Atomistic computer simulations on the generation of bimetallic nanoparticles.

Computer simulations on the generation of bimetallic nanoparticles are presented in this work. Two different generation mechanisms are simulated: (a) cluster-cluster collision by means of atom dynamics simulations; and (b) nanoparticle growth from a previous seed through grand canonical Monte Carlo (gcMC) calculations. When two metal nanoparticles collide, different structures are found: core/shell, alloyed and three-shell (A-B-A). On the other hand, the growth mechanism at different chemical potentials by means of gcMC reveals the same results as atom dynamics collisions do.

Externally applied electric fields on immiscible lipid monolayers: repulsion between condensed domains precludes domain migration.

Lipid and protein molecules anisotropically oriented at a hydrocarbon-aqueous interface configure a dynamic array of self-organized molecular dipoles. Electrostatic fields applied to lipid monolayers have been shown to induce in-plane migration of domains or phase separation in a homogeneous system. In this work, we have investigated the effect of externally applied electrostatic fields on different lipid monolayers exhibiting surface immiscibility. In the monolayers studied, lipids in the condensed state segregate in discontinuous round-shaped domains, with the lipid in the liquid-expanded state forming the continuous phase. The use of fluorescent probes with selective phase partitioning allows analyzing by epifluorescence microscopy the migrations of the domains under the influence of inhomogeneous electric fields applied to the surface. Our observations indicate that a positive potential applied to an electrode placed over the monolayer promotes a repulsion of the domains until a steady state is reached, indicating the presence of a force opposed to the externally applied electric force. The experimental results were modeled by considering that the opposing force is generated by the dipole-dipole repulsion between the domains.

First principles calculations of mechanical properties of 4,4'-bipyridine attached to Au nanowires.

A first attempt is made to calculate the forces involved in the breaking of nanowires consisting of a molecule attached to nanosized metallic pieces. As a model system, we consider different Au nanowires connected by a 4,4(')-bipyridine or pyrazine molecule, for which density functional calculations were performed at different elongations. The geometry of the system was optimized for different forces applied. In all cases the calculated maximum forces were close to 1 nN, which is of the order of the experimental values, and smaller than the corresponding to the rupture of the Au-Au chain (1.5-1.6 nN). When 4,4(')-bipyridine is attached to Au monoatomic nanowire, the maximum force required to break the Au-Au bond may be lowered to values close to that obtain to break the Au-N bond, but when 4,4(')-bipyridine is attached to small Au clusters, the breaking of the nanowire takes place at the Au-N bond only.