Content uploaded by Ahmad J Ghandour
Author content
All content in this area was uploaded by Ahmad J Ghandour on Aug 24, 2020
Content may be subject to copyright.
Content uploaded by Emmanuel Flahaut
Author content
All content in this area was uploaded by Emmanuel Flahaut
Content may be subject to copyright.
Raman Gband in double-wall carbon nanotubes combining pdoping and high pressure
Pascal Puech,1,*Ahmad Ghandour,2Andrei Sapelkin,2Cyril Tinguely,3Emmanuel Flahaut,3David J. Dunstan,2and
Wolfgang Bacsa1
1Cemes, Université Paul Sabatier, 29 rue Jeanne Marvig, 31055 Toulouse, France
2Department of Physics, Queen Mary, University of London, London E1 4NS, United Kingdom
3CIRIMAT-LCMIE, UMR CNRS 5085, Université Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse, France
共Received 22 January 2008; published 10 July 2008兲
We use sulfuric acid as pressure medium to extrapolate the G-band position of the inner and outer tubes of
double-wall carbon nanotubes. Keeping the G-band position of the inner and outer tubes constant, we can
determine the fraction of double-wall and single-wall tubes in samples containing a mixture of the two.
A-band-related electronic interwall interaction at 1560 cm−1 is observed, which is associated with the outer
tube walls. This band is observed to shift with pressure at the same rate as the Gband of outer tubes and is not
suppressed with chemical doping. Differences in the interwall interaction is discussed for double-wall carbon
nanotubes grown by the catalytic chemical-vapor method and double-wall carbon nanotubes obtained through
transformation of peapods.
DOI: 10.1103/PhysRevB.78.045413 PACS number共s兲: 63.22.⫺m, 62.25.⫺g, 81.07.De
I. INTRODUCTION
Double-wall carbon nanotubes 共DWs兲are the simplest
form of multiwall carbon nanotubes, while single-wall car-
bon nanotubes 共SWs兲can be either semiconducting or me-
tallic, depending on the orientation of the graphene sheet
with respect to the tube axis. Multiwall carbon nanotubes
共MWs兲are metallic. The electrical conductivity of graphene
is highly anisotropic. Perpendicular to the graphene layer, the
conductivity of
bonds is less than 1% of the in-plane elec-
trical conductivity.1The electronic conductivity in MWs has
been extensively studied, and the observed intershell conduc-
tance is consistent with tunneling through orbitals of neigh-
boring walls.2DWs are suitable for the study of interwall
coupling.
Two main synthesis methods for DWs are known today:
conversion of peapods leading to DWs with a narrow diam-
eter distribution,3and the use of the catalytic chemical-vapor
deposition 共CCVD兲method resulting in 80%–100% of DWs
with a broader diameter distribution.4,5Raman spectroscopy
is routinely used to screen the carbon nanotubes 共CNTs兲. The
diameter distribution can be obtained from the low-
frequency radial breathing mode as a function of excitation
wavelength and the quality can be assessed by measuring
defect-induced scattering 共Dband兲.6
The Gband in DWs contains contributions from the in-
ternal and external tubes, which depend on external param-
eters such as pressure, temperature, and applied electric
field.7The Gbands of the inner and outer tubes do not fall on
the same spectral position since the pressures experienced by
the inner and outer tubes are different.
We combine the influence of the G-band shape as a func-
tion of chemical doping and hydrostatic pressure to separate
contributions from inner and outer tubes and to assign an
additional spectral band at the lower-energy side of the G
band. Internal tubes in DWs are subject only to the pressure
from the outer tube and are less affected by doping 共10%兲.8
We therefore use the contributions of the internal tubes as a
reference.9The DWs are not open in general and contain no
larger wall defects.10 Filling of DWs would also imply
changes in the G-band position of the internal tubes under
hydrostatic pressure, which we have not observed. We there-
fore assume that the DWs are not filled, which is consistent
with what is observed using high-resolution electron micros-
copy.
The spectral G-band position of the outer tube in DWs
falls at the same frequency range of the G+band in SWs,
which makes it impossible to separate different G-band con-
tributions. Kim et al.11 recently proposed a scheme to deter-
mine the purity in DW samples which contain DWs and SWs
using chemical doping. Chemical doping with sulfuric acid
has a large effect on the Gband of SWs 共Ref. 12兲and DWs.
