National Institute Of Materials Physics - Romania

In this study, we compared the structural, morphological, and optical properties of a series of Zn1-xFexO (x = 0.00, 0.01, and 0.03) powders synthesized via a hydrothermal route with and without cetyltrimethylammonium bromide as a cationic surfactant. Our results highlighted the critical effects of the surfactant and the iron concentration on the structure, morphology, and optical properties of ZnO. X-ray diffraction, Mossbauer spectroscopy, and X-ray photoelectron spectroscopy analyses indicated the presence of a single phase comprising ZnO with a hexagonal wurtzite structure in all samples and a single oxidation state (+3) for the iron (Fe3+) that replaced Zn2+ in the ZnO structure. Morphological investigations by scanning electron microscopy showed that the surfactant and Fe3+ dopant greatly affected the shape of the ZnO grains, which varied between sheets and rod-like flowers. We found that the morphological and photocatalytic properties of the two series of samples comprising iron-doped ZnO with and without the surfactant were in opposition. We propose a possible growth mechanism for the ZnO polycrystalline grains in the presence of the Fe dopant and/or surfactant. The photocatalytic properties increased for the samples prepared with surfactant as the iron content increased, which was confirmed by ultraviolet-visible reflection measurements. The photocatalytic activities of the samples prepared without surfactant decreased in both the ultraviolet and visible spectral regions as the iron content increased in the samples.

We investigate the effect of the nature of the substrate and the bottom interface on the out-of-plane polarization orientation of ultrathin (10-nm) lead zirconate titanate (PZT) thin films of (001) orientation by photoelectron spectroscopy of samples without surface contamination. The substrate nature is varied between insulator (strontium titanate, STO) and semiconductor (Nb-doped STO, STON) and finally to a metal with a work function lower than that of PZT (strontium ruthenate, SRO). Outward polarization is obtained for PZT/STON(001) and inward polarization is obtained for PZT/STO(001) and PZT/SRO(001). Explanations are given for all these typical cases, the main elements being charge accumulation for compensation of the depolarization field, self-doping of PZT films, and the interface electric field driving the orientation of the polarization of the ferroelectric films. We find p-type self-doping is correlated with the inward polarization, and the driving field is formed between a negatively charged region with negatively ionized acceptors near the interface with the substrate and the p-type degenerate region with holes accumulated inside, toward the surface. This mechanism may be reversed under the assumption of n-type self-doping, positively ionized donors near the interface, and accumulated electrons toward the surface in the case of an interface with a substrate with a higher work function, being in line with recent data (PZT/Pt or BaTiO3/SRO).

InGeNi ohmic contacts on n-type semi-insulating GaAs(110) cleaved surfaces were fabricated. Cleaving semiconductor single crystal ensures the obtaining of semiconductor surfaces almost ideal in terms of chemical purity and stoichiometry. The chemical depth profile of metallic layer was investigated and revealed through multiple alternating steps of Ar+ etchings and XPS (x-ray photoelectron spectroscopy) measurements. The metallic layers made of alloy of Ge, In and Ni (60 nm Ge, 60 nm In and 10 nm Ni), were obtained by thermal evaporation on cleaved surfaces in a high vacuum facility, followed by a thermal annealing at 430-450 degrees C for 5 min. Twelve etching sessions were required until the metal/semiconductor interface has been reached. A total of 14 etchings steps were performed during the experiment, a layer with a thickness of 140 nm being removed from the surface. The atomic concentrations of the constituent chemical elements were determined. In3d, In4d, Ga3d, Ga2p, As3d, As2p, Ni2p(3/2), Ge3d, O1s and C1s spectral lines were recorded and chemical bonds within the layer were analyzed from the fittings. The formation of InxGa1-xAs type compounds and of an intermediate semiconductor layer rich in Ge atoms at the interface was highlighted. X-ray detectors InGeNi/GaAs/Al and InGeNi/GaAs/Ti were fabricated with ohmic contacts based on this contacting scheme and Schottky interfaces prepared also by evaporation on cleaved edges. Electrical characteristics have been investigated and key diode parameters determined - ideality factor, Schottky barrier height, series resistance. Detection capabilities of these devices were studied in an x-rays flux, provided by a Co anode x-ray source.

