U.S. patent application number 12/102302 was filed with the patent office on 2012-07-12 for system and method for low-power nanotube growth using direct resistive heating.
This patent application is currently assigned to Raytheon Company. Invention is credited to DELMAR L. BARKER, Mead M. Jordan, William R. Owens.
Application Number | 20120177808 12/102302 |
Document ID | / |
Family ID | 40863503 |
Filed Date | 2012-07-12 |
United States Patent
Application |
20120177808 |
Kind Code |
A1 |
BARKER; DELMAR L. ; et
al. |
July 12, 2012 |
SYSTEM AND METHOD FOR LOW-POWER NANOTUBE GROWTH USING DIRECT
RESISTIVE HEATING
Abstract
Direct resistive heating is used to grow nanotubes out of carbon
and other materials. A growth-initiated array of nanotubes is
provided using a CVD or ion implantation process. These processes
use indirect heating to heat the catalysts to initiate growth. Once
growth is initiated, an electrical source is connected between the
substrate and a plate above the nanotubes to source electrical
current through and resistively heat the nanotubes and their
catalysts. A material source supplies the heated catalysts with
carbon or another material to continue growth of the array of
nanotubes. Once direct heating has commenced, the source of
indirect heating can be removed or at least reduced. Because direct
resistive heating is more efficient than indirect heating the total
power consumption is reduced significantly.
Inventors: |
BARKER; DELMAR L.; (Tucson,
AZ) ; Jordan; Mead M.; (Tucson, AZ) ; Owens;
William R.; (Tucson, AZ) |
Assignee: |
Raytheon Company
|
Family ID: |
40863503 |
Appl. No.: |
12/102302 |
Filed: |
April 14, 2008 |
Current U.S.
Class: |
427/8 ; 118/620;
118/712; 118/723FI; 118/723R; 427/523; 427/592; 977/843 |
Current CPC
Class: |
Y10S 977/752 20130101;
Y10T 117/1016 20150115; C01B 32/991 20170801; Y10S 977/743
20130101; B82Y 30/00 20130101; C01B 21/0602 20130101; Y10S 977/734
20130101; Y10S 977/742 20130101; C01B 2202/08 20130101; C01B 32/956
20170801; C01B 35/023 20130101; Y10S 977/814 20130101; C01B 33/02
20130101; C30B 29/602 20130101; Y10S 977/813 20130101; C30B 25/00
20130101; Y10S 977/822 20130101; C01B 21/064 20130101; Y10S 977/72
20130101; Y10S 977/75 20130101; C30B 11/12 20130101; Y10T 117/102
20150115; B82Y 40/00 20130101; Y10S 977/751 20130101; C01B 32/16
20170801; C01P 2004/13 20130101; Y10T 117/10 20150115; Y10S 977/721
20130101; Y10S 977/722 20130101 |
Class at
Publication: |
427/8 ; 118/620;
118/712; 118/723.R; 118/723.FI; 427/592; 427/523; 977/843 |
International
Class: |
C23C 16/26 20060101
C23C016/26; C23C 16/48 20060101 C23C016/48; C23C 16/52 20060101
C23C016/52; B05C 11/00 20060101 B05C011/00; C23C 16/44 20060101
C23C016/44 |
Claims
1. An apparatus for growing nanotubes from an element selected from
among Carbon, Germanium, Boron, Boron-Nitride, Boron-Carbide,
BiCiNk, Silicon and Silicon-Carbide, comprising: a chamber; one or
more growth-initiated nanotubes on a substrate inside the chamber,
each nanotube including at least one catalyst; a plate over the one
or more nanotubes; an electrical source connected between the
substrate and plate causing electrical current to flow through and
resistively heat the nanotubes and the catalysts; and a material
source that supplies the heated catalyst with the selected element
to sustain nanotube growth.
2. The apparatus of claim 1, further comprising: a sensor for
sensing a condition indicative of the temperature of the heated
nanotubes, said condition being fed back to control the amount of
electrical current to maintain. the temperature within a desired
range.
3. The apparatus of claim 1, wherein the plate is physically bonded
to the one or more nanotubes and said electrical source is a
current source that supplies the electrical current.
4. The apparatus of claim 3, further comprising: an actuator that
lifts the plate to place the nanotubes under tensile strength.
5. The apparatus of claim 1, wherein said electrical source is a
voltage source, further comprising: an actuator that maintains the
plate at a distance above the one or more nanotubes, said voltage
source applies a voltage between the substrate and the plate that
causes field emission to occur across a gap between the nanotubes
and the plate and electrical current to flow through the
nanotubes.
6. The apparatus of claim 5, wherein a plurality of said nanotubes
are grown with different chiralities that exhibit different growth
rates, said actuator maintaining the plate at the distance above a
subset of said nanotubes of one chirality that exhibits the highest
growth rate, field emission being reduced as the gap between the
nanotubes that exhibit lower growth rates and the plate increases
slowing and eventually stopping, their growth to grow an array of
nanotubes of the one chirality having the highest growth rate.
7. The apparatus of claim 5, wherein a plurality of said nanotubes
are grown with different chiralities that exhibit different growth
rates, further comprising a source of oxygen that can be
selectively fed into the sealed chamber, said actuator bringing the
plate in contact with a subset of said nanotubes of one chirality
that exhibits the highest growth rate thereby sourcing sufficient
current in an oxygen environment to burn up those nanotubes, said
actuator maintaining the plate at a distance above another subset
of said nanotubes of another chirality that exhibits the highest
growth rate remaining.
8. The apparatus of claim 1, wherein the material source provides
an element containing growth gas whereby element atoms are absorbed
into the heated catalyst via a chemical vapor deposition
process.
9. The apparatus of claim 1, wherein the material source implants
element ions into the catalysts where the ions recombine with free
electrons in element atoms.
10. The apparatus of claim 1, further comprising: a source of
element-containing gas that can be selectively fed into the
chamber; and an electron beam that bombards the nanotubes in the
gas environment to form additional catalysts at the free end of the
growing nanotubes.
