National Institute Of Materials Physics - Romania

In this work, rapid thermal annealing (RTA) was applied to indium tin oxide (ITO) films in ambient atmosphere, resulting in significant improvements of the quality of the ITO films that are commonly used as conductive transparent electrodes for photovoltaic structures. Starting from a single sintered target (purity 99.95%), ITO thin films of predefined thickness (230 nm, 300 nm and 370 nm) were deposited at room temperature by radio-frequency magnetron sputtering (rfMS). After deposition, the films were subjected to a RTA process at 575 degrees C (heating rate 20 degrees C/s), maintained at this temperature for 10 minutes, then cooled down to room temperature at a rate of 20 degrees C/s. The film structure was modified by changing the deposition thickness or the RTA process. X-ray diffraction investigations revealed a cubic nanocrystalline structure for the as-deposited ITO films. After RTA, polycrystalline compounds with a textured (222) plane were observed. X-ray photon spectroscopy was used to confirm the beneficial effect of the RTA treatment on the ITO chemical composition. Using a Tauc plot, values of the optical band gap ranging from 3.17 to 3.67 eV were estimated. These values depend on the heat treatment and the thickness of the sample. Highly conductive indium tin oxide thin films (rho = 7.4 x 10(-5) Omega cm) were obtained after RTA treatment in an open atmosphere. Such films could be used to manufacture transparent contact electrodes for solar cells.

The competition between interface barrier in the Schottky-Mott limit and polarization driven mechanism is established during gradual formation of metal (Au) - ferroelectric (BaTiO3) interface. X-ray photoelectron spectroscopy provides core level energies and valence band positions in the contact region, to monitor the band alignment from the very first stages of metal deposition on BaTiO3. The band bending at metal/ferroelectric (FE) interface is extracted from the shift of core levels (Ba 3d, Ti 2p) as a function of the metal thickness. It is shown that the interface band alignment mechanism involves a well-defined polarization orientation washing out the bending expected from the work function difference. The sudden modification of the binding energies within ferroelectric at the first 2 angstrom Au indicates that the ferroelectric compensation mechanism triggered by the metal overlayer initiates already at ultrathin top layer, while subsequent growth contributes only at a gradual correction of the potential in the FE. The emerging picture is confirmed in first-principle calculation indicating the preferences of Au to grow preferentially to different terminated regions and to stabilize distinct ferroelectric polarization.

We report on the influence of dilute doping combined with the processing conditions on the morphological, structural, chemical states, photoluminescence and magnetic properties of Zn1-xNixO nanopowders. Ni doping changes the ZnO powder morphology from randomly-aggregated nanocrystals to densely-packed nanocrystals arranged in columnar particles, modifies the high-energy-component of O1 s spectrum and increases the modified Auger parameter in XPS, enhances the blue photoluminescence (PL) emission, suppresses the green PL emission and the intensity of the g = 1.997 EPR signal. Ni-ZnO nanostructures show room-temperature ferro-magnetism (implying they can serve as dilute magnetic semiconductors). The saturation magnetization, crystallite size and microstrain increase with the doping level; the c-axis constant and unit cell volume decrease, however, being unexpectedly higher with respect to a (reference) ZnO powder with a relaxed lattice. We demonstrate that the investigated properties were controlled by both (dilute) doping level and donor native defects produced by non-equilibrium (oxygen-deficiency and high rate of) ZnO formation.

The influence of the Gd3+ co-dopant on the structure, morphology and up-conversion luminescence/magnetic properties of the LiYF4:Gd3+/Yb3+/Er3+ nanocrystals was investigated and compared to those of Gd-free samples. Electron microscopy has indicated an inhibiting effect of the agglomeration and collapsing process of the nanocrystals compared to the Gd-free powder samples. The surface analysis of nanocrystals have not shown oxygen-metal bonds related to the metal oxidation within the surface nanometric layer. The paramagnetic properties are related to the magnetic moment of the Gd3+ ions. The up-conversion luminescence of the LiYF4:Gd3+/Yb3+/Er3+ nanocrystals is about six times stronger than in LiYF4:Yb3+/Er3+ nanocrystals; the enhancement effect is most probably due to the lattice distortion induced by the Gd co-doping.

Up-conversion luminescence and thermoluminescence properties of LaF3 nanocrystals, with sizes of about 20 nm, were studied and discussed in relation to -size and surface related effects. XPS spectra have evidenced the presence of oxidized Er and La ions within a thin layer (of about 1 nm thickness) at the nanocrystals surface, as well as Yb ions bonded with fluorine ions. The green ((H-2(11/2), S-4(3/2)) -> I-4(15/2)) and red (F-4(9/2) -> I-4(15/2)) up-conversion emissions of Er3+ ions are influenced by the relative dominance of Erions that reside within the thin oxidized layer. The broad thermoluminescence curves are assigned to the recombination of trap defects associated with surface states and within the oxidized surface layer. (C) 2019 Elsevier B.V. All rights reserved.

