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1. XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE
Atomization of Reduced Graphene Oxide Ultra-thin
Film for Transparent Electrode Coating
Mohd Rofei Mat Hussin
Nanotechnology & Adv. Device R&D
MIMOS Berhad
Kuala Lumpur, Malaysia
rofei@mimos.my
Nik Mohd Razali Mohd Nor
Semiconductor Tech. Development
MIMOS Semiconductor Sdn Bhd
Kuala Lumpur, Malaysia
zali@mimos.my
Mukter Zaman
Faculty of Engineering
Multimedia University
Cyberjaya, Malaysia
mukter@mmu.edu.my
Siti Aishah Mohamad Badaruddin
Process Engineering R&D
MIMOS Berhad
Kuala Lumpur, Malaysia
aishahb@mimos.my
Yuan Piou Choong
Mechanical Design Engineering
NSW Automation Sdn. Bhd.
Penang, Malaysia
ypchoong@nswautomation.com
Mohd Hilmy Azuan Hamzah
Nanotechnology & Adv. Device R&D
MIMOS Berhad
Kuala Lumpur, Malaysia
hilmy.hamzah@mimos.my
Hin Yong Wong
Faculty of Engineering
Multimedia University
Cyberjaya, Malaysia
hywong@mmu.edu.my
Abstract— In this research, an atomization process was
introduced to deposit carbon-based nanomaterials i.e. graphene
oxide (GO) on 200 mm silicon wafer for high volume production
of a transparent conductive electrode (TCE). An ultrasonic
spray coating process was used to deposit a stable dispersion of
graphene oxide in ethanol on silicon and glass substrates. The
thickness of the thin films can be controlled within a nanometer-
scale with the high uniformity coating process.
Functionalization of GO based on thermal reduction in Rapid
Thermal Process (RTP) and Plasma Enhanced Chemical
Vapour Deposition (PECVD) chambers was performed to make
the GO electrically conductive and optically transparent. AFM
measurements revealed that the thin film thickness has a strong
correlation with the number of spray passes that can be
translated into spray volume. It was discovered that the thin
films demonstrate a good electrical conductivity with high
optical transparency in the visible light. The sheet resistance of
the films can be significantly reduced by rapid thermal
annealing at a high temperature of up to 1100 ℃ and reach 5.3
kΩ/sq with a light transmittance of 77.1% at 550 nm
wavelength. This is a low-cost solution for high volume
production of TCE, which have a high potential for scaling-up.
Keywords—reduced Graphene Oxide, spray coating,
atomization process, transparent conductive electrode, rapid
thermal process
I. INTRODUCTION
Transparent conductive electrodes (TCE) are widely used
in various optoelectronic and photovoltaic applications such
as solar cells, touch panels, optical communication devices,
liquid crystal displays (LCD), etc. The indium tin oxide (ITO)
with transparency of over 95% and sheet resistance as low as
~10 Ω/sq has been the dominant source of TCE [1]. However,
it has some technical disadvantages such as poor mechanical
flexibility, and relatively high deposition temperature that
leads to cracking and scarcity issues. The production cost of
ITO also has risen rapidly in recent years due to the demand
for indium from the explosive growth of LCD based products
like smartphone, laptop, and television sets, etc. Therefore,
alternatively, the carbon-based nanomaterials have been
studied to replace ITO as they have remarkable characteristics,
for example, high optical transmittance and outstanding
electrical properties. There have been many research activities
on graphene, carbon nanotubes, metal nanowires, and hybrid
materials to replace ITO in recent years [2-6]. These new
materials have excellent mechanical and electrical properties
with high chemical stability and low cost of production in
comparison with the conventional ITO electrodes. However,
the major obstacle to this development is the method of
producing good quality of carbon-based nanomaterials and the
manufacturing method to meet high volume requirements.
