U.S. patent application number 12/170014 was filed with the patent office on 2009-06-18 for single crystalline graphene sheet and process of preparing the same.
This patent application is currently assigned to SAMSUNG ELECTRONICS CO., LTD.. Invention is credited to Jae-young CHOI, Jai-yong HAN, Hyeon-Jin SHIN, Seon-mi YOON.
Application Number | 20090155561 12/170014 |
Document ID | / |
Family ID | 40753662 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090155561 |
Kind Code |
A1 |
CHOI; Jae-young ; et
al. |
June 18, 2009 |
SINGLE CRYSTALLINE GRAPHENE SHEET AND PROCESS OF PREPARING THE
SAME
Abstract
A single-crystal graphene sheet includes a polycyclic aromatic
molecule wherein a plurality of carbon atoms are covalently bound
to each other, the single-crystal graphene sheet comprising between
about 1 layer to about 300 layers; and wherein a peak ratio of a
Raman D band intensity to a Raman G band intensity is equal to or
less than 0.2. Also described is a method for preparing a
single-crystal graphene sheet, the method includes forming a
catalyst layer, which includes a single-crystal graphitizing metal
catalyst sheet; disposing a carbonaceous material on the catalyst
layer; and heat-treating the catalyst layer and the carbonaceous
material in at least one of an inert atmosphere and a reducing
atmosphere. Also described is a transparent electrode including a
single-crystal graphene sheet.
Inventors: |
CHOI; Jae-young; (Yongin-si,
KR) ; SHIN; Hyeon-Jin; (Yongin-si, KR) ; YOON;
Seon-mi; (Yongin-si, KR) ; HAN; Jai-yong;
(Yongin-si, KR) |
Correspondence
Address: |
CANTOR COLBURN, LLP
20 Church Street, 22nd Floor
Hartford
CT
06103
US
|
Assignee: |
SAMSUNG ELECTRONICS CO.,
LTD.
Suwon-si
KR
|
Family ID: |
40753662 |
Appl. No.: |
12/170014 |
Filed: |
July 9, 2008 |
Current U.S.
Class: |
428/220 ; 117/8;
423/447.2 |
Current CPC
Class: |
B82Y 30/00 20130101;
C01B 2204/04 20130101; C30B 1/10 20130101; B82Y 40/00 20130101;
C01B 32/188 20170801; C30B 29/02 20130101 |
Class at
Publication: |
428/220 ;
423/447.2; 117/8 |
International
Class: |
D01F 9/12 20060101
D01F009/12; C30B 1/02 20060101 C30B001/02; B32B 9/00 20060101
B32B009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 17, 2007 |
KR |
10-2007-0132682 |
Claims
1. A single-crystal graphene sheet comprising: a polycyclic
aromatic molecule wherein a plurality of carbon atoms are
covalently bound to each other to form a single-crystal, wherein
the single-crystal graphene sheet comprises between about 1 layer
to about 300 layers, and wherein a peak ratio of a Raman D band
intensity to a Raman G band intensity is equal to or less than
about 0.2.
2. The single-crystal graphene sheet of claim 1, wherein the peak
ratio of a Raman D band intensity to a Raman G band intensity is
about 0.
3. The single-crystal graphene sheet of claim 1, wherein the
single-crystal graphene sheet comprises between about 1 layer to
about 60 layers.
4. The single-crystal graphene sheet of claim 1, wherein each of a
width and a length of the single-crystal graphene sheet are between
about 1 mm to about 1,000 mm.
5. A method for preparing a single-crystal graphene sheet, the
method comprising: forming a catalyst layer, the catalyst layer
comprising a single-crystal graphitizing metal catalyst sheet;
disposing a carbonaceous material on the catalyst layer; and
heat-treating the catalyst layer and the carbonaceous material in
at least one of an inert atmosphere and a reducing atmosphere to
form a single-crystal graphene sheet.
6. The method of claim 5, wherein the carbonaceous material is
solid-solubilized in the catalyst layer.
7. The method of claim 5, wherein the catalyst layer comprises a
metal 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, a combination
comprising at least one of the foregoing metals, and an alloy
comprising at least one of the foregoing metals.
8. The method of claim 5, wherein the disposing a carbonaceous
material on the catalyst layer comprises coating a
carbon-containing polymer on a surface of the catalyst layer.
9. The method of claim 8, wherein the carbon-containing polymer is
a self-assembling polymer.
10. The method of claim 8, wherein the carbon-containing polymer is
a polymer selected from the group consisting of an amphiphilic
polymer, a liquid crystal polymer, a conductive polymer, and a
combination comprising at least one of the foregoing polymers.
11. The method of claim 8, wherein the carbon-containing polymer
comprises a polymerizable functional group.
12. The method of claim 5, wherein the disposing a carbonaceous
material on the catalyst layer comprises disposing a
carbon-containing gas on the catalyst layer.
13. The method of claim 12, wherein the carbon-containing gas is a
gas selected from the group consisting of carbon monoxide, ethane,
ethylene, ethanol, acetylene, propane, propylene, butane,
butadiene, pentane, pentene, cyclopentadiene, hexane, cyclohexane,
benzene, toluene, and a combination comprising at least one of the
foregoing gases.
14. The method of claim 5, wherein the disposing a carbonaceous
material on the catalyst layer is performed by immersing the
catalyst layer in a liquid, wherein the liquid comprises the
carbonaceous material.
15. The method of claim 5, wherein the heat-treating the catalyst
layer comprises: a first heat-treating at a temperature and for a
time sufficient to solid-solubilize the carbonaceous material in
the catalyst layer; and a second heat-treating at a temperature and
for a time sufficient to precipitate the solid-solubilized
carbonaceous material and form a single-crystal graphene sheet.
