Au – MoS 2 nanoflowers sensors on interdigitated electrode for monitoring human respiration

Abstract

Respiratory humidity sensors are essential for monitoring breath conditions in a variety of applications such as respiratory disease diagnosis, sleep apnea screening, and personalized medicine. With the increasing prevalence of respiratory diseases, such as asthma, chronic obstructive pulmonary disease (COPD), and sleep apnea, they have become essential tools for healthcare professionals, researchers and individuals. However, existing humidity sensors often suffer from poor sensitivity, slow response, and limited flexibility. In this paper, we report a novel Au – MoS2 respiratory humidity sensor that exhibits high resistivity, fast response, and excellent flexibility. The sensor is fabricated by drop casting a thin layer of Au decorated MoS2 on copper interdigitated electrodes (fabricated using copper clad PCB). The increase in the intensity of the peak was observed in Au – MoS2 hybrids in comparison to MoS2 analyzed using micro–Raman Spectroscopy. Using scanning electron microscopy (SEM), the morphology of MoS2 was observed, confirming its flower like structure. The Au nanoparticles were detected on the edges of MoS2, confirmed by transmission electron microscope (TEM).
The interfacial electronic interaction suggests that there is the formation of Schottky barrier between Au and MoS2 as confirmed by X–Ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The improved electronic interaction results in better sensing properties. The high dielectric constant and excellent surface-to-volume ratio of the MoS2 layer make it extremely sensitive to humidity; the Au nanoparticles offer good electrical conductivity and mechanical flexibility in its place. The study tested a respiratory sensor in a simulated environment, finding it more responsive to breathing and quick to return to rest. This suggests potential for the Au-MoS 2 sensor in tracking breath states due to its sensitivity and adaptability, promising applications across various fields.

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Au – MoS2 nanoflowers sensors on interdigitated electrode for
monitoring human respiration
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2024
Nano Ex.
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1
Au – MoS2 nanoflowers sensors on interdigitated electrode for monitoring
human respiration
Sakshi Pujari1¥, Himadri Tripathi1¥, Anshuman Dobhal1, Saurabh Rawat1, Shivani Dangwal1,
Shashi Bala2, Chanchal Rani3, Charu Dwivedi4, Rajesh Kumar3, Mohit Sharma5, Himani
Sharma1*
1Functional nanomaterials Research Laboratory, Department of Physics, Doon University Uttarakhand-
248001, India
2Government Arya Degree College Nurpur-176202, Kangra, Himanchal Pradesh, India
3Materials and Device Laboratory, Department of Physics, Indian Institute of Technology, Indore-453552,
India
4Department of Chemistry, Doon University Uttarakhand-248001, India
5Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research),
2 Fusionopolis Way, Innovis, #08-03, Singapore 138634, Singapore
Corresponding Author: Himani Sharma
Email ID: himanitiet427@gmail.com
Abstract
Respiratory sensors are emerging as powerful tools for continuous, non-invasive monitoring of
breathing patterns. These sensors hold immense potential for applications in various healthcare
scenarios. In this paper, we report a novel transition metal dichalcogenides (TMDC’s) based
respiratory sensor that exhibits high resistivity, fast response, and excellent flexibility. TMDC
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turned out to be the most suited of all materials as they offer tunable bandgap with wider surface
area resulting in higher sensitivity to specific targets. MoS2 (Molybdenum Disulfide), a TMDC,
has emerged as a promising material for developing next-generation respiratory sensing devices
while its hybrids with Au nanoparticles enhances the electrical conductivity of the sensor,
providing better performance than either material alone. This paper explores the potential
advantages of Au-MoS2 based sensors for respiratory monitoring applications.
In context to that, present work focuses on the resistivity and flexibility of hybrids for
respiratory sensing. The MoS2 nanoflowers are synthesized by hydrothermal method further
using inert gas deposition method for the decoration of Au nanoparticles over MoS2. To form
Au-MoS2 hybrids, Au nanoparticles were decorated over MoS2 using inert gas deposition. The
crystallinity of the hybrids is studied using Raman Spectroscopy and X-Ray Diffraction
techniques. The morphology of the samples was studied through scanning electron
transmission (SEM) while Au nanoparticles were detected on the edges of MoS2 by
transmission electron microscope (TEM). The interfacial electronic interaction and binding
energies were detected using X–Ray photoelectron spectroscopy (XPS) and ultraviolet
photoelectron spectroscopy (UPS). The study tested a respiratory sensor in a simulated
environment, finding it more responsive to breathing and quick to return to rest. This suggests
potential for the Au-MoS2 sensor in tracking breath states due to its sensitivity and adaptability,
promising applications across various fields.
