Cao Z. (Ed.) Thin Film Growth: Physics, materials science and applications

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218 Thin ﬁlm growth

9.3 Roll-to-roll production of 30-inch graphene

ﬁlms for transparent electrodes

9.3.1 Synthesis of 30-inch graphene ﬁlms and roll-to-roll

transfer of graphene ﬁlms

Figures 9.4a–c present photographs of the roll-based synthesis and transfer

process. An 8-inch-wide tubular quartz reactor (Fig. 9.4a) is used in the

CVD system, allowing a monolayer graphene lm to be synthesized on a

roll of copper foil with dimensions as large as 30 inches in the diagonal

direction (Fig. 9.4c). A temperature gradient usually exists that depends

on the radial position inside the tubular reactor. In our preliminary work,

this sometimes resulted in inhomogeneous growth of the graphene on the

copper foils. To solve this problem, a ~7.5-inch quartz tube wrapped with

a copper foil was inserted and suspended inside the 8-inch quartz tube. In

this way, the radial inhomogeneity in the reaction temperature could be

minimized. In the rst step of synthesis, the roll of copper foil is inserted

into a tubular quartz tube and then heated to 1000°C with owing 8 standard

cubic centimetres per minute) (sccm) H

at 90 mtorr. After reaching 1000°C,

the sample is annealed for 30 minutes without changing the ow rate or

pressure. The copper foils are heat-treated to increase the grain size from

a few micrometers to ~100 mm, as we have found that the copper foils

with larger grain size yield higher-quality graphene lms, as suggested by

Li and colleagues.

The gas mixture of CH

and H

is then owed at 460

mtorr at rates of 24 and 8 sccm for 30 minutes, respectively. Finally, the

sample is rapidly cooled to room temperature (~10°C s

–1

) with owing H

under a pressure of 90 mtorr.

After growth, the graphene lm 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 lled with copper etchant, the

transferred graphene lm on the tape is rinsed with deionized water to remove

residual etchant, and is ready to be transferred to any kind of at or curved

surface on demand. The graphene lm on the thermal release tape is inserted

between the rollers together with a target substrate (Fig. 9.4b). By repeating

these steps on the same substrate, multilayered graphene lms can be prepared

that exhibit enhanced electrical and optical properties, as demonstrated by Li

and colleagues using wet-transfer methods at the centimeter scale.

Figure

9.4(c) shows the 30-inch multilayer graphene lm transferred to a roll of 188

mm thick polyethylene terephthalate (PeT) substrate. Figure 9.4(d) shows

a screen-printing process used to fabricate four-wire touchscreen panels

based on graphene/PET transparent conducting lms. After printing electrodes

and dot spacers, the upper and lower panels are carefully assembled and

connected to a controller installed in a laptop computer (Figs 9.4e and f),

ThinFilm-Zexian-09.indd 218 7/1/11 9:42:29 AM

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219Growth of graphene layers for thin ﬁlms

(a)

(b)

(c)

(d)

(e)

(f)

8 inch

39 inch

Stencil mask

Screen

printer

After

heating

Before

heating

1st

2nd

30 inch

9.4 Photographs of the roll-based production of graphene ﬁlms. (a)

Copper foil wrapping around 7.5 inch quartz tube to be inserted into

an 8 inch quartz reactor. The lower image shows the stage in which

the copper foil reacts with CH

and H

gases at high temperatures.

(b) Roll-to-roll transfer of graphene ﬁlms from a thermal release tape

to a PET ﬁlm at 120°C. (c) A transparent ultra-large-area graphene

ﬁlm transferred on a 35 inch PET sheet. (d) Screen printing process

of silver paste electrodes on graphene/PET ﬁlm. The inset shows 3.1

inch graphene/PET panels patterned with silver electrodes before

assembly. (e) An assembled graphene/PET touch panel showing

outstanding ﬂexibility. (f) A graphene-based touchscreen panel

connected to a computer with control software. (From Ref. [2].)

ThinFilm-Zexian-09.indd 219 7/1/11 9:42:29 AM

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220 Thin ﬁlm growth

which shows extraordinary exibility. The scalability and processability of

CVD graphene and the roll-to-roll methods presented here are expected to

enable the continuous production of graphene-based electronic devices at

large scales.

The graphene lms seem to be predominantly composed of monolayers

when analysed using Raman spectra (Fig. 9.5a). However, atomic force

microscope (AFM) and transmission electron microscope (TeM) images often

show bilayer and multilayer islands. 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 signicantly. This is because the

hexagonal lattices of the upper and lower layers are randomly oriented,

unlike the graphite, so the original properties of each monolayer remain

unchanged, even after stacking into multilayers;

18,19

this is clearly different

from the case of multilayer graphene exfoliated from graphite crystals.

The

randomly stacked layers behave independently without signicant change in

the electronic band structures, and the overall conductivity of the graphene

lms appears to be proportional to the number of stacked layers.

The

optical transmittance is usually reduced by ~2.2–2.3% for an additional

transfer, implying that the average thickness is approximately a monolayer

(Fig. 9.5b).

