Effects of electron beam irradiation on tin dioxide gas sensors

In this paper, the effects of electron beam irradiation on the gas sensing performance of tin dioxide thin films toward H₂ are studied. The tin dioxide thin films were prepared by ultrasonic spray pyrolysis. The results show that the sensitivity increased after electron beam irradiation. The electron beam irradiation effects on tin dioxide thin films were simulated and the mechanism was discussed.     

Textured tin dioxide films for gas recognition microsystems

We have studied the microstructure, electrical properties, and gas sensor characteristics of thin tin dioxide (SnO₂) films obtained by RF magnetron sputtering of an oxide target. The synthesized films are composed of crystalline rods with a diameter of 10–60 nm and a length of up to 1000 nm oriented perpendicularly to the substrate plane. This morphology facilitates the access of gas molecules to the side surfaces of crystallites. Use of such SnO₂ films in a multisensor microsystem expanded the spectrum of recognized gases.     

Titanium dioxide thin films for high temperature gas sensors

Titanium dioxide (TiO₂) thin film gas sensors were fabricated via the sol-gel method from a starting solution of titanium isopropoxide dissolved in methoxyethanol. Spin coating was used to deposit the sol on electroded aluminum oxide (Al₂O₃) substrates forming a film 1 μm thick. The influence of crystallization temperature and operating temperature on crystalline phase, grain size, electronic conduction activation energy, and gas sensing response toward carbon monoxide (CO) and methane (CH₄) was studied. Pure anatase phase was found with crystallization temperatures up to 800 °C, however, rutile began to form by 900 °C. Grain size increased with increasing calcination temperature. Activation energy was dependent on crystallite size and phase. Sensing response toward CO and CH₄ was dependent on both calcination and operating temperatures. Films crystallized at 650 °C and operated at 450 °C showed the best selectivity toward CO.     

Laser-deposited thin hafnium dioxide condensates: Electron-microscopic study

Thin films of hafnium dioxide (HfO₂) have been obtained by pulsed laser ablation of hafnium targets in oxygen atmosphere and characterized by transmission electron microscopy and electron diffraction. Conditions ensuring the formation of an amorphous phase, tetragonal and monoclinic modifications of HfO2 have been determined. It is established that the crystalline phase is formed on orienting substrates at lower temperatures than on neutral ones. The phenomenon of epitaxy has been observed for tetragonal modification of HfO₂. Annealing in air leads to crystallization of an initially amorphous film with the formation of a monoclinic HfO₂ modification.     

Mesoporous tin dioxide nanopowders based sensors to selectively detect ethanol vapor

In the paper, mesoporous SnO₂ nanopowders were synthesized via a simple and mild SnCl₄ hydrolysis process using cationic surfactant (cetyltrime thylammonium bromide, CTAB: CH₃(CH₂)₁₅N⁺(CH₃)₃Br^?) as structure directing agent and ammonia as an alkali source at room temperature, combined with a subsequent calcination process. The products were characterized by X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM) and nitrogen adsorption–desorption experiment. A gas sensor was fabricated from the as-prepared mesoporous SnO₂ nanopowders and used to test the response to different concentrations of ethanol, methanol, hexane, NH₃, H₂ and CO at different operating temperatures. The results showed that the mesoporous SnO₂ sensor exhibited high sensitivity, good selectivity and quick response–recovery characteristics to ethanol, implying the potential application of the sensor for detecting ethanol.     

Nitrogen dioxide gas sensors based on titanium dioxide thin films deposited on langasite

Conductometric sensors, based on pure and Au-doped TiO₂ thin films on langasite (LGS) substrates have been investigated for detecting low concentrations (510 ppb and 1060 ppb) of NO₂ in synthetic air at a temperature range between 220 °C and 320 °C. Thin films of TiO₂ were deposited using the radio frequency (RF) magnetron sputtering method. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were employed for the characterization of thin films. It was observed that the Au-doped TiO₂ sensor has fast response and recovery time of 10 and 29 s respectively towards 510 ppb of NO₂ at 273 °C.     

