National Institute Of Materials Physics - Romania

By platinum deposition on a 150 nm thick film of lead zirco-titanate oriented PZT(001), grown on strontium titanate (001) single crystals with a strontium ruthenate buffer layer, which did not show initial preferential out-of-plane orientation of its ferroelectric polarization, a band bending near the interface towards lower energies is observed using photoelectron spectroscopy, by following all core levels from the substrate (Pb 4f, Zr 3d, Ti 2p, O 1s). This is unexpected given the fact that platinum has a larger work function than PZT and a rectifying contact for electrons is expected to be built at the interface. This observation may have two explanations: (i) platinum forms an alloy with elements from PZT yielding a metal with considerable lower work function; (ii) platinum provides electrons to the substrate which are able to compensate the depolarization field generated by the outwards polarization state. Several arguments are brought in favor of the second hypothesis, especially the attenuation of core levels from the substrate which is well described by exponential functions with reasonable values of the photoelectron inelastic mean free path, suggesting the formation of a sharp interface. High resolution transmission electron microscopy confirmed the sharpness of the interface. (C) 2017 Elsevier B.V. All rights reserved.

Nowadays, the advances materials science have created the possibility of using nanomaterials as adsorbents in environmental technologies. This study is focused on the evaluation of C-HAp powders capacity in lead ions removal from contaminated solutions. The C-HAp powders after removal of lead (Pb2+) ions from contaminated water were characterized by X-Ray Photoelectron Spectroscopy (XPS) and Energy Dispersive X-ray analysis (EDX). The presence of lead in the hydroxyapatite powders after removal experiments was observed in the XPS general spectrum and also in the EDX spectra. In addition, the AAS studies revealed that the C-HAp powders possess a high affinity towards lead ions with an effective removal rate of 80% for an initial lead concentration of 20 mg.L-1.

Sodium dodecyl sulfate (SDS) effects at the electrified p- and n-GaAs(100)/H2SO4 interfaces were investigated by EIS, XPS and AFM. XPS data revealed that under the open circuit conditions, SDS adsorption on GaAs(100) results in a protective overlayer which prevents the further oxidation in air of both types of semiconductor surfaces. The dopant nature is, however, decisive for the way of bonding the surfactant molecule to the surface. At the p-doped substrate, SDS adsorbs mainly at As sites by its hydrocarbon tail and by the anion head to the Ga sites at the n-doped one. Although the surfactant behaves as a dipole under the applied potential control, the dopant type plays a key role in the SDS interaction with GaAs(100) electrodes too. EIS data evidenced that SDS interaction with n-GaAs(100) electrode brings a pronounced decrease of the capacitive contribution of the surface states and a shift of the flatband potential to less negative values, unlike the p-doped one, where no significant change in its electronic properties was observed. These results were rationalized in terms of surface states- and field-effects operating at the electrified interfaces under discussion. (C) The Author(s) 2017. Published by ECS.

Pd-Ge based ohmic contacts on III-V semiconductor e.g.Pd-Ge on n-GaAs are viewed as a viable alternative to low contact resistance metallization. As it was remarked [11, together with the device dimensions decrease, the AuGeNi metallization system becomes inadequate for shallow-junction devices. This characteristic is related to the formation of a low melting point beta- AuGa phase that leads to a poor contact thermal stability. Gallium Antimonide is anIII-V semiconductor compound that can be used in a photovoltaic convertor of GaAs/GaSb tandem stack With a predicted efficiency of 30%. Reduced series resistance on GaSb cells can be achieved by the improving of contact metallization properties. The present study is dedicated to the preparation conditions and structural analyzing of PdGe based contacts on n-GaSb, namely: Pd/Au/Ge. There are presented the depth profiling for PdGe metallization obtained from XPS measurements, and morphologic studies arisen from SEM technique and AFM technique.

In this chapter we discuss the capabilities of X-ray photoemission and absorption spectroscopies for the investigation of the electronic, magnetic and electric properties of multiferroic materials and heterostructures. As complementary techniques providing element selective information on both occupied and empty states, their combination delivers a comprehensive picture of the chemical state of individual species, magnetic moments, bulk and surface band structure, and local atomic environment at the interface between dissimilar materials. By directly probing the electronic structure at the atomic level, unique insights can be learned about the mechanisms responsible for the magnetoelectric couplings in this fascinating class of materials.

Upon reduction of the film thickness we observe a metal-insulator transition in epitaxially stabilized, spin-orbit-coupled SrIrO3 ultrathin films. By comparison of the experimental electronic dispersions with density functional theory at various levels of complexity we identify the leading microscopic mechanisms, i.e., a dimensionality-induced readjustment of octahedral rotations, magnetism, and electronic correlations. The astonishing resemblance of the band structure in the two-dimensional limit to that of bulk Sr2IrO4 opens new avenues to unconventional superconductivity by "clean" electron doping through electric field gating.

