U.S. patent application number 14/971922 was filed with the patent office on 2016-04-14 for systems and methods for production of graphene by plasma-enhanced chemical vapor deposition.
The applicant listed for this patent is LOCKHEED MARTIN CORPORATION. Invention is credited to Peter V. BEDWORTH, Steven W. SINTON.
Application Number | 20160102402 14/971922 |
Document ID | / |
Family ID | 51488140 |
Filed Date | 2016-04-14 |
United States Patent
Application |
20160102402 |
Kind Code |
A1 |
SINTON; Steven W. ; et
al. |
April 14, 2016 |
SYSTEMS AND METHODS FOR PRODUCTION OF GRAPHENE BY PLASMA-ENHANCED
CHEMICAL VAPOR DEPOSITION
Abstract
Production of bulk quantities of graphene for commercial
ventures has proven difficult due to scalability issues in certain
instances. Plasma-enhanced chemical vapor deposition of graphene
can address at least some of these issues. Methods for production
of graphene by plasma-enhanced chemical vapor deposition can
include: providing a metal substrate and a carbonaceous electrode,
at least a portion of the metal substrate being located proximate
to the carbonaceous electrode with a gap defined therebetween;
applying a potential between the metal substrate and the
carbonaceous electrode; exciting a plasma-forming gas in the gap
between the metal substrate and the carbonaceous electrode in the
presence of the applied potential, thereby forming a plasma;
ablating a reactive carbon species from the carbonaceous electrode
in the presence of the plasma; and growing graphene on the metal
substrate from the reactive carbon species.
Inventors: |
SINTON; Steven W.; (Palo
Alto, CA) ; BEDWORTH; Peter V.; (Los Gatos,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LOCKHEED MARTIN CORPORATION |
Bethesda |
MD |
US |
|
|
Family ID: |
51488140 |
Appl. No.: |
14/971922 |
Filed: |
December 16, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14192796 |
Feb 27, 2014 |
9242865 |
|
|
14971922 |
|
|
|
|
61773051 |
Mar 5, 2013 |
|
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|
Current U.S.
Class: |
118/723R |
Current CPC
Class: |
C01B 32/186 20170801;
C23C 16/505 20130101; C23C 16/455 20130101 |
International
Class: |
C23C 16/505 20060101
C23C016/505; C23C 16/455 20060101 C23C016/455 |
Claims
1. A system comprising: a reaction chamber; a reel-to-reel
processing line configured to convey a metal substrate within the
reaction chamber between a pay-out reel and a take-up reel; a
carbonaceous electrode housed within the reaction chamber and
disposed proximate to a location through which the metal substrate
is conveyed; wherein the carbonaceous electrode and the
reel-to-reel processing line are electrically connected so as to be
configured to apply a potential between the metal substrate and the
carbonaceous electrode; and a gas inlet configured to flow a
plasma-forming gas in a gap between the metal substrate and the
carbonaceous electrode.
2. The system of claim 1, wherein the pay-out reel and the take-up
reel are located outside the reaction chamber.
3. The system of claim 1, wherein the pay-out reel and the take-up
reel are located inside the reaction chamber.
4. The system of claim 1, wherein the carbonaceous electrode
comprises a graphite electrode, a glassy carbon electrode, a carbon
fiber electrode, an organic polymer electrode embedded with
electrically conductive particles, or any combination thereof.
5. The system of claim 1, wherein the metal substrate comprises
copper.
6. The system of claim 1, wherein the potential comprises a
radiofrequency voltage.
7. The system of claim 6, wherein the radiofrequency voltage
comprises an underlying waveform whose polarity alternates as a
function of time.
8. The system of claim 1, wherein the potential comprises a DC
voltage.
9. The system of claim 1, wherein the system is configured to
operate at atmospheric pressure.
10. The system of claim 1, wherein the system is configured to
operate at a sub-atmospheric pressure.
11. The system of claim 1, wherein the system is configured to
ablate a reactive carbon species from the carbonaceous electrode in
the presence of a plasma.
12. The system of claim 11, wherein the system is configured to
generate the plasma and apply plasma energy to both the
carbonaceous electrode and the metal substrate.
13. The system of claim 12, wherein the system is configured to
apply the plasma energy alternately to the carbonaceous electrode
and the metal substrate.
