.

1. Surface modification and properties modulation of rGO film by short
duration H2 and NH3 plasma treatment
Firzalaila Syarina Md Yakin ⇑
, Mohd Faizol Abdullah, Siti Aishah Mohamad Badaruddin,
Mohd Ismahadi Syono, Nurhidaya Soriadi
Advanced Devices Lab, MIMOS Berhad, Technology Park Malaysia, Kuala Lumpur 57000, Malaysia
a r t i c l e i n f o
Article history:
Available online 17 February 2021
Keywords:
rGO film
H2 plasma
NH3 plasma
Film properties
a b s t r a c t
Reduced graphene oxide (rGO) is a versatile material due to the presence of oxygen functionalization.
One of the methods to tune the properties of rGO for the specific application is plasma treatment.
Plasma H2 and NH3 are known introduced surface defects for increasing surface reactivity and inducing
substitutional doping on rGO film. They need to be further discussed with the changes in electrical and
optical properties of the film for the construction of devices such as electrical, optical, and electro-optical
sensors. This article reports the effect of H2 and NH3 plasma treatment on the surface and properties of
rGO film using a variety in plasma power and temperature. We found low-power H2 plasma at low- and
medium-temperature induced further de-oxygenation of rGO. A tiny etching effect by H2 plasma slightly
reduces the conductivity, r to 54.01 S/cm from the reference rGO value of 448.90 S/cm. Low-power H2
plasma at high-temperature left the high defective sites due to plasma etching. The r was reduced to
8.04 S/cm. Medium-temperature medium-power NH3 plasma did not effective for N-doping. An etching
effect by NH3 plasma noticeably reduces the r to 5.79 S/cm. Medium-temperature high-power and high-
temperature medium-power NH3 plasma induce N-doping but at the same time significantly etch the
rGO. They significantly reduce the hole concentration and lower the r to 0.48 S/cm. High-power high-
temperature NH3 plasma rigorously induces N-doping but extreme plasma etching results in poor surface
condition and film discontinuity. The film exhibited electron as a majority carrier but with the lowest r of
0.05 S/cm.
Ó 2021 Elsevier Ltd. All rights reserved.
Selection and peer-review under responsibility of the scientific committee of the 3rd International Con-
ference on Materials Engineering & Science.
1. Introduction
Graphene oxide (GO) and reduced graphene oxide (rGO) are the
2-dimensional materials derived from graphene. Conductive rGO
film is often regarded as a low-cost alternative for graphene that
is grown by a high-temperature chemical vapor deposition process
[1]. Nevertheless, rGO is a versatile material and has several advan-
tages compared to graphene due to the presence of oxygen func-
tionalization on its basal plane and edges. Some advantages of
rGO for example; ease of coating on the hydrophobic surface [2],
sensitive to adsorbates [3], tunable bandgap for controlling the
hydrophilicity [4], and tunable bandgap for controlling the electri-
cal conductivity [5]. One of the methods to tune the properties of
rGO is by plasma treatment. In the previous report, H2 plasma
treatment was investigated in generating carbon vacancies and
holes on rGO [6]. They established a relationship between the
power of the H2 plasma treatment and the exposure time with
the CAC bond hydrogenation. The creation of defects and holes
by plasma irradiation on graphene enabled higher hydrogen
adsorption capacity and higher hydrogenation activity. In another
work, NH3 plasma treatment was reported on GO to achieve simul-
taneous reduction and N-doping on rGO [7]. They concluded the
short duration of NH3 plasma treatment up to 5 min results in an
increase of sp2
carbon content along with significant incorporation
of the graphitic-N form, forming the n-type rGO.
Undeniably, treatment of rGO film either using H2 or NH3
plasma does physically modify the surface condition by the intro-
duction of defects. Those defects are desirable for increasing sur-
face reactivity and inducing substitutional doping on rGO film.
However, they need to be further discussed with the changes in
the electrical and optical properties of the film. This is important
https://doi.org/10.1016/j.matpr.2020.12.811
2214-7853/Ó 2021 Elsevier Ltd. All rights reserved.
Selection and peer-review under responsibility of the scientific committee of the 3rd International Conference on Materials Engineering & Science.
⇑ Corresponding author.
E-mail address: firzalaila.yakin@mimos.my (F.S. Md Yakin).
