U.S. patent application number 11/374386 was filed with the patent office on 2007-01-11 for methods for assembly and sorting of nanostructure-containing materials and related articles.
Invention is credited to Huaizhi Geng, Lu-Chang Qin, Jie Tang, Guang Yang, Jian Zhang, Otto Z. Zhou.
Application Number | 20070007142 11/374386 |
Document ID | / |
Family ID | 32511597 |
Filed Date | 2007-01-11 |
United States Patent
Application |
20070007142 |
Kind Code |
A1 |
Zhou; Otto Z. ; et
al. |
January 11, 2007 |
Methods for assembly and sorting of nanostructure-containing
materials and related articles
Abstract
A method for depositing a nanostructure-containing material onto
an object or substrate includes one or more of the following: (1)
forming a solution or suspension of nanostructure-containing
material, (2) selectively adding "chargers" to the solution, (3)
immersing electrodes in the solution, the substrate or object upon
which the nanostructure material is to be deposited acting as one
of the electrodes, (4) applying a direct and/or alternating current
electrical field between the two electrodes for a certain period of
time thereby causing the nanostructure materials in the solution to
migrate toward and attach themselves to the substrate electrode,
and (5) subsequent optional processing of the coated substrate.
Associated objects and devices are also provided. A method for
separating nanostructures based on their properties and/or geometry
is also described.
Inventors: |
Zhou; Otto Z.; (Chapel Hill,
NC) ; Tang; Jie; (Chapel Hill, NC) ; Geng;
Huaizhi; (Carrboro, NC) ; Qin; Lu-Chang;
(Chapel Hill, NC) ; Zhang; Jian; (Carrboro,
NC) ; Yang; Guang; (Carrboro, NC) |
Correspondence
Address: |
JENKINS, WILSON, TAYLOR & HUNT, P. A.
3100 TOWER BLVD
SUITE 1200
DURHAM
NC
27707
US
|
Family ID: |
32511597 |
Appl. No.: |
11/374386 |
Filed: |
March 13, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10730068 |
Dec 9, 2003 |
7014743 |
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11374386 |
Mar 13, 2006 |
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60431719 |
Dec 9, 2002 |
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60461401 |
Apr 10, 2003 |
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Current U.S.
Class: |
205/78 |
Current CPC
Class: |
G01Q 70/12 20130101;
B82Y 15/00 20130101; H01J 9/025 20130101; B82Y 30/00 20130101; B82Y
35/00 20130101; Y10T 29/4913 20150115; Y10T 29/49169 20150115; H01J
2201/30469 20130101; Y10S 977/84 20130101; C25D 13/00 20130101;
Y10S 977/845 20130101; B82Y 10/00 20130101; G01Q 70/16
20130101 |
Class at
Publication: |
205/078 |
International
Class: |
C25D 1/00 20060101
C25D001/00 |
Goverment Interests
GOVERNMENT INTEREST
[0002] This invention was made with Government support under Grant
Nos. N00014-98-1-05907 and NAG-1-01061. The Government has certain
rights in the present subject matter.
Claims
1. A method of attaching a nanostructure-containing material onto a
sharp tip of an object, the method comprising: (i) forming a
suspension of nanostructure-containing material in a liquid medium;
(ii) immersing at least one electrode in the suspension; (iii)
placing the sharp tip into the suspension; and (iv) applying a
direct or alternating current to the immersed electrode and the
sharp tip and causing at least a portion of the
nanostructure-containing material in the suspension to become
attached to the object proximate an apex of the sharp tip.
2. The method of claim 1, wherein the object comprises a point
electron field emission source, a probe of an atomic force
microscope, a probe of a scanning tunneling microscope, an electron
source of a transmission electron microscope, an electron source of
a scanning electron microscope, a probe of a magnetic force
microscope, or a profilometer.
3. The method of claim 1, further comprising the step of
functionalizing the nanostrucutre-containing material prior to step
(i).
4. The method of claim 1, wherein steps (iii) and (iv) further
comprise moving the tip toward the surface of the suspension until
electrical contact is established with the suspension, maintaining
the electrical contact for a predetermined period of time, and
withdrawing the tip from the suspension.
5. The method of claim 4, wherein the nanostructure-containing
material comprises carbon nanotubes, and wherein step (iv) further
comprises attaching an individual carbon nanotube, a nanotube
bundle, or a nanowire to the sharp tip or attaching a fibril
comprising a plurality of carbon nanotubes, nanotube bundles, or
nanowires.
6. The method of claim 5, wherein the longitudinal axis of the
individual carbon nanotube, nanotube bundle, or nanowire is aligned
along a cone axis of the sharp tip.
7. The method of claim 5, wherein the longitudinal axis of the
fibril comprising nanotube, nanotube bundle or nanowire is aligned
along a cone axis of the sharp tip.
8. The method of claim 5, wherein the longitudinal axis of the
nanotube, nanotube bundle or nanowire within the fibril is aligned
along the longitudinal axis of the fibril.
9. The method of claim 5, wherein the fibril has a
cylindrical-shaped body and two ends, wherein a first end of the
fibril is attached to the apex of the sharp tip and a second end of
the fibril has a tapered geometry and wherein the tip diameter of
the tapered end is 0.5 nm to 100 nm.
10. The method of claim 5, further comprising annealing the tip
after the nanotube, nanotube bundle, or nanowire is attached.
11. The method of claim 1, wherein step (iii) comprises placing a
plurality of sharp tips into the suspension.
12. The method of claim 1, wherein step (iv) comprises applying an
alternating current with a frequency of 10 Hz to 10 GHz.
13. The method of claim 1, wherein step (iv) comprises applying a
direct current.
14. The method of claim 13, wherein step (i) further comprises
adding at least one compound to the suspension in order to impart a
charge characteristic to the nanostructure-containing material.
15. The method of claim 14, wherein the at least one compound
comprises one or more of: MgCl.sub.2, NaOH, Mg(NO.sub.3).sub.2,
La(NO.sub.3).sub.3, AlCl.sub.3.
16. The method of claim 5, wherein the nanostructure-containing
material comprises at least one of: a single-walled carbon
nanotube; a multi-walled carbon nanotube; silicon; silicon oxide;
germanium; germanium oxide; carbon nitride; boron; boron nitride,
dichalcogenide; silver; gold; iron; titanium oxide; gallium oxide;
indium phosphide; magnetic particles enclosed within a nanotube; a
nanotube with a composition B.sub.xC.sub.yN.sub.z; and a nanotube
or concentric fullerene with a composition of MS.sub.2 (M=tungsten,
molybdenum, or vanadium oxide).
17. The method of claim 4, wherein the tip is gradually withdrawn
from the surface of the suspension while under the applied current
such that nanostructure-containing material is first assembled
proximate an apex of the tip and then assembled onto previously
attached nanostructure-containing material thereby forming a wire
of nanostructure-containing material.
18. The method of claim 17, wherein the wire is formed with a
diameter of 0.5 nm to 100 .mu.m.
19. The method of claim 1, further comprising annealing the sharp
tip and nanostrucutre-containing material after step (iv).
20. A wire having a diameter of 0.5 nm to 100 .mu.m comprising
nanostructure-containing material.
21. The wire of claim 20, having a diameter of 0.5 nm to 1
.mu.m.
22. The wire of claim 21, having a length of 50 nm to 50 .mu.m.
23. The wire of claim 20, wherein the nanostructure-containing
material comprises carbon nanotubes.
24. The wire of claim 23, wherein the carbon nanotubes comprise
single-walled carbon nanotubes.
25. The wire of claim 20, formed by the method of claim 17.
26. The method of claim 1, wherein step (iv) comprises attaching
one carbon nanotube on the apex of a sharp tip, wherein a
longitudinal axis of the carbon nanotube is aligned along a cone
axis of the sharp tip, and wherein an end of the carbon nanotube
pointing away from the apex of the sharp tip contains a magnetic
particle.
27. The method of claim 26, wherein the sharp tip is a probe of an
atomic force microscope.
28. The method of claim 26, wherein the magnetic particle is
encapsulated by the carbon nanotube.
29. A device comprising: a generally conical sharp tip having a
cone axis; and a fibril comprising nanostructure-containing
material attached to the sharp tip and generally aligned along the
cone axis of the sharp tip, the fibril having a diameter of 0.5 nm
to 1.0 .mu.m.
