Publications

Latest developments in the field of high power ultra-short pulse lasers have led to intensive studies dedicated to the fabrication possibility of new antireflective coatings which exhibit high damage threshold. Therefore, this study is focused on the deposition and characterization of metal oxide heterostructures followed by laser-induced damage threshold tests which evidence their application in high power laser optics. Al2O3, SiO2, and HfO2 layers are combined to obtain different heterostructures, i.e. HfO2/Al2O3/HfO2/Al2O3/HfO2 and HfO2/SiO2/HfO2/SiO2/HfO2. The metal oxide heterostructures are deposited in a controllable oxygen atmosphere, either at room temperature or high temperatures (600 degrees C) by pulsed laser deposition (PLD). The morphological, structural and optical properties of the as-deposited heterostructures are first investigated. Atomic force microscopy and spectroscopic ellipsometry investigations reveal a lower roughness of the heterostructures based on HfO2/Al2O3 layers grown at 600 degrees C as compared to those grown at room temperature. Furthermore, following the laser-induced damage threshold (LIDT) tests carried out with a Ti-Sapphire laser, higher LIDT values are obtained for the HfO2/Al2O3-based heterostructures than for the HfO2/SiO2-based heterostructures. The ability to control the morphological and structural properties of the antireflective coatings by modifying the deposition parameters of the metal oxide heterostructures demonstrates that PLD is a suitable technique for the manufacturing of antireflective coatings for high power ultra-short laser systems.

We present an extension of the dynamic electrical model, which enable us to explain some important features of the perovskite solar cells (PSC), like the shape of the hysteresis and the appearance of the ?bump? in the so called reverse scan, without requiring any additional assumptions. We give analytical expressions in terms of the Lambert?s function W for the open circuit voltage, the stationary current, and the instantaneous current, which can be written also in terms of elementary functions for the most part of the ranges of the physical parameters. The initial polarization of the cell, modeled as the charging of a capacitor with voltage dependent capacitance, is consistently determined in the model, from the initial stationary conditions. This is inline with a previously observed sharp increase of the PSC capacitance beyond the open-circuit voltage. Besides the known features, we obtain characteristics that were not yet analyzed experimentally, like the change of the bump from the reverse scan branch of the J?V characteristic to the forward scan, with the increase of the poling voltage (or the increase of the PSC capacitance).

Single-phase Ce3+-doped BaTiO3 powders described by the nominal formula Ba1-xCexTi1-x/4O3 with x = 0.005 and 0.05 were synthesized by the acetate variant of the sol-gel method. The structural parameters, particle size, and morphology are strongly dependent on the Ce3+ content. From these powders, dense ceramics were prepared by conventional sintering at 1300 degrees C for 2 h, as well as by spark plasma sintering at 1050 degrees C for 2 min. For the conventionally sintered ceramics, the XRD data and the dielectric and hysteresis measurements reveal that at room temperature, the specimen with low cerium content (x = 0.005) was in the ferroelectric state, while the samples with significantly higher Ce3+ concentration (x = 0.05) were found to be in the proximity of the ferroelectric-paraelectric phase transition. The sample with low solute content after spark plasma sintering exhibited insulating behavior, with significantly higher values of relative permittivity and dielectric losses over the entire investigated temperature range relative to the conventionally sintered sample of similar composition. The spark-plasma-sintered Ce-BaTiO3 specimen with high solute content (x = 0.05) showed a fine-grained microstructure and an almost temperature-independent colossal dielectric constant which originated from very high interfacial polarization.

MgB2 green bodies were prepared by magnetic field slip casting in ethyl alcohol with added polyethyleneimine dispersing agent under a high magnetic field, mu H-0(0) = 12 T. Samples were further processed by spark plasma sintering (SPS) and characterized for superconducting properties. Slip casting provides texturing of MgB2 (the degree of c-axis orientation is approximately 3.5%), which is further increased significantly (to about 21%) in the SPSed sample. The critical current density (J(c)) displays anisotropy relative to the orientation of the measuring magnetic field. Specific features of J(c)(H, T) and of the pinning force extracted from magnetic measurements with the field parallel and perpendicular to H-0 are discussed.

Staggered gap radial heterojunctions based on ZnO-CuxO core-shell nanowires are used as water stable photocatalysts to harvest solar energy for pollutants removal. ZnO nanowires with a wurtzite crystalline structure and a band gap of approximately 3.3 eV are obtained by thermal oxidation in air. These are covered with an amorphous CuxO layer having a band gap of 1.74 eV and subsequently form core-shell heterojunctions. The electrical characterization of the ZnO pristine and ZnO-CuxO core-shell nanowires emphasizes the charge transfer phenomena at the junction and at the interface between the nanowires and water based solutions. The methylene blue degradation mechanism is discussed taking into consideration the dissolution of ZnO in water based solutions for ZnO nanowires and ZnO-CuxO core-shell nanowires with different shell thicknesses. An optimum thickness of the CuxO layer is used to obtain water stable photocatalysts, where the ZnO-CuxO radial heterojunction enhances the separation and transport of the photogenerated charge carriers when irradiating with UV-light, leading to swift pollutant degradation.

