U.S. patent application number 11/622610 was filed with the patent office on 2007-09-20 for directed assembly of highly-organized carbon nanotube architectures.
This patent application is currently assigned to Rensselaer Polytechnic Institute. Invention is credited to Pulickel M. Ajayan, Anyuan Cao, Yung Joon Jung, G. Ramanath, Bingqing Wei.
Application Number | 20070218202 11/622610 |
Document ID | / |
Family ID | 27737519 |
Filed Date | 2007-09-20 |
United States Patent
Application |
20070218202 |
Kind Code |
A1 |
Ajayan; Pulickel M. ; et
al. |
September 20, 2007 |
DIRECTED ASSEMBLY OF HIGHLY-ORGANIZED CARBON NANOTUBE
ARCHITECTURES
Abstract
A method of controllably aligning carbon nanotubes to a template
structure to fabricate a variety of carbon nanotube containing
structures and devices having desired characteristics is provided.
The method allows simultaneous, selective growth of both vertically
and horizontally controllably aligned nanotubes on the template
structure but not on a substrate in a single process step.
Inventors: |
Ajayan; Pulickel M.;
(Clifton Park, NY) ; Ramanath; G.; (Clifton Park,
NY) ; Wei; Bingqing; (Troy, NY) ; Cao;
Anyuan; (Troy, NY) ; Jung; Yung Joon; (Troy,
NY) |
Correspondence
Address: |
FOLEY AND LARDNER LLP;SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
Rensselaer Polytechnic
Institute
|
Family ID: |
27737519 |
Appl. No.: |
11/622610 |
Filed: |
January 12, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10361640 |
Feb 11, 2003 |
7189430 |
|
|
11622610 |
Jan 12, 2007 |
|
|
|
60356069 |
Feb 11, 2002 |
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60385393 |
Jun 3, 2002 |
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Current U.S.
Class: |
427/249.1 ;
977/742 |
Current CPC
Class: |
C01B 32/16 20170801;
C04B 2235/528 20130101; C30B 33/00 20130101; C04B 35/62802
20130101; H01M 4/583 20130101; Y10T 428/139 20150115; C04B
2235/3206 20130101; C01B 2202/08 20130101; C04B 2235/3217 20130101;
C04B 35/14 20130101; B82Y 40/00 20130101; B82Y 30/00 20130101; C04B
35/62839 20130101; C04B 2235/5288 20130101; C04B 2235/3418
20130101; C04B 2235/5264 20130101; C30B 29/605 20130101; Y02E 60/10
20130101; C30B 33/00 20130101; C30B 29/605 20130101 |
Class at
Publication: |
427/249.1 ;
977/742 |
International
Class: |
C23C 16/00 20060101
C23C016/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The U.S. Government may have certain rights in this
invention pursuant to grant number N00014-00-1-2050 from the Office
of Naval Research.
Claims
1. A method of making carbon nanotubes, comprising: providing a
substrate containing a template structure having at least two
surfaces; providing a nanotube source gas onto the template
structure; and selectively and simultaneously growing the carbon
nanotubes on the at least two surfaces of the template structure
but not on exposed portions of the substrate such that the grown
carbon nanotubes are controllably aligned in a direction
perpendicular to a respective surface of the template
structure.
2. The method of claim 1, wherein the nanotube source gas comprises
xylenes and ferrocene provided onto the template structure in a
chemical vapor deposition apparatus.
3. The method of claim 2, wherein: the carbon nanotubes comprise
multiwalled carbon nanotubes; carbon nanotubes are grown at a
temperature of 600 to 1100.degree. C.; the substrate comprises a
silicon substrate; the template structure comprises a silicon
dioxide template structure that is not coated with a carbon
nanotube growth catalyst; and the carbon nanotubes simultaneously
grow in orthogonal directions on orthogonal template structure
surfaces.
4. The method of claim 1, wherein the carbon nanotubes
simultaneously grow on plural side surfaces of the template
structure in a direction parallel to a surface of the
substrate.
5. The method of claim 1, wherein the carbon nanotubes
simultaneously grow on an upper surface and on at least one side
surface of the template structure.
6. The method of claim 1, wherein the carbon nanotubes
simultaneously selectively grow in different directions on plural
surfaces of the template structure.
7. A method of making carbon nanotubes, comprising: providing a
substrate containing a template structure, wherein the template
structure comprises at least one inclined surface with an oblique
inclination which is neither orthogonal nor parallel with respect
to an upper plane of the substrate; providing a nanotube source gas
onto the template structure; and selectively growing the carbon
nanotubes on the inclined surface of the template structure but not
on exposed portions of the substrate, such that the grown carbon
nanotubes comprise a membrane having an open truncated cone
shape.
8. The method of claim 7, wherein: the nanotube source gas
comprises xylenes and ferrocene provided onto the template
structure in a chemical vapor deposition apparatus at a temperature
of 600 to 1100.degree. C.; the carbon nanotubes comprise
multiwalled carbon nanotubes; the substrate comprises a silicon
substrate; and the template structure comprises a silicon dioxide
template structure.
9. The method of claim 7, wherein the carbon nanotubes
simultaneously grow on an upper surface and on the inclined surface
of the template structure.
10. The method of claim 7, further comprising forming a masking
layer on an upper surface of the template structure, such that the
carbon nanotubes selectively grow on the inclined surface of the
template structure but not on the upper surface of the template
structure.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] The present application is a Divisional of U.S. application
Ser. No. 10/361,640, filed Feb. 11, 2003, which claims benefit
under 35 U.S.C. .sctn. 119(e) of U.S. provisional application
60/356,069, filed Feb. 11, 2002 and 60/385,393, filed Jun. 3, 2002,
both of which are incorporated herein by reference in their
entirety.
BACKGROUND OF THE INVENTION
[0003] The present invention relates generally to carbon nanotubes
and more particularly to selective growth of carbon nanotubes on
template materials.
[0004] It is likely that future devices containing organized
structures of various functional materials with new properties will
be built from nanoscale building blocks. These nanoscale building
blocks can be produced by a variety of synthesis routes. The novel
properties of the nanoscale building blocks arising from their low
dimensions are known for a wide range of materials.
[0005] Carbon nanotubes are a nanostructured material which
promises to have a wide range of applications. However, the present
techniques used to controllably build organized architectures of
nanotubes with predetermined orientations have several
deficiencies. For example, vertically aligned nanotubes has been
fabricated on catalyst printed planar substrates by chemical vapor
deposition. See, for example, W. Z. Li, et al. Science 274, 1701
(1996); R. Sen, A. Govindaraj, C. N. R. Rao, Chem. Phys. Lett. 267,
276 (1997); M. Terrones, et al. Nature 388, 52 (1997); Z. F. Ren,
et al. Science 282, 1105 (1998); S. S. Fan, et al. Science 283, 512
(1999); H. Kind, et al. Adv. Mater. 11, 1285 (1999); R. R.
