S. Gibilisco

Quantum size effects in Raman spectra of Si nanocrystals

First order Raman spectra of Si nanocrystals exhibit large shift and width, strongly exceeding the theoretical calculated values. The disagreement between theory and experiment is dramatically increasing for the smallest nanocrystal sizes. Here, we present a novel theoretical approach showing one-, two-, or three-dimensional quantum effects, due to the limited size of the corresponding coordinate of the quantum structure. The method can be applied to nanosized layers, quantum wires and/or quantum dots. The agreement of the present theory with experimental data is dramatically improved.

Crystallization of ion amorphized Ge2Sb2Te5 thin films in presence of cubic or hexagonal phase

The crystallization kinetics of amorphous Ge2Sb2Te5 (GST) thin films, generated by ion implantation, on top of crystalline GST, either in the cubic or hexagonal phase, was investigated by means of time resolved reflectivity measurements, x-ray diffraction, in situ transmission electron microscopy, and Raman analyses. The crystallization occurred at a lower temperature with respect to a fully amorphous film and in both cases the crystalline phase started growing at the underlying amorphous-crystalline (a-c) interface. However, it was not a solid phase epitaxial growth since cubic GST was always obtained, independent of the phase of the underlying crystal. We speculate that the a-c interface behaves as a continuous region of potential nucleation sites in the crystallization making the crystallization process more efficient.

Order and disorder of carbon deposit produced by arc discharge in liquid nitrogen

We have investigated the formation and the morphology of carbon deposit on the cathode, after arc discharge between two graphite electrodes immersed in liquid nitrogen. We show that the deposit is mainly composed of disordered carbon structures at the bottom and at the external part of the deposit, forming like a bowl, whereas randomly oriented carbon nanotubes (CNTs) aggregated as pillars are found at the center of the deposit. The spatial distribution of the two kinds of structures in the deposit, the size and the sharpness of the CNT pillars are found to depend both on the applied voltage and the current values used for the arc discharge. A correlation between the plasma temperature field and the morphology of the deposit is reported.

Light absorption and electrical transport in Si:O alloys for photovoltaics

Thin films (100–500 nm) of the Si:O alloy have been systematically characterized in the optical absorption and electrical transport behavior, by varying the Si content from 43 up to 100 at. %. Magnetron sputtering or plasma enhanced chemical vapor deposition have been used for the Si:O alloy deposition, followed by annealing up to 1250 °C. Boron implantation (30 keV, 3–30×1014 B/cm2) on selected samples was performed to vary the electrical sheet resistance measured by the four-point collinear probe method. Transmittance and reflectance spectra have been extracted and combined to estimate the absorption spectra and the optical band gap, by means of the Tauc analysis. Raman spectroscopy was also employed to follow the amorphous-crystalline (a-c) transition of the Si domains contained in the Si:O films. The optical absorption and the electrical transport of Si:O films can be continuously and independently modulated by acting on different parameters. The light absorption increases (by one decade) with the Si ...

Anomalous and normal Hall effect in hydrogenated amorphous Si prepared by plasma enhanced chemical vapor deposition

The double sign anomaly of the Hall coefficient has been studied in p-doped and n-doped hydrogenated amorphous silicon grown by plasma enhanced chemical vapor deposition and annealed up to 500 °C. Dark conductivity as a function of temperature has been measured, pointing out a conduction mechanism mostly through the extended states. Anomalous Hall effect has been observed only in the as-deposited n-doped film, disappearing after annealing at 500 °C, while p-doped samples exhibit normal Hall effect. When Hall anomaly is present, a larger optical band gap and a greater Raman peak associated with Si–H bond are measured in comparison with the cases of normal Hall effect. The Hall anomaly will be related to the hydrogen content and implication on photovoltaic applications will be discussed.

Micro-Raman spectroscopy and SIMS characterization of oxykinoshitalite in an olivine nephelinite from the Hyblean Plateau (Sicily, Italy)

A Ba-Ti rich oxymica occurs in an olivine nephelinite from S. Demetrio High in the northern margin of the Hyblean Plateau (Sicily, Italy). The rock sample exhibits a porphyritic texture formed by olivine, clinopyroxene, nepheline, titanian magnetite, apatite, and rare subhedral tabular mica crystals, which have a perfect cleavage on {001}, and a strong pleochroism with X pale brown and YZ brown. The studied oxykinoshitalite, characterized by micro-Raman spectroscopy, electron microprobe WDS (wavelength- dispersive system), and secondary-ion mass spectrometry (SIMS), was compared with the type material from the Fernando de Noronha island (Pernambuco, Brazil). Structural formula of the Hyblean oxykinoshitalite, calculated on the basis of 7 (Si, Al, Fe, Mg, Ti), is (Ba0.51K0.41Na0.04Ca0.01)0.97(Mg1.98Fe0.55Ti0.48)3.01(Si2.42Al1.56)3.98 O10(O1.17F0.62(OH)0.21)2.00. The lack of chemical zoning and the enrichment in Zr and Nb in the groundmass crystals of the Hyblean oxykinoshitalite suggest formation during the final crystallization stage of a basaltoid magma with ocean island basalt (OIB) affinity. Most likely, the magma originated by partial melting of metasomatized ultramafic rocks in the Hyblean crustal basement.