The Berlin researchers examined clusters with 7, 19 and 20 atoms. In the uncharged clusters with 19 and 20 atoms they observed the same structures that are familiar from their negative counterparts: 20 gold atoms stacked into a tetrahedron, a pyramid with a triangular base. With one atom fewer, the pyramid loses its tip. "Seven uncharged gold atoms form a triangle with an additional vertex," says Andr Fielicke. In a positively charged cluster, on the other hand, seven atoms form a hexagon with one atom in its centre. In uncharged form, there are three gold atoms positioned on each edge of the triangle. On one edge, two atoms are bridged by another, which creates the extra vertex. "Uncharged gold atoms probably prefer this structure as it is easier for the electrons to avoid each other," says Fielicke.
In order to scan the structure of the uncharged nano particles, the scientists in Berlin had to solve several problems at once. "The clusters are rather unstable; you can't just buy them as a powder," explains Philipp Gruene, who carried out a large portion of the experiment. Therefore, the scientists had to create the gold clusters in the same apparatus in which they determine the particle structure. To this end, they use a laser to vaporize small quantities of the precious metal from a gold bar that creates gold clusters in different shapes and sizes. In this confusion of particles, it is not possible to discern any particular structures, which poses the next problem.
Normally, chemists separate a mixture of particles like this in a mass spectrometer. This device first ionizes the particles, which means it provides them with an electrical charge. Then it separates them according to their mass in an electrical field, as the field accelerates light
|Contact: Dr. Andr Fielicke|