Publications

We investigate the spectral and transport properties of a diatomic square lattice with hopping to the next-nearest-neighbors and broken time-reversal symmetry, which behaves as a Chern insulator. In a finite-size approach, the attention is paid to the formation of chiral edge states in the topological insulating phase, but also in the semimetallic one. The edge states are revealed in the ribbon and plaquette geometries by analytical and numerical methods, significant differences being produced by the specific atomic connectivity at the boundary. The Hall resistance R-H is calculated in the plaquette geometry using the Landauer-Mittiker approach. The chiral edge states located in the unique gap of the energy spectrum manifest themselves by quantized values R-H = +/- h/e(2) specific to the Chern insulator. The semimetallic system containing chiral edge states embedded in the quasicontinuum of bulk states shows a disorder-driven AQHE as a consequence of the Anderson localization process.

Co-doped CeO2 thin films were grown from a bulk target with starting composition Ce0.95Yb0.04Er0.01O2 by pulsed laser deposition (PLD) on a p(+)-n-n(+) single crystal silicon diode. The PLD laser fluence was varied between 1.7 J/cm(2) and 3.7 J/cm(2). The device with the film grown for a laser fluence of 2.3 J/cm(2) delivers the highest performance taking advantage of the up conversion (UC) effect provided by this film. Namely, the increase in the relative power conversion efficiency of the device is 12.1% and 39.2% for illumination under 1 and 2.1 sun, respectively, and its relative external quantum efficiency is 8.2% when illuminated with 980 nm light. The film grown for the optimum 2.3 J/cm(2) fluence shows good target-film composition transfer and a granular morphology with a low roughness. The UC mechanism consists of efficient energy transfer between spatially separated Yb3+ and Er3+ ions, i.e. the absorption of infrared light photons by the Yb3+ ions (F-2(7/2) -> F-2(5/2) transition) is followed by a two-step energy transfer process to neighboring Er3+ ions and by their characteristic luminescent emissions ((H-2(11/2), S-4(3/2)) -> I-4(15/2)) and (F-4(9/2) -> I-4(15/2)).

Organic heterostructures based on zinc phthalocyanine (ZnPc) and perylene tetracarboxylic dianhydride (PTCDA) were deposited by matrix-assisted pulsed laser evaporation (MAPLE) technique on conductive flexible substrate (ITO/PET) in three configurations: ZnPc/PTCDA (stacked layers), ZnPc:PTCDA (blend) and ZnPc/ZnPc:PTCDA/PTCDA. The effect of the configuration on the optical and electrical properties of the obtained heterostructures was investigated. For all heterostructures was observed an improved optical absorption in visible domain. The I-V characteristics recorded under illumination, revealed higher short circuit current (I (SC)) values for the ZnPc:PTCDA and ZnPc/ZnPc:PTCDA/PTCDA structures in comparison with that of the ZnPc/PTCDA structure. The results proved that by MAPLE can be obtained flexible organic heterostructures (in different configurations) with properties adequate for applications in flexible electronics and solar cell fields.

Chitosan-coated cobalt ferrites were synthesized by wet ferritization method through two different approaches. The nanostructured chitosan-coated cobalt ferrites were characterized by: X-ray diffraction (XRD), scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM/HR-TEM), thermal analysis, infrared spectroscopy (IR) and Mossbauer spectroscopy. X-ray diffraction patterns confirmed the formation of CoFe2O4 cubic spinel structure with an average crystallite size of 1.75 and 5.5 nm, respectively. Mossbauer spectra consist in a central quadrupole pattern. The deconvolution in the hypothesis of Lorentzian line shape evidences the prevailing presence of Fe3+ ions at the nanoparticle surface. The antimicrobial activity of chitosan-coated cobalt ferrites against Gram-positive and Gram-negative bacteria, as well as fungal strains was investigated. Based on the results, chitosan-coated cobalt ferrites are thought to be suitable candidates for the development of novel anti-biofilm agents.

During conflict situations, the combat staff is exposed to a wide variety of aggressions, such as temperature and pressure variations and dynamic impacts (from ammunition or fragments). Textiles used in the manufacture of the military uniforms and devices have always played an important role in defending the military against these hazards, and an adequate level of individual protection equipment is required. In this respect, novel fibre-reinforced polymer composite materials for military application, such as reducing blunt trauma for ballistic protection equipment, have been studied in terms of thermal and mechanical properties and ballistic protection, obtaining very good results.

This work involves the synthesis and characterization of Cu2ZnSnS4 (CZTS) layers. The films were prepared on ITO/glass substrate by ecofriendly and simple single-step electrodeposition method followed by sulfurization and annealing at 500 degrees C under Argon flow. By using two different complexing agents, the electrodeposition process can give better results. Therefore, the effect of combining the trisodium citrate - TC to multiple cornplexing agents (cetyl trimethyl ammonium bromide - CTAB, ethylene diamine tetra acetic acid - EDTA, Boric Acid - BA, Glutamic Acid - GA and Tartaric Acid - TA) is investigated. The characterization of the absorber films was done by X-ray diffraction (XRD) analysis, Raman spectroscopy, Scanning Electron Microscopy and Diffuse Reflectivity. The combination of TC and CTAB is suggested to be the best pair of complexing agents within the combinations used in this work.

