interaction between single-wall carbon nanotubes...
TRANSCRIPT
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: [email protected]; 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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