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Modification of N-doped carbon induced by 30 MeV C60 ions
Nuclear Instruments and Methods in Physics Research B 230 (2005) 262–268 www.elsevier.com/locate/nimb
Modification of N-doped carbon induced by 30 MeV C60 ions q Z.G. Wang a,*, A. Dunlop b, Z.M. Zhao a, Y. Song a, Y.F. Jin a, C.H. Zhang a, Y.M. Sun a, J. Liu a a
Institute of Modern Physics, Chinese Academy of Sciences, P.O. Box 31, Lanzhou 730000, PR China b Laboratoire des Solides Irradie´s, CEA, Ecole Polytechnique, 91128 Palaiseau Cedex, France Available online 22 January 2005
Abstract In the present work, 120 keV N-ion doped and 30 MeV C60 ion irradiated graphite-like-carbon samples were characterized by RBS, micro-FTIR, micro-Raman, XPS spectroscopy and the variation of the properties of the samples with the N-dopant and/or C60 irradiation fluence have been studied. The RBS spectra showed that C60 irradiation can induce a partial diffusion of N atoms to the surface and the amount of the diffused N atoms increases slightly with increasing C60 irradiation fluence. The FTIR and Raman spectra exhibit characteristic bands of carbon nitrogen bonds showing that the C and N atoms are chemically bonded. The amount of chemically bonded C and N atoms increases with increasing N-dopant. By deconvolution of the XPS spectra, the atomic concentration of N and C atoms were obtained and it was identified that the samples mainly consist of three phases, namely, C3N4, CNx and tetrahedral amorphous carbon. The effect of N-dopant and C60 irradiation fluence on the modification of the properties of the samples is also discussed. 2004 Elsevier B.V. All rights reserved. PACS: 61.80.Jh; 61.10. i; 82.80.Yc Keywords: Ion implantation and irradiation; Graphite-like-carbon; Phase change; Carbon-nitrides
1. Introduction
q
Supported by NSFC (Project Nos. 10125522, 10175084) and Chinese Academy of Sciences. * Corresponding author. Tel.: +86 931 496 9330/9331; fax: +86 931 827 2100. E-mail address: [email protected] (Z.G. Wang).
Experimental results showed that energetic ion induced phase change in a solid could be realized not only by irradiation to high fluences [1–3] but also by huge electronic excitations induced by individual ion impacts [4]. The phase change produced in the latter condition is just along the ion latent
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.12.052
Z.G. Wang et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 262–268
tracks. Recently, we have proposed a novel technique [5], ‘‘low energy ion implantation + swift heavy ion irradiation’’, for synthesizing new structures in atom mixed materials in which more attention was paid to the dense electronic excitations effect induced by the incident ions. By use of this technique, swift heavy ion irradiation induced formation of blue-violet emission bands in C-doped SiO2 films [5,6] and some evidences of new chemical bonds and new phase of carbon-nitrides in N-doped diamond-like-carbon films [7] have been observed. In the present work, the novel technique was used to study the modification of N-doped graphite-like-carbon (NGC) under 30 MeV C60 ion irradiations, especially to look for evidence for formation of the theoretically predicted superhard carbon nitrides [8,9]. In the following, the experimental procedure is described first, then the obtained results are presented and the modification of NGC induced by dense electronic excitations is discussed.
2. Experimental The schematic experimental procedure is shown in Fig. 1. The original samples used in the experiments were high purity (>99.99%) graphitelike-carbon (GC). The GC samples were first implanted at room temperature (RT) with 120 keV N-ions and then irradiated at RT with 30 MeV C60 ions. The implantation was performed at the
263
200 KV heavy ion implanter of IMP (Lanzhou, China) and the implantation doses were from 2 · 1017 to 1.2 · 1018 N/cm2. The irradiation was carried out at the 15 MV Tandem of IPNO (Orsay, France) and the irradiation fluence ranged from 1 · 1010 to 6 · 1011 C60/cm2. Using the TRIM code [10], we got that the theoretical peak position of Nconcentration distribution is about 216 nm and the projectile range of C60 ions is about 768 ± 65 nm; thus the C60 ions pass through the region of Nion dopant. The electronic and nuclear energy losses of the C60 ions, Se and Sn, are 56.0 keV/ nm and 1.0 keV/nm at the ion bombarded surface and 43.1 keV/nm and 1.4 keV/nm in the N-ion doped region. After the 30 MeV C60 ion irradiations, the samples were investigated at RT by use of the following techniques: Rutherford backscattering spectroscopy (RBS) analysis using a 2 MeV 4 He++ beam was performed at the 2 · 1.7 MV Tandem of Pekin University. The beam spot was 0.8 · 1.2 mm2 and the scattering angle was 164; Micro-Raman spectra were carried out on a JYT64000 Raman Spectrometer, with 488 nm excitation light from an Ar-laser, with a beam spot of / 2 lm; Micro-Fourier transform infrared spectroscopy (micro-FTIR) analysis was performed on a Spectrum GX (Perkin ElmerTM) with a beam spot
3. Results and discussion 3.1. Micro-FTIR analysis
Fig. 1. Scheme of experimental procedure.
