Long-ranged attraction between charged polystyrene spheres at aqueous interfaces

We report an optical and atomic force microscopic study of interactions between charged polystyrene spheres at a water-air interface. Optical observations of bonded particle clusters and formation of circular chainlike structures at the interface demonstrate that the interaction potential is of dipole origin. Atomic force microscope phase images show patchy domains on the colloidal surface, indicating that the surface charge distribution is not uniform as is commonly believed. Such surface heterogeneity introduces in-plane dipoles, leading to an attraction at short interparticle distances.     

Electronic structure at the PTCDA/GaAs and NTCDA/GaAs interfaces

The formation of the interface between the GaAs(100) single-crystal surface and PTCDA and NTCDA organic semiconductors is investigated. The method of total current spectroscopy makes it possible to trace the formation of the interfacial electronic structure. The two organic materials and the GaAs substrate are bonded together when the π electron cloud of an aromatic ring spreads toward the substrate. This modifies the electronic states of interfacial organic molecules and generates a dipole at the interface.     

Electronic structure at rubrene metal interfaces

Many studies have been done on low energy (1–200 keV) and high dose (10¹⁶–10¹⁷) implantation of Mn in GaAs. This study is an attempt to incorporate Mn ions in GaAs through implantation of 1 MeV Mn⁺¹ ions in semi-insulating GaAs substrates at doses of 3×10¹⁵/cm² and subsequent annealing. This was done to find out if any alloy of Mn–Ga–As, or binary compounds of Mn–Ga or Mn–As form due to annealing of Mn⁺¹ ions implanted in GaAs substrates. High Resolution XRD (HRXRD) performed before annealing shows a possibility of Ga–Mn–As alloy formation, and after annealing at 800°C, except for GaAs main peaks no other phase peaks were detected. Scanning electron microscopy (SEM) shows nanostructures of various dimensions which are thought to be formed due to the defects generated due to implantation. Fourier Transform Infrared (FTIR) study shows the shift in bandgap due to Mn doping. Raman spectroscopy shows the red shift in LO and TO peak positions of GaAs after annealing, which indicates the presence of disorder and damage due to implantation. Resistivity measurement shows a thermally activated semiconductor character of charge conduction with an activation energy of 51 meV and this activation may have occurred through the transitions from impurity band to valence band. Large positive (∼25%) magnetoresistance and a mixture of ferromagnetic and paramagnetic behavior obtained in the magnetization measurement indicate the presence of ferromagnetic MnAs nanoclusters embedded in paramagnetic GaAs:Mn matrix.     

Molecular and supramolecular structure of sulfonated polystyrene ionomers in solutions

The structure of sulfonated polystyrene ionomers in chloroform was studied by molecular hydrodynamics and neutron scattering methods. The ionomer containing 1.35 mol % of ionogenic SO₃Na groups undergoes the coil-globule transition and transforms to a micellar structure due to attraction of polar groups concentrated in the core surrounded by nonpolar chain fragments. At a sulfonation degree of 2.6 mol %, selfassembly of SO₃Na groups with adjacent chain segments to the structure such as a spherical layer, i.e., vesicle (hollow micelle with solvent) with boundaries formed by nonpolar chain fragments, is observed. Vesicular structures in solution associate in pairs. Intra- and interchain associations are significantly enhanced by hydrogen bonds in ionomers with acid groups SO₃H (2.6 and 4.47 mol %) which stabilize micellar structures, promoting their assembly into chain clusters and fragments of the simple cubic lattice.     

Electronic structure and adhesive energies at bimetallic interfaces

The electronic structure at a bimetallic interface is obtained using the density functional formalism and a gradient expansion. The charge distribution near the surface is obtained variationally using a simple parameterised form. Application is made to calculate the adhesive energies due to microscopic contacts of pairs of alkali metals.     

