Thin Film Gas Research Articles

Effect of reduction temperature on structure and hydrogen sensitivity of graphene oxides at room temperature

As precursors exfoliated from graphite oxide gels, graphene oxide thin films are annealed in a temperature range of 100 ℃ to 350 ℃ to obtain a series of reduced graphene oxide samples with different reduction degrees. For the gas sensing experiments, the reduced graphene oxide thin film gas sensing element is prepared by spin coating with Ag-Pd integrated electronic device (Ag-Pd IED). The functional groups, structures, and gas sensing performance of all the samples are investigated by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and gas sensing measurement. The results show that the structure of the graphene oxide samples are transformed to the graphitic structure after reduction at different thermal treatment temperatures. When the reduction temperature is lower than 150 ℃, materials exhibit features of graphite oxide. When the reduction temperature reaches about 200 ℃, the samples show characteristics transformed from graphite oxide to reduced graphite oxide gradually. When the temperature is higher than 250 ℃, materials show features of reduced graphite oxide. During the reduction process, the disorder degree increases from 0.85 to 1.59, and then decreases slightly to 1.41 with the rise of temperature. Additionally, the oxygen containing functional groups are removed with the increasing reduction temperature, and these functional groups can be removed at specific temperatures. In the lower temperature stage (100-200 ℃), the first kind of oxygen containing functional group removed is the hydroxyl group (C-OH) and the epoxy group (C-O-C) is the second. In the higher temperature stage (250-350 ℃), the main removed oxygen containing functional groups are the epoxy group (C-O-C) and the carbonyl group (C=O). The materials treated at 150, 200, 350 ℃ exhibit n-type, ambipolar, and p-type behaviors, respectively, while rGO-200 exhibits considerable increase in resistance upon exposure to hydrogen gas. rGO-200 exhibits very small decrease of resistance at room temperature and moderate increase of resistance at elevated temperatures upon exposure to hydrogen gas, while rGO-350 exhibits considerable decrease of resistance at room temperature upon exposure to hydrogen gas. These results indicate that the reduction temperature affects the distribution of density of states (DOS) in the band gap as well as the band gap size. The graphene oxide and the reduced products at low temperature show good sensitivity to hydrogen gas. With the increasing reduction temperature, the sensitivity fades while the response time and recovery time increases. The gas sensor exhibits high sensitivity (88.56%) and short response time (30 s) when exposed to the 10-4 hydrogen gas at room temperature.

Modeling the sensing characteristics of chemi-resistive thin film semi-conducting gas sensors.

For chemi-resistive thin film gas sensors a generic theoretical model is proposed to predict the variation of sensor response with the operating temperature and thickness of the sensing film. A diffusion equation is formulated assuming that inflammable target gases move through the sensing film by Knudsen diffusion and react with the adsorbed oxygen following first-order kinetics. We have assumed a realistic non-linear variation between the conductance and test gas concentration and derived a general expression relating the sensor response to the operating temperature and thickness of the film. Assuming Langmuir adsorption kinetics, we have theoretically predicted the response and recovery transients during gas sensing using thin film sensing elements. It is predicted that for irreversible type sensing, the response time is reduced with an increase in test gas concentration, whereas for reversible sensing, the response time is independent of test gas concentration. For zinc oxide thin film sensors, an excellent match is obtained between the model prediction and experimental data for their thickness (122 nm to 380 nm) and temperature variation (200 °C to 325 °C) in 500 ppm carbon monoxide (CO) sensing. The maximum CO response% (∼53%) was achieved in 320 nm thick ZnO films. The conductance transients for response and recovery for CO sensing closely follow Langmuir adsorption kinetics and as predicted theoretically, indeed for irreversible sensing, the response time reduces from 350 s to 220 s with an increase in test gas concentration from 20 to 550 ppm. In the case of reversible sensing we found that the response time is ∼55 s irrespective of the CO gas concentration in the range of 5-500 ppm. The models developed in the present work are quite generic in nature and we have discussed their applicability to a wide variety of sensing materials with various types of surface morphologies.