We find that Raman spectra of DWs doped with H2SO4as
reported in literature11,13 are interpreted differently. We use
hydrostatic pressure–induced spectral changes to separate the
different spectral contributions to the Raman Gband and to
clarify the interpretation of doping-induced spectral changes.
Hydrostatic pressure has been used to separate the contri-
butions of the inner and outer tubes to the Gband.14,15 The
G-band position of the inner tube when applying pressure is
found to be close to the Gband observed for graphite, while
for the outer tube the Gband shifts to higher frequency with
increasing pressure. We combine chemical doping with hy-
drostatic pressure using sulfuric acid as pressure transmitting
medium to find the experimental parameters to determine the
fraction of DWs and SWs in DW samples. The differences
observed between the two types of DW samples presents us
with the possibility of exploring interwall interaction in
double-wall carbon nanotubes. We apply the extracted and
experimentally deduced parameters in fitting the Gband
when heating DWs in air.
II. SAMPLE AND EXPERIMENTS
We have used DWs prepared by the CCVD method16 and
for comparison, we have used SWs with 0.8 nm diameter
共HiPco兲and SWs with 1.4 nm diameter 共NanoCarbLab兲.
High-resolution transmission electron microscopy 共HRTEM兲
PHYSICAL REVIEW B 78, 045413 共2008兲
1098-0121/2008/78共4兲/045413共6兲©2008 The American Physical Society045413-1
images show the presence of individual and small bundles of
DWs with diameters ranging from 0.6 to 3 nm 共see Fig. 1兲.
In the CCVD method individual CNTs are grown from cata-
lytic particles formed in situ through selective reduction of
cobalt oxide to cobalt nanoparticles. After the chemical etch-
ing of the catalyst, the tubes agglomerate into bundles
through van der Waals interaction. Frequent formation of in-
terstitial channels is expected due to the broad diameter dis-
tribution 共2–3 nm兲. The diameter has been measured in HR-
TEM images of a hundred isolated CNTs to obtain a
diameter distribution.16 The tubes are found to be single
共⬇15%兲, double 共80%兲, or triple walled 共⬍5%兲. A typical
transmission electron microscope image is shown in Fig. 1.
We use a Renishaw Raman microprobe instrument to
record spectra for the high-pressure experiment at room tem-
perature. A microscope objective 共⫻20兲was used to focus
the laser beam 共633 nm兲on the sample inside the pressure
anvil cell. The laser output power was kept low to avoid
heating. Heating effects are relatively less important at 633
nm than at shorter wavelengths.17 In general we have used
laser powers at least four times smaller than what is needed
to cause heat-induced spectral changes.
The high-pressure Raman measurements were performed
in a diamond anvil cell. The pressure was monitored using
the luminescence of a ruby chip inside the cell and the DWs
were dispersed in sulfuric acid by sonication.
For the 647 nm excitation wavelength, the Raman spectra
were recorded using a XY-Dilor spectrometer. The beam
power had been measured at the laser. To compare laser
power values accurately, one needs to take into account the
Gaussian beam profile, the transmission factor of the micro-
scope 共1/10 for visible XY-Dilor system兲, tube orientation,
tube bundling, and surrounding medium 共gas, liquid, or sub-
strate兲.
III. USING H2SO4AS PRESSURE MEDIUM
A. p-doping effect at atmospheric pressure
Zhou et al.12 showed that the spectral G-band position of
SWs depends on the diameter and the exciting wavelength
when doping. The spectral shift is attributed to doping-
induced strain and charge transfer, which can be estimated
with corresponding shifts in graphite intercalation
compounds.18
The Gband shifts by 16 cm−1 for the first H2SO4inter-
calation stage and shifts two or three times this value for the
second and third intercalation stages.19 High-pressure
experiments20 on intercalated graphite show that the lattice
parameter
␦
a/achanges by 8⫻104for each stage, resulting
in a strain-induced shift of 4 cm−1. The remaining shift of
12 cm−1 can be attributed to modification of the electron-
phonon interaction due to charge transfer.21 Consequently,
doping-induced shifts in DWs can be attributed to changes in
the electron-phonon coupling after subtracting strain-induced
shifts as observed for intercalated graphite.