From micro-FTIR measurements it was found that a broad IR absorption band covering 750– 2300 cm 1 formed in the samples as-implanted with and without C60 irradiations. For the as-implanted samples, the peak position of the main IR absorption band shifted to lower frequency with an increase of the N-ion implantation dose (see Fig. 2(a)). For the C60 ion irradiated 5 · 1017 N/cm2 doped samples, the peak position of the main IR absorption band was nearly constant (centered at about 1700 cm 1) but the
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Fig. 2. Typical micro-FTIR spectra varying with N-ion implantation dose (a) and C60-ion irradiation fluence ((b) and (c)). The sample conditions are given in the figure.
relative IR absorption intensity decreased with an increase of the C60 ion irradiation fluence (Fig. 2(b)). When the irradiation fluence is larger than 1 · 1011 C60/cm2, a shoulder within 970– 1260 cm 1 and an IR absorption peak around 2220 cm 1 were observed. For the C60 ion irradiated 1.2 · 1018 N/cm2 doped samples, the peak intensity of the IR absorption centered at about 2220 cm 1 increased with the C60 irradiation fluence and dual IR absorption peaks were observed when the irradiation fluence was larger than 1 · 1011 C60/cm2 (Fig. 2(c)). The barycenter of the main IR absorption band shifted to lower frequency with an increase of the C60 irradiation fluence. From the literature [11–18] we known that the peak at 2200 cm 1 is from the C„N triple bond,
the main IR absorption band covering 800 and 1800 cm 1 can be attributed to a mixture of N– sp3C, N–sp2C, sp2C@C and C@N bonds and C– O bond. For the irradiation with 1 · 1011 C60/ cm2, a large ratio of N–sp3C, N–sp2C and C–O bonds was obtained. 3.2. Micro-Raman spectroscopy measurement The obtained micro-Raman spectra showed that Raman active but IR prohibited G (ÔGraphiticÕ or sp2–hybridized carbon, at 1580 cm 1) and D (ÔdisorderÕ or sp3–hybridized carbon, at 1360 cm 1) bands were clearly observed from the original GC sample. For N-doped GC samples without C60 ion irradiation and the 5 · 1017 N/cm2 doped GC samples with C60 ion irradiation, there
Z.G. Wang et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 262–268
265
18
1. 2x10 N/ cm 2 doped GC
400
(b)
200
(a)
(0) Original GC (1) unirradiated 10
(2) 5 x10 C60/cm2 11
(3) 1 x10
150
C60/cm2
Intensity (a.u.)
Intensity (a.u.)
300
11
200
(4) 6 x10 C60/cm2
(4 )
100
(3 )
100
50
(2 )
(0 )
0 500
1000
1500
(1 )
2000
0 500
2500
1000
-1
1500
2000
2500
-1
Wavenumber (cm )
Wavenumber (cm ) 400
(C)
Intensity (a.u.)
300
200
100
0 500
1000
1500
2000
2500
-1
Wavenumber (cm )
Fig. 3. Typical micro-Raman spectra of 1.2 · 1018 N/cm2 doped GC samples varying with C60-ion irradiation fluence (a). The sample conditions are given in the figure. (b) and (c) are the deconvolution of the curves (3) and (4) shown in (a), respectively.
