U.S. patent application number 12/536250 was filed with the patent office on 2010-03-25 for methods for sorting nanotubes by wall number.
Invention is credited to Alexander A. Green, Mark C. Hersam.
Application Number | 20100072458 12/536250 |
Document ID | / |
Family ID | 41478472 |
Filed Date | 2010-03-25 |
United States Patent
Application |
20100072458 |
Kind Code |
A1 |
Green; Alexander A. ; et
al. |
March 25, 2010 |
Methods For Sorting Nanotubes By Wall Number
Abstract
The present teachings provide methods for sorting nanotubes
according to their wall number, and optionally further in terms of
their diameter, electronic type, and/or chirality. Also provided
are highly enriched nanotube populations provided thereby and
articles of manufacture including such populations.
Inventors: |
Green; Alexander A.;
(Evanston, IL) ; Hersam; Mark C.; (Evanston,
IL) |
Correspondence
Address: |
K&L Gates LLP
STATE STREET FINANCIAL CENTER, One Lincoln Street
BOSTON
MA
02111-2950
US
|
Family ID: |
41478472 |
Appl. No.: |
12/536250 |
Filed: |
August 5, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61086302 |
Aug 5, 2008 |
|
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|
Current U.S.
Class: |
257/24 ;
257/E29.168; 423/447.1; 423/447.2; 977/750; 977/752; 977/845;
977/938 |
Current CPC
Class: |
B82Y 30/00 20130101;
C01B 32/172 20170801; C01B 2202/06 20130101; B82Y 40/00 20130101;
B82Y 15/00 20130101; B03D 3/00 20130101; B01D 21/262 20130101 |
Class at
Publication: |
257/24 ;
423/447.1; 423/447.2; 977/845; 977/750; 977/752; 257/E29.168;
977/938 |
International
Class: |
H01L 29/66 20060101
H01L029/66; D01F 9/12 20060101 D01F009/12 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The United States Government has certain rights to this
invention pursuant to Grant Nos. EEC-0647560 and DMR-0706067 from
the National Science Foundation, both to Northwestern University.
Claims
1. A method of separating carbon nanotubes by wall number, the
method comprising: centrifuging a carbon nanotube composition in
contact with a first fluid medium comprising a first density
gradient; and separating the carbon nanotube composition into two
or more separation fractions, wherein the carbon nanotube
composition comprises one or more surface active components and a
carbon nanotube population comprising double-walled carbon
nanotubes and carbon nanotubes having a wall number other than two,
and at least one of the two or more separation fractions comprises
a carbon nanotube subpopulation comprising a higher percentage of
double-walled carbon nanotubes than the carbon nanotube
population.
2. The method of claim 1, wherein the two or more separation
fractions are visibly distinguishable by the human eye.
3. The method of claim 1 or 2, wherein the carbon nanotube
population comprises double-walled carbon nanotubes having a mean
outer wall diameter of less than about 1.7 nm.
4. The method of claim 1, wherein the carbon nanotube population
comprises double-walled carbon nanotubes and single-walled carbon
nanotubes having overlapping outer wall diameters.
5. The method of claim 4, wherein at least one of the two or more
separation fractions comprises a carbon nanotube subpopulation
comprising a higher percentage of single-walled carbon nanotubes
than the carbon nanotube population.
6. The method of claim 4, wherein the carbon nanotube composition
is introduced into the first fluid medium at a density less than
about 1.095 g/mL.
7. The method of claim 6, comprising centrifuging an enriched
separation fraction in contact with a second fluid medium
comprising a second density gradient, wherein the enriched
separation fraction comprises a carbon nanotube subpopulation
comprising a higher percentage of double-walled carbon nanotubes
than the carbon nanotube population, wherein the enriched
separation fraction is introduced into the second fluid medium at a
density greater than about 1.115 g/mL.
8. The method of claim 1, wherein the carbon nanotube population
comprises multi-walled carbon nanotubes comprising three or more
walls.
9. A carbon nanotube population comprising greater than about 95%
double-walled carbon nanotubes.
10. A method of separating carbon nanotubes by electronic type, the
method comprising: centrifuging a carbon nanotube composition in
contact with a first fluid medium comprising a first density
gradient; and separating the carbon nanotube composition into two
or more separation fractions, wherein the carbon nanotube
composition comprises two or more surface active components and a
carbon nanotube population comprising double-walled carbon
nanotubes having a semiconducting outer wall (s-DWCNTs), and
double-walled carbon nanotubes having a metallic outer wall
(m-DWCNTs), and the two or more separation fractions comprise a
first separation fraction comprising a carbon nanotube
subpopulation comprising a higher percentage of s-DWCNTs than the
carbon nanotube population, and a second separation fraction
comprising a carbon nanotube subpopulation comprising a higher
percentage of m-DWCNTs than the carbon nanotube population.
11. The method of claim 10, wherein the two or more surface active
components comprise a planar surface active component and a linear
surface active component.
12. The method of claim 10, wherein the carbon nanotube population
comprises semiconducting single-walled carbon nanotubes (s-SWCNTs)
and metallic single-walled carbon nanotubes (m-SWCNTs), and wherein
after the centrifuging step, the carbon nanotube composition is
separated by wall number and by outer wall electronic type into at
least four separation fractions, the at least four separation
fractions comprising the first separation fraction, the second
separation fraction, a third separation fraction comprising a
carbon nanotube subpopulation comprising a higher percentage of
s-SWCNTs than the carbon nanotube population, and a fourth
separation fraction comprising a carbon nanotube subpopulation
comprising a higher percentage of m-SWCNTs than the carbon nanotube
population.
13. The method of claim 12, wherein the relative ratio of the two
or more surface active components is selected to cause metallic
carbon nanotubes to have a lower buoyant density than
semiconducting carbon nanotubes regardless of wall number.
14. The method of claim 12, wherein the relative ratio of the two
or more surface active components is selected to cause
semiconducting carbon nanotubes to have a lower buoyant density
than metallic carbon nanotubes of the same wall number.
15. The method of claim 12, where the relative ratio of the two or
more surface active components is selected to cause metallic carbon
nanotubes to have a lower buoyant density than semiconducting
carbon nanotubes of the same wall number.
16. The method of claim 12, wherein the relative ratio of the two
or more surface active components is selected to cause
semiconducting carbon nanotubes to have a lower buoyant density
than metallic carbon nanotubes regardless of wall number.
17. A population of double-walled carbon nanotubes, wherein greater
than about 50% of the double-walled carbon nanotubes comprise a
metallic outer wall.
18. A transparent conductive film comprising the population of
claim 17.
19. A population of double-walled carbon nanotubes, wherein greater
than about 70% of the double-walled carbon nanotubes comprise a
semiconducting outer wall.
20. A thin film transistor comprising a semiconductor component
comprising the population of claim 19.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of U.S.
Provisional Patent Application Ser. No. 61/086,302, filed on Aug.
5, 2008, the entire disclosure of which is incorporated by
reference herein.
BACKGROUND
[0003] Carbon nanotubes are one-dimensional nanomaterials
consisting of cylinders of graphene. Depending on their diameter
and helicity, carbon nanotubes consisting of a single graphene
tubule, known as single-walled carbon nanotubes (SWCNTs), can
behave either as metals or semiconductors whose band gap varies
inversely with tube diameter. Multi-walled carbon nanotubes, which
consist of multiple concentric graphene cylinders, typically
possess much larger diameters than SWCNTs. Accordingly, these
carbon nanotubes demonstrate metallic or small band gap
semiconducting behavior, and are mechanically stronger than SWCNTs.
Developments in carbon nanotube synthesis have enabled the
preferential production of multi-walled carbon nanotubes consisting
of two walls. These double-walled carbon nanotubes (DWCNTs) can be
synthesized using methods including chemical vapor deposition,
electric arc discharge, and coalescence of chains of C.sub.60
inside SWCNTs. DWCNTs have garnered increasing attention for
applications because their structure provides them with
characteristics situated between those of SWCNTs and multi-walled
carbon nanotubes having three or more walls (MWCNTs). Compared to
SWCNTs and MWCNTs, DWCNTs have demonstrated better performance
parameters in field-effect transistors, improved spatial resolution
and longer scanning lifetimes as atomic force microscope (AFM)
tips, and more desirable field emission characteristics.
[0004] Despite their promising applications, current methods of
synthesizing DWCNTs also produce significant quantities of unwanted
SWCNTs and MWCNTs. Multiple groups have succeeded in increasing the
proportion of DWCNTs following synthesis using high temperature
oxidation, which preferentially destroys the more thermally
unstable SWCNT impurities. However, these oxidative treatments can
degrade the electrical and optical properties of DWCNTs, and are
ineffective at removing MWCNTs because DWCNTs and MWCNTs exhibit
similar thermal stabilities.
[0005] As a result, more refined separation methods are required to
provide nanotube populations that are highly enriched with DWCNTs.
Furthermore, beyond separation by wall number, practical
applications also require DWCNT materials that are enriched
according to chirality, diameter and/or electronic type to ensure
that their electrical and optical properties are uniform.
SUMMARY
[0006] In light of the foregoing, the present teachings relate to
methods of separating nanotubes by wall number. In particular, the
present methods can provide nanotube subpopulations that are
selectively enriched with double-walled nanotubes (DWNTs),
multi-walled nanotubes having three or more walls (MWNTs), or
single-walled nanotubes (SWNTs) from a mixed (or polydisperse)
nanotube population including DWNTs, MWNTs, and SWNTs. Because
MWNTs generally have larger mean outer wall diameters than DWNTs,
and DWNTs generally have larger mean outer wall diameters than
SWNTs, in part, the present methods can be directed to separating
nanotubes by diameter. For nanotubes of different wall numbers that
may have overlapping outer wall diameter ranges, the present
methods are able to separate among them by wall number. In some
embodiments, the present methods also can provide DWNT
subpopulations that are selectively enriched by one or more
characteristics such as a certain outer wall diameter range, a
selected outer wall electronic type, and/or a selected outer wall
chirality (n,m) type.
[0007] Accordingly, in some embodiments, the present methods allow
separation of double-walled carbon nanotubes (DWCNTs) from a mixed
population of carbon nanotubes having different wall numbers,
thereby providing at least one subpopulation of carbon nanotubes
enriched with DWCNTs, that is, the enriched subpopulation has a
higher percentage of DWCNTs compared to the mixed population.
Because of the non-destructive nature of the present methods,
carbon nanotubes other than DWCNTs, e.g., SWCNTs and/or MWCNTs,
also can be separated and recovered from the mixed population,
thereby providing at least one subpopulation of carbon nanotubes
that is enriched with SWCNTs or MWCNTs. Concurrently (i.e., in a
single separation cycle) or in different separation cycles, the
present methods further allow separating the DWCNTs by one or more
characteristics such as diameter, band gap, chirality and/or
electronic type, thereby providing DWCNT subpopulations that are
enriched by selected characteristic(s). The high-purity DWCNT
populations according to the present teachings can offer various
improved properties over prior art DWCNT populations. These
improved properties can be realized when incorporated into devices
and applications such as light emitting diodes, photovoltaics,
electrostatic discharge coatings, atomic force microscope tips,
flat panel displays, touch screens, electromagnetic screening
applications, field-emission displays, biosensors, field-effect
transistors, transparent conductors or functional glass, and other
functional composite materials including drug delivery
materials.
[0008] The present methods can be applied to a mixed population of
carbon nanotubes that include DWCNTs and one or more types of
carbon nanotubes having a wall number other than two. In some
embodiments, the mixed population can include DWCNTs and SWCNTs. In
other embodiments, the mixed population can include DWCNTs and
MWCNTs. In yet other embodiments, the mixed population can include
DWCNTs and both SWCNTs and MWCNTs. The mixed population generally
includes a certain percentage of DWCNTs but also a significant
quantity of SWCNTs and/or MWCNTs. However, in certain embodiments,
the mixed population of carbon nanotubes can include a high
percentage of DWCNTs, where the DWCNT population exhibits
polydispersity in electronic type, diameter, and/or chirality. In
such embodiments, the present methods can be applied to separate
the polydisperse DWCNT population by one or more of electronic
type, diameter, and chirality.