In Fig. 2, we show Raman spectra excited at 633 nm in
the spectral region of the Dand Gbands and the G2D
⬘band of
SWs 共diameters: 1.4 and 0.8 nm兲and DWs. All spectra have
been recorded in air form samples before and after doping
using 1 mW.
We note that SWs with an average diameter of 1.4 nm
show a doping-induced shift of the Gband to higher frequen-
cies. Contributions from the G−band in SWs are strongly
reduced in intensity or absent when doped for 1.4 nm diam-
eter SWs. The average tube diameter in DWs and SWs are
larger than 1.4 nm in our DW sample and we expect a large
upshift as a result of chemical doping. However, we observe
a narrower Gband after doping for DWs containing ⬇15%
of SWs and no upshift. To understand this difference, we
examine in Sec. III B the influence of hydrostatic pressure on
the Gband of doped DWs.
B. Hydrostatic pressure
At normal pressure, i.e., without anvil cell, the Gband
shows no apparent contributions from the inner and outer
tubes. When applying pressure, as seen in Fig. 3, the Gband
from the outer tube shifts to higher frequencies at a larger
rate and the intensity of the Gband of the inner and outer
FIG. 1. 共Color online兲Transmission electron microscopy images
of DWs.
1300 1400 1500 1600 2500 2600 2700
DW ∆ω=0cm-1
∆ω=20cm-1
∆ω=20 cm-1
300K - 633nm
SW 0.8nm
SW 1.4nm
D
G’2D
G
H2SO4
Air
RAMAN INTEN
S
ITY (arb. u.)
WAVENUMBER (cm
-1
)
FIG. 2. Effect of doping with H2SO4on Raman spectra of SW
and DWs excited at 633 nm.
PUECH et al. PHYSICAL REVIEW B 78, 045413 共2008兲
045413-2
tubes is comparable at high pressure 共⬎4 GPa兲. The contri-
bution to the Gband from SWs in the DW sample is also
present but is less intense and not spectrally resolved at high
pressure. At around 5 GPa we observe a decrease in the
intensity of all the contributions to the Gband, which we
attribute to a possible freezing of the medium.
In Fig. 4, we have plotted the spectral position of the G
band corresponding to the inner and outer tubes and the rela-
tive intensity of the two main bands as a function of hydro-
static pressure. We can clearly notice the pressure-induced
transition at 5–6 GPa when the spectral shift and the inten-
sity saturates.
The relative small pressure-induced spectra shifts for both
inner and outer tubes 共see Table I兲using H2SO4as pressure
medium is explained by the formation of ordered molecular
shells on the surface of the tubes, which has the effect of
reducing the pressure experienced by the tubes.9,22 Figure 4
shows how the Gband of the outer tubes increases signifi-
cantly in intensity with increasing pressure when compared
to the Gband of the inner tubes. From the G-band shift with
pressure, we extrapolate the G-band position at zero pres-
sure. Interestingly, the intensity associated with the outer
tube is vanishingly small at zero pressure when excited at
633 nm. It is expected that doping moves the Fermi level
into the valence band, which changes the resonant transition
energies of the outer tubes.
C. Deducing the purity of DW samples
To determine the fraction of DWs and SWs in the sample,
we use the spectral G-band position extrapolated in Fig. 4.
Figure 5shows the G-band spectra of DW samples recorded
at three different locations 共A, B, and C兲and recorded at
different laser power levels. The left-hand side shows Stokes
and anti-Stokes spectra at location B and a spectrum of DWs
in H2SO4. To match the background level for Stokes and
anti-Stokes spectra, we have corrected the anti-Stokes part
by
4and included the Bose-Einstein factor using T
=775 K. This high temperature is attributed to single-
particle excitations.23 We note that no significant Gband is
recorded for the Stokes spectrum. On the right side of Fig. 5,
we have subtracted a linear background for each spectrum
for the three locations A, B, and C.
