U.S. patent application number 11/897129 was filed with the patent office on 2008-09-11 for monodisperse single-walled carbon nanotube populations and related methods for providing same.
Invention is credited to Michael S. Arnold, Mark C. Hersam, Samuel I. Stupp.
Application Number | 20080217588 11/897129 |
Document ID | / |
Family ID | 39512246 |
Filed Date | 2008-09-11 |
United States Patent
Application |
20080217588 |
Kind Code |
A1 |
Arnold; Michael S. ; et
al. |
September 11, 2008 |
Monodisperse single-walled carbon nanotube populations and related
methods for providing same
Abstract
The present teachings provide methods for providing populations
of single-walled carbon nanotubes that are substantially
monodisperse in terms of diameter, electronic type, and/or
chirality. Also provided are single-walled carbon nanotube
populations provided thereby and articles of manufacture including
such populations.
Inventors: |
Arnold; Michael S.; (Ann
Arbor, MI) ; Hersam; Mark C.; (Evanston, IL) ;
Stupp; Samuel I.; (Chicago, IL) |
Correspondence
Address: |
Kirkpatrick & Lockhart Preston Gates Ellis LLP;(FORMERLY KIRKPATRICK &
LOCKHART NICHOLSON GRAHAM)
STATE STREET FINANCIAL CENTER, One Lincoln Street
BOSTON
MA
02111-2950
US
|
Family ID: |
39512246 |
Appl. No.: |
11/897129 |
Filed: |
August 29, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60840990 |
Aug 30, 2006 |
|
|
|
Current U.S.
Class: |
252/502 ;
423/414; 423/445B |
Current CPC
Class: |
B82Y 40/00 20130101;
B82Y 30/00 20130101; C01B 2202/22 20130101; C01B 32/172 20170801;
C01B 2202/02 20130101; C01B 2202/36 20130101 |
Class at
Publication: |
252/502 ;
423/445.B; 423/414 |
International
Class: |
H01B 1/04 20060101
H01B001/04; C01B 31/00 20060101 C01B031/00 |
Goverment Interests
[0002] The United States Government has certain rights to this
invention pursuant to Grant Nos. EEC-0118025 and DMR-0134706 from
the National Science Foundation and Grant No. DE-FG02-00ER54810
from the Department of Energy, all to Northwestern University.
Claims
1. A population of single-walled carbon nanotubes wherein greater
than about 75% of the single-walled carbon nanotubes have a
diameter within less than about 0.5 .ANG. of the mean diameter of
the population.
2. The population of single-walled carbon nanotubes of claim 1
wherein greater than about 90% of the single-walled carbon
nanotubes have a diameter within less than about 0.5 .ANG. of the
mean diameter of the population.
3. The population of single-walled carbon nanotubes of claim 1
wherein greater than about 75% of the single-walled carbon
nanotubes have a diameter within less than about 0.2 .ANG. of the
mean diameter of the population.
4. The population of single-walled carbon nanotubes of claim 1
wherein greater than about 90% of the single-walled carbon
nanotubes have a diameter within less than about 0.2 .ANG. of the
mean diameter of the population.
5. A population of single-walled carbon nanotubes having a diameter
greater than about 10 .ANG., wherein greater than about 70% of the
single-walled carbon nanotubes are semiconducting.
6. The population of claim 5 wherein the single-walled carbon
nanotubes have diameter dimensions ranging from about 11 .ANG. to
about 20 .ANG..
7. The population of claim 5 wherein the single-walled carbon
nanotubes have diameter dimensions ranging from about 11 .ANG. to
about 16 .ANG..
8. The population of claim 5 wherein the single-walled carbon
nanotubes are synthesized by a laser ablation process.
9. The population of claim 5 wherein greater than about 75% of the
single-walled carbon nanotubes are semiconducting.
10. The population of claim 5 wherein greater than about 80% of the
single-walled carbon nanotubes are semiconducting.
11. The population of claim 5 wherein greater than about 85% of the
single-walled carbon nanotubes are semiconducting.
12. (canceled)
13. (canceled)
14. A population of single-walled carbon nanotubes wherein greater
than about 50% of the single-walled carbon nanotubes are
metallic.
15. The population of single-walled carbon nanotubes of claim 14
wherein greater than about 75% of the single-walled carbon
nanotubes are metallic.
16. The population of single-walled carbon nanotubes of claim 14
wherein greater than about 90% of the single-walled carbon
nanotubes are metallic.
17. The population of single-walled carbon nanotubes of claim 14
wherein greater than about 97% of the single-walled carbon
nanotubes are metallic.
18. The population of single-walled carbon nanotubes of claim 14
wherein greater than about 99% of the single-walled carbon
nanotubes are metallic.
19-22. (canceled)
23. An article of manufacture comprising the population of
single-walled carbon nanotubes of claim 1.
24. The article of manufacture of claim 23 wherein the article of
manufacture is an electronic device, an optical device, or an
optoelectronic device.
25-50. (canceled)
51. An article of manufacture comprising the population of
single-walled carbon nanotubes of claim 5.
52. An article of manufacture comprising the population of
single-walled carbon nanotubes of claim 14.
Description
[0001] This application claims priority to and the benefit of the
filing date of U.S. Provisional Application Ser. No. 60/840,990,
filed on Aug. 30, 2006, the entire disclosure of which is
incorporated by reference herein.
INTRODUCTION
[0003] Carbon nanotubes have recently received extensive attention
due to their nanoscale dimensions and outstanding materials
properties such as ballistic electronic conduction, immunity from
electromigration effects at high current densities, and transparent
conduction. However, as-synthesized carbon nanotubes vary in their
diameter and chiral angle, and these physical variations result in
striking changes in their electronic and optical behaviors. For
example, about one-third of all possible single-walled carbon
nanotubes (SWNTs) exhibit metallic properties while the remaining
two-thirds act as semiconductors. Moreover, the band gap of
semiconducting SWNTs scales inversely with tube diameter. For
instance, semiconducting SWNTs produced by the laser-ablation
method range from about 11 .ANG. to about 16 .ANG. in diameter and
have optical band gaps that vary from about 0.65 eV to about 0.95
eV. The unavoidable structural heterogeneity of the currently
available as-synthesized SWNTs prevents their widespread
application as high-performance field-effect transistors,
optoelectronic near-infrared emitters/detectors, chemical sensors,
materials for interconnects in integrated circuits, and conductive
additives in composites. Accordingly, the utilization of SWNTs will
be limited until large quantities of monodisperse SWNTs can be
produced or otherwise obtained.
[0004] While several SWNT purification methods have been recently
demonstrated, no pre-existing technique has been reported that
simultaneously achieves diameter and band gap selectivity over a
wide range of diameters and band gaps, electronic type (metal
versus semiconductor) selectivity, and scalability. Furthermore,
most techniques are limited in effectiveness, and many are only
sensitive to SWNTs that are less than about 11 .ANG. in diameter.
This is a significant limitation because the SWNTs that are most
important for electronic devices are generally ones that are larger
in diameter, since these form less resistive contacts (i.e. reduced
Schottky barriers). The methods of dielectrophoresis and controlled
electrical breakdown are both limited in scalability and are only
sensitive to electronic type (not diameter or band gap).
Furthermore, the selective chemical reaction of diazonium salts
with metallic SWNTs has only been demonstrated for SWNTs in the
7-12 .ANG. diameter range, and this approach does not provide
diameter and band gap selectivity. More problematically, the
chemistry also results in the covalent degradation of the nanotube
sidewalls. In addition, the use of amine-terminated surfactants in
organic solvents is limited to the production of samples that are
only 92% semiconducting, and the technique has been successfully
applied only to SWNTs having a diameter of less than or about 10
.ANG.. Similarly, while diameter and electronic type selectivity
have been observed using anion exchange chromatography, such
approach has only been demonstrated for SWNTs wrapped by specific
oligomers of DNA ranging from 7-11 .ANG. in diameter.
SUMMARY
[0005] In light of the foregoing, it is an object of the present
teachings to provide compositions including carbon nanotubes that
are substantially monodisperse in their structure and/or
properties, specifically with respect to diameter, band gap,
chirality, and/or electronic type (metallic versus semiconducting).
To provide such substantially monodisperse carbon nanotubes, the
present teachings also relate to one or more methods and/or systems
that can be used to separate structurally and/or characteristically
heterogeneous carbon nanotubes, thereby addressing various
deficiencies and shortcomings of the prior art, including those
outlined above.
[0006] It will be understood by those skilled in the art that one
or more embodiments of the present teachings can meet certain
objectives, while one or more other embodiments can meet certain
other objectives. Each objective may not apply equally, in all its
respects, to every embodiment of the present teachings. As such,
the following objects can be viewed in the alternative with respect
to any one embodiment of the present teachings.
[0007] It can be another object of the present teachings to provide
methods and related systems for carbon nanotube separation,
regardless of diameter or length dimension, which are compatible
with various nanotube production techniques and result in
separation on a practical size-scale.
[0008] It can be another object of the present teachings to provide
methods and related systems for carbon nanotube separation as a
function of electronic type, regardless of diameter and/or
chirality.
[0009] It can be another object of the present teachings to provide
methods and related systems for carbon nanotube separation as a
function of diameter, regardless of chirality and/or electronic
type.
[0010] It can be another object of the present teachings to provide
methods and related systems for carbon nanotube separation as a
function of chirality, which can be associated with specific
diameters and/or an electronic type.
[0011] It can be another object of the present teachings to provide
a range of surface active components and use thereof to engineer
differences in the buoyant densities of the complexes formed by the
surface active component(s) and a heterogeneous sample of carbon
nanotubes, such that the nanotubes can be separated as a function
of structure and/or properties including but not limited to
chiralities, diameter, band gap, and/or electronic type.
[0012] It can be another object of the present teachings to provide
such separation methods and systems which can be used in
conjunction with existing automation and can be scaled for
production of commercially-useful quantities.
[0013] Other objects, features, and advantages of the present
teachings will be apparent from the summary and the following
description of certain embodiments, which will be readily apparent
to those skilled in the art knowledgeable of the production and
properties of carbon nanotubes and related separation techniques.
Such objects, features, benefits and advantages will be apparent
from the above as taken into conjunction with the accompanying
examples, data, figures and all reasonable inferences to be drawn
there from, alone or with consideration of the references
incorporated herein.
[0014] In part, the present teachings are directed to a method of
using a density gradient to separate single-walled carbon
nanotubes, wherein the density gradient is provided by a fluid
medium. Such a method can include centrifuging a fluid medium
including a density gradient and a composition including a first
surface active component, a second surface active component and a
mixture of single-walled carbon nanotubes to separate the mixture
along the density gradient, and isolating from the fluid medium a
separation fraction that includes separated single-walled carbon
nanotubes. More specifically, the mixture of single-walled carbon
nanotubes can include a range of nanotube diameter dimensions,
chiralities and/or electronic types, and the ratio of the first
surface active component to the second surface active component can
be other than 4:1.
[0015] As described herein, it should be understood that isolating
a separation fraction typically provides complex(es) formed by the
surface active component(s) and the mixture of single-walled carbon
nanotubes where post-isolation treatment, e.g., removing the
surface active component(s) from the SWNTs such as by washing,
dialysis and/or filtration, can provide substantially pure or bare
single-walled carbon nanotubes. However, as used herein for
brevity, reference may be made to a mixture of single-walled carbon
nanotubes rather than the complexes and such reference should be
interpreted to include the complexes as understood from the context
of the description unless otherwise stated that non-complexed
single-walled carbon nanotubes, e.g., bare SWNTs, are meant.
[0016] In some embodiments, the first surface active component can
be a bile salt and the second surface active component can be an
anionic alkyl amphiphile. The fluid medium and the composition can
be centrifuged for a time and/or at a rotational rate sufficient to
at least partially separate the mixture along the density gradient.
Such a method is without limitation as to separation by nanotube
diameter dimensions, chiralities and/or electronic type. In some
embodiments, single-walled carbon nanotubes in the mixture can
independently have diameter dimensions up to about 20 .ANG. or
more. In certain embodiments, dimensions can range from about 7
.ANG. to about 11 .ANG., while in certain other embodiments,
dimensions can be greater than about 11 .ANG. (for example, ranging
from about 11 .ANG. to about 16 .ANG.). Without limitation, narrow
distributions of separated single-walled carbon nanotubes can be
provided in the separation fraction and subsequently isolated. For
example, in some embodiments, greater than about 70% of the
separated single-walled carbon nanotubes can be semiconducting. In
other embodiments, greater than about 50% of the separated
single-walled carbon nanotubes can be metallic. In some
embodiments, the method can include post-isolation treatment of the
separated single-walled carbon nanotubes to provide bare
single-walled carbon nanotubes. In certain embodiments, the method
can further include repeating the centrifuging and isolating steps
using the separation fraction.
[0017] In part, the present teachings also are directed to a method
of using a density gradient to separate single-walled carbon
nanotubes based on electronic type, wherein the density gradient is
provided by a fluid medium. Such a method can include centrifuging
a fluid medium including a density gradient and a composition
including a mixture of single-walled carbon nanotubes (including
both semiconducting single-walled carbon nanotubes and metallic
single-walled carbon nanotubes) and at least two surface active
components (e.g., a first surface active component and a second
surface active component) to separate the mixture along the density
gradient, and isolating from the fluid medium a substantially
semiconducting separation fraction or a substantially metallic
separation fraction. As used herein, a substantially semiconducting
separation fraction refers to a separation fraction that includes a
majority of or a high concentration or percentage of semiconducting
single-walled carbon nanotubes. For example, the substantially
semiconducting separation fraction can include a higher
concentration or percentage of semiconducting single-walled carbon
nanotubes than the mixture. Similarly, as used herein, a
substantially metallic separation fraction refers to a separation
fraction that includes a majority of or a high concentration or
percentage of metallic single-walled carbon nanotubes. For example,
the substantially metallic separation fraction can include a higher
concentration or percentage of metallic single-walled carbon
nanotubes than the mixture. In some embodiments, the separation
fraction isolated after centrifugation can be substantially
semiconducting. In other embodiments, the separation fraction
isolated after centrifugation can be substantially metallic. For
example, in some embodiments, greater than about 70% of the
single-walled carbon nanotubes in the separation fraction can be
semiconducting single-walled carbon nanotubes. In other
embodiments, greater than about 50% of the single-walled carbon
nanotubes in the separation fraction can be metallic single-walled
carbon nanotubes. The fluid medium and the mixture can be
centrifuged for a time and/or at a rotational rate sufficient to at
least partially separate the mixture (i.e., complexes) along the
density gradient. In some embodiments, single-walled carbon
nanotubes in the mixture can independently have diameter dimensions
up to about 20 .ANG. or more. In certain embodiments, dimensions
can range from about 7 .ANG. to about 11 .ANG., while in certain
other embodiments, dimensions can be greater than about 11 .ANG.
(for example, ranging from about 11 .ANG. to about 20 .ANG. or from
about 11 .ANG. to about 16 .ANG.).
[0018] In some embodiments, the first surface active component can
be a bile salt and the second surface active component can be an
anionic alkyl amphiphile. In some embodiments, the method can
include post-isolation treatment of the separated single-walled
carbon nanotubes to provide bare single-walled carbon nanotubes. In
certain embodiments, the method can include repeating the
centrifuging and isolating steps using the separation fraction. For
example, centrifugation of a first separation fraction can lead to
a second separation by electronic type. The second separation can
provide a second separation fraction that has a higher
concentration or percentage of the desired electronic type compared
to the first separation fraction. In addition to separation based
on electronic type, the method can include further separation by
nanotube diameter dimensions and/or chiralities, for example, by
repeating the centrifuging and isolating steps using the separation
fraction. In some embodiments, repeating the centrifuging and
isolating steps using a substantially semiconducting separation
fraction can provide subsequent separation fractions that
predominantly include semiconducting single-walled carbon nanotubes
of a predetermined range of narrow diameter dimensions (for
example, a diameter dimension of about 7.6 .ANG., a diameter
dimension of about 8.3 .ANG., a diameter dimension of about
9.8/10.3 .ANG., etc.).
[0019] In part, the present teachings are directed to a method of
enriching a population of single-walled carbon nanotubes with
semiconducting single-walled carbon nanotubes. Such a method can
include isolating semiconducting single-walled carbon nanotubes
from a mixture of semiconducting single-walled carbon nanotubes and
metallic single-walled carbon nanotubes without irreversibly
modifying the metallic single-walled carbon nanotubes. In some
embodiments, the method can include separating the semiconducting
single-walled carbon nanotubes from a mixture of semiconducting
single-walled carbon nanotubes and metallic single-walled carbon
nanotubes without irreversibly modifying the metallic single-walled
carbon nanotubes (i.e., before isolating the semiconducting
single-walled carbon nanotubes from the mixture).
[0020] In some embodiments, the method can include treatment of the
enriched population to provide bare single-walled carbon nanotubes.