Sulfur at nickel-alumina interfaces: Molecular orbital theory

An atom superposition and electron delocalization molecular orbital study has been made of surface atomic layer relaxations in Ni(110) and Ni(111), the binding of p(2 × 2)S to Ni(111), S on Ni(100), and the binding of A1₂O₃ to clean Ni(111) and p(2 × 2)S covered Ni(111). Surface Ni atom relaxations for three-layer thick cluster models are calculated to be close to the experimental results and S heights of 1.68 Å on Ni(111) and 1.40 Å on Ni(100) are also in close agreement with experimental determinations, which indicates cluster models are suitable for determining surface properties. Al³⁺ in the basal plane of Al₂O₃ are predicted to support a low-lying surface state band in the O2p---A13s3p band gap. These orbitals are found to participate in strong Al---Ni bonds and Al---S bonds of intermediate strength at the interfaces with Ni(111) and p(2 × 2) S-covered Ni(111). Should such bonds form, they are expected to be too few in number to lead to strong adhesion. The strongest Al₂O₃---Ni bonding is predicted to occur when the Ni surface is oxidized. In this case, though Ni---O bonds are relatively weak, their number is high. It is concluded from the structure models that are studied that the segregation of impurity S in Ni to the interface will markedly decrease the adhesion of protective Al₂O₃ films that grow on NiAl based alloys.     

A neutron reflectometry study of polystyrene network interfaces

We have used neutron reflectometry to measure interfacial widths between two polystyrene films, where either one or both films are crosslinked. The observed interfacial width between two networks is larger than the size expected for “dangling ends”, which suggests motion of heterogeneous regions of the networks. In the case when one of the networks is replaced by a linear polymer, the interfacial profile can be asymmetric with a diffusion “front” of linear polymer penetrating the network to a length scale of up to 200 ?. In the case of a more densely crosslinked network and a high molecular weight linear polymer the interface is symmetric implying negligible penetration. Received: 4 December 1996 / Revised: 13 October 1997 / Accepted: 23 December 1997     

Dynamics of Polaron at Polymer/Polymer Interface

The migration of a polaron at polymer/polymer interface is believed to be of fundamental importance for the transport and light-emitting properties of conjugated polymer-based light emitting diodes. Based on the one- dimensional tight-binding Su-Schrieffer-Heeger （SSH） model, we have investigated polaron dynamics in a one- dimensional polymer/polymer system by using a nonadiabatic evolution method. In particular, we focus on how a polaron migrates through the conjugated polymer/polymer interface in the presence of external electric field. The results show that the migration of polaron at the interface depends sensitively on the hopping integrals, the potential barrier induced by the energy mismatch, and the strength of applied electric field which increases the polaron kinetic energy.     

Charge-transfer at silver/phthalocyanines interfaces

Valence band photoemission spectroscopy (VB-PES) and inverse photoemission spectroscopy (IPES) were employed to determine the occupied and unoccupied density of states upon silver deposition onto layers of two phthalocyanines (H₂Pc and CuPc). The two different Pc molecules give rise to very distinct behaviour already during the initial stage of silver deposition. While in the CuPc case no shift occurs in the energy levels, the H₂Pc highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are shifting simultaneously by 0.3 eV, i.e., the HOMO shifts away from the Fermi level while LUMO shifts towards the Fermi level. As the silver quantity increases the HOMO levels of both Pcs are shifting towards the Fermi level. When the Fermi level is resolved in the VB spectra, the characteristic features of H₂Pc and CuPc are smeared out to some extent. Shifts in HOMO and LUMO energy positions as well as changes in line shapes are discussed in terms of charge-transfer and chemical reactions at the interfaces.     

Misfit disclinations at crystal/crystal and crystal/glass interfaces

Theoretical concepts have been developed for a new type of misfit defects, misfit disclinations, at crystal/crystal and crystal/glass interfaces. It is shown, in particular, that the formation of misfit disclinations is an efficient physical micromechanism of misfit stress relaxation at crystal/crystal interfaces. A model describing misfit disclinations at crystal/glass interfaces has been constructed. The energy characteristics of phase boundaries with misfit disclination ensembles are estimated. Fiz. Tverd. Tela (St. Petersburg) 41, 1637–1643 (September 1999)     

A method of determining the charge trapped at the interfaces of a metal/ferroelectric/metal thin-film structure

A method is developed for determining the trap density at the metal/ferroelectric interfaces in a completely depleted ferroelectric film with two Schottky barriers. The method is based on the recharging of traps induced by an external pulsed bias. The ranges of the bias fields and of the parameters of the metal/ferro-electric/metal structure for which analytical solution of the Poisson equation is possible are found. Using this method and measurements of the transient current, the density of the charge trapped at the upper and lower interfaces of Pt(Ir)/PZT/Ir(Ti/SiO₂/Si) capacitors is determined. The interface charge as estimated from the trap density proved to be much smaller than the residual polarization of the PZT film. The observed correlation between the symmetry of the interface trap charges and the symmetry of the hysteresis loops and switching currents indicates the reliability of the estimation of the trap density determined using the proposed method.     

