Appearance of radial breathing modes in Raman spectra
of multi-walled carbon nanotubes upon laser illumination
Padmnabh Rai a, Dipti R. Mohapatra a, K.S. Hazra a, D.S. Misra a,*, Jay Ghatak b,
P.V. Satyam b
a
Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
b
Institute of Physics, Sachivalaya Marg, Bhubaneshawar 751 005, India
Abstract
The Raman spectra of the multi-walled carbon nanotubes are studied with the laser power of 5–20 mW. We observe the Raman bands
at 1352, 1581, 1607, and 2700 cm1 with 5 mW laser power. As the laser power is increased to 10, 15 and 20 mW, the radial breathing
modes (RBMs) of the single wall carbon nanotubes (SWNTs) appear in the range 200–610 cm1. The diameter corresponding to the
highest RBM is 0.37 nm, the lowest reported so far. The RBMs are attributed to the local synthesis of the SWNTs at the top surface
of the samples at higher laser power.
1. Introduction
Raman spectroscopy has been used extensively to investigate the two most important parameters; the diameter
and chirality of single wall carbon nanotubes (SWNTs)
[1–5]. A series of Raman bands are evident in the low frequency region (150–450 cm1) corresponding to the radial
breathing modes (RBM) of SWNTs. Recently a number of
groups have investigated the downshift in the position of
the Raman bands when the temperature of the sample is
increased [6–11]. Both SWNTs as well as multi-walled carbon nanotubes (MWNTs) have been studied. The temperature dependence of Raman shift arises mainly due to the
anharmonic terms in the lattice potential energy [12,13]
and lengthening of C–C bonds due to thermal expansion
[6–11]. Recently Tan et al. [13] reported that the temperature dependent Raman shift of highly oriented pyrolytic
graphite (HOPG) is purely a temperature effect and arises
due to the anharmonic coupling to phonons of other
branches and anharmonic force constant. Their results
indicate that the effect of thermal expansion may not be
significant for HOPG samples. Raravikar et al. [6] demonstrated that RBM downshift of SWNTs with temperature
is dominantly due to the softening of intra- and intertubular bonds contributions. The contributions from the
softening of the intra- and intertubular bonds are approximately equal according to their calculations.
In the current Letter we report the results of the shift in
the Raman frequencies of the MWNTs grown at temperatures 800 and 850 °C, respectively with the variation of laser
power of incident beam. We varied the laser power of Ar+
laser of wavelength 514.5 nm in the range 5–20 mW. For
a 5 mW laser power we observed the Raman bands at
1352 cm1 (D-band), 1581 cm1 (G-band), 1607 cm1
(D0 -band) and 2700 cm1 (D*-band) in the samples. As
the laser power is increased in the steps of 5 mW, not only
the frequency of D, G, and D*-bands downshift, RBM corresponding to SWNT signature in the range 200–410 cm1
also appear in the samples. When we repeat the measurement at 5 mW power after exposing the samples to
20 mW laser power, two more RBMs appear at 494 and
604 cm1. The highest RBM corresponds to a diameter of
0.37 nm; the lowest diameter SWNT reported so far
84
[14]. The results show that either a transformation from
MWNTs to SWNTs or a local synthesis of SWNTs in the
presence of catalyst particle takes place. The results indicate
an interesting new phenomenon not reported earlier.
2. Experimental
The vertically aligned MWNT samples were synthesized
on iron coated (thickness 50 nm) p-type silicon (1 0 0)
substrate by thermal chemical vapor deposition technique.
A liquid mixture of toluene/ferrocene was evaporated and
transported into the hot zone of tube furnace by a flow of
hydrogen gas (70 SCCM). The synthesis time was 30 min
and the samples were at two different temperatures: one at
800 and other at 850 °C. The CNTs were observed by using
scanning electron microscopy (SEM) in Hitachi-model
S3400 N and transmission electron microscopy (TEM) in
Technai G2 12. High resolution TEM measurements have
been carried out using 200 keV Jeol UHR machine. The
phase identification of CNTs was examined by microRaman spectroscopy (Jobin Yvon, model HR800) using
514.5 nm Ar+ laser and 100 objective lens. The spectral
resolution and spot size of Raman measurement were
0.5 cm1 and 1 lm, respectively.