Interference effects between hydrogen and ozone in the response of SnO2-based gas sensors

In this work it was studied the influence of cross-interference effects between hydrogen and ozone on the performances of the SnO2-based thin film gas sensors. The SnO2 films with different thickness were deposited by spray pyrolysis. It was shown that the interference of gases was not a simple arithmetic addition of two effects related to the influence of individual gases on the sensor parameters. Interference of gases depends on many factors, including the concentration of the interacting gases, the film structure and the surface properties of the gas-sensitive material. Therefore it is difficult to predict this effect basing on the results obtained during the sensor testing to individual gases. It was also found that the air ozonization could be used to optimize the sensor parameters, i.e. to increase sensitivity and decrease the time constants of response and recovery. In addition, we have proposed temperature conditions, providing an improvement of the sensor selectivity when detecting hydrogen and ozone.

Structural, morphological, optical and gas sensing properties of pure and Ce doped SnO2 thin films prepared by jet nebulizer spray pyrolysis (JNSP) technique

Pure and cerium (Ce) doped tin oxide (SnO2) thin films are prepared on glass substrates by jet nebulizer spray pyrolysis technique at 450 °C. The synthesized films are characterized by X-ray diffraction (XRD), scanning electron microscopy, energy dispersive analysis X-ray, ultra violet visible spectrometer (UV–Vis) and stylus profilometer. Crystalline structure, crystallite size, lattice parameters, texture coefficient and stacking fault of the SnO2 thin films have been determined using X-ray diffractometer. The XRD results indicate that the films are grown with (110) plane preferred orientation. The surface morphology, elemental analysis and film thickness of the SnO2 films are analyzed and discussed. Optical band gap energy are calculated with transmittance data obtained from UV–Visible spectra. Optical characterization reveals that the band gap energy is found decreased from 3.49 to 2.68 eV. Pure and Ce doped SnO2 thin film gas sensors are fabricated and their gas sensing properties are tested for various gases maintained at different temperature between 150 and 250 °C. The 10 wt% Ce doped SnO2 sensor shows good selectivity towards ethanol (at operating temperature 250 °C). The influence of Ce concentration and operating temperature on the sensor performance is discussed. The better sensing ability for ethanol is observed compared with methanol, acetone, ammonia, and 2-methoxy ethanol gases.

Controlling surface functional composition and improving the gas-sensing properties of metal oxide sensors by electron beam processing

It has been established that the electron beam processing of the ZnO–SiO2 and SnO2–SiO2 thinfilm gas sensors synthesized using the sol-gel method at an electron energy of 900 keV and an absorbed dose of 50 to 200 kGy leads to a significant increase in their sensitivity towards acetone and isopropanol vapors. It is found that the observed effect, which is enhanced with an increase in the absorbed dose, is correlated to an increase in the concentration of the Bronsted acid sites which are formed under the influence of accelerated electrons and capable of reacting to the molecules of the detected substances.

Ion-in-Conjugation: Squaraine as an Ultrasensitive Ammonia Sensor Material.

An organic thin-film gas sensor based on squaraine detects ammonia as low as 40 ppb with impressive reversibility and stability. The resonance-stabilized zwitterionic characteristics offer squaraines high affinity and sensitivity toward electron-rich analytes without irreversible chemical binding, while the embedded squaric ring makes SA-CH3 highly sensitive. The symmetric molecular geometry and good crystallinity also contribute to the high performance.

Effects of surface roughness and gas rarefaction on the quality factor of micro-beam resonators

In this paper, the average modified molecular gas lubrication (MMGL) equation is utilized to analyze the squeeze film damping (SFD) of micro-beam resonators. Under thin gas film spacing and low ambient pressure conditions, the effects of surface roughness and gas rarefaction become important due to their larger surface area to volume ratio. The external SFD and the combined structural damping [thermoelastic damping (TED) and anchor loss] are studied on the quality factor (Q-factor) of micro-beam resonators. The coupling effects of surface roughness and gas rarefaction on SFD are discussed. The results show that the effects of surface roughness are diluted by the gas rarefaction. The Q-factor of micro-beam resonators depends significantly on surface roughness (film thickness ratio, Peklenik number) in higher gas rarefaction (low ambient pressure, low accommodation coefficients) and/or higher modes of the resonators.

