Publications

We investigate band bending effects occurring at the interface between atomically clean Ge(001) and molecular beam epitaxy (MBE) deposited copper and platinum. Low energy electron diffraction(LEED) confirmed the crystallinity of the surface, evidenced the formation of (2 x 1) and (1 x 2) reconstructions, and revealed that it is strongly affected with metal deposition. X-ray photoelectron spectroscopy (XPS) data let us assume a Stranski-Krastanov growth mechanism and confirmed that the observed band bending is associated to an ohmic contact in both cases. For the platinum contact, the high values of the apparent inelastic mean free path (IMFP) derived from the evolution of the XPS intensities indicate a prevalence of mixture of Pt with Ge nearby the interface. Pt deposited on Ge(001) does not behave like a Schottky contact, as one may have expected due to the higher work function of platinum. The observed effect is similar to the ease where interfacial Pt had a lower work function by 2.25/1.96 eV than that of metallic Pt. We propose a model to explain this fact by the effective mass variation or to the conduction band broadening due to the strong intermixing of platinum with germanium under the surface. (C) 2016 Elsevier B.V. All rights reserved.

Neodymium and gallium codoped barium titanate (Ba1-x-yNdxGayTiO3, x = 0.03, 0.05, 0.07, 0.1 and y = 0.03) were prepared by sol-gel method in order to investigate its dielectric and photoluminescence properties. The structure and morphology of Ba1-x-yNdxGayTiO3 powders calcined at 1100 A degrees C and sintered ceramics by spark plasma sintering (SPS) technique were analyzed using X-ray diffraction and scanning electron microscope, respectively. The photoluminescence and dielectric properties of Nd doped Ba0.97Ga0.03TiO3, (y = 0.03) as a function of Nd3+ concentration and sintering temperature were also investigated. The influence of Ga3+ and Nd3+ dopants on the crystalline phases, sintering process by SPS and macroscopic properties of Ba1-x-yNdxGayTiO3 ceramics was analyzed, as well as a weak hot emission from the 4F5/2 level. The sintered ceramics showed high dielectric constant and moderate dielectric losses. The highest value of the permittivity obtained for Ba1-x-yNdxGayTiO3 ceramics was E >(r) = 5890 at room temperature, 1 kHz and x = y = 0.03. This study shown that Nd3+ decreases the density of the pellets, increases the dielectric losses and decreases the dielectric constant of Ba0.97Ga0.03TiO3 ceramics. Therefore, the values of dielectric properties, at room temperature, of BaTiO3 doped with 3 at.% Ga and aecurrency sign7 at.% Nd are higher than those of undoped BaTiO3 ceramic.

The development of innovative technologies to modify natural textiles holds an important impact for medical applications, including the prevention of contamination with microorganisms, particularly in the hospital environment. In our study, Fe and N co-doped TiO2 nanoparticles have been obtained via the hydrothermal route, at moderate temperature, followed by short thermal annealing at 400 degrees C. These particles were used to impregnate polyester (PES) materials which have been evaluated for their morphology, photocatalytic performance, antimicrobial activity against bacterial reference strains, and in vitro biocompatibility on human skin fibroblasts. Microscopic examination and quantitative assays have been used to evaluate the cellular morphology and viability, cell membrane integrity, and inflammatory response. All treated PES materials specifically inhibited the growth of Gram-negative bacilli strains after 15 min of contact, being particularly active against Pseudomonas aeruginosa. PES fabrics treated with photocatalysts did not affect cell membrane integrity nor induce inflammatory processes, proving good biocompatibility. These results demonstrate that the treatment of PES materials with TiO2-1% Fe-N particles could provide novel biocompatible fabrics with short term protection against microbial colonization, demonstrating their potential for the development of innovative textiles that could be used in biomedical applications for preventing patients' accidental contamination with microorganisms from the hospital environment.

Disordered Sm3+ doped Y3ScxAl5-xO12 (Sm:YS(x)AG) polycrystalline ceramics with compositional parameter x = (0.05, 0.5, 1, 2) and different Sm3+ ions concentrations were obtained by solid state reaction method. XRD investigations show the linear increase of the lattice constant with compositional parameters x. New spectroscopic data obtained from high resolution optical absorption and emission spectra of Sm3+ doped YS(x)AG ceramics reveal composition effects on Sm3+ optical spectra: modification of the shapes and shifts of the lines, presence of the additional satellites and unresolved multicenter structure, etc. The analysis of these data allowed us to establish a partial structure of Sm3+ energy levels in Sm:YS(x)AG (x = 2) ceramics. The emission kinetics of the (4)G(5/2) level for different Sm3+ concentrations and Sc content in Sm:YS(x)AG were measured and analyzed. (C) 2016 Elsevier B.V. All rights reserved.