Therefore, in this research, we have introduced an atomization
method to deposit carbon-based nanomaterials i.e. graphene
oxide (GO) on a large substrate for high volume production
through ultrasonic spray coating process. The Graphene oxide
(GO) is a graphene derivative material that can be viewed as
a functionalized analog of graphene [7]. In this work, the
functionalization of graphene is part of the nano-engineering
in order to tailor their characteristics for TCE application. The
oxygen-containing functional groups such as carbonyl,
carboxyl, and hydroxyl can be removed from the basal plane
and the edges through chemical or thermal reduction process.
The reduced graphene oxide (rGO) have similar
characteristics to graphene, which is transparent and
conductive [8]. Hence, it can be used for transparent
conductive coating and the photonic properties of rGO can
also be modified by altering the functional groups on the
carbon basal plane [8,9,10].
II. EXPERIMENT
In this work, there are four main process and
characterization steps involved as shown in Fig. 1. The first
step is about the material preparation and atomization process,
followed by the functionalization process, patterning, and
characterization steps. The process details are discussed in the
sub-section A, B, C and D below.

2. Fig. 1. The overall process flow for the experiment of transparent
conductive electrode coating.
A. Material Preparation and Atomization Process
The first step in producing transparent conductive
electrode was the production of graphene oxide (GO) solution
via modified Hummer's method. Graphite powder was used
and mixed with Sodium Nitrate (NaNO3) and concentrated
sulphuric acid (H2SO4). The solution was stirred at a constant
speed and potassium permanganate (KMnO4) was then
gradually added prior to dilution process with distilled water.
This process leads to the formation of active species;
diamanganese heptoxide (Mn2O7), which has the ability to
selectively oxidize the unsaturated double bonds graphitic
structure and reaction pathways during oxidation [11,12].
Exfoliation process was then carried out by sonicating the GO
dispersion in ambient temperature for 20 minutes to destroy
the forces between layers. Finally, the GO was fully exfoliated
to single or few layers. The GO was dispersed in ethanol with
the concentration of 1 mg/ml. Sonication is needed prior to
spray coating process.
The atomization process used an ultrasonic atomizer
system to spray coat the nano-thin film of a graphene layer on
glass slides and 200 mm silicon wafer substrate. The solution
was spray coated using NSW-MIMOS Mi-Atomizer 3.0
system. The thickness of the GO film was controlled by the
number of spray passes of the GO solution. For the purpose of
this experiment, we fixed the number of the spray passes to 1,
2, 3 and 4 passes.
B. Functionalization Process
In this work, two functionalization methods were used to
make the GO film electrically conductive and yet transparent.
For making the GO film electrically conductive, thermal
reduction process was introduced to the film. The first method
is based on the Rapid Thermal Process (RTP) with
temperature of up to 1100°C and continuous flow of N2 gas in
the RTP chamber. The AMAT Centura RTP system was used
for this experiment. This reduction process is to reduce the
oxygen-containing functional groups from the basal plane and
the edge of the GO film on silicon dioxide wafer substrate and
the glass slide. This reduced graphene oxide (rGO) film on
silicon dioxide wafer is needed for the measurement of sheet
resistance by Four Point Probe system to study the uniformity
of the thin film coated on a large substrate (200mm wafer
diameter). The second method is based on the thermal
annealing in plasma enhanced CVD chamber with the
temperature of up to 700°C with continuous flow of N2 gas.
The Oxford Instrument Nanofab-700 PECVD System was
used for the reduction process. The maximum temperature for
the system is 700°C.
C. Patterning Process
The rGO transparent electrode was then patterned using
standard lithography and plasma etching process, which
includes the coating of photoresist on the rGO film, the
photoresist exposure and developing step, and the etching of
unwanted rGO film area using plasma etching in PECVD
system.
