U.S. patent application number 11/414945 was filed with the patent office on 2006-08-31 for low work function material.
This patent application is currently assigned to Nano-Proprietary, Inc.. Invention is credited to Richard Lee Fink, Dongsheng Mao, Igor Pavlovsky, Zvi Yaniv.
Application Number | 20060193972 11/414945 |
Document ID | / |
Family ID | 26675013 |
Filed Date | 2006-08-31 |
United States Patent
Application |
20060193972 |
Kind Code |
A1 |
Mao; Dongsheng ; et
al. |
August 31, 2006 |
Low work function material
Abstract
The present invention is directed toward methods for
incorporating low work function metals and salts of such metals
into carbon nanotubes for use as field emitting materials. The
present invention is also directed toward field emission devices,
and associated components, comprising treated carbon nanotubes that
have, incorporated into them, low work function metals and/or metal
salts, and methods for making same. The treatments of the carbon
nanotubes with the low work function metals and/or metal salts
serve to improve their field emission properties relative to
untreated carbon nanotubes when employed as a cathode material in
field emission devices.
Inventors: |
Mao; Dongsheng; (Austin,
TX) ; Yaniv; Zvi; (Austin, TX) ; Fink; Richard
Lee; (Austin, TX) ; Pavlovsky; Igor; (Austin,
TX) |
Correspondence
Address: |
Kelly K. Kordzik;Winstead Sechrest & Minick P.C.
P. O. Box 50784
Dallas
TX
75201
US
|
Assignee: |
Nano-Proprietary, Inc.
Austin
TX
|
Family ID: |
26675013 |
Appl. No.: |
11/414945 |
Filed: |
May 1, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10715934 |
Nov 18, 2003 |
7057203 |
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11414945 |
May 1, 2006 |
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10005989 |
Dec 5, 2001 |
6885022 |
|
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10715934 |
Nov 18, 2003 |
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60254374 |
Dec 8, 2000 |
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Current U.S.
Class: |
427/122 ;
977/842 |
Current CPC
Class: |
H01J 2329/0455 20130101;
H01J 2209/0223 20130101; H01L 51/0048 20130101; H01J 2329/04
20130101; H01J 2201/30469 20130101; C23C 18/00 20130101; H01J
2329/00 20130101; B82Y 10/00 20130101; H01J 1/304 20130101; H01J
9/025 20130101 |
Class at
Publication: |
427/122 ;
977/842 |
International
Class: |
B05D 5/12 20060101
B05D005/12 |
Claims
1. A method comprising the steps of: a) dispersing carbon nanotubes
in a metal salt solution comprising a solvent; and b) removing the
solvent to yield metal salt-treated carbon nanotubes.
2. The method of claim 1, wherein the carbon nanotubes are ground
into a powder prior to dispersing them in the metal salt
solution.
3. The method of claim 1, wherein the metal salt is selected from
the group consisting of alkali metal salts, alkaline earth metal
salts, transition metal salts, p-block metal salts, rare earth
metal salts, and combinations thereof.
4. The method of claim 1, wherein the metal salt is a cesium
salt.
5. The method of claim 1, wherein the solvent is water.
6. The method of claim 1, further comprising a step of washing the
metal salt-treated carbon nanotubes.
7. The method of claim 1, further comprising a step of drying the
metal salt-treated carbon nanotubes.
8-11. (canceled)
12. A method for making a field emission cathode comprising the
steps of: a) providing a substrate; and b) depositing metal
salt-treated carbon nanotubes onto the substrate.
13. The method of claim 12, wherein the metal salt-treated carbon
nanotubes comprise an alkali metal salt.
14. The method of claim 12, wherein the metal salt-treated carbon
nanotubes comprise a cesium salt.
15. The method of claim 12, wherein the metal salt-treated carbon
nanotubes comprise carbon nanotubes selected from the group
consisting of single-wall carbon nanotubes, double-wall carbon
nanotubes, multi-wall carbon nanotubes, carbon fibrils, buckytubes,
metallic carbon nanotubes, semi-conducting carbon nanotubes,
semi-metallic carbon nanotubes, chiral carbon nanotubes,
chemically-modified carbon nanotubes, capped carbon nanotubes,
open-ended carbon nanotubes, endohedrally-modified carbon
nanotubes, and combinations thereof.
16. The method of claim 12, wherein the metal salt-treated carbon
nanotubes are deposited by a technique selected from the group
consisting of spraying, electrophoretic deposition, dipping,
screen-printing, ink-jet printing, dispensing, brushing, and
combinations thereof.