Schlittler, et al. Science 292, 1136 (2001) and L. Dai, A. W. H.
Mau, J. Phys. Chem. B 104, 1891 (2000). However, this technique
requires deposition and patterning, usually in separate processing
steps, of catalyst material, typically in nanoparticle assemblies
or thin film forms, which complicates the nanotube fabrication
method. This also does not allow growth of nanotubes in more than
one preselected orientation at different locations in a
controllable fashion.
[0006] While growth of vertically aligned nanotubes on planar
substrates by CVD has been reported extensively, obtaining
nanotubes that are exclusively oriented parallel to the substrate
in predetermined orientations has been more difficult. Suspended
nanotubes across elevated structures have been produced recently by
several different methods. One method involves adjusting the gas
flow during CVD. See N. R. Franklin, H. Dai, Adv. Mater. 2000, 12,
890; N. R. Franklin, Q. Wang, T. W. Tombler, A. Javey, M. Shim, H.
Dai, Appl. Phys. Lett. 2002, 81, 913; and Y. Homma, Y. Kobayashi,
T. Ogino, T. Yamashita, Appl. Phys. Lett. 2002, 81, 2261. Another
method involves applying an electrical field during CVD. See Y.
Zhang, A. Chang, J. Cao, Q. Wang, W. Kim, Y. Li, N. Morris, E.
Yenilmez, J. Kong, H. Dai, Appl. Phys. Lett. 2001, 79, 3155; and A.
Ural, Y. Li, H. Dai, Appl. Phys. Lett. 2002, 81, 3464. However,
these approaches require pre-deposition and pre-patterning of
nanoscale catalyst particle assemblies. Also, the probability of
nanotubes bridging across different catalyst islands is difficult
to predict and control. Several articles also note that nanotubes
can be aligned in horizontal configurations through electric fields
or microfluidic forces. See A. Star, et al., Angewandte Chem.
International Edition 40, 1721 (2001) and T. Rueckes, et al.,
Science 289, 94 (2000). However, these methods are also
complicated, and are difficult to scale (e.g., create them
reproducibly on an 8 inch Si wafer) and to control, for developing
devices for applications. Moreover, in these cases the nanotubes
are not rooted to the substrate (i.e. they are just lying on them,
and hence not very robust).
[0007] Several methods have also been suggested for controlled
placement of carbon nanotubes onto electrode pairs, including AC
bias-enhanced deposition and chemically modified adsorption. See L.
A. Nagahara, I. Amiani, J. Lewenstein, R. K. Tsui, Appi. Phys.
Lett. 2002, 80, 3826 and M. Burgard, G. Buesberg, G. Philipp, J.
Muster, S. Roth, Adv. Mater. 1998, 10, 584.
[0008] Some of the present inventors have also previously suggested
to selectively grow carbon nanotubes on silica templates located on
a silicon substrate without growing the nanotubes on the silicon
substrate. See Z. J. Zhang, B. Q. Wei, G. Ramanath, P. M. Ajayan,
Appl. Phys. Lett. 77, 3764 (2000). The use of this template
structure is advantageous in that it does not require the
deposition and patterning of the catalyst material. However, as can
be seen in FIG. 4 of the Z. J. Zhang et al. article, while roughly
vertical and horizontal nanotubes were simultaneously grown on the
template structures, it was not possible to controllably align
nanotubes during growth in a direction perpendicular to the silica
template structure surfaces. For example, as shown in the insert in
FIG. 4 of this article, the nanotubes were not aligned precisely
and controllably.
SUMMARY OF THE INVENTION
[0009] A preferred embodiment of the invention provides a method of
making carbon nanotubes, comprising providing a substrate
containing a template structure having at least two surfaces and
providing a nanotube source gas onto the template structure. The
nanotube source gas preferably comprises a mixture of nanotube
forming precursor gas, such as xylenes, and a catalyst gas, such as
ferrocene. However, other suitable gases or a single source gas may
be used instead. The method further provides for selectively and
simultaneously growing the carbon nanotubes on the at least two
surfaces of the template structure but not on exposed portions of
the substrate (i.e., in exclusion to the substrate material which
supports the template structure). The grown carbon nanotubes are
controllably aligned in a direction perpendicular to the respective
surfaces of the template structure.
[0010] Another preferred embodiment of the invention provides a
structure, comprising a substrate and a template structure located
on the substrate, wherein the template structure comprises at least
two surfaces. The structure also comprises a first plurality of
carbon nanotubes disposed on a first surface of the template
structure, wherein the first plurality of carbon nanotubes are
controllably aligned in a first direction perpendicular to the
first surface of the template structure, and a second plurality of
carbon nanotubes disposed on a second surface of the template
structure, wherein the second plurality of carbon nanotubes are
controllably aligned in a second direction perpendicular to the
second surface of the template structure, such that the first
direction is different than the second direction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIGS. 1A-B, 2A-D, 4A-C, 5, 6A-G, 8A-F, 9C, 11A-B and 12C are
SEM images of carbon nanotube structures according to preferred
embodiments of the present invention.
[0012] FIGS. 1C, 4D, 6H, 11C and 12A-B are three dimensional
schematics of carbon nanotube structures according to preferred
embodiments of the present invention. The distance between the
nanotubes in FIG. 1C is shown by the arrows marked "d".
[0013] FIGS. 3A-3C are SEM images of porous carbon nanotube films
according to a preferred embodiment of the present invention.
[0014] FIGS. 7A-B are side cross sectional schematic views of a
method of making carbon nanotube structures according to a
preferred embodiment of the present invention.
[0015] FIG. 9A is a plot of carbon nanotube length as a function of
gold masking material coverage on a SiO.sub.2 template structure
according to a preferred embodiment of the present invention.
[0016] FIG. 9B is an SEM image of gold masking material islands on
a surface of a SiO.sub.2 template structure according to a
preferred embodiment of the present invention.
[0017] FIG. 10A is side cross sectional schematic view of carbon
nanotube structures according to a preferred embodiment of the
present invention.
[0018] FIG. 10B is a plot of measured resistance versus a number of
carbon nanotube bridges according to a preferred embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] The present inventors have developed a method of
controllably aligning carbon nanotubes to a template structure to
fabricate a variety of carbon nanotube containing structures and
devices having desired characteristics. For example, by selecting a
template structure with a sufficient thickness allows a
simultaneous growth of both vertically and horizontally
controllably aligned nanotubes in a single process step. In fact,
the nanotubes may be aligned in any set of in-plane and/or out of
plane orientations and grown in a single process step. The level of
control provided by this method provides construction of complex,
nanotube based, highly organized 1-to-3-D architectures for
building or use in nanotube based devices and systems in a scalable
fashion.
[0020] A template structure, pattern or material is a structure,
pattern or material which allows selective growth of carbon
nanotubes on it without growing any detectable amount of carbon
nanotubes on exposed portions of the substrate which supports the
template. Nanotubes grow normal to, and selectively on the template
structure, inheriting the topography of the template structure.
