This document summarizes research on a nanocrystalline graphite humidity sensor fabricated using plasma enhanced chemical vapor deposition. The sensor shows fast response times to changes in humidity, such as those caused by human breath. It is also durable against contaminants like sweat or saliva. The sensor is small and could be integrated into a wearable smart mask to continuously monitor breath humidity for health applications. Experimental results show the sensor resistance decreases sharply during exhalation and recovers during inhalation, demonstrating its ability to detect changes in breath humidity.

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Nanocrystalline graphite humidity sensors for
wearable breath monitoring applications
Ting Yang Ling
University of Southampton Malaysia
Iskandar Puteri, Johor, Malaysia
ivan.ling@soton.ac.uk
Harold M. H. Chong
School of Electronics and Computer
Science
University of Southampton
Southampton, UK
hmhc@ecs.soton.ac.uk
Lee Hing Wah
Nanoelectronics Lab
MIMOS Berhad
Kuala Lumpur, Malaysia
hingwah.lee@mimos.my
Suan Hui Pu
University of Southampton Malaysia
Iskandar Puteri, Johor, Malaysia
suanhui.pu@soton.ac.uk
John W. McBride
School of Engineering
University of Southampton
Southampton, UK
j.w.mcbride@soton.ac.uk
Abstract— A wearable humidity sensing platform based on
nanocrystalline graphite (NCG) resistive sensors for continuous
breath pattern analysis is reported in this paper. The sensor was
fabricated using plasma enhanced chemical vapour deposition
(PECVD) to grow graphitic nanocrystals directly onto SiO2
substrates without metal catalysts. The resulting sensor shows
suitably fast response, and high durability against
contaminants. The film also demonstrates good mechanical
stability and can be integrated with existing CMOS production
technology to produce a complete system on chip (SoC) sensor
for smart wearable applications. When exposed to human
breath (~100% RH) from room conditions (~40%RH), the
sensor experiences a measurable ~100 Ohm drop in resistance,
with a 2 seconds response from 100% RH to 40% RH. The
resulting sensor is small and can easily be integrated into a
wearable smart-mask for breath analysis.
Keywords—NCG, nanocrystals, humidity sensors, wearable
sensors
I. INTRODUCTION
The sampling of various parameters, such as skin
conductivity, body temperature, breath humidity and
breathing rate are helpful indicators of an individual’s health
conditions. To date, various flexible and wearable electronic
sensors have been proposed to continuously monitor these
parameters. For breath monitoring, there are three main
methods of detection, either through direct detection of
humidity or flow based sensor [1]–[3] through the detection of
chest movement due to breathing [4], [5], or through the sound
produced by the breathing person [6], [7]. Amongst them, a
sound-based sensing method suffers from environmental
noise, being applicable only if the user is in a quiet
environment. For chest movement, [5] presented an image
based breath monitoring system utilizing near-IR imagery.
However, this method is only limited to sensing when the
subject is in a known position. Hence, image and sound -based
sensing methods are only suitable for sleep monitoring
applications, or in hospitals, where the subject is not moving
around. Wearable sensors are often preferred for activity
monitoring, or for monitoring on the go. Humidity and flow
sensors are the best, but they are tricky to wear, as they need
to be positioned close to the user’s nose/mouth without being
too intrusive.
These sensors are often subjected to various
environmental conditions, resulting in exposure to sweat,
mechanical abrasions and other contaminants. Therefore, a
durable and robust sensor which could work under such
conditions would be desirable for wearable sensor
development. Amongst the sensing technologies available,
carbon-based sensors are the most promising, due to low
toxicity and much higher chemical stability than polymer-
based sensors. Previously, the sensing effects of NCG towards
different levels of humidity has been reported in [8], while its
mechanical stability was reported in [9].
PECVD deposited NCG is a promising material for
humidity and gas sensing due to the presence of high grain
boundary density. The humidity sensing property of the film
is attributed to the protonic conduction of water molecules
adsorbed on the film, creating regions of lower resistance,
thus, creating a measurable drop in resistance across the film.
