U.S. patent application number 10/372006 was filed with the patent office on 2004-01-29 for metallization of carbon nanotubes for field emission applications.
This patent application is currently assigned to SI Diamond Technology, Inc.. Invention is credited to Mao, Dongsheng.
Application Number | 20040018371 10/372006 |
Document ID | / |
Family ID | 46123434 |
Filed Date | 2004-01-29 |
United States Patent
Application |
20040018371 |
Kind Code |
A1 |
Mao, Dongsheng |
January 29, 2004 |
Metallization of carbon nanotubes for field emission
applications
Abstract
The present invention is directed towards metallized carbon
nanotubes, methods for making metallized carbon nanotubes using an
electroless plating technique, methods for dispensing metallized
carbon nanotubes onto a substrate. The present invention is also
directed towards cold cathode field emitting materials comprising
metallized carbon nanotubes, and methods of using metallized carbon
nanotubes as cold cathode field emitters.
Inventors: |
Mao, Dongsheng; (Austin,
TX) |
Correspondence
Address: |
Winstead Sechrest & Minick P.C.
P. O. Box 50784
1201 Main Street
Dallas
TX
75250-0784
US
|
Assignee: |
SI Diamond Technology, Inc.
Austin
TX
|
Family ID: |
46123434 |
Appl. No.: |
10/372006 |
Filed: |
February 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60417246 |
Oct 9, 2002 |
|
|
|
60372067 |
Apr 12, 2002 |
|
|
|
Current U.S.
Class: |
428/545 |
Current CPC
Class: |
C23C 18/1204 20130101;
C23C 18/1635 20130101; H01J 2201/30469 20130101; B82Y 10/00
20130101; B82Y 30/00 20130101; H01J 9/025 20130101; C23C 18/31
20130101; C23C 18/2066 20130101; Y10T 428/12007 20150115 |
Class at
Publication: |
428/545 |
International
Class: |
B22F 003/26 |
Claims
What is claimed is:
1. A metallized carbon nanotube material comprising carbon
nanotubes which have a metal coating on them.
2. The material of claim 1, wherein the carbon nanotubes are
selected from the group consisting of single-wall carbon nanotubes,
multi-wall carbon nanotubes, buckytubes, carbon fibrils,
derivatized carbon nanotubes, chemically-modified carbon nanotubes,
metallic carbon nanotubes, semiconducting carbon nanotubes, and
combinations thereof.
3. The material of claim 1, wherein the metal coating is uniformly
distributed over an exterior surface of said carbon nanotubes.
4. The material of claim 1, wherein the metal is selected from the
group consisting of nickel, iron, copper, silver, zinc, rhodium,
tin, cadmium, chromium, beryllium, palladium, indium, platinum,
gold, and combinations thereof.
5. The method of claim 1, wherein the metal coating has a thickness
which ranges from at least about 0.1 nanometers to at most about 1
micrometer.
6. A method of making metallized carbon nanotubes comprising the
steps of: a) providing a plurality of carbon nanotubes; b)
preparing an electroless metal plating solution; c) adding said
carbon nanotubes to said electroless plating solution; d)
subjecting said electroless plating solution to a reducing
condition which causes metal ions in solution to be reduced to
metal and nucleate on the carbon nanotubes to produce metallized
carbon nanotubes; and e) removing said metallized carbon nanotubes
from solution.
7. The method of claim 6, further comprising the step washing the
metallized carbon nanotubes.
8. The method of claim 6, further comprising the step of drying the
metallized carbon nanotubes.
9. The method of claim 6, wherein the carbon nanotubes are selected
from the group consisting of single-wall carbon nanotubes,
multi-wall carbon nanotubes, buckytubes, carbon fibrils,
derivatized carbon nanotubes, chemically-modified carbon nanotubes,
metallic carbon nanotubes, semiconducting carbon nanotubes, and
combinations thereof.
10. The method of claim 6, further comprising the step of treating
the carbon nanotubes with hydrochloric acid prior to their
metallization.
11. The method of claim 6, wherein the electroless plating solution
comprises a solvent, a metal salt, and a reducing agent.
