National Institute Of Materials Physics - Romania

In the present work, a magnetodielectric flexible thick film composite with a quaternary Ba12Fe28Ti15O84 ferrite filler (<= 9 vol%) embedded into PVDF polymer matrix was investigated. The role of filler volume on the macroscopic dielectric, magnetic and ferroelectric properties as well as on the nanoscale magnetic and piezoelectric response was analyzed. The formation of small amounts of polar phases besides the majority alpha-phase of PVDF was shown by XRD and FTIR combined analysis. The electrical properties are dominated by the polymer response, while magnetic order is derived as sum property from the ferrite ones, having a predominant soft magnetic character with small coercivity of H-c similar to 60 Oe and high saturation magnetization of M-s = 2.6 emu/g for the highest concentration of 9 vol%. Permittivity and losses slightly increase with ferrite filler addition, and the composite maintains a dielectric character for all the compositions. Local PFM and MFM investigations have shown a combined ferro/piezoelectric character and magnetic order, with magnetoelectric coupling demonstrated by the reorientation of filler particles and modifications of local piezoresponse when applying a static magnetic field.

A facile and cheap surfactant-assisted hydrothermal method was used to prepare mesoporous cobalt ferrite nanosystems with BET surface area up to 151 m(2)/g. These mesostructures with high BET surface areas and pore sizes are made from assemblies of nanoparticles (NPs) with average sizes between 7.8 and 9.6 nm depending on the initial pH conditions. The pH proved to be the key factor for controlling not only NP size, but also the phase purity and the porosity properties of the mesostructures. At pH values lower than 7, a parasite hematite phase begins to form. The sample obtained at pH = 7.3 has magnetization at saturation M-s = 38 emu/g at 300 K (54.3 emu/g at 10 K) and BET surface area S-BET = 151 m(2)/g, whereas the one obtained at pH = 8.3 has M-s = 68 emu/g at 300 K (83.6 emu/g at 10 K) and S-BET = 101 m(2)/g. The magnetic coercive field values at 10 K are high at up to 12,780 Oe, with a maximum coercive field reached for the sample obtained at pH = 8.3. Decreased magnetic performances are obtained at pH values higher than 9. The iron occupancies of the tetrahedral and octahedral sites belonging to the cobalt ferrite spinel structure were extracted through decomposition of the Mossbauer patterns in spectral components. The magnetic anisotropy constants of the investigated NPs were estimated from the temperature dependence of the hyperfine magnetic field. Taking into consideration the high values of BET surface area and the magnetic anisotropy constants as well as the significant magnetizations for saturation at ambient temperature, and the fact that all parameters can be adjusted through the initial pH conditions, these materials are very promising as recyclable anti-polluting agents, magnetically separable catalysts, and targeted drug delivery vehicles.

Our study focused on the long-term degradation under simulated conditions of coatings based on different compositions of polycaprolactone-polyethylene glycol blends (PCL-blend-PEG), fabricated for titanium implants by a dip-coating technique. The degradation behavior of polymeric coatings was evaluated by polymer mass loss measurements of the PCL-blend-PEG during immersion in SBF up to 16 weeks and correlated with those yielded from electrochemical experiments. The results are thoroughly supported by extensive compositional and surface analyses (FTIR, GIXRD, SEM, and wettability investigations). We found that the degradation behavior of PCL-blend-PEG coatings is governed by the properties of the main polymer constituents: the PEG solubilizes fast, immediately after the immersion, while the PCL degrades slowly over the whole period of time. Furthermore, the results evidence that the alteration of blend coatings is strongly enhanced by the increase in PEG content. The biological assessment unveiled the beneficial influence of PCL-blend-PEG coatings for the adhesion and spreading of both human-derived mesenchymal stem cells and endothelial cells.

Single-phase Ba1-xSrxTiO3 (BST) perovskite ceramics with 0.3 <= x <= 0.4 were prepared from powders synthesized via sol-gel route. The compositions have the ferroelectric-paraelectric phase transition close to room temperature. At 20 degrees C the BST ceramics are ferroelectric for 0.3 <= x <= 0.35 and paraelectric for x = 0.375 and x = 0.40. The study follows the relation between the structural changes produced when increasing the Sr content and the dielectric properties at low intensity electric fields. It is found that the grain size and tetragonality decreases as the Sr content increases. Analyses of complex permittivity and impedance spectroscopy reveal the temperature and frequency dependencies of the dielectric properties. The phase transitions seem to be of first order for all compositions, with a thermal hysteresis that decreases with increasing the Sr content, fact attributed to the corresponding increase of the grain boundaries weight allowing a more efficient stress relaxation in the structure during the change of the symmetry from cubic to tetragonal. The diffusiveness degree during the phase transition is increasing with Sr content, suggesting some relaxor-type contribution attributed to smaller grain size. The ac conductivity follows the universal Jonscher law, with an ac component having the power parameter s independent of Sr content, and a dc component that it is thermally activated with an activation energy of about 0.7-0.77 eV attributed to oxygen vacancies acting as donor-like defects. The fit of impedance spectra at different temperatures and frequencies is obtained by using an equivalent circuit accounting the grains, grain boundaries, electrode interfaces and the local contributions produced by reorientation of defect dipoles or defect clusters. All the component circuits have significant variations around phase transitions. These are discussed in relation to structural changes occurring during transition and considering the changes in the distribution of various charges when polarization vanishes.

Tellurium metallic colloids were evidence in the phosphate tellurite glasses obtained by the melt quenching method over 1000 degrees C. The concentration and subsequent color of these glasses strongly depend on the preparation conditions due to the significant differences among the melting point of the component oxides. The presence of Te metallic colloids was evidenced by the magneto-optical measurements at different temperatures, by the shift of the emissions spectra with the excitation wavelength and the thermoluminescence spectra as a result of recombination between the hole centers, created during x-ray irradiation, with the quasi coupled electrons from the surface of the metallic colloids. The peak position in the absorption spectra of these metallic colloids was modeled based on generalized Mie light scattering theory on these metallic structures, taking into account the size and shape of colloids but also the dielectric constant of the phosphate tellurite glasses in which these colloids are embedded.

