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Highlights: [•] Dispersion state of CoO on Al2O3 was studied using positronium as a new probe. [•] CoO causes spin conversion of positronium when its content is below 3wt%. [•] Inhibition of positronium formation occurs when the CoO content is above 3wt%. [•] The monolayer dispersion capacity of CoO on Al2O3 is about 3wt% detected by Ps. [•] Positronium is a sensitive probe for the dispersion state of active components. [ABSTRACT FROM AUTHOR]

Abstract: NiO/γ-Al2O3 catalysts with NiO content of 9wt% and 24wt% were prepared by solid state reaction method. They are annealed in air at temperatures from 100°C to 1000°C. Positron lifetime spectra were measured to study the microstructure variation during annealing process. Four positron lifetime components were resolved with two long lifetime τ 3 and τ 4, which can be attributed to the ortho-positronium lifetime in microvoids and large pores, respectively. It was found that the longest lifetime τ 4 is rather sensitive to the chemical environment of the large pores. The NiO active centers in the catalysts cause decrease of both τ 4 and its intensity I 4, which is due to the spin-conversion of positronium induced by NiO. However, after heating the catalysts above 600°C, abnormal increase of the lifetime τ 4 is observed. This is due to the formation of NiAl2O4 spinel from the reaction of NiO and γ-Al2O3. The generated NiAl2O4 weakens the spin-conversion effect of positronium, thus leads to the increase of o-Ps lifetime τ 4. Formation of NiAl2O4 is further confirmed by both X-ray diffraction and X-ray photoelectron spectroscopy measurements. [Copyright &y& Elsevier]

Abstract: The oxidation of graphene layer on Ru(0001) has been investigated by means of scanning tunneling microscopy. Graphene overlayer can be formed by decomposing ethyne on Ru(0001) at a temperature of about 1000K. The lattice mismatch between the graphene overlayer and the substrate causes a moiré pattern with a superstructure in a periodicity of about 30Å. The oxidation of graphene/Ru(0001) was performed by exposure the sample to O2 gas at 823K. The results showed that, at the initial stage, the oxygen intercalation between the graphene and the Ru(0001) substrate takes place at step edges, and extends on the lower steps. The oxygen intercalation decouples the graphene layer from the Ru(0001) substrate. More oxygen intercalation yields wrinkled bumps on the graphene surface. The oxidation of graphene, or the removal of carbon atoms can be attributed to a process of the combination of the carbon atoms with atomic oxygen to form volatile reaction products. Finally, the Ru(0001)–(2×1)O phase was observed after the graphene layer is fully removed by oxidation. [ABSTRACT FROM AUTHOR]

Abstract: Fe2O3/Al2O3 catalysts were prepared by solid state reaction method using -Fe2O3 and -Al2O3 nano powders. The microstructure and surface properties of the catalyst were studied using positron lifetime and coincidence Doppler broadening annihilation radiation measurements. The positron lifetime spectrum shows four components. The two long lifetimes and are attributed to positronium annihilation in two types of pores distributed inside Al2O3 grain and between the grains, respectively. With increasing Fe2O3 content from 3wt% to 40wt%, the lifetime keeps nearly unchanged, while the longest lifetime shows decrease from 96ns to 64ns. Its intensity decreases drastically from 24% to less than 8%. The Doppler broadening S parameter shows also a continuous decrease. Further analysis of the Doppler broadening spectra reveals a decrease in the p-Ps intensity with increasing Fe2O3 content, which rules out the possibility of spin-conversion of positronium. Therefore the decrease of is most probably due to the chemical quenching reaction of positronium with Fe ions on the surface of the large pores. [Copyright &y& Elsevier]

Abstract: The results of first-principles calculations about the two possible terminations of (001) surface of SrTiO3 perovskite and adsorption of NO on SrTiO3 (001) surface were presented. Both surface parameters (atomic structures and electronic configurations) and adsorption parameters (bond, energy and charge) of NO on SrTiO3 (001) surface, which have never been investigated before as far as we know were investigated using density functional theory calculations with the local-density approximation (DFT-LDA). It was found that the two possible terminations of SrTiO3 (001) surface have large surface relaxation, which leads to surface polarization and exhibits different reactivity toward the dissociative adsorption of NO. The electronic states of TiO2-terminated surface have larger difference than that of bulk, so it is more favorable for adsorption of NO than SrO-terminated surface. [Copyright &y& Elsevier]

Abstract: Adsorption structures of tetracene on the Ru surface has been investigated by means of ab initio density functional theory (DFT) calculations. Several adsorption geometries of tetracene on Ru were examined in details. The optimized structures have an adsorption energy of 4.23eV for tetracene adsorbed between the top and the second Ru atoms rows with its long molecular axis along the direction, and a slightly smaller adsorption energy of 4.19eV for tetracene adsorbed with its long axis perpendicular to , consisting well with the adsorption structures observed in the scanning tunneling microscopy (STM) measurements for tetracene overlayer on Ru. By Comparing DOS before and after tetracene adsorption, we suggest that the coupling of the substrate d-band and the filled p-orbitals of tetracene determines tetracene adsorption on the Ru surface. [Copyright &y& Elsevier]

