Mono-sized single-wall carbon nanotubes formed in channels of AlPO4-5 single crystal

Article abstract of DOI:10.1063/1.121704

An alternative approach to the synthesis of mono-sized and parallel-aligned single-wall carbon nanotubes (SWCNs) is reported. The SWCNs are formed in 0.73 nm sized channels of microporous aluminophosphate crystallites by pyrolysis of tripropylamine molecules in the channels. They are characterized through transmission electron microscopy, polarized Raman scattering, and electrical transport measurements. Our results would open a door to further detailed studies on the intrinsic properties of carbon nanotubes now in progress.

Full text of DOI:10.1063/1.121704

Mono-sized single-wall carbon nanotubes formed in channels of AlPO4-5
single crystal
Z. K. Tang, H. D. Sun, J. Wang, J. Chen, and G. Li
Citation: Appl. Phys. Lett. 73, 2287 (1998); doi: 10.1063/1.121704
View online: http://dx.doi.org/10.1063/1.121704
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v73/i16
Published by the AIP Publishing LLC.
Additional information on Appl. Phys. Lett.
Journal Homepage: http://apl.aip.org/
Journal Information: http://apl.aip.org/about/about_the_journal
Top downloads: http://apl.aip.org/features/most_downloaded
Information for Authors: http://apl.aip.org/authors
Downloaded 07 Jul 2013 to 141.161.91.14. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

APPLIED PHYSICS LETTERS
VOLUME 73, NUMBER 16
19 OCTOBER 1998
Mono-sized single-wall carbon nanotubes formed in channels
of AlPO -5 single crystal
4
Z. K. Tang,a) H. D. Sun, and J. Wang
Physics Department, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,
Hong Kong, People’s Republic of China
J. Chen and G. Li
Department of Chemistry, Jilin University, Changchun 130023, People’s Republic of China
͑
Received 8 June 1998; accepted for publication 13 August 1998͒
An alternative approach to the synthesis of mono-sized and parallel-aligned single-wall carbon
nanotubes ͑SWCNs͒ is reported. The SWCNs are formed in 0.73 nm sized channels of microporous
aluminophosphate crystallites by pyrolysis of tripropylamine molecules in the channels. They are
characterized through transmission electron microscopy, polarized Raman scattering, and electrical
transport measurements. Our results would open a door to further detailed studies on the intrinsic
properties of carbon nanotubes now in progress. © 1998 American Institute of Physics.
͓
S0003-6951͑98͒00442-2͔
Carbon nanotubes, originally discovered as a byproduct
of fullerene research, are attracting increasing interest as
tures of 350–450 °C and the formation of carbon nanotubes
at 500–800 °C. The formation of the SWCNs is very sensi-
tive to the quality of the AFI crystal, the encapsulation of
hydrocarbon molecules in the AFI channels, and the pyroly-
sis conditions. The carbon nanotube contained AFI crystal
͑C-AFI͒ behaves as a good polarizer with high absorption for
1
they offer new prospects to fundamental as well as nanotech-
2