Introduction
One of the most important aspects of evaluating health is breath monitoring. Conventional
approaches are less efficient in particular situations because they frequently depend on visual
observation or substantial equipment[2]. Respiratory sensors are useful in this situation. These
non-invasive tools are intended to observe, record, and evaluate breathing patterns.
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Respiratory sensors are instruments that can be used to monitor a variety of respiratory
parameters, including respiratory rate, respiratory volume, and exhaled gas composition[3,4].
These variables can be used to diagnose respiratory disorders and evaluate the condition of the
respiratory system. They are made to detect, measure, and interpret respiratory data in real-
time outfitted with cutting-edge technology [5].
Respiratory sensors can incorporate in fitness and wellness tracking. Additionally, it can aid
in the early detection of chronic diseases, post-operative care, and the identification of
possible sleep apnea or other breathing variations. With the rapid advancement of technology,
breathing sensors may soon find use in the monitoring of mental health[6].
Recently Transition metal dichalcogenides (TMDCs) based respiratory sensors have attracted
great attention. TMDCs have a tunable bandgap, which allows for the adjustment of their light
absorption and emission properties, enabling the development of sensors that respond to certain
light wavelengths, in contrast to graphene, which has a zero bandgap that makes it challenging
to target specific molecules that are necessary for sensing[7]. The target analyte, or the
substance being detected, might interact with the sensor material more readily due to its larger
surface area, which could result in a higher sensitivity when detecting the target's existence and
quantity[8]. When it comes to sensors that need to be highly sensitive to certain target
molecules, TMDCs present a strong solution[9].
Molybdenum disulfide (MoS2) is the most promising of all the TMDCs because of its
multilayered structure and special electrical and optical qualities, which have a potential future
in sensing[10]. The layered structure of MoS2 is like that of graphite and consists of sandwich
units with three atom layers (S-Mo-S) piled through Vander Waals forces and possess
exceptional electrical properties which has multiple possible usage for gas adsorption. The
detection of some gases by intrinsic MoS2 is still limited after several applications. In the field
of gas sensing, its low selectivity makes it easily triggered by other gases, such as water vapour
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or carbon dioxide[11]. Also, it is difficult to fabricate large area sensors which makes resist its
application in commercial purposes. However, its doping with other elements like Au, Cu, Pd
have been successful in enhancing the gas sensing performance[12]. Yan et al. (2016) provided
evidence of the effectiveness of enhancing the surface of metal oxide-based gas sensors with
noble metals (Au) to increase their gas detecting capability. They reported that Au particles
assist the detection of ammonia at low temperature because the presence of Au increased the
material's capabilities as a novel gas sensing material. Furthermore, Zhou et al. [13] in their
research how a layered MoS2 nanosheet with an Au nanoparticle could detect a NO2 gas in a
recoverable, sensitive, and selective manner at ambient temperature. He claimed that the added
Au particles boosted both the number of active adsorption sites and the diffusion rate in
addition to speeding up the transfer of adsorbed species from active to inactive sites. This opens
the door to a unique approach for room-temperature Au-MoS2 based hybrids to be used in the
construction of high-performance respiratory sensors[14].
In this work, we fabricated a mask that combines the benefits of gold and is built as a respiratory
sensor on MoS2.[11] The sensor is a resistive type which measures the human breath. The
sensor showed enhanced breathing patterns with a high rate of recovery when combined with
gold particles[15]. It is determined to be a portable system for monitoring human respiration
because of its quick response time and sensitivity[13,16–18].
Materials and Methods
Synthesis of MoS2 Nanoflowers based sensor:
MoS2 nanoflowers were synthesized using hydrothermal synthesis.[19] The standard precursor
concentration was determined to be 5 gm of thiourea (Sisco Research Laboratory) and 0.8 gm of
ammonium molybdate (Sigma Aldrich 99.9% pure), were made transparent by diluting them in
50 millilitres of deionized water (DI) and stirring them for 30 minutes with a magnetic stirrer.
