U.S. patent application number 10/164891 was filed with the patent office on 2002-12-05 for carbon nanotubes and methods of fabrication thereof using a catalyst precursor.
This patent application is currently assigned to The Board of Trustees of the Leland Stanford Junior University. Invention is credited to Chen, Robert J., Dai, Hongjie, Quate, Calvin F..
Application Number | 20020178846 10/164891 |
Document ID | / |
Family ID | 23854325 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020178846 |
Kind Code |
A1 |
Dai, Hongjie ; et
al. |
December 5, 2002 |
Carbon nanotubes and methods of fabrication thereof using a
catalyst precursor
Abstract
Carbon nanotubes including single-walled carbon nanotubes
(SWNTS) are grown in a manner that facilitates the formation of
distinct, individual nanotubes. In one example embodiment of the
present invention, SWNT probe-tips for applications such as atomic
force microscopy (AFM) are synthesized on silicon pyramids for
integration, for example, onto AFM cantilevers. In another
implementation, the growth of SWNTs involves dip coating of silicon
pyramids with a liquid phase catalyst followed by chemical vapor
deposition (CVD) using methane for growing SWNTs. In another
implementation, SWNTs are shortened in an inert atmosphere to
achieve desirable lengths, for instance, as used in AFM tips. With
these approaches, large-scale arrays of nanotubes can be
manufactured, for example, using contact printing for catalyst
deposition and controllably shortening the nanotubes via an inert
discharge.
Inventors: |
Dai, Hongjie; (Sunnyvale,
CA) ; Quate, Calvin F.; (Stanford, CA) ; Chen,
Robert J.; (Palo Alto, CA) |
Correspondence
Address: |
Attn: Robert J. Crawford
Crawford PLLC
Suite 390
1270 Northland Drive
St. Paul
MN
55120
US
|
Assignee: |
The Board of Trustees of the Leland
Stanford Junior University
|
Family ID: |
23854325 |
Appl. No.: |
10/164891 |
Filed: |
June 7, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10164891 |
Jun 7, 2002 |
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09467096 |
Dec 10, 1999 |
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6401526 |
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Current U.S.
Class: |
73/866.5 |
Current CPC
Class: |
B82Y 15/00 20130101;
B82Y 30/00 20130101; Y10S 977/875 20130101; C01B 2202/02 20130101;
B82Y 35/00 20130101; Y10S 977/857 20130101; C01B 32/162 20170801;
D01F 9/127 20130101; B82Y 40/00 20130101; Y10S 977/843 20130101;
D01F 9/1272 20130101; Y10S 977/876 20130101; Y10S 977/742 20130101;
G01Q 70/12 20130101; Y10S 977/847 20130101; Y10S 977/863
20130101 |
Class at
Publication: |
73/866.5 |
International
Class: |
G01B 005/28 |
Goverment Interests
[0002] This invention was supported in part by NSF grant number
ECS-9871947, DARPA/ONR grant number 184NO18.
Claims
What is claimed is:
1. A carbon nanotube probe tip comprising: a support structure
comprising a tip protruding from a substrate; and a carbon nanotube
extending from the tip.
2. The carbon nanotube probe tip of claim 1, wherein the tip is in
the shape of a pyramid.
3. The carbon nanotube probe tip of claim 1, wherein the tip is in
the shape of a cone.
4. The carbon nanotube probe tip of claim 1, wherein the support
structure includes a wider base portion and a narrower tip portion
above the base portion and wherein the carbon nanotube extends from
the lower base portion, along the upper tip portion and extending
therefrom.
5. The carbon nanotube probe tip of claim 4, wherein the lower base
portion has a catalyst material disposed thereon, the catalyst
material being configured and arranged for growing the carbon
nanotube.
6. The carbon nanotube probe tip of claim 1, wherein the support
structure has a catalyst material disposed thereon, the catalyst
material being configured and arranged for growing the carbon
nanotube.
7. The carbon nanotube probe tip of claim 6, wherein the catalyst
material comprises a metal-containing salt, a long-chain molecular
compound and a solvent.
8. The carbon nanotube probe tip of claim 7, wherein the catalyst
material includes a calcined material that includes metal oxide
particles and at least one of: alumina and silica.
9. The carbon nanotube probe tip of claim 1, wherein the carbon
nanotube is a single-walled carbon nanotube.
10. The carbon nanotube probe tip of claim 1, wherein the support
structure includes a cantilever and wherein the tip protrudes from
the cantilever.
11. The carbon nanotube probe tip of claim 1, wherein the support
structure includes a tower and wherein the tip is disposed on the
tower.