30. The device of claim 29, wherein the device comprises a point
electron field emission source.
31. The device of claim 29, wherein the device comprises a probe of
an atomic force microscope, a scanning probe microscope, a
transmission electron microscope, a scanning electron microscope, a
magnetic force microscope, or a profilometer.
32. The device of claim 29, wherein the sharp tip is formed of
tungsten.
33. The device of claim 29, wherein the nanostrucutre-containing
material comprises at least one of: a single-walled carbon
nanotube; a multi-walled carbon nanotube; silicon; silicon oxide;
germanium; germanium oxide; carbon nitride; boron; boron nitride,
dichalcogenide; silver; gold; iron; titanium oxide; gallium oxide;
indium phosphide; magnetic particles enclosed within a nanotube; a
nanotube with a composition B.sub.xC.sub.yN.sub.z; and a nanotube
or concentric fullerene with a composition of MS.sub.2 (M=tungsten,
molybdenum, or vanadium oxide).
34. The device of claim 29, wherein the fibril comprises: a single
carbon nanotube, a plurality of carbon nanotubes, a single carbon
nanotube bundle, or a plurality of carbon nanotube bundles.
35. The device of claim 34, wherein the carbon nanotube or carbon
nanotubes comprise single-walled carbon nanotubes.
36. The device of claim 29, wherein the fibril has a length of 50
nm to 50 .mu.m.
37. The device of claim 29, wherein the devices exhibits an emitted
electron current of greater than 0.5 microamperes, and a current
decay after 10 hours of continuous operation of less than 15%.
38. The device of claim 29, wherein the devices exhibits an emitted
electron current of greater than 1.0 microamperes, and a current
decay after 10 hours of continuous operation of less than 15%.
39. The device of claim 29, wherein the devices exhibits an emitted
electron current of greater than 3.0 microamperes, and a current
decay after 10 hours of continuous operation of less than 15%.
40. The device of claim 29, wherein the devices exhibits an emitted
electron current of greater than 5.0 microamperes, and a current
decay after 10 hours of continuous operation of less than 15%.
41. The device of claim 29, wherein the attached fibril has a
substantially longitudinal axis which defines an angle with the
cone axis of the sharp tip, the angle being less than 15
degrees.
42. The device of claim 29, wherein the attached fibril has a
substantially longitudinal axis which defines an angle with the
cone axis of the sharp tip, the angle being less than 10
degrees.
43. A method of making an electrical connection between a plurality
of components, the method comprising: (i) forming a suspension of
nanostructure-containing material in a liquid medium; (ii) bringing
the suspension into contact with the components; and (iii) applying
a direct or alternating current to the components thereby
establishing an electrical field therebetween causing a wire to be
formed from the nanostructure-containing material connecting the
components.
44. The method of claim 43, wherein the plurality of components
comprises two components.
45. The method of claim 43, wherein the plurality of components
comprise four components, step (ii) comprises brining the
suspension into contact with all four components, and step (iii)
comprises applying direct or alternating current to a first pair of
components to form a first connection therebetween, and then
applying direct or alternating current to a second pair of the
components to form a second connection therebetween.
46. The method of claim 43, wherein the components comprise
components disposed on a circuit board.
47. The method of claim 43, further comprising annealing the tip
after the nanotube, nanotube bundle, or nanowire is attached.
48. The method of claim 43, wherein step (iii) comprises applying
an alternating current with a frequency of 10 Hz to 10 GHz.
49. The method of claim 43, wherein step (iii) comprises applying a
direct current.
50. The method of claim 49, wherein step (i) further comprises
adding at least one compound to the suspension in order to impart a
charge characteristic to the nanostructure-containing material.
51. The method of claim 50, wherein the at least one compound
comprises one or more of: MgCl.sub.2, NaOH, Mg(NO.sub.3).sub.2,
La(NO.sub.3).sub.e, AlCl.sub.3.
52. The method of claim 43, wherein the nanostructure-containing
material comprises at least one of: a single-walled carbon
nanotube; a multi-walled carbon nanotube; silicon; silicon oxide;
germanium; germanium oxide; carbon nitride; boron; boron nitride,
dichalcogenide; silver; gold; iron; titanium oxide; gallium oxide;
indium phosphide; magnetic particles of Fe, Co or Ni enclosed
within a nanotube; a nanotube with a composition
B.sub.xC.sub.yN.sub.z; and a nanotube or concentric fullerene with
a composition of MS.sub.2 (M=tungsten, molybdenum, or vanadium
oxide).
53. An arrangement comprising: a first component; a second
component; and a first wire comprising nanostructure-containing
material, the wire attached to both the first and second components
and providing an electrical connection therebetween.
54. The arrangement of claim 53, wherein the wire is formed by the
method of claim 43.
55. The arrangement of claim 53, further comprising: a third
component; a fourth component; and a second wire comprising
nanostructure-containing material, the wire attached to both the
first and second components and providing an electrical connection
therebetween.
56. The arrangement of claim 55, wherein the second wire is formed
by the method of claim 43.
57. The arrangement of claim 53, wherein the components are
disposed on a circuit board.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/730,068, filed Dec. 9, 2003, which claims the benefit
of U.S. Provisional Patent Application Ser. No. 60/461,401, filed
Apr. 10, 2003 and further claims the benefit of U.S. Provisional
Patent Application Ser. No. 60,431,719, filed Dec. 9, 2002, the
disclosures of which are incorporated herein by reference in their
entireties.
TECHNICAL FIELD
[0003] The present subject matter relates to methods of depositing
a nanostructure or nanotube-containing material onto an object,
methods of sorting nanostructure or nanotube-containing materials,
methods of forming articles from nanostructure or
nanotube-containing materials, methods for sorting nanostructure or
nanotube-containing materials, and associated structures and
devices.
BACKGROUND ART
[0004] In the description of the background that follows reference
is made to certain structures and methods, however, such references
should not necessarily be construed as an admission that these
structures and methods qualify as prior art under the applicable
statutory provisions. Applicants reserve the right to demonstrate
that any of the referenced subject matter does not constitute prior
art with regard to the present disclosure.
[0005] The term "nanostructure" material is used by those familiar
with the art to designate materials including nanoparticles such as
C.sub.60 fullerenes, fullerene-type concentric graphitic particles,
metal, compound semiconductors such as CdSe, InP;
nanowires/nanorods such as Si, Ge, SiO.sub.x, GeO.sub.x, or
nanotubes composed of either single or multiple elements such as
carbon, B.sub.xN.sub.y, C.sub.xB.sub.yN.sub.2, MoS.sub.2, and
WS.sub.2. One of the common features of nanostructure materials is
their basic building blocks. A single nanoparticle or a carbon
nanotube has a dimension that is less than 500 nm at least in one
direction. These types of materials have been shown to exhibit
certain properties that have raised interest in a variety of
applications and processes.
[0006] U.S. Pat. Nos. 6,280,697 and 6,422,450 to Zhou et al. (both
entitled "Nanotube-Based High Energy Material and Method"), the
disclosures of which are incorporated herein by reference, in their
entirety, disclose the fabrication of carbon-based nanotube
materials and their use as a battery electrode material.
[0007] U.S. Pat. No. 6,630,772 to Bower et al. (entitled "Device
Comprising Thin Film Carbon Nanotube Electron Field Emitter
Structure"), the disclosure of which is incorporated herein by
reference, in its entirety, discloses a carbon-nanotube field
emitter structure having a high emitted current density.
[0008] U.S. Pat. No. 6,277,318 to Bower et al. (entitled "Method
for Fabrication of Patterned Carbon Nanotube Films"), the
disclosure of which is incorporated herein by reference, in its
entirety, discloses a method of fabricating adherent, patterned
carbon nanotube films onto a substrate.
[0009] U.S. Pat. No. 6,334,939 (entitled "Nanostructure-Based High
Energy Material and Method"), the disclosure of which is
incorporated herein by reference, in its entirety, discloses a
nanostructure alloy with alkali metal as one of the components.
Such materials are described as being useful in certain battery
applications. U.S. Pat. No. 6,553,096 to Zhou et al. (entitled
"X-Ray Generating Mechanism Using Electron Field Emission
Cathode"), the disclosure of which is incorporated herein by
reference, in its entirety, discloses an X-ray generating device
incorporating a nanostructure-containing material.
