Surface effects on photoluminescence of single ZnO nanowires

The influence of surface effects on the temperature dependent photoluminescence (PL) spectra from individual ZnO nanowires has been studied. It is found that the surface effects of the nanowire are very important in both ultraviolet (UV) and visible emission. We propose a new luminescence mechanism based on the recombination related to oxygen vacancies to explain the temperature dependent visible emission, which is significantly influenced by the carrier depletion and band bending caused by surface effects. In addition, the observed attenuation of UV emission with increasing temperature is ascribed to the decreasing depletion region and the increasing surface states related nonradiative recombination.     

Investigation of chemisorbed oxygen, surface segregation and effect of post-treatments on La0.8Sr0.2MnO3 powder and screen-printed layers for solid oxide fuel cell cathodes

In order to better understand the mechanism of the reaction of oxygen reduction at the surface of strontium doped lanthanum manganites (LSM) cathodes in solid electrolyte fuel cells (SOFC), the surface properties of La_0.8Sr_0.2MnO₃ powders and screen-printed layers have been characterised by various techniques.Strontium enrichment at the surface has been evidenced by X-ray photoelectron spectroscopy according to the conditions of annealing (temperature, oxygen pressure) and polarisation treatments of the samples.The interaction between oxygen and La_0.8Sr_0.2MnO₃ for SOFC cathodes has been studied by thermo-programmed desorption, in situ infrared spectrometry and calorimetry. The results indicate that various adsorbed oxygen species may exist on the surface of LSM depending on temperature.The presence of various adsorbed oxygen species and the surface Sr segregation are important factors to consider in the mechanism of oxygen reduction at LSM SOFC cathodes since they could be responsible for many discrepancies between the interpretations that can be found in the literature data.     

Growth mechanisms for wire-like epitaxial gold silicide islands on Si(1 1 0) surfaces

Epitaxial islands grown on various substrates are usually strained because of differences in lattice constants of the materials of the island and the substrate. Shape transition in the growth of strained islands has been proposed as a mechanism for strain relief and a way to form self-organized quantum wires. Shape transition usually leads to an elongated island growth. However, an elongated island growth may also be due to an anisotropic diffusion of material, the anisotropy being imposed by the symmetry of the substrate surface. In the present example, growth of gold silicide wire-like nanostructures on a Si(1 1 0) surface has been investigated by photoemission electron microscopy (PEEM). Growth of elongated unidirectional gold silicide islands, with an aspect ratio as large as 12:1, has been observed by PEEM following gold deposition on the Si substrate and subsequent annealing at the Au-Si eutectic temperature. Distribution of the width and the length of the gold silicide islands as a function of island area shows a feature similar to that for the shape transition. However, detailed investigations reveal that the elongated growth of gold silicide islands is rather mainly due to anisotropic diffusion of gold due to the twofold symmetry of the (1 1 0) surface of the Si substrate.     

Chemical vapor deposition of diamond

In the recent decade a multitude of diamond thin film production methods has been developed, generally based on chemical vapor deposition processes from thermally or plasma activated gas phases. Diagnostic studies, growth experiments and numerical kinetic investigations have in recent years lead to an improved understanding of the prerequisites of continuous diamond growth and of the chemical processes involved. While the mechanism of carbon incorporation into the diamond surface is not yet known completely, the gas-phase species which are essential in a diamond-growth atmosphere can be narrowed to a small number, whose role in the gas-phase chemistry is quite well known.     

Growth of Ag thin films on ZnO(0 0 0 −1) investigated by AES and STM

The growth of Ag films on ZnO(0 0 0 −1) has been investigated by Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). A high density of islands is nucleated at the earliest stages of the growth. An upstepping mechanism causes these islands to coalesce while the uncovered fraction of the ZnO surface remains constant (30%).     

Advanced design of periodical architectures in bulk metals by means of Laser Interference Metallurgy

Patterning of high-resolution features on large-area metallic substrates has been performed by means of the Laser Interference Metallurgy method. Due to the intensity distribution of the interference pattern, this technique allows to locally and periodically heat the material surface to temperatures higher than the melting point with a long-range order. In this study, commercial stainless steel, copper and aluminum substrates were irradiated using single pulses of a nanosecond Nd:YAG laser with two and three laser-beam configurations operating at 355 nm of wavelength. Thermal simulations have been performed by finite element method and compared to the experiments. The results indicate that the structuring is produced by a surface tension driven mechanism induced by the thermal gradient. Moreover, metals with short thermal diffusion lengths present very homogeneous structures and the structure depth that can be achieved at relatively high laser fluences during single-pulse experiments is on the order of the diffusion length.     

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.     

