J. Phys. Chem. C 2009, 113, 1751–1757
1751
Large Variety of Behaviors for the Raman G′ Mode of Single Walled Carbon Nanotubes
upon Electrochemical Gating Arising from Different (n,m) of Individual Nanotubes
Martin Kalbac,*,†,§ Ladislav Kavan,† Hootan Farhat,‡ Jing Kong,§ and
Mildred S. Dresselhaus§,|
J. HeyroVský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, V.V.i., DolejškoVa 3,
CZ-18223 Prague 8, Czech Republic, and Department of Materials Science and Engineering, Department of
Electrical Engineering and Computer Science, and Department of Physics, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139
ReceiVed: October 5, 2008; ReVised Manuscript ReceiVed: NoVember 27, 2008
The dependence of the second-order Raman G′ mode of individual single walled carbon nanotubes (SWCNTs)
as a function of electrochemical gating has been studied using in situ Raman spectroelectrochemistry. We
show that the change of the frequency of the G′ mode with electrode potential (δ(ωG′)/δ(V)) is specific for
different (n,m) of individual SWCNTs. The study of the G′ mode at the single nanotube level allowed us to
observe many effects that get averaged in observations for the SWCNT bundles. These effects included an
unusual dependence of the G′ mode frequency on electrode potential (V) for particular individual tubes, namely
a decrease of the G′ mode frequency with increasing magnitude of electrode potential. This occurs for both
increasing and decreasing V relative to V ) 0. It is demonstrated that there is no simple dependence of
δ(ωG′)/δ(V) on tube diameter or excitation laser energy. Furthermore, the G′ mode intensity drops with an
increase or decrease of the electrode potential relative to V ) 0 and this decrease in the G′ mode intensity
was found to be gradual and not abrupt, which suggests a more complicated mechanism than the simple
bleaching of electronic transitions. These observations give guidance to future theoretical work on this topic.
Introduction
Single walled carbon nanotubes (SWCNTs) have outstanding
electronic properties with a large number of prospective
applications in nanoscale electronics and devices such as
displays, sensors, and supercapacitors. Resonance Raman
spectroscopy is one of the most important methods for the study
of SWCNTs. The main components of the Raman spectra of
SWCNTs are the radial breathing mode (RBM), the tangential
displacement band (TG), the disorder-induced mode (D), and
the high-frequency second-order mode (G′). The tangential
displacement band (also called the G band) is observed in the
region of 1450-1600 cm-1, and in the case of metallic tubes,
one of the peaks exhibits a pronounced Breit-Wigner-Fano
(BWF) spectral line shape. The RBM frequency of SWCNTs
is inversely proportional to the tube diameter. The D and G′
modes are observed in all kinds of polycrystalline carbon
materials with sp2 hybridization. However, their physical origin
has been explained only recently in terms of double-resonance
theory.1-3 The D and G′ modes are observed in the spectral
regions of 1250-1450 and 2500-2900 cm-1, respectively. The
one-phonon second-order Raman D band appears only if there
is a breakdown in translational crystal symmetry, which can be
caused by defects in the structure. On the other hand, the twophonon second-order Raman G′ feature occurs independently
of the presence of structural defects. The two phonons contributing to the G′ feature have wave vectors q and -q, and thus
the momentum conservation constraint is automatically preserved.
* Corresponding author. Telephone: 420 2 6605 3804. Fax: 420 2 8658
2307. E-mail: [email protected].
†
Academy of Sciences of the Czech Republic.
‡
Department of Materials Science and Engineering, MIT.
§
Department of Electrical Engineering and Computer Science, MIT.
|
Department of Physics, MIT.
Tuning and gaining an understanding of the electronic
structure of SWCNTs are crucial for their application in
nanoelectronic devices. Doping of SWCNTs can be carried out
chemically or electrochemically. Electrochemical charging is
more favorable for fundamental studies due to the precise and
easy control of the doping level.4 This inspired several spectroelectrochemical studies on SWCNTs,5-7 double walled carbon
nanotubes (DWCNTs),8 and fullerene peapods.9 However, these
measurements have been typically performed on bundled
samples. The use of bundled samples complicated the interpretation of the results since the individual properties of specific
tubes were averaged and modified by bundling.10 Therefore, it
is desirable to focus on individual carbon nanostructures in
fundamental studies.