3. Results and discussion
Fig. 1a and b shows the SEM micrographs at the top
surface of vertically aligned MWNTs grown at temperatures 800 and 850 °C, respectively. The top surface of the
sample is covered with iron catalyst particle (white dots)
and bunches of MWNTs. The size of catalyst is smaller
in sample deposited at 800 °C as compared to that in
850 °C; probably at higher temperature catalyst particles
agglomerate to form larger size particles. The TEM images
of MWNTs grown at temperatures 800 and 850 °C are
shown in Fig. 2a and b, respectively. The diameters of
MWNTs grown at temperature 800 °C are in the range
10–15 nm, while the diameters of MWNTs grown at
850 °C are 20–30 nm. This also conforms to the size of catalyst particles. Smaller particles observed in the sample
grown at 800 °C result in smaller diameter tubes and
vice-versa. The MWNT samples grown in our laboratory
are of high purity with no evidence of the other carbonaceous impurities.
Fig. 3a and b shows the Raman spectra in the range
100–3500 cm1 of MWNTs grown at 800 and 850 °C,
excited with laser powers of 5–20 mW. Our samples show
five major bands in the Raman spectra: (i) the radial
breathing modes (RBMs) in the frequency range 200–
610 cm1; which originates due to coherent vibration of
carbon atoms in radial direction of SWNTs, (ii) the
D-band in the range 1300–1400 cm1; the disorder induced
band in the graphite lattice or defects in the carbon nanotubes, (iii) the G-band lying in the rage 1500–1600 cm1;
due to the tangential vibration of carbon atoms, (iv) the
D0 -band at 1610 cm1; is associated with the maximum
Fig. 1. SEM micrograph recorded at top surface of vertically aligned
MWNT samples grown at temperatures (a) 800 and (b) 850 °C.
in the two-dimensional phonon density of states in graphene [15], and (v) the second harmonic of D-band in the
range of 2600–2800 cm1, denoted by D*. A qualitative
estimate of the structural purity of the nanotubes can be
obtained by calculating the ratio ID/IG.
The Raman spectra recorded at 5 mW laser power in the
sample deposited at 800 °C reveal that there is no signature
of SWNTs in the sample and only D, G, D0 , and D*-bands
are present in the pristine sample at frequencies 1352, 1581,
1607, and 2700 cm1, respectively (Table 1). The values
given in the parentheses in Table 1 are the full width at half
maximum (FWHM) of the corresponding peaks. The ratio
ID/IG is 0.77 for 5 mW laser power. As the laser power
increases from 5 to 10 mW, three RBM peaks of SWNTs
appear at 218, 280, and 393 cm1, respectively, along with
the other bands. Increasing the laser power further to
15 mW, the RBM frequencies, D, G and D*-bands of
SWNTs downshift monotonously, whereas at 20 mW laser
power, there is no significant change in the frequencies of
RBM, D, G and D*-bands. When we record the Raman
spectra of the sample again at laser power of 5 mW
85
Fig. 2. Typical TEM images of MWNT synthesized at temperatures
(a) 800 and (b) 850 °C.
(spectrum 5-a in Fig. 3a), the RBMs, D, G, and D*-bands
of SWNTs upshift as expected and two more new RBMs
appear at 494 and 604 cm1. The details of Raman analysis
at different laser power for the samples grown at 800 °C are
given in Table 1. It is noteworthy from Table 1 that the
effect of laser power on the downshift of Raman bands is
not completely reversible. The frequencies of all Raman
bands when measured at 5 mW laser power after exposure
to higher laser power (5-a in Table 1) appear at positions
different from originals. For instance the D-band which
appears at 1352 cm1 in the pristine samples shifts to
1355 cm1 after exposure to 20 mW laser power.
