U.S. patent application number 13/265427 was filed with the patent office on 2012-02-09 for multiple walled nested coaxial nanostructures.
This patent application is currently assigned to OLD DOMINION UNIVERSITY RESEARCH FOUNDATION. Invention is credited to Tarek Abdel-Fattah, Helmut Baumgart, Diefeng Gu, Gon Namkoong.
Application Number | 20120034410 13/265427 |
Document ID | / |
Family ID | 43011782 |
Filed Date | 2012-02-09 |
United States Patent
Application |
20120034410 |
Kind Code |
A1 |
Baumgart; Helmut ; et
al. |
February 9, 2012 |
MULTIPLE WALLED NESTED COAXIAL NANOSTRUCTURES
Abstract
Multiple walled nested coaxial nanostructures, methods for
making multiple walled nested coaxial nanostructures, and devices
incorporating the coaxial nanostructures are disclosed. The coaxial
nanostructures include an inner nanostructure, a first outer
nanotube disposed around the inner nanostructure, and a first
annular channel between the inner nanostructure and the first outer
nanotube. The coaxial nanostructures have extremely high aspect
ratios, ranging from about 5 to about 1,200, or about 300 to about
1200.
Inventors: |
Baumgart; Helmut; (Yorktown,
VA) ; Namkoong; Gon; (Yorktown, VA) ; Gu;
Diefeng; (Newport News, VA) ; Abdel-Fattah;
Tarek; (Yorktown, VA) |
Assignee: |
OLD DOMINION UNIVERSITY RESEARCH
FOUNDATION
Norfolk
VA
|
Family ID: |
43011782 |
Appl. No.: |
13/265427 |
Filed: |
April 23, 2010 |
PCT Filed: |
April 23, 2010 |
PCT NO: |
PCT/US2010/032306 |
371 Date: |
October 20, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61172632 |
Apr 24, 2009 |
|
|
|
Current U.S.
Class: |
428/80 ; 264/344;
427/237; 427/58; 73/23.2; 73/865.8; 977/891; 977/893 |
Current CPC
Class: |
B01D 61/427 20130101;
B01D 69/02 20130101; B01D 67/0072 20130101; B01D 71/02 20130101;
B01D 71/022 20130101; B01D 2313/345 20130101; B01D 2325/08
20130101; C23C 16/01 20130101; B01D 67/0062 20130101; B01D 71/024
20130101; F04B 19/006 20130101; C23C 16/403 20130101 |
Class at
Publication: |
428/80 ; 427/237;
427/58; 264/344; 73/23.2; 73/865.8; 977/891; 977/893 |
International
Class: |
B32B 3/02 20060101
B32B003/02; C23C 16/44 20060101 C23C016/44; B29C 71/00 20060101
B29C071/00; G01N 7/00 20060101 G01N007/00; G01D 21/00 20060101
G01D021/00; C23C 16/04 20060101 C23C016/04; C23C 16/56 20060101
C23C016/56 |
Claims
1. A multiple walled nested coaxial nanostructure comprising: an
inner nanostructure; a first outer nanotube disposed around the
inner nanostructure; and a first annular channel between the inner
nanostructure and the first outer nanotube, wherein the aspect
ratio of the coaxial nanostructure ranges from about 5 to about
1200.
2. The coaxial nanostructure of claim 1, wherein the inner
nanostructure is a nanorod or a nanotube.
3. The coaxial nanostructure of claim 1, wherein the inner
nanostructure is a nanotube.
4. The coaxial nanostructure of claim 1, wherein the inner
nanostructure, the first outer nanotube, or both comprise a
conductor.
5. The coaxial nanostructure of claim 4, wherein the conductor is a
metal or a conducting metal nitride or conducting metal oxide.
6. The coaxial nanostructure of claim 4, wherein the conductor is
selected from Ti, Au, Pt, Al, Cu, Ag, W or a nitride thereof.
7. The coaxial nanostructure of claim 1, wherein the inner
nanostructure, the first outer nanotube, or both comprise an
insulator.
8. The coaxial nanostructure of claim 7, wherein the insulator is a
metal oxide or insulating metal nitride or semiconductor oxide.
9. The coaxial nanostructure of claim 7, wherein the insulator is
selected from SiO.sub.2, HfO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, or
TiO.sub.2.
10. The coaxial nanostructure of claim 1, wherein the inner
nanostructure, the first outer nanotube, or both comprise a
semiconductor.
11. The coaxial nanostructure of claim 10, wherein the
semiconductor is ZnO or TiO.sub.2.
12. The coaxial nanostructure of claim 1, wherein the inner
nanostructure and the first outer nanotube independently comprise a
metal oxide selected from ZnO, SiO.sub.2, HfO.sub.2, ZrO.sub.2,
Al.sub.2O.sub.3, or TiO.sub.2.
13. The coaxial nanostructure of claim 1, wherein the inner
nanostructure and the first outer nanotube comprise the same metal
oxide selected from ZnO, SiO.sub.2, HfO.sub.2, ZrO.sub.2,
Al.sub.2O.sub.3, or TiO.sub.2.
14. The coaxial nanostructure of claim 1, wherein the inner
nanostructure comprises ZrO.sub.2, and the first outer nanotube
comprises ZrO.sub.2.
15. The coaxial nanostructure of claim 1, wherein the inner
nanostructure comprises HfO.sub.2, and the first outer nanotube
comprises HfO.sub.2.
16. The coaxial nanostructure of claim 1, further comprising a
second outer nanotube disposed around the first outer nanotube and
a second annular channel between the first outer nanotube and the
second outer nanotube.
17. The coaxial nanostructure of claim 16, wherein the inner
nanostructure, the first outer nanotube, and the second outer
nanotube independently comprise a metal oxide selected from ZnO,
SiO.sub.2, HfO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, or TiO.sub.2.
18. The coaxial nanostructure of claim 16, wherein the inner
nanostructure, the first outer nanotube, and the second outer
nanotube comprise the same oxide or metal oxide selected from ZnO,
SiO.sub.2, HfO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, or TiO.sub.2.
19. The coaxial nanostructure of claim 16, wherein the inner
nanostructure comprises ZrO.sub.2, the first outer nanotube
comprises ZrO.sub.2, and the second outer nanotube comprises
ZrO.sub.2.
20. The coaxial nanostructure of claim 16, wherein the inner
nanostructure comprises HfO.sub.2, the first outer nanotube
comprises HfO.sub.2, and the second outer nanotube comprises
HfO.sub.2.
21. The coaxial nanostructure of claim 1, wherein the cross-section
of the coaxial nanostructure is substantially circular upon release
from the nanoporous template.
22. The coaxial nanostructure of claim 1, wherein the coaxial
nanostructure is coupled to a substrate.
23. The coaxial nanostructure of claim 1, wherein the coaxial
nanostructure is coupled to a nanoporous substrate, and further
wherein the coaxial nanostructure is disposed within a pore of the
nanoporous substrate.
24. The coaxial nanostructure of claim 1, wherein the coaxial
nanostructure is coupled to a porous anodic aluminum oxide
substrate, and further wherein the coaxial nanostructure is
disposed within a pore of the porous anodic aluminum oxide
substrate.
25. An array comprising two or more coaxial nanostructures
according to claim 1.
26. A device comprising the coaxial nanostructure of claim 1.
27. The device of claim 26, wherein the device is a chemical
sensor, a photovoltaic cell, or a photonic crystal.
28. The coaxial nanostructure of claim 1, wherein the inner
nanostructure, the first outer nanotube, or both are substantially
free of carbon.
29. The coaxial nanostructure of claim 1, wherein the width of the
annular channel ranges from about 5 nm to about 30 nm.
30. The coaxial nanostructure of claim 1, wherein the annular
channel comprises air.
31. The coaxial nanostructure of claim 1, wherein the annular
channel comprises a sacrificial material disposed within the
annular channel.
