Large-area graphene on polymer film for flexible and transparent anode in field emission device

Article abstract of DOI:10.1063/1.3431630

We present the fabrication and electrical characterization of large graphene structure on polyethylene terephthalate (PET) flexible substrate. Graphene film was grown on Cu foil by thermal chemical vapor deposition and transferred to PET by using hot pres

Full text of DOI:10.1063/1.3431630

Large-area graphene on polymer film for flexible and transparent anode in
field emission device
Ved Prakash Verma, Santanu Das, Indranil Lahiri, and Wonbong Choi
Citation: Appl. Phys. Lett. 96, 203108 (2010); doi: 10.1063/1.3431630
View online: http://dx.doi.org/10.1063/1.3431630
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APPLIED PHYSICS LETTERS 96, 203108 ͑2010͒
Large-area graphene on polymer film for flexible and transparent anode
in field emission device
a͒
Ved Prakash Verma, Santanu Das, Indranil Lahiri, and Wonbong Choi
Department of Mechanical and Materials Engineering, Florida International University, Miami,
Florida 33174, USA
͑
Received 25 March 2010; accepted 28 April 2010; published online 20 May 2010͒
We present the fabrication and electrical characterization of large graphene structure on
polyethylene terephthalate ͑PET͒ flexible substrate. Graphene film was grown on Cu foil by thermal
chemical vapor deposition and transferred to PET by using hot press lamination. The graphene/PET
film shows high quality, flexible transparent conductive structure with unique electrical-mechanical
properties; ϳ88.80% light transmittance and ϳ1.1742 k⍀/sq sheet resistance. We demonstrate
application of graphene/PET film as flexible and transparent electrode for field emission displays.
Our proposed techniques can be tailored for any flexible substrate and large scale production, which
could open up exciting device applications in foldable electronics. © 2010 American Institute of
Physics. ͓doi:10.1063/1.3431630͔
Graphene is a two-dimensional ͑2D͒ carbon material
Graphene film was synthesized by CVD of hydrocarbon
having unique band structure and outstanding thermal, me-
on copper foil. Commercially available, cold rolled Cu foil
of 50 to 200 ␮m thickness and large area ͑6 cm width and
15 cm length͒ was first annealed at 1000 °C for 1 h under Ar
atmosphere. After annealing, Cu foil was acid-treated for 10
min using 1 M acetic acid at 60 °C. This acid treatment
helps in removing oxide layer generated at the Cu foil sur-
face during annealing process. Copper foils were thoroughly
washed with de-ionized water and dried at the ambient con-
ditions. Graphene films were grown on copper foils in a
1
–3
chanical, and electrical properties. Some of the potential
applications of graphene are for sensors, transistors, superca-
4
–8
pacitors, solar cells, and flexible displays. It is well known
that graphene has high mechanical strength with flexibility,
8
,9
high transmittance, and high electron mobility. These prop-
erties make graphene an emerging alternate for transparent
conductive metal oxides electrodes, in particular, indium tin
oxide ͑ITO͒ which contains indium as a costly and scarce
element.
1
2
similar way to the previously reported CVD process. In
short, graphene on the Cu foil was synthesized at 1000 °C
and 1 atm pressure, using a 5 min flow of CH and H gases
In order to make a transparent conductive graphene film,
most of the researchers have used liquid solution of graphene
flakes ͑obtained by reducing graphene oxide flakes͒ for
4
2
in 1:4 ratios. After graphene growth, the foil was cooled
down to room temperature before being taken out from the
furnace. Graphene growth on Cu foil has been reported as a
surface-catalyzed process which indicates that Cu act as cata-
lyst for CVD of graphene. A detailed discussion about
growth mechanism of graphene formation on copper foil has
7
,10
deposition of transparent conductive film.
Yamaguchi et al.
Recently,
11
has deposited chemically derived
graphene solution on flexible substrate for large area trans-
parent flexible electrode which contains 2 to 30 layers of
graphene. Application of graphene in flexible electronics will
need synthesis of continuous graphene film on substrates and
1
2
been presented elsewhere.
