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keV Ag ion irradiation induced damage on multiwalled carbon nanotubes
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 264 (2007) 36–40 www.elsevier.com/locate/nimb
keV Ag ion irradiation induced damage on multiwalled carbon nanotubes S. Mathew a, U.M. Bhatta a, B. Joseph a, B.N. Dev b
a,b,*
a Institute of Physics, Sachivalaya Marg, Bhubaneswar 751 005, India Department of Materials Science, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick Road, Jadavpur, Kolkata 700 032, India
Received 13 April 2007; received in revised form 10 July 2007 Available online 10 August 2007
Abstract The stability of carbon nanotubes (CNTs) under ion irradiation is an important parameter for the performance of CNT devices under extreme conditions of heat, radiation, etc. In order to investigate the stability and the evolution of nature of bonding, multiwalled carbon nanotubes were irradiated using 21 keV Ag ions with fluences of 1 · 1013, 5 · 1013, 7 · 1013 and 1 · 1014 ions/cm2. The samples were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. The graphene wall structure of the nanotube is found to sustain up to a fluence of 7 · 1013 ions/cm2. Further increase of ion fluence leads to the destruction of the graphene wall structure. Increasing ion fluence in irradiation is found to lead to an increased number of sp3 hybridized carbon atoms compared with the pristine sample. 2007 Elsevier B.V. All rights reserved. PACS: 81.07.De; 61.80.Jh; 61.48.+c; 68.37.Lp; 79.60.i Keywords: Nanotubes; Ion irradiation effects; Fullerene and related materials; Transmission electron microscopy (TEM); X-ray photoelectron spectroscopy (XPS)
1. Introduction Carbon nanotubes (CNTs) have generated enormous interest among scientists and engineers since the pioneering work by Iijima [1]. These carbon structures find a lot of applications in nano-electromechanical systems [2], molecular electronics [3], biological sensors [4], etc. Multiwalled carbon nanotubes (MWCNTs) consist of several concentric cylindrical layers of rolled up graphene sheets. The combination of excellent properties inherited from the parent graphene and a set of unique properties due to small dimensions, closed topology and lattice helicity makes * Corresponding author. Address: Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India. Tel.: +91 674 2301058; fax: +91 33 2483 6561. E-mail addresses: [email protected], [email protected] (B.N. Dev).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.08.003
CNTs a novel material [5]. MWCNTs can be used in field effect transistors [6], nano tips for field emission display [7], supercapacitor [8], power electronic devices [9], etc. The interaction of charged particles with CNTs is of technological as well as fundamental interest [10]. Ion irradiation introduces a wide range of defects in a controlled manner and is used to tailor material properties. The existence of carbon in sp, sp2 and sp3 hybridization with the possibility to obtain systems having different percentages of carbon bonding makes these systems interesting for both basic as well as applied fields of research [11]. Recent reports on magnetism in carbon based materials, such as proton-irradiated highly oriented pyrolytic graphite (HOPG) [12], nitrogenand carbon-implanted nanodiamond [13] have stimulated renewed interests in ion-irradiated carbon systems. In one of our recent studies, we have observed soft ferromagnetic ordering in proton-irradiated C60 films [14]. Further, ion
S. Mathew et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 36–40
irradiation can also induce coalescence [15], welding of CNTs [16] and changes in intra tube bonding [17] which are important for technological applications. Ion irradiation was also found to be useful in fabricating tunneling barriers in CNT [18]. The ability of ion irradiation to tailor the size and hybridization in carbon systems makes ion beam treatment of carbon materials a very promising field [11,19]. The mechanical properties, especially the stability, have been found to be modified with ion irradiation [20]. The stability of CNTs under ion and other charged particle irradiation is an important parameter for the reactor based and space applications of nanotube devices. Also ion beam irradiation of carbonaceous materials is a powerful technique to obtain non-hydrogenated amorphous carbon (a-C) and new metastable carbon structures [19]. Adhikari et al. [20] recently reported the irradiation-mediated improvements in thermal stability of single-walled CNTs under a few MeV H and He ions. Deterioration of MWCNT into nano-rod structure consisting of amorphous carbon has been reported [21]. Kim et al. [22] showed the appearance of bamboo-like structures along with an expansion of MWCNT diameter under 3 MeV Cl2+ irradiation. Takahiro et