In some embodiments, the method can include centrifuging the
mixture of semiconducting single-walled carbon nanotubes and
metallic single-walled carbon nanotubes. In certain embodiments,
the method can provide a population of single-walled carbon
nanotubes that includes at least 70% semiconducting single-walled
carbon nanotubes. In addition to providing a population enriched
with semiconducting single-walled carbon nanotubes, the method can
further enrich the substantially semiconducting population with a
predetermined range of nanotube diameter dimensions and/or
chiralities. For example, the method can provide substantially
semiconducting populations further enriched with a diameter
dimension of about 7.6 .ANG., a diameter dimension of about 8.3
.ANG., a diameter dimension of about 9.8/10.3 .ANG., etc. In some
embodiments, single-walled carbon nanotubes in the mixture (i.e.,
before separation) can independently have diameter dimensions up to
about 20 .ANG. or more. In certain embodiments, dimensions can
range from about 7 .ANG. to about 11 .ANG., while in certain other
embodiments, dimensions can be greater than about 11 .ANG. (for
example, ranging from about 11 .ANG. to about 20 .ANG. or from
about 11 .ANG. to about 16 .ANG.).
[0021] In part, the present teachings are directed to a method of
enriching a population of single-walled carbon nanotubes with
metallic single-walled carbon nanotubes. Such a method can include
isolating metallic single-walled carbon nanotubes from a mixture of
semiconducting single-walled carbon nanotubes and metallic
single-walled carbon nanotubes. As previously mentioned, current
methods for separating metallic single-walled carbon nanotubes from
an electronically heterogeneous mixture were reported to cause
degradation of the nanotube sidewalls. Accordingly, the present
teachings further relate in part to a method of separating
single-walled carbon nanotubes based on electronic type, wherein
the method can provide a substantially metallic separation fraction
that predominantly includes metallic single-walled carbon nanotubes
that are structurally intact. In some embodiments, the method can
include separating the metallic single-walled carbon nanotubes from
a mixture of semiconducting single-walled carbon nanotubes and
metallic single-walled carbon nanotubes (i.e., before isolating the
metallic single-walled carbon nanotubes from the mixture).
[0022] In some embodiments, the method can include treatment of the
enriched population to provide bare single-walled carbon nanotubes.
In some embodiments, the method can include centrifuging the
mixture of semiconducting single-walled carbon nanotubes and
metallic single-walled carbon nanotubes. In certain embodiments,
the method can provide a population of single-walled carbon
nanotubes that includes at least 50% metallic single-walled carbon
nanotubes. In addition to providing a population enriched with
metallic single-walled carbon nanotubes, the method can further
enrich the substantially metallic population with a predetermined
range of nanotube diameter dimensions and/or chiralities. In some
embodiments, single-walled carbon nanotubes in the mixture can
independently have diameter dimensions up to about 20 .ANG. or
more. In certain embodiments, dimensions can range from about 7
.ANG. to about 11 .ANG., while in certain other embodiments,
dimensions can be greater than about 11 .ANG. (for example, ranging
from about 11 .ANG. to about 20 .ANG. or from about 11 .ANG. to
about 16 .ANG.).
[0023] In part, the present teachings also are directed to a method
of using a density gradient to isolate metallic single-walled
carbon nanotubes from a mixture of semiconducting single-walled
carbon nanotubes and metallic single-walled carbon nanotubes. The
method can include providing a surface active component system,
centrifuging a fluid medium including a density gradient and a
composition including the surface active component system and a
mixture of semiconducting single-walled carbon nanotubes and
metallic single-walled carbon nanotubes to separate the mixture
along the density gradient, and isolating from the fluid medium a
substantially metallic separation fraction. More specifically, the
surface active component system can include a first surface active
component and a second surface active component, wherein the ratio
of the first surface active component to the second surface active
component is adjusted so that when the surface active component
system is contacted and centrifuged with a mixture of single-walled
carbon nanotubes, a substantially metallic SWNT-containing
separation fraction that has a different density (e.g., is less
dense or more dense) than another separation fraction that contains
substantially semiconducting SWNTs. The fluid medium and the
mixture can be centrifuged for a time and/or at a rotational rate
sufficient to at least partially separate the mixture along the
density gradient.
[0024] In some embodiments, the first surface active component can
be a bile salt and the second surface active component can be an
anionic alkyl amphiphile. In some embodiments, the ratio of the
first surface active component to the second surface active
component can be less than about one. In some embodiments, the
method can include treatment, e.g., washing, of the substantially
metallic separation fraction to provide bare metallic single-walled
carbon nanotubes. In some embodiments, the method can include
repeating the centrifuging and isolating steps using the
substantially metallic separation fraction. For example,
centrifugation of a first separation fraction can lead to a second
separation by electronic type. The second separation can provide a
second separation fraction that has a higher concentration or
percentage of metallic single-walled carbon nanotubes compared to
the first separation fraction. In addition to providing a
substantially metallic separation fraction, the method can include
further separation by nanotube diameter dimensions and/or
chiralities, for example, by repeating the centrifuging and
isolating steps using the substantially metallic separation
fraction. In some embodiments, single-walled carbon nanotubes in
the mixture can independently have diameter dimensions up to about
20 .ANG. or more. In certain embodiments, dimensions can range from
about 7 .ANG. to about 11 .ANG., while in certain other
embodiments, dimensions can be greater than about 11 .ANG. (for
example, ranging from about 11 .ANG. to about 16 .ANG.). In some
embodiments, greater than about 50% of the single-walled carbon
nanotubes in the separation fraction can be metallic.
[0025] In part, the present teachings are directed to a method of
using a density gradient to isolate semiconducting single-walled
carbon nanotubes from a mixture of metallic single-walled carbon
nanotubes and semiconducting single-walled carbon nanotubes. The
method can include providing a surface active component system,
centrifuging a fluid medium including a density gradient and a
composition including the surface active component system and a
mixture of semiconducting single-walled carbon nanotubes and
metallic single-walled carbon nanotubes to separate the mixture
along the density gradient, and isolating from the fluid medium a
substantially semiconducting separation fraction. More
specifically, the surface active component system can include a
first surface active component and a second surface active
component, wherein the ratio of the first surface active component
to the second surface active component is adjusted so that when the
surface active component system is contacted and centrifuged with a
mixture of single-walled carbon nanotubes, a substantially
semiconducting SWNT-containing separation fraction that has a
different density (e.g., is less dense or more dense) than another
separation fraction that contains substantially metallic SWNTs. The
fluid medium and the mixture can be centrifuged for a time and/or
at a rotational rate sufficient to at least partially separate the
mixture along the density gradient.
[0026] In some embodiments, the first surface active component can
be a bile salt and the second surface active component can be an
anionic alkyl amphiphile. In some embodiments, the ratio of the
first surface active component to the second surface active
component can be greater than about one. In some embodiments, the
method can include treatment of the substantially semiconducting
separation fraction to provide bare semiconducting single-walled
carbon nanotubes. In some embodiments, the method can include
repeating the centrifuging and isolating steps using the
substantially semiconducting separation fraction. For example,
centrifugation of a first separation fraction can lead to a second
separation by electronic type. The second separation can provide a
second separation fraction that has a higher concentration or
percentage of semiconducting single-walled carbon nanotubes
compared to the first separation fraction. In addition to providing
a substantially semiconducting separation fraction, the method can
include further separation by nanotube diameter dimensions and/or
chiralities, for example, by repeating the centrifuging and
isolating steps using the substantially semiconducting separation
fraction, to provide subsequent separation fractions that
predominantly contain semiconducting single-walled carbon nanotubes
of a predetermined range of diameter dimensions (e.g., a diameter
dimension of about 7.6 .ANG., a diameter dimension of about 8.3
.ANG., a diameter dimension of about 9.8/10.3 .ANG., etc.).
[0027] As demonstrated elsewhere herein, the nanotubes selectively
separated can be identified spectrophotometrically and/or
fluorimetrically, with such identification including comparison of
absorbance and/or emission spectra respectively with a
corresponding reference spectrum.
[0028] In part, the present teachings also are directed to a method
of using a surface active component to alter carbon nanotube
buoyant density. Such a method can include providing a fluid medium
including a density gradient; contacting a mixture of single-walled
carbon nanotubes varying by structure and/or electronic type with
at least one surface active component, to provide differential
buoyant density; contacting the medium and the composition mixture;
centrifuging the medium and the composition for a time and/or at a
rotational rate at least partially sufficient to separate the
mixture (i.e., complexes) by buoyant density along the gradient;
and selectively separating by structure and/or electronic type one
group or portion of the nanotube mixture from the medium. Useful
fluid medium and substances incorporated therein, together with
surface active components, can be as described elsewhere herein.
With regard to the latter, differential buoyant density can,
optionally, be altered or modulated by a combination of two or more
surface active components, where such contact and/or interaction
can be a function of structure and/or electronic type.
[0029] The nanotubes can be of a diameter dimension increasing with
gradient density and their position therealong. Those nanotubes
selectively separated can include at least one chirality and/or at
least one electronic type. Where such nanotubes include at least
two chiralities, the selection can include iterative separation, as
demonstrated elsewhere herein, to further partition the chiralities
along a gradient. Where such nanotubes include a mixture of
electronic types, the invention can include iterative separation,
as demonstrated elsewhere herein, to further partition the
electronic types along a gradient. In so doing, at least one such
separation can vary by change in surface active component, medium
composition or identity, medium density gradient, and/or medium pH,
from one or more of the preceding separations.
[0030] In part, the present teachings can also be directed to a
system for separation of carbon nanotubes. Such a system can
include a fluid density gradient medium, and a composition
including at least one surface active component and carbon
nanotubes including a range of chiralities, diameter dimensions
and/or electronic types, with the complexes of the surface active
component(s) and nanotubes positioned along the gradient of the
medium. Diameter dimensions are limited only by synthetic
techniques used in nanotube production. Without limitation,
diameter dimension can range from less than or about 4 .ANG. to
about 7 .ANG., to about 16 .ANG., or to about 20 .ANG., or greater.
Likewise, the nanotubes in such a system are not limited by
chirality or electronic type. Without limitation, such chiralities
can be selected from any one or combination discussed herein.
Independent of chirality, diameter or any other structural or
physical characteristic, the nanotubes in such a system can be
semiconducting and/or metallic. Regardless, a fluid density
gradient medium and one or more surface active components, with or
without a co-surfactant, can be selected in view of the
considerations discussed elsewhere herein.
[0031] In certain embodiments, the nanotubes of such a system can
be selectively separated by diameter and/or electronic type, such a
characteristic as can correspond, by comparison using techniques
described herein, to a respective manufacturing process and/or
commercial source. Accordingly, carbon nanotubes separated in
accordance with the present teachings (e.g., without limitation,
single-walled carbon nanotubes) can be of and identified as
substantially or predominantly semiconducting or metallic, or by a
diameter ranging from about 7 .ANG. to about 16 .ANG.. Without
limitation, selectivity available through use of methods of the
present teachings can be indicated by separation of carbon
nanotubes differing by diameters less than about 0.6 .ANG.. As a
further indication, the nanotubes of such an electronic type or
within such a diameter range can be of substantially one (n,m)
chirality or a mixture of (n,m) chiralities, where n and m denote
chiral centers.
[0032] The present teachings further relate to populations of
single-walled carbon nanotubes that are substantially monodisperse
in terms of their structures and/or properties. In other words,
such populations generally have narrow distributions of one or more
predetermined structural or functional characteristics. For
example, in some embodiments, the population can be substantially
monodisperse in terms of their diameter dimensions (e.g., greater
than about 75%, including greater than about 90% and greater than
about 97%, of the single-walled carbon nanotubes in a population of
single-walled carbon nanotubes can have a diameter within less than
about 0.5 .ANG. of the mean diameter of the population, greater
than about 75%, including greater than about 90% and greater than
about 97%, of the single-walled carbon nanotubes in a population of
single-walled carbon nanotubes can have a diameter within less than
about 0.2 .ANG. of the mean diameter of the population, greater
than about 75%, including greater than about 90% and greater than
about 97%, of the single-walled carbon nanotubes in a population of
single-walled carbon nanotubes can have a diameter within less than
about 0.1 .ANG. of the mean diameter of the population). In some
embodiments, the population can be substantially monodisperse in
terms of their electronic type (e.g., greater than about 70%,
including greater than about 75%, greater than about 80%, greater
than about 85%, greater than about 90%, greater than about 92%,
greater than about 93%, greater than about 97% and greater than
about 99%, of the single-walled carbon nanotubes in a population of
single-walled carbon nanotubes can be semiconducting, or greater
than about 50%, including greater than about 75%, greater than
about 90%, greater than about 97%, and greater than about 99%, of
the single-walled carbon nanotubes in a population of single-walled
carbon nanotubes can be metallic). In some embodiments, the
population can be substantially monodisperse in terms of their
chiralities (e.g., greater than about 30%, including greater than
about 50%, greater than about 75%, and greater than about 90%, of
the single-walled carbon nanotubes in a population of single-walled
carbon nanotubes can include the same chirality (n, m) type).
[0033] It should be understood that populations of carbon nanotubes
of the present teachings are loose or bulk carbon nanotubes, which
are different from carbon nanotubes that are grown on and adhered
to a substrate for a particular end use thereon.
[0034] Also embraced within the scope of the present teachings are
articles of manufacture that include a population of single-walled
carbon nanotubes according to the present teachings, and those
articles that include isolated or bare single-walled carbon
nanotubes provided by the methods of the present teachings.
Examples of such articles of manufacture include, but are not
limited to, various electronic devices, optical devices, and
optoelectronic devices. Examples of such devices include, but are
not limited to, thin film transistors (e.g., field effect
transistors), chemical sensors, near-infrared emitters, and
near-infrared detectors. Other examples of articles of manufacture
according to the present teachings include transparent conductive
films, interconnects in integrated circuits, and conductive
additives in composites.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawing(s) will be provided by the Office
upon request and payment of the necessary fee.
[0036] It should be understood that certain drawings are not
necessarily to scale, with emphasis generally being placed upon
illustrating the principles of the present teachings. The drawings
are not intended to limit the scope of the present teachings in any
way.
[0037] FIG. 1 illustrates different physical structures of carbon
nanotubes.
[0038] FIG. 2 is a schematic of density gradient
centrifugation.
[0039] FIGS. 3(a)-(c) are schematic diagrams illustrating
surfactant encapsulation and sorting via density.
[0040] FIG. 4 illustrates the layering of a density gradient and
its redistribution during ultracentrifugation. FIG. 4(a) is a
schematic depicting typical, initial density gradient. FIG. 4(b)
shows graphically the redistribution of a density profile.
[0041] FIG. 5 is a photographic representation that illustrates how
SWNTs can be concentrated via density gradient ultracentrifugation
using a large step density gradient.
[0042] FIG. 6 shows the fitting of absorbance spectrum for
determination of relative SWNT concentration.
[0043] FIG. 7 illustrates the separation of SC-encapsulated
CoMoCAT-synthesized SWNTs (which have a diameter range of 7-11
.ANG.) via density gradient ultracentrifugation. FIG. 7(a) is a
photograph of the centrifugation tube after a one-step separation.
FIG. 7(b) shows the optical absorbance spectra (1 cm path length)
after separation using density gradient ultracentrifugation.
[0044] FIG. 8 illustrates the separation of SDBS-encapsulated
CoMoCAT-synthesized SWNTs via density gradient ultracentrifugation.
FIG. 8(a) is a photograph of the centrifugation tube after a
one-step separation. FIG. 8(b) shows the optical absorbance spectra
(1 cm path length) after separation using density gradient
ultracentrifugation.
[0045] FIG. 9 shows optical spectra of (a) deoxycholate
encapsulated SWNTs, (b) taurodeoxycholate encapsulated SWNTs, and
(c) SDS-encapsulated SWNTs separated in single surfactant density
gradients.
[0046] FIG. 10 illustrates the separation of SC-encapsulated laser
ablation-synthesized SWNTs via density gradient
ultracentrifugation. FIG. 10(a) is a photograph of the
centrifugation tube after a one-step separation. FIG. 10(b) shows
the optical absorbance spectra (1 cm path length) after separation
using density gradient ultracentrifugation.
[0047] FIG. 11 shows the fitting of photoluminescence spectrum for
determination of relative SWNT concentration. FIG. 11(a) plots
photoluminescence intensity as a function of excitation and
emission wavelengths (vertical and horizontal axes, respectively).
FIG. 11(b) plots photoluminescence intensity versus excitation
wavelength at 740 nm. FIGS. 11(c) and 11(d) plot the partial
derivatives of photoluminescence intensities as a function of
excitation and emission wavelengths (vertical and horizontal axes,
respectively), and versus excitation wavelength at 740 nm,
respectively.
[0048] FIG. 12 plots photoluminescence intensities as a function of
excitation and emission wavelengths for increasing refinement.
