U.S. patent application number 11/285313 was filed with the patent office on 2006-10-26 for method, arrangement and use of an arrangement for separating metallic carbon nanotubes from semi-conducting carbon nanotubes.
This patent application is currently assigned to Forschungszentrum Karlsruhe GmbH. Invention is credited to Frank Hennrich, Ralph Krupke.
Application Number | 20060242741 11/285313 |
Document ID | / |
Family ID | 32892320 |
Filed Date | 2006-10-26 |
United States Patent
Application |
20060242741 |
Kind Code |
A1 |
Krupke; Ralph ; et
al. |
October 26, 2006 |
Method, arrangement and use of an arrangement for separating
metallic carbon nanotubes from semi-conducting carbon nanotubes
Abstract
A method for separating metallic carbon nanotubes and
semi-conducting carbon nanotubes includes providing a suspension
containing a plurality of individual metallic carbon nanotubes and
semi-conducting carbon nanotubes in a liquid, for which the
dielectric constant .epsilon..sub.L meets the requirement:
.epsilon..sub.M>.epsilon..sub.L>.epsilon..sub.H, wherein
.epsilon..sub.M is the dieletric constant of the metallic carbon
nanotubes and .epsilon..sub.H is the dielectric constant of the
semi-conducting carbon nanotubes. A a non-homogeneous electric
alternating field is applied to the suspension to create spatially
separate species of the metallic carbon nanotubes and the
semi-conducting carbon nanotubes. At least one of the separate
species is then removed.
Inventors: |
Krupke; Ralph; (Stutensee,
DE) ; Hennrich; Frank; (Karlsruhe, DE) |
Correspondence
Address: |
VENABLE LLP
P.O. BOX 34385
WASHINGTON
DC
20045-9998
US
|
Assignee: |
Forschungszentrum Karlsruhe
GmbH
Karlsruhe
DE
|
Family ID: |
32892320 |
Appl. No.: |
11/285313 |
Filed: |
November 23, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10815977 |
Apr 2, 2004 |
|
|
|
11285313 |
Nov 23, 2005 |
|
|
|
Current U.S.
Class: |
209/12.1 ;
209/12.2; 977/845 |
Current CPC
Class: |
B03C 5/005 20130101;
B82Y 40/00 20130101; C01B 2202/22 20130101; C01B 32/172 20170801;
B82Y 30/00 20130101 |
Class at
Publication: |
977/845 ;
209/012.2 |
International
Class: |
B07B 15/00 20060101
B07B015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 8, 2003 |
DE |
103 15 897.9 |
Claims
1. An arrangement for separating metallic carbon nanotubes from
semi-conducting carbon nanotubes, comprising: a semi-conducting
substrate; an insulating layer deposited on the semi-conducting
substrate; and metal electrodes deposited on the insulating layer
which are connectable via contacts to an alternating voltage
source.
2. The device according to claim 1, wherein the semi-conducting
substrate comprises silicon, the insulation layer comprises silicon
dioxide and the metal electrodes comprise gold.
3. The device according to claim 2, wherein the silicon is doped
with boron.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 10/815,977, filed on Apr. 2, 2004 which claims the priority of
German Patent Application No. 10315897.9, filed on Apr. 8, 2003,
the subject matter of both foregoing applications being
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The invention relates to a method, an arrangement and the
use of an arrangement for separating metallic carbon nanotubes from
semi-conducting carbon nanotubes.
[0003] Macro-molecules in which carbon atoms form the outside wall
of a tube are called carbon nanotubes. For the prototype, a
single-wall carbon nanotube is described with the aid of a planar
ribbon of hexagonally arranged carbon atoms, which is rolled up
seamlessly to form a tube. Several concentric tubes, arranged one
inside the other, are referred to as multi-wall carbon
nanotubes.
[0004] Typical single-wall carbon nanotubes have a diameter of 0.5
nm to 100 nm while multi-wall carbon nanotubes have a
correspondingly larger diameter. Typical carbon nanotubes ranges in
length from 100 nm to a few 10 micrometers, wherein the carbon
nanotubes can be cut into smaller sections as well as extended by
fitting them together for some methods.