1
2
transfer it to polymeric substrate in large scale. Li et al. has
grown high quality, predominately monolayer graphene films
on copper foils by chemical vapor deposition ͑CVD͒ method.
We have used hot press lamination and chemical etching
process for transferring graphene grown over the Cu foils to
the transparent flexible substrates. Figure 1͑a͒ shows flow
diagram of graphene transfer technique. Cu foils with
graphene were hot press rolled with a transparent flexible
PET film having thickness ϳ50 ␮m. For complete removal
of Cu from the graphene and laminated film, we have used
8
Kim et al. have demonstrated two different techniques
͑
stamping and scooping͒ for transferring graphene from
nickel substrate to other arbitrary substrates. These tech-
niques are not effective for industrial application which will
require low cost, high quality, and large area production of
graphene flexible electrodes.
concentrated FeCl solution. Laminated polymer film with
3
graphene and Cu foil underneath was floated over the FeCl3
acid bath at room temperature. After 40 min of etching pro-
cess Cu was completely dissolved into the solution leaving
graphene film with the PET substrate. This transparent flex-
ible film was then thoroughly washed with de-ionized water
and dried in air at room temperature. Figure 1͑b͒ demon-
strates a flexible, transparent graphene film with diagonal
length of ϳ16 cm. This hot press lamination process pro-
vides a very adherent graphene film on the flexible substrate
which can be deformed easily into various geometries ͓Fig.
Here we present a direct and effective method for syn-
thesis of large graphene film on copper foils and transferring
it to polyethylene terephthalate ͑PET͒ flexible substrate by
hot press lamination process. This method provides an effec-
tive way to handle large area of graphene film with minimal
physical damage to it. The resulting graphene polymer film is
flexible and remains conductive under high tensile strains.
The application of this graphene film as flexible transparent
conductive anode has been demonstrated in carbon nanotube
͑
CNT͒ field emission devices ͑FEDs͒.
1
͑c͔͒ without damaging the film.
Characterization of graphene over Cu and flexible sub-
a͒Electronic mail: choiw@fiu.edu.
003-6951/2010/96͑20͒/203108/3/$30.00
strate were done by Raman spectroscopy. The Raman spec-
0
96, 203108-1
© 2010 American Institute of Physics
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203108-2
Verma et al.
Appl. Phys. Lett. 96, 203108 ͑2010͒
1
3
was in compliance with a small D peak. The transfer of
graphene film on PET substrate was evident from Raman
spectrum, showing characteristic G and 2D bands.
Optical transmission spectroscopy measurement was
performed using Scinco, SD-1000 Ultraviolet-visible spec-
trophotometer. Figure 2͑b͒ shows the transmittance of the
graphene film with optical range of wavelengths. Percent
transmission of graphene film on PET substrate is 88.80 at
5
50 nm wavelength. The graphene film shows higher trans-
1
5
mittance compared to CNT ͑Ref. 14͒ and ITO films. Sheet
resistance of the graphene film measured by four-point probe
method was ϳ1.1742 k⍀/sq which is comparable to other
8
,16
reported graphene films.
Graphene has an advantage over
ITO, as it shows superior electrical-mechanical properties.
To investigate the electromechanical properties of graphene/
PET film, bending and stretching tests was performed.
2
Graphene/PET film ͑dimension 1ϫ1 cm ͒ was mounted on
the lower jaws of a Vernier caliper which helps in bending
and stretching the film in desired amount. The linear resis-
tance of the film was measured at two edges across the film.
