mixture of polyamic acid (PAA) and boron-doped polyamic acid (PAA-B) which was prepared by polymerizing pyromellitic dianhydride (PMDA) and 4,4diaminodiphenylether having a boron containing functional group at 2-position (DDE-B). The mixture was cast on a glass plate and thermally imidized at 2008C in a vacuum, in the same way as normal Kapton films. Details are described elsewhere [11]. Hereafter the symbol PI–SB is used for boron-containing films and PI for normal Kaptontype films. Boron content of PI–SB is 0.9 at% and film thickness is about 25 mm for both. The film was cut into a square shape, mostly with a size of 10310 mm 2 . Heat treatment at 1200, 1400, and 16008C was carried out for 1 h by an electric furnace with a heating rate of 300 K h 21 in a flow of pure Ar gas. Heat treatment in the range of 2000 to 26008C was done for 30 mins for specimens pretreated at 10008C for 1 h. The specimens were sandwiched between two polished artificial graphite plates and heated in a graphite resistance furnace in an Ar atmosphere. XPS measurements were carried out for as formed specimens with a VG Scientific ESCALAB MKII by MgKa irradiation (15 kV, 20 mA). Details of the calibration of binding energy, EB , the peak separation method, and the peak intensity measurements by area are described elsewhere [11–13]. The mole ratio of N / C was determined by using the peak intensity ratio of I[N 1s] /I[C 1s] by area and experimentally obtained relative sensitivity factor s[N 1s] /s[C 1s]51.75 [13]. The mole ratio of B / C was only approximately obtained with an empirical value of s[B 1s] /s[C 1s]50.52 [14] because of the lack of reliable sensitivity factors based on the standard samples. Scanning electron microscopic observation (SEM) was performed for the surface and a cross-section of films using JEOL JSM-6300F under acceleration voltage of 2.0 kV. Measurements and analysis of XRD patterns were carried out in conformity with the specification standardized by JSPS (Japan Society for Promotion of Science) [15]. CuKa radiation and a specially designed sample holder were used, and the 002, 004, 006 diffraction patterns were measured in the reflection mode. The values of the interlayer spacing d 002 and crystallite size along c-axis Lc002 were determined after correcting the diffrac-
tion profiles for Lorentz-polarization, atomic scattering and absorption factors, referring to the outer standard of thin HOPG specimen. The value of Lc was determined by the Scherrer’s formula with the shape factor K50.9. The Raman spectra were taken by using a JASCO NRS-2000 having a triple-type monochromator and CCD detectors under the irradiation of an argon–ion laser (514.5 nm) of 12 mW. The electric conductivity parallel to the film surface was measured by van der Pauw’s method [16] at room temperature: the method makes it possible to measure the dc conductivity of flat samples having irregular shapes. For these measurements, silver leads were fixed at four corners of the square-shaped film specimen by silver paste. Measurements of Hall coefficient, R H , and maximum transverse magnetoresistance, (Dr /r ) max , were carried out in the magnetic fields up to 4.2 T applied perpendicular to the film surface at a temperature of 3.0 K by immersing the specimens directly into coolant. Details of these measurements are described elsewhere [17,18].
3. Results and discussion
3.1. Chemical states of nitrogen and boron in the films after heat treatment N 1s XPS spectra for PI–SB heat-treated at 1200– 16008C are shown in Fig. 1. The N 1s spectrum for 12008C film can be separated to five components [11]. Three components from lower EB to higher EB are (i) terminal nitrogen like in pyridine (pyridine-type, EB 5398.660.3 eV), (ii) nitrogen bonding to both boron and carbon atoms (.B–N, type, EB 5398.860.3 eV), and (iii) nitrogen substituted for carbon in a hexagonal plane structure (substituted nitrogen, EB 5400.960.3 eV), respectively. Two components at much higher EB side are the nitrogen bonding to oxygen atoms and / or shake-up satellites. Previously, type (iii) nitrogen was referred to as ‘tertiary nitrogen (pyrrole type)’ [11], but it may mislead one into erroneous images on the structure. In our peak separation procedure, peak positions (EB ) are not fixed but changeable parameters. Accordingly, when a peak intensity is very low like the 12008C film in Fig. 1, it is inevitable that EB value drifts to some extent. For the spectrum of 12008C in Fig. 1, the drifts of type (i) and (ii) are apparently large but still they are within the experimental and calculated errors. With increasing heat treatment temperature, the peaks other than .B–N, type diminished and at 16008C, most of the nitrogen atoms were .B–N, type (Fig. 1). The surface composition of nitrogen and boron by XPS is plotted against temperature in Fig. 2: experimental points here are the averages of two to three data for different specimens, except for total B / C at 14008C. As reported previously [11], the total nitrogen on the surface of carbonized PI decreases drastically up to 10008C, while
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Fig. 2. Surface composition of nitrogen and boron for carbonized PI (broken line) and PI–SB (solid line) films by XPS. Points are the averages of two to three data for different specimens except for the point of the total B / C at 14008C. Fig. 1. N 1s XPS spectra for PI–SB.