II. THEORY
The conduction mechanism is believed to be dominated by
the Grotthuss mechanism, where a charge is conducted by the
adsorbed water via protonic conduction on the adsorbed
surface of the film. The protonic transfer process can be
thought of as the exchange of covalent OH-
bonds with the
hydrogen bonds of the neighbouring water molecules [10],
[11]. Due to the granular structure of the NCG films, the inter-
grain boundaries create regions of higher resistivity. As water
molecules adsorb on the surface of the film, shunts are created
by protonic conduction, thus causing a measurable drop in the
film resistance.
Fig. 1. The resistance model for NCG film; red circle shows the grain
boundary of the film, where the resistance can be modulated.

2. The bulk of the current should flow through the crystal-
grain boundary pathway. Fig. 1 shows a resistance model
developed for thin-film NCG. Since the resistance for the
current to flow between the grain (Rintergrain) is assumed to be
much greater than the crystal-grain boundary pathway, it can
be neglected, and the resistance change in parts of the film
where shunts are created by water molecules can be
expressed as shown in Equation (1).
∑ ∑ (1)
When measured with a multimeter and needle probes, the
overall contact resistance due to the probes and multimeter
must be considered. The overall NCG film resistance can then
be shown in Equation 2, while the measured resistance is
shown in Equation 3.
(2)
(3)
The overall magnitude of Rvariable is being modulated due to
the presence of shunts which lowers the overall magnitude of
Rboundary. As the relative humidity of the environment
increases, the frequency of water vapour adsorption exceeds
that of the desorption frequency. Hence, the resistance will
drop. If the relative humidity of the environment decreases,
the desorption rate will be higher, causing an increase in
resistance.
III. FABRICATION PROCESS
The NCG films were deposited directly into a 6-inch,
thermally oxidized (450nm SiO2 grown at 1000 o
C), p-Si
wafers using an Oxford Instruments Nanofab Agile. The
substrate was placed on the lower electrode of the deposition
chamber and grounded, while the upper electrode is excited
with an RF-source to produce plasma at 100 W. The carbon
source used for the deposition is methane (CH4) with
hydrogen (H2) added as diluent. For the NCG deposition, the
chamber pressure was maintained at 1500 mTorr, while the
lower electrode temperature was set to 850 o
C. The deposition
process is illustrated in Fig. 2.
Fig. 2. Illustration of the PECVD process
The mixed process gas is introduced into the PECVD
chamber via the upper electrode. The upper electrode consists
of a showerhead structure designed to introduce gas
uniformly across the whole deposition area. Waste gas is
pumped out from the chamber via the exhaust port located
underneath the bottom electrode. Hydrocarbons that are
deposited onto the sample will desorb due to the high surface
temperature and be purged by the flow of waste gas flowing
out of the chamber.
IV. EXPERIMENTAL PROCEDURE
The resistance of the strip is measured using a Keithley
2001 high-precision multimeter before and after exposure to
exhalation. Human breath is channelled through a 6mm
diameter polyurethane tubing to the top of the sensor module.
The pipe is fixed perpendicularly 2 cm above the sensor. The
measurement configuration is shown in Fig. 3.
Fig. 3. Illustration of the experimental setup for breath sensing test
The resistance measurement was carried out using a 2-
point probe resistive measurement on a meandered sensor
(Chip A) measuring 1000 µm by 10 µm, as shown in Fig. 4.
The heating pads on the side of the sensor were not activated
at the time of measurement to obtain preliminary results. The
overall footprint of the sensor is 400 x 900 µm2,
, including the
Ti-Al heating pads.
Fig. 4. SEM image of the sensor used for breath sensing, referred to as Chip
A (Length = 1000µm, Width = 10µm, Thickness = 200nm)
To simulate the effect of sweat or saliva on the sensor,
1.0 M KCl solution was dripped onto a larger NCG humidity
sensor with SU8 coating to isolate the Ti-Al contact layer from
the liquid (see Fig 5). The baseline resistance before and after
exposure to the KCl solution was recorded.

3. Fig. 5. Test device with KCl solution drop on Chip B. Chip B was wire
bonded to a customized PCB for ease of measurement.
Fig. 6. Picture of the small sensor chip (Chip A) with a British pound for
scale. Red circle shows region where the SEM image was taken.
The larger device (Chip B) has a width of 600 µm and
length of 3000 µm, and is wire bonded to a customized PCB
board for ease of measurement. Both Chip A and Chip B were
diced to the same dimensions (1cm by 1cm), as shown in Fig.