D. Characterization Methods
The transparency percentage (% Transparency) of the rGO
film on the glass slides were then measured using the Agilent
Cary 7000 Universal Measurement Spectrophotometer (UV-
Vis & UV-Vis-NIR Systems). The sheet resistance of rGO
film on SiO2/Si wafers was measured using KLA Tencor
Prometrix RS75 Four Point Probe system. The Bruker Innova
Atomic Force Microscope (AFM) is also being used for the
measurement of the rGO film thickness coated on the silicon
wafer. The results were then analyzed to study the spray
passes and the functionalization effects.
III. RESULTS AND DISCUSSION
The number of spray passes can control the thickness of
thin films. The number of spray passes can be translated into
the volume of the GO solution. Four samples with different
spray passes, which the spray volume varies from 500 μL,
1000 μL, 1500 μL, and 2000 μL were prepared to study the
correlation between process parameters and the physical
characteristics of the thin films. Table I showed the sheet
resistance and the uniformity of rGO films for different
functionalization methods (RTP and PECVD). From the
analysis, the RTP based reduction method clearly
outperformed the PECVD method. The RTP samples have
shown much better performance, which one decade lower
sheet resistance as compared to PECVD based reduction
method. Therefore, the RTP samples were chosen for further
analysis to study their characteristics. However, there is no
value of sheet resistance for the sample with only one spray
pass regardless of their functionalization method. This is
because of the rGO film was not properly formed on the
substrate and the rGO flakes were not fully connected. The
results also indicated that the sheet resistance and the non-
uniformity of the thin films decreased with an additional
number of spray passes demonstrating that the thickness of the
rGO thin films increases with increasing spray passes. The
uniformity of the thin film is also improved when the
thickness of the thin film increased. In order to achieve 90%
uniformity of the rGO thin films, at least 3-spray passes are
required to deposit the nanomaterial on silicon substrate.
PECVD samples are not stable and there is no data available
for the non-uniformity of these samples.
TABLE I. SHEET RESISTANCE AND UNIFORMITY OF RGO (RTP &
PECVD)
Reduction
Method
Sheet Resistance, Rs (kΩ/sq) & Non-Uniformity (%)
1 Pass 2 Pass 3 Pass 4 Pass
RTP
N/A Rs: 12.3 Rs: 5.3 Rs: 3.3
N/A NU: 14.22% NU: 9.38% NU: 6.98%
PECVD
N/A 700 160 70
N/A N/A N/A N/A

3. The rGO thin films pattern on glass substrate were shown
in Fig. 2 and Fig. 3. These all are the RTP samples that have
lower sheet resistance as compared to PECVD samples. The
images clearly showed that the rGO films were successfully
patterned on SiO2/Si substrate and glass slides by using
standard lithography process and plasma etching. The
darkness of the rGO thin films increased with increasing spray
passes as clearly shown in Fig. 2. There is a strong correlation
between the densities of rGO flakes with the number of spray
passes. In Fig. 3, the microscope images showed the density
of rGO flakes increased with increasing spray passes.
Fig. 2. rGO films pattern on glass slides for different spray passes.
Fig. 3. Optical microscope images for rGO film coated on a glass substrate.
The thickness of the rGO thin film for different spray
passes was measured using AFM analysis. Fig. 4 showed the
3D model of AFM measurement for a single spray pass, which
points out that the thickness of the rGO thin film is about 4.631
nm. According to Shi et al. [3], the thickness of a single layer
rGO is about 1.2 nm. This means that a single spray pass may
result in a thin film deposition consist of a few layers of rGO
flakes. The correlation study of thin film thickness and the
number of spray passes (Fig. 5) showed the linear correlation
with the R-squared (R2
) about 0.9547. It is based on the
average thickness of rGO thin film measured at several
locations on the silicon substrate. In average, the thickness of
the rGO thin films for 1, 2 and 3-spray passes are 5 nm, 9 nm,
and 11 nm respectively.
Fig. 4. Atomic Force Microscopy (AFM) measurement for a single spray
pass.
The value of the average thickness is much larger than that
of pristine graphene, which was reported as about 0.3 nm [13].