[0009] A mixed or polydisperse population of carbon nanotubes can
be first contacted with one or more surface active components in a
solvent to form a dispersion. The one or more surface active
components can be selected for their ability to associate
differentially with carbon nanotubes of different wall numbers such
that individual carbon nanotubes of different wall numbers, upon
association with the surface active components, can exhibit
different buoyant densities in the solvent.
[0010] In certain embodiments, the one or more surface active
components can be substantially insensitive to carbon nanotubes of
different electronic types such that semiconducting carbon
nanotubes and metallic carbon nanotubes of the same wall number can
have substantially similar buoyant densities in the solvent when
associated with the one or more surface active components. In
embodiments where the mixed population includes SWCNTs and DWCNTs
having overlapping outer wall diameter ranges, the one or more
surface active components can confer sufficiently different buoyant
densities to the encapsulated SWCNTs and DWCNTs to allow
separation.
[0011] Subsequently, the carbon nanotube dispersion is subjected to
density gradient ultracentrifugation (DGU) by introducing the
dispersion into a fluid medium including a density gradient. The
density gradient can include a linear gradient including three or
more layers of different densities. The layers can be prepared with
different concentrations of a density gradient medium such as
iodixanol. In certain embodiments, the dispersion can be introduced
into the density gradient at a density that is different from the
respective buoyant densities of the various types of carbon
nanotubes in the mixed population as they are associated with the
one or more surface active components. For example, the dispersion
can be introduced into the fluid medium at a density that is lower
than the respective buoyant densities of the various types of
carbon nanotubes in the mixed population as associated with the one
or more surface active components. In certain embodiments, the
dispersion can be layered on top of the linear density
gradient.
[0012] Once introduced, the fluid medium can be agitated, for
example, by ultracentrifugation, to allow separation of the carbon
nanotubes by wall number along the density gradient. After
sufficient agitation, nanotubes of different wall numbers are
allowed to settle into a plurality of separation fractions, where
at least one of the separation fractions is enriched with nanotubes
of a specific wall number. The separation fractions can be visibly
distinguishable among each other by human eye. For example, two or
more separation fractions can be distinguishable by different
colors and/or different shades of a particular color in the visible
spectrum.
[0013] A single separation cycle according to the present methods
often leads to enrichment that is satisfactory for most
applications. Accordingly, following one separation cycle, one or
more separation fractions that have been enriched with DWCNTs can
be collected from the density gradient. For example, where the
mixed population includes 100 carbon nanotubes of any kind, 50 or
50% of the carbon nanotubes can be DWCNTs. In accordance with the
present teachings, after a single separation cycle, the mixed
population can separate into multiple separable subpopulations,
wherein at least one of them is enriched with DWCNTs. For example,
in a subpopulation enriched with DWCNTs, there can be 50 carbon
nanotubes, among which 40 (or 80%) can be DWCNTs. The proportion of
DWCNTs, therefore, has increased from 50% to 80% via one separation
cycle, or an enrichment factor of 0.6. In practice, the enrichment
factor can be calculated from spectroscopic measurements such as
optical absorbance, fluorescence, and raman spectroscopy.
[0014] Accordingly, a DWCNT-enriched fraction can be enriched with
at least 50% more (an enrichment factor of 0.5), at least 75% more
(an enrichment factor of 0.75), at least 100% more (an enrichment
factor of 1.0), or at least 200% more (an enrichment factor of 2.0)
DWCNTs compared to the percent composition of DWCNTs in the initial
mixed population. In certain embodiments, the DWCNT-enriched
separation fraction can include greater than about 60%, greater
than about 70%, greater than about 80%, greater than about 85%,
greater than about 87%, greater than about 90%, greater than about
92%, greater than about 95%, greater than about 97%, or greater
than about 99% encapsulated DWCNTs. In some embodiments, the
DWCNT-enriched separation fraction can be substantially free of
encapsulated SWCNTs. For example, the separation fraction can
include less than about 10%, less than about 5%, less than about
3%, less than about 2%, or less than about 1% encapsulated SWCNTs.
In particular embodiments, the DWCNT-enriched separation fraction
can be substantially free of SWCNTs having diameter ranges
overlapping with the outer wall diameter ranges of the DWCNTs in
the enriched separation fraction.
[0015] Performing one or more additional sorting or separation
cycles can improve the quality of the separation and provide
increasingly enriched separation fractions. Because bundles of
carbon nanotubes tend to form over time, the DWCNT-enriched
separation fraction can include impurities such as bundled SWCNTs.
To remove these impurities, the DWCNT-enriched separation fraction
can be introduced into a second fluid medium including a second
density gradient and centrifuged. In particular embodiments, the
DWCNT-enriched separation fraction can be introduced at a density
that is higher than the buoyant density of DWCNTs in the second
fluid medium. In certain embodiments, the DWCNT-enriched separation
fraction can be introduced into the fluid medium at the bottom of
the density gradient. After sufficient agitation, the DWCNTs and
bundled SWCNTs can settle into a plurality of separation fractions
that are visibly distinguishable by human eye, allowing collection
of a highly DWCNT-enriched separation fraction that is
substantially free of bundled SWCNTs. For example, at least about
80% of all the nanotube materials in the highly enriched-DWCNTs
separation fraction can include DWCNTs.
[0016] In addition to broad distributions of chiralities and
diameters, current synthetic methods typically produce a 1:2 ratio
of metallic-to-semiconducting carbon nanotubes. The present
teachings provide methods of separating carbon nanotubes by wall
number and by electronic type, and methods of separating
double-walled carbon nanotubes by electronic type. As used herein,
when a DWCNT is specified to be of a particular electronic type,
the electronic type should be understood as that of the outer wall
or shell only unless stated otherwise.
[0017] For example, a mixed population of carbon nanotubes that
includes semiconducting SWCNTs (s-SWCNTs), metallic SWCNTs
(m-SWCNTs), semiconducting DWCNTs (s-DWCNTs) and metallic DWCNTs
(m-DWCNTs), or a population of DWCNTs including s-DWCNTs and
m-DWCNTs, can be contacted with two or more surface active
components in a solvent to form a dispersion. As described above
with respect to methods of separating carbon nanotubes by wall
number and by diameter, the two or more surface active components
typically associate with the carbon nanotubes non-covalently,
providing debundled individual carbon nanotubes encapsulated by the
two or more surface active components.
[0018] The two or more surface active components can be selected
for their ability to associate differentially with carbon nanotubes
by electronic type as well as simultaneously by wall number and by
electronic type such that encapsulated individual carbon nanotubes
of different electronic types (and different wall numbers) exhibit
different buoyant densities in the solvent. In certain embodiments,
the two or more surface active components can include a planar
surface active component (e.g., a salt of cholic acid) and a linear
surface active component (e.g., a surfactant having an anionic or
cationic head group and a linear (flexible or rigid) aliphatic tail
group). In some embodiments, the relative ratio of the two or more
surface active components can be selected to cause metallic carbon
nanotubes to have a different (higher or lower) buoyant density
than semiconducting carbon nanotubes regardless of wall number. In
certain embodiments, the relative ratio of the two or more surface
active components can be selected to cause metallic carbon
nanotubes to have a different (higher or lower) buoyant density
than semiconducting carbon nanotubes of the same wall number.
[0019] The dispersion including the encapsulated carbon nanotubes
then can be subjected to density gradient ultracentrifugation (DGU)
processing similar to what is described above. After sufficient
agitation, carbon nanotubes of different electronic types (and
different wall numbers) can settle into a plurality of separation
fractions that are visibly distinguishable by human eye, allowing
collection of separation fractions that include primarily DWCNTs
having an outer wall of a particular electronic type. For example,
in some embodiments, the present teachings can provide a population
of DWCNTs where greater than about 70%, greater than about 80%,
greater than about 85%, greater than about 93%, or greater than
about 97% of the DWCNTs have a semiconducting outer wall. In some
embodiments, the present teachings can provide a population of
DWCNTs where greater than about 50%, greater than about 75%,
greater than about 90%, greater than about 97%, or greater than
about 99% of the DWCNTs have a metallic outer wall. In some
embodiments, the separation fraction can consist essentially of
DWCNTs having an outer wall of a particular electronic type. In
various embodiments, the DWCNT populations described herein can
have a mean outer wall diameter of less than about 1.7 nm.
[0020] Using the appropriate surface active component(s), the
present teachings also can provide populations of DWCNTs where
greater than about 30%, greater than about 50%, greater than about
75%, or greater than about 90% of the DWCNTs have outer walls
having the same chirality (n,m) type.
[0021] The various methods described herein can be used to sort
bulk quantities of nanotubes, for example, populations of nanotubes
that include more than about 10.sup.12 nanotubes, more than about
10.sup.13 nanotubes, more than about 10.sup.14 nanotubes, more than
about 10.sup.15 nanotubes, more than about 10.sup.16 nanotubes or
more than about 10.sup.17 nanotubes; or equivalently, more than
about 10 .mu.g, more than about 100 .mu.g, more than about 1 mg,
more than about 10 mg, more than about 100 mg, or more than about 1
gram of nanotubes by mass, in a single separation cycle.
[0022] The DWCNT populations of the present teachings can be
dispersed in a solvent, for example, by contacting them with one or
more surface active components including those described herein, to
provide colored translucent solutions. The present DWCNT
populations can be incorporated into various articles of
manufacture including various electronic, optical, or
optoelectronic devices such as field-effect transistors,
transparent conductors, interconnect devices, sensors,
light-emitting diodes, and solar cells, to improve one or more
electronic and/or optical properties of these devices.
[0023] Other objects, features, and advantages of the present
teachings will be more fully understood from the following figures,
description, examples, and claims.
BRIEF DESCRIPTION OF DRAWINGS
[0024] 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.
[0025] FIG. 1 are schematic cross-sectional diagrams showing how
surface-active components can associate differentially with a
small-diameter single-walled nanotube (FIG. 1a), a large-diameter
single-walled nanotube (FIG. 1b), and a double-walled nanotube
(FIG. 1c).
[0026] FIG. 2 illustrates an embodiment of the present methods by
which carbon nanotubes can be sorted by wall number. FIG. 2a is a
photograph showing separation of double-walled carbon nanotubes and
single-walled carbon nanotubes (visible to human eye as distinct
bands of materials) from a polydisperse sample after one separation
cycle according to the present methods. FIG. 2b compares the
optical absorbance spectra of sorted nanotube materials (collected
from the centrifuge tube shown in FIG. 2a from the bands indicated)
with the unsorted starting nanotube material.
[0027] FIG. 3 shows Raman spectra of single-walled carbon nanotubes
produced by high-pressure carbon monoxide catalysis (HiPco-SWCNTs),
single-walled carbon nanotubes produced by arc discharge
(AD-SWCNTs), and two subpopulations from a mixed nanotube
population including double-walled carbon nanotubes and
single-walled carbon nanotubes. Using methods according to the
present teachings, one of the subpopulations is enriched with
DWCNTs (DGU-DWCNTs), whereas the other subpopulation is enriched
with SWCNTs (DGU-SWCNTs). FIG. 3a and FIG. 3c display the radial
breathing mode (RBM) regions for all four nanotube materials at
excitation wavelengths of 514.5 nm and 750 nm, respectively. FIG.
3b and FIG. 3d show spectra from the carbon nanotube tangential
modes before and after acid treatment.
[0028] FIG. 4 shows optical absorbance spectra of thin films of
HiPco-SWCNTs, AD-SWCNTs, DGU-SWCNTs, and DGU-DWCNTs before (solid
curves) and after (dashed curves) exposure to thionyl chloride.
[0029] FIG. 5 shows a histogram summarizing the results of a
statistical analysis of the length distribution of single-walled
carbon nanotubes (open bars) and double-walled carbon nanotubes
(shaded bars) after sorting according to the present teachings.
