Microstructure, chemical composition, and dielectric response of CaCu3Ti4O12 ceramics doped with F, Al, and Mg ions.

Ceramics with nominal chemical composition CaCu3Ti4O12 (CCTO), CaCu3Ti3.96Al0.04O11.96F0.04 (CCTOAF), and Ca0.98Mg0.08Cu2.94Ti3.96Al0.04O11.96F0.04 (CCTOMAF) were prepared by the solid-state reactions technique. Using SEM, EDX, XPS, EPR, NMR, and complex impedance spectroscopy, the microstructure, elements distribution, chemical composition of grains and grain boundaries, and the dielectric response of ceramics were investigated. In the ССТО, CCTOAF, and CCTOMAF series, the average grain size increases, the degree of copper segregation at the grain boundaries is inversely related to grain size, and the dielectric loss decreases from 0.071 to 0.047 and 0.030, respectively, while dielectric permittivity ε' at 1 kHz is 5.6 × 104, 7.1 × 104, and 4.3 × 104, respectively. Additives of Al, Mg, F and milled particles (ZrO2, Al2O3, and SiO2) can either partially introduce into the perovskite structure or form low-melting eutectics at the grain boundaries, causing abnormal grain growth. The presence of copper ions in various oxidation states, as well as evidence of exchange spin interactions between them, was confirmed in all samples.

Morphological, optical and photovoltaic characteristics of MoSe2/SiOx/Si heterojunctions.

This work reports the effect of different processing parameters on the structural and morphological characteristics of MoSe2 layers grown by chemical vapour deposition (CVD), using MoO3 and Se powders as solid precursors. It shows the strong dependence of the size, shape and thickness of the MoSe2 layers on the processing parameters. The morphology of the samples was investigated by field emission scanning electron microscopy (FESEM) and the thickness of the deposited layers was determined by atomic force microscopy (AFM). Raman and photoluminescence (PL) spectroscopies were used to confirm the high quality of the MoSe2 layers. Surface composition was examined by photoelectron spectroscopy (XPS). Moreover, the MoSe2/SiOx/Si heterojunctions exhibit diode behaviour, with a rectification ratio of 10, measured at ±2.0 V, which is due to the p-i-n heterojunctions formed at the p-Si/SiOx/MoSe2 interface. A photovoltaic effect was observed with a short circuit current density (Jsc), open circuit voltage (VOC) and efficiency of -0.80 mA/cm2, 1.55 V and 0.5%, respectively. These results provide a guide for the preparation of p-i-n heterojunctions based on few-layer MoSe2 with improved photovoltaic response.

Super-bandgap light stimulated reversible transformation and laser-driven mass transport at the surface of As2S3 chalcogenide nanolayers studied in situ.

The super-bandgap laser irradiation of the in situ prepared As-S chalcogenide films was found to cause drastic structural transformations and unexpected selective diffusion processes, leading to As enrichment on the nanolayer surface. Excitation energy dependent synchrotron radiation photoelectron spectroscopy showed complete reversibility of the molecular transformations and selective laser-driven mass transport during "laser irradiation"-"thermal annealing" cycles. Molecular modeling and density functional theory calculations performed on As-rich cage-like clusters built from basic structural units indicate that the underlying microscopic mechanism of laser induced transformations is connected with the realgar-pararealgar transition in the As-S structure. The detected changes in surface composition as well as the related local and molecular structural transformations are analyzed and a model is proposed and discussed in detail. It is suggested that the formation of a concentration gradient is a result of bond cleavage and molecular reorientation during transformations and anisotropic molecular diffusion.

Sn/Pt(110) bimetallic surfaces: formation and oxygen adsorption.

Submonolayer coverage of Sn on a Pt(110) surface was studied by photoemission and low-energy electron diffraction. Deposition of less than 0.6 ML at 300 K gives rise to a c(2 × 2) surface reconstruction with weak diffraction spots at the very beginning of growth, and no other LEED patterns were found at this temperature. A new (4 × 1) Sn/Pt(110) surface structure was observed after flashing to 570 K a coverage of 0.64 ML. The total Sn coverage decreased to 0.58 ML after flashing as some of the atoms diffused into deeper layers. Different Sn phases were identified on the (4 × 1) Sn/Pt(110) surface: two types of surface Sn atoms in different adsorption sites, a subsurface Sn-Pt intermetallic layer and Sn-Pt surface islands. To investigate chemical reactivity, 0.25 ML Sn/Pt(110) and 0.58 ML (4 × 1) Sn/Pt(110) surfaces were exposed to 1000 L of O(2) at 300 K. Analyses of the photoemission data provide evidence for the formation of tin oxide. The interaction with oxygen of the two surfaces is similar, independent of surface structure and the composition of the subsurface layers. The Sn concentration in the interface intermetallic layer is the main factor which influences the oxygen adsorption.