12. The method of claim 11, wherein the electroless plating
solution further comprises an optional component selected from the
group consisting of a promoter species, an inhibiting agent, a
balancing agent, and combinations thereof.
13. The method of claim 11, wherein the metal salt comprises a
metal selected form the group consisting of nickel, iron, copper,
silver, zinc, rhodium, tin, cadmium, chromium, beryllium,
palladium, indium, platinum, gold, and combinations thereof.
14. The method of claim 6, wherein the step of adding said carbon
nanotubes to said electroless plating solution further comprises
ultrasonicating the carbon nanotubes in a solvent just prior to
addition.
15. The method of claim 6, wherein the step of removing said
metallized carbon nanotubes from solution further comprises a
separation technique selected from the group consisting of
filtration, centrifugation, and combinations thereof.
16. Metallized carbon nanotubes made by a process comprising the
steps of: a) providing a plurality of carbon nanotubes; b)
preparing an electroless metal plating solution; c) adding said
carbon nanotubes to said electroless plating solution; d)
subjecting said electroless plating solution to a reducing
condition which causes metal ions in solution to be reduced to
metal and nucleate on the carbon nanotubes to produce metallized
carbon nanotubes; and e) removing said metallized carbon nanotubes
from solution.
17. A cathode for field emission applications comprising: a) a
substrate; and b) metallized carbon nanotubes.
18. A method of making cathodes for field emission applications
comprising the steps of: a) providing a suitable substrate; and b)
dispensing metallized carbon nanotubes onto said substrate using an
applicator means.
19. The method of claim 18, wherein the applicator means comprises
a spraying technique whereby a suspension of metallized carbon
nanotubes suspended in a suitable solvent is sprayed onto said
substrate.
20. The method of claim 19, wherein the suspension of metallized
carbon nanotubes is generated using ultrasonic assistance.
21. A field emission display device comprising: a) an anode which
includes a phosphor deposited on a substrate; and b) a cathode
comprising a layer of metallized carbon on a substrate.
22. A field emission display device comprising: a) an anode
assembly; and b) a cathode assembly, wherein the cathode assembly
comprises: 1) a substrate; 2) an electically conducting layer
deposited on the substrate; and 3) a layer of metallized carbon
nanotubes deposited over the electrically conducting layer.
23. The field emission display device of claim 22, wherein the
metallized carbon nanotubes comprise single-wall carbon nanotubes.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to the following
U.S. Provisional Patent Application, Serial No. 60/417,246.
TECHNICAL FIELD
[0002] The present invention relates in general to nanostructured
materials, and in particular, to using modified carbon nanotubes
for field emission applications.
BACKGROUND INFORMATION
[0003] Carbon nanotubes (CNTs) are currently being investigated for
use as cold electron sources in a variety of applications. These
include displays, microwave sources, x-ray tubes, etc. For CNTs to
be used as a cold cathode, they must be placed on a conductive
surface (conductive substrate or conductive film on a
non-conductive substrate). This has led some to place catalysts on
the substrate surface and grow the carbon nanotubes in situ using
CVD techniques (Kim et al., J. Appl. Phys., 90(5), 2591 (2001)).
However, this has several draw-backs. This technique typically
grows multi-wall carbon nanotubes (MWNTs). However, MWNTs have
poorer field emission quality compared to single-wall carbon
nanotubes (SWNTs). The substrate is subjected to high temperature,
typically above 600.degree. C., limiting the substrates that can be
used. Uniformity is difficult to achieve because of the high
temperature growth processes required. As a result, the manufacture
of cathodes using this process will be very expensive due to the
number and complexity of post-processing steps needed to generate a
material capable of producing the desired level of field
emission.