However, the average thickness of the rGO thin film is
comparable to a few-layers rGO, which is about 1.3 nm in ref.
[14] and 1.2 nm in ref. [15]. The rGO is thicker than pristine
graphene is because of the existence of residual covalently
bonded reducing agent i.e. oxygen-containing functional
groups such as carbonyl, carboxyl, and hydroxyl groups [14].
Fig. 5. Correlation study of rGO films thickness with spray passes.
Theoretically, pristine graphene has excellent
optoelectronic properties such as high conductivity (Rs < 102
Ω/sq) at transparency higher than 90% for optical transmission
at 550 nm wavelength [6]. However, for the graphene
derivative materials, these optoelectronic properties are
normally degraded and very much dependent on the
preparation and functionalization methods. Fig. 6 showed the
GO and rGO transparency percentage for a various number of
spray passes. The percentage of transparency of GO thin films
is higher than rGO thin films. Increasing the number of spray
passes would also reduce the transparency percentage. This is
because of the thickness of the thin films become thicker when
more GO flakes drop on the substrate. There is a trade-off
between the optical conductivity and electrical conductivity.
Therefore, optimization of the reduction process is needed to
ensure good quality of rGO thin films formed on the substrate,
which can produce low sheet resistance and high light
transmittance.

4. Fig. 6. GO and rGO light transmittance percentage at different wavelength
with different spray passes.
Table II below summarized the percentage of transparency
of the GO and rGO at 550 nm wavelength optical
transmission. The GO thin films showed degradation of
%Transparency with an increasing number of spray passes but
the reduction of light transmittance percentage is much lower
as compared to rGO thin film materials. All GO samples with
1, 2 and 3 spray passes have more than 95% transparency but
when the layer went through the reduction RTP process, then
the degradation rate becomes more severe resulted the rGO
thin films with 3 spray passes to reduced down to 77%
transparency. In comparison with other types of materials as
reported by Hofmann et al. [16], the %Transparency of rGO
in this study is comparable to the ITO, silver nanowire
(AgNWs), single-wall nanotubes (SWNTs) and polystyrene
sulfonate (PEDOT:PSS) but with one decade higher sheet
resistance. However, it is noted that the FOM (Figure-of-
Merit) of the rGO thin films from this study is higher than the
previously reported rGO by Shi et al. [3], which the
transmittance (%) is only about 68.7% to 79.6% with the sheet
resistances between 58 kΩ/sq to 3140 kΩ/sq.
TABLE II. TRANSPARENCY PERCENTAGE OF GO AND RGO (RTP)
#Pass
% Transparency
Graphene Oxide, GO
Reduced Graphene Oxide,
rGO (RTP)
1 98.6% 91.6%
2 97.1% 84.9%
3 96.2% 77.1%
For the transparent conductive electrode (TCE) to be
considered for industrial application, it should have a sheet
resistance of less than 100 Ω/sq at more than 90%
transparency [17]. However, in a real application, the
requirements on the TCE strongly depends on the specific
application [18].
IV. CONCLUSION
Ultrasonic atomization process was introduced as one of
the methods to deposit nanomaterials coating on large
diameter substrate i.e. silicon/glass wafer for the transparent
conductive electrode. The thickness of the thin films can be
controlled within a nanometer-scale with high uniformity
coating process. This is one the low-cost solution for high
volume production of TCE and it has high potential for
scaling-up production. As a proof-of-concept, we have
successfully fabricated the transparent electrode on 200 mm
silicon wafers and glass slides with good transparency
percentage (>77%). RTP reduction method with temperature
of up 1100 ℃ has shown better performance in comparison
with PECVD reduction method.
ACKNOWLEDGMENT
The authors acknowledge funding from the Government
of Malaysia under the 11MP program (P11-Establishment of
National Nanoelectronics Shared Infrastructure). This work
was performed with the collaboration between MIMOS
Berhad, NSW Automation Sdn. Bhd, and Faculty of
Engineering, Multimedia University.
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