National Institute Of Materials Physics - Romania

The pyroelectric coefficient of some Pb(Zr,Ti)O-3 type ceramics was estimated using two methods: a direct method, based on the temperature dependence of the charge/current generated by pyroelectric effect; an indirect method based on the frequency dependence of the pyroelectric signal generated by a pyroelectric element under modulated irradiation with infrared light. The purpose was to compare the results and to assess if the indirect method is enough accurate for rapid estimation of the pyroelectric coefficient of a material envisaged for infrared detection. The direct method is more accurate and requires accurate control of temperature and heating rate, while the indirect method is less accurate, with an error up to 20%, and requires the knowledge of quantities such as dielectric constant, electric conductance, and incident power. However, the indirect method can be more easily implemented and can provide a rapid assessment on the pyroelectric quality of a material with potential use in infrared detectors.

This work involves the synthesis and characterization of Cu2ZnSnS4 (CZTS) layers. The films were prepared on Mo/glass substrates by single-step electrodeposition method followed by sulfurization at 500 degrees C under argon flow. The effect of boric acid concentration on the crystallographic structure, compositional and morphological properties of CZTS films was investigated, with the objective to understand the growth behavior and to enhance the film properties. Cyclic Voltammetry was used in order to estimate the adequate deposition potential for the CZT alloy. The x-ray diffraction analysis showed the formation of the kesterite phase in all the samples. The Raman and x-ray photoelectron spectroscopy studies confirmed the existence of the CZTS phase. The scanning electron microscopy was employed to inspect the films structure. The results indicated that increasing the concentration of boric acid affects the physico-chemical properties of the films.

In view of the HL-LHC upgrade, radiation-tolerant silicon sensors containing low-resistivity p-type implants or substrates, like LGAD or HV-CMOS devices, are being developed in the framework of ATLAS, CMS, RD50 and other sensor R&D projects. These devices are facing a particular problem - the apparent deactivation of the doping due to the irradiation, the so-called acceptor removal effect. In the present work proton and neutron fluence-dependent radiation damage effects, including the change in leakage current and effective doping concentration, space charge sign-inversion, but also introduction and annealing of point- and cluster-like defects have been studied in Si pad diodes fabricated from p-type EPI material of different resistivity (10-1000 Omega cm). Standard electrical characterisations (IV, CV), TCT (Transient Current Technique) and TSC (Thermally Stimulated Current) techniques were applied. A correlation between effective doping concentration obtained from CV measurements and defect concentration N-t extracted from TSC measurements for both - neutron and proton - irradiations was observed pointing towards the microscopic origin of the acceptor removal. A detailed analysis of the dominant TSC peaks - E(30), BiOi and three main deep acceptor levels H(116), H(140) and H(152) - responsible for the changes in the effective space charge is performed. The origin and annealing behaviour of E(30) and H(40) and other cluster-related defects are discussed as well.

Superconductors are key materials for shielding quasi-static magnetic fields. In this work, we investigated the shielding properties of an MgB2 cup-shaped shield with small aspect-ratio of height/outer radius. Shape and aspect-ratio were chosen in order to address practical requirements of both high shielding factors (SFs) and space-saving solutions. To obtain large critical current densities (J(c)), which are crucial for achieving high magnetic-mitigation performance, a high-purity starting MgB2 powder was selected. Then, processing of the starting MgB2 powder into high density bulks was performed by spark plasma sintering. The as-obtained material is fully machinable and was shaped into a cup-shield. Assessment of the material by scaling of the pinning force showed a non-trivial pinning behaviour. The MgB2 powder selection was decisive in enlarging the range of external fields where efficient shielding occurs. The shield's properties were measured in both axial- and transverse-field configurations using Hall probes. Despite a height/outer radius aspect ratio of 2.2, shielding factors higher than 10(4) at T = 20 K up to a threshold field of 1.8 T were measured in axial-field geometry at a distance of 1 mm from the closed extremity of the cup, while SFs > 10(2) occurred in the inner half of the cup. As expected, this threshold field decreased with increased temperature, but SFs still exceeding the above mentioned values were found up to 0.35 T at 35 K. The shield's shape limits the SF values achievable in transverse-field configuration. Nevertheless, the in-field J(c) of the sample supported SFs over 40 at T = 20 K up to a field of 0.8 T, 1 mm away from the cup closure.

In this work, the effect of aliovalent substitution of Pb2+ by Eu3+ on structural, morphological and optical properties of CH3NH3PbI3 (MAPbI(3)) was studied, aiming to improve the properties of perovskite films used in solar cells application. The surface morphology, the microstructure and the optical properties of the obtained films containing different Europium (Eu) concentrations were characterized by atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, UV-vis spectroscopy and photoluminescence spectroscopy.