17. The method of claim 12, wherein the metal salt-treated carbon
nanotubes are deposited by a technique comprising spraying
solvent-dispersed metal salt-treated carbon nanotubes onto the
substrate.
18. The method of claim 17, wherein the substrate is heated during
the deposition.
19. The method of claim 12, further comprising a step of reduction
whereby at least some of the metal salt is reduced to metal.
20. The method of claim 12, further comprising a step of tape
activation.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation-in-part of U.S.
patent application Ser. No. 10/005,989 filed Dec. 5, 2001, which
claims priority to U.S. Provisional Patent Application Ser. No.
60/254,374 filed Dec. 8, 2000.
TECHNICAL FIELD
[0002] The present invention relates in general to field emission
devices, and in particular to field emission devices comprising
carbon nanotubes.
BACKGROUND INFORMATION
[0003] Carbon films, including carbon nanotube (CNT) materials, are
being developed for cold cathode applications. These applications
include field emission displays, x-ray tubes, microwave devices,
CRTs, satellite thrusters, or any applications requiring a source
of electrons. There are many types of carbon films that are being
considered. The emission mechanism believed to be responsible for
the emission of electrons from these carbon films is the
Fowler-Nordheim theory; this is especially true for the carbon
films that are conducting. Included in this emission mechanism is
an electrical barrier at the surface of the conductor that prevents
electrons from exiting the metal. However, if a strong field is
applied, this barrier is lowered or made thin such that electrons
can now "tunnel" through the barrier to create a finite emission
current. The height of this barrier is partially determined by the
work function at the particular surface of the material. The work
function is dependent on the material, which surface of the
material an attempt to extract electrons is being made, whether or
not there are impurities on this surface and how the surface is
terminated. What is important is that the lower the work function,
the lower the barrier becomes and the easier it is to extract
electrons from the carbon film. If a means or treatment is
developed that lowers the value of the work function, then it
becomes easier to extract electrons from the film; easier in the
sense that lower extraction fields are required and higher currents
can be obtained from treated films as opposed to untreated films
operated at the same extraction field.
[0004] In analyzing field emission data, there are four unknowns in
the Fowler-Nordheim (F-N) equation. These are n, .alpha., .beta.,
and .PHI. with n the number of emission sites (e.g., tips), .alpha.
the emission area per site, .beta. the field enhancement factor and
.PHI. the work function. The F-N equation is given by:
I=.alpha.AexpB
[0005] with A=1.54 10.sup.-6E.sup.2/.PHI. B=-6.87
10.sup.7.PHI..sup.1.5v/E v=0.95-y.sup.2
[0006] and y=3.79 10.sup.-4E.sup.0.5/.PHI.
[0007] The field at an emission site is E=.beta.E.sub.0 with
E.sub.0=V/d where V is the extraction voltage and d the
cathode-to-anode distance.
[0008] To see the effect that work function has on the field
emission current, the graph in FIG. 1 shows how lowering the work
function from 4.6 eV to 2.4 eV significantly lowers the threshold
electric field and allows much higher current densities (orders of
magnitude) at a given electrical field.
[0009] Single wall carbon nanotubes (SWNTs) and multiwall carbon
nanotubes (MWNTs) can be used as carbon materials for field
emission applications because they are tall and thin and have sharp
features. These sharp features enhance the electric field at these
points (large .beta.), thus a larger field can be achieved with a
given applied voltage. Being made of carbon, they are also
conductive, mechanically very strong, and chemically robust. The
work function of the SWNT material (4.8 eV) is slightly higher than
graphite (4.6-4.7 eV), as disclosed in Suzuki et al., APL, Vol. 76,
p. 4007, Jun. 26, 2000, which is hereby incorporated by reference
herein.
[0010] What is needed is a means of optimizing the field emission
properties of a carbon material by lowering the work function of
this material. This would improve the emission characteristics of
the carbon nanotube material, both SWNT and MWNT.
SUMMARY OF THE INVENTION
[0011] The present invention is directed toward methods for
incorporating low work function metals and salts of such metals
into carbon nanotubes for use as field emitting materials. The
present invention is also directed toward field emission devices,
and associated components, comprising treated carbon nanotubes that
have, incorporated into them, low work function metals and/or metal
salts, and methods for making same. The treatments of the carbon
nanotubes with the low work function metals and/or metal salts
serve to improve their field emission properties relative to
untreated carbon nanotubes when employed as a cathode material in
field emission devices.
