Low energy ion assisted atomic assembly of metallic superlattices

Metallic superlattices with planar, unalloyed (unmixed) interfacial structures are difficult to fabricate by all conventional vapor deposition methods. Molecular dynamics simulations have been used to explore the ways in which inert gas ions can be used to control the atomic assembly of a model Cu/Co metallic super lattice system. High energy, high atomic weight ions are shown to smooth rough interfaces but introduce undesirable intermixing at interfaces. Light ions with very low energies fail to flatten the rough surfaces that are naturally created during deposition at ambient temperature where surface atom mobility is kinetically constrained. The optimum energies for achieving the lowest combination of interfacial roughness and interlayer mixing have been found for each inert gas ion species and the key mechanisms of surface structure reorganization activated by ion impacts have been identified over the range of ion masses and energies studied. Optimum ion energies that maximize the interface structural perfection have been identified.     

Surface/interface issues in THz electronics

Basic THz elements are produced by standard semiconductor science and technology. Therefore, three main material systems are used. These are first of all semiconductors, for active and passive layer formation; metals, for interconnect and contact formation; and insulators, for passivation and isolation purposes. Additionally, these materials are structured in order to produce a device with desired dimensions and characteristics. Semiconductor surfaces, in particular suffer considerable changes during technological processing. Thus, surface and interface issues are essential here to be considered. Semiconductor–dielectric, semiconductor–metal, and semiconductor–semiconductor interfaces as well as surface effects for the case of GaAs are discussed in detail from the point of view of Schottky diodes and heterostructure-based devices for THz applications.     

Development of novel materials through interface engineering

The impact of interfaces in the processing and properties of polycrystalline material is briefly discussed. The local properties of the interfacial layer are considered in terms of composition, structure and related properties that differ substantially from those of the bulk phase. It has been postulated that novel materials with desired properties for specific industrial applications may be processed through interface engineering rather than through bulk chemistry. This paper considers the impact of interfaces on the properties of materials for technological applications, such as electrochemical devices for reduction of greenhouse gases, through energy conversion and environmental monitoring. The procedures that may be applied for the modification of interfacial chemistry are considered.     

Modeling Three-Dimensional Multiphase Flow Using a Level Contour Reconstruction Method for Front Tracking without Connectivity

Three-dimensional multiphase flow and flow with phase change are simulated using a simplified method of tracking and reconstructing the phase interface. The new level contour reconstruction technique presented here enables front tracking methods to naturally, automatically, and robustly model the merging and breakup of interfaces in three-dimensional flows. The method is designed so that the phase surface is treated as a collection of physically linked but not logically connected surface elements. Eliminating the need to bookkeep logical connections between neighboring surface elements greatly simplifies the Lagrangian tracking of interfaces, particularly for 3D flows exhibiting topology change. The motivation for this new method is the modeling of complex three-dimensional boiling flows where repeated merging and breakup are inherent features of the interface dynamics. Results of 3D film boiling simulations with multiple interacting bubbles are presented. The capabilities of the new interface reconstruction method are also tested in a variety of two-phase flows without phase change. Three-dimensional simulations of bubble merging and droplet collision, coalescence, and breakup demonstrate the new method's ability to easily handle topology change by film rupture or filamentary breakup. Validation tests are conducted for drop oscillation and bubble rise. The susceptibility of the numerical method to parasitic currents is also thoroughly assessed.     

Analytic Approximations for the Velocity of Field-Driven Ising Interfaces

We present analytic approximations for the field, temperature, and orientation dependences of the interface velocity in a two-dimensional kinetic Ising model in a nonzero field. The model, which has nonconserved order parameter, is useful for ferromagnets, ferroelectrics, and other systems undergoing order–disorder phase transformations driven by a bulk free-energy difference. The solid-on-solid (SOS) approximation for the microscopic surface structure is used to estimate mean spin-class populations, from which the mean interface velocity can be obtained for any specific single-spin-flip dynamic. This linear-response approximation remains accurate for higher temperatures than the single-step and polynuclear growth models, while it reduces to these in the appropriate low-temperature limits. The equilibrium SOS approximation is generalized by mean-field arguments to obtain field-dependent spin-class populations for moving interfaces, and thereby a nonlinear-response approximation for the velocity. The analytic results for the interface velocity and the spin-class populations are compared with Monte Carlo simulations. Excellent agreement is found in a wide range of field, temperature, and interface orientation.     

