Organometallic clusters as precursors for metallic nanoparticles: effect of cluster size, ligand set, and decomposition method

The pyrolysis of organometallic clusters containing osmium and/or ruthenium on a silica support leads to metallic nanoparticles in the 1 to 10 nm size range. The particle size obtained is correlated to the ligand set, and there are also indications that the shape of the cluster may have some effect on the shape of the nanoparticles obtained. In comparison, the size of nanoparticles obtained through thermolysis in a coordinating solvent are fairly independent of the nature of the organometallic precursor and tends to lead to aggregation of smaller, spherical nanoparticles into larger particles.     

Surface tension-driven self-folding polyhedra

We discuss finite element simulations and experiments involving the surface tension-driven self-folding of patterned polyhedra. Two-dimensional (2D) photolithographically patterned templates folded spontaneously when solder hinges between adjacent faces were liquefied. Minimization of interfacial free energy of the molten solder with the surrounding fluidic medium caused the solder to ball up, resulting in a torque that rotated adjacent faces and drove folding. The simulations indicate that the folding process can be precisely controlled, has fault tolerance, and can be used to fold polyhedra composed of a variety of materials, ranging in size from the millimeter scale down to the nanometer scale. Experimentally, we have folded metallic, arbitrarily patterned polyhedra ranging in size from 2 mm to 15 microm.     

Au(i)-benzimidazole/imidazole complexes. Liquid crystals and nanomaterials

Three series of Au(I)-imidazole complexes with stoichiometries of [Au(Cn-bim)Cl], [Au(Cn-im)Cl], and [Au(Cn-im)2][NO3] x 2H2O (Cn-bim = N-CnH2n+1 -substituted benzimidazole and Cn-im = N-CnH2n+1-substituted imidazole) together with the compound of [Au(C18-bim)2][NO3] are synthesiszed. Typical structures of each series are determined by single crystal X-ray diffraction. The last series of compounds, are liquid crystals, and exhibit a wider mesophase range than their Ag(I) analogues. These Au(I) complexes form Au nanostructures both through chemical reduction or thermolysis. For the first time, N-long chain imidazole is utilized to stabilize colloidal Au in solution. Also for the first time, unique examples of simple thermolysis to produce large Au plates of nanothickness are demonstrated. Formation of a plate-like morphology through fusion of sphere-like nanoparticles at an early stage is evidenced by TEM images.     

Pricing vulnerable European options with stochastic default barriers

CF Lo and KC Ku Institute of Theoretical Physics and Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, China Email: cho-hoi_hui{at}hkma.gov.hk Received on 31 July 2006. Accepted on 15 March 2007. This paper develops a valuation model of European options incorporatinga stochastic default barrier, which extends a constant defaultbarrier proposed in the Hull–White model. The defaultbarrier is considered as an option writer's liability. Closed-formsolutions of vulnerable European option values based on themodel are derived to study the impact of the stochastic defaultbarriers on option values. The numerical results show that negativecorrelation between the firm values and the stochastic defaultbarriers of option writers gives material reductions in optionvalues where the options are written by firms with leverageratios corresponding to BBB or BB ratings.     

Downstream processing of virus-like particles with aqueous two-phase systems: Applications and challenges

The advancement of recombinant virus-like particle-based vaccines has attracted global attention owing to substantially safety and high efficacy in provoking a protective immunity against various chronic and infectious diseases in humans and animals. A robust, low-cost, and scalability separation and purification technology is of utmost importance in the downstream processing of recombinant virus-like particles to produce affordable and safe vaccines. Being a relatively simple, environmentally friendly, and efficient biomolecules recovery approach, aqueous two-phase systems have received great attention from researchers worldwide. This review aims to highlight the challenges and outlook in addition to the current applications of aqueous two-phase systems in downstream processing of virus-like particles. The efforts will confidently reinforce scholars’ knowledge and fill in the valuable research gap in the aspect of concerning recombinant virus-like particle-based vaccines development, particularly related to the virus-like particles downstream production processes.     

A restarting approach for the symmetric rank one update for unconstrained optimization

Two basic disadvantages of the symmetric rank one (SR1) update are that the SR1 update may not preserve positive definiteness when starting with a positive definite approximation and the SR1 update can be undefined. A simple remedy to these problems is to restart the update with the initial approximation, mostly the identity matrix, whenever these difficulties arise. However, numerical experience shows that restart with the identity matrix is not a good choice. Instead of using the identity matrix we used a positive multiple of the identity matrix. The used positive scaling factor is the optimal solution of the measure defined by the problem—maximize the determinant of the update subject to a bound of one on the largest eigenvalue. This measure is motivated by considering the volume of the symmetric difference of the two ellipsoids, which arise from the current and updated quadratic models in quasi-Newton methods. A replacement in the form of a positive multiple of the identity matrix is provided for the SR1 update when it is not positive definite or undefined. Our experiments indicate that with such simple initial scaling the possibility of an undefined update or the loss of positive definiteness for the SR1 method is avoided on all iterations.     

Synthesis and catalysis of location-specific cobalt nanoparticles supported by multiwall carbon nanotubes for Fischer-Tropsch synthesis

Cobalt nanoparticles located on the concave internal surface of multiwalled carbon nanotubes (Co-in-MW-CNTs) and the convex external surface of MW-CNTs (Co-on-MW-CNTs) were synthesized. Their catalytic performances in Fischer-Tropsch synthesis (FTS) were investigated. A correlation between the location, pretreatment, and surface chemistry of the cobalt nanoparticles and the catalytic selectivity in FTS was built. It is found that the selectivity in production of C(5+) molecules through FTS on cobalt catalysts supported by MW-CNTs depends on activation temperatures and surface chemistry of the cobalt nanoparticles. A pretreatment at 300 °C in H(2) flow results in a different surface chemistry for Co-in-MW-CNTs than for Co-on-MW-CNTs, which leads to a difference in selectvity to the production of C(5+) molecules. Pretreatment at a relatively high temperature, 400 °C, in H(2) flow produces completely reduced Co nanoparticles in Co-in-MW-CNTs and Co-on-MW-CNTs. There is no signifcant difference in catalytic selectivity between the two catalysts upon pretreatment at 400 °C. The absence of a significant difference in catalytic selectivity of metallic Co-on-MW-CNTs and metallic Co-in-MW-CNTs suggests that the electronic effect of the MW-CNT support does not significantly affect the C(5+) selectivity of cobalt catalysts in FTS.     

Dynamics of wicking in silicon nanopillars fabricated with interference lithography and metal-assisted chemical etching

The capillary rise of liquid on a surface, or "wicking", has potential applications in biological and industrial processes such as drug delivery, oil recovery, and integrated circuit chip cooling. This paper presents a theoretical study on the dynamics of wicking on silicon nanopillars based on a balance between the driving capillary forces and viscous dissipation forces. Our model predicts that the invasion of the liquid front follows a diffusion process and strongly depends on the structural geometry. The model is validated against experimental observations of wicking in silicon nanopillars with different heights synthesized by interference lithography and metal-assisted chemical etching techniques. Excellent agreement between theoretical and experimental results, from both our samples and data published in the literature, was achieved.