A. R. Pennisi

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.

Al Intermediate Oxidation-States Observed by Core-Level Photoemission Spectroscopy

Aluminum oxidation states in stoichiometric or substoichiometric configuration are studied by core level photoemission spectroscopy on different substrates (SiO2, graphite). They are compared with recent results reported for the interface Si–Aln+O. Three Al oxidation states have been identified and their space distribution (binding energy, intensity, and width) is determined in the region from the interface with the substrate up to the surface of a thick overlayer. The Al2+–O intermediate oxidation state is shown to be confined at the interface; on the contrary, the Al1+–O oxidation state and the stoichiometric oxide (alumina) are present beyond the interface region. From the attenuation of the substrate core level peak, the deposition morphology and the attenuation length of the photoelectrons have been deduced.

Giant photoluminescence emission in crystalline faceted Si grains

Empowering an indirect band-gap material like Si with optical functionalities, firstly light emission, represents a huge advancement constantly pursued in the realization of any integrated photonic device. We report the demonstration of giant photoluminescence (PL) emission by a newly synthesized material consisting of crystalline faceted Si grains (fg-Si), a hundred nanometer in size, assembled in a porous and columnar configuration, without any post processing. A laser beam with wavelength 632.8 nm locally produce such a high temperature, determined on layers of a given thickness by Raman spectra, to induce giant PL radiation emission. The optical gain reaches the highest value ever, 0.14 cm/W, representing an increase of 3 orders of magnitude with respect to comparable data recently obtained in nanocrystals. Giant emission has been obtained from fg-Si deposited either on glass or on flexible, low cost, polymeric substrate opening the possibility to fabricate new devices.

Raman and photoluminescence spectroscopy of Si nanocrystals: Evidence of a form factor

We investigated the quantum confinement in Si nanocrystals embedded in a SiO2 matrix. The size was accurately controlled in the range 3–8 nm by annealing at high temperature Si/SiO2 multilayers fabricated by chemical vapour deposition. Raman shift and line width were compared with existing theoretical models for each cluster size. We found evidence of uni-dimensional confinement in 3 nm crystals, whereas for 4.5 nm crystals the confinement appears three-dimensional. This conclusion is supported by the luminescence spectra shifting towards higher wavelengths for the smaller size, in opposite direction for larger sizes.

Binding-Energies and Cluster Formation at Low Metal-Deposition - Ag on Si and Sio2

A comparison of photoemission spectra from Ag clusters deposited on silicon oxide and on cleaved silicon substrate has allowed us to discriminate initial and final state effects in the binding energies of the valence band and of the 3d core level of the metal.

Strong infrared photoluminescence in highly porous layers of large faceted Si crystalline nanoparticles.

Almost all physical processes in solids are influenced by phonons, but their effect is frequently overlooked. In this paper, we investigate the photoluminescence of large silicon nanoparticles (approximately 100 nm size, synthesized by chemical vapor deposition) in the visible to the infrared detection range. We find that upon increasing laser irradiance, an enormous photoluminescence emission band appears in the infrared. Its intensity exhibits a superlinear power dependence, increasing over four orders of magnitude in the investigated pump power range. Particles of different sizes as well as different shapes in porous layers are investigated. The results are discussed taking into account the efficient generation of phonons under high-power pumping, and the reduced capability, porosity dependent, of the silicon nanoparticles to exchange energy with each other and with the substrate. Our findings are relevant for heat management strategies in silicon.

Octahedral faceted Si nanoparticles as optical traps with enormous yield amplification

We describe a method for the creation of an efficient optical scatter trap by using fully crystalline octahedral Silicon nanoparticles (Si-NPs) of approximately 100 nanometres in size. The light trapping, even when probing an isolated nanoparticle, is revealed by an enormous amplification of the Raman yield of up to 108 times that of a similar Si bulk volume. The mechanism conceived and optimised for obtaining such a result was related to the capability of a Si octahedron to trap the light because of its geometrical parameters. Furthermore, Si-NPs act as very efficient light scatterers not only for the direct light beam but also for the trapped light after it escapes the nanoparticle. These two effects are observed, either superimposed or separated, by means of the Raman yield and by photoluminescence enhancements. The inductively coupled plasma synthesis process performed at a temperature of only 50°C allows for the ubiquitous use of these particles on several substrates for optical and photovoltaic applications.

Evidence for Pd Bonding with Si Intermediate Oxidation-States

The bonding of Pd atoms evaporated on a thin layer of silicon intermediate oxidation states has been studied by core level and valence band photoemission spectroscopy. The particular substrate was obtained by controlled exposure of a cleaved Si surface to oxygen in order to have few angstroms of silicon oxidation states with a small percentage of SiO2. The deposition of Pd on this substrate was investigated studying the Pd 3d and Si 2p core level spectra as well as the valence band as a function of the metal coverage. Our results indicate evidence that the pristine Si intermediate oxidation states modify their configuration and bind metal atoms. In fact in the Si 2p spectrum changes in the chemical shift are observed for these states, and new peaks arise whose areas increase with metal coverage. The three‐dimensional growth characteristics of the metal are deduced from the behavior of the relative intensity for the different components of the Si 2p core spectrum. Furthermore, the shifts of the Pd 3d core ...

Polarization correlation of a photon pair

Abstract We discuss the twofold polarization correlation of photons generated by triplet positronium with ideal polarizers, to show the large difference which can be found, in some cases, between quantum and local hidden variable predictions. Possible experimental applications are indicated.