Interaction between single-wall carbon nanotubes and encapsulated

C60 probed by resonance Raman spectroscopyw

Soon-Kil Joung,a Toshiya Okazaki,*ab Susumu Okadac and Sumio Iijimaa

Received 5th January 2010, Accepted 7th April 2010

First published as an Advance Article on the web 7th June 2010

DOI: 10.1039/c000102c

The effects of C60 encapsulation on the radial breathing mode (RBM) frequencies of single-wall

carbon nanotubes (SWCNTs) are investigated over a wide range of diameters (dt B1.25–1.5 nm).

The observed frequency shifts show a characteristic behavior depending on the inter-spacing

between C60 and SWCNTs. The present findings clearly indicate the van der Waals nature of the

SWCNT-C60 interaction and an importance of hybridization between the electronic states of

C60 and SWCNTs.

Introduction

Single-wall carbon nanotubes (SWCNTs) have been expected

for the building blocks in future nanodevices due to their

superior electronic properties.1 Because of their tubular

structures, various molecules and atoms can be encapsulated

inside SWCNTs.2–5 The electronic and transport properties of

SWCNTs frequently undergo considerable modification upon

molecular encapsulation, which allows us to finely control

these parameters by alternating the doping species.3,5 A typical

example for such doped SWCNTs is nanopeapods (NPDs),

i.e., SWCNTs encapsulating fullerenes.3 Indeed encapsulated

fullerenes and metallofullerenes have been reported to

change the transport properties of SWCNTs from p-type to

ambipolar in field-effect transistor (FET) structures.6,7

To predict the doping effects on the physical properties of

SWCNTs, it is particularly important to understand the

interaction between SWCNTs and the encapsulated molecules.

Raman spectroscopy is one of the most powerful and

convenient techniques for monitoring interaction with the

encapsulated molecules in SWCNT systems.8 Recently, C60

encapsulation effects on the radial breathing mode (RBM)

frequencies of SWCNTs have been investigated by the present

research group.9 It has been found that the frequency shifts of

RBM phonon strongly depends on the tube diameter (dt);

higher and lower frequency shifts are observed in the case of

smaller diameter tubes (dt o B1.3 nm) and larger diameter

tubes (dt 4 B1.3 nm), respectively.

In the previous study, SWCNTs with dt = 1.2–1.4 nm were

used for investigation. To fully comprehend the interaction

between C60 and SWCNTs, however, SWCNTs having a larger

diameter should be examined. We here report the RBM

frequency shifts upon C60 encapsulations of SWCNTs whose

diameter ranges from 1.3 to 1.5 nm. Further investigation over

a wide range of diameters clearly uncovers the van der Waals

nature of the SWCNT-C60 interaction and the importance of

hybridization between the electronic states of C60 and SWCNTs

in the larger diameter region, which is consistent with the

previous photoluminescence (PL) and theoretical results.10–12

Experimental

Sample preparation

The arc-SWCNTs (Meijo Arc APJ-type, Meijo Nano Carbon

Co. Ltd) were heated at 350 1C for 30 min in air to remove

most of the amorphous carbons and other carbon materials

which coat catalyst metal particles. The obtained SWCNTs

were treated in methanol solution of sodium hydroxide for

30 min and washed by isopropanol for several times. Then we

washed the remaining metal particles with hydrochloric acid

and heated them at 600 1C for 2 h in vacuum.

To open the cap of SWCNTs, purified arc-SWCNTs were

heated at 570 1C for 30 min in air. The treated SWCNTs and

fullerenes were sealed under vacuum (B3.5 � 10�4 Pa) in

quartz tubes and heated at 600 1C for 96 h. The obtained

nanopeapods were washed with toluene to remove the

fullerenes adsorbed on the outside of the walls. After the filtration,

we obtained a dark, paper-like sheet, so-called buckypaper.

