U.S. patent application number 10/509575 was filed with the patent office on 2005-05-19 for method for preparing monolayer carbon nanotube.
This patent application is currently assigned to JAPAN SCIENCE AND TECHNOLOGY AGENCY. Invention is credited to Hongo, Hiroo, Iijima, Sumio, Yudasaka, Masako.
Application Number | 20050106093 10/509575 |
Document ID | / |
Family ID | 28671927 |
Filed Date | 2005-05-19 |
United States Patent
Application |
20050106093 |
Kind Code |
A1 |
Iijima, Sumio ; et
al. |
May 19, 2005 |
Method for preparing monolayer carbon nanotube
Abstract
A combination of a metal-based catalyst having a function as a
catalyst for formation of graphite and a single-crystal substrate
having a certain correspondence to the metal-based catalyst with
respect to the crystal grain size and the crystal orientation
thereof is used; the metal-based catalyst is dispersed on the
single-crystal substrate; and a carbon material is fed to the
substrate at any temperature not lower than 500.degree. C. to
thereby form single single-walled carbon nanotubes through vapor
phase thermal decomposition growth on the substrate. More
precisely, the invention of this application enables production of
single-walled carbon nanotubes with controlled diameter, requiring
neither a porous material nor catalyst particles for use as a
catalyst carrier. One example of the combination of the metal-based
catalyst and the single-crystal substrate is a combination of Fe
and sapphire substrate.
Inventors: |
Iijima, Sumio; (Aichi,
JP) ; Yudasaka, Masako; (Ibaraki, JP) ; Hongo,
Hiroo; (Chiba, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Assignee: |
JAPAN SCIENCE AND TECHNOLOGY
AGENCY
1-8, Hon-cho 4-chome, Kawaguchi-shi
Saitama
JP
NEC Corporation
7-1, Shiba 5-chome, Minato-ku
Tokyo
JP
|
Family ID: |
28671927 |
Appl. No.: |
10/509575 |
Filed: |
December 8, 2004 |
PCT Filed: |
March 27, 2003 |
PCT NO: |
PCT/JP03/03884 |
Current U.S.
Class: |
423/447.1 ;
423/447.3 |
Current CPC
Class: |
C01B 2202/36 20130101;
B82Y 30/00 20130101; C01B 2202/02 20130101; C01B 32/162 20170801;
B82Y 40/00 20130101 |
Class at
Publication: |
423/447.1 ;
423/447.3 |
International
Class: |
D01F 009/12; D01C
005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 29, 2002 |
JP |
2002-097768 |
Claims
1. A method for producing single-walled carbon nanotubes, which
comprises using a combination of a metal-based catalyst having a
function as a catalyst for formation of graphite, and a
single-crystal substrate having a certain correspondence to the
metal-based catalyst with respect to the crystal grain size and the
crystal orientation thereof, dispersing the metal-based catalyst on
the single-crystal substrate, and feeding a carbon material to the
substrate at any temperature not lower than 500.degree. C. to
thereby grow single-walled carbon nanotubes through vapor phase
thermal decomposition.
2. The method for producing single-walled carbon nanotubes as
claimed in claim 1, wherein the single-crystal substrate is coated
with a thin film of metal-based catalyst.
3. The method for producing single-walled carbon nanotubes as
claimed in claim 1, wherein the thin film of metal-based catalyst
has a thickness of from 0.1 to 10 nm.
4. The method for producing single-walled carbon nanotubes as
claimed in claim 1, wherein the metal-based catalyst is any one or
a mixture of two or more components of the group consisting of iron
group metals, platinum group metals, rare earth metals, transition
metals and their metal compounds.
5. The method for producing single-walled carbon nanotubes as
claimed in claim 1, wherein the single-crystal substrate is formed
of a substance stable at 500.degree. C. or higher.
6. The method for producing single-walled carbon nanotubes as
claimed in claim 5, wherein the single-crystal substrate is any of
sapphire (Al.sub.2O.sub.3), silicon (Si), SiO.sub.2, SiC or
MgO.
7. The method for producing single-walled carbon nanotubes as
claimed in claim 1, wherein hydroxyapatite is used in place of the
single-crystal substrate.
