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Schottky barrier diodes, developed by low-cost techniques and low temperature processes (LTP-SBD), have gained attention for different kinds of novel applications, including flexible electronic fabrication. This work analyzes the behavior of the I–V characteristic of solution processed, ZnO Schottky barrier diodes, fabricated at a low temperature. It is shown that the use of standard extraction methods to determine diode parameters in these devices produce significant dispersion of the ideality factor with values from 2.2 to 4.1, as well as a dependence on the diode area without physical meaning. The analysis of simulated I–V characteristic of LTP-SBD, and its comparison with experimental measurements, confirmed that it is necessary to consider the presence of a density of states (DOS) in the semiconductor gap, to understand specific changes observed in their performance, with respect to standard SBDs. These changes include increased values of Rs, as well as its dependence on bias, an important reduction of the diode current and small rectification values (RR). Additionally, it is shown that the standard extraction methodologies cannot be used to obtain diode parameters of LTP-SBD, as it is necessary to develop adequate parameter extraction methodologies for them.

Undoped and nickel-doped zinc oxide thin films were deposited on sodalime glass substrates by utilizing dip coating and ultrasonic spray pyrolysis deposition techniques. In both cases zinc acetate and nickel acetylacetonate were used as zinc precursor and nickel dopant source, respectively. XRD analysis confirms the ZnO wurtzite structure with (002) as the preferential orientation.SEM studies show the formation of two types of morphologies, primarily a porous spherical grains with a grain size distribution from 40 to 150 nm and another, rose-like structures with size distribution from 30 to 200 nm, based on different deposition techniques utilized. The elemental depth profiles across the films were investigated by the secondary-ion mass spectrometry (SIMS). Different gas sensing responses of all ZnO films were obtained for both propane and carbon monoxide gases, at different gas concentrations and operating temperatures. The highest sensing response (~6) for undoped ZnO films was obtained for films deposited by ultrasonic spray pyrolysis (USP). Nevertheless, the highest sensing response (~4 × 104) for doped ZnO films was obtained for films deposited by dip coating method. The behavior of sensing responses is explained in detail based on the morphological properties and the amount of Ni impurities incorporated into the crystal lattice.

The physical properties and the effect of effective surface area (ESA) on the sensing properties of tin dioxide [SnO2] thin films in air and propane [C3H8] atmosphere as a function of operating temperature and gas concentration have been studied in this paper. SnO2 thin films with different estimated thicknesses (50, 100 and 200 nm) were deposited on glass substrates by the chemical spray technique. Besides, they were prepared at two different deposition temperatures (400 and 475 °C). Tin chloride [SnCl4∙5H2O] with 0.2 M concentration value and ethanol [C2H6O] were used as tin precursor and solvent, respectively. The morphological, and structural properties of the as-prepared films were analyzed by AFM and XRD, respectively. Gas sensing characteristics of SnO2 thin solid films were measured at operating temperatures of 22, 100, 200, and 300 °C, and at propane concentration levels (0, 5, 50, 100, 200, 300, 400, and 500 ppm). ESA values were calculated for each sample. It was found that the ESA increased with the increasing thickness of the films. The results demonstrated the importance of the achieving of a large effective surface area for improving gas sensing performance. SnO2 thin films deposited by spray chemical were chosen to study the ESA effect on gas sensing properties because their very rough surfaces were appropriate for this application.

Chromium and ruthenium-doped zinc oxide (ZnO:Cr) and (ZnO:Ru) thin solid films were deposited on soda-lime glass substrates by the sol-gel dip-coating method. A 0.6 M solution of zinc acetate dihydrate dissolved in 2-methoxyethanol and monoethanolamine was used as basic solution. Chromium (III) acetylacetonate and Ruthenium (III) trichloride were used as doping sources. The Ru incorporation and its distribution profile into the films were proved by the SIMS technique. The morphology and structure of the films were studied by SEM microscopy and X-ray diffraction measurements, respectively. The SEM images show porous surfaces covered by small grains with different grain size, depending on the doping element, and the immersions number into the doping solutions. The sensing properties of ZnO:Cr and ZnO:Ru films in a propane (C3H8) atmosphere, as a function of the immersions number in the doping solution, have been studied in the present work. The highest sensitivity values were obtained for films doped from five immersions, 5.8 and 900, for ZnO:Cr and ZnO:Ru films, respectively. In order to evidence the catalytic effect of the chromium (Cr) and ruthenium (Ru), the sensing characteristics of undoped ZnO films are reported as well.