Materials Today: Proceedings 42 (2021) 2996–3001
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2. to evaluate the suitability of the plasma treatment process in mod-
ifying the surface with desired properties for the construction of
devices such as electrical, optical, and electro-optical sensors. In
this article, we report our work on H2 and NH3 plasma treatment
on rGO film. We investigate the consequence of short duration
plasma treatment with different power and temperature on the
highly de-oxygenated rGO film. The variations on H2 and NH3
plasma treatment conditions on properties of rGO film were com-
pared in a single report based on fast and non-destructive optical
and electrical measurements.
2. Experimental
Single-layer GO in the aqueous solution was synthesized by the
improved Hummers method [8]. GO dispersion in ethanol 1 mg/
ml was used for 2-times spin-coating onto 1 1 cm2
SiO2/Si
(100 nm SiO2) substrate at 3000 rpm for 60 sec. After the coating,
the film was left dried for 3 min. The GO film was subjected to
thermal reduction at 1100 °C in the inert N2 furnace (Applied
Materials Centura RTP) for 5 min for the de-oxygenation process
into rGO. After the reduction, rGO film was subjected to either
H2 or NH3 plasma treatment according to details in Table 1. Each
treatment process was carried out in the inductively coupled
plasma chamber (Oxford Instruments Plasmalab System 100) at
400 mTorr for 1 min.
The surface condition was investigated using field emission
scanning electron microscopy (FESEM) at 5000 magnification
(JEOL JSM-7500F). The crystal condition was studied using Raman
spectroscopy with an excitation laser of 432 nm (NTEGRA Spectra
MT-MDT) at room temperature (RT). Single spectrum Raman mea-
surement was done with the periodic calibration of Si peak at
520 cm 1
for sensitive monitoring in spectra shift from the rGO
film. The optical properties of the sample were studied using fixed
angle 75° UV–visible spectral ellipsometry (Semilab SE-2000). The
electrical properties of the sample were obtained using Hall effect
measurement (Ecopia HMS-5300) in van der Pauw configuration at
RT.
3. Results and discussion
3.1. Surface morphology comparison
Spin coating of hydrophilic GO dispersion on SiO2/Si substrate
formed a continuous GO film with thickness 14 nm. Fig. 1(a)
shows the FESEM image of GO after thermal reduction into rGO
film. The de-oxygenation process that occurred on rGO sheets did
not affect the continuity of this material as a film. Fig. 1(b)–(d) is
the surface condition of rGO after subjected to low-power H2
plasma treatment for the S2, S3, and S4, respectively. The surface
appeared to be rougher for every increase in H2 plasma tempera-
ture from RT up to 700 °C. Fig. 1(e) and (f) show the FESEM image
of rGO film after medium-temperature NH3 plasma treatment for
the S5 and S6. We found the effect of increasing plasma power
from 20 to 50 WRF during medium-temperature NH3 plasma treat-
ment results in a tiny change on the rGO surface. Fig. 1(g) and (h) is
the surface condition of rGO after high-temperature NH3 plasma
treatment of S7 and S8. The surface of S7 appeared to be rough
in a similar manner to the previous S4, indicating high-
temperature plasma either H2 or NH3 did attack the surface of
rGO regardless of low or medium plasma power. Provided the
high-temperature and high-power NH3 plasma, we observed the
film continuity of S8 was severely affected by the rigorous physical
etching that occurred on the surface.
3.2. Raman spectra analysis
The comparison in Raman spectra of GO before and after ther-
mal reduction into rGO is provided in Fig. 2(a). Lorentz fitting on
the spectra enables us determining the exact position, intensity
ratio, and full width half maximum (FWHM) of the peak. Two
prominent peaks were identified from GO film at 1593 cm 1
(FWHM = 82 cm 1
) and 1362 cm 1
(FWHM = 112 cm 1
), which is
respectively corresponding to the I(G) and I(D). The I(G) is origi-
nated from the first-order scattering of the E2g phonons at the C-
point, which arises from the CAC bond in graphitic materials [9].
Meanwhile, I(D) is coming from the disorder in sp2
-hybridized car-
bon systems and associated with the amount of defect on graphene
lattice. An increase in I(D)/I(G) ratio from 0.817 to 1.253 in GO film
after subjected to thermal reduction indicates the high-degree of
de-oxygenation occurred and leaves the lattice with a large
amount of defective sites. Thermal reduction of GO at 1100 °C in
inert N2 removed the sp2
- and sp3
-attached groups i.e. hydroxyl,
ether, epoxide, carboxyl, and carbonyl groups from the graphene
lattice. The reduction process slightly shift the position of I(G) to
1591 cm 1
(FWHM = 91 cm 1
) and I(D) to 1378 cm 1
(FWHM = 109 cm 1
).
