U.S. patent application number 14/754983 was filed with the patent office on 2016-02-04 for growth of graphene films from non-gaseous carbon sources.
This patent application is currently assigned to William Marsh Rice University. The applicant listed for this patent is Zhiwei Peng, Gedeng Ruan, Zhengzong Sun, James M. Tour, Zheng Yan. Invention is credited to Zhiwei Peng, Gedeng Ruan, Zhengzong Sun, James M. Tour, Zheng Yan.
Application Number | 20160031711 14/754983 |
Document ID | / |
Family ID | 51351326 |
Filed Date | 2016-02-04 |
United States Patent
Application |
20160031711 |
Kind Code |
A1 |
Tour; James M. ; et
al. |
February 4, 2016 |
GROWTH OF GRAPHENE FILMS FROM NON-GASEOUS CARBON SOURCES
Abstract
In various embodiments, the present disclosure provides methods
of forming graphene films by: (1) depositing a non-gaseous carbon
source onto a catalyst surface; (2) exposing the non-gaseous carbon
source to at least one gas with a flow rate; and (3) initiating the
conversion of the non-gaseous carbon source to the graphene film,
where the thickness of the graphene film is controllable by the gas
flow rate. Additional embodiments of the present disclosure pertain
to graphene films made in accordance with the methods of the
present disclosure.
Inventors: |
Tour; James M.; (Bellaire,
TX) ; Sun; Zhengzong; (Houston, TX) ; Yan;
Zheng; (Houston, TX) ; Ruan; Gedeng; (Houston,
TX) ; Peng; Zhiwei; (Houston, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Tour; James M.
Sun; Zhengzong
Yan; Zheng
Ruan; Gedeng
Peng; Zhiwei |
Bellaire
Houston
Houston
Houston
Houston |
TX
TX
TX
TX
TX |
US
US
US
US
US |
|
|
Assignee: |
William Marsh Rice
University
Houston
TX
|
Family ID: |
51351326 |
Appl. No.: |
14/754983 |
Filed: |
June 30, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13561889 |
Jul 30, 2012 |
9096437 |
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14754983 |
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PCT/US2011/027575 |
Mar 8, 2011 |
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13561889 |
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61311615 |
Mar 8, 2010 |
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61347700 |
May 24, 2010 |
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61433702 |
Jan 18, 2011 |
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61513300 |
Jul 29, 2011 |
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Current U.S.
Class: |
428/408 ;
264/317; 423/448 |
Current CPC
Class: |
C01B 2204/02 20130101;
C01B 32/184 20170801; C01B 2204/04 20130101; C01B 32/194
20170801 |
International
Class: |
C01B 31/04 20060101
C01B031/04 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with government support under Sandia
National Laboratory Grant No. 1100745, awarded by the U.S.
Department of Energy; Office of Naval Research Grant No.
N00014-09-1-1066, awarded by the U.S. Department of Defense; and
Air Force Office of Scientific Research Grant No. FA
9550-09-1-0581, awarded by the U.S. Department of Defense. The
government has certain rights in the invention.
Claims
1. A graphene film made by the process of: a. depositing a
non-gaseous carbon source onto a catalyst surface; and b. exposing
the non-gaseous carbon source to at least one gas, wherein the at
least one gas comprises a gas flow rate; and c. initiating the
conversion of the non-gaseous carbon source to the graphene film,
wherein the formed graphene film comprises one or more layers of
graphene, and wherein the thickness of the graphene film is
controllable by adjusting the gas flow rate.
2. The graphene film of claim 1, wherein the non-gaseous carbon
source is selected from the group consisting of polymers,
non-polymeric carbon sources, raw carbon sources, small molecules,
organic compounds, fullerenes, fluorenes, carbon nanotubes,
phenylene ethynylenes, sucrose, sugars, polysaccharides, proteins,
carbohydrates and combinations thereof.
3. The graphene film of claim 1, wherein the non-gaseous carbon
source comprises a raw carbon source.
4. The graphene film of claim 3, wherein the raw carbon source is
selected from the group consisting of food sources, plants,
insects, waste products, and combinations thereof.
5. The graphene film of claim 1, wherein the catalyst surface
comprises one or more atoms selected from the group consisting of
Ni, Co, Fe, Pt, Au, Al, Cr, Cu, Mg, Mn, Mo, Rh, Si, Ta, Ti, W, U,
V, Zr and combinations thereof.
6. The graphene film of claim 1, wherein the non-gaseous carbon
source is doped with a doping reagent before, during or after the
initiating step, and wherein the doping results in the formation of
a doped graphene film.
7. The graphene film of claim 6, wherein the doping reagent is
selected from the group consisting of melamines, boranes,
carboranes, aminoboranes, ammonia boranes, phosphines, aluminum
hydroxides, silanes, polysilanes, polysiloxanes, phosphites,
phosphonates, sulfides, thiols, ammonia, pyridines, phosphazines,
borazines, and combinations thereof.
8. The graphene film of claim 1, wherein the graphene film is a
monolayered graphene.
9. The graphene film of claim 1, further comprising a step of
adjusting the thickness of the graphene film by adjusting the gas
flow rate.
10. The graphene film of claim 1, wherein the at least one gas is
selected from the group consisting of nitrogen, hydrogen, argon,
and combinations thereof.
11. The graphene film of claim 1, wherein the non-gaseous carbon
source is deposited on a first surface of the catalyst, and wherein
the graphene film forms on a second surface of the catalyst.
12. The graphene film of claim 11, wherein the first surface and
the second surface are on opposite sides of the catalyst.
13. The graphene film of claim 1, wherein the graphene film has
from about 2 layers of graphene to about 9 layers of graphene.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 13/561,889, which is a continuation-in-part application of
PCT Application No. PCT/US2011/027575, filed on Mar. 8, 2011, which
claims priority to United States Provisional Patent Application
Nos. 61/311,615, filed on Mar. 8, 2010; 61/347,700, filed on May
24, 2010; and 61/433,702, filed on Jan. 18, 2011. This application
also claims priority to U.S. Provisional Patent Application No.
61/513,300, filed on Jul. 29, 2011. This application is also
related to PCT Application No. PCT/US2011/027556, entitled
"Transparent Electrodes Based on Graphene and Grid Hybrid
Structures", filed on Mar. 8, 2011. The entirety of each of the
above-referenced applications is incorporated herein by
reference.
BACKGROUND
[0003] Graphene films find many applications in various fields,
including optoelectronics. Current methods to form graphene films
suffer from various limitations, including the inability to use a
variety of carbon sources to yield graphene films with desirable
thicknesses, sizes, patterns and electrical properties. Therefore,
there is currently a need to develop more optimal methods of
forming graphene films.
BRIEF SUMMARY
[0004] In some embodiments, the present disclosure provides methods
of forming graphene films by: (1) depositing a non-gaseous carbon
source (e.g., a poly(methyl methacrylate)) onto a catalyst surface
(e.g., a copper surface); and (2) initiating the conversion of the
non-gaseous carbon source to the graphene film. In some
embodiments, the methods of the present disclosure may also include
a step of exposing the non-gaseous carbon source to at least one
gas with a gas flow rate. In some embodiments, the gas may include
at least one of hydrogen, nitrogen, argon, or combinations thereof.
In some embodiments, the thickness of the graphene film may be
controllable by the gas flow rate. In some embodiments, the methods
of the present disclosure may also include a step of adjusting the
thickness of the graphene film by adjusting the gas flow rate.
[0005] In some embodiments, graphene film formation is initiated
under vacuum. In some embodiments, graphene film formation is
initiated by heating. In some embodiments, the heating occurs at
reaction temperature ranges between about 400.degree. C. to about
1200.degree. C. In some embodiments, the heating may also occur in
a reductive environment (e.g., environments with H.sub.2/Ar gas
streams).
[0006] In some embodiments, the non-gaseous carbon source may
include at least one of polymers, non-polymeric carbon sources, raw
carbon sources, small molecules, organic compounds, fullerenes,
fluorenes, carbon nanotubes, phenylene ethynylenes, sucrose,
sugars, polysaccharides, carbohydrates, proteins, and combinations
thereof. In some embodiments, the non-gaseous carbon source is
doped with a doping reagent (e.g., melamine, carborane or
aminoborane) before, during or after the initiating step to result
in the formation of doped graphene films.
[0007] Additional embodiments of the present disclosure pertain to
graphene films made by the methods of the present disclosure. In
some embodiments, the formed graphene films are monolayers. In some
embodiments, the formed graphene films are utilized in electric
devices, such as transparent electrodes.
[0008] As set forth in more detail below, the methods of the
present disclosure provide numerous advantages, including the
ability to form graphene films with low defects, low sheet
resistance, and ambipolar field effects. The methods of the present
disclosure also enable the formation of easily transferable
graphene films with desirable sizes, thicknesses and patterns from
a variety of non-gaseous carbon sources. As also set forth in more
detail below, the graphene films formed by the methods of the
present disclosure can find numerous applications in various
fields, including optoelectronics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 illustrates synthetic protocols, spectroscopic
analyses and electrical properties of a graphene derived from
poly(methyl methacrylate) (PMMA-derived graphene or PG). FIG. 1A
shows a schematic of how a monolayered PG can be derived from the
solid PMMA films on Cu substrates. FIG. 1B shows a Raman spectrum
(514 nm excitation) of a monolayered PG obtained at 1000.degree. C.