(Invited) Development of the Battery-Driven SnO2 Thin Film Gas Sensor

We have developed a micro methane sensor obtained by laminating “nano-columnar structure SnO2 thin film" and catalyst layer on the circular diaphragm support layer with a built-in thin film heater. Ultra-low power consumption (0.06mW) required for 5 year battery operation was achieved by separating the functional elements of gas sensors, optimizing the materials and structures in charge of each function, microminiaturizing, and driving with low power ultra-short pulse. Furthermore, this sensor has realized higher sensitivity and higher selectivity than conventional methane sensors. In May 2015, Osaka Gas released the world's first battery-driven city gas alarm equipped with this sensor.

Fabrication of nanostructured ZnO thin films based NO2 gas sensor via SILAR technique

Zinc oxide (ZnO) thin films have been widely used as an effective gas sensor element. In the present study, nanostructured thin films of ZnO were prepared by using the simplistic and economical successive ion layer adsorption and reaction (SILAR) technique. The effects of SILAR cycles on the structural, optical, surface morphological and electrical properties of nanostructured ZnO thin films were investigated. Characterization techniques such as XRD, UV-vis, PL, FESEM, and Hall measurement were utilized to study the physical and chemical properties of the synthesized films. XRD confirms the formation of hexagonal phase structural ZnO thin films. FE-SEM analysis reveals the formation of well-dispersed ZnO nanoparticles having sizes of ∼18–40nm. The SILAR cycles play a key role in the synthesis of nanostructured ZnO thin films and it is found that, with increasing SILAR cycles, the grain size continues increasing. Optical studies confirm the presence of oxygen vacancies in synthesized ZnO thin films. Finally, the ZnO thin films were exposed to NO2 gas with a concentration of 100ppb–200ppm and the resulting resistance transient was recorded. The nanostructured ZnO thin films synthesized at 30 SILAR cycles displays an enhancement of gas sensing performance and exhibit significantly higher responses (∼5%per ppm). Moreover, our ZnO thin-film-based gas sensor is sensitive to very low concentrations of dangerous NO2 (100ppb). The sensitive gas sensor used to trace level NO2 detection, synthesized via simple SILAR route proves the novelty of our work. The present report provides a new direction in fabricating nanostructured ZnO thin films for low-cost and efficient gas sensing applications.

The effects of surface conditions of TiO2 thin film on the UV assisted sensing response at room temperature

ABSTRACT Thin film oxides have attracted attention in implementations of gas sensors, notably NO 2 , owing to their unique physical, optical, and chemical properties. While the properties are presumed to be strongly dependent on the surface conditions of the thin films, it is not yet clear how surface properties of the thin film gas sensor affect its analyte sensing response. Here, we report the influence of surface carbon contamination and roughness on the NO 2 sensing properties of TiO 2 thin film sensors. The TiO 2 thin films were prepared by rf-sputtering. The surface of the films were intentionally contaminated and damaged with organic polymers (photolithography resist) and microwave plasma, respectively. The surface chemistry of the films was assessed by high resolution X-ray photoelectron spectroscopy, and atomic force microscopy was exploited to obtain the morphology of the fabricated sensors. The work aims to improve the long-term efficacy of gas sensors by studying the reasons for degradation in performance. Our results indicate that the carbon residue and surface roughness of the TiO 2 based sensor prolong the NO 2 response time by roughly threefold and fivefold, respectively. The recovery rate of the sensor is deteriorated by the poor surface conditions as well.

Dielectric Dispersion in Ga2S3 Thin Films

In this work, the structural, compositional, optical, and dielectric properties of Ga2S3 thin films are investigated by means of X-ray diffraction, scanning electron microscopy, energy dispersion X-ray analysis, and ultraviolet—visible light spectrophotometry. The Ga2S3 thin films which exhibited amorphous nature in its as grown form are observed to be generally composed of 40.7 % Ga and 59.3 % S atomic content. The direct allowed transitions optical energy bandgap is found to be 2.96 eV. On the other hand, the modeling of the dielectric spectra in the frequency range of 270–1,000 THz, using the modified Drude-Lorentz model for electron-plasmon interactions revealed the electrons scattering time as 1.8 (fs), the electron bounded plasma frequency as ~0.76–0.94 (GHz) and the reduced resonant frequency as 2.20–4.60 ×1015 (Hz) in the range of 270–753 THz. The corresponding drift mobility of electrons to the terahertz oscillating incident electric field is found to be 7.91 (cm 2/Vs). The values are promising as they nominate the Ga2S3 thin films as effective candidates in thin-film transistor and gas sensing technologies.