[0049] FIG. 13 are the corresponding optical spectra to the
photoluminescence spectra in FIG. 12.
[0050] FIG. 14 plots the concentration of the (6, 5), (7, 5) and
(9, 5)/(8, 7) chiralities of CoMoCAT-grown SWNTs (indicated by open
triangles, open circles, and open star symbols, respectively)
against density: (a) SC, no buffer, pH=7.4; (b) SC, 20 mM Tris
buffer, pH, 8.5; (c) SC with the addition of SDS as a co-surfactant
(1:4 ratio by weight, SDS:SC).
[0051] FIG. 15 plots photoluminescence intensities as a function of
excitation and emission wavelengths. FIG. 15(a) was obtained with a
heterogeneous population of HiPCO-grown SWNTs before separation.
FIGS. 15(b) and 15(c) were obtained with a heterogeneous population
of HiPCO-grown SWNTs after separation using a co-surfactant system
(1:4 ratio by weight, SDS:SC).
[0052] FIG. 16 shows the optimization of separation by electronic
type by using competing mixture of co-surfactants. FIG. 16(a) is a
photograph showing isolation of predominantly semiconducting
laser-ablation-synthesized SWNTs using a co-surfactant system (1:4
SDS:SC). FIG. 16(b) shows the optical absorbance spectra (1 cm path
length) after separation using density gradient
ultracentrifugation.
[0053] FIG. 17 shows the optical absorbance spectra of
laser-ablation-synthesized SWNTs separated in co-surfactant systems
optimized for separating predominantly metallic SWNTs (3:2 SDS:SC)
and predominantly semiconducting SWNTs (3:7 SDS:SC).
[0054] FIG. 18 compares the optical absorbance spectra of the
isolated predominantly metallic SWNT fraction using a 3:2 SDS:SC
co-surfactant system (optimized, as open circles, FIG. 16) versus a
1:4 SDS:SC co-surfactant system (unoptimized, as open star symbols,
FIG. 15(b)).
[0055] FIG. 19 compares the optical absorbance spectra of unsorted
laser-ablation-synthesized SWNTs with sorted semiconducting
laser-ablation-synthesized SWNTs, where the
laser-ablation-synthesized SWNTs were obtained from three different
sources: (a) raw, unpurified laser ablation-synthesized SWNTs
obtained from Carbon Nanotechnologies, Inc. (Batch A); (b) nitric
acid purified laser ablation-synthesized SWNTs obtained from IBM
(Batch B); and (c) nitric acid purified laser ablation-synthesized
SWNTs obtained from IBM (Batch C).
[0056] FIG. 20 shows the optical absorption spectra of unsorted (as
open star symbols), sorted metallic (as open triangles), and sorted
semiconducting (as open diamond symbols) laser-ablation-synthesized
SWNTs obtained with improved signal-to-noise ratio. The asterisk
symbol at about 900 nm identifies optical absorption from spurious
semiconducting SWNTs. The asterisk symbol at about 600 nm
identifies optical absorption from spurious metallic SWNTs.
[0057] FIG. 21 shows the baseline subtraction for measuring the
amplitudes of absorption for sorted metallic SWNTs. FIG. 21(a)
shows the measurement of absorption from metallic SWNTs. FIG. 21(b)
shows the measurement of absorption from spurious semiconducting
SWNTs.
[0058] FIG. 22 shows the baseline subtraction for measuring the
amplitudes of absorption for sorted semiconducting SWNTs. FIG.
22(a) shows the measurement of absorption from metallic SWNTs. FIG.
22(b) shows the measurement of absorption from spurious
semiconducting SWNTs.
[0059] FIG. 23 shows the baseline subtraction for measuring the
amplitudes of absorption for unsorted SWNTs. FIG. 23(a) shows the
measurement of absorption from metallic SWNTs. FIG. 23(b) shows the
measurement of absorption from spurious semiconducting SWNTs.
[0060] FIG. 24 shows typical yields of sorting experiments by
plotting the percentage of starting SWNTs against fraction number.
The data points in FIG. 24(a) correspond to the starting
material-normalized absorbance at 942 nm (S22) in the 1:4 SDS:SC
sorting experiment for semiconducting laser-ablation-synthesized
SWNTs (FIGS. 16(a)-(b)). The left-most arrow points to the orange
band of semiconducting SWNTs (FIG. 16(a)) and the right-most arrow
points to the black aggregate band (towards the bottom of the
centrifuge in FIG. 16(a)). The data points in FIG. 24(b) correspond
to the starting material-normalized absorbance at 982 nm (the first
order transition for the (6, 5) chirality) in the SC sorting
experiment for CoMoCAT-grown SWNTs (FIGS. 7(a)-(b)) based on
diameter. The arrow points to the magenta band (FIG. 7(b)).
[0061] FIG. 25 shows electrical devices of semiconducting and
metallic SWNTs. FIG. 25(a) is a periodic array of source and drain
electrodes (single device highlighted). FIG. 25(b) shows a
representative atomic force microscopy (AFM) image of thin film,
percolating SWNT network. FIG. 25(c) shows a field-effect
transistor (FET) geometry (s=source; g=gate; d=drain). The SWNT
networks were formed on a 100 nm, thermally-grown SiO.sub.2 layer,
which served as the gate dielectric. FIG. 25(d) shows the inverse
of sheet resistance as a function of gate bias for semiconducting
(open triangles) and metallic (open circles) SWNTs purified in
co-surfactant density gradients. The electronic mobility of the
semiconducting SWNT networks was estimated by fitting the
source-drain current versus the gate bias for a fixed source-drain
bias in the "on" regime of the FETs to a straight line (inset).
[0062] FIG. 26(a) is an image of semiconducting network acquired by
AFM (scale bar 0.5 .mu.m). FIG. 26(b) shows the same image with
conducting pathways due to SWNTs traced in black.
DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS
[0063] Throughout the description, where compositions are described
as having, including, or comprising specific components, or where
processes are described as having, including, or comprising
specific process steps, it is contemplated that compositions of the
present teachings also consist essentially of, or consist of, the
recited components, and that the processes of the present teachings
also consist essentially of, or consist of, the recited processing
steps. It should be understood that the order of steps or order for
performing certain actions is immaterial so long as the method
remains operable. Moreover, two or more steps or actions can be
conducted simultaneously.
[0064] In the application, where an element or component is said to
be included in and/or selected from a list of recited elements or
components, it should be understood that the element or component
can be any one of the recited elements or components and can be
selected from a group consisting of two or more of the recited
elements or components. Further, it should be understood that
elements and/or features of a composition, an apparatus, or a
method described herein can be combined in a variety of ways
without departing from the spirit and scope of the present
teachings, whether explicit or implicit herein.
[0065] The use of the terms "include," "includes", "including,"
"have," "has," or "having" should be generally understood as
open-ended and non-limiting unless specifically stated
otherwise.
[0066] The use of the singular herein includes the plural (and vice
versa) unless specifically stated otherwise. In addition, where the
use of the term "about" is before a quantitative value, the present
teachings also include the specific quantitative value itself,
unless specifically stated otherwise.
[0067] It should be understood that reference herein to "carbon
nanotubes" refers to single-walled carbon nanotubes (SWNTs) unless
otherwise stated or inferred from the description. As used herein,
the terms "carbon nanotubes," "single-walled carbon nanotubes," or
"SWNTs" should be understood to include single-walled carbon
nanotubes synthesized by any current or future techniques and
having any physical properties (e.g., electronic type or chirality)
or dimensions (e.g., individual diameter or length) achieved by
such current or future techniques unless otherwise stated or
inferred from the description. For example, depending on the
synthetic method used to prepare the SWNTs, SWNTs can have
individual lengths ranging from about 1-10.sup.7 nm (about 10 .ANG.
to about 1 cm), and individual diameters ranging from about 0.5-10
nm (about 5-100 .ANG.). To date, single-walled carbon nanotubes
have been synthesized by processes including high pressure carbon
monoxide decomposition ("HiPCO"), Co--Mo catalysis ("CoMoCAT"),
laser ablation, arc discharge, and chemical vapor deposition, and
the individual diameter of the SWNTs synthesized by one or more of
these techniques can be up to about 10 .ANG. (e.g., from about 5
.ANG. to about 10 .ANG.), up to about 20 .ANG. (e.g., from about 5
.ANG. to about 20 .ANG., from about 5 .ANG. to about 16 .ANG., from
about 5 .ANG. to about 11 .ANG., from about 7 .ANG. to about 20
.ANG., from about 7 .ANG. to about 16 .ANG., from about 7 .ANG. to
about 11 .ANG., from about 11 .ANG. to about 20 .ANG., or from
about 11 .ANG. to about 16 .ANG.), and up to about 50 .ANG. (e.g.,
from about 5 .ANG. to about 50 .ANG., from about 7 .ANG. to about
50 .ANG., from about 11 .ANG. to about 50 .ANG., from about 16
.ANG. to about 50 .ANG., or from about 20 .ANG. to about 50 .ANG.).
Because the concepts and principles of the present teachings do not
depend on the individual physical dimensions of the SWNTs to be
separated, the present methods and systems can be applied to
separate SWNTs regardless of their individual diameters, including
SWNTs having individual diameters greater than those achieved by
currently available synthesis methods.
[0068] In one aspect, the present teachings relate to methods for
separating structurally and/or characteristically heterogeneous
SWNTs. Methods of the present teachings can allow separation of
SWNTs as a function of structure and/or one or more other
properties without modifying the nanotubes chemically or
structurally. Methods of the present teachings can achieve
simultaneous selectivity of diameter and chirality, diameter and
electronic type, electronic type and chirality, or diameter,
electronic type, and chirality, and can be applied to separate
SWNTs of a wide range of diameters. Furthermore, methods of the
present teachings are broadly general and scalable, and can be used
in conjunction with existing automation.
[0069] More specifically, the present teachings provide methods for
separating carbon nanotubes by at least one selected property. The
at least one selected property can be one or more of chirality,
diameter, band gap, and electronic type (metallic versus
semiconducting). Some of these properties can be independent of the
other properties, while others can be interrelated. For example,
the diameter and the electronic type of a particular carbon
nanotube can be determined if its chiral indices are known, as
shown in FIG. 1. The physical structure (chirality) of a carbon
nanotube is specified by two integers (n, m), the chiral indices,
such that C=na1+ma2 where is C is the roll-up vector that defines
the circumference of a nanotube, and a1 and a2 are the primary
lattice vectors that define a graphene sheet. In FIG. 1, metallic
SWNTs are labeled green, and mod(n, m)=1 and mod(n, m)=2
semiconducting SWNTs are labeled red and blue, respectively. The
methods can include contacting the carbon nanotubes with an agent
that interacts differentially with carbon nanotubes that vary by
the at least one selected property. In some embodiments, the agent
can affect differentially the density of carbon nanotubes as a
function of the at least one selected property.
[0070] Accordingly, methods of the present teachings can be
directed to using a density gradient to separate carbon nanotubes,
e.g., by means of density gradient centrifugation. Methods of the
present teachings can include creating or enhancing a density (mass
per volume) difference among carbon nanotubes, e.g., SWNTs, of
varying structures and/or properties (e.g., chirality, diameter,
band gap, and/or electronic type). The density difference can be a
buoyant density difference. The buoyant density of a SWNT in a
fluid medium can depend on multiple factors, including the mass and
volume of the carbon nanotube itself, its surface
functionalization, and electrostatically bound hydration layers.
For example, surface functionalization of the carbon nanotubes can
be non-covalent, and can be achieved by encapsulating the carbon
nanotubes with one or more surface active components (e.g.,
surfactants). Accordingly, in some embodiments, methods of the
present teachings can include contacting single-walled carbon
nanotubes of varying structures and/or properties with at least one
surface active component (e.g., surfactant), to provide a
differential buoyant density among the single-walled carbon
nanotubes when the complexes formed by the surface active
component(s) and the single-walled carbon nanotubes are placed in a
fluid medium that includes a density gradient. The differential
buoyant density can be a function of nanotube diameter, band gap,
electronic type and/or chirality, thereby allowing separation of
the single-walled carbon nanotubes by diameter, band gap,
electronic type and/or chirality.
[0071] Generally, density gradient centrifugation uses a fluid
medium with a predefined variation in its density as a function of
position within a centrifuge tube or compartment (i.e. a density
gradient). A schematic of the density gradient centrifugation
process is depicted in FIG. 2. Species of different densities
sediment through a density gradient until they reach their
respective isopycnic points, i.e., the points in a gradient at
which sedimentation stops due to a matching of the buoyant density
of the species with the buoyant density of the fluid medium.
[0072] Fluid media useful with the present teachings are limited
only by carbon nanotube aggregation therein to an extent precluding
at least partial separation. Accordingly, without limitation,
aqueous and non-aqueous fluids can be used in conjunction with any
substance soluble or dispersible therein, over a range of
concentrations so as to provide the medium a density gradient for
use in the separation techniques described herein. Such substances
can be ionic or non-ionic, non-limiting examples of which include
inorganic salts and alcohols, respectively. In certain embodiments,
as illustrated more fully below, such a medium can include a range
of aqueous iodixanol concentrations and the corresponding gradient
of concentration densities. Likewise, as illustrated below, the
methods of the present teachings can be influenced by gradient
slope, as affected by the length of the centrifuge tube or
compartment and/or the angle of centrifugation.
[0073] As understood by those in the art, aqueous iodixanol is a
common, widely used non-ionic density gradient medium. However,
other media can be used with good effect, as would also be
understood by those individuals. More generally, any material or
compound stable, soluble or dispersible in a fluid or solvent of
choice can be used as a density gradient medium. A range of
densities can be formed by dissolving such a material or compound
in the fluid at different concentrations, and a density gradient
can be formed, for instance, in a centrifuge tube or compartment.
More practically, with regard to choice of medium, the carbon
nanotubes, whether or not functionalized, should also be soluble,
stable or dispersible within the fluids/solvent or resulting
density gradient. Likewise, from a practical perspective, the
maximum density of the gradient medium, as determined by the
solubility limit of such a material or compound in the solvent or
fluid of choice, should be at least as large as the buoyant density
of the particular carbon nanotubes (and/or in composition with one
or more surface active components, e.g., surfactants) for a
particular medium.
[0074] Accordingly, with respect to the present teachings, any
aqueous or non-aqueous density gradient medium can be used
providing the single-walled carbon nanotubes are stable; that is,
do not aggregate to an extent precluding useful separation.
Alternatives to iodixanol include but are not limited to inorganic
salts (such as CsCl, Cs.sub.2SO.sub.4, KBr, etc.), polyhydric
alcohols (such as sucrose, glycerol, sorbitol, etc.),
polysaccharides (such as polysucrose, dextrans, etc.), other
iodinated compounds in addition to iodixanol (such as diatrizoate,
nycodenz, etc.), and colloidal materials (such as but not limited
to percoll). Other media useful in conjunction with the present
teachings would be understood by those skilled in the art made
aware of the present teachings and/or by way of co-pending U.S.
patent application Ser. No. 11/368,581, filed on Mar. 6, 2006, the
entirety of which is incorporated herein by reference.
[0075] Other parameters which can be considered upon choice of a
suitable density gradient medium include, without limitation, the
diffusion coefficient and the sedimentation coefficient, both of
which can determine how quickly a gradient redistributes during
centrifugation. Generally, for more shallow gradients, a larger
diffusion coefficient and a smaller sedimentation coefficient are
desired. For instance, Percoll.RTM. is a non-ionic density gradient
medium having a relatively small water affinity compared to other
media. However, it has a large sedimentation rate and a small
diffusion coefficient, resulting in quick redistribution and steep
gradients. While cost can be another consideration, the methods of
the present teachings tend to mitigate such concerns in that the
media can be repeatedly recycled and reused. For instance, while
aqueous iodixanol is relatively expensive as compared to other
density gradient media, it can be recycled, with the iodixanol
efficiently recovered at high yield, for reuse in one separation
system after another.
[0076] Regardless of medium identity or density gradient, a
heterogeneous sample of carbon nanotubes (e.g., a mixture of carbon
nanotubes of varying structures and/or properties) can be
introduced into the fluid medium on or at any point within the
gradient before centrifugation. In certain embodiments, the
heterogeneous sample of carbon nanotubes (or a composition
including the heterogeneous sample of carbon nanotubes and at least
one surface active component) can be introduced at a spatial point
along the gradient where the density remains roughly constant over
time even as the density gradient becomes steeper over the course
of centrifugation. Such an invariant point can be advantageously
determined to have a density corresponding to about the buoyant
density of the nanotube composition(s) introduced thereto.