FIG. 1C shows a room temperature I.sub.ds-V.sub.G curve on a
PG-based back-gate field effect transistor (FET) device. The upper
inset shows the I.sub.ds-V.sub.ds characteristics as a function of
V.sub.G. V.sub.G changes from 0 V (bottom) to -40 V (top). The
lower inset in (c) is the scanning electron microscopy (SEM)
(JEOL-6500 microscope) image of this device, where the PG is
perpendicular to the Pt leads. FIG. 1D shows a selected area
electron diffraction (SAED) pattern of PG. FIGS. 1E-G show high
resolution transmission electron microscopy (HRTEM) images of PG
films. Black arrows in FIG. 1G indicate the Cu atoms.
[0010] FIG. 2 shows data relating to the controllable growth of
pristine graphene films. FIG. 2A illustrates differences in Raman
spectra from PG samples with controllable thicknesses derived from
different flow rates of H.sub.2. FIG. 2B shows the
ultraviolet-visible (UV) absorption spectra of monolayered graphene
and bilayered graphene. The UV transmittance (T %) of the
corresponding PG is measured at 550 nm. FIG. 2C shows the Raman
spectra of graphene derived from sucrose, fluorene and PMMA. FIG.
2D shows HRTEM picture of PG grown on a Ni film. The PG was 3-5
layers at the edges.
[0011] FIG. 3 shows spectroscopic analysis and electrical
properties of PG and N-doped PG. FIG. 3A shows XPS analysis from
the C1s peak of PG (black) and N-doped PG (red). The shoulder can
be assigned to the C--N bond. FIG. 3B shows XPS analysis of the N1s
peak (black line) and its peak fitting (square points) of N-doped
PG. The atomic concentration of N for this sample is about 2% (C is
98%). No N1s peak was observed for PG. FIG. 3C shows Raman spectra
for PG and N-doped PG. FIG. 3D shows room temperature,
I.sub.ds-V.sub.G curves with n-type behavior obtained from three
different N-doped graphene-based back-gate FET devices.
[0012] FIG. 4 shows two representative pristine graphene FETs atop
200 nm SiO.sub.2 with highly doped p.sup.++ Si back gate measured
after storage at 10.sup.-6 Torr for 7 days. Under vacuum, the Dirac
point recovers from positive gate voltages and stabilizes at zero
as surface adsorbents are removed. Mobilities of .about.400
cm.sup.2V.sup.-1s.sup.-1 at room temperature were achieved.
[0013] FIG. 5 shows Raman 2D peak fittings of different layered
PGs. Monolayered PG's 2D band is fitted with a single Lorentz peak.
Bilayered and few-layered graphene 2D bands are splitting into 4
components: 2D.sub.1B, 2D.sub.1A, 2D.sub.2A, 2D.sub.2B (green
peaks, from left to right). Solid lines are from the original data.
Square points are the fitting curves.
[0014] FIG. 6 shows Raman spectrum of PG grown at 800.degree.
C.
[0015] FIG. 7 shows Raman spectra of PMMA films that were heated on
Ni, Si<100> with native oxide, or 200-nm-thick thermally
grown SiO.sub.2.
[0016] FIG. 8 shows various attributes of melamine, a doping
reagent with about 66% of nitrogen in atomic concentration compared
to C. FIGS. 8A-8B shows the x-ray photoelectron spectroscopy (XPS)
spectra of melamine. FIG. 8C shows the chemical structure of
melamine (C.sub.3H.sub.6N.sub.6).
[0017] FIG. 9 shows two-dimensional Raman spectral mapping of
monolayered (FIG. 9A) and bilayered (FIG. 9B) PG graphene films
(75.times.75 .mu.m.sup.2) at 514 nm. The color gradient bar to the
right of each map represents the G/2D peak ratio. The green and
black areas in FIG. 9A are monolayer graphene with an IG/I2D
<0.4, suggesting at least 95% monolayer coverage. The blue area
in FIG. 9B represents bilayered graphene with an IG/I2D .about.0.8,
suggesting more than 85% bilayer coverage. The lateral scale bars
are 20 .mu.m.
[0018] FIG. 10 shows an atomic force microscopy (AFM) image (left
panel) and height profile (right panel) of a monolayer PG on a
SiO.sub.2/Si substrate. Specifically, Step 1 (red) represents the
height profile of the SiO.sub.2/Si substrate. Step 2 (green) is the
height profile of the graphene film edge. The step height is about
.about.0.7 nm, which reflects the thickness of the PG. The AFM
scale bar is 1 .mu.m.
[0019] FIG. 11 illustrates the growth of graphene films from raw
carbon sources. FIG. 11A shows a diagram of an experimental
apparatus suitable for the growth of graphene films from various
raw carbon sources (e.g., food, insects or waste) in a tube
furnace. On the left, the Cu foil with the carbon source contained
in a quartz boat is placed at the hot zone of a tube furnace. The
growth is performed at 1050.degree. C. under low pressure with a
H.sub.2/Ar gas flow. On the right is a cross view that represents
the formation of pristine graphene film on the backside of the Cu
substrate. FIG. 11B shows the growth of graphene film from a
cockroach leg. FIG. 11B(a) shows one roach leg on top of the Cu
foil. FIG. 11B(b) shows the roach leg under vacuum. FIG. 11B(c)
shows the residue from the roach leg after annealing at
1050.degree. C. for 15 min. The pristine graphene film grew on the
bottom side of the Cu film (not shown).
[0020] FIG. 12 shows scanning electron microscopy (SEM) images of
the Cu foil after growth of graphene films from a GIRL SCOUT
cookie. FIG. 12A shows the original front side of the Cu foil,
where there was a large quantity of particle residue after the
pyrolysis of the cookie. FIG. 12B shows the backside of the Cu
foil.
[0021] FIG. 13 shows a representative Raman spectrum of amorphous
carbon grown on the backside of Cu foil when the GIRL SCOUT cookie
fragments were placed 5 cm ahead of the Cu foil in the tube furnace
shown in FIGS. 11A-B.
[0022] FIG. 14 shows Raman spectra of monolayer graphene films from
six different carbon sources. The Raman spectra graphene were
derived from GIRL SCOUT cookies (FIG. 14A); chocolate (FIG. 14B);
grass (FIG. 14C); plastic (polystyrene Petri dish) (FIG. 14D); dog
feces (FIG. 14E) and a cockroach leg (FIG. 14F). There was only a
trace D peak in some of the spectra, and the 2D to G peak intensity
ratios were .about.4, indicating monolayer graphene.
[0023] FIG. 15 shows Raman spectral mapping of graphene films from
dog feces. The scanning was performed at every 5 .mu.m over an area
of 100 .mu.m.times.100 .mu.m. FIG. 15A shows Raman spectral mapping
of 2D/G ratio, over 95% of the scanning area has the signature of
I.sub.2D/I.sub.G>1.8. FIG. 15B shows Raman spectral mapping of
D/G ratio, indicating that over 95% of the scanning area has the
signature of I.sub.D/I.sub.G<0.1. This is confirmation of
high-quality monolayer graphene film formation.
[0024] FIG. 16 shows x-ray photoelectron spectroscopy (XPS) spectra
of graphene films from six raw carbon sources. The C1s XPS spectra
of the randomly selected detection spots on graphene films were
derived from the various raw carbon sources.
[0025] FIG. 17 shows UV-Vis spectra of graphene films derived from
six carbon sources. The absorbance of each monolayer graphene film
at 550 nm is approximately (2.4%.+-.0.1%). On the right top of each
spectrum is the photographic image of the monolayer graphene film
of .about.1 cm.times.1 cm in size on a 1-mm-thick quartz slide,
labeled with a dashed square.
[0026] FIG. 18 shows diffraction pattern and transmission electron
microscopy (TEM) images of a cookie-derived graphene film. FIG. 18A
shows the selected area electron diffraction (SAED) pattern. FIG.
18B shows the suspended graphene film on a 1 .mu.m diameter hole.
FIG. 18C shows the edge of monolayer graphene film.
[0027] FIG. 19 shows Raman spectrum of a control sample. The
annealing conditions are the same as the growth conditions
described in Example 2, except that there is no solid carbon source
added to the growth system. As shows in the spectrum, no graphene
film was present on the backside of the Cu foil in the control
sample.
DETAILED DESCRIPTION
[0028] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only, and are not restrictive of the invention, as
claimed. In this application, the use of the singular includes the
plural, the word "a" or "an" means "at least one", and the use of
"or" means "and/or", unless specifically stated otherwise.
Furthermore, the use of the term "including", as well as other
forms, such as "includes" and "included", is not limiting. Also,
terms such as "element" or "component" encompass both elements or
components comprising one unit and elements or components that
comprise more than one unit unless specifically stated
otherwise.
[0029] The section headings used herein are for organizational
purposes only and are not to be construed as limiting the subject
matter described. All documents, or portions of documents, cited in
this application, including, but not limited to, patents, patent
applications, articles, books, and treatises, are hereby expressly
incorporated herein by reference in their entirety for any purpose.
In the event that one or more of the incorporated literature and
similar materials defines a term in a manner that contradicts the
definition of that term in this application, this application
controls.