The C60 NPDs and the empty SWCNTs were individually

dispersed in the micelle D2O solutions for photoluminescence

(PL) and Raman measurements. Briefly, nanopeapods or

SWCNTs (B1 mg) were dispersed for 10 min. in B20 ml of

D2O containing 1 wt% of dodecylbenzene sulfonate (SDBS)

using 500 W homogenizer (SONICS VCX500) equipped with

a titanium alloy tip (TI-6AL-4V). Each solution was then

centrifuged at 168 000 g for 2.5 h (HITACHI CP 100MX) and

the supernatant of the upper B2/3 volume was used.

Spectroscopic characterizations

The two-dimensional (2D) photoluminescence (PL) mapping

was performed with a Shimadzu NIR-PL system utilizing an

aNanotube Research Center, National Institute of Advanced IndustrialScience and Technology (AIST), Tsukuba 305-8565, Japan.E-mail: toshi.okazaki@aist.go.jp; Fax: +81-29-861-6241;Tel: +81-29-861-4173

b PRESTO, Japan Science and Technology Agency (JST),4-1-8 Honcho, Kawaguchi 332-0012, Japan

c CREST, Japan Science and Technology Agency (JST),4-1-8 Honcho, Kawaguchi 332-0012, Japanw Electronic supplementary information (ESI) available: The observedPL peak positions of the (17, 3) C60 nanopeapods in micelle solution.See DOI: 10.1039/c000102c

8118 | Phys. Chem. Chem. Phys., 2010, 12, 8118–8122 This journal is �c the Owner Societies 2010

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

IR-enhanced InGaAs detector (Princeton instruments

OMA-V2.2) for detection and a tunable Ti-sapphire laser

(Spectra Physics 3900S) for excitation. The slit width for

emission was 10 nm. Typical scan steps were 5 and 2 nm for

excitation and emission, respectively. The raw data were

corrected for wavelength-dependent instrumental factors and

excitation laser intensities.

The resonance Raman spectroscopy was performed with

a triple-grating monochromator (Bunko-Keiki Co. Ltd.

BRM-900) equipped with an InGaAs diode array (Princeton

Instruments OMA-V1.7). A tunable Ti-sapphire laser (Spectra

Physics 3900S) was used as the excitation source.

Theoretical calculations

Details of theoretical calculations were described previously.13

Briefly, the electronic-structure calculation and the geometry

optimizations were performed using the local-density approxi-

mation in the density functional theory (DFT) with the plane

wave basis set and the norm-conserving pseudo-potential.

Results and discussion

Photoluminescence (PL) spectra

Fig. 1A shows a 2D PL map of the unfilled arc-SWCNTs in

SDBS D2O solution as a function of emission (l11) and

excitation (l22) wavelengths. The PL peaks on the map are

clearly seen in the second interband (E22) excitation region

(l22 = 870–1070 nm) and the first interbands (E11) emission

region (l11 = 1500–1800 nm) of SWCNTs with B1.3–1.5 nm

in diameter, which can be assigned to the specific (n, m)

SWCNTs by using the empirical relations of Weisman

et al.14 The well-known ‘‘2n + m’’ family pattern of SWCNTs

is clearly seen in Fig. 1A in which the PL peaks with the same

values (similar dt) are connected.

On the other hand, the PL spectra of C60 NPDs are shown

in Fig. 1B. The totally different PL pattern is an evidence of

high-yield encapsulations. The observed PL positions are

consistent with the previous reports.10,11 The (n, m) SWCNTs

classified as 2n + m = 31, 32, 34, 35 and 37 family members

were contained in the solution. Furthermore, the PL peak

from (17, 3) nanopeapods is newly identified at l11 = 1756 nm

and l22 = 1068 nm in the present study (ESI).w

Resonance Raman spectra

It is well-known that the Raman intensity of SWCNTs

strongly changes with the tube chirality and the excitation

energy due to the resonance effect.8,15 Fig. 2 shows the

resonance Raman spectra of SWCNTs and the corresponding

C60 NPDs in the RBM phonon region under the excitation

from 1.16 to 1.25 eV. Strong excitation energy dependence is a

natural consequence of the chirality distribution of electronic

transitions.

