U.S. patent application number 10/171760 was filed with the patent office on 2003-05-15 for field emission devices using modified carbon nanotubes.
This patent application is currently assigned to Hyperion Catalysis International, Inc.. Invention is credited to Biebuyck, Hans, Fischer, Alan B., Hoch, Robert, Niu, Chunming, Takai, Mikio, Tennent, Howard G..
Application Number | 20030090190 10/171760 |
Document ID | / |
Family ID | 28791793 |
Filed Date | 2003-05-15 |
United States Patent
Application |
20030090190 |
Kind Code |
A1 |
Takai, Mikio ; et
al. |
May 15, 2003 |
Field emission devices using modified carbon nanotubes
Abstract
The present invention relates to a field emission device
comprising an anode and a cathode, wherein said cathode includes
carbon nanotubes nanotubes which have been subjected to energy,
plasma, chemical, or mechanical treatment. The present invention
also relates to a field emission cathode comprising carbon
nanotubes which have been subject to such treatment. A method for
treating the carbon nanotubes and for creating a field emission
cathode is also disclosed. A field emission display device
containing carbon nanotube which have been subject to such
treatment is further disclosed.
Inventors: |
Takai, Mikio; (Hyogo,
JP) ; Fischer, Alan B.; (Cambridge, MA) ; Niu,
Chunming; (Lexington, MA) ; Tennent, Howard G.;
(Kennett Square, PA) ; Hoch, Robert; (Hensonville,
NY) ; Biebuyck, Hans; (Rockville, MD) |
Correspondence
Address: |
Barry Evans, Esq.
Kramer Levin Naftalis & Frankel LLP
919 Third Avenue
New York
NY
10022
US
|
Assignee: |
Hyperion Catalysis International,
Inc.
|
Family ID: |
28791793 |
Appl. No.: |
10/171760 |
Filed: |
June 14, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60298193 |
Jun 14, 2001 |
|
|
|
Current U.S.
Class: |
313/311 |
Current CPC
Class: |
H01J 2329/00 20130101;
B82Y 10/00 20130101; H01J 29/04 20130101; H01J 1/304 20130101; H01J
29/481 20130101; H01J 2201/30469 20130101; H01J 9/025 20130101 |
Class at
Publication: |
313/311 |
International
Class: |
H01J 001/05; H01J
001/38 |
Claims
We claim:
1. A field emission device comprising: a cathode; and an anode
spaced from the cathode, wherein said cathode includes emitters
comprising carbon nanotubes which have been subjected to energy,
plasma, chemical, or mechanical treatment.
2. The field emission device of claim 1, wherein said nanotubes are
substantially cylindrical carbon fibrils having one or more
graphitic layers concentric with their cylindrical axes, said
carbon fibrils being substantially free of pyrolytically deposited
carbon overcoat, having a substantially uniform diameter between 1
nm and 100 nm and having a length to diameter ratio greater than
5.
3. The field emission device of claim 1, wherein said nanotubes are
in the form of aggregates selected from the group consisting of
cotton candy aggregates.
4. The field emission device of claim 1, wherein said nanotubes
have a morphology resembling a fishbone.
5. The field emission device of claim 1, wherein said nanotubes are
single wall nanotubes.
6. The field emission device of claim 1, wherein said nanotubes are
in the form of a film or mat.
7. The field emission device of claim 1, wherein said nanotubes
have been treated with an ion beam.
8. The field emission device of claim 1, wherein said nanotubes
have been treated with a gallium ion beam.
9. The field emission device of claim 1, wherein said nanotubes
have been treated with a beam of ions selected from the group
consisting of hydrogen, helium, argon, carbon, oxygen, and xenon
ions.
10. The field emission device of claim 1, wherein said chemical
treatment is selected from the group consisting of acid treatment,
metal vapor treatment, chemical vapor transport and chemical
sorption.
11. The field emission device of claim 1, wherein said chemical
treatment is performed with chemical reagents selected from the
group consisting of oxidizing agents, electrophiles, nucleophiles,
reducing agents, strong acids, strong bases and mixtures
thereof.
12. The field emission device of claim 1, wherein said chemical
treatment is performed with phthalocyanines or porphyrins.
13. The field emission device of claim 1 wherein said energy
treatment is performed with an energy source selected from a group
consisting of electromagnetic radiation, ionizing radiation, atomic
beams, electron beams, ultraviolet light, microwave radiation,
gamma ray, x-ray, neutron beam, molecular beams and laser beam.
14. The field emission device of claim 1 wherein said plasma
treatment is performed with a plasma selected from a group
consisting of oxygen, hydrogen, ammonia, helium, argon, water,
nitrogen, ethylene, carbon tetrafluoride, sulfur hexafluoride,
perfluoroethylene, fluoroform, difluoro-dichloromethane,
bromo-trifluoromethane, chlorotrifluoromethane and mixtures
thereof.
15. The field emission device of claim 1, wherein said treatment
results in the introduction of metal atoms onto the carbon
nanotubes.
16. The field emission device of claim 1, wherein said treatment
results in the introduction of functional groups onto the carbon
nanotubes.
17. The field emission device of claim 16, wherein said functional
groups have been introduced by chemical sorption.
18. The field emission device of claim 1, wherein said treatment
comprises heating the carbon nanotubes in the presence of metal
vapor.
19. The field emission device of claim 1, wherein said treatment
comprises chemisorption followed by heat treatment.
20. The field emission device of claim 1, wherein said treatment
includes annealing said nanotubes.
21. The field emission device of claim 1, wherein said cathode
further includes a binder.
22. The field emission device of claim 21, wherein said binder is a
conductive carbon paste, conductive metal paste or carbonizable
polymer.
23. A process for treating carbon nanotubes comprising the step of
bombarding carbon nanotubes with ions.
24. The process for treating carbon nanotubes of claim 23, wherein
the nanotubes are bombarded with gallium ions.
25. The process of treating carbon nanotubes of claim 23, wherein
the nanotubes are bombarded with ions selected from the group
consisting of hydrogen, helium, argon, carbon, oxygen, and xenon
ions.
26. Carbon nanotubes formed by the process of claim 23.
27. A field emission cathode comprising carbon nanotubes, wherein
said nanotubes have been subjected to energy, plasma, chemical, or
mechanical treatment.
28. The field emission cathode of claim 27, wherein said nanotubes
are substantially cylindrical carbon fibrils having one or more
graphitic layers concentric with their cylindrical axes, said
carbon fibrils being substantially free of pyrolytically deposited
carbon overcoat, having a substantially uniform diameter between 1
nm and 100 nm and having a length to diameter ratio greater than
5.
29. The field emission cathode of claim 27, wherein said nanotubes
are in the form of aggregates selected from the group consisting of
cotton candy aggregates or bird nest aggregates.
30. The field emission cathode of claim 27, wherein said nanotubes
have a morphology resembling a fishbone.
31. The field emission cathode of claim 27 wherein said nanotubes
are single wall nanotubes.
32. The field emission cathode of claim 27, wherin said nanotubes
are in the form of a film or mat.
33. The field emission cathode of claim 27, wherein said nanotubes
have been treated with an ion beam.
34. The field emission cathode of claim 27, wherein said nanotubes
have been treated with a gallium ion beam.
35. The field emission cathode of claim 27, wherein said nanotubes
have been treated with a beam of ions selected from the group
consisting of hydrogen, helium, argon, carbon, oxygen, and xenon
ions.
36. The field emission cathode of claim 27, wherein said chemical
treatment is selected from the group consisting of acid treatment,
metal vapor treatment, chemical vapor transport, and chemical
sorption.
37. The field emission cathode of claim 27, wherein said chemical
treatment is performed with chemical reagents selected from the
group consisting of oxidizing agents, electrophiles, nucleophiles,
reducing agents, strong acids, strong bases and mixtures
thereof.
38. The field emission cathode of claim 27, wherein said chemical
treatment is performed with phthalocyanines or porphyrins.
39. The field emission cathode of claim 27, wherein said energy
treatment is performed with an energy source selected from a group
consisting of electromagnetic radiation, ionizing radiation, atomic
beams, electron beams, ultraviolet light, microwave radiation,
gamma ray, x-ray, neutron beam, molecular beams and laser beam.