[0030] Since the discovery of graphene in 2004, many methods were
developed to obtain large sheets of monolayered or bilayered
graphene. Such methods have included chemical vapor deposition
(CVD), mechanical peeling, liquid exfoliation, and reduction of
graphene oxide. However, current methods of making graphene films
suffer from various limitations that necessitate the development of
new techniques.
[0031] For instance, with respect to exfoliation methods,
researchers originally used adhesive tape to mechanically peel away
the graphite crystals into few-layer or monolayer graphene. Later,
liquid exfoliation methods were reported. Such methods generally
consisted of chemical oxidation and dispersion of graphite,
reduction of graphite oxide, and annealing in Ar/H.sub.2. However,
the quality of the liquid exfoliated graphene was still lower than
mechanically exfoliated graphene due to the destruction of the
basal plane structure during the oxidation, and incomplete removal
of the functional groups.
[0032] Recently, many research groups have published several CVD
methods for growing large-sized graphene on wafers. However, CVD
methods can be expensive. For instance, the growth of epitaxial
graphene on single-crystal silicon carbide (SiC) can be costly due
to the high price of the 4H--SiC substrates. Moreover, CVD is
limited to the use of gaseous raw materials. Such limitations make
it difficult to apply CVD to a wider variety of non-gaseous carbon
sources that are more readily available at lower costs.
Furthermore, many CVD-based methods utilize volatile gaseous
precursors that present safety issues.
[0033] In addition, many graphene-based electronic devices require
that graphene films be grown in large size with controllable
thickness and electrical properties. However, the methods of the
prior art fail to address these requirements.
[0034] Accordingly, Applicants have developed novel methods of
forming graphene films that address the aforementioned needs and
limitations. Such methods generally involve: (1) depositing a
non-gaseous carbon source onto a catalyst surface; and (2)
initiating the conversion of the non-gaseous carbon source to a
graphene film. In some embodiments, the methods of the present
disclosure may also include a step of exposing the non-gaseous
carbon source to at least one gas with a gas flow rate. In some
embodiments, the thickness of the graphene film may be controllable
by the gas flow rate. In some embodiments, the methods of the
present disclosure may also include a step of adjusting the
thickness of the graphene film by adjusting the gas flow rate.
[0035] In some embodiments, the methods of the present disclosure
also include steps for separating the formed graphene film from the
catalyst surface by coating the graphene film with a protecting
layer, separating the catalyst surface from the coated graphene
film, and transferring the coated graphene film to a different
surface. Various embodiments of the present disclosure allow the
non-gaseous carbon source to be doped with a doping reagent before,
during or after the initiating step to result in the formation of a
doped graphene film. Additional embodiments of the present
disclosure pertain to graphene films made by the methods of the
present disclosure.
[0036] An example of a method of forming graphene films is depicted
in FIG. 1A. In this example, poly(methyl methacrylate) (PMMA) is
the non-gaseous carbon source, and a copper foil is the catalyst
surface. In some embodiments, the copper foil (or other metal
catalyst surface being used) is first cleaned with diluted acid
(i.e., to remove copper oxide), acetone, and deionized water. The
copper foil is then dried with N.sub.2 gas purging. In some
embodiments, the cleaning method could be either acid cleaning or
high temperature annealing under reductive atmospheres.
[0037] Next, PMMA (with or without a doping reagent) is spin-coated
or drop-casted on one side of the copper foil (though it could be
used to coat both sides of a foil or other catalysts structure for
conformal growth). The PMMA layer is then vacuum dried to remove
the solvent. Thereafter, the copper foil is placed in an H.sub.2/Ar
purged furnace. Next, the conversion of PMMA to graphene is
initiated by utilizing a reaction temperature of about 800.degree.
C.-1000.degree. C. (e.g., by moving the samples stored in a furnace
column into a "hot zone"). This results in the catalytic conversion
of the non-gaseous carbon source to a graphene film on the copper
foil.
[0038] Optionally, the formed graphene film may then be separated
from the copper foil by spin-coating the graphene with a thin layer
of polymer (e.g., PMMA) as a protecting layer for the next step.
This is followed by vacuum-drying to remove the solvent. Next, the
copper foil is dissolved in a Marble's reagent
(CuSO.sub.4:HCl:H.sub.2O=10 g:50 ml:50 ml). The polymer and
graphene film are then lifted off and transferred into deionized
water to remove the metal ion and other inorganic contaminations.
Next, the obtained film is transferred on different substrates and
vacuum dried to remove the water. The polymer is then removed by
rinsing with organic solvent or pyrolysis cleaning.
[0039] Another example of a method of forming graphene films is
depicted in FIG. 11A. In this example, a cockroach leg is the
non-gaseous carbon source, and a copper foil is the catalyst
surface. The cockroach leg is deposited on a copper foil that is
placed on a quartz boat. The assembly is then placed at the hot
zone of a tube furnace. The growth is initiated at 1050.degree. C.
under low pressure with a H.sub.2/Ar gas flow. As shown on the
cross view of FIG. 11A, pristine graphene forms on the backside of
the copper foil.
[0040] Various aspects of the aforementioned methods of making
graphene films will now be discussed in more detail herein.
However, Applicants note that the description herein pertains to
non-limiting examples of how a person of ordinary skill in the art
can make and use the graphene films of the present disclosure.
[0041] Non-Gaseous Carbon Sources
[0042] In the present disclosure, non-gaseous carbon sources
generally refer to any non-gaseous compositions that can be
converted to graphene films. As used herein, the term non-gaseous
carbon sources refers to carbon sources that are in liquid state,
solid state, or combinations thereof, and without a substantial
amount of carbon sources that are in gaseous state. However,
Applicants note that, in some embodiments, there may be trace or
minimal amounts of gaseous carbon sources in the non-gaseous carbon
sources of the present disclosure (e.g., without limitation,
.about.0.001% to 10%). In some embodiments, the non-gaseous carbon
sources of the present disclosure may be in a dry state. In some
embodiments, the non-gaseous carbon sources of the present
disclosure may be vacuum dried.
[0043] Various non-gaseous carbon sources may be used to make
graphene films. Non-limiting examples of such non-gaseous carbon
sources include solid carbon sources, polymers, non-polymeric
carbon sources, raw carbon sources, small molecules, organic
compounds, fullerenes, fluorenes, carbon nanotubes, phenylene
ethynylenes, sucrose, sugars, polysaccharides, carbohydrates,
proteins, and combinations thereof. In more specific embodiments,
the non-gaseous carbon source comprises one or more
carbon-containing small molecules with molecular weights of less
than 500 grams/mole.
[0044] In more specific embodiments, the non-gaseous carbon source
may include a polymer. Suitable polymers that can be used as
non-gaseous carbon sources include, without limitation, hydrophilic
polymers, hydrophobic polymers, amphiphilic polymers, homopolymers,
copolymers, polymer blends, thermoplastic polymers, thermosetting
polymers, and combinations thereof. More specific but non-limiting
examples of suitable polymers that can be used as non-gaseous
carbon sources include PMMA, polystyrenes, polyacrylonitriles,
polycarbonates, poly(phenylene ethynylene)s, and cellulose. Other
suitable polymers can also be envisioned. In more specific
embodiments, the non-gaseous carbon source is PMMA.
[0045] In additional embodiments, the non-gaseous carbon source may
include a carbon nanotube. Non-limiting examples of carbon
nanotubes that can be used as non-gaseous carbon sources include
single-walled carbon nanotubes, multi-walled carbon nanotubes,
double-walled carbon nanotubes, ultrashort carbon nanotubes, and
combinations thereof. In some embodiments, the carbon nanotubes may
be functionalized. In some embodiments, the carbon nanotubes may be
in pristine or non-functionalized form.
[0046] In some embodiments, the non-gaseous carbon sources may
include one or more raw carbon sources. In various embodiments, raw
carbon sources may generally refer to carbon sources that are
unprocessed, unpurified, or mixed with other materials. For
instance, in some embodiments, the raw carbon sources may include
at least one of food sources, plants, insects, waste products,
parts thereof, or combinations thereof. In some embodiments, raw
carbon sources may include a food source, such as cookies (e.g.,
GIRL SCOUT cookies), chocolates, and the like. In some embodiments,
the raw carbon sources may include a plant, such as grass, wood,
flowers, leaves, mixtures of organic vegetation, and the like. In
some embodiments, the raw carbon sources may include an insect,
such as an ant or a cockroach. In some embodiments, the raw carbon
sources may include waste products, such as feces or pre-used
plastics (e.g., bulk polystyrene plastics). Other suitable
non-gaseous carbon sources can also be used in accordance with the
methods of the present disclosure.
[0047] Catalyst Surfaces
[0048] In the present disclosure, catalyst surfaces generally refer
to surfaces that are capable of converting non-gaseous carbon
sources to graphene films. In various embodiments, the catalyst
surfaces could be made of porous or non-porous materials. In some
embodiments, the catalyst surface is a solid surface. Non-limiting
examples of suitable catalyst surfaces can include surfaces that
contain one or more of the following atoms: Ni, Co, Fe, Pt, Au, Al,
Cr, Cu, Mg, Mn, Mo, Rh, Si, Ta, Ti, W, U, V, Zr, or combinations
thereof.
[0049] In some embodiments, the catalyst surface is a metal
catalyst. In more specific embodiments, the metallic atoms in the
catalyst surface may be in reduced and/or oxidized forms. In
further embodiments, the metals may be associated with alloys.
