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The paper presents the results of a study on the sensitivity of gas-sensing zinc oxide ZnO film nanostructures at room temperature. The aim of the research was to investigate the effect of ambient humidity on the conductivity of ZnO samples and their response to ethanol vapor in the presence of water vapor. It has been discovered that zinc oxide films are responsive to both water and ethanol vapor at room temperature, across a broad range of concentrations (5% to 50% saturated vapor). The study found that repeated exposure to water vapor can lead to changes in the conductivity of zinc oxide samples when they are exposed to dry air. Additionally, pre-annealing the samples at 400°C can help to replicate the concentration-dependent response of gas-sensitive structures to water vapor. Hysteresis was observed in the relationship between concentration and response to water vapor, in the range of 5% to 90% of the relative humidity. This can be explained by the capillary condensation of water vapor within the mesopores of zinc oxide layers. As the humidity of a gas sample containing ethanol increased, the response values and detection limit for ethanol decreased in the gas-air mixture for ZnO samples. Statistical analysis using the principal component method showed the potential for classifying dry and humid gas samples with ethanol vapor in air. Data processing was used to eliminate the influence of the background humidity on the calibration curve for gas-sensitive ZnO samples, demonstrating the effectiveness of this method.

An experimental and theoretical study was conducted on the temperature dependence of the conductivity of zinc oxide (ZnO) layers in dry air and ethanol. The study found that the conductivity of ZnO depends non-linearly on temperature, and its type varies depending on experimental conditions. Scanning speed directly affects the type of the temperature dependence, with a local minimum in conductivity between 250±5°C at a scanning rate of 0,4°C/min. However, monotonic dependencies are observed at slower scanning rates, such as 30°C/min, and a mechanism is proposed to explain the effect of the scanning velocity on the type of dependency. This mechanism involves the process of the thermal activation of oxygen molecules to form atomic oxygen at adsorption sites. The key idea behind this mechanism is that atomic oxygen forms deeper acceptor levels on zinc oxide surfaces compared to molecular oxygen. At a high scanning rate, the relaxation time for filling the surface with adsorbed oxygen is significantly longer than the experimental time. Therefore, metastable «frozen» states of atomic forms of adsorbed oxygen appear on the surface of zinc oxide layers. As a result, metastable «frozen» states of adsorbed atoms appear on the surface of zinc oxide layers. Based on calculations using this model, we propose a method for reducing the number of nonequilibrium atomic oxygen states by heating samples in a reducing medium.

Temperature conductivity studies of films based on Cu:SnO2 made by magnetron sputtering of the mixed target CuO/SnO2 have been carried out. Temperature conductivity dependencies were substantially nonlinear. It was found that the local conductivity minimum was observed near the temperature of 330°C. To explain the results, a mathematical model is proposed of oxygen adsorption in various forms on the surface of wide-bandgap semiconductors. It was assumed that oxygen particle adsorption resulted in energy levels of the acceptor type localized near the surface of the semiconductor. The simulation carried out within the proposed model showed qualitative and quantitative consistency of the calculation results and experimental data of the temperature dependence of conductivity of the formed gas-sensitive Cu:SnO2 layers in oxygen-containing atmosphere. An analysis of experimental temperature dependence showed that the local conductivity minimum is due to the process of dissociation of oxygen particles adsorbed in molecular form. The desorption energies of each form of adsorbed oxygen and the depth of their surface acceptor level are assessed.

The results of the response study of gas sensors based on gas-sensitive layers Cu:SnO2 to ethanol, acetone, and 2-propanol vapors are presented. Sensor response concentration dependencies are measured at different temperatures to determine optimal operating modes. Experimental temperature dependence of the sensors response to the vapors of the analyzed substances are determined. The dependence is theoretically substantiated by a proposed mathematical model based on consideration of the processes of transfer of charge carriers through the potential barrier grains of a polycrystalline semiconductor film in an oxygen-containing medium and the reducing gas to be analyzed. It is assumed that each type of gas on the surface of SnO2 has its own adsorption centers. The work shows that the model can be used to calculate and predict the gas sensitivity measurements of sensors based on the active layers of polycrystalline semiconductors. The results of the calculations confirm that such energy parameters as the depth of the energy level and the desorption energies are individual for each substance.