40. The field emission cathode of claim 27, wherein said plasma
treatment is performed with a plasma selected from a group
consisting of oxygen, hydrogen, ammonia, helium, argon, water,
nitrogen, ethylene, carbon tetrafluoride, sulfur hexafluoride,
perfluoroethylene, fluoroform, difluoro-dichloromethane,
bromo-trifluoromethane, chlorotrifluoromethane and mixtures
thereof.
41. The field emission cathode of claim 27, wherein said treatment
results in the introduction of metal atoms onto the carbon
nanotubes.
42. The field emission cathode of claim 27, wherein said treatment
results in the introduction of functional groups onto the carbon
nanotubes.
43. The field emission cathode of claim 42, wherein said functional
groups have been introduced by chemical sorption.
44. The field emission cathode of claim 27, wherein said treatment
comprises heating the carbon nanotubes in the presence of metal
vapor.
45. The field emission cathode of claim 27, wherein said treatment
comprises chemisorption followed by heat treatment.
46. The field emission cathode of claim 27, wherein said treatment
includes annealing said nanotubes.
47. The field emission cathode of claim 27, wherein said cathode
further includes a binder.
48. The field emission cathode of claim 47, wherein said binder is
a conductive carbon paste, a conductive metal paste or a
carbonizable polymer.
49. The field emission cathode of claim 27, wherein the nanotubes
are deposited onto a substrate.
50. A method for making a field emission cathode comprising the
steps of: dispersing carbon nanotubes into a liquid vehicle to form
a solution; forming an electrophoresis bath, said bath including an
anode and a cathode immersed therein; applying a voltage to said
anode and said cathode, thereby causing said carbon nanotubes to
deposit onto said cathode; removing said cathode from said bath;
and subjecting the nanotubes deposited on said cathode to an
energy, plasma, chemical, or mechanical treatment.
51. The method for making a field emission cathode of claim 50,
wherein said nanotubes are substantially cylindrical carbon fibrils
having one or more graphitic layer concentric with their
cylindrical axes, said carbon fibrils being substantially free of
pyrolytically deposited carbon overcoat, having a substantially
uniform diameter between 1 nm and 100 nm and having a length to
diameter ratio greater than 5.
52. The method for making a field emission cathode of claim 50,
wherein said nanotubes are in the form of aggregates selected from
the group consisting of cotton candy aggregates or bird nest
aggregates.
53. The field emission display device of claim 50, wherein said
nanotubes have a morphology resembling a fishbone.
54. The method of making a field emission cathode of claim 50,
wherein said nanotubes are single wall nanotubes.
55. The method of making a field emission cathode of claim 50,
wherein said nanotubes are in the form of a film or mat.
56. The method for making a field emission cathode of claim 50,
wherein said cathode is bombarded with ions.
57. The method for making a field emission cathode of claim 50,
wherein said cathode is bombarded with a gallium ions.
58. The method for making a field emission cathode of claim 50,
wherein said cathode is bombarded with ions selected from the group
consisting of hydrogen, helium, argon, carbon, oxygen, and xenon
ions.
59. The method for making a field emission cathode of claim 50,
wherein said chemical treatment is selected from the group
consisting of acid treatment, metal vapor treatment, chemical vapor
transport, and chemical sorption.
60. The method for making a field emission cathode of claim 50,
wherein said chemical treatment is performed with chemical reagents
selected from the group consisting of oxidizing agents,
electrophiles, nucleophiles, reducing agents, strong acids, strong
bases and mixtures thereof.
61. The method for making a field emission cathode of claim 50,
wherein said chemical treatment is performed with phthalocyanines
or porphyrins.
62. The method for making a field emission cathode of claim 50,
wherein said energy treatment is performed with an energy source
selected from a group consisting of electromagnetic radiation,
ionizing radiation, atomic beams, electron beams, ultraviolet
light, microwave radiation, gamma ray, x-ray, neutron beam,
molecular beams and laser beam.
63. The method for making a field emission cathode of claim 50,
wherein said plasma treatment is performed with a plasma selected
from a group consisting of oxygen, hydrogen, ammonia, helium,
argon, water, nitrogen, ethylene, carbon tetrafluoride, sulfur
hexafluoride, perfluoroethylene, fluoroform,
difluoro-dichloromethane, bromo-trifluoromethane,
chlorotrifluoromethane and mixtures thereof.
64. The method for making a field emission cathode of claim 50,
wherein said treatment results in the introduction of metal atoms
onto the carbon nanotubes.
65. The method for making a field emission cathode of claim 50,
wherein said treatment results in the introduction of functional
groups onto the carbon nanotubes.
66. The method for making a field emission cathode of claim 65,
wherein said functional groups have been introduced by chemical
sorption.
67. The method for making a field emission cathode of claim 50,
wherein said treatment comprises heating said cathode in the
presence of metal vapor.
68. The method for making a field emission cathode of claim 50,
wherein said treatment comprises chemisorption followed by heat
treatment.
69. The method for making a field emission cathode of claim 50,
wherein said treatment includes annealing said nanotubes.
70. The method for making a field emission cathode of claim 50,
further comprising the step of adding a binder to said solution
before applying said voltage.
71. The method for making a field emission cathode of claim 70,
wherein said binder is a conductive carbon paste, a conductive
metal paste or a carbonizable polymer.
72. A field emission display device comprising: a cathode including
carbon nanotubes which have been subjected to energy, plasma,
chemical, or mechanical treatment; an insulating layer on said
cathode; a gate electrode on said insulating layer; an anode spaced
from said cathode, said anode comprising a phosphor layer, an anode
conducting layer, and a transparent insulating substrate; and a
power supply.
73. A method for making a field emission cathode comprising the
steps of: screen printing an ink onto a substrate, said ink
comprising a carrier liquid and carbon nanotubes in as-made form or
which have been subjected to energy, plasma, chemical, or
mechanical treatment; and evaporating said carrier liquid.
74. A method for making a field emission cathode comprising the
steps of: ink jet printing an ink onto a substrate, said ink
comprising a carrier liquid and carbon nanotubes which have been in
as-made form or which have been subjected to energy, plasma,
chemical, or mechanical treatment; and evaporating said carrier
liquid.
75. A method for making a field emission cathode comprising the
steps of: spray painting an ink through a stencil onto a substrate,
said ink comprising a carrier liquid and carbon nanotubes which
have been subjected to energy, plasma, chemical, or mechanical
treatment; and evaporating said carrier liquid.
76. A method for making a field emission cathode comprising the
steps of: screen printing, ink-jet printing or spray painting an
ink onto a substrate, said ink comprising a carrier liquid and
carbon nanotubes in as-made form; and subjecting said screen
printed nanotubes to an energy, plasma, chemical-or mechanical
treatment.
77. A field emission cathode made by the method of claim 107.
78. A field emission display device comprising: a baseplate; an
electron emitter array, said array including carbon nanotubes which
have been subjected to energy, plasma, chemical, or mechanical
treatment; a gate on said baseplate; a phosphor coated faceplate
spaced from said gate; a faceplate on said phosphor coated
faceplate; and a power supply.
79. A field emission device comprising: a substrate, a porous top
layer on said substrate, a catalyst material on said layer; and a
cathode on said catalyst material, said cathode including a bundle
of carbon nanotubes which have been subjected to energy, plasma,
chemical, or mechanical treatment.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/298,193, filed Jun. 14, 2001, hereby
incorporated by reference.
FIELD OF INVENTION
[0002] The present invention relates to field emission cathodes
which use carbon nanotubes.
BACKGROUND OF THE INVENTION
[0003] Field emission devices are devices that capitalize on the
movement of electrons. A typical field emission device includes at
least a cathode, emitter tips, and an anode spaced from the
cathode. A voltage is applied between the cathode and the anode
causing electrons to be emitted from the emitter tips. The
electrons travel in the direction from the cathode to the
anode.
[0004] These devices can be used in a variety of applications
including, but not limited to, microwave vacuum tube devices, power
amplifiers, ion guns, high energy accelerators, free electron
lasers, and electron microscopes, and in particular, flat panel
displays. Flat panel displays can be used as replacements for
conventional cathode ray tubes. Thus, they have application in
television and computer monitors.