[0050] The catalyst surfaces of the present disclosure can also
have various shapes and structures. For instance, in various
embodiments, the catalyst surfaces are circular, square-like, or
rectangular. In some embodiments, the catalyst surface can be
pre-patterned. In such embodiments, the graphene can be grown
following those patterns.
[0051] Furthermore, the catalyst surfaces of the present disclosure
may have various sizes. In various embodiments, such sizes can be
in the nanometer, millimeter, centimeter or meter ranges. For
instance, in some embodiments, the catalyst surface can be as small
as 1-nanometer on a face, or as a sphere. In some embodiments, the
catalyst surface can be as large as 100 square meters on a face, or
as a sphere. However, the latter embodiments may require a large
furnace. For the latter embodiments, roll-to-roll films of metal
could also be used as the catalyst surface as the metal passes
though a furnace's hot-zone.
[0052] Deposition of Non-Gaseous Carbon Sources onto Catalyst
Surfaces
[0053] Various methods may also be used to deposit non-gaseous
carbon sources onto catalyst surfaces. Such methods can include,
without limitation, spin-coating, drop-casting, spray coating, dip
coating, physical application, sublimation, blading, inkjet
printing, screen printing, direct placement, or thermal
evaporation.
[0054] The above-mentioned step can also be used to control the
thickness of graphene films. For instance, as discussed in more
detail below, a non-gaseous carbon source may be deposited onto a
catalyst surface until a desired thickness for the graphene film is
achieved. In some embodiments, such desired thickness can be
anywhere from about 0.6 nm to about 10 .mu.m.
[0055] Furthermore, the above-mentioned step can be used to form a
carbon layer with a uniform or non-uniform thickness. This in turn
can result in the formation of a graphene film with the desired
thicknesses.
[0056] Doping of Non-Gaseous Carbon Sources
[0057] The non-gaseous carbon sources deposited onto the catalyst
surface may be doped or un-doped. In some embodiments, the
non-gaseous carbon sources are un-doped. This can result in the
formation of pristine graphene films. In additional embodiments,
the non-gaseous carbon source deposited onto the catalyst surface
may doped with a doping reagent. This can result in the formation
of doped graphene films.
[0058] Various doping reagents may be used in non-gaseous carbon
sources. In some embodiments, the doping reagents may be
heteroatoms of B, N, O, Al, Au, P, Si, and/or S. In more specific
embodiments, the doping reagents may include, without limitation,
melamines, boranes, carboranes, aminoboranes, ammonia boranes,
phosphines, aluminum hydroxides, silanes, polysilanes,
polysiloxanes, phosphites, phosphonates, sulfides, thiols, ammonia,
pyridines, phosphazines, borazines, and combinations thereof. In
some embodiments, the doping reagents may be HNO.sub.3 or
AuCl.sub.3. In some embodiments, HNO.sub.3 or AuCl.sub.3 may be
applied after the graphene film growth rather than during the
growth. In more specific embodiments, the doping reagent is
melamine.
[0059] In some embodiments, the doping reagent may be added
directly to the non-gaseous carbon source. In various embodiments,
the doping can occur before, during or after the initiation step of
graphene film formation. For instance, in some embodiments, the
doping can occur during the conversion of the non-gaseous carbon
source to graphene films.
[0060] In more specific embodiments, the doping reagent is added to
the non-gaseous carbon source as a gas during the conversion of the
non-gaseous carbon source to graphene films. In such embodiments,
the doping reagent may comprise at least one of ammonia, pyridine,
phosphazine, borazine, borane, and ammonia borane.
[0061] In some embodiments, the doping may occur after the
completion of graphene film formation. In some embodiments, the
doping reagent may be covalently bound to the non-gaseous carbon
source. For instance, a doping reagent may be covalently linked to
a polymer's backbone or exogenous additives.
[0062] Furthermore, the doping reagents of the present disclosure
can have various forms. For instance, in various embodiments, the
doping reagents could be in gaseous, solid and/or liquid phases. In
addition, the doping reagent could be one reagent or a combination
of different reagents. Moreover, various doping reagent
concentrations may be used. For instance, in some embodiments, the
final concentration of the doping reagent in the non-gaseous carbon
source could be from about 0% to about 25%.
[0063] In some embodiments that are discussed in more detail below,
as doping reagents are added to the molecular structure or carbon
source material, doped graphene films can form through the
insertion of doping reagents (e.g., heteroatoms) into the graphene
network, or along the graphene network
[0064] Initiation of Graphene Film Formation
[0065] Various methods may also be used to initiate the formation
of graphene films on catalyst surfaces. In some embodiments, the
initiating step may include a heating step, where suitable reaction
temperatures are utilized. In some embodiments, the suitable
reaction temperature may be between about 400.degree. C. to about
1200.degree. C. In more specific embodiments, the suitable reaction
temperature is about 800.degree. C.
[0066] In some embodiments, suitable reaction temperatures are
attained by elevating the environmental temperature. For instance,
a sample containing a carbon source on a catalyst surface may be
placed in a furnace. The furnace temperature may then be elevated
to a suitable temperature, such as about 800.degree. C.
[0067] In some embodiments, suitable reaction temperatures may be
attained by moving a sample to a suitable environment. For
instance, a sample containing a carbon source on a catalyst surface
may be in a furnace column. Thereafter, the sample may be moved
into a "hot zone" of the furnace that has a suitable temperature
(e.g., about 800.degree. C.). See, e.g., FIG. 11A.
[0068] Various environmental conditions may also be used to
initiate graphene film formation. For instance, in some
embodiments, graphene film formation may occur in a closed
environment, such as an oven or a furnace. In some embodiments, the
initiating step may occur under vacuum. In some embodiments, the
vacuum may have a pressure of about 10.sup.-6 Torr.
[0069] In some embodiments, the initiating step may occur under a
stream of one or more gases. For instance, in some embodiments, the
non-gaseous carbon sources may be exposed to one or more gases
prior to or during the initiating step. In some embodiments, the
one or more gases may have one or more individual gas flow rates.
In some embodiments, the thickness of the graphene film may be
controllable by the one or more individual gas flow rates.
Accordingly, various embodiments of the present disclosure also
include a step of adjusting the thickness of the graphene film by
adjusting the one or more individual gas flow rates.
[0070] In some embodiments, the one or more individual gas flow
rates may range from about 1 sccm (standard cubic centimeters per
minute) to about 2,000 sccm. In some embodiments, the one or more
individual gas flow rates may range from about 10 sccm to about 500
sccm.
[0071] In some embodiments, the one or more individual gas flow
rates may range from about 10 cm.sup.3 STP min.sup.-1 to about
1,000 cm.sup.3 STP min.sup.-1. In some embodiments, the one or more
individual gas flow rates may range from about 100 cm.sup.3 STP
min.sup.-1 to about 500 cm.sup.3 STP min.sup.-1.
[0072] In some embodiments, the one or more gases may include,
without limitation, hydrogen, nitrogen, argon, or combinations
thereof. In some embodiments where more than one gas is used, the
different gases may have different individual gas flow rates. For
instance, in some embodiments, the flow rate of one gas (e.g.,
hydrogen) may range from about 1 sccm to about 50 sccm, while the
flow rate of another gas (e.g., argon) may range from about 100
sccm to about 500 sccm.
[0073] In some embodiments, graphene film formation may occur in a
reductive environment. A specific example of a reductive
environment is an environment that contains a stream of a reductive
gas, such as a stream of H.sub.2 or Ar gases. In more specific
embodiments, graphene film formation may occur in a furnace that
contains a stream of H.sub.2/Ar gas.
[0074] Various time periods may also be used to initiate and
propagate graphene film formation. For instance, in some
embodiments, the heating can occur in a time period ranging from
about 1 minute to about 10 hours. In more specific embodiments, the
heating may occur in a time period ranging from about 1 minute to
about 60 minutes. In more specific embodiments, the heating may
occur for about 10 minutes.
[0075] Various methods may also be used to heat graphene films. For
instance, in some embodiments, the heating may be performed by
induction heating. In some embodiments, the energy source for the
heating could be derived from radiating energy (e.g., laser),
infrared rays, microwave or X-rays.
[0076] Graphene film formation can also occur under various
pressures. In some embodiments, such pressure ranges can be from
about 0.01 mm Hg to about 10 atmospheres of pressure. In more
specific and preferred embodiments, pressure ranges can be from
about 1 mm Hg to about 1 atmosphere.
[0077] Separation of Graphene Films from Catalyst Surfaces
[0078] Various embodiments of the present disclosure also include
methods of separating the formed graphene films from the catalyst
surfaces. In some embodiments, such methods may include: (1)
coating the graphene film with a protecting layer; (2) separating
the catalyst surface from the coated graphene film; and (3)
transferring the graphene film to a different surface.
[0079] In some embodiments, the protecting layer may be a polymer,
such as PMMA or polycarbonate (PC). In some embodiments, the
catalyst surface is separated from the graphene film by dissolving
the catalyst surface in a solvent. In some embodiments, the solvent
may include a Marble's reagent (as previously described). In more
specific embodiments, the graphene film may be separated from the
catalyst surface by acid-etching.
[0080] As set forth in more detail below, the isolated graphene
films may then be applied to various surfaces and used in numerous
applications. As also set forth in more detail below, the formed
graphene films can have numerous advantageous properties.