Arrays of magnetic Ni-Cu alloy nanowires with different compositions were prepared by a template-replication technique using electrochemical deposition into polycarbonate nanoporous membranes. Photolithography was employed for obtaining interdigitated metallic electrode systems of Ti/Au onto SiO2/Si substrates and subsequent electron beam lithography was used for contacting single nanowires in order to investigate their galvano-magnetic properties. The results of the magnetoresistance measurements made on single Ni-Cu alloy nanowires of different compositions have been reported and discussed in detail. A direct methodology for transforming the magnetoresistance data into the corresponding magnetic hysteresis loops was proposed, opening new possibilities for an easy magnetic investigation of single magnetic nanowires in the peculiar cases of Stoner-Wohlfarth-like magnetization reversal mechanisms. The magnetic parameters of single Ni-Cu nanowires of different Ni content have been estimated and discussed by the interpretation of the as derived magnetic hysteresis loops via micromagnetic modeling. It has been theoretically proven that the proposed methodology can be applied over a large range of nanowire diameters if the measurement geometry is suitably chosen.

In this work, the ferroelectric characteristics of 0.5Ba(Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3 (BCZT) thin films grown on 0.7 wt. % Nb-doped (001)-SrTiO3 (Nb:STO) single-crystal have been investigated. High-resolution transmission electron microscopy and electron energy loss spectroscopy revealed a very sharp Nb:STO/BCZT interface, while selected area electron diffraction revealed the epitaxial growth of the BCZT layer on the Nb:STO substrate. The ferroelectric nature of the BCZT films have been investigated by piezoresponse force microscopy and hysteresis loops. The effect of electric field on polarization switching kinetics has been investigated and has been analyzed by the nucleation limited switching model with a Lorentzian distribution function. The local field variation was found to decrease with the increase in the electric field, and thus, the switching process becomes faster. The peak value of the polarization current and the logarithmic characteristic switching time exhibited an exponential dependence on the inverse of electric field. This model gave an excellent agreement with the experimental polarization reversal transients throughout the whole time range. Published by AIP Publishing.

Bio-waste eggshell membranes (ESM) present a unique micro-architecture consisting in an interwoven fibrous network which can be functionalized with metal oxides resulting in hybrid materials. ESM were covered with ZnO nanostructures by electroless deposition using Au as catalyst. The structural, optical, morphological and wetting properties of the pristine ESM and ESM/ZnO were evaluated. The ESM fibers were uniformly coated by ZnO hexagonal prisms, the hybrid ESM/ZnO preserving the water absorption characteristic of the pristine ESM. Combining an abundant bio-waste with a simple wet chemical synthesis method, flexible organic/inorganic hybrid networks based on ZnO-functionalized ESM can be designed for various applications. (C) 2018 Elsevier B.V. All rights reserved.

The production and thermal stability of the irradiation paramagnetic point defects (IPPDs) in the as-grown, standard float-zone silicon (STFZ) and oxygen doped float-zone silicon (DOFZ) irradiated at room temperature with high fluence 3.5 MeV (1 x 10(17) cm(-2)) low energy and 27 MeV (2 x 10(16) cm(-2)) high energy electrons were investigated by Electron Spin Resonance (ESR). The nature and concentration of the IPPDs identified in the as-irradiated and isochronally annealed up to 300 degrees C samples by ESR measurements under intense above the gap 1.06 mu m in-situ laser illumination, were found to depend on oxygen concentration and electrons energy. While irradiation of STFZ with electrons produced as main IPPDs, besides divacancies, larger tetravacancy and pentavacancy cluster defects, irradiation of DOFZ resulted mainly in divacancies, vacancy-oxygen and interstitial oxygen-carbon impurity pairs. The observed variation in the nature of the main resulting IPPDs with the concentration of incorporated oxygen is explained by differences in the dominant defects production mechanisms. Thus in STFZ with lower oxygen concentration (1 x 10 16 cm(-3)) the dominant production mechanisms are the direct defects formation from a chain of neighboring vacancies by a cascade of secondary recoils across the path of the high energy irradiating particle and the divacancies diffusion and trapping/aggregation. Meanwhile, in DOFZ with larger oxygen content (1.2 x 10(17) cm(-3)) the primary vacancy and interstitial trapping by the oxygen and carbon impurities is the dominant defect production mechanism. Variations in the concentration and nature of the IPPDs observed during isochronal annealing are discussed in terms of defects thermal activated diffusion and recombination processes.