Similarly the sample grown at temperature 850 °C has
no RBM signature at 5 and 10 mW laser powers, whereas
the D, G, and D*-bands downshift. Again increasing the
laser power to 15 mW, the RBM of SWNTs starts appearing but the positions of D, G, and D*-bands remain
unchanged. The positions of these RBM peaks of SWNTs
are at 216, 278, and 387 cm1, respectively. At 20 mW laser
power, the RBM, D, G, and D*-bands downshift and it
upshift as the laser power decreases to 5 mW. In this sample also two more RBM peaks appear at 495 and 606 cm1
(Fig. 3b, 5-a) after repeating the experiment at 5 mW laser
Fig. 3. Micro-Raman spectra recorded at laser power 5–20 mW on top
surface of the MWNT samples synthesized at (a) 800 and (b) 850 °C. The
laser energy at which the spectra are recorded is mentioned in the figures.
Spectrum 5-a is recorded at 5 mW power again after exposing the sample
to 20 mW laser power.
power. In contrast to the sample grown at 800 °C, the
intensity of D, G and G*-bands decrease in samples grown
at 850 °C with the increase in laser power and after exposure to 20 mW laser power, the G, D0 and D* Raman bands
disappear almost entirely. Table 2 shows the details of
Raman analysis at different laser power for the sample
grown at 850 °C.
Results of Raman spectroscopy discussed above clearly
show the formation of SWNTs in the samples after the
exposure to higher laser power. The exposure of MWNTs
grown at 800 °C to a laser power of 10 and 15 mW results
in the appearance of new RBMs at 217, 279, and 389 cm1,
whereas the exposure to 20 mW laser power results in additional RBMs at 494 and 604 cm1. Similarly the MWNTs
grown at 850 °C show the appearance of RBMs at 216,
278, and 387 cm1 after exposure to laser power of 10
and 15 mW, and two additional RBMs appear at 495
86
Table 1
Position of RBM, D, G, D0 , and D*-bands, values of ID/IG, and diameters of SWNT corresponding to RBM at different laser power for the MWNTs
sample grown at 800 °C
Power
(mW)
RBM
(cm1)
D-band
(cm1)
G-band
(cm1)
D0 -band
(cm1)
D*-band
(cm1)
ID/IG
Diameter
(nm)
5
–
1352 (53)
1581 (35)
1607 (26)
2700 (65)
0.77
–
10
218
280
393
1348 (53)
1577 (31)
1607 (20)
2697 (66)
0.81
–
15
217
279
389
1347 (51)
1575 (31)
1607 (20)
2694 (71)
0.85
–
20
217
279
389
1347 (51)
1575 (31)
1607 (20)
2694 (71)
0.89
–
5-a
224
292
408
494
604
1355 (39)
1583 (37)
1620 (16)
2705 (61)
0.99
1.06
0.8
0.56
0.46
0.37
Table 2
Position of RBM, D, G, D0 , and D*-band, values of ID/IG, and diameters of SWNT corresponding to RBM at different laser power for the MWNTs
sample grown at 850 °C
Power
(mW)
RBM
(cm1)
D-band
(cm1)
G-band
(cm1)
D0 -band
(cm1)
D*-band
(cm1)
ID/IG
Diameter
(nm)
5
–
1354 (66)
1583 (37)
1614 (26)
2705 (60)
0.86
–
10
–
1351 (66)
1580 (38)
1612 (26)
2700 (61)
0.78
–
15
216
278
387
1351 (63)
1581 (39)
1612 (26)
2700 (62)
0.92
–
20
210
270
378
1345 (41)
1580 (44)
1612 (63)
2700 (47)
0.99
–
5-a
225
290
407
495
606
1314 (75)
1591 (65)
–
2704 (46)
1.23
1.06
0.81
0.56
0.46
0.37
and 606 cm1 after exposure to 20 mW. The results on two
samples are similar and reproducible. An interesting facet
of the above results is a RBM at 604 cm1 which corresponds to a SWNT of diameter of 0.37 nm which is lowest
reported so far.
Our MWNTs samples as well as SWNTs produced by
laser exposure show the downshift of the Raman bands
as the power of the incident laser beam increases. For
instance, the RBMs at 224 cm1 and 292 cm1 in SWNTs
downshift to positions 217 and 279 cm1, respectively
when the laser power is increased from 5 to 20 mW (Table
1). Similar downshifts in the position of RBMs with the
higher laser power are seen in SWNTs produced in the
samples grown at 850 °C (Table 2). In MWNTs the downshift in the position of Raman bands D, G, D0 and D* is
not as spectacular as in SWNTs.
