U.S. patent application number 11/441261 was filed with the patent office on 2008-08-21 for method for assembling nano objects.
This patent application is currently assigned to University of North Carolina at Chapel Hill. Invention is credited to Soojin Oh, Hideo Shimoda, Otto Z. Zhou.
Application Number | 20080199626 11/441261 |
Document ID | / |
Family ID | 28040470 |
Filed Date | 2008-08-21 |
United States Patent
Application |
20080199626 |
Kind Code |
A1 |
Zhou; Otto Z. ; et
al. |
August 21, 2008 |
Method for assembling nano objects
Abstract
A method for the self assembly of a macroscopic structure with a
pre-formed nano object is provided. The method includes processing
a nano object to a desired aspect ratio and chemical functionality
and mixing the processed nano object with a solvent to form a
suspension. Upon formation of the suspension, a substrate is
inserted into the suspension. By evaporation of the solvent,
changing the pH value of the suspension, or changing the
temperature of the suspension, the nano objects within the
suspension deposit onto the substrate in an orientational order. In
addition, a seed crystal may be used in place of the substrate
thereby forming single-crystals and free-standing membranes of the
nano-objects.
Inventors: |
Zhou; Otto Z.; (Chapel Hill,
NC) ; Shimoda; Hideo; (Chapel Hill, NC) ; Oh;
Soojin; (Dublin, CA) |
Correspondence
Address: |
JENKINS, WILSON, TAYLOR & HUNT, P. A.
Suite 1200 UNIVERSITY TOWER, 3100 TOWER BLVD.,
DURHAM
NC
27707
US
|
Assignee: |
University of North Carolina at
Chapel Hill
|
Family ID: |
28040470 |
Appl. No.: |
11/441261 |
Filed: |
May 25, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10103803 |
Mar 25, 2002 |
7147894 |
|
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11441261 |
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Current U.S.
Class: |
427/430.1 ;
313/310; 427/115; 427/402; 427/435; 427/58; 427/77 |
Current CPC
Class: |
B05D 1/18 20130101; B82Y
30/00 20130101; Y10S 427/102 20130101; Y10S 977/847 20130101; Y10S
977/882 20130101; B05D 1/204 20130101; B82Y 40/00 20130101; Y10S
977/848 20130101 |
Class at
Publication: |
427/430.1 ;
427/402; 427/115; 427/58; 427/435; 427/77; 313/310 |
International
Class: |
B05D 1/18 20060101
B05D001/18; B05D 1/36 20060101 B05D001/36; B05D 5/12 20060101
B05D005/12; H01J 9/02 20060101 H01J009/02 |
Goverment Interests
GOVERNMENT INTEREST
[0002] At least some aspects of this subject matter were made with
Government support under the sponsorship of the Office of Naval
Research, Contract No. N00014-98-1-0597 and by a grant from the
National Aeronautics and Space Administration (NAG-1-01061). The
Government may have certain rights in this subject matter.
Claims
1. A method for assembling nano objects onto a macroscopic
structure, the method comprising: (a) processing nano objects to a
desired aspect ratio and chemical functionality; (b) admixing the
processed nano objects with a solvent to form a suspension, the
suspension having a concentration, a temperature and a pH level;
(c) submersing a substrate into the suspension; and (d) changing
the concentration to control deposition of the nano objects onto
the substrate, thereby assembling the macroscopic structure onto
the substrate.
2. The method of claim 1, wherein the nano objects comprise single
wall carbon nanotubes (SWNTs) or single wall carbon nanotube
bundles.
3. The method of claim 1, wherein the nano objects comprise multi
wall carbon nanotubes (MWNTs).
4. The method of claim 1, wherein the nano objects comprise a
mixture of single wall carbon nanotubes (SWNTs) and multi wall
carbon nanotubes (MWNTs).
5. The method of claim 2, wherein the operation of processing the
nano objects further comprises: (e) synthesizing the carbon
nanotubes; (f) purifying the carbon nanotubes; and (g) modifying
both a length of the carbon nano tubes and at least one chemical
property of the carbon nano tubes.
6. The method of claim 2, wherein the operation of processing the
nano objects further comprises: (e) synthesizing the SWNTs by a
process selected from the group consisting of laser ablation, arc
discharge, chemical vapor deposition, and pyrolysis; (f) purifying
the SWNTs by selective oxidation and/or filtration; and (g)
reducing an aspect ratio of the SWNTs by sonication in acid or
mechanical cutting.
7. The method of claim 2, wherein the operation of processing the
nano objects further comprises: (e) synthesizing the SWNTs by a
process selected from the group consisting of laser ablation, arc
discharge, chemical vapor deposition, and pyrolysis; (f) purifying
the SWNTs by selective oxidation and/or filtration; and (g)
chemically modifying the SWNTs.
8. The method of claim 6, wherein a length of the processed SWNTs
is in a range between about 0.1 micron and about 10 microns.
9. The method of claim 2, wherein the solvent is selected from the
group consisting of water, alcohol, and combinations thereof.
10. The method of claim 9, wherein the concentration of the
suspension of carbon nanotubes in water is in a range between about
0.01 grams of carbon nanotube per liter of water and about 10.0
grams of carbon nanotube per liter of water.
11. The method of claim 2, wherein the substrate comprises a
hydrophilic region and a hydrophobic region such that the SWNTs
deposit on the hydrophilic region of the substrate.
12. The method of claim 11, wherein the substrate comprises
hydrophilic glass patterned with a hydrophobic material.
13. The method of claim 12, wherein the hydrophobic material is
selected from the group consisting of a polystyrene, a photoresist,
a mono-layer of hydrophobic functional groups, and combinations
thereof.
14. The method of claim 1, wherein the method further comprises
patterning a surface of the substrate such that the substrate
surface comprises a first region and a second region where the
first region has an affinity to the nano objects and the second
region has no affinity to the nano objects, where changing the
concentration of the suspension deposits the nano objects on the
first region of the substrate surface.
15. The method of claim 1, wherein the substrate has a planar
configuration.
16. The method of claim 1, wherein the substrate has a curved
configuration.
17. The method of claim 1, wherein the operation of changing the
concentration of the suspension further comprises changing the
concentration of the suspension by gradual evaporation of the
solvent such that the nano objects deposit on the substrate along
an air/liquid/substrate triple line of the substrate.
18. The method of claim 17, wherein the operation of submersing the
substrate into the suspension further comprises submerging the
substrate into the suspension in a vertical orientation relative to
the suspension.
