1 Graphene-based transparent conductive thin films prepared
using surface wave plasma CVD[12-4] A low-temperature process is
particularly essential for the Si-based device industry and would
pave the way for the direct synthesis of graphene-based films onto
glass and plastic substrates. Two methods for synthesizing
graphene-based films have been previously proposed; the thermal
chemical vapor deposition CVD method on metal surfaces and the
chemical reduction method of graphite oxides. the former method is
restricted to a high deposition temperature limit of around 1000 C
and the latter method requires time consuming procedures and
complicated liquid waste treatment. The large-area surface wave
plasma SWP CVD apparatus with a plasma area of 40 x 60 cm2 by
adopting an array configuration of microwave slot antennas for
synthesizing carbon nanomaterials was proposed. Slide 2 2 The
original large-area SWP operated at gas pressures less than 5 Pa
was performed. Such a low-pressure process allows to keep a
substrate temperature lower due to low neutral gas temperatures and
also uniform CVD growth over large areas due to the enhanced
diffusion of plasma. SWP is able to provide high- density plasmas
and radicals and has relatively low electron temperature below 2 eV
and also low plasma space potential below 10 V in bulk region even
at the low gas pressures. As a result SWP can reduce ion
bombardment on the substrate surface. Fig. 3. Corona and dielectric
barrier discharge (DBD) arrangements. (a) Corona with streamers;
(b) dielectric barriers on electrodes; (c) DBD arrangement for
treatment of large-area planar substrates; (d) DBD arrangement for
two-sided treatments; and (e) multiple grid-type DBD arrangement
(200 m200 m windows). The arrows in (c), (d), and (e) indicate the
direction of substrate motion. Slide 3 3 The polycrystalline Cu and
Al foils were treated to clean their surfaces by using Ar/H2
plasmas at 5 Pa for 20 min before the CVD process. After the
treatment, we turned off the microwave generators, evacuated the
discharge chamber to base pressure of 103 Pa, introduced a reaction
gas mixture into the discharge chamber. We then performed CVD using
these foils as substrates at 3 or 5 Pa with microwave powers of 3
to 4.5 kW for each microwave generator for various growth times
ranging from 30 to 180 s. The formation of the graphene-based films
using CH4 gas mixed with Ar and/or H2 was examined. Although
graphene can be grown using CH4 alone in our plasma CVD process,
the addition of Ar improves the stability and uniformity of the
plasma at low gas pressure. The addition of H2 is expected to
improve the quality of the films because hydrogen is known to
selectively etch amorphous carbon defects. The composition CH4
/Ar=30/10 SCCM and CH4 /Ar/H2=30/20/10 SCCM standard cubic
centimeters per minute in this experiment. The reaction gas
mixtures enable the synthesis of graphene-based films on Cu and Al
foils at substrate temperatures lower than 400 C. Slide 4 4 FIG. 1.
A Raman spectrum of a typical graphene-based film deposited on Cu
foil CVD conditions: 5 Pa, CH4 /Ar/H2=30/20/10 SCCM, b Raman
spectrum of a graphene-based film deposited on Al foil CVD
conditions: 3 Pa, CH4 /Ar/H2 =30/20/10 SCCM, c Substrate
temperature profile during the Ar/H2 plasma treatment and the
plasma CVD for the synthesis of the film on Al foil shown in Fig.
