Design and elaboration of colloidal molecules: an overview

The concept of colloidal molecules was first evoked by van Blaaderen in 2003 for describing small non-spherical colloids made of the aggregation of a small number of particles. He predicted original properties to the complex assemblies of such colloids, in particular in optics. This critical review deals with the different strategies reported for creating robust clusters of spherical particles which could mimic the space-filling models of simple conventional molecules. These routes concern either the controlled clustering of preformed colloids directed by coalescence, physical routes, chemical routes, or 2-D/3-D geometrical confinement, or strategies starting from a single colloid which is decorated by satellite colloids by taking advantage of controlled phase separation or nucleation and growth phenomena. These routes are compared from the viewpoint of the accessible shapes, their tunability and scalability (146 references).     

A comparison of lower bounds for the symmetric circulant traveling salesman problem

When the matrix of distances between cities is symmetric and circulant, the traveling salesman problem (TSP) reduces to the so-called symmetric circulant traveling salesman problem (SCTSP), that has applications in the design of reconfigurable networks, and in minimizing wallpaper waste. The complexity of the SCTSP is open, but conjectured to be NP-hard, and we compare different lower bounds on the optimal value that may be computed in polynomial time. We derive a new linear programming (LP) relaxation of the SCTSP from the semidefinite programming (SDP) relaxation in [E. de Klerk, D.V. Pasechnik, R. Sotirov, On semidefinite programming relaxation of the traveling salesman problem, SIAM Journal of Optimization 19 (4) (2008) 1559-1573]. Further, we discuss theoretical and empirical comparisons between this new bound and three well-known bounds from the literature, namely the Held-Karp bound [M. Held, R.M. Karp, The traveling salesman problem and minimum spanning trees, Operations Research 18 (1970) 1138-1162], the 1-tree bound, and the closed-form bound for SCTSP proposed in [J.A.A. van der Veen, Solvable cases of TSP with various objective functions, Ph.D. Thesis, Groningen University, The Netherlands, 1992].     

Stoichiometry effects on surface properties of GaAs{100} grown in situ by MBE

In this paper, we report results dealing with the effects of stoichiometry on surface properties of GaAs(001) layers grown by MBE. Three aspects of surface properties were investigated: crystallography, electronic properties and chemical reactivity. Surface crystallography was studied mainly by LEED. The reconstruction of the surface was found to be drastically dependent on the composition of the uppermost atomic layer, i.e. the surface stoichiometry. According to the arsenic surface coverage, many structures from the c(8 × 2) Ga rich to the (1 × 1) arsenic saturated surface have been observed. The influence of stoichiometry on surface electronic properties has been studied by electron loss spectroscopy (ELS) and contact potential difference (CPD) measurements. In the electron loss spectra, two peaks, at about 10.3 and 20.2 eV are very sensitive to the surface composition: they gradually disappear when the arsenic coverage increases, and consequently are associated with surface states on gallium atoms. On the other hand, the CPD measurements have shown that the variation of the work function with the arsenic surface coverage is not monotonic: in particular, an abrupt change of work function of about 300 meV occurs between the (1 × 6) and c(2 × 8) structures which are very similar as far as the arsenic surface coverage (about 0,5 and 0,6 respectively) is concerned. Therefore, it seems that the work function is strongly dependent on the atomic reconstruction occurring at the surface, and not only on its stoichiometry. The connection between stoichiometry and chemical reactivity of the surface is illustrated by the study of H₂S adsorption: a large difference (factor of 10³) in sticking coefficient has been found between surfaces with different arsenic coverages.     

Zone–edge optical transitions in GaAs/AlAs lateral superlattices grown on vicinal surfaces by molecular beam epitaxy

We report on new features in the photoluminescence excitation (PLE) spectra and PLE linear polarization spectra of GaAs/AlAs lateral superlattices grown by molecular beam epitaxy (MBE). These lines appear systematically as the tilt angle of the lateral superlattice is varied. They are identified as zone–edge excitonic transitions by comparison between experimental data and detailed numerical calculations of optical transitions including valence-band mixing and tilt effects.     

Evaluation of the activation energy for the oligomerization of ethylene using a microreactor in a DTA

The theoretical considerations and the experimental data to calculate the activation energy of the ethylene oligomerization using a microreactor in a differential thermal analyzer are presented.     

An advanced implementation of the Dantzig—Wolfe decomposition algorithm for linear programming

Since the original work of Dantzig and Wolfe in 1960, the idea of decomposition has persisted as an attractive approach to large-scale linear programming. However, empirical experience reported in the literature over the years has not been encouraging enough to stimulate practical application. Recent experiments indicate that much improvement is possible through advanced implementations and careful selection of computational strategies. This paper describes such an effort based on state-of-the-art, modular linear programming software (IBM's MPSX/370).