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After that, the solution was autoclaved for eighteen hours at 200 °C in a 50-cc stainless steel
autoclave lined with Teflon [19]. Furthermore, solution was filtered to produce the black
precipitates, which were then five times, separately, cleaned with ethanol and distilled water. The
MoS2 nanoflowers was then formed by drying the resulting black precipitate at 70 °C in a hot air
oven [[20].
Synthesis of Au-MoS2 hybrids-based sensor:
Au nanoparticles were decorated via Inert gas deposition to form Au-MoS2 hybrids. Prior to being
placed inside the growing chamber, the substrate (glass slides) and Printed Circuit Board (PCB)
copper electrodes were fully washed with DI water. For gas sensing performance, PCB copper
electrodes were utilised, and tiny glass slides with a standard dimension of 10 mm×10 mm were
employed as the characterisation substrate as well. By resistive heating, 99% pure Au foil was
evaporated using a molybdenum boat [[19]. The deposition was carried out at a base pressure of
4×10-6 Torr by providing 80 A to the boat [[21]. A digital thickness monitor (DTM) was used to
measure the thickness of Au films. To prevent Au nanoparticles aggregation, these deposited
samples were subsequently annealed at 150°C to 200°C in an Argon gas environment. The size
of the nanoparticles is influenced by the difference in vapor pressure between the Au melting
surface and the film (which is being deposited) during a metallic film deposition process. Au-
MoS2 was then drop casted on the copper electrodes with thicknesses of 2 nm separately, at
deposition pressures of 4×10-6 Torr. During inert gas evaporation, the higher areal density of
nanoparticles at lower pressure is linked to the higher atom concentration, which encourages
accelerated nucleation and tends to create bigger nanoparticles. At lower pressures, compared to
higher pressures, the mean free path results in fewer collisions between Au atoms and gas
molecules, producing smaller nanoparticles. As a result, larger, denser Au nanoparticles would
form on the MoS2 nanoflowers as more Au atoms would reach the substrate's surface [[19].
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Results and Discussions
The crystalline structure of MoS2 and Au-MoS2 nanoflowers were confirmed by x-ray
diffraction (XRD). The diffraction pattern of bare MoS2 nanoflowers and Au-MoS2
nanoflowers can be observed in the figure 1. With an XRD pattern indexed at 13.11∘,
26.44∘,32.26∘, 35.53∘, and 57.07∘, corresponding to the (002), (004), (100), (103), and (110)
crystal planes of the MoS2 structure, both samples demonstrate the crystallite nature of MoS2
materials[22]. These results are consistent with the relevant standard card (JCPDS card number
371492). Additional minor peaks at 37.7° exist in the case of Au-MoS2 hybrids, and they
correlate to the Au (111) plane (JCPDS card number 040784)[23].
The strength of the peak at the (002) and (004) planes was lowered by the addition of gold
nanoparticles. This is because there are less well-defined layers that contribute to the signal
when gold particles are added, disrupting the ordered stacking of MoS2 layers and lowering
intensity. Because the addition of Au altered the material's crystallinity, frequently indicating
smaller crystallite sizes or a wider spread of crystallite sizes, the peak likewise became more
widely distributed at (004), (100), (103) planes [22].
