Inverse Ostwald ripening
Ensembles of equal-sized nanoparticles are desirable for many applications because, for instance, the wavelength of the optical luminescence and the potential of the Coulomb blockade are strongly size-dependent. Thus, a broad size distribution hinders the development of novel devices like integrated CMOS compatible lasers based on tiny crystals of silicon or the fabrication of non-volatile nanodot-memories.
Technologically effective methods of nanoparticle fabrication like ion beam synthesis yield usually broad size distributions. The reasons are fundamental physical principles. Nanoparticles are generated by spatiotemporally and statistically distributed nucleation. Subsequent heat treatment leads to Ostwald ripening resulting in the typical, broad LSW distribution.
Inverse Ostwald ripening under ion irradiation:The thermally activated conventional Ostwald ripening is based on the Gibbs-Thomson relation, i.e. the solubility of atoms of a nanoparticle in the surrounding matrix depends on its size (e.g. for Gold nanoclusters embedded in quartz the Au solubility in SiO2). The ripening is driven by differences of the nanoclusters' solubilities: Dissolved atoms diffuse von small to large nanoparticles (see arrows in (a)), which leads to a growth of the mean particle radius (b). Nanoparticles can change their ripening behaviour under continuous ion irradiation (c). In an ion irradiated surface layer collision cascades develop with roughly isotropic directions of low-energy recoils (green traces in (d)). At the FZD it has been discovered that ion irradiation can reverse the size-induced diffusion current, i.e. small particles grow on the expense of large ones (see arrows in (d)). For this aim, a mathematical model was developed and solved analytically, and kinetic Monte-Carlo simulations were performed. Under moderate ion irradiation the particle radius remains constant, and the size distribution becomes rather narrow (inverse Ostwald ripening (e)).