The effect of inactive impurities on a surface in NO-CO reaction: A Monte Carlo simulation

The interaction among the reacting species in the NO-CO reaction on a metal catalytic surface that proceeds according to the Langmuir-Hinshelwood thermal mechanism is studied by means of Monte Carlo simulations. The study of this system is essential for the understanding of the influence of impurities on the catalytic oxidation of NO by CO. It is found that this complex system exhibits irreversible phase transitions between active states with sustained reaction and poisoned states without reaction. The same system has also been investigated by non-thermal (Eley-Rideal) mechanism. Both the phase diagrams of the surface coverage and the steady state production of CO₂ and N₂ are evaluated as a function of the partial pressures of the reactants in the gas phase. From this study, it is observed that with the increase of impurities, the production rate reduces and the reaction stops at a certain point. Moreover, the first order transition in the phase diagram converts into second order phase transition that is in accordance with the experimental findings. Therefore, the first order phase transition, which is a characteristic of such catalytic reactions, is eliminated.     

Density functional study of the adsorption of K on the Cu(111) surface

The adsorption of potassium on the Cu(111) surface in a (2×2) pattern has been simulated with all-electron full-potential density functional calculations. The top site is found to be the preferred adsorption site, with the other highly symmetric adsorption sites being nearly degenerate. The bond length from potassium to the nearest copper atom is computed to be 2.83 ?. Population analysis and density of states indicate that there is no evidence for covalent bonding so that the binding mechanism appears to be a metallic bond. Received 11 April 2001     

Grain segregation mechanism in aeolian sand ripples

Many sedimentary rocks are formed by migration of sand ripples. Thin layers of coarse and fine sand are present in these rocks, and understanding how layers in sandstone are created has been a longstanding question. Here, we propose a mechanism for the origin of the most common layered sedimentary structures such as inverse graded climbing ripple lamination and cross-stratification patterns. The mechanism involves a competition between three segregation processes: (i) size-segregation and (ii) shape-segregation during transport and rolling, and (iii) size segregation due to different hopping lengths of the small and large grains. We develop a discrete model of grain dynamics which incorporates the coupling between moving grains and the static sand surface, as well as the different properties of grains, such as size and roughness, in order to test the plausibility of this physical mechanism. Received 19 July 1999 and Received in final form 4 August 1999     

Growth of thin Si oxide in a cyclic oxygen plasma environment below 200 °C

The growth of Si oxide by means of a cyclic radio-frequency (rf) plasma oxidation process has been explored in a low temperature range of 100-200 °C. The growth mechanism exhibits Cabrera-Mott (CM) oxidation, that is, the transport of mobile ionic species is assisted by an electric field. The low activation energy of 0.3 eV is attributed to the small size of O⁻ and the assistance of the electric field. The oxide becomes off-stoichiometric as one approaches to the exterior surface of the oxide layer.     

An energy approach to the modelling of particle removal by pulsed laser irradiation

An energy model to explain particle removal mechanism has been developed. This model is based on a detailed investigation of contact deformation of a particle on a solid surface, as well as particle motion during the process of substrate surface expansion under uniform laser irradiation. Calculation results show that small particles mainly gain kinetic energy during pulsed laser irradiation, whereas large particles mainly gain elastic deforming potential energy. The particle removal condition is derived from the viewpoint of energy. The relationship of particle removal efficiency with laser fluence and particle size is discussed. Theoretical results are compared with experimental results. Received: 30 July 1998 / Accepted: 14 December 1998 / Published online: 17 March 1999     

Oscillatory zoning caused by oscillating surface relaxations

Oscillatory zoning is a spatial variation in the composition of minerals. It has been observed in many different minerals and a variety of mechanisms have been proposed to explain it. We propose an equilibrium model of oscillatory zoning in which the variations in composition stabilise a ferroelastic phase. This results in a sinusoidal variation in composition. We expect that this mechanism could account for oscillatory zoning found in minerals with oscillatory surface relaxations. Received 9 March 1999     

Structural investigation of n-hexadecanoic acid multilayers on mica surface: Atomic force microscopy study

The structure of n-hexadecanoic acid (HA) multilayers formed by spreading an ethanol solution containing this molecule onto a freshly cleaved mica surface has been studied by atomic force microscopy (AFM). AFM images of multilayers obtained with different coating time showed that HA molecules first formed some sporadic domains on mica surface. With the proceeding of the coating process, these domains gradually enlarged and coalesced, until formed a continuous film finally. It was observed that HA molecules were always adsorbed on mica surface with tilted even-numbered layers structure. The height of the repeated tilted bilayer film was measured to be approximately 3.8 ± 0.2 nm, which implied a ∼60° tilt molecular conformation of the HA bilayers on mica surface. Phase image confirmed that the HA multilayers terminated with the hydrophilic carboxylic acid groups. The formation mechanism of the HA multilayers was discussed in detail. Thus, resulted hydrophilic surfaces are of special interest for further study in biological or man-made member systems.     