14. The system of claim 1, wherein the carbonaceous electrode
further comprises a carbon nanomaterial.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 14/192,796, filed on Feb. 27, 2014, which claims the
benefit of priority under 35 U.S.C. .sctn.119 from U.S. Provisional
Patent Application 61/773,051, filed on Mar. 5, 2013, each of which
is incorporated herein by reference in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
FIELD
[0003] The present disclosure generally relates to carbon
nanomaterials, and, more specifically, to methods and systems for
production of graphene.
BACKGROUND
[0004] Graphene represents an atomically thin layer of carbon in
which the carbon atoms reside at regular two-dimensional lattice
positions within a single sheet or a few stacked sheets (e.g.,
about 10 or less) of fused six-membered carbon rings. In its
various forms, this material has garnered widespread interest for
use in a number of applications, primarily due to its favorable
combination of high electrical and thermal conductivity values,
good mechanical strength, and unique optical and electronic
properties. Of particular interest to industry are large-area
graphene films for applications such as, for example, special
barrier layers, coatings, large area conductive elements (e.g., RF
radiators or antennas), and flexible electronics. A number of
contemplated graphene applications have also been proposed for
carbon nanotubes, since these two materials have certain properties
that are comparable to one another. However, an advantage of
graphene over carbon nanotubes is that graphene can generally be
produced in bulk much more inexpensively than can the latter,
thereby addressing perceived supply and cost issues that have been
commonly associated with carbon nanotubes.
[0005] Despite the fact that graphene is generally synthesized more
easily than are carbon nanotubes, there remain issues with
production of graphene in quantities sufficient to support various
commercial operations. Scalability to produce large area graphene
films represents a particular problem. The most scalable processes
developed to date for making graphene films utilize chemical vapor
deposition (CVD) technology. In typical CVD processes, a
carbon-containing gas is decomposed at high temperatures into
various reactive carbon species, which then deposit upon a suitable
growth catalyst and reorganize to form a graphene film. In typical
CVD graphene syntheses, a carbon-containing gas and a
copper-containing substrate are heated to a high temperature (e.g.,
about 900.degree. C.-1000.degree. C.) that is just below the
melting point of the copper (i.e., 1061.degree. C.). Both metallic
copper substrates and copper-coated substrates can be used (e.g.,
nickel or silicon carbide substrates coated with copper). The CVD
growth process can take place at either atmospheric pressure or a
sub-atmospheric pressure. Due to the high temperatures employed in
typical CVD processes, as well as the common use of reduced
pressures during growth, scaling to afford graphene growth over
large substrate areas can be expensive and complex. Further, since
CVD growth processes often operate in the near-melting point regime
of the substrate, substrate deformation can commonly occur, which
can be undesirable for precision applications. CVD growth of
graphene may not be possible at all on certain low melting
substrates.
[0006] In view of the foregoing, improved processes for producing
bulk quantities of graphene, particularly deposition of graphene
films over a large surface area, would represent a substantial
advance in the art. The present disclosure satisfies the foregoing
need and provides related advantages as well.
SUMMARY
[0007] In some embodiments, methods for production of graphene by
plasma-enhanced chemical vapor deposition can include providing a
metal substrate and a carbonaceous electrode, applying a potential
between the metal substrate and the carbonaceous electrode,
exciting a plasma-forming gas in a gap between the metal substrate
and the carbonaceous electrode in the presence of the applied
potential to form a plasma, ablating a reactive carbon species from
the carbonaceous electrode in the presence of the plasma, and
growing graphene on the metal substrate from the reactive carbon
species. At least a portion of the metal substrate is located
proximate to the carbonaceous electrode with a gap defined
therebetween.
[0008] In some embodiments, methods for production of graphene by
plasma-enhanced chemical vapor deposition can include providing a
metal substrate and a carbonaceous electrode, conveying the metal
substrate by the carbonaceous electrode via a reel-to-reel
processing line between a pay-out reel and a take-up reel, applying
a potential between the metal substrate and the carbonaceous
electrode via the reel-to-reel processing line, flowing a
plasma-forming gas in a gap between the metal substrate and the
carbonaceous electrode in the presence of the applied potential to
form a plasma upon exciting the plasma-forming gas, ablating a
reactive carbon species from the carbonaceous electrode in the
presence of the plasma, and growing graphene on the metal substrate
from the reactive carbon species while the metal substrate is being
conveyed. At least a portion of the metal substrate is located
proximate to the carbonaceous electrode with a gap defined
therebetween.