[0081] Control of Graphene Film Thickness
[0082] A specific advantage of the methods of the present
disclosure is the ability to control graphene film thickness.
Thickness of graphene films can be controlled by adjusting one or
more conditions during graphene film formation. Such adjustable
conditions include, without limitation: (1) non-gaseous carbon
source type; (2) non-gaseous carbon source concentration; (3) gas
flow rate (e.g., H.sub.2/Ar flow rate); (4) pressure; (5)
temperature; and (6) catalyst surface type.
[0083] In some embodiments, the thickness of the graphene film may
be adjusted by adjusting a flow rate of one or more gases. For
instance, in some embodiments, a decrease in a gas flow rate may
lead to the formation of thicker graphene films. In some
embodiments, an increase in a gas flow rate may lead to the
formation of thinner graphene films.
[0084] In some embodiments, the thickness of the graphene film can
range from about 0.6 nm to about 10 wn. In some embodiments, the
formed graphene film is a monolayer with a thickness of about 0.7
nm. See, e.g., FIGS. 9A and 10. In some embodiments, the formed
graphene film is a bilayer. See, e.g., FIG. 9B. In some
embodiments, the graphene films can have from about 2 layers to
about 9 layers. In some embodiments, the graphene films may have up
to 100 layers.
[0085] Modes of Graphene Film Growth
[0086] The methods of the present disclosure may also be used to
grow graphene films on various surfaces of a catalyst. For
instance, in some embodiments, a non-gaseous carbon source may be
deposited on a first surface of the catalyst. Subsequently, the
graphene film may form on a second surface of the catalyst. In some
embodiments, the first surface and the second surface are on
opposite sides of the catalyst. See, e.g., FIG. 11A.
[0087] In some embodiments, a non-gaseous carbon source may be
deposited on a first surface of the catalyst. Subsequently, the
graphene film may form on the first surface and a second surface of
the catalyst. In some embodiments, the first surface and the second
surface are on opposite sides of the catalyst.
[0088] Additional Advantages
[0089] The graphene films and methods of the present disclosure can
provide numerous additional advantages. Such advantages can
include, without limitation: (1) low defects and low sheet
resistance; (2) ambipolar field effects; (3) low temperature
growth; (4) patterned growth; (5) growth from different non-gaseous
carbon sources; (6) large area growth; (7) easy transferability;
and (8) low costs.
[0090] Low Defects and Low Sheet Resistance
[0091] In general, the graphene films produced by the methods of
the present disclosure can have low defects and low resistance. For
instance, as indicated in more detail in the Examples below, Raman
spectrum shows that PG's are highly crystalline. See FIG. 2B. In
addition, the corresponding monolayer PG's sheet resistance is
about 1200 .OMEGA./sq. In some embodiments, the methods of the
present disclosure may be used to form graphene films with sheet
resistance that range from about 300 .OMEGA./sq to about 5,000
.OMEGA./sq.
[0092] Ambipolar Field Effects
[0093] The graphene films produced by the methods of the present
disclosure can also show ambipolar behavior. See, e.g., FIG.
1C.
[0094] Low Temperature Growth
[0095] The methods of the present disclosure can also be used to
grow graphene films at relatively low temperatures. For instance,
as discussed in more detail in the Examples below, Applicants have
been able to obtain high quality graphene films at reaction
temperatures of about 800.degree. C. See, e.g., FIGS. 1A-1B and
FIG. 6. Such temperatures are lower than the original report of CVD
growth temperatures on copper foils. In some embodiments, this
lower temperature growth could be critical for various
applications, such as compatibility with embedded doped silicon
electronics applications. In some embodiments, graphene films can
also be formed at about 750.degree. C., even though they may have
larger D bands. In some embodiments, the methods of the present
disclosure may be used to grow graphene films at temperatures that
range from about 500.degree. C. to about 1070.degree. C.
[0096] Patterned and Tunable Growth
[0097] Applicants have also observed that graphene films have
effective growth rates when doped or un-doped non-gaseous carbon
sources are used in accordance with the methods of the present
disclosure. See, e.g., FIG. 3. Furthermore, the dopant
concentration in the final graphene films can be tuned by the
concentration of the doping reagent in the starting polymer
solutions. Therefore, Applicants envision that the methods of the
present disclosure have the potential to be used for tunable growth
of graphene films. Likewise, the graphene films can be grown on
various patterned surfaces to attain patterned growth.
[0098] Growth from Different Non-Gaseous Carbon Sources
[0099] Another advantage of the present disclosure is that numerous
non-gaseous carbon sources can be used to produce graphene films.
For instance, as illustrated in FIG. 2C and discussed in more
detail in the Examples below, high quality monolayered PG can be
grown from different solid non-gaseous carbon sources, including
precursors containing potential topological defect generators
(e.g., the five-member ring in fluorene) or high concentrations of
heteroatoms (i.e., oxygen in sucrose) Likewise, as illustrated in
FIGS. 11-19 and discussed in more detail in the Examples below,
high quality graphenes can be grown from raw non-gaseous carbon
sources that are readily available at minimal costs.
[0100] Large Area Growth
[0101] As discussed previously, various sizes of catalyst surfaces
can be used in various embodiments of the present disclosure.
Therefore, large graphene films may be generated by the methods of
the present disclosure. For instance, graphene films with areas in
the centimeter range or square meter range (as discussed) can be
obtained by using the methods of the present disclosure.
[0102] Easy Transferability
[0103] In some embodiments, the present disclosure also provides
effective methods of transferring the formed graphene films onto
different substrates in a non-destructive manner. This provides an
effective way of maintaining the integrity and efficacy of the
graphene films for many applications, including use in transparent
electrodes.
[0104] Low Costs
[0105] The methods of the present disclosure can be used to grow
graphene films from numerous non-gaseous carbon sources that are
readily available at little or no cost. For instance, many raw
non-gaseous carbon sources are readily available in nature.
Therefore, the methods of the present disclosure also provide a
cost-effective method of forming high quality graphene films.
[0106] Applications
[0107] The graphene films formed by the methods of the present
disclosure can have numerous applications. For instance, in some
embodiments, the graphene films formed by the methods of the
present disclosure can be used as electrodes for optoelectronics
applications, such as organic photovoltaics, organic light emitting
devices, liquid crystal display devices, touch screens, "heads-up"
displays, goggles, glasses and visors, and smart window panes. In
some embodiments, the graphene films of the present disclosure may
also find applications in flexible solar cells and organic light
emitting diodes (OLEDs).
[0108] Furthermore, the graphene films of the present disclosure
can find applications in various transparent electrode hybrid
structures. Such structures have been disclosed in Applicants'
co-pending PCT Application No. PCT/US2011/027556, entitled
"Transparent Electrodes Based on Graphene and Grid Hybrid
Structures", filed on Mar. 8, 2011. The entirety of this
application is incorporated herein by reference.
ADDITIONAL EMBODIMENTS
[0109] Reference will now be made to more specific embodiments of
the present disclosure and experimental results that provide
support for such embodiments. However, Applicants note that the
disclosure below is for exemplary purposes only and is not intended
to limit the scope of the claimed invention in any way.
Example 1
Growth of Graphene from Various Non-Gaseous Carbon Sources
[0110] In this Example, Applicants demonstrate that large area,
high-quality graphene films with controllable thickness can be
grown from different solid non-gaseous carbon sources, such as
polymer films or small molecules, at temperatures as low as
800.degree. C. Both pristine graphene and doped graphene were grown
with this one-step process using the same experimental set-up.
Temperatures of 800.degree. C. are attractive because underlying
silicon chips can contain dopants. The dopants will minimally
migrate at 800.degree. C. However, at more typical temperatures of
1000.degree. C., dopant migration can make the use of silicon
devices exceedingly difficult.
[0111] With its extraordinary electronic and mechanical properties,
graphene is showing promise in a plethora of applications. Graphene
can now be obtained by several different approaches. The original
mechanical peeling method from highly oriented pyrolytic graphite
(HOPG) yields small amounts of high quality graphene. Liquid
exfoliation and reduction of graphene oxide have been used to
produce chemically converted graphene in large quantities.
Annealing SiC and CVD are efficient methods to synthesize
large-size graphene on wafers. By introducing Ni and Cu as the
substrates for CVD growth, the size, thickness and quality of the
produced graphene is approaching industrially useful
specifications. However, intrinsic graphene is a zero band gap
material which shows a weak ambipolar behavior. These graphene
based transistors show small ON/OFF ratios, so they are too
metallic for many designed electronics applications. In order to
manipulate the Fermi level of graphene, having bilayer
configurations may be needed. Alternatively, doping the graphene
matrix with heteroatoms is a straightforward way to make an n-type,
p-type or hybrid doped graphene.
[0112] In the present Example, the growth of monolayered pristine
graphene from solid non-gaseous carbon sources atop metal catalysts
is demonstrated. See FIG. 1A. The first solid non-gaseous carbon
source used was a spin-coated poly(methyl methacrylate) (PMMA) thin
film. The metal catalyst substrate was Cu film. At a range as low
as 800.degree. C. or as high as 1000.degree. C. (tested limit) for
10 min, with a reductive gas flow (H.sub.2/Ar) under low pressure
conditions, a single uniform layer of graphene was formed on the
substrate. The graphene material thus produced was successfully
transferred to different substrates for further characterization
(as discussed in more detail below).