[0005] Conventional emitter tips are made of metal, such as
molybdenum, or a semiconductor such as silicon. The problem with
metal emitter tips is that the control voltage required for
emission is relatively high, e.g., around 100 V. Moreover, these
emitter tips lack uniformity resulting in non-uniform current
density between pixels.
[0006] More recently, carbon materials, have been used as emitter
tips. Diamond has negative or low electron affinity on its
hydrogen-terminated surfaces. Diamond tips, however, have a
tendency for graphitization at increased emission currents,
especially at currents about thirty mA/cm.sup.2. Carbon nanotubes,
also known as carbon fibrils, have been the latest advancement in
emitter tip technology. Although much work has been done in the
area of carbon nanotubes as emitter tips in field emitting
technologies, substantial improvement is still needed,
specifically, in three areas. These areas are reducing work
voltage, increasing emission current, and increasing emission
sites.
[0007] Reducing the work voltage increases the ease of electron
emission and also increases the longevity of the emitter tips.
Increasing both the emission current and the number of emission
sites increase the brightness.
OBJECTS OF THE INVENTION
[0008] It is an object of the present invention to provide improved
field emission cathodes comprising carbon nanotubes as the
emitters, which operate at reduced working voltage, have increased
emissions and more emission sites.
[0009] It is a further object of this invention to provide improved
field emission cathodes where the emitters comprise treated carbon
nanotubes.
[0010] It is yet a further object of this invention to provide
methods for manufacturing improved field emission cathodes by
screen or ink-jet printing of substrates with inks containing
treated or untreated carbon nanotubes.
[0011] It is still a further object of this invention to provide
improved field emission display devices having improved properties
such as reduced working voltage, increased emissions and more
emission sites.
SUMMARY OF THE INVENTION
[0012] The present invention relates to a field emission cathode
comprising carbon nanotubes, wherein the nanotubes have been
subjected to an energy, chemical, plasma or mechanical treatment.
The carbon nanotubes may form the cathode or may be deposited onto
a substrate to form the cathode.
[0013] This invention also relates to a field emission device
comprising an anode and a cathode which has been subject to such a
treatment.
[0014] In one embodiment, the field emission device comprises a
substrate, a porous top layer positioned on said substrate, a
catalyst material positioned on said layer and a cathode positioned
on said catalyst material, said cathode including a bundle of
carbon nanotubes which have been subjected to a treatment as
described above.
[0015] The present invention also includes various field emission
display devices. In one embodiment, the field emission
display-device comprises a first substrate, a first metal film on
said first substrate; a conductive polymer film on said first metal
film, said conductive polymer film including emitter tips
comprising carbon nanotubes which have been subject to a treatment
as described above; a dielectric film on said first metal film; a
second metal film on said dielectric film; a spacer; a transparent
electrode separated from said second metal film by said spacer; a
fluorescent material on one side of said transparent electrode; a
second substrate on the other side of said transparent electrode;
and a power supply.
[0016] In an alternative embodiment, the field emission display
device comprises a cathode including carbon nanotubes which have
been subjected to a treatment as described above; an insulating
layer on said cathode; a gate electrode on said insulating layer;
an anode spaced from said cathode comprising a phosphor layer, an
anode conducting layer and a transparent insulating substrate; and
a power supply.
[0017] The carbon nanotubes used in the cathodes and field emission
devices of the invention may be single wall or multi-wall. They
comprise substantially cylindrical carbon fibrils having one or
more graphitic layers concentric with their cylindrical axes, are
substantially free of pyrolytically deposited carbon overcoat, have
a substantially uniform diameter between 1 nm and 100 nm and have a
length to diameter ratio greater than 5. The carbon nanotubes may
be in form of aggregates such as cotton candy aggregates or bird
nest aggregates, as well as in the form of a mat or a film.
[0018] Energy treatments may include ion beams, ionizing radiation,
atomic beams, electron beams, ultraviolet light, microwave
radiation, gamma ray, x-ray, neutron beam, molecular beams and
laser beam. Plasma treatment may be performed with a plasma
selected from a group consisting of oxygen, hydrogen, ammonia,
helium, argon, water, nitrogen, ethylene, carbon tetrafluoride,
sulfur hexafluoride, perfluoroethylene, fluoroform,
difluoro-dichloromethane, bromo-trifluoromethane,
chlorotrifluoromethane and mixtures thereof. Chemical treatment may
include acid treatment, metal vapor treatment, chemical vapor
transport, and chemical sorption.
[0019] The field emission cathode may be formed by dispersing
carbon nanotubes into a liquid vehicle to form a solution;
transferring said solution to an electrophoresis bath, said bath
including an anode and a cathode immersed therein; applying a
voltage to said anode and said cathode, thereby causing said carbon
nanotubes to deposit onto said cathode; removing said cathode from
said bath; heating said cathode; and subjecting such cathode to a
treatment as described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The accompanying drawings, illustrate an exemplary
embodiment of the present invention.
[0021] FIG. 1 is a cross-sectional view of a field emission display
device using an modified carbon nanotube cathode according to an
exemplary embodiment of the present invention;
[0022] FIG. 2 is a cross-sectional view of a field emission display
device using modified carbon nanotubes according to another
exemplary embodiment of the present invention;
[0023] FIG. 3 is a cross-sectional view of a field emission display
device using modified carbon nanotubes according to another
exemplary embodiment of the present invention;
[0024] FIG. 4 is a cross-sectional view of a field emission display
device using modified carbon nanotubes according to another
exemplary embodiment of the present invention;
[0025] FIG. 5 illustrates an electrophoresis bath used to fabricate
a carbon nanotube film (electrode);
[0026] FIG. 6 illustrates another electrophoresis bath used to
fabricate a carbon nanotube film (electrode);
[0027] FIG. 7 illustrates a schematic to measure the differences
between treated (modified) and untreated field emission
characteristics;
[0028] FIG. 8 is a plot showing cathode current as a function of
voltage for modified carbon nanotubes versus untreated nanotubes in
a field emission device;
[0029] FIG. 9 is a Fowler-Nordheim plot for modified carbon
nanotubes and untreated nanotubes in a field emission device.
[0030] FIG. 10 illustrates a classical field emitter;
[0031] FIG. 11 illustrates a field emitting device using ion
bombarded carbon nanotubes;
[0032] FIG. 12 is a SEM view of the carbon nanotubes on the
aluminum substrate.
[0033] FIG. 13 illustrates a carbon nanotube mat;
[0034] FIG. 14 illustrates the electron emission behavior of
electrophoretically deposited carbon nanotubes, screen printed
carbon nanotubes and carbon nanotube mats in the form of plots of
current density as a function of the electric field.
[0035] FIG. 15 is a series of photographs of electron emission
patterns of electrophoretically deposited carbon nanotubes, screen
printed carbon nanotubes and carbon nanotube mats.
[0036] FIG. 16 is a series of plots of emission characteristics of
inkjet printed carbon nanotubes.
[0037] FIG. 17 is a series of photographs of electron emission
patterns from sample inkjet printed carbon nanotubes.
[0038] FIG. 18 is a photograph of several inkjet printed carbon
nanotube cathodes made from sample 262-67-01.
[0039] FIG. 19 is a photograph of several inkjet printed carbon
nanotube cathodes made from sample 262-67-02.
[0040] FIG. 20 is a photograph of several inkjet printed carbon
nanotube cathodes made from sample 262-67-04.
[0041] FIG. 21 is a photograph of several inkjet printed carbon
nanotube cathodes made from sample 262-68-01.
DETAILED DESCRIPTION OF THE INVENTION
[0042] All referenced patents, patent applications, and
publications are incorporated by reference herein.
Definitions
[0043] "Aggregate" refers to a microscopic particulate structures
of nanotubes.
[0044] "Assemblage" refers to nanotube structures having relatively
or substantially uniform physical properties along at least one
dimensional axis and desirably having relatively or substantially
uniform physical properties in one or more planes within the
assemblage, i.e. having isotropic physical properties in that
plane. The assemblage can comprise uniformly dispersed individual
interconnected nanotubes or a mass of connected aggregates of
nanotubes. In other embodiments, the entire assemblage is
relatively or substantially isotropic with respect to one or more
of its physical properties.
[0045] "Carbon fibril-based ink" refers to an electroconductive
composite in which the electroconductive filler is carbon
fibrils.