Individual SWCNTs can be obtained by (i) sonication of
SWCNT bundles in an aqueous solution of surfactants such as
sodium dodecyl sulfate (SDS),11 (ii) detachment of the nanotubes
from bundles using surface adhesion forces,12 or (iii) their direct
growth by chemical vapor deposition (CVD).13 The dispersion
of bundles in SDS provides a sample suitable for photoluminescence measurements,11 which proves that successful debundling has occurred. Nevertheless, it is very difficult to remove
the surfactant quantitatively, and thus this method is not favored
for spectroelectrochemical measurements. The detaching of
individual SWCNTs from a bundle is a clean and easy
technique, and can be applied also to other nanostructures such
as peapods and DWCNTs.12 On the other hand, the debundling
is not perfect in this case. Although it provides some isolated
tubes, most of the sample consists of substrate-bonded nanoribbons of interacting nanotubes. Furthermore, the localization
of nanotubes on a substrate is random in both of the abovementioned techniques, and thus contacting nanotubes by external
electrical leads is difficult. The CVD approach, however, can
10.1021/jp808797c CCC: $40.75  2009 American Chemical Society
Published on Web 01/13/2009
1752 J. Phys. Chem. C, Vol. 113, No. 5, 2009
Kalbac et al.
Figure 1. (a) Scanning electron micrograph of as-grown SWCNTs on a SiO2/Si substrate. The arrow indicates the direction of nanotube growth.
(b) AFM image of an as-grown individual SWCNT (the measured height of the nanotube is 1.23 nm, and the RMS background level is 0.3 nm).
provide clean long isolated tubes with a defined orientation on
a substrate.14 Therefore this method is the most favorable for
preparing samples for spectroelectrochemical measurements.
Here we present in situ Raman spectroelectrochemical data
on the G′ mode of individual SWCNTs. We focused our study
on the change of the frequency of the G′ mode during
electrochemical charging of SWCNTs. For bundled SWCNTs,
depending on the nanotube sample, the laser excitation energy,
and the conditions of electrochemical charging, various positive
or negative shifts of the G′ mode frequency have been reported.
6,15-17
Our detailed analysis of the dependence of the G′ feature
on the electrode potential for several individual SWCNTs
showed that the wide variety of the behaviors of the G′ feature
is intrinsic to different (n,m) SWCNTs. Furthermore, our data
on individual SWCNTs uncovered effects that were not previously observed in SWCNT bundles and are not explained by
any current model. We also show here that the behavior of the
G′ mode is very complex and further experiments must be
performed to gain a more complete understanding of the
observed phenomena.
The results reported here contribute to a rationalization of
the results obtained previously for SWCNT bundles,6,16-18 and
serve to guide theorists in their development of models to
explain these phenomena.
Results and Discussion
The scanning electron micrograph and an atomic force
microscopy (AFM) image in Figure 1a and Figure 1b, respectively, show the morphology of our individual SWCNTs grown
on a SiO2/Si substrate. The SWCNTs are parallel to each other,
and they are up to several millimeters in length (see Figure 1a).
The typical distance between two neighboring SWCNTs is larger
than 50 µm. The spacing between tubes is important, since it
determines the number of tubes measured simultaneously during
acquisition of a Raman spectrum. Since the size of the focused
laser spot was about 1 µm, our Raman measurement is almost
always addressing just one individual SWCNT only. However,
it is difficult to distinguish between one SWCNT and a small
bundle containing two or three parallel SWCNTs, and thus the
Behaviors of Raman G′ Mode of SWCTs
J. Phys. Chem. C, Vol. 113, No. 5, 2009 1753
Figure 3. Dependence of the frequency of the G′ feature ωG′ on
electrode potential for 11 different SWCNTs (identical symbols in
different curves correspond to one tube, measured with two different
Elaser values). The spectra are excited by 2.33 (a, i, k), 2.07 (b, f, h),
2.11 (c), 2.13 (d), 2.03 (e), 2.00 (g), 2.17 (j), 1.91 (l), or 1.76 (m) eV
laser radiation. The error of the evaluation of the frequency is estimated
as (2 cm-1.