[0113] The Raman spectrum of this monolayered PMMA-derived graphene
(PG) is shown in FIG. 1B. The spectrum is characteristic of more
than 10 locations recorded over 1 cm.sup.2 of the sample. The two
most pronounced peaks in this spectrum are the G peak at 1580
cm.sup.-1 and the 2D peak at 2690 cm.sup.-1. The I.sub.2D/I.sub.G
intensity ratio is about 4, and the full width at half maximum
(FWHM) of the 2D peak is about 30 cm.sup.-1. These results indicate
that the graphene is a monolayer. The D peak (.about.1350
cm.sup.-1) is in the noise level for PG, indicating the presence of
few sp.sup.3 carbon atoms or defects.
[0114] The electrical properties of the PG were evaluated with a
back-gated field-effect transistor (FET) device atop a 200 nm thick
SiO.sub.2 dielectric. Typical data for the FET devices is shown in
FIG. 1C. For this particular device, the estimated carrier (hole)
mobility is .about.410 cm.sup.2V.sup.-1S.sup.-1 at room
temperature, and the ON/OFF ratio is .about.2, which is expected in
graphene-based FET devices of this size. Although the graphene was
pristine without any doping atoms, it still shows a weak p-type
behavior with the neutrality point moved to positive gate voltage,
probably arising from the physisorption of small molecules like
H.sub.2O. Placing these graphene FETs under high vacuum (10.sup.-5
Torr) for several days moves the neutrality point to zero. See FIG.
4. This observation confirms that the weak p-type behavior was due
to physisorption of volatile molecules.
[0115] Transmission electron microscopy (TEM) images of the
pristine PG and its diffraction pattern are shown in FIGS. 1D-G.
The selected area electron diffraction (SAED) pattern in FIG. 1D
displays the typical hexagonal crystalline structure of graphene.
The layer count on the edges of the images indicates the thickness
of the PG. The PG edges in FIGS. 1E-G were randomly imaged under
TEM, and most were monolayered or bilayered PG, which corroborates
with the Raman data. Although most of the PG surface was continuous
and crystalline according to its diffraction pattern, there is
adsorbed PMMA resulting from the transfer step. Metal atoms or ions
were also found to be trapped on the PG surface (see black arrows
in FIG. 1G) as charge impurities that should increase the charge
density but decrease the PG mobility. Similar phenomena have been
observed with CVD-generated graphene.
[0116] AFM was used to characterize the surface profile of PG on a
SiO.sub.2/Si substrate. In FIG. 10, the thickness of the PG is
about 0.7 nm, which confirms the monolayer nature of this material.
However, limited by the wet-transfer technique, graphene's
intrinsic corrugation is still apparent in the AFM image.
[0117] Graphene's electronic properties are strongly associated
with its thickness. Therefore, it would be useful to be able to
control the thickness when producing the graphene by tuning the
growth parameters. Applicants have found that PG's thickness can be
controlled from monolayer, to bilayer to a few layers by changing
the Ar and H.sub.2 gas flow rate. Typical thicknesses were
evaluated by Raman spectroscopy and UV transmittance of the
graphene. See FIGS. 2A-2B. At 1000.degree. C., a bilayered or
few-layered PG was obtained when the Ar flow rate was 500 sccm and
the H.sub.2 flow rate was 10 sccm or less. When the H.sub.2 flow
rate increased to 50 sccm or higher, only monolayered graphene was
formed on the Cu substrate. Also see FIG. 9A.
[0118] Monolayered graphene showed a transmittance of about 97.1%.
See FIG. 2B. It had a sheet resistance (R.sub.s) of 1200 .OMEGA./sq
by the 4-probe method, which makes it a transparent electrode
material of interest. The bilayer graphene's transmittance is about
94.3%, which shows linear enhancement in the UV absorption. The
few-layered PG sheet in FIG. 2A has a transmittance of 83% at 550
nm, which can be estimated as a 6-layered PG.
[0119] Both the shape and the positions of the 2D peak are
significantly different from monolayered graphene to bilayered
graphene and few-layered graphene. See FIG. 5. For monolayered
graphene, the 2D peak can be fitted with single sharp Lorentz peak.
The observed 2D splitting in bilayered and few-layered graphene can
be assigned to the electronic band splitting caused by the
interaction of the PG planes. H.sub.2 acts as both the reducing
reagent and the carrier gas to remove C atoms that are extruded
from the decomposing PMMA during growth. Some metal catalysts, such
as Ni, are known to reverse graphene growth by converting graphene
to hydrocarbon products, therefore cutting graphene along specific
directions. This reverse reaction does not appear to occur on the
PG which is atop the Cu.
[0120] High quality monolayered PG was obtained at 800.degree. C.
by this method, lower than the original report for CVD growth
temperature on Cu. See FIG. 6. For the semiconductor industry, the
lower processing temperature is favorable because temperatures as
high as 1000.degree. C. would be problematic in the fabrication of
the multi-layered stacks of heterogeneous materials. Therefore, in
addition to changing the Ar/H.sub.2 flow rate, the graphene growth
process was conducted using different temperatures. The quality of
the graphene films was monitored by the D/G peak ratio from Raman
spectroscopic analysis. The D/G ratio for graphene sheets obtained
at 800.degree. C. is less than 0.1. At 750.degree. C., the D/G peak
ratio was .about.0.35. Hence, 800.degree. C. may be the lower limit
for high quality graphene from PMMA in some embodiments. See FIG.
6.
[0121] Applicants also used other solid non-gaseous carbon sources
including fluorene (C.sub.13H.sub.10) and sucrose (table sugar,
C.sub.12H.sub.22O.sub.11) to grow monolayered graphene on Cu
catalyst under the same growth conditions as was used for the PG.
Because these precursors are powders not films, 10 mg of each as a
finely grinded powder was placed directly on a 1 cm.sup.2 Cu foil.
After subjecting the powder-coated Cu films to the same reaction
conditions as used for PG, Raman spectra indicated that all of the
solid non-gaseous carbon sources have been transformed into
monolayered graphene with no D peak observed. See FIG. 2C. Although
these solid carbon precursors contain potential topological defect
generators (the five-member ring in fluorene) or high concentration
of heteroatoms (oxygen in sucrose), they produce high quality
pristine graphene. Without being bound by theory, it is possible
that at elevated temperatures under vacuum, C has a higher affinity
for the metal catalyst surface than the heteroatoms; atom
rearrangement occurs and most of the topological defects are
self-healed as the graphene is formed.
[0122] Other substrates such as Ni, Si<100> with native oxide
and thermally grown SiO.sub.2 were also tested to determine if they
would grow graphene when coated with PMMA. FIG. 2D is the high
resolution TEM image of PG grown on a Ni catalyst, which clearly
illustrates the few-layered structure around the edges of PG. The
Raman spectra in FIG. 7 confirm that Ni is an efficient catalytic
substrate to convert PMMA into highly crystalline graphene
materials with no D peak around 1350 cm.sup.-1. Under the same
growth conditions, neither graphene nor amorphous carbon was
obtained on Si or SiO.sub.2 substrates according to the Raman
spectroscopic analysis of the surface after the reaction. This
demonstrates the potential to grow patterned graphene from a thin
film of shaped Ni or Cu deposited directly on Si/SiO.sub.2 wafers
without post lithography treatment since PG will not grow on the Si
or SiO.sub.2 surfaces.
[0123] Pristine graphene can show weak p-type or n-type behavior
due to physisorption of small molecules, such as H.sub.2O or
NH.sub.3. However, this chemical doping effect induced by
physisorption is labile because it can be easily desorbed under
heat or vacuum. Therefore, intrinsically nitrogen-doped (N-doped)
graphene is more challenging to make compared to pristine graphene.
Intrinsically, N-doped graphene has been obtained by two methods:
introducing a doping gas (NH.sub.3) into the CVD systems during the
graphene growth; or treatment of synthesized graphene or graphene
oxide with NH.sub.3 by annealing or plasma. Here, by using the
solid carbon sources and solid doping reagents, doped graphene can
be grown in one step without any changes to the CVD system.
[0124] A doping reagent (melamine, C.sub.3N.sub.6H.sub.6) was mixed
with PMMA and deposited onto the Cu surface. In order to keep the
nitrogen-atom concentration in the systems, Applicants used
conditions similar to the PG growth except that the growth was done
under atmospheric pressure. See Examples below. The prepared
polymer films were successfully converted into N-doped graphene,
with an N content of 2-3.5%. The XPS data shows the difference of
the C1s peaks between PG and N-doped PG. See FIG. 3A. The shoulder
around 287 eV can be assigned to the C--N bonding. The N1s peak of
N-doped PG indicates that only one type of N is present, at 399.8
eV, corresponding to quaternary N in graphene. See FIG. 3B. This
new N1s peak also has a 2 eV shift from that in melamine which
shows an N1s peak at 397.8 eV. See FIG. 8. The new N1 s peak
suggests that the N1 s signal does not come from the melamine but
that the N atoms are uniformly bound into the graphene structure.
The D peak of this material is always present in the Raman spectra
because the heteroatoms break the graphene symmetry and thereby
introduce defects that are detected by Raman analysis. See FIG. 3C.
