U.S. patent application number 09/333876 was filed with the patent office on 2002-01-10 for carbon nanotubes on a substrate.
Invention is credited to GAO, YUFEI, LIU, JUN.
Application Number | 20020004136 09/333876 |
Document ID | / |
Family ID | 23304630 |
Filed Date | 2002-01-10 |
United States Patent
Application |
20020004136 |
Kind Code |
A1 |
GAO, YUFEI ; et al. |
January 10, 2002 |
CARBON NANOTUBES ON A SUBSTRATE
Abstract
The present invention includes carbon nanotubes whose hollow
cores are 100% filled with conductive filler. The carbon nanotubes
are in uniform arrays on a conductive substrate and are
well-aligned and can be densely packed. The uniformity of the
carbon nanotube arrays is indicated by the uniform length and
diameter of the carbon nanotubes, both which vary from nanotube to
nanotube on a given array by no more than about 5%. The alignment
of the carbon nanotubes is indicated by the perpendicular growth of
the nanotubes from the substrates which is achieved in part by the
simultaneous growth of the conductive filler within the hollow core
of the nanotube and the densely packed growth of the nanotubes. The
present invention provides a densely packed carbon nanotube growth
where each nanotube is in contact with at least one
nearest-neighbor nanotube. The substrate is a conductive substrate
coated with a growth catalyst, and the conductive filler can be
single crystals of carbide formed by a solid state reaction between
the substrate material and the growth catalyst. The present
invention further provides a method for making the filled carbon
nanotubes on the conductive substrates. The method includes the
steps of depositing a growth catalyst onto the conductive substrate
as a prepared substrate, creating a vacuum within a vessel which
contains the prepared substrate, flowing H2/inert (e.g. Ar) gas
within the vessel to increase and maintain the pressure within the
vessel, increasing the temperature of the prepared substrate, and
changing the H2/Ar gas to ethylene gas such that the ethylene gas
flows within the vessel. Additionally, varying the density and
separation of the catalyst particles on the conductive substrate
can be used to control the diameter of the nanotubes.
Inventors: |
GAO, YUFEI; (KENNEWICK,
WA) ; LIU, JUN; (WEST RICHLAND, WA) |
Correspondence
Address: |
NATHAN R RIETH K1-53
BATTELLE MEMORIAL INSTITUTE
PO BOX 999
RICHLAND
WA
99352
|
Family ID: |
23304630 |
Appl. No.: |
09/333876 |
Filed: |
June 14, 1999 |
Current U.S.
Class: |
428/367 ;
428/364 |
Current CPC
Class: |
C01B 2202/08 20130101;
B82Y 30/00 20130101; Y10S 977/742 20130101; C01B 2202/36 20130101;
Y10S 977/842 20130101; Y10T 428/2913 20150115; Y10T 428/12438
20150115; Y10T 428/2918 20150115; C01B 32/162 20170801; B82Y 40/00
20130101; Y10S 977/843 20130101; Y10S 427/102 20130101; C01B
2202/34 20130101 |
Class at
Publication: |
428/367 ;
428/364 |
International
Class: |
B32B 027/14; B32B
003/00 |
Goverment Interests
[0001] This invention was made with Government support under
Contract DE-AC0676RLO1830 awarded by the U.S. Department of Energy.
The Government has certain rights in the invention.
Claims
We claim:
1. An array of a plurality of carbon nanotubes, each of said carbon
nanotubes having a first end attached to a substrate, and a second
end extending from said substrate, each of said carbon nanotubes
having a closed outer wall defining a core that is hollow, wherein
said core is filled more than 10% with a conductive filler.
2. The array as recited in claim 1, wherein each of said carbon
nanotubes is in physical contact with at least one nearest-neighbor
carbon nanotube.
3. The array as recited in claim 1, wherein said substrate is a
conductive substrate.
4. The array as recited in claim 3, wherein said conductive
substrate is selected from the group consisting of titanium,
titanium carbide, vanadium, tantalum, and combinations thereof.
5. The array as recited in claim 1, wherein said conductive filler
comprises carbon and an element from the group of elements
consisting of titanium, vanadium, tantalum, and combinations
thereof.