11. An apparatus for growing, nanotubes from an element selected
from among Carbon, Germanium, Boron, Boron-Nitride, Boron-Carbide,
BiCjNk, Silicon and Silicon-Carbide, comprising: a chamber; a
substrate in the chamber; one or more catalysts on the substrate; a
plate over the catalysts; growth-inititiation system including, an
energy source for indirectly heating the catalysts; and a first
material source that supplies the heated catalysts with the
selected element to initiate growth of an array of nanotubes from
the catalysts, and a low-power growth system including, an
electrical source connected between the substrate and plate to
cause electrical current to flow through and resistively heat the
one or more nanotubes; and a second material source that supplies
the heated catalysts with the selected element to continue growth
of the array of nanotubes.
12. The apparatus of claim 11, wherein the plate is physically
bonded to the one or more nanotubes and said first source is a
current source that supplies the electrical current.
13. The apparatus of claim 11, wherein said electrical source is a.
voltage source, further comprising: an actuator that maintains the
plate at a distance above the one or more nanotubes, said voltage
source applies a voltage between the substrate and the plate
causing field emission to occur across a gap between the nanotubes
and the plate and electrical current to flow through the
nanotubes.
14. The apparatus of claim 11, wherein said first and second
material sources are the same said source for providing an
element-containing growth gas whereby element atoms are absorbed
into the catalysts and the nanotubes grown via a chemical vapor
deposition process.
15. The apparatus of claim 14, wherein the substrate forms a seal
that separates the chamber into a feedstock chamber and a growth
chamber in which the growth-gas is confined to the feedstock
chamber, said catalysts embedded in the substrate with portions of
catalyst surface exposed to the feedstock chamber for absorbing
element atoms from the growth gas and different portions of
catalyst surface exposed to the growth chamber to grow nanotubes in
an environment devoid of said growth gas.
16. The apparatus of claim 11, wherein said first and second
material sources are the same said source for providing a beam of
ions that cause element ions to he implanted into the catalysts
where they recombine with free electrons to form element atoms.
17. The apparatus of claim 16, wherein the substrate separates the
chamber into an implantation region and a growth region, each said
catalyst having a first portion of catalyst surface towards the
implantation region for receiving ions and a different second
portion of catalyst surface directly exposed to the growth region
on which the nanotubes are grown.
18. The apparatus of claim 17, wherein the source emits element
ions that are directly implanted through the first portion of
catalyst surface into the catalyst where they recombine with free
electrons to form element atoms.
19. The apparatus of claim 17, wherein said substrate comprises an
element containing layer between the catalysts and the implantation
region and separated from the catalysts, said source emitting ions
into said element-containing layer that releases a larger number of
element ions that are implanted through the first portion of
catalyst surface into the catalyst where they recombine with free
electrons to form element atoms.
20. The apparatus of claim 11, wherein the substrate forms a seal
that separates the chamber into an implantation region and a
feedstock/growth region in which the gas compositions are
independently controllable, each said catalyst having a first
portion of catalyst surface towards the implantation region and a
different second portion of catalyst surface directly exposed to
the growth region, said first source provides an element-containing
growth gas in said feedstock/growth chamber for absorbing element
atoms into the second portion of the catalyst surfaces to initiate
nanotube growth on the second portion of the catalyst surface via a
chemical vapor deposition process, said second source provides a
beam of ions that cause element ions to be implanted into the first
portion of catalyst surfaces where they recombine with free
electrons to form element atoms to sustain nanotube growth.
21. A method for low-power growth of nanotubes from an element
selected from among Carbon, Germanium, Boron, Boron-Nitride,
Boron-Carbide, BiCiNk, Silicon and Silicon-Carbide, comprising:
providing one or more growth-initiated nanotubes supported on a
substrate, each nanotube including a catalyst: positioning a plate
over the one or more nanotubes; connecting an electrical source
between the substrate and plate to cause electrical current to flow
through and resistively heat the nanotubes and the catalysts; and
supplying the heated catalysts with the selected element to
continue growth of array of nanotubes.
22. The method of claim 21. further comprising: sensing a condition
indicative of the temperature of the heated catalysts, and feeding
back the sensed condition to control the amount of electrical
current to maintain the temperature within a desired range.
23. The method of claim 21, wherein the plate is bonded to the one
or more nanotubes and said electrical source is a current source
that supplies the electrical current.
24. The method of claim 23, further comprising: growing a plurality
of said nanotubes with different chiralities that exhibit different
growth rates; and breaking the bonds between the nanotubes having
slower growth rates and the substrate as the plate is lifted by the
nanotubes having faster growth rates.
25. The method of claim 21, wherein the electrical source is a
voltage source that applies voltage between said substrate and
plate, further comprising: maintaining the plate at a distance
above the one or more nanotubes, where application of the voltage
causes field emission to occur across a gap between a free end of
the growing nanotubes and the plate and electrical current to flow
through the nanotubes.
26. The method of claim 25, further comprising: growing a plurality
of said nanotubes with different chiralities that exhibit different
growth rates; and maintaining the plate at the distance above a
subset of said nanotubes of one chirality that exhibits the highest
growth rate so that field emission is reduced as the gap between
the nanotubes that exhibit lower growth rates and the plate
increases, slowing and eventually stopping their growth.
27. The method of claim 25, further comprising: growing a plurality
of said nanotubes with different chiralities that exhibit different
growth rates; introducing oxygen into the chamber; bringing the
plate in contact with a subset of said nanotubes of one chirality
that exhibits the highest growth rate thereby sourcing current in
an oxygen environment to burn up those nanotubes; and maintaining
the plate at a distance above another subset of said nanotubes of
another chirality that exhibits the highest growth rate
remaining.
28. The method of claim 21, wherein the step of providing the
growth initiated array comprises: providing the substrate with the
catalysts; and initiating nanotube growth using a hot chemical
vapor deposition process, wherein once direct resistive heating of
the nanotubes has commenced, nanotube growth continues by supplying
the selected elements using a cold chemical vapor deposition
process.