[0010] U.S. Pat. No. 6,965,199 (entitled "Coated Electrode With
Enhanced Electron Emission And Ignition Characteristics"), the
disclosure of which is incorporated herein by reference, in its
entirety, discloses an electrode including a first electrode
material, an adhesion-promoter, and a carbon nanotube-containing
material disposed on at least a portion of the adhesion promoting
layer, as well as associated devices incorporating such an
electrode.
[0011] U.S. Pat. No. 6,787,122 (entitled "Method of Making
Nanotube-Based Material With Enhanced Field Emission"), the
disclosure of which is incorporated herein by reference, in its
entirety, discloses a technique for introducing a foreign species
into the nanotube-based material in order to improve the properties
thereof.
[0012] U.S. Pat. No. 6,876,724 (entitled "Large-Area Individually
Addressable Multi-Beam X-Ray System and Method of Forming Same"),
the disclosure of which is incorporated herein by reference, in its
entirety, discloses a structure to generate x-rays having a
plurality of stationary and individually electrically addressable
field emissive electron sources, such as carbon nanotubes.
[0013] U.S. Patent Publication No. US 2003/0180472 (entitled
"Method for Assembling Nano-objects"), the disclosure of which is
incorporated herein by reference, in its entirety, discloses a
technique for the self assembly of a macroscopic structure with
preformed nano-objects, which may be processed to render a desired
aspect ratio and/or chemical functionality.
[0014] As evidenced by the above, nanostructure materials,
especially those such as carbon nanotubes and other nano-objects
having a large aspect ratio (i.e.--a length which is substantially
larger than its diameter) possess promising properties that make
them attractive for a variety of applications, such as lighting
elements, field emission devices such as flat panel displays, gas
discharge tubes for over voltage protection, x-ray generating
devices, small conduction wires, sensors, actuators and high
resolution probes such as those used in scanning microscopes.
[0015] The effective incorporation of such materials into such
devices has been hindered by difficulties encountered in the
processing of such materials. For instance, nanostructured
materials can be formed by techniques such as laser ablation, and
arc discharge methods, solution synthesis, chemical etching,
molecular beam epitaxy, chemical vapor deposition, laser ablation,
etc. However, processing techniques to assemble these nanostructure
materials have posed certain difficulties.
[0016] Post-formation methods such as screen printing and spraying
have been utilized to deposit pre-formed nano-objects such as
carbon nanotubes on a substrate. However, such techniques pose
certain drawbacks. For instance, screen printing requires the use
of binder materials as well as an activation step. Spraying can be
inefficient and is not practical for large-scale fabrication.
Moreover, these techniques typically result in randomly distributed
nanostructure materials on the substrate.
[0017] Carbon nanotubes have been grown directly upon substrates by
use of chemical vapor deposition (CVD) techniques. See, e.g.--J.
Hafner et al., Nature, Vol. 398, pg. 761, 1999 and U.S. Pat. Nos.
6,457,350 and 6,401,526. One potential application of this
technique is the formation of conducting wires made from
nanostructure materials, such as carbon nanotubes and electrical
circuitry. The CVD process can be used to form the conducting wires
which are attached to electrodes at specific locations using CVD
techniques to form the conducting wires. However, such techniques
require relatively high temperatures (e.g. -600-1,000.degree. C.)
as well as reactive environments, and the use of catalysts in order
to effectively grow the nanotubes. The requirement for such harsh
environmental conditions severely limits the types of substrate
materials which can be utilized. In addition, the CVD technique
often results in mutli-wall carbon nanotubes. These mutli-wall
carbon nanotubes generally do not have the same level of structural
perfection and thus have inferior electronic emission properties
when compared with single-walled carbon nanotubes. Also, direct
growth of nanotubes onto the substrate by such techniques makes it
difficult to control the length, orientation and number of the
nanotubes deposited thereby.
[0018] Other techniques have involved efforts to precisely control
the deposition of individual or small groups of nano-objects, such
as carbon nanotubes, onto a substrate, such as sharp tips or
projections. See, e.g.--Dai, Nature, Vol. 384, pgs. 147-150 (1996);
and R. Stevens et al., Appl. Phys. Lett., Vol. 77, pg. 3453.
However, such techniques are painstaking and time-consuming, and do
not lend themselves to efficient large-scale production, or batch
processing. For example, U.S. Pat. No. 6,528,785 describes a
process by which plate-like electrodes are placed in an
electrophoretic solution and nanotubes are deposited on at least
one of the electrodes. The electrode(s) is withdrawn from the
solution and nanotubes deposited thereon are transferred to a sharp
tip in a further processing step. The nanotube(s) is then "fusion
welded" to the tip by yet another processing step which may include
the deposition of a coating material over at least the portion of
the nanotube attached the sharp tip. The process is slow and lacks
of control of the orientation. The tips formed usually comprise one
carbon nanotube (CNT) per tip. The interfacial bonding between the
tip and CNT tends to be weak. It is difficult to fabricate many
tips at one time. This process is undesirably complicated and
tedious, and thus is impractical for commercial scale
production.
[0019] Another consideration in the art is that in the construction
of electrical devices using nanostructured materials, it is often
necessary to have materials with the same properties, such as their
electronic properties. This has not been achieved. For example,
single wall carbon nanotubes materials synthesized by the laser
ablation methods contain materials that are both metallic and
semiconducting by nature. Currently, there is no effective method
to separate the nanotubes based on their properties. For instance,
separation of metallic and semiconducting nanotubes is necessary
for many device applications.
SUMMARY
[0020] Techniques, and their associated articles, have been
developed which permit efficient and effective assembly of
nanostructure-containing materials under relatively mild
conditions, separation of nanostructure-containing materials based
on their properties wherein such methods are suitable for
batch-scale processing.
[0021] According to one aspect, a method of attaching a
nanostructure-containing material onto a sharp tip of an object,
the method comprising: (i) forming a suspension of
nanostructure-containing material in a liquid medium;
[0022] (ii) immersing at least one electrode in the suspension;
(iii) placing the sharp tip into the suspension; and (iv) applying
a direct or alternating current to the immersed electrode and the
sharp tip and causing at least a portion of the
nanostructure-containing material in the suspension to become
attached to the object proximate an apex of the sharp tip.
[0023] According to another aspect, a wire is provided having a
diameter of 0.5 nm-100 .mu.m and a length of 10 nanometers (nm) to
greater than 1 centimeter (cm) comprising nanostructure-containing
material.
[0024] According to another aspect, an object is provided having a
sharp tip coated with a single carbon nanotube, carbon nanotube
bundle, or nanowire, the object comprising at least one of: a point
electron field emission source, a probe of an atomic force
microscope (AFM), a probe of a scanning tunneling microscope (STM),
an electron source of a transmission electron microscope (TEM), an
electron source of a scanning electron microscope (SEM), a probe of
a magnetic force microscope (MFM), or a profilometer.
[0025] According to one aspect, a method of attaching a
nanostructure-containing material onto a sharp tip of an object is
provided comprising (i) forming a suspension of
nanostructure-containing material in a liquid medium, (ii)
immersing at least one electrode in the suspension, (iii) placing
the sharp tip into the suspension, and (iv) applying a direct or
alternating current to the immersed electrode and the sharp tip and
causing at least a portion of the nanostructure-containing material
in the suspension to become attached to the object proximate an
apex of the sharp tip.
[0026] According to another aspect, a device is provided comprising
a generally conical sharp tip having a cone axis, and a fibril
comprising nanostructure-containing material attached to the sharp
tip and generally aligned along the cone axis of the sharp tip, the
fibril having a diameter of 0.5 nm to 10 .mu.m.
[0027] According to yet another aspect, a method of making an
electrical connection between a plurality of components is provided
comprising (i) forming a suspension of nanostructure-containing
material in a liquid medium, (ii) bringing the suspension into
contact with the components and (iii) applying a direct or
alternating current to the components thereby establishing an
electrical field therebetween causing a wire to be formed from the
nanostructure-containing material connecting the components.
[0028] According to another aspect, an arrangement is provided
comprising a first component; a second component; and a first wire
comprising nanostructure-containing material, the wire attached to
both the first and second components and providing an electrical
connection therebetween.