16. The method of claim 8, wherein the disposing a carbonaceous
material on the catalyst layer is performed by coating a
carbon-containing polymer on the surface of the catalyst layer,
wherein the heat-treating is performed at a temperature between
about 300.degree. C. to about 2,000.degree. C. and for a time
between about 0.001 hours to about 1000 hours.
17. The method of claim 12, wherein when the disposing a
carbonaceous material on the catalyst layer is performed by
disposing a carbon-containing gas on the catalyst layer, and the
heat-treating is performed at a temperature between about
300.degree. C. to about 2,000.degree. C. for a time between about
0.001 hours to about 1000 hours.
18. The method of claim 5, further comprising separating the
single-crystal graphene sheet from the catalyst layer by removing
the catalyst layer by treating the single-crystal graphene sheet
and the catalyst layer with an acid after the heat-treating.
19. A transparent electrode comprising the single-crystal graphene
sheet according to claim 1.
20. A method for preparing a single-crystal graphene sheet,
comprising: forming a catalyst layer, the catalyst layer comprising
a single-crystal graphitizing metal catalyst sheet; disposing a
self-assembling polymer on the catalyst layer; heat-treating the
catalyst layer and the self-assembling polymer in at least one of
an inert atmosphere and a reducing atmosphere to solid-solubilize a
carbonaceous material in the catalyst layer; and heat-treating at a
temperature and for a time sufficient to precipitate the
solid-solubilized carbonaceous material and form a single-crystal
graphene sheet.
Description
[0001] This application claims priority to Korean Patent
Application No. 10-2007-0132682, filed on Dec. 17, 2007, and all
the benefits accruing therefrom under 35 U.S.C. .sctn. 119, the
contents of which in their entirety are herein incorporated by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This disclosure relates to a single-crystal graphene sheet
and a process of preparing the same.
[0004] 2. Description of the Related Art
[0005] Generally, graphite is a stack of two-dimensional graphene
sheets formed from a planar array of carbon atoms bonded to form
hexagonal structures. Recently, testing of graphene sheets revealed
beneficial properties of single or multiple-layered graphene
sheets. One beneficial property of graphene is that electrons flow
in an entirely unhindered fashion in a graphene sheet, which is to
say that the electrons flow at the velocity of light in a vacuum.
In addition, graphene sheets exhibit an unusual half-integer
quantum Hall effect for both electrons and holes.
[0006] The electron mobility of conventional graphene sheets is
about 20,000 to 50,000 cm.sup.2/Vs.
[0007] In some applications carbon nanotubes can be used as a
conductor. However, carbon nanotubes are expensive due to low
yields during synthesis and purification processes. Also single
wall carbon nanotubes exhibit different metallic and semiconducting
characteristics according to their chirality and diameter.
Furthermore, single wall carbon nanotubes having identical
semiconducting characteristics have different band gap energies
depending on their chirality and diameter. Thus, single wall carbon
nanotubes are preferably separated from each other in order to
obtain the desired semiconducting or metallic properties. However,
separating single wall carbon nanotubes can be problematic.
[0008] On the other hand, it is advantageous to use graphene sheets
in a device, because graphene sheets can be engineered to exhibit
the desired electrical characteristics by arranging the graphene
sheets so that their crystallographic orientation is in a selected
direction since the electrical characteristics of graphene depend
on crystallographic orientation. It is envisaged that the
characteristics of graphene sheets can be applied to future
carbonaceous electrical devices or carbonaceous electromagnetic
devices.
[0009] Graphene sheets can be prepared using a micromechanical
method or by SiC thermal decomposition. According to the
micromechanical method, a graphene sheet can be separated from
graphite attached to the surface of Scotch.TM. tape by attaching
the tape to a graphite sample and detaching the tape. In this case,
the separated graphene sheet does not include a uniform number of
layers, and the ripped portions do not have a uniform shape.
Furthermore, a large-sized graphene sheet cannot be prepared using
the micromechanical method. Meanwhile, in SiC thermal
decomposition, a SiC single crystal is heated to remove Si by
decomposition of the SiC on the surface thereof, the residual
carbon then forming a graphene sheet. However, the SiC single
crystal material used as a starting material in SiC thermal
decomposition is very expensive, and formation of a large-sized
graphene sheet can be problematic.
[0010] Accordingly, a process to economically and reproducibly
prepare a large-size graphene sheet that has the desired electrical
properties is needed.
BRIEF SUMMARY OF THE INVENTION
[0011] Disclosed is a single-crystal graphene sheet.
[0012] Also disclosed is a process of preparing the single-crystal
graphene sheet.
[0013] In addition, disclosed is a transparent electrode comprising
the single-crystal graphene sheet.
[0014] In an embodiment, there is provided a single-crystal
graphene sheet, including a polycyclic aromatic molecule wherein a
plurality of carbon atoms are covalently bound to each other to
form a single crystal, wherein the single-crystal graphene sheet
comprises between about 1 to about 300 layers, and wherein a peak
ratio of the Raman D band intensity to a Raman G band intensity is
equal to or less than 0.2.
[0015] The peak ratio of the Raman D band/G band can be 0.
[0016] The single-crystal graphene sheet can have between about 1
to about 60 layers.
[0017] The single-crystal graphene sheet can have between about 1
to about 15 layers.
[0018] Each of a width and a length of the single-crystal graphene
sheet can be between about 1 mm to about 1,000 mm.
[0019] According to another embodiment, there is provided a method
of preparing a single-crystal graphene sheet, the method
comprising: forming a catalyst layer, the layer comprising a
single-crystal graphitizing metal catalyst sheet; disposing a
carbonaceous material on the catalyst layer; and heat-treating the
catalyst layer and the carbonaceous material in at least one of an
inert atmosphere or reducing atmosphere.
[0020] The carbonaceous material can be solid-solubilized in the
single-crystal graphitizing metal catalyst sheet.
[0021] Disposing a carbonaceous material on the catalyst layer can
be performed by coating a carbon-containing polymer on a surface of
the catalyst layer, disposing a carbon-containing gas on the
catalyst layer, or immersing the catalyst layer in a
carbon-containing liquid solution.
