Investigation on oxygen vacancies influence on reduced WO3 sensing properties

Nowadays, the development of innovative and low-cost smart gas sensors is required in many applications, including medical screening, environmental monitoring and precision farming. Chemoresistive gas sensors are the most widely studied solid state gas sensors in this perspective, due to their small size, low production cost and high sensitivity [1]. However, the lack of selectivity of the nanostructured metal oxides (MOX), i.e. the most widely used class of sensing material so far, limited the effective and widespread adoption of these devices in many applications. In the last few years, great attention has been paid on the development of innovative sensing materials with advanced chemoresistive properties, able to overcome the shortcomings of the typical MOX, seeking the optimization of the sensing performance. Modified MOX (doped or functionalised) proved to be good candidates, owing to the right combination of typical MOX stability and improved selectivity due to surface sensitisation [2]. Considering doping, the investigation of the influence of intrinsic dopants on the MOX sensing properties has attracted considerable attention recently, particularly with regard to oxygen vacancies (Ov), which have shown to have huge impact on the MOX electrical properties and surface reactivity. In this work, a specific reducing treatment at high temperature has been investigated in order to develop reduced WO3 with controlled Ov concentration. Nanostructured WO3 has been synthesised by using a simple sol gel method. Then, a calcination treatment at 650ºC in air has been carried out in order to obtain a nanocrystalline and stoichiometric WO3. A rapid thermal annealer (RTP) has been employed for the controlled reduction of the WO3 nanoparticles, by using H2 (4% in N2) as reducing agent. Different times (15 and 30 minutes) and temperatures (from 300 to 800ºC) were investigated, in order to study their impact on the Ov formation. The Ov in the reduced samples were characterized by using SEM-EDX, XRD and XPS. The XPS characterization has revealed a strong increase in the 5+, 4+ and 3+ oxidation states of W as the treatment temperature rises, due to a strong increase in the surface concentration of Ov. Both in-plane and bridging Ov were formed. An increase of the bulk Ov has been observed as well by XRD analysis. On the other hand, the concentration of Ov did not change significantly with treatment time. The reduced powders were deposited on silicon substrates and their sensing performances were investigated vs. NH3. WO3 reduced at 700ºC showed the best sensing performance towards NH3 at near room working temperature, showing an impressive increase of the sensitivity and selectivity compared to stoichiometric WO3. The role of surface Ov in the sensing mechanism is under investigation

Caratterizzazione SIMS ToF-SIMS e XPS di Ossidonitruri

L’obiettivo del presente lavoro è stato la caratterizzazione di ossidi sottili nitrurati tramite tecniche di microanalisi differenti e complementari. Lo sviluppo della tematica si è svolto attraverso tre linee guida la cui identificazione permette di esporre al meglio il lavoro svolto e definire in modo completo i risultati raggiunti: - Sviluppo di una metodologia necessaria a caratterizzare in modo completo ed accurato questa tipologia di campioni. - Caratterizzazione chimico-fisica di ossidonitruri al fine di definirne le proprietà fondamentali quali distribuzione dell’azoto, auqntificazione del contenuto di azoto, legami chimici. - Applicazione della metodologia sviluppata a campioni di produzione e impiego dei risultati ottenuti per monitorare ed aiutare a risolvere le problematiche di process

Characterization of RTP Oxynitrides by SIMS and XPS Analyses

Aim of this work is the characterization of oxynitride films grown by Rapid Thermal Processing (RTP) using nitrous oxide and nitric oxide precursors. The thickness of the oxynitrided layers is about 7nm. Secondary Ions Mass Spectrometry (SIMS) and X-rays Photoemission Spectroscopy (XPS) have been employed to obtain a complete chemical characterization. XPS analyses have been performed at different depths after removal of oxynitride layers by chemica etching. SIMS and XPS analyses have been also performed on the same samples after a reoxidation treatment. Depending on the precursors used, the oxynitrides show different characteristic

Characterisation of RTA Oxynitrides by SIMS and XPS Analyses

Aim of this work is the characterization of oxynitride films grown by Rapid Thermal Processing (RTP) using nitrous oxide and nitric oxide precursors. The thickness of the oxynitrided layers is about 7nm. Secondary Ions Mass Spectrometry (SIMS) and X-rays Photoemission Spectroscopy (XPS) have been employed to obtain a complete chemical characterization. XPS analyses have been performed at different depths after removal of oxynitride layers by chemica etching. SIMS and XPS analyses have been also performed on the same samples after a reoxidation treatment. Depending on the precursors used, the oxynitrides show different characteristic

ALD growth, thermal treatments and characterisation of Al2O3 layers

Al2O3 films with thickness ranging from 20 to 300 nm are grown in a home-made reactor using atomic layer deposition with trimethylaluminum and water on different semiconductor substrates. The deposited films are investigated in terms of composition and electrical properties. Annealing treatments appear necessary in order to obtain films with good insulating properties, although the same treatments do not affect the stoichiometry of the main components