that of carbonized PI–SB decreases monotonously with increasing heat treatment temperature. On the PI–SB films, .B–N, bonds started to appear in the surface layer from about 8008C and saturated in the range of 1000–14008C. Fig. 3a shows B 1s spectra for the films treated at 1200–16008C. Reproducibility of these spectra was not very high because the peak of B 2 O 3 and / or H 3 BO 3 appeared slightly above 193 eV and the peak intensity changed from specimen to specimen. Anomalous high value of the total B / C at 14008C in Fig. 2 is due to B 2 O 3 and / or H 3 BO 3. Boron oxides on the surface may be due to the sublimation of borate components originated from the boron-containing functional groups in the molecule. As XPS spectra are very sensitive to the surface contamination, only a slight difference in the contamination level affects the intensity, though we have no explanation why it was so large at 14008C. Surface treatment to remove the contamination was not carried out, since an appropriate treatment method is lacking at present: ion-sputtering seriously damages the surface and changes the chemical state of element. For 1400 and 16008C films, however, three other components were distinguishable in addition to B 2 O 3 and / or H 3 BO 3 ; type (i) boron substituted for carbon in a hexagonal plane structure (substituted boron, EB 5 188.160.3 eV), type (ii) boron bonding to both nitrogen
and carbon atoms (.B–N, type, EB 5190.060.3 eV), and type (iii) boron bonding to both oxygen and carbon atoms (EB 5191.060.3 eV). Different EB [B 1s] values, EB 5 188.5–191.4 eV, are reported for the boron atoms substituted in a hexagonal plane structure [19–21], but EB [B 1s]5188.1 eV reported by Shirasaki et al. [22] is the most reliable on account of the experimental techniques and supporting data. The B / C mole ratio for 14008C film was estimated to be 0.0054 for type (i) (boron substituted in carbon hexagonal layer) and 0.004 for type (ii) (.B–N, type) from XPS spectra (Fig. 2). The latter is smaller than the corresponding value for nitrogen in Fig. 2, probably due to the sensitivity factor used and the peak separation which has some arbitrary factors. For 14008C film, nearly a half of the total boron is attributed to oxides, and this temperature is not the average but single data. For the film treated at 22008C in Fig. 3b three types of boron components are still distinguishable, though rather marked noise is observed due to low concentration on the surface. The peak at 186.4 eV is assigned to boron clusters. The peaks around 188.3 and 190.3 eV correspond to type (i) (substituted boron) and type (ii) (.B–N, type). At 24008C the peak of substituted boron became more pronounced but that for .B–N, type is not distinguishable. This is consistent with the results for PI (film without boron) that nitrogen disappeared from the surface at around 23008C [23]. Though the information by XPS is confined only to the surface, the surface composition by XPS for PI
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Fig. 3. B 1s XPS spectra for PI–SB films heat-treated at (a) 1200–16008C and (b) 2200–24008C.
films heat-treated up to 12008C paralleled the matrix composition by elemental analysis [12,13]. Accordingly, the present results by XPS are considered to represent the changes in the film. The present results suggest that boron atoms start to substitute into carbon structure around 14008C. In the samples formed by chemical vapor deposition around 11008C, formation of substituted boron was reasonably indicated [22]. Therefore, it is possible that the boron substitution takes place around 14008C in the present sample, though it is not concluded exclusively by only one type of experimental evidence, namely XPS data.