[0009] In some embodiments, systems for continuous production of
graphene by plasma-enhanced chemical vapor deposition can include:
a reaction chamber, a reel-to-reel processing line configured to
convey a metal substrate within the reaction chamber between a
pay-out reel and a take-up reel, a carbonaceous electrode housed
within the reaction chamber and disposed proximate to a location
through which the metal substrate is conveyed, and a gas inlet
configured to flow a plasma-forming gas in a gap between the metal
substrate and the carbonaceous electrode. The carbonaceous
electrode and the reel-to-reel processing line are electrically
connected so as to apply a potential between the metal substrate
and the carbonaceous electrode.
[0010] The foregoing has outlined rather broadly the features of
the present disclosure in order that the detailed description that
follows can be better understood. Additional features and
advantages of the disclosure will be described hereinafter. These
and other advantages and features will become more apparent from
the following description taken in conjunction with the
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] For a more complete understanding of the present disclosure,
and the advantages thereof, reference is now made to the following
descriptions to be taken in conjunction with the accompanying
drawings describing specific embodiments of the disclosure,
wherein:
[0012] FIGS. 1 and 2 show schematics of illustrative systems for
producing graphene by plasma-enhanced chemical vapor deposition
according to one or more embodiments described herein.
DETAILED DESCRIPTION
[0013] The present disclosure is directed, in part, to methods for
production of graphene by plasma-enhanced chemical vapor deposition
(PECVD), particularly continuous or semi-continuous graphene
production processes. The present disclosure is also directed, in
part, to systems configured for continuous production of graphene
by PECVD.
[0014] As discussed above, the high temperatures associated with
conventional CVD processes for producing graphene can be
problematic in many aspects, not the least of which is thermal
deformation or damage of a substrate under the growth conditions.
In addition, the sub-atmospheric pressures utilized during many
common CVD processes can be problematic from an engineering
standpoint in large scale graphene production. Conversion
efficiency of a carbon-containing gas into graphene can also be
relatively poor in many common CVD processes, thereby necessitating
use of large quantities of the carbon-containing gas and resulting
in increased costs.
[0015] The present inventors recognized that improved PECVD
processes could be used to provide a number of advantages in the
production of graphene, particularly in terms of scalability to
generate large area graphene films. Foremost, PECVD processes can
produce reactive carbon species at lower temperatures than can
related CVD processes, due to the reactivity of the plasma, thereby
lowering the risk of substrate deformation or damage in PECVD
processes. Further, by judicious orientation of the plasma within a
CVD reaction chamber, the plasma can convey energy to both a
carbonaceous source material and to a substrate, including a
catalyst disposed thereon, which can reduce the need to externally
raise the temperature during the deposition process. In this
regard, applying plasma energy to a carbonaceous source material
can decrease the decomposition temperature needed to form a
reactive carbon species for graphene production, and applying
plasma energy to a graphene growth substrate and its deposited
catalyst is believed to facilitate reorganization of the reactive
carbon species on the substrate to form graphene. Both DC and
radiofrequency plasmas can be used to accomplish the foregoing.
[0016] In addition, the inventors recognized that improved PECVD
processes might significantly reduce the quantities of a
carbon-containing gas needed to produce graphene, thereby lowering
production costs. Even more specifically, the inventors identified
various synthetic configurations in which the input of a
carbon-containing gas to a graphene synthesis process is largely
replaced with a carbonaceous electrode as a source of a reactive
carbon species. The carbonaceous electrode can be disposed in close
proximity to the substrate, which can result in a decreased
incidence of premature recombination of the reactive carbon species
and increased graphene production. Moreover, the inventors
recognized that by establishing a suitable potential between the
carbonaceous electrode and the substrate in close proximity
thereto, a plasma could be established in the gap defined between
the two in the presence of a plasma-forming gas. Due to the close
proximity of the plasma to both the carbonaceous electrode and the
substrate, the advantages described above can be realized.