was one broadened Raman peak covering the G and D bands and the shape of the Raman peak was nearly the same for different treatments. For the 1.2 · 1018 N/cm2 doped GC samples with C60 ion irradiation, the shape of the Raman spectra change from a broadened Raman peak to two peaks with an increase of the C60 irradiation fluence (see Fig. 3(a)). The Raman spectra were analyzed by deconvolutions (e.g. Fig. 3(b) and (c)) and the deconvolved peaks and FWHMs (full width at half maximum) are given in Table 1. The deconvolved Raman peak at about 630–710 cm 1 can be assigned to the out-of-plane bending mode for graphite-like domains in which a number of C atoms are substituted by N atoms or to the C–N bond in b-C3N4 [19]. The broad luminescence peak
centered at about 2050–2100 cm 1 may be from water absorption or sp1 N triple bonds. It is well known that the G band originates from the symmetric E2g vibration mode in graphite-like materials, while the D band arises from the limitations in the graphite domain size induced by grain boundaries or imperfections [20], e.g. substitutional N atoms, sp3C, or other impurities. From the literature we may think that the deconvolved Raman peaks in the ranges of 1094– 1260 cm 1, 1300–1380 cm 1, 1400–1500 cm 1 and 1550–1650 cm 1 correspond to N–sp3C, sp3C–C, N–sp2C and sp2C@C bonds, respectively. With an increase of the C60 irradiation fluence, the relative amount of N–sp3C and sp3C–C bonds increases, and that of the N–sp2C and sp2C@C
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Table 1 Deconvolution Raman peaks and FWHMs (full width at half maximum) of some typical micro-Raman spectra Element
Peak position
A B C D E F
634 685 688 706 645
(750) (588) (437) (508) (600)
1360 1335 1303 1258 1206 1094
(40) (456) (352) (320) (348) (250)
18
(FWHM)
(cm 1)
1501 1449 1427 1406 1371
1580 1577 1561 1557 1564 1554
(196) (243) (277) (344) (281)
(22) (134) (149) (162) (152) (104)
2
18
1600 (70) 2
Element A is the original GC sample, B is 1.2 · 10 N/cm doped GC sample, C, D, E and F are 1.2 · 10 N/cm doped GC samples with 1 · 1010, 5 · 1010, 1 · 1011 and 6 · 1011 C60/cm2 irradiations, respectively.
bonds decreases. The increase of single bonds is a key factor for the formation of C3N4. It must be pointed out that no irradiation induced C„N bonds formation (Raman peak at 2200 cm 1) was detected clearly in any of the samples.
Table 2 Atomic concentration of N-dopant, cN, at the near surface of the C60 ion irradiated 1.2 · 1018 N/cm2 doped GC samples Element
cN (at.%)
U (C60/cm2) 0
1 · 1010
5 · 1010
1 · 1011
6 · 1011
1.10
0.91
0.97
1.08
1.29
3.3. XPS measurement Fig. 4 shows C1s (a) and N1s (b) X-ray photoelectron spectra of 1.2 · 1018 N/cm2 doped GC samples after C60-ion irradiation to different fluences. By deconvolution of the C1s and N1s core level spectra, the atomic concentration of N and C atoms were obtained. Table 2 shows the atomic concentration of N-dopant, cN, at the near surface of the samples, obtained from the intensities of the
Fig. 4. C1s (a) and N1s (b) XPS spectra of 1.2 · 1018 N/cm2 doped GC samples varying with C60-ion irradiation fluence. The sample conditions are the same as those given in Fig. 2(c).
N1s to C1s lines with a correction for sensitivity factors for N and C atoms. The cN values given in Table 2 imply that C60-ion irradiation results in release of the implanted N atoms and enhanced the N-dopant diffusion toward to the sample surface. Furthermore, the N1s core peak located at 398–402 eV which implies that CNx formed in the NGC samples (N1s peaks for N–C, N@C and N„C bonds are around 398.3–399.3 eV, 399.8–400.9 eV and 398.1–398.4 eV, respectively [21]). Table 3 shows the deconvolved peaks and the FWHMs of C1s core level spectra. Three peaks were identified at 285.0 ± 0.1 eV (C1), 286.3 ± 0.3 eV (C2) and 288.3 ± 0.4 eV (C3, weak) which are believed to come from the tetrahedral amorphous carbon (ta-C), N@sp2C (triagonal CN) and N–sp3C (tetrahedral CN, e.g. C–N in b-C3N4) bonding, respectively [16,22]. There is an optimum C60 irradiation fluence for sp2/sp3 hybridized carbon atoms formation. Below this fluence, the relative amount of sp2/sp3 bonds increases with the increase of C60 fluence but above it decreases, which implies that too high irradiation could destroy the previously formed sp2/sp3 bonds. Therefore, the variation of sp2/sp3 bonds with the irradiation fluence should be a key factor for the appearance of the shoulder within
Z.G. Wang et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 262–268 Table 3 Deconvolution peaks and FWHMs of C1s core level spectra of 1.2 · 1018 N/cm2 doped GC samples Element
Peak
Eb (eV)
FWHM (eV)
Relative density (at.%)
(a)
C1 C2 C3
284.9 286.4 287.7
1.6 1.4 1.0
76.9 19.0 5.1
(b)
C1 C2 C3
285.0 286.5 288.7
1.9 3.5 2.6
74.0 21.8 4.2
(c)
1
C C2 C3
284.9 286.0 288.2
2.0 3.2 3.2
61.7 33.7 4.6
(d)
C1 C2 C3
285.0 286.3 288.2
2.0 2.6 2.9
66.3 27.4 6.3
(e)
C1 C2 C3
285.0 286.5 288.5
1.8 2.2 2.5
73.3 22.1 4.6
Element (a) is the N-doped GC sample, (b), (c), (d) and (e) are N-doped GC samples with 1 · 1010, 5 · 1010, 1 · 1011 and 6 · 1011 C60/cm2 irradiations, respectively.
