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Article

Synthesis and Characterization of Indium-Doped SnO2-Based Impedance Spectroscopy Sensor for Real-Time Humidity Sensing Applications

by
Birhanu Alemayehu
1,
Eunsung Shin
1,
Vladimir Vasilyev
2 and
Guru Subramanyam
1,*
1
Center of Excellence for Thin-Film Research and Surface Engineering, Department of Electrical and Computer Engineering, University of Dayton, Dayton, OH 45469, USA
2
Air Force Research Laboratory, Sensors Directorate, Wright Patterson AFB, Fairborn, OH 45433, USA
*
Author to whom correspondence should be addressed.
Crystals 2024, 14(1), 82; https://doi.org/10.3390/cryst14010082
Submission received: 21 December 2023 / Revised: 12 January 2024 / Accepted: 12 January 2024 / Published: 15 January 2024
(This article belongs to the Section Inorganic Crystalline Materials)
Browse Figures
Versions Notes

Abstract

:
Metallic transition-metal dichalcogenides are emerging as promising electrode materials for applications such as 2D electronic devices owing to their good electrical conductivity. In this study, a high-performance humidity sensor based on NbTe2 electrode material and an indium-doped SnO2 thin film sensing layer was fabricated using a pulsed laser deposition system. The morphology, structural, elemental compositions, and electrical properties of the as-deposited samples were characterized. Additionally, the humidity sensing response of the fabricated sensor with In-doped SnO2 (8:92 wt%) sensing film was evaluated in a wide range of relative humidity at room temperature. The results demonstrated that the humidity sensor based on In-doped SnO2 exhibited a high sensitivity of 103.1 Ω/%RH, fast response and recovery times, a low hysteresis value, good linearity, and repeatability. In addition, the sensor had good long-term stability, with a variation in impedance of less than 3%. The results indicated that the humidity sensor could be suitable for practical humidity sensing applications.