[0022] The catalyst layer can include a metal 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 a combination comprising at least one
of the foregoing metals.
[0023] The heat-treating can be performed at a temperature between
about 300.degree. C. to about 2,000.degree. C. for a time between
about 0.001 hours to about 1000 hours.
[0024] The method can further include separating the single-crystal
graphene sheet from the catalyst layer by removing the catalyst
layer by treating the single-crystal graphene sheet and the
catalyst layer with an acid after the heat-treating.
[0025] According to another embodiment, there is provided a
transparent electrode comprising the single-crystal graphene
sheet.
[0026] The transparent electrode can be flexible.
[0027] Also disclosed is a method for preparing a single-crystal
graphene sheet, including forming a catalyst layer, the catalyst
layer including a single-crystal graphitizing metal catalyst sheet;
and disposing a self-assembling polymer on the catalyst layer;
heat-treating the catalyst layer and the self-assembling polymer in
at least one of an inert atmosphere and a reducing atmosphere to
solid-solubilize a carbonaceous material in the catalyst layer; and
heat-treating at a temperature and for a time sufficient to
precipitate the solid-solubilized carbonaceous material and form a
single-crystal graphene sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The above and other aspects, features and advantages will
become more apparent by describing in further detail exemplary
embodiments thereof with reference to the attached drawings in
which:
[0029] FIG. 1 schematically shows a process to prepare a graphene
sheet according to the prior art;
[0030] FIG. 2 schematically shows a process to prepare a
polycrystalline graphene sheet;
[0031] FIG. 3 schematically shows a polymer coated on a catalyst
layer;
[0032] FIG. 4 schematically shows a structure of a single-crystal
graphene sheet formed on a catalyst layer;
[0033] FIG. 5 schematically shows a stack of polymers having a
hydrophilic part and a hydrophobic part;
[0034] FIG. 6 is a graph illustrating Raman spectra of
single-crystal graphene sheets prepared according to Examples 1 and
2; and
[0035] FIG. 7 is a graph illustrating Raman spectra of graphene
sheets prepared according to Comparative Examples 1 and 2.
DETAILED DESCRIPTION OF THE INVENTION
[0036] Hereinafter, embodiments are described more fully with
reference to the accompanying drawings, in which exemplary
embodiments are shown.
[0037] The terms "the", "a" and "an" do not denote a limitation of
quantity, but rather denote the presence of at least one of the
referenced item. The suffix "(s)" as used herein is intended to
include both the singular and the plural of the term that it
modifies, thereby including at least one of that term (e.g., the
colorant(s) includes at least one colorants).
[0038] Unless defined otherwise, technical and scientific terms
used herein have the same meaning as is commonly understood by one
of skill in the art.
[0039] As used herein, approximating language can be applied to
modify any quantitative representation that can vary without
resulting in a change in the basic function to which it is related.
Accordingly, a value modified by a term or terms, such as "about"
and "substantially," cannot to be limited to the precise value
specified, in some cases. In at least some instances, the
approximating language can correspond to the precision of an
instrument for measuring the value. Thus the modifier "about" used
in connection with a quantity is inclusive of the stated value and
has the meaning dictated by the context (e.g., includes the degree
of error associated with measurement of the particular
quantity).
[0040] All ranges disclosed herein are inclusive of the endpoints
and are independently combinable. The endpoints of all ranges
directed to the same component or property are inclusive and
independently combinable (e.g., ranges of "less than or equal to
about 25 wt %, or, more specifically, about 5 wt % to about 20 wt
%," is inclusive of the endpoints and all intermediate values of
the ranges of "about 5 wt % to about 25 wt %," etc.).
[0041] "Optional" or "optionally" means that the subsequently
described event or circumstance can or cannot occur, and that the
description includes instances where the event occurs and instances
where it does not. As used herein, "substrate" or "substrates" can
be used interchangeably with "surface" or "surfaces."
[0042] Disclosed is a single-crystal graphene sheet prepared by
epitaxial growth using a catalyst layer comprised of a
single-crystal graphitizing metal catalyst. Since the
single-crystal graphene sheet has uniform shape and is
substantially without defects, it can be efficiently applied in a
variety of electrical devices or electromagnetic devices, including
display devices, solar cells, or the like.
[0043] The single-crystal graphene sheet is prepared by disposing a
carbonaceous material on the catalyst layer, the catalyst layer
optionally disposed on a substrate, and heat-treating the catalyst
layer and carbonaceous material in selected conditions to form
graphene.
[0044] The process of graphene growth will now be described in more
detail. A carbonaceous material is disposed on a catalyst layer,
and the combination heat-treated so that the carbonaceous material
thermally decomposes on the surface of the catalyst layer. The
carbonaceous material can be a carbon containing gas. Carbon atoms
derived from the carbonaceous material are infiltrated into and
solid-solubilized in to the catalyst layer. When the amount of the
infiltrated carbon atoms exceeds a solubility limit of the catalyst
layer, which is an intrinsic property of the catalyst layer,
graphene nucleation occurs and graphene grows to form a
single-crystal graphene sheet.
[0045] The catalyst layer has a single-crystal structure, not a
polycrystalline structure. Because the catalyst layer is a single
crystal, it can solve problems, such as defect formation, which can
result if a polycrystalline graphitizing metal catalyst is used. A
polycrystalline graphitizing metal catalyst can comprise a
substrate 100 and plurality of grains of metal catalyst 110, thus
can have a granular structure with boundaries between the grains,
as is shown in FIG. 1. Formation of graphene on a catalyst with a
plurality of grains can result in polycrystalline graphene 120. As
a result, the solid-solubilized carbon can precipitate at grain
boundaries during the growth of graphene, thereby creating defects
in the single-crystal graphene sheet. In addition, since each of
the grains can have a different crystallographic orientation, the
rate of carbon precipitation in the grains will not be equal, thus
decreasing the uniformity of the graphene sheet. However, when the
graphitizing metal catalyst has a single-crystal structure, it has
been observed that the resulting graphene sheet does not have a
substantial number of defects. While not wanting to be bound by
theory, it is believed the absence of defects is because a single
grain is formed, thus a uniform single-crystal graphene sheet can
be formed because the rate of graphene formation is the same
throughout the entire surface of the catalyst layer.