Non-ionising energy loss of radiation produces point defects and defect clusters in silicon, which result in a significant degradation of sensor performance. In this contribution results from TSC (Thermally Stimulated Current) defect spectroscopy for silicon pad diodes irradiated by electrons to fluences of a few 10(14) cm(-2) and energies between 3.5 and 27 MeV for isochronal annealing between 80 and 280 degrees C, are presented. A method based on SRH (Shockley-Read-Hall) statistics is introduced, which assumes that the ionisation energy of the defects in a cluster depends on the fraction of occupied traps. The difference of ionisation energy of an isolated point defect and a fully occupied cluster, Delta E-a is extracted from the TSC data. For the VOt (vacancy-oxygen interstitial) defect Delta E-a = 0 is found, which confirms that it is a point defect, and validates the method for point defects. For clusters made of deep acceptors the Delta E-a values for different defects are determined after annealing at 80 degrees C as a function of electron energy, and for the irradiation with 15 MeV electrons as a function of annealing temperature. For the irradiation with 3.5 MeV electrons the value Delta E-a = 0 is found, whereas for the electron energies of 6-27 MeV Delta E-a > 0. This agrees with the expected threshold of about 5 MeV for cluster formation by electrons. The Delta E-a values determined as a function of annealing temperature show that the annealing rate is different for different defects. A naive diffusion model is used to estimate the temperature dependencies of the diffusion of the defects in the clusters.

We have prepared Eu2+ (1%) doped BaCl2 nanopowders by using a hybrid sol-gel/thermal decomposition route using barium acetate and trichloracetic acid as in situ chlorination agent. Structural and thermal analysis data have shown that orthorhombic BaCl2 phase crystallization occurs at 260 degrees C. BaCl2:Eu2+ nanocrystals of about 100-200 nm in size were produced after annealing (in air) at 500 degrees C and they displayed Eu2+ d -> f type luminescence at 396 nm. X-ray photoelectron spectroscopy indicated the presence of both Eu2+, Eu3+ ions species and a BaO passivating layer at the surface of the nanocrystals. A polymer nanocomposite (PVP@BaCl2:Eu2+) was also processed and its optical response under x-ray irradiation was found to be similar to that of the nanopowder. X-ray excited luminescence showed Eu2+ luminescence at 396 nm, whereas the thermoluminescence peak at 130 degrees C was assigned to the F(Cl)-center recombination with Eu2+ related hole centers.

A series of Fe3O4@SiO2@Re catalysts were prepared by deposition of rhenium by means of the precipitation-deposition and impregnation procedures. Characterization of the catalysts confirmed that the deposition of silica protected the magnetite nanoparticles, resulting in a stable SiO(2)pFe(3)O(4) composite, which was not affected by the treatments during the further deposition of rhenium. Rhenium was silent in XRD over the range of concentrations, at which it was deposited. Furthermore, XPS detected rhenium, only in the impregnation series; this may confirm that dispersion was high. As expected, rhenium was not reduced to the metallic state and generated weakly acidic Bronsted-type centers as detected by NH3-TPD. H-2-TPD and chemisorption experiments demonstrate the capacity of these catalysts to chemisorb hydrogen. In line with these properties, rhenium catalyzed both C-C hydrogenolysis and C-O hydrolysis in successive steps. The performance of these catalysts was checked for a series of lignins of different origin and by means of different separation procedures. A very important finding is that these catalysts were highly stable and easy to recover.