[0046] "Graphenic" carbon is a form of carbon whose carbon atoms
are each linked to three other carbon atoms in an essentially
planar layer forming hexagonal fused rings. The layers are
platelets having only a few rings in their diameter or ribbons
having many rings in their length but only a few rings in their
width.
[0047] "Graphenic analogue" refers to a structure which is
incorporated in a graphenic surface.
[0048] "Graphitic" carbon consists of layers which are essentially
parallel to one another and no more than 3.6 angstroms apart.
[0049] "Nanotube", "nanofiber" and "fibril" are used
interchangeably. Each refers to an elongated hollow carbon
structure having a diameter less than 1 .mu.m. The term "nanotube"
also includes "bucky tubes" and graphitic nanofibers in which the
graphene planes are oriented in herring bone pattern.
[0050] The terms "emitter tips" and "emitters" are interchangeable.
The use of the word "tip" is not meant to limit the emission of the
electrons only to the tips of the carbon nanotubes. The electrons
can be emitted from any part of the carbon nanotubes.
Carbon Nanotubes
[0051] Carbon nanotubes (CNTs) are vermicular carbon deposits
having diameters of less than five hundred nanometers. They exist
in a variety of forms, and have been prepared through the catalytic
decomposition of various carbon-containing gases at metal surfaces,
by high temperature carbon arc processes, where solid carbon is
used as the carbon feed stock, and by simultaneous laser
vaporization of graphite rods and a transition metal. Tennent, U.S.
Pat. No. 4,663,230, succeeded in growing small diameter nanotubes
having cylindrical ordered graphite cores and an ordered "as grown"
graphitic surface uncontaminated with pyrolytic carbon. Tennent,
describes carbon nanotubes that are free of a continuous thermal
carbon overcoat and have multiple graphitic outer layers that are
substantially parallel to the fibril axis. As such they may be
characterized as having their c-axes, the axes which are
perpendicular to the tangents of the curved layers of graphite,
substantially perpendicular to their cylindrical axes. They
generally have diameters no greater than 0.1 micron and length to
diameter ratios of at least five. Such nanotubes having graphitic
layers that are substantially parallel to the fibril axis and
diameters between 3.5 and 75 nanometers, are described in Tennent
et al., U.S. Pat. No. 5,165,909 and Tennent et al, U.S. Pat. No.
5,171,560.
[0052] The graphitic planes may also be oriented at an angle to the
fibril axis. Such structures are often called "fishbone" fibrils or
nanotubes because of the appearance of the two dimensional
projection of the planes. Such morphologies and methods for their
production are discussed in U.S. Pat. No. 4,855,091 to Geus, hereby
incorporated by reference.
[0053] Assemblages and composites consisting of multiwall nanotubes
have been described in Tennent et al, U.S. Pat. No. 5,691,054. Such
assemblages and composites are composed of randomly oriented carbon
fibrils having relatively uniform physical properties. Furthermore,
these multiwall nanotubes are substantially free of pyrolytically
deposited carbon.
[0054] The carbon nanotubes disclosed in U.S. Pat. Nos. 4,663,230,
5,165,909, and 5,171,560, may have diameters that range from about
3.5 nm to 70 nm and lengths greater than 100 times the diameters,
an outer region of multiple essentially continuous layers of
ordered carbon atoms and a distinct inner core region. Simply for
illustrative purposes, a typical diameter for a carbon fibril may
be approximately between about 7 and 25 nm, and a typical range of
lengths may be 1 .mu.m to 10 .mu.m.
[0055] As disclosed in U.S. Pat. No. 5,110,693 and references
therein, two or more individual carbon fibrils may form microscopic
aggregates of entangled fibrils. These aggregates can have
dimensions ranging from 5 nm to several cm. Simply for illustrative
purposes, one type of microscopic aggregate ("cotton candy or CC")
resembles a spindle or rod of entangled fibers with a diameter that
may range from 5 nm to 20 .mu.m with a length that may range from
0.1 .mu.m to 1000 .mu.m. Again for illustrative purposes, another
type of microscopic aggregate of fibrils ("birds nest, or BN") can
be roughly spherical with a diameter that may range from 0.1 .mu.m
to 1000 .mu.m. Larger aggregates of each type (CC and/or BN) or
mixtures of each can be formed.
[0056] Recently carbon nanotubes having a single wall comprising
graphite have been produced. These single wall carbon nanotubes
have been described in Bethune et al., U.S. Pat. No. 5,424,054;
Guo, et al., Chem. Physics Lett., 243:1-12 (1995); Thess, et al,
Science, 273:483-487 (1996); Journet et al., Nature 388 (1997) 756;
Vigolo, et al., Science 290 (2000) 1331. They are also described in
U.S. patent application Ser. No. 08/687,665, entitled "Ropes of
Single-Walled Carbon Nanotubes" herein incorporated by
reference.
[0057] Additional methods of producing single wall nanotubes
production have been described in PCT Application No.
PCT/US99/25702 and PCT Application No. PCT US98/16071 herein
incorporated by reference.
[0058] Single wall nanotubes are useful in a variety of
applications. The tubular structure imparts superior strength, low
weight, stability, flexibility, thermal conductivity, large surface
area and a host of electronic properties. They can be used as
reinforcements in fiber reinforced composite structures or hybrid
composite structures, i.e., composites containing reinforcements
such as continuous fibers in addition to single wall nanotubes.
[0059] The carbon nanotubes may be treated in their as-made form or
may be deposited as a film on a suitable substrate and then
treated.
Preparation of Films Containing Carbon Nanotubes
[0060] The carbon nanotubes used were obtained from Hyperion
Catalysis International, Cambridge Mass. They had the designations
#1100 and #1100 L. Sample #1100 L comprised carbon nanotubes having
a so-called BN macromorphology that had been ball milled in a Red
Devil Shaking Ball Mill for approximately four hours. Some samples
were treated with an acid wash of twelve grams of H.sub.3PO.sub.4
in 1.5-liters of water at atmospheric reflux before ball milling.
The carbon nanotubes were dried in an oven before ball milling.
The Solution of Nanotubes
[0061] The nanotubes were dispersed by known methods in a suitable
solvent as is well known in the art, e.g. isopropyl alcohol.
The Substrate
[0062] Aluminum substrates were prepared by vapor depositing
aluminum onto glass flats that were approximately 55 mm.times.45
mm.times.1 mm in its dimensions. Aluminum adhesion may be enhanced
with the addition of an underlying vapor deposited adhesion layer.
A dielectric mask can be applied to pattern the aluminum surface
into a plurality of electodes prior to nanotube deposition.
[0063] The aluminum can also be pretreated to promote the adhesion
of the carbon nanotubes. This can be done with any known
pretreatments of aluminum. The carbon nanotubes can also adhere to
other substrates, e.g., SnO.sub.2-in/Sb
The Electrophoresis Bath
[0064] The elecrophorectic deposition of the carbon nanotubes was
conducted in an electrophoresis bath. The bath consists of a
chamber to contain the solution of carbon nanotubes and means for
immersing two opposing electrodes separated by some distance with
the carbon nanotubes between the opposing electrodes. A DC power
supply, external to the bath, is used to apply a voltage between
the two electrodes immersed in the bath. The cathode lead is
connected to the patterned aluminum substrate and the anode lead is
connected to the other electrode. Tantalum was used for the second
metal. The voltage applied to the two electrodes can be adjusted to
a suitable level or the voltage can be adjusted to obtain a
suitable current between the two electrodes.
[0065] The attachment of carbon nanotubes to the aluminum can be
enhanced by a binder. The binders can be a mixture of Ag paste,
carbon nanotubes and ethanol. Or the binders can be a conductive
carbon paste, a conductive metal paste or a carbonizable
polymer.
Electrophoretic Deposition of Carbon Nanotubes on the Substrate
[0066] A field emitter substrate is loaded into the electrophoresis
bath. A plurality of cathodes are arranged on a glass substrate,
and a dielectric film is formed with holes over the cathodes. Metal
gates with openings which are located over the holes of the
dielectric film are formed to expose the surface of the cathodes.
Then, the carbon nanotubes are uniformly deposited onto the
obtained substrate, on the surface of the cathodes exposed through
the holes by electrophoretic deposition at room temperature.