12. A liquid phase precursor for fabricating one or more carbon
nanotubes comprising: a metal-containing salt; a long-chain
molecular compound; and a solvent.
13. The precursor of claim 12, wherein the metal-containing salt is
selected from the group of: chloride, sulfate and nitrate.
14. The precursor of claim 13, wherein the chloride is an inorganic
chloride.
15. The precursor of claim 14, wherein the inorganic chloride
includes AlCl.sub.3, SiCl.sub.4, FeCl.sub.3 and
MoO.sub.2Cl.sub.2.
16. The precursor of claim 15, wherein the long chain molecule is a
surfactant or a polymer.
17. The precursor of claim 16, wherein the long chain molecule is a
tri-block copolymer comprising pluronic P-123 poly(alkylene oxide)
HO(CH.sub.2CH.sub.2O).sub.20--(CH.sub.2CH(CH.sub.3)O).sub.70--(CH.sub.2CH-
.sub.2O).sub.20OH.
18. The precursor of claim 12, wherein the solvent is selected from
the group of: alcohol, acetone and water.
19. The precursor of claim 12, wherein the metal-containing salt,
the long-chain molecular compound and the solvent are configured
and arranged to form a catalyst material for growing a carbon
nanotube in the presence of a carbon-containing gas.
20. The precursor of claim 19, wherein the metal-containing salt,
the long-chain molecular compound and the solvent are configured
and arranged to form a catalyst material when calcined.
21. A method for manufacturing a single-walled nanotube (SWNT), the
method comprising: introducing an inert gas to the SWNT; and
applying a voltage between the SWNT and a substrate and shortening
the SWNT.
22. The method of claim 21, further comprising contacting the SWNT
to the substrate.
23. The method of claim 22, wherein the substrate is a
heavily-doped silicon substrate.
24. The method of claim 22, wherein applying a voltage between the
SWNT and the substrate includes gradually increasing the voltage
applied between the SWNT and the substrate until a loss of
nanotube-substrate contact occurs.
25. The method of claim 21, further comprising: using a force
calibration curve to detect a stiffness characteristic of the SWNT;
and in response to detecting a stiffness characteristic that
indicates that the SWNT is not sufficiently stiff, repeating the
steps of introducing an inert gas to the SWNT and applying a
voltage between the SWNT and a substrate and shortening the
SWNT.
26. The method of claim 21, wherein the SWNT is disposed on an AFM
cantilever and wherein applying a voltage between the SWNT and a
substrate includes manipulating the cantilever to contact the SWNT
to the substrate.
27. The method of claim 21, wherein applying a voltage between the
SWNT and a substrate includes using the voltage to align the SWNT
in a direction generally normal to the substrate.
Description
RELATED PATENT DOCUMENTS
[0001] This is a continuation/divisional of U.S. patent application
Ser. No. 09/467,096, filed on Dec. 10, 1999 (STFD.016PA/S99-134),
to which Applicant claims priority under 35 U.S.C. .sctn.120 for
common subject matter.
FIELD OF THE INVENTION
[0003] This invention relates generally to carbon nanotubes and,
more particularly, to carbon nanotubes and their fabrication using
a catalyst precursor.
BACKGROUND
[0004] Carbon nanotubes are unique carbon-based, molecular
structures that exhibit interesting and useful electrical
properties. There are two general types of carbon nanotubes,
referred to as multi-walled carbon nanotubes (MWNTs) and
single-walled carbon nanotubes (SWNTs). SWNTs have a cylindrical
sheet-like, one-atom-thick shell of hexagonally-arranged carbon
atoms, and MWNTs are typically composed of multiple coaxial
cylinders of ever-increasing diameter about a common axis. Thus,
SWNTs can be considered to be the structure underlying MWNTs and
also carbon nanotube ropes, which are uniquely-arranged arrays of
SWNTs.
[0005] Due to their unique electrical properties, carbon nanotubes
are being studied for development in a variety of applications.
These applications include, among others, chemical and bio-type
sensing, atomic force microscopy (AFM), field-emission sources,
selective-molecule grabbing, nano-electronic devices, and a variety
of composite materials with enhanced mechanical and
electromechanical properties. For general information regarding
carbon nanotubes, and for specific information regarding SWNTs and
its applications, reference may be made generally to the
above-mentioned patent documents, and also to: "Carbon Nanotubes:
Synthesis, Structure, Properties and Applications," M. S.
Dresselhaus, G. Dresselhaus and Ph. Avouris (Eds.), Springer-Verlag
Berlin Heidelberg, New York, 2001; and "T. Single-shell Carbon
Nanotubes of 1-nm Diameter," Iijima, S. & Ichihashi, Nature
363, 603-605 (1993).