[0012] The foregoing has outlined rather broadly the features and
technical advantages of the present invention in order that the
detailed description of the invention that follows may be better
understood. Additional features and advantages of the invention
will be described hereinafter which form the subject of the claims
of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] For a more complete understanding of the present invention,
and the advantages thereof, reference is now made to the following
descriptions taken in conjunction with the accompanying drawings,
in which:
[0014] FIG. 1 illustrates a graph of current density versus
electric field;
[0015] FIG. 2 illustrates a graph of work function versus surface
concentration of alkali or metallic atoms;
[0016] FIG. 3 illustrates an apparatus configured in accordance
with an embodiment of the present invention;
[0017] FIG. 4 illustrates a display;
[0018] FIG. 5 illustrates a data processing system;
[0019] FIG. 6 illustrates a method of making in accordance with an
embodiment of the present invention;
[0020] FIG. 7 illustrates a ball milling device used to grind
carbon nanotubes;
[0021] FIG. 8 illustrates how metal ions (e.g., Cs.sup.+) are
adsorbed onto the surface of carbon nanotubes;
[0022] FIG. 9 illustrates a spraying technique used to deposit
metal salt-treated carbon nanotubes onto a surface;
[0023] FIG. 10 illustrates a screen printing device, which can be
used in the depositing of a metal salt-treated carbon nanotube
dispersion onto a substrate;
[0024] FIG. 11 illustrates how dispensing or ink jet printing can
be used to deposit a dispersion of metal salt-treated carbon
nanotubes on a substrate; and
[0025] FIG. 12 depicts a graph of the emission current vs. electric
field for untreated carbon nanotubes (CNT) and Cs salt-treated
carbon nanotubes (Cs-CNT).
DETAILED DESCRIPTION
[0026] The work function of a surface can be lowered by depositing
metal materials (e.g., alkali metals) or metal salts (e.g., cesium
salts) on the surface, or in some cases such as carbon, by
intercalating or doping the metal atoms or ions into the structure
of the carbon material.
[0027] The present invention is directed toward methods for
lowering the work function of carbon nanotube surfaces by the
addition and/or incorporation of metal materials and/or metal
salts. Such addition and/or incorporation may comprise deposition,
coating, adsorption, doping, intercalation, and combinations
thereof.
[0028] Carbon nanotubes (CNTs), according to the present invention,
include, but are not limited to, single-wall carbon nanotubes
(SWNTs), double-wall carbon nanotubes, multi-wall carbon nanotubes
(MWNTs), carbon fibrils, buckytubes, metallic carbon nanotubes,
semi-conducting carbon nanotubes, semi-metallic carbon nanotubes,
chiral carbon nanotubes, chemically-modified carbon nanotubes,
capped carbon nanotubes, open-ended carbon nanotubes,
endohedrally-modified carbon nanotubes, and combinations thereof
CNTs, according to the present invention, can be made by any known
method. Such methods, some of which require metal catalysts,
include, but are not limited to, arc-synthesis, chemical vapor
deposition, chemical vapor deposition with either a supported or an
unsupported catalyst, laser-oven synthesis, flame synthesis, and
combinations thereof.
[0029] A metal material, according to the present invention, can be
comprised of any metallic element or combination of elements of the
periodic table which has a work function generally less than about
4 eV, typically less than about 3.5 eV, and more typically less
than about 3 eV. Examples of suitable metals include, but are not
limited to, alkali metals, alkaline earth metals, transitional
metals, rare-earth metals, p-block metals, metal alloys, and
combinations thereof. A metal salt, according to the present
invention, can be any salt of any of the metal materials described
herein. Examples of such salts include, but are not limited to,
metal halides, metal nitrates, metal carbonates, metal nitrides,
metal oxides, and combinations thereof.
[0030] As an example of how metals can affect the work function of
a surface, Cs (cesium) atoms lower the work function when they are
attached to carbon material, but it has been found that the
resistance of a SWNT is a non-monotonous function of the Cs uptake.
Resistance decreases initially with Cs uptake, goes through a
minimum, then increases with further doping and finally
saturates.
[0031] Furthermore, Cs can also be used to make a negative electron
affinity surface of GaAs. In this case, a monolayer of Cs bonded
with oxygen on the surface of GaAs leads to an optimum bending of
the conduction and valence band at the surface, making a negative
electron surface. Increasing the Cs concentration on the surface
leads to a metallic surface with increased work function and highly
unstable, very chemically reactive.
[0032] When this same principle is applied to SWNTs and MWNTs, the
work function of the CNT materials is decreased, and this process
may be optimized to obtain an optimal situation in the decrease of
the work function material (see FIG. 2).