8. The method for producing single-walled carbon nanotubes as
claimed in claim 1, wherein single-walled carbon nanotubes with
controlled diameter are grown through vapor phase thermal
decomposition, the diameter depending on the combination of the
metal-based catalyst and the single-crystal substrate and its
crystal plane.
9. The method for producing single-walled carbon nanotubes as
claimed in claim 8, wherein the combination of the metal-based
catalyst, the single-crystal substrate and the crystal plane
connecting the two is a combination of Fe and any of A-plane,
R-plane or C-plane of sapphire.
10. The method for producing single-walled carbon nanotubes as
claimed in claim 1, wherein the carbon material is a
carbon-containing substance that is gaseous at any temperature not
lower than 500.degree. C.
11. The method for producing single-walled carbon nanotubes as
claimed in claim 10, wherein the carbon material is methane,
ethylene, phenanthrene or benzene.
Description
TECHNICAL FIELD
[0001] The invention of this application relates to a method for
producing single-walled carbon nanotubes. More precisely, the
invention of this application relates to a method for producing
single-walled carbon nanotubes, which do not require a porous
material or catalyst particles as a catalyst carrier and which
enable production of single-walled carbon nanotubes with controlled
diameter.
BACKGROUND ART
[0002] Heretofore, chemical vapor deposition (CVD) methods have
been specifically given attention in production of high-quality
single-walled carbon nanotubes (SWNTs), that are extremely useful
in various industries. This is because CVD methods may enable
industrial mass-production of SWNTs and have the potential for
controlling the vapor phase thermal decomposition growth of SWNTs
by skillfully controlling the type and the particle size of the
catalyst to be used.
[0003] Many researchers have made various studies relating to the
production of SWNTs through chemical vapor deposition, and some
reports have been made by them. For example, J. King. et al. have
reported that SWNTs can be obtained by heating to 1000.degree. C. a
substrate coated with a mixture of Fe(NO.sub.3).sub.3.9H.sub.2O,
Mo(acac).sub.2 and alumina nanoparticles in a methane gas
atmosphere. J. H. Hafner, et al. have reported that SWNTs grow when
nanometer-size metal particles supported on alumina nanoparticles
are heated with circulated CO gas. In these experiments, a salt of
Fe and/or Mo is used as a metal-based catalyst, and alumina
nanoparticles are used as the carrier.
[0004] There are other reports of production of SWNTs through
chemical vapor deposition, reporting that use of porous material
such as zeolite, silica or anodized silicon oxide as a carrier
enables production of SWNTs.
[0005] However, it is to be noted that, when chemical vapor
deposition is carried out without use of such nanoparticles or
porous material as a carrier in the above-mentioned experiments,
then SWNTs could not be formed but only multi-walled carbon
nanotubes are obtained irrespective of the amount and the size of
the metal-based catalyst used.
[0006] Specifically, in production of SWNTs through conventional
vapor phase deposition, use of a metal-based catalyst and
nanoparticles or porous material as a carrier of the metal-based
catalyst is an indispensable requirement. Taking industrial
mass-production of SWNTs into consideration, a substrate that has a
fine structure comparable to that of nanoparticles or porous
material and has a broad surface area will be needed as the
carrier.
[0007] The invention of this application has been made in
consideration of the above-mentioned situation, and its object is
to provide a method for producing single-walled carbon nanotubes,
which does not require nanoparticles and porous material as a
carrier and which enables production of single-walled carbon
nanotubes with controlled diameter.
DISCLOSURE OF THE INVENTION
[0008] To solve the above-mentioned problems, the invention of this
application provides the following:
[0009] In the first aspect thereof, the invention provides a method
for producing single-walled carbon nanotubes, which comprises using
a combination of a metal-based catalyst having a catalytic function
in formation of graphite and a single-crystal substrate having a
certain correspondence to the metal-based catalyst with respect to
the crystal grain size and the crystal orientation thereof,
dispersing the metal-based catalyst on the single-crystal
substrate, and feeding a carbon material to the substrate at any
temperature not lower than 500.degree. C. to thereby grow
single-walled carbon nanotubes through vapor phase thermal
decomposition.