[0046] The defects formed on the graphene sheet caused by the
polycrystalline structure can be identified using Raman
spectroscopy, in particular by the existence of a D band. The D
band in Raman spectrum indicates the existence of defects in the
graphene, and the intensity of the D band is believed to be
proportional to the number of defects in the single-crystal
graphene sheet.
[0047] A peak ratio can be defined as a ratio of the peak Raman D
band intensity to the peak Raman G band intensity. A single-crystal
graphene sheet prepared by epitaxial growth using the catalyst
layer has a peak ratio equal to or less than about 0.2,
specifically equal to or less than about 0.01, more specifically
equal to or less than about 0.001, and most specifically "0"
(zero). The catalyst layer is believed to assist carbon atoms in
the carbonaceous material to be bound to each other to form a
planar hexagonal structure. A catalyst that is suitable for
synthesis of graphite, inducing carbonization, or preparing carbon
nanotubes, can be used as the catalyst layer. Exemplary catalysts
can have a single-crystal structure, and comprise a metal 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 the like, and a combination
comprising at least one of the foregoing metals. The catalyst can
comprise an alloy of at least one of the foregoing metals. The
catalyst layer can be prepared by forming a single-crystal of the
metal or the alloy, or a commercially available single-crystal
metal material can be used. The commercially available
single-crystal metal can be in a rod shape, which can then be cut
into thin films to form sheets. The area of the final
single-crystal graphene sheet can vary and depend on the area of
the catalyst layer. Thus, a large-sized graphene sheet can be
obtained by enlarging the unit area of the catalyst layer.
Accordingly, a large-sized graphene sheet can be prepared.
[0048] The catalyst layer can be used alone or disposed on a
substrate comprised of silicon, or the like. The catalyst layer can
be bound to or in intimate contact with the substrate. Using the
catalyst layer without a substrate can be more efficient since
graphene can be formed on both sides of the catalyst layer.
[0049] The carbon atoms can be formed on the catalyst layer by
disposing the carbonaceous material on the catalyst layer. The
disposing can be performed by various methods including, for
example, coating a carbonaceous material on a surface of the
catalyst layer, contacting catalyst layer with a gaseous
carbonaceous material, or immersing the catalyst layer in a liquid
solution comprising the carbonaceous material.
[0050] The disposing processes will now be described.
[0051] <Coating a Carbonaceous Material on the Surface of a
Single Crystalline Graphitizing Metal Catalyst>
[0052] The carbonaceous material coated on the catalyst layer can
be a polymer that can be solid-solubilized in the catalyst layer,
or any other polymer containing carbon, without limitation to its
structure or composition, as is shown in FIG. 2. A polymer that
forms a dense coating can be used in order to form a dense graphene
layer. Polymers that are irregularly arranged on the catalyst layer
when coated on the catalyst layer by spin coating, dip coating, or
the like, can form an irregular network structure, and thus cannot
have a dense structure. As is shown in FIG. 2, a polymer 220 can be
disposed on a catalyst layer 210, which in turn is disposed on a
substrate 200. Heat treating the assembly can cause the polymer to
decompose to provide a catalyst layer comprising solid-solubilized
carbon 230. If the polymer is irregularly arranged on the catalyst
layer 230, further heat-treatment can result in polycrystalline
graphene 240. On the other hand, when a self-assembling polymer is
coated in the form of a polymer layer on the catalyst layer, the
polymer is regularly arranged on the surface of the catalyst layer
as is shown in FIG. 3. Shown in FIG. 3 is a polymer having a
hydrophobic portion 330 and a hydrophilic portion 320 disposed on a
catalyst layer 310 which is in turn disposed on a substrate 300.
Heat-treatment of this assembly can provide a single-crystal
graphene sheet having a dense structure as shown in FIG. 4, which
shows the resulting graphene sheet 410.
[0053] Any self-assembling polymer can be used herein without
limitation. For example, the self-assembling polymer can be a
polymer selected from the group consisting of an amphiphilic
polymer, a liquid crystal polymer, a conductive polymer, and the
like, and a combination comprising at least one of the foregoing
polymers.
[0054] The amphiphilic polymer includes a hydrophilic group and a
hydrophobic group, and thus can be arranged in a selected direction
when disposed in a water soluble solution. For example,
Langmuir-Blodgett arrangements, dipping arrangements, spin
arrangements, or the like, are possible.
[0055] The amphiphilic polymer includes a hydrophilic group and a
hydrophobic group. Exemplary hydrophilic groups include an amino
group, a hydroxyl group, a carboxyl group, a sulfate group, a
sulfonate group, a phosphate group or the like, or a combination
comprising at least one of the foregoing groups, or salts thereof.
Exemplary hydrophobic groups include a halogen atom, a
C.sub.1-C.sub.30 alkyl group, a C.sub.1-C.sub.30 halogenated alkyl
group, a C.sub.2-C.sub.30 alkenyl group, a C.sub.2-C.sub.30
halogenated alkenyl group, a C.sub.2-C.sub.30 alkynyl group, a
C.sub.2-C.sub.30 halogenated alkynyl group, a C.sub.1-C.sub.30
alkoxy group, a C.sub.1-C.sub.30 halogenated alkoxy group, a
C.sub.1-C.sub.30 heteroalkyl group, a C.sub.1-C.sub.30 halogenated
heteroalkyl group, a C.sub.6-C.sub.30 aryl group, a
C.sub.6-C.sub.30 halogenated aryl group, a C.sub.7-C.sub.30
arylalkyl group, a C.sub.7-C.sub.30 halogenated arylalkyl group, or
the like, or a combination comprising at least one of the foregoing
groups. Exemplary amphiphilic polymers include capric acid, lauric
acid, palmitic acid, stearic acid, myristoleic acid, palmitolic
acid, oleic acid, stearidonic acid, linolenic acid, capryl amine,
lauryl amine, stearyl amine, oleyl amine, or the like, or a
combination comprising at least one of the foregoing polymers.