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C3N4, CNx and ta-C. Furthermore, an optimized C60 irradiation fluence for sp2/sp3 C–N bonds formation was observed and it was seen that too high irradiation can destroy the previously formed sp2/ sp3 bonds.
Acknowledgments We are very grateful to Dr. Limin Zhao (IMP, Lanzhou) for N ion implantation, to Prof. S. Della-Negra and the Tandem group of INPO (Orsay) and Dr. M. Toulemonde (CIRIL, Caen) for C60 ion irradiation, to Dr. Hongji Ma (Peking University, Beijing) and the support of Institute of Heavy Ion Physics of Pekin University for RBS analysis, to Prof. E.Q. Xie (Lanzhou University, Lanzhou) for helpful discussions. One of the authors (Z.G. Wang) gratefully acknowledges the support of K.C. Wong Education Foundation, Hong Kong.
References 970–1260 cm in Fig. 2(c).
1
in the obtained FTIR spectra given
3.4. RBS analysis The obtained RBS spectra showed that part of the implanted N atoms diffused to the sample surface with or without C60 irradiation. After C60 irradiation, the N-peak content decreased slightly with increasing irradiation fluence. This is in agreement with the FTIR and XPS analysis.
4. Conclusion Modification of N-doped GC samples induced by 30 MeV C60 ion irradiations has been studied. The results show that C60 irradiation can induce CN bonds formation, N atoms partially diffuse to the sample surface and the amount of diffused N atoms increases slightly with increasing C60 irradiation fluence. By deconvolution of the XPS and Raman core level spectra, it was identified that the samples mainly consist of three phases, namely,
[1] T.L. Daulton, M.A. Kirk, R.S. Lewis, L.E. Rehn, Nucl. Instr. and Meth. B 175–177 (2001) 12. [2] H. Dammak, A. Barbu, A. Dunlop, D. Lesusueur, N. Lorenzelli, Philos. Mag. 67 (1993) 253. [3] H. Dammak, A. Dunlop, D. Lesueur, Nucl. Instr. and Meth. B 107 (1996) 204. [4] M. Angiolini, H. Dammak, A. Dunlop, Philos. Mag. Lett. 82 (2) (2002) 81. [5] Z.G. Wang, E.Q. Xie, Y.F. Jin, et al., Nucl. Instr. and Meth. B 179 (2001) 289. [6] Z.G. Wang, J. Liu, Z.Y. Zhu, Y.F. Jin, E.Q. Xie, T.W. Zhu, X.X. Chen, Y.M. Sun, M.D. Hou, Nucl. Instr. and Meth. B 191 (2002) 396. [7] Z.G. Wang, Z.M. Zhao, Y. Song, J. Liu, Y.M. Sun, C.H. Zhang, J.L. Duan, Y.F. Jin, Nucl. Instr. and Meth. B 218 (2004) 472. [8] A.Y. Liu, M.L. Cohen, Science 245 (1989) 841. [9] A.Y. Liu, M.L. Cohen, Phys. Rev. B 41 (1990) 10727. [10] TRIM96, see: J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [11] J.H. Kaufman, S. Metin, D.D. Saperstein, Phys. Rev. B 39 (1989) 13053. [12] M.R. Wixom, J. Am. Cerem. Soc. 73 (1990) 1973. [13] N. Nakayama, Y. Tsuchiya, S. Tamada, K. Kosuge, S. Nagata, K. Takahiro, S. Yamaguchi, Jpn. J. Appl. Phys. 32 (1993) L1465. [14] A.R. Merchant, D.G. McCulloch, D.R. McKenzie, Y. Yin, L. Hall, E.G. Gerstner, J. Appl. Phys. 79 (1996) 6914.
