Publications

The frequency and temperature dependence of dielectric properties of CH3NH3PbI3 (MAPI) crystals have been studied and analyzed in connection with temperature-dependent structural studies. The obtained results bring arguments for the existence of ferroelectricity and aim to complete the current knowledge on the thermally activated conduction mechanisms, in dark equilibrium and in the presence of a small external a.c. electric field. The study correlates the frequency-dispersive dielectric spectra with the conduction mechanisms and their relaxation processes, as well as with the different transport regimes indicated by the Nyquist plots. The different energy barriers revealed by the impedance spectroscopy highlight the dominant transport mechanisms in different frequency and temperature ranges, being associated with the bulk of the grains, their boundaries, and/or the electrodes' interfaces.

This work focuses on the temperature evolution of the martensitic phase epsilon (hexagonal close packed) induced by the severe plastic deformation via High Speed High Pressure Torsion method in Fe57Mn27Si11Cr5 (at %) alloy. The iron rich alloy crystalline structure, magnetic and transport properties were investigated on samples subjected to room temperature High Speed High Pressure Torsion incorporating 1.86 degree of deformation and also hot-compression. Thermo-resistivity as well as thermomagnetic measurements indicate an antiferromagnetic behavior with the Neel temperature (T-N) around 244 K, directly related to the austenitic gamma-phase. The sudden increase of the resistivity on cooling below the Neel temperature can be explained by an increased phonon-electron interaction. In-situ magnetic and electric transport measurements up to 900 K are equivalent to thermal treatments and lead to the appearance of the bcc-ferrite-like type phase, to the detriment of the epsilon(hcp) martensite and the gamma (fcc) austenite phases.

Our study focuses on an advanced exploratory research based on the development of the new IT-SOFCs anode compositions in conjunction with surfactants as template to obtain mesoarchitectured catalysts. We designed NiO/Sr doped Ce0.85Pr0.10 Er0.05O2-o (2.5 and 5 mol.% Sr) mesoarchitectures with robust crystalline framework following a hydrothermal synthesis route. NiO was deposited by wet impregnation and calcined in air at 900.C, in order to assure a good interaction between Ni and mesoporous matrix. The obtained materials exhibit a cubic CeO2 fluorite phase with Pr2O3 and SrCeO3 as secondary phase traces and these were assessed as catalyst for the partial oxidation of CH4 over time and IT-SOFCs anode under reducing atmosphere. The electrochemical behavior as anode fed by wet H2 (2.5% vol. H2O) was investigated in the NiO/CPS/YSZ/GDC/Pt electrochemical cells. The compositional effect on the structural properties, surface chemistry, catalytic and electrochemical performance were highlighted. Sr incorporated in the lattice, proved by X-ray Photoelectron Spectroscopy, guarantees an excellent stability of the catalysts over time for 20h, during partial oxidation of CH4 with high CH4 conversion and CO selectivity (90% in the range 700-800.C) as well as a better performance as IT-SOFCs anode.

The crystalline structure and Fe local environment in a Co-doped Ni-Fe-Ga Heusler alloy, prepared by the melt-spinning technique, were investigated by X-ray diffraction (XRD) and EXAFS at room and low temperatures. The characteristic temperatures of the austenite-martensite phase transitions were determined by differential scanning calorimetry via cooling and heating cycles of the alloy ribbons. As shown by room-temperature XRD, the austenitic phase of the alloy has the chemically ordered L2(1) Heusler structure. This was confirmed by EXAFS, although this technique was not able to conclusively distinguish between the L2(1) and B2 structures of the austenite for the analyzed alloy. The low-temperature martensitic phase and its structural evolution towards austenite with increasing temperature were studied by high-energy X-ray diffraction, which evinced the martensite modulation. However, the Fe environment could be fitted by EXAFS with the tetragonal L1(0) structure of the non-modulated martensite. This proves that the martensite modulation has structural effects on a long-range scale, without significant changes in the short-range order around the atoms. The changes in the local structure around iron on martensitic transformation were correlated with changes in the electronic structure, described by XANES spectroscopy at the Fe K edge.

In this work, synthesis and optical properties of a new composite based on poly(o-phenylenediamine) (POPD) fiber like structures, poly(vinylidene fluoride) (PVDF) spheres and double-walled carbon nanotubes (DWNTs) are reported. As increasing the PVDF weight in the mixture of the chemical polymerization reaction of o-phenylenediamine, the presence of the PVDF spheres onto the POPD fibers surface is highlighted by scanning electron microscopy (SEM). The down-shift of the Raman line from 1421 cm(-1) to 1415 cm(-1) proves the covalent functionalization of DWNTs with the POPD-PVDF blends. The changes in the absorbance of the IR bands peaked around 840, 881, 1240 and 1402 cm(-1) indicate hindrance steric effects induced of DWNTs to the POPD fiber like structures and the PVDF spheres, as a consequence of the functionalization process of carbon nanotubes with macromolecular compounds. The presence of the PVDF spheres onto the POPD fiber like structures surface induces a POPD photoluminescence (PL) quenching process. An additional PL quenching process of the POPD-PVDF blends is reported to be induced in the presence of DWNTs. The studies of anisotropic PL highlight a change of the angle of the binding of the PVDF spheres onto the POPD fiber like structures surface from 50.2 degrees to 38 degrees when the carbon nanotubes concentration increases in the POPD-PVDF/DWNTs composites mass up to 2 wt.%.