1. Introduction

Since the discovery of graphene [1], the family of two-dimensional (2D) atomically thin materials has recently attracted considerable research interest, offering exciting potential in the fields of nanoelectronics, optoelectronics, and photovoltaics [2,3,4,5,6]. Among 2D materials, transition metal dichalcogenides (TMDs), formulated as MX2 (M = transition metal (e.g., Mo, W, etc.) and X = chalcogen (S, Se, and Te)), have gained a lot of attention due to their unique structures, such as good flexibility, and novel physical or chemical properties [7,8,9,10]. Depending on the variation in the band structure, TMDs can be semiconducting TMDs or metallic TMDs (MTMDs) [11]. According to [12,13,14,15,16], MTMDs exhibited various fascinating physical phenomena, such as ferromagnetism, charge density waves, and superconductivity. In particular, conductive MTMD materials have been used in multiple fields of applications, such as energy storage and electronic devices [17,18]. For example, vanadium dichalcogenides were used as electrode materials to manufacture sensors, supercapacitors, and lithium-ion batteries [19,20,21]. MTMDs generally have high electrical conductivity comparable to that of conventional metals, making them excellent electrode materials for the fabrication of electronic devices. In this context, the metal tellurides of the VB group (VTe2, NbTe2, and TaTe2) offer significant potential in applications requiring good electrical conductivity and structural advantages such as flexibility and conformal electrodes for wearable sensors.
Exploring the aforementioned physical properties and versatile applications requires the controlled synthesis of MTMD materials with different thicknesses, large-area films/nanosheets, and high-crystalline structures. To date, numerous techniques have been applied to synthesize 2D materials. For instance, liquid-based and mechanical exfoliations have been widely used to synthesize a few layers of 2D materials due to their simplicity [22,23]. However, these techniques have some drawbacks, such as the multiple steps involved in synthesis, contamination, and the difficulty of precisely controlling the thickness and size of nanosheets [24,25]. Alternatively, chemical vapor deposition (CVD) has been proven to be one of the most effective techniques for synthesizing a wide range of highly scalable TMD nanosheets [1,26,27,28,29]. It is well known that the CVD process is typically performed at high temperatures, which instantly crystallizes the film. Normally, photolithography is used to pattern the crystallized film or fabricate electronic devices.
However, the development of an adequate photolithography process for 2D materials is still in its early stages [30]. This circumstance limits the widespread use of the CVD technique in the fabrication of electrical devices based on MTMD material [31,32]. Therefore, the controllable synthesis of MTMD materials with a reduced deposition temperature has become a hot topic. Pulsed laser deposition (PLD) is an impressive alternative to the above techniques due to its unique advantages, such as easily controllable deposition parameters, high scalability, room-temperature deposition, precise stoichiometry, and control of film thickness and size [33,34,35]. Grangeon et al. reported controllable growth of thin films of niobium telluride (NbTe2) at elevated deposition temperatures using the PLD technique. They demonstrated the effect of adjusting the deposition parameters on the formation of the different phases of NbTe2 [36]. However, after that, reports on how to synthesize NbTe2 using PLD were unexpectedly absent for a long time. Here, we report a controllable synthesis of a thin film of NbTe2 using PLD at room temperature, as well as the application of the fabricated film as an electrode in the form of an interdigitated transducer (IDT) in the development of an impedance spectroscopy-type humidity sensor.
Monitoring the humidity level in the surrounding environment has become a major concern in many sectors, such as manufacturing industries, health care centers, homes, the environment, and agriculture [37]. An excellent humidity sensor must meet several criteria, including fast response time, short recovery time, high sensitivity, good reproducibility, and long-term stability [38,39]. Several sensing materials/films, such as carbon-based materials, conductive polymers, and semiconducting materials, have been used to develop humidity sensors. Among these, metal oxide semiconductors, including zinc oxide (ZnO), iron (III) oxide (Fe2O3), tin oxide (SnO2), nickel oxide (NiO), and titanium oxide (TiO2), have continued to be the focus of significant attention in the planning and development of highly sensitive humidity sensors [40,41,42,43,44]. Specifically, SnO2 is the most promising candidate for humidity sensors due to its fascinating chemical and physical properties, such as chemical sensitivity to oxygen and water vapor, high chemical stability, non-toxicity, and low cost [45]. SnO2 is an n-type semiconductor with a wider band gap. However, the pristine SnO2 humidity sensors exhibited insufficient response, poor linearity, and long response and recovery times [46,47]. Thus, several effective strategies have been implemented to improve the sensing performance of SnO2-based sensors. For example, doping of sensing materials with other metal oxides or metal ions is predominant, resulting in increased surface areas/active sites and heterogeneous interfaces between the host and the dopant that facilitate an effective water adsorption/desorption process [48,49,50].
In this study, we present the synthesis of an indium (In)-doped SnO2 sensing film using the PLD technique and investigate its application for humidity sensing at room temperature. The effect of the different weight percentages of In in SnO2 was investigated [51,52,53,54]. To our knowledge, the use of In-doped SnO2 film for humidity sensing applications at room temperature has not yet been reported. An impedance spectroscopy-based humidity sensor was fabricated by depositing the sensing film on the interdigitated transducer. The crystalline structure, morphology, chemical composition, and electrical characteristics of the In-doped SnO2 and NbTe2 films were examined with X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and a four-point probe sheet resistance measurement method. The humidity sensing performance of the fabricated impedance spectroscopy-based sensor was evaluated over a wide range of relative humidity (RH) with a simple test setup. The sensor based on In-doped SnO2 exhibited high sensitivity, good linearity, fast response and recovery times, and low hysteresis. In-doped SnO2 has good potential for health monitoring and environmental detection, given its humidity-sensing capability.

2. Materials and Methods

2.1. Materials

In-doped SnO2 (SnO2 92/In 8 wt%) and NbTe2 ceramic targets were purchased from Kurt J. Lesker Co., Jefferson Hills, PA, USA, and high-resistivity silicon substrates were obtained from University Wafer Inc., South Boston, MA, USA. All chemicals were ensured to be analytical grade, applied directly without further purification, and distilled water was used in the experiments.

