Conductometric Gas Sensors Research Articles

Improved reversibility in aged porous silicon NO 2 sensors

Porous silicon (PS) conductometric gas sensors can exhibit large sensitivity to gases, due to the large surface versus volume ratio of porous silicon. A possible application is the detection of traces of nitrogen dioxide (NO 2), an air pollutant. Δ G/ G signals in excess of 10 in the presence of concentrations as low as 50 ppb in dry air can be demonstrated. Unfortunately, such high sensitivity to NO 2 is achieved, in fresh samples, with poor reversibility. Another problem is the interference of water vapor, which also affects the porous silicon conductivity. However, we show that reversibility is complete in aged samples, and sensitivity to water vapor is lowered. Although in aged samples large Δ G/ G signals are harder to achieve, we show that concentration levels of NO 2 at few tens of ppb are still detectable.

Nanoepitaxy of SnO 2 on α-Al 2O 3(0 1 2)

Thin tin dioxide films have been grown on r-cut sapphire substrates by atomic-layer chemical vapor deposition (ALCVD), using SnI 4 and O 2 as precursors. The films prepared at 600 °C were characterized in some detail by X-ray diffraction (XRD), reflection high-energy electron diffraction (RHEED), scanning force microscopy (SFM), and conductivity measurements. The XRD data demonstrated that the films grew epitaxially, had a cassiterite structure, and were (1 0 1) cassiterite∥(0 1 2) sapphire oriented. The in-plane orientation relationships for two mutually perpendicular directions have been determined to be [0 1 0] cassiterite∥[1 0 0] sapphire and [1 0 1 ̄ ] cassiterite∥[ 1 ̄ 2 ̄ 1] sapphire. Evidence was found for different epitaxial subdomains. The smoothness of the films has been verified by the RHEED and SFM measurements. The latter showed that ultrathin films with a thickness of ∼10 nm were as smooth as the substrates. The atomic alignment and surface morphology worsened when the ALCVD growth proceeded. It was revealed that the conductivity of the ultrathin films considerably increased under the action of reducing gases in air. This makes the films suitable for application in conductometric gas sensors.

Organotin films deposited by laser-induced CVD as active layers in chemical gas sensors

In this work, an organotin film obtained by L-CVD at 193 nm from tetramethyltin was used as active layer in a conductometric gas sensor. As it was stated by in situ surface analysis, the `as deposited' material contained Sn and C. However, exposure to a large dose of O 2 or air determined the formation of a thin SnO 2 layer covering the remaining film, which was less oxidised and had an oxygen concentration gradient in its depth. Electrical tests of the sensors showed high sensitivity and short response time for the detection of NO 2. Satisfactory performances were observed also with reducing gases. In this case, the high sensitivity measured for H 2 and ethanol, in comparison to CO and hydrocarbons, indicated the possibility to use the sensor for selective revelation.

Fabrication of conductometric gas-sensing films by selected area chemical-vapor deposition

A new selected-area chemical-vapor deposition (CVD) process to fabricate thin sensing films on a micromachined silicon-based, conductometric gas-sensor structure is reported. The CVD method is self-lithographic and does not require the use of masks to define the thin-film region. Since the films are deposited after silicon processing, the selection of film materials is not constrained by their compatibility with the etchants used in silicon micromachining. The presence of conductance electrodes underneath the growing film permits in-situ monitoring of film resistance. The rapid thermal response of the device allows precise termination of film growth once the film resistance has reached a value suitable for gas sensing. The application of this new CVD method is demonstrated for growth of thin platinum films, which are used for sensing of O 2 and H 2 gases.

Composite polyaniline/calixarene Langmuir - Blodgett films for gas sensing

Mixtures of the polyaniline (emeraldine base) and phosphorylated calix[4]resorcinolarene derivative (CA) are proposed to prepare LB films for conductometric gas sensors. They are quite stable at the air - water interface and give LB films of high quality. The average thickness of the mixed monolayers is found to be 1.6 nm. The as-deposited films are insulating. Doping with HCl increases the conductivity up to between and which depends on the component ratio. The films containing more than 20 wt% of CA are doped reversibly in part. Thus, the films which are highly sensitive to either or HCl films are prepared by choosing the component ratio. Detection of and HCl in the ppm range is demonstrated.

A silicon micromachined conductometric gas sensor with a maskless Pt sensing film deposited by selected-area CVD

Inexpensive, conductometric gas micro-sensor platforms have been micromachined using conventional silicon processing. The front end of these sensors is a conducting film that undergoes resistance changes when exposed to certain chemical species. A variety of sensing film materials needs to be explored to obtain a significant sensor response above noise level for selected gases and vapors over wide ranges of temperature and pressure. However, only a few sensing film materials are compatible with the etchants used for silicon micromachining. To open up a wider range of materials for the sensing element, a new selected area CVD process is used to deposit thin conducting films on sensor platforms that have fully functioning conductance probes and integrated microheaters. This paper describes this selected-area CVD process for maskless deposition of thin platinum films with in-situ monitoring of film resistance during growth. Film growth is terminated at a desired value of resistance suitable for sensing of gases and vapors. The structure of the deposited films is characterized by scanning force microscopy. The response of Pt CVD films to H 2, O 2, CO, and propylene gases is reported.

Selected-area deposition of multiple active films for conductometric microsensor arrays

A new generation of planar conductometric gas sensors is being developed which combines active semiconducting oxide films with Si-micromachined ‘micro-hotplate’ array structures. These devices are tailored for a variety of applications by tuning both the composition of multiple types of active films and the temperature cycles programmed for individual elements within an array. In this paper we described and demonstrate the approach, and present results for, a chemical vapor deposition (CVD)-based self-lithographic microfabrication method. This method is being examined as a highly efficient and compatible means of depositing oxide films and low-coverage catalytic metal overlayers on the microsensor elements of micro-hotplate arrays. Real-time monitoring of the film growth processe is provided by conductance measurements, and surveys of processing/property relationships can be performed in a very effective manner to determine optimal fabrication methods for multiple-active material arrays.

Fuzzy neural computing of coffee and tainted-water data from an electronic nose

In this paper we compare the ability of a fuzzy neural network and a common back-propagation network to classify odour samples that were obtained by an electronic nose employing semiconducting oxide conductometric gas sensors. Two different sample sets have been analysed: first, the aroma of three blends of commercial coffee, and secondly, the headspace of six different tainted-water samples. The two experimental data sets provide an excellent opportunity to test the ability of a fuzzy neural network due to the high level of sensor variability often experienced with this type of sensor. Results are presented on the application of three-layer fuzzy neural networks to electronic nose data. They demonstrate a considerable improvement in performance compared to a common back-propagation network.

Intelligent gas sensing using an integrated sensor pair

Abstract The performance of conductometric gas sensors (e.g. semiconducting oxide) is often reduced by the poisoning of the active film or by poor long-term stability of the output signal. In this paper we propose the use of an integrated pair of sensing elements, one with a narrow electrode gap and the other with a wide electrode gap compared with the film thickness, to recognize or overcome these advantages. Equations are derived that described the steady-state response of such a device for the two limiting cases of a uniform gas concentration within the film (type I) and a distance-dependent one with a boundary (type II). The characteristic response, or operating curve, of a sensor pair for a type I material can then be used as a real-time diagnostic for poisoning of the film, whereas the response for a type II material can be used to improve sensor stability by eliminating undesirable common-mode signals. The basic principles apply to several classes of materials including both organic semiconductors (usually type I) and inorganic semiconductors (usually type II).