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Carbon 45 (2007) 2659–2664

The effects of 2 MeV Ag ion irradiation on multiwalledcarbon nanotubes

S. Mathew a, U.M. Bhatta a, J. Ghatak a, B.R. Sekhar a, B.N. Dev a,b,*

a Institute of Physics, Sachivalaya Marg, Bhubaneswar 751 005, Indiab Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

Received 18 January 2007; accepted 2 August 2007Available online 8 August 2007

Abstract

Multiwalled carbon nanotubes were irradiated using 2 MeV Ag2+ ions with fluences 1 · 1015 and 1 · 1016 ions/cm2. The samples werecharacterized by transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The graphene structureof the nanotube wall is found to be damaged upon irradiation. Although at a fluence of 1 · 1015 ions/cm2 the outer morphology of thesamples remain unchanged, samples irradiated with a fluence of 1 · 1016 ions/cm2 show complete destruction of the graphene structure.The amorphous structure produced due to irradiation is found to show an increased number of sp3 hybridized carbon atoms.� 2007 Published by Elsevier Ltd.

1. Introduction

Carbon nanotubes (CNTs) have been the subject ofmany interesting studies since the pioneering work by Iij-ima [1]. These carbon structures find tremendous applica-tions in nano-electromechanical systems [2], molecularelectronics [3] biological sensors [4], etc. Apart from theirapplications, these quasi-one-dimensional systems showmany physical phenomena interesting from a fundamentalscientific point of view [5]. Different types of nanotubes,particularly, single-walled carbon nanotubes (SWCNTs)and multiwalled carbon nanotubes (MWCNTs) attractedconsiderable interest in applications due to their extraordi-nary mechanical and electrical properties along with theone-dimensional structure [6]. MWCNTs consist of severalconcentric cylindrical layers of rolled up graphene sheets.The existence of carbon in sp, sp2 and sp3 hybridizationwith the possibility to obtain systems having different per-centages of carbon bonding makes these systems interest-

0008-6223/$ - see front matter � 2007 Published by Elsevier Ltd.doi:10.1016/j.carbon.2007.08.001

* Corresponding author. Address: Department of Materials Science,Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India. Fax: +91 33 2483 6561.

E-mail addresses: bhupen@iopb.res.in, msbnd@iacs.res.in (B.N. Dev).

ing for both basic as well as applied fields of research [7].The ability of ion irradiation to tailor the size and hybrid-ization in carbon systems makes ion beam treatment of car-bon materials a very promising field [7,8].

The interaction of charged particles with CNTs is oftechnological as well as fundamental interest [5]. Recentreports on magnetism in carbon based materials, such asproton-irradiated highly oriented pyrolytic graphite(HOPG) [9], nitrogen- and carbon-implanted nanodia-mond [10] have stimulated renewed interests in ion irradi-ated carbon systems. In one of our recent studies, wehave observed soft ferromagnetic ordering in proton-irra-diated C60 films [11]. There are many theoretical modelsalso pointing towards magnetism in CNTs [12], some ofthem considering mainly the defects induced in the graph-ene network [13]. The vacancies and adatoms produced byirradiation can induce magnetism in carbon nanotubes.Further, ion irradiation can also induce coalescence [14],welding of CNTs [15] and changes in intra-tube bonding[16] which are important for technological applications.Ion irradiation was also found to be useful in fabricatingtunneling barriers in CNT [17]. A recent study has pointedout the use of ion irradiation in inserting atoms inside aCNT which does not have open ends [18]. When Ni-filled

Fig. 1. A HRTEM image showing the graphene walls of pristineMWCNT and the corresponding transmission electron diffraction(TED) pattern is shown at the inset.

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MWCNT is irradiated with 100 MeV Au7+ ions, latticedamage have been reported for both CNTs and Ni [19].

The mechanical properties, especially the stability alsohave been found to be modified with ion irradiation [20].The stability of CNTs under ion and other charged particleirradiation is an important parameter for the reactor basedand space applications of nanotube devices. Also ion beamirradiation of carbonaceous materials is a powerful tech-nique to obtain non-hydrogenated amorphous carbon (a-C) and new metastable carbon structures [8]. Deteriorationof MWCNT into nano-rod structure consisting of amor-phous carbon have been reported [21].

Adhikari et al. [20] recently reported the thermal stabil-ity of single-walled ion irradiated CNTs, where a few MeVH, He and Ne ions were used. Kim et al. [22] showed theappearance of bamboo-like structures along with an expan-sion of MWCNT diameter under 3 MeV Cl2+ irradiation.Considering these aspects we have irradiated MWCNTswith 2 MeV Ag2+ ions. Apart from the above experimentalworks, there are also a few fundamental interests in ionirradiated CNT systems. It has been theoretically shownthat CNTs can be used for GeV proton channeling [23].Channeling phenomena are also associated with a fractionof dechanneled ions which can cause lattice damage. Thusthe study of ion induced damage in CNTs is important.The behavior and mobility of irradiation induced defectsis a subject of recent theoretical interest [5].