[0017] The systems and methods described herein for PECVD growth of
graphene are further advantageous in that they allow for further
regulation of the graphene growth rate to be readily realized. In
this regard, a carbon-containing gas (e.g., a hydrocarbon) can also
be introduced to the plasma being produced proximate to the
substrate and the carbonaceous electrode, thereby supplementing the
reactive carbon species produced from the carbonaceous electrode
alone. Introduction of a carbon-containing gas to the plasma can
increase the graphene deposition rate if the native deposition rate
is not sufficiently high. At the very least, the synthetic
configurations identified by the inventors can allow decreased
quantities of a carbon-containing gas to be used compared to
related CVD processes. More advantageously, the synthetic
configurations identified by the inventors can allow for more
efficient production and reaction of reactive carbon species to
occur.
[0018] Control of the plasma through electronic and spatial means
can further provide an additional degree of process control
compared to the parameter variation of conventional CVD processes
(e.g., time, pressure and temperature). In addition, various dopant
species can also be introduced into the plasma that can alter its
chemistry and the production of graphene therefrom. In some
embodiments, the dopant species can become incorporated in the
graphene lattice to change one or more of its electrical, thermal,
optical, or mechanical properties.
[0019] The embodiments described herein can also offer advantageous
high throughput capabilities. Specifically, in some embodiments,
the processes described herein can be compatible with reel-to-reel
processing strategies, which can allow large substrate areas to be
exposed to a reactive carbon species per unit time. Reel-to-reel
processing capabilities can allow synthetic configurations with a
small "footprint" to be implemented. As further described
hereinbelow, the particular embodiments depicted and described
herein can offer further advantages as well.
[0020] As used herein, the term "atmospheric pressure" refers to
the magnitude of the air pressure at sea level, approximately 760
torr.
[0021] As used herein, the term "sub-atmospheric pressure" refers
to a pressure that is less than atmospheric pressure.
[0022] As used herein, the term "metal substrate" refers to a
substance having a metal surface, including monolithic metal
structures, metal surface coatings on a metallic or non-metallic
substrate, and the like. The metal surface on such substrates can
be continuous or discontinuous as needed for practicing one or more
embodiments described herein.
[0023] As used herein, the term "reactive carbon species" refers to
internally excited and/or ionized carbon atoms or molecular
fragments produced upon gasification of a carbonaceous material at
high temperatures.
[0024] As used herein, the term "proximate" generally refers to the
condition of two entities being in a close spatial relation with
one another. In the context of the present embodiments, two
entities can be considered proximate to one another if they are
about 10 cm apart or less, particularly about 1 mm to about 10 cm
apart from one another.
[0025] In various embodiments, systems for production of graphene
by plasma-enhanced chemical vapor deposition can include a reaction
chamber, a reel-to-reel processing line configured to convey a
metal substrate within the reaction chamber between a pay-out reel
and a take-up reel, a carbonaceous electrode housed within the
reaction chamber and disposed proximate to a location through which
the metal substrate is conveyed, and a gas inlet configured to flow
a plasma-forming gas in a gap between the metal substrate and the
carbonaceous electrode. The carbonaceous electrode and the
reel-to-reel processing line are electrically connected so as to
apply a potential between the metal substrate and the carbonaceous
electrode.
[0026] Depending on whether one wants to operate the system at
atmospheric pressure or a sub-atmospheric pressure, the
reel-to-reel processing line can be located inside or outside the
reaction chamber. For example, when it is desired to grow graphene
on a metal substrate at substantially atmospheric pressure, the
pay-out reel and the take-up reel can be located outside the
reaction chamber. In locating the pay-out reel and the take-up reel
outside the reaction chamber, the reaction to form graphene can be
confined to that portion of the metal substrate housed within the
reaction chamber at a given time, particularly nearby a generated
plasma within the reaction chamber. In such embodiments, the
reaction chamber can be open to the atmosphere, thereby allowing
the metal substrate to easily pass therethrough. In contrast, when
it is desired to grow graphene on a metal substrate at a
sub-atmospheric pressure, the reaction chamber can be closed. In
such embodiments, the pay-out reel and the take-up reel can both be
located within the reaction chamber. Even with the pay-out reel and
the take-up reel being housed within the reaction chamber, graphene
growth can be confined to only a particular portion of the metal
substrate at any given time in some embodiments. Specifically, in
some embodiments, graphene growth can be confined in or near a
plasma-forming region within the reaction chamber (e.g., in a gap
between the carbonaceous electrode and the metal substrate). For
completeness, it should be also noted that even when the pay-out
reel and the take-up reel are both housed within the reaction
chamber and the reaction chamber is closed, graphene growth can
still take place at atmospheric pressure in such embodiments.