[0005] Carbon nanotubes are divided into two categories because of
their electronic characteristics: metallic carbon nanotubes and
semi-conducting carbon nanotubes. Metallic carbon nanotubes are
suitable for use as molecular wires with extremely high
current-carrying capacity which are resistant to electro-migration.
Semi-conducting carbon nanotubes are particularly suitable as
molecular transistors. Both types represent promising components
for nano-electronic circuits because of their nanoscale
dimensions.
[0006] It is absolutely necessary for the production of nanoscale
circuits from carbon nanotubes that metallic and semi-conducting
carbon nanotubes can be manipulated separately, thus requiring
metallic carbon nanotubes or semi-conducting carbon nanotubes to be
produced as type-specific as possible and/or to separate these.
SUMMARY OF THE INVENTION
[0007] Starting with this premise, it is the object of the present
invention to provide a method, an arrangement and the use of an
arrangement for separating metallic carbon nanotubes from
semi-conducting carbon nanotubes.
[0008] The above and other objects are accomplished according to
the invention by the provision of a method for separating metallic
carbon nanotubes and semi-conducting carbon nanotubes, which
according to an exemplary embodiment comprises: providing a
suspension containing a plurality of individual metallic carbon
nanotubes and semi-conducting carbon nanotubes in a liquid, for
which the dielectric constant .epsilon..sub.L meets the
requirement: .epsilon..sub.M>.epsilon..sub.L>.epsilon..sub.H,
wherein .epsilon..sub.M is the dieletric constant of the metallic
carbon nanotubes and .epsilon..sub.H is the dielectric constant of
the semi-conducting carbon nanotubes; applying a non-homogeneous
electric alternating field to the suspension to create spatially
separate species of the metallic carbon nanotubes and the
semi-conducting carbon nanotubes; and removing at least one of the
separate species.
[0009] With the method according to the invention, metallic carbon
nanotubes and semi-conducting carbon nanotubes, which are both
present in a liquid as suspension, can be separated from each other
in such a way that they can respectively be processed further.
[0010] The method according to the invention is based on the fact
that carbon nanotubes are initially placed into a liquid in such a
way that they do not adhere to each other. If the carbon nanotubes
are not present separately, but agglomerate into bundles between
the tubes as a result of a van-der-Waals interaction, no separation
of the species would occur because a single bundle as a rule is
formed from metallic carbon nanotubes as well as semi-conducting
carbon nanotubes. Statistically, the overwhelming number of bundles
therefore contains at least one metallic carbon nanotube, so that
nearly all bundles on the whole would behave like a metallic carbon
nanotube. A method for separating the carbon nanotubes is disclosed
in Bachilo S. M. et al, Science, Volume 298 (2002), page 2361.
[0011] For the method according to the invention, on the other
hand, it is not important how the carbon nanotubes are produced.
Known methods for this are, among others, the laser ablation, the
disproportioning of carbon monoxide (HiPCO), the arc discharging
methods (arc-discharge) or the chemical vapor deposition (CVD).
[0012] For the method according to the invention, it is critical
that the dielectric constant of liquid .epsilon..sub.L, fulfils the
following requirement:
.epsilon..sub.M>.epsilon..sub.L>.epsilon..sub.H, (1) wherein
.epsilon..sub.M is the dielectric constant of the metallic carbon
nanotube and .epsilon..sub.H is the dielectric constant of the
semi-conducting carbon nano tube. It is possible to deduce from
theoretical reflections that the value of the dielectric constant
for the metallic carbon nanotubes is .epsilon..sub.M>1000,
meaning it is very high, while the dielectric constant for the
semi-conducting carbon nanotubes assumes a low value of
.epsilon..sub.H.apprxeq.10. The use of polar liquids and in
particular a watery solution with a dielectric constant
.epsilon..sub.L=81 is consequently preferred as a starting point
for the method according to the invention.