al. very recently reported Ar ion irradiation induced amorphization of various sp2 hybridized carbon systems [23]. In order to investigate the influence of ion beam induced changes on the structure and hybridization which causes the changes in magnetic and physical properties of CNTs, we have performed this study of CNTs as a function of the fluence of ion irradiation. Here we use ions of relatively lower energy (tens of keV), where the efficiency of causing structural damage is higher compared to MeV ions. Using high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy we have investigated the structural changes of MWCNTs under irradiation. X-ray photoelectron spectroscopy (XPS) has been employed for understanding the nature and strength of the bonding of carbon species formed after irradiation. 2. Experimental We have used MWCNT samples synthesized by arc discharge method (supplied by Ion Arc Machines India Ltd.). A suspension of MWCNT powder in toluene was made and ultrasonicated for 60 min. To make samples for TEM one drop of the above solution was kept on a carbon coated copper grid and dried. For Raman spectroscopy and X-ray photoelectron spectroscopy study three drops of the MWCNT suspension was poured on a pre-cleaned silicon wafer with native oxide and dried. Ion irradiations were carried out using the low energy negative ion accelerator facility in our laboratory [24]. The MWCNT samples were irradiated uniformly with a 21 keV Ag beam. The ion fluences used for irradiation were 1 · 1013, 5 · 1013, 7 · 1013 and 1 · 1014 ions/cm2 respectively. The beam current during irradiation was kept at
37
20 nA. TEM measurements were carried out using a 200 kV high resolution transmission electron microscope (JEOL 2010) with point to point resolution of 0.19 nm and lattice resolution of 0.14 nm. Raman spectra were recorded at room temperature in the backscattering geometry using photons of 514.5 nm wavelength as exciting radiation, U1000 monochromator and CCD detector. XPS measurements were made using an X-ray photoelectron spectrometer (ESCA 2000) operating with Mg Ka radiation at a base pressure of 2 · 1010 mbar during the measurements. The binding energy positions in the XPS spectrum were calibrated using the Si 2p core-level line (99.4 eV) of an uncovered portion of the Si substrate used for the study. 3. Results and discussions HRTEM images of pristine and irradiated MWCNTs with fluences of 1 · 1013, 5 · 1013, 7 · 1013 and 1 · 1014 ions/cm2 are shown in Fig. 1. The presence of graphene wall structure can be seen in pristine sample as well as in samples irradiated up to a fluence of 7 · 1013 ions/cm2. In Fig. 1(d) the graphene walls seem to be broken. In Fig. 1(e), for a fluence of 1 · 1014 ions/cm2 the graphene walls of the nanotube are found to be destroyed. XPS spectra of the pristine and the irradiated samples with fluences of 1 · 1013 and 1 · 1014 ions/cm2 are shown in Fig. 2. The fitting of the spectrum is done by a chi-square iteration programme using a convolution of Lorentzian– Gaussian function with a Sherly background. The core level spectra of C 1s in all the samples were fitted with two components positioned around 284.3 and 285.2 eV and a shake-up satellite peak at 287.4 eV. The peaks observed at 284.3 and 285.2 eV correspond to sp2 and sp3 hybridized carbon atoms in graphite, respectively [25]. We do not observe a noticeable shift of binding energies of C 1s peak position after irradiation. The FWHM of the sp2 hybridized peak for both the irradiated samples are found to be increased by 0.3 eV in comparison with the pristine sample. The binding energy positions and FWHMs of all the other constituent peaks were kept constant for fitting the spectra of the pristine and irradiated samples as shown in Fig. 2. The line shapes of the XPS peaks give information about the chemical bonding environments in the sample. An estimate of the sp3 hybridization in each of these samples can be obtained from the ratio of the corresponding sp3 peak area to the total C 1s peak area [26,25]. An increase of sp3 hybridization with irradiation fluence can be seen in Fig. 2. For the pristine sample, the sp3 content is estimated to be 20% and for the sample irradiated with a fluence of 1 · 1014 ions/cm2 it has increased to 29%. Along with the above increase of the sp3 peak, the intensity of the shake-up satellite peak is found to be decreasing with irradiation fluence, as barely noticed in Fig. 2. The intensity of the shake-up satellite peak in the pristine sample is 11% of the total intensity of the C 1s peak. This is reduced
38
S. Mathew et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 36–40
Fig. 1. HRTEM images of (a) pristine MWCNT sample and samples irradiated with 21 keV Ag ions at fluences of (b) 1 · 1013 ions/cm2, (c) 5 · 1013 ions/cm2, (d) 7 · 1013 ions/cm2 and (e) 1 · 1014 ions/cm2.