FeRAM memories constitute themselves as a robust counterpart to the mainstream NVRAM memories. Their unique properties, such as radiation resilience and low voltage operation, together with high speed writing and reading, high and long lasting remanence and very good endurance to cycling endorse them in the data storage domain. However price, poor scalability, the destructive way of reading and the necessary consequent re-writing represent technical drawbacks that diminish their practical value. This paper presents a new paradigm of completing previous efforts towards fast nondestructive reading of FeRAMs. An intermittent low intensity IR (infrared) laser diode generates a pyroelectric signal in the ferroelectric material. The amplitude and phase of this response depends on the state of polarisation of the ferroelectric. Comparing the phase of the electric response with the phase of the illuminating radiation (on/off), a decisive indication of the poling sense in the ferroelectric is given. Aiming to circumvent the notorious slowness of the pyroelectric response the array is continuously illuminated with the pulsating laser, which ensures a prompt answer from every bit of memory. The reading speed is, practically, limited by the electronics.

Achieving thermoelectric devices with high performance based on low-cost and nontoxic materials is extremely challenging. Moreover, as we move toward an Internet-of-Things society, a miniaturized local power source such as a thermoelectric generator (TEG) is desired to power increasing numbers of wireless sensors. Therefore, in this work, an all-oxide p-n junction TEG composed of low-cost, abundant, and nontoxic materials, such as n-type ZnO and p-type SnOx thin films, deposited on borosilicate glass substrate is proposed. A type II heterojunction between SnOx and ZnO films was predicted by density functional theory (DFT) calculations and confirmed experimentally by X-ray photoelectron spectroscopy (XPS). Moreover, scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDS) show a sharp interface between the SnOx and ZnO layers, confirming the high quality of the p-n junction even after annealing at 523 K. ZnO and SnOx thin films exhibit Seebeck coefficients (alpha) of similar to 121 and similar to 258 mu V/K, respectively, at 298 K, resulting in power factors (PF) of 180 mu W/m K-2 (for ZnO) and 37 mu W/m K-2 (for SnOx). Moreover, the thermal conductivities of ZnO and SnOx films are 8.7 and 1.24 W/m K, respectively, at 298 K, with no significant changes until 575 K. The four pairs all-oxide TEG generated a maximum power output (P-out) of 1.8 nW (approximate to 126 mu W/cm(2)) at a temperature difference of 160 K. The output voltage (V-out) and output current (I-ou(t)) at the maximum power output of the TEG are 124 mV and 0.0146 mu A, respectively. This work paves the way for achieving a high-performance TEG device based on oxide thin films.

In this work, the photodegradation process of atorvastatin calcium (ATC) is reported as depending on: (1) the presence and the absence of excipients in the solid state; (2) the chemical interaction of ATC with phosphate buffer (PB) having pH equal to 7 and 8; and (3) hydrolysis reaction of ATC in the presence of aqueous solution of NaOH. The novelty of this work consists in the monitoring of the ATC photodegradation by photoluminescence (PL). The exposure of ATC in solid state to UV light induces the photo-oxygenation reactions in the presence of water vapors and oxygen from air. According to the X-ray photoelectron spectroscopic studies, we demonstrate that the photo-oxygenation reaction leads to photodegradation compounds having a high share of C=O bonds compared to ATC before exposure to UV light. Both in the presence of PB and NaOH, the photodegradation process of ATC is highlighted by a significant decrease in the intensity of the PL and photoluminescence excitation (PLE) spectra. According to PLE spectra, the exposure of ATC in the presence of NaOH to UV light leads to the appearance of a new band in the spectral range 340-370 nm, this belonging to the photodegradation products. Arguments concerning the chemical compounds, that resulted in this last case, are shown by Raman scattering and FTIR spectroscopy.

Energy harvesting for Internet of Things applications, comprising sensing, life sciences, wearables, and communications, requires efficient thermoelectric (TE) materials, ideally semiconductors compatible with Si technology. In this work, we investigate the potential of GeSn/Ge layers, a group IV material system, as TE material for low-grade heat conversion. We extract the lattice thermal conductivity, by developing an analytical model based on Raman thermometry and heat transport model, and use it to predict thermoelectric performances. The lattice thermal conductivity decreases from 56 W/(m.K) for Ge to 4 W/(m.K) by increasing the Sn atomic composition to 14%. The bulk cubic Ge0.86Sn0.14 alloy features a TE figure of merit of ZT similar to 0.4 at 300 K and an impressive 1.04 at 600 K. These values are extremely promising in view of the use of GeSn/Ge layers operating in the typical on-chip temperature range.