Figure 2. In situ Raman spectroelectrochemical data on an individual
SWCNT in the range from -1.5 to 1.5 V (from bottom to top). The
spectra are excited by 1.92 eV laser radiation. The bold spectrum
corresponds to 0 V. The electrochemical potential change between
adjacent curves is 0.1 V. The spectra are offset for clarity, but the
intensity scale is the same for all spectra. The vertical solid line is a
guide to the eye and corresponds to the position of the G′ phonon feature
at 0 V.
possible occurrence of such small bundles in our samples cannot
be excluded. Some tubes in a small bundle are not detected in
Raman spectra, because they are out of resonance, and thus
mimics the behavior of an individual tube.
Figure 2 shows in situ Raman spectroelectrochemical data
measured on an individual SWCNT using 1.92 eV laser
radiation. The electrochemical doping of a SWCNT shifts the
frequency of the G′ mode. The original frequency of the G′
mode at 0 V (which is close to the open-circuit potential) is
around 2620 cm-1. Increasing the potential to 1.5 V causes the
frequency to downshift toward 2612 cm-1. On the other hand,
decreasing the potential to -1.5 V results in a more pronounced
downshift of the G′ mode to a frequency around 2600 cm-1.
Previous experiments on SWCNT bundles also showed a change
of the G′ mode frequency during electrochemical doping, but
never showed a downshift for both positive and negative
potentials.6,15-17 For positive doping of nanotube bundles an
upshift of the G′ mode frequency has been found, while almost
no shift was observed for negative doping using the 1.96 eV
laser excitation energy.6 This behavior for bundled tubes is
obviously different from our observation at the individual
nanotube level (Figure 2). The interpretation of effects in
nanotube bundles is complicated by the fact that many interacting tubes contribute to the actual acquired spectra and the doping
level is inhomogeneous across the bundle. Nevertheless, the
dominating argument for the potential-driven change of the G′
mode frequency was the hardening/softening of the C-C bond.6
On the other hand, it was found for SWCNT bundles that the
frequency shifts are small with 488 nm laser excitation, but a
strong upshift of the G′ mode with increasing electrode potential
is found for the 568 nm laser excitation with small changes in
electrode potential.17 This effect cannot be explained by
hardening/softening of the C-C bond. Thus it was suggested
that it is a consequence of the change of the shape of the highenergy TO phonon branch in the phonon dispersion of graphene.
It was assumed, that the increased electrode potential leads to
a flattening of this branch.17 This causes a change in the slope
of the TO branch for lower laser energy excitations, while the
“upper” part of this branch (accessed by higher energy excitation) remains unaffected.17 In other words, at lower energy
excitation, the TO mode energy changes with applied potential,
and therefore the frequency of the Raman G′ mode is changed.17
However, this model explains only the increase in the frequency
of the G′ mode with increasing electrode potential. This is
obviously in contrast to our experimental results at the single
nanotube level, where ωG′ decreases with both increasing V and
decreasing V (Figure 2). Note, in contrast, that for SWCNT
bundles an upshift of the G′ mode frequency with increasing
electrode potential is usually observed. Only for doping at
negative potentials was a downshift sometimes found in prior
work on bundle SWCNT samples.18
Considering only the results obtained for a bundled sample,
it is impossible to identify the effects which are responsible for
the G′ mode frequency variations with applied electrochemical
potential shown in Figure 2. Various phenomena might be
considered in interpreting the G′ mode frequency/potential
dependencies: (i) the dependence on the particular nanotube
(n,m), (ii) the dependence on the laser excitation energy and,
consequently, on the particular Eii transition involved in the
resonance enhancement, (iii) the changes of the C-C bond
length as a result of doping, (iv) the dependence on the
electrolyte solution and/or on other experimental conditions, and
(v) the dependence on the difference between the laser excitation
energy and the particular Eii of the nanotube that is in resonance
with Elaser.
