Highly sensitive wide range linear integrated temperature compensated humidity sensors fabricated using Electrohydrodynamic printing and electrospray deposition

Journal Pre-proof Highly sensitive wide range linear integrated temperature compensated humidity sensors fabricated using Electrohydrodynamic printing and electrospray deposition H.M. Zeeshan Yousaf, Soo Wan Kim, Gul Hassan, Khasan Karimov, Kyung Hyun Choi, Memoon Sajid

PII:

S0925-4005(20)30027-7

DOI:

https://doi.org/10.1016/j.snb.2020.127680

Reference:

SNB 127680

To appear in:

Sensors and Actuators: B. Chemical

Received Date:

5 November 2019

Revised Date:

22 December 2019

Accepted Date:

5 January 2020

Please cite this article as: Zeeshan Yousaf HM, Kim SW, Hassan G, Karimov K, Choi KH, Sajid M, Highly sensitive wide range linear integrated temperature compensated humidity sensors fabricated using Electrohydrodynamic printing and electrospray deposition, Sensors and Actuators: B. Chemical (2020), doi: https://doi.org/10.1016/j.snb.2020.127680

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Highly sensitive wide range linear integrated temperature compensated humidity sensors fabricated using Electrohydrodynamic printing and electrospray deposition H. M. Zeeshan Yousaf1, 4†, Soo Wan Kim2†, Gul Hassan3, Khasan Karimov1, Kyung Hyun Choi2, Memoon Sajid1,* 1, *

Faculty of Electrical Engineering, Ghulam Ishaq Khan Institute of Engineering Sciences and

Technology, Topi, Swabi, K.P. Pakistan. E-Mail: [email protected], Phone: +923468710421 Department of Mechatronics Engineering, AMM Lab, Jeju National University, Korea.

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Department of Electrical Engineering, Islamic International University Islamabad, Pakistan.

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Department of Electrical & Computer Engineering, COMSATS University Islamabad, Sahiwal Campus.

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† Both authors are considered as the first contributing authors of this manuscript

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Graphical Abstract

Highlights 1

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Integrated micro temperature and humidity sensors

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Real time temperature compensation for humidity sensors output

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Liquid mechanical exfoliation of MoS2 to achieve 2D flakes and particles suspended in PEO polymer matrix

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Detailed investigation of working principle and sensing mechanism

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Micron level size compatible with integration in MEMS based commercial devices

ABSTRACT Percentage relative humidity is dependent on environmental temperature but the fact is ignored in

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most of research on humidity sensors. Secondly, the physical size of sensors should be small enough to enable integration on a commercial microchip. In this work, an integrated micro temperature plus humidity sensor system is presented that compensates for the effect of surrounding temperature on the output

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resistance of humidity sensor. The sensors were fabricated using Electrohydrodynamic drop-on-demand (EHD-DOD) printing for the electrodes, and Electrospray deposition (ESD) for active layer fabrication of

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humidity sensors. Electrode line widths were 10 µm while the combined area of both sensors was ~2 mm2. Meander type silver patterns were used as resistive temperature sensors and interdigitated transducer (IDT)

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electrodes were used for humidity sensors. The active layer of humidity sensors was fabricated using a novel composite of Polyethylene Oxide (PEO) and 2D Molybdenum disulfide (MoS 2) flakes to achieve a highly sensitive (85 kΩ/%RH) and almost linear response for a wide detection range (0-80% RH) of relative

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humidity. A mathematical model relating the outputs of both sensors was developed to compensate for the effects of temperature. The system presents optimal solution for commercialization ready temperature

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compensated integrated micro temperature and humidity sensors fabricated through all printing techniques.

Key Words: EHD-DOD; Micro Printing; Integrated Sensors; Temperature Compensation; Relative Humidity