29. The method of claim 21, wherein the steps of providing the
growth-initiated array and continuing growth comprise: providing
the substrate with the catalysts; providing a source of energy to
indirectly heat the catalysts for initiating growth; and directing
a beam of ions that cause element ions to be implanted into the
catalysts to initiate nanotube growth to lift the thin-film;
wherein once direct resistive heating of the nanotubes has
commenced, at least reducing the source of energy that indirectly
heats the catalysts and directing the beam of ions to continue
nanotube growth.
30. The method of claim 21, wherein the steps of providing the
growth-initiated array and continuing growth comprise: providing
the substrate with the catalysts; forming a seal around the
substrate that separates the chamber into a feedstock/growth
chamber and an implantation chamber in which the gas compositions
are independently controllable, each said catalyst having a first
portion of catalyst surface towards the implantation region and a
different second portion of catalyst surface directly exposed to
the growth region; providing a source of energy to indirectly heat
the catalysts for initiating growth; and introducing a growth-gas
into the feedstock/growth chamber for absorbing element atoms into
the second portion of the heated catalyst :surfaces to initiate
nanotube growth thereon via a chemical vapor deposition process;
and wherein once direct resistive heating of the nanotubes has
commenced, at least reducing the source of energy that indirectly
heats the catalysts and directing a beam of ions through the
implantation chamber that cause element ions to be implanted
through the first portion of catalyst surfaces to continue nanotube
growth.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to nanotube (NT) growth of Carbon and
other materials such as Germanium, Boron, Boron-Nitride,
Boron-Carbide, B.sub.iC.sub.jN.sub.k, Silica and Silica-Carbide,
and more particular to a low-power approach to growing
nanotubes.
[0003] 2. Description of the Related Art
[0004] Carbon nanotubes (CNTs) have stimulated a great deal of
interest in the microelectronic and other industries because of
their unique properties including tensile strengths above 35 GPa,
elastic modulus reaching 1 TPa, higher thermal conductivity than
diamond, ability to carry 1000.times. the current of copper,
densities below 1.3 g/cm.sup.3 and high chemical, thermal and
radiation stability. CNTs have great promise for devices such as
field effect transistors, field emission displays, single electron
transistors in the microelectronic industry, and uses in other
industries. Commercialization of CNTs will depend in large part on
the ability to grow and network CNTs on a large cost-effective
scale without compromising these properties.
[0005] As illustrated in FIG. 1, a CNT 10 is a hollow cylindrical
shaped carbon molecule. The cylinderical structure is built from a
hexagonal lattice of sp.sup.2 bonded carbon atoms 12 with no
dangling bonds. The properties of single-walled nanotubes (SWNTs)
are determined by the graphene structure in which the carbon atoms
are arranged to form the cylinder. Multi-walled nanotubes (MWNTs)
are made of concentric cylinders around a common central
hollow.
[0006] CNTs are commonly grown using several techniques such as arc
discharge, laser ablation and chemical vapour deposition (CVD). In
CVD the growth ofa CNT is determined by the presence of a catalyst,
usually a transition metal such as Fe, Co or Ni, which causes the
catalytic dehydrogenation of hydrocarbons and consequently the
formation of a CNT. CVD generally produces MWNTs or SWNTs of
relatively poor quality due mostly to the poorly controlled
diameters of the nanotubes. However, CVD is relatively easy to
scale up and can be integrated with conventional microelectronic
fabrication, which favors commercialization.
[0007] The way in which nanotubes are formed at the atomic scale is
not precisely known. The growth mechanism is still a subject of
scientific debate, and more than one mechanism might be operative
during the formation of CNTs. As shown in FIGS. 2a and 2b, a
catalyst 20 is deposited on a support such as silicon, zeolite,
quartz, or inconel 22. At elevated temperatures, exposure to a
carbon containing gas causes the catalyst to take in carbon, on
either the surfaces, into the bulk, or both. This thermal diffusion
process of neutral carbon atoms occurs at energies of a few
electronvolts (eV). A precursor to the formation of nanotubes and
fullerenes, C.sub.2, is formed on the surface of the catalyst. From
this precursor, a rodlike carbon 24 is formed rapidly, followed by
a slow graphitization of its wall. The CNT can form either by
`extrusion` (also know as `base growth` or `root growth`) shown in
FIG. 2a, in which the CNT grows upwards from the catalyst that
remains attached to the support, or the particles can detach from
the substrate and move at the head of the growing nanotube,
labelled `tip-growth`, as shown in FIG. 2b. Depending on the size
of the catalyst particle either SWNT or MWNT are grown. A typical
catalyst may contain an alloy of Fe, Co or Ni atoms having a total
diameter of 1 to 100 nm (on the order of 1,000 atoms for 1 nm
diameter of catalyst).
[0008] The application of thermal energy or heat is essential to
stimulate the growth mechanism of CNTs. Heat is required to break
the hydrocarbon molecules in the carbon containing gas upon
colliding with the catalyst so they attach to the catalysts. Heat
is required to transport these carbon atoms via diffussion
processes to the interface of the catalyst and the carbon nanotubes
to obtain higher growth rates. Heat is required for the CNT to
attach the carbon atoms quickly for fast growth. The thermal energy
must be controlled to provide sufficient heating to stimulate these
growth processes without melting the catalyst of breaking the CNT.
Typically heating is provided by induction, plasma discharge,
substrate or wall heating. The power consumption required by these
methods of indirect heating of the catalyst is a significant factor
in the manufacturing cost.