To clarify these issues, we first accessed the behavior of the
Raman spectra of different tubes using different laser excitation
energies. Figure 3 demonstrates the large variety of behaviors
1754 J. Phys. Chem. C, Vol. 113, No. 5, 2009
Kalbac et al.
found for the dependence of the G′ band frequency on electrode
potential for different individual SWCNTs. For example tubes
b, e, and f show a very weak or no dependence on electrode
potential, much less than is typical for a bundle sample. Tubes
d, i, j, and l exhibit a decrease of the G′ mode frequency with
applying both negative and positive potentials as for the case
of the tube in Figure 2. However, for tube d the effects were
small, while for tubes i and l the effects were large. For tubes
d and j the effects were symmetric with positive and negative
potentials, while for tubes i and l the behaviors were strongly
asymmetric for positive and negative potentials. On the other
hand, tubes a and k show an increase of the G′ mode frequency
with applying both negative and positive potentials. Tube m
does not reflect the changes with a negative applied potential;
rather the G′ mode frequency increases with increasing positive
potential. Finally, tube h shows a strong decrease of the G′ mode
frequency during negative doping. For positive doping of tube
h, the behavior of the G′ mode frequency is more complex
since it decreases up to a potential of about 0.8 V and then it
again increases in going to a potential of 1.5 V. This richness
in behavior requires detailed studies involving many (n,m) tubes
to tease out the detailed dependence of this variety of behaviors
on tube type, never seen before for bundled samples.
Obviously ωG′ at 0 V is different for each curve. This reflects
the dependence of ωG′ on the laser energy used for the excitation
of the Raman spectra, on the diameter (and perhaps also the
chirality) of the studied SWCNTs, and perhaps on whether the
tube is metallic or semiconducting.
The dependence of ωG′ on Elaser has been previously fitted to
the equation
ωG’ ) k1 + k2Elaser
(1)
where k1 ) 2420 cm-1 and k2 ) 106 cm-1/eV for individual
SWCNTs on a SiO2/Si substrate.19 A similar value of k2 ) 96
cm-1/eV has been found for SWCNTs dispersed in SDS20 or
for the outer tubes in DWCNT bundles (k2 ) 103 cm-1/eV).15
On the other hand, the narrower inner tubes of DWCNTs
exhibited a dependence with a smaller slope.15 Here the values
of k1 and k2 were 2448 cm-1 and 76 cm-1/eV, respectively, for
the inner tubes in DWCNTs. The different slope for the inner
tubes (relative to the outer tubes) is a consequence of the
different values of Eii, which are involved in the resonant process
for the inner and outer tubes.15,21
The dependence of ωG′ on Elaser is also demonstrated in Figure
3 for individual tubes. The curves measured on the same tube
but at a different Elaser are shifted by a constant value, confirming
the dependence of ωG′ on Elaser according to eq 1. Curve b in
Figure 3 is upshifted by 5 cm-1 from curve c, and curve f is
upshifted by ca. 8 cm-1 from curve g in Figure 3 (considering
the values of ωG′ to be taken at 0 V, see Figure 2). Hence, the
δ(ωG′)/δ(Elaser) values for curves f and g resemble a linear
dependence of eq 1. On the other hand, the dependence δ(ωG′)/
δ(Elaser) for curves b and c has an opposite sign; that is, k2 is
negative. This behavior may be indicative of a steplike character
of the δ(ωG′)/δ(Elaser) dependence at the individual tube level
(different Eii values are employed for Elaser ) 2.07 and 2.11
eV).19,22 Note that the difference in ωG′ for curves b and c
reaches the resolution limit of the spectrometer. In order to
evaluate the dependence of δ(ωG′)/δ(V) on excitation laser
energy, we fitted the δ(ωG′)/δ(V) for each tube shown in Figure
3 for both negative and positive doping. For simplicity we used
a linear fit even though some tubes showed a much more
complex behavior. The results (not shown) confirm that there
is no obvious dependence of δ(ωG′)/δ(V) on excitation laser
energy for the individual SWCNTs studied in this work. The
results were obtained for 11 different nanotubes using eight laser
lines. Nevertheless, more in-depth study on a much larger
number of tubes would be necessary to establish our preliminary
finding on a sound experimental basis.