nological applications. An important recent advance in car-
bon nanotube science is the synthesis of single-wall carbon
nanotubes ͑SWCNs͒ in high yield using the laser ablation
3
4
method and the electric arc technique. In each case, a small
amount of transition metal was added to the carbon target as
a catalyst. However, it is still a big challenge to produce
mono-sized SWCNs with well-defined symmetry, and to
make a good electrical contact to a single nanotube for car-
rying out experimental investigations. Here we report an al-
ternative approach to the synthesis of mono-sized and
parallel-aligned SWCNs in channels of microporous single
crystal by pyrolysis of tripropylamine ͑TPA͒. The nanotube
structure is confirmed using the transmission electron mi-
croscopy ͑TEM͒ observation. The SWCNs are characterized
by polarized Raman scattering and electrical transport mea-
surements. The fabrication of monodispersed SWCNs in
three-dimensional ordered and nano-sized channels repre-
sents an important step towards the development and appli-
cations of carbon nanotube materials.
the light polarized parallel to the channel direction (E c) and
ʈ
with a high transparency for EЌc, consisting of the one-
dimensional characteristics of the nanotubes.
Figure 1 shows the TEM image of the nanotubes ex-
posed in free space after removing the AFI framework using
HCl acid. The image was taken using a Philips CM200 at
2
00 keV. Smoothly bent wire-like carbon specimens, with a
separation distance of about 7 Å, are seen in the image. We
attribute the wire-like carbon specimen to SWCNs. The di-
ameter of the SWCNs is expected to be smaller than 7 Å.
The structure of the small SWCNs is stable when they are
confined in the channels of AFI crystal, but they are unstable
Microporous aluminophosphate AlPO -5 crystals ͑AFI͒
4
with elongated hexagonal prisms of dimensions 110 ␮m in
cross-section diameter and 300 ␮m in length were used as
the hosts to encapsulate the SWCNs. The framework of the
AFI crystal is constructed of alternative tetrahedra of
Ϫ
ϩ
(
AlO4) and (PO ) , which form parallel open channels
4
packed in the hexagonal structure. The coordinate diameter
of the channels is 10.1 Å ͑inner diameter 7.3 Å͒, and the
separation distance between two neighboring channels is
13.7 Å. The AFI host was synthesized according to the
method given in Ref. 5. TPA molecules were encapsulated in
the channels during the crystal growth. The synthesis proce-
dure of the SWCNs involves the pyrolysis of TPA molecules
Ϫ4
in the AFI channels in a vacuum of 10 Torr at tempera-
FIG. 1. Transmission electron microscope ͑TEM͒ image shows the SWCNs
which are slightly bent. The TEM image was taken after the AFI framework
was removed using HCl acid, and the SWCNs were exposed in free space.
a͒Electronic mail: PHZKTANG@usthk.ust.hk
0003-6951/98/73(16)/2287/3/$15.00
2287
© 1998 American Institute of Physics
Downloaded 07 Jul 2013 to 141.161.91.14. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