Pressures greater than atmospheric pressure can also be used in a
closed reaction chamber in some embodiments.
[0027] In general, the carbonaceous electrode of the embodiments
described herein can contain any electrically conductive material
containing carbon. Illustrative electrically conductive carbon
materials can include, for example, graphite, glassy carbon, carbon
fibers, and the like. That is, in some embodiments, the
carbonaceous electrode can include a graphite electrode, a glassy
carbon electrode, a carbon fiber electrode, or any combination
thereof.
[0028] In other embodiments, the carbonaceous electrode can include
an insulating material, such as organic polymer, that has been
embedded with particles or inclusions that render it electrically
conductive under an applied DC or RF voltage. Such embedded
particles or inclusions can include nanomaterials such as carbon
nanotubes. Although carbon nanotubes represent a rather expensive
carbon source with present carbon nanotube production techniques,
the high electrical conductivity of carbon nanotubes can
potentially afford some advantages when practicing the embodiments
described herein. In related embodiments, graphene platelets can be
included in a carbonaceous electrode formed from an organic
polymer. When utilizing a carbon nanomaterial and an organic
polymer in an electrode, the carbon nanomaterial can primarily be
used to convey electrical conductivity to the bulk polymer, rather
than representing the predominant carbon source for producing a
reactive carbon species. That is, the carbon nanomaterial can be
used to the extent necessary to convey electrical conductivity to
an organic polymer used for producing a reactive carbon species,
although at least a portion of the reactive carbon species may also
be formed from the carbon nanomaterial as the electrode is
consumed.
[0029] The systems and methods described herein will now be
described with further reference to the drawings. FIGS. 1 and 2
show illustrative schematics of systems for producing graphene by
plasma-enhanced chemical vapor deposition according to one or more
embodiments described herein. The depicted systems can produce
graphene in a continuous or semi-continuous (i.e., periodic) manner
on a mobile metal substrate. In the configuration of FIG. 1, the
system is open for operation at atmospheric pressure, and in FIG.
2, the system is closed for atmospheric pressure operation,
sub-atmospheric pressure operation, or elevated pressure operation.
That is, the systems depicted in FIGS. 1 and 2 differ primarily in
the placement of their reel-to-reel processing line.
[0030] Referring to FIG. 1, system 10 includes reaction chamber 12
within which a reel-to-reel processing line conveys metal substrate
14. The direction of conveyance is indicated with arrows in FIG. 1.
Reel-to-reel processing line includes pay-out reel 16 and take-up
reel 18, each of which is located outside of reaction chamber 12.
Tensioning bars 20 and 20' help to provide further guidance of
metal substrate 14 as it is being conveyed.
[0031] Located proximate to at least a portion of metal substrate
14 within reaction chamber 12 is carbonaceous electrode 22.
Carbonaceous electrode 22 is electrically connected to metal
substrate 14 by circuit 24, through which a potential is applied,
particularly a radiofrequency voltage or a DC voltage for producing
a plasma in gap 26. Although FIG. 1 has depicted circuit 24 as
establishing an electrical connection to metal substrate 14 via
tensioning bar 20', it is to be recognized that an electrical
connection can be made to metal substrate 14 at any appropriate
location. Moreover, circuit 24 need not necessarily extend to the
exterior of reaction chamber 12, as depicted in FIG. 1.
[0032] Gap 26 is defined between metal substrate 14 and
carbonaceous electrode 22. Gas inlet 28 enters reaction chamber 12
and is configured to supply a gas to gap 26. Although gas inlet 28
has been depicted as entering reaction chamber 12 from the bottom,
it is to be recognized that it may enter in any suitable manner to
project a gas into gap 26. Upon the application of a suitable
potential between metal substrate 14 and carbonaceous electrode 22,
a plasma can form from a suitable plasma-forming gas within gap 26.
Because metal substrate 14 is located proximate to the location
where the plasma is formed, graphene can grow on metal substrate 14
by plasma-enhanced chemical vapor deposition when a catalyst
suitable for forming graphene is present on metal substrate 14, or
metal substrate 14 itself can catalyst the formation of
graphene.
