Study of the nucleation and growth of TiO2 and ZnO thin films by means of molecular dynamics simulations

The nucleation and growth of titanium dioxide (TiO₂) and zinc oxide (ZnO) thin films on Fe₂O₃ (hematite), Al₂O₃ (α-alumina) and SiO₂ (α-quartz) are studied by molecular dynamics simulations. The results show the formation of a strong interface region between the substrate and the film in the six systems studied here. A combination of polycrystalline and amorphous phases are observed in the TiO₂ films grown on the three substrates. ZnO deposition on the Fe₂O₃ and Al₂O₃ crystals yields a monocrystalline film growth. The ZnO film deposited on the SiO₂ crystal exhibits less crystallinity. The simulation results are compared with experimental results available in the literature.     

Co-Adsorption of CO in NO-CO Reaction on a Metal Catalytic Surface Studied by Computer Simulation

The effect of co-adsorption of CO molecules in the NO-CO reaction on a metal catalytic surface like Pt（001） is studied by applying the Langmuir-Hinshelwood mechanism using the Monte Carlo simulations. The system is investigated by two approaches of NO adsorption; dissociatively at two empty surface sites and molecularly at a single vacant site. The elementary steps are the same as those in the conventional Ziff-Gulari-Barshad model. With the additional reaction step of co-adsorption, the sustained production of CO2 is obtained, which has never been seen on a square lattice without introducing additional parameters. The most interesting result is the elimination of continuous second order phase transition, i.e. the production of CO2 starts as soon as the partial pressure of CO departs from zero, which is in accordance with the experimental observations. The effect of co-adsorption probability on the phase diagrams has also been studied.     

Electronic transport in double-strand poly(dG)-poly(dC) DNA segments

We study the electronic properties of a double-strand quasiperiodic DNA molecule modeled by a one-dimensional effective Hamiltonian, which includes contributions from the nucleobasis system as well as the sugar-phosphate backbone. Our theoretical approach makes use of Dyson's equation together with a transfer-matrix treatment, considering an electronic tight-binding Hamiltonian model to investigate the electronic density of states (DOS) and the electronic transmissivity of sequences of DNA finite segments. To mimic the DNA segments, we consider the finite quasiperiodic sequences of Fibonacci's type, in a poly(dG)-poly(dC) configuration, whose building blocks are the bases guanine G and cytosine C. We compared the electronic transport found for the quasiperiodic structure to those using a sequence of natural DNA, as part of the human chromosome Ch22.     

Exciton formation with interchain couplings in organic polymers

Based on the framework of the tight binding approach and the nonadiabatic dynamics, the formation of an exciton in inter-coupled polymer chains is studied. Both a localized exciton in one single chain and a spread exciton between chains are obtained. It is found that an excited electron-hole pair is more inclined to evolve into a localized exciton, and the long range Coulomb e-e interaction is favorable to the exciton formation. By calculating the formation time of an exciton, we show that the optical response time is faster in a dilute solution than that in a solid film of polymer molecules.     

Photoemission and coincidence studies on gas-phase molecules

This article describes Young’s double-slit experiment using high-energy core-level photoemission from N₂ molecules and experimental identification of interatomic Coulombic decay in Ar₂ dimers after Auger decay using k-resolved electron–ion–ion coincidence spectroscopy, aiming to illustrate the leading edge of gas-phase experiments using synchrotron radiation.     

Synthesis and characterization of (h 0 h)-oriented silicalite-1 films on α-Al2O3 substrates

A simple and well-designed synthesis procedure is proposed to fabricate silicalite-1 films on porous α-Al₂O₃ substrates on purpose of preventing the aluminum leaching. The continuous and 2 μm thick seed layer of silicalite-1 crystals is fabricated by using a spin coater. The first-time seeded growth is performed to synthesize a thin layer of intergrown ZSM-5 crystals on the silicalite-1 seed layer, where the use of low alkalinity and short synthesis time is to reduce the aluminum leaching. The intergrown layer of ZSM-5 crystals serves as a barrier to block the aluminum leaching from porous α-Al₂O₃ substrates in the second-time seeded growth, leading to the formation of ca. 11 μm thick intergrown and oriented silicalite-1 films with an extremely high Si/Al ratio. According to SEM images and XRD measurements, the as-synthesized silicalite-1 film is dense, continuous, and (1 0 1)-oriented. The electron probe microanalysis (EPMA) of the resulting film demonstrates that there is no aluminum leaching in the second-time seeded growth. The leaking tests confirm that non-zeolitic pores in the silicalite-1 film are negligible.     

Inelastic scattering of oppositely charged polarons in conjugated polymers

Within an one-dimensional tight-binding model, we investigate the inelastic scattering processes of oppositely charged polarons in conjugated polymers under the influence of an external electric field, by using a nonadiabatic evolution method. It is found that the polaron pair does not necessarily scatter into an entity(neutral exciton), but a mixed state composed of both polarons and excitons. The yield of the neutral exciton depends sensitively on the strength of applied fields. Additionally, effects of interchain coupling on the scattering processes are also discussed, which shows that the interchain coupling is of fundamental importance and facilitates the formation of the polaron-exciton.     

Relationships between a microscopic parameter and the stochastic equations for interface's evolution of two growth models

The relationship between a microscopic parameter p, that is related to the probability of choosing a mechanism of deposition, and the stochastic equation for the interface's evolution is studied for two different models. It is found that in one model, that is similar to ballistic deposition, the corresponding stochastic equation can be represented by a Kardar-Parisi-Zhang (KPZ) equation where both λ and ν depend on p in the following way: ν(p) = νp and λ(p) = λp ^3/2. Furthermore, in the other studied model, which is similar to random deposition with relaxation, the stochastic equation can be represented by an Edwards-Wilkinson (EW) equation where ν depends on p according to ν(p) = νp ². It is expected that these results will help to find a framework for the development of stochastic equations starting from microscopic details of growth models. Received 26 August 2002 / Received in final form 20 November 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: ealbano@inifta.unlp.edu.ar