280 research outputs found
Adsorption and Diffusion of Pt and Au on the Stoichiometric and Reduced TiO2 Rutile (110) Surfaces
A comparative first principles pseudopotential study of the adsorption and
migration profiles of single Pt and Au atoms on the stoichiometric and reduced
TiO2 rutile (110) surfaces is presented. Pt and Au behave similarly with
respect to (i) most favorable adsorption sites, which are found to be the
hollow and substitutional sites on the stoichiometric and reduced surfaces,
respectively, (ii) the large increase in their binding energy (by ~1.7 eV) when
the surface is reduced, and (iii) their low migration barrier near 0.15 eV on
the stoichiometric surface. Pt, on the other hand, binds more strongly (by ~2
eV) to both surfaces. On the stoichiometric surface, Pt migration pattern is
expected to be one-dimensional, which is primarily influenced by interactions
with O atoms. Au migration is expected to be two-dimensional, with Au-Ti
interactions playing a more important role. On the reduced surface, the
migration barrier for Pt diffusion is significantly larger compared to Au.Comment: 3 figures, 1 table, submitted to PR
Microdomain Formation, Oxidation, and Cation Ordering in LaCa\u3csub\u3e2\u3c/sub\u3eFe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e8+y\u3c/sub\u3e
The compound LaCa2Fe3O8+y, also known as the Grenier phase, is known to undergo an order–disorder transformation (ODT) at high temperatures and oxidation has been observed when the compound is cooled in air after the ODT. In this study, we have synthesized the Grenier compound in air using traditional solid-state reactions and investigated the structure and composition before and after the ODT. Thermal analysis showed that the material undergoes an ODT in both oxygen and argon atmospheres with dynamic, temperature dependent, oxidation upon cooling. Results from scanning transmission electron microscopy (STEM) suggest that the Grenier phase has preferential segregation of Ca and La on the two crystallographic A sites before the ODT, but a random distribution above the ODT temperature. Furthermore, STEM images suggest the possibility that oxygen excess may exist in La-rich regions within microdomains rather than at microdomain boundaries
Potential barrier lowering and electrical transport at the LaAlO/SrTiO heterointerface
Using a combination of vertical transport measurements across and lateral
transport measurements along the LaAlO/SrTiO heterointerface, we
demonstrate that significant potential barrier lowering and band bending are
the cause of interfacial metallicity. Barrier lowering and enhanced band
bending extends over 2.5 nm into LaAlO as well as SrTiO. We explain
origins of high-temperature carrier saturation, lower carrier concentration,
and higher mobility in the sample with the thinnest LaAlO film on a
SrTiO substrate. Lateral transport results suggest that parasitic
interface scattering centers limit the low-temperature lateral electron
mobility of the metallic channel.Comment: 10 pages, 3 figures, and 1 tabl
The Electronic and Superconducting Properties of Oxygen-Ordered MgB2 compounds of the form Mg2B3Ox
Possible candidates for the Mg2B3Ox nanostructures observed in bulk of
polycrystalline MgB2 (Ref.1) have been studied using a combination of
Z-contrast imaging, electron energy loss spectroscopy (EELS) and
first-principles calculations. The electronic structures, phonon modes, and
electron phonon coupling parameters are calculated for two oxygen-ordered MgB2
compounds of composition Mg2B3O and Mg2B3O2, and compared with those of MgB2.
We find that the density of states for both Mg2B3Ox structures show very good
agreement with EELS, indicating that they are excellent candidates to explain
the observed coherent oxygen precipitates. Incorporation of oxygen reduces the
transition temperature and gives calculated TC values of 18.3 K and 1.6 K for
Mg2B3O and Mg2B3O2, respectively.Comment: Submitted to PR
High temporal-resolution scanning transmission electron microscopy using sparse-serpentine scan pathways
Scanning transmission electron microscopy (STEM) provides structural analysis with sub-angstrom resolution. But the pixel-by-pixel scanning process is a limiting factor in acquiring high-speed data. Different strategies have been implemented to increase scanning speeds while at the same time minimizing beam damage via optimizing the scanning strategy. Here, we achieve the highest possible scanning speed by eliminating the image acquisition dead time induced by the beam flyback time combined with reducing the amount of scanning pixels via sparse imaging. A calibration procedure was developed to compensate for the hysteresis of the magnetic scan coils. A combination of sparse and serpentine scanning routines was tested for a crystalline thin film, gold nanoparticles, and in an in-situ liquid phase STEM experiment. Frame rates of 92, 23 and 5.8 s-1 were achieved for images of a width of 128, 256, and 512 pixels, respectively. The methods described here can be applied to single-particle tracking and analysis of radiation sensitive materials
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