Simulation of wetting and drying at solid-fluid interfaces on the Delft Molecular Dynamics Processor

The adsorption is studied of a fluid at a structured solid substrate by means of computer simulations on the Delft Molecular Dynamics Processor. Two types of particles are present, 2904 of one type for building a three-layer substrate and about 8500 of another type for composing the fluid. Interactions between like and unlike atoms are modeled by pair potentials of Lennard-Jones form cut off at 2.5. Simulations are performed at constant temperature and variable ratio of substrate-adsorbate to adsorbate-adsorbate attraction. On the basis of measurements of density profiles, coverages, surface tensions, and contact angles, a wetting as well as a drying phase transition have been identified. Both transitions are of first order.     

Molecular dynamics simulation of binary hard sphere crystal/melt interfaces

The structure and thermodynamics of the (100) and (111) disordered face-centred cubic (fcc) crystal/melt interfaces for a binary hard sphere system have been examined using molecular dynamics simulation. This study is an extension of previous work (Davidchack, R. L., and Laird, B. B., 1996, Phys. Rev. E, 54, R5905), in which preliminary data for the (100) interface were reported. Density and diffusion profiles on both fine and coarse grained scales are calculated and analysed, leading to the conclusion that equilibrium interfacial segregation is minimal in this system.     

Low temperature reactions at Si/metal interfaces; What is going on at the interfaces?

Si-metal contacts play one of the most important roles in Si-device or integrated circuit technology represented by IC, LSI and VLSI. The role of the contacts is to interconnect individual devices (transistors, diodes and so on) in a Si chip and connect them as a whole to the outer circuit. These contacts are numerous in the chip and can easily total more than one million. Therefore, the establishment of criteria for providing stable and reproducible electrical (ohmic or Schottky barrier) contacts has been a key problem in Si-LSI technology. Si is a typical covalent semiconductor with a large bond energy (≈eV/ bond) and consequently its melting point is high (≈1440 °C). However,crucial to the Si-metal contact problem is that when Si is in contact with metal it readily reacts with it at a temperature as low as ≈100 °C. This interfacial reaction induces large scale transport of materials across the Si/metal interface to give rise to several interesting phenomena. Examples of this are thick (≈1000 Å) SiO₂ growth for a short time (≈10 min) due to Si-Au reaction and uniform silicide layer formation at Si/Pd, Pt, Ni interfaces. Interesting point is that since the Si-Si covalent bonding is very strong without the presence of such effect of metal to weaken the Si bond adjacent to the metal, the above contact or interfacial reactions can rarely occur. As possible mechanism of this bond-weakening, several models have been proposed. One of them is by the present author postulating electronic screening of Coulomb interaction responsible for the covalent bonding due to mobile electrons in the metal films. In the present article, this “screening model” is critically discussed and compared with other models on the basis of experiments done by several laboratories on microscopic or atomic-scale observation of initial stages of the Si-Au and Si-Pd reactions by both electron and ion scattering spectroscopies. In addition new usage of the channeling effect of MeV He⁺ ions is demonstrated to be a powerful tool for interface and surface studies.     

Origin of fine structure in si photoelectron spectra at silicon surfaces and interfaces

Using a first-principles approach, we investigate the origin of the fine structure in Si 2p photoelectron spectra at the Si(100)-(2 x 1) surface and at the Si(100)-SiO2 interface. Calculated and measured shifts show very good agreement for both systems. By using maximally localized Wannier functions, we clearly identify the shifts resulting from the electronegativity of second-neighbor atoms. The other shifts are then found to be proportional to the average bond-length variation around the Si atom. Hence, in combination with accurate modeling, photoelectron spectroscopy can provide a direct measure of the strain field at the atomic scale.