The change in Raman spectra for rGO film after subjected to
plasma treatment is shown in Fig. 2(b). Details for the plot are pro-
vided in Table 2. For the rGO film after low-power H2 plasma treat-
ment of S2 – S4, an increase in temperature from low up to high-
temperature simply increase the value of I(D)/I(G). We notice the
position of I(G) negligibly affected by the H2 plasma treatment.
Instead, the FWHM of I(G) was narrowed, indicating the improved
quality of crystalline C–C lattice regardless of the increase in I(D)/I
(G). The position of I(G) exhibited shifting to higher wavenumber
after H2 plasma treatment. The FWHM of I(D) narrowed corre-
sponding to the peak shifting. Such a situation can be explained
by considering plasma H2 contributing to the further de-
oxygenation occurred especially at high-temperature, terminating
the bond with CAH, while at the same time plasma attacking the
crystalline CAC sites.
For the medium-temperature NH3 plasma treatment of S5 and
S6, medium-power results in higher I(D)/I(G) compared to the
high-power plasma. The I(G) was insignificantly shifted for the
S5, but significant for the S6. Shifting of I(G) to lower wavenumber
is associated with the change in carrier concentration or doping on
Table 1
Assigned sample no. and its plasma treatment condition on rGO film.
Sample no. Plasma gas Temp. (°C) Power (WRF) Remark
1 Reference rGO film rGO without plasma treatment
2 H2 RT 10 H2 plasma, low-temp., low-power
3 H2 200 10 H2 plasma, medium-temp., low-power
4 H2 700 10 H2 plasma, high-temp., low-power
5 NH3 200 20 NH3 plasma, medium-temp., medium-power
6 NH3 200 50 NH3 plasma, medium-temp., high-power
7 NH3 700 20 NH3 plasma, high-temp., medium-power
8 NH3 700 50 NH3 plasma, high-temp., high-power
Firzalaila Syarina Md Yakin, Mohd Faizol Abdullah, Siti Aishah Mohamad Badaruddin et al. Materials Today: Proceedings 42 (2021) 2996–3001
2997

3. the rGO sheet, specifically indicates the reduction in carrier con-
centration [10]. We believe medium-temperature NH3 plasma
treatment using either medium or high-power did not effectively
dope the rGO with N atom, contributes to a small degree of further
de-oxygenation, and induces more defect sites due to the plasma.
For the high-temperature NH3 plasma treatment of S7 and S8,
medium-power also results in higher I(D)/I(G) compared to the
high-power plasma. Differently, I(G) was shifted to a higher
wavenumber. Shifting of I(G) in the case of high-temperature
NH3 plasma treatment suggests the increase in electron concentra-
tion in rGO film contributed by pyridinic-N and quaternary-N dop-
ing [11]. The narrowing of I(G) accompanied by the broadening of I
(D) was the sign of the improved quality of crystalline C–C sites.
The crystallite size, La can be estimated from Raman spectra using
the empirical formula of Tuinstra-Koenig relation [12], where La =
(2.4 10 10
)k4
/[I(D)/I(G)]. The value of k is according to Raman
laser which is 432 nm. Since La was dependent on I(D)/I(G), there
were only small changes in La after plasma treatment, where it
shrunk to a range of 5.51–6.64 nm from the original value of
6.67 nm.
3.3. Spectral ellipsometry analysis
The change in the polarization state of the reflected light from
the film is measured in terms of ellipsometry W and D. They are
related to the complex number of q = rp/rs = tanW.exp(iD), where
rp and rs are the Fresnel amplitude reflection coefficients for p-
and s-polarized light [13]. We use two-phase structures of ‘air/
rGO/SiO2/substrate’ to fit the W and D. Fig. 3 shows the measured
and fitted W and D from the rGO film. A sharp D dip indicates the
measurement angle 75° is near Brewster’s angle. We fit the W and
D spectra using a combination of the Tauc-Lorentz (T-L) and Tan-
guy model, tuned by the Levenberg-Marquardt algorithm accord-
ing to our previous report [14]. The T-L model is typically applied
to the parametrization of dielectric functions of amorphous semi-
conductors and insulators in the interband region, while the Tan-
guy model provides a possibility to calculate the optical
properties of highly excited semiconductor structures near the
bandgap when a large carrier density is present. High fitting qual-
ity with R2
exceeding 0.999 was obtained for rGO samples before
and after plasma treatment.
Fig. 1. Surface condition of: (a) S1. (b) S2. (c) S3. (d) S4. (e) S5, (f) S6. (g) S7. (h) S8.