[0077] Prior to introduction into the density gradient medium, the
heterogeneous sample of carbon nanotubes can be provided in
composition with one or more surface active components. Generally,
such components can function, in conjunction with the fluid medium,
to reduce nanotube aggregation. In some embodiments, the one or
more surface active components can include one or more surfactants
selected from a wide range of non-ionic or ionic (cationic,
anionic, or zwitterionic) amphiphiles. In certain embodiments, the
surface active component can include an anionic surfactant. In some
embodiments, a surface active component can include one or more
sulfates, sulfonates, carboxylates, and combinations thereof. In
some embodiments, a surface active component can include one or
more bile salts (including but not limited to cholates,
deoxycholates, taurodeoxycholates and combinations thereof), or
other amphiphiles with anionic head groups and flexible alkyl tails
(referred interchangeably herein below as anionic alkyl
amphiphiles; such as but not limited to dodecyl sulfates and
dodecylbenzene sulfonates). Examples of such bile salts can include
but are not limited to sodium cholate (SC), sodium deoxycholate,
and sodium taurodeoxycholate. Examples of amphiphiles with anionic
head groups and flexible alkyl tails can include, but are not
limited to, sodium dodecyl sulfate (SDS) and sodium dodecylbenzene
sulfonate (SDBS). More generally, such bile salts can be more
broadly described as a group of molecularly rigid and planar
amphiphiles with a charged face opposing a hydrophobic face. As
such, these bile salts (or other surface active components having
characteristics similar to these bile salts) are capable of
providing a planar and/or rigid structural configuration about and
upon interaction with carbon nanotubes, which can induce
differential nanotube buoyant density. In other embodiments, the
surface active component can include a cationic surfactant. For
example, such a component can be selected from amphiphiles with
cationic head groups (e.g., quaternary ammonium salts) and flexible
or rigid tails.
[0078] Without wishing to be bound to any particular theory, a
study on graphene, which is the closest analog to a SWNT, has
reported that while anionic-alkyl surfactants organize into
hemicylindrical micelles with liquid-like hydrophobic cores (EM. F.
Islam, E. Rojas, D. M. Bergey, A. T. Johnson, A. G. Yodh, Nano
Lett. 3, 269 (2003); E. J. Wanless, W. A. Ducker, J. Phys. Chem.
100, 3207 (1996)), bile salts form well-structured monolayers with
their less polar sides facing the hydrophobic surface (Y. Sasaki et
al., Colloids Surf., B 5, 241 (1995)). It also has been reported
that bile salts order to form well defined guest-host structures
around small hydrophobic molecules (S. Mukhopadhyay and U. Maitra,
Curr. Sci. 87, 1666 (2004); J. Tamminen, E. Kolehmainen, Molecules
6, 21 (2001)). Accordingly, the rigidity and planarity of bile
salts, in contrast with anionic-alkyl surfactants, can be expected
to result in encapsulation layers that are sensitive to subtle
changes in the underlying SWNT. Other effects, such as
charge-transfer between metallic SWNTs and the surfactants also
could be important.
[0079] Density gradient centrifugation can be used with comparable
effect for the separation of a wide range of
surfactant-encapsulated SWNTs. Without limitation to any one theory
or mode of operation, surfactant-based separation via density
gradient centrifugation is believed to be largely driven by how
surfactants organize around SWNTs of different structures and
electronic types. FIGS. 3(a)-(c), for example, illustrate how a
single type of surfactant encapsulates carbon nanotubes of
different structures (in this case, diameters) differentially. As
such encapsulation contributes to a density difference proportional
to the diameter of the carbon nanotubes, separation of such
surfactant encapsulated SWNTs is possible via density gradient
ultracentrifugation. The energetic balance among nanotube-, water-
and surfactant-surfactant interactions as well as their packing
density, orientation, ionization, and the resulting hydration of
these surfactants can all be critical parameters affecting buoyant
density and the quality of separation and purification.
[0080] While density gradient centrifugation has been employed to
separate DNA-wrapped SWNTs by diameter and band gap, DNA
functionalization has not been optimized for all embodiments. For
instance, due to limited stability in aqueous density gradients,
DNA-wrapped SWNTs may not be amenable to the refinements in
purification gained from repeated centrifugation in density
gradients. In addition, the complete removal of the DNA wrapping
after enrichment can be problematic. Furthermore, the availability
and cost of specific, custom oligomers of single-stranded DNA can
be prohibitive. Sensitivity to electronic type (metallic versus
semiconducting) also has yet to be fully explored.
[0081] Accordingly, the methods of the present teachings can be
directed to use of a surface active component that does not include
DNA or DNA fragments. For example, in embodiments where the surface
active component includes a single surfactant, an anionic
amphiphile such as an anionic-alkyl surfactant or any of the bile
salts described above can be used. In particular, many surfactants
contemplated for use with the present teachings cost orders of
magnitude less than single-stranded DNA. The difference is
significant when comparing, for instance, sodium cholate (98%
purity) from Sigma-Aldrich (St. Louis, Mo.) on a 100 g scale,
quoted at $0.62/g, with single-stranded DNA of sequence
d(GT).sub.20 produced on the largest scale offered (150 mg scale,
much less than 98% purity) by Alpha-DNA (Montreal, Canada) at
$2242.80/g. Furthermore, the adsorption of the surface active
components disclosed herein to SWNTs is reversible and compatible
with a wide range of tube diameters (e.g., SWNTs having a diameter
in the range of about 7 .ANG. to about 16 .ANG.. More importantly,
by using such a surface active component, the structure-density
relationship for SWNTs can be easily controlled by varying the
surfactant(s) included in the surface active component.
[0082] As demonstrated herein, successful separation by the present
method(s) has been achieved using surfactants such as salts of bile
acids, e.g. cholic acid, including sodium cholate, sodium
deoxycholate, and sodium taurodeoxycholate. Separation in density
gradients also can be achieved using other surface active
components, such as surfactants, consistent with the principles and
concepts discussed herein and the knowledge of those skilled in the
art. For the case of single surfactant separations, distinct
structure-density relationships were observed for anionic-alkyl
surfactants and bile salts as described in examples herein below.
Use of a single surfactant can be especially useful for separation
by diameter. Without wishing to be bound by any particular theory,
it is believed that the use of single surface active component
results in a substantially uniform thickness of the surface active
component around the differently dimensioned SWNTs in a mixture and
accordingly, a substantially uniform density for SWNTs of a
specific diameter.
[0083] In some embodiments, the heterogeneous sample of carbon
nanotubes can be provided in composition with at least two surface
active components, where the at least two surface active components
can be of the same type or of different types. In some embodiments,
the at least two surface active components can competitively adsorb
to the SWNT surface. For example, the at least two surface active
components can be two different surfactants. Such a competitive
co-surfactant system can be used to achieve optimal separation
between metallic and semiconducting single-walled carbon nanotubes.
For example, the at least two surface active components can include
two bile salts, or alternatively, a bile salt with a surfactant. In
certain embodiments, the use of sodium cholate with sodium dodecyl
sulfate in a ratio between about 4:1 and about 1:4 by weight, and
particularly, 7:3 by weight, was observed to afford good selective
separation of SWNTs by electronic type. The metal-semiconductor
selectivity observed using the present methods indicates a certain
degree of coupling of the surfactant(s) and/or their hydration with
the electronic nature of the underlying SWNTs. Additionally, the
packing density of the surfactants and their hydration likely may
be sensitive to electrostatic screening by the underlying
SWNTs.
[0084] Upon sufficient centrifugation (i.e., for a selected period
of time and/or at a selected rotational rate at least partially
sufficient to separate the carbon nanotubes along the medium
gradient), at least one separation fraction including separated
single-walled carbon nanotubes can be separated from the medium.
Such fraction(s) can be isopycnic at a position along the gradient.
An isolated fraction can include substantially monodisperse
single-walled carbon nanotubes, for example, in terms of at least
one characteristic selected from nanotube diameter dimensions,
chiralities, and electronic type. Various fractionation techniques
can be used, including but not limited to, upward displacement,
aspiration (from meniscus or dense end first), tube puncture, tube
slicing, cross-linking of gradient and subsequent extraction,
piston fractionation, and any other fractionation techniques known
in the art.
[0085] The medium fraction and/or nanotube fraction collected after
one separation can be sufficiently selective in terms of separating
the carbon nanotubes by the at least one selected property (e.g.
diameter). However, in some embodiments, it can be desirable to
further purify the fraction to improve its selectivity.
Accordingly, in some embodiments, methods of the present teachings
can include iterative separations. Specifically, an isolated
fraction can be provided in composition with the same surface
active component system or a different surface active component
system, and the composition can be contacted with the same fluid
medium or a different fluid medium, where the fluid medium can have
a density gradient that is the same or different from the fluid
medium from which the isolated fraction was obtained. In certain
embodiments, fluid medium conditions or parameters can be
maintained from one separation to another. In certain other
embodiments, at least one iterative separation can include a change
of one or more parameters, such as but not limited to, the identity
of the surface active component(s), medium identity, medium density
gradient and/or medium pH with respect to one or more of the
preceding separations. Accordingly, in some embodiments of the
methods disclosed herein, the choice of the surface active
component can be associated with its ability to enable iterative
separations, which, for example, is considered not possible for DNA
wrapped SWNTs (due to, in part, the difficulties in removing the
DNA from the SWNTs).
[0086] In certain embodiments, such as separations by chirality or
electronic type, the present methods can include multiple
iterations of density gradient centrifugation, whereby the degree
of separation by physical and electronic structure can improve with
each iteration. For instance, removal of undesired chiralities can
be effected by successively repetitive density gradient
centrifugation. Additionally, the surfactant(s) encapsulating the
SWNTs can be modified or changed between iterations, allowing for
even further refinement of separation, as the relationship between
density and the physical and electronic structure will vary as a
function of any resulting surfactant/encapsulation layer.
Separation fractions isolated after each separation can be washed
before further complexation and centrifugation steps are
performed.
[0087] The selectivity of the fraction(s) collected can be
confirmed by various analytical methods. For example, optical
techniques including but not limited to spectroscopic techniques
such as spectrophotometric analysis and fluorimetric analysis can
be used. Such techniques generally include comparing one or more
absorbance and/or emission spectra with a corresponding reference
spectrum. The isolated nanotube fraction generally has a narrower
distribution in the variance of the at least one selected
property.
[0088] As described above, carbon nanotubes synthesized by
currently known techniques including, without limitation, high
pressure carbon monoxide ("HiPCO") process, Co--Mo catalysis
("CoMoCAT") process, and laser ablation process, typically have
heterogeneous structures and properties. For example, both the
CoMoCAT and the HiPCO methods typically yield SWNTs having a
diameter in the range of about 7 .ANG. to about 11 .ANG., while the
laser-ablation growth method typically yields SWNTs having a
diameter in the range of about 11 .ANG. to about 16 .ANG..
Accordingly, before separation by the methods disclosed herein, the
heterogeneous sample of carbon nanotubes can have varying
chiralities, diameter, and/or electronic type. In some embodiments,
the diameter dimensions of the carbon nanotubes can range from
about 7 .ANG. to about 20 .ANG., from about 7 .ANG. to about 16
.ANG., from about 7 .ANG. to about 15 .ANG., from about 7 .ANG. to
about 12 .ANG., from about 7 .ANG. to about 11 .ANG., from about 7
.ANG. to about 10 .ANG., from about 11 .ANG. to about 20 .ANG.,
from about 11 .ANG. to about 16 .ANG., from about 11 .ANG. to about
15 .ANG., from about 12 .ANG. to about 20 .ANG., from about 12
.ANG. to about 16 .ANG., or from about 12 .ANG. to about 15 .ANG..
In some embodiments, the heterogeneous sample of carbon nanotubes
can include metallic carbon nanotubes and semiconducting carbon
nanotubes.
[0089] As demonstrated by the examples herein below, selectivity
made possible by the present teachings can be indicated by
separation of carbon nanotubes differing by diameters less than
about 0.6 .ANG.. For example, in some embodiments, the present
teachings can provide a population of carbon nanotubes (e.g.,
SWNTs) in which >99.9%, >99%, >97%, >95%, >90%,
>85%, >80%, >75%, or >50% of the carbon nanotubes can
have a diameter differing by less than about 0.6 .ANG. or that
>99.9%, >99%, >97%, >95%, >90%, >85%, >80%,
>75%, or >50% of the carbon nanotubes can have a diameter
within about 0.6 .ANG. of the mean diameter of the population. In
some embodiments, the present teachings can provide a population of
carbon nanotubes in which >99.9%, >99%, >97%, >95%,
>90%, >85%, >80%, >75%, or >50% of the carbon
nanotubes can have a diameter differing by about 0.5 .ANG. or that
>99.9%, >99%, >97%, >95%, >90%, >85%, >80%,
>75%, or >50% of the carbon nanotubes can have a diameter
within about 0.5 .ANG. of the mean diameter of the population. In
some embodiments, the present teachings can provide a population of
carbon nanotubes in which >99.9%, >99%, >97%, >95%,
>90%, >85%, >80%, >75%, or >50% of the carbon
nanotubes can have a diameter differing by about 0.2 .ANG. or that
>99.9%, >99%, >97%, >95%, >90%, >85%, >80%,
>75%, or >50% of the carbon nanotubes can have a diameter
within about 0.2 .ANG. of the mean diameter of the population. In
some embodiments, the present teachings can provide a population of
carbon nanotubes in which >99.9%, >99%, >97%, >95%,
>90%, >85%, >80%, >75%, or >50% of the carbon
nanotubes can have a diameter differing by about 0.1 .ANG. or that
>99.9%, >99%, >97%, >95%, >90%, >85%, >80%,
>75%, or >50% of the carbon nanotubes can have a diameter
within about 0.1 .ANG. of the mean diameter of the population. In
certain embodiments, the present teachings can provide a population
of carbon nanotubes in which >75% of the carbon nanotubes can
have a diameter within about 0.5 .ANG. of the mean diameter of the
population.
[0090] Selectivity made possible by the present teachings can also
be indicated by separation of carbon nanotubes where >33% of
such separated carbon nanotubes are metallic or >67% of such
separated carbon nanotubes are semiconducting. For example, in some
embodiments, the present teachings can provide a population of
carbon nanotubes (e.g., SWNTs) in which >99.9%, >99%,
>97%, >95%, >92%, >90%, >85%, >80%, >75%,
>50%, or >33% of the carbon nanotubes can be metallic. In
other embodiments, the present teachings can provide a population
of carbon nanotubes in which >99.9%, >99%, >97%, >95%,
>92%, >90%, >85%, >80%, >75%, or >67% of the
carbon nanotubes can be semiconducting. In certain embodiments, the
present teachings can provide a population of carbon nanotubes in
which >50% of the carbon nanotubes can be metallic. In certain
embodiments, the present teachings can provide a population of
carbon nanotubes in which >70% of the carbon nanotubes can be
semiconducting.
[0091] Similarly, selectivity made possible by the present
teachings can be indicated by separation of carbon nanotubes where
>15% of such separated carbon nanotubes are of the same
chirality (n,m) type. For example, in some embodiments, the present
teachings can provide a population of carbon nanotubes (e.g.,
SWNTs) in which >99.9%, >99%, >97%, >95%, >90%,
>85%, >80%, >75%, >50%, >30%, or >15% of the
carbon nanotubes can be of the same chirality (n,m) type. In
certain embodiments, the present teachings can provide a population
of carbon nanotubes in which >30% of the carbon nanotubes can
include the same chirality (n,m) type.
[0092] As described herein, density gradient ultracentrifugation
can provide a scalable approach for the bulk purification of carbon
nanotubes by diameter, band gap, and electronic type. As
demonstrated in the examples below, the present teachings can
purify heterogeneous mixtures of SWNTs and provide sharp diameter
distributions in which greater than 97% of semiconducting SWNTs are
within 0.2 .ANG. of the mean diameter. Furthermore, the
structure-density relationship for SWNTs can be engineered to
achieve exceptional metal-semiconductor separation, for example, by
using mixtures of competing co-surfactants, thus enabling the
isolation of bulk quantities of SWNTs that are predominantly a
single electronic type.
[0093] Because SWNTs purified by methods of the present teachings
are highly compatible with subsequent processing techniques and can
be integrated into devices, the present teachings also provide
articles of manufacture (including electronic devices, optical
devices, and combinations thereof) and other technological
applications that require SWNTs with monodisperse structure and
properties.
EXAMPLES OF THE INVENTION
[0094] The following non-limiting examples and data illustrate
various aspects and features relating to the methods and/or systems
of the present teachings, including the preparation and use of
density gradient media for carbon nanotube separation, confirmation
of which is available using spectroscopic techniques of the sort
described herein and known to those skilled in the art. In
comparison with the prior art, the present methods and systems
provide results and data which are surprising, unexpected and
contrary thereto. While the utility of the present teachings is
illustrated through the use of several methods and the density
gradient media and surface active components which can be used
therewith, it will be understood by those skilled in the art that
comparable results are obtainable with various other media and
surface active components, as are commensurate with the scope of
the present teachings. Other non-limiting examples are provided
upon consideration of the examples, figures and corresponding
discussion in the aforementioned, incorporated application.
