Thin Film Based Gas Sensor Research Articles

Facile synthesis of nanogranular PPy thin films for sensitive and selective detection of toxic NO gas

The pyrrole monomers have been synthesized by in situ oxidative polymerization and followed by the drop casting of polypyrrole thin films onto glass. The optimized PPy thin film for gas sensor showed the highest NO 2 sensitivity of 40.08 % at room temperature with a stable response even after 45 days with a retention of stability of around 86 %. The gas sensing results suggested that the nanogranular PPy sensor exhibited an excellent response toward NO 2 and also showed high sensitivity even at low concentrations up to 10 ppm. • Synthesis of pyrrole monomers by in situ oxidative polymerization for NO 2 gas sensor. • Achieve sensitivity of 40.08 % at room temperature with a stable response even after 45 days. • Selective sensing towards NO 2 within a wide range of concentrations from 10 to 100 ppm. • The sensor exhibits reliable long-term stability, reproducibility and reversibility. Nanogranular polypyrrole (PPy) have been synthesized by in-situ oxidative polymerization of pyrrole monomer followed by deposition of PPy thin films onto a glass substrate by drop casting method. The deposited thin films were characterized for their structural, morphological, compositional studies using x-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), and x-ray photoelectron spectroscopic (XPS) studies. The XRD patterns revealed that the synthesized films are amorphous in nature. FE-SEM microscopic images exhibit agglomerated granular-like morphology of PPy thin films with high degree of roughness. The XRD patterns confirm the amorphous nature of PPy thin films. XPS analysis reveals the formation of PPy. The gas sensing properties of PPy thin film based gas sensor were studied in terms of sensitivity, response-recovery time, selectivity and stability by varying the concentration of nitrogen dioxide (NO 2 ) gas. The optimized PPy thin film gas sensor showed highest sensitivity of 40.08 % at room temperature with stable response even after 45 days with retention of stability around 86%. The gas sensing results suggested that nanogranular PPy sensor exhibited an excellent response toward NO 2 and also showed high sensitivity even at low concentrations upto 10 ppm level.

Growth and NO2 sensing properties of Cs2SnI6 thin film

In this study, Cs2SnI6 thin film was synthesized on a glass substrate through a two-step process using the chemical vapor deposition (CVD) method and was subsequently used as a NO2 gas sensor. First, the CsI spheres with an average diameter of 360 nm were fabricated on the glass substrate using a CVD system at 240 °C. Second, Cs2SnI6 film was formed through the reaction of CsI spheres with SnI4 vapor at 200 °C for 1 h. Finally, the as-synthesized Cs2SnI6 thin film based gas sensor was then fabricated to explore its potential applications. The results indicated that the device showed good selectivity, stability, and detection for NO2 with low concentration at room temperature. In addition, the effect of humidity on NO2-sensing performance of the device was further studied. Furthermore, the NO2 sensing mechanism of the Cs2SnI6 thin film was discussed. Hence, this study helps understand the gas sensing mechanism of halide perovskite Cs2SnI6 and the room temperature development of NO2 gas sensors.

Detection of hydrogen sulfide using polyaniline incorporated with graphene oxide aerogel

This paper presents the fabrication of polyaniline and graphene oxide aerogel composite (PGA) thin film-based sensors for H2S gas sensing application. The optical and morphological properties of these composite films were characterised correspondingly by UV–visible spectroscopy and scanning electron microscopy. To prepare the gas sensors, thin films of PANI and PGA composites were deposited onto interdigitated electrodes by spin coating method. The response of these sensors towards H2S gas was evaluated by monitoring the change in electrical conductivity at room temperature under two relative humidity values of 60% RH and 80% RH. The PGA composite thin film-based gas sensors showed a higher response compared with the pure PANI-based sensor. The gas sensor made of PGA-3 showed a good response of 3.87 in 67 s (60% RH), which is 4.6 times higher than that of pure PANI and 2.5 times higher than that of PGA-1 at 1 ppm. The first-principles simulations were carried out to investigate the H2S sensing mechanism of PGA heterostructure. The adsorption of H2S at different locations of pure PANI and PGA composites was analysed. The band gap for PGA showed more significant change after adsorption than that of pure PANI, which indicated the higher sensitivity for detecting H2S.

