U.S. patent application number 12/525699 was filed with the patent office on 2009-12-24 for method for the electrochemical deposition of carbon nanotubes.
This patent application is currently assigned to E.I. DU PONT DE NEMOURS AND COMPANY. Invention is credited to Lap-Tak Andrew Cheng, Walter Mahler, David Herbert Roach, Ming Zheng.
Application Number | 20090314647 12/525699 |
Document ID | / |
Family ID | 39710667 |
Filed Date | 2009-12-24 |
United States Patent
Application |
20090314647 |
Kind Code |
A1 |
Zheng; Ming ; et
al. |
December 24, 2009 |
METHOD FOR THE ELECTROCHEMICAL DEPOSITION OF CARBON NANOTUBES
Abstract
This invention relates to the electrochemical deposition of
carbon nanotubes ("CNTs") on a substrate using an electrochemical
cell. A dispersion of a complex of CNTs and an anionic polymer is
neutralized and thereby caused to deposit on the anode plate of the
cell.
Inventors: |
Zheng; Ming; (Wilmington,
DE) ; Cheng; Lap-Tak Andrew; (Newark, DE) ;
Roach; David Herbert; (Hockessin, DE) ; Mahler;
Walter; (Wilmington, DE) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1122B, 4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Assignee: |
E.I. DU PONT DE NEMOURS AND
COMPANY
Wilmington
DE
|
Family ID: |
39710667 |
Appl. No.: |
12/525699 |
Filed: |
February 22, 2008 |
PCT Filed: |
February 22, 2008 |
PCT NO: |
PCT/US2008/002344 |
371 Date: |
August 4, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60903260 |
Feb 24, 2007 |
|
|
|
Current U.S.
Class: |
205/50 ;
205/316 |
Current CPC
Class: |
H01J 9/025 20130101;
C25D 13/02 20130101; H01J 2201/30469 20130101; C09D 7/61 20180101;
H01J 1/304 20130101; C25D 13/04 20130101; C09D 5/448 20130101; C08K
3/04 20130101; C25D 15/02 20130101; C09D 5/4484 20130101; C09D
5/4407 20130101; C09D 7/70 20180101 |
Class at
Publication: |
205/50 ;
205/316 |
International
Class: |
C25D 9/04 20060101
C25D009/04; C25D 7/00 20060101 C25D007/00 |
Claims
1. A method for the deposition of carbon nanotubes, comprising: (a)
providing an electrochemical cell that comprises a cathode, an
anode plate, a first electrically conducting pathway connecting the
cathode to an electrical power supply, and a second electrically
conducting pathway connecting the electrical power supply to the
anode plate; (b) providing as an aqueous electrolyte disposed
between the cathode and the anode a dispersion of a complex formed
from carbon nanotubes and a first anionic polymer; and (c) applying
a voltage to the electrochemical cell to deposit the complex on the
anode.
2. A method according to claim 1 wherein the aqueous electrolyte
further comprises a coagulant.
3. A method according to claim 2 wherein coagulant residue is
deposited on the anode together with the complex.
4. A method according to claim 1 wherein the first polymer
comprises a nucleic acid molecule.
5. A method according to claim 1 wherein the first polymer
comprises RNA.
6. A method according to claim 1 wherein the electrolyte further
comprises a second anionic polymer.
7. A method according to claim 6 wherein the second ionic polymer
comprises a styrenic ionomer or an ionized ethylene/(meth)acrylic
acid copolymer.
8. A method according to claim 6 wherein the complex, as deposited
on the anode, is deposited in a matrix of the second anionic
polymer.
9. A method according to claim 7 wherein the first polymer
comprises a nucleic acid molecule.
10. A method according to claim 7 wherein the first polymer
comprises RNA.
11. A method according to claim 1 further comprising a step of
removing the anode plate from the cell, and installing it in a
field emission device.
12. A film comprising a substrate and, disposed on the substrate,
(a) coagulant residue, and (b) a complex formed from carbon
nanotubes and a first anionic polymer.
13. A method according to claim 12 wherein the first polymer
comprises a nucleic acid molecule.
14. A method according to claim 12 wherein the first polymer
comprises RNA.
15. A film according to claim 12 wherein there is further disposed
on the substrate a second anionic polymer.
16. A method according to claim 15 wherein the second ionic polymer
comprises a styrenic ionomer or an ionized ethylene/(meth)acrylic
acid copolymer.
17. A method according to claim 15 wherein the first polymer
comprises a nucleic acid molecule.
18. A method according to claim 15 wherein the first polymer
comprises RNA.
19. A cathode assembly for a field emission device comprising a
film according to claim 1.
20. A field emission device comprising a cathode assembly according
to claim 19.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/903,260, filed 24 Feb. 2007, which is by this
reference incorporated in its entirety as a part hereof for all
purposes.
TECHNICAL FIELD
[0002] This invention relates to the electrochemical deposition of
carbon nanotubes ("CNTs") on a substrate.
BACKGROUND
[0003] U.S. Pat. No. 6,902,658 describes an electrophoretic
deposition method in which a separate step of depositing a binder
material onto a substrate is performed prior to deposition thereon
of CNTs. A need thus remains for a method in which CNTs and one or
more accompanying materials may be deposited onto a substrate
simultaneously.