2.2. Methods

2.2.1. Interdigitated Transducer Fabrication Process

A standard positive photoresist lift-off process with S1813 photoresist was used to pattern the NbTe2 film to fabricate the interdigitated transducer (IDT) structure. The thin film of NbTe2 was deposited onto the pattered wafer at room temperature using a KrF excimer laser (λ = 248 nm) to ablate the NbTe2 target [55]. After the lift-off process, the wafer was annealed in a conventional furnace to crystallize the film. Thin films of 20 nm Ti and 330 nm Au were deposited using e-beam evaporation as contacts using another lift-off lithography process. Figure 1 illustrates the main process for fabricating the humidity sensor based on the metallic NbTe2 film. IDTs were used to measure the impedance changes of In-doped SnO2 during exposure to various levels of relative humidity.

2.2.2. Sensing Film Fabrication

The sensing film, a 100 nm thick sensing In-doped SnO2 film, was also deposited using a positive photoresist lithography process on top of the NbTe2 IDT structures. The In-doped SnO2 target (SnO2 92/In 8 wt%) was used for the synthesis of In-doped SnO2 thin films [55] using a 248 nm KrF excimer laser. 600 laser pulses were used to obtain a 100 nm film. After the lift-off process, the wafer was annealed in a furnace at 330 °C to crystallize the In-doped SnO2 film. The wafer was diced and then cleaned. Figure 2 shows the microscopic picture of the NbTe2-based fabricated impedance-type sensor with the In-doped SnO2 sensing film. The width of the finger and the spacing between the fingers were 160 and 80 µm, respectively.

2.2.3. Characterization

The crystal structure of the NbTe2 and In-doped SnO2 samples was carried out on a PANalytical X-Pert diffractometer with a hybrid monochromator for Cu Kα1 radiation (λ = 1.554056 Å). The diffraction patterns were collected at a scan angle (2θ) ranging from 8 to 90° with a step size of 0.02°. The morphology, structure, and elemental composition of the NbTe2 and In-doped SnO2 samples were studied using a high-resolution scanning electron microscope (HRSEM, Hitachi S-4800, Hitachi, Japan) at an acceleration voltage of 10 kV and energy-dispersive spectroscopy (EDS, Zeiss EVO-50XVP, Jena, Germany) instruments. The surface electrical conductivity of the prepared thin film samples was measured using the four-point probe sheet resistance tester (Ossila).

2.2.4. Humidity Sensing

The humidity sensing of the fabricated sensor was evaluated at room temperature using a custom-made experimental setup for the measurement of relative humidity, where the sensor was enclosed in a test chamber to maintain consistent sensing conditions, as shown in Figure 3. The sensor was then connected to a handheld impedance analyzer (AD5940, Analog Devices, Wilmington, MA, USA) to measure the impedance response of the sensor at different RH levels. For data logging, the impedance analyzer was connected directly to the PC. Additionally, a commercially available humidity sensor (HTU21D, humidity precision: 2% RH, humidity resolution: 0.1% RH, SparkFun Electronics, Boulder, CO, USA), which serves as a reference, was placed side by side with the fabricated sensors to measure the relative humidity level within the test chamber. Analog data from this device was logged onto the PC through an Arduino microcontroller. The system also included a mass flow controller (GA50A, MKS, Andover, MA, USA) and a humidifier. Initially, nitrogen gas (N2) was introduced into the test chamber to purge the entire test setup and establish a baseline. The humid air or moisture was generated by the humidifier. Different levels of RH were achieved by adjusting the flow rate of N2 through the mass flow controller and mixing it with humid air in the mixer. Then, the impedance changes of the In-doped SnO2 film were measured during cyclic exposure to nitrogen and moist air using the impedance analyzer. Sensor impedance values were measured at an operating frequency of 35 kHz due to the improved sensitivity of the device between 30 and 40 kHz [55].