Current-voltage characteristics of thin-film gas sensor structures based on tin dioxide

The experimental current-voltage (I–U) curves of thin-film structures based on tin dioxide (SnO₂) exhibit nonlinearity in the range of strong applied electric fields. The results of I-U measurements are interpreted within the framework of a model that assumes the drift of adsorbed ions over the film surface. The observed phenomenon can be used both for detecting the impurities in air and for recognizing the types of adsorbed species.     

Micro-structure of graphite-intercalated tin oxide and its influence on SnO2-based gas sensors

A nano-scaled graphite oxide (GO) was prepared with a micro-layer structure for intercalation. Graphite-intercalated SnO₂ was obtained at temperatures lower than 100vH. The morphology, microstructure, crystalline phases and thermal property of this intercalative composite were studied by atomic force microscopy (AFM), field-emission scanning electron microscopy (FE-SEM), X-ray diffractometry (XRD) and differential scanning calorimetrythermogravimetry (DSC-TG) doped with a proper amount of graphite-intercalated composites (GITs), GIT-SnO2 composite was obtained after heat treatment. This combined gas sensor reveals low resistance and high sensitivity to butane between 200vH and 300vH.     

Effect of laser treatment on the gas sensitivity of tin dioxide films

It is shown that the sensitivity of antimony-doped polycrystalline SnO₂ films can be increased by treatment with laser pulses. Pis’ma Zh. Tekh. Fiz. 24, 57–60 (April 12, 1998)     

Work function measurements for gas detection using tin oxide layers with a thickness between 1 and 200 nm

Tin oxide films, 1, 2, 20, 50, 80 and 200 nm thick, were deposited on platinum and titanium/tungsten surfaces by reactive sputter deposition. The structure and composition of tin oxide as well as the film homogeneity were investigated by scanning electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry and atomic force microscopy.

The 200 nm SnO₂ films are polycrystalline, with a grain size between 40 and 80 nm. For layers between 20 and 200 nm the composition is characteristic for stoichiometric SnO₂. The 1 and 2 nm layers show a homogeneous coverage of the metal. For all sensitive systems gas measurements were carried out with a Kelvin probe. Testing gases were humidity (40% r.h.), carbon monoxide (1000 ppm), hydrogen (100 ppm), nitrogen dioxide (8 ppm) and ammonia (10 ppm).

For thinner SnO₂ adlayers only surface-related processes contribute to the response, which is still material specific. It turns out that the thin layers are more stable.     

Materials for solid-state gas sensors

The major problems in the development of inorganic gas-sensor materials are discussed. The general principle of semiconductor gas sensors is considered, and the band-structure parameters sensitive to the gas-phase composition are determined. Ways of improving sensor selectivity are examined. The composition, microstructure, and gas sensitivity of nanocrystalline SnO₂ and ZnO are investigated. The dopant content and grain size of Ni-doped SnO₂ are optimized for H₂S detection. The prospects of employing systems of two or more nanocrystalline oxides (nanocomposites) for gas detection are discussed     

Micro-structure of graphite-intercalated tin oxide and its influence on SnO2-based gas sensors

A nano-scaled graphite oxide (GO) was prepared with a micro-layer structure for intercalation. Graphite-intercalated SnO₂ was obtained at temperatures lower than 100°C. The morphology, microstructure, crystalline phases and thermal property of this intercalative composite were studied by atomic force microscopy (AFM), field-emission scanning electron microscopy (FE-SEM), X-ray diffractometry (XRD) and differential scanning calorimetry-thermogravimetry (DSC-TG) doped with a proper amount of graphite-intercalated composites (GITs), GIT-SnO₂ composite was obtained after heat treatment. This combined gas sensor reveals low resistance and high sensitivity to butane between 200°C and 300°C. Translated from Journal of Materials Science & Engineering, 2006, 24(4): 582–587 (in Chinese)     

Fabrication and characterization of nanostructured indium tin oxide film and its application as humidity and gas sensors