Bovine hydroxyapatite (BHA) and BHA blended with clinoptilolite (CLIN) and alumina (Al2O3) coatings were synthesized using pulsed laser deposition (PLD) with a KrF* excimer laser source (lambda = 248 nm, tau(FWHM) <= 25 ns). Physical-chemical characteristics and the potential use of coatings for preventing bacteria adhesion and biofilm formation were investigated. Optimized PLD conditions were selected for coatings with rough morphologies, suitable for good cell adhesion and implant anchorage and good replication of the source target composition. The crystallinity of composite coatings was progressively decreasing with the augment of the Al2O3 and CLIN contents, which in turn can facilitate an efficacious release of active components. Al2O3- and CLIN-containing coatings exhibited high cytocompatibility and specific antibiofilm profiles, preventing the initiation and maturation of bacterial biofilms. Optimum biological activity profiles associated with the use of sustainable and/or inexpensive materials are, in our opinion, of key importance for the future development of performant implant coatings, which should he perfectly compatible with the surrounding tissue while preventing postsurgical endogenous or nosocomial infections.

Ba0.6Sr0.4TiO3 (BST) ferroelectric thick films were grown on MgO(001) and Al2O3 (0 0 0 1) single-crystal substrates by using a pulsed laser deposition method. Structural, morphological, optical, and terahertz characterization of the BST films were performed by X-ray Diffraction, Atomic Force Microscopy, Spectroscopic Ellipsometry (SE), and Terahertz Time-Domain Spectroscopy (THz-TDS). Single-phase samples with strong preferred (1 1 1) orientation and surface roughness lower than 1.5% of their thicknesses have been obtained for both types of substrates. SE was employed to extract the thickness and optical properties by using a 3-layer optical model (substrate/thin film/roughness). The inferred refractive index @630 nm is around 2.05, while the optical interference is visible until 3.3 eV. The THz-TDS measurements in transmission set-up were carried out one after the other on substrates before and after the BST film deposition. The standard THz-TDS analysis of double-layer samples proved difficult to complete in the cases in which a thin or thick film is deposited on a much thicker substrate of known dielectric properties. However, we have been able to extract the complex dielectric permittivity in the THz domain for BST samples with thicknesses of few microns, by developing a specific procedure of data analysis.

A complex investigation of the morpho-structural, magnetic, magneto-optical and magneto-transport properties of amorphous Fe-Gd thin films crossing the magnetization compensation point is reported and the unexpected observed magneto-functionalities are discussed. A tendency of magnetic domain formation with increasing the Fe content over the compensation concentration is observed. The switch from a reversed Magneto-optical Kerr Effect loop to a direct loop when increasing the Fe content over the compensation point is explained via the specific contribution to the rotation of the polarization vector from each magnetic sublattice, belonging to Fe and Gd, respectively. Local atomic configurations and magnetic interactions ascertained the amorphous character and revealed an out-ofplane orientation of the magnetic moment of Fe above the compensation point. The thermomagnetic curves prove a concentration dependent behavior, explained by weakly coupled magnetization relaxation processes of the two magnetic sub-lattices. On the other hand, the magnetic hysteresis loops gave evidence of two exchange coupled magnetic phases with different coercive fields. According to structural and Fe-57 Mossbauer Spectroscopy results, the two phases correspond to definite nanosized volumes of two different average concentrations (one of them closer to the compensation point) which are randomly distributed in the film. The unexpected single step-like behavior of the magneto-resistivity curves was explained by dissimilar switching of the spins in these two magnetic phases distributed in nano-sized volumes.

We explore the cross coupling between the ferroelectric and ferromagnetic phases in Ni/BaTiO3(001) heterostructures and demonstrate the modulation of the magnetism and incidence of exchange bias in the ultrathin metallic Ni overlayer, depending on the ferroelectric state of the bottom layer. We establish that 5-nm-thick monocrystalline Ni film deposited on BaTiO3 with ferroelectric polarization pointing towards the surface (P+) favors the organization of Ni into uniform ferromagnetic domains. Ni grown on BaTiO3 with opposite ferroelectric polarization is featured by emerging exchange-bias coupling between the ferromagnetic Ni top layers and the antiferromagnetic reacted interface, as theoretically explained by first-principles calculations. We explicitly obtain the morphology of the magnetic domains of the crystalline Ni layer in atomic and magnetic force microscopy measurements (AFM/MFM). The resemblance of AFM and MFM images indicate that, although with radically different morphologies, in both cases all spins orient in the Ni plane. Consequently, the distinct signature of the ferroelectric-ferromagnetic coupling extracted from the magneto-optical Kerr effect measurements encodes all the information of sample magnetism. The peculiar magnetic coupling depending on the ferroelectric state indicates new ways of engineering the functionality of metal/ferroelectric interfaces.

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.