Gas sensing properties of palladium-modified zinc oxide nanofilms: A DFT study

The improved gas-detecting ability of ZnO nanostructures has generated considerable attention for further development based on a unique and practical approach. However, reduced gas sensitivity and selectivity, as well as the sophisticated synthesis requiring high-cost technology restricts its efficient use. In this regard, the enhancement of gas sensing properties can be brought about by incorporating the noble metal palladium (Pd) to modify the ZnO surface. This work applied density functional theory (DFT) calculations to study such sensing materials to the target gases (i.e., hydrogen (H2) and ethanol (C2H5OH)), which are extensively applied in energy production, thereby requiring rapid and accurate detection. The modified ZnO thin film-based gas sensor was modeled into Pd-substitutional doped and Pd-decorated ZnO surfaces. The charge transport phenomenon at the atomic scale, binding strength, and other electronic and structural properties were elucidated as the varied number of Pd. Pd-decorated ZnO surfaces enhance the sensitivity of both molecules. The calculation results could provide an atomistic insight and a better understanding of altered electrical and sensing properties of ZnO surface modification by Pd atoms.

Room temperature ammonia gas sensing characteristics of copper oxide-tin oxide composite thin films prepared by radio frequency magnetron sputtering technique

In this work, thin films of composite copper oxide-tin oxide [CuO:SnO2 (1:1)] were prepared by radio frequency magnetron sputtering technique at room temperature on quartz glass substrates. X-ray diffraction study revealed that the as-deposited films were amorphous in nature and the crystallinity of the films was obtained by annealing the films at 1000 °C. The hexagon rod-like structure, dews-like particles and cylindrical-shaped particles were observed in surface morphological study. The X-ray photoelectron spectroscopic study confirmed the formation of Cu2+ and Sn4+ states in the deposited films. The decrease in optical energy band gap with increase in RF power and annealing temperature may be due to the creation of localized states near the band edges of CuO:SnO2. The gas sensing characteristics of the films were analysed by recording the electrical resistance variation of the films in the presence/absence of various concentrations of NH3 gas at room temperature. The CuO:SnO2 film exhibited a highest sensing response of 3838 for 125 ppm of NH3 gas at room temperature. The film sustained its initial sensor response even after 6 months period for 5 repeated cycles, which ascertained the stability and repeatability of CuO:SnO2 thin film based gas sensor.

Rationalization in the ethanol-gas recognition mechanism of Al-doped ZnO nanoparticulate thin film defined on a highly integrated micro hot plate

The gas sensor integration in smart devices, represents a huge step forward for health and environmental sensing applications. Several technical problems as well as the insufficient rationalization in the gas-recognition mechanisms still impede their development. Addressing this challenge, an ethyl alcohol/Al-doped ZnO interaction experiment is designed, exploiting a reliable chemiresistive thin film-based gas sensor. An ink-jet printing technique is chosen to deposit the nanostructured active layer on the highly integrated micro hot plate. Deposited sensing layer, is investigated with scanning electron microscopy, energy dispersive x-ray spectroscopy and atomic force microscopy. Conformal coating and profiled signals according to the covered sensor structure were achieved. The sensing results demonstrate a reproducible dynamic gas response with a rapid initial decrease in resistance, followed by a steady rise in conductivity at a different rate, until it plateaus. The well-defined signal profile, with the peculiar inflection points, has enabled an interpretation of the chemical reaction mechanism.