[0013] An arrangement for generating non-homogeneous electric
alternating fields is furthermore needed to realize the method
according to the invention. Arrangements of this type are known to
some extent from the dielectrophoresis. For example, U.S. Patent
Application Publication No. 2003/0048619 A1 discloses an
arrangement and a method for producing electrically-conducting
micro-wires, in particular made of nanoscale gold particles ranging
in size from 15 nm to 30 nm, which are formed between a
spaced-apart electrode pair with a gap of several micrometers to a
few centimeters by applying an alternating field of 50V to 250V
with a frequency of 50 Hz to 1 kHz.
[0014] According to the present invention, the suspension
containing both species of carbon nanotubes is subjected to a
non-homogeneous electrical alternating field. Alternatively, the
suspension is introduced into an alternating field of this type. In
the non-homogeneous electrical alternating field, a carbon nanotube
is subjected to dielectrophoretic forces F(.omega.) as follows: F
.function. ( .omega. ) .varies. Re [ T .function. ( .omega. ) ]
.times. Re .function. [ T .function. ( .omega. ) - I .function. (
.omega. ) T .function. ( .omega. ) - I .function. ( .omega. ) ]
.times. .gradient. E rms 2 , ( 2 ) ##EQU1## wherein .epsilon..sub.T
is the dielectric constant of the carbon nanotube under
consideration, .epsilon..sub.L is the dielectric constant of the
liquid, .epsilon..sub.rms is the effective value of the electrical
field intensity and .omega. is the frequency of the electrical
alternating field.
[0015] As long as the suspension is subject to a non-homogeneous
electrical alternating field, the metallic carbon nanotubes will
move according to equation (2), with
.epsilon..sub.T=.epsilon..sub.M, in the direction of the field
gradient, meaning they experience forces with positive mathematical
signs which take them to areas of higher field intensity.
Accordingly, semi-conducting carbon nanotubes are subjected to
forces, where .epsilon..sub.T=.epsilon..sub.H, with negative
mathematical sign and, in contrast to the metallic species, are
pushed in the opposite direction and out of the electrical
alternating field.
[0016] In this way, metallic carbon nanotubes and semi-conducting
carbon nanotubes can be separated spatially and can subsequently be
separated in different ways. For example, a substrate on which the
metallic carbon nanotubes have been deposited can be washed off
with the ultrasound effect.
[0017] Following the deposit of all metallic carbon nanotubes on a
substrate which is subsequently removed from the suspension, it is
possible according to a special embodiment and an additional step
to separate additional components from the semi-conducting carbon
nanotubes as suspension in a second liquid with a dielectric
constant .epsilon..sub.L'<.epsilon..sub.H. Since the dielectric
constant of the semi-conducting carbon nanotubes has a low value of
.epsilon..sub.H.apprxeq.10, non-polar substances with a low
dielectric constant are suitable as a second liquid, e.g. toluene
(.epsilon.=5), cyclohexane (.epsilon.=2.0), benzene
(.epsilon.=2.3), or carbon tetrachloride (.epsilon.=2.2).
[0018] The frequency level of the applied electrical alternating
field must exceed a minimum value, derived from the consideration
that carbon nanotubes frequently carry an inherent static
electrical charge, that is to say independent of whether they
belong to the metallic or semi-conducting species. As a result of
this charge, the electrophoretic forces in the constant field
dominate the movement of the carbon nanotubes. To avoid this
effect, which is undesirable for separating the species, electric
alternating fields with sufficiently high frequencies, meaning
higher than 10 kHz, must therefore be used.
[0019] The non-homogeneous electric alternating field applied to
the suspension has a peak-to-peak field intensity that is selected
from the range between 10.sup.3 V/m and 10.sup.9 V/m, preferably
10.sup.4 V/m to 10.sup.6 V/m and especially preferred 10.sup.5 V/m.
With field intensities above 10.sup.9 V/m, marked changes in the
band structure of the carbon nanotubes occur, whereas with field
intensities below 10.sup.3 V/m, the separating effect of the
alternating field is too low.
[0020] The time scale for separating the two species is in the
minute range. However, the separation can also last several hours,
depending on the desired degree of separation. Following a time
period that is sufficient for the separation, the alternating
voltage remains connected or is turned off and the liquid
(suspension) containing the semi-conducting species in the
above-described, preferred embodiment is removed. The metallic
carbon nanotubes, on the other hand, which adhere to the surfaces
of the electrodes can be removed, for example with the aid of
ultrasound, to a second liquid as suspension.