[0033] In some embodiments of the present invention, a CNT layer is
grown in situ on a substrate, then metal materials or metal salts
are deposited on this layer. In some embodiments, however, the
metal materials or salts are deposited during the in situ growth
process of the carbon nanotubes. In still other embodiments, metal
materials or metal salts are incorporated with the carbon nanotubes
after the nanotube growth process, but prior to depositing the
nanotubes on a surface.
[0034] In some embodiments of the present invention, the CNTs are
first grown on a substrate, with subsequent incorporation of metal
material and/or metal salts to alter the work function. Some
exemplary components of these embodiments are described below.
Substrate Preparation
[0035] In some embodiments, the substrate can be considered as a
material on which the nanotubes are deposited, and having three
constituent parts (layers): substrate base, catalyst, and interface
layer in between them.
[0036] For many applications, the substrate base is a dielectric
material withstanding the temperatures on order of 700.degree. C.
(e.g., Corning 1737F glass, B3-94 Forsterite ceramic material). It
has been determined that carbon forms on Forsterite substrates over
a broader range of deposition conditions than it does on the
glass.
[0037] In this particular example, a catalyst is consumed during
the deposition of the nanotubes (the feature of the CNT formation
when carbon grows only on the catalyst interface thus lifting the
Ni particle and giving rise to CNTs). The roles of the interface
layer are (1) to provide feedlines to the emitter and (2) to be a
bonding layer between the glass and the catalyst or nanotubes.
Ti--W (10%-90%) successfully fulfills the two functions. In some
embodiments, the thickness of the Ti--W coating may be 2000
.ANG..
[0038] The catalyst materials used were Ni and Fe. In typical
deposition conditions for Ni, no carbon is formed on the iron
catalyst. Ni was likely to have a lower temperature of cracking
C--H bonds, though not many experiments have been done with Fe.
[0039] The thickness of the Ni catalyst layer is important. If the
thickness is too small (<.about.70 .ANG.), the crystalline
structure of the formed carbon is rather amorphous. So also with a
thick Ni coating, 200 .ANG. or more. The advantageous thickness
value lies in the range of about 130-170 .ANG..
Deposition Conditions
[0040] Carbon was deposited in a gaseous mixture of ethylene,
C.sub.2H.sub.4, and hydrogen, with the use of a catalyst. The flow
rates of the gases are of the order of a standard liter per minute,
and have comparable values. Typical flow rates for H.sub.2 are 600
to 1000 sccm (standard cubic centimeters per minute), and 700 to
900 sccm for ethylene. The currently used ratio of the gas flows is
H.sub.2:C.sub.2H.sub.4=600:800 sccm. The gases used for carbon
deposition were H.sub.2, C.sub.2H.sub.4, NH.sub.3, N.sub.2, He.
Ethylene is a carbon precursor. The other gases can be used to
dilute ethylene to get carbon growth. The temperature was set to
660-690.degree. C. Suitable heating devices include tube furnaces
such as a 6-inch Mini Brute quartz tube furnace.
The Deposition Procedure and Timing (Steps 601-602 in FIG. 6)
[0041] 1. Loading the sample and air evacuation. This step takes
about 3 to 5 minutes, until the pressure reaches its base value of
about 15-20 mTorr. [0042] 2. Back fill. This step replaces a He
purge stage used to be a first deposition step in small furnace.
The gas used for back filling is H.sub.2. In a large furnace, it
takes 10 minutes to get atmospheric pressure within the tube. The
temperature in the tube decreases since the hydrogen effectively
transfers heat to the distant parts of the tube that have lower
temperature. [0043] 3. Push to deposition zone. [0044] 4. Preheat.
It takes about 15 minutes for the substrate to reach equilibrium
temperature inside the tube. [0045] 5. Deposition. While the
hydrogen is on, ethylene is turned on for another 15 minutes to
obtain carbon growth. [0046] 6. Purge. In fact, purging can be
considered a part of the deposition due to slow gas flows along the
tube. This step requires the ethylene to be turned off, and lasts 5
minutes with H.sub.2 on. [0047] 7. Pull to load
zone--Evacuate--Vent--Unload. Preparing the Metal Activation
[0048] Once the carbon (CNT) film is prepared (steps 601-602 in
FIG. 6), the sample can be activated by coating it with a layer of
alkali metal (step 603). These metals include lithium (Li), sodium
(Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).
Cs lowers the work function more than the other alkali metals. This
result is an optimized carbon film with alkali material 302 on a
substrate 301 (FIG. 3).
Variations on the Above Embodiments
[0049] In some embodiments, the carbon film is placed in a vacuum
chamber and a source of Cs is placed with the carbon film such that
Cs atoms can be deposited onto the carbon film by evaporation,
sputtering, or other physical vapor deposition methods. The
thickness of the Cs film is optimized such that the work function
of the carbon film is at its lowest.