to 4% in the sample irradiated at a fluence of 1 · 1014 ions/cm2. A decrease of the shake-up satellite peak of CNTs with Ar ion irradiation has also been reported earlier [21]. C 1s shake-up satellite peaks in an sp2 hybridized carbon system originate from the p–p* transition of p electrons in the valence region. From an experiment on photopolymerization of C60, Onoe et al. [27] suggested that the transition probability of the shake-up process remained almost unchanged and the decrease in the intensity of the shake-up satellite is determined by the reduction of p
electrons on a C60 molecule. The intensity of the shakeup satellite is proportional to the product of the transition probability (p–p*) and the number of p electrons on a C60 molecule. In our experiment, the increase in the observed sp3 hybridized component of the C 1s core peak is thus consistent with the decrease in the intensity of the shakeup satellite in the irradiated samples; both indicate an enhancement of C–C r bonding. The first order Raman spectrum of MWCNTs consists of bond stretching out-of-plane phonon modes in the low
S. Mathew et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 36–40
Fig. 2. XPS spectra of C 1s from a pristine MWCNT sample (a) and irradiated MWCNT samples at fluences of 1 · 1013 ions/cm2 (b) and 1 · 1014 ions/cm2 (c).
frequency region (radial breathing modes) [28], in-plane bond stretching motion of pairs of sp2 hybridized carbon atoms (G mode) and in-plane breathing mode of A1g symmetry due to the presence of sixfold aromatic rings (D mode) [29]. The dominant feature in the second order Raman spectrum is the appearance of G 0 band, which is the overtone of the D band [30]. The second order G 0 band is an intrinsic property of two-dimensional (2D) graphene lattice, i.e. this peak is observed even in the case of crystalline graphite where the disorder induced D band is absent because the G 0 band is symmetry-allowed by momentum
39
conservation requirements whereas the D band involves a break down of the in-plane translational symmetry [30]. Raman spectra from the pristine and the irradiated samples are shown in Fig. 3. The characteristic G peak at 1580 cm1 and the D peak at 1350 cm1 of MWCNT are visible both in pristine and the irradiated samples [29]. In the second order spectra, the G 0 band appears at 2700 cm1 which is shown in Fig. 3 (panel c). The D band is induced in the first order scattering process by the presence of finite size effects, vacancies, grainboundaries etc. all of which lower the symmetry of the quasi infinite lattice [30,31]. The ratio of the intensities of D peak (I(D)) to G peak (I(G)) is related to the in-plane crystallite size (La) as I(D)/I(G) / 1/La until the material becomes nanocrystalline graphite [31]. An increase of I(D)/I(G) ratio is expected when a perfect graphene structure breaks down to nano crystallites. In Fig. 3, although the spectra are shown in different panels for three frequency ranges, they were recorded under identical conditions of incident intensity and duration of scan. The I(D)/I(G) ratio for the pristine sample is 0.36 and it is found to be increasing with irradiation up to a fluence of 1 · 1014 ions/cm2. The increase of I(D)/I(G) ratio can be due to an increase of disorder in the graphene structure. These results are apparently correlated with the XPS spectra of the irradiated samples in Fig. 2, which indicates an enhancement of sp3 hybridization. The variation of sp2/ sp3 and I(D)/I(G) ratios with irradiation fluence is shown in Table 1. The presence of 2D graphene structure in the sample can be monitored by the nature of the G 0 band in the second order Raman spectra [20]. In the case of pristine sample
Fig. 3. Raman spectrum showing the D, G and G 0 peaks of (1) pristine MWCNT sample and the samples irradiated at fluences of (2) 1 · 1013 ions/cm2, (3) 5 · 1013 ions/cm2 and (4) 1 · 1014 ions/cm2.