3. Results

3.1. Film Characterization

Before fabricating the NbTe2-based humidity sensor, a preliminary investigation was performed to determine the optimal PLD deposition parameters and the annealing temperature to grow thin films of both NbTe2 and In-doped SnO2. Once these parameters were identified, a thin film of NbTe2 was deposited onto a silicon substrate, and the crystal structure of the sample was analyzed using XRD. Figure 4 shows the XRD pattern obtained for the as-deposited NbTe2 sample. As can be seen in the figure, the diffraction peaks of the sample were sharp and strong in intensity, indicating complete crystallization of NbTe2. Moreover, the XRD peaks were well-matched with those of the monoclinic crystal structure of NbTe2. For example, the 2θ diffraction peaks of the sample at ~13.1°, 26.4°, 40°, and 54.4° were indexed to the (0 0 1), (0 0 2), (3 1 1), and (−7 1 5) planes of NbTe2 (JCPDS No. 21-0605). Furthermore, the absence of other diffraction peaks confirmed the sample’s phase purity.
Figure 5 shows the SEM images of the surface of the NbTe2 film deposited onto the silicon substrate. As can be seen in Figure 5a, a thin film of NbTe2 with no known defects was grown using PLD. The cross-sectional view (Figure 5b) reveals that the film had a dense and integrated structure with good adhesion that can reflect the quality of the film. In addition, the formation of nanopores was observed. The EDS spectrum of the as-deposited NbTe2 film grown on the silicon substrate (Figure 6) contains the peaks of niobium and tellurium. From the percentage of atomic weight, the atomic ratio of Te to Nb was calculated and found to be 1.94, which was very close to the theoretical value of NbTe2, suggesting that the sample was stoichiometric NbTe2. The electrical conductivity of NbTe2 was measured and found to be ~28.4 S/m.
Similarly, the crystalline structure, morphology, and elemental composition of In-doped SnO2 were studied. A thin film of In-doped SnO2 (8:92 wt%) was deposited onto the silicon substrate and crystallized in the furnace. Figure 7 shows the XRD pattern of the In-doped SnO2 film. The diffraction peaks of In-doped SnO2 at 2θ of 26.58°, 33.86°, and 51.76° were indexed to the (1 1 0), (0 1 1), and (1 2 1) planes of the tetragonal cassiterite phase of SnO2 (JCPDS No: 41-1445). Moreover, it is clear that In was incorporated into the SnO2 lattice because no other secondary phases, such as In or In2O3, were observed.
The SEM image of the as-deposited In-doped SnO2 sample presented in Figure 8 shows that a uniform and smooth surface was obtained. Also, no known defects were observed on the surface of the film. The analysis of the EDS spectrum of the In-doped SnO2 film presented in Figure 9 confirmed the existence of In, Sn, and O elements in the deposited thin film. Furthermore, the elementary mappings demonstrated that all elements, such as In, Sn, and O, were uniformly distributed on the film’s surface.

