Molecular Crystals and Liquid Crystals, 340 (2000) pp. 757-762.

Resonance Raman Scattering of Br2 Doped Single-Walled Carbon
Nanotube Bundles

HIROMICHI KATAURAa, YOSHINORI KUMAZAWAa,NORIAKI KOJIMAa, YUTAKA MANIWAa, IKUROU UMEZUb,
SHINICHI MASUBUCHIc, SHIGEO KAZAMAc, YOHSUKE OHTSUKAa,
SHINZO SUZUKIa, and YOHJI ACHIBAaaTokyo Metropolitan University, Hachiohji, Tokyo 192-0397, JAPAN; bKonan
University, Higashinadaku, Kobe 658-8501, JAPAN; cChuo University,Bunkyo-ku, Tokyo 112-8551, JAPANWe have measured Raman spectra of bromine doped single-walled carbon
nanotubes (SWNTs) using various laser lines to clarify the electronic states ofthe doped SWNT. In the case of evacuated sample after full doping, twobreathing mode peaks were observed simultaneously by visible laser
excitations. We assigned the higher frequency peak to the doped SWNT
bundles, and the other peak to the undoped portions in the sample. Intensity
ratio between them decreased with decreasing excitation energy, and in theinfrared region, the breathing mode band of the doped bundle was not observed.
These results can be explained by a simple rigid band model.

Keywords: nanotubes; single-walled carbon nanotubes; doping; Raman
scattering; electronic states

INTRODUCTION
Lee et al.[1] have reported that the bromine doped single-walled carbon
nanotube (SWNT) bundles show a remarkable enhancement on conductivity
and a typical metallic character in temperature dependence. At the same time,
Rao et al.[2] have shown that the shift of Raman peak can be used as a effective
scale of the charge transfer between SWNTs and dopants. When we discuss
about Raman spectra of SWNTs, however, we have to take into account thefact that the resonance effect is dominant in this system. Because the sample

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contains many kinds of SWNTs with different chiral indexes that havedifferent electronic states [3], we have to measure the Raman spectra for various
excitation energies to see the detail of the electronic states of SWNTs anddoping effects.

EXPERIMENTAL

SWNTs were prepared by a conventional electric arc method using NiY
catalyst [4]. Transmission electron microscope (TEM) observation revealed thatthe bundle size of the SWNTs was much smaller than SWNTs by laser ablation
method using NiCo catalyst [5] and that considerable amount of individual
SWNTs existed in the sample. The sample for doping was homogenized by theultra sonic agitation. Then it was dried and installed into a quartz ampoule.
Saturated bromine vapor was introduced to the ampoule at room temperature
after evacuation. Then the ampoule was sealed off. We prepared several
samples with different depth of doping. In this report, however, we will showthe results about the sample of which the ampoule was evacuated after doping.
Raman spectra were measured using a JOBIN YVON U1000 double
monochromator and HAMAMATSU PHOTONICS photon counting systeminterfaced to a personal computer. Ar+, dye and Ti-sapphire lasers were usedfor the excitation.

RESULTS AND DISCUSSION

Low frequency resonance Raman spectra for visible to near infrared excitation
are shown in Fig. 1 (a). For visible excitation, two structures around 180 cm-1
and 240 cm-1 were observed. The structure around 180 cm-1 is similar to thebreathing mode of the pristine sample. When we measured the fully doped
sample, we observed only one peak around 260 cm-1, which is consistent with758

RESONANCE RAMAN SCATTERING OF Br2 DOPED SWNT(a)e xcitation
(eV )
2 .73

2.5

Intensity (arb . units )sem icondu ctor
22 .66

O ptical density (arb. unit)m e tal

2 .54

2 .41
2 .08
1 .96
1 .90

pristine
sam ple

1.5

2∆ EF

10.5

1 .78

Intensity ratio I(doped) / I(undoped )

(b)