In order to investigate the influence of ion beam inducedchanges on the structure and hybridization, which causechanges in magnetic and physical properties of CNTs, wehave performed this study of CNTs as a function of the flu-ence of 2 MeV Ag2+ ion irradiation. Using high resolutiontransmission electron microscopy (HRTEM) and RamanSpectroscopy we have investigated the structural changesin our MWCNTs. X-ray photoelectron spectroscopy(XPS) has been employed for understanding the natureand strength of the bonding of carbon species formed afterirradiation.

Fig. 2. A HRTEM micrograph of the MWCNT sample irradiated with2 MeV Ag2+ at a fluence of 1 · 1015 ions/cm2 and the inset shows the TEDpattern of the irradiated sample.

2. Experiment

We have used MWCNT samples synthesized by arc discharge method(supplied by Ion Arc Machines India Ltd). A suspension of MWCNTpowder in toluene was made and ultrasonicated for 60 min. To make sam-ples for TEM one drop of the above solution was kept on a carbon coatedcopper grid and dried. For Raman spectroscopy and X-ray photoelectronspectroscopy study three drops of the MWCNT suspension was poured ona pre-cleaned silicon wafer with native oxide and dried.

Ion irradiations were carried out using the 3 MV 9SDH2 tandem Pelle-tron accelerator facility in our laboratory. The MWCNT samples wereirradiated uniformly with a 2 MeV Ag2+ beam. The ion fluences usedfor irradiation were 1 · 1015 and 1 · 1016 ions/cm2. The beam currentduring irradiation was kept at �50 nA. Plan view TEM measurementswere carried out using 200 kV (JEOL 2010) HRTEM with point to pointresolution of 0.19 nm and lattice resolution of 0.14 nm. Raman spectrawere recorded at room temperature in the backscattering geometry usingphotons of 514.5 nm wavelength as exciting radiation, U1000 monochro-mator and CCD detector. XPS measurements were made using an X-rayphotoelectron spectrometer (ESCA 2000) operating with MgKa radiationat a base pressure of 2 · 10�10 mbar during the measurements.

3. Results and discussion

A TEM image of a pristine MWCNT is shown in Fig. 1.The outer diameter of the tube is 13.5 nm with 14 grapheneshells. The graphene walls of a pristine nanotube are seenin the HRTEM image.

The reciprocal space of CNTs consists of annular ringsand tubes. The (100) and (00 2) diffraction spots are themost intense ones. The appearance of other spots like(hk0) depends mainly on the orientation of electron beamwith respect to the tube axis [24]. A selected area diffraction(SAD) pattern from the pristine sample is shown in theinset of Fig. 1. The (002) and (100) spots are clearly visiblein the figure. Crystalline nature of the sample is evidentfrom the presence of lattice images.

A TEM micrograph from a sample, irradiated with2 MeV Ag2+ ions at a fluence of 1 · 1015 ions/cm2, is shownin Fig. 2. The presence of graphene layers are evident from

Fig. 3. HRTEM image of a 2 MeV Ag2+ irradiated MWCNT sample at afluence of 1 · 1016 ions/cm2 and the TED pattern of the irradiated sampleis shown at the inset.

Fig. 4. Raman spectrum from a (a) pristine MWCNT sample andirradiated MWCNT samples at fluences of (b) 1 · 1015 Ag2+ ions/cm2 and(c) 1 · 1016 Ag2+ ions/cm2.

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the HRTEM image although the layers appear to be lessordered. It can be seen that the diameter [Fig. 2] and theouter morphology (from the low resolution image, notshown here) of the tubes remain unchanged. Distortionof graphene layers in comparison with the unirradiatedsample is evident from the above figure, although the over-all graphene structure remains intact. The correspondingTED pattern is shown at the inset of Fig. 2, which showsonly (002) spots.

A TEM micrograph of samples irradiated at a fluence of1 · 1016 ions/cm2 is shown in Fig. 3. A TED pattern fromthis irradiated sample is shown in the inset of Fig. 3.Absence of graphene walls in HRTEM image along withthe appearance of a ring pattern in TED clearly indicatesthe disappearance of the graphene structure.