[0050] Another means of coating the carbon film with an metal or
alkali metal can be done by depositing a compound of the metal or
alkali metal, such as a salt (e.g., CsCl), oxide, nitride or
similar compound, onto the carbon film by physical vapor deposition
methods (e.g., evaporation), or by painting, spraying or soaking in
a wet solution. This compound can then be optionally decomposed
(e.g., reduced to a metal with a reducing agent) in a plasma or by
heat to leave only the Cs metal on the carbon film. The amount of
Cs can be controlled by metering the amount of the compound placed
on the carbon film or by controlling the means of
decomposition.
[0051] Another means of activating the carbon film is to put the
substrate with the carbon film together with a source of alkali
metal in a sealed furnace having a vacuum or inert gas atmosphere
(e.g. helium, nitrogen, etc.). The sample and source of Cs is
heated to high temperatures under high pressures such that the
alkali metal atoms intercalate into the carbon film. Intercalation
means that the Cs atoms diffuse into the carbon film but do not
replace the carbon atoms in the film, and instead fit into
positions between layers of the carbon film. The optimization can
be controlled by controlling the alkali metal intercalation
parameters.
[0052] Another means of activating the carbon film is to dope the
carbon film with alkali metal atoms. This means that some of the
carbon atoms in the CNT matrix are replaced with atoms of alkali
metal. This can be done during the growth of the carbon film or
after the film is grown.
[0053] In some embodiments wherein metal salts are incorporated
with CNT material, the step of decomposition or reduction of these
metal salts to a metal is unnecessary. In other embodiments, metal
incorporated with CNTs is actually converted to a salt material
(this can occur spontaneously, for example, when an alkali metal is
exposed to air). In some embodiments it is desirous to have metal
salts incorporated with the CNTs. In some embodiments, micro- or
nano-porous crystals of metal salts incorporated into a CNT
material (e.g., adsorbed onto the CNT surface(s)), because of their
polar nature, can be oriented in an electric field.
Optimization of Metal Deposition
[0054] The optimization of the alkali metal deposition can be
performed in at least a couple of different ways.
[0055] Several samples can be made and tested for optimal
performance. Each sample can have a measured amount of material
that is different from the other samples. By correlating the
results to the amount of coating or activation, the optimal amount
can be defined for the type of sample investigated.
[0056] Another method is similar to above, except the emission
measuring tools are in the same vacuum chamber as the alkali
source. In this way, the sample can be measured for emission at the
same time the alkali metal is coating the sample. This has the
advantage in that the feedback is in real time and exposure to air
does not complicate the results. Once the results are known, then
the same amount of material can be applied to other samples without
having to monitor the results. The results are expected to be
reproducible such that they do not have to be monitored for every
sample.
[0057] Thereafter, the carbon film with alkali material (302) can
be used on a cathode for many applications where emitted electrons
are useful, including x-ray equipment and display devices, such as
in U.S. Pat. No. 5,548,185, which is hereby incorporated by
reference.
[0058] FIG. 4 illustrates a portion of a field emission display 538
made using a cathode, such as created above and illustrated in FIG.
3. Included with the cathode is a conductive layer 401. The anode
may be comprised of a glass substrate 402, and indium tin layer
403, and a phosphor layer 404. An electrical field is set up
between the anode and the cathode. Such a display 538 could be
utilized within a data processing system 513, such as illustrated
with respect to FIG. 5.
[0059] A representative hardware environment for practicing the
present invention is depicted in FIG. 5, which illustrates an
exemplary hardware configuration of data processing system 513 in
accordance with the subject invention having central processing
unit (CPU) 510, such as a conventional microprocessor, and a number
of other units interconnected via system bus 512. Data processing
system 513 includes random access memory (RAM) 514, read only
memory (ROM) 516, and input/output (I/O) adapter 518 for connecting
peripheral devices such as disk units 520 and tape drives 540 to
bus 512, user interface adapter 522 for connecting keyboard 524,
mouse 526, and/or other user interface devices such as a touch
screen device (not shown) to bus 512, communication adapter 534 for
connecting data processing system 513 to a data processing network,
and display adapter 536 for connecting bus 512 to display device
538. CPU 510 may include other circuitry not shown herein, which
will include circuitry commonly found within a microprocessor,
e.g., execution unit, bus interface unit, arithmetic logic unit,
etc. CPU 510 may also reside on a single integrated circuit.
