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The synthesis of α-MoO3 nanoplates with a unique well-faceted rectangular morphology produced by a polymeric solution route is presented. This versatile chemical route allowed the growth of layered nanoplates on different crystalline substrates to manufacture electronic/optoelectronic nanodevices. The α-MoO3 crystalline phase was identified by both X-ray diffraction (XRD) and Raman spectroscopy. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that the well-faceted rectangular layered nanoplates can be obtained depending on the fine control of the synthesis parameters due to the MoO3 sublimation. Gas sensing measurements revealed that the α-MoO3 nanoplates exhibit an enhanced response to NO2, with sensor signals up to 50, combined with a low optimum operating temperature. In addition, the devices exhibited the highest selectivity to NO2 relative to H2, CO, and CH4, which can be a consequence of the specific exposed surface plane. These results reveal that layered α-MoO3 nanoplates are promising for use in the manufacturing of high-performance NO2 gas sensor nanodevices. [ABSTRACT FROM AUTHOR]

In this work, electric transport properties of CaCu3Ti4O12 (CCTO) thin films were investigated for resistive switching, rectifying and gas sensor applications. Single phase CCTO thin films were produced by polymeric precursor method (PPM) on different substrates and their electrical properties were studied. Films produced on LNO/Si substrates have symmetrical non-ohmic current-voltage characteristics, while films deposited on Pt/Si substrates have a highly asymmetrical non-ohmic behavior which is related to a metal-semiconductor junction formed at the CCTO/Pt interface. In addition, results confirm that CCTO has a resistive switching response which is enhanced by Schottky contacts. Sensor response tests revealed that CCTO films are sensitive to oxygen gas and exhibit n-type conductivity. These results demonstrate the versatility of CCTO thin film prepared by the PPM method for gas atmosphere or bias dependent resistance applications. [ABSTRACT FROM AUTHOR]

An analysis of the admittance and dielectric response of SnO2·MnO-based varistors in the frequency range from 100 Hz to 15 MHz and a temperature range from 293–573 K allowed the formation of electronic transitions in space-charge regions to be characterized as a function of the Nb2O5 dopant concentration in SnO2· MnO polycrystalline-based ceramics. This analysis revealed that the formation of trapping states in space-charge regions was indirectly dependent on the concentration of Nb2O5 as a result of a microstructural modification, as discussed in Part I of this paper. Moreover, the presence of type I SnO2-SnO2 grain boundaries, which are thin and Mn rich, was found to favor the formation of space-charge regions in this kind of device. In the systems in question, oxygen vacancies or Mn′Sn-like defects were suggested as the source of trap states situated ∼0.49 eV below the conduction band. The samples were also found to display very dissimilar dielectric responses in terms of the power law exponents of the peak loss, which were attributed to the influence of the conductance terms of type-II junctions whose frequency cannot be separated from the trapping relaxation process. As a result, Mott-Schottky plots could not be constructed for the SnO2· MnO-based systems studied here. [ABSTRACT FROM AUTHOR]

This report discusses important microstructural features of SnO2.MnO-based polycrystalline ceramics. The influence of the sintering time and the concentration of donor Nb2O5 on the microstructure of these ceramics are investigated, and the correlation between the microstructural features and nonohmic behavior are also discussed. High resolution analytical electron microscopy was used for a detailed characterization of the microstructure and grain boundary chemistry of the compositions, revealing that SnO2-MnO dense ceramics consist of two phases, SnO2 grains and Mn2SnO4, precipitated mainly at triple grain points. In addition, two types of SnO2-SnO2 grain boundary were identified: type I, Mn-rich and thin, and type II, Mn-poor and thick. Changes in Mn concentrations at the grain boundaries are ascribed to both grain misorientation and Mn diffusivity along the grain boundary. The identification of two kinds of junctions in SnO2-MnO has significant implications in the material’s nonohmic behavior, as will be discussed in detail here and in Part II, and is important in understanding the sintering mechanism and microstructural formation of SnO2 ceramics. [ABSTRACT FROM AUTHOR]

Important microstructural features of SnO2-MnO-based polycrystalline ceramics are discussed. The influence of the sintering time and the concentration of donor Nb2O5 on the microstructure of these ceramics are investigated.

The addition of different dopants affects the densification, mean grain size and electrical properties of TiO2-based varistor ceramics. This paper discusses the microstructural and electrical properties of (Ta, Co, Pr) doped TiO2 systems, demonstrating that some of these systems display electrical properties that allow for their use as low voltage varistor. Dopants such as Ta2O5 play a special role in the formation of barriers at the grain boundary and in the nonlinear behavior in TiO2-based systems. The higher values of nonlinear coefficient and breakdown electric field were obtained in the system just doped with Ta2O5 and CoO. [ABSTRACT FROM AUTHOR]

This paper discuss the qualitative use of electrostatic force microscopy to study the grain boundary active potential barrier present in dense SnO2-based polycrystalline semiconductors. The effect of heat treatment under rich- and poor-oxygen atmospheres was evaluated while especially considering the number of active barriers at grain boundary regions. The results show that the number of active barriers decrease after heat treatment in an oxygen-poor atmosphere and increase after heat treatment in oxygen-rich atmospheres. The observed effect was explained by considering the presence of oxidized transition metal elements segregated at grain boundary regions which leads to the p-type character of this region, in agreement with the atomic barrier formation mechanism in metal oxide varistor systems. [ABSTRACT FROM AUTHOR]

In the present work, electroactive grain boundaries of highly dense metal oxide SnO2-based polycrystalline varistors were determined by electrostatic force microscopy (EFM). The EFM technique was applied to identify electroactive grain boundaries and thus estimate the amount of active grain boundary, which, in the metal oxide SnO2-based varistor, was calculated at around 85%, i.e., much higher than that found in traditional metal oxide ZnO-based varistors. The mean potential barrier height value obtained from the EFM analysis was in complete agreement with the values calculated from the C-V measurements, together with a complex capacitance plane analysis that validates the methodology proposed here. [ABSTRACT FROM AUTHOR]

Copper tungstate (CuWO 4 ) crystals were synthesized by the sonochemistry (SC) method, and then, heat treated in a conventional furnace at different temperatures for 1h. The structural evolution, growth mechanism and photoluminescence (PL) properties of these crystals were thoroughly investigated. X-ray diffraction patterns, micro-Raman spectra and Fourier transformed infrared spectra indicated that crystals heat treated and 100°C and 200°C have water molecules in their lattice (copper tungstate dihydrate (CuWO 4 ·2H 2 O) with monoclinic structure), when the crystals are calcinated at 300°C have the presence of two phase (CuWO 4 ·2H 2 O and CuWO 4 ), while the others heat treated at 400°C and 500°C have a single CuWO 4 triclinic structure. Field emission scanning electron microscopy revealed a change in the morphological features of these crystals with the increase of the heat treatment temperature. Transmission electron microscopy (TEM), high resolution-TEM images and selected area electron diffraction were employed to examine the shape, size and structure of these crystals. Ultraviolet-Visible spectra evidenced a decrease of band gap values with the increase of the temperature, which were correlated with the reduction of intermediary energy levels within the band gap. The intense photoluminescence (PL) emission was detected for the sample heat treat at 300°C for 1h, which have a mixture of CuWO 4 ·2H 2 O and CuWO 4 phases. Therefore, there is a synergic effect between the intermediary energy levels arising from these two phases during the electronic transitions responsible for PL emissions., (Copyright © 2017 Elsevier B.V. All rights reserved.)