Publications

Polymer-based nanocomposites have recently received considerable attention due to their unique properties, which makes them feasible for applications in optics, sensors, energy, life sciences, etc. The present work focuses on the synthesis of nanocomposites consisting of a polytetrafluorethylene-like matrix in which metallic nano-silver are embedded, by using multiple magnetron plasmas. By successively exposing the substrate to separate RF magnetrons using as combination of target materials polytetrafluorethylene (PTFE) and silver, individual control of each deposition process is insured, allowing obtaining of structures in which silver nanoparticles are entrapped in-between two PTFE layers with given thicknesses. The topographical and morphological characteristics investigated by means of Scanning Electron Microscopy and Atomic Force Microscopy techniques evidenced the very presence of Ag nanoparticles with typical dimension 7 nm. The chemical composition at various depositing steps was evaluated through X-ray Photoelectron Spectroscopy. We show that the presence of the top PTFE layer prevents silver oxidation, while its thickness allows the tailoring of optical properties, as evidenced by spectroellipsometry. The appearance of chemical bonds between silver atoms and PTFE atoms at interfaces is observed, pointing out that despite of pure physical deposition processes, a chemical interaction between the polymeric matrix and metal is promoted by plasma.

Iron oxides such as magnetite and maghemite coated with cetyltrimethylammonium bromide (CTAB) are very promising materials for wastewater treatment because iron oxide can be easily separated from solutions using the magnetic separation procedure Iron oxide (IO) coated CTAB was synthesized by an adapted co-precipitation method. In the present study, the IO-CTAB was used for removing arsenic from water for the first time. In the present study, the performance of iron oxide coated CTAB biocomposites as an adsorbent for arsenic removal from aqueous solutions was examined. X-ray diffraction (XRD) analysis and the results revealed a cubic phase Fd-3 m of Fe(3)O(4)with lattice a = 8.40 angstrom and average crystal size equal to 17.26 +/- 3 nm. The mean particle size calculated from transmission electron microscopy (TEM) was 19.86 +/- 1.7 nm. The results of the adsorption batch experiments and the data determined using the Langmuir and Freundlich models emphasized that IO-CTAB nanoparticles were favorable for the adsorption of As(III) ions from aqueous solutions. Ultrasound measurements have shown that IO-CTAB is a cost-effective biocomposite for removing arsenic from contaminated solutions. Moreover, x-ray photoelectron spectroscopy (XPS) has shown that during the process of arsenic absorption, there is oxidation from As(III) to As(V), which leads to a decrease in toxicity during this process. The results of the cytotoxic assays confirmed that the IO-CTAB nanoparticles did not induce any morphological changes in the HeLa cells and did not affect their proliferation after 24 h of incubation.

GeSn alloys have the potential of extending the Si photonics functionality in shortwave infrared (SWIR) light emission and detection. Epitaxial GeSn layers were deposited on a relaxed Ge buffer on Si(100) wafer by using high power impulse magnetron sputtering (HiPI-MS). Detailed X-ray reciprocal space mapping and HRTEM investigations indicate higher crystalline quality of GeSn epitaxial layers deposited by Ge HiPI-MS compared to commonly used radio frequency magnetron sputtering (RF-MS). To obtain a rectifying heterostructure for SWIR light detection, a layer of GeSn nanocrystals (NCs) embedded in oxide was deposited on the epitaxial GeSn one. Embedded GeSn NCs are obtained by cosputtering deposition of (Ge1-xSnx)(1-y)(SiO2)(y) layers and subsequent rapid thermal annealing at a low temperature of 400 degrees C. Intrinsic GeSn structural defects give p-type behavior, while the presence of oxygen leads to the n-character of the embedded GeSn NCs. Such an embedded NCs/epitaxial GeSn p-n heterostructure shows superior photoelectrical response up to 3 orders of magnitude increase in the 1.2-2.5 mu m range, as compared to performances of diode based only on embedded NCs.