[0009] As shown in FIG. 3, to synthesize CNTs 24 using CVD the
support 22 and catalytic material 20 are placed inside an
environmentally-controlled chamber 32. The sample is heated until
the temperature is great enough (400.degree. C.) that the
introduction of hydrogen along with a buffer gas (Argon) "reduces"
(removes the oxide) the particle. A plurality of gas feeds 34
introduce a process gas including a mixture of a carbon-containing
growth gas 36, typically a hydrocarbon C.sub.xH.sub.y such as
Ethylene (C.sub.2H.sub.4), Methane (CH.sub.4), Ethanol
(C.sub.2H.sub.5OH), or Acetylene (C.sub.2H.sub.2) or possibly a
non-hydrocarbon such as carbon-monoxide (CO), an inert buffer gas
38 such as Argon (Ar) to control pressure inside the chamber and
prevent released hydrogen atoms from exploding and possibly a
scrubber gas 40 such as H.sub.2O or O.sub.2 to periodically or
continuously clean the surface of the catalyst. An energy source 42
such as induction, plasma discharge, substrate or wall heating
provides the energy necessary (e.g. a few eV) to heat the catalyst
to a temperature which allows it to `crack` the hydrocarbon
molecules into reactive atomic carbon 44 upon colliding with the
catalyst, to heat the catalyst to increase the transport of carbon
to the catalysts/CNT interface and to heat the CNT itself. The
reactive carbon 44 is absorbed into the surface of catalytic
material 20 causing the CNT to grow from the same catalytic
surface. A pump system 46 including a vacuum and/or pressure pump
controls the pressure inside the chamber to produce conditions both
conducive to absorption of carbon atoms into the catalytic material
and growth of CNTs from the catalytic material. A number of
electrical ports 48 are provided to accommodate pressure sensors,
thermocouples and the like to monitor conditions inside the
chamber.
[0010] As shown in FIGS. 4a and 4b, CVD can be used to synthesize
an array of vertically aligned CNTs 50 between a Si substrate 52
and a metal thin-film 54, suitably nickel, via a lift-off process.
The thin-film is formed over Fe particles 56 on substrate 52 that
serve as catalysts. The CVD process initiates nanotube growth that
`lifts` thin-film 54 off of the substrate. The fabrication of
three-dimensional networks of CNTs with controlled orientation will
be essential for building large-scale function devices integrated
with microelectronics circuits. Bingqing Wei et al. "Lift-up growth
of aligned carbon nanotube patterns" Applied Physics Letters Volume
77, Number 19 6 November 2000 and JacquelinMerikhi et al. "Sandwich
growth of carbon nanotubes" Diamond & Related materials 15
(2006) pp. 104-106.
SUMMARY OF THE INVENTION
[0011] The present invention provides a low-power system and method
for growing nanotubes out of carbon and other materials using a
CVD, ion implantation or hybrid process with direct resistive
heating of the nanotubes.
[0012] This is accomplished by providing a growth-initiated array
of nanotubes in which the nanotubes and their respective catalysts
are supported on a substrate. An electrical source is connected
between the substrate and a plate over the nanotubes to cause
electrical current to flow through and resistively heat the
nanotubes and their catalysts. The process of nanotube growth
continues using a CVD or ion implantation process through
completion. The direct resistive heating of the nanotubes replaces
or reduces the indirect heating typically used thereby improving
heating efficiency and reducing overall power consumption. A sensed
condition indicative of the temperature of the nanotubes is
suitably fed back to control the electrical source to maintain a
temperature within a desired range for optimal growth.
[0013] In an embodiment, opposite ends of the nanotubes are
physically bonded to the substrate and the plate. The electrical
source is a current source that supplies the electrical current to
the nanotubes. The plate may be lifted by the growth of nanotubes.
Alternately, a mechanical actuator can lift the plate. The actuator
can be controlled to either match the growth rate or to exert a
small pulling force on the nanotubes to increase the growth rate.
If the nanotubes exhibit the same chirality they should grow at the
same rate. Statistically some nanotubes will grow slower than
others. Those nanotubes will exhibit a lower resistance and thus
draw a higher proportion of the sourced current. This additional
heating should further stimulate growth to keep the growth rate of
the entire array fairly uniform. If the nanotubes exhibit different
chiralities they will grow at different rates. The bonds of the
slower growing nanotubes will likely break thereby producing an
array of only nanotubes having one chirality with the fastest
growth rate.
[0014] In another embodiment, a mechanical actuator maintains the
plate at a small distance above the nanotubes. The electrical
source is a voltage source, whereby application of a voltage across
the gap between the free end of the nanotubes and the plate causes
field emission to occur and electrical current to flow through the
nanotubes. If the nanotubes exhibit the same chirality they should
grow at the same rate. If the nanotubes exhibit different
chiralities some of them will grow slower than the others. The
actuator maintains the distance to the tallest fastest growing
nanotubes. This increases the gap to the shorter nanotubes which
reduces the amount of current to those nanotubes further slowing
their growth. This approach can be used to filter the nanotubes by
chirality, particularly the fastest growing nanotubes. To select a
subset of nanotubes having a slower growth rate, the actuator may
contact the plate to the tallest nanotubes in an oxygen environment
to burn up the nanotubes. The actuator then maintains the plate at
a distance above another subset of nanotubes having a chirality
that exhibits the highest growth rate among the remaining
nanotubes.
[0015] In another embodiment, a conventional hot CVD process is
used to form the growth-initiated array of nanotubes. Once direct
resistive heating of the nanotubes is initiated the CVD process is
run cold to improve energy efficiency. The CVD process can be
configured with a single feedstock/growth chamber as per convention
or the substrate can be used to separate the chamber into a
feedstock chamber on one side and a growth chamber on the other.
The latter approach separates nanotube growth from the noxious
feedstock gases which tend to deteriorate the catalyst with
byproducts over time.
[0016] In another embodiment, an ion implantation process is used
to form the growth-initiated array of nanotubes. The requisite
heating can be provided indirectly by wall or substrate heating or
by the energy in the ion beam itself. Once direct resistive heating
of the nanotubes is initiated the indirect heat source can be
removed or reduced (reduced beam energy) to improve energy
efficiency. The ion implantation process can be configured with a
single implantation/growth chamber or the substrate can be
configured to provide an implantation region on one side and a
growth region on the other. The two chambers may be held in the
same vacuum or the substrate may provide an environmental seal for
independent control. This approach separates nanotube growth from
the ion beam.
[0017] In another embodiment, a hybrid CVD and ion implantation
process is used. The substrate forms a seal creating two separate
chambers. A feedstock/growth chamber is formed on one side of the
substrate and an implantation chamber on the other side of the
substrate. A CVD process initiates growth of the nanotube array.
Current is passed through the nanotubes to provide the direct
resistive heating. At this point, either the CVD process can be run
cold for awhile before switching to the ion implantation process or
the ion implantation process can start immediately. The hybrid
approach combines the fast growth capability of the CVD process to
initiate growth with the sustained growth capability of ion
implantation to grow nanotubes of arbitrary length.