Example 1
Separation of SWNTs Using Different Single-Surfactant Systems
Raw SWNT Material
[0095] SWNTs of various diameters were explored by utilizing SWNTs
produced by the CoMoCAT method (which yields tubes about 7-11 .ANG.
in diameter), and the laser-ablation growth method (which yields
tubes about 11-16 .ANG. in diameter). CoMoCAT material was
purchased from Southwest Nanotechnologies, Inc. (Norman, Okla.) as
raw material purified only to remove silica. The laser-ablation
grown SWNTs were manufactured by Carbon Nanotechnologies Inc.
(Houston, Tex.) and received in their raw form.
Surfactant Encapsulation
[0096] To disperse SWNTs in solutions of bile salts or other
surfactants, 1 mg/mL SWNTs were dispersed in solutions of 2% w/v
surfactant via ultrasonication. Sodium dodecyl sulfate,
electrophoresis grade, minimum 99%, was purchased from Fisher
Scientific. Dodecylbenzene sulfonic acid, sodium salt, an 80% (CH)
mixture of homologous alkyl benzenesulfonates; sodium cholate
hydrate, minimum 99%; deoxycholic acid, minimum 99%; and sodium
taurodeoxycholate hydrate, minimum 97% TLC, were purchased from
Sigma-Aldrich, Inc. The sodium salt of deoxycholic acid was used in
experiments and was formed by addition of equal molar
concentrations of NaOH. Ultrasonication (Sonic Dismembrator 500,
Fisher Scientific) was implemented by immersing an ultrasonic probe
(microtip extension, Fisher Scientific) into 3-15 mL of the SWNT
solution. The probe was driven at 40% of the instrument's maximum
amplitude for 60 minutes at 20 kHz. During sonication, the solution
was immersed in a bath of ice-water to prevent heating. In some
instances, after ultrasonication, large aggregations of insoluble
material were removed via ultracentrifugation at 54 krpm for 14
minutes in a TLA100.3 rotor (Beckman-Coulter).
Methods for Creating Density Gradients
[0097] Density gradients were formed from aqueous solutions of a
non-ionic density gradient medium, iodixanol, purchased as
OptiPrep.RTM. 60% w/v iodixanol, 1.32 g cm.sup.-3, (Sigma-Aldrich
Inc.). Gradients were created directly in centrifuge tubes by one
of two methods, by layering and subsequent diffusion or by using a
linear gradient maker. See J. M. Graham, Biological centrifugation,
(BIOS Scientific Publishers, Limited, ebrary, Inc., 2001). In the
layering and subsequent diffusion method, 3-6 layers, each
consisting of discrete, decreasing iodixanol concentrations, were
layered in a centrifuge tube. Initially, this resulted in a density
gradient that increased step-wise in density from the top to the
bottom of a centrifuge tube. The centrifuge tube was then capped
and the gradient was allowed to diffuse for 1-18 hours, depending
on the length of the centrifuge tube and its angle of tilt during
the diffusion step, until it was approximately linear. In an
alternative method for creating density gradients, a linear
gradient maker was utilized (SG 15 linear gradient maker, Hoefer
Inc.) to directly create linear gradients in centrifuge tubes
without having to wait for diffusion.
[0098] In some instances, an under-layer of 60% weight per volume
iodixanol was inserted at the bottom of the gradient to raise the
linear portion of the gradient in the centrifuge tube. Also, in
some instances, centrifuge tubes were filled with an over-layer
consisting of only surfactant (0% w/v iodixanol). All the layers
initially consisted of the same concentration of surfactant, which
was typically 2% w/v.
[0099] For the inclusion of SWNTs in linear gradients, several
methods were utilized: (i) SWNTs, dispersed in aqueous solutions of
surfactants (typically 2% w/v), were layered on top of the gradient
before centrifugation; (ii) iodixanol was added to an aqueous
solution of dispersed SWNTs to adjust its density and this solution
was then inserted into a linear gradient via a syringe at the point
in which the density of the preformed gradient matched that of the
solution; and (iii) iodixanol was added to an aqueous solution of
dispersed SWNTs and this solution was used as a layer of a step
gradient. Due to the slower diffusion rate of the SWNTs compared
with that of iodixanol, the SWNTs were observed to remain in their
initial position during the diffusion step.
Centrifugation
[0100] Centrifugation was carried out in two different rotors, a
fixed angle TLA1100.3 rotor and a swing bucket SW41 rotor
(Beckman-Coulter), at 22 degrees Celsius and at 64 krpm and 41
krpm, respectively, for 9-24 hours, depending on the spatial extent
and initial slope of a gradient.
Typical Slopes and Densities of Initial Gradients (Before
Centrifugation)
[0101] FIGS. 4(a)-(b) illustrate layering of a density gradient and
its redistribution during ultracentrifugation. FIG. 4(a) is a
schematic depicting a typical, initial density gradient. In between
a dense underlayer and buoyant overlayer, a linear gradient of
iodixanol is created and SWNTs are inserted into that layer before
centrifugation. FIG. 4(b) shows graphically the redistribution of a
density profile. During ultracentrifugation, the density gradient
media (e.g., iodixanol) undergoes diffusion while simultaneously
sedimenting towards the bottom of the centrifuge tube in response
to the centripetal force, as governed by the Lamm equation.
[0102] In TLA100.3 centrifuge tubes (inner diameter 1.1 cm,
capacity 3 mL), typical gradients varied from 5% w/v iodixanol at
the top to 40% w/v iodixanol at the bottom (1.03 to 1.21 g
cm.sup.-3). Surfactant encapsulated SWNTs were initially seeded
anywhere in the top 2/3 of the gradient. Typical centrifugation
conditions were 9 hours at 64 krpm.
[0103] In SW41 centrifuge tubes (inner diameter 1.3 cm, capacity
.about.12 mL), typical gradients were constrained to less than the
full height of the centrifuge tubes (FIG. 4). First, 1.5 mL of 60%
w/v iodixanol (1.32 g cm.sup.-3) was added to the bottom of the
centrifuge tube. This layer was used to raise the height of the
gradient in the centrifuge tube. On top of that underlayer, 5 mL of
linear gradient was added. Then, 0.88 mL of SWNT solution (density
already adjusted by addition of iodixanol) was inserted into that
gradient. On top of the gradient, surfactant solution (no
iodixanol) was added to completely fill the centrifuge tube to
prevent its collapse at large centripetal forces (FIG. 4). For
sodium cholate separations, the gradient-portion of the centrifuge
tube linearly varied from 7.5% w/v (1.04 g cm.sup.-3) at the top to
22.5% w/v (1.12 g cm.sup.-3) at the bottom or from 10% w/v (1.05 g
cm.sup.-3) at the top to 25% w/v (1.13 g cm.sup.-3) at the bottom.
Typical centrifugation conditions were 12 hours at 41 krpm.
[0104] The chosen density and slope of a gradient are parameters
that can be varied to optimize the effectiveness of density
gradient ultracentrifugation. It is preferred that a density
gradient be constructed to minimize the distance that the SWNTs
must sediment before reaching their isopycnic point. Furthermore,
it should be understood that during ultracentrifugation, the
density profile (density as a function of height in the centrifuge
tube) will redistribute as the density gradient medium responds to
the centripetal force. Typically, this means that the density
gradient will become steeper with time.
[0105] To aid in the formation of optimal density gradients, the
re-distribution iodixanol and the separation of SWNTs during
ultracentrifugation can be roughly predicted via numerical
solutions to the Lamm equation if the buoyant densities of the
SWNTs and their sedimentation coefficients are known. See J. M.
Graham, Biological centrifugation, (BIOS Scientific Publishers,
Limited, ebrary, Inc., 2001).
Concentration of SWNTs in Step Gradients
[0106] In some instances, after dispersion and isolation of SWNTs
but before separation in density gradients, SWNT solutions were
concentrated by ultracentrifugation in a step density gradient.
FIG. 5 is a photographical representation showing the concentration
of SWNTs via density gradient ultracentrifugation using a large
step density gradient. The photograph on the left hand side shows
the distribution of the SWNT solution (a), which includes sodium
cholate, the encapsulating agent, and no iodixanol, and the stop
layer (b), which includes 60% w/v iodixanol with the encapsulating
agent added at the same concentration as layer (a), before
concentration. The photograph on the left hand side shows the
concentrated SWNT solution after ultracentrifugation at
.about.200,000 g. The sodium cholate-encapsulated SWNTs, which have
a buoyant density between .rho..sub.a and .rho..sub.b, have
sedimented to the interface between layer (a) and layer (b).
[0107] To form a step gradient and subsequently concentrate a SWNT
solution, the SWNT solution (.rho..about.1 g/mL) was layered
directly on top of an OptiPrep.RTM. solution (60% w/v iodixanol
solution, 1.32 g/mL). Surfactant was added to the OptiPrep.RTM.
solution at the same weight per volume as in the SWNT solution
(usually 2% w/v surfactant). During ultracentrifugation, the
isolated SWNTs, with a buoyant density between 1.00 and 1.32 g/mL,
sedimented to the interface between both layers. The SWNTs at the
interface were then withdrawn from the centrifuge tube via
fractionation. This enabled the concentration of SWNTs by a factor
of 3-5, as determined from optical spectrophotometry. The
concentrated SWNTs can be removed via fractionation.
Fractionation
[0108] After centrifugation, the separated SWNTs were removed from
their density gradients, layer by layer, by fractionation. To
fractionate TLA100.3 tubes, a modified Beckman Fractionation System
(Beckman-Coulter Inc.) was utilized in an upward displacement mode
using Fluorinert.RTM. FC-40 (Sigma-Aldrich, Inc.) as a dense chase
media. 25 .mu.L fractions were collected. To fractionate SW41
centrifuge tubes, a Piston Gradient Fractionator system was
utilized (Biocomp Instruments, Inc., Canada). 0.5-3.0 mm fractions
were collected (70-420 .mu.L in volume). In both cases, fractions
were diluted to 1 mL in 2% w/v surfactant solution for optical
characterization.
Measurement of Density Profile
[0109] To measure the density profile of a redistributed gradient
after centrifugation, 100-300 .mu.L fractions were collected and
their densities were determined by measuring the mass of a known
volume of those fractions using a calibrated micropipette and
electronic balance. With increasing centrifugation time, the
iodixanol redistributed towards the bottom of the centrifuge tube,
resulting in steeper gradients, as governed by the Lamm equation
(FIG. 4(b)).
Measurement of Optical Absorbance Spectra
[0110] The optical absorbance spectra of collected fractions of
separated SWNTs were measured using a Cary 500 spectrophotometer
(Varian, Inc.) from 400 to 1340 nm at 1 nm resolution for
0.066-0.266 s integration time. Samples of similar optical index of
refraction (similar iodixanol and surfactant concentrations) were
used as reference samples for subtraction of background absorbance
(due to water, surfactant, iodixanol, etc.), using the two-beam
mode of the Cary 500 (lamp illumination split between the sample of
interest and the reference sample, with reference absorption
subtracted from that of the sample). A baseline correction was
utilized to correct for varying instrument sensitivities with
wavelength.
[0111] To subtract the effects of the slowly varying background
absorption from the measured optical absorption spectra, the
derivative of the measured optical absorption with respect to
wavelength was used. FIG. 6 shows the fitting of absorbance
spectrum for determination of relative SWNT concentration. The
absorbance spectrum is plotted as open triangles (left axis). The
derivative of absorbance with respect to wavelength is plotted as
open circles (right axis). The effects of background absorbencies
are minimized by using the amplitude of the derivative (depicted by
arrows) rather than the absolute absorbance.
[0112] In addition to using the derivative of the measured optical
absorption with respect to wavelength as opposed to the absolute
absorbance, it is assumed that the background absorption (from
residual carbonaceous impurities, the tail of .pi.-plasmon
resonances, and off-resonance, neighboring absorbance peaks) was
slowly varying with respect to wavelength in comparison with the
variation near a first order, optical transition. This is a
reasonable assumption because the line-width of a first order,
optical transition of an isolated, semiconducting SWNT has been
measured to be relatively narrow--about 25 meV. Furthermore, the
spacing between the six transitions studied here is significantly
greater than 25 meV (Table 1). A slowly varying background implies
that the derivative of the background absorption is sufficiently
small and can be ignored. It is also assumed that the line-shape of
these transitions remain constant with concentration and buoyant
density, as expected from Beer's law. An invariant line-shape
implies that the derivative will be directly proportional to the
amplitude of absorption. In this case, the relative amplitude of
absorption can be measured using the derivative. To further
eliminate small linear variations of the background absorbance with
respect to wavelength, the maximum absolute value of the derivative
to the right and left of each peak in optical absorption were
averaged, and the averaged value was reported as the amplitude of
absorbance and it is proportional to concentration (Beer's law).
Referring to Table 1, it can be seen that three of the six optical
transitions originate from two different chiralities of
nanotubes.
TABLE-US-00001 TABLE 1 Assignment of near infrared absorption
peaks. .lamda..sub.11s (nm) Chiralities Diameters (.ANG.) 929 (9,
1) 7.57 991 (6, 5), (8, 3) 7.57, 7.71 1040 (7, 5) 8.29 1134 (8, 4),
(7, 6) 8.40, 8.95 1199 (8, 6) 9.66 1273 (9, 5), (8, 7) 9.76,
10.32
Analysis of Optical Spectra
[0113] A. Separation of CoMoCAT-Grown, SC-Encapsulated SWNTs
[0114] Initial SWNT dispersion: 6.2 mg raw CoMoCAT SWNTs were
dispersed in 6.2 mL of 2% w/v sodium cholate (SC) via horn
ultrasonication for 1 hour as described previously. Coarse
aggregates and insoluble materials were then removed by a short
ultracentrifugation step. This was implemented by filling two
polycarbonate centrifuge tubes (Beckman-Coulter) with 3.0 mL of the
ultrasonicated solution and separating at 54 krpm for 14 minutes
(TLA100.3, 22.degree. C.). Following the short ultracentrifugation,
the top 2.5 mL of each centrifuge tube was decanted and saved for
later separation in density gradients.
[0115] Density gradient centrifugation: The Beckman SW41 rotor was
utilized for this sorting experiment. Gradients were formed
directly in SW41-sized polyclear centrifuge tubes (Beckman-Coulter)
using the linear gradient maker by the following procedure. First,
the bottom of a centrifuge tube was filled with 1.5 mL of an
underlayer consisting of 60% w/v iodixanol, 2% w/v SC, as described
previously. Then, 3 mL of 7.5% w/v iodixanol, 2% w/v SC and 3 mL of
22.5% w/v iodixanol, 2% w/v SC were prepared and 2.5 mL of each was
added to the mixing and reservoir chambers of the linear gradient
maker, respectively. The linear gradient was delivered from the
output of the gradient maker to slightly above (<2 mm) the
underlayer in the centrifuge tube using a piece of glass tubing
(inner diameter .about.1 mm, length .about.10 cm). The glass
segment and the output of the linear gradient maker were connected
via flexible tubing. Using this procedure, it was expected that the
gradient maker would create an approximately linear density
gradient that would vary from top to bottom from 7.5% w/v iodixanol
to 22.5% w/v iodixanol, with equal concentrations of 2% w/v SC
throughout. This expectation was confirmed by fractionating and
measuring the density profile of a gradient immediately after
formation.
[0116] After formation of the gradient, a 1.1 mL solution
consisting of already dispersed SWNTs (as described above), 2% w/v
SC, and 20% w/v iodixanol was created. To make this solution, 367
.mu.L of 60% w/v iodixanol, 2% w/v SC and 733 .mu.L of CoMoCAT
SWNTs dispersed in 2% w/v SC were mixed. Then, 0.88 mL of this SWNT
solution was slowly inserted (at a rate of 0.1 mL min.sup.-1 using
a syringe pump, PhD 2000, Harvard Apparatus, Inc.) into the
previously made density gradient via a syringe needle inserted of
the way down the gradient. The height of the syringe needle was
adjusted such that the SWNT solution was inserted where its density
matched that of the previously formed gradient. Following insertion
of the SWNT solution, the remainder of the centrifuge tube was
filled with an overlayer consisting of 2% w/v SC (no iodixanol).
The centrifuge tube was filled to .about.4 mm from its top. Sorting
occurred via ultracentrifugation at 41 krpm for 12.0 hours at
22.degree. C.
[0117] Fractionation: After sorting via density gradient
ultracentrifugation, the gradient was fractionated into 0.5 mm
segments (70 .mu.L). Each fraction was diluted to 1 mL and
optically characterized as described previously.
[0118] FIG. 7 illustrates the separation of SC-encapsulated
CoMoCAT-synthesized SWNTs (which have a diameter range of 7-11
.ANG.) via density gradient ultracentrifugation. FIG. 7(a) is a
photograph of the centrifugation tube after a one-step separation.
