Publications

Carbon layers are deposited on 100 nm thick atomically clean (001) lead zirconate titanate (PZT) in ultrahigh vacuum, ruling out the presence of any contaminants. X-ray photoelectron spectroscopy is used to assess the substrate surface or interface composition, substrate polarization and the thickness of carbon layers, which ranges from less than one monolayer (1 ML) of graphene to several monolayers. Atomically clean PZT(001) exhibit inwards polarization, and this polarization reverses the sign upon carbon deposition. Cationic vacancies are detected near the PZT surface, consistent with heavy p doping of these films near the surface. The carbon layers exhibited a consistent proportion of atoms forming in-plane sp(2) bonds, as detected by near-edge absorption fine structure (NEXAFS) analysis and confirmed partially by scanning tunneling microscopy (STM). In situ poling with simultaneous in-plane transport measurements revealed the presence of resistance anti-hysteresis versus the polarization orientation for films with less than 1 ML carbon amount, evolving towards 'normal' hysteresis for thicker carbon films. The anti-hysteresis is explained in terms of a mixed screening mechanism, involving charge carriers from the sp(2) carbon layers together with holes or ionized acceptors in PZT(001) near the interface. For thicker films, the compensation mechanism becomes extrinsic, involving mostly electrons and holes from carbon, yielding the expected hysteresis.

There is significant research showing that essential oils extracted from the plants have antibacterial effects. The purpose of this study was to develop a biocomposite based on hydroxyapatite coated with Artemisia absinthium essential oil and to highlight its antibacterial activity. Therefore, present studies are aimed at developing new materials combining hydroxyapatite with Artemisia absinthium essential oil, in order to avoid postoperative infections. The purpose of this work is to highlight the antimicrobial properties of the Artemisia absinthium essential oil-hydroxyapatite composites obtained by a simple method and at low costs. The structural properties and antimicrobial efficiency of the Artemisia absinthium essential oil-hydroxyapatite composite have been studied. The samples based on Artemisia absinthium essential oil analyzed in this study showed that wormwood essential oil presented the highest efficacy against the fungal strain of C. parapsilosis. It has been shown that wormwood essential oil has a strong antimicrobial effect against the microbial strains tested in this study. Furthermore, the antimicrobial properties of the biocomposites based on hydroxyapatite and essential oil are due to the presence of the essential oil in the samples.

In this work, ZnO-CdS composite powders synthesized by a simple chemical precipitation method were thoroughly characterized. The morphological, structural, compositional, photocatalytical, and biological properties of the prepared composites were investigated in comparison with those of the pristine components and correlated with the CdS concentration. ZnO-CdS composites contain flower-like structures, their size being tuned by the CdS amount added during the chemical synthesis. The photocatalytic activity of the composites was analyzed under UV irradiation using powders impregnated with methylene blue; the tests confirming that the presence of CdS along the ZnO in composites can improve the dye discoloration. The biological properties such as antioxidant capacity, antibacterial activity, and cytotoxicity of the ZnO, CdS, and ZnO-CdS composites were evaluated. Thus, the obtained composites presented medium antioxidant effect, biocidal activity against Escherichia coli, and no toxicity (at concentrations less than 0.05 mg/mL for composites with a low CdS amount) for human fibroblast cells. Based on these results, such composites can be used as photocatalytic and/or biocidal additives for photoactive coatings, paints, or epoxy floors, which in their turn can provide a cleaner and healthier environment.

Hematite-titania thin films with various ratios of Ti/Fe have been prepared on FTO substrate via cathodic and anodic deposition of amorphous Ti oxyhydroxide gel (TiOx) and iron oxyhydroxide (FeOOH) films, respectively followed by annealing processes. TiOx underlayer behaved as a template for FeOOH electrodeposition. The photocatalytic behavior of the photoanodes based on the prepared structures was studied. An increased photocurrent density was obtained without anodic shifting of the photocurrent onset potential. Mott-Schottky studies showed that Ti contributes little to donor density in hematite-titania photoanodes prepared by this technique. The impedance measurements suggest that both separation and transfer of charge carriers are promoted in these hematite-titania photoanodes. For comparative studies, hematite-titania photoanodes have been prepared from TiOx and FeOOH precursors formed by spin-coating technique and anodic deposition method, respectively. The photoelectrochemical performance of these photoanodes in the presence of cobalt-phosphate catalyst was also evaluated.

The dextran-thyme magnesium-doped hydroxyapatite (10MgHAp-Dex-thyme) composite layers were prepared by a dip-coating procedure from stable suspensions and further analyzed for the first time. Different characterization techniques were employed to explore the physical-chemical features of the 10MgHAp-Dex-thyme suspensions and derived coatings. Information regarding the 10MgHAp-Dex-thyme suspensions was extracted on the basis of dynamic light scattering, zeta potential, and ultrasound measurements. The crystalline quality of the biocomposite powders-resulting after the centrifugation of suspensions-and the layers deposited on glass was assessed by X-ray diffraction in symmetric and grazing incidence geometries, respectively. The chemical structure and presence of functional groups were evaluated for both powder and coating by Fourier transform infrared spectroscopy in attenuated total reflectance mode. The extent of the antimicrobial effect range of the biocomposite suspensions and coatings was tested against different Gram-positive and Gram-negative bacteria (Staphylococcus aureus, Enterococcus faecalis, Escherichia coli, and Pseudomonas aeruginosa) and fungus (Candida albicans) strains with promising results.