Thus, the nanotubes are controllably aligned in a direction
perpendicular to the surface of the template structure from which
they grow such that all the nanotubes which grow from a particular
template structure surface are oriented in the same direction. The
precise control of nanotube orientation allows the fabrication of a
wide variety of organized architectures of differing complexities,
shapes, densities, dimensions and orientation. The bottom-up
fabrication approach is easy, scalable, and compatible with silicon
microfabrication techniques and processes.
[0021] In a preferred aspect of the present invention, the template
structure comprises a silicon dioxide structure located on a
silicon substrate. However, other suitable template and substrate
materials may be used instead. For example, other silicon oxide and
metal oxide template structures materials, such as silicon
oxynitride, magnesium oxide, aluminum oxide or indium tin oxide,
may be used instead. Thus, a carbon nanotube growth catalyst
material, such as a metal nanoparticle layer, is not necessary to
selectively grow carbon nanotubes, and is preferably omitted to
simplify processing. In alternative aspects of the present
invention, a metal that does not catalyze nanotube growth, such as
gold or copper, can also be used to mask part of the template
structure material on the substrate. The substrate material can be
any material that does not catalyze nanotube growth. Examples of
preferred substrate materials are a single crystal silicon wafer,
epitaxial silicon and polysilicon layers. Furthermore, other
semiconductor materials, such as II-VI and III-V semiconductor
materials, and non-semiconductor materials, such as ceramics,
metals, glasses and plastics that do not catalyze nanotube growth
may be used as a substrate by appropriately selecting the template
material which allows selective growth of carbon nanotubes.
[0022] In another alternative aspect of the present invention, the
substrate may be omitted entirely, and the nanotubes may be
fabricated on free standing template structures. For example carbon
nanotubes may be formed on the surfaces of oxide particles, such as
silica, alumina, MgO and other silicon and metal oxide particles.
Preferably, the particles are substantially spherical oxide
particles, such as spherical, oval and roughly spherical (i.e.,
spherical particles with chiseled surfaces) particles. The
particles may be micro sized (preferably 1 to 1,000 micron
diameter) or macro sized (preferably 1,000 microns to 1 cm
diameter). The carbon nanotubes are aligned perpendicular to the
oxide particle surfaces.
[0023] In preferred embodiments of the present invention,
controllably aligned multiwalled carbon nanotubes are selectively
and simultaneously grown in patterns and in multiple directions on
lithographically patterned silica templates in a single process.
This process is preferably carried out through a CVD method that
delivers the nanotube-forming precursor and the catalyst material
(in compound or elemental form) from the gas phase either
simultaneously or sequentially.
[0024] The specific examples of nanotube structures of the present
invention shown are illustrated in SEM images in the Figures.
However, the present invention should not be considered limited by
the structures and methods of the specific examples, which are
provided for illustration of the present invention.
[0025] The nanotube structures shown in the SEM images in the
Figures were selectively grown on non-planar patterns composed of
SiO.sub.2 and Si surfaces. The substrates were Si (100) wafers
capped with thermally grown or plasma-enhanced chemical vapor
deposition (PECVD) deposited SiO.sub.2 template structures having a
thickness of 100 nm to several microns, such as 2 to 8.5 microns.
For example, thick silica layers (up to about 8.5 microns) were
deposited by PECVD to create high-aspect-ratio silica features.
Patterns of Si/SiO.sub.2 of various shapes were generated by
photolithography followed by a combination of wet and/or dry
etching.
[0026] Patterned nanotube growth was achieved without metal
catalyst predeposition and patterning, thereby simplifying the
template preparation by eliminating 2 processing steps. Instead,
CVD nanotube growth was stimulated by exposing the substrate and
the template structure on the substrate to vapor mixtures
comprising xylenes (C.sub.8H.sub.10), a nanotube-forming precursor,
and ferrocene (Fe(C.sub.5H.sub.5).sub.2), a nanotube catalyst, at
about 600 to 1100.degree. C., preferably at about 800 to
900.degree. C. However, other suitable source gases and
temperatures may be used instead. Ferrocene was dissolved in
xylenes (which preferably contains different isomers) at
concentrations of about 0.01 g/ml, the mixture was pre-heated,
co-evaporated and fed into the CVD chamber. Ferrocene preferably
comprises 0.001 to 1 percent of the ferrocene/xylenes mixture.
Prolonged growth in the temperature range of 600-1100.degree. C.,
produced films of densely packed multiwalled carbon nanotubes on
the template structures but not on the substrate. Uniform,
vertically aligned nanotube films having a thickness of a few
microns to several tens of microns were produced in few minutes at
growth rates of about 10 microns/minute. The nanotubes in the
nanotube films were about 30 nm diameter multiwalled carbon
nanotubes. No nanotube growth was observed on pristine Si surfaces,
or on the native oxide layer formed on the silicon substrate.
Aligned nanotubes grew readily on SiO.sub.2 templates in a
direction normal to the template growth surface, and the
selectivity was retained down to micron size SiO.sub.2
templates.
[0027] Using the substrate-selective and normal direction growth
process, the nanotubes structures of various shapes and
orientations with respect to the Si surface were fabricated on
active SiO.sub.2 template surfaces. Thus, carbon nanotubes were
selectively grown in desired patterns and architectures without
further photolithographic patterning.
[0028] The method and structure of the first preferred embodiment
will now be described. In the first preferred embodiment, the
carbon nanotubes were selectively grown in one direction on various
template structure shapes to produce various nanotube structure
shapes having the footprint of the template surfaces. These
nanotube structures can then be optionally removed from over the
substrate and placed into a suitable device.
[0029] FIGS. 1A-B show a striking example of aligned nanotube
pillars placed on specific sites on the substrate according to one
aspect of the first embodiment. FIG. 1A illustrates an SEM image of
pillars of aligned carbon nanotube (CNT) arrays within trenches
surrounded by a thick aligned nanotube film grown vertically on the
SiO.sub.2 pattern on a Si substrate. The nanotube film was
selectively grown on a SiO.sub.2 pattern or template structure. In
the center of box-like regions defined by the nanotube film walls,
micro-sized cylindrical blocks or pillars of vertically oriented
nanotubes are grown from the underlying template structure
constituted by SiO.sub.2 patterns in this case. Within each block,
the nanotubes are highly oriented and densely packed. Three
different pillar packing densities are shown in FIG. 1A, where the
separation between pillars in each group is 10 microns (top), 5
microns (bottom) and 2 microns (center).
[0030] FIG. 1B is an enlarged image of the middle array shown in
FIG. 1A showing the alignment of the nanotubes in each of the
pillars. FIG. 1C illustrates a schematic of the nanotube pillars on
the SiO.sub.2 patterns located a Si substrate. Specifically, FIG.