32. The coaxial nanostructure of claim 1, wherein the annular
channel comprises a sacrificial material disposed within the
annular channel, the sacrificial material capable of being
dissolved by a chemical etchant.
33. The coaxial nanostructure of claim 1, wherein the annular
channel comprises Al.sub.2O.sub.3 disposed within the annular
channel.
34. A method of making a coaxial nanostructure comprising: forming
a layer of a first material on an inner surface of a nanopore of a
nanoporous substrate using atomic layer deposition; forming a first
layer of a sacrificial material on the layer of the first material
using atomic layer deposition; and forming a layer of a second
material on the first layer of the sacrificial material using
atomic layer deposition, wherein a coaxial nanostructure is
provided, the coaxial nanostructure having an aspect ratio ranging
from about 300 to about 1200.
35. The method of claim 34, wherein the nanoporous substrate is
anodic aluminum oxide.
36. The method of claim 34, wherein the sacrificial material
comprises Al.sub.2O.sub.3.
37. The method of claim 34, wherein the first material, the second
material, or both comprise a conductor.
38. The method of claim 34, wherein the first material, the second
material, or both comprise an insulator.
39. The method of claim 34, wherein the first material, the second
material, or both comprise a semiconductor.
40. The method of claim 34, wherein the first material and the
second material independently comprise a metal oxide selected from
ZnO, SiO.sub.2, HfO.sub.2, ZrO.sub.2, or TiO.sub.2.
41. The method of claim 34, further comprising removing the
sacrificial layer by chemical etching.
42. The method of claim 34, further comprising removing the
nanoporous substrate.
43. The method of claim 34, further comprising removing the
nanoporous substrate by chemical etching.
44. The method of claim 34, further comprising forming a second
layer of a sacrificial material on the layer of the second material
and forming a layer of a third material on the second layer of the
sacrificial material.
45. An oxygen sensor comprising the coaxial nanostructure of claim
1.
46. A sensor capable of simultaneously detecting a plurality of
chemicals comprising the coaxial nanostructure of claim 1.
47. The coaxial nanostructure of claim 1, wherein the coaxial
nanostructure is disposed within a pore of a porous substrate.
48. The coaxial nanostructure of claim 1, wherein the porous
substrate is porous silicon or porous anodized aluminum oxide.
49. The multiple walled nested coaxial nanostructure, wherein the
aspect ratio is about 300 to about 1,200.
50. A method of making a coaxial nanostructure comprising: forming
a layer of a first material on an inner surface of a nanopore of a
nanoporous substrate using atomic layer deposition; forming a first
layer of a sacrificial material on the layer of the first material
using atomic layer deposition; and forming a layer of a second
material on the first layer of the sacrificial material using
atomic layer deposition, wherein a coaxial nanostructure is
provided, the coaxial nanostructure having an aspect ratio ranging
from about 5 to about 1200.
51. A multiple walled nested coaxial nanostructure comprising: an
inner nanostructure; a first outer nanotube disposed around the
inner nanostructure; and a first annular channel between the inner
nanostructure and the first outer nanotube, wherein the aspect
ratio of the coaxial nanostructure ranges from about 300 to about
1200.
52. A method of making a coaxial nanostructure comprising: forming
a layer of a first material on an inner surface of a nanopore of a
nanoporous substrate or a macroporous substrate using atomic layer
deposition; forming a first layer of a sacrificial material on the
layer of the first material using atomic layer deposition; and
forming a layer of a second material on the first layer of the
sacrificial material using atomic layer deposition, wherein a
coaxial nanostructure is provided, the coaxial nanostructure having
an aspect ratio ranging from about 5 to about 1200.
53. The method of claim 52, wherein the macroporous substrate is
used.
54. The method of claim 52, wherein the substrate is silicon.
Description
RELATED APPLICATIONS
[0001] This application claims priority to U.S. provisional
application Ser. No. 61/172,632 filed Apr. 24, 2009 to Baumgart et
al., which is hereby incorporated by reference in its entirety.
BACKGROUND
[0002] Nanostructures, including nanotubes, exhibit novel physical
properties and play an important role in fundamental research. In
addition, nanostructures and nanotubes find many practical
applications because of their restricted size and high surface
area. See R. Kelsall et al., Nanoscale Science and Technology,
Wiley, Chichester, (2006); C. R. Martin, Acc. Chem. Mater. 28, 61
(1995); J. Goldberger et al., Nature, 422 599 (2003); and S. B. Lee
et al., Science, 296, 2198 (2002). Nanotubes may be formed from a
variety of materials, including different classes of materials such
as insulators, semiconductors, and metals, including transition
metal oxides. In particular, hafnium oxide (hafnia, HfO.sub.2),
aluminum oxide (alumina, Al.sub.2O.sub.3), titanium oxide
(TiO.sub.2) and zirconium oxide (zirconia, ZrO.sub.2) are important
materials widely used in ceramics, chemical sensors, catalysts,
opto-electronics, and as high-k dielectrics in microelectronics.
The semiconductor, zinc oxide (ZnO), is also used in chemical
sensors. However, a need exists for nanotubes formed from metal
oxides and other materials that have more complex structures,
higher aspect ratios, and higher surface areas.
SUMMARY
[0003] Provided herein are multiple walled nested coaxial
nanostructures, methods for making the multiple walled nested
coaxial nanostructures, and devices incorporating the multiple
walled nested coaxial nanostructures. The disclosed multiple walled
nested coaxial nanostructures have extremely high aspect ratios and
surface areas. Consequently, devices incorporating these multiple
walled nested coaxial nanostructures exhibit superior and novel
properties as compared with conventional devices. These advantages
are further discussed below with respect to specific devices
incorporating the coaxial nanostructures. The disclosed multiple
walled nested coaxial nanostructures may be formed using atomic
layer deposition (ALD) or other suitable chemical vapor deposition
(CVD) techniques to deposit different materials by coating the
inner walls of the pores of various nanoporous substrates (also
referred to herein as nanoporous templates or nanoporous
membranes), one atomic layer at a time. Nanoporous substrates or
templates may be formed from nanoporous alumina, polycarbonate
membranes, porous silicon, or any other suitable porous substrate.
The ability to achieve multiple walled nested coaxial
nanostructures with such high aspect ratios is derived, in part,
from the use of long ALD deposition dwell times and the use of
sacrificial spacer layer technology to open up all surfaces of such
multiple walled nested coaxial nanostructures. The use of long ALD
deposition dwell times is contrary to conventional wisdom, since
longer ALD deposition times can clog the pores of the underlying
porous substrates.
[0004] In one aspect, multiple walled nested coaxial nanostructures
are provided. In one embodiment, the multiple walled nested coaxial
nanostructure may include an inner nanostructure, a first outer
nanotube disposed around the inner nanostructure, and a first
annular channel between the inner nanostructure and the first outer
nanotube. In another embodiment, the coaxial nanostructure may
further include a second outer nanotube disposed around the first
outer nanotube and a second annular channel between the first outer
nanotube and the second outer nanotube. In other embodiments, a
third outer nanotube may be disposed around the second outer
nanotube, a fourth outer nanotube may be disposed around the third
outer nanotube, and so forth, up to an n.sup.th outer nanotube. The
aspect ratio of the coaxial nanostructures may range from about 5
to about 1,200, or about 300 to about 1200, although other aspect
ratios are possible.
[0005] The materials used to form the inner nanostructure and the
outer nanotubes may vary and may include a conductor, an insulator,
or a semiconductor. Specific examples of conductors, insulators,
and semiconductors are provided herein. A sacrificial spacer
material, including Al.sub.2O.sub.3, may be disposed within the
annular channel of any of the coaxial nanostructures in order to
create annular channels to open up all surfaces (inner and outer
wall) of these multiple walled nested coaxial nanostructures.
