Alberto Tagliaferri

One of the greatest challenges in the monolithic integration of micro-magnets in Si-based microelectromechanical systems (MEMS) is the release of suspended structures through exposure to hydrofluoric acid. Ni80Fe20 (Py) has been mainly used in prototypes of these hybrid devices, essentially because of its high resistance to exposure to hydrofluoric acid (HF). However, a detailed investigation of the physical mechanisms leading to its resistance to corrosion, as well as a characterization of the impact of HF exposure on the magnetic properties of Py, is still missing. Here, we present a combined surface science and magnetic investigation of Py films exposed to HF vapors in the typical conditions of MEMS release. X-ray photoemission spectroscopy and scanning Auger microscopy measurements show the tendency of Py to self-passivation by the creation of a surface fluoride layer (a few nm thick), accompanied by the formation of blisters with diameter on the order of 10–20 μm. A decrease in the average static magnetization is observed by vibrating sample magnetometry upon HF exposure, compatible with the creation of an interfacial fluoride capping behaving as a magnetic dead layer. A slight increase in the Gilbert damping (from 0.0079 to 0.0095) is found by vector network analyzer-ferromagnetic resonance, which could be compatible with blistering. Overall, this analysis confirms that Py displays excellent intrinsic resistance to HF so that it can be effectively used for integration of thick magnetic elements in MEMS without protective layers. For thin films to be used in spintronic/magnonic devices, instead, an additional capping layer should be used to fully preserve dynamic magnetic properties.

A bilayer WO₃ photoelectrode was obtained by radio frequency (RF) plasma sputtering in a reactive 40%O₂/Ar atmosphere by depositing two successive WO₃ coatings on a tungsten foil at two different total gas pressures (3 Pa and 1.7 Pa, respectively), followed by calcination at 600 °C. Two monolayer samples deposited at each of the two pressures and a bilayer sample deposited at inverted pressures were also prepared. Their photoelectrocatalytic (PEC) activity was evaluated by both Incident Photon-to-Current Efficiency (IPCE) measurements and separate evolution of H₂ and O₂ by water splitting in a two-compartment PEC cell. SEM analysis revealed that the photoanodes have a nanostructured porous double layer surmounting a columnar basement (Staffa-like morphology, after the name of the Scottish island). Mott–Schottky analysis showed that the single layer deposited at 3 Pa has a conduction flat band potential 0.1 V more positive than that deposited at 1.7 Pa. The equivalent n–n heterojunction at the interface of the double-layer creates a built-in electric field that facilitates the photopromoted electron transfer toward the lower lying conduction band material, while the columnar innermost layer introduces percolation paths for efficient electron transport toward the conductive tungsten foil. Both phenomena contribute to decrease the interfacial charge transfer resistance (R_ct) and lead up to a ca. 30% increase in the PEC performance compared to the monolayer and the inverted bilayer coatings and to a 93% faradaic efficiency, which is among the highest reported so far for WO₃ photoanodes. Upon methanol addition an outstanding 4-fold photocurrent density increase up to 6.3 mA cm⁻² was attained over the bilayer WO₃ photoanode, much larger than the usually observed current doubling effect.

A full exploitation of the unique features of graphene and related 2D materials in micro and nano-scaled optoelectronic devices for high-data rate photonic systems calls for a deep understanding of their fundamental structural, electronic, photophysical properties and fast dynamics. We introduce some advanced spectroscopic techniques developed to this aim, while reviewing their specific potentialities and highlighting recent results. To provide absolute local nanoscale thickness metrology of as-grown 2D materials, Scanning Auger Electron Microspectroscopy has been calibrated. Results obtained for graphene and graphene oxide (GO) are reported, showing sub-monolayer resolution. Ultrafast optical transient absorption spectroscopy, with temporal resolution down to 10fs, unveils the relaxation dynamics of hot electrons in single layer graphene and the formation and radiative recombination via stimulated emission of biexcitons in ultra-narrow, structurally well-defined nanoribbons (GNRs). By recently developed ultrafast Time- and Angle-resolved Photo-Electron Spectroscopy (TR-ARPES) we investigate electronic states and spin dynamics close to the Dirac cone, in graphene and metal chalcogenide topological insulators (TI) under fs strong laser excitation, in view of spintronic applications.