40
S. Mathew et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 36–40
Table 1 The sp2/sp3 and I(D)/I(G) ratios for the pristine sample and their variation for irradiated samples with ion fluence Irradiation fluence (ions-cm2)
sp2/sp3
I(D)/I(G)
0 (pristine) 1 · 1013 1 · 1014
4.0 3.35 2.45
0.36 0.57 1.2
a well defined G 0 band is visible. As the irradiation fluence increases, the intensity of G 0 peak decreases and at a fluence of 1 · 1014 ions/cm2 this peak does not survive as shown in Fig. 3 (panel 4c). This shows the amorphous nature of the samples irradiated at a fluence of 1 · 1014 ions/ cm2. The nuclear and the electronic energy loss of 21 keV Ag ions in an amorphous carbon target with the density of graphite is estimated (using SRIM [32]) to be 1.8 keV/nm and 0.27 keV/nm, respectively. When a charged particle bombards CNT, due to the high thermal and electrical conductivity of graphene shells, the dominant mechanism for defect creation is the knock-on atomic displacements due to kinetic energy transfer [10]. The irradiation induced structural transformation in CNTs are due to defects mainly in the form of vacancies and interstitials. The threshold energy (Ed) required to produce a Frenkel pair in graphene system is estimated to be 20 eV [33]. Those recoiled atoms with energy slightly above Ed can travel more distance in CNTs than in other solids due to the peculiar structure, i.e. the open structure inside the graphene walls of CNTs. Because of the anisotropy of the atomic network along with high thermal and electrical conductivity along the tube axis, the mechanisms of defect production and annealing in CNTs are different from other regular solids and are yet to be understood. 4. Conclusions Structural stability of multiwalled carbon nanotubes under 21 keV Ag irradiation has been investigated. With increasing ion fluence formation of defects leads to the destruction of the graphene structure of the nanotube wall; the breakage of the graphene walls starts at a fluence of 7 · 1013 ions/cm2 leading to the disintegration of graphene walls at 1 · 1014 ions/cm2 fluence. The amorphous carbon structure produced at the highest fluence is found to have 29% sp3 hybridized carbon atoms. Further, experiments to estimate the thermal stability, band gaps, etc. of the ion-irradiated MWCNTs are essential for both fundamental and application points of view. Acknowledgments We thank Mr. Janardhanan Nair for providing the MWCNT powder. We acknowledge Prof. S.N. Sahu and Mr. S.N. Sarangi for Raman spectroscopy measurements and Prof. S. Varma and Mr. S.K. Choudhury for XPS measurements.
References [1] S. Iijima, Nature 354 (1991) 56. [2] P. Poncharal, Z.L. Wang, D. Ugarte, W.A. de Heer, Science 283 (1999) 1513. [3] S.J. Tans, A.R.M. Verschueren, C. Dekker, Nature 393 (1998) 49. [4] A. Star, J.C.P. Gabriel, K. Bradley, G. Gruner, NanoLetters 3 (2003) 459. [5] P.M. Ajayan, Chem. Rev. 99 (1999) 1787; M. Endo, T. Hayashi, Y.A. Kim, H. Muramatsu, Jpn. J. Appl. Phys. 45 (2006) 4883. [6] J. Derakhshandeh, Y. Abdi, S. Mohajerzadeh, H. Hosseinzadegan, E. Asl Soleimani, H. Radamson, Mater. Sci. Eng. B 124–125 (2005) 354. [7] Q.H. Wang, A.A. Setlur, J.M. Lauerhaas, J.Y. Dai, E.W. Seelig, R.P.H. Chang, Appl. Phys. Lett. 72 (1998) 2912. [8] K.H. An, W.S. Kim, Y.S. Park, Y.C. Choi, S.M. Lee, D.C. Chung, D.J. Bae, S.C. Lim, Y.H. Lee, Adv. Mater. 13 (2001) 497. [9] R.P. Raffaelle, B.J. Landi, J.D. Harris, S.G. Bailey, A.F. Hepp, Mater. Sci. Eng. B 116 (2005) 233. [10] For a recent review see A.V. Krasheninnikov, K. Nordlund, Nucl. Instr. and Meth. B 216 (2004) 355. [11] G. Compagnini, S. Battiato, O. Puglisi, G.A. Baratta, G. Strazzulla, Carbon 43 (2005) 3025. [12] P. Esquinazi, D. Spemann, R. Hohne, A. Setzer, K.H. Han, T. Butz, Phys. Rev. Lett. 91 (2003) 227201. [13] S. Talapatra, P.G. Ganesan, T. Kim, T.R. Vajtai, H. Huang, M. Shima, G. Ramanath, D. Srivastava, S.C. Deevi, P.M. Ajayan, Phys. Rev. Lett. 95 (2005) 97201. [14] S. Mathew, B. Satpati, B. Joseph, B.N. Dev, R. Nirmala, S.K. Mallik, R. Kesavamoorthy, Phys. Rev. B 75 (2007) 75426. [15] M. Terrones, H. Terrones, F. Banhart, J.C. Charlier, P.M. Ajayan, Science 288 (2000) 1226. [16] M. Terrones, F. Banhart, N. Grobert, J.C. Charlier, H. Terrones, P.M. Ajayan, Phys. Rev. Lett. 89 (2002) 75505. [17] B. Ni, S. Sinnott, Phys. Rev. B 61 (2000) R16343. [18] M. Suzuki, K. Ishibashi, K. Toratani, D. Tsuya, Y. Aoyagi, Appl. Phys. Lett. 81 (2002) 2273. [19] D.L. Baptista, F.C. Zawislak, Diamond Relat. Mater. 