Phosphate-tellurite glasses exhibit magnetic properties, due to the presence of the small metallic Te colloids which were revealed in low field magnetic circular dichroism. These metallic colloids induce the red coloring of these glasses together with the absorbance in the visible region. The temperature dependence of the absorption spectrurn and the A-term in magnetic circular dichroism are specific for Te metallic nanoparticles, which results during the melting procedure over 1000 degrees C due to the conversion of Te4+ in Te-0 atoms. The X-ray photoelectron spectroscopy supports this fact due to the presence of small peaks as satellites in the region of Te 3d core-level spectrum. Quantification of these satellites compared with Te 3d(3/2) and 3d(5/2) peaks suggests a 14% concentration of Te metallic nanoparticles in these phosphate-tellurite glasses. The presence of metallic particles induces the crystallization effects of Te micrograins upon thermal treatments at higher temperatures.

The bone regeneration field targeted lately the development of new products based on precursors of natural origin. This study aimed to obtain the optimal design of polymer-ceramic composites for guided bone regeneration application from cellulose acetate (CA) and hydroxyapatite (HA) by varying three relevant parameters: the amount of HA powder added to the CA matrix (in the 20-40 wt% range), the HA particles size (max. 20 mu m vs. max. 40 mu m) and the homogenization time required for HA powder dispersion in the CA matrix (1 min vs. 4 min). For polymer-ceramic film structures preparation, the phase inversion by immersion in water method was used. This involved the deposition of composite solution (i.e. CA with 20-40 wt% HA) on a glass support, followed by sizing it at a thickness of 0.2 mm. The obtained film structures were investigated in terms of morphocompositional and structural properties. The surface features evaluation was achieved by surface wettability, roughness, water permeation, protein retention and in vitro evaluation of MC3T3-E1 morphology and viability. Further, ceramic particle distribution throughout samples volume was provided by computed tomography methods. These investigations targeted the validation of the prepared composite film structures as viable solutions for guided bone regeneration.

Recently, a large spectrum of biomaterials emerged, with emphasis on various pure, blended, or doped calcium phosphates (CaPs). Although basic cytocompatibility testing protocols are referred by International Organization for Standardization (ISO) 10993 (parts 1-22), rigorous in vitro testing using cutting-edge technologies should be carried out in order to fully understand the behavior of various biomaterials (whether in bulk or low-dimensional object form) and to better gauge their outcome when implanted. In this review, current molecular techniques are assessed for the in-depth characterization of angiogenic potential, osteogenic capability, and the modulation of oxidative stress and inflammation properties of CaPs and their cation- and/or anion-substituted derivatives. Using such techniques, mechanisms of action of these compounds can be deciphered, highlighting the signaling pathway activation, cross-talk, and modulation by microRNA expression, which in turn can safely pave the road toward a better filtering of the truly functional, application-ready innovative therapeutic bioceramic-based solutions.

Three novel fluorescent mesoporous silica composites were obtained through the covalent immobilization of 7-amino-4-(trifluoromethyl)coumarin, 6-amino-chromen-2-one and 7-amino-4-methyl-3-coumarinylacetic acid, respectively, inside the channels of mesoporous silica SBA-15. Presence of fluorescent moieties was assessed by elemental analysis, thermal analysis, infrared, UV-Vis, Si-29- and C-13-CP/MAS NMR, and fluorescence spectroscopy. Reduction of specific surface area of the composites by 50-60% and also the average pore size diameter by 0.5-0.55 nm compared to unfunctionalized SBA-15 was evidenced by N-2 adsorption desorption analysis. Their antioxidant, antimicrobial activity and cytotoxicity on HeLa-2 cells were evaluated in order to formulate some potential applications of the obtained compounds. The obtained results recommend the obtained fluorescent mesoporous nanocomposites as potential candidates for the development of novel probes for the in situ tracking of oxidative stress, as well as for antimicrobial applications.

Trilayer memory capacitors of control HfO2/floating gate of Ge nanoparticles in HfO2/tunnel HfO2/Si substrate deposited by magnetron sputtering and subsequently annealed are investigated for the first time for applications in radiation dosimetry. In the floating gate (FG), amorphous Ge nanoparticles (NPs) are arranged in two rows inside the HfO2 matrix. The HfO2 matrix is formed of orthorhombic/tetragonal nanocrystals (NCs). The adjacent thin films to the FG are also formed of orthorhombic/tetragonal HfO2 NCs. This phase is formed during annealing, in samples with thick control HfO2, in the presence of Ge, being induced by the stress. In the rest of the control oxide, HfO2 NCs are monoclinic. Orthorhombic HfO2 has ferroelectric properties and therefore enhances the memory window produced by charge storage in Ge NPs to above 6 V. The high sensitivity of 0.8 mV Gy(-1) to a particle irradiation from a Am-241 source was measured by monitoring the flatband potential during radiation exposure after electrical writing of the memory.