1C shows a silicon substrate 10 containing cylindrical SiO.sub.2
template structures 12 on the upper surface of the substrate 10.
Carbon nanotube pillars 14 are selectively grown and are located on
the upper surfaces of the template structures 12. Since the height
(i.e., thickness) of the template structures 12 is less than a
cutoff thickness for growth of nanotubes (such as less than 1
micron, preferably 200 nm or less), the nanotubes do not grow from
the sidewalls of the template structures 12. The cutoff thickness
varies with the template material and with the nanotube growth
parameters.
[0031] The structures shown in FIGS. 1A-C were made by the
following method. The SiO.sub.2 template structures or patterns 12
were formed on a silicon substrate 10 by conventional
photolithography. The template structures 12 were exposed to
xylenes/ferrocene gas mixtures at temperatures ranging from 600 to
1100.degree. C., preferably 800 to 900.degree. C. in a CVD tube
furnace. The nanotubes 14 grew extremely selectively on the
SiO.sub.2 patterns 12, but did not grow on the silicon substrate
10, leaving the exposed silicon substrate blank. Alignment of
nanotubes can be easily identified from the sidewalls, which
separate the micro pillar arrays of nanotubes. The template
structures had a diameter of about 10 microns to form cylindrical
nanotube pillars having a diameter of about 10 microns in diameter
and separations between individual pillars 14 of 10, 5 and 2
microns for the top, bottom and middle pillar sets, respectively.
In addition to the cylindrical pillars 14 and cylindrical template
structures 12, other suitable pillar 14 shapes, such as polygonal
(triangular, rectangular, trapezoidal, etc.), oval or irregular
shapes, can be fabricated by shaping the template structures 12
accordingly. Furthermore, while SiO.sub.2 template structures 12 on
a silicon 10 substrate were illustrated, other suitable materials
for the template structures 12 and the substrate 10 may be used
instead, as described herein.
[0032] FIG. 2A is top view of several "sandwich" or rectangular
block structures of platelets of aligned nanotubes grown from
parallel silica lines (i.e., template structures) on a silicon
substrate according to another preferred aspect of the first
embodiment. The different sandwich structures show different
nanotube platelet thickness (2-10 microns, decreasing from left to
right in FIG. 2A) and separation between the platelets (2-10
microns, decreasing from left to right). The SiO.sub.2 line
patterns or template structures were each 100 microns in length.
They were fabricated using standard photolithography. The nanotube
platelets or lines were selectively grown on the template
structures by the CVD method described above. The result shows the
excellent control of the placement of the nanotube platelet
structures at the desired locations, their thickness, height and
the separation between them (density). The height of these
structures can be precisely controlled to within a few tenths of
microns to several hundreds of microns by tuning the nanotube
deposition time. Of course shapes other than rectangular platelets,
template materials other than SiO.sub.2 and substrate materials
other than silicon may be used instead.
[0033] FIG. 2B is a higher magnification SEM image showing the
alignment of nanotubes within the nanotube platelets. The
coordinated deformation and displacement of the individual
platelets on the substrate indicate substantial attractive forces
between individual nanotubes, suggesting that each platelet may be
manipulated individually.
[0034] FIGS. 2C and 2D illustrate very long, ordered micro fibers
of aligned nanotubes grown from silica patterns of two different
shapes, resulting in two different fiber cross-sections, according
to another aspect of the first embodiment. FIG. 2C shows fibers
with a circular cross section and FIG. 2D shows fibers with a
square cross section. Thus, the cross-sections of these nanotube
fibers are controlled by the shape of the underlying template
patterns on which they grow. Plural nanotubes within each
micro-fiber can grow simultaneously to hundreds of micrometers,
aligned all along the length. The template structures or patterns
12 used to produce these fibers are essentially the same as those
used for structures in FIGS. 1A-C, but the growth time is longer
resulting in flexible ordered fibers, compared to stiffer, shorter
pillars of FIGS. 1A-C. The fibers can have a length of 100 to 500
microns, such as 150 to 300 microns, depending on the length of the
nanotube deposition time.
[0035] In all cases of the nanotube pillars shown in FIGS. 1A-C,
the nanotube platelets shown in FIGS. 2A-B and the nanotube fibers
shown in FIGS. 2C-D, the present inventors observed good adhesion
between individual nanotubes within each geometrical block, and
between the nanotubes and the substrate. The nanotube blocks,
however, can be detached from the substrates by ultrasonic
agitation, and can be manipulated individually using any suitable
nanotube manipulation method. For example, the removed nanotube
pillars, platelets and/or fibers may be selectively placed into a
desired device, such as an electronic device.
[0036] The method and structure of the second preferred embodiment
will now be described. In the second preferred embodiment, porous
carbon nanotube films were fabricated. These porous films were
fabricated with a high degree of control over pore sizes, shapes
and separations. This is illustrated in FIGS. 3A-C, which show
nanotube films with regular arrays of pores. Three different pore
features imprinted in the nanotube film are shown in FIGS. 3A-C.
For example, nanotube films with square and rectangular pores
arranged in a rectangular grid are shown in FIGS. 3A and 3B,
respectively. A nanotube film with randomly shaped pores arranged
in a random orientation is shown in FIG. 3C. The porous carbon
nanotube films shown in FIGS. 3A-C include a plurality of carbon
nanotubes aligned lengthwise in a direction away from the template
layer and a plurality of first pores extending through the film in
the same direction. The bottom ends of the carbon nanotubes are
attached to the porous template material such that the pores in the
nanotube film have a controlled size and are aligned with the
respective pores in the template material. The carbon nanotubes are
not disposed on portions of the substrate exposed through the
pores.
[0037] The porous nanotube films were obtained by using a porous
template structure or layer formed over a substrate. For example, a
porous silica template layer was obtained by forming a silica layer
on the silicon substrate and then photolithographically masking the
layer and etching pores or holes in this layer. Since the pores or
holes are formed by photolithography and etching, they have a
controlled size. The pores or holes extend down to the silicon
substrate and may extend into the silicon substrate if desired.
Thus, portions of the substrate are exposed through pores in the
porous template layer. Nanotube-forming gases or mixtures were
provided onto the porous template layer and the carbon nanotubes
were selectively grown on the porous template layer. However, the
nanotubes were not formed on portions of the substrate exposed
through pores in the porous template layer. Furthermore, the
nanotubes did not form on the template layer pore sidewalls because
the template layer thickness is not sufficient to allow nanotube
growth on side surfaces of the pores in the template layer. For
example, the template layer may have a thickness of less than about
200 nm, depending on the exact processing conditions and the
template material used, to avoid nanotube growth on the side
surfaces of the pores. If desired, after the nanotube film is
grown, the substrate may be selectively removed, such as by
polishing or by silicon selective etching, to form a free standing
porous nanotube film. In an alternative aspect of the second
embodiment, the porous nanotube film is produced by a different
method. The template material can be selectively covered by
non-catalytic masking materials (e.g., gold) through lithography
and deposition, at locations where pores are desired. The template
material has a plurality of first regions masked by a masking
material which does not catalyze nanotubes, such as gold or copper
masking material. The ends of the nanotubes are attached to the
template material such that the plurality of pores in the nanotube
film have a controlled size and are aligned with the plurality of
first regions on the template material masked by the masking
material. When the nanotubes are grown on the masked template
material, the nanotubes selectively grow on the exposed template
material but not on the masked first regions of the template
material.