The integration of ferroelectrics in perovskite solar cells is proposed as possible way to enhance charge collection efficiency. First results on solar cellmanufactured with PbTiO3 (PTO) instead of TiO2 have shown negligible values for the power conversion efficiency (PCE). This is explained by the high serial resistance of sol-gel deposited PTO on F:SnO2 electrodes (FTO). Although PTO layer has remnant polarization of 22 mu C/cm(2), the high potential barrier (0.25 +/- 0.05 eV) at the FTO/PTO interface and lowcarriermobility (10(-8) cm(2) V-1 s(-1)) compared to TiO2 leads to high serial resistance. Better results were obtained with thinner PTO layers grown by pulsed laser deposition, with PCE values up to 0.6%. Further enhancement was obtained by replacing PTO with BaTiO3 (BTO), with PCE value reaching about 0.8% after poling the cell with +3 V. The most important finding was that the magnitude of the short circuit current increases with the amplitude of the poling voltage while the value of the open-circuit voltage remains about the same, around 0.9 V. This is explained through more efficient collection of the charges generated under illumination in the absorber layer due to the polarization that is present in the ferroelectric film. (C) 2017 Elsevier Ltd. All rights reserved.

Ni films of 20 nm nominal thickness were grown on Ge(001) substrates by molecular beam epitaxy at several different temperatures from room temperature up to 400 C. X-ray diffraction and X-ray photoelectron spectroscopy reveal the nucleation of Ni-Ge compounds (NiGe, Ni2Ge, Ni5Ge2) as well as a departure from the fcc Ni structure exhibited by the films at and beyond a temperature of 100 C. The binding energy of the Ni 2p peak increases from the RT value (852.7 eV) by 0.51.1 eV for the Ni/Ge(001) samples, while the Ge 2p binding energy changes by 0.60.7 eV after Ni growth compared to a clean Ge(001) substrate (there is only a +/- 0.15 eV shift among the samples grown on substrates at higher temperatures). By increasing substrate temperature, we obtained higher intermixing of Ni and Ge, but rather than both Ni and Ge interdiffusing, we find that Ni diffuses further into the germanium with higher substrate temperature, forming increasingly Ni-rich Ni-Ge compounds diluted into the Ge matrix. Based on Magneto-optic Kerr Effect measurements, Ni/Ge(001) grown on substrates at 100 and 200 C does not exhibit a hysteresis loop, while the samples on 300 and 400 C substrates show magnetic behavior, which we attribute to the magnetic character of hexagonal Ni5Ge2 (which is determined here for the first time to be a ferromagnetic phase). (c) 2017 Elsevier B.V. All rights reserved.

We investigate a possible route for optimization of organic P3HT:ICBA photovoltaic cells. In order to ensure a more efficient charge separation and collection at the electrodes, two- and three-layer structures are produced, where additional P3HT and ICBA single layers are placed adjacent to the mixed layer. The J-V characteristics are modeled using Monte-Carlo simulations in a flexible computational framework, reproducing the typical morphologies of the active layers. We discuss the implications of the structural modifications, in particular the enhancement of the open circuit voltage. Qualitative features of the theoretical simulations are validated by experiment. The proposed fabrication technique of using solvents with different boiling points for successive deposition of the individual layers may constitute an accessible route for producing optimized solar cell structures. (C) 2017 Elsevier B.V. All rights reserved.

Thin films of chalcogenide, Cu(In, Ga) Se-2 have been obtained using a single quaternary target by radio frequency magnetron sputtering method, with thickness in the range 750 nm to 1200 nm. X-ray photoelectron spectroscopy investigations showed, that the composition of Cu(In, Ga) Se-2 thin films was very similar to that of the used target CuIn0.75Ga0.25Se2. Identification of the chemical composition of Cu(In, Ga) Se-2 thin films by XPS performed in high vacuum, emphasized that the samples exhibit surface features suitable to be integrated into the structure of solar cells. Atomic Force Microscopy and Scanning Electron Microscopy investigations showed that surface morphology was influenced by the increase in thickness of the Cu(In, Ga) Se-2 layer. From X-Ray Diffraction investigations it was found that all films were polycrystalline, having a tetragonal lattice with a preferential orientation along the (112) direction. The optical reflectance as a function of wavelength was measured for the studied samples. The increase in thickness of the Cu(In, Ga) Se-2 absorber determined a decrease of its optical bandgap value from 1.53 eV to 1.44 eV. The results presented in this paper showed an excellent alternative of obtaining Cu(In, Ga) Se-2 compound thin films from a single target. (C) 2016 Elsevier B.V. All rights reserved.