13 (2004) 1791. [20] A.R. Adhikari, M. Huang, H. Bakhru, R. Vajtai, C.Y. Ryu, P.M. Ajayan, J. Appl. Phys. 100 (2006) 64315. [21] Y. Zhu, T. Yi, B. Zheng, L. Cao, Appl. Surf. Sci. 137 (1999) 83. [22] H.M. Kim, H.S. Kim, S.K. Park, J. Joo, T.J. Lee, C.J. Lee, J. Appl. Phys. 97 (2005) 26103. [23] K. Takahiro, A. Terai, S. Oizumi, K. Kawatsura, S. Yamamoto, H. Naramoto, Nucl. Instr. and Meth. B 242 (2006) 445. [24] B. Joseph, A.K. Behera, S. Mohapatra, P. Kuiri, D.P. Mahapatra, Solid State Phys. (India) 46 (2003) 271. [25] J. Daz, G. Paolicelli, S. Ferrer, F. Comin, Phys. Rev. B 54 (1996) 8064. [26] R. Haerle, E. Riedo, A. Pasquarello, A. Baldereschi, Phys. Rev. B 65 (2001) 45101. [27] J. Onoe, A. Nakao, K. Takeuchi, Phys. Rev. B 55 (1997) 10051. [28] J.M. Benoit, J.P. Buisson, O. Chauvet, C. Godon, S. Lefrant, Phys. Rev. B 66 (2002) 73417. [29] M.S. Dresselhaus, G. Dresselhaus, R. Saito, A. Jorio, Phys. Rep. 409 (2005) 47; W.S. Bacsa, D. Ugarte, A. Chatelain, V.A. de Heer, Phys. Rev. B 50 (1994) 15473. [30] M.S. Dresselhaus, P.C. Eklund, Adv. Phys. 49 (2000) 705. [31] F. Tuinstra, J.L. Koening, J. Chem. Phys. 53 (1970) 1126; A.C. Ferrari, J. Robertson, Phys. Rev. B. 61 (2000) 14095. [32] J.F. Ziegler, J.P. Biersack, U. Littmark, SRIM 2003 – A Version of the TRIM Program: The Stopping and Range of Ions in Matter, Pergamon, New York, 2003. [33] F. Banhart, Rep. Prog. Phys. 62 (1999) 1181.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 264 (2007) 36–40 www.elsevier.com/locate/nimb
keV Ag ion irradiation induced damage on multiwalled carbon nanotubes S. Mathew a, U.M. Bhatta a, B. Joseph a, B.N. Dev b
a,b,*
a Institute of Physics, Sachivalaya Marg, Bhubaneswar 751 005, India Department of Materials Science, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick Road, Jadavpur, Kolkata 700 032, India
Received 13 April 2007; received in revised form 10 July 2007 Available online 10 August 2007
Abstract The stability of carbon nanotubes (CNTs) under ion irradiation is an important parameter for the performance of CNT devices under extreme conditions of heat, radiation, etc. In order to investigate the stability and the evolution of nature of bonding, multiwalled carbon nanotubes were irradiated using 21 keV Ag ions with fluences of 1 · 1013, 5 · 1013, 7 · 1013 and 1 · 1014 ions/cm2. The samples were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. The graphene wall structure of the nanotube is found to sustain up to a fluence of 7 · 1013 ions/cm2. Further increase of ion fluence leads to the destruction of the graphene wall structure. Increasing ion fluence in irradiation is found to lead to an increased number of sp3 hybridized carbon atoms compared with the pristine sample. 2007 Elsevier B.V. All rights reserved. PACS: 81.07.De; 61.80.Jh; 61.48.+c; 68.37.Lp; 79.60.i Keywords: Nanotubes; Ion irradiation effects; Fullerene and related materials; Transmission electron microscopy (TEM); X-ray photoelectron spectroscopy (XPS)
1. Introduction Carbon nanotubes (CNTs) have generated enormous interest among scientists and engineers since the pioneering work by Iijima [1]. These carbon structures find a lot of applications in nano-electromechanical systems [2], molecular electronics [3], biological sensors [4], etc. Multiwalled carbon nanotubes (MWCNTs) consist of several concentric cylindrical layers of rolled up graphene sheets. The combination of excellent properties inherited from the parent graphene and a set of unique properties due to small dimensions, closed topology and lattice helicity makes * Corresponding author. Address: Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India. Tel.: +91 674 2301058; fax: +91 33 2483 6561. E-mail addresses: [email protected], [email protected] (B.N. Dev).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.08.003