The first order Raman spectrum of MWCNTs consistsof bond stretching out-of-plane phonon modes in the lowfrequency region (radial breathing modes) [25], in-planebond stretching motion of pairs of sp2 hybridized carbonatoms (G mode) and in-plane breathing mode of A1g sym-metry due to the presence of six fold aromatic rings (Dmode) [26]. Raman spectra from the pristine sample andthe samples irradiated with fluences of 1 · 1015 ions/cm2

and 1 · 1016 ions/cm2 (here after sample A, B and C,respectively) are shown in Fig. 4a–c, respectively. The char-acteristic G peak at 1581 cm�1 and the D peak at1356 cm�1 of MWCNT are clearly visible both in pristineand irradiated samples [26]. A low intensity D peak is usu-ally observed in the Raman spectra of CNTs [26]. The spec-trum is fitted using multiple Lorentzians and a linearbackground as shown in Fig. 4. The D band is inducedby disorder due to finite particle size effect or lattice distor-tion of graphene structure. The ratio of the intensities of Dpeak (I(D)) to G peak (I(G)) is related to the in-plane crys-tallite size as (I(D)/I(G) / 1/La) until the material becomesnanocrystalline graphite [27]. An increase of I(D)/I(G)ratio is expected when a perfect graphene structure breaksdown to nano crystallites. Further increase of disorder candestroy the aromatic ring structure and this ratio of peakintensities found to decrease as suggested by Ferrari and

Robertson [28] in their three stage model for the transfor-mation of graphite to tetrahedral amorphous carbon(ta-C). The I(D)/I(G) ratio increases for the sample B in

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comparison with pristine sample and then decreases afterthe second irradiation (shown later in Fig. 6b). The Gmode in Fig. 4c shows a doublet nature, a mode at1555 cm�1 is seen with 46% of the intensity of the G mode.A broad peak at 1459 cm�1 is also visible in Fig. 4c. Thepossible origins of these peaks will be discussed in the laterpart of the article. The D modes in the irradiated samplesshow a doublet structure. Similar doublet structure of Dmode is reported in 100 keV carbon-irradiated highly ori-ented pyrolytic graphite samples [29]. Nemanich et al.[30] observed double peaks at the D and G positions in acomposite film of graphite and diamond. The decrease ofI(D)/I(G) ratio along with the observation of an extra peakwith considerable intensity near the G peak position indi-cates an increase in the percentage sp3 hybridization inthe irradiated samples [28,31]. To estimate the nature ofhybridization in our samples we have carried out XPSmeasurements.

XPS spectra of the pristine and the irradiated samplesare shown in the Fig. 5. The fitting of the spectrum is doneby a chi-square iteration programme using a convolutionof Lorenzian–Gausian function with a Sherly background.The core level spectra of carbon 1s in all the samples werefitted with three components positioned around 283.6,284.6, and 285.7 eV and a shake up satellite peak at287.7 eV. The peaks observed at 284.6 and 285.7 corre-spond to sp2 and sp3 hybridized carbon atoms in graphite,respectively [32]. We do not observe a noticeable shifting ofbinding energies of C 1s peak position after irradiation.The binding energy positions and FWHMs of all the con-stituent peaks were kept constant for fitting the spectrumfrom the pristine and the irradiated samples as shown inFig. 5. The peak at 283.6 eV can be due to surface contam-inants from the toluene solution used to prepare the sample[33]. Intensity of this peak is found to be decreasing withincreasing ion fluence; the reduction in the intensity canbe due to sputtering of this surface contaminants due toion bombardment.

Fig. 5. XPS spectrum of C 1S from a (a) pristine MWCNT sample andirradiated MWCNT samples at fluences of (b) 1 · 1015 Ag2+ ions/cm2 and(c) 1 · 1016 Ag2+ ions/cm2.

The line shapes of the XPS peaks give informationabout the chemical bonding environments in the sample.An estimate of the sp3 hybridization in each of these sam-ples can be obtained from the ratio of the correspondingsp3 peak area to the total C 1s peak area (excluding thecontribution at 283.6 eV from the contaminant) and isshown in Fig. 6a. An increase of sp3 hybridization withirradiation fluence can be seen from the figure. The inten-sity of the shake up satellite peak is found to be decreasingin the irradiated samples.

The nuclear and the electronic energy loss of 2 MeV Agions in an amorphous carbon target with the density ofgraphite is estimated (using SRIM [34]) to be 0.94 keV/nm and 1.45 keV/nm, respectively. When a charged particlebombards CNT, due to the high thermal and electrical con-ductivity of graphene shells, the dominant mechanism fordefect creation is the knock on atomic displacements dueto kinetic energy transfer [5]. The irradiation induced struc-tural transformation in CNTs are due to defects mainly inthe form of vacancies and interstitials. The thresholdenergy (Ed) required to produce a Frenkel pair in graphenesystem is estimated to be 20 eV [35]. The mechanisms ofdefect production and annealing in CNTs are differentfrom other materials. Those recoiled atoms with energyslightly above Ed can travel more distance in CNTs thanin other solids due to the peculiar structure, i.e. the openstructure inside the graphene walls of CNTs.