This paper reports the synthesis and characterization of nanocrystalline indium tin oxide (ITO) and its application as humidity and gas sensors. The structure and crystallite size of the synthesized powder were determined by X-ray diffraction. The minimum crystallite size was found 5 nm by Debye–Scherrer equation and confirmed by transmission electron microscopy image. Optical characterizations of ITO were studied using UV–visible absorption spectroscopy and Fourier transforms infrared spectroscopy. Thermal analysis was carried out by differential scanning calorimetry. Further, the ITO thin film was fabricated using sol–gel spin coating method. The surface morphology of the fabricated film was investigated using scanning electron microscopy images. For the study of humidity sensing, the thin film of ITO was exposed with humidity in a controlled humidity chamber. The variations in resistance of the film with relative humidity were observed. The average sensitivity of the humidity sensor was found 0.70 MΩ/%RH. In addition, we have also investigated the carbon dioxide (CO₂) and liquefied petroleum gas sensing behaviour of the fabricated film. Maximum sensitivity of the film was ~17 towards CO₂. Its response and recovery times were ~5 and 7 min respectively. Sensor based on CO₂ is 97 % reproducible after 3 months of its fabrication. Better sensitivity, small response time and good reproducibility recognized that the fabricated sensor is challenging for the detection of carbon dioxide.     

Laser alloyed tin layers on GaAs

A Q-switched ruby laser has been used to alloy deposited layers of tin on (100) GaAs. The tin concentration in the GaAs substrates has been investigated by electrical measurements, electron probe microanalysis, Rutherford backscattering and transmission electron microscopy. The results show that a high concentration of tin diffuses into the GaAs for an energy density up to 0.6 J cm^–2 and the electrical properties improve with increasing energy density. However, at high energy densities this leads to the introduction of damage near the GaAs surface. At the highest energy density of 2 J cm^–2, very complex dislocation networks are produced and a cellular structure results along with microcracking of the surface. This produces high levels of residual strain in the surface.     

Model for the potential barrier depth along the grain boundaries of an ultrathin tin dioxide gas sensor

A potential barrier model along the grain boundaries in ultrathin tin dioxide (SnO₂) gas sensors is presented. It is assumed that the negatively charged oxygens are adsorbed only on the grain boundaries. The potential barrier depth is expressed as functions of grain size, donor concentration and surface coverage of adsorbed oxygen ions at the boundaries. A direct consequence is that the conduction electrons are effectively confined in a grain when the film thickness becomes smaller than a critical value. This indicates a drastic increase in resistivity with decreasing film thickness in air, and thus an occurrence of an extremely high gas sensitivity.     

Effect of processing and palladium doping on the properties of tin oxide based thick film gas sensors

In the present work, solid-state reaction and sol–gel route derived pure tin oxide (SnO₂) powders have been used to develop the palladium (Pd)-doped SnO₂ thick film sensors for detection of liquefied petroleum gas (LPG). Efforts have been made to study the gas sensing characteristics i.e., sensor response, response/recovery time and repeatability of the thick film sensors. The response of the sensors has been investigated at different operating temperatures from 200 to 350 °C in order to optimise the operating temperature which yields the maximum response upon exposure to fixed concentration of LPG. The optimum temperature is kept constant to facilitate the gas sensing characteristics as a function of the various concentration (0.25–5 vol%) of LPG. The structural and microstructural properties of Pd-doped SnO₂ powder and developed sensors have been studied by performing X-ray diffraction and field emission electron microscopy measurements. The improvement in the response along with better response and recovery time have been correlated to the reduction in crystallite size of SnO₂ powder and morphology of printed sensor in thick film form. It is found that the thick film sensor developed by using sol–gel route derived SnO₂ powder with an optimum doping of 1 wt% Pd is extremely sensitive (86 %) to LPG at 350 °C.     

MEMS气敏传感器

随着MEMS技术的飞速发展，各种MEMS器件和系统相继问世，MEMS气敏传感器是其中之一。本文重点介绍了7种MEMS气敏传感器。