3.2. Structural changes in the film SEM images of the surface and cross-section of PI–SB films heat-treated at 2000–26008C are shown in Fig. 4. At 20008C a lot of pores less than 100 nm in diameter were observed but they are confined to the surface. These pores were formed below this temperature when boron atoms
were released as B 2 O 3 from the surface. Pores disappeared at 22008C and small flower-like clusters were observed instead. Above 24008C these clusters grew up and were formed even inside the film, as can be seen in the cross section. Considering the results by XPS in Fig. 3b, these may be boron clusters, though diffraction peaks corresponding to metallic boron were not observed in XRD patterns. In Fig. 5, d 002 and Lc002 determined from the XRD patterns of PI and PI–SB are plotted against heat treatment temperature. The d 002 of PI slowly decreased up to 24008C and then abruptly decreased to 0.3363 nm. With commercially available Kapton films, this change started around 21008C [23]. The difference may be due to the degree of the orientation of molecules in the film: our PI films were prepared by casting on a glass plate, so that it is less orientated. The d 002 of PI–SB decreased more steadily and the value at 26008C was 0.3359 nm, apparently indicating graphitization. The Lc002 of PI–SB was small up to 24008C but slightly larger than that of PI. At 26008C, however,
H. Konno et al. / Carbon 39 (2001) 1731 – 1740
Fig. 4. SEM images of the surface and cross-section of PI–SB films heat-treated at 2000–26008C.
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Fig. 5. d 002 and Lc002 calculated from XRD data for PI and PI–SB films heat-treated at 2000–26008C.
formed around 25008C by diffusion doping to have d 002 5 0.33527 nm and 0.4 at% B (estimated from XRD data) by Hishiyama et al. [10], except for R(5ID /IG in Raman spectra) value: R¯0.1 for 26008C PI–SB and R¯0.14 for the boronated graphite. Hishiyama et al. showed that R value increases with increasing boron content in the boronated graphite [10], suggesting that the substituted boron in 26008C PI–SB is smaller than 0.4 at%. The information depth by Raman spectroscopy is less than 1 mm but it is deep enough compared with the film thickness (15–17 mm). In addition, the cross-sectional views of film by SEM indicate no significant difference between the outer layer and the central part, except for the 20008C film (Fig. 4). The present data, XRD and Raman spectra, and those by Hishiyama et al. [10] clearly indicate that boron doping into graphite decreases d 002 but it does not mean the development of graphite structure, irrespective of the doping method. This can be singly attributed to the presence of substituted boron atoms in the matrix: they were detected on the surface by XPS. Disappearance of .B–N, type bonds from the surface between 22008C and 24008C by XPS (Fig. 3b) and the development of boron clusters at 24008C (Figs. 3b and 4) suggest that most of the nitrogen was released below this temperature, that is, .B–N, type bonds were broken and boron atoms remained. The results that no significant change was observed in XRD and Raman spectra between 22008C and 24008C do not deny the presence of substituted boron atoms in the matrix of 24008C film. There is a possibility that structural change could not catch up with compositional change due to the degree of the orientation of molecules in the film. Supporting evidences are given by the measurements of electric properties as follows.
3.3. Electromagnetic properties of the films both Lc002 increased suddenly and the value of PI was much larger than that of PI–SB. First and second order Raman spectra for PI and PI–SB are shown in Fig. 6a and b, respectively. In case of PI, the G line (around 1580 cm 21 ) became strong and sharp with increasing heat treatment temperature, while the D line (around 1360 cm 21 ) and the D’ line (around 1620 cm 21 ) became weak. At 26008C both D and D’ lines were very weak and a clear shoulder appeared on the lower wave number side of 2D line (around 2700 cm 21 ). Although PI film at 26008C was not completely graphitized from XRD (d 002 50.3363 nm), the above spectra indicate that the ordered structure was well developed. In contrast to PI, the D and D’ lines were clearly appeared up to 26008C for PI–SB and the split of 2D line was not very sharp, though the intensity of D line decreased suddenly at 22008C. It is clear that the disordered structure remained significantly in the PI–SB film at 26008C in spite of the small d 002 value (0.3359 nm). The feature of the Raman spectra for 26008C PI–SB is very similar to that of the boronated graphite
The electric conductivity parallel to the surface, s, is plotted against heat treatment temperature in Fig. 7. The conductivity for both films increases very slightly up to 24008C, that of PI–SB being a little larger than that of PI. At 26008C, however, the conductivity of PI increases to the value more than ten times of that at 24008C, while that of PI–SB remains low. The difference is not explained only by the difference in the crystalline structure (Fig. 5). It suggests that the boron atoms in the hexagonal carbon structure reduce the mobility of electrons. The results of Hall coefficient, R H , and magnetoresistance, (Dr /r ) max , measurements at 3.0 K for 24008C and 26008C films are shown as a function of applied magnetic field, B, in Fig. 8a and b, respectively. The values of R H for PI and PI–SB heat-treated at 24008C are positive and almost constant against B. (Dr /r ) max for both films are negative and decreasing with increasing B. These results in Fig. 8a indicate that 24008C films, both PI and PI–SB, are hole-carrier type, similar to the carbon films derived from
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Fig. 6. First and second order Raman spectra for (a) PI and (b) PI–SB films heat-treated at 2000–26008C.