SUMMARY
[0004] In one embodiment, this invention provides a method for the
deposition of carbon nanotubes by:
[0005] (a) providing an electrochemical cell that comprises a
cathode, an anode plate, a first electrically conducting pathway
connecting the cathode to an electrical power supply, and a second
electrically conducting pathway connecting the electrical power
supply to the anode plate;
[0006] (b) providing as an aqueous electrolyte disposed between the
cathode and the anode a dispersion of a complex formed from carbon
nanotubes and a first anionic polymer; and
[0007] (c) applying a voltage to the electrochemical cell to
deposit the complex on the anode.
[0008] In another embodiment, this invention provides a film that
includes a substrate and, disposed on the substrate, (a) coagulant
residue, and (b) a complex formed from carbon nanotubes and a first
anionic polymer.
[0009] In a further embodiment, this invention provides a cathode
assembly for a field emission device comprising a film as described
above.
[0010] In yet another embodiment, this invention provides a field
emission device comprising a cathode assembly as described
above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 shows a schematic representation of the mechanism of
deposit in one embodiment of the methods of this invention.
[0012] FIG. 2 shows deposited material on a film as prepared in
Example 1.
[0013] FIG. 3 shows the configuration of an electrochemical cell as
used in the examples.
[0014] FIG. 4 shows deposited material on a film as prepared in
Example 2.
[0015] FIG. 5 shows an image of phosphor illumination from a film
as tested in Example 3.
[0016] FIG. 6 shows a plot of recorded anode current and anode
voltage values as obtained in Example 4.
[0017] FIG. 7 shows an image of phosphor illumination from a film
as tested in Example 4.
[0018] FIG. 8 shows an image of phosphor illumination from a film
as tested in Example 5.
DETAILED DESCRIPTION
[0019] CNTs are well known to have unique and useful electrical
properties, and are frequently used in the fabrication of the
cathode of a field emission device. However, adoption of these
materials is constrained by their high cost. Therefore, an
objective of the invention is to provide a process for making a
uniform CNT film on a substrate such as a conducting substrate with
good uniformity and low material consumption. A further objective
is to pattern the CNT film thus prepared to be useful in electronic
applications. The CNT film so made may be used in a cathode
assembly that is installed in a field emission device.
[0020] A CNT film is made by the method of this invention by the
deposition of CNTs on a substrate by electrochemical means, and for
such purpose the method hereof involves the use of an
electrochemical cell. The cell contains a cathode, an anode plate,
a first electrically conducting pathway connecting the cathode to
an electrical power supply, and a second electrically conducting
pathway connecting the electrical power supply to the anode plate.
An aqueous electrolyte is provided to the cell and is disposed
between the cathode and the anode. Contained in the electrolyte is
a dispersion of a complex formed from CNTs and a first anionic
polymer, and optionally a coagulant.
[0021] CNTs as used herein are generally about 0.5-2 nm in diameter
where the ratio of the length dimension to the narrow dimension,
i.e. the aspect ratio, is at least 5. In general, the aspect ratio
is between 10 and 2000. CNTs are comprised primarily of carbon
atoms, however may be doped with other elements, e.g. metals. The
carbon-based nanotubes of the invention can be either multi-walled
nanotubes (MWNTs) or single-walled nanotubes (SWNTs). A MWNT, for
example, includes several concentric nanotubes each having a
different diameter. Thus, the smallest diameter tube is
encapsulated by a larger diameter tube, which in turn, is
encapsulated by another larger diameter nanotube. A SWNT, on the
other hand, includes only one nanotube.
[0022] CNTs may be produced by a variety of methods, and are
additionally commercially available. Methods of CNT synthesis
include laser vaporization of graphite [A. Thess et al, Science
273, 483 (1996)], arc discharge [C. Journet et al, Nature 388, 756
(1997)] and HiPCo (high pressure carbon monoxide) process [P.
Nikolaev et al, Chem. Phys. Lett. 313, 91-97 (1999)]. Chemical
vapor deposition (CVD) can also be used in producing carbon
nanotubes [J. Kong et al, Chem. Phys. Lett. 292, 567-574 (1998); J.
Kong et al, Nature 395, 878-879 (1998); A. Cassell et al, J. Phys.
Chem. 103, 6484-6492 (1999); H. Dai et al, J. Phys. Chem. 103,
11246-11255 (1999)]. Additionally CNTs may be grown via catalytic
processes both in solution and on solid substrates [Yan Li et al,
Chem. Mater.; 2001; 13(3); 1008-1014); (N. Franklin and H. Dai,
Adv. Mater. 12, 890 (2000); A. Cassell et al, J. Am. Chem. Soc.
121, 7975-7976 (1999)].
[0023] A major obstacle to the use of CNTs has been the diversity
of tube diameters, chiral angles, and aggregation states in
nanotube samples obtained from the various preparation methods.
Aggregation is particularly problematic because the highly
polarizable, smooth-sided fullerene tubes readily form parallel
bundles or ropes with a large van der Waals binding energy. This
bundling perturbs the electronic structure of the tubes, and it
confounds almost all attempts to separate the tubes by size or type
or to use them as individual macromolecular species.