The unique electronic band structure of graphene allows modulation of the

charge carrier concentrations, depending on an electric eld 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 is very effective for p-doping of graphene lms. Figure 9.5(c) shows

Raman spectra of the graphene lms before and after doping with 63 wt%

Hno

for 5 minutes. The large peak shift (Du = 18 cm

–1

) indicates that the

graphene lm is strongly p-doped. The shifted g peak is often split near the

randomly stacked bilayer islands, as shown in Fig. 9.5(c). We hypothesize

that 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 C 1s peaks corresponding to sp

and sp

hybridized

states are shifted to lower energy, similar to the case for p-doped carbon

nanotubes.

However, multilayer stacking results in blueshifted C1s peaks.

We suppose that weak chemical bonding such as p-p stacking interaction

causes descreening of nucleus charge, leading to an overall increase in core

electron binding energies. We also nd that the work functions of graphene

lms as estimated by UV photoelectron spectroscopy (UPS) are blueshifted

by ~130 meV with increasing doping time (Fig. 9.5d, inset). The multiple

stacking also changes the work functions (Fig. 9.5d, inset) which could be

very important in controlling the efciency of photovoltaic

or light-emitting

devices based on graphene transparent electrodes.

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221Growth of graphene layers for thin ﬁlms

9.5 Optical characterizations of the graphene ﬁlms prepared using

layer-by-layer transfer on SiO

/silicon and PET substrates. (a) Raman

spectra of graphene ﬁlms with different numbers of stacked layers.

The left inset shows a photograph of transferred graphene layers

on a 4 inch SiO

(300 nm)/silicon wafer. The right inset is a typical

optical microscope image of the monolayer graphene, showing >95%

monolayer coverage. A PMMA-assisted transfer method is used for

this sample. (b) UV-vis spectra of roll-to-roll layer-by-layer transferred

graphene ﬁlms on quartz substrates. The inset shows the UV spectra

of graphene ﬁlms with and without HNO

doping. The right inset

shows optical images for the corresponding number of transferred

layers (1 ¥ 1 cm

). The contrast is enhanced for clarity. (c) Raman

spectra of HNO

-doped graphene ﬁlms, showing ~18 cm

–1

blueshift

both for G and 2D peaks. D-band peaks are not observed before or

after doping, indicating that HNO

treatment is not destructive to the

chemical bonds of graphene. (d) XPS peaks of monolayer graphene

ﬁlms transferred on SiO

/Si substrates, showing typical redshift

and broadening of carbon 1s peaks (C1s) caused by p-doping. The

inset shows work function changes (DF) with respect to doping time

(lower x-axis), measured by UPS. (From Ref. [2].)

1500 2000 2500 3000

Raman shift (cm

–1

)

(a)

10 µm

No. of layers = 4

Intensity (a.u.)

20,000

15,000

10,000

5000

200 400 600 800 1000

Wavelength (nm)

(b)

Transmittance (%)

100

No. of layers (T

at 550 nm) = 1 (97.4%)

2 (95.1%)

3 (92.9%)

4 (90.1%)

Before HNO

After HNO

Transmittance (%)

100

200 400 600 800

Wavelength (nm)

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222 Thin ﬁlm growth

9.3.2 Optical and electrical properties of graphene ﬁlms

The electrical properties of graphene lms formed using layer-by-layer

staking methods were also investigated. Usually, the sheet resistance of

graphene lm with 97.4% transmittance is as low as ~125 W/sq (Fig. 9.6a)

when it is transferred by a soluble polymer support such as polymethyl

methacrylate (PMMA).

16,24,25

The transferrable size achievable using a

wet transfer method is limited to less than a few inches of wafer because

of the weak mechanical strength of spin-coated PMMA layers. However,

the scale of roll-to-roll dry transfer assisted by a thermal release tape is in

principle unlimited. In the process of roll-to-roll dry transfer, the rst layer

sometimes shows approximately two to three times larger sheet resistance

Intensity (a.u.)

6000

5000

4000

3000

2000

1000

Before HNO

After HNO

Intensity (a.u.)

4000

2000

No D peaks

1200 1300 1400 1500

Raman shift (cm

–1

)

1582

1600

1520 1560 1600 1640 2560 2640 2720 2800

Raman shift (cm

–1

)

(c)

2678

2695

292 290 288 286 284 282

Binding energy (eV)

(d)

Intensity (a.u.)