19. The method of claim 18, wherein longitudinal axes of the nano
objects align in a direction of the air/liquid/substrate triple
line.
20. The method of claim 1, further comprising: (e) removing the
substrate from the suspension; (f) depositing a second material
onto the nano objects assembled on the substrate; and (g) repeating
operations (c) through (e) thereby forming a multi layer
structure.
21. The method of claim 20, wherein the second material is selected
from the group consisting of a metal, a semiconductor, a polymer,
an inorganic material, an organic material, a biological material,
and combinations thereof.
22. A multilayer structure prepared by the method of claim 20.
23. The multilayer structure of claim 22, wherein the multi layer
structure is selected from the group consisting of an electrode, an
electrolyte for a battery, and a fuel cell.
24. The multilayer structure of claim 22, wherein the multi-layer
structure is selected from the group consisting of a capacitor, a
super-capacitor, an electronic device, and a sensor.
25. A method for assembling pre-formed nano objects into a
macroscopic structure, the method comprising: (a) processing the
nano objects, wherein the processing of the nano object comprises:
(i) synthesizing the pre-formed nano objects, where the pre-formed
nano objects are single wall carbon nanotube (SWNT) bundles; (ii)
purifying the SWNT bundles by reflux in a hydrogen peroxide
solution and filtering the SWNT bundles; and (iii) cutting the
purified SWNT bundles by reacting the SWNTs with HNO.sub.3 and/or
H.sub.2SO.sub.4, and with ultra-sonication; (b) forming a
suspension by admixture of the nano objects with a solution; (c)
inserting a substrate into the suspension; and (d) gradually
removing the substrate from the suspension during which the nano
objects from the suspension deposit on the surface of the
substrate.
26. The method of claim 25, wherein the substrate comprises a
material selected from the group consisting of glass, quartz,
aluminum, chromium, tin, and silicon or any other substrate with a
hydrophilic coating on a surface of the substrate.
27. The method of claim 25, the substrate further comprising a
hydrophobic coating, the hydrophilic coating and the hydrophobic
coating forming a pattern on the substrate wherein the processed
pre-formed nano objects form onto the substrate at the hydrophilic
coating thereby forming a pattern corresponding to the pattern
formed by the hydrophilic coating and the hydrophobic coating.
28. A method for assembling pre-formed nano objects into a
macroscopic structure, the method comprising: (a) processing the
nano objects; (b) forming a suspension by admixture of the nano
objects with a solution; (c) inserting a substrate into the
suspension; and (d) gradually removing the substrate from the
suspension during which the nano objects from the suspension
deposit on the surface of the substrate, wherein the nano objects
form on an air/liquid/substrate triple line of the substrate.
29. A method for assembling nano objects into a free-standing
macroscopic structure, the method comprising: (a) processing the
nano objects such that the nano objects disperse or dissolve in a
solvent; (b) admixing the processed nano objects with the solvent
to form a stable suspension or a solution in a container that does
not attract the processed nano objects; (c) submersing a
seed-crystal into the suspension; and (d) changing a concentration
of the suspension to bring the suspension into super-saturation
such that the processed nano objects assemble into a free-standing
macroscopic structure.
30. The method of claim 29, wherein the nano objects assemble into
the macroscopic structure around the seed crystal such that a
structure of the macroscopic structure is the same as a structure
of the seed crystal.
31. The method of claim 29, wherein a thickness of the freestanding
macroscopic structure is in a range between about 1 nanometer to
about 10 microns.
32. The method of claim 29, wherein an area of the free-standing
macroscopic structure is in a range between about 1 micron.times.1
micron and about 10 cm.times.10 cm.
33. The method of claim 29, wherein the nano objects are selected
from the group consisting of single wall carbon nanotubes and multi
wall carbon nanotubes.
34. The method of claim 29, wherein the nano objects comprise
nanowires/nanorods, and wherein the nanowires/nanorods comprise at
least one material selected from the group consisting of carbon,
silicon, germanium, oxygen, boron, nitrogen, sulfur, phosphorus,
and metal.
35. A free-standing macroscopic structure assembled by the method
of claim 29.
36. A method for fabricating electron field emission cathodes by
self-assembly of pre-formed nano objects, the method comprising:
(a) processing the nano objects such that the nano objects disperse
or dissolve in a solvent; (b) admixing the processed nano objects
with the solvent to form a suspension; (c) submersing a substrate
into the suspension; and (d) changing a concentration of the
suspension to assemble the processed nano objects on certain
regions of the substrate surface thereby fabricating the electron
field emission cathode, wherein the substrate comprises a region A
and a region B where the region A attracts the processed nano
objects and the region B does not attract the processed nano
objects where the nano objects deposit on the region A upon
changing the concentration of the suspension.
37. The method of claim 36, wherein the nano objects comprise
carbon nanotubes.
38. The method of claim 36, wherein a smallest lateral dimension of
the region A is at least about 100 nm.
39. The method of claim 36, wherein the substrate comprises
hydrophilic glass.
40. The method of claim 39, further comprising coating the
substrate with a region of a hydrophobic polymer such that the
region of the hydrophobic polymer forms the region B and an
uncoated region of the substrate forms the region A.
41. The method of claim 40, further comprising removing the region
of the hydrophobic polymer after deposition of the nano
objects.
42. The method of claim 40, wherein the hydrophobic polymer is
removed by washing in a solvent selected from the group consisting
of acetone, methanol, ethanol, buffered hydrofluoric acid, and
combinations thereof.
43. The method of claim 36, wherein the process further comprises
annealing the substrate deposited with the nano-objects at a
temperature in a range between about 100.degree. C. and about
500.degree. C. in a vacuum.
44. The method of claim 36, wherein the nano objects are single
wall carbon nanotube bundles with an aspect ratio larger than about
10 and a bundle length in a range between about 300 nm and about 1
micron.
45. The method of claim 36, wherein the field emission cathode has
a threshold electrical field in a range between about 1 V/micron
and about 5V/micron for an emission current density of about 1
mA/cm.sup.2.
46. A field emission cathode fabricated by the method of claim
36.
47. A method for assembling nano objects onto a macroscopic
structure, the method comprising: (a) processing nano objects to a
desired aspect ratio and chemical functionality; (b) admixing the
processed nano objects with a solvent to form a suspension, the
suspension having a concentration, a temperature and a pH level;
and (c) coating the suspension onto a substrate thereby assembling
the macroscopic structure onto the substrate.
