[1] We investigate the longitudinal distribution of the vertical E Â B drift velocity and ion density in the lowlatitude ionosphere using the first Republic of China Satellite (ROCSAT-1) data acquired during 1999 -2004. The ROCSAT-1 observations during daytime demonstrate the presence of the longitudinally periodic patterns of the vertical E Â B drift and plasma density on the topside F region (600 km). The four longitude sectors where the peaks in the plasma density are found are coincident with the peaks in the E Â B drift. This observation may indicate the association of the large-scale longitudinal density structure with the daytime E-region dynamo electric field. The density structure exists before the occurrence of the pre-reversal enhancement (PRE) and therefore the PRE is not directly related to this phenomenon. Citation:

No abstract

No abstract

Selectivity in chemical reactions is a major objective in industrial processes to minimize spurious byproducts and to save scarce resources. In homogeneous catalysis the most important factor which determines selectivity is structural symmetry. However, a transfer of the symmetry concept to heterogeneous catalysis still requires a detailed comprehension of the underlying processes. Here, we investigate a ring-closing reaction in surface-confined meso-substituted porphyrin molecules by scanning tunneling microscopy, temperature-programmed desorption, and computational modeling. The identification of reaction intermediates enables us to analyze the reaction pathway and to conclude that the symmetry of the porphyrin core is of pivotal importance regarding product yields.

The fabrication and control of coordination compounds or architectures at well-defined interfaces is a thriving research domain with promise for various research areas, including single-site catalysis, molecular magnetism, light-harvesting, and molecular rotors and machines. To date, such systems have been realized either by grafting or depositing prefabricated metal-organic complexes or by protocols combining molecular linkers and single metal atoms at the interface. Here we report a different pathway employing metal-organic chemical vapor deposition, as exemplified by the reaction of meso-tetraphenylporphyrin derivatives on atomistically clean Ag(111) with a metal carbonyl precursor (Ru3(CO)12) under vacuum conditions. Scanning tunneling microscopy and X-ray spectroscopy reveal the formation of a meso-tetraphenylporphyrin cyclodehydrogenation product that readily undergoes metalation after exposure to the Ru-carbonyl precursor vapor and thermal treatment. The self-terminating porphyrin metalation protocol proceeds without additional surface-bound byproducts, yielding a single and thermally robust layer of Ru metalloporphyrins. The introduced fabrication scheme presents a new approach toward the realization of complex metal-organic interfaces incorporating metal centers in unique coordination environments.

Materials showing reversible resistance switching between high-resistance state and low-resistance state at room temperature are attractive for today’s semiconductor technology. In this letter, the reproducible hysteresis and resistive switching characteristics of metal-CuxO-metal (M-CuxO-M) heterostructures driven by low voltages are demonstrated. The fabrication of the M-CuxO-M heterostructures is fully compatible with the standard complementary metal-oxide semiconductor process. The hysteresis and resistive switching behavior are discussed. The good retention characteristics are exhibited in the M-CuxO-M heterostructures by the accurate controlling of the preparation parameters.

We demonstrate that terminal alkynyl moieties represent powerful functional groups for driving thermally stable, on-surface supramolecular structure formation on a reactive substrate. Through a combination of scanning tunneling microscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption-fine-structure spectroscopy and density functional theory calculations, we investigate the molecule–surface interaction and self-assembly of two prototypical hydrocarbon species on Cu(111). For 1,3,5-tris­(4-ethynylphenyl)­benzene (Ext-TEB) adsorption at low temperature (200 K) results in nonassembling, conformationally adapted intact species. Deprotonation of the terminal alkyne moieties, taking place at temperatures ranging from 300 to 350 K, triggers the formation of room-temperature stable, close-packed supramolecular islands. Through DFT calculations, the stabilizing interaction is identified as a trifurcated ionic C–H···π–δ hydrogen bonding between the π-system of the ionic alkynyl groups and methine moieties of nearby benzene rings, providing an energy gain of 0.26 eV/molecule upon network formation. Robust assemblies result from the combination of this weak directional attraction with the strong surface anchoring also provided by the alkynyl groups. The generality of this novel ionic hydrogen-bonding type is demonstrated by the observation of low-dimensional assemblies of 9,10-diethynyl-anthracene on the same surface, consistently explained with the same type of interaction.

We investigated the synthesis of one-dimensional nanostructures via Schiff base (imine) formation on three close-packed coinage metal (Au, Ag, and Cu) surfaces under ultrahigh vacuum conditions. We demonstrate the feasibility of forming pyrene-fused pyrazaacene-based oligomers on the Ag(111) surface by thermal annealing of tetraketone and tetraamine molecules, which were designed to afford cyclocondensation products. Direct visualization by scanning tunneling microscopy of reactants, intermediates, and products with submolecular resolution and the analysis of their statistical distribution in dependence of stoichiometry and annealing temperature together with the inspection of complementary X-ray photoelectron spectroscopy signatures provide unique insight in the reaction mechanism, its limitations, and the role of the supporting substrate. In contrast to the reaction on Ag(111), the reactants desorb from the Au(111) surface before reacting, whereas they decompose on the Cu(111) surface during the relevant thermal treatment.