Post Deposition Heat Treatment
[0067] After the deposition of carbon nanotube particles by
electrophoresis, low-temperature heating is performed to sustain
the deposition of the carbon nanotubes on the cathodes and ensure
easy removal of impurities which are incorporated into the field
emitter during the deposition.
EXAMPLE I
Preparation of Nanotube Film on Aluminum Substrate
[0068] With reference to FIG. 5, a solution is formed that contains
150 ml i-propyl alcohol (IPA) and 0.44 grams of acid washed carbon
nanotubes. This solution is placed in an electrophoresis bath
5000.
[0069] A patterned, aluminum coated glass substrate 5002 serves as
one electrode in electrophoresis bath 5000. The pattern forms the
pixel size. The smallest feature size can be ca. 1 micron. The
aluminum coated glass 5002 is about 55 mm.times.45 mm.times.1 mm in
its dimensions. The aluminum pattern size is about 9 mm.times.9 mm.
The other electrode, tantalum (Ta) electrode 5004 is also inserted
into the electrophoresis bath 5000. A spacer 5006 separates the
aluminum coated glass 5002 from the tantalum electrode 5004. A DC
voltage, for example between 40 to 120 volts, e.g., 100 volts is
applied to the electrodes. A current between 1.0 to 5 mA, e.g., 3.8
mA, is observed between the electrodes. The duration of the
preparation time can be between about 30 to about 90 minutes, e.g.,
60 minutes.
[0070] FIG. 6 illustrates an alternative electrophoretic method of
creating the film according to the method disclosed in UK patent
application 2,353,138 described below. First, a carbon nanotube
suspension is created. The carbon nanotube particles can have
lengths from about 0.1 to about 1 .mu.m. The suspension can also
include a surfactant, e.g. an anionic, ionic, amphoteric or
nonionic, or other surfactant known in the art. Examples of
suitable surfactants include octoxynol, bis(1-ethylhexyl)sodium
sulfosuccinate, and nitrates of Mg(OH).sub.2, Al(OH).sub.3 and
La(OH).sub.3.
[0071] The suspension is then sonicated to charge the carbon
nanotube particles. The intensity of the electric field and the
time for which the electric field is applied define the thickness
of the carbon nanotube layer. Greater intensity and longer time
yield thicker layers.
[0072] With reference to FIG. 6 the field emitter substrate 6030 is
loaded into the electrophoresis bath 6000 containing a carbon
nanotube suspension 6010. An electrode plate 6020 is also installed
in the electrophoresis bath 6000 spaced apart from the field
emitter substrate 6030. The cathode of a DC power supply 6040,
which is installed outside of the electrophoresis bath 6000, is
connected to the other cathodes of the field emitter substrate 6030
and the anode of the DC power supply 6040 is connected to the
electrode plate 6020. Then, a bias voltage of about 1 to-about 1000
volts is applied from the DC power supply 6040 between the
electrode plate 6020 and the cathodes of the field emitter
substrate 6030.
[0073] As a positive voltage of the DC power supply 6040 is applied
to the electrode plate 6020, carbon nanotube particles charged by
positive ions in the carbon nanotube suspension 6010 migrate to and
are attached to the exposed cathodes of the field emitter substrate
6030, which results in the formation of a carbon nanotube film in
the pattern of the exposed cathodes.
[0074] The height of the printed carbon nanotube film, also known
as the ink, coating, or paste, may be less than 10 microns and the
space which isolates carbon nanotube cathodes from the indium tin
oxide anode with indium tin oxide and phosphor is about 125
microns.
[0075] The electrophoresis process can be applied to both diodes
and triodes. For applications to a diode, an electric field having
opposite charges to those on the surfaces of the carbon nanotube
particles is applied to exposed electrode surface of a field
emitter substrate for selective deposition of carbon nanotube
particles thereon. For application to a triode having gates, a weak
positive electric field is applied to the gates while a positive
electric field is applied to the electrodes of the field emitter
substrate, which avoids deposition of carbon nanotube particles on
the gates. In particular, the electrode plate is connected to the
anode of the DC power supply and the cathodes of the field emitter
substrate are connected to the cathode of the DC power supply. As a
positive potential is applied to the gates, the gates repel
positive ions in the carbon nanotube suspension at the surface,
while the exposed cathodes of the field emitter substrate, which
are connected to the cathode of the DC power supply, pull positive
ions of the suspension through the holes. As a result, the carbon
nanotubes are deposited only on the entire exposed surface of the
cathodes, not on the gates of the field emitter substrate. At this
time, carbon nanotube particles are attracted to the field emitter
substrate and are oriented substantially horizontal, or
substantially parallel to the substrate, which allows the
carbon-nanotube particles to smoothly migrate through the holes to
the cathodes, and thus the carbon nanotubes can be deposited.
[0076] The film can also be prepared similarly to the carbon ink
disclosed in European Patent Application EP 1 020 888 A1--Carbon
ink, electron-emitting element, method for manufacturing and
electron-emitting element and image display device
Alternative Methods to Prepare Carbon Nanotube Films
[0077] In addition to electrophoresis, other processes such as
screen printing can be used for creating the patterns. A screen
printing process is disclosed in U.S. Pat. No. 6,270,369. In
addition to screen printing, the carbon nanotubes can be applied to
a substrate by ink jet printing. Ink printing is accomplished with
carbon nanotube based liquid media or inks in which the fibrils are
nearly individualized. Inks typically contain a carrier liquid and
carbon nanotubes, and may be dried (i.e. evaporate the carrier
liquid).
[0078] The carbon nanotubes can also be deposited in the form of a
mat. Such porous mats, having densities between 0.10 and 0.40 gm/cc
are conveniently formed by filtration of suspensions of nanotubes
as described in U.S. Pat. Nos. 6,099,965 and 6,031,711. Oxidized
nanotubes are easily dispersed in and then filtered from aqueous
media. The mats may be subjected to a rigidization or cross linking
step as discussed in the aforecited patents.
[0079] Carbon nanotube mat cathodes have uniform emission sites at
relatively low applied field and may obtain a current density of
more than 10 mA/cm.sup.2.
[0080] A comparison of the electron emission behavior of
electrophoretically deposited carbon nanotubes, screen printed
carbon nanotubes and carbon nanotube mats in the form of plots of
current density as a function of the electric field is displayed in
FIG. 14. A further comparison of the electron emission patterns of
electrophoretically deposited carbon nanotubes, screen printed
carbon nanotubes and carbon nanotube mats is displayed in FIG.
15.
Carbon Nanotube Based Inks
[0081] In yet another method, fibril based inks can be formulated
for use in spray equipment. When combined with a masking
technology, spray painting of fibril based inks offers a suitable
method for depositing fibril ink patterns of either simple or
complex designs. Spray painting can also be used to apply a uniform
coating over a large area, with or without a masking technology.
Spraying equipment can accommodate inks/paints with a wide range of
viscosity and thixotropy. Airbrushes are a type of sprayer widely
used in the graphic arts industry and areas where fine detailed
spraying is desired.
[0082] The inks are sprayed through a stencil (i.e., mask, layer
with cut out pattern, etc.) to form the corresponding pattern on
the substrate and the carrier fluid is allowed to evaporate. The
ratio of air to ink and the distance from the substrate can be
adjusted to allow the optimum amount of drying of the aerosol
droplets before they impinge on the substrate surface. In this way
the adhesion of the droplets to the substrate and the tendency of
the ink to run or spread can be controlled. Once dried the dried
ink can have conductivity approaching that of a bare fibril mat
depending on the level of any binder that may have been included in
the ink formulation.
[0083] The compositions are prepared by dispersing oxidized fibrils
in water first, then adding other additional ingredients if so
desired.
[0084] The formation of thin fibril films with these compositions
can be achieved by both printing and dip coating. Text and patterns
have been printed with an Epson.RTM. ribbon printer. The surface
resistivity of printed pattern was measured about
3.5.times.10.sup.5 .OMEGA.-cm (sample 4 in Table 1). The thickness
of the pattern is in the range of few tens of nanometers,
corresponding few layers of fibrils. Papers with .about.2.5 mm
fibril coating on both sides have been prepared by dip-coating
method. Measured surface resistivity for the coated paper is
between 200-300 .OMEGA.-cm. Bulk resistivity of the fibril coating
is .about.5.times.10.sup.-2 .OMEGA.-cm, a number very close to that
measured for a freestanding fibril mat. Furthermore, adhesion of
fibril films to the paper is excellent due to the strong
interaction between functional groups on the fibril surface and
groups associated with cellulose paper.