[0004] Other investigations have centered on processes for making
CNT cathodes in a separate process, collecting them, and then
dispensing them onto a substrate using a variety of techniques (Kim
et al., Diamond and Related Materials, 9, 1184 (2000)). This has
several advantages over the in situ method described above. First,
the fabrication of the CNT material is decoupled from the
fabrication of the cathode. This permits choosing the optimal CNT
material for the application (single-wall, double-wall, multi-wall,
purified, non-purified, etc.). Second, the dispensing process is
carried out a relatively low-temperatures, permitting greater
flexability in the choice of substrates. Third, uniform deposition
over large area substrates is far more feasable using
currently-available, low-cost equipment. Current dispensing
processes, however, have their disadvantages. One of these is that
the CNT fibers are often dispensed such that they clump together or
are imbedded inside another material. These factors limit the
performance of the CNT material. "Activation" processes are often
employed after dispensing the CNT material, and these processes
recover some of the performance of the virgin CNT (Chang et al.,
U.S. Pat. No. 6,436,221 B1). These "activation" process steps,
however, can add cost to the product and may lead to non-uniform
performance. Yet another disadvantage of current dispensing
techniques is that the dispensed CNT fibers may not have
sufficiently good contact to the substrate or the substrate's
conductive layer such that this impedes their ability to supply the
electrons needed for field emission.
[0005] It has been recently found that by mixing CNT material with
other nanoparticle materials, the field emission properties of the
CNT were improved (Mao et al., U.S. Provisional Application No.
60/417,246, incorporated herein by reference). Because neighboring
nanotubes shield the extracted electric fields from each other
(Bonard et al., Adv. Mat., 13, 184 (2001)), it is believed that
this improvement is a result of induced separation of the CNT
material by the nanoparticles. In situations where the CNT fibers
are too close, they may electrically screen the applied electric
field from each other. By increasing the separation between the
fibers, the effective applied field strength at the emission sites
is higher.
[0006] Many SWNT fibers are semiconducting with a bandgap that is
dependent upon the chiral indices (n,m) of the SWNT. Choi et al.
(U.S. Pat. No. 6,504,292 B1) teach that, for field emission
applications, this bandgap can be overcome by depositing a metal
film on CNT fibers that are already attached to a substrate. Choi
et al. teach that the CNT fibers are coated after the fibers are
grown using CVD techniques. This method has the inherent
aforementioned disadvantages of growing CNTs on the substrate.
Furthermore, were the CNT fibers to be dispensed onto the substrate
and then coated, the problems of separating the CNT fibers for
improved emission would still remain.
[0007] Consequently, there is a demonstrated need for a method of
dispensing SWNTs onto a substrate in such a way as to inhibit
clumping, provide for sufficiently good contact to the substrate,
overcome the limitations imposed by semiconducting CNTs, and which
obviates the need for activation processes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] For a more complete understanding of the present invention,
and the advantages thereof, reference is now made to the follwing
descriptions taken in conjunction with the accompanying drawings,
in which:
[0009] FIG. 1 illustrates metallized carbon nanotubes on
indium-tin-oxide (ITO)/glass, wherein the metal coating is not
necessarily uniform over all of the carbon nanotubes (CNTs);
[0010] FIG. 2 illustrates an electroless plating bath used to coat
carbon nanotubes with metal;
[0011] FIG. 3 illustrates an optical microscope image of metallized
CNT material dispensed on a substrate surface;
[0012] FIG. 4 illustrates an optical microscope image of non-coated
CNT material dispensed on a substrate surface;
[0013] FIG. 5 illustrates field emission current vs. electric field
for cobalt-coated and non-coated carbon nanotubes; and
[0014] FIG. 6 illustrates a field emission display device
incorporating the present invention.
DETAILED DESCRIPTION
[0015] The present invention is directed towards metallized carbon
nanotubes, methods for making metallized carbon nanotubes, methods
for dispensing metallized carbon nanotubes onto a substrate, cold
cathode field emitting materials comprising metallized carbon
nanotubes, and methods of using metallized carbon nanotubes as cold
cathode field emitters.
[0016] Metallized carbon nanotubes, according to the present
invention, are carbon nanotubes which have been at least partially
coated with one or more metals. Carbon nanotubes, according to the
present invention, include, but are not limited to, single-wall
carbon nanotubes, multi-wall carbon nanotubes, buckytubes, carbon
fibrils, derivatized carbon nanotubes, chemically-modified carbon
nanotubes, metallic carbon nanotubes, semiconducting carbon
nanotubes, and combinations thereof. Purity of the carbon nanotube
reactant materials (i.e., the carbon nanotubes prior to being
metallized) ranges generally from at least about 1 percent to at
most about 100 percent, specifically from at least about 10 percent
to at most about 100 percent, and more specifically from at least
about 20 percent to at most about 100 percent. Carbon nanotubes, as
described herein, can exist in bundles or as individual entities.