The main objective of this paper is to develop silver-doped hydroxyapatite suspensions (HApAg) with different concentrations of silver (x(Ag) = 0.05 and x(Ag) = 0.2) in order to obtain uniform and homogenous layers by spin-coating procedure. The colloidal properties of HApAg suspensions are evaluated by dynamic light scattering (DLS) analysis, zeta-potential (ZP), and ultrasound measurements. The ultrasound studies show that the HApAg20 sample revealed better stability than the HApAg5 sample. The structural and morphological analysis on suspensions and thin layers is also conducted. It is observed that the particles of the two samples have a similar shape and are uniform. The layers obtained present a homogeneous appearance of the surface without evidence of cracks or interruption of the coatings. The in vitro antifungal studies conducted on the two thin layers at two different time intervals (24 and 48 h) show that both HApAg5 (x(Ag) = 0.05) and HApAg20 (x(Ag) = 0.05) nanoparticles suspensions and composite layers inhibit the development of colony forming units (CFU) even after 24 h of incubation comparative to the control, represented by the Candida albicans (C. albicans) culture in a proper medium. The fungicidal effect was evident after 48 h of incubation in the case of both HApAg20 nanoparticles suspensions and composite layers.

Magnetic imaging of superconductors typically requires a soft-magnetic material placed on top of the superconductor to probe local magnetic fields. For reasonable results the influence of the magnet onto the superconductor has to be small. Thin YBCO films with soft-magnetic coatings are investigated using SQUID magnetometry. Detailed measurements of the magnetic moment as a function of temperature, magnetic field and time have been performed for different heterostructures. It is found that the modification of the superconducting transport in these heterostructures strongly depends on the magnetic and structural properties of the soft-magnetic material. This effect is especially pronounced for an inhomogeneous coating consisting of ferromagnetic nanoparticles.

The present paper reports the dependence of structural and functional properties on the porosity level with values down to 50% in BaTiO3 ceramics. Micro-porosity (pore size below 15 pm) with (0-3) connectivity has been produced by using dried pollen particles as sacrificial template. The properties of such porous ceramics are mostly affected by porosity, but also by possible small doping with foreign ions resulted from the template and by small variations of Ba/Ti stoichiometry at the ceramic-pore interfaces, as observed by a shift of the Curie temperature towards lower values when porosity level increases. The dielectric relaxation evidenced a few processes characterised by different activation energies which seem to be not affected by porosity, i.e. they might be assigned to the ceramic part and not to the ceramic-pore interfaces. When increasing porosity, a regular tilting of P(E) loops and increasing of coercivity and reducing polarization were observed, as result of reduction of active ferroelectric component and to the field inhomogeneity in such ceramics.

Magnetic iron oxides have been used in biomedical applications, such as contrast agents for magnetic resonance imaging, carriers for controlled drug delivery and immunoassays, or magnetic hyperthermia for the past 40 years. Our aim is to investigate the effect of pressure and temperature on the structural, thermal, and magnetic properties of iron oxides prepared by hydrothermal synthesis at temperatures of 100-200 degrees C and pressures of 20-1000 bar. It has been found that pressure influences the type of iron oxide crystalline phase. Thus, the results obtained by Mossbauer characterization are in excellent agreement with X-ray diffraction and optical microscopy characterization, showing that, for lower pressure values (100 bar, the major crystalline phase is goethite. In addition, thermal analysis results are consistent with particle size analysis by X-ray diffraction, confirming the crystallization of the synthesized iron oxides. One order of magnitude higher magnetization has been obtained for sample synthesized at 1000 bar. The same sample provides after annealing treatment, the highest amount of good quality magnetite leading to a magnetization at saturation of 30 emu/g and a coercive field of 1000 Oe at 10 K and 450 Oe at 300 K, convenient for various applications.

A study on clay mineral polymer nanocomposites (CPN), namely polyaniline/montmorillonite-cetyltrimethylammonium bromide (PANI/Mt-CTAB), poly o-anisidine/montmorillonite-cetyltrimethylammonium bromide (poly(o-ANIS)/Mt-CTAB) and poly o-anisidine-co-aniline/montmorillonite-cetyltrimethylammonium bromide (poly(o-ANIS-co-ANI)/Mt-CTAB), synthesized by oxidative chemical polymerization method is presented. The nanocomposites have been characterized by Fourier transform infrared spectroscopy, UV-vis spectroscopy, and cyclic voltammetry, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, cyclic voltammetry and thermogravimetry and differential scanning calorimetry analysis. By UV-vis measurements different electronic transitions for the CPNs were pointed out. The voltammograms indicate that the synthetized materials are electroactive. The FTIR analysis reveals the characteristic bands of the polymers and of the Mt-CTAB. The shift of the bands to higher/lower wavenumbers demonstrate the interaction between the pristine polymers macromolecular chains and the montmorillonite (Mt). The intercalation of the polymers inside the mineral clay was confirmed by the increased interlayer distance connected with the position of the 011 diffraction plane of the Mt., and the intercalation and exfoliation states were highlighted in the scanning and transmission electron microscopy images. The obtained results are encouraging in respect with the purpose to use them in the field of photovoltaic applications.