Dynamics of molybdenum nano structure formation on the TiO2(1 1 0) surface: A kinetic Monte Carlo approach

The rutile TiO₂(1 1 0) surface is a highly anisotropic surface exhibiting “channels” delimited by oxygen rows. In previous experimental and theoretical DFT works we could identify the molybdenum adsorption sites. They are located inside the channels. Moreover, experimental studies have shown that during subsequent annealing after deposition, special molybdenum nano structures can be formed, especially two monolayer high pyramidal chains of atoms.In order to better understand the dynamics of nano structure formation, we present a kinetic Monte Carlo study on diffusion and adsorption of molybdenum atoms on a TiO₂(1 1 0) surface. A quasi one-dimensional lattice gas model has been used which describes the possible adsorption sites of a Mo atom in a single channel of the surface. The atomic positions of a 1.5 monolayer thick Mo film formed of pyramidal chains define the lattice sites of the model. Thereby the formation of three-dimensional clusters could be studied. Here we have studied the cluster formation as a function of parameters that can be controlled in a growth experiment by physical vapor deposition: deposition and annealing temperature, flux and total amount of deposited Mo. Good qualitative agreement with recent experiments is obtained.     

Electromagnetic properties of strange baryons in a relativistic quark model

We present some of our results for the electromagnetic properties of excited Σ hyperons, computed within the framework of the Bonn constituent-quark model, which is based on the Bethe-Salpeter approach. The seven parameters entering the model are fitted against the best-known baryon masses. Accordingly, the results for the form factors and helicity amplitudes are genuine predictions. We compare with the scarce experimental data available and discuss the processes in which Σ ^*'s may play an important role.     

Preparation of activated carbons from cherry stones by activation with potassium hydroxide

Using cherry stones, the preparation of activated carbon has been undertaken in the present study by chemical activation with potassium hydroxide. A series of KOH-activated products was prepared by varying the carbonisation temperature in the 400-900 °C range. Such products were characterised texturally by gas adsorption (N₂, −196 °C), mercury porosimetry, and helium and mercury density measurements. FT-IR spectroscopy was also applied. The carbons prepared as a rule are microporous and macroporous solids. The degree of development of surface area and porosity increases with increasing carbonisation temperature. For the carbon heated at 900 °C the specific surface area (BET) is 1624 m² g⁻¹, the micropore volume is 0.67 cm³ g⁻¹, the mesopore volume is 0.28 cm³ g⁻¹, and the macropore volume is 1.84 cm³ g⁻¹.     

Catalytic supports on the base of activated anthracites and synthetic carbons

Selective adsorption of platinum group metals (PMG) on activated carbons from a multi-component model and technological solutions was proposed for the preparation of heterogeneous-supported catalysts. Activated natural anthracites and a nitrogen-containing synthetic carbon are considered as carriers for Pd-supported catalysts. Their catalytic activity was studied in the Pd-catalyzed reactions of hydrogen peroxide decomposition and chloride ions oxidation by manganese(III). On the base of the obtained results, novel high sensitive analytical methods both for direct determination of supported-metal quantity and palladium oxidation states on the surface of spent adsorbents are suggested.     

Superheating in linear polymers studied by ultrafast nanocalorimetry

To study phase transition kinetics on submillisecond time scale a sensitive ultrafast nanocalorimeter was constructed. Controlled ultrafast cooling, as well as heating, up to 10⁶K/s was attained. The method was applied for the measurements of the superheating phenomenon in a set of linear polymers: iPS, PBT, PET, and iPP. A power law relation between the superheating and the heating rate holds in the heating rate range 10^-2-10⁴K/s. A limiting superheating of about 10% of the melting temperature was observed at rates above 10⁴-10⁵K/s. This limit depends on annealing conditions before sample melting. The observed superheating limit, as well as the power law, can be accounted for the internal stresses near the crystalline amorphous interface in semicrystalline polymers induced by heating, which are related to the thermal expansion gradients inherent in a semicrystalline material.     

Theory of spin transfer phenomena in magnetic metals and semiconductors

We propose a general theory of the spin-transfer effects that occur when current flows through inhomogeneous magnetic systems. Our theory does not rest on an appeal to conservation of total spin, can assess whether or not current-induced magnetization precession and switching in a particular geometry will occur coherently, and can estimate the efficacy of spin-transfer when spin-orbit interactions are present. We illustrate our theory by applying it to a toy-model twodimensional-electron-gas ferromagnet with Rashba spin-orbit interactions.     

Initial growth of Boltzmann entropy and chaos in a large assembly of weakly interacting systems

We introduce a high-dimensional symplectic map, modeling a large system, to analyze the interplay between single-particle chaotic dynamics and particles interactions in thermodynamic systems. We study the initial growth of the Boltzmann entropy, S_B, as a function of the coarse-graining resolution (the late stage of the evolution is trivial, as the system is subjected to no external drivings). We show that a characteristic scale emerges, and that the behavior of S_B vs t, at variance with the Gibbs entropy, does not depend on the resolution, as far as it is finer than this scale. The interaction among particles is crucial to achieve this result, while the rate of entropy growth, in its early stage, depends essentially on the single-particle chaotic dynamics. It is possible to interpret the basic features of the dynamics in terms of a suitable Markov approximation.     

Metastability of gold-carbonyl cluster complexes, Au N(CO) M -

Smaller gold-cluster anions, typified by Au ₇ ^- , adsorb multiple CO molecules in a high-pressure, room-temperature flow-reactor, tending toward previously unknown saturation compositions, Au₇(CO) ₄ ^- . The weakness of the gold-carbonyl adsorption bond is evidenced indirectly by the high CO partial pressure required and more directly by the high probability of fragmentation in the field-free flight region of the reflectron-type time-of-flight mass spectrometer. The analysis of this metastability reveals that the actual distribution f_N,M of products Au₇(CO) _M ^- in the reactor may be highly non-statistical, e.g. with only even-M species present. Received 17 April 2001     

Cluster size dependence of SiO2 thin film formation by O2 gas cluster ion beams

Cluster size effects of SiO₂ thin film formation with size-selected O₂ gas cluster ion beams (GCIBs) irradiation on Si surface were studied. The cluster size varied between 500 and 20,000 molecules/cluster. With acceleration voltage of 5 kV, the SiO₂ thickness was close to the native oxide thickness by irradiation of (O₂)_20,000 (0.25 eV/molecule), or (O₂)_10,000 (0.5 eV/molecule). However, it increased suddenly above 1 eV/molecule (5000 molecules/cluster), and increased monotonically up to 10 eV/molecule (500 molecules/cluster). The SiO₂ thickness with 1 and 10 eV/molecule O₂-GCIB were 2.1 and 5.0 nm, respectively. When the acceleration voltage was 30 kV, the SiO₂ thickness has a peak around 10 eV/molecule (3000 molecules/cluster), and it decreased gradually with increasing the energy/molecule. At high energy/molecule, physical sputtering effect became more dominant process than oxide formation. These results suggest that SiO₂ thin film formation can be controlled by energy per molecule.     

Charge transport in a polypeptide chain

Charge transport is one important example of signal transduction in a protein which is responsible for action at a distance, and is a fundamental process in biochemical action. A model is presented in which electronic effects interact with motional processes to combine into a bifunctional model. This model is investigated with new detailed molecular dynamics calculations and successfully explains such action at a distance. Received 1st February 2002 / Received in final form 26 May 2002 Published online 13 September 2002