Furthermore, the carbon nanotubes from which the metallized carbon
nanotubes are derived can be produced by any process which suitably
provides for carbon nanotubes according to the present
invention.
[0017] Metal coatings (also termed "films") on the carbon nanotubes
comprise one or more metal layers and range generally in thickness
from at least about 0.1 nanometer (nm) to at most about 10
micrometers (.mu.m), specifically from at least about 0.1 nanometer
to at most about 1 micrometer, and more specifically from at least
about 0.5 nanometers to at most about 1 micrometer. Metal coatings
on the carbon nanotubes include, but are not limited to nickel
(Ni), iron (Fe), copper (Cu), silver (Ag), zinc (Zn), rhodium (Rh),
tin (Sn), cadmium (Cd), chromium (Cr), beryllium (Be), palladium
(Pd), indium (In), platinum (Pt), gold (Au), and combinations
thereof. In some embodiments, the metal coating comprises an alloy
of two or more metals. The weight percent of metal in the
metallized carbon nanotube product ranges generally from at least
about 0.1 percent to at most about 99 percent, specifically from at
least about 1 percent to at most about 99 percent, and more
specifically from at least about 5 percent to at most about 99
percent. In some embodiments of the present invention, these metal
coatings are highly uniform over individual carbon nanotubes. In
some embodiments, these metal coatings are non-uniform,
non-continuous, and/or incomplete, as depicted in FIG. 1 wherein
metal coating 105 is shown on carbon nanotubes 104 to form
metallized carbon nanotubes 106. In some embodiments these metal
coatings are deposited primarily on the exterior of carbon nanotube
bundles. In some embodiments, bundles of carbon nanotubes are
metallized within the interior of the bundle. In some embodiments,
the carbon nanotubes are metallized endohedrally, inside the tube
structure. Some embodiments comprise metallized carbon nanotubes
with any combination(s) of the aforementioned metallized carbon
nanotubes.
[0018] Exemplary methods of making metallized carbon nanotubes
comprise the steps of: a) providing a plurality of carbon
nanotubes; b) preparing an electroless metal plating solution; c)
adding said carbon nanotubes to said electroless metal plating
solution to form a reaction solution; d) subjecting said reaction
solution to a reducing condition which causes metal ions in
solution to be reduced to metal and nucleate on the carbon
nanotubes to produce metallized carbon nanotubes; and e) removing
said metallized carbon nanotubes from the reaction solution. In
some embodiments of the present invention, the metallized carbon
nanotubes are washed and dried after being removed from the
reaction solution.
[0019] Carbon nanotubes, as described herein, can be carbon
nanotubes of any dimension, chirality, and number of walls that
suitably provides for carbon nanotubes of the present invention and
include, but are not limited to, single-wall carbon nanotubes
(SWNTs), multi-wall carbon nanotubes (MWNTs), buckytubes, carbon
fibrils, derivatized carbon nanotubes, chemically-modified carbon
nanotubes, metallic carbon nanotubes, semiconducting carbon
nanotubes, and combinations thereof. In some embodiments of the
present invention, the carbon nanotubes are treated with
hydrochloric acid prior to the metallization step.
[0020] An electroless plating solution (commonly referred to as a
plating bath), according to the present invention, comprises a
solvent, a metal salt, and a reducing agent (See Ranney et al.,
Electroless Plating and Coating of Metals," Noyes, Park Ridge, N.J.
(1972), incorporated herein by reference, for a detailed
description of electroless plating techniques). In some embodiments
of the present invention, there is a promoter species which helps
to dissolve the metal salt. In some embodiments, there may be a
balancing agent to control the pH. The solvent can be any solvent
which suitably provides for the solvation of the electroless
plating solution components. An exemplary solvent is water. The
metal salt can be any metal salt that suitably provides for
electroless metal plating according to the present invention and
includes, but is not limited to, salts of the following: nickel,
iron, copper, silver, zinc, rhodium, tin, cadmium, chromium,
beryllium, palladium, indium, platinum, and combinations thereof.