[0021] Raman scattering provides proof that the method according to
the invention in reality effectively separates the two species of
carbon nanotubes. Carbon nanotubes show resonant Raman scattering
when admitted with electromagnetic radiation having a wavelength
for which the energy corresponds to the excitations between
electronic bands with high density of state, the so-called van Hove
Singularities. See for example Dresselhaus M. S. and Eklund P. C.,
Advances in Physics, Volume 6 (2000), pages 705-814.
[0022] Viewed geometrically, different vibration modes of the
single-wall carbon nanotubes are excited in that case. In the
process, the frequency of the radial breathing mode (in short
called rbm) represents a direct measure for the diameter of the
excited carbon nanotube.
[0023] If the diameter of the carbon nanotube is known, it can be
shown which of the two species is Raman active (see Ding et al.
Phys. Rev. B, Volume 66 (2002), page 73401) in the respective
wavelength range by taking into consideration the excitation energy
with the aid of ribbon-structure calculations for the metallic
and/or semi-conducting carbon nanotubes. The breathing modes of
metallic and semi-conducting carbon nanotubes below the selected
conditions (tube material, excitation wavelength) are far enough
apart so that theoretical deficiencies do not change the data
interpretation.
[0024] According to a further aspect of the invention, there is
provided a method of separating metallic carbon nanotubes from
semi-conducting carbon nanotubes comprising utilizing a cell which
is suitable for realizing a dielectrophoresis, whererin the cell
comprises at least two electrodes as well as a liquid suitable as a
dielectric and which meets the requirements of equation 1.
[0025] An arrangement according to the invention for separating
metallic carbon nanotubes and semi-conducting carbon nanotubes,
using a non-homogeneous electrical alternating field, comprises
electrode pairs that are separated from each other by a gap.
Suitable for this is a semi-conducting substrate, preferable
consisting of silicon and preferably boron-doped, onto which an
insulating layer preferably made of silicon dioxide is deposited.
Electrodes, preferably made of gold, are then deposited on the
silicon dioxide layer by means of electron-beam lithography and
subsequent metallizing, and an alternating voltage is applied to
the electrodes. As a result of the field distribution in this
arrangement, the metallic species drifts in the direction of the
electrodes and adheres to it--orientation of the induced dipole
moments in the metallic particles along the field lines--parallel
and side-by-side as well as at a right angle to the surface of the
electrodes. The semi-conducting carbon nanotubes, on the other
hand, remain in liquid suspension.
[0026] The separation of metallic carbon nanotubes and
semi-conductor carbon nanotubes is a precondition for a purposeful
generation of nano electronic structures, such as molecular wires
and/or field effect transistors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] In the following, the invention is explained in further
detail with an exemplary embodiment and with the aid of
Figures.
[0028] FIG. 1 is a schematic representation of an arrangement for
separating metallic carbon nanotubes from semi-conducting carbon
nanotubes in an alternating field with the aid of
dielectrophoresis.
[0029] FIG. 2 is a graph showing Raman spectra at different points
of a reference probe.
[0030] FIG. 3 is a graph showing Raman spectra at different points
between the metallic electrodes.
[0031] FIG. 4 is a graph showing a correlation between possible
excitation energies and a breathing mode frequency in relation to
the diameter of the carbon nanotubes.
DETAILED DESCRIPTION OF THE INVENTION
[0032] In a first step, a suspension of separated carbon nanotubes
is provided. Used as starting material are carbon nanotubes
produced with the so-called HiPco method as described in, for
example, P. Nikolaev, Chem. Phys. Lett., Volume 313 (1999), page
91. For this, 50 mg HiPco pipe soot is placed into a solution of
100 ml D.sub.2O with 1% sodium lauryl sulphate (SLS), a surfactant,
and is subjected to an ultrasound treatment for 10 minutes with an
ultrasound finger having a diameter of 13 mm and a capacity of 200
W. Subsequently, this suspension is centrifuged at 180000 g for 4
hours. Finally, the supernatant is carefully decanted from the
solid material.