[0056] The self-assembling polymer molecules can be arranged in a
selected direction in a liquid state. A conductive polymer can also
be used. The conductive polymer can be dissolved in a solvent to
form a membrane, and evaporation of the solvent can cause alignment
of the polymer molecules to form a crystalline structure. Thus, the
polymers can be aligned by dipping arrangement, spin coating
arrangement, or the like. Exemplary polymers include polyacetylene,
polypyrrole, polythiophene, polyanilline, polyfluorene,
poly(3-hexylthiophene), polynaphthalene, poly(p-phenylene sulfide),
poly(p-phenylene vinylene), or the like, or a combination
comprising at least one of the foregoing polymers.
[0057] In addition, a polymer that can self-align in a direction
when disposed from a vapor, for example, a conductive polymer
disposed using vapor deposition, can also be used. Exemplary
conductive polymers include acene and its derivatives, anthracene
and its derivatives, heteroanthracene (e.g., benzodithiophene and
dithienothiophene) and its derivatives, tetracene and its
derivatives (e.g., halogenated tetracene, tetracene derivatives
having a polar substituent, tetracene-thiophene hybrid materials,
rubrene and alkyl-, and alkoxy-substituted tetracene), hetero
tetracene and its derivatives, pentacene and its derivatives (e.g.,
alkyl- and halogen-substituted pentacene, aryl-substituted
pentacene, alkynyl-substituted pentacene, alkynyl-substituted alkyl
and alkynyl pentacene and alkynyl-substituted pentacene ether),
heteropentacene and its derivatives, heteroacene and its
derivatives, or the like, or a combination comprising at least one
of the foregoing conductive polymers.
[0058] The polymer can include a polymerizable functional group
such as a carbon-carbon double bond or triple bond capable of
polymerizing to form a crosslink. The polymerizable functional
group can be polymerized to crosslink the polymers after formation
of the polymer layer by, for example, exposing the polymer layer to
UV irradiation after the polymer layer is formed. Since the
polymerized polymer has a large molecular weight, evaporation of
carbon during the heat treatment of the polymer can be
substantially prevented.
[0059] The polymerization of the polymer can be performed before or
after coating the polymer on the catalyst layer. That is, when
polymerization is induced in the polymer before coating, a
self-supporting polymer layer can be formed, and the polymer layer
on the catalyst layer can be formed by transferring a polymer
membrane to the catalyst layer. The polymerization and transfer
processes can be repeated several times to control the thickness of
the graphene.
[0060] The polymer can be aligned on the surface of the catalyst
layer using various coating methods, including Langmuir-Blodgett,
dip coating, spin coating, vacuum deposition, or the like, or a
combination comprising at least one of the foregoing coating
methods to form an aligned polymer. The molecular weight of the
aligned polymer, thickness of the polymer layer, or the number of
self-assembled polymer layers can be selected depending on a
desired number of layers in the resulting graphene sheet. That is,
use of a polymer having a large molecular weight increases the
number of layers of the graphene sheet since the polymer has a
large amount of carbon. As the thickness of the polymer layer is
increased, the number of layers of graphene formed is increased,
and thus the thickness of the graphene sheet is also increased. The
thickness of the graphene can be controlled using the molecular
weight and the amount of the polymer.
[0061] In addition, the amphiphilic polymer, which can be a
self-assembling polymer, can include a hydrophilic part and a
hydrophobic part in one molecule. As shown in FIG. 5, the
hydrophilic part 510 of the polymer combines with the hydrophilic
catalyst layer 500 so that it is substantially uniformly aligned on
the catalyst layer, and the hydrophobic part 520 of the amphiphilic
polymer aligns in the opposite direction so that it is
substantially combined with the hydrophilic part of another
amphiphilic polymer that is not combined with the catalyst layer.
When the amount of the amphiphilic polymer is sufficient, layers of
the amphiphilic polymer can be caused to stack on the catalyst
layer by the hydrophilic-hydrophobic bonds. The layers of
amphiphilic polymer can be covered by water 530. The stacked
layers, formed from a plurality of the amphiphilic polymers, can
form a graphene layer upon heat-treatment. Thus, a graphene sheet
having a desired thickness can be prepared since the number of
layers of the graphene in the graphene sheet can be controlled by
selecting an appropriate amphiphilic polymer and selecting the
amount of the amphiphilic polymer.
[0062] The polymer coated on the catalyst layer is heat-treated to
graphitize the polymer. The heat-treatment can be performed in
stages to control the growth of the graphene sheet. For example,
the carbonaceous material can be decomposed by a metal catalyst in
a first heat-treatment to form carbon, and the carbon infiltrated
into the catalyst layer to form a solid solution therein. Then, the
carbon solid-solubilized in the metal catalyst can be precipitated
to form a single-crystal graphene sheet on the surface of the
catalyst layer in a second heat-treatment performed at a
temperature lower than solubility limit of the metal catalyst. The
first and second heat-treatments can be performed independently,
simultaneously, or combined and performed in series.
[0063] In addition, the process can further include removing
impurities, such as amorphous carbon formed on the surface of the
catalyst layer, by polishing the surface of a carbon-containing
catalyst layer formed after solid-solubilizing carbon atoms in the
catalyst layer in the first heat-treatment.