[0006] Many electronic devices benefit from small-scale electronic
circuits and arrangements, and also play an important role in a
variety of applications. The size and electrical properties of
nanotubes including carbon nanotubes make them potentially useful
for such small-scale devices.
[0007] As discussed above, atomic force microscopy (AFM) is one
particular application for which carbon nanotubes are useful. AFM
has been a powerful tool for a wide range of fundamental research
and technological applications. Atomic force microscopes employ a
probe tip used to obtain an image of a specimen, with the size and
shape of the probe tip being related to the lateral resolution and
fidelity of images obtained using the AFM arrangement. Carbon
nanotube tips present ideal characteristics for enhancing the
capabilities of AFM in imaging, manipulation and nanofabrication
due to their sharpness, high aspect ratios, high mechanical
stiffness and resilience, and chemical characteristics. AFM tips
employing carbon nanotubes exhibit advantages over conventional AFM
tips, such as longer durability, deep structure probing capability,
and high lateral resolution in imaging and lithographic
applications.
[0008] In view of the above, carbon nanotubes exhibit
characteristics that make them useful for a variety of
implementations. However, such nanotubes have been difficult to
manufacture and implement in a variety of such applications. For
instance, obtaining individual, high quality, single-walled
nanotubes has proven to be a difficult task. Existing methods for
the production of nanotubes, including arc-discharge and laser
ablation techniques, yield bulk materials with tangled nanotubes.
The nanotubes in the bulk materials are mostly in bundled forms.
These tangled nanotubes are extremely difficult to purify, isolate,
manipulate, shorten and use as discrete elements for making
functional devices. Furthermore, many previously-available
nanotubes have exhibited molecular-level structural defects, which
can result in relatively weak nanotubes with poor electrical
characteristics.
SUMMARY
[0009] The present invention is directed to overcoming the
above-mentioned challenges and others related to the types of
devices and applications discussed above and in other
implementations. The present invention is exemplified in a number
of implementations and applications, some of which are summarized
below.
[0010] According to one example embodiment of the present
invention, carbon nanotubes including SWNTs are grown on a support
structure, such as a silicon tip and/or a pyramid or cone shaped
tip on top of a tower structure. In one particular implementation,
the SWNTs are formed having radii less than about 1 nanometer and
are implemented for use in AFM. In another implementation, arrays
of the SWNTs are formed. In still another implementation,
nanostructures having a feature size less than about 10 nanometers
are fabricated using the SWNTs.
[0011] In another example embodiment of the present invention,
carbon nanotubes are fabricated using a catalyst precursor
material, such as a liquid phase catalyst material. A support
structure is formed and a portion thereof is coated with the
catalyst precursor material. In one implementation, the catalyst
precursor material comprises a metal-containing salt and a
long-chain molecular compound dissolved in a solvent. The catalyst
precursor material is calcined to form a catalyst material, and the
support structure with the catalyst is then exposed to a carbon
containing gas while heated, for example, in a tube furnace. One or
more nanotubes are grown from the support structure as carbon from
the carbon-containing gas is deposited thereon using the catalyst
material to catalyze a reaction of the carbon-containing gas.
[0012] In another example embodiment of the present invention, an
array of carbon nanotube probe tips are grown on a substrate having
an array of support structures with a catalyst precursor material
thereon. In one implementation, a stamp is coated with a liquid
phase precursor material and contacted to the support structure
array to transfer the catalyst precursor material thereto. The
array is then exposed to a carbon containing gas and heated to form
carbon nanotubes on the support structures, such as discussed
above. In one implementation, the catalyst precursor material is
applied to the support structures without coating portions of the
substrate between the support structures.
[0013] In still another example embodiment of the present
invention, carbon nanotubes are shortened in a controllable fashion
using an inert discharge. A voltage is applied to the carbon
nanotubes in an atmosphere having an inert gas. The inert discharge
may, for example, be used to shorten nanotubes grown in connection
with one or more of the example embodiments and implementations
thereof discussed above. In one implementation, a nanotube is
shortened via inert discharge shortening by contacting the nanotube
to a heavily-doped silicon substrate and applying a voltage between
the nanotube and the substrate.