[0033] FIG. 1 shows an exemplary embodiment of an arrangement for
separating metallic carbon nanotubes from semi-conducting carbon
nanotubes according to the invention. Metallic electrodes 3 for
generating non-homogeneous electric alternating fields are produced
on a substrate 1, 2 and are connected connected via terminals to a
an alternating voltage source 4. As shown in FIG. 1, a boron-doped
silicon with a specific electric resistance of p>1.OMEGA.cm is
used as substrate 1, onto which is deposited a 600 nm thick,
thermally oxidized insulating layer 2 of silicon dioxide
(SiO.sub.2), thus resulting in a total thickness of 525 .mu.m. The
lateral dimensions of the probe are 8 mm.times.4 mm.
[0034] An electrode structure is written into a resist of
polymethyl methacrylate (PMMA) by means of standard electron-beam
lithography and is subsequently developed. The metallizing of the
electrode structure according to FIG. 1 takes place in a
high-vacuum atomizer. For this, a 2 nm thick layer of titanium as
adhesion promoter and subsequently a 30 nm thick layer of gold are
deposited on the prepared electrode structure with the sputtering
technique. The contacts of the electrodes 3 are connected to a
function generator which may serve as alternating voltage source
4.
[0035] To separate the metallic carbon nanotubes from the
semi-conducting carbon nanotubes, 2 ml of the suspension containing
individual carbon nanotubes are initially dripped onto electrodes
3. The function generator 4, which is operated with a starting
frequency of 10 MHz and an alternating voltage with peak-to-peak
amplitude V.sub.p-p=10 V is initially turned on for 10 minutes for
the alternating field dielectrophoresis. Following the shutting
down of the alternating field, the suspension around the electrodes
3 is removed with a pipette and the surface blown dry with the aid
of nitrogen. Metallic carbon nanotubes 5 are deposited on the
surface between the metallic electrodes, and are removed with an
ultrasound treatment in a new suspension consisting of D.sub.2O
with 1% sodium lauryl sulphate (SLS) The new suspension contains
metallic carbon nanotubes 5 and is practically free of the
semi-conducting species.
[0036] FIG. 2 shows Raman spectra recorded at different points on a
reference probe. The different spectra with their intensity are
applied in random units as compared to the wave number in
cm.sup.-1, relative to the wavelength for the excitation wave. The
reference probe is produced by dripping the original suspension
consisting of separated carbon nanotubes onto a silicon substrate.
Two lines dominate the spectra, one of which is in the spectral
region below 200 cm.sup.-1 and one is around 275 cm.sup.-1.
[0037] FIG. 3 shows Raman spectra recorded at different points
between metallic electrodes. The spectra are deposited in the same
way as in FIG. 2. A line around 275 cm.sup.-1 in this case
dominates the spectra.
[0038] FIG. 4 is used for interpreting the data and contrasts the
computed excitation energies and the measures lines, which can be
interpreted as breathing modes for different tube diameters. Owing
to theoretical uncertainties in .gamma..sub.o, meaning the overlap
integral with tight-binding calculations, carbon nanotubes having
the diameters as shown must be considered for the observed Raman
scattering for the excitation wavelength .lamda.=514 that is used.
The clearly different frequencies of the breathing modes (radial
breathing modes, in short rbm), however, permit the clear statement
that metallic tubes are responsible for breathing frequencies at
274 cm.sup.-1 and that breathing frequencies close to 192 cm.sup.-1
stem from semi-conducting tubes.
[0039] From this statement we can draw the conclusion that the
metallic species of the carbon nanotubes is present almost
exclusively between the metallic electrodes, as can be seen from a
comparison of the Raman spectra shown in FIGS. 2 and 3.
[0040] The invention has been described in detail with respect to
exemplary embodiments, and it will now be apparent from the
foregoing to those skilled in the art, that changes and
modifications may be made without departing from the invention in
its broader aspects, and the invention, therefore, as defined in
the appended claims, is intended to cover all such changes and
modifications that fall within the true spirit of the
invention.
* * * * *