1TABLE 1 Composition and Properties of Fibril-Based Ink Resistivity
Sam- Composition(%) V .rho..sub.sur .rho. ple Fibril H.sub.2O EG SS
DIOP (cps) t(.mu.m) (.OMEGA.-cm) (.OMEGA.-cm) 1 2 98 -- -- -- 0 2.5
200- 5 .times. 300 10.sup.-2 2 4 96 -- -- -- 19.2 -- -- -- 3 2.5
77.5 20 -- -- 0 -- -- -- 4 2.5 77.17 20 0.03 0.3 0 -- 3.5 .times. 5
.times. 10.sup.5 10.sup.-2
Modification of Carbon Nanotube Films
[0085] The carbon nanotubes, or film, may be modified by chemical
or mechanical treatment. The surface may be treated to introduce
functional groups. Techniques that may be used include exposing the
carbon nanotubes to electromagnetic radiation, ionizing radiation,
plasmas or chemical reagents such as oxidizing agents,
electrophiles, nucleophiles, reducing agents, strong acids, and
strong bases and/or combinations thereof. Of particular interest is
plasma treatment.
Plasma Treatment of Nanotube Films
[0086] Plasma treatment is carried out in order to alter the
surface characteristics of the carbon fibrils, fibril structures
and/or the matrix, which come in contact with the plasma during
treatment; by this means the fibril composite treated can be
functionalized or otherwise altered as desired. Once equipped with
the teaching herein, one of ordinary skill in the art will be able
to adapt and utilize well-known plasma treatment technology to the
treatment of such composite materials. Thus, the treatment can be
carried out in a suitable reaction vessel at suitable pressures and
other conditions and for suitable duration, to generate the plasma,
contact it with the composite material, and effect the desired kind
and degree of modification. Plasmas such as those based on oxygen,
hydrogen, ammonia, helium, or other chemically active or inert
gases can be utilized.
[0087] Examples of other gases used to generate plasmas include,
argon, water, nitrogen, ethylene, carbon tetrafluoride,
sulfurhexafluoride, perfluoroethylene, fluoroform,
difluoro-dicholoromethane, bromo-trifluoromethane,
chlorotrifluoromethane, and the like. Plasmas may be generated from
a single gas or a mixture of two or more gases. It may be
advantageous to expose a composite material to more than one type
of plasma. It may also be advantageous to expose a composite
material to a plasma multiple times in succession; the conditions
used to generate the plasma, the duration of such successive
treatments and the duration of time between such successive
treatments can also be varied to accomplish certain alterations in
the material. It is also possible to treat the composite material,
e.g., coat the material with a substance, wash the surface of the
material, etc., between successive treatments.
[0088] Plasma treatment of a composite material may effect several
changes. For example, a composite material comprising a polymer and
a plurality of carbon fibrils dispersed therein can be exposed to
plasma. Exposure to plasma may etch the polymer and expose carbon
fibrils at the surface of the composite, thus increasing the
surface area of exposed carbon fibrils, e.g., so that the surface
area of the exposed fibrils is greater than the geometric surface
area of the composite. Exposure to plasma may introduce chemical
functional groups on the fibrils or the polymer.
[0089] Treatment can be carried out on individual fibrils as well
as on fibril structures such as aggregates, mats, hard porous
fibril structures, and even previously functionalized fibrils or
fibril structures. Surface modification of fibrils can be
accomplished by a wide variety of plasmas, including those based on
F.sub.2, O.sub.2, NH.sub.3, He, N.sub.2 and H.sub.2, other
chemically active or inert gases, other combinations of one or more
reactive and one or more inert gases or gases capable of
plasma-induced polymerization such as methane, ethane or acetylene.
Moreover, plasma treatment accomplishes this surface modification
in a "dry" process as compared to conventional "wet" chemical
techniques involving solutions, washing, evaporation, etc. For
instance, it may be possible to conduct plasma treatment on fibrils
dispersed in a gaseous environment.
[0090] Once equipped with the teachings herein, one of ordinary
skill in the art will be able to practice the invention utilizing
well-known plasma technology. The type of plasma used and length of
time plasma is contacted with fibrils will vary depending upon the
result sought. For instance, if oxidation of the fibrils' surface
is sought, an O.sub.2 plasma would be used, whereas an ammonia
plasma would be employed to introduce nitrogen-containing
functional groups into fibril surfaces. Once in possession of the
teachings herein, one skilled in the art would be able to select
treatment times to effect the degree of
alteration/functionalization desired.
[0091] More specifically, fibrils or fibril structures are plasma
treated by placing the fibrils into a reaction vessel capable of
containing plasmas. A plasma can, for instance, be generated by (1)
lowering the pressure of the selected gas or gaseous mixture within
the vessel to, for instance, 100-500 mTorr, and (2) exposing the
low-pressure gas to a radio frequency which causes the plasma to
form. Upon generation, the plasma is allowed to remain in contact
with the fibrils or fibril structures for a predetermined period of
time, typically in the range of approximately 10 minutes more or
less depending on, for instance, sample size, reactor geometry,
reactor power and/or plasma type, resulting in functionalized or
otherwise surface-modified fibrils or fibril structures. Surface
modifications can include preparation for subsequent
functionalization.
[0092] Treatment of a carbon fibril or carbon fibril structure as
indicated above results in a product having a modified surface and
thus altered surface characteristics which are highly advantageous.
The modifications can be a functionalization of the fibril or
fibril structure such as chlorination, fluorination, etc., or a
modification which makes the surface material receptive to
subsequent functionalization, optionally by another technique or
other chemical or physical modification as desired.
Chemical Treatments of Nanotube Films
[0093] Chemical treatment can be also used. Acid treatments,
particularly severe acids treatments, result in cutting the lengths
of nanotubes, sharpening the ends of nanotubes, creating defects on
the surface of nanotubes and introducing functional groups on the
surface of nanotubes. Acid treated nanotubes are water dispersible,
so chemical treatment offers advantages for the formation of
nanotube film electrodes. Functional groups can be mostly removed
by thermal treatment. The process for doing this is disclosed in
U.S. Pat. No. 6,203,814. The Raman effect or titration can be used
to measure the effect of the acid treatment. Raman can be used to
measure the degree of structure imperfection after removing oxygen
groups introduced during acid treatment. The effect of the
treatment can be measured by electron spin resonance or simple
titration as disclosed therein. See also, R. Khan et al. Electron
Delocalization in Amorphous Carbon by Ion Implantation, 63 PHYSICAL
REVIEW B 121201-1 (2001).
[0094] In metal vapor treatment procedures metal atoms can be
introduced into nanotube films by heating films under vapor of a
metal. For example, Cs atoms, which have been shown to enhance
field emission, can be introduced into a nanotube film by placing
the film in a vacuum chamber which has a Cs source held at
200.degree. C. above (Vapor pressure of Cs at 373.degree. C. is 10
mm Hg).
[0095] Chemical vapor transport methods can be used. Most metals
vaporize at very high temperature. Metal atoms of these metals,
such as Ga, can be introduced into nanotube films by chemical vapor
transport. The nanotube film is placed in an evacuated glass tube,
at one end. a A metal particle is placed at the other end. A
chemical vapor transport agent, such as Cl.sub.2, I.sub.2, Br.sub.2
and HCl is also included. The tube is placed into a three-zone
furnace. The temperature of nanotube is held lower that that of the
metal, so that metal atoms are transported to the nanotube films by
the transport agent.
[0096] Chemical sorption followed by heat treatment can be used.
Metal atoms can be introduced into nanotube films by first
absorbing a metal compound like metal halides or organometallic
compounds on the surface of the nanotube films, followed by heating
then under inert gas atmosphere to convert metal halides or
organometallic compounds to metal atoms. For example Ge atoms may
be introduced into a nanotube film by absorbing GeBr.sub.2 on the
surface of nanotubes from a GeBr2 alcohol solution, then heating
the nanotube film between 200 and 400.degree. C. to decompose
GeBr2.
