Publications

The structural, optical and electrical conduction properties of (Li/Cu,N):ZnO codoped thin films synthesized by the sol-gel method were investigated by field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), transmission and absorption, photoluminescence (PL) and I-V measurements in order to bring evidence of the formation of acceptor centers by dual-acceptor codoping processes. The (Li 3%, N 5%): ZnO films consist of crystallites with average size of 15 nm, show 95% transmission in the visible region, and an optical band gap of 3.22 eV. The PL spectra show emission maxima at 3.21 and 2.96 eV which are related to the emission of acceptor centers and the presence of defects, respectively. Li occupies interstitial sites and may form Lii-N(O) defect complexes that act as acceptor centers. The (Cu 3%, N 5%): ZnO films consist of crystallites with average size of 12 nm, and exhibit 90% transmission in the visible region. The PL spectra reveal band edge emission at 3.23 eV and defect related emission at 2.74 eV. In the (Cu, N) codoped films, copper substitutes zinc and adopts mainly the Cu1+ state. A possible defect complex involving Cu and N determines the transition from n-to p-type conductivity. These findings are in agreement with results of electronic structure calculations at the GGA-PBE level.

Raman studies performed on different morphologies of CdS samples indicate an exciton-phonon interaction (EPI) that manifests at low temperatures by different enhancements in intensity of the Raman spectra in the Stokes and anti-Stokes branches. This effect, interpreted as stimulated Raman effect, is conditioned on the appearance and strength by the degree of overlap of the excitation laser light with the excitonic photoluminescence band. EPI is greatest in samples characterized by high light diffusing power, which means a long path of light into the sample and a great overlapping of the two optical fields, i.e., the pump laser and excitonic light. (C) 2016 Optical Society of America

The strain in Si containing group-V impurities is a topical subject of study due to its potential applications in quantum computing. In this paper we study Bi-209 implanted Si concerning the correlation between the strain produced by stopped Bi ions and trapping characteristics of the defects resulted from implantation. The depths distributions of stopped ions and primary defects are simulated and the distributions of permanent defects are modelled for Si implanted with low fluence Bi-209 ions of 28 MeV kinetic energy. For comparison, these depths distributions were similarly calculated for I-127 ions with the same fluence and energy, implanted in Si. The results are compared with each other and correlated with the characteristics of traps in these systems, previously obtained. We demonstrate that the intensity of the strain field is the most important factor in changing of trap parameters, while the superposition between the region with strain and the region where defects are located is a second order effect. (C) 2016 Elsevier Ltd. All rights reserved.

The intrinsic pyroelectric properties of thick, coarse grained Ba1-xSrxTiO3 (BST) ceramics were studied in relation with Sr content. The intrinsic values of the pyroelectric coefficient were determined directly from the frequency dependence of the measured pyroelectric signal by adapting the formalism developed for thin films deposited on thick substrates to thick and non-uniformly heated ceramics with uniform composition. Ceramic pellets, of about 1 mm thickness, with relative density of 89-95% were prepared using powder synthesized by the sol gel method. It was found that the intrinsic pyroelectric properties are enhanced by increasing the Sr content, while other features (e.g. tetragonality, grain size, polarization, transition temperature) decrease. The best intrinsic pyroelectric properties were obtained for 30% Sr content. The present results open the possibility to use large and thick BST ceramic pellets for manufacturing detectors useful for monitoring high power lasers. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

A novel glassy nanocomposite containing Er3+-doped BaCl2 nanocrystals of about tens of nm embedded in a silicate matrix has been prepared by using sol-gel route followed by controlled crystallization of the xerogel. Under 810 nm laser light pumping it shows green ((H-2(11/2), S-4(3/2)) -> I-4(15/2)) and red (F-4(9/2) -> I-4(15/2)) up-conversion luminescences ascribed by a two-photon processes. The luminescence signal recorded in the oxychloride nano-glass ceramic is only about 10% compared to the NaYF4:Er3+ (18%). In the nano glass ceramic there are effective non-radiative decay channels, most probably related to hydroxyl ions and surface defects acting as centers for the non-radiatively energy dissipation. (C) 2016 Elsevier Ltd. All rights reserved.