1b. The high-intensity D peak and low-intensity D shoulder in the
Raman spectrum shown in Fig. 1a are attributed to the edges and the
boundaries of the flakes, which suggests that the film contains
submicrometer sized flakes of graphene- based materials. Figure 1a
suggests that Raman peaks are assigned to the peaks of few-layer
graphene in the deposited film. Slide 5 5 (a) Optical transmittance
of a graphene-based film transferred on a glass plate. The inset
shows a picture of the film with 81% transparency. (b) Plot of
sheet resistance vs optical transmittance average between 400 and
800 nm of the films obtained by SWP-CVD with various CVD
conditions. FIG. 3. Color online Synthesis of large-area graphene-
based films with an area of 23 x 20 cm using SWP-CVD CVD
conditions: 3 Pa, CH4 /Ar/H2=30/20/10 SCCM, 4.5 kW per a MW
generator. (a) Cu film after plasma CVD. (b) Transferred film on an
acrylic plate deposited for 180 s. (c) Transferred film on an
acrylic plate deposited for 90 s. (d) Spatial distribution of the
sheet resistance of the graphene-based film transferred on an
acrylic plate shown in Fig. 3b. Slide 6 6 12.2 Large area flexible
transparent conductive thin films produced through
Langmuir-Blodgett Assembly A LB film contains one or more
monolayers of an organic material, deposited from the surface of
liquid onto a solid by immersing the solid substrate into (or from)
the liquid. A monolayer is adsobbed homogeneously with each
immersion or emersion step. GO sheets are strongly hydrophilic and
can produce stable and homogeneous colloidal suspensions in aqueous
and various polar organic solvents due to the electrostatic
repulsion between the negatively charged GO sheets. Before the LB
deposition, the as-prepared GO dispersion was screened to separate
the dispersions containing the smallest and the largest groups of
GO sheets, namely, the small graphene oxide (S-GO, several m 2 )
and ultra large graphene oxide (UL-GO, 1~10000 m 2 ) dispersions.
Slide 7 7 The work involves pre-exfoliation of natural graphite
flakes to produce gram quantities of ultral arge graphene oxide
(UL-GO) sheets, up to 50~200 m in lateral size with a yield
exceeding 50% by weight. The LB assembly technique is then used to
transfer the GO monolayers onto substrates and produce highly
conducting transparent LB thin films. After thermal reduction and
chemical doping treatments, the transparent conductor made from the
UL-GO sheets shows a sheet resistance of 459 /sq at a transmittance
of 90% along with a remarkable DC / OP ratio of 7.29. where Z 0
=377 ohm is the impedance of free space. A high ratio represents a
high transmittance and a low sheet resistance, and thus high
opto-electrical properties of transparent conductors, Slide 8 8
Parameters affect LB films. Surface pressure The typical surface
pressure-area isotherm shown in Figure 3a presents the change in
the slope corresponding to the phase transition of GO sheets from
gas to condensed liquid and to solid state. There existed an
initial gas phase where the surface pressure remained essentially
constant (stage a). The pressure began to increase as the
compression continued (stage b) and the GO sheets were about to
touch one another, tiling( ) over the entire surface. The increase
in surface pressure was likely due to the electrostatic repulsion
between the GO sheets. A further increase in surface pressure
followed (stage c) when the monolayer was compressed beyond the
close-packed stage. This occurred because the GO sheets started to
fold at the touching points along their edges instead of
overlapping on top of another. At a higher pressure (stage d),
partial overlapping and wrinkling happened, leading to a nearly
complete monolayer of interlocked GO. Slide 9 9 The LB transfer of
flat GO sheets is a self-assembly process whose quality depends
largely on the evaporation of water molecules present between UL-GO
sheets and substrate. The relatively small size of S-GO allowed
water to evaporate easily, and thus wrinkle-free S-GO films were
obtained (Figure 4). However, the water droplets are often trapped
between the UL-GO sheets, and the capillary force induced by water
evaporation causes wrinkling of the sheets. In particular, if a
high pulling speed is applied, there would be too short a time for
the UL-GO sheets to relax into a flat state and transfer onto the
substrate. In this case, water may not be fully evaporated, causing
wrinkling to occur during the transfer process due to the capillary
force and gravity. Pulling rate and evaporation rate Figure 4. SEM
images of S-GO sheets collected on a silicon substrate at different
stages of surface pressure (see Figure 3a). The packaging density
increases from (a) isolated S-GO sheets to (b) close-packed S-GO
sheets, (e) over packed S-GO sheets with folded edges, and (f) over
packed S- GO sheets with overlapping. Slide 10 10 Figure 5. SEM
images of UL-GO sheets collected on a silicon wafer at different
stages of surface pressure at a pulling speed of 0.1 mm/min. The
packaging density increases from (a) isolated UL-GO sheets to (b)
close-packed UL-GO sheets, (c) overlapped UL-GO sheets with some
wrinkles, and (d) overlapped UL-GO sheets with extensive wrinkles.