[0014] The above summary of the present invention is not intended
to describe each illustrated embodiment or every implementation of
the present invention. The figures and detailed description that
follow more particularly exemplify these embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The invention may be more completely understood in
consideration of the detailed description of various embodiments of
the invention that follows in connection with the accompanying
drawings, in which:
[0016] FIGS.1A-1C show the growth of oriented SWNTs on pyramidal
AFM tips in connection with one or more example embodiments of the
present invention, in which:
[0017] FIG. 1A shows a tip structure having a catalyst being formed
thereon; and
[0018] FIGS. 1B and 1C show images of carbon nanotubes grown from a
tip structure similar to that shown in FIG. 1A;
[0019] FIGS. 2A-2D show the growth of single-walled carbon
nanotubes on a large-scale support structure array, according to
another example embodiment of the present invention, in which:
[0020] FIG. 2A shows a substrate having tip structures and a stamp
having a catalyst precursor thereon;
[0021] FIG. 2B shows the stamp being applied to the tip structures
of FIG. 2A;
[0022] FIG. 2C shows the tip structures of FIG. 2B having a
catalyst precursor material formed thereon; and
[0023] FIG. 2D shows carbon nanotubes grown from the tip structures
of FIG. 2C;
[0024] FIG. 3 shows a support structure for growing a nanotube tip,
according to another example embodiment of the present
invention;
[0025] FIG. 4A shows a SEM image of a silicon tip-on-tower array,
according to another example embodiment of the present
invention;
[0026] FIG. 4B shows a schematic of a catalyst transfer process
involving contact-printing, according to another example embodiment
of the present invention;
[0027] FIG. 4C shows SEM images of SWNTs grown off silicon tips in
accordance with one or more of the example embodiments discussed
herein;
[0028] FIGS. 5A-5B show force plots of a SWNT before and after
inert discharge shortening thereof, according to another example
embodiment of the present invention;
[0029] FIG. 6A shows an AFM image of .lambda.-DNA molecules on a
mica surface recorded by an individual SWNT probe tip fabricated in
connection with an example embodiment of the present invention;
and
[0030] FIG. 6B shows TiO.sub.2 lines (bright structures) fabricated
on a Titanium thin film using a SWNT probe tip in connection with
another example embodiment of the present invention.
[0031] While the invention is amenable to various modifications and
alternative forms, specifics thereof have been shown by way of
example in the drawings and will be described in detail. It should
be understood, however, that the intention is not to limit the
invention to the particular embodiments described. On the contrary,
the intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the
invention.
DETAILED DESCRIPTION
[0032] The present invention is believed to be applicable to a
variety of different types of devices, and the invention has been
found to be particularly suited for nanotube devices, such as AFM
probe tips, and the growth of nanotubes using a catalyst material
disposed on a tip-type structure. While the present invention is
not necessarily limited to such applications, various aspects of
the invention may be appreciated through a discussion of various
examples using this context.
[0033] According to an example embodiment of the present invention,
carbon nanotubes are grown using methane CVD synthesis of SWNTs
using a liquid-phase catalyst precursor material. In one
implementation, the liquid-phase precursor material includes three
components: a metal containing salt, a long-chain molecular
compound, and a solvent. The salt may, for example, include a
chloride, sulfate or nitrate material. The long-chain molecule may,
for example, include a surfactant, such as soap, or a polymer. The
solvent may include one or more of a variety of organic or
inorganic solvents, e.g., an alcohol, acetone or water. The
liquid-phase catalyst precursor is deposited on a structure and
used to form a catalyst for reacting the methane for CVD growth of
the SWNTs.
[0034] In a specific example implementation, the salt is a mixture
of inorganic chlorides, the long-chain molecular compound is a
polymer that serves as a structure-directing agent for the
chlorides and the solvent is an alcohol, such as ethanol, methanol
or butanol. Exemplary block copolymers that may be used in
connection with this example implementation include tri-block
copolymers such as pluronic P-123 poly(alkylene oxide)
HO(CH.sub.2CH.sub.2O).sub.20--(CH.sub.2CH(CH.sub.3)O-
).sub.70--(CH.sub.2CH.sub.2O).sub.20OH (available from BASF, Inc.,
of Mount Olive, N.J.). Exemplary inorganic chlorides that may be
used in connection with this example implementation include
AlCl.sub.3, SiCl.sub.4, FeCl.sub.3 and MoO.sub.2Cl.sub.2. In
addition, sulfates, nitrates or other types of salt may be used in
place of or in addition to the organic chloride.