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[15] Y.K. Yap, S. Kida, T. Aoyama, Y. Mori, T. Sasaki, Appl. Phys. Lett. 73 (1998) 915. [16] A.X. Wei, D.H. Chen, N. Ke, S.Q. Peng, S.P. Wong, Thin Solid Films 323 (1998) 217. [17] G.Q. Yu, S.H. Lee, J.J. Lee, Diamond Relat. Mater. 11 (2002) 1633. [18] Y.H. Cheng, X.L. Qiao, J.G. Chen, Y.P. Wu, C.S. Xie, Y.Q. Wang, D.S. Xu, S.B. Mo, Y.B. Sun, Diamond Relat. Mater. 11 (2002) 1511.
[19] A. Bousetta, M. Lu, A. Bensaoula, A. Schultz, Appl. Phys. Lett. 65 (1994) 696. [20] N. Hellgren, M.P. Johansson, E. Broitman, L. Hultman, J.E. Sundren, Phys. Rev. B 59 (1999) 5162. [21] E. DÕAnna, M.L. De Giorgi, A. Luches, M. Martino, A. Perrone, A. Zocco, Thin Solid Films 347 (1999) 72. [22] G.Q. Yu, S.H. Lee, D.G. Lee, H.D. Na, H.S. Park, J.J. Lee, Surf. Coat. Technol. 154 (2002) 68.
Modification of N-doped carbon induced by 30 MeV C60 ions q Z.G. Wang a,*, A. Dunlop b, Z.M. Zhao a, Y. Song a, Y.F. Jin a, C.H. Zhang a, Y.M. Sun a, J. Liu a a
Institute of Modern Physics, Chinese Academy of Sciences, P.O. Box 31, Lanzhou 730000, PR China b Laboratoire des Solides Irradie´s, CEA, Ecole Polytechnique, 91128 Palaiseau Cedex, France Available online 22 January 2005
Abstract In the present work, 120 keV N-ion doped and 30 MeV C60 ion irradiated graphite-like-carbon samples were characterized by RBS, micro-FTIR, micro-Raman, XPS spectroscopy and the variation of the properties of the samples with the N-dopant and/or C60 irradiation fluence have been studied. The RBS spectra showed that C60 irradiation can induce a partial diffusion of N atoms to the surface and the amount of the diffused N atoms increases slightly with increasing C60 irradiation fluence. The FTIR and Raman spectra exhibit characteristic bands of carbon nitrogen bonds showing that the C and N atoms are chemically bonded. The amount of chemically bonded C and N atoms increases with increasing N-dopant. By deconvolution of the XPS spectra, the atomic concentration of N and C atoms were obtained and it was identified that the samples mainly consist of three phases, namely, C3N4, CNx and tetrahedral amorphous carbon. The effect of N-dopant and C60 irradiation fluence on the modification of the properties of the samples is also discussed. 2004 Elsevier B.V. All rights reserved. PACS: 61.80.Jh; 61.10. i; 82.80.Yc Keywords: Ion implantation and irradiation; Graphite-like-carbon; Phase change; Carbon-nitrides
1. Introduction
q
Supported by NSFC (Project Nos. 10125522, 10175084) and Chinese Academy of Sciences. * Corresponding author. Tel.: +86 931 496 9330/9331; fax: +86 931 827 2100. E-mail address: [email protected] (Z.G. Wang).
Experimental results showed that energetic ion induced phase change in a solid could be realized not only by irradiation to high fluences [1–3] but also by huge electronic excitations induced by individual ion impacts [4]. The phase change produced in the latter condition is just along the ion latent
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.12.052
Z.G. Wang et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 262–268
tracks. Recently, we have proposed a novel technique [5], ‘‘low energy ion implantation + swift heavy ion irradiation’’, for synthesizing new structures in atom mixed materials in which more attention was paid to the dense electronic excitations effect induced by the incident ions. By use of this technique, swift heavy ion irradiation induced formation of blue-violet emission bands in C-doped SiO2 films [5,6] and some evidences of new chemical bonds and new phase of carbon-nitrides in N-doped diamond-like-carbon films [7] have been observed. In the present work, the novel technique was used to study the modification of N-doped graphite-like-carbon (NGC) under 30 MeV C60 ion irradiations, especially to look for evidence for formation of the theoretically predicted superhard carbon nitrides [8,9]. In the following, the experimental procedure is described first, then the obtained results are presented and the modification of NGC induced by dense electronic excitations is discussed.