3.2. Humidity Sensing Performance

The humidity sensing performance of the In-doped SnO2 sensing film was investigated in a wide range of relative humidity levels (15–90% RH) at an operating frequency of 35 kHz. The dynamic impedance response curve versus time with increasing relative humidity is shown in Figure 10. The impedance of the sensor returned to its baseline state as the sensor switched between the different relative humidity levels. At a specific % RH, the impedance of the film initially decreased due to the adsorption of water molecules; however, when exposed to N2 gas, the desorption caused the impedance to return to its baseline state.
Figure 11 illustrates the variation in the impedance of In-doped SnO2 as a function of the relative humidity level at room temperature. The fitting function of impedance (Z) and the relative humidity of the In-doped SnO2 humidity sensor was Z = (−0.13 × RH + 24.6) kΩ with a regression coefficient R2 of 0.980. As shown in the figure, the impedance decreased linearly with increasing relative humidity, but the sensor response increased with increasing relative humidity. This result demonstrated the excellent linearity of the sensor response to RH, which is essential for practical applications. Furthermore, sensitivity (S) is also an important parameter in quantifying the performance of humidity sensors. It is defined in [46] as the variation in impedance for relative humidity and is mathematically expressed as
S = ∆ Z ∆ % R H k Ω / % R H ,
where ∆ Z and ∆ % R H represent the change in impedance and relative humidity, respectively. The sensitivity value was 103.15 Ω / % R H in the range of 10 to 90% RH. In addition, response time, recovery time, repeatability, and hysteresis are significant indicators to evaluate humidity sensor performance [51,52].
To investigate the repeatability of the humidity sensor, the sensor was exposed to a level of 45% RH for five successive cycles. Figure 12 shows the impedance variations of the In-doped SnO2 sensor in five cycles switching between N2 gas and different relative humidity. As shown in the figure, at a relative humidity of 45%, the change in baseline and final impedance values was almost negligible after multiple cycles. Moreover, the standard deviation of the impedance value to 45% RH was calculated and found to be 0.002, indicating excellent dynamic stability and repeatability of the sensor. Response time (tres) is defined as the time it takes for the impedance to reach 90% of its final value after exposure to humid air, and recovery time (trec) is defined as the time it takes for the impedance to return 90% of its baseline value [56]. Based on the dynamic impedance-time curve in Figure 12, the response and recovery times of the In-doped SnO2-based humidity sensor were estimated to be 10 and 21 s, respectively.
To study the hysteresis characteristic of the sensor, the sensor was consecutively exposed to different levels of relative humidity (adsorption process) and then followed by a downward one (desorption process). Figure 13 shows the sensor hysteresis during adsorption and desorption. In the adsorption process, the impedance decreased as the relative humidity increased. However, in the desorption process, the impedance increased as the relative humidity decreased. Hysteresis was calculated by measuring the impedance difference between the adsorption and desorption measurements at a particular RH value and then divided by the full output range [57], mathematically expressed as H = ΔHmax/2FFS, where ΔHmax and FFS are the maximum hysteresis value and the full-scale output, respectively. The hysteresis of the sensor was about 2.58%, further demonstrating the reversibility of the sensor.
Finally, the long-term stability of the In-doped SnO2-based humidity sensor was investigated by exposing it to humid air (45% RH) for one month. Figure 14 shows the variation in the impedance of the humidity sensor to 45% RH as a function of time during that period. The variation in impedance was less than 2%, suggesting that the long-term stability of the sensor is suitable for use in practical applications.
Table 1 compares the humidity sensing performance of the In-doped SnO2-based humidity sensor used in this study with other previously reported SnO2-based humidity sensors. Compared to others, the In-doped SnO2-based humidity sensor exhibited fast response and recovery times. In general, the In-doped SnO2-based sensor has great potential in practical humidity sensing applications.