[0038] Thus, the template layer may be considered to be the
negative pattern of the one used to make free-standing nanotube
blocks shown in FIGS. 1 and 2. Both the number of nanotubes in each
block (e.g., pillar), and the lateral separation between the blocks
or pores are limited only by the smallest dimension of silica
patterns that can be produced, and these can conceivably be made
smaller by using electron-beam lithographed templates or templates
made by other submicron lithography or patterning methods.
[0039] The method and structure of the third preferred embodiment
will now be described. In the third embodiment, selective nanotube
growth on the template structure having at least one, and
preferably two or more surfaces. The nanotube growth occurs in a
direction normal to the respective surfaces and hence this approach
can be harnessed to simultaneously grow nanotubes in several
predetermined directions. For example, nanotube growth in mutually
orthogonal directions may be carried out by using template
structures comprising of deep etched trenches, drilled all the way
to silicon substrate, separating several thick SiO.sub.2 template
structure towers. Preferably, the nanotubes are grown on side
surfaces of the template structures, which are preferably thicker
than about 200 nm, preferably thicker than 2 microns. For example,
the template structure may be several microns thick, such as 2 to
10 microns thick, preferably 5 to 8.5 microns thick. However,
thickness of the template structure for growing controllably
aligned nanotubes on the side surfaces of the template structure
varies based on the template material and the nanotube growth
conditions. The preference of nanotubes to grow normal to, and
selectively on, silica surfaces, forces the nanotubes to inherit
the topography of the substrate templates, enabling the
premeditation of both nucleation sites and growth direction.
[0040] Preferably, the template structure that is located on the
substrate has at least one, and preferably two or more surfaces. A
first plurality of carbon nanotubes are disposed on a first surface
of the template structure. These first plurality of carbon
nanotubes are controllably aligned in a first direction
perpendicular to the first surface of the template structure.
Furthermore, a second plurality of carbon nanotubes are disposed on
a second surface of the template structure. This second plurality
of carbon nanotubes are controllably aligned in a second direction
perpendicular to the second surface of the template structure, such
that the first direction is different than the second direction.
The same applies for other surfaces of the template structures.
[0041] FIGS. 4A-D illustrate vertically and horizontally aligned
nanotube arrays adjacent to each other produced in a single-step
growth process. FIG. 4A is an SEM image of simultaneously
vertically and horizontally aligned nanotube arrays of nanotubes
grown on a template structure comprising a thick SiO.sub.2 film
with deep, etched trenches that separate alternating layers of
SiO.sub.2 and silicon. FIG. 4B is an enlarged area marked by the
white box in FIG. 4A. FIG. 4B illustrates three dimensional aligned
vertical and horizontal growth of nanotubes. The length of
nanotubes in both vertical and horizontal growth is about 60
microns. The thickness of SiO.sub.2 patterns was 8.5 microns, which
is thicker than a threshold required to grow controllably aligned
nanotubes on side surfaces. FIG. 4C shows a cross-section of the
specimen of FIG. 4A cut along the white line shown in FIG. 4A. The
inset in FIG. 4C is a higher magnification image that shows the
perfect, controlled orthogonal alignment of the nanotube
arrays.
[0042] FIG. 4D is a schematic illustration of the SEM images of
FIGS. 4A-C. The rectangular silica template structure 12 is formed
on a silicon substrate 10. The controllably aligned nanotube
structures or arrays 14 are formed on the upper surface of the
template structure 12 as well as in two orthogonal directions in
the substrate plane itself (i.e., the nanotubes 14 are aligned
parallel to the upper plane or surface of the substrate 10). Thus,
the nanotubes 14 on the upper template structure 12 surface are
perpendicular to the nanotubes 14 on plurality of side surfaces of
the template structure 12, and the nanotubes 14 on different
template side surfaces are located in different (i.e.,
perpendicular) directions from each other. Thus, the nanotubes 14
are controllably aligned in directions perpendicular to the
plurality of the side surfaces of the template structure 12.
[0043] Another preferred aspect of the third embodiment is
illustrated in FIG. 5. In this aspect, the template structure 12
shown in FIG. 5 comprises a cylindrical pillar having a height of
about 5 microns containing one continuous side surface. The carbon
nanotubes are controllably aligned in a direction perpendicular to
this side surface of the template structure. The nanotube domains
or structures 14 in FIG. 5 includes vertical and horizontal arrays
of nanotubes. The short vertical block of nanotubes grown in the
center of each pattern is surrounded by radiating nanotube arrays
(wings) grown on the side surface of the silica template structure.
If wet etching of the thick silica template structure 12 creates
inclined side surfaces (forming a frustum rather than a perfect
cylinder), then the nanotube wings would not be perfectly
horizontal (i.e., parallel to the substrate surface), but still
perpendicular to the template surface.
[0044] The method and structure of the fourth preferred embodiment
will now be described. In the fourth embodiment, the structure
contains nanotubes packed with oblique inclinations, neither
orthogonal nor planar or parallel with respect to the upper
substrate plane, by using deep-trench template structures, such as
silica structures, with inclined side surfaces. FIGS. 6A-6G show
illustrative examples where nanotubes grow normal to the walls of
such circular trenches, resulting in membrane iris-shaped
structures having the shapes of open truncated cones.
[0045] The term "cone" includes the right circular cone with a
circular base or directrix as well as structures having a curved
(open or closed curve) or polygonal base or directrix to form
pyramidal type structures with irregular or trapezoidal faces
extending toward a common vertex. Thus, "cone" includes any
structure having a surface generated by a straight line passing
though a fixed point and moving along the intersection with a fixed
curve.
[0046] FIG. 6H is a schematic illustration of the cone shaped
nanotube structures. The template structure 12 on the substrate 10
contains an inclined side surface 16 and a flat upper surface. The
inclined surface 16 has an oblique inclination which is neither
orthogonal nor parallel with respect to an upper plane of the
substrate 10. The nanotubes 14 on the upper surface grow
perpendicular to this surface of the template structure 12. The
nanotubes 18 on the inclined side surface 16 of the template
structure extend perpendicular to this surface 16 to form the cone
shaped membrane.