Sol-gel assisted nano-structured SnO2 sensor for low concentration ammonia detection at room temperature

SnO2 nanostructured thin film based gas sensor is fabricated by sol-gel spin coating technique. The performance of the fabricated sensor has been investigated for analytes viz. ammonia (NH3) solution, acetone (C3H6O), methanol (CH3OH) and 2-propanol (C3H8O) at room temperature (RT) with humidity level ∼55% RH for concentration range 500 ppb-500 ppm. High response and good selectivity towards ammonia are observed with very fast response and recovery time at RT, for extreme low concentrations. Upon exposure to 500 ppb and 1 ppm of NH3, sensor manifests appreciable response ∼28% and ∼31.5%, respectively. High response of the sensor at RT is attributed due to porous nanograins (with average particle size ∼50 nm) based SnO2 thin film layer. Nanograins structure is obtained due to addition of glycerine in the sol solution, which also leads to porosity enhancement of the sensing layer. Good reproducibility and appreciable immunity to drift behavior are other attributes of the fabricated device. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscopy (SEM) results are used to study the structural, chemical composition, topography and morphological characteristics of the prepared SnO2 thin film, respectively.

CuO Thin Films Functionalized with Gold Nanoparticles for Conductometric Carbon Dioxide Gas Sensing

Metal oxides (MOx) are a well-established material for gas sensing. MOx-based gas sensors are sensitive to a wide variety of gases. Furthermore, these materials can be applied for the fabrication of low-cost and -power consumption devices in mass production. The market of carbon dioxide (CO 2 ) gas sensors is mainly dominated by infra-red (IR)-based gas sensors. Only a few MOx materials show a sensitivity to CO 2 and so far, none of these materials have been integrated on CMOS platforms suitable for mass production. In this work, we report a cupric oxide (CuO) thin film-based gas sensor functionalized with gold (Au) nanoparticles, which exhibits exceptional sensitivity to CO 2 . The CuO-based gas sensors are fabricated by electron beam lithography, thermal evaporation and lift-off process to form patterned copper (Cu) structures. These structures are thermally oxidized to form a continuous CuO film. Gold nanoparticles are drop-coated on the CuO thin films to enhance their sensitivity towards CO 2 . The CuO thin films fabricated by this method are already sensitive to CO 2 ; however, the functionalization of the CuO film strongly increases the sensitivity of the base material. Compared to the pristine CuO thin film the Au functionalized CuO film shows at equal operation temperatures (300 ∘ C) an increase of sensitivity towards the same gas concentration (e.g., 2000 ppm CO 2 ) by a factor of 13. The process flow used to fabricate Au functionalized CuO gas sensors can be applied on CMOS platforms in specific post processing steps.

TiO2 thin film based gas sensors for CO-detection

Pure titanium dioxide (TiO2) thin films were deposited and investigated as gas sensors for carbon monoxide (CO) detection. TiO2 thin films were deposited by ultrasonic spray pyrolysis technique, starting from titanium(IV) oxyacetilacetonate, onto soda-lime glass substrates. Structural, morphological, and compositional properties of the TiO2 films were obtained utilizing X-ray diffractometry, energy dispersive X-ray, scanning electron microscopy, and secondary ion mass spectroscopy techniques, respectively. Films deposited above 400 °C were polycrystalline, and its X-ray pattern fit well to the TiO2 anatase structure, and no other phases were detected, whereas films deposited at lower temperatures presented an amorphous structure. The sensitivity of the TiO2 films is analyzed by varying both thickness and deposition temperature. In addition, the sensor response was measured for CO concentrations from 1 to 300 ppm at different operation temperatures, 100, 200, and 300 °C. The highest sensitivity (~ 300) was obtained for TiO2 thin films deposited with the lowest film thickness, at a substrate temperature of 350 °C. The results obtained in this work show that the TiO2 films processed by ultrasonic spray pyrolysis exhibit very promising results for detection of CO.