48. The method of claim 47, wherein the operation of coating the
suspension onto a substrate further comprises a coating procedure
selected from the group consisting of spin coating, spraying, and
electrophoresis.
49. The method for assembling nano objects onto a macroscopic
structure as recited in claim 47, wherein the substrate has a first
region and a second region where the first region attracts the nano
objects and the second region does not attract nano objects.
Description
RELATED APPLICATIONS
[0001] This application is a divisional patent application from and
which claims the benefit to the filing date of U.S. patent
application Ser. No. 10/103,803, filed Mar. 25, 2002, the
disclosure of which is incorporated herein by reference in its
entirety.
TECHNICAL FIELD
[0003] The present subject matter relates generally to methods to
assemble nano objects into functional structures.
BACKGROUND ART
[0004] In the description of the background of the present subject
matter that follows reference is made to certain structures and
methods. 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 subject
matter.
[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;
inorganic and organic nanowires/nanorods composed of either single
or multiple elements such as Si, Ge, metals, oxides such as
SiO.sub.x, GeO.sub.x; carbides such as silicon carbides; nitrides,
borides, or hollow 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. One of the common features of
nanostructure materials is the dimension of their basic building
blocks. A single nanoparticle or a nanotube or a nanowire has a
dimension that is less than 1 micron in at least 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. No. 6,280,697 to Zhou et al. entitled
"Nanotube-Based High Energy Material and Method," the disclosure of
which is incorporated herein by reference, in its entirety,
discloses the fabrication of carbon-based nanotube materials and
their use as a battery electrode material.
[0007] application Ser. No. 09/296,572 entitled "Device Comprising
Carbon Nanotube Field Emitter Structure and Process for Forming
Device" the disclosure of which is incorporated herein by
reference, in its entirety, discloses a carbon nanotube-based
electron emitter structure.
[0008] application Ser. No. 09/351,537 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.
[0009] 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.
[0010] U.S. Pat. No. 6,334,939 to Zhou et al. (application Ser. No.
09/594,844) 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.
[0011] application Ser. No. 09/679,303 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.
[0012] application Ser. No. 09/817,164 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-promoting layer 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.
[0013] application Ser. No. 09/881,684 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 emission
properties thereof.
[0014] As evidenced by the above, nanostructure materials, such as
carbon nanotubes, possess promising properties. Carbon nanotubes
(CNTs) are one type of nano objects. CNTs are cylindrical carbon
structures with a length between 0.1 .mu.m and 100 .mu.m and a
diameter between 0.4 nm and 50 nm (see, e.g. M. S. Dresselhaus, G.
Dresselhaus, and P. Avouris, eds. Carbon Nanotubes: Synthesis,
Structure, Properties, and Applications. Topics in Applied Physics.
Vol. 80. 2000, Springer-Verlag). CNTs can have either a single
graphite shell per nanotube in which case CNTs are called
single-wall carbon nanotubes (SWNTs). CNTs may also have concentric
multi-shell graphite structures in which case CNTs are called
multi-wall carbon nanotubes (MWNTs). Carbon nanotubes have
exceptional mechanical properties with high elastic modulus, high
ductility, high electrical and high thermal conductivity, thermal
stability and chemical stability. CNTs are excellent electron field
emitters since CNTs have a large aspect ratio and a sharp tip.
(See, e.g. P. M. Ajayan and O. Zhou, in "Topics in Applied Physics,
80," M. S. Dresselhaus, G. Dresselhaus, and P. Avouris, Editors.
2000, Springer-Verlag). In particular, carbon-nanotube materials
exhibit low emission threshold fields as well as large emission
current densities. Such properties make them attractive electron
field emitters for microelectronic applications, such as lighting
elements, field emission flat panel displays, gas discharge tubes
for over voltage protection and x-ray generating devices. Other
applications of carbon nanotubes include but limited to: sensors,
composites, shielding materials, detectors, electrodes for
batteries, fuel cells, small conduction wires, small cylinders for
storage, etc.
[0015] Carbon nanotubes, nanowires and nanorods, nanoparticles are
typically fabricated by techniques such as laser ablation, arc
discharge, and chemical vapor deposition methods. In some cases
they can also be made via solution or electrochemical synthesis.
However, in most cases, the as-synthesized materials cannot be
utilized without further processing. For example, carbon nanotubes
produced by the laser ablation and arc discharge techniques are in
the form of porous mats and powders. Device applications require
assembling these nano objects into ordered, patterned films,
membranes, crystals on desired supporting surfaces and the
pre-determined locations. In addition, it is often advantageous to
assemble elongated nano objects such as the carbon nanotubes into
orientationally ordered macroscopic structures which provide
properties such as anisotropic electrical, mechanical, thermal,
magnetic and optical properties.
[0016] The conditions used to assemble the nano objects need to be
compatible with the conditions used for device fabrications. For
example, nano objects which are to be used as field emission
cathodes in field emission displays should not have a fabrication
temperature which exceeds the melting point of glass substrates
(about 650.degree. C.). Also, the temperature should be
substantially lower when supporting surfaces of the nano objects
are polymer. For such applications, direct growth of nano objects
using chemical vapor deposition (CVD) techniques are not feasible
since CVD techniques generally require relatively high temperatures
(800.degree. C.-1200.degree. C.) as well as reactive environments.
In addition, CVD techniques often result in defective multi-wall
carbon nanotubes.
[0017] As such, a more desirable approach for fabrication of
macroscopic structures of nano objects for applications is
post-processing which includes synthesis of nano objects by arc
discharge, laser ablation, or chemical vapor deposition techniques
and assembly of these "pre-formed" nano objects into a macroscopic
structure. Post-deposition processes that have been employed in the
past include screen printing. (see, e.g. W. B. Choi, et al., Appl.
Phys. Lett., 75, 3129 (1999)), spraying, and electrophoretic
deposition (see, e.g. B. Gao et al. Adv. Mater., 13 (23), 1770,
2001) However, such techniques pose certain drawbacks. For
instance, screen printing includes admixture of pre-formed nano
objects with an organic or inorganic paste in order to form a thick
paste. The thick paste is then placed on a substrate. After
placement of the thick paste onto the substrate, the organic binder
resides at an exposed surface of the paste. Therefore, an
additional step is required to expose the nano objects within the
thick paste. Typically, a plasma etching process or similar
chemical process is used to expose the nano objects. In addition,
the use of thick paste limits the size of a structure can be
formed. In general it is difficult to form structures less than 20
microns-50 microns using the screen printing methods. Furthermore,
screen printing methods requires considerable amount of materials.