Printed thin-film transistors and NO2 gas sensors based on sorted semiconducting carbon nanotubes by isoindigo-based copolymer

In this work, a new poly(9,9-dioctylfluorene) (PFO) derivative (named PFIID) with the large conjugated planar surfaces was designed and synthesized, and the ability to sort sc-SWCNTs was investigated in detailed. The absorption spectra of sorted SWCNT solutions demonstrated that PFIID exhibited the ability to selectively and efficiently sort sc-SWCNTs from commercial arc discharge carbon nanotubes. The underlying separation mechanism was studied with the aid of photoluminescence excitation (PLE) spectra and the molecular dynamic simulation. It was found that the simulation results agreed well with the experimental data. PFIID-sorted sc-SWCNTs were acted as the channel materials to construct printed thin-film transistors (TFTs) and nitrogen dioxide gas sensors. Printed SWCNT TFTs exhibited superior performance with good uniformity, high on/off ratios (∼106), high effective hole mobility (∼29.8 cm2 V−1 s−1) and small subthreshold swing (SS) (142–163 mV/dec) at low operating voltage (±2 V). NO2 gas sensors also showed good performance with high sensitivity (ΔR/Ro∼96%), fast response (30 s), good stability and the recovery time of 30 s when exposed to 60 ppm NO2 at room temperature. Both of the sensitivity and recovery time are one of the best performances reported for NO2 gas sensors at room temperature.

Effect of gas rarefaction on the quality factors of micro-beam resonators

The external squeeze film damping (SFD) of microelectromechanical systems (MEMS) resonators is a dominant factor to lower the quality factor (Q-factor) due to their large surface area to volume ratio and small spacing. To improve the Q-factor of MEMS resonators, the effect of gas rarefaction (low gas ambient pressure in thin gas film thickness) or operating in higher mode should be considered in SFD analysis. The modified molecular gas lubrication (MMGL) equation is applied for modeling the SFD with gas rarefaction effects taken into consideration. The effects of inverse Knudsen number, surface accommodation coefficients (ACs) and operating frequency on SFD are discussed. The combined effects of SFD, thermoelastic damping (TED) and anchor loss on the total Q-factors of MEMS resonators are considered. The contribution of SFD on the total Q-factor (weighting of SFD) is also discussed. The results show that weighting of SFD could be decrease at low inverse Knudsen number or low ACs or operating at high resonant frequencies.

Optimization of Gas Sensing Performance of Nanocrystalline SnO2 Thin Films Synthesized by Magnetron Sputtering**Supported by the Bandar Abbas Branch of the Islamic Azad University.

Tin oxide (SnO2) is one of the most promising transparent conducting oxide materials, which is widely used in thin film gas sensors. We investigate the dependence of the deposition time on structural, morphological and hydrogen gas sensing properties of SnO2 thin films synthesized by dc magnetron sputtering. The deposited samples are characterized by XRD, SEM, AFM, surface area measurements and surface profiler. Also the H2 gas sensing properties of SnO2 deposited samples are performed against a wide range of operating temperature. The XRD analysis demonstrates that the degree of crystallinity of the deposited SnO2 films strongly depends on the deposition time. SEM and AFM analyses reveal that the size of nanoparticles or agglomerates, and both average and rms surface roughness is enhanced with the increasing deposition time. Also gas sensors based on these SnO2 nanolayers show an acceptable response to hydrogen at various operating temperatures.

A fast response/recovery of hydrophobic Pd/V2O5 thin films for hydrogen gas sensing

We have developed a fast and highly sensitive gas sensor based on the Pd/V2O5 thin films for the low temperature detection of hydrogen in the range 2–500ppm in air. Sensor response, selectivity, and stability studies reveal excellent sensing of the thin films. The formation of vanadium bronze was studied by spectroscopic ellipsometry. The low temperature operation under ambient conditions and the wide range sensing indicates that these hydrophobic nanocrystalline thin films can be explored for optical as well as chemiresistive gas sensing applications.