6. The array as recited in claim 3, wherein said conductive
substrate is a prepared substrate having a catalyst coating.
7. The array as recited in claim 6, wherein said catalyst coating
is a growth catalyst.
8. The array as recited in claim 7, wherein said growth catalyst is
selected from the group consisting of iron, iron oxide and
combinations thereof.
9. The array as recited in claim 1, wherein said carbon nanotubes
are of uniform length such that the length of each of said carbon
nanotubes varies no more than 5%.
10. The array as recited in claim 9, wherein said length ranges
from 1 to 2 .mu.m.
11. The array as recited in claim 1, wherein said carbon nanotubes
are of uniform diameter such that the diameter of each of said
carbon nanotubes varies no more than 5%.
12. The array as recited in claim 1, wherein said diameter is an
outside nanotube diameter ranging from 50 to 400 nm and an inside
nanotube diameter ranging from 10 to 100 nm.
13. A method for synthesizing an array of a plurality of carbon
nanotubes, each of said carbon nanotubes having a first end
attached to a substrate, and a second end extending from said
substrate, each of said carbon nanotubes having a closed outer wall
defining a core that is hollow, wherein said core is filled more
than 10% with a conductive filler, comprising the steps of: (a)
depositing a growth catalyst onto said substrate forming a prepared
substrate; (b) creating a vacuum within a vessel containing said
prepared substrate; (c) flowing H2/Ar gas within said vessel and
increasing the pressure within said vessel; (d) increasing the
temperature of said prepared substrate; and (e) changing said H2/Ar
gas to ethylene gas such that said ethylene gas flows within said
vessel.
14. The method as recited in claim 13, wherein said depositing
comprises depositing a layer of said growth catalyst by electron
beam evaporation.
15. The method as recited in claim 14, wherein said layer is a thin
layer between about 1 to 30 nanometers in thickness.
16. The method as recited in claim 13, wherein said growth catalyst
is selected from the group consisting of iron, iron oxide and
combinations thereof.
17. The method as recited in claim 13, wherein said vacuum is a
pressure at or below 1 torr.
18. The method as recited in claim 13, wherein said vessel is a
quartz reactor.
19. The method as recited in claim 13, wherein said pressure is a
pressure within the range of pressures between 200 torr and 400
torr.
20. The method as recited in claim 13, wherein said vessel is
within a tube furnace for said heating.
21. The method as recited in claim 13, wherein said temperature is
a temperature within the range of temperatures between 650.degree.
C. and 800.degree. C.
22. The method as recited in claim 13, wherein said substrate is a
conductive substrate.
23. The method as recited in claim 22, wherein said conductive
substrate is selected from the group consisting of titanium,
titanium carbide, vanadium, tantalum, and combinations thereof.
24. The method as recited in claim 13, wherein said conductive
filler comprises carbon and an element from the group of elements
consisting of titanium, vanadium, tantalum, and combinations
thereof.
25. The method as recited in claim 13, wherein said growth catalyst
is selected from the group consisting of iron, iron oxide and
combinations thereof.
26. The method as recited in claim 13, wherein said carbon
nanotubes are of uniform length such that the length of each of
said carbon nanotubes varies no more than 5%.
27. The method as recited in claim 26, wherein said length ranges
from 1 to 2 .mu.m.
28. The method as recited in claim 13, wherein said carbon
nanotubes are of uniform diameter such that the diameter of each of
said carbon nanotubes varies no more than 5%.
29. The method as recited in claim 28, wherein said diameter is an
outside nanotube diameter ranging from 50 to 400 nm and an inside
nanotube diameter ranging from 10 to 100 nm.
Description
FIELD OF THE INVENTION
[0002] The present invention relates generally to the synthesis of
carbon nanotubes on substrates. More specifically, the invention
relates to dense arrays of well-aligned carbon nanotubes filled
with conductive filler synthesized on conductive substrates and a
method for making these carbon nanotubes.