[0029] According to yet another aspect, a method of separating
groups of nanostructure-containing materials is provided comprising
(i) forming a mixture comprising the groups of
nanostructure-containing materials to be separated and a liquid
medium, (ii) introducing a plurality of electrodes into the
mixture; (iii) establishing an asymmetrical electrical field within
the mixture, (iv) polarizing the groups of nanostructure-containing
materials in the mixture, thereby causing at least a first group to
migrate to a first electrode and causing a second group to migrate
to a second electrode, and (v) recovering at least the first group
from the liquid medium at the first electrode.
[0030] According to a further aspect, a method of separating a
first group of particles from a second group of particles contained
in a mixture, at least one of the first and second groups of
particles comprising a nanostructure-containing material, is
provided comprising (i) forming an arrangement of electrodes, (ii)
applying an alternating current power source to the arrangement,
(iii) bringing the mixture into proximity with the arrangement,
(iv) polarizing the first group of particles differently than the
second group of particles, (v) separating the first group of
particles from the second group of particles based on differences
in polarity, and (vi) recovering at least one of the first or
second group of particles.
[0031] As used herein, the term "nanostructure-containing material"
means materials which are composed, at least in part, from
nanoparticles such as C.sub.60 fullerenes, fullerene-type
concentric graphitic particles, metal, compound semiconductors such
as CdSe, InP; nanowires/nanorods such as Si, Ge, SiO.sub.x,
GeO.sub.x, or nanotubes composed of either single or multiple
elements such as carbon, B.sub.xN.sub.y, C.sub.xB.sub.yN.sub.z,
MoS.sub.2, and WS.sub.2.
[0032] The term "generally conical" encompasses geometries which
are mainly conical in nature, but may include various structural
deviations from purely conical shapes. The extent of these
variations are those typically observed in the formation of sharp
tips for uses such as in the formation of probes for atomic force
microscopes, scanning probe microscopes, magnetic force
microscopes, profilometers, transmission electron microscopes,
scanning electron microscopes, and the like, utilizing conventional
techniques for forming such sharp tips.
[0033] The term "generally aligned" encompasses differences in
alignment on the order of up to 15 degrees, or slightly more.
[0034] The term "fibril" encompasses a single nanostructure
particle, nanowire or nanotube; a plurality of such particles,
nanowires or nanotubes; a single bundle of these objects; or a
plurality of bundles of the same.
[0035] Some of the objects having been stated hereinabove, and
which are addressed in whole or in part by the present subject
matter, other objects will become evident as the description
proceeds when taken in connection with the accompanying drawings as
best described hereinbelow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a schematic illustration of a
nanostructure-containing material.
[0037] FIG. 2 is a TEM image of a nanostructure-containing
material.
[0038] FIG. 3 is a schematic illustration of the
nanostructure-containing materials of FIG. 1 and FIG. 2 assembled
onto an object by disclosed methods.
[0039] FIGS. 4A and 4B are schematic illustrations of one
embodiment of a process conducted according to disclosed
methods.
[0040] FIGS. 5A and 5B are schematic illustration of a further
embodiment of a process conducted according to disclosed
methods.
[0041] FIG. 6 is a schematic illustration of an embodiment of an
apparatus for conducting the process.
[0042] FIG. 7 is a schematic illustration of another embodiment of
an apparatus for conducting the process.
[0043] FIGS. 8A and 8B are schematic illustrations of a further
embodiment of a process conducted according to the principles of
the present invention.
[0044] FIGS. 9A to 9E are magnified SEM images tips with attached
fibrils of carbon nanotubes or nanowires.
[0045] FIG. 10 is a schematic illustration of a fibril showing the
relationship of the various orientations when attached to a
tip.
[0046] FIG. 11 is a schematic illustration of a further object
formed according to another embodiment of the present subject
matter.
[0047] FIG. 12 is a schematic illustration of another object formed
according to an alternative embodiment of the present subject
matter.
[0048] FIG. 13 is still a further object formed according to
another embodiment of the present subject matter.
[0049] FIG. 14 is a schematic illustration of a separation
arrangement and/or technique according to the present subject
matter.
[0050] FIG. 15 is a schematic illustration of yet another
separation arrangement and/or technique according to the present
subject matter.
[0051] FIG. 16 is a plot of the emitted electron current vs.
applied voltage for a device formed according to the present
subject matter.
[0052] FIG. 17 is a plot of emitted electron current over time for
a device formed according to the present subject matter.
[0053] FIG. 18 is an illustration of a point electron field
emission source and emission pattern for a device formed according
to the present subject matter.
DETAILED DESCRIPTION
[0054] Methods performed consistent with the principles of the
present invention, and according to illustrative embodiments, along
with corresponding structures and devices, are described as
follows.
[0055] Generally, a method performed according to the principles of
the present invention can include a combination of some or all of
the following steps: (1) forming a suitable
nanostructure-containing material; (2) purifying the
nanostructure-containing material; (3) functionalizing the
nonstructural-containing material; (4) forming a solution or
suspension containing the nanostructure material; (5) selectively
adding "chargers" to the solution; (6) immersing electrodes in the
solution, the substrate or object upon which the nanostructure
material is to be deposited acting as one of the electrodes; (7)
applying a direct and/or alternating current thus creating an
electrical field between the electrodes for a certain period of
time thereby causing the nanostructure materials in the solution to
migrate toward and attach themselves to the substrate electrode;
(8) gradually drawing the object away from the liquid while keeping
the foremost surface of the deposited materials in contact with the
liquid to allow continuous deposition of the nanostructure
materials on the substrate; and (9) optional subsequent processing
of the coated substrate. It is within scope of the present
invention to limit the process solely to a combinations of some or
all of these general steps. However, it is also contemplated that
additional steps and processes may be included. Also comprehended
within the scope of the invention is to perform the method in the
specific sequence of all or a combination of some of the steps
described. Alternatively, the sequence of steps can differ from
that described herein.
[0056] The process begins with pre-formed, "raw"
nanostructure-containing material, preferably with a high aspect
ratio, or a nanotube-containing material, such as a carbon
nanotube-containing material. This raw material can comprise at
least one of single-walled carbon nanotubes, multi-walled carbon
nanotubes, silicon, silicon oxide, germanium, germanium oxide,
carbon nitrides, boron, boron nitride, dichalcogenide, silver,
gold, iron, titanium oxide, gallanium oxide, indium phosphide, or
magnetic particles such as Fe, Co, and Ni enclosed within
nanostructures. According to a preferred embodiment, the raw carbon
nanotube-containing material comprises single-walled carbon
nanotubes. The carbon nanotubes can be formed by any suitable
technique, such as the above-mentioned arc-discharge, laser
ablation and chemical vapor deposition methods.
[0057] It is also contemplated by the present invention that raw
materials be in the form of nanotube structures with a composition
of B.sub.xC.sub.yN.sub.z (B=boron, C=carbon, and N=nitrogen), or
nanotube or concentric fullerene structures with a composition
MS.sub.2 (M=tungsten, molybdenum, or vanadium oxide) can be
utilized. These raw materials can be formed by any suitable
technique, such as the above-mentioned arc-discharge technique.
[0058] The raw material can be fabricated according to a number of
different techniques familiar to those in the art, such as those
mentioned above in the Background
[0059] According to a further embodiment of the present invention,
magnetic tubes or nanowires can be used as the starting materials.
For example, as illustrated in FIG. 1, nanostructured materials
100, such as carbon nanotubes, can contain magnetic particles 110.
Several techniques to form these magnetic particles are envisioned.
One such technique involves the chemical vapor deposition method.
Magnetic particles such as Fe, Co, and/or Ni are used as catalysts
in the CVD technique. Hydrocarbons are used as the carbon source.
Under appropriate conditions, the magnetic particles will function
as nucleation sites for nanotube formation. The magnetic particles
110 are trapped either at the ends or tips of the nanotubes.
Alternatively, nanotubes, such as single or multi-walled carbon
nanotubes are formed as a starting material using a suitable
technique. The starting material is then processed to open the ends
of the nanotubes. The magnetic particles are then inserted into the
core of the nanotubes by either solution, electrochemical, vapor
phase, or solid state processing techniques. The ends can then be
closed by suitable methods, such as the formation of passivation
layers. Such techniques are described in greater detail in U.S.
Patent Publication No. US 2003/0180472, the disclosure of which is
incorporated herein by reference, in its entirety.
