A. Giberti

Near-infrared photoluminescence in titania: Evidence for phonon-replica effect

The photoluminescence of rutile and anatase TiO2 has been investigated in the range of 360–890 nm at several temperatures. An unexpected intense near-infrared (1.53 eV) photoluminescence band was recorded for both phases. At low temperatures, the resulting bands form a substructure of equally spaced peaks irrespective of the phase. The spectra were interpreted in the framework of the single-configuration-coordinate model as the phonon-replica effect, originating from ionization of oxygen vacancies.

Role of intragrain oxygen diffusion in polycrystalline tin oxide conductivity

Resistivity transients of tin oxide films at step isothermal changes in oxygen pressure are investigated. It is expected that, after exposing the samples to oxygen, the resistivity would increase monotonically as barriers become higher to finally reach a plateau at steady state. Here we present experimental results showing a nonmonotonic resistivity transient response that cannot be explained by only considering changes in the Schottky barrier heights. We provide an explanation based on the effects of intragrain oxygen diffusion that accounts for the observed main features of conduction in this polycrystalline material. Oxygen diffuses into the grains annihilating vacancies; the donor concentration is then reduced affecting the sample conductivity.

Metal Sulfides as Sensing Materials for Chemoresistive Gas Sensors

This work aims at a broad overview of the results obtained with metal-sulfide materials in the field of chemoresistive gas sensing. Indeed, despite the well-known electrical, optical, structural and morphological features previously described in the literature, metal sulfides present lack of investigation for gas sensing applications, a field in which the metal oxides still maintain a leading role owing to their high sensitivity, low cost, small dimensions and simple integration, in spite of the wide assortment of sensing materials. However, despite their great advantages, metal oxides have shown significant drawbacks, which have led to the search for new materials for gas sensing devices. In this work, Cadmium Sulfide and Tin (IV) Sulfide were investigated as functional materials for thick-film chemoresistive gas-sensors fabrication and they were tested both in thermo- and in photo-activation modes. Furthermore, electrical characterization was carried out in order to verify their gas sensing properties and material stability, by comparing the results obtained with metal sulfides to those obtained by using their metal-oxides counterparts. The results highlighted the possibility to use metal sulfides as a novel class of sensing materials, owing to their selectivity to specific compounds, stability, and the possibility to operate at room temperature.

Resonant photoactivation of cadmium sulfide and its effect on the surface chemical activity

Photo-enhanced surface chemical activity of cadmium sulfide gives rise to a wide class of surface-dependent phenomena, such as heterogeneous photocatalysis, chemoresistivity, and chemiluminescence, which have several technological and scientific applications. In this work, the photochemical properties of nanostructured cadmium sulfide films are investigated by means of electrical conductance measurements in controlled atmosphere, while irradiated by light of wavelengths ranging from 400 to 645 nm. Chemisorption of benzene, carbon monoxide, methane, ethanol, and hydrogen sulfide onto CdS surface has been analyzed as a function of the wavelength, in a gas concentration range of the order of parts per million. It resulted that the increase of photoconductance with gas adsorption is resonant with the bandgap energy. It turns out that this resonant enhancement of the surface chemical activity can be of advantage for all the optical and chemical mechanisms that depend upon it. An interpretation of these results...

Electrical and spectroscopic analysis in nanostructured SnO2: “Long-term” resistance drift is due to in-diffusion

A model for conductance in n-type non-degenerate semiconductors is proposed and applied to polycrystalline SnO2 used as a gas sensor. Particular attention is devoted to the fundamental mechanism of Schottky barrier formation due to surface states in nanostructured grains. Electrical and absorption infra-red spectroscopic analysis constitutes strong evidence for oxygen diffusion into the tin oxide grains. The model is then extended to include oxygen in- and out-diffusion. Thus, it is possible to explain the “long-term” resistance drift in oxygen for fully depleted grained samples in terms of tunneling through the double barrier.

Semiconductor Gas Sensors for Environmental Monitoring

The main semiconductor oxides for gas sensing have been examined, describing the synthesis processes and the morphological and structural properties. The powders have been deposited as thick films through screen-printing technique to achieve gas sensors. The sensing layers have been studied with regard to semiconductor behaviour, surface barrier potential heights and gas sensing properties. The interaction of humidity with semiconductor oxides has been considered with the aim to compensate, using an algorithm, its effect in gas detection. Finally, nitrogen oxides monitoring, contemporaneously performed in several sites of Italy, has been described.

Tin (IV) Sulfide chemoresistivity: A possible new gas sensing material

In the last years, the research in the gas sensor field had a significant upward thrust. Regarding the chemoresistive gas sensors, this has produced a remarkable study of metal oxides semiconductors which, however, have shown different limits. In particular their low selectivity and lack of stability take them to an unreliable responses over time. For this reason, in this work it was decided to study the chemoresistive behavior of a non metal oxide semiconductor as Tin (IV) Sulfide (SnS2). SnS2 nanoparticles was synthetized by precipitation reaction in aqueous solution. Then, structural chemical and morphological characterizations were carried out by means of X-Ray Diffraction and SEM techniques. Furthermore, the thermal stability of the powder was studied with thermogravimetric analysis. The sensitive films were obtained by preparing a screen-printing paste and then depositing it on alumina substrates by means of screen-printing technique. The sensing properties of the obtained devices were tested with several gases at different working temperatures. At the best working temperature, a high selectivity to ketones and aldehydes, with respect to different types of molecules, was observed.

Silicon Carbide: A Gas Sensing Material for Selective Detection of SO2

Silicon carbide (SiC) is a long-time known material with exceptional mechanical properties. Ceramics obtained by sintering SiC grains are very hard and find application in car brakes, bulletproof vests and, in general, in high endurance applications. [...]

Mesoporous silicon gas sensors: design, fabrication and conduction model

Most chemoresistive gas sensors are supported by an insulating substrate, not integrable into silicon IC platforms, and need very high temperature to reach operating performance, this implies energy consumption and a risk factor in the presence of flammable gases. Therefore, porous silicon substrates represent a good choice, thanks to its chemical and physical properties. In this work we designed mesoporous silicon as substrate for gas sensors, and provided a theoretical investigation about the p-Si/PS/gas interface, by analysing the semiconductor band bending at the interface, the formation of a Schottky barrier and the consequent pinning of the Fermi level, due to the high density of surface states in porous silicon. The theoretical considerations have been verified through the experimental measurements with sensors based on p-Si substrate.

Relative Permittivity of Nanostructured Solid Solutions of Tin and Titanium Oxides

Impedance Spectroscopy technique was employed in order to characterize nanostructured powders based on solid solutions of SnO2 and TiO2, employed in thick-film chemoresistive gas sensors. The measurements were performed with a cylindrical capacitor, manufactured for this specific purpose, with the powders used as dielectric between the plates. Results were interpreted with the support of a theoretical model, which describes the electrical properties of the powders and allows us to estimate their relative permittivity.