Determining Carbon Nanotube Properties from Raman
Scattering Measurements
Ying Geng1, David Fang2, and Lei Sun3
1
2
3
The Institute of Optics, Electrical and Computer Engineering, Laboratory for Laser Energetics
Rochester, NY 14627
Abstract
Carbon nanotubes (CNT) have fascinating optical, electrical and mechanical
properties which stem from their unique geometry. Current fabrication techniques are
limited in their control of CNT size.
Therefore, a post-process characterization
technique such as Resonant Confocal Raman Spectroscopy (RCRS) becomes useful for
highlighting nanotubes with properties of interest. In our study, we grow carbon
nanotubes of varying diameters using a chemical vapor deposition (CVD) process and
subsequently determine chirality and electronic transport properties using RCRS.
Introduction
Research in carbon nanotubes (CNT) has progressed rapidly since their discovery
in 1991 by Iijima from the NEC laboratory in Tsukuba, Japan
[1]
.
CNTs are
quasi-one-dimensional structures with a length on the order of microns and diameter of
1-2 nanometers. A single-walled Nanotube (SWNT) is comprised of a single layer of
carbon (graphene) with its ends folded to form a seamless hollow cylinder.
Multi-walled nanotubes are constructed from multiple SWNT of varying radii arranged
concentrically. Nanotubes attract a great amount of interest because of their size and
unique physical, electrical, and optical properties [2]. A significant barrier in nanotube
research, however, stems from limited control of nanotube size distribution and
chirality (n,m) during fabrication. Therefore, post-fabrication characterization
techniques become critical. Resonant Confocal Raman Spectroscopy (RCRS) is one
such technique used to characterize CNTs
[3]
. In this study, we determine the size,
chirality, and electrical transport properties of several CNTs using RCMRS.
Nanotube preparation
A chemical vapor deposition (CVD) system was used in the synthesis of CNTs for
our study. A quartz substrate is pre-baked in the oven at 500 ˚C at 10-5 mbar for 12
hours to eliminate moisture which can hinder the growth process. A methanol solution
containing the catalyst bis(acetylacetonato)-dioxomolybdenum(VI) and cobalt
acetylacetonate (0.1% each by weight) was placed on the substrate and then baked
under atmospheric pressure for 20 minutes at 500 ˚C causing the cobalt to oxidize.
Next, the sample was placed into the tube furnace which was evacuated to 10-5 mbar
and purged with Ar to bring the pressure inside the furnace to 10 mbar. Pressure at 10
mbar was maintained with hydrogen gas flow at a rate of 200 sccm. The sample was
then heated to 800˚C reducing the Cobalt Oxide to metal Cobalt. In the final step,
ethanol gas was flown into the tube at a rate of 1000 gm/min at 8 mbar for 30 mintues.
At 800˚C, ethanol is decomposed into carbon which reacts with the cobalt catalyst to
form CNTs. The sample is allowed to cool for several hours under Ar flow before
Raman measurements were taken.
Raman scattering spectra
CNTs are described by their tube diameter and chirality. The chirality is defined
r
r
r
r
r
by the vector C h = na1 + ma 2 ≡ ( n, m) , where a1 and a 2 are unit vectors of the
hexagonal lattice. This vector connects two crystallographically equivalent sites O
and A on a two-dimensional plane shown in Figure 1.
Figure 1 The chirality vector describing an unrolled CNT
The
diameter
of
a
(n,m)
nanotube
can
be
determined
by
d t = C h / π = 3aC −C ( m 2 + mn + n 2 )1 / 2 / π , where aC-C is the nearest-neighbor center
r
to center distance (1.42 Å for graphite) and Ch is the length of the chiral vector C h .
SWNT are classified as metallic or semiconducting based on their chiral indices (n, m).
Metallic tubes have n − m = 3q and semiconducting CNTs have n − m ≠ 3q where q
is an integer.
Raman measurements were performed using a He-Ne laser operating at a
wavelength of 632.8 nm (1.96 eV) and input power of ~300 μW at the sample surface.
Figure 2 shows a schematic of the confocal microscope experimental setup used for
Raman signal acquisition.
Sample on XY Translation Stage
Oil immersion
NA = 1.4 (60X)
632.8 nm ~300 μW
10/90 BS
Linearly Polarized He-Ne
APD
FM
LP, BP Filter
Notch filter
Spectrometer
Figure 2 Confocal microscope experimental setup
Figure 3 shows a 40 μm x 40 μm scanning confocal microscope image of a sample area.
The encircled areas indicate selected areas where the laser excitation was in resonance
with the CNT structures.