Plasma processing

Low pressure, non-equilibrium, weakly to partially ionized gas discharge plasmas are used for a variety of surface materials processing applications. The most extensive applications are in microelectronics manufacturing, where plasma sputtering, etching, stripping, cleaning and film deposition play key roles in this growing industry. Up to 30% of all process steps in integrated circuit manufacture involve low pressure plasmas in one way or another. The rapid pace of process and product technological change in this industry, coupled with the unique capabilities of plasma processing for extremely finely controlled surface modification, offers new opportunities to plasma scientists     

Ion-induced secondary electron and negative ion emission from Mo/O

Absolute yields of secondary electrons and negative ions resulting from collisions of Na⁺ with Mo(100) and a polycrystalline molybdenum surface have been measured as a function of the oxygen coverage of the surface for impact energies below 500 eV. The sputtered negative ions have been identified with mass spectroscopy, and O⁻ is found to be the dominant sputtered negative ion for the surfaces at all oxygen coverages and impact energies. Both the electron and O⁻ yields have an impact energy threshold at about 50 eV and exhibit a strong dependence on oxygen coverage. The kinetic energy distributions of the secondary electrons and sputtered O⁻ were determined as functions of the oxygen coverage and impact energy. The distributions for O⁻ are characterized by a narrow low-energy peak (at 1–2 eV) followed by a low-level high-energy tail. The secondary electrons have a narrow (FWHM 1–2 eV) kinetic energy distribution, centered approximately at 1–2 eV. The shapes of the distributions and their most probable energies are essentially invariant with impact energy, oxygen coverage and the nature of the Mo surface. The emission is explained and analyzed in terms of a simple model which involves a collision-induced electronic excitation of the MoO⁻ surface state. The decay of this excited state leads to the production of both secondary electrons and O⁻ with energy distributions and yields comparable to those observed.     

Low-Voltage Membrane Interface for Ion Extraction from Polar Solutions

Electric field that extracts ions from polar solutions to gas phase is calculated for membrane interface with electrode located on the vacuum surface of the membrane. Such a structure can be used to significantly decrease the voltage that provides efficient emission of ions from solution filling the membrane channel and, hence, improve the characteristics of membrane interface.     

A front-tracking/ghost-fluid method for fluid interfaces in compressible flows

A front-tracking/ghost-fluid method is introduced for simulations of fluid interfaces in compressible flows. The new method captures fluid interfaces using explicit front-tracking and defines interface conditions with the ghost-fluid method. Several examples of multiphase flow simulations, including a shock–bubble interaction, the Richtmyer–Meshkov instability, the Rayleigh–Taylor instability, the collapse of an air bubble in water and the breakup of a water drop in air, using the Euler or the Navier–Stokes equations, are performed in order to demonstrate the accuracy and capability of the new method. The computational results are compared with experiments and earlier computational studies. The results show that the new method can simulate interface dynamics accurately, including the effect of surface tension. Results for compressible gas–water systems show that the new method can be used for simulations of fluid interface with large density differences.     

Improvement of Mechanical Properties and Ultraviolet Resistance of Polyethylene Pipe Materials Using High Density Polyethylene Matrix Grafted Carbon Black

In recent years, high grade high density polyethylene (HDPE) pipe materials are being more and more widely used for water and gas supply. Carbon black (CB) is usually used as an anti-UV-light reagent for pipe materials. However, homogeneous dispersion of CB in the HDPE matrix and modification of the interface has always been a great challenge. In this work, HDPE matrix grafted CB (HDPE-g-CB) was successfully prepared through HDPE radicals formation by a thermo-mechanical method and subsequent radical capture by the CB surface. The weight percentage of grafted HDPE approached 10 wt% and the modification sharply reduced the surface free energy of the CB. The SEM (scanning electron micrographs) and TEM (transmission electron microscopy) results showed that HDPE-g-CB was uniformly dispersed in the HDPE pipe materials and the domain size of the dispersed phase was remarkably decreased from that in HDPE/CB. Therefore, compared with the HDPE/CB, the mechanical properties and ultraviolet (UV) resistance of HDPE/HDPE-g-CB were significantly improved, positively influencing the expected life span of pipelines.     

Interfacial strengthening of high-impact polypropylene compounds by reactive modification

The effects of reactive modification on the interfacial strength in high-impact polypropylene materials have been investigated using a model system and demonstrated in two specific examples on high flow materials for thin wall injection moulding and reinforced compounds for engineering applications. Using Borealis' proprietary modification technology, the modification degree was controlled by varying the free radical initiator and co-agent content, respectively. The influence of the modification degree on the formation of ethylene-propylene copolymer grafted to polypropylene, which is believed to be the main reason for the strengthened interface between the PP matrix and EPR particles, the phase morphology and material properties was investigated. Depending on practical application requirements, a well defined material performance such as toughness, flowability, surface stability, etc. could be adjusted.     

Band bending at the Si(1 1 1)–SiO2 interface induced by low-energy ion bombardment

Experiments employing photoreflectance spectroscopy have uncovered band bending due to electrically active defects at the Si(1 1 1)–SiO₂ interface after sub-keV Ar⁺ ion bombardment. The band bending of about 0.5 eV resembles that for Si(1 0 0)–SiO₂, and both interfaces exhibit two kinetic regimes for the evolution of band bending upon annealing due to defects healing. The healing takes place about an order of magnitude more quickly at the (1 1 1) interface, however, probably because of less fully saturated bonding and higher compressive stress.     