[0018] These and other features and advantages of the invention
will be apparent to those skilled in the art from the following
detailed description of preferred embodiments, taken together with
the accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1, as described above, is a diagram of a carbon
nanotube;
[0020] FIGS. 2a-2b, as described above, are diagrams illustrating
root and tip CNT growth;
[0021] FIG. 3, as described above, is a diagram of a conventional
CVD process using a single feedstock-growth chamber to grow CNTs on
a substrate;
[0022] FIGS. 4a and 4b, as described above, are diagrams of a CVD
"lift-off" process for growing an array of CNTs that lifts a metal
thin-film;
[0023] FIGS. 5a and 5b are physical and electrical schematic
diagrams of a current source connected across a growth-initiated
CNT array to provide direct resistive heating of the nanotubes and
their respective catalysts;
[0024] FIGS. 6a through 6c are diagrams of carbon nanotubes
illustrating armchair, zig-zag and chiral orientations,
respectively;
[0025] FIGS. 7a and 7b are diagrams of a voltage source connected
between a growth-initiated CNT array and a plate to stimulate field
emission to provide direct resistive heating of the nanotubes and
their catalysts for single and multiple chirality growth,
respectively;
[0026] FIG. 8 is a diagram of a feedstock/growth chamber for a
low-power CVD process;
[0027] FIG. 9 is a diagram of a low-power CVD process in which the
substrate separates the feedstock and growth chambers;
[0028] FIG. 10 is a diagram of an implantation/growth chamber for a
low-power ion implantation process;
[0029] FIG. 11 is a diagram of a low-power ion implantation process
in which the substrate separates implantation and growth
regions;
[0030] FIG. 12 is a diagram of a low-power hybrid CVD-ion
implantation process in which the substrates isolates an
implantation chamber from a feedstock/growth chamber; and
[0031] FIG. 13 is a diagram of a single nanotube in which a second
catalyst has been formed within the nanotube.
DETAILED DESCRIPTION OF THE INVENTION
[0032] The present invention provides a low-power system and method
for growing nanotubes out of carbon and other materials such as
Germanium, Boron, Boron-Nitride, Boron-Carbide,
B.sub.iC.sub.jN.sub.k where i, j and k are any non-negative
integers, Silicon and Silicon-Carbide using a CVD, ion implantation
or hybrid process with direct resistive heating of the nanotubes.
This is accomplished by providing a growth-initiated array of
nanotubes. An electrical source is connected between the substrate
and a plate over the nanotubes (in contact with or separated by a
small gap) to cause electrical current to flow through the
nanotubes producing direct resistive heating of the nanotubes and
their catalysts. The process of nanotube growth continues using a
CVD or ion implantation process through completion. The direct
resistive heating of the nanotubes replaces or reduces the indirect
heating typically used thereby improving heating efficiency and
reducing overall power consumption. A sensed condition indicative
of the temperature of the nanotubes is suitably fed back to control
the electrical source to maintain a temperature within a desired
range for optimal growth.
[0033] As shown in FIGS. 5a and 5b, some process such as CVD or ion
implantation is used to provide a growth-initiated array of
nanotubes 60 in which the nanotubes and their respective catalysts
62 are supported between and bonded to a substrate 64 and a plate
66. Plate 66 is suitably a metal thin-film such as nickel provided
via a lift-off process. In this embodiment, the nanotubes are grown
via lip growth'. The nanotubes may be alternately grown via `root
growth` or both. Either growth process uses some type of indirect
heating to heat the catalysts to initiate nanotube growth. Indirect
heating is an inefficient approach to heating the catalysts because
much energy is expended to heat the environment inside the chamber,
substrate, chamber walls etc. However, it is needed to initiate
nanotube growth.
[0034] To reduce the go-forward, hence total power consumption, a
current source 68 is connected across the substrate 64 and
thin-film 66, which are configured to provide electrical contacts
at opposite ends of the nanotubes, to close an electrical circuit.
The substrate and thin-film typically conduct electrical current.
Alternately, conductive traces or paths could be formed in either
or both if non-conductive. The current source sources electrical
current i.sub.S 70 that flows through the nanotubes as i.sub.NT 72
producing direct resistive heating 74 of the nanotubes and their
catalysts (and the nearby surrounding gas in a CVD process). A
controller 76 suitably controls the amount of current i.sub.S 70 to
maintain the nanotube temperature in a desired range for optimal
growth. Typical ranges for carbon nanotube growth are 400 to 1000
degrees Celsius. Closer tolerances in temperature may be required
in certain process controls. The initial current is set based on a
calculation or empirical evidence of the estimated number of
nanotubes and average resistance. The control may operate open-loop
depending on the temperature tolerances. Alternately, one or more
sensors 78 suitably sense a condition indicative of the temperature
of the nanotubes that is fed back to the controller 76 to control
the current source to maintain the temperature within the desired
range. The sensed condition may be the temperature of the nanotubes
or another parameter correlated to temperature. In one embodiment,
an optical pyrometer outside the chamber is used to directly sense
the temperature inside the chamber. An optical pyrometer generally
senses the maximum temperature in an imaged area. The thin-film may
be lifted by the growth of nanotubes. Alternately, a mechanical
actuator 80 such as a piezo actuator can lift the thin-film 66. The
actuator can be controlled to either match the growth rate or to
exert a small pulling force on the nanotubes to place them under
tensile stress and increase the growth rate. The process of
nanotube growth continues using a growth process such as CVD or ion
implantation through completion. The direct resistive heating of
the nanotubes replaces or reduces the indirect heating typically
used thereby improving heating efficiency and reducing overall
power consumption.