[0035] In one implementation, aluminum and silicon chlorides are
processed via hydrolyses, gellation and polymerization into a
network of oxides with the block-copolymer phase directing the
structure of a network. Small amounts of iron and molybdenum
chlorides, relative to the aluminum and silicon chlorides, are
added to the precursor mixture, which leads to a material
containing finely dispersed metal oxide catalytic nanoparticles on
an alumina/silica matrix. The catalytic nanoparticles serve as
active sites for SWNT synthesis. The liquid phase catalyst
precursor is formed in one or more thin layers, for example,
through dip-coating, spin-coating, patterning transferring, or
contact printing techniques. For information regarding a general
approach of using block copolymers and inorganic chlorides to
produce high surface area mesoporous materials, and for specific
information regarding such applications that may be used in
connection with one or more example embodiments of the present
invention, reference may be made to Yang et al, Nature, 1998, 396,
152 and to Yang et al, Science, 1998, 282, 2244, both of which are
fully incorporated herein by reference.
[0036] In another implementation, the catalyst precursor material
is formed by first dissolving AlCl.sub.3.6H.sub.2O (about 1.2 g) in
methanol (about 9 mL) followed by the addition of SiCl.sub.4 (about
0.6 mL) to form a first solution. A second solution is prepared by
dissolving FeCl.sub.3.6H.sub.2O (about 0.09 g) and
MoCl.sub.20.sub.2 (about 0.006 g, mmol) in ethanol (about 2 mL). A
third solution of P-123 (about 1.0 g) in ethanol (about 5 mL) is
then combined with the first and second solutions and stirred for
about 24 hours. The mixture is aged at room temperature for about
1-3 days to form the catalyst precursor. Solvent is removed from
the catalyst precursor, followed by calcination of the catalyst
precursor in air to form a precursor material containing iron-oxide
nanoparticles supported on a mixed alumina/silica oxide. Such a
material is active for the synthesis of SWNTs by chemical vapor
deposition (CVD). Although the above example recites specific
quantities of chlorides and solvents, a variety of quantities of
precursor material may be formed using appropriate (similar) ratios
of the above materials based on the above quantities. For instance,
the ratio of FeCl.sub.3 to ethanol can be increased or decreased by
a factor of 10 with respect to the ratio described above, while
achieving a catalyst precursor suitable for the growth of carbon
nanotubes.
[0037] Referring to FIG. 1A, a liquid phase catalyst precursor 105
is formed on a tip 100 disposed on a structure 110, and
subsequently calcined to form a catalyst material 108, according to
another example embodiment of the present invention. The catalyst
material 108 is then used to grow a carbon nanotube from the tip
100, with example nanotubes grown using this approach shown in
scanning electron microscope (SEM) and transmission electron
microscope (TEM) images respectively in FIGS. 1B and 1C, which can
be grown substantially free of amorphous carbon. The tip 100 may,
for example, include a silicon pyramidal tip integrated onto
commercially available AFM cantilevers (e.g., structure 100).
Cantilevers that may be used in connection with the present
invention include the FESP type cantilever having a spring constant
of about 1N/m (Newton/meter) and manufactured by Digital
Instruments of Santa Barbara, Calif. The support structure 110 and
tip 100 may be formed by any suitable technique such as
microlithography, ion milling or etching.
[0038] In one implementation, under an optical microscope equipped
with mechanical manipulators, a silicon pyramid tip 100 is dipped
into a precursor solution (e.g., contained in a micro-pipette) and
pulled out, leaving a drop of liquid catalyst precursor 105 on the
silicon pyramid tip 100. Due to high surface tension at the tip
100, the droplet tends to retract from the tip of the pyramid and
settle around the base of the pyramid, as shown. This dip-coating
process may be repeated multiple times (e.g., 5-10) until the
catalyst precursor 105 in a gel-like state covers approximately
half of the pyramid height as shown in FIG. 1A. The tip 100 is then
calcined in air at 400.degree. C. for about 1 hour and at
700.degree. C. for about 1/2 hour evaporate solvent and remove a
polymer component from the catalyst precursor to form a thin layer
of catalyst 108 surrounding the base of the tip 100 and partially
coating the sides thereof. The calcining may include, for example,
heating a solid or gel-state material to a temperature below its
melting point to create a condition of thermal decomposition.
[0039] The nanotubes 120 and 130 shown in FIGS. 1B and 1C are grown
from tips 122 and 132, respectively, by forming a catalyst thereon
as discussed in connection with FIG. 1A and subjecting the tips to
chemical vapor deposition using a carbon-containing gas. In one
implementation, the carbon containing gas includes methane and the
tips 122 and 132 are heated to approximately 900.degree. C. for
about 15 minutes in a 1-inch tube furnace under a methane flow rate
of about 1000 mL/min. In another implementation, a carbon
containing gas that does not pyrolize at temperatures between about
800.degree. C. and about 1000.degree. C. is used in addition to or
in replacement of the methane. In addition, the carbon containing
gas may include small concentrations of hydrocarbons such as
ethane, butane, propane or benzene.
