"Cluster" are the smallest nanoparticles with diameters in the range of 1-3 nanometers. In this size range, the properties may vary with each additional atom. At this extreme end of the scale, the door to a new type of material science opens. However, it is diffcult to obtain access to this nano world is difficult and requires the development of new experimental techniques.
An example: the Al13H cluster
A cluster consisting of 13 aluminum atoms and a single hydrogen atom. This nanoparticle is double magic: it has a closed electronic shell (40 valence electrons) and a closed geometric shell (13 atoms, the H atom is too small to change the geometry). Picture by Gotthard Seifert, TU Dresden.
Technical progress is due to the improvement of manufacturing accuracy. It all started more than 100, 000 years ago with the hand ax manufactured with an accuracy of ± 5 mm. Today, photolithography, the technique used for the mass production of computer ships, reaches an accuracy of 20 nm.
Highend of nanoscience: Design of clusters with atomic precision
Towards the end of the Middle Ages manufacturing accuracy based on hand-held tools and the naked eye reached the limit of one-tenth of a millimeter. With light microscopes and computer-controlled machine tools, the accuracy rose within 150 years to a tenth of a micron (= 100 nm). Beyond this limit the nano world begins. So far, this world is only accessible with expensive high-resolution electron microscopes, lasers and electron beams. Nanoparticles and their smaller brethren, the clusters, are the building blocks of this world.