1600

1200

800

400

Number of layers

1 2 3 4

= –130 meV

~0.83 eV

~0.12 eV

After doping

Before doping

C=O C–O C–C

0 60 120 180 240 300 360

Doping time (s)

0.20

0.15

0.10

0.05

0.00

DF (eV)

9.5 Continued

C1s

ThinFilm-Zexian-09.indd 222 7/1/11 9:42:30 AM

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223Growth of graphene layers for thin ﬁlms

Roll-to-roll transfer

Wet transfer with PMMA

Roll-to-roll + HNO

doping

1 2 3 4

Number of layers

(a)

Sheet resistance (W

–1

)

300

250

200

150

100

9.6 Electrical characterization of layer-by-layer transferred and HNO

doped graphene ﬁlms. (a) Sheet resistances of transferred graphene

ﬁlms using roll-to-roll (R2R) dry transfer method combined with

thermal release tapes and a PMMA-assisted wet transfer method. (b)

Comparison of sheet resistance from this research and transmittance

plots taken from other references. The dashed arrows indicate the

expected sheet resistances at lower transmittance. (c) Electrical

properties of a monolayer graphene Hall bar device in vacuum. Four-

probe resistivity (left inset) is measured as a function of gate voltage

in the monolayer graphene Hall bar shown in the right inset at room

temperature (solid curve) and T = 6 K (dashed curve). The QHE

effect at T = 6 K and B = 9 T is measured in the same device. The

longitudinal resistivity r

and Hall conductivity s

are plotted as a

function of gate voltage. The sequence of the ﬁrst three half-integer

plateaux corresponding to u = 2, 6 and 10, typical for single-layer

graphene, are clearly seen. The Hall effect mobility of this device is

Hall

= 7350 cm

–2

–1

at 6 K (~5100 cm

–2

–1

at 295 K). Scale

bar (inset), 3 mm. (d) Electromechanical properties of graphene-based

touchscreen devices compared with ITO/PET electrodes under tensile

strain. The inset shows the resistance changes with compressive and

tensile strain applied to the upper and lower graphene/PET panels,

respectively. (From Ref. [2].)

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. 9.6a). We postulate that the adhesion of the rst layer with the

substrate is not strong enough for complete separation of the graphene lms

from the thermal release tape. As a result, there can be mechanical damage

on the graphene lms, leading to an increase in the overall sheet resistance.

Because additional layers are not directly affected by the adhesion with the

substrate surface, the sheet resistance of multilayers prepared by the roll-

to-roll method does not differ much from that for the wet transfer case. The

p-doping with Hno

clearly enhances the electrical properties of graphene

ThinFilm-Zexian-09.indd 223 7/1/11 9:42:30 AM

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224 Thin ﬁlm growth

70 75 80 85 90 95 100

Transmittance (%)

(b)

Sheet resistance (W

–1

)

This work

Theory

Ref. 6

Ref. 19

Ref. 25

Ref. 28

Wet-transferred

graphene

(ref. 19)

Graphene grown on Ni

Carbon nanotubes

R2R graphene

R2R graphene + doping

ITO (calculation)

Graphene (calculation)

9.6 Continued

1 2 3 4 5 6

Strain (%)

(d)

DR/R

200

150

100

ITO

R2R graphene

Tensile

Compressive

0 2 4

Strain (%)

–30 –20 –10 0 10 20 30 40

(V)

(c)

Magnetoresistance (kW)

–5

–10

–15

R (kW)

–20 0 20 40

(V)

T=295K

T=6K

ThinFilm-Zexian-09.indd 224 7/1/11 9:42:31 AM

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225Growth of graphene layers for thin ﬁlms

lms, and is more effective in roll-to-roll processes. The sheet resistance of

the p-doped four-layer graphene lm with ~90% optical transmittance is as

low as ~30 W/sq, which is superior to common transparent electrodes such

as indium tin oxide (ITO) and carbon-nanotube lms (Fig. 9.6b).

Standard electron-beam lithography has been used to fabricate graphene

Hall bars on conventional 300 nm Sio

/Si substrates (Fig. 9.6c). The left

inset of Fig. 9.6c shows the four-terminal resistance of such samples as a

function of backgate voltage (V

) at both room temperature (solid curve)

and at a low temperature (T = 6K) and zero magnetic eld. We observe the

graphene specic gate bias dependence of the resistance with a sharp Dirac

peak and an effective Hall mobility of 7350 cm

–1

at low temperatures.

This allows the observation of the quantum Hall effect (QHe)

at 6K and

a magnetic eld of B = 9T (Fig. 9.6c, right). The ngerprint of single-layer

graphene, the half-integer quantum Hall effect, is observed with plateaux at

lling factors of u = 2,6 and 10 at R

= 1/2, 1/6 and 1/10 (h

), respectively.

Although the sequence of the plateaux remains for both the electron side

and the hole side, there is a slight deviation from the fully quantized values

on the hole side.

Finally, the electromechanical properties of graphene/PeT touchscreen

panels were tested (Fig. 9.6d). Unlike an ITo-based touch panel, which

easily breaks under ~2–3% strain, the graphene-based panel resists up to

6% strain; this is limited not by the graphene itself, but by the printed silver

electrodes (Fig. 9.6d).

9.4 Conclusions

In conclusion, we showed a method for graphene growth on nickel and copper

layers using chemical vapour deposition and further described a method to

transfer graphene lms to stretchable substrates. This will lead to numerous

applications including use in large-scale exible, stretchable, foldable

transparent electronics. In addition, given the scalability and processability

of roll-to-roll and CVD methods and the exibility and conductivity of

graphene lms, we anticipate that the commercial production of large-scale

transparent electrodes, replacing ITo, will be realized in the near future.

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227Growth of graphene layers for thin ﬁlms

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