[0040] In connection with another example embodiment of the present
invention, it has been discovered that a conditioning step of the
CVD growth chamber significantly increases the yield of SWNTs grown
on pyramidal silicon tips, such as discussed in connection with
FIGS. 1A-1C above. In one implementation, the conditioning involves
suspending a supported catalyst material in methanol and coating an
inner wall of a 1 inch quartz tube reactor with a layer of the
catalyst. In connection with this implementation, it has been
discovered that SWNT tips grow at a success rate of approximately
20% using a non-conditioned quartz tube reactor, whereas the
conditioning step increases the success rate of growing SWNTs on
pyramids to approximately 90%. In another implementation, reactive
hydrocarbon species (e.g., higher order hydrocarbons and/or
radicals such as CH.sub.n or C.sub.mH.sub.n) are generated by the
reactions between CH.sub.4 and the catalyst in the chamber
environment. These species feed the growth of SWNTs on the pyramids
more effectively than CH.sub.4 alone.
[0041] In another implementation, during CVD growth of a nanotube
from the pyramid tip 100 in FIG. 1A, SWNTs nucleate near the base
portion of the tip and lengthen in various directions. As growth
terminates, the nanotube (or bundle of nanotubes) adheres to the
surface of the pyramid tip 100 and extends off the tip, thereby
maximizing tube-surface van der Waals interactions. The overlapping
section between the pyramid tip 100 and the SWNT can be as great as
about 10 micrometers in length, or the height of the pyramid tip. A
substantial number of SWNTs may also be grown in the plane of the
structure 105 and situated away from the tip of the pyramid such
that they generally do not interfere with an extended SWNT (e.g.,
nanotube 120 and/or 130), which is useful, for example, when using
the SWNT as a probe.
[0042] FIGS. 2A-2D shows various stages of manufacture of an array
of carbon nanotubes, according to another example embodiment of the
present invention. The array may be grown using, for example, one
or more of the nanotube growth approaches discussed above, using a
liquid phase precursor material to facilitate the production of the
array. In FIG. 2A, an array 210 of a plurality of support
structures including structure 220 is shown formed on a substrate
230. The support structure 220 may, for example, include pyramidal
or conical silicon tip arrays obtained by microfabrication
processes such as photolithography, dry etching and
oxidation-sharpening. A layer of catalyst precursor material 240 is
coated onto a flat stamp 255. In one implementation, stamp 250 is
made from an elastic material such as polydimethylsilane (PDMS).
Stamp 250 is coated with precursor material 240 by any suitable
means. In one implementation, the stamp 250 is coated via spin
coating, wherein the stamp 240 spins at a rate of about 5000
revolutions per minute (rpm) for about 10 seconds.
[0043] In FIG. 2B, the coated stamp 250 is pressed against
substrate 230 to transfer the catalyst precursor material 240 to
the array 210 of support structures. The coated support structure
array 210 is then calcined as shown in FIG. 2C, for example, in a
manner consistent with the examples discussed above in connection
with FIG. 1A. Using support structure 220 as an example, the
calcining results in calcined catalyst material 240 extending over
at least a portion of the support structure 220. In FIG. 2D,
nanotubes including nanotubes 260 and 261 are grown on the array
210 of support structures using, for example, the CVD approach
discussed above.
[0044] FIG. 3 shows support structures 310 and 311 in a tower-tip
form and extending from a substrate 316, according to another
example embodiment of the present invention. The support structures
310 and 311 may be used, for example, in connection with the
example embodiments discussed above with FIGS. 2A-2D (e.g., as a
replacement for or in addition to the array 210 of support
structures). The structures 310 and 311 include cone-shaped tips
312 and 313 disposed on top of towers 314 and 315, respectively.
This approach is particularly conducive to deposition of catalyst
precursor material onto sides of the cone-shaped tips 312 and 313,
for example, using a stamping approach similar to that discussed
above. This tower-tip approach also inhibits catalyst material from
collecting on portions 318 of the substrate 316 between support
structures 310 and 311 during stamping.
[0045] FIG. 4A shows an SEM image of columns 406 and 408 of an
array of silicon tip-on-tower support structures that may, for
example, be formed in connection with one or more of the example
embodiments discussed above. The height of the cone-shaped tips
(e.g., tip 402) is about 16 micrometers and the height of the
towers (e.g. 404) is 55 micrometers.
