Experimental and theoretical studies of instabilities of metal nanoparticles: a new kind of quasimelting

In this work we report experimental and theoretical studies of structural instabilities of gold nanoparticles supported on a carbon substrate using high-resolution transmission microscopy and molecular dynamics simulations. It is shown that particles undergo structural fluctuations in which a particle exhibits a change in orientation whilst maintaining the overall structure. These fluctuations are of a different kind than the ones reported in previous work, in which transitions between different structures or the appearance, movement or vanishing of twin boundaries are observed. Computer calculations were performed using a model that included both the particle and the substrate. It is found that during the fluctuations there is a collective displacement of the atoms at the interface between the gold and the carbon substrate that results in a larger contact area. Calculations of particle stability that include the interaction with the carbon substrate show that different orientations of the particle have similar energies. As a result, during the fluctuations the particle becomes trapped between different orientations of the same structure. This kind of phenomenon can be considered as a new kind of quasimelting. Received: 26 September 2000 / Accepted: 27 January 2001 / Published online: 23 May 2001     

Quasiperiodic Bloch-like states in a surface-wave experiment

Bloch-like surface waves associated with a quasiperiodic structure are observed in a classic wave propagation experiment which consists of pulse propagation with a shallow fluid covering a quasiperiodically drilled bottom. We show that a transversal pulse propagates as a plane wave with quasiperiodic modulation, displaying the characteristic undulatory propagation in this quasiperiodic system and reinforcing the idea that analogous concepts to Bloch functions can be applied to quasicrystals under certain circumstances.     

Gold Nanoparticles Obtained by Bio-precipitation from Gold(III) Solutions

The use of metal nanoparticles has shown to be very important in recent industrial applications. Currently gold nanoparticles are being produced by physical methods such as evaporation. Biological processes may be an alternative to physical methods for the production of gold nanoparticles. Alfalfa biomass has shown to be effective at passively binding and reducing gold from solutions containing gold(III) ions and resulting in the formation of gold(0) nanoparticles. High resolution microscopy has shown that five different types of gold particles are present after reaction with gold(III) ions with alfalfa biomass. These particles include: fcc tetrahedral, hexagonal platelet, icosahedral multiple twinned, decahedral multiple twinned, and irregular shaped particles. Further analysis on the frequency of distribution has shown that icosahedral and irregular particles are more frequently formed. In addition, the larger particles observed may be formed through the coalescence of smaller particles. Through modification of the chemical parameters, more uniform particle size distribution may be obtained by the alfalfa bio-reduction of gold(III) from solution.