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1. Introduction

Monitoring of basic environmental parameters is crucial for weather information, industry, food,

health, and even space related applications. Humidity and temperature are amongst the most elementary weather parameters that play a significant role in human life. Humidity sensors give information about the amount of water vapors present in the atmosphere either as an absolute quantity or as a relative percentage of the saturation point at a certain temperature. This information can be used in the prediction of floods, food processing and preserving, protection of plants and agriculture, establishing the optimum and reliable conditions in manufacturing processes, health monitoring, weather telemetry applications, and electronics 2

industry [1–5]. In the current era of digitization and ultrafast computer controlled systems, the demand for efficient and multifunctional sensors having exceptional performance parameters such as high sensitivity, low cost, fast transient response, and greater durability has increased many folds [6–9]. Current research on humidity sensors is targeting to improve these parameters while reducing the fabrication cost and keeping the fabrication processor easy. All the parameters can’t be optimized at the same time and so, a compromise has to be made and only few parameters are optimized at a time depending upon the target application [3,10–12]. A major chunk of the research on humidity sensors is based on development on new materials to address these issues. The vast types of materials that have been utilized for humidity sensors fabrication include ceramics, transition metals, polymers, 2D materials, metal oxides, carbon and its derivatives in multiple phases, bio-derived materials, composites of materials from same category and

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between different categories, and so on [13–18]. Humidity sensors based on polymeric materials are the most common and are mostly resistive or capacitive [11]. 2D materials are also becoming popular to

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fabricate different types of sensors including biosensors, humidity sensors, and gas sensors because of the high surface area to volume ratio of their thin films resulting in good sensitivity and enhanced stability

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[4,19–23]. In addition to materials based research, various structures, transduction methods, and fabrication techniques are also being developed to optimize the required parameters of humidity sensors [13,24]. Different types of structures and transduction methods including surface acoustic waves [25,7],

been discussed in the previous works.

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interdigitated transducers , field effect Transistors, and other techniques [10,14] and structures [17,24] have

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Another crucial factor that is often ignored in majority of the research reported in literature on humidity sensors is the effect of surrounding temperature on the output of relative humidity sensors. Ignoring the effect of temperature can result in a significant error in real life applications [26]. A couple of

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researchers have attempted to develop mathematical model for temperature compensation of humidity sensors using artificial intelligence (AI) and neural networks [27,28] but a complete integrated solution is missing. Also, employing AI for a simple humidity sensor is not feasible as it requires a processor with high processing power resulting in higher cost, size, and power consumption. This study focuses on both the chemical and physical aspects for the fabrication of an integrated humidity and temperature sensor system. Flakes of few layered MoS2, a 2D transition metal dichalcogenide

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(TMD), suspended in aqueous solution of Polyethylene Oxide (PEO) polymer were used to fabricate the active layer of humidity sensors. The integrated micro temperature sensor used for compensation of the effects of temperature was a simple RTD (temperature dependent resistor) [29,30] based on meander type electrode structure of silver. All the fabrication steps were carried out using printing electronics techniques. A mathematical model was developed relating the surrounding temperature measured using the integrated sensor to both the relative humidity and the resistance output of the humidity sensor. The final expression was used to calculate real life relative humidity in changing surrounding temperature with real time

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compensation of temperature effects. The sensors showed excellent performance in terms of sensitivity, range, transient response time, and linearity.

2. Experimental 2.1 Materials and methods Ultrafine powder of Molybdenum disulfide (MoS2) with a purity >99.0% was purchased from Graphene Supermarket. Isopropyl alcohol (IPA) having purity of >99.7% and Polyethylene oxide (PEO) powder with molecular weight (MW) of 200,000 were purchased from Sigma Aldrich. Commercially available ink of silver nanoparticles (Ag-NP) sold by PARU was used for the printing of conductive

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electrode patterns. The physical and electrical properties of the ink play a vital role in the determination

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and optimization of printing parameters. The detailed ink properties are given in Table 1.

Table 1: Properties of silver ink used for electrode fabrication using EHD-DOD. Values

Conductivity (S/m)

3.5 x 10-8

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Properties

Viscosity (mPa.s)

300

0.05

Average particle size (nm)

Sintering condition

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Nanoparticles weight (%)

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Surface Tension (N/m)

66.2 150°C, 10 min

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Pristine MoS2 was liquid exfoliated to make aqueous solution having mono to few layers of MoS2. The exfoliation process was adopted from our previous works [19–21]. Liquid assisted mechanical exfoliation was performed by adding 1 ml of ethanol in 20 g of pristine MoS2 followed by wet grinding in a mortar for 1.5 hours and the resultant mixture was dried at room temperature. 100 ml ethanol was added to 20 g of the dried MoS2 powder which was obtained after wet grinding. Then probe sonication of the

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suspension was done for 1 hour followed by centrifugation at 4000 rpm for 30 min. Supernatant was extracted as the final suspension of 2D MoS2 flakes. The complete process is shown in Figure 1. PEO ink was prepared by dissolving the powder in deionized water by 5 wt%/vol. The mixture was put on a magnetic stirrer at 40ºC overnight until a clear homogenous solution was formed. After the two solutions were prepared, the final ink for the active layer deposition was prepared by homogenously mixing them in equal volume ratio of 1:1. The resultant ink was placed on a magnetic stirrer for 5 hours at 1500 rpm.