[0046] FIG. 4B shows tip-on-tower structures 412 and 413, such as
those shown in FIG. 4A, undergoing a catalyst transfer process by
contact-printing in a manner not inconsistent with the example
embodiments discussed in connection with FIGS. 2A-2D. The printing
is effected using a pad 450 having a layer of catalyst material
440, which is transferred to the tip-on-tower structures 412 and
413 for subsequent growth of nanotubes therefrom.
[0047] FIG. 4C shows SEM images of SWNTs grown off silicon tips
472, 473 and 474, such as those shown in FIG. 4A, with the SWNTs
oriented about normal to a substrate, according to another example
embodiment of the present invention. In one implementation, the
silicon tip-on-tower structures 472, 473 and 474 (or, e.g., similar
structures formed in an array as shown in FIG. 4A) are arranged on
an array of AFM cantilevers fabricated on a silicon wafer. With
this approach, CVD synthesis can be combined with the carbon
nanotube growth approaches discussed herein to form molecular probe
tips in a large-scale approach.
[0048] In another example embodiment of the present invention,
carbon nanotubes are shortened using an inert discharge shortening
approach. In many applications, including those discussed above,
SWNTs extend between 1-20 micrometers in length beyond growth
structures, such as the pyramid tip 100. It is sometimes desirable
to shorten the nanotubes to about 30-100 nanometers in length, such
as for forming rigid AFM probe tips (e.g., as shown in FIG. 1C). In
order to shorten the nanotubes, a voltage is applied thereto in the
presence of inert gas, which results in an electrical arc
discharge. Inert atmospheres that can be implemented in connection
with this example embodiment include nitrogen (N.sub.2) and noble
gases such as helium, neon, argon, xenon and krypton. In one
implementation, SWNTs are shortened in Argon using between about
20-50 volts applied between the SWNT and a substrate. In such an
inert atmosphere, the concentrations of oxygen and other reactive
molecules are relatively low compared to ambient conditions, which
significantly limits the probability and length scale of a
discharge event near the end of a sharp tip under high electric
fields. With this approach, the length of nanotubes including SWNTs
can be controlled.
[0049] FIGS. 5A and 5B show force calibration curves before and
after, respectively, inert discharge shortening of a relatively
long (about 5 micrometers prior to shortening) SWNT on a cantilever
tip, according to another example embodiment of the present
invention. In FIG. 5A, cantilever amplitude is plotted vs. distance
for an as-grown carbon nanotube. The cantilever is oscillated at
the amplitude shown, with the as-grown nanotube contacting, or
tapping, across a surface. During extension (solid curve 510), the
SWNT is too soft to cause a noticeable decrease in the cantilever
amplitude as it crashes into a substrate (e.g., the nanotube bends
rather than causing a decrease in the amplitude). During retraction
(dashed curve 520), however, the amplitude response curve shows
oscillating variations, indicating `stick-slip` motions of the
nanotube as it is pulled off the substrate (e.g., the nanotube
stick and slips at atom sites or unit cells on the substrate). In
the amplitude vs. distance curve of FIG. 5B, the nanotube has been
shortened to less than about 40 nanometers, resulting in a stiff
SWNT that can be used as a probe tip. The cantilever tip is again
oscillated, with the amplitude recovery beyond full tube-substrate
contact being due to buckling of the SWNT. Little hysteresis exists
between extension 530 and retraction 540 curves, which is a desired
characteristic of high quality probe tips for tapping mode AFM.
[0050] In another example embodiment of the present invention, a
stream of Argon is directed over a cantilever mounted in an AFM
with a SWNT extending from a structure on the cantilever. The SWNT
is brought into contact with a heavily doped silicon substrate and
monitored by a cantilever amplitude vs. distance curve, such as
that discussed above in connection with FIGS. 5A and 5B. A voltage
is then applied between the SWNT and the substrate and gradually
increased until the loss of nanotube-substrate contact occurs as a
result of nanotube shortening. This approach is particularly
useful, for example, for reducing the length of SWNTs in steps of
about 30 nanometers, providing an excellent control of the length
of SWNT probes.
[0051] In another example embodiment of the present invention,
SWNTs are aligned to a substrate using an inert discharge
shortening approach, such as discussed above. The shortening
process results in strong electrostatic forces between an SWNT and
a substrate to which the nanotube is brought into contact, which
has also been discovered to promote normal alignment of the SWNT to
the substrate. The orientation of the substrate, relative to the
SWNT, is selected to achieve a desired SWNT orientation. For
instance, when shortening SWNTs extending from a cantilever, it is
often desirable for the SWNT to be perpendicular to the cantilever.