Slide 11 11 Surface morphology When the second GO layer was
deposited on top of the first layer, these two layers were likely
to experience both electrostatic repulsion and van der Waals
attraction. Since the GO sheets are brought together on top of
another, their van der Waals potential can be scaled with (1/d 2 ).
The residual -conjugated domains can also contribute to the
attraction between GO sheets. While these attractive forces
dominate and lead to successful layer-by-layer deposition of GO
sheets, the GO sheets also experience electrostatic repulsion from
both their neighbors and those already deposited, causing wrinkling
to occur. Figure 6. AFM images of GO films consisting of two layers
(a,b) and eight layers (c,d) of monolayer GO sheets taken before
(a,c) and after thermal treatment (b,d). It is also worth noting
that the surface roughness was consistently reduced after the
thermal treatment due to the removal of oxygenated functional
groups and graphitization of the films at an elevated temperature.
Slide 12 12 Comparison of optical and electrical properties between
ULGO films with different number of layers obtained at different
stages of treatment is presented in Figure 8. A thicker film
resulted in a higher degree of absorption of light and thus a lower
transparency at all treatment stages studied. The transparency was
significantly deteriorated after the thermal treatment, whereas
part of the lost transparency was restored after the chemical
treatments (Figure 8a,b).The films darkened after the thermal
treatment due to the reduction of GO and the adsorption of impurity
particles on the other side of quartz substrates, which was an
artifact. The removal of these impurities by the subsequent acid
treatment contributed to the improvement of transparency. Optical
Transmittance and Electrical Conductivity. Figure 8. Comparison of
optical and electrical properties between UL-GO films of different
number of layers taken at different stages of treatment: (a)
transmittance measured at 550 nm wavelength; (b) transmittance
spectra of C-rUL-GO films as a function of wavelength; and (c)
sheet resistance at different stages. Slide 13 13 (f) Figure 9.
(a,c) Raman spectra for natural graphite, UL-GO, rUL-GO, and
C-rUL-GO; and (b,d) the corresponding D/G and 2D/(D+G) intensity
ratios. Schematic illustrations of (e) molecular structure of SOCl2
molecules absorbed onto graphene surface; and (f) chemical
structure of graphene sheet after chemical doping treatments. Slide
14 14 Besides the G- and D-bands, there are two other Raman bands,
called 2D and D+G at 2600-3000 cm -1 (Figure 9c), which are often
ignored due to their weak intensities compared to D- and G-bands.
The 2D-band is Raman-active for crystalline graphitic materials and
is sensitive to the -band in the graphitic electronic structure,
while the combination mode of D+G is induced by disorder.43 The
intensity ratio I 2D /I D+G shown in Figure 9d indicates that the I
2D /I D+G ratio was more sensitive to the change in electronic
conjugation from UL-GO to rUL-GO than the I D /I G ID/IG ratio
(Figure 9b), as a reflection of the recovery of graphitic
electronic conjugation. The reduction of the I 2D /I D+G ratio
corresponding to the modification from rUL-GO to C-rULGO indicates
that the newly doped functional groups, such as -Cl, -SOCl, and
-COOH, introduced disorder again. Slide 15 15 12.2 Large area
flexible transparent conductive thin films produced through the
roll- to-roll method [12-6, 12-7] There are three essential steps
in the roll-to-roll transfer (Fig. 1a): (i)adhesion of polymer
supports to the graphene on the copper foil; (ii)etching of the
copper layers; (iii)release of the graphene layers and transfer
onto a target substrate. In the adhesion step, the graphene film,
grown on a copper foil, is attached to a thin polymer film coated
with an adhesive layer by passing between two rollers. In the
subsequent step, the copper layers are removed by electrochemical
reaction with aqueous 0.1 M ammonium persulphate solution (NH 4 ) 2
S 2 O 8. Finally, the graphene films are transferred from the
polymer support onto a target substrate by removing the adhesive
force holding the graphene films. Slide 16 16 In the first step of
synthesis, the roll of copper foil is inserted into a tubular
quartz tube and then heated to 1,000C with flowing 8 sccm. H 2 at
90 mtorr. After reaching 1,000C, the sample is annealed for 30 min
without changing the flow rate or pressure. The copper foils are
heat-treated to increase the grain size from a few micrometers to
100 m, as we have found that the copper foils with larger grain
size yield higher-quality graphene films. The gas mixture of CH 4
and H 2 is then flowed at 460 mtorr with rates of 24 and 8 sccm.