To fit the Gband, we take into account four spectral con-
tributions: the spectral positions for inner and outer tubes of
DWs as extrapolated from high-pressure experiments, the G+
band of SWs, and an electronic coupling interwall 共EI兲–
induced band located at 1568 cm−1. We keep the spectral
positions constant and take the intensities of the four contri-
butions as free parameters. The fitting parameters, as de-
duced from the hydrostatic experiment, are reported in Table
II. We find that even if a small spectral shift is allowed for
the four bands, the intensity ratio remains unchanged. For
location B, we show the spectra for two different laser pow-
ers. The fit remains stable and the higher power has the ef-
fects of simply reducing the spectral noise.
From the result of the fit, we can take the ratio of the
intensity of the two G-band contributions corresponding to
SWs and the inner tubes of DWs. We assume that the ratio of
the number of SW tubes and DW tubes, NSW/NDWi, is pro-
1
500 1550 1600 1650 1700
9.5
8.7
6.3
4.6
3.7
3.3
0.9
2.5
P(GPa)
H2SO4- 633nm - 300K
WAVENUMBER (cm-1)
FIG. 3. Gband of DW sample for pressures up to 9.5 GPa using
H2SO4as pressure transmitting medium.
012345678910
1580
1590
1600
1610
1620
1630
01234567891
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
11cm-1
Very small
signal
Acurate
Inner
Outer
WAVENUMBER (cm
-
1
)
PRESSURE
(
GPa
)
DW
- 633nm - 300
K
I(outer)/I(inner)
PRESSURE (GPa)
FIG. 4. G-band position of inner and outer tubes in function of
pressure 共left兲and intensity ratio 共right兲.
1500 1550 1600 1500 1550 1600
A
633nm - DW 2.2nm -
S
W 1.6nm
H2SO4air
Without background
S 100mW/4x25mW - B
AS 100mW B - corrected
RAMAN INTEN
S
ITY (arb. u.)
WAVENUMBER
(
cm
-1
)
0.41
0.52
0.52
0.29
I(SW)/I(DWi)
25mW - A
25mW - C
100mW - B
25mW - B
0
WAVENUMBER (cm
-1
)
FIG. 5. Doped and undoped Raman spectra of Gband of DW
sample on three different locations 共A, B, and C兲: Stokes 共S兲and
anti-Stokes 共AS兲spectra after including the Bose-Einstein 共T
=775 K兲and
4factors.
RAMAN GBAND IN DOUBLE-WALL CARBON…PHYSICAL REVIEW B 78, 045413 共2008兲
045413-3
portional to the ratio of intensities of the Gband for SWs and
inner tubes of DWs, ISW /IDWi. Taking the experimentally
determined purity 共80%兲by analyzing transmission electron
microscopy images and the average value of ISW /IDWire-
ported in Fig. 5, we find an empirical proportionality factor
共633 nm excitation兲:
NSW
NDW
⬇0.3 ISW
IDWi
.
To understand the proportionality factor of 0.3, we should
keep in mind that the spectral G-band position for the outer
tube is at a higher frequency for DWs than that for SWs 共26
and 16 cm–1兲and that at atmospheric pressure, the outer
tube does not contribute to the Gband significantly.
We note that considerable reduction in the G-band inten-
sity through doping of up to 50% has been reported in the
literature,12 and we observe that the intensity of the Gband
falls exponentially with pressure as I=I0exp共−0.03⌬
outer兲.
IV. PRESSURE MEDIUM- AND TEMPERATURE-INDUCED
CHANGES IN THE RAMAN GBAND
A. Pressure medium and excitation wavelength
Table Ilists the spectral G-band positions, pressure coef-
ficients, and relative intensities for the inner and outer tubes
and for different pressure media. We find that the pressure
medium itself influences the exact spectral position of the G
band. The larger effect of oxygen on the G-band position
compared to that of alcohol or argon can be explained by p
doping of the tubes by oxygen which upshifts the G
band.24,25 Chen et al.8investigated DWs grown from pea-
pods doped with Br2共pdoping兲and showed that the charge
transfer is dominated by the outer tube with only 10% of the
total charge originating from the inner tube.
