AFM and STM investigation of carbon nanotubes produced byhigh energy ion irradiation of graphite

L.P. Bir�o a,b,*, G.I. M�ark a, J. Gyulai a,c, K. Havancs�ak d, S. Lipp c,e, Ch. Lehrer c,L. Frey c, H. Ryssel c

a Research Institute for Technical Physics and Materials Science, P.O. Box 49, H-1525 Budapest, Hungaryb Facultes Universitaires Notre Dame de la Paix, LASMOS, Rue de Bruxelles 61, 5000 Namur, Belgium

c FhG-Institut f�ur Integrierte Schaltungen, Schottkystr. 10, 91058 Erlangen, Germanyd E�otv�os University, Institute for Solid State Physics, H-1088, M�uzeum Krt. 6-8, Hungary

e FEI-Europe GmbH, Bretonischer Ring 16, 85630 Grasbrunn, Germany

Abstract

Carbon nanotubes (CNT) were produced by high energy, heavy ion irradiation (215 MeV Ne, 246 MeV Kr, 156 MeV

Xe) of graphite. On samples irradiated with Kr and Xe ions large craters were found by atomic force microscopy, these

are attributed to sputtering. Frequently one or several CNTs emerge from the craters. Some of the observed CNTs

showed a regular vibration pattern. No other carbon based materials, like amorphous carbon or fullerenes were evi-

denced. Focused ion beam cuts were used to compare CNTs with surface folds on graphite. Ó 1999 Elsevier Science

B.V. All rights reserved.

PACS: 61.48.+c; 61.16.Ch; 61.80.Jh; 81.05.Tp; 81.05.Ys; 85.40.Ux

Keywords: Carbon nanotubes; Atomic force microscopy; Scanning tunneling microscopy; Ion irradiation; Vibrating

carbon nanotube; Focused ion beam

1. Introduction

The investigation of carbon nanotubes emergedfrom the ®eld of fullerene research originated bythe work of Smalley, Kroto, and Curl in 1985 [1].In 1991, Iijima observed the ®rst carbon nanotubes

using high resolution transmission electron mi-croscopy [2]. The experimental and theoreticalwork showing the remarkable electronic, me-chanical and quantum properties of these nano-objects has been summarized recently [3,4]. Mostrecently, the room temperature operation of anelectronic device, based on a single wall carbonnanotube ± called ``TUBEFET'' ± was demon-strated [5].

A single wall carbon nanotube is a single sheetof carbon atoms arranged like in graphite andfaultlessly rolled into a cylinder. The typical

Nuclear Instruments and Methods in Physics Research B 147 (1999) 142±147

* Corresponding author. Address: KFKI ± Research Insti-

tute for Materials Sciences, P.O. Box 49, H-1525 Budapest,

Hungary. Tel.: 36-1-395-9220/1968; fax: 36-1-395-9284; e-mail:

biro@mfa.kfki.hu

0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 5 6 5 - 5

diameters of single wall nanotubes are in the rangeof 1 nm, while the multi-wall ones ± built by``encapsulating'' one into to other several single-wall tubes of increasing diameter, in a way that thedistance between the concentric walls is 3.3 �A (i.e.,the same like between the c planes of graphite) ±may have exterior diameters up to 100 nm.

There are three well established methods toproduce large quantities of carbon nanotubes: theelectric arc method [6], the laser ablation method[7], and the catalytic procedure [8]. The ®rst twoare based on the generation of carbon vapors attypically 2000±3000°C, while in the catalytic pro-cedure the building material for the growth ofnanotubes is produced by dehydrogenation ofhydrocarbons at 700°C. These methods yield to-gether with the entangled, single-, or multi-wallcarbon nanotubes other forms of carbon: fullere-nes, bucky-onions, amorphous carbon, graphiticmaterial, etc. The separation of carbon nanotubesis achieved by tedious chemical procedures.

In the present paper we describe a new methodfor the production of carbon nanotubes, which isbased on the irradiation of highly orientedpyrolytic graphite (HOPG) targets with high en-ergy (E>100 MeV) heavy ions. This method yieldsindividual carbon nanotubes of several micronslength without producing amorphous carbon orother unwanted carbon based materials.

2. Experimental results and discussions

HOPG was irradiated with low dose, high en-ergy, heavy ions: 1012 cmÿ2, 215 MeV Ne or 246MeV Kr, or with 1011 cmÿ2 156 MeV Xe. Duringthe same run, HOPG, muscovite mica, and Sisamples were irradiated. HOPG was freshly cleavedbefore irradiation, a special care was taken to avoidany post-irradiation contamination of the samples.