In some embodiments, alloys of two or more metals are plated on the
carbon nanotubes with this process. The reducing agent can be any
reducing agent that suitably provides for the reduction of the
metal salt according to the present invention and includes, but is
not limited to NaH.sub.2PO.sub.2.H.sub.2O, N.sub.2H.sub.4.2HCl,
N.sub.2H.sub.4.xH.sub.2O- , and combinations thereof. The optional
promoter species can be any species which suitably promotes the
electroless metal plating process of the present invention by
facilitating the dissolution of the metal salt in the solution.
Suitable promoter species include, but are not limited to
C.sub.4H.sub.4O.sub.6KNa.4H.sub.2O, Na.sub.2C.sub.4H.sub.4O.sub.6,
Na.sub.3C.sub.6H.sub.5O.sub.7.2H.sub.2O, and combinations thereof.
The optional balancing agent can be any species which suitably
provides for the control of pH according to the present invention.
Suitable balancing agents include, but are not limited to NaOH,
KOH, NH.sub.4OH, and combinations thereof.
[0021] In some embodiments of the present invention, the process of
adding the carbon nanotubes to the electroless plating solution is
carried out by first ultrasonicating the carbon nanotubes in a
suitable solvent just prior to addition. This enhances their
dispersal in the electroless plating solution to form a reaction
solution. This reaction solution is subjected to a reducing
condition which causes metal ions in solution to be reduced to
metal and nucleate on the carbon nanotubes to produce metallized
carbon nanotubes. Reducing conditions, according to the present
invention, are any conditions which suitably provide for a
reduction of the metal ions in solution. Such reducing conditions
induce this reduction and include, but are not limited to, heating,
irradiation, chemical activation, and combinations thereof. In some
embodiments, the electroless plating solution is subjected to the
reducing condition prior to the addition of the carbon
nanotubes.
[0022] In some embodiments of the present invention, the degree of
carbon nanotube metallation (i.e., the amount of metal coated on
the carbon nanotubes) is modulated by the amount of carbon
nanotubes present in the reaction solution. In other embodiments,
the degree of carbon nanotube metallation is modulated by the
concentration of metal salts and reducing agents present in the
reaction solution. In other embodiments, the degree of carbon
nanotube metallation is modulated by the time the carbon nanotubes
spend in the reaction solution. In still other embodiments, a
combination of one or more of the aforementioned methods of
modulating the degree of carbon nanotube metallation is used to
produce a metallized carbon nanotube product with certain desired
characteristics dependent upon the degree in which is has been
metallized.
[0023] In some embodiments of the present invention, prior to the
step of removing the metallized carbon nanotubes from the reaction
solution, a stabilizing agent is added to slow the reduction of the
metal ions. A stabilizing agent can be any species which suitably
provides for the slowing of the reduction process of the present
invention and includes, but is not limited to, H.sub.3BO.sub.3,
C.sub.3H.sub.6O.sub.3, and combinations thereof. Such slowing of
the reaction facilitates greater control over the nature of the end
product. Suitable methods of removing the metallized carbon
nanotubes from the reaction solution include, but are not limited
to, centrifugation (and subsequent decantation), filtration, and
combinations thereof. In some embodiments of the present invention,
after the step of removing the metallized carbon nanotubes from the
reaction solution, there is a step of washing the metallized carbon
nanotube product. Suitable washing solvents include any solvent
which suitably removes unwanted reactants or reaction products from
the final product. Suitable solvents include, but are not limited
to, water, isopropyl alcohol, acetone, and combinations thereof.
Optional drying of the metallized carbon nanotube product can be
carried out by any drying process which suitably provides for the
drying of the metallized carbon nanotubes according to the present
invention and includes, but is not limited to, heating, exposure to
vacuum, vacuum heating, irradiation, and combinations thereof.