[0006] The multiple walled nested coaxial nanostructures may be
coupled to other components, including various substrates. In some
embodiments, the substrate may be a porous anodic aluminum oxide
(AAO) substrate. In other embodiments, a porous silicon substrate
or any other suitable porous template may be used. The substrate
can be macroporous. Also provided herein are arrays comprising two
or more of any of the disclosed coaxial nanostructures and devices
incorporating any of the disclosed coaxial nanostructures.
[0007] In another aspect, methods for making the multiple walled
coaxial nanostructures are provided. In one embodiment, the method
may include forming a layer of a first material on an inner surface
of a nanopore of a nanoporous substrate using atomic layer
deposition, forming a first layer of a sacrificial material on the
layer of the first material using atomic layer deposition, and
forming a layer of a second material on the first layer of the
sacrificial material using atomic layer deposition. In another
embodiment, the method may further include forming a second layer
of a sacrificial material on the layer of the second material and
forming a layer of a third material on the second layer of the
sacrificial material (until the n.sup.th layer in the most general
case). The aspect ratio of the coaxial nanostructures provided by
such a method may range from about 5 to about 1,200, or about 300
to about 1200, although other aspect ratios are possible. The
methods may further include removing the nanoporous template and/or
the layers of the sacrificial spacer material by a variety of
methods, including by chemical etching. The nanoporous substrates
and the compositions of the first material, the second material,
the third material, the n.sup.th material and the sacrificial
spacer material may vary as described above with respect to the
multiple walled nested coaxial nanostructures.
[0008] In yet another aspect, an electroosmotic pump is provided.
In one embodiment, an electroosmotic pump may include a nanoporous
membrane having one or more nanopores, a layer of a first material
deposited on an inner surface of the nanopore, a first electrode
(anode) coupled to a first side of the nanoporous substrate, and a
second electrode (cathode) coupled to a second side of the
nanoporous template. In another embodiment, the electroosmotic pump
may further include a layer of a second material deposited on the
layer of the first material. A variety of nanoporous substrates and
compositions for the first material, the second material, and the
electrodes may be used. Specific examples are provided herein. The
performance of the disclosed electroosmotic pumps is superior to
conventional pumps.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIGS. 1A and 1B show SEM images of a porous anodic aluminum
oxide (AAO) substrate. FIG. 1A shows the surface of the substrate
after ion milling. FIG. 1B shows a cross-section of a cleaved AAO
substrate.
[0010] FIG. 2 shows a cross-sectional SEM image of ALD (atomic
layer deposited) zirconia coated AAO substrate (A) and a
corresponding EDS Zr mapping showing uniform distribution of
zirconia throughout the entire thickness of the 60 .mu.m AAO
substrate (B).
[0011] FIG. 3 shows a top-down SEM image of an uncoated AAO
substrate (A); the same AAO substrate with a thin film ALD coating
of ZrO.sub.2 (B); and the same coated AAO substrate after the
alumina substrate walls have been removed to provide single
ZrO.sub.2 nanotubes (C).
[0012] FIG. 4 shows a SEM micrograph of HfO.sub.2 tube-in-tube
coaxial nanostructures. A top-down view of the sample surface and a
partial side-view from a cleavage site is shown by tilting the
sample.
[0013] FIG. 5 is a TEM micrograph of a separated HfO.sub.2
tube-in-tube coaxial nanostructure shown in FIG. 4.
[0014] FIG. 6 is a top-down SEM image showing three concentric
metal oxide (ZrO2) nanotubes inside large AAO pores following the
dissolution of the 2 separating spacer Al.sub.2O.sub.3 layers in
order to expose the sidewalls of the coaxial (ZrO2) nanotubes.
These five coaxial nanostructures were formed by layering
ZrO.sub.2/Al.sub.2O.sub.3/ZrO.sub.2/Al.sub.2O.sub.3/ZrO.sub.2 and
removing the Al.sub.2O.sub.3 layers by chemical etching.
[0015] FIG. 7 is a top-down SEM image of hafnia/zirconia coaxial
nanostructures disposed in the nanopores of a AAO substrate showing
the simultaneous coating of the AAO surface and the inner walls of
the nanopores.
[0016] FIG. 8 is an illustration of an exemplary coaxial
nanostructure having an inner nanotube of ZnO, a first outer
nanotube of ZrO.sub.2 disposed around the inner nanotube, and a
first annular channel between the inner nanotube and the outer
nanotube.
[0017] FIG. 9 is an illustration of an exemplary structure that may
be used to provide a chemical sensor.
[0018] FIG. 10 is an SEM image of free-standing single walled
nanotubes.
[0019] FIG. 11 is a schematic depiction of a process sequence for
synthesizing free-standing HfO2 nested coaxial tube-in-tube
nanostructures.
[0020] FIG. 12 shows coaxial HfO.sub.2 nanotubes formed by, for
example, the method depicted in FIG. 11.
[0021] FIG. 13 provides thermodynamic modeling diagrams showing the
distributions of ionic species to represent, for example, the
removal of AAO substrate from ZnO nanotubes using NaOH of various
pH values.
[0022] FIG. 14 is an SEM micrograph showing large numbers of high
aspect ratio coaxial ALD HfO.sub.2 nanotubes.
[0023] FIG. 15 is a top-down SEM image showing five nested coaxial
ALD layers such as nested nanotube structures.
DETAILED DESCRIPTION
[0024] Provided herein are multiple walled nested coaxial
nanostructures, methods for making the multiple walled nested
coaxial nanostructures, and devices incorporating the multiple
walled nested coaxial nanostructures.
Multiple Walled Nested Coaxial Nanostructures
[0025] The multiple walled nested coaxial nanostructures include an
inner nanostructure and at least one outer nanotube disposed around
the inner nanostructure. The multiple walled nested coaxial
nanostructures may include multiple outer nanotubes (up to n outer
nanotubes) arranged concentrically around the inner nanostructure.
This includes embodiments in which the coaxial nanostructure
includes a first outer nanotube disposed around an inner
nanostructure, a second outer nanotube disposed around the first
outer nanotube, a third outer nanotube disposed around the second
outer nanotube, and so forth. In any of these embodiments, the
inner nanostructure may also be a nanotube. However, the innermost
nanostructure may also be a nanorod.
[0026] The multiple walled nested coaxial nanostructures may also
include an annular channel between the inner nanostructure and the
at least one outer nanotube. For those embodiments having more than
one outer nanotube, the coaxial nanostructure may include
additional annular channels between the additional outer nanotubes.
By way of example only, a multiple walled nested coaxial
nanostructure may include a first outer nanotube disposed around an
inner nanostructure, a first annular channel between the inner
nanostructure and the first outer nanotube, a second outer nanotube
disposed around the first outer nanotube, a second annular channel
between the first outer nanotube and the second outer nanotube, and
so forth. In some embodiments, the annular channel comprises air,
after the sacrificial spacer material has been removed from the
annular channel. In other embodiments, the annular channel may
comprise a sacrificial material. Sacrificial materials are further
described below.
[0027] The materials used to form the coaxial nanostructures may
vary. By way of example only, the inner nanostructure and any of
the outer nanotubes may comprise a conductor, an insulator, or a
semiconductor. A variety of conductors may be used, including
metals or nitrides of metals. Non-limiting examples of metals
include Ti, Au, Pt, Al, Cu, Ag, and W. Non-limiting examples of
conducting metal nitrides include TiN and TaN and conducting metal
oxides include ITO (indium tin oxide) and RuO.sub.2. Similarly, a
variety of insulators may be used, including metal oxides.
Non-limiting examples of insulating oxides and metal oxides include
SiO.sub.2, HfO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, Ta.sub.2O.sub.5,
La.sub.2O.sub.3, Y.sub.2O.sub.3, MoO.sub.2, In.sub.2O.sub.3,
V.sub.2O.sub.5, A variety of semiconductors may also be used,
including, but not limited to ZnO, TiO.sub.2, WO.sub.3, NiO, GaAs,
GaP, GaN, InP, InAs, AlAs, and Ge. In some embodiments, the inner
nanostructure and any of the outer nanotubes are substantially free
of carbon. By "substantially free of carbon," it is meant that the
nanostructures do not include, and are not formed of, carbon.