Metal Salt Treatment of Carbon Nanotubes
[0060] Further to the present invention, Applicants have found that
treatment of carbon nanotubes with metal salt solutions can
significantly improve the field emission properties of such
materials. Such treatment generally involves immersion of a CNTs in
a metal salt solution and subsequent removal of CNTs from the metal
salt solution, and optionally washing and/or drying the CNTs.
[0061] CNTs and metal salts can be any of those described above.
CNTs, especially SWNTs, may be ground into powder form prior to
immersion in the metal salt solution using, for example, a simple
ball mill like that shown in FIG. 7. Such grinding may serve to
facilitate dispersion of the CNTs in the metal salt solution and
further dispersal aids, such as surfactants, may also be used.
[0062] While not intending to be bound by theory, it is believed
that the field emission improvement described above is a result of
adsorption of metal ions on the surface of the CNTs which in turn
can lower the work function of the CNTs, which if left untreated is
about 5.5 eV. Alkali metals for which this works include Li(2.93
eV), Na(2.36 eV), K(2.29 eV), Rb(2.261 eV), Cs(1.95 eV) (Handbook
of Chemistry and Physics, p12-124, 78.sup.th Edition 1997-1998, CRC
Press). Besides these alkali elements, some other materials whose
work function is generally less than about 4 eV, typically less
than about 3.5 eV, and more typically less than about 3 eV, may
also be considered effective at improving the field emission
properties of the carbon nanotubes. Examples of such metals
include, but are not limited to, Ba(2.52 eV), Ca(2.87 eV), Ce(2.9
eV), Gd(2.9 eV), Sm(2.7 ev), Sr(2.58 eV).
[0063] This process has several advantages in that 1) large
quantities of CNTs can be easily and efficiently surface-treated
with metal ions in this manner at relatively low cost; 2) metal
ions do not have to be decomposed (e.g., reduced) to their
corresponding pure metal; and uniform deposition over large area
substrates is possible using currently available low-cost
equipment. Such deposition methods include, but are not limited to,
spraying a dispersion of such treated CNTs onto a substrate
surface.
[0064] Variations on the abovementioned embodiments include an
optional reduction of the metal salt to a metal. While not
intending to be bound by theory, when metal salt treated CNTs are
used as the cathode material in field emission devices (see FIG.
4), it is possible that some or all of the metal cations adsorbed
onto the CNT surface become reduced when a potential is applied
between the cathode and the anode and an emission current begins to
flow between them. Further, in some embodiments, micro- or
nano-crystals of such metal salts which coat CNTs used in such
emission devices, may become oriented when an electric field is
generated as such.
[0065] The following example is provided to more fully illustrate
some of the embodiments of the present invention. The example
illustrates methods by which metal salt-treated CNTs can be made
and prepared for field emission applications. It should be
appreciated by those of skill in the art that the techniques
disclosed in the examples which follow represent techniques
discovered by the inventor to function well in the practice of the
invention, and thus can be considered to constitute exemplary modes
for its practice. However, those of skill in the art should, in
light of the present disclosure, appreciate that many changes can
be made in the specific embodiments which are disclosed and still
obtain a like or similar result without departing from the spirit
and scope of the invention.
EXAMPLE
[0066] This Example describes a method used to make Cs salt
treated-CNTs and their preparation for field emission
applications.
1. Treating Carbon Nanotubes with Cs Salts
[0067] This process provides a way of contacting Cs ions to the
surface of carbon nanotube powders using an Alkali salt/water
solution.
A) Source of Carbon Nanotube and Cs Salt
[0068] Purified single wall carbon nanotubes (SWNTs) were purchased
from Carbon Nanotechnologies, Inc., Houston, Tex., USA. These SWNTs
were about 1-2 nm in diameter and about 5-20 .mu.m in length. It is
believed that other kinds of carbon nanotubes such as single wall,
double-wall or multiwall carbon nanotubes (MWNTs) with different
diameters and lengths from other venders could be substituted in
this example with similar results.
[0069] Cesium nitrate (CsNO.sub.3) was obtained from Spectrum
Laboratory Products, Inc. Gardena, Calif., USA. Applicants suggest
that other kinds of Cs salt such as CsCl, Cs.sub.2CO.sub.3, and
Cs.sub.2SO.sub.4 could be substituted for CsNO.sub.3 in this
example with similar results.