In this instance, the substrate is oriented parallel with the
cantilever, such that the SWNT is oriented perpendicular to the
cantilever. With this approach, the inert discharge shortening
process helps to orient the SWNT for use in a variety of
applications.
Experimental Results
[0052] In connection with one or more of the various example
embodiments and implementations discussed above, a variety of
experimental results were obtained, with summaries below. These
experimental results include AFM imaging of double-stranded
.lambda.-DNA (from Life Technologies, Gaithersburg, Md.) absorbed
onto freshly cleaved mica surfaces. FIG. 6A shows the structure of
linear DNA molecules probed by an individual SWNT in air. The
imaging was conducted using SWNT AFM tips fabricated as described
above. The SWNT radius was approximately 1 nanometer. The inset 610
of FIG. 6A shows the topography along a line cut across a DNA
molecule. The full-width-at-half-maximum (FWHM) of the DNA molecule
is measured to be 3 nm along the molecule, closely approaching the
true width (about 2 nanometers) of double-stranded DNA. The DNA
strands also exhibit fine structures along the length, with
quasi-periodic corrugations that are spaced at approximately 3-4
nanometer distance, close to the about 3.4 nanometer helical
pitch.
[0053] Imaging in aqueous solutions with individual SWNT probe tips
should allow high order DNA structures to be clearly resolved with
the molecules in native environments and make small probing forces
possible due to the absence of undesired capillary forces.
Systematic imaging in air with the SWNT tips described above
consistently gave apparent widths in the range of 3-5 nanometers
for .lambda.-DNA. In comparison, the apparent widths of DNA
molecules were about 15.+-.5 nanometers observed by using
conventional pyramidal tips, and were about 10 nanometers by
multi-walled nanotube probes. These results show that individual
SWNT tips are promising in improving the lateral resolution of AFM
imaging of biological systems to the submolecular level.
[0054] Scanning probe lithographic fabrication of oxide
nanostructures on metal substrates was also carried out with SWNT
tips synthesized in connection with one or more of the examples
discussed herein. Miniaturization by existing microfabrication
methods has previously been limited to the sub-micron scale.
Scanning probe lithography may provide a viable route to future
nanoscale devices with high throughput in imaging and
nanofabrication achievable through AFM equipped with parallel probe
arrays. Previous scanning tunneling microscopy work fabricated
structures with near atomic resolution under ultra-high vacuum
conditions. However, feature sizes obtainable with AFM operating
under ambient conditions have been less than about 10 nanometers,
limited by the size of the probe tips.
[0055] The SWNT tips fabricated as described above were able to
fabricate TiO.sub.2 nanostructures with feature size below about 10
nanometers. The substrate used was a smooth Titanium film deposited
onto atomically flat single crystal .alpha.-Al.sub.20.sub.3.
TiO.sub.2 lines (bright structures) 6 nm wide (FWHM) were
fabricated on a Titanium thin film deposited onto an
.alpha.-Al.sub.20.sub.3 substrate. The apparent height of the
TiO.sub.2 is about 0.8 nanometers. The oxide structures were
obtained by applying bias voltages modulated between a-8.5 volt
(tip relative to substrate) and +0.5 volt during tapping mode
scanning at a rate of about 120-160 micrometers/s. The average
tip-substrate distance was maintained at approximately 1 nanometers
during the lithographic scan.
[0056] An AFM image recorded by the same SWNT tip after
lithographic writing is shown in FIG. 6B. The FWHM of TiO.sub.2
lines is about 6 nanometers. Using SWNT tips, about 6 nanometer
TiO.sub.2 nanodots spaced at about a 10 nanometer pitch with a
packing density of about 1 Tera-bit per square inch (data not
shown) was also fabricated. SWNT AFM tips thus enable the
fabrication of nanostructures with feature sizes below the previous
10-nanometer barrier for AFM operation under ambient conditions.
This advancement is useful in electronic, recording and other types
of miniaturized devices.
[0057] The various embodiments described above are provided by way
of illustration only and should not be construed to limit the
invention. Based on the above discussion and illustrations, those
skilled in the art will readily recognize that various
modifications and changes may be made to the present invention
without strictly following the exemplary embodiments and
applications illustrated and described herein. For instance, such
changes may include modifying the nanotubes for one or more
particular applications, altering the nanotube arrangements, and
where appropriate, using SWNTs as building blocks for more complex
devices. Moreover, the nanotubes may be made of materials other
than carbon, such as silicon and boron, which can also be grown
using a synthesis process similar to that described above. Such
modifications and changes do not depart from the true spirit and
scope of the present invention, which is set forth in the following
claims.
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