[0010] In the second aspect thereof, the invention provides a
method for producing single-walled carbon nanotubes, wherein the
single-crystal substrate is coated with a thin film of metal-based
catalyst; in the third aspect thereof, the invention provides a
method for producing single-walled carbon nanotubes, wherein the
thin film of metal-based catalyst has a thickness of from 0.1 to 10
nm; in the fourth aspect thereof, the invention provides a method
for producing single-walled carbon nanotubes, wherein the
metal-based catalyst is any one or a mixture of elements of a group
consisting of iron group metals, platinum group metals, rare earth
metals, transition metals, and their metal compounds; in the fifth
aspect thereof, the invention provides a method for producing
single-walled carbon nanotubes, wherein the single-crystal
substrate is formed of a substance stable at 500.degree. C. or
higher; in the sixth aspect thereof, the invention provides a
method for producing single-walled carbon nanotubes, wherein the
single-crystal substrate is sapphire (Al.sub.2O.sub.3), silicon
(Si), SiO.sub.2, SiC, or MgO; in the seventh aspect thereof, the
invention provides a method for producing single-walled carbon
nanotubes, wherein hydroxyapatite is used in place of the
single-crystal substrate; in the eighth aspect thereof, the
invention provides a method for producing single-walled carbon
nanotubes, wherein single-walled carbon nanotubes with controlled
diameter are grown through vapor phase thermal decomposition, the
diameter depending on the combination of the metal-based catalyst,
the single-crystal substrate, and the crystal plane where the two
contact; in the ninth aspect thereof, the invention provides a
method for producing single-walled carbon nanotubes, wherein the
combination of the metal-based catalyst, the single-crystal
substrate, and the crystal plane where the two contact is a
combination of Fe and the A-plane, R-plane or C-plane of sapphire;
in the tenth aspect thereof, the invention provides a method for
producing single-walled carbon nanotubes, wherein the carbon
material is a carbon-containing substance that is gaseous at any
temperature not lower than 500.degree. C.; in the eleventh aspect
thereof, the invention provides a method for producing
single-walled carbon nanotubes, wherein the carbon material is
methane, ethylene, phenanthrene, or benzene.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 includes photographs of SEM images, showing a deposit
grown at 800.degree. C. on the A-plane (a), the R-plane (b) and the
C-plane (c) of sapphire coated with a thin Fe film having a
thickness of 2 nm.
[0012] FIG. 2 includes photographs of SEM images, showing a deposit
grown at 800.degree. C. on the A-plane (a), the R-plane (b) and the
C-plane (c) of sapphire coated with a thin Fe film having a
thickness of 5 nm.
[0013] FIG. 3 includes photographs of TEM images, showing a deposit
grown on (a) A (2 nm), (b) R (2 nm), and (c) C (5 nm).
[0014] FIG. 4 shows the Raman scattering spectrum of single-walled
carbon nanotubes produced in Example; (a) indicate a range of up to
500 cm.sup.-1, and (b) indicates a range of from 1200 to 1800
cm.sup.-1.
BEST MODE FOR CARRYING OUT THE INVENTION
[0015] The invention of this application has the characteristics as
above, and its embodiments are described hereinunder.
[0016] The method for producing single-walled carbon nanotubes of
the invention of this application comprises using a combination of
a metal-based catalyst that has a catalytic function in the
formation of graphite, and a single-crystal substrate that has a
certain correspondence to the metal-based catalyst with respect to
the crystal grain size and the crystal orientation thereof,
dispersing the metal-based catalyst on the single-crystal
substrate, and feeding a carbon material to the substrate at any
temperature not lower than 500.degree. C. to thereby grow
single-walled carbon nanotubes through vapor phase thermal
decomposition.
[0017] In the invention of this application, various types of
metals having a catalytic function in formation of graphite, that
is, in vapor phase thermal decomposition growth of single-walled
carbon nanotubes, may be used for the metal-based catalyst.
Concretely, for example, any one or a mixture of components of the
group consisting of iron group metals such as Ni, Fe, Co; platinum
group metals such as Pd, Pt, Rh; rare earth metals such as La, Y;
transition metals such as Mo, Mn; and their metal compounds may be
used herein.
[0018] For the single-crystal substrate, any of various materials
that are stable at the treatment temperature of 500.degree. C. or
higher can be used. For example, they include sapphire
(Al.sub.2O.sub.3), silicon (Si), SiO.sub.2, SiC, and MgO. In
contrast to those in the related art, these materials are not
always required to be porous structures or nanoparticles, and they
may be flat. In the invention of this application, the
single-crystal substrate may be replaced, for example by pillar
crystals such as hydroxyapatite.