The temperature dependencies of the response of Cu:SnO2 films to ethanol, 2-propanol vapours, as well as ethanol with 3% vol. background impurities of 2-prophanol in the working temperature range of 250-375°C. The aim of the study was to determine the effect of the 2-propanol background impurities on the sensor response to ethanol, as well as to assess the possibility of distinguishing a steam-air mixture containing pure ethanol vapors from a gas mixture of ethanol/2-propanole by means of single-sensor signal statistical processing. The temperature dependencies analysis of the sensor response showed that the temperature at which the maximum response is observed is individual for each substance. A selective response to substances was detected. It was found that the 3% vol. isopropyl alcohol compound decreases the response to ethanol in the tested concentration and temperature range. Statistical processing of experimental data by the principal component analysis (PCA) and cross-validation of the model by the ellipsoid and nearest-neighbor showed the fundamental ability to recognize ethanol, 2-propanol and their mixture.

Composite thin films based CuO:SnO2 and ZnO:SnO2 with different impurity concentrations was synthesized by sol-gel method by mixing sols. The films were annealed in oxygen-containing atmosphere at 550°C. A contact system was formed on top of the films by thermal evaporation of chromium, followed by deposition through a mask onto the sample. Based on the temperature dependence of conductivity for layers based on pure tin dioxide, a local maximum of conductivity is observed in the range of 250°C, which is probably due to the thermal activation process of oxygen desorption. Conductivity of SnO2 layers was decreased as the concentration of Cu dopant. 2% Cu additive reduces the long-term conductivity drift by an order of magnitude. Arguably it’s the consequence of occurrence the low mobility complexes created by copper atoms and oxygen vacancies. The best sensitivity to ethanol vapors in all concentration range was shown by 6%Zn doped samples. Accordingly, supplementations of Cu and Zn boost sensitivity of SnO2 layers to ethanol and decrease response time by its impact at 150-300°C range.

Tin dioxide nanowhiskers were grown by physical vapor deposition and transferred to the contact system by the frozen drop method. The sensors demonstrate gas-sensitivity. Exposing sensors to the atmosphere contained vapors of reducing-gases leads to an increasing of their conductivity. A long-term drift of the sensor conductivity during reducing-gas exposition was shown. A sensitivity response vs concentration for ethanol, acetone, and propanol-2 vapors at temperature 300°C was investigated. Desorption energies of the particles and the positions of the donor levels induced by adsorption of reducing gases particles were calculated by analysis of the conductivity vs concentration dependence. The calculated parameters had a significantly smaller time drift in comparison with the conductivity. Using of these parameters makes possible to recognize gas-air mixtures: classify the each of three studied samples to one of classes.

Nanostructured gas-sensitive tin dioxide films have been synthesized by sol-gel technology. A conductivity vs temperature dependence of a gas sensor into atmosphere of synthetic air has been investigated. A response vs temperature dependence of a gas sensor into atmosphere of isopropanol vapors with various concentrations has been investigated. Local minimum on the temperature dependence of the tin dioxide film conductivity in clean air atmosphere were observed. A decrease in conductivity with increase temperature in the range of 300...350 °C can be associated with a dissociation molecular form of the adsorbed oxygen. At temperatures above 350 °C, conductivity increases because of desorption of the atomic form of the adsorbed oxygen on the surface of gas-sensitive tin dioxide film. The greatest response value is achieved at a sensor temperature equal to 350 °C. It is proposed that the reason is a domination of the atomic form of the chemisorbed oxygen on the surface. The analysis of response time vs concentration and response time vs temperature of gas sensor has been carried out. Sensor response time decreases monotonically with increase admixture substance in gas-probes, apparently because of increase in adsorption rate admixture particles on the surface of gas-sensitive film. It was found that the dependence of the response time on the operating temperature has an Arrhenius form. This may be associated with thermally activated adsorption-desorption processes and heterogeneous chemical reactions on the surface of sensor active layer.

