This document summarizes research on an inkjet-printed graphene-based flexible humidity sensor. Key points: - Researchers from MIMOS Berhad developed a humidity sensor consisting of an interdigitated electrode made from reduced graphene oxide and silver nanoparticle ink, with graphene oxide as the sensing material, printed on a flexible polyethylene terephthalate substrate. - The graphene oxide sensing material's dielectric constant changes with humidity, allowing the sensor to detect humidity levels based on capacitance measurements. - Testing showed the sensor achieved a highest sensitivity of 0.15pF/%RH at 50°C, with a linear response at low humidity and exponential increase above 60%RH. It also demonstrated good temperature dependence below

1. 2020 IEEE International Conference on Semiconductor Electronics (ICSE)
978-1-7281-5968-3/20/$31.00 ©2020 IEEE
Inkjet-Printed Graphene-based Flexible Humidity
Sensor for Environmental Applications
Suraya Sulaiman
Nanoelectronics Lab
MIMOS Berhad
Kuala Lumpur, Malaysia
suraya.sulaiman@mimos.my
Nora’zah Abdul Rashid
Nanoelectronics Lab
MIMOS Berhad
Kuala Lumpur, Malaysia
norazah@mimos.my
Aiman Sajidah Abdul Aziz
Nanoelectronics Lab
MIMOS Berhad
Kuala Lumpur, Malaysia
aiman.sajidah@mimos.my
Liew Qiao Jun
Nanoelectronics Lab
MIMOS Berhad
Kuala Lumpur, Malaysia
qj.liew@mimos.my
Syed Muhammad Hafiz Syed Mohd Jaafar
Nanoelectronics Lab
MIMOS Berhad
Kuala Lumpur, Malaysia
hafiz.jaafar@mimos.my
Abstract—In this study, a fully flexible inkjet-printed
graphene-based capacitive humidity sensor is investigated
using graphene oxide (GO) as sensing material mixed with
Polyacrylic Acid, PAA. The printed humidity sensor consists
of i) an optimally designed interdigitated electrode (IDE)
made from reduced graphene oxide conductive ink material
decorated with silver nanoparticle (rGO/Ag), ii) inkjet-
printed GO as sensing material and iii) polyethylene
terephthalate, PET as flexible substrate. The conductive ink
is synthesize in house with a stable and consistent
temperature setting which has successfully improve the yield
performance of the graphene-based inkjet and the method of
preparation has prevent the ink from agglomeration during
printing. The multilayer inkjet-printed GO on top of the
rGO/Ag IDE is employ to increase the contact area between
the sensing layer and water vapor, thus leading to a high
sensitivity sensor. The results shows that as the relative
humidity is varies in the range of 30%RH to 90%RH, the
proposed humidity sensor is able to achieve a highest
sensitivity of 0.15pF/%RH at 50°C. The sensor show a stable
capacitance value with a linear output at low humidity and
increase exponentially at above 60%RH. The developed
sensor also show a good temperature dependence at
temperature below 60°C and humidity lower than 70%RH.
Keywords—flexible; printed; multilayer; graphene;
humidity
I. INTRODUCTION
The high demand for future electronic devices,
particularly on wearable device, seek a huge demand for
new production and manufacturing methods beyond the
traditional semiconductor cleanroom processes, which are
typically achieved at vacuum and high temperature
condition. In view of that, inkjet printing with the use of
nano-based conductive inks offers a high potential to
support the development of the next generation wearable
electronic devices due to its lower cost, low temperature,
less complex and vacuum-less process technologies.
The development of printing technology with the
formulation of a new simple, low-cost conductive ink
material will enable the production of a range of functional
sensors and electronics devices including humidity
sensors. Humidity sensors have been widely used in our
daily lives and become increasingly important in many
applications such as meteorology, agriculture, smart
homes, medical equipment, and biotechnology [1–4].
Various types of humidity sensors are available, based on
different measurement principles such as resistive,
capacitive, optical, acoustic, and thermal techniques.
Among them all, the capacitive devices are preferred due
to their high sensitivity, linearity, accuracy, fast response
time, and negligible temperature dependent. Most of these
capacitive sensors are based on the dielectric changes in
the sensing layer upon water vapour uptake as a measure
of the water vapour content in the environment.
II. SENSING PRINCIPLES
This paper propose a flexible, high-sensitivity and low-
hysteresis printed capacitive humidity sensor consists of
inkjet-printed IDE and GO as the sensing materials. This
capacitive humidity sensors utilize GO as the sensing
material which reacts to the environmental changes when
the surfaces adsorb water molecules. The change in
capacitance is directly in relation to the water
concentration or humidity in the environment. The IDE,
acting as the electrical conductor, is printed using
conductive rGO/Ag materials while the sensing material is
printed on top of the IDE to form the high-sensitivity
flexible humidity sensor as shown in Fig. 1 below. Here,
the sensor platform is equipped by inserting the inkjet-
printed sensor into a connector which is soldered to PCB
board that was attached with Harwin connector for use in
sensing test later.
Fig. 1. Samples of inkjet-printed graphene-based flexible humidity
sensor.
