Diffusion in Nanostructures

Frank Cichos,
TU Chemnitz
Nanostructured materials become complex compared to macroscopic bulk phases due to interfaces or surfaces, which are influencing the physics of these structures to a large extent. There are numerous examples of such a finite size effect. The talk will focus on two examples of drastically different materials where the finite size of the system determines the physics. In both cases the detailed physical processes are hidden in the ensemble average of common experimental techniques. To unravel these details one has to observe individual members of the ensemble, which we achieve for both systems by applying single molecule microscopy techniques.
The first example concerns a liquid nanostructure - an ultrathin liquid film deposited on a solid wall. Such films are of large interest as lubricants. Ultrathin liquid films decrease i.e. the friction between the magnetic head and the recording media in computer hard disks. The physics in such films is largely determined by near order effects, which cause a stacking of the molecules in the liquid in layers parallel to the surface. The motion of molecules in this layered structure is then anisotropic. Experiments with single fluorescent molecules as probes in this liquid nanostructure reveal an extremely slow exchange between these layers - an experimental observation which has also impacts on surface chemistry.
The second example focuses on silicon nanocrystals, which have typically a few nanometers in diameter. The small diameter of the nanocrystal squeezes the electron-hole pair in an excited electronic state and turns silicon into an efficient light emitter, which is extremely important for integrated optoelectronics based on silicon. The small size of the nanocrystal allows further an escape of electron or hole from the nanocrystal. The motion of this charge outside the nanocrystal determines many effects observed for the ensemble and connects the nanocrystals with the liquid film, since in both cases diffusion processes are important even though one of the materials is liquid and the other solid.