[0024] There is provided by this invention a method for dispersing
a population of bundled carbon nanotubes by contacting the bundled
nanotubes with an aqueous solution of an anionic polymer. A complex
containing the anionic polymer and the CNTs is thereby formed, but
the association between the anionic polymer and the CNTs in the
complex is a loose association, is formed essentially by van der
Waals bonds or some other non-covalent means, and is not formed
through the interaction of specific functionalized groups. The
structural integrity of the CNTs is therefore retained, but the
complexes they form with the anionic polymers become suspended in a
dispersion in the electrolyte.
[0025] A variety of anionic polymers may thus be used as
dispersants for the purpose of dispersing CNTs in an aqueous
solution by facilitating the formation of the polymer/CNT complex,
but a preferred polymer for use for such purpose is a nucleic acid,
particularly a stabilized solution of nucleic acid molecules.
Nucleic acids are very effective in dispersing CNTs by the
formation of nanotube-nucleic acid complexes based on non-covalent
interactions between the nanotube and the nucleic acid molecule.
The method of this invention therefore includes a method for the
dispersion of bundled CNTs by contacting the nanotubes with a
solution of anionic polymers such as nucleic acid molecules.
[0026] In the following discussion of the use of nucleic acid
molecules to form complexes with and thereby disperse CNTs, the
following terms and abbreviations are used:
[0027] "cDNA" means complementary DNA
[0028] "PNA" means peptide nucleic acid
[0029] "SEM" means scanning electron microscopy
[0030] "ssDNA" means single stranded DNA
[0031] "tRNA" means transfer RNA
[0032] "CNT" means carbon nanotube
[0033] "MWNT" means multi-walled nanotube
[0034] "SWNT" means single walled nanotube
[0035] "TEM" means transmission electron microscopy
[0036] A "nucleic acid molecule" is defined as a polymer of RNA,
DNA, or peptide nucleic acid (PNA) that is single- or
double-stranded, optionally containing synthetic, non-natural or
altered nucleotide bases. A nucleic acid molecule in the form of a
polymer of DNA may be comprised of one or more segments of cDNA,
genomic DNA or synthetic DNA.
[0037] The letters "A", "G", "T", "C" when referred to in the
context of nucleic acids will mean the purine bases adenine
(C.sub.5H.sub.5N.sub.5) and guanine (C.sub.5H.sub.5N.sub.5O) and
the pyrimidine bases thymine (C.sub.5H.sub.6N.sub.2O.sub.2) and
cytosine (C.sub.4H.sub.5N.sub.3O), respectively.
[0038] The term "peptide nucleic acids" refers to a material having
stretches of nucleic acid polymers linked together by peptide
linkers.
[0039] A "stabilized solution of nucleic acid molecules" refers to
a solution of nucleic acid molecules that are solubilized and in a
relaxed secondary conformation.
[0040] A "nanotube-nucleic acid complex" means a composition
comprising a carbon nanotube loosely associated with at least one
nucleic acid molecule. Typically the association between the
nucleic acid and the nanotube is by van der Waals bonds or some
other non-covalent means.
[0041] The term "agitation means" refers to a devices that
facilitate the dispersion of nanotubes and nucleic acids. A typical
agitation means is sonication.
[0042] The term "denaturant" refers to substances effective in the
denaturation of DNA and other nucleic acid molecules.
[0043] Standard recombinant DNA and molecular biology techniques
used here are well known in the art and are described by Sambrook,
J., Fritsch, E. F. and Maniatis, T., Molecular Cloning: A
Laboratory Manual, Second Edition, Cold Spring Harbor Laboratory
Press, Cold Spring Harbor, N.Y. (1989) (hereinafter "Maniatis"); by
Silhavy, T. J., Bennan, M. L. and Enquist, L. W., Experiments with
Gene Fusions, Cold Spring Harbor Laboratory Cold Press Spring
Harbor, N.Y. (1984); and by Ausubel, F. M. et al., Current
Protocols in Molecular Biology, published by Greene Publishing
Assoc. and Wiley-Interscience (1987).
[0044] Nucleic acid molecules as used in a method of this invention
may be of any type and from any suitable source and include but are
not limited to DNA, RNA and peptide nucleic acids. The nucleic acid
molecules used herein may be generated by synthetic means or may be
isolated from nature by protocols well known in the art (Sambrook
supra). The nucleic acid molecules may be either single stranded or
double stranded and may optionally be functionalized at any point
with a variety of reactive groups, ligands or agents.
Functionalization of nucleic acids is not, however, required for
their association with CNTs for the purpose of dispersion, and most
of the nucleic acids used herein for dispersion do lack functional
groups and are therefore referred to herein as
"unfunctionalized".
[0045] Peptide nucleic acids (PNA) are particularly useful herein
for dispersion as they possess the double functionality of both
nucleic acids and peptides. Methods for the synthesis and use of
PNA's are well known in the art, see for example Antsypovitch, S.
I., Peptide nucleic acids: Structure, Russian Chemical Reviews
(2002), 71(1), 71-83.