It was shown recently that some of the features of the TG
band (A1LO of semiconducting and metallic tubes and A1TO of
semiconducting tubes) exhibit a change of frequency with
applied gate voltage (V).23 The slope of this change is dependent
on the diameter of the SWCNT.23 Therefore, a similar dependence can be suggested for the slope of the change of the G′
mode frequency with potential (δ(ωG′)/δ(V)). Unfortunately, it
is very difficult to obtain Raman spectra for SWCNTs where
both the RBM and the G′ modes appear with a reasonable
intensity at the same time for the same tube. This complicates
the evaluation of the tube diameter based on the frequency of the
RBM band. Nevertheless, it was previously reported that the
frequency of the G′ band (ωG′) scales with the inverse tube
diameter d according to the equation
ωG′ ) C1 - C2 /dn
(2)
where C1 ) 2708.1 cm-1, n ) 1, and the values of C2 in the
literature vary from 35.4 to 67 cm-1 · nmn.15,24 (Note that the
samples in these references are very different from each other.
Our results lend credibility to the relatively broad range of values
found in the literature for C2. The values of C1 and C2 are
expected to be dependent on which Eii a particular tube is in
resonance with, and whether the particular tube is semiconducting or metallic.) In addition, an alternative dependence with n
) 2 and C1 ) 2645 cm-1 was recently suggested by Cardenas
et al.25 However, for our sample eq 2 gives the value of ωG′ up
to 2675 cm-1 (Figure 3). Thus the original fit with C1 ) 2708.1
cm-1, n ) 1, and 35.4 < C2 < 67 cm-1 · nm15,24 seems to explain
the experimental results more accurately. In our study here, we
measured the tubes on a SiO2/Si substrate, which is similar to
ref 24. However, the data in the latter reference were obtained
using 2.41 eV laser excitation energy. Since ωG′ is dependent
on Elaser, it is necessary to combine eqs 1 and 2 to estimate the
diameters of the tubes. (For this purpose we used k2 ) 100 cm-1/
eV, C1 ) 2708.1 cm-1, C2 ) 35.4 cm-1 · nm, and n ) 1. The
parameter k1 was calculated for each tube.) It is important to
note that here we are neglecting the steplike behavior of
δ(ωG′)/δ(Elaser)19,22 and we assume that C1 and C2 are the same
for all tubes in our experiment. Thus the resulting tube diameters
may exhibit some error for particular tubes. Nevertheless, we
believe that this approximate procedure is sufficient to evaluate
the dependence of the slope δ(ωG′)/δ(V) on tube diameter. In
this way, we evaluated a plot of the slope δ(ωG′)/δ(V) for each
of the 11 tubes for both positive and negative doping. Our
experimental results showed no obvious dependence of δ(ωG′)/
δ(V) on tube diameter, which is a new result.
Figure 4 shows the Raman spectra of the TG mode region
for the same tubes that are presented in Figure 3 (at a potential
0 V). The broadening of the G- component (centered at ∼1550
cm-1) of the TG mode for curves e, f, g, and m indicates that
these tubes are metallic. On the other hand, the spectra for the
rest of the tubes do not exhibit such a broadening and thus they
probably correspond to semiconducting tubes. The development
of the TG band during electrochemical charging (not shown)
confirms that curves e, f, g, and m in Figure 4 correspond to
metallic tubes.26 (The intensity of the G- component is increased
Behaviors of Raman G′ Mode of SWCTs
J. Phys. Chem. C, Vol. 113, No. 5, 2009 1755
Figure 5. Dependence of the relative intensity of the G′ feature on
the electrode potential for different SWCNTs (identical symbols for
different curves correspond to one tube, measured with two different
laser excitation energies). For all curves, the intensity at 0 V is set to
unity. The spectra are excited by 1.92 (g), 2.00 (d), or 2.07 (a, c) eV
laser radiation.