2. Experimental The schematic experimental procedure is shown in Fig. 1. The original samples used in the experiments were high purity (>99.99%) graphitelike-carbon (GC). The GC samples were first implanted at room temperature (RT) with 120 keV N-ions and then irradiated at RT with 30 MeV C60 ions. The implantation was performed at the
263
200 KV heavy ion implanter of IMP (Lanzhou, China) and the implantation doses were from 2 · 1017 to 1.2 · 1018 N/cm2. The irradiation was carried out at the 15 MV Tandem of IPNO (Orsay, France) and the irradiation fluence ranged from 1 · 1010 to 6 · 1011 C60/cm2. Using the TRIM code [10], we got that the theoretical peak position of Nconcentration distribution is about 216 nm and the projectile range of C60 ions is about 768 ± 65 nm; thus the C60 ions pass through the region of Nion dopant. The electronic and nuclear energy losses of the C60 ions, Se and Sn, are 56.0 keV/ nm and 1.0 keV/nm at the ion bombarded surface and 43.1 keV/nm and 1.4 keV/nm in the N-ion doped region. After the 30 MeV C60 ion irradiations, the samples were investigated at RT by use of the following techniques: Rutherford backscattering spectroscopy (RBS) analysis using a 2 MeV 4 He++ beam was performed at the 2 · 1.7 MV Tandem of Pekin University. The beam spot was 0.8 · 1.2 mm2 and the scattering angle was 164; Micro-Raman spectra were carried out on a JYT64000 Raman Spectrometer, with 488 nm excitation light from an Ar-laser, with a beam spot of / 2 lm; Micro-Fourier transform infrared spectroscopy (micro-FTIR) analysis was performed on a Spectrum GX (Perkin ElmerTM) with a beam spot
3. Results and discussion 3.1. Micro-FTIR analysis
Fig. 1. Scheme of experimental procedure.
From micro-FTIR measurements it was found that a broad IR absorption band covering 750– 2300 cm 1 formed in the samples as-implanted with and without C60 irradiations. For the as-implanted samples, the peak position of the main IR absorption band shifted to lower frequency with an increase of the N-ion implantation dose (see Fig. 2(a)). For the C60 ion irradiated 5 · 1017 N/cm2 doped samples, the peak position of the main IR absorption band was nearly constant (centered at about 1700 cm 1) but the
264
Z.G. Wang et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 262–268
Fig. 2. Typical micro-FTIR spectra varying with N-ion implantation dose (a) and C60-ion irradiation fluence ((b) and (c)). The sample conditions are given in the figure.
relative IR absorption intensity decreased with an increase of the C60 ion irradiation fluence (Fig. 2(b)). When the irradiation fluence is larger than 1 · 1011 C60/cm2, a shoulder within 970– 1260 cm 1 and an IR absorption peak around 2220 cm 1 were observed. For the C60 ion irradiated 1.2 · 1018 N/cm2 doped samples, the peak intensity of the IR absorption centered at about 2220 cm 1 increased with the C60 irradiation fluence and dual IR absorption peaks were observed when the irradiation fluence was larger than 1 · 1011 C60/cm2 (Fig. 2(c)). The barycenter of the main IR absorption band shifted to lower frequency with an increase of the C60 irradiation fluence. From the literature [11–18] we known that the peak at 2200 cm 1 is from the C„N triple bond,
the main IR absorption band covering 800 and 1800 cm 1 can be attributed to a mixture of N– sp3C, N–sp2C, sp2C@C and C@N bonds and C– O bond. For the irradiation with 1 · 1011 C60/ cm2, a large ratio of N–sp3C, N–sp2C and C–O bonds was obtained. 3.2. Micro-Raman spectroscopy measurement The obtained micro-Raman spectra showed that Raman active but IR prohibited G (ÔGraphiticÕ or sp2–hybridized carbon, at 1580 cm 1) and D (ÔdisorderÕ or sp3–hybridized carbon, at 1360 cm 1) bands were clearly observed from the original GC sample. For N-doped GC samples without C60 ion irradiation and the 5 · 1017 N/cm2 doped GC samples with C60 ion irradiation, there
Z.G. Wang et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 262–268
265
18
1. 2x10 N/ cm 2 doped GC
400
(b)
200
(a)
(0) Original GC (1) unirradiated 10
(2) 5 x10 C60/cm2 11
(3) 1 x10
150
C60/cm2
Intensity (a.u.)