[0060] FIG. 2 is illustrative of such a magnetic particle described
above. As illustrated, for example, in FIG. 2, a multi-walled
carbon nanotube 200 is provided and includes a Co particle 210
deposed therein. As illustrated in FIG. 3, magnetic particles 310
can also be assembled or attached to a sharp tip, such as a probe
for an atomic force microscope, according to methods disclosed in
greater detail herein.
[0061] As further explained and illustrated herein, these magnetic
particles can be assembled or attached to a sharp tip, such as a
probe for an atomic-force microscope, according to methods
disclosed in greater detail below. By measuring the interaction
force between the magnetic tip fabricated as described above and an
object to be imaged, the magnetic structure of the object can be
imaged. One such object comprises magnetic data storage media.
Because the small size of the above-described magnetic particle, a
much higher resolution can be expected.
[0062] The raw nanostructure-containing material can be subjected
to purification. A number of techniques for purifying the raw
materials are envisioned. According to one preferred embodiment,
the raw material can be purified by reflux in a suitable solvent,
such as a combination of peroxide (H.sub.2O.sub.2) and water, with
an H.sub.2O.sub.2 concentration of 1-40% by volume, preferably
about 20% by volume H.sub.2O.sub.2, with subsequent rinsing in
CS.sub.2 and then in methanol, followed by filtration. According to
an exemplary technique, approximately 10 to 100 ml of peroxide is
introduced into the medium for every 1 to 10 mg of nanotubes in the
medium, and the reflux reaction is carried out at a temperature of
20-100.degree. C. (see, e.g. U.S. Pat. No. 6,553,096 (Ser. No.
09/679,303)).
[0063] According to another alternative, the raw
nanostructure-containing material is placed in a suitable liquid
medium, such as an acidic medium, an organic solvent, or an
alcohol; preferably methanol. The raw materials are kept in
suspension within the liquid medium for several hours using a
high-powered ultrasonic horn, while the suspension is passed
through a microporous membrane. In another embodiment, the raw
materials can be purified by oxidation in air or an oxygen
environment at a temperature of 200-700.degree. C. The impurities
in the raw materials are oxidized at a faster rate than the
nanotubes.
[0064] In yet another embodiment, the raw materials can be purified
by liquid chromatography to separate the nanotubes/nanowires from
the impurities.
[0065] The raw material is then optionally subjected to further
processing to shorten the nanotubes and nanotube bundles, such as
chemical etching or milling.
[0066] The purified raw material, regardless of whether subjected
to the above-described shortening process, can also optionally be
annealed at a suitable temperature, such as 100.degree.
C.-1200.degree. C. According to a preferred embodiment, the
annealing temperature is 100.degree. C.-600.degree. C. The material
is annealed for a suitable time period, such as approximately 1 to
60 minutes. According to a preferred embodiment, the material is
annealed for approximately 1 hour. The material is annealed in a
vacuum of about 10.sup.-2 torr, or at an even higher vacuum
pressure. According to a preferred embodiment, the vacuum is about
5.times.10.sup.-7 torr.
[0067] The above described "raw" or preformed material can now be
introduced into a solution for deposition onto an object or
substrate and/or for forming articles such as conducting wires.
[0068] According to an alternative embodiment, the purified
nanostructure-containing material is functionalized before
placement into the above-mentioned solution or suspension. For
example, the purified nanostructure-containing material can be
subjected to oxidation in a strong acid to render the material
hydrophilic. One exemplary process for doing this includes
introducing the purified nanostructure material into a solution of
H.sub.2SO.sub.4 and HNO.sub.3 for 10-24 hours while being subjected
to ultrasonic energy. Alternatively, purified
nonstructural-containing material can be chemically functionalized
by chemically or physically attaching chemical species to the outer
surfaces of the nanostructures so as to render the nanostructures
soluble or facilitate the formation of stable suspensions.
[0069] A suitable liquid medium is selected which will permit the
formation of a stable suspension of the raw nanostructure material
therein. According to a preferred embodiment the liquid medium
comprises at least one of water, methanol, ethanol, alcohol, and
dimethylformamide (DMF), and glycerol. Upon adding the raw material
to the liquid medium, the mixture can optionally be subjected to
ultrasonic energy or stirring using, for example, a magnetic
stirrer bar, in order to facilitate the formation of a stable
suspension. The amount of time that the ultrasonic energy is
applied can vary, but it has been found that approximately two
hours at room temperature is sufficient.
[0070] The concentration of raw material in the liquid medium can
be varied, so long as a stable suspension is formed. For example,
with a liquid medium comprising methanol, approximately 0.01 mg of
the raw material, such as single-walled carbon nanotubes, can be
present per ml of the liquid medium (0.01 mg/ml) and provide a
stable suspension. When the liquid medium comprises DMF,
approximately 0.4 to 0.5 mg of the raw material, such as
single-walled carbon nanotubes, can be present per ml of the liquid
medium (0.4 to 0.5 mg/ml) and provide a stable suspension. When
shortened carbon nanotubes are used, stable suspension can be
obtained at a higher concentration. For example, a stable
dispersion of approximately 0.1 mg/ml of shortened nanotubes in
water can be formed. In another example, single-walled carbon
nanotube bundles rendered hydrophilic by chemical treatments are
dispersed in water. The concentration of the nanotubes is adjusted
according to the desired deposition rate.
[0071] According to one embodiment, a charger is added to the
suspension in order to facilitate electrophoretic deposition. One
such preferred charger is MgCl.sub.2. Other possible chargers
include Mg(NO.sub.3).sub.2, La(NO.sub.3).sub.3, Y(NO.sub.3).sub.3,
AlCl.sub.3, and sodium hydroxide. Any suitable amount can be
utilized. Amounts ranging from less than 1% up to 50%, by weight,
as measured relative top to the amount of nanostructure-containing
material, are feasible. According to a preferred embodiment, the
suspension can contain less than 1% of the charger.
[0072] According to another embodiment of this invention, a small
amount of carbon nanotubes are dispersed in water. The suspension
is sonicated to achieve a uniform dispersion of the carbon
nanotubes. No charger is added to the suspension. Alternating
current dielectrophoresis technique is used to deposit the carbon
nanotubes onto the desired object. Under the AC field, the carbon
nanotubes are polarized and will migrate to certain field direction
depending on the field concentration, the frequency of the AC
field, and the dielectric constants of the liquid and the carbon
nanotubes.
[0073] A plurality of electrodes are then introduced into the
suspension. According to a preferred embodiment, two electrodes are
utilized. One of the electrodes comprises the object upon which the
nanostructure material is to be deposited. Any suitable object or
substrate material is envisioned, so long as it possesses the
requisite degree of electrical conductivity. According to a
preferred embodiment, the object is either metal or doped silicon.
According to another preferred embodiment at least one of the
electrodes comprises one or more sharp tips.
[0074] An alternating current, or a direct current, is applied to
the electrodes thereby producing an electrical field between the
electrodes. This causes the nanostructure material in the
suspension to migrate toward and attach to the substrate electrode.
According to one embodiment, the electrical field applied between
electrodes is 0.1 to 1000 V/cm, and a direct current of 0.1 to 200
mA/cm.sup.2 is applied for 1 second to 1 hour. According to a
further embodiment, an alternating current at a frequency of 10 Hz
to 10 GHz is applied.
[0075] According to a preferred embodiment, the above-described
electrophoretic deposition is carried out at room temperature.
[0076] The rate of deposition, as well as the structure and
morphology of the deposited nanostructure material can be
influenced by many factors. Such factors include: the concentration
of nanostructure material in the suspension, the concentration of
the charger material (e.g. MgCl.sub.2), if any, in the suspension,
the conductivity of the object, and control of the power source
(i.e. applied current, voltage conditions and frequency when
alternating current is applied).
[0077] For instance, when direct current is applied, the particular
electrode (i.e. anode or the cathode) to which the nanostructure
material migrates can be controlled through the selection of the
charger material. For example, the use of a negative charger, such
as sodium hydroxide (NaOH) imparts a negative charge to the
nanostructure material, thereby creating a tendency for the
nanostructure material to migrate towards the positive electrode
(cathode). Conversely, when a positive charger material is used,
such as MgCl.sub.2, a positive charge is imparted to the
nanostructure material, thereby creating a tendency for the
nanostructure material to migrate toward the negative electrode
(anode).
[0078] When an alternating current field is used, the particular
electrode to which the nanostructure material migrates can be
controlled through the selection of alternating current frequency,
dielectric constant of the liquid comparing to the nanostructure
material, the field concentration, and the geometry of the
electrodes.