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2. III. MATERIAL CHARACTERIZATION AND SENSOR
DESIGN
Transmission electron microscopy (TEM) measurement
is use to determine the morphology and size of the Ag
nanoparticle. It is also able to confirm the presence of Ag
in anchoring onto the rGO nanoflakes matrix in the
synthesised conductive ink. In Fig. 2, it is shown that the
reduced GO flakes were decorated by abundant of well-
disperse spherical Ag nanoparticles. Fig. 3 shows the
measured lattice fringe spacing of Ag nanoparticles is
0.239 nm which corresponds to the (111) crystal plane of
the Face Centered Cubic, FCC lattice. Next, the rGO/Ag
ink was further investigated using Energy-Dispersive X-
ray Spectroscopy (EDS) equipment. The peaks obtained in
EDS spectra have been identified as carbon (C), oxygen
(O), copper (Cu) and silver (Ag) as the main elemental
constituent of the synthesized rGO/Ag conductive ink.
The elemental chemical analysis revealed that a higher
share for C (94.11%), Cu (5.08%) O (0.18%) and Ag
(0.61%) is due to the basic elemental composition of
rGO/Ag. The weight and atomic proportion of identified
elements is given in Table 1.
The size of this sensor is 10,000 μm by 4,800 μm with
a sensing area of 4200 μm by 4800 μm. The finger length
of the interdigitated electrode is 500 μm with the width
and the gap both being 200 μm and 300 μm. A total of 4
pairs of IDEs are used. The rGO/Ag ink deposition on the
PET substrate was performed using a Fujifilm Dimatix
Materials Printer (DMP-2831) as shown in Fig. 4. For this
paper, 10 pL cartridges having 16 nozzles of diameter 21
μm that are driven by the piezoelectric element of the
Dimatix printer are used.
Table II shows the experimental printing parameters of
the IDE platform design. The parameters that control the
printing quality are jetting waveform, jetting voltage,
jetting frequency, cartridge temperature, platen
temperature and resolution of the pattern. To generate
optimum printing quality, these parameters were
optimized after multiple test printing. Table III shows the
optimal printing parameters used to print the humidity
sensors. Fig. 5 shows the final printing outcome of the
flexible IDE platform using the printing parameters stated
in Table 2 and Table 3.
Fig. 2. TEM view of rGO/Ag showing 0.61 wt.% of Ag.
Fig. 3. Silver nanoparticles with fringe spacing confirming the
attachment to the rGO sheets.
TABLE 1. ELEMENTAL COMPOSITION OF GRAPHENE-BASED INK
OBTAINED FROM EDS ANALYSIS
Element Weight % Atomic %
C (K) 94.11 98.77
O (K) 0.18 0.14
Cu (K) 5.08 1.00
Ag (L) 0.61 0.07
Fig. 4. Fujifilm Dimatix Materials Printer.
Fig. 5. Flexible IDE Platform.
TABLE II. EXPERIMENTAL PRINTING PARAMETERS
TABLE III. OPTIMAL PRINTING PARAMETERS
Jetting Voltage 15.4 V
Jetting Frequency 20 kHz
Drop Size 10 pL
Printing Print Height 200 μm
Cartridge Temperature 27°C
Platen Temperature 30°C
IV. RESULTS AND DISCUSSIONS
Capacitive humidity sensor respond to humidity change
by varying its dielectric permittivity. The variation is
directly proportional to the water vapour change. The
capacitance of the capacitive humidity sensor is expressed
as
C = H0Hr (A/d) (1)
where H0 is the permittivity of vacuum, Hr is the relative
permittivity of the sensing material, A is the effective
sensing area of the humidity sensor, and d is the thickness
of the sensing layer. The permittivity of the sensing
material changes proportionally with water vapour. By
measuring the changes in the capacitance, humidity
change is detected. Graphene's permittivity varies with
frequency. At frequencies near direct current, it is near
6.9. Water has dielectric permittivity of ~ 80. As humidity
Parameter Value Unit
Drop Spacing (DS) 16 μm
Number of printing layers (rGO/Ag) 2 layers
Number of printing layers (GO) 1 layer
Sintering Temperature None °C
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3. increases, more water will be absorbed by the GO hence
higher permittivity will be observed.
Before testing took place, initially four samples
of flexible printed humidity sensors were placed inside the
bench top of Votsch Climatic Environmental Chamber
Model 7010 at humidity and surrounding temperature of
30%RH and 25°C, respectively to allow sensors to
stabilize. Testing frequency of the LCR meter was set at
150 Hz and the direct current voltage of 1V was used.
After that the capacitive response were varied towards
relative humidity at varying temperatures of 25°C, 40°C,
50°C and 60°C. All the measurement were recorded
manually using laptop. The examples of the results are
shown in Fig. 6 and Fig. 7. The response of the device
towards increasing humidity was recorded from 30%RH
to 90%RH with increment/decrement steps of 10%RH
while the temperature of the chamber was maintained at
constant temperatures of 25°C, 40°C, 50°C and 60°C. At
every temperature, the relationship of capacitance versus
%RH was observed. For all samples the increase in
capacitance was linear up to 60%RH and then increased
rapidly at higher humidity levels. This can be attributed to
the fact that GO absorb more water molecules at higher
humidity levels.
