Publications

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.

Passivated zinc oxide nanowires (NW) were used to improve the charge injection in organic lightemitting diode (OLED) structures. Conducting polymers, deposited on the well-dispersed ZnO NW, were used to modify the electrical conductivity across the OLED structure because the charge transport is influenced by the interface interactions. Passivation with polymers improves the transport characteristics of the device due to the interaction between ZnONWand PEDOT:PSS polymer. The hole current density increases with the ZnO NW concentration, which made the current injection more balanced and therefore enhanced the electroluminescence efficiency. A templateless electrochemical deposition method was used to grow zinc oxide nanowires on an ITO/glass substrate because parameters such as the densities and dimensions of the nanowires can be controlled to produce thin and well dispersed structures.

We demonstrate the fabrication and characterization of epitaxial W2C Schottky contacts into 4H-SiC via sputtering deposition of ultra-thin W followed by thermal annealing. The alloying reaction occurs below 600 degrees C, yielding a stable layer up to 1200 degrees C. The epitaxial layer is hexagonal, unexpectedly forming W2C phase even at the lowest annealing temperature, which is predicted to occur at 1500 degrees C in bulk systems. Schottky contacts based on epitaxial W2C films exhibit smoother interface morphology, higher thermal stability, lower turn-ON voltage and about similar to 50% reduction in Schottky-contact resistance compared with thick Ti Schottky contacts.

High density bulks (97%-99%) of MgB2 were prepared by spark plasma sintering (SPS) in nitrogen (N-2) atmosphere for different heating rates (10, 20 and 100 degrees C min(-1)) and compared with reference samples processed in vacuum and Ar. N-2 reacts with MgB2 and forms MgB9N along the MgB2 grain boundaries. The high-field critical current density is enhanced for the sample processed in N-2 with a heating rate of 100 degrees C min(-1). At 2-35 K, this sample shows the strongest contribution of the grain boundary pinning (GBP). All samples are in the point pinning (PP) limit and by increasing temperature the GBP contribution decreases.

We report on output power performances obtained by diode-laser pumping of buried cladding-wave guides that were inscribed with a femtosecond-laser beam writing technique in several Nd:YVO4 media. Continuous-wave output power of 3.4 W at 1.06 mu m for an absorbed pump power at 808 nm of 10.3 W was obtained from a circular waveguide of 100-mu m diameter that was realized in a 6.9-mm long, 0.5-at% Nd:YVO4 crystal; the slope efficiency with respect to the absorbed pump power was 0.36. The pump at 880 nm, directly into the F-4(3/2) emitting level, was used to improve the waveguide output characteristics. With an absorbed pump power of 9.8 W at 880 nm, the same waveguide yielded 4.4 W at 1.06 mu m, whereas for emission at 1.34 mu m the output power reached 1.7 W; the slope efficiency improved to 0.47 for laser emission at 1.06 mu m and reached 0.24 for operation at 1.34 mu m. Results recorded from similar waveguides that were inscribed in 0.7-at% Nd:YVO4 and 1.0-at% Nd:YVO4 crystals are presented. (C) 2016 Elsevier Ltd. All rights reserved.