[0035] As mentioned above, the carbon nanotubes 60 grow as a hollow
cylindrical shaped carbon molecule built from a hexagonal lattice
of sp.sup.2 bonded carbon atoms with no dangling bonds. As shown in
FIGS. 6a through 6c, the orientation of the hexagonal lattice can
exhibit different `chirality` e.g. armchair 82, zig-zag 84, and
chiral 86. The different chiralities exhibit different electrical
and thermal conductivities and different growth rates. Typically,
the array of carbon nanotubes will exhibit different chiralities
somewhat randomly across the array. The bonds of the slower growing
nanotubes will likely break (the CNT being much stronger than the
bond between the CNT and substrate or thin-film) thereby producing
an array of only nanotubes having one chirality with the fastest
growth rate. If the growth process can be controlled so that all
nanotubes exhibit the same chirality they should grow at the same
rate. Statistically some nanotubes will grow slower than others
even if they are the same chirality. Those nanotubes will exhibit a
lower resistance R.sub.NT and thus draw a higher proportion of the
sourced current. This additional heating should further stimulate
growth to keep the growth rate of the entire array fairly uniform
for one chirality. A system and method for growing carbon nanotube
arrays of one chirality is disclosed in co-pending U.S. application
Ser. No. ______ entitled "System and Method of Cloning an
Epitaxially Generated Precursor Chiral Nanotube" filed on ______,
2008, the contents of which are incorporated by reference.
[0036] As shown in FIG. 7a, a process such as CVD or ion
implantation is used to provide a growth-initiated array of
nanotubes 90 in which the nanotubes 90 and their respective
catalysts 92 are supported on a substrate 94. In this embodiment,
the nanotubes are grown via `root growth`. The nanotubes may be
alternately grown via `tip growth` or both. Either growth process
uses some type of indirect heating to heat the catalysts to
initiate nanotube growth. Indirect heating is an inefficient
approach to heating the catalysts because much energy is expended
to heat the environment inside the chamber, substrate, chamber
walls etc. However, it is needed to initiate nanotube growth.
[0037] Once nanotube growth is initiated, the array is heated using
direct resistive heating. A mechanical actuator 96 maintains a
plate 98 at a small distance above the nanotubes. A voltage source
100 connected across the substrate 94 and plate 98 applies a
voltage across a gap 102 between the free end 104 of the nanotubes
90 and the plate 98 causing field emission of electrons 106 to
occur and electrical current i.sub.NT 108 to flow through the
nanotubes 90 producing direct resistive heating 110 of the
nanotubes and their catalysts (and the surrounding gas in a CVD
process). A controller 112 controls the voltage level and/or the
actuator 96 controls the gap to adjust the current level to
maintain the nanotube temperature in a desired range for optimal
growth. The initial voltage is set based on a calculation or
empirical evidence of the estimated number of nanotubes and average
resistance. The controller may simply fix the voltage level or vary
it based on calculations or empirical evidence. Alternately, one or
more sensors 114 suitably sense a condition indicative of the
temperature of the nanotubes, which is fed back to the controller
112 to control the voltage source and/or mechanical actuator to
maintain the temperature within the desired range. The sensed
condition may be the temperature of the nanotubes or another
parameter correlated to temperature. The process of nanotube growth
continues using a CVD or ion implantation process through
completion. The direct resistive heating of the nanotubes replaces
or reduces the indirect heating typically used to improve heating
efficiency and reduce overall power consumption.
[0038] As mentioned above, the carbon nanotubes may exhibit
different chiralities. If, as depicted in FIG. 7a, the nanotubes 90
exhibit the same chirality they should grow at the same rate. If,
as depicted in FIG. 7b, the nanotubes 90 exhibit different
chiralities some of them will grow slower than the others. The
actuator 96 maintains the distance to the tallest fastest growing
nanotubes. This increases the gap to the shorter nanotubes which
reduces the amount of current to those nanotubes further slowing
their growth. If the gap is large enough field emission, hence
current flow will cease. This approach can be used to filter the
nanotubes by chirality, particularly the fastest growing nanotubes.
To select a subset of nanotubes having a slower growth rate, oxygen
gas 116 is fed into the chamber via line 118 and the actuator
contacts the plate to the tallest nanotubes to burn up the
nanotubes. The oxygen is pumped out of the chamber and actuator
then maintains the plate at a distance above another subset of
nanotubes having a chirality that exhibits the highest growth rate
among the remaining nanotubes.
[0039] Direct resistive heating to grow nanotubes out of carbon and
other materials can be implemented with, for example, CVD, ion
implantation or hybrid growth processes. Both the current and
voltage source embodiments can be used with any of these or other
growth processes. By way of example only, each of these growth
processes will be described in context of the current source
embodiment.
Direct Resistive Heating in CVD Processes
[0040] A conventional hot CVD process can be used to form the
growth-initiated array of nanotubes. Once direct resistive heating
of the nanotubes is initiated the CVD process is run cold to
improve energy efficiency. The CVD process can be configured with a
single feedstock/growth chamber as per convention (FIG. 8) or the
substrate can be configured with catalyst material embedded therein
that provides a feedstock chamber on one side and a growth chamber
on the other (FIG. 9). The latter approach separates nanotube
growth from the noxious feedstock gases. The latter approach is
detailed in co-pending U.S. application Ser. No. 11/969,533
entitled "Carbon Nanotube Growth via Chemical Vapor Deposition
using a Catalytic Transmembrane to Separate Feedstock and Growth
Chambers" filed on Jan. 4, 2008, the contents of which are
incorporated by reference.
[0041] As shown in FIG. 8, to synthesize CNTs 124 using CVD the
substrate 122 and catalyst 120 with a thin-film 126 thereon are
placed inside an environmentally controlled chamber 132. A
plurality of gas feeds 134 introduce a process gas including a
mixture of a carbon-containing growth gas 136, typically a
hydrocarbon C.sub.xH.sub.y such as Ethylene (C.sub.2H.sub.4),
Methane (CH.sub.4), Ethanol (C.sub.2H.sub.5OH), or Acetylene
(C.sub.2H.sub.2) or possibly a non-hydrocarbon such as
carbon-monoxide (CO), an inert buffer gas 138 such Argon (Ar) to
control pressure inside the chamber and prevent released hydrogen
atoms from exploding and possibly a scrubber gas 140 such as
H.sub.2O or O.sub.2 to periodically or continuously clean the
surface of the catalyst. An energy source 142 such as induction,
plasma discharge, substrate or wall heating provides the energy
necessary (e.g. a few eV) for a hot CVD process to heat the
catalyst to a temperature which allows it to `crack` the
hydrocarbon molecules into reactive atomic carbon 144, to heat the
catalyst to increase the transport of carbon to the catalysts/CNT
interface and to heat the CNT itself. The reactive carbon 144 is
absorbed into the surface of catalyst 120 causing the CNT 124 to
grow from the same catalytic surface and lift-thin film 126. A pump
system 146 including a vacuum and/or pressure pump controls the
pressure inside the chamber to produce conditions both conducive to
absorption of carbon atoms into the catalytic material and growth
of CNTs from the catalytic material. A number of ports 148 are
provided to accommodate pressure sensors, thermocouples and the
like to monitor conditions inside the chamber.