Figure 4. Raman spectra in the TG mode region of tubes from Figure
3 (at a potential 0 V). The spectra are excited by 2.33 (a, i, k), 2.07 (b,
f, h), 2.11 (c), 2,13 (d), 2.03 (e), 2.00 (g), 2.17 (j), 1.91 (l), or 1.76 (m)
eV laser radiation. The arrows point to components of the TG band
with the BWF line shape.
as the potential is changed from 0 V to either 1.5 or -1.5 V.)
The Raman bands below 1520 cm-1 and above 1650 cm-1
correspond to the electrolyte. The features of the electrolyte do
not change with a change of the electrode potential, and they
can therefore be easily recognized.
It has been shown that the G′ mode splits for a particular
nanotube due to a resonance enhancement employing both the
incident and the scattered phonons.27 Closer inspection of
the development of the decrease of the frequency of the G′ mode
with changing electrode potential in Figure 3 shows that there
is a small plateau, followed by a steeper decrease of the
frequency, and finally there is a second plateau. Considering
this, the development of ωG′ with changing potential could be
alternatively rationalized in the following way. First, we consider
that the spectral intensity of a particular tube is enhanced by
the incident light via the resonance with the E44 transition for
semiconducting tubes. Second, we assume that the change of
the electrode potential can change the energy of Eii.6 Consequently, the change of the energy of E44 decreases the resonance
enhancement. However, the electrochemical doping also causes
changes in E33. Therefore for some tubes the next lower energy
singularity E33 may come into resonance with the scattered light
and a downshifted G′ feature appears in the spectra.27 This
mechanism will be analogous to that reported previously for a
steplike character of the δ(ωG′)/δ(Elaser) dependence on Elaser19,22
and is an effect that can only be studied at the individual
nanotube level.
The resonance with the scattered light always seems to be
weaker than that with the incident light.27 In nanotube bundles,
many tubes with different diameters are present and thus the
G′ feature coming from the resonance with the scattered light
can be easily hidden by stronger bands coming from the G′
feature excited by the incident light.
The intensity of the G′ mode is expected to be dependent on
the electrode potential. This change is attributed to the filling/
depletion of the states within the van Hove singularities. The
electrochemical charging leads to a change of the Fermi level
energy, and subsequently the electronic states are filled or
depleted with electrons. The depletion of electronic charge is
believed to suppress the resonance enhancement, and the
spectrum is therefore bleached. In other words, the intensity
should be constant if the Fermi level is below the energy of the
van Hove singularity which is involved in resonance enhancement and should drop suddenly to zero when the Fermi level
achieves the energy of this particular van Hove singularity. In
contrast to these expectations, at the single SWCNT level we
see cases where the intensity of the G′ mode bleaches continuously (as shown in Figures 2 and 5). A continuous bleaching
of the spectral intensity was observed previously in nanotube
bundles.6 In the latter case, the effect could be explained by
the presence of many different tubes, with slightly different Eii
values causing resonance enhancement. Furthermore, one can
also argue that the interaction of the electrolyte counterion is
different for a tube within the bulk of a bundle and with a tube
at the surface of a bundle. These arguments are ruled out for
individual nanotubes (Figure 5). Thus, for individual tubes, the
bleaching of the spectrum is expected to be sudden and only at
the potential corresponding to half the energy of a particular
Eii. In our case this should occur at an electrode potential above
1 V. (In an ideal case, the electrode potential and the Fermi
level shifts would be numerically identical. However, the
experimental data show that the potential of 1 V corresponds
to the Fermi level shift by 0.4-0.9 eV.28,29) In contrast to these
expectations, the experimental results show a gradual decrease
of the G′ mode intensity with increasing magnitude of the electrode potential (Figure 5). Therefore studies at the single
SWCNT level show that a different explanation must be found.