The D' peak is also found in doped graphene materials obtained by
the other doping methods. The 2D peak position and I.sub.2D/I.sub.G
intensity ratio reveals that this N-doped PG is monolayered.
Compared to PG, the I.sub.2D/I.sub.G decreased from 4 to 2,
implying a successful doping according to the electrostatically
gated Raman results.
[0125] Doping effects were also demonstrated by N-doped PG-based
FETs. The n-type behavior shown in FIG. 3D with the neutrality
point shifted to negative gate voltage is consistently observed for
devices on the same piece of N-doped PG. After keeping the N-doped
PG-based FET devices under vacuum (10.sup.-6 Torr) for 24 h, their
neutrality point did not move to zero, indicative of the covalent
bonding between carbon and nitrogen rather than just physisorption.
Applicants envision that the dopant N atoms donate free electrons
to graphene. Meanwhile, the N-doped graphene's mobility calculated
from the N-doped FETs was about 1 order of magnitude lower than in
PG. Due to the broken symmetry of the N-doped graphene's lattice
structure, the N atoms act as scattering centers that suppress its
mobility. Patterned hydrogenation on graphene already shows its
band gap opening. Similarly, if the doping atoms are periodically
dispersed in graphene's matrix, they cannot only tune the Fermi
level of graphene, but tailor its band gap. However, in the present
N-doped graphene, the ON/OFF ratio does not increase, which
suggests that the N atoms are randomly incorporated into the
graphene matrix.
[0126] In conclusion, Applicants have demonstrated in the above
study a one-step method for the controllable growth of both
pristine graphene and doped graphene using non-gaseous carbon
sources. This stands as a complementary method to CVD growth while
permitting growth at more acceptable temperature ranges (i.e.,
lower temperatures). The Examples below provide additional
information about the aforementioned study.
[0127] Utilized Equipment
[0128] Raman Spectroscopy was performed on a transferred graphene
films on 100 nm Si/SiO.sub.2 wafer with a Renishaw Raman microscope
using 514-nm laser at ambient temperature. The electrical
properties were measured in a probe station (Desert Cryogenic
TT-probe 6 system) under vacuum (10.sup.-5.about.10.sup.-6 Torr).
The IV data were collected by an Agilent 4155C semiconductor
parameter analyzer. The HRTEM images were taken using a 2100F Field
Emission Gun Transmission Electron Microscope with graphene samples
directly transferred on a C-flat TEM grid (Protochips, Inc.). XPS
was performed on a PHI Quantera SXM scanning X-ray microprobe with
45.degree. takeoff angle and a 100 .mu.m beam size.
[0129] PG Growth and Transfer
[0130] 200 .mu.L PMMA (MicroChem Corp. 950 PMMA A4, 4% in anisole)
solution was deposited on a 25 .mu.m thick Cu foil (Alfa Aesar,
item No. 13382, cut to 1 cm.times.1 cm squares) by spin coating at
5000 rpm for 1 min. The obtained PMMA/Cu film was cured at
180.degree. C. for 1 min and then dried in a vacuum oven at
70.degree. C. for 2 h to remove the solvent. A typical process for
thermally converting the PMMA films to monolayer graphene was: (1)
evacuate a standard 1-inch quartz tube furnace to 100 mTorr and
maintain the temperature at 1000.degree. C.; (2) introduce the
PMMA/Cu film into the furnace and anneal it under the H.sub.2 (50
sccm) and Ar (500 sccm) flow for 10-20 min, maintaining the total
pressure <30 Torr; and (3) cool the Cu foil with the graphene to
room temperature under a H.sub.2/Ar atmosphere. Then temperature
could also be changed from 1000.degree. C. to 800.degree. C.
[0131] To transfer high-quality graphene films to Si/SiO.sub.2
substrates, the same procedure was used that was developed to
transfer graphene films for high performance transparent conductive
electrodes, except that Marble's reagent (CuSO.sub.4:HCl:H.sub.2O::
10 g:50 mL:50 mL) was used as the etchant. See Kim, K. S. et al.,
Large-scale pattern growth of graphene films for stretchable
transparent electrodes. Nature 457, 706 (2009). The graphene film
was recovered from the graphene/Cu foil by (1) spin-coating a PMMA
layer (200 .mu.L, 3000 rpm for 1 min) onto the graphene film; (2)
etching Cu foil with Marble's reagent for 2 h and lifting off the
PMMA/graphene film; (3) submerging a clean glass substrate into the
etchant, picking up the floating film and transferring it into
deionized (DI) water for 10 min (3 times) to remove the etchant
ions; (4) dipping a new substrate into the deionized water and
picking up the film; (5) vacuum drying the film on the substrate at
70.degree. C. for 2 h to remove the water; (6) rinsing the film
with acetone twice to remove the PMMA layer; and (7) drying the
graphene film with blowing N.sub.2 gas.
[0132] N-Doped Graphene Growth
[0133] 100 mg melamine (Acros Organics, 98%) was dissolved into 10
mL 4% PMMA anisole solution to prepare the precursor for the
N-doped graphene. 200 .mu.L of the precursor solution was
spin-coated on the catalyst surface at 5000 rpm for 1 min. The
obtained films were cured at 180.degree. C. for 1 min and then
dried in a vacuum oven at 70.degree. C. for 2 h to remove the
solvent. The films were heated in a tube furnace at 1000.degree. C.
for 10 min at atmospheric pressure with a flow of H.sub.2 (100
sccm) and Ar (500 sccm) to grow the doped graphene atop the
catalyst substrate. The transfer of the N-doped graphene to the
Si/SiO.sub.2 surface is the same as the procedure used to transfer
pristine graphene.
[0134] Fabrication Procedure for FET Devices (Shown in FIG. 4)
[0135] PG was deposited on a highly doped p.sup.++ substrate with
200 nm thermal oxide. A PMMA mask on top of the graphene was
defined by conventional electron beam lithography. In the exposed
areas, PG was removed by reactive ion etching with O.sub.2/Ar flow
(flow rate ratio of 1:2 and a total flow rate of 35 sccm) for 30 s
at room temperature. The PMMA mask was removed with acetone to
reveal undamaged PG stripes. Pt electrodes were defined by e-beam
lithography.
Example 2
Growth of Graphene from Raw Carbon Sources
[0136] In this work, Applicants have developed a less expensive
approach of growing graphene films by using six easily obtained,
low or negatively valued raw carbon sources without
pre-purification (cookies, chocolate, grass, plastics, roaches, and
dog feces) to grow graphene directly on the backside of a Cu foil
at 1050.degree. C. under H.sub.2/Ar flow. The non-volatile
pyrolyzed species were easily removed by etching away the frontside
of the Cu. Analysis by Raman spectroscopy, X-ray photoelectron
spectroscopy, ultraviolet-visible spectroscopy and transmission
electron microscopy indicates that the monolayer graphene derived
from these carbon sources is of high quality.
[0137] Specifically, Applicants demonstrate in this Example that
much less expensive carbon sources, such as food, insects and
waste, can be used without purification to grow high-quality
monolayer graphene directly on the backside of Cu foils under the
H.sub.2/Ar flow. For food, a GIRL SCOUT cookie and chocolate were
investigated. For waste with low or negative monetary value,
Applicants used bulk polystyrene plastic, a common solid waste,
blades of grass and dog feces. For insects, another often negative
value carbon source, a cockroach leg was used. Growing high-quality
graphene from these raw carbon sources opens a new way to convert
the raw carbon into a high-value-added product, as graphene is one
of the most expensive materials in the world.
[0138] Without being bound by theory, Applicants propose a possible
purification and growth mechanism. For instance, it is envisioned
that the graphene film forms as solid carbon sources decompose and
diffuse to the backside of the Cu foil, leaving the other elemental
residues on the original side. Using this procedure, only high
quality pristine graphene with few defects and .about.97%
transparency was grown on the backside of the Cu foil, as confirmed
by Raman and UV-Vis spectroscopy. No heteroatoms were detected in
the monolayer graphene according to X-ray photoelectron
spectroscopy (XPS), suggesting its pristine nature. Analysis by
selected area diffraction pattern (SAED) in transmission electron
microscopy (TEM) confirms the hexagonal lattice structure of the
graphene.
[0139] In a typical growth experiment, as shown in FIG. 11, 10 mg
of the dry carbon source was placed atop a Cu foil, and the foil
was introduced into a 1050.degree. C. tube furnace. The sample was
annealed for 15 min under low pressure with H.sub.2 and Ar at a
flow rate of 100 cm.sup.3 STP min.sup.-1 and 500 cm.sup.3 STP
min.sup.-1, respectively. For the grass and dog feces, the samples
were heated in a 65.degree. C. vacuum (102 Torr) oven for 10 h to
remove excess moisture. The experimental setup and procedures are
similar to the method used to grow PMMA-derived graphene, as
illustrated in Example 1. A difference in this work is that the
high quality monolayer graphene only forms on the backside of Cu
foil, while the PMMA-derived graphene derived from Example 1 grows
on both sides of the Cu foil.
[0140] Since the carbon sources contain non-carbon elements,
non-volatile residue may remain on the Cu foil after annealing.