Interfacing different transition-metal oxides opens a route to functionalizing their rich interplay of electron, spin, orbital, and lattice degrees of freedom for electronic and spintronic devices. Electronic and magnetic properties of SrTiO3-based interfaces hosting a mobile two-dimensional electron system (2DES) are strongly influenced by oxygen vacancies, which form an electronic dichotomy, where strongly correlated localized electrons in the in-gap states (IGSs) coexist with noncorrelated delocalized 2DES. Here, we use resonant soft-X-ray photoelectron spectroscopy to prove the e(g) character of the IGSs, as opposed to the t(2g) character of the 2DES in the paradigmatic LaAlO3/SrTiO3 interface. We furthermore separate the d(xy) and d(xz)/d(xz) orbital contributions based on deeper consideration of the resonant photoexcitation process in terms of orbital and momentum selectivity. Supported by a self-consistent combination of density functional theory and dynamical mean field theory calculations, this experiment identifies local orbital reconstruction that goes beyond the conventional e(g)-vs-t(2g) band ordering. A hallmark of oxygen-deficient LaAlO3/SrTiO3 is a significant hybridization of the e(g) and t(2g) orbitals. Our findings provide routes for tuning the electronic and magnetic properties of oxide interfaces through "defect engineering" with oxygen vacancies.

Nanostructures based on buried interfaces and heterostructures are at the heart of modern semiconductor electronics as well as future devices utilizing spintronics, multiferroics, topological effects, and other novel operational principles. Knowledge of electronic structure of these systems resolved in electron momentum k delivers unprecedented insights into their physics. Here we explore 2D electron gas formed in GaN/AlGaN high-electron-mobility transistor heterostructures with an ultrathin barrier layer, key elements in current high-frequency and high-power electronics. Its electronic structure is accessed with angle-resolved photoelectron spectroscopy whose probing depth is pushed to a few nanometers using soft-X-ray synchrotron radiation. The experiment yields direct k-space images of the electronic structure fundamentals of this system-the Fermi surface, band dispersions and occupancy, and the Fourier composition of wavefunctions encoded in the k-dependent photoemission intensity. We discover significant planar anisotropy of the electron Fermi surface and effective mass connected with relaxation of the interfacial atomic positions, which translates into nonlinear (high-field) transport properties of the GaN/AlGaN heterostructures as an anisotropy of the saturation drift velocity of the 2D electrons.

We present a study on the growth and characterization of high-quality single-layer MoS2 with a single orientation, i.e. without the presence of mirror domains. This single orientation of the MoS2 layer is established by means of x-ray photoelectron diffraction. The high quality is evidenced by combining scanning tunneling microscopy with x-ray photoelectron spectroscopy measurements. Spin-and angle-resolved photoemission experiments performed on the sample revealed complete spin-polarization of the valence band states near the K and -K points of the Brillouin zone. These findings open up the possibility to exploit the spin and valley degrees of freedom for encoding and processing information in devices that are based on epitaxially grown materials.

Highly defected SiO2 nanotubes (SiO2-NT) were obtained by a simple sol-gel procedure followed by calcination. Boron-doped diamond (BDD) polycrystalline films coated with SiO2-NT were used as working electrodes and, unexpectedly, cyclic voltammetric experiments have shown that the concentration of both positive and negative defects at the surface is high enough to enable redox processes involving positively charged Ru(bpy)(3)(2+/3+) to occur. Conversely, no electrochemical activity was put into evidence for Fe (CN)(6)(3/4) species, most likely as a result of the strong electrostatic repulsion exerted by the negatively charged SiO2 surface. The concentration of surface defects was further increased by a subsequent thermal treatment in a hydrogen atmosphere which, as EIS measurements have shown, significantly promotes Ru (bpy)(3)(2+) anodic oxidation. Digital simulation of the voltammetric responses demonstrated that this treatment does not lead to a similar increase of the number of electron-donor sites. It was also found that methanol anodic oxidation at hydrogenated SiO2-NT-supported platinum results in Tafel slopes of 116-220 mV decade(-1), comparable to those reported for both conventional PtRu and Pt-oxide catalysts. (C) 2018 Elsevier B.V. All rights reserved.