The influence of organic vapors (isopropanol, ethanol, and acetone) with different concentrations in air on a response gas sensor based on nanostructured tin dioxide films synthesized by the sol-gel method was studied. It was found experimentally that inject of the gas mixtures containing organic vapors into measure chamber results to an increase of the conductivity of nanostructured tin dioxide films. In the area of high concentrations (more than 50% of saturated vapor), the concentration dependence of conductivity trends to saturation. The recognizing possibility of species gas admixture by using statistical processing of response only single sensor at different analyzed gas mixture concentrations is validated. The surface donor level of gas-reducing relative to acceptor level oxygen and desorption heat of gas particle were determined. A new method based multiparametric recognition gas mixtures is proposed based on using concentration-independent physical and chemical parameters of analyzed gases as classification properties. It is shown that proposed multiparametric recognition method has higher reliability in comparison with standard methods based on the analysis gas sensor response.

Tin dioxide nanowhiskers were grown by the vapor deposition. The effect of technological parameters on the morphology of the grown nanostructures was investigated. It is shown that an increase in the partial pressure of oxygen in the range from 0,06 mbar to 9,3 mbar and the temperature of the deposition zone in the range from 940°C to 995°C leads to an increase in the diameter of the formed nanocrystals. Experimentally it was shown that the average diameter of nanowhiskers varied from 80 nm to 250 nm. A gas sensor was formed with an active layer, based on the nanocrystals, sensitive to acetone vapor. The value of the gas sensitivity and the characteristic times of the sensor are sufficient for practical use in gas analyzers and systems for recognizing gas mixtures and odors.

The concentration dependence of the gas sensitivity of a gas sensor based on a nanostructured tin dioxide film was studied under various operating conditions in a wide range of concentrations of gases of various nature (ammonia, acetone, propanol, and ethanol). It has been experimentally shown that the concentration dependence of the gas sensitivity is power-law, the exponent n is different for different gas samples, and the value of the power-law exponent n depends on the operating mode of the sensor and the chemical nature of the sample being analyzed. The studied phenomenon makes it possible to recognize gas samples containing vapors of different chemical substances. The probability of classification of gas samples of ammonia, acetone, propanol and ethanol by the method of nearest neighbors was more than 95 %.

Using standard methods of data classification, the possibility of recognizing gas mixtures using a multisensor microsystem with a gas-sensitive layer based on whisker nanocrystals is shown. The statistical analysis of the experimental results showed that the illumination of a gas-sensitive layer of tin dioxide with a LED in the ultraviolet range significant increases the median Euclidean-Mahalanobis distance between the classes (recognition ability) of various gas samples compared to the results of measurements in the dark. Suggested that the increase in the recognition ability of a multisensor microsystem is associated with the selective effect of illumination on surface chemical reactions of oxygen species and particles of analyzed gases.

The results of experimental studies of sensitivity to organic vapors in air of the tin dioxide gas sensor are presented. It is shown that there is a fundamental possibility of recognizing organic vapors in air using a single chemoresistive sensor by varying temperatures of the operation heater.

In this work cyclical I−V characteristics of a gas sensor based on tin dioxide thin films in wet air at room temperature has been studied. It was demonstrated that cyclical voltage variation leads to redox reactions on the surface of the gas sensor active layer. Redox reaction potential does not depend on the gas concentration in an analyzed gas probe. Therefore, the redox potential can be used as an identifying agent.

Investigation results of water vapour and illumination influence on the conductivity of tin dioxide thin film based gas sensors are presented. According to literature it is known that water vapor has a donor effect on the conductivity of the tin dioxide (n-type semiconductor), i.e. its conductivity in the water vapor atmosphere increases. It was experimentally found that the presence of water vapor in the measuring chamber leads to an increase in the conductivity of the sensor (in the dark) or to a decrease in conductivity (when the active layer of the sensor is illuminated).

The purpose of the present study is temperature dependence of the conductivity of nanostructured tin dioxide films on temperature after treatment in an argon plasma. It is shown that thermal cycling conducted after treatment in an argon plasma returns temperature dependence of conductivity in the initial state.