Figure 5 shows a typical Raman spectrum from a
semiconducting nanotube. The peaks at 1300, 1592 and 2600 cm-1 correspond to the
D-band (defect), G-band and G’-band, respectively. The D-band is a result of a
photon-defect interaction. The G-band is due to the longitudinal and to a lesser extent
the circumferential, stretching of the nanotube.
The G’-band stems from a
photon-second phonon interaction. These aforementioned peaks are characteristic of
both CNTs and graphite sheets.
5 μm
Figure 3 Confocal microscope image with selected nanotubes circled. The contrast is
acquired by integrating over the intensity of the G-band in the Raman spectrum. A
632.8 nm He-Ne laser was used for the source.
The peak at 300 cm-1 corresponds to the radial breathing mode (RBM) and is
unique to CNTs. The RBM also provides information on chirality and thus the
electronic properties of the nanotube. Because a single excitation energy was used in
our experiment, only nanotubes resonant with this particular energy will demonstrate a
peak at the RBM frequency.
The nanotube diameter can be determined by
ν RBM = 248 / d t [3], where ν RBM is the Raman frequency shift of the RBM in cm-1.
The ratio of the D-band to G-band intensity ID/IG is used as a measure of the defect
density of a particular nanotube and can provide information on the crystalline quality
of the CNTs. Because the nanotubes were grown at low temperature by CVD, it is
possible that some of the structures did not fully crystallize due to insufficient thermal
energy during the annealing process. It is also possible that stray atoms may have
infiltrated the carbon lattice thus creating a defect in the periodic structure of the
nanotube.
RBM
Pixel
(cm-1)
1
285.6
7
5
2
195.5
14
2
3
173.4
10
10
4
200.4
15
5
198.0
6
7
m
d (Å)
ID / IG
Class
8.18
0.2043
SC
11.83
0.1549
M
13.57
N/A
M
0
11.75
N/A
M
14
2
11.83
0.1186
M
193.0
13
4
12.06
0.1404
M
193.0
13
4
12.06
0.3944
M
8
198.0
14
2
11.83
0.1042
M
9
193.1
13
4
12.06
0.1925
M
10
283.2
7
5
8.18
0.2246
SC
11
N/A
N/A
N/A
N/A
N/A
12
193.1
13
4
12.06
0.2285
M
13
264.2
7
6
8.83
0.1312
SC
Nanotube Size Distribution
5
4
Occurrence
n
3
2
1
0
8
M/SC
9
10
11
12
Diameter (Angstroms)
Figure 4 Histogram showing CNT diameter distribution
Table 1 CNT properties of thirteen samples
The diameter and (n, m) chirality of the 13 CNT samples are listed in Table 1.
Characteristic CNT properties were determined based on the frequency of the RBM and
laser excitation energy. Our experimental values were compared with measurements
performed by Maultzsch et. al to determine nanotube chirality and diameter
[4]
. A
distribution of CNT diameter is shown in Figure 4. Because only thirteen samples
were probed, no conclusions can be made about the general distribution of nanotube
diameter of this growth process. From literature, it is predicted that nanotube size
distributions approach a Gaussian
[2]
.
Figure 5 and Figure 6 demonstrate typical
spectra from semiconducting and metallic nanotubes, respectively. We note that the
G-band lineshape is different for these two samples as well as the position of the RBM
frequency.
13
Figure 5 Raman spectra of semiconducting sample
Figure 6 Raman spectra of metallic sample
Summary
Fabrication of carbon nanotubes was carried out using the CVD method and the
structure of CNTs identified using Raman scattering spectra. Both semiconducting
and metallic nanotubes were probed. Of the thirteen samples, four could be classified
as semiconducting. In general, the defect density of our samples was low (< %25).
Acknowledgements
This project could not have been possible without the guidance of Graham Marsh
(fabrication) and Neil Anderson (Raman measurements).
References
[1] S. Iijima, “Helical microtubules of graphitic carbon”, Nature (London) 354: 56,
1991.
[2] M. S. Dresselhaus, G. Dresselhaus, Ph. Avouris, “Carbon Nanotubes: Synthesis,
Structure, Properties and Applications”, Springer 2001.
[3] A. Jorio, R. Saito, J. H. Hafner, et al, “Structural ( n, m) Determination of Isolated
Single-Wall Carbon Nanotubes by Resonant Raman Scattering”, Phys Rev Lett. 86: 6,
2001.
[4] A. Maultzsch, H. Telg, S. Reich, C. Thomsen, “Radial breathing mode of
single-walled carbon nanotubes: Optical transition energies and chiral-index
assignment”, Phys Rev B. 72: 205438, 2005.