The upshift and downshift of the Raman bands in CNTs
as a function of laser power and temperature had been
reported recently [6–11,16]. The appearance of RBMs as
the sample is exposed to higher laser power is, however,
a new and interesting phenomenon. The increase in temperature of MWNTs due to the laser exposure is generally
calculated using the intensity ratio of anti stokes and stokes
lines of the spectra and temperature at 5 mW laser power
was estimated to be 725 K [8,9]. The temperatures at
Raman location of our samples due to 10 and 15 mW laser
power can be calculated by using the expression of intertubular contribution in the RBM shift as given below [6]
dwint er ðT Þ
¼ 3wint er ðT R Þca
dT
where c is the Gruneisen constant of value 3.6 and TR is
room temperature. The value of the coefficient of thermal
expansion of SWNT bundles a = 0.75 105/K.
The estimated temperatures for 10 and 15 mW laser
powers are approximately 900 and 980 K. In the samples,
87
The above experiment has also been done in the same
conditions as described above, on the side walls of the
MWNTs and we observe no appearance of RBM of
SWNTs. Only the downshifts of D and G and D*-bands
are observed. We also performed the same experiment on
separately produced SWNTs and again only downshift of
RBM and G-band was seen. Appearance of new RBM frequencies takes place only when the top surface of sample is
exposed to laser beam.
4. Conclusions
Fig. 4. HRTEM image of the locally synthesized SWNT of diameter
0.37 nm.
the rise of temperature to 980 K would soften the C–C
bond considerably resulting in large downshift as observe
red in the sample. In contrast due to multi shell structure
of MWNTs the C–C bond may not soften considerably
and the resulting downshift is not spectacular. Also the
Vander-Wall interaction (intertubular) may not play dominant role in MWNTs sample due to the entanglement. The
disappearance of G-band in sample grown at 850 °C at
higher laser power is due to the defective nature of
MWNTs in the sample. The values of FWHM of G-band
in the sample grown at 850 °C are significantly higher
and support the above conclusion. Most surprising result
is the appearance of the RBM frequencies of SWNTs corresponding to the diameters 0.37 and 0.46 nm. The high
resolution transmission microscopy (HRTEM) image of a
sample prepared by replica technique after exposure of
MWNTs to 20 mW laser power is shown in Fig. 4. Indeed
a SWNT of diameter 0.37 nm is visible in the image. It is
surprising that a SWNT of 0.37 nm can form yet it may
be possible due to the high temperatures in the focused
spot. As shown in Fig. 1, the surface of our samples
contains iron catalyst particles and bunches of MWNTs.
Typical spot size diameter in our micro-Raman spectrometer may be of the order of 0.5 lm and it would imply a
laser density 107 W/cm2. In the focused spot thus the temperature may reach considerably higher than 980 K, the
estimated value, and from the saturated vapors of C atoms
in the presence of iron catalyst leading to the synthesis of
SWNTs. Alternatively, SWNTs could also form by transformation of MWNTs due to higher temperature. This,
however, looks remote as the diameters of SWNTs are
too small.
In conclusions, we have described the downshift of D,
G, D*-bands of MWNTs at laser power in the range
5–20 mW. The appearance of RBMs in the Raman spectra
of MWNT with the exposure of the samples to the laser
powers of 10–20 mW is also reported. The highest RBM
occurs at 604 cm1 corresponding to a SWNT diameter
of 0.37 nm; the lowest diameter reported so far. We explain
the appearance of RBMs due to the local synthesis of
SWNTs in presence of iron catalyst particles at higher laser
power. The downshift of D, G, D*-bands arises from the
temperature dependence of the C–C stretching force constant. The current results could have important implications for the ambient synthesis of SWNTs.
Acknowledgements
We acknowledge gratefully the discussion with Prof.
A.K. Sood, Department of Physics, Indian Institute of
Science Bangalore, India.
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