The present work reports for the first time, the employment of ferroelectric ZrO(2)films as energy storage capacitors utilized in pulsed power systems. Furthermore, the effect of insertion of a low permittivity dielectric HfO2:Al2O3(HAO) layer, with a thickness ranging from 2 to 8 nm, on the tunability of ferroelectric and energy storage characteristics of ZrO(2)films is assessed. The increase in thickness of the HAO layer gave rise to distorted ferroelectric loops with decreased polarization, coercive field, and hysteresis loss of the films. These results are correlated with the depolarization field induced by the insertion of the dielectric HAO layer. An optimum combination of high energy density of 54.3 J cm(-3)and good storage efficiency of 51.3% are obtained for the ZrO(2)film capacitors with 2 nm-thick HAO insert layer. These values correspond to an increase of similar to 55% and similar to 92%, from the respective values of pure ZrO(2)film capacitors. In addition, the HAO/ZrO(2)films showed a good fatigue endurance of energy storage performance over 10(9)electric field cycles. The energy storage density obtained from HAO(2 nm)/ZrO(2)film capacitor is found to be higher than that reported for several Pb-based as well as Pb-free ferroelectric ceramic films with complex compositions. The present study demonstrates that simple binary oxides such as ZrO(2)with ferroelectric behavior could be potential candidates for developing high performance energy storage capacitors.

The negative-capacitance effect in devices based on combined ferroelectric-dielectric gate oxides is thought to be a potential solution to break free from the so-called Boltzmann tyranny. To lower the power consumption in field-effect transistors, the subthreshold swing factor S should be reduced below the ther-modynamic limit of 60 mV per decade. Yet, despite numerous studies dedicated to this effect in the past decade, its origin in ferroelectric capacitors or ferroelectric-based superlattices remains unclear, being considered either a transitory product of polarization switching or an intrinsic phenomenon related to the presence of ferroelectric polarization. In this study it is shown, starting from simple electrostatic con-siderations, that negative capacitance is present during polarization switching and is accompanied by a significant increase of the current flowing through the ferroelectric capacitor. Coupled with piezo-force microscopy results, it is shown that the polarization orientation suddenly changes at the coercive voltage, accompanied by a complete reconfiguration of the potential barriers at the Schottky-like contacts present at the electrode-ferroelectric interfaces. A method to estimate the polarization-switching time, as the time associated with the presence of the negative-capacitance effect, is proposed. Values in the range from 100 to 1000 ns are obtained for epitaxial PbZr0.2Ti0.8O3 films. These findings suggest that negative capacitance may be an intrinsic effect in ferroelectrics but that it is a transitory effect, present only when ferroelectric polarization passes through zero (switching).

We have grown, by Pulsed Laser Deposition (PLD), a large number of YBa2Cu3O7 films with artificial pinning centers, with various impurities, various architectures and thickness, and various techniques of nano-scale pinning engineering: substrate decoration, BaZrO3 (BZO) nano-inclusions, and (quasi) multilayer architecture. Here we will present the results regarding vortex matter, dynamics, and pinning, in some of our best samples. Magnetic relaxation studies with magnetic field perpendicular to the film show that in both cases the splayed defects and/or nanoscale inclusions help reduce the dissipation in certain field-temperature range, by inhibiting the detrimental double vortex-kink formation. The response of the vortex system to AC excitation is rather complex but extremely useful for the characterization of the vortex dynamics. We have measured AC susceptibility in various DC fields, with various AC fields and frequencies and found that the effective vortex activation energy U-eff has a logarithmic dependence on the AC-field-induced current density J. Synergetic pinning centers assured a low anisotropy of the critical current for various field orientations, a property desired in coil/solenoid applications.

