Structure, Surface Morphology, Chemical Composition, and Sensing Properties of SnO2 Thin Films in an Oxidizing Atmosphere

We conducted experiments on SnO2 thin layers to determine the dependencies between the stoichiometry, electrochemical properties, and structure. This study focused on features such as the film structure, working temperature, layer chemistry, and atmosphere composition, which play a crucial role in the oxygen sensor operation. We tested two kinds of resistive SnO2 layers, which had different grain dimensions, thicknesses, and morphologies. Gas-sensing layers fabricated by two methods, a rheotaxial growth and thermal oxidation (RGTO) process and DC reactive magnetron sputtering, were examined in this work. The crystalline structure of SnO2 films synthesized by both methods was characterized using XRD, and the crystallite size was determined from XRD and AFM measurements. Chemical characterization was carried out using X-ray photoelectron (XPS) and Auger electron (AES) spectroscopy for the surface and the near-surface film region (in-depth profiles). We investigated the layer resistance for different oxygen concentrations within a range of 1–4%, in a nitrogen atmosphere. Additionally, resistance measurements within a temperature range of 423–623 K were analyzed. We assumed a flat grain geometry in theoretical modeling for comparing the results of measurements with the calculated results.

Tuning the Optical Properties of WO3 Films Exhibiting a Zigzag Columnar Microstructure

Tungsten oxide WO3 thin films are deposited by DC reactive magnetron sputtering. The Reactive Gas Pulsing Process (RGPP) associated with the GLancing Angle Deposition method (GLAD) are implemented to produce zigzag columnar structures. The oxygen injection time (tON time) and the pulsing period are kept constant. Three tilt angles α are used: 75, 80, and 85° and the number of zigzags N is progressively changed from N = 0.5, 1, 2, 4, 8 to 16. For each film, refractive index, extinction coefficient, and absorption coefficient are calculated from optical transmission spectra of the films measured in the visible region from wavelength values only. Absorption and extinction coefficients monotonously drop as the number of zigzags increases. Refractive indices are the lowest for the most grazing tilt angle α = 85°. The highest refractive index is nevertheless obtained for a number of zigzags close to four. This optimized optical property is directly correlated to changes of the microstructure, especially a porous architecture, which is favored for high tilt angles, and tunable as a function of the number of zigzags.

Effect of Structural Characteristics on Magnetic Behavior of Highly-Pure NiFe2O4 Nanoparticles

In this work, the sol-gel and dc reactive magnetron sputtering techniques were used to prepare nickel ferrite nanoparticles (NiFe2O4 NPs). In the sol-gel method, citric acid was used as a fuel to decompose metal complexes at a low temperature. The nanoparticle size of NiFe2O4 structures prepared by the dc reactive magnetron sputtering was smaller (∼31 nm) than those prepared by the sol-gel method (∼47 nm). The effects of the structural characteristics on the magnetic behavior of these nanoparticles were introduced by the vibrating sample magnetometer (VSM) measurements performed at room temperature. It was found that the nanoparticles synthesized by the sol-gel process exhibit ferromagnetic properties with a saturation magnetization 30.02 emu/g, while those synthesized by dc reactive magnetron sputtering exhibit superparamagnetic properties with a saturation magnetization of 27.05 emu/g. This difference is attributed to the quantum size effect resulted from the decrease in nanoparticle size.

Preparation and gas-sensing properties of very thin sputtered NiO films

We present results on very thin NiO films which are able to detect 3 ppm of acetone, toluene and n-butyl acetate in synthetic air and to operate at 300°C. NiO films with 25 and 50 nm thicknesses were prepared by dc reactive magnetron sputtering on alumina substrates previously coated by Pt layers as heater and as interdigitated electrodes. Annealed NiO films are indexed to the (fcc) crystalline structure of NiO and their calculated grain sizes are in the range from 22 to 27 nm. Surface morphology of the examined samples was influenced by a rough and compact granular structure of alumina substrate. Nanoporous NiO film is formed by an agglomeration of small grains with different shapes while they are created on every alumina grain.