B) Grinding the SWNTs
[0070] CNTs can easily agglomerate and form clusters and
bundles--held together by van der Waals attractive forces. Thus, it
is often beneficial to disperse them before they are immersed in
the Cs salt/water solution. A simple ball mill was used to grind
SWNT bundles. FIG. 7 is a diagram of this ball mill comprising a
motor 701 to which a wheel 702 is attached to a belt 703 which
drives a second wheel 704. This second wheel 704, via a turbine
705, gear 706, and chain 707 assembly, drives a shaft 708 which
spins a milling chamber 709. It is in this milling chamber 709 that
the CNTs and/or particles are placed. The rate at which this
machine is typically run is about 50-60 revolutions per minute.
[0071] In this particular example, approximately 0.5 g of SWNTs,
together with tens of Al.sub.2O.sub.3 balls used for grinding
(.about.5-10 mm in diameter), were mixed with about 100 ml water.
The CNT powders were ground for between 1-14 days in order to fully
disperse the carbon nanotubes. Optionally, a surfactant such as
sodium dodecylbenzene sulfonate (M. F. Islam, E. Rojas, D. M.
Bergey, A. T. Johnson, and A. G. Yodh, Nano Lett. 3(2),
269-273(2003), incorporated herein by reference), or similar
materials, can also be added to the mixture in order to achieve
better dispersion of the carbon nanotubes.
C) Preparation of the Cs Salt Solution
[0072] In this example, approximately 2 g of CsNO.sub.3 powder was
put into a beaker with approximately 400 ml of water and was
stirred using a glass stirbar until the powder was dissolved. The
solution can be optionally filtered one or more times to extract
impurities and/or difficult to dissolve particles.
D) Immersing the Ground CNT Dispersion into the Cs Salt
Solution
[0073] The ground Carbon nanotubes, dispersed in approximately 100
ml of water, was added to the CsNO.sub.3 solution to form a
mixture. The total solution volume was 500 ml and the concentration
of CsNO.sub.3 was 0.02 M (other concentrations may be more optimal
for achieving the best field emission properties from CNTs). Then,
the solution was placed onto a hot plate/magnetic stirrer and
stirred using a magnetic stir bar for approximately 15-20 hours
(the time can be varied in other embodiments such that a the CNTs
surface becomes saturated with metal ions. FIG. 8 illustrates how
Cs ions are allowed to adsorb onto the surface of CNTs. Referring
to FIG. 8, beaker 801 contains CsNO.sub.3/water solution 802, CNTs
803, and a magnetic stir bar 804. The mixture is heated/stirred by
a stirring hot plate 805.
E) Washing the CNTs
[0074] After the mixture was stirred for approximately 15-20 hours,
the CNTs were washed several times using deionized water in order
to remove the salt residue in the solution. The water was removed
until all the CNTs sank down to the bottom of the beaker. Then, new
water was added. After the CNTs were washed, they could be dried in
a oven at approximately 40-100.degree. C. for a certain time. In
this experiment, the CNT solution was further washed several times
with isopropyl alcohol (IPA) in order to then prepare a CNT
cathode.
2. Applying Cs Salt-Treated Carbon Nanotubes to the Surface of a
Substrate
[0075] In this Example, a spraying technique was employed to
deposit Cs salt-treated CNTs onto a substrate. Because CNTs can
easily clump together if they are, an ultrasonic horn or bath was
used to redisperse them in an IPA solution just prior to spraying
them onto the substrate. In this Example, the CNT-IPA solution was
sprayed onto a conventional silicon (Si) substrate comprising an
area of approximately 2.times.2 cm.sup.2 (such a solution could
also be sprayed onto various other substrates such as metal,
ceramic, glass, semiconductors and plastics). In order to get
better coating uniformity and dispersal on the substrates, more IPA
can be added into the above solution prior to spraying. In this
Example, the solution used for spraying was a mixture of
approximately 0.2 g of Cs salt-treated CNTs in approximately 100 ml
of IPA. In alternative embodiments, this solution can be applied to
a selective area or areas using a shadow mask. In order to prevent
the IPA from flowing to unexpected area, the substrate was heated
to approximately 70.degree. C. on both the front side and back side
during the spraying process, in order to evaporate the IPA quickly.
The substrate was sprayed back and forth and/or up and down several
to tens of times until the entire surface was coated with the
mixture. The thickness of the mixture was about 1-10 .mu.m. The
surface was then dried in air. FIG. 9 illustrates the spraying
technique employed in this Example, wherein a condensed gas 901 is
used to charge an atomizer 902 containing a solvent-suspended
mixture of metal salt-treated carbon nanotubes 903. Mixture 903 is
sprayed onto a substrate 904, optionally in contact with heater 905
and/or infrared (IR) heat lamp 906, to form cathode material layer
907 comprising metal salt-treated CNTs.