In the spectra of the empty SWCNT sample (Fig. 2A),

prominent RBM phonon peaks are observed at around 170

and 180 cm�1 under excitations ofB1.17 eV (= 1060 nm) and

B1.25 eV (= 990 nm), respectively. Based on the previous

two-dimensional RBM phonon intensity map9 and the PL

spectra (Fig. 1A), it is highly likely that SWCNTs with the

2n + m = 34 family largely contribute to the RBM spectrum

under an excitation energy of B1.25 eV. Indeed, curve fitting

analyses show that the Raman spectrum for 1.25 eV was

mainly composed of the 2n +m = 34 ((13, 8), (14, 6),

(15, 4), (16, 2) and (17, 0)) tubes (Fig. 3A, lower panel), where

the obtained RBM frequencies of (14, 6), (15, 4), (16, 2)

and (17, 0) match those reported in ref. 9. In addition to

the RBM peaks of the 2n + m = 34 tubes, the other

peaks were observed in the lower frequency (o 170 cm�1),

belonging to another 2n + m branch. These peaks are

assignable to 2n + m = 37 ((18, 1) and (17, 3)) tubes

according to the previous Raman9,16 and the present PL

results (Fig. 1A).

Fig. 1 2D PL contour maps of (A) SWCNTs and (B) the corresponding

C60 nanopeapods in SDBS-D2O solutions.

Fig. 2 Resonance Raman spectra in RBM phonon region of (A)

SWCNTs and (B) C60 nanopeapods in SDBS-D2O solution under

excitation from 1.16 to 1.25 eV.

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Under the excitation of lower energy of 1.17 eV (Fig. 3B,

lower panel), the Raman intensities of the 2n + m = 37

((18, 1) and (17, 3)) tubes become more prominent due to the

resonance effects.9 Concequently, the overall peak position

shifts to the lower frequency (Fig. 2A).

The other Raman spectra obtained by the other excitation

energies were also reasonably attributed to each (n, m) tube

(Fig. 4, lower panels). The obtained RBM frequencies of each

(n, m) SWCNT are exactly the same within experimental error

in all these excitation energies and consistent with a previous

report (see Table 1).9 Moreover, all Raman spectra were fitted

with Lorentzian functions and their linewidths were 4–8 cm�1.

A previous study reported that the RBM phonon intensity

of type I tubes ((mod (2n + m, 3) = 1) is much higher than

those of type II tubes ((mod (2n + m, 3) = 2)) for second

interband transition (E22).9 Also in this study, the Raman

signals of 2n + m = 32 and 35 tubes were hardly observed

from the sample solution even though the E22 locate between

the excitation energies used here (Fig. 1A). This characteristic

behavior can be explained by the electron–phonon coupling

model.17–19 Ab initio calculations show that the electron–

phonon coupling strength of type I is larger than that of type

II tubes for E22 transition, resulting in higher Raman intensity

of type I such as 2n + m = 31, 34 and 37 tubes.

The Raman spectral shapes and peak positions change due

to C60 encapsulations (Fig. 2B). On the basis of a previous

Raman study9 and the PL spectra (Fig. 1B), the observed

peaks around 1.25 eV excitation are assignable to 2n + m =

34 and 31 families (Fig. 3A, upper panel). The obtained RBM

frequencies of (14, 6), (15, 4), (16, 2), (17, 0) and (13, 5) tubes

are identical to those reported previously.9 The appearance of

(13,5) tube is a consequence of the change in the optical

transition energies upon C60 encapsulations.9,10 The two

RBM peaks at o170 cm�1 are assignable to (17, 3) and

(18, 1) tubes because E22 of these tubes do not significantly

change by the C60 insertions (Fig. 1B).