From Transparent Conducting Material to Gas-Sensing Application of SnO2:Sb Thin Films

Transparent conductive thin films of nanocrystalline tin oxide:antimony (SnO2:Sb) were deposited on a preheated glass substrate at 400°C via spray pyrolysis technique. The effects of Sb doping concentration on morphological, structural and optical properties of the films were investigated by ultraviolet (UV)-visible absorption, x-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The XRD study revealed that all the films had a polycrystalline nature, which increased with the Sb doping up to 4 wt.% and then decreased with further Sb doping. Similarly, the SEM images highlighted that the grain size of the SnO2:Sb thin films increased with Sb doping from 2 wt.% to 4 wt.% and then decreased for 6 wt.%. UV–visible study demonstrated that the average transmission in the visible region was found to vary from 35% to 75% depending on the Sb doping concentration. As a proof of concept, we implemented the SnO2:Sb thin films with different Sb doping for gas-sensing applications. To measure the selectivity of the SnO2:Sb thin films, the Sb-doped and -undoped films were exposed to different types of gases with varied concentration. The results of this work demonstrated that the SnO2:Sb thin film-based gas sensor had a high potential for NH3 at a low temperature (100°C). In addition, long-term stability of the SnO2:Sb thin film-based sensor was measured at 100 ppm NH3 for 90 days.

Enhancement in alcohol vapor sensitivity of Cr doped ZnO gas sensor

In this work, we have demonstrated that Cr doping enhances the ethanol sensitivity of ZnO thin film based gas sensor. Cr (0 and 5wt%) substituted ZnO thin films have been grown on Al2O3 substrate using pre-doped target through RF magnetron sputtering technique. XRD and Raman analyses confirm single phase wurtzite structure of the films and average crystallite size reduces on Cr substitution . SEM images exhibit particles of Cr doped ZnO are more uniform and smaller than the undoped thin film. EDS and XPS spectra revealed successful doping of Cr3+ ions and formation of oxygen vacancy on the surface of ZnO film. Moreover, the doping effect enhances the sensitivity of ZnO sensor almost two times and an improvement in the response time/recovery time was also observed. This may be due to increase in surface to volume ratio which leads adsorption of more numbers of ethanol gas molecules at the surface.

Versatile approaches to tune a nanocolumnar structure for optimized electrical properties of In2O3 based gas sensor

In this study, we demonstrate tuning of electrical properties and sensing responses of In2O3 nanocolumnar structure via varying glancing angle (GLAD) deposition conditions by e-beam evaporator. The varied deposition conditions include glancing angle, vacuum level and deposition rate. The electrical property of In2O3 nanostructured thin films, demonstrated by the base resistance, change up to 3 orders of magnitude from 110Ω to 103104Ω depending on the porosity of nanocolumnar structure and oxygen vacancy concentration. This variation in electrical property transfers to the tuning of gas sensing response, and we achieve tuning the same material (In2O3) based gas sensors to better perform for specific type of gases (either oxidizing or reducing). The highest responses achieved in this work reached up to 176 for oxidizing gases (5ppm NO2, Rgas/Rair) and 929 for reducing gases (50ppmC2H5OH, Rair/Rgas). Therefore, we demonstrate that gas sensors can be optimized for specific type of target gases with the same material, via simple control of deposition conditions. Along with the high reproduciblility and sensitivity, this puts the nanocolumnar thin film based gas sensors by GLAD with huge potential for further miniaturization and mass production, suitable for the upcoming IoT era.

A tunable volatile organic compound sensor by using PtOx/GQDs/TiO2 nanocomposite thin films at room temperature under visible-light activation

Pt/GQDs/TiO2 nanocomposite thin film-based gas sensors show tunable VOC sensing behaviour at room temperature under visible-light activation.