BACKGROUND OF THE INVENTION
[0003] Carbon nanotubes constitute a new class of materials with a
broad range of potential applications. Their unique properties make
carbon nanotubes ideal candidates for novel application in areas
such as vacuum microelectronics, flat panel displays, scanning
probes and sensors, field emission devices and nanoelectronics.
[0004] A wide range of techniques has been used to prepare carbon
nanotubes. For example, carbon nanotubes can now be produced in
high yield and with reasonable quality as reported by C. Journet et
al., Nature 388, 756 (1997), using arc discharge, by A. Thess et
al., Science 273, 483 (1996), using laser ablation, and by R. T.
Baker, Carbon 27, 315 (1989), using thermal decomposition of
hydrocarbons.
[0005] Alignment of carbon nanotubes is particularly important for
their use in applications such as flat panel displays. Ajayan et
al., Science 265, 1212 (1994) report manufacturing a composite with
carbon nanotubes randomly dispersed inside a polymer resin matrix
and found that slicing the composite caused partial alignment of
the nanotubes on the cut surface. De Heer et al., Science 268, 845
(1995) fabricated partially aligned nanotube films by drawing a
nanotube suspension through a micropore filter.
[0006] More recently, well-aligned carbon nanotube arrays have been
synthesized on solid substrates. W. Z. Li et al., Science 274,1701
(1996), report well-aligned carbon nanotube arrays synthesized by
thermal decomposition of acetylene gas in nitrogen on porous silica
containing iron nanoparticles, and Z. F. Ren et al., Science 282,
1105 (1998), report well-aligned carbon nanotube arrays synthesized
by hot-filament plasma-enhanced thermal decomposition of acetylene
gas on nickel-coated glass. All of these preparations however,
result in isolated carbon nanotubes on substrates where all the
nanotubes are separated by distances on the order of 100 nanometers
within the arrays. Disadvantages of these separations between the
carbon nanotubes include decreased nanotube capacity on the
substrate and a decreased ability to maintain alignment as the
nanotubes grow longer.
[0007] Although hollow carbon nanotubes have substantial utility,
it is recognized that filling the hollow core of carbon nanotubes
with materials having useful physical, chemical, and electronic
properties significantly broadens the range of potential
applications for carbon nanotubes. Early attempts to fill carbon
nanotubes were based on electric arc or laser ablation methods
using metal/carbon composites as reported for example by Zhang et
all., Science 281, 973 (1998), or on capillary-force infiltration
of open-ended nanotulbes as reported by Ugarte et al., Science 274,
1897 (1996). In addition, Dia et al., Nature 375, 769 (1995),
reported an attempt to fill carbon nanotubes resulting in the
reaction of the carbon nanotubes with titanium oxide (TiO) which
converted all the nanotubes into titanium carbide (TiC) nanorods.
In these and other prior experiments the carbon nanotubes were
found to be only partially filled to a level of approximately 10%.
The disadvantage of having carbon nanotubes that can only be
partially filled is a decrease in the benefit sought to be gained
through the useful properties of the materials filling the nanotube
cores.
[0008] In view of the current and potential applications for carbon
nanotubes, there remains a need in carbon nanotube technology for a
method of synthesizing dense arrays of well-aligned carbon
nanotubes on conductive substrates where the nanotubes are
simultaneously and completely filled with conductive materials.
SUMMARY OF THE INVENTION
[0009] The present invention includes carbon nanotubes whose hollow
cores are 100% filled with conductive filler. The carbon nanotubes
are in uniform arrays on a conductive substrate and are
well-aligned and can be densely packed. The uniformity of the
carbon nanotube arrays is indicated by the uniform length and
diameter of the carbon nanotubes, both which vary from nanotube to
nanotube on a given array by no more than about 5%. The alignment
of the carbon nanotubes is indicated by the perpendicular growth of
the nanotubes from the substrates which is achieved in part by the
simultaneous growth of the conductive filler within the hollow core
of the nanotube and the densely packed growth of the nanotubes. The
present invention provides a densely packed carbon nanotube growth
where each nanotube is in contact with at least one
nearest-neighbor nanotube. The substrate is a conductive substrate
coated with a growth catalyst, and the conductive filler can be
single crystals of carbide formed by a solid state reaction between
the substrate material and the growth catalyst.