However, such nanostructure may include trace amounts of carbon
that may be unavoidable due to the methods used to form the
nanostructures. The structures can be different from and not
comprise carbon nanotubes including multi-walled carbon nanotubes,
single walled carbon nanotubes, and other types of carbon
nanotubes. In still other embodiments, the inner nanostructure and
any of the outer nanotubes are completely free of carbon. The inner
nanostructure and each of the outer nanotubes may be formed of the
same material. Alternatively, the inner nanostructure and each of
the outer nanotubes may be each formed of different materials.
Finally, some of outer nanotubes and the inner nanostructure may be
formed of the same material while others are formed of different
materials.
[0028] The dimensions of the coaxial nanostructures may also vary.
The diameter of the coaxial nanostructures may range from about 50
nm to about 300 nm for alumina templates and at the upper range
pore diameters may range as large as several micrometers for porous
silicon templates. The pore diameter range that is achievable
depends on the material parameters of the porous template material
and the electro-chemical parameters of the fabrication method used.
This includes embodiments in which the diameter is about 60 nm, 75
nm, 90 nm, 125 nm, 150 nm, 175 nm, 200 nm, 250 nm, or 300 nm
(including 300 nm for AAO case). The length of the coaxial
nanostructures may range from about 15 .mu.m to about 75 .mu.m.
This includes embodiments in which the length is about 20 .mu.m, 30
.mu.m, 40 .mu.m, 50 .mu.m, 60 .mu.m, or 70 .mu.m. The aspect ratio
(the ratio of the length of the coaxial nanostructure to the
diameter of the coaxial nanostructure) may also vary. In some
embodiments, the aspect ratio ranges from about 5 to about 1,200,
or about 300 to about 1200. This includes embodiments in which the
aspect ratio is about 400, 500, 600, 700, 800, 900, or 1000.
Finally, the cross-sectional shape of the coaxial nanostructures
may vary. In some embodiments, the cross-sectional shape is a
polyhedron, such as an octahedron. In other embodiments, the
cross-sectional shape is substantially circular. By "substantially
circular," it is meant that shape is circular-elliptical, but not
necessarily perfectly circular.
[0029] Similarly, the dimensions of the outer nanotubes and the
inner nanorod or nanotube forming the multiple walled nested
coaxial nanostructures may vary, depending upon number of such
structures present in the coaxial nanostructure and the overall
dimensions of the coaxial nanostructure itself. The width of the
walls of the nanotubes and the width of the annular spacer channel
(if present) may also vary. In some embodiments, the width ranges
from about 5 nm to about 30 nm. This includes embodiments in which
the width is about 10 nm, 15 nm, 20 nm, 25 nm and 35 nm.
[0030] Multiple walled nested coaxial nanostructures may be coupled
to other elements. In some embodiments, the multiple walled nested
coaxial nanostructure is coupled to a substrate. A variety of
substrates may be used, including any of the metals described
above. In some embodiments, the substrate is an Al substrate. In
such embodiments, the coaxial nanostructure may be attached to the
substrate at one of the ends of the coaxial nanostructure. In other
embodiments, the substrate may be a nanoporous substrate and the
multiple walled nested coaxial nanostructure may be disposed within
a pore of the nanoporous substrate. A variety of nanoporous
substrates may be used, including, but not limited to, porous
anodic aluminum oxide (AAO) substrates, polycarbonate nanoporous
templates (membranes), and porous silicon. Such nanoporous
(substrates) templates are known and AAO is commercially available.
In still other embodiments, the multiple walled nested coaxial
nanostructure may be coupled to both a metal substrate, such as an
Al substrate, and a nanoporous substrate, such as an AAO substrate.
In such an embodiment, the coaxial nanostructure may be disposed
within a pore of the nanoporous substrate and attached to the metal
substrate at one of the ends of the coaxial nanostructure.
[0031] Regarding AAO substrates, anodic aluminum oxide can be
formed by electrochemical oxidation of aluminum in acidic solutions
to form regular porous channels, which are parallel to each other.
See H. Masuda and K. Fukuda, Science, 268, 1466 (1995); V. P. Menon
and C. R. Martin, Anal. Chem., 67, 1920 (1995); and M. A. Cameron,
I. P. Gartland, J. A. Smith, S. F. Diaz and S. M. George, Langmuir,
16, 7435 (2000). The individual pore diameters inside the porous
alumina membrane are mainly defined by the anodization voltage. The
diameter of the pore depends on the electrolyte nature, its
temperature and concentration, the current density and other
parameters of the anodization process. Aside from the modulation of
the pore diameters by variation of the electrolyte composition and
anodization conditions, it is possible to further enlarge the pore
diameters by another subsequent selective etching of the porous
template walls. The Examples below provide an exemplary method for
making a suitable AAO substrate.
[0032] Also provided herein are arrays of two or more of any of the
coaxial nanostructures described above. The arrays of coaxial
nanostructures may be coupled to any of the substrates described
above.
[0033] A non-limiting exemplary multiple walled nested coaxial
nanostructure is illustrated in FIG. 8. In FIG. 8, a first layer of
a metal oxide (e.g., ZrO.sub.2) is deposited on the inner surfaces
of the nanopores of an AAO substrate. Next, a second layer of a
metal oxide (e.g., Al.sub.2O.sub.3) is deposited on the first layer
of the metal oxide. Next, a third layer of a metal oxide (e.g.,
ZnO) may be deposited on the second layer of the metal oxide.
Finally, both the AAO substrate and the second layer of the metal
oxide may be removed by etching to provide a coaxial nanostructure
comprising an inner nanotube of ZnO, a first outer nanotube of
ZrO.sub.2 disposed around the inner nanotube, and a first annular
channel between the inner nanotube and the outer nanotube.
Methods
[0034] The multiple walled nested coaxial nanostructures described
above may be prepared according to the following methods. The
methods can use atomic layer deposition or other suitable chemical
vapor deposition (CVD) techniques to deposit layers (also referred
to as films herein) of the types of materials described above on
the inner surface of the nanopores of a nanoporous substrate. ALD
is a known technique. Briefly, ALD technology deposits thin films
using pulses of chemical precursor gases to adsorb at the target
surface one atomic layer at a time. ALD is based on the sequential
deposition of individual monolayers or fractions of a monolayer in
a controlled fashion. More specifically, in ALD the growth
substrate surface is alternately exposed to the vapors of one of
two chemical reactants (complementary chemical precursors), which
are supplied to the reaction chamber one at a time. The exposure
steps are separated by inert gas purge or pump-down steps in order
to remove any residual chemical precursor or its by-product before
the next chemical precursor can be introduced into the reaction
chamber. Thus, ALD involves a repetition of individual growth
cycles. See also Ritala, M., "Atomic Layer Deposition", p. 17-64,
in Institute of Physics Series in Materials Science and Engineering
"High-k Gate Dielectrics" edited by Michel Houssa, Institute of
Physics Publishing, Bristol and Philadelphia 2003.; Leskala, M.,
and Ritala, M., "ALD Precursor Chemistry: Evolution and Future
Challenges," J. Phys. IV 9, p. 837-852, 1999.
[0035] Since a film deposited by ALD is grown in a layer-by-layer
fashion and the total film thickness is given by the sum of the
number of ALD cycles, it is possible to calculate the number of
cycles necessary to reach a desired final film thickness.
Conversely the thickness of a film can be set digitally by counting
the number of reaction cycles. In general, ALD achieves deposition
rates on the order of 0.1-1.0 .ANG. per cycle, with cycle times
ranging from one to ten seconds. Due to the self-limiting nature of
the surface reactions, accidental overdosing with precursors does
not result in increased film deposition. Thus, ALD is able to
achieve very precise across-wafer film thickness uniformity,
unmatched step coverage and exceptional conformality. Because of
the nature of ALD, film thickness is immune to variations caused by
non-uniform distribution of reactant vapor or temperature in the
reaction chamber. See Niinisto, L., Paivasaari, J., Niinisto, J.,
Putkonen, M., and Mieminen, M., "Advance electronic and
optoelectronic materials by Atomic Layer Deposition: An overview
with special emphasis on recent progress in processing high-k
dielectrics and other oxide materials", Phys. Stat. Solid. (a) 201,
p. 1443-1452, (2004); and Ritala, M., "Atomic layer deposition,"
Editors Michel Houssa, High-k Gate Dielectrics, p. 17-64, Publisher
Institute of Physics Publishing, Bristol, UK, 2004.
[0036] A variety of chemical precursors may be used with ALD,
depending upon the desired film. The general requirements and
properties of useful chemical precursors are known. See Sneh, O.,
Clark-Phelps, R. B., Londergan, A. R., Winkler J., and Seidel, T.,
"Thin film atomic layer deposition equipment for semiconductor
processing," Thin Solid Films, Vol. 402, Issues 1-2, p. 248-261,
2002 and Leskela, M., and Ritala, M., "Atomic Layer Deposition
(ALD): from precursor to thin film structures," Thin Solid Films,
409, p. 138-146, 2002. Specific chemical precursors are provided in
the Examples below.
[0037] In one embodiment of the disclosed methods, the method
comprises forming a layer of a first material on an inner surface
of a nanopore of a nanoporous substrate using atomic layer
deposition and forming a layer of a second material on the layer of
the first material using atomic layer deposition. In another
embodiment, a layer of a third material may be formed on the layer
of the second material, a layer of a fourth material may be formed
on the layer of the third material, and so forth. In each of these
embodiments, the layer of the first material corresponds to an
outer nanotube of the coaxial nanostructures described above. The
layer of the second material provides either an additional outer
nanotube, or an inner nanostructure, depending upon the number of
layers of materials deposited. The first material, second material,
and third material may include any of the conductors, insulators,
and semiconductors described above. Similarly, any of the
nanoporous substrates described above may be used with the
disclosed method.
[0038] The method may further comprise removing the nanoporous
(substrate) template after the multiple walled nested coaxial
nanostructure is formed. A variety of methods may be used to remove
the nanoporous (substrate) template, including, but not limited to
chemical etching. A variety of chemical etchants may be used,
depending upon the composition of the nanoporous substrate. By way
of example only, when the nanoporous substrate is AAO, NaOH may be
used to remove the porous template (substrate).
[0039] In another embodiment of the disclosed methods, the method
comprises forming a layer of a first material on an inner surface
of a nanopore of a nanoporous substrate using atomic layer
deposition, forming a first layer of a sacrificial material on the
layer of the first material using atomic layer deposition, and
forming a layer of a second material on the first layer of the
sacrificial material using atomic layer deposition. Other
sacrificial spacer layers and layers of additional materials may be
deposited. For example, a second layer of a sacrificial spacer
material may be formed on the layer of the second material, a layer
of a third material may be formed on the second layer of the
sacrificial material, and so forth. By "sacrificial spacer
material," it is meant a material that is capable of being
substantially removed (i.e., removed, but not necessarily
completely removed) by a chemical etchant. A non-limiting example
of a sacrificial material is Al.sub.2O.sub.3, which is capable of
being substantially removed by a variety of chemical etchants,
including NaOH. However, the sequence of synthesizing the multiple
walled nested coaxial nanostructures comprises alternating
sacrifical spacer material annular rings with the next nested
coaxial nanotube material of choice. As discussed above, the first
material, second material, and third material may include any of
the conductors, insulators, and semiconductors described above.
Similarly, any of the nanoporous templates (substrates) described
above may be used with the disclosed method.
[0040] In the method involving deposition of a layer or layers of a
sacrificial material, the method may further comprise removing any
or all of the sacrificial layers by chemical etching. Such a method
provides the multiple walled nested coaxial nanostructures having
one or more annular channels comprising air, as described above.
The method may further comprise removing the nanoporous substrate
after the coaxial nanostructure is formed, as described above.
[0041] The description of the coaxial nanostructures, AAO
substrates, and ALD process make clear that the dimensions of the
coaxial nanostructures are both a function of the pore sizes of the
AAO substrates as well as the number of cycles and length of each
cycle of the ALD process. In order to make the highest aspect ratio
coaxial nanostructures for a given AAO substrate, the length of the
cycle may be maximized to ensure deposition along the entire length
of the nanopore. Long cycle times, however, are contrary to the
conventional wisdom that cycle times should be minimized to prevent
clogging the pores of the AAO substrates.
DEVICES AND APPLICATIONS
[0042] The multiple walled nested coaxial nanostructures described
above may be incorporated into a variety of devices for use in a
variety of applications. By way of example only, the multiple
walled nested coaxial nanostructures may be used in electroosmotic
pumps, chemical sensors, photovoltaic devices, and photonic
crystals. The multiple walled nested coaxial nanostructures may
also find use as extremely hard and highly durable nanometer-sized
pipette tips for various medical applications. Although many of
these devices are known, devices incorporating the disclosed
coaxial nanostructures are expected to exhibit superior properties
over conventional devices due to the high aspect ratio and high
surface area of the coaxial nanostructures. These devices are
further described below.
Electroosmotic Pumps
[0043] Electroosmosis is the motion of ionized liquid relative to a
stationary charged surface by an externally applied electric field.
Electroosmotic (EO) flows are useful in microfluidic systems, since
they enable fluid pumping and flow control without the need for
mechanical pumps or valves, and they also minimize the sample
dispersion effects. See Karniadakis, G. E., Beskok, A., and Alum,
N., Microflows and Nanoflows: Fundamentals and Simulation,
Springer, N.Y., 2005. However, conventional EO pumps suffer from a
number of drawbacks, including the need for large operating
voltages (on the order of 1 kV to 10 kV), electrolysis of water,
oxidation of electrode surfaces, and Joule heating. The need for a
high voltage supply limits the use of conventional EO pumps in
lab-on-a-chip (LoC) type portable devices, designed for
bio-medical, pharmaceutical, environmental monitoring and
homeland-security applications.
[0044] In one embodiment, a two-terminal electroosmotic pump
comprises a nanoporous substrate having one or more nanopores and a
layer of a first material deposited on an inner surface of the
nanopore. The layer of the first material provides a nanotube
disposed within the nanopore of the nanoporous substrate.
Electrodes may be coupled to both sides of the nanoporous
substrate. Any of the nanoporous substrates described above may be
used. In some embodiments, the aspect ratio of the nanopores of the
nanoporous substrate ranges from about 5 to about 1,200, or about
300 to about 1200. This includes embodiments in which the aspect
ratio is about 400, 500, 600, 700, 800, 900, or 1000. The
composition of the first material may vary. In some embodiments,
the first material comprises a metal oxide or a metal nitride. Any
of the metal oxides disclosed above may be used, including, but not
limited to HfO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, ZnO, TiO.sub.2,
TiN, or SiO.sub.2. Similarly, the composition of the electrodes may
vary. In some embodiments, the electrodes comprise a metal.
Examples of useful metals, include, but are not limited to, Au, Pt,
and W. As noted above, the performance of the disclosed
two-terminal electroosmotic pump exceeds that of conventional
electroosmotic pumps.
[0045] In another embodiment, a three-terminal electroosmotic pump
comprises a nanoporous substrate having one or more nanopores, a
layer of a first material deposited on an inner surface of the
nanopore, and a layer of a second material deposited on the layer
of the first material. The layer of the first material provides an
outer nanotube and the second material provides an inner nanotube,
resulting in a coaxial nanostructure disposed within the nanopore
of the nanoporous substrate. Electrodes may be coupled to both
sides of the nanoporous substrate. Any of the nanoporous substrates
described above may be used. In some embodiments, the aspect ratio
of the nanopores of the nanoporous substrate ranges from about 5 to
about 1,200, or about 300 to about 1200. This includes embodiments
in which the aspect ratio is about 400, 500, 600, 700, 800, 900, or
1000.
[0046] The composition of the first material, the second material,
and the electrodes may vary. In some embodiments, the first
material comprises a metal, a metal nitride, or a semiconductor.
Non-limiting examples of metals and metal nitrides include Ti, Au,
Pt, Al, Cu, Ag, and nitrides thereof. A non-limiting example of a
semiconductor includes ZnO. In some embodiments, the second
material comprises a metal oxide. Non-limiting examples of oxides
and metal oxides include HfO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3,
TiO.sub.2, and SiO.sub.2. Other possible metals, metal nitrides,
semiconductors, or metal oxides include, but are not limited to,
those described above. In some embodiments, the electrodes comprise
a metal. Examples of useful metals, include, but are not limited
to, Au, Pt, and W. Similar to the two-terminal electroosmotic pumps
described above, the performance of the three-terminal
electroosmotic pumps exceeds that of conventional electroosmotic
pumps.
[0047] The methods for forming these and other electroosmotic pumps
is similar to the methods described above, involving the use of
atomic layer deposition to deposit the desired number of layers of
materials in nanoporous substrates. Methods for depositing
electrodes and patterning contacts on the electroosmotic pumps
using photolithography or wire bonding techniques are known.
Chemical Sensors
[0048] Sensors that are capable of detecting dangerous chemicals
and hazardous gases are known. See P. Grundler, Chemical Sensors:
An Introduction for Scientists and Engineers, Springer (2007).
However, conventional sensors often use potentially hazardous
radioactive materials and may only be able to detect a single type
of chemical. In addition, conventional sensors often have limited
capacity and lifetime.
[0049] Any of the multiple walled nested coaxial nanostructures
described above may be incorporated into a sensor. By way of
example only, a sensor may include a coaxial nanostructure having
an inner nanotube formed of ZnO and an outer nanotube formed of
ZrO.sub.2, wherein the inner and outer nanotubes are separated by
an annular channel. ZnO is an ideal material for detecting carbon
monoxide and ZrO.sub.2 is an ideal material for detecting oxygen.
Accordingly, such a sensor is capable of detecting multiple
chemicals simultaneously. In addition, the "tube-in-tube" or
"nested" design increases the reactive surface area by at least
four times, thereby providing a sensor with a greater capacity and
lifetime than conventional sensors. As outlined above, the nested
coaxial nanotube design can be extended to include up to n-times
nested detector nanotubes each separated by empty annular spacer
channel, where each coaxial nanotube is custom tailored to sense a
different chemical. In this fashion, multi-functional broadband
sensors and detectors can be prepared.
Photovoltaic Cells and Photonic Crystals
[0050] Photovoltaic cells and the components used to form the cells
are known. See Luque, A., et al., Handbook of Photovoltaic Science
and Engineering, Wiley (2003). Any of the coaxial nanostructures
described above, including the multiple walled nested coaxial
nanostructures comprising an annular channel, may be incorporated
into a photovoltatic cell and coupled to components such as an
anode, cathode, and supporting substrate.
[0051] Similarly, photonic crystals and the components used to form
the crystals are known. See Lourtioz, J. M., et al., Photonic
Crystals: Towards Nanoscale Photonic Devices, Springer (2008). Any
of the coaxial nanostructures described above, including the
multiple walled nested coaxial nanostructures comprising an annular
channel, may be incorporated into a photonic crystals and coupled
to various components such as a supporting metal substrate.
Two-dimensional photonic crystals may be formed from coaxial
nanostructures having an outer nanotube disposed around an inner
nanotube, wherein the nanotubes are separated by an annular
channel. Three-dimensional photonic crystals may be similarly
formed, using a nanoporous substrate having branched channels
connecting the main nanopores.
[0052] Non-limiting exemplary devices are illustrated in FIG. 9. In
FIG. 9, a layer of a metal oxide (e.g., ZrO.sub.2) is deposited on
the inner surfaces of the nanopores of an AAO substrate. Next, a
layer of a metal (e.g., Pt) is deposited on the top surface and the
bottom surface of the AAO substrate. Finally, the AAO substrate may
be dissolved by chemical etching. The structure shown may be used
as an oxygen sensor.
[0053] Additional embodiments and descriptions may be found in
co-pending application Ser. No. ______ filed on Apr. 23, 2010
("Electroosmotic Pump"; Baumgart et al.), and in a publication to
Gu, et al, "Synthesis of Nested Coaxial Multiple-Walled Nanotubes
by Atomic Layer Deposition," ACS Nano, Vol. 4 No. 2, 753-758, 2010,
both of which are hereby incorporated by reference in their
entireties.
Other Embodiments
[0054] Additional embodiments of the present invention include
single-walled nanotubes of insulating, semiconducting and metallic
materials. For example, insulating material may be high-k ZrO2,
semiconducting material may be ZnO and metallic material may be Pt.
FIG. 10A shows an SEM micrograph of partially released,
single-walled, and ALD synthesized nanotubes of insulating high-k
ZrO2. FIG. 10B shows SEM views of cleaved samples of partially
released, single-walled, and ALD synthesized nanotubes of
semiconductor ZnO. FIG. 10C shows SEM views of cleaved samples of
partially released, single walled, and ALD synthesized metallic Pt
nanotubes obtained by dissolving an alumina template in an NaOH
solution.
[0055] In other embodiments, a sensor capable of simultaneously
detecting a plurality of chemicals comprises any of the coaxial
nanostructures described herein. For example, in an additional
embodiment, a sensor based on multiple walled nested nanotubes,
such as a multiple walled nanotube comprising an inner
nanostructure, at least one of an outer nanotube disposed around
the inner nanostructure, and a first annular channel between the
inner nanostructure and the at least one first outer nanotube, is
capable of detecting several different chemicals, for example
several different hazardous or dangerous gases. In this embodiment,
each of the at least one of an outer nanotubes may comprise a
material capable of targeting specific chemical compounds. The at
least one outer nanotube may be a plurality of nanotubes, wherein
each subsequent nanotube is disposed around the previous nanotube,
and an annular channel is formed between each of the plurality of
nanotubes. In this embodiment, each of the nanotubes is capable of
targeting one or more chemical compounds, and may be capable of
targeting the same or different chemical compound as subsequent
nanotubes. Accordingly, a sensor having broadband sensing
capabilities can be engineered by substituting a specific sensor
material for one of the multiple tubes.
[0056] Other embodiments of multi-layered tube-in-tube
nanostructures described herein may be used in applications
including sensors and detectors, MEMS, nano-capacitors, photonic
crystals, Microfluidic electroosmotic pumps for drug delivery and
general medical applications and photovoltaic devices. Additional
embodiments include the use of the methods described herein in
applications such as commercial fabrication and assembly of
extremely hard and durable ZnO.sub.2 nanometer pipette tips for
medical research needed for injecting chemicals from aqueous
solutions into cancer cells, or for fertilization of egg cells in
reproductive medicine.
[0057] Non-limiting exemplary methods of forming nested coaxial
tube-in-tube nanostructures are illustrated in FIG. 11. In FIG. 11A
a nanoporous AAO substrate is formed. FIG. 11B shows a subsequent
step of using ALD to coat the inner pore walls of the AAO substrate
of FIG. 11A with HfO.sub.2. An ALD deposition of a sacrifical
spacer layer consisting of, for example, Al.sub.2O.sub.3 over the
HfO.sub.2 layer is shown FIG. 11C. Next, as shown in FIG. 11D,
using an ALD process, a second layer of HfO.sub.2 is coated on the
sacrificial spacer layer and AAO template walls. FIG. 11E shows an
ion milling sputter removal step of the ALD composite layers from
surfaces of the AAO template in order to expose the sacrifical
spacer layer and the AAO template walls. FIG. 11F shows a step of
releasing and separating formed coaxial HfO.sub.2 nanotubes by
chemical dissolution of the alumina AAO template walls and the
sacrifical ALD Al.sub.2O.sub.3 spacer layers using an Aqueous NaOH
solution.
[0058] FIG. 12A is a high-magnification tilted SEM top view of
resultant coaxial HfO.sub.2 nanotubes following release from an AAO
template and after removing an sacrifical spacer Al.sub.2O.sub.3
layer as in the method of FIG. 11. FIG. 12B is a schematic model
depicting an array of free standing coaxial nested nanotubes.
[0059] While NaOH solution may be used to dissolve the AAO
template, as discussed above, the dissolution process may be
predetermined depending on the various deposited and sacrificial
layers, and the type of material of the nanotubes. For example,
various process parameters and etch chemistry characteristics
determine the release characteristics of ALD ZnO nanotubes from AAO
templates. FIG. 13a represents a thermodynamic model showing the
distributions of the fraction of all Al.sup.3+ species at different
pH values calculated at 298K for 0.001 mM Al.sup.3+ solution. In
FIG. 13a, the formation of solid alumina is in the pH range of 4.2
to 9.8 and the maximum solubility of Al.sub.2O.sub.3 is at pH below
4.2 and above pH 9.8. FIG. 13b represents a thermodynamic model
showing the distributions of the fraction of all Zn.sup.2+ species
at different pH values calculated at 298 K for 0.001 mM Zn.sup.2+
solution. In FIG. 13b, the thermodynamic modeling of Zn.sup.2+
species indicates that zinc always has soluble species at any pH
value, the existence of crystalline ZnO is in the pH range of 9.2
to 11.5 and the maximum solubility of crystalline ZnO is at pH
below 9.2 and above pH 11.5. Thus, an NaOH solution at pH 13
partially eteches and degrades a ZnO surface. In embodiments
described herein, an NaOH solution in the range of pH 10.3-11.0,
for example a pH of 11.0 may be used to successfully remove
AAO.
[0060] Upon further removal of a template structure, for example,
an AAO template the free-standing coaxial nanotubes may be released
from the template as shown in FIG. 14. For many practical
applications, the embedded nanotubes have to be released in order
to collect and incorporate them into device structures. Certain
applications call for attached upright standing coaxial nanotubes,
while other application require completely chemically released and
detached coaxial nanotubes. Large numbers of completely detached
and individual nanotubes of the present embodiments can be
harvested by, for example, sonication in an aqueous solution. Such
nanotubes may be completely detached and may have high aspect
ratios of about 5 or above, or about 300 and above.
[0061] FIGS. 5, and 15A-B show that the annular channel between the
nested coaxial nanotubes provides sufficient space to continue a
process of growing additional nanotubes by ALD. For example, a
synthesis and assembly of nested multiple tube-in-tube
nanostructures can be extended to n-layers, where n is more than
one. For example, as shown in FIG. 15, a total number of five
nested coaxial nanotube structures may be provided. Upon reducing
the thickness of the nanotube walls, by varying ALD growth
parameters, the number of nested nanotubes may be increased. The
structures shown in FIG. 15 consist of triple coaxial HfO.sub.2
nanotubes separated by a gap and two sacrifical ALD AL.sub.2O.sub.3
spacer layers.
[0062] Additional description is provided with use of the following
non-limiting examples.
EXAMPLES
[0063] The following examples made use of an ALD reactor from
Cambridge Nanotech, Model Savannah 100.
Example 1
Formation of a Nanoporous AAO Substrate
[0064] The nanoporous AAO substrate was prepared by a two-step
anodization procedure. High purity aluminum sheets (0.5 mm thick)
were degreased in acetone. The Al sheets were then electropolished
in a solution of HClO.sub.4 and ethanol (1:4, v/v) at 20 V for 5-10
min or until a mirror like surface was achieved. The first
anodization step was carried out in a 0.3 M oxalic acid solution
electrolyte under a constant direct current (DC) voltage of 80 V at
17.degree. C. for 24 h. The porous alumina layer was then stripped
away from the Al substrate by etching the sample in a solution
containing 6 wt % phosphoric acid and 1.8 wt % chromic acid at
60.degree. C. for 12 h. The second anodization step was carried out
in a 0.3 M oxalic acid solution under a constant direct current
(DC) voltage of 80 V at 17.degree. C. for 24 h. The AAO substrates
with highly ordered arrays of nanopores were then obtained by
selectively etching away the unreacted Al in a saturated HgCl.sub.2
solution.
[0065] FIG. 1A shows the SEM image of the pore structure of the AAO
after the surface was planarized by ion milling. The pore size is
in the range of 200-300 nm and the wall width between pores is
around 50 nm. Some of the pores were connected through thinning of
the wall. The cross-sectional SEM image shown in FIG. 1B reveals
that the pores are all parallel to each other and across the whole
substrate of 60 .mu.m thickness. The inset to FIG. 1B shows the
formation of branches in some of the pores. These branches may be
eliminated with shorter anodization times, which results in a
shorter pore length. A closer view of tube opening showed that the
side connected to the cathode has smaller pore size, to a depth of
a few micrometers. This thin layer can be removed by etching to
achieve uniform pore diameter across the entire substrate depth.
High magnification FE-SEM of a cleavage sample highlights the
microstructure of partially split open nanopores of AAO. The smooth
morphology of the inside walls of the AAO nanopores can be clearly
seen. Excellent surface finish of the inner pore walls of the
template is useful for obtaining highly ordered tube-in-tube
nanostructures, since the ALD thin film coating technique
replicates the surface finish on an Angstrom scale.
Example 2
Formation of HfO.sub.2, ZrO, and ZnO Nanotubes
[0066] The AAO substrates were subsequently transferred to the ALD
chamber for ZrO.sub.2, HfO.sub.2 and ZnO coating of the inside
surfaces of the nanopores. The ZrO.sub.2 and HfO.sub.2 deposition
was performed at 250.degree. C. using water vapor as the oxidant
and tetrakis (dimethylamido) hafnium (IV) and tetrakis
(dimethylamido) zirconium (IV) as the precursor, respectively. The
deposition rate is about 1 .ANG./cycle at this temperature. ZnO was
grown with diethyl zinc (DEZ) as precursor and water vapor as
oxidation source. The optimum ALD process window for ZnO was
determined to be in the temperature range between 110.degree. C.
and 160.degree. C.
[0067] Due to the extremely high (60 .mu.m) depth of the nanopores
and the diffusivity of the chemical precursors, the entire
nanopores may not be coated uniformly unless an extended ALD cycle
time is used. For AAO pores coated with 20 nm HfO.sub.2, cross
sectional energy dispersive spectroscopy (EDS) mapping demonstrated
that Hf signal was detected up to a depth of about 15 .mu.m from
the sample surface without any added ALD exposure time. For AAO
pores coated with 20 nm ZrO.sub.2, the surface pore diameter was
reduced after ZrO.sub.2 deposition, indicating that ZrO.sub.2 was
also deposited on AAO template. Increased ALD exposure times were
used for the Zr precursor to reach saturation of precursor species
on the inside walls of the pores and ensure uniform coating along
the length of the pores.
[0068] FIGS. 2A and 2B show the cross-sectional SEM image and EDS
mapping of the AAO substrate coated with 20 nm ZrO.sub.2 using 30 s
additional ALD exposure time. It can be observed that there is
still a gradient in the Zr signal following the length of the metal
oxide the nanotubes. This is because the AAO substrate was placed
in the ALD chamber flat on one side so that access of the Zr
precursor to the backside opening was blocked. The uniformity of
coating can be improved by lifting the AAO substrate so that the
precursor can access both sizes of the pore opening during ALD
deposition.
[0069] FIGS. 3A and 3B show an AAO substrate before being coated
with ZrO.sub.2 (A) and after being coated with 20 nm ZrO.sub.2 (B).
A comparison of the figures shows that the pore size has been
reduced because the wall thickness has been increased by growing a
ZrO.sub.2 film. In order to fabricate free-standing ZrO.sub.2
nanotubes (i.e., nanotubes unsupported along their lengths by the
AAO substrate), the alumina walls between the pores were dissolved
by a 6M NaOH solution. The porous AAO surface was first cleared of
its ZrO.sub.2 films by ion milling to expose the AAO wall to the
etchant. FIG. 3C shows the free-standing ZrO.sub.2 nanotubes after
ion milling and chemical dissolution of alumina walls. The SEM
image clearly shows the empty trenches in place of the former
alumina side walls. The dimensions of the nanotubes are dependent
upon the thickness and pore diameter of the AAO substrate and the
ALD deposition time. Smaller tubes or even rods can be fabricated
using this method by using AAO substrates with smaller pores.
Different materials can still be deposited inside of the nanotubes
depending on the application.
Example 3
Formation of a HfO.sub.2 Tube-in-Tube Coaxial Nanostructure
[0070] In this example, a second nanotube having a smaller
dimension was deposited inside of the aforementioned HfO.sub.2
nanotubes. To fabricate this tube-in-tube structure, two layers of
10 nm HfO.sub.2 films were deposited inside of the AAO pores and
separated by 25 nm of a layer of Al.sub.2O.sub.3, which was
deposited by ALD at 300.degree. C. using [Al(CH.sub.3).sub.3] (TMA)
and water vapor as the aluminum and oxygen source, respectively.
Al.sub.2O.sub.3 is the same material as the AAO substrate and can
be easily etched away. Following the three layer coating, the
sample surface was again polished by ion milling and then dipped
into NaOH solution to etch both the AAO substrate and
Al.sub.2O.sub.3 layer between HfO.sub.2 layers. FIG. 4 shows a
double-walled HfO.sub.2 tube-in-tube structure after wet etching in
NaOH solution. The HfO.sub.2 tube thicknesses are very uniform from
both the top and cross section. The expected wall thicknesses for
both tubes are 10 nm, as determined from the number of ALD cycles.
However, the HfO.sub.2 tubes look much thicker from the SEM image
due to the gold coating for charging release.
[0071] Transmission electron microscopy (TEM) was used to examine
the HfO.sub.2 tube-in-tube structure and tube wall thickness using
the following processing sequence. After NaOH etching the HfO.sub.2
nanotubes were suspended in isopropanol solution and separated by
sonicating. The HfO.sub.2 nanotubes in isopropanol were
subsequently poured onto the TEM copper grid. FIG. 5 shows TEM high
magnification micrographs of double-walled HfO.sub.2 tube-in-tube
structure. The tube-in-tube structure shown in FIG. 5 was achieved
even from AAO pores with branches or dead ends. FIG. 5 also reveals
that upon release of the nanotubes from the AAO template
(substrates), the coaxial nanostructures have undergone a shape
transformation from an irregular octahedral shaped cross-section
(compare FIGS. 3 and 4) to a circular cross-section. Since this
shape transformation takes place at room temperature during the
chemical release of the coaxial nanostructures, temperature
activated diffusion processes are ruled out. This spontaneous
snap-back upon release from the template of otherwise very hard
ceramics like ZrO.sub.2 and HfO.sub.2 into the energetically most
favorable circular shape is clearly a nanotechnology phenomenon.
This snap-back process of shape transformation cannot occur at
macroscopic sizes of the same hard ceramic materials. Coaxial
nanostructures having circular cross-sections are desirable for
pipette tips for various medical applications.
Example 4
Formation of a ZrO, Tube-in-Tube-in-Tube Coaxial Nanostructure
[0072] The method of Example 3 was modified to provide two
nanotubes having a smaller dimension deposited inside of the
aforementioned ZrO.sub.2 nanotubes. Three layers of ZrO.sub.2 films
were deposited inside of AAO pores, separated by a layer of
Al.sub.2O.sub.3. The Al.sub.2O.sub.3 layers were removed as
described above. The resulting tube-in-tube-in-tube coaxial
nanostructure is shown in FIG. 6.
Example 5
Formation of HfO.sub.2/ZrO.sub.2 Coaxial Nanostructure
[0073] ALD was used to deposit a layer of HfO.sub.2 inside the
nanopore of an AAO substrate followed by a layer of ZrO.sub.2 on
the layer of HfO.sub.2 to provide a double-walled coaxial
nanostructure. FIG. 7 shows the surface morphology and tube size
after the two layer coating.
[0074] In addition, the AAO substrate is transferred to an ALD
reaction chamber in order to grow nested multiple-walled nanotubes
within the AAO pores. Pt may be used for metal nanotubes and ZnO
and TiO.sub.2 may be used for semiconducting metal oxide nanotubes.
As insulating materials, the transition metal oxides of ZrO.sub.2,
HfO.sub.2, and Al.sub.2O.sub.3 may be used. ALD is a thin film
growth technique that requires the sequential exposure of the
sample to two chemical precursors to saturate the sample surface
and to react with each other. The technical details of the ALD
process conditions and the different chemical precursors and
deposition parameters utilized for all of the nested nanotubes
investigated in this study are listed in Table 1 below.
[0075] Following the ALD deposition of the aforementioned
materials, the AAO sample surfaces may be polished by ion milling
to expose the template surface and the ALD grown alumina spacer to
the NaOH solution. A 1M NaOH solution is used to etch alumina for
all ALD nanotube materials except for ZnO nanotubes. For the case
of ZnO, 0.1 M NaOH is used to achieve etching of the alumina
template while minimizing the etch attack of the ZnO nanotubes. It
is also essential to perform a post-ALD deposition annealing
procedure for ZnO nanotubes at 600.degree. C. for 10 min in air, in
order to obtain high quality smooth surface morphologies of the ZnO
nanotubes.
TABLE-US-00001 TABLE 1 Growth Rate Deposition Precursor (Angstrom/
Materials Temp. Precursor I II cycle) ZrO.sub.2 250
tetrakis(dimethylamido)zirconium H.sub.2O 1 Vapor ZnO 150 diethyl
zinc H.sub.2O 2.3 Vapor Pt 300 (trimethyl)methylcyclopentadienyl
oxygen 0.5 platinum TiO.sub.2 250 titanium isopropoxide H.sub.2O
0.3-0.4 Vapor HfO.sub.2 250 tetrakis(dimethylamido)hafnium H.sub.2O
1 Vapor Al.sub.2O.sub.3 300 trimethylaluminmum H.sub.2O 1 Vapor
[0076] As will be understood by one skilled in the art, for any and
all purposes, particularly in terms of providing a written
description, all ranges disclosed herein also encompass any and all
possible subranges and combinations of subranges thereof. Any
listed range can be easily recognized as sufficiently describing
and enabling the same range being broken down into at least equal
halves, thirds, quarters, fifths, tenths, etc. As a non-limiting
example, each range discussed herein can be readily broken down
into a lower third, middle third and upper third, etc. As will also
be understood by one skilled in the art all language such as "up
to," "at least," "greater than," "less than," and the like include
the number recited and refer to ranges which can be subsequently
broken down into subranges as discussed above.
[0077] All publications, patent applications, issued patents, and
other documents referred to in this specification are herein
incorporated by reference as if each individual publication, patent
application, issued patent, or other document were specifically and
individually indicated to be incorporated by reference in its
entirety. Definitions that are contained in text incorporated by
reference are excluded to the extent that they contradict
definitions in this disclosure.
[0078] For the purposes of this disclosure and unless otherwise
specified, "a" or "an" means "one or more."
* * * * *