<Emphasis Type="Italic">Ab initio</Emphasis> simulation of interface reactions as a foundation of understanding polymorphism

Chemical reactions at solid/liquid or solid/gas hybrid interfaces govern the morphogenesis of growing solid state phases. In order to understand the polymorphism of solids, the understanding of the mechanisms and kinetics of these reactions is of crucial importance, as is an insight into the morphology of said surfaces on an atomistic scale. Ab initio simulations are a valuable tool to obtain these data, especially in materials design, where many possible materials for an application may have to be examined. In such cases the fabrication of all these materials for examination and characterization can be extremely time consuming and expensive. We present methods to determine surface properties and reaction energetics from ab initio simulations and demonstrate their potential using three examples: reaction energetics for the adsorption of adhesive component molecules on alumina surfaces, the functionalization of a silica surface with a fluorocarbon layer and the investigation of the wetting behavior of the functionalized surface, and the calculation of reaction energetics for a proton transfer process in the bacterial reaction centre of photosynthesis, including quantum mechanical effects as well as solvent and continuum electrostatic influences from the surrounding medium.     

Reflection anisotropy spectroscopy: A new probe for the solid-liquid interface

Introducing reflection anisotropy spectroscopy (RAS) as a new probe for solid-liquid interfaces, we present results for the Au(110)/electrolyte interface which serves as a model system. We demonstrate that RAS is sensitive to surface phase transitions, step morphology, and electronic surface states. Using an empirical approach, the RA spectra are reproduced and features are identified which reflect the known character of the bias voltage driven (2x1) to (1x1) phase transition. RAS is established as an experimental technique to probe the electronic structure of solid-liquid interfaces in real time to study a wide range of interface properties.     

Effect of elastic characteristics of the surface layer on the deformation properties of materials with an interface-controllable structure

Computer simulation is used for analyzing the possibility of changing the ultimate strain in samples of “interface” materials whose mechanical behavior is determined by strain localization at the interfaces of structural elements (blocks, grains, etc.) by controlled modification of surface layers. It is shown that a considerable improvement of the deformability of samples subjected to cyclic loading can be attained by reducing the Young modulus and the elastic limit of interfaces in the surface layer. This effect can be explained by the large volume of the material involved in irreversible strain accumulation, which suppresses strain localization in the vicinity of stress macroconcentrators and delays the crack formation.     

Structural phase diagram for ZnS nanocrystalline thin films under swift heavy ion irradiation

The synthesis of nanocrystalline ZnS thin films by pulsed laser deposition and their modification by swift heavy ions are presented. The irradiations with 150 MeV Ni ions at fluences of 1×10¹¹, 1×10¹² and 1×10¹³ ions/cm² have been used for these studies. Irradiation results in structural phase transformation and bandgap modification of these films are investigated by using X-ray diffraction and UV-visible absorption measurements, respectively. Since stoichiometry changes induced by irradiation can contribute to the modification of these properties, elastic recoil detection analysis has been performed on pristine and 150 MeV Ni ions irradiated ZnS thin films using a 120 MeV Ag ion beam. The stoichiometry of the films has been found to be similar for pristine and ion irradiated samples. A structural phase diagram based on thermal and pressure spikes has been constructed to explain the structural phase transformation.     

Interfacial reactions and microstructure control in metal and intermetallic matrix composite processing

The achievement of the potential of composites for demanding applications where high specific strength and stiffness are required together with useful toughness is often limited by the available materials processing capabilities to develop specific reinforcement-matrix interface characteristics. Similarly, the elevated temperature stability of advanced composites is dominated by the behavior of internal interfaces. In order to develop effective processing strategies and stable composite designs, it is essential to consider the relevant phase diagrams which for most composites are at least of ternary order. On the basis of these diagrams, it is possible to select compositions of phases which possess desirable properties. In addition to phase diagram data, kinetic data such as the interdiffusion pathway and rates are required to understand and to control the possible interfacial chemical reactions in a composite system. From this basis, reinforcement coatings and barrier layers can be developed to control reactions and allow for in-service lifetime analysis. With solidification processing of composites, melt-reinforcement interactions involved in wetting, solidification reactions, and matrix microstructure evolution can also be evaluated in terms of the phase equilibria and kinetic pathways. Some applications of this approach have been demonstrated in the development of Ti- and A1-based composite systems.     

Mechanisms of electron-stimulated desorption of protons from water: Gas,chemisorbed and ice phases

The stimulated desorption of ions from gas phase and condensed phase H₂O on Ni(111) has been examined theoretically and experimentally for the near threshold excitation region, 15 to 40 eV. The excited state potential energy curves have been calculated using configuration interaction for H₂O and a restricted Hartree-Fock (RHF) approach for a variety of small clusters including (H₂O)₅ and NiH₂O. Both proton yield and kinetic energy distributions have been measured for chemisorbed, ice phase, and gas phase water and are discussed in terms of specific electronic excitations corresponding to possible desorption pathways. For condensed phase water, the major proton desorption threshold occurs at 20–21 eV and is due to surface predissociation. The final state potential energy curves reached in this process are, in general, described by two electron excitations from the ground state and are thus not dipole allowed. At threshold, these potential energy curves correspond to the excited states of the neutral rather than the ionized molecule. Above 28–29 eV, predissociation or shake-up involving excitations from the O 2s orbital contributes to the ion yield and can give rise to protons of high (7–8 eV) kinetic energy.