The XPS spectra of the irradiated sample show anincrease of sp3 hybridization with increasing fluence[Fig. 6a]. The sample irradiated with 1 · 1016 ions/cm2

has 42% sp3 hybridization. Correspondingly there arechanges in the Raman spectra also. There is an increasein the I(D)/I(G) ratio for the sample irradiated with a flu-ence of 1 · 1015 ions/cm2 as compared to the pristine sam-ple. This increase of I(D)/I(G) ratio can be due to anincrease of disorder in the graphene structure. Although

Fig. 6. The variation of (a) hybridization estimated from XPS spectra(Fig. 5) and (b) the I(D)/I(G) ratio from the Raman spectra (Fig. 4) withirradiation fluence.

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there is disorder present in the system, HRTEM measure-ments on this sample showed that the graphene wall struc-ture has not been destroyed in irradiation at this fluence.Further increase of the irradiation fluence to 1 ·1016 ions/cm2 results in a decrease in the I(D)/I(G) ratio[Fig. 6b]. A peak at 1555 cm�1, near the G peak position,and an extra peak at 1459 cm�1 can be seen in the sampleirradiated with 1 · 1016 ions/cm2 [Fig. 4c]. As suggested byFerrari and Robertson in their three stage model [28], thedecrease in the I(D)/I(G) ratio along with the appearanceof a peak at 1555 cm�1 with considerable intensity clearlyindicates an increase of sp3 hybridization in the sampleirradiated with 1 · 1016 ions/cm2. Earlier observationsregarding the appearance of the Raman modes around1555 cm�1 and 1459 cm�1 in amorphous carbon/dia-mond-like carbon systems are given below. Schroderet al. reported the down shifting of G mode to 1550 cm�1

in diamond-like carbon thin films produced by chemicalvapor deposition [36]. There are reports on the down shift-ing of G peak to lower wavenumber with disorder andfourfold coordination in amorphous carbon samples [37].Anders et al. observed the position of G band at1558 cm�1 for cathodic-arc deposited amorphous hard car-bon [38]. Diaz et al. reported broad peaks at 1555 and1470 cm�1 in carbon films grown by pulsed laser evapora-tion [39]. The appearance of a peak at 1459 cm�1 in theirradiated sample may be due to distortion of the graphenenetwork [36]. Ferrari and Robertson [40] pointed out thatthe appearance a peak at 1450 cm�1 in amorphous dia-mond sample is due to the presence of transpolyacetylenepresent in the sample. Another possible reason for theappearance of Raman modes at 1555 and 1459 cm�1 insample C could be the hydrogenation of the amorphouscarbon produced due to irradiation [37,38,40]. An ideaabout the amount of hydrogen in the irradiated samplein comparison with pristine sample can be estimated fromthe photoluminescence (PL) background for the first orderRaman spectra. The ratio of the slope of the fitted linearbackground of the Raman spectrum to the intensity ofthe G peak can be used as a measure of the bonded H con-tent in the film [41]. An increase in the PL background isnot observed in the Raman spectra of the irradiated sam-ples in Fig. 4. This clearly eliminates the possibility ofhydrogenation in our irradiated samples. XPS and Ramanspectra analysis clearly shows an increase of sp3 hybridiza-tion in the samples irradiated with 1 · 1016 ions/cm2 flu-ence. This, in conjunction with the analysis of the PLbackground of Raman spectra, indicates that the observedenhancement of sp3 hybridization in our irradiated samplescan be due to carbon–carbon bond.

4. Conclusions

Structural stability of multiwalled carbon nanotubesunder 2 MeV Ag2+ irradiation has been investigated. Withincreasing ion fluence, formation of defects leads to thedestruction of the graphene structure of the nanotube wall;

a fluence of 1 · 1016 ions/cm2 causes disintegration ofgraphene structure and the associated amorphous carbonproduced is found to have �42% sp3 hybridized carbonatoms. The diameter of the tubes remains unchanged upto this fluence. The observation of mixed sp2 and sp3

hybridization points towards the possibility of magneticordering in the irradiated samples, a ferromagnetic phaseof mixed sp2 and sp3 pure carbon has been predicted theo-retically [42]. Further experiments to estimate the magneticproperties, thermal stability, band gaps, etc of the ion irra-diated MWCNTs are essential from both fundamental andapplication points of view.

Acknowledgments

We thank Mr. Janardhanan Nair for providing theMWCNT powder. We acknowledge Prof. S. N. Sahu andMr. S. N. Sarangi for Raman spectroscopy measurements,Prof. S. Varma, and Mr. S. K. Choudhury for XPS mea-surements and Prof. P. V. Satyam for useful suggestions.

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