Native Silicon Oxide Properties Determined by Doping.

The physico-chemical properties of native oxide layers, spontaneously forming on crystalline Si wafers in air, can be strictly correlated to the dopant type and doping level. In particular, our investigations focused on oxide layers formed upon air exposure in a clean room after Si wafer production, with dopant concentration levels from ≈1013 to ≈1019 cm-3. In order to determine these correlations, we studied the surface, the oxide bulk, and its interface with Si. The surface was investigated using the contact angle, thermal desorption, and atomic force microscopy measurements which provided information on surface energy, cleanliness, and morphology, respectively. Thickness was measured with ellipsometry and chemical composition with X-ray photoemission spectroscopy. Electrostatic charges within the oxide layer and at the Si interface were studied with Kelvin probe microscopy. Some properties such as thickness, showed an abrupt change, while others, including silanol concentration and Si intermediate-oxidation states, presented maxima at a critical doping concentration of ≈2.1 × 1015 cm-3. Additionally, two electrostatic contributions were found to originate from silanols present on the surface and the net charge distributed within the oxide layer. Lastly, surface roughness was also found to depend upon dopant concentration, showing a minimum at the same critical dopant concentration. These findings were reproduced for oxide layers regrown in a clean room after chemical etching of the native ones.

Design of a Metal-Oxide Solid Solution for Sub-ppm H2 Detection.

Hydrogen is largely adopted in industrial processes and is one of the leading options for storing renewable energy. Due to its high explosivity, detection of H2 has become essential for safety in industries, storage, and transportation. This work aims to design a sensing film for high-sensitivity H2 detection. Chemoresistive gas sensors have extensively been studied for H2 monitoring due to their good sensitivity and low cost. However, further research and development are still needed for a reliable H2 detection at sub-ppm concentrations. Metal-oxide solid solutions represent a valuable approach for tuning the sensing properties by modifying their composition, morphology, and structure. The work started from a solid solution of Sn and Ti oxides, which is known to exhibit high sensitivity toward H2. Such a solid solution was empowered by the addition of Nb, which─according to earlier studies on titania films─was expected to inhibit grain growth at high temperatures, to reduce the film resistance and to impact the sensor selectivity and sensitivity. Powders were synthesized through the sol-gel technique by keeping the Sn-Ti ratio constant at the optimal value for H2 detection with different Nb concentrations (1.5-5 atom %). Such solid solutions were thermally treated at 650 and 850 °C. The sensor based on the solid solution calcined at 650 °C and with the lowest content of Nb exhibited an extremely high sensitivity toward H2, paving the way for H2 ppb detection. For comparison, the response to 50 ppm of H2 was increased 6 times vs SnO2 and twice that of (Sn,Ti)xO2.

Nanostructured SmFeO3 Gas Sensors: Investigation of the Gas Sensing Performance Reproducibility for Colorectal Cancer Screening.

Among the various chemoresistive gas sensing properties studied so far, the sensing response reproducibility, i.e., the capability to reproduce a device with the same sensing performance, has been poorly investigated. However, the reproducibility of the gas sensing performance is of fundamental importance for the employment of these devices in on-field applications, and to demonstrate the reliability of the process development. This sensor property became crucial for the preparation of medical diagnostic tools, in which the use of specific chemoresistive gas sensors along with a dedicated algorithm can be used for screening diseases. In this work, the reproducibility of SmFeO3 perovskite-based gas sensors has been investigated. A set of four SmFeO3 devices, obtained from the same screen-printing deposition, have been tested in laboratory with both controlled concentrations of CO and biological fecal samples. The fecal samples tested were employed in the clinical validation protocol of a prototype for non-invasive colorectal cancer prescreening. Sensors showed a high reproducibility degree, with an error lower than 2% of the response value for the test with CO and lower than 6% for fecal samples. Finally, the reproducibility of the SmFeO3 sensor response and recovery times for fecal samples was also evaluated.