[0019] One characteristic feature of the invention of this
application resides in the combination of the metal-based catalyst
and the single-crystal substrate. In the invention of this
application, the metal-based catalyst and the single-crystal
substrate have a specific correspondence to each other.
Specifically, the metal-based catalyst may be combined with the
single-crystal substrate in such a manner that the substrate may
act on the specific correspondence to the catalyst in point of the
crystal grain size of the recrystallized grains thereof formed
through solid-phase reaction such as deposition or
recrystallization of the metal-based catalyst at the treatment
temperature of 500.degree. C. or higher, and of the crystal
orientation between the neighboring non-recrystallized grains. More
concretely, for example, it is desirable that the single-crystal
substrate may control the crystal grain size of the metal-based
catalyst to fall within a range of from 0.1 to 10 nm or so, at the
treatment temperature of 500.degree. C. or higher, and may have a
relation with the metal-based catalyst which acts to make the
crystal plane of the catalyst specifically oriented relative to the
single-crystal substrate. One preferred embodiment of the
combination of the metal-based catalyst and the single-crystal
substrate in the invention of this application is a combination of
Fe and sapphire.
[0020] The mode of dispersing the metal-based catalyst on the
single-crystal substrate is not specifically defined. For example,
it may be realized by dispersing fine particles of a metal-based
catalyst on a single-crystal substrate, or by coating a
single-crystal substrate with a thin film of a metal-based
catalyst. The latter method is preferred, as it is simple in actual
production lines. Various methods may be utilized for the
dispersion, concretely including, for example, a dry process of
vacuum evaporation or sputtering, and a wet process of
liquid-dropping, spraying or spin-coating.
[0021] The amount of the metal-based catalyst to be dispersed on
the single-crystal substrate is not specifically defined, and may
be any desired one. For example, the catalyst may be partially or
wholly dispersed on the single-crystal substrate to a thickness of
one atomic layer or so. When it is desired that the single-walled
carbon nanotubes are obtained at a relatively high yield, then for
example a metal-based catalyst is dispersed on a single-crystal
substrate as a thin film thereof and the thickness of the film is
controlled to fall within a range of from 0.1 to 10 nm or so,
though this may not always be desirable depending on the
combination of the metal-based catalyst and the single-crystal
substrate. If the film of metal-based crystal is too thick, then it
is unfavorable since the film may not be able to interact with the
single-crystal substrate locally in some surface part thereof and,
as a result, there may be a possibility that the metal-based
catalyst particles cannot be controlled by the substrate.
[0022] The single-crystal substrate thus having the metal-based
catalyst dispersed thereon is heated at any temperature not lower
than 500.degree. C., and then a carbon material is fed to it.
[0023] Heating the single-crystal substrate to any temperature not
lower than 500.degree. C. may be done in an inert atmosphere. The
carbon material may be any of the carbon-containing substances that
are gaseous at any temperature not lower than 500.degree. C. More
concretely, it includes, for example, methane (CH.sub.4), ethylene
(C.sub.2H.sub.4), carbon monoxide (CO) and others that are gaseous
at room temperature; and phenanthrene, benzene and others that are
solid or liquid at room temperature but become gaseous when heated
at 500.degree. C. or higher. With it, single-walled carbon
nanotubes may be grown on the surface of the single-crystal
substrate through vapor phase thermal decomposition thereon.
[0024] When the metal-based catalyst and the single-crystal
substrate are suitably combined in the manner as above, then
single-walled carbon nanotubes may be produced, not requiring
porous-structured or granular-shaped single-crystal substrates as
in the related art.
[0025] Having noted the interaction between the metal-based
catalyst and the single-crystal substrate in the invention of this
application, we, the present inventors have made more detailed
studies into this, and as a result have found that not only the
combination of the metal-based catalyst and the single-crystal
substrate as above but also the crystal plane of the single-crystal
substrate should be taken into consideration in determining the
interaction between the metal-based catalyst and the single-crystal
substrate, and that the diameter of the single-walled carbon
nanotubes to be formed may be specifically controlled depending on
the combination of all the above. The technology of controlling the
diameter of single-walled carbon nanotubes in producing them in a
mode of vapor phase thermal decomposition growth thereof has not
been known at all up to now, and we, the present inventors, are the
first to have realized it. Specifically, the method for producing
single-walled carbon nanotubes which the invention of this
application provides herein is characterized in that the specific
combination of the metal-based catalyst, the single-crystal
substrate, and its crystal plane connecting the two realizes vapor
phase thermal decomposition growth of single-walled carbon
nanotubes with a specific diameter.