By fitting the Gband of DWs, we find a contribution on
the lower-frequency side 共1560 cm−1兲, which is also ob-
served for SWs. This band persists with increasing pressure
for DWs in contrast with what is observed for SWs.10 The
two Gbands for both type of DWs 共CCVD and peapods兲
shift at a similar rate with pressure, i.e., 3 and 6 cm−1 GPa−1
for the inner tubes and the outer ones, when using ethanol-
methanol as pressure transmitting medium. In the low-
pressure regime 共⬍3 GPa兲, the two bands overlap and the
numerical fit is not stable if the Gbands of the inner and
outer walls are not known. A change in shape can be either
due to intensity variation or change in spectral position. We
find that there are clear differences between the two types of
DWs for the Gband of the inner tubes when extrapolating
from pressure-induced shifts. The spectral position of the
inner tube at zero pressure deduced from linear fitting is at
1579 cm−1 for the DWs grown from peapods and at
1581 cm−1 for DWs grown by CCVD. We also find that the
shifting of the Gband of the inner tube with pressure is
delayed for DWs grown by CCVD. This can be explained by
differences in the interwall spacing between the two types of
DWs with different diameter distributions. This is consistent
when deducing the interwall distance in DWs from radial
breathing modes . This implies that the coupling of the two
walls is not the same, which is consistent with the differ-
ences observed for the band at 1560 cm−1 attributed to in-
terwall interaction.
When excited at 514 nm as compared to at 633 nm, the
spectral position of the Gband of the inner tube remains the
same, while changes in the position of the Fermi level with
respect with the excitation energy increases the intensity
from the outer tubes 共1608 cm−1兲. In the report of Kim et
al.,11 we can see a slight change in the DW G-band shape as
the signal from the outer tube is reduced, while the signal
from the inner tube and the band due to electronic interwall
interaction is upshifted 共CCVD tubes兲.ndoping on DW
from peapods show a more complex G-band shape.26 It ap-
pears that without doping, the walls are less coupled. ndop-
ing reduces coupling between the walls due to lattice exten-
sion of the outer tube, while pdoping increases coupling
between the walls due to lattice compression of the outer
tube.13
B. DWs in air and temperature variation
In this section, we use the parameters deduced here and
from previous studies27 on CCVD grown DWs to fit G-band
spectra as a function of temperature.
TABLE I. Spectral G-band positions of inner and outer tubes of DWs, corresponding pressure coefficients, relative intensities, and
spectral positions of the shoulder for four different pressure transmitting media 共excitation wavelength, 633 nm兲共i: inner tube; o, outer tube;
EI, electronic interwall兲.
Medium
i共P=0兲
共cm−1兲
o共P=0兲
共cm−1兲
d
i/dP
共cm−1 GPa−1兲
d
o/dP
共cm−1 GPa−1兲d
i/d
oI共i兲/I共o兲
⌫i-o共P=0兲
共cm−1兲
EI共P=0兲
共cm−1兲
⌫EI
共cm−1兲
Me-Et 1582 1594 3.3 5.8 0.57 1.04 9 1560 35
O21584 1598 4.1 6.9 0.59 0.64 10 1560 35
Argon 1581 1592 5.1 8.6 0.59 0.54 13 1560 35
H2SO41587 1618 2.2 ⬇2.1共⫾30%兲Linear 10
TABLE II. Parameters for fitting SW/DW spectra in H2SO4共red
wavelength excitation兲.
Wave number
共cm−1兲
HWHM
共cm−1兲
DW-inner =1587 ⌫=10
DW-outer =1618 ⌫=10
SW =1606 ⌫=10
DWEI =1568 ⌫=35
PUECH et al. PHYSICAL REVIEW B 78, 045413 共2008兲
045413-4
We have previously determined the variation in the half
width at half maximum 共HWHM兲and the spectral G-band
position with temperature, and we have determined the
G-band position and the HWHM of DWs in Ar and O2at
zero pressure using 633 nm excitation.
When excited in the ultraviolet or red spectral region, we
find that the HWHMs of the Gband for the inner and outer
tubes range between 8 and 10 cm−1, while the electronic
coupling–induced band ranges from 25 to 35 cm−1.
In the absence of any doping, the Gband of the outer
tubes falls in the same spectral range as the G+ band of the
SWs and we consider only three bands at constant spectral
positions and constant HWHM for a given temperature. The
parameters used for the three bands including the
temperature-induced shifts are listed in Table III.
For the fitting, we use two parameters for the linear back-
ground and three parameters for the intensities of the three
bands and we include temperature-induced shifts 共Table III兲.