[0047] The free standing membrane structures 18 made of aligned
nanotubes created in the shapes of open truncated cones may have
any desired size. The width of the membrane or the size of the
opening can be controlled by the pattern diameter, the angle of
trench wall and the time of growth. Typical examples of different
diameter, cone angle, size of opening of the nanotube membrane are
shown in FIGS. 6A-F. The diameter of the pattern structures are
300, 250, 200, 150, 100 and 50 microns, respectively. FIG. 6G shows
an enlarged view of the 100 .mu.m diameter structure in FIG. 6E to
demonstrate the geometry of the nanotube membrane in this 3-D
architecture.
[0048] This cone shaped membrane is formed the same way as the
previous nanotube structures. The nanotube source gas is provided
onto the template structure 12 and the carbon nanotubes 18 are
selectively grown on the inclined surface 16 of the template
structure but not on exposed portions of the substrate 10, such
that the grown carbon nanotubes comprise a membrane having an open
truncated cone shape. The nanotubes 14 may also be grown on the
upper surface of the template structure 12 or the upper surface of
the template structure may be masked, as will be described below,
to form only the nanotube membrane 18 on the template structure
12.
[0049] FIGS. 6A-G also show the flexibility of the method of the
fourth embodiment to obtain radially oriented nanotubes with the
entire spectrum of in-plane orientations relative to the substrate
plane. The present inventors produced structures with different
diameter, cone angle, size of opening and film thickness by
tailoring the pattern dimensions, and growth time to control
nanotube length. Similarly, complex shapes of nanotube structures
can be generated by altering the trench geometry and depth.
[0050] The method and structure of the fifth preferred embodiment
will now be described. In the fifth embodiment, a masking layer or
material is formed on a portion of the template structure to
selectively grow nanotubes only on an exposed portion of the
template structure. A masking layer is a layer which inhibits
growth of nanotubes on the template structure. For example, a 2 to
20-nm-thick gold layer can be used as a masking material for silica
template structures. Other examples of masking materials include
polysilicon, copper, or any other material that does not catalyze
nanotube growth. The masking material preferably has a thickness at
which it forms a continuous layer. Thus, in a preferred aspect of
the fifth embodiment, a template structure is located on the
substrate. A masking material covers a first portion of the
template structure. A plurality of carbon nanotubes are located on
a second portion of the template structure not covered by the
masking material. The carbon nanotubes are not located on the
masking material or on exposed portions of the substrate.
Preferably, a plurality of template structures are located on the
substrate.
[0051] The method of the fifth preferred embodiment includes
providing a substrate containing a template structure having at
least a portion covered by a masking material and providing a
nanotube source gas onto the template structure. The method also
includes selectively growing the carbon nanotubes on an exposed
portion of the template structure but not on exposed portions of
the substrate and not on portions of the template structure covered
by the masking material.
[0052] The method provides selective growth of nanotube bundles in
controlled directions parallel to the substrate surface, by
inhibiting nanotube growth from certain template structure portions
or surfaces, such as the upper and/or some of the side surfaces.
The lengths of CNTs growing in different directions can be tuned by
adjusting the masking material coverage and thickness respective
template structure surfaces, the deposition time, and the SiO.sub.2
pattern spacing. This allows fabrication of arrays of
low-electrical resistance nanotube-metal contact structures that
could be used to create nanotube-based electrical devices.
[0053] FIGS. 7A and 7B are schematic illustrations of a method of
forming the masking material on the template structures. As shown
in FIG. 7A, silicon ridges 11 were photolithographically formed on
a silicon substrate 10. The substrate 10 and the ridges 11 were
covered with a thermally grown SiO.sub.2 layer to form
thermally-oxidized SiO.sub.2 template structures 12. Other suitable
methods to form the template structures may be used if desired. For
example, a CVD SiO.sub.2 layer may be patterned into template
structures, as in the first embodiment.
[0054] FIG. 7B illustrates selective deposition of the gold masking
layer on some surfaces of the template structure 12. The substrate
10 containing the template structures 12 was placed into a
sputtering apparatus containing a sputtering target 20 and was
tilted at 15 to 40 degrees, such as at 20 degrees relative to the
target 20. A gold masking layer was then selectively sputtered onto
the upper surfaces 22 and the left side surfaces 24 of the template
structures. The right side surfaces 26 of the template structures
were not covered by the masking layer. In the specific example of
forming a gold masking layer, a gold layer was sputtered onto
SiO.sub.2 covered Si substrates in a 50 mTorr Ar plasma at an anode
voltage of 7 V and a current of 25 mA, leading to an average
deposition rate of about 1 nm s.sup.-1. Inside the sputter chamber,
the substrates with SiO.sub.2 patterns were placed in a
near-vertical configuration, at a tilt angle of about 20 degrees,
with a sample-cathode distance of 30 mm. This sample placement
geometry results in Au coverage on the top surface and one sidewall
of each SiO.sub.2 template structure that faces the Au flux, while
the opposite sidewall is protected from Au coverage. Rutherford
Backscattering Spectrometry (RBS) measurements carried out using a
2 MeV He.sup.++ probe beam show that SiO.sub.2 surfaces are covered
with 1.1.times.10.sup.17 atoms/cm.sup.2 of Au (nominal thickness of
about 20 nm) when exposed to the Au sputter-flux for 20
seconds.
[0055] Of course, other masking layer fabrication methods may be
used instead. For example, the masking layer may be deposited by
evaporation, sputtering, CVD or plating on the entire surface of
the substrate and the template structures. The masking layer may
then be photolithographically patterned to cover only the desired
portions of the template structures 12. Alternatively the masking
layer can be deposited only at desired locations by using a shadow
mask, or through lift-off lithography techniques.
[0056] The Au-coated SiO.sub.2 template structures were placed into
a CVD tube furnace to grow carbon nanotubes selectively on exposed
SiO.sub.2 surfaces using a xylenes-ferrocene mixture. Substrates
without Au coating were also loaded in the CVD furnace. A solution
of 0.4 g ferrocene dissolved in 40 ml xylenes was fed into the CVD
furnace at temperatures between 600 to 1100.degree. C. for time
intervals ranging from 5 to 30 minutes. This method produced
aligned nanotubes, which were typically multiwalled nanotubes with
diameters of about 20 to 50 nm.
[0057] FIGS. 8A-8B illustrate the growth behavior of carbon
nanotubes (CNTs) on SiO.sub.2 template structures that were not
coated with Au before CVD. FIG. 8A illustrates star-like growth of
aligned CNTs on a row of SiO.sub.2 template structures without Au
coverage. The inset shows a schematic sketch of the morphology.
FIG. 8B is a high magnification view of FIG. 8A showing the CNT-CNT
interface (see arrow) between SiO.sub.2 patterns. The nanotubes
grow from all five surfaces of these template structure, i.e., the
upper surface and four side surfaces, forming a star-like
structure. The CNTs in each direction are well aligned, and have
the same length of about 25 microns. CNTs growing in opposite
directions, towards each other between patterns, cease to grow when
they meet each other and form CNT-CNT interfaces as shown in FIG.
8B.