Micro-Raman spectroscopy (µRS) was used to analyze the atomic vibrational modes of the bare
and Au decorated MoS2 hybrids [19]. The excitation wavelength used for Raman spectroscopy is
533 nm. Figure 2 represents µRS of bare MoS2 and Au-MoS2 hybrids. The peaks were observed
at 377 cm-1 and 405.4 cm-1 [24]. The gap of 28.4 cm-1 indicates that 2D flakes of the nano petals
of MoS2 are present in bulk. In addition to the frequency change, the Au-MoS2 hybrids show a
rise in peak intensity with increasing Au NPs size. The addition of Au nanoparticles slightly
shifted the peak at 383 cm-1 and 408 cm-1 inferring an increase in the layers of MoS2 from the
sample [[25]. Because a locally amplified electric field is created on the surface of the Au
nanoparticles, the Raman intensity increases dramatically when the plasmon oscillation frequency
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is in resonance with the photon frequency of the input light. There are side peaks for both E and
A mode in Au -MoS2 sample (Figure 2). The introduction of gold nanoparticles was the source of
the side origin of side peaks at the A1g and E12g modes [24]. The coupling of strain and lattice
vibrations, as well as the interactions of various phonon modes within the Au -MoS2 compound,
caused by the addition of Au, resulted in new peaks and altered surface morphology [22]. The
Raman spectra of MoS2 nanoflowers exhibit a third peak at 452 cm-1, which can be attributed to
the second-order longitudinal acoustic phonon (2LA(M) mode) at the M point [26]. Remarkably,
both modes' intensities increased as the reaction time increased, indicating that MoS2 nanoflowers'
crystallinity had improved. First-layer effect arising from the emergence of extra Raman modes
around 379 cm-1 and 397 cm-1 (the latter is called the A1g reduced mode in the following) is
specific to MoS2 in direct contact with the Au surface. The normal Raman modes get more intense
as the number of layers increases, while the extra modes nearly always stay at the same position
and intensity [22]. The mode around 379 cm-1 can be interpreted as the E12g mode of strained
MoS2.
The morphology of MoS2 was imaged using FESEM which confirmed the flower like structure
[26]. The findings showed that no additional nanoparticles were confirmed and that their
existence had no impact on the characteristics of MoS2 nanoflowers [22]. Figure 3(a) and 3(b)
depicts the same. Furthermore, TEM imaging [23] of bare MoS2 and Au-MoS2 hybrids was
performed as shown in figure 3(c) and 3(d) respectively. The HRTEM images attest to the
presence of Au nanoparticles and a structure like a flower. Au nanoflowers typically have a more
uniform and spherical morphology compared to MoS2 nanoflowers, which have a more irregular
and plate-like structure. Thus, as per SEM images and TEM analysis Au nanoparticles are
uniformly distributed across the surface of MoS2.
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XPS characterisation was used to further investigate the electrical interaction at the interface
between the Au-MoS2 hybrid and the bare MoS2 and Au-MoS2 hybrids, as well as the elemental
composition and chemical states of both samples (Figure 4). The obtained samples contain Mo,
S and Au, which are verified by the examination of the XPS patterns. The Mo3d XPS scan is
displayed in Figure 4(a) where two major peaks are observed at 228.3 and 232.5 eV,
respectively, and are attributed to Mo3d5/2 and Mo 3d3/2, the presence of the Mo4+ oxidation
state.
The lower side change in the binding energy is caused by the electrical interaction between Au
and MoS2 at the interface. The increased work function of Au could be an indication of the
formation of a Schottky barrier at the contact that transfers electrons from MoS2 to Au. As a
result of this contact, the interface is modified, which increases electron availability since Au
particles have a higher electronegativity and hence decreases hybrid resistivity and improves
the capacity to sense gases.
Figure 4(b) displays the Au XPS spectra for MoS2 and Au-MoS2 hybrids. The in situ generated
Au particles have been identified as the source of two unique peaks, Au 4f7/2 and Au 4f5/2,
respectively, at 84 eV and 87.7 eV. Figure 4(c) shows XPS spectra of S 2p. The S peak breaks
into two peaks at 162.8 eV and 161.6 eV, which correspond to the S 2p3/2 and S 2p1/2 orbitals
of S 2p, respectively[19]. These peaks are observed to be slightly wider and lower shift when
Au particles are decorated on the surface, indicating surface modification. The S 2p
nanoparticles were stabilized by the presence of Au, which also changed the electron charge
density surrounding the S atom and the binding energy and peak shape of the XPS spectrum.
Figure 4(d) displays the XPS spectra of the O 1s. Its peak was observed at 531 eV and 532.4
eV [19,27].
Figure 5 represents the valence band edge and secondary electron cutoff for MoS2 and Au-
MoS2 acquired from the UPS. As shown in figures 4(a)-(b), the valence band maxima for MoS2
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(MS) and Au- MoS2 (Au-MS) were 3.68 and 3.655 eV, respectively, which is 0.025 less than
that of MoS2. This discrepancy suggests that the Au-MoS2 material is slightly more reactive
and sensitive than MoS2 alone because it has a little inclination to receive electrons. The work
function obtained for bare MoS2 and Au-MoS2 hybrids were found to be 4.45eV and 4.23eV
respectively, which tells us that the work function is decreased by 0.22eV when the sensor was
decorated with Au on the surface. Moreover, it confirms that the Schottky barrier creation at
the interface is causing an electron transfer from MoS2 to Au nanoparticles. As a result, this
interaction improves the gas detecting capability[19,21].