[0033] The configuration depicted in FIG. 2 differs from that of
FIG. 1 in the placement of its pay-out reel 16 and take-up reel 18.
Specifically, these elements are both within reaction chamber 12 in
the depicted configuration of FIG. 2. As the remaining elements of
FIG. 2 are similar to those of FIG. 1, they will not be described
again in detail.
[0034] In general, any catalyst suitable for forming graphene by
chemical vapor deposition can be used in the embodiments described
herein. In some embodiments, the catalyst can be a metallic
catalyst. Suitable metallic catalysts can include, for example,
copper, nickel, ruthenium, iridium, platinum, alloys thereof, and
the like. The metal substrate itself can be formed from the
metallic catalyst or metal alloy, or a coating of the metallic
catalyst can be applied to a non-metallic substrate in some
embodiments. In still further embodiments, a metallic catalyst
suitable for forming graphene can be coated upon a metal substrate
that is not itself suitable for directly growing graphene. In
particular embodiments, the metal substrate can include copper,
such as a copper foil.
[0035] In various embodiments, methods for production of graphene
by plasma-enhanced chemical vapor deposition on a metal substrate
are described herein. In various embodiments, the metal substrate
can be fixed or conveyed while growing graphene thereon, as
discussed hereinafter.
[0036] In some embodiments, the methods can include providing a
metal substrate and a carbonaceous electrode, applying a potential
between the metal substrate and the carbonaceous electrode,
exciting a plasma-forming gas in a gap between the metal substrate
and the carbonaceous electrode in the presence of the applied
potential to form a plasma, ablating a reactive carbon species from
the carbonaceous electrode in the presence of the plasma, and
growing graphene on the metal substrate from the reactive carbon
species. At least a portion of the metal substrate is located
proximate to the carbonaceous electrode with a gap defined
therebetween.
[0037] In some embodiments, the methods can further include
conveying the metal substrate while growing graphene thereon. In
such embodiments, the metal substrate can be conveyed on a
reel-to-reel processing line between a pay-out reel and a take-up
reel. The pay-out reel, the take-up reel, or both can be disposed
within a reaction chamber, as depicted in FIG. 2 above, or they can
both be disposed outside a reaction chamber, as depicted in FIG. 1
above.
[0038] As discussed above, the pressure at which it is desired to
produce graphene can dictate the chosen configuration of the
pay-out and take-up reels. Considerations to be taken into account
when choosing a growth pressure include the number of graphene
layers to be grown. Fewer graphene layers can be favored by
sub-atmospheric deposition pressures. In some embodiments, growing
graphene on the metal substrate can take place substantially at
atmospheric pressure. In embodiments where atmospheric pressure
graphene growth takes place, the reaction chamber can be either
open or closed, corresponding to FIGS. 1 and 2 above, respectively.
In other embodiments, growing graphene on the metal substrate can
take place at a sub-atmospheric pressure. In such embodiments, the
reaction chamber in which graphene growth takes place can be
closed, as depicted in FIG. 2 above. Suitable sub-atmospheric
pressures for graphene growth can include, for example, pressures
ranging between about 700 torr and about 750 torr, or between about
650 torr and about 700 torr, or between about 600 torr and about
650 torr, or between about 500 torr and about 600 torr, or between
about 400 torr and about 500 torr.
[0039] In various embodiments, the plasma-forming gas can include a
mixture of hydrogen and a noble gas. Particularly suitable noble
gases for inclusion in the plasma-forming gas can include heavy
noble gases such as, for example, argon, krypton, xenon and any
combination thereof. Light noble gases such as helium and neon can
also be included in the plasma-forming gas in other various
embodiments. In various embodiments, the plasma-forming gas can
include about 0.1% to about 50% hydrogen by volume.