Structural and Optical Properties of Ultra-high Pure Hot Water Processed Ga2O3 Thin Film

Thin film based gas sensor is an advanced application of thin film especially Ga2O3 (GO) thin film gas sensor is useful for high temperature gas sensor. The effect of moisture or environment on thin film properties has more influence on gas sensing properties. Radio Frequency sputtered Ga2O3 thin film was synthesized and processed in ultra-high pure hot water at 95 °C for different time durations. The structural properties were verified by the Xray Diffraction technique and the observed spectra revealed the formation of hydroxyl compound of Gallium (Gallium Oxide Dueterate – GOD) on the surface of the thin film and evidenced for structural defects as an effect of moisture. Decreased crystallite size and increased dislocation density was showed the crystal defects of prepared film. From the Ultra Violet – Visible spectra, decreased optical transmittance was noticed for various processing time. The formation of needle like GOD was confirmed using Field Emission Secondary Electron Microscope (FESEM) images.DOI: http://dx.doi.org/10.5755/j01.ms.22.2.7186

Integrated Micro-Heater Solid State Based NO2 Gas Sensors with Enhanced Response

Microelectromechanical systems (MEMS) based integrated gas sensors offer many potential advantages for sensing applications including low power consumption, low fabrication cost, high quality, and reliability. A conductance type gas sensor system can be implemented in a cost effective manner using MEMS technology. In the present work an effort has been made to develop highly sensitive gas sensors for trace level detection of NO2 gas. SnO2 thin film was deposited by RF sputtering technique under 30% O2 and 70% Ar in the reactive gas mixture using a metallic tin (Sn) target at an optimized deposition pressure of 16 mTorr. Bare SnO2 thin film based gas sensor showed a very high sensitivity of ~1.4x104 towards 10 ppm of NO2 gas at a low operating temperature of 100 oC, but with very slow response and recovery times of ~4 min. and ~33min. respectively. Platinum micro-heater has been fabricated using micromachining technique on silicon dioxide membrane of about 1.8 mm thickness which provided improved thermal isolation of the active area. This mircrohotplate has been integrated with the SnO2 based gas sensor for the fast detection of 1 ppm NO2 gas. The temperature of mircrohotplate could be controlled by varying the applied dc voltage. The microhoheater has obtained a maximum temperature of ~800 oC at a dc voltage of 3 V. The SnO2 sensor based on MEMS structure exhibits the maximum sensing response of about 56 towards 1 ppm of NO2 gas at an operating temperature of 70 oC. Thus to attain an operating temperature of 70 oC, the power consumption of MEMS sensor was found to be about 1.2 mW. The studies indicate that the fabricated NO2 gas sensor having integrated micro-heater and SiO2 membrane can be utilized efficiently for the fabrication of a commercial gas sensor with low power consumption.

Low resistive gallium doped nanocrystalline zinc oxide for gas sensor application via sol–gel process

This paper reports the effect of low concentration of Ga doping on the performance of ZnO thin-film based gas sensors. The ZnO films were synthesized on the alkali-free glass substrates by employing a sol–gel process. Different concentrations of Ga were added to the sol–gel mixture for systematic comparison of doping effect. The effect of Ga doping on the microstructure, optical properties, surface morphology, electrical conductivity and gas sensing characteristics was investigated comparing undoped and Ga-doped ZnO thin films. The results revealed that the grain size decreased after the doping of Ga in ZnO. The optical transmittance and bandgap of ZnO films were not affected by the doping of Ga. The Ga doping resulted in a significant increase in electrical conductivity of ZnO thin films. The gas sensing behavior was studied under different concentrations of H2 exposure in the air at an operating temperature of 130°C. The results indicated that the sensing response for H2 was enhanced after the doping of a trace amount of Ga. It was found that 0.3at.% was the optimum Ga doping concentration for ZnO sensors in the low Ga doping concentrations, which exhibited both high response and short response time. The selectivity of 0.3at.% Ga-doped ZnO sensor was investigated for important industrial gas mixtures. It was found that the Ga-doped ZnO sensors exhibited a better detection capability of H2 than NH3 and CH4.