Kapton at 1600–22008C as reported by Hishiyama et al. [24]. The electric conductivity, s, for one carrier type conductor is inversely proportional to R H so that the results of Fig. 8a are parallel to Fig. 7 that 24008C PI–SB is more conductive than 24008C PI. The carrier density, n, and
mean carrier mobility, m, for one carrier type conduction can be determined by n 5 1 /R H q and
(1)
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On the contrary, R H value for PI–SB remains positive and decreases with increasing B, and (Dr /r ) max value is small and negative up to B52.6 T, and then increases slightly to positive value. This behavior of the field dependence of (Dr /r ) max was also observed for the boronated graphite with low boron concentration by diffusion doping [9]. These results suggest that the electric conduction in the 26008C treated PI–SB film is also two-carrier type, but hole carriers are predominant to electrons because of the doping of boron.
4. Conclusion
Fig. 7. Electric conductivity of PI and PI–SB films measured at room temperature.
m 5 uR H us
(2)
where q is the charge of carriers and 1e for holes and 2e for electrons. From R H in Fig. 8a the hole concentration in 24008C films is calculated to be n PI – SB 55.7310 25 m 23 and n PI 52.5310 25 m 23. Similarly, mPI – SB ¯8310 23 m 2 V 21 s 21 and mPI ¯1.3310 22 m 2 V 21 s 21 . The values of n PI and mPI are nearly the same with those of 22008C Kapton film [24] but the mean mobility in PI–SB is much smaller than that in PI. These differences in the electronic properties between PI and PI–SB cannot be attributed to the clusters formed in the films (Fig. 4). The small mPI – SB suggests that substituted boron atoms act as strong scattering centers. If the hole concentration is assumed to be attributed to boron atoms, the boron concentration in 24008C PI–SB is estimated at about 0.03 at%. The negative magnetoresistance observed for 24008C PI is characteristic of carbon materials with turbostratic structure, and is ascribed to a two dimensional weak localization of carrier [25]. In contrast, the behavior of 24008C PI–SB resembles that of boronated graphite obtained by diffusion doping [7–9], and it can be attributed to a three dimensional weak localization of carriers [8]. The results for 24008C PI–SB in Fig. 8a, XRD (Fig. 5) and Raman data (Fig. 6) support that boron atoms can substitute for carbon even in turbostratic structure. After the heat treatment at 26008C, however, R H value for PI shifts to negative and become less negative with increasing B, while (Dr /r ) max value shifts to positive and increases with increasing B, as shown in Fig. 8b. It implies that 26008C treated PI films have two carriers, holes and electrons. This conduction behavior is common to graphite.
Kapton-type polyimide film having boron-bearing functional groups in the molecule was synthesized. The films carbonized and graphitized up to 26008C were characterized by XPS, SEM, XRD, Raman spectrometry, and the measurements of electronic properties. Results are summarized as follows: 1. .B–C, type bonds started to form in the films around 8008C and these bonds were broken above 12008C and boron atoms started to substitute carbon atoms in the turbostratic structure. 2. Graphitization was recognized at 26008C for the present films but the boron-doped film had smaller Lc (by XRD) and more disordered structure (confirmed by Raman spectroscopy), which implies that the boron doping decreases d 002 of graphite film but it does not contribute to the development of graphite structure. 3. Electronic property measurements revealed that boron could be substituted even in turbostratic structure. 4. The boron-doped film formed at 26008C showed twocarrier (holes and electrons) type conduction but hole carriers were predominant to electrons because of the doping of boron, leading to less conductive than the undoped film. 5. Boron doping to turbostratic carbon film and graphite film was found to disturb electric conduction. 6. Below 26008C, two doping methods, a diffusion doping and a intrinsic doping, provided no significant difference in the structure and electronic properties. 7. The results of surface analysis by XPS and Raman spectroscopy were consistent with those by XRD and electric properties (Hall coefficient and magnetoresistance), which represent the matrix nature.
