Proceeding of IWEPNM 2000

Bundle Effects of Single-Wall Carbon Nanotubes
H. Katauraa, Y. Maniwaa, S. Masubuchib, S. Kazamab, X. Zhaoc, Y. Andoc,
Y. Ohtsukad, S. Suzukid, Y. Achibad and R. Saitoe
aDepartment of Physics, Tokyo Metropolitan University, Hachiohji, Tokyo 192-0397, Japanb
Department of Physics, Chuo University, Kasuga Bunkyo-ku, Tokyo 112-8551, Japanc
Department of Physics, Meijo University, Shiogamaguchi Tempaku-ku, Nagoya 468-8502, Japand
Department of Chemistry, Tokyo Metropolitan University, Hachiohji, Tokyo 192-0397, Japane
Department of Electronics Engineering, University of Electro-Communications,Chofu, Tokyo, 182-8585, JapanAbstract. To see the bundle effects on the electronic and the vibrational properties of singlewall carbon nanotubes (SWNTs), we have measured the resonance Raman scattering of isolated
SWNTs and thick bundles. For the measurements of the isolated SWNTs, we used an evacuatedsample after bromine doping. A broad and asymmetric tangential mode band, which is a sign ofthe resonance of the metallic SWNTs and can be fitted by a Fano line shape, is not observed in
the isolated SWNTs. This suggests the inter-tube interactions play an important role to the Fanointerference. On the other hand, the purified sample shows very thick acquired bundles. Weobserved 4% higher breathing mode frequencies than in the pristine sample. Further, in the caseof multi-wall nanotubes, we observed 5% higher breathing mode frequencies than the SWNTs.
These results can be explained by the interlayer interactions.

INTRODUCTION
Pristine arc samples contain many isolated single-wall carbon nanotubes (SWNTs)
as well as bundles. (1) Thus, in greater or less degree, measured optical responses areregarded as mixtures of signals from the isolated SWNTs and the bundles. If it ispossible to wipe off the signal from the bundles, we can get information about opticalproperties of the isolated SWNTs. To realize this, we used bromine doped SWNTs. If
the isolated SWNTs do not have any stable site for bromine molecules, the evacuation
can remove the bromine molecules surrounding isolated SWNTs. Recently, Kazaoui etal. have shown that the bromine doped SWNTs do not show any absorption bandbelow 1.8 eV. (2) If we measure the Raman spectra of the evacuated sample byexcitations lower than 1.8 eV, we will obtain the Raman spectra only of the isolated
SWNTs. Since the charge transfer suppresses the optical transitions in bundles, theRaman signals from the bundles can be wiped off.
Some theoretical works predicted the radial breathing mode (RBM) frequencies arestrongly modified by the bundle effects.(3–5) We know that highly purified samples
show very thick bundles. The bundle effects should be observed more clearly in thepurified sample than in the pristine ones. Further, it is expected that the interlayerinteractions in multi-wall nanotubes should be stronger than the inter-tube interactions
in SWNT bundles. We will discuss about these results.

Proceeding of IWEPNM 2000

EXPERIMENTAL
Samples for a doping were prepared by a conventional electric arc method using
NiY catalyst.(6) HRTEM revealed that a considerable amount of isolated SWNTs areexisting in the sample. To avoid a doping inside SWNTs, pristine soot was installed
into a quartz ampoule without any purification. Saturated bromine vapour wasintroduced to the ampoule at room temperature. After keeping one hour, the ampoule
was evacuated by a rotary pump. Then the ampoule was sealed off.
Thick bundles of SWNTs were prepared by the purification of a SWNT sample
fabricated by the laser ablation method using NiCo catalyst.(7) The purification wasdone by 15% H2O2 reflux for six hours.(8) Multi-wall nanotube sample was preparedby the carbon arc in hydrogen gas.
Raman spectra were measured using a JOBIN YVON U1000 double
monochromator and HAMAMATSU PHOTONICS photon counting system interfacedto a personal computer. Ar+, dye and Ti-sapphire lasers were used for the excitation.

RESULTS AND DISCUSSION
For 488 nm excitation, the fully bromine doped SWNTs show only one RBM bandat 260 cm-1, which is consistent with the result by Rao et al.(9) After the evacuation,
we observe two RBM bands at 180 and 240 cm-1. We confirmed that the two-bandstructure is stable up to 200 °C in vacuum. The band at 180 cm-1 is nearly the same to
the RBM of the pristine sample and can be observed by any excitation energies incontrast with the 240 cm-1 band which has no intensity for excitations lower than 1.8eV. This means that the sample consists of two kinds of SWNTs. One has stable

In te nsity (a rb. units)

1.78 eVS W N T bu ndle s+Isolated S W N T s
P ristin e

E va cu ated a fte r
B r d oping
2Isolate d S W N T s

1300

1400

1500

1600

1700

1800

-1

R am an sh ift (cm )FIGURE 1. Near infrared Raman spectraof pristine and bromine doped SWNTs.
Dashed curve indicates the fitted Fanoline shape. Here, ω0 = 1548 cm-1, 1/q =−0.25 and Γ = 27 cm-1.