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Figure 1: Step by step liquid mechanical exfoliation process used to make suspension of 2D MoS2 flakes in ethanol.

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2.2 Sensors fabrication

After the selection and formulation of materials in solution form, sensors were fabricated through

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all printing techniques. Glass substrates were first cleaned by washing with ethanol followed by DI water. After drying at room temperature, they were treated with UV ozone plasma for 5 minutes to remove any

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contamination and improve hydrophilicity. A high resolution Electrohydrodynamic drop on demand (EHDDOD) printing system was used for the fabrication of electrodes. The system was completely developed inhouse as presented in Figure 2(a). The detailed specifications of the system components are presented in Table 2.

Components

Items

Features

AC Power Supply

Trek, 10/10B-HS

Output: 0 ~ ±10 kV

Function generator

Agilent, 33210A

10 MHz Arbitrary

X-Y Stage

Aerotech, DL-200

Accuracy: ±0.5 µm

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Table 2: Detailed specifications of the EHD-DOD printing system components.

Z Stage

Aerotech ANT130

Accuracy: ±3 µm

Process Camera

VC 4MC M180

5.5 µm x 5.5 µm, 20X

Monitoring Camera

VCC G20E20

8.4 µm x 9.8 µm, 10X

Further details about the printing process and fabrication system are available in our previous work [31]. Meander type micro electrodes for temperature sensor were printed through EHD-DOD as presented in Figure 2(c). Line width of the electrodes was 10 µm with gaps also of 10 µm. The total sensor width was 5

500 µm while the total height was 300 µm. IDT (Interdigital Transducer) based electrodes with 11 finger pairs were used for the micro-humidity sensor. Line width of each finger was kept at 5 µm with a gap of 10 µm between fingers. Total sensor width was 650 µm while the total height was 350 µm. The dimensions of the sensors were selected to keep the overall surface area of the sensors within 2 mm2 ensuring its seamless

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integration inside a MEMS based chip that is crucial for commercialization of such sensors.

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Figure 2: Fabrication Process: Step-by-step detailed schematic of all printing fabrication process showing different stages of (a) high voltage pulse pattern supplied to the EHD head, (b) schematic of EHD-DOD printing system, (c) images of fabricated integrated micro electrodes, and (d) schematic of ESD process for active layer thin film deposition.

After the fabrication of electrodes, electrospray deposition (ESD) was used to fabricate the

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composite active layer for humidity sensor. The sensors were cured at 60ºC for 2 hours after the active layer deposition. 60ºC was selected as the curing temperature because it is below the glass transition temperature (65ºC) of PEO. PEO changes from crystalline to amorphous phase if heated beyond the transition temperature and the structure becomes un-stable for long term use in humidity sensor applications. The schematic of the in-house developed ESD printing/deposition system is presented in Figure 2(d). More details of the ESD printing system and process can be found in our previous works [4,19,20]. Thin films of few hundred nano-meters thickness were deposited on the active region of IDT based humidity sensors.

2.3 Sensor characterizations 6

Morphological, chemical, and electrical characterizations of the active materials and the devices were carried out to determine the properties and sensing mechanism. Carl Zeiss Supra 55VP operating at 20 kV field emission scanning electron microscope (FESEM) was used to investigate the physical morphology of the humidity sensing active layer. Energy dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy (BruklerIFS666/S-Germany), and Raman spectroscopy (LabRAM HR Raman) were used for in depth investigation of the chemical composition of the sensing film. To measure the electrical response of sensors towards the changing humidity and temperature, an in-house developed measurement setup was used [3]. The schematic diagram of the setup is presented in Figure 3. A commercial MEMS based high accuracy temperature and humidity sensor Bosch BME280 was used as the reference sensor. Applent AT-825 digital LCR meter with 0.6 Vrms AC output was used to

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measure and log the output resistance of the devices under test in real time. Dry Argon gas was used as purging gas of the chamber for dehumidification. For humidification, a stream of atomized water vapors

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from an ultrasonic humidifier was used. The humidity level was changed very slowly and with idle intervals

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to enable settling time for the output of the sensors.