3.3. Humidity Sensing Mechanism

The humidity sensing mechanism of the sensor was associated with the adsorption of water molecules on the surface of the In-doped SnO2 film, which caused a change in the electrical properties of the sensing film. Figure 15 shows the schematic diagram of the adsorption process of water molecules on the surface of In-doped SnO2, which resulted in the formation of chemisorbed and physisorbed layers. Briefly, at low RH levels, upon exposure to humid air, water molecules decomposed into protons (H+) and hydroxide ions (OH−) as a result of self-ionization and surface collision with the film surface [63,64]. Subsequently, OH− interacted with cations on the surface of In-doped SnO2 and formed a chemisorption layer. This phenomenon caused a decrease in the potential barrier and the disappearance of the depletion layer by reducing the electron affinity of the cations and removing the electrons captured in a dry environment [62,63,64]. In turn, the bulk conductivity and the dielectric constant increased. Therefore, the sensor was quite sensitive, even at low RH levels. However, at low humidity, only a small amount of water was absorbed, leading to the appearance of a discontinuous water layer on the film surface [65]. It makes the conduction of protons to neighboring OH− ions challenging. Therefore, electron conduction was more dominant than ion conduction.
In mid-RH, as the humidity increased, the adsorption of water molecules also increased. The adsorption of more water molecules led to the formation of a physisorbed layer on the surface of the film through hydrogen bonds [47]. Water molecules dissociated and produced hydronium ions (H3O+) under a strong electrostatic field in the chemisorption layer [66]. Subsequently, H3O+ was converted back to H2O, and more protons were released as a result of the applied electric fields, acting as a source of protons. Due to the formation of additional layers, protons (H+) can move to adjacent water molecules [67]. As a result, ionic conduction becomes effective, and the impedance decreases further. In addition, as more and more water molecules were absorbed, subsequent physisorbed layers gradually formed. At high RH, as the humidity increases further, a large volume of water molecules could be absorbed, creating a continuous water layer. This layer accelerated the free movement of protons toward adjacent water molecules. This leads to the formation of a constant dipole and electrolyte layer between the electrodes. As a result, the dielectric constant and bulk conductivity increased further [68]. The porous structure of In-doped SnO2 can also play a substantial role in facilitating the absorption of water molecules and, in turn, improving the sensor’s performance.

4. Conclusions

In summary, we demonstrated the controllable synthesis of large-area and thickness-tunable NbTe2 nanosheets by optimizing the deposition parameters during the PLD process. The X-ray diffraction pattern exhibited the pure NbTe2 phase. The stoichiometric composition of the NbTe2 film was confirmed using EDS. The PLD process was also used for the direct synthesis of the In-doped SnO2 sensing film on the substrate. The X-ray diffraction pattern showed the crystal structure of In-doped SnO2. Besides, the SEM image revealed the presence of nanopores on the surface of the film. An impedimetric humidity sensor based on In-doped SnO2 (8:92 wt%) was developed, and its sensing response was investigated in a wide range of relative humidity (15–90% RH) at room temperature. The In-doped SnO2 humidity sensor showed a superb linear response, low hysteresis, long-term stability, a remarkable sensitivity of 103.15 Ω/%RH, and a short response and recovery times of 10 and 21 s, respectively. In addition, the sensor had good repeatability. Finally, the humidity sensing mechanism of In-doped SnO2 was discussed in detail. Overall, the results highlighted that In-doped SnO2 has a good prospect as a sensing material for practical humidity sensing applications. Additionally, NbTe2 is a promising electrode material that can serve as a building block for the construction of electronic devices.

Author Contributions

Conceptualization, B.A. and G.S.; methodology, B.A., E.S. and V.V.; formal analysis, B.A.; investigation, B.A., E.S., V.V. and G.S.; resources, G.S., V.V. and E.S.; writing—original draft preparation, B.A.; writing—review and editing, G.S., E.S. and V.V.; supervision, G.S. and E.S; project administration, G.S.; funding acquisition, G.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data is contained within the article.