for 30 min, respectively. Finally, the sample is rapidly cooled to
R.T. (~10 C/s) with flowing H 2 under a pressure of 90 mtorr. Slide
17 17 After growth, the graphene film grown on copper foil is
attached to a thermal release tape by applying soft pressure (0.2
MPa) between two rollers. After etching the copper foil in a
plastic bath filled with copper etchant, the transferred graphene
film on the tape is rinsed with deionized water to remove residual
etchant, and is then ready to be transferred to any kind of flat or
curved surface on demand. The graphene film on the thermal release
tape is inserted between the rollers together with a target
substrate and exposed to mild heat (~90120C), achieving a transfer
rate of ~150200 mm min -1 and resulting in the transfer of the
graphene films from the tape to the target substrate (Fig. 2b). By
repeating these steps on the same substrate, multilayered graphene
films can be prepared that exhibit enhanced electrical and optical
properties, As the graphene layers are transferred one after
another, the intensities of the G- and 2D- band peaks increase
together, but their ratios do not change significantly. This is
because the hexagonal lattices of the upper and lower layers are
randomly oriented, unlike in graphite, so the original properties
of each monolayer remain unchanged, even after stacking into
multilayers; this is clearly different from the case of multilayer
graphene exfoliated from graphite crystals. Slide 18 18 The
randomly stacked layers behave independently without significant
change in the electronic band structures, and the overall
conductivity of the graphene films appears to be proportional to
the number of stacked layers. The optical transmittance is usually
reduced by 2.22.3% for an additional transfer, implying that the
average thickness is approximately a monolayer The unique
electronic band structure of graphene allows modulation of the
charge carrier concentrations in dependence on an electric field
induced by gate bias or chemical doping, resulting in enhancement
of sheet resistance. We tried various types of chemical doping
methods, and found that nitric acid (HNO 3 ) is very effective for
p-doping of graphene films. Slide 19 19 Figure 3c shows Raman
spectra of the graphene films before and after doping with 63 wt%
HNO3 for 5 min. The large peak shift ( =18 cm -1 ) indicates that
the graphene film is strongly p-doped. The shifted G peak is often
split near the randomly stacked bilayer islands. as shown in Fig.
3c. The lower graphene layer, which is screened by top layers,
experiences a reduced doping effect, leading to G- band splitting.