[0030] FIG. 6 shows sheet resistance versus transmittance graphs
for transparent conductive films prepared from DGU-SWCNTs
(triangles), DGU-DWCNTs (squares), and the starting unsorted DWCNT
population (circles) at a wavelength of 550 nm. FIG. 6a shows the
transparent conductor performance data for pristine (updoped)
populations. FIG. 6b shows the transparent conductor performance
data following doping with thionyl chloride.
[0031] FIG. 7 illustrates certain embodiments of the present
teachings by which carbon nanotubes can be sorted by both wall
number and outer wall electronic type. FIG. 7a and FIG. 7c are
photographs showing separation of double-walled carbon nanotubes
having a semiconducting outer wall, double-walled carbon nanotubes
having a metallic outer wall, semiconducting single-walled carbon
nanotubes, and metallic single-walled carbon nanotubes (visible to
human eye as distinct bands of materials) from a polydisperse
sample after one separation cycle according to the present methods.
In FIG. 7a, the sorting conditions are adapted to favor separation
of DWCNTs having a semiconducting outer wall. In FIG. 7b, the
sorting conditions are adapted to favor separation of DWCNTs having
a metallic outer wall. FIG. 7b and FIG. 7d show the optical
absorbance data of sorted materials collected from the centrifuge
tubes shown in FIG. 7a and FIG. 7c, respectively, from the bands
indicated.
[0032] FIG. 8 shows Raman spectra of DGU-DWCNTs (FIG. 8b) as
compared to state-of-the-art high-purity as-synthesized DWCNTs
(FIG. 8a).
[0033] FIG. 9 shows the optical absorbance of sorted DWCNT samples
in aqueous solution that are highly enriched by the outer wall
electronic type, specifically, a sample highly enriched with DWCNTs
having a semiconducting outer wall (s-DWCNT, solid curve), and a
sample highly enriched with DWCNTs having a metallic outer wall
(m-DWCNT, dashed curve).
[0034] FIG. 10a shows the optical absorbance of a DWCNT sample in
thin film form that is highly enriched with s-DWCNT before (solid
curve) and after (dashed curve) doping treatment with thionyl
chloride. FIG. 10b shows the optical absorbance of a DWCNT sample
in thin film form that is highly enriched with m-DWCNT before
(solid curve) and after (dashed curve) doping treatment with
thionyl chloride.
[0035] FIG. 11 shows Raman spectra in the RBM region obtained from
s-DWCNTs (solid curve) and m-DWCNTs (dashed curve) at an excitation
wavelength of 514.5 nm.
[0036] FIG. 12 shows data from field-effect transistors made from
s-DWCNT and m-DWCNT thin films. FIG. 12a shows the source-drain
current (I.sub.DS) as a function of the gate bias (V.sub.G) at
different source-drain voltages (V.sub.DS) for field-effect
transistors having a 4 .mu.m.times.250 .mu.m channel and including
a thin film of s-DWCNT (s-DWCNT FET, solid curves) or a thin film
of m-DWCNT (m-DWCNT FET, dashed curves) as the active layer. FIG.
12b shows the maximum current as a function of on/off ratio for the
s-DWCNT FET (open symbols) and the m-DWCNT FET (closed symbols).
Squares are from devices with a channel length of 2 .mu.m and
triangles are from devices with a channel length of 4 .mu.m. All
devices had a channel width of 250 .mu.m.
DETAILED SPECIFICATION
[0037] 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.
[0038] 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 or 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.
[0039] The use of the terms "include," "includes," "including,"
"have," "has," "having," "contain," "contains," or "containing"
should be generally understood as open-ended and non-limiting
unless specifically stated otherwise.
[0040] 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.
[0041] It should be understood that the order of steps or order for
performing certain actions is immaterial so long as the present
teachings remain operable. Moreover, two or more steps or actions
may be conducted simultaneously.
[0042] As used herein, a "nanotube" refers to an elongated hollow
structure, for example, a cylindrical tube, having a nanoscale
diameter, e.g., less than about 50 nm and usually less than about
10 nm, and typically a high aspect ratio. The present teachings can
be useful for separating various elemental or molecular nanotubes
including, but not limited to, carbon, boron, BN, WS.sub.2, and
MoS.sub.2. As such, while the description and examples herein may
refer specifically to carbon nanotubes, the present teachings are
intended to encompass separation of nanotubes in general regardless
of their composition.
[0043] As used herein, a "population" of nanotubes can include
about greater than about 10.sup.8 nanotubes, greater than about
10.sup.9 nanotubes, greater than about 10.sup.10 nanotubes, greater
than about 10.sup.11 nanotubes, greater than about 10.sup.12
nanotubes, greater than about 10.sup.13 nanotubes, greater than
about 10.sup.14 nanotubes, greater than about 10.sup.15 nanotubes,
greater than about 10.sup.16 nanotubes, or greater than about
10.sup.17 nanotubes. Further, by weight, a population of nanotubes
can have a mass of about 0.001 .mu.g, greater than about 0.01
.mu.g, greater than about 0.1 .mu.g, greater than about 1 .mu.g,
greater than about 10 .mu.g, greater than about 100 .mu.g, greater
than about 1 mg, greater than about 10 mg, greater than about 100
mg, or greater than about 1 g. In certain embodiments of the
present teachings, a separation cycle can be used to sort bulk
quantities of nanotubes, for populations of nanotubes that include
more than about 10.sup.12 nanotubes, more than about 10.sup.13
nanotubes, more than about 10.sup.14 nanotubes, more than about
10.sup.15 nanotubes, more than about 10.sup.16 nanotubes or more
than about 10.sup.17 nanotubes; or equivalently, more than about 10
.mu.g, more than about 100 .mu.g, more than about 1 mg, more than
about 10 mg, more than about 100 mg, or more than about 1 gram of
nanotubes by mass.
[0044] As used herein, "enrichment" or "enriched" refers to an
increase in the statistical proportion of nanotubes comprising one
or more specific characteristics in a fraction obtained from a
sample as compared to the sample as a whole. As described herein, a
nanotube subpopulation that is "enriched" according to the present
teachings by one or more properties, such as wall number, diameter,
electronic type, chirality, or combinations thereof, means that the
subpopulation (i.e., the enriched population) has a higher
percentage of nanotubes having the one or more properties when
compared to the starting population (i.e., the mixed population)
from which the subpopulation is derived.
[0045] Various methods have been used to different degrees of
success for separating SWCNTs by one or more characteristics such
as chirality, diameter, and electronic type. However, the extension
of separation methods for SWCNTs to DWCNTs and MWCNTs can be
described as unpredictable at best. Because of their more
complicated structures and intershell interactions, DWCNTs and
MWCNTs can be expected to respond differently to a particular
separation method when compared to SWCNTs. Using DWCNTs as an
example, four distinct outer shell-inner shell combinations are
possible, namely, metallic (M)-semiconducting (S), M-M, S-M, and
S-S. It is not well known how the electronic type of the inner
shell affects the electronic properties of a DWCNT as a whole. See
e.g., Wang et al., J. Phys. Chem. B, vol. 109(37): 17361-17365
(2005). Similarly, it is not well known how the inner wall diameter
and the interwall spacing modify the properties of a DWCNT. See
e.g., Okada et al., Phys. Rev. Lett., vol. 91(21):
216801(1)-216801(4) (2003). For example, it is often unpredictable
how a SWCNT and an otherwise similar DWCNT, e.g., the outer shell
of the DWCNT has similar characteristics as the SWCNT but for the
presence of the inner shell, react differently to a particular
functionalizing group. See e.g., Pfeiffer et al., Phys. Rev. B,
vol. 72, 161404(R)(1)-161404(R)(4) (2005). To the inventors'
knowledge, there has been no reported method for separating carbon
nanotubes by wall number that allows isolation of DWCNTs and/or
MWCNTs, or for separating DWCNTs and/or MWCNTs by one or more
desirable characteristics of the outer shell such as chirality,
diameter, and/or electronic type. In addition, it is unclear
whether any of the separation methods that have been used with
SWCNTs can distinguish between SWCNTS and DWCNTs and/or MWCNTs
having similar outer shell characteristics.
[0046] The present teachings provide methods related to sorting
nanotubes, in particular, carbon nanotubes, according to their wall
number. More specifically, it has been found that density gradient
ultracentrifugation (DGU) can be used to separate carbon nanotubes
according to their wall number, thereby providing, for example, a
separation fraction that includes primarily DWCNTs from an initial
mixed population that includes DWCNTs, SWCNTs and/or MWCNTs.
Furthermore, it has been found that with the appropriate choice of
surface active component(s), the present teachings can lead to
simultaneous separation (i.e., in a single separation cycle) of
carbon nanotubes by wall number and at least one other property,
e.g., diameter or electronic type. Accordingly, certain embodiments
of the present methods can enable sorting of carbon nanotubes by
both wall number and (outer wall) electronic type, while certain
embodiments of the present methods can enable sorting of carbon
nanotubes by both wall number and (outer wall) diameter. In
addition, various embodiments of the present methods can separate
carbon nanotubes of a specified wall number, for example, DWCNTs,
based on one or more other properties such as electronic type,
diameter and/or chirality.
[0047] In particular, as-synthesized DWCNT samples typically
include unwanted SWCNTs having diameter ranges that overlap with
the outer wall diameter ranges of at least a portion of the DWCNTs.
Certain embodiments of the present teachings are directed to
sorting small-diameter DWCNTs, i.e., DWCNTs having a mean outer
wall diameter of less than about 2.0 nm (e.g., less than about 1.7
nm), from large-diameter SWCNTs, e.g., those having a mean diameter
of greater than or about 1.4 nm, which has not been possible with
prior art methods. For ease of comparison with DWCNTs, the diameter
of SWCNTs sometimes may be referred herein as the outer wall
diameter. It is understood by a person skilled in the art that a
SWCNT consists of a single carbon tubule and therefore can have
only one diameter.
[0048] In addition, while one of the advantages of the present
teachings relates to simultaneous separation of carbon nanotubes by
wall number and at least one other properties, sorting carbon
nanotubes by multiple properties via more than one separation cycle
is within the scope of the present teachings.
[0049] Accordingly, in some embodiments, the present teachings
provide a method of separating carbon nanotubes by wall number. The
present method can be used to sort as-synthesized samples of
DWCNTs, which often include a mixed population of carbon nanotubes
including DWCNTs and SWCNTs, DWCNTs and MWCNTs, or DWCNTS and both
SWCNTs and MWCNTs. In addition, the mixed population can include
SWCNTs and DWCNTs having overlapping outer wall diameter ranges.
For example, the mixed population can include less than about 50%,
less than about 60%, less than about 70%, less than about 75%, less
than about 80%, less than about 85%, less than about 90%, or less
than about 95% DWCNTs.
[0050] The mixed population of carbon nanotubes can be produced by
one or more methods known in the art including chemical vapor
deposition such as high-pressure carbon monoxide conversion
(HiPco), laser ablation, arc discharge, coalescence, and specific
catalytic processes such as CoMoCAT.RTM. (SouthWest
NanoTechnologies Inc., Norman, Okla.). In most embodiments, the
mixed population includes pristine or as-synthesized carbon
nanotubes when applied to the present methods. However, in certain
embodiments, carbon nanotubes that have been chemically treated
post-synthesis (e.g., doped or otherwise irreversibly
functionalized) can be used as well.