On the other hand, under a lower excitation energy of

1.17 eV (Fig. 3B, upper panel), the Raman intensities of

2n + m = 37 tubes significantly increase compared with

2n + m = 34 tubes (Fig. 3B) due to the resonance effect.9

The fitting results of the RBM profiles for the other excita-

tion energies are shown in Fig. 4 (upper panels). Note that the

whole RBM peaks can be reasonably assigned to each (n, m)

C60 NPD. It was unnecessary to assume the additional RBM

peaks originated from another species. The obtained RBM

phonon frequencies of C60 NPDs (opeapods) and the corres-

ponding SWCNTs (oSWCNTs) are summarized in Table 1.

Fig. 5 shows G-band spectra of nanopeapods and SWCNTs

at different excitation energies; 1.33 eV (930 nm), 1.29 eV

(960 nm), 1.28 eV (970 nm), and 1.25 eV (990 nm). It is

Fig. 3 Resonance Raman spectra in RBM phonon region of C60

NPDs (upper panel) and SWCNTs (lower panel) at excitation energies

of (A) 1.25 eV and (B) 1.17 eV, respectively.

Fig. 4 Resonance Raman spectra in the RBM phonon region of C60

NPDs (upper panel) and SWCNTs (lower panel) at excitation energies

of (A) 1.24 eV, (B) 1.19 eV, (C) 1.18 eV and (D) 1.16 eV, respectively.

Table 1 RBM frequencies of SWCNTs and C60 NPDs, and thedifference between them (Do)

(n, m) dt/nm

oRBM/cm�1

DoRBM/cm�1SWCNTs C60 nanopeapod

(14, 3)a 1.2148 194.3 198.0 3.7(13, 5)a 1.278 188.2 194.2 6.0(17, 0) 1.350 184.8 182.9 �1.9(16, 2) 1.357 182.9 180.5 �2.4(15, 4) 1.377 180.8 175.2 �5.6(14, 6) 1.411 176.9 170.5 �6.4(13, 8) 1.457 172.7 168.9 �3.8(18, 1) 1.470 170.2 167.1 �3.1(17, 3) 1.483 167.5 164.2 �3.3a Ref. 9.

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well-known that the charge transfer between encapsulated

molecules and SWCNTs causes a peak shift of the G-band.

However, the peaks shift was not observed in this system

within the experimental error (B1 cm�1), which is consistent

with a previous report of C60 nanopeapods.20

Interaction between SWCNT and C60

To investigate the interaction between SWCNT and C60

in detail, the RBM frequency shifts upon C60 insertions

(Do = opeapods � oSWCNTs) are plotted as a function of the

distance (d) between the graphitic surfaces of SWCNTs and

C60 (Fig. 6, solid circles). The inter-surface distance between

SWCNTs and C60 was calculated by d = (dCNT � dC60)/2

where dCNT and dC60 are diameters of SWCNTs and C60

(= 0.710 nm),21 respectively. The Do values for (14, 3) and

(13, 5) NPDs are taken from ref. 9.

Naturally, the observed Do is a result of the interaction

between SWCNT and C60. For small inter-spacing with

d o B0.3 nm, the upshifts of the RBM frequencies were

observed upon C60 encapsulations (Fig. 6). The positive Do at

d o B0.3 nm can be explained by steric hindrance of the

radial motion of SWCNTs by C60. A small spacing interrupts

the vibration of SWCNTs, resulting in the upshift of the RBM

frequency.12

On the other hand, with increasing the inter-spacing, Dodecreases towards about �6 cm�1 at around 0.35 nm and then

increases again, approaching zero. Interestingly, the value of

0.35 nm is very similar to the interlayer distance of graphite

(0.335 nm).22 The similarity of the equilibrium distances

strongly suggests that the downshift in Do is mainly caused

by hybridization between SWCNT and C60. In fact, DFT

calculations with local-density approximation (LDA)

