Nucleation,morphology, and structure of sub-nm thin ceria islands on Rh(111)

The early stages of ceria growth on Rh(111) at high temperature have been investigated by low-energy electron microscopy and photoemission electron microscopy. Ceria was deposited by reactive Ce deposition at substrate temperatures between 700°C and 900°C in an oxygen ambient of 5 × 10⁻⁷ Torr. At 700°C, we observe a high nucleation density of 100-nm-sized islands. With elevated temperature, the average island size increases, and the nucleation density decreases. Triangularly shaped islands nucleate preferentially at step edges, with seemingly abrupt interfaces between Ce and Rh. At 900°C, the island edges are still straight, but during growth the islands lose their triangular form. Instead, growth along the substrate step edges becomes favorable, leading to a maze-like morphology. Atomic force microscopy reveals islands of 0.3 to 0.6-nm height, consistent with ceria islands formed by one or two trilayers (O―Ce―O) of ceria. Moreover, the second layer of the islands is also triangularly shaped, with lateral dimensions of 50 nm and similar step heights. IV-LEEM analysis leads to the conclusion that the rhodium surface is covered by a layer of reduced cerium oxide, which is partially overgrown by smaller islands of CeO₂.     

Silver: a novel growth catalyst for Ge nanoislands on Si(113)

The impact of silver pre‐adsorption on germanium growth on Si(113) was investigated using in‐situ low‐energy electron microscopy (LEEM) as well as low‐energy electron diffraction (LEED). The adsorption of silver leads to the formation of a regular pattern of nanofacets along the [1 0] direction. The periodicity of this pattern in [33 ] direction was determined to (44 ± 4) nm. From LEED series at different energies the facets were identified to be of (111) and (115) orientation. While the (111) facets show a (√3 × √3)‐R30° reconstruction, the (115) facets exhibit a (2 × n) superstructure. The subsequent growth of Ge results in the formation of nanoislands that are aligned along the facets. These Ge islands have an anisotropic shape with typical sizes of about 100 nm in [33 ] direction and 400 nm in [1 0] direction. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Untersuchung des Harns

Analytical and Bioanalytical Chemistry -     

Temperature dependent low energy electron microscopy study of Ge growth on Si(1 1 3)

We investigated the initial Ge nucleation and Ge island growth on a Si(1 1 3) surface using low energy electron microscopy and low energy electron diffraction. The sample temperature was varied systematically between 380 °C and 590 °C. In this range, a strong temperature dependence of the island shape is observed. With increasing temperature the Ge islands are elongated in the direction. Simultaneously, the average island size increases while their density decreases. From the Arrhenius-like behaviour of the island density, a Ge adatom diffusion barrier height of about 0.53 eV is deduced.     

Bioaccessibility of selected trace metals in urban PM<Subscript>2.5</Subscript> and PM<Subscript>10</Subscript> samples: a model study

Bioaccessibility of trace metals originating from urban particulate matter was assessed in a worst case scenario to evaluate the uptake and thus the hazardous potential of these metals via gastric juice. Sampling was performed over a period of about two months at the Getreidemarkt in downtown Vienna. Concentrations of the assayed trace metals (Ti, Cr, Mn, Co, Ni, Cu, Zn, Mo, Ag, Cd, Sn, Sb, Tl and Pb) were determined in PM_2.5 and PM₁₀ samples by ICP-MS. The metal concentrations in sampled air were in the low picogram to high nanogram per cubic metre range. The concentrations in PM_2.5 samples were generally lower than those in PM₁₀ samples. The average daily intake of these metals by inhalation for a healthy adult was estimated to be in the range of <1 ng (Tl) to >1,000 ng (Zn). To estimate the accessibility of the inhaled and subsequently ingested metals (i.e. after lung clearance had taken place) in the size range from 2.5- to 10-μm aerodynamic equivalent diameter, a batch-extraction with synthetic gastric juice was performed. The data were used to calculate the bioaccessibility of the investigated trace metals. Extractable fractions ranged from 2.10% (Ti in PM_2.5) to 91.0% (Cd in PM_2.5), thus yielding bioaccessible fractions (PM_2.5–10) from 0.16 ng (Ag) to 178 ng (Cu).     

Alignment of Ge nanoislands on Si(111) by Ga-induced substrate self-patterning

A novel mechanism is described which enables the selective formation of three-dimensional Ge islands. Submonolayer adsorption of Ga on Si(111) at high temperature leads to a self-organized two-dimensional pattern formation by separation of the 7 x 7 substrate and Ga/Si(111)-(square root[3] x square root[3])-R30 degrees domains. The latter evolve at step edges and domain boundaries of the initial substrate reconstruction. Subsequent Ge deposition results in the growth of 3D islands which are aligned at the boundaries between bare and Ga-covered domains. This result is explained in terms of preferential nucleation conditions due to a modulation of the surface chemical potential.     

Gravitational wave emission from the single-degenerate channel of Type Ia supernovae

The thermonuclear explosion of a C/O white dwarf as a Type Ia supernova (SN Ia) generates a kinetic energy comparable to that released by a massive star during a SN II event. Current observations and theoretical models have established that SNe Ia are asymmetric, and therefore--like SNe II--potential sources of gravitational wave (GW) radiation. We perform the first detailed calculations of the GW emission for a SN Ia of any type within the single-degenerate channel. The gravitationally confined detonation (GCD) mechanism predicts a strongly polarized GW burst in the frequency band around 1 Hz. Third-generation spaceborne GW observatories currently in planning may be able to detect this predicted signal from SNe Ia at distances up to 1 Mpc. If observable, GWs may offer a direct probe into the first few seconds of the SNe Ia detonation.     

Growth and decay of a two‐dimensional oxide quasicrystal: High‐temperature in situ microscopy

The recently discovered two‐dimensional oxide quasicrystal (OQC) derived from BaTiO₃ on Pt(111) is the first material in which a spontaneous formation of an aperiodic structure at the interface to a periodic support has been observed. Herein, we report in situ low‐energy electron microscopy (LEEM) studies on the fundamental processes involved in the OQC growth. The OQC formation proceeds in two steps via of an amorphous two‐dimensional wetting layer. At 1170 K the long‐range aperiodic order of the OQC develops. Annealing in O₂ induces the reverse process, the conversion of the OQC into BaTiO₃ islands and bare Pt(111), which has been monitored by in situ LEEM. A quantitative analysis of the temporal decay of the OQC shows that oxygen adsorption on bare Pt patches is the rate limiting step of this dewetting process.