[0046] The nucleic acid molecules used herein may have any
composition of bases and may even consist of stretches of the same
base (poly A or polyT for example) without impairing the ability of
the nucleic acid molecule to disperse the bundled CNTs. Preferably
the nucleic acid molecules will be less than about 2000 bases where
less than 1000 bases is preferred and where from about 5 bases to
about 1000 bases is most preferred. Generally the ability of
nucleic acids to disperse CNTs appears to be independent of
sequence or base composition, however there is some evidence to
suggest that the less G-C and T-A base-pairing interactions in a
sequence, the higher the dispersion efficiency, and that RNA and
varieties thereof is particularly effective in dispersion and is
thus preferred herein. Nucleic acid molecules suitable for use
herein include without limitation those having the general formula:
[0047] 1. An wherein n=1-2000; [0048] 2. Tn wherein n=1-2000;
[0049] 3. Cn wherein n=1-2000; [0050] 4. Gn wherein n=1-2000;
[0051] 5. Rn wherein n=1-2000, and wherein R may be either A or G;
[0052] 6. Yn wherein n=1-2000, and wherein Y may be either C or T;
[0053] 7. Mn wherein n=1-2000, and wherein M may be either A or C;
[0054] 8. Kn wherein n=1-2000, and wherein K may be either G or T;
[0055] 9. Sn wherein n=1-2000, and wherein S may be either C or G;
[0056] 10. Wn wherein n=1-2000, and wherein W may be either A or T;
[0057] 11. Hn wherein n=1-2000, and wherein H may be either A or C
or T; [0058] 12. Bn wherein n=1-2000, and wherein B may be either C
or G or T; [0059] 13. Vn wherein n=1-2000, and wherein V may be
either A or C or G; [0060] 14. Dn wherein n=1-2000, and wherein D
may be either A or G or T; and [0061] 15. Nn wherein n=1-2000, and
wherein N may be either A or C or T or G.
[0062] In addition to the combinations listed above, any of these
sequences may have one or more deoxyribonucleotides replaced by
ribonucleotides (i.e. RNA or RNA/DNA hybrid) or one or more
sugar-phosphate linkages replaced by peptide bonds (i.e. PNA or
PNA/RNA/DNA hybrid).
[0063] Nucleic acid molecules as used herein may be stabilized in a
suitable solution. It is preferred that the nucleic acid molecules
be in a relaxed secondary conformation and only loosely associated
with each other to allow for the greatest contact by individual
strands with the CNTs. Stabilized solutions of nucleic acids are
common and well known in the art (see Sambrook, supra) and
typically include salts and buffers such as sodium and potassium
salts, and TRIS (Tris(2-aminoethyl)amine), HEPES
(N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid), and
MES(2-(N-Morpholino)ethanesulfonic acid. Preferred solvents for
stabilized nucleic acid solutions are those that are water miscible
where water is most preferred. The process of dispersion may be
improved with the optional addition of nucleic acid denaturing
substances to the solution. Common denaturants include but are not
limited to formamide, urea and guanidine. A non-limiting list of
suitable denaturants may be found in Sambrook, supra.
[0064] To prepare a dispersion according to a method hereof, one or
more nucleic acid molecules may be contacted with a population of
bundled carbon nanotubes. It is preferred, although not required,
contact be made in the presence of an agitation means of some sort.
Typically the agitation means employs sonication, but may also
include devices that produce high shear mixing of the nucleic acids
and CNTs (i.e. homogenization), or any combination thereof. Upon
agitation, the CNTs will become dispersed and will form
nanotube-nucleic acid complexes comprising at least one nucleic
acid molecule loosely associated with the CNT by hydrogen bonding
or some other non-covalent means.
[0065] Temperature during the process of contacting CNTs with a
nucleic acid may have an effect on the efficacy of the dispersion.
Mixing at room temperature or higher has been seen to give longer
dispersion times whereas mixing at temperatures below room
temperature (23.degree. C.) has been seen to give more rapid
dispersion times, and temperatures of about 4.degree. C. are
preferred. The dispersion of CNTs by contact with nucleic acid
molecules is also described in US 2004/0132072 and US 2004/0146904,
each of which is by this reference incorporated in its entirety as
a part hereof for all purposes.
[0066] In addition to the nucleic acid molecules described above,
one or more other anionic polymers may be used for the purpose of
preparing an aqueous dispersion of CNTs. Examples of other anionic
polymers that have been found useful in the preparation of a
dispersion of CNTs include without limitation ionized poly(acrylic
acid) ("PAA") or ionized ethylene/(meth)acrylic acid copolymer
("EAA" or "EMAA"), either of which may be neutralized with cations
such as Na.sup.+, K.sup.+, NH.sub.4.sup.+ or Cr.sup.+; styrenic
ionomers such as styrene/sodium styrene sulfonate copolymer (PSS)
or styrene/sodium styrene methacrylate copolymer; and ionized
tetrafluoroethylene/sulfonic acid copyolymers such as Nafion.TM.
copolymer (from DuPont) in which the sulfonic acid group in a
tetrafluoroethylene/perfluorovinyl ether copolymer may be sodium
neutralized. As indicated above with respect to nucleic acid
molecules, ultrasonication or other mixing means may be applied to
facilitate the dispersion of CNTs in an aqueous solution of one or
more of the anionic polymers discussed in this paragraph.