[0079] The coated object, with nanostructure-containing material
deposited thereon may optionally be subjected to further
processing. For example, the coated object may be annealed to
remove the liquid medium. Such an annealing procedure may be
preferable, since removal of impurities such as residual suspension
medium improves the electrical and thermal contact and bonding
between the nanostructure material and the object. By way of
example, the coated substrate can be heated to a temperature of
approximately 100 to 1200.degree. C. for approximately 1 hour, and
then at approximately 800.degree. C. for 2 hours, both at a vacuum
of approximately 5.times.10.sup.-7 torr.
[0080] According to a further embodiment, the adhesion of carbon
nanotubes to the substrate can be further improved by incorporation
of adhesion promoting materials such as binders, carbon-dissolving
or carbide-forming metal and high temperature annealing. These
materials can be introduced by, for example, one of the following
processes: co-deposition of the nanostructures and particles of
adhesion promoting materials, sequential deposition, pre-deposition
of a layer of adhesion promoting materials, etc.
[0081] In one embodiment, binders such as polymer binders are added
to the suspension of the nanostructure-containing material, which
is then either stirred or sonicated to obtain a uniform suspension.
Suitable polymer binders include poly(vinyl butyral-co vinyl
alcohol-co-vinyl acetate) and poly(vinylidene fluoride). Suitable
chargers are chosen such that under the applied electrical field,
either DC or AC, the binder and the nanostructures would migrate to
the same electrodes to form a coating with an intimate mixing of
the nanostructures and the binder.
[0082] In another embodiment, small metal particles such as
titanium, iron, lead, tin, cobalt are mixed into the suspension of
the nanostructure-containing material. Suitable chargers are chosen
such that under the applied electrical field, the metal particles
and the nanostructures will migrate to the desired electrode to
form a uniform coating with an intimate mixing of the metal
particles and the nanostructures. After deposition, the coated
object is annealed in vacuum with a base vacuum pressure of
10.sup.-3 torr or greater for 0.1 to 10 hours. Preferably, the
diameter of the particles is smaller than 1 micrometer.
[0083] The binders or adhesion promoting materials can be added in
any suitable amount. Amounts ranging from 0.1 to 20% by weight,
measured relative to the amount of nanostructure-containing
material is envisioned.
[0084] In another embodiment, the object to be coated with the
nanostructures is first coated with at least one layer of
adhesion-promoting metal such as titanium, iron, lead, tin, cobalt,
nickel, tantalum, tungsten, niobium, zirconium, vanadium, chromium
or hafnium. The layer can be applied by techniques such as
electrochemical plating, thermal evaporation, sputtering or pulsed
laser deposition. After electrophoretic deposition of the
nanostructures, the film is annealed in vacuum with a base vacuum
pressure of 10.sup.-3 torr or greater for 0.1 to 10 hours.
[0085] The above-described processes are advantageously
well-adapted for high output and automation. These processes are
also very versatile. The methods of the present invention are
useful in producing articles including nanostructure-containing
materials, which have properties that make them beneficial for use
in a number of different applications. The processes can be used to
produce articles of micrometer or larger size as well as larger or
macromolecules including DNA.
[0086] Generally, methods of the present invention are especially
beneficial in providing nanotube and/or nanorod material for
incorporation into profilometers and probes for electron
microscopes, electron field emission cathodes for devices such as
x-ray generating devices, gas discharge tubes, lighting devices,
microwave power amplifiers, ion guns, electron beam lithography
devices, high energy accelerators, free electron lasers and flat
panel displays. For example, the electrophoresis method disclosed
can be used to deposit an individual or a bundle of carbon
nanotubes or nanowires selectively onto a sharp tip. This sharp tip
can be, for example, the tip used for microscopes including atomic
force microscopes (AFM), scanning tunneling microscopes (STM),
magnetic force microscopes (MFM), and chemical force microscopes
(CFM). The method can also lead to the production of continuous or
long fibers of carbon nanotubes and other nanostructure materials.
The method can further be used to make electrical interconnects
using nanostructure materials in electrical circuitry.
[0087] Specific techniques and their associated articles which are
consistent with the principles of the present invention, as
outlined above, will now be described for purposes of
illustration.
[0088] One such technique is illustrated in FIGS. 4A and 4B, where
a dilute suspension of nanostructure-containing material such as
nanotube or nanowire is first prepared. A counter electrode 410 is
first immersed into the suspension 420. A metal tip 430 is used as
the second electrode. A metal tip can be formed, for example, by
chemically etching a standard metal wire, such as a tungsten wire.
Metal tip 430 is first placed perpendicular to the surface of
suspension 420, with the sharp tip where the
nanostructure-containing material is to be deposited, just slightly
above the top surface of the suspension 420. The tip 430 is then
gradually moved towards the surface of the suspension 420. A power
source 440 is electrically connected to electrode 410 and tip 430.
A meter such as a current meter can be used to monitor the
electrical current between the counter electrode and the metal tip.
In addition, an appropriate optical magnification device can be
used to monitor the gap between the metal tip 430 and the
suspension surface 420. When the tip 430 touches the surface of the
suspension 420, the electrical current passing between the two
electrodes is detected. Depending on the concentration of the
nanostructure-containing material in the suspension and the
electrical field used, the tip 430 is allowed to stay in contact
with for a pre-determined time. Voltage is applied by direct or
alternating current via power source 440 between the two
electrodes. The voltage is then turned off after the predetermined
time and the tip 430 is raised to be above the suspension to stop
the deposition process. Alternatively, as illustrated in FIG. 4B,
the voltage can remain on while the tip 430 is gradually withdrawn
from suspension 420 thereby forming a wire or strand of nanotubes
450 attached to the tip 430. A device 460 which could be a stage or
a roller, for imparting rotational and/or linear movement to the
tip 430 may be incorporated. The speed by which the tip is
withdrawn is controlled according to the deposition rate of the
nanostructure material 450 such that only the outermost surface of
the nanostructure material that attached to the tip 430 is in
contact in the suspension 420. The metal tip 430 with a carbon
nanotube(s) 450 or other nanostructure attached thereto is vacuum
can also be annealed to increase the bonding between the tip and
the nanostructure.
[0089] Another embodiment of the invention is described by
reference to FIGS. 5A and 5B. A nanostrucutre-containing material
such as any of the materials previously disclosed, and preferably
carbon nanotubes, is dispersed in a suitable solvent such as water
at a concentration of about 0.01 g/L. The suspension is sonicated
such that the material is dispersed uniformly inside the liquid. A
droplet of the nanotube/water suspension 510 is applied to a metal
o-ring 520. The metal o-ring 520 is movable, for example, mounted
on a translation stage (not shown). The metal tip 530 is
electrically connected to a power supply 540 that is also connected
to the metal o-ring 520. Initially the metal tip 530 is not in
contact with droplet 510. In a preferred embodiment, the set-up is
placed under an optical microscope. A 20 V and 10 MHz AC signal is
established by an AC power source 540. Under the microscope, tip
530 is moved gradually towards droplet 510. After they are in
physical contact, they are held still for approximately 1 to 60
seconds under the electrical field. Tip 530 is then gradually
removed from the liquid. While the metal tip 530 is in physical
contact with the droplet 510, an electrical circuit is formed, with
metal tip 530 and the metal o-ring acting as the two electrodes. As
illustrated in FIG. 5B, under the AC field, the nanostructures 550
are polarized. Because the electrodes are asymmetric, the field is
not evenly distributed and concentrates around the tip. The
nanostructures 550, e.g.--nanotubes, are thereby drawn to the tip
530 as illustrated in FIG. 5B. By adjusting the nanostructure
concentration and the deposition time, a single or group of
nanostructures, such as a single carbon nanotube or carbon nanotube
bundle(s) can be attached to the metal tip.
[0090] As one possible variation on the above-described procedure,
a direct-current electrical field rather than an
alternating-current field is established between the tip 530 and
the counter electrode or o-ring 520. A suitable "charger" such as
MgCl.sub.2 is added to the suspension. Under the electrical field,
the nanostructures 550 or nanotubes migrate toward the tip 530 and
attached to the surface to the tip.