CNTs a novel material [5]. MWCNTs can be used in field effect transistors [6], nano tips for field emission display [7], supercapacitor [8], power electronic devices [9], etc. The interaction of charged particles with CNTs is of technological as well as fundamental interest [10]. Ion irradiation introduces a wide range of defects in a controlled manner and is used to tailor material properties. The existence of carbon in sp, sp2 and sp3 hybridization with the possibility to obtain systems having different percentages of carbon bonding makes these systems interesting for both basic as well as applied fields of research [11]. Recent reports on magnetism in carbon based materials, such as proton-irradiated highly oriented pyrolytic graphite (HOPG) [12], nitrogenand carbon-implanted nanodiamond [13] have stimulated renewed interests in ion-irradiated carbon systems. In one of our recent studies, we have observed soft ferromagnetic ordering in proton-irradiated C60 films [14]. Further, ion
S. Mathew et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 36–40
irradiation can also induce coalescence [15], welding of CNTs [16] and changes in intra tube bonding [17] which are important for technological applications. Ion irradiation was also found to be useful in fabricating tunneling barriers in CNT [18]. The ability of ion irradiation to tailor the size and hybridization in carbon systems makes ion beam treatment of carbon materials a very promising field [11,19]. The mechanical properties, especially the stability, have been found to be modified with ion irradiation [20]. The stability of CNTs under ion and other charged particle irradiation is an important parameter for the reactor based and space applications of nanotube devices. Also ion beam irradiation of carbonaceous materials is a powerful technique to obtain non-hydrogenated amorphous carbon (a-C) and new metastable carbon structures [19]. Adhikari et al. [20] recently reported the irradiation-mediated improvements in thermal stability of single-walled CNTs under a few MeV H and He ions. Deterioration of MWCNT into nano-rod structure consisting of amorphous carbon has been reported [21]. Kim et al. [22] showed the appearance of bamboo-like structures along with an expansion of MWCNT diameter under 3 MeV Cl2+ irradiation. Takahiro et al. very recently reported Ar ion irradiation induced amorphization of various sp2 hybridized carbon systems [23]. In order to investigate the influence of ion beam induced changes on the structure and hybridization which causes the changes in magnetic and physical properties of CNTs, we have performed this study of CNTs as a function of the fluence of ion irradiation. Here we use ions of relatively lower energy (tens of keV), where the efficiency of causing structural damage is higher compared to MeV ions. Using high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy we have investigated the structural changes of MWCNTs under irradiation. X-ray photoelectron spectroscopy (XPS) has been employed for understanding the nature and strength of the bonding of carbon species formed after irradiation. 2. Experimental We have used MWCNT samples synthesized by arc discharge method (supplied by Ion Arc Machines India Ltd.). A suspension of MWCNT powder in toluene was made and ultrasonicated for 60 min. To make samples for TEM one drop of the above solution was kept on a carbon coated copper grid and dried. For Raman spectroscopy and X-ray photoelectron spectroscopy study three drops of the MWCNT suspension was poured on a pre-cleaned silicon wafer with native oxide and dried. Ion irradiations were carried out using the low energy negative ion accelerator facility in our laboratory [24]. The MWCNT samples were irradiated uniformly with a 21 keV Ag beam. The ion fluences used for irradiation were 1 · 1013, 5 · 1013, 7 · 1013 and 1 · 1014 ions/cm2 respectively. The beam current during irradiation was kept at
37
20 nA. TEM measurements were carried out using a 200 kV high resolution transmission electron microscope (JEOL 2010) with point to point resolution of 0.19 nm and lattice resolution of 0.14 nm. Raman spectra were recorded at room temperature in the backscattering geometry using photons of 514.5 nm wavelength as exciting radiation, U1000 monochromator and CCD detector. XPS measurements were made using an X-ray photoelectron spectrometer (ESCA 2000) operating with Mg Ka radiation at a base pressure of 2 · 1010 mbar during the measurements. The binding energy positions in the XPS spectrum were calibrated using the Si 2p core-level line (99.4 eV) of an uncovered portion of the Si substrate used for the study. 3. Results and discussions HRTEM images of pristine and irradiated MWCNTs with fluences of 1 · 1013, 5 · 1013, 7 · 1013 and 1 · 1014 ions/cm2 are shown in Fig. 1. The presence of graphene wall structure can be seen in pristine sample as well as in samples irradiated up to a fluence of 7 · 1013 ions/cm2. In Fig. 1(d) the graphene walls seem to be broken. In Fig. 1(e), for a fluence of 1 · 1014 ions/cm2 the graphene walls of the nanotube are found to be destroyed. XPS spectra of the pristine and the irradiated samples with fluences of 1 · 1013 and 1 · 1014 ions/cm2 are shown in Fig. 2. The fitting of the spectrum is done by a chi-square iteration programme using a convolution of Lorentzian– Gaussian function with a Sherly background. The core level spectra of C 1s in all the samples were fitted with two components positioned around 284.3 and 285.2 eV and a shake-up satellite peak at 287.4 eV. The peaks observed at 284.3 and 285.2 eV correspond to sp2 and sp3 hybridized carbon atoms in graphite, respectively [25]. We do not observe a noticeable shift of binding energies of C 1s peak position after irradiation. The FWHM of the sp2 hybridized peak for both the irradiated samples are found to be increased by 0.3 eV in comparison with the pristine sample. The binding energy positions and FWHMs of all the other constituent peaks were kept constant for fitting the spectra of the pristine and irradiated samples as shown in Fig. 2. The line shapes of the XPS peaks give information about the chemical bonding environments in the sample. An estimate of the sp3 hybridization in each of these samples can be obtained from the ratio of the corresponding sp3 peak area to the total C 1s peak area [26,25]. An increase of sp3 hybridization with irradiation fluence can be seen in Fig. 2. For the pristine sample, the sp3 content is estimated to be 20% and for the sample irradiated with a fluence of 1 · 1014 ions/cm2 it has increased to 29%. Along with the above increase of the sp3 peak, the intensity of the shake-up satellite peak is found to be decreasing with irradiation fluence, as barely noticed in Fig. 2. The intensity of the shake-up satellite peak in the pristine sample is 11% of the total intensity of the C 1s peak. This is reduced
38
S. Mathew et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 36–40
Fig. 1. HRTEM images of (a) pristine MWCNT sample and samples irradiated with 21 keV Ag ions at fluences of (b) 1 · 1013 ions/cm2, (c) 5 · 1013 ions/cm2, (d) 7 · 1013 ions/cm2 and (e) 1 · 1014 ions/cm2.
to 4% in the sample irradiated at a fluence of 1 · 1014 ions/cm2. A decrease of the shake-up satellite peak of CNTs with Ar ion irradiation has also been reported earlier [21]. C 1s shake-up satellite peaks in an sp2 hybridized carbon system originate from the p–p* transition of p electrons in the valence region. From an experiment on photopolymerization of C60, Onoe et al. [27] suggested that the transition probability of the shake-up process remained almost unchanged and the decrease in the intensity of the shake-up satellite is determined by the reduction of p
electrons on a C60 molecule. The intensity of the shakeup satellite is proportional to the product of the transition probability (p–p*) and the number of p electrons on a C60 molecule. In our experiment, the increase in the observed sp3 hybridized component of the C 1s core peak is thus consistent with the decrease in the intensity of the shakeup satellite in the irradiated samples; both indicate an enhancement of C–C r bonding. The first order Raman spectrum of MWCNTs consists of bond stretching out-of-plane phonon modes in the low
S. Mathew et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 36–40
Fig. 2. XPS spectra of C 1s from a pristine MWCNT sample (a) and irradiated MWCNT samples at fluences of 1 · 1013 ions/cm2 (b) and 1 · 1014 ions/cm2 (c).