Referring to FIG. 7(a), multiple regions of separated SWNTs are
visible throughout the density gradient. The separation is
evidenced by the formation of colored bands of isolated SWNTs
sorted by diameter and band gap, with at least three different
colored bands being clearly visible (from top to bottom: magenta,
green, and brown). The different color bands correspond to
different band gaps of the semiconducting tubes. Bundles,
aggregates, and insoluble material sediment to lower in the
gradient (as a black band).
[0119] FIG. 7(b) shows the optical absorbance spectra (1 cm path
length) after separation using density gradient
ultracentrifugation. SWNTs before purification are depicted as a
dashed, gray line. As shown by the optical absorbance spectra in
FIG. 7(b), the amplitudes of optical absorbance for different
transitions in the 900-1340 nm range (first order semiconducting
transitions) also indicate separation by diameter and band gap.
More specifically, the spectra illustrate that SWNTs of
increasingly larger diameters are enhanced at increasingly larger
densities.
[0120] The semiconducting first order transitions for SWNTs
produced by the CoMoCAT method are spectrally located between
900-1340 nm, as described in the literature. Specifically, three
diameter ranges of semiconducting SWNTs are highlighted (red,
green, and blue; (6, 5), (7, 5) and (9, 5)/(8, 7) chiralities; 7.6,
8.3, and 9.8/10.3 .ANG. in diameter; maximized in the 3rd, 6th, and
7th fractions, respectively). As described above, absorbance
spectra were fit in this spectral range to determine the
concentration of different semiconducting (n, m) chiralities. In
some cases, several (n, m) chiralities overlap because they have
first order transitions at similar wavelengths (Table 1).
Generally, SWNTs with optical transitions at longer wavelengths are
larger in diameter. Thus, by analyzing the strength of these
transitions at different wavelengths as a function of density, it
is possible to determine the density of SWNTs of different
diameters (FIG. 6). However, the E.sub.11 optical transitions are
on top of a slowly varying background absorbance which was
substrated as described above. The difference in density from the
top fraction to the bottom fraction was measured to be 0.022 g
cm.sup.-3, and the density for the top fraction was measured to be
1.08.+-.0.02 g cm.sup.-3.
[0121] B. Separation of CoMoCAT-Grown, SDBS-Encapsulated SWNTs
[0122] Initial SWNT dispersion: 3.8 mg raw CoMoCAT SWNTs were
dispersed in 3.8 mL of 2% w/v sodium dodecylbenzene sulfonate
(SDBS) via horn ultrasonication for 1 hour. Coarse aggregates and
insoluble materials were then removed by a short
ultracentrifugation step. This was implemented by filling one
polycarbonate centrifuge tube (Beckman-Coulter) with 3.0 mL of the
ultrasonicated solution and separating at 27 krpm for 45 minutes
(TLA100.3, 22.degree. C.). Following the short ultracentrifugation,
the top 2.5 mL of each centrifuge tube was decanted and saved for
later separation in density gradients.
[0123] Density gradient centrifugation: The Beckman TLA100.3 rotor
was utilized for this sorting experiment. A gradient was formed
directly in a TLA100.3-sized polycarbonate centrifuge tube
(Beckman-Coulter) by layering. Three discrete solutions of 1.0 mL
were layered on top of each other in the centrifuge tubes by hand
using a Pasteur pipette. The bottom layer consisted of 40% w/v
iodixanol, 2% w/v SDBS. The middle layer consisted of 20% w/v
iodixanol, 2% w/v SDBS. The top layer consisted of 10% w/v
iodixanol and 2% w/v SDBS. Specifically, this layer was created by
mixing 166 .mu.L 60% w/v iodixanol with 834 .mu.L of SWNTs
dispersed in 2% w/v SDBS.
[0124] After layering, the gradient was tilted to .about.80 degrees
from vertical for 1 hour to allow for diffusion of iodixanol into
an approximately linear profile. After the diffusion step, sorting
was induced by ultracentrifugation at 64 krpm for 9 hours at
22.degree. C.
[0125] Fractionation: After sorting via density gradient
ultracentrifugation, the gradient was fractionated into 25 .mu.L
segments. Each fraction was diluted to 1 mL and optically
characterized as described above.
[0126] FIG. 8 illustrates the separation of SDBS-encapsulated
CoMoCAT-synthesized SWNTs via density gradient ultracentrifugation.
FIG. 8(a) is a photograph of the centrifugation tube after a
one-step separation. Referring to FIG. 8(a), it can be seen that,
in contrast to SC-encapsulated SWNTs, all of the SDBS-encapsulated
SWNTs are compressed into a narrow black band. In the corresponding
optical spectra (FIG. 8(b), it can also be seen that neither
diameter nor band gap separation is indicated. The difference in
density from the top fraction to the bottom fraction was measured
to be 0.096 g cm.sup.-3, and the density for the top fraction was
measured to be 1.11.+-.0.02 g cm.sup.-3.
[0127] C. Separation of CoMoCAT-Grown SWNTs Using Other
Single-Surfactant Systems
[0128] Following procedures similar to those described above but
using three other single-surfactant systems, a similar correlation
between diameter and density was observed for the cases of sodium
deoxycholate (FIG. 9(a)) and sodium taurodeoxycholate (FIG. 9(b)).
However, for the case of sodium dodecyl sulfonate (SDS) (FIG.
9(c)), separation as a function of diameter was absent.
[0129] D. Separation of Laser Ablation-Synthesized SWNTs
[0130] SWNTs in the 11-16 .ANG. diameter range synthesized by the
laser ablation growth method were purified using SC-encapsulations.
Procedures identical to those described in Section A above were
used except for the following changes: (1) SWNTs grown by the
laser-ablation method were used instead of SWNTs grown by the
CoMoCAT method; (2) 10.0% and 25.0% w/v iodixanol solutions were
used instead of the 7.5% and 22.5% w/v iodixanol solutions,
respectively, during linear density gradient formation; (3) the
solution containing SWNTs was prepared as a 24.1% w/v iodixanol
solution rather than a 20.0% w/v iodixanol solution before
insertion into the gradient.
[0131] FIG. 10 illustrates the separation of SC-encapsulated laser
ablation-synthesized SWNTs via density gradient
ultracentrifugation. FIG. 10(a) is a photograph of the
centrifugation tube after a one-step separation. Referring to FIG.
10(a), colored bands of SWNTs are apparent, suggesting separation
by electronic-structure. Specifically, five or more colored bands
are visible (from top to bottom: a first green band, an orange
band, a yellow band, a second green band, and a brown band). Also
the trend of increasing density with increasing diameter also was
observed. The difference in density from the top fraction to the
bottom fraction was measured to be 0.026 g cm.sup.-3, and the
density for the bottom fraction was measured to be 1.08.+-.0.02 g
cm.sup.-3.
[0132] FIG. 10(b) shows the optical absorbance spectra (1 cm path
length) after separation using density gradient
ultracentrifugation. SWNTs before purification is depicted as a
dashed, gray line. In the optical absorbance spectra of FIG. 10(b),
the second and third order semiconducting and first order metallic
optical transitions are labeled S22, S33, and M11, respectively.
The diameter separation was observed as a red-shift in the emphasis
in the S22 optical transitions (second-order optical absorbance
transitions for semiconducting SWNTs, 800-1075 nm) with increasing
density. Moreover, an enrichment of these SWNTs by electronic type
was also detected. In the most buoyant fractions, an enhancement in
concentration of semiconducting SWNTs was observed with respect to
metallic SWNTs, which have first-order optical absorbance
transitions ranging from 525 to 750 nm (the metallic SWNTs (M11)
were depleted in the most buoyant fractions).
Example 2
Multiple Cycles of Density Gradient Ultracentrifugation
[0133] The degree of isolation achieved after a single step of the
technique is limited by the diffusion of SWNTs during
ultracentrifugation, mixing during fractionation, and statistical
fluctuations in surfactant encapsulation. To overcome these
limitations and improve the sorting process, the centrifugation
process can be repeated for multiple cycles. For example, after the
first iteration of density gradient centrifugation, subsequent
fractionation, and analysis of the optical absorbance spectra of
the collected fractions, the fractions containing the largest
concentration of the target chirality or electronic type of
interest can be combined. The density and volume of the combined
fractions can then be adjusted by the addition of iodixanol and
water, both containing surfactant/encapsulation agent (usually at
2% w/v surfactant). This sorted sample can then be inserted into a
second density gradient, centrifuged, and the entire protocol can
be repeated. This process can be repeated for as many iterations as
desired. This enables the optimal isolation of a targeted
electronic type or a specific chirality of SWNT.
[0134] To demonstrate the approach, the enrichment of the (6, 5)
and (7, 5) chiralities of semiconducting SWNTs was targeted (7.6
and 8.3 .ANG. in diameter, respectively), and photoluminescence
spectra were obtained to show quantitatively the improvements in
separation by repeated centrifugation.
[0135] Initial SWNT dispersion: Four solutions, each consisting of
6.2 mg raw CoMoCAT SWNTs and 6.2 mL of 2% w/v sodium cholate, were
created. The SWNTs in each solution were dispersed via horn
ultrasonication for 1 hour as described previously. Coarse
aggregates and insoluble materials were then removed by a short
ultracentrifugation step. This was implemented by filling eight
polycarbonate centrifuge tubes (Beckman-Coulter) with 3.0 mL of the
ultrasonicated solutions and separating at 54 krpm for 14 minutes
(TLA100.3, 22.degree. C.). Following the short ultracentrifugation,
the top 2.5 mL of each of the eight centrifuge tubes was decanted
and saved for concentration.
[0136] After initial dispersion, these SWNTs were then concentrated
in preparation for the first iteration of density gradient
ultracentrifugation. Six SW41 polyclear centrifuge tubes
(Beckman-Coulter) were each filled with 8.62 mL of 60% w/v
iodixanol, 2% w/v SC, which served as stop layers. Then, on top of
each of these dense stop layers, 3.0 mL of initially dispersed
SWNTs was added to fill the centrifuge tubes to .about.4 mm from
their tops. The SWNTs were then concentrated via
ultracentrifugation at 41 krpm at 22.degree. C. for 7.5 hours, as
depicted in FIG. 5. Afterwards, each centrifuge tube was
fractionated and the concentrated SWNTs were extracted in 0.7 cm
(0.98 mL) fractions. The end result was a concentration by a factor
of three. All of the concentrated fractions were combined and the
buoyant density of the combined fractions containing the
concentrated SWNTs measured 1.12 g cm.sup.-3. The density of this
combined solution was then reduced to 1.105 g cm.sup.-3 by adding
2% w/v SC.
[0137] Density gradient centrifugation: The Beckman SW41 rotor was
utilized. Gradients were formed directly in SW41-sized polyclear
centrifuge tubes (Beckman-Coulter) using the linear gradient maker.
Underlayers or overlayers were not used. Stock solutions of
.about.100 mL of 8.9% w/v iodixanol, 2% w/v SC and of 25.9% w/v
iodixanol, 2% w/v SC were prepared. 5.5 mL of each was added to the
mixing and reservoir chambers of the linear gradient maker,
respectively. The linear gradient was delivered from the output of
the gradient maker to the bottom of a centrifuge tube using a piece
of glass tubing.
[0138] After the formation of a gradient, 0.88 mL of SWNT solution
(1.105 g cm.sup.-3) was slowly inserted (0.1 mL min.sup.-1) via a
syringe needle and the height of the syringe needle was adjusted
such that the SWNT solution was inserted where its density matched
that of the local density gradient. Sorting occurred via
ultracentrifugation at 40 krpm for 24 hours at 22.degree. C.
[0139] Fractionation: After sorting via density gradient
ultracentrifugation, each gradient was fractionated into 0.66 mm
segments (93 .mu.L). Some fractions were diluted to 1 mL and
optically characterized. Other fractions were not diluted and were
saved for further sorting in subsequent density gradients.
[0140] Iterations: 1.sup.st iteration: Concentrated tubes were
separated in six gradients. All six were prepared and fractionated
identically. One of the six sets of fractions was diluted for
optical characterization to determine the fractions most enriched
in the (6, 5) or (7, 5) chiralities. Once this determination had
been made, the best six fractions enriched in either the (6, 5) or
(7, 5) chirality from each of the remaining five sets of fractions
were combined. The densities of (6, 5) and the (7, 5) combinations
were adjusted to 1.105 g cm.sup.-3.
[0141] 2.sup.nd iteration: The best (6, 5) and (7, 5) fractions
resulting from the first iteration were then separated in fresh
density gradients. The SWNTs enriched in the (6, 5) chirality were
separated in three gradients and the SWNTs enriched in the (7, 5)
chirality were separated in three gradients. Identical
ultracentrifuge parameters were used for the first and second
iterations. Again after density gradient ultracentrifugation, one
set of fractions was diluted for the measurement of optical
absorbance spectra to determine the fractions that were optimally
enriched in the desired, targeted chirality of interest. Each of
the best (6, 5) fractions and the best (7, 5) fractions were
combined and their density was adjusted to 1.105 g cm.sup.-3.
[0142] 3.sup.rd iteration: The best (6, 5) and (7, 5) fractions
resulting from the second iteration were then separated in fresh
density gradients identical to those used in the first iteration,
except 20 mM Tris was added throughout each gradient to raise the
pH to 8.5 to optimize the isolation of the (7, 5) chirality of SWNT
(FIG. 7.14b). One gradient was run for the (6, 5) SWNTs and another
for the (7, 5) SWNTs. Each gradient was fractionated into 0.066 mm
fractions, and all the fractions were diluted and analyzed using
photoluminescence techniques as described below.
Measurement of Photoluminescence Spectra
[0143] Photoluminescence spectra were measured using a Horiba
Jobin-Yvon (Edison, N.J.) Nanolog-3 fluorimeter with a double
excitation-side and a single emission-side monochromator, both set
to band pass slit widths ranging from of 10-14.7 nm. The
photoluminescence was detected using a liquid nitrogen cooled
InGaAs photodiode. A 3-mm thick RG-850 Schott glass filter (Melles
Griot, Carlsbad, Calif.) was used to block second order Rayleigh
scattering in the emission monochromator. A 495-nm cutoff,
long-pass filter (FGL495S, Thorlabs, Newton, N.J.) was used to
block second order Rayleigh scattering in the excitation
monochromators. Matrix scans in which the excitation wavelength was
varied from 525 to 825 nm in 6 nm increments and the emission
wavelength was varied from 900 to 1310 nm were collected with
integration times ranging from 0.5-2.5 s. To determine
concentration from emission-excitation matrices, excitation scans
were interpolated along the excitation axis through the E.sub.22
transition at an emission wavelength corresponding to the E.sub.11
wavelength. FIG. 11 illustrates the fitting of photoluminescence
spectra for determination of relative SWNT concentration. FIG.
11(a) plots photoluminescence intensity as a function of excitation
and emission wavelengths (vertical and horizontal axes,
respectively). FIG. 11(b) plots photoluminescence intensity versus
excitation wavelength at 740 nm. Both broadly varying background
photoluminescence from off resonance SWNTs and emission from the
(7, 5) semiconducting SWNT were observed (black arrows). To
minimize the effects of the slowly varying background, a derivative
method similar to that applied to analyze absorbance spectra was
then applied to extract the relative concentration of specific (n,
m) chiralities. Specifically, the partial derivative of
photoluminescence intensity versus excitation wavelength was
computed (FIGS. 11(c) and 11(d)). The strength of the (7, 5)
chirality (proportional to concentration) was determined from the
amplitude of the partial derivative, depicted as a black line in
FIG. 11(d). The effects of re-absorption of emitted
photoluminescence and the decay the excitation beam intensities
were also corrected.
Analysis of Photoluminescence Spectra
[0144] The data obtained in this example illustrate how successive
separations of SC-encapsulated SWNTs can lead to much improved
isolation of specific, targeted chiralities and produce
corresponding increasingly narrow diameter distributions of
SWNTs.
[0145] FIG. 12 depicts the photoluminescence intensity of
semiconducting SWNTs as a function of excitation and emission
wavelengths before and after each of three iterations of density
gradient centrifugation. After each iteration, the relative
concentrations of the (6, 5) and (7, 5) chiralities of
semiconducting SWNTs was observed to have increased. After
enriching the (6, 5) chirality (7.6 .ANG.) three times, bulk
solutions of the SWNTs were achieved in which >97% of the SWNTs
are of the (6, 5), (9, 1), and (8, 3) chiralities (7.6 .ANG., 7.6
.ANG., and 7.8 .ANG. in diameter, respectively) (Table 2). In other
words, >97% of the SWNTs isolated from the third iteration were
within 0.2 .ANG. of the mean diameter (compared to 62.3% from the
initial population, 86% after the 1.sup.st iteration, and 88.6%
after the 2.sup.nd iteration). The (7, 5) optimization rendered the
(7, 5) chirality dominant after repeated separations. Further
improvements in purity can be expected with additional cycles.
Table 2 below shows the quantitative concentrations of individual
chiralities of SWNTs as determined through analysis of the
photoluminescence spectra using the partial derivative method
described above.