FIGURE 2. TEM images of (a) pristine and (b)purified SWNTs fabricated by the laser ablation
method using NiCo catalyst.

Proceeding of IWEPNM 2000

bromine sites, and the other one does not have. We think the former should be thebundles and the latter the isolated SWNTs. We conclude that the 240 cm-1 band is theRBM of the partially doped SWNT bundles and the 180 cm-1 band is that of de-doped
portions in the sample namely, the isolated SWNTs.
Now we can measure the Raman spectra of the isolated SWNTs by near infraredlaser excitations. Figure 1 indicates the high-frequency Raman spectra of the pristineand the isolated SWNTs by the excitation in the metallic window of the sample.
(10,11) In the case of the pristine sample, the spectrum can be fitted well by some
Lorentzians and a Fano line shape, which is the sign of the resonance of metallic
SWNTs. Surprisingly, however, the Fano line shape cannot be observed anymore in
the case of the isolated SWNTs. The tangential modes can be fitted simply by three
Lorentzians, instead. If the Fano line shape is originating from the metallic bundles
and three Lorentzians are from the isolated SWNTs, the both Raman spectra can beexplained simply. This result indicates that the Fano interference strongly connectswith the inter-tube interactions. Indeed, the coupled phonon mode in the Fano line
shape is 45 cm-1 lower than the tangential mode of the isolated SWNTs. Thisfrequency shift is the similar value to the inter-tube mode frequency. (3)To see the bundle effect on the RBM frequencies we measured the resonanceRaman spectra of isolated, pristine and purified SWNTs. Typical change in bundle
diameter by the purification is indicated in Fig.2. TEM images show typical bundle
diameters of the purified sample are larger than 50 nm. Figure 3 shows the low
frequency spectra of each sample in the metallic window. Each sample shows similar
resonance feature but RBM frequencies of the purified sample are 4% higher than thepristine one. This is consistent with the theoretical predictions of the bundle effect on
the RBM frequencies.(3-5) Although the RBM frequencies of the isolated SWNTs areslightly higher than those of the pristine sample, this may be the effect of the bromine
(a)

(b)

(c)

E vacuated after B r doping
2181 cm

Isolated

C orrected intensity (arb. un its)

N iC o laser purified

N iC o laser pristineM e diu m bu nd le-1
179 cm-1

T hick bu nd le1.78 eV

1.75

1.72
1.69
1.67

100

150-1

Intensity (a rb. un its)

168 cm

1 64 .5 cm-1

1.78 e V
1.75

1.63

1 .67
1 .65
1.63

250-1-1-1

1.78 e V
1 .75
1.72
1.69

1.69

200

185 cm1.721.65

R am an shift (cm )1 72 .5 cm

Intensity (arb. units)

N iY arc

100

1502001 .67
1 .65
1.63250-1

R am an shift (cm )

1 00

1 50

2 00

2 50
-1

Ram an shift (cm )FIGURE 3. Resonance Raman spectra of (a) isolated SWNTs, (b) mixture of isolated SWNTs andmedium bundles, and (c) thick bundles of SWNTs.

Proceeding of IWEPNM 2000

molecules remaining around the isolated SWNTs. Further, we found a interesting
phenomenon. In the cases of the isolated SWNTs and the purified sample, RBM
frequencies are independent of the excitation energy. In the case of pristine sample,
however, the RBM peak position is strongly depending on the excitation energy. This
strange result can be explained by the bundle effects by taking into account the factthat the pristine sample is a mixture of the isolated SWNTs and the bundles. The
isolated SWNTs have lower RBM frequencies (3) and narrower electronic bandwidththan the bundles.(12) When the excitation energy is just the energy gap of a certain
kind of SWNTs, isolated SWNTs are resonated predominantly. Thus, the RBM peak
position locates the lowest frequency with the highest intensity. When the excitation ishigher or lower than the energy gap, bundles are mainly resonated due to the broader
band feature. Thus, RBM peak shows up-shift with decreasing the intensity. In the caseof the purified sample, most of the SWNTs are inside of thick bundles. Consequently,
the RBM frequency is stably high. Apparently, there is no cause to move the RBM
peak of the isolated SWNTs.
Multi-wall carbon nanotubes produced by carbon arc in hydrogen gas show RBM
peaks.(13,14) The peak frequencies are about 5% higher than the corresponding RBMsof SWNTs. Although there is no theoretical calculation about interlayer interaction in
MWNTs, it is reasonable that the interlayer interaction in the MWNT is larger thaninter-tube interactions in the SWNT bundle.

ACKNOWLEDGEMENTS
The authors thank Mr. Misaki for taking HRTEM photographs of SWNT samples.
This work was supported in part by Japan Society for Promotion of Science, Researchfor the Future Program. This work was supported in part by Grant-in-Aid for Scientific
Research on the Priority Area "Fullerenes and Nanotubes" by the Ministry ofEducation, Science, and Culture of Japan.

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