Two sets of spectra have been used to test the fitting scheme.
The fitted spectra are shown in Fig. 6.
Figure 6shows the Gband as a function of laser power
using two different microscope objectives and recorded at
633 nm. We find that the fits are only satisfactory when the
intensity ratio between inner and outer tube is not kept con-
stant. We find that the intensity associated with the outer tube
is small in comparison to the intensity associated with the
inner tube as observed before. We notice that at low laser
power and without any previous beam exposure, a band ap-
pears at 1610 cm−1. As the laser power is increased, the
downshift and broadening of all bands is consistent with the
increase in temperature as indicated in Table III. At tempera-
tures above 350 ° C, the Gband changes irreversibly, which
we attribute to transformation and oxidation of the tubes.
This is deduced from the fact that the spectral position is
located between 1581 and 1592 cm−1 when the laser power
is reduced, inconsistent with the temperature-induced spec-
tral shifts.
V. CONCLUSION
We find that chemical doping with H2SO4and using hy-
drostatic pressure, we can extrapolate the G-band position of
the inner and outer tubes. The fitted spectral intensity, assum-
ing fixed spectral positions for contributions from inner and
outer walls of DWs and SWs, is used to determine the purity
of CCVD DWs containing SWs. Apart from the contribu-
tions of the Gband of the inner and outer tubes at 1581 and
1592 cm−1, a band related to electronic interwall interaction
at 1560 cm−1 is observed. This band is associated with the
outer tubes due to the fact that it shifts with pressure at the
same rate as the outer tubes. When chemical doping with
H2SO4is considered, this additional band persists, contrary
to what is observed for SWs, where the band disappears
when doping. Gbands from DWs recorded in air and at
different temperatures can be explained by using constant
G-band positions for inner and outer walls.
ACKNOWLEDGMENT
We thank Jenny Patterson, Intel Ireland, for stimulating
discussions.
*pascal.puech@cemes.fr
1K. Matsubara, K. Sugihara, and T. Tsuzuku, Phys. Rev. B 41,
969 共1990兲.
2B. Bourlon, C. Miko, L. Forro, D. C. Glattli, and A. Bachtold,
Phys. Rev. Lett. 93, 176806 共2004兲.
3S. Bandow, M. Takizawa, K. Hirahara, M. Yudasaka, and S.
Iijima, Chem. Phys. Lett. 337,48共2001兲.
4E. Flahaut, A. Peigney, Ch. Laurent, and A. Rousset, J. Mater.
Chem. 10, 249 共2000兲.
5M. Endo, H. Muramatsu, T. Hayashi, Y. A. Kim, M. Terrones,
and M. S. Dresselhaus, Nature 共London兲433, 476 共2005兲.
6C. Thomsen and S. Reich, in Light Scattering in Solids IX, edited
by M. Cardona and R. Merlin 共Springer-Verlag, Berlin Heidel-
berg, 2005兲.
7Ladislav Kavan and Lothar Dunsch, ChemPhysChem 8, 974
共2007兲.
8G. Chen, S. Bandow, E. R. Margine, C. Nisoli, A. N. Kolmog-
orov, V. H. Crespi, R. Gupta, G. U. Sumanasekera, S. Iijima, and
P. C. Eklund, Phys. Rev. Lett. 90, 257403 共2003兲.
9P. Puech, E. Flahaut, A. Sapelkin, H. Hubel, D. J. Dunstan, G.
TABLE III. Parameters for fitting DW spectra in air 共red wave-
length excitation兲.
Gband Electronic contribution
⌫=10⫻共1+ 0.00146⌬T兲⌫EI =35⫻共1+ 0.003⌬T兲
inner =1581− 0.024⌬T
outer =1592− 0.024⌬T
EI =1560− 0.024⌬T
1450 1500 1550 1600 1450 1500 1550 1600 165
0
-140K
550K
360K
360K
330K
20K
λ=647nm x40
1.5mW
backward
35mW
19mW
13mW
7mW
3.5mW
RAMAN INTENSITY (arb. u.)
∆
T
(from fit)
Laser Power
λ
=633nm x20
∆
T
(from fit)
330K
130K
100K
30K
10mW
25mW
50mW
100mW
WAVENUMBER (cm
-1
)
FIG. 6. DW spectra excited with red wavelengths and fitted with
the parameters reported in Table III. Arrows indicate that a band at
1550 or 1610 cm−1 was used. Laser power and objective power
共⫻40 or ⫻20兲are also indicated.
RAMAN GBAND IN DOUBLE-WALL CARBON…PHYSICAL REVIEW B 78, 045413 共2008兲
045413-5
Landa, and W. S. Bacsa, Phys. Rev. B 73, 233408 共2006兲.
10 A. Merlen, N. Bendiab, P. Toulemonde, A. Aouizerat, A. San
Miguel, J. L. Sauvajol, G. Montagnac, H. Cardon, and P. Petit,
Phys. Rev. B 72, 035409 共2005兲.
11 Y. A. Kim, H. Muramatsu, M. Kojima, T. Hayashi, M. Endo, M.
Terrones, and M. S. Dresselhaus, Chem. Phys. Lett. 420, 377
共2006兲.
12 W. Zhou, J. Vavro, N. M. Nemes, J. E. Fischer, F. Borondics, K.
Kamaras, and D. B. Tanner, Phys. Rev. B 71, 205423 共2005兲.
13 E. B. Barros, H. Son, Ge. G. Samsonidze, A. G. Souza Filho, R.
Saito, Y. A. Kim, H. Muramatsu, T. Hayashi, M. Endo, J. Kong,
and M. S. Dresselhaus, Phys. Rev. B 76, 045425 共2007兲.
14 P. Puech, H. Hubel, D. J. Dunstan, R. R. Bacsa, C. Laurent, and
W. S. Bacsa, Phys. Rev. Lett. 93, 095506 共2004兲.
15 J. Arvanitidis, D. Christofilos, K. Papagelis, K. S. Andrikopou-
los, T. Takenobu, Y. Iwasa, H. Kataura, S. Ves, and G. A. Kour-
ouklis, Phys. Rev. B 71, 125404 共2005兲.
16 E. Flahaut, R. Bacsa, A. Peigney, and Ch. Laurent, Chem. Com-
mun. 共Cambridge兲2003, 1442.
17 A. Bassil, P. Puech, L. Tubery, W. Bacsa, and E. Flahaut, Appl.
Phys. Lett. 88, 173113 共2006兲.
18 C. T. Chan, W. A. Kamitakahara, K. M. Ho, and P. C. Eklund,
Phys. Rev. Lett. 58, 1528 共1987兲.
19 R. Nishitani, Y. Sasaki, and Y. Nishina, Phys. Rev. B 37, 3141
共1988兲.
20 M. Hanfland, H. Beister, and K. Syassen, Phys. Rev. B 39,
12598 共1989兲.
21 S. Piscanec, M. Lazzeri, J. Robertson, A. C. Ferrari, and F.
Mauri, Phys. Rev. B 75, 035427 共2007兲.
22 W. Zhou, J. E. Fischer, P. A. Heiney, H. Fan, V. A. Davis, M.
Pasquali, and R. E. Smalley, Phys. Rev. B 72, 045440 共2005兲.
23 D. S. Kim and P. Y. Yu, Phys. Rev. B 43, 4158 共1991兲.
24 P. Corio, A. P. Santos, P. S. Santos, M. L. A. Temperini, V. W.
Brar, M. A. Pimenta, and M. S. Dresselhaus, Chem. Phys. Lett.
383, 475 共2004兲.
25 S. B. Fagan, A. G. Souzafilho, J. Mendes Filho, P. Corio, and M.
S. Dresselhaus, Chem. Phys. Lett. 406,54共2005兲.
26 H. Rauf, T. Pichler, R. Pfeiffer, F. Simon, H. Kuzmany, and V.
N. Popov, Phys. Rev. B 74, 235419 共2006兲.
27 P. Puech, F. Puccianti, R. Bacsa, C. Arrondo, V. Paillard, A.
Bassil, M. Monthioux, E. Flahaut, F. Bardé, and W. Bacsa, Phys.
Rev. B 76, 054118 共2007兲.
PUECH et al. PHYSICAL REVIEW B 78, 045413 共2008兲
045413-6