[0026] More concretely, for example, for the preferred combination
of metal-based catalyst and single-crystal substrate, Fe and
sapphire mentioned above, the combination of Fe with any of the
A-plane, R-plane or C-plane of sapphire may be taken into
consideration, and the diameter of the single-walled carbon
nanotubes to be formed through vapor phase thermal decomposition
growth thereof may be controlled differently in every combination
of these. For example, regarding the combination of Fe with the
A-plane, R-plane or C-plane of sapphire, the diameter of the
single-walled carbon nanotubes to be grown is controlled to
specific values of 1.43 nm, 1.30 nm and 1.20 nm on the A-plane;
1.45 .mu.m, 1.24 nm and 1.18 nm on the R-plane; and 1.49 nm, 1.31
nm and 1.18 .mu.m on the C-plane.
[0027] In the invention of this application, in addition, the
thickness of the thin film of metal-based catalyst may be
controlled differently on each crystal plane of the single-crystal
substrate and the yield of the single-walled carbon nanotubes to be
produced may be thereby increased. More concretely, for example,
regarding the combination of Fe with any of the A-plane, R-plane or
C-plane of sapphire, the yield of the single-walled carbon
nanotubes may be increased on the A-plane and the R-plane by
reducing the thickness of the thin Fe film thereon within the range
mentioned above, while the yield thereof to be formed on the
C-plane may be increased by increasing the thickness of the thin Fe
film thereon within that range.
[0028] On the other hand, existence of single-walled carbon
nanotubes of various symmetry (chirality) is known. The chirality
of single-walled carbon nanotubes may be represented by chirality
indexes (m, n), and it has strong correlation with the diameter of
the single-walled carbon nanotubes. This suggests the possibility
that the method of the invention of this application may control
not only the diameter but also the chirality of single-walled
carbon nanotubes.
[0029] As described hereinabove, the invention of this application
indicates that the interaction between the metal-based catalyst and
the single-crystal substrate material plays an important part in
vapor phase thermal decomposition growth of single-walled carbon
nanotubes, and, when a single-crystal substrate with such a
metal-based catalyst dispersed thereon is used, then single-walled
carbon nanotubes may be grown through vapor phase thermal
decomposition thereon. In addition, when the combination of the
metal-based catalyst with the single-crystal substrate and its
crystal plane is suitably selected, the diameter of the
single-walled carbon nanotubes is controlled. Further, when the
crystal plane of the single-crystal substrate and the thickness of
the thin catalyst layer formed thereon are suitably controlled,
then the yield of the single-walled carbon nanotubes to be formed
on the substrate may be increased.
[0030] Example of the invention is described below with reference
to the drawings attached hereto, and the embodiments of the
invention are described in more detail.
EXAMPLE
[0031] Producing SWNTs was tried, using a tube furnace having an
inner diameter of 2 inches and using methane gas as the carbon
material. For the single-crystal substrate, the A-plane, R-plane
and C-plane of sapphire were used. On the single-crystal substrate,
a thin Fe film serving as the metal-based catalyst was formed
through electron beam deposition in a vacuum of about
4.times.10.sup.-6 Torr so that its thickness would be from 2 to 5
nm.
[0032] These substrates were introduced into a tube furnace and
heated in an argon atmosphere; and after they reached a
predetermined temperature between 600.degree. C. and 800.degree.
C., methane (99.999%) used here as the carbon material was fed
thereinto at a flow rate of 0.6 liters/min. The methane
introduction continued for 5 minutes, and then argon was again
introduced into the furnace. Then, the tube furnace was cooled to
room temperature.