1 .72
1 .67
01 .63100

150

2002500300

0.5

11.5

22.5

3

P hoton energy (eV )-1

R am a n shift (cm )FIGURE 1. (a) Resonance Raman spectra of bromine doped SWNTs.
Sample was evacuated after full doping. (b) Intensity ratio of breathingmodes between doped and undoped portions. Dotted curve indicates anoptical absorption spectrum of pristine sample. Background absorption
was subtracted.
]

the results of Rao et al.[2 Further, it was found that heating in vacuum up to250 ºC was required to remove the bromine completely. Thus, we concluded
that the structure around 240 cm-1 is the breathing mode of the partially doped
SWNT bundles and the structure around 180 cm-1 is that of undoped portions
namely, individual SWNTs in the sample.
Relative intensity of the breathing mode bands of doped bundles decreased
with decreasing the excitation energy. The peak around 240 cm-1 disappeared
for the near infrared excitation. Intensity ratio between doped and undoped
portions is shown in Fig. 1 (b) as a function of the excitation energy. Opticalabsorption spectrum of the pristine sample is also indicated. It is clearly seen759

H. KATAURA et al.
that the Raman intensity of doped portion arises around the third absorption
peak of the pristine sample. It is already known that the first and second
absorption peaks are due to the optical transition between spike like density ofstates of semiconductor SWNTs in the sample and that the third peak isoriginating from the metallic SWNTs. Further, the integrated intensity ofbreathing mode is roughly proportional to the optical absorption [6]. Thus, ourpresent results indicate that optical transitions lower than 1.8 eV disappear bythe bromine doping. Indeed, Chen et al.[7] have shown that the absorption
peaks of SWNT solution at 0.67 and 1.2 eV disappeared after the bromine
doping.
This result can be explained by a simple rigid band model. Initially, pristine

Intens ity (arb. un its)

1.78 eVSW N T bundles+
Ind ividual S W N T s

P ristine

Eva cuated afterB r dop ing

Ind ividual S W N T s

1 300

1 400

21 500

1 600

1 700

1 800-1

R am a n shift (cm )FIGURE 2. Near infrared Raman spectra of pristine and bromine doped
SWNTs. Conditions of the measurement were the same. Dotted curve
indicates the fitted Fano line shape. Here, ω0 = 1548 cm-1, 1/q = −0.25
and Γ = 27 cm-1.

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RESONANCE RAMAN SCATTERING OF Br2 DOPED SWNT
sample contains both the metallic and semiconducting SWNTs. From theRaman and TEM observations, it was known that the diameters of constituent
SWNTs spread from 1.1 to 1.5 nm and were weighted on 1.3 nm. Energy bandsof SWNTs can be calculated by zone folding method [3]. For example, the firstand second energy gaps of (10,10) SWNT are 1.7 and 3.23 eV, respectively.
On the other hand, energy gaps of semiconductor (14,6) are 0.57, 1.09, 2.29
and 2.58 eV. Charge transfer from SWNT to bromine causes a change of Fermi
energy for both kinds of SWNTs. If the energy shift ∆EF is about –0.9 eV,
optical transitions lower than 1.8 eV will disappear due to the lack of density ofstates. Consequently, we cannot observe any resonance Raman scattering. On
the other hand, energy gaps larger than 1.8 eV are still effective for theresonance Raman scattering. The zone folding calculation indicates thatobserved resonance Raman peaks, excitaed in a range from 2 to 2.7 eV, areoriginating from the doped semiconductor SWNTs [6].
Now we pay attention to the individual SWNTs remaining in the doped
sample. In the case of pristine sample, Fano line shape was observed when theexcitation was at the optical transition of metallic SWNTs [6]. However, we did
not observe Fano line shape for any excitation energy on the bromine doped
SWNTs. Figure 2 shows infrared high frequency Raman spectra of the pristineand bromine doped SWNTs. As we mentioned above, the Raman signal fromthe bromine doped bundle could not be observed at this photon energy. Thus,
the Raman spectrum of the doped sample indicates that of the individualSWNTs. On the other hand, the pristine sample contains both the bundles andindividual SWNTs. Its Raman spectrum seems to be reproduced with asuperposition of asymmetric Fano line shape and sharp Raman lines ofindividual SWNTs. If the Fano line shape is originating only from the metallic
bundles, this result is quite reasonable. However, scanning tunnelingspectroscopy measurements have indicated that armchair SWNT showed
metallic behavior whether it was in bundle [8] or isolated [9]. It is still unknown
why the individual SWNTs do not show Fano line shape.

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Acknowledgements
This work was partially supported by Japan Society for Promotion of Science
Research for the Future Program. This work was partially supported by Grantin-Aid for Scientific Research on the Priority Area "Fullerenes and Nanotubes"
by the Ministry of Education, Science, and Culture of Japan.
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