5 and Fig. 6.
V. RESULTS AND DISCUSSION
A. Response towards human breath
The response of the NCG humidity sensor towards human
breath moisture was recorded for 2 deep inhalation and
exhalation cycles over a period of 40 s (~20 s per cycle). Each
inhalation and exhalation event takes 2 s, with an
approximately 10 s pause between each inhalation/exhalation
event.
Fig. 7. Time response of the sensor to human breath. (a) Exhalation (b)
Hold (c) Inhalation (d) Hold. The resistance is normalized to baseline at room
relative humidity (RH) level, 40% RH, to the maximum RH level,
~100%RH. The baseline resistance is 56.32 kOhm.
Fig.7 shows the response time of the sensor during a
human breathing cycle. For each exhalation cycle (a), the
sensor’s resistance drops sharply. However, during the hold-
time before exhalation (b), the resistance of the sensor
increases. This is due to the desorption of moisture from the
surface of the film to the surrounding. During the inhalation
cycle (c), the resistance of the film rose slightly faster due to
the faster desorption of water molecules from the film surface,
as dry air is being sucked towards the opening of the pipe.
Nearing the end of (c), the signal saturates, as most of the
adsorbed water molecules has desorbed, and the film returns
to its baseline state.
B. Sensor recovery after KCl exposure
The sensor baseline for Chip B was established by taking
the average resistance in air at ~40% RH for 80 s. Then, a
single 20 µL drop of 1.0 M KCl solution was dripped onto
Chip B and left on the chip for 60 seconds. After that, the KCL
solution was removed using absorbent filter paper before
another drop was added.
Fig. 8. Resistance drop of test structure in Chip B due to KCl solution. (a)
1.0 M KCl drip applied (b) Hold time (c) Drip cleaned up with filter paper
before another drip was applied. Normalized to baseline resistance in air
(13.89 kOhm).
As shown in Fig. 8, the strip shows an almost instant drop
in resistance when the KCl solution was dripped, which is
expected, due to the higher conductivity of the KCl solution.
However, when the solution was left on the chip, it was
observed that the resistance was slowly increasing with time.
This was not expected. If the droplet evaporates with time, one
would expect the concentration to increase, and thus,
increasing the conductivity of the solution, as seen in [12].
Upon closer inspection, it was observed that the contact area
between the droplet and the strip decreases with time, hence,
causing a slight increase in the strip’s measured resistance.
When the KCl drip was removed, the resistance returned
to its original baseline value. No visible signs of scratching
can be found on the film after droplet removal using clean
filter paper.
C. Film characterization
Fig. 9. SEM image of the NCG film

4. The structure of the film was characterized with SEM and
Raman spectroscopy to verify its morphological structure.
The Raman spectrum was carried out with a Nano Raman
(NDT-MT Integra Spectra). The SEM image of the NCG film
shows clear granular structure of graphitic nanocrystals of
around ~30nm, surrounded by grain boundaries.
The Raman spectra shows high D-peak, which can be
attributed to the high inherent defect density of the NCG film.
The high defect density is due to the high density of grain
boundaries, resulting in a material which is well-suited for gas
sensing. As the intensity of G peak and D peak corresponds to
the presence of carbon pairs and rings in a film, it is possible
to determine the degree of amorphization of a film by
observing the change in ratio between the two peaks, and the
shift in the G peak. The Raman spectra of the film corresponds
to the nanocrystalline phase of the three-stage model of carbon
amorphization proposed in [13].
VI. CONCLUSION
In this study, NCG is shown to have desirable traits for
moisture sensing making it suitable for integration in a
wearable smart mask system for breath monitoring. The
PECVD NCG was shown to experience a clearly measurable
drop in resistance when exposed to human breath. Exposure
to a high-concentration KCl led to a distinguishable drop in
resistance, simulating the presence of sweat or saliva on the
surface of the sensor. Upon cleaning, the sensor regains its
baseline resistance level and humidity sensing ability.
ACKNOWLEDGMENT
This work is supported by studentships from the School of
Engineering at the University of Southampton, and the
Malaysian Ministry of Education under the MyBRAIN15
MyPhD scholarship. SEM and Raman spectroscopy
characterisation were performed in collaboration with
MIMOS Bhd.
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