2288
Appl. Phys. Lett., Vol. 73, No. 16, 19 October 1998
Tang et al.
FIG. 3. Polarized Raman spectra of the SWCNs measured in ͑a͒ ZZ con-
figurations, and ͑b͒ XZ configuration. In the ZZ configuration, the polariza-
tions of the incident and the scattered lights are parallel to each other and are
polarized along the nanotube direction; in the XZ configuration the polar-
izations of the incident and the scattered lights are perpendicular to each
other.
FIG. 2. Raman spectrum of the SWCNs measured at room temperature
using the 632.8 nm line of a He–Ne laser as the excitation.
when they are exposed into free space especially under
electron-beam irradiation. This fact indicates that the one-
dimensional channels of AFI crystal plays an important role
in stabilizing small carbon nanotubes.
ization selection rules for a zigzag (n,0) or an armchair
6
(
n,m) nanotube, the A1g mode is allowed but the E1g and
It is difficult to determine the configuration of the
SWCNs accurately from electron microscopy, but the nano-
tube configuration can be studied in detail using Raman
spectroscopy. Figure 2 shows Raman spectra of the SWCNs
using a 632.8 nm line of a He–Ne laser as excitation. The
Raman spectrum exhibits three main zones at low
E2g modes are forbidden in the ZZ configuration. The E1g is
allowed and the A1g and E2g modes are forbidden in the XZ
configuration. Comparing the polarization behavior of Ra-
man spectra in Fig. 3 with the polarization selection rules,
Ϫ1
the bands at 534 and 1615 cm
can be assigned to A1g
Ϫ1
Ϫ1
Ϫ1
modes, and the band at 1598 cm to the E1g mode. The
presence of the E1g as a shoulder in the ZZ configuration
͑curve a͒ might be due to a slight relaxation of the E1g mode
selection rule because of some possible structure defects in
the SWCNs.
(
200–800 cm ), intermediate (1000–1500 cm ), and
Ϫ1
high (1500–1620 cm ) frequencies. The strongest low-
frequency Raman mode at 534 cm is expected to be the
Ϫ1
radial breathing A1g mode. This frequency is in the silent
region for graphite and the other carbon materials. Hence, it
is a good marker for specifying the carbon nanotube struc-
ture. The radial breathing A1g mode is not sensitive to nano-
tube structure but to the nanotube radius. The observed fre-
The electric transport properties are also studied for the
SWCNs. In order to carry out electrical measurements, the
sample was vertically fixed using epoxy inside a small hole
drilled on a machinable ceramic. Both sides of the sample
were then polished until the hexagon ends of the crystal were
exposed. Electrical contacts were made by evaporating a thin
layer of gold on both end surfaces, as shown in Fig. 4͑a͒.
Current–voltage (I–V) properties are measured in the tem-
perature range from 0.3 to 300 K. The inset of Fig. 4͑b͒
shows the conductivity ␴ of the SWCNs measured near zero
voltage, plotted in logarithm scale as a function of the tem-
perature T. At room temperature the conductivity of the
Ϫ1
quency of 534 cm is, however, a little higher than that
6
expected for a nanotube with a radius of about 3 Å. This
high-frequency shift may result from the interaction between
the nanotube wall and the rigid wall of the AFI channel. In
the high-frequency zone, there is a main Raman band at
Ϫ1
Ϫ1
1
615 cm with a shoulder at 1598 cm . According to the
6
,7
calculation, some modes with symmetries of A1g , E1g
,
and E2g are expected in this frequency region. These Raman
modes are sensitive to polarization configuration. Thus, mea-
surement of polarized Raman scattering can specify the dif-
ferent vibration modes. Figure 3 shows the polarized Raman
spectra. Curve ͑a͒ is measured in a ZZ configuration in
which the incident and the scattered polarizations are parallel
to each other along the tube direction, while curve ͑b͒ is
measured in a XZ configuration in which the incident and the
scattered polarizations are perpendicular to each other. The
intensity of the Raman signal in the ZZ configuration is
much stronger than those in all other polarization configura-
tions, indicating the one-dimensional characteristics of the
carbon specimen accommodated in the AFI channels. As
seen in Fig. 3, the Raman spectrum in the ZZ configuration
Ϫ1
Ϫ1
Ϫ1
SWCN is on the order of 10
⍀
cm , which is lower
than the reported conductivity of metallic single-wall carbon
nanotubes.8 It is worthwhile to point out that the backbone
of the AFI crystal with or without TPA in the channels is
highly insulating. It contributes a current at least five orders
of magnitude smaller than that of C-AFI samples. The con-
ductivity of the SWCNs is monotonically increased with in-
creasing temperatures, indicating that the SWCN is of semi-
conductor characteristics. The typical dc I–V curve ͑open
circles͒ measured at different temperatures is plotted in log–
log scale in Fig. 4͑b͒. The solid lines are fittings with IϰV
and IϰV3 as indicated in Fig. 4. The current starts out as a
–10
/2
Ϫ1
3/2
is dominated by the 534 cm band in the low-frequency
linear function of the voltage, but switches to a V behavior
Ϫ1
zone and the 1615 cm band in the high-frequency zone.
when the bias voltage is high or when the temperature is low.
Since the SWCNs in our experiment are not intentionally
doped, the conducting carriers are expected to be due to the
thermally generated free carriers. Because the electrode
These two bands are absent in the XZ configuration. Instead,
the spectrum in the XZ configuration is dominated by the
Ϫ1
1
598 cm band. According to the symmetry and the polar-
Downloaded 07 Jul 2013 to 141.161.91.14. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