Intensity (a.u.)
300
11
200
(4) 6 x10 C60/cm2
(4 )
100
(3 )
100
50
(2 )
(0 )
0 500
1000
1500
(1 )
2000
0 500
2500
1000
-1
1500
2000
2500
-1
Wavenumber (cm )
Wavenumber (cm ) 400
(C)
Intensity (a.u.)
300
200
100
0 500
1000
1500
2000
2500
-1
Wavenumber (cm )
Fig. 3. Typical micro-Raman spectra of 1.2 · 1018 N/cm2 doped GC samples varying with C60-ion irradiation fluence (a). The sample conditions are given in the figure. (b) and (c) are the deconvolution of the curves (3) and (4) shown in (a), respectively.
was one broadened Raman peak covering the G and D bands and the shape of the Raman peak was nearly the same for different treatments. For the 1.2 · 1018 N/cm2 doped GC samples with C60 ion irradiation, the shape of the Raman spectra change from a broadened Raman peak to two peaks with an increase of the C60 irradiation fluence (see Fig. 3(a)). The Raman spectra were analyzed by deconvolutions (e.g. Fig. 3(b) and (c)) and the deconvolved peaks and FWHMs (full width at half maximum) are given in Table 1. The deconvolved Raman peak at about 630–710 cm 1 can be assigned to the out-of-plane bending mode for graphite-like domains in which a number of C atoms are substituted by N atoms or to the C–N bond in b-C3N4 [19]. The broad luminescence peak
centered at about 2050–2100 cm 1 may be from water absorption or sp1 N triple bonds. It is well known that the G band originates from the symmetric E2g vibration mode in graphite-like materials, while the D band arises from the limitations in the graphite domain size induced by grain boundaries or imperfections [20], e.g. substitutional N atoms, sp3C, or other impurities. From the literature we may think that the deconvolved Raman peaks in the ranges of 1094– 1260 cm 1, 1300–1380 cm 1, 1400–1500 cm 1 and 1550–1650 cm 1 correspond to N–sp3C, sp3C–C, N–sp2C and sp2C@C bonds, respectively. With an increase of the C60 irradiation fluence, the relative amount of N–sp3C and sp3C–C bonds increases, and that of the N–sp2C and sp2C@C
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Table 1 Deconvolution Raman peaks and FWHMs (full width at half maximum) of some typical micro-Raman spectra Element
Peak position
A B C D E F
634 685 688 706 645
(750) (588) (437) (508) (600)
1360 1335 1303 1258 1206 1094
(40) (456) (352) (320) (348) (250)
18
(FWHM)
(cm 1)
1501 1449 1427 1406 1371
1580 1577 1561 1557 1564 1554
(196) (243) (277) (344) (281)
(22) (134) (149) (162) (152) (104)
2
18
1600 (70) 2
Element A is the original GC sample, B is 1.2 · 10 N/cm doped GC sample, C, D, E and F are 1.2 · 10 N/cm doped GC samples with 1 · 1010, 5 · 1010, 1 · 1011 and 6 · 1011 C60/cm2 irradiations, respectively.
bonds decreases. The increase of single bonds is a key factor for the formation of C3N4. It must be pointed out that no irradiation induced C„N bonds formation (Raman peak at 2200 cm 1) was detected clearly in any of the samples.
Table 2 Atomic concentration of N-dopant, cN, at the near surface of the C60 ion irradiated 1.2 · 1018 N/cm2 doped GC samples Element
cN (at.%)
U (C60/cm2) 0
1 · 1010
5 · 1010
1 · 1011
6 · 1011
1.10
0.91
0.97
1.08
1.29
3.3. XPS measurement Fig. 4 shows C1s (a) and N1s (b) X-ray photoelectron spectra of 1.2 · 1018 N/cm2 doped GC samples after C60-ion irradiation to different fluences. By deconvolution of the C1s and N1s core level spectra, the atomic concentration of N and C atoms were obtained. Table 2 shows the atomic concentration of N-dopant, cN, at the near surface of the samples, obtained from the intensities of the
Fig. 4. C1s (a) and N1s (b) XPS spectra of 1.2 · 1018 N/cm2 doped GC samples varying with C60-ion irradiation fluence. The sample conditions are the same as those given in Fig. 2(c).