In X-ray photoelectron spectra (XPS), the C1s peaks corresponding
to sp2 and sp3 hybridized states are shifted to lower energy,
similar to the case for p-doped carbon nanotubes25. However,
multilayer stacking results in blueshifted C1s peaks. We suppose
that weak chemical bonding such as the pp stacking interaction
causes descreening of nucleus charges, leading to an overall
increase in core electron binding energies. We also find that the
work functions of graphene films as estimated by UV photoelectron
spectroscopy (UPS) are blue-shifted by 130 meV with increasing
doping time. (Fig. 3d, inset). The multiple stacking also changes
the work functions which could be very important in controlling the
efficiency of photovoltaic or light- emitting devices based on
graphene transparent electrodes. Slide 20 20 Usually, the sheet
resistance of graphene film with 97.4% transmittance is as low as
125 / (Fig. 4a) when it is transferred by a soluble polymer support
such as polymethyl methacrylate (PMMA). In the process of
roll-to-roll dry transfer, the first layer sometimes shows
approximately two to three times larger sheet resistance than that
of the PMMA- assisted wet-transfer method. As the number of layers
increases, the resistance drops faster compared to the wet-transfer
method (Fig. 4a). Slide 21 21 Preparation of a 100-m-long graphene
transparent conductive film [12-7] Owing to the small emissivity (e
0.04) and heat conduction of thin copper foil, a large area copper
foil can be selectively heated to 1000 C in the vacuum chamber
equipped with a conventional roll-to-roll system. A cold rolled
copper foil (230mm wide, >100m long, 36 m thick, >99.9% pure)
was enclosed in the vacuum chamber, in which the pressure was
maintained at 1000 Pa under a continuous flow of CH4 (450 sccm) and
H2 (50 sccm). The pressure in the chamber at a higher value than
used in a typical low-pressure CVD synthesis was used to suppress
the sublimation of copper at high temperature. Slide 22 22 A high
methane partial pressure of 900 Pa was used to achieve a high
growth rate of graphene at a relatively low growth temperature. The
copper foil was wound on the roll-to-roll system for 100 m at a
velocity of 0.1 m/min. Owing to the precisely controlled handling
of the copper foil and the small heat load to the roll- to-roll
system. The first 52 m of graphene film was grown at temperature
Tg~950C by applying a constant direct current J=82 A/mm 2 to the
suspended copper foil, and the remaining 48 m of graphene film was
grown at J=83 A/mm 2 (Tg~980C). Since Joule heating of a copper
foil may result in a nonuniform temperature distribution, the
spatial uniformity of the temperature within the copper foil was
investigated by growing graphene films that only partially covered
the copper foil surface, and then measured the coverage
distribution [Fig. 2(a)]. After baking the copper foil at 180 C for
5 min, the graphene-covered regions can be observed with an optical
microscope because of oxidation of the bare copper surface. Near
the center of the foil, the graphene coverages were89% and 98% for
J=82 and 83 A/mm 2, respectively. Slide 23 23 Raman spectroscopy
[Fig. 2(d)] revealed that the graphene film consisted of
predominantly single-layer graphene. A scanning electron microscopy
(SEM) image analysis [Fig. 2(b)] showed that each graphene grain is
nearly hexagonal, and in many cases, a thicker multi-layer grain
existed at the center. We observed more than 800 grains and found
that multi-layer grains with a diameter larger than 100 nm covered
~5% of the film. The lower coverage near the edge, as shown in Fig.
2(c), suggests that there is a non-negligible temperature drop due
to a larger heat loss at the edges. However, uniform graphene
coverage was observed near the center of the copper foil,
indicating that the temperature variation across the majority of
the surface area of the copper foil is negligibly small. These
results demonstrate that selective Joule heating is a reasonable
heating technique for graphene CVD synthesis. Slide 24 24 12-1.
Nano Letters, Watcharotone 2007 7 721888 12-2. J of Materials
Chemistry, Tien 2012 22 2545 12-3. ChemComm, Dominggues 2011 47
2592 12-4. Applied Physics Letters, Kim 2011 98 091502 12-5. ACS
nano, Zheng, 2011 5 7 6039 12-6. Nature Nanotechnology 2010 5 574
12-7. Applied Physics Letters, Kobayashi 2013 102 023112 12-8. Nano
Letters, Wang 2008 8 323 12-9. Chemistry of Materials 2010 22 1392
12-10.Energy Environ. Sci., Pumera 2011 4 668-674 12-11.Nano
Letters, Li 2009 9 12 4359 12-12.Nanoscale, Nguyen 2011
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