[0033] After the heat treatment, the substrates were analyzed in
detail through observation with a scanning electron microscope
(SEM), through Raman spectrometry and through observation with a
transmission electronic microscope (TEM). The samples for SEM
observation were coated each with a thin Pd-Pt film having a
thickness of about 2 nm for more definite observation thereof. For
Raman spectrometry, the samples were exposed to 488 nm light (30
mW) from an Ar laser having a convergent spot size of about 1
.mu.m. The samples for TEM observation were prepared by collecting
the deposit from the sapphire substrate, dispersing it in ethanol,
dropwise applying the resulting dispersion onto a TEM grid and
drying it thereon.
[0034] SEM Observation
[0035] FIG. 1(a)(b)(c) show SEM images of the deposit grown at
800.degree. C. on the A-plane, R-plane and C-plane, respectively,
of sapphire coated with a thin Fe film having a thickness of 2 nm.
It is definitely observed that the amount of the tubular deposit on
the A-plane is larger than that on the R-plane. In addition, it is
also clear that the amount of the tube deposit on the C-plane is
the smallest of the three.
[0036] FIG. 2(a)(b)(c) show SEM images of the deposit grown at
800.degree. C. on the A-plane, R-plane and C-plane, respectively,
of sapphire coated with a thin Fe film having a thickness of 5 nm.
It is confirmed that the same tubular deposit as in the above is
formed on all the three planes. It is understood that these
nanotubes are either thick and short ones (having a diameter of
from 20 to 50 nm and a length of about 1 mm), or thin and long ones
(having a diameter of less than 3 nm and a length of 2 mm or
more).
[0037] In addition, when the sapphire coated with a thin Fe film
having a thickness of 2 nm was heated at 600.degree. C., then
hardly any tubular deposits grew on the A-plane and the R-plane of
the substrate, but a few thicker nanotubes (having a diameter of
from about 30 to 50 nm) were found to have grown on the C-plane.
The structure of these nanotubes was analyzed through TEM
observation and Raman spectrometry thereof.
[0038] TEM Observation
[0039] FIG. 3(a) shows a TEM image of the deposit grown on the
A-plane of sapphire coated with a thin Fe film having a thickness
of 2 nm (this is hereinafter indicated by A (2 nm)). It is
understood that A (2 nm) includes SWNTs and an extremely small
amount of amorphous carbon (this is hereinafter indicated by a-C).
The TEM image of the deposit grown on the R-plane of sapphire
coated with a thin Fe film having a thickness of 2 nm (this is
hereinafter indicated by R (2 nm)), shown in FIG. 3(b), confirms
that R (2 nm) includes SWNTs and a-C. The TEM image of the deposit
grown on the C-plane of sapphire coated with a thin Fe film having
a thickness of 5 nm (this is hereinafter indicated by C (5 nm)),
shown in FIG. 3(c), confirms that the amount of a-C is the largest
on C (5 nm) and there are hardly any SWNTs. Though not shown in
FIG. 3(c), it is confirmed that on C (5 nm) some double-walled
carbon nanotubes were grown.
[0040] The TEM observation confirms that the diameter of SWNTs
bundled on the A-plane, R-plane and C-plane of the substrate falls
approximately between 1.0 and 1.7 nm.
[0041] Raman Spectrometry
[0042] FIG. 4(a)(b) show the Raman scattering spectrum of the
deposit formed on the A-plane, R-plane and C-plane of sapphire
coated with a thin Fe film having a thickness of 2 nm, 3 nm or 5
nm. All the samples showed peaks at about 1592 cm.sup.-1 and 1570
cm.sup.-1, and showed from 1 to 4 fine peaks within a range of from
100 to 230 cm.sup.-1. These peaks are characteristics of SWNTs, and
indicate the presence of SWNTs in the deposit. The peaks appearing
at about 1592 cm.sup.-1 and 1570 cm.sup.-1 correspond to the
tangent mode, and the peaks appearing between 100 and 230 cm.sup.-1
correspond to Raman bleeding mode (RBM) of the SWNTs.
[0043] For example, SWNTs formed on R (2 nm) gave a strong RBM peak
at 167 cm.sup.-1 indicating that these are SWNTs having a diameter
of 1.4 nm, and gave a weak peak at 203 cm.sup.-1 indicating that
these are SWNTs having a diameter of 1.2 nm. However, it was found
that the peaks given by the samples coated with an Fe film thicker
than this are not pronounced. Thus, it is concluded from the
tangent mode and RBM mode peak intensity data that, with the
increase in the thickness of the Fe film from 2 nm to 5 nm, the
amount of SWNTs formed on the A-plane and the R-plane decreases. On
the other hand, however, it is found that the amount of SWNTs
formed on the C-plane increases with the increase in the thickness
of the Fe film from 2 nm to 5 nm.