[0051] Current synthetic methods of DWCNTs such as those listed
above not only produce significant quantities of unwanted SWCNTs
and/or MWCNTs, they also produce broad distributions of chiralities
and diameters within the DWCNT population and the unwanted SWCNT
and/or MWCNT population(s). For example, the mixed population can
include DWCNTs having outer wall diameter ranges that are between
about 0.9 nm and about 3.0 nm. The mixed population also can
include SWCNTs having diameter ranges that are between about 0.7 nm
and about 2.0 nm, for example, between about 0.8 nm and about 1.8
nm. As such, the mixed population can include SWCNTs and DWCNTs
having overlapping diameter ranges and outer wall diameter ranges,
in particular, between about 1.0 nm and about 2.0 nm. The present
methods can separate DWCNTs having an outer wall diameter that is
less than about 2.0 nm effectively from SWCNTs having similar
diameters. For example, the present methods can separate DWCNTs
having an outer wall diameter that is less than about 1.7 nm from
SWCNTs having a diameter that falls within the same range, i.e.,
between about 1.1 nm and about 1.8 nm.
[0052] In some embodiments, a mixed (or polydisperse) nanotube
population can be contacted with one or more surface active
components in a solvent (e.g., water) to provide a dispersion. The
one or more surface active components typically associate with the
carbon nanotubes non-covalently, providing debundled individual
carbon nanotubes "encapsulated" by the one or more surface active
components. For example, the one or more surface active components
can associate with the carbon nanotubes by one or more of ionic
interaction, .pi.-.pi. orbital interaction, hydrogen bonding, and
Van Der Waals interaction. As used herein, "encapsulate,"
"encapsulated," or "encapsulating" refers to non-covalent
association with a target such as nanotubes. For example, the one
or more surface active components can wrap around the sidewall
(circumference) of the carbon nanotubes, yet not be present at the
two ends of the nanotubes. In some embodiments, the surface active
components can arrange themselves around the sidewall of a carbon
nanotube as a helical monolayer. However, other arrangements, e.g.,
longitudinal or annular arrangement, are possible.
[0053] In various embodiments, nanotubes having different
properties exhibit different buoyant densities upon association
with (e.g., encapsulation by) the surface active components. The
encapsulated nanotube complexes are introduced into a density
gradient provided by a fluid medium and centrifuged. Over the
course of the ultracentrifugation, the complexes move within the
density gradient to their respective isopycnic points, that is,
where their respective buoyant density matches the density of a
particular layer of the density gradient. 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), the complexes
settle into multiple bands of materials according to the desirable
characteristic(s) and can be removed layer by layer from the
density gradient to provide separation fractions that primarily
contain nanotubes having the desirable characteristic(s). The
success of a separation can be defined as having the complexes
settle into distinct bands of materials at different locations in
the density gradient that are visible to human eye. For example,
each band of materials can differ in colors or shades of similar
colors.
[0054] Without limitation to any one theory or mode of operation,
separation via density gradient centrifugation is believed to be
driven largely by how the surface active component(s), for example,
surfactant(s), organize around nanotubes of different structure and
electronic type. The energetic balance among inter-nanotubes,
solvent particles, and surface active components interactions as
well as their packing density, orientation, ionization, and the
resulting hydration of these surface active components can all be
parameters affecting buoyant density and the quality of separation
and purification. The buoyant density of an encapsulated nanotube
in a fluid medium can depend on multiple factors, including the
density and electronic character of the nanotube itself, the
structure and composition of the surface active component(s)
surrounding the nanotube, and the spatial extent of any
electrostatically bound hydration layers. For DWCNTs and MWCNTs,
additional parameters such as intershell interaction and interwall
spacing are expected to contribute to unpredictable variations of
the buoyant density of an encapsulated nanotube.
[0055] FIG. 1 illustrates how a particular surface active component
(or a particular system including two or more surface active
components) can associate with and confer different buoyant
densities to a small-diameter single-walled nanotube, a
large-diameter single-walled nanotube, and a double-walled nanotube
having an outer wall diameter similar to the diameter of the
large-diameter single-walled nanotube. For example, the surface
active component can be sensitive to both wall number and diameter,
such that upon association with the surface active component, the
nanotubes can be separated according to their different buoyant
densities, for example, the nanotubes can have increasing buoyant
density in the order of small-diameter single-walled nanotube,
large-diameter single-walled nanotube, and double-walled nanotube.
It should be understood that a surface active component system
comprises one or more surface active agents.
[0056] The one or more surface active components can be selected
from a wide range of non-ionic or ionic (cationic, anionic, or
zwitterionic) amphiphiles. In some embodiments, the surface active
component can include an anionic surfactant. For example, a surface
active component can include one or more sulfates, sulfonates,
carboxylates, and combinations thereof. In certain embodiments, the
one or more surface active components can include a compound having
a planar polycyclic (e.g., carbocyclic) core. For example, the one
or more surface active components can include a compound having a
sterane core. In particular embodiments, the one or more surface
active components can include one or more bile salts. Bile salts
can be more broadly described as a group of molecularly rigid and
planar amphiphiles with a charged face opposing a hydrophobic face.
Examples of bile salts include salts (e.g., sodium or potassium
salts) of conjugated or unconjugated cholates and cholate
derivatives including deoxycholates, chenodeoxycholates,
taurodeoxycholates, glycochenodeoxycholates, ursodeoxycholates, and
glycoursodeoxycholates.
[0057] In some embodiments, amphiphiles with anionic head groups
and flexible alkyl tails (interchangeably referred to herein as
anionic alkyl amphiphiles) can be used. Examples of anionic alkyl
amphiphiles include dodecyl sulfates and dodecylbenzene sulfonates
such as sodium dodecyl sulfate (SDS) and sodium dodecylbenzene
sulfonate (SDBS). In certain 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. In some embodiments, deoxyribonucleic acid (DNA) such as
single-stranded DNA or DNA fragments also can be used as the
surface active component as described in U.S. patent application
Ser. No. 11/368,581, the disclosure of which is incorporated by
reference herein in its entirety.
[0058] However, to separate effectively nanotubes by wall number
and by diameter, or by wall number and by electronic type, it has
been found that the surface active component(s) should not interact
with the nanotubes in a way that is simultaneously sensitive to
both diameters and electronic types. For example, to enable
effective separation by wall number and by diameter, the surface
active component(s) need to be sensitive to diameter but
sufficiently insensitive to electronic type, such that, for
example, DWCNTs encapsulated by the appropriate surface active
component(s) will have significantly higher buoyant densities than
SWCNTs having similar diameters regardless of the electronic
properties of the respective shells of the nanotubes. In this
regard, planar surface active components have been found useful. As
used herein, a planar surface active component refers to a surface
active agent that includes a geometrically planar portion (e.g., a
sterane core) which can have small substituent(s) thereon. In
certain embodiments, the surface active components can consist of
sodium cholate (SC).
[0059] Similarly, to enable effective separation by wall number and
by electronic type, the surface active component(s) need to be
sensitive to electronic type but sufficiently insensitive to
diameter. It has been found that two or more surface active
components (e.g., a co-surfactant system) can be used to effect
such separation by selecting a relative ratio of the two or more
surface active components that can cause nanotubes of a first
electronic type (e.g., semiconducting) to have a lower buoyant
density than nanotubes of a second electronic type (e.g.,
metallic), while the nanotubes are simultaneously sorted by wall
number.
[0060] The two or more surface active components can be of the same
type or of different types. In some embodiments, the two or more
surface active components can competitively adsorb to the nanotube
surface. For example, the at least two surface active components
can have different molecular geometries and/or different functional
groups. Such a competitive co-surfactant system can be used to
achieve optimal separation between metallic and semiconducting
nanotubes. In certain embodiments, the two or more surface active
components can include two different bile salts. In some
embodiments, the two or more surface active components can include
a planar surface active component and a linear surface active
component. As used herein, a linear surface active component refers
to a surface active agent that includes a linear aliphatic group,
for example, a linear aliphatic group with at least six carbon
atoms. Examples include the various amphiphiles described above
such as SDS and SDBS that include a flexible or rigid alkyl tail
and a cationic or anionic head group.
[0061] In some embodiments, the two or more surface active
components can comprise a co-surfactant system including varying
amounts of SDS and SC. In certain embodiments, the present method
can include using a co-surfactant system that includes a lower
ratio of the linear surface active component relative to the planar
surface active component to achieve simultaneous separation of
nanotubes by wall number and by electronic type, particularly, with
the separation by wall number having a dominant effect over the
separation by electronic type. For example, using a co-surfactant
system that includes an SDS:SC ratio of 1:4 (by weight), a mixed
population that includes SWCNTs and DWCNTs of both electronic types
can be expected to separate into four visibly distinct bands of
materials (from low to high density) in the order of semiconducting
SWCNTs (s-SWCNTs), metallic SWCNTs (m-SWCNTs), DWCNTs having a
semiconducting outer wall (s-DWCNTs), and DWCNTs having a metallic
outer wall (m-DWCNTs) in a single separation cycle. In other
embodiments, the present method can include using a co-surfactant
system that includes a higher ratio of the linear surface active
component relative to the planar surface active component to
achieve simultaneous separation of nanotubes by wall number and by
electronic type, particularly, with the separation by electronic
type having a dominant effect over the separation by wall number.
For example, using a co-surfactant system that includes an SDS:SC
ratio of 3:2 (by weight), a mixed population that includes SWCNTs
and DWCNTs can be expected to separate into four visibly distinct
bands of materials (from low to high density) in the order of
m-SWCNTs, m-DWCNTs, s-SWCNTs, and s-DWCNTs in a single separation
cycle.
[0062] 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). Fluid media useful with the present teachings are
limited only by nanotube aggregation therein to an extent
precluding at least partial separation. Accordingly, 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. Such a medium can
include a range of aqueous iodixanol concentrations and the
corresponding gradient of concentration densities. As understood by
those skilled in the art, aqueous iodixanol is a common, widely
used non-ionic density gradient medium. However, other media can be
used in methods of the present teachings, as would be understood by
those skilled in the art.
[0063] 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 nanotubes, whether or not functionalized
(e.g., by means of association with one or more surface active
components), also should 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 nanotubes (and/or in
composition with one or more surface active components) for a
particular medium. Accordingly, any aqueous or non-aqueous density
gradient medium can be used provided that the nanotubes are stable;
that is, do not aggregate to an extent precluding useful
separation. Alternatives to iodixanol include 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 Percoll.RTM.). Other parameters which can be
considered upon choice of a suitable density gradient medium
include 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.
[0064] It has been discovered that the point at which the
encapsulated nanotubes are introduced into the density gradient in
the fluid medium can be important to the quality of the separation.
In some embodiments, the encapsulated nanotubes are introduced into
the density gradient at a density that is different from the
buoyant density of any of the encapsulated nanotubes.
[0065] In particular embodiments, a two-iteration DGU process can
be used to isolate DWCNTs from SWCNTs of roughly the same outer
wall diameter. For example, in the first separation cycle, the
carbon nanotubes can be loaded at the top of a linear gradient and
made to sediment from lower to higher densities during the
ultracentrifugation. By having the nanotubes sediment to higher
densities, all the individually encapsulated SWCNTs inside the
gradient should be unable to reach the dense isopycnic position of
the DWCNTs. In contrast, a separation in which the nanotubes
sediment from higher to lower densities can result in a poor
separation, with a number of slow-moving SWCNTs reaching only the
DWCNT equilibrium position. After ultracentrifugation, it can be
expected that the SWCNTs and DWCNTs of similar outer wall diameters
(e.g., having a mean outer wall diameter of about 1.6 nm) will
settle into two different separation fractions, with the separation
fraction enriched with SWCNTs settling above the separation
fraction enriched with DWCNTs. If the mixed population further
includes small-diameter SWCNTs (e.g., SWCNTs having a mean outer
wall diameter of less than about 1.4 nm), these small-diameter
SWCNTs can be expected to settle above the separation fraction
enriched with the larger-diameter SWCNTs having similar diameters
as the DWCNT outer walls. Any carbon nanotube bundles, MWCNTs, and
carbonaceous impurities, if present, can be expected to settle
below the separation fraction enriched with DWCNTs.