[0097] Functionalization of nanotubes by chemical sorption can be
used, some molecules, like metal phthalocyanines may have the
effect of lowing work function of nanotubes and lead to an
enhancement of field emission when absorbed on the surface of
nanotubes. Absorption can be carried out by placing a nanotube film
electrode in a phthalocyanine, porphyrin or metalloporphyrin
solution; this procedure is disclosed in U.S. Pat. No. 6,203,814.
The functionalization is carried out by phthalocyanines,
metalloporphyrins, porphyrins or other organometallics.
[0098] The treatment can also include annealing the film after
functionalization. The annealing temperature can be carried out
between 200 and 900 degrees Celsius in inert gas or under 360
degrees Celsius in air.
Ion Bombardment of Carbon Nanotube Films
[0099] The carbon nanotube films, are treated by ion bombardment
before use in a field emission device or field emitting
cathode.
[0100] The settings used to bombard the carbon nanotubes were as
follows:
[0101] energy: 30 keV. Other ranges appropriate for the present
invention can be from about 5 eV to about 1 MeV, e.g., 10-50
keV.
[0102] ion: Ga. Although Ga was used as the ion, any type of ion
can be used. Other types of ions, for example, include H, He, Ar,
C, O, and Xe.
[0103] spot size: defocused, 500 nm. Other ranges appropriate for
the present invention include from about 1 nm to about 1 micron.
Appropriate spot size can also be based on desired resolution and
dose.
[0104] scan area: 760 microns.times.946 microns Rasterscanned for
about twenty seconds. Any appropriate scan area will suffice.
[0105] dose: 2.times.10.sup.14/cm.sup.2 ranges include from about
10.sup.2/cm.sup.2 to about 10.sup.20/cm.sup.2
Additional Methods for Treatments of Nanotube Films
[0106] Other energetic beams/sources, including atomic beams,
electron beams, neutron beams, molecular beams, lasers, plasmas, UV
light, x-ray and gamma rays can be used to treat nanotube films
instead of ion bombardment. Mechanical treatment resulting in
mechanical disruption, for example, ball milling can be used.
[0107] Other characteristics of carbon nanotubes can be modified by
the above treatments. For example, the treatments can remove
surface oxygen, remove insulating oxidation residues, generate
edges, points, and singularities, recrystallize the tubes, generate
non-tube carbon nanoparticles. A treatment can also be used to
clean the carbon nanotubes, for example cleaning to remove the
insulation coating generated by oxidation and cleaning to remove
oxygen.
Characterization of the Treated Film
[0108] By viewing samples in a SEM it is possible to detect
irradiated areas by contrast change, i.e., dark image. FIG. 12
illustrates scanning electron microscope views of carbon nanotubes
on aluminum.
[0109] FIG. 7 schematically illustrates an apparatus used to make
the emission measurements. FIG. 7 illustrates the top view, FIG.
7a, and side views, FIG. 7b. FIG. 7a shows a 6 mm.times.6 mm
phosphor on indium tin oxide (ITO). In FIG. 7b, the phosphor is
shown to be spaced from the patterned carbon nanotubes by a
distance of 125 .mu.m. The entire system is evacuated with a vacuum
of 5.times.10.sup.-9 Torr in the emission chamber.
[0110] The degree of improvement achieved by ion beam treatment are
summarized in Table 2.
2 TABLE 2 Treated/Modified Untreated CNT Cathode CNT Cathode
Threshold voltage 350 Volts 140 Volts Threshold Field 2.8 V/.mu.m
1.1 V/.mu.m Emission current see figure 8 6 times increase
[0111] The ion bombardment achieves a reduction in work voltage,
increases emission current and increases the number of emission
sites. With reference to FIG. 8, it is seen that the turn-on
voltage was reduced substantially as the result of ion beam
treatment.
[0112] FIG. 9 is a Fowler-Nordheim (F-N) plot. The shape of the
curves provides the theoretical proof of FE. Shifting the curve
toward the right side--toward lower voltage - indicates an
increased number of emission sites.
[0113] Similar improvements can be obtained by treating the carbon
nanotubes with ultra-violet light, laser beam and plasma.
EXAMPLE II
Emission Characteristics of Ion-Beam-Treated Nanotube Films
[0114] Carbon nanotube films fabricated by electrophoresis on an
aluminum layer deposited on a glass have been locally irradiated
with focused ion beams. A diode structure with a distance of 125
.mu.m between cathodes and anodes was used for emission
measurement. A maximum emission current of 375 microamps with a
turn-on voltage of 2.8 V/.mu.m for carbon nanotube emitters was
found to decrease by focused ion beam irradiation to 1.1 V/.mu.m
with increase in emission current by a factor of six.
[0115] The current range that was used in the test was in the low
range with an anode voltage of about 400 to 500 volts, close to the
turn-on (threshold) voltage for field emission. The change was from
0.05 to about 0.18 microamps to more than 0.9 microamps with a
drastic change in the F-N plot of FIG. 9.
[0116] The physical and chemical effects of ion bombardment on
carbon nanotubes are not entirely known. While not wishing to be
bound to any particular theory, it may be that the effect of the
ion bombardment is the creation of surface sites which enhance
field emission. It is believed that the treatment 1) cuts lengths
of nanotubes, in particular, if high energy beams are used, hence
generating more ends; 2) implants ions, like Ga ions, into the
nanotube film, the ions being inside a single tube and outside
tubes; 3) saturates dangling bonds with hydrogen (where a hydrogen
ion beam/plasma is used), resulting in hydrogenated surface; 4)
cleans the surface of nanotubes by removing contaminants, such as
binder residue and oxygenated groups; 5) generates localized and
delocalized regions along the nanotube axis by creating pits and
carbon nanoparticles and recrystallizing amorphous carbons on the
surface of nanotubes, and disrupting carbon layers, leading to an
increasing in emission sites; 6) improves electric contacts between
nanotubes.
[0117] The surface sites generated by ion bombardment can be
defects, which are carbon atoms at edges, carbon atoms associated
with other atoms, like a hydrogen atom, and an implanted Ga atom,
and carbon atoms with a sp3 configuration or configurations between
sp2 and sp3. The defects can be at the ends (exposed) of a
nanotube, and on the surface of a single nanotube associated with a
nanoparticle, a pit and a disrupted carbon layer.
Construction of a Field Emission Display Device Using Treated
Carbon Nanotube Cathodes
[0118] Generally, field emission display devices are based on the
emission of electrons in a vacuum. Emitter tips emit electrons that
are accelerated in a strong electric field. The electrons
ultimately collide with fluorescent materials that emit light. The
advantages of this type of display over other types, such as
cathode ray tubes, are that they are very thin and light and yield
high brightness and resolution. Processes for constructing these
devices are disclosed in EP No. 1,073,090 A2.
[0119] FIG. 1 shows an exemplary embodiment of a field emission
display device using an treated carbon nanotube cathode. The field
emission display 1000 includes, for example, a first substrate
1010, first metal film 1020, a conductive high polymer film 1030, a
dielectric film 1040, a second metal film 1050, a spacer 1060, a
transparent electrode 1070, a second substrate 1080, and emitter
tips, the treated carbon nanotube cathode, 1090.
[0120] The substrate 1010 is, for example, made of glass quartz,
silicon, or alumina (Al.sub.2O.sub.3). Other substrates include
silica, platinum, iron and its alloys, cobalt and its alloys,
nickel and its alloys, and ceramics.
[0121] The first metal film 1020 functions as the cathode and is,
for example, made of chrome, titanium, tungsten, or aluminum. The
first metal film 1020 has a thickness form about 0.2 to about 0.5
.mu.m.
[0122] On the first metal film 1020 is, for example, the dielectric
film 1040. The dielectric film 1040 has a thickness from about one
to about five .mu.m.
[0123] On the dielectric film 1040 is the second metal film 1050.
The second metal film 1050 functions as a gate electrode and is
made from, for example, chrome, titanium, or palladium. The
thickness of the second metal film is from about 0.2 to 0.5 .mu.m.
The second metal film 1050 can also be patterned, for example, by
using a photoresist film that has a thickness from about 1.5 to
about 2.0 .mu.m. The photoresist film is later developed forming a
photoresist pattern. The accelerating gate electrode should be in
close proximity to the emitting source approximately one to ten
.mu.m.