Respiratory Sensors and Mask Design
The hybrid Au-MoS2 solution was evenly drop casted onto the PCB copper electrode of size
2.2cm ×3.2cm as shown in figure 6(a). The sensor was then mounted on a cotton mask so that
the wearer could easily operate it. To protect the copper-plated threads that functioned as
electrodes, an electrode cover made of a hydrophobic material was used [[27]. The thyristor
(sensor) was mounted in the mask so that it sits between the nose and the mouth and detects
breathing. This allowed the sensor to be stabilized at the correct location. Further, human breath
was recorded using the source metre (Keithley 2450) as shown in figure 6(b), and graphs were
created utilising the data [[8]. A schematic representation of the sensor incorporated into the
mask is shown in Figure 6(c).
Respiration Monitoring
The sensors work by detecting variations in electrical resistance when exposed to different
breathing patterns. The mechanism involves how the interaction between the oxygen molecules
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and the Au-MoS2 changes the electrical resistivity of the sensor [[17]. During inhalation,
oxygen molecules present in the air may interact with the surface of the sensor. The interaction
between them alters the sensor conductivity [[18]. This change in conductivity is thus measured
as a change in resistance. While during exhalation, a decrease in the oxygen level is observed,
leading to change in the sensor’s conductivity, which is again detected and processed [[28].
The respiratory levels were not measured independently along with a change in the
conductivity for the mask measurements because variations in the total charge of respiration
levels inside a mask depend on the moisture content of each user's breath and the fit of the
mask for each user [4]. This arrives to a speculative conclusion that variations in the breathing
within the face mask induced by the user's breathing modify the resistance of the final mask
platform as they breathe in and out [[29].
The sensor is capable of precisely detecting the moisture in a breath and recovering quickly
enough to detect the next breath. The signal quality was steady even when measuring breaths
outside. The lingering moisture in the mask between these breaths provides an explanation for
this. The resistance was observed to be continuous and capable of returning to its initial value
during each breathing study [[30,31]. Figure 7 displays the response curve that was created by
repeatedly measuring the user's breathing patterns in order to assess the sensor's repeatability.
The amplitude variations of the various breathing patterns were shown by the graphs in Figure
8. Normal breathing, as depicted in figure 8(b), lies in the amplitude range between shallow
breathing in figure 8(a) and deep breathing in figure 8(b) [32]. The increase in the electrode
resistance also suggested that the humidity level had increased. As a result, it concluded that
Au-MoS2 solution, rather than MoS2, was a superior choice for the sensor since it was very
sensitive and hence more easily able to capture changes in the breathing pattern [33].
The breathing response curve at various parameters is shown in Figure 9. Talking while
walking curve in figure 9(b) exhibits a pattern with considerable consistent fluctuations, while
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no talking while walking curve in figure 9(a) shows a dramatic decline in resistance in addition
to various variations. Furthermore, as shown in figure 9(c), the sensor was able to differentiate
between nasal breathing and regular breathing. It shows a smoother curve for nasal breathing
as opposed to oral breathing since the slopes obtained were varied based on the amount of
moisture expelled [[31].
Additionally, figure 9(d) presents a comparison between coughing and normal breathing,
revealing the relative changes observed in the device signals. When we cough, the resistance
changes more dramatically, making the slope steeper than it would be when we breathe
normally. [34]. A tabular representation of the latest research on respiratory sensors, with an
emphasis on the many metrics depicted in figure 10.