[0067] In one embodiment, deposition on the anode plate of the cell
of the complexes formed from CNTs and molecules of an anionic
polymer, as dispersed in the electrolyte solution contained in the
cell, will be facilitated by the presence therein of the optional
coagulant. The coagulant will neutralize the negative charge on the
anionic polymer in the complex. As the population of anionic
polymer/CNT complexes has been maintained in dispersion primarily
by the repulsion of one negatively charged complex from another (or
by the repulsion of positively charged double layers surrounding
the complexes), neutralization of those negative charges (or
compression of the double layer) by the coagulant will remove the
force enabling the population of complexes to remain in dispersion
in the electrolyte solution. As the action of the coagulant to
neutralize the complexes occurs in close proximity to the anode
plate, the complexes (as no longer dispersed) will in varying
degrees undergo a transition from solution phase to solid phase,
become progressively aggregated and agglomerated (similar to the
formation of floccules and flocs), and then be collected and
deposited on the surface of the anode plate. In addition to the CNT
complexes, the material as deposited on the plate may include
coagulant residue.
[0068] If first and second anionic polymers are present in the
electrolyte solution, such as a first polymer that forms a complex
with CNTs and a second polymer that does not or is more loosely
associated with CNTs than the first polymer, they may become
deposited on the surface of the anode at the same time. The first
polymer may, for example, be deposited in a matrix of the second
polymer. If additional materials needed to enhance the usefulness
and performance of the anode plate in a field emission device, such
as conductive or functionalized particles, are present in the
electrolyte solution, they may be deposited on the anode plate at
the same time as the anionic polymer/CNT complexes. FIG. 2 shows a
typical example of the type of film formed by such deposition on
the anode plate, which film has good uniformity of evenly
deposited, well-adhered material all across its surface.
[0069] Coagulants suitable for use herein for the purpose of
neutralizing an anionic polymer/CNT complex include inorganic
coagulants such as trivalent cations formed from metals including
Group VIII/VIIIA metals such as iron, cobalt, ruthenium or osmium.
As a trivalent cation can be up to as much as ten times more
effective in neutralizing the complex than a divalent cation, a
convenient way to provide the coagulant is to supply a divalent
cation such as tris(2,2'-bipyridyl)dichloro-ruthenium (II) to the
electrolyte solution wherein the 2.sup.+ cation is oxidized to a
3.sup.+ valence by the loss of electrons to the anode plate. A
schematic representative of this mechanism is shown in FIG. 1.
[0070] In an alternative embodiment, however, a coagulant is not
used where the anode plate is formed from a metal such as silver or
nickel. In such case, the metal on the plate dissolves in the
electrolyte solution, and the charge on an anionic polymer/CNT
complex is neutralized by cations formed from metal atoms that have
gone into solution from the solid metal from which the plate if
formed.
[0071] The method hereof is generally performed by operation of the
cell at lower potential such as less than about 5 volts, or from
about 2 to less than about 5 volts, or from about 2 volts to about
3 volts. Thickness of the deposited film is to a large extent
directly related to length of deposition time. A deposition time in
the range of about 1 to about 10 minutes, or in the range of about
1 to about 2 minutes, may be used. A positive potential is
maintained at the anode plate relative to the cathode of the
cell.
[0072] The method hereof may be used to produce a film in which the
deposited material is deposited in a pre-determined pattern. This
may be accomplished by patterning the surface of the plate used as
the anode using conventional photoimaging techniques. Thus a
photoresist may be activated through a mask and then developed to
provide on the surface of the anode a pattern such as an array of
circular wells. As the anionic polymer/CNT complexes are aggregated
and settle out of solution, they are deposited only in the holes,
and the photoresist may be removed. This provides a patterned CNT
film, with the anode plate serving as a substrate for the film, for
use by installation in a field emission device.
[0073] After completion of the deposition of CNT complex material
on the anode plate in the cell, the plate may be removed from the
cell, rinsed, dried and installed in such condition in a field
emission device for use as part of the cathode assembly therein to
provide electron emission. Alternatively, however, the plate may be
baked and/or fired before installation in a field emission device
to melt the deposited polymer(s) and utilize them in that form as
an adhesive to more securely anchor the CNTs to the surface of the
plate, resulting in a CNT-containing film with excellent abrasion
resistance.
[0074] In the field emission device into which the above described
plate coated with deposited material may be installed, an electron
emitting material is disposed on a cathode and, when energized,
bombards an anode with electrons. The electron emitting material
may be an acicular substance such as carbon, a semiconductor, a
metal or mixtures thereof. As used herein, "acicular" means
particles with aspect ratios of 10 or more. Typically, glass frit,
metallic powder or metallic paint or a mixture thereof is used to
attach the electron emitting material to a substrate in the cathode
assembly.
[0075] Acicular carbon as used as the electron emitting material
may be of various types, but carbon nanotubes are the preferred
acicular carbon and single wall CNTs are especially preferred.
Carbon fibers grown from the catalytic decomposition of
carbon-containing gases over small metal particles are also useful
as acicular carbon, and other examples of acicular carbon are
polyacrylonitrile-based (PAN-based) carbon fibers and pitch-based
carbon fibers.