Spraying can be inefficient and is not practical for large-scale
fabrication. Neither of these processes can control the orientation
of the nano objects.
[0018] Therefore, a need exists for a process/method to assemble
nano objects with a controlled structure, morphology, thickness,
orientation, and ordering. In addition, a need exists for a method
that operates at mild conditions acceptable for device
fabrications. In addition, a need exists for an efficient and
scalable assembly process.
SUMMARY
[0019] The present subject matter provides a method for forming
microscopic and macroscopic structures using nano objects. The
method of the present subject matter allows self assembly of nano
objects onto a supporting surface, into a freestanding structure,
or into a crystal. In addition, the present subject matter provides
a method for assembling the nano objects into patterned structures
with a controlled thickness, density and a controlled orientation
of the nano objects. In addition, the present subject matter
provides an efficient process to assemble pre-formed nano objects
under mild conditions that are acceptable for a wide range of
substrates and devices. The resulting structures are useful in a
variety of devices including electron field emission cathodes for
devices such as field emission displays, cold-cathode x-ray tubes,
microwave amplifiers, ignition devices; electrodes batteries, fuel
cells, capacitors, supercapacitors; optical filters and polarizers;
sensors; and electronic inter-connects.
[0020] In one embodiment of the present subject matter, a method
for assembling a macroscopic structure with pre-formed nano objects
is disclosed. The method comprises processing the nano objects such
that they form a stable suspension or solution in a solvent. Once
the nano objects are processed, the nano objects are admixed with a
solvent to form a stable suspension or a solution. Upon formation
of the stable suspension or solution, a substrate is submersed into
the suspension or solution. Upon changing either the concentration,
temperature, or pH value of the suspension, the nano objects
deposit on the surface of the substrate.
[0021] In a further embodiment of the present subject matter, a
method for assembling pre-formed nano objects into a patterned
structure is disclosed. The method comprises processing pre-formed
nano objects such that they form a stable suspension or solution in
a suitable solvent. After processing the nano objects, the
processed pre-formed nano object is mixed with a solvent to form a
stable suspension or a solution. A patterned substrate is then
inserted into the liquid. Upon changing either the concentration,
temperature, or the pH value of the liquid, the nano objects
assemble on certain regions of the substrate surface to form a
patterned structure comprising the nano objects.
[0022] In yet another embodiment of the present subject matter, a
method for assembling pre-formed nano objects into a crystal or a
membrane is disclosed. The method comprises processing the nano
objects so that they form a stable suspension or solution in a
solvent. The processed nano objects are mixed with a solvent to
form a suspension or a solution in a container that does not
attract the nano objects. Upon changing a concentration,
temperature or a pH value of the liquid, the nano objects
crystallize in the liquid. In this embodiment of the present
subject matter, a seed crystal may be used to form the crystal.
[0023] In another embodiment of the present subject matter, a
method for assembling pre-formed nano objects into multi-layered
structures is disclosed. The method comprises first processing the
nano objects so that they form a stable suspension or solution in a
solvent. The processed nano object is then mixed with a solvent to
form a suspension or a solution. After formation of the stable
suspension or solution, a substrate is submersed into the
suspension or solution. Upon changing the concentration,
temperature or the pH value of the suspension or solution, the nano
objects assemble on the surface of the substrate. The substrate is
then removed from the suspension or solution. After removal, a
second type of material is coated on the surface of the
self-assembled nano objects on the substrate. The substrate is then
submersed into the suspension or solution containing the nano
objects. The process is repeated until a multi-layer structure with
a desired thickness and number of repeating layers are
obtained.
[0024] In yet another embodiment of the present subject matter, a
method for assembling elongated nano objects into orientational
ordered structures is disclosed. The method comprises forming a
stable suspension or solution of the nano objects in a suitable
solvent. A substrate is then submersed into the liquid and the
solvent gradually evaporated. Upon evaporation of the solvent, the
nano objects deposit on the surface of the substrate such that
longitudinal axes of the nano objects align parallel to the
substrate surface. The process can further comprise the use of an
external field such as either an AC or a DC electrical field or a
magnetic field during the assembly process.
[0025] In a further embodiment of the present subject matter, a
method for assembling elongated nano objects into a vertically
aligned structure on a supporting surface is disclosed. The method
comprises first processing the nano objects such that a tail and a
body of the nano objects have different affinities toward certain
types of solvents. For example, the tails are hydrophobic and the
bodies are hydrophilic. The processed nano objects are dispersed in
a suitable solvent where the solvent has an affinity towards the
tails of the nano objects. A substrate with one of the surfaces
having the same affinity towards the solvent and the tails of the
nano objects is submersed into the liquid. Upon changing the
concentration, temperature, or pH value of the solvent, the nano
objects deposit on the substrate with their tails bonded to the
surface and longitudinal axes of the nano objects vertically
aligned with respect to the surface of the substrate.
[0026] As may be appreciated, the present subject matter provides a
method for self assembly of nano objects, such as carbon nanotubes,
nanowires and nanorods, onto a substrate, into free-standing
membranes, into a crystal, or a into multi-layer structure. The
nano objects form into functional structures having long range
ordering. In addition, the present subject matter provides a method
for controlling the functionality of formed macroscopic structures.
Some of the objects of the subject matter having been stated
hereinabove, and which are addressed in whole or in part by the
present subject matter, other objects may become evident as the
description proceeds when taken in connection with the accompanying
drawings as best described hereinbelow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Objects and advantages of the subject matter may become
apparent from the following detailed description of preferred
embodiments thereof in connection with the accompanying drawings in
which like numerals designate like elements and in which:
[0028] FIG. 1A illustrates a substrate within a suspension where
the suspension includes nano objects for deposition onto the
substrate in accordance with an embodiment of the present subject
matter.
[0029] FIG. 1B shows an embodiment of the present subject matter
where the substrate shown with reference to FIG. 1A includes
hydrophobic regions and hydrophillic regions.
[0030] FIG. 2 illustrates the evaporation of the suspension during
submersion of the substrate shown with respect to FIG. 1A in
accordance with an embodiment of the present subject matter.
[0031] FIG. 3 is an embodiment of the present subject matter
illustrating a air/liquid/substrate triple line on the substrate
shown with respect to FIG. 1A where nano objects deposit on the
substrate along the air/liquid/substrate triple line.