FIG. 12 shows SEM images of both sides of the Cu foil after a
growth experiment. On the original frontside, many residual
particles were found, as shown in FIG. 12A, while almost no
particles were observed on the backside of the Cu foil where the
graphene is formed (FIG. 12B). In FIG. 11B, photographic images of
different growth stages are shown. A black residue is present after
the growth in FIG. 11B(c). Based on the experimental evidence
during the growth, most of the carbon segments from the
decomposition of the solids are carried away as gases by the
H.sub.2/Ar flow. However, a portion of the carbon source diffuses
to the backside of the Cu foil, forming a monolayer graphene film.
It is not known whether the diffusion is through the Cu foil or via
the edges. As a comparison experiment, if the solid carbon sources
were placed 5 cm ahead of the Cu substrate (but still in the quartz
boat) and both were introduced into the hot furnace at the same
time, only amorphous carbon formed on both sides of the Cu foil.
The representative Raman spectrum of the film displays a large D
peak, as shown in FIG. 13.
[0141] After the monolayer graphene samples on the backside of the
Cu foil were transferred onto a 100 nm SiO.sub.2/Si wafer using
standard protocols, the product was analyzed using Raman
spectroscopy at 514 nm laser excitation. As shown in FIG. 14, all
of the graphene samples grown have small or no D peaks in their
Raman spectra, an indication of few graphene defects. The large
2D/G ratio suggests that it is high quality monolayer graphene. The
exact G and 2D peak positions and their full-width at half-maximum
(FWHM) for each spectrum were measured. The results are summarized
Table 1.
TABLE-US-00001 TABLE 1 The wavelength number of the G and 2D peak,
and their FWHM for graphene samples derived from six different
carbon sources Carbon G peak G peak 2D peak 2D peak source (cm-1)
FWHM (cm-1) (cm-1) FWHM (cm-1) Cookie 1585.5 14.1 2682.6 32.0
Chocolate 1591.4 15.9 2693.9 32.6 Grass 1585.7 16.0 2692.1 33.1
Plastic 1587.7 15.8 2685.7 34.8 Dog feces 1589.6 16.3 2689.7 35.1
Roach 1588.4 14.6 2687.4 33.5
[0142] The G and 2D peaks are located at 1585.5-1591.4 cm.sup.-1
and 2682.6-2693.9 cm.sup.-1, respectively. The FWHM of the G peak
and 2D peak are 14.1-16.3 cm.sup.-1 and 32.0-35.1 cm.sup.-1,
respectively. In order to investigate the uniformity of the
graphene film, a Raman mapping over a 100.times.100 .mu.m.sup.2
area (graphene derived from dog feces) was acquired. Over 95% of
the scanned area had a signature of I.sub.2D/I.sub.G >1.8 and
I.sub.D/I.sub.G<0.1, which further demonstrated the high quality
of the monolayer graphene, as shown in FIG. 15.
[0143] XPS analysis of the graphene films was performed to confirm
the elemental composition and the chemical environment of the C
atoms. In FIG. 16, only a sharp peak at 284.5 eV with an asymmetric
tailing towards high bonding energy is observed for C1s peak,
suggesting a sp.sup.2 graphitic peak. The FWHM was .about.1.1 eV
for each C1s peak. Although the raw carbon sources contain other
elements such as oxygen, nitrogen, iron, sulfur, or phosphorus, the
obtained graphene consisted of carbon, with none of these other
elements found in the XPS survey spectra, confirming the graphene's
pristine composition.
[0144] In the growth system, the H.sub.2 gas might act as both a
reducing reagent and a carrier gas. Since carbon is the most
abundant element in these materials and graphene is the most
thermodynamically stable form of carbon, only pristine graphene
forms on the Cu. According to the C--C bond length (0.142 nm) in
the hexagonal lattice of graphene, the surface area of one side of
a monolayer of graphene is about 1315 m.sup.2/g. Theoretically, it
only takes 228 ng of carbon to cover one side of a 2 cm.times.3 cm
Cu foil with monolayer graphene. In the growth system, the size of
the graphene is ultimately limited by the size of the tube furnace,
which limits the size of the Cu substrate that can be used. With a
larger furnace, larger-sized graphene could be produced with 10 mg
of the carbon source. Therefore, the limiting reagent in this
Example is the Cu foil, though scrolled Cu foil could provide
enhanced surface areas.
[0145] All the graphene films were transferred to quartz slides
before UV-Vis analysis. In the spectra, each graphene film exhibits
a peak at 268 nm, a typical .pi..fwdarw..pi.* transition for the
aromatic C--C bond in graphene, and the typical (2.4%.+-.0.1%)
absorption at 550 nm corresponding to the monolayer nature of
graphene, as shown in FIG. 17. In the photographic images, the
graphene films on quartz slides are unifom and transparent. Also,
the sheet resistance (R.sub.s) of the graphene was in the range of
1.5-3.0 k.OMEGA./square by the four-probe method.
[0146] TEM images and the selected area electron diffraction (SAED)
pattern were taken to determine the crystal structure of a
representative graphene sample derived from the cookie. The
graphene was transferred to a c-flat TEM grid (Protochips), where
most of the area of the graphene was determined to be crystalline
by its hexagonal diffraction pattern (FIG. 18A) and was continuous,
as shown in FIG. 18B. A randomly chosen monolayer edge of the
graphene was imaged in FIG. 18C. The thickness of the graphene
corresponds to monolayer graphene, corroborating the UV-Vis spectra
and Raman data. The dark spots in the image in FIG. 18C might arise
from the PMMA residues introduced during the etching and
transferring step.
[0147] In sum, Applicants have demonstrated in this Example a
general method to grow high-quality graphene from various raw
carbon materials at 1050.degree. C. under vacuum and H.sub.2/Ar
flow. The carbon sources were foods (cookie and chocolate), waste
(grass, plastic, dog feces) and insect-derived. With this
technique, many kinds of solid materials that contain carbon can
potentially be used without purification as the feedstocks to
produce high-quality graphene without pre-purification.
Furthermore, through this method, low-valued foods and
negative-valued solid wastes are successfully transformed into
high-valued graphene which brings new solutions for recycling of
carbon from impure sources.
[0148] Growth and Transfer of Graphene Samples
[0149] Six different carbon sources were used: GIRL SCOUT Cookie
(shortbread flavor), chocolate (Chocolate Kennedy Half Dollar Gold
Coins), grass (Ophiopogon picked at Rice University), plastic
(Fisherbrand polystyrene Petri dishes, catalog #08-757-12), dog
feces (Miniature Dachsund) and a cockroach leg (American cockroach
caught in a house). The grass and the dog feces were dehydrated in
a vacuum oven (102 Torr) at 65.degree. C. for 10 h before being
used in the growth process.
[0150] The CVD system was evacuated to 10 mTorr for 10 h before
growth. For the growth of graphene, 10 mg of a carbon source was
placed atop the Cu foil (99.8% purity) and annealed at 1050.degree.
C. for 15 min with Ar flow at 500 cm.sup.3 STP min.sup.-1 and
H.sub.2 flow at 100 cm.sup.3 STP min.sup.-1. The system was then
fast cooled (moved to the cool zone using a magnetic transfer rod)
to room temperature under the H.sub.2/Ar flow. A 100 nm-thick PMMA
film was deposited on the backside of the foil using a 4% PMMA
anisole solution spin-coated at 3000 rmp for 40 s. The frontside of
the Cu foil was etched away by floating the foil metal-down on an
acidic CuSO.sub.4 solution (made with CuSO.sub.4.5H.sub.2O (15.6
g), con. HCl (50 mL), H.sub.2O (50 mL) and H.sub.2SO.sub.4 (2 mL))
for .about.5 s, then dipping the foil in DI water. This process was
repeated at least two times in order to wash away the residue left
on the frontside of the Cu foil. If the water washes did not remove
the residue from the front side of the Cu foil, a Chemwipe was used
to carefully brush the residue away before all of the Cu was
removed. The PMMA-coated graphene was transferred to different
substrates, such as 100 nm SiO.sub.2/Si wafers and quartz. After
the film was completely dried in a vacuum oven at 65.degree. C. for
2 h, the film was rinsed with acetone 3.times. before
characterization.
[0151] Characterization
[0152] Raman spectra were obtained by the single scan generated by
the WiRE spectral acquisition wizard using a 514.5 nm laser in a
Renishaw Raman RE02 microscope. UV-Vis spectroscopy was done using
a 1-mm-thick quartz slide on which the sample was placed in a
Shimadzu UV-3101 system. The XPS were obtained using a 100 .mu.m
X-ray beam of the 45.degree. take-off angle and 26.00 eV pass
energy in a PHI Quantera SXM scanning X-ray microprobe system. TEM
imaging was obtained in a 2100F field emission gun transmission
electron microscope. The graphene samples were transferred to a
C-flat TEM grid (Protochips).
[0153] Without further elaboration, it is believed that one skilled
in the art can, using the description herein, utilize the present
disclosure to its fullest extent. The embodiments described herein
are to be construed as illustrative and not as constraining the
remainder of the disclosure in any way whatsoever. While the
preferred embodiments have been shown and described, many
variations and modifications thereof can be made by one skilled in
the art without departing from the spirit and teachings of the
invention. Accordingly, the scope of protection is not limited by
the description set out above, but is only limited by the claims,
including all equivalents of the subject matter of the claims. The
disclosures of all patents, patent applications and publications
cited herein are hereby incorporated herein by reference, to the
extent that they provide procedural or other details consistent
with and supplementary to those set forth herein.
* * * * *