Silvia Maria Pietralunga

Origin of femtosecond laser induced periodic nanostructure on diamond

We study the evolution of periodic nanostructures formed on the surface of diamond by femtosecond laser irradiation delivering 230 fs pulses at 1030 nm and 515 nm wavelengths with a repetition rate of 250 kHz. Using scanning electron microscopy, we observe a change in the periodicity of the nanostructures by varying the number of pulses overlapping in the laser focal volume. We simulate the evolution of the period of the high spatial frequency laser induced periodic surface structures at the two wavelengths as a function of number of pulses, accounting for the change in the optical properties of diamond via a generalized plasmonic model. We propose a hypothesis that describes the origin of the nanostructures and the principal role of plasmonic excitation in their formation during multipulse femtosecond laser irradiation.

Enhanced photopromoted electron transfer over a bilayer WO3 n–n heterojunction prepared by RF diode sputtering

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 (Rct) 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.

Polycrystalline indium phosphide on silicon by indium assisted growth in hydride vapor phase epitaxy

Polycrystalline InP was grown on Si(001) and Si(111) substrates by using indium (In) metal as a starting material in hydride vapor phase epitaxy (HVPE) reactor. In metal was deposited on silicon substrates by thermal evaporation technique. The deposited In resulted in islands of different size and was found to be polycrystalline in nature. Different growth experiments of growing InP were performed, and the growth mechanism was investigated. Atomic force microscopy and scanning electron microscopy for morphological investigation, Scanning Auger microscopy for surface and compositional analyses, powder X-ray diffraction for crystallinity, and micro photoluminescence for optical quality assessment were conducted. It is shown that the growth starts first by phosphidisation of the In islands to InP followed by subsequent selective deposition of InP in HVPE regardless of the Si substrate orientation. Polycrystalline InP of large grain size is achieved and the growth rate as high as 21 μm/h is obtained on both s...

Charge dynamics in aluminum oxide thin film studied by ultrafast scanning electron microscopy

The excitation dynamics of defects in insulators plays a central role in a variety of fields from Electronics and Photonics to Quantum computing. We report here a time-resolved measurement of electron dynamics in 100 nm film of aluminum oxide on silicon by Ultrafast Scanning Electron Microscopy (USEM). In our pump-probe setup, an UV femtosecond laser excitation pulse and a delayed picosecond electron probe pulse are spatially overlapped on the sample, triggering Secondary Electrons (SE) emission to the detector. The zero of the pump-probe delay and the time resolution were determined by measuring the dynamics of laser-induced SE contrast on silicon. We observed fast dynamics with components ranging from tens of picoseconds to few nanoseconds, that fits within the timescales typical of the UV color center evolution. The surface sensitivity of SE detection gives to the USEM the potential of applying pump-probe investigations to charge dynamics at surfaces and interfaces of current nano-devices. The present work demonstrates this approach on large gap insulator surfaces.

Advanced spectroscopies of graphene and 2D materials

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.