[0032] FIG. 4 illustrates the deposition of nano objects onto a
substrate to form a nano object film, in accordance with an
embodiment of the present subject matter.
[0033] FIG. 5 illustrates a method 200 assembling a macroscopic
structure with preformed carbon nanotubes onto a substrate in
accordance with an embodiment of the present subject matter.
[0034] FIG. 6A illustrates a method of submersing a seed crystal
into a solution for assembling nano objects into a crystal in
accordance with an embodiment of the present subject matter.
[0035] FIG. 6B is an embodiment of the present subject matter
illustrating the formation of a crystal formed with the nano
objects shown with reference to FIG. 6A.
[0036] FIG. 7A illustrates a method of assembling elongated nano
objects on a surface such at they are aligned vertically in respect
to the supporting surface in accordance with an embodiment of the
present subject matter.
[0037] FIG. 7B shows nano objects shown with respect to FIG. 7A in
accordance with an embodiment of the present subject matter.
[0038] FIG. 8A illustrates a multi-layer structure fabricated in
accordance with an embodiment of the present subject matter.
[0039] FIG. 8B shows a tri-layer multi-layer structure made in
accordance with an embodiment of the present subject matter.
[0040] FIG. 9A illustrates a glass substrate patterned with
photoresist and hydrophilic regions in accordance with an
embodiment of the present subject matter.
[0041] FIG. 9B illustrates an embodiment of the present subject
matter where the substrate shown with respect to FIG. 9A includes a
SWNTs.
[0042] FIG. 9C shows a phosphor screen for placement over the glass
substrate shown with reference to FIG. 9B in accordance with an
embodiment of the present subject matter.
DETAILED DESCRIPTION
[0043] The present subject matter discloses a method for assembling
nano objects. The nano objects formed in accordance with the
present subject matter may be formed onto a supporting surface,
into free-standing membranes and into multi-layer structures. The
nano objects may be a variety of materials, including hollow nano
tubes which are composed of single or multiple elements. The single
or multiple elements may be carbon, boron, nitrogen, solid
inorganic or organic nanowires/nanorods. Prior to assembling the
nano objects, the nano objects are processed such that they form a
stable suspension or a solution in a suitable solvent. The
processing operation includes attaching chemical groups to the
surface of the nano objects and reducing aspect ratios of elongated
nano objects. After processing, a suspension or a solution is
formed by admixture of the processed nano objects with a solvent.
Upon formation of the suspension, a substrate, such as glass, is
submersed into the substrate. After submersion of the substrate,
the nano objects self assemble into uniform thin films on the
substrate. The nano objects self assemble when either of the
following occurs: evaporation of the suspension, a concentration
change of the suspension, a temperature change of the suspension or
a pH level change of the concentration. The nano objects which self
assemble onto the uniform film may be any nano object, such as
carbon nanotubes, silicon nanowires or the like.
[0044] Now making reference to the Figures, and more particularly
FIG. 1A, FIG. 1A illustrates a substrate 100 within a suspension
102 having nano objects 104, in accordance with an embodiment of
the present subject matter. The substrate 100 may be any substrate
which allows deposition of nano objects such as carbon nanotubes
onto the substrate, such as hydrophilic glass, gold (Au), a silicon
wafer, aluminum, chromium, tin, a polymer, a metal or the like. In
accordance with an alternative embodiment of the present subject
matter, the substrate may include regions with alternating chemical
properties such as hydrophilic and hydrophobic regions, as shown in
greater detail with reference to FIG. 1B.
[0045] FIG. 1B illustrates an embodiment of the present subject
matter where the substrate 100 includes hydrophobic regions 100a
and hydrophilic regions 100b. In this embodiment, the nano objects
deposit on one of these regions. Where the nano objects deposit
depend on the properties of the nano objects and how they are
processed. In the case where nano objects are hydrophilic, the nano
objects deposit on the hydrophilic regions 100b and resist the
hydrophobic regions 100a during deposition. Therefore, a user may
control deposition formation on a substrate and functionality of
the substrate by controlling the location of hydrophilic and
hydrophobic regions on the surface of the substrate. As may be
appreciated, controlling the functionality of the substrate enables
the fabrication of patterned nano object films. It should be noted
that the hydrophobic regions 100a may be formed using any suitable
technique, such as spin coating a thin layer of hydrophobic
polymers such as polystyrene over the surface of the substrate 100,
by deposition of a monolayer of hydrophobic chemical groups or the
like. Moreover, after nano object deposition, the hydrophobic
regions 100a may be removed by washing the substrate 100 in any
suitable solvent, such as acetone, methanol, ethanol, buffered
hydrofluoric acid or the like. Therefore a user may pattern the
substrate such that the formed carbon nanotube film may have
different uses, such as the basic field emission pixels for field
emission displays.
[0046] Returning attention to FIG. 1A, the substrate 100 is
submersed into the suspension 102. The suspension 102 includes nano
objects 104 admixed with suitable solvents. In the case of
processed carbon nanotubes, the solvent can be either de-ionized
water or alcohol. Some of the nano objects 104 may be dispersed or
dissolved in a suitable solvent after fabrication without further
processing. Other materials, such as carbon nanotubes or Si
nanowires, are processed in order to form a stable suspension. The
processing operation includes the following: attaching chemicals to
the surfaces of the nano objects and/or reducing the aspect ratios
of the elongated nano objects. In one embodiment of this subject
matter, the nano objects 104 are single-wall carbon nanotube (SWNT)
bundles which are produced by either arc-discharge, laser ablation,
or other suitable techniques. They are then purified by a refluxing
operation in hydrogen peroxide and filtration. After purification,
the nano objects 104 are etched to uniform lengths. The nano
objects 104 can be cut by a variety of techniques including
sonication in concentrated acids such as sulfuric and nitric acids,
or a mechanical process such as ball-milling or mechanical cutting.
Upon etching, the SWNT bundles are rinsed in de-ionized water and
annealed at 200.degree. C. in a 10.sup.-6 torr dynamic vacuum. The
etching operation changes a morphology of the SWNT bundles from
highly entangled to a rigid-rod structure after reducing the aspect
ratio of the SWNT bundles to less than 100. In one embodiment of
the present subject matter, SWNT bundles processed for 30 hours had
an indicated bundle length of 0.5 .mu.m. Moreover, the etched SWNTs
are metallic like and have less than 2.0% hydrogen (H). In
addition, the above method used for processing the SWNT bundles
maintains the frequency of a Raman-active SWNT breathing mode at a
same amount while reducing the aspect ratio and changing the
morphology of the SWNT bundles. In accordance with an alternative
embodiment of the present subject matter, the nano objects 104 may
also be multi-walled nano tube bundles and nanowires/nanorods.