Sample evaluation was done by scanning tun-neling microscopy (STM) and atomic force mi-croscopy (AFM). Although, for several years weinvestigated by STM and AFM in great detail thesurface structures produced on all of these mate-rials [9±11], we never found carbon nanotubes onSi, or muscovite mica, but we regularly foundcarbon nanotubes on the HOPG samples. STM

examination was carried out in ambient atmo-sphere, in constant current mode, using mechani-cally prepared Pt tips. Typical tunneling currentswere in the range of 1 nA, with biases of 100 mV, awide range of scan speeds were used, but slowscans were preferred to obtain better resolution.Contact mode (CM) and tapping mode (TM)AFM were used in ambient atmosphere to imagethe irradiated samples. Tips with large radii ofcurvature, R>100 nm were preferred, which dueto convolution e�ects between the tip shape andthe tube shape facilitated the ®nding of carbonnanotubes in scan windows of 100 lm2.

On the HOPG samples irradiated with Kr andXe ions, craters with diameters in the lm rangeand depths of 20±80 nm were found. A typicalexample is shown in Fig. 1. The volume of themissing material is estimated to be 2 ´10ÿ14 cm3,this corresponds to 2.28´109 carbon atoms. In thevicinity of the crater the height ± i.e., the diameter± of the tubular object which emerges towards theupper side of Fig. 1(a), is 7 nm, its total length is25.4 lm. Assuming that this object has at least twowalls ± a single wall nanotube would collapse atthis diameter [12] ± and taking in account that thediameter decreases continuously towards the endof the tube, one can estimate that the missing Catoms are su�cient for building 20 nanotubes ofsimilar size. A smaller nanotube is seen in Fig. 1(a)emerging towards the lower, left corner.

The crater itself is attributed to sputteringproduced by simultaneous, dense nuclear cascades± Brinkman type cascades [13] ± propagating fromthe bulk of the sample towards the irradiatedsurface. This kind of a cascade may originate as ahigher order event, i.e., after several collisionssu�ered by one of the knocked-on target atoms.To get more insight in this kind of process, theTRIM code [14] was used for modeling the craterproduction. 1 MeV C atoms slowing down incarbon will have a projected range Rp� 1.2 lm,and a radial spread of 1.16 lm at Rp. The energiesdeposited during the slowing down of a 1 MeV Catom in carbon at a location at 1.2 lm from thepoint where the C atom started moving, are asfollows: energy transferred to recoils, Er� 12 eV/ion/�A; energy of lattice vibrations, Ev� 1.5´10ÿ2

eV/ion/�A; and energy deposited by ionization,

L.P. Bir�o et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 142±147 143

Ei� 7.9´10ÿ2 eV/ion/�A. These values show thatwhen the region where the 1 MeV C atoms stop isclose to the sample surface, crater production maytake place, mainly, due to kinematic e�ects. In thecase of simultaneous, dense, nuclear cascades, dueto cooperative e�ects between neighboring sub-cascades, material corresponding to several atomiclayers may be ejected from the sample surface. Thefastest particles immediately leave the region whilethe slower ones generate an ``expanding cloud'' inwhich they may aggregate into carbon nanotubes.After the growth is terminated the nanotubes willcollapse onto the surface of the target and theymay vibrate if excited by the tip of the TM-AFM.The lighter region in Fig. 1 indicates that the

cascade intersected the surface under a directiondi�erent from the normal.

In the case of heavier ions like Kr and Xe thesurface densities of the nanotubes observed byAFM were in the range of 2±3 ´10ÿ3 nanotube/lm2 for the ion doses used. In the case of Ne ionirradiation with the same dose as Kr, the nano-tubes were only rarely observed, but they werefound on every sample investigated in detail. Thesputtering rates given by TRIM simulation, ex-pressed as the ratio of sputtered C atoms to thenumber of incident ions: 6´ 10ÿ5 for Ne, 8.5´10ÿ3

for Kr, and 2.3´ 10ÿ2 for Xe show a similar trend.In Fig. 2 a TM-AFM image of a carbon

nanotube is presented, which originates from a

Fig. 1. TM-AFM image of a sputtering crater on the surface of HOPG irradiated with 246 MeV Kr ions. (a) plane view, the line cut

taken along the white line is shown in (c); (b) three-dimensional view of the same area, the lighter color region extending from the crater

to the lower right corner in (a) is shown as an elevation over the ¯at surface; (c) line cut along the white line in (a), the height di�erence

of the two markers is 22 nm.

