Since the development of the first chemoresistive metal oxide based gas sensors, transducers with innovative properties have been prepared by a variety of wet- and dry-deposition methods. Among these, direct assembly of nanostructured films from the gas phase promises simple fabrication and control and with the appropriate synthesis and deposition methods nm to μm thick films, can be prepared. Dense structures are achieved by tuning chemical or vapor deposition methods whereas particulate films are obtained by deposition of airborne, mono- or polydisperse, aggregated or agglomerated nanoparticles. Innovative materials in non-equilibrium or sub-stoichiometric states are captured by rapid cooling during their synthesis. This Review presents some of the most common chemical and vapor-deposition methods for the synthesis of semiconductor metal oxide based detectors for chemical gas sensors. In addition, the synthesis of highly porous films by novel aerosol methods is discussed. A direct comparison of structural and chemical properties with sensing performance is given.

Semiconductor Gas Sensors: Dry Synthesis and Application

Effect of substrate temperature on the structural and optical properties of ZnO and Al-doped ZnO thin films prepared by dc magnetron sputtering

This Review highlights basic and transition metal conducting and semiconducting oxides. We discuss their material and electronic properties with an emphasis on the crystal, electronic, and band structures. The goal of this Review is to present a current compilation of material properties and to summarize possible uses and advantages in device applications. We discuss Ga2O3, Al2O3, In2O3, SnO2, ZnO, CdO, NiO, CuO, and Sc2O3. We outline the crystal structure of the oxides, and we present lattice parameters of the stable phases and a discussion of the metastable polymorphs. We highlight electrical properties such as bandgap energy, carrier mobility, effective carrier masses, dielectric constants, and electrical breakdown field. Based on literature availability, we review the temperature dependence of properties such as bandgap energy and carrier mobility among the oxides. Infrared and Raman modes are presented and discussed for each oxide providing insight into the phonon properties. The phonon properties also provide an explanation as to why some of the oxide parameters experience limitations due to phonon scattering such as carrier mobility. Thermal properties of interest include the coefficient of thermal expansion, Debye temperature, thermal diffusivity, specific heat, and thermal conductivity. Anisotropy is evident in the non-cubic oxides, and its impact on bandgap energy, carrier mobility, thermal conductivity, coefficient of thermal expansion, phonon modes, and carrier effective mass is discussed. Alloys, such as AlGaO, InGaO, (Al xIn yGa1− x− y)2O3, ZnGa2O4, ITO, and ScGaO, were included where relevant as they have the potential to allow for the improvement and alteration of certain properties. This Review provides a fundamental material perspective on the application space of semiconducting oxide-based devices in a variety of electronic and optoelectronic applications.

A review of band structure and material properties of transparent conducting and semiconducting oxides: Ga2O3, Al2O3, In2O3, ZnO, SnO2, CdO, NiO, CuO, and Sc2O3

SnO2 thin films morphological and optical properties in terms of the Boubaker Polynomials Expansion Scheme BPES-related Opto-Thermal Expansivity ψAB

In this work, we report for the first time a simple two-step route to fabricate a novel porous metal oxide film composed of polycrystalline nanowires with ultra-small nanoparticles, good interconnectivity between nanoparticles, and a high density of ultra-fine nanopores. The as-prepared metal oxide films combine the advantages of small crystal size, high surface-to-volume ratio, and one-dimensional-nanowire-induced unique charge transport paths (with correspondingly high interconnectivity). Taking In2O3 as an example, porous In2O3 films, composed of polycrystalline In2O3 nanowires with ultra-small nanocrystals (less than 10 nm) and a high density of ultra-fine nanopores (1.6–3.1 nm), have shown very high sensitivity and good reproducibility towards ethanol gas, which are 10–20 times higher than for In2O3 octahedra and commercial SnO2 thick films. The response/recovery speeds of the as-prepared porous In2O3 films are also 5–6 times higher than for In2O3 octahedra, SnO2 nanobelts, and commercial SnO2 thick films. We believe that such metal oxide flexible films made from highly porous nanowires will replace their traditional thick film counterparts, not only in gas sensors but also in other functional devices, such as batteries, supercapacitors, solar cells, etc.

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Highly porous metal oxide polycrystalline nanowire films with superior performance in gas sensors

Tin oxide (SnO 2 ) thin films were grown on Si (1 0 0) substrates using pulsed laser deposition (PLD) in O 2 gas ambient (10 Pa) and at different substrate temperatures (RT, 150, 300 and 400 °C). The influence of the substrate temperature on the structural and morphological properties of the films was investigated using X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). XRD measurements showed that the almost amorphous microstructure transformed into a polycrystalline SnO 2 phase. The film deposited at 400 °C has the best crystalline properties, i.e. optimum growth conditions. However, the film grown at 300 °C has minimum average root mean square (RMS) roughness of 3.1 nm with average grain size of 6.958 nm. The thickness of the thin films determined by the ellipsometer data is also presented and discussed.

Effects of deposition temperature on the structural and morphological properties of SnO2 films fabricated by pulsed laser deposition

ZnO thin films were grown on Si(1 0 0) substrates using pulsed laser deposition in O 2 gas ambient (10 Pa) and at different substrate temperatures (25, 150, 300 and 400 °C). The influence of the substrate temperature on the structural and morphological properties of the films was investigated using XRD, AFM and SEM. At substrate temperature of T =150 °C, a good quality ZnO film was fabricated that exhibits an average grain size of 15.1 nm with an average RMS roughness of 3.4 nm. The refractive index and the thickness of the thin films determined by the ellipsometry data are also presented and discussed.

Rakhi Khandelwal

Papers

Effects of deposition temperature on the structural and morphological properties of SnO2 films fabricated by pulsed laser deposition

Effects of deposition temperature on the structural and morphological properties of thin ZnO films fabricated by pulsed laser deposition