[0064] The first and second heat-treatments can be performed in an
inert or reducing atmosphere in order to prevent oxidization of the
polymer. The first and second heat-treatments can each be performed
at a temperature between about 300.degree. C. to about
2,000.degree. C., about 500.degree. C. to about 1800.degree. C., or
about 600.degree. C. to about 1700.degree. C. When the temperature
is lower than about 300.degree. C., the graphitization cannot be
sufficiently performed. On the other hand, when the temperature is
higher than about 2,000.degree. C., carbon can be evaporated. The
heat-treatment may be performed for a time between about 0.001
hours to about 1000 hours, about 0.01 hours to about 100 hours, or
about 0.1 hours to about 10 hours. When the heat-treatment time is
not between about 0.001 hours to about 1000 hours, the
graphitization can be insufficient, or efficiency can be
decreased.
[0065] The heat-treatment can be performed by induction heating,
radiant heating, laser heating, infrared heating (IR), microwave
heating, plasma heating, ultraviolet (UV) ray heating, surface
plasmon heating, or the like, or a combination comprising at least
one of the foregoing heating methods without limitation. In
particular, the graphitizing catalyst on which the polymer is
coated can be activated by selectively heating the catalyst by the
induction heating or using microwave radiation. Thus, a selected
region can be graphitized, and a single-layered graphene sheet can
be prepared by graphitizing a polymer having a short length. The
carbon atoms from the carbonaceous material can be covalently bound
to each other by the heat-treatment. For example, the carbon atoms
can form a planar hexagonal structure to form a graphene sheet on
the catalyst layer.
[0066] <Contacting a Single Crystalline Graphitizing Metal
Catalyst with a Gaseous Carbonaceous Material>
[0067] Alternatively, a catalyst layer can be contacted with a
gaseous carbon source containing a carbonaceous material, in
addition to coating a carbonaceous material on the surface of the
catalyst layer described above. Any material that can supply carbon
and be in the gas phase at about 300.degree. C. or higher can be
used as the carbon source, without limitation. The gaseous carbon
source can be a compound containing carbon atoms, specifically
equal to or less than about 6 carbon atoms, more specifically equal
to or less than about 4 or carbon atoms, and most specifically
equal to or less than about 2 or carbon atoms. The carbon source
can include at least one compound selected from the group
consisting of carbon monoxide, ethane, ethylene, ethanol,
acetylene, propane, propylene, butane, butadiene, pentane, pentene,
cyclopentadiene, hexane, cyclohexane, benzene, toluene, and the
like, and a combination comprising at least one of the foregoing
compounds.
[0068] The carbon source can be added at a constant pressure to a
chamber that comprises the catalyst layer. The pressure of the
carbon source in the chamber can be about 10.sup.-6 to about
10.sup.4 toss, specifically about 10.sup.-3 to about 10.sup.4 torr.
The carbon source can further include an inert gas such as helium,
argon, or the like, or a combination comprising at least one of the
foregoing inert gases.
[0069] In addition, hydrogen can be used with the gaseous carbon
source in order to control gaseous reactions by cleaning the
surface of the catalyst layer. Thus the carbon source can also
include hydrogen. The amount of the hydrogen can be between about
0.1% to about 99.9% by volume, specifically about 10% to about 90%
by volume, and more specifically about 15% to about 90% by volume
based on the total volume of the chamber.
[0070] When the gaseous carbon source is added to the chamber
containing the catalyst layer, and the chamber, including the
catalyst layer heat-treated, graphene is formed on the surface of
the catalyst layer. The heat-treatment temperature is an important
factor for the formation of the graphene and can be between about
300.degree. C. to about 2000.degree. C., specifically about
700.degree. C. to about 1200.degree. C. When the heat-treatment
temperature is less than about 300.degree. C., the rate of graphene
formation can be insufficient. On the other hand, when the
heat-treatment temperature is greater than about 2000.degree. C.,
graphene can overgrow or grow as particles or fibers, not as a
single-crystal sheet.
[0071] The graphene formation can be controlled by the temperature
and duration of the heat-treatment. That is, as the heat-treatment
time is increased, the amount of graphene formed is increased,
thereby increasing the thickness of the graphene sheet. As the
heat-treatment time is reduced, the ultimate thickness of the
formed graphene sheet is reduced. Accordingly, the heat-treatment
time can be an important factor to control in order to obtain a
desired thickness of the single-crystal graphene sheet. Other
factors that can control the thickness of the graphene sheet
include the type of the carbon source, the pressure of the carbon
source, the type of graphitizing metal catalyst, and the size of
the chamber. The heat-treatment can be performed for a time between
about 0.001 hours to about 1000 hours. When the heat-treatment is
performed for less than about 0.001 hours, the rate of graphene
formation can be insufficient. On the other hand, when the
heat-treatment is performed for longer than 1000 hours, formation
of too much graphene can cause graphitization.
[0072] The heat-treatment can be performed by induction heating,
radiant heating, laser heating, infrared (IR) heating, microwave
heating, plasma heating, ultraviolet (UV) heating, surface plasmon
heating, or the like, or a combination comprising at least one of
the foregoing heating methods, without limitation. The heat source
can be attached to the chamber to increase the temperature inside
the chamber to a selected temperature.
[0073] The single-crystal graphene sheet obtained by the
heat-treatment is cooled. Cooling is a process for uniformly
growing the formed graphene and arranging the crystallographic
orientation of the graphene in a selected direction. Since rapid
cooling can cause cracks in the graphene sheet, the cooling can be
performed slowly at a constant speed. For example, the cooling can
be performed at a rate between about 0.1.degree. C./minute to about
10.degree. C./minute, or about 0.5.degree. C./minute to about
5.degree. C./minute, or about 1.degree. C./minute to about
4.degree. C./minute, or natural cooling can be used. Natural
cooling is performed by removing the heat source used in the
heat-treatment from the chamber, thus a sufficient cooling speed
can be obtained by merely removing the heat source.