Figure 2͑c͒ shows the resistance of graphene/PET film with
tensile strain ranging from 0% to 60%. Surprisingly,
graphene/PET film remains conductive and shows only one
order of magnitude change by ϳ60% stretching. Foldability
of graphene/PET film was evaluated by measuring resistance
with respect to bending radii ͓Fig. 2͑d͔͒. The graphene is
only few nanometers thick and PET thickness ͑ϳ50 ␮m͒
alone is used in tensile strain calculation. Bending the film
upto a radius of curvature of 1.52 mm ͑approximate tensile
strain of 6.5%͒ changes the resistance of graphene from 2.09
to 2.68 k⍀, which was recovered completely after unbend-
ing the graphene/PET film. These stretching and bending
tests shows that graphene film has superior mechanical and
electrical properties compared to brittle ITO electrode, which
generate microcracks inside the film under mechanical
FIG. 1. ͑Color online͒ ͑a͒ Process flow for graphene transfer from Cu foil to
PET substrate. Hot press lamination and chemical etching processes were
used in this method. ͑b͒ Large area graphene film transferred over PET
substrate. ͑c͒ Flexibility of graphene/PET film.
trum of graphene film on Cu foil and PET film are shown in
Fig. 2͑a͒. A symmetric and sharp 2D-band indicates mono-
layer of graphene film over Cu foil. The I /I ratio for
G
2D
17
stress. The excellent mechanical properties of graphene
graphene on Cu foil was 0.4, conforming monolayer
1
2
film can be attributed to atomically perfect lattice of hexago-
graphene film. A high quality of graphene film over Cu foil
1
8
nally arranged strong carbon–carbon bonds.
This transparent and conductive graphene film was char-
acterized for its possible application in FEDs, where
graphene/PET film was used as a flexible anode. Green phos-
phor ͑Phosphor Tech; DPG01͒ material was deposited on the
graphene/PET flexible substrate by dip coating process. This
graphene/PET anode was assembled over bent ͑radius of cur-
vature ϳ13.65 mm͒ CNT field emitters grown on copper
foil ͓Fig. 3͑a͔͒. Details of synthesis and field emission prop-
erties of CNT field emitters are discussed in our past
1
9
publication. The interelectrode distance was kept constant
at 600 ␮m and the device was tested for its performance and
−
7
stability under dc bias and high vacuum ͑ϳ1ϫ10 Torr͒
condition. Figure 3͑b͒ shows the field emission response of
the device in bent configuration. The emission current behav-
ior of the device was analyzed using Fowler–Nordheim
2
2
3/2
͑
=
F–N͒ equation I=͑aA␤ E /⌽͒exp͑−b⌽ /␤E͒, where a
1.54ϫ10− A eV V and b=6.83ϫ10 eV V cm , re-
6
−2
7
3/2
−1
spectively, A is the emission area, ␤ is the field enhancement
factor, E is the applied electric field in volt per centimeter,
2
0
FIG. 2. ͑Color online͒ ͑a͒ Raman spectra from graphene on copper foil and
PET substrates. ͑b͒ Transmittance of graphene film over PET substrate. Inset
shows large area transparent graphene/PET film. ͑c͒ Variation in resistance
of graphene/PET film uniaxial stretched by 60%. ͑d͒ Resistance of
graphene/PET film with different bending radii. Insets show schematic of
stress modes applied to graphene/PET film.
and ⌽ is the work function in electron volt. Inset in Fig.
͑b͒ shows the corresponding F–N plot. Straight line nature
3
of the F–N plot indicates that the emission current was due to
field emission. Turn-on field for the device was found to be
1.75 V/␮m and field enhancement factor ͑␤͒, as calculated
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203108-3
Verma et al.
Appl. Phys. Lett. 96, 203108 ͑2010͒
niques can be tailored for any flexible substrate and large
scale production at low-cost, which could open up exciting
applications in foldable electronics and electromechanical
devices.
The authors thank Dr. Chang-soo Han at Korea Institute
of Machinery and Materials for characterizing optical trans-
mittance. V.V. acknowledges Dissertation Year Fellowship
from University Graduate School, Florida International Uni-
versity for financial support. This research was partly sup-
ported by AFOSR ͑Grant No. FA9550-09-1-0544͒.
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In conclusion, we have synthesized large area graphene
film and transferred it to PET flexible substrate by using hot
press lamination technique. Graphene film on PET substrate
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this graphene /PET film as a flexible transparent anode in
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