[0091] In another embodiment of the invention, a wire comprising
nanostructures, preferably carbon nanotubes, is fabricated by
techniques similar to the ones described above in connection with
FIGS. 4A, 4B, 5A and 5B. As shown in FIGS. 4B and 5B, when an AC
field is established, the nanostructures 450, 550 or carbon
nanotubes dispersed in the liquid are polarized and aligned to the
field direction. Under the asymmetrical electrical field, the
nanostructures 450, 550 then migrate toward the sharp electrode tip
430, 530 where the field is higher. They will then attach to the
tip 430, 530, which is initially the outermost surface of the
electrodes. As more nanostructures 450, 550 are drawn in and
attached, the outermost nanostructure 450, 550 or carbon nanotube
that is attached becomes the outermost surface of the electrodes. A
long continuous wire of nanostructure material can be formed by the
above process (see, e.g.--FIG. 9). By optimizing the rotation rate,
the electrical field, the frequency of the electrical field, and
the concentration of the liquid dispersed with the nanostructure
material, a roll of continuous fiber of nanostructure materials can
be fabricated. The length of the wire is controlled by parameters
such as the current and the concentration of nanostructures in the
suspension used. After the deposition, the wire can be further
processed, such as by annealing, to increase the mechanical
strength or modify the functionality.
[0092] It is further envisioned that the processes for further
treatment of the fiber can be integrated into the deposition
process described above. For example, it is possible that the wire
or fiber drawn from the liquid can run through a furnace that is
placed between device 460 and the liquid such that the properties
of the fiber is enhanced during the formation process in one step.
It is also envisioned that the fiber can run through a bath
containing a different type of material such as a polymer melt or a
polymer solution. By drawing the fiber of nanostructure material
first from the liquid then moving through the liquid, the polymer
will impregnate and coat the fiber and thus form a composite of
nanostructure material and polymer.
[0093] An illustrative embodiment of a process and/or apparatus of
the type generally described above is illustrated in FIG. 6.
According to the illustrated embodiment, an electrode 610 is placed
in a container 620. The container 620 is then filled with a medium
containing nanostructure materials 630. A continuous string or
strand of nanostructure-containing material 640 can be drawn from
the medium 630 as follows. A counterelectrode in the form of a roll
650 is initially located in contact with the surface of the medium
630. The counterelectrode 650 can then moved in a direction away
from the top surface of the medium 630, while a power source 660
supplies an alternating current potential between the electrode 610
and the counterelectrode 650. As described in detail herein, the
nanostructurecontaining materials contained in the medium 630
attach themselves to the counterelectrode 650, and then to each
other and can form a continuous string or strand 640. The string or
strand of nanostructure-containing material 640 can optionally be
passed through a heating device or furnace 670 which can be
utilized to drive off liquids, etc. which were contained in the
medium 630. According to further alternatives, a series of rolls
680 can be utilized to further transport the string or strand 640
through various additional processing steps. For example, as
illustrated in FIG. 6, the string or strand 640 can be passed
through a second medium 690 which can contain a second material,
which is desired to be combined with the nanostructure-containing
string or strand 640. For example, the medium 690 can contain a
polymer melt, which can impregnate the nanostructure-containing
material string or strand 640, thus forming a composite fiber. The
medium 690 is located in a container 695. The strand 690 is then
withdrawn from the container 695 and transported to additional
steps of the process, or is a accumulated on a final roll 680. The
nanostructure-containing material string or strand 640 can
comprise, or consist essentially of, single-walled carbon
nanotubes. Alternatively, the nanostructure-containing material can
comprise or consist essentially of any of those
nanostructure-containing materials previous mentioned in the
present disclosure.
[0094] In yet another embodiment of the present subject matter, a
batch-type process for simultaneous processing of multiple sharp
tips at the same time is envisioned. As illustrated in FIG. 7, a
group of tips 710 are either formed directly by etching from a
starting material or mounted on a holder 720. A suspension of the
nanostructures 730, such as carbon nanotubes is placed inside a
container 740. A flat metal electrode 750 is inside the same
container. An alternating current power source 760 is connected to
the holder 720 and the flat electrode 750. An AC field is
established between the tips 710 and the flat electrode 750. An
example of the frequency is 1 KHz to 10 GHz. The container 740 is
raised (or the tips 710 are lowered) until the tips 710 are in
contact with the surface of the suspension 730. After a fixed dwell
time (such as 1 second to 10 second), the tips 710 are separated
from the suspension 730. By controlling the current, the
concentration nanostructure-containing material in the suspension,
and the dwell time, it is possible that one nanostructure or carbon
nanotube or a group of nanostructures or carbon nanotube bundle(s)
is attached to each tip. After the deposition, the tips 710 can be
further processed to increase the bonding strength. The coated tips
can be used as the probes for AFM, STM or other scanning probe
microscopes, as microelectrodes for sensors, or as electrochemical
cells.
[0095] In another embodiment, wires comprising nano-objects are
formed in an assembled into predetermined pattern to make
electrical connections between different components in an
electrical circuitry. One particular example is illustrated in
FIGS. 8A and 8B. A droplet 850 of liquid containing nano-objects
such as carbon nanotubes is placed in the area defined by four
components 810, 820, 830 and 840, which can be components on a
circuit board. An electrical field such as an AC field is first
established between two components, such as 810 and 830 until a
wire 860 comprising the nano-objects or nanotubes is formed and
connects 810 and 830. A field is then established between 820 and
840 until a wire comprising the said nano-objects is formed and
connects 820 and 840. As a result, a cross-electrical connection
between the four components 810-840 is established.
[0096] FIGS. 9A to 9E are SEM images of sharp metallic tips 910
with attached carbon nanotubes or nanowire 920 formed according to
the present subject matter. FIG. 9C is a scanning electron
microscope (SEM) image of a wire comprising single-wall carbon
nanotube (SWNT) bundles that are attached to a tungsten tip by a
process of the present subject matter. According to exemplary
embodiments, the diameter of the fibril can be less than 1.0
micron, and preferably 0.5 nm to 10 microns, preferably less than 1
micron. The length of the fibril can be 50 nm to 50 microns,
preferably less than 30 microns. Tips constructed according to the
present subject matter are capable of stable electron emission
current on the order of 1 to 10 microamperes FIG. 10 is a schematic
illustration of a fibril F having a generally longitudinal axis
A.sub.F is attached to the generally conical sharp tip T having
cone axis A.sub.C, in a manner such that the fibril F is generally
aligned along the cone axis A.sub.C. That is, the axis A.sub.F and
the axis A.sub.C define an angle .alpha. relative to each other
that is less than approximately 15 degrees, and preferably, less
than approximately 10 degrees.
[0097] Tips with attached carbon nanotubes fabricated according to
the present subject matter can be used in for scanning probe
microscopes including atomic force microscopes, scanning tunneling
microscopes, magnetic force microscopes, and chemical force
microscopes. Because of the large aspect ratios and small diameters
of nanostructured materials, such as the carbon nanotubes, a better
imaging quality can be obtained. For example, FIG. 11 illustrates a
typical atomic force microscope probe 1110 with nanostructures 1120
assembled at the tip thereof. FIG. 12 illustrates a scanning
electron microscope tip 1210 having nanostructures 1220 disposed
thereon. FIG. 13 is a Spindt-type electron field emitter 1310
having nanostructures 1320 assembled at the tip thereof.
[0098] According to another aspect of the present subject matter,
techniques and arrangements for separating nanostructure materials
based on differences in their properties is provided. Materials
containing a mixture of one or more nanostructure-containing
materials can be separated. For example, metallic and
semiconducting nanostructures, such as carbon nanotubes, can be
separated into two sub-groups. For example, one sub-group contains
only metallic carbon nanotubes, the other sub-group contains only
semiconducting carbon nanotubes. In one particular example,
pre-fabricated single wall carbon nanotubes are dispersed in water.
More preferably the single wall carbon nanotubes are first purified
by a suitable technique to remove impurity phases. An asymmetric AC
field is established in the liquid by applying an AC voltage
between two electrodes. One way to establish the asymmetric
electrical field is to use two electrodes with different
geometries. For example, one electrode has a planar geometry and
the other has a sharp protrusion. The frequency of the AC field is
adjusted. Because of the difference in electronic properties, the
polarization of the metallic and semiconducting carbon nanotubes
will have a different frequency dependence. Under an optimal
frequency, the metallic carbon nanotubes will move faster towards
the electrode with a protrusion than the semiconducting carbon
nanotubes. In another situation, it is possible to adjust the
frequency and the liquid used such that under certain conditions,
the metallic carbon nanotubes move towards one electrode while the
semiconducting carbon nanotubes move toward the opposite electrode.