[0066] To achieve further enrichment of DWCNTs, the DWCNT-enriched
separation fraction can be subjected to a second iteration of DGU
processing. In the second separation cycle, the encapsulated DWCNTs
can be introduced at the bottom (rather than at the top as in the
first separation cycle) of the linear density gradient and made to
sediment from high to low densities. Because nanotube bundles can
be expected to form over time in nanotube dispersions, this second
iteration helps remove SWCNT bundles that may have formed over time
in addition to any slow-moving MWCNTs that did not sediment fast
enough in the first iteration. Of course the second separation
itself can be practiced as a method of the present teachings, where
the starting population of nanotubes is a sample comprising
substantially DWCNTs.
[0067] The separation fractions collected according to the
procedures described herein can be sufficiently selective for most
current applications of nanotubes. However, it can be desirable to
purify further the separation fractions to improve their
selectivity by performing additional iterations of the present
methods. Specifically, a separation fraction can be provided in a
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
separation fraction was obtained. In certain embodiments, fluid
medium conditions or parameters can be maintained from one
separation to another. In other embodiments, at least one iterative
separation can include a change of one or more parameters including
the identity of the surface active component(s), medium identity,
medium density gradient, and/or medium pH, as well as the duration
and the rotational speed of the centrifugation process, with
respect to one or more of the preceding separations. In certain
embodiments, the surfactant(s) encapsulating the nanotubes can be
modified or changed between iterations, allowing for even further
refinement of separation. Separation fractions isolated after each
separation can be washed before further complexation and
centrifugation steps are performed.
[0068] The selectivity of the fraction(s) collected can be
confirmed by various analytical methods including optical
absorbance, Raman spectroscopy, transmission emission spectroscopy
(TEM), fluorescence spectroscopy, and other methods known in the
art.
[0069] As described herein, the present teachings provide nanotube
populations that are substantially monodisperse in terms of wall
number as well as populations of double-walled 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 double-walled
nanotubes in a population of double-walled 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 double-walled carbon
nanotubes in a population of double-walled 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 double-walled
nanotubes in a population of double-walled 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 outer wall 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 double-walled
nanotubes in a population of double-walled nanotubes can have a
semiconducting outer wall, 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 double-walled nanotubes in
a population of double-walled nanotubes can have a metallic outer
wall). In some embodiments, the population can be substantially
monodisperse in terms of their outer wall chiralities (e.g.,
greater than about 30%, including greater than about 50%, greater
than about 75%, and greater than about 90%, of the double-walled
nanotubes in a population of double-walled nanotubes can have outer
walls having the same chirality (n, m) type).
[0070] The nanotube populations of the present teachings can be
incorporated into various electronic, optical, or optoelectronic
devices such as field-effect transistors, transparent conductors,
interconnect devices, sensors, light-emitting diodes, and solar
cells, to improve one or more electronic and/or optical properties
of these devices. The present nanotube populations can be processed
or analyzed `as is,` i.e., with the individual nanotubes
encapsulated by one or more surface active components;
alternatively, the surface active components can be removed prior
to or during analysis or processing. The one or more surface active
components can be removed according to methods known in the art,
for example, as disclosed in Meitl et al., Nano Lett., 4: 1643
(2004); and Zhou et al., Appl. Phys. Lett., 88: 123109 (2006).
[0071] The following examples are provided to illustrate further
and to facilitate the understanding of the present teachings and
are not in any way intended to limit the invention.
Example 1
Dispersion of Carbon Nanotubes and Concentration
[0072] The starting carbon nanotube material (Batch #: DW331UA) was
obtained from Carbon Nanotechnologies Inc. (Houston, Tex.). The
sample batch was indicated by the manufacturer to comprise
approximately 70% DWCNTs with outer wall diameters ranging from
about 1.5 nm to about 3.0 nm based on transmission electron
microscopy (TEM). Optical absorbance data further characterized
that about 22% to about 35% of the DWCNTs in the sample batch have
an outer wall diameter of less than about 1.7 nm.
[0073] The carbon nanotube material was added to 110 mL of a 1% w/v
sodium cholate (SC) aqueous solution at a loading of about 2 mg/mL
in a steel beaker. This mixture was cooled in an ice bath, then
subjected to horn ultrasonication using a 13 mm-diameter tip for 90
minutes at a power of 160 W. Following sonication, the dispersion
was incorporated directly into a step gradient for concentration.
The step gradient consisted of a dense underlayer of 2 mL of a 1%
w/v SC aqueous solution with 60% w/v iodixanol topped by
approximately 10 mL of the carbon nanotube dispersion. The step
gradient was ultracentrifuged in an SW41 Ti swing-bucket rotor
(Beckman-Coulter Inc.) at a rotational frequency of 41 krpm for 12
hours. Concentrated fractions or bands of carbon nanotube materials
were removed from the centrifuge tube in a two step fractionation
procedure. First, a 1 mL displacement layer consisting of a 1% w/v
SC aqueous solution with 30% w/v iodixanol was slowly infused into
the gradient to separate poorly dispersed carbon nanotubes from the
buoyant, individually encapsulated carbon nanotubes. After upward
displacement, the resulting concentrated band, with a density less
than 30% w/v iodixanol was removed using a piston gradient
fractionator (Biocomp Instruments Inc.).
Example 2
Sorting by Wall Number--First Iteration
[0074] Carbon nanotubes in the concentrated fraction from Example 1
were sorted by wall number in a gradient containing 1% w/v SC
throughout. The density gradient consisted of a 1.5 mL underlayer
containing 60% w/v iodixanol, followed by a 5 mL linear gradient
(SG15, Hoefer Inc.) with a density varying from 32.5% to 17.5% w/v
iodixanol. On top of this linear gradient was added a 1 mL layer
containing the concentrated dispersion of carbon nanotubes diluted
to a density of 15% w/v iodixanol. The remaining .about.4.5 mL of
the centrifuge tube was filled with 0% w/v iodixanol solution. This
linear gradient was ultracentrifuged for 12 hours at a rotational
frequency of 41 krpm in an SW41 Ti rotor, at the end of which
isopycnically separated SWCNTs and DWCNTs were produced.
[0075] FIG. 2a is a photograph of a centrifuge tube 100 containing
a dispersion of a polydisperse carbon nanotube sample taken after
one separation cycle according to the present methods, which
resulted in the dispersion separating into four colored bands
located at different buoyant densities. Band 102 corresponds to
small-diameter SWCNTs; band 104 corresponds to large-diameter
SWCNTs; band 108 corresponds to DWNTs; and the thick black band 110
includes very dense MWNTs, carbonaceous impurities, and bundled
nanotubes. The transparent band 106 is substantially free of carbon
nanotube materials.
[0076] To confirm these qualitative assignments, these bands of
separated materials were extracted from the centrifuge tube using a
piston gradient fractionator (Biocomp Instruments, Inc.) and
characterized by optical absorbance (FIG. 2b). The topmost band 102
exhibits multiple discrete peaks in the visible and near infrared
that can be assigned to optical transitions associated with SWCNTs
having a diameter of about 0.7 nm to about 1.2 nm (curve 112). For
the carbon nanotubes located in the band below (band 114), the
optical absorbance (curve 114a) contains two broad peaks ranging
from about 640 nm to about 860 nm and from about 900 nm to about
1300 nm that can be attributed to metallic and semiconducting
SWCNTs, respectively, with average diameters of about 1.6 nm. For
the DWCNT band (band 108), the optical absorbance (curve 118a) is
similar to that of the large diameter SWCNTs indicating the DWCNT
outer walls are of comparable diameter to the SWCNTs. In addition,
a transition at about 1190 nm, discussed in more detail below, is
indicative of absorbance from the inner walls of the DWCNTs. The
optical absorbance of the dense, broad black band 110 exhibits
further red-shifting of the semiconducting and metallic transitions
as well as increased background absorbance (data not shown) which
is consistent with nanotubes of larger diameters, as well as
increased bundling and carbonaceous impurity content. Curves 116
and 120 correspond to the absorbance spectra of the transparent
band 106 and the starting material, respectively.
[0077] Of these materials, a 2 mm thick band (band 104 in FIG. 2a)
with an average density of 17.8% iodixanol (1.095 g/mL) was
selected for a second iteration of sorting for its high
large-diameter SWCNT content. Furthermore, a 2.5 mm thick band
(band 108 in FIG. 1a) with an average density of 21.5% iodixanol
(1.115 g/mL) was selected for a second iteration of sorting as a
result of its large DWCNT content.
Example 3
Sorting by Wall Number--Second Iteration
[0078] The SWCNT- and DWCNT-enriched fractions were isolated and
introduced into separate gradients consisting of the following
layers, each with 1% w/v SC content. First, a dense, 1.5 mL
underlayer of 60% w/v iodixanol was added to the bottom of the
centrifuge tube, followed by a 1 mL layer of the enriched fraction
diluted to a density of 33.5% w/v iodixanol. A 5 mL linear gradient
with density running from 31% to 16% w/v iodixanol was added above
the carbon nanotube layer and was topped by a .about.4.5 mL layer
with a density of 1 g/mL. This gradient was then ultracentrifuged
in an SW41 Ti for 12 hours at 41 krpm. Following centrifugation,
bands of isolated carbon nanotubes were recovered from the
centrifuge tube using a piston gradient fractionator.
[0079] Absorbance spectra of the highly enriched SWCNT fraction
(DGU-SWCNTs, curve 114b) and the highly enriched DWCNT fraction
(DGU-DWCNTs, curve 118b) show significant improvement in sorting
quality after two iterations (FIG. 2b). Compared to curves 114a and
118a, both materials exhibit decreased peak widths and lower
background absorbance levels, which can be attributed to decreased
numbers of bundled carbon nanotubes and smaller amounts of
carbonaceous impurities.
Example 4
Characterization by Transmission Electron Microscopy
[0080] DGU-DWCNTs, initially dispersed in water with SC and
iodixanol, were precipitated by diluting the nanotube dispersion
with water to bring the SC concentration to less than 0.1% w/v, and
subsequently diluted with isopropanol to completely withdraw the SC
from the carbon nanotube sidewalls. The precipitates were filtered
through anodized aluminum oxide membranes (Whatman Anodisc), and
rinsed with copious amounts of water to remove the remaining SC and
iodixanol. The resulting carbon nanotube films were immersed in
acetone and freed from the membranes using bath sonication. The
acetone in turn was removed by heating at 90.degree. C. for two
hours leaving solid, surfactant-free sorted carbon nanotube
material.
[0081] The DGU-DWCNTs were redispersed in 3 ml of a 1% w/v sodium
dodecyl sulfate (SDS) solution in deuterated water (D.sub.2O) using
a horn ultrasonicator (Fisher Scientific Model 500 Sonic
Dismembrator). The ultrasonicator was equipped with a 3 mm diameter
tip and operated at 15% amplitude for 90 minutes while the sample
was cooled in an ice bath. Poorly dispersed carbon nanotube bundles
were removed by ultracentrifugation for 14 minutes at 38 krpm in a
Beckman Coulter TLA100.3 rotor. A 5 .mu.L, droplet from the top 2.5
mL layer of the dispersion was deposited on TEM grids coated with
an ultrathin (<3 nm) carbon film (Prod. #01824, Ted Pella,
Inc.). After .about.30 seconds, the grid was dried using filter
paper and rinsed in deionized water. The grid was then dried again
using filter paper.
[0082] TEM images taken on JEOL JEM-2100F Fast TEM confirmed that
the DGU-DWNTs predominantly consisted of double-walled species.
Example 5
Characterization and Diameter Determination Using Raman
Spectroscopy
[0083] Previous studies of DWCNTs have revealed substantial
differences in the Raman spectra of DWCNTs and SWCNTs following
treatment with concentrated sulfuric acid and have been used to
assess DWCNT content (see Kim et al., Chem. Phys. Lett., 420: 377
(2006); Banos et al., Phys. Rev. B, 76: 045425 (2007); and Filho et
al., Nano Lett., 7: 2383 (2007)).