have predicted that C60 molecules are most stabilized at

d B 0.33 nm.23 In this situation, the p orbital of SWCNTs is

effectively mixed with that of C60. Fig. 7 shows the occupied

states of C60 encapsulated (10, 10) SWCNT (d = 0.33 nm)

near the Fermi level. The squared wave function originating

from SWCNT clearly exhibits their hybridized nature between

CNT and C60. Such hybridization of the electronic states

reduces the total electron density on the wall of SWCNTs,

which induces the bond softening of SWCNTs (i.e. Doo 0).12

Additional long-range interactions such as London

dispersion force should be included in the attractive inter-

action, which cannot be reproduced by DFT. A recent

theoretical study reported that these two interactions coexist

in the graphitic materials.24 Normally, the intermolecular

distance dependence of the long-range attractive interaction

is much gentler than that of the repulsive part. However, the

gradient for increasing of Do at d 4 0.35 nm seems to be

almost the same as that for decreasing of Do at d o 0.35 nm

(Fig. 5). This means that the hybridization of the p orbital of

SWCNTs and C60 makes significant contribution to the

vibrational frequency shifts, at least, at around the equilibrium

inter-surface distance.

Surprisingly, the diameter dependence of the RBM phonon

frequency shifts is in good agreement with the shifts in the

optical transition energies upon C60 encapsulations.10,11 The

difference between the first transition energies before and after

C60 encapsulation (DE11 = E11peapods� E11

SWCNTs) for corres-

ponding Type I tubes is also shown in Fig. 6 (open circles)10,11

along with the present RBM results (Do, solid circles). The

Fig. 5 Raman spectra in the G-band region of C60 nanopeapod and

SWCNTs at different excitation energies (1.25B1.33 eV).

Fig. 6 Experimentally obtained Do (solid circles) and DE11 of type I

tubes (open circles)10,11 as a function of inter-space distance.

Fig. 7 Squared wave function of the occupied state of C60

encapsulated (10, 10) SWCNTs near the Fermi level. Apparently,

the p state of the SWCNTs mixes with that of the encapsulated C60

(inside the tube wall).

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remarkable similarity strongly suggests that the diameter

dependence of Do and DE11 can be explained by the

common mechanisms and ensures that the above-mentioned

interaction is essential to describe the physical properties of

nanopeapods.9–11

Here we found that the interaction between C60 and

SWCNTs strongly depends on the inter-spacing between them.

Analogously, the interaction between inner and outer tubes

of double-wall carbon nanotubes (DWCNTs) should also

depend on the inter-spacing. For example, the RBM of the

inner tubes of DWCNTs synthesized from nanopeapods are

composed of many narrow peaks. This characteristic behavior

is reasonably explained by the inter-spacing dependent inter-

action between inner and outer tubes.25 Further, the presence

of photoluminescence from DWCNTs is still controversial.

Several groups have reported bright PL from inner tubes of

DWCNTs.26–28 In contrast, a recent report suggested that

observable PL from DWCNTs sample is attributed to

SWCNT impurities.29 These totally opposite behaviors may

be explainable by the distance dependent interaction between

outer and inner tubes.30

Conclusions

The interaction between SWCNTs and the encapsulated C60

were systematically investigated by resonance Raman spectro-

scopy. The observed RBM frequency shifts strongly depend

on the inter-spacing between them. When the inter-spacing is

smaller than B0.3 nm, the interaction is dominated by steric

hindrance for the radial motion of SWCNTs by C60.

On the other hand, in the case of larger spacing (d4B0.3 nm),

hybridization becomes effective and gives rise to an equili-

brium distance of B0.35 nm. The information obtained here

provides important insights into an accurate description of

inter-spacing interactions in nanopeapod systems and their

electronic and transport properties.

Acknowledgements

We thank R. Takano (AIST) for her experimental help. We

also thank Dr T. Nakanishi (AIST) for fruitful discussions.

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