Spark Plasma Sintering (SPS) and High Pressure-High Temperature (HPHT) methods were chosen in order to study the behaviour of metastable cubic boron nitride (cBN) during sintering of Al2O3 + 30 vol.% cBN composites at low and high pressure. Relatively low sintering temperatures were used to avoid the undesired reverse transformation of cBN to hexagonal, graphite-like form. The microstructure, and phase composition as well as physical and mechanical properties of the resulting Al2O3-cBN composites were investigated. In the ball-on-disc tests, the coefficient of friction and the specific wear rate of the sintered samples were determined by rotating the sample against a stationary alumina ball. The best composites obtained by SPS (at 1300 degrees C/75 MPa) had slightly lower density, Young's modulus and hardness than those obtained by HPHT but their wear resistance was much better. Formation of hBN was suppressed up to 1300 degrees C during SPS consolidation at the pressure of 75 MPa. (C) 2016 Elsevier Ltd. All rights reserved.

Vortex activation energy U-AC in the critical-state related AC magnetic response of superconductors (appearing in the vicinity of the DC irreversibility line) takes large values, as often reported, which is not yet understood. This behavior is essentially different from that of the vortex-creep activation energy at long relaxation time scales, and may become important for AC applications of superconductors. To elucidate this aspect, we investigated the AC signal of almost decoupled [YBa2Cu3O7](n)/[PrBa2Cu3O7](4) superlattices (with n = 11 or 4 units cells) in perpendicular DC and AC magnetic fields. In these model samples, the length of the hopping vortex segment is fixed by the thickness of superconducting layers and vortices are disentangled, at least at low DC fields. It is shown that the high U-AC values result from the large contribution of the pinning enhanced viscous drag in the conditions of thermally activated, non-diffusive vortex motion at short time scales, where the influence of thermally induced vortex fluctuations on pinning is weak. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

5 mol% Ce3+-doped barium titanate nanoshell tubes were prepared via sol gel method using as template a polycarbonate membrane with channels of 200 nm diameter. FE-SEM analyses showed that continuous and uniform green tubes with length up to 15 mu m, an average outer diameter of 188.6 nm and wall thickness of 15.1 nm, were obtained. After calcination at 700 degrees C for 1 h, these amorphous 1D nanostructures were converted into polycrystalline tubes with an average outer diameter of 157.4 nm and a grain size of 43.4 nm, as high resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) indicated. Thermo-Raman investigations pointed out diffuse phase transitions, inducing the preservation of a stable polar state at higher temperatures (up to 200 degrees C). Piezoresponse force microscopy (PFM) investigations on 5 mol% Ce3+-doped BaTiO3 nanoshell tubes revealed ferroelectric and piezoelectric behaviour which recommends these tubes for microelectronic devices. (C) 2016 Elsevier Ltd. All rights reserved.

Thin amorphous films based on gallium-chalcogen (Ga-Ch), namely Ga2S3, Ga2Se3, Ga2Te3, and GaTe have been prepared by pulsed laser deposition (PLD). The films were characterized by X-ray diffraction, extended X-ray absorption fine structure (EXAFS), energy-dispersive X-ray spectroscopy (EDX), optical transmission spectroscopy, ellipsometry, and electrical measurements. Structural measurements showed that Ga is threefold coordinated, except the Te-based alloys were, it seems, only twofold coordinated, while the chalcogen is usually twofold coordinated. In all the compositions, layered and chain-like structures are assumed. The bandgaps range between 1.09 eV for Ga2Te3 and 2.21 eV for Ga2Se3. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Ex-situ spark plasma sintering (SPS) was used to obtain dense MgB2-based tapes in a Fe sheath with the starting composition (MgB2)(0.975) + (SiC)(0.025) + Te-0.01. Prior to the SPS procedure of tape formation, the samples were submitted to a series of cold working processes typical for the powder-in-tube technique. The tapes were compared with optimal doped bulk samples (having the same starting composition) and a pristine MgB2 tape. The morphology of the composite samples, the phase structure of both the core and the inner face of the metallic sheath shows the formation of a plethora of traces as a result of interaction between MgB2, additives, and the Fe sheath. Important critical parameters, like critical current density and the irreversibility field, show that there is a field and temperature range where the SiC and Te-added tapes display better critical parameters comparative to either pristine MgB2 tapes in the Fe sheath or SiC and Te doped MgB2 bulk samples.