Figure 3: Detailed schematic of the automatic humidity and temperature measurement setup with controlled environment chamber and auto data logging. For temperature measurements of the individual micro RTD, the integrated sensor was placed on a hot plate and the temperature response was recorded from room temperature up to 100ºC. To measure the temperature effect on humidity sensors, the overall chamber temperature was changed by adding a Peltier heater at the base of the chamber without any direct contact to any of the devices under test or to the 7

reference sensor. Transient response curves for the output of the sensor were recorded by using two air streams of dry Argon and concentrated humid air directly from the humidifier. The active area of the sensors was placed perpendicularly to the joint opening of the two streams. Valves controlling humid stream and dry Argon stream were quickly opened and closed to switch between high and low humidity [19,32].

3. Results and Discussion 3.1 Morphological and chemical characterizations For the investigation of the effect of exfoliation, and to visualize the 2D nano flakes of MoS2, surface SEM was used. The recorded images of the thin films of active layer materials deposited on glass

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using ESD are presented in Figure 4. Physical structures of the MoS 2 flakes before and after exfoliation are presented in Figure 4 (a, b) respectively. It can be observed that the bulky chunks of pristine MoS2 powder have been successfully exfoliated into 2D nano-sheets with sizes ranging from hundreds of nanometers to

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a few micrometers. The exfoliated 2D flakes possess semiconducting properties that are absent in the pristine form of bulk MoS2. Also, mechanical exfoliation results in surface and edge defects giving rise to

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active bonding sites for the hydroxyl ions and water molecules that are crucial in humidity sensing application. The active layer thin film for humidity sensor was fabricated using a composite ink of MoS 2

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and PEO. Figure 4(c) shows the surface SEM image of this composite active thin film. It can be observed that visible MoS2 flakes are embedded inside the polymer matrix. Some flakes are visible from the surface while other are possibly buried inside the thin film. Figure 4(d) shows an agglomerated drop of MoS2+PEO

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composite used for EDS. It can be confirmed that 2D MoS2 flakes are present in the polymer matrix and they seem to be non-uniformly distributed when an agglomerated drop is observed. But, in case of spray deposition, the ink solutions were first sonicated and the 2D flakes and particles were thoroughly distributed

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throughout the polymer solution and all across the thin film.

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Figure 4: SEM images of the materials used in the humidity sensor active layer showing (a) SEM of bulk MoS2, (b) exfoliated 2D MoS2 flakes, (c) thin film of composite of MoS 2 & PEO with visible embedded MoS2 flakes, and (d) an agglomerated drop of MoS 2+PEO composite used for EDS showing 2D MoS2 flakes in polymer.

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The results of Energy Dispersive Spectroscopy (EDS) of the composite material are presented in Figure 5(a-c). The results clearly indicate the presence of Molybdenum and Sulfur atoms in 1:2 in the composite. Other elements visible in the spectrum are Carbon and Oxygen. Both elements are a part of PEO (C2nH4n+2On+1) structure. The reason for higher percentage of Oxygen is the presence of moisture in the surroundings adsorbed by the surface. Hydrogen element could not be detected as EDS cannot detect the

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lightest elements. Further confirmation of chemical composition of the humidity sensing layer was done through FTIR spectroscopy. Figure 5(d) shows the FTIR spectroscopy results confirming the chemical structure of PEO [33]. Characteristic peaks at 3486 cm-1 and 1640 cm-1 correspond to the presence of OH groups showing physical adsorption and hydrogen bonded hydroxyl ions respectively. The peak at 2874 cm-1 correspond to C-H stretching vibrations. The peaks at 1454 cm-1 and 1248 cm-1 can be attributed to the CH2 twisting while the one at 1350 cm-1 is because of CH2 Wagging. The absorption bands at 1198 cm-1 and 1100 cm-1 correspond to the C-O-C stretching vibrations confirming the semi crystalline phase of PEO. The absorption bands below 1000 cm-1 predominantly visible at 944 cm-1 are associated to CH2 rocking 9

band. MoS2 peak was not visible as its probable location is near 470 cm-1 [34]. The current spectrum range does not include the mentioned region but even after extending the range, the peak was not visible due to its relatively lower intensity. The presence of MoS2, however, had already been confirmed through EDS

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and SEM.