[0024] Exemplary methods of dispensing metallized carbon nanotubes
onto a substrate comprise: a) dispersing the metallized carbon
nanotubes in a solvent to form a suspension; and b) applying the
suspension to a substrate using an "applicator means." Solvents
into which the metallized carbon nanotubes are dispersed include,
but are not limited to, isopropanol, methanol, acetone, water,
ethanol, and combinations thereof. Methods of dispersing the
metallized carbon nanotubes in the solvent include, but are not
limited to, stirring, shaking, ultrasonic assistance, and
combinations thereof. FIG. 1 illustrates one embodiment of
metallized carbon nanotubes 106 on a substrate 103.
[0025] An applicator means, according to the present invention, can
be any method which suitably dispenses the suspension of metallized
carbon nanotubes onto a substrate in a controlled manner. Such
application can be uniform or non-uniform, and can vary
considerably in terms of the thickness of the resulting film, or
layer, of metallized carbon nanotubes on the substrate. An
exemplary applicator means comprises a spraying technique whereby
the suspension of metallized carbon nanotubes is sprayed onto a
surface using a sprayer. While not intending to be bound by theory,
a sprayer, according to the present invention, can be a pump
sprayer which rapidly pushes the suspension through a small orifice
and, upon exiting said orifice, the suspension becomes an aerosol
of small suspension droplets which are directed toward the
substrate surface. Optionally, the substrate can be heated during
the application process to prevent the running of excess solvent.
Typically, the substrate, after having applied the metallized
carbon nanotubes to its surface, is dried to remove any excess
solvent. A substrate, as described herein, can be any substrate
which suitably provides for a surface on which to dispense
metallized carbon nanotubes according to the present invention and
includes, but is not limited to, metals, ceramics, glass,
semiconductors, coated surfaces, layered materials, and
combinations thereof.
[0026] In some embodiments of the present invention, the metallized
carbon nanotubes are used for field-emission application. In some
embodiments, these metallized carbon nanotubes are more suitable
for field-emission applications than carbon nanotubes without a
metal coating. While not intending to be bound by theory, it is
likely that, when incorporated into a device for field-emission
applications, the metallized carbon nanotubes are better separated
from one another, creating a carbon nanotube arrangement of lower
density that reduces the shielding effects contributed by
neighboring carbon nanotubes. Furthermore, said metal coatings
likely enhance the flow of electrons in semiconducting carbon
nanotubes and at the nanotube-substrate junction. In some
embodiments of the present invention involving field emission
applications, the metallized carbon nanotubes are dispensed onto a
substrate using an applicator means, and the resulting substrate
(with the metallized carbon nanotubes) is used as the cathode in,
for example, a field emission display. Other field emission
applications in which metallized carbon nanotubes can be used
include, but are not limited to X-ray sources, electron sources, rf
arrays, microwave tubes, and combinations thereof.
[0027] Thus, as disclosed herein, the present invention is also
directed towards an improved field emission cathode using carbon
nanotube emitters that are first coated with a metal film and then
dispensed onto the cathode. This field emission cathode is
illustrated in FIG. 1. Referring to FIG. 1, metallized carbon
nanotubes 106 are shown on a substrate 103 which comprises a
conductive layer 102 and an optional layer 101, which can be either
conductive or non-conductive. Collectively, this forms
field-emission cathode 100. This cathode has advantages over the
current art in that: a) the metal layer provides a high level of
electrical conductivity along the length of the CNT fiber even if
the fiber is semiconducting; b) the metal layer provides an
additional means of separating the CNT fibers from each other,
decreasing the mutual electrical shielding and eliminating the need
for post-deposition activation steps; c) metal-coated carbon fibers
adhere to metal layers on the substrate much more strongly than do
bare carbon nanotubes (adhesion forces between metals are much
stronger than the adhesion forces between the substrate and the
un-metallized carbon nanotubes); and the metal coatings can be
applied to SWNTs and MWNTs, semiconducting or metallic CNTs,
purified or non-purified CNTs--all using standard electrolytic
techniques permitting selection from a large variety of available
CNT fibers.
[0028] Referring to FIG. 6, the field emission cathode described
above can be incorporated into field emission display 600. On
substrate 601, conductive layer 602 is deposited and metallized
carbon nanotube layer 603 is deposited on top thereof. The anode
includes substrate 604, which may be a glass substrate, conductive
layer 605, which may be indium-tin-oxide, and a phosphor layer 606
for receiving electrons emitted from metallized carbon nanotube
layer 603. Electrons are emitted from layer 603 in response to an
appropriate electric field between the anode and the cathode.