Obtaining high-quality materials, based on nanocrystals, at low temperatures is one of the current challenges for opening new paths in improving and developing functional devices in nanoscale electronics and optoelectronics. Here we report a detailed investigation of the optimization of parameters for the in situ synthesis of thin films with high Ge content (50 %) into SiO2. Crystalline Ge nanoparticles were directly formed during co-deposition of SiO2 and Ge on substrates at 300, 400 and 500 degrees C. Using this approach, effects related to Ge-Ge spacing are emphasized through a significant improvement of the spatial distribution of the Ge nanoparticles and by avoiding multi-step fabrication processes or Ge loss. The influence of the preparation conditions on structural, electrical and optical properties of the fabricated nanostructures was studied by X-ray diffraction, transmission electron microscopy, electrical measurements in dark or under illumination and response time investigations. Finally, we demonstrate the feasibility of the procedure by the means of an Al/n-Si/Ge:SiO2/ITO photodetector test structure. The structures, investigated at room temperature, show superior performance, high photoresponse gain, high responsivity (about 7 AW(-1)), fast response time (0.5 mu s at 4 kHz) and great optoelectronic conversion efficiency of 900% in a wide operation bandwidth, from 450 to 1300 nm. The obtained photoresponse gain and the spectral width are attributed mainly to the high Ge content packed into a SiO2 matrix showing the direct connection between synthesis and optical properties of the tested nanostructures. Our deposition approach put in evidence the great potential of Ge nanoparticles embedded in a SiO2 matrix for hybrid integration, as they may be employed in structures and devices individually or with other materials, hence the possibility of fabricating various heterojunctions on Si, glass or flexible substrates for future development of Si-based integrated optoelectronics.

The direct synthesis from elements of tungsten semicarbide, W2C, taking place in a narrow concentration range and at high temperatures, is unsuited for some applications requiring small particle sizes. We report an efficient synthesis method for obtaining pure phase W2C at low temperatures (< 1050 degrees C), through the carbothermal reduction of tungsten oxide species in the presence of metallic palladium nanoparticles obtained in situ. A 1:5 (at.) Pd:W ratio or higher yields pure phase W2C, while increasing the Pd content decreases the carburization temperature. The resulting composite materials contain Pd(0) and W2C particles well-dispersed on carbon, with sizes between 50 and 500 nm. The samples have been tested using cyclic voltammetry and chronoamperometry as potential anode materials for the electrooxidation of formic acid and show improved stability in comparison with commercial Pd and comparable current densities, up to 2 times higher than the commercial catalyst. (C) 2016 Elsevier B.V. All rights reserved.

Atomically clean lead zirco-titanate PbZr0.2Ti0.8O3 (001) layers exhibit a polarization oriented inwards P(-), visible by a band bending of all core levels towards lower binding energies, whereas as introduced layers exhibit P(+) polarization under air or in ultrahigh vacuum. The magnitude of the inwards polarization decreases when the temperature is increased at 700 K. CO adsorption on P(-) polarized surfaces saturates at about one quarter of a monolayer of carbon, and occurs in both molecular (oxidized) and dissociated (reduced) states of carbon, with a large majority of reduced state. The sticking of CO on the surface in ultrahigh vacuum is found to be directly related to the P(-) polarization state of the surface. A simple electrostatic mechanism is proposed to explain these dissociation processes and the sticking of carbon on P(-) polarized areas. Carbon desorbs also when the surface is irradiated with soft X-rays. Carbon desorption when the polarization is lost proceeds most probably in form of CO2. Upon carbon desorption cycles, the ferroelectric surface is depleted in oxygen and at some point reverses its polarization, owing to electrons provided by oxygen vacancies which are able to screen the depolarization field produced by positive fixed charges at the surface.

Multiple and complex functionalities are a demand nowadays for almost all materials, including common day-to-day materials such as paper, textiles, wood, etc. In the present report, the surface temperature control of different types of materials, including paper and textiles, was demonstrated by Joule heating of metallic-web transparent electrodes both by direct current and by RF induced eddy currents. Polymeric submicronic fiber webs were prepared by electrospinning, and metal sputtering was subsequently performed to transform them into flexible transparent electrodes. These electrodes were thermally attached to different substrates, including paper, textiles and glass. Using thermochromic inks, we demonstrated a high degree of control of the substrates' surface temperature by means of the Joule effect. Metallic fiber webs appear to be excellently suited for use as transparent electrodes for controlling the surface temperature of common materials, their highly flexible nature being a major advantage when dealing with rough, bendable substrates. This kind of result could not be achieved on bendable substrates with rough surfaces such as paper or textiles while employing classical transparent electrodes i.e. metal oxides. Moreover, contactless heating with induced currents is a premiere for transparent electrodes and opens up a score of new application fields.

We present detailed spectral calculations for small Lieb lattices having up to N = 4 number of cells, in the regime of half-filling, an instance of particular relevance for the nanomagnetism of discrete systems such as quantum dot arrays, due to the degenerate levels at midspectrum. While for the Hubbard interaction model-and even number of sites-the ground state spin is given by the Lieb theorem, the inclusion of long-range interaction-or odd number of sites-makes the spin state not known a priori, which justifies our approach. We calculate also the excitation energies, which are of experimental importance, and find significant variation induced by the interaction potential. One obtains insights on the mechanisms involved that impose as ground state the Lieb state with lower spin rather than the Hund one with maximum spin for the degenerate levels, showing this in the first and second orders of the interaction potential for the smaller lattices. The analytical results agree with the numerical ones, which are performed by exact diagonalization calculations or by a combined mean-field and configuration interaction method. While the Lieb state is always lower in energy than the Hund state, for strong long-range interaction, when possible, another minimal spin state is imposed as ground state.