[0010] The present invention further provides a method for making
the filled carbon nanotubes on the conductive substrates. The
method includes the steps of depositing a growth catalyst onto the
conductive substrate as a prepared substrate, creating a vacuum
within a vessel which contains the prepared substrate, flowing
H2/inert (e.g. Ar) gas within the vessel to increase and maintain
the pressure within the vessel, increasing the temperature of the
prepared substrate, and changing the H2/Ar gas to ethylene gas such
that the ethylene gas flows within the vessel. Additionally,
varying the density and separation of the catalyst particles on the
conductive substrate can be used to control the diameter of the
nanotubes.
[0011] It is an object of the present invention to provide a method
for the synthesis of dense arrays of well-aligned carbon nanotubes
on conductive substrates prepared with a growth catalyst where each
carbon nanotube is simultaneously and completely filled with a
conductive filler.
[0012] The subject matter of the present invention is particularly
pointed out and distinctly claimed in the concluding portion of
this specification. However, both the organization and method of
operation, together with further advantages and objects thereof,
may best be understood by reference to the following description
taken in connection with accompanying drawings wherein like
reference characters refer to like elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1a is a photograph of a scanning electron microscopy
micrograph illustrating the dense and well-aligned morphology of
the carbon nanotube films.
[0014] FIG. 1b is a photograph of a scanning electron microscopy
micrograph illustrating the structure of the carbon nanotubes.
[0015] FIG. 1c is a photograph of a scanning electron microscopy
micrograph illustrating a tilted carbon nanotube revealing the
structure of a nanowire enclosed within.
[0016] FIG. 2a is a photograph of a cross-sectional transmission
electron microscopy illustrating the nature of the conductive
fillers (nanowires) filling the core of carbon nanotubes.
[0017] FIG. 2b is an energy-dispersive x-ray spectra illustrating
that the core of the conductive filler (nanowire) is comprised of
both titanium and carbon.
[0018] FIG. 2c is a photograph of a high resolution transmission
electron microscopy illustrating that the carbon walls are
disordered graphite.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0019] The present invention is an array of a plurality of carbon
nanotubes where each nanotube is attached to a substrate and
extends from the substrate and has a closed outer wall defining a
hollow core that is simultaneously filled more than 10% with a
conductive filler while the carbon nanotube grows. The present
invention further provides a method for making this array of carbon
nanotubes which includes the steps of depositing a growth catalyst
onto the substrate to form a prepared substrate, creating a vacuum
within a vessel which contains the prepared substrate, flowing
H2/inert (e.g. Ar) gas within the vessel and increasing the
pressure within the vessel, increasing the temperature of the
prepared substrate, and changing the H2/Ar gas to ethylene gas such
that the ethylene gas flows within the vessel.
[0020] The arrays of the carbon nanotubes filled with conductive
filler are fabricated by first depositing a thin layer of growth
catalyst onto a substrate. Depositing the growth catalyst is
preferably by electron beam evaporation, and preferably results in
a thin layer on the substrate which is about 1 to 30 nanometers in
thickness. The substrate is an electrically conductive substrate,
preferably made of metals including but not limited to transition
elements appearing in groups IIIB, IVB, VIB, VIIB, VIII, and IB of
the periodic table. Preferred metals are titanium, vanadium,
tantalum, and combinations thereof. Non-metals such as carbides may
also be used for the substrate, for example, titanium carbide. The
growth catalyst is preferably iron, but may also be iron oxide and
combinations thereof.
[0021] After the conductive substrate is prepared with the growth
catalyst coating, the vessel which contains the prepared substrate
is evacuated to a first pressure below 2 torr. The vessel is
preferably a quartz reactor placed within a tube furnace. The
pressure within the vessel is then increased to a second pressure
within the range from about 200 torr to about 400 torr by flowing
H2/inert (e.g. Ar) gas within the vessel. After the pressure
stabilizes within the vessel, the prepared substrate is heated
using the tube furnace. Once the prepared substrate temperature
reaches the growth temperature, which can range from about
650.degree. C. to about 800.degree. C., but which is preferably
from about 700.degree. C. to 775.degree. C., the H.sub.2/Ar flow is
stopped and ethylene gas, preferably but not necessarily with a
purity of about 99.5%, is introduced into the reactor. A preferable
introduction flow rate of 200 cm.sup.3/min. Typical growth periods
range from about 10 minutes to 3 hours.