[0084] To confirm that the increased buoyant densities and
absorbance characteristics of DGU-DWCNTs did not arise from SWCNTs
with bimodal diameter distributions, Raman spectra were obtained on
thin film samples of DGU-SWCNTs and DGU-DWCNTs. Specifically,
DGU-SWCNTs and DGU-DWCNTs were processed into thin films of
.about.60% transmittance on glass and quartz substrates (see Wu et
al., Science, 305: 1273 (2004)). For comparison, thin films of
HiPco-SWCNTs (Carbon Nanotechnologies Inc.) and arc-discharge-grown
SWCNTs (AD-SWCNTs, Carbon Solutions Inc.) having similar diameters
to the inner walls and the outer walls of DGU-DWCNTs also were
prepared on the same transparent substrates. Based on the optical
absorbance of the DGU-DWCNTs, Raman spectra were measured at two
different excitation wavelengths: 514.5 nm to probe DWCNTS having
semiconducting outer walls and metallic inner walls, and 750 nm to
probe DWCNTS having metallic outer walls and semiconducting inner
walls (FIG. 3). The radial breathing mode (RBM) and G band spectra
obtained from these samples (FIG. 3a and FIG. 3c) are consistent
with the assignments derived from optical absorbance (FIG. 2b).
[0085] Because SWCNT diameters can be related to the RBM
frequencies with the equation .omega..sub.RBM=A/d.sub.t+B (see
Bachilo et al., Science, 298: 2361-2366 (2002)), the values of A
and B were first determined using the Raman spectra of thin films
of as-produced HiPco-SWCNTs having a known chirality distribution.
The RBM frequencies and diameters of the HiPco-SWCNTs were well
described by a fit with A=218.2 and B=19.6.
[0086] To evaluate the mean diameters of the DGU-DWCNTs, the
average RBM frequencies of the peaks associated with the inner wall
and the outer wall of DWCNTs were calculated. These RBM frequencies
were then converted to carbon nanotube diameters and corrected to
account for differences in the laser power. The calculations
revealed that the DGU-DWCNTs have mean inner wall diameters of
.about.0.86 nm and mean outer wall diameters of .about.1.61 nm.
[0087] To ensure that signals arising from small-diameter carbon
nanotubes were due to the inner wall of DWCNTs and not
small-diameter SWCNT impurities, optical measurements were
performed before and after the DGU-DWCNTs were subjected to
chemical treatments selected to strongly affect the optical
properties of the outer wall of the DWCNTs. Because the inner wall
of DWCNTs should be isolated from adsorbed species by the
protective outer wall, the optical characteristics of the inner
nanotubes would be largely unaffected by the chemical treatment,
unlike any SWCNT impurities.
[0088] Thin film samples of DGU-SWCNTs and DGU-DWCNTs were
chemically treated by coating them with concentrated sulfuric acid
(95-98%) for 10 minutes, after which any excess acid was removed
using a jet of nitrogen gas, and the films were left to dry over
several days.
[0089] FIG. 3a shows the Raman spectra of the four nanomaterials
(HiPco-SWCNTs--122, AD-SWCNTs--124, DGU-SWCNTs--126, and
DGU-DWCNTs--128) measured as pristine films (solid curves, a) and
films treated with sulfuric acid (dashed curves, b) at 514.5 nm
excitation. FIG. 3b shows spectra from the carbon nanotube
tangential modes (HiPco-SWCNTs--132, AD-SWCNTs--134,
DGU-SWCNTs--136, and DGU-DWCNTs--138) before (solid curves, a) and
after (dashed curves, b) acid treatment at 514.5 nm excitation.
FIG. 3c shows the Raman spectra of the four nanomaterials
(HiPco-SWCNTs--142, AD-SWCNTs--144, DGU-SWCNTs--146, and
DGU-DWCNTs--148) measured as pristine films (solid curves, a) and
films treated with sulfuric acid (dashed curves, b) at 750 nm
excitation. FIG. 3d shows spectra from the carbon nanotube
(HiPco-SWCNTs--152, AD-SWCNTs--154, DGU-SWCNTs--156, and
DGU-DWCNTs--158) tangential modes before (solid curves, a) and
after (dashed curves, b) acid treatment at 750 nm excitation.
[0090] As described above, the Raman spectra 128a, 148a of
DGU-DWCNTs exhibit RBMs corresponding to inner walls (162, 166)
having an average diameter of .about.0.86 nm and outer walls (160,
164) having an average diameter of .about.1.61 nm. For DGU-SWCNTs
126a, 146a, the RBMs attributable to small diameter carbon
nanotubes (c.f. HiPco-SWCNTs 122a, 142a) are completely absent at
514.5 nm and weak in intensity at 750 nm excitation. Moreover, the
RBMs associated with large diameter nanotubes (c.f. AD-SWCNTs 124a,
144a) indicate the DGU-SWCNTs have an average diameter of
.about.1.60 nm, similar to the outer wall diameter of DGU-DWCNTs
(128a, 148a).
[0091] The G-bands of the pristine SWCNT samples have peak shapes
that can be adequately described by two Lorentzians corresponding
to the G.sup.+ and G.sup.- bands (FIG. 3b and FIG. 3d). Conversely,
the DGU-DWCNT sample exhibits a G-band (FIG. 3b and FIG. 3d) with a
finer structure with two pairs of G.sup.+ and G.sup.- bands
attributable to both the inner and outer tubes (see Kim et al.,
Chem. Phys. Lett., 420: 377 (2006)).
[0092] Following acid treatment, all carbon nanotube films showed
decreased Raman signal intensity and marked changes in their RBMs
and G-band. Referring back to FIG. 3, pristine carbon nanotube
films (solid curves) display significantly higher intensity than
films treated with sulfuric acid (dashed curves). For both the
control SWCNT samples (i.e., HiPco-SWCNTs and AD-SWCNTs) and
DGU-SWCNTs, RBMs were almost completely suppressed by the sulfuric
acid treatment. However, as shown in FIG. 3, for DGU-DWCNTs,
protection protected by the outer wall (as indicated by the shaded
regions 160 and 164) preserves much of the RBM intensity of the
inner wall (as indicated by the shaded regions 162 and 166)
following acid doping. Meanwhile, RBMs associated with the outer
walls were significantly reduced. In addition, the degree of
inner-wall RBM attenuation was similar to that observed for the
G-band of DGU-DWCNTs, indicating that much of the decrease in RBM
intensity was due to changes in film morphology and thickness
following acid treatment, not from large amounts of impurity
SWCNTs.
[0093] For the tangential modes, the G.sup.+ components for the
SWCNTs and the DWCNT outer walls were upshifted by at least 5
cm.sup.-1 following acid treatment. In contrast, the G.sup.+
component corresponding to the inner walls of the DWCNTs was
essentially fixed in frequency with a small upshift of .about.1
cm.sup.-1 as a result of the chemical resistance afforded by the
outer wall.
[0094] Differences in the optical properties of pristine and
thionyl chloride doped SWCNTs and DWCNTs were studied to provide
further evidence of sorting by wall number. Thin films of
HiPco-SWCNTs, AD-SWCNTs, DGU-SWCNTs, and DGU-DWCNTs were immersed
in the acceptor-type dopant thionyl chloride (see U.
Dettlaff-Weglikowska et al., J. Am. Chem. Soc. 127, 5125 (2005))
for 12 hours followed by drying in air for another 12 hours.
[0095] FIG. 4 shows optical absorbance spectra of thin films of
HiPco-SWCNTs (162), AD-SWCNTs (164), DGU-SWCNTs, (166), and
DGU-DWCNTs (168) before (solid curves, a) and after (dashed curves,
b) exposure to thionyl chloride. The absorbencies of the four
pristine spectra were normalized to the .pi.-plasmon at about 280
nm with the spectra of the doped films scaled by the same factor.
The first-order, second-order, and third-order semiconducting
optical transitions are shaded and labeled S11, S22, and S33; and
the first-order and second-order metallic optical transitions are
shaded and labeled M11 and M22.
[0096] Before doping, all four sets of films exhibited strong
absorbance as a result of metallic and semiconducting chiralities
as labeled in FIG. 4. For the three SWCNT samples and the outer
wall of the DGU-DWCNTs, thionyl chloride treatment completely
suppressed the first-order semiconducting transitions (S11) as a
result of large shifts in the Fermi level induced by the dopant
molecules. Other peaks associated with the second-order
semiconducting transitions (S22) and first-order metallic
transitions (M11) also were attenuated and broadened but could
still be identified following treatment. In contrast, the DGU-DWCNT
film maintains several sharp and intense absorption peaks in the
S22, S33, and M11 regions as indicated by arrows in FIG. 4 before
and after chemical treatment. Furthermore, the peaks corresponding
to inner semiconducting carbon nanotubes in the 1,050 nm to 1,250
nm wavelength region displayed considerably smaller line widths
than those of semiconducting HiPco-SWCNTs of the same diameter.
Peak broadening typically occurs for SWCNTs as a result of
increased nanotube-nanotube interactions when SWCNTs form bundles
in thin films. The absence of peak broadening observed for the
transitions in DGU-DWCNTs is yet further evidence that the inner
walls of the DWCNTs are protected from the external environment by
the outer carbon nanotube shell.
Example 6
Length Distribution of Sorted Carbon Nanotubes
[0097] Following optical characterization, the length distribution
of DGU-SWCNTs and DGU-DWCNTs were quantified using atomic force
microscopy (AFM). AFM samples were prepared as follows.
[0098] Preparation of AFM Samples: Surfactant-Encapsulated Carbon
Nanotubes were Deposited on SiO.sub.2 capped Si wafers via a
(3-aminopropyl)triethoxysilane (APS; Sigma-Aldrich) self-assembled
monolayer. The monolayer was formed by immersing the Si wafers in
2.5 mM APS aqueous solution for at least 30 minutes. Following
immersion, the wafers were dried under nitrogen gas, rinsed in
deionized water, and dried again. To improve the yield of the
deposition, the carbon nanotube dispersions were diluted into a 2%
w/v SDS aqueous solution (Sigma-Aldrich). A 10 uL drop of the
diluted dispersion was then placed on the silanized substrate and
allowed to dry over 10 minutes. The remaining solution was blown
off with nitrogen gas, and the wafer was rinsed in deionized water
to remove adsorbed surfactant molecules. After drying with
nitrogen, the wafers covered with carbon nanotubes at sub-monolayer
coverage were heated at 250.degree. C. in air for one hour to
remove any remaining surfactants and iodixanol.
[0099] AFM images were obtained using a Thermo Microscopes
Autoprobe CP-Research AFM operating in tapping mode. Conical AFM
probes with a Cr--Au backside coating were used for all
measurements (MikroMasch, NSC36/Cr--Au BS). Images 5 .mu.m.times.5
.mu.m in size were analyzed for determining the DGU-SWCNT and
DGU-DWCNT length distributions and typically contained 10 to 30
carbon nanotubes each. Intersecting nanotubes whose paths could not
be clearly identified and those that were clearly part of bundles
were not included in the analysis.
[0100] The lengths of 356 individual DGU-SWCNTs and 392 individual
DGU-DWCNTs were measured over multiple AFM images and compiled into
the histogram shown in FIG. 5. The DGU-SWCNTs (open bars) were
found to have an average length of about 626 nm, while the
DGU-DWCNTs (shaded bars) were determined to be about 44% longer on
average, with a mean length of about 904 nm. Both length
distributions were found to follow log-normal distributions as
indicated by the solid and dashed curves in FIG. 5. The difference
in carbon nanotube length as a function of wall number can be
attributed to improved mechanical properties of DWCNTs compared to
SWCNTs. In particular, the sonication used to disperse carbon
nanotubes into surfactant solution is known to decrease nanotube
length as a result of tube cutting. However, DWCNTs, with the
mechanical reinforcement of their inner wall, are expected to be
more resistant to sonication-induced cutting compared to SWCNTs.
Because both the DGU-SWCNTs and DGU-DWCNTs were exposed to
identical sonication conditions, it follows that the DWCNT material
should exhibit longer average lengths than the SWCNTs, assuming
both classes of material had the same initial length
distribution.
