Introduction

An important reason why NT seems so interesting for technical developments is that properties of materials in the nano-range depend on their size and shape (particle, fiber, platelets) - a behavior that is not known from the macroscopic world. For material developers, this offers very interesting possibilities, since in addition to the molecular structure and the composition, the size / shape of the subunits is also available as an additional parameter for the control of desired properties.

Do special laws govern the nano world? No, all physical and chemical laws are in place. They are only applied to units that are located between atomic / molecular and macroscopic systems. Nanoscale systems are therefore often referred to as mesoscopic.

The number of atoms of a material, which are at its surface, increases very strongly with decreasing particle sizes. Thus, in the extreme case of a 2 nm platinum particle, more than 50 percent of all atoms can be found at the surface / interface and the specific surface area of the particle increases sharply. Also in bulk materials with nanoscale grains such as nanoscale ceramics or metals especially the mechanical properties are determined significantly by the grain sizes.

Since surface atoms have fewer neighbors than those inside the material, they have higher energy and are more reactive. This results in lowering of the melting point, thermodynamically stable intermediate phases or especially by an increased catalytic activity of materials.

The electronic and optical properties of metals and semiconductors also depend very much on the system size (picture left). For example, discrete molecular orbitals with defined LUMO (lowest unoccupied molecular orbital) and HOMO (highest occupied molecular orbital) states dominate in molecules. With increasing size e.g. in clusters the energy states change e.g. the band gap is lowering. A further increase in the system size leads to the known energy states of semiconductors or metals. In the nanoscale intermediate region between atomic and macroscopic states, optical properties can therefore be changed in a controlled manner. This results in the change of absorption (color) or fluorescence of semiconductor particles (quantum dots). The confinement, ie the limitation of the charge carrier movement in nanostructures, as in thin films, wires or in all three spatial directions of nanoparticles, is the basis for many applications of nanoscale systems in optics (lasers, biosensors, etc.).