National Institute Of Materials Physics - Romania

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.

Memristors characterized by non-volatile memory resistance switching are promising candidates for building brain inspired computing architectures. However, existing memristive devices are still far from the energy efficiency of petaflops per joule exhibited by biological neural networks. Therefore, to achieve the goal of ultra-low power operation, it is necessary to develop new materials for the active layer in memristors. Here, we show highly energy efficient memristive devices built from liquid-exfoliated 2D WS2 and MoS2 nanosheets, enriched in monolayers using a cascade centrifugation method. Lateral devices with electrochemically inert electrodes were built using the drop casting method. The devices show non-volatile resistive switching with a remarkable low energy consumption. This work contributes to the realization of energy efficient and high performance neuromorphic computing applications. (c) 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

The emerging field of electronics based on ferro-functional materials relies on driving effectively and predictably a ferroelectric system between different polarization states through bias applied to metallic contacts. This requires detailed understanding of the growth mechanisms and electronic properties of the interface, including ferroelectric and material - dependent band alignment and Schottky barrier heights. Whether the major contribution at the interface band alignment comes from the work function difference or from the ferroelectric state is still under debate. Here, using X-ray photoemsion and ab-initio calculations, we derive the complex microscopic picture of metal/ferroelectric interface formation, including growth mechanism, valence alteration, ferroelectric-dependent electrostatic potential and thickness - dependent compensation mechanisms of ferroelectricity, starting from the ultrathin growth of Cu up to 100 angstrom on BaTiO3. One establishes the evolution of the band bending and of the build-in potential from the initial probed thickness of the ferroelectric in the range of 3 lambda (lambda - the inelastic mean free path) while gradually approaching the contact region with the metal at higher thickness of the top layer. We find that the well-defined orientation of the ferroelectric polarization lead to a band bending at the interface, which add at the bending expected from the work function difference of the two joining materials.

Iridium thin films are grown by direct-current plasma magnetron sputtering, on MgO single-crystal substrates with various surface orientations, i.e. (100), (111), and (110). The surface morphology, the crystalline properties of the films, and the substrate-thin-film interface are investigated by atomic force microscopy, X-ray diffraction (XRD), focused ion beam scanning electron microscopy, and high-resolution transmission electron microscopy, respectively. The results reveal that hetero-epitaxial thin films with different crystallographic orientation and notable atomic scale smooth surface are obtained. From the XRD analysis, the following epitaxial relations are obtained: (1) (100)(Ir)||(100)(MgO) out-of-plane and [001](Ir)||[001](MgO) in-plane for Ir grown on MgO(100), (2) (110)(Ir)||(110)(MgO) out-of-plane and [1-10](Ir)||[1-10](MgO) in-plane for Ir grown on MgO(110), and (3) (111)(Ir)||(111)(MgO) out-of-plane and two variants for in-plane orientation [1-10](Ir)||[1-10](MgO) and [1-10](Ir)||[10-1](MgO), respectively, for Ir grown on MgO(111). Because of the large misfit strain (9.7%), the thin films are found to grow in a strain-relaxed state with the formation of geometrical misfit dislocations with a similar to 2.8-nm spacing, whereas thermal strain is stored upon cooling down from the growth temperature (600 degrees C). The best structural characteristics are obtained for the (111)-oriented films with a mosaicity of 0.3 degrees and vanishingly small lattice distortions. The (100)- and (110)-oriented films exhibit mosaicities of similar to 1.2 degrees and lattice distortions of similar to 1% which can be explained by the larger surface energy of these planes as compared to (111).

Powders of beta-tricalcium phosphate [beta-TCP, beta-Ca-3(PO4)(2)] and composite powders of beta-TCP and polyvinyl alcohol (PVA) were synthesized by using wet precipitation methods. First, the conditions for the preparation of single phase beta-TCP have been delineated. In the co-precipitation procedure, calcium nitrate and diammonium hydrogen phosphate were used as calcium and phosphorous precursors, respectively. The pH of the system was varied in the range 7-11 by adding designed amounts of ammonia solution. The filtered cakes were desiccated at 80 degrees C and subsequently calcined at different temperatures in the range between 700-1100 degrees C. Later on, rifampicin form II was used to produce drug-loaded beta-TCP and PVA/beta-TCP powders. All the synthesized materials have been characterized from morphological (by scanning electron microscopy) and structural-chemical (by X-ray diffraction and Fourier transform infrared spectroscopy) point of view. The drug loading capacity of the selected pure beta-TCP powder has been assessed. The biological performance (cytocompatibility in fibroblast cell culture and antibacterial efficacy against Escherichia coli and Staphylococcus aureus) has been tested with promising results. Application perspectives of the designed drug-bioceramic-polymer blends are advanced and discussed. [GRAPHICS] .

We developed a new type of chalcohalide glasses with physicochemical and nonlinear optical properties that are tunable by composition. It is found that more than 60 mol.% CdCl2 heavy metal halide can be dissolved into the ternary Ge Sb S system and forming stable glasses. The visible-light transparency range is extended to shorter wavelengths with the addition of CdCl2, which is beneficial for the optical quality control and infra-red (IR) system alignment. The third-order optical nonlinearity (TONL) is studied using the femtosecond Z-scan method. The results show that both the nonlinear refractive index and two photon absorption co-efficient decrease with CdCl2. Benefiting from the favorable property-tailoring effects of CdCl2, the TONL figure of merit (FOM) can be improved to meet the requirement (FOM < 1) for all-optical switching and IR photonic applications.

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.

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.

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.

Development of reproducible, low-cost fabrication technologies that are readily adaptable to large-scale production, is one of the main challenges in the field of perovskite solar cells (PSCs). So far, for all the other layers in a solar cell, large-area deposition methods have been adapted, except for mesoporous fabrication. Herein, the fabrication of mesoporous TiO2 scaffolds using a large-area deposition technique, such as spray coating, is shown. Moreover, this technique induces the formation of a very specific reticulated structure with well-delimited, oval-shaped cavities. The cavities have irregular dimensions, with diameters in the range of 3-7 mu m, approximate to 350 nm height, resulting in an overall increase in roughness of one order of magnitude, compared with the spin-coated mesoporous scaffold. Using this rough structured mesoporous TiO2 in PSCs not only does not affect the efficiency of solar cells but actually improves it from an average of 10% to 12% in comparison with the devices containing a spin-coated mesoporous scaffold.