Enhancement of gas sensor response of nanocrystalline zinc oxide for ammonia by plasma treatment

Abstract The effect of oxygen plasma treatment on nanocrystalline ZnO thin film based gas sensor was investigated. ZnO thin films were synthesized on alkali-free glass substrates by a sol–gel process. ZnO thin films were treated with oxygen plasma to change the number of vacancies/defects in ZnO. The effect of oxygen plasma on the structural, electrical, optical and gas sensing properties was investigated as a function of plasma treatment time. The results suggest that the microstructure and the surface morphology can be tuned by oxygen plasma treatment. The optical transmission in the visible range varies after the oxygen plasma treatment. Moreover, it is found that the oxygen plasma has significant impact on the electrical properties of ZnO thin films indicating a variation of resistivity. The oxygen plasma treated ZnO thin film exhibits an enhanced sensing response towards NH 3 in comparison with that of the as-deposited ZnO sensor. When compared with the as-deposited ZnO film, the sensing response was improved by 50% for the optimum oxygen plasma treatment time of 8 min. The selectivity of 8 min plasma treated ZnO sensor was also examined for an important industrial gas mixture of H 2 , CH 4 and NH 3 .

Plasma-produced phase-pure cuprous oxide nanowires for methane gas sensing

Phase-selective synthesis of copper oxide nanowires is warranted by several applications, yet it remains challenging because of the narrow windows of the suitable temperature and precursor gas composition in thermal processes. Here, we report on the room-temperature synthesis of small-diameter, large-area, uniform, and phase-pure Cu2O nanowires by exposing copper films to a custom-designed low-pressure, thermally non-equilibrium, high-density (typically, the electron number density is in the range of 1011–1013 cm−3) inductively coupled plasmas. The mechanism of the plasma-enabled phase selectivity is proposed. The gas sensors based on the synthesized Cu2O nanowires feature fast response and recovery for the low-temperature (∼140 °C) detection of methane gas in comparison with polycrystalline Cu2O thin film-based gas sensors. Specifically, at a methane concentration of 4%, the response and the recovery times of the Cu2O nanowire-based gas sensors are 125 and 147 s, respectively. The Cu2O nanowire-based gas sensors have a potential for applications in the environmental monitoring, chemical industry, mining industry, and several other emerging areas.

Sputtered Nickel Oxide Films for NO<sub>2 </sub>Gas Sensors

In recent years, lot of efforts was made to develop many types of nitrogen oxide gas sensors. Among them, most interesting are WO3, V2O5, TiO2, SnO2 and NiO thin films as gas sensing layers. Relatively small effort has been done to examine nickel oxide. But reasonably good electrical properties and stability in air make it feasible for the fabrication of nickel oxide thin film based gas sensors. Hence, in the present study NiO thin films were deposited by dc reactive magnetron sputtering technique from a nickel metal target in argon and oxygen mixed atmosphere and studied its gas sensing properties towards NO2 gas. The effect of process parameters on the morphological and electrical properties of NiO was studied by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Hall effect studies respectively. The films prepared at optimum conditions showed superior electrical properties and exhibited fine and uniform grains with RMS roughness 9.4 nm. These films were tested for gas sensing characteristics of NO2 gas. The sensitivity of NiO thin film was investigated in the temperature range 373 to 573 K. The dynamic response for the NiO films was observed at an operating temperature of 473 K and gas concentration of 50 ppm for NO2 gas.

Fabrication and characterization of ZnO thin film for hydrogen gas sensing prepared by RF-magnetron sputtering

A ZnO thin film-based gas sensor was fabricated using a SiO2/Si substrate with an integrated platinum comb-like electrode and heating element. The structural characteristics, morphology, and surface roughness of the as-grown ZnO nanostructure were investigated. The optical properties were examined by UV–vis spectrophotometry. The film revealed the presence of a c-axis oriented (002) phase of 20.8nm grain size. The sensor response was tested for hydrogen concentrations of 50, 70, 100, 200, 400, and 500ppm at operating temperatures ranging from 250°C to 400°C. The sensitivity toward 50 and 200ppm of hydrogen at the optimum operating temperature of 350°C were about 78% and 98%, respectively. The response was linear within the range of 50–200ppm of hydrogen concentration. Our results demonstrated the potential application of ZnO nanostructure for fabricating cost-effective and high-performance gas sensors.