144 L.P. Bir�o et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 142±147

crater and shows a regular vibration pattern alongthe tube axis as seen in the line cut. The amplitudeof the oscillation is 0.7 nm, while the associatedspatial period is 194 nm. It has to be emphasizedthat the line cut running on the ¯at HOPG doesnot show any vibration, nor any pro®le takenacross the crater, these observations exclude thatthe vibration pattern observed on the nanotubemay be a consequence of improper imaging con-ditions.

To get a better insight in the way how does theTM-AFM generates the image of a vibrating ob-ject, computer modeling was used. As a ®rst ap-proximation a simple model consisting of acylindrical rod of 36 nm diameter which vibrateswith a period of 51.2 s was used. The rod wasscanned in a window of 50´50 lm2 at a scan fre-

quency of 1 Hz by a tip with a radius of 300 nm.The window was sampled in 256´ 256 pixels, withan averaging of 64 kHz, the value used in the TM-AFM. Similar vibration patterns like those mea-sured experimentally were obtained.

By high resolution STM imaging carbon nano-tubes with an apparent height of 4 �A were found.Taking in account the reduction in the apparentheight of a carbon nanotube laying on HOPG dueto the di�erences in the electronic structure ofHOPG and of the nanotube, and due to the exis-tence of the second tunneling gap between thenanotube and its support, these objects may beidenti®ed with single wall carbon nanotubes.

When cleaving HOPG, surface folds may beproduced. Although such folds are not expected tovibrate when scanned with the AFM, to clearlydistinguish the nanotubes from these folds we usedfocused ion beam (FIB) milling for cutting bothcarbon nanotubes and surface folds. In Fig. 3 twothick, inter-crossing carbon nanotubes are pre-sented. In the left upper inset three locations areshown where cutting attempts were done. One maynote the bright rims around the rectangular holesmilled in the surface. These rims are produced bythe re-condensation of the sputtered material. Inthe lower right inset the rectangular hole milledright at the inter-crossing is shown. A small hole inthe section of the nanotube is marked by A, a moredetailed image is presented in the upper right inset.Unfortunately, the hollows expected inside thecarbon nanotubes are regularly ®lled with thesputtered material. However, the comparison witha partial FIB cut through a surface fold, Fig. 4, veryclearly shows the di�erence between the structureof the nanotubes and that of a surface fold.

3. Conclusions

Carbon nanotubes were evidenced on HOPGirradiated with high energy, low dose, heavy ions.In the case of heavier ions like Kr and Xe largearea, shallow surface craters were found fromwhich frequently emerge one or several nanotubes.Some of the observed nanotubes vibrate duringscanning with an AFM probe. The formation ofcarbon nanotubes from the C atoms sputtered

Fig. 2. TM-AFM image of a sputtering crater and a vibrating

nanotube on HOPG irradiated with 156 MeV Xe ions. The line

cut taken along the white line is shown. The averaged tube

height decreases by 1.2 nm over a distance of 2 lm, the am-

plitude of the vibration is 0.7 nm with a relatively constant

spatial period of 190 nm.

L.P. Bir�o et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 142±147 145

Fig. 4. Scanning electron microscope image of a fold on the graphite surface partially cut by FIB.

Fig. 3. Scanning electron microscope images of carbon nanotubes cut by FIB. Main image: two inter-crossing carbon nanotubes; left

upper inset: locations where FIB cuts were done; right lower inset: inclined view of the cut at the inter-crossing, note the dark region

labeled by A; right upper inset: detail around A.

146 L.P. Bir�o et al. / Nucl. Instr. and Meth. in Phys. Res. B 147 (1999) 142±147

from the shallow craters is proposed. Cross-sec-tional examination of carbon nanotubes and sur-face folds of HOPG clearly showed di�erentstructures for these two kinds of features.

No amorphous deposits were observed, this in-dicates that the method used may prove useful toproduce carbon nanotubes without producingother, unwanted materials like, amorphous carbon.

Acknowledgements

Helpful discussions with Prof. Ph. Lambin andProf. P.A. Thiry of FUNDP Namur, are gratefullyacknowledged. This work was supported in Hun-gary by OTKA T025928, AKP 96/2-637 and AKP96/2-462 grants. L.P. Bir�o gratefully acknowledgesa fellowship from the Belgian SSTC for S&T Co-operation with Central and Eastern Europe.

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