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Figure 5: Chemical characterization of the humidity sensing active layer showing (a-c) Energy dispersive spectroscopy (EDS) of the PEO+MoS2 composite showing different elements present in the composite and (d) FTIR spectrum for PEO+MoS2 composite.

3.2 Electrical response and behavior Four identical samples of the integrated twin sensor pairs were prepared under the same fabrication conditions and using the same system parameters. Each sensor pair consisted of a meander type microelectrode pattern acting as a resistive temperature sensor (RTD) and an Interdigitated Transducer (IDT) electrode for the micro humidity sensor. Active layers of bulk MoS2, 2D MoS2 flakes, pure PEO, and 10

PEO+MoS2 composite were deposited as thin films on the interdigitated electrodes of humidity sensors. All the sensors of both types (temperature and humidity) had their output response recorded in terms of change in device resistance versus the parameter under test (temperature and relative humidity). The response of the sensors was recorded through a semi-automatic controlled environment setup explained earlier. Figure 6 shows the resistive response of different types of humidity sensors with different active layer materials

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as the sensing elements.

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Figure 6: Response of humidity sensors showing resistance vs %RH with active layer of (a) Bulk MoS 2 flakes, (b) Exfoliated 2D MoS2 flakes, (c) Pure PEO thin film, and (d) thin film of PEO + 2D MoS 2 flakes composite showing linear response. Figure 6(a) shows the response of the humidity sensor with bulk MoS2 powder as the sensing layer

active material. The response curve shows that the sensor’s resistance changes non-linearly with the changing relative humidity. Sensitivity towards low %RH levels is very low while it increases exponentially after 55% RH. The reason behind this is the sensing mechanism of bulk MoS 2 based thin film. It detects humidity solely based on physical adsorption phenomenon and there are no other conduction principles associated with it. When the %RH is low, it is insufficient to create conductive water paths in the thin film and thus little to no change in resistance is observed. At higher %RH, the adsorbed water molecules create 11

physical water based micro conductive paths in the thin films resulting in current flow, and thus, lowering of the output resistance of sensor. In contrast, Figure 6(b) shows the response of a sensor based on exfoliated 2D MoS2 flakes. It can be clearly observed that an opposite trend of resistance vs relative humidity is being followed here. As soon as the relative humidity increases, water vapors are readily adsorbed by the highly porous thin film of exfoliated 2D MoS2 flakes. They attach to the surface and edge defects created during the mechanical liquid exfoliation of 2D flakes and result in an ionic plus proton hopping based current flow [35]. Another factor resulting in the drop of resistance is due to the increase in charge career mobility of semiconducting 2D MoS2 sheets upon water adsorption [36]. This phenomenon occurs due to the n-type semiconducting properties of 2D MoS2 and the electron donor behavior of hydroxyl group [37]. Finally, water adsorption also results in the formation of conductive layers between 2D sheets resulting in ionic

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conduction. This is why the resistance of the sensor based on exfoliated 2D MoS2 flakes quickly drops and reaches lower saturation resistance at around 35% RH. This means that the issue of poor responsivity of the

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sensor towards low humidity levels in case of bulk MoS 2 was solved through exfoliation but it ended up in poor responsivity towards higher %RH. To fabricate a sensor with excellent response towards the full range

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of relative humidity levels, the responses of two sensors showing high responsivity in different regions could be ideally combined. Bulk and exfoliated flakes of MoS2 could not be combined as a composite because that would have resulted in stacking and agglomeration of the same material. A composite of

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exfoliated 2D MoS2 flakes was prepared with a hydrophilic polymer (PEO). The response of PEO based sensor towards relative humidity is presented in Figure 6(c) showing the relatively lower sensitivity towards

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low %RH and higher sensitivity towards high %RH, thus giving an exponential output somehow similar to that of bulk MoS2. The intrinsic resistance of both PEO based sensor and exfoliated MoS 2 based sensor were comparable at 0% RH. That resulted in a perfect composite of the two materials with net intrinsic