N1s to C1s lines with a correction for sensitivity factors for N and C atoms. The cN values given in Table 2 imply that C60-ion irradiation results in release of the implanted N atoms and enhanced the N-dopant diffusion toward to the sample surface. Furthermore, the N1s core peak located at 398–402 eV which implies that CNx formed in the NGC samples (N1s peaks for N–C, N@C and N„C bonds are around 398.3–399.3 eV, 399.8–400.9 eV and 398.1–398.4 eV, respectively [21]). Table 3 shows the deconvolved peaks and the FWHMs of C1s core level spectra. Three peaks were identified at 285.0 ± 0.1 eV (C1), 286.3 ± 0.3 eV (C2) and 288.3 ± 0.4 eV (C3, weak) which are believed to come from the tetrahedral amorphous carbon (ta-C), N@sp2C (triagonal CN) and N–sp3C (tetrahedral CN, e.g. C–N in b-C3N4) bonding, respectively [16,22]. There is an optimum C60 irradiation fluence for sp2/sp3 hybridized carbon atoms formation. Below this fluence, the relative amount of sp2/sp3 bonds increases with the increase of C60 fluence but above it decreases, which implies that too high irradiation could destroy the previously formed sp2/sp3 bonds. Therefore, the variation of sp2/sp3 bonds with the irradiation fluence should be a key factor for the appearance of the shoulder within
Z.G. Wang et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 262–268 Table 3 Deconvolution peaks and FWHMs of C1s core level spectra of 1.2 · 1018 N/cm2 doped GC samples Element
Peak
Eb (eV)
FWHM (eV)
Relative density (at.%)
(a)
C1 C2 C3
284.9 286.4 287.7
1.6 1.4 1.0
76.9 19.0 5.1
(b)
C1 C2 C3
285.0 286.5 288.7
1.9 3.5 2.6
74.0 21.8 4.2
(c)
1
C C2 C3
284.9 286.0 288.2
2.0 3.2 3.2
61.7 33.7 4.6
(d)
C1 C2 C3
285.0 286.3 288.2
2.0 2.6 2.9
66.3 27.4 6.3
(e)
C1 C2 C3
285.0 286.5 288.5
1.8 2.2 2.5
73.3 22.1 4.6
Element (a) is the N-doped GC sample, (b), (c), (d) and (e) are N-doped GC samples with 1 · 1010, 5 · 1010, 1 · 1011 and 6 · 1011 C60/cm2 irradiations, respectively.
267
C3N4, CNx and ta-C. Furthermore, an optimized C60 irradiation fluence for sp2/sp3 C–N bonds formation was observed and it was seen that too high irradiation can destroy the previously formed sp2/ sp3 bonds.
Acknowledgments We are very grateful to Dr. Limin Zhao (IMP, Lanzhou) for N ion implantation, to Prof. S. Della-Negra and the Tandem group of INPO (Orsay) and Dr. M. Toulemonde (CIRIL, Caen) for C60 ion irradiation, to Dr. Hongji Ma (Peking University, Beijing) and the support of Institute of Heavy Ion Physics of Pekin University for RBS analysis, to Prof. E.Q. Xie (Lanzhou University, Lanzhou) for helpful discussions. One of the authors (Z.G. Wang) gratefully acknowledges the support of K.C. Wong Education Foundation, Hong Kong.
References 970–1260 cm in Fig. 2(c).
1
in the obtained FTIR spectra given
3.4. RBS analysis The obtained RBS spectra showed that part of the implanted N atoms diffused to the sample surface with or without C60 irradiation. After C60 irradiation, the N-peak content decreased slightly with increasing irradiation fluence. This is in agreement with the FTIR and XPS analysis.
4. Conclusion Modification of N-doped GC samples induced by 30 MeV C60 ion irradiations has been studied. The results show that C60 irradiation can induce CN bonds formation, N atoms partially diffuse to the sample surface and the amount of diffused N atoms increases slightly with increasing C60 irradiation fluence. By deconvolution of the XPS and Raman core level spectra, it was identified that the samples mainly consist of three phases, namely,
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