It was previously suggested for nanotube bundles that Eii
changes with applied potential.6 In this case one would expect
that, due to the change of the electrode potential, some of the
tubes move out from the resonance window while other tubes
move into resonance. Consequently, the overall spectral intensity
should not change. However, for nanotube bundles the superposition of other effects mentioned above cannot be excluded,
and thus the resulting dependence of the spectral intensity on
electrode potential is a very complex issue. Here, in this study,
the measured individual nanotubes have to match the resonance
condition (otherwise there is no signal in the Raman spectra.)
However, some of the tubes might not be in perfect resonance,
and in principle, the shift of the van Hove singularity might
1756 J. Phys. Chem. C, Vol. 113, No. 5, 2009
lead to an improvement of the resonance condition. This effect
could explain some of the “fluctuations” of the G′ mode intensity
close to E ) 0 V in Figure 5.
It has been shown previously that the substitutional doping
of SWCNTs with boron or nitrogen causes a perturbation to
the electronic structure.30 This is represented by new electronic
states shifted from their unperturbed van Hove singularities.30
For example, it has been shown that the isolated nitrogen
impurity forms a flat energy level lying inside the band gap.31
Implanting boron or nitrogen into nanotubes can furthermore
lead to a change of the energy gap. We assume that electrochemical doping may cause analogous (but not the same) effects.
The application of an electrode potential to SWCNTs leads to
a stronger interaction of the SWCNTs with electrolyte ions
which can result in a perturbation of the electronic structure of
SWCNTs. Consequently, the density of states in van Hove
singularities can be reduced. The “broadening” of van Hove
singularities will extend the region where the SWCNTs can be
in resonance, but the resonance effect would be weaker. This
will explain a simultaneous resonance through E33 or E44
transitions and also a continuous bleaching of the Raman spectra
during electrochemical doping.
The interaction of the SWCNT with the SiO2 substrate may
also contribute to the variety of behaviors of the G′ mode. A
charge transfer between the substrate and the SWCNT together
with an enhanced adsorption of air oxygen on the SWCNT due
to the SWCNT-SiO2 substrate interaction are considered to be
the important effects of the SiO2 substrate on the SWCNT.32,33
However, this charge transfer can be compensated by electrochemical doping, as has been shown previously.4,32 Our
measurements are referred to the potential of the Ag pseudoreference electrode, and thus the doping level of an individual
SWCNT is fully controlled in our experiment. Furthermore, the
magnitude of the charge transfer between the SiO2 and SWCNT
is very small compared to the potentials applied in our study.
Other effects of the substrate such as substrate-induced strain
or deformation of the nanotube shape are not expected to have
a significant effect on our conclusions, since the effects observed
in our study are fairly robust. Furthermore, we always follow a
relative change of the G′ mode (for example, as a function of
V or normalized to the G+ band intensity) and therefore the
substrate should not influence the conclusions drawn from our
experimental results. Therefore, we believe that the large variety
of behaviors observed for the G′ mode is an intrinsic property
of SWCNTs with different chiral indices (n,m). Only small
perturbations of the G′ mode frequency and intensity are
expected to be caused by interactions with the substrate and
are also dependent on whether the SWCNT is semiconducting
or metallic.
Conclusion
Raman spectroscopy and in situ Raman spectroelectrochemistry of the G′ mode were measured on individual SWCNTs, in
order to evaluate the effect of electrochemical doping on this
double-resonance feature.