Acknowledgements This work was partly supported by the ‘Research for the Future’ program of JSPS (JSPS RFTF96R11701). We thank Prof. Norio Miyaura (Graduate School of Engineer-
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Fig. 8. Hall coefficient, R H , and magnetoresistance, (Dr /r ) max measured at 3.0 K for (a) 24008C films and (b) 26008C films.
ing, Hokkaido University) who has synthesized DDE-B for this work.
References [1] [2] [3] [4]
Lowell CE. J Am Ceram Soc 1967;50:142–4. Murty HM, Biederman DL, Heintz EA. Fuel 1977;56:305. Oya A, Yamashita R, Otani S. Fuel 1979;58:495. Hagio T, Nakamizo M, Kobayashi K. Carbon 1989;27:259– 63. [5] Marinkovic S. In: Thrower PA, editor, Chemistry and physics of carbon, Vol. 19, New York: Dekker, 1984, pp. 1–63. [6] Soule DE. In: Proceedings of 5th conference on carbon, Vol. 1, New York: Pergamon, 1963, pp. 13–21. [7] Hishiyama Y, Kaburagi Y, Kobayashi K, Inagaki M. Mol Cryst Liq Cryst 1998;310:279–84.
[8] Sugihara K, Hishiyama Y, Kaburagi Y. Mol Cryst Liq Cryst 2000;340:367–71. [9] Hishiyama Y, Kaburagi Y, Sugihara K. Mol Cryst Liq Cryst 2000;340:337–42. [10] Hishiyama Y, Kaburagi Y, Soneda Y. In: Eurocarbon 2000, Berlin, Germany, 2000, pp. 831–2, Extended abstracts. [11] Konno H, Oka H, Shiba K, Tachikawa H, Inagaki M. Carbon 1999;37:887–95. [12] Konno H, Nakahashi T, Inagaki M. Carbon 1997;35:669–74. [13] Konno H, Inagaki M. Anal Sci 1999;15:799–801. [14] Wagner CD. In: Briggs D, Seah MP, editors, Practical surface analysis, New York: Wiley, 1990, p. 635. [15] JSPS 117 Committee. Tanso 1963;36:25–34. [16] van der Pauw LJ. Philips Res Repts 1958;13:1–9. [17] Kaburagi Y. Tanso 1998;185:332–5. [18] Hishiyama Y, Kaburagi Y, Inagaki M. In: Thrower PA, editor, Chemistry and physics of carbon, Vol. 23, New York: Dekker, 1991, pp. 13–20. [19] Cermignani W, Paulson TE, Onneby C, Pantano CG. Carbon 1995;33:367–74.
1740
H. Konno et al. / Carbon 39 (2001) 1731 – 1740
[20] Jacques S, Guette A, Bourrat X, Langlais F, Guimon C, Labrugere C. Carbon 1996;34:1135–43. ´ C, Trinquecoste M, Chadeyron [21] Maqin B, Derre´ A, Labrugere ´ P. Carbon 2000;38:145–56. P, Delhaes ´ ´ [22] Shirasaki T, Derre´ A, Menetrier M, Tressaud A, Flandrois S. Carbon 2000;38:1461–7. [23] Inagaki M, Tachikawa H, Nakahashi S, Konno H, Hishiyama Y. Carbon 1998;36:1021–5.
[24] Hishiyama Y, Igarashi K, Kanaoka I, Fujii H, Kaneda T, Koidesawa T, Shimazawa Y, Yoshida A. Carbon 1997;35:657–68. [25] Bayot V, Piraux L, Michnaud J-P, Issi J-P, Lelaurain M, Moore A. Phys Rev B 1990;41:11770–9.