Achieving selectivity from the synergistic effect of Cr and Pt activated SnO2 thin film gas sensors

The enhanced selectivity due to the synergistic effect of Cr and Pt co-doping on gas sensing properties of SnO2 thin films, is reported. The undoped and Cr/Pt co-doped SnO2 thin films are deposited on the glass substrates by Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM). The as-deposited films are nanocrystalline in nature with crystallite size ∼3nm, as seen from x-ray diffraction and transmission electron microscopy (TEM). The chemical state of dopants is investigated using x-ray photoelectron spectroscopy. Further, the presence of separate Pt additives in SnO2 film is also confirmed using TEM studies. The gas sensing properties show that although Pt addition enhances the overall sensitivity, it does not improve the selectivity of the sensor. Nonetheless, Cr and Pt co-doped SnO2 films exhibit greater selectivity towards CO gas, while only Cr doped film exhibits selectivity towards NO2 gas at 275°C. The improved selectivity is attributed to the change in carrier concentration, oxygen stoichiometry and specific catalytic effect of the dopant ions.

An Experimental Study on the Gas Entrainment in Horizontally and Vertically Installed Centrifugal Pumps

In this work, we have studied how gas accumulates in an industrial centrifugal pump under various steady-state two-phase flow conditions. Thereby, we considered both horizontal and vertical pump installation positions. Phase fractions within the impeller region of the pump have been quantitatively disclosed using high-resolution gamma-ray computed tomography (HireCT) and applying time-averaged rotation-synchronized CT scanning technique. The study was made for inlet volumetric gas flow rates between 0% and 5%. To account for different inlet flow conditions, which are assumed to occur during unwanted gas entrainment by hollow vortices, we produced disperse and swirling gas–liquid inlet flows. In this way, the influence of inlet flow boundary conditions on the pump performance as well as gas fraction distributions and gas holdup within the impeller wheel region could be successfully analyzed and compared with respect to the impeller alignment. It was shown that the installation position offers only a minor effect on the pump performance in comparison to the inlet flow conditions. In addition, for the first time, thin gas films at the pressure side of the impeller wheel blades could be visualized in an industrial centrifugal pump.

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

Slot coating minimum film thickness in air and in rarefied helium

This study assesses experimentally the role of gas viscosity in controlling the minimum film thickness in slot coating in both the slot over roll and tensioned web modes. The minimum film thickness here is defined with respect to the onset of air entrainment rather than rivulets, the reason being that rivulets are an extreme form of instabilities occurring at much higher speeds. The gas viscosity effects are simulated experimentally by encasing the coaters in a sealed gas chamber in which various gases can be admitted. An appropriate choice of two gases was used to compare performances: air at atmospheric pressure and helium at sub-ambient pressure (25mbar), which we establish has a significantly lower “thin film” viscosity than atmospheric air. A capacitance sensor was used to continuously measure the film thickness on the web, which was ramped up in speed at a fixed acceleration whilst visualizations of the film stability were recorded through a viewing port in the chamber. The data collected show clearly that by coating in rarefied helium rather that atmospheric air we can reduce the minimum film thickness or air/gas entrainment low-flow limit. We attribute this widening of the stable coating window to the enhancement of dynamic wetting that results when the thin film gas viscosity is reduced. These results have evident practical significance for slot coating, the coating method of choice in many new technological applications, but it is their fundamental merit which is new and one that should be followed with further data and theoretical underpinning.

New Insight into H2S Sensing Mechanism of Continuous SnO2–CuO Bilayer Thin Film: A Theoretical Macroscopic Approach

SnO2–CuO is one the most promising systems for detection of detrimental H2S gas. Although previous experimental research has suggested a sulfidation reaction to explain selectivity toward H2S, little is known about the origin of change of electrical response of this system by changing the H2S gas concentration. In this study the relation between sensing response of continuous SnO2–CuO bilayer thin film and H2S gas concentration is computed based on changeability of chemical composition of covellite CuxS. For this purpose, chemical activity of sulfur as a function of atomic fraction in covellite copper sulfide is estimated using Gibbs energies of formation and chemical thermodynamics. By considering equilibrium between covellite and H2S, a relationship between chemical composition of the copper sulfide and concentration of the gas at 150 °C is obtained. Utilizing electronic physics and finite difference method, electrical response of the sensor as a function of H2S concentration is computed. Results show t...