The work describes the development of a flexible, hydrogel embedded pH-sensor that can be integrated in inexpensive wearable and non-invasive devices at epidermal level for electrochemical quantification of H+ ions in sweat. Such a device can be useful for swift, real time diagnosis and for monitoring specific conditions. The sensors' working electrodes are flexible poly(methyl methacrylate) electrospun fibers coated with a thin gold layer and electrochemically functionalized with nanostructured palladium/palladium oxide. The response to H+ ions is investigated by cyclic voltammetry and electrochemical impedance spectroscopy while open circuit potential measurements show a sensitivity of aprox. -59 mV per pH unit. The modification of the sensing interface upon basic and acid treatment is characterized by scanning and transmission electron microscopy and the chemical composition by X-ray photoelectron spectroscopy. In order to demonstrate the functionality of the pH-sensor at epidermal level, as a wearable device, the palladium/palladium oxide working electrode and silver/silver chloride reference electrode are embedded within a pad of polyacrylamide hydrogel and measurements in artificial sweat over a broad pH range were performed. Sensitivity up to -28 mV/pH unit, response time below 30 s, temperature dependence of approx. 1 mV/degrees C as well as the minimum volume to which the sensor responses of 250 nanoliters were obtained for this device. The proposed configuration represents a viable alternative making use of low-cost and fast fabrication processes and materials.

The present work describes a new simple procedure for the direct immobilization of biomolecules on Ni electrodes using magnetic Ni nanoparticles (NiNPs) as biomolecule carriers. Ni electrodes were fabricated by electroplating, and NiNPs were chemically synthesized. The chemical composition, crystallinity, and granular size of Ni electrodes, NiNP, and NiNP-modified Ni electrodes (NiNP/Ni) were determined by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). The electrochemical characterization of Ni electrodes by cyclic voltammetry and electrochemical impedance spectroscopy confirmed the existence of nickel oxides, hydroxides, and oxohydroxide films at the surface of Ni. Magnetic characterization and micromagnetic simulations were performed in order to prove that the magnetic force is responsible for the immobilization process. Further, Ni electrodes were employed as amperometric sensors for the detection of hydrogen peroxide because it is an important performance indicator for a material to be applied in biosensing. The working principle for magnetic immobilization of the enzyme-functionalized NiNP, without the use of external magnetic sources, was demonstrated for glucose oxidase (GOx) as a model enzyme. XPS results enabled to identify the presence of GOx attached to the NiNP (GOx-NiNP) on Ni electrodes. Finally, glucose detection and quantification were evaluated with the newly developed GOx-NiNP/Ni biosensor by amperometry at different potentials, and control experiments at different electrode materials in the presence and absence of NiNP demonstrated their importance in the biosensor architecture.

Powders of molybdenum disulfide platelets strongly grafted on graphene have been prepared by pyrolysis of ammonium alginate containing adsorbed various proportions of (NH4)(2)MoS4. After pyrolysis, formation of MoS2 supported on graphene was determined by XRD and electron microscopy and spectroscopic techniques. MoS2/G exhibits catalytic activity for the methanation of CO2, the performance being optimal at intermediate loadings. The catalytic activity of sharply contrasts with that of bulk MoS2 that promotes the reverse water gas shift, affording CO as the main product. Characterization of the spent MoS2/G catalyst shows the partial conversion of external MoS2 into MoO3. Comparison of the catalytic activity of MoS2/G with that of MoO3/G shows that the latter is less efficient, but more selective for CO2 methanation.

A hard templating method, using SBA-15 in combination with glucose solution and different heteroatom precursors, has been employed to investigate the influence of the different heteroatom dopants nitrogen, boron, sulfur, and phosphorus on carbon electrocatalysts for the oxygen reduction reaction. Samples were synthesized under the same conditions and resulted in a similar morphology and surface areas around 1000 m(2)/g. Incorporating nitrogen into the carbon matrix was found to be easier than for boron or phosphorus, while sulfur doping proved problematic and only yielded 2 at% of sulfur or less. Different dopant concentrations as well as a combination of dopants suggested that nitrogen was the only heteroatom exerting an actual influence on the catalytic activity, resulting in higher electron transfer numbers. The other dopants exhibited a similar performance regardless of the dopant content, though slightly improved when compared to an undoped control sample. These findings indicate that incorporated nitrogen can act as catalytic sites, while boron, sulfur and phosphorus can enhance the catalytic activity by possibly creating defects in the carbon matrix.

Growth of Ag films of up to 30 nm thickness on Si(1 1 1) 7 x 7 at room temperature is investigated by low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). LEED revealed the coexistence of Ag and Si spots starting with 1 monolayer (ML) of Ag deposited. The Ag lattice constant, starting with 25 ML, is slightly higher than for bulk Ag and increase linearly with Ag thickness, reaching about 4.2 nm for the thickest films. The average terrace widths detected from LEED spot profile analysis are about 30 nm for clean Si(1 1 1) 7 x 7 and about 5.5 nm for the thickest Ag(1 1 1) film, in agreement with STM observations. The intensity variation of core levels analyzed by XPS is taken into account by a model assuming the initial formation of Ag islands with linear variation of coverage vs. the amount of Ag deposited, followed by growth in a quasi layer-by-layer mode. The interface barrier is in the range of 0.4 eV, lower than all values reported previously. Ag deposited on Si(1 1 1) 7 x 7 at room temperature provides flat Ag(1 1 1) for synthesis of 2D materials, and may be used for low barrier Schottky diodes.