Acknowledgments

The authors thank the funding support from CETRASE, University of Dayton, for this research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic showing the main fabrication process for the In-doped SnO2 thin film-based humidity sensor with the NbTe2 electrode.
Figure 1. Schematic showing the main fabrication process for the In-doped SnO2 thin film-based humidity sensor with the NbTe2 electrode.
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Figure 2. A microscopic picture of a fabricated impedimetric humidity sensor.
Figure 2. A microscopic picture of a fabricated impedimetric humidity sensor.
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Figure 3. Block diagram of the experimental setup for humidity sensing.
Figure 3. Block diagram of the experimental setup for humidity sensing.
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Figure 4. XRD pattern of the NbTe2 film deposited using PLD. XRD peaks represent the crystalline phase of NbTe2 (the monoclinic crystal structure).
Figure 4. XRD pattern of the NbTe2 film deposited using PLD. XRD peaks represent the crystalline phase of NbTe2 (the monoclinic crystal structure).
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Figure 5. SEM images of the NbTe2 film: (a) top view; (b) cross-sectional view.
Figure 5. SEM images of the NbTe2 film: (a) top view; (b) cross-sectional view.
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Figure 6. EDS spectrum profile of the as-grown NbTe2 film.
Figure 6. EDS spectrum profile of the as-grown NbTe2 film.
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Figure 7. XRD spectroscopy of the In-doped SnO2 film deposited using PLD. XRD peaks represent the presence of the crystalline phase of SnO2 (the tetragonal SnO2 cassiterite).
Figure 7. XRD spectroscopy of the In-doped SnO2 film deposited using PLD. XRD peaks represent the presence of the crystalline phase of SnO2 (the tetragonal SnO2 cassiterite).
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Figure 8. SEM image of the surface of In-doped SnO2 thin film.
Figure 8. SEM image of the surface of In-doped SnO2 thin film.
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Figure 9. EDS spectrum of the as-deposited In-doped SnO2 film.
Figure 9. EDS spectrum of the as-deposited In-doped SnO2 film.
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Figure 10. Dynamic impedance response curve versus time with gradually increasing % RH.
Figure 10. Dynamic impedance response curve versus time with gradually increasing % RH.
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Figure 11. Impedance vs. %RH for In-doped SnO2 humidity sensor with the trend line.
Figure 11. Impedance vs. %RH for In-doped SnO2 humidity sensor with the trend line.
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Figure 12. Dynamic impedance measurements for determining the response and recovery times of the humidity sensor.
Figure 12. Dynamic impedance measurements for determining the response and recovery times of the humidity sensor.
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Figure 13. Plot of the impedance vs. relative humidity for the adsorption and desorption cycles showing hysteresis characteristics.
Figure 13. Plot of the impedance vs. relative humidity for the adsorption and desorption cycles showing hysteresis characteristics.
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Figure 14. Long-term stability of the humidity sensor measured for 30 days.
Figure 14. Long-term stability of the humidity sensor measured for 30 days.
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Figure 15. The humidity sensing mechanism of In-doped SnO2 thin film.
Figure 15. The humidity sensing mechanism of In-doped SnO2 thin film.
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Table 1. Comparison of humidity sensors based on SnO2 from this and previously reported work.
Table 1. Comparison of humidity sensors based on SnO2 from this and previously reported work.
Sensing Materialtres (s)trec (s)Ref.
Co-doped SnO2/rGO 522100[58]
rGO/SnO26–1026–9[59]
rGO-SnO21060[60]
ZnO/SnO241198[61]
Sb-doped SnO24064[62]
Ni-doped SnO2154[49]
MoS2/SnO2176[47]
WS2/SnO2100100[57]
In-doped SnO21021This work
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Alemayehu, B.; Shin, E.; Vasilyev, V.; Subramanyam, G. Synthesis and Characterization of Indium-Doped SnO2-Based Impedance Spectroscopy Sensor for Real-Time Humidity Sensing Applications. Crystals 2024, 14, 82. https://doi.org/10.3390/cryst14010082

AMA Style

Alemayehu B, Shin E, Vasilyev V, Subramanyam G. Synthesis and Characterization of Indium-Doped SnO2-Based Impedance Spectroscopy Sensor for Real-Time Humidity Sensing Applications. Crystals. 2024; 14(1):82. https://doi.org/10.3390/cryst14010082

Chicago/Turabian Style

Alemayehu, Birhanu, Eunsung Shin, Vladimir Vasilyev, and Guru Subramanyam. 2024. "Synthesis and Characterization of Indium-Doped SnO2-Based Impedance Spectroscopy Sensor for Real-Time Humidity Sensing Applications" Crystals 14, no. 1: 82. https://doi.org/10.3390/cryst14010082

APA Style

Alemayehu, B., Shin, E., Vasilyev, V., & Subramanyam, G. (2024). Synthesis and Characterization of Indium-Doped SnO2-Based Impedance Spectroscopy Sensor for Real-Time Humidity Sensing Applications. Crystals, 14(1), 82. https://doi.org/10.3390/cryst14010082

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