Firzalaila Syarina Md Yakin, Mohd Faizol Abdullah, Siti Aishah Mohamad Badaruddin et al. Materials Today: Proceedings 42 (2021) 2996–3001
2998

4. Fig. 2. Raman spectra for: (a) GO de-oxygenated into rGO. (b) rGO film after plasma treatment.
Table 2
Raman peak analysis of rGO film before and after plasma treatment.
Sample no. I(D) pos. (cm 1
) I(D) FWHM (cm 1
) I(G) pos. (cm 1
) I(G) FWHM (cm 1
) I(D)/I(G) ratio La (nm)
1 1378 109 1591 91 1.253 6.67
2 1374 103 1590 92 1.259 6.64
3 1373 100 1592 88 1.323 6.32
4 1368 94 1592 74 1.448 5.77
5 1373 99 1592 84 1.366 6.12
6 1370 97 1588 82 1.312 5.45
7 1372 93 1594 73 1.534 6.37
8 1372 106 1595 68 1.517 5.51
Fig. 3. Ellipsometry W and D for rGO using T-L and Tanguy model.
Firzalaila Syarina Md Yakin, Mohd Faizol Abdullah, Siti Aishah Mohamad Badaruddin et al. Materials Today: Proceedings 42 (2021) 2996–3001
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5. The thickness and optical constants in terms of refractive index
(RI) and extinction coefficient (EC) can be obtained from the fitting
W and D spectra with the Kramers-Kronig consistency check. The
absorption coefficient, a can be expressed as a = 4p(EC)/k. This
allows us to extract the optical bandgap, Eg by Tauc plot method
of (ahv)2
versus hv [15], with an assumption of direct band transi-
tion in every sample. Table 3 compares the optical constants (at
632.8 nm) and the thickness of the rGO film from the ellipsometry
fitting of the structure.
Thermal reduction greatly reduced the thickness from 14 nm
GO to only 2.73 nm rGO. Initially, the value of RI and EC of GO
was 1.782 and 0.008, respectively. The increase in optical constants
after thermal reduction represents stronger light absorption in
rGO, which resembles graphene [16]. Considerably high Eg of
3.755 eV measured from rGO is coming from the remaining oxi-
dized sites on the lattice. However, optical Eg will not directly
determine the electrical conduction on the film as the carrier trans-
port on rGO is by variable-range-hopping mechanism [17]. We
learned that low-power H2 plasma treatment at low- and
medium-temperature in S2 and S3 lowered the Eg without adjust-
ing much on the optical constants. This suggests further de-
oxygenation occurred on the residual oxidized sites by replacing
several oxygen functional groups with C–H termination. For the
S5 with slightly lower Eg than reference rGO, medium-
temperature medium-power NH3 plasma treatment with ineffec-
tive N-doping just induces a tiny further de-oxygenation process.
Low-power high-temperature H2 plasma in S4 etch the rGO
sheets, therefore results in high Raman I(D)/I(G) and large Eg of
3.978 eV. For both medium-temperature high-power S6 and
high-temperature medium-power S7 NH3 plasma treatment, N-
atom doping and defect by plasma etching results in high Eg. Sub-
stitutional doping by N-atom into graphene lattice is known to
open the Eg of graphene [18]. We suggest the effect of Eg widening
by N-atom doping and defect by plasma etching are auxiliary pro-
cesses to each other. Therefore, S8 after high-temperature high-
power NH3 plasma treatment exhibited the largest Eg of 4.360 eV
while at the same with the lowest optical constants due to exces-
sive thinning by plasma etching.
3.4. Hall effect measurement analysis
Table 4 compares the electrical properties of each sample
obtained from the Hall effect measurement. Bulk values of carrier
concentration, n and conductivity, r were calculated from sheet
values concentration, ns and resistance, Rs with optical thickness
data from the ellipsometry. It follows Drude’s relation for 3-
dimensional conductivity r = qnm, where q is the elementary
charge constant and m is the carrier mobility. Reference rGO film
before plasma treatment was a p-type conductor (hole majority
carrier) high r approximately 448.9 S/cm. For the low-power H2
plasma treatment S2 – S4, the values of r were steadily reduced
down to 8.04 S/cm. This situation occurred due to the surface
roughening of rGO film by H2 plasma, which is affecting both n
and m. We notice the effect of surface etching and film thinning
was severe for all NH3 plasma-treated samples S5 – S8 and the r
eventually degraded to only 0.05 S/cm.
We can actually separate the discussion between the sheet and
bulk properties for the rGO sample after NH3 plasma treatment.