Estimation of surface charges on dielectric materials for high power rf windows

The surface discharges observed at rf windows and vacuum circuit breakers (VCBs) are one of the difficulties faced when developing high-power rf windows or compact VCBs. The surface discharge is considered to take place due to the release of the surface charges. Despite the importance of the surface charging/discharging, these phenomena have not been well evaluated. In this paper, the surface charges are estimated using the multipulse method, where electron beam irradiates a sample up to the saturation condition of surface charges. The amount of surface charges on alumina and TiN coated alumina are compared and the charging mechanism is discussed.     

Theoretical study of breaking and slipping processes for HMX/graphite interface

Interface dynamics is an important issue to understand the hot spot formation mechanism in high energy explosives. We have studied the interface between octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and graphite. The former is a good performance explosive but has high sensitivity. The latter is the desensitizer for the former. Two kinds of dynamic processes have been investigated: breaking and slipping. The structure evolution, energy variation, and breaking/slipping stresses were calculated. We found that different interface processes lead to different energy dissipation ways. For breaking, it is by surface relaxation. For slipping, it is by interface friction. Both the two ways contribute to the hot spot formation and shock sensitivity of explosives.     

CO-H2-O2 reaction on a catalytic surface: A computer simulation study

The oxidation of carbon monoxide to form carbon dioxide and the oxidation of hydrogen to form water are the reactions of environmental and industrial importance. These two reactions have been studied independently by Monte Carlo computer simulation using Langmuir-Hinshelwood mechanism but no effort has been made to study the combined CO-H₂-O₂ reaction on these lines. Keeping in view the importance of this 3-component system, the surface coverages and production rates are studied as a function of CO partial pressure for different ratios of H₂ and O₂. The diffusion of reacting species on the surface as well as their desorption from the surface is also introduced to include temperature effects. The phase diagrams of the system are drawn to observe the behavior of these atoms/molecules on the surface and the production of CO₂ and H₂O are determined at different concentrations of H₂. The results are compared with 2-component systems.     

Structural and optical properties of thin films porous amorphous silicon carbide formed by Ag-assisted photochemical etching

In this work, we present the formation of porous layers on hydrogenated amorphous SiC (a-SiC: H) by Ag-assisted photochemical etching using HF/K₂S₂O₈ solution under UV illumination at 254 nm wavelength. The amorphous films a-SiC: H were elaborated by d.c. magnetron sputtering using a hot pressed polycrystalline 6H-SiC target. Because of the high resistivity of the SiC layer, around 1.6 MΩ cm and in order to facilitate the chemical etching, a thin metallic film of high purity silver (Ag) has been deposited under vacuum onto the thin a-SiC: H layer. The etched surface was characterized by scanning electron microscopy, secondary ion mass spectroscopy, infrared spectroscopy and photoluminescence. The results show that the morphology of etched a-SiC: H surface evolves with etching time. For an etching time of 20 min the surface presents a hemispherical crater, indicating that the porous SiC layer is perforated. Photoluminescence characterization of etched a-SiC: H samples for 20 min shows a high and an intense blue PL, whereas it has been shown that the PL decreases for higher etching time. Finally, a dissolution mechanism of the silicon carbide in 1HF/1K₂S₂O₈ solution has been proposed.     

Preparation of flower-like and rod-like boehmite via a hydrothermal route in a buffer solution

Flower-like and rod-like boehmite has been synthesized using a hydrothermal route in a buffer solution. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS) and Fourier transform infrared (FTIR). The SEM and the TEM images of products obtained after different processing times reveal that the buffer solution plays an important role in the rod-like boehmite formation. This approach may allow us to have a better control of the size and morphology of the crystalline boehmite. And the surface area of boehmite hollow microspheres was calculated using Brunauer-Emmett-Teller (BET) model. The possible formation mechanism was proposed and discussed.     

Fabrication and characterization of CuO-core/TiO2-shell one-dimensional nanostructures

We have fabricated CuO-core/TiO₂-shell one-dimensional nanostructures by coating the CuO nanowires with MOCVD-TiO₂. The structure of the core/shell nanowires has been investigated by using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis techniques. The CuO-cores and the TiO₂-shells of the as-synthesized nanowires have been found to have crystalline monoclinic CuO and crystalline tetragonal anatase TiO₂ structures, respectively. The CuO-core/TiO₂-shell nanowires are winding and has rougher surface, whereas the CuO nanowires are straight and have smoother surface.Influence of the substrate temperature and the growth time on the structure such as the morphology, size, and crystallographic orientation of CuO nanowires synthesized by thermal oxidation of Cu foils have also been investigated. All the nanowires have only the CuO phase synthesized at 600 °C, whereas those synthesized at 400 °C have both CuO and Cu₂O phases. The highest density of CuO nanowires with long thin straight morphologies can be obtained at 600 °C. In addition, the growth mechanism of the CuO nanowires has been discussed.