[0074] <Immersing a Single-Crystal Graphitizing Metal Catalyst
in a Carbonaceous Material-Containing Solution> [0075] A
carbonaceous material can be disposed on the surface of a catalyst
layer, or the carbon obtained by decomposing a carbonaceous
material can be carburized in a catalyst layer by immersing
catalyst layer in a liquid that comprises the carbonaceous
material, and heating the catalyst layer. [0076] The liquid can be
an alcohol, a polyol, a mixture thereof, or a solution comprising
at least one of an alcohol and a polyol. The polyol is a substance
having a plurality of hydroxyl groups, for example a polyhydric
alcohol. Exemplary polyols include ethylene glycol, propylene
glycol, butylene glycol, tetraethyl glycol, glycerin, or the like,
or a combination comprising at least one of the foregoing polyols.
[0077] When the catalyst layer is immersed in a solution comprising
the carbonaceous material, for a selected period of time, the
carbonaceous material can be bound to the surface of the catalyst
layer or carburized in the catalyst layer. The catalyst layer, to
which the carbonaceous material is bound, or in which carbon is
carburized, is then heat-treated, as described above, to prepare a
graphene sheet.
[0078] The heat-treatment can be performed in the same manner as
described above in conjunction with the method of coating the
polymer as the carbonaceous material.
[0079] The graphene sheet prepared by the above described coating,
contacting or immersion processes and heat-treatment processes can
have between about 1 layer to about 300 layers, specifically about
1 layer to about 60 layers, and more specifically about 1 layer to
about 15 layers. A graphene sheet having over about 300 layers is
regarded as graphite, which is distinct from graphene.
[0080] The catalyst layer can be used alone, or a plurality of
catalyst layers can be stacked on a substrate. The substrate can be
an inorganic substrate, such as a Si substrate, a glass substrate,
a GaN substrate, a silica substrate, or the like, or a combination
comprising at least one of the foregoing inorganic substrates, or
the substrate can be a metal substrate comprising Ni, Cu, W, or the
like, or a combination comprising at least one of the foregoing
metals. In the case of a silica substrate, the surface of the
silica substrate can be coated with a blocking layer in order to
prevent unnecessary reactions between the substrate and the
graphitizing metal catalyst. The blocking layer can be interposed
between the substrate and the graphitizing catalyst to inhibit
reduction in graphene formation efficiency caused by reactions
between the graphitizing catalyst and the substrate. The blocking
layer can be formed of SiO.sub.x, TiN, Al.sub.2O.sub.3, TiO.sub.2,
Si.sub.3N, or the like, or a combination comprising at least one of
the foregoing materials, and can be disposed on the substrate by
sputtering, or the like. The blocking layer can have a selected
thickness, preferably between about 0.1 nanometers (nm) to about
1000 micrometers (.mu.m). When the thickness of the blocking layer
is less than about 0.1 nm, the desired effect of the blocking layer
may not be obtained. On the other hand, when the thickness of the
blocking layer is greater than about 1000 .mu.m, costs can be
increased.
[0081] The graphene sheet can be identified using Raman
spectroscopy. That is, since pure graphene has a G' peak in the
vicinity of about 1594 cm.sup.-1, the formation of graphene can be
identified by the presence of an absorption at this wavenumber. In
particular, the graphene sheet formed using the catalyst layer can
have minimized defects or no defects. The existence of defects in a
graphene sheet can be identified by the presence of a D band in a
Raman spectrum. The existence of the D band can indicate the
existence of defects of the graphene sheet, and a high intensity of
the D band peak can indicate a large number the defects.
[0082] The disclosed graphene sheet has few or no defects, and thus
the D band may not be observed or only a very weak D band
absorption may be observed. The single-crystal graphene sheet
prepared by epitaxial growth using the catalyst layer has a peak
ratio of D band/G band of equal to or less than about 0.2,
specifically equal to or less than about 0.01, more specifically
equal to or less than about 0.001, and most specifically "0"
(zero).
[0083] The single-crystal graphene sheet can thus be formed on a
catalyst layer which can be disposed on a substrate. The
single-crystal graphene sheet can be used with the catalyst layer,
or the graphene sheet can separated from the catalyst layer by
dissolving and removing the catalyst layer using an acid-treatment,
if desired. If desired, the single-crystal graphene sheet can be
separated from the substrate.
[0084] The separated single-crystal graphene sheet can be processed
in a variety of ways according to its desired use. That is, the
single-crystal graphene sheet can be cut into a selected shape, or
the single-crystal graphene sheet can be wound to form a tube. The
processed single-crystal graphene sheet can also be combined with
various articles to be applied in various ways.
[0085] The single-crystal graphene sheet can be applied in various
fields and applications. The graphene sheet can be efficiently used
as a transparent electrode since it has excellent conductivity and
high uniformity. An electrode that is used as a substrate for a
solar cell, or the like, is desirably formed to be transparent to
allow light to penetrate therethrough. A transparent electrode
formed from the single-crystal graphene sheet has excellent
conductivity and flexibility due to the flexibility of the graphene
sheet. A flexible solar cell can be prepared by using a flexible
plastic as a substrate and the graphene sheet as a transparent
electrode.
[0086] In addition, when the graphene sheet is used in the form of
a conductive thin film in a display device, the desired
conductivity can be obtained using only a small amount of the
single-crystal graphene sheet and light penetration can thus be
improved.
[0087] In addition, the single-crystal graphene sheet formed in the
form of a tube can be used as an optical fiber, a hydrogen storage
medium, or as a membrane that selectively allows hydrogen to
penetrate.
[0088] The disclosure will now be described in greater detail with
reference to the following examples. The following examples are for
illustrative purposes only and are not intended to limit the scope
of the claims.