[0042] A direct resistive heating system includes a current source
150 that is electrically connected through ports 148 between
substrate 122 and thin-film 126 to source current through the
parallel-combination of nanotubes 124, a temperature sensor 152
such as an optical pyrometer that that senses the temperature of
the nanotubes through a port 148 and a controller 154 that
processes the temperature data to adjust the total source current
to maintain the temperature in a desired range for optimal nanotube
growth. Once growth is initiated, energy source 142 is suitably
turned off and the heat required to crack the hydrocarbon molecules
colliding with the catalyst, heat the catalyst for more rapid
diffussion and to heat the CNT is provided by the direct resistive
heating 156. The energy and power required to operate the current
source is far less than the energy required to operate indirect
energy source 142.
[0043] As shown in FIG. 9, direct resistive heating can also be
used in conjunction with a modified CVD process in which the
substrate 122 is secured by a gasket 160 to separate the chamber
into a feedstock chamber 162 and a growth chamber 164 in which the
growth-gas is confined to the feedstock chamber. The catalysts 120
are embedded in the substrate with portions 166 of catalyst surface
exposed to the feedstock chamber for absorbing carbon atoms 144
from the growth gas and different portions of catalyst surface 168
exposed to the growth chamber to grow nanotubes 124 in an
environment devoid of said growth gas. A vacuum or pressure pump
170 controls the pressure in the growth chamber. A buffer gas 172
may be fed into the chamber through lines 174 if desired. Substrate
122 is suitably quite thin, a few millimeters thick. Consequently
the direct heating of the CNTs and catalysts in the growth chamber
efficiently heats the gases in the feed stock chamber providing
sufficient energy to `crack` the hydrocarbon molecules which come
into contact with the hot tubes and catalysts.
Direct Resistive Heating in Ion Implantation Processes
[0044] An ion implantation process can be used to form the
growth-initiated array of nanotubes. Once direct resistive heating
of the nanotubes is initiated the indirect heat source used to
initiate growth is turned off or at least reduced to improve
overall energy efficiency. The ion implantation process can be
configured with a single implantation/growth chamber (FIG. 10) or
the substrate can be configured with catalyst material embedded
therein or thereon that provides an implantation region on one side
and a growth region on the other (FIG. 11). The latter approach
separates nanotube growth from the ion beam. The ion implantation
approach is detailed in co-pending U.S. application Ser. No.
12/061,317 entitled "System and Method for Nanotube Growth via Ion
Implantation using a Catalytic Transmembrane" filed on Apr. 2, 2008
the contents of which are incorporated by reference. Growth rates
via direct implantation are expected to be considerably slower than
CVD but sustainable and may be increased by indirect implantation
via "knock on" or "sputtering" processes that amplify the number of
carbon ions transferred into the catalyst. Ion implantation is a
more precise and controllable process than CVD that facilitates
closer spacing of CNTs in an array and control of CNT length.
[0045] As shown in FIG. 10, a substrate 200 having one or more
catalysts 202 supported thereon and covered by a thin-film 204 is
placed inside an environmentally controlled chamber 206 held at
vacuum by a vacuum pump 207. A source 208 directs a beam of carbon
ions 210 through the thin-film to implant the ions into the
catalysts 202. The energy of the ion beam itself or an indirect
energy source 212 provides the thermal energy necessary to heat the
catalysts for proper diffussion and attachment of carbon atoms to
initiate growth of CNTs 214.
[0046] A direct resistive heating system includes a current source
220 that is electrically connected through ports 222 between
substrate 200 and thin-film 204 to source current through the
parallel-combination of nanotubes 214, a temperature sensor 224
such as an optical pyrometer that that senses the temperature of
the nanotubes through a port 222 and a controller 226 that
processes the temperature data to adjust the total source current
to maintain the temperature in a desired range for optimal nanotube
growth. Once growth is initiated, energy source 212 is suitably
turned off or reduced (e.g. if ion beam provides heating, reduce
beam energy) and the heat required to heat the catalyst for more
rapid diffussion and to heat the CNT is provided by the direct
resistive heating 228. The energy and power required to operate the
current source 220 is far less than the energy required to operate
indirect energy source 212 and/or to operate the ion beam at higher
energy levels.
[0047] As shown in FIG. 11, direct resistive heating can also be
used in conjunction with a modified ion implant process in which
substrate 200 physically separates chamber 206 into an implantation
region 230 and a growth region 232. Catalysts 202 supported on the
underside of substrate 200 (or embedded in the substrate) provide
an implantation surface 234 to receive carbon ions from beam 210
with sufficient energy to reach, penetrate and stop in the catalyst
and a growth surface 236 directly exposed to the growth region to
grow carbon nanotubes 214. This configuration protects the CNTs
from the ion beam. In addition, "knock-on" processes can be used to
increase the flux of carbon ions implanted into the catalysts. A
spacer layer 240 separates a knock on layer 242 (e.g. Graphite)
from the catalyst material. An anti-sputtering layer 244 (e.g. Ti,
Mo, etc.) is deposited over the knock-on layer. Source 208 directs
ion beam 210 through the anti-sputtering layer onto knock-on layer
242. Through a "knock-on" process, each ion knocks multiple carbon
ions forward through the substrate into catalyst 202 thereby
providing gain. In this configuration, the source does not have to
emit carbon ions, it could, for example, emit heavier ions to
improve knock-on efficiency. In an alternate embodiment, a gasket
is fitted around substrate 200 to isolate an implantation chamber
from a growth chamber. Consequently, the pressure and gas
environment of the growth chamber can be independently controlled
as desired.