[0076] Various processes can be used to attach an electron emitting
material to a substrate. The means of attachment must withstand and
maintain its integrity under the conditions of manufacturing the
apparatus into which the field emitting cathode is placed and under
the conditions surrounding its use, e.g. typically vacuum
conditions and temperatures up to about 450.degree. C. A preferred
method is to screen print a paste comprised of the electron
emitting material and glass frit, metallic powder or metallic paint
or a mixture thereof onto a substrate in the desired pattern and to
then fire the dried patterned paste. For a wider variety of
applications, e.g. those requiring finer resolution, the preferred
process comprises screen printing a paste which further comprises a
photoinitiator and a photohardenable monomer, photopatterning the
dried paste and firing the patterned paste.
[0077] The substrate can be any material to which the paste
composition will adhere. If the paste is non-conducting and a
non-conducting substrate is used, a film of an electrical conductor
to serve as the cathode electrode and provide means to apply a
voltage to the electron emitting material will be needed. Silicon,
a glass, a metal or a refractory material such as alumina can serve
as the substrate. For display applications, the preferable
substrate is glass and soda lime glass is especially preferred. For
optimum conductivity on glass, silver paste can be pre-fired onto
the glass at 500-550.degree. C. in air or nitrogen, but preferably
in air. The conducting layer so-formed can then be over-printed
with the emitter paste.
[0078] The paste used for screen printing typically contains the
electron emitting material, an organic medium, solvent, surfactant
and either low softening point glass frit, metallic powder or
metallic paint or a mixture thereof. The role of the medium and
solvent is to suspend and disperse the particulate constituents,
i.e. the solids, in the paste with a proper rheology for typical
patterning processes such as screen printing. There are many
organic media known for use for such purpose including cellulosic
resins such as ethyl cellulose and alkyd resins of various
molecular weights. Butyl carbitol, butyl carbitol acetate, dibutyl
carbitol, dibutyl phthalate and terpineol are examples of useful
solvents. These and other solvents are formulated to obtain the
desired viscosity and volatility requirements.
[0079] A glass frit that softens sufficiently at the firing
temperature to adhere to the substrate and to the electron emitting
material is also used. A lead or bismuth glass frit can be used as
well as other glasses with low softening points such as calcium or
zinc borosilicates. If a screen printable composition with higher
electrical conductivity is desired, the paste may also contain a
metal, for example, silver or gold. The paste typically contains
about 40 wt % to about 80 wt % solids based on the total weight of
the paste. These solids include the electron emitting material and
glass frit and/or metallic components. Variations in the
composition can be used to adjust the viscosity and the final
thickness of the printed material.
[0080] The emitter paste is typically prepared by three-roll
milling a mixture of the electron emitting material, organic
medium, surfactant, solvent and either low softening point glass
frit, metallic powder or metallic paint or a mixture thereof. The
paste mixture can be screen printed using, for example, a
165-400-mesh stainless steel screen. The paste can be deposited as
a continuous film or in the form of a desired pattern. When the
substrate is glass, the paste is then fired at a temperature of
about 350.degree. C. to about 550.degree. C., preferably at about
450.degree. C. to about 525.degree. C., for about 10 minutes in
nitrogen. Higher firing temperatures can be used with substrates
which can endure them provided the atmosphere is free of oxygen.
However, the organic constituents in the paste are effectively
volatilized at 350-450.degree. C., leaving a layer of the composite
of the electron emitting material and glass and/or metallic
conductor. The electron emitting material appears to undergo no
appreciable oxidation or other chemical or physical change during
the firing in nitrogen.
[0081] If the screen-printed paste is to be photopatterned, the
paste may also contain a photoinitiator, a developable binder and a
photohardenable monomer comprised, for example, of at least one
addition polymerizable ethylenically unsaturated compound having at
least one polymerizable ethylenic group. Typically, a paste
prepared from an electron emitting material such as CNTs, silver
and glass frit will contain about 0.01-6.0 wt % nanotubes, about
40-75 wt % silver in the form of fine silver particles and about
3-15 wt % glass frit based on the total weight of the paste.
[0082] The anode of the field emission device is an electrode
coated with an electrically conductive layer. When the field
emission device is used in a display device where the cathode
contains an array of pixels of the thick film paste deposits
described above, the anode in the display device may comprise
phosphors to convert incident electrons into light. The substrate
of the anode would also be selected to be transparent so that the
resulting light could be transmitted. From the cathode assembly and
anode, a sealed unit is constructed in which the cathode assembly
and anode are separated by spacers, and there is an evacuated space
between the anode and the cathode. This evacuated space needs to be
under partial vacuum so that the electrons emitted from the cathode
may transit to the anode with only a small number of collisions
with gas molecules. Frequently the evacuated space is evacuated to
a pressure of less than 10.sup.-5 Torr.
[0083] Such a field emission device is useful in a variety of
electronic applications, e.g. vacuum electronic devices, flat panel
computer and television displays, back-light source for LCD
displays, emission gate amplifiers, and klystrons and in lighting
devices. For example, flat panel displays having a cathode using a
field emission electron source, i.e. a field emission material or
field emitter, and a phosphor capable of emitting light upon
bombardment by electrons emitted by the field emitter have been
proposed. Such displays have the potential for providing the visual
display advantages of the conventional cathode ray tube and the
depth, weight and power consumption advantages of the other flat
panel displays. The flat panel displays can be planar or curved.