Spinel-type ZnMn2O4 nanoparticles were synthesized via a simple and inexpensive microwave-assisted colloidal route. Structural studies by X-ray diffraction showed that a spinel crystal phase of ZnMn2O4 was obtained at a calcination temperature of 500 °C, which was confirmed by Raman and UV-vis characterizations. Spinel-type ZnMn2O4 nanoparticles with a size of 41 nm were identified by transmission electron microscopy. Pellet-type sensors were fabricated using ZnMn2O4 nanoparticles as sensing material. Sensing measurements were performed by exposing the sensor to different concentrations of propane or carbon monoxide at temperatures in the range from 100 to 300 °C. Measurements performed at an operating temperature of 300 °C revealed a good response to 500 ppm of propane and 300 ppm of carbon monoxide. Hence, ZnMn2O4 nanoparticles possess a promising potential in the gas sensors field.

In this work, we report the synthesis of Cu, Pt and Pd doped SnO2 powders and a comparative study of their CO gas sensing performance. Dopants were incorporated into SnO2 nanostructures using chemical and impregnation methods by using urea and ammonia as precipitation agents. The synthesized samples were characterized using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM). The presence of dopants within the SnO2 nanostructures was evidenced from the HR-TEM results. Powders doped utilizing chemical methods with urea as precipitation agent presented higher sensing responses compared to the other forms, which is due to the formation of uniform and homogeneous particles resulting from the temperature-assisted synthesis. The particle sizes of doped SnO2 nanostructures were in the range of 40–100 nm. An enhanced sensing response around 1783 was achieved with Cu-doped SnO2 when compared with two other dopants i.e., Pt (1200) and Pd:SnO2 (502). The high sensing response of Cu:SnO2 is due to formation of CuO and its excellent association and dissociation with adsorbed atmospheric oxygen in the presence of CO at the sensor operation temperature, which results in high conductance. Cu:SnO2 may thus be an alternative and cost effective sensor for industrial applications.

Spinel ZnCo2O4 nanoparticles were synthesized by means of the microwave-assisted colloidal method. A solution containing ethanol, Co-nitrate, Zn-nitrate, and dodecylamine was stirred for 24 h and evaporated by a microwave oven. The resulting solid material was dried at 200 °C and subsequently calcined at 500 °C for 5 h. The samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy, confirming the formation of spinel ZnCo2O4 nanoparticles with average sizes between 49 and 75 nm. It was found that the average particle size decreased when the dodecylamine concentration increased. Pellets containing ZnCo2O4 nanoparticles were fabricated and tested as sensors in carbon monoxide (CO) and propane (C3H8) gases at different concentrations and temperatures. Sensor performance tests revealed an extremely high response to 300 ppm of CO at an operating temperature of 200 °C.

Pure and copper (Cu)-incorporated tin oxide (SnO2) pellet gas sensors with characteristics provoking gas sensitivity were fabricated and used for measuring carbon monoxide (CO) atmospheres. Non-spherical pure SnO2 nano-structures were prepared by using urea as the precipitation agent. The resultant SnO2 powders were ball milled and incorporated with a transition metal, Cu, via chemical synthesis method. The incorporation is confirmed by high-resolution transmission electron microscope (HRTEM) analysis. By utilizing Cu-incorporated SnO2 pellets an increase in the CO sensitivity by an order of three, and a decrease in the response and recovery times by an order of two, were obtained. This improvement in the sensitivity is due to two factors that arise due to Cu incorporation: necks between the microparticles and stacking faults in the grains. These two factors increased the conductivity and oxygen adsorption, respectively, at the pellets’ surface of SnO2 which, in turn, raised the CO sensitivity.

Bystromite (MgSb2O6) nanorods were prepared using a colloidal method in the presence of ethylenediamine, after a calcination step at 800 °C in static air. From X-ray powder diffraction analyses, a trirutile-type structure with lattice parameters a = 4.64 Å and c = 9.25 Å and space group P42/mnm was identified. Using scanning electron microscopy (SEM), microrods with sizes from 0.2 to 1.6 μm were observed. Transmission electron microscopy (TEM) analyses revealed that the nanorods had a length of ~86 nm and a diameter ~23.8 nm. The gas-sensing properties of these nanostructures were tested using pellets elaborated with powders of the MgSb2O6 oxide (calcined at 800 °C) at temperatures 23, 150, 200, 250 and 300 °C. The pellets were exposed to different concentrations of carbon monoxide (CO) and propane (C3H8) at these temperatures. The results showed that the MgSb2O6 nanorods possess excellent stability and high sensitivity in these atmospheres.