After applying the electrical field for certain time, each group of
material can be collected from the liquid on the surfaces of the
opposing electrodes. The method described herein can also be used
to separate carbon nanotubes from magnetic particles that are
present in the raw materials. The method is not limited to carbon
nanotubes. It can be used to separate other nano-objects according
to their electronic properties and/or geometry.
[0099] Another illustrative embodiment of a separation
technique/arrangement according to the present subject matter is
illustrated in FIG. 14.
[0100] The arrangement 1400 includes a plurality of electrodes
1402, 1404 arranged according to a particular pattern. According to
the illustrative embodiment, the electrodes 1402, 1404 are disposed
at 90.degree. angles relative to one another. However, it should be
recognized, that numerous other electrode patterns are
contemplated. A power source 1406 supplies an alternating current
to the electrodes 1402, 1404. Non-uniform electrical fields results
due to fringing effects caused by the flow of alternating current
through the electrodes 1402, 1404. In the illustrative embodiment,
an area of a relatively high strength electrical field 1408 is
produced. Further, areas of relatively low electrical field
strength 1410 are also produced.
[0101] The arrangement 1400 can be placed within a channel, or
chamber, schematically illustrated at 1412. A mixture containing a
first type of particle 1416, and a second type of particle 1418,
can be introduced into the chamber or channel 1412. The mixture
containing at least two distinct types of particles can take many
forms. For example, at least one of particles 1416 and 1418 can
comprise a nanostructure-containing material. According to one
example, the particles 1416 and 1418 are disposed within a liquid
medium. According to a further example, the mixture includes a
first type of nanostructure-containing material particle 1416, a
second distinct type of nanostructure-containing material particle
1418, disposed in a liquid medium. The liquid medium can be
introduced into the chamber or channel 1412 at one end thereof, and
caused to flow, for example, generally in the direction indicated
by the arrows F. Due to the differences in their properties, the
particles 1416 and 1418 are polarized, or affected by the applied
electrical fields in different manners. For example, the first type
of particle 1416 may be negatively polarized by the applied
electrical field, while the second type of particle 1418 may be
positively polarized via application of the electrical field. Thus,
due to the difference in polarities, the second type of particle
1418 is retained and trapped within the relatively high strength
electrical field area 1408, and may adhere to the electrodes 1402,
1404. By contrast, the first type of particle 1416, due to its
polarity is not attracted to the relatively high strength
electrical field 1408, but instead is allowed to flow within the
relatively low strength electrical field areas 1410 in the
direction indicated by arrows F of FIG. 14. In this manner, the
first particles 1416 can flow downstream within the chamber or
channel 1412 and can be collected at a location remote from the
electrodes. The second type of particles 1418, are contained within
the electrical field 1408, and/or adhered to the electrodes 1402,
1404, and can thus be recovered therefrom.
[0102] Another illustrative embodiment of a separation
technique/arrangement is contained in FIG. 15. The arrangement 1500
includes a plurality of patterned electrodes 1502, 1504, 1506, and
1508. An alternating current is applied to these electrodes, which
according to one embodiment is of equal magnitude with respect to
all of these electrodes. However, the current is applied in a
manner such that it is out of phase with respect to the other
electrodes by 90.degree.. The arrangement 1500 may then be placed
in a mixture including at least a first and second type of particle
1510, 1512. The mixture may comprise any of those types of mixtures
previously described. Due to the difference and effects caused by
the applied electrical field on the first and second particles
1510, 1512, the interaction between these particles and the
electrodes 1502, 1504, 1506 and 1508 is such that relative motion
is imparted to each of the particles. The particular direction of
motion depends on the polarizability of each of the two different
types of particles 1510, 1512. Thus, one of the particles,
e.g.--1510 is caused to move toward the center of the arrangement
1500, as illustrated in FIG. 15, while the other particle 1512 is
caused to move in a direction away from the center of the
arrangement 1500. Thus, the two groups of particles are separated
in this manner. The first group of particles 1510 can be collected
from the center of the arrangement 1500, and may actually adhere to
the electrodes 1502, 1504, 1506, and 1508. The second group of
particles 1512 migrate in a direction away from the arrangement
1500, and thus can be recovered at a location remote therefrom.
[0103] The principles of the present subject matter will now be
further described through reference to the following exemplary,
non-limiting example.
EXAMPLE
[0104] A tungsten tip prepared by chemical etching is used as the
working electrode and a small metal plate or ring as the counter
electrode. They are mounted on separate translation stages and were
placed under an optical microscope. Single-wall carbon nanotubes
were first purified, etched to .about.2 .mu.m in bundle length and
rendered hydrophilic by chemical oxidation. They were then
dispersed in de-ionized water. A droplet of the nanotube suspension
was placed inside the metal ring. To begin the deposition, an AC
field (10V at 2 MHz) was first applied between the two electrodes.
The tip electrode was translated horizontally to contact the
nanotube/water suspension and was gradually withdrawn under the
electrical field until a fibril with the desired length was
formed.
[0105] After deposition, the metal tip with the attached nanotube
fibril is annealed at 10.sup.-6 torr vacuum at 800.degree. C. for
one hour to remove residual solvent and to increase the adhesion of
the nanotubes to the metal tip and the bonding between the
nanotubes.
[0106] The electron field emission characteristics of these tips
were measured using pointplane geometry with a tip mounted on a
micrometer head as the cathode and an opposing metal plate as the
anode. The distance between the two was adjusted by translating the
micrometer head and was fixed to .about.200 mm. The set-up was
placed inside a vacuum chamber at 5.times.10.sup.-7 Torr base
pressure. The total emission current from the tip was collected as
a function of the applied voltage. FIG. 16 and inset show the data
collected from a tip with a .about.50 nm tip diameter. The emission
current-voltage characteristics exhibited the classic
Fowler-Nordheim type behavior with a linear relation between
In(I/V.sup.2) and 1/V. The emission current reached 5 mA at less
than 1000 V applied voltage, which gives an estimated current
density of 2.5.times.10.sup.5 A/cm.sup.2.
[0107] The emission stability was measured by monitoring the
variation of the emission current with time for 10 hours
continuously in DC mode at a fixed voltage. FIG. 17 shows the data
from a tip (.about.50 nm in diameter) measured sequentially at
three different voltages. The emission current was stable at 1 mA
and 2.8 mA when the applied voltage was fixed at 800 V and 1000 V,
respectively, with no overall decay of the emission current. With a
ballast resistor in series with the cathode, the local current
fluctuation was calculated to be 5% at 1 mA and 2.5% at 2.8 mA. The
emission current from the same sample reached 5.5 mA when the
voltage was further increased to 1200 V, but was then suddenly
dropped to zero after one hour. No current was detected even at a
higher voltage. SEM examinations revealed catastrophic failure from
fracture of the W wire and disappearance of the entire tip as shown
in FIG. 17. This is attributed to resistive heating at the W and
fibril interface. In another prepared tip, a stable current of 6.5
mA was obtained. The current fluctuation of this sample without the
ballast resistor was about 12%, comparable to value reported from
an un-ballasted individual single-wall carbon nanotube.
[0108] As illustrated in FIG. 18, to record the emission pattern
from the tip 1800, the metal anode used in the above measurements
was replaced with a phosphor-coated ITO (indium-tin-oxide) glass
1810. The image 1820 formed on the phosphor screen by the emitted
electrons was recorded by a CCD camera placed outside the vacuum
chamber. The emission images 1820 were obtained from a multi-walled
nanotube/tip. A single emission spot was observed in all operating
voltages, consistent with the SEM results which showed that there
is only one close-packed carbon nanotube anchored on each tungsten
tip. At 1400 V the diameter of the bright spot on the phosphor
screen is about 200 mm giving a beam divergence angle of
50.degree.. The large spread angle results from the electrical
field distribution in the point-plane geometry rather than the
intrinsic divergence angle of the field-emitted electrons from the
nanotubes.
[0109] It will be understood that various details of the present
subject matter may be changed without departing from the scope of
the present subject matter. Furthermore, the foregoing description
is for the purpose of illustration only, and not for the purpose of
limitation.
* * * * *