Interaction of oxygen with Au/Ti(0001) surface alloys studied by photoelectron spectroscopy.

The interaction of oxygen with gold adsorbed on Ti(0001) was studied by synchrotron radiation photoelectron spectroscopy. Two kinds of surfaces were explored: as-deposited 0.38, 1.16 and 1.85 monolayer (ML) thick Au overlayers on the Ti(0001) surface, and the same samples after thermal treatment, which resulted in the formation of Au-Ti intermetallic surfaces. The Ti 3p core level was strongly affected by reaction with oxygen, while the Au 4f core level showed only minor changes other than a decrease in intensity. The Ti 3p peak was fitted with several components which were identified as Ti atoms in different oxidation states, namely TiO, Ti(2)O(3), TiO(2) and Ti-OH. Titanium oxide phase formation is accompanied by Au-Ti bond dissociation and outward diffusion of Ti. The presence of an Au-Ti intermetallic phase on the Ti(0001) surface promotes oxidation of the Ti atoms.

Interaction of Au with CeO2(111): A photoemission study.

We have studied the adsorption of low dimensional gold on ceria, produced by evaporation onto the surface. The interaction of gold with CeO(2)(111) layers was investigated with x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and resonant photoelectron spectroscopy (RPES). Gold was deposited in steps onto a 1.5 nm thick CeO(2)(111) layer epitaxially grown on a Cu(111) substrate. The RPES showed a partial Ce(4+)-->Ce(3+) reduction, observed as a resonant enhancement of the 4f level of the Ce(3+) species. This can be explained by possible creation of a new Au(+) ionic state. The observed effects are stronger for Au deposition at room temperature than at 250 degrees C. The obtained results are in agreement with already published density functional theory calculations reporting weakening of bond between the oxygen and the Ce atoms in ceria caused by the presence of gold.

Low pressure oxidation of ordered Sn/Pd(110) surface alloys.

The reaction of oxygen at low pressure with the Sn/Pd(110) system has been examined by photoelectron spectroscopy using synchrotron radiation. The c(2 × 2) and (3 × 1) reconstructions of the Sn/Pd(110) surface at 0.5 and 0.7 monolayers (ML) Sn coverage and a 1.75 ML Sn overlayer on the Pd(110) surface after flashing to 470 K were studied. The Sn 4d core level is strongly affected by O(2) adsorption while the Pd 3d core level shows very little change other than a decrease in intensity. Starting with a 10 L dose of oxygen, prominent changes in the spectra were observed for all Sn/Pd(110) surface alloys. Analysis of the Sn 4d core levels indicates that oxidation proceeds with the formation of well-defined states of Sn, which were identified as a Pd-Sn-O interface layer, SnO and SnO(2) oxides. The valence band spectra confirm this assignment. The Sn(2+) and Sn(4+) component signals originate from the topmost surface layer, i.e. tin atoms in more highly oxidized states constitute the topmost surface layer on top of the Pd-Sn-O interface. The presence of a sub-surface PdSn intermetallic alloy facilitates the tin oxide formation; the Sn-O phase formation is accompanied by Pd-Sn bond dissociation.

Surface-stabilized nonferromagnetic ordering of a layered ferromagnetic manganite.

An outstanding question regarding the probing or possible device applications of correlated electronic materials (CEMs) with layered structure is the extent to which their bulk and surface properties differ or not. The broken translational symmetry at the surface can lead to distinct functionality due to the charge, lattice, orbital, and spin coupling. Here we report on the case of bilayered manganites with hole doping levels corresponding to bulk ferromagnetic order. We find that, although the hole doping level is measured to be the same as in the bulk, the surface layer is not ferromagnetic. Further, our low-energy electron diffraction and x-ray measurements show that there is a c-axis collapse in the outermost layer. Bulk theoretical calculations reveal that, even at fixed doping level, the relaxation of the Jahn-Teller distortion at the surface is consistent with the stabilization of an A-type antiferromagnetic state.

Photoelectron spectroscopy characterization of diamond-like carbon films.

The electronic states of diamond-like hydrogenated carbon (DLC) films were studied by synchrotron radiation photoelectron spectroscopy. The valence band spectra measured at different excitation energies show the gradual emergence of the p-pi band in relation to the sample annealing and ion bombardment amorphization. The p-pi band of the annealed DLC was characterized by localized p(z) states, while the formation of the amorphous carbon surface was accompanied by appearance of the delocalized p(z) states, which reduce the optical gap. A simple approach permitting the extraction of the 2p band shape from the photoelectron spectra is proposed.