[0040] In some embodiments, the plasma-forming gas can further
include a hydrocarbon in addition to the hydrogen and noble gas
mentioned above. Inclusion of a hydrocarbon in the plasma-forming
gas can be desirable if a sufficient growth rate or growth density
of graphene on the metal substrate is not achieved using only
ablated reactive carbon species formed from the carbonaceous
electrode. Accordingly, the techniques described herein can offer
significant flexibility toward tuning the properties of the
synthesized graphene. Suitable hydrocarbons that can be included in
the plasma-forming gas include, but are not limited to, methane,
ethane, propane, ethylene, acetylene, and the like. Natural gas can
also be used as an inexpensive hydrocarbon source in some
embodiments. When present, the hydrocarbon can generally constitute
about 20% or less of the plasma-forming gas by volume, more
typically about 1% to about 10% hydrocarbon by volume, or about 5%
to about 15% hydrocarbon by volume. In some embodiments, the ratio
of hydrogen to the hydrocarbon can be greater than about 1:1.
[0041] In some embodiments, the methods described herein can
include flowing the plasma-forming gas in the gap between the metal
substrate and the carbonaceous electrode. Suitable mass velocities
across the metal substrate are desirably kept as low as possible,
but can include mass velocities up to about 0.1 gram/cm.sup.2/sec.
In alternative embodiments, a plasma-forming gas can be charged
statically to a reaction chamber housing a carbonaceous electrode
and a proximately disposed metal substrate. That is, in some
embodiments, a plasma can be generated from a non-flowing load of
the plasma-forming gas. In various embodiments, the potential
applied between the metal substrate and the carbonaceous electrode
can be a radiofrequency voltage. Frequencies of electromagnetic
radiation suitable for forming a plasma from a plasma-forming gas
can include electromagnetic radiation ranging between about 10 MHz
to about 30 GHz. In some or other embodiments, the applied
potential can be a DC voltage. That is, both radiofrequency and DC
plasmas can be used in the various embodiments described
herein.
[0042] As described above, a desirable feature of the present
embodiments is that the energy from a generated plasma can be
directed onto the metal substrate and/or the carbonaceous electrode
to provide significant advantages, particularly by varying the
applied potential such that the plasma is alternately directly
between the two as a function of time. Accordingly, in some
embodiments, a radiofrequency voltage can further include an
underlying waveform whose polarity alternates as a function of
time. That is, a background voltage profile can accompany a
radiofrequency voltage that can periodically alter the magnitude of
the potential between the metal substrate and the carbonaceous
electrode. In such embodiments, the plasma energy can be
alternately directed onto the metal substrate and the carbonaceous
electrode in order to advantageously provide energetic enhancement
to both. That is, providing plasma energy to the carbonaceous
electrode can advantageously expedite the ablation of reactive
carbon species therefrom, and providing plasma energy to the metal
substrate can facilitate the reorganization of the reactive carbon
species into graphene upon the surface of the metal substrate. In
addition, direction of the plasma energy onto the metal substrate
can result in further cleaning or etching of the substrate, as well
as smoothing of the catalyst surface, thereby further promoting the
growth of graphene. It should be noted that these benefits can also
be realized without utilization of an underlying waveform in the
applied potential. Given the benefit of the present disclosure, one
of ordinary skill in the art will be able to determine an
appropriate waveform to alternate the magnitude of the applied
potential as a function of time to produce a desired outcome during
the growth of graphene on the metal substrate.
[0043] In more specific embodiments, methods described herein can
include providing a metal substrate and a carbonaceous electrode,
conveying the metal substrate by the carbonaceous electrode via a
reel-to-reel processing line between a pay-out reel and a take-up
reel, applying a potential between the metal substrate and the
carbonaceous electrode via the reel-to-reel processing line,
flowing a plasma-forming gas in a gap between the metal substrate
and the carbonaceous electrode in the presence of the applied
potential to form a plasma upon exciting the plasma-forming gas,
ablating a reactive carbon species from the carbonaceous electrode
in the presence of the plasma, and growing graphene on the metal
substrate from the reactive carbon species while the metal
substrate is being conveyed. At least a portion of the metal
substrate is located proximate to the carbonaceous electrode with a
gap defined therebetween.
[0044] Although the invention has been described with reference to
the disclosed embodiments, one skilled in the art will readily
appreciate that these are only illustrative of the invention. It
should be understood that various modifications can be made without
departing from the spirit of the invention. The invention can be
modified to incorporate any number of variations, alterations,
substitutions or equivalent arrangements not heretofore described,
but which are commensurate with the spirit and scope of the
invention. Additionally, while various embodiments of the invention
have been described, it is to be understood that aspects of the
invention may include only some of the described embodiments.
Accordingly, the invention is not to be seen as limited by the
foregoing description.
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