[0029] The following examples are provided to more fully illustrate
some of the embodiments of the present invention. The examples
illustrate methods by which metal-coated (metallized) CNTs can be
made and prepared for field emission applications. It should be
appreciated by those of skill in the art that the techniques
disclosed in the examples which follow represent techniques
discovered by the inventor to function well in the practice of the
invention, and thus can be considered to constitute exemplary modes
for its practice. However, those of skill in the art should, in
light of the present disclosure, appreciate that many changes can
be made in the specific embodiments which are disclosed and still
obtain a like or similar result without departing from the spirit
and scope of the invention.
EXAMPLES
Example 1
[0030] Coating Single-Wall Carbon Nanotubes with a Cobalt Thin
Film
[0031] This process provides a way of depositing a metal thin film
or coating on the surface of carbon nanotubes using an electroless
plating technique. Using this relatively inexpensive and simple
process, metallized carbon nanotubes can be made efficiently in
relatively large amounts.
[0032] The single-wall carbon nanotube (SWNT) material used here
was purchased from Iljin Nanotech, Inc. (Korea). The length of the
SWNTs ranged from approximately several micrometers to
approximately 20 micrometers, and the diameters were generally less
than about 2 nanometers.
[0033] Referring to FIG. 2, electroless plating apparatus 200
comprises an electroless plating solution 204 contained in a beaker
203 which in turn is immersed in a water bath 202. Water bath 202
is heated by a magnetic stirring hotplate 201 and temperature is
monitored by thermometer 206. Stirring is accomplished with stir
bars 205 activated by the magnetic stirring hotplate 201 and the
stirring motor 207. In the present example, electroless plating
solution 204 comprises water and the following chemicals:
[0034] 1. A cobalt (Co) salt (CoSO.sub.4.7H.sub.2O) to provide Co
ions (Note that other salts may be used, e.g.,
CoCl.sub.2.6H.sub.2O). Concentration of this component is
approximately 20-28 grams per liter.
[0035] 2. A reducing agent (NaH.sub.2PO.sub.2.H.sub.2O) to reduce
Co ions to Co(0). Concentration of this component is approximately
18-25 grams per liter.
[0036] 3. A promoter species to facilitate dissolution of the Co
salt into the solution (C.sub.4H.sub.4O.sub.6KNa.4H.sub.2O).
Concentration of this component is approximately 140-160 grams per
liter.
[0037] 4. A stabilizing agent (H.sub.3BO.sub.3), to slow the
reducing reaction. Concentration of this component is 27-35 grams
per liter.
[0038] 5. A balancing agent (NaOH). This is used to control the pH
value of the solution. The amount of this material that is used is
that needed to maintain a pH of 8-10 for the metal plating
solution.
[0039] The above chemicals were dissolved in deionized water up to
900 milliliters.
[0040] The cobalt ions in this solution undergo reduction under a
reducing condition of approximately 85-95.degree. C. The pH of the
solution needs to be controlled before and during the reaction. In
this example, the pH value was maintained at about 9. NaOH was
added during the plating process to control the pH of the
solution.
[0041] Approximately 3-4 grams of carbon nanotube powder is
ultrasonicated in a beaker containing approximately 100 milliliters
of water for several minutes before being introduced into the
electroless plating solution (after addition, total solution is
1000 milliliters). After the solution is prepared, it is heated in
a water bath to 85-95.degree. C. and the ultrasonicated SWNTs are
then added to the electroless plating solution quickly while the
solution was stirred. Because the carbon nanotubes easily clump
together, the water+CNT mixture should be ultrasonicated
immediately before adding it to the plating solution. The typical
reaction time in the plating solution is about 5-10 minutes. Longer
times do not appear to affect the results greatly. During the
reaction, gas is evolved from the solution. The solution is pink at
the beginning but gradually turns colorless. At the end of the
reaction, little or no gas is evolved from the solution.