[0047] Upon processing to the form of the nano objects, the nano
objects 104 are admixed with a solution such as de-ionized water to
form the suspension 102. In this embodiment, when the nano objects
104 are carbon nanotubes etched using the method described above,
the nano objects 104 and the de-ionized water admix to form a
homogeneous suspension which is stabilized with a carbon nanotube
concentration up to 1.0 mg/mL without flocculation for several
days. In accordance with alternative embodiments of the present
subject matter, other solvents, such as alcohol, may also be used
in the suspension 102.
[0048] Upon insertion of the substrate 100 into the suspension 102,
the solvent 102 evaporates as shown with reference to FIG. 2. FIG.
2 illustrates the evaporation of the suspension 102 during
submersion of the substrate 100 in accordance with an embodiment of
the present subject matter. As may be seen with reference to FIG.
2, the nano objects 104 transfer to the substrate as the suspension
102 evaporates. In some cases, deposition occurs along an
air/liquid/substrate line or triple line 106, as shown with
reference to FIG. 3. In accordance with an embodiment of the
present subject matter, the triple line 106 is an area on the
substrate 100 where the suspension 102 ends on the substrate 102 as
indicated by a point A. As may be appreciated, the point A moves in
a downward direction Y along the substrate 100 as the suspension
102 evaporates. It should be noted that the rate of evaporation may
be controlled through controlling the ventilation of an area
immediately surrounding the substrate 100 and the suspension 102
and increasing or decreasing the temperature of the suspension 102.
It should also be noted that in an embodiment of the present
subject matter, the suspension evaporates at room temperature.
[0049] Now turning attention to FIG. 4, FIG. 4 illustrates the
deposition of the nano objects 104 onto the substrate 100 to form a
macroscopic structure, such as a film 108 in accordance with an
embodiment of the present subject matter. As the triple line 106
moves in the downward direction Y (shown with reference to FIG. 3),
the nano objects 104 continue to deposit on the substrate 102
forming the film 108. In this embodiment, the film 108 has a
uniform thickness, as demonstrated by the image shown in FIG. 1,
which varies in a range between about 1 nm to about 10 microns. The
thickness of the deposited film may be controlled through
controlling the concentration of the nano objects 104 in the
suspension 102. To further illustrate, for a carbon nanotube film
thickness of 0.1 micron, a concentration in a range preferably
between about of 0.2 mg/mL of nanotube/water and about 0.5 mg/mL of
nanotube/water suspension may be used. When a carbon nano object
film having a thickness of 1.0 micron is desired, a concentration
preferably in a range between about 0.5 mg/mL and about 1 mg/mL may
be used.
[0050] It should be noted that carbon nanotube film deposition
occurs for the film 108 when the suspension 102 is super saturated.
To further illustrate, when a concentration C.sub.o of the
suspension 102 is less than a critical concentration C* of the
suspension 102, deposition occurs during evaporation of the
suspension, as previously described. For example, if the
concentration C* of the suspension is 0.5 mg/mL and the critical
concentration C* of the suspension 102 is 1 mg/mL deposition occurs
at the triple line 106. Nonetheless, if the concentration C.sub.o
of the suspension 102 is substantially smaller than a critical
concentration of the suspension, even when the suspension 102 has a
high evaporation rate (i.e., an evaporation rate of 1 mm/hour), the
nano objects 104 may not deposit on the substrate. For example,
when Co<0.1 mg/mL and Co=1 mg/mL, no deposition of SWNTs occurs
on the glass substrate.
[0051] The nano objects 104 in the self-assembled film 108 are
orientationally ordered such that longitudinal axes of the nano
objects 104 which deposit on the substrate 100 lie along the triple
line 106 direction. This is demonstrated in FIG. 4, which shows a
TEM image of the aligned SWNT bundles after assembly. Moreover, a
degree of ordering of the nano objects in the film 108 maybe
controlled by length and length distribution of the nano objects
104 and the thickness of the film 108 deposited on the substrate
100. To further illustrate, when the nano objects 104 are
preferably in a range between about 0.01 .mu.m and about 1 .mu.m
with a thin deposited film 108 in a range preferably between about
10 nm and about 1 micron, the film 108 has a higher degree of
ordering. Furthermore, if the nano objects 104 are longer (i.e. 2
.mu.m or more in length), a nano object film having a
polycrystalline structure tends to form with well-ordered domains
and partial alignment of neighboring domains.
[0052] Upon formation, the carbon nanotube film 108 displays
anisotropic polarization of individual carbon nanotubes. The
individual carbon nanotubes also demonstrate long-range
orientational ordering. As those skilled in the art will
appreciate, the electrical conductivity of the film 108 is higher
when measured parallel to the alignment direction as opposed to
being perpendicular with the alignment direction.
[0053] In addition, the substrate 100 may have a plurality of
shapes in addition to the planar configuration shown with respect
to the Figures. For example, the substrate 100 may also include a
curved surface, a sandwich structure or the like. In embodiments
where a multi-planar substrate is used, electrophoresis may be used
to deposit the film 108 onto the substrate 100, as more fully
discussed with reference to commonly owned application Ser. No.
09/996,695 filed on Nov. 30, 2001, the specification of which is
herein incorporated by reference in its entirety.
[0054] One advantage of present subject matter includes strong
bonding and selectivity of the nano objects 104 to the substrate
100. The nano objects 104 are both mechanically and chemically
stable in certain solvents. The stability and selectivity of the
self assembled nano objects are attractive from a fabrication
standpoint and for use in device applications. To further
illustrate, when nano tubes are carbon nanotubes and a substrate
which includes glass and chromium is used, the nano objects bond
strongly to the substrate. In this example, the nano objects may
not be removed by mechanical scratching or through the use of a
solvent such as acetone. Nonetheless, the nano objects may be
removed by washing or sonication in water. When the nano objects
are removed through the use of water, water is stirred into a
suspension, such as the suspension 102, and the nano objects, such
as the film 108, separate into smaller free standing membranes
which float on a surface of the water.