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resistance unchanged and the final response with the qualities of both sensors. Figure 6(d) shows that the response of this composite based humidity sensor is nearly linear showing high sensitivity and responsivity of the sensor towards both very low and high %RH levels. After recording and optimization of the response of relative humidity sensor and final selection of active layer materials, the response of integrated temperature and humidity sensor was tested. The temperature sensor is a simple metallic resistive sensor (RTD) that had an output in terms of increase in

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resistance with increasing temperature as shown in Figure 7(a). It can be easily observed that a linear relationship exists between changing temperature and output resistance of the sensor. The output resistance of the temperature changes from ~282 Ω to 352 Ω with the change in temperature of 0ºC to 100ºC. This results in a slope or sensitivity of ~0.7 Ω/ºC with an intercept of 285. An error of just ~1.5 % was observed after three trails of readings. High accuracy, small volume, capability to be mass-produced, and high sensitivity are the advantages of RTD based temperature sensors [29,38]. Figure 7(b) shows the dependence of the resistance of relative humidity sensor on change in surrounding temperature also in addition to change in relative humidity. Ideally, the resistance of humidity sensor should not be affected by the change in 12

temperature but this is a general limitation in case of relative humidity sensors where the active layer material cannot be insulated from the effects of temperature. Relative humidity itself is dependent on the surrounding temperature, and hence, the relative humidity percentage itself changes with change in surrounding temperature. Even if the active layer material shows no change in resistance towards change in surrounding temperature, the resistance at temperature 1 will correspond to a different relative humidity when compared to the resistance at temperature 2. This makes it impossible to find an accurate independent relationship between resistance of the sensor and relative humidity. The fact is usually ignored in most of the research on relative humidity sensors and an assumption is taken where all readings were recorded at a fixed nominal room temperature. This assumption results in high inaccuracies and is critical in applications

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like medicine and food industry.

Figure 7: Response towards change in temperature of (a) Resistive temperature sensor, (b) change in resistance of humidity sensor with changing temperature, and (c) mathematical relationships showing dependence of humidity on surrounding temperature. The fact has been considered in this work and an attempt has been made to find the mathematical model relating the dependence of humidity sensor’s resistance on surrounding temperature also in addition to relative humidity. It can be observed from Figure 7(b) that an increase in surrounding temperature results in decrease in resistance of the humidity sensor. This behavior is due to the intrinsic properties of the active layer materials and can be different for different materials. In this particular case, the effect is due to the 13

change in conductivity of PEO with changing temperature [33]. Humidity vs resistance response curves for the full range of detection were recorded at different temperatures. It was observed that the general mathematical equation relating the dependence of sensor’s resistance on relative humidity can be kept the same (first order exponential in this case) even for different temperatures. Change in temperature only causes the change in the values of constants in the general relationship. This change in the values of constants was plotted against temperature in Figure 7(c) to find the exact mathematical relationship between change in temperature vs change in resistance of the relative humidity sensor. The equations were combined to get a single general relationship presented in equation (1) that relates the percentage relative humidity with the resistance of the sensor and the surrounding temperature. {𝑅 − (𝑎𝑦𝑜 + 𝑏𝑦𝑜 𝑇)} 𝑏𝑡1 ⁄ {(𝑎𝐴1 + 𝑏𝐴1 𝑇)}] × [𝑎𝑡1 𝑇 ]

(1)

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%𝑅𝐻 = ln [

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Here, %𝑅𝐻 refers to the percentage relative humidity; 𝑅 is the sensor resistance; 𝑇 is the surrounding temperature; and 𝑎𝑦𝑜 , 𝑏𝑦𝑜 , 𝑎𝐴1 , 𝑏𝐴1, 𝑎𝑡1, and 𝑏𝑡1 are the different constants that relate the

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change in resistance of the sensor simultaneously to both temperature and relative humidity change. The numerical values of these constants have been calculated for this particular sensor and the final equation has been presented in Figure 7. The only limitation of this approach is the need to individually calibrate

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every individual sensor and define the exact values of the constants listed above. The values of constants can be easily substituted into the general relationship presented in equation (1) to get the exact compensation

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relationship for each individual device. This relationship compensates for the effect of surrounding temperature in relative humidity measurements if the value of temperature is known. Once the calibration is finished, there are no further steps required and the sensors are ready to be used. The real time value of

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surrounding temperature in this study was recorded using the integrated printed temperature sensor discussed above. The end result was a complete integrated solution for relative humidity measurement with temperature compensation. The response of the final integrated device was recorded by interfacing it to a microcontroller. The results of the performance response parameters of the final integrated device are

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presented in Figure 8.