TABLE-US-00002 TABLE 2 Concentration of (n, m) chiralities of SWNTs
as determined from photoluminescence spectra depicted in FIG. 12.
(6, 5) optimization (7, 5) optimization Initial 1.sup.st 2.sup.nd
3.sup.rd 1.sup.st 2.sup.nd 3.sup.rd (6, 5) 43.1% 70.2% 69.7% 83.6%
37.4% 26.6% 24.3% (9, 1) 2.4% 2.5% 3.0% 2.4% 1.8% 1.5% 2.2% (8, 3)
16.8% 13.3% 15.9% 11.0% 12.7% 10.5% 10.3% (9, 2) 0.9% 0.5% 0.7%
0.0% 1.3% 0.8% 1.3% (7, 5) 21.1% 8.1% 4.0% 0.7% 27.3% 40.5% 58.6%
(8, 4) 4.9% 3.5% 4.7% 1.5% 6.5% 6.7% 0.9% (10, 2) 1.6% 1.4% 1.6%
0.6% 2.0% 3.0% 0.1% (7, 6) 5.0% 0.4% 0.3% 0.1% 5.2% 6.8% 1.8% (9,
4) 1.6% 0.0% 0.0% 0.0% 3.5% 0.9% 0.0% (8, 6) 1.6% 0.0% 0.0% 0.0%
1.5% 1.8% 0.0% (9, 5) 0.3% 0.0% 0.0% 0.0% 0.3% 0.6% 0.0% (8, 7)
0.7% 0.1% 0.1% 0.0% 0.4% 0.3% 0.4%
[0146] FIG. 13 shows optical spectra corresponding to the
photoluminescence spectra in FIG. 12. FIG. 13(a) shows absorbance
spectra from the (6, 5) optimization. Starting from the unsorted
material (dashed grey line, unsorted), the relative strengths of
the (6, 5) chirality optical transitions at 471 nm and 982 nm
(highlighted) are increasingly reinforced with each iteration. FIG.
13(b) shows absorbance spectra from the (7, 5) optimization. Over
three iterations of sorting, the (7, 5) optical transition at 1031
nm (highlighted) is strongly enhanced compared to the unseparated
material (dashed grey line, unsorted).
Example 3
Adjustment of pH and Addition of Co-Surfactants
[0147] While the purification of SWNTs can be significantly
enhanced via multiple cycles of ultracentrifugation as demonstrated
in Example 2 above, further improvements can be realized by
optimizing the effectiveness of a single cycle through tuning of
the structure-density relationship for SWNTs. For example, by
adjusting the pH or by adding competing co-surfactants to a
gradient, the purification of a specific diameter range or
electronic type can be targeted. In this example, improvements in
isolating SWNTs of specific, targeted diameters and electronic
types were demonstrated by separating SC-encapsulated
CoMo-CAT-grown SWNTs at pH 7.4 versus at pH 8.5, and using a
co-surfactant system (1:4 SDS:SC (by weight) and 3:2 SDS:SC (by
weight)) to separate CoMoCAT-grown and laser ablation-synthesized
SWNTs. Co-surfactant systems having other ratios also can be used.
For example, the ratio (by weight) of an anionic alkyl amphiphile
(e.g., SDS, SDBS, or combinations thereof) to a bile salt (e.g.,
SC, sodium deoxycholate, sodium taurodeoxycholate, or combinations
thereof) can be about 1:10 to about 2:1, such as about 1:8, about
1:6, about 1:4, about 1:3, about 1:2, about 3:4, about 1:1, about
5:4, about 6:5, about 3:2, about 7:4, about 2:1. In certain
embodiments, the ratio can be about 1:10 to about 1:2, such as
about 1:8 to about 1:3. In other embodiments, the ratio can be
about 5:4 to about 2:1, such as about 6:5 to about 7:4.
A. Effect of pH
Procedures
[0148] Separation of SC-encapsulated CoMoCAT-grown SWNTs at pH 7.4:
Same procedures as those described in Example 1, Section A were
used.
[0149] Separation of SC-encapsulated CoMoCAT-grown SWNTs at pH 8.5:
Same procedures as those described in Example 1, Section A were
used except 20 mM Tris was added throughout the gradient to raise
the pH to 8.5 (but not during the initial SWNT dispersion
phase).
Analysis
[0150] The relative concentration of several different diameters
(7.6 .ANG.-(6, 5) as open triangles, 8.3 .ANG.-(7, 5) as open
circles, and 9.8/10.3 .ANG.-(9, 5)/(8, 7) as open star symbols) of
SWNTs is plotted against density for the cases of SC-encapsulated
SWNTs at pH 7.4 in FIG. 14(a) and of SC-encapsulated SWNTs at pH
8.5 in FIG. 14(b). Concentrations were determined from absorbance
spectra via the derivative method described above (FIG. 6 and FIG.
7(b)). The density for the fractions with the highest (6, 5)
chirality relative concentration was measured to be 1.08.+-.0.02 g
cm.sup.-3.
[0151] Comparing FIG. 14(b) with FIG. 14(a), it can be seen that by
increasing the pH to 8.5, the SWNTs near 8.3 .ANG. in diameter
shifted to more buoyant densities, enabling optimal separation of
SWNTs in the 9.8/10.3 .ANG. range ((9, 5)/(8, 7) chiralities).
B. Use of Co-Surfactant Systems
Procedures
[0152] Separation of CoMoCAT-grown SWNTs based on nanotube diameter
dimensions using a co-surfactant system including 1:4 SDS:SC (by
weight): Same procedures as those described in Example 1, Section
A, were used except for the following changes: (1) 15.0% and 30.0%
w/v iodixanol solutions were used instead of the 7.5% and 22.5% w/v
iodixanol solutions, respectively, during linear density gradient
formation; (2) the solution containing SWNTs was prepared as a
27.5% w/v iodixanol solution rather than a 20.0% w/v iodixanol
solution before insertion into the gradient; and (3) a 1:4 ratio by
weight of SDS:SC, 2% w/v overall, was utilized during density
gradient ultracentrifugation instead of a single surfactant
solution of only 2% w/v SC. Thus, each part of the gradient
contained 0.4% w/v SDS and 1.6% w/v SC. However, the SWNTs were
still initially dispersed via ultrasonication in single surfactant
solutions of SC and that co-surfactant, in all cases SDS, was only
introduced at the density gradient ultracentrifugation stage.
[0153] Separation of HiPCO-grown SWNTs based on nanotube diameter
dimensions using a co-surfactant system including 1:4 SDS:SC (by
weight): Same procedures as those described immediately above for
separation of CoMoCAT-grown SWNTs were followed except that
HiPCO-grown SWNTs (raw, not purified) from Carbon Nanotechnologies,
Inc. were used rather than CoMoCAT-grown SWNTs.
[0154] Separation of laser ablation-synthesized SWNTs based on
electronic type (semiconducting) using a co-surfactant system
including 1:4 SDS:SC (by weight): Same procedures as those
described in Example 1, Section A were used except for the
following changes: (1) SWNTs grown by the laser-ablation method
were used instead of SWNTs grown by the CoMoCAT method; (2) 15.0%
and 30.0% w/v iodixanol solutions were used instead of the 7.5% and
22.5% w/v iodixanol solutions, respectively, during linear density
gradient formation; (3) the solution containing SWNTs was prepared
as a 27.5% w/v iodixanol solution rather than a 20.0% w/v iodixanol
solution before insertion into the gradient; and (4) a 1:4 ratio by
weight of SDS:SC, 2% w/v overall, was utilized during density
gradient ultracentrifugation instead of a single surfactant
solution of only 2% w/v SC. Thus, each part of the gradient
contained 0.4% w/v SDS and 1.6% w/v SC.
[0155] Separation of laser ablation-synthesized SWNTs based on
electronic type (semiconducting) using a co-surfactant system
including 3:7 SDS:SC (by weight): Same procedures as those
described immediately above were followed, except that a 3:7 ratio
by weight of SDS:SC, 2% w/v overall, was utilized during density
gradient ultracentrifugation instead of the 1:4 SDS:SC, 2% w/v
overall, co-surfactant system. Thus, each part of the gradient
contained 0.6% w/v SDS and 1.4% w/v SC.
[0156] Separation of laser ablation-synthesized SWNTs based on
electronic type (metallic) using a co-surfactant system including
3:2 SDS:SC (by weight): Same procedures as those described in
Example 1, Section A were used except for the following changes:
(1) SWNTs grown by the laser-ablation method were used instead of
SWNTs grown by the CoMoCAT method; (2) 20.0% and 35.0% w/v
iodixanol solutions were used instead of the 7.5% and 22.5% w/v
iodixanol solutions, respectively, during linear density gradient
formation; (3) the solution containing SWNTs was prepared as a
32.5% w/v iodixanol solution rather than a 20.0% w/v iodixanol
solution before insertion into the gradient; and (4) a 3:2 ratio by
weight of SDS:SC, 2% w/v overall, was utilized during density
gradient ultracentrifugation instead of a single surfactant
solution of only 2% w/v SC. Thus, each part of the gradient
contained 1.2% w/v SDS and 0.8% w/v SC.
[0157] Separation of laser ablation-synthesized SWNTs of three
different origins based on electronic type (semiconducting) using a
co-surfactant system including 1:4 SDS:SC (by weight): Same
procedures as those described above in connection with separation
of laser ablation-synthesized SWNTs based on electronic type
(semiconducting) using a co-surfactant system including 1:4 SDS:SC
(by weight) were followed except that SWNTs of three different
origins were tested: (1) raw, unpurified laser ablation-synthesized
SWNTs obtained from Carbon Nanotechnologies, Inc.; (2) nitric acid
purified laser ablation-synthesized SWNTs obtained from IBM (Batch
A); and (3) nitric acid purified laser ablation-synthesized SWNTs
obtained from IBM (Batch B).
[0158] For co-surfactant based separation by electronic type, the
gradient-portion linearly varied from 15% w/v (1.08 g cm.sup.-3) at
the top to 30% w/v (1.16 g cm.sup.-3) at the bottom or from 20% w/v
(1.11 g cm.sup.-3) at the top to 35% w/v (1.19 g cm.sup.-3) at the
bottom.
Analysis
[0159] 1. Separation of CoMoCAT-Grown SWNTs Based on Nanotube
Diameter Dimensions Using a Co-Surfactant System
[0160] Similar to FIGS. 14(a) and 14(b), the relative concentration
of several different diameters (7.6, 8.3, and 9.8/10.3 .ANG.) of
SWNTs is plotted against density for a mixture of 1:4 SDS:SC (by
weight) in FIG. 14(c). Comparing FIG. 14(c) to FIG. 14(a), it can
be seen that by adding SDS to compete with the SC for non-covalent
binding to the nanotube surface, the SWNTs in the 8.3 and 9.8/10.3
.ANG. diameter regime shifted to significantly larger buoyant
densities, enabling optimal separation of SWNTs near 7.6 .ANG. in
diameter ((6, 5) chirality).
[0161] 2. Separation of HiPCO-Grown SWNTs Based on Nanotube
Diameter Dimensions Using a Co-Surfactant System
[0162] FIG. 15(a) depicts the photoluminescence intensity of a
heterogeneous population of HiPCO-grown SWNTs as a function of
excitation and emission wavelengths before density gradient
centrifugation. As shown in FIG. 15(a), one of the strongest
signals were observed at an emission wavelength of about 980 nm
(and an excitation wavelength of about 570 nm), which corresponds
to a nanotube diameter dimensions of about 7.5 .ANG.. A barely
noticeable signal was observed at an emission wavelength of about
1190 nm (and an excitation wavelength of about 800 nm), and at an
emission wavelength of about 1210 nm (and an excitation wavelength
of about 790 nm), both of which correspond to a nanotube diameter
dimensions of about 10.5 .ANG..
[0163] Following density gradient centrifugation using a
co-surfactant system including 1:4 SDS:SC (by weight), two
separation fractions were obtained. The photoluminescence spectra
of the two separation fractions are shown in FIGS. 15(b) and 15(c),
respectively. As shown in FIG. 15(b), one of the two separation
fractions contained predominantly nanotubes that emit at an
emission wavelength in the range of about 960 nm to about 980 nm.
More specifically, the strongest signal was observed at an emission
wavelength of about 980 nm (and an excitation wavelength of about
570 nm). The spectra indicate that this separation fraction
contained predominantly single-walled carbon nanotubes having a
diameter dimension of about 7.5 .ANG.. By comparison, in the
spectra shown in FIG. 15(c), a number of signals were observed at
different emission and excitation wavelengths. However, the signals
within the emission wavelength range of about 960 nm to about 980
nm were highly suppressed, while the signals at the
emission/excitation wavelengths of about 1190/800 nm and about
1210/790 nm (which were barely noticeable in FIG. 15(a)) have
become the strongest, indicating that in this separation fraction,
the concentration of single-walled carbon nanotubes having a
diameter dimension of about 10.5 .ANG. has considerably increased
compared to the pre-sorted sample. Accordingly, the spectra of FIG.
15 together show that separation by nanotube diameter dimensions
also was possible with HiPCO-grown SWNTs, and can be achieved with
good results using, for example, the co-surfactant system described
above.
[0164] 3. Separation of Laser Ablation-Synthesized SWNTs Based on
Electronic Type Using a Co-Surfactant System
[0165] Co-surfactant populations were observed to have an even
greater effect on the optimization of metal-semiconductor
separation for SWNTs in the 11-16 .ANG. diameter regime. FIG. 16(a)
is a photograph of laser-ablation-synthesized SWNTs separated in a
co-surfactant system (1:4 SDS:SC). As shown in FIG. 16(a), only
three bands were observed. The difference in density between the
two bands was measured to be 0.006 g cm.sup.-3, and the density for
the top band was measured to be 1.12.+-.0.02 g cm.sup.-3. From the
measured optical absorbance spectra (FIG. 16(b)), it appears that
the top band (orange hue) consists of predominantly semiconducting
SWNTs (plotted in blue in FIG. 16(b)), and that the band just below
the top band (green hue) is highly enriched in metallic SWNTs,
although some semiconducting SWNTs remain (plotted in red in FIG.
16(b)). The absorbance spectrum of the heterogeneous mixture before
sorting is plotted as a dashed grey line in FIG. 16(b).
[0166] It was observed that further tuning of the co-surfactant
mixture to a 3:2 SDS:SC ratio permitted significantly improved
isolation of metallic laser ablation synthesized SWNTs.
Improvements with isolation of semiconducting laser ablation
synthesized SWNTs also were observed when the 1:4 SDS:SC
co-surfactant mixture was replaced with a 3:7 SDS:SC co-surfactant
mixture. In FIG. 17, spectra corresponding to primarily metallic
(3:2 SDS:SC, plotted as open circles) SWNTs and primarily
semiconducting (3:7 SDS:SC, plotted as open triangles) SWNTs are
shown. Improvements in the absorption signal in the M11 range can
be more clearly seen in FIG. 18 (S6), which includes the
unoptimized spectrum from FIG. 16(b) using the co-surfactant
mixture of 1:4 SDS:SC (as open star symbols) and the optimized
spectrum from FIG. 17 using the co-surfactant mixture of 3:2 SDS:SC
(as open circles). The arrows highlight the strengthening of the
signal in the M11 range, and the suppression of the signals in the
S33 and S22 ranges.
[0167] 4. Separation Based on Electronic Type Demonstrated by Laser
Ablation-Synthesized SWNTs of Different Sources
[0168] FIG. 19 compares the optical absorbance spectra of unsorted
laser-ablation-synthesized SWNTs with sorted semiconducting
laser-ablation-synthesized SWNTs, where the
laser-ablation-synthesized SWNTs were further obtained from three
different sources: raw, unpurified laser ablation-synthesized SWNTs
obtained from Carbon Nanotechnologies, Inc. (Batch A); nitric acid
purified laser ablation-synthesized SWNTs obtained from IBM (Batch
B); and nitric acid purified laser ablation-synthesized SWNTs
obtained from IBM (Batch C). The three sorted spectra are
comparable in their general profiles to the sample shown in FIG.
16. A strong isolation of semiconducting SWNTs was observed in each
of the sorted spectra regardless of the source of the samples.
However, while all the results were similar, subtle differences in
the suppression of the metallic SWNTs are apparent. In addition,
the enrichment of semiconducting SWNTs and the removal of metallic
SWNTs appear to be better when nitric acid purified laser
ablation-synthesized SWNTs were used (Batches B and C), and worse
when raw, unpurified laser ablation-synthesized SWNTs were used
(Batch A).
Example 4
Quantitative Analysis of Separation by Electronic Type
[0169] In this example, new spectra of primarily semiconducting and
metallic laser ablation-synthesized SWNTs were obtained with
improved signal-to-noise ratio. The sorted solutions were prepared
using procedures analogous to those described in Example 3, Section
B, but at a higher concentration which led to an improvement in the
signal-to-noise ratio given a fixed background noise level.