[0022] The heat treatment of the prepared substrate can be
controlled to vary the density and separation of catalyst particles
on the prepared substrate. A higher heat results in more coalescing
of the catalyst particles, and thus, fewer and larger catalyst
sites, which results in separation distances between these sites on
the substrate. The density of the catalyst sites controls the
diameter of the carbon nanotubes with a higher density resulting in
a greater diameter carbon nanotube. As the ethylene gas flows, it
decomposes as a carbon source and diffuses into the catalyst
particles causing precipitation and growth of the carbon nanotubes.
At the same time, the substrate material diffuses into the catalyst
particles resulting in the growth of a carbide core (conductive
filler) within the hollow carbon nanotubes. The carbide core within
the carbon nanotubes is a conductive filler preferably made up of
carbon and titanium. However, the conductive filler may also be
made up of carbon and whichever metal makes up the conductive
substrate, which includes but is not limited to transition elements
appearing in groups IIIB, IVB, VIB, VIIB, VIII, and IB of the
periodic table. Preferred metals are titanium, vanadium, tantalum,
and combinations thereof. The hollow core of each carbon nanotube
is filled with the conductive filler to a point which is greater
than 10% full, but which is preferably greater than 50% full, and
which is more preferably greater than 75% full, and which is most
preferably about 100% full, where "about 100%" means 100%, plus or
minus 5%.
[0023] The carbon nanotubes have lengths ranging from about 1 to 2
.mu.m, varying no more than about 5%, which provides uniform
lengths. The carbon nanotubes also have uniform diameters such that
their diameters vary no more than about 5%. The outside nanotube
diameter ranges from less than 40 to about 400 nm and the inside
nanotube diameter ranges from about 10 to about 100 nm. The
diameters of the carbon nanotubes and filled cores can be
controlled by varying the thickness of the catalyst (iron) layer.
In general, the thicker the iron catalyst layer, the bigger the
tube diameter. However, when the tube diameter is less than 40 nm,
the carbon nanotubes are curved and only partially filled.
EXAMPLE 1
[0024] A number of substrates were selected to investigate their
effects on the formation of the filled carbon nanotubes. The
formation of the filled nanotubes depends on the solubility of the
iron (the catalyst) in the substrate and the free energy of
formation for the respective carbide phase. The substrates selected
included tantalum, silicon, and molybdenum. All of these materials
can form stable carbides. Carbon nanotubes were deposited on the
substrates under the same growth conditions used for growth of
carbon nanotubes on titanium substrates. While dense arrays of
filled carbon nanotubes were observed on tantalum substrates
similar to those shown in FIG. 1a, only curved hollow carbon
nanotubes were formed on silicon substrates. No carbon nanotubes
were observed on molybdenum substrates. For molybdenum substrates,
X-ray photoelectron spectrometry and backscattering electron SEM
indicated formation of Fe--Mo solid solutions in the surface region
of the substrates. The high solubility of Fe in Mo depleted the
catalytic material required to grow the carbon nanotubes. For
silicon substrates, the driving force to form SiC is much lower
than that for the formation of either TiC or TaC. The free energy
of formation of these carbides is on the order of -43 kcal/mol for
TiC, -35 kcal/mol for TaC, and -15 kcal/mol for SiC. Although
carbon nanotubes are formed on silicon, the growth rate of SiC is
very low compared to that of TiC or TaC, resulting in hollow carbon
nanotubes. These tubes tend to be tilted or curved.