U.S. Pat. Nos. 4,857,799 and 5,015,912 disclose matrix-addressed
flat panel displays using micro-tip cathodes constructed of
tungsten, molybdenum or silicon. WO 94-15352, WO 94-15350 and WO
94-28571 disclose flat panel displays wherein the cathodes have
relatively flat emission surfaces. These devices are also described
in US 2002/0074932, which is by this reference incorporated in its
entirety as a part hereof for all purposes.
[0084] Materials as used in the process hereof may be made by
processes known in the art, or are available commercially from
suppliers such as Alfa Aesar (Ward Hill, Mass.), City Chemical
(West Haven, Conn.), Fisher Scientific (Fairlawn, N.J.),
Sigma-Aldrich (St. Louis, Mo.) or Stanford Materials (Aliso Viejo,
Calif.).
[0085] The advantageous attributes and effects of this invention
may be seen in a series of examples (Examples 1.about.5), as
described below. The embodiments on which the examples are based
are representative only, and the selection of those embodiments to
illustrate the invention does not indicate that materials,
conditions, specifications, components, reactants, techniques and
protocols not described in these examples are not suitable for
practicing this invention, or that subject matter not described in
these examples is excluded from the scope of the appended claims
and equivalents thereof.
EXAMPLES
[0086] 150 mg of laser-ablated CNTs (from CNI, Houston, Tex.) was
mixed with 30 mg yeast RNA (from Sigma Aldrich) in 15 mL of
1.times.TBE [tris borate (ethylenediaminetetraacetic acid)] buffer
(from Sigma Aldrich). The mixture was sonicated with a probe
sonicator at a power level of 20 W for 30 min. The resulting
dispersion was mixed with two other components according to the
following table (Table 1) to make up 100 mL of deposition solution.
Ru.sup.2+(bipy).sub.3 as used in the deposition solution is
tris(2,2'-bipyridyl)dichloro-ruthenium (II) and is obtained from
Sigma Aldrich. EMMA is ethylene/methacrylic acid ionomer obtained
from DuPont as Surlyn.TM. ionomer.
TABLE-US-00001 TABLE 1 Composition of deposition solution Component
Stock conc. Volume added Final conc. CNT 10 mg/mL 4 mL 0.04%
dispersion EMMA 10 mg/mL 2 mL 0.02% Ru.sup.2+(bipy).sub.3 10 mM 2
mL 0.2 mM water 92 mL
Example 1
[0087] A 2'.times.2' stainless steel plate (used as the cathode)
and a 2'.times.2' indium tin oxide ("ITO") plate (used as the
anode) were inserted in a parallel fashion into a rectangular
electrochemical cell (the configuration for which is shown in FIG.
3). The cell was charged with 15 mL of the deposition solution as
the electrolyte. A potential difference of 3.2 V was applied
between the two electrodes. After 1 minute, the deposition was
stopped, and the ITO plate was taken out of the cell, rinsed with
deionized water and dried in air. Uniform deposition of material on
the plate was obtained as shown in FIG. 2.
Example 2
[0088] A photoresist (PR) patterned indium tin oxide (ITO)
substrate (2'.times.2') was used as the anode. The PR layer defines
an array of circular wells with 20 um diameter. The wells expose
the surface of the ITO plate for CNT deposition. Before
electrodeposition, the PR coated ITO plate was dipped into a
solution of 0.01% Triton X-100, taken out and dried by blowing
N.sub.2 gas. This assists with coating the hydrophobic PR layer
with a thin hydrophilic layer for better wetting. After this
treatment, a 2'.times.2' stainless steel plate (used as the
cathode) and the PR-coated ITO plate (used as the anode) were
inserted in a parallel fashion into the same type of
electrochemical cell as used in Example 1. The cell was charged
with 15 mL of the deposition solution. An AC potential (100 Hz
square wave with 0 to 3.5 V peak-to-peak voltage and 50% duty
cycle) was applied between the two electrodes. After 1 minute, the
deposition was stopped, and the ITO plate was taken out of the
cell, rinsed with deionized water and dried in air. The PR layer
was removed by treatment with an acetone solvent. Good uniformity
of deposition of CNT material in the exposed wells was obtained as
shown in FIG. 4.
Example 3
[0089] The dried plate obtained from Example 1 and depicted in FIG.
2 was then fired in nitrogen for 10 minutes at 420.degree. C. A
piece of adhesive tape was then laminated over the CNT film and
subsequently removed. This process, commonly referred to as
"activation", is known to fracture the film surface exposing and
lifting the CNT filaments off the substrate surface to dramatically
enhance electron field emission. A diode field emission device was
then assembled by using the CNT film coated ITO substrate as a
cathode. Opposite to this "activated" cathode, an anode plate
consisting of an ITO coated glass substrate with a phosphor coating
was mounted. Electrically insulating spacers 1 mm thick were used
to maintain a distance between the cathode and anode substrates.
