Growth and optical properties of Ge/Si quantum dots formed on patterned SiO2/Si(0 0 1) substrates

Single and stacked layers of Ge/Si quantum dots were grown in SiO₂ windows patterned by electron-beam lithography on oxidized Si (0 0 1) substrates. The growth of a silicon buffer layer prior to Ge deposition is found to be an additional parameter for adjusting the Ge-dot nucleation process. We show that the silicon buffer layer evolves towards [1 1 3]-faceted pyramids, which reduces the area of the topmost (0 0 1) surface available for Ge nucleation. By controlling the top facet area of the Si buffer layers, only one dot per circular window and a high cooperative arrangement of dots on a striped window can be achieved. In stacked layers, the dot homogeneity can be improved through the adjustment of the Ge deposited amount in the upper layers. The optical properties of these structures measured by photoluminescence spectroscopy are also reported. In comparison with self-assembled quantum dots, we observed, both in single and stacked layers, the absence of the wetting-layer component and an energy blue shift, confirming therefore the dot formation by selective growth.     

Viscoelastic behaviour of ferro- and antiferroelectric phases

We have investigated the viscoelastic behaviour exhibited by chiral smectic C* phases in homeotropic orientation. For an appropriate scattering geometry and using the light scattering technique, the orientational diffusivities associated with the Goldsone mode in ferro- and antiferroelectric phases of C₁₁thiobenzoate and C₈tolane compounds have been measured. (SmC_A*) is about 100 times weaker than (SmC*). This divergence may be largely attributed to the mode of the molecular arrangement in ferro- and antiferroelectric phases, in spite of the same helical structure observed in the two phases.     

Liquid crystalline complexes of 4-alkoxystilbazoles with silver 1,12-dodecylene disulphate

A new series of metal complexes of silver(I) 1,12-dodecylene disulphate with 4-alkoxystilbazole ligands has been synthesized. These difunctional silver stilbazole complexes are mesomorphic, showing a nematic phase at short chain lengths and SmA and SmC phases at longer chain lengths. The single crystal X-ray structure of the hexyloxy homologue shows a polymerized arrangement of sulphate-bridged dimers.     

SYNTHESIS AND SPECTROSCOPIC DIFFERENTIATION OF 2- AND 4-ALKOXYTHIOTETRONIC ACIDS

O-Alkylation of thiotetronic acids gives a mixture of 2- and 4-position enol ether products. Comparison of the physical data revealed that UV spectroscopy was the most reliable method of distinguishing between these related ethers. We have determined that 4-position ethers have a distinct absorption between 235-240 nm, while 2-position ethers have two absorbance peaks, one between 205-220 nm and the other between 305-310 nm. This report describes the synthesis and unambiguous characterization of 2- and 4-methoxy-3,5-dimethylthiotetronic acids. The UV absorption properties of several other pairs of thiotetronic acid ethers confirm that these differences are general features that provide a simple method for distinguishing between 2- and 4-substituted isomers.     

Stabilization of discotic liquid organic thin films by ITO surface treatment

Discotic liquid crystals (LCs) are promising materials in the field of electronic components and, in particular, to make efficient photovoltaic cells due to their good charge transport properties. These materials generally exhibit a mesophase in which the disk-shaped molecules can self-assemble into columns, which favorize charge displacement, and may align themselves uniformly on surfaces to form well-oriented thin films. In order to orientate such a columnar thin film on an indium tin oxide (ITO) substrate, the film is heated up to the temperature range of the isotropic liquid phase and subsequently cooled down again. This treatment may lead not only to the desired alignment, but also to dewetting, which leads to an appreciable inhomogeneity in film thickness and to short circuits during the realization of photovoltaic cells. In this article, we describe how this dewetting and the film morphology can be influenced by ITO surface treatments. The chemical modifications of the surface by these treatments were studied by X-ray photoelectron spectroscopy (XPS). Such ITO treatments are shown to be efficient to prevent thin film dewetting when combined with rapid cooling through the isotropic-to-LC phase transition.     

On the use of exchange biased top electrodes in magnetic tunnel junctions

We have investigated the magnetic behavior and the structural properties of ferromagnetic–antiferromagnetic systems (NiFe–IrMn and Co–IrMn) deposited directly on a thin tantalum buffer layer (bottom configuration) or above a thin Al₂O₃ tunnel barrier layer (top configuration). In the bottom configuration, the bilayer system exhibits higher magnetic performances than in the top configuration in terms of thermal stability. We have performed a detailed structural study by X-ray diffraction and high-resolution transmission electron microscopy which allow us to establish a clear correlation between the situation of the bilayer with respect to the tunnel barrier, its texture and its magnetic properties.     

A higher-order finite element scheme for incompressible lubrication calculations

A higher-order finite element scheme is formulated for incompressible lubrication calculations, based on the energy functional of the lubricating system (derived from the variational technique) and the hierarchical approximation concept. The current formulation ensures the pressure continuity across inter-element mating boundaries. Since this is a hierarchical formulation, it facilitates convergence studies of results. Numerical examples are provided to demonstrate the accuracy of the proposed method, the simplicity of modeling, applications, and the convergence characteristics of numerical solutions.     

Local convergence analysis for the REQP algorithm using conjugate basis matrices

The REQP algorithm solves constrained minimization problems using a sequential quadratic programming technique based on the properties of penalty functions. The convergence of REQP has been studied elsewhere (Refs. 1, 2). This paper uses a novel approach to the analysis of the method near to the solution, based on the use of conjugate subspaces. The stepp taken by a constrained minimization algorithm can be thought of as having two components,h in the subspace tangential to the constraints andv in the subspace spanned by the constraint normals. It is usual forh andv to be orthogonal components. Recently, Dixon (Ref. 3) has suggested constructingp from components which are not orthogonal. That is, we writep=h + v, whereh is in the subspace tangential to the constraints and wherev andh are conjugate with respect to the Hessian of the Lagrangian function. By looking at the conjugate components of the REQP search directions, it is possible to simplify the analysis of the behavior near the solution and to obtain new results about the local rate of convergence of the method.This work was supported by a SERC Studentship (TTN).