We show here that the G′ band electrochemical behavior of
individual SWCNTs is sensitive to the nanotube structure (n,m)
and that the details of this behavior are very complex. Some
(n,m) SWCNTs exhibit a very weak dependence of the G′ mode
on electrode potential, while other (n,m) SWCNTs show either
a strong decrease or increase of the G′ mode frequency with
either an increase or decrease in the potential. Such behaviors
can only be studied at the single nanotube level. In the present
work, we did not find any correlation between δ(ωG′)/δ(V) and
Kalbac et al.
either Elaser or nanotube diameter. On the other hand, we have
observed a wide variety of behaviors, which require detailed
study for many tubes in order to establish the differences
between metallic and semiconducting tubes, and the varieties
of each subset of metallic and semiconducting tubes exhibiting
the various behaviors described in this paper. Our findings can
rationalize contradictory results previously observed for the
behavior of the frequency shift with changing electrode potential
of the G′ mode in nanotube bundles. The nanotube bundles
contain many tubes with different (n,m) indices. Using different
excitation energies leads to a resonance enhancement of only a
fraction of these tubes. The latter observation was previously
interpreted as the dependence of the G′ band frequency shift
with changing electrode potential on laser excitation energy.
However, our results here show that the effect is instead caused
by a large variety of behaviors of the G′ band frequency for
individual (n,m) tubes presumably associated with their metallicity and tube type.
In agreement with previous studies on SWCNT bundles, we
have found that the intensity of the G′ mode is decreased in
going from an electrode potential of 0 V to either +1.5 or -1.5
V. The relative change of the G′ mode intensity is monotonic
and is qualitatively similar for all individual nanotubes that were
studied. The monotonic change of the intensity is in contrast to
our general expectation for observations at the individual tube
level, and it was explained here by a broadening of the van
Hove singularities as the electrode potential is changed in
magnitude and sign.
Last but not least, our data demonstrate the importance of
the measurements at the single nanotube level, since the
bundling effect together with the averaging of data measured
for different nanotubes may lead to an incorrect interpretation
of the observed effects. The variety of new phenomena that have
been identified in the present work provides guidance for the
development of theoretical models to account for the rich G′
band spectra that are observed in Raman spectroelectrochemical
measurements, depending on the particular SWCNT (n,m)
structure, on whether the tube is semiconducting or metallic,
and on the magnitude and sign of the applied potential.
Experimental Section
SWCNTs were synthesized directly on a SiO2/Si substrate
using Fe as a catalyst.14 The as-grown SWCNTs were contacted
by Cr (5 nm)/Au (50 nm) evaporated on a part of the substrate,
and the SWCNTs served as the working electrodes. The
electrode potential was varied from +1.5 to -1.5 V. The cell
was completed with a Pt counter electrode and an Ag wire
pseudoreference electrode. The electrolyte medium was 0.1 M
LiClO4 in propylene carbonate/poly(methyl methacrylate) (w/w
3:1) (Aldrich). Electrochemical doping was carried out potentiostatically (EG&G PAR potentiostat). The Raman spectra were
excited by a dye laser (R6G dye, Coherent), a Ti:sapphire laser
(Coherent), or a Kr+ laser (Coherent). The spectrometer was
interfaced to a microscope (Carl-Zeiss, objective 100×). The
diameter of the focused laser spot was about 1 µm. The AFM
measurements were made with a Veeco Nanoscope instrument.
Acknowledgment. This work was supported by the Academy
of Sciences of the Czech Republic (Contract Nos. 203/07/J067,
IAA400400804, and KAN200100801) and by the Czech
Ministry of Education, Youth and Sports (Contract No. LC510 and ME09060). M.S.D. acknowledges support from NSF/
DMR-07-04197, H.F. and J.K. acknowledge the Materials,
Structure and Devices (MSD) center, one of the five programs
Behaviors of Raman G′ Mode of SWCTs
in the focus center research program (FCRP). Work was carried
out using the Raman facility in the Spectroscopy Laboratory
supported by Grant NSF-CHE 0111370 and Grant NIHRR02594. Authors are grateful to M. Hofmann from MIT,
Department of Electrical Engineering and Computer Science,
for making the AFM measurements.
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