Next, hysteresis test was performed on Sample 3
to further investigate the sensor behaviour. Fig. 8 and Fig.
9 show typical absorption-desorption characteristic of the
humidity device at 40°C and 60°C. The sensors was
placed inside the humidity chamber and were subjected to
30%RH to 90%RH at 40°C and 60°C. Next, the same
sensors were subjected to decrease in humidity from
90%RH to 30%RH. Fig. 8 and Fig. 9 show that the
hysteresis increases at the beginning, reaches its
maximum value at 90%RH, and then decreases with a
further decrease in the %RH. We can conclude that the
sensors was able to absorb and desorb the water vapour
effectively at all selected humidity condition.
Next, the temperature dependence test of the sensor with
relative humidity variation from 30%RH to 90%RH were
investigated at temperature 25°C, 40°C, 50°C and 60°C.
The results for humidity range in between 30%RH to
90%RH are shown at Fig. 10 (a-b) and Fig. 11 (a-b). For
example, for Sample 1 and Sample 3, the slope of the
curves does not interfere with the adjacent temperatures at
below 60°C and below 70%RH, which means that for
these sensors the temperature does not influence on the
sensitivity of the humidity sensors. This result is a good
indication for temperature-independent sensor
applications. However, when the sensors are tested at
60°C and above 70%RH, the sensors behaviour are no
longer acted as temperature dependence.
Next, the temperature dependence test of the
sensor with relative humidity variation from 30%RH to
90%RH were investigated at temperature 25°C, 40°C,
50°C and 60°C. The results for humidity range in between
30%RH to 90%RH are shown at Fig. 10 (a-b) and Fig. 11
(a-b). For example, for Sample 1 and Sample 3, the slope
of the curves does not interfere with the adjacent
temperatures at below 60°C and below 70%RH, which
means that for these sensors the temperature does not
influence on the sensitivity of the humidity sensors. This
result is a good indication for temperature-independent
sensor applications. However, when the sensors are tested
at 60°C and above 70%RH, the sensors behaviour are no
longer acted as temperature dependence.
Fig. 6. Measured capacitance of the humidity sensors during relative
humidity changes from 30%RH to 90%RH at 25°
C.
Fig. 7. Measured capacitance of the humidity sensors during relative
humidity changes from 30%RH to 90%RH at 40°
C.
Fig. 8. Hysteresis characteristics of the humidity sensor in relative
humidity variation from 30%RH to 90%RH at 40°
C.
Fig. 9. Hysteresis characteristics of the humidity sensor in relative
humidity variation from 30%RH to 90%RH at 60°
C.
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4. Fig. 10 (a)
Fig. 10 (b)
Fig.10. (a) Temperature dependence of the humidity sensor measured at
below 70%RH, at 25°C, 40°C, 50°C and 60°C for Sample 1 and (b)
Temperature dependence of the humidity sensor measured at below
70%RH, at 25°C, 40°C, 50°C and 60°C for Sample 3.
Fig. 11 (a)
Fig. 11 (b)
Fig.11. (a) Temperature dependence of the humidity sensor measured
over 70%RH, at 25°C, 40°C, 50°C and 60°C for Sample 1 and (b)
Temperature dependence of the humidity sensor measured over 70%RH,
at 25°C, 40°C, 50°C and 60°C for Sample 3.
Next, the sensitivity, S of the humidity sensors can be
expressed as;
S = C90 – C30 (pF/%RH) (2)
90 – 30
where C90 and C30 denote the capacitance obtained at
90%RH and 30%RH, respectively [5]. The 90 and 30
values are the highest and lowest relative humidity values
in the variation range, respectively. Fig. 12 shows the
sensitivity of humidity sensors at different temperature
variation for all sensors. From the histogram, sample 4
can be concluded as the most sensitive sensor. The results
show that as the relative humidity ranges from 30%RH to
90%RH, the proposed humidity sensors are able to
achieve a high sensitivity of 0.15pF/%RH at 50°C.
Fig. 12. Sensitivity of humidity sensor at temperature range between
25°C to 60°C.
V. CONCLUSIONS
In summary, a fully flexible ink-jet printed graphene-
based capacitive humidity sensor is investigated and
successfully demonstrated working at certain humidity
and temperature range. The humidity-sensitive layer
composed of a GO layer is successfully printed on IDE
platform which manage to simplify the fabrication process
of a conventional humidity sensor. During actual testing,
this paper demonstrated a sensor with a high sensitivity of
0.15pF/%RH at 50°C for Sample 4 with stable
capacitance value with an exponential output ranging
from 30%RH to 90%RH. It has been shown that the
developed sensor is also independent of temperature
change at temperature less than 60°C and humidity level
below 70%RH. More process optimization and
investigation need to be carried out later to improve the
sensor performance which include sensing thickness
optimization and optimum printing quality.
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151
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