[0076] Techniques other than spraying may also be used to apply the
mixture to a surface. Such techniques include, but are not limited
to, electrophoretic deposition, dipping, screen-printing, ink-jet
printing, dispensing, brushing, and combinations thereof. Other
solvents, such as acetone or methanol, may also be used as the
carrier (in lieu of IPA) when applying the Cs salt-treated CNTs to
a surface.
[0077] FIGS. 10A-C illustrate a screen printing method by which a
dispersion of metal salt-treated carbon nanotubes can be deposited
onto a substrate according to some embodiments of the present
invention. Referring to FIG. 10A, a substrate 1001 is placed on a
substrate stage/chuck 1002 and brought in contact with an image
screen stencil 1003. A dispersion 1004 comprising metal
salt-treated carbon nanotubes (dispersion 1004 may also comprise
insulating or conducting particles such as alumina, silica, or
silver, and also standard paste vehicles and thinners to control
the viscosity and curing properties of the paste) is then "wiped"
across the image screen stencil 1003 with a squeegee 1005, as shown
in FIG. 10B. The dispersion 1004 then contacts the substrate 1001
only in the regions directly beneath the openings in the image
screen stencil 1003. The substrate stage/chuck 1002 is then lowered
to reveal the patterned cathode material 1006 on substrate 1001, as
shown in FIG. 10C. The patterned substrate is then removed from the
substrate stage/chuck.
[0078] FIG. 11 illustrates an embodiment wherein a dispensor or an
ink jet printer is used to deposit metal salt-treated carbon
nanotubes onto a substrate. Referring to FIG. 11, printing head
1101 is translated over a substrate 1104 in a desired manner. As it
is translated over the substrate 1104, the printing head 1101
sprays droplets 1102 comprising metal salt-treated carbon nanotubes
dispersed in a solvent. As these droplets 1102 contact substrate
1104, they form printed cathode material 1103 comprising metal
salt-treated carbon nanotubes. In some embodiments, the substrate
1104 is heated so as to effect rapid evaporation of solvent within
said droplets. Heat and/or ultrasonic energy may be applied to the
printing head 1101 during dispensing.
3. Activation of the CNTs
[0079] Once the Cs salt-treated CNTs were deposited onto the
surface of the substrate, an activating technique, referred to
herein as "tape activation," can be applied to the CNT film by
applying an adhesive tape material to the film and then pealing the
adhesive tape away (Yang Chang, Jyh-Rong Sheu, Cheng-Chung Lee,
Industrial Technology Research Institute, Hsinchu, TW, "Method of
Improving Field Emission Efficiency for Fabrication Carbon Nanotube
Field Emitters", U.S. Pat. No. 6,436,221 B1, incorporated herein by
reference).
[0080] After the carbon nanotubes were sprayed on to the substrate,
an adhesive tape process may be needed to remove the top layer of
material from the surface. In this Example, clear tape (3M, Catalog
number #336) was optionally used to remove the top layer of
material. The tape was applied to the Cs salt-treated CNT layer
using a laminating process. Care was taken to ensure that there was
no air between the tape and the CNT layer (if a bubble is exists,
the mixture at that area will not be removed or treated as the
other areas are). A rubber roll was used to further press the tape
in order to further eliminate air at the intersection of the tape
and the Cs salt treated CNT layer. Finally, the tape is
removed.
4. Field Emission Test of the Cs Salt-Treated CNTs in a Field
Emission Device
[0081] To compare field emission properties, untreated CNTs were
also made using the same spray and activation conditions as the Cs
salt treated sample. Both samples were then tested by mounting them
with a phosphor screen in a diode configuration, like that shown in
FIG. 4, with a gap of about 0.63 mm between the anode and the
cathode. The test assembly was placed in a vacuum chamber and
pumped to about 10.sup.-7 Torr. The electrical properties of the
cathode were then measured by applying a negative, pulsed voltage
(AC) to the cathode and holding the anode at ground potential and
measuring the current at the anode (a DC potential could also be
used for the testing, but this may damage the phosphor screen). A
graph of the emission current vs. electric field for the two
samples is shown in FIG. 12.
[0082] It can be seen from FIG. 12 that the Cs salt-treated CNTs
have significantly better field emission properties than untreated
CNTs. A threshold field of less than 0.9 V/.mu.m and emission
current of 30 mA at 1.84 V/.mu.m was achieved for the Cs
salt-treated CNTs, whereas the untreated CNTs exhibited a threshold
field of about 1.3 V/.mu.m and required a field of approximately
2.80 V/.mu.m to generate an emission current of 30 mA.
[0083] Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions and alterations can be made herein without departing
from the spirit and scope of the invention as defined by the
appended claims.
* * * * *