Medium-temperature medium-power S5 was having lower Rs
and ns compared to medium-temperature high-power S6. How-
ever, the r measured from S5 is higher than S6. This occurred
due to the thinning of rGO by medium-temperature NH3 plasma
overwhelmed the contribution of de-oxygenation. Since medium-
temperature NH3 plasma did not effectively dope the rGO with N
atom, the de-oxygenation only can mobilize hole carrier on con-
ductive C–C sp2
sites and increase hole ns. For the high-
temperature NH3 plasma treatment with effective N-doping, hole
ns decreased in S7 and eventually, the rGO became electron major-
ity carrier in S8 due to N-atom substitutional doping on graphene
lattice. Unfortunately, it still cannot overwhelm the effect of etch-
ing by plasma. Thinning of S8 results in low n and most probably
the continuity of rGO film across the sample surface cannot be
maintained, therefore justify its worst r value.
4. Conclusion
The reduction of GO into rGO does change the properties of the
materials towards conductive graphene, except with the lattice
defects. Plasma treatment on rGO film results in several distinct
conditions from the reference film, depending on the types and
parameters of plasma treatment. Low-power H2 plasma at low-
and medium-temperature just induces further de-oxygenation of
rGO with lower Eg of CAH termination. A little etching effect by
H2 plasma slightly reduces the m but noticeably affects the r.
Meanwhile, low-power H2 plasma at high-temperature left the
rGO film with many defective sites and large Eg due to plasma etch-
Table 3
Optical properties of rGO film before and after plasma treatment.
Sample no. Thickness (nm) RI (632.8 nm) EC (632.8 nm) Eg (eV) Fitting R2
1 2.73 2.516 1.329 3.755 0.99970
2 2.99 2.619 1.333 3.542 0.99971
3 2.79 2.552 1.301 3.518 0.99971
4 2.84 2.428 1.169 3.978 0.99973
5 2.79 2.451 1.244 3.701 0.99972
6 2.67 2.396 1.197 3.938 0.99972
7 2.93 2.443 1.223 3.854 0.99971
8 2.33 1.997 0.776 4.360 0.99966
Table 4
Electrical properties of rGO film before and after plasma treatment.
Sample no. Rs (kO/h) m (cm2
/Vs) ns (cm 2
) n (cm 3
) r (S/cm)
1 8.16 6.17 1.32 1014
4.84 1020
448.90
2 6.25 4.67 2.27 1014
2.78 1020
196.01
3 8.13 3.27 2.36 1014
1.04 1020
54.01
4 19.23 3.49 9.48 1013
1.47 1019
8.04
5 9.58 5.49 1.23 1014
6.82 1018
5.79
6 11.12 3.93 1.49 1014
3.09 1018
1.87
7 14.70 4.29 1.12 1014
7.93 1017
0.48
8 60.23 10.31 3.83 1013
1.16 1017
0.05
Firzalaila Syarina Md Yakin, Mohd Faizol Abdullah, Siti Aishah Mohamad Badaruddin et al. Materials Today: Proceedings 42 (2021) 2996–3001
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6. ing. The reduction in r is significant compared to the low- and
medium-temperature H2 plasma treatment. For the medium-
temperature medium-power NH3 plasma treatment, it is not effec-
tive for N-doping. It contributes to further de-oxygenation without
changing much on Eg. A tiny etching effect by NH3 plasma slightly
reduces the m but noticeably affects the r. Medium-temperature
high-power and high-temperature medium-power NH3 plasma
induce N-doping but at the same time significantly etch the rGO.
Therefore, the number of defective sites and Eg were simultane-
ously increased. N-doping contributes to more electron on rGO
sheets, but the hole still the majority carrier across the film. They
significantly reduce the hole concentration and lower the r.
High-power high-temperature NH3 plasma rigorously induces N-
doping, but the extreme plasma etching results in film discontinu-
ity. It lefts large defect sites and high Eg. The film exhibited electron
as a majority carrier and the process severely degrades the r.
CRediT authorship contribution statement
Firzalaila Syarina Md Yakin: Methodology, Investigation, For-
mal analysis, Writing - original draft. Mohd Faizol Abdullah: Con-
ceptualization, Methodology, Writing - review editing. Siti
Aishah Mohamad Badaruddin: Validation, Resources, Project
administration. Mohd Ismahadi Syono: Validation, Supervision,
Funding acquisition. Nurhidaya Soriadi: Methodology,
Investigation.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgment
This project is internally funded by MIMOS Berhad.
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