Appl. Phys. Lett., Vol. 73, No. 16, 19 October 1998
Tang et al.
2289
and n and n are the free carrier density injected from the
0
electrode and the thermally generated carrier density, respec-
tively. At high temperature or low bias voltage, n ӷn, the
0
current is simply given by Eq. ͑1͒: I ϭI ␮(nϩn )V,
⍀
⍀0
0
which is bulk limited and is a linear function of bias voltage
V ͑ohmic current͒. At low temperature or under high bias, n
is comparable with n . Solving Eqs. ͑1͒ and ͑2͒ yields
0
1/2 3/2
ISCLϭI ␮n0
V , which is the so-called space-charge-
S0
limited ͑SCL͒ current. At a given temperature, the total cur-
rent of the sample is due to the superposition of the I⍀ and
the ISCL . We expect the current to be dominated by I⍀ at
low bias and high temperature and by ISCL at high bias and
low temperature. The solid lines in Fig. 4͑b͒ show the calcu-
lated currents at different temperatures. The good agreement
between experimental data and calculation also indicates that
the SWCN confined in the AFI channel is an intrinsic semi-
conductor.
In summary, we have fabricated mono-sized SWCNs
with well-defined structure symmetry. They were character-
ized through TEM, Raman scattering, and electrical transport
measurements. Our results would open a door to further de-
tailed studies on the intrinsic properties of semiconducting
carbon nanotubes presently underway.
The authors are grateful to Professor L. L. Chang, Pro-
fessor M. M. T. Loy, Professor P. Sheng, and Professor G.
K. L. Wong for their encouragement and valuable comments.
In particular, H. D. Sun is grateful to Professor G. K. L.
Wong for introducing him to this research area. The authors
thank Dr. C. T. Chen for his theoretical advice and useful
discussion. The TEM image was taken by Dr. N. Wang. This
research was supported by the RGC Committee of Hong
Kong, and the EHIA program from HKUST. J. Chen and G.
Li would like to acknowledge the support from the National
Natural Science Foundation of China.
FIG. 4. ͑a͒ Schematic diagram illustrating the experimental setup for elec-
trical measurements for the SWCNs, and ͑b͒ the experimental dc I–V
curves ͑open circles͒ at different temperatures plotted on a log–log scale.
The fittings are shown as solid lines with IϰV in the low bias region and
3
/2
IϰV in the high bias region. The inset of ͑b͒ shows the conductivity
measured near zero bias as a function of temperature.
1
S. Iijima, Nature ͑London͒ 345, 56 ͑1991͒.
See, for example, a recent review book by M. S. Dresselhaus, G. Dressel-
metal and the SWCN have different work functions, a
Schottky barrier can be formed at the lead–sample contact.
When a bias is applied, the barrier prevents one type of the
thermally excited carriers ͑say electrons͒ to pass through, but
allows the other type of carriers ͑say holes͒ to pass through
freely. Hence, electrical conduction in the SWCN is ex-
pected to be a single-carrier process. A quantitative descrip-
tion of the experimental I–V curves can then be obtained by
evaluating the current across the sample by using the trans-
2
haus, and P. C. Eklund, Science of Fullerenes and Carbon Nanotubes
͑Academic, New York, 1996͒.
T. Guo, P. Nikolaev, A. Thess, D. T. Colbert, and R. E. Smalley, Chem.
Phys. Lett. 243, 49 ͑1995͒.
C. Journet, W. K. Maser, P. Bernier, A. Loiseau, M. Lamy de la Chapelle,
S. Lefrant, P. Deniard, R. Lee, and J. E. Fischer, Nature ͑London͒ 388,
756 ͑1997͒.
S. Qiu and W. Pang, Zeolites 9, 440 ͑1989͒.
R. Saito, T. Takeya, T. Kimura, G. Dresselhaus, and M. S. Dresselhaus,
Phys. Rev. B 57, 4145 ͑1998͒.
3
4
5
6
1
1,12
7
port equation
P. C. Eklund, J. M. Holden, and R. A. Jishi, Carbon 33, 959 ͑1995͒.
A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H.
Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tombnek,
8
Jϭq␮͑nϩn ͒E
͑1͒
0
J. E. Fischer, and R. E. Smalley, Science 273, 483 ͑1996͒.
T. W. Ebbesen, H. J. Lezec, H. Hiura, J. W. Bennet, H. F. Ghaemi, and T.
and the Poisson equation
9
2
Thio, Nature ͑London͒ 382, 54 ͑1996͒.
A. Yu Kasumov, I. I. Khodos, P. M. Ajayan, and C. Colliex, Europhys.
Lett. 34, 429 ͑1996͒.
M. A. Lampert, A. Many, and P. Mark, Phys. Rev. 135, A1444 ͑1964͒.
K. C. Kao and W. Hwang, Electrical Transport in Solids ͑Pergamon,
London, 1981͒.
1
d͑r E͒ qn
1
1
0
1
ϭ
,
͑2͒
r
dr
⑀
where q is the carrier charge, ␮ is the carrier drift mobility, E
is the applied electric field, ⑀ is the static electric constant,
12
Downloaded 07 Jul 2013 to 141.161.91.14. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions