U.S. patent application number 11/511886 was filed with the patent office on 2011-02-17 for porous substrates, articles, systems and compositions comprising nanofibers and methods of their use and production.
This patent application is currently assigned to Nanosys, Inc.. Invention is credited to Chunming Niu.
Application Number | 20110039690 11/511886 |
Document ID | / |
Family ID | 39136483 |
Filed Date | 2011-02-17 |
United States Patent
Application |
20110039690 |
Kind Code |
A1 |
Niu; Chunming |
February 17, 2011 |
Porous substrates, articles, systems and compositions comprising
nanofibers and methods of their use and production
Abstract
Porous and/or curved nanofiber bearing substrate materials are
provided having enhanced surface area for a variety of applications
including as electrical substrates, semipermeable membranes and
barriers, structural lattices for tissue culturing and for
composite materials, production of long unbranched nanofibers, and
the like. A method of producing nanofibers is disclosed including
providing a plurality of microparticles or nanoparticles such as
carbon black particles having a catalyst material deposited
thereon, and synthesizing a plurality of nanofibers from the
catalyst material on the microparticles or nanoparticles.
Compositions including carbon black particles having nanowires
deposited thereon are further disclosed.
Inventors: |
Niu; Chunming; (Palo Alto,
CA) |
Correspondence
Address: |
NANOSYS INC.
2625 HANOVER ST.
PALO ALTO
CA
94304
US
|
Assignee: |
Nanosys, Inc.
Palo Alto
CA
|
Family ID: |
39136483 |
Appl. No.: |
11/511886 |
Filed: |
August 29, 2006 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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11331445 |
Jan 11, 2006 |
7553371 |
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11511886 |
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10941746 |
Sep 15, 2004 |
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11331445 |
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60541463 |
Feb 2, 2004 |
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Current U.S.
Class: |
502/184 ;
252/182.32; 252/502; 252/514; 420/507; 423/335; 423/348; 502/180;
502/182; 502/185; 502/439 |
Current CPC
Class: |
D06M 11/45 20130101;
H01L 21/02164 20130101; H01L 21/02645 20130101; H01L 21/02603
20130101; B01J 35/026 20130101; B01D 2239/0492 20130101; B01D
2239/064 20130101; B01D 2239/1225 20130101; B01J 35/0013 20130101;
B01D 2239/0421 20130101; D06M 11/46 20130101; H01L 21/0262
20130101; B01D 2239/10 20130101; G01N 30/52 20130101; B82Y 30/00
20130101; H01L 21/02551 20130101; H01L 21/02653 20130101; B01D
2239/0208 20130101; B01D 2239/1216 20130101; H01B 1/00 20130101;
B01J 21/18 20130101; B01D 39/2065 20130101; B01D 39/083 20130101;
B01J 37/0221 20130101; H01L 21/02538 20130101; H01L 21/02532
20130101; H01B 1/04 20130101; H01L 21/02524 20130101; H01L 21/02639
20130101; B01D 2239/0428 20130101; B01D 2239/025 20130101; D06M
11/49 20130101; Y10T 428/2991 20150115; D06M 11/83 20130101; B01D
2239/0216 20130101; B01D 2239/065 20130101; B01D 2239/0407
20130101; B01D 2239/0654 20130101; B01D 39/086 20130101; B01J 23/42
20130101; D06M 11/44 20130101; H01L 21/02664 20130101; H01L
21/02606 20130101; H01L 21/0237 20130101; B01J 23/52 20130101 |
Class at
Publication: |
502/184 ;
420/507; 252/514; 423/348; 423/335; 252/502; 252/182.32; 502/439;
502/180; 502/182; 502/185 |
International
Class: |
B01J 23/52 20060101
B01J023/52; C22C 5/02 20060101 C22C005/02; H01B 1/02 20060101
H01B001/02; C23C 16/24 20060101 C23C016/24; C23C 16/40 20060101
C23C016/40; H01B 1/04 20060101 H01B001/04; C09K 3/00 20060101
C09K003/00; B01J 21/18 20060101 B01J021/18; B01J 23/42 20060101
B01J023/42 |
Claims
1. A method of producing nanofibers, the method comprising:
providing a plurality of microparticles or nanoparticles having a
catalyst material deposited thereon; and synthesizing a plurality
of nanofibers from the catalyst material on the microparticles or
nanoparticles.
2. The method of claim 1, wherein the nanofibers comprise
nanowires.
3. The method of claim 2, wherein synthesizing the plurality of
nanowires comprises: depositing a gold colloid on at least a
portion of the overall surface area of the microparticles or
nanoparticles; and growing the nanowires from the gold colloid with
a VLS synthesis technique.
4. The method of claim 1, wherein the microparticles or
nanoparticles comprise carbon black particles.
5. The method of claim 1, wherein the microparticles or
nanoparticles comprise glass or quartz microspheres.
6. The method of claim 1, wherein the plurality of nanofibers
comprises a semiconductor material selected from group IV, group
II-VI and group III-V semiconductors.
7. The method of claim 1, wherein the plurality of nanofibers are
comprised of silicon or silicon oxide.
8-10. (canceled)
11. A composition comprising: a population of substrates, wherein
the substrates each comprise a carbon particle; and a population of
nanowires deposited each individual carbon particle substrate.
12. The composition of claim 11, wherein the composition is
conductive.
13. The composition of claim 11, wherein the nanowires are
comprised of silicon or silicon oxide.
14. The composition of claim 11, wherein the substrates are
porous.
15. The composition of claim 11, further comprising one or more
catalyst particles deposited on the carbon particle substrates.
16. The composition of claim 15, wherein the one or more catalyst
particles comprise metal.
17. The composition of claim 16, wherein the metal comprises gold
or platinum.
18. The composition of claim 11, wherein the carbon particles are
suspended in a colloidal solution.
19. The composition of claim 11, wherein the carbon particles are
spherical.
20. The composition of claim 11, wherein the carbon particles are
microparticles or nanoparticles.
21. The composition of claim 11, wherein one or more surfaces of
the carbon particles are curved.
22. The composition of claim 15, wherein the catalyst particles are
deposited on multiple surfaces of the carbon particles.
23. The composition of claim 11, wherein the nanowires are grown in
situ on the carbon particles.
24. The composition of claim 11, wherein the carbon particles
comprise carbon black.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation in-part of U.S. patent
application Ser. No. 11/331,445 filed Jan. 11, 2006, which is a
continuation-in-part of U.S. patent application Ser. No.
10/941,746, filed Sep. 15, 2004, entitled "POROUS SUBSTRATES,
ARTICLES, SYSTEMS AND COMPOSITIONS COMPRISING NANOFIBERS AND
METHODS OF THEIR USE AND PRODUCTION" by Dubrow and Niu, which
claims priority to and benefit of provisional U.S. Patent
Application Ser. No. 60/541,463, filed Feb. 2, 2004, the full
disclosures of which are hereby incorporated by reference in their
entirety for all purposes.
FIELD OF THE INVENTION
[0002] The invention relates primarily to the field of
nanotechnology. More specifically, the invention pertains to
nanofibers, including methods of synthesizing or stabilizing
nanofibers, articles comprising nanofibers, and use of nanofibers
in various applications.
BACKGROUND OF THE INVENTION
[0003] Nanotechnology has been simultaneously heralded as the next
technological evolution that will pave the way for the next
societal evolution, and lambasted as merely the latest batch of
snake oil peddled by the technically overzealous. Fundamentally,
both sides of the argument have a number of valid points to support
their position. For example, it is absolutely clear that
nanomaterials possess very unique and highly desirable properties
in terms of their chemical, structural and electrical capabilities.
However, it is also clear that, to date, there is very little
technology available for integrating nanoscale materials into the
macroscale world in a reasonable commercial fashion and/or for
assembling these nanomaterials into more complex systems for the
more complex prospective applications, e.g., nanocomputers,
nanoscale machines, etc. A variety of researchers have proposed a
number of different ways to address the integration and assembly
questions by waving their hands and speaking of molecular self
assembly, electromagnetic assembly techniques and the like.
However, there has been either little published success or little
published effort in these areas.
[0004] In certain cases, uses of nanomaterials have been proposed
that exploit the unique and interesting properties of these
materials more as a bulk material than as individual elements
requiring individual assembly. For example, Duan et al., Nature
425:274-278 (September 2003), describes a nanowire based transistor
for use in large area electronic substrates, e.g., for displays,
antennas, etc., that employs a bulk processed, oriented
semiconductor nanowire film or layer in place of a rigid
semiconductor wafer. The result is an electronic substrate that
performs on par with a single crystal wafer substrate, but that is
manufacturable using conventional and less expensive processes that
are used in the poorer performing amorphous semiconductor
processes. In accordance with this technology, the only new process
requirement is the ability to provide a film of nanowires that are
substantially oriented along a given axis. The technology for such
orientation has already been described in detail in, e.g.,
International Patent Application Publications. WO 03/085700, WO
03/085701, and WO 2004/032191, as well as U.S. Pat. No. 7,067,328,
(the full disclosures of each of which are hereby incorporated by
reference herein, in their entirety for all purposes) and is
readily scalable to manufacturing processes.
[0005] In another exemplary case, bulk processed nanocrystals have
been described for use as a flexible and efficient active layer for
photoelectric devices. In particular, the ability to provide a
quantum confined semiconductor crystal in a hole conducting matrix
(to provide type-II bandgap offset), allows the production of a
photoactive layer that can be exploited either as a photovoltaic
device or photoelectric detector. When disposed in an active
composite, these nanomaterials are simply processed using standard
film coating processes that are available in the industry. See,
e.g., U.S. Pat. No. 6,878,871, and incorporated herein by reference
in its entirety for all purposes.
[0006] In accordance with the expectation that the near term value
of nanotechnology requires the use of these materials in more of a
bulk or bulk-like process, certain aspects of the present invention
use nanomaterials not as nanomaterials per se, but as a
modification to larger materials, compositions and articles to
yield fundamentally novel and valuable materials compositions and
articles.
BRIEF SUMMARY OF THE INVENTION
[0007] The present invention is directed, in one aspect, to a novel
presentation of nanomaterials that enables a broader use and
application of those materials while imparting ease of handling,
fabrication, and integration that is lacking in previously reported
nanomaterials. In particular, one aspect of the present invention
provides a porous substrate upon which is attached a plurality of
nanofibers. The nanofibers may be attached to any portion or over
the entire overall surface of the substrate or may be localized
primarily or substantially upon the interior wall surfaces of the
apertures that define the pores that are disposed through the
porous substrate.
[0008] The articles of the invention may be employed as filtration
media to filter gas, fluids or the like, or they may be employed as
semipermeable barriers, e.g., breathable moisture barriers for
outerwear, bandages, or the like. The articles of the invention may
also be employed to integrate nanomaterials into electronic
devices, in which the nanomaterials impart useful characteristics,
e.g., as electrodes and or other active elements in photovoltaic
devices and the like, or they may be used to integrate these
nanomaterials into physical structures, e.g., composites, or
biological structures, e.g., tissue. Synthesis of nanofibers on a
porous or curved substrate can facilitate production of large
numbers and/or a high density of long, unbranched nanofibers for
use in any of a variety of applications.
[0009] Thus, a first general class of embodiments provides methods
of producing nanofibers. In the methods, a substrate comprising a)
a plurality of apertures disposed therethrough, the substrate
comprising an overall surface area that includes an interior wall
surface area of the plurality of apertures, or b) a curved surface
is provided. A plurality of nanofibers is synthesized on the
substrate, wherein the resulting nanofibers are attached to at
least a portion of the overall surface area of the substrate of a)
or to at least a portion of the curved surface of b).
[0010] The substrate can comprise a solid substrate with a
plurality of pores disposed through it, a mesh (e.g., a metallic
mesh, e.g., a mesh comprising a metal selected from the group
consisting of: nickel, titanium, platinum, aluminum, gold, and
iron), a woven fabric (e.g., an activated carbon fabric), or a
fibrous mat (e.g., comprising glass, quartz, silicon, metallic, or
polymer fibers). As other examples, the substrate can comprise a
plurality of microspheres (e.g., glass or quartz microspheres), a
plurality of fibers, e.g., glass or quartz fibers (e.g.,
microfibers, fiberglass, glass or quartz fiber filters), or a foam.
In certain embodiments, the plurality of apertures in the substrate
of a) have an effective pore size of less than 10 .mu.m, less than
1 .mu.m, less than 0.5 .mu.m, or even less than 0.2 .mu.m. In other
embodiments, the plurality of apertures in the substrate of a) have
an effective pore size of at least 25 .mu.m, at least 50 .mu.m, at
least 100 .mu.m, or more.
[0011] The nanofibers can comprise essentially any type of
nanofibers. In certain embodiments, the nanofibers comprise
nanowires, and the methods can include synthesizing the plurality
of nanowires by depositing a gold colloid on at least a portion of
the overall surface area of the substrate of a) or on at least a
portion of the curved surface of b) and growing the nanowires from
the gold colloid, e.g., with a VLS synthesis technique. The
plurality of nanofibers optionally comprises a semiconductor
material selected from group IV, group II-VI and group III-V
semiconductors (e.g., silicon).
[0012] The methods optionally include surrounding or at least
partially encapsulating the substrate and the resulting attached
nanofibers with a matrix material; dissolving a soluble substrate
following synthesis of the nanofibers on the substrate; forming a
coating on the resulting nanofibers, wherein the coating is
contiguous between adjacent nanofibers; disposing a layer of porous
material on the resulting nanofibers (and optionally disposing the
substrate on a second layer of porous material, sandwiching the
nanofiber-bearing substrate); and/or functionalizing the nanofibers
(e.g., by attaching a chemical moiety or nanocrystal to their
surface).
[0013] In one class of embodiments, yield of the resulting
nanofibers having a length greater than 10 .mu.m (e.g., greater
than 20 .mu.m, 30 .mu.m, 40 .mu.m, 50 .mu.m, or 60 .mu.m) is at
least 10% greater than yield of nanofibers of that length
synthesized on a planar non-porous substrate of the same surface
area using substantially the same growth process. The yield from
the methods is optionally at least 25%, 50%, 75%, or even 100%
greater than the yield from growth on the planar non-porous
substrate.
[0014] The nanofibers are optionally removed from the surface area
of the substrate of a) or the curved surface of b) following
synthesis of the nanofibers, e.g., by sonicating the substrate, to
produce a population of detached nanofibers. In one class of
embodiments, at least 10% of the nanofibers in the population of
detached nanofibers have a length greater than 10 .mu.m, 20 .mu.m,
30 .mu.m, 40 .mu.m, 50 .mu.m, or 60 .mu.m, while at most 50% of the
nanofibers have a length less than 10 .mu.m.
[0015] Articles or populations of nanofibers produced by the
methods form another feature of the invention. Thus, one exemplary
class of embodiments provides an article comprising a substrate
having a curved surface, and a plurality of nanofibers (e.g.,
nanowires) attached to at least a portion of the curved surface of
the substrate. The substrate can comprise, e.g., a plurality of
microspheres or one or more glass fiber, quartz fiber, metallic
fiber, polymer fiber, or other fiber.
[0016] As for the embodiments above, the plurality of nanofibers
optionally comprises a semiconductor material selected from group
IV, group II-VI and group semiconductors (e.g., silicon).
Optionally, at least 10% of the nanofibers present on the curved
surface have a length greater than 10 .mu.m, 20 .mu.m, 30 .mu.m, 40
.mu.m, 50 .mu.m, or 60 .mu.m, while at most 50% of the nanofibers
present on the curved surface have a length less than 10 .mu.m. The
nanofibers can be preformed and deposited on the substrate to
produce the article, or the plurality of nanofibers can be attached
to the portion of the curved surface by having been grown on the
portion of the curved surface. The article optionally includes a
matrix material surrounding at least a portion of the curved
surface and plurality of nanofibers. Devices or compositions
including the article form another feature of the invention, for
example, an implantable medical device comprising an article of the
invention, e.g., attached to and covering at least a portion of the
surface of the implantable medical device.
[0017] Another general class of embodiments provides methods of
stabilizing nanofibers (e.g., nanowires). In the methods, a
population of nanofibers is provided, and a coating is formed on
the nanofibers. The coating is contiguous between adjacent
nanofibers in the population. A first material comprising the
nanofibers is optionally different from a second material
comprising the coating. In one class of embodiments, the coating
comprises a carbide, a nitride, or an oxide, e.g., an oxide of
silicon, titanium, aluminum, magnesium, iron, tungsten, tantalum,
iridium, or ruthenium, or an oxide of the material comprising the
nanofibers. In another class of embodiments, the nanofibers are
comprised of silicon and the coating is comprised of
polysilicon.
[0018] The population of nanofibers is optionally provided by
synthesizing the nanofibers on a surface of a substrate. The
methods can include functionalizing the coating with a chemical
binding moiety, a hydrophobic chemical moiety, a hydrophilic
chemical moiety, or the like.
[0019] Populations of nanofibers formed by the methods are another
feature of the invention. One general class of embodiments provides
a population of nanofibers that includes nanofibers (e.g.,
nanowires) and a coating on the nanofibers, wherein the coating is
contiguous between adjacent nanofibers in the population. As for
the methods described above, a first material comprising the
nanofibers is optionally different from a second material
comprising the coating. In one class of embodiments, the coating
comprises a carbide, a nitride, or an oxide, e.g., an oxide of
silicon, titanium, aluminum, magnesium, iron, tungsten, tantalum,
iridium, or ruthenium, or an oxide of the material comprising the
nanofibers. In another class of embodiments, the nanofibers are
comprised of silicon and the coating is comprised of polysilicon.
The coating can be functionalized with a chemical binding moiety, a
hydrophobic chemical moiety, a hydrophilic chemical moiety, or the
like. The nanofibers are optionally attached to a substrate. In one
embodiment, the nanofibers are attached to and cover at least a
portion of a surface of an implantable medical device.
[0020] Yet another general class of embodiments provides an article
comprising a substrate having a plurality of apertures disposed
therethrough, the substrate comprising an overall surface area that
includes an interior wall surface area of the plurality of
apertures, and a plurality of nanofibers attached to at least a
portion of the overall surface area of the substrate. The substrate
can comprise, for example, a solid substrate (e.g., a silica based
wafer, a metallic plate, or a ceramic sheet or plate) while the
plurality of apertures comprises a plurality of pores disposed
through the solid substrate. As another example, the substrate can
comprise a mesh, e.g., a polymer mesh or a metallic mesh
(comprising, e.g., nickel, titanium, platinum, aluminum, gold, or
iron). As yet another example, the substrate can comprise a woven
fabric, e.g., a fabric comprising fiberglass, carbon fiber, or a
polymer (e.g., polyimide, polyetherketone, or polyaramid). As yet
another example, the substrate can comprise a fibrous mat, e.g., a
fibrous mat comprising silica based fibers (e.g., glass and
silicon), metallic fibers, or polymer fibers.
[0021] In certain embodiments, the plurality of apertures have an
effective pore size of less than 10 .mu.m, for example, less than 1
.mu.m, less than 0.5 .mu.m, or less than 0.2 .mu.m. In other
embodiments, for example, embodiments in which synthesis of long
unbranched nanofibers are desired, the plurality of apertures have
an effective pore size of at least 25 .mu.m, at least 50 .mu.m, at
least 100 .mu.m, or more.
[0022] The nanofibers (e.g., nanowires) can comprise essentially
any suitable material. For example, the plurality of nanofibers can
comprise a semiconductor material selected from group IV, group
II-VI and group III-V semiconductors, e.g., silicon. The nanofibers
can be pre-formed and deposited on the substrate, or they can be
attached to the portion of the overall surface area of the
substrate by having been grown on the portion of the surface area.
The plurality of nanofibers is optionally electrically coupled to
the substrate. The plurality of nanofibers can be functionalized
with a chemical binding moiety, e.g., a hydrophobic chemical
moiety.
[0023] In one class of embodiments, a matrix material surrounds or
at least partially encapsulates the substrate and plurality of
nanofibers. The matrix material can at least partially intercalate
into the apertures. In one embodiment, the matrix material and the
plurality of nanofibers have a type-II energy band-gap offset with
respect to each other. The matrix material optionally comprises a
polymer, for example, a polyester, an epoxy, a urethane resin, an
acrylate resin, polyethylene, polypropylene, nylon, or PFA. In one
aspect, the invention provides implantable medical devices. For
example, an implantable medical device can include an article of
the invention attached to and covering at least a portion of a
surface of the implantable medical device.
[0024] In one class of embodiments, the substrate comprises
activated carbon, e.g., an activated carbon fabric. At least a
first population of nanocrystals can be attached to the nanofibers,
for example, nanocrystals comprising a material selected from the
group consisting of: Ag, ZnO, CuO, Cu.sub.2O, Al.sub.2O.sub.3,
TiO.sub.2, MgO, FeO, and MnO.sub.2. At least a second population of
nanocrystals is optionally also attached to the nanofibers, where
the nanocrystals of the second population comprise a different
material than do the nanocrystals of the first population. In
certain embodiments, the nanofibers are functionalized with a
chemical moiety, e.g., a chemical moiety that absorbs or decomposes
a non-organic gas. Preferred nanofibers in these embodiments
include carbon nanotubes and silicon nanowires. An article of
clothing can comprise the nanofiber-enhanced substrate of the
invention.
[0025] Various techniques can be used to protect the nanofiber
bearing substrate. For example, in one class of embodiments, the
substrate (e.g., a woven fabric) comprises a first surface, and the
article further comprises a first layer of porous material disposed
on the first surface of the substrate. Optionally, the substrate
comprises a second surface, and the article also includes a second
layer of porous material disposed on the second surface of the
substrate, whereby the substrate is sandwiched between the first
and second layers of porous material. As another example, the
article can include a coating on the nanofibers, which coating is
contiguous between adjacent nanofibers.
[0026] Yet another general class of embodiments provides methods of
producing a vapor absorbing fabric. In the methods, a porous fabric
substrate that comprises a plurality of apertures disposed
therethrough is provided. The substrate comprises an overall
surface area that includes an interior wall surface area of the
plurality of apertures. A plurality of nanofibers attached to at
least a portion of the overall surface area of the fabric substrate
is also provided, and the nanofibers are functionalized with a
moiety that absorbs or decomposes at least one organic or
non-organic gas, thereby producing a vapor absorbing fabric.
[0027] The fabric is preferably an activated carbon fabric. The
nanofibers can be functionalized with a chemical moiety that
absorbs or decomposes at least one non-organic gas. Preferably, the
nanofibers are functionalized by attaching at least a first
population of nanocrystals to the nanofibers, which first
population of nanocrystals comprises a first material that absorbs
or decomposes at least one non-organic gas. The vapor absorbing
fabric can be incorporated into an article of clothing or other
protective apparatus.
[0028] A related class of embodiments also provides methods of
producing a vapor absorbing fabric. In the methods, a porous fabric
substrate that comprises a plurality of apertures disposed
therethrough is provided (e.g., a mesoporous carbon fabric). The
substrate comprises an overall surface area that includes an
interior wall surface area of the plurality of apertures. A
plurality of nanocrystals is attached to at least a portion of the
overall surface area of the fabric substrate, which nanocrystals
absorb or decompose at least one non-organic gas, thereby producing
the vapor absorbing fabric.
BRIEF DESCRIPTION OF THE FIGURES
[0029] FIG. 1 Panels A and B show a schematic illustration of a
porous substrate having nanowires attached to its surfaces.
[0030] FIG. 2 Panels A and B show a schematic illustration of
nanowires attached to the interior wall portions of a porous
substrate material.
[0031] FIG. 3 Panels A and B show a schematic illustration of the
articles of the invention incorporated in a filtration
cartridge.
[0032] FIG. 4 Panels A and B show a schematic illustration of a
layered textile that incorporates a substrate of the invention as a
semi-permeable moisture barrier for use in, e.g., outdoor
clothing.
[0033] FIG. 5 Panels A and B show a schematic illustration of the
articles of the invention incorporated in a self adhesive, moisture
repellant bandage.
[0034] FIG. 6 Panels A and B are schematic illustrations of the
substrate material of the invention incorporated into a
photovoltaic device.
[0035] FIG. 7 is a schematic illustration of an article of the
invention used as a lattice for incorporation into a composite
matrix for use as, e.g., a dielectric layer.
[0036] FIG. 8 Panels A and B schematically illustrate separation
media incorporating the substrates of the invention in conjunction
with column apparatus for performing chromatographic
separations.
[0037] FIG. 9 Panels A and B show electron micrographs of
cross-fused or linked nanowires creating an independent mesh
network as used in certain aspects of the present invention.
[0038] FIG. 10 schematically illustrates a process for producing a
cross-linked nanowire mesh network for use either in conjunction
with or independent from an underlying porous, e.g., macroporous,
substrate.
[0039] FIG. 11 illustrates a composite material that employs the
porous substrates of the invention disposed within a matrix
material.
[0040] FIG. 12 Panels A and B illustrate an example of the
nanofiber bearing, porous substrates of the invention.
[0041] FIG. 13 schematically illustrates a nanofiber-enhanced
fabric for use, e.g., in protective clothing or apparatus.
[0042] FIG. 14 schematically illustrates protection of a
nanofiber-bearing substrate by disposing it between layers of
porous material.
[0043] FIG. 15 Panel A depicts an electron micrograph of
reticulated aluminum. Panel B depicts an electron micrograph of
nanowires grown on a reticulated aluminum substrate.
[0044] FIG. 16 depicts silicon nanowires grown on quartz fiber
filters (Panels A-B), grown on quartz fiber filters and removed
from the substrate by sonication (Panels C-D), and grown on a glass
fiber (Panel E).
[0045] FIG. 17 Panel A depicts simulated nanowires growing on a 5
.mu.m diameter fiber. Panel B depicts a graph of the collisions per
nanowire as a function of fiber radius for simulated nanowire
growth on a fiber.
[0046] FIG. 18 Panel A depicts an electron micrograph of carbon
black powdered particles having gold colloid particles deposited
thereon.
[0047] FIG. 18 Panels B and C depict an electron micrograph of
silicon nanowires grown from the carbon black supported gold
colloid particles of FIG. 18 Panel A.
DETAILED DESCRIPTION
I. General Description of the Invention
[0048] The present invention generally provides, inter alia, novel
articles and compositions that employ nanowire surfaces or surface
portions to impart unique physical, chemical and electrical
properties. In particular, the present invention is directed, in
part, to porous substrates that have nanowires attached to at least
a portion of the overall surfaces of the porous substrates in order
to provide materials that have a wide range of unique and valuable
properties for a wide range of different applications.
[0049] The application of nanowires to the various surfaces of
porous substrates not only improves the performance of porous
substrates in applications where they are already used, but also
improves performance of substrate materials in a number of other
different applications, where such porous substrates may or may not
conventionally be employed.
[0050] By way of example, incorporation of nanowire enhanced
surfaces in membranes or other semi-permeable barriers can enhance
filtration efficiencies. In particular, by providing nanowires
within the pores of existing membranes or other permeated layers,
one can provide higher filtration efficiencies without the expected
increase in pressure drop across the filter (see Grafe et al.,
Nanowovens in Filtration-Fifth International Conference, Stuttgart,
Germany, March 2003). Relatedly, such nanofibers may be used to
impart alternate properties to such barriers, e.g., breathable
moisture repellant barriers, antibacterial/antiseptic barriers.
Such barriers would be widely applicable in the outdoor clothing
industry but would also be particularly useful as bandages or
surgical dressings due to their permeability to oxygen but
impermeability to moisture or particles including bacteria, as well
as the use of antimicrobial nanofibers. This latter application is
particularly interesting in light of the dry adhesive
characteristics of nanowire/nanofiber enhanced surfaces (see, e.g.,
U.S. Pat. No. 7,056,409, incorporated herein by reference in its
entirety for all purposes). Relatedly, such nanofiber enhanced
surfaces can also be used in the construction of chemical and/or
biological protective barriers, e.g., clothing, optionally
permeable to moisture but absorbing chemical vapors.
[0051] While some researchers have proposed depositing nanofibers
onto membranes to achieve higher surface areas, the ability to
attach fibers to the surface, and particularly to grow such fibers
in situ, provides numerous advantages over simple deposition of
fibers. In particular, in merely depositing fibers on membranes, it
is difficult to get uniform or complete, e.g., penetrating,
coverage of the fibers over the total surface area of the membrane,
whereas in situ growth methods give far better coverage of interior
surfaces, and thus provide much greater surface area for the
membrane or barrier. Additionally, such methods provide for varied
orientations of such fibers from the surfaces to which they are
attached, i.e., having fibers extend from the surface as opposed to
laying flat against the surface.
[0052] In addition to improving the function of porous substrates,
the use of porous substrates in conjunction with
nanofibers/nanowires also provides a unique, ultra high surface
area material that can be used in a wide variety of applications
that may have little to do with the use of porous substrates, per
se. For example, ultra high surface area electrical components may
have a variety of applications as electrodes for interfacing with,
e.g., biological tissue (e.g., in pacemakers), coverings for other
biological implants as tissue lattice or anti-infective barriers
for catheters, or the like.
[0053] In still other applications, porous substrates provide a
unique synthesis lattice for providing dense populations of
nanofibers/nanowires for use in a variety of different
applications, e.g., for use in composite films, etc. Such films may
generally be applied as semiconductive composites, dielectric
films, active layers for electronic or photoelectric devices,
etc.
[0054] In still other applications, porous substrates provide a
unique synthesis lattice for synthesizing nanofibers, particularly
long, unbranched nanowires at high yield and/or density.
[0055] A broad range of potential applications exists for these
techniques, materials, and articles and will be apparent to one of
ordinary skill in the art upon reading the instant disclosure.
II. Articles of the Invention, Structure and Architecture
[0056] As noted above, in one aspect, the articles of the invention
incorporate porous substrates as a foundation of the article. The
porous substrates used in accordance with the present invention
typically include any of a variety of solid or semisolid materials
upon which the nanowires may be attached, but through which
apertures exist. As such, these substrates may include solid
contiguous substrates, e.g., plates, films, or wafers, that may be
flexible or rigid, that have apertures disposed through them, e.g.,
stamped or etched metal or inorganic perforated plates, wafers,
etc., porated or perforated films, or the substrate may include
aggregates of solid or semisolid components e.g., fibrous mats,
mesh screens, amorphous matrices, composite materials, woven
fabrics, e.g., fiberglass, carbon fiber, polyaramid or polyester
fabrics, or the like. As will be apparent, any of a wide variety of
different types of materials may comprise the substrates, including
organic materials, e.g., polymers, carbon sheets, etc., ceramics,
inorganic materials, e.g., semiconductors, insulators, glasses,
including silica based materials (e.g., silicon, SiO.sub.2), etc.,
metals, semimetals, as well as composites of any or all of
these.
[0057] Additionally, substrates, e.g., rigid or solid substrates,
may be engineered to have additional topographies, e.g., three
dimensional shapes, such as wells, pyramids, posts, etc. on their
surface to further enhance their effectiveness, e.g., provide
higher surface areas, channel fluids or gases over them, provide
prefiltration in advance of the filtration provided by the porous
substrate, per se, etc. Additionally, although referred to as
including a porous substrate, it will be appreciated that in
application, multiple substrates may be provided together in a
single article, device or system. Further, although described and
exemplified primarily as planar porous substrates, it will be
appreciated that the porous substrates may be fabricated into any
of a variety of shapes depending upon the application, including
non-planar three dimensional shapes, spheres, cylinders, disks,
cubes, blocks, domes, polyhedrons, etc. that may be more easily
integrated into their desired application. Substrates, e.g., planar
sheet substrates, are optionally rigid or flexible.
[0058] Examples of metal substrates include steel/iron, nickel,
aluminum, titanium, silver, gold, platinum, palladium, or virtually
any metal substrate that imparts a desirable property to the
finished article, e.g., conductivity, flexibility, malleability,
cost, processibility, etc. In certain preferred aspects, a metal
wire mesh or screen is used as the substrate. Such meshes provide
relatively consistent surfaces in a ready available commercial
format with well defined screen/pore and wire sizes. A wide variety
of metal meshes are readily commercially available in a variety of
such screen/pore and wire sizes. Alternatively, metal substrates
may be provided as perforated plates, e.g., solid metal sheets
through which apertures have been fabricated. Fabricating apertures
in metal plates may be accomplished by any of a number of means.
For example, relatively small apertures, e.g., less than 100 .mu.m
in diameter, as are used in certain aspects of the invention, may
be fabricated using lithographic and preferably photolithographic
techniques. Similarly, such apertures may be fabricated using laser
based techniques, e.g., ablation, laser drilling, etc. For larger
apertures, e.g., greater than 50-100 .mu.m, more conventional metal
fabrication techniques may be employed, e.g., stamping, drilling or
the like.
[0059] Polymeric and inorganic substrates may be similarly
structured to the metal substrates described above, including mesh
or screen structures, fibrous mats or aggregates, e.g., wools, or
solid substrates having apertures disposed through them. In terms
of polymeric substrates, again, the primary selection criteria is
that the substrate operate in the desired application, e.g., is
resistant to chemical, thermal or radiation or other conditions to
which it will be exposed. In preferred aspects the polymeric
substrate will also impart other additional useful characteristics
to the overall article, such as flexibility, manufacturability or
processibility, chemical compatibility or inertness, transparency,
light weight, low cost, hydrophobicity or hydrophilicity, or any of
a variety of other useful characteristics. Particularly preferred
polymeric substrates will be able to withstand certain elevated
environmental conditions that may be used in their manufacturing
and/or application, e.g., high temperatures, e.g., in excess of 300
or 400.degree. C., high salt, acid or alkaline conditions, etc. In
particular, polymers that tolerate elevated temperatures may be
particularly preferred where the nanowires are actually grown in
situ on the surface of the substrate, as such synthetic processes
often employ higher temperature synthetic processes, e.g., as high
as 450.degree. C. Polyimide polymers, polyetherketone, polyaramid
polymers and the like are particularly preferred for such
applications. Those of skill in the art will recognize a wide range
of other polymers that are particularly suitable for such
applications. Alternatively, lower temperature fiber synthesis
methods may also be employed with a broader range of other
polymers. Such methods include that described by Greene et al.
("Low-temperature wafer scale production of ZnO nanowire arrays",
L. Greene, M. Law, J. Goldberger, F. Kim, J. Johnson, Y. Zhang, R.
Saykally, P. Yang, Angew. Chem. Int. Ed. 42, 3031-3034, 2003), or
through the use of PECVD, which employs synthesis temperatures of
approximately 200.degree. C. In the case where the porous substrate
is merely the recipient of nanofibers already synthesized, e.g.,
where the substrate is either to be coupled to the nanowires or is
to act as a macroporous support for the nanowires, a much wider
variety of porous substrates may be employed, including organic
materials, e.g., organic polymers, metals, ceramics, porous
inorganics, e.g., sintered glass, which would include a variety of
conventionally available membrane materials, including cellulosic
membranes, e.g., nitrocellulose, polyvinyl difluoride membranes
(PVDF), polysulfone membranes, and the like.
[0060] In some cases, the porous substrate may comprise a soluble
material, e.g., cellulose, or the like. Following attachment of the
nanofibers, and optionally placement of the overall substrate into
its ultimate device configuration, the supporting porous substrate
may be dissolved away, leaving behind an interwoven mat or
collection of nanofibers. For example, a soluble mesh may be
provided with nanofibers attached to its overall surfaces or
interior wall surfaces as described herein. The mesh may then be
rolled into a cylindrical form and inserted into a cylindrical
housing, e.g., a column for separations applications. The
supporting mesh is then dissolved away to yield the column packed
with nanofibers. Further, as described above, the porous matrix may
comprise any of a number of shapes, and be soluble as well, so as
to yield any of a variety of shapes of aggregations of fibers, once
the substrate is dissolved.
[0061] As noted above, the apertures of the substrates used herein
typically are defined in terms of their effective pore size or
"effective porosity". Although described as apertures or pores, it
will be appreciated that the term "aperture" or "pore" when used in
the context that it is disposed through a substrate, refers simply
to a contiguous pathway or passage through a substrate material,
whether that material be a single solid piece of substrate material
or a mesh or mat of aggregated pieces of substrate material. Thus,
such "apertures" or pores do not need to represent a single
passage, but may constitute multiple passages strung together to
form the contiguous path. Likewise, an aperture or pore may simply
represent the space between adjacent portions of substrate
material, e.g., fibers, etc. such that the spaces provide a
contiguous path through the material. For purposes of the
invention, pore or aperture size, in the absence of any nanofibers
disposed thereon, will typically vary depending upon the nature of
the application to which the material is to be put.
[0062] For example, filtration applications will typically vary
pore size depending upon the nature of the particles or other
material to be filtered, ranging from tens to hundreds of microns
or larger for coarser filtration operations to submicron scale for
much finer filtration applications, e.g., bacterial sterilizing
filters. Similarly for semi-permeable barrier applications, such
pores will typically vary depending upon the type of permissible
permeability is sought. For example, breathable moisture barriers
may have pore sizes from tens of microns to the submicron range,
e.g., 0.2 .mu.m, or smaller. In some cases, it may be desirable to
have an effective pore size that is less than 100 nm, and even less
than 20 nm, so as to block passage of biological agents, e.g.,
bacteria and viruses.
[0063] The articles and substrates described herein may include
nanowires substantially on any and all surfaces of the substrate
material including both exterior surfaces and the surfaces that are
within the pores. Together, these surfaces upon which nanowires may
be disposed are referred to herein as the "overall surface" of the
substrate material, while the wall surfaces that are disposed upon
the interior walls of the pores are generally referred to herein as
the "interior wall surfaces" of the substrate material or pores. As
will be clear to one of ordinary skill in reading the instant
disclosure, a reference to a surface as an interior wall surface
for certain embodiments, e.g., in the case of a fibrous mat or wool
like substrate does not necessarily denote a permanent status of
that surface as being in the interior portion of a pore or aperture
as the basic flexibility and/or malleability of certain substrate
materials may provide the ability to shift or move the various
portions of the substrate material's overall surface around.
[0064] As noted above, the substrates of the invention gain
significant unique properties by incorporating nanofibers or
nanowires on their surfaces. For most applications, the terms
"nanowire" and "nanofiber" are used interchangeably. However, for
conductive applications, e.g., where the nanofibers' conductive or
semiconductive properties are of interest, the term "nanowire" is
generally favored. In either instance, the nanowire or nanofiber
generally denotes an elongated structure having an aspect ratio
(length:width) of greater than 10, preferably greater than 100 and
in many cases 1000 or higher. These nanofibers typically have a
cross sectional dimension, e.g., a diameter that is less than 500
nm and preferably less than 100 nm and in many cases, less than 50
nm or 20 nm.
[0065] The composition of the nanofibers employed in the invention
typically varies widely depending upon the application to which the
resulting substrate material is to be put. By way of example,
nanofibers may be comprised of organic polymers, ceramics,
inorganic semiconductors and oxides, carbon nanotubes, biologically
derived compounds, e.g., fibrillar proteins, etc. or the like. For
example, in certain embodiments, inorganic nanofibers are employed,
such as semiconductor nanofibers. Semiconductor nanofibers can be
comprised of a number of Group IV, Group III-V or Group II-VI
semiconductors or their oxides. Particularly preferred nanofibers
include semiconductor nanowires or semiconductor oxide
nanofibers.
[0066] Typically, the nanofibers or nanowires employed are produced
by growing or synthesizing these elongated structures on substrate
surfaces. By way of example, Published U.S. Patent Application No.
US-2003-0089899-A1 discloses methods of growing uniform populations
of semiconductor nanowires from gold colloids adhered to a solid
substrate using vapor phase epitaxy. Greene et al.
("Low-temperature wafer scale production of ZnO nanowire arrays",
L. Greene, M. Law, J. Goldberger, F. Kim, J. Johnson, Y. Zhang, R.
Saykally, P. Yang, Angew. Chem. Int. Ed. 42, 3031-3034, 2003)
discloses an alternate method of synthesizing nanowires using a
solution based, lower temperature wire growth process. A variety of
other methods are used to synthesize other elongated nanomaterials,
including the surfactant based synthetic methods disclosed in U.S.
Pat. Nos. 5,505,928, 6225,198 and 6,306,736, for producing shorter
nanomaterials, and the known methods for producing carbon
nanotubes, see, e.g., US-2002/0179434 to Dai et al. As noted
herein, any or all of these different materials may be employed in
producing the nanofibers for use in the invention. For some
applications, a wide variety of group III-V, H-VI and group IV
semiconductors may be utilized, depending upon the ultimate
application of the substrate or article produced. In general, such
semiconductor nanowires have been described in, e.g.,
US-2003-0089899-A1, incorporated herein above. In certain preferred
embodiments, the nanowires are selected from a group consisting of:
Si, Ge, Sn, Se, Te, B, diamond, P, B--C, B-P(BP6), B--Si,
Si--C,
[0067] Si--Ge, Si--Sn, Ge--Sn, BN, BP, BAs, AlN, AlP, AlAs, AlSb,
GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, ZnO, ZnS, ZnSe, ZnTe,
CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe, MgS, MgSe, GeS,
GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,
CuI, AgF, AgCl, AgBr, AgI, BeSiN.sub.2, CaCN.sub.2, ZnGeP.sub.2,
CdSnAs.sub.2, ZnSnSb.sub.2, CuGeP.sub.3, CuSi.sub.2P.sub.3, (Cu,
Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te).sub.2, Si.sub.3N.sub.4,
Ge.sub.3N.sub.4, Al.sub.2O.sub.3, (Al, Ga, In).sub.2(S, Se,
Te).sub.3, AI.sub.2CO, and an appropriate combination of two or
more such semiconductors. The nanofibers optionally comprise a gold
tip.
[0068] In the cases of semiconductor nanofibers, and particularly
those for use in electrical or electronic applications, the
nanofibers may optionally comprise a dopant from a group consisting
of: a p-type dopant from Group III of the periodic table; an n-type
dopant from Group V of the periodic table; a p-type dopant selected
from a group consisting of: B, Al and In; an n-type dopant selected
from a group consisting of: P, As and Sb; a p-type dopant from
Group II of the periodic table; a p-type dopant selected from a
group consisting of: Mg, Zn, Cd and Hg; a p-type dopant from Group
IV of the periodic table; a p-type dopant selected from a group
consisting of: C and Si; or an n-type dopant is selected from a
group consisting of: Si, Ge, Sn, S, Se and Te.
[0069] In some cases, it may be desirable to utilize nanofibers
that have a self sterilizing capability, e.g., in semipermeable
bandage, clothing, filtration or other applications. In such cases,
the nanofibers may be fabricated from, e.g., TiO.sub.2, which upon
exposure to UV light oxidizes organic materials to provide a self
cleaning functionality (See, e.g., US Patent Application
Publication No. 20060159916, and incorporated herein by reference
in its entirety for all purposes).
[0070] Additionally, such nanofibers may be homogeneous in their
composition, including single crystal structures, or they may be
comprised of heterostructures of different materials, e.g.,
longitudinal heterostructures that change composition over their
length, or coaxial heterostructures that change composition over
their cross section or diameter. Such coaxial and longitudinal
heterostructured nanowires are described in detail in, e.g.,
Published International Patent Application No. WO 02/080280, which
is incorporated herein by reference for all purposes.
[0071] The nanowire portion of the articles of the invention are
preferably synthesized in situ, e.g., on the desired surface of the
porous substrate. For example, in preferred aspects, inorganic
semiconductor or semiconductor oxide nanofibers are grown directly
on the surface of the porous substrate using a colloidal catalyst
based VLS (vapor-liquid-solid) synthesis method such as those
described above. In accordance with this synthesis technique, the
colloidal catalyst is deposited upon the desired surface of the
porous substrate (which in some cases may include the overall
surface of the porous substrate). The porous substrate including
the colloidal catalyst is then subjected to the synthesis process
which generates nanofibers attached to the surface of the porous
substrate. Other synthetic methods include the use of thin catalyst
films, e.g., 50 nm, deposited over the surface of the porous
substrate. The heat of the VIS process then melts the film to form
small droplets of catalyst that form the nanofibers. Typically,
this latter method may be employed where fiber diameter homogeneity
is less critical to the ultimate application. Typically, catalysts
comprise metals, e.g., gold, and may be electroplated or evaporated
onto the surface of the substrate or deposited in any of a number
of other well known metal deposition techniques, e.g., sputtering
etc. In the case of colloid deposition, the colloids are typically
deposited by first treating the surface of the substrate so that
the colloids adhere to the surface. Such treatments include those
that have been described in detail previously, e.g., polylysine
treatment, etc. The substrate with the treated surface is then
immersed in a suspension of colloid.
[0072] Alternatively, the nanofibers may be synthesized in another
location and deposited upon the desired surface of the porous
substrate using previously described deposition methods. For
example, nanofibers may be prepared using any of the known methods,
e.g., those described above, and harvested from their synthesis
location. The free standing nanofibers are then deposited upon the
relevant surface of the porous substrate. Such deposition may
simply involve immersing the porous substrate into a suspension of
such nanofibers, or may additionally involve pretreating all or
portions of the porous substrate to functionalize the surface or
surface portions for fiber attachment. A variety of other
deposition methods are known, e.g., as described in U.S. Pat. Nos.
7,067,328, and 6,962,823, the full disclosures of which are
incorporated herein by reference in their entirety for all
purposes.
[0073] Where nanofibers are desired to be attached primarily to the
interior wall portions of the surface of the porous substrate, such
deposition may be accomplished by growing the nanofibers in such
locations or by selectively depositing the nanofibers in such
locations. In the case of in situ grown nanofibers, this may be
accomplished by depositing a layer of another material on all of
the exterior surfaces of the substrate, e.g., a resist, before
depositing the colloids. Following immersion in colloid, the resist
layer may be developed and removed to yield substrate having
colloid substantially only deposited on the interior wall surfaces
of the substrate.
[0074] FIGS. 1 and 2 schematically illustrate substrates according
to the present invention. In particular, FIG. 1 shows a schematic
illustration of a porous nanowire carrying substrate of the
invention. As shown in FIGS. 1A and 1B, a porous substrate 102 is
provided. For purposes of exemplification, a mesh or screen is
employed as the porous substrate, although fibrous mats are also
useful in such applications. As shown in FIG. 1B, nanofibers 104
are provided that are, at least in part, disposed on the internal
wall portions 106 of the apertures or pores, and which extend into
the void area 108 of the pores, yielding openings or passages
through the overall material that are somewhat more restrictive or
narrow than those provided by the underlying substrate, itself. As
shown in FIG. 1, the nanofibers 104 are also disposed on other
surface portions of the mesh (the overall surface).
[0075] FIGS. 2A and 2B schematically illustrate the case where
nanofibers are primarily disposed only on the interior wall
portions of the apertures that define the pores. As shown, a
perforated substrate 202 forms the underlying porous substrate. A
plurality of apertures 208 are fabricated through the substrate
202, e.g., by punching etching or other known fabrication methods.
As shown in FIG. 2B, an expanded view of the aperture 208 is
provided that details the presence of nanofibers 204 attached to
the interior wall portions 206 of the aperture. As shown, the
nanofibers generally protrude away from the interior wall surface
206. This is typically accomplished by growing the nanofibers, in
situ, using a catalytic growth CVD process, whereupon the fibers
grow away from the surface upon which the catalyst is initially
deposited. Other methods may also be employed to deposit nanofibers
on these interior wall portions that may or may not result in the
fibers protruding into the void space of the apertures, including
immersing the porous substrate in a suspension of nanofibers that
are chemically able to attach to the surfaces of interest.
[0076] FIG. 12 shows a photograph of a silicon substrate that has
pores or apertures disposed through it. Silicon nanofibers were
grown over the surface of the substrate, including within the
pores. The substrate was a 0.1 mm thick silicon wafer with
regularly spaced 100 .mu.m holes disposed through it. FIG. 12A
shows a view of a larger area of the substrate, while FIG. 12 B
shows a closer up view of the pore and substrate surface, as well
as the nanofibers on those surfaces.
[0077] In alternative arrangements, the porous substrates may be
employed in steps that are discrete from the synthesis process, and
that employ the porous substrate as a capture surface for the
nanofibers. In particular, nanofibers may be produced as
suspensions or other collections or populations of free-standing,
e.g., a population of discrete and individual members, nanofibers.
Such free standing nanofibers are generally produced from any of
the aforementioned processes, but including a harvest step
following synthesis whereby the nanofibers are removed from a
growth substrate and deposited into a suspending fluid or other
medium or deposited upon a receiving substrate, or otherwise moved
from a growth or synthesis environment into a manipulable
environment, e.g., a fluid suspension. The population of nanowires
is then deposited over a porous substrate to yield a mat of
deposited nanofibers that form a micro or nanoporous network over
the underlying porous substrate. In accordance with this aspect of
the invention, the pores in the porous substrate are typically
selected so that they are smaller than the largest dimensions of
the nanofibers to be deposited thereon, e.g., the length of the
nanofiber. For example, where nanofibers in a particular population
have an average length of approximately 10 .mu.m, the pores in the
substrate will typically be smaller in cross section than 10 .mu.m,
e.g., less than 5 .mu.m, less than 2 .mu.m, or smaller. To ensure
sufficient capture of nanofibers, the largest cross section of the
pore in the porous substrate will typically be less than 50% of the
average largest dimension of the nanofiber population, generally
the length, in some cases, less than 20% of such dimension, and in
many cases, less than 10% of such dimension.
[0078] The nanofiber mat is then optionally fused or cross-linked
at the points where the various fibers contact each other, to
create a more stable, robust and potentially rigid fibrous
membrane. The void spaces between the interconnected nanofibers
form the porous network of the nanofibrous mat. The effective pore
size of the mat will generally depend upon the density of the
deposited nanofiber population that is deposited, as well as the
thickness of that layer, and to some extent, the width of the
nanofibers used. All of these parameters are readily varied to
yield a mat having a desired effective porosity.
[0079] FIGS. 9A and 9B show electron micrographs of cross-linked
nanofibrous mats that illustrate certain aspects of the invention.
FIG. 9A shows a population of semiconductor nanofibers that were
cross-linked through vapor deposition of inorganic material, e.g.,
silicon. In particular, a population of silicon nanowires was
prepared by a conventional synthesis scheme, e.g., silicon
nanowires were grown at 480.degree. C., from a gold colloid
catalyst, under SiH4 partial pressure, 1 torr, total pressure, 30
torr for 40 minutes. After the growth was terminated by pumping out
the process gasses, the temperature of the substrate was ramped up
to 520.degree. C. under 30 torr He. The process gases (SiH.sub.4)
were switched on again once temperature was reached, and the
resulting silicon deposition cross-linked the adjacent or
contacting nanowires. The deposition time was 10 minutes. As will
be appreciated, separately harvested and deposited nanofibers may
be similarly crosslinked using this technique.
[0080] The nanofibers in FIG. 9B, on the other hand, were linked
using a polymer deposition process that at least partially coated
or encased the nanofibers to link them together. In particular, a
PVDF polymer was suspended along with the nanowires in acetone and
sonicated. The acetone was then evaporated to yield the
encapsulated or crosslinked nanowires or nanofibrous mat. As can be
seen in each case, the network of silicon nanofibers, or nanowires,
shows cross-linking at the intersections of various nanofibers.
Also as shown, the pores created by the interwoven nanofibers are
defined by the void space between the nanofibers.
[0081] As noted above, the alternative aspects of the invention may
be accomplished by simply depositing nanowires upon a receiving or
supporting substrate such that the nanofibers are overlaying each
other to form a mat, and preferably a dense nanofiber mat. In
general, this process is simplified by using a porous supporting
substrate such that the nanofibers may be captured upon the upper
surface of the porous supporting substrate while the medium in
which the nanofibers were originally disposed is allowed to pass
through the pores, essentially filtering the nanofibers with the
substrate and densely depositing the nanofibers on the surface of
the substrate. The resulting fibrous mat is then treated to
crosslink the fibers at the points where they contact or are
sufficiently proximal to each other.
[0082] The process for such mat formation is schematically
illustrated in FIG. 10. In particular, a nanofiber population 1000
is provided as a suspension 1002, where the nanofibers may be
suspended in liquid, gas, or simply provided as a free flowing
population or powder. The nanofiber population is then deposited or
poured onto a porous substrate 1004. The nanofiber population 1000
is then retained upon the upper surface 1006 of the porous
substrate 1004, at which point it forms an overlaying mat 1008 of
nanofibers supported by substrate 1004. The mat 1008 is added to by
depositing additional nanofibers onto the substrate. As noted
previously, any medium in which the nanofibers are suspended freely
passes through pores 1010 in the porous substrate 1004, allowing
the nanofibers to pack densely against the upper surface 1006 of
the porous substrate 1004.
[0083] Once the nanofiber mat 1008 is of the desired thickness and
fiber density, the mat may be readily employed upon its supporting
macroporous substrate, e.g., as a filter membrane or other
semipermeable layer. However, in preferred aspects, the nanofibrous
mat is treated (as indicated by arrows 1014) to crosslink the
nanofibers at their respective contact points to form couplings
1012 between the nanofibers in the mat, as shown in the expanded
view. The use of crosslinked nanofibers has been described for
ultra high surface area applications (See, e.g., commonly owned
U.S. Patent Application Publication No. 20060159916, and
incorporated herein by reference in its entirety for all purposes).
Crosslinking, as noted previously, may be accomplished by a number
of means, including thermal fusing, chemical surface
modification/crosslinking, encapsulation or coating. Thermal fusing
methods may vary depending upon the makeup of the nanofibers, with
polymeric nanofibers being fused at substantially lower
temperatures than metal or inorganic semiconductor nanofibers.
[0084] Nanofibers may also include surface chemical groups that may
form chemical crosslinks in order to cross-link the underlying
nanofibers. For example, polymeric materials, such as
polyacrylamide or polyethylene glycol groups, may be readily
coupled to the surfaces of nanofibers, e.g., through well known
silane and/or pegylation chemistries. Well known polymer
crosslinking techniques are then used to crosslink the nanofibers.
Similarly, epoxide, acrylate or other readily available reactive
groups may be provided upon the surface of the nanofibers that
allow thermal curing, optical curing, e.g., UV, or other chemical
interaction and coupling between adjacent, contacting nanofibers to
provide the crosslinking.
[0085] In another aspect, the nanofibrous mat may be crosslinked
together using a polymer coating or encapsulation technique that
locks the various nanofibers into position. For example, vapor
deposition techniques may be employed to vapor deposit thin polymer
layers over the nanofiber portions of the mat, effectively
cementing the nanofibers into position. Examples of such polymers
include, e.g., PTFE, PVDF, parylene, and the like. A wide variety
of other polymeric materials may optionally be employed using a
liquid deposition or an in situ polymerization and/or crosslinking
techniques, e.g., as described above. As will be appreciated,
polymeric crosslinking may provide certain benefits over thermal
and/or chemical crosslinking in terms of pliability of the
resulting mat of material.
[0086] Once the nanofibrous mat is crosslinked, it may be employed
along with the underlying macroporous substrate, e.g., as a
backing, or it may be separated from the substrate to yield an
independent nanofibrous membrane, e.g., membrane 1016. As will be
readily appreciated, larger area nanofiber layers may be produced
using conventionally available processes, including drum or belt
filter techniques where a large area, continuous macroporous
substrate layer, e.g., in a belt or as a surface of a drum, is used
to retain nanofiber layers, which layers are crosslinked or
otherwise treated as described herein. Such processes may be
configured in a continuous or large area batch mode operation in
order to provide extremely large amounts of the fibrous layer
material, e.g., for use in clothing, outdoor fabrics, e.g., tents,
and other high volume applications.
[0087] In one embodiment, an article of the invention includes a
nanofibrous mat that comprises a plurality of overlaid nanofibers,
wherein said plurality of nanofibers are crosslinked together at
points where such nanofibers contact or are proximal to others of
said nanofibers, to form a semipermeable layer. The nanofibrous mat
is optionally deposited upon a surface of a porous substrate, with
the porous substrate and nanofibrous mat forming a semipermeable
layer. At least a portion of the nanofibers optionally comprise an
attached hydrophobic moiety.
[0088] In one embodiment, methods of producing a contiguous
population of nanofibers are provided. In the methods, a porous
substrate having an overall surface area is provided, as is a
plurality of nanofibers attached to the overall surface area of the
porous substrate. A related embodiment also provides methods of
producing a contiguous population of nanofibers. The methods
include providing a porous substrate having an upper surface and a
plurality of pores disposed through the porous substrate, wherein
each of said pores has an effective pore size; depositing a
plurality of nanofibers onto the upper surface of the porous
substrate, said nanofibers having at least one dimension greater
than the effective pore size, such that the nanofibers are retained
upon the upper surface as a nanofibrous mat; and crosslinking
individual nanofibers in the plurality of nanofibers with other
individual nanofibers of the plurality of nanofibers to produce a
contiguous nanofiber population.
III. Applications
[0089] As alluded to herein, the porous substrates of the invention
having nanofibers attached to portions of their surfaces have
myriad applications that take advantage of a wide variety of
particularly interesting properties of such materials. In certain
applications, the presence of nanowires provides porous materials
with enhanced properties. In other applications, the combination of
porous substrates and nanowires provides materials having
substantially new properties and usefulness.
[0090] A. Semi-Permeable barriers
[0091] In a first particularly preferred application, the porous
substrates of the invention are useful as semi-permeable barriers.
Semi-permeable barriers, in general, also find a wide variety of
different applications depending upon their level of permeability,
cost, etc. For example, such barriers may be permeable to gas and
not liquid, or to air or gas and not particulate matter. Still
further, such semi-permeable barriers may provide antiseptic or
antibacterial properties to their applications.
[0092] In one aspect, the invention provides a semipermeable
membrane including a porous substrate having a plurality of
apertures disposed therethrough, the porous substrate having an
overall surface that includes an interior wall surface of the
apertures, and a plurality of nanofibers deposited upon or attached
to at least a portion of the overall surface of the porous
substrate, wherein the nanofibers and apertures together define a
pore through the semipermeable membrane, the pore being permeable
to one or more materials and not permeable to one or more different
materials. The nanofibers can be attached to or deposited on at
least a portion of the overall surface of the porous substrate.
Individual nanofibers can be crosslinked with other individual
nanofibers to provide a crosslinked nanofibrous mat on the surface
of the porous substrate. The nanofibers optionally comprise a
hydrophobic moiety coupled thereto, rendering the pore permeable to
gas but not permeable to liquid water. The pore can have an
effective pore size that excludes particles of a first particle
size while permitting passage of particles of a second particle
size, smaller than the first particle size. The effective pore size
can be, e.g., smaller than 10 .mu.m, smaller than 1 .mu.m, less
than 0.2 .mu.m, less than 100 nm, or even less than 20 nm. The
membrane can be incorporated into a variety of articles. For
example, a filter cartridge can include a semipermeable membrane of
the invention disposed within a housing having an inlet passage in
fluid communication with a first side of the semipermeable membrane
and an outlet side in fluid communication with a second side of the
semipermeable membrane. In one embodiment, the housing is coupled
to a breathing mask to filter breathing air. As another example, an
article of clothing can include a semipermeable membrane of the
invention, e.g., layered with at least a second fabric layer.
[0093] 1. Filtration
[0094] In their simplest aspect, semi-permeable barriers are used
as filtration media for separating gases or liquids from
particulate matter. For example, there are a wide range of
different filtration options available for e.g., air filtration,
from simple consumer filtration needs, e.g., home furnaces, air
conditioners, air purifiers, to more demanding filtration needs,
e.g., HEPA filtration for industrial use, hazardous materials
filtration for protective gear, clean room applications, automotive
applications, etc. For liquid applications, such filters may
provide water purification, particulate separation for fuels and
lubricants for industrial or consumer machinery, e.g., automobiles,
etc.
[0095] In accordance with the filtration applications of the
present invention, porous substrates are used as the foundation for
the filtration media to be produced. Nanofibers are then provided
on the surfaces overall or substantially only interior surfaces to
further enhance the filtration capabilities of the underlying
porous foundation. In particular, one of the key areas that are
sought for improvement in filtration media is the ability to
increase the filtration efficiency, e.g., reduce the pore size or
increase the overall capacity/lifetime of a filter, without
yielding a substantial increase in the pressure drop, which could
lead to early filter failure/clogging, higher energy demands,
etc.
[0096] The enhancements brought by the present invention include
effectively decreasing the pore size of the filter without
substantially increasing the pressure drop across a filter. In
particular, the present invention provides porous substrates having
nanofibers disposed within the pores of the substrate to provide
additional filtration by modifying the effective pore size.
Nanofibers, because of their extremely small size, are particularly
useful in these applications due to their ability to substantially
increase the surface area within a pore without substantially
increasing the volume of material disposed within that pore, this
increasing the filtration efficiency without decreasing the flow
through the filter media. Fluids or gases are then passed through
the porous substrate to separate particulate materials from the
carrier liquid or gas.
[0097] One example of a filtration cartridge, e.g., for a filter
mask, gas line or the like, is illustrated in FIGS. 3A and 3B. As
shown in FIG. 3A, a filter cartridge 300 includes a main housing
302 having a filter layer 304 disposed within the housing. A filter
support 306 is typically also included on the low pressure side of
the filter layer to provide structural support to the filter layer.
The filter layer typically includes an inlet or high pressure side
308 and a low pressure or outlet side 310. Gas or liquid is
filtered through the cartridge by passing from the high pressure or
inlet side to the low pressure or outlet side of the filter. The
filter cartridge thus includes an inlet passage 312 or passages for
passing gas or fluid to the inlet side of the filter layer to be
filtered, and an outlet passage 314 or passages for passing gas or
fluid that has been filtered through the filter layer 304. Filter
3B shows an end view of the outlet side of the particular filter
cartridge shown in FIG. 3A.
[0098] As noted, the filtration cartridge may be incorporated into
larger systems depending upon the ultimate application. For
example, air filters may be incorporated into heating and air
conditioning or other environmental control systems to provide
purified air for, e.g., commercial or industrial, i.e., clean room,
applications. Filtration cartridges of the invention may optionally
be incorporated into fluid filtration systems as well, for water,
fuel or chemical filtration applications.
[0099] In accordance with the filtration applications, effective
pore sizes of the filter media may be varied depending upon
application, e.g., from coarse particle filtration, e.g., effective
pore sizes of 1, 10 or more microns, to antibacterial filtration,
e.g., effective pore sizes of 0.2 .mu.m or less, e.g., down to 20
nm or less. As alluded to elsewhere herein, the phrase "effective
pore size" does not necessarily reflect the size of a discrete
passage through the substrate, but instead may reflect the cross
sectional dimensions of a contiguous path through which fluid, gas
or particles may pass, or be blocked from passing. In addition, the
"effective pore size" of a given passage does not necessarily
define the absolute dimensions of the contiguous passage, but
instead defines the size of the particles that are effectively
blocked from passing through the passage. Typically, such varied
pore sizes will be of a function of nanowire density disposed
within the larger apertures that exist in the underlying substrate,
the diameter and length of the nanofibers, as well as a result, to
some extent, of the size of such apertures to begin with.
[0100] Similarly, the composition or make up of the filtration
media, both in terms of nanofibers and the underlying substrate,
may depend upon the application to which the material is to be put,
with materials being generally selected to withstand the conditions
to which they will be exposed. Such conditions might include
extremes of temperature, alkalinity or acidity, high salt content,
etc.
[0101] In one aspect the invention includes methods of filtering a
fluid or gas. In the methods, a porous substrate is provided and
the gas or liquid is passed through the porous substrate to filter
the gas or liquid. The substrate has a plurality of apertures
disposed therethrough to provide a porous substrate that has an
overall surface area that includes an interior wall surface area of
the apertures, and the substrate comprises a plurality of nanowires
attached to at least a portion of the overall surface area of the
porous substrate.
[0102] 2. Breathable Moisture Barriers
[0103] In a related aspect, the substrate is configured to be
permeable to gas, e.g., air, while remaining impermeable to liquid.
For example, such barriers are particularly useful as breathable
moisture barriers for clothing and medical applications, allowing
moisture vapor, oxygen and other gases to pass through the barrier
freely, but preventing liquid from passing. In accordance with the
invention, this is accomplished by providing nanofibers within the
apertures that are disposed through the porous substrate. In
contrast to other aspects of the invention, however, the nanofibers
for the moisture barrier applications are selected or treated to
have increased hydrophobicity. Treatment of nanofibers surfaces to
increase hydrophobicity was described in detail in U.S. Patent
Application Publication No. 20050181195, which is hereby
incorporated herein by reference in its entirety for all purposes.
In particular, the nanofibers and or the substrate surface may be
derivatized to attach hydrophobic chemical moieties to their
surfaces to increase the hydrophobicity of the material. Those of
ordinary skill in the art are well versed in the coupling of
hydrophobic chemical moieties to substrates, including, e.g.,
silane chemistries for treating silica based substrates, and the
like. By providing such super hydrophobic nanofiber surfaces on
porous underlying substrates, one can prevent passage of liquids,
e.g., liquid water or other aqueous solutions, while permitting
air, water vapor or other gases to pass. Typically, such barriers
will be substantially impermeable to moisture, e.g., preventing
passage of the substantial majority of moisture that comes into
contact with the surface under ambient conditions.
[0104] Such moisture permeable barriers are particularly useful in
outdoor gear, such as clothing, shelters, etc. where it is
desirable to eliminate moisture generated from within, while not
permitting liquid water to enter. FIGS. 4A and 4B schematically
illustrate a layered textile product, e.g., coat 400, that is
comprised of a layered textile 402 (shown in exploded view in FIG.
4B). As shown, a layer of the porous nanofiber bearing substrate
material 408 is provided between layers of other material, e.g., a
nylon outer shell 404 and cotton or polypropylene fabric lining
406, which provides external protection from wind and cold and
internal comfort against the skin or clothing of the wearer.
[0105] In one aspect the invention includes methods of producing a
gas permeable moisture barrier. In the methods, a porous substrate
that comprises a plurality of apertures therethrough and a
plurality of nanofibers attached to at least a portion of an
overall surface area of the porous substrate are provided. The
porous substrate and nanofibers together provide a gas permeable
barrier. At least the nanofibers are treated to increase
hydrophobicity of the nanofibers attached to the overall surface of
the porous substrate, to provide a gas permeable moisture
barrier.
[0106] 3. Bandages
[0107] In another preferred embodiment, these moisture permeable
barriers are useful as bandages or wound dressings, as they allow
oxygen to reach wound areas, gas and vapor to escape wound areas,
all the while preventing liquid water and other harmful
forces/abrasions, etc., from contacting the wound areas. In
addition to their benefits as semi-permeable barriers, the
nanofiber coated surfaces also may provide adhesion, to maintain
the bandage in place, e.g., adhering the bandage to itself or the
skin around the wound area. Use of nanofiber surfaces as dry
adhesives or high friction materials is described in detail in U.S.
Pat. No. 7,056,409, and incorporated herein by reference in its
entirety for all purposes. Additionally, such nanofiber coatings
may comprise antimicrobial materials, e.g., ZnO or the like, to
help prevent any infections in the wound areas.
[0108] FIG. 5 schematically illustrates a self adhesive, semi
permeable, moisture repellant bandage as described above. As shown,
a bandage 500 includes a flexible porous substrate strip 502 of the
invention, e.g., a woven fabric or soft mesh material, i.e., a
polymer or cloth mesh, having nanofibers that are appropriately
treated to provide a hydrophobic barrier, e.g., a moisture barrier
(represented by hatching on strip 502). The substrate strip 502
functions both as a breathable moisture impermeable cover and as an
adhesive strip. A protective pad 504 is provided upon a portion of
one side of the substrate strip 502 to provide protection for a
wound that is covered by the bandage. When applied to a wound, the
protective pad overlays the wound to provide protection from
rubbing or other contact, while the portions 506 and 508 of the
strip provide adhesion to the surface tissues adjacent a wounded
area (or when wrapped completely around a wounded appendage, to an
opposing surface of the other end of the substrate strip, e.g.,
region 506 adheres to the back side of region 508).
[0109] B. Vapor Barriers
[0110] In a related aspect, the porous substrates of the invention
are useful in breathable chemical/biological protective clothing
and apparatus, to adsorb or decompose various organic and inorganic
vapors. In accordance with the vapor barrier applications of the
present invention, porous substrates (e.g., fabrics or flexible
meshes) are used as the foundation. Preferably, an activated carbon
fabric is used. The activated carbon fabric absorbs organic vapors
and acts as a support structure. Activated carbon fabric is
commercially available, e.g., from Spectracorp.
[0111] Nanofibers (e.g., silicon nanowires, carbon nanotubes, or
polymeric nanofibers) are embedded, disposed on, or grown in situ
on the activated carbon fabric or other substrate. The nanofibers
reduce the permeability of the activated carbon fabric such that
adsorption of vapors is more efficient, permitting the layer to be
thinner. A hydrophilic surface is optionally present on the
nanofibers (e.g., due to the nanofiber composition or to surface
treatment of the nanofibers with a hydrophilic material), such that
a high degree of moisture vapor transmission can be maintained
through the layer despite the reduced air permeability.
[0112] The nanofibers are functionalized with moieties that bind or
decompose vapors. For example, the nanofibers can be functionalized
with moieties that bind or decompose non-organic gases such as
phosgene which are not absorbed by activated carbon. The nanofibers
can be functionalized with a chemical moiety, e.g., a chemical
moiety that absorbs or decomposes a non-organic gas, such as a
carboxylic acid moiety which binds ammonia or a moiety selected
from Table 1. In a preferred aspect, the nanofibers are
functionalized by attachment of nanocrystals to the nanofibers. The
nanocrystals can comprise a material such as Ag, ZnO, CuO,
Cu.sub.2O, Al.sub.2O.sub.3, Ti0.sub.2, MgO, FeO, MnO.sub.2, Zn, or
a material selected from Table 1, for example. Multiple
functionality can be conveniently imparted by using nanocrystals of
different compositions, attaching two or more populations of
nanocrystals or other nanostructures comprising different materials
adsorbing or binding different gases to the nanofibers (e.g., two,
three, four, five, six, or more populations). In a related aspect,
two or more batches of nanofibers can be chemically derivatized,
mixed, and incorporated into the fabric. Similarly, two or more
populations of nanofibers can be synthesized from different
materials, e.g., selected from Table 1, mixed, and incorporated
into the fabric (with or without additional functionalization), or
nanofiber longitudinal heterostructures (where a single nanofiber
comprises two or more materials) can be used. In a related aspect,
nanocrystals or other nanostructures can be attached to a
mesoporous carbon fabric or other fabric, particularly another high
surface area fabric, even in the absence of nanofibers.
[0113] A nanocrystal generally denotes a structure that is
substantially monocrystalline (or the core of which is
substantially monocrystalline). Nanocrystals typically have a
diameter that is less than 500 nm and preferably less than 100 nm
or 50 nm and in many cases, less than 20 nm or 15 nm. Nanocrystals
optionally have an aspect ratio of less than 10, for example, less
than 5 or 2, and in some cases, between about 0.1 and about 1.5.
Exemplary nanocrystals include, but are not limited to,
substantially spherical nanocrystals (e.g., spherical nanocrystals
having a diameter between 1 nm and 6 nm), rod shaped crystals
(e.g., rods that are about 5.times.50 nm), and tetrapods (also
called nanotetrapods, which have four rod-like arms coming out of a
central core). Techniques for chemically modifying nanofibers
and/or attaching nanocrystals the nanofibers are well known in the
art; for example, coating the nanofibers with polylysine or a
polymer. Attachment can be covalent or noncovalent.
[0114] FIG. 13 schematically illustrates a nanostructure-enhanced
fabric as described above. As shown, woven fabric 1301 includes
apertures 1302. Nanofibers 1304 are attached to the surface of the
fabric, and nanocrystals 1303 are attached to the nanofibers.
[0115] The fabric or other substrate can be incorporated into an
article of clothing, for example, a protective suit, or into other
protective apparatus. The nanofiber-enhanced fabric is optionally
protected by application of a layer of porous fabric or other
porous material on one or both sides.
[0116] The vapor-absorbing fabrics of the invention offer a number
of advantages over current protective layers. For example,
chemical/biohazard protection suits for the U.S. military are
currently based on activated carbon pellets embedded in a urethane
polymer. Although the pellets absorb many hazardous vapors, the
suits have a low moisture vapor transmission rate and are
susceptible to many non-organic vapors. Due to its extremely high
surface area and high binding affinity for organics, activated
carbon cloth has been used commercially in hazardous protection
suits. However, the weakness of these fabrics is their inability to
absorb non-organic molecules. By filling in the interstices of the
fabric with nanofibers that are functionalized to decompose or
absorb the non-organic vapors, a single layer fabric such as those
described above can provide full functionality while maintaining
good moisture vapor transmission.
TABLE-US-00001 TABLE 1 Exemplary materials for functionalization of
nanofibers. Toxic Industrial Compounds Absorber/Decomposer Chemical
Inorganic Acid Name Compound form Warfare Agents Gasses Organic
Biological Aluminum Oxide Al.sub.2O.sub.3 Ammonia, VX, SO.sub.2
4-Vinylpyridine, p- GD, HD, Cresol Simulants: 2CEES, DMMP Titanium
Dioxide TiO.sub.2 Ammonia, HCl, Formaldehyde, VX, GD, HD, Nicotine,
p-Cresol Simulants: 2CEES, DMMP, Paraoxon Magnesium Oxide MgO
Ammonia, HCl, SO.sub.2, CO.sub.2 Acetaldehyde, 4-
Antimicrobial/biocidal VX, GD, HD, Vinylpyridine, (MgO*I.sub.2,
MgO*Cl.sub.2) Simulants: Formaldehyde, DMMP, Paraoxon Nicotine
Copper Oxide, Cu Salts Cu.sub.2O, CuO, CuSO.sub.4*5H.sub.2O,
Ammonia, H.sub.2S, SO.sub.2, NO.sub.2 CuCl.sub.2*2H.sub.2O,
Phosgene, HCN, Cu(NO.sub.3).sub.2*3H.sub.2O Chlorine, Arsine Zinc
Oxide, Salts ZnO, Na.sub.2ZnO.sub.2, Paraoxon, H.sub.2S
Antimicrobial/biocidal Ammonia, Phosgene, HCN, Arsine Silver,
Silver Oxide, Ag Salts Ag, Maglon, Ag.sub.2O, Arsine,
Antimicrobial/biocidal Surfacine, AgNO.sub.3 Phosphine, Phosgene,
Chlorine, Diphosgene Iron Oxides FeO, Fe.sub.2O.sub.3 H.sub.2S
Mercaptans (CH.sub.3SH), Dimethyl Sulfide, Halogenated Hydrocarbons
Manganese Oxide, Salt MnO.sub.2, Mn.sub.xO.sub.y, KMnO.sub.4 CO
(w/CuO), Aldehydes NO.sub.2, H.sub.2S Chromium Salts CuCrO.sub.4,
Cyanogen NO.sub.2, H.sub.2S CuCrO.sub.4*NH.sub.3*5H.sub.2O,
Chloride (ClCN) Na.sub.2Cr.sub.2O.sub.7, (NH.sub.4)2Cr.sub.2O.sub.7
Polyoxometallates (POMs) H5PV2Mo10O40 type GD, HD heteropoly acids,
PW9O37 Sulfur Mercury Potassium Salts K.sub.2CO.sub.3, KI,
K.sub.2MnO.sub.4 Phosphine, SO.sub.2, NO.sub.2, H.sub.2S, Methyl
Iodide Arsine Carbon Disulfide, (radioactive) Mercury Phosphoric
& Sulfuric acid H.sub.3PO.sub.4, H.sub.2SO.sub.4 Ammonia
Mercury Amine, Acetaldehyde Pyridine C.sub.5H.sub.5N Cyanogen
Chloride Triethylenediamine (TEDA) Methyl Iodide (radioactive)
Para-Aminobenzoic acid H.sub.2S (PABA) Ortho-Iodosobenzoic acid GB,
GD, GA, (IBA) simulant Dimebu
[0117] C. High Contact Surface for, e.g., Electrical
Interfacing
[0118] As alluded to previously, the above described applications
typically employ nanofibers disposed upon a porous substrate to
provide enhanced properties to the porous substrates, e.g.,
enhanced porosity for filtration, moisture repulsion, etc. However,
in a number of applications, applying nanofibers to a porous
substrate provides a unique material that is not employed simply
for its porosity, but for other properties that are enhanced by the
synergistic structural characteristics of small dimension materials
coupled to a high surface area underlying substrate.
[0119] In particular, application of nanofibers to a porous
substrate, as a result of its higher surface area, provides for
higher packing levels of nanofibers per square centimeter of
projected area. In particular, dense mats of nanofibers may be
provided joined together on the porous substrate, which density
levels would not be readily achieved on flat surfaces. The higher
surface area of nanofibers is also readily accessible via the
apertures or pores in the underlying substrate.
[0120] In addition to the increase in nanofiber densities and/or
higher surface areas, porous substrates, e.g., meshes and fibrous
mats, tend to be flexible by comparison to more rigid, solid
substrates, e.g., silicon wafers, metal plates, or the like.
Additionally, depending upon the relative porosity of the
substrate, the overall article may benefit from being partially or
even substantially translucent or transparent, e.g., like a window
screen.
[0121] D. High Surface Area/High Density Fiber Applications
[0122] Separate and apart from the properties set forth above,
porous substrates can also provide a lightweight, high density
lattice for maintaining, handling, storing and otherwise using
nanofibers. Nanofibers may be harvested from this lattice, or
portions of the lattice may be used in their entirety to be applied
in more nanofiber specific applications, e.g., as semiconductive
elements, composite filler materials for structural or electrical
enhancement, high surface area matrices, e.g., for separations, or
the like.
[0123] In still other applications, the porous substrates having
nanowires disposed thereon may provide electrical integratability
to the nanofibers (or in this case, specifically nanowires) that
are attached thereto. Specifically, use of conductive porous
substrates may provide at least a portion of the electrical
connection to the nanowires necessary for the given application.
For example, semiconductor nanowires coupled to a metallic or other
conductive or semiconductive mesh are already partially integrated
into an electrical circuit, e.g., the mesh becomes an electrode,
e.g., source or drain, in the overall device.
[0124] The following description includes a number of such specific
examples of applications that benefit from the aforementioned
properties for illustration purposes alone. However, a much larger
number of specific uses and applications of the substrates and
articles of the invention will be readily apparent to those of
skill in the art upon the realization of the above-described
benefits, and the following description should not be viewed as
limiting and in no way excludes such applications.
[0125] In a first exemplary application, the substrate of the
invention is used as one electrode in a diode configuration. In
particular, a conductive mesh is used as the underlying porous
substrate with nanowires attached to its surfaces, e.g., overall
surface. The composition of the mesh is selected to have a work
function that promotes conduction of the major carriers in the
nanowire portion. For its part, the nanowire portion is selected to
provide one half of the diode circuit, and may include, e.g., p
doped nanowires. In accordance with this architecture, the nanowire
coated porous substrate functions as a portion of the diode
circuit. The other portion of the diode circuit may be provided as
either a conventional semiconductor substrate or as a mirror image
of the first, except with the materials being selected to conduct
the other carriers, e.g., holes, e.g., by providing n-doped
nanowires and appropriate electrode compositions for the underlying
substrate. The two substrates are then mated to interface the
nanowires at the surface to provide the functioning diode.
Additional elements may also be provided to ensure proper contact
between the nanowires, including conductive elements, annealing
steps, etc.
[0126] In another exemplary application, high surface area nanowire
coated substrates may be used as electrodes for interfacing with
other elements, e.g., electrical or non-electrical, such as human
tissue for electrical stimulation of the tissue. By way of example,
electrodes for pace makers typically benefit from having high
surface areas, and thus making more complete contact to the tissue
they are stimulating. Relatedly, where the nanofiber coated article
is being used as a tissue lattice, e.g., to facilitate
bioincorporation, higher surface areas and greater porosity are
highly beneficial in providing adherence points without blocking
access to such tissue by nutrients etc. Specifically, as described
in U.S. Patent Application Publication 20060159916, nanofiber
coated surfaces on medical implants provide `non-tortuous path`
enhanced surface areas that can provide enhanced tissue adhesion
and bioincorporation. It is expected that by providing such
nanofiber surfaces over porous underlying substrates, these
properties will be further enhanced.
[0127] In a particular application, such a diode arrangement is
employed as a photoactive element, e.g., as a photovoltaic or
photodiode device. The use of partially or substantially
translucent porous substrates facilitates this application in
letting light pass through the electrode components to impinge on
the semiconductor nanowires, thus generating charge separation at
the heterojunction of the opposing nanowires. Selection of
materials for the opposing underlying substrates may follow the
same criteria as used in conventional photovoltaic devices. For
example, one underlying substrate mesh may be comprised of aluminum
while the other is comprised of another metal having a different
work function, e.g., ITO, or a similar conductive material.
[0128] Previously described nanocomposite photovoltaics have
employed an active layer of a nanocomposite material sandwiched
between two conductive layers that function as electrodes. The
upper electrode typically comprises a transparent conductive
coating on the active layer, e.g., indium tin oxide (ITO). These
nanocomposite photovoltaic devices employed a first component in
which initial charge separation occurs. This typically employed a
nanocrystal in which an exciton was created upon exposure to light.
This nanocrystal component typically conducts one charge carrier
better than the other, e.g., electrons. The nanocrystals are
typically disposed in a matrix of another material which conducts
the other charge carrier, e.g., holes, away from the nanocrystal
component. By conducting the two carriers to opposite electrodes,
one generates an electric potential. Typically, the hole conducting
component comprises an organic semiconducting polymer, e.g.,
poly-3-hexylthiophene (P3HT), although the hole conducting
component can be another nanocrystal of a different composition.
The overall architecture of a nanocomposite photovoltaic device is
described in detail in, e.g., U.S. Pat. No. 6,878,871.
[0129] In accordance with the present invention, the overall
photovoltaic device 600 includes one (as shown in FIG. 6A), or two
(as shown in FIG. 6B) porous substrates 602 and 604 upon which
semiconducting nanowires are deposited. The first porous substrate
602 typically comprises a first conductive mesh 602a or other
porous material (as described previously herein) that functions as
one electrode in the system, e.g., the lower electrode, and
includes a first population of nanowires 606 of a first composition
attached to its overall surfaces. In a first embodiment shown in
FIG. 6A, the first porous substrate 602 and its associated
nanofibers are coated with a conductive matrix material 608 that
has a type-H band gap offset from the nanowire population 606, so
as to affect charge separation. A transparent electrode 610 is then
provided over the matrix layer 608.
[0130] In a second exemplary embodiment, the upper, transparent
electrode 610 in FIG. 6A is replaced by the second porous substrate
604, which is again fabricated from a conductive mesh 604a, but
which includes a different work function from that of the first
porous substrate 602, to facilitate charge separation. A second
population of nanowires 612 is provided attached to the second
porous substrate 604. The composition of the first and second
nanowire populations 606 and 612, respectively, is selected to
provide a type-II bandgap energy offset, again, so as to facilitate
charge separation and differential conduction. The first and second
porous substrates (602 and 604) are then mated together such that
their respective nanowire populations 606 and 612, respectively,
are in electrical communication so as to permit charge separation
between the two layers. As noted elsewhere, herein, in some cases,
the opposing nanowire populations may be further processed to
permit such electrical communication, including, e.g., thermal
annealing, or the like. The use of a dual semiconductor system,
e.g., as shown in FIG. 6B, may obviate the need for any organic
species within the active layer, e.g., conductive polymers, or the
like, and is expected to improve charge separation efficiencies by
speeding conduction of their respective carriers to their
respective electrodes, and thus prevent recombination of the
charges within the active layer.
[0131] In one embodiment, an electrical device of the invention
includes a porous substrate having a plurality of apertures
disposed therethrough, the porous substrate having an overall
surface area that includes an interior wall surface area of the
plurality of apertures, and a plurality of conductive or
semiconductive nanowires attached to and electrically coupled to
the porous substrate. An exemplary photovoltaic device includes a
first such electrical device, wherein the plurality of nanowires
comprises a first energy band gap, and a second such electrical
device, wherein the plurality of nanowires comprises a second
energy band gap. The first and second energy band gaps display a
type-II band gap offset relative to each other, and the nanowires
of the first electrical device are in electrical communication with
the nanowires of the second electrical device, so as to allow
charge separation between the first and second electrical devices
upon exposure to light.
[0132] As will be readily appreciated, the photovoltaic devices
described above are primarily for the illustration of the
applicability of the substrates of the invention to certain
electronic or optoelectronic applications. Those of skill in the
art will recognize a broad range of other electronic devices in
which such substrates would be useful.
[0133] In still another exemplary application, porous nanofiber or
nanowire coated substrates are encased in matrix components, e.g.,
a polymer matrix, for use as a composite matrix, including the
underlying mesh. Such applications are particularly useful where
the nanofibers are being employed as a bulk material to enhance the
functionality of the composite matrix. Such enhancements include
electrical enhancements, e.g., where the composite is being used as
a dielectric material, or to partially orient the nanofibers in
optoelectric applications, e.g., photovoltaics, structural
enhancements where the presence of the nanofibers imparts unique
structural characteristics to the matrix, e.g., tensile strength,
elasticity etc.
[0134] FIG. 7 schematically illustrates a composite matrix
incorporating the materials of the invention. As shown, a film of
composite material 702 includes within a matrix material, e.g., a
polymer, ceramic, glass or the like, a porous substrate 706 that
includes nanofibers 708 disposed upon its surface 710, including
within pores or apertures 710. The porous substrate is generally
immersed or impregnated with matrix material 704 to provide film
702. As noted above, these composite films are then applied in a
variety of applications, e.g., as conductive films, dielectric
films, etc.
[0135] FIGS. 8A and 8B schematically illustrate the use of porous
substrates to provide high surface area matrices for separation
applications, e.g., chromatography. In particular, as shown in FIG.
8A, a porous substrate 802 that has nanofibers attached to its
surface is provided. As noted, this porous substrate may be
provided in a number of different forms. For example, substrate 802
may comprise a mesh or screen that is rolled into a cylinder,
either before or after the fibers are attached or grown upon it.
Alternatively, the substrate may comprise a solid, but sintered or
fritted material, e.g., metal or glass. In still other aspects, the
substrate may comprise a fibrous material, e.g., glass wool, woven
fabric etc., that is shaped into the desired shape, e.g., a
cylinder 802 as shown, either by forming the material as such or
packing the material into a cylindrical (or other shaped) housing.
Again, such shaping may take place either before or after the
nanofibers have been grown or otherwise attached to the surface of
the porous material.
[0136] The substrate 802 is then placed into a column 804, which
includes an inlet 806 and an outlet 808 through which fluids are
flowed into and out of the column during a separation operation. As
shown in FIG. 8B the column 804 is then connected to appropriate
liquid handling equipment, e.g., gradient makers 810, pumps 812,
detectors 814, fraction collectors 816, and the like, for carrying
out chromatographic separations.
[0137] As will be apparent, the separation matrices incorporating
the substrate materials of the invention may encompass any of a
variety of the different substrate structures and conformations,
employ any number of a variety of different types of nanofibers, as
described elsewhere herein. Such structures, conformations and
compositions are generally selected depending upon the particular
application to which they are to be put and which will generally be
appreciated by those of ordinary skill in the art.
[0138] E. Reinforcing Lattice for Composite Materials
[0139] In still another aspect of the invention, the porous,
nanofiber bearing substrates of the invention form the lattice of a
composite material to enhance the integration of the lattice and
improve the structural characteristics of the overall composite
material. In particular, a number of composite materials include a
lattice that provides the underlying structural integrity that
supports an additional material, e.g., epoxies or other polymers,
ceramics, glasses, etc. For example, composites of fiberglass cloth
encased in epoxy resins, or other polymers are routinely used in a
variety of different applications, including, e.g., furniture,
surfboards and other sporting goods, auto body repairs, and the
like. Likewise, carbon fiber cloths or substrates are also
generally encased in a polymer or epoxy resin before they are
formed into the desired shape. Ultimately, these composite
materials generally possess structural characteristics, e.g.,
strength to weight ratios, that are better than most other
materials. Without being bound to a particular theory of operation,
it is believed that the interaction of the encasing material and
the lattice material is of significant importance in these
structural characteristics. Specifically, it is believed that by
enhancing the interaction of the two components of the composite,
e.g., improving integration of one into the other, will improve the
strength of the ultimate composite material. Because the nanofiber
bearing porous substrates of the invention benefit from extremely
high surface areas, as compared to that of the porous substrate
alone, it is expected that they will possess substantially greater
interactivity with the surrounding encasing material, e.g., the
epoxy. As such, another aspect of the invention includes the use of
the porous substrates having nanofibers deposited thereon, as a
lattice material for a composite material.
[0140] A general illustration of this aspect of the invention is
shown in FIG. 11. As shown, a porous substrate 1100 having a
surface that includes nanofibers 1102, is immersed within a matrix
material, e.g., hardened polymer 1104 to provide a composite
material 1106, that may be fabricated into a variety of different
materials or articles of manufacture.
[0141] As noted, a variety of fabrics are generally incorporated
into composite matrices as a supporting lattice for the ultimate
material. For example, carbon fiber composites typically employ a
woven carbon fiber material which is then intercalated with a
resin, e.g., an epoxy or other polymeric material. The composite
material is then formed into a desired shape and allowed to cure.
Alternatively, the desired shape may be formed post curing, e.g.,
by sanding or otherwise sculpting the hardened material. Similarly,
woven glass fabrics are used in fiberglass composite materials by
intercalating the fabric with an appropriate matrix, e.g., an
epoxy, etc.
[0142] In the context of the present invention, a porous substrate
that has nanofibers deposited upon it or attached to its surface(s)
is used as the lattice material for the resultant composite. The
nanofiber material is intercalated with a matrix material that
substantially or at least partially fills the voids within the
material. Because of the extremely high surface area, the matrix
binds to and integrates the lattice material extremely well,
resulting in a stronger composite material than those based simply
on a porous substrate alone, e.g., in the absence of the nanofiber
surfaces. Such composite materials may be generally employed in a
number of applications where high strength to weight ratios are
desired, such as in lightweight engineered parts, e.g., bicycles,
tennis rackets, automotive parts, aviation parts, satellite and
other extraterrestrial equipment and parts, etc.
[0143] While virtually any porous substrate material, e.g., as
described elsewhere herein, may be employed as the supporting
lattice, for a number of applications, a flexible lattice material
is more desirable, as it may be later conformed to a desired shape,
e.g., molded or sculpted, for a particular application. In at least
a first preferred aspect, flexible mesh materials are used as the
supporting lattice. Such materials include porous polymeric sheets,
porous metal sheets, flexible porous glass sheets, e.g., sintered
glass sheets, and the like. In other preferred aspects, porous
woven cloth-like materials are employed as the lattice, including,
e.g., woven polymeric fabrics, (e.g., polyesters, nylons,
polyetherketones, polyaramid, etc.), woven glass fabrics (e.g.,
fiberglass fabrics, glass wool, etc), carbon or graphite fiber
fabrics, Kevlar fabrics, and metallic fiber fabrics (e.g.,
titanium, stainless steel, nickel, platinum, gold, etc.). The wide
range of different porous, flexible substrates for use as the
lattice material will generally be appreciated by those of ordinary
skill in the art, and may generally be varied to accomplish the
needs of the ultimate application, e.g., light weight and/or
enhanced strength, materials compatibility, and the like.
[0144] Like the lattice material, the type of material used as the
intercalating matrix for the lattice will generally depend upon the
nature of the application to which the material is to be put. By
way of example, inorganic materials may be employed as the matrix,
including glass, ceramics or the like. Alternatively, and
preferably, polymeric matrices are employed, including thermosets,
such as polyester, epoxy, urethane and acrylate resins, and the
like, thermoplastics and/or thermoplastic elastomers, such as
polyethylene, polypropylene, nylon, PFA, and the like. Typically
any of these matrix materials may be deposited as a polymer over
the lattice substrate and allowed to intercalate throughout the
nanofiber mesh. Subsequently, the matrix material is allowed to or
caused to cure in situ. Alternatively, polymeric matrices may be
intercalated as a monomeric solution and polymerized in situ to
"cure" the matrix in place. In still further alternate aspects, the
polymeric matrix may be deposited over the porous substrate bearing
the nanofibers, using a vapor phase or solvent deposition process,
e.g., as described above for the cross-linking of nanofibrous mats.
The full range of different polymers and their utility in a wide
range of different applications will be readily apparent to those
of ordinary skill in the art.
[0145] F. Protected Nanofiber Surfaces
[0146] A variety of applications exist for articles with
nanofiber-enhanced surfaces, as noted above; for example,
implantable medical devices such as stents, nanofiber-enhanced
fabrics, nanofiber arrays, membranes, and the like. However, such
nanofiber-based devices frequently present a relatively fragile
nanofiber surface at which even light contact with other objects
(e.g., packaging material, skin, etc.) can cause nanofiber
breakage, matting, and removal. Techniques for strengthening and/or
protecting nanofiber surfaces are thus desirable.
[0147] As described above, nanofibrous mats can be fused, coated,
or crosslinked at points of nanofiber contact. Similar techniques
can be used to protect essentially any population of nanofibers,
whether grown in situ or deposited on a substrate, that would
benefit from protection from abrasion, breakage, etc.
[0148] In one aspect, the invention provides methods of stabilizing
nanofibers (e.g., nanowires). In the methods, a population of
nanofibers is provided, and a coating is formed on the nanofibers.
The coating is contiguous between adjacent nanofibers in the
population.
[0149] In one class of embodiments, a first material comprising the
nanofibers is different from a second material comprising the
coating. The first and second materials are optionally unrelated or
related. Thus, in certain embodiments, the second material is an
oxide of the first material. For example, the nanofibers can
comprise silicon and the coating silicon oxide, the nanofibers can
comprise titanium and the coating titanium oxide, etc. Generally,
regardless of the composition of the nanofibers, the coating
optionally comprises an oxide, e.g., an oxide of silicon, titanium,
aluminum, magnesium, iron, tungsten, tantalum, iridium, or
ruthenium. For example, titanium nanowires can be oxidized to form
the titanium oxide coating, or other nanowires can be coated with
titanium which is then oxidized to form the coating. The nanofibers
are optionally sintered or oxidized in situ during synthesis, or
after synthesis and/or deposition by a rapid thermal oxidation
(RTO) technique, e.g., in embodiments in which any substrate to
which the nanofibers are attached is compatible with the high
temperatures required for RTO. As another example, as described
above, silicon nanowires (nanofibers) can be synthesized (e.g., at
about 480.degree. C.) and then coated with polysilicon (e.g., at
about 600.degree. C.) to thicken and strengthen the nanowires and
to fuse wires together at wire-wire junctions.
[0150] It will be evident that the coating is not limited to
comprising an oxide or polysilicon, but can include essentially any
material that imparts a desirable property to the resulting coated
nanofiber population, e.g., stability or a desirable
electrochemical or dielectric property. Additional exemplary
coatings include polymers, carbon, carbides, and nitrides. For
example, silicon nanowires can be synthesized and then coated with
carbon and silicon carbide (resulting, e.g., from conversion of
some of the silicon nanowire) or with TaN (e.g., by atomic layer
deposition).
[0151] In one aspect, the population of nanofibers is provided by
synthesizing the nanofibers on a surface of a substrate. Exemplary
substrates have been described herein, e.g., porous, curved, woven,
and/or flexible substrates, but it will be evident that the methods
are not limited to nanofibers synthesized on or attached to such
substrates. The nanofibers can be preformed and deposited on a
substrate. The substrate is optionally non-porous. The substrate
optionally comprises or covers at least a portion of a surface of
an implantable medical device.
[0152] The coated population of nanofibers preferably retains any
desirable surface properties of the original nanofiber population,
for example, minimal contact area with objects, a highly porous
three-dimensional structure, superhydrophobicity, low biological
growth and attachment, or the like. Relatedly, the coating can
provide desirable surface characteristics; for example, the coating
can comprise a material that is readily functionalized (e.g.,
silicon oxide). In one class of embodiments, the methods include
functionalizing the coating with a chemical binding moiety, a
hydrophobic chemical moiety, a hydrophilic chemical moiety, a drug
(e.g., to inhibit cell or bacterial growth), or the like.
[0153] Populations of nanofibers (e.g., nanowires) formed by the
methods are another feature of the invention. One general class of
embodiments provides a population of nanofibers that includes
nanofibers and a coating on the nanofibers, wherein the coating is
contiguous between adjacent nanofibers in the population. A device
bearing such a population, e.g., an implantable medical device, is
likewise a feature of the invention.
[0154] In a related aspect, a nanofiber surface is protected by a
layer of porous (or alternatively, non-porous) material. A
nanofiber bearing substrate can have a first layer of porous
material disposed on its surface. For example, a nanowire coated
fabric, flexible mesh, or other flexible and/or porous substrate
can be protected with a porous material layer that can, e.g., be
heat sealed or ultrasonically welded to the substrate, on one or
both sides of the substrate. This protects the delicate nanofiber
bearing substrate from abrasion or similar damage, while still
allowing vapors and/or liquids to penetrate to the substrate.
Exemplary porous materials include, but are not limited to,
non-woven polypropylene porous material (for example, similar to
that used in Celgard.TM. laminates, Hoechst Celanese). Additional
weldable materials include polyethylene, polystyrene, acetate, and
thin pretreated Teflon.TM. layers. The protective layer(s) are
optionally flexible or inflexible.
[0155] The substrate is optionally sandwiched between two layers of
porous or non-porous material; for example, between two layers of
porous material, or between a porous layer on one side and a
non-porous layer on the other side. It will be evident that choice
of material(s) for the protective layer(s) can depend on the
desired application. For example, a flexible fabric substrate can
be sandwiched between flexible porous layers for certain
applications, or between a porous layer and an inflexible
non-porous layer for other applications. As noted, a protective
layer can be heat sealed or welded to the substrate. Similarly, the
protective layer can be attached to the substrate by sewing or use
of an adhesive.
[0156] FIG. 14 schematically illustrates protection of a nanofiber
bearing substrate by a porous layer. As shown in FIG. 14, substrate
1402 bearing nanofibers 1403 is provided. The substrate is
typically a flexible porous substrate, but need not be; in certain
embodiments, it is a rigid and/or non-porous substrate. First layer
1404 of porous material is disposed on the first surface of the
substrate and second layer 1405 of porous material is disposed on
the second surface of the substrate, providing article 1401.
Nanofibers 1403 are protected from abrasion, matting, removal,
breakage, etc. by being sandwiched between the porous material
layers.
[0157] G. Nanofiber Synthesis
[0158] In one aspect, the porous substrates of the invention are
useful as lattices for synthesis of large quantities and/or high
densities of long, unbranched nanofibers, particularly nanowires.
Silicon nanowires, for example, are desirable materials for inter
alia a variety of macroelectronic applications. Such nanowires are
typically required to be long (e.g., at least about 40, 50, or 60
.mu.m in length), straight, and branch free. However, large
quantities of such long and unbranched nanowires are not readily
obtained through current synthesis techniques
[0159] Currently, nanowires are typically grown on flat wafer
substrates. When non-oriented wire growth methods (e.g.,
SiH.sub.4), which are simpler and less expensive to implement than
oriented growth methods, are used to produce nanowires, collisions
between growing wires lead to growth termination, branched wire
formation, and the like. Collisions between nanowires can be
limited by reducing nanowire density (e.g., by reducing gold
colloid particle density) and/or length;
[0160] however, these tactics obviously do not result in
inexpensive, simple production of large quantities of nanowires
and/or long nanowires. For example, for non-oriented growth on a
flat substrate, to obtain 40 .mu.m long nanowires with only 10%
collisions per wire, gold colloid particle density can be at most
0.01 particle/.mu.m.sup.2, based on both theoretical predictions
and experimental results. Yield of long, straight, unbranched
nanowires from even non-oriented growth techniques can be improved
by growing the nanowires on high surface area porous substrates or
curved substrates.
[0161] In other embodiments, nanofibers are grown on porous
surfaces to provide the semipermeable membranes and other articles
noted above, for example, and need not be long, straight, and/or
unbranched.
[0162] Accordingly, one general class of embodiments provides
methods of producing nanofibers. In the methods, a substrate
comprising a) a plurality of apertures disposed therethrough, the
substrate comprising an overall surface area that includes an
interior wall surface area of the plurality of apertures, or b) a
curved surface is provided. A plurality of nanofibers is
synthesized on the substrate, wherein the resulting nanofibers are
attached to at least a portion of the overall surface area of the
substrate of a) or to at least a portion of the curved surface of
b). The curved surface is preferably convex when used for growth of
long unbranched nanofibers, but can alternatively or additionally
be concave. The curved surface optionally has a nonzero mean radius
of curvature over a significant fraction of the substrate's surface
(e.g., a cylindrical fiber-shaped substrate) or over its entire
surface (e.g., a microsphere or similar substrate). It will be
evident that many substrates can be described as either or both
porous and/or curved; for example, a fibrous mat can be described
as a whole as a porous substrate or on the level of individual
constituent fibers as a curved substrate.
[0163] A number of exemplary substrates have been described above.
For example, the substrate can comprise a solid substrate with a
plurality of pores disposed through it, a mesh, a woven fabric, or
a fibrous mat. As other examples, the substrate can comprise a
plurality of microspheres or other microparticles or nanoparticles
(e.g., glass or quartz microspheres or nanospheres), a powder
(e.g., carbon black powder), a plurality of glass or quartz fibers
(e.g., microfibers, fiberglass, glass or quartz fiber filters), or
a foam (e.g., a polymer or metal foam such as reticulated
aluminum). FIG. 15A shows reticulated aluminum, while FIG. 15B
illustrates nanowires grown on a reticulated aluminum substrate.
FIG. 18 Panel A depicts an electron micrograph of carbon black
powder having gold catalyst colloid particles deposited
thereon.
[0164] FIG. 18 Panels B and C depict an electron micrograph of
silicon nanowires grown from the carbon black supported gold
catalyst colloid of FIG. 18 Panel A. In this particular method of
growing silicon nanowires, the nanowires are grown directly from
the catalyst supported carbon black powder using a colloidal
catalyst based VLS (vapor-liquid-solid) synthesis method such as
those described above. In accordance with this synthesis technique,
the colloidal catalyst (e.g., gold) is first deposited upon the
surface of the carbon black powder. The carbon black including the
colloidal catalyst is then subjected to the synthesis process which
generates nanofibers (e.g., nanowires) attached to the surface of
the carbon black particles. Typically, catalysts comprise metals,
e.g., gold, platinum, and the like, and may be deposited from
solution onto the surface of the carbon black powder which is
treated (e.g., oxidized) so that the catalyst colloids adhere to
the surface of the carbon black powder. Other surface treatments
include those that have been described in detail previously, e.g.,
polylysine treatment, etc. The catalyst colloids may also be
deposited in any of a number of other well known metal deposition
techniques, e.g., sputtering etc. Nanowires are then prepared by
feeding, either by gravity or gas injection (e.g., using an inert
gas), the colloid-containing carbon black particles and a reactive
gas such as silane (SiH4) in a vertical tube reactor at about 480
degrees Celsius. The reactor includes a quartz tube equipped with
an internal quartz wool plug for receiving the carbon black
particles and a thermocouple for monitoring the reactor
temperature. Inlet ports through which the catalyst, reactant gas,
and purge gas, e.g., argon, are added are also provided, as well as
an outlet port for venting the reactor. Following growth of the
nanowires from the carbon black supported metal colloid, the
nanowires may optionally be heated in the quartz reactor to a
temperature greater than about 500 degrees Celsius, e.g., between
about 500 and 700 degrees Celsius, to evaporate the carbon black
powder, thereby leaving free-standing, detached nanowires. The
nanowires are then harvested from the tube and can be filtered to
remove any wires of poor quality and prepared for further
manipulation and processing (e.g., integration into functional
devices such as the membrane electrode assembly of a fuel cell or
for other catalyst applications).
[0165] Essentially any other porous or curved substrate can also be
employed in the methods; preferred substrates are chemically
compatible with any chemicals used to synthesize the nanofibers
(e.g., have low levels of magnesium), can withstand the synthesis
temperatures, can have catalyst dispersed on them (if required),
and facilitate clean harvest of nanofibers from the substrate (if
desired). In certain embodiments, e.g., for various filtration
applications noted above, the porous substrate preferably has an
effective pore size of less than 10 .mu.m, less than 1 .mu.m, less
than 0.5 .mu.m, or even less than 0.2 .mu.m. In other embodiments,
e.g., for synthesis of long nanowires, the porous substrate
preferably has an effective pore size of at least 25 .mu.m, at
least 50 .mu.m, at least 100 .mu.m, or more, depending on the
desired nanowire length (for example, the width of the apertures in
a mesh used for nanowire synthesis would be at least about twice
the desired length of the nanowires).
[0166] The nanofibers can comprise essentially any type of
nanofibers, e.g., silicon nanowires, carbon nanotubes, or any of
the other nanofibers noted above. In certain embodiments, the
nanofibers comprise nanowires, and the methods include synthesizing
the plurality of nanowires by depositing a gold colloid on at least
a portion of the overall surface area of the substrate of a) or on
at least a portion of the curved surface of b) and growing the
nanowires from the gold colloid with a VLS synthesis technique.
[0167] The methods optionally include surrounding or at least
partially encapsulating the substrate and the resulting attached
nanofibers with a matrix material; dissolving a soluble substrate
following synthesis of the nanofibers on the substrate; forming a
coating on the resulting nanofibers, wherein the coating is
contiguous between adjacent nanofibers; disposing a layer of porous
material on the resulting nanofibers (and optionally disposing the
substrate on a second layer of porous material, sandwiching the
nanofiber-bearing substrate); and/or functionalizing the nanofibers
(e.g., by attaching a chemical moiety or nanocrystal), as described
above.
[0168] FIG. 16 shows electron and optical micrographs that
illustrate certain aspects of the invention. FIG. 16 E shows a
single glass fiber covered by silicon nanowires. The glass fiber
substrate was chemically treated by soaking in a polylysine
solution for 20 minutes to attract gold colloid to its surface.
Silicon nanowires were grown from the gold colloid on the substrate
using a chemical vapor deposition (CVD) method; a silane reaction
at a moderately low temperature (480.degree. C., much lower than
the high temperatures of approximately 900.degree. C. or more used
by the industry to grow bulk single crystalline silicon) resulted
in single crystalline silicon nanowires. Quartz fiber filters have
also been used as substrates for growth of silicon nanowires. Two
quartz fiber filters (AQFA04700 from Millipore and QF-200 from
F&J Specialty Products) were used to grow silicon nanowires. As
described for growth on glass fiber substrates, nanowires were
grown on the quartz fiber filters at 480.degree. C. for 90 minutes
(FIG. 16 A-B). The silicon nanowires were removed from the filters
by sonication (see, e.g., FIG. 16 C-D, which depict the resulting
detached nanowires).
[0169] Growth of nanowires on porous or curved substrates such as
meshes, microfibers, microbeads, or microporous glass or quartz
materials offers a number of advantages over growth on planar
substrates. For example, compared to a flat wafer surface,
microfibers, microbeads, or microporous surfaces can grow nanowires
with fewer collisions per wire at the same wire length and catalyst
particle density due to the surface curvature of the substrate.
Therefore, it is possible to obtain straight longer wires free of
branches at higher yield on curved or porous substrates than on
flat surfaces. The total surface area of the micro substrate can be
easily controlled, for example, by varying the diameter and the
volume of the micro-materials (e.g., the thickness and pore size
for fiber membranes, or the diameter of beads). The high yield of
nanowires from the micro substrates can significantly reduce the
cost of making nanowires. Straight branch-free silicon nanowires
can be produced using a silane method or other un-oriented nanowire
growth method on micro substrates. If desired, the resulting
nanowires can be easily removed by sonication after synthesis, due
to the flexibility and/or small size of the substrate materials. In
addition, when a porous substrate is employed, reacting gases can
readily reach substantially all of the catalyst particles deposited
on the substrate.
[0170] Simulations of nanowire growth provide an additional
illustration of the advantages offered by curved substrates as
lattices for nanowire growth. FIG. 17A shows simulated randomly
oriented 10 .mu.m long nanowires growing on a 5 .mu.m diameter
fiber at a density of 0.5 nanowires/.mu.m.sup.2. Light squares mark
collisions between wires. FIG. 17B graphs the number of collisions
per nanowire as a function of the radius of the fiber, at two
different nanowire densities: 0.05 nanowires/.mu.m.sup.2 and 0.5
nanowires/.mu.m.sup.2. As the radius of the fiber increases (and
therefore, the curvature of the fiber's surface decreases), the
number of collisions per wire approaches that observed on a flat
surface. As is evident from the graph, the number of collisions per
wire is affected by wire density and also by the diameter of the
fiber. A higher density of high-quality nanowires (long and
unbranched nanowires having few collisions per wire) can be
obtained on a curved surface than a flat one. Furthermore, at a
constant density of nanowires (wires/area), the number of
collisions per wire decreases with decreasing fiber radius.
[0171] Accordingly, the yield of long and/or unbranched nanofibers
produced by the methods is optionally greater than the yield of
comparable nanofibers produced by synthesis on flat substrates. In
one class of embodiments, yield of the resulting nanofibers having
a length greater than 10 .mu.m (e.g., greater than 20 .mu.m, 30
.mu.m, 40 .mu.m, 50 .mu.m, or 60 .mu.m) is at least 10% greater
than yield of nanofibers of that length synthesized on a planar
non-porous substrate (i.e., a solid planar substrate with no
apertures or pores therethrough) of the same surface area, using
substantially the same growth process. The yield from the methods
is optionally at least 25%, 50%, 75%, or even 100% greater than the
yield from growth on the planar non-porous substrate. For example,
growth of nanowires using a non-oriented synthesis technique, e.g.,
VLS growth from a gold colloid, can produce more long nanofibers on
the substrate of a) or b) than on a flat non-porous substrate of
comparable surface area using substantially the same growth process
(e.g., the same temperature, colloid deposition density, growth
times, process gases, and the like).
[0172] In one class of embodiments, the curved substrate of b) has
at least one dimension (typically, a cross-sectional diameter) that
is less than 1000, less than 500, less than 100, or less than 50
times an average cross-sectional diameter of the nanofibers. The at
least one dimension of the substrate is optionally greater than 2,
greater than 5, greater than 10, or greater than 20 times the
average cross-sectional diameter of the nanofibers. The substrate
optionally comprises a different material than the nanofibers.
[0173] As in the example above, the nanofibers are optionally
removed from the surface area of the substrate of a) or the curved
surface of b) following synthesis of the nanofibers, e.g., by
sonicating the substrate, to produce a population of detached
nanofibers. As noted, the method can produce long nanowires. Thus,
in one class of embodiments, at least 10% (e.g., at least 20%, 30%,
40%, 50%, 60%, 70%, 80%, or even 90%) of the nanofibers in the
population of detached nanofibers have a length greater than 10
.mu.m, 20 .mu.m, 30 .mu.m, 40 .mu.m, 50 .mu.m, or 60 .mu.m, while
at most 50% (e.g., at most 40%, 30%, 20%, or 10%) of the nanofibers
have a length less than 10 .mu.m. Optionally, at least 50%, 60%,
70%, 80%, or 90% of the nanofibers in the population of detached
nanofibers are unbranched, while at most 50%, 40%, 30%, 20%, or 10%
of the nanofibers are branched. The nanofibers can have similar
length and branched/unbranched distributions when attached to the
substrate.
[0174] The methods optionally include characterizing the
nanofibers, attached to the substrate and/or after their removal
from the substrate, by determining one or more of: their length,
diameter, percent branched/unbranched, collision with other
nanowires per nanowire or per unit length of the nanowires, and the
like, per nanowire or as an average or distribution.
[0175] Articles or populations of nanofibers produced by the
methods form another feature of the invention. Thus, one exemplary
class of embodiments provides an article comprising a substrate
having a curved surface, and a plurality of nanofibers (e.g.,
nanowires) attached to at least a portion of the curved surface of
the substrate. The substrate can comprise, e.g., a plurality of
microspheres or one or more glass fiber, quartz fiber, metallic
fiber, or polymer fiber. An implantable medical device comprising
an article of the invention, e.g., attached to and covering at
least a portion of the surface of the implantable medical device,
is also a feature of the invention.
[0176] All publications, patents, patent applications, and/or other
documents cited in this application are incorporated by reference
in their entirety for all purposes to the same extent as if each
individual publication, patent, patent application, and/or other
document were individually indicated to be incorporated by
reference for all purposes. Although the present invention has been
described in some detail by way of illustration and example for
purposes of clarity and understanding, it will be apparent that
certain changes and modifications may be practiced within the scope
of the appended claims.
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