Organometallic compounds embedded in thin films are widely used for Organic Light-Emitting Diodes (OLED), but their functionalities are strongly correlated with the intrinsic properties of those films. Controlling the concentration of the organometallics in the active layers influences the OLED performances through the aggregation processes. These aggregations could lead to crystallization processes that significantly modify the efficiency of light emission in the case of electroluminescent devices. For functional devices with organometallic-based thin films, some improvements, such as the optimization of the charge injection, are needed to increase the light output. One dual emitter IrQ(ppy)2 organometallic compound was chosen for the aggregation correlations from a multitude of macromolecular organometallics that exist on the market for OLED applications. The choice of additional layers like conductive polymers or small molecules as host for the active layer may significantly influence the performances of the OLED based on the IrQ(ppy)2 organometallic compound. The use of the CBP small molecule layer may lead to an increase in the electroluminescence versus the applied voltage.

A versatile, modular in situ high-intensity monochromatic illumination set-up installed on a standard Q-band ESR spectrometer equipped with a cryostat and probe head for measurements at cryogenic temperatures, which can be easily assembled from commercially available optical components is presented. Using as monochromatic light sources pig-tailed laser diodes (LDs) or fiber-coupled light-emitting diodes (LEDs), a high efficiency of the light transfer (more than 95%) through an optical guide inserted in the sample holder is achieved in the sample area of the microwave cavity. With various LEDs and LDs, one can perform ESR in situ illumination experiments from UV to far-IR, in both cw and pulse mode. Its operation is illustrated with an experiment revealing the presence of certain ESR silent defects in oxygen-doped floating-zone ultrapure Si samples irradiated at room temperature with high-energy-high-fluence electron beams and pulse annealed up to 300 degrees C. New information is obtained by comparing the ESR spectra recorded at T = 120 K, without and with 1.06 mu m across-the-gap in situ illumination.

This study aims to obtain uniform and homogeneous bismuth germanate oxides thin films by spin coating and using the sol-gel technique with different precursors, followed by low-temperature annealing at 560 degrees C. By using Bi(NO3)(3) precursors, we have obtained transparent, yellowish thin films with a 200 nm thickness. The structural analysis of the initial sol-gel powder has shown the presence of two crystalline structures, the cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5 crystallites, which evolves towards the BGO structure after annealing. The elemental analysis confirmed the composition of the desired compound Bi4Ge3O12 with 60 wt % GeO2 and 40 wt % Bi2O5. On the other hand, by changing the precursor to (Bi(CH3COO)(2), the film thickness increased to 500 nm thicker due to the high viscosity of the sol, and a dominant monoclinic Bi2GeO5 crystalline structure appeared. The elemental analysis revealed a nonstoichiometric composition with 38 wt % GeO2 and 62 wt % Bi2O3. Due to the low GeO2 phase content that reacted with metastable Bi2GeO5, we obtained cubic Bi4Ge3O12 as a secondary phase, with Bi2GeO5 as a dominant crystalline phase. The redshifts of both absorptions and emissions spectra peaks confirmed a different disorder structure as an interplay between the cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5 phases.

A complete characterization from optical, morphological, electrical, photo-electrical and thermo-electrical point of view was done for PEDOT:PSS thin films and DMSO and EG sensitized PEDOT:PSS films. The studies on the electrical conductivity and electrical photoconductivity allowed the calculation of different relaxation times. The relaxation time of the electrical conductivity is of order of femtosecond and is multiplied by a factor 10 when PEDOT: PSS thin films were deposited on surfaces sensitized with DMSO and EG. Besides, the photoconduction excitation and relaxation times are of order of seconds. An increase in the relaxation photoconduction time by 2 was observed for films deposited on surfaces sensitized with DMSO and by 1.2 for films deposited on surfaces sensitized with EG. The time response of electrical conductivity after exposure to light adds supplementary knowledge for the understanding of the inertial processes, or hysteresis behavior of organic or perovskite solar cells involving PEDOT:PSS films. The electrical conductivity increases with the temperature and the use of DMSO and EG as surfactants lead to higher values of electrical conductivity and Seebeck coefficient. A better stability of the electrical conductivity with the temperature increase, was noticed for films deposited on surfaces sensitized with DMSO.

Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NC s composition and size. In this study, SiGe-SiO2 amorphous films were deposited by magnetron sputtering on Si substrate followed by rapid thermal annealing at 700, 800 and 1000 degrees C. We investigated films with Si:Ge:SiO2 compositions of 25:25:50 vol.% and 5:45:50 vol.%. TEM investigations reveal the major changes in films morphology (SiGe NCs with different sizes and densities) produced by Si:Ge ratio and annealing temperature. XPS also show that the film depth profile of SiGe content is dependent on the annealing temperature. These changes strongly influence electrical and photoconduction properties. Depending on annealing temperature and Si:Ge ratio, photocurrents can be 10(3) times higher than dark currents. The photocurrent cutoff wavelength obtained on samples with 25:25 vol% SiGe ratio decreases with annealing temperature increase from 1260 nm in SWIR for 700 degrees C annealed films to 1210 nm for those at 1000 degrees C. By increasing Ge content in SiGe (5:45 vol%) the cutoff wavelength significantly shifts to 1345 nm (800 degrees C annealing). By performing measurements at 100 K, the cutoff wavelength extends in SWIR to 1630 nm having high photoresponsivity of 9.35 AW(-1).