Conclusions
To summarise, the analysis of decorated copper electrodes covered with MoS2 and Au-MoS2 is
done to explore the possible benefits of Au over MoS2 based sensors concerning respiratory
monitoring applications. After being exposed to gold particles, the sensor shows a higher
recovery rate, which enhances our breathing patterns[35]. This makes the development of a
high-performing respiratory sensor possible. In this work, we constructed a mask that functions
as a respiratory sensor on MoS2 and combines the advantages of gold. X-ray diffraction (XRD)
was used to confirm the crystalline structure of MoS2 and Au-MoS2 nanoflowers, and the
results were consistent with the JCPDS. It was noticed that the ordered stacking of MoS2 layers
at (002) and (004) was disrupted by the addition of Au, decreasing its intensity and similarly
widely dispersed peaks at planes (004), (100), and (103). The SEM images confirmed the
successful formation of Au-MoS2 hybrids with distribution of Au nanoparticles on the MoS2
nanosheets. According to micro-Raman spectroscopy, the vibrational modes were confirmed
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with the peak positions of 377 cm-1 and 405.4 cm-1, indicating a change in frequency and an
increase in peak strength. Due to a locally enhanced electric field on the Au surface, the
inclusion of Au nanoparticles caused the peak to shift to 383 cm-1 and 408 cm-1. Strong
interactions with incident light caused by surface plasmon resonance on the gold nanoparticle
split the Raman Peak at 452 cm-1. It also demonstrates that as reaction times grew in tandem
with mode intensities, the crystallinity of MoS2 nanoflowers improved. Moreover, the Schottky
barrier formation at the interface was verified by UPS and XPS, providing us with evidence
that the Au-MoS2 sample has undergone modifications. The availability of electrons increased
as a result of the Schottky barrier formation raising Au's work function[36]. The slightly wider
and lower shift in the S peaks, which indicates the surface change, was also caused by the
addition of Au. Because the Au-MoS2 material has a little inclination to receive electrons, the
valence band maxima for both MoS2 and Au-MoS2 were 0.025 less, suggesting that the Au-
MoS2 material is somewhat more reactive. Similarly, the work function was reduced by 0.22
eV when the sensor's surface was decorated with Au. Au thus improved the sensor's properties
to some extent and had a high rate of recovery. Its sensitivity and fast response time lead to the
conclusion that it is a portable system for monitoring human breathing. With the speed at which
technology is developing, improving breathing sensors can help with post-operative care, early
chronic illness detection, and the diagnosis of potential sleep apnea or other breathing
abnormalities. Equipped with state-of-the-art technology, it may be made to detect, measure,
and interpret respiratory data in real-time.
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Figure1: XRD pattern of bare MoS2 and Au-MoS2 nanoflowers confirming their crystalline pattern.
Figure 2: Raman Spectra of bare MoS2 and Au decorated MoS2 confirming the formation of MoS2 in
bare MoS2 and Au-MoS2 hybrids.
10 15 20 25 30 35 40 45 50 55 60
110 110
103
100
004
002
004
200
110
103
002
100
Intensity (a.u.)
MoS2 Nanoflowers
2θ
Au-MoS2 Nanoflowers
200 300 400 500 600
Intensity (a.u.)
Raman Shift (cm-1)
MoS2
MoS2 -Au
E12g
A1g
2LA(M)
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Figure3: FESEM images of (a)-(b) bare MoS2 nanoflower, HRTEM images of (c) bare MoS2 (d)
Lattice fringes of MoS2-Au with Au particle size 2nm
Figure4: X-ray photoelectron spectroscopy of bare MoS2
and Au-MoS2 hybrids: (a)Mo 3d (b)Au 4f
(c)S 2p (d)O 1s. The peaks confirmed the presence of Mo, S and Au. The Schottky barrier
formation and electrical interaction at the Au-MoS2 hybrid interface are shown by a lower shift in
binding energy.
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Figure5: UPS spectra of MoS2 hybrids. By projecting the low binding energy edge to the baseline, the
intercept on the abscissa was used to estimate the valence band maximum for each spectrum.
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Figure 6: Schematic representation of: (a) Drop-casting on PCB Copper electrodes (b) monitoring
human respiration by Keithley source-meter (c) Respiratory sensor integrated into a mask.
Figure 7: Repeatability performance of the sample.
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Figure8: Response curve of the respiratory sensor (a)Slow breathing (b)Relaxed breathing (c)Fast
breathing.
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Figure9: Response curve at different parameters: (a) No talking while walking (b) Talking while
walking (c) Coughing (d) Oral-Nasal Breathing. These parameters showed a variety of patterns as
observed by Au-MoS2 based respiratory sensors.
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Figure10: Table showing the variation of sensors on previous work.
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