[0058] In contrast, FIGS. 8C-F illustrate selective CVD in-plane
growth of aligned CNT bundles on exposed portions of template
structures whose remaining portions were covered by a masking
material. The selective growth is clearly in FIG. 8C, which shows
50 micron long CNT bridges grown from the left sidewalls of
SiO.sub.2 template structures whose upper and right sidewall
surfaces were coated with Au. The CNTs form continuous nanotube
bridges across a row of template structures.
[0059] The CNTs grow straight, in a direction perpendicular to the
silica surface seeding them, until they arrive at the next template
structure, as shown in FIG. 8D. The CNT growth terminates abruptly
when the nanotube tips reach the Au masking material covered
sidewall of the next template structure, which serves as a barrier
to further growth. This feature is illustrated in FIG. 8E. FIG. 8E
is a close-up view of the rectangular box in FIG. 8D, and shows a
sharp interface between CNT tips and the template structure
sidewall covered by Au where CNT growth stops. The inset in FIG. 8E
is a schematic illustration of the side-view of this interface.
Thus, when the masking material is a metal, a metal-nanotube
contact structure is formed where the carbon nanotubes contact the
metal masking material. Horizontal arrows in FIGS. 8C, D, E and F
indicate the CNT growth direction.
[0060] The process of selectively coating Au on different SiO.sub.2
surfaces can be harnessed to controllably grow CNTs along any sets
of in-plane directions. For example, the nanotube growth direction
was reversed by coating Au on the upper surface and left sidewall
of each template structure, as shown in FIG. 8F, compared to the
direction shown in FIG. 8B.
[0061] The CNT length is solely controlled by deposition time in
the absence of physical obstacles in the growth direction. The CNTs
grown from the last template structure in FIG. 8F are about 80
microns long, while those grown between template structures are
only 50 microns long, identical to the inter structure distance or
spacing. This is a useful attribute in order to simultaneously
bridge patterns with different inter-pattern distances by a single
CVD process, where the growth time can be chosen to allow CNTs to
bridge the longest inter structure gaps. Thus, nanotube bridges may
be formed between different template structures in a single step
even when a distance or spacing between a first and a second
template structure differs from a distance or spacing between a
second and a third template structure. The nanotube bridges which
connect the first template structure to a second template structure
differ in length from those which connect the second template
structure to the third template structure. These nanotube bridges
extend parallel to a surface of the substrate.
[0062] Thus, the nanotube source gas is provided for a sufficient
time to allow the carbon nanotubes growing on the exposed portions
of a one template structure to contact the masking material
covering the covered portion of an adjacent template structure to
form a nanotube bridge between these template structures.
Furthermore, the nanotube source gas is provided for a sufficient
time to simultaneously grow carbon nanotubes of a different length
to allow the carbon nanotubes growing on the exposed portions of
template structures to contact the masking material covering the
covered portions of adjacent template structures. Thus, the growth
time is selected to allow the nanotubes to bridge the longest
desired structure spacing or distance. Thus, by varying the growth
time of nanotubes on an array of differently spaced template
structures, only the template structures with the desired maximum
inter structure spacing are connected by the bridges.
[0063] FIGS. 8B and 8F also show that CNTs grown on the two
side-walls of SiO.sub.2 patterns indicated by two tilted arrows in
FIG. 8F are curled and shorter compared to the straight nanotube
bundles between patterns. This result suggests that a lower Au
coverage due to line-of-sight Au deposition on these SiO.sub.2
surfaces leads to short nanotubes with less alignment.
[0064] In order to examine this correlation, the present inventors
measured the length of CNTs grown on SiO.sub.2 surfaces with
different Au coverages. The Au coverages were measured by RBS and
varied between 0 and 1.1.times.10.sup.17 atoms/cm.sup.2.
[0065] FIG. 9A is a plot of CNT length as a function of Au coverage
on SiO.sub.2. The nominal Au film thickness is also shown on the
top axis. For all the cases, CVD conditions were identical and the
reaction time was 30 min. FIG. 9B is an SEM image of Au islands
(white contrast) on SiO.sub.2 surface. FIG. 9C is an SEM image of
CNTs grown from the two sidewalls of a template structure with
different lengths, 40 microns leftward from a SiO.sub.2 surface
partially covered by Au and 100 microns downward from a surface not
covered by Au during the same deposition step.
[0066] The plot in FIG. 9A shows that CNT lengths decrease
monotonically with increasing Au-atom coverage, and no nanotube
growth is observed for Au coverages above 1.1.times.10.sup.17
atoms/cm.sup.2 (nominal Au thickness of about 20 nm). While present
inventors do not want to be bound by any particular theory, they
propose that the nanotube length decreases with increasing masking
layer thickness according to the following mechanism. For Au
coverages less than about 1.1.times.10.sup.17 atoms/cm.sup.2, the
as-deposited Au layer on SiO.sub.2 is discontinuous and has an
island morphology, as shown in FIG. 9B. It is assumed that the Au
islands decrease the intake and interactions of carbon/catalyst
flux with SiO.sub.2, and alter the nanotube growth rate. At higher
coverages, the Au layer is continuous and masks the entire
SiO.sub.2 surface, thus completely preventing CNT growth. The CNT
length-dependence on the Au-coverage is useful for obtaining CNTs
of tunable lengths along different directions through control of Au
coverage on different faces of SiO.sub.2 patterns, as shown in FIG.
9C.
[0067] Thus, in a method of a preferred aspect of the fifth
embodiment, a first material which facilitates growth of carbon
nanotubes, such as a silica or other suitable material layer or
structure is provided. At least a portion of the first material is
covered with a masking material, such as Au or another material
which inhibits CNT growth. A nanotube source gas is provided onto
the first material. The carbon nanotubes of a first length are
selectively grown on an exposed portion of the first material. If
the masking material is thicker than a critical thickness at which
it comprises a continuous layer (e.g., Au layer with nominal
thickness of greater than about 20 nm), then no CNTs are grown on
the masking material. If the masking material is thinner than a
critical thickness at which it comprises a continuous layer (i.e.,
such as an about 20 nm or thinner Au layer), then shorter CNTs of a
second length less than the first length are growth on the masking
material. Thus, if plurality of different masking materials and/or
a plurality of masking material portions of different thickness are
formed over the first material, then a plurality of CNTs with
different lengths may be grown in the same growth step.
[0068] Therefore, by varying the masking material thickness and/or
type and/or by varying the spacing the template structure spacing,
a plurality of CNTs of different length may be controllably grown
during the same deposition step. The growth method includes
providing a growth surface which facilitates growth of carbon
nanotubes, providing a nanotube source gas onto the growth surface,
and controllably growing the carbon nanotubes of different length
during the same deposition step.