[0042] After reaction/deposition of metal, the reaction beaker is
taken out of the water bath and allowed to cool down to room
temperature. After several minutes, the metallized carbon nanotube
powders collect at the bottom of the beaker and the solution is
decanted from the powder. The powder is washed several times, each
time being careful to not disturb the powder. Washing dilutes the
concentration of any electroless plating reactants still remaining
on the powder after the reaction. The powder is then removed and
dried in a furnace at about 60.degree. C.-100.degree. C. for
several hours. The carbon nanotube powder is now coated with a thin
layer or film of metal.
Example 2
[0043] Dispensing Carbon Nanotubes onto a Substrate
[0044] In this example, cobalt-metallized SWNT powder was mixed
with isopropyl alcohol (IPA) to form a suspension. The suspension
comprised approximately 1 gram of metallized SWNTs in 1000 ml IPA.
Because the SWNTs clump together readily, ultrasonic agitation was
used to disperse the nanotubes in the IPA before spraying the
solution onto cathode substrates. The SWNT/IPA suspension was
sprayed onto conductive indium-tin-oxide (ITO)/glass substrate with
an area of 2.times.2 cm.sup.2. In order to prevent the IPA from
flowing uncontrollably, the substrate was heated up to
approximately 30-70.degree. C. on both the front side and back side
during the spraying process. The substrate was sprayed back and
forth several to tens of times until the carbon nanotubes covered
on the entire surface. The thickness of the carbon nanotube layer
was about 1-20 .mu.m. The film was then dried in air.
[0045] FIGS. 3 and 4 show optical images of CNT material after
having been dispensed onto a substrate surface (FIG. 3:
metal-coated CNTs, FIG. 4: non-coated CNTs). Isolated, metallized
CNT islands/clusters are formed during the dispensing spray
process. The size of these islands is approximately 10-30
micrometers in diameter. A density of approximately 40,000 islands
per square centimeter was achieved by the spaying process, and a
considerable number of small clusters (approximately 0.5-5 .mu.m in
diameter) between these islands. If it is assumed that every island
and cluster provides one emission site each, this would provide
1,000,000 emission sites per square centimeter.
[0046] 3. Field Emission Test of the Samples
[0047] Substrates with metallized SWNT material coated on them were
prepared as cathodes and tested for field emission properties as
illustrated in FIGS. 1 and 6. Non-metallized SWNT coated substrates
were also prepared in an identical fashion by the spray process for
comparison purposes. The cathodes were tested by mounting them with
a phosphor screen in a diode configuration with a gap of about 0.5
mm. The test assembly was placed in a vacuum chamber and pumped to
10.sup.-7 torr. The electrical properties of the cathodes were then
measured by applying a negative, pulsed voltage to the cathode and
holding the anode at ground potential and measuring the current at
the anode. A pulsed voltage was used to prevent damage to the
phosphor screen at the high current levels (duty factor: 2%). FIG.
5 illustrates the results of these tests. In each case, the
cathodes were not "activated," they were tested as they were
deposited. It was found that the metallized CNT cathodes were very
stable and very uniform. The non-metallized cathodes typically were
unstable during the turn-on process (several arcing events
occurred). From FIG. 5 it can be seen that metallized SWNTs yield
much better field emission properties than the non-metallized
SWNTs. Tests on the cathodes show threshold extraction fields of
about 2 V/.mu.m and emission current of 30 mA at 4 V/.mu.m for
Co-coated CNT compared with extraction fields of 3.5 V/.mu.m and
emission current of 30 mA at 6.5 V/.mu.m for non-metallized
CNTs.
[0048] All of the compositions and methods disclosed and claimed
herein can be made and executed without undue experimentation in
light of the present disclosure. While the compositions and methods
of this invention have been described in terms of preferred
embodiments, it will be apparent to those of skill in the art that
variations may be applied to the compositions and methods and in
the steps or in th esequence of steps of the methods described
herein without departing from the concept, spirit, and scope of the
invention. More specifically, it will be apparent that certain
agents which are both chemically and physiologically related may be
substituted for the agents described herein while the same or
similar results would be achieved. All such similar substitutes and
modifications apparent to those skilled in the art are deemed to be
within the spirit, scope, and concept of the invention as defined
by the appended claims.
* * * * *