Abstract: In situ photoluminescence spectroscopy (PL) measurements of tris(8-hydroxyquinoline) aluminum (Alq3) film were carried out. Upon deposition of Alq3 on the glass substrate, the PL intensity changes dramatically, while the peak position of Alq3 emission shows a sharp red-shift from 524nm at the initial deposition of Alq3, and tends to a saturation value of 536nm for the film thickness range from 2 to 500nm. This red-shift is associated with the change from the 2D to 3D exciton state with increasing Alq3 film thickness. Temperature dependent PL spectra of Alq3 films showed, besides the changes in the PL intensity, clearly a blue-shift of Alq3 emission about 9nm for the film annealing up to 150°C, while no any shift of Alq3 emission was observed for the film annealing below 130°C. Both changes in PL intensity, and especially in the peak position of Alq3 emission were attributed to crystallization (thermal) effect of Alq3 film upon annealing. [Copyright &y& Elsevier]

Abstract: Thin film growth of tetracene on Ag(110) has been investigated by using scanning tunneling microscopy (STM). In the monolayer regime, the tetracene molecules formed islands of ordered structure, which is commensurate (4-fold periodicity) in the direction, but incommensurate in the [001] azimuth, with the substrate. All the tetracene molecules align regularly with their longer axes parallel to the [001] direction, and with their molecular planes parallel to the surface. At multilayer coverages, the tetracene molecules form a crystalline film in a novel 3D orthorhombic structure, with their orientations remained unchanged. Growth of tetracene on Ag(110) is in the layer-by-layer growth mode. [Copyright &y& Elsevier]

Abstract: Coverage-dependent adsorption energy of the Ge/Ru(0001) growth system and the geometrical distortions of the most stable adsorption structure are investigated through first-principles calculations within density functional theory. A local minimum in adsorption energy is found to be at a Ge coverage of 1/7 monolayer with a Ru(0001)- symmetry. Based on this stale superstructure, the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) images are simulated by means of surface local-density of states (LDOS). The results are consistent well with the STM measurements on the phase for Ge overlayer on Ru(0001). From this stimulation, the relations between the STM images and the lattice distortion are also clarified. [Copyright &y& Elsevier]

Abstract: The adsorption of fluorescein on the Ag(110) surface has been investigated by the first-principles pseudopotential method. Various adsorption geometries have been calculated and the energetically most favorable structure of fluorescein/Ag(110) was identified. The fluorescein molecule, in most favorable structure, is on hollow site, and the adsorption energy is 2.34eV. Here the adsorption sites refer to the positions at the first layer of the substrate where the middle carbon atom of the fluorescein molecule is located. The bonding strength of the fluorescein molecule to the Ag substrate is site selective, being determined by electron transfer to the oxygen atoms of the molecule and local electrostatic attraction between the oxygen atoms and the silver atoms. [Copyright &y& Elsevier]

Abstract: The thin fluorescein films grown on Ag (110) have been studied by the ultraviolet photoemission spectroscopy (UPS). Four emission features are located at 2.7, 3.8, 7.4 and 9.8eV below the Fermi level, respectively. The angle-resolved ultraviolet photoemission spectroscopy (ARUPS) measurements show that the Triring plane of fluorescein is nearly parallel to the substrate and the axis of Ce to the azimuth. [Copyright &y& Elsevier]

Ag/SiO 2 /Si multilayer samples with different SiO 2 layer thicknesses were prepared through deposition of Ag layer onto SiO 2 /Si substrates at different substrate temperatures (T sub) by RF magnetron sputtering method. Microstructure of the multilayer was characterized by XRD θ /2 θ scan and φ -scan analysis, depth-resolved XPS analysis, and Doppler broadening of annihilation radiation (DBAR) spectra of positron measurements. The experimental results indicate that, at T sub = 25 °C, the Ag deposition forms Ag/SiO 2 /Si multilayer structure. With T sub increasing to 200 °C, Ag atoms are found to diffuse into the SiO 2 layer. This thermal diffusion process plays a critical role in removing SiO 2 layer on the Si substrate. At T sub of 600 °C, the 31-nm-thick SiO 2 layer is partially desorbed from the Si surface as a result of Ag diffusion, and eventually results in a deposition of Ag layer directly on the Si substrate. This is also confirmed by the XRD φ -scan measurements, which indicates epitaxial growth of Ag atoms directly on the surface of Si substrate at T sub of 600 °C. • The deposition of Ag on SiO 2 /Si substrate at 25 °C is easily forming Ag/SiO 2 /Si multilayer structure. • Compared with Ag single crystal, Ag films have the smaller positron effective diffusion lengths due to its higher density of structural defects. • Depth-resolved XPS results show the high diffusion nature of surface O when the substrate temperature up to 600 °C. [ABSTRACT FROM AUTHOR]