EXAMPLE 1
[0089] A single-crystal Ni thin film having a diameter of 1.2 cm
and a thickness of 0.2 mm (having a (111) surface, Matec, Inc.) was
placed in a reaction chamber, and heat-treated at 700.degree. C.
for 1 hour while flowing hydrogen into the chamber at 60 sccm to
remove an oxide formed on the surface of the single-crystal Ni thin
film. Then, the single crystal Ni thin film was heat-treated at
750.degree. C. for 2 minutes using a halogen lamp heat source while
introducing acetylene gas and hydrogen gas into the chamber at 5
sccm and 45 sccm respectively to form graphene on the
single-crystal Ni thin film graphitizing catalyst.
[0090] Then, the heat source was removed and the chamber was
naturally cooled to grow the graphene to a constant thickness,
thereby forming a graphene sheet having a diameter of 1.2 cm and
about 7 layers.
[0091] Then, the single-crystal Ni thin film on which the graphene
sheet formed was dissolved by treatment in 0.1 M HCl for 24 hours
to remove the Ni thin film and yield the single-crystal graphene
sheet.
[0092] FIG. 6 is a graph illustrating a Raman spectrum of the
graphene sheet. Referring to FIG. 6, the formation of graphene was
identified by G peak shown at 1594 cm.sup.-1. In addition, a peak
ratio of the D band to the G band was observed to be=0.193 using
the D peak shown at 1360 cm.sup.-1.
EXAMPLE 2
[0093] A Ni single crystal having a diameter of 1.2 cm and a
thickness of 0.2 mm (having a (111) surface, Matec, Inc.) was
placed in a reaction chamber on a _ substrate, and heat-treated at
700.degree. C. for 1 hour while flowing hydrogen into the chamber
at 60 sccm to remove an oxide formed on the surface of the single
crystalline Ni. Then, the Ni single-crystal was heat-treated at
900.degree. C. for 2 minutes using a halogen lamp heat source while
introducing acetylene gas and hydrogen gas into the chamber at 5
sccm and 45 sccm respectively to form graphene.
[0094] Then, the heat source was removed and the chamber was
naturally cooled to grow the graphene to a constant thickness,
thereby forming a graphene sheet having a diameter of 1.2 cm and
about 7 layers.
[0095] Then, the substrate on which the graphene sheet is formed
was dissolved by treatment in 0.1 M HCl for 24 hours to remove the
Ni thin film and yield the single-crystal graphene sheet.
[0096] FIG. 6 is a graph illustrating a Raman spectrum of the
graphene sheet. Referring to FIG. 6, the formation of graphene was
identified by the G peak shown at 1594 cm.sup.-1. In addition, a D
peak was not observed at 1360 cm.sup.-1.
COMPARATIVE EXAMPLE 1
[0097] A polycrystalline Ni thin film having a width and a length
of 1.2 cm and a thickness of 0.2 mm (Matec, Inc.) was placed in a
reaction chamber, and the polycrystalline Ni thin film heat-treated
at 700.degree. C. for 1 hour while flowing hydrogen into the
chamber at 60 sccm to remove an oxide formed on the surface of the
polycrystalline Ni thin film. Then, the polycrystalline Ni catalyst
was heat-treated at 750.degree. C. for 2 minutes using a halogen
lamp heat source while introducing acetylene gas and hydrogen gas
into the chamber at 5 sccm and 45 sccm respectively to form
graphene.
[0098] Then, the heat source was removed and the chamber was
naturally cooled to grow the graphene to a constant thickness,
thereby forming a graphene sheet having a width and a length of 1.2
cm and having about 7 layers.
[0099] Then, the polycrystalline Ni thin film on which the graphene
sheet formed was dissolved by treatment in 0.1 M HCl for 24 hours
to remove the polycrystalline Ni thin film to separate the graphene
sheet.
[0100] FIG. 7 is a graph illustrating a Raman spectrum of the
graphene sheet. Referring to FIG. 7, the formation of graphene was
identified by a G peak at 1594 cm.sup.-1. In particular, the
existence of defects in the graphene sheet was identified by a D
peak at 1360 cm.sup.-1. The D peak/G peak peak ratio was 0.261.
COMPARATIVE EXAMPLE 2
[0101] A polycrystalline Ni thin film having a width and a length
of 1.2 cm and a thickness of 0.2 mm (Matec, Inc.) was placed in a
reaction chamber, and heat-treated at 700.degree. C. for 1 hour
while flowing hydrogen into the chamber at 60 sccm to remove an
oxide formed on the surface of the polycrystalline Ni thin film.
Then, the polycrystalline Ni thin film was heat-treated at
900.degree. C. for 2 minutes using a halogen lamp heat source while
introducing acetylene gas and hydrogen gas into the chamber at 5
sccm and 45 sccm respectively to form graphene.
[0102] Then, the heat source was removed and the chamber was
naturally cooled to grow the graphene to a constant thickness,
thereby forming a graphene sheet having a width and a length of 1.2
cm and about 7 layers of graphene.
[0103] Then, the polycrystalline Ni thin film on which the graphene
sheet formed was dissolved by treatment in 0.1 M HCl for 24 hours
to remove the Ni thin film and separate the graphene sheet.
[0104] FIG. 7 is a graph illustrating Raman spectrum of the
graphene sheet. Referring to FIG. 7, the formation of graphene was
identified by [G']G peak at 1594 cm.sup.-1. In particular, the
existence of defects in the graphene sheet was identified by D peak
at 1360 cm.sup.-1, and the D peak/G peak peak ratio was observed to
be 0.348.
[0105] Disclosed is an economical process for preparing a
large-size single-crystal graphene sheet without defects, and a
process for control of the thickness of the graphene sheet. The
large-size single-crystal graphene sheet can have a desired
thickness and can be applied to various fields and
applications.
[0106] While the disclosed embodiments have been particularly shown
and described with reference to exemplary embodiments thereof, it
will be understood by those of ordinary skill in the art that
various changes in form and details may be made therein without
departing from the spirit and scope of the disclosure as defined by
the following claims.
* * * * *