V. V. Sysoev

Highly selective gas sensor arrays based on thermally reduced graphene oxide

The electrical properties of reduced graphene oxide (rGO) have been previously shown to be very sensitive to surface adsorbates, thus making rGO a very promising platform for highly sensitive gas sensors. However, poor selectivity of rGO-based gas sensors remains a major problem for their practical use. In this paper, we address the selectivity problem by employing an array of rGO-based integrated sensors instead of focusing on the performance of a single sensing element. Each rGO-based device in such an array has a unique sensor response due to the irregular structure of rGO films at different levels of organization, ranging from nanoscale to macroscale. The resulting rGO-based gas sensing system could reliably recognize analytes of nearly the same chemical nature. In our experiments rGO-based sensor arrays demonstrated a high selectivity that was sufficient to discriminate between different alcohols, such as methanol, ethanol and isopropanol, at a 100% success rate. We also discuss a possible sensing mechanism that provides the basis for analyte differentiation.

A comparative study of SnO2 and SnO2:Cu thin films for gas sensor applications

SnO2 thin film gas sensors were prepared by r.f. magnetron sputtering. It was shown that the gas-sensitive properties of SnO2 thin films are affected substantially by the conditions of preparation which determine the oxygen content in the films. Oxygen-deficient films were fabricated in two manners: (1) deposition in Ar atmosphere followed by annealing in an oxygen atmosphere, and (2) deposition in Ar/O2 mixture. Both methods facilitated the fabrication of polycrystalline films with a high sensitivity to reducing gases. However, the long-term instability of the film conductance caused by oxygen diffusion at advanced operating temperatures did not appropriately meet the sensor requirements. Doping of the films with copper atoms was shown to be a useful method of stabilizing the electrical characteristics. The mechanism of stabilization is proposed.

Conductivity of SnO2 thin films in the presence of surface adsorbed species

Abstract The influence of oxygen and/or ethanol vapors on the SnO 2 :Cu thin film conductance has been studied both in vacuum and in the air-based mixtures under the atmospheric pressure. We show that the dependence of the film conductance on the gas pressure can be approximated by the power low with a constant exponent only in a restricted range of pressure values. The film conductivity is discussed in the framework of electron adsorption theory suggesting that oxygen and ethanol create, correspondingly, acceptor-like and donor-like surface states in the energy gap of the oxide. We demonstrate that the gas-sensitivity of the fully depleted films may be successfully accounted for by the compensation of the semiconductor conductivity with surface impurities. The results of the calculations are in good agreement with our experimental data.

Oxygen flow effect on gas sensitivity properties of tin oxide film prepared by r.f. sputtering

Abstract SnO2 thin film gas sensors were prepared by r.f. magnetron sputtering. It was shown that the gas-sensitive properties of SnO2 thin films are managed substantially by the conditions of preparation which determine the oxygen content in the films. Oxygen-deficient films were fabricated in two manners: (1) deposition in Ar atmosphere followed by annealing in an oxygen atmosphere and; (2) deposition in Ar/O2 mixture with a variable oxygen concentration. Both methods facilitated with the same success the fabrication of polycrystalline films having a high sensitivity to reducing gases. A quantitative model was developed in order to describe the sputtering process of thin metal oxide films. It has been shown how the stoichiometry of as-deposited films is managed by an oxygen concentration in a reactive chamber. The calculations performed could be matched with the experimental results.

Intrinsic device-to-device variation in graphene field-effect transistors on a Si/SiO2 substrate as a platform for discriminative gas sensing

Arrays of nearly identical graphene devices on Si/SiO2 exhibit a substantial device-to-device variation, even in case of a high-quality chemical vapor deposition (CVD) or mechanically exfoliated graphene. We propose that such device-to-device variation could provide a platform for highly selective multisensor electronic olfactory systems. We fabricated a multielectrode array of CVD graphene devices on a Si/SiO2 substrate and demonstrated that the diversity of these devices is sufficient to reliably discriminate different short-chain alcohols: methanol, ethanol, and isopropanol. The diversity of graphene devices on Si/SiO2 could possibly be used to construct similar multisensor systems trained to recognize other analytes as well.

Employment of Electric Potential to Build a Gas-Selective Response of Metal Oxide Gas Sensor Array

This paper presents an approach to design multisensor microarrays for electronic nose instruments employing a metal oxide thin film whose spatial gas-sensitive properties are locally differentiated by electric potential. The measurement of local potential as a sensor signal over the metal oxide film serves to build a gas discriminating pattern which can be selectively identified by pattern recognition techniques. The gas sensitivity studies have revealed a higher gas response on the film areas, which are activated by positive potential, than that at earth potential. The gas-recognition capability of potential patterns is, at least, comparable with the one of conductance patterns recorded at the same microarrays differentiated by nonhomogeneous spatial heating. The experimental data are discussed in terms of oxide surface charging by adspecies coming through the ambient air. The suggested approach could be considered for designing reproducible multisensor systems on a single-chip platform.

Effect of oxygen adsorption on the conductivity of thin SnO2 films

The effect of oxygen adsorption on the conductivity of a thin homogeneous film of tin dioxide was studied. The concentration of free electrons in the film as a function of the partial oxygen pressure in the atmosphere is described using a model formulated within the framework of a flat band approximation. The results of calculations are compared to experimental data.

Individual and collective effects of oxygen and ethanol on the conductance of SnO2 thin films

We study the changes in the conductance of the fully depleted SnO2 films exposed both to oxygen or ethanol vapors in vacuum and to their mixture in the air. The dependence of free carrier concentration on the acceptor-like and/or donor-like gas pressure is discussed assuming the flat-band condition. We show that the gas sensitivity of the depleted semiconductor layer can be explained taking into account the compensation of the sample conductivity by the surface adsorbed species. The results of calculations are in good agreement with our experimental data.

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.

Discrimination of acetone and ammonia vapor using an array of thin-film sensors of the same type

It is shown that a gas mixture can be discriminated by using an array of thin-film semiconductor sensors having variable internal parameters. It is established that a multisensor system consisting of an array of sensors, fabricated by the same technological process but having a spread of active-layer parameters, can discriminate between ammonia and acetone vapor impurities in air.