[0101] The observed resistance to tube cutting could be
particularly beneficial in thin film applications of carbon
nanotubes. The performance of thin film devices consisting of
networks of carbon nanotubes previously has been hindered by two
interrelated factors: nanotube polydispersity and nanotube length.
Most applications of carbon nanotube thin films, such as flexible
field effect transistors and transparent conductors, can benefit
greatly from the use of carbon nanotubes that are monodisperse in
both electronic type and diameter (see Arnold et al., Nature
Nanotech., 1:60 (2006); and Green et al., Nano Lett., 8: 1417
(2008)). However, to allow processing, carbon nanotube bundles need
to be separated into individual carbon nanotubes on a large scale.
Unfortunately, ultrasonication, the most common separation
technique, reduces carbon nanotube length and as a result,
increases the number of nanotube-nanotube junctions required for
charge transport across the network, thereby impairing nanotube
network performance. In view of the above, DWCNTs could prove to be
an ideal class of carbon nanotubes for thin film device
applications because they can maintain relatively long average
lengths even after aggressive sonochemistry.
Example 7
Transparent Conductive Coatings Including Sorted Carbon
Nanotubes
[0102] To test the thin film advantages of DGU-DWCNTs, DGU-DWCNTs
were incorporated into a series of transparent conductive coatings
and their performance was compared to that of DGU-SWCNTs films and
films prepared from unsorted DWCNT materials. Specifically, the
unsorted DWNT film samples were prepared from the same dispersion
of sonicated carbon nanotubes used to produce the DGU-SWCNT and
DGU-DWCNT materials (Example 1). A 1.5 mL volume of this starting
dispersion was centrifuged for 30 minutes at 16,000 relative
centrifugal force (Eppendorf Microcentrifuge 5424). The top 1.0 mL
of solution, free of large bundles and poorly solubilised material,
was decanted carefully and incorporated into control films of
unsorted DWCNTs using vacuum filtration and transferred to glass
substrates (see Wu et al., Science, 305: 1273 (2004)).
[0103] The sheet resistance and optical transmittance of the films
were measured using a four-point probe and a spectrophotometer
(Cary 500, Varian Inc.), respectively. For quantitative assessment
of the behavior of each transparent conductive material, the
experimental data was fit to the following equation:
T = ( 1 + 1 2 R s .mu. 0 0 .sigma. op .sigma. dc ) - 2 , ( 1 )
##EQU00001##
where R.sub.S is the sheet resistance, T is the wavelength
dependent transmittance, .sigma..sub.op is the optical conductivity
which varies as a function of wavelength, .sigma..sub.dc is the
direct current conductivity, and .mu..sub.0 and .epsilon..sub.0 are
the permeability and permittivity of free space, respectively.
Equation 1 has been used previously to describe the behavior of
carbon nanotube transparent conductors (see Hu et al., Nano Lett.,
4: 2513 (2004); and Zhou et al., Appl. Phys. Lett., 88: 123109
(2006)) and is particularly valuable in assessing the performance
of DGU-processed material, because the sole fitting parameter
.sigma..sub.op/.sigma..sub.dc can be used to concisely quantify the
performance improvement offered by different carbon nanotube
materials (see Green et al., Nano Lett., 8: 1417 (2008)).
[0104] For pristine DGU-DWCNTs at a wavelength of about 550 nm,
.sigma..sub.op/.sigma..sub.dc is 0.12 compared to 0.29 for
DGU-SWCNTs and 0.58 for unsorted materials (FIG. 6a). This
difference implies that transparent conductors containing
DGU-DWCNTs are approximately 2.4 times and 4.8 times more
conductive than those containing DGU-SWCNTs and unsorted nanotubes,
respectively. As a result, a pristine DGU-SWCNT transparent
conductor would have a sheet resistance of
.about.352.OMEGA./.quadrature. at 75% transmittance while a
DGU-DWCNT film at the same transmittance would provide a sheet
resistance of .about.146.OMEGA./.quadrature..
[0105] To further enhance the electrical conductivity of the films,
the carbon nanotube thin films were doped with thionyl chloride
using the same procedure employed when studying the effects of
doping on nanotube optical absorbance. Doping increased the thin
film conductivity of the DGU-DWCNTs, DGU-SWCNTs and unsorted
nanotubes by factors of 2.2, 3.8 and 2.0, respectively, at 550 nm
(FIG. 6b). The differences in the effects of doping between the
sorted materials could be due in part to higher metallic carbon
nanotube content in the DGU-DWCNTs compared to the DGU-SWCNTs as
indicated by differences in the relative intensities of first-order
metallic and semiconducting transitions in the DGU-SWCNTs and
DGU-DWCNTs (FIG. 4). In addition, the presence of inner walls in
the DWCNTs could mitigate some of the effects of the adsorbed
dopants. The lower conductivity of unsorted nanotubes compared to
DGU-DWCNTs and DGU-SWCNTs can be explained by the presence of
carbonaceous impurities in unsorted films that are typically
removed during sorting. These impurities not only reduce the
conductivity of pristine unsorted films, but also do not respond as
strongly to chemical doping. Despite the larger conductivity
increase demonstrated by the DGU-SWCNTs after thionyl chloride
exposure, doped DGU-DWCNT films still exhibited .about.42% higher
conductivity than doped DGU-SWCNT networks. Furthermore, doped
films generated from more weakly sonicated, longer DGU-DWCNTs
produced sheet resistances as low as 40.OMEGA./.quadrature. at 70%
transmittance at a wavelength of 550 nm. The exemplary properties
of the DGU-DWCNT films make them promising candidates for
high-performance transparent conductor applications.
Example 8
Sorting of Carbon Nanotubes by Both Wall Number and Outer Wall
Electronic Type
[0106] Sorting by both wall number and (outer wall) electronic type
were conducted in gradients with density profiles identical to
those described in Examples 2 and 3 but with different surfactant
loadings. The density profiles consisted of a 1.5 mL, 60% iodixanol
underlayer, followed by a 1 mL carbon nanotube layer with a density
of 35% w/v iodixanol. On top of this layer was a 5 mL linear
gradient with densities ranging from 32.5% to 17.5% w/v iodixanol
that was topped by a .about.4.5 mL overlayer with 0% w/v
iodixanol.
[0107] FIG. 7a and FIG. 7c show photographs of centrifuge tubes 200
and 300 each containing a dispersion of a polydisperse carbon
nanotube sample (including a mixture of large-diameter SWCNTs and
DWCNTs) after one separation cycle according to the present
methods. The samples shown in FIG. 7a and FIG. 7c were sorted using
surface active components designed to optimize separation of DWCNTs
having a semiconducting outer wall (s-DWCNTs) and DWCNTs having a
metallic outer wall (m-DWCNTs), respectively. For semiconductor
enrichment, the overall surfactant loading was set to 1% w/v with a
1:4 SDS/SC ratio (by weight); hence, the gradient contained 0.2%
w/v SDS and 0.8% w/v SC. For metallic enrichment, the overall
surfactant loading was also 1% w/v with a 3:2 SDS/SC ratio. The two
density gradients were ultracentrifuged in a SW41 Ti rotor for 12
hours at a rotational frequency of 41 krpm. Following
ultracentrifugation, the carbon nanotubes enriched by electronic
type were removed using a piston gradient fractionator.
[0108] Referring to FIG. 7a, two pairs of colored bands of sorted
material appeared in the centrifuge tube 200. The upper bands were
produced from SWCNTs sorted by electronic type (s-SWCNT band 202 in
red, m-SWCNT band 204 in green) while the lower pair of bands
corresponded to DWCNTs sorted by electronic type which possessed
higher buoyant densities as a result of the density of the inner
DWCNT wall (s-DWCNT band 206 in red, m-DWCNT band 208 in
green).
[0109] Referring to FIG. 7c, again two pairs of colored bands of
sorted material appeared in the centrifuge tube 300. However, the
upper pair of bands are both green in color this time (m-SWCNTs
302, m-DWCNTs 304) suggesting metallic nanotubes, while the lower
pair of bands are both red in color, suggesting semiconducting
nanotubes (s-SWCNTs 306, s-DWCNTs 308).
[0110] Optical absorbance spectra confirmed the band assignment
(FIGS. 6b and 6d). Enrichment by electronic type is evidenced by
changes in the relative amplitude of transitions associated with
metallic and semiconducting chiralities in each of the bands.
Enrichment in metallic carbon nanotubes (214, 218, 312, 314) is
indicated by strong absorbance in the wavelength ranges (M11, M22)
associated with metallic carbon nanotubes and weak absorbance in
the ranges (S22, S33) associated with semiconducting carbon
nanotubes, while enrichment in semiconducting species (212, 216,
316, 318) is evidenced by the converse. DWCNTS (solid curves) are
marked by characteristic optical absorbance peaks associated with
their inner walls (labeled with arrows) that are absent in the
absorbance of SWCNTs (dashed curves). The absorbance of the
starting material is represented by curves 220 and 320.
Example 9
Comparison with State-of-the-Art High-Purity, As-Synthesized
Double-Walled Carbon Nanotubes
[0111] The literature has reported as-synthesized DWCNTs that are
nominally about 95% pure. See Endo et al., Nature, vol. 433, 476
(2005), and Kim et al., Chem. Vap. Deposition, vol. 12, 327-330
(2006).
[0112] To compare the purity of these state-of-the-art materials
with those of the sorted DWCNTs of the present teachings
("DGU-DWCNTs"), Raman spectra of DGU-DWCNTs were collected at
nearly the same excitation energies and compared to the data
reported in Barros et al., Phys. Rev. B, 76: 045425 (2007) ("Barros
data"). The Barros data include Raman spectra of undoped DWCNTS
(solid curves) and DWCNTs doped with H.sub.2SO.sub.4 (dashed
curves), and are shown in FIG. 8a. Data of DGU-DWCNTs are shown in
FIG. 8b. As can be seen from these spectra, the Barros DWCNTs and
DGU-DWCNTs have very similar diameters (about 1.6 nm outer diameter
and about 0.9 nm inner diameter) as indicated by similar
frequencies of RBM peaks.
[0113] The DGU-DWCNTs only have two small peaks that can be
identified as SWCNTs (as a result of their extreme diameters;
indicated by arrows in FIG. 8b). In contrast, many peaks that can
be associated with SWCNTs are apparent in the Barros data (see FIG.
8a). For instance, the shaded region covering frequencies ranging
from about 200 cm.sup.-1 to about 225 cm.sup.-1 contains strong RBM
peaks at multiple excitation energies. These peaks are associated
with nanotubes of about 1 nm to about 1.2 nm. Because the interwall
spacing of DWCNTs is about 0.7 nm, if these 1-1.2 nm diameter
nanotubes are the inner walls of DWCNTs, their outer walls must be
about 1.7 nm to about 1.9 nm in diameter. However, no RBM peaks
corresponding to nanotubes having this outer wall diameter can be
observed in the Barros data (marked by the shaded region covering
frequencies ranging from about 125 cm.sup.-1 to about 145
cm.sup.-1). Consequently, these Raman data suggest that DGU-DWCNTs
according to the present teachings are of higher purity than the
state-of-the-art as-synthesized DWCNTs reported in the
literature.
Example 10
Further Enrichment of Double-Walled Carbon Nanotubes by Electronic
Type
[0114] Carbon nanotubes (Batch #: DW411UA) obtained from Carbon
Nanotechnologies, Inc. (Houston, Tex.) were dispersed into a 1% w/v
SC aqueous solution as described in Example 1. The dispersion was
concentrated and coarsely sorted by wall number in a single
centrifugation step. This processing was accomplished by forming a
7 mL linear density gradient containing 1% w/v SC that varied from
25% w/v iodixanol to 40% w/v iodixanol and adding approximately 31
mL of the carbon nanotube solution on top. The resulting density
gradient was ultracentrifuged in an SW28 rotor (Beckman-Coulter
Inc.) for 22 hours at a rotational frequency of 28 krpm. The sorted
carbon nanotubes were then removed from the centrifuge tube in a
two step fractionation procedure. First, a 4 mL displacement layer
consisting of 1% w/v SC aqueous solution with 40% w/v iodixanol was
slowly infused into the gradient to separate poorly dispersed
carbon nanotubes from the buoyant, individually encapsulated
materials. After upward displacement, the sorted carbon nanotube
fractions with densities less than 40% w/v iodixanol were removed
using a piston gradient fractionator (Biocomp Instruments Inc.).
