Publications

A two dimensional photonic crystal with hexagonal symmetry is investigated by finite-difference time-domain and finite difference frequency-domain theoretical calculations. The system consist in a germanium substrate patterned with holes of different radii, varying from 0.1 alpha to 0.45 alpha, The photonic band gaps (PBGs) and the reflectance spectra allow a complete characterization of the structure. It was shown the presence of PBGs at telecommunication wavelengths which makes the present systems suitable for optoelectronic applications, representing a potential alternative to Si-based technology

The direct detection of dark matter candidates, for which semiconductor materials are used, is based on the detection of low energy selfrecoils. We show that the corrections to the energy partition curves due to the energy exchange during the transient processes modify the predictions of the standard Lindhard theory. These effects, calculated in the frame of a new model developed by the authors, depend on the initial temperature at which the detector operates, and on the energy of the recoil. Most of the available experimental data accumulated during more than fifty years. support these results.

The experimental demonstration of fundamental direct bandgap, group IV GeSn alloys has constituted an important step towards realization of the last missing ingredient for electronic-photonic integrated circuits, i.e. the efficient group IV laser source. In this contribution, we present electroluminescence studies of reduced-pressure CVD grown, direct bandgap GeSn light emitting diodes (LEDs) with Sn contents up to 11 at.%. Besides homojunction GeSn LEDs, complex heterojunction structures, such as GeSn/Ge multi quantum wells (MQWs) have been studied. Structural and compositional investigations confirm high crystalline quality, abrupt interfaces and tailored strain of the grown structures. While also being suitable for light absorption applications, all devices show light emission in a narrow short-wave infrared (SWIR) range. Temperature dependent electroluminescence (EL) clearly indicates a fundamentally direct bandgap in the 11 at.% Sn sample, with room temperature emission at around 0.55 eV (2.25 mu m). We have, however, identified some limitations of the GeSn/Ge MQW approach regarding emission efficiency, which can be overcome by introducing SiGeSn ternary alloys as quantum confinement barriers.

We report the applicability of a hybrid system comprising a La3+-based catalyst and an Au/TiO2 photocatalyst in the decomposition of chemical weapons. This system is able to perform complete degradation of soman, sarin and VX in less than 1 minute under low basic conditions and visible light irradiation.

The structure and charge storage properties of a trilayer structure with Ge nanocrystals embedded in HfO2 oxide were studied. The trilayer structure HfO2/Ge-HfO2/HfO2/p-Si was prepared by magnetron sputtering and subsequent rapid thermal annealing at 600 degrees C. The TEM investigations reveal the formation of Ge NCs embedded in crystalline HfO2 at the position of the Ge-HfO2 layer. The capacitors were made by Al evaporation on both front and backside of the trilayer structure. The C-V characteristics show a counterclockwise hysteresis with large memory window of 0.85 V which is given only by the contribution of the Ge NCs embedded in HfO2. The I-V characteristics show an asymmetric behavior, the currents are three times higher for the negative voltage than the positive one.

One of the methods of direct detection of the massive component of dark matter is the registration of self-recoils in semiconductor detectors at cryogeniC temperatures after the WIMPS scattering. In this contribution, after a short presentation of Lindhard curves, derived from the integro-differential equations of the general theory, we discuss the discrepancies between experimental data and theoretical predictions, as well as the corrections suggested in the literature. A new correction is proposed, which is due to the energy transfer between the electronic and atomic subsystems in the solid during the transient phenomena following the primary interaction, and some numerical results are presented.

Laser-induced chemical conversion of graphene oxide (GO) is an effective way to modify its properties and expand its potential use for numerous applications. In this work, a mechanically stable and flexible freestanding GO membrane is synthesized and further processed by ultraviolet laser radiation in gas and liquid ammonia-rich environments. Electron and atomic force microscopy, as well as X-ray photoelectron spectroscopy analysis, reveal that laser irradiation in gas leads to a large defect-induced morphology modification and high deoxygenation process, accompanied by the slight incorporation of nitrogen functionality to the reduced GO structure. Conversely, irradiation in liquid provokes significant integration of nitrogen groups, essentially amines, into a partially reduced GO structure, without evident modification of the morphology. Electrical measurements on the macro-and nano-scale point to a complex contribution of morphology and oxidized regions to the overall resistance of the rGO.

We report the conversion electron Mossbauer spectroscopy (CEMS) used to analyze the iron valence states and magnetic ordering properties in Fe-57 low doped (0.1-1% at.) as prepared and anneale d anatase TiO2 nanoparticles. The spectra evidence the static and dynamic diversity of local defects around the Fe ions, localized mainly on the surface samples.

The effect of "in situ" thermal treatments (by DSC measurements) on the martensitic transformation in two representative Ni-Fe-Ga and Ni-Mn-Ga alloys has been studied and discussed by correlating the structural and magnetic properties. The alloys were prepared from high purity elements, by arc melting under argon protective atmosphere as bulk and also as melt-spun ribbons - an alternative preparation route that also allows to assess the influences of grains size and strain induced by this processing method. All samples presented reversible thermo-elastic transformations. The thermal treatments promote a reduction of the martensitic transformation temperatures in the Ni-Fe-Ga investigated samples, as opposed to the stoichiometric Ni2MnGa where the temperatures increase with increasing the annealing temperatures. Interestingly however, the off-stoichiometric Ni-Mn-Ga with increased Ni content recovers the behaviour with reduction of transformation temperatures by thermal treatments. The precipitation of the secondary FCC (gamma) phase is inherently found in Ni-Fe-Ga alloys with Ga <= 27% at, and also-although in lower amounts- in the off-stoichiometric Ni-Mn-Ga. The gamma phase is considered to contribute to the decrease of the MT temperatures (via valence electrons concentration depletion of the main matrix) and of the transformation heat as well as to the final structural degradation if the temperature of the thermal treatments is further increased. In addition, this phase, located mainly at the grain boundaries, is responsible for the improved ductility of Ni-Fe-Ga based alloys. Changes in the transformation heat due to thermal treatments are observed and discussed in both types of alloys, the maxima of the transformation heat being associated with the highest atomic order. Thermo-magnetic measurements show that Ni-Fe-Ga alloys have close magnetic and structural transitions temperatures, with promising applications for magnetic refrigeration. Copyright (C) 2016 The Authors. Published by Elsevier B.V.

By coupling two spectral techniques, namely, X-ray photoelectron and specular reflection infrared spectroscopy at high reflection angles, we identified, in this paper,. the formation of SiO2 respectively SiOx (1 < x < 2) structures on the surface of polydimethylsiloxane thin films. These layers have been obtained as a result of placing a thin liquid film precursor on a germanium substrate under corona charge injection in multipoints to plane electrodes configuration in air at atmospheric pressure for 2 hours.