[0055] Now with reference to FIG. 5, FIG. 5 illustrates a method
200 for assembling a macroscopic structure with pre formed carbon
nanotubes onto a substrate in accordance with an embodiment of the
present subject matter. Initially, in an operation 202, starting
materials used for forming the macroscopic structures are
processed. For example, turning attention to FIG. 1A and both the
suspension 102 and the nano objects 104 shown with respect to the
Figure, prior to forming the suspension 102, a user processes the
nano objects 104 and the substrate 100. During the operation 202,
the SWNT bundles 104 are etched to controlled lengths by sonication
in concentrated sulfuric and nitric acids after purification by
reflux in hydrogen peroxide and filtration. Upon etching, the SWNT
bundles are rinsed in de-ionized water and annealed at 200.degree.
C. in a 10.sup.-6 torr dynamic vacuum to form the nano objects 104.
It should also be noted that if the user desires to pattern the
carbon nano object film deposition onto a substrate, the user
patterns hydrophobic regions and hydrophilic regions onto the
substrate, as discussed with reference to FIG. 1B and the substrate
100. Once a user processes the starting materials to be used for
the formation of the carbon nano object film, an operation 204 is
performed, as shown with reference to FIG. 5.
[0056] During the operation 204, the suspension is formed with the
starting materials. The user admixes the processed starting
materials at a given concentration with a solution in order to form
the suspension. Returning to the example and FIG. 1A, the user
admixes the nano objects 104 with de-ionized water such that the
concentration of the nano objects 104 within the suspension 102 is
1.0 mg/mL in this example. Upon formation of the suspension with
the starting materials, the method 200 performs an operation
206.
[0057] During the operation 206, a user inserts a substrate into
the suspension. Once the user inserts the substrate into the
suspension, an operation 208 is performed. During the operation
208, the suspension evaporates, thereby forming a nano object film
on a surface of the substrate. Turning back to the example and FIG.
2, upon submersion of the substrate 100 into the suspension 102 in
the operation 206, the suspension 102 begins evaporation in the
operation 208. As previously discussed, as the suspension 102
evaporates, the film 108 deposits on the substrate 100, thereby
forming a macroscopic structure with the preformed nano objects 104
in accordance with an embodiment of the present subject matter. In
one embodiment of the present subject matter, the substrate 100 may
be annealed in a vacuum at a temperature in a range preferably
between 100.degree. C. and about 500.degree. C.
[0058] Now making reference to FIG. 6A, FIG. 6A illustrates a
method for assembling pre-formed nano objects into a crystal or a
membrane in accordance with an embodiment of the present subject
matter. The method comprises processing the nano objects so that
they form a stable suspension or solution in a suitable solvent as
discussed earlier. A processed nano object 610 is mixed with a
solvent 600 to form a suspension or a solution in a container 620
that does not attract the nano objects 610, such as TEFLON.RTM. or
the like if the nano objects are hydrophilic. Preferably, a seed
crystal 630 comprising the same nano objects or similar materials
as the nano object 610 is either submersed in the solution or
touches the surface of the solution. When the processed nano
objects 610 are carbon nanotubes, the solvent 600 can be water and
preferably de-ionized water. The temperature or the pH value of the
suspension or solution is changed after contact of the seed crystal
620 with the solution to bring it to super-saturation. The nano
objects assemble in the liquid or around the seed crystal 630 to
form a crystal 640 (shown with reference to FIG. 6B). It should be
noted that when a seed crystal is used, the seed crystal may be
slowly withdrawn from the solution such that nano objects in the
solution assemble around a lower surface of the seed crystal 630.
The nano objects 610 assemble around the seed crystal 630 thereby
forming a crystal 640, as shown with respect to FIG. 6B. The nano
objects 610 form around the seed crystal 630 such that a structure
of the crystal 640 is the same as a structure of the seed crystal
630. Additionally, the crystal 640 may have a thickness in a range
preferably between about 1 nanometer and about 10 microns.
Moreover, the area of the freestanding membrane is in a range
between 1 micron.times.1 micron and 10 cm.times.10 cm.
[0059] In a further embodiment of the present subject matter, nano
objects may be formed in a vertical structure, as shown with
reference to FIG. 7A. FIG. 7A illustrates a method for vertically
aligning nano objects 711 into a vertically aligned structure on a
substrate surface 760. In this embodiment, nano objects 711 are
processed such that the nano objects 711 have a tail 720 and a body
710 which have different affinities toward certain types of
solvents. For example, the tails 720 are hydrophobic and the bodies
710 are hydrophilic. In an embodiment of the present subject
matter, the tails 720 may be a chemical group comprising
hydrocarbons such as C17H35. Additionally, the bodies 710 may be a
chemical group comprising carboxylic acid --CO.sub.2H. The
processed nano objects 711 are dispersed in a solvent 750 which
attracts the tail 720 but repels the bodies 710 of the nano objects
711. As such, the nano objects float on a surface of the solvent
750 and more preferably with the tails 720 in contact with the
solvent and bodies 710 away from the solvent 750. An example of a
solvent which may be used when the tail 720 is composed of
hydrocarbons is toluene or the like. It is also possible through
applying pressure or external electrical or magnetic field to
assist with the assembly of the nano objects 711 on the surface of
the solvent 750.
[0060] Upon dispersion of the nano objects 711 within the solvent
750, a substrate 760 with a surface 730 having the same affinity as
the tails 720 is submersed into a container 740 that includes the
solvent 750. After submersing the substrate 760 into the solvent
750, the substrate 760 is withdrawn from the solvent 750. Upon
withdrawing the substrate 760 from the solvent 750, the nano
objects 711 deposit on the substrate 760 with the tails 720 bonded
to the surface 730 and longitudinal axes of the nano objects 711
vertically aligned with respect to the surface 730 of the substrate
760.
[0061] In one example, the nano objects 711 can be carbon nano
tubes made hydrophilic by oxidation in acid. A hydrophobic chemical
group can be attached to the ends of the carbon nano tubes 711 that
are open after the oxidation process. In this example, the
substrate may be glass coated with a layer of hydrophobic chemicals
such that the carbon nano tubes vertically align with the
structure. The vertically aligned structure is useful, for example,
as sensors which detect biological systems, chemicals or gases. The
vertically aligned structure may also be useful as an electron
field emission cathode. It should be noted that it is also possible
to make bodies of carbon nanotubes hydrophobic and tails of carbon
nanotubes hydrophilic. A vertically aligned structure can also be
formed using this type of carbon nanotube. In addition, in this
embodiment, the solvent 650 may be hydrophilic. Thus, the
hydrophilic tails attach to the surface 730 such that longitudinal
axes of the nano objects are perpendicular to the surface 730.