[0170] FIG. 20 shows the optical absorption spectra of unsorted (as
open star symbols), sorted metallic (as open triangles), and sorted
semiconducting (as open diamond symbols) SWNTs. The asterisk symbol
at about 900 nm identifies optical absorption from spurious
semiconducting SWNTs. The asterisk symbol at about 600 nm
identified optical absorption from spurious metallic SWNTs.
[0171] The amplitude of absorption from the M 11 transitions
(475-700 nm) and the S22 transitions (800-1150 nm) was used to
determine the relative concentration of semiconducting and metallic
SWNTs, respectively, in each sample (FIG. 20). The measured
amplitude of absorption was determined by subtracting the
background absorption, which was determined by linearly
interpolating the background underneath an absorption peak. FIGS.
21-23 show the background baseline from which the amplitude of
absorption was subtracted to obtain the measured amplitude. Because
equal masses or concentrations of metallic and semiconducting SWNTs
will have different strength of optical absorbance, the amplitude
of absorption of metallic SWNTs first had to be scaled for relative
comparison with the amplitude of absorption of semiconducting
SWNTs. The scaling coefficient was determined from the unsorted
sample, which was known to be composed of 66.7% semiconducting
SWNTs and 33.3% metallic SWNTs.
[0172] Additionally, in determining the relative concentration of
semiconducting and metallic SWNTs in each sample, three assumptions
were made: (i) the mass of SWNTs is linearly proportional to the
amplitude of optical absorption; (ii) the background absorption can
be linearly interpolated; (iii) similar diameter ranges of SWNTs
exist before and after sorting (dissimilar diameter ranges would
affect width of absorption in the M11 and S22 ranges, invalidating
assumption (i)).
[0173] Table 3 below shows that in the sample optimized for
separation of metallic SWNTs (FIG. 20), 99.3% of the SWNTs were
metallic and 0.7% of the SWNTs were semiconducting. In the sample
optimized for separation of semiconducting SWNTs (FIG. 20), 97.4%
of the SWNTs were semiconducting and 2.6% of the SWNTs were
metallic.
TABLE-US-00003 TABLE 3 Relative concentration of sorted metallic
and semiconducting SWNTs as determined from optical absorption
spectra depicted in FIG. 20. Calculated compositions (scaled by
metallic Data from measured optical spectra (measured amplitude of
renormalization absorbance by linearly interpolating background
absorbance) coefficient) SORTED METALLIC Metallic Semiconducting
Semiconducting Absorbance Absorbance nanotubes (mass) .lamda. (nm)
A .lamda. (nm) A 0.7% Bkgd 1 425 0.395 839 0.126 Metallic nanotubes
(mass) Bkgd 2 750 0.166 1069 0.075 99.3% Peak 602 1 0.271 878 0.126
0.117 Amplitude 0.729 Amplitude 0.009 SORTED SEMICONDUCTING
Metallic Semiconducting Semiconducting Absorbance Absorbance
nanotubes (mass) .lamda. (nm) A .lamda. (nm) A 97.4% Bkgd 1 591
0.508 623 0.491 Metallic nanotubes (mass) Bkgd 2 620 0.491 1182
0.296 2.6% Peak 602 0.511 0.502 943 0.100 0.379 Amplitude 0.009
Amplitude 0.620 UNSORTED Metallic Semiconducting Semiconducting
Absorbance Absorbance nanotubes (mass) .lamda. (nm) A .lamda. (nm)
A 66.7% Bkgd 1 569 0.673 759 0.528 Metallic nanotubes (mass) Bkgd 2
740 0.541 1150 0.347 33.3% Peak 647 0.735 0.612 943 0.875 0.443
Amplitude 0.122 Amplitude 0.432 * Metallic renormalization
coefficient (calculated from unsorted sample to produce a 2:1
semiconducting to metallic ratio) = 1.77; .lamda. = wavelength; A =
Absorbance; Bkgd = background.
Example 5
Determination of Typical Yields and Scales
[0174] Typical yields of sorting experiments can be estimated
through optical absorbance spectra taken before and after each step
of the separation process. During the initial dispersion of SWNTs
in SC, roughly one quarter of the as-produced SWNT material is
successfully encapsulated as either individual SWNTs or small
bundles of SWNTs, with the remaining carbonaceous impurities, large
SWNT aggregates, and insoluble species removed after the short
centrifugation step. The solution processed SWNTs can then be
incorporated into density gradients for sorting.
[0175] For each gradient, an average of 400 .mu.L of SWNT solution
(.about.250 .mu.g mL.sup.-1 SWNT loading) is infused into each
centrifuge tube, resulting in .about.100 .mu.g of SWNT starting
material per experiment. It is important to note, however, that
this starting material consists of a mixture of individually
encapsulated SWNTs, which can be sorted by diameter and electronic
type, and of small bundles of SWNTs, for which such separation is
unlikely. As a result, the yield of the separation experiments is
highly dependent on the efficient encapsulation of individual SWNTs
by surfactant.
[0176] The allocation of the starting SWNT material to points in
the density gradient after sorting can be estimated by optical
absorbance spectra of the fractionated material. This approximate
yield is calculated by collecting the absorbance of each fraction
at a wavelength of interest and normalizing by the absorbance of
the starting solution at the same wavelength. For instance, for
laser-ablation-grown SWNTs, we can assess the yield of
semiconducting nanotubes in the 1:4 SDS:SC sorting experiment
(FIGS. 16(a)-(b)) by tracking the starting material-normalized
absorbance at 942 nm, which corresponds to the peak of the second
order semiconductor transitions (FIG. 24(a)). The peak
semiconducting fraction contains >9% of the starting material
(.about.9 .mu.g), corresponding to an overall yield of
approximately 2.3%. An analogous analysis for CoMoCAT diameter
separation in sodium cholate (FIGS. 7(a)-(b)) scanning the optical
absorbance at 982 nm (FIG. 24(b)), the first order transition for
the (6, 5) chirality, reveals that >6% of the starting material
(.about.6 .mu.g) is contained in the fraction with the highest
overall yield of approximately 1.5%.
[0177] Despite the modest yields reported above, a more reasonable
measure of the experimental outcome taking into account only
individually encapsulated SWNTs, excluding bundles incapable of
being sorted, could increase the stated yields by factors of two to
five. Additionally, fractions with highly isolated distributions of
SWNTs are generally located above and below the fractions with the
peak yields; thus, combining this sorted material can further
improve the sorting efficiency. Moreover, the mass of sorted
material produced can be increased three to five times by
concentrating the SWNT solution prior to separation as described in
the Concentration of SWNTs in step gradients section in Example
1.
[0178] Although the methods described herein only succeed in
producing microgram quantities of sorted SWNT materials, there are
definite ways in which the methods of the present teachings could
be expanded to an industrial scale. For instance, by employing a
large-volume, industrial centrifuge capable of g-forces comparable
to the centrifuge used, it could be possible to sort over a gram of
SWNTs at a time. Such centrifuges can accommodate 8 L of solution,
enabling 1 L of SWNT solution to be sorted in a 7 L density
gradient. If the efficiency of individual SWNT encapsulation is
increased and/or the solution is strongly concentrated prior to
sorting, the 1 L of solution could be loaded with 4 g of isolated
SWNTs. Thus, in a single 12 hour centrifugation, gram quantities of
SWNTs could be sorted according to diameter and/or electronic type.
Multiple centrifugations can be run in parallel and/or in series,
and their resultant yields can be added together to achieve
kilogram quantities or more of sorted SWNTs.
Example 6
Fabrication of FETs Using Sorted Metallic and Semiconducting
SWNTs
[0179] In order to demonstrate the applicability of SWNTs separated
in density gradients and to confirm their purification by
electronic type, field-effect transistors (FETs) were fabricated
consisting of percolating networks of thousands of metallic or
semiconducting SWNTs. FIG. 25(a) shows a periodic array of source
and drain electrodes (scale bar 40 .mu.m, gap 20 .mu.m). FIG. 25(b)
is a representative atomic force microscopy (AFM) image of a
percolating SWNT network (scale bar=1 .mu.m). The density of SWNTs
per unit area is >10 times the percolation limit. FIG. 25(c)
shows the geometry of the field-effect transistors (FETs)
fabricated (s=source; g=gate; d=drain).
Fabrication of Electrical Devices
[0180] Electrical devices were fabricated from percolating networks
of semiconducting and metallic SWNTs. The percolating networks were
formed via vacuum filtration of the purified SWNTs dispersed in
surfactant solutions through porous mixed cellulose ester (MCE)
membranes (0.02 .mu.m, Millipore Corporation) following the methods
of Wu et al. (Z. C. Wu et al., Science 305, 1273 (2004)). After
filtration of the SWNT solution, the network was allowed to dry for
30 minutes to set and then was rinsed by 10-20 mL of deionized
water to remove residual surfactant and iodixanol from the network,
leaving a network of bare SWNTs behind.
[0181] The networks on top of the MCE membranes were then
transferred to Si (100) substrates capped with 100 nm
thermally-grown SiO.sub.2 (Silicon Quest International). The MCE
membrane was wet with deionized water and pressed into the
SiO.sub.2 surface (SWNTs in contact with SiO.sub.2) for 2 minutes
between two glass slides. The slides were removed and the MCE
membranes were allowed to dry for several minutes on the SiO.sub.2
substrates. The substrates were then rinsed in 3 sequential acetone
baths for 15 minutes each to dissolve the MCE membranes, followed
by a rinse in methanol. Then, the networks of SWNTs on the
substrates were blown dry in a stream of N.sub.2 gas.
[0182] The densities (SWNTs per unit area) of the networks were
controlled by adjusting the volume of the fractions of SWNTs that
were filtered. Quantitative measurements of the network densities
were determined by measuring the optical density of the SWNTs in
solution before filtration and via atomic force microscopy (AFM)
after filtration and subsequent transfer to substrates.
[0183] Arrays of electrodes (Au, 30 nm) were lithographically
defined on top of the percolating networks using a TEM grid as a
shadow mask (300 mesh, Cu, SPI Supplies, West Chester, Pa.; pitch
83 .mu.m, bar width 25 .mu.m) in an e-beam evaporator. After
evaporation, the substrates were then rinsed in acetone,
2-propanol, and then water, followed by annealing at 225.degree. C.
in air for 20 minutes.
[0184] The percolating networks of metallic and semiconducting
SWNTs were electrically characterized in a field-effect transistor
(FET) geometry using two source-meter units (KE2400, Keithley,
Inc.). A gate bias was applied to the underlying Si substrate,
which served as the gate electrode, to modulate the carrier
concentration in the SWNT network. A bias of up to 5 V was applied
between two of the neighboring electrodes, created from the TEM
grid shadow mask, which served as the source and drain. The gate
leakage current and the source-drain current were both measured. In
all cases, the source-drain current significantly exceeded the gate
leakage current. Sweeps of the gate bias were made from negative to
positive bias. Hysteresis was observed depending on the sweep
direction due to the presence of mobile charge, an effect routinely
observed in SWNT FET devices fabricated on 100 nm thick SiO.sub.2
gate dielectrics.
Measurement of Percolation Density of the SWNT Networks
[0185] For each percolating network, several devices were
characterized via contact mode AFM (512 512 resolution, 3-20 .mu.m
image sizes, contact force <10 nN). During imaging, the contact
force was kept at a minimum to limit the mechanical perturbation of
the network. The images of the networks were analyzed to determine
the percolation density (SWNTs per unit area). Each percolating
pathway was traced to determine the total pathway length per unit
area of the network (FIG. 26). In FIGS. 26(a)-(b), an image and
trace, respectively, of the thin film, semiconducting network
(electrically characterized in FIG. 25(d)) are shown. The trace
corresponds to 22.1 .mu.m of conducting pathway per square .mu.m of
the substrate. For an average SWNT length of 0.45 .mu.m (average
length determined from additional AFM studies of laser-ablation
grown SWNTs separated in density gradients and then isolated on
substrates), this corresponds to a percolation density of .about.50
SWNTs/.mu.m.sup.2, about 10 times larger than the percolation
threshold, .about.5 SWNTs/.mu.m.sup.2. The measured percolation
density of .about.50 SWNTs/.mu.m.sup.2 is an underestimate because
it does not account for multiple SWNTs per pathway due to the
possibility of overlapping SWNTs or small bundles. Such effects are
anticipated as a result of the large van der Waals attraction
expected among SWNTs once their encapsulating surfactant has been
rinsed away during the film formation. The semiconducting networks
were created first and then characterized electrically and via AFM.
Then, to make comparison between the metallic and semiconducting
networks equitable, the metallic networks were created such that
their percolation densities were equal to or less than the
semiconducting network.
[0186] Their average characteristics are plotted in (FIG. 25(d)).
Error bars depict two standard deviations. (For semiconducting
devices n=4; metallic devices n=3).
[0187] The electronic mobility of the semiconducting SWNT networks
was estimated by fitting the source-drain current versus the gate
bias for a fixed source-drain bias in the "on" regime
(V.sub.g<V.sub.T) of the FETs to a straight line (FIG. 25(d),
inset). The following relationship was used:
I.sub.ds=.mu.C.sub.ox*(W/L)*(V.sub.g-V.sub.t)*V.sub.ds where
I.sub.ds is the source-drain current, .mu. is the mobility,
C.sub.ox is the oxide capacitance, W is the channel width, L is the
channel thickness, V.sub.g is the gate bias, V.sub.t is the gate
threshold bias, and V.sub.ds is the source-drain bias.
[0188] An upper bound on the capacitance between the SWNT networks
and the Si substrate was determined by assuming a parallel plate
capacitor geometry (L, W of 20, 63 .mu.m). The linear fit yields a
lower bound for mobility .mu. of >20 cm.sup.2 V.sup.-1 s.sup.-1
(which is comparable to previously reported mobilities for thin
films of as-synthesized mixtures of metallic and semiconducting
SWNTs near their percolation threshold) and a gate threshold
voltage of -20 V. The fit on the mobility is a lower bound because
the assumption of parallel plate capacitance is drastically
overestimating the capacitance, as the SWNT network occupies only a
fraction of the channel area. Furthermore, resistive losses at the
contacts were not taken into account.
Distinctive Behaviors of the Semiconducting and Metallic Films
[0189] At negative gate biases, it was observed that both networks
exhibited similar sheet resistances of about 500 k.OMEGA.
square.sup.-1. However, by varying the voltage applied across the
gate dielectric capacitor (100 nm SiO.sub.2), the resistivity of
the semiconducting network was increased by over 4 orders of
magnitude (on/off ratio >20,000). In contrast, the metallic
networks were significantly less sensitive to the applied gate bias
characterized by on/off ratios of less than two (switching ratios
larger than 1 may indicate perturbations to the electronic
band-structure of the metallic SWNTs at tube-endpoints or tube-tube
contacts or resulting from tube-bending or chemical defects). The
two distinct behaviors of the semiconducting and metallic films
independently confirm the separation by electronic type initially
observed by optical absorption spectroscopy (FIG. 17).
Additionally, the two films establish the applicability of the
method of the present teachings in producing usable quantities of
purified, functional material. For example, a single fraction of
purified semiconducting SWNTs (150 .mu.L) contains enough SWNTs for
20 cm.sup.2 of a thin film network similar to that demonstrated in
FIG. 25, corresponding to >10.sup.11 SWNTs. According to the
present teachings, a population of SWNTs can include about 10 or
more SWNTs, such as >10 SWNTs, >50 SWNTs, >100 SWNTs,
>250 SWNTs, >500 SWNTs, >10.sup.3 SWNTs, >10.sup.4
SWNTs, >10.sup.5 SWNTs, >10.sup.6 SWNTs, >10.sup.7 SWNTs,
>10.sup.8 SWNTs, >10.sup.9 SWNTs, >10.sup.10 SWNTs, or
>10.sup.11 SWNTs. Further, by weight, a population of SWNTs can
have a mass of about 0.01 .mu.g, such as >0.01 .mu.g, >0.1
.mu.g, >1 .mu.g, >0.01 mg, >0.1 mg, >1 g, >10 g, or
>100 g. Such thin film networks have applications as flexible
and transparent semiconductors and conductors. As would be
understood by those skilled in the art, such characterization,
under conditions of the sort described herein, can reflect SWNT
quantities in accordance herewith. Such quantities are
representative of bulk SWNTs available through the present
teachings, and can be a further distinction over prior art methods
and materials.
[0190] The present teachings can be embodied in other specific
forms, not delineated in the above examples, without departing from
the spirit or essential characteristics thereof. The present
teachings can be embodied in other specific forms without departing
from the spirit or essential characteristics thereof. The foregoing
embodiments are therefore to be considered in all respects
illustrative rather than limiting on the present teachings
described herein. Scope of the invention is thus indicated by the
appended claims rather than by the foregoing description, and all
changes that come within the meaning and range of equivalency of
the claims are intended to be embraced therein.
* * * * *