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Figure 8: Response of the final integrated relative humidity sensor showing (a) Transient response time of the device, (b) reproducibility after 90 days, and (c) real life relative humidity measurement with and without temperature compensation. Transient response of the humidity sensors was recorded to find the response and recovery times of the sensor. The results presented in Figure 8(a) show an impressive response time of 0.6 sec for the sensor’s output to change from 10% to 90% of the maximum value with a change of relative humidity from 0% RH to 90% RH. An even better recovery time of 0.3 sec was observed for the sensor’s output to return

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from 90% to 10% of the maximum for a change in relative humidity from 90% RH to 0% RH. The stability and reproducibility of the sensor were tested by recording the full range response curves at day one and day 90. Results presented in Figure 8(b) indicate excellent reproducibility with an absolute error of just ~5%. It is important to mention here that the reference sensor used in this study itself had a likely error of ~3% as indicated by the manufacturer. This makes an error of 5% to be considered as excellent. Finally, the integrated sensors were deployed for real life relative humidity measurement in an open environment with continuously changing temperature and humidity for consecutive 2 hours. The temperature of the surrounding kept varying in range of ~31.5ºC to ~34.5ºC while the relative humidity as measured by the 15

reference (temperature compensated) commercial humidity sensor varied between ~60% RH to ~53% RH. The results of the fabricated sensor presented in Figure 8(c) indicate that the independent humidity sensor (without integrated temperature sensor) showed the relative humidity to change from ~67.5% RH to ~59% RH if the temperature compensation was not taken into account. This is an error of approximately 13% with a change in temperature of just 7ºC on average. The plot presented as an orange curve in Figure 8(c) shows the end result after temperature compensation calculations were considered using the same humidity sensor’s output resistance value but taking the temperature data also from the integrated micro temperature sensor. The results show outstanding improvement and it can be clearly observed that the orange line shifts to closely follow the reference green line. This ascertains that the developed mathematical equations for temperature compensation work flawlessly for continuously varying relative humidity and temperature

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environment. A detailed comparison of various parameters of the developed sensor and those available in market and published in literature is presented in Table 3. The competitors were selected based on the latest

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results with best performance parameters. The finest in each category has been highlighted and the overall best sensor based on the maximum number of best performance parameters has been indicated in the table

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showing clear superiority of our work over the previous reported sensors in literature and market.

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Table 3: Comparison of various performance parameters of the developed sensor with those in market and literature. Response/ Recovery Time

% Error

Curve Shape

Temperature Compensate d

Size (mm x mm)

Reference

1 MΩ/%RH

13/17 s

NA

Nonlinear

NO

9x3

[39]

10-90% RH

1 kΩ/%RH

21/78 s

NA

Nonlinear decay

NO

6x5

[24]

Resistive

10100% RH

27 µV/%RH

19/10 s

NA

~ Linear

NO

~5x 5

[13]

Impedance

15-92% RH

120 kΩ/%RH

NA

3%

Expon ential decay

NO

NA (big)

[6]

Range

Sensitivit y

Ce-doped ZnO

Impedance

11-95% RH

QCP4VP,RG O

Impedance

Graphene oxide, Silicon PDEB

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Materials

Sensing Mechanism

Zinc Oxide

Chemristor

5-85% RH

42.678%

3/12 s

NA

Nonlinear decay

NO

10 x 5

[40]

BEHP-coMEH:PP V + PAA.PSS

Capacitive

0-80% RH

NA

3.5/5 s

NA

Expon ential

NO

10 x 10

[3]

PAM/Cr3 C2 bi-layer

Impedance

0-90% RH

0.66 kΩ/%RH

1-1.9 s

NA

~ Linear

NO

10 x 10

[21]

16

Resistive

0-90% RH

35 kΩ/%RH

2.8/5.7 s

2.4%

Linear

NO

10 x 10

[33]