Electrical contact was made to the cathode and anode electrodes
using silver paint and copper tape to complete the diode device.
The device was mounted in a vacuum chamber which was evacuated to a
pressure of <1.times.10.sup.-5 Torr. A pulsed square wave with a
repetition rate of 60 Hz and a pulse width of 60 .mu.s was applied
to the anode electrode. The cathode electrode was maintained at
ground potential. At an anode voltage of 2 kV, an anode current of
200 .mu.A was obtained. An image of phosphor illumination by
electrons emitted by this device is shown in FIG. 5.
Example 4
[0090] The dried plate obtained from Example 2 and depicted in FIG.
4 was fired in nitrogen for 10 minutes at 420.degree. C. as
described in Example 3. The CNT dot surfaces were activated with a
piece of adhesive tape as described in Example 3. A diode field
emission device was then assembled by using the CNT dot covered ITO
substrate as a cathode and an ITO coated glass substrate with a
phosphor coating as anode. Glass spacers 0.22 mm thick were used in
this example to maintain a distance between the cathode and anode
substrates. The device was mounted in a vacuum chamber which was
evacuated to a pressure of <1.times.10.sup.-5 Torr. A pulsed
square wave with a repetition rate of 60 Hz and a pulse width of 60
.mu.s was applied to the anode electrode. The cathode electrode was
maintained at ground potential. When the pulsed anode voltage
reached 800 V, an average anode current of 5 .mu.A was measured. As
the pulse anode voltage was increased, increasing anode current was
measured. At an anode voltage of 925 V, an anode current of 40
.mu.A was obtained. FIG. 6 shows a plot of the recorded anode
current and anode voltage values from this field emission device.
An image of phosphor illumination by electrons emitted by this
device, operating at 975 V anode voltage and 80 .mu.A anode
current, is shown in FIG. 7. Each rectangular illuminated pixel on
the anode was produced by an array of multiple CNT dots on the
cathode.
Example 5
[0091] Instead of a plain ITO coated glass substrate as was made in
Example 2 and used in Example 4, the method hereof was used to
deposit CNT dots on a top-gate triode substrate. A top-gate triode
substrate typically consists of two conductive layers between which
is disposed an insulating layer. In this example, an ITO coated
glass substrate was used as the substrate for the top-gate triode,
using the ITO layer as the cathode. An insulating dielectric layer
was deposited on top of the ITO layer. A metallic gate electrode
layer was deposited on the dielectric layer. In addition, an array
of circular wells was etched through the metal and dielectric
layers, using a photoresist ("PR") and mask, exposing the ITO
surface. As in Example 2, the array of circular wells defined a
pattern on the PR layer that covered the triode assembly. The
openings of the wells in the PR had a smaller diameter than the
diameter of the wells that extended through the metal and
dielectric layer, but the circumference of the smaller well was
concentric with that of the larger. Using procedures similar to
those described in Examples 2 and 4, CNT dots were deposited on the
ITO surface, fired and activated.
[0092] Opposite the activated triode cathode, an anode plate
consisting of an ITO coated glass substrate with a phosphor coating
was mounted. Spacers 3 mm thick were used to maintain the distance
between the cathode and anode substrates. Electrical contact was
made to the ITO cathode electrode, metal gate electrode, and ITO
anode electrode using silver paint and copper tape to complete a
top-gate triode device. The device was mounted in a vacuum chamber
which was evacuated to a pressure of <1.times.10.sup.-5 Torr. A
DC voltage of 3 kV was applied to the anode electrode. A pulsed
square wave with a repetition rate of 120 Hz and a pulse width of
30 .mu.s was applied to the gate electrode. The cathode electrode
was maintained at ground potential. When the pulsed gate voltage
reached 70 V, an average anode current density of 5.0
.mu.A/cm.sup.2 was measured. An image of phosphor illumination by
electrons emitted by this triode device is shown in FIG. 8.
[0093] Features of certain of the devices of this invention are
described herein in the context of one or more specific embodiments
that combine various such features together. The scope of the
invention is not, however, limited by the description of only
certain features within any specific embodiment, and the invention
also includes (1) a subcombination of fewer than all of the
features of any described embodiment, which subcombination may be
characterized by the absence of the features omitted to form the
subcombination; (2) each of the features, individually, included
within the combination of any described embodiment; and (3) other
combinations of features formed by grouping only selected features
of two or more described embodiments, optionally together with
other features as disclosed elsewhere herein.
[0094] In this specification, unless explicitly stated otherwise or
indicated to the contrary by the context of usage, where an
embodiment of the subject matter hereof is stated or described as
comprising, including, containing, having, being composed of or
being constituted by or of certain features or elements, one or
more features or elements in addition to those explicitly stated or
described may be present in the embodiment. An alternative
embodiment of the subject matter hereof, however, may be stated or
described as consisting essentially of certain features or
elements, in which embodiment features or elements that would
materially alter the principle of operation or the distinguishing
characteristics of the embodiment are not present therein. A
further alternative embodiment of the subject matter hereof may be
stated or described as consisting of certain features or elements,
in which embodiment, or in insubstantial variations thereof, only
the features or elements specifically stated or described are
present.
* * * * *