Following characterization using optical absorbance, the fractions
with the highest DWCNT content were selected for electronic type
sorting.
Selective Separation of Double-Walled Carbon Nanotubes Having a
Metallic Outer Wall
[0115] Fractions highly enriched in DWCNTs having a metallic outer
wall ("highly enriched m-DWCNTs") were produced in a two-iteration
sorting process with density gradient parameters summarized in
Table 1. In the first iteration, the DWCNT enriched material was
injected at the bottom of the linear density gradient and the
nanotubes moved upward in the density gradient during
ultracentrifugation. This step removed dense carbon nanotube
bundles and multi-walled carbon nanotubes, and resulted in pairs of
metallic carbon nanotube and semiconducting carbon nanotube bands
similar to those shown in FIG. 7c. Fractions enriched in m-DWCNTs
from the first iteration were then placed at the top of the linear
density gradient for the second iteration. As a result, buoyant,
slow sedimenting SWCNTs were removed from the material during this
separation leaving highly enriched m-DWCNT material in the denser
regions of the gradient.
TABLE-US-00001 TABLE 1 m-DWCNT DGU Parameters First Iteration
Second Iteration Surfactant 1% w/v 3:2 SDS/SC throughout 1% w/v 3:2
SDS/SC throughout Overlayer 4.5 mL, 0% w/v iodixanol 8 mL, 0% w/v
iodixanol Position of Bottom of linear gradient Top of linear
gradient Nanotube Layer Nanotube Layer 1 mL, 37.5% w/v iodixanol;
10 mL, 3% w/v iodixanol; m- DWCNT enriched material DWCNT enriched
material from first iteration Linear Density 5 mL, 20% to 35% w/v
iodixanol 15 mL, 25% to 40% w/v iodixanol Gradient Underlayer 1.5
mL, 60% w/v iodixanol 5 mL, 60% iodixanol Ultracentrifugation SW41
Ti, 12 hours at 41 krpm and 22.degree. C. SW28, 22 hours at 28 krpm
and 22.degree. C. Parameters
Selective Separation of Double-Walled Carbon Nanotubes Having a
Semiconducting Outer Wall
[0116] Fractions highly enriched in DWCNTs having a semiconducting
outer wall ("highly riched s-DWCNTs") were produced in a
three-iteration sorting process with density gradient parameters
summarized in Table 2. In the first iteration, the DWCNT enriched
material was injected at the bottom of the linear density gradient
and the nanotubes moved upward in the density gradient during
ultracentrifugation. This step removed dense carbon nanotube
bundles and multi-walled carbon nanotubes, and resulted in a pair
of SWCNT and DWCNT bands similar to those shown in FIG. 7a.
Fractions enriched in s-DWCNTs from the first iteration were then
placed at the top of the linear density gradient for the second
iteration. As a result, most buoyant, slow sedimenting SWCNTs were
removed from the material during this separation leaving enriched
s-DWCNT material in the denser regions of the gradient. Optical
absorbance of these s-DWCNTs revealed the presence of >1.7 nm
diameter m-SWCNTs. A third iteration in 1% w/v 3:2 was utilized to
remove these impurity species, which banded in a lower density
region of the gradient following ultracentrifugation.
TABLE-US-00002 TABLE 2 s-DWCNT DGU Parameters First Iteration
Second Iteration Third Iteration Surfactant 1% w/v 1:4 SDS/SC 1%
w/v 1:4 SDS/SC, 1% w/v 3:2 SDS/SC throughout except nanotube layer
throughout Overlayer 4.5 mL, 0% w/v 8 mL, 0% w/v 2 mL, 0% w/v
iodixanol iodixanol iodixanol Position of Bottom of linear Top of
linear Top of linear Nanotube Layer gradient gradient gradient
Nanotube Layer 1 mL, 37.5% w/v 10 mL, 3% w/v 3.5 mL, 3% w/v
iodixanol; iodixanol; iodixanol; DWCNT enriched s-DWCNT enriched
s-DWCNT enriched material material from material from first
iteration; 1% second iteration w/v SC with small amount of SDS
Linear Density 5 mL, 20% to 35% w/v 15 mL, 25% to 40% w/v 5 mL, 25%
to 40% w/v Gradient iodixanol iodixanol iodixanol Underlayer 1.5
mL, 60% w/v 5 mL, 60% iodixanol 1.5 mL, 60% iodixanol iodixanol
Ultracentrifugation SW41 Ti, 12 hours at SW28, 22 hours at SW41 Ti,
12 hours at Parameters 41 krpm and 22.degree. C. 28 krpm and
22.degree. C. 41 krpm and 22.degree. C.
Characterization of Highly Enriched s-DWCNTs and m-DWCNTs
[0117] Highly enriched s-DWCNTs and m-DWCNTs were characterized by
optical absorbance and Raman spectroscopy to confirm enrichment by
wall number and electronic type. FIG. 9 shows the optical
absorbance from these materials in aqueous solution. The spectrum
acquired from s-DWCNTs exhibits strong absorption peaks in the S22
and S33 regions associated with outer wall semiconducting
transitions, while the absorption in the M11 region arising from
metallic outer wall species is strongly suppressed. These features
are consistent with outer wall semiconductor enrichment. The
opposite behavior in the S22, S33, and M11 regions can be seen in
the m-DWCNT material, which is consistent with outer wall metal
enrichment. Weak peaks marked by arrows in FIG. 9 can be attributed
to the DWCNT inner walls.
[0118] The optical absorbance of thin films of highly enriched
s-DWCNTs and m-DWCNTs were measured to more conclusively identify
optical transitions from the inner wall of the DWCNTs. Pristine
thin films of the highly enriched s-DWCNTs (FIG. 10a, solid curve)
and highly enriched m-DWCNTs (FIG. 10b, solid curve) exhibited
strong absorbance peaks in the outer wall semiconductor (S11, S22,
and S33) and metal (M11) transition ranges, respectively. Upon
doping with thionyl chloride (dashed curves in FIG. 10a and FIG.
10b), the absorption peaks arising from the DWCNT outer walls were
suppressed, leaving the inner wall transitions largely unaffected
due to the protection afforded by the outer wall. These inner wall
transitions are labeled with arrows and confirm that these
materials contain predominantly double-walled species.
[0119] For further evidence of isolation by outer wall electronic
type, Raman spectra were obtained from s-DWCNT and m-DWCNT films
using a laser excitation wavelength of 514.5 nm (FIG. 11). This
wavelength was selected because it is in resonance with the
semiconducting DWCNT outer walls and the metallic DWCNT inner
walls. For both materials, RBM peaks in the 240 cm.sup.-1 to 360
cm.sup.-1 range corresponding to nanotube diameters of 0.64 nm to
0.99 nm can be observed, a diameter range expected for DWCNT inner
walls. In contrast, the strong RBM peak in the 150 cm.sup.-1 to 190
cm.sup.-1 range corresponding to outer wall diameters of 1.28 nm to
1.67 nm observed in the s-DWCNT material is suppressed by
.about.85% in the m-DWCNTs. Because predominantly s-DWCNTs are
excited at 514.5 nm, the weakness of this particular RBM peak in
the m-DWCNTs confirms that the material contains only low levels of
s-DWCNTs.
Calculation of Purity Levels of Highly Enriched s-DWCNTs and
m-DWCNTs
[0120] The level of enrichment by electronic type of the m-DWCNTs
and s-DWCNTs was calculated following an established method (see
Green et al., Nano Lett., 8: 1417 (2008)). In this analysis,
optical absorbance spectra were used to calculate the areas in
energy space of the first order metallic (M11) and first order
semiconducting (S11) absorbance peaks of the m-DWCNTs and s-DWCNTs,
as well as a control sample of unsorted AD-SWCNTs with known
one-third metallic carbon nanotube content. The absorbance areas of
the AD-SWCNT sample were then used to determine the extinction
coefficient of the metallic carbon nanotubes, which was found to be
a factor of 1.5 less than that of semiconducting carbon nanotubes.
With this relative extinction factor, the percent metallic purities
of the sorted DWCNTs were calculated using the following
equation:
% Metallic Purity=1.5.times.(M11 Area)/[1.5.times.(M11 Area)+(S11
Area)]
[0121] The absorbance peak areas and calculated metallic purity
levels are presented in Table 3. Intermediate levels of percent
metallic purity can be obtained by combining different amounts of
m-DWCNTs and s-DWCNTs.
TABLE-US-00003 TABLE 3 m-DWCNT and s-DWCNT Electronic Type Purity
Levels Area of Absorbance Peak % Metallic Sample M11 S11 Purity
AD-SWCNTs 2.44 7.31 33.3% m-DWCNTs 41.6 2.62 96.0% s-DWCNTs 0.532
19.1 4.0%
Example 11
Field-Effect Transistors Produced from Thin Films of Highly
Enriched s-DWCNTs and Highly Enriched m-DWCNTs
[0122] Thin film field-effect transistors were fabricated with the
highly enriched s-DWCNT materials ("s-DWCNT devices") and the
highly enriched m-DWCNT materials ("m-DWCNT devices") as the active
layer in the devices, respectively. To prepare these devices,
gold/palladium electrodes were first defined on degenerately doped
silicon wafers capped by a 100 nm thick dry thermal oxide.
Percolating thin film networks of s-DWCNTs and m-DWCNTs produced by
vacuum filtration were transferred onto the wafers and annealed in
air for 20 minutes at 225.degree. C. The devices were tested with a
probe station (M-150, Cascade Microtech Inc.) inside a light-tight
enclosure using two source meter units (KE2400, Keithley Inc.).
Gate voltages were applied through the silicon substrate and gate
leakage currents were monitored to ensure that they did not
contribute significantly to the measured source-drain current. The
devices have channel lengths and widths of 4 .mu.m and 250 .mu.m,
respectively.
[0123] Transfer curves for the s-DWCNT devices (solid curves) and
the m-DWCNT devices (dashed curves) are shown in FIG. 12a. While
both types of devices were modulated in response to the gate
voltage and had similar on currents, the s-DWCNT network exhibited
an on/off ratio .about.100 larger than that of the m-DWCNT network.
The higher switching ratio of the s-DWCNT devices was expected as a
result of its high semiconductor content. The switching ratio of a
film of metallic single-walled carbon nanotubes was reported to be
.about.2 (see Arnold et al., Nature Nanotech., 1: 60 (2006)). The
higher on/off ratio observed in the m-DWCNT devices could be due to
current modulation through the semiconducting inner walls of the
m-DWCNTs.
[0124] FIG. 12b presents the variations in on/off ratios and on
currents for the s-DWCNT devices (open symbols) and the m-DWCNT
devices (closed symbols) at 2 .mu.m (squares) and 4 .mu.m
(triangles) channel lengths. A significant increase in on/off
ratios (10.sup.2) for the s-DWCNT devices was observed at various
on currents and for both channel lengths. Treating the s-DWCNT
network as a uniform semiconducting film (see Arnold et al., Nature
Nanotech., 1: 60 (2006)), it is possible to extract a lower bound
for the mobility of these devices from the linear region of the
transfer curve. This analysis results in effective mobilities of
.about.7.6 cm.sup.2/V-s and .about.4.2 cm.sup.2/V-s for the s-DWCNT
devices of 2 .mu.m and 4 .mu.m channel lengths, respectively.
[0125] The present teachings encompass embodiments 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 present
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.
* * * * *