2D hBN + PEO

Impedance

0-90% RH

24 kΩ/%RH

2.6/2.8 s

2%

~ Linear

Partial

10 x 10

[19]

2D MoS2/PE DOT:PSS Series

Impedance

0-80% RH

50 kΩ/%RH

0.5/0.8 s

2%

Linear by parts

NO

30 x 10

[4]

HTU-21D

MEMS

0-100% RH

10-bit digital data

7/10 s

5%

Calibr ated

YES

On chip

Commercial

DHT22

Polymeric

0-95% RH

Digital output

5/20 s

2%

Calibr ated

YES

On chip

Commercial

Si7021

MEMS

0-80% RH

12-bit digital data

5/18 s

3%

Calibr ated

YES

On chip

Commercial

HS3 Probe

MEMS

0-100% RH

0.1 V/%RH

NA

1%

Calibr ated

NO

5x5

Commercial

PEDOT:P SS/Methly Red/Grap hene Series

Impedance

0-100% RH

100 & 260 kΩ/%RH

1/3.5 s

2.2 %

Linear by parts

NO

50 x 10

[32]

2D MoS2 + PEO

Resistive

0-80% RH

85 kΩ/%RH

0.6/0.3 s

~ Linea r

YES

1.5 x 0.8

This Work

ro

-p

re

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4. Conclusions

5.4%

of

Amorpho us PEO

This work successfully presented the fabrication and characterization of a temperature compensated

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integrated sensor set with micro temperature and relative humidity sensors printed side-by-side on a glass substrate. The sensor electrodes were fabricated using Electrohydrodynamic (EHD) drop-on-demand printing technique while the active thin film of humidity sensor was deposited using Electrospray deposition (ESD). Both methods are 100% compatible with additive manufacturing and printing techniques. The resulting integrated sensor pair has total dimensions of ~1.8 mm by 0.8 mm that can be seamlessly integrated on a commercial microchip. The integrated sensor is capable to measure temperature in range of

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0ºC to 100ºC with a sensitivity of 0.7 Ω/ºC and an average error of ~1.5%. The second part of the integrated sensor is capable to measure relative humidity from 0% RH to 80% RH with a sensitivity of 85 kΩ/%RH, and response and recovery times of 0.6 sec and 0.3 sec respectively.. A detailed mathematical model was developed to relate the interdependence of temperature, relative humidity, and the output resistance of humidity sensor. The model was then used to compensate in real time, the resistive output response of relative humidity sensor for temperature variations. The twin sensors proved capable of temperature compensation in the final output of measured relative humidity with excellent accuracy. In conclusion, the 17

developed solution is a big-leap forward for all printed highly sensitive temperature compensated cheap commercial relative humidity sensors.

5. Conflict of Interest The authors declare no conflict of interest.

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Biographies

Hafiz M. Zeeshan Yousaf is pursuing his PhD at Semiconductor devices lab, Ghulam Ishaq Khan Institute, Pakistan under the supervision of Prof. Dr. Khasan Karimov and Dr. Memoon Sajid. His research interests include sensors fabrication through printed electronics techniques.

Soo Wan Kim did his B.S. and M.S. from Jeju National University, Korea and is now pursuing his PhD at the same research laboratory. His major area of research is development of printed electronics systems for high resolution conductive patterns for electrodes and displays repairing. 21

Dr. Gul Hassan is serving as an assistant professor in the department of electronics engineering in International Islamic University Islamabad. His areas of interest include MEMS devices, their fabrication, characterization, and performance optimization.

Prof. Khasan Karimov is leading the semiconductor devices lab in GIK Institute. His interest includes fabrication and design of devices like OLED, Sensors, OPV, TFT, Memristors, and RFID etc.

Prof. Kyung Hyun Choi is leading the Advanced Micro Mechatronics (AMM) Lab in Jeju National University since 2005 working on printed electronics based fabrication of devices like OLED, Sensors,

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OPV, TFT, Memristors, and RFID etc.

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Dr. Memoon Sajid is serving as an assistant professor of electronic engineering in GIK Institute. He has a vast experience in fabrication and performance optimization of environmental sensors. His major area

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of research covers fabrication of sensors using functional electronic materials and their applications in environmental and bio-monitoring. His research interests include fabrication of printed electronic devices

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including sensors, OLEDs, TFTs, and Memristors.

22