Direct Resistive Heating in a Hybrid CVD/Ion Implantation
Process
[0048] Direct resistive heating can be similarly used in a hybrid
CVD/ion implantation process. The substrate forms a seal creating
two separate chambers. A feedstock/growth chamber is formed on one
side of the substrate and an implantation chamber on the other side
of the substrate. A CVD process initiates growth of the nanotube
array. Current is passed through the nanotubes to provide the
direct resistive heating. At this point, either the CVD process can
be run cold for awhile before switching to the ion implantation
process or the ion implantation process can start immediately. The
hybrid approach combines the fast growth capability of the CVD
process to initiate growth with the sustained growth capability of
ion implantation to grow nanotubes of arbitrary length.
[0049] As shown in FIG. 12, to initiate nanotube growth using CVD a
substrate 300 with one or more catalysts 302 on the underside of
the substrate or embedded therein and with a thin-film 304 over the
catalysts is placed inside an environmentally controlled chamber
306. A gasket 308 holds the substrate 300 to form a seal that
separates the chamber into a feedstock and growth chamber 310 for
CVD and an implantation chamber 312 for ion implantation.
[0050] A plurality of gas feeds 314 introduce a process gas
including a mixture of a carbon-containing growth gas 316,
typically a hydrocarbon C.sub.xH.sub.y such as Ethylene
(C.sub.2H.sub.4), Methane (CH.sub.4), Ethanol (C.sub.2H.sub.5OH),
or Acetylene (C.sub.2H.sub.2) or possibly a non-hydrocarbon such as
carbon-monoxide (CO), an inert buffer gas 318 such Argon (Ar) to
control pressure inside the chamber and prevent released hydrogen
atoms from exploding and possibly a scrubber gas 320 such as
H.sub.2O or O.sub.2 to periodically or continuously clean the
surface of the catalyst. An energy source 322 such as induction,
plasma discharge, substrate or wall heating provides the energy
necessary (e.g. a few eV) for a hot CVD process to heat the
catalyst to a temperature which allows it to `crack` the
hydrocarbon molecules into reactive atomic carbon 323, to heat the
catalyst to increase the transport of carbon to the catalysts/CNT
interface and to heat the CNT itself. The reactive carbon is
absorbed into the exposed surface of catalyst 302 to initiate
growth of CNT 324 to grow from the same catalytic surface and
lift-thin film 304. A pump system 326 including a vacuum and/or
pressure pump controls the pressure inside the chamber to produce
conditions both conducive to absorption of carbon atoms into the
catalyst and growth of CNTs from the catalyst.
[0051] A direct resistive heating system includes a current source
330 that is electrically connected through ports 332 between
substrate 300 and thin-film 304 to source current through the
parallel-combination of nanotubes 324, a temperature sensor 334
such as an optical pyrometer that that senses the temperature of
the nanotubes through a port 332 and a controller 336 that
processes the temperature data to adjust the total source current
to maintain the temperature in a desired range for optimal nanotube
growth. Once growth is initiated, energy source 332 is suitably
turned off and the heat required to heat the catalyst for more
rapid diffussion and to heat the CNT is provided by the direct
resistive heating 338.
[0052] At this point, either the CVD process can be run cold for
awhile before switching to the ion implantation process or the ion
implantation process can start immediately. A vacuum pump 340 holds
the implantation chamber 312 at vacuum. A source 342 directs a beam
of ions 344 towards the substrate to cause carbon ions 346 to be
implanted into catalyst 302. The beam may inject carbon ions
directly into the catalysts or amplify them, as shown, using
`knock-on` processes. A spacer layer 348 separates a knock on layer
350 (e.g. Graphite) from the catalyst material. An anti-sputtering
layer 352 (e.g. Ti, Mo, etc.) is deposited over the knock-on layer.
Source 342 directs ion beam 244 through the anti-sputtering layer
onto knock-on layer 350. Through a "knock-on" process, each ion
knocks multiple carbon ions forward through the substrate into
catalyst 302 thereby providing gain.
[0053] Direct resistive heating is used to effectively and energy
efficiently grow one or more nanotubes. In addition to proper and
efficient heating, nanotube growth can be further stimulated by the
formation of additional catalysts within the nanotubes as they
grow. As shown in FIG. 13, root growth of a catalyst 400 on
substrate 402 produces a nanotube 404. An element-containing gas or
mist 406, for example, is introduced into the chamber environment
through a gas feed 408. An electron beam 410 bombards the nanotubes
in the element-containing environment to form an additional
catalyst 412 at the tip of the growing nanotubes. In this
embodiment, the tip is attached to a thin-film 414 to support the
direct resistive heating. In the field emission embodiment, the tip
may be unattached. The additional catalyst 412 may grow the
nanotube via root or tip growth. The process can be repeated for
another catalyst 414 on the free end of the nanotube.
[0054] This process for forming additional catalysts within the
nanotubes to further stimulate and speed growth is not limited to
direct resistive heating, the process can be used in any of the CVD
or ion implantation processes with or without direct resistive
heating.
[0055] Although the description of the invention has focused on the
growth of carbon nanotubes the approach is viable for growing
nanotubes from other materials such as Germanium (Ge), Boron (B),
Boron-Nitride (BN), Boron-Carbide, B.sub.iC.sub.jN.sub.k, Silicon
(Si) or Silicon-Carbide (SiC). The interest in and development of
carbon nanotube technology is well beyond that of other materials,
hence the focus on carbon nanotubes. However, the approach of using
direct resistive heating to grow nanotubes from these other or yet
to be discovered materials is equally applicable.
[0056] While several illustrative embodiments of the invention have
been shown and described, numerous variations and alternate
embodiments will occur to those skilled in the art. Such variations
and alternate embodiments are contemplated, and can be made without
departing from the spirit and scope of the invention as defined in
the appended claims.
* * * * *