Hematite (alpha-Fe2O3) is a promising and Earth-abundant material for solar fuel production, and Si-doping has been employed as a general strategy to improve its performance. However, an atomistic description that reconciles the modifications that Si-doping induces on the morphology, crystalline lattice, and electronic and magnetic properties of alpha-Fe2O3 has remained elusive. Here we report on the role of electron small polarons in driving the morphological transition from nearly rounded-shaped to nanowire nanocrystals in Si-doped hematite alpha-Fe2O3. Electron small polaron formation is evidenced by the formation of Fe2+ and the increase of FeO6 distortion at increasing Si content. Local analysis via pair distribution function highlights an unreported crossover from small to large polarons, which affects the correlation length of the polaronic distortion from short to average scales. Ferromagnetic double exchange interactions between Fe2+/Fe3+ species are found to be the driving force of the crossover, constraining the chaining of chemical bonds along the [110] crystallographic direction. This promotes the increase in the reticular density of Fe atoms along the hematite basal plane only, which boosts the anisotropic growth of nanocrystals with more extended [110] facets. Our results show that magnetic and electronic interactions drive preferential crystallographic growth in Si-doped alpha-Fe2O3, thus providing new insights for the nanoscale structural design of efficient solar fuel devices.

We have measured the two-dimensional image potential states (IPS) of a germanene layer synthesized on a Ge2Pt crystal using scanning tunnelling microscopy and spectroscopy. The IPS spectrum of germanene exhibits several differences as compared to the IPS spectrum of pristine Ge(001). First, then = 1peak of the Rydberg series of the IPS spectrum of germanene has two contributions, labelledn = 1(-)andn = 1(+), respectively. The peak at the lower energy side is weaker and is associated to the mirror-symmetric state with opposite parity. The appearance of this peak indicates that the interaction between the germanene layer and the substrate is very weak. Second, the work function of germanene is about 0.75 eV lower in energy than the work function of Ge(001). This large difference in work function of germanene and pristine Ge(001) is in agreement with first-principles calculations.

Melanin is a natural pigment present in almost all biological groups, and is composed of indolic polymers and characterized by black-brown colorization. Furthermore, it is one of the pigments produced by extremophiles including those living in the Antarctic desert, and is mainly involved in their protection from high UV radiation, desiccation, salinity and oxidation. Previous studies have shown that melanized species have an increased capability to survive high level of radiation compared with the non-melanized counterpart. Understanding the molecular composition of fungal melanin could help to understand this peculiar capability. Here, we aimed to characterize the melanin pigment extracted from the Antarctic black fungus Cryomyces antarcticus, which is a good test model for radioprotection researches, by studying its chemical properties and spectral data. Our results demonstrated that, in spite of having a specific type of melanin as the majority of fungi, the fungus possesses the ability to produce both 1,8-dihydroxynaphthalene (DHN) and l 3-4 dihydroxyphenylalanine (L-DOPA) melanins, opening interesting scenarios for the protection role against radiation. Researches on fungal melanin have a huge application in different fields, including radioprotection, bioremediation, and biomedical applications.

The effects of 4-(2-hydroxyphenyl)-2-(morpholin-4-yl)-1,3-thiazole (Pr02), 1-(3,5-dibromo-2-hydroxyphenyl)-1-oxoethan-2-yl-N,N-diethyldithiocarbamate (Pr04) and 1-(5-bromo-2-hydroxy-3-methylphenyl)-1-oxoethan-2-yl-O- ethyl xanthate (Pr06) on the aqueous oxidation of chalcopyrite (CuFeS2) in air-equilibrated solution at a temperature of 25 degrees C and a pH of 2.5 were studied. The effects were investigated by using potentiodynamic polarization, electro-chemical impedance spectroscopy (EIS), scanning electron microscopy coupled with energy dispersive X-ray (SEM/EDX) analysis, aqueous batch experiments, Fourier transform infrared (FTIR) spectroscopy, Raman scattering and quantum chemical calculations. It is found that the anodic current densities decrease in the order of EtOH Pr02 > Pr04 > Pr06. These results, along with those of the EIS measurements, show that Pr02, Pr04 and Pr06 are effective anodic inhibitors of chalcopyrite aqueous oxidation. Both Raman scattering and FTIR spectroscopy indicate that the elemental sulfur, polysulfide and ferric oxyhydroxides that form on the surface of the mineral are not responsible when it comes to the aqueous oxidation inhibition of chalcopyrite. Quantum chemical calculations show that the adsorption of the tested compounds on the chalcopyrite surface is energetically favorable and so, it can explain the inhibiting effects that were observed.