[0069] The present inventors carried out electrical resistance
measurements with a two-point probe on Au-coated SiO.sub.2 template
structures with and without CNT bridges to explore their use in
meso-scale circuits. FIG. 10A is a schematic sketch of
configuration used to measure the electrical resistance. The
unbridged sample was prepared using a short CVD time so that CNTs
did not grow long enough to connect adjacent SiO.sub.2 template
structure. Two bridged samples were also prepared. During the
measurements, one probe was fixed at P.sub.0, while another probe
was moved to different locations from P.sub.1, to P.sub.10, to
record the resistances across different number of CNT bridges. FIG.
10B is a plot of resistance as a function of number of CNT bridges.
Circles and squares show the resistance behavior for structures
with CNT bridges, while triangles show data obtained from unbridged
structures. R.sub.unbridged is due to the Au layer sputtered on the
substrate area between SiO.sub.2 patterns.
[0070] For all three samples, the resistance increases linearly
with the number of patterns. The circuit contact resistance between
the probe and the Au layer on top of SiO.sub.2 patterns
(=2R.sub.probe-Au), obtained from the ordinate intercept of each
plot, is also nearly constant at about 200-300 ohms for all
samples. However, the average resistance between two adjacent
patterns, denoted by the slope of the plots, is more than a factor
of two smaller for patterns bridged by CNTs
(R.sub.bridged=54.+-.2.OMEGA.) compared with that of the unbridged
patterns (R.sub.unbridged=120.OMEGA.). This result indicates that
the CNT bridges provide a low-resistance electronic transport
pathway between the SiO.sub.2 patterns. The low resistance of the
pattern-CNT-pattern units in the prepared samples is consistent
with the good contact between CNT tips and Au layers observed in
FIG. 8E, and the well-graphitized multi-walled tube structures.
[0071] The fifth preferred embodiment provides in-plane growth of
CNTs in predefined directions and with tunable lengths by selective
masking of SiO.sub.2 template structures with a metal that does not
catalyze nanotube growth compared to silica. This method can be
used to create nanotube bridges that interconnect SiO.sub.2
patterns and provide low-resistance pathways for electrical
transport. The methods which combine CVD with topographical masking
of patterned substrates may be used for controllably growing
nanotube-metal architectures for electronic switching, memory
storage, sensing and actuation devices.
[0072] The method and structure of the sixth preferred embodiment
will now be described. A structure of the sixth preferred
embodiment includes at least one suspended template material layer.
A first aligned carbon nanotube layer is located on a first surface
of the template material layer. A second aligned carbon nanotube
layer is located on a second surface of the template material
layer, located opposite to the first surface. Thus, a bilayer of
aligned carbon nanotubes contains a suspended template layer
between them. The nanotubes extend away from the respective surface
of the template layer. If desired, three or more layers of aligned
carbon nanotubes may be formed on two or more template layers.
[0073] The suspended template material layer may be a template
material cantilever or membrane supported on a portion of a
substrate that does not catalyze carbon nanotube growth. If the
template layer has sufficient thickness, then a third aligned
carbon nanotube layer is located on an edge surface of the template
material layer, such that the third aligned carbon nanotube layer
is located perpendicular to the first and the second aligned carbon
nanotube layers.
[0074] FIG. 11A illustrates a low magnification SEM image showing a
bilayer of aligned carbon nanotubes grown at a 180 degree angle
with respect to each other (up and down) from the top and bottom
surfaces of a suspended, circular SiO.sub.2 cantilever layer
suspended on a Si base pillar. Any other suitable suspended
template material and substrate/base may also be used. If desired,
the template layer may comprise a membrane (i.e., a suspended layer
supported on two or more sides or on two or more portions of a
surface) rather than a cantilever (i.e., a suspended layer
supported on one side or on one portion of a surface). The template
layer may also contain both edge cantilever portions and a central
membrane portion, if desired.
[0075] The two growth directions from the SiO.sub.2 surfaces
(inset) and a schematic illustration of nanotube bilayer growth are
shown in FIG. 11B. The SiO.sub.2 layer (transparent) is located on
the silicon base pillar (opaque), as illustrated in FIGS. 11B and
11C. This pattern of suspended SiO.sub.2 templates with two exposed
surfaces was generated by undercutting the silica layer located on
a silicon substrate into the substrate during deep etching of about
40-50 microns. The template layer in FIGS. 11A-B does not have
sufficient thickness to grow nanotubes on its edge surface. The
nanotube growth directions are shown by the arrows in FIG. 11C.
[0076] FIGS. 12A-B are schematic sketches showing simultaneous
multilayer and multidirectional growth of oriented nanotubes from
thick (e.g., 2 microns or more) SiO.sub.2 layer suspended on deep
etched Si pillars. FIG. 12A is a schematic of a silicon substrate
with a thick cantilever SiO.sub.2 layer suspended on a Si base
before nanotube deposition. FIG. 12B is a schematic of the nanotube
growth on the suspended SiO.sub.2 layer in three orientations.
Since the silica layer is sufficiently thick, a nanotube layer
growth from the edge surface of the suspended silica layer. FIG.
12C is an SEM micrograph showing the aligned nanotubes extending in
three directions marked by arrows. Thus, as shown in FIGS. 11 and
12, the nanotubes may extend from a suspended template layer toward
an upper surface of the substrate at a 90 degree angle. However,
the nanotubes may extend towards the substrate at any suitable
angle, such as at a 30-160 degree angle, depending on the
orientation of the template layer.
[0077] Simultaneous multidirectional growth of highly oriented
nanotubes with tunable lengths of the preferred embodiments allows
formation of complex three dimensional CNT networks, where
vertically and horizontally aligned nanotube arrays enable the
construction of hierarchical multilevel architectures.
Specifically, the nanotube networks may be used in diverse
applications such as nanotube-based electronic devices, micro and
nano-electromechanical systems, micro- and nano-size porous
supports and membranes for catalysis, fluidics and separation, and
skeletal reinforcements for composites.
[0078] The present inventors have achieved excellent control and
flexibility in designing and fabricating a wide variety of carbon
nanotube architectures, as discussed in the preferred embodiments.
Several ordered nanotube based structural elements of different
orientations may be integrated onto one substrate by combining
standard lithography techniques with a substrate-selective CNT CVD
growth process effected by gas-phase delivery of catalysts.
Structures such as micro-fibers and membranes containing highly
aligned nanotubes can fabricated and could find use in
nanocomposites and electrode systems. The fabrication method
described here is far easier and more versatile than the prior art
methods. The preferred method does not require a metal catalyst
patterning step, thus simplifying the method to a great degree and
providing better control, particularly for the catalyst particle
size, for the growth process. The preferred method is also scalable
over large areas for commercial production with the aid of
fabrication techniques commonly used in silicon microfabrication
technology.
[0079] The foregoing description of the invention has been
presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise
form disclosed, and modifications and variations are possible in
light of the above teachings or may be acquired from practice of
the invention. The description was chosen in order to explain the
principles of the invention and its practical application. It is
intended that the scope of the invention be defined by the claims
appended hereto, and their equivalents.
* * * * *