[0044] The peak position and the RBM intensity of these SWNTs
differ among individual deposits thereof at different positions.
Each deposit was more carefully analyzed in at least 10 different
points thereof, and, as a result, the following tendency was clear.
Specifically, the RBM peak width is narrow, falling between 7 and
12 cm.sup.-1; the number of the peaks is from 1 to 4; and the peak
position depends on the plane of the substrate sapphire.
[0045] More concretely, for example, the Raman spectra obtained at
10 sites on the A-plane (2 nm), the R-plane (2 nm) and the C-plane
(2 nm) were separately averaged, and the RBM peak and the
calculated diameter of SWNTs are shown in Table 1.
1 TABLE 1 RBM Peak (cm.sup.-1) Sample SWNTs diameter (nm) A (2 nm)
170 188 203 1.43 1.30 1.20 R (2 nm) 168 194 207 1.45 1.24 1.18 C (2
nm) 164 186 206 1.49 1.31 1.18
[0046] As in the above, when the crystal plane of the sapphire
substrate is specifically selected, then SWNTs can be formed
thereon with their diameter controlled to be a specific value.
Comparative Example 1
[0047] In place of the substrate sapphire in the above-mentioned
Example, a silicon single-crystal plane (or SiO.sub.2 plane
thermally grown on silicon) was used, but SWNTs could not be formed
thereon through CVD at 800.degree. C. irrespective of the thickness
of the thin Fe film formed thereon.
Comparative Example 2
[0048] A sapphire substrate was coated with a thin Ni film in place
of the thin Fe film as in the above-mentioned Example, and then
processed in the same manner as above. However, SWNTs could not be
formed on it.
Comparative Example 3
[0049] A silicon wafer with a mixture of
Fe(NO.sub.3).sub.3.H.sub.2O and alumina nanoparticles (with no
Mo(acac).sub.2) applied thereto was prepared as a substrate. This
was subjected to the same heat treatment as in the above-mentioned
Example, and SWNTs were formed on it. The Raman spectrum of these
SWNTs is shown in FIG. 4. The RBM peak of the Raman spectrum is
broad, falling between 120 and 200 cm.sup.-1, and this means that
the diameter of SWNTs formed herein varies to fall within a broad
range of from 1.2 to 2.0 nm.
[0050] From this, it is understood that, though the alumina
nanoparticles of the metal-based catalyst carrier are formed of
Al.sub.2O.sub.3, the same as that of sapphire, the alumina
nanoparticles have various crystal planes and amorphous
characteristics because of their morphology, and therefore, SWNTs
could grow thereon but the diameter of SWNTs grown thereon could
not be controlled, and, as a result, the diameter of SWNTs grown
thereon fell within a broad range.
[0051] Because of the reasons above, the production of SWNTs in
conventional vapor phase thermal decomposition growth on a catalyst
carrier unavoidably requires a porous material and nanoparticles as
the catalyst carrier. In the invention of this application,
however, when the crystal to be the substrate, the crystal plane of
the substrate, the metal-based catalyst, the film thickness of the
catalyst and the crystal growing temperature are suitably selected,
then SWNTs may be formed even on a flat crystal substrate.
Accordingly, it is surmised that these requirements stipulated by
the invention have influence on the catalyst metal diffusion
coefficient and on the crystal grain size and the crystal
orientation of the catalyst metal incidental to it, and, as a
result, SWNTs having a specific diameter may be formed on the
substrate.
[0052] Needless-to-say, the invention is not limited to the
embodiments described hereinabove, and its details may undergo
various changes and modifications.
INDUSTRIAL APPLICABILITY
[0053] As described in detail hereinabove, the invention relates to
a method for producing single-walled carbon nanotubes. More
precisely, the invention of this application provides a method for
producing single-walled carbon nanotubes, which does not require a
porous material and catalyst particles and which enables production
of single-walled carbon nanotubes with controlled diameter.
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