Metal Clusters and Nanoparticles One Contribution of 13 to a Theme Issue 'metal Clusters and Nanoparticles'

There is still no sharp discrimination between the terms ‘cluster’ and ‘nanoparticle’ in the literature. Clusters are mostly considered as species, exactly defined in chemical composition and structure, whereas the expression ‘nanoparticle’ usually means particles of less precise characterization, often linked with a certain size distribution and so to some extent substitutes the historical expression ‘colloid’. Examples for typical and long-known clusters are Co4(CO)12 or Rh6(CO)16, exhibiting bridging and terminal CO groups as well as direct metal–metal bonds. Gold nanoparticles (gold colloids), for instance, were already used in ancient times to colour glass (ruby glass) without any scientific understanding. The first who scientifically investigated gold colloid formation was Michael Faraday in the mid-nineteenth century. He reduced a solution of HAuCl4 with elemental phosphorus resulting in the formation of ruby-red gold sols. In the meantime, we learned that the reason for this colour is due to the interaction of visible light with surface electrons of the gold nanoparticles (Mie theory). As we know today, the colour depends on size, shape and surrounding medium. Faraday’s original gold sols contained sub-30 nm gold particles with a rather broad size distribution. Nowadays, the number of precise metal clusters and of less precise metal nanoparticles is immense. From physical and chemical properties, large metal clusters and nanoparticles represent a bridge between the molecular and solid state. As such, they often exhibit properties belonging to both of these classical disciplines and therefore are of immense scientific and practical interest. The aspect that the property of a metallic particle can be designed by the number of atoms is a driving force in modern physics and chemistry. For the size dependence of physical and chemical properties, the expression ‘size quantization’ has been introduced. Clusters or nanoparticles, consisting of metal atoms only, change their physical properties spontaneously from a distinct size on. Metal particles below about 2 nm show typical quantum size behaviour, even at room temperature owing to the existence of discrete electronic energy levels and the loss of overlapping electronic bands, the characteristic of a bulk metal. Therefore, they are also called ‘artificial big atoms’ and are to be considered as links between bulk metals and small molecular clusters. Besides the exciting electronic changes on the way from bulk to molecule or vice versa, other properties change too. For instance, small metal nanoparticles show a significantly lower melting point than the corresponding bulk metal. Magnetic and optical properties also show a typical dependence on the particle size, and the band gap of semiconductor clusters also varies significantly with size.