[0025] The carbon nanotubes of the present invention are examined
by scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). FIG. 1a reveals the dense, well-aligned
morphology of the carbon nanotube films. The SEM images were
recorded using 70% secondary electron signals and 30% back
scattering electron signals. The intensity is therefore
proportional to the atomic number of the elements that comprise the
material. A magnified SEM image (FIG. 1b) shows that the structure
of the carbon nanotubes of the present invention is different from
that of oriented carbon nanotubes previously reported. First, the
carbon tubes are densely packed, rather than well separated as with
prior reported carbon nanotubes. Secondly, the tube tips as shown
in FIG. 's 1a, 1b, & 1c, appear brighter at the center of the
carbon nanotubes, indicating that the cores of the carbon nanotubes
are filled with a material having elements of higher atomic number
than carbon.
[0026] Most of the carbon nanotubes filled with conductive filler
of the present invention in FIG. 1a have similar length and are
approximately perpendicular to the substrate surface, although in a
few cases the conductive filler is tilted and extended above the
film surface. A SEM image (FIG. 1c) of a tilted nanotube reveals a
structure of conductive filler enclosed by carbon. The filled
material has a large head near the tube tip, and its diameter is
about one quarter of the carbon nanotube diameter. Further evidence
of filled cores and carbon caps is provided by the cross-sectional
TEM image in FIG. 2a and energy-dispersive xray (EDX) spectra
illustrated in FIG. 2b. The cross-sectional TEM image in FIG. 2a
shows that individual conductive filler grows continuously from the
bottom to the top of the films. The filled nature of the carbon
nanotubes can be clearly seen by the distinct contrast between the
filled core and carbon wall. The cross-sectional TEM image in FIG.
2a reveals that the cores have a large head near the tube tip, in
agreement with the SEM image (FIG. 1c). The core diameter in the
cross-sectional TEM image in FIG. 2a appears slightly smaller than
that observed in the SEM image (FIG. 1c), presumably due to the
off-center cut of the TEM specimen. EDX analysis revealed that the
walls and caps of the conductive fillers are carbon. A
high-resolution TEM image (FIG. 2c) reveals that the carbon walls
are disordered graphite.
[0027] EDX analysis of the carbon nanotube and conductive filler of
the present invention also indicates that the core of the
conductive filler is comprised of titanium and carbon except for
the region near the substrate where iron was found. Electron
diffraction patterns obtained from the cores reveal that the cores
are cubic TiC. The convergent beam electron diffraction pattern
(inset in FIG. 2a) recorded along the <001> zone axis
parallel to the electron beam exhibits a lattice spacing
(.about.0.43 nm) and four-fold symmetry corresponding to the (100)
planes of cubic TiC. In addition, the electron diffraction patterns
reveal that the TiC cores are single crystals. In the
high-resolution TEM image (FIG. 2c), the interface between the
graphite wall and TiC core is abrupt and free of any intermediate
phase. The magnified images show well-ordered lattice fringes of
the TiC core (right inset of FIG. 2c) and disordered lattice
fringes of the graphite wall (left inset of FIG. 2c). It should be
pointed out that this is the first time that carbon nanotubes are
completely filled with metallic TiC cores.
[0028] Moreover, all TiC cores show a big head near the tube tip
(FIG. 1c and 2a), indicating that the initial growth of the TiC
cores is faster. This may be due to an initially high concentration
of titanium dissolved in the iron particle during heating to the
growth temperature (2.7 to 3 hours) before introducing ethylene gas
into the reactor. After the initial growth, the dissolution and
precipitation process reaches an equilibrium condition under which
the consumed rate of titanium for the precipitation of TiC is
approximately equal to the mass transfer rate of titanium into the
iron particle, resulting in uniform core diameters. Carbon
diffusion is not the limiting step in the formation of TiC because
interstitial diffusion of carbon in iron is much faster than
substitutional diffusion of titanium in iron.
[0029] Therefore it can be concluded that growth of oriented and
filled carbon nanotubes requires stable catalytic particles and low
free energy of formation of a reaction product in the core.
[0030] While a preferred embodiment of the present invention has
been shown and described, it will be apparent to those skilled in
the art that many changes and modifications may be made without
departing from the invention in its broader aspects. The appended
claims are therefore intended to cover all such changes and
modifications as fall within the true spirit and scope of the
invention.
* * * * *