frequency region (radial breathing modes) [28], in-plane bond stretching motion of pairs of sp2 hybridized carbon atoms (G mode) and in-plane breathing mode of A1g symmetry due to the presence of sixfold aromatic rings (D mode) [29]. The dominant feature in the second order Raman spectrum is the appearance of G 0 band, which is the overtone of the D band [30]. The second order G 0 band is an intrinsic property of two-dimensional (2D) graphene lattice, i.e. this peak is observed even in the case of crystalline graphite where the disorder induced D band is absent because the G 0 band is symmetry-allowed by momentum
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conservation requirements whereas the D band involves a break down of the in-plane translational symmetry [30]. Raman spectra from the pristine and the irradiated samples are shown in Fig. 3. The characteristic G peak at 1580 cm1 and the D peak at 1350 cm1 of MWCNT are visible both in pristine and the irradiated samples [29]. In the second order spectra, the G 0 band appears at 2700 cm1 which is shown in Fig. 3 (panel c). The D band is induced in the first order scattering process by the presence of finite size effects, vacancies, grainboundaries etc. all of which lower the symmetry of the quasi infinite lattice [30,31]. The ratio of the intensities of D peak (I(D)) to G peak (I(G)) is related to the in-plane crystallite size (La) as I(D)/I(G) / 1/La until the material becomes nanocrystalline graphite [31]. An increase of I(D)/I(G) ratio is expected when a perfect graphene structure breaks down to nano crystallites. In Fig. 3, although the spectra are shown in different panels for three frequency ranges, they were recorded under identical conditions of incident intensity and duration of scan. The I(D)/I(G) ratio for the pristine sample is 0.36 and it is found to be increasing with irradiation up to a fluence of 1 · 1014 ions/cm2. The increase of I(D)/I(G) ratio can be due to an increase of disorder in the graphene structure. These results are apparently correlated with the XPS spectra of the irradiated samples in Fig. 2, which indicates an enhancement of sp3 hybridization. The variation of sp2/ sp3 and I(D)/I(G) ratios with irradiation fluence is shown in Table 1. The presence of 2D graphene structure in the sample can be monitored by the nature of the G 0 band in the second order Raman spectra [20]. In the case of pristine sample
Fig. 3. Raman spectrum showing the D, G and G 0 peaks of (1) pristine MWCNT sample and the samples irradiated at fluences of (2) 1 · 1013 ions/cm2, (3) 5 · 1013 ions/cm2 and (4) 1 · 1014 ions/cm2.
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Table 1 The sp2/sp3 and I(D)/I(G) ratios for the pristine sample and their variation for irradiated samples with ion fluence Irradiation fluence (ions-cm2)
sp2/sp3
I(D)/I(G)
0 (pristine) 1 · 1013 1 · 1014
4.0 3.35 2.45
0.36 0.57 1.2
a well defined G 0 band is visible. As the irradiation fluence increases, the intensity of G 0 peak decreases and at a fluence of 1 · 1014 ions/cm2 this peak does not survive as shown in Fig. 3 (panel 4c). This shows the amorphous nature of the samples irradiated at a fluence of 1 · 1014 ions/ cm2. The nuclear and the electronic energy loss of 21 keV Ag ions in an amorphous carbon target with the density of graphite is estimated (using SRIM [32]) to be 1.8 keV/nm and 0.27 keV/nm, respectively. When a charged particle bombards CNT, due to the high thermal and electrical conductivity of graphene shells, the dominant mechanism for defect creation is the knock-on atomic displacements due to kinetic energy transfer [10]. The irradiation induced structural transformation in CNTs are due to defects mainly in the form of vacancies and interstitials. The threshold energy (Ed) required to produce a Frenkel pair in graphene system is estimated to be 20 eV [33]. Those recoiled atoms with energy slightly above Ed can travel more distance in CNTs than in other solids due to the peculiar structure, i.e. the open structure inside the graphene walls of CNTs. Because of the anisotropy of the atomic network along with high thermal and electrical conductivity along the tube axis, the mechanisms of defect production and annealing in CNTs are different from other regular solids and are yet to be understood. 4. Conclusions Structural stability of multiwalled carbon nanotubes under 21 keV Ag irradiation has been investigated. With increasing ion fluence formation of defects leads to the destruction of the graphene structure of the nanotube wall; the breakage of the graphene walls starts at a fluence of 7 · 1013 ions/cm2 leading to the disintegration of graphene walls at 1 · 1014 ions/cm2 fluence. The amorphous carbon structure produced at the highest fluence is found to have 29% sp3 hybridized carbon atoms. Further, experiments to estimate the thermal stability, band gaps, etc. of the ion-irradiated MWCNTs are essential for both fundamental and application points of view. Acknowledgments We thank Mr. Janardhanan Nair for providing the MWCNT powder. We acknowledge Prof. S.N. Sahu and Mr. S.N. Sarangi for Raman spectroscopy measurements and Prof. S. Varma and Mr. S.K. Choudhury for XPS measurements.
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