[0124] Both the first metal film 1020 and the dielectric film 1040
have a plurality of fine holes. The holes have, for example, a
diameter of 0.5 to 10.0 .mu.m and are separated from each other by
about 2.0 to about 15.0 .mu.m.
[0125] Formed within the fine holes of the dielectric film 1040 and
the second film 1050, is the conductive high polymer film 1030. The
conductive high polymer film 1030 can be, for example, made from
carbon adhesive or silver adhesive. To attach the conductive high
polymer film 1030 to the first metal film 1020, the conductive high
polymer film 1030 is liquefied by heating and poured to fill
approximately one-third of each of the fine holes.
[0126] Arranged vertically or horizontally within the conductive
high polymer film 1030 are carbon nanotubes used as emitter tips
1090. The emitter tips 1090 are made from the ion bombarded carbon
nanotubes discussed previously. These emitter tips 1090 can obtain
a great amount of emission current at a low operating voltage, for
example, about 1.5 V/.mu.m. The range can be from about 0.1 to
about 2.0 V/.mu.m, e.g., about 0.8 V/.mu.m to about 1.5V/.mu.m.
[0127] Above the second metal film 1050 is the spacer 1060. The
spacer 1060 is installed to about 100 to about 700 .mu.m. on the
second metal film 1050.
[0128] The transparent electrode 1070 is on top of the spacer 1060.
The transparent electrode 1060 functions as an anode and is made of
a conducting oxide, such as indium oxide, indium tin oxide, tin
oxide, copper oxide, or zinc oxide.
[0129] The second substrate 1080 is on the transparent electrode
1070 and can be made of glass. Fluorescent material, attached to
the transparent electrode 1070, emits red, blue, or green light
when electrons contact it.
[0130] The emitter tips 1090 are made of the ion bombarded carbon
nanotubes. The geometrical features of the emitter tips 1090 should
be small. For example, the diameters of each emitter tip 1090
should be as small as 1.3 nm. The average height of the nanotubes
is from about 0.1 to about 1000 .mu.m, preferably between 0.1 to
about 100 .mu.m. The average diameter is between 1.3 to 200 nm
depending on whether the nanotubes are single walled or
multi-walled.
[0131] More than 10.sup.4 emitting tips are needed per pixel of
100.times.100 .mu.m.sup.2 assuming 50% of nanotube density with a
tubule diameter of about 10 to about 100 nanometers. The emitter
density is preferably at 1 /.mu.m.sup.2, in particularly at least
10/.mu.m.sup.2. The entire field emission display 1000 is
evacuated.
[0132] In FIG. 2, a field emission display 2000 is shown. The field
emission display 2000, includes, for example, a baseplate 2010, a
spaced-apart phosphor coated faceplate 2020, and an electron
emitter array 2030 positioned on the baseplate 2010 for emitting
electrons that collide with the phosphor causing illumination. The
components of the field emission display 2000 are in a vacuum. The
electron emitter array (cathode) 2030 is composed of treated carbon
nanotubes that can have either an orientation parallel,
perpendicular, or any angle between zero and ninety degrees to the
baseplate 2010. (See PCT/US 99/13648--Free Standing and Aligned
Carbon Nanotubes and Synthesis thereof).
[0133] FIG. 3 shows yet another embodiment of the field emission
device. The device 3000, has, for example, a substrate 3010, a
porous top layer 3020, a catalyst material 3030, and bundles of
treated carbon nanotubes 3040 as the cathode.
[0134] The substrate 3010 and the porous top layer 3020 can be made
of, for example, silicon. The catalyst material 3030 can be a thin
film of iron oxide that is formed in a particular pattern. The
carbon nanotube bundles 3040 serve as the cathode. The bundles 3040
are oriented substantially perpendicular to the substrate 3010.
Alternatively, the bundles 3040 can also be oriented substantially
parallel to the substrate 3010.
[0135] The carbon nanotube bundles 3040 may be about 10-250 .mu.m
wide, and up to or greater than three hundred .mu.m in height. The
bundles 3040 are of the same size and shape as the patterns of
catalyst material 3030, for example. The nanotube bundles 3040 can
have flat tops or bowl-shaped tops as shown in the figure. The
sharp edges of the nanotube bundles 3040 function as field emission
regions. Each bundle 3040 provides the field emission for a single
pixel in a flat panel display.
[0136] The device is evacuated to from about 10.sup.-3 Torr to
about 10.sup.-9 Torr, e.g., from about 10.sup.-7 Torr to about
10.sup.-8 Torr.
[0137] The calculation of any electrical field within the device
3000 is made by taking the applied voltage and dividing it by the
distance from the emitter tips to the anode. See (PCT appln.
PCT/US99/26332)
[0138] FIG. 4 shows another embodiment of a flat panel field
emission display 4000. The display 4000, for example, includes
cathode 4010 that contains a plurality of treated carbon nanotube
emitting tips 4020 and an anode 4030. The anode 4030 further
includes an anode conductor 4040 and a phosphor layer 4050. Between
the cathode 4010 and the anode 4030 is a perforated conductive gate
electrode 4060. Between the gate electrode 4060 and the cathode
4010 is an insulating layer 4070. The space between the anode 4030
and the carbon nanotube emitting tips are sealed and evacuated. The
voltage is supplied by a power supply. The electrons emitted from
the emitting tips 4020 are accelerated by the gate electrode 4060,
and move toward the anode conductor layer 4080 which is a
transparent conductor such as indium-tin oxide. The gate electrode
4060 should be within 10 .mu.m of the emitting tips 4020. As the
emitted electrons hit the phosphor layer 4050, light is given off.
(see, EP 1,022,763 A1). The colors of the emitted light depend on
the phosphors that are used. For example Zn:Scu, Al for green,
Y.sub.2O.sub.3:Eu for Red, and ZnS:Ag for blue.
[0139] The cathodes and anodes can be referred to as sources and
drains respectively.
Operation of a Field Emission Device
[0140] To operate the field emission display device, the treated
carbon nanotube cathode is held at a negative potential relative to
the anode. As a result of this potential difference, electrons are
emitted from the emitter tips and travel to the anode. The gate
electrode can be used to accelerate the emitted electrons.
Field Emission Display Devices
[0141] Using the ion bombarded carbon nanotube cathode, various
devices can be created, such as a field emitter array. An array can
include a single nanotube, a single bundle, or many carbon
nanotubes and field emission display, e.g., a flat panel
television. The treated carbon nanotube can constitute the array.
FIG. 10 is an illustration of a classical field emitter.
[0142] Table 3 shows example characteristics of a field emitter
display
3 TABLE 3 emission type low & high voltage brightness
(cd/m.sup.2) 150, 600 viewing angle (degrees) 160 emission
efficiency (lm 10-15 response time 10-30 contrast ratio >100:1
number of colors 16 million number of pixels 640/480 resolution (mm
pitch) 0.31 power consumption (W) 2 max screen size (cm) 26.4 panel
thickness (mm) 10 operating temp range (.degree. C.) -5 to 85
[0143] The advantages of field emission display over other types of
displays such as cathode ray tubes include: high brightness, peak
brightness, full viewing angle, high emission efficiency, high
dynamic range, fast response time and low power consumption.
Bibliography
Use of Carbon Nanotubes in Field Emission Cathodes for Light
Sources
[0144] PCT Appln. PCT/SE00/015221--A Light Source, and a Field
Emission Cathode
[0145] Other Uses
[0146] PCT Appln. PCT/US99/13648--Free-Standing and Aligned Carbon
Nanotubes and Synthesis Thereof (scanning electron microscope,
alkali metal batteries, electromagnetic interference shield, and
microelectrodes).
[0147] [Articles further describing the invention incorporated
herein by reference:
[0148] Yahachi Saito et al., Cathode Ray Tube Lighting Elements
with Carbon Nanotube Field Emitters, 37 JAPAN. J. APPLIED PHYSICS
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[0164] O. Yavas et al., Maskless Fabrication of Field-Emitter Array
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[0165] A. Seidl et al., Geometry Effects Arising from Anodization
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[0166] O. Yavas et al., Pulsed Laser Deposition of Diamond Like
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