[0062] Now turning attention to FIG. 8A, FIG. 8A discloses
assembled preformed nano objects which form a multi-layered
structure in accordance with an embodiment of the present subject
matter. A self-assembled nano object film 810A is first deposited
on a substrate 830. After deposition, the substrate 830 is removed
from a solution having nano objects which formed the self-assembled
nano object film 810A. A second material 820A is then coated on the
surface of the self-assembled nano objects 810A on the substrate.
The second material may be coated onto the self assembled nano
object film 810A using a variety of techniques such as
spin-coating, spray, electrophoresis, evaporation or sputtering.
The material 820A has the same affinity to the self-assembled nano
object film 810A as a surface of the substrate. An example of the
type of material which may be used for the material 820A includes
polymers, metals, ceramics, semiconductors, inorganic materials,
organic materials, biological materials or the like.
[0063] The substrate 830 is re-submersed into the solution
containing the nano objects to form a nano object film 810B similar
to the nano object film 810A. The process is repeated until a
multi-layer structure with a desired thickness and a desired number
of repeating layers, e.g., layers 820B and 810C, are obtained as
shown with reference to FIG. 8A. Alternatively, a third material
860 can be deposited on top of the second layer 850 to form a
tri-layer structure as shown with respect to FIG. 8B, the structure
of which may be used as a thin film re-chargeable battery.
[0064] A first layer electrode of the re-chargeable battery is
carbon nanotubes 840 which are deposited on a conducting surface
830. The second layer 850 is an electrolyte material that can be
deposited over the carbon nanotubes 840 using any suitable
technique such as evaporation, pulsed laser deposition, sputtering
or the like. A third layer 860 is a second electrode of the
rechargeable battery which can be Li.sub.xMnO.sub.4 or
Li.sub.xCO.sub.2. The third layer 860 can be deposited by any
suitable technique including evaporation, pulsed laser deposition,
sputtering or the like. The multi-layer structure can then be used
as a re-chargeable battery or as a fuel cell.
[0065] In another embodiment of this subject matter, carbon
nanotubes are assembled into a structure which can be used as an
electron field emission cathode for applications such as field
emission flat panel displays. SWNTs are first synthesized by the
laser ablation method and then raw materials are purified. The
average bundle length is then reduced from >10 .mu.m to
.about.0.5 .mu.m by, for example, sonication in a mixture of
HNO.sub.3 and H.sub.2SO.sub.4 for 30 hours. The short SWNTs are
then rinsed in deionized water and annealed at 200.degree. C. in
10.sup.6 torr dynamic vacuum before use. A homogeneous suspension
of shortened SWNTs is stabilized in de-ionized water at a nanotube
concentration of 1.0 mg/mL.
[0066] Now making reference to FIG. 9A, FIG. 9A illustrates a
patterned substrate 900 in accordance with an embodiment of the
present subject matter. Initially, a thin layer of photoresist is
spin-coated onto a top surface of the glass slide 900. A photo mask
with periodic lines (less than 100 micron width) is placed on top
of the glass surface coated with the photoresist. After placement
of the photo mask over the glass surface, an ultraviolet light
source is used to expose the glass. The exposed glass is then
developed in chemicals to remove the photoresist materials that are
exposed to UV light. The developed glass forms a patterned glass
substrate with periodic hydrophobic regions which are covered by
the photoresist 910 and hydrophilic regions 920 which are free of
the photoresist. The glass with patterned hydrophobic and
hydrophilic regions is submersed into the previously described
SWNT/water suspension at room temperature. It should be noted that
SWNTs having an aspect ratio preferably of about 10 and a bundle
length in a range preferably between about 300 nm and about 1
micron may be used. As the water evaporates, SWNTs 930 deposit on
the hydrophilic region of the glass slide.
[0067] In a next operation, the glass slide coated with the SWNTs
930 is washed in a suitable solvent such as acetone, methanol,
ethanol, buffered hydrochloric acid or the like. During the washing
process, the remaining photoresist is removed and the SWNTs 930
remain on the glass surface. After removing from the remaining
photoresist, the glass slide is heated in either air or vacuum at
200.degree. C. to remove the residual solvent to achieve SWNTs 930,
as shown with reference to FIG. 9B. The width of the SWNTs 930 can
be as small as 0.1 micron and as wide as 1 cm or larger. It should
be noted that the SWNTs 930 may have other patterns in addition to
that shown with reference to FIG. 9B, such as squares, circles,
dots or any other geometry that can be patterned by
photolithography.
[0068] Electrical contacts 950 are coupled with each of the SWNTs
930 on the glass substrate 900 as shown with reference to FIG. 9B.
When the structure shown with respect to FIG. 9B is placed inside a
vacuum system and subjected to an electrical field in the order of
1-10V/micron, electrons emit from the carbon nanotubes 930. When a
phosphor screen 960 (FIG. 9C) is placed above the carbon nanotube
structure, images can be obtained by controlling where the
electrons emit and at which location the electrons strike the
phosphor thereby forming a field emission flat display. It should
be noted that a field emission cathode formed in accordance with
the present subject matter may have a threshold electrical field in
a range between about 1V/micron to about 5V/micron for an emission
current density of 1 mA/cm.sup.2.
[0069] The present subject matter provides a method for the self
assembly of preformed nano objects onto a substrate. As may be
appreciated, the present subject matter allows for higher packing
densities than those techniques used in the prior art. Forming nano
structures with filtration techniques form nanotube papers which
have a lower packing density than that of nano structures formed in
accordance with the present subject matter. In addition, the
present subject matter may be performed at room temperature as
previously mentioned. The efficient room temperature deposition
process provides an attractive alternative to chemical vapor
deposition techniques, more specifically in display applications
having low melting temperatures.
[0070] Variations of the above-described exemplary method, as well
as additional methods, are evident in light of the above-described
devices of the present subject matter. Although the present subject
matter has been described in connection with preferred embodiments
thereof, it will be appreciated by those skilled in the art that
additions, deletions, modifications, and substitutions not
specifically described may be made without department from the
spirit and scope of the subject matter as defined in the appended
claims. Furthermore, the foregoing description is for the purpose
of illustration only, and not for the purpose of limitation.
* * * * *